Neurons in Superficial Trigeminal Subnucleus Caudalis Responsive to Oral Cooling, Menthol, and Other Irritant Stimuli

Separation of Oral Cooling and Warming Requires TRPM8

Cooling sensations arise inside the mouth during ingestive and homeostasis behaviors. Oral presence of cooling temperature engages the cold and menthol receptor TRPM8 (transient receptor potential melastatin 8) on trigeminal afferents. Yet, how TRPM8 influences brain and behavioral responses to oral temperature is undefined. Here we used in vivo neurophysiology to record action potentials stimulated by cooling and warming of oral tissues from trigeminal nucleus caudalis neurons in female and male wild-type and TRPM8 gene deficient mice. Using these lines, we also measured orobehavioral licking responses to cool and warm water in a novel, temperature-controlled fluid choice test. Capture of antidromic electrophysiological responses to thalamic stimulation identified that wild-type central trigeminal neurons showed diverse responses to oral cooling. Some neurons displayed relatively strong excitation to cold <10°C (COLD neurons) while others responded to only a segment of mild cool temperatures below 30°C (COOL neurons). Notably, TRPM8 deficient mice retained COLD-type but lacked COOL cells. This deficit impaired population responses to mild cooling temperatures below 30°C and allowed warmth-like (≥35°C) neural activity to pervade the normally innocuous cool temperature range, predicting TRPM8 deficient mice would show anomalously similar orobehavioral responses to warm and cool temperatures. Accordingly, TRPM8 deficient mice avoided both warm (35°C) and mild cool (≤30°C) water and sought colder temperatures in fluid licking tests, whereas control mice avoided warm but were indifferent to mild cool and colder water. Results imply TRPM8 input separates cool from warm temperature sensing and suggest other thermoreceptors also participate in oral cooling sensation.

Menthol dural application alters meningeal arteries tone and enhances excitability of trigeminocervical neurons in rats

Headaches, including migraines, can have a causal relationship to exposure to cold, and this relationship may be both positive and negative, as cold can both provoke and alleviate cephalgia. The role of thermoreceptors responsible for transduction of low temperatures belongs to the transient receptor potential cation channel subfamily melastatin member 8 (TRPM8). These channels mediate normal cooling sensation and have a role in both cold pain and cooling-mediated analgesia; they are seen as a potential target for principally new anti-migraine pharmaceuticals. Using a validated animal migraine models, we evaluated effects of menthol, the TRPM8-agonist, on trigeminovascular nociception. In acute experiments on male rats, effects of applied durally menthol solution in various concentrations on the neurogenic dural vasodilatation (NDV) and firing rate of dura-sensitive neurons of the trigeminocervical complex (TCC) were assessed. Application of menthol solution in concentrations of 5 % and 10 % was associated with NDV suppression, however amplitude reduction of the dilatation response caused not by the vascular dilatation degree decrease, but rather due to the significant increase of the meningeal arterioles' basal tone. In electrophysiological experiments the 1 % and 30 % menthol solutions intensified TCC neuron responses to the dural electrical stimulation while not changing their background activity. Revealed in our study excitatory effects of menthol related to the vascular as well as neuronal branches of the trigeminovascular system indicate pro-cephalalgic effects of TRPM8-activation and suggest feasibility of further search for new anti-migraine substances among TRPM8-antagonists.

TRPA1: Pharmacology, natural activators and role in obesity prevention

Transient receptor potential ankyrin 1 (TRPA1) channel is a calcium permeable, non-selective cation channel, expressed in the sensory neurons and non-neuronal cells of different tissues. Initially studied for its role in pain and inflammation, TRPA1 has now functionally involved in multiple other physiological functions. TRPA1 channel has been extensively studied for modulation by pungent compounds present in the spices and herbs. In the last decade, the role of TRPA1 agonism in body weight reduction, secretion of hunger and satiety hormones, insulin secretion and thermogenesis, has unveiled the potential of the TRPA1 channel to be used as a preventive target to tackle obesity and associated comorbidities including insulin resistance in type 2 diabetes. In this review, we summarized the recent findings of TRPA1 based dietary/non-dietary modulation for its role in obesity prevention and therapeutics.

Cinnamaldehyde elicits itch behavior via TRPV1 and TRPV4 but not TRPA1

Introduction:

Cinnamaldehyde (CA) elicits itch sensation in humans. We investigated if CA elicits scratching behavior in mice and determined the roles for TRPV1, TRPA1, and TRPV4.

Materials and Methods:

Scratching behavior elicited by intradermal injection of CA was assessed in wildtype (WT) mice and knockout (KO) mice lacking TRPV1, TRPA1, TRPV4, or deficient in mast cells. We also assessed scratching and wet dog shakes elicited by low-threshold mechanical stimulation of skin treated topically with CA or vehicle. Using calcium imaging we tested if CA activates dorsal root ganglion (DRG) neurons of each genotype.

Results:

Intradermal cheek injection of CA elicited dose-dependent hindlimb scratch bouts, with fewer forelimb wipes and facial groom bouts that were not dose-dependent. CA elicited significantly fewer scratch bouts in TRPV1 and TRPV4 KO mice, but not TRPA1KOs, compared with WTs. There were no sex differences across genotypes. The histamine H1 antagonist cetirizine did not affect CA-evoked scratching, which was normal in mast cell deficient mice, indicating lack of histamine involvement. Scores for alloknesis were significantly greater following topical application of CA compared with vehicle. Post-CA alloknesis scores were significantly higher in TRPV4KOs of both sexes and in female TRPV1 and TRPA1KOs, compared with WTs. Low threshold mechanical stimuli also elicited significantly more wet dog shakes in mice treated topically with 20% CA, with significantly fewer in TRPV1, TRPA1, and TRPV4KOs compared with WTs. In calcium imaging studies, CA excited 24% of WT DRG cells, significantly fewer (11.5%) in cells from TRPV4KOs, and none in TRPA1KOs. Responses of cells of all genotypes exhibited significant sensitization to repeated CA stimulation. Sensitization was significantly enhanced by IL-4, which itself excited 16% of WT DRG cells and none from TRPA1KOs.

Discussion:

The results indicate that TRPA1 is dispensable for CA-evoked scratching, which depends partly on TRPV1 and TRPV4.

