Autoradiographic evidence for decrease in binding of μ- and δ-opioid receptors after subchronic [D-Ala²,D-Leu⁵]enkephalin treatment in rats

Tolerance to the antinociceptive effect of [D-Ala²,D-Leu⁵]enkephalin (DADLE) develops in Sprague-Dawley rats given the peptide chronically. δ-Opioid receptor binding is significantly reduced in P₂ membranes from various brain areas after 1-3 days' treatment. μ-Opioid receptor binding, however, is reduced only in striatum, and only after 5 days. To study this finding further, receptor autoradiography was used to quantity μ- and δ-opioid binding sites in rat brain sections after subchronic DADLE treatment. Autoradiograms were made following equilibrium binding of the highly selective opioid radioligands, [³H]Tyr-D-Ala-Gly-MePhe-Gly-ol ([³H]DAMGO) and [³H][D-Pen^2,5]enkephalin ([³H]DPDPE) to brain sections. Computerized grain counting was applied to discrete regions of the autoradiograms corresponding to caudate and interpeduncular nuclei. We found that [³H]DAMGO binding decreased in caudate after 3 days of DADLE treatment and [³H]DPDPE binding decreased in the interpeduncular nucleus, rostal portion, after 1-day DADLE treatment. These autoradiographic changes are consistent with our earlier results for biochemical binding, although we now detected an earlier change in μ-opioid receptor binding, by using autoradiography.