Thermosensitive midbrain neurons in the cat.

Responses of single neurons in the midbrain of cats anesthetized with chloral hydrate were studied during manipulations of midbrain temperature produced with a bilateral water-perfused thermode. Temperatures of the thermodes and the anterior hypothalamus were monitored while single neuron activity was recorded between the thermodes and correlated with the midbrain temperature. Q10's and thermal coefficients were calculated from the estimated temperature at the neuron itself. A surprisingly high percentage (72%) of the 72 neurons recorded in the caudal paramedian midbrain of 11 cats were thermoresponsive. Most of these were heat sensitive and exhibited a variety of frequency/temperature curves. Explorations of more rostral regions of the midbrain in 9 cats yielded only 18% thermoresponsive units out of the 99 neurons sampled. We suggest that the concentrated pool of warm-sensitive neurons in the caudal midbrain is part of an extensive system of brain stem thermosensors which are involved in establishing and controlling normal brain temperature.