The effect of the selective 5-HT1A agonists alnespirone (S-20499) and 8-OH-DPAT on extracellular 5-hydroxytryptamine in different regions of rat brain

1. We have examined the effects of the systemic administration of the selective 5-HT_1A agonist alnespirone (S-20499) on in vivo 5-hydroxytryptamine (5-HT) release in the dorsal raphe nucleus, the median raphe nucleus and four forebrain areas innervated differentially by both (dorsal striatum, frontal cortex, ventral hippocampus and dorsal hippocampus).
2. Alnespirone (0.1–3 mg kg⁻¹, s.c.) dose-dependently reduced extracellular 5-HT in the six areas examined. In forebrain, the maximal reductions occurred in striatum and frontal cortex (maximal reduction to 23 and 29% of baseline, respectively). Those in dorsal and ventral hippocampus were more moderate (to ca 65% of baseline). In contrast, the decrease in 5-HT elicited in the median raphe nucleus was more marked than that in the dorsal raphe nucleus (to ca 30 and 60% of baseline, respectively). The selective 5-HT_1A antagonist WAY-100635 (0.5 mg kg⁻¹, s.c.) prevented the decrease in 5-HT induced by alnespirone (0.3 mg kg⁻¹, s.c.) in frontal cortex.
3. 8-OH-DPAT (0.025, 0.1 and 0.3 mg kg⁻¹, s.c.) also reduced extracellular 5-HT in a regionally-selective manner (e.g., to 32% of baseline in striatum and to 69% in dorsal hippocampus at 0.1 mg kg⁻¹, s.c.). In midbrain, 8-OH-DPAT reduced the dialysate 5-HT slightly more in the median than in the dorsal raphe nucleus at all doses examined.
4. Doses of both compounds close to their respective ED₅₀ values (0.3 mg kg⁻¹ alnespirone, 0.025 mg kg⁻¹ 8-OH-DPAT) reduced 5-HT to a comparable extent in all regions examined. However, the reductions attained at higher doses were more pronounced for 8-OH-DPAT.
5. These data show that the reduction of 5-HT release elicited by alnespirone and 8-OH-DPAT is more important in forebrain areas innervated by 5-hydroxytryptaminergic neurones of the dorsal raphe nucleus. This regional selectivity seems unlikely to be accounted for by differences in the sensitivity of 5-HT_1A autoreceptors controlling 5-HT release, given the dissimilar effects of these two 5-HT_1A agonists in regions rich in cell bodies and nerve terminals. This suggests the presence of complex mechanisms of control of 5-HT release by 5-HT_1A receptors.

     

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     

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.

     

Reduced effect of morphine in midbrain raphe lesioned rats

The reaction of painful stimulation was studied in rats by means of three different techniques.

Lesions of the midbrain raphe in rats, which reduced the levels of brain 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) decreased the analgesic activity of morphine. In some tests, an increase in sensitivity to painful stimulation was noted in lesioned animals.

The results obtained support the view that a decrease of brain 5-HT concentration antagonizes morphine analgesia. The possible role of the other brain monoamines in the analgesic activity of morphine is also discussed.     

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.     

5-Hydroxytryptamine 5-HT1D receptors mediating inhibition of cyclic AMP accumulation in Madin-Darby canine kidney (MDCK) cells

