Assessment of Intranasal Function of the Trigeminal Nerve in Daily Clinical Practice

BACKGROUND

The trigeminal nerve is a mixed cranial nerve responsible for the motor innervation of the masticatory muscles and the sensory innervation of the face, including the nasal cavities. Through its nasal innervation, we perceive sensations, such as cooling, tingling, and burning, while the trigeminal system mediates the perception of airflow. However, the intranasal trigeminal system has received little attention in the clinical evaluation of patients with nasal pathology.

SUMMARY

Testing methods that enable the clinical assessment of intranasal trigeminal function have recently been developed. This study aims to present the current clinical methods that can be utilised in everyday practice, as described in the literature. These methods include four assessment techniques: (1) the quick screening test of trigeminal sensitivity involves patients rating the intensity of ammonium vapour presented in a lipstick-like container. (2) The lateralisation test requires subjects to identify which nasal cavity is being stimulated by a trigeminal stimulus, such as eucalyptol or menthol, while the other side receives an odourless stimulus. (3) The trigeminal sticks test evaluates the trigeminal function similarly to the olfactory function using sticks filled with trigeminal stimulant liquids. (4) The automated CO2 stimulation device is used for measuring trigeminal pain thresholds, utilising intranasal CO2 stimuli to define the pain threshold.

KEY MESSAGES

Assessing intranasal trigeminal function clinically may prove useful in evaluating rhinology patients, particularly those who encounter nasal obstruction without anatomical blockage and those experiencing olfactory disorders with suspected trigeminal dysfunction. Despite their limitations, the presented methods may provide useful information about nasal patency, chemosensitivity, and pain sensation in the daily clinical practice of such patients, leading to better therapeutic decisions.

Morphology of GNAT3-immunoreactive chemosensory cells in the nasal cavity and pharynx of the rat

Solitary chemosensory cells and chemosensory cell clusters are distributed in the pharynx and larynx. In the present study, the morphology and reflexogenic function of solitary chemosensory cells and chemosensory cell clusters in the nasal cavity and pharynx were examined using immunofluorescence for GNAT3 and electrophysiology. In the nasal cavity, GNAT3-immunoreactive solitary chemosensory cells were widely distributed in the nasal mucosa, particularly in the cranial region near the nostrils. Solitary chemosensory cells were also observed in the nasopharynx. Solitary chemosensory cells in the nasopharyngeal cavity were barrel like or slender in shape with long lateral processes within the epithelial layer to attach surrounding ciliated epithelial cells. Chemosensory cell clusters containing GNAT3-immunoreactive cells were also detected in the pharynx. GNAT3-immunoreactive cells gathered with SNAP25-immunoreactive cells in chemosensory clusters. GNAT3-immunoreactive chemosensory cells were in close contact with a few SP- or CGRP-immunoreactive nerve endings. In the pharynx, GNAT3-immunoreactive chemosensory cells were also attached to P2X3-immunoreactive nerve endings. Physiologically, the perfusion of 10 mM quinine hydrochloride (QHCl) solution induced ventilatory depression. The QHCl-induced reflex was diminished by bilateral section of the glossopharyngeal nerve, suggesting autonomic reflex were evoked by chemosensory cells in pharynx but not in nasal mucosa. The present results indicate that complex shape of nasopharyngeal solitary chemosensory cells may contribute to intercellular communication, and pharyngeal chemosensory cells may play a role in respiratory depression.

Photoactivation of olfactory sensory neurons does not affect action potential conduction in individual trigeminal sensory axons innervating the rodent nasal cavity

