Effect of destruction of central noradrenergic and serotonergic nerve terminals by systemic neurotoxins on the long-term effects of antidepressants on beta-adrenoceptors and 5-HT2 binding sites in the rat cerebral cortex

Animal models of depression in drug discovery: a historical perspective

Over the course of the last 50 years many models of major depressive disorder have been developed on the basis of theoretical aspects of this disorder. These models and procedures have been crucial in the discovery and development of clinically-effective drugs. Notwithstanding, there is presently great concern about the discrepancy between positive outcomes of new candidate drugs in animal models and apparent lack of efficacy in humans i.e., the predictive validity of animal models. Some reasons for this concern lie in the over-reliance in the face value of behavioural models, design of clinical trials, placebo responses, genetic variations in response to drugs, species differences in bioavailability and toxicology, and not least, disinterest of pharmaceutical sponsors to continue developing certain drugs. Present model development is focusing on endophenotypic aspects of behaviours rather than trying to model whole syndromes. This essay traces the origins and theoretical bases of our animal models of depression or depressed-like behaviours in humans and indicates how they have evolved from behavioural assays used to measure the potency and efficacy of potential candidate drugs to tools by which endophenotypes of depression may be identified and verified pharmacologically. A cautionary note is included though to indicate that the true predictive validity of our models will not be fully assessed until we can determine the attrition rate of molecules discovered from new drug targets translating into clinically-effective drugs.

5-HT2A and 5-HT2C receptors and their atypical regulation properties

The 5-HT(2A) and 5-HT(2C) receptors belong to the G-protein-coupled receptor (GPCR) superfamily. GPCRs transduce extracellular signals to the interior of cells through their interaction with G-proteins. The 5-HT(2A) and 5-HT(2C) receptors mediate effects of a large variety of compounds affecting depression, schizophrenia, anxiety, hallucinations, dysthymia, sleep patterns, feeding behaviour and neuro-endocrine functions. Binding of such compounds to either 5-HT(2) receptor subtype induces processes that regulate receptor sensitivity. In contrast to most other receptors, chronic blockade of 5-HT(2A) and 5-HT(2C) receptors leads not to an up- but to a (paradoxical) down-regulation. This review deals with published data involving such non-classical regulation of 5-HT(2A) and 5-HT(2C) receptors obtained from in vivo and in vitro studies. The underlying regulatory processes of the agonist-induced regulation of 5-HT(2A) and 5-HT(2C) receptors, commonly thought to be desensitisation and resensitisation, are discussed. The atypical down-regulation of both 5-HT(2) receptor subtypes by antidepressants, antipsychotics and 5-HT(2) antagonists is reviewed. The possible mechanisms of this paradoxical down-regulation are discussed, and a new hypothesis on possible heterologous regulation of 5-HT(2A) receptors is proposed.

Noradrenergic lesion antagonizes desipramine-induced adaptation of NMDA receptors

Repeated administration of the tricyclic antidepressant, desipramine, for 28 days to mice effected a decrease in the potency of glycine to displace [3H]5,7-dichlorokynurenic acid (5,7-DCKA) in mouse cortical homogenates. Pre-treatment with the noradrenergic neurotoxin DSP-4, while having no effect alone, attenuated the desipramine-induced effect. The present findings support a norepinephrine-dependent adaptation of the NMDA receptor complex in vivo following chronic desipramine treatment. The inter-relationship of norepinephrine and glutamate transmission may provide insight into the mechanism underlying the action of antidepressant drugs.

Modulatory role of 5-HT3 receptors in mediation of apomorphine-induced aggressive behaviour in male rats

We have studied the effects of serotonin (5-HT) 5-HT3 receptor agonists 1-phenylbiguanide (1-PBG) and 1-(m-chlorophenyl)biguanide (mCPBG), and antagonists 3-tropanyl-3,5-dichlorobenzoate (MDL-72222) and tropisetron (3-tropanyl-indole-3-carboxylate HCl; ICS-205930) on apomorphine-induced aggressive behaviour in normal or DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride] pre-treated male Wistar rats. DSP-4 (50 mg/kg) pre-treatment significantly accelerated the development of apomorphine-induced aggressive behaviour. mCPBG (1.0 and 10 mg/kg) did not modify the aggressiveness, but 1-PBG (3.0 and 30 mg/kg) attenuated the aggressiveness in normal but not DSP-4 pre-treated rats. MDL-72222 (0.4 and 4.0 mg/kg) attenuated the aggressive behaviour in normal rats, tropisetron (0.3 mg/kg) had an antiaggressive effect only by citalopram (10 mg/kg) challenge. MDL-72222 and tropisetron were ineffective in DSP-4 pre-treated rats. In conclusion, our results indicate that the 5-HT3 receptors modulate the apomorphine-induced aggressive behaviour and the 5-HT3 receptor antagonists have moderate antiaggressive effect in this test.

Reduction of substance P after chronic antidepressants treatment in the striatum, substantia nigra and amygdala of the rat

We investigated the effects of chronic treatment with food containing one of five antidepressants on substance P (SP) content in the rat brain using radioimmunoassay and enzyme-immunoassay. The antidepressants used were imipramine, desipramine, clomipramine, amoxapine and mianserin. Following 40 days of treatment, all the antidepressants decreased SP concentrations in the striatum, substantia nigra and amygdala. Only imipramine and desipramine reduced the peptide content in the hippocampus, and only mianserin reduced it in the septum. We further examined the acute effects of antidepressants one hour after a single intraperitoneal administration. Acute imipramine and desipramine treatment reduced SP in the striatum, whereas acute mianserin decreased it in the striatum and substantia nigra. These results demonstrate that all antidepressants on chronic treatment had a common effect, a reduction of SP content in the striatum, substantia nigra and amygdala. This raises the possibility that such a decrease may contribute to the therapeutic action of antidepressants in affective disorders.

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Interactions between 5-hydroxytryptamine receptor subtypes: is a disturbed receptor balance contributing to the symptomatology of depression in humans?

The purpose of this review is to describe the consequences of antidepressant treatment on the behaviour of rodents after activation of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes. In a summary table, the involvement of 5-HT receptors in inducing behavioural changes are described. It is emphasized that these effects are not always only exclusively linked to serotonergic functions nor that they are only initiated by central 5-HT receptors. Hereafter, the complex mutual inhibitory effects of 5-HT receptor subtype-mediated processes are discussed by interpreting effects of antagonists and describing the different effects of low and high doses of mixed 5-HT1C/5-HT2 receptor agonists. Mutual influences are seen particularly with 5-HT1A, 5-HT1C and 5-HT2, but not with 5-HT1B, 5-HT1D or 5-HT3 receptor-mediated effects. It is shown that the behavioural consequences of 5-HT1A, 5-HT1C and 5-HT2 receptor stimulation may be changed by brain lesions or chronic treatment with drugs. Among these drugs are the antidepressants. Finally, 5-HT receptor function in depressed patients is discussed, and the hypothesis is proposed that an important function of antidepressants is to restore a disturbed balance between 5-HT1A, 5-HT1C and 5-HT2 receptors in depressed patients.

