Effect of pindolol pretreatment on MK-212-induced plasma cortisol and prolactin responses in normal men

Serotonergic 5-HT2A/2C receptors are involved in prolactin secretion in hyperestrogenic rats

Serotonin (5-HT) has been shown to participate in prolactin secretion through a complex process resulting in both positive and negative effects. Estrogen has also been recognized as being involved in this serotonergic effect on prolactin release. Therefore, the aim of the present study was to assess whether estradiol modulates serotonergic involvement in prolactin secretion though 5-HT1A and/or 5-HT2A/2C receptors. Ovariectomized female Wistar rats, treated for 2 weeks with estrogen to induce a hyperprolactinemic status (hyperestrogenic rats) or with sunflower oil vehicle (hypoestrogenic rats), were injected daily with normal saline solution or 6-chloro-2-(1-piperazinyl)pyrazine (MK-212), an 5-HT2A/2C receptor agonist, for 4 days. Other groups of ovariectomized animals received 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) or pindolol, an agonist and antagonist of the 5-HT1A receptor respectively, on the last day of treatment with estrogen or vehicle. Prolactin levels were compared among groups in each experiment under the distinct drug treatments. MK-212 was found to increase prolactin concentrations both in hyper- and hypoestrogenic females compared to controls (p<0.05). In contrast, prolactin levels remained similar to those of controls both in hyperestrogenic animals treated with 8-OH-DPAT and pindolol and in hypoestrogenic rats administered the same treatments. In conclusion, our findings indicate the involvement of 5-HT2A/2C receptors on prolactin release through serotonergic pathways in female animals, especially in hyperestrogenic states.

5-HT(2C) agonists as therapeutics for the treatment of schizophrenia

The 5-HT(2C) receptor is a highly complex, highly regulated receptor which is widely distributed throughout the brain. The 5-HT(2C) receptor couples to multiple signal transduction pathways leading to engagement of a number of intracellular signaling molecules. Moreover, there are multiple allelic variants of the 5-HT(2C) receptor and the receptor is subject to RNA editing in the coding regions. The complexity of this receptor is further emphasized by the studies suggesting the utility of either agonists or antagonists in the treatment of schizophrenia. While several 5-HT(2C) agonists have demonstrated clinical efficacy in obesity (lorcaserin, PRX-000933), the focus of this review is on the therapeutic potential of 5-HT(2C) agonists in schizophrenia. To this end, the preclinical profile of 5-HT(2C) agonists from a neurochemical, electrophysiological, and a behavioral perspective is indicative of antipsychotic-like efficacy without extrapyramidal symptoms or weight gain. Recently, the selective 5-HT(2C) agonist vabicaserin demonstrated clinical efficacy in a Phase II trial in schizophrenia patients without weight gain and with low EPS liability. These data are highly encouraging and suggest that 5-HT(2C) agonists are potential therapeutics for the treatment of psychiatric disorders.

Augmentation by citalopram of risperidone-induced monoamine release in rat prefrontal cortex

RATIONALE

A typical antipsychotics (APDs), e.g. olanzapine and risperidone, have been reported to be effective adjunctive treatment for depression if selective serotonin (5-HT) reuptake inhibitors (SSRIs) alone are ineffective.

OBJECTIVES AND METHODS

We utilized microdialysis in awake, freely moving rats to study the effect of risperidone in combination with citalopram, an SSRI, on extracellular 5-HT, dopamine (DA), and norepinephrine (NE) efflux in rat medial prefrontal cortex (mPFC).

RESULTS

Risperidone (1.0 mg/kg, s.c.), given alone, significantly increased 5-HT, DA, and NE concentrations in the mPFC. Citalopram (10 mg/kg, s.c.), by itself, produced a significant increase in 5-HT levels only. The combination of risperidone and citalopram produced significantly greater increases in efflux of both DA and NE than risperidone alone. However, the effect of this combination on extracellular 5-HT concentrations was not significantly different than that of citalopram alone. The augmentation of DA and NE efflux induced by risperidone plus citalopram could be partially blocked by the selective 5-HT1A antagonist, WAY 100635 (0.2 mg/kg, s.c.).

