1. To study the basic mechanisms of neuronal thermosensitivity, rat hypothalamic tissue slices were used to record and compare intracellular activity of temperature-sensitive and -insensitive neurones. This study tested the hypothesis that different neuronal types have thermally dependent differences in the transient potentials that determine the interspike interval. 2. Most spontaneously firing neurones displayed depolarizing prepotentials that preceded each action potential. In warm-sensitive neurones, warming increased the rate of rise of the depolarizing prepotential which, in turn, decreased the interspike interval and increased the firing rate. In contrast, temperature had little or no effect on the rate of rise in prepotentials of temperature-insensitive neurones. 3. Prepotential depolarization can be due to increasing depolarizing conductances or decreasing hyperpolarizing conductances. These are differences in the ionic conductances responsible for prepotentials in temperature-sensitive and -insensitive neurones. In warm-sensitive neurones, the net ionic conductance decreased as the prepotential depolarized towards threshold, suggesting that the prepotential is primarily determined by a decrease in outward potassium conductances. In contrast, in low-slope temperature-insensitive neurones, the net conductance remained constant during the interspike interval, suggesting a more balanced combination of both depolarizing and hyperpolarizing conductances. 4. Transient outward potassium currents, including A-currents, are important determinants of neuronal firing rates. These currents were identified in all warm-sensitive neurones tested, as well as in many temperature-insensitive and silent neurones. Since warming increased the rates of inactivation of these currents, transient K+ currents may contribute to the temperature-dependent prepotentials of some hypothalamic neurones.

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