Behavioural and neuroendocrine responses to D-fenfluramine in rats treated with neurotoxic amphetamines

The amphetamine derivatives p-chloroamphetamine (pCA), 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') and D-fenfluramine can, if given repeatedly in high doses to rats, produce a degeneration of serotonergic nerve terminals which we have previously shown to result in a reduction in D-fenfluramine-evoked release of 5-HT in vivo. It is therefore possible that fenfluramine-evoked responses may have value as a probe of 5-HT neurodegeneration in man. The present study examined the effect of pre-treatment with these three agents (pCA 12 mg/kg×2; MDMA 20 mg/kg×8; D-fenfluramine 12.5 mg/kg×8, 14 days prior to testing) on behavioural (5-HT syndrome) and neuroendocrine [prolactin and adrenocorticotrophin (ACTH)] responses in rats to acute administration of D-fenfluramine and other serotonergic agonists. All three pre-treatments attenuated the D-fenfluramine-evoked behavioural syndrome, but did not affect the prolactin or ACTH responses to acute challenge with D-fenfluramine (apart from a small effect of pre-treatment with pCA on the ACTH response to D-fenfluramine). For comparison, the effect of pCA pre-treatment on the behavioural responses to acute administration of pCA and the 5-HT(1A) and 5-HT(2) receptor agonists 8-hydroxy-2-(di- n- propylamino)tetralin (8-OH-DPAT) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), respectively, were also examined. pCA pre-treatment attenuated all components of the behavioural response to pCA but had little or no effect on the behavioural responses to 8-OH-DPAT or DOI, suggesting that there was no alteration in post-synaptic 5-HT(1A) or 5-HT(2) receptor function. While the loss of behavioural effect of D-fenfluramine on rats pre-treated with neurotoxic amphetamines can be understood in terms of the loss of D-fenfluramine's 5-HT-releasing action following 5-HT neurodegeneration, the lack of change in the neuroendocrine responses to D-fenfluramine is not easily explicable in this way. These results emphasise the need for further research into the actions of D-fenfluramine before carrying it forward as a probe of neurodegeneration in man.

Involvement of noradrenergic and 5-hydroxytryptaminergic systems in allylnitrile-induced head twitching

Allylnitrile induces in rats persistent behavioral abnormalities, including head twitching, following a single administration. We studied the role of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) systems in the brain of rats in inducing and maintaining the head twitching. Allynitrile (1.49 mmol/kg) induced 5-HT system activation in all areas of the brain studied 1-4 days after oral administration, and a reduction in the content of NA in the hippocampus, cortex and hypothalamus 1 day after dosing, in the hippocampus, cortex, hypothalamus and midbrain 2 days after dosing, and in the hypothalamus 4 days after dosing. Allylnitrile induced no change in the content of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA) or NA 7-28 days after dosing. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT) suppressed the allylnitrile-induced head twitching, and decreased the contents of 5-HT and 5-HIAA in almost all areas of the brain throughout the observation period, as well as the ratio of 5-HIAA/5-HT in the medulla oblongata plus pons from 1 to 30 days after dosing with allylnitrile. No change in NA was observed in any areas of the brain. Pretreatment with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) increased the head twitching induced by allylnitrile, and decreased the content of NA in all areas of the brain throughout the observation period, without any change in the contents of 5-HT or 5-HIAA or in the ratio of 5-HIAA/5-HT. The present results suggest the involvement of 5-HT and NA systems in allylnitrile-induced head twitching.

Up-regulation of 5-hydroxytryptamine2 and neurokinin-1 receptors associated with serotonin/substance P hyperinnervation in the rat inferior olive

The fate of serotonin and substance P receptors following serotonin/substance P hyperinnervation of CNS tissue was investigated in the inferior olivary complex of adult rats subjected to earlier intraventricular administration of 5,6-dihydroxytryptamine. [3H]8-hydroxy-2-(Dl-n-propylamino)tetralin, [3H]5-hydroxytryptamine, [3H]ketanserin and [125I]Bolton-Hunter-substance P were respectively used to label 5-hydroxytryptamine1A, 5-hydroxytryptamine1B, 5-hydroxytryptamine2 and neurokinin-1 receptor sites for quantitative ligand binding autoradiography. Only 5-hydroxytryptamine2 and neurokinin-1 sites were detected in the normal or serotonin/substance P-hyperinnervated inferior olivary complex. In the normal inferior olivary complex, the density of [3H]ketanserin binding (5-hydroxytryptamine2 receptors) was relatively low, being the highest in pars a of the caudal medial accessory olive and the principal olive; moderate in pars c of the caudal medial accessory olive; truly low in the medial and the lateral dorsal accessory olive, nucleus b and pars b of the caudal medial accessory olive; and negligible in the middle medial accessory olive, rostral medial accessory olive and the smaller subnuclei. [125I]Bolton-Hunter-substance P binding (neurokinin-1 receptors) appeared denser, being highest in nucleus beta and the middle medial dorsal accessory olive; moderate in the three portions of the caudal medial accessory olive, the lateral dorsal accessory olive and the dorsal cap of Kooy; low in the rostral medial accessory olive, the ventrolateral outgrowth and the dorsomedial cell column; and very low or null in the principal olive and the medial dorsal accessory olive. After serotonin/substance P hyperinnervation, there were striking increases in the apparent density of both populations of receptor. [3H]Ketanserin binding was now stronger in the most olivary subnuclei, including some in which it had not been found in the normal, such as the middle and the rostral medial accessory olive. [125I]Bolton-Hunter-substance P binding showed even greater elevations in a few subnuclei, such as the principal olive and the dorsomedial cell column; it was now detectable in the medial dorsal accessory olive, unchanged in the dorsal cap of Kooy and the ventrolateral outgrowth, and slightly decreased in the lateral dorsal accessory olive. The normal and altered distributions of both ligands did not match the respective patterns of serotonin and substance P innervation and hyperinnervation previously demonstrated with immunocytochemistry. In some sectors of the inferior olivary complex where both transmitters are presumably co-localized, there was no overlap in the distribution of the respective binding sites either in the normal or in the hyperinnervated state.(ABSTRACT TRUNCATED AT 400 WORDS)

Ziskind-Somerfeld research Award. The involvement of guanine nucleotide binding proteins in the pathogenesis and treatment of affective disorders

