Noxious inputs to supraoptic neurosecretory cells in the rat.

The effects of noxious stimuli were studied on discharge activity of the neurosecretory cells identified in the supraoptic nucleus by antidromic excitation after pituitary stimulation, in anaesthetized rats. Tail pinching excited 24% and inhibited spontaneous discharge of 6% of the 91 cells tested. Noxious heat stimuli (44-63 degrees C) applied to the hindlimb paw produced a transient excitation in 26% of the 23 cells tested. Electric stimulation of either the sciatic or cutaneous nerve with 20-Hz pulses for 1 s, at an intensity 5 times stronger than the threshold for evoking the changes in respiratory movements and blood pressure similar to those after tail pinching or noxious heat stimuli, excited about 30% of the cells tested. The excitation produced by these noxious stimuli preceded, on some occasions, the respiratory movement and blood pressure decrease which occurred concomitantly. Peristimulus time histograms of spontaneous discharges constructed during stimulation of either nerve at 1 Hz, revealed the presence of excitatory synaptic inputs in about 35% of the neurosecretory cells tested. These data indicate the existence of direct neural pathways which mediate excitatory synaptic inputs originating from nociceptors to supraoptic neurosecretory cells. Since 9 of the 22 cells which were excited by tail pinching exhibited a "phasic" pattern of spontaneous discharge which is known to characterize certain vasopressin-secreting neurones in rats, it is suggested that these excited cells were, at least in part, vasopressinergic.