5-Hydroxytryptamine 5-HT_1B/5-HT_1D receptors are members of the same receptor subfamily, but display a different pharmacology (Hartig et al. (1992) Trends Pharmacol Set 13:152–159). Whereas several cell lines have been reported to contain 5-HT_1B receptors, none has been described, however, that endogenously expresses well-characterized 5-HT_1D receptors. The present study deals with the identification of 5-HT_1D receptors inhibiting cyclic AMP accumulation in Madin-Darby canine kidney (MDCK) cells. 5-HT (1 nM– 10 M) induced a concentration-dependent inhibition of the cyclic AMP accumulation stimulated by prostaglandin E₁ (1 M) in MDCK cells. The maximal effect of 5-HT averaged 50% inhibition and was abolished after a pre-treatment of the cells with pertussis toxin. Other agonists mimicked the effects of 5-HT, with the following rank order of potency (pEC₅₀ ± SEM, n 3): 5-carboxamidotryptamine (8.36 ± 0.48) > PAPP (p-aminophenylethyl-m-trifluoromethylphenyl piperazine, 7.89 ± 0.23) > 5-HT (7.35 ± 0.05) > sumatriptan (6.65 ± 0.27). PAPP behaved as a partial agonist. 8-OH-DPAT (8-hydroxy-2(di-n-propylamino)tetralin) was less potent, its maximal effect being not reached at 0.1 mM. Methiothepin, GR127935, (–)propranolol, rauwolscine and ketanserin were all devoid of intrinsic activity (up to 10 M or 0.1 mM). Methiothepin (10 nM, 0.1 M and 1 M) antagonized 5-HT effect (pA₂ 8.57 ± 0.44, Schild slope 1.17 ± 0.21, n = 3). GR127935 (1 nM, 10 nM and 0.1 M) shifted the curve of 5-HT to the right, but the antagonism was not fully surmountable (apparent pK_B value, 9.80 ± 0.16, n = 9). From the shifts obtained with rauwolscine (1 M) and (–)propranolol (10 M), respective pK_B values were estimated 6.68 ± 0.30 and 5.4 (n = 3 each). PAPP, when tested as an antagonist at 1 M, also shifted the curve of 5-HT to the right, with a pK_B of 8.27 ± 0.16 (n = 3). Finally, ketanserin (10 M) also antagonized the effects of 5-HT, the pK_B being 6.54 ± 0.16 (n = 9). The rank orders of agonist and antagonist potencies strongly suggest 5-HT receptors mediating inhibition of cyclic AMP accumulation in MDCK cells to be 5-HT_1D receptors. This is the first report of a cell line expressing endogenous, well-characterized, 5-HT_1D receptors. With regard to the 5-HT_1D receptor subtype involved, the relatively high potency of ketanserin would suggest it to be a 5-HT_1D subtype or a mixture of 5-HT_1D/5-HT_1D\ subtypes. However, caution must be exercised here, owing to the poor knowledge of canine 5-HT_1D receptor subtypes.     

Differential effects of clomipramine given locally or systemically on extracellular 5-hydroxytryptamine in raphe nuclei and frontal cortex

Summary The antidepressant drug clomipramine (CIM) blocks 5-hydroxytryptamine (5-HT) uptake in vitro. Electrophysiological studies have shown that CIM also reduces the firing of serotonergic neurons in the dorsal raphe nucleus. In order to assess the effects of CIM on serotonergic transmission in vivo, the technique of intracerebral microdialysis was used. CIM was administered either through the dialysis probe or i. p., and dialysate 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) were determined in frontal cortex and/or raphe nuclei. In addition, the action of extracellular 5-HT in raphe nuclei on the release of 5-HT in frontal cortex was studied.The administration of CIM through the dialysis probe increased dialysate 5-HT in frontal cortex in a dose-dependent fashion. An actual ED₅₀ of 3.15 M CIM for the in vivo inhibition of 5-HT uptake can be calculated in this brain area. When given systemically (10 or 20 mg/kg i. p.), CIM did not increase dialysate 5-HT in the frontal cortex. The occurrence of extracellular 5-HT in the raphe area was demonstrated. This pool of 5-HT increased markedly after local (10 or 40 M) or systemic (20 mg/kg i. p.) administration of CIM. We also examined the effect of CIM applied locally in the raphe nuclei on extracellular 5-HT in the frontal cortex. The increased dialysate 5-HT in raphe after 10 or 40 M CIM paralleled a decrease of dialysate 5-HT in the frontal cortex. Values of dialysate 5-HT in the two areas correlated negatively. The administration of CIM through the dialysis probe slightly decreased dialysate 5-HIAA in the frontal cortex. When given systemically, CIM also decreased dialysate 5-HIAA in the frontal cortex, but significantly only after the highest dose tested (20 mg/kg i.p.). Furthermore, the local application of CIM into the raphe nuclei produced a decrease of dialysate 5-HIAA in the frontal cortex.These results prove the in vivo inhibitory activity of CIM on 5-HT uptake. This effect is much more pronounced in the raphe nuclei than in the frontal cortex when CIM is given systemically. However, both areas are equally sensitive to the local application of the drug. The effect of CIM on raphe nuclei partly antagonizes that on the frontal cortex, providing evidence for a functional link between these two brain regions.This work was presented in part at the 6th Meeting of the International Study Group For Tryptophan Research, Baltimore (USA), 1989     

