Lack of quinine-evoked activity in rat trigeminal subnucleus caudalis

Bitter-Induced Salivary Proteins Increase Detection Threshold of Quinine, But Not Sucrose

Exposures to dietary tannic acid (TA, 3%) and quinine (0.375%) upregulate partially overlapping sets of salivary proteins which are concurrent with changes in taste-driven behaviors, such as rate of feeding and brief access licking to quinine. In addition, the presence of salivary proteins reduces chorda tympani responding to quinine. Together these data suggest that salivary proteins play a role in bitter taste. We hypothesized that salivary proteins altered orosensory feedback to bitter by decreasing sensitivity to the stimulus. To that end, we used diet exposure to alter salivary proteins, then assessed an animal's ability to detect quinine, using a 2-response operant task. Rats were asked to discriminate descending concentrations of quinine from water in a modified forced-choice paradigm, before and after exposure to diets that alter salivary protein expression in a similar way (0.375% quinine or 3% TA), or 1 of 2 control diets. Control animals received either a bitter diet that does not upregulate salivary proteins (4% sucrose octaacetate), or a nonbitter diet. The rats exposed to salivary protein-inducing diets significantly decreased their performance (had higher detection thresholds) after diet exposure, whereas rats in the control conditions did not alter performance after diet exposure. A fifth group of animals were trained to detect sucrose before and after they were maintained on the 3% TA diet. There was no significant difference in performance, suggesting that these shifts in threshold are stimulus specific rather than task specific. Taken together, these results suggest that salivary proteins reduce sensitivity to quinine.

Altering salivary protein profile can increase acceptance of a novel bitter diet

Bitter taste is often associated with toxins, but accepting some bitter foods, such as green vegetables, can be an important part of maintaining a healthy diet. In rats and humans, repeated exposure to a bitter stimulus increases acceptance. Repeated exposure allows an individual the opportunity to learn about the food's orosensory and postingestive effects. It also alters the salivary protein (SP) profile, which in turn alters taste signaling. We have hypothesized that altering the salivary proteome plays a role in the increased acceptance after repeated exposure. Here we test this and attempt to disentangle the contribution of learning during dietary exposure from the contribution of SPs in increased acceptance of bitter diet. Dietary exposure to quinine or tannic acid and injection of isoproterenol (IPR) result in similar salivary protein profiles. Here we used either the bitter stimulus tannic acid or IPR injection to upregulate a subset of SPs before exposing animals to a novel diet containing quinine (0.375%). Control animals received either a control diet before being exposed to quinine, or a diet containing sucrose octaacetate, a compound that the animals avoid but does not alter SP profiles. The treatments that alter SP expression increased rate of feeding on the quinine diet compared to the control treatments. Additionally, tannic acid exposure altered intake and meal size of the quinine diet. These data suggest that SPs, not just learning about bitter food, increase acceptance of the bitter diet.

Mouse Parabrachial Neurons Signal a Relationship between Bitter Taste and Nociceptive Stimuli

Taste and somatosensation both mediate protective behaviors. Bitter taste guides avoidance of ingestion of toxins while pain sensations, such as noxious heat, signal adverse conditions to ward off harm. Although brain pathways for taste and somatosensation are typically studied independently, prior data suggest that they intersect, potentially reflecting their common protective role. To investigate this, we applied electrophysiologic and optogenetic techniques in anesthetized mice of both sexes to evaluate relationships between oral somatosensory and taste activity in the parabrachial nucleus (PbN), implicated for roles in gustation and pain. Spikes were recorded from taste-active PbN neurons tested with oral delivery of thermal and chemesthetic stimuli, including agonists of nocisensitive transient receptor potential (TRP) ion channels on somatosensory fibers. Gustatory neurons were also tested to follow electrical pulse stimulation of an oral somatosensory region of the spinal trigeminal subnucleus caudalis (Vc), which projects to the PbN. Neurons composed classic taste groups, including sodium, electrolyte, appetitive, or bitter cells. Across groups, most neurons spiked to Vc pulse stimulation, implying that trigeminal projections reach PbN gustatory neurons. Among such cells, a subpopulation responsive to the bitter taste stimuli quinine and cycloheximide, and aversive concentrations of sodium, cofired to agonists of nocisensitive TRP channels, including capsaicin, mustard oil, and noxious heat. Such neurons populated the lateral PbN. Further, nociceptive activity in PbN bitter taste neurons was suppressed during optogenetic-assisted inhibition of the Vc, implying convergent trigeminal input contributed to such activity. Our results reveal a novel role for PbN gustatory cells in cross-system signaling related to protection.SIGNIFICANCE STATEMENT Prior data suggest that gustatory and trigeminal neural pathways intersect and overlap in the parabrachial area. However, no study has directly examined such overlap and why it may exist. Here we found that parabrachial gustatory neurons can receive afferent projections from trigeminal nuclei and fire to oral nociceptive stimuli that excite somatosensory receptors and fibers. Activation to aversive nociceptive stimuli in gustatory cells was associated with responding to behaviorally avoided bitter tastants. We were further able to show that silencing trigeminal projections inhibited nociceptive activity in parabrachial bitter taste neurons. Our results imply that in the parabrachial area, there is predictable overlap between taste and somatosensory processing related to protective coding and that classically defined taste neurons contribute to this process.

