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Neuropsychopharmacology: The Fifth Generation of Progress |
Neuroendocrine Interactions
Bruce S. McEwen
One
of the key problems in biology is understanding the connection between genetic
(nature) and environmental (nurture) influences.
During the past hundred years, there have been many debates as to which
is more important in explaining the origins of the traits of individuals, nature
or nurture. Neither side has been able to win such arguments,
because the dispute itself reflects a fundamental misunderstanding, namely,
a failure to acknowledge that genes are continually being regulated by the external
environment from conception throughout life. Modern cell and molecular biology is elucidating
this fact at an accelerating pace. One of the purposes of this chapter is to provide a sense of what
is meant by hormonal regulation of gene expression in the brain and how it relates
to behavior and pathophysiology.
The
discussion will focus on circulating hormones, because endocrine secretions
represent one of the principal links between the environment and the genes (see
Fig. 1). Hormone secretion is controlled
by the brain acting through the hypothalamus, and it is coordinated by, or
in some cases, triggered by the external environment. Furthermore, hormones act on many tissues and
cells, including the brain, to promote appropriate responses to environmental
change. Illustrative examples include
the coordination by gonadal hormones of reproductive processes with reproductive
behavior during reproductive cycles that are synchronized to diurnal or seasonal
rhythms, and the synchronization by adrenal steroids of energy metabolism with
food-seeking behavior and cognitive alertness during the diurnal sleep-wake
cycle. Another important example of
hormone involvement is the adaptive responses of the organism to stressful
environmental challenges, which are mediated principally by epinephine and adrenal
glucocorticoids.
Besides these cyclic or otherwise reversible
processes, hormones also mediate important permanent developmental events,
such as sexual differentiation and the growth-coordinating effects of thyroid
hormone. Furthermore, stressful experiences
early in life can have long-lasting effects on emotionality.
Not all of the actions of hormones are beneficial to the organism, and pathophysiological
changes are also an important component of environmental influences on the adult
and developing organism. In some cases, irreversible changes ensue, such as neural damage
that results from severe chronic stress. In
other cases, stress exacerbates existing pathophysiology in the form of atherosclerosis
or worsening of diabetes, gastrointestinal pathology, or asthma.
The genetic constitution plays an important role by providing certain
individuals with systems that are vulnerable to
the impact of external forces in promoting a disease process.
Studies of various diseases in identical twins reveal that it is rare
for concordances to be higher than 50% 1, indicating that there is
considerable latitude for exerting external control and even prevention when
we can only know the underlying mechanisms.
This chapter therefore discusses a full range
of topics concerning hormone action on the nervous system, beginning with cellular
mechanisms and ending with a discussion of how hormone effects may culminate
in individual differences in brain function and malfunction (see Electrophysiology,
Electrophysiology
of Serotonin Receptor Subtypes and Signal Transduction Pathways, The
Neurobiology of Neurotensin, Intracellular
Messenger Pathways as Mediators of Neural Plasticity, Clinical
Study Design-Critical Issues, and Short-and-Long-Term
Psychopharmacological, this volume).
CELLULAR MECHANISMS OF ACTION OF NEUROACTIVE
HORMONES
The brain is a target for the actions of circulating
hormones and is the principal controller of hormone secretion. Hormones act on brain cells as they do on other
cells, through receptors that mediate both cell-surface phenomena and intracellular
events, including gene expression. Steroid hormones and thyroid hormones act via intracellular receptors
that bind to deoxyribonucleic acid (DNA) and regulate gene expression 2.
In some cases, steroid hormones also act via cell-membrane receptor sites
(see Fig. 2) 3.
The
brain also produces some steroids from cholesterol, notably pregnenolone, and
it can convert pregnenolone to dehydroepiandrosterone.
Such steroids have been called neurosteroids (see Fig. 3)
4,5. It is important to distinguish between neurosteroids, meaning steroids produced
in the brain, and the broader class of neuroactive
steroids, which includes any steroid with actions upon neural tissue. It should also be emphasized that the term
neuroactive steroids is not restricted to any one cellular mechanism of action.
Intracellular steroid hormone receptors were
the first proteins recognized to be capable of regulating gene expression by
binding to specific nucleotide sequences in the promotor region of various genes.
Such proteins are called transacting factors. Since their discovery, many other DNA-binding
proteins have been identified. Some
of these are regulated in their ability to bind to the DNA site (called a response
element) by phosphorylation in a number of different second messenger systems;
others, such as the immediate early genes, are induced by second-messenger-system
activation, and they then promote or inhibit the transcription of other genes
by binding to response elements located in the promotor regions of some genes.
A schematic diagram of some of these interactions is presented in Figure
4.
There are two types of response elements: simple
and composite 2. Simple response
elements bind only one transacting factor, whereas composite response elements
bind one of several types: for example, a steroid receptor and a transacting
factor, such as CREB, which is phosphorylated by second messenger systems.
An alternative mode of interactions between transacting factors is that
they sometimes bind to each other, and, in so doing, inactivate each others
ability to bind to the DNA response element.
Membrane actions of steroid hormones are not
as well characterized as those mediated by the intracellular genomic receptors.
Representative nongenomic effects of steroids 6 are listed
below:
1. GABAa receptor. A-ring reduced
metabolites of progesterone and deoxycorticosterone facilitate opening of
chloride channel; this is a property of a variety of combinations of GABAa receptor
subunits expressed in cells that normally do not express such receptors 7.
2. Corticosterone receptor. A
G-protein coupled membrane site in the newt brain is linked to rapid inhibition
of sexual behavior 8,9.
3. Progesterone receptor. A membrane
progesterone receptor mediates mobilization of calcium stores in spermatozoa,
leading to capacitation of the sperm 10.
4. Aldosterone
receptor. Monocytes respond to aldosterone
in altering ionic balance via a receptor distinct from the intracellular Type
I adrenal steroid receptor 11.
5. Estrogen
receptor. Estradiol rapidly facilitates
non-NMDA excitatory amino acid effects on hippocampal CA1 neurons 12. Other
examples of estrogen action may be found in a recent review 13.
Binding
studies have been successful in only one case, namely, a corticosteroid receptor
present in the forebrain of a newt that is coupled to a G-protein and appears
to mediate a rapid inhibition of sexual behavior.