The TRPA1 Ion Channel Contributes to Sensory-Guided Avoidance of Menthol in Mice

The flavoring agent menthol elicits complex orosensory and behavioral effects including perceived cooling at low concentrations and irritation and ingestive avoidance at higher intensities. Oral menthol engages the cold-activated transient receptor potential (TRP) ion channel TRP melastatin 8 (TRPM8) on trigeminal fibers, although its aversive feature was discussed to involve activation of TRP ankyrin 1 (TRPA1) associated with nociceptive processing. Here, we studied the roles of TRPM8 and TRPA1 in orosensory responding to menthol by subjecting mice gene deficient for either channel to brief-access exposure tests, which measure immediate licking responses to fluid stimuli to capture sensory/tongue control of behavior. Stimuli included aqueous concentration series of (−)-menthol [0 (water), 0.3, 0.5, 0.7, 1.0, 1.5, and 2.3 mM] and the aversive bitter taste stimulus quinine-HCl (0, 0.01, 0.03, 0.1, 0.3, 1, and 3 mM). Concentration-response data were generated from daily brief-access tests conducted in lickometers, which recorded the number of licks water-restricted mice emitted to a randomly selected stimulus concentration over a block of several 10-s stimulus presentations. Wild-type mice showed aversive orosensory responses to menthol above 0.7 mM. Oral aversion to menthol was reduced in mice deficient for TRPA1 but not TRPM8. Oral aversion to quinine was similar between TRPA1 mutant and control mice but stronger than avoidance of menthol. This implied menthol avoidance under the present conditions represented a moderate form of oral aversion. These data reveal TRPA1 contributes to the oral sensory valence of menthol and have implications for how input from TRPA1 and TRPM8 shapes somatosensory-guided behaviors.

Modulation of taste processing by temperature

Taste stimuli have a temperature that can stimulate thermosensitive neural machinery in the mouth during gustatory experience. Although taste and oral temperature are sometimes discussed as different oral sensory modalities, there is a body of literature that demonstrates temperature is an important component and modulator of the intensity of gustatory neural and perceptual responses. Available data indicate that the influence of temperature on taste, herein referred to as "thermogustation," can vary across taste qualities, can also vary among stimuli presumed to share a common taste quality, and is conditioned on taste stimulus concentration, with neuronal and psychophysical data revealing larger modulatory effects of temperature on gustatory responding to weakened taste solutions compared with concentrated. What is more, thermogustation is evidenced to involve interplay between mouth and stimulus temperature. Given these and other dependencies, identifying principles by which thermal input affects gustatory information flow in the nervous system may be important for ultimately unravelling the organization of neural circuits for taste and defining their involvement with multisensory processing related to flavor. Yet thermal effects are relatively understudied in gustatory neuroscience. Major gaps in our understanding of the mechanisms and consequences of thermogustation include delineating supporting receptors, the potential involvement of oral thermal and somatosensory trigeminal neurons in thermogustatory interactions, and the broader operational roles of temperature in gustatory processing. This review will discuss these and other issues in the context of the literature relevant to understanding thermogustation.

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.

Expression of vesicular glutamate transporters in transient receptor potential melastatin 8 (TRPM8)-positive dental afferents in the mouse

Transient receptor potential melastatin 8 (TRPM8) is activated by innocuous cool and noxious cold and plays a crucial role in cold-induced acute pain and pain hypersensitivity. To help understand the mechanism of TRPM8-mediated cold perception under normal and pathologic conditions, we used light microscopic immunohistochemistry and Western blot analysis in mice expressing a genetically encoded axonal tracer in TRPM8-positive (+) neurons. We investigated the coexpression of TRPM8 and vesicular glutamate transporter 1 (VGLUT1) and VGLUT2 in the trigeminal ganglion (TG) and the dental pulp before and after inducing pulpal inflammation. Many TRPM8+ neurons in the TG and axons in the dental pulp expressed VGLUT2, while none expressed VGLUT1. TRPM8+ axons were dense in the pulp horn and peripheral pulp and also frequently observed in the dentinal tubules. Following pulpal inflammation, the proportion of VGLUT2+ and of VGLUT2+/TRPM8+ neurons increased significantly, whereas that of TRPM8+ neurons remained unchanged. Our findings suggest the existence of VGLUT2 (but not VGLUT1)-mediated glutamate signaling in TRPM8+ neurons possibly underlying the cold-induced acute pain and hypersensitivity to cold following pulpal inflammation.

Thermosensitive transient receptor potential (TRP) channel agonists and their role in mechanical, thermal and nociceptive sensations as assessed using animal models

INTRODUCTION

The present paper summarizes research using animal models to investigate the roles of thermosensitive transient receptor potential (TRP) channels in somatosensory functions including touch, temperature and pain. We present new data assessing the effects of eugenol and carvacrol, agonists of the warmth-sensitive TRPV3, on thermal, mechanical and pain sensitivity in rats.

METHODS

Thermal sensitivity was assessed using a thermal preference test, which measured the amount of time the animal occupied one of two adjacent thermoelectric plates set at different temperatures. Pain sensitivity was assessed as an increase in latency of hindpaw withdrawal away from a noxious thermal stimulus directed to the plantar hindpaw (Hargreaves test). Mechanical sensitivity was assessed by measuring the force exerted by an electronic von Frey filament pressed against the plantar surface that elicited withdrawal.

RESULTS

Topical application of eugenol and carvacrol did not significantly affect thermal preference, although there was a trend toward avoidance of the hotter surface in a 30 vs. 45°C preference test for rats treated with 1 or 10% eugenol and carvacrol. Both eugenol and carvacrol induced a concentration-dependent increase in thermal withdrawal latency (analgesia), with no significant effect on mechanosensitivity.

CONCLUSIONS

The analgesic effect of eugenol and carvacrol is consistent with previous studies. The tendency for these chemicals to increase the avoidance of warmer temperatures suggests a possible role for TRPV3 in warmth detection, also consistent with previous studies. Additional roles of other thermosensitive TRP channels (TRPM8 TRPV1, TRPV2, TRPV4, TRPM3, TRPM8, TRPA1, TRPC5) in touch, temperature and pain are reviewed.

Inhibition of TRPM8 channels reduces pain in the cold pressor test in humans

The transient receptor potential (subfamily M, member 8; TRPM8) is a nonselective cation channel localized in primary sensory neurons, and is a candidate for cold thermosensing, mediation of cold pain, and bladder overactivity. Studies with TRPM8 knockout mice and selective TRPM8 channel blockers demonstrate a lack of cold sensitivity and reduced cold pain in various rodent models. Furthermore, TRPM8 blockers significantly lower body temperature. We have identified a moderately potent (IC50 = 103 nM), selective TRPM8 antagonist, PF-05105679 [(R)-3-[(1-(4-fluorophenyl)ethyl)(quinolin-3-ylcarbonyl)amino]methylbenzoic acid]. It demonstrated activity in vivo in the guinea pig bladder ice water and menthol challenge tests with an IC50 of 200 nM and reduced core body temperature in the rat (at concentrations >1219 nM). PF-05105679 was suitable for acute administration to humans and was evaluated for effects on core body temperature and experimentally induced cold pain, using the cold pressor test. Unbound plasma concentrations greater than the IC50 were achieved with 600- and 900-mg doses. The compound displayed a significant inhibition of pain in the cold pressor test, with efficacy equivalent to oxycodone (20 mg) at 1.5 hours postdose. No effect on core body temperature was observed. An unexpected adverse event (hot feeling) was reported, predominantly periorally, in 23 and 36% of volunteers (600- and 900-mg dose, respectively), which in two volunteers was nontolerable. In conclusion, this study supports a role for TRPM8 in acute cold pain signaling at doses that do not cause hypothermia.

