The influence of naloxone on the effects of adenosine receptor agonists in analgesic tests and binding studies.

Naloxone (1 mg/kg ip) reduced analgesic effect of R-phenylisopropyladenosine (R-PIA-0.2 mg/kg sc) in hot plate and tail-immersion tests in mice and in tail-immersion test in rats. Also the effect of 2-chloroadenosine (2-CADO-2 mg/kg sc) was significantly reduced by naloxone in mice in both nociceptive tests. Naloxone induced partial reduction of analgesic effects of 5'-N-ethylcarboxamideadenosine (NECA-0.02-0.05 mg/kg sc) in mice and rats. Binding studies revealed that the affinity of adenosine agonists (R-PIA and NECA) to opioid receptors was about 5000 times weaker than the corresponding affinity of naloxone.