Unilateral knife cuts were performed in the midbrain of lactating rats and the activities of oxytocin neurones were recorded extracellularly from the supraoptic nuclei (SON) in order to investigate the location of the neural mechanism responsible for the synchronization of milk-ejection bursts of oxytocin neurones in different magnocellular nuclei of the hypothalamus. The lesions involved the mesencephalic lateral tegmentum, the intermedial tegmentum and the central grey. Ninety-six SON neurones were antidromically activated by neurohypophyseal stimulation and were also identified as oxytocin neurones, which included 17 pair-recorded neurones. First, the response of oxytocin neurones recorded from the unilateral SON to bilateral or unilateral suckling was tested. During bilateral suckling, not only the oxytocin neurones recorded from the SON on the intact side (n=34) but also those recorded from the SON on the lesioned side (n=58) displayed milk-ejection bursts. When only the nipples ipsilateral to the lesion were suckled (ipsilateral suckling), bursts were induced in most of the oxytocin neurones on the intact (83·3%, n=12) and lesioned side (88·9%, n=27). In contrast, none of the oxytocin neurones (n=37) produced bursts and none of the rats tested (n=23) showed milk ejections during contralateral suckling. Secondly, some characteristics of the bursts of pair-recorded neurones during bilateral suckling and their response to different modes of suckling were investigated. When oxytocin neurones on both sides displayed milk-ejection bursts, they were always well synchronized but the mean burst amplitude of the neurones on the lesioned side (55·6 ±4·9 spikes, n=43) was significantly (P<0·05) lower than that of the neurones on the intact side (65·7 ±5·6 spikes, n=43). Late-recruited neurones were observed in 6 pairs of oxytocin neurones, and these mainly occurred in the neurones on the lesioned side (5/6). In 5 pair-recorded oxytocin neurones, bursts could also be induced synchronously by ipsilateral suckling but not by contralateral suckling. Thus it is very likely that the major mechanism synchronizing the milk-ejection bursts of oxytocin neurones in the bilateral SON is located in the region rostral to the midbrain.
Journal of Endocrinology (1995) 144, 463–470