The cannabinoid CB1 receptor antagonists rimonabant (SR141716) and AM251 directly potentiate GABA(A) receptors

BACKGROUND AND PURPOSE

Rimonabant (SR141716) and the structurally related AM251 are widely used in pharmacological experiments as selective cannabinoid receptor CB₁ antagonists / inverse agonists. Concentrations of 0.5–10 µM are usually applied in in vitro experiments. We intended to show that these drugs did not act at GABA_A receptors but found a significant positive allosteric modulation instead.

EXPERIMENTAL APPROACH

Recombinant GABA_A receptors were expressed in Xenopus oocytes. Receptors were exposed to AM251 or rimonabant in the absence and presence of GABA. Standard electrophysiological techniques were used to monitor the elicited ionic currents.

KEY RESULTS

AM251 dose-dependently potentiated responses to 0.5 µM GABA at the recombinant α₁β₂γ₂ GABA_A receptor with an EC₅₀ below 1 µM and a maximal potentiation of about eightfold. The Hill coefficient indicated that more than one binding site for AM251 was located in this receptor. Rimonabant had a lower affinity, but a fourfold higher efficacy. AM251 potentiated also currents mediated by α₁β₂, α_xβ₂γ₂ (x = 2,3,5,6), α₁β₃γ₂ and α₄β₂δ GABA_A receptors, but not those mediated by α₁β₁γ₂. Interestingly, the CB₁ receptor antagonists {"type":"entrez-nucleotide","attrs":{"text":"LY320135","term_id":"1257555575","term_text":"LY320135"}}LY320135 and O-2050 did not significantly affect α₁β₂γ₂ GABA_A receptor-mediated currents at concentrations of 1 µM.

CONCLUSIONS AND IMPLICATIONS

This study identified rimonabant and AM251 as positive allosteric modulators of GABA_A receptors. Thus, potential GABAergic effects of commonly used concentrations of these compounds should be considered in in vitro experiments, especially at extrasynaptic sites where GABA concentrations are low.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7     

Suppression of firing activity of 5-HT neurons in the dorsal raphe by alpha-adrenoceptor antagonists

Previous pharmacological studies have suggested that the firing activity of 5-HT cells of the dorsal raphe nucleus is dependent on a tonically active, central adrenergic system. In this study, a wide variety of alpha-adrenoceptor antagonists, WB-4101 (41 ± 20 μg/kg; ED₅₀ ± SD), piperoxan (0.64 ± 0.20 mg/kg), thymoxamine (0.42 ± 0.31 mg/kg) and phenoxybenzamine (3.0 mg/kg) were found to suppress firing when administered sytemically. These alpha-adrenoceptor antagonists, as well as phentolamine and dihydroergocryptine, also reduced 5-HT cell firing when applied iontophoretically. The order of potency of the drugs when applied systemically was WB-4101 ? piperoxan ～- thymoxamine > phenoxybenzamine. This ranking correlates well with their activity at classical peripheral postsynaptic α-adrenoceptors. In addition, the order of potency of microiontophoretically applied adrenergic agonists (norepinephrine > phenylephrine >α-methylnorepinephrine > isoproterenol > salbutamol) in restoring 5-HT cell firing during competitive alpha-adrenoceptor blockade suggests that this receptor should be classified in the alpha-1-adrenoceptor category. Previous anatomical studies have demonstrated that the dorsal raphe receives an adrenergic input. Taken together, these findings suggest that NE terminals, present in the dorsal raphe, mediate a tonically active adrenergic influence upon which the firing of 5-HT cells depends.     

Cannabinoid CB1 and CB2 receptors antagonists AM251 and AM630 differentially modulate the chronotropic and inotropic effects of isoprenaline in isolated rat atria

Background

Drugs targeting CB₁ and CB₂ receptors have been suggested to possess therapeutic benefit in cardiovascular disorders associated with elevated sympathetic tone. Limited data suggest cannabinoid ligands interact with postsynaptic β-adrenoceptors. The aim of this study was to examine the effects of CB₁ and CB₂ antagonists, AM251 and AM630, respectively, at functional cardiac β-adrenoceptors.

