The Serotonin Hypothesis of Major Depression

Michael Maes and Herbert Y. Meltzer

The serotonin (5-HT) hypothesis of major depression has been formulated in three distinct ways. One version of this hypothesis is that a deficit in serotonergic activity is a proximate cause of depression. A second theory is that a deficit in serotonergic activity is important as a vulnerability factor in major depression. A third hypothesis, now of historical interest only, attributed increased vulnerability to major depression to enhanced serotonergic activity (51).

The last review of these hypotheses in this series (51) concluded that the available data on the role of 5-HT in major depression favored the hypothesis that a deficiency in brain serotonergic activity increases vulnerability to major depression. This review (51) summarized the following evidence: (a) Disorders in serotonergic activity could contribute to many of the symptoms of major depression, for example, mood, appetite, sleep, activity, suicide, sexual, and cognitive dysfunction. (b) Interference with 5-HT synthesis or storage may induce depression in some vulnerable individuals. (c) Abnormalities in serotonergic activity in depression could occur at one or more of several levels, for example, diminished availability of L-tryptophan (L-TRP), the precursor of 5-HT, impaired 5-HT synthesis, release, reuptake, or metabolism, or 5-HT postsynaptic receptor abnormalities. (d) Finally, antidepressant drugs may act, in part, by enhancing central serotonergic activity.

Since that review, there have been numerous developments shedding further light on the role of 5-HT in depression. This chapter reviews the highlights of serotonergic research in major depression since 1987, aiming to elucidate the role of 5-HT activity in the pathogenesis or pathophysiology of that illness. Because of limitations on references, secondary sources were frequently cited and some sources were left out entirely (see Serotonin and Behavior: A General Hypothesis and Indoleamines: The Role of Serotonin in Clinical Disorders).

This chapter discusses new findings on the role of 5-HT in the pathogenesis or pathophysiology of major depression and the mechanism of action of antidepressant drugs. Clinical studies of 5-HT metabolism in major depression that provide evidence for an abnormality of the 5-HT system are reviewed. New evidence that the availability of L-TRP, the rate-limiting step in the synthesis of 5-HT, is an important factor in the pathophysiology of depression and the response of antidepressant drugs are discussed. The evidence for abnormalities in the 5-HT uptake system in major depression as well as abnormalities in some of the numerous types of postsynaptic 5-HT receptors are reviewed. The 5-HT1A and 5-HT2 postsynaptic receptors appear to be of particular importance. Cooperative and competitive interactions may be important to the function of the 5-HT system and abnormalities in this regard are possible factors in the pathophysiology of major depression. The function of these postsynaptic receptors in depression has been assessed with a variety of pharmacological probes as well as postmortem studies. The evidence that antidepressants may act via their long-term ability to modulate pre- and postsynaptic serotonergic function is discussed. The important relationships between serotonergic activity and the hypothalamic– pituitary–adrenal (HPA) axis are reviewed. The possibility that disorders in the functional relationships between both systems and gender differences in 5-HT function may be involved in the pathophysiology of major depression are also discussed. Finally, the importance of studying interactions among 5-HT and other neurotransmitter systems in depression is stressed. Indeed, it seems doubtful that any one neurotransmitter is entirely responsible for the pathogenesis or pathophysiology of depression because of the extensive interactions between neurotransmitters at the levels of cell bodies as well as terminal regions. Nevertheless, 5-HT appears to be the most important monoamine relevant to the pathophysiology of depression and the action of antidepressant drugs (see Molecular Biology of Serotonin Receptors: A Basis for Understanding and Addressing Brain Function, Serotonin Receptor Subtypes and Liagands, Serotonin Receptors: Signal Transduction Pathways, Anatomy, Cell Biology, and Maturation of the Serotonergic System: Neurotrophic Implications for the Actions of Psychotropic Drugs, Electrophysiology of Serotonin Receptor Subtypes and Signal Tranduction Pathways, Serotonin and Behavior: A General Hypothesis, and Indoleamines: The Role of Serotonin in Clinical Disorders, for related discussion and background).

Total or free L-TRP and the ratio of L-TRP to the sum of competing amino acids (CAA), known to compete for the same cerebral uptake mechanism, provide measures of the availability of L-TRP to the brain and hence for 5-HT synthesis in the brain (42). There are several reports that plasma L-TRP availability is significantly lower in subjects with major depression than in normal controls or subjects with minor depression (42, 51). The data suggest that a lower plasma L-TRP/CAA ratio in depression is related to decreased concentrations of plasma L-TRP rather than to increases in CAA (42). Plasma L-TRP levels following ingestion of large oral doses of L-TRP or intravenous L-TRP have been reported to be lower in depressed patients (15, 28).

One hypothesis to explain lower plasma L-TRP concentrations and altered L-TRP pharmacokinetics in depression is enhanced catabolism of L-TRP in the liver by induction of pyrrolase, the first rate-limiting enzyme of the kynurenine-nicotinamide pathway (39). The activity of this pathway can be quantified by measuring 24-hr urinary excretion of xanthurenic acid after loading with 5 g of L-TRP (25, 39). In depression, L-TRP–induced xanthurenic acid excretion was significantly and negatively related to plasma L-TRP concentrations (39). The above findings lend support to the hypothesis that increased activity of this pathway may contribute to the lower plasma L-TRP concentrations and increased clearance in major depression (39).

