Multiple routes of chemosensitivity to free fatty acids in humans

Selected free fatty acids (FFAs) are documented effective somatosensory and olfactory stimuli whereas gustatory effects are less well established. This study examined orthonasal olfactory, retronasal olfactory, nasal irritancy, oral irritancy, gustatory, and multimodal threshold sensitivity to linoleic, oleic, and stearic acids. Sensitivity to oxidized linoleic acid was also determined. Detection thresholds were obtained using a three-alternative, forced-choice, ascending concentration presentation procedure. Participants included 22 healthy, physically fit adults sensitive to 6-n-propylthiouracil. Measurable thresholds were obtained for all FFAs tested and in 96% of the trials. Ceiling effects were observed in the remaining trials. Greater sensitivity was observed for multimodal stimulation and lower sensitivity for retronasal stimulation. There were no statistically significant correlations for linoleic acid thresholds between different modalities, suggesting that each route of stimulation contributes independently to fat perception. In summary, 18-carbon FFAs of varying saturation are detected by multiple sensory systems in humans.     

Orthonasal and Retronasal Odorant Identification Based upon Vapor Phase Input from Common Substances

Subjects were trained to identify by assigned number commonsubstances presented as vapor phase stimuli via an orthonasalor a retronasal route. Following training, odorant identificationlearning was evaluated by measuring ability to correctly identifyto a criterion. Those who met the criterion were then testedfirst with the stimuli presented to the nares that differedin location from the nares used in training, and second to thenares that corresponded in location to the nares used in training.It was found that, under conditions of natural retronasal breathing,orthonasally trained subjects made correct identifications on{small tilde}80% of the trials upon retronasal testing, butfor the following orthonasal testing identifications were significantlymore frequent, approaching 100% correct. After subsequent retronasaltraining, the same subjects' orthonasal identifications remainedsignificantly higher, although identifications improved to {smalltilde}92% correct on retronasal trials. Other subjects wereinstructed in a breathing technique designed to enhance retronasalstimulation. After orthonasal training, retronasal testing ofthese subjects still gave significantly fewer correct identificationsthan orthonasal testing, notwithstanding the modified retronasalbreathing, but after subsequent retronasal training correctidentifications by these subjects no longer differed significantlybetween orthonasal and retronasal testing. Efficacy of modifiedretronasal breathing was confirmed in two subsequent experiments.The observed substantial positive transfers between retronasaland orthonasal odorant identification training and testing locidemonstrate that these odorant pathways do not subserve completelyindependent olfactory systems, while the less accurate identificationsvia the retronasal route, unless instruction in retronasal breathingwas given, suggest a difference in the efficiency with whichodorants are normally delivered to the olfactory mucosa. Chem.Senses 21: 529–543, 1996.     

Real-time monitoring of nasal mucosal pH during carbon dioxide stimulation: implications for stimulus dynamics

Carbon dioxide is a commonly employed irritant test compound in nasal chemesthetic studies because it is essentially free of olfactory stimulus properties. CO(2) is thought to act via hydration to H(2)CO(3) and dissociation to H(+) in nasal mucus, with resulting activation of acid sensors. However, transient changes in nasal mucosal pH have not been documented during CO(2) stimulation in humans. We placed a small pH probe on the floor of the right anterior nasal cavity during CO(2) stimulation in eight human subjects with historically high (>30%) and low (< or =20%) CO(2) detection thresholds. Three second pulses of CO(2) (15-45% v/v) paired with air in random order (12-15 s inter-stimulus interval; 60 s inter-trial interval) were administered by nasal cannula at 5 l/min. in an ascending series. For each subject, both a CO(2) detection threshold and suprathreshold psychophysical ratings [psi; labeled magnitude scale] were generated. All subjects showed phasic drops in pH associated with CO(2) stimulation (DeltapH). For all subjects combined, a positive correlation was apparent between applied [CO(2)] and both DeltapH and psi, as well as between DeltapH and psi themselves (P < 0.0001 for each comparison). Subjects with historically low CO(2) thresholds showed steeper dose-response curves for psi as a function of both applied [CO(2)] and DeltapH, but not for DeltapH as a function of applied [CO(2)]. For the six of eight subjects with measurable pH changes at threshold, DeltapH was positively related to log [CO(2) threshold] (P < 0.01). These data imply that variability in CO(2) detection thresholds and suprathreshold rating may derive from intrinsic differences in neural sensitivity, rather than differences in stimulus activation to hydrogen ion.     

