Rat trigeminal, olfactory and taste responses after capsaicin desensitization

Capsaicin burn increases thickness discrimination thresholds independently of chronic chili intake

The trigeminal nerve transduces both chemical irritation and textural sensations suggesting that perception in one may influence perception in the other. Little is known about how the oral burn of capsaicin may affect texture sensitivity. The aim of this study was to determine the effect of burning sensations on thickness discrimination thresholds in liquid foods assessed by consumers who vary in habitual spicy food intake. Forty-seven Caucasian participants (31 females and 16 males; mean age: 25.0 ± 5.7 yrs; mean BMI: 21.5 ± 2.6 kg/m²) were recruited in the Netherlands. Chili pepper intake frequency and preference for chili peppers and spicy foods were assessed using questionnaires. Perceived burn and disliking/liking of bouillon soups thickened with xanthan gum (concentrations ranging from 0.06 to 0.21 g/mL; viscosity at 50 s^-1 (η_50s-1) ranging from 11 to 48 mPas) containing varying amounts of capsaicin (0, 1, or 10 ppm) were determined using generalized scales (gLMS and gDOL). Estimates of thickness discrimination thresholds were determined using the 2-Alternative Forced Choice ascending staircase method. Capsaicin was applied in two ways: (i) capsaicin was added directly to the soups or (ii) a pre-rinse of a capsaicin solution was held in mouth before evaluating soups without capsaicin. As expected, frequent chili pepper consumers reported significantly lower burn intensity and higher hedonic ratings compared to infrequent consumers. Thickness discrimination thresholds (i.e., BET expressed as Δη_50s-1) increased significantly from 11.3 mPas at 0 ppm to 16.1 mPas at 1 ppm (42% increase) to 21.4 mPas at 10 ppm capsaicin (89% increase) on average across all participants. Similar modification of thickness discrimination thresholds were observed regardless of whether capsaicin was added to the soup or was applied as a pre-rinse. No significant differences in thickness discrimination thresholds were observed between infrequent and frequent chili consumers. We conclude that oral burn caused by capsaicin affects thickness discrimination independently of reported chili pepper intake. Also, we suggest the ability of capsaicin to alter thickness discrimination may be due to increased neural noise, attentional effects or cross-modal interactions.

The intranasal trigeminal system

Many odors activate the intranasal chemosensory trigeminal system where they produce cooling and other somatic sensations such as tingling, burning, or stinging. Specific trigeminal receptors are involved in the mediation of these sensations. Importantly, the trigeminal system also mediates sensitivity to airflow. The intranasal trigeminal and the olfactory system are closely connected. With regard to central nervous processing, it is most interesting that trigeminal stimuli can activate the piriform cortex, which is typically viewed as the primary olfactory cortex. This suggests that interactions between the two systems may form at a relatively early stage of processing. For example, there is evidence showing that acquired olfactory loss leads to reduced trigeminal sensitivity, probably on account of the lack of interaction in the central nervous system. Decreased trigeminal sensitivity may also be responsible for changes in airflow perception, leading to the impression of congested nasal airways.

Chronic Oral Capsaicin Exposure During Development Leads to Adult Rats with Reduced Taste Bud Volumes

INTRODUCTION

Cross-sensory interaction between gustatory and trigeminal nerves occurs in the anterior tongue. Surgical manipulations have demonstrated that the strength of this relationship varies across development. Capsaicin is a neurotoxin that affects fibers of the somatosensory lingual nerve surrounding taste buds, but not fibers of the gustatory chorda tympani nerve which synapse with taste receptor cells. Since capsaicin is commonly consumed by many species, including humans, experimental use of this neurotoxin provides a naturalistic perturbation of the lingual trigeminal system. Neonatal or adults rats consumed oral capsaicin for 40 days and we examined the cross-sensory effect on the morphology of taste buds across development.

METHODS

Rats received moderate doses of oral capsaicin, with chronic treatments occurring either before or after taste system maturation. Tongue morphology was examined either 2 or 50 days after treatment cessation. Edema, which has been previously suggested as a cause of changes in capsaicin-related gustatory function, was also assessed.

RESULTS

Reductions in taste bud volume occurred 50 days, but not 2 days post-treatment for rats treated as neonates. Adult rats at either time post-treatment were unaffected. Edema was not found to occur with the 5 ppm concentration of capsaicin we used.

CONCLUSIONS

Results further elucidate the cooperative relationship between these discrete sensory systems and highlight the developmentally mediated aspect of this interaction.

IMPLICATIONS

Chronic exposure to even moderate levels of noxious stimuli during development has the ability to impact the orosensory environment, and these changes may not be evident until long after exposure has ceased.

Activation of olfactory and trigeminal cortical areas following stimulation of the nasal mucosa with low concentrations of S(-)-nicotine vapor--an fMRI study on chemosensory perception

Applied to the nasal mucosa in low concentrations, nicotine vapor evokes odorous sensations (mediated by the olfactory system) whereas at higher concentrations nicotine vapor additionally produces burning and stinging sensations in the nose (mediated by the trigeminal system). The objective of this study was to determine whether intranasal stimulation with suprathreshold concentrations of S(-)-nicotine vapor causes brain activation in olfactory cortical areas or if trigeminal cortical areas are also activated. Individual olfactory detection thresholds for S(-)-nicotine were determined in 19 healthy occasional smokers using a computer-controlled air-dilution olfactometer. Functional magnetic resonance images were acquired using a 1.5T MR scanner with applications of nicotine in concentrations at or just above the individual's olfactory detection threshold. Subjects reliably perceived the stimuli as being odorous. Accordingly, activation of brain areas known to be involved in processing of olfactory stimuli was identified. Although most of the subjects never or only rarely observed a burning or painful sensation in the nose, brain areas associated with the processing of painful stimuli were activated in all subjects. This indicates that the olfactory and trigeminal systems are activated during perception of nicotine and it is not possible to completely separate olfactory from trigeminal effects by lowering the concentration of the applied nicotine. In conclusion, even at low concentrations that do not consistently lead to painful sensations, intranasally applied nicotine activates both the olfactory and the trigeminal system.