Olfactory and trigeminal chemosensory systems reside in parallel within the mammalian nose. Psychophysical studies in people indicate that these two systems interact at a perceptual level. Trigeminal sensations of pungency mask odour perception, while olfactory stimuli can influence trigeminal signal processing tasks such as odour localization. While imaging studies indicate overlap in limbic and cortical somatosensory areas activated by nasal trigeminal and olfactory stimuli, there is also potential cross-talk at the level of the olfactory epithelium, the olfactory bulb and trigeminal brainstem. Here we explored the influence of olfactory and trigeminal signaling in the nasal cavity. A forced choice water consumption paradigm was used to ascertain whether trigeminal and olfactory stimuli could influence behaviour in mice. Mice avoided water sources surrounded by both volatile TRPV1 (cyclohexanone) and TRPA1 (allyl isothiocyanate) irritants and the aversion to cyclohexanone was mitigated when combined with a pure odorant (rose fragrance, phenylethyl alcohol, PEA). To determine whether olfactory-trigeminal interactions within the nose could potentially account for this behavioural effect we recorded from single trigeminal sensory axons innervating the nasal respiratory and olfactory epithelium using an isolated in vitro preparation. To circumvent non-specific effects of chemical stimuli, optical stimulation was used to excite olfactory sensory neurons in mice expressing channel-rhodopsin (ChR2) under the olfactory marker protein (OMP) promoter. Photoactivation of olfactory sensory neurons produced no modulation of axonal action potential conduction in individual trigeminal axons. Similarly, no evidence was found for collateral branching of trigeminal axon that might serve as a conduit for cross-talk between the olfactory and respiratory epithelium and olfactory dura mater. Using direct assessment of action potential activity in trigeminal axons we observed neither paracrine nor axon reflex mediated cross-talk between olfactory and trigeminal sensory systems in the rodent nasal cavity. Our current results suggest that olfactory sensory neurons exert minimal influence on trigeminal signals within the nasal cavity.

[АNTINOCICEPTIVE PROTECTION ON A STAGE OF CONCLUDING OF SURGICAL INTERVENTION AND EARLY POSTOPERATIVE PERIOD]

Efficacy of antinociceptive defense at the terminal period of operation and in early (6 h) postoperative period, using additional injection of phentanil, paracetamol and nalbufin in anesthesiological support, and applying sevofluran in 107 patients, оperated on facial skull, in 2 stage of operative risk in accordance to ASA, was a nalyzed. Insufficient antinociceptive protection at the end of operation and in early postoperative period while using phentanil and nonsteroidal antiinflammatory medicines only for anesthesia, was established, basing on analysis of hemodynamic indices, pain syndrome severity and indices of metabolic stress. Application of paracetamol have promoted raising of the antinociceptive protection efficacy during short period (up to 2 h) only. Prescription of nalbufin have had guaranteed enhanced efficacy and duration of antinociceptive protection in early postoperative period, that's why its wide application is recommended.

Phagocytosis of bacteria by olfactory ensheathing cells and Schwann cells

Opportunistic bacterial infections of the nasal cavity could potentially lead to infection of the brain if the olfactory or trigeminal nerves are colonised. The olfactory nerve may be a more susceptible route because primary olfactory neurons are in direct contact with the external environment. Peripheral glia are known to be able to phagocytose some species of bacteria and may therefore provide a defence mechanism against bacterial infection. As the nasal cavity is frequently exposed to bacterial infections, we hypothesised that the olfactory and trigeminal nerves within the nasal cavity could be subjected to bacterial colonisation and that the olfactory ensheathing cells and Schwann cells may be involved in responding to the bacterial invasion. We have examined the ability of mouse OECs and Schwann cells from the trigeminal nerve and dorsal root ganglia to phagocytose Escherichia coli and Burkholderia thailandensis in vitro. We found that all three sources of glia were equally able to phagocytose E. coli with 75-85% of glia having phagocytosed bacteria within 24h. We also show that human OECs phagocytosed E. coli. In contrast, the mouse OECs and Schwann cells had little capacity to phagocytose B. thailandensis. Thus subtypes of peripheral glia have similar capacities for phagocytosis of bacteria but show selective capacity for the two different species of bacteria that were examined. These results have implications for the understanding of the mechanisms of bacterial infections as well as for the use of glia for neural repair therapies.

Retronasal perception of odors

We perceive odors orthonasally during sniffing; in contrast, we perceive odors retronasally during eating when they enter the nose through the pharynx. There are clear differences between orthonasal and retronasal olfaction in neuronal processing and perception, so that these two pathways convey two distinct sensory signals. The perception of foods is based on the interaction between ortho- and retronasal smell, taste, trigeminal activation and texture, so it is difficult to investigate one of these factors in isolation. Specific clinical aspects include effects of retronasal olfaction on satiation and swallowing.

Nasal tactile sensitivity in elderly

CONCLUSION

Although older people varied widely in tactile sensitivity, our results show that tactile thresholds increased with age.