Trimetazidine modulates AMPA/kainate receptors in rat vestibular ganglion neurons

Trimetazidine (1[2,3,4-trimethoxy-benzyl] piperazine, 2 HCl) is an anti-ischemic agent frequently administered as a prophylactic treatment for episodes of angina pectoris and chorioretinal disturbances. It is also employed as a symptomatic treatment of vertigo but its mechanism of action is yet to be defined. Using Fura-2 fluorescence photometry and whole-cell patch-clamp recordings we investigated the effect of trimetazidine on the [Ca(2+)](i) and current responses induced by the application of non-N-methyl-D-aspartate (NMDA) receptor agonists on low density vestibular ganglion neuronal cultures explanted from 3 day s postnatal rats. Trimetazidine blocked the [Ca(2+)](i) and current responses induced by 100 microM applications of both kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). These responses were dependent on external Ca(2+) and were blocked by the voltage-dependent Ca(2+) channel blockers Ni(2+) and Cd(2+) . Trimetazidine only acts on the AMPA/kainate receptors and had no effect on K(+)-induced depolarizations. Dose-dependent curves were obtained for the inhibition by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and trimetazidine (IC(50) 7 microM and 0.7 microM) of kainate stimulations. After AMPA stimulation, dose-response inhibition curves showed an IC(50) of 3 microM for CNQX and 25 microM for trimetazidine. These results indicate that trimetazidine could be a potent antagonist of AMPA/kainate receptors in vestibular ganglion neurons. This may explain the protective role of trimetazidine in the inner ear suggesting an anti-excitotoxic activity.     

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)     

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.     

Investigations of cardiovascular 5-hydroxytryptamine receptor subtypes in the rat

Summary Peripheral 5-HT receptor-mediated responses were examined in pithed spontaneously hypertensive rats and normotensive wistar rats. Responses examined were: Pressor and depressor responses, tachycardia and inhibition of stimulation-evoked tachycardia. In pithed spontaneously hypertensive rats, 5-HT, but not the 5-HT₁-selective agonist 5-carboxamidotryptamine, produced pressor responses, and these were potently antagonised by the 5-HT₂-selective antagonists ketanserin and LY 53857. In pithed spontaneously hypertensive rats, the tachycardia to 5-HT was abolished by a combination of the 5-HT₂ receptor antagonist LY 53857 and propranolol, suggesting that the tachycardia is mediated by 5-HT₂ receptors and by release of noradrenaline. In pithed spontaneously hypertensive rats, 5-carboxamidotryptamine, 5-HT, and to a lesser extent the 5-HT₁ receptor agonist RU 24969, but not the 5-HT_1A receptor agonist 8-OH-DPAT, produced depressor responses which were antagonised by methysergide and metitepin, but which do not clearly fit with any of the 5-HT, ligand binding sites. In pithed normotensive wistar rat, 5-carboxamidotryptamine was approximately 100 times more potent than 5-HT and 8-OH-DPAT at inhibiting the cardio-acceleration produced by single pulse electrical stimulation and this inhibition was antagonised by metitepin, so that the response is mediated by 5-HT₁ receptors.     

中缝背核投射到基底外侧杏仁核的5-羟色胺能纤维对睡眠的调节作用

目的　观察大鼠中缝背核 (DRN)到基底外侧杏仁核(BLA)的 5 HT能纤维投射在睡眠 -觉醒调节中的作用。方法　采用脑立体定位 ,核团微量注射和多导睡眠描记 (PSG)方法。结果　DRN内微量注射L Glu ,可使觉醒 (W )增加 ,慢波睡眠 (SWS)和异相睡眠 (PS)明显减少。在双侧BLA微量注射非选择性 5 HT受体阻断剂麦角新碱 (MS)可以逆转DRN内微量注射L Glu的效应 ,SWS增加 ,W减少 ,但PS没有变化 ;DRN内微量注射PCPA ,导致SWS增加 ,W减少 ,但当在DRN内微量注射PCPA后 ,双侧BLA内微量注射 5 HTP可以逆转PCPA所引起的睡眠增加效应 ,使SWS减少 ,W增加 ,但PS没有变化。结论　DRN对睡眠 -觉醒的调节作用部分通过DRN到BLA的 5 HT能纤维投射介导的     