Mechanisms of nicotine actions on dorsal raphe serotoninergic neurons

Nicotine, locally administered into the dorsal raphe nucleus (DRN) of rat midbrain slices, increased the discharge rate of 70% of serotoninergic neurons, decreased it in 30% and induced reciprocal oscillatory increases in serotonin (5-hydroxytryptamine, 5-HT) and gamma-aminobutyric acid (GABA) release. All of nicotine's stimulatory effects were maximal at 2.15 microM. Bicuculline, a GABA(A) receptor antagonist, increased the firing rate in 64% of serotoninergic neurons, decreased it in 36% and augmented serotonin and GABA release. Bicuculline increased nicotine's stimulatory effects on firing rate but did not reverse the inhibitory ones. N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinil-cyclohexanecarboxamide (WAY-100635), a 5-HT(1A) receptor antagonist, increased the firing rate of 88% of serotoninergic neurons, as well as serotonin and GABA release and reversed nicotine's inhibitory action on serotoninergic neurons. These data suggest that nicotine decreases the firing rate of one third of serotoninergic neurons through serotonin release and increases the firing rate of the remaining two thirds, due to stronger stimulatory than indirect inhibitory effects.     

Receptor-mediated regulation of serotonin output in the rat dorsal raphe nucleus: effects of risperidone

OBJECTIVES: The present study was undertaken to characterize the regulation of serotonin (5-HT) efflux and neuronal activity in the dorsal raphe nucleus (DRN) as well as to examine the potential ability of the antipsychotic drug risperidone to interfere with these mechanisms. METHODS AND RESULTS: By using microdialysis in freely moving rats, it was found that administration of the alpha2 adrenoceptor antagonist idazoxan (0.25 mg/kg, SC), the 5-HT1B/D receptor antagonist GR 127,935 (1.0 mg/kg, SC) and risperidone (0.6 or 2.0 mg/kg, SC) increased 5-HT output in the DRN. Local DRN perfusion with GR 127,935 or risperidone via reversed dialysis (100 or 10-100 microM, respectively) enhanced 5-HT efflux in this area, whereas idazoxan (10-100 microM) failed to affect this parameter. Both systemic administration and reversed DRN dialysis of the D2/3 and 5-HT2A receptor antagonists raclopride (2.0 mg/kg, SC or 10-100 microM) and MDL 100,907 (1.0 mg/kg, SC or 10-100 microM), respectively, were without effect. Intraraphe dialysis of the 5-HT1B/D receptor agonist CP 135,807 (0.2 microM) decreased the efflux of 5-HT in the DRN, an effect which was antagonized by co-administration of either GR 127,935 or risperidone (10 and 3.3 microM, respectively). By using single-cell recording, it was found that administration of GR 127,935 (50-400 microg/kg, IV) decreased, whereas CP 135,807 (2.5-20 microg/kg, IV) increased firing of 5-HT cells in the DRN. CONCLUSIONS: Our findings suggest a regulatory role of local 5-HT1B/D receptors on 5-HT efflux as well as cell firing in the DRN and indicate that risperidone may interfere with the regulation of 5-HT availability in this area primarily via blockade of 5-HT1D receptors.     

5-HT1A receptor-mediated autoinhibition does not function at physiological firing rates: evidence from in vitro electrophysiological studies in the rat dorsal raphe nucleus

