Comparison of cannabinoid binding sites in guinea-pig forebrain and small intestine

1 We have investigated the nature of cannabinoid receptors in guinea-pig small intestine by establishing whether this tissue contains cannabinoid receptors with similar binding properties to those of brain CBI receptors. The cannabinoids used were the CB1-selective antagonist SR141716A, the CB,-selective antagonist SR144528, the novel cannabinoid receptor ligand, 6'-azidohex-2'-yne-Delta 8-tetrahydrocannabinol (O-1184), and the agonists CP55940, which binds equally well to CBI and CB2 receptors, and WIN55212-2, which shows marginal CB2 selectivity. 2 [[H-3]-CP55940 (1 nM) underwent extensive specific binding both to forebrain membranes (76.3%) and to membranes obtained by sucrose density gradient fractionation of homogenates of myenteric plexuslongitudinal muscle of guinea-pig small intestine (65.2%). 3 Its binding capacity (B-max) was higher in forebrain (4281 fmol mg(-1)) than in intestinal membranes (2092 fmol mg(-1)). However, the corresponding K-D values were not significantly different from each other (2.29 and 1.75 nM respectively). Nor did the K-i values for its displacement by CP55940, WIN55212-2, O-1184, SR141716A and SR144528 from forebrain membranes (0.87, 4.15, 2.85, 5.32 and 371.9 respectively) differ significantly from the corresponding Ki values determined in experiments with intestinal membranes (0.99, 5.03, 3.16, 4.95 and 361.5 nM respectively). 4 The B-max values of [H-3]-CP55940 and [H-3]-SR141716A in forebrain membranes did not differ significantly from each other (4281 and 5658 fmol mg-1) but were both greater than the B-max of [H-3](-) WIN55212-2 (2032 fmol mg(-1)). 5 O-1184 (10 or 100 nM) produced parallel dextral shifts in the log concentration-response curves of WIN55212-2 and CP55940 for inhibition of electrically-evoked contractions of the myenteric plexuslongitudinal muscle preparation, its K-D values being 0.20 nM (against WIN55212-2) and 0.89 nM (against CP55940). 6 We conclude that cannabinoid binding sites in guinea-pig small intestine closely resemble CB1 binding sites of guinea-pig brain and that 0-1184 behaves as a cannabinoid receptor antagonist in the guinea-pig myenteric plexus-longitudinal muscle preparation.