Effect of chronic tricylic antidepressant treatment on the serotoninergic autoreceptor: a microiontophoretic study in the rat

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Les temps de la dépression

Revue à travers les données de la littérature, la dépression apparaît comme un sujet de recherche très important, qui a déjà suscité de nombreuses théories chronobiologiques, souvent fondées sur des ≪critères diagnostiques objectifs≫. En revanche, l’impact thérapeutique de ces approches reste souvent limité à de rares services ultraspécialisés où les patients déprimés sont adressés en dernier recours.

Dans cet article, les auteurs passent en revue quelques-unes des principales données de la littérature, en particulier celles qui concernent les aspects chronobiologiques de la dépression. Les investigations sophistiquées réalisées dans les unités de recherche peuvent avoir certaines conséquences cliniques et pratiques qui sont également évoquées. En effet, une évaluation clinique soigneuse, orientée sur des bases chronobiologiques suffit bien souvent à proposer des traitements individuels assez simples. Parmi ceux-ci, la privation partielle de sommeil (PPS) représente un moyen utile, permettant une amélioration rapide, mais malheureusement souvent imprévisible et transitoire, des troubles de l’humeur. Il semble possible de rendre plus fiables les PPS grâce à des interventions sur la température, la prise de nourriture et l’exposition à la lumiére.

De plus, les auteurs décrivent un moyen possible de pérenniser les effets antidépresseurs de la PPS par des «microprivations de sommeil» réalisées pendant 15 min, à un moment spécifique de la nuit. Les résultats concernant 11 patients déprimés graves soumis à cette méthode sont exposés, 7 de ces sujets étant maintenus depuis 6 à 20 mois dans un état euthymique.

Serotonin and beyond: therapeutics for major depression

The serotonin (5-HT, 5-hydroxytryptamine) system has been implicated in the pathogenesis of major depressive disorder (MDD). The case for its contribution to the therapeutic efficacy of a wide variety of antidepressant treatments is, however, much stronger. All antidepressant strategies have been shown to enhance 5-HT transmission in the brain of laboratory animals. Catecholamines, norepinephrine (NE) and dopamine (DA) can also play a pivotal role in the mechanism of action of certain antidepressant strategies. The enhancement of 5-HT transmission by selective serotonin reuptake inhibitors, which leads to a dampening of the activity of NE and DA neurons, may account in part for the low remission rate achieved with these medications and/or the residuals symptoms after remission is achieved. The functional connectivity between the 5-HT, NE and DA systems can be used to understand the mechanism of action of a wide variety of augmentation strategies in treatment-resistant MDD. Proof-of-concept studies have shown that antidepressant medications with complementary mechanisms of action on monoaminergic systems can double the remission rate achieved in a trial of standard duration. Novel approaches are also being used to treat MDD, which also appear to involve the monoaminergic system(s) to a varying extent.

Selective deletion of forebrain glycogen synthase kinase 3β reveals a central role in serotonin-sensitive anxiety and social behaviour

Serotonin (5-HT) neurotransmission is thought to underlie mental illnesses, such as bipolar disorder, depression, autism and schizophrenia. Independent studies have indicated that 5-HT or drugs acting on 5-HT neurotransmission regulate the serine/threonine kinase glycogen synthase kinase 3β (GSK3β). Furthermore, GSK3β inhibition rescues behavioural abnormalities in 5-HT-deficient mice with a loss-of-function mutation equivalent to the human variant (R441H) of tryptophan hydroxylase 2. In an effort to define neuroanatomical correlates of GSK3β activity in the regulation of behaviour, we generated CamKIIcre-floxGSK3β mice in which the gsk3b gene is postnatally inactivated in forebrain pyramidal neurons. Behavioural characterization showed that suppression of GSK3β in these brain areas has anxiolytic and pro-social effects. However, while a global reduction of GSK2β expression reduced responsiveness to amphetamine and increased resilience to social defeat, these behavioural effects were not found in CamKIIcre-floxGSK3β mice. These findings demonstrate a dissociation of behavioural effects related to GSK3 inhibition, with forebrain GSK3β being involved in the regulation of anxiety and sociability while social preference, resilience and responsiveness to psychostimulants would involve a function of this kinase in subcortical areas such as the hippocampus and striatum.

Substance P Neurokinin 1 Receptor Activation within the Dorsal Raphe Nucleus Controls Serotonin Release in the Mouse Frontal Cortex

Preclinical studies suggest that substance P (SP) neurokinin 1 (NK1) receptor antagonists are efficient in the treatment of anxiety and depression. This therapeutic activity could be mediated via stimulation of serotonin (5-HT) neurons located in the dorsal raphe nucleus (DRN), which receive important SP-NK1 receptor immunoreactive innervations. The present study examined the effects of intraraphe injection of SP on extracellular 5-HT levels in the frontal cortex, ventral hippocampus, and DRN by using intracerebral microdialysis in conscious mice. Intraraphe SP injection dose dependently decreased cortical 5-HT release, whereas no effects were detected in the ventral hippocampus. Cortical effects were blocked by the selective NK1 receptor antagonist N-[[2-methoxy-5-[5-(trifluoromethyl)tetrazol-1-yl]phenyl]methyl]-2-phenylpiperidin-3-amine (GR205171) and completely dampened in mice lacking NK1 receptors. Furthermore, genetic (in knockout 5-HT1A(-/-) mice) or pharmacological inactivation of 5-HT1A autoreceptors blocked cortical responses to SP. Contrasting with its cortical effects, intraraphe SP injection increased 5-HT outflow in the DRN in wild-type mice; this effect was potentiated by a local perfusion of the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635). Finally, SP-induced changes in frontal cortex and DRN dialysate 5-HT levels were blocked by the DRN perfusion of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate ionotropic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These data support the hypothesis that SP-induced over-activation of 5-HT1A autoreceptors within the DRN limits cortical 5-HT release. A better knowledge of the complex relationship between tachykininergic, serotonergic, and glutamatergic systems within the DRN might help better understand the pathophysiology and subsequent treatment of depression.

