Effects of 8-OHDPAT and 5-HT1A antagonists WAY100135 and WAY100635, on guinea-pig behaviour and dorsal raphe 5-HT neurone firing.

1. The effects of 5-HT1A antagonists on guinea-pig behaviour and dorsal raphe neuronal activity were investigated. 2. WAY100135 (10 mg kg-1, s.c.) and WAY100635 (1 mg kg-1, s.c.) significantly reduced the behaviours induced by 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) (1 mg kg-1, s.c.) indicative of post-synaptic 5-HT1A receptor antagonism. WAY100635 (10 mg kg-1, s.c.) alone induced ear twitches, which were antagonized by ketanserin (1 mg kg-1, s.c.), but no other overt behaviours. 3. WAY100635 (0.125 mg kg-1, i.v.) produced a right-ward shift in the dose-related inhibition of neuronal firing by 8-OHDPAT (5-100 micrograms kg-1, i.v.) but did not affect the maximum inhibition induced by 8-OHDPAT indicating competitive antagonism between 8-OHDPAT and WAY100635 at the 5-HT1A somato-dendritic autoreceptor in the dorsal raphe nucleus of the guinea-pig. WAY100635 also produced a dose-related increase in the basal firing of 5-HT neurones in the dorsal raphe nucleus and restored the firing of dorsal raphe neurones previously inhibited by 8-OHDPAT (10 micrograms kg-1, i.v.). 4. The results indicate that WAY100635 is a competitive 5-HT1A antagonist in the guinea-pig. Furthermore WAY100635 can increase 5-HT neuronal firing, suggesting that it blocks a 5-HT1A receptor-mediated inhibitory tone acting on guinea-pig 5-HT neurones resulting in increased 5-HT release and 5-HT2 receptor-mediated behaviours.     

Electrophysiological actions of nicotine on substantia nigra single units.

Extracellular recordings of single unit activity were made in the substantia nigra (SN) of chloral hydrate-anaesthetized rats. Dopaminergic neurones of the pars compacta (SNC) were stimulated by (-)-nicotine bitartrate (1.0 mg kg-1) given subcutaneously (s.c.). This action was prevented by the secondary amine mecamylamine HCl (2.0 mg kg-1 i.v.) but not by a ganglion-blocking dose of the bisquaternary compound chlorisondamine Cl (0.1 mg kg-1 i.v.). Mecamylamine reduced the spontaneous activity of dopaminergic neurones. Nicotine, when administered intravenously (2-128 micrograms kg-1 cumulative dose), also stimulated dopamine cells and this action was dose-related. Nicotine, administered intravenously, (2-128 micrograms kg-1 cumulative dose) markedly excited non-dopamine cells in the pars reticulata (SNR) in a dose-related manner. In rats pretreated with chlorisondamine (0.1 mg kg-1 i.v.), nicotine induced a small excitatory or depressant action, but the marked excitation was not seen. Mecamylamine (2 mg kg-1 i.v.) completely prevented the actions of nicotine. The results are consistent with a direct excitatory action of nicotine on dopaminergic neurones of the substantia nigra pars compacta. The pronounced excitatory action of systemically administered nicotine on non-dopamine cells of the pars reticulata appears to be of peripheral origin.     

SB 242084, a selective serotonin2C receptor antagonist, increases dopaminergic transmission in the mesolimbic system.

