Pentobarbital Antagonizes the A1 Adenosine Receptor-Mediated Inhibition of Hippocampal Neurotransmitter Release

Barbiturates have been shown to be competitive antagonists at A1 adenosine receptors in radioligand binding studies. The present study investigates the effects of pentobarbital on the A1 receptor-mediated inhibition of neurotransmitter release from rabbit hippocampal slices. The inhibition of the electrically evoked release of 3H]noradrenaline by the A1 receptor agonist (R)-N6-phenylisopropyladenosine (R-PIA) was antagonized by pentobarbital with an apparent pA2 value of 3.5. Low concentrations of pentobarbital alone altered neither basal nor evoked release of 3H]noradrenaline, whereas 1,000 microM pentobarbital enhanced the basal and reduced the evoked release. In the presence of 8-phenyltheophylline, pentobarbital (200 microM and 1,000 microM) reduced the evoked noradrenaline release. Pentobarbital also antagonized the inhibition of 3H]acetylcholine release by R-PIA. In contrast to the noradrenaline release model, the evoked release of acetylcholine was enhanced by the presence of pentobarbital (50-500 microM), an effect that was lost in the presence of 8-phenyltheophylline. These results indicate that pentobarbital, in addition to a direct inhibitory action at higher concentrations, has a facilitatory effect on neurotransmitter release by blocking presynaptic A1 adenosine receptors. The possible relevance of these findings for the excitatory effects of barbiturates is discussed.