Serotonergic responsivity in male young adults with autistic disorder. Results of a pilot study

New evidence for a loss of serotonergic nerve terminals in rats treated with d,l-fenfluramine

Fenfluramine has been classified as a neurotoxin because animals treated with this anorectic lose 5-HT uptake sites located on serotonergic nerve terminals. However, there are two possible bases for this finding: either uptake sites are lost because the terminals themselves have been destroyed (neurotoxicity); or uptake sites are lost from otherwise intact terminals. To distinguish between these possibilities, we established an animal model in which male Wistar rats were injected (intraperitoneally) with an irreversible 5-HT uptake site antagonist (EEDQ). Since their 5-HT sites were inhibited (blocked) non-competitively, by this agent, such animals had effectively lost 5-HT uptake sites from intact serotonergic terminals. Synaptosomes prepared from such animals showed the predicted reduction in the Bmax of [3H]paroxetine binding to the 5-HT uptake site, and a reduction in the Vmax of [14C]5-HT uptake. However, they showed no significant reduction in maximal [14C]5-HT loading (alpha) compared with synaptosome from sham-injected controls. In contrast, fenfluramine-treated animals showed reduced [3H]paroxetine binding, reduced maximal [14C]5-HT uptake and significantly (P < 0.02) reduced synaptosomal [14C]5-HT loading. Therefore, the results suggest that fenfluramine does indeed cause the destruction of serotonergic nerve terminals.

Prolactin response to fenfluramine and placebo challenge following maintenance pharmacotherapy withdrawal in remitted depressed patients

Plasma prolactin (PRL) response to fenfluramine (FF) (60 mg orally) and placebo challenge was examined in eight remitted depressed patients who were withdrawn for 14 days from maintenance pharmacotherapy with clomipramine (CMI) plus lithium carbonate (Li) (n = 6) or Li alone (n = 2), 6 months after recovering from their major depressive episode. The patients had undergone identical FF challenge tests while drug free prior to commencing treatment with electroconvulsive therapy (ECT) (n = 4) or CMI supplemented with Li (n = 4) and after completing the above treatments. PRL response to FF in the remitted, drug-free state was significantly enhanced compared to the response prior to treatment (while depressed and drug-free) and not significantly different from the response following treatment with ECT (n = 4) or CMI plus Li (n = 4) 6 months before. Other work of a similar nature supports the view that enhanced serotonergically mediated hormone release in drug-withdrawn, remitted depressives, represents a long-standing change in central serotonergic responsiveness and not a continued effect of antidepressant treatment or a manifestation of medication withdrawal.

Receptor inhibition by immunoglobulins: specific inhibition by autistic children, their relatives, and control subjects

Forty-two parents of children with autistic disorder, 15 children with autistic disorder, 17 siblings of children with autistic disorder, and 12 unrelated normal adult controls were studied to determine if immunoglobulins isolated from their plasma would inhibit binding of the 5HT1A agonist, [3H]-8-hydroxy-N,N-dipropyl-2-aminotetralin (DPAT) to 5HT1A receptors in human hippocampal membranes. There were no significant differences among the means of percentage inhibition of DPAT binding of parents, children with autistic disorder, siblings, or unrelated controls. In addition, there were no differences in the proportion of subjects with > 15% DPAT inhibition among autistic children, their parents, their siblings, or unrelated controls. Immunoglobulin inhibition was not specific for the 5HT1A receptor binding site, since immunoglobulins inhibited binding to 5HT2, D1, D2, and alpha 2-adrenergic binding sites. The immunoglobulins isolated from normal controls inhibited [3H]-rauwolscine binding at alpha 2-adrenergic sites less than immunoglobulins of children with autistic disorder and their parents and siblings. This study did not support the hypothesis that autoantibodies to 5HT1A or 5HT2 receptors are characteristic of autistic disorder.