Electrophysiological techniques and in vivo microdialysis were used to investigate the effect of SB 242084, a potent and selective 5-HT2C receptor antagonist in the control of nigro-striatal and mesolimbic dopaminergic function. Thus, extracellular single unit recordings were performed from neurochemically-identified dopamine (DA) neurons in the substantia nigra, pars compacta (SNc) and the ventral tegmental area (VTA), as well as monitoring of striatal and accumbal basal DA release in anesthetized rats following the administration of SB 242084 and RO 60-0175. Administration of SB 242084 (160-640 microg/kg, i.v.) caused a dose-dependent increase in the basal firing rate of VTA DA neurons, reaching its maximum (27.8+/-6%, above baseline) after 640 microg/kg. Moreover, bursting activity was significantly enhanced by SB 242084 in the VTA. On the other hand, SB 242084 (160-640 microg/kg, i.v.) did not cause any significant change in the basal firing rate and bursting activity of DA neurons in the SNc. Injection of the 5-HT2C receptor agonist RO 60-0175 (80-320% microg/kg, i.v.) dose-dependently decreased the basal firing of DA neurons in the VTA but not in the SNc. RO 60-0175 exerted its maximal inhibitory effect (53.9+/-15.1%, below baseline) in the VTA at the dose of 320 microg/kg. Basal DA release (34.8+/-9%, above baseline) and dihydroxyphenylacetic acid (DOPAC) efflux (19.7+/-7%, above baseline) were significantly enhanced in the nucleus accumbens following the intraperitoneal administration of 10 mg/kg SB 242084. Intraperitoneal injection of 5 mg/kg SB 242084 significantly increased DA release (16.4+/-6%, above baseline) in the nucleus accumbens, but did not affect DOPAC efflux. In the striatum, SB 242084 (5 and 10 mg/kg, i.p.) only slightly increased DA release above baseline (3.5+/-4 and 11.2+/-6%, respectively), without affecting DOPAC efflux in this area. However, the effect of SB 242084 in the striatum was rendered more evident by the fact that injection of the vehicle used to dissolve the drug in a group of control rats, significantly reduced basal DA output by 19.6+/-7%. Stimulation of 5-HT2C receptors by RO 60-0175 (1 mg/kg, i.p.) significantly decreased DA release in the nucleus accumbens by 26.1+/-4% (below baseline) 60 min after injection. On the other hand, RO 60-0175 (1 mg/kg, i.p.) did not cause any significant change of DA release in the striatum. However, DOPAC efflux was reduced by RO 60-0175 (1 mg/kg, i.p.) both in the striatum and the nucleus accumbens. Taken together, these data indicate that the central 5-HT system exerts a tonic and phasic inhibitory control on mesolimbic DA neuron activity and that 5-HT2C receptor subtypes are involved in this effect. Moreover, these findings might open new possibilities for the employment of 5-HT2C receptor antagonists in the treatment of neuropsychiatric disorders related to a hypofunction of central DA neurons.     

TC-2559 excites dopaminergic neurones in the ventral tegmental area by stimulating alpha4beta2-like nicotinic acetylcholine receptors in anaesthetised rats

1. The in vivo effects of a selective partial agonist for neuronal nicotinic acetylcholine receptor (nAChRs) alpha4beta2 subtype, TC-2559, characterised recently in in vitro preparations, have been profiled. The brain bioavailability of TC-2559 and its effects on the spontaneous firing and bursting properties of the dopaminergic (DAergic) neurones recorded extracellularly in the ventral tegmental area (VTA) were studied following systemic administration in anaesthetised rats. 2. Cumulative doses of TC-2559 (0.021-1.36 mg kg(-1), i.v.) increased both the firing and bursting activities of VTA DA neurones. The effect of bolus doses of TC-2559 of 0.66 or 1.32 mg kg(-1), i.v., was approximately equivalent to that of 0.0665 mg kg(-1), i.v. nicotine. 3. The excitation evoked by both nicotine and TC-2559 was fully reversed by DHbetaE (0.39-0.77 mg kg(-1), i.v.), an alpha4beta2-subtype-preferring nicotinic antagonist, and application of nicotine after DHbetaE failed to evoke any excitation. MLA (0.23 mg kg(-1), i.v.), an alpha7 selective antagonist, failed to alter TC-2559-evoked excitation and bursting activities, and a novel alpha7 agonist (PSAB-OFP; 0.23 mg kg(-1), i.v.) was also without effect. 4. The present results indicated that TC-2559 fully mimics nicotine by increasing both the excitability and bursting behaviour of VTA DA neurones, effects that are predominantly due to activation of alpha4beta2-like nAChRs. 5. TC-2559 has been demonstrated to be a useful in vivo pharmacological tool for studying the alpha4beta2 subtype of nicotinic receptor.     

The role of serotonin and dopamine in brain in the antidepressant-like effect of clonidine in the forced swimming test.

