Endotoxin-Induced Uveitis Causes Long-Term Changes in Trigeminal Subnucleus Caudalis Neurons

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.

Neuropathic symptoms of the ocular surface: dryness, pain, and itch

PURPOSE OF REVIEW

This review aims to describe the recent findings on epidemiology, pathophysiology, and management of neuropathic symptoms of the ocular surface, with a focus on potential similarities between sensations of dry eye, pain and itch.

RECENT FINDINGS

A narrative review of the literature was undertaken. Key references from research in dry eye, neuropathic symptoms of the ocular surface, ocular pain and itch, as well as general references on itch and pain neurobiology were included. Recent findings suggest aspects of dry eye, chronic ocular pain and itch symptomatology are driven by neuropathic pain mechanisms involving peripheral and central sensitization processes.

SUMMARY

Ocular dryness, pain, and itch are prevalent complaints with several of shared features. Multiple lines of evidence suggest that peripheral and central neuronal sensitization processes are involved in generating and maintaining ocular sensory symptoms. Research is warranted on the epidemiology of ocular sensations, molecular mechanisms involved in nociception and pruriception in the eye, electrophysiological alterations in animal models of eye conditions, and therapeutic modalities that can alleviate unpleasant ocular sensations.

Noninvasive Electrical Stimulation for the Treatment of Chronic Ocular Pain and Photophobia

INTRODUCTION

"Dry eye" or "keratoconjunctivitis sicca" is a multifactorial disease estimated to have a worldwide prevalence of 5-33%. Conventional therapies targeting the ocular surface with artificial tears, anti-inflammatories, punctal closure, eyelid hygiene, and antibiotics do not provide relief in all patients, especially those with neuropathic-like ocular complaints (wind hyperalgesia and photophobia). We anticipated that ocular transcutaneous electrical nerve stimulation (TENS) would alleviate symptoms of ocular pain, photophobia, and dryness in these latter individuals.

METHODS

All individuals who received electrical stimulation between May 10, 2016 and April 6, 2017 for the treatment of chronic ocular pain at the oculofacial pain clinic of the Miami Veterans Administration Hospital were included in this retrospective review. All patients had symptoms of dryness along with other neuropathic-like symptoms (e.g., photophobia) and minimal signs of tear dysfunction. Ocular pain intensity, symptoms of dryness, and light sensitivity were compared pre-treatment and five min post-treatment via a two-tailed paired Student's t-test.

RESULTS

The use of TENS significantly reduced the mean pain intensity in both the right and left eyes five min after treatment compared to prior to treatment (p < 0.05, paired t-test). The use of TENS significantly decreased light sensitivity in both eyes (p < 0.05). The findings for symptoms of dryness, however, were equivocal with a significant decrease in the left eye but not the right (p < 0.05, paired t-test).

DISCUSSION

Our data indicate that TENS may similarly provide analgesia in patients with dry eye symptoms as it does for many other chronic pain conditions. Furthermore, the noted effect on symptoms of photophobia and dryness suggest that all may be linked by similar trigeminal-thalamic-cortical pathways. Prospective studies with electrical stimulation of dry eye are needed to further elucidate its benefit and mechanism of action.

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.

What Causes Eye Pain?

Eye pain is an unpleasant sensory and emotional experience including sensory-discriminative, emotional, cognitive, and behavioral components and supported by distinct, interconnected peripheral and central nervous system elements. Normal or physiological pain results of the stimulation by noxious stimuli of sensory axons of trigeminal ganglion (TG) neurons innervating the eye. These are functionally heterogeneous. Mechano-nociceptors are only excited by noxious mechanical forces. Polymodal nociceptors also respond to heat, exogenous irritants, and endogenous inflammatory mediators, whereas cold thermoreceptors detect moderate temperature changes. Their distinct sensitivity to stimulating forces is determined by the expression of specific classes of ion channels: Piezo2 for mechanical forces, TRPV1 and TRPA1 for heat and chemical agents, and TRPM8 for cold. Pricking pain is evoked by mechano-nociceptors, while polymodal nociceptors are responsible of burning and stinging eye pain; sensations of dryness appear to be mainly evoked by cold thermoreceptors. Mediators released by local inflammation, increase the excitability of eye polymodal nociceptors causing their sensitization and the augmented pain sensations. During chronic inflammation, additional, long-lasting changes in the expression and function of stimulus-transducing and voltage-sensitive ion channels develop, thereby altering polymodal terminal's excitability and evoking chronic inflammatory pain. When trauma, infections, or metabolic processes directly damage eye nerve terminals, these display aberrant impulse firing due to an abnormal expression of transducing and excitability-modulating ion channels. This malfunction evokes 'neuropathic pain' which may also result from abnormal function of higher brain structures where ocular TG neurons project. Eye diseases or ocular surface surgery cause different levels of inflammation and/or nerve injury, which in turn activate sensory fibers of the eye in a variable degree. When inflammation dominates (allergic or actinic kerato-conjunctivitis), polymodal nociceptors are primarily stimulated and sensitized, causing pain. In uncomplicated photorefractive surgery and moderate dry eye, cold thermoreceptors appear to be mainly affected, evoking predominant sensations of unpleasant dryness.