OBJECTIVES

The aim of this study was to evaluate the effects of aging on nasal tactile sensitivity.

METHODS

A total of 160 healthy patients aged between 50 and 90 years were included. According to their age, patients were divided into groups (A, B, C, D, E, F, G, and H). From the age of 50, each group included subjects with an age range of 5 years (i.e. group A, 50-55 years; group B, 56-60 years, etc.). Each patient's outcome was assessed through the nasal monofilament test: a set of 20 Semmes-Weinstein monofilaments was used to detect nasal sensitivity for both nasal cavities. The sensitivity threshold was recorded as the minimum monofilament size from which patients could detect at least two of three stimuli.

RESULTS

In groups D (66-70 years), E (71-75 years), F (76-80 years), G (81-85 years), and H (86-90 years) a significantly (p < 0.05) higher stimulus (171.1 ± 0.34 mg vs 67.7 mg, 167.01 ± 0.31 mg 67.7 mg, 166.54 ± 0.28 mg 67.7 mg, 201.24 ± 0.43 mg 67.7 mg, 165,87 ± 0.27 mg 67.7 mg) was required to trigger a touch response in the monofilament test.

Unmyelinated fibers of the anterior ethmoidal nerve in the rat co-localize with neurons in the medullary dorsal horn and ventrolateral medulla activated by nasal stimulation

The anterior ethmoidal nerve (AEN) innervates the nasal passages and external nares, and serves as the afferent limb of the nasopharyngeal and diving responses. However, although 65% of the AEN is composed of unmyelinated fibers, it has not been determined whether this afferent signal is carried by unmyelinated or myelinated fibers. We used the transganglionic tracers WGA-HRP, IB4-HRP, and CTB-HRP to trace the central projections of the AEN of the rat. Interpretation of the labeling patterns suggests that AEN unmyelinated fibers project primarily to the ventral tip of the ipsilateral medullary dorsal horn (MDH) at the level of the area postrema. Other unmyelinated projections were to the ventral paratrigeminal nucleus and ventrolateral medulla, specifically the Bötzinger and RVLM/C1 regions. Myelinated AEN fibers projected to the ventral paratrigeminal and mesencephalic trigeminal nuclei. Stimulating the nasal passages of urethane-anesthetized rats with ammonia vapors produced the nasopharyngeal response that included apnea, bradycardia and an increase in arterial blood pressure. Central projections of the AEN co-localized with neurons within both MDH and RVLM/C1 that were activated by nasal stimulation. Within the ventral MDH the density of AEN terminal projections positively correlated with the rostral-caudal location of activated neurons, especially at and just caudal to the obex. We conclude that unmyelinated AEN terminal projections are involved in the activation of neurons in the MDH and ventrolateral medulla that participate in the nasopharyngeal response in the rat. We also found that IB4-HRP was a much less robust tracer than WGA-HRP.

Peripheral and central levels in nasal trigeminal sensitization and desensitization

In order to investigate the role of central and peripheral mechanisms in nasal trigeminal sensitization/desensitization processes, the present work recorded psychophysical (intensity ratings) and psychophysiological (skin conductance) responses to allyl isothiocyanate volatile nasal stimulation--during normal breathing--in monorhinal condition after a controlateral stimulation of the other nostril. Insofar as both nostrils are anatomically separated, modifications in responses can be interpreted as a central regulation process. Results showed that sensitization was clearly related to central mechanisms contrarily to desensitization which depended only of peripheral level.

Prevention of ultrasonic coagulator-mediated mucoperiosteal flap injury and defects by using a clip manipulation in the resection of the posterior nasal nerve

We previously reported on the clinical effectiveness of functional inferior turbinosurgery utilizing modified vidian neurectomy, the resection of the posterior nasal nerve (PNN), combined with inferior turbinoplasty. In order to prevent re-innervation of the PNN after resection and to avoid postoperative massive hemorrhage--presumably resulting from insufficient fixation and unexpected exposure of the bony or cartilaginous fragments covered on the resected neurovascular bundle containing the sphenopalatine vessels and the PNN--we designed a surgical technique during which a vascular clip was used in order to provide traction of the mucoperiosteal flap. Then we compared it with the previous procedure (without the use of the clip). The injury and defects of the mucoperiosteal flap were evaluated by the degree of exposure to the bony or cartilaginous fragments and scored on a scale of 0 to 2 points. The defects of the mucoperiosteal flap were reduced by using a vascular clip. The average score of the defects was 0.97 +/- 0.73 (n = 64) in the conventional procedure without any manipulation and 0.27 +/- 0.45 (n = 60) in the procedure using a vascular clip. The difference observed between the two gropups was statistically significant (p < 0.001). These results demonstrated that this is a safe technique to prevent injury and defects of the mucoperiosteal flap in gaining access to expose the PNN. This should promote early wound healing, reduce the chance of recurrence and of postoperative massive hemorrhage.