Recent studies suggest that a reduction of dietary L-TRP can induce depressive symptomatology under some circumstances. In normal men, lowering of plasma L-TRP by dietary means has been reported to cause an acute lowering of mood, which was inversely related to postingestion plasma L-TRP levels (75). In recently remitted depressed patients receiving selective 5-HT reuptake inhibitors (SSRIs), acute L-TRP depletion coupled with ingestion of large concentrations of CAA led to a rapid clinically significant return of depressive symptoms, such as depressed mood, terminal insomnia, decreased appetite, loss of energy, loss of interest, anhedonia, decreased concentration, ruminative thinking, and a sense of worthlessness (16, 23). Remitted depressed patients maintained with tricyclic antidepressants, however, were less prone to a depressive relapse following L-TRP depletion. Because SSRIs enhance central presynaptic 5-HT activity (see the section below on neuroendocrine probes and antidepressive treatments), the findings strongly suggest that the synthesis of 5-HT from plasma L-TRP is necessary for the maintenance of remission induced by those drugs. Decreased plasma L-TRP concentrations are most likely related to lower central presynaptic 5-HT activity, because diets causing a decrease in plasma L-TRP availability also reduce cerebrospinal fluid (CSF) levels of 5-hydroxyindoleacetic acid (5-HIAA) in humans. Therefore, the behavioral and cognitive changes observed may be due to a deficiency in central presynaptic 5-HT elements, which may result from lower plasma L-TRP availability. The above results are consistent with the findings that anorexia, anergy, sleep disorders, cognitive disturbances, and depressed mood are psychopathological correlates of lowered L-TRP availability in depression (43). However, it is noteworthy that unmedicated depressed patients do not worsen following L-TRP depletion. The lack of worsening by further interference with 5-HT synthesis may suggest that decreased serotonergic activity is not the limiting factor in the severity of depression in untreated major depression. This is consistent with the hypothesis of diminished serotonergic activity as a vulnerability factor in depression.

Møller et al. (56) reported that the plasma ratio of L-TRP/CAA was inversely related to the response to antidepressive treatment, such as L-TRP, imipramine, amitryptiline, and lithium + L-TRP, and that major depressed subjects with lower L-TRP availability responded preferentially to treatment with SSRIs, such as citalopram and paroxetine. These results suggest that (a) plasma L-TRP availability may influence the therapeutic response to antidepressive treatment and may predict a favorable response to SSRIs; (b) lower plasma L-TRP availability may be related to lower central 5-HT activity; and (c) that antidepressive treatment with serotonergic drugs (L-TRP, L-TRP + lithium, SSRIs) may compensate for this deficit.

Blood platelets are able to take up, store, and release 5-HT by mechanisms that are sufficiently similar to those of central 5-HT neurons to render platelets a model for the 5-HT neuron (50). Several dozen studies of platelet 5-HT uptake in major depression have been published between 1971 and 1992; approximately 65% to 75% have reported significantly lower Vmax values in patients with major depression than in normal controls (50). However, studies on platelet 5-HT uptake lack specificity and sensitivity for clinical use on their own. The question of whether lower platelet 5-HT uptake indicates lower 5-HT uptake in the brain remains elusive.

There are now several reports on lower imipramine binding (Bmax) in platelets of depressed patients compared to normal controls (50). However, 3[H]imipramine binding is rather heterogeneous since this substance labels two separate binding sites: one high-affinity binding site that corresponds to the 5-HT uptake site and one low-affinity site that is unrelated to the 5-HT transporter (49). Paroxetine is a potent and selective inhibitor of 5-HT uptake in platelets and brain. Paroxetine is a superior ligand for labeling the 5-HT transporter in both platelets and brain compared to imipramine, because 3[H]paroxetine labels the 5-HT transporter more selectively than does 3[H]imipramine, while exhibiting a higher affinity (49). Several groups were unable to find significant differences in platelet paroxetine binding between depressed patients and controls (31).

Concentrations of 5-HIAA, the major 5-HT metabolite, in CSF have been extensively studied in depressed subjects, both in basal conditions and after blockade of its reabsorption by probenecid treatment (51). Meltzer and Lowy (51) concluded that it is very difficult to draw any valid conclusions on 5-HT turnover in depression on the basis of CSF 5-HIAA data. Several papers published after their 1987 review were also unable to find significant differences in CSF 5-HIAA between major depressed subjects and normal controls (63). In fact, there is more evidence that low CSF 5-HIAA levels are related to (violent) suicidal behavior and to violence-impulsivity rather than to depression per se (17).

