PARTIAL SCIATIC-NERVE LIGATION RESULTS IN AN ENLARGEMENT OF THE RECEPTIVE-FIELD AND ENHANCEMENT OF THE RESPONSE OF DORSAL HORN NEURONS TO NOXIOUS-STIMULATION BY AN ADENOSINE AGONIST

In this study we examined the effect of partial sciatic nerve ligation (PSNL) on the receptive field size, the baseline firing rate (BFR) an d the response of spinal dorsal horn (DH) neurons to mechanical stimul ation. In addition, we tested the effect of adenosine agonist, 5'-N-et hylcarboxamide-adenosine (NECA), and the adenosine antagonist caffeine on these parameters. Adult male Sprague-Dawley animals were used. One -third to one-half of the right sciatic nerve was tightly ligated. Una nesthetized animals were tested for their response to mechanical stimu lation using Von Prey filaments and a blunt probe. The mean force that produced a paw withdrawal response in the operated animals was signif icantly less than the force that produced withdrawal in unoperated ani mals (median: 103.5 vs. 259.7; P < 0.001 for the paw ipsilteral to the ligation). Extracellular recordings were made from nociceptive-specif ic DH neurons located in laminal I-V of chloral hydrate-anesthetized r ats. Recordings were made from 38 neurons in the right and 29 cells in the left DH of unoperated and 40 cells in right and 41 cell in the le ft DH of operated animals. The BFRs of neurons recorded in the operate d animals were not significantly different from those recorded in norm al animals. The mean receptive field size (RFS) of neurons (both ipsil ateral and contralateral to the ligation) in the operated animals was significantly larger than the RFS of unoperated animals (right side: 1 80 +/- 2.8 mm(2) compared to 66 +/- 2.3 mm(2); left side: 93 +/- 31 co mpared to 65 +/- 21). Twenty-four percent of all neurons in the operat ed group had bilateral receptive fields; in contrast, only 3% of the n eurons in the control animals showed bilateral receptive fields. To ex amine the effects of adenosine agonist and antagonist, NECA and caffei ne were applied next to the recording electrode. The BFR and the RFS o f neurons in control and operated animals were not affected by either NECA or caffeine. However, NECA significantly increased the duration o f the response of neurons in the operated animals to noxious stimulati on. These effects of NECA were blocked by caffeine. It is concluded th at PSNL produces hyperalgesia in Sprague-Dawley rats that is associate d with an increase in the RFS. In addition, in animals with PSNL, an a denosine agonist potentiates the response of nociceptive-specific neur ons to noxious stimulation. Together these results suggest that PSNL a lters the functional characteristics of DH neurons in part by changing purinergic transmission within the spinal cord.