In Vivo 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

Abstract: The characteristics of the serotonin (5‐HT) output in the dorsal and median raphe nuclei of the rat were studied using in vivo microdialysis. The basal output of 5‐HT increased after KC1 was added to the perfusion fluid. In contrast, neither the omission of calcium ions nor the addition of 0.5 nM tetrodotoxin affected dialysate 5‐HT or 5‐hy‐droxyindoleacetic acid (5‐H1AA). Reserpine did not decrease the output of 5‐HT 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‐pro‐pylamino)tetralin (8‐OH‐DPAT), at 1 or 10 μM, perfused into the raphe did not change the outputs of 5‐HT or 5‐HIAA. Higher doses (0.1, Land 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 vaM dose of 8‐OH‐DPAT perfused into the raphe decreased the hippocampal output of 5‐HT and 5‐HIAA. The systemic 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 extracellular 5‐HT in raphe nuclei originates from a cytoplasmic pool and is not dependent on either nerve impulse of 5‐HT neurons or local activation of 5‐HT1A receptors.