The possibility that peripheral abnormalities in 5-HT metabolism occur in melancholic patients has been further investigated by the study of the 5-HT content of plasma and platelets. Platelet 5-HT is considered to represent a reserve pool of peripheral 5-HT (64). Plasma 5-HT has a turnover rate considerably higher than the platelet pool and represents the equilibrium between 5-HT secretion, 5-HT catabolism (by monoamine oxidase enzyme activity in liver and lung), and platelet uptake mechanisms (64). Lowered plasma and platelet 5-HT contents in depression have been reported by several groups (64). Celada et al. (7) reported that chronic treatment with fluvoxamine decreased platelet 5-HT content and that responders had significantly lower platelet 5-HT content pretreatment. These authors suggest that SSRIs may be most useful in patients with low platelet 5-HT. The significance of the above findings for central serotonergic activity is unknown.

Reports of 5-HT and 5-HIAA concentrations in the brain of suicide victims have yielded conflicting results: some found decreased levels or no changes in 5-HIAA concentrations in the brain of depressed suicides, whereas others reported increased 5-HIAA levels in the hippocampus or amygdala of (depressed) suicide victims (10). Taken together, the above results offer little support that 5-HT turnover is reduced in depressed subjects who have committed suicide. Moreover, the effect of drug treatments, substance abuse, glucocorticoid elevations, and receptor sensitivity alterations make it difficult to interpret the results.

Some, but not all, groups described a reduction in imipramine binding in the frontal cortex of suicides (20). The latter group also found an increase in imipramine binding sites in the hippocampus of suicide victims. No significant differences in paroxetine binding sites of several brain areas could be detected between normal controls and suicide victims in whom a diagnosis of depression was made (30). Leake et al. (32) found lower 3[H]citalopram binding in the brains from depressed subjects.

Three studies of 5-HT2 binding in the blood platelets of depressed patients and normal controls, found increased 5-HT2 binding (Bmax) in the former, but one study did not (2). Increased 5-HT2 binding (Bmax) in depressed patients is also supported by the increased 5-HT2 receptor functional response as measured by phosphoinositide turnover and 5-HT–induced platelet aggregation (55). Mikuni et al. (55) reported that the effects of 5-HT to increase intracellular calcium in platelets was greater in depressed patients than in controls, which is consistent with the hypothesis of 5-HT2 receptor up-regulation in depression.

A new and original method to assess central 5-HT2 function is the assessment of slow-wave sleep (SWS) after challenge with 5-HT2 receptor antagonists. Blockade of 5-HT2 receptors is normally accompanied by an increase in SWS (69). Sharpley et al. (67) found that the normal increase in SWS following treatment with cyproheptadine, a nonspecific 5-HT receptor antagonist, was absent in major depressed patients maintained on tricyclic antidepressants. Staner et al. (69) found that ritanserin enhanced SWS stage 3 in normal controls and depressed subjects, but the latter group showed significantly lower SWS in stage 4. These findings are consistent with 5-HT2 receptor up-regulation in patients with major depression (69).

There are now several reports of increased 5-HT2 receptor-binding sites in the frontal cortex of (depressed) suicide victims or depressed subjects who died from natural causes (1, 2). Other laboratories found a trend toward or a significant decline in 5-HT2 binding in membrane homogenates from the prefrontal cortex of suicide victims (11). However, differences among the above studies may be due to drug effects (treatment with antidepressants and antipsychotic agents reducing 5-HT2 binding), use of different ligands, the postmortem interval, or the heterogeneity of psychiatric illnesses (e.g., schizophrenia, alcoholism) and personality (e.g., borderline and antisocial) disorders associated with suicide.

Increased 5-HT1A binding in the prefrontal cortex of nonviolent suicides compared to violent suicides and controls has been reported (48). Several other studies reported no significant differences in the number or affinity of 5-HT1 binding sites in the frontal or temporal cortex between drug-free depressed suicide victims and controls (12). The number of 5-HT1 binding sites was significantly lower in hippocampus, whereas the affinity of 5-HT1 binding sites was significantly lower in the amygdala (12). These results may provide some evidence of increased cortical 5-HT1 receptors in (depressed) suicides, and decreased density of 5-HT1 receptors in the hippocampus and amygdala of depressed subjects. These findings need to be further explored using more specific ligands, autoradiography, and with attention to variables such as type and severity of depression, concomitant alcoholism or other drug abuse, the effects of suicide per se, and so on.

One strategy to assess central serotonergic neurotransmission in vivo is the measurement of HPA-axis hormone, prolactin, growth hormone, and other responses following the administration of 5-HT precursors and direct or indirect 5-HT agonists (38, 54). Secretion of these hormones is, in part, regulated by 5-HT inputs, and their responses to the acute administration of 5-HT agents are mediated at least in part by 5-HT mechanisms (54).

There is strong evidence suggesting that 5-HT1A, 5-HT1C, and 5-HT2 receptors may stimulate cortisol and prolactin secretion in man (52). Various neuroendocrine (behavioral and electrophysiological) studies with 5-HT agonists and antagonists have provided evidence for important interactions between 5-HT1A and 5-HT2/1C receptors (52). In rats, there is some evidence that 5-HT2/ 5-HT1C receptors may modulate 5-HT1A-related behaviors. 5-HT1A receptors may provide inhibitory effects on 5-HT2 receptor-mediated functions. These findings suggest the possibility of a signal transduction from 5-HT1C/5-HT2 to 5-HT1A receptors in the expression of 5-HT1A-mediated behaviors, together with an inhibitory effect of 5-HT1A receptors on 5-HT2-mediated functions. Cooperation among those receptors has important implications for the interpretation of neuroendocrine challenge studies with serotonergic agents (52). In particular, our laboratory has provided some evidence that blunted prolactin responses to challenge with 5-HT precursors or agonists may be due to diminished 5-HT1A or 5-HT1C/5-HT2 receptor sensitivity, whereas enhanced cortisol responses might be from increased sensitivity of 5-HT1C/2 receptors which would be unaffected by 5-HT1A receptor subsensitivity (52).

