Regional differences of CGRP-immunoreactive nerve fibers in nasal epithelium of the rat

Hot topic: Mapping of the human intranasal mucosal thermal sensitivity: A clinical study on thermal threshold and trigeminal receptors

INTRODUCTION

The olfactory and trigeminal system are closely interlinked. Existing literature has primarily focused on characterizing trigeminal stimulation through mechanical and chemical stimulation, neglecting thermal stimulation thus far. The present study aimed to characterize the intranasal sensitivity to heat and the expression of trigeminal receptors (transient receptor potential channels, TRP).

METHODS

A total of 20 healthy participants (aged 21-27 years, 11 women) were screened for olfactory function and trigeminal sensitivity using several tests. Under endoscopic control, a thermal stimulator was placed in 7 intranasal locations: anterior septum, lateral vestibulum, interior nose tip, lower turbinate, middle septum, middle turbinate, and olfactory cleft to determine the thermal threshold. Nasal swabs were obtained in 3 different locations (anterior septum, middle turbinate, olfactory cleft) to analyze the expression of trigeminal receptors TRP: TRPV1, TRPV3, TRPA1, TRPM8.

RESULTS

The thermal threshold differed between locations (p = 0.018), with a trend for a higher threshold at the anterior septum (p = 0.092). There were no differences in quantitative receptor expression (p = 0.46) at the different sites. The highest overall receptor RNA expression was detected for TRPV1 over all sites (p<0.001). The expression of TRPV3 was highest at the anterior septum compared to the middle turbinate or the olfactory cleft. The thermal sensitivity correlated with olfactory sensitivity and results from tests were related to trigeminal function like intensity ratings of ammonium, a questionnaire regarding trigeminal function, nasal patency, and CO2 thresholds. However, no correlation was found between receptor expression and psychophysical measures of trigeminal function.

DISCUSSION

This study provided the first insights about intranasal thermal sensitivity and suggested the presence of topographical differences in thermal thresholds. There was no correlation between thermal sensitivity and trigeminal mRNA receptor expression. However, thermal sensitivity was found to be associated with psychophysical measures of trigeminal and olfactory function.

Technique of flat-mount immunostaining for mapping the olfactory epithelium and counting the olfactory sensory neurons

The pathophysiology underlying olfactory dysfunction is still poorly understood, and more efficient biomolecular tools are necessary to explore this aspect. Immunohistochemistry (IHC) on cross sections is one of the major tools to study the olfactory epithelium (OE), but does not allow reliable counting of olfactory sensory neurons (OSNs) or cartography of the OE. In this study, we want to present an easy immunostaining technique to compensate for these defects of IHC. Using the rat model, we first validated and pre-screened the key OSN markers by IHC on cross sections of the OE. Tuj-1, OMP, DCX, PGP9.5, and N-cadherin were selected for immunostaining on flat-mounted OE because of their staining of OSN dendrites. A simple technique for immunostaining on flat-mounted septal OE was developed: fixation of the isolated septum mucosa in 0.5% paraformaldehyde (PFA) preceded by pretreatment of the rat head in 1% PFA for 1 hour. This technique allowed us to correctly reveal the olfactory areas using all the 5 selected markers on septum mucosa. By combining the mature OSN marker (OMP) and an immature OSN marker (Tuj-1), we quantified the mature (OMP+, Tuj-1-), immature (OMP-, Tuj-1+), transitory (OMP+, Tuj-1+) and total OSN density on septal OE. They were respectively 42080 ± 11820, 49384 ± 7134, 14448 ± 5865 and 105912 ± 13899 cells per mm2 (mean ± SD). Finally, the same immunostaining technique described above was performed with Tuj-1 for OE cartography on ethmoid turbinates without flat-mount.

Transient Receptor Potential Ankyrin 1 Mediates Hypoxic Responses in Mice

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel that is broadly expressed in sensory pathways, such as the trigeminal and vagus nerves. It is capable of detecting various irritants in inspired gasses and is activated during hypoxia. In this study, the role of TRPA1 in hypoxia-induced behavioral, respiratory, and cardiovascular responses was examined through four lines of experiments using TRPA1 knockout (KO) mice and wild type (WT) littermates. First, KO mice showed significantly attenuated avoidance behavior in response to a low (15%) oxygen environment. Second, the wake-up response to a hypoxic ramp (from 21 to 10% O₂ in 40 s) was measured using EEG electrodes. WT mice woke up within 30 s when oxygen was at 13–14%, but KO mice did not wake up until oxygen levels reached 10%. Histological analysis confirmed that mild (13% O₂) hypoxia resulted in an attenuation of trigeminal neuronal activation in KO mice. Third, the ventilatory response to hypoxia was measured with whole body plethysmography. KO mice showed attenuated responses to mild hypoxia (15% O₂) but not severe hypoxia (10% O₂). Similar responses were observed in WT mice treated with the TRPA1 blocker, AP-18. These data clearly show that TRPA1 is necessary for multiple mild hypoxia (13–15% O₂)-induced physiological responses. We propose that TRPA1 channels in the sensory pathways innervating the airway can detect hypoxic environments and prevent systemic and/or cellular hypoxia from occurring.