Effects of triiodothyronine on 5-HT(1A) and 5-HT(1B) autoreceptor activity, and postsynaptic 5-HT(1A) receptor activity, in rat hypothalamus: lack of interaction with imipramine

Triiodothyronine (T3) is effective in both augmenting and accelerating the therapeutic response to antidepressant drugs, especially tricyclics, and there is evidence from both human and animal studies that it acts on serotonergic neurotransmission. In this work we examined the effects of T3 alone and together with imipramine on 5-HT levels in the hypothalamus and on 5-HT(1A) and 5-HT(1B) autoreceptor sensitivity, using in vivo microdialysis in the rat. The effects of T3 on postsynaptic 5-HT(1A) receptor activity in the hypothalamus were also determined using a neuroendocrine challenge procedure. T3 administered daily at 20 microg/kg s.c. for 2 weeks reduced the sensitivity of 5-HT(1A) autoreceptors which control 5-HT release, as measured by the effect of 8-OH-DPAT to decrease 5-HT in the hypothalamus, and also the sensitivity of hypothalamic 5-HT(1B) receptors as measured by the effect of the 5-HT(1B) receptor agonist CP 93129 to decrease 5-HT release. Imipramine at 10 mg/kg daily for 4 weeks by osmotic minipump reduced 5-HT(1A) autoreceptor activity, as measured by the effect of 8-OH-DPAT in the hypothalamus, but the combination of T3 and imipramine given for 2 weeks did not affect either 5-HT(1A) or 5-HT(1B) autoreceptor activity. T3 at 20 microg/kg s.c. given daily for 1 week also reduced the sensitivity of postsynaptic 5-HT(1A) receptors in the hypothalamus, as measured by injection of 8-OH-DPAT and determination of the plasma ACTH and corticosterone responses. Animals which received T3 for 7 days showed a dose-dependent reduction in plasma free T4 levels but no change in total T3 levels. We conclude that while T3 alone affects both presynaptic and postsynaptic components of the serotonergic system, these effects may not be responsible for the therapeutic acceleration action seen with a combination of a tricyclic drug and T3.

Shared mechanisms of antidepressant and antiepileptic treatments: drugs and devices

Many treatments for the epilepsies and affective disorder share the properties of seizure suppression and mood stabilization. Moreover, affective disorders and the epilepsies appear to share partially similar pathogenic mechanisms. A component of the shared predisposition appears to arise from noradrenergic and serotonergic deficits. Increasing evidence supports the hypothesis that noradrenergic and/or serotonergic elevation is a mechanism of therapeutic benefit shared by most antidepressants and many antiepileptic medications. Medication induced alterations in GABAergic, glutamatergic, and CRH (corticotropin releasing hormone) containing neurons may also contribute to the shared therapeutic properties of antidepressant and antiepileptic medications.

Selective decrease in serotonin synthesis rate in rat brainstem raphe nuclei following chronic administration of low doses of amitriptyline: an effect compatible with an anti-migraine effect

The effects of chronic, low-dose amitriptyline on serotonin (5-HT) synthesis rate were measured in rat brain using autoradiography and the trapping of alpha-[14C]-methyl-L-tryptophan (alpha-[14C]-MTrp). Rats received amitriptyline (2 mg/kg per day) or saline via intraperitoneal osmotic minipumps for 21 days. Amitriptyline had no effect on any physiological parameters measured, or on free or total plasma tryptophan levels. However, amitriptyline exerted selective decreases of 15% and 17% (P < 0.001) in serotonin synthesis rates in the dorsal and median raphe nuclei, respectively. There was no reduction in any of the projection areas studied, including the cerebral cortex, hippocampus, thalamus, hypothalamus or striatum. The data suggest that chronic low doses of amitriptyline can lead to sustained 5-HT re-uptake inhibition selectively in the raphe nuclei, an effect compatible with tonic activation of 5-HT(1A) autoreceptors and inhibition of 5-HT synthesis. The failure of chronic amitriptyline treatment to affect 5-HT synthesis rate in the projection areas may ensure an adequate regulation of pain pathways implicated in migraine headache, an effect possibly related to amitriptyline anti-migraine efficacy.

Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration

Adaptive changes in the serotonergic system are generally believed to underlie the therapeutic effectiveness of the azapirone anxiolytics and a variety of antidepressant drugs. The serotonin-1A (5-HT(1A)) receptor has been implicated in affective disorders. Thus, studies of the regulation of 5-HT(1A) receptor function may have important implications for our understanding the role of this receptor in the mechanism of action of these therapeutic agents. This review focuses on the regulation of central 5-HT(1A) receptor function following administration of 5-HT(1A) receptor agonists or antidepressant drugs expected to increase the synaptic concentration of the neurotransmitter 5-HT. The majority of evidence supports regional differences in the regulation of central 5-HT(1A) receptor function following repeated agonist or antidepressant administration, which may be due to differences in processes involved in desensitization of the receptor at the cellular level. Region-specific differences in the regulation of 5-HT(1A) receptor function may be based on compensatory changes distal to the receptor, such as regulatory changes at the level of effector (e.g. adenylyl cyclase or ion channel), or at the level of the G protein such as changes in G protein expression, or phosphorylation of the G protein. It may be that the increase in serotonin neurotransmission, due to somatodendritic autoreceptor desensitization following agonist or antidepressant treatment, to normo-sensitive 5-HT(1A) receptors in certain brain regions (e.g. hippocampus or cortex) and to sub-sensitive 5-HT(1A) receptors in other brain regions (e.g. amygdala or hypothalamus) underlies the therapeutic efficacy of these drugs.

Repeated administration of milnacipran induces rapid desensitization of somatodendritic 5-HT1A autoreceptors but not postsynaptic 5-HT1A receptors

The effects of the repeated administration of milnacipran, a serotonin (5-HT)-noradrenaline reuptake inhibitor (SNRI), on the functional status of somatodendritic 5-HT1A receptors, and postsynaptic 5-HT1A receptors were explored using electrophysiological approaches in rats. In-vitro electrophysiological recordings in the dorsal raphe nucleus showed that 5-HT inhibited the firing of serotonergic neurones, and the selective 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY 100635), reversed the inhibitory effect of 5-HT. The potency of 5-HT to inhibit the firing of serotonergic neurones was slightly attenuated after 3 days of treatment with milnacipran (30 mg/kg, p.o., twice daily), and significantly attenuated after 7 or 14 days treatment at the same dose. The tricyclic antidepressant, imipramine, did not significantly modify the inhibitory effect of 5-HT. After 7 days treatment at 30 mg/kg, p.o., once daily, milnacipran reduced the potency of 5-HT to inhibit the firing of serotonergic neurones, whereas the selective serotonin reuptake inhibitors, fluvoxamine and fluoxetine (60 and 30 mg/kg, p.o., once daily, respectively), did not modify it under these conditions. Treatment with milnacipran (30 mg/kg, p.o., twice daily) for 14 days did not change the inhibition of the CA1 field potential in rat hippocampal slices by 5-HT. These data suggest that somatodendritic 5-HT1A receptors, but not postsynaptic 5-HT1A receptors, rapidly desensitize in response to the repeated administration of milnacipran.

Functional and pharmacological characterization of the modulatory role of serotonin on the firing activity of locus coeruleus norepinephrine neurons

Previous studies, using in vivo extracellular unitary recordings in anaesthetized rats, have shown that the selective 5-HT(1A) receptor antagonist WAY 100,635 suppressed the firing rate of locus coeruleus (LC) norepinephrine (NE) neurons and that this effect was abolished by lesioning 5-HT neurons. In the present experiments, the selective 5-HT(2A) receptor antagonist MDL 100,907, while having no effect on the spontaneous firing activity of LC neurons in controls, was able to restore NE neuronal discharges following the injection of WAY 100,635. The 5-HT(1A) receptor agonist 8-OH-DPAT enhanced the firing activity of NE neurons and this action was entirely dependent on intact 5-HT neurons, unlike the inhibitory effect of the 5-HT(2) receptor agonist DOI. Taken together, these data indicate that 5-HT(2A) but not 5-HT(1A) receptors controlling LC firing activity are postsynaptic to 5-HT neurons. Prolonged, but not subacute, administration of selective 5-HT reuptake inhibitors (SSRIs) produces a decrease in the spontaneous firing activity of LC NE neurons. MDL 100,907 partially reversed this suppressed firing activity of LC neurons in paroxetine-treated rats. Although the alpha(2)-adrenoceptor antagonist idazoxan also enhanced the firing activity of NE neurons in paroxetine-treated rats, this increase was similar to that obtained in controls. In conclusion, prolonged SSRI treatment enhances a tonic inhibitory influence by 5-HT on LC neurons through postsynaptic 5-HT(2A) receptors that are not located on NE neurons. A speculative neuronal circuitry accounting for these phenomena on LC NE activity is proposed.

Effect of the selective noradrenergic reuptake inhibitor reboxetine on the firing activity of noradrenaline and serotonin neurons

