Litter sex composition influences plasma prolactin levels but not the melanin-concentrating hormone immunoreactive neurons in the medial preoptic area of late lactating Long-Evans rats.

This study examines the influence of litter sex composition on melanin-concentrating hormone immunoreactive (MCH-ir) neurons in the ventromedial medial preoptic area (vmMPOA) and on plasma prolactin levels in lactating rats. MCH is a critical regulator of maternal behavior and displays sexual dimorphism within the MPOA, making it an important target for understanding neuroendocrine adaptations in lactation. Prolactin, a pivotal hormone in lactation and maternal care, was also assessed to elucidate its interaction with litter sex composition. Thirty lactating female rats were divided into five experimental groups based on litter sex composition: all-male (10 male pups), all-female (10 female pups), balanced control (five male and five female pups), predominantly male (seven male and three female pups), and predominantly female (three male and seven female pups). On post-partum day 19 (PPD19), the dams were euthanized for biological analysis. Blood samples were collected for plasma prolactin quantification, and the brains were processed to analyze MCH-ir neurons in the vmMPOA. Results showed no significant differences in food and water intake or the number of MCH-ir neurons in the vmMPOA among experimental groups. However, significant variation in prolactin levels was observed, with the all-male offspring group exhibiting the highest levels (mean prolactin level 23.9 ng/mL, p < .001), followed by the all-female group (20.3 ng/mL, p < .01), compared to the control group (14.3 ng/mL). Additionally, the all-male group showed a reduction in body weight gain. These results suggest that although litter sex composition does not alter the number of MCH-ir neurons in the vmMPOA, it significantly impacts maternal prolactin levels. This differential prolactin regulation may reflect distinct physiological demands or caregiving behaviors imposed by homogeneous litters, which could, in turn, influence maternal energy balance, lactation efficiency, and adaptive maternal responses. Understanding these sex-specific influences on maternal neuroendocrine function has important implications for comprehending maternal care dynamics and energy allocation during lactation.