Utility of organotypic raphe slice cultures to investigate the effects of sustained exposure to selective 5-HT reuptake inhibitors on 5-HT release

BACKGROUND AND PURPOSE

Selective 5-hydroxytryptamine (5-HT, serotonin) reuptake inhibitors (SSRIs) are widely used antidepressants and their therapeutic effect requires several weeks of drug administration. The delayed onset of SSRI efficacy is due to the slow neuroadaptive changes of the 5-hydroxytryptaminergic (5-HTergic) system. In this study, we examined the acute and chronic effects of SSRIs on the 5-HTergic system using rat raphe slice cultures.

EXPERIMENTAL APPROACH

For organotypic raphe slice cultures, mesencephalic coronal sections containing dorsal and median raphe nuclei were prepared from neonatal Wistar rats and cultured for 14–16 days.

KEY RESULTS

Acute treatment with citalopram, paroxetine or fluoxetine (0.1–10 µM) in the slice cultures slightly increased extracellular 5-HT levels, while sustained exposure for 4 days augmented the elevation of 5-HT level in a time-dependent manner. Sustained exposure to citalopram had no effect on tissue contents of 5-HT and its metabolite, expression of tryptophan hydroxylase or the membrane expression of 5-HT transporters. The augmented 5-HT release was attenuated by Ca²⁺-free incubation medium or treatment with tetrodotoxin. Experiments with 5-HT_1A/B receptor agonists and antagonists revealed that desensitization of 5-HT₁ autoreceptors was not involved in the augmentation of 5-HT release. Finally, co-treatment with an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, but not an N-methyl-d-aspartate, receptor antagonist, suppressed this augmentation.

CONCLUSION AND IMPLICATIONS

These results suggest that sustained exposure to SSRIs induces the augmentation of exocytotic 5-HT release, which is caused, at least in part, by the activation of AMPA/kainate receptors in the raphe slice cultures.     

5-Hydroxytryptamine 5-HT1B receptors inhibiting cyclic AMP accumulation in rat renal mesangial cells

A clonal cell line derived from rat renal mesangial cells was shown to express endogenous 5-hydroxytryptamine (serotonin, 5-HT) receptors that mediate inhibition of cyclic AMP accumulation. These receptors were characterized as being of the 5-HT_1B receptor subtype. 5-HT₁ receptor agonists inhibited forskolin-stimulated cyclic AMP accumulation in rat renal mesangial cells (60–70% maximal inhibition) with the following rank order of potency (mean pEC₅₀ values±SEM, n 3): ergotamine (9.58±0.51)>RU 24969 (8.67±0.23)5-CT (8.42±0.06)CP 93129 (8.15±0.27)>5-HT (7.75±0.11) > sumatriptan (6.29±0.30) > 8-OH-DPAT (4.32±0.15). 5-HT₂ and 5-HT₄ receptor agonists were without effect. 5-HT-induced inhibition of cyclic AMP accumulation was abolished by a pre-treatment of the cells with pertussis toxin. (-)Propranolol was a partial agonist (27% maximal inhibition, pEC₅₀ 7.19±0.24, n = 3); when used as an antagonist at 1 M, it shifted the concentration-response curve of 5-HT to the right (pKB 7.22±0.35, n = 3). Methiothepin was a competitive antagonist of 5-HT (pA₂ 8.04±0.10, Schild slope 0.87±0.21, n = 3). Rauwolscine (10 M) had no antagonist activity. There was a significant correlation (r = 0.98, P = 0.0001) between the cyclic AMP data obtained in rat mesangial cells and 5-HT_1B binding data reported in rat brain cortex. The same pattern of responses was observed in early passages of primary cultures of rat mesangial cells. This study shows that rat mesangial cells can be used as a convenient source of functional 5-HT_1B receptors. It also constitutes further evidence for the widespread distribution of 5-HT_1B receptors outside the brain.     

