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

The roles of neural adaptation and sensitization in contact lens discomfort

PURPOSE

To investigate the roles of neural adaptation and sensitization in contact lens discomfort (CLD).

METHODS

Cooling stimuli (20 °C) were applied to the cornea in a group comprising 24 symptomatic and 25 asymptomatic contact lens (CL) wearers as well as 15 non-CL wearing controls, using a computerized Belmonte esthesiometer. The adaptation paradigm consisted of 20 repetitive stimuli at threshold, sub- and supra-threshold levels. The sensitization paradigm involved five levels of suprathreshold stimuli ranging between 1x to 2x threshold. Following each stimulus, participants rated the sensation magnitude regarding intensity, coolness and irritation. Measurements were taken with habitual CL (BL_CL), after 2 weeks of no-CL (No_CL) and after restarting habitual CL wear (ReSt_CL).

RESULTS

The symptomatic subjects exhibited a lower threshold but reported enhanced sensations during the adaptation and sensitization paradigm, compared to the asymptomatic and control groups (all p ≤ 0.021). At the BL_CL and ReSt_CL visits, they showed increased ratings to repeated subthreshold stimuli (p = 0.025) and greater irritation during the sensitization paradigm (p ≤ 0.032). Ratings in asymptomatic and control groups were relatively unchanged over time (p ≥ 0.181). Logistic regression revealed a link between the augmented sensory responses and increased likelihood with CLD.

CONCLUSION

The maladaptive sensory responses seen in CLD subjects, with reduced adaptation and heightened sensitization to ocular surface stimulation, suggest an imbalance between sensitization and adaptation in CLD. As CLD may represent a reversible subcategory of dry eye, it can serve as a human dry eye model for studying the neurosensory effect of ocular surface stimulation.

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.

Cyclosporine A Decreases Dryness-Induced Hyperexcitability of Corneal Cold-Sensitive Nerve Terminals

Cyclosporine A (CsA) is used for the treatment of dry eye (DE) with good clinical results, improving tear secretion and decreasing subjective symptoms. These effects are attributed to the improved tear film dynamics, but there are no data on the effect of CsA on the abnormal sensory nerve activity characteristic in DE. Our purpose was to evaluate the CsA effect on the enhanced activity of corneal cold thermoreceptors in a tear-deficient DE animal model using in vitro extracellular recording of cold thermoreceptors nerve terminal impulses (NTIs) before and in the presence of CsA. NTI shape was also analyzed. Blinking frequency and tearing rate were also measured in awake animals before and after topical CsA. CsA increased the tearing and blinking of treated animals. CsA significantly decreased the peak response to cold of cold thermoreceptors. Neither their spontaneous NTIs discharge rate nor their cooling threshold were modified. CsA also seemed to reverse some of the changes in NTI shape induced by tear deficiency. These data suggest that, at least in part, the beneficial clinical effects of CsA in DE can be attributed to a direct effect on sensory nerve endings, although the precise mechanisms underlying this effect need further studies to be fully clarified.

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.

Evidence for a phenotypic switch in corneal afferents after lacrimal gland excision

Dry eye is a common cause of ocular pain. The aim of this study was to investigate corneal innervation, ongoing pain, and alterations in corneal afferent phenotypes in a mouse model of severe aqueous tear deficiency. Chronic dry eye was produced by ipsilateral excision of the extra- and intraorbital lacrimal glands in male and female mice. Tearing was measured using a phenol thread and corneal epithelial damage assessed using fluorescein. Changes in corneal ongoing ocular pain was evaluated by measuring palpebral opening ratio. Corneal axons were visualized using Nav1.8-Cre;tdTomato reporter mice. Immunohistochemistry was performed to characterize somal expression of calcitonin gene-related peptide (CGRP), the capsaicin sensitive transient receptor potential vanilloid 1 (TRPV1), and activating transcription factor-3 (ATF-3) in tracer labeled corneal neurons following lacrimal gland excision (LGE). LGE decreased tearing, created severe epithelial damage, and decreased palpebral opening, indicative of chronic ocular irritation, over the 28-day observation period. Corneal axon terminals exhibited an acute decrease in density after LGE, followed by a regenerative process over the course of 28 days that was greater in male animals. Corneal neurons expressing CGRP, TRPV1, and ATF3 increased following injury, corresponding to axonal injury and regeneration processes observed during the same period. CGRP and TRPV1 expression was notably increased in IB4-positive cells following LGE. These results indicate that dry eye-induced damage to corneal afferents can result in alterations in IB4-positive neurons that may enhance neuroprotective mechanisms to create resiliency after chronic injury.

