Effects of noxious hindpaw immersion on evoked and spontaneous firing of contralateral convergent dorsal horn neurons in both intact and spinalized rats.

The effects of immersing a hindpaw in 60 degrees C water for 15 s on spontaneous and pinch-evoked activity recorded from 77 convergent neurons in the contralateral lumbar enlargement were examined. Cells were recorded from intact rats anesthetized with either ketamine or pentobarbital, or in rats that were decerebrated-intact or decerebrated-spinalized to determine if the heterotopic modulation was altered under these different conditions. In 47 neurons with background firing, immersion of the contralateral hindpaw inhibited the on-going activity in 6 of 19 cells recorded from ketamine anesthetized rats, 4 of 12 from pentobarbital anesthetized rats, 7 of 12 from decerebrate-spinalized rats, and 0 of 4 from decerebrate-intact rats. However, this inhibition was variable, both between and within neurons, because not every convergent neuron was inhibited, and there were trials in which immersion had no effect on the on-going activity of a neuron that had been previously inhibited by identical stimulation. In all 77 neurons, across all groups, discharges evoked by pinching the ipsilateral hindpaw were unaffected by contralateral immersion, even in cells where on-going activity was inhibited. Immersion of a hindpaw inhibited on-going, but not evoked, activity of contralateral convergent neurons. This inhibition was observed in rats under varied anesthetic conditions and in spinalized and intact rats. However, not all convergent neurons were inhibited by the remote stimulation even in intact rats, an observation inconsistent with the DNIC model of supraspinally mediated heterotopic modulation. This indicates that the effect of remote noxious stimulation in intact animals is not, under every experimental condition, a widespread inhibition of all convergent neurons.