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ou,,sil of Nes,oc/,emjvtrv I ippiucotI-Riven Publishers. Philadelphia I 906 lnterriaiioiisl Society or NeurochemistryAbstract: Release of endogenous serotonin [5-hydroxytryptamirie (5-HT)] in the cerebellum of awake rats was characterized using in vivo microdialysis. 5-HT output was increased (-70%) by local application of KCI (100 mM) and was reduced (--60%) by both tetrodotoxin (0.5 pM) and omission of Ca 2 from the perfusion fluid. 5-HT release was decreased (-70%) by the selective 5-HT 1A agonist 8-hydroxy-2-(di-n -propylamino)tetralin (0.25 mg/kg, s.c.), and this effect was rapidly reversed by a selective 5-HT1A antagonist, N-carboxamide trihydrochloride (WAY-i 00635; 0.1 mg /kg, i.p.). These results indicate that a large portion of the measurable 5-HT output in the cerebellum is of neuronal origin, is dependent on impulse flow, and is sensitive to 5-HT1A autoreceptor activation. Further studies examined the relationship between 5-HT levels and general activity of the animals across the light-dark transition and during behavioral manipulations. Both 5-HT levels and behavioral activity were significantly elevated during the dark period, with changes in 5-HT efflux closely paralleling changes in activity. Similar increases (~-40%)in 5-HT output were observed during both feeding and feeding in the presence of a stressor (tail pinch). These findings suggest that behavioral state is an important factor determining neuronal 5-HT release in cerebellum under 6/7

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