naltrindole and enadoline

The effects of a daily injection of the delta selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on antinociceptive and behavioral responses to the mu selective agonist alfentanil (65 microg/kg) and the kappa selective agonist CI-977 (50 microg/kg) in 20-day-old male rats were investigated. Antinociception was assessed using the tail immersion test and behavioral testing was performed by employing an open field. The functional blockade of the delta receptor by naltrindole blocked the antinociceptive response to alfentanil but did not affect the antinociception induced by CI-977. The effects of alfentanil (increased exploration) and CI-977 (a marked hypoactivity) in the open field were not modified by neonatal naltrindole treatment. The results suggest a functional interaction between delta and mu receptors in the postnatal period but not between delta and kappa receptors. The data also suggest differences in the delta and mu receptors interacting in the modulation of antinociception and those involved in behavioral responses in the open field.

Topics: Alfentanil; Animals; Animals, Newborn; Benzofurans; Male; Naltrexone; Narcotic Antagonists; Narcotics; Pain Measurement; Psychomotor Performance; Pyrrolidines; Rats; Rats, Wistar; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu

In mammals, opioids act by interactions with three distinct types of receptors: mu, delta, or kappa opioid receptors. Using a novel assay of antinociception in the Northern grass frog, Rana pipiens, previous work demonstrated that selective mu, delta, or kappa opioids produced a potent antinociception when administered by the spinal route. The relative potency of this effect was highly correlated to that found in mammals. Present studies employing selective opioid antagonists, beta-FNA, NTI, or nor-BNI demonstrated that, in general, these antagonists were not selective in the amphibian model. These data have implications for the functional evolution of opioid receptors in vertebrates and suggest that the tested mu, delta, and kappa opioids mediate antinociception via a single type of opioid receptor in amphibians, termed the unireceptor.

Topics: Acetic Acid; Analgesics; Animals; Benzofurans; Female; Injections, Spinal; Male; Naltrexone; Narcotic Antagonists; Oligopeptides; Pain Measurement; Pyrrolidines; Rana pipiens; Receptors, Opioid; Time Factors