C57BL/6J-bgJ (beige) mice: Differential sensitivity in the tail flick test to centrally administered MU- and delta-opioid receptor agonists

Abstract

The antinociceptive effects of two mu-opioid receptor agonists, morphine and [DAla2, MePhe4, Gly-o15]enkephalin (DAGO), and a selective delta-receptor agonist, [D-Pen2, L-Pen5]enkephalin (DPLPE), were determined in C57BL/6J-bgJ (beige) and control mice (CRS-CD1 and C57BL/6By) using a standard tail-flick assay. The antinociceptive response of C57BL/6J-bgJ mice to intracerebroventicularly administered morphine and DAGO was significantly reduced compared to controls, but there was no difference in the antinociceptive response to DPLPE. These results suggest (a) that there is a genetic deficit of mu-opioid receptor number or a genetically-induced alteration in receptor function in regions of C57BL/6J-bgJ brains involved in antinociception, (b) that delta-opioid receptors can mediate antinociception in mice, and (c) that the C57BL/6J-bgJ strain may offer a practical new animal model for studying the function of opioid receptor subtypes.