Antinociception: interaction between adenosine and GABA systems.

The interaction of GABA agonists and antagonists with the antinociceptive effects of drugs active on adenosine receptors, was examined in mice using the tail-flick test. Baclofen, but not muscimol (in the doses used), induced antinociception. The adenosine agonist NECA (5'-N-ethylcarboxamidoadenosine) produced a greater degree of antinociception than did PIA (N6-phenylisopropyladenosine). The antinociceptive effect of NECA and PIA was decreased by the GABA antagonists bicuculline, picrotoxin and phaclophen. Combination of theophylline with baclofen, but not with muscimol, induced a high antinociceptive response. Muscimol decreased the antinociceptive effect of NECA, and baclofen increased the antinociceptive action of PIA. It is concluded that there may be an interaction between antinociception induced by GABA and that induced by (an) adenosine mechanism(s).