Fig. 2.

Assays of pain-depressed behavior can dissociate analgesics from drugs that produce motor impairment. (A and B) Effects produced in mice by the μ opioid receptor agonist (morphine) and a κ opioid receptor agonist (U50,488 or U69,593) in assays of pain-stimulated unconditioned behavior (A; intraperitoneal acid-stimulated stretching) or pain-depressed unconditioned behavior (B; intraperitoneal acid-induced depression of nesting). Abscissae show the dose in milligrams per kilogram. Ordinates are the number of acid-stimulated stretches (A) or zones cleared (B; a measure of nesting). Points above “V” indicate effects of vehicle administration. Filled points indicate significant differences from V (P < 0.05). Both morphine and the κ agonist blocked acid-stimulated stretching, whereas only morphine was effective to block acid-induced depression of nesting. These results are consistent with clinical analgesic effectiveness of centrally acting μ but not κ agonists. IP, intraperitoneal; V, vehicle. Data adapted from Bagdas et al. (2016) and Negus et al. (2015).