Enhancement of 5-hydroxytryptamine-induced behavioral effects following chronic administration of antidepressant drugs

The effect of IV L-tryptophan on prolactin, growth hormone, and mood in healthy subjects

In order to assess the effects of increased CNS serotonergic function in humans on prolactin (PRL), growth hormone (GH), and mood, IV L-tryptophan (TRP) was administered to ten healthy subjects. The TRP infusion induced robust increases in PRL in all ten subjects. A significant increase in GH concentration was also observed, although the response was more variable. The subjects reported feeling significantly more 'high', 'mellow', and 'drowsy' following the TRP infusion in comparison to placebo. These findings indicate an important role for serotonin in PRL and GH secretion, as well as in mood regulation. The IV TRP challenge may be of use in the study of serotonergic function in a variety of neurologic and psychiatric diseases.

The effect of intravenous L-tryptophan on prolactin and growth hormone and mood in healthy subjects

In order to assess the effects of increased central nervous system serotonergic function in humans on prolactin (PRL), growth hormone (GH) and mood, intravenous L-tryptophan (TRP) was administered to ten healthy subjects. The TRP infusion induced robust increases in PRL in all ten subjects. A significant increase in GH concentration was also observed, although the response was more variable. The subjects reported feeling significantly more high, mellow, and drowsy following the TRP infusion in comparison to placebo. These findings indicate an important role for serotonin in PRL and GH secretion, as well as in mood regulation. The intravenous TRP challenge may be of use in the study of serotonergic function in a variety of neurologic and psychiatric diseases.

Physical exercise as a novel antidepressant agent: possible role of serotonin receptor subtypes

Long-term exercise is associated with an antidepressant effect in patients with mild to moderate forms of nonbipolar depression and appears to be a promising new approach to its treatment. Adaptive changes in serotonin (5-HT) receptor functioning appears to play an important role in mediating the action of various antidepressant treatments. We investigated the adaptive changes in behavioral sensitivity of the 5-HT receptor subtype following 4 weeks of swimming exercise in normal rats, as well as in an animal model of depression (3 week, variety of chronic stressors). 5-HT1A autoreceptor sensitivity was assessed by hyperphagic response induced by 8-OH-DPAT (0.25 mg/kg, IP); 5-HT1A postsynaptic receptor by 5-HT syndrome induced by 8-OH-DPAT (0.75 mg/kg, IP), and 5 Me-ODMT (5 mg/kg, IP); and 5-HT2 receptor by wet dog shakes response induced by quipazine (1 mg/kg, IP) and 5MeODMT (5 mg/kg, IP). It was observed that exercise training in normal rats resulted in enhanced sensitivity of the 5-HT2 receptors along with subsensitivity of 5-HT1A autoreceptors. Exercise, given prophylactically along with chronic stressors, was able to prevent the development of behavioral deficit in the open-field test, and the animals developed remarkably enhanced sensitivity of 5-HT2 receptors. This adaptive supersensitivity of 5-HT2 receptor is also seen after various antidepressant treatments and may play an important role in mediating the antidepressant action of exercise.

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

Long-term imipramine treatment potentiates m-chlorophenylpiperazine-induced changes in prolactin but not corticosterone or growth hormone levels in rats

Intravenous administration of m-chlorophenylpiperazine (m-CPP, a selective 5-HT agonist) to rats produced increases in plasma prolactin and corticosterone and a decrease in plasma growth hormone concentrations. Long-term but not short-term imipramine treatment potentiated m-CPP's effect on plasma prolactin, but not its effects on corticosterone or growth hormone. Short-term or long-term imipramine treatment did not produce significant changes in baseline levels of prolactin, corticosterone or growth hormone. These findings are compatible with development of functional supersensitivity of 5-HT receptors mediating prolactin release. Lack of potentiation of m-CPP's effects on corticosterone and growth hormone following long-term imipramine treatment suggests either differential regulation of these hormones by serotonergic and possibly other mechanisms, or different 5-HT receptor subtypes mediating the release of these hormones. Alternatively, adaptive changes in other aminergic neurotransmitter mechanisms such as the noradrenergic system may account for the differential effect of long-term imipramine treatment on m-CPP-induced neuroendocrine changes.