Evidence for TRPA1 involvement in central neural mechanisms in a rat model of dry eye

Dry eye (DE) disease is commonly associated with ocular surface inflammation, an unstable tear film and symptoms of irritation. However, little is known about the role of central neural mechanisms in DE. This study used a model for persistent aqueous tear deficiency, exorbital gland removal, to assess the effects of mustard oil (MO), a transient receptor potential ankyrin (TRPA1) agonist, on eyeblink and eyewipe behavior and Fos-like immunoreactivity (Fos-LI) in the trigeminal brainstem of male rats. Spontaneous tear secretion was reduced by about 50% and spontaneous eyeblinks were increased more than 100% in DE rats compared to sham rats. MO (0.02-0.2%) caused dose-related increases in eyeblink and forelimb eyewipe behavior in DE and sham rats. Exorbital gland removal alone was sufficient to increase Fos-LI at the ventrolateral pole of trigeminal interpolaris/caudalis (Vi/Vc) transition region, but not at more caudal regions of the trigeminal brainstem. Under barbiturate anesthesia ocular surface application of MO (2-20%) produced Fos-LI in the Vi/Vc transition, in the mid-portions of Vc and in the trigeminal caudalis/upper cervical spinal cord (Vc/C1) region that was significantly greater in DE rats than in sham controls. MO caused an increase in Fos-LI ipsilaterally in superficial laminae at the mid-Vc and Vc/C1 regions in a dose-dependent manner. Smaller, but significant, increases in Fos-LI also were seen in the contralateral Vc/C1 region in DE rats. TRPA1 protein levels in trigeminal ganglia from DE rats ipsilateral and contralateral to gland removal were similar. Persistent tear reduction enhanced the behavioral and trigeminal brainstem neural responses to ocular surface stimulation by MO. These results suggested that TRPA1 mechanisms play a significant role in the sensitization of ocular-responsive trigeminal brainstem neurons in this model for tear deficient DE.

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.

Trigeminal interpolaris/caudalis transition neurons mediate reflex lacrimation evoked by bright light in the rat

Abnormal sensitivity to bright light can cause discomfort or pain and evoke protective reflexes such as lacrimation. Although the trigeminal nerve is probably involved, the mechanism linking luminance to somatic sensory nerve activity remains uncertain. This study determined the effect of bright light on second-order ocular neurons at the ventral trigeminal interpolaris/caudalis transition (Vi/Vc) region, a major termination zone for trigeminal sensory fibers that innervate the eye. Most Vi/Vc neurons (80.9%) identified by responses to mechanical stimulation of the ocular surface also encoded bright light intensity. Light-evoked neural activity displayed a long latency to activation (> 10 s) and required transmission through the trigeminal root ganglion. Light-evoked neural activity was inhibited by intravitreal injection of phenylephrine or l-N(G) -nitro-arginine methyl ester (L-NAME), suggesting a mechanism coupled to vascular events within the eye. Laser Doppler flowmetry revealed rapid light-evoked increases in ocular blood flow that occurred prior to the increase in Vi/Vc neural activity. Synaptic blockade of the Vi/Vc region by cobalt chloride prevented light-evoked increases in tear volume, whereas blockade at the more caudal spinomedullary junction (Vc/C1) had no effect. In summary, Vi/Vc neurons encoded bright light intensity and were inhibited by drugs that alter blood flow to the eye. These results support the hypothesis that light-responsive neurons at the Vi/Vc transition region are critical for ocular-specific functions such as reflex lacrimation, whereas neurons at the caudal Vc/C1 junction region probably serve other aspects of ocular nociception.

Ultraviolet irradiation of the eye and Fos-positive neurons induced in trigeminal brainstem after intravitreal or ocular surface transient receptor potential vanilloid 1 activation