CONCLUSIONS

The results suggest that the ability of atypical APDs to augment the therapeutic efficacy of SSRIs in major depression and treatment-resistant depression may be due, at least in part, to potentiation of SSRI-induced increases in cortical DA and NE. The contributions of 5-HT1A receptor stimulation and 5-HT2A and alpha2 adrenergic receptor antagonism to this augmentation are discussed.

Hormonal and temperature responses to flesinoxan in normal volunteers: an antagonist study

RATIONALE

Flesinoxan is a highly potent and selective 5-HT1A agonist. In a recent study, in normal volunteers, flesinoxan induced a significant and dose-dependent increase in adrenocorticotropic hormone (ACTH), cortisol, prolactin (PRL), growth hormone (GH) and a decrease in body temperature.

OBJECTIVES

In order to better define the role of 5-HT receptor subtypes in response to flesinoxan, we assessed the influence of 5-HT1A and 5-HT2 antagonists on hormonal and temperature responses to flesinoxan.

METHODS

Hormonal and temperature responses were studied in 6 volunteers with or without pretreatment with pindolol (30 mg p.o.), a 5-HT1A antagonist, or ritanserin (10 mg p.o.), a selective 5-HT2 antagonist, using a double-blind crossover design.

RESULTS

Pindolol significantly antagonized ACTH, PRL, GH and temperature responses to flesinoxan and ritanserin exhibited similar activity on PRL and ACTH responses.

CONCLUSIONS

These results show the role of 5-HT1A mechanisms in the PRL, ACTH, GH, and temperature responses to flesinoxan, and the role of 5-HT2 mechanisms in PRL and ACTH responses. Therefore, they confirm the interest of flesinoxan as a 5-HT neuroendocrine probe.

The pharmacology and clinical pharmacology of 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy")

The amphetamine derivative (+/-)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is a popular recreational drug among young people, particularly those involved in the dance culture. MDMA produces an acute, rapid enhancement in the release of both serotonin (5-HT) and dopamine from nerve endings in the brains of experimental animals. It produces increased locomotor activity and the serotonin behavioral syndrome in rats. Crucially, it produces dose-dependent hyperthermia that is potentially fatal in rodents, primates, and humans. Some recovery of 5-HT stores can be seen within 24 h of MDMA administration. However, cerebral 5-HT concentrations then decline due to specific neurotoxic damage to 5-HT nerve endings in the forebrain. This neurodegeneration, which has been demonstrated both biochemically and histologically, lasts for months in rats and years in primates. In general, other neurotransmitters appear unaffected. In contrast, MDMA produces a selective long-term loss of dopamine nerve endings in mice. Studies on the mechanisms involved in the neurotoxicity in both rats and mice implicate the formation of tissue-damaging free radicals. Increased free radical formation may result from the further breakdown of MDMA metabolic products. Evidence for the occurrence of MDMA-induced neurotoxic damage in human users remains equivocal, although some biochemical and functional data suggest that damage may occur in the brains of heavy users. There is also some evidence for long-term physiological and psychological changes occurring in human recreational users. However, such evidence is complicated by the lack of knowledge of doses ingested and the fact that many subjects studied are or have been poly-drug users.

Dopamine and serotonin function in untreated schizophrenia: clinical correlates of the apomorphine and d-fenfluramine tests

This study examined the prolactin (PRL), adrenocorticotropin (ACTH) and cortisol responses to the direct DA receptor agonist apomorphine (APO) and the selective 5HT-releasing agent d-fenfluramine (d-FEN) in 20 untreated inpatients with DSM-IV schizophrenia and without a history of suicide attempt, compared to 23 hospitalized healthy controls. We hypothesized that different patterns of responsiveness of the DA and 5-HT systems might be associated with specific schizophrenic symptom clusters. A positive correlation was observed between pituitary-adrenal response to APO and d-FEN tests (i.e. deltaACTH and deltacortisol) in the overall population and in schizophrenic patients. Pituitary-adrenal response to APO was lower in patients than in normal controls. Moreover, lower pituitary-adrenal response to APO and d-FEN was associated with increased severity of BPRS thought disturbance score. Lower pituitary-adrenal responses to APO (and to a lesser degree to d-FEN) differentiated paranoid from disorganized schizophrenic patients. Neither PRL suppression to APO, nor PRL stimulation to d-FEN were altered in schizophrenic patients. Our results suggest that decreased hypothalamic DA receptor activity (possibly secondary to increased presynaptic DA release) together with relatively decreased 5-HT tone characterize paranoid SCH, while normal hypothalamic DA receptor activity together with relatively increased 5-HT tone characterize the disorganized SCH subtype.

