An in vitro autoradiographic analysis of mu and delta opioid binding in the hippocampal formation of kindled rats

Recent studies have shown that opioid peptide levels are altered in hippocampal formation of kindled animals. We therefore studied the distributions of mu and delta opioid binding sites in hippocampal formation of kindled and control rats using quantitative in vitro autoradiography. Animals received daily stimulations of the amygdala until they experienced 3 class 5 seizures. Paired control animals underwent implantation of electrodes but were not stimulated. Mu binding sites were labeled with ¹²⁵I-FK-33824. Twenty-four hours after the last kindled seizure, mu binding was decreased by 32% in stratum pyramidale of CA₁ and stratum radiatum of CA₂ and by 17-27% throughout most of the rest of CA₁, CA₂ and CA₃. Few, if any, differences were seen between kindled and control animals at 7 or 28 days after the last kindled seizure. Delta binding sites were labeled with ¹²⁵I-[d-Ala², d-Leu⁵]enkephalin in the presence of the morphiceptin analog PL-032. Twenty-four hours after the last kindled seizure, delta binding was decreased only in stratum moleculare of the dentate gyrus. Seven days after the last kindled seizure, delta binding was decreased by 11-17% throughout CA₁, CA₃, and the dentate gyrus. At 28 days after the last seizure, however, no differences were found between kindled and control animals. Since the decreases in mu and delta opioid binding are transient, they are unlikely to be the molecular basis of the permanent kindling phenomenon. Rather, these changes in opioid binding may represent responses to repeated seizures.