The other well-characterized steroid site is on the g-aminobutyric
acid Type a (GABAa) receptor, specifically the site to which A-ring-reduced
steroids (see Fig. 3) bind and facilitate
opening of the chloride channel. This site is recognized for its functional
activity rather than its binding, and its existence on the GABA receptor has
been demonstrated by showing that GABAa receptors have such steroid response
sites when they are expressed from DNA in cells that normally do not express
the GABAa receptor. Inferences about
other membrane receptors for steroids come from the actions of various steroids
on membrane-based events: for example, the facilitation of non-N-methyl-D-aspartate
(non-NMDA) receptor activity by estradiol in hippocampal neurons; the promotion
of calcium mobilization by progesterone in sper-matozoa.
From these
and other examples, it appears that membrane actions of steroids may be quite
common. Nevertheless, there are also actions of steroids
on neural excitability that are rapid and yet which may be mediated genomically.
For example, androgens act on muscle cells to increase acetylcholine-activated
ion channels 14, whereas estradiol and progesterone act on myometrial
cells to increase potassium and calcium currents 15,16; these effects
require hours or days of exposure and are blocked by antagonists of intracellular
steroid receptors. Moreover, adrenal
steroids have rapid actions (within a number of minutes) to modulate excitability
of hippocampal pyramidal neurons and dentate gyrus granule neurons 17,18. These effects are antagonized by steroid antagonists
of the intracellular type I and type II adrenal steroid receptors, implying
that they involve a genomic mechanism.
CIRCULATING HORMONES
AS MEDIATORS OF CHANGE IN ADULT BRAIN
One of the most important roles of circulating
hormones is to coordinate cellular and organ responses during cyclic processes
such as reproduction and daily sleep-wake activity by synchronizing neural activity
and behavior with processes throughout the body.
Circulating hormones complement the actions of neural connections and
release of neurotransmitters, although neural actions are rapid, hormone actions
have longer lasting influences and can affect tissues independently of whether
they are innervated, provided that they have hormone receptors.
Gonadal hormones act both in the brain and peripherally to coordinate
the timing of mating with the optimal chance for pregnancy, and adrenal hormones
play a key role both peripherally and centrally in coordinating energy metabolism
with food-seeking behavior and cognitive alertness.
Reproductive Behavior
The capacity for reproduction is usually tied to cycles: seasonal in some species,
monthly in humans and primates, and on the order of a few days in rats and
mice. The ovarian cycle of the female
is the key for successful reproduction. For
the rat, synchronizing the time of ovulation with the time of behavioral sexual receptivity and preparation of
the reproductive tract for pregnancy is the function of circulating estradiol
and progesterone. Although many aspects
of reproduction have been intensively investigated by endocrinologists, reproductive
behavior has been the province of behavioral scientists, that is, until the
past two decades, when behavioral neuroscience has emerged and moved ever closer
to the cellular, molecular, anatomical, and neurophysiological aspects of neuroscience
and to cellular and molecular endocrinology. Studies of the rat have been particularly useful
in elucidating brain mechanisms underlying reproductive behavior 19.
The
4- or 5-day estrous cycle of the female rat
is tightly coupled to the diurnal clock, and increasing levels of estradiol
during the initial phase of the cycle serve to amplify a diurnal peak of luteinizing
hormone (LH) secretion so that, on the afternoon of proestrus, this peak becomes
the LH surge, which is the immediate stimulus for ovulation.
The LH surge also stimulates a surge of ovarian progesterone release;
the progesterone synergizes with the prior secretion of estradiol to trigger
neural mechanisms subserving the mating response of the female rat, namely,
lordosis behavior 19.
What makes the rat particularly advantageous
for the study of brain mechanisms underlying reproductive behavior is the importance
of a pair of hypothalamic nuclei for the hormonal control of lordosis.
Lordosis behavior is primed by estradiol acting in various neural sites,
principally the ventromedial nuclei (VMN) of the hypothalamus.
Estradiol priming acts via genomic receptors in the cell nuclei of VMN neurons
to induce a number of fundamental changes in these cells 20:
Cellular Processes in Ventromedial Nuclei Regulated by Estradiol and Progesterone in Relation
to Lordosis Behavior
1. Rapid
increase of RNA synthesis. Within
2 h after estradiol, VMN neurons increase their capacity to make proteins by
making more ribosomal RNA 21.
2.
Rapid
increase in new synapses. In the
ensuing 24 h to 48 h, these neurons produce new synaptic contacts with as yet
unknown afferent neurons; the production of new synapses and their postsynaptic
structures, including spines on dendrites, must require some of the new protein
synthetic capability of the VMN neurons. However, these new synaptic contacts disappear
rapidly during the 24-h period following proestrus and after the progesterone
surge, and thus they appear and disappear cyclically during the estrous cycle
22.
3. Induction
of oxytocin receptors. Concurrently
with item 2. above, estradiol also induces VMN neurons to make new oxytocin
receptors, and oxytocin appears to be an important neurotransmitter that helps
to trigger the lordosis behavior 23.
4. Induction
of progesterone receptors. Concurrently
with these changes, estradiol induced VMN neurons to make more progesterone
receptors, and these receptors play a key role in the acute triggering of lordosis
behavior by rapid induction of as-yet unidentified proteins 24.
5. Nongenomic
actions of progesterone. Progesterone
also plays another important role in the triggering of lordosis through nongenomic
mechanisms: in the VMN, it acts at the membrane level to activate oxytocin
receptors and make them functionally active in the right place within the ventromedial
hypothalamus 23; and in the midbrain, progesterone produces a nongenomic
facilitation of the mating response 25.
6. Mating
activates c-fos expression. The
mating stimulus by the male activates oxytocin-containing neurons in two other
hypothalamic regions, the paraventricular nucleus (PVN) and the bed nuclei of
the stria terminalis. This activation
can be seen by the induction of the protooncogene transcription factor, c-fos,
in the small (parvoocellular) oxytocin neurons of the PVN 26.
7. Shutdown
of lordosis after proestrus. Following
the proestrus occurrence of ovulation and mating, the lordosis system shuts
down, and the progesterone surge is believed to play a major role in this event.
Progesterone administration produces, sequentially, facilitation and
then inhibition of lordosis responding, and protein synthesis inhibitors were
shown to block both phases of action 27.
A possible correlate of the inhibition is clearance of synapses formed
as a result of estrogen secretion,
and recent evidence indicates that progesterone plays an active role in synaptic
down-regulation during the estrous cycle 28.