5-HT(1A)-mediated autoinhibition of neurones in the dorsal raphe nucleus (DRN) is considered to be the principal inhibitory regulator of 5-HT neuronal activity. The activation of this receptor by endogenous 5-HT was investigated using electrophysiological recordings from the rat DRN in vitro. At a concentration which blocked the inhibitory effect of exogenous 5-HT, the 5-HT(1A) antagonist WAY 100635 did not alter basal firing rate or modulate the excitatory response to the alpha(1)-agonist phenylephrine. Blockade of 5-HT reuptake by a concentration of fluoxetine, which enhanced the inhibitory effect of exogenous 5-HT, lowered phenylephrine-induced basal firing presumably due to potentiation of the effect of endogenous 5-HT. However, this effect was not firing rate dependent and neither the proportional increase nor the time-course of the response to a higher concentration of phenylephrine were altered in the presence of fluoxetine. These data suggest that the inhibitory 5-HT(1A) receptor on raphe neurones is neither tonically activated nor plays any role in modulating the response to excitatory transmitters. Thus, at physiological firing rates this receptor does not appear to function as an autoreceptor of serotonergic neurones of the DRN.     

The role of serotonergic neurons in dorsal raphe, median raphe and anterior hypothalamic pressor mechanisms

The role of serotonergic neurons in the dorsal raphe and median raphe in the pressor response to electrical stimulation of these areas, and the contribution of these neurons to the pressor response to serotonin (5-HT) in the anterior hypothalamus-preoptic area (AH/PO) have been studied by the use of local injections of 5,7-dihydroxytryptamine (5,7-DHT), a neurotoxin selective for 5-hydroxytryptamine (5-HT). When blood pressure was recorded in urethane-anesthetized rats, selective lesions of 5-HT-containing neurons in the dorsal raphe nucleus reduced by 60% the pressor response to electrical stimulation (50 Hz, 100-150 microA, 0.3 msec pulse duration) of this nucleus. On the other hand, selective lesion of 5-HT-containing neurons in the median raphe nucleus had no effect on the pressor response to electrical stimulation of this area. Injection of 5,7-dihydroxytryptamine into the anterior hypothalamus/preoptic area resulted in an increased pressor response to the injection of 5-HT (5 nmol) into the lesioned area 10 days later. Furthermore, the destruction of 5-HT-containing neurons in the dorsal raphe nucleus resulted in an enhanced pressor response to the injection of 5-HT (5 nmol) into the anterior hypothalamus/preoptic area, while the destruction of 5-HT-containing neurons in the median raphe nucleus had no effect on the pressor response to the injection of 5-HT (5 nmol) into the anterior hypothalamus/preoptic area. Therefore, it appears that 5-HT neurons in the dorsal raphe nucleus are important in the pressor response to electrical stimulation and are involved in a pressor mechanism in the anterior hypothalamus/preoptic area.(ABSTRACT TRUNCATED AT 250 WORDS)     

Benzodiazepines: potentiation of a GABA inhibitory response in the dorsal raphe nucleus.

Based on evidence that the dorsal raphe nucleus (DR) has specific and independent receptors for 5HT, GABA and glycine (Gallager and Aghajanian, 1976; Wang and Aghajanian, 1977), alterations in the firing rate of DR neurons following the administration of benzodiazepines (BZ) were evaluated to determine whether they were the result of a direct interaction with 5HT receptors or due to interactions of these drugs with GABA and/or glycine. The effects of BZs after both direct and systemic application were tested in rats using microiotophoretic and single-cell recording techniques. Although the BZs did not alter the spontaneous firing rate of the DR, both the systemic and iontophoretic administration of these drugs were found to potentiate the inhibitory response produced by GABA. The data suggest that this potentiation is mediated postsynaptically. Since the effects of BZs on the spontaneous activity of the DR are only apparent following pretreatments with AOAA, it is speculated that these drugs may only have pronounced effects when GABAergic input is prominent.     

In vivo control of 5-hydroxytryptamine release by terminal autoreceptors in rat brain areas differentially innervated by the dorsal and median raphe nuclei

