Facilitation of serotonin (5-HT) release in the rat brain cortex by cAMP and probable inhibition of adenylate cyclase in 5-HT nerve terminals by presynaptic α2-adrenoceptors

Summary Stimulation-evoked tritium overflow was examined in superfused rat brain cortex slices (stimulus: electrical impulses; 3 Hz) and synaptosomes (stimulus: potassium 12 mmol/l) preincubated with ³H-5-HT. 1. In slices and synaptosomes, the evoked ³H overflow was facilitated by forskolin and 8-Br-cAMP, but was not affected by AH 21-132 (an inhibitor of cAMP phosphodiesterase; cis-6-(p-acetamidophenyl)-1, 2, 3, 4, 4 a,10b-hexahydro-8, 9-dimethoxy-2-methylbenzo [c] [1,6]-naphthyridine). In the presence of AH 21-132, the facilitatory effect of forskolin on evoked overflow was increased. 2. In slices, AH 21-132 or combined administration of forskolin plus AH 21-132 did not change the percentage of basal or evoked ³H overflow represented by unmetabolized ³H-serotonin (about 30% and 60%, respectively). 3. In slices, cocaine or 6-nitroquipazine, an inhibitor of serotonin uptake, did not influence the increase in evoked overflow produced by forskolin plus AH 21-132. Forskolin plus AH 21-132 did not alter the inhibitory effect of serotonin (examined in the presence of 6-nitroquipazine) and the facilitatory effect of metitepin (a serotonin receptor antagonist) on evoked ³H overflow, but considerably decreased the inhibitory effect of clonidine or B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[5,4-d]-azepine). The present results suggest that the serotoninergic nerve terminals in the rat brain cortex are endowed with an adenylate cyclase, which is negatively coupled to the presynaptic ₂-adrenoceptors, but is not linked to the presynaptic autoreceptors.This study was supported by a grant of the Deutsche Forschungsgemeinschaft. Part of the present results was reported at the 9th International Congress of Pharmacology, London 1984 (Schlicker et al. 1984) Send offprint requests to M. Göthert     

The 5-HT3 receptor agonist 1-(m-chlorophenyl)-biguanide facilitates noradrenaline release by blockade of α2-adrenoceptors in the mouse brain cortex

We analyzed the facilitatory effect of the 5-HT₃ receptor agonist 1-(m-chlorophenyl)-biguanide (mCPBG) on the electrically evoked noradrenaline release in superfused mouse brain tissue. In addition, we determined the affinities of mCPBG and two other 5-HT receptor ligands, namely, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT; also a 5-HT₃ receptor agonist) and 5-carboxamidotryptamine (5-CT; a 5-HT₁ receptor agonist) for ₂ binding sites. The latter two 5-HT receptor agonists were included because of the claimed involvement of ₂-adrenoceptors in their effects on noradrenaline release.In superfusion experiments on mouse brain cortex slices preincubated with ³H-noradrenaline, tritium overflow evoked by 2-min periods of electrical field stimula tion (3 Hz) was facilitated by mCPBG and, in addition, by rauwolscine (₂-adrenoceptor antagonist) and tetraethylammonium (K⁺ channel blocker) (which were examined for comparison). The effect of mCPBG was not affected by the 5-HT3 receptor antagonist tropisetron or by desipramine but was abolished by rauwolscine. In slices superfused with medium containing desipramine, the concentration-response curve of unlabelled noradrenaline for its inhibitory effect on the electrically (0.3 Hz) evoked overflow was shifted to the right by mCPBG and rauwolscine (apparent pA₂ 5.35 and 7.88, respectively). In another series of superfusion experiments, 4 electrical pulses, administered at 100 Hz, were used to evoke tritium overflow. Tritium overflow evoked by this stimulation procedure (under which an endogenous tone of noradrenaline does not develop) was not affected by mCPBG and rauwolscine but still increased by tetraethylammonium. The specific binding of 3H-rauwolscine to rat brain cortex homogenates was displaced monophasically by unlabelled rauwolscine, mCPBG, 2-methyl-5-HT and 5-CT (pK_i 8.59, 5.84, 5.05 and 5.86, respectively).In conclusion the present results indicate that mCPBG acts as a low-affinity antagonist at ₂-adrenoceptors. This property has to be considered in functional studies of 5-HT₃ receptor-mediated effects in tissues containing ₂-adrenoceptors as well.Abbreviations mCPBG 1-(m-chlorophenyl)-biguanide - 5-CT 5-carboxamidotryptamine - 2-methyl-5-HT 2-methyl-5-hydroxytryptamine - POP pseudo-one-pulse - TEA tetraethylammonium Correspondence to: E. Schlicker at the above address     

Biochemical and behavioural effects of isamoltane,a β-adrenoceptor antagonist with affinity for the 5-HT1B receptor of rat brain

Summary The biochemical and behavioural effects of isamoltane, a \-adrenoceptor and 5-HT_1B receptor antagonist that has higher affinity for 5-HT_1B receptors than for 5-HTIA receptors, on 5-HT neurotransmission in the rat brain were examined. In binding experiments isamoltane was found to be about five times more potent as a ligand for the 5-HT_1B receptor than for the 5-HT_1A receptor (K_i values 21 and 112 nmol/l, respectively). Isamoltane increased the K⁺-evoked overflow of ³H from ³H-5-HT loaded slices of rat occipital cortex at 0.1 mol/l, consistent with inhibition of the terminal 5HT autoreceptor. In vivo, isamoltane significantly increased the concentration of 5-hydroxyindoleacetic acid in hypothalamus and hippocampus indicating an increased 5-HT turnover with a maximal effect at 3 mg/kg s.c. A higher dose produced a less pronounced effect. This effect did not seem to be due to the -adrenoceptor blocking action of isamoltane since the -adrenoceptor antagonists, (–)-alprenolol, betaxolol or ICI 118,551 had no significant effects on 5-HT turnover at 5 mg/kg s.c. Isamoltane at 3 mg/kg s.c. induced the wet-dog shake response which was blocked by the tryptophan hydroxylase inhibitor p-chlorophenylalanine. In contrast, the same response induced by the 5-HT₂ receptor agonist quipazine was not blocked by pretreatment with p-chlorophenylalanine. The wet-dog shakes evoked by isamoltane and quipazine were blocked by ritanserin, which indicates that 5-HT₂ receptors are involved in their expression. These observations indicate that isamoltane, by inhibiting the terminal 5-HT autoreceptors, increased the synaptic concentration of 5-HT to a level that induced a behavioural response. Send of offprint requests to S. B. Ross at the above addressThe present results have been presented in part at the Second IUPHAR Satellite Meeting on Serotonin, Basel, Switzerland, July 11–13, 1990