Mechanisms of Peripheral and Central Pain Sensitization: Focus on Ocular Pain

Persistent ocular pain caused by corneal inflammation and/or nerve injury is accompanied by significant alterations along the pain axis. Both primary sensory neurons in the trigeminal nerves and secondary neurons in the spinal trigeminal nucleus are subjected to profound morphological and functional changes, leading to peripheral and central pain sensitization. Several studies using animal models of inflammatory and neuropathic ocular pain have provided insight about the mechanisms involved in these maladaptive changes. Recently, the advent of new techniques such as optogenetics or genetic neuronal labelling has allowed the investigation of identified circuits involved in nociception, both at the spinal and trigeminal level. In this review, we will describe some of the mechanisms that contribute to the perception of ocular pain at the periphery and at the spinal trigeminal nucleus. Recent advances in the discovery of molecular and cellular mechanisms contributing to peripheral and central pain sensitization of the trigeminal pathways will be also presented.

Photophobia: shared pathophysiology underlying dry eye disease, migraine and traumatic brain injury leading to central neuroplasticity of the trigeminothalamic pathway

BACKGROUND

Photophobia is a potentially debilitating symptom often found in dry eye disease (DE), migraine and traumatic brain injury (TBI).

METHODS

We conducted a review of the literature via a PubMed search of English language articles with a focus on how photophobia may relate to a shared pathophysiology across DE, migraine and TBI.

RESULTS

DE, migraine and TBI are common conditions in the general population, are often comorbid, and share photophobia as a symptom. Across the three conditions, neural dysregulation of peripheral and central nervous system components is implicated in photophobia in various animal models and in humans. Enhanced activity of the neuropeptide calcitonin gene-related peptide (CGRP) is closely linked to photophobia. Current therapies for photophobia include glasses which shield the eyes from specific wavelengths, botulinum toxin, and inhibition of CGRP and its receptor. Many individuals have persistent photophobia despite the use of these therapies, and thus, development of new therapies is needed.

CONCLUSIONS

The presence of photophobia in DE, migraine and TBI suggests shared trigeminothalamic pathophysiologic mechanisms, as explained by central neuroplasticity and hypersensitivity mediated by neuropeptide CGRP. Treatment strategies which target neural pathways (ie, oral neuromodulators, transcutaneous nerve stimulation) should be considered in patients with persistent photophobia, specifically in individuals with DE whose symptoms are not controlled with traditional therapies.

Role of diurnal variation of corneal sensory processing in contact lens discomfort

PURPOSE

To examine the diurnal variation of corneal threshold and suprathreshold sensory processing, symptoms, and tear secretion in symptomatic and asymptomatic contact lens (CL) wearers and controls.

METHODS

26 symptomatic and 25 asymptomatic CL wearers and 15 asymptomatic non-CL wearing controls participated. Cooling thresholds, symptoms and tear meniscus height (TMH) were measured on each of 3 measurement days (random order) on the following schedules; Day-1 within 1 h of awakening (Baseline) and 3, 6 and 9 h later, Day-2 baseline and 9 h later (CLs worn in CL group) and Day-3 baseline and 9 h later. Magnitudes estimates for threshold-scaled suprathreshold stimuli were also estimated on Day-3. Data were analyzed using mixed models and repeated measures ANOVA.

RESULTS

Cooling thresholds for the symptomatic group were lower and decreased over Day-1 (p < 0.008) and after 8 h of CL wear on Day-2 (p < 0.001) and were paralleled by increased symptoms (all p < 0.001), whereas minimal variations were found in the asymptomatic and control groups. Magnitude estimates for suprathreshold stimuli were higher (p ≤ 0.002) in the symptomatic group but did not differ significantly over the day. TMH varied little over time and was lower in the symptomatic group, but the difference was not statistically significant.

CONCLUSION

Corneal sensitivity and symptoms, but not TMH, increased diurnally irrespective of CL wear in symptomatic CL wearers. These results reveal the essential role of neurosensory abnormalities in CL discomfort and suggest involvement of a central mechanism in the diurnally increased symptoms of these patients.

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.

Effects of Tear Film Instability on Sensory Responses to Corneal Cold, Mechanical, and Chemical Stimuli

Purpose

To investigate the effects of tear film instability (TFI) induced by sustained tear exposure (STARE) on sensory responses to corneal cold, mechanical, and chemical stimuli.