Expression of cGMP signaling elements in the Grueneberg ganglion

The Grueneberg ganglion (GG) is a cluster of neurons localized to the vestibule of the anterior nasal cavity. Based on axonal projections to the olfactory bulb of the brain, as well as expression of olfactory receptors and the olfactory marker protein, it is considered a chemosensory subsystem. Recently, it was observed that in mice, GG neurons respond to cool ambient temperatures. In mammals, coolness-induced responses in highly specialized neuronal cells are supposed to rely on the ion channel TRPM8, whereas in thermosensory neurons of the nematode worm Caenorhabditis elegans, detection of environmental temperature is mainly mediated by cyclic guanosine monophosphate (cGMP) pathways, in which cGMP is generated by transmembrane guanylyl cyclases. To unravel the molecular mechanisms underlying coolness-induced responses in GG neurons, potential expression of TRPM8 in the murine GG was investigated; however, no evidence was found that this ion channel is present in the GG. By contrast, a substantial number of GG neurons was observed to express the transmembrane guanylyl cyclase subtype GC-G. In the nose, GC-G expression appears to be confined to the GG since it was not detectable in other nasal compartments. In the GG, coolness-stimulated responses are only observed in neurons characterized by the expression of the olfactory receptor V2r83. Interestingly, expression of GC-G in the GG was found in this V2r83-positive subpopulation but not in other GG neurons. In addition to GC-G, V2r83-positive GG cells also co-express the phosphodiesterase PDE2A. Thus, in summary, coolness-sensitive V2r83-expressing GG neurons are endowed with a cGMP cascade which might underlie thermosensitivity of these cells, similar to the cGMP pathway mediating thermosensation in neurons of C. elegans.

Odor detection ability and thallium-201 transport in the olfactory nerve of traumatic olfactory-impaired mice

Although olfactory nerve damage is a contributing factor in the diagnosis of posttraumatic olfactory loss, at present, there are no methods to directly assess injury to these nerves. We have shown that following olfactory nerve injury in mice, thallium-201 (201 Tl) transport from the nasal cavity to the olfactory bulb decreases. To determine if olfactory function after nerve injury could be assessed with nasal administration of 201 Tl, we measured the correlation between odor detection ability (ODA) and the rate of transport of 201 Tl in olfactory nerves. Both ODA and 201 Tl transport were measured after bilateral olfactory nerve transection for a 4-week period. Cycloheximide solution was used for ODA against tap water. 201 Tl transport was measured as the ratio of radioactivity in the nasal cavity and olfactory bulb with gamma spectrometry. There was a significant correlation between ODA and the rate of 201 Tl transport in the olfactory nerve. These findings suggest that olfactory function after nerve injury can be objectively evaluated with the nasal administration of 201 Tl.

Recordings of the optical intrinsic signal from the middle turbinate in response to olfactory and trigeminal stimulation: a pilot study

Responses from the middle turbinate elicited by olfactory and trigeminal stimuli were studied using the intrinsic optical signal (IOS) recording technique. Nasal cavity was illuminated by 617 nm light. Olfactory (H2S) or trigeminal (CO2) stimuli of 5-s duration were presented using a computer-controlled olfactometer; IOS responses were captured by a special camera. Averages across five individual IOS recordings were analyzed. When the nasal cavity was exposed to H2S, a significant change of the IOS was found; responses to CO2 were even more pronounced. The present results argue for the idea that the IOS is an indicator of intranasal chemosensory activation.