Reboxetine is a non-tricyclic antidepressant with selective noradrenergic (NA) reuptake-blocking effects. The effects of acute and sustained administration of reboxetine, on the firing activity of locus coeruleus NA neurons and dorsal raphe 5-HT neurons, were assessed using in vivo extracellular unitary recording in rats anaesthetized with chloral hydrate. Reboxetine (0.1-1.25 mg/kg, i.v.) dose-dependently decreased the firing activity of NA neurons (ED50 = 480 +/- 14 microg/kg). A 2-day treatment with reboxetine at 1.25, 2.5, 5, or 10 mg/kg per day (using osmotic minipumps implanted subcutaneously) produced significant decreases of 52%, 68%, 81%, and 83%, respectively, of NA firing activity. When the reboxetine treatment (2.5 mg/kg per day) duration was prolonged to 7 days, a 66% decrease in NA firing activity was observed which further decreased to 80% after 21 days of treatment. In contrast, 5-HT neuron firing rate remained unaltered following short- and long-term reboxetine treatments. The suppressant effect of the alpha2-adrenoceptor agonist clonidine on the firing activity of NA neurons was unchanged in long-term reboxetine-treated rats, but its effect on the firing activity of 5-HT neurons was blunted. The enhancement of NA firing activity by the 5-HT1A agonist 8-OH-DPAT was abolished in long-term reboxetine-treated rats, whereas, the inhibitory effect of the 5-HT2 agonist DOI was attenuated by about three-fold. In conclusion, sustained NA reuptake blockade by reboxetine lead to profound alterations in the function of NA neurons and of 5-HT receptors modulating their firing activity.

Role of the medial prefrontal cortex in 5-HT1A receptor-induced inhibition of 5-HT neuronal activity in the rat

1. We examined the involvement of the frontal cortex in the 5-HT2A receptor-induced inhibition of 5-HT neurones in the dorsal raphe nucleus (DRN) of the anaesthetized rat using single-unit recordings complemented by Fos-immunocytochemistry. 2. Both transection of the frontal cortex as well as ablation of the medial region of the prefrontal cortex (mPFC) significantly attenuated the inhibition of 5-HT neurones induced by systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.5-16 microg kg(-1), i.v.). In comparison, the response to 8-OH-DPAT was not altered by ablation of the parietal cortex. The inhibitory effect of 8-OH-DPAT was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.) in all neurones tested. 3. In contrast, cortical transection did not alter the sensitivity of 5-HT neurones to iontophoretic application of 8-OH-DPAT into the DRN. Similarly, cortical transection did not alter the sensitivity of 5-HT neurones to systemic administration of the selective 5-HT reuptake inhibitor, paroxetine (0.1-0.8 mg kg(-1) , i.v.). 4. 8-OH-DPAT evoked excitation of mPFC neurones at doses (0.5-32 microg kg(-1), i.v.) in the range of those which inhibited 5-HT cell firing. At higher doses (32-512 microg kg(-1), i.v.) 8-OH-DPAT inhibited mPFC neurones. 8-OH-DPAT (0.1 mg kg(-1), s.c.) also induced Fos expression in the mPFC. The neuronal excitation and inhibition, as well as the Fos expression, were antagonized by WAY 100635. 5. These data add further support to the view that the inhibitory effect of 5-HT1A receptor agonists on the firing activity of DRN 5-HT neurones involves, in part, activation of a 5-HT1A receptor-mediated postsynaptic feedback loop centred on the mPFC.

Effect of acute, short- and long-term milnacipran administration on rat locus coeruleus noradrenergic and dorsal raphe serotonergic neurons

The effect of milnacipran on the firing activity of dorsal raphe serotonin (5-HT) neurons and locus coeruleus norepineprine (NE) neurons was assessed using extracellular unitary recording in chloral hydrate anesthetized rats. A 2-day treatment with milnacipran (20 or 60 mg/kg/day, s.c.) markedly decreased the firing rate of NE neurons, and it remained reduced after a 7- or a 14-day treatment. Although the suppressant effect of the alpha2-adrenergic agonist clonidine on the firing rate of NE neurons was markedly reduced following long-term milnacipran (60 mg/kg/day x 14 days, s.c.), that of NE remained unchanged. The firing rate of 5-HT neurons was reduced following a 2-day treatment with milnacipran (20 mg/kg/day, s.c.), but there was a partial recovery after a 7-day treatment (20 mg/kg/day, s.c.) and a complete one after a 14-day treatment (20, 40 or 60 mg/kg/day, s.c.). The suppressant effect of 5-HT and of the 5-HT1A agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) on the firing rate of 5-HT neurons was also unaltered after milnacipran (60 mg/kg/day x 14 days, s.c.). The latter milnacipran treatment did not affect the uptake of [3H]5-HT but it inhibited that of [3H]NE by 30% in hippocampal slices. The NE system was thus investigated in an attempt to explain the effects of milnacipran on the firing activity of 5-HT neurons. Acute injection of milnacipran suppressed the firing activity of 5-HT neurons (with an ED50 of 5.7+/-1.5 mg/kg, i.v.), but not in NE-denervated rats. Furthermore, the inhibitory effect of clonidine on 5-HT neuron firing activity was markedly reduced by the long-term milnacipran treatment, whereas the inhibition of electrically evoked release of [3H]NE as well as that of [3H]5-HT produced by the alpha2-adrenoceptor agonist UK 14.304 from preloaded mesencephalic slices containing the dorsal raphe was unaltered. The latter results indicate that the alpha2-adrenergic autoreceptor and heteroreceptor were unaffected in the raphe area by milnacipran. In conclusion, milnacipran had profound effects on the function of 5-HT and NE neurons, and the mechanism by which 5-HT neurons regained their normal firing during milnacipran treatment appeared to implicate the NE system.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

Alterations in responses to LSD in humans associated with chronic administration of tricyclic antidepressants, monoamine oxidase inhibitors or lithium

