Detailed Radiomorphometric Analysis of the Surgical Corridor for the Suprageniculate Approach

BACKGROUND

The suprageniculate fossa (SGF) is located between the geniculate ganglion, the middle cranial fossa (MCF) and the anterior semicircular canal (ASCC). An endoscopic transcanal approach has been recently proposed to treat the different lesions in this area. The aim of the study is to describe the anatomical pathway of this approach by measuring the dimensions of its boundaries while checking their correlation with the pneumatization of the SGF area.

METHODS

This is a retrospective anatomical analysis of Cone Beam CT scans of 80 patients, for a total of 160 temporal bones analyzed. Two checkpoints were measured for the SGF route, as an internal and an external window. These are triangles between the MCF dura, the geniculate ganglion and the ASCC on parasagittal and axial planes. The pneumatization of the SGF was also assessed, classified and correlated with the measured dimensions.

RESULTS

The depth of the SGF was 7.5 ± 1.8 mm. The width of the external window was 7.5 ± 1.9, 5.6 ± 2.4 and 1.6 ± 1.6 mm for the posterior, middle and anterior points of measurement, respectively. The height of the internal window was 7.6 ± 1.2, 4.5 ± 1.5 and 1.7 ± 1.7 mm for the posterior, middle and anterior points of measurement, respectively. Type A pneumatization was found in 87 cases, type B in 34 and type C in 39. The degree of pneumatization directly correlated to the depth and height of the fossa.

CONCLUSIONS

The suprageniculate approach route is defined by the internal and external windows which should be evaluated during a pre-surgery imaging assessment. The detailed anatomy of the approach and the novel classification of the pneumatization of the SGF are here described which may be useful to plan a safer procedure with minimal complications.

EGR4 is critical for cell-fate determination and phenotypic maintenance of geniculate ganglion neurons underlying sweet and umami taste

The sense of taste starts with activation of receptor cells in taste buds by chemical stimuli which then communicate this signal via innervating oral sensory neurons to the CNS. The cell bodies of oral sensory neurons reside in the geniculate ganglion (GG) and nodose/petrosal/jugular ganglion. The geniculate ganglion contains two main neuronal populations: BRN3A+ somatosensory neurons that innervate the pinna and PHOX2B+ sensory neurons that innervate the oral cavity. While much is known about the different taste bud cell subtypes, considerably less is known about the molecular identities of PHOX2B+ sensory subpopulations. In the GG, as many as 12 different subpopulations have been predicted from electrophysiological studies, while transcriptional identities exist for only 3 to 6. Importantly, the cell fate pathways that diversify PHOX2B+ oral sensory neurons into these subpopulations are unknown. The transcription factor EGR4 was identified as being highly expressed in GG neurons. EGR4 deletion causes GG oral sensory neurons to lose their expression of PHOX2B and other oral sensory genes and up-regulate BRN3A. This is followed by a loss of chemosensory innervation of taste buds, a loss of type II taste cells responsive to bitter, sweet, and umami stimuli, and a concomitant increase in type I glial-like taste bud cells. These deficits culminate in a loss of nerve responses to sweet and umami taste qualities. Taken together, we identify a critical role of EGR4 in cell fate specification and maintenance of subpopulations of GG neurons, which in turn maintain the appropriate sweet and umami taste receptor cells.

Taste arbor structural variability analyzed across taste regions

Taste ganglion neurons are functionally and molecularly diverse, but until recently morphological diversity was completely unexplored. Specifically, taste arbors (the portion of the neuron within the taste bud) vary in structure, but the reason for this variability is unclear. Here, we analyzed structural variability in taste arbors to determine which factors determine their morphological diversity. To characterize arbor morphology and its relationship to taste bud cells capable of transducing taste stimuli (taste-transducing cell) number and type, we utilized sparse cell genetic labeling of taste ganglion neurons in combination with whole-mount immunohistochemistry. Reconstruction of 151 taste arbors revealed variation in arbor size, complexity, and symmetry. Overall, taste arbors exist on a continuum of complexity, cannot be categorized into discrete morphological groups, and do not have stereotyped endings. Arbor size/complexity was not related to the size of the taste bud in which it was located or the type of taste-transducing cell contacted (membranes within 180 nm). Instead, arbors could be broadly categorized into three groups: large asymmetrical arbors contacting many taste-transducing cells, small symmetrical arbors contacting one or two taste-transducing cells, and unbranched arbors. Neurons with multiple arbors had arbors in more than one of these categories, indicating that this variability is not an intrinsic feature of neuron type. Instead, we speculate that arbor structure is determined primarily by nerve fiber remodeling in response to cell turnover and that large asymmetrical arbors represent a particularly plastic state.