Thallium Transport and the Evaluation of Olfactory Nerve Connectivity between the Nasal Cavity and Olfactory Bulb

Little is known regarding how alkali metal ions are transported in the olfactory nerve following their intranasal administration. In this study, we show that an alkali metal ion, thallium is transported in the olfactory nerve fibers to the olfactory bulb in mice. The olfactory nerve fibers of mice were transected on both sides of the body under anesthesia. A double tracer solution (thallium-201, (201)Tl; manganese-54, (54)Mn) was administered into the nasal cavity the following day. Radioactivity in the olfactory bulb and nasal turbinate was analyzed with gamma spectrometry. Auto radiographic images were obtained from coronal slices of frozen heads of mice administered with (201)Tl or (54)Mn. The transection of the olfactory nerve fibers was confirmed with a neuronal tracer. The transport of intranasal administered (201)Tl/(54)Mn to the olfactory bulb was significantly reduced by the transection of olfactory nerve fibers. The olfactory nerve transection also significantly inhibited the accumulation of fluoro-ruby in the olfactory bulb. Findings indicate that thallium is transported by the olfactory nerve fibers to the olfactory bulb in mice. The assessment of thallium transport following head injury may provide a new diagnostic method for the evaluation of olfactory nerve injury.

Fos expression in the brainstem nuclei evoked by nasal air-jet stimulation in rats

BACKGROUND

Noxious stimulation of the nasal mucosa may induce protective reflexes in the upper airway in rats. Previously, we have reported that nasal air-jet stimulation increases the activities of the laryngeal muscles in decerebrate cats; however, the neuronal mechanism of this phenomenon still is not clarified.

METHODS

After the application of nasal air-jet stimulation for 2 hours, we investigated the distribution of Fos-positive cells (FPCs) throughout the medulla compared with sham-operated rats using Fos immunoreactivity.

RESULTS

FPCs in the spinal trigeminal nucleus, the parvocellular reticular nucleus, and the nucleus of the solitary tract were more frequent than the sham-operated rats.

CONCLUSION

These results suggest that the afferents induced by air-jet stimulation were conveyed to these FPCs and that some of these cells might participate in the augmentation of laryngeal muscle activities during nasal air-jet stimulation.

Chemosensory properties of murine nasal and cutaneous trigeminal neurons identified by viral tracing

Background

Somatosensation of the mammalian head is mainly mediated by the trigeminal nerve that provides innervation of diverse tissues like the face skin, the conjunctiva of the eyes, blood vessels and the mucouse membranes of the oral and nasal cavities. Trigeminal perception encompasses thermosensation, touch, and pain. Trigeminal chemosensation from the nasal epithelia mainly evokes stinging, burning, or pungent sensations. In vitro characterization of trigeminal primary sensory neurons derives largely from analysis of complete neuronal populations prepared from sensory ganglia. Thus, functional properties of primary trigeminal afferents depending on the area of innervation remain largely unclear.

Results

We established a PrV based tracing technique to identify nasal and cutaneous trigeminal neurons in vitro. This approach allowed analysis and comparison of identified primary afferents by means of electrophysiological and imaging measurement techniques.

Neurons were challenged with several agonists that were reported to exhibit specificity for known receptors, including TRP channels and purinergic receptors. In addition, TTX sensitivity of sodium currents and IB4 binding was investigated.

Compared with cutaneous neurons, a larger fraction of nasal trigeminal neurons showed sensitivity for menthol and capsaicin. These findings pointed to TRPM8 and TRPV1 receptor protein expression largely in nasal neurons whereas for cutaneous neurons these receptors are present only in a smaller fraction. The majority of nasal neurons lacked P2X₃ receptor-mediated currents but showed P2X₂-mediated responses when stimulated with ATP. Interestingly, cutaneous neurons revealed largely TTX resistant sodium currents. A significantly higher fraction of nasal and cutaneous afferents showed IB4 binding when compared to randomly chosen trigeminal neurons.

Conclusion

In conclusion, the usability of PrV mediated tracing of primary afferents was demonstrated. Using this technique it could be shown that compared with neurons innervating the skin nasal trigeminal neurons reveal pronounced chemosensitivity for TRPM8 and TRPV1 channel agonists and only partially meet properties typical for nociceptors. In contrast to P2X₃ receptors, TRPM8 and TRPV1 receptors seem to be of pronounced physiological relevance for intranasal trigeminal sensation.

