Vagus Nerve Stimulation is used to treat a variety of medical conditions including stroke, tinnitus, and opioid withdrawal. However, because implantation of the device is invasive and involves surgical risks, stimulation of the auricular branch of the vagus nerve (ABVN) has been proposed as a non-invasive alternative. The ABVN is a purely sensory nerve that innervates a small portion of the external ear. Unfortunately, little work has been done to understand the function of the complex, overlapping nerves of the external ear, partly because methods to target the sensory nerves of the external ear have been limited. The goal of this study was to test whether we could selectively label sensory nerves of the external ear in mice using a novel adeno-associated virus (AAV). A Cre-dependent fluorescent reporter was packed in AAV-PHP.S, an AAV with enhanced tropism for peripheral sensory neurons, and was injected retro-orbitally into 9 adult mice. The mice transgenically expressed Cre recombinase under the control of a promoter (Pirt) which is expressed primarily in peripheral sensory neurons. One week after the injection, we transcardially perfused the mice and harvested the sensory ganglia for cranial nerves V, VII, IX, X, cervical dorsal root ganglia, and external ears. Immunohistochemistry was used to visualize reporter expression within the ganglia and the peripheral nerve fibers. We used anti-RFP to visualize the tdTomato reporter and β-III tubulin to label the nerve fibers within the external ear. After clearing the external ear, we imaged via lightsheet microscopy and confocal microscopy. We quantified the extent of nerve fibers with tdTomato expression in the external ear by dividing the virus labeled area by the β-III tubulin labeled area. The jugular ganglion (which houses the cell-bodies of the ABVN), nodose-petrosal ganglia, and nodose-petrosal-jugular complex all showed relatively similar incidence of virus labeling, specifically an average of 16%, 25%, and 22% of neurons in these ganglia were labeled, respectively. On the other hand, approximately 44% of the sensory afferents of the ear displayed labeling with tdTomato. Overall, these findings are a key step to a better understanding of the innervation of the rodent external ear and understanding the function of the ABVN. Future directions include targeted transfection of each ganglion that innervates the external ear. Funding Sources: This study was supported by NIH/NINDS (R01NS131493) and NIH/NIDCD (R01DC018733), and The Buoniconti Fund This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Hypothesis: A novel adeno-associated virus (AAV) can be used to label the sensory nerves of the external ear in mice. Background: Stimulation of the auricular branch of the vagus nerve (ABVN) has been proposed as a noninvasive approach to vagus nerve stimulation. Here, we use a recently introduced AAV with enhanced tropism for peripheral sensory neurons (AAV-PHP.S) to label the somatosensory nerves of the external ear, including the ABVN. Methods: AAV-PHP.S capsids with a Cre-dependent fluorescent reporter cargo were injected retro-orbitally into 9 adult transgenic mice. Sensory ganglia for cranial nerves V, VII, IX, X, along with cervical dorsal root ganglia (DRG) and the external ear, were harvested. Immunohistochemistry was used to visualize expression of the reporter, tdTomato, in ganglion neurons and in sensory afferent nerve fibers in the external ear. Results: Eight of the 9 injected animals showed expression in all the relevant ganglia (CN V, VII, IX, X, and DRG). Expression was quantified in the jugular ganglion of the vagus nerve (which houses the cell bodies of the ABVN); 15.8%±1.3% of the neurons in this ganglion were successfully transduced with a fluorescent reporter (tdTomato). In 8 of the 9 animals, we also detected reporter expression in sensory nerve fibers in the external ear. We quantified the expression of the reporter in the external ear and found that the majority of nerve fiber area (66%±18%) strongly expressed tdTomato. Conclusion: AAV-PHP.S can be used to transduce the sensory nerves of the external ear in mice, including the ABVN.

In taste buds, the GPCR heterodimer TAS1R2 + TAS1R3 is considered the canonical taste receptor for sugars and noncaloric sweeteners. Nevertheless, evidence has accumulated for the presence of an alternative noncanonical transduction pathway that detects sugars, particularly at high concentration. Sodium-glucose transporter 1 (SGLT1) has been proposed as this transducer, selectively transporting glucose into a subset of taste bud cells which then transmit the signal to taste afferent neurons. To test for TAS1R-independent sweet taste detection, we conducted in vivo Ca2+ imaging on geniculate ganglion gustatory afferent neurons of Plcb2 knock-out (KO) mice of both sexes. These mice lack an essential signaling effector for TAS1R2 + TAS1R3, thus permitting visualizing signals for an alternative pathway. Indeed, glucose, sucrose, and other sugars evoked responses in Plcb2 KO gustatory afferent neurons but only when presented orally at 1 M. However, glucose, a known substrate for SGLT1, and fructose, not a substrate, elicited equivalent responses. Furthermore, response amplitudes for glucose and fructose were unaffected by varying Na+ concentration from 0 to 100 mM NaCl, again inconsistent with SGLT1. We also detected sugar-evoked responses in gustatory neurons from normal (heterozygous) mice that were consistent with a "noncanonical" pathway. Such responses were detected in separate neurons from those showing responses mediated by TAS1R2 + TAS1R3. Our results provide neural evidence for noncanonical taste transduction for many sugars but suggest that it may rely on mechanisms other than SGLT1. Most importantly, our data suggest that at least two separate, parallel neural pathways convey information on sweet taste detection from taste buds into the brainstem.

