The heterotopic effects of visceral pain: Behavioural and electrophysiological approaches in the rat

Abstract

The heterotopic effects of peritoneo-visceral pain were investigated in behavioral and electrophysiological experiments performed on rats. The i.p. administration of acetic acid (i.p. AA), an algesic agent commonly used to induce writhing behavior in rodents, was used as a conditioning stimulus in 2 parallel series of experiments involving 3 behavioral tests and recordings of dorsal horn convergent neurons. The responses to a nociceptive stimulus applied to the tail or paw as were lowered by i.p. AA but these depended on the behavioral test used: in the tail-flick test, AA produced a transient low magnitude increase in latencies; the threshold for vocalization induced by electrical stimulation of the tail was clearly (25%) and sustainedly (full recovery taking up to 1 h) increased; the jump latency in hot plate was markedly increased (100% at 15 min). I.p AA strongly depressed the C fiber evoked responses of coccygeal convergent neurons to suprathreshold transuctaneous electrical stimulation applied on their tail excitatory receptive fields. The time course of these inhibitory effects roughly paralleled the behaivoral hypoalgesic effect observed in the vocalization test. These results are discussed with reference to diffuse noxious inhibitory controls (DNIC). Analogies with counter-irritation phenomena are emphasized.