In the present study, we investigated the role of prolactin (PRL) in the suckling-induced increase in hypothalamic neuropeptide Y (NPY) gene expression in the dorsomedial nucleus of the hypothalamus (DMH) and the caudal portion of the arcuate nucleus of the hypothalamus (ARH-C). Lactating rats were deprived of their eight-pup litters on d 9 postpartum. After 48 h, the animals were randomly divided into two groups: nonsuckled controls and eight pups suckling for 24 h. In addition, some of the suckled animals received two injections of bromocriptine (0.5 mg/rat per injection) to inhibit suckling-induced PRL secretion. Some bromocriptine-treated rats also received ovine PRL (1 mg/rat per injection). In situ hybridization was performed to measure NPY mRNA levels. Suckling for 24 h induced a significant increase in NPY mRNA levels in the DMH and ARH-C. Bromocriptine treatment greatly attenuated the increase of NPY mRNA in the DMH but not in the ARH. Injections of ovine PRL in bromocriptine-treated rats greatly restored DMH NPY mRNA levels but had no additional effects on the ARH NPY expression. Double-label in situ hybridization for NPY and PRL receptor (PRL-R) in the lactating rat brains showed that NPY-positive neurons in the DMH also express PRL-R mRNA. On the contrary, few ARH NPY neurons expressed PRL-R. These data suggest that PRL could act directly on DMH NPY neurons to modulate NPY gene expression during lactation. Thus, the results from the present study demonstrate that NPY neurons in the DMH and ARH are differentially regulated by PRL during lactation.
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