The interior structures of the eye are well supplied by the trigeminal nerve; however, the function of these afferent fibers is not well defined. The aim of this study was to use c-fos like immunohistochemistry (Fos-LI) to map the trigeminal brainstem complex after intravitreal microinjection or ocular surface application of capsaicin, a selective transient receptor potential vanilloid 1 (TRPV1) agonist in male rats under barbiturate anesthesia. The effect of ocular inflammation on Fos-LI was tested 2 or 7 days after UV irradiation of the eye. In non-inflamed controls, intravitreal capsaicin produced peaks of Fos-LI at the trigeminal subnucleus interpolaris/caudalis (Vi/Vcvl) transition and in superficial laminae at the caudalis/upper cervical cord (Vc/C1) junction regions. At the Vc/C1 junction intravitreal capsaicin induced Fos-LI in a dose-dependent manner, while at the Vi/Vcvl transition responses were similar after vehicle or capsaicin injections. Two days, but not 7 days, after UV irradiation intravitreal and ocular surface capsaicin-evoked Fos-LI at the Vc/C1 junction and nucleus tractus solitarius (NTS) were markedly enhanced, whereas the responses at the Vi/Vcvl transition were not different from non-inflamed controls. More than 80% of trigeminal ganglion neurons labeled after intravitreal microinjection of Fluorogold also expressed immunoreactivity for the TRPV1 receptor. These findings suggested that most intraocular trigeminal sensory nerves serve as nociceptors. The similar pattern and magnitude of Fos-LI after capsaicin suggested that TRPV1-responsive trigeminal nerves that supply intraocular and ocular surface tissues form a unified integrative circuit in the caudal brainstem. Intensity coding of capsaicin concentration and facilitation of Fos-LI expression after UV irradiation strongly supported the hypothesis that the Vc/C1 junction was critical for nociceptive processing related to ocular pain, whereas the Vi/Vcvl transition region likely served other functions in ocular homeostasis under naïve and inflamed conditions.

Behavioral and neurophysiological correlates of nociception in an animal model of photokeratitis

Ocular exposure to ultraviolet irradiation (UVR) induces photokeratitis, a common environmental concern that inflames ocular tissues and causes pain. The central neural mechanisms that contribute to the sensory aspects of photokeratitis after UVR are not known. In awake male rats, ocular surface application of hypertonic saline evoked eye wipe behavior that was enhanced 2-3 days after UVR and returned to control levels by 7 days. Similarly, under isoflurane anesthesia, hypertonic saline-evoked activity of ocular neurons in superficial laminae at the trigeminal subnucleus caudalis/cervical (Vc/C1) region was enhanced 2 days, but not 7 days, after UVR. By contrast, the response of neurons at the interpolaris/caudalis (Vi/Vc) transition region to hypertonic saline was not affected by UVR. The background activity and convergent cutaneous receptive field areas of Vc/C1 or Vi/Vc neurons were not affected by UVR. Aqueous humor protein levels were elevated 2 and 7 days after UVR. UVR enhanced nociceptive behavior, after a latent period, with a time course similar to that of ocular neurons in superficial laminae at the Vc/C1 region. The Vc/C1 region plays a key role in primary hyperalgesia induced by UVR, whereas the Vi/Vc region likely mediates other aspects of ocular function.

Central sensitization induced in trigeminal and upper cervical dorsal horn neurons by noxious stimulation of deep cervical paraspinal tissues in rats with minimal surgical trauma

OBJECTIVE

This study investigated if central sensitization is induced in the trigeminal subnucleus caudalis (also termed the medullary dorsal horn) and C1 and C2 dorsal horns by noxious stimulation of deep upper cervical paraspinal tissues in a preparation relatively free of surgical trauma.

METHODS

Adult male Sprague-Dawley rats (275-450 g) were anesthetized intraperitoneally. Animals were then placed in a stereotaxic frame; a small cutaneous incision was made 3 to 4 mm near the bregma in the midline, and an opening into the skull was prepared by a 1/32-inch drill, 1 mm to the left from the midline. An epoxylite-coated tungsten microelectrode was introduced at an 18 degrees angle to enter this small opening on the skull and was then carefully advanced about 16 mm through cortex, cerebellum, and brainstem to reach subsequently histologically confirmed sites in the Vc and upper cervical (C1 and C2) dorsal horn region. Thirty-three, 27, and 15 neurons recorded in medullary, C1, and C2 dorsal horns, respectively, of chloralose/urethane-anesthetized rats were activated by noxious stimulation of mechanoreceptive fields involving V1, V2, and/or V3 trigeminal nerve territories. The inflammatory irritant mustard oil was injected into the deep paraspinal tissues at the level of the left C1-C2 joint. Pre and postinjection receptive field (RF) sizes were mapped by nonnoxious mechanical stimuli and noxious mechanical and heat stimuli.

RESULTS

A 30- to 50-minute increase (mean, 165% +/- 38.1%) in RF size postinjection for 62% of neurons tested was demonstrated, suggesting central sensitization; for most (>70%) neurons, the RF expanded caudally into cervically innervated tissues.

CONCLUSIONS

These findings provide the first documentation that deep cervical nociceptive inputs can induce central sensitization in medullary and C1/C2 dorsal horns and suggest that these effects may reflect mechanisms contributing to deep cervical pain and its referral.