Assessment of Upper Respiratory Tract and Ocular Irritative Effects of Volatile Chemicals in Humans

Accurate assessment of upper respiratory tract and ocular irritation is critical for identifying and remedying problems related to overexposure to volatile chemicals, as well as for establishing parameters of irritation useful for regulatory purposes. This article (a) describes the basic anatomy and physiology of the human upper respiratory tract and ocular mucosae, (b) discusses how airborne chemicals induce irritative sensations, and (c) reviews practical means employed for assessing such phenomena, including psychophysical (e.g., threshold and suprathreshold perceptual measures), physiological (e.g., cardiovascular responses), electrophysiological (e.g., event-related potentials), and imaging (e.g., magnetic resonance imaging) techniques. Although traditionally animal models have been used as the first step in assessing such irritation, they are not addressed here since (a) there are numerous reviews available on this topic and (b) many rodents and rabbits are obligate nose breathers whose nasal passages differ considerably from those of humans, potentially limiting generalization of animal-based data to humans. A major goal of this compendium is to inform the reader of procedures for assessing irritation in humans and to provide information of value in the continued interpretation and development of empirical databases upon which future reasoned regulatory health decisions can be made.

Solitary chemoreceptor cell survival is independent of intact trigeminal innervation

Nasal solitary chemoreceptor cells (SCCs) are a population of specialized chemosensory epithelial cells presumed to broaden trigeminal chemoreceptivity in mammals (Finger et al. [2003] Proc Natl Acad Sci USA 100:8981-8986). SCCs are innervated by peptidergic trigeminal nerve fibers (Finger et al. [2003]) but it is currently unknown if intact innervation is necessary for SCC development or survival. We tested the dependence of SCCs on innervation by eliminating trigeminal nerve fibers during development with neurogenin-1 knockout mice, during early postnatal development with capsaicin desensitization, and during adulthood with trigeminal lesioning. Our results demonstrate that elimination of innervation at any of these times does not result in decreased SCC numbers. In conclusion, neither SCC development nor mature cell maintenance is dependent on intact trigeminal innervation.

Multiple types of sensory neurons respond to irritating volatile organic compounds (VOCs): Calcium fluorimetry of trigeminal ganglion neurons

Many volatile organic compounds (VOCs) are significant environmental irritants that stimulate somatosensory nerve endings to produce pain and irritation. We measured intracellular calcium in cultured trigeminal ganglion neurons to characterize the cellular mechanisms and chemical structural determinants underlying sensitivity to VOCs. Trigeminal neurons responded to homologous series of alcohols (C4-C7) as well as saturated and unsaturated aldehydes in a concentration dependent manner. Ranked in terms of threshold to recruit neurons by compounds of the same carbon chain length, enaldehyde<aldehyde<alcohol. Unlike aldehydes and alcohols that displayed ascending concentration curves, recruitment of neurons by enaldehydes (C4-C7) appeared to saturate, consistent with a mechanism that is restricted in its neural distribution. Using pentanol, pentanal and pentenal as model compounds, we found that many but not all cool/cold-sensitive and capsaicin-sensitive neurons responded with increases in intracellular calcium. These VOCs also stimulated other neurons that were insensitive to cooling and capsaicin. Because not all cooling- and all capsaicin-sensitive neurons responded to the model VOCs, it is highly unlikely that known nociceptive ion channels such as TRPV1 or TRPA1 mediate sensitivity to these compounds. For pentanol, pentanal and pentenal, induced calcium influx was dependent on the presence of extracellular calcium. Responses of all neurons to pentanal and pentenal were also dependent upon extracellular sodium. Responses to pentanol were variably dependent on sodium. The distribution of sensitivity suggests that VOC irritation may be mediated by an as yet unidentified mechanism(s) that is/are distributed across different modalities of neurons.

Suppression of central taste transmission by oral capsaicin

Because intraoral capsaicin is reported to reduce the perceived intensity of certain taste qualities, we investigated whether it affects the central processing of gustatory information. The responses of gustatory neurons in the nucleus tractus solitarius (NTS) to tastant stimuli were recorded before and after lingual application of capsaicin in anesthetized rats. Thirty-four NTS units were characterized as responding best to sucrose (0.3 m), NaCl (0.1 m), citric acid (0.03 m), monosodium glutamate (0.2 m), or quinine (0.001 m). During lingual application of 330 microm capsaicin for 7 min, the firing rate increased for five units and decreased for four units; the remainder were unaffected. Immediately after capsaicin, responses to each tastant were in nearly all cases depressed (mean, 61.5% of control), followed by recovery in most cases. NTS tastant-evoked unit responses were unaffected by lingual application of vehicle (5% ethanol). Capsaicin elicited an equivalent reduction (to 64.5%) in tastant-evoked responses of nine additional NTS units recorded in rats with bilateral trigeminal ganglionectomy, arguing against a trigeminally mediated central effect. Furthermore, capsaicin elicited a puncate pattern of plasma extravasation in the tongue that matched the distribution of fungiform papillae. These results support a peripheral site of capsaicin suppression of taste possibly via direct or indirect effects on taste transduction or taste receptor cell excitability. The depressant effect of capsaicin on gustatory transmission might underlie its ability to reduce the perceived intensity of some taste qualities.

Odorants elicit evoked potentials in the human amygdala

Electroencephalographical (EEG) recording studies have shown that odorants produce olfactory evoked potentials (OEPs) on the scalp surface. However, EEGs can only provide limited information about the intracerebral sources from where the OEPs are generated. By contrast, intracerebral EEG recordings enable direct examination of the electrophysiological activity from a given cerebral area. In the present study, neural activity was recorded from the amygdala of seven epileptic patients undergoing intracerebral EEG recordings prior to surgical treatment for relief of intractable seizures. Two olfactory tests were used: a passive-stimulation test consisting of the successive presentation of 12 common odorants and a suprathreshold detection test including both odorant and non-odorant stimulations. Recordings from the amygdala revealed that all odorant stimulations induced large and reproducible OEPs, whereas the non-odorant stimulations did not. It was also found that repetition of the same odorant stimulation led to a decrease in the latency of the first OEP component. This modulation, which corresponds to a faster olfactory processing, strongly suggests that the amygdala is involved in early olfactory attentional processes. In conclusion, it appears that the human amygdala discriminates the incoming information from the nasal airflow as being odorant or not and, additionally, that its speed of processing is sensitive to recent experience with an odor.

