Efficient males (A, C) have lower levels of binding in the rostral MPOA compared to inefficient (B, D) rats. Sections containing the caudal MPN that were incubated with oxytocin receptor radioligand show high levels of binding (E). Adjacent sections also containing the caudal MPN, from the same animal, that were incubated with the OTR radioligand and an OTA showed almost no binding (F). MPOA, medial preoptic area; MPN, medial preoptic nucleus; BNST, bed nucleus of the stria terminalis.