These changes illustrate the types of cellular
responses that may characterize the response of other neuronal systems to circulating
hormones of the adrenals, gonads, and thyroid gland.
Coordination of Food-seeking, Metabolism, Salt Intake,
and Cognition
Adrenal steroid secretion provides an important
coordinating mechanism for the sleep-wake transition and for behavior and brain
function associated with the waking state. The peak of endogenous adrenocortical secretion
precedes the daily waking period in rats and humans - in the early morning
for humans and in the late afternoon for nocturnally active rats. Endogenous adrenocortical activity is driven
by two largely independent entrainable clocks: one located in the suprachiasmatic
nuclei (SCN) is synchronized by the light-dark cycle; the other, whose location
is unknown, is entrained by the availability of food 29. When food is available for a restricted time
of each day, corticosterone levels anticipate the appearance of food and appear
to act as a wake-up signal. When food
becomes available continuously, the anticipatory corticosterone (CORT) peak
disappears, and the normal light-entrainable rhythm emerges without any evidence
of phase shifting 29.
Adrenal steroids are of two types, glucocorticoids,
such as corticosterone and cortisol, and mineralocorticoids, such as aldosterone,
because of actions to promote gluconeogenesis and sodium retention, respectively. A summary of the effects of adrenal steroids
on brain function 30,31 are listed below:
Behavioral and Neurologic Effects
1. Stimulates food intake, especially at the beginning
of the waking period.
2. Promotes obesity in genetically prone or brain-lesioned
animals.
3. Biphasically modulates hippocampal long-term
potentiation and primed-burst potentiation.
4. Increases salt hunger and biphasically regulates
blood pressure.
Structural Effects
1. Stabilizes dentate gyrus granule neurons -
prevents increased cell death and de novo neurogenesis.
2. Facilitates atrophy of dendrites of hippocampal CA3 pyramidal neurons.
3. Promotes pyramidal neuronal loss during aging
and as a result of chronic stress.
4. Promotes hippocampal CA1 and subiculum damage
after ischemia.
However, in spite of their names, both classes
of steroids play an important role in stimulating food intake as well as facilitating
salt intake and regulating blood pressure. Glucocorticoids also promote food intake and
metabolism leading to obesity in genetically obese-prone rats and mice, as
well as in rodents with hypothalamic
lesions that cause obesity. Adrenalectomy
markedly reduces the body weight gain and obesity in both situations (for review,
see ref. 31). Finally, adrenal steroids play a role in promoting neural activity
that optimizes waking cognitive function. Synaptic activity and long-term potentiation in the hippocampus,
a brain region involved in learning and memory, are greatest during the waking
period of the light-dark cycle in squirrel monkeys and in rats, and bilateral
adrenalectomy abolishes this dark-light difference in rats 30. Adrenal steroids exert a biphasic influence
on the magnitude of long-term potentiation (LTP) and primed-burst potentiation
(PBP), two neurophysiologic processes related to learning: low levels of adrenal
steroid potentiate the response, whereas high levels of adrenal steroids inhibit
LTP and PBP 32. Two types
of adrenal steroid receptors are involved in these effects. The adrenal steroid potentiation of the LTP-PBP
response is blocked by an inhibitor of intracellular type I adrenal steroid
receptors, whereas the adrenal steroid inhibition of the LTP-PBP response is
blocked by an inhibitor of intracellular type 11 adrenal steroid receptors 18.
The symptoms of jet lag may
be a consequence, at least in part, of the failure of adrenal steroid secretion
to rise at the time of day that would be appropriate to the time zone in which
the traveler finds him or herself; as a result, the normal stimulation of metabolism
and cognitive function that results from the diurnal cortisol rise early in
the morning does not take place until later in the day (see Electrophysiology,
Monoamine
Oxidase: Basic and Clinical Perspectives, and The
Neurobiology of Neurotensin, this volume, for related LTP issues).
Steroids Regulate Neuronal Morphology, Neurogenesis and Neuronal
Survival
Adrenal and gonadal steroids exert surprising
effects during adult life on the formation and destruction of synapses, on the survival of certain nerve cell types,
and on structual remodelling as well as damage to neurons produced by
chronic stress, transient ischemia or aging
(see the above listing). Most of these effects have been studied in
the hippocampal formation, which is important in declarative and spatial learning
and memory. Steroid hormones also participate in regulating the formation of
new nerve cells in a brain region important for cognitive function, namely,
the dentate gyrus region of the hippocampal formation. All of these forms of structural plasticity
involve an interaction between steroid hormones and neurotransmitters, particularly
excitatory amino acids and serotonin.
Dentate gyrus neurogenesis and neuronal turnover
The formation
of new nerve cells in the adult brain was first appreciated in the song bird
in conjunction with seasonal cycles of learning new song 33 and has
been recognized in the olfactory bulbs 34. Recently,
neurogenesis has been found in the lining of the cerebral ventricles
35,36 and in the dentate gyrus (DG) of the hippocampal
formation, where it may subserve important functions in learning and memory
37. The recent finding of neurogenesis in the
human dentate gyrus 38 indicates the relevance to human cognitive
function and neuropsychiatric disorders. Among the salient features of the dentate gyrus neurogenesis are
the following points (for review, see 37:
1. DG neurogenesis has been found in adult mouse,
rat, tree shrew, marmoset, macaque and human brains.
2. DG neurogenesis is regulated negatively by
excitatory amino acids and adrenal steroids and positively by serotonin. Estrogens increase neurogenesis in the female
DG.
3. Neuronal
survival is increased by hippocampus-specific learning and by an enriched environment.
4. Acute psychosocial stressors inhibit neurogenesis,
and chronic stress appears to decrease the volume of the dentate gyrus.
Dentate gyrus granule neurons not only can be
formed in adulthood, but they are also destroyed by a process of programmed
cell death. Type I adrenal steroid receptors,
which are important in regulating LTP, also play an important role in stabilizing
the neuronal population of the dentate gyrus, since adrenalectomy has been found
to cause death of granule neurons and aldosterone replacement of adrenalectomized
rats blocks this neuronal death (for review, see ref. 39). When adrenalectorny
stimulates granule neuron death it also accelerates the process of neurogenesis
in the dentate gyrus of the adult rats, and new granule neurons are born 39. Besides possible involvement in learning and
memory, the granule neuron turnover in the dentate gyrus of the adult mammal
may be part of a seasonal or even diurnal mechanism by which the number of neurons
is changed to adjust the functional capacity of the hippocampus for memory storage
40.
