A microdialysis study of the in vivo release of 5-HT in the median raphe nucleus of the rat

1 The present study has examined several characteristics of the release of 5-HT in the median raphe nucleus in terms of its dependence of nerve impulse, provenance of a vesicular storage fraction as well as the regulatory role played by 5-HT,A receptors. 2 Tetrodotoxin (1 mu M) and reserpine (5 mg kg(-1), i.p.) virtually suppressed the output of 5-MT. 3 The administration of EEDQ (10 mg kg(-1), i.p.) did not alter the basal release of 5-HT but abolished the reduction of 5-HT release induced by 8-OH-DPAT (0.1 mg kg(-1), s.c.). 4 The perfusion of 1-100 mu M of 8-OH-DPAT or the novel 5-HT1A, agonist BAY x 3702 decreased the efflux of 5-MT, whereas the perfusion of the 5-HT1A antagonist WAY-100635 failed to alter 5-HT release. 5 The decrease in dialysate 5-HT induced by 100 mu M 8-OH-DPAT was reversed by the concurrent perfusion of 100 mu M WAY-100635. Also, the perfusion of 100 mu M WAY-100635 for 2 h inhibited partly the reduction of 5-HT release evoked by the systemic administration of 8-OH-DPAT (0.1 mg kg(-1)). 6 These results indicate that extracellular 5-HT in the median raphe nucleus is stored in vesicles and released in an impulse-dependent manner. Also, the basal release of 5-HT in the median raphe nucleus does not appear to be under the tonic control of somatodendritic 5-HT1A receptors by endogenous 5-HT1A Instead, this feedback mechanism seems to be triggered when an excess of the transmitter or a 5-HT1A agonist is present in the extracellular space of the median raphe nucleus.