Regional Changes in Brain Oxytocin Receptors Post‐Partum: Time‐Course and Relationship to Maternal Behaviour

In a previous report, receptors for oxytocin in rat brain were reported to increase during the early post‐partum period. The current study set out to replicate this finding using the novel, highly selective oxytocin receptor ligand, [ 125 l]d(CH 2 ) 5 [Tyr(Me) 2 , Thr 4 , Tyr‐NH 2 9 ]OVT ([ 125 I]‐OTA). Binding was measured using in vitro receptor autoradiography in rat brain on Day 15 of pregnancy, on Days 1 and 6 post‐partum, and at least 6 days following the end of lactation. Relative to pregnancy, oxytocin receptor binding was increased on Day 1 (but not Day 6) post‐partum in the bed nucleus of the stria terminalis and the ventromedial nucleus of the hypothalamus. In several other regions, including the anterior olfactory nucleus, the central nucleus of the amygdala, and the ventral subiculum binding was equivalent across the groups. Most surprising, binding in the bed nucleus of the stria terminalis was lowest in the post‐lactating group (post‐partum Day 1 group was 87% higher than post‐lactating group). Saturation studies suggested that binding differences reflected changes in number and not affinity of sites. These findings are consistent with earlier studies of changes in brain oxytocin receptors following exogenous gonadal steroid administration. To determine if maternal behaviour elicited by nonhormonal means (i.e. concaveation) was also associated with increased oxytocin receptors, virgin females with extensive pup exposure were studied. No change in oxytocin receptor binding in the bed nucleus of the stria terminalis was noted in virgin, maternal females—demonstrating that the increase observed post‐partum was not essential for the onset of maternal behaviour.