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

The heterotopic effects of peritoneo-visceral pain were investigated in behavioural and electrophysiological experiments performed on rats. The intraperitoneal administration of acetic acid (i.p. AA), an algesic agent commonly used to induce writhing behaviour in rodents, was used as a conditioning stimulus in two parallel series of experiments involving 3 behavioural tests and recordings of dorsal horn convergent neurones. The responses to a nociceptive stimulus applied to the tail or paws were lowered by i.p. AA but these effects depended on the behavioural 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). Intraperitoneal AA strongly depressed the C fibre evoked responses of coccygeal convergent neurones to suprathreshold transcutaneous electrical stimulation applied on their tail excitatory receptive fields. The time course of these inhibitory effects roughly paralleled the behavioural 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.