The effect of clonidine (0.1 mg/kg, i.p.), as a three-injection course, on behaviour in the forced swimming test was studied in rats injected intracerebroventricularly (i.c.v.) with 150 micrograms 5,7-dihydroxy-tryptamine (5,7-DHT) to destroy serotonin (5-HT) neurones or treated with 100 mg/kg (i.p.) (+/-)-sulpiride or 0.5 micrograms/0.5 microliter (-)-sulpiride in the nucleus accumbens. Clonidine significantly increased struggling and reduced floating and the effects were antagonized by both treatments with sulpiride but not by 5,7-DHT which markedly depleted 5-HT in brain. The results suggest that the mesolimbic dopaminergic system but not 5-HT neurones, plays a permissive role in the antidepressant-like effect of clonidine in the forced swimming test.     

Parthenolide Blocks Cocaine's Effect on Spontaneous Firing Activity of Dopaminergic Neurons in the Ventral Tegmental Area

Chronic cocaine administration leads to catecholamine reuptake inhibition which enhances reward and motivational behaviors. Ventral Tegmental Area dopaminergic (VTA DA) neuronal firing is associated with changes in reward predictive signals. Acute cocaine injections inhibit putative VTA DA cell firing in vertebrates. Parthenolide, a compound isolated from the feverfew plant (Tanacetum parthenium), has been shown to substantially inhibit cocaine's locomotion effects in a planarian animal model (Pagán et al., 2008). Here we investigated the effects of parthenolide on the spontaneous firing activity of putative VTA DA neurons in anesthetized male rats (250-300g). Single-unit recordings were analyzed after intravenous (i.v.) parthenolide administration followed by 1mg/kg i.v. cocaine injection. Results showed that parthenolide at 0.125 mg/kg and 0.250mg/kg significantly blocked cocaine's inhibitory effect on DA neuronal firing rate and bursting activity (p< 0.05, two way ANOVA). We propose that parthenolide might inhibit cocaine's effects on VTA DA neurons via its interaction with a common binding site at monoamine transporters. It is suggested that parthenolide could have a potential use as an overdose antidote or therapeutic agent to cocaine intoxication.     

Effect of fluoxetine on the spontaneous electrical activity of fronto-cortical neurons

The effect of fluoxetine on spontaneous extracellular activity of fronto-cortical neurons of chloral hydrate-anesthetized rats was investigated. Fluoxetine significantly increased the basal firing rate of cortical neurons in a dose-dependent manner (0.1–1000 μg kg⁻¹ i.v.), with a maximum excitatory effect of 53% at 1000 μg kg⁻¹. Selective destruction of ascending serotoninergic pathways induced by intracerebroventricular injections of 150 μg 5,7-dihydroxytryptamine, in desipramine-pretreated rats, antagonized the excitatory effect of fluoxetine. The present results suggest that fluoxetine significantly increases the electrical activity of the fronto-cortical neurons acting on serotoninergic uptake mechanisms localized at the level of raphe nuclei.     

Enhancement of D2 receptor agonist-induced inhibition by D1 receptor agonist in the ventral tegmental area.

1. A microiontophoretic study was performed on chloral hydrate-anaesthetized rats to examine the role of D1 receptors in the ventral tegmental area (VTA) neurones, which are inhibited by autoreceptor and D2 receptor agonists. 2. Inhibition by microiontophoretic application of quinpirole (a D2 agonist) of antidromic spikes elicited by stimulation of the nucleus accumbens in dopaminergic neurones of the VTA, was significantly enhanced by simultaneous application of SKF 38393 (D1 agonist), although SKF 38393 alone had little effect on the neurones. 3. In addition, quinpirole-induced inhibition was antagonized by iontophoretic application of domperidone (D2 antagonist), but was not affected by SCH 23390 (D1 antagonist). 4. Furthermore, SKF 38393-induced enhancement of inhibition by quinpirole was antagonized by simultaneous application of SCH 23390. 5. These results suggest that activation of D1 receptors located on the VTA dopaminergic neurones or on non-dopaminergic nerve terminals is not essential for inducing inhibition of the dopaminergic neurones, but enhances D2 receptor-mediated inhibition directly or indirectly via inhibitory neurones.     

