[Effects of lactation, lactation-cessation and lactation-cessation-lactation paradigms on hypothalamic orexin-A immunoreactive neurons in rats].

ABSTRACT

Orexin-A is a novel neuropeptide produced by neurons mainly located in lateral hypothalamic area that potently facilitates appetite and food intake. The purpose of this study was to investigate the possible change in orexin-A immunoreactivity in suckling-induced hyperphagia. By using immunohistochemistry and image analysis techniques we examined orexin-A-like immunoreactivity in a series of rat brain sections corresponding to the hypothalamus in groups of non-lactating, lactating, lactating with overnight cessation of suckling, lactating and cessation followed by resumed short-term sucklings. Long-term lactation significantly increased daily food intake on day 3 (81%) and day 11 (180%) postpartum compared to that in non-lactating postpartum rats, whereas daily food intake was significantly decreased by overnight cessation of suckling on day 11 postpartum in long-term lactating rats (45%). Moreover, long-term lactating rats on day 12 postpartum exhibited significantly greater number and higher mean staining intensity of orexin-A immunoreactive neurons than those of non-suckling postpartum rats (P<0.001 and P<0.05, respectively). Overnight cessation of lactation in rats on day 12 postpartum significantly decreased both the number and mean staining intensity of orexin-A immunoreactive neurons compared to those in long-term lactating group of rats (P<0.001 and P<0.05, respectively), similar to the levels in the non-lactating postpartum rats. Resumed lactation for 2 and 5 h after overnight cessation of lactation significantly increased the number (P<0.001 and P<0.05, respectively) and mean staining intensity (P<0.05) of orexin-A immunoreactive neurons compared to those in the rats without resumed lactation. Both long-term lactation and short-term resumed suckling enhanced orexin-A immunoreactivity in the hypothalamus in rats, and overnight cessation of lactation down-regulated the increased orexin-A immunoreactivity induced by long-term lactation. Suckling may regulate orexin-A expression in the hypothalamus and the increased orexin-A may be involved in hyperphagia in lactating rats, suggesting the possibility of the existence of some neural-humoral links between suckling and hypothalamic orexin-A-immunoreactive neurons.