Comprehensive microsurgical anatomy of the middle cranial fossa: Part II-neurovascular anatomy

In order to master the surgical approaches to the middle cranial fossa, the surgeon needs to understand the relevant bony anatomy. However, she/he also needs to have a clear and sound understanding of the neural and vascular anatomy because, oftentimes, the osseous anatomy (except for the optic apparatus) should be removed to expose and protect the neurovascular anatomy. This is the second of a two-part article discussing the neurovascular anatomy of the middle cranial fossa. A brief discussion of the surgical approaches follows.

Ramsay Hunt Syndrome: An Introduction, Signs and Symptoms, and Treatment

Ramsay Hunt syndrome is the complication of the virus varicella-zoster and the infection caused by it, which shows apparent geniculate ganglion involvement. This article discusses the etiology, epidemiology, and pathology of Ramsay Hunt syndrome. Clinically it may be presented as a vesicular rash on the ear or even in the mouth, pain in the ear, and facial paralysis. Some other rare symptoms may also be present, which are also discussed in this article. Skin involvement is also seen in some cases as patterns due to anastomoses between cervical and cranial nerves. This article provides an overview of how the varicella-zoster virus causes facial paralysis and other neurological symptoms. Knowing about this condition and its clinical features is essential to make an early diagnosis and, thus, provide a good prognosis. A good prognosis is required to reduce the nerve damage, prevent further complications, and start an early therapy of acyclovir and corticosteroid. This review also presents a clinical picture of the disease and its complications. The incidence of Ramsay Hunt syndrome has gradually decreased over time because of the development of the varicella-zoster vaccine and better health facilities. The paper also talks about how the diagnosis of Ramsay Hunt syndrome is made and the various treatment options available. Facial paralysis in Ramsay Hunt syndrome presents differently than Bell's Palsy. If not treated for too long, it may cause permanent muscle weakness and may also cause a loss of hearing. It may be confused with simple herpes simplex virus outbreaks or contact dermatitis.

Advances in Optical Tools to Study Taste Sensation

Taste sensation is the process of converting chemical identities in food into a neural code of the brain. Taste information is initially formed in the taste buds on the tongue, travels through the afferent gustatory nerves to the sensory ganglion neurons, and finally reaches the multiple taste centers of the brain. In the taste field, optical tools to observe cellular-level functions play a pivotal role in understanding how taste information is processed along a pathway. In this review, we introduce recent advances in the optical tools used to study the taste transduction pathways.

Oral Sensory Neurons of the Geniculate Ganglion That Express Tyrosine Hydroxylase Comprise a Subpopulation That Contacts Type II and Type III Taste Bud Cells

Oral sensory neurons of the geniculate ganglion (GG) innervate taste papillae and buds on the tongue and soft palate. Electrophysiological recordings of these neurons and fibers revealed complexity in the number of unique response profiles observed, suggesting there are several distinct neuronal subtypes. Molecular descriptions of these subpopulations are incomplete. We report here the identification of a subpopulation of GG oral sensory neurons in mice by expression of tyrosine hydroxylase (TH). TH-expressing geniculate neurons represent 10-20% of oral sensory neurons and these neurons innervate taste buds in fungiform and anterior foliate taste papillae on the surface of the tongue, as well as taste buds in the soft palate. While 35-50% of taste buds on the tongue are innervated by these TH+ neurons, 100% of soft palate taste buds are innervated. These neurons did not have extragemmal processes outside of taste buds and did not express the mechanosensory neuron-associated gene Ret, suggesting they are chemosensory and not somatosensory neurons. Within taste buds, TH-expressing fibers contacted both Type II and Type III cells, raising the possibility that they are responsive to more than one taste quality. During this analysis we also identified a rare TH+ taste receptor cell type that was found in only 12-25% of taste buds and co-expressed TRPM5, suggesting it was a Type II cell. Taken together, TH-expressing GG oral sensory neurons innervate taste buds preferentially in the soft palate and contact Type II and Type III taste bud receptor cells.

Facial Nerve Schwannoma: The Rare/Great Mimicker of Vestibular Schwannoma/Neuroma

Schwannomas are benign tumors arising from Schwann cells which are a protective casing of nerves, composing myelin sheath and can develop in any nerve where Schwann cells are present. Most common are vestibulocochlear nerve schwannomas. Facial nerve schwannomas (FNSs) are uncommon tumors involving seventh nerve of which geniculate ganglion involvement is most common. Clinical presentations and the imaging appearances of FNSs are influenced by the topographical anatomy of the facial nerve and vary according to the segments involved. We report a case of 73-year-old man presenting with right side facial weakness of lower motor neuron type involvement. Computed tomography and magnetic resonance imaging are clinching the diagnosis. An early diagnosis is important in containing the disease facilitating early surgical intervention.

