SEROTONIN NEURONAL RELEASE FROM DORSAL HIPPOCAMPUS FOLLOWING ELECTRICAL-STIMULATION OF THE DORSAL AND MEDIAN RAPHE NUCLEI IN CONSCIOUS RATS

We have studied 5-hydroxytryptamine (5-HT) release in the hippocampal formation following electrical stimulation of the dorsal and median ra phe nuclei in the behaving rat. The primary finding in this study is a decrease in neuronal release of serotonin in the dorsal hippocampal f ormation following electrical stimulation of either the dorsal or medi an raphe nucleus in conscious rats. At no time did electrical stimulat ion of either raphe nucleus result in behavioral, including vigilance state, changes. The amount of 5-HT released was found to be frequency dependent with higher frequencies (20 Hz) producing larger decreases i n release of 5-HT However, the pattern of release differs between the two raphe nuclei. Extracellular levels of 5-HT decrease during stimula tion of the dorsal raphe, whereas levels decrease only following cessa tion of stimulation of the median raphe nucleus. This may relate to th e patterns of innervation of the dorsal hippocampal formation by these two midbrain raphe nuclei and also may reflect an inhibition of media n raphe cell firing during stimulation of the dorsal raphe. Electrical stimulation of the dorsal raphe in anesthetized animals resulted in a n enhanced release of 5-HT. The suppression of 5-HT release in the dor sal hippocampal formation in behaving animals was long-lasting (over 2 h), suggesting that the control mechanisms that regulate 5-HT release operate over a long time-course. This difference in release between n on-anesthetized and anesthetized animals may relate to anesthesia bloc king long- and/or short-loop serotonin recurrent axonal collaterals ne gatively feeding back onto 5-HT1A and 5-HT1D somatodendritic autorecep tors on raphe neurons. Further, the anesthetized animal has diminished monoaminergic ''gating'' influences on the hippocampal formation, whe reas the behaving animal is more complex with behavioral (vigilance) s tates associated with different patterns of gating of information flow through the hippocampal formation. Hippocampus 1998,.8:282-273. (C) 1 998Wiley-Liss, Inc.