Anti-hyperalgesic effects of a novel TRPM8 agonist in neuropathic rats: a comparison with topical menthol

Menthol has historically been used topically to alleviate various pain conditions. At low concentrations, this non-selective TRPM8 agonist elicits a cooling sensation, however higher concentrations result in cold hyperalgesia in normal subjects and paradoxically analgesia in neuropathic patients. Through behavioural and electrophysiological means, we examined whether this back-translated into a pre-clinical rodent model. Menthol was applied topically to the hind paws of naive and spinal nerve-ligated (SNL) rats. In behavioural assays, menthol did not affect withdrawal thresholds to mechanical stimulation and 10% and 40% menthol rarely sensitised withdrawals to innocuous cooling in naïve rats. However, in SNL rats, 10% and 40% menthol alleviated cold hypersensitivity. This was partly corroborated by in vivo electrophysiological recordings of dorsal horn lamina V/VI neurones. As several studies have implicated TRPM8 in analgesia, we examined whether a novel systemically available TRPM8 agonist, M8-Ag, had more potent anti-hyperalgesic effects than menthol in neuropathic rats. In vitro, M8-Ag activates TRPM8, expressed in HEK293 cells, with an EC50 of 44.97 nM. In vivo, M8-Ag inhibited neuronal responses to innocuous and noxious cooling in SNL rats with no effect in sham-operated rats. This effect was modality selective; M8-Ag did not alter neuronal responses to mechanical, heat or brush stimulation. In addition, M8-Ag attenuated behavioural hypersensitivity to innocuous cooling but not mechanical stimulation. These data suggest that menthol induced hyperalgesia is not consistently replicable in the rat and that the analgesic properties are revealed by injury. Systemic TRPM8 agonists might be beneficial in neuropathy without affecting normal cold sensitivity.

Eugenol and carvacrol excite first- and second-order trigeminal neurons and enhance their heat-evoked responses

Eugenol and carvacrol from clove and oregano, respectively, are agonists of the warmth-sensitive transient receptor potential channel TRPV3 and the irritant-sensitive transient receptor potential ankyrin (TRPA)-1. Eugenol and carvacrol induce oral irritation that rapidly desensitizes, accompanied by brief enhancement of innocuous warmth and heat pain in humans. We presently investigated if eugenol and carvacrol activate nociceptive primary afferent and higher order trigeminal neurons and enhance their heat-evoked responses, using calcium imaging of cultured trigeminal ganglion (TG) and dorsal root ganglion (DRG) neurons, and in vivo single-unit recordings in trigeminal subnucleus caudalis (Vc) of rats. Eugenol and carvacrol activated 20-30% of TG and 7-20% of DRG cells, the majority of which additionally responded to menthol, mustard oil and/or capsaicin. TG cell responses to innocuous (39°) and noxious (42 °C) heating were enhanced by eugenol and carvacrol. We identified dorsomedial Vc neurons responsive to noxious heating of the tongue in pentobarbital-anesthetized rats. Eugenol and carvacrol dose-dependently elicited desensitizing responses in 55% and 73% of heat-sensitive units, respectively. Responses to noxious heat were briefly enhanced by eugenol and carvacrol. Many eugenol- and carvacrol-responsive units also responded to menthol, cinnamaldehyde and capsaicin. These data support a peripheral site for eugenol and carvacrol to enhance warmth- and noxious heat-evoked responses of trigeminal neurons, and are consistent with the observation that these agonists briefly enhance warmth and heat pain on the human tongue.

Central connectivity of transient receptor potential melastatin 8-expressing axons in the brain stem and spinal dorsal horn

Transient receptor potential melastatin 8 (TRPM8) ion channels mediate the detection of noxious and innocuous cold and are expressed by primary sensory neurons, but little is known about the processing of the TRPM8-mediated cold information within the trigeminal sensory nuclei (TSN) and the spinal dorsal horn (DH). To address this issue, we characterized TRPM8-positive (+) neurons in the trigeminal ganglion and investigated the distribution of TRPM8+ axons and terminals, and their synaptic organization in the TSN and in the DH using light and electron microscopic immunohistochemistry in transgenic mice expressing a genetically encoded axonal tracer in TRPM8+ neurons. TRPM8 was expressed in a fraction of small myelinated primary afferent fibers (23.7%) and unmyelinated fibers (76.3%), suggesting that TRPM8-mediated cold is conveyed via C and Aδ afferents. TRPM8+ axons were observed in all TSN, but at different densities in the dorsal and ventral areas of the rostral TSN, which dominantly receive sensory afferents from intra- and peri-oral structures and from the face, respectively. While synaptic boutons arising from Aδ and non-peptidergic C afferents usually receive many axoaxonic contacts and form complex synaptic arrangements, TRPM8+ boutons arising from afferents of the same classes of fibers showed a unique synaptic connectivity; simple synapses with one or two dendrites and sparse axoaxonic contacts. These findings suggest that TRPM8-mediated cold is conveyed via a specific subset of C and Aδ afferent neurons and is processed in a unique manner and differently in the TSN and DH.

Physiological temperatures drive glutamate release onto trigeminal superficial dorsal horn neurons

Trigeminal sensory afferent fibers terminating in nucleus caudalis (Vc) relay sensory information from craniofacial regions to the brain and are known to express transient receptor potential (TRP) ion channels. TRP channels are activated by H(+), thermal, and chemical stimuli. The present study investigated the relationships among the spontaneous release of glutamate, temperature, and TRPV1 localization at synapses in the Vc. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from Vc neurons (n = 151) in horizontal brain-stem slices obtained from Sprague-Dawley rats. Neurons had basal sEPSC rates that fell into two distinct frequency categories: High (≥10 Hz) or Low (<10 Hz) at 35°C. Of all recorded neurons, those with High basal release rates (67%) at near-physiological temperatures greatly reduced their sEPSC rate when cooled to 30°C without amplitude changes. Such responses persisted during blockade of action potentials indicating that the High rate of glutamate release arises from presynaptic thermal mechanisms. Neurons with Low basal frequencies (33%) showed minor thermal changes in sEPSC rate that were abolished after addition of TTX, suggesting these responses were indirect and required local circuits. Activation of TRPV1 with capsaicin (100 nM) increased miniature EPSC (mEPSC) frequency in 70% of neurons, but half of these neurons had Low basal mEPSC rates and no temperature sensitivity. Our evidence indicates that normal temperatures (35-37°C) drive spontaneous excitatory synaptic activity within superficial Vc by a mechanism independent of presynaptic action potentials. Thus thermally sensitive inputs on superficial Vc neurons may tonically activate these neurons without afferent stimulation.

