Characterization of A-2 Receptors in Postmortem Human Pineal Gland

We have examined the binding of the adenosine agonist radioligands 3N]N6-cyclohexyladenosine (3H]CHA) and 3H]5'-N-ethylcarboxamidoadenosine (3H]NECA) to membranes prepared from postmortem human pineal glands. The results showed that the A-1-specific ligand CHA did not bind to membranes. By contrast, 3H]NECA, a nonselective A-1/A-2 ligand, gave 68% specific binding of the total binding. This specific binding was nearly insensitive to the N-ethyl-maleimide pretreatment method. To characterize this binding, we used cyclopentyladenosine (50 nM). Under those conditions 3H]NECA binding at 30 degrees C was rapid and reversible; the KD determined from the kinetic studies was 141 nM. In postmortem human pineal gland, the rank order of potency of adenosine analogues and drugs competing with 3H]NECA showed the specificity for an A-2 receptor: NECA greater than 2-chloroadenosine greater than L-N6(2-phenylisopropyl)adenosine greater than 8-phenyltheophylline greater than 3-isobutyl-1-methylxanthine greater than caffeine. Guanylylimidodiphosphate (100 microM) induced a decrease in the affinity of 3H]NECA, a result suggesting the involvement of a G protein mechanism in the coupling of the adenosine receptor to other components of the receptor complex. Scatchard analysis revealed one class of binding sites for 3H]NECA with KD and Bmax ranging from 175 to 268 nM and 11.0 to 14.1 pmol/mg protein, respectively. The binding of 3H]NECA was not affected by age, sex, or postmortem delay. 3H]NECA should be a useful tool to assess brain A-2 receptor density in a variety of neuropsychiatric disorders.