This study sought to investigate possible interactions between antidepressant agents and lysergic acid diethylamide (LSD) in humans through the use of retrospective questionnaires. Ten subjects were identified who used LSD during chronic (3 weeks or longer) periods of antidepressant administration. These subjects were asked to describe the phenomenological effects of self-administered hallucinogens prior to and during antidepressant treatment; a structured, standardized questionnaire was used to evaluate LSD experiences. Chronic tricyclic antidepressant administration was associated with subjective increases in physical, hallucinatory and psychological responses to LSD. Similarly, subjects receiving lithium chronically also reported increases in their responses to LSD. In contrast, subjects who had been chronically taking an monoamine oxidase (MAO) inhibitor reported subjective decreases in the effects of LSD. This is similar to a previous report by our group of a decreased response to LSD in individuals who were chronically taking serotonin-selective antidepressants. These altered responses to LSD most likely involve differential changes in central serotonin and dopamine receptor systems and are consistent with other recent data suggesting that the clinical efficacy of different classes of antidepressants may not necessarily rely on a common mechanism of action in the brain.

Effect of repeated electroconvulsive shocks on serotonergic neurons

The hypothermia induced by the serotonin (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was attenuated in rats that had received a course of six electroconvulsive shocks (ECS) over a two-week period. The firing activity of dorsal raphe 5-HT neurons, as well as their responsiveness to microiontophoretic applications of 5-HT and 8-OH-DPAT, was unaltered in ECS-treated rats. The electrically evoked overflow of [3H]5-HT from preloaded slices of guinea pig hypothalamus was unchanged after the same ECS treatment. The concentration-effect curves of the 5-HT autoreceptor agonist 5-carboxyamidotryptamine (0.1-100 nM) were similar in slices prepared from control and ECS-treated guinea pigs. In addition, the reduction in the evoked [3H]5-HT overflow obtained by increasing the stimulation frequency from 1 to 5 Hz, which is due to a greater activation of terminal 5-HT autoreceptors at the higher frequency, was not altered by the ECS treatment. The enhancing effects of the 5-HT autoreceptor antagonist methiothepin (0.1-1 microM) and of the 5-HT3 agonist 2-methyl-5-HT (0.1-1 microM) on the evoked [3H]5-HT overflow were unaltered by the ECS treatment. These results thus indicate that repeated ECS attenuates the 8-OH-DPAT-induced hypothermia in rats, as previously reported, but does not affect the firing activity of 5-HT neurons and the sensitivity of their somatodendritic 5-HT1A autoreceptors in the dorsal raphe. The function of 5-HT terminals in the guinea pig hypothalamus was also unaffected by repeated ECS. In conclusion, repeated ECS does not affect the function of 5-HT neurons at the cell body and nerve terminal.

Psychoactive drugs and human sexual behavior: the role of serotonergic activity

A wide range of both prescription and nonprescription drugs has been reported to affect human sexual functioning. While the sexual side effects resulting from drug use have often been attributed to adrenergic, anticholinergic or dopaminergic activity, the present review considers the potential role of serotonin. Based on animal studies, serotonin has been shown to either facilitate or inhibit sexual activity depending on which serotonin receptor subtype is activated. However, few studies have been done in the human that assess the effects of drugs that bind selectively to serotonin receptors. Consequently, little is known about the role of serotonin in human sexual functioning. In this review, a wide range of drugs that affect both brain serotonergic systems and human sexual behavior is examined in an effort to determine the possible role of serotonin in human sexual behavior. A review of the literature is consistent with the hypothesis that the 5-HT1A and the 5-HT2 receptor subtypes play a facilitatory role in human sexual behavior. The evidence suggests that drugs that act as agonists on these receptor sites enhance sexual functioning in the human, while those that act as antagonists inhibit sexual functioning.

Differential effects of ovarian steroid hormones on beta-adrenoceptor downregulation caused by the antidepressants imipramine and rolipram

Downregulation of beta-adrenoceptors in response to repeated administration of imipramine or rolipram was investigated in the brain of pregnant rats and ovariectomized animals treated with estrone, progesterone or a combination of both hormones. Ovariectomy alone was without influence on drug responses. Pregnancy and complete hormone replacement of animals prior to drug treatment halved the response to imipramine and obliterated that to rolipram. Administration of only estrone to ovariectomized rats did not affect the extent of adrenergic downregulation caused by imipramine, but fully suppressed the rolipram action. Castrated males with or without treatment with estrone and progesterone were used as further controls. Castration alone diminished the effects of both antidepressants. Treatment of gonadectomized rats with both ovarian steroid hormones voided the actions of rolipram. The data indicate that imipramine and rolipram share a common pathway for their mechanism of action which can be attenuated by ovarian steroid hormones. In addition, imipramine has a second major site of action which is not subject to modulation by female steroids.

Correlative changes of serotonin and catecholamines with pharmacokinetic alterations of imipramine in rat brain

Rats were given i.p. imipramine (20 mg/kg), acutely or chronically, and the levels of serotonin (5-HT), norepinephrine (NE), dopamine (DA) and their metabolites in the brain at different times were compared with the concentrations of imipramine and desipramine. The levels of 5-HT, DA, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain did not appear to be affected by quantitative alterations in the concentrations of imipramine and desipramine. The level of 5-hydroxyindole acetic acid (5-HIAA) was reduced and the level of 3-methoxy-4-hydroxyphenylglycol (MHPG) tended to decrease 3 h after imipramine administration in acutely treated rats. The reduced level of 5-HIAA was maintained during the chronic treatment with imipramine, whereas the MHPG level increased and the NE level decreased. The decrease in 5-HIAA depended on the concentration of imipramine in the brain, whereas the changes in the levels of NE and MHPG appeared to be caused by desipramine. The present studies show that pharmacokinetic variations of imipramine in the brain might correlate with the altered levels of 5-HIAA, NE and MHPG.

Effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system

1. A review of the effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system is presented. 2. The effects of antidepressants on adenylate cyclase activity and on receptor binding in brain tissue are discussed. Effects on a variety of receptor types are considered. 3. The utilization of electrophysiological, behavioral, and neurochemical studies to assess receptor function after chronic antidepressant administration is discussed, as is the use of peripheral receptor estimations in clinical studies. 4. Animal studies on the actions of chronic administration of neuroleptics on pre- and postsynaptic dopamine receptors are reviewed. Effects of these drugs on dopamine receptors in humans are considered from the following perspectives: postmortem and in vivo binding studies in schizophrenia, tardive dyskinesia, and central versus peripheral receptor estimation.

Autoradiographic quantification of serotonin1A receptors in rat brain following antidepressant drug treatment

There is growing evidence that the serotonergic (5-HT) system is involved in the pathogenesis and treatment of major depression. The 5-HT receptor subtype involved in the enhancing effect of antidepressant treatments, however, has not been identified. The present study was undertaken to quantify 5-HT1A sites in the rat brain by autoradiography and membrane binding, using the selective ligand [3H]8-hydroxy-N,N-dipropyl-2-aminotetralin (8-OH-DPAT), following long-term antidepressant treatment. Following a 21-day treatment with amitriptyline (10 mg/kg/day), there was a significant increase of [3H]8-OH-DPAT binding measured by autoradiography in the dorsal hippocampus, but there was no change in the nucleus raphe dorsalis; whole brain membrane binding revealed an increase in the number of binding sites, with no change in the affinity for [3H]8-OH-DPAT. Conversely, fluoxetine (10 mg/kg/day), a selective blocker of 5-HT reuptake, and gepirone (10 mg/kg/day), a 5-HT1A agonist, both administered for 21 days, significantly reduced [3H]8-OH-DPAT binding measured by autoradiography in the nucleus raphe dorsalis without altering hippocampal binding sites. The control active treatment with diazepam (2 mg/kg/day) did not alter [3H]8-OH-DPAT binding in the hippocampus or in the nucleus raphe dorsalis. All groups were compared to a 21-day vehicle-treated control group. These results are fully consistent with previous electrophysiological and behavioral studies and suggest that alterations of 5-HT1A receptors might underlie the enhancement of 5-HT neurotransmission by antidepressant treatments.

The effect of 8-OH-DPAT on temperature in the rat and its modification by chronic antidepressant treatments

Administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to rats produced a dose-dependent hypothermia. Pretreatment with the receptor antagonist methiothepin abolished this effect, and pretreatment with haloperidol, propranolol and pindolol partially attenuated it, although methiothepin and pindolol had hyperthermic actions of their own. Other receptor antagonists including ritanserin, naloxone, clonidine, phenoxybenzamine and metergoline did not significantly modify the response elicited by subsequent 8-OH-DPAT challenge. In antidepressant studies, chronic treatment (22 days) with clorgyline attenuated the hypothermic response to 8-OH-DPAT, whereas similar duration of treatment with the tricyclics clomipramine and imipramine did not significantly modify it. Also, acute treatment for three days with each of the antidepressants did not modify 8-OH-DPAT-induced hypothermia. We conclude that rat rectal temperature can be a useful model to help assess the functional state of serotonergic mechanisms, including the adaptational changes induced by long-term antidepressant treatment.

Cocaine and central monoaminergic neurotransmission: a review of electrophysiological studies and comparison to amphetamine and antidepressants

Psychomotor stimulants (e.g. cocaine and amphetamine) and many antidepressants are believed to elicit their psychotropic actions by interacting primarily with central monoaminergic neurons. The acute central neuronal effects of amphetamine and antidepressants have been extensively investigated in rats utilizing extracellular single unit electrophysiological and microiontophoretic techniques in vivo. In recent years the chronic effects of these compounds on the above neuronal systems have also been reported. Such investigations have proliferated because of the realization that the mechanisms underlying the psychotomimetic effects (e.g. amphetamine and cocaine) and mood elevation (i.e. antidepressants) observed with the administration of these drugs are more accurately reflected in chronic studies. For many years it has been assumed that cocaine and amphetamine produce very similar if not identical psychotropic effects through their actions on central monoaminergic neurotransmission. In terms of effects on single monoaminergic neurons, this assumption had gone by untested until two years ago, when the first report of the electrophysiological effects of cocaine on central monoaminergic (locus ceruleus) neurons appeared in the literature (61). This review discusses recent electrophysiological studies with cocaine at the level of single identified monoaminergic neurons and compares such data with that previously reported for amphetamine and antidepressants. In addition to identifying some of the similarities and differences between these compounds, this review also highlights some of the gaps in our knowledge regarding the effects of these drugs on central monoaminergic neurotransmission.

