A lack of potency for the delta-opioid antagonist naltrindole after microinjection into the rostral ventrolateral medulla of rabbits.

1. Naltrindole is a novel delta-opioid antagonist which is highly potent in vitro. We examined the effects on arterial pressure of naltrindole (0.3-300 pmol) after microinjection into the pressor area of the rostral ventrolateral medulla of chloralose-anaesthetized rabbits. 2. Naltrindole itself had no significant effects on arterial pressure, and only slightly attenuated the hypotensive effect of an exogenous agonist ( [D-Ala2, D-Leu5]-enkephalin). This is in contrast to previous demonstration of a marked pressor effect following another delta-antagonist, ICI 174,864. 3. Thus, naltrindole, tested in vivo against endogenous opioid effects, does not appear to be as potent as it is in vitro, and in this case was virtually ineffective.