Ambient temperature influences core body temperature response in rat lines bred for differences in sensitivity to 8-hydroxy-dipropylaminotetralin

Agonist-induced decrease in core body temperature has commonly been used as a measure of serotonin1A (5-HT(1A)) receptor sensitivity in mood disorder. The thermoregulatory basis for 5-HT(1A) receptor agonist-induced temperature responses in humans and rats remains unclear. Therefore, the influence of ambient temperature on 5-HT(1A) receptor-mediated decreases in core body temperature were measured in rat lines bred for high (HDS) or low (LDS) sensitivity to the selective 5-HT(1A) receptor agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT). HDS and LDS rats were injected with either saline, 0.25 or 0.50 mg/kg 8-OH-DPAT at ambient temperatures of 10.5, 24, 30, or 37.5 degrees C, and core temperature was measured by radiotelemetry. For both lines, the thermic response to acute 8-OH-DPAT was greatest at 10.5 degrees C and decreased in magnitude as ambient temperature increased to 30 degrees C, consistent with hypothermia. HDS rats displayed a greater hypothermic response than LDS rats at 10.5, 24, and 30 degrees C. At 37.5 degrees C, LDS rats showed a lethal elevation of temperature in response to 0.50 mg/kg 8-OH-DPAT. All thermic responses to 8-OH-DPAT, including the lethality, were effectively blocked by pretreatment with the 5-HT(1A) receptor antagonist WAY100635, suggesting line differences in thermoregulatory circuits that are influenced by 5-HT(1A) receptor activation. Following repeated injection of 8-OH-DPAT, the magnitude of the hypothermic response decreased in both lines at 10.5 degrees C, but increased in HDS rats treated with 0.50 mg/kg 8-OH-DPAT at 30 and 37.5 degrees C. This pattern was reversed in HDS rats following 8-OH-DPAT challenge at 24 degrees C, suggesting that a compensatory thermoregulatory response accounts for changes in the hypothermic response to chronic 8-OH-DPAT.

Lack of effect of HPA axis hyperactivity on hormonal responses to d-fenfluramine in major depressed patients: implications for pathogenesis of suicidal behaviour

There is evidence for inhibitory effects of adrenocorticosteroids on serotonergic (5-HT) activity. However, in depression the relationship between altered cortisol levels and brain 5-HT function remains to be clarified. The aim of this study was to investigate whether hypothalamic-pituitary-adrenal (HPA) axis hyperactivity is associated with 5-HT dysfunction in depressed patients, especially in those with suicidal behaviour. Cortisol levels following the dexamethasone suppression test (DST, 1 mg PO) and prolactin, corticotropin and cortisol responses to the d-fenfluramine test (d-FEN, 45 mg PO) - a specific 5-HT releaser/uptake inhibitor - were measured in 71 drug-free DSM-IV major depressed inpatients (40 with a history of suicide attempt, 31 without) and 34 hospitalized healthy control subjects. Depressed patients showed higher post-DST cortisol levels but similar responses to d-FEN compared with control subjects. Hormonal responses to d-FEN were not correlated with cortisol levels (basal or post-DST). Among the depressed patients, DST suppressors and DST nonsuppressors exhibited no significant difference in endocrine responses to d-FEN. However, patients with a history of suicide attempt, when compared with patients without such a history, showed lower hormonal responses to d-FEN but comparable basal and post-DST cortisol levels. Taken together these results suggest that, in depression, HPA axis hyperactivity is not responsible for the reduced 5-HT activity found in patients with a history of suicidal behavior.