Salivary proteins alter taste-guided behaviors and taste nerve signaling in rat

Taste stimuli are normally dissolved in saliva prior to interacting with their respective receptor targets. There are hundreds of proteins in saliva, and it has been hypothesized that these proteins could interact with either taste stimuli or taste receptors to alter taste signaling and diet acceptance. However, the impact of these proteins on feeding has been relatively unexplored using rodent models. We have developed a novel technique for saliva collection that allows us to link salivary protein expression with feeding behavior. First, we monitored the microstructure of rats' feeding patterns on a 0.375% quinine diet (Q-diet) while tracking changes in salivary protein expression. We found 5 protein bands were upregulated by diet exposure to Q-diet and upregulation of a subset of these bands were statistically related to increased diet acceptance, including changes in behavioral measures that are thought to represent both orosensory and postingestive signaling. In a second experiment, we measured the licking to a range of quinine solutions (0.01-1.0mM) before and after the animals were exposed to a tannic acid diet that altered salivary protein expression. Rats found the quinine solutions less aversive after salivary protein altering diets. In a third experiment we recorded the response of the chorda tympani (CT) nerve while delivering quinine solutions (0.3-30mM) to the front of the tongue dissolved in either "donor saliva" containing salivary proteins or donor saliva which has had the salivary proteins removed. Donor saliva was collected from a separate group of animals using isoproterenol and pilocarpine. The samples containing salivary proteins resulted in lower nerve responses than those without salivary proteins. Together these data suggest that salivary proteins are capable of altering taste-guided behaviors and taste nerve signaling.

Separate functions for responses to oral temperature in thermo-gustatory and trigeminal neurons

Oral temperature is a component and modifier of taste perception. Both trigeminal (V) and taste-sensitive cells, including those in the nucleus of the solitary tract (NTS), can respond to oral temperature. However, functional associations in thermal sensitivity between V and gustatory neurons are poorly understood. To study this we recorded electrophysiological responses to oral stimulation with cool (9, 15, 25, 32, and 34 °C) and warm (40 and 45 °C) temperatures from medullary V (n = 45) and taste-sensitive NTS (n = 27) neurons in anesthetized mice. Results showed temperatures below 34 °C activated the majority of V neurons but only a minority of NTS units. V neurons displayed larger responses to cooling and responded to temperatures that poorly stimulated NTS cells. Multivariate analyses revealed different temperatures induced larger differences in responses across V compared with NTS neurons, indicating V pathways possess greater capacity to signal temperature. Conversely, responses to temperature in NTS units associated with gustatory tuning. Further analyses identified two types of cooling-sensitive V neurons oriented toward innocuous or noxious cooling. Multivariate analyses indicated the combined response of these cells afforded distinction among a broad range of cool temperatures, suggesting multiple types of V neurons work together to represent oral cooling.

Effects of gustatory nerve transection and/or ovariectomy on oral capsaicin avoidance in rats

The incidence of chronic oral pain such as burning mouth syndrome is greater in peri-menopausal females, and was postulated to be associated with gustatory nerve damage. We investigated whether bilateral transection of the chorda tympani, with or without accompanying ovariectomy, affected oral capsaicin avoidance in rats. Female rats had restricted access to 2 bottles, 1 bottle containing capsaicin (concentration range: 0.33-33 μM/L) and the other vehicle. Percent volume of capsaicin consumption and lick counts were measured. The concentration series was tested before and 0.5, 3, 6, 9, and 12 months after the following surgical procedures: (a) bilateral transection of the chorda tympani (CTx); (b) ovariectomy (OVx); (3) CTx plus OVx; or (4) sham CT surgery. Before surgery there was a concentration-dependent decrease in licks and volume of capsaicin consumed, with a threshold between 0.1 and 0.3 ppm. The majority of drink licks occurred during the first 9 minutes of access. Over the 12-month test period, the CTx group did not exhibit reduced capsaicin consumption, and consumed significantly more capsaicin at 6 and 9 months postsurgery. Rats in the OVx group consistently consumed significantly less capsaicin and exhibited significantly higher counts of capsaicin-evoked Fos-like immunoreactivity in the dorsomedial trigeminal subnucleus caudalis (Vc) compared to all other treatment groups. That CTx, with or without OVx, did not enhance capsaicin avoidance indicates that damage to the gustatory system does not disinhibit trigeminal nociceptive transmission.

