Postsynaptic and extrasynaptic localization of kappa-opioid receptor in selected brain areas of young rat and chick using an anti-receptor monoclonal antibody.

kappa-opioid receptors were visualized by light and electron microscopical immunohistochemistry in young rat and chick brains, using a monoclonal antibody KA8 (IgG1, kappa) raised against a kappa-opioid receptor preparation from frog brain, which recognizes selectively the kappa-type receptor with preference for the kappa-2 subtype. The most pronounced kappa-opioid receptor-like immunoreactivity was observed in the hypothalamic nuclei of the rat brain and in the chick optic tectum, in regions where the functional significance of kappa-opioid receptors is well documented. Both neurons and glia were stained, the former on both somata and dendrites. At the ultrastructural level, the receptor-like immunoreactivity was similar in both species. Immunoprecipitate decorated the inner surface of the plasma membrane of glial cells, neuronal somata and dendrites, in a discontinuous arrangement. In the cytoplasm, labelling was associated with ribosomes, polyribosomes and rough endoplasmic reticulum membranes but not with Golgi cisternae. In the neuropil, the immunoprecipitate was observed along the dendritic microtubules and was also associated with postsynaptic sites. Nuclei and axons were devoid of label and immunoreactivity was never visible presynaptically. Our findings indicate that the antibody used in the present study marks various forms of the kappa-opioid receptor protein including those synthesised in ribosomes, transported along dendritic microtubules and incorporated into postsynaptic and non-synaptic membranes. The antibody also recognizes glial opioid receptors. The observed subcellular distribution appears to be conserved in phylogenetically distant species.