Neuropeptides in sensory signal processing

Peptides released from trigeminal fibers fulfill well-understood functions in neuroinflammatory processes and in the modulation of nociceptive signal processing. In particular, calcitonin gene-related peptide (CGRP) and substance P (SP), released from afferent nerve terminals, exert paracrine effects on the surrounding tissue and this has been recently highlighted by the prominent parcrine role of CGRP in the development of headache and migraine. Some recent communications suggest that these sensory neuropeptides may also modulate the workings of sensory organs and influence afferent signals from nose, tongue, eyes and ears. Here, we briefly review the evidence for modulatory effects of CGRP and SP in the sensory periphery.

Intranasal trigeminal function in patients with empty nose syndrome

OBJECTIVES/HYPOTHESIS

Trigeminal nerve mediates the perception of nasal airflow. This study examines whether impaired intranasal trigeminal function is a part of the paradoxical nasal obstruction sensation in patients with empty nose syndrome (ENS).

STUDY DESIGN

Prospective case-control study in a tertiary hospital.

METHODS

Three groups were examined: 1) ENS patients with previous bilateral near total inferior turbinectomy, 2) patients who underwent near total inferior turbinate removal (ITR) without ENS symptoms, and 3) control participants. All participants examined with active anterior rhinomanometry, olfactory testing (extended Sniffin' Sticks test), and trigeminal testing (lateralization task using menthol and odorless solvent).

RESULTS

Seventy-one participants were included (21 ENS patients, 18 ITR patients, and 31 controls). Analyses revealed that ENS patients had significantly lower scores on trigeminal lateralization testing than the ITR group and controls. The ENS group had also significantly lower scores in olfactory testing than controls. No statistical differences were found in rhinomanometry between groups. The gender factor was not associated with the chemosensory testing; however, this was not the case with the age factor, as trigeminal test results were negatively correlated.

CONCLUSIONS

This study demonstrates significantly impaired intranasal trigeminal function in ENS patients when compared with ITR patients and controls. Further prospective studies are needed to clarify the role of preoperative trigeminal function of these patients and the contribution of surgery to this impairment.

LEVEL OF EVIDENCE

3b. Laryngoscope, 127:1263-1267, 2017.

Topographical differences in distribution and responsiveness of trigeminal sensitivity within the human nasal mucosa

The study was designed to provide a topographical map of the sensitivity of the human nasal respiratory epithelium towards trigeminal chemosensory stimuli. As an electrophysiological measure of intranasal trigeminal activation at the level of the epithelium, we used the so-called negative mucosa potential (NMP), a measure that represents the sum of generator potentials of trigeminal receptor neurons after chemical stimulation. Sixty subjects participated (30 men and 30 women; mean age 23.5 years). Measurements were made in response to stimulation with menthol, CO(2), ethanol, and cinnamaldehyde, which are known to activate trigeminal receptors to various degrees. Recordings of the NMP were made from five intranasal sites: the anterior septum, the posterior septum, the tip of the middle turbinate, the tip of the lower turbinate, and the lateral side wall of the posterior nasal cavity. The recording electrode was positioned under endoscopic control. The largest NMP amplitudes were recorded at the anterior septum in response to stimulation with CO(2). Comparing all recording sites, significant differences were observed between responses at the posterior septum and the lateral side wall of the posterior nasal cavity in response to stimulation by ethanol, menthol, and CO(2). These findings suggest that the presence of topographical and chemosensory differences in the responsiveness of the nasal mucosa to irritants.

Responses to trigeminal irritants at different locations of the human nasal mucosa

BACKGROUND

Previous work suggests differences in the distribution of human intranasal trigeminal receptors. The aim of this study was to investigate these topographic differences for different concentrations of different trigeminal irritants using an electrophysiologic measure of trigeminal activation, the negative mucosa potential (NMP).

MATERIALS AND METHODS

A total of 15 healthy volunteers participated. Presented by a computer-controlled olfactometer CO2 (30% and 40% v/v), ethyl acetate (5.5% and 9.3% v/v) and acetic acid (205 and 40% v/v) were used for stimulation. NMP was recorded at the middle septum, the middle turbinate, and the floor of the nasal cavity.

