Heat production associated with synaptic transmission in the bullfrog spinal cord.

By using heat detectors made with pyroelectric film, rapid heat production by the bullfrog spinal cord in response to dorsal root stimulation has been demonstrated. The heat production rises to its peak in about 100 ms after the arrival of afferent impulses and falls slowly with a time course comparable to that of the dorsal root potential. Stimulation of the ventral roots produces no detectable heat. The heat production was reversibly suppressed by immersion of the cord in a low Ca2+, high Mg2+ salt solution, indicating that the underlying exothermic process is associated with intraspinal synaptic transmission. The source of this 'synaptic heat' is located near the boundary between the dorsal column and the substantia gelatinosa in the vicinity of the stimulated dorsal roots.