Ultrastructural study of gap junctions between dendrites of parvalbumin-containing GABAergic neurons in various neocortical areas of the adult rat.

Parvalbumin (PV)-containing GABAergic neurons in the hippocampus form dual networks linked by both dendrodendritic gap junctions and mutual inhibitory synapses. Recent physiological studies have demonstrated similar functional connectivity among cortical GABAergic neurons, but the corresponding structures have not been fully analyzed at the electron microscopic level. In this study we examined detailed ultrastructural features of gap junctions between PV neurons in the mature neocortex. Light microscopic observations and confocal laser scanning microscopy revealed frequent dendrodendritic contacts between PV neurons. Electron microscopic analysis provided direct morphological evidence for the existence of gap junctions between 22 pairs of PV-immunoreactive dendrites in the visual, auditory, and somatosensory cortices. Their ultrastructural features that were characteristic of immunolabeled profiles were consistent with the general structure of gap junctions. In one case a gap junction coexisted with a dendrodendritic chemical synapse, making a mixed synapse. Importantly, we also encountered a gap junction between PV positive and negative, presumptive non-principal cell-derived, dendrites. Quantitative analysis was made in 16 pairs of PV positive dendrites forming gap junctions in the infragranular layers of the somatosensory cortex. Diameters of these dendrites ranged from 0.3 to 2.7 microm, suggesting diverse locations of gap junctions along the proximal-distal axis of dendritic trees, but the majority (81%) were less than 1 microm. The mean size of gap junctions along apposing membranes was 0.22+/-0.09 microm. By using this size, the theoretical value of a junctional conductance was estimated to be 2.1-5.3 nS. Dendrites of PV neurons in the infragranular layers of the somatosensory cortex were reconstructed light microscopically and the sites of contacts with other PV neurons were mapped. Although these contacts do not necessarily imply gap junctional coupling, their number (5.3+/-2.3 per cell, n=11) suggested the degree of connectivity of less than 10 coupling from single PV neurons with others. Sholl analysis revealed that only 38% of their dendrites occurred within 200 microm from the soma. The present study demonstrated detailed ultrastructural features of gap junctions between mature cortical PV neurons. These features will facilitate not only identification of gap junctions in variously labeled neurons but also analysis of their functional aspects by enabling theoretical estimate of their junctional conductances.