Selective serotonin reuptake inhibitors (SSRIs) reduce the 5-HT release in vivo. This effect is due to the activation of somatodendritic 5-HT_1A receptors and it displays a regional pattern comparable to that of selective 5-HT_1A agonists, i.e., preferentially in forebrain areas innervated by the dorsal raphe nucleus (DRN). However, despite a comparatively lower 5-HT_1A-mediated inhibition of 5-HT release and a greater density of serotonergic uptake sites in hippocampus, the net elevation produced by the systemic administration of SSRIs is similar in various forebrain areas, regardless of the origin of serotonergic fibres. As terminal autoreceptors may also limit the SSRI-induced elevations of 5-HT in the extracellular brain space, we reasoned that a differential control of 5-HT release by terminal autoreceptors in DRN- and median raphe-innervated areas might be accountable. To examine this possibility, we have conducted a regional microdialysis study in the DRN, MRN and four forebrain regions preferentially innervated either by the DRN (frontal cortex, striatum) or the median raphe nucleus (MRN; dorsal and ventral hippocampus) using freely moving rats. Dialysis probes were perfused with 1 μM of the SSRI citalopram to augment the endogenous tone on terminal 5-HT autoreceptors. The non-selective 5-HT₁ antagonist methiothepin (10 and 100 μM, dissolved in the dialysis fluid) increased extracellular 5-HT in frontal cortex and dorsal hippocampus in a concentration-dependent manner. The 5-HT_1B/1D antagonist GR 127935 was ineffective at 10 μM and tended to reduce 5-HT in dorsal hippocampus at 100 μM. The local infusion of 100 μM methiothepin significantly elevated the extracellular 5-HT concentration to 142–173% of baseline (mean values of 260 min post-administration) in the DRN, MRN, frontal cortex, striatum and hippocampus (dorsal and ventral). Comparable elevations were noted in the four forebrain regions examined. As observed in frontal cortex and dorsal hippocampus, the perfusion of 10 μM GR 127935 did not elevate 5-HT in DRN, MRN, striatum or ventral hippocampus. Because the stimulated 5-HT release in the DRN has been suggested to be under control of 5-HT_1B/1D receptors, we examined the possible contribution of these receptor subtypes to the effects of methiothepin in the DRN. The perfusion of sumatriptan (0.01–10 μM) or GR 127935 (0.01–10 μM) did not significantly modify the 5-HT concentration in dialysates from the DRN. Thus, the present data suggest that the comparable effects of SSRIs in DRN- and MRN-innervated forebrain regions are not explained by a preferential attenuation of 5-HT release by terminal 5-HT_1B autoreceptors in hippocampus, an area with a low inhibitory influence of somatodendritic 5-HT_1A receptors. Methiothepin-sensitive autoreceptors (possibly 5-HT_1B) appear to play an important role not only in the projection areas but also with respect to the control of 5-HT release in the DRN and MRN. In addition, our findings indicate that GR 127935 is not an effective antagonist of the actions of 5-HT at rat terminal autoreceptors. Received: 27 February 1998 / Accepted: 12 June 1998     

中缝背核参与大鼠吗啡依赖和戒断的形成机制研究

目的观察大鼠脑内神经核団中缝背核(DRN)在一氧化氮(NO)介导的吗啡依赖和戒断形成的作用机制。方法雄性成年SD大鼠随机分为5组:戒断组(腹腔注射吗啡+纳洛酮);依赖组(注射吗啡+生理盐水);生理盐水组(注射生理盐水);纳洛酮组(注射生理盐水+纳洛酮);抑制剂组(注射吗啡加NOS抑制剂+纳洛酮)。戒断组和依赖组,用剂量递增法经腹腔注射吗啡10～100mg.kg-1,每天3次,连续5天,建立吗啡依赖与戒断模型,并进行行为学观测与评分后,用神经元型一氧化氮合酶(nNOS)免疫组织化学标记,计数各组动物相同层面脑片上nNOS标记细胞的表达情况。结果戒断组,戒断症状及总评分较对照组和依赖组均差异显著(P<0.01);NOS抑制剂组,戒断症状评分较戒断组明显降低(P<0.05)。于中缝背核相应区域计数到部分nNOS标记神经元,生理盐水组及纳洛酮组比较无显著性差异;而依赖组与戒断组,其神经元计数明显增加(P<0.05);而NOS抑制剂组,神经元数量较戒断组明显减少(P<0.05)。结论脑内中缝背核可能通过一氧化氮信号通路参与了大鼠吗啡依赖与戒断的形成。     

