INDUCTION OF ENKEPHALIN IN TUBEROINFUNDIBULAR DOPAMINERGIC-NEURONS OFPREGNANT, PSEUDOPREGNANT, LACTATING AND AGED FEMALE RATS

The tuberoinfundibular dopaminergic (TIDA) neurons projecting to the e xternal zone of the median eminence arise in the dorsomedial and ventr olateral subdivisions of the arcuate nucleus. In cycling female rates these regoins contain only scattered enkephalin-immunoreactive (ENK-i) neurons some of which coexpress dopamine, detected by immunostaining for tyrosine hydroxylase (TH). The present immunocytochemical, in situ hybridization and retrograde-labeling studies show that each TIDA neu ron of pregnant, pseudopregnant, lactating, and aged female rats conta ins ENK-like immunoreactivity and pro-ENK mRNA and projects to the hyp ophysial portal circulation. Ovariectomy of lactating and aged rats di d not change ENK staining within TIDA neurons, suggesting that ovarian steroids do not play a critical role in the colocalization of ENK and dopamine. Since prolactin levels are elevated in each of these experi mental animals, a possible role for prolactin in the induction of the ENK gene in TIDA neurons is suggested. Prolactin stimulates dopamine a nd its own secretion via a short-loop feedback mechanism. The sensitiv ity of this regulatory mechanism is altered in these experimental anim als, resulting in elevated prolactin secretion. ENK, which has prolact in-releasing activity and is colocalized with dopamine, could mediate the positive short-loop feedback regulation and sustain elevated level s of prolactin in pregnant, pseudopregnant, lactating, and aged animal s.