Additional related information may be found at: |
Neuropsychopharmacology: The Fifth Generation of Progress |
Central Norepinephrine Neurons and Behavior
Trevor W. Robbins and Barry J. Everitt
I. Introduction
The
neurobiological data reviewed in past and present articles in the Generation
of Progress series (50; see chapters by Foote and Aston-Jones and by Valentino
and Aston-Jones in this volume) and summarized in Table
1 provide several clues to the functions of the LC in the behaving animal.
However, extrapolations from such data to psychological processes must be made
with care. Clearly, the widespread nature of the ceruleo-cortical projection
indicates that activation of this noradrenergic cell group will have pervasive
effects in diverse terminal regions such as neocortex, hippocampus and the amygdala.
It is perhaps no surprise that the LC has been implicated in
such distinct processes as learning, memory, attention and anxiety, which
clearly depend to different degrees on these discrete regions of the forebrain.
Two critical questions are: (i) what
effect, if any, does such activation have on cognitive and behavioral processes
? and (ii) under what circumstances does such activation normally occur? The
first question can only be answered by studying the behaving animal and inferring
changes in psychological function. Thus it is predicted that manipulations of
ceruleo-cortical function will affect various psychological processes including
associative learning, different forms of memory and attention depending, in
large part, on which terminal domains of the noradrenergic projection are especially
engaged during the task under study. Presumably, the distributed nature of these
effects represents an integrated, adaptive response to the environmental and
behavioral setting. The electrophysiological data and also neurochemical indices,
show that LC neurons are especially active during relatively high states of
arousal, including exposure to stressful environments and salient, phasic stimuli
(see chapter by Foote and Aston-Jones in this volume). Thus, in stressful circumstances
it may be useful not only to consolidate memories more efficiently, but also
to focus attention on salient features of the environment: the ceruleo-cortical
noradrenergic system is clearly suited to such a function.
II. Behavioural functions of the locus coeruleus
(LC)
A. Electrophysiological
studies in the behaving animal
Measuring
LC activity concomitantly with behavior provides useful information on the functions
of the LC norepinephrine (NE) neurons, although it must be borne in mind that
any correlative study of this sort does not establish the overall significance
of the LC in particular behavioral processes. The mere presence of neuronal
activity, even if highly correlated with environmental events, do not necessarily
bear implications for causal factors in behaviour ,which are best assessed by
interventions which affect NE function more directly.
The
observation that LC noradrenergic neurons are active during stressful circumstances
is clearly consistent with suggestions that they may be involved in the learning
of aversively motivated tasks (see below). It is noteworthy, therefore, that
Jacobs (50) has demonstrated marked increases in NE neuron unit activity during
presentation of a CS previously paired with an aversive air puff to the face
of a cat. The selectivity of this response of LC neurons was emphasized by the
observation that the same CS previously paired with a rewarding stimulus did
not evoke increases in NE neuronal firing. While these data strongly suggest
LC involvement in learning associated with stressful stimuli, for which there
is considerable support from behavioral studies (see below), they do not account
for the observation that learning of some appetitively motivated tasks is
significantly retarded following lesions of the ceruleo-cortical noradrenergic
projection (38).
Indeed,
recent data suggest that LC neurons
are active during appetitive learning. In rats, there was an immediate response
of LC cells to any change in stimulus-reinforcer contingencies in appetitive
and aversive conditioning, often prior to the behavioral expression of conditioning
(75). The changes were even more reliable
than the responses to novelty and were
unrelated to movements, disappearing when the behavioral response was well established.
In primates, LC neurons are active during the performance of a vigilance or
"oddball" visual discrimination task that is appetitively motivated.
Monkeys were trained to release a lever after a target cue light that occurred
randomly on 10% of trials and to withhold responding during non-target cues.
LC neurons responded selectively to the target cues on this task,
and rapidly reversed this salience if the non-target stimuli became relevant
instead (10,11,12 ). High levels of LC discharge were related to decreased foveation,
restlessness and impaired task performance (10). Importantly, cortical, event-related
potentials were elicited in this task selectively by the same stimuli that evoked
LC responses, suggesting that the LC activation was associated with cortical
processing mechanisms and in learning the significance of behaviorally important
stimuli.
At
the cortical level, P-300 like potentials
have been recorded in squirrel monkeys in other "oddball"
paradigms where the animal responds to low probability auditory stimuli (65). The generation and modulation
of the P-300 like auditory potential was
shown to be impaired by electrolytic lesions of the LC, although more specific destruction of ascending NE
fibres failed to have the same effects.
In
general these electrophysiological studies are consistent with a role for LC
NE neurons in processes related to the processing of novelty, including new
contingencies that require learning.