Effects of ketanserin on neuronal responses to serotonin in the prefrontal cortex, lateral geniculate and dorsal raphe nucleus

The ability of the putative serotonin2 (5-HT2) antagonist ketanserin, to alter serotonin (5-HT)-induced responses in cell firing was examined in the prefrontal cortex, the lateral geniculate nucleus and the dorsal raphe nucleus of the rat by microiontophoretic extracellular single unit recording techniques. In the prefrontal cortex, ketanserin failed to antagonize the inhibitory effects of 5-HT recorded in cerveau isolé or preparations anesthetized with chloral hydrate (pure excitatory responses to 5-HT were not observed in either of these preparations). Paradoxically, the inhibitory response produced by 5-HT (but not gamma-aminobutyric acid, tryptamine or norepinephrine) was potentiated, even in cells where ketanserin alone did not alter spontaneous firing rates. The systemic administration of ketanserin (5 mg/kg, i.p.) had effects similar to those observed in the microiontophoretic experiments in the prefrontal cortex. In the dorsal raphe nucleus of animals anesthetized with chloral hydrate, ketanserin neither attenuated nor potentiated the inhibition of serotonergic neurons by 5-HT. In the lateral geniculate nucleus, as in the prefrontal cortex, ketanserin potentiated rather than attenuated, the inhibitory effect of 5-HT. Ketanserin was found to attenuate the excitatory responses produced by norepinephrine, an alpha 1-adrenoceptor-mediated response, in the lateral geniculate nucleus. The observed potentiation by ketanserin of inhibitory responses to 5-HT but not those of gamma-aminobutyric acid, tryptamine or norepinephrine, recorded in the prefrontal cortex, may be consistent with the proposed interaction between ketanserin and a specific 5-HT2 binding site.(ABSTRACT TRUNCATED AT 250 WORDS)     

Unilateral 5,7-dihydroxytryptamine lesions of the dorsal raphe nucleus (DRN) and rat rotational behaviour

A unilateral lesion in the dorsal raphe nucleus (DRN) resulted in a decreased concentration of 5-hydroxytryptamine (5-HT) in the ipsilateral striatum (CS), anterior cortex and substantia nigra (SN), a loss of [³H]5-HT uptake sites in the cortex and striatum and a selective in 5-HT turnover in the substantia nigra. The directly acting 5-HT agonist 5-methoxy-N,N-dimethyltryptamine induced contralateral turning behaviour in the lesioned animals, whilst the 5-HT releasing agent, p-chloroamphetamine, induced ipsilateral rotation. All rotational behaviour was blocked by haloperidol and the turning behaviour in response to 5-MeODMT was blocked by methysergide. The data presented suggest that the DRN innervates the SN and CS differentially and that nigral 5-HT receptors become supersensitive after denervation of the DRN-SN pathway.     

Effect of antipsychotic drugs on the firing of dorsal raphe cells. II. Reversal by picrotoxin.

As reported in the preceding study, the ability of certain antipsychotic and adrenolytic agents to inhibit the spontaneous firing of serotonergic 5HT neurons in the dorsal raphe nucleus appeared to be related to adrenergic blocking efficacy. However, the interaction between adrenergic and serotonergic systems was apparently indirect. In this phase of the study we investigated the hypothesis that another transmitter system could mediate this interaction. We examined the effects of two inhibitory amino acid transmitters (GABA and glycine) for possible effects on dorsal raphe cell firing using single cell recording and microiontophoretic techniques. In addition, the ability of the GABA antagonist, picrotoxin and the glycine antagonist, strychnine to reverse the effects of the antipsychotic and alpha-blocking drugs on dorsal raphe firing was tested. Both GABA and glycine were found to inhibit raphe cell firing selectively, allowing for a possible neurotransmitter function for these amino acids within the dorsal raphe nucleus. However, picrotoxin but not strychnine was found to reverse the effects of the antipsychotic and alpha-blocking drugs on raphe firing. Based on these results, we propose that the adrenergic input may influence 5HT neurons indirectly via a GABAergic interneuron or interposed GABA neuron.     

