Regulation of serotonin synthesis and release in rat striatum

Abstract

Evidence is presented indicating that cAMP stimulates tryptophan hydroxylase (TrpH), the rate-limiting enzyme for serotonin (5-HT) synthesis. Forskolin, an activator of adenylate cyclase, stimulated 5-HT formation in synaptoneurosomes prepared from rat brain striatum. 8-thio-methyl cAMP enhanced soluble TrpH activity, supporting a direct role of cAMP, probably via protein kinase A (PK-A), in regulating TrpH. In addition to a direct activation of TrpH by PK-A, it was considered whether cAMP induced changes in 5-HT release and thereby indirectly influenced TrpH. The synthesis and release of {dollar}\sp3{dollar}H-5-HT newly synthesized from {dollar}\sp3{dollar}H-tryptophan was evaluated. Forskolin increased the absolute amount of {dollar}\sp3{dollar}H-5-HT released; however the fractional efflux of {dollar}\sp3{dollar}H-5-HT was unchanged. Furthermore, forskolin did not influence 5-HT release from synaptosomes prelabeled with {dollar}\sp3{dollar}H-5-HT. Thus, forskolin increases the pool of releasable 5-HT, but does not stimulate 5-HT synthesis by triggering 5-HT release.;Both basal and forskolin-stimulated 5-HT formation were inhibited by calmidazolium, and anti-calmodulin (CaM) agent. Calmidazolium did not inhibit cAMP formation or 5-HT release. It is concluded that basal and forskolin-stimulated 5-HT synthesis requires CaM-dependent processes independent 5-HT release.;5-HT itself inhibited 5-HT formation; zimelidine, which inhibits 5-HT uptake, diminished this effect. The 5-HT agonists 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole succinate (RU-24969), trimethylphenylpiperazine (TFMPP), and 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT) did not inhibit 5-HT synthesis. These results suggest that 5-HT-inhibited 5-HT synthesis is independent of 5-HT{dollar}\sb{lcub}\rm 1a \ or \ 1b{rcub}{dollar} receptors and may involve intraneuronal entry of 5-HT.;The 5-HT agonists RU-24969, TFMPP, and DPAT stimulated efflux of prelabeled {dollar}\sp3{dollar}H-5-HT. Chlorimipramine, a 5-HT uptake inhibitor, reversed these effects, suggesting that the 5-HT uptake carrier interacts with 5-HT agonists to regulate 5-HT release. In the presence of chlorimipramine, RU-24969 diminished K{dollar}\sp+{dollar}-evoked 5-HT release, presumably by an action at the 5-HT{dollar}\sb{lcub}\rm 1b{rcub}{dollar} autoreceptor. These studies, as well as studies of {dollar}\sp3{dollar}H-DPAT and {dollar}\sp3{dollar}H-5-HT uptake, indicate that the agonist-induced efflux of 5-HT involves the binding of agonist to the uptake carrier without carrier-specific neuronal entry of the agonist and subsequent exchange of agonist for intraneuronal 5-HT. Thus, certain 5-HT{dollar}\sb1{dollar} agonists may act on the outside surface of the uptake carrier to regulate 5-HT release and uptake, but not synthesis, similar to the actions of tricyclic and related antidepressants.