Adenosine antagonists increase spontaneous and evoked transmitter release from neuronal cells in culture

Abstract

To examine the role played by endogenous adenosine in the modulation of transmitter release in the CNS, the effect of adenosine antagonists has been studied. Two systems have been used: (1) EPSPs recorded from pyramidal cells in organotypic hippocampal cultures; and (2) release of newly synthesized [³H]glutamate from cerebellar granule cells in dissociated culture. Bath application of 0.1–1 μM 8-phenyltheophylline (8-PT) reversibly increased both the number and size of spontaneous EPSPs and caused bursting activity in some cells. This effect was blocked by the glutamate antagonist γ-d-glutamylglycine (DGG) (1 mM) but not by atropine (10 μM) or bicuculline (100 μM). Another adenosine antagonist isobutylmethylxanthine (IBMX, 10 μM) had a similar effect to 8-PT. Spontaneous activity in pyramidal cells and that induced by adenosine antagonists was blocked by the adenosine agonist 2-chloroadenosine (2-CA) (0.2–20 μM). 8-PT (10 μM) markedly potentiated K⁺-stimulated release of newly synthesized glutamate, and also enhanced basal glutamate release. The agonist (−)-phenylisopropyladenosine ((−)-PIA, 2 μM) which is relatively selective for A₁ receptors, reduced by19 ± 5% the 8-PT-induced enhancement, and reduced K⁺-stimulated glutamate release in the absence of 8-PT to a similar extent. In contrast 5′-N-ethylcar☐amido adenosine (NECA, 2 μM), which is a relatively selective A₂ agonist, slightly enhanced glutamate release. From these results it is likely that 8-PT potentiates glutamate release in both systems by blocking the effect of endogenous adenosine on presynaptic A₁ receptors.