An optimized approach to study endocannabinoid signaling: Evidence against constitutive activity of rat brain adenosine A 1 and cannabinoid CB 1 receptors

Abstract

1. At nanomolar concentrations, SR141716 and AM251 act as specific and selective antagonists of the cannabinoid CB 1 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 1 receptors in native tissue. We show here, using [ 35S]GTPγS 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 ≈cirsimarin>caffeine), adenosine deaminase (ADA) was not fully capable in eliminating basal A 1 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 CB 1-mediated responses (10 -6 M), did not reduce basal G-protein activity, indicating that CB 1 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 CB 1 antagonists right-shifted A 1 agonist dose-response curves without affecting maximal responses, suggesting competitive mode of antagonist action. The CB 1 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 1 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.