Effect of prolonged administration of tianeptine on 5-HT neurotransmission: an electrophysiological study in the rat hippocampus and dorsal raphe

Extracellular unitary recordings of dorsal hippocampus CA₃ pyramidal neurons and of dorsal raphe 5-hydroxytryptamine (5-HT) neurons were used to assess the effect of tianeptine, a putative antidepressant, on the efficacy of 5-HT neurotransmission. Sustained tianeptine administration (20 mg/kg/day, s.c. × 14 days) did not modify the firing activity of 5-HT neurons in the dorsal raphe. Their responsiveness to the intravenous injection of LSD, an agonist of the somatodendritic 5-HT autoreceptor, and of 8-OH-DPAT, a selective 5-HT_1A agonist, was also unaffected by this treatment. The responsiveness of CA₃ pyramidal neurons to microiontophoretic application of 5-HT remained unchanged after sustained tianeptine administration, but it was markedly enhanced in rats treated with repeated electroconvulsive shocks. Finally, the duration of suppression of firing activity of CA₃ pyramidal neurons produced by electrical stimulation of the ascending 5-HT pathway, delivered at 1 Hz and 5 Hz, was not modified in rats treated with tianeptine. Methiothepin, an antagonist of the terminal autoreceptor enhanced the effectiveness of 5-HT pathway stimulation to the same extent in control and tianeptinetreated rats. The present results indicate that, administered at a dose known to stimulate 5-HT reuptake (20 mg/kg/day, s.c.; by minipump), and for a period of time (14 days) for which other antidepressant treatments have been shown to enhance 5-HT function, tianeptine does not modify the efficacy of 5-HT synaptic transmission in the rat hippocampus.     

Acute and chronic effects of corticosterone on 5-HT1A receptor-mediated autoinhibition in the rat dorsal raphe nucleus

Corticosteroid modulation of 5-HT(1A) receptor function may contribute to the aetiology of affective disorders. To examine this modulation, the effects of acute and chronic corticosterone administration on 5-HT(1A) autoreceptor function were investigated using in vitro electrophysiology in the rat dorsal raphe nucleus (DRN). The magnitude and time course of the inhibitory response to a submaximal dose of 5-HT was not affected by acute application of either corticosterone (30-200 nM) or dexamethasone (100 nM) in vitro, when tested either in slices from control rats or rats adrenalectomised two weeks prior to recording. For chronic treatment, rats were supplied with drinking water containing corticosterone (50 microg/ml) or ethanol vehicle (0.5%) for 25-31 days. The autoinhibitory response to 5-HT was significantly attenuated in the corticosterone-treated group; vehicle EC(50)=48+/-8 microM vs. corticosterone EC(50)=121+/-20 microM. Furthermore a subpopulation of 5-HT neurones from corticosterone-treated animals exhibited marked insensitivity to 5-HT. In situ hybridisation histochemistry showed that corticosterone did not affect the expression of mRNA encoding the 5-HT(1A) receptor or either the type 1 and type 3 subunits of the G-protein linked inwardly rectifying K+ (GIRK) channel. However, GIRK2 subunit mRNA expression was significantly reduced. Thus, 5-HT(1A) autoreceptor function in the DRN is attenuated following chronic, but not acute, exposure to elevated corticosterone levels, and this effect may involve changes to the receptor-effector coupling mechanism.     