Demonstration of abnormal cyclic AMP production in platelets from patients with fragile X syndrome

Cyclic AMP production was studied in platelets from 31 patients with fragile X syndrome, 16 patients with mental retardation, 4 patients with autistic disorder, and 57 control individuals. 1-isobutyl-3-methylxanthine (IBMX) was used to inhibit phosphodiesterase; prostaglandin E1 (PGE1), to stimulate cAMP production via a receptor-dependent mechanism, and forskolin (FSK), to directly activate the catalytic subunit. Cyclic AMP production in IBMX, PGE1 + IBMX, and FSK + IBMX was 50% (P < 0.05), 65% (P = 0.001), and 53% (P = 0.001), respectively, in fragile X platelets relative to controls. Cyclic AMP production was not statistically different from controls in patients with mental retardation or autistic disorder. There was no effect of age or sex on cAMP production. Dose response curves suggested that abnormal cAMP production was due to diminished maximal response rather than altered potency of stimulating agents. The data presented here demonstrate that diminished cAMP production exists in platelets from patients with fragile X syndrome. Thus, defective functioning of cAMP-mediated regulatory signalling pathways in fragile X brain may contribute to the mental deficiency in these patients.

Neuroendocrine probes of serotonergic function: a critical review

Neuroendocrine probes of serotonergic function are based on the premise that serotonin (5-HT) exhibits an excitory influence on the release of certain anterior pituitary hormones and that the extent of release of these hormones following a challenge with a 5-HT agonist would provide an index of central 5-HT activity. This paper critically reviews studies published to date on healthy volunteers to assess the validity of tests of serotonergic function. Using standardized criteria, it is concluded that although extensive data is available on 5-HT-neuroendocrine probes, there is no test that is currently available that would fulfil all the proposed requirements for a test to be acceptable, although ipsapirone, m-chlorophenylpiperazine (m-CPP), and possibly fenfluramine challenge tests come very close. The rapid development and availability of more specific and stereo-selective 5-HT agonists/antagonists seem to indicate that it will not be too long before a valid neuroendocrine test for the assessment of serotonergic function emerges.

Platelet serotonin studies in hyperserotonemic relatives of children with autistic disorder

Platelet serotonin (5-HT) studies were conducted with 12 hyperserotonemic and 12 normoserotonemic age-, sex-, and relationship-matched relatives of autistic probands. Each group consisted of 7 mothers, 4 fathers, and 1 sister of autistic children and adolescents. The density (Bmax) of platelet 5-HT2 receptor binding sites, labelled with [3H]-lysergic acid diethylamide (LSD), was significantly lower in 11 hyperserotonemic subjects compared to 12 normoserotonemic subjects (40.9 +/- 13.5 fmol/mg protein, 59.6 +/- 13.2; p < 0.004). The affinity (Kd) for [3H]-LSD binding did not differ. Although the density (Bmax) of [3H]-paroxetine binding did not differ between groups, there was a small difference in the affinity (Kd) for [3H]-paroxetine binding (hyperserotonemic 47.6 +/- 9.0 pM, normoserotonemic 54.8 +/- 12.1; p < 0.05). There were no significant differences in platelet 5-HT uptake, or in thrombin-stimulated 5-HT release. Basal, 5-HT-stimulated, and arginine-vasopressin (AVP)-stimulated inositol phosphate production, as well as basal, prostaglandin E1 (PGE1)-, and forskolin-stimulated cAMP production did not differ. There were significant correlations between whole blood 5-HT levels and LSD Bmax (rs = -0.63, N = 23, p < 0.002) and whole blood 5-HT levels and 5-HT uptake Vmax (rs = 0.56, N = 18, p < 0.02). However, [3H]-LSD labelled 5-HT2 binding and 5-HT uptake were not correlated with each other. Hyperserotonemia of autism may be heterogeneous with one subgroup of subjects with increased 5-HT uptake and another subgroup with decreased 5-HT2 binding.