Reduced oral ethanol avoidance in mice lacking transient receptor potential channel vanilloid receptor 1

Ethanol is a known oral trigeminal stimulant and recent data indicate that these effects are mediated in part by transient receptor potential channel vanilloid receptor 1 (TRPV1). The importance of this receptor in orally mediated ethanol avoidance is presently unknown. Here, we compared orosensory responding to ethanol in TRPV1-deficient and wild type mice in a brief-access paradigm that assesses orosensory influences by measuring immediate licking responses to small stimulus volumes. TRPV1(-/-) and control mice were tested with six concentrations of ethanol (3, 5, 10, 15, 25, 40%), capsaicin (0.003, 0.01, 0.03, 0.1, 0.3, 1 mM), sucrose (0.003, 0.01, 0.03, 0.1, 0.3, 1 M), and quinine (0.01, 0.03, 0.1, 0.3, 1, 3 mM) and psychophysical concentration-response functions were generated for each genotype and stimulus. TRPV1 knockouts displayed reduced oral avoidance responses to ethanol regardless of concentration, insensitivity to capsaicin, and little to no difference in sweet or bitter taste responding relative to wild type mice. These data indicate that the TRPV1 channel plays a role in orosensory-mediated ethanol avoidance, but that other receptor mechanisms likely also contribute to aversive oral responses to alcohol.

The Psychophysical Relationship between Bitter Taste and Burning Sensation: Evidence of Qualitative Similarity

Although it has long been studied as a pure sensory irritant, the ability of capsaicin to evoke, mask, and desensitize bitter taste suggests that burning sensations and bitter taste might be closely related perceptually. The current study investigated the psychophysical relationship between bitterness and burning using 2 different approaches. In Experiment 1, spatial discrimination of 4 taste stimuli was measured in the presence or absence of capsaicin. The subjects' task was to report which of 3 swabs, spaced 1 cm apart and presented to the tongue tip, contained a taste stimulus when 1) water was presented on the other 2 swabs or 2) when 10 muM capsaicin was presented on all 3 swabs. The presence of capsaicin did not change performance on the 3 alternative forced-choice (3-AFC) task for sweet, sour, and salty stimuli, while the localization error for 1.8 mM quinine sulfate (QSO(4)) increased significantly. In Experiment 2, the perceptual similarity/dissimilarity of taste stimuli and capsaicin was measured directly using pairs of stimuli applied to opposite sides of the tongue tip on swabs separated by 2 cm. Multidimensional scaling analyses showed that capsaicin fell nearer to QSO(4) than to any other taste stimulus. Cluster analysis corroborated this finding: capsaicin was closely linked with QSO(4) and the capsaicin-QSO(4) group was separated from the other taste stimuli. The latter result indicated that bitterness was more similar to burning than to the other tastes. These findings imply that despite being mediated by different sensory modalities, bitterness and burn are qualitatively similar. We speculate that this similarity reflects a common function of these 2 sensations as sensory signals of potentially harmful stimuli.

Mustard oil has differential effects on the response of trigeminal caudalis neurons to heat and acidity

Topical application of mustard oil (allyl isothiocyanate) to the skin or injection into joints induces hyperalgesia, allodynia, and neuroinflammation. However, when applied to the oral or nasal mucosa, mustard oil evokes a desensitizing pattern of irritation. Presently we investigated the responses of neurons in superficial laminae of trigeminal subnucleus caudalis (Vc) to noxious thermal (53 degrees C) and chemical (pentanoic acid; 200 mM) stimuli prior to and following lingual mustard oil application. A low concentration of mustard oil (0.125%) applied by constant flow (0.5 ml/min; 15 min), initially excited Vc neurons followed by partial desensitization. Responses to noxious heat were unchanged following mustard oil. A high concentration of mustard oil (1.25%) initially excited Vc neurons followed quickly (within 20 s) by nearly complete desensitization. The desensitization was transient since reapplication of mustard oil approximately 20 min later elicited a comparable response that also rapidly desensitized. Mustard oil also transiently cross-desensitized Vc responses to pentanoic acid (to 52%), in striking contrast to noxious heat-evoked responses which were significantly sensitized to approximately 160% of pre-mustard oil levels. The data suggest that the effect of mustard oil on subsequent lingual nociceptive responses is concentration dependent, transient, and modality specific.