The milk-ejection reflex was studied in anaesthetized, lactating Wistar rats in order to evaluate the contribution of the paraventricular nucleus (PVN) to the patterning of milk ejection and the facilitatory action of centrally administered oxytocin. In the first series of experiments, radiofrequency lesions were performed and centred: (1) antero-dorsal to the PVN, damaging parts of the medial septum and anterior hypothalamus; (2) in the PVN, such that much of the parvocellular division was

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... royed, but parts of the magnocellular division remained intact; or (3) in the PVN, destroying both parvocellular and magnocellular divisions. Suckling tests performed before and after lesioning showed that the milk-ejection interval was significantly increased (decreased frequency) after lesioning in groups 2 and 3, but that milk-ejection amplitude was significantly decreased only in group 3. These results suggest that damage to the parvocellular division of the PVN affects milk-ejection frequency, but that damage to the magnocellular PVN only affects amplitude. Subsequent tests on rats injected into the PVN with the neurotoxin N-methyl-d,l-aspartate revealed a fall in the amplitude and frequency of milk ejection, similar to that after complete radiofrequency lesions of the PVN. In the second series of experiments, the facilitatory action of centrally administered oxytocin (1 mU, 2.2 ng) was examined in animals bearing either sham or complete PVN lesions. In both groups, intracerebroventricular injection of oxytocin was able to increase the frequency of milk ejections, although the incidence of milk ejection was lower in the pre- and post-injection period in the PVN-lesioned animals. In conclusion, the parvocellular component of the PVN may be an important site for regulating milk-ejection frequency, possibly mediated by its centrally projecting oxytocin neurones. However, the PVN does not appear to be the principle target site by which central oxytocin exerts its facilitatory effect on the frequency of milk ejection. Journal of Endocrinology (1990) 125, 467–475