Social isolation prompts maternal behavior in sexually naïve male ddN mice

A comprehensive approach to modeling maternal immune activation in rodents

Prenatal brain development is a highly orchestrated process, making it a very vulnerable window to perturbations. Maternal stress and subsequent inflammation during pregnancy leads to a state referred to as, maternal immune activation (MIA). If persistent, MIA can pose as a significant risk factor for the manifestation of neurodevelopmental disorders (NDDs) such as autism spectrum disorder and schizophrenia. To further elucidate this association between MIA and NDD risk, rodent models have been used extensively across laboratories for many years. However, there are few uniform approaches for rodent MIA models which make not only comparisons between studies difficult, but some established approaches come with limitations that can affect experimental outcomes. Here, we provide researchers with a comprehensive review of common experimental variables and potential limitations that should be considered when designing an MIA study based in a rodent model. Experimental variables discussed include: innate immune stimulation using poly I:C and LPS, environmental gestational stress paradigms, rodent diet composition and sterilization, rodent strain, neonatal handling, and the inclusion of sex-specific MIA offspring analyses. We discuss how some aspects of these variables have potential to make a profound impact on MIA data interpretation and reproducibility.

Social Network Plasticity of Mice Parental Behavior

Neural plasticity occurs during developmental stages and is essential for sexual differentiation of the brain and the ensuing sex-dependent behavioral changes in adults. Maternal behavior is primarily affected by sex-related differences in the brain; however, chronic social isolation even in mature male mice can induce maternal retrieving and crouching behavior when they are first exposed to pups. Social milieus influence the inherent behavior of adults and alter the molecular architecture in the brain, thereby allowing higher levels of associated gene expression and molecular activity. This review explores the possibility that although the development of neural circuits is closely associated with maternal behavior, the brain can still retain its neuroplasticity in adults from a neuromolecular perspective. In addition, neuronal machinery such as neurotransmitters and neuropeptides might influence sociobehavioral changes. This review also discusses that the neural circuits regulating behaviors such as parenting and infanticide (including neglect behavior), might be controlled by neural relay on melanin concentrating hormone (MCH)–oxytocin in the hypothalamus during the positive and negative mode of action in maternal behavior. Furthermore, MCH–oxytocin neural relay might contribute to the anxiolytic effect on maternal behavior, which is involved with reward circuits.

Neural Contributions of the Hypothalamus to Parental Behaviour

Parental behaviour is a comprehensive set of neural responses to social cues. The neural circuits that govern parental behaviour reside in several putative nuclei in the brain. Melanin concentrating hormone (MCH), a neuromodulator that integrates physiological functions, has been confirmed to be involved in parental behaviour, particularly in crouching behaviour during nursing. Abolishing MCH neurons in innate MCH knockout males promotes infanticide in virgin male mice. To understand the mechanism and function of neural networks underlying parental care and aggression against pups, it is essential to understand the basic organisation and function of the involved nuclei. This review presents newly discovered aspects of neural circuits within the hypothalamus that regulate parental behaviours.

The social network: Neural control of sex differences in reproductive behaviors, motivation, and response to social isolation

Social animal species present a vast repertoire of social interactions when encountering conspecifics. Reproduction-related behaviors, such as mating, parental care, and aggression, are some of the most rewarding types of social interactions and are also the most sexually dimorphic ones. This review focuses on rodent species and summarizes recent advances in neuroscience research that link sexually dimorphic reproductive behaviors to sexual dimorphism in their underlying neuronal circuits. Specifically, we present a few possible mechanisms governing sexually-dimorphic behaviors, by hypothalamic and reward-related brain regions. Sex differences in the neural response to social isolation in adulthood are also discussed, as well as future directions for comparative studies with naturally solitary species.

Involvement of MCH-oxytocin neural relay within the hypothalamus in murine nursing behavior

Multiple sequential actions, performed during parental behaviors, are essential elements of reproduction in mammalian species. We showed that neurons expressing melanin concentrating hormone (MCH) in the lateral hypothalamic area (LHA) are more active in rodents of both sexes when exhibiting parental nursing behavior. Genetic ablation of the LHA-MCH neurons impaired maternal nursing. The post-birth survival rate was lower in pups born to female mice with congenitally ablated MCH neurons under control of tet-off system, exhibiting reduced crouching behavior. Virgin female and male mice with ablated MCH neurons were less interested in pups and maternal care. Chemogenetic and optogenetic stimulation of LHA-MCH neurons induced parental nursing in virgin female and male mice. LHA-MCH GABAergic neurons project fibres to the paraventricular hypothalamic nucleus (PVN) neurons. Optogenetic stimulation of PVN induces nursing crouching behavior along with increasing plasma oxytocin levels. The hypothalamic MCH neural relays play important functional roles in parental nursing behavior in female and male mice.

With a little help from her friends (and family) part II: Non-maternal caregiving behavior and physiology in mammals

The diversity of competing frameworks for explaining the evolution of non-maternal care in mammals (Part I, this issue) reflects the vast range of behaviors and associated outcomes these theories attempt to subsume. Caretaking comprises a wide variety of behavioral domains, and is mediated by an equally large range of physiological systems. In Part II, we provide an overview of how non-maternal care in mammals is expressed, the ways in which it is regulated, and the many effects such care has on both recipients and caretakers. We also discuss the two primary ways in which closer integration of ultimate and proximate levels of explanation can be useful when addressing questions about non-maternal caretaking. Specifically, proximate mechanisms provide important functional clues, and are key to testing theory concerning evolutionary tradeoffs. Finally, we highlight a number of methodological and publication biases that currently shape the literature, which provide opportunities for knowledge advancement in this domain going forward. In this conclusion to our two-part introduction, we provide a broad survey of the behavior and physiology that the contributions to this special issue represent.

Vomeronasal signal deficiency enhances parental behavior in socially isolated male mice

We previously reported that social isolation promotes parental care in sexually naïve male mice. This effect was blocked by exposure to chemosensory and auditory social signals derived from males in an adjacent compartment. In the present study, we examined whether the chemosensory signals detected in the vomeronasal organ (VNO) are involved in parental behavior by using mice deficient for a VNO-specific ion channel (Trpc2^-/-) and thus impaired in VNO-input signaling. We housed virgin homozygous Trpc2^-/- and heterozygous Trpc2^± males for 3weeks during puberty (5-8weeks old) alone or in groups of 3-5 males. At 8weeks of age, the mice were placed with three pups in an observation cage and tested for parental behavior. The Trpc2^-/- males housed under isolated conditions spent significantly longer in the vicinity of pups than did the Trpc2^-/- males than had been group housed, whereas no isolation effect was observed in heterozygous Trpc2^± males. Both Trpc2 knockout and isolation housing significantly increased the time males spent licking pups and crouching (arched back posture over pups to enable nursing), whereas only isolation housing increased the incidence of retrieval behavior. These results demonstrated that social signals transmitted not only through the VNO but also from other modalities, independent of each other, suppress the expression of parental behavior during puberty in sexually naïve males.