Guanine nucleotide binding (G) proteins play a pivotal role in postreceptor information transduction. An important characteristic of G proteins is their increased guanine nucleotide binding following agonist stimulation, which in turn leads to their activation. We have developed a method that enables the measurement of early events in signal transduction beyond receptors, through activated receptor-coupled guanine nucleotide exchange on G proteins. Using this method, lithium was recently demonstrated to inhibit the coupling of both muscarinic cholinergic and beta-adrenergic receptors to pertussis toxin-sensitive and cholera toxin-sensitive G proteins, respectively, thus suggesting alteration of the function of G protein by lithium, as the single site for both the antimanic and antidepressant effects of this drug. One of the most puzzling aspects of the ability of lithium to ameliorate the manic-depressive condition is its relatively selective action upon the central nervous system (CNS). It was previously shown that lithium selectively attenuated the function of Gs proteins in the CNS. In the present study, we show that inhibition by lithium of muscarinic receptor-coupled G protein function is also selective to the CNS. The clinical profile of lithium, carbamazepine, and electroconvulsive treatment (ECT), agents that are effective in the prevention and treatment of bipolar affective disorder, differs from that of purely antidepressant drugs. Antidepressant drugs are effective in the acute treatment and prevention of depression only, and can even precipitate hypomanic or manic "switches," or "rapid cycling" between mania and depression. We have investigated and compared the effects of chronic antibipolar and antidepressant treatments on receptor-coupled G protein function. Antibipolar treatments (lithium, carbamazepine, ECT) attenuate both receptor-coupled Gs and non-Gs (i.e., Gi, Go) proteins function; in contrast, only Gs protein function is inhibited by antidepressant drugs [either tricyclics or monoamine oxidase (MAO) inhibitors]. Moreover, an integral adrenergic neuronal system is required for antidepressant inhibition of Gs protein function, as pretreatment with the noradrenergic neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) specifically abolishes the effects of antidepressant drugs on Gs protein, whereas antibipolar drug effects on G protein function are unaffected by DSP-4. Our results suggest that attenuation of beta-adrenergic receptor-coupled Gs protein function, which is common to both antidepressant and antibipolar treatments, may be the mechanism underlying their antidepressant therapeutic efficacy.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of ACTH, adrenalectomy and the combination treatment on the density of 5-HT2 receptor binding sites in neocortex of rat forebrain and 5-HT2 receptor-mediated wet-dog shake behaviors

The effect of ACTH and/or adrenalectomy on serotonin (5-HT)2 receptor binding sites was evaluated in the neocortex of rat forebrain. One day after the adrenalectomy or sham operation, ACTH (50 micrograms/day) was injected subcutaneously into adult male SD rats for 10 consecutive days. Saturation analysis showed that subchronic ACTH treatment significantly increased the Bmax values for 3H-ketanserin binding without any change in the Kd values. Moreover, this ACTH-induced increase in the Bmax values was prevented by adrenalectomy. The concentrations of 5-HT and 5-hydroxyindole acetic acid (5-HIAA) measured by HPLC-ECD were not altered by these manipulations. Ten-day administration of corticosterone (20 and 50 mg/kg) also increased 5-HT2 receptor density in the neocortex of rat forebrain. 5-HT2 (and 5-HT1C) receptor agonist, (+/-)DOI-induced wet-dog shakes in ACTH and/or adrenalectomy-treated rats were also examined. Ten-day administration of ACTH enhanced (+/-)DOI-induced wet-dog shakes and this increase was prevented by adrenalectomy. These results indicate that subchronic adrenocorticotropin-adrenal axis activation of rats increases both the number of 5-HT2 receptors in neocortex of forebrain and the wet-dog shake responses induced by (+/-)DOI.

Determination of the role of noradrenergic and 5-hydroxytryptaminergic neurones in postsynaptic alpha 2-adrenoceptor desensitization by desipramine and ECS

1. Experiments were conducted to determine the respective roles which noradrenergic and 5-hydroxytryptaminergic neurones play in the down-regulation of postsynaptic alpha 2-adrenoceptors by desipramine and electroconvulsive shock (ECS). The functional status of these receptors was monitored by use of clonidine-induced mydriasis in conscious mice. 2. Mydriasis to clonidine (0.1 mg kg-1, i.p.) was markedly attenuated by administration of either desipramine (10 mg kg-1, i.p.) for 14 days or ECS (200 V, 2s) given five times over ten days confirming our previous observations. 3. The neurotoxin, DSP-4 (100 mg kg-1, i.p. X 2), reduced brain noradrenaline levels by 64% and abolished the mydriasis induced by the noradrenaline releasing agent and reuptake inhibitor, methamphetamine, without significantly altering the response to clonidine, confirming our earlier results. This lesion prevented the attenuation of clonidine mydriasis by repeated administration of desipramine, but not ECS. 4. Lesioning of central 5-hydroxytryptaminergic neurones with 5,7-dihydroxytryptamine (75 micrograms, i.c.v.) had no influence on the reduction in clonidine mydriasis produced by repeated administration of either desipramine or ECS. 5. Since noradrenergic neurones are essential for the desensitization of postsynaptic alpha 2-adrenoceptors by desipramine, it indicates that this effect is probably the result of increased synaptic noradrenaline levels. This mechanism is not responsible for the change induced by ECS because this adaptation is independent of an intact noradrenergic input. 5-HT-containing neurones do not play a permissive role in the down-regulation of postsynaptic alpha 2-adrenoceptors by either antidepressant treatment.

Depletion of brain serotonin differently affects behaviors induced by 5HT1A, 5HT1C, and 5HT2 receptor activation in rats

5-hydroxytryptamine (5HT)-depleted rats were subjected to behavioral experiments in which the response to activation of 5HT1A, 5HT1c, and 5HT2 receptor subtypes was measured. Depletion of 5HT was produced by unilateral intracerebroventricular injection of 5,7-dihydroxytryptamine (100 micrograms/rat) or by systemic injection of p-chlorophenylalanine (150 mg/kg injected intraperitoneally 72, 48, and 24 h before the test). The dose-response curve of the 5HT1A-mediated, 8-hydroxy-2-(di-n-propylamino)tetralin (0.022-0.46 mg/kg)-induced lower lip retraction was not changed after depletion, nor was the dose-response curve of the 5HT2 receptor-mediated (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (0.046-1.0 mg/kg)-induced head shake response. The dose-response curve for penile erections, a 5HT1c receptor-mediated response after mCPP (0.1-1.0 mg/kg), a direct 5HT1c agonist, is shifted to the left after 5HT depletion, whereas the response to indirect activation of the 5HT1c receptor with the 5HT reuptake inhibitors citalopram (2.2-4.6 mg/kg) and paroxetine (0.22-2.2 mg/kg) was inhibited after 5HT depletion. These results suggest that 5HT1c receptors are more subject to denervation supersensitivity than 5HT1A and 5HT2 receptors. This lesion model may be useful to discriminate behaviorally between direct and indirect activation of the 5HT1c receptor.