B. Effects
of neurotoxic lesions of the dorsal bundle on behavioral function
Another
approach to investigating the functions of the LC NE system has been to define
the behavioral effects of selectively removing the rostral, noradrenergic projections
of the LC, and the environmental circumstances under which such effects occur.
Profound levels of forebrain NE depletion can be achieved by injecting the selective,
catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) into the trajectory of the
LC axons in the midbrain - namely, into the dorsal noradrenergic bundle (DB).
The rationale underlying this neurochemically highly specific approach to studying
LC function has been discussed extensively elsewhere (61,70) and will not be
labored here. With optimal parameters (see Figure
1) it is feasible to deplete the telencephalic projection zones of NE to
less than 10% of control values without any effect on any other known neurotransmitter
system Indeed, such potent
degrees of depletion, as assessed by tissue levels of NE, are essential
if comparable reductions of extracellular NE are to be achieved (1). Smaller
degrees of depletion clearly enhance possible plastic and recuperative responses
of noradrenergic neurons to 6-OHDA lesions, including up-regulation of adrenoceptor
populations (2,34). Such problems may also make difficult
interpretation of the effects of the alternative noradrenergic neurotoxin DSP-4,
which can lead to cortical NE loss of less than 70% (e.g. 91). Conclusions concerning the lack
of involvement of central NE in particular psychological processes such as memory
and learning are clearly unsafe if depletions are substantially less than 90%.
On the other hand, several groups have consistently demonstrated robust and
long-lasting behavioral deficits associated with depletion over 90% of telencephalic
NE , which would seem hard to explain in terms of changes leading to over-compensation
of the damaged system (see 70). However, there are two major problems specifically
related to the effects of DB lesions: (i) assessing the possible contaminating
effects of hypothalamic NE depletion caused unavoidably by diffusion of the
neurotoxin to fibres projecting from the cell bodies of the medulla oblongata
contributing to the so-called ventral noradrenergic bundle (VB). This can be
addressed by making control lesions of this structure which, of course, also
contribute to our understanding of the role of hypothalamic NE.
(ii) Isolating the critical terminal region for a particular effect;
this problem will be addressed in section IIC. While there are undoubtedly problems
associated with the use of neurotoxic lesions of the DB or VB to understand
the behavioral functions of the central NE systems, these have to be weighed
against the difficulties of interpreting effects of
systemic treatments with adrenergic receptor agents, for example because of lack of pharmacological specificity
and peripheral factors.
However, such experiments can be helpful in
providing converging sources of evidence and will be mentioned as appropriate.
1. Effects
on unconditioned behavior. DB lesions have no obvious effect on gross behavior
in the rat, such as eating, drinking or spontaneous locomotor activity. This
contrasts markedly with the changes produced in these forms of behavior by central
dopamine depletion, as well as the increases in body weight and feeding often
observed following hypothalamic NE depletion (48,73). This lack of effect of
DB lesions on gross ingestive and locomotor responses simplifies the interpretation
of many of the other effects of the lesion. However, it is evident that DB lesions
do affect some forms of unconditioned behavior, for example, behavioral responses
to novelty, as might be expected from the single unit studies that show stimulus
novelty to be an effective trigger for LC neurons. As will become obvious for
many of the effects of these lesions, however the sequelae appear to depend
on subtle features of the testing situation (see 23 for a review). For example,
DB lesions can reduce feeding overall in the threatening situation of a highly
illuminated open field, and yet attenuate the suppression of eating normally
shown to a novel food. Steketee et al have replicated the attenuation of this
food neophobia in novel situations following DB lesions and have found that it can be attenuated by icv
treatment with the beta receptor agonist isoproterenol (86).
2. Effects
on conditioning and conditioned behavior One
of the most important generalizations to have emerged about the effects of DB lesions is that they tend to impair the
acquisition of new behavior to a greater extent than previously established
performance. The concept that the DB is implicated in learning is by now quite
venerable, but some of the original experiments were not convincing and there
are many forms of learning that are not reliably affected by DB lesions. However, there is some evidence that under
well-defined conditions, DB lesions (and DSP-4 induced NE depletion, 3) reliably
retard the gradual learning of an appetitive, conditional discrimination task,
in which the rat is required to learn a rule (press right or left lever) to
guide its response to one of two discriminative stimuli (e.g. lights flashing
at one of two different frequencies). On the other hand such lesions do not
impair its performance if the lesion is made following the establishment of
the discrimination by pre-operative training (38). This clearly rules out many
explanations of the effects of DB lesions based on simple performance factors
such as changes in sensory capacity or motivation.