Decreased trigeminal sensitivity in anosmia

The present study aimed to investigate intranasal trigeminal sensitivity in a large sample of patients with anosmia due to different etiologies. We investigated the trigeminal detection threshold for formic acid in healthy controls (n = 96) and patients with anosmia due to head trauma (n = 18) or sinonasal disease (n = 54). Anosmics exhibited higher thresholds compared with normosmics (p < 0.001). In addition, thresholds were found to be higher in patients with posttraumatic anosmia compared to anosmics with sinonasal disease (p < 0.001). The data indicate that (1) loss of olfactory sensitivity in humans may be associated with a decreased sensitivity towards trigeminal stimuli and (2) alteration of intranasal trigeminal function is stronger in patients with posttraumatic anosmia compared to patients with sinonasal disease. This may have implications for the medicolegal investigation of anosmic patients where trigeminal stimuli are frequently used to assess the patient's response bias.

Morphology of the nerve endings in laryngeal mucosa of the horse

To discuss the significance of laryngeal sensation on various disorders of the horse, we studied the morphological and topographical characteristics of sensory structures in the laryngeal mucosa using immunohistochemistry and immunoelectron microscopy. Various sensory structures, i.e. glomerular endings, taste buds and intraepithelial free nerve endings, were found in the laryngeal mucosa by immunohistochemistry for protein gene product 9.5 (PGP 9.5) and neurofilament 200kD (NF200). Glomerular nerve endings were distributed mainly in the epiglottic mucosa; some endings were also found in the arytenoid region arising from thick nerve fibres running through the subepithelial connective tissue. Some terminals directly contacted the epithelial cells. Taste buds were distributed in the epithelium of the epiglottis and aryepiglottic fold. In the whole mount preparation, the taste buds were supplied by the terminal branching of the thick nerve fibres. In some cases, the taste buds were arranged around the opening of the duct of the epiglottic glands. The intraepithelial free nerve endings were found to be immunoreactive for substance P (SP) and calcitonin gene-related peptide (CGRP). These nerve endings were surrounded by the polygonal stratified epithelial cells in the supraglottic region, and by the ciliated cells in the subglottic region. The density of the intraepithelial free nerve endings was highest in the corniculate process of the arytenoid region and lowest in the vocal cord mucosa. The densities of CGRP- and SP-immunoreactive nerve endings in the arytenoid region were (mean +/- s.d.) 30.6+/-12.0 and 10.0+/-4.9 per unit epithelial length (1 mm), respectively and in the vocal fold mucosa, 1.1+/-0.9 and 0.8+/-0.7, respectively. Approximately one half of the CGRP immunoreactive nerve endings were immunoreactive for SP, and most SP-immunoreactive nerve endings were also immunoreactive for CGRP. Well-developed subepithelial plexus with numerous intraepithelial fibres were observed in flat or round mucosal projections that existed on the corniculate process of the arytenoid region. In conclusion, the laryngeal mucosa of the horse seems to have morphology- and/or location-dependent sensory mechanisms against various endo-and exogenious stimuli.

Capsaicin-treated rats permanently overingest low- but not high-concentration sucrose solutions

The effect of capsaicin-induced chemical ablation of visceral afferents on 1-h liquid sucrose consumption was investigated in food-deprived rats. We first show that although 10% sucrose is permanently overconsumed by capsaicin-treated (CAPs) compared with vehicle-treated (VEHs) control rats, 40% sucrose is only overconsumed during the first but not subsequent 1-h exposures. Furthermore, one group of CAPs lost the overconsumption response at 20% when exposed to progressively increasing sucrose concentrations of 10-40%, and another group recovered the overconsumption response at 10% when exposed to a series of decreasing concentrations. Control rats ingested relatively constant volumes of sucrose over the range of 10, 15, and 20%, resulting in significantly different energy intakes. In contrast, CAPs generally showed a concentration-dependent decrease in volume intake, resulting in relatively constant energy intake. These results suggest that capsaicin-sensitive visceral afferents, likely from gastric distension and other preabsorptive sensors, provide major control over volume ingested. In the absence of these signals, rats initially overconsume, but rapidly learn to use other signals from capsaicin-resistant preabsorptive or postabsorptive sites, to control future intake. This redundant satiety system appears to be sensitive to the osmotic value or caloric content of the unfamiliar food, but only if this is above a threshold of about 15% sucrose.

Selective attention to the chemosensory modality

Previous studies have shown that behavioral responses to auditory, visual, and tactile stimuli are modulated by expectancies regarding the likely modality of an upcoming stimulus (see Spence & Driver, 1997). In the present study, we investigated whether people can also selectively attend to the chemosensory modality (involving responses to olfactory, chemical, and painful stimuli). Participants made speeded spatial discrimination responses (left vs. right) to an unpredictable sequence of odor and tactile targets. Odor stimuli were presented to either the left or the right nostril, embedded in a birhinally applied constant airstream. Tactile stimuli were presented to the left or the right hand. On each trial, a symbolic visual cue predicted the likely modality for the upcoming target (the cue was a valid predictor of the target modality on the majority of trials). Response latencies were faster when targets were presented in the expected modality than when they were presented in the unexpected modality, showing for the first time that behavioral responses to chemosensory stimuli can be modulated by selective attention.

Assessment of intranasal trigeminal function

Intranasal trigeminal function is more and more understood as an integral part of human chemosensory perception. Sensations like burning, stinging, warmth, coolness, or itching are produced by almost all odorants so that they can be perceived by anosmics. Electrophysiological responses to trigeminal stimuli allow the specific assessment of trigeminally mediated information at different levels of processing including the periphery or the cortex. Information regarding the localization of these processes can be derived from magnetoencephalographic recordings or functional imaging data. When using these techniques in combination with psychophysical measures, it seems to be possible to specifically describe how and where the processing of irritation takes place, how it may interact with olfactory mediated sensations, and how it is modulated, e.g. by environmental influences or analgesic drugs.