Influence of genioglossus tonic activity on upper airway dynamics assessed by phrenic nerve stimulation.

Upper airway (UA) dynamics can be evaluated during wakefulness by using electrical phrenic nerve stimulation (EPNS) applied at end-expiration during exclusive nasal breathing by dissociating twitch flow and phasic activation of UA muscles. This technique can be used to quantify the influence of nonphasic electromyographic (EMG) activity on UA dynamics. UA dynamics was characterized by using EPNS when increasing tonic EMG activity with CO(2) stimulation in six normal awake subjects. Instantaneous flow, esophageal and nasopharyngeal pressures, and genioglossal EMG activity were recorded during EPNS at baseline and during CO(2) ventilatory stimulation. The proportion of twitches presenting an inspiratory-flow limitation pattern decreased from 100% at baseline to 78.7 +/- 21.4% (P = 10(-4)) during CO(2) rebreathing. During CO(2) stimuli, maximal inspiratory twitch flow (VI(max)) of flow-limited twitches significantly rose, with the driving pressure at which flow limitation occurred being more negative. For the group as a whole, the increase in VI(max) and the decrease in pressure were significantly correlated with the rise in end-expiratory EMG activity. UA stability assessed by EPNS is dramatically modified during CO(2) ventilatory stimulation. Changes in tonic genioglossus EMG activity significantly contribute to the improvement in UA stability.     

Effect of upper airway carbon dioxide on ventilation and blood gases in the awake pony

Carbon dioxide concentrations were increased during expiration in the upper one-half of the trachea, pharynx, and nasal sinuses to determine if elevation of upper airway CO2 would alter breathing or arterial blood gases in the awake pony. Carbon dioxide (100%) was injected into the midcervical trachea via a chronically implanted transcutaneous cannula during the first part of the animal's expiration. This maneuver elevated upper airway expiratory CO2 concentrations but prevented any exogenous CO2 from entering the lung and being absorbed into the arterial blood. Twelve experiments were performed on six ponies in which upper airway CO2 was elevated 2, 4, and 6% above the normal expired CO2 concentrations. Tidal volume increased in a dose dependent manner during upper airway CO2 exposure, but total ventilation was unchanged from base-line measurements made while the animal breathed room air. Arterial Po2 also increased during upper airway CO2 administration, reaching a mean value 6 Torr (1 Torr = 133.322 Pa) greater than the base-line values at the +6% CO2 exposure. We conclude that upper airway CO2 exposure alters breathing pattern slightly (increases tidal volume) and increases arterial PO2 in the awake pony.     

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.     

Retronasal perception of odors

Odors often produce different sensations when presented in front of the nose or intraorally, when eaten. It is a long-standing question whether these differences in sensations are due, for example, to the additional mechanical sensations elicited by the food in the mouth or additional odor release during mastication. To study this phenomenon in detail, a stimulation technique has been developed that allows controlled ortho- or retronasal presentation of odorous stimuli. Results from psychophysical, electrophysiological, and imaging studies suggest that there are clear differences in the perception of ortho- and retronasal stimuli. This 'duality of the sense of smell' is also observed in a clinical context where some patients exhibit good retronasal olfactory function with little or no orthonasal function left, and vice versa. The differences between ortho- and retronasal perception of odors are thought to be, at least partly, due to absorption of odors to the olfactory epithelium, which appears to differ in relation to the direction of the airflow across the olfactory epithelium.     

