Intracellular analysis of inherent and synaptic activity in hypothalamic thermosensitive neurones in the rat.

1. Intracellular neuronal activity was recorded in rat preoptic-anterior hypothalamic tissue slices. Thirty neurones were classified as warm sensitive, cold sensitive or temperature insensitive, based on their firing rate response to temperature changes. Seventy-seven per cent of the neurones were temperature insensitive, which included both spontaneously firing and silent neurones. Of all neurones, 10% were warm sensitive and 13% were cold sensitive. 2. Silent temperature-insensitive neurones had lower input resistances (126 +/- 21 M omega) than thermosensitive neurones (179 +/- 24 M omega). Regardless of neuronal type, however, resistance was inversely related to temperature. 3. Warm-sensitive neurones were characterized by a slow, depolarizing pre-potential, whose rate of rise was temperature dependent. This depolarizing potential disappeared during current-induced hyperpolarization, suggesting that intrinsic mechanisms are responsible for neuronal warm sensitivity. 4. Spike activity in cold-sensitive neurones correlated with putative excitatory and inhibitory postsynaptic potentials, whose frequency was thermosensitive. This suggests that cold sensitivity in these neurones depends on synaptic input from nearby neurones. 5. Like cold-sensitive neurones, action potentials of temperature-insensitive neurones often were preceded by short duration (less than 20 ms), rapidly rising pre-potentials, whose rates of rise were not affected by temperature. In some temperature-insensitive neurones, depolarizing current injection increased both firing rate (by 5-8 impulses s-1) and warm sensitivity, with pre-potentials having temperature-dependent rates of rise. We suggest that temperature-insensitive neurones employ two opposing, thermally dependent mechanisms: a voltage-dependent depolarizing conductance and a hyperpolarizing sodium-potassium pump.