Morphine and corticotrophin-releasing factor potentiate maternal acceptance in multiparous ewes after vaginocervical stimulation.

The effects of intracerebroventricular infusions of morphine, oxytocin, and corticotrophin-releasing factor (CRF) on maternal behaviour alone, or in conjunction with vaginocervical stimulation were determined in both nulliparous and multiparous ovariectomized ewes, primed with oestrogen for 3 days prior to testing. Both plasma and cerebrospinal fluid levels of oxytocin were measured under similar conditions, following CRF and morphine treatments. In nulliparous ewes, i.c.v. oxytocin and morphine reduced rejection behaviours, but neither these treatments nor vaginocervical stimulation promoted proceptive acceptance behaviour. Ewes that were maternally experienced showed increases in acceptance and a reduction of rejection behaviours following vaginocervical stimulation or i.c.v. oxytocin. Morphine and CRF potentiated the effects of vaginocervical stimulation on these behaviours. No treatment influenced oxytocin release per se, but i.c.v. morphine increased both peripheral and central release of oxytocin in response to vaginocervical stimulation. The results show that maternal experience is essential for oxytocin to promote proceptive behaviours and that morphine and CRF potentiate the effects of vaginocervical stimulation on acceptance behaviour. Since morphine also potentiated the release of oxytocin in response to vaginocervical stimulation it is difficult to separate the relative contribution of direct versus indirect effects of opiates in potentiating maternal bonding.