Rapid changes in functional responsiveness of the 5-HT system after single-dose and multiple-dose treatment with antidepressants: effects of maprotiline and oxaprotiline and its enantiomers

Functional responsiveness to 5-HTP (L-5-hydroxytryptophan) and quipazine 24 h after one single dose and 24 h and 72 h after repeated administration (2.5 and 10 mg/kg twice daily for 7 days) of maprotiline, oxaprotiline and its (+) and (-) enantiomers was tested in the rat. All compounds markedly increased the behavioural effects of 5-HTP 24 h after one single injection. This effect was further enhanced after repeated treatment. The response to quipazine was less consistently affected than the response to 5-HTP. Neither drug increased the response to quipazine 24 h after the last injection. However, maprotiline and oxaprotiline enhanced the quipazine-induced head-twitch 72 h after the withdrawal of subchronic treat ment, enantiomers failed to do so. This differential effect of drugs in respect to the interactions with 5-HTP and quipazine is, however, not understood at present. The results of this study indicate that only one single dose of antidepressants may change the functional respon siveness of the 5-HT system and that such changes occur rapidly. The results also provide a possible rationale for the suggested antidepressant activity of levoprotiline, the (-) enantiomer of oxaprotiline and otherwise biochemically inactive compound.

Differential effects of long-term antidepressant treatments on 8-OHDPAT-induced increases in plasma prolactin and corticosterone in rats

Intravenous administration of 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT) to rats produced increases in plasma prolactin and corticosterone concentrations. Long-term or short-term treatment with the MAO type A inhibiting antidepressant, clorgyline, or tricyclic antidepressants (imipramine and clomipramine), did not change baseline levels of either prolactin or corticosterone. Long-term but not short-term clorgyline treatment attenuated 8-OHDPAT's effect on plasma prolactin but not on corticosterone. On the other hand, long-term but not short-term treatment with clomipramine and to some extent imipramine also, accentuated 8-OHDPAT's effect on plasma prolactin but not on corticosterone. These findings demonstrate that long-term antidepressant treatment in rats produces a differential effect on 8-OHDPAT-induced increases in plasma prolactin and corticosterone, which is consistent with other clinical and animal studies demonstrating a differential effect of long-term antidepressant treatment on two different 5-HT-mediated neuroendocrine functions.

Long-term imipramine treatment differentially affects fenfluramine-induced suppression of food intake and locomotor activity

Administration of fenfluramine to rats produced decreases in one-hour food intake and locomotor activity. Short-term (2-6 days) or long-term (21-25 days) treatment with the tricyclic antidepressant, imipramine, did not affect daily food intake, body weight gain or baseline locomotor activity when compared to saline treatment. However, long-term but not short-term imipramine treatment attenuated fenfluramine-induced decreases in one-hour food intake. On the other hand, neither short-term nor long-term imipramine treatment affected fenfluramine-induced decreases in locomotor activity. These findings demonstrate a differential effect of long-term imipramine treatment on fenfluramine-induced suppression of food intake and locomotor activity.

Long-term imipramine treatment enhances locomotor and food intake suppressant effects of m-chlorophenylpiperazine in rats

1 Administration of the 5-HT1B receptor agonist m-chlorophenylpiperazine (m-CPP) to rats produces dose-dependent decreases in locomotor activity and food intake. 2 The locomotor suppressant effect of m-CPP was inhibited by the 5-hydroxytryptaminergic antagonist, metergoline, but not by phentolamine, propranolol, clonidine, or haloperidol. 3 The locomotor suppressant effects of m-CPP were enhanced following long-term (but not short-term) treatment with imipramine, possibly reflecting the postulated development of a functional supersensitivity of 5-HT1B receptors mediating locomotion during longer-term antidepressant drug treatment. 4 The food intake suppressant effects of m-CPP were enhanced following both short (3-5 days) and longer-term (21 days) treatment with imipramine. Rapidly developing 5-hydroxytryptamine uptake inhibition may be responsible for this change, or it may represent an earlier adaptive change in the 5-HT1B receptors mediating food intake compared to more complexly modulated motor responses.

