The Role of Oxytocin Release in the Paraventricular Nucleus in the Control of Maternal Behaviour in the Sheep

Oxytocin (OT) release within the brain is thought to play a major role in inducing maternal behaviour in a number of mammalian species but little is known about the sites of release which are important in this respect. We have investigated whether the paraventricular nucleus of the hypothalamus (PVN) is a site of OT action on maternal behaviour in the sheep. In vivo microdialysis and retrodialysis was used to determine whether OT is released in the region of the PVN during the post-partum induction of maternal behaviour and if its release at this site can stimulate maternal behaviour in non-pregnant animals. In vivo sampling showed that OT concentrations increased significantly in the region of PVN at birth. When OT was retrodialysed bilaterally into the PVN (1 or 10 mM) of multiparous ewes treated with progesterone and oestradiol to stimulate lactation, maternal behaviour was induced in a significant number of animals (1 mM, 6/8 and 10 mM, 5/8) compared with controls (0/8 ewes). Similar infusions of the ring structure of OT, tocinoic acid (TOC— 10 mM), also induced maternal behaviour in a significant propor tion of animals (5/6 ewes) as did intracerebroventricular (ICV) OT (6/8 ewes) and artificial stimulation of the vagina and cervix (VCS, 8/9 ewes). On the other hand, vasopressin (AVP) 1 mM did not induce maternal behaviour in any ewes and a 10 mM dose only induced it in 2/8 animals. The neurochemical changes accompanying the above treatments were also investigated. Noradrenaline concentrations increased in the PVN after the retrodialysis administration of OT 1 mM and 10 mM, TOC 10 mM and AVP 1 mM, OT ICV and VCS. Dopamine concentrations were also increased by OT 10 mM, TOC 10 mM, AVP 1 mM and OT ICV. Aspartate and glutamate concentrations were significantly reduced by retrodialysis infusions of OT 1 mM and AVP 1 and 10 mM but not by any other treatment. Finally, the retrodialysis infusions of OT and TOC, as well as ICV OT, significantly increased plasma OT release whereas AVP infusions did not. These results provide evidence that OT is released in the PVN during parturition and is important for the induction of maternal behaviour. It seems probable that OT release at this site has a positive feedback effect on both parvocellular and magnocellular OT neurones to facilitate co-ordinated OT release both in central OT terminal regions (to facilitate maternal behaviour) and peripherally into the blood (to facilitate uterine contractions/milk let down). The potential functional roles for the actions of OT on monoamine and amino acid transmitter release in the PVN are discussed.