CYCLOPENTYLADENOSINE-INDUCED HOMOLOGOUS DOWN-REGULATION OF A(1) ADENOSINE RECEPTORS (A(1)AR) IN INTACT NEURONS IS ACCOMPANIED BY RECEPTOR SEQUESTRATION BUT NOT A REDUCTION IN A(1)AR MESSENGER-RNA EXPRESSION ORG-PROTEIN ALPHA-SUBUNIT CONTENT

We showed previously that exposure of cerebellar granule cells to the A(1) adenosine receptor (A(1)AR)selective agonist, cyclopentyladenosin e, decreases A(1)AR density and G protein coupling corresponding to bl unted agonist-induced adenylyl cyclase (EC 4.6.1.1) inhibition. We hav e now determined that A,AR-mediated adenylyl cyclase inhibition was de sensitized in a homologous manner. Carbachol- and baclofen-induced inh ibition of adenylyl cyclase was unaffected by 48-h exposure to 10 mu M cyclopentyladenosine. Expression of G protein alpha-subunits was not affected dramatically by agonist exposure. The fraction of sequestered A(1)AR was increased significantly at 4, 24, and 48 h of cyclopentyla denosine exposure (35, 57, and 81 % increase over control, respectivel y). The time course of agonist-induced A(1)AR 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. Neithe r steady-state A(1)AR mRNA levels nor transcript stability was affecte d by 48-h agonist exposure. We determined that A(1)AR half-life in cer ebellar granule cells is 20.9 h, which is considerably longer than tha t reported for several other G protein-coupled receptors. The slow tim e course of A(1)AR sequestration and the stability of the correspondin g mRNA may be a reflection of the tonic inhibitory tone exerted by ade nosine in brain.