Rats smell in stereo

It has been hypothesized that rats and other mammals can use stereo cues to localize odor sources, but there is limited behavioral evidence to support this hypothesis. We found that rats trained on an odor-localization task can localize odors accurately in one or two sniffs. Bilateral sampling was essential for accurate odor localization, with internasal intensity and timing differences as directional cues. If the stimulus arrived at the correct point of the respiration cycle, internasal timing differences as short as 50 milliseconds sufficed. Neuronal recordings show that bulbar neurons responded differentially to stimuli from the left and stimuli from the right.

The olfactory organ modulates gonadotropin-releasing hormone types and nest-building behavior in the tilapia Oreochromis niloticus

Direct olfactory inputs to any of the known gonadotropin-releasing hormone (GnRH) containing neurons have not been demonstrated. Therefore, the rationale of this study was to examine whether olfactory inputs might in some way interact with the GnRH system(s) to synchronize reproductive behaviors. In order to establish this, we used anosmic mature male tilapia to investigate changes in reproductive behaviors, gonadal morphology, and GnRH1, GnRH2, and GnRH3 cellular morphology and change in GnRH mRNA levels by real-time polymerase chain reaction. Bilateral removal of the olfactory rosettes followed by occlusion of the nasal cavity (ORX) inhibited nest-building behavior, but had no effect on aggressive and sexual behaviors or gonadal morphology. ORX failed to alter the morphological features of GnRH1, GnRH2, and GnRH3 (cell number, size, GnRH optical density), but significantly decreased copies of GnRH1 and GnRH2 mRNAs. GnRH immunoreactive fibers were not evident in the olfactory nerve and rosettes. DiI application to the olfactory nerve labeled inputs primarily to the glomerular layer of the olfactory bulbs and extrabulbar inputs to the forebrain but not to GnRH neurons. These results provide evidence that the olfactory rosette is crucial for modulating nest-building behavior through second-order olfactory pathways interacting with GnRH1 and GnRH2 neuronal systems.

Physical stresses at the air-wall interface of the human nasal cavity during breathing

The nose is the front line defender of the respiratory system and is rich with mechanoreceptors, thermoreceptors, and nerve endings. A time-dependent computational model of transport through nasal models of a healthy human has been used to analyze the fields of physical stresses that may develop at the air-wall interface of the nasal mucosa. Simulations during quiet breathing revealed wall shear stresses as high as 0.3 Pa in the noselike model and 1.5 Pa in the anatomical model. These values are of the same order of those known to exist in uniform large arteries. The distribution of temperature near the nasal wall at peak inspiration is similar to that of wall shear stresses. The lowest temperatures occur in the vicinity of high stresses due to the narrow passageway in these locations. Time and spatial gradients of these stresses may have functional effects on nasal sensation of airflow and may play a role in the well-being of nasal breathing.

Fgf8 expression defines a morphogenetic center required for olfactory neurogenesis and nasal cavity development in the mouse

In vertebrate olfactory epithelium (OE), neurogenesis proceeds continuously, suggesting that endogenous signals support survival and proliferation of stem and progenitor cells. We used a genetic approach to test the hypothesis that Fgf8 plays such a role in developing OE. In young embryos, Fgf8 RNA is expressed in the rim of the invaginating nasal pit (NP), in a small domain of cells that overlaps partially with that of putative OE neural stem cells later in gestation. In mutant mice in which the Fgf8 gene is inactivated in anterior neural structures, FGF-mediated signaling is strongly downregulated in both OE proper and underlying mesenchyme by day 10 of gestation. Mutants survive gestation but die at birth, lacking OE, vomeronasal organ (VNO), nasal cavity, forebrain, lower jaw, eyelids and pinnae. Analysis of mutants indicates that although initial NP formation is grossly normal, cells in the Fgf8-expressing domain undergo high levels of apoptosis, resulting in cessation of nasal cavity invagination and loss of virtually all OE neuronal cell types. These findings demonstrate that Fgf8 is crucial for proper development of the OE, nasal cavity and VNO, as well as maintenance of OE neurogenesis during prenatal development. The data suggest a model in which Fgf8 expression defines an anterior morphogenetic center, which is required not only for the sustenance and continued production of primary olfactory (OE and VNO) neural stem and progenitor cells, but also for proper morphogenesis of the entire nasal cavity.

