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.

     

Differential involvement of GABAA and GABAB receptors in propofol self-administration in rats

Aim:

Propofol has shown abuse potential. The aim of the present study is to investigate the effects of GABA_A antagonist and GABA_B agonist on propofol reinforcement.

Methods:

Sprague-Dawley rats were trained to self-administer propofol at a dose of 1.7 mg/kg per infusion under a fixed ratio (FR1) schedule of reinforcement for 14 d. In a separate set of experiments, food-maintained self-administration under a fixed ratio (FR5) schedule and locomotor activities of Sprague-Dawley rats were examined.

Results:

GABA_A receptor antagonist bicuculline (0.25 mg/kg, ip) significantly increased the number of injections and active responses. Pretreatment with GABA_B receptor agonist baclofen (3 mg/kg, ip) significantly decreased the number of active responses and total infusions of propofol during the training session. Moreover, microinjection of baclofen (50 and 100 ng/side) into the ventral tegmental area (VTA) significantly decreased the number of active responses and total infusions of propofol. Neither baclofen (1-3 mg/kg, ip) nor bicuculline (0.25-1 mg/kg, ip) affected food-maintained responses or motor activities.

Conclusion:

Propofol maintains its reward properties partially through GABA_A receptor activation. Stimulation of GABA_B receptors in VTA may counteract the reinforcing properties of propofol.     

Intraspecific aggressiveness after lesions of midbrain raphe nuclei in rats.

Serotonin-depleting lesions of midbrain raphe (extensive lesions located within dorsal and partly median raphe muclei) induced intraspecific agressiveness in grouped Wistar male rats but failed to increase mouse-killing behaviour. Rats which had lesions in the lateral midbrain did not display intraspecific aggression nor mouse-killing activity. These animals showed unchanged levels of both serotonin and noradrenaline in the forebrain.     

Monoamine oxidase inhibitors increase preferentially extracellular 5-hydroxytryptamine in the midbrain raphe nuclei. A brain microdialysis study in the awake rat

Summary We have examined the local and systemic effects of clorgyline, tranylcypromine and deprenyl on extracellular serotonin (5-HT) and 5-hydroxyindoleacetic acid in the raphe nuclei and in frontal cortex of awake, freely-moving rats using microdialysis. When administered through the dialysis probe, monoamine oxidase (monoamine: oxygen oxidoreductase (deaminating), E.C. 1.4.3.4., MAO) inhibitors increased 5-HT output in a dose-dependent manner in both brain areas. The effects were more pronounced in the raphe nuclei for the three MAO inhibitors at all doses assayed.When the monoamine oxidase inhibitors were given i.p., dialysate 5-HT increased dramatically, after tranylcypromine (15 mg/kg), in raphe nuclei and frontal cortex (area under the curve (AUC) to 4 h post-treatment: 63-fold and 11-fold, respectively) whereas the effects of clorgyline (10 mg/kg) were much less pronounced (+ 47% increase in the AUC for raphe nuclei, P < 0.09; + 18% increase in the AUC for frontal cortex, n.s.). Deprenyl (2.5 mg/kg, i.p.) induced a moderate (+ 22%) increase of dialysate 5-HT from the raphe nuclei but did not cause a change in dialysate 5-HT from the frontal cortex (+ 4%). However, clorgyline, or deprenyl, dramatically increased dialysate 5-HT in animals which had been pre-treated with the above dose of deprenyl, or clorgyline, respectively, showing that the blockade of both forms of MAO results in much larger increases of extracellular 5-HT than does the blockade of either form alone.These results indicate that: (a) deamination by MAO participates actively in the control of the extracellular concentration of 5-HT in those areas of the brain that are rich in serotoninergic nerve terminals as well as in cell bodies, (b) in vivo, brain 5-HT is deaminated preferentially by MAO-A but its full inhibition does not result in an increased release of 5-HT, in spite of a large accumulation of 5-HT in the brain tissue, (c) MAO-B deaminates 5-HT when the A-form is inhibited (in this situation, MAO-B participates actively in the control of a releasable pool of 5-HT), (d) the raphe nuclei appears to be a preferential site of action of MAO inhibitors, administered either locally or systemically. These results may help to understand the model of action of MAO inhibitors as antidepressant drugs.Correspondence to F. Artigas at the above address     

The importance of 5-hydroxytryptamine for the induction of harmine tremor and its antagonism by dopaminergic agonists assessed by lesions of the midbrain raphe nuclei.

