EVIDENCE THAT INHIBITORY MECHANISMS MASK INAPPROPRIATE SOMATOTOPIC CONNECTIONS IN THE SPINAL-CORD OF NORMAL RAT

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 L(2) and L(5)-L(6) 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 L(2) and to sciatic stimulation in L(5)-L(6) were selected for this study. The responses consisted of bursts of greater than or equal to 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 L(5)-L(6) 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 L(5)-L(6)(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 L(5)-L(6) (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 L(5)-L(6) (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 L(2). 8. In rats with a local block of the sciatic nerve, the responses evoked in L(5)-L(6) 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 L(2). 9. We conclude that the sciatic nerve exerts a tonic inhibitory control on inputs from saphenous afferents, and that this inhibitory control may be disrupted by locally antagonizing inhibitory transmitters or by peripherally blocking the sciatic input. Furthermore, we suggest that there are both pre- and postsynaptic bundles of saphenous projections to the sciatic fields.