Gap Junctions among Dendrites of Cortical GABAergic Neurons Establish a Dense and Widespread Intercolumnar Network

Abstract

Gap junctions are common between cortical GABAergic interneurons but little is known about their quantitative distribution along dendritic profiles. Here, we provide direct morphological evidence that parvalbumin-containing GABAergic neurons in layer 2/3 of the cat visual cortex form dense and far-ranging networks through dendritic gap junctions. Gap junction-coupled networks of parvalbumin neurons were visualized using connexin36 immunohistochemistry and confocal laser-scanning microscopy (CLSM). The direct correspondence of connexin36-immunopositve puncta and gap junctions was confirmed by examining the same structures in both CLSM and electron microscopy. Single parvalbumin neurons with large somata (Ն200 m 2 ) formed 60.3 Ϯ 12.2 (mean Ϯ SD) gap junctions with other cells whereby these contacts were not restricted to proximal dendrites but occurred at distances of up to 380 m from the soma. In a Sholl analysis of large-type parvalbumin neurons, 21.9 Ϯ 7.9 gap junctions were within 50 m of the soma, 21.7 Ϯ 7.6 gap junctions in a segment between 50 and 100 m, 11.2 Ϯ 4.7 junctions between 100 and 150 m, and 5.6 Ϯ 3.6 junctions were in more distal segments. Serially interconnected neurons could be traced laterally in a boundless manner through multiple gap junctions. Comparison to the orientation-preference columns revealed that parvalbumin-immunoreactive cells distribute randomly whereby their large dendritic fields overlap considerably and cover different orientation columns. It is proposed that this dense and homogeneous electrical coupling of interneurons supports the precise synchronization of neuronal populations with differing feature preferences thereby providing a temporal frame for the generation of distributed representations.

Changes in parvalbumin immunoreactivity with aging in the central auditory system of the rat

“…mutually connected interneuronal network interconnected with gap junctions (Fukuda et al, 2006). Decreases in the presence of parvalbumin-ir neurons have been described in several major illnesses of the human brain such as schizophrenia and epilepsy (for review see Woo and Lu, 2006;Lewis et al, 2005).…”

Connexin36 vs. connexin32, “miniature” neuronal gap junctions, and limited electrotonic coupling in rodent suprachiasmatic nucleus

“…749.11, 2002) for immunocytochemical precedent, Long and coworkers (Long et al, 2005) proposed Cx36 as the primary or sole connexin in neuronal gap junctions in SCN, and suggested that Cx36 deletion disrupted circadian rhythms. Among the 21 connexins known to be present in mammals (Condorelli et al, 1998;Söhl and Willecke, 2003;Willecke et al, 2002), both Cx36 and Cx45 have been demonstrated in ultrastructurally-defined neuronal gap junctions (Rash et al, 2000(Rash et al, , 2001Fukuda et al, 2006;Kamasawa et al, 2006), and there is immunocytochemical evidence for neuronal expression of Cx50 and Cx57 in retina (Massey et al, 2003;Hombach et al, 2004;O'Brien et al, 2006). In view of controversies surrounding the existence and connexin composition of neuronal gap junctions in SCN, we used confocal immunofluorescence microscopy and freeze-fracture replica immunogold labeling (FRIL) to document gap junctions in SCN neurons and to identify the constituent connexins in neurons vs. glia, and whole-cell recordings from hypothalamic slices to re-assess tracer and electrotonic coupling.…”

Identification of connexin36 in gap junctions between neurons in rodent locus coeruleus

“…In addition to coupling between LC neurons, recent studies (Alvarez-Maubecin et al, 2000;Van Bockstaele et al, 2004) suggested widespread but weak gap junctional coupling between LC neurons and astrocytes in neonatal and adult rats, including strong dye-transfer from astrocytes to some neurons. Using silver-intensified immunogold-labeling electron microscopic immunocytochemistry, they described detection of connexin32 (Cx32) and connexin26 (Cx26) protein in a very high percentage of dendro-dendritic profiles and proposed even greater abundance of these two connexins at membrane appositions between dendrites and glial cells (primarily astrocytes).While clear evidence exists for coupling between neurons in developing LC, several challenging issues remain concerning documentation of gap junctional coupling in this nucleus: 1) No ultrastructural data exist for the presence of connexin-containing membrane appositions between neurons in the LC that meet accepted criteria for designation as gap junctions (Brightman and Reese, 1969;Sotelo and Korn, 1978;Rash et al, 1998); 2) Unlike the reported expression of abundant Cx26 and Cx32 in both neurons and astrocytes in LC, all other locations in the CNS examined by freeze-fracture replica immunolabeling (FRIL) electron microscopy have revealed cell-type-specific expression of connexin proteins, with Cx32 found only in gap junctions formed by oligodendrocytes (Rash et al, 2001a;Nagy et al, 2003a;Li et al, 2004a;Kamasawa et al, 2005), Cx26 only in gap junctions formed by pia mater and by subpopulations of astrocytes [(Nagy et al, 2001[(Nagy et al, ,2003bRash et al, 2001a); also see Mercier and Hatton (2001) and Altevogt et al, (2004)], and Cx36 only in neuronal gap junctions (Rash et al, ,2001bKamasawa et al, 2006) [also see Fukuda et al (2006)]; 3) Although proposals for coupling of neurons to astrocytes based on propagation of "calcium waves" (Nedergaard, 1994) were extended to dye coupling between astrocytes and neurons in LC slice preparations (Alvarez-Maubecin et al, 2000;Van Bockstaele et al, 2004), other nongap-junctional mechanisms appear to account for initiation and propagation of calcium waves (Charles, 1994(Charles, ,1996Parpura et al, 1994;Hassinger et al, 1995); and 4) ...…”