Serotonergic and noradrenergic function in depression: clinical correlates

The present study was conducted in order to investigate the relationships between central noradrenergic (NA) and serotonergic (5-HT) function and clinical characteristics of a major depressive episode according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. We measured growth hormone response (ΔGH) to clonidine (CLO) (an α2 NA agonist), as an index of central NA function, and prolactin response (APRL) to d-fenfluramine (d-FEN) (a specific 5-HT releaser/uptake inhibitor), as an index of central 5-HT function, in 53 medication-free depressed inpatients. On the basis of their CLO and d-FEN test responses, patients were classified into 4 groups. Group 1 (blunted ΔPRL_d-FEN alone [11 %]) was characterized by a recent violent suicide attempt, a high degree of medical damage, and mild anxiety. Group 2 (blunted ΔGH_CLO alone [32%]) was characterized by an absence of a history of suicide attempt and by severe anxiety. Group 3 (combination of blunted ΔGH_CLO and APRL_d-FEN [18%]) was characterized by a history of suicide attempts, total duration of the illness of over W years, age over 40 years, and more than 3 previous hospitalizations. Group 4 (no abnormality [39%]) had no specific clinical profile. These results suggest that, in depression, specific psychopathological features may be linked to 5-HT and/or NA dysfunction. However, our results also suggest that NA and/or 5-HT dysfunction are less likely to be the primary cause of mood disorders but are more indicative of failure of compensatory mechanisms involved in affective homeostatic processes.

Inhibition of rat nucleus tractus solitarius neurones by activation of 5-HT2C receptors

In vivo, nucleus tractus solitarius (NTS) neurones receiving monosynaptic vagal input and inactive intermediate neurones were inhibited by both DOI and a selective 5-HT2C receptor agonist, MK-212. Cells receiving a more polysynaptic input were excited by DOI and although MK-212 also excited a few of these cells, the majority of cells in these groups were unaffected by MK-212. The inhibitory, but not the excitatory actions of both MK-212 and DOI were prevented by a selective 5-HT2C receptor antagonist, RS-102221. In contrast, most dorsal vagal preganglionic neurones were unaffected by application of either DOI or MK-212, the few remaining cells being excited by both agonists. These data demonstrate that DOI-evoked inhibition of NTS cells activated by vagal afferent input and DOI-evoked excitation of vagal preganglionic neurones is mediated by 5-HT2C receptors.

Thyroid axis activity and serotonin function in major depressive episode

Recent studies in depression have reported alterations in both hypothalamic-pituitary-thyroid (HPT) axis activity and serotonin (5-HT) function; however, the functional relationships between the two systems have not been well defined in patients with major depressive episode. Thyrotropin (TSH) response to 0800 and 2300 h protirelin (TRH) challenges, and adrenocorticotropic hormone (ACTH), cortisol, and prolactin (PRL) responses to D-fenfluramine (D-FEN), a specific 5-HT releasing/uptake-inhibiting agent, were examined in 60 drug-free DSM-IV major depressed inpatients and 20 hospitalized controls. Compared with controls, patients showed lower basal serum 2300 h TSH, 2300 h maximum increment in serum TSH above baseline (delta TSH) and difference between 2300 h delta TSH and 0800 h delta TSH (delta delta TSH) levels. The hormonal responses to D-FEN (i.e. delta ACTH, delta cortisol and delta PRL) were interrelated. No significant difference in basal and post-D-FEN ACTH, cortisol or PRL values were found between controls and patients. A negative relationship between hormonal responses to D-FEN and 2300 h delta TSH and delta delta TSH values was observed in the depressed group. When patients were classified on the basis of their delta TSH test status, patients with reduced delta delta TSH values (i.e. with HPT axis abnormality) had hormonal D-FEN responses comparable to those of controls. Patients with normal delta delta TSH values (i.e. without HPT axis abnormality) showed lower ACTH, cortisol and PRL responses to D-FEN than controls and patients with abnormal delta delta TSH values. These results suggest that: (1) pathophysiological mechanisms other than 5-HT dysregulation may be involved in TSH blunting in major depressed patients; (2) 5-HT function is reduced in some depressed patients, especially those without HPT axis abnormality; and (3) HPT dysregulation may be regarded as a compensatory mechanism for diminished central 5-HT activity.

