OSMOLARITY MODULATES K+ CHANNEL FUNCTION ON RAT HIPPOCAMPAL INTERNEURONS BUT NOT CA1 PYRAMIDAL NEURONS

1. Whole-cell and single-channel recording methods were used in conjun ction with infrared video microscopy techniques to examine the propert ies of voltage-activated potassium channels in hippocampal neurons dur ing the application of hyposmolar solutions to hippocampal slices from rats. 2. Hyposmolar external solutions (osmolarity reduced by 10% to 267 mosmol l(-1)) produced a significant potentiation of voltage-activ ated K+ current on lacunosum/moleculare (L/M) hippocampal interneurons , but not on CAl and subiculum pyramidal neurons. Hyperpolarization-ac tivated (I-H) and leak currents were not altered during the applicatio n of hyposmolar solutions in all cell types. 3. Mean channel open time and the probability of channel opening were dramatically increased un der hyposmolar recording conditions for outside-out patches from L/M i nterneurons; no changes were observed for patches from CAl pyramidal n eurons. Mean current amplitude and the threshold for channel activatio n were not affected by hyposmotic challenge. 4. Hyposmolar external so lutions produced a significant reduction in the firing frequency of L/ M interneurons recorded in current-clamp mode. Hyposmolar solutions ha d no effect on resting membrane potential, action potential amplitude or duration, and spike afterhyperpolarization amplitude. 5. These resu lts indicate that selective modulation of interneuron ion channel acti vity may be a critical mechanism by which osmolarity can regulate exci tability in the central nervous system.