Identification of air phase retronasal and orthonasal odorant pairs

Identifications (IDs) of paired retronasal and orthonasal odorants were studied, with stimuli limited to air phase. Odorants were liquid extracts of plant materials, sold as food flavorings, matched by each subject both for retronasal-only and orthonasal-only air phase intensities and then learned to 100% correct veridical name retronasal-only and orthonasal-only IDs. Subjects were tested for ID of (a) retronasal-only and orthonasal-only odorants, (b) homogeneously paired odorant (the same odorant in retronasal and orthonasal locations), and (c) heterogeneously paired odorants (different odorants in retronasal and orthonasal locations). Paired odorants were presented in two different sequences: retronasal location odorant smelled first or orthonasal location odorant smelled first. IDs were reported after odorants were removed. Results were as follows: (a) no significant differences between correct ID of odorants when in retronasal-only versus orthonasal-only locations, although percent correct IDs were lower for half the retronasal-only location odorants; (b) correct ID of a homogeneously paired odorant equaled or exceeded its unpaired ID, with two successive, identical IDs reported on the majority of its trials; (c) with heterogeneous pairs, for all odorants when in the orthonasal location of a pair, correct ID occurred less often than when these odorants were presented orthonasal-only, but for odorants in the retronasal location, correct ID equaled or exceeded retronasal-only correct ID; and (d) perceived order of presentation of heterogeneous pairs was the reverse of the physically presented sequence for both retronasal-first and orthonasal-first conditions. The heterogeneous odorant ID outcome supports the concept that processing of retronasal and orthonasal odorants differ, and the perceived reversal of the presented sequence is in agreement with the importance of recency in odorant memory.     

Temporal synchrony and integration of sub-threshold taste and smell signals

The importance of stimulus timing and location on the perceptual integration of taste and odour was studied based on a sub-threshold methodology. From a panel of 16 people, 12 showed the integration effect previously reported while 4 showed no effect. The experiment was repeated using retronasal and orthonasal delivery of the odour and with tastant present or absent in the mouth. Integration of taste and odour only occurred when both stimuli were present at the same time. Retro- or orthonasal presentation both produced integration providing that tastant delivery was synchronous but the threshold values for the two presentation methods were different. The relevance of these findings to flavour perception under realistic conditions is considered.     

Modification of Electrical Pain Threshold by Voluntary Breathing-Controlled Electrical Stimulation (BreEStim) in Healthy Subjects

Background

Pain has a distinct sensory and affective (i.e., unpleasantness) component. BreEStim, during which electrical stimulation is delivered during voluntary breathing, has been shown to selectively reduce the affective component of post-amputation phantom pain. The objective was to examine whether BreEStim increases pain threshold such that subjects could have improved tolerance of sensation of painful stimuli.

Methods

Eleven pain-free healthy subjects (7 males, 4 females) participated in the study. All subjects received BreEStim (100 stimuli) and conventional electrical stimulation (EStim, 100 stimuli) to two acupuncture points (Neiguan and Weiguan) of the dominant hand in a random order. The two different treatments were provided at least three days apart. Painful, but tolerable electrical stimuli were delivered randomly during EStim, but were triggered by effortful inhalation during BreEStim. Measurements of tactile sensation threshold, electrical sensation and electrical pain thresholds, thermal (cold sensation, warm sensation, cold pain and heat pain) thresholds were recorded from the thenar eminence of both hands. These measurements were taken pre-intervention and 10−min post-intervention.

Results

There was no difference in the pre-intervention baseline measurement of all thresholds between BreEStim and EStim. The electrical pain threshold significantly increased after BreEStim (27.5±6.7% for the dominant hand and 28.5±10.8% for the non-dominant hand, respectively). The electrical pain threshold significantly decreased after EStim (9.1±2.8% for the dominant hand and 10.2±4.6% for the non–dominant hand, respectively) (F_{[1, 10]} = 30.992, p = .00024). There was no statistically significant change in other thresholds after BreEStim and EStim. The intensity of electrical stimuli was progressively increased, but no difference was found between BreEStim and EStim.