Ipsapirone administration significantly increases HPA-axis hormone secretion in normal men and rodents (34). Major depressed subjects show blunted HPA-axis hormone [adrenocorticotropic hormone (ACTH) or cortisol] responses following ipsapirone challenge compared to normal controls (34; also Meltzer and Maes, unpublished). Therefore, these attenuated responses may be interpreted to indicate that major depression is characterized by a down-regulation or hyporesponsivity of postsynaptic 5-HT1A receptors.

Buspirone is another azapirone which is a partial agonist at 5-HT1A receptors; its acute administration evokes dose-related HPA-axis and prolactin responses (53). The prolactin response is blocked by pindolol, suggesting it is 5-HT1A-mediated. Our laboratory found no significant differences in buspirone-induced cortisol or prolactin responses between major depressed subjects and normal controls. The reason for the discrepancy between the cortisol responses to ipsapirone and buspirone in major depression requires further study. It is possible that differences in the intrinsic activity as partial 5-HT1A agonist may be relevant. It is likely that the prolactin response to buspirone is mediated by DA2-receptor blockade rather than 5-HT1A agonism. Further efforts to employ buspirone-induced cortisol or prolactin responses as probes of 5-HT1A function in depression appear to be of limited value.

Intraperitoneal or oral administration of high doses of 5-HT precursor (5-HTP) causes a marked increase in corticosterone secretion in rodents, whereas the effects of oral 5-HTP on HPA-axis hormone secretion in humans are somewhat more variable in studies using lower doses of the racemic mixture (D, L) and/or enteric coated tablets (52). There is now evidence that 200 mg L-5-HTP, in nonenteric coated tablets, reliably stimulates HPA-axis hormones and prolactin secretion in normal humans, and that 5-HTP-induced activation of both HPA-axis and prolactin secretion are probably related to 5-HT mechanisms (52). Significantly higher 5-HTP (D, L: 200 mg; L: 125 to 200 mg) -induced cortisol responses were observed in major depressed subjects than in normal controls or minor depressed subjects (38, 54). The use of 5-HTP as a 5-HT probe was challenged, because administration of very high doses of 5-HTP in rodents may lead to 5-HT synthesis in central catecholaminergic neurons and may increase synthesis of catecholamines (73). The dose of L-5-HTP used in human studies, however, is much lower than that needed to increase catecholamine turnover in animal studies. Because 5-HT1A postsynaptic receptors are probably down-regulated in major depression, the above findings may be explained either by supersensitive 5-HT2 or 5-HT1C receptors. Since several types of studies (reviewed here) indicate increased 5-HT2 receptor binding or disorders in 5-HT2–related behaviors in major depression or suicide, whereas there is no specific evidence as yet for 5-HT1C receptor supersensitivity in depression, it may be suggested that the results of the studies with 5-HTP as challenger are compatible with up-regulation or supersensitivity of 5-HT2 postsynaptic receptors (52).

Administration of L-TRP reliably increases prolactin secretion (58). Several papers reported blunted prolactin responses to intravenous L-TRP in depression (15, 28). The blunted prolactin responses to L-TRP may reflect abnormalities in the synthesis of 5-HT from L-TRP, its release or reuptake, or decreased responsivity of postsynaptic 5-HT receptors that may mediate the serotonergic influence on prolactin secretion (e.g., 5-HT1A postsynaptic receptors). There is as yet no evidence from studies with direct-acting 5-HT agonists for a blunted prolactin response in depression.

Higher doses of L-TRP also increase corticosterone secretion in rodents (18). Our laboratory has reported significantly increased 24-hr urinary free cortisol (UFC) secretion in melancholic subjects than in normal controls or minor depressed patients after loading with L-TRP (2 or 5 g orally) (46). This finding is consistent with hyperresponsivity of 5-HT2 postsynaptic receptors.

However, the use of L-TRP as a 5-HT probe was challenged: (a) One study found that, after controlling for differences in L-TRP plasma concentrations, the differences in prolactin responses between depressed subjects and normal controls disappeared (28). Thus, blunted TRP-induced prolactin responses, in fact, may result from disorders in L-TRP disposition in major depression. (b) L-Tryptophan may be acting nonspecifically, that is, not via 5-HT (73). Indeed, administration of 2 to 5 g of L-TRP significantly decreased various indices of catecholaminergic turnover (26, 46, 73). This may be explained by the capacity of L-TRP to decrease L-tyrosine availability to the brain and the transport of tyrosine through the blood–brain barrier, which may cause a decrease in brain noradrenergic turnover (46). (c) Finally, administration of L-TRP produces only small increases in 5-HT formation but very important increments in metabolites of the nicotinamide pathway, which may exert pharmacological actions (26, 27).