Hormone responses to fenfluramine and placebo challenge in endogenous depression

Plasma prolactin and cortisol levels after oral administration of d-l fenfluramine hydrochloride (60 mg) and placebo were examined in 24 endogenously depressed patients and 21 age- and sex-matched normal control subjects in a randomized, double-blind study. Prolactin levels were significantly increased by fenfluramine in both groups, but the response was significantly blunted in the depressed patients compared with the controls. This effect was partially dependent upon elevated baseline cortisol levels in the depressed group and was also influenced by a history of weight loss. Plasma cortisol levels were not increased by fenfluramine in either group. These findings confirm previous reports and suggest that patients with endogenous major depression are characterized by central serotonergic hyporesponsivity. The need to account for baseline effects on hormonal responses to putative serotonergic agents is supported by the findings; however, these effects appear to be less striking when endogenicity is a prominent clinical feature of the depressive syndrome. The apparently complex influence of weight loss on prolactin response to serotonergic challenge remains to be clarified as well as the role played by the bioavailability of the challenge drug and its metabolite.

Fenfluramine stimulation of prolactin in obsessive-compulsive disorder

The success of serotonergic reuptake inhibitors in the treatment of obsessive-compulsive disorder (OCD) has suggested that serotonergic neurotransmission may play a role in the pathogenisis of this disorder. Prolactin responses to a 60-mg oral dose of fenfluramine in 26 medication-free patients with a DSM-III-R diagnosis of OCD were compared with those of 20 controls subjects. Fenfluramine produced a significant elevation of prolactin levels in both OCD patients and controls. Prolactin responses were significantly blunted in OCD patients compared with responses in control subjects. Female subjects in both groups showed greater prolactin responses to fenfluramine than did their male counterparts. There was a significant interaction between sex and the presence of OCD such that female patients had lower prolactin responses than their controls, while the difference between male patients and controls was not significant. Prolactin responses were not correlated with age, weight, drug level, depression, anxiety, or degree of OCD symptoms. The results are consistent with a relative reduction in serotonergic efficacy in the setting of OCD.

Enhanced serotonergic responsivity following electroconvulsive therapy in patients with major depression

Prolactin release in response to fenfluramine hydrochloride (60 mg orally) and placebo was evaluated in 18 medication-free patients with RDC major depressive disorder, endogenous subtype, before and after a series of bilateral treatments with ECT. Before ECT, fenfluramine induced a twofold increase in plasma prolactin levels. This response was significantly enhanced after the ECT series, while baseline prolactin levels and response to the placebo challenge were not altered. There was no significant difference in plasma fenfluramine and norfenfluramine levels during the pre- and post-ECT challenges. These findings suggest that ECT enhances central serotonergic responsivity and extend to depressed patients pre-clinical observations regarding the effect of electroconvulsive shock on serotonergic function.

Effects of fenfluramine on plasma homovanillic acid in healthy subjects

The specificity of fenfluramine as a pharmacological probe of the serotonin system has been questioned, since animal studies with high dose l-fenfluramine show increases in striatal levels of the dopamine metabolite homovanillic acid. To test the specificity of fenfluramine in humans with clinical doses, we compared plasma homovanillic acid (pHVA) concentration in healthy volunteers after administration of fenfluramine (60 mg) and placebo. There were no significant effects on pHVA, which supports previous findings that at doses used in pharmacological challenge paradigms, the effect of fenfluramine on the dopamine system is insufficient to alter measures of its change.

Neuroendocrine and behavioral responses to challenge with the indirect serotonin agonist dl-fenfluramine in adults with obsessive-compulsive disorder

Neuroendocrine and behavioral responses to a single 60-mg oral dose of the indirect serotonin agonist dl-fenfluramine were assessed in unmedicated adults with obsessive-compulsive disorder (OCD) and neuroendocrine results contrasted with those in normal control subjects. Net fenfluramine-induced prolactin release did not differ significantly between OCD patients and normal controls. Prolactin responses in the OCD group were not significantly correlated with baseline Yale-Brown Obsessive Compulsive Scale scores for either obsessions or compulsions, but were positively correlated with the baseline Hamilton Depression Scale score and Hamilton Anxiety Scale score. No clear difference in the severity of patients' obsessions or compulsions was found following challenge with fenfluramine versus placebo. Although the present study does not demonstrate a serotonergic abnormality in OCD, this may be more a reflection of limitations of the test procedures than evidence that central nervous system (CNS) serotonergic function is normal in the disorder.

