Anti-Inflammatory effects of antidepressants through suppression of the interferon-gamma/interleukin-10 production ratio

There is some evidence that major depression--in particular, treatment-resistant depression (TRD)--is accompanied by activation of the inflammatory response system and that proinflammatory cytokines may play a role in the etiology of depression. This study was carried out to examine the effects of antidepressive agents, i.e., imipramine, venlafaxine, L-5-hydroxytryptophan, and fluoxetine on the production of interferon-gamma (IFN-gamma), a proinflammatory cytokine, and interleukin-10 (IL-10), a negative immunoregulatory cytokine. Diluted whole blood of fluoxetine-treated patients with TRD (mean age, 50.6+/-3.9 years) and age-matched healthy controls (mean age, 51.6+/-1.7 years) and younger healthy volunteers (mean age, 35.4+/-9.6 years) was stimulated with phytohemagglutinin (1 microg/mL) and lipopolysaccharide (5 microg/mL) for 48 hours with and without incubation with the antidepressants at 10-6 M and 10(-5) M. IFN-gamma and IL-10 were quantified by means of enzyme-linked immunoassays. The ratio of IFN-gamma to IL-10 production by immunocytes was computed because this ratio is of critical importance in determining the capacity of immunocytes to activate or inhibit monocytic and T-lymphocytic functions. All four antidepressive drugs significantly increased the production of IL-10. Fluoxetine significantly decreased the production of IFN-gamma. All four antidepressants significantly reduced the IFN-gamma/IL-10 ratio. There were no significant differences in the antidepressant-induced changes in IFN-gamma or IL-10 between younger and older healthy volunteers and TRD patients. Tricyclic antidepressants, selective serotonin reuptake inhibitors, and serotonin-noradrenaline reuptake inhibitors, as well as the immediate precursor of serotonin, have a common, negative immunoregulatory effect by suppressing the IFN-gamma/IL-10 production ratio. It is suggested that the therapeutic efficacy of antidepressants may be related to their negative immunoregulatory effects.     

Negative immunoregulatory effects of antidepressants: inhibition of interferon-gamma and stimulation of interleukin-10 secretion.

There is now some evidence that major depression is accompanied by activation of the inflammatory response system. There is also some evidence that antidepressants may suppress the release of cytokines, such as interleukin-1 beta (IL-1 beta) and IL-6 by activated monocytes and IL-2 and interferon-gamma (IFN gamma) by activated T cells. This study was carried out to examine the effects of clomipramine, sertraline, and trazodone on the stimulated production of IFN gamma, a pro-inflammatory cytokine, and IL-10, a negative immunoregulatory cytokine. Whole blood of nine healthy volunteers was stimulated with PHA, 5 micrograms/mL and LPS, 25 micrograms/mL for 72 hr with and without incubation with clomipramine, 10(-6) and 10(-9) M, sertraline, 10(-6) and 10(-8) M, and trazodone, 10(-6) and 10(-8) M. All three antidepressants significantly reduced IFN gamma secretion, whereas clomipramine and sertraline significantly increased IL-10 secretion in culture supernatant. All three antidepressants significantly reduced the IFN gamma/IL-10 ratio. The results suggest that antidepressants, at concentrations in the therapeutical range, have negative immunoregulatory effects through inhibition of IFN gamma and stimulation of IL-10 release.     

