Endogenous adenosine-receptive systems do not mediate the discriminative stimulus properties of ethanol

Adenosine is an endogenous neuromodulator with depressant effects on CNS neurons. Adenosine agonists produce biphasic effects on activity, decreases in operant response rate, and anticonvulsant effects. These effects are similar to some of the behavioral effects of ethanol. In addition, it has recently been shown that relative sensitivities to some of the behavioral effects of ethanol and purinergic drugs are similar in inbred strains of mice. These findings have prompted the speculation that ethanol's behavioral effects may be mediated by an agonist action on adenosine-receptive neurons. The present study provided a direct test of this hypothesis with respect to the discriminative stimulus properties of ethanol. In this study, neither the A1 receptor agonist N6-cyclohexyladenosine nor the A2 receptor agonist N6-ethylcarboxamide adenosine produced significant generalization to the ethanol stimulus. In addition, neither the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)-adenine nor the adenosine uptake inhibitor dipyridamole were able to enhance the level of ethanol-appropriate responding seen after a low dose of ethanol. Both caffeine and 8-phenyltheophylline partially but significantly antagonized the stimulus properties of ethanol. However, the doses required to achieve these effects were much higher than those needed to block adenosine receptors. These findings strongly suggest that the discriminative stimulus properties of ethanol are not mediated through an agonist action on adenosine-receptive neurons.