Differentiation between Aδ and C fibre evoked nociceptive reflexes by TTX resistance and opioid sensitivity in the cat

The contribution of nociceptive Aδ-fibres and C-fibres of the central pad of the foot to nociceptive spinal flexor reflex pathways (FRA-type) and to nociceptive excitatory reflex pathways to foot extensors (non-FRA type) was investigated in high spinal cats. Persisting effects after complete blocking of A-fibres by tetrodotoxin (TTX) application were thus attributed to nociceptive C-fibres. The results revealed that both Aδ and C-fibre afferents contributed to nociceptive reflexes of a FRA-pattern and of a non-FRA pattern. Thereby, the effects of Aδ-fibres were evoked with a distinctly shorter delay than those of C-fibres. Furthermore, Aδ-fibres partly exerted a significant inhibitory influence on the C-fibre action in FRA and non-FRA pathways. Treatment with different opioid receptor agonists (DAMGO and DSLET) and subsequently with naloxone revealed that a distinct part of the opioid action on nociceptive reflex pathways of the FRA-type and of the non-FRA type was evidently exerted via Aδ-fibres.

Research highlights
► Aδ and C-fibre contribute to nociceptive reflexes of FRA and non-FRA pattern.
► Aδ-fibres exert inhibitory influence on C-fibre action in FRA and non-FRA pathways.
► Part of the opioid action on nociceptive reflex pathways is exerted via Aδ-fibres.