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

Ocular surface information seen from the somatosensory thalamus and cortex

Ocular Surface (OS) somatosensory innervation detects external stimuli producing perceptions, such as pain or dryness, the most relevant symptoms in many OS pathologies. Nevertheless, little is known about the central nervous system circuits involved in these perceptions, and how they integrate multimodal inputs in general. Here, we aim to describe the thalamic and cortical activity in response to OS stimulation of different modalities. Electrophysiological extracellular recordings in anaesthetized rats were used to record neural activity, while saline drops at different temperatures were applied to stimulate the OS. Neurons were recorded in the ophthalmic branch of the trigeminal ganglion (TG, 49 units), the thalamic VPM-POm nuclei representing the face (Th, 69 units) and the primary somatosensory cortex (S1, 101 units). The precise locations for Th and S1 neurons receiving OS information are reported here for the first time. Interestingly, all recorded nuclei encode modality both at the single neuron and population levels, with noxious stimulation producing a qualitatively different activity profile from other modalities. Moreover, neurons responding to new combinations of stimulus modalities not present in the peripheral TG subsequently appear in Th and S1, being organized in space through the formation of clusters. Besides, neurons that present higher multimodality display higher spontaneous activity. These results constitute the first anatomical and functional characterization of the thalamocortical representation of the OS. Furthermore, they provide insight into how information from different modalities gets integrated from the peripheral nervous system into the complex cortical networks of the brain. KEY POINTS: Anatomical location of thalamic and cortical ocular surface representation. Thalamic and cortical neuronal responses to multimodal stimulation of the ocular surface. Increasing functional complexity along trigeminal neuroaxis. Proposal of a new perspective on how peripheral activity shapes central nervous system function.

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.

Tear secretion by Diquafosol suppresses the excitability of trigeminal brainstem nuclear complex neurons by reducing excessive P2Y2 expression in the trigeminal ganglion in dry eye rats

The P2Y₂ receptor agonist, diquafosol sodium, is commonly used to treat the signs and symptoms of dry eye disease (DE) patients. Although diquafosol improves tear film stability, the neural mechanisms underlying the reduction in ocular pain are not well defined. This study determined if repeated application of diquafosol reduces the sensitization of nociceptive neurons in the lower trigeminal brainstem nuclear complex (TBNC) via peripheral P2Y₂ mechanisms in a rat model for DE. Diquafosol was applied to the ocular surface daily for 28 days, starting at day 0 or day 14, after exorbital gland removal. The number of eyeblinks, P2Y₂-immunoreactive neurons in the trigeminal ganglion (TG), and correlates of TBNC neural excitability (i.e., cFos protein and phosphorylated extracellular signal-regulated kinase (pERK) expression) were assessed in male rats. Diquafosol increased spontaneous tear volume and reduced the number of ocular surface-evoked eyeblinks in DE rats. Fluorogold-labeled TG neurons that supply the cornea expressed P2Y₂. The number of P2Y₂-immunoreactive neurons was increased in DE rats and suppressed by diquafosol. Diquafosol also reduced the number of cFos- and pERK-immunoreactive neurons in the TBNC in DE rats. These findings suggest that diquafosol, regardless of late-phase treatment, relieves ocular nociception in DE by reducing peripheral P2Y₂ expression.

Validation of Six Commercial Antibodies for the Detection of Heterologous and Endogenous TRPM8 Ion Channel Expression

TRPM8 is a non-selective cation channel expressed in primary sensory neurons and other tissues, including the prostate and urothelium. Its participation in different physiological and pathological processes such as thermoregulation, pain, itch, inflammation and cancer has been widely described, making it a promising target for therapeutic approaches. The detection and quantification of TRPM8 seems crucial for advancing the knowledge of the mechanisms underlying its role in these pathophysiological conditions. Antibody-based techniques are commonly used for protein detection and quantification, although their performance with many ion channels, including TRPM8, is suboptimal. Thus, the search for reliable antibodies is of utmost importance. In this study, we characterized the performance of six TRPM8 commercial antibodies in three immunodetection techniques: Western blot, immunocytochemistry and immunohistochemistry. Different outcomes were obtained for the tested antibodies; two of them proved to be successful in detecting TRPM8 in the three approaches while, in the conditions tested, the other four were acceptable only for specific techniques. Considering our results, we offer some insight into the usefulness of these antibodies for the detection of TRPM8 depending on the methodology of choice.

Role of ocular surface neurobiology in neuronal-mediated inflammation in dry eye disease

Inflammation is the consequence of dry eye disease regardless of its etiology. Several injurious or harmless processes to the ocular surface neurons promote ocular surface neurogenic inflammation, leading to the vicious cycle of dry eye disease. These processes include the regular release of neuromediators during the conduction of ocular surface sensations, hyperosmolarity-induced ocular surface neuronal damage, neuro-regenerative activities, and neuronal-mediated dendritic cell activities. Neurogenic inflammation appears to be the main culprit, instigating the self-perpetuating inflammation observed in patients with dry eye disease.

Transient Receptor Potential Channels: Important Players in Ocular Pain and Dry Eye Disease

Dry eye disease (DED) is a multifactorial disorder in which the eyes respond to minor stimuli with abnormal sensations, such as dryness, blurring, foreign body sensation, discomfort, irritation, and pain. Corneal pain, as one of DED’s main symptoms, has gained recognition due to its increasing prevalence, morbidity, and the resulting social burden. The cornea is the most innervated tissue in the body, and the maintenance of corneal integrity relies on a rich density of nociceptors, such as polymodal nociceptor neurons, cold thermoreceptor neurons, and mechano-nociceptor neurons. Their sensory responses to different stimulating forces are linked to the specific expression of transient receptor potential (TRP) channels. TRP channels are a group of unique ion channels that play important roles as cellular sensors for various stimuli. These channels are nonselective cation channels with variable Ca²⁺ selectivity. TRP homologs are a superfamily of 28 different members that are subdivided into 7 different subfamilies based on differences in sequence homology. Many of these subtypes are expressed in the eye on both neuronal and non-neuronal cells, where they affect various stress-induced regulatory responses essential for normal vision maintenance. This article reviews the current knowledge about the expression, function, and regulation of TRPs in ocular surface tissues. We also describe their implication in DED and ocular pain. These findings contribute to evidence suggesting that drug-targeting TRP channels may be of therapeutic benefit in the clinical setting of ocular pain.

Lacripep for the Treatment of Primary Sjögren's-Associated Ocular Surface Disease: Results of the First-In-Human Study

PURPOSE

The purpose of this study was to assess the safety, tolerability, dosing, and efficacy of the active 19 amino acid fragment of lacritin (Lacripep), a broad regulator of ocular surface homeostasis, in the treatment of ocular surface disease associated with primary Sjögren's syndrome.

METHODS

Two hundred four subjects were randomized to receive vehicle, 22 μM Lacripep, or 44 μM Lacripep 3 times daily for 28 days, preceded by a 14-day run-in and followed by 14-day washout. Outcome measures were corneal fluorescein staining (CFS), lissamine conjunctival staining, Schirmer with anesthesia, tear break-up time, SANDE scoring, and visual analog scale assessment of symptoms.

RESULTS

This study established the safety and tolerability of topical treatment with Lacripep in patients with primary Sjögren's syndrome. There were few adverse events: Only mild irritation was found in less than 3 percent of patients dosed with Lacripep. Total CFS and Eye Dryness Score were not significantly changed at day 28. Post hoc analysis of patients with Eye Dryness Severity scores of 60 or greater at baseline revealed significant improvements in inferior CFS at 14 and 28 days and complaints of burning and stinging at 14 days. Significant improvement in regional lissamine conjunctival staining was seen at 14 and 28 days.

CONCLUSIONS

This first-in-human study of Lacripep in patients with primary Sjögren's syndrome demonstrated clinically significant improvements in specific signs and symptoms on which to base future studies. This study established safety and tolerability and potential metrics of efficacy in patients with moderate to severe disease. Further work on appropriate dosing and concentration is ongoing.