Effect of chronic administration of antidepressant drugs on 5-HT2-mediated behavior in the rat following noradrenergic or serotonergic denervation

Chronic (14 day) administration of several pharmacologically-distinct antidepressant drugs resulted in marked reductions in the serotonin2 (5-HT2)-mediated quipazine-induced head shake response which were accompanied by significant reductions in the density of cortical beta-adrenergic and 5-HT2 binding sites. Noradrenergic (DSP4[N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine]-induced) and serotonergic (5,7-DHT[5,7-dihydroxytryptamine]-induced) lesions either attenuated or blocked antidepressant-induced reductions in 5-HT2-mediated behavior. DSP4- and 5,7-DHT lesions did not alter the down-regulation of 5-HT2 binding sites produced by imipramine, desipramine, phenelzine or iprindole. To a large extent, the antagonism of antidepressant-induced reductions in 5-HT2-mediated behavior was coincident with the blockade of down-regulation of beta-adrenergic binding sites by both noradrenergic and serotonergic denervation. The functional interrelationship of 5-HT2 and beta-adrenergic receptors suggested by the present findings may provide insight into a common mechanism underlying the action of pharmacologically-distinct antidepressant drugs.

Destruction of the nucleus raphe obscurus and potentiation of serotonin-mediated behaviors following administration of the neurotoxin 3-acetylpyridine

Systemic administration of the neurotoxin 3-acetylpyridine (3-AP) to rats produced spontaneous episodes of spasmodic movement involving the trunk and limbs including torticollis, contortions of the trunk and rigid extension of the limbs. Because the neurotransmitter serotonin (5-HT) has been implicated in various human involuntary movement disorders, the functional and anatomical integrity of the 5-HT system in rats treated with 3-AP were examined. 5-HT-containing neurons in the brain stem were studied using immunohistochemical labeling with antiserum to 5-HT. Cells in the nucleus raphe obscurus were found to be altered following 3-AP treatment as shown by a decrease in 5-HT immunoreactivity as compared to control rats. No changes in 5-HT immunoreactivity were observed in any other region containing 5-HT cell bodies. Behaviorally, rats treated with 3-AP were 2.5-fold more sensitive to the ability of the 5-HT1A agonist 8-OH-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.33-3.3 mg/kg) to produce the 5-HT syndrome. Similarly, 3-AP-treated rats were 2-fold more sensitive to the selective 5-HT2 agonist 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane (DOB; 0-1.0 mg/kg) at producing the head shake response. Although these behaviors associated with brain stem 5-HT receptors were potentiated by 3-AP, the hypothermic effect of 8-OH-DPAT which involves ascending mesencephalic 5-HT neurons was unchanged following 3-AP treatment. Treatment with 3-AP did not produce significant alterations of 5-HT or 5-hydroxyindoleacetic acid (5-HIAA) content in any brain region studied. Quantitative autoradiographic analysis of the density of 5-HT1A receptors labeled with [3H]8-OH-DPAT revealed that these sites were unchanged in regions of the brain (frontal cortex, hippocampus and brain stem) and in the spinal cord. Similarly, few changes in the density of 5-HT2 receptors measured with [3H]ketanserin were observed in various brain regions. These results suggest that neurons from the nucleus raphe obscurus are involved in the elicitation of 5-HT-mediated behavioral responses by 5-HT1A and 5-HT2 receptor agonists that are though to be mediated through brain stem and spinal cord mechanisms. In addition, because of the close neuroanatomical relationship of the nucleus raphe obscurus with various brain regions known to be involved in motor control, the destruction of this region by 3-AP may contribute to the spasmodic motor behaviors observed following 3-AP treatment.

An investigation of the role of 5-hydroxytryptamine in the attenuation of presynaptic alpha 2-adrenoceptor-mediated responses by antidepressant treatments

Changes in the function of presynaptic alpha 2-adrenoceptors in the brain were assessed by rating the hypoactivity (sedation) response of mice to clonidine (0.1 mg/kg). A single injection of 5,7-dihydroxytryptamine (5,7-DHT, 75 micrograms ICV) or administration of p-chlorophenylalanine (PCPA; 200 mg/kg) daily for 11-15 days produced 62-77% reductions in brain 5-HT concentrations and marked supersensitivity of 5-HT2 receptor function, as indicated by the enhancement of the head-twitch response to 5-methoxy-N,N-dimethyltryptamine (2 mg/kg). Clonidine-induced hypoactivity was moderately enhanced after 5,7-DHT lesioning, but not after repeated PCPA injection. In addition, 5,7-DHT lesioning prevented the adaptive attenuation of this alpha 2-adrenoceptor-mediated response produced by daily injection of desipramine (10 mg/kg) for 14 days, but had no effect on the reduction caused by five electroconvulsive shocks (ECS, 200 V, 2 s) spread over 10 days. In contrast, repeated PCPA treatment did not prevent the reduction of clonidine-induced hypoactivity produced by repeated desipramine or ECS administration. Together, these results indicate that 5-HT (or possibly a cotransmitter contained within 5-hydroxytryptamine neurones) influences presynaptic alpha 2-adrenoceptor function. Furthermore, an intact 5-HT neuronal input is a prerequisite for the attenuation of clonidine-induced hypoactivity by desipramine, but not ECS. The probable explanation for a contrasting requirement for a functional 5-HT input is that desipramine and ECS induce this common adaptive response by different pharmacological mechanisms.

Sertraline-induced desensitization of the serotonin 5HT-2 receptor transmembrane signaling system

Sertraline is a new, selective serotonin (5HT) uptake inhibitor with antidepressant activity. The effect of chronic administration of sertraline on 5HT-2 receptors in rat cortex was compared with that of the tricyclic antidepressant, amitriptyline. 5HT-2 receptors were evaluated in binding assays using [3H]-ketanserin and in functional assays of transmembrane signaling, hydrolysis of phosphoinositides. The daily injection of 17 mg/kg sertraline induced a desensitization of 5HT-2-mediated phosphoinositide hydrolysis after 28, but not 21, days. The administration of 1.2 mg/kg/day via continuous release pumps caused a more rapid desensitization. Amitriptyline administered chronically also produced a desensitization of the 5HT-2-mediated phosphoinositide hydrolysis response. A decrease in the density of 5HT-2 binding sites accompanies the functional desensitization after amitriptyline, but changes in 5HT-2 binding sites were not detected after chronic sertraline administration. Studies of the mechanism of action of sertraline show that the desensitization of the phosphoinositide hydrolysis response is homologous in nature, and that it is not secondary to changes in the synthesis of precursor lipids. Other possibilities such as alterations in coupling efficiency or in the activity of effector enzymes are currently being considered. The present results suggest a new postsynaptic action of antidepressant drugs at central 5HT-2 receptors (i.e., changes in 5HT-2 signal transduction at a site distal to the cell surface binding site) and illustrate the importance of studies of receptor signaling pathways to complement radioligand binding.

