Attenuation of the gerbil writhing response by mu-, kappa- and delta-opioids, and NK-1, -2 and -3 receptor antagonists

Mu-, kappa- and delta-opioid receptor agonists are reported to attenuate the acetic-acid-induced abdominal constriction response in mice. NK-1, -2 and -3 receptor antagonists also display activity in several visceral pain models. As the gerbil NK-1 receptor is comparable to the human receptor, we evaluated the efficacy of NK-1, -2 and -3 receptor antagonists and opioids (both alone and in combination) in the writhing test in this species. The effects of a selective L-type calcium (Ca2+) channel antagonist on the writhing response were also assessed to determine the contribution of Ca2+ channel antagonism to the antinociceptive effects of the NK-1 antagonists. Gerbils received subcutaneous injections of either the mu-opioids morphine or fentanyl, the kappa-opioid U50,488-H, the delta-opioid SNC80, NK-1 antagonists R116301, CP-96,345 or GR203040, the NK-2 antagonist SR-48968, the NK-3 antagonist SR-142801 or the Ca2+ channel antagonist nimodipine. Writhing was evoked 1 h after treatment by intraperitoneal injection of 0.2 ml 1% acetic acid solution and the frequency was recorded. Morphine, fentanyl and U50,488-H attenuated the writhing response dose dependently with complete inhibition occurring at the highest doses. SNC80 did not significantly attenuate the writhing response even at a dose of 40 mg/kg. The tachykinin NK-1 antagonists CP-96,345 and GR203040, the NK-2 antagonist SR-48968 and the NK-3 antagonist SR-142801 reduced the writhing frequency although without complete inhibition. The NK-1 antagonist R116301 displayed limited activity at doses up to 40 mg/kg. Nimodipine did not exhibit any antinociceptive efficacy in this assay. Adding the NK-1, -2 or -3 antagonists to the opioids did not improve the efficacy of the opioids. Selective NK antagonists may therefore be effective in a visceral nociception assay in gerbils but do not modulate opioid action.