Role of serotonin and catecholamines in brain in the feeding suppressant effect of fluoxetine.

The hypophagic effect of fluoxetine was studied in rats, injected intracerebroventricularly with 150 micrograms/20 microliters 5,7-dihydroxytryptamine, to destroy serotonin-containing neurones or 250 micrograms/20 microliters 6-hydroxydopamine, to destroy catecholamine-containing neurones. The effect of various serotonin receptor antagonists was assessed as well. Neither neurotoxin significantly modified the effect of 20 mg/kg (i.p.) fluoxetine on food intake. Metergoline (1-5 mg/kg), (-)-propranolol (16 mg/kg) and ICS 205-930 (0.1 and 1 mg/kg) did not modify the hypophagic effect of fluoxetine, while mianserin (1 and 5 mg/kg), ritanserin (0.5 and 1 mg/kg) and xylamidine (3 mg/kg) slightly but significantly reduced it. While the mechanism by which some 5-HT receptor antagonists modify the effect of fluoxetine remains to be elucidated, it seems clear that 5-HT receptors hardly have any significant role in the ability of the drug to suppress food intake.     

The involvement of endothelium-derived relaxing factor in the regulation of renal cortical blood flow in the rat.

1. In the present study the role of endogenous nitric oxide (NO) was investigated, in the regulation of renal cortical blood flow (RCBF) in vivo in anaesthetized rats under conditions in which prostacyclin involvement had been eliminated. 2. Infusions of the NO synthesis inhibitor NG-monomethyl-L-arginine (MeArg) at 1 or 3 mg kg-1 min-1, i.v., produced significant decreases in RCBF of 29 +/- 7% and 35 +/- 5%, respectively. These effects were reversed by co-infusion of a 3 fold excess of L-arginine (L-Arg). 3. Similarly, intravenous infusion of N omega-nitro-L-arginine methyl ester (NO2Arg) at 30 or 300 micrograms kg-1 min-1 attenuated RCBF by 21 +/- 4% or 53 +/- 4%, respectively, and these effects were reversed by L-Arg (3 or 10 mg kg-1 min-1, i.v.). Most importantly, a low dose of NO2Arg (30 micrograms kg-1 min-1, i.v.), while having no pressor effect, considerably reduced RCBF, indicating that basal release of NO is important for the maintenance of renal cortical blood flow. 4. MeArg (3 mg kg-1 min-1, i.v.) or NO2Arg (300 micrograms kg-1 min-1, i.v.) inhibited endothelium-dependent acetylcholine (ACh, 10 micrograms kg-1 min-1, i.v. for 3 min) increases in RCBF in an L-Arg reversible manner, but did not affect endothelium-independent (dopamine 10 micrograms kg-1 min-1, i.v., for 3 min) increases.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of idazoxan on dorsal raphe 5-hydroxytryptamine neuronal function.

The effects of the alpha 2-adrenoceptor antagonist idazoxan on 5-hydroxytryptamine (5-HT) neuronal firing and release have been investigated. Idazoxan, administered i.v. (10 micrograms/kg and 0.5 mg/kg) increased dorsal raphe nucleus (DRN)-5-HT neuronal firing rate in a dose-dependent fashion. At the higher dose, a voltammetric study revealed increases in extracellular 5-HT and 5-hydroxyindole acetic acid (5-HIAA) levels, there was no effect with the lower dose. Intra-raphe administration of idazoxan (1 ng) also elevated the firing rate of 5-HT neurones in the dorsal raphe, suggesting that idazoxan may produce the increase in firing by a direct effect in the DRN. However, microiontophoretic application of idazoxan did not increase the firing rate of 5-HT neurones in the DRN. Thus the increase in the firing rate of 5-HT neurones in the DRN observed with systemic and local administration of idazoxan is probably not due to a direct action of idazoxan on the 5-HT neurone. Possibly the idazoxan acted at alpha 2-adrenoceptors located on noradrenergic terminals thus stimulating noradrenaline release and consequently increased 5-HT activity. Chronic administration of idazoxan (0.8 mg/kg per h for 14 days), using osmotic mini-pumps, caused an elevation in basal firing rate and an attenuation of the inhibitory response of DRN 5-HT neurones to the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) (10 micrograms/kg i.v.). This finding suggests that chronic infusion with idazoxan leads to desensitisation of the 5-HT1A somatodendritic autoreceptor.     