Methods

Fifteen normal subjects were enrolled. TFI was induced during 10 repeated trials of STARE. Pneumatic cold, mechanical, and chemical stimuli were delivered using a computer-controlled Belmonte esthesiometer on three separate visits. The magnitude of the sensory responses to threshold and suprathreshold (1.25 and 1.50 times threshold levels) stimuli were assessed for intensity, coolness or warmness, irritation and pain, using a 0 (none) to 100 (very strong) scale, before and after STARE trials. Symptoms of ocular discomfort were evaluated using the Current Symptom Questionnaire (CSQ). Repeated measures ANOVA was used for data analysis.

Results

Following STARE trials, the intensity and coolness ratings to cooling stimuli decreased (P = 0.043 and 0.044 for intensity and coolness, respectively), while rated irritation to mechanical stimuli was increased (P = 0.024). The CSQ scores also increased regardless of visits (all P < 0.001). Intensity ratings, coolness to room temperature stimuli and irritation to mechanical and chemical stimuli increased for all suprathreshold stimuli with increasing stimulus levels (P ≤ 0.005).

Conclusions

Repeated TFI induced by STARE affects neurosensory function of the ocular surface. The decrease in reports of cooling and increase in irritation after repeated TFI suggest a complex interaction of neural mechanisms (particularly nonnociceptive cold and nociceptive mechanical) giving rise to ocular surface sensation in humans.

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.

Alkali Burn Induced Corneal Spontaneous Pain and Activated Neuropathic Pain Matrix in the Central Nervous System in Mice

PURPOSE

To explore whether alkali burn causes corneal neuropathic pain and activates the neuropathic pain matrix in the central nervous system in mice.

METHODS

A corneal alkali burn mouse model (grade II) was used. The mechanical threshold in the cauterized area was tested using Von Frey hairs. Spontaneous pain behavior was investigated with conditioned place preference. Phosphor extracellular signal-regulated kinase (ERK), which is a marker for neuronal activation in chronic pain processing, was investigated in several representative areas of the neuropathic pain matrix: the 2 regions of the spinal trigeminal nucleus (subnucleus interpolaris/caudalis, Vi/Vc; subnucleus caudalis/upper cervical cord, Vc/C1), insular cortex, anterior cingulated cortex (ACC), and the rostroventral medulla (RVM). Furthermore, pharmacologically blocking pERK activation in the ACC of alkali burn mice was performed in a separate study.

RESULTS

Corneal alkali burn caused long-lasting damage to the corneal subbasal nerve fibers, and mice exhibited spontaneous pain behavior. By testing in several representative areas of the neuropathic pain matrix in the higher nervous system, phosphor ERK was significantly activated in Vc/C1, but not in Vi/Vc. Also, ERK was activated in the insular cortex, ACC, and RVM. Furthermore, pharmacologically blocking ERK activation in the ACC abolished alkali burn induced corneal spontaneous pain.

CONCLUSIONS

Alkali burn could cause corneal spontaneous pain and activate the neuropathic pain matrix in the central nervous system. Furthermore, activation of ERK in the ACC is required for alkali burn induced corneal spontaneous pain.

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.

Retrospective study to identify trigeminal-cervical ocular referred pain as a new causative entity of ocular pain

Purpose

To determine the prevalence and clinical characteristics of trigeminal–cervical (TC) ocular referred pain.

Methods

A retrospective study of 1,680 patients seen during 2002–2010 was performed in an ocular surface specialty center to identify patients with or without TC pain defined as ocular pain with ipsilateral trigger points located at the occipital region. Patients with refractory TC pain despite topical anesthetics and conventional treatments received interventional injection to each trigger point.

Results

A total of 81 (4.8%) patients (study group) with TC pain and 241 patients (control group) without TC pain were identified out of the 1,680 patients over an 8 year period. There was no difference in age, gender, prior surgeries, medications, non-pain symptoms, pain laterality, and concomitant ocular diseases between the 2 groups. Multivariate regression analysis showed that patients with TC pain had a significant correlation with persistent deep ocular pain, ipsilateral trigger points (f²=99, p<0.001) but not headaches (f²=0.09, p=0.5). Injection at the trigger points achieved complete or partial pain resolution with a low recurrence rate in 43 of 45 (96%) patients with TC pain.

Conclusion

TC pain defined herein may be a different entity of ocular pain and can indeed be differentiated from other ocular pain by the referral character so that one may avoid mislabeling it as undetermined or as a reason to unnecessarily overtreat concomitant ocular diseases.

