Ionic mechanism for the osmotically-induced depolarization in neurones of the guinea-pig supraoptic nucleus in vitro.

1. Effects of hypertonic solutions prepared by adding NaCl or sucrose to Krebs solution on intracellular potentials were observed in neurones of the supraoptic nucleus using brain slices of the guinea-pig hypothalamus. 2. Hypertonic solutions (306-488 m-osmole/kg) depolarized the membrane, increased the input resistance and augmented the spontaneous firing rate in supraoptic neurones, whereas cells in the hippocampus and anterior or ventromedial hypothalamus were not affected by the hypertonic solutions. 3. The excitatory action of hypertonic solutions on supraoptic neurones was retained in the medium containing 0 mM-Ca2+ and 12 mM-Mg2+. 4. Amplitude of the depolarization induced by superfusion of hypertonic solutions was voltage-dependent. The reversal potential for the depolarization was about -90 mV. 5. The reversal potential for the depolarization induced by hypertonic solution shifted as a function of [K+]0. 6. These results suggest that the supraoptic neurones are themselves osmosensitive and that the local osmotic-related information is transduced to neural signals in these cells by alteration in the membrane ionic permeability, probably due to suppression of K+ conductance.