Transient overconsumption of novel foods by deafferentated rats: effects of novel diet composition

We recently demonstrated that capsaicin-treated rats consume more of an unfamiliar high-fat diet than vehicle-treated controls, but only on initial exposure (Chavez et al, 1997). We hypothesized that negative feedback signals carried by capsaicin-sensitive visceral afferents are critical for the regulation of intake of novel foods, but redundant pathways take over during subsequent exposures. To examine the role of nutrient content of the novel diet, rats were systemically treated with capsaicin (n = 15) or vehicle (n = 10), and exposed to 1) a fat/olestra diet that was isocaloric with chow; 2) a readily accepted fat-free cake; and 3) pure corn oil. Each 3-h feeding trial was preceded by 24-h food deprivation. Treated rats did not overconsume familiar chow, but did consume 50% more than controls of both the fat/olestra diet and the corn oil on first exposure; this suggests that capsaicin eliminated visceral afferents that normally carry satiety signals. However, the effect with the fat/olestra mixture was due primarily to depressed intake by controls, unlike the pure fat diets; this apparent neophobic response was blunted in treated rats. Because treated rats failed to overconsume the fat-free cakes, the neural system damaged by capsaicin appears to be linked to energy or fat sensory mechanisms, and possibly to hedonic responsiveness.

Roles of chemical mediators in the taste system

Recent advances in neural mechanisms of taste are reviewed with special reference to neuroactive substances. In the first section, taste transduction mechanisms of basic tastes are explained in two groups, whether taste stimuli directly activate ion channels in the taste cell membrane or they bind to cell surface receptors coupled to intracellular signaling pathways. In the second section, putative transmitters and modulators from taste cells to afferent nerves are summarized. The candidates include acetylcholine, catecholamines, serotonin, amino acids and peptides. Studies favor serotonin as a possible neuromodulator in the taste bud. In the third section, the role of neuroactive substances in the central gustatory pathways is introduced. Excitatory and inhibitory amino acids (e.g., glutamate and GABA) and peptides (substance P and calcitonin gene-related peptide) are proved to play roles in transmission of taste information in both the brainstem relay and cortical gustatory area. In the fourth section, conditioned taste aversion is introduced as a model to study gustatory learning and memory. Pharmacobehavioral studies to examine the effects of glutamate receptor antagonists and protein kinase C inhibitors on the formation of conditioned taste aversion show that both glutamate and protein kinase C in the amygdala and cortical gustatory area play essential roles in taste aversion learning. Recent molecular and genetic approaches to disclose biological mechanisms of gustatory learning are also introduced. In the last section, behavioral and pharmacological approaches to elucidate palatability, taste pleasure, are described. Dopamine, benzodiazepine derivatives and opioid substances may play some roles in evaluation of palatability and motivation to ingest palatable edibles.

Cutaneous wounds produced by capsaicin treatment of newborn rats are due to trophic disturbances

The purpose of this study was to show that the occurrence of skin ulcers observed in animals neonatally treated with the neurotoxin capsaicin coincide with trophic disturbances. In addition, cutaneous lesions increased when self-grooming and scratching behaviors reached maturity. The temporal course of cephalic cutaneous wounds in neonatally capsaicin-treated rats was evaluated in animals wearing and not wearing plastic collars from postnatal day (P) 21 until P45. The collars were used to prevent self-grooming and scratching. Beginning on P21, capsaicin-treated rats under both conditions showed transient skin ulcers distributed throughout the head and neck regions. In the capsaicin-treated group without collars, lesions reached their greatest severity by P40, when self-grooming and scratching behaviors obtained adult characteristics. Furthermore, no lesions were detected after 25 days. In the capsaicin-treated rats that wore plastic collars, the widest distribution of skin lesions occurred on P55, after which time lesions vanished detection by 25 days. In this latter group, the cutaneous lesions were exacerbated when collars were removed. Data suggest that transient cutaneous wounds associated with neonatal capsaicin administration may be mediated via capsaicin-sensitive sensory neurons that are involved in trophic and regenerating neural mechanisms.

Agonistic sensory effects of airborne chemicals in mixtures: odor, nasal pungency, and eye irritation

Thresholds responses of odor, nasal pungency (irritation), and eye irritation were measured for single chemicals (1-propanol, 1-hexanol, ethyl acetate, heptyl acetate, 2-pentanone, 2-heptanone, toluene, ethyl benzene, and propyl benzene) and mixtures of them (two three-component mixtures, two six-component mixtures, and one nine-component mixture). Nasal pungency was measured in subjects lacking a functional sense of smell (i.e., anosmics) to avoid interference from olfaction. Various degrees of stimulus agonism (additive effects) were observed for each of the three sensory channels when testing mixtures. As the number of components and the lipophilicity of such components in the mixtures decreased, so did the degree of agonism. Synergistic stimulus agonism characterized the eye-irritation response for the most complex (the nine-component) and the most lipophilic (one of the six-component) mixtures. Physicochemical properties play a large role in the determination of sensitivity to airborne chemicals, particularly to their ability to evoke irritation. While this has revealed itself previously with respect to single chemicals, it seems to have relevance to mixtures as well.

Innervation of taste buds in the canine larynx as revealed by immunohistochemistry for the various neurochemical markers

The distribution and innervation of the canine laryngeal taste buds were observed using immunohistochemistry with antibodies against protein gene product 9.5 (PGP 9.5) and neurofilament protein (NFP). We also observed the immunohistochemical distribution of serotonin, tyrosine hydroxylase (TH) and various neuropeptides including calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP), galanin, methionine enkephalin (ENK) and neuropeptide Y (NPY). The taste buds in the canine larynx were densely distributed in the mucosa at the basal portion of the epiglottis and cuneiform process of the arytenoid cartilage. The taste cells were immunoreactive for PGP 9.5 and serotonin. The nerve fibers with immunoreactivity for PGP 9.5 in the taste buds were observed in the perigemmal region and intra- and subgemmal plexuses, and these were classified into two types based on their diameter. The thick nerve fibers corresponded to the fibers immunoreactive for NFP, while the thin nerve fibers corresponded to the fibers immunoreactive for TH and various neuropeptides. Numerous nerve fibers immunoreactive for SP and CGRP were observed in the perigemmal region, and intra- and subgemmal plexuses. A few galanin- and ENK-immunoreactive nerve fibers were also observed in the taste buds, whereas NPY-immunoreactive nerve fibers were noted beneath them. All peptide-containing fibers except for VIP-immunoreactive nerves were situated in the subgemmal regions. In conclusion, the multiple innervation to the laryngeal taste buds were documented. Thick nerve fibers are likely to be irritant receptors, while thin varicose nerve fibers seem to regulate taste buds themselves. The laryngeal taste buds may be among the important structures which are sensitive to exogeneous chemical and/or mechanical stimuli, for the protection of the airway and the regulation of the respiratory function.