The brain lesion technique was used to destroy the ascending 5-hydroxytryptamine (5-HT) system at its cell bodies in the dorsal and medial raphe nuclei in order to assess the importance of 5-HT for the induction of harmine tremor and its antagonism by the dopaminergic agonists, L-DOPA, apomorphine and d-amphetamine. Lesions of the medial or dorsal raphe nucleus reduced the intensity of harmine tremor. The remaining tremor was generally resistant to further reduction by the dopaminergic agonists. 5-hydroxytryptophan was shown to enhance tremor: this effect was reduced both by the raphe lesions and by treatment with L-DOPA. The data are discussed in terms of the possible relationship between 5-HT and dopamine.     

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     

Presynaptic regulation of the release of acetylcholine by 5-hydroxytryptamine.

1 The effect of 5-hydroxytryptamine (5-HT) on the release of acetylcholine (ACh) from bullfrog sympathetic preganglionic nerve terminals and frog sciatic nerve terminals was studied with intra-cellular microelectrodes. 2 The change in transmitter release was measured from the mean quantal content calculated by the variance method from evoked fast e.p.s.ps or e.p.ps in low Ca2+-high Mg2+ Ringer solution. 3 5-HT facilitated the release of ACh in low concentrations and depressed it in relatively high concentrations at both preganglionic and motor nerve terminals. 4 These results suggest the possibility that 5-HT may play a role in regulating cholinergic transmission in general.     

Central GABAA excitatory and GABAB inhibitory receptors regulate gastric acid secretion in rats

The present study investigates the effects of GABAA- and GABAB-receptor agonists and antagonists on gastric secretion after their i.c.v. administration to conscious pylorus-ligated rats and anaesthetized stomach lumen-perfused rats. Muscimol was without effect in pylorus-ligated rats, whereas baclofen produced a significant decrease in acid secretion, which was fully prevented by phaclofen. Under these conditions, a significant decrease in acid secretion was also obtained with bicuculline. In stomach lumen-perfused rats, muscimol caused a marked, dose-dependent increase in acid secretion, which was antagonized by bicuculline. Under the same conditions, baclofen induced a moderate, but significant, bicuculline-sensitive increase in acid secretion. Overall, our results suggest the presence of two central GABA pathways which mediate opposite effects: (a) a bicuculline-sensitive GABAA-receptor, the stimulation of which increases acid secretion under pharmacological depression of central vagal tone (anaesthetized rats); (b) a phaclofen-sensitive GABAB-receptor, the activation of which decreases vagally stimulated acid secretion (pylorus-ligated rats).     

Effects of morphine, physostigmine and raphe nuclei stimulation on 5-hydroxytryptamine release from the cerebral cortex of the cat.

1. The release of 5-hydroxytryptamine (5-HT) from the cerebral cortex and caudate nucleus of brainstem-transected cats and from the cerebral cortex of rats anaesthetized with urethane was determined by radioenzymatic and biological assay. 2. The stimulation of nucleus linearis intermedius of raphe doubles the basal 5-HT release in the caudate nucleus and increases it 3 fold in the cerebral cortex. The effects of the electrical stimulation of the raphe are potentiated by chlorimipramine. 3. Brain 5-HT release is greatly increased by morphine hydrochloride (6 mg/kg i.v.) and by physostigmine (100 microgram/kg i.v.), but not by DL-DOPA (50 mg/kg i.v.). 4. It is suggested that the 5-HT releasing action of physostigmine can contribute to some of its pharmacological effects such as the analgesic effect so far attributed exclusively to its indirect cholinomimetic activity. 5. The 5-HT releasing action of physostigmine seems unrelated to its anticholinesterase activity.     

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     

Dual population of GABAA and GABAB receptors in rat pars intermedia demonstrated by release of alpha MSH caused by barium ions.

We have studied the effects of selective GABAA and GABAB agonists on alpha-melanophore stimulating hormone (alpha MSH) release from intact rat neurointermediate lobes (NIL) in vitro. Agonist effects were tested against either basal alpha MSH output or BaCl2 (5 mM)-evoked release. GABA (50 microM) produced a biphasic effect on basal release, with an enhancement followed by inhibition of release. The enhancement but not the inhibition was blocked by bicuculline methiodide (100 microM). Baclofen (10 microM), a specific GABAB agonist, reduced the basal and Ba2+-evoked hormonal release in a stereospecific manner. (-)-Baclofen (5 microM) was active whereas the (+)-isomer was inactive at the same concentration. Isoguvacine (50 microM) a specific GABAA agonist, potentiated the Ba2+-evoked release of alpha MSH. GABA (50 microM) mimicked this effect, and its action was antagonized by bicuculline methiodide (200 microM). The results suggest that both GABAA and GABAB receptors are present on the endocrine cells of the intermediate lobe.     

