Maternal behaviour in sheep and its neuroendocrine regulation.

Non-parturient sheep, hormonally primed and presented with newborn lambs are, at best, indifferent to them and if approached by the lamb may show violent rejection. However, non-gestant ewes primed with oestrogen and progesterone, but given vaginocervical stimulation, do show a rapid onset in maternal behaviour. This stimulation is ineffective in promoting maternal behaviour with epidural anaesthesia. Vaginocervical stimulation increases the release of oxytocin into cerebrospinal fluid and in-vivo microdialysis has revealed high levels of oxytocin release in limbic brain areas known to be important for maternal behaviour. Oxytocin, when given intraventricularly, produces the full complement of acceptance and suckling behaviour in non-gestant ewes. Although ineffective when given alone, opioids potentiate the release of oxytocin in the limbic brain and increase the intensity of maternal responding, while the opioid receptor blocker, naltrexone, prevents both maternal induction and oxytocin release. This neural basis for maternally motivated behaviour may be equally relevant to human behaviour, although the mechanisms available for addressing these peptidergic systems have clear differences.