The dichotomy between acquisition and performance has also been observed for aversive
conditioning: thus, conditioned suppression of food-maintained operant responding
in the presence of a light acting as an aversive conditioned stimulus (CS) is
attenuated in acquisition following surgery, but is unaffected if established
prior to surgery (21). This dissociation argues against a simple explanation
of the acquisition deficit in terms of an intervening variable such as "anxiety",
and this is supported by demonstrations of a lack of effect of DB lesions on
response suppression in the Geller-Seifter conflict paradigm (54).
Furthermore, the acquisition of a possible oral coping response (e.g.
gnawing, eating) to an unconditioned aversive event, tail-pinch, is impaired
following DB lesions, but not if the
response is established by experience prior to surgery (73). However, we should
emphasize that associative processes are not always impaired by such lesions.
For example, simultaneous visual discriminations,
where the rat simply has to approach the rewarded stimulus and the effects of
reversing the contingencies so that the animal has to approach the formerly
non-reinforced cue, are not affected by DB lesions (37). Conditioned taste aversion
has also proved particularly resistant to disruption following DB lesions (35).
Moreover, the effects on other forms of aversive learning and performance are complex
and may depend on such factors as the nature and intensity of the aversive reinforcer,
time elapsed between the DB lesion and testing, and the precise conditioning
contingencies and test situation used (see review, 80). In general, VB lesions
do not affect acquisition of appetitive or aversive conditioning, although they
have been found to increase resistance to extinction of the latter, both for
conditioned suppression and conditioned taste aversion (21,35). It seems plausible
that these effects of VB lesions on extinction represent possible interactions
with neuroendocrine mechanisms in the hypothalamic-pituitary axis, and that
at least some of the earlier reports of extinction deficits following DB lesions
(61) might be attributable to effects on neurons of the VB.
Contextual
aversive conditioning (that is conditioning to the background cues rather than
to an explicit conditioned stimulus
(CS)) is also spared by DB lesions; in fact such conditioning may be enhanced
by ceruleo-cortical NE loss, either when an explicit CS is also present (77)
or when it is not (80). The difference between aversive conditioning to discrete,
explicit cues and to the wider contexts in which they occur has obvious applicability
to various forms of clinical anxiety. Plasma corticosterone is also affected by such lesions, but only
in a manner predicted by the conditioned behavioral response (77). However,
the complementary pattern of impaired CS conditioning and enhanced contextual
conditioning seen in rats with DB lesions is another piece of evidence against
a simple anxiolytic view of their effects, and may instead argue for effects
on attentional function (see below).
A possibly related effect of DB lesions on contextual
conditioning is seen in spatial learning in the Morris swim maze, where the
rat is required to learn to use distal cues around the room to locate a safe,
but invisible platform. DB lesions again may actually enhance acquisition of
the task (55), especially if the rat is swimming in cold water (78). In this
situation the DB lesion appears to protect the rat against the deleterious effects
on learning in water sufficiently cold to lead to hypothermia, while having no effect
by itself on body temperature in the swim-maze, or on plasma corticosterone levels (78). In contrast, VB lesions have no
effects on spatial (78) or contextual (77) conditioning, although they do affect
plasma corticosteroid levels to the shock (81), thereby showing a dissociation
between effects on conditioning (by
DB lesions) and endocrine status (by VB lesions ) which probably corresponds
to the relative roles of these systems in cognitive and vegetative functions,
respectively.
In
attempting to resolve the, at first sight, bewildering, pattern of effects of ceruleo-cortical NE depletion
on these different forms of learning, strong unifying themes can in fact be found. One such theme is the
difficulty or sensitivity of the learning procedure, another is the task-associated
level of arousal; either factor might explain the relatively greater sensitivity
of aversive paradigms to learning deficits following DB lesions. Thus, the conditional
appetitive discrimination task which has been repeatedly shown (3,38,70) to
show deficits in DB lesioned rats typically takes many sessions to learn even
for a normal rat. Another factor is the attentional requirements of the tasks;
it is possible that some of the DB effects result not from deficits in associative
processes, but in the input to the associative mechanisms. Thus, the dissociation
between CS and contextual learning may suggest that DB rats are utilizing more
distal cues than the normal rat, which, in some situations may produce a paradoxical
enhancement of acquisition. This prediction was tested directly in the water-maze task with
a discrimination between two local sets of cues (platforms with vertical
or horizontal stripes). The DB lesion impaired acquisition, despite facilitating
acquisition of the spatial variant of the task (78), thus again showing a dissociation of effects
on local versus distal cues, that possibly results from shifts in attention.
3. Effects
on attention Since the original Segal and Bloom (76)
findings of enhanced S/N ratios following iontophoretically applied NE
in hippocampus, there have been a rash of hypotheses concerning the role of
the LC in selective attention (61,70). An earlier review (70) made it clear
that direct tests of this hypothesis employing paradigms including latent inhibition,
blocking and non-reversal shift have not generally
confirmed earlier positive findings (61, but see 72). However, there have been
some interesting findings that probably require further investigation (e.g.57,89).