5-HT3 receptors augment neuronal, cholinergic contractions in guinea pig trachea European Journal of Pharmacology, 234 (1993) 109–112

Serotonin (5-HT) and the selective 5-HT₃ receptor agonist, 2-methyl-5-hydroxytryptamine enhanced electrical field stimulated contractions of the isolated guinea pig trachea. 5-HT (EC₅₀ = 3.5 μM) was twice as potent as 2-methyl-5-hydroxytryptamine (EC₅₀ = 7.4 μM). The effects of 5-HT and 2-methyl-5-hydroxytryptamine were antagonized by the selective 5-HT₃ receptor antagonist, zacopride (apparent pA₂ = 7.60 against 2-methyl-5-hydroxytryptamine). 2-Methyl-5-hydroxytryptamine (10 μM) had no effect on contractile responses to exogenous acetylcholine. Furthermore, the increase in electrical field stimulated contraction by 2-methyl-5-hydroxytryptamine was unchanged by hexamethonium (100 μM) but contractions were blocked by atropine (1 μM). These results suggest that excitatory 5-HT₃ receptors exist on postganglionic cholinergic nerves in the isolated guinea pig trachea.     

5-HT2/5-HT1C receptor-mediated facilitatory action on unit activity of ventral horn cells in rat spinal cord slices.

5-Methoxy-N,N-dimethyltryptamine (5-McODMT) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) facilitate motoneuron excitability through 5-HT_1C/5-HT₂ receptors in rats. Using spinal cord slices prepared from adult rals, we recorded unitary cell discharges, evoked by local stimulation of the adjacent site, extracellularly in the motor nuclei of the ventral horn. 5-MeODMT, DOI, 5-hydroxytryptamme (5-HT), 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) and tandospirone facilitated the probability of firing in the motor nuclei, with 5-MeODMT and DOI being the most potent. The effect of 5-MeODMT was significantly suppressed by ketanserin (a 5-HT₂ receptor-selective antagonist), spinerone (a 5-HT_1A/5-HT₂ receptor antagonist) and cyproheptadine (a 5-HT_1A/5-HT₂ receptor antagonist), but not by 3-tropanyl-3,5-dichlorobenzoate (MDL 72222, a 5-HT₃ receptor-selective antagonist) or pindolol (a 5-HT_1A/5-HT_1B receptor antagonist). This suggests that 5-HT₂ and/or 5-HT_1C receptors are involved in the facilitatory effects of 5-HT receptor agonists on the synaptic activity of ventral horn cells.     

Overview on 5-HT receptors and their role in physiology and pathology of the central nervous system

The present review gives an overview on the serotonin (5-hydroxytryptamine; 5-HT) system, its receptors and their relationship to central nervous system physiology and disorders. Additionally, we also introduce the recent knowledge about the 5-HT receptor ligands in preclinical research, clinical trials and as approved drugs.     

Peripheral 5-HT1B and 5-HT2A receptors mediate the nociceptive response induced by 5-hydroxytryptamine in mice

While the role of 5-hydroxytryptamine (5-HT, serotonin) in the nociceptive processing has been widely investigated in the central nervous system, information regarding its role in peripheral tissues is still lacking. Noteworthy, 5-HT induces phenotypic changes of nociceptors and peripheral injection induces pain in humans and nociceptive response in rodents. However, local receptors involved in 5-HT effects are not well characterized. Thus, we aimed to investigate the role of 5-HT and some of its receptors in the peripheral nociceptive processing in mice. Intraplantar injection of 5-HT (10, 20 or 40 μg) into the hind-paw of mice induced paw licking behavior, which was inhibited by previous intraplantar treatment with cyproheptadine (5-HT₁ and 5-HT₂ antagonist; 0.5 or 5 μg), mianserin (5-HT₂ and 5-HT₆ antagonist; 0.1 μg), isamoltane (5-HT_1B antagonist; 0.5 or 5 μg) and ketanserin (5-HT_2A antagonist; 0.1 or 1 μg), but not by BRL 15572 (5-HT_1D antagonist; 1 or 10 μg), ondansetron (5-HT₃ antagonist; 1, 5, 10 or 20 μg) and SB 269970 (5-HT₇ antagonist; 2.5 and 25 μg). Altogether, these results indicate the local involvement of 5-HT₁, 5-HT₂ and 5-HT₆, especially 5-HT_1B and 5-HT_2A, in the nociceptive response induced by 5-HT in mice, thus contributing to a better understanding of 5-HT role in the peripheral nociceptive processing. In addition, they also point to important species differences and the need of a wide evaluation of the peripheral nociceptive processing in mice as these animals have been increasingly used in studies investigating the cellular and molecular mechanisms mediating the nociceptive response.     