Corneal nerves and their role in dry eye pathophysiology

As the cornea is densely innervated, its nerves are integral not only to its structure but also to its pathophysiology. Corneal integrity depends on a protective tear film that is maintained by corneal sensation and the reflex arcs that control tearing and blinking. Furthermore, corneal nerves promote epithelial growth and local immunoregulation. Thus, corneal nerves constitute pillars of ocular surface homeostasis. Conversely, the abnormal tear film in dry eye favors corneal epithelial and nerve damage. The ensuing corneal nerve dysfunction contributes to dry eye progression, ocular pain and discomfort, and other neuropathic symptoms. Recent evidence from clinical studies and animal models highlight the significant but often overlooked neural dimension of dry eye pathophysiology. Herein, we review the anatomy and physiology of corneal nerves before exploring their role in the mechanisms of dry eye disease.

Acute Inhibitory Effects of Antidepressants on Lacrimal Gland Secretion in the Anesthetized Rat

Purpose

Patients that medicate with antidepressants commonly report dryness of eyes. The cause is often attributed to the anticholinergic properties of the drugs. However, regulation of tear production includes a substantial reflex-evoked component and is regulated via distinct centers in the brain. Further, the anticholinergic component varies greatly among antidepressants with different mechanisms of action. In the current study it was wondered if acute administration of antidepressants can disturb production of tears by affecting the afferent and/or central pathway.

Methods

Tear production was examined in vivo in anesthetized rats in the presence or absence of the tricyclic antidepressant (TCA) clomipramine or the selective serotonin reuptake inhibitor (SSRI) escitalopram. The reflex-evoked production of tears was measured by challenging the surface of the eye with menthol (0.1 mM) and cholinergic regulation was examined by intravenous injection with the nonselective muscarinic agonist methacholine (1–5 µg/kg).

Results

Acute administration of clomipramine significantly attenuated both reflex-evoked and methacholine-induced tear production. However, escitalopram only attenuated reflex-evoked tear production, while methacholine-induced production of tears remained unaffected.

Conclusions

This study shows that antidepressants with different mechanisms of action can impair tear production by attenuating reflex-evoked signaling. Further, antimuscarinic actions are verified as a likely cause of lacrimal gland hyposecretion in regard to clomipramine but not escitalopram. Future studies on antidepressants with different selectivity profiles and mechanisms of action are required to further elucidate the mechanisms by which antidepressants affect tear production.

Efficacy and Safety of 1% Progesterone Gel to the Forehead for Ocular Chronic Graft-versus-Host Disease

There is no Food and Drug Administration-approved treatments for ocular chronic graft-versus-host disease (oGVHD) to date, and current therapeutic options are limited. Forehead application of 1% progesterone gel provides corneal antinociception in preclinical models, suggesting it may be useful in alleviating ocular irritations. This study was conducted to evaluate the efficacy and safety of 1% progesterone gel in treating moderate to severe symptomatic oGVHD. Thirty-three patients with oGVHD following allogeneic stem cell transplantation were enrolled in this single-center, sponsor-initiated, prospective exploratory randomized double-masked placebo-controlled phase II clinical trial. The inclusion criteria included a National Institutes of Health consensus score of ≥2, moderate to severe ocular discomfort level, and receipt of a stable immunosuppression regimen. Twenty-one of the 22 patients in the progesterone arm and all 11 patients in the placebo arm completed the course of twice-daily forehead drug application for 10 weeks. The changes from baseline of self-reported ocular symptom scores and physician-recorded cornea fluorescein staining scores were analyzed using mixed-model repeated-measures regression model in an intention-to-treat population. The 33 patients included 12 women and 21 men, with a median age of 66 years (range, 24 to 75 years). At 10 weeks, there was a significant reduction in ocular symptoms from baseline in the progesterone group compared with the placebo group in symptom frequency (-30.7 versus -2.2; P < .001) and severity (-19.8 versus +1.6; P = .005). At 10 weeks, there was also greater reduction of cornea fluorescein staining centrally (-1.2 versus +.1; P = .001) and inferiorly (-1.4 versus -0.2; P = .005). No difference was noted in superior cornea staining. There were no severe adverse events in the progesterone group. Forehead application of 1% progesterone gel significantly improved ocular signs and symptoms within 10 weeks. It appears to be a safe and effective new therapy for oGVHD, and a novel mechanism for neuroaxis drug delivery. A multicenter phase III clinical trial is planned for further validation.

Morphological and Functional Changes of Corneal Nerves and Their Contribution to Peripheral and Central Sensory Abnormalities

The cornea is the most densely innervated and sensitive tissue in the body. The cornea is exclusively innervated by C- and A-delta fibers, including mechano-nociceptors that are triggered by noxious mechanical stimulation, polymodal nociceptors that are excited by mechanical, chemical, and thermal stimuli, and cold thermoreceptors that are activated by cooling. Noxious stimulations activate corneal nociceptors whose cell bodies are located in the trigeminal ganglion (TG) and project central axons to the trigeminal brainstem sensory complex. Ocular pain, in particular, that driven by corneal nerves, is considered to be a core symptom of inflammatory and traumatic disorders of the ocular surface. Ocular surface injury affecting corneal nerves and leading to inflammatory responses can occur under multiple pathological conditions, such as chemical burn, persistent dry eye, and corneal neuropathic pain as well as after some ophthalmological surgical interventions such as photorefractive surgery. This review depicts the morphological and functional changes of corneal nerve terminals following corneal damage and dry eye disease (DED), both ocular surface conditions leading to sensory abnormalities. In addition, the recent fundamental and clinical findings of the importance of peripheral and central neuroimmune interactions in the development of corneal hypersensitivity are discussed. Next, the cellular and molecular changes of corneal neurons in the TG and central structures that are driven by corneal nerve abnormalities are presented. A better understanding of the corneal nerve abnormalities as well as neuroimmune interactions may contribute to the identification of a novel therapeutic targets for alleviating corneal pain.

TRPM8: A Therapeutic Target for Neuroinflammatory Symptoms Induced by Severe Dry Eye Disease

Dry eye disease (DED) is commonly associated with ocular surface inflammation and pain. In this study, we evaluated the effectiveness of repeated instillations of transient receptor potential melastatin 8 (TRPM8) ion channel antagonist M8-B on a mouse model of severe DED induced by the excision of extra-orbital lacrimal and Harderian glands. M8-B was topically administered twice a day from day 7 until day 21 after surgery. Cold and mechanical corneal sensitivities and spontaneous ocular pain were monitored at day 21. Ongoing and cold-evoked ciliary nerve activities were next evaluated by electrophysiological multi-unit extracellular recording. Corneal inflammation and expression of genes related to neuropathic pain and inflammation were assessed in the trigeminal ganglion. We found that DED mice developed a cold allodynia consistent with higher TRPM8 mRNA expression in the trigeminal ganglion (TG). Chronic M8-B instillations markedly reversed both the corneal mechanical allodynia and spontaneous ocular pain commonly associated with persistent DED. M8-B instillations also diminished the sustained spontaneous and cold-evoked ciliary nerve activities observed in DED mice as well as inflammation in the cornea and TG. Overall, our study provides new insight into the effectiveness of TRPM8 blockade for alleviating corneal pain syndrome associated with severe DED, opening a new avenue for ocular pain management.

Role of transient receptor potential melastatin 8 activity in menthol-induced cold sensitivity and its qualitative perception in dry eye

PURPOSE

To evaluate the role of transient receptor potential melastatin 8 (TRPM8) activity in menthol-induced cold sensitivity and its qualitative perception in patients with dry eye (DE).

METHODS

This prospective, cross-sectional, comparative study included 52 eyes of 52 subjects (mean age: 66.8 ± 9.2 years; range: 44-86) with a tear break-up time (TBUT) of ≤5 s. The participants were classified into three groups: 17 patients with DE symptoms and keratoconjunctival (KC) staining scores of ≥3 points (positive KC-DE group), 18 patients with DE symptoms and KC staining scores of <3 points (negative KC-DE group), and 17 individuals with KC staining scores of <3 points and no symptoms (non-DE control group). The menthol-induced cool sensation (M-cool) and TBUT were measured after administration of 2 μl of 1.0 mM menthol eye drops. Furthermore, participants answered a questionnaire regarding their stimulus perception (pleasant, unpleasant, or neither).

