Central mechanisms underlying the neural and neuroendocrine determinants of maternal behaviour

The neuroendocrine and neurochemical events which occur in the brain in the context of maternal behaviour fall into two main categories: 1) Those which simultaneously address wide areas of the brain and lack any specific "coding" for maternal behaviour but are nevertheless essential for it to occur. The steroid hormones and rostrally projecting catecholamine systems fall into this category. The steroid hormones may be viewed as primers, but not just for maternal behaviour, while the amines, specifically noradrenaline, act to synchronise a variety of neural systems associated with maternal behaviour. These include the activation of neuroendocrine mechanisms important for the peripheral changes associated with maternal behaviour, an interaction with the oxytocin and beta-endorphin peptidergic systems which are the specific addressing systems, and enhancement of sensory signals and learning contingent upon parturition. Indeed, parturition is the physiological event which activates the noradrenergic system in the context of maternal behaviour. 2) Those which address restricted areas of the brain and may be viewed as specific to maternal behaviour. Their action is dependent on estrogen priming and the noradrenergic synchronisation of other neural events in order for complete maternal behaviour to ensue. The oxytocinergic and beta-endorphin peptidergic systems fall into this category, promoting maternal behaviour and the neural reinforcement associated with this. In order to ensure specificity, these peptidergic systems may be inhibitory to potentially competing behaviours such as sexual behaviour and the reproductive neuroendocrine response associated with it. Although not discussed in this review, the nigrostriatal dopamine projection may be considered both specific, in that it relates only to motor events, and non-specific, in that this relationship exists in all behavioural contexts. Finally, as maternal behaviour progresses, other parts of the brain are called upon to coordinate the different behaviours that become associated with maternalism and ensure protection and feeding of the offspring. The suckling stimulus appears to be important for activating these neural systems by way of the peripeduncular nucleus.