The immunoregulatory effects of antidepressants

There is some evidence that major depression is accompanied by activation of the inflammatory-response system (IRS). It has been hypothesized that increased production of proinflammatory cytokines may play a role in the etiology of major depression. If increased production of proinflammatory cytokines is at all involved in the etiology of depression, one would expect antidepressive treatments to have negative immunoregulatory effects. This paper reviews the effects of antidepressants, such as tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), heterocyclic antidepressants (HCAs), serotonin-noradrenaline reuptake inhibitors (SNRIs), lithium, l-5-hydroxytroptophan (L-5-HTP), reversible inhibitors of MAO-A (RIMA) on the production of proinflammatory cytokines, e.g. interferon-gamma (IFNgamma), and negative immunoregulatory cytokines and agents, e.g. interleukin-10 (IL-10). In depressed patients, prolonged treatment with antidepressants and mood stabilizers normalizes signs of activation of the IRS, such as increased serum IL-6 and acute phase protein concentrations. In vitro, it has been shown that various types of antidepressive drugs, including TCAs (imipramine; clomipramine); SSRIs (citalopram, fluoxetine, sertraline); lithium; SNRIs (venlafaxine); HCAs (trazodone); RIMAs (moclobemide) and L-5-HTP significantly suppress the ratio of IFNgamma/IL-10 production by peripheral blood immunocytes. These antidepressant drugs appear to have a common effect on the IRS, i.e. in vitro they increase the production of IL-10 by peripheral blood leukocytes. Thus, the results suggest that antidepressants have negative immunoregulatory effects. It may be speculated that antidepressants exert some of their antidepressant effects through their negative immunoregulatory capacities. Copyright 2001 John Wiley & Sons, Ltd.     

The 5-HT1A receptor agonist flesinoxan shares discriminative stimulus properties with some 5-HT2 receptor antagonists.

Ten homing pigeons were trained to discriminate the selective 5-HT1A receptor agonist flesinoxan (0.25 mg/kg p.o.) from its vehicle in a fixed-ratio (FR) 30 two-key operant drug discrimination procedure. The 5-HT2 receptor antagonist mianserin (ED50 = 4.8 mg/kg) fully substituted for flesinoxan, whereas ketanserin, ritanserin, mesulergine, and SB200646A substituted only partially, suggesting an interaction between 5-HT1A and 5-HT2 receptors. However, the 5-HT2 receptor agonists [DOI (0.6 mg/kg), TFMPP (10 mg/kg), mCPP (4 mg/kg)] were unable to antagonize the flesinoxan cue. The 5-HT1A receptor antagonists DU125530 (0.5-13 mg/kg) and WAY100,635 (0.1-1 mg/kg) partially antagonized the generalization of mianserin to flesinoxan. Taken together, these results are in accordance with the hypothesis that 5-HT1A receptor activation exerts an inhibitory effect on activation of 5-HT2 receptors. These results are in broad agreement with existing theories on 5-HT1A and 5-HT2 receptor interaction. Furthermore, it is argued that the discriminative stimulus properties of a drug may undergo qualitative changes with prolonged training.     

Ritanserin antagonism of m-chlorophenylpiperazine effects in neuroleptic-free schizophrenics patients: support for serotonin-2 receptor modulation of schizophrenia symptoms

RATIONALE: Serotonin-2 (5-HT(2)) receptor antagonism has been hypothesized to have antipsychotic activity. However, there has been limited evidence directly linking 5-HT(2) receptor antagonism to symptom control in schizophrenic patients. Objectives: In order to test this hypothesis this study evaluated the capacity of pretreatment with the 5-HT(2) receptor antagonist ritanserin to attenuate the effects of the 5-HT agonist, m-chlorophenylpiperazine (mCPP). METHODS: Twenty-two male inpatients who met DSM-III-R criteria for schizophrenia or schizoaffective disorder completed 4 test days during which 10 mg ritanserin or matched placebo was administered orally 50 min prior to the intravenous infusion of 0.1 mg/kg mCPP or saline. The test days were conducted in a randomized order under double-blind conditions. RESULTS: mCPP mildly and transiently increased positive symptoms and behavioral activation but not negative symptoms, as assessed by the Brief Psychiatric Rating Scale. mCPP also raised plasma prolactin and cortisol levels. All of these effects were attenuated by ritanserin pretreatment. CONCLUSIONS: These data support a contribution of 5-HT(2) receptor stimulation to symptom exacerbation in schizophrenic patients and a role for 5-HT(2) receptor antagonism in the prevention of this effect.     