RESULTS

M-cool values were similar in the three groups. TBUT significantly increased in the negative KC-DE and control groups (P < 0.05) and remained unchanged in the positive KC-DE group (P > 0.05) after menthol administration. DE patients reported the sensation as pleasant or unpleasant, whereas most control participants were indifferent (P < 0.05).

CONCLUSIONS

While M-cold sensitivity was similar in DE and control groups, its qualitative perception differed between these groups. Thus, TRPM8 activation at the peripheral level alone may not be sufficient to account for the manifestation of discomfort symptoms associated with DE.

Lacrimal gland excision in male and female mice causes ocular pain and anxiety-like behaviors

Lacrimal gland excision (LGE) induced dry eye produces more severe corneal damage in female mice, yet signs of LGE-induced ocular pain and anxiety in male and female mice have not been characterized. Excision of either the extraorbital gland (single LGE), or both the extraorbital and intraorbital glands (double LGE) was performed in male and female C57BL/6J mice to induce moderate and severe dry eye. Ongoing pain was assessed by quantifying palpebral opening and evoked nociceptive responses after corneal application of capsaicin and menthol. The open-field and plus maze were used to assess anxiety. Single LGE caused a reduction in palpebral opening and an increase in capsaicin and menthol-evoked responses only in female mice. Furthermore, single LGE produced signs of increased anxiety in female but not male mice. Overall, female mice appear more susceptible to signs of ocular pain, irritation, and anxiety in response to aqueous tear deficiency.

Improving Tolerance and Compliance with Topical Immunomodulators Using Micro-Emulsion Lipid Layer Artificial Tears

Purpose

To investigate the efficacy of a micro-emulsion (ME) lipid layer artificial tear in improving tolerance of immunomodulator eye drops for the treatment of dry eye disease.

Patients and Methods

A total of 33 patients with previously diagnosed dry eye disease were given the micro-emulsion lipid layer artificial tear in conjunction with either lifitegrast or cyclosporine. Patients were queried on their tolerance of the regimen by reporting VAS scores before starting the adjunctive eye drop, immediately after starting, and 2–3 weeks later. Tolerance was statistically compared over time and with respect to previous medication compliance, timing of the adjunctive eye drop, age, gender, and ethnicity.

Results

Across all patients, the VAS pre-treatment score (6.8 ± 0.6) was significantly higher than both the VAS 1-day post ME lipid tear instillation time point (3.0 ± 0.7) (post hoc Bonferroni, p < 0.001) and the VAS 2–3-week post instillation time point (1.7 ± 0.7) (post hoc Bonferroni, p < 0.001), with the mean VAS score improving over time (post hoc Bonferroni, p < 0.028). Average VAS scores did not vary with respect to specific medical therapy or the timing of instillation of this artificial tear. Both the “at-risk” and “conversion” groups independently had significant improvements at both 1-day and 2–3-week time points compared to baseline.

Conclusion

The micro-emulsion lipid layer artificial tear was effective as an adjunctive eye drop to improve tolerance of lifitegrast and cyclosporine for patients with dry eye disease who were at risk of failing or had previously failed immunomodulatory therapy.

Novel RvD6 stereoisomer induces corneal nerve regeneration and wound healing post-injury by modulating trigeminal transcriptomic signature

The high-density corneal innervation plays a pivotal role in sustaining the integrity of the ocular surface. We have previously demonstrated that pigment epithelium-derived factor (PEDF) plus docosahexaenoic acid (DHA) promotes corneal nerve regeneration; here, we report the mechanism involved and the discovery of a stereospecific Resolvin D6-isomer (RvD6si) that drives the process. RvD6si promotes corneal wound healing and functional recovery by restoring corneal innervation after injury. RvD6si applied to the eye surface elicits a specific transcriptome signature in the trigeminal ganglion (TG) that includes Rictor, the rapamycin-insensitive complex-2 of mTOR (mTORC2), and genes involved in axon growth, whereas genes related to neuropathic pain are decreased. As a result, attenuation of ocular neuropathic pain and dry eye will take place. Thus, RvD6si opens up new therapeutic avenues for pathologies that affect corneal innervation.

TRPV1 activity and substance P release are required for corneal cold nociception

As a protective mechanism, the cornea is sensitive to noxious stimuli. Here, we show that in mice, a high proportion of corneal TRPM8⁺ cold-sensing fibers express the heat-sensitive TRPV1 channel. Despite its insensitivity to cold, TRPV1 enhances membrane potential changes and electrical firing of TRPM8⁺ neurons in response to cold stimulation. This elevated neuronal excitability leads to augmented ocular cold nociception in mice. In a model of dry eye disease, the expression of TRPV1 in TRPM8⁺ cold-sensing fibers is increased, and results in severe cold allodynia. Overexpression of TRPV1 in TRPM8⁺ sensory neurons leads to cold allodynia in both corneal and non-corneal tissues without affecting their thermal sensitivity. TRPV1-dependent neuronal sensitization facilitates the release of the neuropeptide substance P from TRPM8⁺ cold-sensing neurons to signal nociception in response to cold. Our study identifies a mechanism underlying corneal cold nociception and suggests a potential target for the treatment of ocular pain.

The eye shows protective responses to noxious stimuli including cold. Here, the authors show that TRPV1, found co-expressed on TRPM8 + fibres in the cornea, is necessary for cold nociception in the eye.

Progesterone Application to the Rat Forehead Produces Corneal Antinociception

Purpose

Ocular pain and discomfort are the most defining symptoms of dry eye disease. We determined the ability of topical progesterone to affect corneal sensitivity and brainstem processing of nociceptive inputs.

Methods

Progesterone or vehicle gel was applied to the shaved forehead in male Sprague Dawley rats. As a site control, gel also was applied to the cheek on the side contralateral to corneal stimulation. Corneal mechanical thresholds were determined using the Cochet-Bonnet esthesiometer in intact and lacrimal gland excision–induced dry eye animals. Eye wipe behaviors in response to hypertonic saline and capsaicin were examined, and corneal mustard oil-induced c-Fos immunohistochemistry was quantified in the brainstem spinal trigeminal nucleus.

Results

Progesterone gel application to the forehead, but not the contralateral cheek, increased corneal mechanical thresholds in intact and lacrimal gland excision animals beginning <30 minutes after treatment. Subcutaneous injection of the local anesthetic bupivacaine into the forehead region before application of progesterone prevented the increase in corneal mechanical thresholds. Furthermore, progesterone decreased capsaicin-evoked eye wipe behavior in intact animals and hypertonic saline evoked eye wipe behavior in dry eye animals. The number of Fos-positive neurons located in the caudal region of the spinal trigeminal nucleus after corneal mustard oil application was reduced in progesterone-treated animals.

Conclusions

Results from this study indicate that progesterone, when applied to the forehead, produces analgesia as indicated by increased corneal mechanical thresholds and decreased nociceptive responses to hypertonic saline and capsaicin.

Ectodysplasin-A signaling is a key integrator in the lacrimal gland-cornea feedback loop

A lack of ectodysplasin-A (Eda) signaling leads to dry eye symptoms, which have so far only been associated with altered Meibomian glands. Here, we used loss-of-function (Eda ^-/-) mutant mice to unravel the impact of Eda signaling on lacrimal gland formation, maturation and subsequent physiological function. Our study demonstrates that Eda activity is dispensable during lacrimal gland embryonic development. However, using a transcriptomic approach, we show that the Eda pathway is necessary for proper cell terminal differentiation in lacrimal gland epithelium and correlated with modified expression of secreted factors commonly found in the tear film. Finally, we discovered that lacrimal glands present a bilateral reduction of Eda signaling activity in response to unilateral corneal injury. This observation hints towards a role for the Eda pathway in controlling the switch from basal to reflex tears, to support corneal wound healing. Collectively, our data suggest a crucial implication of Eda signaling in the cornea-lacrimal gland feedback loop, both in physiological and pathophysiological conditions. Our findings demonstrate that Eda downstream targets could help alleviate dry eye symptoms.