Computed tomographic features of the proximal petrous facial nerve canal in recurrent Bell's palsy

OBJECTIVES

The primary objective was to determine whether the narrowest dimensions of the labyrinthine facial nerve (LFN) canal on the symptomatic side in patients with unilateral recurrent Bell's palsy (BP) differ from those on the contralateral side or in asymptomatic, age- and gender-matched controls on computed tomography (CT). The secondary objectives were to assess the extent of bony covering at the geniculate ganglion and to record inter-observer reliability of the CT measurements.

METHODS

The dimensions of the LFN canal at its narrowest point perpendicular to the long axis and the extent of bony covering at the geniculate ganglion were assessed by two radiologists. Statistical analysis was performed using the Wilcoxon signed-rank and Mann-Whitney U tests (LFN canal dimensions) and the Chi-squared test (bony covering at the geniculate ganglion). Inter-observer reliability was evaluated using Intra-Class Correlation (ICC) and Cohen's kappa.

RESULTS

The study included 21 patients with unilateral recurrent BP and 21 asymptomatic controls. There was no significant difference in the narrowest dimensions of the ipsilateral LFN canal when compared to the contralateral side or controls (P = .43-.94). Similarly, there was no significant difference in the extent of bony covering at the geniculate ganglion when compared to either group (P = .19-.8). Good inter-observer reliability was observed for LFN measurements (ICC = 0.75-0.88) but not for the bony covering at the geniculate ganglion (Cohen's kappa = 0.53).

CONCLUSION

The narrowest dimensions of the LFN canal and the extent of bony covering at the geniculate ganglion do not differ in unilateral recurrent BP, casting doubt over their etiological significance.

LEVEL OF EVIDENCE

Level IV.

The Fungiform Papilla Is a Complex, Multimodal, Oral Sensory Organ

When solid or liquid stimuli contact the tongue tip during eating, the sensations of taste, touch and temperature are immediately evoked, and tongue function relies on these simultaneous multimodal responses. We focus on the fungiform papilla of the anterior tongue as a complex organ for taste, tactile and thermal modalities, all via chorda tympani nerve innervation from the geniculate ganglion. Rather than a review, our aim is to revise the classic archetype of the fungiform as predominantly a taste bud residence only and instead emphasize an amended concept of the papilla as a multimodal organ. Neurophysiological maps of fungiform papillae in functional receptive fields demonstrate responses to chemical, stroking and cold lingual stimuli. Roles are predicted for elaborate extragemmal nerve endings in tactile and temperature sensations, and potential functions for keratinocytes in noncanonical sensory signaling. The fungiform papilla is presented as a polymodal lingual organ, not solely a gustatory papilla.

Euclidean Relationship between the Superior Semicircular Canal and the Arcuate Eminence

Objective  This study was aimed to better characterize the surgical anatomy of the floor of the middle cranial fossa using three dimensional Euclidean relationships between the arcuate eminence (AE), the superior semicircular canal (SSC), and the geniculate ganglion (GG). Study Design  Submillimeter distances were recorded from computed tomography (CT) scans of 50 patients (100 sides). The AE, apex of the SSC, and the GG were identified and three dimensional distances measured. Setting  The study was conducted at a tertiary academic teaching hospital. Main Outcome Measures  In this study, Euclidean distance was obtained from AE to SSC by using a fixed anatomical landmark (GG) as the origin. Results  On average, the AE is 2.1 ± 0.3 mm lateral, 2.5 ± 0.1 mm superior, and 2.1 ± 0.3 posterior to the SSC. Thirty percent (30/100) of patients had an AE that was less than 2 mm superior to SSC. The AE was medial to the SCC in 13% samples and anterior to the SSC in 18% samples. The results also show that there was no difference in mean distance between sides (1.08 mm; 95% confidence interval [CI] =  - 2.67-0.52; p -value = 0.29) or gender (0.56 mm; 95% CI =  - 1.34, 2.45; p -value = 0.86). Conclusions  This study represents a comprehensive analysis of the relational anatomy of the floor of the middle fossa to date. In quantifying relationships between the AE, SSC, and GG, and by understanding the variability of these relationships in some planes, the middle fossa surgeon can feel more comfortable with this most challenging approach.