Devauges and Sara (33) have suggested that activation of central NE mechanisms
by the alpha-2 receptor antagonist, idazoxan, can lead to apparent improvements
in shifting of attention between different types of cue, although when state-dependent
controls were included it was shown that this drug impaired non-reversal
shift learning in a maze between spatial and visual cues (72).
In
assessing the role of the LC in attentional mechanisms, it is notable that there
are several different forms of attention, including selective (focused) attention,
divided attention and sustained attention, including vigilance. Deficits can
be observed following DB lesions in
a continuous performance task which requires some of these attentional capacities,
under certain conditions (17,25). As might be predicted, these deficits only
occurred under very specific test conditions, namely when bursts of loud white
noise were interpolated immediately prior to expected visual targets,
when the rats were treated with amphetamine, or when the stimuli were
temporally unpredictable, a manipulation that increases arousal. There were
no such deficits in attention when the task was merely made more difficult by,
for example, reducing the brightness of the discriminanda or by increasing the
frequency of their presentation. The effect of white noise is manifestly consistent
with the demonstration that LC neurons fire in response to such phasic stimuli
(50, see chapter by Foote and Aston-Jones in this volume) and also with demonstrations
of enhanced distractibility in the maze situation (71), and possibly the enhanced
reactivity of DB lesioned rats in the open field setting (15) , as described
above. The effect of temporal unpredictability is important in that it suggests
the LC can be activated in quite complex ways, viz. not only in response to
the initial occurrence of a novel stimulus, but also to the omission of an expected
event. One of the concomitant effects of white noise is to produce behavioral
activation, which can be manifested as quicker reaction times and a propensity
to impulsive responding (17). Such behavioral activation is also produced by
the dopaminergic agonist, D-amphetamine, especially when infused into the region
of the nucleus accumbens septi (22,24). Thus, it would be expected that discriminative
accuracy in the visual vigilance task would also be impaired in rats with DB
lesions following intra-accumbens infusions of amphetamine and this result has
been found (22). The
degree of behavioral activation produced by amphetamine was unaffected
by the DB lesion (22). Thus, under conditions of equivalent behavioral
activation resulting from the endogenous cue of dopamine release, DB-lesioned
rats became less efficient at detecting brief visual signals. The parallel with
the effects of white noise is enhanced by evidence that dopamine depletion within
the ventral striatum attenuates the activating effects of this stimulus as well
as those of D-amphetamine (24). The lack of effect of DB lesions in a test well-validated
to measure sustained attention or behavioral vigilance has supported the general
lack of effects of DB lesions on the basic version of the continuous performance
test analogue (59). Perhaps the advent of relatively specific alpha-1 and alpha-2
adrenoceptor agents will enable further tests of these hypotheses if the drugs
are administered centrally: thus far, however, it is evident that systemic administration
of adrenoceptor agonists or antagonists do not strongly mimic effects of DB
lesions (e.g. 83).
C. Interactions
of LC with specific terminal regions
It
is clear that DB lesions can affect
a variety of behavioral processes in carefully defined conditions, but it is
less clear how readily these may be
attributed to the different projection fields of the LC. There are several strategies
available for addressing this problem; one is to attempt to mimic the effects
of a DB lesion with a manipulation of
a discrete terminal zone, including local NE depletion or the acute modulation
of NE function via intracerebral administration of specific adrenoceptor agonists
or antagonists. In the former case, there are problems posed by the considerable
plasticity following damage to terminal regions, which is undoubtedly
greater than following damage at the level of the cell body or fibre bundle.
In the latter instance, the problems of interpretation resulting from diffusion
and local concentration are equally challenging, but they leave open the possibility
of boosting local NE function and predicting an opposite effect to that of DB
lesions. Not surprisingly, progress in this area has been limited.
When
interpreting the effects of DB lesions on aversive conditioning, there is considerable
evidence of an involvement of the amygdala in learning about aversive CSs (32,44,58),
whereas there is complementary evidence for a role of the hippocampus in
aversive conditioning to context (79). Local depletion of NE from the
region of the amygdala impairs conditioning to aversive CSs in the same way
as does DB lesions (77) although local
depletion of hippocampal NE on contextual conditioning has not been studied. The behavioral consequences of NE depletion
from both regions following DB lesions may depend, therefore, on the relative
degree of engagement of hippocampal versus amygdaloid mechanisms in any particular
task or situation.