Reproducibility and temporal stability of ADP-induced platelet aggregation: comparison of the anticoagulants sodium citrate and D-phenylalanyl-L-prolyl-L-arginyl-chloromethyl ketone

We compared the reproducibility and temporal stability of ADP-induced aggregation of platelet-rich plasma anticoagulated with citrate or with D-phenylalanyl-L-prolyl-L-arginyl-chloromethyl ketone (PPACK), a thrombin inhibitor. Citrate- or PPACK-platelet-rich plasma (PRP) was stored at room temperature in capped plastic tubes or in plastic syringes from which all air was expelled. At intervals over 1-5 hr after venipuncture, platelet aggregation was induced by 1-1.5 microM ADP. Initially, the aggregation of PPACK-PRP was nearly twice that of citrate-PRP. The response of PPACK-PRP stored in the syringe remained essentially constant over the interval of study, in contrast to the responses of the other samples which declined with time. The improved stability of the response obtained from samples anticoagulated with PPACK was due to the absence of citrate, since PRP containing both citrate and PPACK became less responsive over time in a manner similar to PRP which contained only citrate. Anticoagulation with PPACK rather than citrate results in a more reproducible and stable aggregation response and more closely reproduces the in vivo environment of the platelet.

Platelet 5-HT2 serotonin receptor binding sites in autistic children and their first-degree relatives

We examined platelet serotonin2 [5-hydroxytryptamine2 (5-HT2)] receptor binding sites, whole blood serotonin (5-HT), and plasma norepinephrine (NE) in male autistic children and their first-degree relatives. Saturation studies utilizing 125I-spiroperidol labeled the 5-HT2 sites with an affinity of 224.6 +/- 84.4 pmol/L (Kd). No group differences, i.e., autistic (n = 12), siblings (n = 6), parents (n = 22), control (adult; n = 7: child; n = 10), were seen for either the Kd or the total number of sites (Bmax: 14.3 +/- 10.9 fmol/mg protein). No correlations were found in any group between binding parameters (Kd or Bmax) and whole blood 5-HT. For the parental group, inverse correlations were found between NE and Bmax (standing NE, rs = -0.67, n = 21, p = 0.001; supine NE, rs = -0.49, n = 22, p = 0.021). In the autistic group, no correlation was seen between plasma NE and Bmax. A correlation between the autistic boys' Bmax and their fathers' Bmax was observed (rs = 0.79, n = 11, p = 0.004). These findings suggest (1) circulating NE may be involved in heterologous regulation of 5-HT2 receptors in the platelet and (2) genetic (paternal-filial) factors may play a role in the expression of 5-HT2 binding sites in the platelet. These preliminary findings are discussed in relation to heterologous receptor regulation. The relationships between these findings and either the pathophysiology of autism or hyperserotonemia in autism are unknown.

Clinical and genetic relationships between autism-pervasive developmental disorder and Tourette syndrome: a study of 19 cases

Children with autism or pervasive developmental disorder (PDD) and Tourette syndrome (TS) share a number of symptoms. Forty-one cases have been reported in which PDD patients subsequently developed TS. We term this PDD----TS. We describe an additional 16 such patients plus 3 families where a close relative of a TS proband had autism. There was a high frequency of alcoholism, drug abuse, obsessive-compulsive, and other behavior disorders in the relatives of these patients. This frequency was virtually identical to that observed in relatives of individuals with TS only. We suggest there is an intimate genetic, neuropathologic relatedness between some cases of PDD and TS. Many observations have led us to suggest that the genetic defect in TS may be a mutation of tryptophan oxygenase and that TS is inherited as a semidominant semirecessive trait, i.e., homozygosity for a common gene which shows some expression in the heterozygous state. We propose that some types of PDD are inherited in the same fashion and by the same gene. This would explain the similarity of symptoms, frequent evolution of PDD into TS, the apparent recessive inheritance of PDD despite no increase in consanguinity, the high frequency of behavior problems in the relatives of PDD----TS patients and the serotonin abnormalities.