The Effects of Diabetes and High-Fat Diet on Polymodal Nociceptor and Cold Thermoreceptor Nerve Terminal Endings in the Corneal Epithelium

Purpose

There is a substantial body of evidence indicating that corneal sensory innervation is affected by pathology in a range of diseases. However, there are no published studies that have directly assessed whether the nerve fiber density of the different subpopulations of corneal sensory neurons are differentially affected. The present study explored the possibility that the intraepithelial nerve fiber density of corneal polymodal nociceptors and cold thermoreceptors are differentially affected in mice fed with a high-fat high cholesterol (HFHC; 21% fat, 2% cholesterol) diet and in those that also have diabetes.

Methods

The mice were fed the HFHC diet for the duration of the experiment (up to 40 weeks). Mice in the diabetes group had hyperglycaemia induced with streptozotocin after 15 weeks on the HFHC diet. Age-matched control animals were fed a standard diet. All corneal nerve fibers were labeled with a pan neuronal antibody (antiprotein gene product 9.5), and polymodal nociceptors and cold thermoreceptors were labeled with antibodies directed against transient receptor potential cation channel, subfamily V, member 1 and transient receptor potential cation channel subfamily M member 8, respectively.

Results

The mice fed a HFHC diet and those that in addition have hyperglycemia have similar reductions in corneal nerve fiber density consistent with small fiber neuropathy. Importantly, both treatments more markedly affected the intraepithelial axons of cold thermoreceptors than those of polymodal nociceptors.

Conclusions

The results provide evidence that distinct subpopulations of corneal sensory neurons can be differentially affected by pathology.

Lacrimal Gland Denervation Alters Tear Protein Composition and Impairs Ipsilateral Eye Closures and Corneal Nociception

Purpose

To evaluate spontaneous and evoked ocular sensory responses in rats after denervation of the lacrimal gland, as well as protein changes in tears that may mediate functional changes.

Methods

Sprague-Dawley rats served as subjects. The left lacrimal gland was partially denervated with saporin toxin conjugated to p75. Unilateral and bilateral eye closures (winks and blinks) and grooming behaviors were measured weekly. Nociceptive responses were evoked by ocular application of menthol; tear production was assessed using the phenol thread test. Relative changes in tear protein abundances were measured using a Tandem Mass Tagging approach.

Results

Denervation of the lacrimal gland reduced eye closure behavior, particularly in the ipsilateral eye, and eye wipe responses to noxious menthol were also reduced. Tear volume did not change, but tear protein composition was altered. Proteins implicated in the structural integrity of epithelial cells and in protective functions were reduced by lacrimal denervation, including keratins, serotransferrin, and beta-defensin. Other proteins that may modulate TRPM8 channels and alter sensory neuronal function were reduced, including arachidonate 15-lipoxygenase B. A low-abundance protein that responds to oxidative stress and injury, proteasome subunit beta type 10, was upregulated in denervated rats.

Conclusions

Denervation of the lacrimal gland causes long-lasting hypoalgesia, impairs the blink response, and alters tear proteins. Tear proteins were altered without changing tear volume. We speculate that impaired TRPM8 function in corneal sensory nerves may contribute to ocular hypoalgesia, supporting growing evidence that this transduction molecule is important for both nociceptive and spontaneous blinking behaviors.

Dry eye sensitizes cool cells to capsaicin-induced changes in activity via TRPV1

Corneal cool cells are sensitive to the ocular fluid status of the corneal surface and may be responsible for the regulation of basal tear production. Previously, we have shown that dry eye, induced by lacrimal gland excision (LGE) in rats, sensitized corneal cool cells to the transient receptor potential melastatin 8 (TRPM8) agonist menthol and to cool stimulation. In the present study, we examined the effect of dry eye on the sensitivity of cool cells to the transient receptor potential vanilloid 1 (TRPV1) agonist capsaicin. Single-unit recordings in the trigeminal ganglion were performed 7-10 days after LGE. At a concentration of 0.3 μM, capsaicin did not affect ongoing or cool-evoked activity in control animals yet facilitated ongoing activity and suppressed cool-evoked activity in LGE animals. At higher concentrations (3 μM), capsaicin continued to facilitate ongoing activity in LGE animals but suppressed ongoing activity in control animals. Higher concentrations of capsaicin also suppressed cool-evoked activity in both groups of animals, with an overall greater effect in LGE animals. In addition to altering cool-evoked activity, capsaicin enhanced the sensitivity of cool cells to heat in LGE animals. Capsaicin-induced changes were prevented by the application of the TRPV1 antagonist capsazepine. With the use of fluorescent in situ hybridization, TRPV1 and TRPM8 expression was examined in retrograde tracer-identified corneal neurons. The coexpression of TRPV1 and TRPM8 in corneal neurons was significantly greater in LGE-treated animals when compared with sham controls. These results indicate that LGE-induced dry eye increases TRPV1-mediated responses in corneal cool cells at least in part through the increased expression of TRPV1. NEW & NOTEWORTHY Corneal cool cells are known to detect drying of the ocular surface. Our study is the first to report that dry eye induced alterations in cool cell response properties, including the increased responsiveness to noxious heat and activation by capsaicin. Along with the changes in cell response properties, it is possible these neurons also function differently in dry eye, relaying information related to the perception of ocular irritation in addition to regulating tearing and blinking.

The Immunosuppressant Macrolide Tacrolimus Activates Cold-Sensing TRPM8 Channels

TRPM8 is a polymodal, nonselective cation channel activated by cold temperature and cooling agents that plays a critical role in the detection of environmental cold. We found that TRPM8 is a pharmacological target of tacrolimus (FK506), a macrolide immunosuppressant with several clinical uses, including the treatment of organ rejection following transplants, treatment of atopic dermatitis, and dry eye disease. Tacrolimus is an inhibitor of the phosphatase calcineurin, an action shared with cyclosporine. Tacrolimus activates TRPM8 channels in different species, including humans, and sensitizes their response to cold temperature by inducing a leftward shift in the voltage-dependent activation curve. The effects of tacrolimus on purified TRPM8 in lipid bilayers demonstrates conclusively that it has a direct gating effect. Moreover, the lack of effect of cyclosporine rules out the canonical signaling pathway involving the phosphatase calcineurin. Menthol (TRPM8-Y745H)- and icilin (TRPM8-N799A)-insensitive mutants were also activated by tacrolimus, suggesting a different binding site. In cultured mouse DRG neurons, tacrolimus evokes an increase in intracellular calcium almost exclusively in cold-sensitive neurons, and these responses were drastically blunted in Trpm8 KO mice or after the application of TRPM8 antagonists. Cutaneous and corneal cold thermoreceptor endings are also activated by tacrolimus, and tacrolimus solutions trigger blinking and cold-evoked behaviors. Together, our results identify TRPM8 channels in sensory neurons as molecular targets of the immunosuppressant tacrolimus. The actions of tacrolimus on TRPM8 resemble those of menthol but likely involve interactions with other channel residues.SIGNIFICANCE STATEMENT TRPM8 is a polymodal TRP channel involved in cold temperature sensing, thermoregulation, and cold pain. TRPM8 is also involved in the pathophysiology of dry eye disease, and TRPM8 activation has antiallodynic and antipruritic effects, making it a prime therapeutic target in several cutaneous and neural diseases. We report the direct agonist effect of tacrolimus, a potent natural immunosuppressant with multiple clinical applications, on TRPM8 activity. This interaction represents a novel neuroimmune interface. The identification of a clinically approved drug with agonist activity on TRPM8 channels could be used experimentally to probe the function of TRPM8 in humans. Our findings may explain some of the sensory and anti-inflammatory effects described for this drug in the skin and the eye surface.