Influence on 5-[3H]hydroxytryptamine binding site development in chick embryo by serotonergic compounds

Saturable and specific binding sites for 5-[3H]hydroxytryptamine (5-HT, serotonin) characterized by a KD of 3.5-4.5 nM were detected in the chick embryo brain and were shown to develop linearly as a function of age, weight, and protein content. Saturation and displacement studies using unlabeled 5-HT as the displacing ligand suggested a single population of binding sites. However, displacement studies using 5-methoxytryptamine, lysergic acid diethylamide (LSD), 2-bromo-lysergic acid diethylamide (BOL), methysergide, and spiperone as competing ligands suggested the existence of subclasses of [3H]5-HT binding sites because the Hill coefficients were less than unity. When compared with the reported [3H]5-HT binding sites (5-HT1) in the rat forebrain, the IC50 values of the competing ligands were similar. However, the Hill coefficients for LSD and methysergide were less than unity which suggested that the [3H]5-HT binding sites in the chick embryo brain may be more similar to those found in rat spinal cord than rat forebrain. To study [3H]5-HT binding site regulation and development, various serotonergic compounds were injected into the chorioallantoic fluid of the eggs at different times during embryonic development. Multiple pretreatments with d,l-5-hydroxytryptophan, 5-HT, or BOL were found to have no significant effects on either the affinity (KD) or number (Bmax) of specific [3H]5-HT binding sites. Multiple pretreatments with p-chlorophenylalanine were found to increase the Bmax of specific [3H]5-HT binding by 23% (p less than 0.01) whereas multiple pretreatments with LSD were found to decrease the Bmax of specific binding by 45% (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Antidepressants and serotonergic neurotransmission: an integrative review

The effects of acute and chronic antidepressant treatment on various aspects of 5-HT neurotransmission are reviewed, in order to assess the net effect of antidepressants on transmission across 5-HT synapses. Events considered include presynaptic effects of antidepressants (on autoreceptor function, uptake and turnover) and effects on postsynaptic receptor function (assessed by electrophysiological, neuroendocrine, behavioural, and receptor binding methods). Acute antidepressant treatment has variable effects: transmission may be enhanced, unchanged or reduced, depending mainly upon the relative contributions of 5-HT uptake blockade and 5-HT receptor antagonism. However, on chronic administration, most antidepressants appear to enhance 5-HT transmission. This effect is clearest in the case of ECS, which has little effect on 5-HT turnover, but reduces uptake and increases postsynaptic receptor function. MAOIs may be an exception: there is little evidence that MAOIs enhance 5-HT transmission following chronic treatment. Most other antidepressant drugs, including some which are powerful receptor antagonists on acute administration, reduce 5-HT receptor function briefly, but enhance receptor function if several hours elapse between the final injection and testing. Zimelidine has little effect on postsynaptic receptor function, but enhances 5-HT transmission by its powerful blockade of 5-HT uptake. Chronic treatment with antidepressant drugs has usually been found to reduce binding to 5-HT2 receptors; it is difficult to reconcile these observations with the functional studies. In general, with the possible exception of MAOIs, chronic administration of antidepressants may enhance 5-HT transmission by both pre- and post-synaptic effects, and the relative contributions vary. This conclusion supports the classical "indoleamine hypothesis of depression" rather than the more recent "hypersensitive serotonin receptor" theory.

Long-term effects of two antidepressants on copulatory and exploratory behaviour in the female rat

The immediate and long-term effects of antidepressant drugs on two different behaviours--copulatory and exploratory--were compared in hormone-treated, ovariectomized rats. The drugs used were desmethylimipramine (DMI 2 and 5 mg/kg) and zimelidine (ZIM 5 and 25 mg/kg). The acute effects were studied 1-4 hours after drug injection, and the long-term effects after four weeks of daily injections. The acute treatment with DMI and ZIM significantly suppressed the lordosis behaviour compared with that in saline-treated controls. After long-term administration of DMI, the 5 mg/kg dose of this drug was still able to inhibit the lordosis response, while the low dose of DMI and ZIM did not produce a significant effect after chronic administration. We suggest that subsensitivity to certain effects of the antidepressant drugs develops during the long-term treatment.

Attenuation by antidepressant drugs of alcohol intake in rats

Ethanol preferring rats were selected and showed a constant voluntary intake of a 12% ethanol solution during 14 days (about 5 g/kg body weight daily). These alcohol preferring rats were daily IP injected during two weeks with different antidepressant drugs, according to their specificity of action: nomifensine (5 and 10 mg/kg) and maprotiline (2.5 and 5 mg/kg) (dopamine uptake inhibitors), desipramine and metapramine (5 and 10 mg/kg) (noradrenaline uptake inhibitors) clomipramine and doxepin (5 and 10 mg/kg) (serotonin uptake inhibitors). Only desipramine, 5 and 10 mg/kg, metapramine, 10 mg/kg, clomipramine, 5 and 10 mg/kg and doxepin, 10 mg/kg, were able to reduce significantly the ethanol intake. These drugs specifically inhibit noradrenaline or serotonin uptake. These data lead us to think that norepinephrine and/or serotonin, but not dopamine, are involved in the voluntary intake of alcohol.