This
principle may also apply to the case of choosing between familiar and novel
food in a novel environment, especially as both amygdala and hippocampal lesions
probably affect the responses to, perhaps different, aspects of novelty There
has been relatively little investigation of the effects of terminal NE manipulations
on the response to novelty. However, Borsini and Rolls (14) found that 6-OHDA
lesions of, and also NE infusions into, the amygdala affected the response to
novel foods, although these two manipulations did not have the expected opposite
pattern of effects. In assessing a locomotor response to a novel environment,
Flicker and Geyer (40) found that chronic infusions of NE into the dentate gyrus
retarded the habituation of spatial exploration. These two sets of results are
intriguing in that they suggest that terminal NE manipulations can affect different
aspects of the response to novelty at distinct neural sites, a conclusion fully
consistent with findings of the different effects of DB lesions
on response to different aspects of novelty (see above).
With
respect to the neocortex, there has similarly been little direct investigation
of the role of NE in behavioral paradigms. Performance of the 5-choice attentional
task described above. is known to depend on the rat neocortex, especially implicating
anterior regions (63). Other evidence
has shown the involvement of NE mechanisms in processes of working memory in
the primate prefrontal cortex, using the delayed response paradigm.
The decline in working memory in aged rhesus monkeys is significantly
ameliorated by systemic treatment with the alpha-2 adrenergic agonist, clonidine.
Arnsten and Goldman-Rakic (7) further showed that prefrontal cortical
ablation (around the sulcus principalis),
or local noradrenergic denervation of this area induced by 6-OHDA, both disrupt
performance on the task - confirming its dependence on this area of neocortex.
Clonidine was able to reduce the NE lesion-induced deficit, but not the cortical
ablation-induced deficit, emphasizing the interpretation that not only does
delayed response performance depend on the prefrontal cortex, but that NE interacting
with postsynaptic alpha-2 receptors in this site appears to be an important
component of the processes occurring there (7). Guanfacine, a more specific
alpha-2A
agonist with less marked sedative and cardiovascular effects, was even more
potent than clonidine in improving memory in this spatial delayed response paradigm
in young (41) as well as old (9) adult monkeys .
While
beneficial effects of clonidine have also been observed in a delayed matching
to sample task in monkeys (49), other investigators have failed to observe effects
of systemic clonidine on delayed response
performance in aged monkeys (31); this discrepancy might depend on subtle differences
in test setting and procedure. For example, the delayed response task in these
two studies was implemented in different ways; notably the latter investigation
(31) employed an automated procedure whereas the studies by Arnsten and colleagues
(e.g. 7,9) utilize a manual procedure in an environment susceptible to distraction.
The possible importance of the attentional requirements of the task is provided
by the study of Arnsten and Contant (6) showing that
alpha-2 receptor agonists have
strong protective effects against extraneous distraction in the
aged animals during performance of the delayed response task, and that
these effects can be blocked by treatment with a an alpha-2 receptor antagonist
acting predominantly at post-synaptic receptors. These findings are compatible
with evidence reviewed above suggesting that DB lesions enhance distractibility
in certain settings in the rat.
In
recent work, the disruptive effects of an alpha-1 adrenergic/imidazoline agonist
(cirazoline) has been shown to impair spatial working memory performance in
the delayed response task at very low doses, effects that were antagonized by
the alpha-1 receptor antagonist prazosin (8). Similar findings have been reported
in the rat when alpha-1 agents are infused directly into the prefrontal cortex
(see 5). Arnsten has synthesized these findings to suggest that post-synaptic alpha-1 and alpha-2
receptors may have opposing functions in the prefrontal cortex, just as they
do in other brain regions (5).
Overall,
these results provide evidence that the effects of noradrenergic transmission
through the ceruleo-cortical system are mediated through quite specific areas
of its termination, whether neocortical, archaecortical (hippocampus) or involving
subcortical structures of the limbic forebrain such as the amygdala, although
the basolateral component of this nuclear complex may, in fact, be more cortical
than subcortical in terms of both structure and connections. Overall, it is
apparent that very different effects, e.g. of adrencoceptor agonists have been
found, possibly as a function of the behavioral task under study, which in turn
would relate to fundamental differences in the types of processing occurring
in the terminal domains.
D. Role
of ceruleo-cortical NE in memory and
other forms of plasticity
Early
theories of LC function emphasized its possible role in memory consolidation,
which may contribute to some of the selective effects of DB lesions on acquisition
(see 70). This can be contrasted with
the considerable evidence, in the rat, of spared short term memory function
following DB lesions, for example, in delayed matching to position (49) and
delayed alternation (66) tasks. It appears that, as well as acute effects on
attentional mechanisms, the DB may also be implicated in rather longer term
plastic changes in synaptic function that contribute to learning, perhaps particularly
in tasks requiring lengthy training (see above).