TRPM8 Channels and Dry Eye

Transient receptor potential (TRP) channels transduce signals of chemical irritation and temperature change from the ocular surface to the brain. Dry eye disease (DED) is a multifactorial disorder wherein the eyes react to trivial stimuli with abnormal sensations, such as dryness, blurring, presence of foreign body, discomfort, irritation, and pain. There is increasing evidence of TRP channel dysfunction (i.e., TRPV1 and TRPM8) in DED pathophysiology. Here, we review some of this literature and discuss one strategy on how to manage DED using a TRPM8 agonist.

TRPM8 Activation via 3-Iodothyronamine Blunts VEGF-Induced Transactivation of TRPV1 in Human Uveal Melanoma Cells

In human uveal melanoma (UM), tumor enlargement is associated with increases in aqueous humor vascular endothelial growth factor-A (VEGF-A) content that induce neovascularization. 3-Iodothyronamine (3-T₁AM), an endogenous thyroid hormone metabolite, activates TRP melastatin 8 (TRPM8), which blunts TRP vanilloid 1 (TRPV1) activation by capsaicin (CAP) in human corneal, conjunctival epithelial cells, and stromal cells. We compare here the effects of TRPM8 activation on VEGF-induced transactivation of TRPV1 in an UM cell line (92.1) with those in normal primary porcine melanocytes (PM) since TRPM8 is upregulated in melanoma. Fluorescence Ca²⁺-imaging and planar patch-clamping characterized functional channel activities. CAP (20 μM) induced Ca²⁺ transients and increased whole-cell currents in both the UM cell line and PM whereas TRPM8 agonists, 100 μM menthol and 20 μM icilin, blunted such responses in the UM cells. VEGF (10 ng/ml) elicited Ca²⁺ transients and augmented whole-cell currents, which were blocked by capsazepine (CPZ; 20 μM) but not by a highly selective TRPM8 blocker, AMTB (20 μM). The VEGF-induced current increases were not augmented by CAP. Both 3-T₁AM (1 μM) and menthol (100 μM) increased the whole-cell currents, whereas 20 μM AMTB blocked them. 3-T₁AM exposure suppressed both VEGF-induced Ca²⁺ transients and increases in underlying whole-cell currents. Taken together, functional TRPM8 upregulation in UM 92.1 cells suggests that TRPM8 is a potential drug target for suppressing VEGF induced increases in neovascularization and UM tumor growth since TRPM8 activation blocked VEGF transactivation of TRPV1.

Advances in Functional Restoration of the Lacrimal Glands

The lacrimal glands produce tears to support a healthy homeostatic environment on the ocular surface. The lacrimal gland dysfunction characteristic of dry eye disease causes ocular discomfort and visual disturbances and in severe cases can result in a loss of vision. The demand for adequate restoration of lacrimal gland function has been intensified due to advances in stem cell biology, developmental biology, and bioengineering technologies. In addition to conventional therapies, including artificial tears, tear alternatives (such as autologous serum eye drops) and salivary gland transplantation, a regenerative medicine approach has been identified as a novel strategy to restore the function of the lacrimal gland. Recent studies have demonstrated the potential of progenitor cell injection therapy to repair the tissue of the lacrimal glands. A current three-dimensional (3D) tissue engineering technique has been shown to regenerate a secretory gland structure by reproducing reciprocal epithelial-mesenchymal interactions during ontogenesis in vitro and in vivo. A novel direct reprogramming method has suggested a possibility to induce markers in the lacrimal gland developmental process from human pluripotent stem cells. The development of this method is supported by advances in our understanding of gene expression and regulatory networks involved in the development and differentiation of the lacrimal glands. Engineering science has proposed a medical device to stimulate tearing and a bio-hybrid scaffold to reconstruct the 3D lacrimal gland structure. In this review, we will summarize recent bioengineering advances in lacrimal gland regeneration toward the functional restoration of the lacrimal glands as a future dry eye therapy.

3-Iodothyronamine Activates a Set of Membrane Proteins in Murine Hypothalamic Cell Lines

3-Iodothyronamine (3-T1AM) is an endogenous thyroid hormone metabolite. The profound pharmacological effects of 3-T1AM on energy metabolism and thermal homeostasis have raised interest to elucidate its signaling properties in tissues that pertain to metabolic regulation and thermogenesis. Previous studies identified G protein-coupled receptors (GPCRs) and transient receptor potential channels (TRPs) as targets of 3-T1AM in different cell types. These two superfamilies of membrane proteins are largely expressed in tissue which influences energy balance and metabolism. As the first indication that 3-T1AM virtually modulates the function of the neurons in hypothalamus, we observed that intraperitoneal administration of 50 mg/kg bodyweight of 3-T1AM significantly increased the c-FOS activation in the paraventricular nucleus (PVN) of C57BL/6 mice. To elucidate the underlying mechanism behind this 3-T1AM-induced signalosome, we used three different murine hypothalamic cell lines, which are all known to express PVN markers, GT1-7, mHypoE-N39 (N39) and mHypoE-N41 (N41). Various aminergic GPCRs, which are the known targets of 3-T1AM, as well as numerous members of TRP channel superfamily, are expressed in these cell lines. Effects of 3-T1AM on activation of GPCRs were tested for the two major signaling pathways, the action of Gαs/adenylyl cyclase and Gi/o. Here, we demonstrated that this thyroid hormone metabolite has no significant effect on Gi/o signaling and only a minor effect on the Gαs/adenylyl cyclase pathway, despite the expression of known GPCR targets of 3-T1AM. Next, to test for other potential mechanisms involved in 3-T1AM-induced c-FOS activation in PVN, we evaluated the effect of 3-T1AM on the intracellular Ca2+ concentration and whole-cell currents. The fluorescence-optic measurements showed a significant increase of intracellular Ca2+ concentration in the three cell lines in the presence of 10 μM 3-T1AM. Furthermore, this thyroid hormone metabolite led to an increase of whole-cell currents in N41 cells. Interestingly, the TRPM8 selective inhibitor (10 μM AMTB) reduced the 3-T1AM stimulatory effects on cytosolic Ca2+ and whole-cell currents. Our results suggest that the profound pharmacological effects of 3-T1AM on selected brain nuclei of murine hypothalamus, which are known to be involved in energy metabolism and thermoregulation, might be partially attributable to TRP channel activation in hypothalamic cells.

Acute hyperalgesia and delayed dry eye after corneal abrasion injury

Introduction:

Corneal nerves mediate pain from the ocular surface, lacrimation, and blinking, all of which protect corneal surface homeostasis and help preserve vision. Because pain, lacrimation and blinking are rarely assessed at the same time, it is not known whether these responses and their underlying mechanisms have similar temporal dynamics after acute corneal injury.

Methods:

We examined changes in corneal nerve density, evoked and spontaneous pain, and ocular homeostasis in Sprague-Dawley male rats after a superficial epithelial injury with heptanol. We also measured changes in calcitonin gene-related peptide (CGRP), which has been implicated in both pain and epithelial repair.

Results:

Hyperalgesia was seen 24 hours after abrasion injury, while basal tear production was normal. One week after abrasion injury, pain responses had returned to baseline levels and dry eye symptoms emerged. There was no correlation between epithelial nerve density and pain responses. Expression of both ATF3 (a nerve injury marker) and CGRP increased in trigeminal ganglia 24 hours after injury when hyperalgesia was seen, and returned to normal one week later when pain behavior was normal. These molecular changes were absent in the contralateral ganglion, despite reductions in corneal epithelial nerve density in the uninjured eye. By contrast, CGRP was upregulated in peripheral corneal endings 1 week after injury, when dry eye symptoms emerged.

Conclusion:

Our results demonstrate dynamic trafficking of CGRP within trigeminal sensory nerves following corneal injury, with elevations in the ganglion correlated with pain behaviors and elevations in peripheral endings correlated with dry eye symptoms.

