Lack of antinociceptive efficacy of intracerebroventricular [D-Ala²,Glu⁴]deltorphin, but not [D-Pen²,D-Pen⁵]enkephalin, in the μ-opioid receptor deficient CXBX mouse

The antinociceptive efficacy of [D-Pen²,D-Pen⁵]enkephalin (DPDPE) (δ₁agonist) and [D-Ala²,Glu⁴]deltorphin (δ₂agonist) was evaluated following intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration in CD-1 and CXBK strains of mice using the radiant heat tail-flick test. Following i.c.v. administration, [D-Ala²,Glu⁴]deltorphin was effective in CD-1, but not CXBK, mice; DPDPE was approximately equiactive in both strains. While i.c.v. [D-Ala²,Glu⁴]deltorphin did not produce antinociception in the CXBK mouse, it effectively antagonized the antinociceptive actions if i.c.v. DPDPE. [D-Ala²,Glu⁴]deltorphin was effective following i.t. administration in both strains. These data suggest possible differences in the supraspinal populations of opioid δ receptor subtypes in the CXBK strain. On the basis of previously established selectivity of these agonists, the CXBK mouse may have a predominate population of supraspinal opioid δ₁, rather than δ₂, receptors.