Effects of imipramine and serotonin-2 agonists and antagonists on serotonin-2 and beta-adrenergic receptors following noradrenergic or serotonergic denervation

The effects of chronic (14 day) administration of the tricyclic antidepressant imipramine, the serotonin-2 (5-HT2) antagonist ketanserin, and the serotonin agonist quipazine on 5-HT2 receptor binding parameters and 5-HT2-mediated behavior were examined in rats with or without prior serotonergic denervation [via 5,7-dihydroxytryptamine (5,7-DHT)] or noradrenergic denervation [via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)]. Chronic administration of imipramine, ketanserin, or quipazine produced a marked reduction in the number of 5-HT2 binding sites which was accompanied by reductions in the 5-HT2-mediated quipazine-induced head shake response. In animals receiving DSP4 or 5,7-DHT lesions and continuous vehicle treatment, beta-adrenergic receptor binding sites were significantly up-regulated while 5-HT2 receptor binding sites did not change. Imipramine normalized the lesion-induced increases in beta-adrenergic binding observed in DSP4 and 5,7-DHT-lesioned rats but failed to down-regulate beta-adrenergic binding sites below non-lesioned control levels. Chronic imipramine, ketanserin, and quipazine reduced quipazine-induced head shakes and down-regulated 5-HT2 binding sites in rats with noradrenergic denervation. While imipramine, ketanserin, and quipazine all down-regulated 5-HT2 binding sites in animals with serotonergic denervation, only imipramine's ability to reduce quipazine-induced head shakes was attenuated in 5,7-DHT-lesioned rats. The present results suggest that imipramine-induced down-regulation of 5-HT2 receptors may not involve presynaptic 5-HT mechanisms, and imipramine-induced alterations in 5-HT2 sensitivity as reflected in the quipazine-induced head shake may, in part, be influenced by beta-adrenergic receptors.

Modulation of second messenger function in rat brain by in vivo alteration of receptor sensitivity: relevance to the mechanism of action of electroconvulsive therapy and antidepressants

1. The second messengers cyclic AMP and inositol triphosphate are the intracellular mediators for a number of neurotransmitters for which receptors exist on brain neurons. 2. Up- or down-regulation of these receptors in general produce corresponding changes in the associated second messenger systems. 3. Chronic administration of antidepressants including electroconvulsive shock to rats produces a number of changes in cerebral receptors, notably down-regulation of beta-adrenergic and serotonin 5-HT2 receptors and up-regulation of alpha-1 adrenergic receptors. 4. The changes in receptor number induced by such antidepressant treatments are in general accompanied by corresponding changes in the associated second messenger reactions. 5. Antidepressant administration has also been shown to induce increased post-receptor mediated adenylate cyclase activity in cortical membranes, and similar effects have also been reported in striatum after chronic administration of neuroleptics. The relevance of these effects to the mechanism of action of the drugs is discussed.

Role of serotonergic input in the down-regulation of beta-adrenoceptors following long-term clorgyline treatment

Administration of the selective monoamine oxidase (MAO) type A-inhibiting antidepressant clorgyline (1 mg/kg per day) to rats for 21 days caused a significant decrease in cortical [3H]dihydroalprenolol binding. Selective lesioning of central serotonergic axons by 5,7-dihydroxytryptamine (5,7-DHT; confirmed by the presence of the serotonin syndrome in response to a 40 mg/kg dose of 5-hydroxytryptophan (5-HTP) or inhibition of 5-HT synthesis by parachlorophenylalanine (PCPA) caused significant 5-HT and 5-HIAA depletions in the cortex without much effect on NE and DA concentrations, but did not have any significant effect on beta-adrenoceptor density, and furthermore failed to attenuate clorgyline-induced decreases in beta-adrenoceptor density. Clorgyline treatment partially antagonized 5-HT depletion by the 5,7-DHT lesion or PCPA treatment. These findings suggest that due to their ability to raise 5-HT concentrations, MAO-inhibiting antidepressants may be a better alternative than the tricyclics in treating depressed patients with reduced 5-HT if down-regulation of beta-adrenoceptors is critical for antidepressant efficacy.

Serotonergic and adrenergic receptors in alcohol-preferring and non-preferring rats

Serotonergic and adrenergic receptors in brain areas of the alcohol-preferring P and alcohol-nonpreferring NP rats were compared by radioligand-binding assays. Binding of 3H-serotonin (3H-5HT) to 5HT-1 receptors in membranes of cerebral cortex and hippocampus was significantly higher in density (B max values) and affinity (Kd values) in the P than in the NP rats, whereas B max values in membranes from the brain stem of the P rats were lower than those of the NP rats. No significant difference between the P and NP lines was observed when the binding of 3H-ketanserin to 5HT-2 receptors and of 3H-WB4101, 3H-clonidine and 3H-dihydoalprenolol to alpha-1, alpha-2 and beta-adrenergic receptors was compared. The increase of 3H-5HT binding probably indicates up-regulation or supersensitivity of 5HT-1 receptors as a compensatory mechanism to the lower levels of 5HT in brain areas of the P rats (Murphy, et al., 1982).

Evaluation of the role of pre- and postsynaptic serotonergic receptors in electroconvulsive shock therapy

Despite the well-established clinical efficacy of electroconvulsive therapy (ECT) in the treatment of affective disorders, there is no generally accepted theory regarding its mode of action. Pre- and postsynaptic serotonin (5-hydroxytryptamine, 5-HT) receptors were studied following treatment of rabbits with electroconvulsive therapy (8 shocks). No change was observed in the binding at the postsynaptic 5-HT2 receptor in frontoparietal cortex, as indicated by an unchanged affinity and by the total number of receptors. In control rabbits, exogenously added serotonin and metitepin modulated the electrically evoked 3H-serotonin release, probably by acting on a presynaptic serotonin autoreceptor. Following electroconvulsive therapy, exogenous serotonin still inhibited the 3H-serotonin release, whereas metitepin had lost its enhancing effect. This observation provides evidence for a decreased amount of endogenous neurotransmitter present in the synaptic cleft at stimulation. The results indicate that postsynaptic receptor changes following antidepressant treatments are related to the presynaptic autoreceptor changes.