Effects of acute and chronic treatment with imipramine on 5-hydroxytryptamine nerve cell groups and on bulbospinal 5-hydroxytryptamine/substance P/thyrotropin releasing hormone immunoreactive neurons in the rat. A morphometric and microdensitometric analysis

Groups of male rats were treated for a period of 14 days with imipramine (10 mumol/kg) given twice daily. Separate groups of rats received a single dose treatment using the same dose and experimental design as for the repeated treatment. Employing the avidin-biotin immunoperoxidase technique for immunohistochemistry 5-hydroxytryptamine (5-HT)-, substance P(SP)- and thyrotropin releasing hormone (TRH)-like immunoreactivities (IRs) were visualized in consecutive coronal sections of the brain stem and of the spinal cord. The IRs were studied by means of morphometric and microdensitometric procedures using automatic image analysis on profiles representing nerve terminal networks of the ventral horn of the cervical and lumbar enlargements of the spinal cord as well as their coexistence (5-HT/SP and 5-HT/TRH). With the same technique 5-HT IR was measured in the 5-HT nerve cell groups of the medulla oblongata (B1, B2, B3) and of the nucleus raphe dorsalis (B7) of the midbrain. In addition 5-HT and 5-hydroxyindolacetic acid (5-HIAA) levels were measured in the ventral and dorsal horns of the cervical and lumbar enlargements of the spinal cord using high performance liquid chromatography (HPLC). In the same parts of the spinal cord SP IR was studied by means of radioimmunoassay (RIA). The microdensitometric studies showed that chronic, but not acute, imipramine treatment selectively increased SP IR in the 5-HT/SP/TRH costoring nerve terminals of the medial part of the ventral horn in both the cervical and the lumbar enlargements. Furthermore, quantitative analysis of the entity of coexistence in the 5-HT nerve terminals networks of these areas showed that all the 5-HT nerve terminals contained SP and TRH IRs and that this phenomenon remained after acute and chronic imipramine treatment. The microdensitometric studies on the 5-HT nerve cell groups of the medulla oblongata and of the nucleus raphe dorsalis demonstrated that chronic, but not acute, imipramine treatment selectively increased 5-HT IR in the nerve cell bodies of the lateral part of group B3 as evaluated from the median grey values. Acute, but not chronic, imipramine treatment significantly increased the field area of 5-HT IR of nerve cell bodies in group B7, reflecting an increase in the mean profile area of the 5-HT IR nerve cell body profiles. Instead, the mean profile area of 5-HT IR cell bodies of group B1 was acutely reduced by imipramine.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of nialamide and clomipramine on serotonin efflux and autoreceptors

Serotonin (5-HT) activity in vivo and in vitro was evaluated in rats following acute and chronic administration of the antidepressants nialamide (NMD) and clomipramine (CMI). The 5-HT motor syndrome was used as an index of in vivo serotonergic function. In vitro, 3H-5-HT uptake, potassium-evoked 3H-5-HT release and 5-HT autoreceptor activity were evaluated as measures of presynaptic function. Repeated injections of NMD abolished the 5-methoxy-N, N-dimethyltryptamine (5-MeODMT)-induced motor syndrome and the ability of 5-methoxytryptamine (5-MEOT) to attenuate the potassium-evoked release of 3H-5HT. Autoreceptor subsensitivity was associated with a marked increase in basal and potassium-evoked 3H-5-HT release. In contrast, acute NMD, and acute and chronic CMI did not affect the expression of the motor syndrome or alter 3H-HT release or autoreceptor activity. Acute and chronic injections of NMD enhanced 3H-5-HT uptake. The results suggest that the antidepressant efficacy of monoamine oxidase inhibitor (MAOI) antidepressants may be related to their ability to increase endogenous levels of 5-HT and thereby produce a subsensitivity of 5-HT1 type receptors. This subsensitivity is reflected both by attenuation of the motor syndrome and enhanced 5-HT neurotransmission resulting in part from autoreceptor down-regulation.

The effect of prolonged treatment with tricyclic antidepressants on the actions of 5-hydroxytryptamine in the hippocampal slice of the rat

The effect of long-term treatment with the tricyclic antidepressants imipramine (IMI) and desmethylimipramine (DMI) on neuronal responsiveness to 5-hydroxytryptamine (5-HT) was examined in the hippocampal slice preparation from the rat. Population spikes, evoked by electrical stimulation of the stratum radiatum, were recorded in the pyramidal cell layer of the CA1 region of the isolated hippocampus. When 5-HT (10(-7) to 2 X 10(-5) M) was applied there was an initial increase followed by a decrease in the amplitude of the population spike. On washout of 5-HT the amplitude increased transiently above control levels. Daily injection of 10 mg/kg of imipramine or desmethylimipramine, intraperitoneally, into rats for 4-5 weeks was found to produce a significant decrease in the inhibitory effect of 10(-5) M 5-HT, whereas there was no apparent change in the excitatory effects. The acute application of 10(-5) M imipramine or desmethylimipramine antagonized the inhibitory effect of 10(-5) M 5-HT without affecting the excitatory effects. Acute application of the 5-HT receptor antagonists cyproheptadine (10(-5) M) and ketanserin (7.5 X 10(-6) M) completely prevented the appearance of the inhibitory effect of 10(-5) M 5-HT without affecting the excitatory effects. It was concluded that the decreased inhibitory effect of 5-HT which was produced by chronic treatment with imipramine or desmethylimipramine was probably due to a reduction in the number of 5-HT receptors or a reduction in the post-receptor effector mechanisms for 5-HT.

