Adenosine A1 receptor inhibition of glutamate exocytosis and protein kinase C-mediated decoupling

The adenosine modulation of glutamate exocytosis from guinea pig cerebrocortical synaptosomes is investigated. Endogenously leaked adenosine is sufficient to cause a partial tonic inhibition of 4-aminopyridine-evoked glutamate release, which can be relieved by adenosine deaminase. The adenosine A1 receptor is equally effective in mediating inhibition of glutamate exocytosis evoked by 4-aminopyridine (where K(+)-channel activation would inhibit release) and by elevated KCl (where K(+)-channel activation would have no effect), arguing for a central role of Ca(2+)-channel modulation. In support of this, the plateau phase of depolarization-evoked free Ca2+ elevation is decreased by adenosine with both depolarization protocols. No effect of adenosine agonists is seen on membrane potential in polarized or KCl- or 4-aminopyridine-stimulated synaptosomes. The interaction of protein kinase C with the A1 receptor-mediated inhibition is examined. Activation of protein kinase C by 4 beta-phorbol dibutyrate has been shown previously by this laboratory to modulate glutamate release via K(+)-channel inhibition, and is shown here to have an additional action of decoupling the adenosine inhibition of glutamate exocytosis.