EFFECTS OF HEMITRANSECTION OF THE MIDBRAIN ON MILK-EJECTION BURST OF OXYTOCIN NEURONS IN LACTATING RAT

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-e jection bursts of oxytocin neurones in different magnocellular nuclei of the hypothalamus. The lesions involved the mesencephalic lateral te gmentum, the intermedial tegmentum and the central grey. Ninety-six SO N neurones were antidromically activated by neurohypophyseal stimulati on and were also identified as oxytocin neurones, which included 17 pa ir-recorded neurones. First, the response of oxytocin neurones recorde d from the unilateral SON to bilateral or unilateral suckling was test ed. During bilateral suckling, not only the oxytocin neurones recorded from the SON on the intact side (n=34) but also those recorded from t he SON on the lesioned side (n=58) displayed milk-ejection bursts. Whe n only the nipples ipsilateral to the lesion were suckled (ipsilateral suckling), bursts were induced in most of the oxytocin neurones on th e intact (83.3%, n=12) and lesioned side (88 9%, n=27). In contrast, n one of the oxytocin neurones (n=37) produced bursts and none of the ra ts 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 su ckling were investigated. When oxytocin neurones on both sides display ed milk-ejection bursts, they were always well synchronized but the me an 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 neuron es on the intact side (65.7 +/- 5.6 spikes, n=43). Late-recruited neur ones were observed in 6 pairs of oxytocin neurones, and these mainly o ccurred in the neurones on the lesioned side (5/6). In 5 pair-recorded oxytocin neurones, bursts could also be induced synchronously by ipsi lateral suckling but not by contralateral suckling. Thus it is very li kely that the major mechanism synchronizing the milk-ejection bursts o f oxytocin neurones in the bilateral SON is located in the region rost ral to the midbrain.