Involvement of 5-hydroxytryptamine1A receptors in Delta9-tetrahydrocannabinol-induced catalepsy-like immobilization in mice

The present study investigated the involvement of 5-hydroxytryptamine_1A (5-HT_1A) receptors in Δ⁹-tetrahydrocannabinol (THC)-induced catalepsy-like immobilization in mice. THC (10 mg/kg, i.p.) induced catalepsy-like immobilization but had no effect on motor coordination in the rota-rod test. The selective cannabinoid CB₁ receptor antagonist rimonabant (3 mg/kg, i.p.) completely antagonized THC-induced catalepsy-like immobilization. The 5-HT_1A/5-HT₇ receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; 0.3 and 1 mg/kg, i.p.) and 5-HT_1A receptor partial agonist buspirone (0.06 and 0.1 mg/kg, i.p.) inhibited this THC-induced catalepsy-like immobilization. Moreover, the selective 5-HT_1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohezane carboxamide dihydrochloride (WAY100635; 0.3 or 1 mg/kg, i.p.) reversed the inhibition of THC-induced catalepsy-like immobilization by 8-OH-DPAT (1 mg/kg) or buspirone (0.06 mg/kg). In contrast, the selective 5-HT₇ receptor antagonist (R)-3-[2-[2-(4-methylpiperidin-1-yl)ethyl]pyrrolidine-1-sulfonyl]phenol hydrochloride (SB269970) had no effect on this inhibitory effect of 8-OH-DPAT. On the other hand, WAY100635 (0.3 and 1 mg/kg, i.p.) enhanced the catalepsy-like immobilization induced by THC (6 mg/kg, i.p.). These findings suggest that the 5-HT_1A receptors are involved in THC-induced catalepsy-like immobilization.     

Influence of LY 300164, an antagonist of AMPA/kainate receptors, on the anticonvulsant activity of clonazepam

LY 300164 [7-acetyl-5-(4-aminophenyl)-8,9-dihydro-8-methyl-7H-1,3-dioxolo(4,5H)-2,3-benzodiazepine], an antagonist of AMPA/kainate receptors, at 5 mg/kg exerted a significant anticonvulsant effect, as regards seizure and afterdischarge durations in amygdala-kindled seizures in rats. At lower doses, LY 300164 did not exert anticonvulsant activity. Clonazepam alone (0.003–0.1 mg/kg) significantly diminished seizure severity, seizure and afterdischarge durations. Coadministration of LY 300164 (2 mg/kg) with clonazepam (0.001 mg/kg) resulted in the significant anticonvulsant activity. Seizure severity score, seizure and afterdischarge durations were reduced from 5 to 4, from 32.6 s to 12.3 s, and 42.7 s to 23.2 s. LY 300164 (2 mg/kg), clonazepam (0.001–0.1 mg/kg) and the combination of clonazepam (0.001 mg/kg) with LY 300164 (2 mg/kg) did not affect long-term memory evaluated in the passive avoidance task in rats. LY 300164 (at the subprotective dose of 2 mg/kg) significantly potentiated the anticonvulsant action of clonazepam against maximal electroshock but not against pentylenetetrazol-induced convulsions in mice. The results indicate that blockade of glutamate-mediated events at AMPA/kainate receptors may differently affect the protection offered by clonazepam, which seems dependent upon the model of experimental seizures.