Whole blood serotonin in juvenile obsessive-compulsive disorder

Whole blood serotonin (5-HT) concentration was assessed in 16 children and adolescents with severe obsessive-compulsive disorder (OCD) and in 14 normal adolescent controls. There was no difference in blood 5-HT content between the OCD patients and the normal controls. However, the OCD patients with a family history of OCD had significantly higher blood 5-HT levels than did either the OCD patients without a family history of OCD or the normal controls. Blood 5-HT content was not associated with a history of major depressive disorder or chronic tic disorder. These preliminary results suggest that studies of serotonergic functioning in OCD may need to control for family history of OCD and that blood 5-HT may be a useful biochemical measure in family-genetic studies of OCD.

Reduced thyroid-stimulating hormone response to thyrotropin-releasing hormone in autistic boys

The thyroid-stimulating hormone (TSH) and prolactin responses to thyrotropin-releasing hormone (TRH) were compared among four groups of boys--41 autistic, 12 mentally retarded (MR), 12 with minimal brain dysfunction (MBD) and five controls. The autistic boys were divided into two groups: DQ(IQ) greater than or equal to 80 and DQ(IQ) less than 80. Mean TSH basal and peak levels were significantly lower in both autistic groups than in the MR, MBD and control groups. Mean TSH peak value minus basal value (p-b) was significantly lower in both autistic groups than in the control group. Mean prolactin levels and p-b value did not differ among groups. It is suggested that there may be enhanced dopaminergic and/or reduced serotonergic activity in the central nervous system of autistic children, together with hypothalamic dysfunction.

Signal-to-noise optimization of HPLC-fluorometric systems and their application to the analysis of indoles

The signal-to-noise optimization of high performance liquid chromatographic (HPLC) flow-cell fluorometric systems is described and the possibilities for further improving limits of detection for indoles is discussed. Application of HPLC-fluorometry to analyses in rat cerebrospinal fluid (CSF), human lumbar CSF, and human blood is presented. Finally, the intriguing clinical finding of hyperserotonemia in autism is discussed.

Functional consequences of fenfluramine neurotoxicity

Male Sprague-Dawley rats were trained to discriminate the anorectic drug d,l-fenfluramine (2.0 mg/kg intraperitoneally administered) from its vehicle using a food-motivated (fixed-ratio 10 schedule) two-lever operant task. Once trained, doses of 0.5, 1.0 and 1.5 mg/kg fenfluramine tested 20 min after IP administration produced dose-responsive discrimination performance. Subsequently, noncontingent twice-a-day administrations of 1 ml/kg saline were made for 4 days and the dose-effect relationship redetermined on the 13th to 15th day after initiation of the chronic saline regimen. Results of these dose-response experiments indicated that there was no significant effect upon fenfluramine discrimination after multiple saline injections or after 10 days without training. Following four days of retraining, 6.25 mg/kg fenfluramine twice-a-day for four days was followed 10 days later by another dose-response determination. This purportedly neurotoxic regimen of fenfluramine significantly increased the rats' ability to discriminate fenfluramine. These results suggest the possibility that chronic release of serotonin or selective damage to serotonin-containing neurons produced by fenfluramine may lead to postsynaptic supersensitivity as manifested by the functionally increased discriminative performance observed.

Fluvoxamine treatment of coincident autistic disorder and obsessive-compulsive disorder: a case report

This is a single-case report of fluvoxamine treatment of comorbid autistic disorder (AD) and obsessive-compulsive disorder (OCD). Psychological, neuroanatomical, and neurochemical parallels are drawn between AD and OCD. The implications of this case of coincident AD and OCD, as well as the response to fluvoxamine, are discussed with respect to nosology, pathophysiology, and treatment of these disorders.

