Wiring the Ocular Surface
The cornea is one of the most densely innervated tissues in the human body. Stimulation of afferent nerve endings can elicit a collection of protective reflex responses including blinking, tearing or gross motor movements such as flinching.
The endings originate from cell bodies in the ipsilateral trigeminal ganglion; these nerve endings project via the ophthalmic nerve, which is composed of multiple axonal fibers including the high speed, myelinated A-delta fibers and the larger, slower-conducting C fibers. In the cornea, nerves bifurcate and project toward the corneal surface through the basal and more superficial layers of the corneal epithelium. These endings are the nerve fingertips, transducing mechanical, chemical and thermal inputs into an integrated neural output directing appropriate tearing and blink responses.
A number of recent studies have compiled confocal images of the ocular surface to map the overall pattern of corneal innervation. In one study, cholinesterase staining revealed perforation sites at which nerves crossed through Bowman’s membrane prior to their final bifurcation and projection into the epithelial layer; these sites were relatively diffuse in the central cornea (20 to 30 sites), and more common in the mid-peripheral cornea (130 to 160 sites).1 The density of this corneal neuropil fluctuates according to the overall health of the ocular surface.
The nerve cells that comprise this sensory network are heterogeneous. Polymodal nerves, which constitute approximately 70 percent of the total nerves in the cornea, are activated by mechanical, chemical and/or thermal stimuli.2 Another 20 percent of corneal sensory nerve cells respond exclusively to mechanical inputs such as physical displacement, and the remaining 10 percent are sensitive only to the sensation of cold. In addition to their role in detecting environmental threats, the sensory nerves of the cornea are a vital part of basal tear-film regulation. This role is demonstrated in patients with familial dysautonomia (Riley-Day syndrome), a genetic disorder in which parts of the autonomic nervous system, including corneal sensory nerves, fail to develop normally.3 These patients exhibit a congenital corneal anesthesia, and suffer from severe dry eye due to lack of tear production. Thus the concept of a nociceptor, or nerve that senses pain, is more about how these specific sensory inputs are interpreted and utilized than it is about the specific stimuli to which each responds.
Like nociceptors in other tissues, corneal nerves respond in a graded fashion that is proportional to the stimulus, and this response can be enhanced by repeated stimuli in a process referred to as sensitization. Unlike other nociceptors, however, those in the cornea have an inherently lower activation threshold and so will trigger afferents with stimuli that would not activate receptors in other tissues.4 Changes associated with sensitization can include a further reduced threshold for activation, increased magnitude in response, or both. Altered responsivity of nociceptors can establish a hyperalgesic state in which neurons are spontaneously active and sensitive to stimuli that are not considered to be noxious. Sensitization most typically occurs as a consequence of tissue injury and inflammation, and can occur within minutes of an initial offense.5
The high density and heterogeneity of nociceptors in the cornea underlie an exquisite ability to respond to diverse environmental stimuli of varying intensities. Sensory activation requires simultaneous participation of multiple nerve ending response elements including ion channels, g-proteins and other intracellular signaling molecules. The key transducers of sensory stimuli are the non-specific cation channels (also called transient receptor potential channels, or TRPs). There are 28 human TRP genes, and the sensitivity to thermal, mechanical and chemical stimuli displayed by individual nerve endings is, in large part, a reflection of their specific TRP channel repertoire.4 Stimulus-dependent TRP channel opening causes local membrane depolarizations that are amplified and propagated by voltage-gated sodium channels (NAV) from the nerve terminals toward the nerve cell bodies, and from there on to the CNS.
The TRP channel family includes receptors for capsaicin, menthol and mustard oil, and while responses to these compounds are more pharmacological than physiological, there is an inherent logic to the revelation that the menthol receptor, TRPM8, is the initial sensory moiety responsible for cold sensation. This receptor has recently been established as an exquisite sensor of ocular surface dryness, and plays a key role in the production of basal tears.6 In particular, the TRPM8 variant expressed in the cornea is particularly sensitive to very small changes (±0.3 C) in surface temperature, and plays a key role in maintenance of tear-film homeostasis.7 Basal tearing is dramatically diminished in TRPM8 knockout mice, and pharmacological modulation of the TRPM8 channel has also been shown to affect tearing.8
By monitoring subtle temperature changes, TRPM8 functions as a reporter of ocular surface hydration. During the interblink period, when the tear film evaporates, ocular surface temperature decreases approximately 0.3 C per second.9 In addition, cold thermoceptors can be activated by hyperosmotic tears, which is relevant to interblink evaporative effects.10 TRPM8 channels sense the temperature and osmotic changes, causing them to depolarize and send signals to the central nervous system that result in tear secretion by the lacrimal gland and goblet cells. Patients with reduced corneal innervation, either as a result of chronic inflammation or aging, are at greater risk of dry eye as the feedback from TRPM8-containing receptors diminishes.
