Effects of tranylcypromine treatment on neuroendocrine, behavioral, and autonomic responses to tryptophan in depressed patients

Looking beyond 5-HT(3) receptors: a review of the wider role of serotonin in the pharmacology of nausea and vomiting

The concept that 5-hydroxytryptamine (5-HT; serotonin) is involved in the emetic reflex was revealed using drugs that interfere with its synthesis, storage, release and metabolism ahead of the discovery of selective tools to modulate specific subtypes of receptors. This review comprehensively examines the fundamental role of serotonin in emesis control and highlights data indicating association of 5-HT1-4 receptors in the emetic reflex, whilst leaving open the possibility that 5-HT5-7 receptors may also be involved. The fact that each receptor subtype may mediate both emetic and anti-emetic effects is discussed in detail for the first time. These discussions are made in light of known species differences in emesis control, which has sometimes affected the perception of the translational value of data in regard to the development of novel anti-emetic for use in man.

Antidepressant-induced hyperprolactinaemia: incidence, mechanisms and management

Prolactin, a polypeptide hormone, is responsible, amongst other things, for milk production during lactation and breast enlargement during pregnancy. Numerous drugs can affect prolactin levels. Most commonly, conventional antipsychotics are associated with hyperprolactinaemia but there have also been reports of antidepressants causing hyperprolactinaemia. This review sets out to establish the incidence of antidepressant-induced hyperprolactinaemia, its possible mechanism and to determine appropriate remedial actions. Nearly all antidepressants are reported to be associated with hyperprolactinaemia. Incidence rates were not clearly established and symptoms were very rare. The mechanism by which antidepressants may cause hyperprolactinaemia is not fully understood, though several theories have been postulated, such as serotonin stimulation of GABAergic neurons and indirect modulation of prolactin release by serotonin. Patients taking antidepressants presenting to their clinician with symptoms potentially related to hyperprolactinaemia, such as galactorrhoea, should have their plasma prolactin level measured and their antidepressant changed if an increased prolactin level is confirmed. Routine monitoring of prolactin levels is otherwise not appropriate.

Hyperprolactinemia associated with psychotropics--a review

INTRODUCTION

Different classes of psychotropics can cause hyperprolactinemia to varying degrees. Among antipsychotics, typical agents and risperidone are the most frequent and significant offenders. In this review we discuss the pathophysiology, offending medications, assessment and management of hyperprolactinemia.

METHODS

We did a literature review between 1976 and 2008 using PubMed, MEDLINE, PsychINFO and Cochrane database. Search terms used were prolactin, hyperprolactinemia, psychotropics, antipsychotics, typical antipsychotics, atypical antipsychotics, antidepressants and SSRIs.

RESULTS

Prolactin elevation is more common with antipsychotics than with other classes of drugs. Typical antipsychotics are more prone to cause hyperprolactinemia than atypical agents. Management options include discontinuation of offending medication, switching to another psychotropic, supplementing concurrent hormonal deficiencies and adding a dopamine agonist or aripiprazole.

CONCLUSION

Clinicians need to be alert about the potential for hyperprolactinemia and its manifestations with these medications. Prolactin levels need to be monitored and other causes of hyperprolactinemia ruled out in suspected cases.

Drugs and prolactin

Medications commonly cause hyperprolactinemia and their use must be differentiated from pathologic causes. The most common medications to cause hyperprolactinemia are the antipsychotic agents, although some of the newer atypical antipsychotics do not do so. Other medications causing hyperprolactinemia include antidepressants, antihypertensive agents, and drugs which increase bowel motility. Often, the medication-induced hyperprolactinemia is symptomatic, causing galactorrhea, menstrual disturbance, and erectile dysfunction. In the individual patient, it is important differentiate hyperprolactinemia due to a medication from a structural lesion in the hypothalamic-pituitary area. This can be done by stopping the medication temporarily to determine if the prolactin (PRL) levels return to normal, switching to another medication in the same class which does not cause hyperprolactinemia (in consultation with the patient's physician and/or psychiatrist), or by performing an MRI or CT scan. If the hyperprolactinemia is symptomatic, management strategies include switching to an alternative medication which does not cause hyperprolactinemia, using estrogen/testosterone replacement, or cautiously adding a dopamine agonist.

