An optimized approach to study endocannabinoid signaling: evidence against constitutive activity of rat brain adenosine A(1) and cannabinoid CB1 receptors

1 At nanomolar concentrations, SR141716 and AM251 act as specific and selective antagonists of the cannabinoid CB1 receptor. In the micromolar range, these compounds were shown to inhibit basal G-protein activity, and this is often interpreted to implicate constitutive activity of the CB, receptors in native tissue. We show here, using [S-35]GTPgammaS binding techniques, that micromolar concentrations of SR141716 and AM251 inhibit basal G-protein activity in rat cerebellar membranes, but only in conditions where tonic adenosine A(1) receptor signaling is not eliminated. 2 Unlike lipophilic A(1) receptor antagonists (potency order DPCPX>>N-0840 approximate to cirsimarin>caffeine), adenosine deaminase (ADA) was not fully capable in eliminating basal A, receptor-dependent G-protein activity. Importantly, all antagonists reduced basal signal to the same extent (20%), and the response evoked by the inverse agonist DPCPX was not reversed by the neutral antagonist N-0840. These data indicate that rat brain A(1) receptors are not constitutively active, but that an ADA-resistant adenosine pool is responsible for tonic A(1) receptor activity in brain membranes. 3 SR141716 and AM251, at concentrations fully effective in reversing CBI-mediated responses (10(-6)M), did not reduce basal G-protein activity, indicating that CB1 receptors are not constitutively active in these preparations. 4 At higher concentrations (1 -2.5 x 10(-5) M), both antagonists reduced basal G-protein activity in control and ADA-treated membranes, but had no effect when A(1) receptor signaling was blocked with DPCPX. Moreover, the CB1 antagonists right-shifted A(1) agonist dose-response curves without affecting maximal responses, suggesting competitive mode of antagonist action. The CB, antagonists did not affect muscarinic acetylcholine or GABA(B) receptor signaling. 5 When further optimizing G-protein activation assay for the labile endocannabinoid 2-arachidonoylglycerol (2-AG), we show, by using HPLC, that pretreatment of cerebellar membranes with methyl arachidonoyl fluorophosphonate (MAFP) fully prevented enzymatic degradation of 2-AG and concomitantly enhanced the potency of 2-AG. In contrast to previous claims, MAFP exhibited no antagonist activity at the CB, receptor. 6 The findings establish an optimized method with improved signal-to-noise ratio to assess endocannabinoid-dependent G-protein activity in brain membranes, under assay conditions where basal adenosinergic tone and enzymatic degradation of 2-AG are fully eliminated.