ADENOSINE A(1) RECEPTORS IN RAT-BRAIN SYNAPTOSOMES - TRANSDUCTIONAL MECHANISMS, EFFECTS ON GLUTAMATE RELEASE, AND PRESERVATION AFTER METABOLIC INHIBITION

Synaptosomes from various rat brain areas show saturable and specific binding to an adenosine A(1) receptor ligand [Dagani et al. (1993): Dr ug Dev Res 28:359-363]. in this study, the functional correlates of th ese receptors were characterized in rat hippocampal synaptosomes by ev aluating the ability of the adenosine analogue cyclo-pentyl-adenosine (CPA) to modulate KCl- and veratridine-induced glutamate outflow. CPA concentration-dependently reduced both depolarization-induced glutamat e release and the associated increases of intrasynaptosomal Ca2+ conce ntrations. Maximal reduction by 100 mu M CPA (30% and 40% with respect to control for veratridine- and KCI-induced release, respectively) wa s completely antagonized by the xanthine adenosine receptor blocker ba myfilline, confirming the involvement of adenosine A(1) receptors. CPA selectively affected outflow of glutamate, with no significant influe nce on release of aspartate or GABA. Experiments performed in the abse nce of extrasynaptosomal Ca2+ ions suggested that adenosine selectivel y modulates glutamate Ca2+-dependent vesicular pool. Transductional st udies showed that mobilization of Ca2+ from intrasynaptosomal inositol -phosphate-sensitive stores does not contribute to adenosine effects o n release, therefore implying that CPA reduction of Ca2+ influx is due to direct effects on membrane Ca2+ conductance. Conversely, activatio n of A(1) receptors resulted in inhibition of forskolin-stimulated cAM P production in the same agonist concentration range effective on modu lation of release, suggesting that this second messenger may play a ro le in the presynaptic effects of adenosine analogues. Finally, CPA red uced both glutamate efflux and the increases of internal Ca2+ concentr ations associated with block of synaptosomal energy metabolism with ro tenone and iodoacetic acid, suggesting that presynaptic A, receptors r epresent a strategic site of action for adenosine also under experimen tal conditions resembling brain ischemia and hypoxia. (C) 1995 Wiley-L iss, Inc.