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.

     

Panicolytic-like effect induced by the stimulation of GABAA and GABAB receptors in the dorsal periaqueductal grey of rats

Activation of GABA(A) and benzodiazepine receptors within the dorsal periaqueductal grey inhibits the escape behaviour evoked by the electrical stimulation of this midbrain area, a defensive reaction that has been related to panic. Nevertheless, there is no evidence indicating whether the same antiaversive effect is also observed in escape responses evoked by species-specific threatening stimuli. In the present study, male Wistar rats were injected intra-dorsal periaqueductal grey with the benzodiazepine receptor agonist midazolam (10, 20 and 40 nmol), the GABA(A) receptor agonist muscimol (2, 4 and 8 nmol), the GABA(B) receptor agonist baclofen (2, 4 and 8 nmol), or with the benzodiazepine inverse agonist FG 7142 (20, 40 and 80 pmol) and tested in an ethologically-based animal model of anxiety, the elevated T-maze. Besides escape, this test also allows the measurement of inhibitory avoidance which has been related to generalised anxiety disorder. Midazolam, muscimol and baclofen impaired escape, a panicolytic-like effect, without altering inhibitory avoidance. FG 7142, on the other hand, facilitated both avoidance and escape reactions, suggesting an anxiogenic and panicogenic-like effect, respectively. The data suggest that GABA(A)/benzodiazepine and GABA(B) receptors within the dorsal periaqueductal grey are involved in the control of escape behaviour and that a failure in this regulatory mechanism may be of importance in panic disorder.     

Presynaptic modulation of the release of GABA by GABAA receptors in pars compacta and by GABAB receptors in pars reticulata of the rat substantia nigra

The effect of GABA agonists and antagonists on K⁺-stimulated [³H]GABA release was studied to assess how presynaptic GABA receptors modulate GABA release. The release was affected in a quite different manner in the pars compacta and in the pars reticulata. Muscimol markedly inhibited the release from the pars compacta but had no effect on the release from the pars reticulata. Baclofen inhibited the release from the pars reticulata without affecting the release from the pars compacta. Bicuculline itself facilitated the release from the pars compacta but inhibited the release from the pars reticulata. Picrotoxin facilitated the release from the pars compacta and had no effect in the pars reticulata. The results suggest that the release of GABA from GABAergic terminals in the substantia nigra of the rat brain is modulated by GABA_A autoreceptors in the pars compacta and by GABA_B receptors in the pars reticulata.     

Fluvoxamine preferentially increases extracellular 5-hydroxytryptamine in the raphe nuclei: an in vivo microdialysis study.

The effects of systemic administration of fluvoxamine on extracellular serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the frontal cortex and raphe nuclei of freely moving rats were examined. Fluvoxamine significantly increased extracellular 5-HT concentrations in both regions at the two doses used (1 and 10 mg/kg i.p.). However, the increase in the raphe nuclei was several-fold that in the frontal cortex. Dialysate 5-HIAA concentrations decreased after treatment with fluvoxamine. These results confirm that 5-HT uptake inhibitors preferentially increase extracellular concentrations of 5-HT in the vicinity of cell bodies and dendrites of serotonergic neurones.     

Interactions of ginsenosides with ligand-bindings of GABAA and GABAB receptors

• 1.1. Total saponin fraction decreased the affinity of specific [³H]muscimol binding without changes in B_max. Ginsenoside Rb₁ Rb₂, Rc, Re, Rf and Rg₁ inhibited the specific [³H]muscimol binding to the high-affinity site.
• 2.2. Total saponin fraction increased the affinity of specific [³H]flunitrazepam binding. Ginsenoside Re and Rf enhanced specific [³H]flunitrazepam binding.
• 3.3. Total saponin fraction decreased the affinity of specific [³⁵S]TBPS binding without changes in B_max. Ginsenosides did not affect specific or non-specific [³⁵S]TBPS binding.
• 4.4. Total saponin fraction decreased the affinity of specific [³H]baclofen binding without changes in B_max. Ginsenoside Rc inhibited specific [³H]baclofen binding.