Responses of intact and injured sural nerve fibers to cooling and menthol

Intact and injured cutaneous C-fibers in the rat sural nerve are cold sensitive, heat sensitive, and/or mechanosensitive. Cold-sensitive fibers are either low-threshold type 1 cold sensitive or high-threshold type 2 cold sensitive. The hypothesis was tested, in intact and injured afferent nerve fibers, that low-threshold cold-sensitive afferent nerve fibers are activated by the transient receptor potential melastatin 8 (TRPM8) agonist menthol, whereas high-threshold cold-sensitive C-fibers and cold-insensitive afferent nerve fibers are menthol insensitive. In anesthetized rats, activity was recorded from afferent nerve fibers in strands isolated from the sural nerve, which was either intact or crushed 6-12 days before the experiment distal to the recording site. In all, 77 functionally identified afferent C-fibers (30 intact fibers, 47 injured fibers) and 34 functionally characterized A-fibers (11 intact fibers, 23 injured fibers) were tested for their responses to menthol applied to their receptive fields either in the skin (10 or 20%) or in the nerve (4 or 8 mM). Menthol activated all intact (n = 12) and 90% of injured (n = 20/22) type 1 cold-sensitive C-fibers; it activated no intact type 2 cold-sensitive C-fibers (n = 7) and 1/11 injured type 2 cold-sensitive C-fibers. Neither intact nor injured heat- and/or mechanosensitive cold-insensitive C-fibers (n = 25) and almost no A-fibers (n = 2/34) were activated by menthol. These results strongly argue that cutaneous type 1 cold-sensitive afferent fibers are nonnociceptive cold fibers that use the TRPM8 transduction channel.

The role of trigeminal nasal TRPM8-expressing afferent neurons in the antitussive effects of menthol

The cold-sensitive cation channel TRPM8 is a target for menthol, which is used routinely as a cough suppressant and as an additive to tobacco and food products. Given that cold temperatures and menthol activate neurons through gating of TRPM8, it is unclear how menthol actively suppresses cough. In this study we describe the antitussive effects of (-)-menthol in conscious and anesthetized guinea pigs. In anesthetized guinea pigs, cough evoked by citric acid applied topically to the tracheal mucosa was suppressed by menthol only when it was selectively administered as vapors to the upper airways. Menthol applied topically to the tracheal mucosa prior to and during citric acid application or administered continuously as vapors or as an aerosol to the lower airways was without effect on cough. These actions of upper airway menthol treatment were mimicked by cold air delivered to the upper airways but not by (+)-menthol, the inactive isomer of menthol, or by the TRPM8/TRPA1 agonist icilin administered directly to the trachea. Subsequent molecular analyses confirmed the expression of TRPM8 in a subset of nasal trigeminal afferent neurons that do not coincidently express TRPA1 or TRPV1. We conclude that menthol suppresses cough evoked in the lower airways primarily through a reflex initiated from the nose.

Corneal dry-responsive neurons in the spinal trigeminal nucleus respond to innocuous cooling in the rat

Corneal primary afferent neurons that respond to drying of the ocular surface have been previously characterized and found to respond to innocuous cooling, menthol, and hyperosmotic stimuli. The purpose of the present study was to examine the receptive field properties of second-order neurons in the trigeminal nucleus that respond to drying of the ocular surface. Single-unit electrophysiological recordings were performed in anesthetized rats, and dry-responsive corneal units were isolated in the brain stem at the transition zone between the spinal trigeminal subnucleus caudalis and subnucleus interpolaris. Corneal units were characterized according to their responses to changes in temperature (cooling and heating), hyperosmotic artificial tears, menthol, and low pH. All dry-responsive neurons (n = 18) responded to cooling of the ocular surface. In addition, these neurons responded to hyperosmotic stimuli and menthol application to the cornea. One-half of the neurons were activated by low pH, and these acid-sensitive neurons were also activated by noxious heat. Furthermore, neurons that were activated by low pH had a significantly lower response to cooling and menthol. These results indicate that dry-responsive neurons recorded in the trigeminal nucleus receive input from cold, sensitive primary afferent neurons, with a subset of these neurons receiving input from corneal primary afferent neurons sensitive to acid and noxious heat. It is proposed that acid-insensitive corneal neurons represent a labeled line for lacrimation in response to evaporation of tears from the ocular surface, whereas acid-sensitive neurons are involved in tearing, elicited by damaging or potentially damaging stimuli.

Menthol activation of corneal cool cells induces TRPM8-mediated lacrimation but not nociceptive responses in rodents

PURPOSE

Stimulation to the cornea via noxious chemical and mechanical means evokes tearing, blinking, and pain. In contrast, mild cooling of the ocular surface has been reported to increase lacrimation via activation of corneal cool primary afferent neurons. The purpose of our study was to determine whether menthol induces corneal cool cell activity and lacrimation via the transient receptor potential melastatin-8 (TRPM8) channel without evoking nociceptive responses.

METHODS

Tear measurements were made using a cotton thread in TRPM8 wild type and knockout mice after application of menthol (0.05-50 mM) to the cornea. In additional studies, nocifensive responses (eye swiping and lid closure) were quantified following cornea menthol application. Trigeminal ganglion electrophysiologic single unit recordings were performed in rats to determine the effect of low and high concentrations of menthol on corneal cool cells.

RESULTS

At low concentrations, menthol increased tear production in TRPM8 wild type and heterozygous animals, but had no effect in TRPM8 knockout mice, while nocifensive responses remained unaffected. At the highest concentration, menthol (50 mM) increased tearing and nocifensive responses in TRPM8 wild type and knockout animals. A low concentration of menthol (0.1 mM) increased cool cell activity, yet a high concentration of menthol (50 mM) had no effect.

CONCLUSIONS

These studies indicated that low concentrations of menthol can increase lacrimation via TRPM8 channels without evoking nocifensive behaviors. At high concentrations, menthol can induce lacrimation and nocifensive behaviors in a TRPM8 independent mechanism. The increase in lacrimation is likely due to an increase in cool cell activity.

Topical hindpaw application of L-menthol decreases responsiveness to heat with biphasic effects on cold sensitivity of rat lumbar dorsal horn neurons

Menthol is used in pharmaceutical applications because of its desired cooling and analgesic properties. The neural mechanism by which topical application of menthol decreases heat pain is not fully understood. We investigated the effects of topical menthol application on lumbar dorsal horn wide dynamic range and nociceptive-specific neuronal responses to noxious heat and cooling of glabrous hindpaw cutaneous receptive fields. Menthol increased thresholds for responses to noxious heat in a concentration-dependent manner. Menthol had a biphasic effect on cold-evoked responses, reducing the threshold (to warmer temperatures) at a low (1%) concentration and increasing threshold and reducing response magnitude at high (10%, 40%) concentrations. Menthol had little effect on responses to innocuous or noxious mechanical stimuli, ruling out a local anesthetic action. Application of 40% menthol to the contralateral hindpaw tended to reduce responses to cooling and noxious heat, suggesting a weak heterosegmental inhibitory effect. These results indicate that menthol has an analgesic effect on heat sensitivity of nociceptive dorsal horn neurons, as well as biphasic effects on cold sensitivity, consistent with previous behavioral observations.

