CHARACTERIZATION OF ANANDAMIDE-INDUCED AND FLUOROANANDAMIDE-INDUCED ANTINOCICEPTION AND CROSS-TOLERANCE TO DELTA(9)-THC AFTER INTRATHECAL ADMINISTRATION TO MICE - BLOCKADE OF DELTA(9)-THC-INDUCED ANTINOCICEPTION

The antinociceptive effects of the putative endogenous cannabinoid lig and anandamide (ANA) and its fluorinated analog, fluoroanandamide (FA) , were determined as measured by the tail-flick and p-phenylquinone (P PQ) stretch tests, The ED(50) values (confidence limits) for ANA and F A were 77 (52-13) and 7 (2-21) mu g/mouse, respectively, for the tail- flick test and 30 (23-41) and 0.5 (0.1-2) mu g/mouse, respectively, fo r the PPQ test after intrathecal (i.t.) administration. ANA was not si gnificantly less potent than hg-tetrahydrocannabinol (THC) in the tail -flick test, but it was less potent in the PPQ test. FA was more poten t than either ANA or THC in tail-flick test, The antinociceptive effec ts of all drugs (administered i.t.) were blocked significantly or near ly abolished by the pretreatment of the mice with pertussis toxin (i.t .). Pretreatment of the mice with 5 and 25 mu g forskolin per mouse or 10 mu g (4-chlorophenylthio)-adenosine-3',5'-monophosphate cyclic mon osodium salt per mouse (both i.t.) significantly attenuated the antino ciception produced by THC but not by ANA or FA. Various calcium modula tors were tested in combination with THC, ANA, and FA, but they failed to alter the antinociceptive effects of the drugs. Various potassium channel blockers were tested in combination with the drugs. Apamin, a blocker of small (low)-conductance calcium-gated potassium channels th at attenuates THC-induced antinociception, failed to alter ANA- or FA- induced antinociception. In contrast to THC, which is blocked by the k appa antagonist nor-binaltorphimine, ANA- and FA-induced antinocicepti on was not altered by classic opioid antagonists. Also in contrast to THC, which enhances mu and delta opioid-induced antinociceptive effect s, ANA failed to significantly alter opioid antinociception, ANA signi ficantly shifted the THC dose-effect curve to the right. Thus, ED(50) for DMSO/THC in the tail-flick test was shifted from 14 (7-29) to 54 ( 38-77) mu g/mouse and was shifted in the hot-plate test from 22 (12-42 ) to 63 (43-92) mu g/mouse. The magnitude of the shift in the ED(50) w as 3.8-fold in the tail-flick test and 2.9-fold in the hot-plate test, The shifts were parallel and significant. The K-i for the displacemen t of H-3-CP 55,940 binding by ANA and FA was 214 nM (+/-45 S.E.M.) and 72 nM (+/-5 S.E.M.), respectively, in pure spinal cord synaptosomes f rom the rat. ANA and FA were significantly cross-tolerant to THC. Alth ough similarities between ANA and cannabinoids were shown, several mar ked differences were observed between ANA and the classic cannabinoids . ANA appears to function as both a cannabimimetic and a blocker of ca nnabinoid-induced antinociception.