D,L-Fenfluramine promotes a rapid release of 5-HT, inhibits its reuptake, and may function as an indirect 5-HT receptor agonist. In rats and humans, D,L-fenfluramine produces a dose-dependent increase in prolactin secretion. Most but not all laboratories (36, 41) have reported significantly blunted D,L-fenfluramine-induced prolactin responses in major depressed subjects compared with controls. However, because the L-isomer may block striatal DA receptors and increase DA turnover, a combination of serotonergic and dopaminergic effects could account for blunted D,L-fenfluramine–induced prolactin responses in major depression (58).

D-Fenfluramine stimulates the serotonergic system more specifically than the racemic mixture; D-fenfluramine–induced prolactin secretion may be mediated via 5-HT1A receptors as well as 5-HT2/5-HT1C receptors. Maes et al. (41) were unable to detect any significant differences in post–D-fenfluramine (45 mg orally) prolactin responses between healthy controls and major depressed subjects. O'Keane and Dinan (60), on the other hand, found that plasma prolactin responses after D-fenfluramine (30 mg orally) were significantly lower in major depressed patients than in control groups. In conclusion, there is some agreement that prolactin responses to D,L- and D-fenfluramine administration may be blunted in major depressed subjects compared to normal controls. These findings could be explained by a combination of decreased availability of 5-HT, increased inactivation or diminished 5-HT1A postsynaptic receptor sensitivity.

The clomipramine probe assesses central 5-HT activity through the assay of prolactin responses following acute, intravenous challenge with clomipramine. There are now several publications reporting blunted clomipramine-induced prolactin responses in major depressed subjects compared to healthy controls (19). Blunted prolactin responses to clomipramine are not attributable to diminished prolactin secretory capacity in anterior pituitary, because prolactin responses to thyrotropin-releasing hormone, which acts directly on the pituitary to release prolactin, were normal in those patients. In conclusion, the clomipramine-challenge findings are in agreement with the blunted prolactin responses after challenge with L-TRP, D,L-fenfluramine, or D-fenfluramine.

Enhanced prolactin responsivity to TRP challenge has been found following various antidepressive treatments, for example, amitriptyline, desipramine, fluvoxamine, clomipramine, tranylcypromine, and tricyclics with lithium (9). Fenfluramine-induced prolactin responses were significantly increased following therapy with imipramine, clomipramine, and amitryptiline or fluoxetine (61). Electroconvulsive therapy may or may not (61) enhance the prolactin responses to fenfluramine. Prolactin responses to clomipramine were significantly enhanced by short-term lithium treatment. Shapira et al. (66) and Upadhyaya et al. (72) provided some evidence that this enhancement of serotonergic function by antidepressive treatments represents a true correction of an underlying serotonergic deficit and not a continued effect of antidepressant treatment or a manifestation of medication withdrawal. In conclusion, it appears that various antidepressive treatments share the capacity to enhance central pre- or postsynaptic 5-HT activity.

Several studies have shown that long-term treatment with tricyclic antidepressants or monoamine oxidase inhibitors leads to down-regulation in the number of 5-HT2 receptor binding sites in the brains of rodents (13). A further observation is that the time course of this 5-HT2 receptor down-regulation following antidepressive treatment is similar to that of clinical response in depressed patients. Some, but not all (e.g., citalopram) SSRIs produce adaptive changes that manifest themselves by a decreased responsiveness of 5-HT2 receptors (35). Chronic treatment with 5-HT1A agonists also may induce 5-HT2 receptor down-regulation (35). Electroconvulsive therapy, on the other hand, has been shown to increase the number of 5-HT2 binding sites in the brain (33). Most antidepressive drugs reduce 5-HT2 receptor mediated behaviors in the rodent (e.g., head-twitch response), whereas electroconvulsive therapy may increase 5-HT2-related behavior (13). One study has shown that increased 5-HTP–induced cortisol responses in depressed patients were normalized after chronic antidepressive treatment (54). It may be argued that the above effects of antidepressives on 5-HT2 receptors are probably due to rapid desensitization or down-regulation following agonist stimulation or to the 5-HT2 receptor-blocking properties of some antidepressive drugs (35).

Effects of antidepressive treatments on 5-HT1–receptor binding or functioning are rather conflicting. Chronic treatment with some monoamine oxidase inhibitors, SSRIs, typical antidepressants or electroconvulsive therapy may decrease 5-HT1 binding sites, responsiveness of 5-HT1A presynaptic receptors, or 5-HT1A postsynaptic receptor-mediated behaviors (47, 70). Hayakawa et al. (22) found that repeated treatment with electroconvulsive therapy causes an upregulation of postsynaptic 5-HT1A receptors in the hippocampus. Chronic treatment with desipramine or amitryptiline resulted in a functional up-regulation of 5-HT1A-receptor-mediated behaviors in rats or increased postsynaptic 5-HT1A receptor binding in the hippocampus (37, 74).

