The 'tonic' pain-related behaviour seen in mononeuropathic rats is modulated by morphine and naloxone.
This study investigated the sensitivity to pharmacological manipulations of a rating method, adapted from the formalin test, to measure the tonic component of the pain-related behaviour induced by creating a peripheral mononeuropathy with 4 loose ligatures around the common sciatic nerve. Although the adequacy of opioid substances in alleviating neuropathic pain is highly controversial, the effects of morphine (1 mg/kg i.v.) and naloxone (1 mg/and 3 micrograms/kg i.v.) were tested 1-2 weeks after the nerve ligatures were established, when pain-related behaviours were well developed. Morphine (1 mg/kg i.v.) induced a potent and prolonged decrease in the pain-rating score at week 2 after surgery. Either at week 1 or week 2, naloxone elicited a bidirectional dose-dependent action: a further increase in the pain-rating score with the high dose (1 mg/kg i.v.), and a paradoxical decrease in the score with the low dose of 3 micrograms/kg i.v. These effects are comparable to those already described in several rat models of inflammatory pain and, in the same model of neuropathy, using a phasic nociceptive test, the measure of the vocalization to paw pressure. A few differences in the effects of naloxone on tonic and phasic pain are noted and discussed.
Jazat
F
F
Unité de Recherches de Physiopharmacologie du Système Nerveux, INSERM U161, 2 Rue d'Alésia, 75014 ParisFrance.
Guilbaud
G
G
D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology*; D009422:Nervous System Diseases / Q000523:psychology*; D010146:Pain / Q000523:psychology*; D010147:Pain Measurement; D051381:Rats; D011919:Rats, Inbred Strains; D012584:Sciatic Nerve / Q000502:physiology
D009270:Naloxone; D009020:Morphine