Cyclopentyladenosine-induced homologous down-regulation of A₁ adenosine receptors (A₁AR) in intact neurons is accompanied by receptor sequestration but not a reduction in A₁AR mRNA expression or G protein α-subunit content

We showed previously that exposure of cerebellar granule cells to the A1 adenosine receptor (A₁AR)selective agonist, cyclopentyladenosine, decreases A₁AR density and G protein coupling corresponding to blunted agonist- induced adenylyl cyclase (EC 4.6.1.1) inhibition. We have now determined that AiAR-mediated adenylyl cyclase inhibition was desensitized in a homologous manner. Carbachol- and baclofen-induced inhibition of adenylyl cyclase was unaffected by 48-h exposure to 10 μM cyclopentyladenosine. Expression of G protein α subunits was not affected dramatically by agonist exposure. The fraction of sequestered A₁AR was increased significantly at 4, 24, and 48 h of cyclopentyladenosine exposure (35, 57, and 81% increase over control, respectively). The time course of agonist-induced A₁AR sequestration was slower than that reported for other G protein-coupled receptors. Incubation with the adenosine receptor antagonist, 8-p-sulfophenyltheophylline or adenosine deaminase did not alter sequestration significantly. Neither steady-state AiAR mRNA levels nor transcript stability was affected by 48-h agonist exposure. We determined that A₁AR half-life in cerebellar granule cells is 20.9 h, which is considerably longer than that reported for several other G protein-coupled receptors. The slow time course of A₁AR sequestration and the stability of the corresponding mRNA may be a reflection of the tonic inhibitory tone exerted by adenosine in brain.