Intercellular interactions in the mammalian olfactory nerve

Abstract

The small, unmyelinated axons of olfactory sensory neurons project to the olfactory bulb in densely packed fascicles, an arrangement conducive to axo-axonal interactions. We recently demonstrated ephaptic interactions between these axons in the olfactory nerve layer, the layer of the olfactory bulb in which the axon fascicles interweave and rearrange extensively. In the present study, we hypothesized that the axons, which express connexins, may have another mode of communication: gap junctions. Previous transmission electron microscopy (TEM) studies have failed to demonstrate such junctions. However, the definitive method for detecting gap junctions, freeze fracture, has not been used to examine the interaxonal connections of the olfactory nerve layer. Here, we apply a combined approach of TEM and freeze fracture to determine if gap junctions are present between the olfactory axons. Gap junctions involving olfactory axons were not found. However, by freeze fracture, P faces of both the axons and ensheathing cells (glia that surround the axon fascicles) contained distinctive linear arrays of particles, aligned along the small columns of extracellular space. In axons, few intramembranous particles were present outside of these arrays. Multi-helix proteins, including ion channels and connexin hemichannels, have been shown to be visible as particles by freeze fracture. This suggests that the proteins important for signal transmission are confined to the linear arrays. Such an arrangement would facilitate ephaptic transmission, calcium waves, current oscillations, and paracrine communication and may be important for olfactory neural code processing.Keywords freeze fracture; ultrastructure; olfactory bulb; ephaptic; connexins; hemichannels Olfactory receptor neurons project from the olfactory epithelium to the olfactory bulb by dense bundles of thin, unmyelinated olfactory axons (Marin-Padilla and Amieva, 1989;Daston et al., 1990;Mori, 1993). The high packing density of these axons and the absence of intervening glia suggest that olfactory coding may be affected by axoaxonal interactions.We recently demonstrated (Bokil et al., 2001) that neighboring axons are able to influence each other by ephaptic interactions-current spread through the small extracellular space. We have argued (Bokil et al., 2001) that ephaptic interactions, as well as interneuronal interactions mediated by extruded potassium ions (Bliss and Rosenberg, 1979) First, olfactory receptors of mature mammals are constantly regenerated (Graziadei and Graziadei, 1979;Graziadei and Monti Graziadei, 1983), and developing neurons of the central nervous system are often extensively coupled by gap junctions (reviewed in Kandler and Katz, 1995;Naus and Bani-Yaghoub, 1998;Chang and Balice-Gordon, 2000). Second, olfactory receptor neurons express connexins, the proteins that form gap junctions. Olfactory receptor neurons express mRNA for connexins 43 (Zhang et al., 2000), 45 (Zhang andRestrepo, 2002), and 36 (Zhang andRestrepo, 2003). Furthermore, in immun...

Ultrastructural localization of connexins (Cx36, Cx43, Cx45), glutamate receptors and aquaporin-4 in rodent olfactory mucosa, olfactory nerve and olfactory bulb

“…3), which produce most of the aqueous nasal secretions that bathe the sensory cilia. Consistent with previous ultrastructural studies (Miragall et al, 1992(Miragall et al, ,1996aBlinder et al, 2003), areas of close membrane apposition identified as gap junctions linked fibroblasts of the lamina propria ( Fig. 2(B)), Bowman's gland cells (Fig.…”

“…Even with high visibility and high detective efficiency of FRIL, no axonal gap junctions were detected, labeled or unlabeled; nor was immunogold labeling above background for any connexin. Similarly, other investigators using conventional freeze fracture did not detect gap junctions between axons or between axons and ensheathing cells (Blinder et al, 2003). Thus, we conclude that gap junctions do not occur (or are extremely rare) between adjacent olfactory axons within the olfactory mucosa, cribriform plate, or olfactory neuron layer of the OB.…”

“…Thus, Zhang and coworkers concluded that gap junctions containing these connexins were relatively abundant in most ORNs over major areas of the OE. However, numerous freeze-fracture studies did not detect gap junctions between ORNs or between their unmyelinated axons (Menco, 1980b;Miragall et al, 1996b;Blinder et al, 2003). These conflicting data have led to confusion regarding possible involvement of Cx36, Cx43 and/or Cx45 in proposed ORN/ORN gap junctions in olfactory signaling (Zufall, 2005;Copelli et al, 2005).…”

Epithelial and endothelial barriers in the olfactory region of the nasal cavity of the rat

“…This has prompted investigations suggesting ephaptic interactions between adjacent axons (Bokil et al 2001;Blinder et al 2003). Thus, tight junctions between OEC processes might serve to conWne interactions between axons.…”

Axon Physiology

“…They are primarily observed when the extracellular conductance is reduced (37,280). This condition is fulfilled, for instance, in bundles of unmyelinated axons where the periaxonal space is minimal, as in olfactory nerves (62,68). Ephaptic interactions between axons have also been observed in frog sciatic nerve (288) and in demyelinated spinal axons of dystrophic mice (436).…”