Freeze‐fracture organization of hair cell synapses in the sensory epithelium of guinea pig organ of Corti

Abstract

The fine structure of both the afferent and efferent hair cell synapses in the sensory epithelium of guinea pig organ of Corti was examined by freeze-fracture electron microscopy. In the afferent synapse, barlike aggregates of intramembrane particles (IMPs) of about 10 nm in diameter were seen on the P-face of the afferent presynaptic membrane directly beneath the presynaptic dense projection which is located in the active zone of the presynaptic membrane. Small and large depressions have been seen on the presynaptic membrane. The former were observed in the proximity of the barlike aggregates, while the latter were observed some distance from the aggregate. In outer hair cells, IMPs of about 10 nm in diameter were seen on the P-face of the afferent postsynaptic membrane at a density of 3,000/microns 2. In the efferent synapse, many aggregates composed of from several to tens of large IMPs of 13 nm in diameter were observed on the presynaptic membrane. These aggregates were localized to small membrane depressions, which tended to be deeper as particle number per aggregate increased. Dense populations of IMPs of about 9 nm in diameter were observed on the P-face of the efferent postsynaptic membrane at a density of 4,000/microns 2. A fenestrated subsynaptic cistern completely covers the efferent postsynaptic membrane. Moreover, the subsynaptic cistern spans several efferent postsynaptic membranes when efferent synapses are gathered in a group. In the afferent and efferent synapses of hair cells, specializations of the synaptic membranes were represented by marked aggregates characteristic of IMPs. In the efferent synapse, IMP movement inside the synaptic membrane was proposed in relationship to retrival of synaptic vesicle membrane. Structural relationship between the subsynaptic cistern and efferent postsynaptic membrane was revealed.

Expression of neurotrophin receptor trkB in rat cochlear hair cells at time of rearrangement of innervation

“…With regard to IHC, trkB expression in the cells occurs concomitantly with a switch in the pattern of efferent innervation, i.e., from the base of the IHC to a more distal position below the IHC, correlated probably with the morphological switch of efferents from axosomatic to axodendritic fields (Pujol 1986). Moreover, axosomatic terminating efferents can innervate the IHC at the supranuclear level within this developmental period, as shown previously (Angelborg and Engström 1973;Hashimoto et al 1990;Saito 1990;Sobkowicz 1992;Yamashita et al 1993;Elgoyhen et al 1994;Sobkowicz and Slapnick 1994;Knipper et al 1995). Thus, trkB expression in IHC and OHC seems to be limited to the time when these sensory cells undergo a massive rearrangement of their innervation pattern.…”

Developmental expression of ?9 acetylcholine receptor mRNA in the rat cochlea and vestibular inner ear

“…However, in the adult, efferent fibers in the IHC region terminate primarily upon the dendrites of afferent neurons. The majority of authors report that synapses between olivocochlear efferent fibers and adult IHCs are rare (Lenoir et al, 1980;Saito, 1980Saito, , 1990Schwartz and Ryan, 1986), although nonsynaptic contacts between olivocochlear axons and IHCs are observed. Although others suggest that synapses are not uncommon (Hashimoto et al, 1990;Sobkowicz and Slapnick, 1994), there is evidence that the efferents that do end directly on adult IHCs are ␥-aminobutyric acid (GABA)ergic (Schwartz and Ryan, 1986;Sobkowicz and Slapnick, 1994).…”

Extracellular chloride regulation of Kv2.1, contributor to the major outward Kv current in mammalian outer hair cells

“…To estimate the Cl Ϫ concentration change made by a small flux of Cl Ϫ ions at the base of the OHC, we assessed the volume within which Cl Ϫ concentration may change from published electron microscopy descriptions (9,22,29,39). As an example, we evaluate a typical mature OHC in the apical region, which has a diameter of ϳ7 m. The distance from just above the nucleus to the bottom round end of the OHC, where …”