In vitro immunoregulatory effects of antidepressants in healthy volunteers

Major depression is accompanied by an activation of the inflammatory response system (IRS) and antidepressants may have immunoregulatory activities. This study was carried out to compare the effect of imipramine, mianserin and lithium on the in vitro production of Th1-like cytokines, such as IL-2, IFN-gamma, lymphotoxin and Th2-like cytokines such as IL-4, IL-10 as well as IL-12 and TGF-beta. Peripheral blood mononuclear cells (PBMC) of 16 healthy volunteers were stimulated with polyclonal activators (phytohemagglutinin with lipopolysaccharide PHA + LPS) with or without incubation with imipramine, mianserin (1 microM) or lithium (1 mM). Imipramine and mianserin exhibited similar activities enhancing unstimulated IFN-gamma and IL-10 production. In PHA + LPS-stimulated PBMC both antidepressants inhibited IFN-gamma, IL-2 and lymphotoxin production (Th1-like cytokines) as well as IL-12 and IL-4 production. Under the same in vitro conditions, both antidepressants stimulated production of negative immunoregulatory cytokines such as IL-10 and TGF-beta. Lithium differed significantly from imipramine and mianserin, as it enhanced IL-2, IFN-gamma, IL-10 and TGF-beta production and inhibited only IL-4. All three examined antidepressants reduced IFN-gamma/IL-10 ratio. None of the antidepressants at the used concentrations induced apoptosis in PBMC so those changes in cytokine production were not the result of selective killing of certain cell subpopulations. Imipramine and mianserin at high concentrations negatively influenced reactive oxygen species (ROS) production in neutrophils, however, at concentrations in the therapeutical range none of the antidepressants used influenced "oxidative burst" in neutrophils. The results indicate that antidepressants exert immunoregulatory effects on human leukocyte functions, especially on cytokine production.     

The 5-HT2C/2B receptor agonist m-chlorophenylpiperazine (mCPP) inhibits 2-deoxy-D-glucose (2-DG)-induced hyperphagia in rats.

Effects of the 5-HT2C2/2B receptor agonist m-chlorophenylpiperazine (mCPP) on hyperphagia elicited by 2-deoxy-D-glucose (2-DG) were investigated in rats. mCPP apparently reduced 2-DG-induced hyperphagia. Suppressive effects of mCPP on hyperphagia induced by 2-DG were inhibited by the 5-HT2A/2B/2C receptor antagonist, ritanserin, although the 5-HT2, receptor antagonist ketanserin was without effect. Thus, inhibitory effects of mCPP on 2-DG-induced hyperphagia are mediated by the 5-HT2C/2B receptor. Our results demonstrate that mCPP can inhibit the bulimia model, 2-DG-induced hyperphagia.     

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

RATIONALE: Flesinoxan is a highly potent and selective 5-HT1A agonist. In a recent study, in normal volunteers, flesinoxan induced a significant and dose-dependent increase in adrenocorticotropic hormone (ACTH), cortisol, prolactin (PRL), growth hormone (GH) and a decrease in body temperature. OBJECTIVES: In order to better define the role of 5-HT receptor subtypes in response to flesinoxan, we assessed the influence of 5-HT1A and 5-HT2 antagonists on hormonal and temperature responses to flesinoxan. METHODS: Hormonal and temperature responses were studied in 6 volunteers with or without pretreatment with pindolol (30 mg p.o.), a 5-HT1A antagonist, or ritanserin (10 mg p.o.), a selective 5-HT2 antagonist, using a double-blind crossover design. RESULTS: Pindolol significantly antagonized ACTH, PRL, GH and temperature responses to flesinoxan and ritanserin exhibited similar activity on PRL and ACTH responses. CONCLUSIONS: These results show the role of 5-HT1A mechanisms in the PRL, ACTH, GH, and temperature responses to flesinoxan, and the role of 5-HT2 mechanisms in PRL and ACTH responses. Therefore, they confirm the interest of flesinoxan as a 5-HT neuroendocrine probe.     