Medication-induced hyperprolactinemia

Medication use is a common cause of hyperprolactinemia, and it is important to differentiate this cause from pathologic causes, such as prolactinomas. To ascertain the frequency of this clinical problem and to develop treatment guidelines, the medical literature was searched by using PubMed and the reference lists of other articles dealing with hyperprolactinemia due to specific types of medications. The medications that most commonly cause hyperprolactinemia are antipsychotic agents; however, some newer atypical antipsychotics do not cause this condition. Other classes of medications that cause hyperprolactinemia include antidepressants, antihypertensive agents, and drugs that increase bowel motility. Hyperprolactinemia caused by medications is commonly symptomatic, causing galactorrhea, menstrual disturbance, and impotence. It is Important to ensure that hyperprolactinemia in an Individual patient is due to medication and not to a structural lesion in the hypothalamic/pituitary area; this can be accomplished by (1) stopping the medication temporarily to determine whether prolactin levels return to normal, (2) switching to a medication that does not cause hyperprolactinemia (in consultation with the patient's psychiatrist for psychoactive medications), or (3) performing magnetic resonance imaging or computed tomography of the hypothalamic/pituitary area. If the patient's hyperprolactinemia is symptomatic, treatment strategies include switching to an alternative medication that does not cause hyperprolactinemia, using estrogen or testosterone replacement, or, rarely, cautiously adding a dopamine agonist.

Monoamine depletion in unmedicated depressed subjects

BACKGROUND

Although significant evidence suggests that diminished monoamine function is associated with clinical depression, catecholamine or indoleamine depletion alone has not been associated with significant mood changes in unmedicated depressed subjects or never-depressed control subjects. This study assesses the integrated role of these monoamine systems in depressed patients.

METHODS

Unmedicated depressed subjects underwent a 2-week, double-blind, random-ordered crossover study consisting of the following active and control conditions respectively: indoleamine (via tryptophan depletion) plus catecholamine (via alpha-methyl-paratyrosine administration) depletion and, separately, indoleamine plus sham (via diphenhydramine administration) catecholamine depletion. Ten subjects completed both conditions; two subjects were withdrawn after active testing and one after control testing.

RESULTS

Mean Hamilton Depression Rating Scale (HDRS) scores decreased progressively throughout the study days (baseline 26.7 points +/- 1.7 SEM and termination 20.0 +/- 2.4, active depletion; baseline 26.1 points +/- 2.3 SEM and termination 23.2 +/- 2.6, control testing) but did not differ between groups. Only three patients demonstrated 20% or greater increases from baseline HDRS at any point during the observation days.

CONCLUSIONS

Overall, results show that simultaneous disruptions of indoleamine and catecholamine function do not exacerbate symptoms in unmedicated depressed subjects, thus lending further support to the notion that monoamines regulate mood in actively depressed patients via indirect mechanisms.

Serotonergic neurons projecting to hippocampus activate locomotion

Although the role of brain serotonergic neurons in locomotion has been extensively studied, their influence may vary depending upon the terminal areas. Thus, using microdialysis and microinjection techniques, we examined the relationship between serotonin (5-HT) levels in striatum, hippocampus or prefrontal cortex (PFC) and motor activity in rats. The systemic injection (10 mg/kg i.p.) of monoamine oxidase inhibitor, tranylcypromine (TC), significantly elevated 5-HT levels in the striatum, hippocampus and PFC accompanied by a parallel increase in motor activity of the rats. This effect was mimicked by microinfusions of TC (1.0 mM) or 5-HT (1. 0 mM) into the hippocampus and to some extent into PFC (the response delayed in time), but not into striatum. The increase in motor activity produced by local infusions of TC either into the hippocampus or PFC could be prevented by pretreatment with 10 microM tetrodotoxin infused into the hippocampus. However, tetrodotoxin infused to PFC failed to prevent hyperlocomotion produced by intrahippocampal infusion of TC, although the response was delayed in time. Thus, we conclude that serotonergic neurons projecting to the hippocampus are involved in locomotor activity and PFC serotonergic fibers may facilitate hippocampal control of locomotion.

Effects of single and chronic treatment with tranylcypromine on extracellular serotonin in rat brain

We have examined the effects of tranylcypromine, a monoamine oxidase inhibitor used as antidepressant, on the tissue and extracellular concentration of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in frontal cortex and dorsal raphe nucleus using microdialysis in conscious rats. Single treatment with tranylcypromine sulphate (0.5, 3 and 15 mg/kg, i.p.) dose dependently elevated dialysate 5-HT in both areas but more markedly in the DRN. Extracellular and tissue 5-HT concentrations were affected by the drug in a different manner. The former increased sharply when tissue 5-HT reached a plateau. This may have reflected saturation of intracellular stores and overflow of the amine. In contrast, tissue and extracellular 5-HIAA concentrations--that indicate metabolic effects of tranylcypromine--were affected similarly. A 2-week treatment with 0.5 mg/kg.day of tranylcypromine sulphate increased basal extracellular 5-HT in frontal cortex and dorsal raphe nucleus (ca. 220%) whereas a further injection of 0.5 mg/kg was without effect in both areas. Thus, chronic, but not acute, treatment with low doses of tranylcypromine increases extracellular 5-HT concentration, suggesting that clinical effects of this monoamine oxidase inhibitor are related to its capacity to enhance serotonergic transmission.

