SPATIAL-DISTRIBUTION OF NOCICEPTIVE PROCESSING IN THE RAT SPINAL-CORD

1. Quantitative 2-deoxyglucose (2-DG) experiments were undertaken to determine the spatial distribution of nociceptive responses in the rat spinal cord. Twenty unanesthetized, paralyzed rats with T2 transected spinal cords were divided into groups (n = 4) and stimulated with non-noxious (35-degrees-C) or graded noxious temperatures (45-degrees, 47-degrees, 48-degrees, and 49-degrees-C). Stimulation was delivered by cyclical immersion of one hind paw in a temperature-controlled water bath. 2. When stimulation began, 50-mu-Ci of 2-Dg was injected into the rat, and timed sequential blood samples were drawn to monitor plasma glucose and 2-DG levels. On termination of stimulation, spinal cords were removed, sectioned, and prepared for autoradiography. Local rates of spinal cord metabolism were obtained by microcomputer analysis of autoradiographs. 3. Nociceptive stimulation produced increases in glucose utilization in some of the grey matter laminae previously implicated in nociceptive processing. Within the nociceptive range, 49-degrees and 48-degrees-C intensities elicited significantly greater responses than did 45-degrees-C stimulation. 4. The rostrocaudal spatial distribution of nociceptive responses was most extensive within laminae V-VI with stimulus-evoked metabolic activity extending 9 mm from L2-L5. Similarly, metabolic increases occurred over 7 mm within lamina VII. In contrast, metabolic activity within the upper dorsal horn (laminae I-IV) extended only 3 mm and was limited to L4. Metabolic patterns within laminae VIII-IX sharply differed from those within other laminae in that no increases occurred within L4, although regions of L3 and L5 were responsive to stimulation. 5. Metabolic increases were somatotopically specific; the ipsilateral dorsal horn was more active than the contralateral dorsal horn, and medial portions of the dorsal laminae (regions that represent the distal portions of the limb) were more active than lateral regions. Within the ventral horn, a band of activation corresponding with the location of flexor motoneuron pools was clearly evident at stimulus intensities sufficient to elicit the flexion reflex. 6. These results obtained by the use of 2-DG analysis indicate that noxious stimulation produces extensive spatial recruitment of neural populations and that this model of nociception is sensitive to both sensory and motor consequences of noxious stimulation.