Experiments utilizing post-trial noradrenergic manipulations on the retention
of one-trial aversive conditioning tasks have examined possible direct or modulatory
roles of NE in the consolidation of memory traces (44,56,58). In general, the
data suggest that low doses of NE infused into the amygdala facilitate retention
(56), but higher doses are either without effect (56) or are amnesic (36). Furthermore,
intra-amygdala beta-blockers are also amnesic in their effects (44) Evidence
suggests that central NE mechanisms, probably within the amygdala, are a final
common pathway for a variety of amnestic and promnestic treatments, for example,
adrenaline infusions (58) and treatment with CRF (18), respectively.
A
further locus of plastic change is the hippocampus, a structure typically
associated with aspects of learning and memory, and there is evidence that depletion
of NE can reduce LTP in the dentate gyus, a finding supported by demonstrations
of an initiation of short- and long-term potentiation of the dentate gyrus response
to perforant path input by the exogenous or endogenous application of NE (see
review, 47). New evidence suggests that noradrenergic depletion (e.g. by DSP-4)
can exacerbate working memory deficits in the rat produced by intra-hippocampal
blockade of muscarinic or NMDA receptors (e.g. 64).
In
parallel with the early theorising on memory consolidation were suggestions
that manipulation of cortical NE could
affect visual development (51). Destruction of central NE systems in neonatal
rats does influence the degree of recovery from frontal cortex lesions when
assessed behaviorally during adulthood (53). However, these effects are controversial
and still being re-evaluated (67). A recent study has shown that the DSP-4 induced
disruption of the imprinting response in the chick which has a large visual
component and depends on a critical period early in development, is reversed
by centrally administered NE or the beta-2 agonist salbutamol (30). Perhaps
most surprisingly, destruction of the DB, apparently, via cortical alpha-1 receptors,
prevents the adaptive changes in resulting from damage to the mesolimbicocortical
dopamine system, leading to an abolition of both neurochemical changes (in D1
dopamine receptors) and behavioral deficits (hyperactivity, impaired delayed
alternation performance) normally associated with such depletion (87). In this
context, it is of considerable interest that the disruption by fornix lesions
of performance of an 8 arm radial maze task, which is correlated with reductions
in cholinergic hippocampal markers, is completely ameliorated by central NE depletion
induced by DSP-4 (74). Thus, again central NE depletion actually benefits behavioral
recovery, and this emphasizes the importance of considering interactions and
balances with activity in other neurotransmitter
systems.
There
have been several dominant ideas in theories of LC NE function, based largely
on experiments in animals, ranging from notions of reinforcement and arousal
(see 70), to the mediation of anxiety and stress (46,68), and the control of
selective attention (61,70). Each theory commands its own set of supporting
behavioral phenomena, but it is probably fair to say that no single construct
can adequately explain the functions of the LC. While it is natural (though
sometimes insightful) to ignore evidence that
does not fit into a particular theory (and we are aware that a brief
chapter such as this is unlikely to be free of this tendency), it is desirable
that the theory be as precise but as all-embracing as possible. It seems likely,
in fact, that the LC has rather general functions which bear on aspects of attention,
learning and anxiety. These functions are based on two very clear points. First,
activity in LC neurons is monotonically
related to increased arousal. Second, this
activity probably improves the processing of salient events in diverse
forebrain sites, whether these events are novel, salient because of conditioning,
or even internalized, as representations of stimulus events receiving further
processing in memory consolidation and retrieval.
An
early theoretical suggestion (4) was that the LC functioned rather like the
cognitive arm of a central sympathetic ganglion, and this notion has more recently been taken up by others (10). "Thus activation of the peripheral
sympathetic system prepares the animal physically for adaptive phasic responses
to urgent stimuli, while parallel activation of LC increases attention and vigilance,
preparing the animal cognitively for adaptive responses to such stimuli"
(10, p516). The notion of adaptive preparation or coping with the consequences
of sympathetic arousal is also to be found in the earlier review (4), and has
also been stated by us (69,70) previously in terms such as the DB functions
to preserve attentional selectivity especially under elevated levels of arousal
(69). This is related to psychological theories such as that of Easterbrook
(see 39) who argues that high levels of arousal cause attentional focusing.