A truffle in the mouth is worth two in the bush: odor localization in the human brain

It is widely thought that locating the source of a smell is an ability best left to nonhuman members of the animal kingdom. In this issue of Neuron, two complementary articles highlight the neural mechanisms underlying the localization of an odor, either to the left or right side of the nose (Porter et al.) or to the inside or outside of the mouth (Small et al.). Together, these studies validate the idea that the human brain is equipped with the apparatus necessary to pinpoint the location of an odor source.

Differential neural responses evoked by orthonasal versus retronasal odorant perception in humans

Odors perceived through the mouth (retronasally) as flavor are referred to the oral cavity, whereas odors perceived through the nose (orthonasally) are referred to the external world. We delivered vaporized odorants via the orthonasal and retronasal routes and measured brain response with fMRI. Comparison of retronasal versus orthonasal delivery produced preferential activity in the mouth area at the base of the central sulcus, possibly reflecting olfactory referral to the mouth, associated with retronasal olfaction. Routes of delivery produced differential activation in the insula/operculum, thalamus, hippocampus, amygdala, and caudolateral orbitofrontal cortex in orthonasal > retronasal and in the perigenual cingulate and medial orbitofrontal cortex in retronasal > orthonasal in response to chocolate, but not lavender, butanol, or farnesol, so that an interaction of route and odorant may be inferred. These findings demonstrate differential neural recruitment depending upon the route of odorant administration and suggest that its effect is influenced by whether an odorant represents a food.

Developmental changes in GABA receptor subunit composition within the gonadotrophin-releasing hormone-1 neuronal system

It is becoming increasingly evident that GABA plays an important role in the regulation of gonadotrophin-releasing hormone (GnRH)-1 neurones via the GABAA receptor. The aim of the present study was to characterise expression of the GABAA receptor within the GnRH-1 system across development. The expression pattern of five GABAAalpha subunits and one GABAAbeta subunit was first examined within individual GnRH-1 neurones by the polymerase chain reaction. A significant increase in the expression of GABAAalpha2 and a significant decrease in the expression of GABAAalpha6 over time were found. Of the other subunits examined, two (alpha1 and alpha3) showed no differences in expression and two (alpha4 and beta3) showed variable low incidence of expression. Given the reciprocal relationship of alpha2 and alpha6 expression, we hypothesised that there is a developmental switch in the expression of these subunits in GnRH-1 neurones. To investigate this hypothesis, single- and double-label immunocytochemistry for GABAAalpha2 and alpha6 and GnRH-1 was performed in tissue from ages E12.5 to adulthood, as well as in nasal explants. We show that GABAAalpha2 and alpha6 are present in the GnRH-1 neuronal system both in vivo and in vitro and that the levels of expression are altered as a function of age.

Trigeminal sensitization and desensitization in the nasal cavity: a study of cross interactions

Chemical irritation in the human nasal cavity is poorly documented. In this field, an important issue concerns the differential responses produced by successive stimulation. Repeated identical chemical irritant stimuli can produce increases or decreases in responses (two phenomena known as self-sensitization or self-desensitization). In the same way, different molecules can interact and produce cross-sensitization or cross-desensitization. The aim of this study was to contribute to this question using two specific molecules, acetic acid (AA) and allyl isothiocyanate (AIC). As the self-sensitization and -desensitization for AIC is known, a first experiment in the present work investigated the response, acute effects and time course of sensitization or desensitization to acetic acid. A second experiment tested the responses of acetic acid after a previous stimulation with allyl isothiocyanate (mustard oil) and inversely with a short inter-stimulus interval (ISI of 45 s). A third experiment similar to the second used a long inter-stimulus interval (ISI of 3 min 30). Twelve healthy subjects participated in the study using psychophysical (intensity ratings) and psychophysiological (skin conductance response) measurements. Firstly, the results showed that repeated nasal stimulation with acetic acid produced a self-desensitization whatever the ISI. Secondly, the results showed a cross-desensitization of allyl isothiocyanate by previous acetic acid stimulation. In contrast, previous stimulation with allyl isothiocyanate had no effect on the following acetic acid response. These findings confirm that trigeminal sensitization and desensitization in the nasal cavity do not follow the same processes in relation to molecules used.