Fig. 4.

Assays of pain-related CPA can dissociate analgesics from nonanalgesics. (A and B) Effects produced in mice by the μ opioid receptor agonist analgesic morphine (A) or the κ opioid receptor agonist nonanalgesic U50,488 (B) in mice also conditioned with intraperitoneal (IP) acetic acid (AA) or in vehicle-treated controls (V). Abscissae show the dose in milligrams per kilogram. Ordinates are the aversion score expressed as time in seconds in the acid- or vehicle control-paired compartment after conditioning minus time in that compartment before conditioning. AA administered alone produced a CPA, whereas V did not. Morphine blocked acid-induced CPA in acid-treated mice more potently than it produced a conditioned place preference in vehicle-treated mice. Filled points in (A) indicate a significant difference from V or AA after morphine treatment (P < 0.05). Conversely, U50,488 failed to alter either acid-induced CPA in acid-treated mice or preference in the vehicle-treated mice. U50,488 was tested up to doses that blocked acid-stimulated stretching (see Fig. 1). Data adapted from Bagdas et al. (2016).