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Smith FL, Cichewicz D, Martin ZL, Welch SP. 
“The enhancement of morphine antinociception in mice by delta9-tetrahydrocannabinol”. 
Pharmacol Biochem Behav. 1998 Jun;60(2):559-66.
We have previously reported that intracerebroventricular or intrathecal administration of inactive doses of delta9-tetrahydrocannabinol (THC) greatly enhance the antinociceptive potency of morphine in the mouse tail-flick test. Experiments were conducted to test the hypothesis that morphine's potency would be enhanced in mice receiving THC and morphine by conventional per os (p.o.) and subcutaneously (s.c.) routes of administration. Antinociception was measured in the tail-flick test of radiant heat after administration of different combinations of THC and morphine p.o. and s.c. Subcutaneous administration of THC (4 and 25 mg/kg) increased the potency of s.c. morphine 8.5- and 22.3-fold, respectively, while s.c. THC (25 mg/kg) increased the potency of p.o. morphine 3.1-fold. Per os administration of THC (10 and 20 mg/kg) increased the potency of s.c. and p.o. morphine 11.4-fold and 7.6-fold, respectively. Thus, morphine's potency was significantly increased regardless of the enteral and parenteral routes of THC and morphine administration. The synthetic receptor selective cannabinoid CP-55, 940 (0.1 mg/kg, s.c.) also enhanced morphine's potency. Finally, the ability of the CB1 receptor antagonist SR141716A to antagonize the enhancement of morphine by THC indicates that THC was acting through a cannabinoid receptor mechanism.
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