Capsaicin pretreatment does not inhibit the opioid withdrawal response in guinea-pigs.

The effects of capsaicin pretreatment on withdrawal responses of guinea-pig isolated ileum to [Met5]enkephalin (ME) and morphine and on the locomotor withdrawal response of guinea-pigs following a single dose of morphine, were investigated. In vitro treatment of ileum with capsaicin, 1.5 mumol/l for 1 h, did not significantly affect the response to washout following 2 min contact with ME, 1 mumol/l, or the withdrawal response precipitated by naloxone, 1 mumol/l, following 2 min contact with morphine, 1 mumol/l, or the response to naloxone of tolerant-dependent ileum obtained from guinea-pigs treated with a total dose of 690 mg/kg of morphine over 3 days. Pretreatment of guinea-pigs with capsaicin 140 mg/kg subcutaneously (s.c.) over 4 days also did not affect the washout withdrawal response of the ileum to ME. Pretreatment of guinea-pigs with capsaicin did not affect the locomotor withdrawal response precipitated by naloxone hydrochloride 15 mg/kg s.c., 2 h after injection of morphine sulphate 15 mg/kg s.c. It was concluded that primary afferent neurones do not play an essential role in opioid withdrawal responses.