The negative immunoregulatory effects of fluoxetine in relation to the cAMP-dependent PKA pathway

Recently, we have shown that various types of antidepressants, including selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine, have negative immunoregulatory effects. These antidepressants suppress the interferon-gamma (IFN-gamma)/interleukin-10 (IL-10) production ratio, which is of critical importance for the determination of the capacity of immunocytes to inhibit or activate monocytic/lymphocytic functions. Since cyclic adenosine monophosphate (cAMP) production is stimulated by some antidepressants, and since cAMP inhibits IFN-gamma and stimulates IL-10 production, we postulate that the negative immunoregulatory effects of antidepressants result from their effects on the cAMP-dependent protein kinase A (PKA) pathway. The aim of the present study was to determine whether the negative immunoregulatory effects of fluoxetine may be blocked by antagonists of the cAMP-dependent PKA pathway, such as, e.g., SQ 22536, an adenylate cyclase inhibitor, and Rp-8-Br-cAMPs (Rp-isomer of 8-bromo-adenosine-3',5'-monophosphorothioate), a PKA antagonist. To this end, diluted whole blood collected from 17 normal volunteers was incubated with fluoxetine (10(-6) and 10(-5) M), with or without SQ 22536 (10(-6) and 10(-4) M) and Rp-8-Br-cAMPs (10(-6) and 10(-4) M), afterwards, IFN-gamma, IL-10 and the tumor necrosis factor alpha (TNF-alpha) were determined. Fluoxetine, 10(-6) and 10(-5) M, significantly reduced the production of IFN-gamma and TNF-alpha, and significantly decreased the IFN-gamma/IL-10 production ratio. SQ 22536 and Rp-8-Br-cAMPs were unable to block the suppressant effects of fluoxetine on the IFN-gamma/IL-10 ratio. Rp-8-Br-cAMPs, 10(-4), but not 10(-6) M, normalized the fluoxetine-induced suppression of TNF-alpha production. It is concluded that the suppressant effect of fluoxetine on the IFN-gamma/IL-10 production ratio is probably not related to the induction of the cAMP-dependent PKA pathway, whereas the suppressant effect on TNF-alpha may be related to the induction of PKA. The obtained results suggest that increased activation of the PKA-dependent pathway may constitute an important molecular basis for some (suppression of TNF-alpha production), but not all (suppression of IFN-gamma production), negative immunoregulatory effects of fluoxetine.     

Evidence for 5-HT(1B/1D) and 5-HT(2A) receptors mediating constriction of the canine internal carotid circulation

The present study has investigated the preliminary pharmacological profile of the receptors mediating vasoconstriction to 5-hydroxytryptamine (5-HT) in the internal carotid bed of vagosympathectomised dogs. One minute intracarotid infusions of the agonists 5-HT (0.1 - 10 microg min(-1)), sumatriptan (0.3 - 10 microg min(-1); 5-HT(1B/1D)), 5-methoxytryptamine (1 - 100 microg min(-1); 5-HT(1), 5-HT(2), 5-HT(4), 5-ht(6) and 5-HT(7)) or DOI (0.31 - 10 microg min(-1); 5-HT(2)), but not 5-carboxamidotryptamine (0.01 - 0.3 microg min(-1); 5-HT(1), 5-ht(5A) and 5-HT(7)), 1-(m-chlorophenyl)-biguanide (mCPBG; 1 - 1000 microg min(-1); 5-HT(3)) or cisapride (1 - 1000 microg min(-1); 5-HT(4)), resulted in dose-dependent decreases in internal carotid blood flow, without changing blood pressure or heart rate. The vasoconstrictor responses to 5-HT, which remained unaffected after saline, were resistant to blockade by i.v. administration of the antagonists ritanserin (100 microg kg(-1); 5-HT(2A/2B/2C)) in combination with tropisetron (3000 microg kg(-1); 5-HT(3/4)) or the cyclo-oxygenase inhibitor, indomethacin (5000 microg kg(-1)), but were abolished by the 5-HT(1B/1D) receptor antagonist, GR127935 (30 microg kg(-1)). Interestingly, after administration of GR127935, the subsequent administration of ritanserin unmasked a dose-dependent vasodilator component. GR127935 or saline did not practically modify the vasoconstrictor effects of 5-MeO-T. In animals receiving GR127935, the subsequent administration of ritanserin abolished the vasoconstrictor responses to 5-MeO-T unmasking a dose-dependent vasodilator component. The vasoconstriction induced by sumatriptan was antagonized by GR127935, but not by ritanserin. Furthermore, ritanserin (100 microg kg(-1)) or ketanserin (100 microg kg(-1); 5-HT(2A)), but not GR127935, abolished DOI-induced vasoconstrictor responses. The above results suggest that 5-HT-induced internal carotid vasoconstriction is predominantly mediated by 5-HT(1B/1D) and 5-HT(2A) receptors.     