Morphological and functional changes in TRPM8-expressing corneal cold thermoreceptor neurons during aging and their impact on tearing in mice

Morphological and functional alterations of peripheral somatosensory neurons during the aging process lead to a decline of somatosensory perception. Here, we analyze the changes occurring with aging in trigeminal ganglion (TG), TRPM8-expressing cold thermoreceptor neurons innervating the mouse cornea, which participate in the regulation of basal tearing and blinking and have been implicated in the pathogenesis of dry eye disease (DED). TG cell bodies and axonal branches were examined in a mouse line (TRPM8^BAC -EYFP) expressing a fluorescent reporter. In 3 months old animals, about 50% of TG cold thermoreceptor neurons were intensely fluorescent, likely providing strongly fluorescent axons and complex corneal nerve terminals with ongoing activity at 34°C and low-threshold, robust responses to cooling. The remaining TRPM8⁺ corneal axons were weakly fluorescent with nonbeaded axons, sparsely ramified nerve terminals, and exhibited a low-firing rate at 34°C, responding moderately to cooling pulses as do weakly fluorescent TG neurons. In aged (24 months) mice, the number of weakly fluorescent TG neurons was strikingly high while the morphology of TRPM8⁺ corneal axons changed drastically; 89% were weakly fluorescent, unbranched, and often ending in the basal epithelium. Functionally, 72.5% of aged cold terminals responded as those of young animals, but 27.5% exhibited very low-background activity and abnormal responsiveness to cooling pulses. These morpho-functional changes develop in parallel with an enhancement of tear's basal flow and osmolarity, suggesting that the aberrant sensory inflow to the brain from impaired peripheral cold thermoreceptors contributes to age-induced abnormal tearing and to the high incidence of DED in elderly people.

Cold-sensing TRPM8 channel participates in circadian control of the brown adipose tissue

Transient receptor potential (TRP) channels are known to regulate energy metabolism, and TRPM8 has become an interesting player in this context. Here we demonstrate the role of the cold sensor TRPM8 in the regulation of clock gene and clock controlled genes in brown adipose tissue (BAT). We investigated TrpM8 temporal profile in the eyes, suprachiasmatic nucleus and BAT; only BAT showed temporal variation of TrpM8 transcripts. Eyes from mice lacking TRPM8 lost the temporal profile of Per1 in LD cycle. This alteration in the ocular circadian physiology may explain the delay in the onset of locomotor activity in response to light pulse, as compared to wild type animals (WT). Brown adipocytes from TrpM8 KO mice exhibited a larger multilocularity in comparison to WT or TrpV1 KO mice. In addition, Ucp1 and UCP1 expression was significantly reduced in TrpM8 KO mice in comparison to WT mice. Regarding circadian components, the expression of Per1, Per2, Bmal1, Pparα, and Pparβ oscillated in WT mice kept in LD, whereas in the absence of TRPM8 the expression of clock genes was reduced in amplitude and lack temporal oscillation. Thus, our results reveal new roles for TRPM8 channel: it participates in the regulation of clock and clock-controlled genes in the eyes and BAT, and in BAT thermogenesis. Since disruption of the clock machinery has been associated with many metabolic disorders, the pharmacological modulation of TRPM8 channel may become a promising therapeutic target to counterbalance weight gain, through increased thermogenesis, energy expenditure, and clock gene activation.

Dynamic Sensitivity of Corneal TRPM8 Receptors to Menthol Instillation in Dry Eye Versus Normal Subjects

PURPOSE

To assess the sensitivity of corneal cold receptors to a known transient receptor potential melastatin 8 (TRPM8) agonist, menthol, in dry eye and normals, and to determine whether factors such as disease duration or age affect responses.

METHODS

Dry eye disease (DED) (N = 33) and normal (N = 15) subjects were randomly assigned to receive Rohto^® Hydra (0.01% menthol) or Systane^® Ultra treatments (OU) in a prospective, double-blind, crossover study. DED subjects had documented disease and symptom response scores >2 on a 0- to 5-point scale. Normals had no history of DED and scores <2 on the same scale. Endpoints included mean cooling score (0 = not cool and 10 = very cool) evaluated at 0, 0.5, 1, 2, 3, and 4 min post-instillation, sum cooling scores (5 time points, range 0-60), and ocular signs and symptoms.

RESULTS

Mean (±SD) ages were similar, 62.2 ± 8.6-year (DED) versus 53.5 ± 7.6-year (normal). Corneal sensitivity scores were not different between groups. Mean cooling scores at 0.5-4 min post-menthol instillation were significantly higher in DED subjects (P ≤ 0.03). Sum cooling scores were significantly higher (P = 0.04) in DED subjects with a disease duration <10 years (N = 18, 28.3 ± 2.58) versus ≥10 years (N = 15, 20.2 ± 2.76). Age did not affect cooling response in either group.

CONCLUSION

DED subjects had greater sensitivity to cold than normal subjects. DED duration, and not age, was critical to cooling sensitivity. The finding that cooling scores were higher in subjects with DED for less than 10 years compared to more than 10 years suggests that corneal cold receptor sensitivity decreases as the duration of DED increases.

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.

A novel TRPM8 agonist relieves dry eye discomfort

Background

Physical cooling of the eye surface relieves ocular discomfort, but translating this event to drug treatment of dry eye discomfort not been studied. Here, we synthesized a water-soluble TRPM8 receptor agonist called cryosim-3 (C3, 1-diisopropylphosphorylnonane) which selectively activates TRPM8 (linked to cooling) but not TRPV1 or TRPA1 (linked to nociception) and tested C3 in subjects with mild forms of dry eye disease.

Methods

A set of 1-dialkylphosphoryalkanes were tested for activation of TRPM8, TRPV1 and TRPA1 receptors in transfected cells. The bioactivity profiles were compared by perioral, topical, and intravenous delivery to anesthetized rats. The selected lead candidate C3 or vehicle (water) was applied with a cotton gauze pad to upper eyelids of patients with dry eye disease (n = 30). Cooling sensation, tear film break-up time (TBUT), basal tear secretion, and corneal staining were evaluated. C3 was then applied four times daily for 2 weeks to patients using a pre-loaded single unit applicator containing 2 mg/mL of C3 in water (n = 20) or water only. TBUT, basal tear secretion, and corneal staining, and three questionnaires surveys of ocular discomfort (VAS scale, OSDI, and CVS symptoms) were analyzed before and at 1 and 2 weeks thereafter.

Results

C3 was a selective and potent TRPM8 agonist without TRPV1 or TRPA1 activity. In test animals, the absence of shaking behavior after C3 perioral administration made it the first choice for further study. C3 increased tear secretion in an animal model of dry eye disease and did not irritate when wiped on eyes of volunteers. C3 singly applied (2 mg/ml) produced significant cooling in <5 min, an effecting lasting 46 min with an increase in tear secretion for 60 min. C3 applied for 2 weeks also significantly increased basal tear secretion with questionnaire surveys of ocular discomfort indices clearly showing improvement of symptoms at 1 and 2 weeks. No complaints of irritation or pain were reported by any subject.

Conclusions

C3 is a promising candidate for study of TRPM8 function on the eye surface and for relief of dry eye discomfort.

Trial registration

ISRCTN24802609 and ISRCTN13359367. Registered 23 March 2015 and 2 September 2015.

Electronic supplementary material

The online version of this article (doi:10.1186/s12886-017-0495-2) contains supplementary material, which is available to authorized users.

The neural pathway for lacrimal gland tear secretion in New Zealand White rabbits

OBJECTIVE

We investigated the neural pathway for tear secretion from the lacrimal gland of New Zealand White rabbits.

METHODS

Nine healthy adult New Zealand White rabbits were randomly divided into three experimental groups, namely, an irritant-stimulated group, a non-stimulated group, and a saline-stimulated group. Sanitized dry cotton swabs with menthol were used to wipe both of the rabbits' eyelids in the irritant-stimulated group, and the non-stimulated group and saline- stimulated group were compared as controls. The animals in the three groups were killed 2h later and the expressions of c-Fos in the frontal cortex, hippocampus, hypothalamus, pons, and medulla oblongata of the rabbits were detected using immunofluorescence labeling. According to the distribution of c-Fos protein expression, 12 healthy adult New Zealand rabbits were similarly divided into three groups for retrograde tract tracing via pseudorabies virus (PRV) injection into the lacrimal gland. Immunofluorescence labeling was used to analyze PRV-infected neurons in the brains of rabbits after survival for 30h, 38h, and 46h.