Immunohistochemical, electrophysiological, and electron microscopical study of rat fungiform taste buds after regeneration of chorda tympani through the non-gustatory lingual nerve

The sensory innervation of fungiform papillae on the rat dorsal tongue is derived from branches of two cranial nerves: the lingual branch of the trigeminal nerve which provides somatosensory innervation and the chorda tympani (CT) branch of the facial nerve, which provides innervation to the taste buds. Removal of the CT results in degeneration of the taste buds. Removal of both nerves results in reduction in size of fungiform papillae and an altered pattern of keratinization in its epithelium. Regeneration of nerves to the epithelium restores the pre-operative condition. Thus, in addition to their sensory functions, both the CT and lingual seem to exert trophic effects on the phenotypic expression of epithelial cells in the fungiform papillae. We severed both the CT and lingual nerves in rats and sutured the proximal stump of the CT to the distal stump of the lingual to promote regeneration of the CT along the lingual nerve pathway. At the same time, we prevented the proximal stump of the lingual from regenerating into the tongue. Our purpose was to determine whether and how the innervation pattern of the regenerated taste bud might be different from normal under these experimental conditions. We found that reinnervation by the CT through the lingual nerve occurs, that this restores the anatomical and functional integrity of the fungiform taste buds and papillae, and that some papillae, but not all, were richly innervated with subgemmal, extragemmal, and perigemmal neuron-specific enolase, calcitonin gene-related peptide, substance P, and neurokinin A-positive fibers. Moreover, responses to taste stimuli were recorded electrophysiologically from the CT.

Olfactory evoked potentials produced by electrical stimulation of the olfactory mucosa

We stimulated the olfactory mucosa electrically and elicited evoked potentials in rabbits. A bipolar stimulating electrode was placed on the olfactory region of the nasal mucosa via an anterior naris non-invasively. Evoked potentials were detected from the surface of a head. In most instances they were composed of triphasic negative-positive-negative peaks, the latencies of these peaks were about 25, 40, and 65 ms, respectively. This peak complex seemed to originate in the olfactory bulb. This method is non-invasive and is applicable to studying the olfactory system in animals and also in humans.

Maternal ingestion of ortho-aminoacetophenone during gestation affects intake by offspring

Ingested flavor chemicals cross the placental barrier and occur in the fetal blood and amniotic fluid. This occurrence is detectable by the fetus, and can influence post parturition feeding. In the present experiment, pregnant mice were offered either 0.1% ortho-aminoacetophenone emulsions (OAP) or water throughout gestation. OAP is normally avoided by mice, apparently on the basis of chemosensory characteristics. Subsequently, offspring were offered 0.5%, 0.25%, or 0.1% OAP in one-bottle tests at 26 or 88 days of age. Offspring of mothers given OAP drank greater amounts of OAP than did offspring of mothers given water. Enhanced acceptance of OAP was not detected in mice exposed to 0.1% OAP as adults for a duration similar to that given during gestation. We conclude that fetal experiences with OAP lowered sensitivity and/or raised tolerance for the compound.

Effects of capsaicin desensitization on taste in humans

The desensitization effects on taste resulting from application of 100 or 10 ppm capsaicin, accompanied by daily testing of a capsaicin series (1-1000 ppm, in log steps), or the desensitization resulting from application of 100 ppm capsaicin without the daily capsaicin testing, were investigated. The taste stimuli were three concentrations each of NaCl, sucrose, citric acid, quinine and 6-n-propylthiouracil. Following either type of 100 ppm desensitization, the magnitude estimates of the two bitter tastes, in particular, and citric acid showed significant decrements. Following 10 ppm capsaicin or an ethanol control procedure, there were no such effects. Recovery was complete in 1-3 days. It seems possible that the taste decrements are due to effects on both the taste and tactile components of taste, though there is a stronger case for effects on the tactile component.

Sensory irritation effects of n-propanol and ethylbenzene after pretreatment with capsaicin or indomethacin

Irritation of upper respiratory tract (sensory irritation) due to two model solvents, ethylbenzene and propanol, were investigated from their reflex-induced decrease in respiratory rate in mice. Intranasal application of capsaicin decreased the sensory irritation response of both solvents, indicating that at least part of the sensory irritation effect must occur as a result of activation of capsaicin sensitive afferent nerves. The vehicle used for the application of capsaicin in itself decreased the respiratory rate strongly, either caused by effects on the upper respiratory tract or on the lungs. This demonstrates the need for a formulation of a pharmacologically inactive vehicle for nasal application of lipophilic substances. Pretreatment with indomethacin had no influence on sensory irritation of any of the solvents. This suggests that sensory irritation is caused by a direct interaction between receptors on the trigeminal nerves and the vapours rather than being an indirect effect of tissue damage giving rise to metabolites from the cyclooxygenase pathway.

