Osmosensitivity of preoptic thermosensitive neurons in hypothalamic slices in vitro

The effects of local osmotic changes on the activity of preoptic thermosensitive neurons were investigated in rat hypothalamic slices in vitro. Thirty-seven (53%) of 70 neurons recorded from the medial preoptic nucleus (MPO) (66% of thermosensitive neurons and 12% of thermally insensitive neurons) changed their firing rates in response to alterations in local osmolality of less than 15 mOsm/kg. The minimum change in osmolality to produce the neuronal response for six neurons tested was found to be less than 5 mOsm/kg. Statistical analysis revealed that there was a higher incidence of warm-sensitive neurons inhibited by hyperosmolality (50% of warm-units) and of thermally insensitive neurons which were osmotically insensitive (88%). None of the four warm-sensitive neurons tested lost either their osmosensitivity or thermosensitivity during synaptic blockade, and were taken to possess an inherent sensitivity to both temperature and osmolality. The phenomenon of reduced evaporative heat loss in dehydrated mammals may be explained, at least in part, by the reduced activity of MPO warm-sensitive neurons in a hyperosmotic environment.