Temperature effects on membrane potential and input resistance in rat hypothalamic neurones.

1. Whole-cell recordings were conducted in rat hypothalamic tissue slices to test the hypothesis that thermal changes in membrane potential contribute to neuronal thermosensitivity. Intracellular recordings of membrane potential and input resistance were made in eighty-two neurones, including twenty-four silent neurones and fifty-eight spontaneously firing neurones (22 warm-sensitive neurones and 36 temperature-insensitive neurones). Fifty-seven of the neurones were recorded in the preoptic and anterior hypothalamus. 2. Warm-sensitive neurones increased their firing rates during increases in temperature (1.07 +/- 0.06 impulses s-1 degree C-1), but their resting membrane potentials were not affected by temperature (0.06 +/- 0.06 mV degree C-1). Similarly, temperature did not affect the membrane potentials of temperature-insensitive neurones or silent neurones. 3. Silent neurones had significantly lower input resistances (256.9 +/- 20.0 M omega), compared with temperature-insensitive (362.6 +/- 57.2 M omega) and warm-sensitive neurones (392.2 +/- 50.0 M omega). Temperature had the same effect on all three types of neurones, such that resistance increased during cooling and decreased during warming. 4. If hyperpolarizing or depolarizing holding currents were applied to neurones, temperature caused changes in the membrane potentials. This spurious effect can be explained by thermally induced changes in the input resistance. 5. Measurements of electrode tip potentials indicated that artificial changes in membrane potential may also be recorded if grounding electrodes are not isolated from the changes in temperature. 6. These results suggest that physiological changes in resting membrane potentials do not determine neuronal warm sensitivity, and thermal changes in input resistance do not determine the primary differences between warm-sensitive and temperature-insensitive hypothalamic neurones.