Peripheral peptidergic fibers of the trigeminal nerve in the olfactory bulb of the rat

Axons immunoreactive for calcitonin gene-related peptide (CGRP) and substance P are present in the olfactory nerve, although few, if any, olfactory receptor cells contain immunocytochemically detectable levels of these peptides. The possible trigeminal origin of these fibers was tested by performing unilateral stereotaxic lesions of the ophthalmic division of the trigeminal nerve, followed 2-25 days later by immunocytochemistry for CGRP and substance P. As reported previously, free nerve endings immunoreactive for both peptides were found transversing the nasal epithelium on the unlesioned side. Also on the unlesioned side, peptidergic axons, immunoreactive for both CGRP and substance P, could be traced from the olfactory nerve into the glomerular layer throughout the olfactory bulb, but especially into its rostral third. Ipsilateral to the trigeminal ganglion lesion, such peptide-immunoreactive fibers were absent or markedly reduced in the bulb, nerve, and epithelium. These results indicate that the peripheral branches of the ophthalmic branch of the trigeminal nerve enter the olfactory bulb along with the olfactory nerve and terminate in the glomerular layer along with the olfactory axons. Ultrastructural analysis of the CGRP-immunoreactive terminals in the glomeruli reveal vesicle-filled axonal processes terminating in the absence of obvious pre- or postsynaptic specializations. Whether the trigeminal fibers in the bulb are functional, e.g., convey information to the olfactory bulb via an axon reflex, or relay information from the olfactory bulb to the brainstem trigeminal nuclei is unclear.

Convergence of olfactory and nasotrigeminal inputs and possible trigeminal contributions to olfactory responses in the rat thalamus

To elucidate the role of trigeminal input on the olfactory system, field-evoked potentials were measured following electrical stimulation of the nasociliary branch of the trigeminal nerve in the olfactory-related structures in the rat brain. Significant potential changes were recorded in the mediodorsal nucleus of the thalamus and the lateral hypothalamic area. In the mediodorsal nucleus of the thalamus, the neurons responding to olfactory bulb electrical stimulation also responded to trigeminal nerve stimulation. Single neuronal responses of mediodorsal thalamic neurons following odorant stimulation were enhanced by blockade of the trigeminal nerve with procaine. These results suggest that olfactory and trigeminal pathways converge on the same neural elements within the mediodorsal nucleus of the thalamus and that the trigeminal input may modulate olfactory input in this nucleus.

The effect of neonatal capsaicin treatment on gustatory behavior in the albino rat

Small-diameter fibers present in gustatory peripheral nerves have historically been suspected of relaying information about the bitter quality of a taste stimulus. Neonatally injected capsaicin irreversibly destroys a proportion of unmyelinated C- and some A delta-fibers. Consummatory responses to increasing concentrations of quinine and other chemical solutions following neonatal capsaicin injection were compared to those of untreated and vehicle-injected control Sabra albino rats. Capsaicin-treated rats significantly increased their withdrawal thresholds to noxious, CO2 laser-generated heat pulses verifying treatment effectiveness. Furthermore, neonatal capsaicin treatment diminished sensitivity to pungent capsaicin solutions in mature rats. However, there were no group differences in quinine intake, suggesting that the full array of unmyelinated fibers associated with taste buds is not essential for the transmission of bitter taste. Capsaicin-treated animals showed a significant reduction in intake of normally highly preferred sodium chloride and sucrose concentrations. These results were probably not due to loss of peripheral unmyelinated afferent fibers per se, but rather to secondary central changes.

Immunohistochemical localization of neuron-specific enolase and calcitonin gene-related peptide in pig taste papillae

Immunoreactivity to neuron-specific enolase (NSE), a specific neuronal marker, and calcitonin gene-related peptide (CGRP) was localized in lingual taste papillae in the pigs. Sequential staining for NSE and CGRP by an elution technique allowed the identification of neuronal subpopulations. NSE-staining revealed a large neuronal network within the subepithelial layer of all taste papillae. NSE-positive fibers then penetrated the epithelium as isolated fibers, primarily in the foliate and circumvallate papillae, or as brush-shaped units formed by a multitude of fibers, especially in the fungiform papillae and in the apical epithelium of the circumvallate papilla. Taste buds of any type of taste papillae were found to express a dense subgemmal/intragemmal NSE-positive neuronal network. CGRP-positive nerve fibers were numerous in the subepithelial layer of all three types of taste papillae. In the foliate and circumvallate papillae, these fibers penetrated the epithelium to form extragemmal and intragemmal fibers, while in the fungiforms, they concentrated almost exclusively in the taste buds as intragemmal nerve fibers. Intragemmal NSE- and CGRP-positive fiber populations were not readily distinguishable by typical neural swellings as previously observed in the rat. The NSE-positive neuronal extragemmal brushes never expressed any CGRP-like immunoreactivity. Even more surprising, fungiform taste buds, whether richly innervated by or devoid of NSE-positive intragemmal fibers, always harboured numerous intragemmal CGRP-positive fibers. Consequently, NSE is not a general neuronal marker in porcine taste papillae. Our observations also suggest that subgemmal/intragemmal NSE-positive fibers are actively involved in synaptogenesis within taste buds. NSE-positive taste bud cells were found in all three types of taste papillae. CGRP-positive taste bud cells were never observed.

Immunohistochemical localization of neuron-specific enolase and calcitonin gene-related peptide in rat taste papillae

Calcitonin gene-related peptide-like and neuron-specific enolase-like immunoreactivity (CGRP-IR and NSE-IR) were surveyed immunohistochemically in the fungi-form, foliate and circumvallate papillae in rats. A dense CGRP-IR network (subgemmal and extragemmal) in the taste papillae is linked to the presence of taste buds, even though CGRP-IR fibers are rarely present in the taste buds. Three typical fiber populations were detected with these two markers. (a) A population of coarse NSE-IR intragemmal fibers characterized by thick neural swellings, never expressing CGRP-immunoreactivity. (b) A population of thin varicose intragemmal NSE/CGRP-IR fibers. (c) A population of subgemmal and extragemmal NSE-/CGRP-IR fibers that partly penetrated the epithelium. The common distribution of CGRP-IR and NSE-IR fibers at the base of taste buds, their differential distribution and morphology within taste buds, added to their restricted nature (gustatory or somatosensory) suggest that a population of CGRP-IR fibers undergoes a target-induced inhibition of its CGRP phenotype while entering the taste buds. The combined use of NSE and CGRP allowed a better characterization of nerve fibers within and between all three types of taste papillae. NSE was also a very good marker for a subtype of taste bud cells in the foliate and in the circumvallate papillae, but no such cells could be observed in the fungiform papillae.

