The developmental psychobiology of behavioural plasticity in mice: the role of social experiences in the family unit.
Small perturbations of young animals' sensory experience or hormonal milieu have been shown to alter ontogenetic pathways and to potentially produce huge effects on CNS functioning and behaviour later in life. From a social point of view, variables such as the expression of affiliative bonding and of playful interactions among littermates, the quantity/quality of maternal care, or episodes of maternal or sibling deprivation during critical phases in development, seem to interfere as epigenetic factors with the rigidly ordered temporal sequences of events that occur during the ontogenesis of CNS. This leads to the onset of adaptive neurodevelopmental changes, which are observable within a continuum that encompasses both "normal" individual variability and potential behavioural disorganisation, which in turn will probably be related to profound alteration in the establishment of adult social competence. The present review summarises the more recent work in mice dealing with short-term, as well as long-term modifications, in naturally occurring species-typical social and non-social responses as a function of the early manipulation of social characteristics of the family unit (such as litter gender composition and time of weaning). These analyses were carried out on infant animals, i.e. during the ontogenetic stage of the establishment of social bonding, as well as on pre-pubertal and adult mice and on lactating adult females. Critical issues, such as the respective roles of sibling-sibling and dam-offspring interactions in the shaping of "sibling effects", are also addressed. Overall, these studies indicate that, within their natural range of variation, early patterns of social stimulation are powerful determinants of subsequent behaviour of developing altricial rodents, and confirm that early social life events warrant attention because they can strongly affect neurobehavioural development. Evidence of a relationship between social events occurring during early rearing (i.e. when dramatic transitions in neuroendocrine and neurochemical CNS systems occur) and individual behavioural variability in the infant and adult response to the effects of psychostimulants abused by humans is presented. A better understanding of the mechanisms that mediate such remarkable plasticity might have great psychobiological as well as clinical importance, especially when considering the issue of vulnerability to drug abuse.