INVIVO BRAIN DIALYSIS STUDY OF THE SOMATODENDRITIC RELEASE OF SEROTONIN IN THE RAPHE NUCLEI OF THE RAT - EFFECTS OF 8-HYDROXY-2-(DI-N-PROPYLAMINO)TETRALIN

The characteristics of the serotonin (5-HT) output in the dorsal and m edian raphe nuclei of the rat were studied using in vivo microdialysis . The basal output of 5-HT increased after KCl was added to the perfus ion fluid. In contrast, neither the omission of calcium ions nor the a ddition of 0.5 muM tetrodotoxin affected dialysate 5-HT or 5-hydroxyin doleacetic acid (5-HIAA). Reserpine did not decrease the output of 5-H T and 5-HIAA 24 h later and p-chloroamphetamine increased 5-HT in both vehicle- and reserpine-treated rats severalfold. 8-Hydroxy-2-(di-n-pr opylamino)tetralin (8-OH-DPAT), at 1 or 10 muM, perfused into the raph e did not change the outputs of 5-HT or 5-HIAA. Higher doses (0. 1, 1, and 10 mM) increased extracellular 5-HT in the raphe, probably via an inhibition of uptake. In animals bearing two probes (raphe nuclei and ventral hippocampus), only the 10 mM dose of 8-OH-DPAT perfused into the raphe decreased the hippocampal output of 5-HT and 5-HIAA. The sys temic injection of 0.1 mg/kg 8-OH-DPAT decreased dialysate 5-HT and 5- HIAA in the raphe and hippocampus. These results suggest that extracel lular 5-HT in raphe nuclei originates from a cytoplasmic pool and is n ot dependent on either nerve impulse of 5-HT neurons or local activati on of 5-HT1A receptors.