Purine involvement in morphine antinociception

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.