Chemosensory information processing between keratinocytes and trigeminal neurons.

Trigeminal fibers terminate within the facial mucosa and skin and transmit tactile, proprioceptive, chemical, and nociceptive sensations. Trigeminal sensations can arise from the direct stimulation of intraepithelial free nerve endings or indirectly through information transmission from adjacent cells at the peripheral innervation area. For mechanical and thermal cues, communication processes between skin cells and somatosensory neurons have already been suggested. High concentrations of most odors typically provoke trigeminal sensations in vivo but surprisingly fail to activate trigeminal neuron monocultures. This fact favors the hypothesis that epithelial cells may participate in chemodetection and subsequently transmit signals to neighboring trigeminal fibers. Keratinocytes, the major cell type of the epidermis, express various receptors that enable reactions to multiple environmental stimuli. Here, using a co-culture approach, we show for the first time that exposure to the odorant chemicals induces a chemical communication between human HaCaT keratinocytes and mouse trigeminal neurons. Moreover, a supernatant analysis of stimulated keratinocytes and subsequent blocking experiments with pyrodoxalphosphate-6-azophenyl-2',4'-disulfonate revealed that ATP serves as the mediating transmitter molecule released from skin cells after odor stimulation. We show that the ATP release resulting from Javanol® stimulation of keratinocytes was mediated by pannexins. Consequently, keratinocytes act as chemosensors linking the environment and the trigeminal system via ATP signaling.

Astringency is a trigeminal sensation that involves the activation of G protein-coupled signaling by phenolic compounds.

Astringency is an everyday sensory experience best described as a dry mouthfeel typically elicited by phenol-rich alimentary products like tea and wine. The neural correlates and cellular mechanisms of astringency perception are still not well understood. We explored taste and astringency perception in human subjects to study the contribution of the taste as well as of the trigeminal sensory system to astringency perception. Subjects with either a lesion or lidocaine anesthesia of the Chorda tympani taste nerve showed no impairment of astringency perception. Only anesthesia of both the lingual taste and trigeminal innervation by inferior alveolar nerve block led to a loss of astringency perception. In an in vitro model of trigeminal ganglion neurons of mice, we studied the cellular mechanisms of astringency perception. Primary mouse trigeminal ganglion neurons showed robust responses to 8 out of 19 monomeric phenolic astringent compounds and 8 polymeric red wine polyphenols in Ca(2+) imaging experiments. The activating substances shared one or several galloyl moieties, whereas substances lacking the moiety did not or only weakly stimulate responses. The responses depended on Ca(2+) influx and voltage-gated Ca(2+) channels, but not on transient receptor potential channels. Responses to the phenolic compound epigallocatechin gallate as well as to a polymeric red wine polyphenol were inhibited by the Gαs inactivator suramin, the adenylate cyclase inhibitor SQ, and the cyclic nucleotide-gated channel inhibitor l-cis-diltiazem and displayed sensitivity to blockers of Ca(2+)-activated Cl(-) channels.

In vivo monitoring of chemically evoked activity patterns in the rat trigeminal ganglion.

Albeit lacking a sense of smell, anosmic patients maintain a reduced ability to distinguish different volatile chemicals by relying exclusively on their trigeminal system (TS). To elucidate differences in the neuronal representation of these volatile substances in the TS, we performed voltage-sensitive dye imaging (VSDI) in the rat trigeminal ganglion (TG) in vivo. We demonstrated that stimulus-specific patterns of bioelectrical activity occur within the TG upon nasal administration of ten different volatile chemicals. With regard to spatial differences between the evoked trigeminal response patterns, these substances could be sorted into three groups. Signal intensity and onset latencies were also dependent on the administered stimulus and its concentration. We conclude that particular compounds detected by the TS are represented by (1) a specific spatial response pattern, (2) the signal intensity, and (3) onset latencies within the pattern. Jointly, these trigeminal representations may contribute to the surprisingly high discriminative skills of anosmic patients.

Transient receptor potential channels encode volatile chemicals sensed by rat trigeminal ganglion neurons.

Primary sensory afferents of the dorsal root and trigeminal ganglia constantly transmit sensory information depicting the individual's physical and chemical environment to higher brain regions. Beyond the typical trigeminal stimuli (e.g. irritants), environmental stimuli comprise a plethora of volatile chemicals with olfactory components (odorants). In spite of a complete loss of their sense of smell, anosmic patients may retain the ability to roughly discriminate between different volatile compounds. While the detailed mechanisms remain elusive, sensory structures belonging to the trigeminal system seem to be responsible for this phenomenon. In order to gain a better understanding of the mechanisms underlying the activation of the trigeminal system by volatile chemicals, we investigated odorant-induced membrane potential changes in cultured rat trigeminal neurons induced by the odorants vanillin, heliotropyl acetone, helional, and geraniol. We observed the dose-dependent depolarization of trigeminal neurons upon application of these substances occurring in a stimulus-specific manner and could show that distinct neuronal populations respond to different odorants. Using specific antagonists, we found evidence that TRPA1, TRPM8, and/or TRPV1 contribute to the activation. In order to further test this hypothesis, we used recombinantly expressed rat and human variants of these channels to investigate whether they are indeed activated by the odorants tested. We additionally found that the odorants dose-dependently inhibit two-pore potassium channels TASK1 and TASK3 heterologously expressed In Xenopus laevis oocytes. We suggest that the capability of various odorants to activate different TRP channels and to inhibit potassium channels causes neuronal depolarization and activation of distinct subpopulations of trigeminal sensory neurons, forming the basis for a specific representation of volatile chemicals in the trigeminal ganglia.