Dendritic remodelling and neuronal loss in Ammon’s horn
Yet another example of morphological plasticity
is the damaging effect of adrenal steroids on hippocampal pyramidal neurons.
Loss of pyramidal neurons in the hippocampus was first described during
aging in the rat. After it was shown that it could be attenuated
by adrenalectomy in midlife 3,41, glucocorticoid treatment for 12
weeks was shown to produce pyramidal neuron loss in the hippocampal formation
of young adult rats 42. Prolonged
social stress was later shown to produce hippocampal neuronal loss in vervet
monkeys, and it has been tempting to conclude that stress-induced secretion
of glucocorticoids is to blame (see refs. 41, and 30 for
review).
However, the probable mechanism for hippocampal
neuronal loss is a good deal more complicated. This was first indicated by the finding that
neuronal damage produced by the excitotoxin, kainic acid, was potentiated by
endogenous and exogenous glucocorticoids. Likewise,
ischemic damage to the hippocampus, which also involves excitatory amino acids,
is potentiated by glucocorticoids, and excitotoxic effects on hippocampal neurons
in cell culture are exacerbated by
glucocorticoids in the medium acting at type II glucocorticoid receptors 43.
There are also important regional differences
within the hippocampal formation, namely, that ischemic damage is worse in CA1
pyramidal neurons and subiculum, whereas age- and stress-related damage are
more pronounced in the CA3 pyramidal layer. One difference between these two regions is
that the CA3 neurons receive the mossy fiber input from the dentate gyrus as
well as a direct perforant pathway input from the entorhinal cortex, whereas
CA1 neurons do not receive the Schaeffer collateral input from the CA3 neurons.
The CA3 neurons are known to be very susceptible to seizure-induced damage.
Another process occuring in the CA3 neurons
involves a remodelling, or atrophy, of the apical dendrites by a process that
is reversible. The CA3 neurons respond
to several weeks of repeated restraint stress or to repeated injections of corticosterone
by showing atrophy of the apical dendrites of the long-shaft pyramidal neurons.
This atrophy does not occur in the basal dendrites and there is no indication
of neuronal death. The stress-induced
atrophy can be blocked by several types of treatments: (i) interference with
release and action of excitatory amino acids by phenytoin; (ii) enhancement
of serotonin reuptake by tianeptine, an atypical tricyclic antidepressant drug
44; and (iii) inhibition of adrenal steroid synthesis in response
to stress 18. Dendritic atrophy has been found in rats and
tree shrews after prolonged psychosocial stress 37.
The atrophy of dendrites of CA3 pyramidal neurons
may represent the first stage of the degenerative process, or, alternatively,
it may represent an adaptive mechanism that is intended to protect the CA3
neurons from overstimulation. Further
research is needed to distinguish between these possibilities, but it is clear
that the hippocampus is a vulnerable brain structure and that permanent damage
can result from severe and prolonged
stress, as well as from seizures and ischemia.
Synapse formation and neuroprotection by estrogens
The hippocampus
is also an important target of action of ovarian steroids and of of androgens. Estrogen and androgen receptors are found
in hippocampal neurons 13, and androgens exert a neuroprotective
towards depolarizing and damaging effects of excitatory amino acids in vitro 45 whereas estrogens
are also known to exert neuroprotective effects and also to induce the formation
of excitatory synapses on hippocampal pyramidal neurons in the CA1 region; these
synapses are formed and broken down in the course of the estrous cycle 13.
Thus ovarian steroids regulate synapse turnover, not only in the VMN in connection
with lordosis behavior, but also in the CA1 region of the hippocampus 28,46.
Estradiol promotes formation of new excitatory
spine synapses on dendrites of CA1 pyramidal neurons, and progesterone actively
stimulates the rapid disappearance of these synapses after ovulation and does
this via intracellular progesterone receptors that are blockable using Ru38486
28. Excitatory amino acids
acting via NMDA receptors play a critical role in synapse formation, since blockade
of NMDA receptors prevents synapse formation 13. At the same time, NMDA receptors are key mediators of neurotoxicity and neuronal damage
via their role in increasing the production of free radicals (see above).
New evidence indicates that estrogen exert neuroprotective effects on
neurons via traditional genomic receptors and also via a novel non genomic mechanism
that is so far uncharacterized as far as the receptors that are involved 13.
Change in function
and volume of the human hippocampus
Hippocampal involvement in declarative memory
is enhanced by estrogens in women who have estrogen deficiency and there are
indications that estrogens may increase neuronal funciton as indicated by glucose
uptake studies using PET 13. There are also indications that estrogens exert neuroprotective
effects towards Alzheimer’s disease. although there are many brain regions besides
the hippocampus, such as the basal forebrain cholinergic system and the serotonergic
system, that are also benefitted by ovarian steroids 13.
The human hippocampus undergoes atrophy relative
to other brain regions in Cushing’s syndrome, recurrent depressive illness,
post-traumatic stress disorder, schizophrenia and in aging prior to the
onset of Alzheimer’s disease; and there are also documented impairments of hippocampal-dependent
cognitive function that accompany these disorders 37,47. It is not yet clear which
of the cellular changes described above are involved in the shrinkage of the
whole hippocampus, whether there is permanent cell damage or reversible dendritic
atrophy or dentate gyrus neuron reduction or changes in glial cells number and
volume (for discussion,see 37). Indeed, determining the underlying mechanism and reversibility is
important for developing therapeutic strategies to either reverse or prevent,
in the case of irreversible damage, the changes in the hippocampus. There is also evidence that atrophy also occurs
in other brain areas in psychiatric illness, including the prefrontal cortex
and amygdala (for discussion,see 37).
Protective
and Damaging Effects of Stress Mediators
An important function of circulating hormones
is to mediate responses to external challenges that are frequently stressful
to the organism. Natural disasters,
man-made disasters such as transportation accidents, military combat, rape,
physical trauma, and stressful life events such as job loss, divorce, and death
of a loved-one are all occurrences that are not cyclic and produce a huge impact
on those who experience them 48. However, the body and brain have considerable
resilience.