Inhibitory control of nociceptive responses of trigeminal spinal nucleus cells by somatosensory corticofugal projection in rat

The caudal division of the trigeminal spinal nucleus (Sp5C) is an important brainstem relay station of orofacial pain transmission. The aim of the present study was to examine the effect of cortical electrical stimulation on nociceptive responses in Sp5C neurons. Extracellular recordings were performed in the Sp5C nucleus by tungsten microelectrodes in urethane-anesthetized Sprague-Dawley rats. Nociceptive stimulation was produced by application of capsaicin cream on the whisker pad or by constriction of the infraorbital nerve. Capsaicin application evoked a long-lasting increase in the spontaneous firing rate from 1.4±0.2 to 3.4±0.6 spikes/s. Non-noxious tactile responses from stimuli delivered to the receptive field (RF) center decreased 5 min. after capsaicin application (from 2.3±0.1 to 1.6±0.1 spikes/stimulus) while responses from the whisker located at the RF periphery increased (from 1.3±0.2 to 2.0±0.1 spikes/stimulus under capsaicin). Electrical train stimulation of the primary (S1) or secondary (S2) somatosensory cortical areas reduced the increase in the firing rate evoked by capsaicin. Also, S1, but not S2, cortical stimulation reduced the increase in non-noxious tactile responses from the RF periphery. Inhibitory cortical effects were mediated by the activation of GABAergic and glycinergic neurons because they were blocked by bicuculline or strychnine. The S1 and S2 cortical stimulation also inhibited Sp5C neurons in animals with constriction of the infraorbital nerve. Consequently, the corticofugal projection from S1 and S2 cortical areas modulates nociceptive responses of Sp5C neurons and may control the transmission of nociceptive sensory stimulus.

Orosensory and Homeostatic Functions of the Insular Taste Cortex

The gustatory aspect of the insular cortex is part of the brain circuit that controls ingestive behaviors based on chemosensory inputs. However, the sensory properties of foods are not restricted to taste and should also include salient features such as odor, texture, temperature, and appearance. Therefore, it is reasonable to hypothesize that specialized circuits within the central taste pathways must be involved in representing several other oral sensory modalities in addition to taste. In this review, we evaluate current evidence indicating that the insular gustatory cortex functions as an integrative circuit, with taste-responsive regions also showing heightened sensitivity to olfactory, somatosensory, and even visual stimulation. We also review evidence for modulation of taste-responsive insular areas by changes in physiological state, with taste-elicited neuronal responses varying according to the nutritional state of the organism. We then examine experimental support for a functional map within the insular cortex that might reflect the various sensory and homeostatic roles associated with this region. Finally, we evaluate the potential role of the taste insular cortex in weight-gain susceptibility. Taken together, the current experimental evidence favors the view that the insular gustatory cortex functions as an orosensory integrative system that not only enables the formation of complex flavor representations but also mediates their modulation by the internal state of the body, playing therefore a central role in food intake regulation.

Facial injections of pruritogens or algogens elicit distinct behavior responses in rats and excite overlapping populations of primary sensory and trigeminal subnucleus caudalis neurons

In the present study, we investigated whether intradermal cheek injection of pruritogens or algogens differentially elicits hindlimb scratches or forelimb wipes in Sprague-Dawley rats, as recently reported in mice. We also investigated responses of primary sensory trigeminal ganglion (TG) and dorsal root ganglion (DRG) cells, as well as second-order neurons in trigeminal subnucleus caudalis (Vc), to pruritic and algesic stimuli. 5-HT was the most effective chemical to elicit dose-dependent bouts of hindlimb scratches directed to the cheek, with significantly less forelimb wiping, consistent with itch. Chloroquine also elicited significant scratching but not wiping. Allyl isothiocyanate (AITC; mustard oil) elicited dose-dependent wiping with no significant scratching. Capsaicin elicited equivalent numbers of scratch bouts and wipes, suggesting a mixed itch and pain sensation. By calcium imaging, ∼ 6% of cultured TG and DRG cells responded to 5-HT. The majority of 5-HT-sensitive cells also responded to chloroquine, AITC, and/or capsaicin, and one-third responded to histamine. Using a chemical search strategy, we identified single units in Vc that responded to intradermal cheek injection of 5-HT. Most were wide dynamic range (WDR) or nociceptive specific (NS), and a few were mechanically insensitive. The large majority additionally responded to AITC and/or capsaicin and thus were not pruritogen selective. These results suggest that primary and second-order neurons responsive to pruritogens and algogens may utilize a population coding mechanism to distinguish between itch and pain, sensations that are behaviorally manifested by distinct hindlimb scratching and forelimb wiping responses.

A possible new relay for tongue thermal sense in the dorsal margin of the trigeminal principal nucleus in rats

Neurons responding to innocuous thermal stimulation of the anterior tongue were newly identified in the dorsal margin of the trigeminal principal nucleus (PV). Connections of the dorsal margin of the PV were neuroanatomically identified using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) with electrophysiological techniques. Injection of WGA-HRP into the dorsal margin of the PV demonstrated anterograde labels distributed from the most dorsomedial portion of the posteromedial ventral nucleus (VPM) to the dorsolateral portion of the parvicellular part of the VPM and retrograde labels in the superficial layers of the caudal subnucleus of the spinal trigeminal nucleus (SpVc). Injection of WGA-HRP in the most dorsomedial portion of the VPM demonstrated retrogradely labeled cells in the dorsal margin of the PV as well as in the superficial layers of the SpVc. Tracer injection into the superficial layers of the SpVc resulted in anterograde labels in the dorsal margin of the PV as well as in the dorsomedial portion of the VPM. These findings show that neurons in the PV respond to innocuous thermal stimulation of the trigeminal field and suggest that the dorsal margin of the PV is a possible new relay for the tongue thermal sense, receiving thermal information from the superficial layers of the SpVc, the primary thermal relay for the trigeminal field, and passing the information to the thalamic relay.

Novel menthol-derived cooling compounds activate primary and second-order trigeminal sensory neurons and modulate lingual thermosensitivity

We presently investigated 2 novel menthol derivatives GIV1 and GIV2, which exhibit strong cooling effects. In previous human psychophysical studies, GIV1 delivered in a toothpaste medium elicited a cooling sensation that was longer lasting compared with GIV2 and menthol carboxamide (WS-3). In the current study, we investigated the molecular and cellular effects of these cooling agents. In calcium flux studies of TRPM8 expressed in HEK cells, both GIV1 and GIV2 were approximately 40- to 200-fold more potent than menthol and WS-3. GIV1 and GIV2 also activated TRPA1 but at levels that were 400 times greater than those required for TRPM8 activation. In calcium imaging studies, subpopulations of cultured rat trigeminal ganglion and dorsal root ganglion cells responded to GIV1 and/or GIV2; the majority of these were also activated by menthol and some were additionally activated by the TRPA1 agonist cinnamaldehyde and/or the TRPV1 agonist capsaicin. We also made in vivo single-unit recordings from cold-sensitive neurons in rat trigeminal subnucleus caudalis (Vc). GIV 1 and GIV2 directly excited some Vc neurons, GIV1 significantly enhanced their responses to cooling, and both GIV1 and GIV2 reduced responses to noxious heat. These novel cooling compounds provide additional molecular tools to investigate the neural processes of cold sensation.