The effects of neonatal capsaicin administration on trigeminal nerve chemoreceptors in the rat nasal cavity

Trigeminal nerve fibers in the nasal cavity respond to a variety of volatile chemical stimuli. Some of these trigeminal nerve fibers have been suggested to be capsaicin-sensitive and thus belong to a class of pain receptor rather than constituting a separate class of chemoreceptor. Our current results confirm this suggestion. Trigeminal nerve responses to volatile chemical stimuli were eliminated in rats which were injected with capsaicin on the second day of life. Animals whose nerves were unresponsive to chemical stimuli also exhibited a loss of intraepithelial peptide-immunoreactive fibers in their nasal cavities. The results of this study suggest that trigeminal nerve fibers in the nasal cavity which respond to chemical stimuli may be polymodal nociceptors which contain substance P, calcitonin gene-related peptide, or perhaps other neuropeptides.

Neonatal capsaicin treatment impairs functional properties of primary olfactory afferents in the rat

Long-lasting influences of neonatal capsaicin treatment on functional properties of the olfactory nerve were studied in 30-day-old rats by determining excitability thresholds of nerve fibres by means of orthodromic field potentials elicited in the main olfactory bulb, as well as by analyzing the pattern of the low-frequency component of the bulbar electroencephalogram. In addition, body, brain and olfactory bulbar weights were measured. Neonatal capsaicin resulted in reduced excitability of primary olfactory afferents and reduced wave amplitude of the bulbar electroencephalogram. Capsaicin treated animals had reduced body, brain and bulbar weights, the latter being the most affected. The result indicates that capsaicin given early in life leads to altered transmission properties of olfactory nerve fibres at later age, suggesting that olfactory afferents are sensitive to the neurotoxic action of capsaicin as occurs in other chemosensory afferent systems.

Ultrastructure of substance P- and CGRP-immunoreactive nerve fibers in the nasal epithelium of rodents

The respiratory and olfactory mucosae of rats and mice were examined at ultrastructural levels for the presence of intraepithelial nerve endings. Immunocytochemical studies utilizing antisera directed against substance P and calcitonin gene-related peptide (CGRP) revealed numerous intraepithelial peptide-immunoreactive fibers near the basal region of the epithelium. Occasional transepithelial fibers were observed to extend outward to nearly reach the epithelial surface. In no cases, however, did the transepithelial fibers reach the surface, but instead, stopped at the line of tight junctions approximately 1 micron from the surface. No specialized contacts between the nerve fibers and the epithelial cells were observed. The transepithelial fibers provide a possible anatomical substrate for the sensitivity of the trigeminal nerve to many air-borne chemical stimuli. That potential chemical stimuli must traverse the tight-junctional barrier may explain why lipid solubility is related to effectiveness for trigeminal stimuli.

Peptide-containing nerve fibers in the circumvallate papillae

The occurrence and distribution of an array of neuropeptides and dopamine-beta-hydroxylase in the circumvallate papillae of monkey, pig, cow, ferret, cat, rat and mouse was studied by immunocytochemistry. The animals were chosen to represent species with different diets. Substance P/neurokinin A- and calcitonin gene-related peptide-containing fibers were numerous in the circumvallate papillae of all animals examined, with the highest frequency in monkey, pig, cow, rat and mouse; in ferret and cat moderate numbers were detected. Vasoactive intestinal peptide/peptide histidine isoleucine amide-containing fibers were numerous in the circumvallate papillae of pig, while they were moderate in number in monkey, ferret and mouse. Neuropeptide Y-containing fibers were few to moderate in number in the circumvallate papillae of all species. Galanin-containing fibers were numerous in the pig circumvallate papillae, while only a few fibers could be detected in monkey, cow, cat, rat and mouse. Somatostatin-containing fibers were seen only in the cat circumvallate papillae, gastrin-releasing peptide-containing fibers in the cow and cat, cholecystokinin/gastrin-containing fibers in the pig and cow. Dopamine-beta-hydroxylase-containing fibers were detected in all animals studied. They were few to moderate in number in the circumvallate papillae. There was no obvious link between the peptidergic innervation pattern and the food habits.

Capsaicin sensitization and desensitization on the tongue produced by brief exposures to a low concentration

The intensity of sensations of burning and stinging produced by repeated exposures to capsaicin (at a nominal concentration of 3 ppm) was measured on a localized area of the tongue as a function of both the number of exposures and the time between them. It was discovered in the initial experiment that stimulation at the rate of 1/min (for up to 25 min) resulted in a monotonic increase in the intensity of burning sensations in a manner consistent with the phenomenon of sensitization. However, the insertion of a 15-min delay in stimulation resulted in a reduction in the intensity of the sensations produced by further stimulation, i.e., desensitization occurred. Desensitization was statistically significant even after exposure to as few as 5 stimuli prior to the delay. A subsequent experiment established that the minimum delay necessary to produce desensitization was between 2.5 and 5 min. Hence, paradoxically, the sensory response to capsaicin on the tongue increased as stimulation continued, then decreased after stimulation had ceased.

The effects of capsaicin on emotional responses to odors in the rat

Capsaicin is described as disturbing the autonomic responses to stress-inducing environments. The effects of capsaicin (130 mg/kg in 2 series of subcutaneous injections) on emotionality responses were studied in 19 Sprague-Dawley male rats using the open-field test. Eleven rats treated with isotonic saline served as controls. Emotionality (E) measured before capsaicin treatment in the open-field ventilated with deodorized air was similar in the 2 groups of rats. Nine out of the 19 treated rats survived. Their E was significantly higher than that of there 10 rats that died from capsaicin. When a frightening odor (fox feces) was added to the open-field E increased in the controls but remained unchanged in the capsaicin-surviving rats. The ability to discriminate palatable food or female odor was similar in the two groups. The results suggest that; 1) Highly emotional rats survived subcutaneous capsaicin injections; 2) Reaction to an emotionality-inducing environment was decreased in the capsaicin-surviving rats; 3) Olfactory discrimination was not impaired by capsaicin treatment.

