Regulation of the release of 5-hydroxytryptamine in the median raphe nucleus of the rat by catecholaminergic afferents.

The present study was conducted in order to examine the influence of catecholaminergic afferents on the release of serotonin in the median raphe nucleus in vivo. To this aim, selective dopamine D1 and D2, and alpha1- and alpha2-adrenergic agonists and antagonists were administered locally (1, 10 and 100 microM) through a dialysis probe implanted in the median raphe nucleus of freely moving rats. The D1 and D2 agonists, (+/-)-1-phenyl-2,3,4, 5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF-38393) and quinpirole, respectively, and the D1 and D2 antagonists, R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine (SCH-23390) and raclopride, respectively, did not alter the release of serotonin in the median raphe nucleus. The alpha1-adrenoceptor agonist phenylephrine did not modify the release of serotonin in this nucleus, although an increased release was observed when the more potent alpha1-adrenoceptor agonist cirazoline was used. In contrast, the alpha1-adrenoceptor antagonist prazosin reduced the release of 5-hydroxytryptamine (5-HT) in a concentration-dependent manner. The release of 5-HT was also reduced by the alpha2-adrenoceptor agonist clonidine and increased by the alpha2-adrenoceptor antagonist 2-methoxy-idazoxan (RX821002). These results indicate that the release of serotonin in the median raphe nucleus does not appear to be regulated by dopaminergic afferents through the activation of dopamine D1 or D2 receptors. On the contrary, it is suggested that endogenous noradrenaline exerts a direct tonic stimulatory control on the release of serotonin through alpha1-adrenoceptors, and an indirect tonic inhibitory influence through alpha2-adrenoceptors located probably in noradrenergic nerve terminals within the median raphe nucleus.