Self- and cross-desensitization of oral irritation by menthol and cinnamaldehyde (CA) via peripheral interactions at trigeminal sensory neurons

Menthol and cinnamaldehyde (CA) are plant-derived spices commonly used in oral hygiene products, chewing gum, and many other applications. However, little is known regarding their sensory interactions in the oral cavity. We used a human psychophysics approach to investigate the temporal dynamics of oral irritation elicited by sequential application of menthol and/or CA, and ratiometric calcium imaging methods to investigate activation of rat trigeminal ganglion (TG) cells by these agents. Irritancy decreased significantly with sequential oral application of menthol and CA (self-desensitization). Menthol cross-desensitized irritation elicited by CA, and vice versa, over a time course of at least 60 min. Seventeen and 19% of TG cells were activated by menthol and CA, respectively, with ∼50% responding to both. TG cells exhibited significant self-desensitization to menthol applied at a 5, but not 10, min interval. They also exhibited significant self-desensitization to CA at 400 but not 200 μM. Menthol cross-desensitized TG cell responses to CA. CA at a concentration of 400 but not 200 μM also cross-desensitized menthol-evoked responses. The results support the argument that the perceived reductions in oral irritancy and cross-interactions between menthol and CA and menthol observed (at least at short interstimulus intervals) can be largely accounted for by the properties of trigeminal sensory neurons innervating the tongue.

Threshold and rate sensitivity of low-threshold thermal nociception

Previous studies have shown that sensations of burning, stinging or pricking can be evoked by warming or cooling the skin to innocuous temperatures [low-threshold thermal nociception (LTN)] below the thresholds of cold- and heat-sensitive nociceptors. LTN implies that some primary afferent fibers classically defined as warm and cold fibers relay stimulation to the nociceptive system. We addressed this question in humans by determining if different adaptation temperatures (ATs) and rates of temperature change would affect thermal sensation and LTN similarly. In Experiment 1 subjects rated the intensity of warmth, cold and nociceptive sensations produced by increasing steps in temperature (+/-0.5 degrees C increments) from ATs of 35, 33 and 31 degrees C for cooling, and 30, 32 and 34 degrees C for heating. Depending upon the AT, thresholds for nociceptive and thermal sensations estimated from the rating data differed by as little as -1.0 degrees C for cooling and +1.5 degrees C for heating. Thresholds of thermal and nociceptive sensations shifted by similar amounts across the three ATs during cooling, whereas during heating the nociceptive threshold was significantly affected only between ATs of 32 and 34 degrees C. In Experiment 2, increasing the rate of temperature change from 0.5 to 4.0 degrees C/s increased the intensity of thermal and nociceptive sensations significantly but the effect was greatest for nociceptive sensations during heating. The results of both experiments are consistent with the mediation of LTN by low-threshold thermoreceptors, although LTN caused by heating may depend on a subset of fibers that express less sensitive TRP channels than those that serve sensations of warmth at the mildest temperatures.

Evidence for an integrated oral sensory module in the human anterior ventral insula

Taste, which is almost always accompanied by other oral sensations, serves to identify potential nutrients and toxins. The present study was designed to determine the influence of sensory modality (chemesthetic vs. gustatory) and physiological significance (potentially nutritive vs. potentially harmful) on insular response to oral stimulation. Sixteen subjects underwent functional magnetic resonance imaging scanning while receiving 2 potentially nutritive solutions (sucrose and NaCl), 2 potentially harmful solutions (quinine and capsaicin, a chemesthetic stimulus), and a tasteless control solution. We identified a region of anterior ventral insula that responded to oral stimulation irrespective of modality or physiological significance. However, when subjects tasted a potentially nutritive stimulus, the connectivity between the insula and a feeding network including the hypothalamus, ventral pallidum, and striatum was greater than when tasting a potentially harmful stimulus. No differential connectivity was observed as a function of modality (gustatory vs. chemesthetic). These results support the existence of an integrated supramodal flavor system in the anterior ventral insula that preferentially communicates with the circuits guiding feeding when the flavor is potentially nutritive.

Localisation of unilateral nasal stimuli across sensory systems

Odor stimuli presented to one nostril can only be localised if they additionally activate the trigeminal nerve's chemosensitive fibers. In this study we aimed to investigate characteristics in the localisation of unilateral trigeminal, olfactory and somatosensory nasal stimuli. We compared the ability of healthy young subjects to localise monorhinally presented (a) pure olfactory stimuli (phenyl ethyl alcohol), (b) mixed olfactory trigeminal stimuli (eucalyptol), and (c) somatosensory stimuli (air puffs). As expected, subjects could localise the air puffs and eucalyptol, but could not phenyl ethyl alcohol. Interestingly, we observed a significant correlation between localisation performance for eucalyptol and phenyl ethyl alcohol but not between the ability to localise somatosensory and trigeminal or olfactory stimuli. These observations show that on a behavioural level, the trigeminal chemosensory system is more intimately connected to the olfactory system than to the somatosensory system despite the fact that anatomically its information is conveyed via same nerve as the latter. Furthermore, they show that the trigeminal chemosensory system should therefore be considered a self-confined contributor to chemosensory perception.

Topical application of L-menthol induces heat analgesia, mechanical allodynia, and a biphasic effect on cold sensitivity in rats

Menthol is used in analgesic balms and also in foods and oral hygiene products for its fresh cooling sensation. Menthol enhances cooling by interacting with the cold-sensitive thermoTRP channel TRPM8, but its effect on pain is less well understood. We presently used behavioral methods to investigate effects of topical menthol on thermal (hot and cold) pain and innocuous cold and mechanical sensitivity in rats. Menthol dose-dependently increased the latency for noxious heat-evoked withdrawal of the treated hindpaw with a weak mirror-image effect, indicating antinociception. Menthol at the highest concentration (40%) reduced mechanical withdrawal thresholds, with no effect at lower concentrations. Menthol had a biphasic effect on cold avoidance. At high concentrations (10% and 40%) menthol reduced avoidance of colder temperatures (15 degrees C and 20 degrees C) compared to 30 degrees C, while at lower concentrations (0.01-1%) menthol enhanced cold avoidance. In a -5 degrees C cold plate test, 40% menthol significantly increased the nocifensive response latency (cold hypoalgesia) while lower concentrations were not different from vehicle controls. These results are generally consistent with neurophysiological and human psychophysical data and support TRPM8 as a potential peripheral target of pain modulation.