D-fenfluramine-induced prolactin and cortisol release in major depression: response to treatment

In order to test the effects of various biological treatments on serotonergic function in depression, twenty-one patients with a diagnosis of major depression underwent neuroendocrine challenge tests before and after treatment with either ECT, fluoxetine or amitriptyline. The serotonin (5-HT) releasing agent d-fenfluramine was used as a challenge drug and cortisol (CORT) and prolactin (PRL) plasma levels were monitored over a 5-h period. Overall PRL responses were significantly enhanced following pharmacotherapy irrespective of therapeutic outcome. Effective treatment in each case lowered baseline CORT levels but CORT response to d-fenfluramine remained blunted. Hypercortisolaemia may be involved in the impaired pretreatment PRL response as a strong inverse relationship was established, for the combined studies, between basal CORT plasma concentrations and PRL responses.

A comparison of trazodone and fluoxetine: implications for a serotonergic mechanism of antidepressant action

Trazodone is an atypical antidepressant drug that is commonly referred to as a serotonin (5-hydroxytryptamine; 5-HT) uptake inhibitor. However, the most potent pharmacological effect of trazodone appears to be antagonist action at 5-HT2/1C receptors. This is in contrast to fluoxetine, for which inhibition of 5-HT uptake is the most potent pharmacological action. The effects of trazodone and fluoxetine on several antidepressant drug screens are mediated by antagonist action at 5-HT2 receptors and inhibition of 5-HT uptake, respectively. While fluoxetine is an effective agent for the treatment of major depression, obsessive-compulsive disorder (OCD) and panic disorder, trazodone does not appear to be effective in the treatment of OCD and panic disorder. In addition, trazodone and fluoxetine differ in humans with respect to their effects on sleep and weight. Taken together, the preclinical and clinical data suggest that trazodone acts as an antidepressant via antagonist action at 5-HT2/1C receptors, while fluoxetine likely acts as an antidepressant via inhibition of 5-HT uptake.

ECT treatment does not enhance neuroendocrine responses to serotonergic challenge

We prospectively investigated the effects of a course of electroconvulsive therapy (ECT) on neuroendocrine responses to serotonergic challenge in five depressed patients. Low dose intravenous chlorimipramine (CMI) challenge produced a modest release of prolactin and significant increases in plasma adrenocorticotrophic hormone (ACTH) and cortisol. Interestingly, ECT did not alter the neuroendocrine responses to serotonergic challenge despite clinical response in four of the five patients. If anything, the modest prolactin (PRL) response to CMI, rather than being enhanced, appeared to be abolished following ECT. Using confidence intervals, we estimate that there is less than a 5% probability of a 78% increase in prolactin response to CMI after ECT. To detect this, a sample size of greater than 35 would be needed. These findings suggest that neither ECT nor the clinical response in severely depressed patients is likely to produce consistent changes in neuroendocrine response to the acute serotonergic effects of CMI infusion. The lack of effect of ECT on prolactin response to serotonergic challenge might be explained by simultaneous enhancement of both serotonergic and dopaminergic neurotransmission.

Blunted growth hormone and prolactin responses to L-tryptophan in depression; a state-dependent abnormality

We have investigated whether attenuated growth hormone (GH) and prolactin (PRL) responses to L-tryptophan in depression return to normal with clinical recovery. Ten patients who had received intravenous infusions of L-tryptophan (100 mg/kg) when depressed were retested at least 3 months after full recovery and cessation of treatment. In recovered depressives growth hormone responses showed considerable recovery, in all but three cases to within a few units of their healthy age- and sex-matched controls. Prolactin responses increased with clinical recovery in all six male subjects. Results in females were inconclusive because of the effect of weight loss on prolactin responses. The results suggest that GH and PRL responses to tryptophan are state-dependent abnormalities rather than indicators of predisposition to depression. This allows the possibility that impaired functioning in systems with a 5HT1A or 5HT1D receptor link may be part of the causal chain in depression.