RESULTS

The most c-Fos-positive immunolabeled cells were observed in the menthol-stimulated group, whereas fewer c-Fos-positive immunolabeled cells were observed in the saline-stimulated group.The non-treated group showed the least c-Fos-positive immunolabeled cells. At 30h after PRV injection, PRV-positive neurons were found only in the superior salivary nucleus of the pons (SSN). At 38h, PRV-infected neurons were observed in the lateral nucleus of the superior olive (LSO) and the medial nucleus of the superior olive (MSO). At 46h, PRV-infected neurons were found in the nucleus of the trapezoid body (Tz) and the hypothalamic paraventricular nucleus (PVN), and their distributions were dense in the LSO and MSO.

CONCLUSIONS

Menthol-induced c-Fos protein expression and PRV-mediated tract tracing suggest that in New Zealand White rabbits, the neural pathway that regulates tear secretion from the lacrimal gland proceeds from the PVN to the superior olivary complex of the pons to the SSN and finally to the lacrimal gland.

Effects of a warm compress containing menthol on the tear film in healthy subjects and dry eye patients

Menthol is thought to stimulate lacrimation via activation of cold-sensitive primary afferent neurons in the cornea. We evaluated a warm compress containing menthol as a potential treatment for dry eye by examining its effects on the tear film in healthy subjects (n = 20) and dry eye patients (n = 35). Disposable eyelid-warming steamers that either did (MH) or did not (HO) contain menthol were applied to one eye of each subject either once only for 10 min or repeatedly over 2 weeks. Single application of MH significantly increased tear meniscus volume (P = 8.6 × 10⁻⁵, P = 1.3 × 10⁻⁵) and tear film breakup time (P = 0.006, P = 0.002) as well as improved meibum condition in healthy subjects and dry eye patients, respectively. Repeated application of MH significantly increased tear meniscus volume (P = 0.004, P = 1.7 × 10⁻⁴) and tear film breakup time (P = 0.037, P = 0.010) in healthy subjects and dry eye patients, respectively. Repeated application of MH thus induced persistent increases in tear fluid volume and tear film stability in dry eye patients, suggesting that repeated use of a warm compress containing menthol is a potential novel treatment for dry eye disease.

Select noxious stimuli induce changes on corneal nerve morphology

The surface of the cornea contains the highest density of nociceptive nerves of any tissue in the body. These nerves are responsive to a variety of modalities of noxious stimuli and can signal pain even when activated by low threshold stimulation. Injury of corneal nerves can lead to altered nerve morphology, including neuropathic changes which can be associated with chronic pain. Emerging technologies that allow imaging of corneal nerves in vivo are spawning questions regarding the relationship between corneal nerve density, morphology, and function. We tested whether noxious stimulation of the corneal surface can alter nerve morphology and neurochemistry. We used concentrations of menthol, capsaicin, and hypertonic saline that evoked comparable levels of nocifensive eye wipe behaviors when applied to the ocular surface of an awake rat. Animals were sacrificed and corneal nerves were examined using immunocytochemistry and three-dimensional volumetric analyses. We found that menthol and capsaicin both caused a significant reduction in corneal nerve density as detected with β-tubulin immunoreactivity 2 hr after stimulation. Hypertonic saline did not reduce nerve density, but did cause qualitative changes in nerves including enlarged varicosities that were also seen following capsaicin and menthol stimulation. All three types of noxious stimuli caused a depletion of CGRP from corneal nerves, indicating that all modalities of noxious stimuli evoked peptide release. Our findings suggest that studies aimed at understanding the relationship between corneal nerve morphology and chronic disease may also need to consider the effects of acute stimulation on corneal nerve morphology.

Peripheral Sensory Neurons Expressing Melanopsin Respond to Light

The ability of light to cause pain is paradoxical. The retina detects light but is devoid of nociceptors while the trigeminal sensory ganglia (TG) contain nociceptors but not photoreceptors. Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are thought to mediate light-induced pain but recent evidence raises the possibility of an alternative light responsive pathway independent of the retina and optic nerve. Here, we show that melanopsin is expressed in both human and mouse TG neurons. In mice, they represent 3% of small TG neurons that are preferentially localized in the ophthalmic branch of the trigeminal nerve and are likely nociceptive C fibers and high-threshold mechanoreceptor Aδ fibers based on a strong size-function association. These isolated neurons respond to blue light stimuli with a delayed onset and sustained firing, similar to the melanopsin-dependent intrinsic photosensitivity observed in ipRGCs. Mice with severe bilateral optic nerve crush exhibit no light-induced responses including behavioral light aversion until treated with nitroglycerin, an inducer of migraine in people and migraine-like symptoms in mice. With nitroglycerin, these same mice with optic nerve crush exhibit significant light aversion. Furthermore, this retained light aversion remains dependent on melanopsin-expressing neurons. Our results demonstrate a novel light-responsive neural function independent of the optic nerve that may originate in the peripheral nervous system to provide the first direct mechanism for an alternative light detection pathway that influences motivated behavior.

Borneol Is a TRPM8 Agonist that Increases Ocular Surface Wetness

Borneol is a compound widely used in ophthalmic preparations in China. Little is known about its exact role in treating eye diseases. Here we report that transient receptor potential melastatin 8 (TRPM8) channel is a pharmacological target of borneol and mediates its therapeutic effect in the eyes. Ca²⁺ measurement and electrophysiological recordings revealed that borneol activated TRPM8 channel in a temperature- and dose-dependent manner, which was similar to but less effective than the action of menthol, an established TRPM8 agonist. Borneol significantly increased tear production in guinea pigs without evoking nociceptive responses at 25°C, but failed to induce tear secretion at 35°C. In contrast, menthol evoked tearing response at both 25 and 35°C. TRPM8 channel blockers N-(3-Aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)benzamide hydrochloride (AMTB) and N-(4-tert-butylphenyl)-4-(3-chloropyridin-2-yl)piperazine-1-carboxamide (BCTC) abolished borneol- and menthol-induced tear secretion. Borneol at micromolar concentrations did not affect the viability of human corneal epithelial cells. We conclude that borneol can activate the cold-sensing TRPM8 channel and modestly increase ocular surface wetness, which suggests it is an active compound in ophthalmic preparations and particularly useful in treating dry eye syndrome.

Denervation of the Lacrimal Gland Leads to Corneal Hypoalgesia in a Novel Rat Model of Aqueous Dry Eye Disease

PURPOSE

Some dry eye disease (DED) patients have sensitized responses to corneal stimulation, while others experience hypoalgesia. Many patients have normal tear production, suggesting that reduced tears are not always the cause of DED sensory dysfunction. In this study, we show that disruption of lacrimal innervation can produce hypoalgesia without changing basal tear production.

METHODS

Injection of a saporin toxin conjugate into the extraorbital lacrimal gland of male Sprague-Dawley rats was used to disrupt cholinergic innervation to the gland. Tear production was assessed by phenol thread test. Corneal sensory responses to noxious stimuli were assessed using eye wipe behavior. Saporin DED animals were compared to animals treated with atropine to produce aqueous DED.

RESULTS

Cholinergic innervation and acetylcholine content of the lacrimal gland were significantly reduced in saporin DED animals, yet basal tear production was normal. Saporin DED animals demonstrated normal eye wipe responses to corneal application of capsaicin, but showed hypoalgesia to corneal menthol. Corneal nerve fiber density was normal in saporin DED animals. Atropine-treated animals had reduced tear production but normal responses to ocular stimuli.

CONCLUSIONS

Because only menthol responses were impaired, cold-sensitive corneal afferents appear to be selectively altered in our saporin DED model. Hypoalgesia is not due to reduced tear production, since we did not observe hypoalgesia in an atropine DED model. Corneal fiber density is unaltered in saporin DED animals, suggesting that molecular mechanisms of nociceptive signaling may be impaired. The saporin DED model will be useful for exploring the mechanism underlying corneal hypoalgesia.