Influence of long-term zimelidine treatment on LSD-induced behavioural effects

The effect of lysergic acid diethylamide (LSD) on copulatory and exploratory behaviour in hormone-treated, ovariectomized rats was investigated 4 weeks after chronic treatment with either saline or zimelidine (ZIM). A decrease in sensitivity to the lordosis-inhibiting effect of LSD was observed in the group treated with ZIM. The two groups also differed in their reaction to LSD in the exploratory test. The ZIM-treated animals seemed to be less sensitive to LSD. We therefore suggest that long-term treatment with ZIM reduces the sensitivity of the postsynaptic serotonin receptor.

The effect of mianserin on alpha-2 adrenergic receptor function in depressed patients

Recent clinical investigations have shown that long term treatment with the tricyclic antidepressants desipramine and amitriptyline reduces the sensitivity of the alpha-2 adrenergic autoreceptor. In order to determine whether the tetracyclic antidepressant mianserin also has this action, the effect of clonidine, an alpha-2 adrenergic receptor agonist, on plasma levels of the norepinephrine metabolite 3-methoxy-4- hydroxyphenlethyleneglycol (MHPG), blood pressure, and patient-rated sedation were measured in fifteen depressed patients before and during mianserin treatment. Postsynaptic alpha-2 adrenergic receptor function was assessed by measuring the growth hormone response to clonidine before and during treatment. Mianserin had little or no effect on the ability of clonidine to lower plasma MHPG and blood pressure, and to increase sedation and growth hormone secretion. The findings of this investigation indicates that long term mianserin treatment does not produce significant subsensitivity of the alpha-2 adrenergic receptor and suggests that a reduction in alpha-2 adrenergic autoreceptor sensitivity is not a necessary action for all effective antidepressant treatments.

Effects of chronic antidepressant treatment on serotonin receptor activity in mice

The effect of acute and chronic treatments with conventional and atypical antidepressant drugs on serotonin receptor activity was assessed by the responsiveness of mice to the serotonin receptor agonist 5-methoxy-N,N-dimethyltryptamine. Acute treatment with 10 mg/kg of amitriptyline, imipramine, trazodone, mianserin or viloxazine reduced the head twitch response measured 1 h following a challenged dose of the serotonin agonist. Acute iprindole and desmethylimipramine, however, had no effect on the serotonergic response. Chronic treatment with the clinically effective antidepressants amitriptyline, imipramine, desmethylimipramine, iprindole, and trazodone produced an enhanced responsiveness to 5-MeODMT. The enhanced responsiveness was first observed 24 h after cessation of treatment with most drugs. The effect lasted for at least 48 h. Chronic treatment with the neuroleptic haloperidol did not result in altered responsivity to the serotonin agonist. Brain accumulation of imipramine and amitriptyline and their deaminated metabolites were measured. Brain drug and metabolite levels peaked 1 h following both acute and chronic treatments. Brain accumulations of amitriptyline and its metabolite were much greater than those of imipramine and its metabolite. This pharmacokinetic data is consistent with an early (1 h) antagonism of the 5-MeODMT response and the emergence of hightened responsiveness to 5-MeODMT after chronic treatment, when brain drug levels are reduced. These findings are also consistent with the greater inhibitory effect found after treatment with amitriptyline than with imipramine. It is concluded that enhanced serotonin neurotransmission which develops during chronic treatment with antidepressant drugs may be related to the clinical action of these drugs.