Selective serotonin reuptake inhibitors and neuroendocrine function

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are effective drugs for the treatment of several neuropsychiatric disorders associated with reduced serotonergic function. Serotonergic neurons play an important role in the regulation of neuroendocrine function. This review will discuss the acute and chronic effects of SSRIs on neuroendocrine function. Acute administration of SSRIs increases the secretion of several hormones, but chronic treatment with SSRIs does not alter basal blood levels of hormones. However, adaptive changes are induced by long-term treatment with SSRIs in serotonergic, noradrenergic and peptidergic neural function. These adaptive changes, particularly in the function of specific post-synaptic receptor systems, can be examined from altered adrenocorticotrophic hormone (ACTH), cortisol, oxytocin, vasopressin, prolactin, growth hormone (GH) and renin responses to challenges with specific agonists. Neuroendocrine challenge tests both in experimental animals and in humans indicate that chronic SSRIs produce an increase in serotonergic terminal function, accompanied by desensitization of post-synaptic 5-HT1A receptor-mediated ACTH, cortisol, GH and oxytocin responses, and by supersensitivity of post-synaptic 5-HT2A (and/or 5-HT2C) receptor-mediated secretion of hormones. Chronic exposure to SSRIs does not alter the neuroendocrine stress-response and produces inconsistent changes in alpha2 adrenoceptor-mediated GH secretion. Overall, the effects of SSRIs on neuroendocrine function are dependent on adaptive changes in specific neurotransmitter systems that regulate the secretion of specific hormones.

Assessment of serotonergic function in major depression using d-fenfluramine: relation to clinical variables and antidepressant response

BACKGROUND

d-Fenfluramine, a specific serotonin (5-HT)-releasing agent without the catecholamine effects of d,l-fenfluramine, was used as a serotonergic neuroendocrine challenge in subjects with unipolar major depression.

METHODS

Patients were given 30 mg of d-fenfluramine orally, and prolactin and cortisol responses were measured over the following 5 hours. Endocrine responses were examined in relation to clinical variables and subsequent response to antidepressant treatment.

RESULTS

5-HT-mediated cortisol responses at baseline were inversely correlated with depression severity on the Montgomery Asberg Depression Rating Scale, Bech Melancholia Scale, and Clinical Global Impression scale. Prolactin responses were inversely correlated with anxiety on the Brief Symptom Inventory. A higher initial cortisol response to d-fenfluramine predicted a subsequent good antidepressant response.

CONCLUSIONS

We conclude that: a) the severity and symptom profile in major depression may be closely related to 5-HT dysfunction; and b) higher cortisol responses to d-fenfluramine predict a preferential response to treatment.

Diminished serotonin-mediated prolactin responses in nondepressed stroke patients compared with healthy normal subjects

BACKGROUND AND PURPOSE

The purpose of this study was to use hormonal responsiveness to d-fenfluramine (d-FEN) challenge as a measure of central serotonin (5-HT) function in a comparative evaluation of serotonergic abnormalities between stroke patients and healthy elderly normal subjects to test the hypothesis that stroke may be associated with diminished serotonergic functioning.

METHODS

Eight nondepressed medically stable stroke patients and 12 healthy volunteers completed a single-blind, placebo-controlled, fixed-order, crossover design challenge test with 30 mg of oral d-FEN. Baseline prolactin (PRL) and cortisol (CORT) and hormonal responses to d-FEN and placebo were measured at hourly intervals over a 4-hour period. Cardiovascular responses (pulse and blood pressure) and behavioral responses were also recorded at the same time points.

RESULTS

The 2 groups were comparable in demographics, body weight, plasma drug concentration, and behavioral and CORT responses. A 3-way ANOVA for repeated measures showed group differences for baseline adjusted PRL responses (change of scores from baseline). Peak PRL responses (maximal PRL change from baseline scores after treatment with d-FEN) in nondepressed stroke patients were attenuated compared with healthy elderly subjects, suggesting diminished serotonergic responsiveness in stroke patients.

CONCLUSIONS

The demonstrated serotonergic hypofunctioning poststroke may contribute to the high incidence of depressive disorders in stroke patients. Serotonergic agents may have a role in augmentation of stroke recovery.

Psychoneuroendocrinology of depression. Prolactin

Prolactin provides us with a window to the brain in our quest for understanding the psychobiology of depression, since the regulation of its release involves some of the monamine neurotransmitter systems that have been implicated in the pathophysiology of depression. Investigation examining basal prolactin plasma concentrations in depressed patients, including assessments of the rhythm of prolactin release, have not provided clear, consistent findings. Further exploration of the precise mechanisms involved in serotonin-stimulated prolactin release should shed light on the pathophysiology of abnormal prolactin responsivity in depression, and by extension, the psychobiologic basis of depression.