Another, probably related, formulation is that the LC is
implicated in "controlled" or "effortful", as distinct
from "automatic", processing (25); that is, the system modulates attentional
capacity. Presumably, LC activity would
be part of that mechanism that effectively focuses attention onto salient events
in threatening or demanding situations. This theory
explains why the LC is more implicated in acquisition than performance of learning
tasks, why aversive situations are more
sensitive to manipulations of the LC, why attentional function is disrupted
under certain conditions in DB lesioned rats, and why the LC is active during
orienting responses to novel stimuli. According to this view, the LC does not
mediate anxiety or stress per se, but rather a state of arousal that is correlated
with anxiety or stress, leading to attentional
and cognitive change. This state of arousal
can be self-regulated to a point, and there is evidence to implicate
LC neurons in some of the phenomena of "learned helplessness". Specifically,
central NE activity is affected by environmental contingencies such as the availability
of effective avoidance or "coping" responses; in situations where
the animal is exposed to inescapable, uncontrollable
stress, NE function is depressed (82, see chapter by Valentino and Aston-Jones
in this volume for review), but can be restored by treatment with adrenoceptor
agents infused in the vicinity of the LC, leading to attenuation of those responses
characteristic of "behavioral despair" (82). From a consideration of the cognitive sequelae
of manipulations of central NE reviewed in this chapter, it is obvious that
some of the major cognitive features not only of learned helplessness, but also
depression, including problems in attention and learning, could result from
LC dysfunction.
Several
of the effects of DB lesions (e.g. reduced food neophobia and conditioned suppression)
would be expected if such depletion had anxiolytic effects and the role for
NE mechanisms of the LC in mediating
certain elements of the opiate withdrawal syndrome (88) is also suggestive of
a role in aversive motivation. However, the limited nature of the conditioned suppression deficits, in particular
the apparent enhancement of contextual conditioning, the lack of effect on punished
responding and the different effects of chlordiazepoxide and DB lesions (23,54)
in the food neophobia setting, are all inconsistent with a simple form of the
anxiety hypothesis. On the other hand, it is clear from studies with humans that anxiety often leads
to enhanced distractibility rather than enhanced focusing, and that there is
evidence from several sources (5,10,74,78) that elevated LC activity, far from
improving performance, may actually impair it. A parsimonious account would
then invoke the inverted U shaped function relating arousal level to efficiency
of performance (see 39, for review ).
The decrements in performance at high levels of arousal have been often been attributed to the attention the
subject begins to pay to visceral cues arising from sympathetic arousal, such
as palpitation, which may become correlated
with subjective feelings of anxiety
(39).
In
contrast, low levels of coeruleo-cortical noradrenergic activity, such as those
produced by low doses of clonidine, can lead to lapses in attention that are
reversed by environmental stressors such as background noise (84). In certain
circumstances, the task itself may theoretically induce arousal that helps to
counteract the reduced arousal associated with low NE activity. One recent study
has shown a correlate of this in terms of clonidine-induced reductions in basal
regional cerebral blood flow being cancelled out by the requirement to perform
an attentional task (29). Such a result obviously adds another dimension to
the view that the coeruleocortical NE system plays a role in behavioral vigilance
(10). While many might consider that
the invocation of a traditional notion such as level of arousal to summarize
the range of effects associated with different degrees of noradenergic activation
to be outmoded, it is quite difficult to find a more parsimonious account. What
has emerged however from more contemporary analyses is that the underlying neural
mechanisms are becoming clearer; for example through analyses of regional changes
in blood flow, and in the involvement of different populations of adrenoceptors
that regulate the effects of altered DB activity in defined ways.
Some
of the experimental work on behavioral functions of the LC in animals appears
to fit with human psychopharmacological experiments on cognitive effects of
adrenoceptor agonists and antagonists. Thus, Clark et al (19) found that the mixed alpha 1/2 agonist clonidine impaired
performance in a set of dichotic listening tasks (both divided and focused attention),
presumably via its presynaptic actions to reduce LC firing. They
suggested that the drug reduced alertness or arousal, and so increased the demands of the tasks on
the volunteers. In contrast, a dopamine receptor blocker, while also impairing
performance, reduced the activational state of the subjects, or their readiness
for action. Clark et al (19), following a similar suggestion (69), make the
useful distinction between the dual roles of the LC NE system and central dopamine
pathways; the former being concerned with regulating the capacity for conscious
registration of external stimuli, whereas the latter regulates the capacity
to respond to them.
Other
effects of clonidine are compatible with the evidence concerning effects of
DB lesions in animals. First, clonidine has been shown to impair the learning
of difficult paired associates (43). Second, the drug apparently reduces the
cost (in reaction time) of shifting attention to
a spatial cue (20); this is perhaps analogous to the changes in attentional
distribution produced by DB lesions. On the other hand, a recent study of a
similar paradigm using two rhesus monkeys found very different effects of clonidine
and gunafacine, mainly to reduce the beneficial ‘alerting’ effects of a spatially
uninformative cue (92). The latter authors attribute the discrepancy to several
possible factors, including the method used to compute the cost of spatial shifting.