Conclusion

Voluntary breathing controlled electrical stimulation selectively increases electrical pain threshold, while conventional electrical stimulation selectively decreases electrical pain threshold. This may translate into improved pain control.     

Topical inhibition of nasal carbonic anhydrase affects the CO2 detection threshold in rats

Previous studies indicate that Long-Evans rats can be operantly trained to discriminate inspired CO(2) concentrations as low as 0.5%. This ability has been proposed to be due to the presence of CO(2)-sensitive olfactory receptors that contain the enzyme carbonic anhydrase (CA). The objectives of the present study were as follows: 1) to determine whether Zucker rats could be operantly conditioned to discriminate low concentrations of CO(2) from control air and 2) to determine the rats' CO(2) detection thresholds before and after nasal perfusion of mammalian Ringers or methazolamide, a CA inhibitor. Rats were operantly trained to discriminate between 25% CO(2) and control air (0% CO(2)) and were then subjected to various CO(2) concentrations (0.5-12.5%) to determine their CO(2) detection thresholds. The average (+/-standard error of mean) baseline CO(2) detection threshold of 7 Zucker rats was 0.48 +/- 0.07% CO(2), whereas the average CO(2) detection thresholds after nasal perfusion of either mammalian Ringers or 10(-2) M methazolamide were 1.41 +/- 0.30% and 5.92 +/- 0.70% CO(2), respectively. The average CO(2) detection threshold after methazolamide was significantly greater (P<0.0001) than the baseline detection threshold. These findings demonstrate that like Long-Evans rats, Zucker rats can be trained to discriminate low concentrations of CO(2) and that inhibition of nasal CA reduces the ability of the rats to detect low concentrations (3.5% and below) but not higher concentrations of CO(2) (12.5%). These results add to the growing evidence that olfactory neurons exhibiting CA activity are CO(2) chemoreceptors sensitive to physiological concentrations of CO(2).     

Control of extrathoracic airway dynamics

The ventilatory controlling factors associated with oral augmentation of nasal breathing were investigated in 25 (14 women, 11 men) healthy adults during an incrementally graded bicycle exercise test. Ventilatory variables were measured by separate oral and nasal pneumotachometers integrated with a valveless oral-nasal face mask and a flexible oral catheter. Inspired and expired breath length, nasal flows, nasal ventilation, transnasal pressures, nasal work of breathing, nasal powers, and nasal resistances were measured simultaneously on a breath-by-breath basis and averaged over the 30-s interval before oral augmentation. Subjects participated in a minimum of two separate tests, with statistical analysis focusing on the correlation obtained for nasal work of breathing (r = 0.870), nasal average power (r = 0.838), and average transnasal pressure (r = 0.819) during inspiration and for average nasal power (r = 0.801) during expiration indicates that these variables were the most reliable predictors of the oral augmentation of nasal breathing.     

Mechanical obstruction of the olfactory cleft reveals differences between orthonasal and retronasal olfactory functions

Following up on recent observations in patients with nasal polyposis (NP), the present study aimed to investigate whether a mechanical obstruction of the anterior olfactory cleft (OC) would produce differential effects on orthonasal and retronasal olfactory functions. To this end, we studied 33 healthy subjects in a randomized trial. Sponges with high content of saline were either placed in the OC or on the respiratory epithelium, such that this was blinded to both subject and observer. The results indicated that orthonasal (P = 0.04) but not retronasal (P = 0.15) olfactory identification ability was lower when the OC was blocked. This confirms the idea that differences between orthonasal and retronasal olfactory functions, as observed in NP patients, are, at least to some degree, due to mechanical obstruction of the anterior portion of the OC. The present data also suggest that mechanical obstruction is a means to induce reversible hyposmia void of side effects which can be performed in a blinded fashion. This might become a valuable model of hyposmia for future investigations.     