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.

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.

Ocular inflammation induces trigeminal pain, peripheral and central neuroinflammatory mechanisms

Ocular surface diseases are among the most frequent ocular pathologies, with prevalence ranging from 20% of the general population. In addition, ocular pain following corneal injury is frequently observed in clinic. The aim of the study was to characterize the peripheral and central neuroinflammatory process in the trigeminal pathways in response to cornea alteration induced by chronic topical instillations of 0.2% benzalkonium chloride (BAC) in male C57BL/6J mice. In vitro BAC induced neurotoxicity and increases neuronal (FOS, ATF3) and pro-inflammatory (IL-6) markers in primary mouse trigeminal ganglion culture. BAC-treated mice exhibited 7days after the treatment reduced aqueous tear production and increased inflammatory cell infiltration in the cornea. Hypertonic saline-evoked eye wipe behavior was enhanced in BAC-treated animals that exhibited increased FOS, ATF3 and Iba1 immunoreactivity in the trigeminal ganglion. Ocular inflammation is associated with a significant increase in IL-6 and TNF-α mRNA expression in the trigeminal ganglion. We reported a strong increase in FOS and Iba1 positive cells in particular in the sensory trigeminal complex at the ipsilateral interpolaris/caudalis (Vi/Vc) transition and Vc/upper cervical cord (Vc/C1) regions. In addition, activated microglial cells were tightly wrapped around activated FOS neurons in both regions and phosphorylated p38 mitogen-activated protein kinase was markedly enhanced specifically in microglial cells during ocular inflammation. Similar data were obtained in the facial motor nucleus. These neuroanatomical data correlated with the increase in mRNA expression of pro-inflammatory (TNF-α, IL-6, CCL2) and neuronal (FOS and ATF3) markers. Interestingly, the suppression of corneal inflammation 10days following the end of BAC treatment resulted in a marked attenuation of peripheral and central changes observed in pathological conditions. This study provides the first demonstration that corneal inflammation induces activation of neurons and microglial p38 MAPK pathway within sensory trigeminal complex. These results suggest that this altered activity in intracellular signaling caused by ocular inflammation might play a priming role in the central sensitization of ocular related brainstem circuits, which represents a significant factor in ocular pain development.

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.

Chronic dry eye symptoms after LASIK: parallels and lessons to be learned from other persistent post-operative pain disorders

Laser in-situ keratomileusis (LASIK) is a commonly performed surgical procedure used to correct refractive error. LASIK surgery involves cutting a corneal flap and ablating the stroma underneath, with known damage to corneal nerves. Despite this, the epidemiology of persistent pain and other long-term outcomes after LASIK surgery are not well understood. Available data suggest that approximately 20-55% of patients report persistent eye symptoms (generally regarded as at least 6 months post-operation) after LASIK surgery. While it was initially believed that these symptoms were caused by ocular surface dryness, and referred to as “dry eye,” it is now increasingly understood that corneal nerve damage produced by LASIK surgery resembles the pathologic neuroplasticity associated with other forms of persistent post-operative pain. In susceptible patients, these neuropathological changes, including peripheral sensitization, central sensitization, and altered descending modulation, may underlie certain persistent dry eye symptoms after LASIK surgery. This review will focus on the known epidemiology of symptoms after LASIK and discuss mechanisms of persistent post-op pain due to nerve injury that may be relevant to these patients. Potential preventative and treatment options based on approaches used for other forms of persistent post-op pain and their application to LASIK patients are also discussed. Finally, the concept of genetic susceptibility to post-LASIK ocular surface pain is presented.

Feedback in the brainstem: an excitatory disynaptic pathway for control of whisking

Sensorimotor processing relies on hierarchical neuronal circuits to mediate sensory-driven behaviors. In the mouse vibrissa system, trigeminal brainstem circuits are thought to mediate the first stage of vibrissa scanning control via sensory feedback that provides reflexive protraction in response to stimulation. However, these circuits are not well defined. Here we describe a complete disynaptic sensory receptor-to-muscle circuit for positive feedback in vibrissa movement. We identified a novel region of trigeminal brainstem, spinal trigeminal nucleus pars muralis, which contains a class of vGluT2+ excitatory projection neurons involved in vibrissa motor control. Complementary single- and dual-labeling with traditional and virus tracers demonstrate that these neurons both receive primary inputs from vibrissa sensory afferent fibers and send monosynaptic connections to facial nucleus motoneurons that directly innervate vibrissa musculature. These anatomical results suggest a general role of disynaptic architecture in fast positive feedback for motor output that drives active sensation.