Effects of chronic oral administration of the antidepressants, desmethylimipramine and zimelidine on rat cortical GABAB binding sites: a comparison with 5-HT2 binding site changes

1. The effects of chronic oral administration of desmethylimipramine (DMI) or zimelidine (1.25 and 5 mg kg-1 twice daily for 21 days) were studied on rat whole cortical gamma-aminobutyric acidB (GABAB) binding sites. No changes in receptor affinity or number were found with either drug. 2. A subsequent study of GABAB binding sites using higher doses of these drugs (5 and 10 mg kg-1) and rat frontal cortex was also without effect, when investigated 24 h after termination of drug administration or 72 h after DMI administration (5 mg kg-1). 3. The number of frontal cortical 5-hydroxytryptamine2 (5-HT2) binding sites was significantly and dose-dependently decreased after both drugs, whereas the number of hippocampal 5-HT2 binding sites was not significantly altered after either drug. 4. As the number of frontal cortical GABAB binding sites was unaltered whereas the number of 5-HT2 binding sites was significantly decreased under identical study conditions, it may be concluded that the effects of antidepressant administration upon GABAB binding sites is a less consistent observation than their effects on 5-HT2 binding sites.

Receptor interactions of dopamine and serotonin antagonists: binding in vitro and in vivo and receptor regulation

The advent of receptor binding techniques has provided new ways of studying the mechanism of action of drugs. In vitro radioligand binding is now currently applied to investigate the specificity or multiple action of compounds. By using the same technique, the binding affinity of a drug can be measured for a variety of neurotransmitter, drug, peptide and ion channel receptor binding sites, providing the drug's receptor binding profile (LEYSEN et al. 1981; LEYSEN 1984). However, in vitro receptor binding is only the initial step in the investigation of drug-receptor interactions. Investigations in vivo are required to allow evaluation of how and where a drug acts. In fact, the study of drug-receptor interactions comprises three main stages: (a) in vitro radioligand receptor binding; (b) in vivo receptor binding, providing information on the accessibility of the drugs to the receptors localized in various central and peripheral tissues, on the drug potency for occupying various receptors, on the duration of receptor occupation and on the relationship between the degree of receptor occupation and pharmacological effects; and (c) the study of receptor regulation: the effect of chronic drug treatment on receptor alterations compared with alterations in functional responses in vivo. In this article, we will illustrate the three stages of investigation of receptor interactions and discuss the relevance and importance of the findings, using as examples three drugs known in psychopharmacological research: (a) the neuroleptic haloperidol, a prototype of a dopamine D2 antagonist: (b) Setoperone, a potential antipsychotic agent with very potent serotonin S2 and moderate D2 antagonistic activity (CEULEMANS et al. 1985; LEYSEN et al. 1986); and (c) ritanserin, a potent and long-acting S2 antagonist (LEYSEN et al. 1985), which has revealed therapeutic activity in dysthymia and negative symptoms of schizophrenia (REYNTJENS et al. 1986; GELDERS et al. 1986). Particular attention will be paid to the problem of receptor regulation. We challenge the general applicability of the receptor regulation theory, which states that persistent receptor stimulation causes desensitisation and receptor downregulation, whereas chronic deprivation of receptor stimulation leads to supersensitivity and receptor upregulation. Recent research has revealed that the theory does not hold for S2 receptor alterations, which were found to downregulate following chronic receptor blockade.

Are increases in GABAB receptors consistent findings following chronic antidepressant administration?

Desmethylimipramine and zimelidine (10 mg/kg) were administered to rats once daily for 21 days. Chronic desmethylimipramine significantly reduced the number of 5HT2 binding sites in the frontal cortex but not hippocampus while chronic zimelidine did not affect 5HT2 binding in either brain region. The number and affinity of GABAB binding sites in the frontal cortex were unaltered by either drug.

Repeated electroconvulsive shock prevents increased neocortical beta 1-adrenoceptor binding after DSP-4 treatment in rats

Repeated electroconvulsive shock (ECS) was administered to rats previously injected with DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride), a noradrenergic neurotoxin. The normal increase in neocortical beta 1-adrenoceptor binding caused by noradrenaline depletion was effectively prevented by ECS. This finding suggests that the plasticity of the beta 1-adrenoceptor may be partially independent of endogenous noradrenaline concentration. Additionally, functional noradrenergic neurons are not necessarily a critical requirement for the antidepressant effect of electroconvulsive treatment in humans.

Opposite regulation of serotonin-S2 and dopamine-D2 receptors in rat brain following chronic receptor blockade

Rats were chronically treated with setoperone, a mixed serotonin and dopamine antagonist. Alterations in serotonin-S2 and dopamine-D2 receptors in the brain and changes in behavioural responses to tryptamine and apomorphine were studied along with duration of treatment and drug withdrawal. As with neuroleptics, behavioural supersensitivity to apomorphine and increase in the number of striatal dopamine-D2 receptor sites were apparent after 2 days setoperone treatment, both effects were maximal with 14 days treatment and were maintained over more than 20 days drug withdrawal. In contrast to the changes in the dopaminergic system, the rats showed a decreased response to tryptamine and serotonin-S2 receptor sites in the frontal cortex were significantly reduced in numbers. Both effects developed in parallel over 14 days treatment and extinguished over 10 days drug withdrawal. KD-values of radioligand binding to dopamine-D2 and serotonin-S2 receptor sites were unchanged by the setoperone treatment. The concomitant development and extinction of the in vivo and in vitro effects suggests a causal relationship between them. Chronic treatment with a selective histamine-H1 antagonist (levocabastine) or the tranquilizer diazepam did not affect dopamine-D2 or serotonin-S2 receptor sites. These observations demonstrate that in contrast to the receptor regulation theory, serotonin-S2 receptors are down regulated following persistent receptor blockade. Implications for the clinical use of serotonin antagonists and possible molecular mechanisms involved in the receptor regulation have been discussed.

Axonal transport of receptors: characterization, role in receptor regulation and possible involvement in learning

There is an increasing amount of evidence indicating that presynaptic receptors move antero- and retrogradely along axons of neurones through axoplasmic transport mechanisms. The main features of this dynamic process will be presented: it is bidirectional, fast, microtubule dependent and associated with vesicles which are presumably recycled in the cell body. Receptor axonal transport is impaired or enhanced in certain pathological conditions. The implications of this process and its possible involvement in long-term memory will be discussed.

