Stimulation of the Serotonin Autoreceptor Prevents the Calcium‐Calmodulin‐Dependent Increase of Serotonin Biosynthesis in Rat Raphe Slices

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Abstract
The role of the serotonin (5-hydroxytryptamine) autoreceptor in the regulation of the activity of tryptophan hydroxylase was investigated in rat raphe slices. The activity of tryptophan hydroxylase was estimated by measuring the accumulation of 5-hydroxytryptophan in the presence of inhibition of aromatic L-amino acid decarboxylase using 3-hydroxy-4-bromobenzyloxyamine by HPLC with fluorescence detection. Serotonin and its agonists N,N-dimethyl-5-methoxytryptamine and 1-(m-chlorophenyl)-piperazine reduced the formatin of 5-hydroxytryptophan to 50-60% at 10-5 M. The effect of serotonin was reversed by 10-5 M methiothepin, an antagonist of the serotonin autoreceptor. The calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5), dose-dependently reduced the basal formation of 5-hydroxytryptophan to 40-50% at 10-6 and 10-4 M, respectively. W-7 also reduced the activated formation by A-23187 or dibutyryl cyclic AMP in a dose-dependent manner. W-7 had no effect on 5-hydroxytryptophan formation reduced by serotonin at 10-5 M. These results suggest that the role of the serotonin autoreceptor was related to the prevention of the calcium-calmodulin-dependent activation of tryptophan hydroxylase.

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