     

GABAA and GABAB receptors in locus coeruleus: effects of blockers

Racemic baclofen, (-)-baclofen and muscimol depressed all spontaneously firing locus coeruleus neurons tested in a slice preparation. Racemic phaclofen (100 microM; 1 mM) moderately antagonized the effects of racemic baclofen without antagonizing those of muscimol. Bicuculline (10, 30, 100 microM) potently antagonized the action of muscimol without affecting the inhibition of baclofen. Phaclofen and bicuculline had no pronounced effect on the spontaneous discharge rate of cells. The results suggests that there are GABAA and GABAB receptors in the locus coeruleus.     

Antagonism of 5-hydroxytryptamine receptors by quipazine.

1 The antagonist actions of quipazine on 5-hydroxytryptamine (5-HT) receptors have been investigated in the rabbit isolated superior cervical ganglion and on the rat isolated spinal cord and stomach strip. 2 Changes in membrane potential induced by 5-HT or by the nicotinic agonist, 1,1-dimethyl-4-phenyl piperazinium (DMPP), were measured in the ganglion by the sucrose-gap technique. At ganglionic receptors, quipazine had little or no agonist activity, but greatly depressed depolarizations evoked by 5-HT but not depolarizations evoked by DMPP or trimethylammonium (TMA). Injections into the superfusion stream to the ganglion of 2 to 5 mumol quipazine in a small volume of Krebs solution prevented all subsequent responses to 5-HT. Superfusion of the ganglion with quipazine at a concentration of 1 microM produced complete blockade of responses to 5-HT in 3 of 6 ganglia and reduced responses by over 90% in 2 others; responses to DMPP were potentiated in amplitude and duration. Superfusion at a concentration of 0.1 microM depressed responses to 5-HT by 75% on average. The threshold concentration for the blocking action was around 0.01 microM, which depressed responses by 42% on average in 6 experiments (range 0 to 75%). 3 5-HT (1 microM or 100 microM) depressed the amplitude of the dorsal root potentials recorded from the isolated, hemisected cord of the neonate rat by 27 +/- 5% (mean +/- s.e. mean, n = 14) and by 45 +/- 6% (n = 14), respectively. In the presence of quipazine (0.01 microM), 5-HT (1 microM or 100 microM) depressed the amplitude by 6 +/- 2% (n = 15) and by 3 +/- 1% (n = 7), respectively. 4 Concentration-response curves of the contractions induced by 5-HT in the fundus of the rat stomach were obtained in the absence and presence of quipazine. Quipazine (1 microM) shifted the concentration-response curve to the right and depressed the maximum, suggesting a non-competitive mode of antagonism. pI50 values were calculated in order to assess the antagonist activity of quipazine at rat fundus 5-HT receptors; the mean pI50 was 6.91 +/- 0.2 (n = 6). 5 It is concluded that quipazine may be an effective antagonist at 5-HT receptors in various tissues.     

Coexistence of GABAA and GABAB receptors on A delta and C primary afferents.

Intracellular recordings from adult rat dorsal root ganglion neurones were performed in vitro and the coexistence of two gamma-aminobutyric acid (GABA) receptors on the membrane of identified A delta and C primary afferents was demonstrated. Transient applications of GABA (10(-6)-10(-2) M) evoked dose-dependent depolarizations and increased membrane conductance. The responses were mimicked by muscimol, isoguvacine, THIP and 3 amino propane sulphonic acid (3 APS); they were blocked by bicuculline and picrotoxin. Pentobarbitone induced an increase of GABA-induced depolarizations. Perfusion of tetraethylammonium (TEA, 7.5 mM) and intracellular injection of Cs+ ions unmasked the Ca2+ component of action potentials, which appeared as long-lasting plateau depolarizations. Such action potentials were shortened in the presence of methoxyverapamil (D600, 5 X 10(-6)-10(-5) M) and in a medium without Ca+ ions. Prolonged (5-10 min) perfusion of GABA (10(-9)-10(-5) M) shortened the Ca2+ component of action potentials. This effect was mimicked by baclofen (10(-7)-5 X 10(-6) M) and muscimol (5 X 10(-7)-10(-5) M) and was not affected by bicuculline perfusion (5 X 10(-6)-10(-5) M). Isoguvacine (2.5 X 10(-5) M) did not affect action potential duration. It is concluded that two GABA receptors coexist on the membrane of slow conducting primary afferents: the bicuculline-sensitive GABAA receptor mediates depolarizations and the bicuculline-insensitive GABAB receptor shortens the calcium component of action potentials.