RESULTS

Maximum amplitudes of the NMP were found at the middle septum and were lowest at the nasal floor. Response amplitudes were related to stimulus concentrations. There was no significant difference between responses to the three different stimuli in relation to three recording sites.

CONCLUSIONS

In agreement with previous work, the present data suggest that there are topographic differences in the responsiveness of the mucosa to chemical irritants.

Topographical differences in the trigeminal sensitivity of the human nasal mucosa

The aim of the study was to investigate differences in the distribution of intranasal trigeminal receptors in humans using an electrophysiological measure of trigeminally induced activation, the negative mucosa potential. A total of 29 young, healthy volunteers participated, results were on the basis of data from 18 participants. The trigeminal irritant CO2 was presented using a computer-controlled olfactometer. Negative mucosa potential recording sites included the anterior olfactory cleft, the anterior septum, and the lower turbinate. Lowest amplitudes of the negative mucosa potential were found in the olfactory cleft, maximum amplitudes at the septum. Intranasal measurements of CO2 concentrations suggested that these differences were not due to the intranasal distribution of CO2. These results are compatible with the idea that the trigeminal system acts as a sentinel of the human airways.

Responsiveness of human nasal mucosa to trigeminal stimuli depends on the site of stimulation

There is evidence that functionally different areas can be distinguished within the nasal mucosa with regard to stimulation site and stimulus properties. The aim of the present study was the comparison of electrophysiological and psychophysical measures obtained in response to mechanical and chemosomatosensory stimulation of two different regions of the nasal mucosa. A total of 40 volunteers participated in this study (age range 21-36 years). Chemosomatosensory event-related potentials (ERPs) were recorded using gaseous CO2 as stimulant, while somatosensory ERPs were recorded in response to intranasal mechanical stimuli (air puffs). Stimuli were released to the anterior portion and to the posterior portion of the nasal cavity. A significant interaction between stimulus properties and site of stimulation could be detected after analysis of ERP parameters and intensity ratings. Thus, the chemosensory stimulus was perceived as stronger in the anterior portion of the nasal cavity whereas this was not the case for mechanosensory stimuli. In addition, mechanosensory stimuli were found to evoke ERPs with shorter latencies. These results underline the idea that the respiratory mucosa should not be seen as a homogeneous tissue. It exhibits varying sensitivities to trigeminal stimulation depending on stimulus quality and site of stimulation. Hence, perception of chemosensory stimuli seems to be most accurate in the anterior portion of the nasal cavity, while sensitivity to mechanical stimuli appears to be highest in the posterior portion. In addition, these differences within the respiratory mucosa may contribute to differences in the perception of orthonasal and retronasal odorous stimulation.

Changes in nasal mucosal innervation in horses with grass sickness

REASONS FOR PERFORMING STUDY

Equine grass sickness is a dysautonomia characterised by widespread destruction of autonomic ganglia, resulting in the clinical signs of dysphagia, constipation, profuse sweating, tachycardia, rhinitis sicca and high mortality rate. Rhinitis sicca is a common finding in horses with the chronic form and we have postulated that alterations in autonomic innervation of the nasal mucosa might underlie this clinical presentation.

OBJECTIVES

In this study, the expression and distribution of nerve fibres immunoreactive for calcitonin gene-related peptide (CGRP), substance P (SP), the general neuronal marker protein gene-product 9.5 (PGP 9.5; ubiquitin) and the intermediate neurofilaments (PAN-N; neurorfilaments L, M and H) in the nasal mucosa of normal horses (n = 10) and horses with EGS (n = 18; acute n = 8, subacute n = 3, chronic n = 7) was assessed.

METHODS

Innervation density and distribution was investigated in the different groups using standard immunohistochemical techniques.

RESULTS

Significant differences were noted when comparing the density and distribution of nerve fibres immunoreactive for PGP 95 and PAN-N, with PGP 95 consistently giving better staining in all groups and at all sites in the nasal mucosa. An apparent increase in the density of innervation was noted for acute vs. normal cases. A significant reduction in the density of innervation was noted only with PAN-N when comparing normal horses and acute cases with the chronic group (P < 0.05). CGRP and SP immunoreactive nerve fibres were typically most abundant in the epithelial and subepithelial layers, but the quality of staining and nerve fibre density was greater for SP, achieving significant difference in several comparisons. The density of innervation for SP was significantly reduced in the chronic group compared to the normal and acute groups (P < 0.01). A significant decrease was noted for CGRP only for the acute and chronic groups (P<0.05).

