Inhibitory effect of the lateral reticular nucleus on neurons of the gigantocellularis nucleus which respond to noxious stimuli.

The effect was assessed of electrical stimulation of 'analgesic sites' in the lateral reticular nucleus (LRN) on the activity (spontaneous and evoked by noxious stimuli) of neurons in the nucleus gigantocellularis (NGC) in rabbits. The neurons (n = 145) were classified as 'nociceptive' (n = 98) and 'non-nociceptive' (n = 47) according to their responsiveness to a peripherally applied noxious heat stimulus (HS). The response to HS consisted in all cases of an increase in firing rate of up to 5 times the resting values. LRN stimulation was ineffective on the background discharge of the 'non-nociceptive' neurons, whereas it induced an inhibition lasting 40-120 msec on both spontaneous and noxious evoked activity in 66% (n = 68) of the 'nociceptive' neurons. Furthermore, prolonged LRN stimulation could induce a 'tonic' inhibition evaluable by the response to HS. This effect, quantified in 24 neurons, significantly decreased the noxious evoked activity to the baseline value. After hemilesion of the spinal cord, the inhibitory effects induced by LRN stimulation on the spontaneous and on the noxious evoked activity of NGC neurons were still present. The possibility that inhibition of the NGC neurons is a supraspinal component of the 'analgesic effects' resulting from LRN stimulation is discussed.