Nerve section alters the interaction between C-fibre activity and intrathecal neuropeptides on the flexor reflex in rat.

The hamstring flexor reflex evoked by C-fibre strength electric shocks to the sural nerve was recorded in unanaesthetised, decerebrate, spinalized rats with sciatic nerves intact or sectioned 14-21 days previously. The effect of a brief C-fibre strength conditioning stimulus (CS) train (20 s, 1 Hz) to the sural nerve or intrathecally applied neuropeptides substance P (SP), vasoactive intestinal polypeptide (VIP) and calcitonin gene-related peptide (CGRP) on reflex excitability were quantified. The sural CS and all of the neuropeptides caused a brief (1-5 min) facilitation of the test reflex with no concomitant changes in the amplitude of the monosynaptic reflex. A synergistic facilitatory interaction between the CS and SP, but not between the CS and VIP was observed in rats with intact nerves. In nerve-sectioned animals no synergism was found between CS and SP, but a very strong synergistic facilitation was seen with CS and VIP. Since SP and VIP both coexist with CGRP in separate populations of primary afferents, the interaction between SP plus CGRP and VIP plus CGRP was tested in animals with intact and sectioned nerves. In animals with intact nerves, SP and CGRP had a strong synergistic effect, whereas VIP and CGRP had a weak facilitatory interaction. In animals with sectioned nerves, SP and CGRP did not interact, whereas VIP and CGRP had a stronger facilitatory interaction. Since it has been shown that there are changes in peptide levels in primary afferents and dorsal horn interneurons following nerve section, it is proposed that such alterations may underly some of the physiological changes in spinal cord function after damage to peripheral nerves.