Tearing in response to noxious stimuli is maintained in TRPM8 knockout mice, emphasizing the role of other types of ocular surface sensors in health and disease.7 This returns us to the discussion of tissue injury, inflammation and nociceptor sensitization. When ocular surface drying is accompanied by proinflammatory mediator production by cells of the cornea and conjunctiva, the conscious perception of discomfort is set into motion by the collective contribution of nociceptors at various levels of sensitization and activity. Carlos Belmonte, MD, PhD, of Alicante, Spain, has proposed that dry eye is essentially a state of ocular surface pain and that corneal nociceptors are central to pathophysiology and probably disease management.6,11 Polymodal neurons are the key participants in the discomfort associated with dry eye. TRPV1 and TRPA1 are cold transducers expressed on corneal polymodal neurons that inherently have higher thresholds for activation than TRPM8 channels. Therefore, noxious cold or hyperosmolarity are required to activate these channels and send signals to the brain that are perceived as ocular surface discomfort.
The Impact of Inflammation
Ocular surface cells, and inflammatory leukocytes that invade the tissue in disease states, produce mediators that can influence sensory neurons. Corneal sensory neurons exhibit a property termed plasticity, alluding to their ability to adapt to local conditions. Receptors for inflammatory mediators such as prostaglandins and bradykinin mediate the sensitization of nociceptors during tissue injury. Inflammation-induced sensory neuron changes include increased expression of transducing components within nerve ending membranes as well as enhanced signaling upon stimulation.1 In addition, neuronal sensitivity is modulated by density and localization of transducers within membrane lipid rafts, post-translational glycosylation and kinase-dependent phosphorylation.
The plasticity of corneal sensory neurons is not only characterized by the ability to increase in sensitivity but also to decrease in sensitivity to stimulation. For example, corneal sensory neurons have been shown to adapt to stimulation in contact lens wearers.14 Lens wear stimulates mechanical nociceptors as well as polymodal sensors. Approximately half of contact lens wearers have been reported to exhibit dry-eye symptoms as a result of stimulation of these sensory transducers.15 Mechanical contact and hypoxia-induced proinflammatory mediator production contribute to this discomfort during contact lens wear. While contact lens wear is another example of corneal nerve sensitization, adaptive desensitization to particular stimuli in a subgroup of contact lens wearers reflects the complex dynamics of ocular surface sensations.14
A New Therapeutic Target
It’s clear that the vital role of corneal sensory function, together with the impact of inflammation on sensory neuron function, suggest that modulators of nociceptors could present a number of therapeutic opportunities. The broadest spectrum ophthalmic anti-inflammatory agents available are the corticosteroids, which have side effects that limit their use in chronic conditions. Alternatively, non-steroidal anti-inflammatory drugs are frequently used successfully post-surgery, but these drugs target only prostaglandin synthesis; other mediators are unaffected. Strategies that directly target corneal nociceptors and their transducers may provide improvements in ocular surface pain management.
Effects achieved with the TRPV1 agonist capsaicin provide evidence that analgesia can be achieved by targeting the nerve directly. Capsaicin initially causes pain via its TRPV1 agonist properties, but sustained channel opening with long exposures ultimately leads to cytotoxicity due to excessive calcium influx. This cytotoxicity results in analgesia due to the death of the sensory neuron. SYL1001 (Sylentia SA), a short interfering RNA (siRNA) targeted at the TRPV1 channel, is in early clinical development for the treatment of dry-eye discomfort. Other studies suggest that TRPM8 may be an attractive target for pain management.16 The rationale for this target is derived from the analgesic properties of menthol, a known TRPM8 agonist. This approach remains speculative since menthol has agonist activity at other TRPs, and can produce a hyperalgesic state at high concentrations. Low concentrations of menthol can induce tearing, which is consistent with the above discussion of the role of TRPM8 in basal tear secretion.17 Therefore, TRPM8 is a particularly attractive target for dry eye due to the potential combined effects on both discomfort and tear secretion.
Our own experiences tell us that corneal sensory inputs operate with exquisite responsiveness. The molecular mechanisms behind the cornea’s ability to sense and respond to sensory input are being elucidated. The ability to selectively manipulate individual components of this response system has the potential to provide life-changing opportunities for therapeutic advances. TRP channels appear to be a central factor in the signaling pathways initiated by corneal insult and leading to a compensative response. The trick will be to untangle the many subtleties of TRP channel variants and their signaling co-conspirators. Until then, keep the artificial tears handy. REVIEW
Dr. Abelson is a clinical professor of ophthalmology at Harvard Medical School. Dr. Gamache is director of ophthalmic pharmaceuticals market research at Market Scope LLC.
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