Effect of Prolactin on Voluntary Running Behaviour and Locomotion

Abstract

The regulation of energy expenditure is crucial for maintenance of energy balance within individuals across variable environments. Neuroendocrine regulation of energy expenditure employs multiple neural populations across diverse regions of the brain. These populations are receptive to, and impacted by, circulating hormones released from the pituitary gland and peripheral tissues. One such hormone that has been implicated in energy regulation is prolactin. This study aimed to investigate a role for prolactin within energy expenditure systems, specifically investigating the role of prolactin in regulating voluntary activity measured through running wheel use. Behavioural and anatomical experiments were performed on male and cycling female mice to elucidate the specific functions of prolactin in such systems. Here, we found that prolactin treatment (5 mg/kg, i.p.) induced a significant, acute suppression of wheel running (measured as distance travelled) compared to saline treatment within non-pregnant, wild-type females across the 12 h active phase (n = 12; p < 0.01, paired t-test). In contrast, when experimental parameters were repeated in wild-type males, this suppressive effect was absent (n = 12; p > 0.05, paired t-test). Using female transgenic mice within which prolactin receptors (PrlR) are deleted from GABAergic neurons, the suppressive effect persisted (n = 9; p < 0.05, paired t-test). Following our initial finding, we made progress in discerning behavioural mechanisms underpinning this function of prolactin. We found that prolactin action was specific to voluntary running and did not translate to home-cage activity, measured through locomotion and beam breaks (n = 10; p > 0.05, unpaired t-test). Nor did the hormone affect exploratory behaviour in novel environments, as measured in open field and elevated plus maze testing (both groups n = 8; p > 0.05, unpaired t-test). To begin to determine how prolactin may mediate this action, we examined the effect of prolactin treatment on brain activation following 2 h of wheel use, as well as analysing activation following manipulations of wheel use in transgenic mice designed to report PrlR-expressing neurons. cFos was used to identify neurons activated in response to wheel use. Based on evidence in the literature, we focused on PrlR and cFos expression within the anterior part of the medial subdivision of the bed nucleus of the stria terminalis (BnSTMA). Wheel use was manipulated using locking instruments in transgenic female cohorts that were grouped as follows: unlocked wheels (n = 5), wheels locked on the night of perfusion (acute lock; n = 5) and chronically locked wheels (n = 4). It was hypothesised that cFos expression would be increased in females with unlocked wheels compared to locked groups. Additionally, if the BnSTMA was the region through which prolactin exerted effects, it was hypothesised that the number of co-localised cFos and PrlR-expressing cells would vary across experimental groups. Through dual-labelling immunofluorescence and z-stack imaging and quantification, we observed no difference in cFos or co-localised staining across groups within the BnSTMA following 2 h of activity (p > 0.05; one way ANOVA with Tukey’s post hoc analysis). Additionally, in wild-type females, it was hypothesised that prolactin treatment, delivered prior to wheel running, would reduce cFos expression in this region compared to saline. We found again, however, that the number of activated neurons did not differ across treatments following ~2 h of running (both n = 5; p > 0.05, unpaired t-test). These data suggest that prolactin action in the BnSTMA is not involved in the prolactin-induced suppression of running activity. These results provide evidence for a novel role for prolactin in the acute regulation of energy balance, as well as the specific conditions under which its suppressive effect presents. Evidence for potential anatomical sites and populations through which prolactin acts are presented in this thesis, with the BnSTMA ruled out as a primary site of action. Overall, these results aid in beginning to understand the role of prolactin in regulating energy expenditure, which is important considering the hormone’s adaptive expression across varying physiological states, and between sexes.