Blier and colleagues (3) have thoroughly investigated the effects of short- and long-term treatment with various antidepressants on pre- and postsynaptic electrophysiological properties of 5-HT neurons. Long-term treatment with tricyclic antidepressants and electroconvulsive therapy appears to increase the sensitivity of postsynaptic 5-HT receptors, although no long-term effects on basal firing rate or autoreceptor-induced inhibition of 5-HT turnover are observed. Postsynaptic sensitization to 5-HT occurs probably in the hippocampus, suprachiasmatic nucleus, and somatosensory cortex (3). Blier et al. (3) have provided some evidence that this postsynaptic sensitization to 5-HT is, at least in part, attributable to the increased number of 5-HT1A binding sites in the hippocampus, cerebral cortex, and septum. Moreover, the time course for developing sensitization to 5-HT is consistent with the delayed activity of tricyclic drugs in relieving the symptoms of depression (3). Administration of monoamine oxidases and SSRIs appear to enhance 5-HT release per impulse from desensitization of the terminal autoreceptor (probably the 5-HT1B in rodents or the 5-HT1D receptor in humans). In conclusion, rodent studies indicate that various antidepressive treatments induce a gradual development of increased 5-HT activity; the mechanisms by which this enhancement is achieved may be different for these treatments.

Increased activity of the HPA-axis has been consistently reported in severe depression. There is converging evidence from various studies that major depression is characterized by a moderately increased spontaneous HPA-axis function and by a failure to suppress plasma intact ACTH (the 1–39 sequence) and cortisol with the 1 mg dexamethasone suppression test (DST) (45). It is assumed that the above disorders are related to (a) central corticotropin releasing hormone (CRH) hypersecretion; (b) potentiating effects of increased arginine vasopressin (AVP) secretion on CRH-induced ACTH secretion; and (c) subsensitivity in negative feedback by glucocorticoids, which may be related to down-regulation of glucocorticoid receptors (GR) or mineralocorticoid receptors (MR) in the hippocampus, which, in turn, may be induced by sustained exposure to high concentrations of glucocorticoids.

One major hypothesis to relate the HPA axis to serotonergic dysfunction in depression is that lowered plasma L-TRP availability may be related to activation of the kynurenine-nicotinamide pathway in the liver due to induction of liver pyrrolase by glucocorticoids (42). This hypothesis is supported by the following findings: in depression there is a significant inverse relationship between plasma L-TRP or L-TRP/CAA values and post-DST cortisol values (44); in rats, administration of a cortisol suppression dose of dexamethasone results in significantly augmented liver pyrrolase activity and a decreased availability of L-TRP to the brain (57); dexamethasone (1 mg, orally) administration also significantly reduces L-TRP plasma levels in normal controls and minor and major depressed subjects (42).

The hippocampus has been demonstrated to be a site of serotonergic innervation associated with CNS control of the HPA-axis. A good correlation exists between the concentrations of cellular receptors for 5-HT and glucocorticoids. There is now compelling evidence that glucocorticoids may accelerate 5-HT synthesis and turnover in the brain of rodents (8). Increased central 5-HT turnover is, in part, caused by glucocorticoid or CRH-mediated induction of tryptophan hydroxylase (68). In humans, glucocorticoids may also augment central 5-HT turnover; some groups found that TRP-induced prolactin responses were significantly higher after dexamethasone administration and found increased levels of CSF 5-HIAA after administration of dexamethasone in a group of psychiatric patients (71).

It has been suggested that glucocorticosteroid hypersecretion in major depressed subjects may down-regulate the sensitivity of postsynaptic 5-HT1A receptor signal transduction or of the 5-HT1A receptor itself (15, 34, Meltzer and Maes, unpublished). Increased baseline cortisol secretion was shown to be related to diminished ipsapirone-induced cortisol responses (34, Meltzer and Maes, unpublished). Hypercortisolemia may also explain the impaired D,L-fenfluramine and L-TRP–induced prolactin responses (15, 61). This hypothesis is supported by experimental data showing that chronic exposure to glucocorticoids may alter 5-HT1A receptor-mediated functions or behaviors in rodents and that adrenalectomy may increase the number of 5-HT1A binding sites (8).

It has been shown that both ACTH and corticosterone administration may increase the number of 5-HT2 receptors in the neocortex of forebrain and 5-HT2–mediated behavioral responses in rodents (29, 62). These findings suggest that HPA-axis hyperactivity in depression may contribute to upregulated 5-HT2 receptor density in the prefrontal cortex (29).