Influence of AMPA/kainate receptors on extracellular 5-hydroxytryptamine in rat midbrain raphe and forebrain

1. The regulation of 5-hydroxytryptamine (5-HT) release by excitatory amino acid (EAA) receptors was examined by use of microdialysis in the CNS of freely behaving rats. Extracellular 5-HT was measured in the dorsal raphe nucleus (DRN), median raphe nucleus (MRN), nucleus accumbens, hypothalamus, frontal cortex, dorsal and ventral hippocampus.
2. Local infusion of kainate produced increases in extracellular 5-HT in the DRN and MRN. Kainate infusion into forebrain sites had a less potent effect.
3. In further studies of the DRN and nucleus accumbens, kainate-induced increases in extracellular 5-HT were blocked by the EAA receptor antagonists, kynurenate and 6,7-dinitroquinoxaline-2,3-dione (DNQX).
4. The effect of infusing kainate into the DRN or nucleus accumbens was attenuated or abolished by tetrodotoxin (TTX), suggesting that the increase in extracellular 5-HT is dependent on 5-HT neuronal activity. In contrast, ibotenate-induced lesion of intrinsic neurones did not attenuate the effect of infusing kainate into the nucleus accumbens. Thus, the effect of kainate in the nucleus accumbens does not depend on intrinsic neurones.
5. Infusion of α-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate (AMPA) into the DRN and nucleus accumbens induced nonsignificant changes in extracellular 5-HT. Cyclothiazide and diazoxide, which attenuate receptor desensitization, greatly enhanced the effect of AMPA on 5-HT in the DRN, but not in the nucleus accumbens.
6. In conclusion, AMPA/kainate receptors regulate 5-HT in the raphe and in forebrain sites.

     

Risperidone inhibits 5-hydroxytryptaminergic neuronal activity in the dorsal raphe nucleus by local release of 5-hydroxytryptamine

1. The effects of risperidone on brain 5-hydroxytryptamine (5-HT) neuronal functions were investigated and compared with other antipsychotic drugs and selective receptor antagonists by use of single cell recording and microdialysis in the dorsal raphe nucleus (DRN).
2. Administration of risperidone (25–400 μg kg⁻¹, i.v.) dose-dependently decreased 5-HT cell firing in the DRN, similar to the antipsychotic drug clozapine (0.25–4.0 mg kg⁻¹, i.v.), the putative antipsychotic drug amperozide (0.5–8.0 mg kg⁻¹, i.v.) and the selective α₁-adrenoceptor antagonist prazosin (50–400 μg kg⁻¹, i.v.).
3. The selective α₂-adrenoceptor antagonist idazoxan (10–80 μg kg⁻¹, i.v.), in contrast, increased the firing rate of 5-HT neurones in the DRN, whereas the D₂ and 5-HT_2A receptor antagonists raclopride (25–200 μg kg⁻¹, i.v.) and MDL 100,907 (50–400 μg kg⁻¹, i.v.), respectively, were without effect. Thus, the α₁-adrenoceptor antagonistic action of the antipsychotic drugs might, at least partly, cause the decrease in DRN 5-HT cell firing.
4. Pretreatment with the selective 5-HT_1A receptor antagonist WAY 100,635 (5.0 μg kg⁻¹, i.v.), a drug previously shown to antagonize effectively the inhibition of 5-HT cells induced by risperidone, failed to prevent the prazosin-induced decrease in 5-HT cell firing. This finding argues against the notion that α₁-adrenoceptor antagonism is the sole mechanism underlying the inhibitory effect of risperidone on the DRN cells.
5. The inhibitory effect of risperidone on 5-HT cell firing in the DRN was significantly attenuated in rats pretreated with the 5-HT depletor PCPA (p-chlorophenylalanine; 300 mg kg⁻¹, i.p., day⁻¹ for 3 consecutive days) in comparison with drug naive animals.
6. Administration of risperidone (2.0 mg kg⁻¹, s.c.) significantly enhanced 5-HT output in the DRN.
7. Consequently, the reduction in 5-HT cell firing by risperidone appears to be related to increased availability of 5-HT in the somatodendritic region of the neurones leading to an enhanced 5-HT_1A autoreceptor activation and, in turn, to inhibition of firing, and is probably only to a minor extent caused by its α₁-adrenoceptor antagonistic action.

     

Effect of antipsychotic drugs on the firing of dorsal raphe cells. I. Role of adrenergic system.

