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The effects of serotonin (5-HT) on the release of endogenous glutamate (GLU) in rat cerebellum were investigated in slices depolarized with 35 mM K+. The Ca2+-dependent release of GLU was potently inhibited by 5-HT in a concentration-dependent way. Release was also inhibited by the 5-HT1 receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). The inhibition by 10 nM 5-HT was partly (35-40%) counteracted by the 5-HT2 receptor antagonist ketanserin but was fully blocked by the mixed 5-HT1/5-HT2 receptor antagonist methiothepin. The effect of 8-OH-DPAT was not affected by ketanserin but was totally antagonized by methiothepin, while the effect of DOI was entirely suppressed by ketanserin. Ketanserin or methiothepin themselves increased (by 23 and 55%, respectively, at 10 nM) the K+-evoked release of GLU. In conclusion the release of endogenous GLU in rat cerebellum can be inhibited by 5-HT through receptors of the 5-HT1 and 5-HT2 type. The enhancement of GLU release by ketanserin or methiothepin could suggest a tonic inhibition. The possible localization of the 5-HT receptors involved in the interaction with the GLU systems is discussed.

The 5-hydroxytryptamine (5-HT) autoreceptors mediating inhibition of [3H]5-HT release in rat hippocampus have been characterized pharmacologically in terms of 5-HT receptor subtype by using superfused synaptosomes depolarized with 15 mM KCl. Exogenous 5-HT inhibited in a concentration-dependent way (pEC30 = 8.74) the K+-evoked release of [3H]5-HT. Methiothepin shifted the concentration-response curve of 5-HT to the right (pA2 = 8.62). The 5-HT2 receptor antagonists, ketanserin, methysergide or spiperone were ineffective against 5-HT. The 5-HT1 receptor agonist, 5-methoxy-3-[1,2,3,6-tetrahydropyridin-4-yl]-1H-indole (RU 24969) mimicked 5-HT and was equipotent as an inhibitor of the release of [3H]5-HT. In contrast, the putative 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was almost ineffective at 1 microM. Finally, (-)propranolol, used as a non-selective 5-HT1A/5-HT1B receptor antagonist, shifted to the right (pA2 = 7.91) the concentration-response curve of 5-HT whereas the 5-HT1C receptor antagonist mesulergine was ineffective. In conclusion, 5-HT nerve terminals of rat hippocampus possess autoreceptors which appear to belong to the 5-HT1B subtype.