Gap junction protein in rat hippocampus: Correlative light and electron microscope immunohistochemical localization

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Abstract
Immunohistochemical techniques and an affinity‐purified antibody directed against the 27‐kD gap‐junctional protein (GJP) from rat liver were used to determine the ultrastructural localization of GJP in the rat hippocampus. At the light microscope level, dense GJP immunoreactivity having a stringlike appearance was seen in a very small percentage of medium‐sized neuronal somata located in the stratum pyramidale, and diffuse immunostaining was seen in many small cell bodies in the stratum pyramidale, stratum oriens, and the alveus. Abundant GJP‐immunoreactive (GJP‐IR) varicose fibers were observed in the strata pyramidale, radiatum, and oriens but were less concentrated in the alveus. Numerous punctate GJP‐IR elements were observed in all hippocampal layers. Upon EM analysis, GJP‐IR neuronal somata in the stratum pyramidale were found to be, without exception, nonpyramidal neurons as judged by such distinguishing features as their fusiform perikarya, indented nucleus, and well‐developed rough endoplasmic reticulum (RER). Immunostaining within these cells was largely localized to the Golgi apparatus and associated vesicular components. Small, diffusely GJP‐IR cells were identified ultrastructurally as protoplasmic and fibrous astrocytes. Immunostaining within these cells was localized to the Golgi apparatus, RER, and small, ribosomelike bodies 15–25 nm in diameter. Among neuronal processes GJP immunoreactivity was found within dendrites, axons, and axonal terminals. The latter structures contained numerous GJP‐IR vesicles having an average diameter of about 40 nm. A frequent observation indicating some degree of specificity of the anti‐GJP antibody employed here was immunostaining of typical gap junctions between dendrites and, more commonly, between processes of glial cells. Ocassionally, however, GJP‐IR dendrodendritic, axodendritic, and axoaxonic contacts were found that could be considered, at best, as being gap‐junction‐like (gj‐L). In these cases, asymmetric immunostaining of adjacent plasma membranes forming gj‐L structures was not uncommon. These results confirm the existence of gap junctions between dendrites in the rat hippocampus and demonstrate that GJP immunoreactivity on cytoplasmic membranes is restricted either to typical neuronal and glial gap junctions or to gj‐L structures at circumscribed sites of contact between various types of neuronal elements where GJP may contribute to a novel mechanism of neural communication.