INDUCTION OF PROENKEPHALIN IN TUBEROINFUNDIBULAR DOPAMINERGIC-NEURONSBY HYPERPROLACTINEMIA - THE ROLE OF SEX STEROIDS

The observation that tuberoinfundibular dopaminergic (TIDA) neurons of pregnant, pseudopregnant, lactating, and aged rats express enkephalin s suggested that chronically elevated PRL levels, which are characteri stic for these animals, are essential for the induction of proenkephal in gene expression in TIDA neurons. The present studies investigated f urther the role of PRL in this phenomenon. Elevated PRL levels were ac hieved either experimentally by implanting anterior pituitaries under the kidney capsule of intact or hypophysectomized female rats or by us ing lactating rats. For controls, the elevated PRL levels were reduced with bromocryptine, a dopamine receptor agonist. The role of sex ster oids in PRL-induced enkephalin gene expression was also studied in cyc ling, sex hormone-treated, hypophysectomized or ovariectomized rats, p ituitary-implanted/sex hormone-treated rats, and ovariectomized mother s. Enkephalin immunoreactivity was detected by immunocytochemistry and enkephalin messenger RNA with in situ hybridization histochemistry us ing S-35- or digoxigenin-labeled riboprobes. Enkephalin or its messeng er RNA was present in TIDA neurons in all experimental animals with el evated PRL levels. Although estradiol had no or only a minor effect on PRL-induced enkephalin gene expression, progesterone supported the ef fect of PRL. The present observations suggest that the expression of e nkephalin in TIDA neurons is PRL dependent and supported by sex steroi ds, primarily progesterone.