The activity of serotonergic (5HT) neurons in the dorsal raphe nucleus was inhibited by the i.v. administration of certain antipsychotic drugs (methiothepin, clozapine and thioridazine). However, other antipsychotic agents (chlorpromazine, haloperidol and pimozide) did not inhibit raphe cell firing. The inhibitory potency of these drugs on raphe activity correlates with reported central noradrenergic blocking efficacy. An alpha-adrenergic blocking agent, piperoxane, but not the beta-blocking agents, propranolol and MJ 1999, inhibited raphe activity when administered systemically. All of these drugs appear to act indirectly since they (and NE) have relatively weak or variable effects when applied microiontophoretically to raphe neurons. The depressant effects of certain antipsychotic drugs and piperoxane on 5HT neurons appears to be mediated by a cnetral adrenergic system since (1) the depression could be reversed by the catecholamine releasing agents 1- and d-amphetamine; (2) the depression could be abolished by destruction of adrenergic pathways in the CNS by chemical, mechanical, or electrothermic lesions. While a precise localization has not yet been obtained, the data suggest that these drug effects may be mediated by an adrenergic pathway ascending from the lower brainstem.     

Pharmacological characterisation of the orexin receptor subtype mediating postsynaptic excitation in the rat dorsal raphe nucleus

Electrophysiological recordings from dorsal raphe nucleus (DRN) neurones in rat brain slices have revealed that the orexins can cause direct and reversible depolarisation of the postsynaptic membrane. Whilst it is known that the membrane depolarisation produced by orexin-A can dramatically increase the firing rate of DRN neurones, quantitative pharmacological analysis that determines the receptor subtype mediating the orexinergic response has not yet been performed. Here, we demonstrate that the rank order of potencies of orexin receptor agonists to excite serotonergic DRN neurones is orexin-A=orexin-B>SB-668875-DM. In contrast, the rank order of potency of these agonists to excite noradrenergic locus coreleus (LC) neurones is orexin-A>orexin-B>SB-668875-DM. We show further that the orexin receptor antagonist, SB-334867-A, inhibits the effects of orexin-A in the LC and DRN with pK_B values of 6.93 and 5.84, respectively, values similar to those calculated for human OX₁ (7.27) and OX₂ (5.60) receptors expressed in CHO cells. These data suggest a differential role for OX₁ and OX₂ receptors in stimulating distinct populations of monoaminergic neurones in the rat CNS with OX₂ receptors exhibiting a more pronounced functional significance in serotonergic neurones and OX₁ in noradrenergic neurones.     

CB(1) cannabinoid receptors inhibit the glutamatergic component of KCl-evoked excitation of locus coeruleus neurons in rat brain slices

CB(1) cannabinoid receptors located at presynaptic sites suppress synaptic transmission in the rat brain. The aim of this work was to examine by single-unit extracellular techniques the effect of the synthetic cannabinoid receptor agonist WIN 55212-2 on KCl-evoked excitation of locus coeruleus neurons in rat brain slices. Short applications of KCl (30 mM) increased by 9-fold the firing rate of locus coeruleus cells. Perfusion with the GABA(A) receptor antagonist picrotoxin (100 microM) increased KCl-evoked effect, whereas NMDA and non-NMDA glutamate receptor antagonists (D-AP5 100 microM and CNQX 30 microM, respectively) were able to decrease KCl-evoked effect only in the presence of picrotoxin (100 microM). Bath application of WIN 55212-2 (10 microM) inhibited KCl-evoked effect; this inhibition was blocked by the CB(1) receptor antagonist AM 251 (1 microM). However, a lower concentration of WIN 55212-2 (1 microM) did not significantly change KCl effect. In the presence of picrotoxin (100 microM), perfusion with D-AP5 (100 microM) or CNQX (30 microM) blocked WIN 55212-2-induced inhibition, although picrotoxin (100 microM) itself failed to affect cannabinoid effect. In conclusion, GABAergic and glutamatergic components are both involved in KCl-evoked excitation of LC neurons, although CB(1) receptors only seem to inhibit the glutamatergic component of KCl effect in the locus coeruleus.