THE ROLE OF OXYTOCIN RELEASE IN THE PARAVENTRICULAR NUCLEUS IN THE CONTROL OF MATERNAL-BEHAVIOR 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 l ittle is known about the sites of release which are important in this respect. We have investigated whether the paraventricular nucleus of t he hypothalamus (PVN) is a site of OT action on maternal behaviour in the sheep. In vivo microdialysis and retrodialysis was used to determi ne whether OT is released in the region of the PVN during the post-par tum induction of maternal behaviour and if its release at this site ca n stimulate maternal behaviour in non-pregnant animals. In vivo sampli ng showed that OT concentrations increased significantly in the region of PVN at birth. When OT was retrodialysed bilaterally into the PVN ( 1 or 10 mu M) Of multiparous ewes treated with progesterone and oestra diol to stimulate lactation, maternal behaviour was induced in a signi ficant number of animals (1 mu M, 6/8 and 10 mu M, 5/8) compared with controls (0/8 ewes). Similar infusions of the ring structure of OT, to cinoic acid (TOC-10 mu M), also induced maternal behaviour in a signif icant proportion of animals (5/6 ewes) as did intracerebroventricular (ICV) OT (6/8 ewes) and artificial stimulation of the vagina and cervi x (VCS, 8/9 ewes). On the other hand, vasopressin (AVP) 1 mu M did not induce maternal behaviour in any ewes and a 10 mu M dose only induced it in 2/8 animals. The neurochemical changes accompanying the above t reatments were also investigated, Noradrenaline concentrations increas ed in the PVN after the retrodialysis administration of OT 1 mu M and 10 mu M, TOC 10 mu M and AVP 1 mu M, OT ICV and VCS. Dopamine concentr ations were also increased by OT 10 mu M, TOC 10 mu M, AVP 1 mu M and OT ICV. Aspartate and glutamate concentrations were significantly redu ced by retrodialysis infusions of OT 1 mu M and AVP 1 and 10 mu M but not by any other treatment. Finally, the retrodialysis infusions of OT and TOC, as well as ICV CT, significantly increased plasma CT release whereas AVP infusions did not. These results provide evidence that OT is released in the PVN during parturition and is important for the in duction of maternal behaviour. It seems probable that OT release at th is site has a positive feedback effect on both parvocellular and magno cellular OT neurones to facilitate co-ordinated OT release both in cen tral OT terminal regions (to facilitate maternal behaviour) and periph erally into the blood (to facilitate uterine contractions/milk let dow n). The potential functional roles for the actions of CT on monoamine and amino acid transmitter release in the PVN are discussed.