The brain vasopressin system mediates maternal behaviour in lactating rats - impact of V1b receptors in hypothalamic and limbic brain regions

Adequate maternal behaviour offers the best chance for the offspring to survive to maturity. This pro-social behaviour is known to be regulated by the nonapeptide arginine-vasopressin (AVP) amongst other peptidergic and non-peptidergic systems in rodents as well as in humans. Most research regarding the AVP system in maternal behaviour has focussed on the peptide itself and its V1a receptor (V1aR), and revealed a pivotal involvement in the regulation of different aspects of maternal behaviour. In addition, the involvement of AVP and its V1aR was investigated in the animal model of rats selected for extremes in anxiety-related behaviour. Highly anxious dams (HAB) have a single nucleotide polymorphism in the AVP promoter-region, resulting in increased expression and release of AVP. Regarding the phenotype, HAB dams show a hyper-protective mothering style in contrast to dams selected for low anxiety-related behaviour (LAB), which only provide a low amount of maternal behaviour. Since the maternal phenotype of HAB and LAB dams can be partly reversed by blockade or infusion of V1aR or synthetic AVP, respectively, these rats provide a good model to observe the behavioural consequences after pharmacological manipulation of the AVP system on maternal behaviour. The influence of V1b receptors (V1bR) on mediating maternal behaviour has only been investigated in a single study in V1bR knockout mice to date. This study showed that V1bR knockout dams show decreased maternal aggression compared to wildtype dams. Therefore, I aimed to investigate in more detail the role of central V1bR on the maternal repertoire via acute central application of V1bR agonist and antagonist during lactation in non-selected rats (Chapter 2) as well as in the animal model of rats selected for extremes in anxiety-related behaviour (Chapter 3). Although the V1bR is widely distributed within the brain, I aimed to investigate the involvement of the receptor subtype in brain-regions known to be involved in maternal behaviour, including the medial preoptic area (MPOA), the bed nucleus of the stria terminalis (BNST) and the paraventricular nucleus (PVN) via acute local infusion of V1bR antagonist. In addition, I determined whether endogenous V1bR protein and/or mRNA expression differ in lactation within these brain regions (Chapter 4 -5). Further, in a pilot study co-expression of AVP and gonadotropin-releasing hormone (GnRH) immune-reactivity in the organum vasculosum of the lamina terminalis (OVLT) in lactating rats was observed. Since mice with a 30 % decrease of GnRH neurons show impaired pup retrieval, and the co-localization suggests an interaction with the AVP system known to be involved in maternal behaviour, I was interested in the influence of the GnRH and the interconnected kisspeptin system in the regulation of maternal behaviour in rodents (Chapter 6). To investigate my aims I used a variety of cellular, molecular and behavioural approaches like quantitative real-time PCR, western blotting, central or local acute pharmacological manipulation of V1bR, GnRH receptors or kisspeptin receptors followed by observations of maternal care, maternal motivation to retrieve pups, maternal aggression and anxiety-related behaviour. In this thesis I showed that central blockade of V1bR in non-selected rats impaired maternal care but did not affect maternal motivation, maternal aggression or anxiety-related behaviour. V1bR manipulation in HAB and LAB dams had no effect on any maternal behaviour. When investigating the role of V1bR in specific brain regions, I could show that blockade of V1bR within the MPOA increased maternal motivation whereas within the BNST maternal aggression, more precisely pup-directed behaviour, was decreased. Additionally, V1bR blockade in both brain regions decreased nursing, but only under stress conditions. Interestingly, in the PVN only V1aR, but not V1bR blockade decreased nursing under non-stress conditions in addition to maternal anxiety. Thus, both V1R subtypes seem to be involved in the regulation of maternal behaviour in a brain region-dependent and context-specific manner, thereby acting exclusively, as complement or as counterpart of the other receptor subtype. Interestingly, this was not associated with alterations in mRNA or protein expression in any brain-region, suggesting that either transport of the receptor-subtype to the membrane, signal transduction or agonist sensitivity may be altered in lactation, which remains to be investigated. Regarding the involvement of GnRH and kisspeptin in the regulation of maternal behaviour, blocking GnRH decreased maternal aggression, whereas kisspeptin receptor (GPR54) blockade resulted in impaired maternal care, in a dose-dependent manner. These results provide evidence, that the GnRH and kisspeptin system are not only involved in reproduction, but also in various aspects of maternal behaviour. Further investigation of their role, especially in maternal brain circuits, is essentially required. In conclusion, the presence of V1bR in brain regions involved in maternal behaviour are mandatory in lactating rodents to show adequate maternal behaviour, especially in stressful situations. Further, maternal behaviour is regulated by a variety of hormones acting centrally within the brain like oxytocin and AVP, whereby hormones like GnRH and kisspeptin, thought to be mainly responsible in reproduction also influence maternal behaviour. Taken all these results into account I could provide further knowledge of the impact of central hormonal systems on the complexity of regulatory mechanism in maternal behaviour