CONCLUSIONS

These results demonstrate a reduction in the expression of the sensory neuropeptides in nasal mucosal innervation as a consequence of equine dysautonomia, and may underlie the clinical presentation of rhinitis sicca noted with this disease.

POTENTIAL RELEVANCE

Nasal biopsy may be of use in antemortem diagnosis of grass sickness and identification of mucosal denervation; and might also be useful in the treatment of rhinitis in EGS cases.

Expression of vasoactive intestinal peptide, calcitonin gene-related peptide, substance P, and intermediate neurofilaments in nasal mucosal nerve fibers of horses without nasal disease

OBJECTIVE

To determine the distribution of nerve fibers containing calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP), and intermediate neurofilaments in nasal mucosa of horses.

ANIMALS

6 horses without evidence of nasal disease.

PROCEDURE

Full-thickness nasal tissue specimens were obtained from the rostral portion of the nasal septum at necropsy, and fluorescence immunohistochemistry was performed to assess mucosal distribution of nerve fibers.

RESULTS

Nerve fibers with CGRP-like immunoreactivity (CGRP-Li) formed a dense subepithelial network, and a large number of fibers were found coursing between epithelial cells. Fibers with CGRP-Li were also associated with blood vessels and mucous glands. Fibers with SP-like immunoreactivity (SP-Li) had a similar distribution and density. In contrast, there were few fibers with VIP-like immunoreactivity. Fibers containing intermediate neurofilaments were prominent and appeared as large nerve fiber bundles mainly adjacent to the nasal septum but also close to mucous glands and within the lamina propria. Intermediate neurofilaments were also identified in single nerve fibers at all sites, but the density of fibers with intermediate neurofilaments did not match that of fibers with CGRP- or SP-Li.

CONCLUSIONS

The density and distribution of nerve fibers containing SP- or CGRP-Li in nasal mucosa of horses was similar to that reported for other species. However, expression of VIP in nerve fibers was low. Antibodies against intermediate neurofilaments identified many nerve fibers in nasal mucosa of horses but did not appear to identify small diameter fibers expressing SP or VIP.

Morphology of intraepithelial corpuscular nerve endings in the nasal respiratory mucosa of the dog

Corpuscular nerve endings in the nasal respiratory mucosa of the dog were investigated by immunohistochemical staining specific for protein gene product 9.5 by light and electron microscopy. In the nasal respiratory mucosa, complex corpuscular endings, which displayed bulbous, laminar and varicose expansions, were distributed on the dorsal elevated part of the nasal septum and on the dorsal nasal concha. The endings were 300-500 microm long and 100-250 microm wide. Some axons gave rise to a single ending while others branched into 2 endings. Cryostat sections revealed that the corpuscular endings were located within the nasal respiratory epithelium. On electron microscopy, immunoreactive nerve terminals that contained organelles, including mitochondria and neurofilaments, were observed within the epithelial layer near the lumen of the nasal cavity. Some terminals contacted the goblet cell. Such terminal regions were covered by the cytoplasmic process of ciliated cells and were never exposed to the lumen of the nasal cavity. These nerve endings are probably activated by pressure changes.

Substance P- and calcitonin gene-related peptide-containing nerve fibers in the nasal mucosa of chronically hypoxic rats

The distribution of substance P-immunoreactive and calcitonin gene-related peptide-immunoreactive nerve fibers in the nasal mucosa was compared between normoxic and chronically hypoxic rats (10% O2 and 3.0-4.0% CO2 for 3 months). In the normoxic nasal mucosa, substance P- and calcitonin gene-related peptide-immunoreactive nerve fibers were found within and under the epithelium and around the glands and blood vessels in the lamina propria. These immunoreactive fibers have many varicosities. In the chronically hypoxic nasal mucosa, the relative density of intra- and subepithelial substance P-immunoreactive and calcitonin gene-related peptide-immunoreactive fibers and those in the lamina propria was higher than in normoxic mucosa. The length of substance P-positive fibers within the chronically hypoxic olfactory and respiratory epithelium was 1.66 and 2.45 times higher than within the normoxic epithelium, respectively. The length of calcitonin gene-related peptide-immunostained fibers within the chronically hypoxic olfactory and respiratory epithelium was 1.56 and 1.84 times higher, respectively. Because substance P and calcitonin gene-related peptide are the predominant signal peptides of primary sensory neurons, the increased number of these fibers may represent enhanced sensory mechanisms in the hypoxic nasal mucosa. In addition, considered together with the findings in chronically hypoxic tracheal mucosa, the increased density of intraepithelial fibers containing substance P and calcitonin gene-related peptide suggests that this is a predominant feature of hypoxic adaptation throughout the upper and lower respiratory tracts.