Age-Related Expression of α-Gustducin in the Rat Larynx

The diffuse chemosensory system: exploring the iceberg toward the definition of functional roles

The diffuse chemosensory system (DCS) is an anatomical structure composed of solitary chemosensory cells (SCCs, also called solitary chemoreceptor cells), which have analogies with taste cells but are not aggregated in buds. The concept of DCS has been advanced, after the discovery that cells similar to gustatory elements are present in several organs. The elements forming the DCS share common morphological and biochemical characteristics with the taste cells located in taste buds of the oro-pharyngeal cavity but they are localized in internal organs. In particular, they may express molecules of the chemoreceptorial cascade (e.g. trans-membrane taste receptors, the G-protein alpha-gustducin, PLCbeta2, TRPM5). This article will focus on the mammalian DCS in apparatuses of endodermic origin (i.e. digestive and respiratory systems), which is composed of an enormous number of sensory elements and presents a multiplicity of morphological aspects. Recent research has provided an adequate description of these elements, but the functional role for the DCS in these apparatuses is unknown. The initial findings led to the definition of a DCS structured like an iceberg, with a mysterious "submerged" portion localized in the distal part of endodermic apparatuses. Recent work has focussed on the discovery of this submerged portion, which now appears less puzzling. However, the functional roles of the different cytotypes belonging to the DCS are not well known. Recent studies linked chemosensation of the intraluminal content to local control of absorptive and secretory (exocrine and endocrine) processes. Control of the microbial population and detection of irritants seem to be other possible functions of the DCS. In the light of these new findings, the DCS might be thought to be involved in a wide range of diseases of both the respiratory (e.g. asthma, chronic obstructive pulmonary disease, cystic fibrosis) and digestive apparatuses (absorptive or secretive diseases, dysmicrobism), as well as in systemic diseases (e.g. obesity, diabetes). A description of the functional roles of the DCS might be a first step toward the discovery of therapeutic approaches which target chemosensory mechanisms.

The role of transient receptor potential vanilloid-1 on neural responses to acids by the chorda tympani, glossopharyngeal and superior laryngeal nerves in mice

The transient receptor potential vanilloid-1 (TRPV1) receptor acts as a polymodal nociceptor activated by capsaicin, heat, and acid. TRPV1, which is expressed in sensory neurons innervating the oral cavity, is associated with an oral burning sensation in response to spicy food containing capsaicin. However, little is known about the involvement of TRPV1 in responses to acid stimuli in either the gustatory system or the general somatosensory innervation of the oropharynx. To test this possibility, we recorded electrophysiological responses to several acids (acetic acid, citric acid and HCl) and other taste stimuli from the mouse chorda tympani, glossopharyngeal and superior laryngeal nerves, and compared potential effects of iodo-resiniferatoxin (I-RTX), a potent TRPV1 antagonist, on chemical responses of the three nerves. The results indicated that in the chorda tympani nerve, I-RTX (1-100 nM) did not affect responses to acids, sucrose and quinine HCl, but reduced responses to NaCl (I-RTX at concentrations of 10 and 100 nM) and KCl and NH(4)Cl (100 nM). In contrast, in the glossopharyngeal nerve, I-RTX significantly suppressed responses to all acids and salts, but not to sucrose and quinine HCl. Responses to acetic acid were suppressed by I-RTX even at 0.1 nM concentration. The superior laryngeal nerve responded in a concentration-dependent manner to acetic acid, citric acid, HCl, KCl, NH(4)Cl and monosodium l-glutamate. The responses to acetic acid, but not to the other stimuli, were significantly inhibited by I-RTX. These results suggested that TRPV1 may be involved in the mechanism for responses to acids presented to the posterior oral cavity and larynx. This high degree of responsiveness to acetic acid may account for the oral burning sensation, known as a flavor characteristic of vinegar.

Eukaryotic vs. prokaryotic chemosensory systems

In the last decades, microbiologists demonstrated that microorganisms possess chemosensory capabilities and communicate with each other via chemical signals. In parallel, it was demonstrated that solitary eukaryotic chemosensory cells are diffusely located on the mucosae of digestive and respiratory apparatuses. It is now evident that on the mucosal surfaces of vertebrates, two chemoreceptorial systems (i.e. eukaryotic and prokaryotic) coexist in a common microenvironment. To date, it is not known if the two chemosensory systems reciprocally interact and compete for detection of chemical cues. This appears to be a fruitful field of study and future researches must consider that the mucosal epithelia possess more chemosensory capabilities than previously supposed.