One of the main roles of hormonal responses
to stressful situations is to protect the organism from further damage 49. The secretion of epinephrine and adrenocortical
hormones are the most general hormonal features of the stress response, whereas
epinephrine secretion is a rapid reaction and is involved in the fight-or-flight
reaction, the role of adrenal steroids is as a second line of defense, helping
to restore and repair systems and also preventing the primary reactions, such
as epinephrine secretion, from gaining the upper hand.
Both inflammation and primary immune responses are examples of rapid
reactions to insults to the body, and glucocorticoids contain and counterregulate
these actions 50.
Neurochemical systems in the brain follow a
similar pattern of primary response to stressors, and glucocorticoids are involved
in mediating effects of repeated stress on the brain, which often take the form
of a counterregulation of the primary effects of stressful stimulation (see
Fig. 5) 30.
The production and secretion of corticotropin releasing hormone is a
primary response mechanism to stressors, leading to behavioral activation and
to the secretion of adrenocorticotropic hormone (ACTH), as well as immunosuppressive
effects. Yet glucocorticoid secretion
counterregulates part of this system, particularly the hypothalamic production
of corticotropin-releasing hormone (CRH), and keeps this part of the CRH system
in check. Likewise, norepinephrine (NE)
is released from the locus coeruleus system in response to arousing and stressful
stimuli. Repeated stress induces a progressively
greater capacity to produce and release NE, because it induces increased production
of the rate-limiting enzyme, tyrosine hydroxylase. Glucocorticoids keep this system in check by
inhibiting release of catecholamines 51 and by reducing the postsynaptic
response to released NE via a suppression of the adenylate cyclase response
to NE (see Figure 5) 52. The adenylate cyclase effect is accomplished by at least two mechanisms:
(i) a suppression of the a-1 adrenergic
receptor-effector system that works cooperatively with beta adrenergic receptors
to regulate cyclic adenosine monophosphate (cAMP) production 53;
(ii) a suppression of calcium-calmodulin adenylate cyclase activity by a mechanism
that does not involve reducing messenger ribonucleic acid (mRNA) production
but does involve the actions of glucocorticoids 30.
Another example of adrenal steroid involvement
in the effects of repeated stress involves serotonin (5HT). The serotonergic system is turned on by stressful
events, and glucocorticoid feedback counterregulates the 5HT1A receptors in
the hippocampus 44, but it also up-regulates the 5HT2 receptor in
the cerebral cortex 30,54.
As noted above for CRH, 5HT, and NE, adrenal
steroid regulation and counterregulation of neurochemical systems occurs in
some brain areas and not in others, resulting in an altered balance of various
neurotransmitter systems. For example,
there are a number of groups of neurons making CRH outside of the hypothalamus that are not counterregulated
by glucocorticoids 55,56. Likewise, the counterregulation of cAMP production and adenylate
cyclase activity is more pronounced in the cerebral cortex than in the hippocampus
30. On the other hand, the
counterregulation of 5HT1A receptors by glucocorticoids is found only in the
hippocampus and not in the cerebral cortex, whereas the up-regulation of 5HT2
receptors is found in the cerebral cortex 30,54.
We have also noted that not all aspects of glucocorticoid
action involve counterregulation. Acute actions of corticosteroids facilitate the stress-induced
activity of the serotonergic system 57 and also potentiate the activity
of GABAa-benzodiazepine receptors 58. Both of these effects may be produced by nongenomic
actions of steroids (Fig. 2).
Whereas the mechanism for activating serotonin formation remains a mystery,
the activation of GABAabenzodiazepine receptors is known to involve the generation
of metabolites of desoxycorticosterone, a steroid released by the adrenal cortex
during stress, and the interaction of the metabolite, tetrohydrodesoxycorticosterone
(THDOC), with a specific site on the chloride channel of the GABAa-benzodiazepine
receptor complex to facilitate chloride flux 58.
Thus the interactions of glucocorticoids with
the neurochemical systems described above represent an interlocking and carefully
balanced system that modulates arousal and excitation and prevents any of these
systems from overresponding to stressful stimuli. The protective role of adrenal steroids in
relation to behavioral consequences of stress is illustrated by the finding
that gluoccorticoids, which are anxiolytic, suppress the development of learned
helplessness in rats in response to inescapable shock 59. Insofar as learned helplessness is a putative
animal model of depressive illness, these results suggest that depression may
be regarded as a failure of normal adapation in the face of stressful experiences,
in which any one of many neurochemical systems becomes disregulated in relation
to other systems 30 (see Central
Norepinephrine Neurons and Behavior, The
Noradrenergic Receptor Subtypes, and Serotonin
Receptor Subtypes and Liagands, this
volume, for related issues).
However, stress hormones also exacerbate certain disease processes
and tissue damage, such as those describe above for the hippocampus.
A new formation of the relationship between environmental challenges,
physiological response and resilience or disease uses two terms “allostasis”
and “allostatic load”.Allostasis, meaning
literally “maintaining stability (or homeostasis) through change” was introduced by Sterling and Eyer 60
to describe how the cardiovascular system adjusts to resting and active states
of the body. This notion was generalized
to other physiological mediators, such as the secretion of cortisol as well
as catecholamines, and the concept of
“allostatic load” was proposed to refer to the wear and tear which the body
experiences due to the repeated use of allostatic responses as well as the inefficient
turning-on or shutting off of these responses 1,49. As an example of allostatic load, the persistent activation of blood pressure
in dominant male cynomologus monkeys vying for position in an unstable dominance
hierarchy has been shown to accelerate atherosclerotic plaque formation 61,
and this can be prevented by beta adrenergic blocking drugs 62.
The concept of allostasis and allostatic load has been broadened
to include all systems of the body and to focus on the protective and damaging
role of primary stress mediators, such
as catecholamines and cortisol. There
are four primary aspects of this formulation. First, the
brain is the integrative center for
coordinating the behavioral and neuroendocrine responses (hormonal, autonomic)
to challenges, some of which qualify as “stressful” but others of which are
related to the diurnal rhythm and its ability to coordinate waking and sleeping
functions with the environment. Second, there are considerable individual differences in coping with challenges that are based upon
interacting genetic, developmental and experiential influences. Third, inherent
within the neuroendocrine and behavioral responses to challenge is the capacity
to adapt (allostasis); and, indeed,
the neuroendocrine responses are set up to be protective in the short-run.
