Hypoalgesia induced by counter-irritation is not affected by pCPA pretreatment

In a previous work (4), it has been described that a noxious visceral stimulation through the intraperitoneal injection of acetic acid (ipAA) induced a transient and low magnitude increase in tail-flick latencies, but a marked increase in the threshold for vocalization and hot-plate latencies. In the present work, this phenomenon of hypoalgesia through counter-irritation was investigated in intact rats with or without pretreatment with the potent serotonin depletor parachlorophenylalanine (pCPA). Three behavioural tests were performed. In two tests (tail flick, vocalization induced by transcutaneous electrical stimulation of the tail), pCPA pretreatment induced an increase of baseline levels, before IP injection of the allogenic agent (ipAA). In the third test, pCPA pretreatment had no effects on jump latencies. Parachlorophenylalanine pretreatment had no effect upon hypoalgesic actions of IP injected AA in all three tests. These results are discussed in terms of pCPA's differential effects upon basal nociception and analgesia induced by various heterotopic nociceptive stimulations.