Induction of enkephalin in tuberoinfundibular dopaminergic neurons during lactation.

Single- and double-labeling immunocytochemistry was used to demonstrate that the tuberoinfundibular dopaminergic (TIDA) neurons of lactating rats, in contrast to those of male or female rats on any day of the estrous cycle, synthesize enkephalin which is colocalized with dopamine. Each enkephalin-immunopositive perikaryon in the dorsomedial and ventrolateral subdivisions of the arcuate nucleus contains dopamine; therefore, the median eminence of lactating rats contains high levels of enkephalin compared to male or female rats. It has been shown that endogenous opiates, including enkephalin, stimulate PRL secretion by reversing the inhibitory action of dopamine at the level of the TIDA neurons. The present findings suggest that enkephalin, coproduced with dopamine in TIDA neurons of lactating rats, may be an endogenous source for this action and maintain elevated PRL and milk secretion during the nonsuckling periods of lactation.