Ultrastructure, histological distribution, and freeze-fracture immunocytochemistry of gap junctions in rat brain and spinal cord.

The historical development of concepts of gap junctions as sites for electrical, ionic, and metabolic coupling is reviewed, from the initial discovery of gap junctions linking heart cells, to the current concepts that gap junctions represent 'electrotonic synapses' between neurons. The ultrastructure and immunocytochemistry of gap junctions in heart, brain, and spinal cord of adult rats is examined using conventional thin sections, negative staining, grid-mapped freeze-fracture replicas, and immunogold-labeled freeze-fracture replicas. We review evidence for neuronal gap junctions at 'mixed' (combined electrical and chemical) synapses throughout adult rat spinal cord. We also show immunogold labeling of connexin43 in astrocyte and ependymocyte gap junctions and of connexin32 in oligodendrocyte gap junctions. Ultrastructural and freeze-fracture immunocytochemical methods have provided for definitive determination of the number, size, histological distribution, and connexin composition of gap junctions between neurons in all regions of the central nervous systems of vertebrate species.