Corticosterone and prolactin response to TFMPP in rats during repeated antidepressant administration

The corticosterone and prolactin response to acute administration of the 5-HT agonist 1-(m-trifluoromethylphenyl) piperazine (TFMPP) (10 mg kg-1) was assessed in rats treated for 10 days with either saline, amitriptyline (20 mg kg-1 day-1) or nialamide (40 mg kg-1 day-1). For all groups, TFMPP significantly increased both serum corticosterone and prolactin concentrations compared with control animals challenged with saline. However, the corticosterone response to TFMPP was attenuated significantly by nialamide pretreatment, while the prolactin response to TFMPP was enhanced significantly by amitriptyline pretreatment. These results support previous reports that antidepressants differentially affect 5-HT-ergic systems involved in the regulation of corticosterone and prolactin secretion.

Effect of amitriptyline on endocrine responses to intravenous L-tryptophan

The endocrine responses to the 5-hydroxytryptamine (5HT) precursor, L-tryptophan, were assessed in 10 depressed patients before and after at least 4 weeks of treatment with amitriptyline. Overall, amitriptyline did not alter either prolactin or growth hormone responses to L-tryptophan. When three subjects with severe pretreatment weight loss were excluded, however, the remaining seven patients showed a significant increase in the prolactin response to L-tryptophan, consistent with other published studies. The findings suggest that severe recent weight loss may alter the effects of tricyclic antidepressants on 5HT-mediated prolactin release.

Depression and antidepressant therapy: receptor dynamics

1. The classical norepinephrine (NE) and serotonin (5-HT) theories of depression have been abandoned in light of recent chronic antidepressant drug studies. 2. The new NE and 5-HT theories of depression focus on the dynamics of receptor subtypes in depression and chronic antidepressant treatments. 3. Recent studies in molecular genetics suggest a reclassification of monoamine receptors based on receptor structural homologies in DNA and amino acid sequences rather than receptor affinity for ligands. 4. Electrophysiologic studies in rats suggest that 5-HT1 receptor function is facilitated by chronic antidepressant treatment. 5. Preclinical studies employing a range of 5-HT1 mediated behavioral models also suggest that chronic antidepressant treatment facilitates transmission at central 5-HT1 receptors. 6. Patient studies, employing a 5-HT1 mediated neuroendocrine model, suggest that depression is associated with decreased transmission at CNS 5-HT1 receptors; and that chronic antidepressant treatment facilitates 5-HT1 receptor responsiveness in depressed patients. 7. New 5-HT1 selective agonists have been developed and found to be clinically effective antidepressants. 8. The above clinical and preclinical data suggest that some forms of depression are related to a decreased responsiveness of 5-HT1 receptors which is reversed by chronic antidepressant treatment. 9. Beta adrenergic and NE-stimulated cyclic AMP studies suggest that chronic antidepressant treatment decreases the responsiveness of central beta-adrenergic receptors, particularly beta-1 receptors. 10. A novel approach to antidepressant drug development focuses on identifying centrally active beta-1 agonists, which like clinically proven antidepressants, decrease beta-1 receptor responsiveness with chronic treatment. 11. 5-HT2 receptor binding studies and initial studies of 5-HT2 receptor coupled PI turnover suggest that chronic antidepressant treatment decreases 5-HT2 receptor number and function. 12. The development of new atypical antidepressants with 5-HT2 receptor related mechanisms of action suggest that 5-HT2 receptors may be associated with certain types of depression and their clinical treatment.

A role for 5-HT in the action of antidepressant drugs

Administration of antidepressant drugs to rodents appears to decrease 5-HT2 receptor function while transmission through postsynaptic 5-HT1 receptor synapses may be enhanced. Antidepressant drugs also alter 5-HT mechanisms in humans; some of these changes are congruent with effects noted in animal studies. Thus certain 5-HT-mediated neuroendocrine responses are enhanced by tricyclic antidepressants and monoamine oxidase inhibitors, and it seems likely that tricyclics may act as 5-HT2 receptor antagonists in the human brain. However, there is presently no firm evidence that any of these changes are necessary for the clinical efficacy of antidepressant drugs. The best evidence that 5-HT neurons may be involved in mediating antidepressant effects is the antidepressant activity of selective 5-HT uptake blockers.

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.

Effects of trazodone treatment on serotonergic function in depressed patients

Changes in serotonergic (5HT) neurotransmission may mediate the therapeutic actions of some antidepressant drugs. In the present study, the 5HT precursor L-tryptophan (L-TRP) was administered intravenously to nine depressed patients before and during treatment with the triazolopyridine antidepressant trazodone (TRZ). Neuroendocrine, subjective mood, and cardiovascular responses to L-TRP were assessed. Unlike tricyclic antidepressants and monoamine oxidase inhibitors, TRZ did not enhance the prolactin response to L-TRP and had little effect on other measures. Since other studies indicate that the TRP-induced increase of prolactin in humans may reflect 5HT function, the present study suggests that TRZ treatment does not enhance net 5HT function in depressed patients.