Pharmacological characterization of adenosine A1 receptors and its functional role in brown trout (Salmo trutta) brain

The adenosine receptor agonist N-6-cyclohexyl[H-3]adenosine ([H-3]CHA) was used to identify and pharmacologically characterize adenosine Al receptors in brown trout (Salmo trutta) brain. In membranes prepared from trout whole brain, the Al receptor agonist [H-3]CHA bound saturably, reversibly and wi th high affinity (K-d = 0.69 +/- 0.04 nM; B-max = 0.624 +/- 0.012 pmol/mg p rotein) to a single class of binding sites, in equilibrium competition expe riments, the adenosine agonists and antagonists all displaced [H-3]CHA from high-affinity binding sites with the rank order of potency characteristic for an adenosine Al receptors. Al receptor density appeared not age-related (from 3 months until 4 years), and was similar in different brain areas. T he specific binding was inhibited by guanosine 5'-triphosphate (IC50 = 0.77 8 +/- 0.067 mu M). GTP (5 mu M) induced a low affinity state of Al receptor s. In superfused trout cerebral synaptosomes, 30 mM K+ stimulated the relea se of glutamate in a calcium dependent manner. Gluramate-evoked release was dose-dependently reduced by CHA, and the inhibition was reversed by the Al antagonist 8-cyclopentyltheophylline (CPT). In the same synaptosomal prepa ration, 30 mM K+ as well as 1 mM glutamate stimulated the release of adenos ine in a Ca2+-independent manner and tetrodotoxin insensitive. These findin gs show that in trout brain adenosine Al receptors are present which are in volved in the modulation of glutamate transmitter release. Moreover, the st imulation of adenosine release by K+ depolarisation or glutamate support th e hypothesis that, as in mammalian brain, a cross-talk between adenosine an d glutamate systems exists also in trout brain. (C) 1999 Elsevier Science B .V. All rights reserved.