Psychophysical evaluation of a sanshool derivative (alkylamide) and the elucidation of mechanisms subserving tingle

Previous studies investigated the neural and molecular underpinnings of the tingle sensation evoked by sanshool and other natural or synthetic alkylamides. Currently, we sought to characterize the psychophysical properties associated with administration of these compounds. Like other chemesthetic stimuli, the synthetic tingle analog isobutylalkylamide (IBA) evoked a sensation that was temporally dynamic. Repeated IBA application at short (30 sec) interstimulus intervals (ISI) resulted in a tingle sensation that increased across trials. Application at longer ISIs (approximately 30 min) resulted in a sensation of decreased intensity consistent with self-desensitization. Prior treatment with the TRPV1 or TRPA1 agonists, capsaicin and mustard oil did not cross-desensitize the tingle sensation evoked by IBA suggesting that neither TRPV1 nor TRPA1 participate in the transduction mechanism sub-serving tingle. When evaluated over 30-min time period, lingual IBA evoked a sensation that was described initially as tingling and pungent but after approximately 15 min, as a cooling sensation. Further, we found that the sensation evoked by lingual IBA was potentiated by simultaneous application of cold (0 degrees C) and cool (21 degrees C) thermal stimuli but was unaffected by warm (33 degrees C) and hot (41 degrees C) temperatures. Finally, to test the hypothesis that the tingling sensation is subserved by the activation of mechanosensitve fibers, we evaluated lingual tactile thresholds in the presence and absence of lingual IBA. The presence of IBA significantly raised lingual tactile thresholds, whereas capsaicin did not, identifying a role for mechanosensitive fibers in conveying the tingle sensation evoked by sanshool-like compounds. Collectively, these results show that lingual alkylamide evokes a complex sensation that is temporally dynamic and consistent with in vitro and in vivo experiments suggesting these compounds activate mechanosensitve neurons via blockade of KCNK two-pore potassium channels to induce the novel tingling sensation.

Nicotine activates the chemosensory cation channel TRPA1

Topical application of nicotine, as used in nicotine replacement therapies, causes irritation of the mucosa and skin. This reaction has been attributed to activation of nicotinic acetylcholine receptors (nAChRs) in chemosensory neurons. In contrast with this view, we found that the chemosensory cation channel transient receptor potential A1 (TRPA1) is crucially involved in nicotine-induced irritation. We found that micromolar concentrations of nicotine activated heterologously expressed mouse and human TRPA1. Nicotine acted in a membrane-delimited manner, stabilizing the open state(s) and destabilizing the closed state(s) of the channel. In the presence of the general nAChR blocker hexamethonium, nociceptive neurons showed nicotine-induced responses that were strongly reduced in TRPA1-deficient mice. Finally, TRPA1 mediated the mouse airway constriction reflex to nasal instillation of nicotine. The identification of TRPA1 as a nicotine target suggests that existing models of nicotine-induced irritation should be revised and may facilitate the development of smoking cessation therapies with less adverse effects.

Temperature perception on the hand during static versus dynamic contact with a surface

Innocuous cooling or heating of the forearm can evoke nociceptive sensations, such as burning, stinging, and pricking (low-threshold thermal nociception, LTN), that are inhibited by dynamic contact. In the present study, I investigated whether LTN can also be perceived on the hand, and if so, whether it is normally suppressed by active touching. Innocuous cold (28 degrees , 25 degrees , and 18 degrees C) and warm (38 degrees , 40 degrees , and 43 degrees C) temperatures were delivered to the distal metacarpal pads and intermediate and distal phalanges of the fingers via a handgrip thermode that subjects either statically held or actively grasped. The same temperatures were delivered to the forearm via another thermode that either rested on the arm or was touched to the arm. Subjects rated the intensity of thermal (warmth, cold) and nociceptive (e.g., burning) sensations and indicated the qualities of sensation experienced. The results showed that LTN can be perceived on the hand, although less frequently and less intensely than on the forearm. Dynamic contact inhibited nociceptive and thermal sensations on the hand, although less strongly than on the forearm. These findings indicate that temperature perception on the hand is attenuated and its quality is changed when thermal stimulation is accompanied by dynamic tactile stimulation, as it is during haptic exploration.

Effects of hot and cold stimulus combinations on the thermal preference of rats

Traditional evaluation of pain in animals has primarily used reflexive withdrawal or nocifensive response from singly presented stimulation. However, daily experience of thermal sensation involves situations in which rapid temperature changes from cold to hot can occur. Therefore, in order to better understand integration of competing stimuli and their role in the motivational character of pain perception, behavioral tasks have been adapted to evaluate treatment-driven changes in hindpaws when exposed to two or more stimuli. However, such assessments of craniofacial sensitivity are lacking. In this study, we sought to characterize thermal preference for facial stimulation when rats are given the option of experiencing a hot or cold stimulus to obtain a milk reward, or abstaining from stimulation. We found that when both cold and hot stimuli were either non-noxious or where both stimuli were noxious the hot stimulus was preferred. When the hot stimulus was noxious, non-noxious cold was preferred. Unstimulated time was dependent on the combined aversiveness of the two stimuli, such that unstimulated time was the greatest with a highly aversive stimulus pair (-4 and 48 degrees C). We also found that pairing stimuli modulated successful task completion for each stimulus, but for nociceptive heat, this was not solely a consequence of thermal preference. Finally, we found that previous preference could both induce and abolish subsequent thermode preference independent of stimulus cues. The findings in this study will allow us to evaluate experimental pain states and analgesic treatments in a manner more relatable to the experience of the patient.

Glutamate and capsaicin effects on trigeminal nociception I: Activation and peripheral sensitization of deep craniofacial nociceptive afferents

We have examined the effect of the peripheral application of glutamate and capsaicin to deep craniofacial tissues in influencing the activation and peripheral sensitization of deep craniofacial nociceptive afferents. The activity of single trigeminal nociceptive afferents with receptive fields in deep craniofacial tissues were recorded extracellularly in 55 halothane-anesthetized rats. The mechanical activation threshold (MAT) of each afferent was assessed before and after injection of 0.5 M glutamate (or vehicle) and 1% capsaicin (or vehicle) into the receptive field. A total of 68 afferents that could be activated by blunt noxious mechanical stimulation of the deep craniofacial tissues (23 masseter, 5 temporalis, 40 temporomandibular joint) were studied. When injected alone, glutamate and capsaicin activated and induced peripheral sensitization reflected as MAT reduction in many afferents. Following glutamate injection, capsaicin-evoked activity was greater than that evoked by capsaicin alone, whereas following capsaicin injection, glutamate-evoked responses were similar to glutamate alone. These findings indicate that peripheral application of glutamate or capsaicin may activate or induce peripheral sensitization in a subpopulation of trigeminal nociceptive afferents innervating deep craniofacial tissues, as reflected in changes in MAT and other afferent response properties. The data further suggest that peripheral glutamate and capsaicin receptor mechanisms may interact to modulate the activation and peripheral sensitization in some deep craniofacial nociceptive afferents.

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.

