Ultrastructural Aspects of Olfactory Signaling

Abstract

The olfactory area of the nasal cavity is lined with olfactory receptor cell cilia that come in contact with incoming odor molecules. Ultrastructural immunocytochemical studies in rodents have shown that these cilia contain all the proteins necessary to transduce the odorous message into an electrical signal that can be transmitted to the brain. These signaling proteins include putative odor receptors, GTP binding proteins, type III adenylyl cyclase and cyclic nucleotide-gated channels. The rest of the cells, including dendrites and dendritic knobs, showed no discernible labeling with antibodies to these signaling proteins. Furthermore, freeze-fracture and freeze-etch studies have shown that the membrane morphology of olfactory cilia differs substantially from that of non-sensory cilia. Olfactory cilia have many more membrane particles. Transmembrane signaling proteins, such as odor receptors, adenylyl cyclase and cyclic nucleotide-gated channels, conceivably appear as membrane particles. Thus, the long-standing supposition that olfactory cilia are peculiarly adapted to deal with the reception and initial transduction of odorous messages has now been verified in terms of both ultrastructural morphology and cytochemistry. Emerging studies on vomeronasal receptor cell microvilli indicate that the same is true for this organ, even though the actual signaling components differ from those of the main olfactory system.

Localization of eNOS in the Olfactory Epithelium of the Rat

“…At the electron microscopic level, NO was suggested to be produced by eNOS in the cilia of the olfactory cells in the present study. In addition, many molecules involved in the signal transduction, such as Gs, Golf, ACIII, and CNG channel, are expressed in the distal parts of cilia of the olfactory cells [29,30]. Their distributions correspond to the localization of eNOS protein observed in the present study.…”

Chloride Accumulation in Mammalian Olfactory Sensory Neurons

“…In mammals, sensory cilia are membrane tubes of ϳ10 m length with a diameter of Ͻ200 nm (Menco, 1997). Moreover, these tiny tubes are immersed in mucus, an extracellular medium of mostly unknown composition that determines ion fluxes and response characteristics of OSNs in vivo.…”

“…The water-filled volume of rat olfactory cilia is estimated to be 3 ϫ 10 Ϫ18 l/m length (Lindemann, 2001), accommodating Ͻ10 4 MQAE molecules at an intracellular MQAE concentration of 5 mM, which is not sufficient to allow fluorescence detection with our optical system. Thus, we visualized the dendritic knobs at the epithelial surface, which are more or less spherical structures with a volume of ϳ10 Ϫ14 l. Electron microscopy studies (Menco, 1997) and patch-clamp measurements (Lowe and Gold, 1991) (Chao et al, 1989). Fluorescence lifetimes decreased from 3.5 to 2.2 nsec when [Cl Ϫ ] i was raised from 30 to 90 mM within the dendritic knobs (Fig.…”

Diffusion of a soluble protein, photoactivatable GFP, through a sensory cilium

“…Defects in ciliary genes lead to anosmia (Kulaga et al, 2004). (Left) Scanning EM of an embryonic rat olfactory receptor dendritic knob with multiple primary cilia (reproduced from Menco, 1997 with permission of Oxford University Press). (Right) Longitudinal section through an olfactory cilium (reproduced from Menco et al, 1997 [fig.…”