Abstract
The effect of conditioning stimulation of the internal capsule (IC) on nociceptive neurons in the rat medullary dorsal horn was investigated The animals were anesthetized with N_2O-O_2 (2 1) and 0 5%-halothane, and immobilized with pancuronium bromide Nociceptive neurons responding to noxious stimulations of the face and oral structures were recorded in the trigemmal caudal nucleus and the medial reticular subnuclei These neurons were classified into wide dynamic range (WDR) and nociceptive-specific (NS) types NS neurons were distributed in the superficial layers of the caudal nucleus and the dorsal part of the reticular subnuclei, whereas WDR neurons were distributed diffusely throughout the area except for the magnocellular layer of the caudal nucleus A test stimulus with a single rectangular pulse (5-70 V) was applied to the center of the receptive field, the nociceptive neurons exhibited short-and/or long-latency responses Both responses in about 53% of the nociceptive neurons were inhibited by conditioning stimuli to the ipsilateral IC with trains of 33 pulses (100-300 μA) at 330Hz The percent of inhibitory effects on the WDR neurons and NS neurons were 20 7±245% (n=16) and 165±137% (n=2), respectively The inhibitory effect continued for conditioning-test intervals of up to 500 msec Effective sites for conditioning stimulation were concentrated in the lateral side of the IC, through which the corticofugal fibers from the somatosensory and motor cortices pass The IC inhibitory effect was not affected by a systemic injection of naloxone Our findings suggest that the transmission of nociceptive information is modulated at the level of the medullary dorsal horn by the corticofugal fiber via the IC and that transmitters other than opioids are involved in the IC antmociception
