Prolactin modulates hypothalamic preproenkephalin, but not proopiomelanocortin, gene expression during lactation.

The aim of this study was to examine prolactin (PRL) regulation of preproenkephalin and proopiomelanocortin (POMC) gene expression in the hypothalamus during lactation. In the first experiment, lactating rats were deprived of pups for 3, 6, 12, or 24 h. Preproenkephalin mRNA levels were decreased in the arcuate nucleus (ARC) to 60 or 53% of suckled levels and in the ventromedial nucleus to 70% of suckled levels after 12 or 24 h but were unchanged in the striatum. POMC mRNA levels in the ARC and periarcuate area were increased to 165% of suckled levels within 3 h and remained elevated two- to threefold for 24 h. Subcutaneous administration of bromocriptine to suckled dams markedly suppressed circulating PRL levels and decreased preproenkephalin mRNA signal levels to 38 and 50% of control levels in the arcuate and ventromedial nuclei, respectively. Intravenous administration of oPRL completely reversed this effect. By contrast, bromocriptine with or without administration of ovine PRL (oPRL) did not alter POMC mRNA signal levels in the ARC. Administration of oPRL to pup-deprived dams increased preproenkephalin mRNA levels in the arcuate and ventromedial nuclei and reduced POMC mRNA levels in the ARC to levels similar to suckled control levels. In conclusion, POMC neurons in the ARC appear to be refractory to PRL regulation in the presence of a suckling stimulus, and other components of the suckling stimulus may contribute to the suppression of POMC mRNA levels during lactation. By contrast, PRL provides a regulatory influence for the suckling-induced increase in preproenkephalin mRNA signal levels in arcuate and ventromedial nuclei.