Differences in olfactory bulb mitral cell spiking with ortho- and retronasal stimulation revealed by data-driven models

The majority of olfaction studies focus on orthonasal stimulation where odors enter via the front nasal cavity, while retronasal olfaction, where odors enter the rear of the nasal cavity during feeding, is understudied. The coding of retronasal odors via coordinated spiking of neurons in the olfactory bulb (OB) is largely unknown despite evidence that higher level processing is different than orthonasal. To this end, we use multi-electrode array in vivo recordings of rat OB mitral cells (MC) in response to a food odor with both modes of stimulation, and find significant differences in evoked firing rates and spike count covariances (i.e., noise correlations). Differences in spiking activity often have implications for sensory coding, thus we develop a single-compartment biophysical OB model that is able to reproduce key properties of important OB cell types. Prior experiments in olfactory receptor neurons (ORN) showed retro stimulation yields slower and spatially smaller ORN inputs than with ortho, yet whether this is consequential for OB activity remains unknown. Indeed with these specifications for ORN inputs, our OB model captures the salient trends in our OB data. We also analyze how first and second order ORN input statistics dynamically transfer to MC spiking statistics with a phenomenological linear-nonlinear filter model, and find that retro inputs result in larger linear filters than ortho inputs. Finally, our models show that the temporal profile of ORN is crucial for capturing our data and is thus a distinguishing feature between ortho and retro stimulation, even at the OB. Using data-driven modeling, we detail how ORN inputs result in differences in OB dynamics and MC spiking statistics. These differences may ultimately shape how ortho and retro odors are coded.     

Color enhances orthonasal olfactory intensity and reduces retronasal olfactory intensity

The effect of color on orthonasal and retronasal odor intensities was investigated. When odorants were smelled orthonasally (i.e., through the nostrils), color enhanced odor intensity ratings, consistent with previous reports. However, when odorants were smelled retronasally (i.e., the odorous solution was put in the mouth), color reduced odor intensity ratings. These different effects of color on odor intensity (i.e., enhancement orthonasally and suppression retronasally) appear to be the result of route of olfactory stimulation rather than of any procedural artifact. This supports previous reports that retronasal and orthonasal odors are perceived differently.     

Measurement of sensitivity to olfactory flavor: application in a study of aging and dentures

Olfaction involves a dual sensory process for perceiving odors orthonasally (through the nostrils) and retronasally (through the mouth). This investigation entailed developing a measure of sensitivity to an odor delivered in an orally sampled food (orange flavoring in a sucrose-sweetened gelatin) and examining sensitivity in the elderly. In experiment 1, olfactory flavor sensitivity was 49 times lower in elderly (n = 21) than in young (n = 28) subjects. In experiment 2, with 73 elderly women, higher olfactory flavor sensitivity correlated significantly with higher orthonasal perception (Connecticut Chemosensory Clinical Research Center test). Some women, however, exhibited low olfactory flavor sensitivity despite high orthonasal perception; none had high olfactory flavor sensitivity and low orthonasal perception. Those who wore complete or palatal covering dentures had lower olfactory flavor sensitivity than those who were dentate or wore dentures that did not cover the palate. Through multiple regression analysis, orthonasal perception and denture status were found to be independent contributors to predicting olfactory flavor sensitivity. In summary, elderly subjects showed depressed olfactory flavor sensitivity (i.e. retronasal sensitivity) that related to poor orthonasal olfactory perception and denture characteristic. Thus, while good orthonasal olfaction may be necessary for good olfactory flavor sensitivity, it is not sufficient. Other factors, some associated with oral conditions, may impede release and retronasal transport of odors from the mouth to the olfactory receptors.     

