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.
INSERM U.205, INSA, Laboratoire de Chimie Biologique, Villeurbanne, France.
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D
Reynaud
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Gharib
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Lagarde
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Sarda
D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D001344:Autopsy; D002462:Cell Membrane / Q000378:metabolism; D005033:Ethylmaleimide / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D010870:Pineal Gland / Q000378:metabolism*; D011869:Radioligand Assay; D011983:Receptors, Purinergic / Q000187:drug effects / Q000378:metabolism*
D011983:Receptors, Purinergic; D006165:Guanylyl Imidodiphosphate; C048599:N(6)-cyclopentyladenosine; D000241:Adenosine; D005033:Ethylmaleimide