Evidence that the medial and dorsal raphe nuclei mediate serotonergically-induced increases in prolactin release from the pituitary

Electrolytic lesions of the medial (MR) or dorsal (DR) raphe nucleus significantly antagonized serum prolactin elevations produced by 5-hydroxytryptophan (5-HTP) in rats pretreated with fluoxetine or citalopram, (serotonin (5-HT) uptake blockers). Lesioned animals in which total blockade of serum prolactin elevations was observed also had total blockade of 5-HT accumulation in the median eminence. However, the increase in serum prolactin levels produced by 5-HTP plus 5-HT reuptake blockade in lesioned rats was not significantly different from sham-operated rats if as little as 15-20% of control median eminence accumulation was present. Serum prolactin elevations produced by quipazine, a direct acting 5-HT agonist, were not significantly affected by MR lesions. On the basis of these results, we suggest that: (1) serum prolactin elevations following 5-HT reuptake blockade plus 5-HTP are correlated with 5-HT concentration in the median eminence; (2) lesion-induced antagonism of 5-HTP-induced prolactin elevation is critically dependent upon complete blockade of median eminence 5-HT accumulation; and (3) 5-HT neurons arising from cell bodies located in the MR and DR are necessary for endogenous serotonergically mediated effects on prolactin secretion in the rat.