Recent advances in taste transduction and signaling

In the last few years, single-cell profiling of taste cells and ganglion cells has advanced our understanding of transduction, encoding, and transmission of information from taste buds as relayed to the central nervous system. This review focuses on new knowledge from these molecular approaches and attempts to place this in the context of previous questions and findings in the field. The individual taste cells within a taste bud are molecularly specialized for detection of one of the primary taste qualities: salt, sour, sweet, umami, and bitter. Transduction and transmitter release mechanisms differ substantially for taste cells transducing sour (Type  III cells) compared with those transducing the qualities of sweet, umami, or bitter (Type II cells), although ultimately all transmission of taste relies on activation of purinergic P2X receptors on the afferent nerves. The ganglion cells providing innervation to the taste buds also appear divisible into functional and molecular subtypes, and each ganglion cell is primarily but not exclusively responsive to one taste quality.

Facial Nerve Decompression After Temporal Bone Fracture-The Bangalore Protocol

Background: Our tertiary otology center treats facial weakness and paralysis after motor vehicle crashes. We evaluate these patients with physical exam, audiogram, Schirmer's test, and CT scan. Our protocol for management of the facial weakness provides good results for our patients.

Methods: Our protocol begins with oral steroids, and serial evaluations. Indications for decompression and our unique transcanal approach to identify the sites for decompression are described. A retrospective review of the medical record presents our patients treated between 1998 and 2017.

Results: One hundred and forty one patients with grade 4 or more weakness underwent decompression. Mean pre-operative and post-operative House-Brackmann (HB) scores were HB5 and HB2, respectively. Fourteen of 104 patients (13%) presenting with HB5 and 6 still had HB5 or HB6 after decompression. Eighty-three of thee 104 patients (80%) achieved HB1 or HB2 at 6 months. Post-operative bone levels were unchanged. Post-operative air levels were improved in cases of perigeniculate fractures (84%).

Conclusion: This Bangalore protocol facilitates advantageous improvement in facial function and conductive hearing loss after traumatic facial nerve crush injuries. The surgical technique, albeit challenging, helps identify the fracture lines, facilitates reconstruction of disrupted ossicles, and avoids craniotomy.

Recognizing Taste: Coding Patterns Along the Neural Axis in Mammals

The gustatory system encodes information about chemical identity, nutritional value, and concentration of sensory stimuli before transmitting the signal from taste buds to central neurons that process and transform the signal. Deciphering the coding logic for taste quality requires examining responses at each level along the neural axis-from peripheral sensory organs to gustatory cortex. From the earliest single-fiber recordings, it was clear that some afferent neurons respond uniquely and others to stimuli of multiple qualities. There is frequently a "best stimulus" for a given neuron, leading to the suggestion that taste exhibits "labeled line coding." In the extreme, a strict "labeled line" requires neurons and pathways dedicated to single qualities (e.g., sweet, bitter, etc.). At the other end of the spectrum, "across-fiber," "combinatorial," or "ensemble" coding requires minimal specific information to be imparted by a single neuron. Instead, taste quality information is encoded by simultaneous activity in ensembles of afferent fibers. Further, "temporal coding" models have proposed that certain features of taste quality may be embedded in the cadence of impulse activity. Taste receptor proteins are often expressed in nonoverlapping sets of cells in taste buds apparently supporting "labeled lines." Yet, taste buds include both narrowly and broadly tuned cells. As gustatory signals proceed to the hindbrain and on to higher centers, coding becomes more distributed and temporal patterns of activity become important. Here, we present the conundrum of taste coding in the light of current electrophysiological and imaging techniques at several levels of the gustatory processing pathway.

Hedgehog Signaling Regulates Taste Organs and Oral Sensation: Distinctive Roles in the Epithelium, Stroma, and Innervation

The Hedgehog (Hh) pathway has regulatory roles in maintaining and restoring lingual taste organs, the papillae and taste buds, and taste sensation. Taste buds and taste nerve responses are eliminated if Hh signaling is genetically suppressed or pharmacologically inhibited, but regeneration can occur if signaling is reactivated within the lingual epithelium. Whereas Hh pathway disruption alters taste sensation, tactile and cold responses remain intact, indicating that Hh signaling is modality-specific in regulation of tongue sensation. However, although Hh regulation is essential in taste, the basic biology of pathway controls is not fully understood. With recent demonstrations that sonic hedgehog (Shh) is within both taste buds and the innervating ganglion neurons/nerve fibers, it is compelling to consider Hh signaling throughout the tongue and taste organ cell and tissue compartments. Distinctive signaling centers and niches are reviewed in taste papilla epithelium, taste buds, basal lamina, fibroblasts and lamellipodia, lingual nerves, and sensory ganglia. Several new roles for the innervation in lingual Hh signaling are proposed. Hh signaling within the lingual epithelium and an intact innervation each is necessary, but only together are sufficient to sustain and restore taste buds. Importantly, patients who use Hh pathway inhibiting drugs confront an altered chemosensory world with loss of taste buds and taste responses, intact lingual touch and cold sensation, and taste recovery after drug discontinuation.