Amphetamine exerts anomalous effects on dopaminergic neurons in neonatal rats in vivo.

The effects of amphetamine, apomorphine and haloperidol on the spontaneous activity of electrophysiologically identified nigral dopaminergic neurons were examined with extracellular recordings in vivo in neonatal rats ranging in age from postnatal day 1 to postnatal day 28, and in adult rats. In postnatal day 1-6 pups amphetamine (5 mg/kg i.p.) produced a paradoxical increase in neuronal firing in 45% and had no effect on 30% of the 20 neurons examined. During the second week half of the neurons recorded were unresponsive to amphetamine. Typical amphetamine-induced inhibition was observed in only 25% of the neurons from postnatal day 1-6 and 50% of those from postnatal day 7-15 rats compared to 81.8% in postnatal day 16-28 pups and 100% in adults. Apomorphine (50-200 micrograms/kg i.p.; 5-20 micrograms/kg i.v.), significantly inhibited the spontaneous activity of dopaminergic neurons, including cells that previously failed to be inhibited by amphetamine, independent of age. The apomorphine-induced inhibition was consistently reversed by administration of haloperidol (0.5-2.0 mg/kg, i.p.; 50-200 micrograms/kg i.v.). The anomalous responses to amphetamine in early neonatal rats may be related to its paradoxical behavioral effects in human children afflicted with attention deficit disorder.     

Inhibition of non-dopamine cells in the ventral tegmental area by benzodiazepines: relationship to A10 dopamine cell activity

Previous electrophysiological studies have demonstrated that non-dopaminergic (non-DA) neurons within the substantia nigra pars reticulata (SNR) are extremely sensitive to the inhibitory effects of GABA and GABA-mimetic drugs, including benzodiazepines, whereas dopaminergic (DA) neurons in the substantia nigra pars compacta (SNC) are less sensitive to these compounds and may be influenced indirectly by SNR neurons. The interactions between A10 DA and non-DA neurons within the adjacent ventral tegmental area (VTA) are not as well characterized. In the present experiments, single unit recording and microiontophoretic techniques were used to determine the effects of benzodiazepines on DA and non-DA neurons in the VTA of chloral hydrate anesthetized rats. Diazepam, administered intravenously (i.v.), potently inhibited non-DA, SNR-like cells within the VTA. The effects of diazepam on A10 DA cells were more variable than those observed on non-DA, SNR-like cells in this region, but 77% of such cells showed moderate to marked excitation. Both of these effects were reversed by the benzodiazepine antagonist Ro 15-1788; on many cells, this agent produced marked rebound effects beyond the original basal firing rates. However, when administered alone, Ro 15-1788 exerted no effect on either cell population. Microiontophoretic administration of the benzodiazepines chlordiazepoxide and flurazepam resulted in marked inhibition of non-DA SNR-like cells, but produced either mild inhibition or no effect on A10 DA cells; excitation of DA cells was never observed even though the same neuron was excited by i.v. diazepam. These findings suggest that benzodiazepines act directly upon non-DA, SNR-like cells in the VTA to produce inhibition of activity and a disinhibition of A10 DA cells. This relationship makes it unlikely that benzodiazepines would enhance feedback inhibition of DA cells following neuroleptic administration. In fact, when administered following haloperidol, i.v. diazepam failed to reverse haloperidol-induced increases of A10 DA cell firing; if anything, diazepam further depolarized the cell. If antipsychotic drugs produce their clinical effects, in part, by inducing depolarization inactivation of DA cells, then benzodiazepines might be a useful adjunctive therapy in the treatment of schizophrenia.     

Evidence that central 5-hydroxytryptaminergic neurones are involved in the anxiolytic activity of buspirone.

