Repeated cannabinoid injections into the rat periaqueductal gray enhance subsequent morphine antinociception

Cannabinoids and opiates inhibit pain, in part, by activating the periaqueductal gray (PAG). Evidence suggests this activation occurs through distinct mechanisms. If the antinociceptive mechanisms are distinct, then cross-tolerance between opioids and cannabinoids should not develop. This hypothesis was tested by measuring the antinociceptive effect of microinjecting morphine into the ventrolateral PAG of rats pretreated with the cannabinoid HU-210 for two days. Male Sprague–Dawley rats were injected twice a day for two days with vehicle (0.4 μL), morphine (5 μg/0.4 μL), HU-210 (5 μg/0.4 μL), or morphine combined with HU-210 into the ventrolateral PAG. Repeated injections of morphine caused a rightward shift in the morphine dose–response curve on Day 3 (i.e., tolerance developed). No tolerance was evident in rats pretreated with morphine combined with HU-210. In rats pretreated with HU-210 alone, morphine antinociception was enhanced. This enhancement was blocked by pretreating rats with the cannabinoid receptor antagonist AM-251, and it also disappeared when rats were tested one week later. Acute microinjection of HU-210 into the PAG antagonized morphine antinociception, suggesting that HU-210-induced enhancement of morphine antinociception is a compensatory response. As hypothesized, there was no evidence of cross-tolerance between morphine and HU-210. In fact, cannabinoid pretreatment enhanced the antinociceptive effect of microinjecting morphine into the ventrolateral PAG. These findings suggest that alternating opioid and cannabinoid treatment could be therapeutically advantageous by preventing the development of tolerance and enhancing morphine antinociception.