Geniculate Ganglion Neurons are Multimodal and Variable in Receptive Field Characteristics

Afferent chorda tympani (CT) fibers innervating anterior tongue fungiform papillae have neuron cell bodies in the geniculate ganglion (GG). To characterize electrophysiological and receptive field properties, we recorded extracellular responses from single GG neurons to lingual application with chemical, thermal and mechanical stimuli. Receptive field size was mapped by electrical stimulation of individual fungiform papillae. Responses of GG neurons to room temperature chemical stimuli representing five taste qualities, and distilled water at 4 °C and mechanical stimulation were used. Based on responses to these stimuli, GG neurons were divided into CHEMICAL, CHEMICAL/THERMAL, THERMAL and TACTILE groups. Neurons in the CHEMICAL group responded to taste stimuli but not to either cold water or stroking stimuli. CHEMICAL/THERMAL neurons responded to both taste stimuli and cold water. THERMAL neurons responded only to cold water and TACTILE neurons responded only to light stroking stimuli. The receptive field sizes for CHEMICAL, and CHEMICAL/THERMAL neurons averaged five papillae exceeding the field size of THERMAL and TACTILE neurons which averaged about two papillae. Detailed analysis of the receptive field of CHEMICAL/THERMAL neurons revealed that within one field only a subset of the fungiform papillae making up the receptive field responded to the cold stimuli, whereas the other papillae responded only to chemical stimuli. These finding demonstrate that fungiform papilla are complex sensory organs with a multisensory function suggesting a unique role in detecting and sampling food components prior to ingestion.

The transcription factor Phox2b distinguishes between oral and non-oral sensory neurons in the geniculate ganglion

Many basic characteristics of gustatory neurons remain unknown, partly due to the absence of specific markers. Some neurons in the geniculate ganglion project to taste regions in the oral cavity, whereas others innervate the outer ear. We hypothesized that the transcription factor Phox2b would identify oral cavity-projecting neurons in the geniculate ganglion. To test this possibility, we characterized mice in which Phox2b-Cre mediated gene recombination labeled neurons with tdTomato. Nerve labeling revealed that all taste neurons projecting through the chorda tympani (27%) and greater superficial petrosal nerves (15%) expressed Phox2b during development, whereas non-oral somatosensory neurons (58%) in the geniculate ganglion did not. We found tdTomato-positive innervation within all taste buds. Most (57%) of the fungiform papillae had labeled innervation only in taste buds, whereas 43% of the fungiform papillae also had additional labeled innervation to the papilla epithelium. Chorda tympani nerve transection eliminated all labeled innervation to taste buds, but most of the additional innervation in the fungiform papillae remained. Some of these additional fibers also expressed tyrosine hydroxylase, suggesting a sympathetic origin. Consistent with this, both sympathetic and parasympathetic fibers innervating blood vessels and salivary glands contained tdTomato labeling. Phox2b-tdTomato labels nerve fascicles in the tongue of the developing embryo and demonstrates a similar stereotyped branching pattern DiI-labeling.

Geniculate Ganglion Tumors: Clinical Presentation and Surgical Results

OBJECTIVE

Facial nerve tumors are rare lesions mostly located in the geniculate ganglion. This study aims to compare those tumors limited to the geniculate ganglion in terms of clinical features and postoperative outcomes.

STUDY DESIGN

Case series with chart review.

SETTINGS

University tertiary reference center.

SUBJECTS AND METHODS

Medical charts were reviewed for 17 patients who had surgery for geniculate ganglion tumor removal (10 hemangiomas, 6 schwannomas, 1 meningioma). Hemangiomas and schwannomas were compared for preoperative facial nerve function, hearing, tumor size, and postoperative outcomes.

RESULTS

Facial palsy was observed in all cases. Regarding the preoperative facial nerve function, severe facial palsy (House-Brackmann grades V and VI) was present in 70% of cases for hemangiomas and for no case of schwannoma (P = .01), although hemangiomas were significantly smaller tumors (P = .01). Hearing loss was observed in 4 cases (23.5%) and was related to tumor volume (P < .0001). A complete excision was achieved in all cases, and a facial nerve graft was performed immediately after interruption in 16 patients (94%). Postoperative facial nerve function was improved or stabilized in 94% of cases. A preoperative House-Brackman grade VI was shown as a negative factor for postoperative facial nerve function.