1. A two-compartment exploratory test was used to assess the role of central 5-hydroxytryptaminergic neurones in the anxiolytic activity of buspirone in rats. 2. Buspirone 0.1 mg kg-1, administered subcutaneously 15 min before testing, significantly increased black-white transitions (BWT) in control rats but had no effect in animals injected intracerebroventricularly one week before with 150 micrograms 5,7-dihydroxytryptamine (in 20 microliters). 3. Infusion of buspirone in the median raphe (but not in the dorsal raphe) significantly enhanced BWT, at doses from 1 micrograms to 10 micrograms (in 0.5 microliters). Buspirone 5 and 10 micrograms, but not 1 microgram, administered in the median raphe, significantly enhanced motor activity of rats during the first 10 min of testing in the activity cages. 4. The effect on BWT of 5 micrograms buspirone in the median raphe was completely antagonized in animals which had received either 5,7-dihydroxytryptamine intraventricularly, 150 micrograms (in 20 microliters), one week before or an infusion of 0.1 microgram (in 0.5 microliter) (-)-propranolol in the same area 5 min before. (-)-Propranolol infused in the median raphe did not modify the effect of buspirone on locomotion. 5. Infusion of 5 micrograms buspirone (in 0.5 microliter) in the median raphe significantly enhanced punished responses in a conflict test with no effect on unpunished responding. Buspirone infused in the dorsal raphe had no effect on punished or unpunished responding over a wide dose range. 6. The results indicate that at the relatively low dose used in the present study buspirone produces an anxiolytic effect by acting on central 5-hydroxytryptaminergic neurones.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of different receptors in the central and peripheral effects of histamine on intestinal motility in the rat

The effects of histamine on intestinal motility have been investigated in conscious rats, fed or fasted, using an electromyographic method. Histamine peripherally administered (10 mg kg-1) in 15 h fasted rats induced an inhibition followed by a period of irregular spiking activity disrupting the duodenojejunal migrating myoelectric complexes (MMC) and suppressed the postprandial motor spiking activity when administered 50 min after a meal. The selective agonist of the H1-receptors, 2-pyridylethylamine (2-PEA) induced an irregular spiking activity while dimaprit acting on H2-receptors, inhibited the MMC pattern. Effects of peripherally administered histamine were antagonized by previous administration of chlorpheniramine (0.5 mg kg-1 i.p.) and in a lesser extent by cimetidine (10 mg kg-1 i.p.). Histamine (1-10 micrograms) administered intracerebroventricularly (i.c.v.) in fasted rats increased the motor cycle frequency and immediately restored the MMC pattern when given to fed rats. Among the three agonists of the H1- H2- and H3-receptors (2-PEA, dimaprit and R-alpha-methylhistamine, respectively) only R-alpha-methylhistamine (1-10 micrograms i.c.v.) was able to reproduce this effect. It is concluded that the effects of histamine on intestinal motility were centrally and peripherally mediated involving mainly H1-receptors at the peripheral level and H3-receptors at the CNS level.     

Effects of a selective 5-HT2 agonist, DOI, on 5-HT neuronal firing in the dorsal raphe nucleus and 5-HT release and metabolism in the frontal cortex.

Systemic administration of the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (50 and 100 micrograms kg-1, i.v.) inhibited dorsal raphe neuronal firing. DOI (100 micrograms kg-1, i.v.) also produced a decrease in extracellular 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex measured by microdialysis. However, local administration of DOI into the frontal cortex produced no change in extracellular 5-HT and 5-HIAA at any dose given (1, 10 and 100ng). The results demonstrate that DOI is a potent inhibitor of 5-HT neuronal firing and terminal release and that the effects on release are not mediated by an action within the terminal region. The site of action and the receptor involved in the inhibition remains to be determined.     

Involvement of 5-HT2 receptors in the behaviours produced by intrathecal administration of selected 5-HT agonists and the TRH analogue (CG 3509) to rats.

