Effect of naloxone upon diffuse noxious inhibitory controls (DNIC) in the rat

Twenty-eight convergent neurones, responding to both noxious and innocuous stimuli applied to their cutaneous receptive fields were recorded at the lumbar level in anaesthetized intact rats. These cells received Aa and C fibre inputs as shown by electrical stimulation of their receptive fields, and were located in the medial part of the dorsal horn. (a) For 15 units, Diffuse Noxious Inhibitory Controls (DNCI)67,68 were investigated by applying noxious thermal stimuli (52 degrees C) to the distal two-thirds of the tail. This conditioning stimulus induced strong inhibition of the responses to both Aa (28%) and C (71%) fibres. Post-effects of long duration were commonly observed after cessation of the conditioning stimulus. The systemic injection of naloxone (0.3 mg/kg, i.v.) resulted in a partial reduction in these inhibitory effects with a decrease of about 50% for both Aa and C fibre response 10 min after naloxone administration. This was followed by a progressive recovery lasting 30 min. (b) 28 convergent units, including the 15 reported above, were recorded to investigate the effect of naloxone upon the unconditioned response. Responses to Aa fibre were unaffected, whereas the responses to C fibre were slightly (17%) but significantly increased by naloxone.