Long lasting supersensitivity to 5-HT mediated behaviour following monoamine depletion in the rat brain

The effects of the depletion of 5-hydroxytryptamine (5-HT) in the rat brain upon the 5-HT behavioural syndrome (ejaculatory response, abduction of hind-limbs, forepaw treading, and Straub tail) induced by 5-methoxy-N,N-dimethyltryptamine (1 mg/kg intraperitoneally) were investigated. The 5-HT depletion was produced by p-chlorophenylalanine (PCPA 2 X 200 mg/kg intraperitoneally) or by reserpine (5 mg/kg subcutaneously). It resulted in rapidly developing and long-lasting supersensitivity of the behavioural responses which were enhanced between 2 and 14 days after the PCPA injections and between 1 and 42 days after a single dose of reserpine. The depletion of catecholamines by alpha-methyltyrosine (250 + 125 mg/kg intraperitoneally) had no effect. These results underline the usefulness of the 5-HT mediated behavioural models for studies of changes in 5-HT receptor activity.

The inhibition of the cage-leaving response--a model for studies of the serotonergic neurotransmission in the rat

It was observed that rats that had been given drugs that enhance serotonergic neurotransmission, e.g. the serotonin releasing compounds p-chloroamphetamine (PCA) and fenfluramine, the MAO-A inhibitors and serotonin releasing agents amiflamine and alpha-ethyltryptamine and the serotonin agonists 5-methoxy-N, N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT), m-chlorophenyl piperazine (m-CPP) and 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl)1H-indole (RU 24969), did not leave their home-cages when the grid-covers were removed in contrast to normal rats who almost immediately left the cages. The association between the serotonin neurotransmission and the inhibitory effect of PCA on the cage-leaving response was indicated by the findings that 1. Serotonin uptake inhibitors (alaproclate and citalopram) antagonized the effect of PCA. 2. High, neurotoxic doses of PCA antagonized the effect of PCA when tested one week after the former administration. The serotonin uptake inhibitor zimeldine counteracted the effect of neurotoxic PCA. 3. Depletion of brain serotonin with p-chlorophenylalanine counteracted the effect of acute PCA. 4. Repeated treatment of rats for 7 days with zimeldine, amiflamine, alpha-ethyltryptamine or clorgyline plus a low dose of PCA counteracted the effect of acute PCA probably due to a functional down-regulation at postsynaptic receptors. Clorgyline or a low dose of PCA by themselves had no effect. 5. Compounds interacting with dopamine or noradrenaline mechanisms, e.g. alpha-methyltyrosine, N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP 4), pimozide, remoxipride and prazosin did not antagonize the effect of PCA nor did (+)-amphetamine inhibit the cage-leaving response. None of the serotonin receptor antagonists (cinanserin, ketanserin, metergoline, methysergide, metitepine, mianserin, pirenperone) blocked the inhibition of the cage-leaving response produced by PCA, indicating that the receptors involved may not be of the S1- and S2- types. Observation of the cage-leaving response may be a valuable technique in studies of drugs that enhance the serotonin neurotransmission in the rat brain.

Rapid down-regulation of S2 serotonin receptors by antidepressants: noradrenergic-serotonergic interactions

Mianserin stereoselectively decreases rat cortical S2 binding, the (+)enantiomer having higher potency. This and other data suggest that an alpha-2 receptor is unlikely to contribute to the mechanism of rapid S2 down-regulation by mianserin. Yohimbine, an alpha-2 antagonist which enhances desipramine-induced S2 decreases, was not dependent on NE or 5-HT release for its effect. Depletion of NE by 75% or 5-HT by 94% did not alter the ability of yohimbine and desipramine to decrease binding. These results raise previously unsuspected mechanisms involved in acute down-regulation of S2 binding by mianserin and yohimbine.

In vivo regulation of the serotonin-2 receptor in rat brain

Serotonin-2 (5-HT-2) receptors in brain were measured using [3H]ketanserin. We examined the effects of amitriptyline, an antidepressant drug, of electroconvulsive shock (ECS) and of drug-induced alterations in presynaptic 5-HT function on [3H]ketanserin binding to 5-HT-2 receptors in rat brain. The importance of intact 5-HT axons to the up-regulation of 5-HT-2 receptors by ECS was also investigated, and an attempt was made to relate the ECS-induced increase in this receptor to changes in 5-HT presynaptic mechanisms. Twelve days of ECS increased the number of 5-HT-2 receptors in frontal cortex. Neither the IC50 nor the Hill coefficient of 5-HT in competing for [3H]ketanserin binding sites was altered by ECS. Repeated injections of amitriptyline reduced the number of 5-HT-2 receptors in frontal cortex. Reserpine, administered daily for 12 days, caused a significant increase in 5-HT-2 receptors, but neither daily injections of p-chlorophenylalanine (PCPA) nor lesions of 5-HT axons with 5,7-dihydroxytryptamine (5,7-DHT) affected 5-HT-2 receptors. However, regulation of 5-HT-2 receptors by ECS was dependent on intact 5-HT axons since ECS could not increase the number of 5-HT-2 receptors in rats previously lesioned with 5,7-DHT. Repeated ECS, however, does not appear to affect either the high-affinity uptake of [3H]5-HT or [3H]imipramine binding, two presynaptic markers of 5-HT neuronal function. 5-HT-2 receptors appear to be under complex control. ECS or drug treatments such as reserpine or amitriptyline, which affect several monoamine neurotransmission systems including 5-HT, can alter 5-HT-2 receptors. While depleting 5-HT alone (5,7-DHT or PCPA) does not alter [3H]ketanserin binding to 5-HT-2 receptors, intact 5-HT axons are necessary for the adaptive up-regulation of the receptor following ECS.

Cortical beta- and alpha 2- adrenoceptor binding, hypothalamic noradrenaline and pineal melatonin concentrations measured at different times of the day after repeated treatment of rats with imipramine, zimeldine, alaproclate and amiflamine

The effect of repeated treatment of rats for 21 days with the monoamine reuptake inhibitors imipramine, zimeldine, alaproclate (in each case 10 mumol/kg b.i.d.) and the reversible monoamine oxidase-A inhibitor amiflamine (3 mumol/kg b.i.d.) on brain noradrenergic mechanisms measured at different times of the day and night was investigated. Imipramine treatment produced a down-regulation of the Bmax for 3H-dihydroalprenolol binding to cortical beta-adrenoceptors that was not dependent upon the time of day the animals were killed. Zimeldine, on the other hand, reduced both Bmax and Kd of binding for day-time, but not night-time samples. Alaproclate and amiflamine were without effect on the binding. Twenty-four hour mean values for 1 nM 3H-p-aminoclonidine binding to alpha 2-adrenoceptors were lower for the zimeldine-treated rats than for the saline-treated rats. Pineal melatonin concentrations, which are regulated by beta-adrenoceptors, showed a pronounced diurnal rhythm, with the highest concentrations being found at 02:00. At this time point, a lower pineal melatonin content was found after amiflamine treatment, whereas imipramine, zimeldine and alaproclate were without significant effect. The importance of the use of more than one time point and the use of more than one biochemical test for the determination of the effects of repeated antidepressant treatment on central noradrenergic systems measured ex vivo is discussed.