CONCLUSIONS

Differences in clinical presentations could help in establishing the good therapeutic option depending on the tumor type. Surgery, when indicated, is safe and effective, and postoperative outcomes are not related to tumor type.

Innervation of taste buds revealed with Brainbow-labeling in mouse

Nerve fibers that surround and innervate the taste bud were visualized with inherent fluorescence using Brainbow transgenic mice that were generated by mating the founder line L with nestin-cre mice. Multicolor fluorescence revealed perigemmal fibers as branched within the non-taste epithelium and ending in clusters of multiple rounded swellings surrounding the taste pore. Brainbow-labeling also revealed the morphology and branching pattern of single intragemmal fibers. These taste bud fibers frequently innervated both the peripheral bud, where immature gemmal cells are located, and the central bud, where mature, differentiated cells are located. The fibers typically bore preterminal and terminal swellings, growth cones with filopodia, swellings, and rounded retraction bulbs. These results establish an anatomical substrate for taste nerve fibers to contact and remodel among receptor cells at all stages of their differentiation, an interpretation that was supported by staining with GAP-43, a marker for growing fibers and growth cones.

Receptive field size, chemical and thermal responses, and fiber conduction velocity of rat chorda tympani geniculate ganglion neurons

Afferent chorda tympani (CT) fibers innervating taste and somatosensory receptors in fungiform papillae have neuron cell bodies in the geniculate ganglion (GG). The GG/CT fibers branch in the tongue to innervate taste buds in several fungiform papillae. To investigate receptive field characteristics of GG/CT neurons, we recorded extracellular responses from GG cells to application of chemical and thermal stimuli. Receptive field size was mapped by electrical stimulation of individual fungiform papillae. Response latency to electrical stimulation was used to determine fiber conduction velocity. Responses of GG neurons to lingual application of stimuli representing four taste qualities, and water at 4°C, were used to classify neuron response properties. Neurons classified as SALT, responding only to NaCl and NH4Cl, had a mean receptive field size of six papillae. Neurons classified as OTHER responded to salts and other chemical stimuli and had smaller mean receptive fields of four papillae. Neurons that responded to salts and cold stimuli, classified as SALT/THERMAL, and neurons responding to salts, other chemical stimuli and cold, classified as OTHER/THERMAL, had mean receptive field sizes of six and five papillae, respectively. Neurons responding only to cold stimuli, categorized as THERMAL, had receptive fields of one to two papillae located at the tongue tip. Based on conduction velocity most of the neurons were classified as C fibers. Neurons with large receptive fields had higher conduction velocities than neurons with small receptive fields. These results demonstrate that GG neurons can be distinguished by receptive field size, response properties and afferent fiber conduction velocity derived from convergent input of multiple taste organs.

The Role of 5-HT3 Receptors in Signaling from Taste Buds to Nerves

Activation of taste buds triggers the release of several neurotransmitters, including ATP and serotonin (5-hydroxytryptamine; 5-HT). Type III taste cells release 5-HT directly in response to acidic (sour) stimuli and indirectly in response to bitter and sweet tasting stimuli. Although ATP is necessary for activation of nerve fibers for all taste stimuli, the role of 5-HT is unclear. We investigated whether gustatory afferents express functional 5-HT3 receptors and, if so, whether these receptors play a role in transmission of taste information from taste buds to nerves. In mice expressing GFP under the control of the 5-HT(3A) promoter, a subset of cells in the geniculate ganglion and nerve fibers in taste buds are GFP-positive. RT-PCR and in situ hybridization confirmed the presence of 5-HT(3A) mRNA in the geniculate ganglion. Functional studies show that only those geniculate ganglion cells expressing 5-HT3A-driven GFP respond to 10 μM 5-HT and this response is blocked by 1 μM ondansetron, a 5-HT3 antagonist, and mimicked by application of 10 μM m-chlorophenylbiguanide, a 5-HT3 agonist. Pharmacological blockade of 5-HT3 receptors in vivo or genetic deletion of the 5-HT3 receptors reduces taste nerve responses to acids and other taste stimuli compared with controls, but only when urethane was used as the anesthetic. We find that anesthetic levels of pentobarbital reduce taste nerve responses apparently by blocking the 5-HT3 receptors. Our results suggest that 5-HT released from type III cells activates gustatory nerve fibers via 5-HT3 receptors, accounting for a significant proportion of the neural taste response.