The role of 5-HT in stimulating the HPA axis encompasses effects on CRH by activation of 5-HT1A and 5-HT2/5-HT1C receptors and on AVP by activation of 5-HT2 receptors (5, 18). Recently, it has been demonstrated that serotonergic structures may modify glucocorticoid negative-feedback effects on HPA-axis function. Seckl and Fink (65) found that depletion of 5-HT in hippocampal structures may attenuate the negative-feedback effects of glucocorticoids on the HPA axis through reduced expression of GR or MR. In depression, a significant negative correlation between plasma L-TRP availability and baseline cortisol-adjusted post-DST cortisol values was found, suggesting that lower presynaptic 5-HT activity is related to escape of negative-feedback inhibition (44). This suggests the possibility that a diminished central 5-HT neurotransmission in major depression may attenuate the hippocampal negative-feedback control over the HPA axis, thus inducing excessive corticosteroid secretion. Treatment with L-TRP (3.5 to 7 g/day) for 1 to 2 weeks has been shown to improve DST nonsuppression in depressed subjects (59). Therefore, it may be hypothesized that TRP treatment may have restored the serotonergic deficit in the hippocampus, thus increasing the negative feedback over the HPA axis. Treatment with fluoxetine and imipramine may also increase the level of MR messenger ribonucleic acid (mRNA), thus increasing the efficacy of the negative feedback on hypothalamic CRH mRNA (4). Other results may indicate that upregulation of postsynaptic 5-HT2 receptors in major depression is related to escape of ACTH/cortisol secretion from negative-feedback effects; compared with patients who had minor depression, those with a diagnosis of major depression exhibited a significant enhancing effect of L-5-HTP (125 to 200 mg) on post-DST ACTH or cortisol values, although L-5-HTP converted DST cortisol or ACTH suppression into nonsuppression in some major depressed subjects (40).

There are several reports suggesting that there are gender differences in peripheral and central 5-HT metabolism. Preclinical data suggest that female rats have a higher activity of 5-HT synthesizing enzymes, a greater storage capacity for 5-HT in brain 5-HT neurons, a more pronounced 5-HT behavioral syndrome in response to 5-HT agonists, and higher brain and CSF levels of TRP, 5-HT, and 5-HIAA compared to males (6). Animal data show that female rats exhibit a larger prolactin response to 5-HT1A receptor agonists than male rats (21).

Significantly lower fasting plasma L-TRP levels are found in female control subjects (42). Delgado et al. (16) found that males maintained their plasma free and total TRP levels closer to baseline values in response to TRP depletion. Our laboratory found that buspirone evoked a significantly greater prolactin response in women than in men (53). Delgado et al. (16) found that males demonstrated smaller prolactin responses to L-TRP infusion than did female subjects.

In depression, plasma total L-TRP levels tend to be more reduced in female than in male patients (42). In female major depressed subjects, but not in males, there is a significant negative correlation between self-rated depression and plasma levels of total L-TRP (42). Depressed females show significantly higher xanthurenic acid excretion following L-TRP loading than depressed males (39). Major depressed females exhibit significantly higher L-5-HTP-induced cortisol responses than male major depressed subjects (38).

Some of these gender related differences in 5-HT metabolism may perhaps be explained by the fact that liver pyrrolase activity is greater in women and that 5-HT receptors appear to be estrogen sensitive. Curzon's group (14) found that female rats, as opposed to male rats, failed to adapt to repeated restraint stress. Maladaption in female rats was associated with greater corticosterone response and defects in 5-HT1A receptor-mediated behavior.

The above review has provided some evidence that among the biological factors predisposing a person to major depression, alterations in presynaptic 5-HT activity, alterations in postsynaptic 5-HT2 and 5-HT1A receptors in the brain, and reciprocal relationships between dysfunctions in these systems and the HPA axis may be of special importance. Table 1 summarizes the various findings on peripheral and central and pre- and postsynaptic 5-HT activity in major depression.

There are several arguments to support a deficient 5-HT presynaptic activity: lower availability of plasma L-TRP to the brain; induction of depressive symptomatology by L-TRP depletion techniques; the relationship between lower L-TRP levels and positive response to serotonergic antidepressive treatments; lower L-TRP, 5-HT, and 5-HIAA in postmortem tissues of some depressed suicide victims; blunted L-TRP, D,L-fenfluramine, D-fenfluramine, or clomipramine-induced prolactin responses; antidepressive-treatment–induced increases in L-TRP-, D,L-fenfluramine-, or electroconvulsive-therapy–stimulated prolactin responses; and antidepressive–treatment–induced increments in presynaptic 5-HT activity.

Major depression is characterized by an increased number, affinity, or responsivity of central postsynaptic 5-HT2 receptors. This evidence comes from higher 5-HT2 receptor binding in platelets of major depressed subjects and in the prefrontal cortex of depressed suicide victims; lower 5-HT2 antagonist-induced SWS; increased HPA-axis responses to L-TRP and (L)-5-HTP; and antidepressive–treatment–induced decrements in 5-HT2 binding and 5-HT2-related behavioral or hormonal responses.

Major depression is accompanied by down-regulated or desensitized postsynaptic 5-HT1A receptors. This hypothesis is corroborated by attenuated ipsapirone-induced HPA-axis hormone responses; lower hippocampal 5-HT1 receptor binding in postmortem brain; blunted prolactin responses to L-TRP, fenfluramine, or clomipramine; and sensitization or up-regulation of 5-HT1A postsynaptic receptors by chronic antidepressive treatment with tricyclic antidepressants and electroconvulsive therapy.