Sensitization of trigeminal brainstem pathways in a model for tear deficient dry eye

Persistent tear deficiency was sufficient to cause sensitization of neurons at multiple regions of the trigeminal brainstem and enhanced orbicularis oculi muscle activity.

Chronic dry eye disease (DE) is associated with an unstable tear film and symptoms of ocular discomfort. The characteristics of symptoms suggest a key role for central neural processing; however, little is known about central neuroplasticity and DE. We used a model for tear deficient DE and assessed effects on eye blink behavior, orbicularis oculi muscle activity (OOemg), and trigeminal brainstem neural activity in male rats. Ocular-responsive neurons were recorded at the interpolaris/caudalis transition (Vi/Vc) and Vc/upper cervical cord (Vc/C1) regions under isoflurane, whereas OOemg activity was recorded under urethane. Spontaneous tear volume was reduced by ∼50% at 14 days after exorbital gland removal. Hypertonic saline–evoked eye blink behavior in awake rats was enhanced throughout the 14 days after surgery. Saline-evoked neural activity at the Vi/Vc transition and in superficial and deep laminae at the Vc/C1 region was greatly enhanced in DE rats. Neurons from DE rats classified as wide dynamic range displayed enlarged convergent periorbital receptive fields consistent with central sensitization. Saline-evoked OOemg activity was markedly enhanced in DE rats compared with controls. Synaptic blockade at the Vi/Vc transition or the Vc/C1 region greatly reduced hypertonic saline–evoked OOemg activity in DE and sham rats. These results indicated that persistent tear deficiency caused sensitization of ocular-responsive neurons at multiple regions of the caudal trigeminal brainstem and enhanced OOemg activity. Central sensitization of ocular-related brainstem circuits is a significant factor in DE and likely contributes to the apparent weak correlation between peripheral signs of tear dysfunction and symptoms of irritation.

Challenges and Strategies for Regenerating the Lacrimal Gland

The lacrimal gland produces the aqueous component of tears, including electrolytes, peptides, and glycoproteins necessary to maintain homeostasis and optical properties of the ocular surface. Stem cells that contribute to the homeostasis of the lacrimal gland are under extensive study. It is still unclear whether such stem cells are of mesenchymal or epithelial origin. It is also possible that a unique epithelial stem cell undergoes epithelial-mesenchymal transition and contributes to the mesenchyme. Developmental studies in mice have shown that a network of growth factors contributes to epithelial-mesenchymal interaction during morphogenesis of the lacrimal gland. Recently, the developmental process was successfully recapitulated in vitro, providing a valuable tool for study of lacrimal gland development and possibly opening doors to regenerative therapy. While further studies are required to identify and appreciate the potential of lacrimal gland stem cells, advances in stem cell biology in general should become a catalyst towards developing regenerative therapy of the lacrimal gland.

Ocular transient receptor potential channel function in health and disease

Transient receptor potential (TRP) channels sense and transduce environmental stimuli into Ca(2+) transients that in turn induce responses essential for cell function and adaptation. These non-selective channels with variable Ca(2+) selectivity are grouped into seven different subfamilies containing 28 subtypes based on differences in amino acid sequence homology. Many of these subtypes are expressed in the eye on both neuronal and non-neuronal cells where they affect a host of stress-induced regulatory responses essential for normal vision maintenance. This article reviews our current knowledge about the expression, function and regulation of TRPs in different eye tissues. We also describe how under certain conditions TRP activation can induce responses that are maladaptive to ocular function. Furthermore, the possibility of an association between TRP mutations and disease is considered. These findings contribute to evidence suggesting that drug targeting TRP channels may be of therapeutic benefit in a clinical setting. We point out issues that must be more extensively addressed before it will be possible to decide with certainty that this is a realistic endeavor. Another possible upshot of future studies is that disease process progression can be better evaluated by profiling changes in tissue specific functional TRP subtype activity as well as their gene and protein expression.

Corneal sensitivity following lacrimal gland excision in the rat

PURPOSE

Dry eye disease (DED) produces ocular pain and irritation, yet a detailed characterization of ocular sensitivity in a preclinical model of DED is lacking. The aim of the present study was to assess nociceptive behaviors in an aqueous tear deficiency model of DED in the rat.

METHODS

Spontaneous blinking, corneal mechanical thresholds, and eye wipe behaviors elicited by hypertonic saline (5.0 M) were examined over a period of 8 weeks following the unilateral excision of either the exorbital lacrimal gland or of the exorbital and infraorbital lacrimal glands, and in sham surgery controls. The effect of topical proparacaine on spontaneous blinking and of systemic morphine (0.5-3.0 mg/kg, subcutaneous [SC]) on spontaneous blinking and eye wipe responses were also examined.

RESULTS

Lacrimal gland excision resulted in mechanical hypersensitivity and an increase in spontaneous blinking in the ipsilateral eye over an 8-week period that was more pronounced after infra- and exorbital gland excision. The time spent eye wiping was also enhanced in response to hypertonic saline (5.0 M) at both 1- and 8-week time-points, but only in infra- and exorbital gland excised animals. Morphine attenuated spontaneous blinking, and the response to hypertonic saline in dry eye animals and topical proparacaine application reduced spontaneous blinking down to control levels.

CONCLUSIONS

These results indicate that aqueous tear deficiency produces hypersensitivity in the rat cornea. In addition, the increase in spontaneous blinks and their reduction by morphine and topical anesthesia indicate the presence of persistent irritation elicited by the activation of corneal nociceptors.

Bioengineered Lacrimal Gland Organ Regeneration in Vivo

The lacrimal gland plays an important role in maintaining a homeostatic environment for healthy ocular surfaces via tear secretion. Dry eye disease, which is caused by lacrimal gland dysfunction, is one of the most prevalent eye disorders and causes ocular discomfort, significant visual disturbances, and a reduced quality of life. Current therapies for dry eye disease, including artificial tear eye drops, are transient and palliative. The lacrimal gland, which consists of acini, ducts, and myoepithelial cells, develops from its organ germ via reciprocal epithelial-mesenchymal interactions during embryogenesis. Lacrimal tissue stem cells have been identified for use in regenerative therapeutic approaches aimed at restoring lacrimal gland functions. Fully functional organ replacement, such as for tooth and hair follicles, has also been developed via a novel three-dimensional stem cell manipulation, designated the Organ Germ Method, as a next-generation regenerative medicine. Recently, we successfully developed fully functional bioengineered lacrimal gland replacements after transplanting a bioengineered organ germ using this method. This study represented a significant advance in potential lacrimal gland organ replacement as a novel regenerative therapy for dry eye disease. In this review, we will summarize recent progress in lacrimal regeneration research and the development of bioengineered lacrimal gland organ replacement therapy.

Ocular neuropathic pain

As the biological alarm of impending or actual tissue damage, pain is essential for our survival. However, when it is initiated and/or sustained by dysfunctional elements in the nociceptive system, it is itself a disease known as neuropathic pain. While the critical nociceptive system provides a number of protective functions, it is unique in its central role of monitoring, preserving and restoring the optical tear film in the face of evaporative attrition without which our vision would be non-functional. Meeting this existential need resulted in the evolution of the highly complex, powerful and sensitive dry eye alarm system integrated in the peripheral and central trigeminal sensory network. The clinical consequences of corneal damage to these nociceptive pathways are determined by the type and location of its pathological elements and can range from the spectrum known as dry eye disease to the centalised oculofacial neuropathic pain syndrome characterised by a striking disparity between the high intensity of symptoms and paucity of external signs. These changes parallel those observed in somatic neuropathic pain. When seen through the neuroscience lens, diseases responsible for inadequately explained chronic eye pain (including those described as dry eye) can take on new meanings that may clarify long-standing enigmas and point to new approaches for developing preventive, symptomatic and disease-modifying interventions for these currently refractory disorders.

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.