Taste Bud-Derived BDNF Is Required to Maintain Normal Amounts of Innervation to Adult Taste Buds

Gustatory neurons transmit chemical information from taste receptor cells, which reside in taste buds in the oral cavity, to the brain. As adult taste receptor cells are renewed at a constant rate, nerve fibers must reconnect with new taste receptor cells as they arise. Therefore, the maintenance of gustatory innervation to the taste bud is an active process. Understanding how this process is regulated is a fundamental concern of gustatory system biology. We speculated that because brain-derived neurotrophic factor (BDNF) is required for taste bud innervation during development, it might function to maintain innervation during adulthood. If so, taste buds should lose innervation when Bdnf is deleted in adult mice. To test this idea, we first removed Bdnf from all cells in adulthood using transgenic mice with inducible CreERT2 under the control of the Ubiquitin promoter. When Bdnf was removed, approximately one-half of the innervation to taste buds was lost, and taste buds became smaller because of the loss of taste bud cells. Individual taste buds varied in the amount of innervation each lost, and those that lost the most innervation also lost the most taste bud cells. We then tested the idea that that the taste bud was the source of this BDNF by reducing Bdnf levels specifically in the lingual epithelium and taste buds. Taste buds were confirmed as the source of BDNF regulating innervation. We conclude that BDNF expressed in taste receptor cells is required to maintain normal levels of innervation in adulthood.

Vascular leiomyoma and geniculate ganglion

Objectives Discussion of a rare case of angioleiomyoma involving the geniculate ganglion and the intratemporal facial nerve segment and its surgical treatment.

Design Case report.

Setting Presence of an expansive lesion englobing the geniculate ganglion without any lesion to the cerebellopontine angle.

Participants A 45-year-old man with a grade III facial paralysis according to the House-Brackmann scale of evaluation.

Main Outcomes Measure Surgical pathology, radiologic appearance, histological features, and postoperative facial function.

Results Removal of the entire lesion was achieved, preserving the anatomic integrity of the nerve; no nerve graft was necessary. Postoperative histology and immunohistochemical studies revealed features indicative of solid vascular leiomyoma.

Conclusion Angioleiomyoma should be considered in the differential diagnosis of geniculate ganglion lesions. Optimal postoperative facial function is possible only by preserving the anatomical and functional integrity of the facial nerve.

Spike rate and spike timing contributions to coding taste quality information in rat periphery

There is emerging evidence that individual sensory neurons in the rodent brain rely on temporal features of the discharge pattern to code differences in taste quality information. In contrast, investigations of individual sensory neurons in the periphery have focused on analysis of spike rate and mostly disregarded spike timing as a taste quality coding mechanism. The purpose of this work was to determine the contribution of spike timing to taste quality coding by rat geniculate ganglion neurons using computational methods that have been applied successfully in other systems. We recorded the discharge patterns of narrowly tuned and broadly tuned neurons in the rat geniculate ganglion to representatives of the five basic taste qualities. We used mutual information to determine significant responses and the van Rossum metric to characterize their temporal features. While our findings show that spike timing contributes a significant part of the message, spike rate contributes the largest portion of the message relayed by afferent neurons from rat fungiform taste buds to the brain. Thus, spike rate and spike timing together are more effective than spike rate alone in coding stimulus quality information to a single basic taste in the periphery for both narrowly tuned specialist and broadly tuned generalist neurons.

Types of taste circuits synaptically linked to a few geniculate ganglion neurons

The present study evaluates the central circuits that are synaptically engaged by very small subsets of the total population of geniculate ganglion cells to test the hypothesis that taste ganglion cells are heterogeneous in terms of their central connections. We used transsynaptic anterograde pseudorabies virus labeling of fungiform taste papillae to infect single or small numbers of geniculate ganglion cells, together with the central neurons with which they connect, to define differential patterns of synaptically linked neurons in the taste pathway. Labeled brain cells were localized within known gustatory regions, including the rostral central subdivision (RC) of the nucleus of the solitary tract (NST), the principal site where geniculate axons synapse, and the site containing most of the cells that project to the parabrachial nucleus (PBN) of the pons. Cells were also located in the rostral lateral NST subdivision (RL), a site of trigeminal and sparse geniculate input, and the ventral NST (V) and medullary reticular formation (RF), a caudal brainstem pathway leading to reflexive oromotor functions. Comparisons among cases, each with a random, very small subset of labeled geniculate neurons, revealed "types" of central neural circuits consistent with a differential engagement of either the ascending or the local, intramedullary pathway by different classes of ganglion cells. We conclude that taste ganglion cells are heterogeneous in terms of their central connectivity, some engaging, predominantly, the ascending "lemniscal," taste pathway, a circuit associated with higher order discriminative and homeostatic functions, others engaging the "local," intramedullary "reflex" circuit that mediates ingestion and rejection oromotor behaviors.