For the neuroendocrine system, turning on and turning off responses efficiently
is vital; inefficiency in allostasis leads to cumulative effects over long time
intervals. Fourth, allostasis
has a price (allostatic load, referring to cumulative negative effects)
that are related to how inefficient the response is, or how many challenges
an individual experiences (i.e., a lot of stressful events). Thus allostatic load is more than “chronic stress” and also reflects genetically- or developmentally-programmed
inefficiency in handling the normal challenges of daily life related to the
sleep-wake cycle and other daily experiences.
For behavioral responses to challenge, there are also protective and damaging aspects. Individuals can act to increase or decrease further risk for harm or disease - for example, antisocial responses such as hostility and aggression vs. cooperation and conciliation; risk taking behaviors such as smoking, drinking, and physical risk-taking vs. self protection; poor diet and health practices vs. good diet, exercise, etc. The linkage of “allostasis” and “allostatic load” probably applies to behavioral responses as well to physiological responses to challenge in so far as the behavioral response, such as smoking or alcohol consumption, may have at least perceived adaptive effects in the short run but damaging effects in the long run.
HORMONES AS MEDIATORS OF
Hormone actions in the adult brain are often
reversible, as in the case of cyclic changes and adaptive responses to stressors,
but they can also be irreversible, as exemplified by the neuronal damage after
prolonged stress or repeated elevation of glucocorticoids. Some developmental effects of hormones are
permanent, and they are characteristically actions that program the organism
to respond in a certain way or within certain limits when it is mature. The actions of thyroid hormone and testosterone
on brain development illustrate this important class of hormone effects.
Thyroid Hormone
Thyroid hormone action early in life plays a
key role in determining the normal timing of neural development. Either excess or insufficiency of thyroid hormone
during early development leads to abnormalities in brain structure. For example, neonatal hyperthyroidism in rats
causes hypertrophied development of pyramidal neurons of the CA3 region of hippocampus
and of astroglial cells within the hippocampus and basal forebrain 63. The basal forebrain cholinergic system is
also permanently increased by transient postnatal hyperthyroidism 64. In contrast, hyperthyroidism in adulthood causes
a totally different alteration, namely, a reduction in dendritic spine density in the CA1 region of the hippocampus
63.
In spite of hypertrophied CA3 pyramidal neurons
and elevated levels of cholinergic neurons in the basal forebrain, developmentally
hyperthyroid rats are actually less efficient in learning a spatial maze 65.
In contrast, there is a mouse strain in which thyroid hormone treatment
at birth improves cognitive performance 66; it may be that this strain
suffers from a congenital insufficiency of thyroid hormone at birth, which
is counteracted by the transient treatment with thyroid hormone at birth.
Having less thyroid hormone during postnatal
life decreases neuronal number in the CA1 pyramidal cell layer of the hippocampus
but not in CA3 region 66, although the dendritic and glial morphology
of the hippocampus and basal forebrain have so far not been investigated in
relation to postnatal hypothyroidism. Based on the effects of hyperthyroidism summarized above, one would
predict opposite effects.
Sexual Differentiation
Sexual differentiation of the brain, reproductive
tract, and secondary sex characteristics is another major developmental event
in which hormones play a determining role. Nurture triumphs over nature, in the sense that testosterone can
turn a genetic female into a phenotypic
male (see Fig. 6).
In mammals, the principle role of the Y chromosome is to determine that
the presumptive gonad differentiates into a testis, whereas the presence of
two X chromosomes means that ovaries will develop.
The testes then secrete testosterone during a specific period of embryonic
or neonatal life, whereas the ovary does not secrete any hormones during this
time. As a result of the actions of testosterone on a variety
of tissues, including the brain, the masculine phenotype becomes differentiated
67.
In the developing
human man, there are three phases of testosterone secretion during early life:
a testosterone peak at midgestation (12 to 20 weeks), which masculinizes the
reproductive tract and probably also the hypothalamus; a second testosterone
surge within the first year after birth, which may act on the developing cerebral
hemispheres; and a third testosterone elevation at the time of puberty. In the rat, the first two peaks are fused into
one, since the rat is born in an immature state; however, there are important
distinctions between what happens prenatally and postnatally. Prenatally, testosterone secretion masculinizes
the reproductive tract and begins to masculinize the brain, particularly those
aspects that govern male sexual and aggressive behavior.
Some of the testosterone can reach females in utero.
In litters of rats, females lying between two males in utero have greater
anogenital distance, indicating exposure to testosterone; in litters of mice,
females lying between two males in utero have higher levels of aggressive behavior.
However, these females are not sterile and still show sexual behavior
and ovulation. Thus, the process of defeminization, which
is designed to suppress both sexual behavior and ovulation, has been programmed
to occur postnatally to protect females in utero from sterilization 68.
How do we know that the brain undergoes sexual
differentiation? Until the late 1960s, the brain was not regarded as different
between males and females, but then studies using light microscopy revealed
effects of neonatal hormonal manipulations on the size of hypothalamic neurons
69. Then in 1971, a landmark
paper was published by Geoffrey Raisman and Pauline Field who used an electron
microscope to show morphologic sex differences that are developmentally programmed
by testosterone early in postnatal life 70. This study opened the floodgates on light microscopic
and neurochemical studies that established numerous sex differences in the brains
of rats, songbirds, and other species, including humans 71,72. One of the earliest examples of a morphologic
sex difference for mammals was the finding by Gorski et al. 73 that
the nucleus of the preoptic areas is sexually dimorphic; Nottebohm and Arnold
74 described sex differences in vocal control areas of the songbird
brain. Another important study 75
concerns the sexual dimorphism of the spinal nucleus of the bulbocavernosus
muscle, which innervates the penis and is present only in males.
The retention of this nucleus during early development is promoted by
the presence of androgens, which promote survival of the muscle and of the motor
neurons that innervate them; an important question is whether the androgen is
acting on the muscle, on the motor neurons, or on both.
Many of the morphologic sex differences have
functional correlates71. In
songbirds, the size of brain nuclei that control the production of song are
larger in males than in females in accordance with the greater and more complex
song production by males. In rats and
other species, the male has a spinal motor control nucleus, which innervates
the penis; as noted, this nucleus is absent in females 75. In humans, the size of parts of the corpus
callosum, as well as the anterior commissure, both of which connect the two
cerebral hemispheres, are on the average greater in women than in men 76-78. This feature correlates with, and may eventually
help to explain, the greater ability of women to overcome congenital defects
or brain damage to one cerebral hemisphere by using the other cerebral hemisphere
to compensate 79.