Evidence that mCPP may have behavioural effects mediated by central 5-HT1C receptors.

1. The effects of 1-(3-chlorophenyl)piperazine (mCPP) and 1-[3-(trifluoromethyl)phenyl] piperazine (TFMPP) on activity of rats in a novel cage, and on the rotorod and elevated bar co-ordination tests was examined. 2. Peripherally administered mCPP and TFMPP dose-dependently reduced locomotion, rearing, and feeding scores but not grooming of freely fed rats placed in a novel observation cage. Yawning behaviour was increased. Similar effects were also observed after injection of mCPP into the 3rd ventricle. 3. Co-ordination on a rotating drum of both untrained and trained rats was impaired following mCPP but co-ordination on an elevated bar was not. 4. The hypoactivity induced by mCPP was opposed by three antagonists with high affinity for the 5-hydroxytryptamine (5-HT1C) site; metergoline, mianserin, cyproheptadine and possibly also by a fourth antagonist mesulergine. Metergoline, mianserin and cyproheptadine also opposed the reduction in feeding scores. However, neither effect of mCPP was antagonized by the 5-HT2-receptor antagonists ketanserin or ritanserin, the 5-HT3-receptor antagonist ICS 205-930, the 5-HT1A and 5-HT1B-receptor antagonists (-)-pindolol, (-)-propranolol and (+/-)-cyanopindolol or the 5-HT1A-, 5-HT2- and dopamine receptor antagonist spiperone. The specific alpha 2-adrenoceptor antagonist idazoxan was also without effect. 5. Hypoactivity induced by TFMPP was similarly antagonized by mianserin but unaffected by (+/-)-cyanopindolol. 6. These results suggest that the hypoactivity is mediated by central 5-HT1C-receptors and that mCPP and possibly TFMPP may be 5-HT1C-receptor agonists. 7. As mianserin, cyproheptadine and mesulergine in the absence of mCPP did not increase locomotion but increased the number of feeding scores, the activation of 5-HT1C-receptors may be of physiological importance in the control of appetite. The possible relevance of these results to the therapeutic and side-effects of clinically used antidepressants (particularly trazodone and mianserin) and anorexigenic drugs is discussed.     

The 5-HT(2C/2B) receptor agonist, m-chlorophenylpiperazine, increases plasma glucagon levels in rats

Effects of the 5-HT(2C/2B) receptor agonist m-chlorophenylpiperazine (mCPP) on plasma glucagon levels were investigated in rats. mCPP dose dependently increased plasma glucagon levels. Hyperglucagonemia elicited by mCPP was prevented by the 5-HT(2A/2B/2C) receptor antagonist, ritanserin, while the 5-HT(2A) receptor antagonist, ketanserin, did not show any effect. Increases in glucagon levels induced by mCPP were inhibited by prior adrenodemedullation. These results indicate that increases in plasma glucagon levels induced by mCPP are mediated by the 5-HT(2C/2B) receptor which in turn facilitates adrenaline release.     