Other studies have shown that clonidine produces
significant decrements in a ‘rapid visual information processing’ test of sustained
attention (28). These effects were selective in several ways: decrements were
much more apparent than in a test of self-ordered spatial working memory, they
contrasted with those produced by diazepam, and unusually they were more evident
after the subjects had some previous experience with the task on an earlier
session under placebo (27,28). Through the study of changes in regional cerebral
blood flow using PET, this has recently been attributed to task-related arousal
which serves to antagonize the de-arousing effects of clonidine more effectively
when the task is novel (29). This study also showed a significant drug X task
interaction in the right thalamus. We can expect many more sophisticated analyses
of this type to pin-point central loci of systemic
adrenoceptor agents on cognition in the future. It is of interest that
a study of the alpha-2 antagonist, idazoxan, actually
produced what appears to be the complementary effect to that of clonidine
in psychological terms, of increased attention to the location of the previous
cue (85).
Such
results are certainly have clinical potential (see also 5). One of the more
remarkable examples of "cognitive enhancement" by psychotropic drugs
has been the improvements in cognitive performance in Korsakoff patients by
clonidine (60). While this result is
not readily predicted by the effects reviewed here on normal volunteers, it
is possible that the presynaptic degeneration of NE neurons may enhance the contribution of post-synaptic effects
of the drug, in a way possibly reminiscent of the effects of clonidine in aged
monkeys reviewed above. These results hold some promise for the treatment of
other neurodegenerative disorders associated with ceruleo-cortical NE loss,
including Parkinson's and Alzheimer's diseases. The former is associated with
significant cognitive deficits, often resembling effects of frontal lobe damage,
but the possible role of the LC and therapeutic possibilities of treatment with
adrenoceptor agents remain largely unexplored (though see 13). While the cognitive
syndrome in Alzheimer's disease is probably multifactorial, it is possible that
a malfunctioning ceruleo-cortical system may make patients more susceptible
to the abrupt decline in cognitive status often associated with transfer to
a new environment such as a nursing institution.
A
major problem for further work in this area is to decide on the optimal method
for enhancing noradrenergic function using
drugs such as clonidine, guanfacine or idazoxan, which exert a balance of effects
at both pre- and post-synaptic receptors and also have additional effects on
other systems (e.g. via imidazoline or 5-HT receptors).
Experimental studies may help to resolve this potentially important clinical
issue. For example, systemic idazoxan has been shown to increase
extracellular NA in the rat prefrontal cortex, a finding that might encourage
the use of this drug to enhance potentially enhance impaired cortical function
via boosting noradrenergic transmission in patients with dementia. In one such study, specifically of three patients
diagnosed with dementia of the fronto-temporal type, improvements were observed
following systemic idazoxan in a number of tests (26), some of which are sensitive
frontal lobe dysfunction, as might have been predicted from the opposite effects
of clonidine in normal volunteers (27,28). However, performance on a test of
spatial working memory was impaired by this treatment, showing that executive
functions of the frontal lobe respond differentially to presumed noradrenergic
activation . The result is however consistent with the improvement seen at certain
doses of clonidine in spatial working memory tasks in both monkeys (7) and humans
(27). Moreover, patients with dementia of the Alzheimer type showed only deficits
with similar idazoxan treatment (45).
In
a more general sense, it is likely that possible
malfunctioning of the LC is important in those human disorders in which there
is an important interface between cognition and emotion, including depression,
post-traumatic stress disorder, anxiety and drug withdrawal states.
In some of these conditions, it is possible that the LC is overactive,
and , as we have seen , this may also result in cognitive problems, which may
exacerbate the emotional state. For example, it is plausible in panic disorder
that noradrenergic overactivity helps
to concentrate attention on dominant events or pathological cognitive schemas,
promoting their consolidation and thus slowing the extinction of their influence
over the subject's behavior. Some striking evidence in favour of this hypothesis,
which again brings out the utility of the earlier animal work on the neuromodulation of aversive memory by
mechanisms in the amygdala, has been the demonstration that the beta-1/2
receptor antagonist propanolol selectively impairs memory for emotional material
in human subjects (16,90), findings of potential clinical utility in the treatment
of post-traumatic stress disorder. A range of other disorders, including cognitive
aging, Korsakoff’s syndrome, depression, and certain forms of Attention Deficit
Hyperactivity Disorder may suffer, alternatively, from an underactivation of
coeruleo-cortical NE activity, requiring a different pharmacotherapeutic strategy that also targets the noradrenergic system (see above and
ref.5 for a fuller discussion).
We commented in the last version of this review
in 1995 (page 370) that “many of these
hypotheses will be tested in the clinical setting in the next generation of
research, when its heuristic value is expected to become apparent”. We believe
this statement to be ever more appropriate.
Acknowledgements
Our own research is supported by the Wellcome Trust and The Medical Research
Council. We thank our colleagues for their efforts and Dr. B.J. Cole for
kindly providing Figure
1.
published 2000