Abstract
Lactating mice display elevated levels of aggressive behaviour towards intruders in comparison with non-pregnant females. Elevated aggression is a vital maternal adaptation to ensure survival of offspring through increased defensive behaviour against potential danger and threats. Both males and females feature neurons expressing estrogen receptor alpha (ERa) in the ventromedial hypothalamus (VMN) which are involved in modulation of aggressive behaviour. However, whether estrogen signalling itself is involved in upregulating aggressive behaviour during lactation is still unknown. The aim was to investigate whether VMN-specific ERa deletion impacts aggressive behaviour in lactating mice. It was hypothesised ERa deletion from the VMN will reduce aggressive behaviour during lactation. Groups of ERalox/lox female mice received bilateral injections of an adeno-associated virus expressing Cre recombinase (AAV-Cre) or mCherry (AAV- mCherry) into the VMN. Estrous cyclicity and body weight were recorded as a measure of fertility and metabolic changes respectively, and three weeks following AAV injection, mice were mated with stud C57BL/6 males. We tested behaviour between lactation day 2-8, using a resident-intruder test (assess aggression), a pup retrieval test (assess maternal behaviour) and an elevated plus maze (assess anxiety-like behaviour).
Immunohistochemistry confirmed significant (p= 0.0007, Student’s T test) knock-down of ERa in the VMN of mice injected with AAV-Cre, compared with controls. Importantly, ERa expression in the arcuate nucleus (ARC), located adjacent to the VMN, was unaffected (p= 0.1859). Surprisingly, estrous cyclicity was significantly disrupted in VMN ERa-knockdown mice. ERa-knockdown mice spent significantly longer (p= 0.0007, Student’s T test) in the estrus phase compared with control mice, with 6 of 8 mice displaying estrus phase for all 14 days of representative monitoring. Additionally, all ERa-knockdown mice failed to become pregnant. Our data suggest that estrogen signalling through ERa in the VMN may be critical for regulating fertility. Due to the resultant infertility in the ERa-deleted mice we could not assess the role of estrogen signalling on behaviour during lactation. Mice with significant ERa-knockdown also gained more body weight over in the virgin state compared with control mice. Therefore, behavioural testing was undertaken in non-pregnant ERa-knockdown mice and additional virgin control mice (C57BL/6 and ERalox/lox) to assess whether ERa signalling contributes to regulation of aggression in the non-lactating state. We found no significant differences between non-pregnant ERa-knockdown mice and virgin control mice in terms of aggressive behaviour (p= 0.8596, one-way ANOVA), maternal behaviour (p= 0.4096, one-way ANOVA) and anxiety-like behaviour (p= 0.1196, one-way ANOVA). This data indicates that aggression, in a non-pregnant/virgin state is not regulated by estrogen signalling through ERa.