Serotonin receptor changes after chronic antidepressant treatments: ligand binding, electrophysiological, and behavioral studies

Early biochemical research on antidepressant treatments provided evidence that the treatments alter catecholaminergic and serotonergic activity. The mechanisms of action proposed by the resulting biogenic amine hypotheses of affective disorders, however, are not consistent with the delayed onset of therapeutic effects of antidepressant treatments nor with the acute effects of more recently developed antidepressant drugs. Recent investigation of chronic antidepressant treatments using ligand binding, electrophysiological, and behavioral techniques have attempted to identify subgroups of receptors that might be affected uniquely and specifically by chronic antidepressant treatments. Such receptor changes have been suggested to form a basis for the mechanism of action of antidepressants. At the present time, however, the data produced by ligand binding experiments and electrophysiological experiments investigating serotonergic functioning do not fit together. In addition, interpretational problems and internal contradictions exist within each of the three bodies of data when straightforward hypotheses regarding a serotonergic role in antidepressant treatment are formulated. In order to clarify the serotonergic role in antidepressant drug and ECS effects the functional significance of observed changes in putative serotonergic receptors must be discovered. Unfortunately, putative receptors identified by ligand binding cannot be directly compared to those identified by electrophysiological techniques, because these two methods require the disassembly of the organism in mutually incompatible ways. In order to prove that either or both techniques do in fact identify functional serotonin receptors, investigators need to proceed both more microscopically and also more globally. Further anatomical and physiological studies are necessary to locate putative receptors and to demonstrate their place in existing serotonergic networks. Further behavioral studies must be done to relate alterations in receptor characteristics to the functioning of the intact organism.

Tryptophan and tyrosine ratios to neutral amino acids in endogenous depression. Relation to antidepressant response to amitriptyline and lithium + L-tryptophan

The plasma ratios of tryptophan and tyrosine to those amino acids that compete with them during transport across the blood-brain barrier have been determined in depressed patients before and after treatment for four weeks with amitriptyline or lithium + L-tryptophan. There was no relation between the absolute plasma concentrations of free or total tryptophan or tyrosine and the clinical response to amitriptyline. There was also no relation between pre-treatment ratio of plasma tyrosine to competing amino acids and response to amitriptyline, but depressives with subnormal tryptophan ratio improved significantly more than patients with supernormal tryptophan ratio with comparable serum drug levels. The therapeutic response to lithium + L-tryptophan was predicted neither by the absolute plasma concentrations of free or total tryptophan or tyrosine nor by the tyrosine ratio, but there was also a trend towards greater improvement in patients with subnormal compared with supernormal tryptophan ratio. The results suggest that the pre-treatment plasma ratio of tryptophan to competing amino acids is a useful predictor of clinical response to amitriptyline. The possible mode of action of amitriptyline and lithium + L-tryptophan is briefly discussed.

Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Acutely administered antidepressants possess a multiplicity of pharmacological actions. However, the fact that agents possessing similar pharmacological actions are devoid of antidepressant activity, together with the lack of correlation between doses required for acute pharmacological effects and clinical efficacy, suggest that the mechanism(s) of action of antidepressants cannot be directly attributed to the acute pharmacological properties of the drugs. The lag phase in onset of clinical effectiveness emphasizes the importance of adaptive changes following chronic antidepressant administration. A rapidly accelerating trend in attempting to delineate the precise molecular mechanisms of action of antidepressants is the shift in emphasis following chronic antidepressant therapies from alterations in uptake, storage, synthesis and release of neurotransmitters to adaptive changes in receptor functioning. These adaptations occur both pre- and postsynaptically. Examples of the former are alpha 2 and DA presynaptic receptors, both being down-regulated by certain forms of chronic antidepressant therapy. The fact that the NE-coupled adenylate cyclase system in rat brain slices is down-regulated by tricyclics, atypical antidepressants, MAO inhibitors and ECT emphasizes the importance of the system. Electrophysiological and behavioral studies point to the up-regulation of central alpha 1 and 5-HT receptor functioning following long-term antidepressant therapy. In contrast to the beta-adrenoceptor, these findings cannot be correlated with data from radioligand binding studies. In general central alpha 1-adrenoceptor binding remains unaltered. This is also true for 5-HT1 binding whereas cortical 5-HT2 binding is both increased and decreased depending on the type of antidepressant therapy being investigated. The relationship of these adaptive changes to the clinical efficacy of antidepressants in man is not clear since there is generally a lack of good models for studying human central receptor functioning. A review of current data from animal studies would tend to disfavour the view that all forms of antidepressant therapy possess a common mechanism of action. Perhaps multiple intervention sites exist. The introduction and evaluation of agents possessing a specificity of pharmacological action will undoubtedly aid psychotherapeutic research. The knowledge that peptides and 'classical' neurotransmitters can co-exist in the same neurone will undoubtedly generate studies of the significance and importance of the co-transmitter function of peptides in the mechanisms of action of antidepressant therapies.