At present, it is difficult to conclude whether presynaptic 5-HT hypoactivity and changes in 5-HT2 or 5-HT1A postsynaptic receptor function are causally related. First, lesioning of serotonergic neurons has been reported to enhance, not decrease, certain responses of 5-HT1A receptors (24). Second, there are now several data that suggest that an increase in 5-HT2 binding does not represent a compensatory up-regulation of postsynaptic elements in response to deficiencies in the presynaptic neurons innervating cortical targets (29, 35). It may be hypothesized that desensitized postsynaptic 5-HT1A receptors could diminish the 5-HT1A–mediated inhibition of 5-HT2 receptor responsivity, leading to an augmentation of 5-HT2 receptor responsivity. In addition, the decrease in presynaptic 5-HT activity may be a factor preventing the restoration of normal 5-HT2 receptor responsivity.

Disorders in both peripheral and central 5-HT metabolism and HPA-axis hyperactivity may be interrelated phenomena, which participate in the pathophysiology of major depression. Diminished central hippocampal serotonergic activity may result in elevated central and peripheral HPA-axis activity due to lowered hippocampal negative feedback by GR or MR on hypothalamic CRH. Increased CRH secretion may stimulate HPA-axis activity and increased glucocorticoid levels may be involved in further down-regulation of GR or MR, defective 5-HT1A postsynaptic receptor signaling pathways and maybe up-regulation of 5-HT2 receptors. Supersensitive 5-HT2 receptors in limbic structures or in the hypothalamus may sustain 5-HT–related HPA-axis hyperactivity, through stimulatory effects on CRH and AVP secretion and an enhanced negative-feedback breakthrough secretion of pituitary ACTH. Increased cortisol secretion may further compromise central serotonergic activity, by lowering L-TRP availability through induction of the liver-pyrrolase pathway. Other putative effects of HPA-axis hormones may be regarded as compensatory mechanisms that try to restore a lowered central presynaptic 5-HT activity, for example, increased 5-HT turnover. The latter could also explain the more conflicting results on central presynaptic 5-HT activity in major depression.

The gender-related differences in peripheral and central 5-HT metabolism, together with the greater susceptibility of 5-HT and HPA-axis systems to environmental stressors in females, could contribute to the higher incidence of major depression in females.

Although much has been learned about serotonergic dysfunction in major depression since 1987, it is clear that there is no simple answer to the question of whether altered 5-HT activity is directly related to the pathogenesis or pathophysiology of major depression or whether it acts as a vulnerability factor in that illness. Future research on serotonergic activity in depression might focus on the following issues.

The availability of L-TRP to the brain, which may be the rate-limiting factor in the synthesis of 5-HT in patients with major depression remains an important issue. Further study is needed of the conversion of a TRP load to 5-HT versus the products of the kynurenine-nicotinamide pathway. The transport of L-TRP into the brain needs to be studied. The findings of Møller (56) of a low-plasma L-TRP to CAA ratio predicting clinical response to serotonergic antidepressive drugs needs confirmation. The basis for the failure of L-TRP depletion to exacerbate major depression in untreated patients should be clarified.

Further studies are needed of postmortem indices of 5-HT function, such as brain 5-HT and 5-HIAA concentrations; 5-HT1A, 5-HT1C, and 5-HT2 receptor binding; and second messenger systems in depressed patients who died of natural as well as suicide causes. They must control for gender, age, drug treatment, substance use or abuse, seasonality in 5-HT function, comorbidity with, for example, anxiety, personality, or impulse control disorders, and glucocorticoid elevation.

There are only a few studies using single photon emission computed tomography (SPECT) or positron emission tomography (PET) with serotonergic markers in depression (e.g., 125I-ketanserin). The results of these studies are difficult to interpret for a variety of reasons. More SPECT or PET scan studies with ligands that are relatively specific for 5-HT2/5-HT1C or 5-HT1A sites and the 5-HT transporter in depressed patients prior to and after remission are needed.

Until now, there have been few neuroendocrine studies using direct agonists at 5-HT2/5-HT1C sites (e.g., MK-212, mCPP) in major depression. The finding of hyporesponsiveness of cortisol to the 5-HT1A agonist ipsapirone needs further replication. More research should be focused on the possible cooperation between 5-HT2/5-HT1C and 5-HT1A receptors. One strategy to investigate this cooperation between 5-HT receptors in major depression is neuroendocrine challenge (HPA-axis hormone and prolactin assays) after administration of L-5-HTP in depressed patients and normal controls pretreated with pindolol or ritanserin.

The relationships between HPA-axis hyperactivity and peripheral and central 5-HT turnover in major depression await further elucidation. More information on the following topics is needed to fully delineate the 5-HT/HPA-axis hypothesis: effects of glucocorticoids on L-TRP transport through the blood–brain barrier, and the uptake of 5-HT, and imipramine and paroxetine binding to blood platelets. Additionally, the effects of glucocorticoids at 5-HT1A and 5-HT2/5-HT1C receptor sites need further exploration through neuroendocrine or imaging studies.

Finally, comprehensive studies of the relevant monoamine systems, such as norepinephrine and dopamine, and g-aminobutyric acid which may interact with 5-HT and each other to cause depression, must be studied using the techniques described above.

Because multiple variables must be measured, large sample sizes and multiple cooperating laboratories will be needed. This could be the goal of a national or international collaboration.

published 2000