Abstract
The effects of various i.v. doses of morphine (0.1, 0.3 and 1 mg/kg) and of thiorphan, an inhibitor of enkephalinase (0.7, 2.5, 5, 10 and 15 mg/kg), were studied upon the vocalization threshold to foot pressure in normal rats and rats with Freund's adjuvant-induced arthritis. The vocalization threshold in arthritic rats was, before any injections, significantly lower than in normal rats (mean pressure threshold for vocalization: 115.2g ± 14.7 (n= 152) for arthritic rats vs 182.5g ± 21.3 for normal rats (n= 152). The various doses of morphine in raising the vocalization threshold were more efficient in arthritic than in normal rats (maximum vocalization threshold (% of control) following 1mg/kg morphine= 225.70 ± 10.21 in arthritic rats vs 140.75 ± 6.87 in normal rats, n= 9 in each case). This effect was dose-dependent, and in every case, naloxone-reversible. Injected at doses of 5–15 mg/kg, thiorphan increased the vocalization threshold (maximum= 223.91% ± 11.96 in arthritic rats vs 223.30% ± 5.93 in normal rats for 15 mg/kg i.v., n= 9 for each group). This effect was not greater in arthritic than in normal rats. The dose of 2.5 mg/kg of thiorphan was insufficient. Administered at 0.7 mg/kg, thiorphan significantly decreased the vocalization threshold in the arthritic rats only. These effects of thiorphan were all naloxone-reversible using doses of naloxone which were one-hundredth of those of thiorphan.
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