IL-18BPa:Fc cooperates with immunosuppressive drugs in human whole blood

The proinflammatory cytokine interleukin (IL)-18 appears to be involved in the pathogenesis of diseases associated with immunoactivation and inflammation. Consequently, blockage of IL-18 bioactivity by use of IL-18 binding protein (IL-18 BP) is likely a promising therapeutic concept. In the present study, we investigated immunomodulatory activities of IL-18 BPa:Fc in human whole blood cultures. We report that IL-18 BPa:Fc (200 ng/mL) significantly inhibited lipopolysaccharide (LPS, 10 ng/mL)/IL-12 (5 ng/mL)-induced release of interferon-gamma (IFNgamma) and matrix metalloproteinase-9 (MMP-9) from whole blood cultures of healthy donors. Notably, IL-18 BPa:Fc (200 ng/mL) further reinforced dexamethasone (5 nM)- or mycophenolic acid (2 microM)-mediated reduction of LPS/IL-12-induced IFNgamma production by an additional 50.5 or 49.9%, respectively. To investigate effects of IL-18 BP:Fc in the context of autoimmune diseases, experiments were performed with whole blood obtained from patients with systemic lupus erythematosus or Wegener's granulomatosis undergoing immunosuppressive therapy. After ex vivo stimulation with LPS (10 ng/mL), production of IFNgamma and MMP-9 was determined. Both mediators likely contribute to renal inflammation frequently seen in these diseases. In accord with the aforementioned data, LPS (10 ng/mL)-induced IFNgamma was significantly reduced by coincubation with IL-18 BPa:Fc at 200 ng/mL. IL-18 BPa:Fc also inhibited production of MMP-9. The present data demonstrate that IL-18 BPa:Fc has the potential to amplify anti-inflammatory actions of immunosuppressive drugs, and thus may prove to be a valuable novel pharmacological component in the treatment of human autoimmune diseases.     

Anxiogenic-like effects of mCPP and TFMPP in animal models are opposed by 5-HT1C receptor antagonists

1-(3-chlorophenyl)piperazine (mCPP) and 1-[3-(trifluoromethyl)phenyl]piperazine (TFMPP) (0.1-1.0 mg/kg) reduce total interaction time in a rat social interaction test under low light familiar conditions and its following components; grooming, following, crawling over, fighting, sniffing. Locomotion was only reduced by the highest dose of mCPP. mCPP also reduced activity in the light but not total locomotion in a light/dark transition test. These results suggest that mCPP (and TFMPP) are anxiogenic but not sedative in these tests. The effect of mCPP on social interaction was blocked by three antagonists which share a high affinity for 5-HT1C and 5-HT2 receptors: mianserin, cyproheptadine and metergoline but not by the 5-HT2 antagonists ketanserin or ritanserin or the 5-HT1A and 5-HT1B antagonists cyanopindolol and (-)-propranolol. It was prevented by a low (0.05 mg/kg) but not by a high (1.0 mg/kg) dose of ICS 205,930 a specific 5-HT3 antagonist reported to be anxiolytic at low doses. It was also prevented by chronic pretreatment with the anxiolytic drug chlordiazepoxide. These results argue for an anxiogenic action of mCPP mediated by 5-HT1C receptors. Since the chronic chlordiazepoxide pretreatment did not prevent the hypolocomotion or hypophagia induced by mCPP at high dosage (5 mg/kg) these latter effects are unlikely to be secondary to anxiety.     

Mediation of 5-HT-induced internal carotid vasodilatation in GR127935- and ritanserin-pretreated dogs by 5-HT7 receptors

