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) and the response of spinal dorsal horn (DH) neurons to mechanical stimulation. In addition, we tested the effect of adenosine agonist, 5'-N-ethylcarboxamide-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. Unanesthetized animals were tested for their response to mechanical stimulation using Von Prey filaments and a blunt probe. The mean force that produced a paw withdrawal response in the operated animals was significantly less than the force that produced withdrawal in unoperated animals (median: 103.5 vs. 259.7; P < 0.001 for the paw ipsilteral to the ligation). Extracellular recordings were made from nociceptive-specific DH neurons located in laminal I-V of chloral hydrate-anesthetized rats. 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 left DH of operated animals. The BFRs of neurons recorded in the operated animals were not significantly different from those recorded in normal animals. The mean receptive field size (RFS) of neurons (both ipsilateral and contralateral to the ligation) in the operated animals was significantly larger than the RFS of unoperated animals (right side: 180 +/- 2.8 mm(2) compared to 66 +/- 2.3 mm(2); left side: 93 +/- 31 compared to 65 +/- 21). Twenty-four percent of all neurons in the operated group had bilateral receptive fields; in contrast, only 3% of the neurons in the control animals showed bilateral receptive fields. To examine the effects of adenosine agonist and antagonist, NECA and caffeine were applied next to the recording electrode. The BFR and the RFS of neurons in control and operated animals were not affected by either NECA or caffeine. However, NECA significantly increased the duration of the response of neurons in the operated animals to noxious stimulation. These effects of NECA were blocked by caffeine. It is concluded that PSNL produces hyperalgesia in Sprague-Dawley rats that is associated with an increase in the RFS. In addition, in animals with PSNL, an adenosine agonist potentiates the response of nociceptive-specific neurons to noxious stimulation. Together these results suggest that PSNL alters the functional characteristics of DH neurons in part by changing purinergic transmission within the spinal cord.