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.

Effects of the two antidepressant drugs mianserin and indalpine on the serotonergic system: single-cell studies in the rat

Several antidepressant treatments enhance serotonergic neurotransmission. The present electrophysiological studies were undertaken to assess the effect of mianserin and indalpine, two antidepressant drugs with different pharmacological profiles, on serotonergic neurotransmission. In a first series of experiments, the responsiveness of hippocampal pyramidal neurons to microiontophoretic applications of serotonin (5-HT), norepinephrine (NE) and gamma-aminobutyric acid (GABA) was assessed following mianserin, imipramine (5 mg/kg/day IP) or saline administration for 14 days. At 48 h after the last dose of mianserin, responsiveness to 5-HT was increased whereas that to NE and GABA was not modified. The degree of sensitization to 5-HT was the same as that produced by imipramine. Acute IV administration of mianserin (up to 10 mg/kg) did not decrease the firing rate of dorsal raphe 5-HT neurons. In a second series of experiments, long-term administration of indalpine (5 mg/kg/day IP for 14 days) did not modify the responsiveness of hippocampal pyramidal neurons to microiontophoretically applied 5-HT, NE and GABA whereas imipramine treatment (5 mg/kg/day IP) increased selectively their sensitivity to 5-HT when compared to indalpine-treated rats. In keeping with its potent reuptake-blocking property, acute IV indalpine produced a marked decrease in the firing rate of dorsal raphe 5-HT neurons (ED50 0.33 mg/kg). The firing rate of dorsal raphe 5-HT neurons was assessed following 2-, 7- and 14-day treatments with indalpine (5 mg/day IP). After 2 days, the firing rate of 5-HT neurons was greatly reduced, after 7 days it had recovered partially and after 14 days it had returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Amitriptyline sensitization of a serotonin-mediated behavior depends on the passage of time and not repeated treatment

Daily treatment for 10 days with either amitriptyline or the tricyclic muscle relaxant, cyclobenzaprine, increased the incidence of head-twitch behavior in response to 5-hydroxytryptophan (5-HTP) when this was examined two days later. Only one day of amitriptyline treatment followed by an 11-day hiatus before administration of 5-HTP also sensitized the head-twitch response whereas similar amitriptyline treatment followed by 5-HTP one hour later failed to do so. These data provide the first evidence for time-dependent sensitization of brain serotonin systems.

The effect of estradiol on the alterations in monoamine-mediated behavioural responses induced by administration of electroconvulsive shocks or imipramine to female rats

Female rats were treated daily with electroconvulsive shocks (ECS) or imipramine (10 mg/kg) for 10 days. Both types of treatment enhanced behavioural responses mediated by 5-hydroxytryptamine (5-HT) and noradrenaline (NA). A behavioural response mediated by dopamine (DA) was enhanced by electroconvulsive shock-treatment alone. On the other hand, rats treated only once with imipramine exhibited reduced DA-mediated behaviour. Priming the rats with estradiol valerate before starting electroconvulsive shock- or imipramine- treatment did not produce any significant effect on the enhancement in the behavioural response mediated by 5-HT. The enhancement in behaviour mediated by NA caused by electroconvulsive shock was also not altered, but that caused by treatment with imipramine was abolished. Enhancement of behaviour mediated by DA following electroconvulsive shock-treatment was also attenuated, while there was a positive reduction in behaviour mediated by DA in imipramine-treated rats. The two therapeutic approaches to depression, viz., electroconvulsive shock and imipramine, thus produced somewhat different effects on the central functions mediated by monoamines. Furthermore, an estrogen given prior to the treatments differentially altered the influence exerted by electroconvulsive shock and imipramine on monoamine functions in brain. The results may be pertinent to the clinical impression that estrogens produce a partial resistance to the antidepressant efficacy of imipramine-like 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.

Effects of quipazine on pre- and postsynaptic serotonin receptors: single cell studies in the rat CNS

Many behavioural and biochemical studies have pointed to an agonistic activity of quipazine on serotonin (5-HT) receptors. In the present electrophysiological study, the effect of quipazine on pre- and postsynaptic 5-HT receptors in the rat was studied. Quipazine, administered intravenously, depressed the firing rate of 5-HT-containing dorsal raphe neurones (ED50 = 0.82 mg/kg). Microiontophoretic applications of quipazine on 5-HT-containing neurones in the dorsal raphe and on neurones of two forebrain regions receiving a 5-HT input (the ventral lateral geniculate nucleus and the dorsal hippocampus) consistently depressed neuronal firing rate as did 5-HT and D-lysergic acid diethylamide (LSD). Quipazine was more potent on 5-HT neurones than on the ventral lateral geniculate nucleus and hippocampal neurones: the post/presynaptic efficacy ratio for quipazine was similar to that of LSD. Following a selective denervation of 5-HT neurones with intraventricular injection of 5,7-di-hydroxy-tryptamine in desipramine-pretreated rats, the responsiveness of neurones in the ventral lateral geniculate nucleus to quipazine, applied microiontophoretically, was increased as was that to 5-HT and to LSD. These results provide direct evidence for the agonistic activity of quipazine on both pre- and postsynaptic 5-HT receptors.

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.