Purine involvement in morphine antinociception

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Abstract
1 The effects of a series of adenosine derivatives on morphine antinoceptive effect were investigated in rats by the ‘tail-flick’ method. 2 2-Chloroadenosine (CADO) and L-N6-phenylisopropyladenosine (L-PIA), given intraperitoneally, caused decreased morphine antinociception. 3 Intracerebroventricular injections of CADO, L-PIA and 5′-N-ethylcarboxamide adenosine (NECA), but not of 2′-deoxyadenosine, antagonized morphine antinociception. 4 The effects of both central and peripheral injections of CADO and L-PIA on morphine antinociception were partially reversed by caffeine. 5 Intracerebroventricular injection of dibutyryl-cyclic 3′,5′ adenosine monophosphate (db cyclic AMP) had no effect on morphine antinociception. 6 These data indicate that adenosine plays a role in morphine-induced antinociception. The results are discussed in terms of postulated effects of adenosine derivatives on adenylate cyclase.