Evidence that inhibitory mechanisms mask inappropriate somatotopic connections in the spinal cord of normal rat

Abstract

1. The responses to stimulation of the sciatic and saphenous nerves have been studied in 65 pairs of spinal dorsal horn neurons simultaneously recorded at the L2 and L5-L6 lumbar segments of the rat's spinal cord. The neurons were recorded in anesthetized and paralyzed animals. 2. Five- or seven-barreled micropipettes were utilized for recording and for the application of drugs with iontophoresis or micropressure techniques. The drugs used were: strychnine, as a selective antagonist at glycine receptors; sodium glutamate and N-methyl-D-aspartate (NMDA), as agonists at excitatory glutamatergic receptors; glycine, as an agonist at the inhibitory glycine receptor; and the local anesthetic lidocaine, as a reversible local conduction blocker both in the periphery and in the spinal cord. 3. All neurons had cutaneous receptive fields in the ipsilateral hindpaw. Neurons responding exclusively to saphenous stimulation in L2 and to sciatic stimulation in L5-L6 were selected for this study. The responses consisted of bursts of > or = 5 spikes, often partially inserted in a field potential, with latencies of 5.0 +/- 1.1 (SD) and 5.2 +/- 1.2 ms, respectively. The thresholds of stimulation and the response latencies were controlled to be stable throughout the experiments. 4. Eighty-five percent (29 of 35) of the neurons tested in L5-L6 exhibited responses to saphenous stimulation during strychnine microejection in the recorded neurons. The neurons became again unresponsive to saphenous stimulation shortly after the end of strychnine ejection. 5. All the neurons tested in L5-L6 (n = 14) showed a significant increase in background activity and remained unresponsive to saphenous stimulation during glutamate microejection on the recorded neurons. 6. All the neurons tested in L5-L6 (n = 17) showed responses to saphenous stimulation after sciatic nerve block with local anesthetic. The responses to saphenous stimulation disappeared after the effect of local anesthetic ceased. 7. All the neurons tested in L5-L6 (n = 6), in rats with a local block of the sciatic nerve, showed a reversible increase in the background activity during NMDA microejection in the spinal saphenous area at L2. 8. In rats with a local block of the sciatic nerve, the responses evoked in L5-L6 neurons (n = 12) by saphenous stimulation were reduced both in field potential amplitude and in number of spikes during microejection of glycine, and were suppressed during microejection of lidocaine in the spinal saphenous area in L2.(ABSTRACT TRUNCATED AT 400 WORDS)