Contribution of TTX-resistant C-fibres and Aδ-fibres to nociceptive flexor-reflex and non-flexor-reflex pathways in cats

The contribution of A delta-fibres and C-fibres activated by noxious heat stimulation 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. A-fibres were completely blocked by tetrodotoxin (TTX), leaving C-fibre conduction intact. Thus, effects persisting after TTX were attributed to nociceptive C-fibres while the contribution of nociceptive A delta-fibres was defined by the difference between those effects and the control effects before TTX. The initial action of noxious stimulation on both types of reflex action was mediated predominantly by A delta-fibres, while the later action was mainly mediated by C-fibres. In two (out of seven) experiments A delta-fibres exerted a significant inhibitory influence on the C-fibre action in FRA pathways, but such an inhibitory interaction between the two fibre groups was absent in the non-FRA reflex pathways. The technique of TTX application at the peripheral nerve proved to be a reliable method for a long-lasting selective investigation of C-fibre effects. The results revealed that both A delta- and C-fibres contributed to nociceptive FRA and non-FRA reflex pathways. (C) 2000 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All rights reserved.