Activation of supraoptic neurosecretory cells by local osmotic stimulation of the median preoptic nucleus

In urethane-anesthetized male rats extracellular recordings were obtained from neurosecretory cells in the supraoptic nucleus (SON) while each of electrical stimulation and local osmotic stimulation produced by pressure injection of hypertonic saline was applied to the median preoptic nucleus (MnPO) or to the medial septal nucleus (MS). Electrical stimulation of the MnPO produced orthodromic excitation or initial inhibition followed by strong excitation in most SON cells ( 59 61 ), and local osmotic stimulation of the MnPO excited majority of cells ( 39 51 ). On the contrary, local osmotic stimulation of the MS did not excite SON cells, although electrical stimulation excited all the cells ( 5 5 ). The results suggest that the MnPO is one of the osmosensitive sites controlling electrical activity of SON neurosecretory cells.