Neurotrophic factor receptor expression and in vitro nerve growth of geniculate ganglion neurons that supply divergent nerves

We investigated which neurotrophic factors may contribute to the divergence of two peripheral nerves emanating from the geniculate ganglion. We compared receptor mRNA profiles of the neurons that supply the nerves, and also the growth of their neurites in response to neurotrophic factors in culture. Three mRNAs, Gfra2, TrkA, and TrkC, were differentially expressed. Only one ligand, Neurturin, promoted substantially different nerve regrowth from the nerves, and therefore may contribute to nerve divergence. Three receptor mRNAs were expressed in 100% of the neurons: TrkB, TrkB.T2 (kinase-lacking isoform), and NCAM-140. Ligands for these Trks and FRalpha-1 promoted more outgrowth than ligands for the other receptors. NT-3 and BDNF synergistically promoted outgrowth. Finally, receptors are coexpressed at random rates, arguing against the existence of neuronal subtypes defined by a combinatorial code of these receptors.

Time course of morphological alterations of fungiform papillae and taste buds following chorda tympani transection in neonatal rats

The time course of structural changes in fungiform papillae was analyzed in rats that received unilateral chorda tympani nerve transection at 10 days of age. Morphological differences between intact and denervated sides of the tongue were first observed at 8 days postsection, with an increase in the number of fungiform papillae that did not have a pore. In addition, the first papilla with a filiform-like appearance was noted on the denervated side at 8 days postsectioning. By 11 days after surgery, the total number of papillae and the number of papillae with a pore were significantly lower on the transected side of the tongue as compared to the intact side. At 50 days postsection, there was an average of 70.5 fungiform papillae on the intact side and a mean of only 20.8 fungiform papillae the denervated side. Of those few remaining papillae on the cut side, an average of 13.5 papillae were categorized as filiform-like, while no filiform-like papillae occurred on the intact side. Significant reduction in taste bud volume was noted at 4 days posttransection and further decrements in taste bud volume were noted at 8 and 30 days postsection. Electron microscopy of the lingual branch of the trigeminal nerve from adult rats that received neonatal chorda tympani transection showed normal numbers of both myelinated and unmyelinated fibers. Thus, in addition to the well-characterized dependence of taste bud maintenance on the chorda tympani nerve, the present study shows an additional role of the chorda tympani nerve in papilla maintenance during early postnatal development.

Alterations in size, number, and morphology of gustatory papillae and taste buds in BDNF null mutant mice demonstrate neural dependence of developing taste organs

Sensory ganglia that innervate taste buds and gustatory papillae (geniculate and petrosal) are reduced in volume by about 40% in mice with a targeted deletion of the gene for brain-derived neurotrophic factor (BDNF). In contrast, the trigeminal ganglion, which innervates papillae but not taste buds on the anterior tongue, is reduced by only about 18%. These specific alterations in ganglia that innervate taste organs make possible a test for roles of lingual innervation in the development of appropriate number, morphology, and spatial pattern of fungiform and circumvallate papillae and associated taste buds. We studied tongues of BDNF null mutant and wild-type littermates and made quantitative analyses of all fungiform papillae on the anterior tongue, the single circumvallate papilla on the posterior tongue, and all taste buds in both papilla types. Fungiform papillae and taste buds were reduced in number by about 60% and were substantially smaller in diameter in mutant mice 15-25 days postnatal. Remaining fungiform papillae were selectively concentrated in the tongue tip region. The circumvallate papilla was reduced in diameter and length by about 40%, and papilla morphology was disrupted. Taste bud number in the circumvallate was reduced by about 70% in mutant tongues, and the remaining taste buds were smaller than those on wild-type tongues. Our results demonstrate a selective dependence of taste organs on a full complement of appropriate innervation for normal growth and morphogenesis. Effects on papillae are not random but are more pronounced in specific lingual regions. Although the geniculate and petrosal ganglia sustain at least half of their normal complement of cell number in BDNF -/- mice, remaining ganglion cells do not substitute for lost neurons to rescue taste organs at control numbers. Whereas gustatory ganglia and the taste papillae initially form independently, our results suggest interdependence in later development because ganglia derive BDNF support from target organs and papillae require sensory innervation for morphogenesis.