Litter size determines the number of melanin-concentrating hormone neurons in the medial preoptic area of Sprague Dawley lactating dams.

Melanin-concentrating hormone [MCH] is an important neuromodulator related to motivated behaviors. The MCH-containing neurons are mainly located in the lateral hypothalamic area, zona incerta, and incerto-hypothalamic area. In the medial preoptic area [MPOA], a key region for the regulation of maternal behavior, Pmch mRNA expression and MCH synthesis can be detected exclusively during the lactation period. As litter size affects different parameters of maternal physiology, the aim of this study was to verify whether litter size can modulate the number of MCH-containing neurons in the MPOA of lactating rats. The dams were divided into the following groups: postpartum day 12, 15, or 19, with a large, small or reduced litter. Our results show that the number of MCH-immunoreactive neurons in the MPOA is positively correlated with the number of pups in the litter and that artificially reducing the number of pups can also decrease the number of MCH-immunoreactive neurons in the MPOA.