Maximum airflow through the nose in humans

Inspiratory and expiratory flow via the nose and via the mouth during maximum-effort vital capacity (VC) maneuvers have been compared in 10 healthy subjects. Under baseline conditions maximum flow via the nose was lower than that via the mouth in the upper 50-60% of the VC on expiration and throughout the VC on inspiration. The mean ratio of maximum inspiratory to maximum expiratory flow at mid-VC was 1.38 during mouth breathing and 0.62 during nasal breathing. Inspiratory flow limitation with no increase in flow through the nose as driving pressure was increased above a critical value (usually between 12 and 30 cmH2O) was found in all six subjects studied. Stenting the alae nasi in seven subjects increased peak flow via the nose from a mean of 3.49 to 4.32 l/s on inspiration and from 4.83 to 5.61 l/s on expiration. Topical application of an alpha-adrenergic agonist in seven subjects increased mean peak nasal flow on inspiration from 3.25 to 3.89 l/s and on expiration from 5.03 to 7.09 l/s. Further increases in peak flow occurred with subsequent alan stenting. With the combination of stenting and topical mucosal vasoconstriction, nasal peak flow on expiration reached 81% and, on inspiration, 79% of corresponding peak flows via the mouth. The results demonstrate that narrowing of the alar vestibule and the state of the mucosal vasculature both influence maximum flow through the nose; under optimal conditions, nasal flow capacity is close to that via the mouth.     

Mechanisms of control of alae nasi muscle activity.

Human upper airway dilator muscles are clearly influenced by chemical stimuli such as hypoxia and hypercapnia. Whether in humans there are upper airway receptors capable of modifying the activity of such muscles is unclear. We studied alae nasi electromyography (EMG) in normal men in an attempt to determine 1) whether increasing negative intraluminal pressure influences the activity of the alae nasi muscle, 2) whether nasal airway feedback mechanisms modify the activity of this muscle, and 3) if so, whether these receptor mechanisms are responding to mucosal temperature/pressure changes or to airway deformation. Alae nasi EMG was recorded in 10 normal men under the following conditions: 1) nasal breathing (all potential nasal receptors exposed), 2) oral breathing (nasal receptors not exposed), 3) nasal breathing with splints (airway deformation prevented), and 4) nasal breathing after nasal anesthesia (mucosal receptors anesthetized). In addition, in a separate group, the combined effects of anesthesia and nasal splints were assessed. Under each condition, EMG activity was monitored during basal breathing, progressive hypercapnia, and inspiratory resistive loading. Under all four conditions, both load and hypercapnia produced a significant increase in alae nasi EMG, with hypercapnia producing a similar increment in EMG regardless of nasal receptor exposure. On the other hand, loading produced greater increments in EMG during nasal than during oral breathing, with combined anesthesia plus splinting producing a load response similar to that observed during oral respiration. These observations suggest that nasal airway receptors have little effect on the alae nasi response to hypercapnia but appear to mediate the alae nasi response to loading or negative airway pressure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Do lizards breathe through their mouths while running?

We measured the partitioning of airflow between nasal and oral circuits in five species of lizards before, during and after exercise. Expired gases were measured separately from the mouth and nose circuits in order to estimate the relative contribution of each circuit to ventilatory airflow. Nasal breathing dominates before exercise; however, during exercise the breathing pattern switched to oronasal expiration. Airflow averaged 30% oral expiration across all species during and after exercise. These results have important implications for the design of appropriate masks for respirometry in lizards. In order to ensure that all gases are captured, it is critically important that both the nose and mouth circuits are sampled.     

Time-dependent responses of expiration reflex in cats

We investigated the effectiveness of the "expiration reflex" in 10 anesthetized spontaneously breathing cats. The expiration reflex was produced by mechanical stimulation of the vocal folds and electrical stimulation of the superior laryngeal nerve at different moments in the respiratory cycle and at various levels of respiratory chemical drive. The effectiveness of the expiration reflex was evaluated from sudden changes in expiratory flow immediately following the stimulation. Both mechanical and electrical stimulations given during early inspiration caused little or no expiratory efforts, whereas stimulations given during early expiration or hypocapnic apnea produced a typical expiration reflex. Changes in arterial CO2 and O2 partial pressures influenced neither the relationships between the stimulation and its effect on the expiration reflex nor the strength of the expiration reflex. These results indicate that the timing of stimulation with relation to the phase of the respiratory cycle is critical to its effect on the expiration reflex and that changes in respiratory chemical drive do not modify the expiration reflex characteristics.