AbstractBecause of their ease of use, adeno-associated viruses (AAVs) are indispensable tools for much of neuroscience. Yet AAVs have been used relatively little to study the identities and connectivity of peripheral sensory neurons, principally because methods to selectively target peripheral neurons have been limited. The introduction of the AAV-PHP.S capsid with enhanced tropism for peripheral neurons (Chan et al., 2017) offered a solution, which we further elaborate here. Using AAV-PHP.S with GFP or mScarlet fluorescent proteins, we show that the mouse sensory ganglia for cranial nerves V, VII, IX, and X are targeted. Pseudounipolar neurons of both somatic and visceral origin, but not satellite glia, express the reporters. One week after virus injection, ≈66% of geniculate ganglion neurons were transduced. Fluorescent reporters were transported along the central and peripheral axons of these sensory neurons, permitting visualization of terminals at high resolution, and in intact, cleared brain using light sheet microscopy. Further, using a Cre-dependent reporter, we demonstrate by anatomic and functional criteria, that expression is in a cell type-selective manner. Finally, we integrate earlier neuroanatomical and molecular data within vivoCa2+imaging to demonstrate the sensory characteristics of geniculate ganglion auricular neurons, which were previously undocumented. Our analyses suggest that the AAV-PHP.S serotype will be a powerful tool for anatomically and functionally mapping the receptive fields and circuits of the expanding numbers of molecular subtypes of many somatosensory and viscerosensory neurons that continue to be defined via single-cell RNA sequencing.

In November 2019, the NIH held the “Sensory Nutrition and Disease” workshop to challenge multidisciplinary researchers working at the interface of sensory science, food science, psychology, neuroscience, nutrition, and health sciences to explore how chemosensation influences dietary choice and health. This report summarizes deliberations of the workshop, as well as follow-up discussion in the wake of the current pandemic. Three topics were addressed: A) the need to optimize human chemosensory testing and assessment, B) the plasticity of chemosensory systems, and C) the interplay of chemosensory signals, cognitive signals, dietary intake, and metabolism. Several ways to advance sensory nutrition research emerged from the workshop: 1) refining methods to measure chemosensation in large cohort studies and validating measures that reflect perception of complex chemosensations relevant to dietary choice; 2) characterizing interindividual differences in chemosensory function and how they affect ingestive behaviors, health, and disease risk; 3) defining circuit-level organization and function that link and interact with gustatory, olfactory, homeostatic, visceral, and cognitive systems; and 4) discovering new ligands for chemosensory receptors (e.g., those produced by the microbiome) and cataloging cell types expressing these receptors. Several of these priorities were made more urgent by the current pandemic because infection with sudden acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the ensuing coronavirus disease of 2019 has direct short- and perhaps long-term effects on flavor perception. There is increasing evidence of functional interactions between the chemosensory and nutritional sciences. Better characterization of this interface is expected to yield insights to promote health, mitigate disease risk, and guide nutrition policy.

In mammalian taste buds, Type I cells comprise half of all cells. These are termed “glial-like” based on morphologic and molecular features, but there are limited studies describing their function. We tested whether Type I cells sense chemosensory activation of adjacent chemosensory (i.e., Types II and III) taste bud cells, similar to synaptic glia. Using Gad2;;GCaMP3 mice of both sexes, we confirmed by immunostaining that, within taste buds, GCaMP expression is predominantly in Type I cells (with no Type II and ≈28% Type III cells expressing weakly). In dissociated taste buds, GCaMP+ Type I cells responded to bath-applied ATP (10-100 μm) but not to 5-HT (transmitters released by Type II or III cells, respectively). Type I cells also did not respond to taste stimuli (5 μm cycloheximide, 1 mm denatonium). In lingual slice preparations also, Type I cells responded to bath-applied ATP (10-100 μm). However, when taste buds in the slice were stimulated with bitter tastants (cycloheximide, denatonium, quinine), Type I cells responded robustly. Taste-evoked responses of Type I cells in the slice preparation were significantly reduced by desensitizing purinoceptors or by purinoceptor antagonists (suramin, PPADS), and were essentially eliminated by blocking synaptic ATP release (carbenoxolone) or degrading extracellular ATP (apyrase). Thus, taste-evoked release of afferent ATP from type II chemosensory cells, in addition to exciting gustatory afferent fibers, also activates glial-like Type I taste cells. We speculate that Type I cells sense chemosensory activation and that they participate in synaptic signaling, similarly to glial cells at CNS tripartite synapses. SIGNIFICANCE STATEMENT Most studies of taste buds view the chemosensitive excitable cells that express taste receptors as the sole mediators of taste detection and transmission to the CNS. Type I “glial-like” cells, with their ensheathing morphology, are mostly viewed as responsible for clearing neurotransmitters and as the “glue” holding the taste bud together. In the present study, we demonstrate that, when intact taste buds respond to their natural stimuli, Type I cells sense the activation of the chemosensory cells by detecting the afferent transmitter. Because Type I cells synthesize GABA, a known gliotransmitter, and cognate receptors are present on both presynaptic and postsynaptic elements, Type I cells may participate in GABAergic synaptic transmission in the manner of astrocytes at tripartite synapses.