The vasoconstrictor effects of 5-hydroxytryptamine (5-HT) in the internal carotid bed of anaesthetised dogs with bilateral vagosympathectomy are mainly mediated by both 5-HT1B and 5-HT2 receptors. The blockade of this vasoconstrictor effect of 5-HT by the combined use of the antagonists, GR127935 (5-HT1B/1D) and ritanserin (5-HT2), unmasks a dose-dependent vasodilator effect of 5-HT, but not of sumatriptan. Therefore, the present study set out to analyse the pharmacological profile of this vasodilator 5-HT receptor in the internal carotid bed of vagosympathectomized dogs systematically pretreated with intravenous (i.v.) injections of GR127935 (30 microg/kg) and ritanserin (100 microg/kg). One-minute (1-min) intracarotid (i.c.) infusions of 5-HT (0.1-10 microg/min), 5-carboxamidotryptamine (5-CT; 0.01-0.3 microg/min), 5-methoxytryptamine (5-MeO-T; 1-100 microg/min) and acetylcholine (ACh; 0.003-0.1 microg/min) resulted in dose-dependent increases in internal carotid blood flow (without changes in blood pressure or heart rate) with a rank order of agonist potency of ACh > 5-CT > 5-HT > or =5-MeO-T. The internal carotid vasodilator responses to 5-HT, 5-CT and 5-MeO-T, which remained unaffected after saline (0.03 ml/kg and 0.1 ml/kg, i.v.), were specifically and dose-dependently blocked by i.v. administration of lisuride (10 microg/kg and 30 microg/kg), clozapine (1000 microg/kg), mesulergine (300 microg/kg and 1000 microg/kg) and LY215840 (300 microg/kg and 1000 microg/kg) with the following apparent rank order of potency: lisuride > mesulergine = LY215840 > or = clozapine. The above results suggest that the 5-HT receptor mediating internal carotid vasodilatation in vagosympathectomized dogs pretreated with GR127935 and ritanserin is operationally similar to other 5-HT7 receptors mediating vascular and non-vascular responses.     

Serotonergic Mechanisms Involved in the Exploratory Behaviour of Mice in a Fully Automated Two-Compartment Black and White Test Box

Abstract A fully automated version of the black and white two-compartment box for mice is presented. The anxiolyticlike effects of the benzodiazepines, diazepam and chlordiazepoxide, were confirmed, and the involvement of serotonergic mechanisms was studied in this animal model of anxiety. The partial 5-HT_1A receptor agonists, buspirone and ipsapirone showed anxiolytic-like effects in a limited dose interval. The full agonist hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) was inactive. The non-selective 5-HT₁ receptor agonist, eltoprazine, induced marked increases of exploratory behaviour in the white compartment over a broad range of doses. Also pindolol a mixed 5-HT_1A/1B and β-adrenergic receptor antagonist showed anxiolytic-like effects, whereas another compound with a similar profile (-)-, penbutolol and the β-adrenoceptor antagonist ICI 118,551, was inactive. The 5-HT_2A/2C receptor antagonist, ritanserin, showed anxiogenic-like, and the 5-HT₃ receptor antagonists, zacopride and ondansetron, showed anixiolytic-like effects. An overall increase of serotonergic activity by means of 5-HT uptake inhibition (citalopram), 5-HT release (fenfluramine) or administration of a 5-HT precursor (1–5-HTP) facilitated exploratory activity in the white compartment. Reduction of serotonergic activity by treatment with the 5-HT depletor p-chloro-phenylalanine methyl ester (PCPA) did not change the exploratory behaviour, but attenuated the response to fenfluramine significantly.     

5-HT1C receptors in the serotonergic control of periaqueductal gray induced aversion in rats

The functional role of brain 5-HT and 5-HT receptor subtypes in periaqueductal gray (PAG) induced aversion has been investigated in rats. Antiaversive effects were found with the serotonin agonists TFMPP, mCPP and DOI but not with RU 24969 which was found to facilitate PAG aversion. The first three serotonin agonists share potent 5-HT_1C activity while RU 24969 differs with a high 5-HT_1A activity. Proaversive effects were found with the mixed 5-HT_1C/5-HT₂ antagonists cyproheptadine and ritanserin; this effect was already reported for the mixed 5-HT_1C/5-HT₂ antagonists metergoline and mianserin and is opposite to the effects of the selective 5-HT₂ antagonists ketanserin, pirenperone, trazodone and spiperone. The antiaversive effects of mCPP (1 mg/kg) could be prevented by pretreatment of the animals with mianserin (1 and 10 mg/kg). These results suggest that 5-HT_1C receptors play an important role in the serotonergic control of PAG aversion. 5-HT_1C receptor activation seems to mediate antiaversive effects whereas acute 5-HT_1C receptor blockade appears to facilitate PAG aversion. Functional interactions take place between several receptor types in the in vivo control of PAG aversion, where 5-HT_1C receptors appear to play a predominant function.     