Brain sex differences arise through the developmental
actions of testosterone (or its metabolite, estradiol) on developing neuronal
systems at intracellular androgen and estrogen receptors 41,71. During pre- or early postnatal development,
depending on the species, these receptors are expressed permanently in the hypothalamus,
preoptic area, and pituitary and transiently in the cerebral cortex and hippocampus
68. The developmental actions
of these hormones are the subject of intensive investigation.
Neuronal migration and survival are among the processes influenced by
hormones that give rise to morphologic sex differences.
Hormones also promote differentiation of specific programs of response
of neuronal systems when they are mature. For
example, the ability of estradiol to induce cyclic synaptogenesis in the ventromedial
hypothalamus of the adult female rat is suppressed early in life by the defeminizing
actions of testosterone in the male 20.
Brain sex differences have been detected in
the human species as differences in size of cell groupings, or nuclei at the
hypothalamic level as well as in the size and shape of the corpus callosum and
anterior commissure, leading to the conclusion that the process of sexual differentiation
operates on the human brain much as it does in lower mammalian species 72,80.
However, it is not clear whether the midgestation testosterone peak or
the postnatal testosterone elevation is responsible for producing these sex
differences.
Moreover, it is difficult to specify the behavioral
or neurologic traits that are associated with morphologic sex differences.
Play behavior of children has been studied with regard to boy-girl differences
in energy level and choice of play styles and toys, by analogy with studies
in rhesus monkeys and rats showing that males engage in more rough-and-tumble
play 81. However, attribution of sex differences in
behavior to developmental hormonal influences is virtually impossible owing
to the many social factors that contribute to play behavior in children. A more objective endpoint has been spatial
learning, as in the mental rotation of figures test. Here, men outperform women significantly. However, just as there is overlap in brain
morphologic traits between men and women, so is there overlap in test scores
on the mental rotation test, so that sex differences in mean measures should
not obscure the fact that there is much overlap between the sexes 54. Nevertheless, some data show a developmental
endocrine influence on this test. In
the adrenogenital syndrome (AGS), genetic females with an enzyme defect in adrenal
steroid production produce androgens that masculinize the fetus, normally the
masculinized genitalia of AGS girls is surgically corrected at birth and these
individuals are raised as girls 81. Nevertheless, there is clearcut data showing
a mean performance by AGS girls on mental rotation of figures tests that approximates
that of normal males 82.
Evidence for normal sex differences in human
brain morphology led three research groups to examine the brains of homosexual
men who had died of AIDS for evidence of differences from heterosexual men.
The first Study revealed that the suprachiasmatic nucleus of homosexual
men was larger than that of heterosexual men, even though there was not a marked
sex difference in the SCN 83. The
second study showed that homosexual men have a smaller interstitital nucleus
of the anterior hypothalamus (INAH3) than heterosexual men, resembling the size
of this nucleus in women 84. The
third study showed that homosexual men have a larger anterior commissure than
heterosexual men, which even exceeds the somewhat larger area found in women. In none of the three studies did AIDS appear
to affect the results, since heterosexual individuals who died of AIDS showed
the same size of these structures typical of their sex 85. It is not possible to say what behavioral features
these differences in three brain structures may serve, but the existence of
such differences between the brains of homosexual and heterosexual men raise
the interesting possibility that a biological substrate exists for the differences
in sexual preference and lifestyle. Needless to say, the results must be taken provisionally until other
investigators have confirmed the essential findings on other groups of brains.
Having covered specific information concerning
the major types of hormone effects on the brain, it is time to note some of
the concepts that these examples illustrate.
First, it is evident from studies of hormone
action that the brain changes all the
time, not just during early development,
but also in adult life. Cyclic changes,
within the 4- to 5-day-estrous cycle of the female rat, in synaptogenesis in
hypothalamus and CA1 neurons of hippocampus, controlled by estradiol and progesterone,
are a dramatic illustration of reversible alterations. Other examples of irreversible changes are
those produced by stress on the hippocampus or the increased cell death of dentate
gyrus granule neurons following adrenalectomy. These structural changes, together with reversible
induction and down-regulation of neurotransmitter and peptide systems and their
receptors by hormones and by other agents, illustrate the dynamic nature of
the brain.
Second,
it is evident from the analysis of hormone effects on brain that many
of these actions differ across developmental
stage as well as among brain regions. Estrogen actions during early postnatal development mediate
the effects of testosterone on brain sexual differentiation, and one of the
consequences of sexual differentiation of the rat hypothalamus is the reprogramming
of how the adult hypothalamus will respond to estradiol in adult life; for example,
the ability of estradiol to induce progesterone receptors or to elicit synapse
formation in the adult hypo-thalamus is markedly suppressed in the male.
An example of brain regional differences in hormone action is the protective
action of adrenal steroids on dentate gyrus neurons, blocking programmed cell
death, whereas neighboring hippocampal pyramidal neurons are not affected in
this way by the presence or absence of adrenal steroids.
Both dentate gyrus granule neurons and hippocampal pyramidal neurons
have both types of adrenal steroid receptors, so one cannot predict what effect
the relevant hormone will have simply on the basis of what hormone receptors
are present.
Third, we have also seen that developmental effects often bias or determine an adult response. One
example, already cited above, is how the process of sexual differentiation suppresses
the ability of the adult hypothalamus to respond to estradiol in showing progesterone
receptor induction and synaptogenesis. Another illustration is how developmental hyperthyroidism
alters not only basal forebrain and hippocampal morphology but also compromises
the efficiency of radial maze learning in adult life 65.
Fourth, as a result of knowing more about hormone actions
in development and adult life, it is possible to suggest that hormones participate in the expression of individual as well as group differences. The thyroid hormone example cited above shows, in principle,
how deviations in normal thyroid hormone levels during early development can
help to determine not only morphology but also learning ability on an individual
basis. Likewise, the effects of prenatal
stress 86 and postnatal handling 87 have shown long-term
influences on emotionality 86 as well as the rate at which the hippocampus
ages 87. There is evidence
in infrahuman primates that prenatal stress can lead to impairments in motor
coordination and attention span and that adrenal steroids may participate in
these effects 23. Finally, sex hormones play a major role in
determination of sex (i.e., group) differences in brain structure, neurochemistry,
and certain features of behavior, and there is the intriguing possibility that
sexual orientation, which can be regarded as an individual trait but also as
a subgroup of human sexual behavior, may also involve a combination of hormonal,
genetic, and experiential factors (see The
Neurobiology of Neurotensin, Arachidonic
Acid, and Nitric
Oxide and Related Substance as Neural Messengers, this volume).
