C-fibre excitation and tonic descending inhibition of dorsal horn neurones in adult rats treated at birth with capsaicin.

Single unit electrical activity has been recorded from dorsal horn neurones in the lumbar cord of rats anaesthetized with sodium pentobarbitone. Three groups of animals were used: normal adult rats, adult rats that had been treated at birth with capsaicin (50 mg kg-1 s.c.) and adult rats that had been injected at birth with the drug vehicle only. Rats treated at birth with capsaicin showed a substantial reduction in the number of afferent C fibres as indicated by the virtual absence of C waves in the compound action potentials evoked in the sural nerve by antidromic stimulation of the L4-L6 dorsal roots. No significant differences were found in any of the parameters measured between the vehicle treated and the untreated animals. Therefore, rats from these two groups are referred to as control animals. All dorsal horn neurones studied were driven by electrical stimulation of the A fibres in the ipsilateral sural nerve and had cutaneous receptive fields in the ipsilateral hind limb. Two groups of neurone were distinguished: those receiving an input from A fibres only (A only) and those neurones that could also be driven by sural C fibres (A + C). In the control group, 56% of the neurones were A only and 44% were A + C. In capsaicin-treated rats these proportions were significantly different: 78% and 22% respectively. No differences were found in receptive field sizes of A-only neurones between those recorded in control rats and those from capsaicin-treated animals. However, a large and significant increase in receptive field size of A + C neurones was observed in capsaicin-treated rats compared to their counterparts in normal animals. In control rats 80% of the A + C neurones showed tonic descending inhibition of their C-fibre-evoked responses as assessed by reversible spinalization. In capsaicin-treated rats this proportion fell to 47% of the A + C neurones. The magnitude of the tonic descending inhibition was also reduced in the fewer A + C neurones of capsaicin-treated rats that were subjected to it. Only 4% of A + C neurones with tonic descending inhibition in capsaicin-treated rats were powerfully inhibited compared to 26% in control animals. The mean number of spikes evoked by C-fibre stimulation of the sural nerve in A + C neurones of control and of capsaicin-treated rats was not significantly different between these two groups of animals in the intact and in the spinalized states.(ABSTRACT TRUNCATED AT 400 WORDS)