Discriminative stimulus properties of flesinoxan

Different groups of rats were trained to discriminate either 0.3 mg/kg of flesinoxan (N = 13) or 0.1 mg/kg of 8-OH-DPAT (N = 7) from saline in a two-lever operant drug discrimination task using a fixed ratio 10 schedule of reinforcement. Once trained, animals in both groups displayed a dose-related decrease in discriminative performance upon administration of lower doses of the drug used in training. In generalization tests, flesinoxan generalized to 8-OH-DPAT in 8-OH-DPAT-trained animals and 8-OH-DPAT substituted for flesinoxan in flesinoxan-trained animals. Buspirone substituted partially for both the flesinoxan and the 8-OH-DPAT cue. The results of the present study indicate similarity between the discriminative stimulus effects of flesinoxan and the stimulus produced by the 5-HT1A agonist 8-OH-DPAT. These results, coupled with the finding that flesinoxan has a significant affinity and selectivity for 5-HT1A binding sites, suggest that the stimulus effects of flesinoxan are mediated by a 5-HT1A mechanism.     

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.     

The atypical 5-HT2 receptor mediating tachycardia in pithed rats: pharmacological correlation with the 5-HT2A receptor subtype

1. In pithed rats, 5-HT mediates tachycardia both directly (by 5-HT(2) receptors) and indirectly (by a tyramine-like effect). The receptor mediating tachycardia directly has been classified as an 'atypical' 5-HT(2) receptor since it was 'weakly' blocked by ketanserin. Moreover, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT(2) agonist, failed to mimic 5-HT-induced tachycardia. Since 5-HT(2) receptors consist of 5-HT(2A), 5-HT(2B) and 5-HT(2C) subtypes, this study investigated if these subtypes mediate the above response. 2. In pithed rats, intraperitoneally (i.p.) pre-treated with reserpine (5 mg kg(-1)), intravenous (i.v.) administration of 5-HT, 5-methoxytryptamine (5-MeO-T), 1-(3-chlorophenyl) piperazine (mCPP) and 5-carboxamidotryptamine (5-CT) (10, 30, 100 and 300 microg kg(-1) each), produced dose-dependent tachycardic responses. Interestingly, DOI (10 - 1000 microg kg(-1), i.v.) induced only slight, dose-unrelated, tachycardic responses, whilst the 5-HT(2C) agonist, Ro 60-0175 (10 - 1000 microg kg(-1), i.v.), produced a slight tachycardia only at 300 and 1000 microg kg(-1). In contrast, sumatriptan and 1-(m-trifluoromethylphenyl)- piperazine (TFMPP) were inactive. The rank order of potency was: 5-HT > or =5-MeO-T> mCPP > or =5-CT > or =DOI > Ro 60-0175. 3. The tachycardic responses to 5-HT, which remained unaffected after i.v. saline (0.3 and 1 ml kg(-1)) or propranolol (3 mg kg(-1)), were selectively blocked by the 5-HT(2A) antagonists ketanserin (30 and 100 microg kg(-1)) or spiperone (10 and 30 microg kg(-1)) as well as by the non-selective 5-HT(2) antagonists, ritanserin (10 and 30 microg kg(-1)) or mesulergine (100 microg kg(-1)). Remarkably, these responses were unaffected by the antagonists rauwolscine (5-HT(2B)), SB204741 (5-HT(2B/2C)) or Ro 04-6790 (5-ht(6)) (300 and 1000 microg kg(-1) each). 4. These results suggest that the 'atypical' 5-HT(2) receptors mediating tachycardia in reserpinized pithed rats are pharmacologically similar to the 5-HT(2A) receptor subtype.