Finally, the
formulation of allostasis and allostatic
load offers a new way of looking
at the relationship between protective and damaging effects of stress mediators
and a new way of integrating behavioral and biological processes in understanding
resilience and vulnerability to disease processes. Allostasis and allostatic load are broader
than and more precise than the term “stress”,
the indiscriminate use of which in popular culture has made this word
a less and less useful term to describe the ways in which the body copes with
psychosocial, environmental and physical
challenges, including the adjustments
to the circadian light-dark cycle. “Stress”, as a subjective experience, does not always correlate
with physiological responses, and therefore
the measurement of the physiological responses of the body to environmental
challenges constitutes the primary means of connecting experience with resilience
or the risk for disease. The “stress
mediators”, hormones such as cortisol
and catecholamines, also vary in their basal secretion according to a diurnal
rhythm that is coordinated by the light-dark cycle and sleep-waking patterns,
and this fact reinforces the inadequacy of the popular use of the term
“stress” as a useful descriptor. Moreover, as describe above, new research has provided
further support for the fact that the “stress mediators” have protective and adaptive as well as damaging
effects, and the search for biological
mechanisms that determine protective versus damaging effects of these mediators
is a new theme in biobehavioral research
49.
A final question remains, namely, what is the
relevance of hormone actions on the brain to human and animal pathophysiology
and, in particular, to nervous and mental disorders? We consider these in order
of some of the major types of hormone effects: cyclic processes, sex differences,
and the effects of stress.
Cyclic disorders include catamenial epilepsy,
premenstrual tension, and jet lag. Catamenial
epilepsy varies according to the menstrual cycle, with the peak frequency of
occurrence corresponding to the lowest ratio of progesterone to estradiol during
the cycle 88,89. Bearing in mind that there is some genetic and/or developmental
predisposition to express seizures, there may be at least three types of hormone
actions involved in the cyclic occurrence of epilepsy: (i) estrogen induction
of excitatory synapses in hippocampus, leading to decreased seizure thresholds
46; (ii) progesterone actions via the steroid metabolites that act
via the GABAa receptor to decrease excitability 6; and (iii) hormone
actions on the liver to increase clearance rates of antiseizure medication
89.
Premenstrual tension is a cyclic mood disorder,
which is referred to as premenstrual syndrome (PMS) in its most severe form,
and its symptoms are eliminated by arresting the menstrual cycle 89. However, specific hormonal causes are unknown
52. Although there are indications
that high luteal-phase estrogen and progesterone levels may exacerbate symptoms
of PMS, the administration of a gonadotropin-releasing hormone agonist along
with low amounts of estradiol and progesterone to prevent ovarian hormone deficiency
has been reported to alleviate symptoms of PMS 90.
As far as
jet lag is concerned, one of the causal factors for malaise and cognitive dysfunction
may be that cortisol is secreted according to the time zone of origin rather
than the time zone of destination, that is, until the diurnal clock is entrained
to the local light-dark cycle 30.
The diurnal cycle involves shifting thresholds of vulnerability, as,
for example, with an increased frequency of heart attacks occurring in the morning
hours 1. Moreover, shift work alters risk factors for cardiovascular disease as well
as the sense of well-being 91.
Sex differences play a significant role in the
occurrence of diseases. Besides the
well-known predominance of cardiovascular disease in men over premenopausal
women, men also have more frequent developmental learning disorders 79.
Also mental disorders differ in occurrence between the sexes according
to type: Anxiety disorders and depression are more common in women; substance
abuse and antisocial behavior are more prevalent in men 92.
Schizophrenia, however, is equally prevalent in men and women 92),
but estrogens play an important role in containing dopaminergic function and
reducing the severity of symptoms in younger women 93. Moreover, different doses of neuroleptic drugs
are required for treating schizophrenia in women and men, depending on the presence
of circulating estrogens, and there are sex differences in the type and severity
of symptoms and response to treatment 94. Likewise, Parkinson's disease is more severe
in women with circulating estrogens, reflecting antidopaminergic actions of
estradiol in the face of dopaminergic hypofunction 93. One of the important lessons of these examples
is that men and women must be studied separately for the actions of psychotropic
drugs, including antidepressants as well as antipsychotics and benzodiazepines,
because differences in circulating hormone levels or intrinsic, developmentally
programmed sex differences may bias the mechanisms by which these pharmaceutical
agents act on the brain.
In addition to the cardiovascular system, the
brain also may be differentially vulnerable to severe stress in men and women.
Both severe social stress in vervet monkeys 82 and cold swim
stress in rats 95 damage the hippocampus of males but not of females.
Again, it may be differences in circulating hormone levels or intrinsic
sex differences that will explain these sex differences.
As describe above, allostatic load (replacing
the term “stress”) is a factor in exacerbating symptoms of a number of diseases,
such as atherosclerosis, asthma, diabetes, gastrointestinal disorders, as well
as resistance to viral infections and metastasis of tumors (1,48,
49). However, it is not always
clear what aspects of stressful experience contribute to the pathological process,
and it is likely that both chronic and acute stress is involved.
For example, chronic stress appears to facilitate development of atherosclerotic
plaques 44, whereas acute stress may precipitate myocardial infarction
1. Yet, acute stress can enhance immune function,
whereas chronic stress can suppress it 49.
We have made the point several times throughout
the article that hormones participate in determining the expression of individual
differences, possibly by regulating the expression of genetic factors that contribute
to or determine the vulnerability to a disease. This attractive notion remains hypothetical;
support for it requires the elucidation of specific mechanisms by which hormones
affect normal brain function as well as mechanisms by which the endocrine system
participates in the development of pathologic states. This chapter has provided glimpses into specific
mechanisms for sex, stress, and thyroid hormone actions on diverse neural mechanisms
and structural changes, which can provide guidelines for future investigations.
Research
in the author's laboratory for work described in this chapter is supported by
NIH Grants NS 07080, MH 41256, and MH 43787 and also by grants from the Health
Foundation (New York) and Servier (France).
published 2000