The response of rat spinal cord cells to unmyelinated afferents after peripheral nerve section and after changes in substance p levels

Abstract

The sciatic nerve on one side was cut and ligated in rats. Seven to nineteen days later the animals were decerebrated and spinalized and recordings made from the cells in the dorsal horn of the spinal cord. The two sciatic nerves, one chronically and the other acutely cut, were stimulated and the cord cell responses measured. Sciatic nerve stimulation sufficient to evoke a peripherally recorded C wave resulted in late responses in cord cells, attributable to the arriving volley in unmyelinated afférents. These late responses could be evoked from both the chronically cut nerve and the previously intact nerve. Local changes in the region of the cut made absolute threshold comparisons difficult, but the ratio of thresholds for A fibre and C fibre evoked responses and the size of the A wave was the same on both sides. The size of sample cell population responses to afferent volleys was measured at matched locations on both sides of the cord ipsilateral to chronically cut and uncut nerves. Three methods of measurement were used and no consistent difference was found in the responses evoked by the two nerves in any of the methods. This was true using single stimuli at 1 Hz or using 100 Hz trains of stimuli. A substantial decrease in the amount of substance P was shown in the region of the terminals of the fine fibres from the cut nerve. We were therefore unable to find any evidence that the central excitatory effect of an afferent volley is changed during the period 7–19 days after the fibres have been cut in the periphery. Since, during this time, there is a marked decrease in the amount of substance P in the terminals, the results do not support the proposal that substance P is an excitatory transmitter.