Neuropathic ocular pain: an important yet underevaluated feature of dry eye

Dry eye has gained recognition as a public health problem given its prevalence, morbidity, and cost implications. Dry eye can have a variety of symptoms including blurred vision, irritation, and ocular pain. Within dry eye-associated ocular pain, some patients report transient pain whereas others complain of chronic pain. In this review, we will summarize the evidence that chronicity is more likely to occur in patients with dysfunction in their ocular sensory apparatus (ie, neuropathic ocular pain). Clinical evidence of dysfunction includes the presence of spontaneous dysesthesias, allodynia, hyperalgesia, and corneal nerve morphologic and functional abnormalities. Both peripheral and central sensitizations likely play a role in generating the noted clinical characteristics. We will further discuss how evaluating for neuropathic ocular pain may affect the treatment of dry eye-associated chronic pain.

The TFOS International Workshop on Contact Lens Discomfort: report of the subcommittee on neurobiology

This report characterizes the neurobiology of the ocular surface and highlights relevant mechanisms that may underpin contact lens-related discomfort. While there is limited evidence for the mechanisms involved in contact lens-related discomfort, neurobiological mechanisms in dry eye disease, the inflammatory pathway, the effect of hyperosmolarity on ocular surface nociceptors, and subsequent sensory processing of ocular pain and discomfort have been at least partly elucidated and are presented herein to provide insight in this new arena. The stimulus to the ocular surface from a contact lens is likely to be complex and multifactorial, including components of osmolarity, solution effects, desiccation, thermal effects, inflammation, friction, and mechanical stimulation. Sensory input will arise from stimulation of the lid margin, palpebral and bulbar conjunctiva, and the cornea.

The role of corneal afferent neurons in regulating tears under normal and dry eye conditions

The cornea is one of several orofacial structures requiring glandular secretion for proper lubrication. Glandular secretion is regulated through a neural reflex initiated by trigeminal primary afferent neurons innervating the corneal epithelium. Corneal sensory afferents must respond to irritating and potentially damaging stimuli, as well as drying that occurs with evaporation of the tear film, and the physiological properties of corneal afferents are consistent with these requirements. Polymodal neurons are sensitive to noxious mechanical, thermal and chemical stimuli, mechanoreceptive neurons are selectively activated by mechanical stimuli, and cool cells respond to innocuous cooling. The central terminations of corneal primary afferents are located within two regions of the spinal trigeminal nucleus. The more rostral region, located at the transition between the trigeminal subnucleus caudalis and interpolaris, represents a critical relay for the regulation of the lacrimation reflex. From this region, major control of lacrimation is carried through projections to preganglionic parasympathetic neurons located in or around the superior salivatory nucleus. Dry eye syndrome may be caused by a dysfunction in the tear secreting glands themselves or in the neuronal circuit regulating these glands. Furthermore, the dry eye condition itself may modify the properties of corneal afferents and affect their ability to regulate secretion, a possibility just now being explored.

Dry eye modifies the thermal and menthol responses in rat corneal primary afferent cool cells

Dry eye syndrome is a painful condition caused by inadequate or altered tear film on the ocular surface. Primary afferent cool cells innervating the cornea regulate the ocular fluid status by increasing reflex tearing in response to evaporative cooling and hyperosmicity. It has been proposed that activation of corneal cool cells via a transient receptor potential melastatin 8 (TRPM8) channel agonist may represent a potential therapeutic intervention to treat dry eye. This study examined the effect of dry eye on the response properties of corneal cool cells and the ability of the TRPM8 agonist menthol to modify these properties. A unilateral dry eye condition was created in rats by removing the left lacrimal gland. Lacrimal gland removal reduced tears in the dry eye to 35% compared with the contralateral eye and increased the number of spontaneous blinks in the dry eye by over 300%. Extracellular single-unit recordings were performed 8-10 wk following surgery in the trigeminal ganglion of dry eye animals and age-matched controls. Responses of corneal cool cells to cooling were examined after the application of menthol (10 μM-1.0 mM) to the ocular surface. The peak frequency of discharge to cooling was higher and the cooling threshold was warmer in dry eye animals compared with controls. The dry condition also altered the neuronal sensitivity to menthol, causing desensitization to cold-evoked responses at concentrations that produced facilitation in control animals. The menthol-induced desensitization of corneal cool cells would likely result in reduced tearing, a deleterious effect in individuals with dry eye.