Differential effects of various doses of morphine and naloxone on two nociceptive test thresholds in arthritic and normal rats

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Abstract
In an attempt to clearly gauge the influence of the test used on opioid effects, the present study systemically compares the effects of various doses of morphine and naloxone with 2 differentially integrated tests: a suprasegmentally integrated test, the vocalization threshold to paw pressure and a spinally coordinated reflex, the paw withdrawal to pressure. In both normal and arthritic rats, clear differential effects of the drugs were observed: low doses of morphine (0.3 and 1 mg/kg i.v.) produced marked effects on the vocalization test, especially in arthritic rats, while it was less effective on the paw withdrawal test. Naloxone and morphine at extremely low doses (3-10 .mu.g/kg, and 6 .mu.g/kg i.v. respectively) clearly produced marked effects on the vocalization test, but failed to modify the paw withdrawal threshold in arthritic rats. By contrast, a high dose of naloxone (1 mg/kg i.v.) induced a comparable decrease in thresholds in both tests. This comparative study clearly shows the interest of using the vocalization threshold to paw pressure as a nociceptive test for evaluation of the antinociceptive effect of opioids. In addition, it provides useful information for a better understanding of the complex effects of morphine and the opioid antagonist naloxone in arthritic rats.