Chronic effect of [D-Pen²,D-Pen⁵]enkephalin on rat brain opioid receptors

In previous studies, we have demonstrated that chronic etorphine or [D-Ala²,D-Leu⁵]enkephalin (DADLE) treatment of rats results in the reduction of μ- and δ-opioid receptor binding activities as tolerance develops. As both etorphine and DADLE are relatively non-specific opioid ligands, interacting with both μ- and δ-rcceptors these studies could not determine whether down-regulation of a specific receptor type occurs. Therefore, in the present studies, animals were rendered tolerant to the δ-opioid receptor-selective agonist [D-Pen²,D-Pen⁵]enkephalin (DPDPE), and receptor binding activities were measured. Treating Sprague-Dawley rats with increasing doses of DPDPE (80-160-240-320 μg/kg) i.c.v. for 1 to 4 days resulted in a time-dependent increase in the ad₅₀ of DPDPE to elicit an antinociceptivc response. When δ-receptor binding was determined by using [³H]DPDPE, a 40-50% decrease in binding in the midbrain and cortex, and 25-35% decrease in binding in the striatum were observed after 3 or 4 days of DPDPE treatment. Scatchard analysis of the [³H]DPDPE saturation binding data revealed a decrease in B_max values and no significant change in k_d values. To our surprise, when μ-receptor hinding was determined by using (³H]Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO), a 10-15% decrease in binding was also observed in the midbrain and cortex after 4 days of DPDPE treatment. Our conclusion is that chronic DPDPE treatment preferentially reduces δ-opioid receptor binding activity. Its minor effect on the μ-opioid receptor maybe due to an interaction between δ_ex and μ_ex binding sites.