Characterization of mouse orofacial pain and the effects of lesioning TRPV1-expressing neurons on operant behavior

Background

Rodent models of orofacial pain typically use methods adapted from manipulations to hind paw; however, limitations of these models include animal restraint and subjective assessments of behavior by the experimenter. In contrast to these methods, assessment of operant responses to painful stimuli has been shown to overcome these limitations and expand the breadth of interpretation of the behavioral responses. In the current study, we used an operant model based on a reward-conflict paradigm to assess nociceptive responses in three strains of mice (SKH1-Hr^hr, C57BL/6J, TRPV1 knockout). We previously validated this operant model in rats and hypothesized in this study that wild-type mice would demonstrate a similar thermal stimulus-dependent response and similar operant pain behaviors. Additionally, we evaluated the effects on operant behaviors of mice manipulated genetically (e.g., TRPV1 k.o.) or pharmacologically with resiniferatoxin (RTX), a lesioning agent for TRPV1-expressing neurons. During the reward-conflict task, mice accessed a sweetened milk reward solution by voluntarily position their face against a neutral or heated thermode (37–55°C).

Results

As the temperature of the thermal stimulus became noxiously hot, reward licking events in SKH1-Hr^hr and C57BL/6J mice declined while licking events in TRPV1 k.o. mice were insensitive to noxious heat within the activation range of TRPV1 (37–52°C). All three strains displayed nocifensive behaviors at 55°C, as indicated by a significant decrease in reward licking events. Induction of neurogenic inflammation by topical application of capsaicin reduced licking events in SKH1-Hr^hr mice, and morphine rescued this response. Again, these results parallel what we previously documented using rats in this operant system. Following intracisternal treatment with RTX, C57BL/6J mice demonstrated a block of noxious heat at both 48 and 55°C. RTX-treated TRPV1 k.o. mice and all vehicle-treated mice displayed similar reward licking events as compared to the pre-treatment baseline levels. Both TRPV1 k.o. and RTX-treated C57BL/6J had complete abolishment of eye-wipe responses following corneal application of capsaicin.

Conclusion

Taken together, these results indicate the benefits of using the operant test system to investigate pain sensitivity in mice. This ability provides an essential step in the development of new treatments for patients suffering from orofacial pain disorders.

Multisensory Processing of Gustatory Stimuli

Gustatory perception is inherently multimodal, since approximately the same time that intra-oral stimuli activate taste receptors, somatosensory information is concurrently sent to the CNS. We review evidence that gustatory perception is intrinsically linked to concurrent somatosensory processing. We will show that processing of multisensory information can occur at the level of the taste cells through to the gustatory cortex. We will also focus on the fact that the same chemical and physical stimuli that activate the taste system also activate the somatosensory system (SS), but they may provide different types of information to guide behavior.

Nociceptive sensations evoked from 'spots' in the skin by mild cooling and heating

It was recently found that nociceptive sensations (stinging, pricking, or burning) can be evoked by cooling or heating the skin to innocuous temperatures (e.g., 29 and 37 degrees C). Here, we show that this low-threshold thermal nociception (LTN) can be traced to sensitive 'spots' in the skin equivalent to classically defined warm spots and cold spots. Because earlier work had shown that LTN is inhibited by simply touching a thermode to the skin, a spatial search procedure was devised that minimized tactile stimulation by sliding small thermodes (16 and 1mm(2)) set to 28 or 36 degrees C slowly across the lubricated skin of the forearm. The procedure uncovered three types of temperature-sensitive sites (thermal, bimodal, and nociceptive) that contained one or more thermal, nociceptive, or (rarely) bimodal spots. Repeated testing indicated that bimodal and nociceptive sites were less stable over time than thermal sites, and that mechanical contact differentially inhibited nociceptive sensations. Intensity ratings collected over a range of temperatures showed that LTN increased monotonically on heat-sensitive sites but not on cold-sensitive sites. These results provide psychophysical evidence that stimulation from primary afferent fibers with thresholds in the range of warm fibers and cold fibers is relayed to the pain pathway. However, the labile nature of LTN implies that these low-threshold nociceptive inputs are subject to inhibitory controls. The implications of these findings for the roles of putative temperature receptors and nociceptors in innocuous thermoreception and thermal pain are discussed.

Cross-desensitization of responses of rat trigeminal subnucleus caudalis neurons to cinnamaldehyde and menthol

Most cold-sensitive subnucleus caudalis (Vc) neurons are also excited by the TRPM8 agonist menthol and the TRPA1 agonist cinnamaldehyde (CA). We investigated how interactions among menthol, CA and noxious cooling and heating of the tongue affected responses of superficial Vc units recorded in thiopental-anesthetized rats. Units responded to 1% CA which enhanced cold- and heat-evoked responses 5 min later. They responded more strongly to 10% CA which initially depressed cold responses, followed by enhancement at 5 min without affecting responses to heat. Following 10% CA, the mean response to 1% menthol was significantly lower than when menthol was tested first. After menthol, the subsequent response to CA was significantly weaker compared to the mean CA-evoked response when it was tested first. These results demonstrate mutual cross-desensitization between CA and menthol. The response to CA was enhanced following prior application of 10% ethanol (menthol vehicle). Prior application of menthol did not prevent the biphasic effect of 10% CA on cold-evoked responses, nor did prior application of CA prevent menthol enhancement of cold-evoked responses. Responses to noxious heat were unaffected by 10% CA and menthol regardless of the order of chemical presentation. These data indicate that superficial Vc neurons receive convergent input from primary afferents expressing TRPM8 and TRPA1. The mutual cross-desensitization between CA and menthol, and differential modulation of cold- vs. heat-evoked responses, suggests a direct inhibition of TRPM8 and TRPA1 expressed in peripheral nerve endings by CA and menthol, respectively, rather than a central site of interaction.

Effects of TRPA1 agonists mustard oil and cinnamaldehyde on lumbar spinal wide-dynamic range neuronal responses to innocuous and noxious cutaneous stimuli in rats

Mustard oil [allyl isothiocyanate (AITC)] and cinnamaldehyde (CA), agonists of the ion channel TRPA1 expressed in sensory neurons, elicit a burning sensation and heat hyperalgesia. We tested whether these phenomena are reflected in the responses of lumbar spinal wide-dynamic range (WDR) neurons recorded in pentobarbital-anesthetized rats. Responses to electrical and graded mechanical and noxious thermal stimulation were tested before and after cutaneous application of AITC or CA. Repetitive application of AITC initially increased the firing rate of 52% of units followed by rapid desensitization that persisted when AITC was reapplied 30 min later. Responses to noxious thermal, but not mechanical, stimuli were significantly enhanced irrespective of whether the neuron was directly activated by AITC. Windup elicited by percutaneous or sciatic nerve electrical stimulation was significantly reduced post-AITC. These results indicate that AITC produced central inhibition and peripheral sensitization of heat nociceptors. CA did not directly excite WDR neurons, and significantly enhanced responses to noxious heat while not affecting windup or responses to skin cooling or mechanical stimulation, indicating a peripheral sensitization of heat nociceptors.