A spinal ventral root response was measured following the activation of peripheral fibres by noxious (heat at 48 degrees C, capsaicin, bradykinin) and innocuous (brush) stimuli in a preparation of the neonatal rat spinal cord-tail maintained in vitro. 2. Following superfusion of the tail with 0.1-1.0 nM of the potent irritant, resiniferatoxin (RTX), brief, irregular depolarization and a selective loss of capsaicin sensitivity was produced. RTX 10-100 nM evoked a tonic response, initiated transient irregular depolarizations and densitization to further applications of RTX and capsaicin but not to other stimuli. Following RTX 1 microM a prolonged loss of sensitivity to all noxious stimuli was produced. 3. When a selective densitization to capsaicin was produced by a long application of capsaicin, RTX was also ineffective. 4. Superfusion of the tail with 4 beta-phorbol, 12, 13-dibutyrate (PDBu), a protein kinase C activator, stimulated capsaicin-sensitive peripheral fibres. Prolonged administration of PDBu attenuated or abolished further responses to PDBu and bradykinin but responses to RTX and capsaicin were unchanged. The protein kinase C inhibitor staurosporine (50-200 nM), attenuated the effects of PDBu and bradykinin but not those of RTX or capsaicin. 5. The present data suggest that neither RTX nor capsaicin act on peripheral nociceptors via a phorbol ester-like stimulation of protein kinase C. Rather, RTX acts on nociceptors by a similar mechanism to capsaicin. These effects may be the basis for the irritant properties of RTX and may further relate to the antinociceptive actions observed in vivo. RTX is therefore a potent new tool with which to investigate the properties of nociceptive neurones and provides a prototype for further development of antinociceptive agents.