Inhibition of head twitch response to quipazine in rats by chronic amitriptyline but not fluvoxamine or citalopram

Chronic (twice daily/14 days), but not acute, treatment with 10 mg/kg PO amitriptyline reduced the number of quipazine (5 mg/kg)-induced head twitches in rats, measured 2 h (but not 72 h) after the last administration of the drug. Similar treatment with fluvoxamine or citalopram, which are more potent and much more specific serotonin uptake inhibitors than amitriptyline, did not affect the quipazine-induced response. In acute experiments, fluvoxamine (10 mg/kg PO) and citalopram (10 mg/kg PO) potentiated the head twitch reaction induced by L-5-hydroxytryptophan (50 mg/kg IP) given together with Ro 4-4602 (25 mg/kg IP), a peripheral decarboxylase inhibitor. Amitriptyline (10 mg/kg PO) slightly decreased the number of L-5-hydroxytryptophan (5-HTP)-induced head twitches. Higher doses of amitriptyline (20-40 mg/kg PO) also inhibited the quipazine-induced head twitch reaction. The brain level of amitriptyline measured 0.5-24 h after the last oral administration of the chronic dose of 10 mg/kg was always much higher than that observed at the same time intervals after an acute oral dose of 20 or 40 mg/kg. The results obtained indicate that a postsynaptic rather then presynaptic mechanism is responsible for the development of subsensitivity of the central serotonin receptors in the course of chronic treatment with amitriptyline.

Intracerebroventricular administration of 5,7-dihydroxytryptamine to mice increases both head-twitch response and the number of cortical 5-HT2 receptors

5-Hydroxytryptamine-containing (5-HT) neurones in brain of the mouse were selectively destroyed by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT, 50 micrograms). Sham-lesioned controls received vehicle (2 microliters, i.c.v.). Two weeks later the head-twitch response induced by 5-methoxy-N,N-dimethyltryptamine (2.0 mg/kg) and mediated by 5-HT2 receptors was markedly enhanced in the lesioned mice. Furthermore, lesioning also increased 5-HT2 binding sites in the cortex, assessed by the binding of [3H]ketanserin in these animals, and decreased levels of 5-HT in the cortex (70%) and mid/hindbrain (64%) regions. A second group of mice, lesioned with less 5,7-DHT (5-20 micrograms, i.c.v.) showed unaltered head-twitch responses to 5-methoxy-N, N-dimethyltryptamine (2.0 mg/kg) and did not have increased 5-HT2 receptor binding in the cortex. Depletions of 5-HT were 32 and 40% in the cortex and mid/hindbrain, respectively. Comparison of the extent of depletion of 5-HT in the mid/hindbrain after lesioning, with the increase in head-twitch response, suggested that depletions of less than 40% did not affect this behaviour. However, depletions greater than 40% produced marked increases in this response and there was a good correlation between these two variables (r=0.701, P less than 0.01). In conclusion, these data suggest that enhanced head-twitch responses occurring after lesioning with 5,7-DHT may result from increased 5-HT2 receptors in brain. Moreover, the magnitude of the observed behavioural enhancement was dependent upon the extent of depletion of 5-HT produced by the lesioning.

The effects of manipulation of presynaptic 5-HT nerve terminals on postsynaptic 5-HT1 and 5-HT2 binding sites of the rat brain

The effects of long-term treatment of rats with alaproclate and amiflamine on the number and kinetics of 5-HT1 and 5-HT2 binding sites were investigated using in vitro receptor binding techniques. Some other studies have reported down-regulatory effects of alaproclate and amiflamine on 5-HT2 binding sites in certain regions of the rat forebrain, but no such effects could be detected in the present study. Induction of a high-affinity binding site for 3H-5-HT after long-term antidepressant treatment, as has been reported elsewhere, was not obtained in the present study. The results are compared to the effects obtained by treatment of rats with para-chloroamphetamine (PCA), which depletes the presynaptic neurons of monoamines. These different types of treatment do not cause any change in the binding properties of the specific 5-HT binding sites. It is thus concluded that such manipulations of the presynaptic 5-HT neurons do not affect the postsynaptic 5-HT1 and 5-HT2 binding sites.

Differential coupling of GABA-A and GABA-B receptors to the noradrenergic system

The GABA-A receptor agonist THIP, or the mixed GABA-A/GABA-B receptor agonist progabide dose dependently increased the release of norepinephrine (as measured by the production of MHPG) in the cerebral cortex and hippocampus. This effect was partially reversed by treatment with the GABA-A receptor antagonist bicuculline. In contrast, the GABA-B receptor agonist baclofen decreased the release of norepinephrine in the cerebral cortex and hippocampus. Pretreatment with the presynaptic noradrenergic neurotoxin DSP4 increased the Bmax for beta-adrenergic receptor binding in the cerebral cortex and hippocampus. This effect was partially prevented by chronic (14 day) treatment with either the beta-adrenergic agonist clenbuterol or the GABA-B receptor agonist baclofen. In contrast, chronic (14 day) administration with either the GABA-A receptor agonist THIP or the antidepressant imipramine failed to alter the increase in beta-adrenergic receptor binding produced by DSP4 pretreatment. These data suggest that the GABA-A receptor may be coupled to the presynaptic noradrenergic neuron and modulate the release of norepinephrine, while the GABA-B receptor is coupled to the postsynaptic noradrenergic neuron and likely functions through the cyclic AMP generating system.

Characterization of the binding of 3H-norzimeldine, a 5-HT uptake inhibitor, to rat brain homogenates

The binding of radiolabelled norzimeldine, a potent selective 5-HT reuptake inhibitor, to rat brain homogenates is described. 3H-Norzimeldine binds to a site with high affinity (KD = 10.5 nM) in a saturable manner (Bmax = 15.4 pmol/g wet weight in the cerebral cortex). The number of binding sites in the various regions of the brain parallels the capacity of the 5-HT reuptake mechanism. Drugs that inhibit the reuptake of 5-HT are also potent inhibitors of the 3H-norzimeldine binding, as are the tricyclic antidepressants, which are non-specific inhibitors of the noradrenaline and the 5-HT reuptake. Lesioning experiments using DSP4 (a NA neurotoxin) and p-chloroamphetamine (a 5-HT neurotoxin) suggest that the binding site is located on the presynaptic 5-HT nerve terminal, although a small component of the binding may be to noradrenergic uptake sites as well.