Growth hormone response to L-dopa and clonidine in autistic children

Studies have shown abnormal pituitary hormone responses to neuroendocrine agonists in autistic subjects. Two probes (clonidine and L-Dopa) were used to investigate neuroendocrine responses through changes in growth hormone levels. Seven medication-free autistic subjects (ages 6.6 to 19.1) were evaluated and compared to 14 normal controls. Growth hormone was collected at 30-min intervals during the entire study. Clonidine was administered first (dose: 0.15 mgm2), and samples were collected for 180 min. L-Dopa was then administered (dose: 250 mg for subjects less than 70 lb and 500 mg for subjects greater than 70 lb), and samples were collected for 120 min. There was no difference in the amplitude of the clonidine or L-Dopa peak growth hormone responses in the control versus the autistic subjects. In the autistic subjects, the L-Dopa-stimulated growth hormone peak was delayed and the clonidine growth hormone peak was premature. A statistical difference with the control subjects was found when consideration was given to both the premature response of growth hormone to clonidine and the delayed response to L-Dopa (p = .01, Fisher's Exact Test). These findings suggest possible abnormalities of both dopaminergic and noradrenergic neurotransmission in subjects with autism.

The hyperserotonemia of autism

The planned and ongoing studies of platelet function and composition should allow us to better define the alteration which we presume to be present in platelets of autistic subjects. Although much of the research focuses on serotonergic aspects, the more general research should permit a better delineation of the extent of the alteration and will protect against a premature narrowing of the inquiry. The methodological development which has been a necessary aspect of the work should contribute to an improved understanding of platelet function and composition, as well as result in improved clinical tools for the assessment of platelet functioning in neuropsychiatric disorders and hematology. As an example, improvements in short-term in vitro storage conditions to stabilize aggregation and shape change responses over time were found to be necessary, and are probably critical to an optimal comparison of these phenomena across groups. The identification of the platelet alteration which is responsible for the hyperserotonemia of autism should prove useful in several ways. It would be expected that assessment of the altered function would provide a marker with less overlap with the normal population than the multidetermined measure of blood 5-HT. Determination of the specific protein(s) involved in the altered platelet should lead directly to gene probes and chromosomal location. These, in turn, should prove useful for neonatal screening, subtyping, and more powerful genetic and family studies. Work of this sort might also allow early intervention and improved treatment. Finally, characterization of the physiological alteration would provide a basis for focusing studies of brain neurochemistry and should, as well, suggest modes of neuropharmacological intervention. The confidence that one can have in the basic finding of hyperserotonemia in autism and the potential benefits to be derived from its explication make further research in this area of great interest.

Effects of age and gender on CNS serotonergic responsivity in normal adults

The effects of age and gender on central nervous system (CNS) serotonergic responsivity were assessed with a neuroendocrine challenge test in 30 normal adults. Subjects greater than or equal to 30 years of age, compared with younger subjects, exhibited decreased prolactin secretion in response to a 60-mg oral dose of dl-fenfluramine hydrochloride, an indirect serotonin agonist. Furthermore, women had greater prolactin responses than men. As prolactin secretory capacity appears to be stable through midlife, the age-associated decrease in fenfluramine-induced prolactin release suggests a decline in CNS serotonergic responsivity. In contrast, the finding of greater prolactin release in women than in men probably reflects the effects of nonserotonergic modulatory influences at the level of the lactotroph. Age and gender effects must be considered in studies of the CNS serotonergic system.