1. The behavioural effects of the intrathecal injection of a thyrotrophin-releasing hormone (TRH) analogue L-orotyl-L-histidyl-prolineamide (CG 3509, 0.5 micrograms), the non-selective 5-HT1 and 5-HT2 receptor agonist 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT, 2-100 micrograms) and the selective 5-HT2 receptor agonist 2,5-dimethoxy-alpha,4-dimethyl-benzene ethamine hydrochloride (DOM, 2-25 micrograms) were compared with the response of systemically administered 5-MeODMT (2 mg kg-1, i.p.) in rats, to establish whether the agonist-induced behaviours were mediated by bulbospinal 5-HT1 or 5-HT2 receptors. 2. Intrathecal injection of 5-MeODMT or DOM produced dose-related back muscle contractions (a previously undocumented behaviour) and wet-dog shakes which were both markedly attenuated by ritanserin pretreatment (1 mg kg-1, i.p.) indicating the involvement of 5-HT2 receptors. In contrast, reciprocal forepaw treading, flat body posture and Straub-tail were evoked by 5-MeODMT but not by DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. However, as ritanserin pretreatment reduced the reciprocal forepaw treading induced by intrathecal 5-MeODMT, this behaviour may be facilitated by 5-HT2 receptor activation. 3. Intrathecal 5,7-dihydroxytryptamine (5,7-DHT, 2 x 150 micrograms) treatment decreased thoraco-lumbar spinal cord 5-HT (-95%) and potentiated the back muscle contractions produced by intrathecal DOM injection without altering the wet-dog shake behaviour. None of the components of the 5-HT syndrome produced by 5-MeODMT (2 mg kg-1, i.p.), with the exception of a small increase in wet-dog shakes, was significantly altered by intrathecal 5,7-DHT (which reduced thoraco-lumbar spinal cord 5-HT by 84%). Taken together these data suggest that the only 5-HT agonist-induced behaviour mediated by the activation of 5-HT2 receptors located postsynaptic to bulbospinal 5-hydroxytryptaminergic (5-HTergic) neurones was back muscle contractions. 4. The wet-dog shake and forepaw licking behaviors produced by intrathecal CG 3509 (0.5 micrograms) were attenuated when ritanserin was administered intrathecally 30 min before, but not when it was given at the same time as CG 3509 and neither behaviour was altered by intrathecal 5,7-DHT. This suggests that bulbospinal 5-HTergic neurones are not involved in the production of these TRH analogue-induced behaviours and that the 5-HT2 receptors which mediate these behaviours are not located in the spinal cord.     

Modulation of non-adrenergic, non-cholinergic neural bronchoconstriction in guinea-pig airways via GABAB-receptors.

1. Evidence suggests that gamma-aminobutyric acid (GABA) and its receptors are present in the peripheral nervous system. We have now investigated the effect of GABA and related substances on non-adrenergic, non-cholinergic (NANC) neurally-evoked bronchoconstriction in the anaesthetised guinea-pig. 2. Bilateral vagal stimulation (5 V, 5 ms, 3 or 5 Hz) for 30 s, after propranolol (1 mg kg-1 i.v.) and atropine (1 mg kg-1 i.v.) evoked a NANC bronchoconstrictor response manifest as a mean tracheal pressure rise of 21.9 +/- 1.04 cmH2O (n = 70). The bronchoconstrictor response was reproducible for any given animal. 3. GABA (10 micrograms-10 mg kg-1 i.v.) did not alter basal tracheal pressure but reduced the NANC bronchoconstrictor response to vagal stimulation in a dose-dependent manner (ED50 = 186 micrograms kg-1 with a maximal inhibition of 74 +/- 3.4% at 10 mg kg-1). Neither the opioid antagonist naloxone (1 mg kg-1 i.v.) nor the alpha-adrenoceptor antagonist phentolamine (2.5 mg kg-1 i.v.) had any significant effect on the inhibitory response produced by GABA (500 micrograms kg-1). 4. GABA-induced inhibition was not antagonised by the GABAA-antagonist bicuculline (2 mg kg-1 i.v.). 5. The GABAB-agonist baclofen (10 micrograms-3 mg kg-1 i.v.) caused a dose-dependent inhibition of the NANC response (ED50 = 100 micrograms kg-1 with a maximal inhibition of 35.5 +/- 2.8% at 3 mg kg-1). The GABAA-agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-C] pyridin-3-ol (THIP), also inhibited the NANC bronchoconstrictor response.(ABSTRACT TRUNCATED AT 250 WORDS)