Calcitonin-gene-related-peptide-immunoreactive innervation of the rat head with emphasis on specialized sensory structures

The distribution of calcitonin-gene-related peptide-like immunoreactivity (CGRP-IR) was studied in sections of decalcified rat head and selected whole-mount preparations in order to address the complex peptidergic innervation patterns in peripheral cephalic specialized zones and to examine neuronal ganglia in situ. Labeled neuron somata in trigeminal, glossopharyngeal, and vagal ganglia comprised a large proportion of small to medium size type B ganglion cells. Parasympathetic ganglia (ciliary, otic, sphenopalatine, submandibular) revealed a small population of labeled somata and numerous perisomatic IR axons, whereas sympathetic ganglion cells (superior cervical) were devoid of label though richly innervated by perisomatic IR axons. The gustatory geniculate ganglion contained only a few labeled neurons and axons. Coarse peripheral CGRP-IR axons were traced to skeletal muscle motor end plates (e.g., lingual, tensor tympani, etc.), and thin sensory axons most densely innervated the cornea, iris, general integument, all mucosal epithelia lining the tympanic, nasal, sinus and oropharyngeal cavities, and the cerebral meninges. Blood vessels, glands, ducts, and their orifices were often heavily innervated, and specific specializations and exceptions are discussed. Distinctive patterns of IR innervation characterized the various specialized sensory systems, including 1) cochlear and vestibular hair cells; 2) lingual, palatal, oropharyngeal, and laryngoepiglottal taste buds; 3) main olfactory epithelium and axons projecting to glomeruli in specific sectors of main olfactory bulb; 4) septal-olfactory organ; 5) vomeronasal organ; and 6) the nervus terminalis system. Secretory epithelia (ciliary body, choroid plexus, and stria vascularis) were notably lacking in CGRP-IR. Despite the multiplicity of functionally distinct CGRP neuronal and axonal populations, certain generalizations merit consideration. The extensive innervation of chemosensory nasal and oral epithelia may contribute to specific chemical sensitivities (e.g., relating to olfactory and gustatory senses) as well as evoking "nociceptive" responses to chemical irritants as part of a "common chemical sense." An efferent role for some of these peptidergic afferent axons may also be inferred from their specific distributions. Sites involved in regulating access to and sensitivity of sense organs to external stimuli (e.g., cochlear and vestibular hair cells, taste bud orifices, and main olfactory epithelium) are heavily innervated. Other IR axons are in position to exert control over airflow through nasal turbinates, glandular secretion, blood circulation, and duct transport systems.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of the discriminatory ability and sensitivity of the trigeminal and olfactory systems to chemical stimuli in the tiger salamander

Trigeminal receptors can respond to a wide variety of chemical stimuli, but it is unknown whether these receptors mediate discrimination between chemical stimuli matched for equal perceptual intensity. The present electrophysiological and behavioral experiments address this issue using tiger salamanders, Ambystoma tigrinum, and four compounds (amyl acetate, cyclohexanone, butanol, and d-limonene). In addition, the relative sensitivities of the trigeminal and olfactory systems to these compounds are compared. In electrophysiological cross-adaptation experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), there was complete cross adaptation such that only concentrations above the background (cross-adapting) stimulus concentration elicited responses, suggesting that chemical stimuli may stimulate trigeminal receptors nonspecifically. In behavioral experiments (amyl acetate vs cyclohexanone; butanol vs d-limonene), only animals with intact olfactory nerves could discriminate between perceptually equivalent concentrations, that is concentrations that elicited the same level of responding. Both electrophysiologically and behaviorally, the trigeminal system exhibited higher thresholds than the olfactory system. We conclude that trigeminal chemoreceptors, at least in salamanders, are unable to discriminate between these two pairs of compounds when matched for equal perceptual intensity, and that trigeminal chemoreceptors are less sensitive than olfactory receptors.

The organization of taste sensibilities in hamster chorda tympani nerve fibers

Electrophysiological measurements of nerve impulse frequencies were used to explore the organization of taste sensibilities in single fibers of the hamster chorda tympani nerve. Moderately intense taste solutions that are either very similar or easily discriminated were applied to the anterior lingual surface. 40 response profiles or 13 stimulus activation patterns were considered variables and examined with multivariate statistical techniques. Three kinds of response profiles were seen in fibers that varied in their overall sensitivity to taste solutions. One profile (S) showed selectivity for sweeteners, a second (N) showed selectivity for sodium salts, and a third (H) showed sensitivity to salts, acids, and other compounds. Hierarchical cluster analysis indicated that profiles fell into discrete classes. Responses to many pairs of effective stimuli were covariant across profiles within a class, but some acidic stimuli had more idiosyncratic effects. Factor analysis of profiles identified two common factors, accounting for 77% of the variance. A unipolar factor was identified with the N profile, and a bipolar factor was identified with the S profile and its opposite, the H profile. Three stimulus activation patterns were elicited by taste solutions that varied in intensity of effect. Hierarchical cluster analysis indicated that the patterns fell into discrete classes. Factor analysis of patterns identified three common unipolar factors accounting for 82% of the variance. Eight stimuli (MgSO4, NH4Cl, KCl, citric acid, acetic acid, urea, quinine HCl, HCl) selectively activated fibers with H profiles, three stimuli (fructose, Na saccharin, sucrose) selectively activated fibers with S profiles, and two stimuli (NaNO3, NaCl) activated fibers with N profiles more strongly than fibers with H profiles. Stimuli that evoke different patterns taste distinct to hamsters. Stimuli that evoke the same pattern taste more similar. It was concluded that the hundreds of peripheral taste neurons that innervate the anterior tongue play one of three functional roles, providing information about one of three features that are shared by different chemical solutions.

Olfactory receptor cell function is affected by trigeminal nerve activity

In the frog, antidromic electrical stimulation of the ophthalmic branch of the trigeminal nerve (NV-ob) evokes a slow potential in the olfactory mucosa, modifies the activity of receptor cells and modulates the responses to odour. Substance P (SP) application evokes similar electrical responses. These results imply that the functioning of the olfactory system might be controlled at the receptor cell level. It is suggested that the trigeminal system could modulate the activity of the olfactory receptor cells via a local axon reflex which may result in the release of SP.