Serotonergic function and negative and depressive symptomatology in schizophrenia and major depression

BACKGROUND

Serotonergic abnormalities are found in both major depressive disorder (MDD) and schizophrenia. Depressive symptoms commonly occur alongside the negative or defect symptoms in schizophrenia and antiserotonergic drugs may be particularly effective in their treatment. We wished to explore whether these symptoms could be distinguished biologically by directly comparing serotonergic function in these two illnesses.

METHOD

Fifteen patients with MDD and 13 patients with schizophrenia underwent testing with the specific serotonin releasing agent D-fenfluramine (D-FEN). Prolactin and cortisol responses were measured to ascertain central serotonergic function. Individual patient results were compared with their own carefully matched control to correct for the effect of age, sex, weight and menstrual cycle, before the two patient groups were then compared.

RESULTS

Prolactin responses differed significantly between the two patient groups, being lower in MDD patients and higher in schizophrenia patients than their individually matched controls. Cortisol responses did not differ. Within the schizophrenia group, increased serotonergic function correlated positively with depressive symptoms, but there was no such correlation with defect symptoms. Depressive scores were negatively correlated with the presence of negative symptoms in the schizophrenic group.

CONCLUSIONS

Schizophrenia and MDD have distinct and opposite neuroendocrine responses to D-FEN. There is no evidence that depressive symptoms in these two conditions have a common serotonergic basis. Moreover, these responses distinguished between negative and depressive symptoms in our schizophrenic group.

Effect of sleep deprivation on neuroendocrine response to a serotonergic probe in healthy male subjects

Neuroendocrine responses to stimulation with a selective serotonin reuptake inhibitor (citalopram) were measured to investigate the effects of all-night sleep deprivation on serotonergic function in healthy male subjects (n = 7). We studied citalopram-stimulated prolactin and cortisol plasma concentrations in a placebo-controlled cross-over protocol following sleep and sleep deprivation. Citalopram infusion (20 mg i.v. at 14:20-14:50 h) after a night of undisturbed sleep prompted robust increases in both plasma prolactin and cortisol concentrations. Following a night of sleep deprivation, by contrast, the citalopram-induced prolactin response was blunted, but the cortisol response was not significantly altered. This differential response pattern relates to the distinct pathways through which serotonin may activate the corticotrophic and the lactotrophic systems. While an unchanged cortisol response does not indicate (but also does not refute the possibility of) an altered serotonergic responsivity following sleep deprivation, the suppressed prolactin response could reflect a downregulation of 5-HT1A or 2 receptors. An alternative, not mutually exclusive, explanation points to the possibility that sleep deprivation activates the tubuloinfundibular dopaminergic system, the final inhibitory pathway of prolactin regulation.

Pindolol pretreatment blocks stimulation by meta-chlorophenylpiperazine of prolactin but not cortisol secretion in normal men

Previous reports from this laboratory have shown that pindolol, a partial serotonin1A receptor agonist, inhibited prolactin, but not cortisol secretion induced by administration of the serotonin (5-HT) precursor L-5-hydroxytryptophan or the direct-acting 5-HT2A/5HT2C receptor agonist MK-212. The findings suggest additive or interactive effects of 5-HT1A and 5-HT2A/5-HT2C receptors in modulating 5-HT-related prolactin, but not cortisol, responsivity. To examine further the role of 5-HT1A and 5-HT2A/5-HT2C receptors in prolactin and cortisol secretion in healthy men, the effects of meta-chlorophenylpiperazine (mCPP), a potent 5-HT receptor agonist, on the above hormones were studied in eight healthy men with and without pindolol pretreatment. It has previously been demonstrated that ketanserin, a 5-HT2A antagonist, and ritanserin, a 5-HT2A/5-HT2C antagonist, block the prolactin and attenuate the hypothalamic-pituitary-adrenal axis responses to mCPP in man or rodents. Administration of mCPP induced a significant increase in plasma concentrations of prolactin and cortisol. The mCPP-induced prolactin concentrations were significantly blocked by pretreatment with pindolol, whereas mCPP-stimulated cortisol levels were not diminished by pindolol pretreatment. Thus, mCPP-induced prolactin secretion appears to require the availability of both 5-HT2C and 5-HT1A receptor activation, since blockade of either of these receptors may diminish the mCPP-induced prolactin response. Cortisol secretion stimulated by mCPP may occur following 5-HT2C receptor stimulation in the presence of 5-HT1A receptor blockade.