Bright light produces Fos-positive neurons in caudal trigeminal brainstem

Excessive discomfort after exposure to bright light often occurs after ocular injury and during headache. Although the trigeminal nerve is necessary for light-evoked discomfort, the mechanisms underlying this phenomenon, often referred to generally as photophobia, are not well defined. Quantitative Fos-like immunoreactivity (Fos-LI) was used to determine the pattern of neuronal activation in the caudal brainstem after bright light stimulation and, secondly, whether a neurovascular mechanism within the eye contributes to this response. Under barbiturate anesthesia, male rats were exposed to low (1 x 10(4) lx) or high intensity (2 x 10(4) lx) light delivered from a thermal neutral source for 30 min (30 s ON, 30 s OFF) and allowed to survive for 90 min. Intensity-dependent increases in Fos-LI were seen in laminae I-II at the trigeminal caudalis/cervical cord junction region (Vc/C1) and nucleus tractus solitarius (NTS). Fos-LI also increased at the trigeminal interpolaris/caudalis transition (Vi/Vc(vl)) and dorsal paratrigeminal (dPa5) regions independent of intensity. Intravitreal injection of norepinephrine greatly reduced light-evoked Fos-LI at the Vc/C1, dPa5 and NTS, but not at the Vi/Vc transition. Lidocaine applied to the ocular surface had no effect on Fos-LI produced in trigeminal brainstem regions. These results suggested that multiple regions of the caudal trigeminal brainstem complex integrate light-related sensory information. Fos-LI produced at the dPa5 and NTS, coupled with norepinephrine-induced inhibition, was consistent with the hypothesis that light-evoked activation of trigeminal brainstem neurons involves an intraocular neurovascular mechanism with little contribution from neurons that supply the ocular surface.

Ocular surface-evoked Fos-like immunoreactivity is enhanced in trigeminal subnucleus caudalis by prior exposure to endotoxin

Endotoxin-induced uveitis (EIU) is a common animal model for anterior uveitis in humans that causes long-term changes in trigeminal brain stem neurons. This study used c-fos immunohistochemistry to assess the effects of different routes of administration of endotoxin on activation of trigeminal brain stem neurons produced by ocular surface stimulation. A single dose of endotoxin (lipopolysaccharide (LPS)) given to male rats by systemic (i.p., 1 mg/kg) or intraocular (ivt, 20 microg) routes increased the number of Fos-positive neurons in rostral (trigeminal subnucleus interpolaris/subnucleus transition (Vi/Vc)) and caudal portions of trigeminal subnucleus caudalis (trigeminal subnucleus caudalis/upper cervical spinal cord transition (Vc/C(1-2))) by 20% mustard oil (MO) applied to the ocular surface 7 days, but not at 2 days, after LPS compared with naïve rats. I.c.v. (20 microg) LPS did not affect MO-evoked Fos. To determine if the pattern of enhanced Fos expression after systemic LPS also depended on the nature of the ocular surface stimulus, additional groups received ocular stimulation by 10% histamine or dry eye conditions. Seven days, but not 2 days, after i.p. LPS both histamine- and dry eye-evoked Fos was increased at the Vi/Vc transition, while smaller effects were seen at other regions. These results suggested that EIU modulation of trigeminal brain stem neuron activity was mediated mainly by peripheral actions of LPS. Enhancement of Fos at the Vi/Vc region after MO, histamine and dry eye conditions supports the hypothesis that this region integrates innocuous as well as noxious sensory information, while more caudal portions of Vc process mainly nociceptive signals from the eye.

Bronchial epilepsy or broncho-pulmonary hyper-excitability as a model of asthma pathogenesis

Over the last 20 years, the prevalence of asthma has nearly doubled in industrialized countries. A similar increase has been predicted for the next two decades. Asthma is major illness in terms of morbidity and suffering, asthma is the leading cause of hospitalizations in children under 15 years of age. According to many top experts, asthma is correctly characterized as a syndrome rather than disease. This lack of definition for asthma makes the search for a cause, prevention and potential cure elusive. Episodic airway obstruction and reversible bronchial hyperresponsiveness to non-specific irritants are the major symptoms of asthma. Airway inflammation is now widely accepted as the key factor underlying the pathogenesis of asthma. However, many patients show no signs of inflammation, yet they still have severe airflow limitation and asthma symptoms. The primary clinical symptoms of asthma are attacks of shortness of breath, wheezing, and coughing resulting from excessive and inappropriate constriction of the airway smooth muscle. Our research suggests a possible epileptic or hyper-excitatory condition of bronchial system in asthma pathogenesis. The paroxysmal, spasmodic character of asthma attacks may be similar to seizures. We propose a unified pathogenetic mechanism of asthma as a syndrome of inducible or genetically predisposed membrane hyper-excitability (bronchial epilepsy).