KAPPA-OPIOID RECEPTOR-LIKE IMMUNOREACTIVITY IN GUINEA-PIG BRAIN - ULTRASTRUCTURAL-LOCALIZATION IN PRESYNAPTIC TERMINALS IN HIPPOCAMPAL-FORMATION

Physiological and pharmacological studies have suggested that kappa op ioid receptors (KORs) may be located presynaptically in the guinea pig hippocampal formation. In the present study, KOR-like immunoreactivit y (-LI) was examined by using a rabbit antibody raised against a synth etic peptide from the carboxyl terminus of a cloned rat kappa receptor (KT). The specificity of affinity-purified KT antibody was confirmed by Western blotting, enzyme-linked immunosorbent assay, immunolabeling of KORs expressed in Xenopus oocytes, and immunocytochemical preadsor ption controls. Specificity also was demonstrated by the light microsc opic distribution of KT-LI in sections through the forebrain and the p ens, which was largely consistent with the distribution of KORs previo usly reported, and resembled that of immunoreactivity for dynorphin B, an endogenous ligand for KORs. Detailed analysis of the hippocampal f ormation revealed that KT-LI was located predominantly in thin process es in the granule cell and inner molecular layers of the dentate gyrus . A few KT-labeled processes were also present in stratum lacunosum-mo leculare of the CA1 region and all layers of the CA3 region of the hip pocampus. By electron microscopy, KT-LI was restricted to unmyelinated axons and axon terminals, and was associated with plasma membranes, l arge dense-core vesicles, and cytoplasmic surfaces of small vesicles. In the dentate gyrus, immunolabeled terminals formed asymmetric synaps es with granule cell perikarya and large unlabeled dendrites. In the C A3 region of hippocampus, KT-LI was present in small unmyelinated axon s. The results of this study 1) demonstrate the specificity of the KT antibody, 2) show that the distribution of KT labeling corresponds wel l with previous KOR and dynorphin localization in many regions, and 3) provide ultrastructural evidence that KORs are located presynapticall y in the guinea pig hippocampal formation. (C) 1996 Wiley-Liss, Inc.