Autism: review of neurochemical investigation

The neurochemistry of autism, the most well-validated childhood neuropsychiatric disorder, has been studied extensively over the past three decades. Autism is of interest neurochemically because it represents a relatively homogeneous disorder with a triad of social, communicative, and intellectual developmental disturbance. Because a sufficient animal model has been lacking and relatively few diagnosed people with autism have died, most investigation has been of peripheral fluids and tissues. The most consistent finding has been that over 25% of autistic children and adolescents are hyperserotonemic. However, after 29 years of investigation, the mechanism of hyperserotonemia has not been determined. Hyperserotonemia has been found to be familial. Elevated plasma norepinephrine has also been a replicated finding. Cerebrospinal fluid (CSF) opiate activity has been found to be elevated in two studies. Plasma cyclic adenosine monophosphate (cAMP) has been found to be elevated in autistic children. A high rate of nonsuppression after dexamethasone and blunted or delayed growth hormone response to L-dopa have been found. Abnormal cell-mediated immunity has been replicated consistently in autism. Although several pharmacological trials have been conducted and shown promise in initial open trials, only "typical" antipsychotic drugs have shown replicable chronic ameliorating effects in double-blind trials. However, chronic neurotoxicity (tardive dyskinesia) has also been revealed. Findings of morphological changes in the cerebellum have been replicated. Findings in need of replication include diminished platelet function, increased baseline CSF homovanillic acid, decreased nerve cell adhesion molecule serum fragment, blunted prolactin response to fenfluramine, amelioration of symptoms by naltrexone and bromocriptine, reduced electroretinographic (ERG) b-wave amplitude, and morphological changes in the hippocampus, amygdala, and septal nuclei. In addition to refining and replicating past findings, future directions that may be fruitful include investigation of neurochemical aspects of platelet function, of interactions between monoaminergic systems, of phosphatidylinositides, and of pharmacological response to "atypical" antipsychotic agents and relatively selective serotonin receptor subtype agonists or antagonists.

Clinical studies of 5-HT function using i.v. L-tryptophan

1. Preclinical studies reveal that long-term treatment with antidepressant drugs induces significant changes in serotonergic (5-HT) receptor sensitivity. Similarly, clinical studies suggest that brain 5-HT function is abnormal in depression. Of the available methodologies for conducting such clinical studies, the pharmacological challenge strategy has proven particularly useful. 2. I.v. L-TRP has emerged as the most frequently used challenge agent in diagnostic and neuropsychopharmacological studies of 5-HT function. I.v. L-TRP increases serum prolactin (PRL) in humans, probably via 5-HT mechanisms. Under carefully standardized conditions, this PRL response to L-TRP appears to be a reasonably sensitive and valid measure of net 5-HT function. 3. The PRL response to L-TRP is blunted in depressed patients compared with healthy controls. Blunting has not been observed in panic disorder, obsessive compulsive disorder, or schizophrenia, although preliminary findings suggest it may occur in bulimia. 4. The PRL response to L-TRP is enhanced by certain classes of thymoleptic drugs (TCAs, MAOIs, 5-HT reuptake inhibitors, lithium) in a differentially time-dependent fashion. So-called "atypical" antidepressants (trazodone, mianserin) and benzodiazepines have no effect. Such findings are generally consistent with preclinical electrophysiological findings. 5. These clinical studies of the PRL response to L-TRP, in conjunction with emerging evidence that experimentally reduced plasma TRP can reverse the therapeutic effects of some antidepressants, suggest that antidepressant drug action may be more accurately conceptualized as 5-HT dependent rather than 5-HT enhancing. The availability of more selective 5-HT-active drugs promises to further clarify 5-HT mechanisms of neuropsychiatric disease and drug action at the clinical level.

Neuroendocrine aspects of the serotonergic hypothesis of depression

This review examines the role of serotonin (5-HT) in depression. Dysfunction of serotonergic neurons has been implicated as one of the causes of endogenous depression. Since serotonergic neurons innervate the hypothalamus and these neurons send collaterals to several other brain areas, it is possible that hypothalamic sites which control hormone secretion receive the same serotonergic afferents that innervate other limbic areas in the brain. Several investigators have devised neuroendocrine challenge tests measuring the effect of 5-HT agonists on plasma cortisol and prolactin in depressed patients. These tests help to identify dysfunctional 5-HT neurons, and are a "window into the brain." The secretion of cortisol and prolactin is increased predominantly by 5-HT1 receptors. However, changes in 5-HT2 receptors have also been implicated in depression. Results from our laboratory and by others suggest that brain serotonergic neurons stimulate renin and vasopressin secretion by activation of 5-HT2 receptors. Therefore, the renin and vasopressin response to 5-HT agonists should be included in neuroendocrine tests of serotonergic function in affective disorders. Since antidepressants produce a decrease in the density of 5-HT2 receptors, renin and vasopressin could be used to evaluate the antidepressant potential of new drugs.