LAUSR

In addition to its critical role in lactation, the anterior pituitary hormone prolactin also influences a broad range of other physiological processes. In particular, widespread expression of prolactin receptor (Prlr) in the brain has highlighted pleiotropic roles for prolactin in regulating neuronal function, including maternal behaviour, reproduction and energy balance. Research into the central actions of prolactin has predominately focused on effects on gene transcription via the canonical JAK2/STAT5; however, it is evident that prolactin can exert rapid actions to stimulate activity in specific populations of neurones. We aimed to investigate how widespread these rapid actions of prolactin are in regions of the brain with large populations of prolactin‐sensitive neurones, and whether physiological state alters these responses. Using transgenic mice where the Cre‐dependent calcium indicator, GCaMP6f, was conditionally expressed in cells expressing the long form of the Prlr, we monitored changes in levels of intracellular calcium ([Ca2+]i) in ex vivo brain slice preparations as a surrogate marker of cellular activity. Here, we surveyed hypothalamic regions implicated in the diverse physiological functions of prolactin such as the arcuate (ARC) and paraventricular nuclei of the hypothalamus (PVN), as well as the medial preoptic area (MPOA). We observed that, in the ARC of males and in both virgin and lactating females, prolactin can exert rapid actions to stimulate neuronal activity in the majority of Prlr‐expressing neurones. In the PVN and MPOA, we found a smaller subset of cells that rapidly respond to prolactin. In these brain regions, the effects we detected ranged from rapid or sustained increases in [Ca2+]i to inhibitory effects, indicating a heterogeneous nature of these Prlr‐expressing populations. These results enhance our understanding of mechanisms by which prolactin acts on hypothalamic neurones and provide insights into how prolactin might influence neuronal circuits in the mouse brain.