Juana Gallar

Excitation by irritant chemical substances of sensory afferent units in the cat's cornea

1. Single‐unit electrical activity was recorded from thin myelinated sensory nerve fibres innervating the cornea of deeply anaesthetized cats. 2. Based on their responses to mechanical (calibrated von Frey hairs), chemical (10 mM‐acetic acid and/or 616 mM‐NaCl) and thermal (ice‐cold or heat up to 51 degrees C) stimuli, corneal A delta fibres were classified as polymodal nociceptors (63%), high‐threshold mechanoceptors (22%) and mechano‐heat nociceptors (15%). Thin myelinated fibres responding only to cold were found in the limbus of the eye. 3. Application of 10 mM‐acetic acid on the corneal surface for 30 s evoked in polymodal fibres a brisk discharge of impulses often followed by a low‐frequency impulse activity. NaCl (616 mM) produced a more gradual and sustained firing response. 4. The responses of polymodal fibres to acid were proportional to extracellular pH values (pH range: 4.5‐6.0). After sensitization to repeated heating, most mechano‐heat units developed a sensitivity to acidic stimulation. 5. Topical 0.33 mM‐capsaicin excited polymodal nociceptors of the cornea; 5 min after capsaicin about 15% of these fibres were inactivated to all subsequent stimuli. In the rest of the fibres, chemical and thermal sensitivity disappeared after 0.33‐3.3 mM‐capsaicin, but mechanosensitivity was preserved. 6. Corneal mechanoceptors and limbal cold receptors were not affected by capsaicin (up to 33 mM). 7. These experiments demonstrate that the cornea of the cat is innervated by polymodal as well as mechanoceptive A delta nociceptors. In polymodal nociceptive fibres, mechanical and chemical sensitivities appear to be subserved by separate transduction mechanisms.

Response of sensory units with unmyelinated fibres to mechanical, thermal and chemical stimulation of the cat's cornea.

1. In the cat anaesthetized with sodium pentobarbitone, electrical activity was recorded from single unmyelinated sensory fibres innervating the cornea. 2. Based on their response to mechanical (calibrated aesthesiometer), chemical (10 mM acetic acid or 616 mM NaCl) and thermal (cooling from 35 to 5 degrees C; heating to 51 degrees C) stimuli, corneal unmyelinated fibres were classified as polymodal (71%) or ‘cold’ nociceptors (29%). 3. Polymodal units responded to mechanical indentation of the cornea and developed fatigue after repeated stimulation. They were excited by temperatures over 37 degrees C and exhibited sensitization to repeated heating. 4. Corneal polymodal units were also activated by topical application of 10 mM acetic acid and hypertonic NaCl (616 mM). Capsaicin (0.33 mM) elicited a discharge of impulses that was followed by inactivation to mechanical, chemical and thermal stimuli. 5. ‘Cold’ nociceptors had small receptive fields, preferentially located at the periphery of the cornea. They were excited by small temperature decreases of the corneal surface in a range between 30 and 8 degrees C, but were not responsive to noxious heat. 6. ‘Cold’ nociceptors encoded temperature changes between 35 and 23 degrees C. The discharge was proportional to the velocity of the temperature drop; sustained temperatures were not signalled by changes in static frequency values. ‘Cold’ nociceptive fibres responded to hypertonic NaCl (616 mM) and weakly to 10 mM acetic acid. Capsaicin (0.33 mM) first excited and then inactivated ‘cold’ nociceptors. 7. Thermoreceptive fibres were found in the episclera. They fired in bursts and responded to small temperature decreases, but were insensitive to irritant chemical and capsaicin.

Neurobiology of ocular pain

Abstract Ocular irritation and pain are associated with many clinical situations (e.g. accidental injury, eye diseases, surgery and contact lens wearing). Pain and related ocular sensations begin with stimulation by injurious stimuli of first-order sensory neurons of the trigeminal ganglion. Neurons responding solely to application in their receptive field of noxious mechanical forces (mechanonociceptive neurons), or of irritant chemicals and heat (polymodal nociceptive neurons), have been identified electrophysiologically in the conjunctiva, cornea, sclera, iris, ciliary body and choroid. The cornea is additionally innervated by neurons responding to low temperatures, which may account for corneal discomfort caused by cold. Also, low-threshold mechanoreceptive and cold-sensitive neurons supply the conjunctiva and sclera, possibly mediating touch and thermal sensations aroused by innocuous stimuli in the front of the eye. Ocular sensory information is transmitted from the trigeminal ganglion to specific higher-order neurons located in the trigeminal brainstem nuclear complex, the thalamus and the cerebral cortex. Local ocular inflammatory responses enhance injury-induced neural activity both in ocular nociceptive terminals and in higher order neurons. In addition to signalling acute lesions, ocular primary sensory neurons participate in post-injury processes, contributing to local inflammatory reactions (neurogenic inflammation) and to the repair of damaged tissues. These effects are mediated at least in part, by substance P and CGRP, two neuropeptides contained in ocular sensory nerve cells that are released peripherally upon tissue damage. Ocular tissues have a trophic interdependence with their sensory neurons. Ocular tissues are the source of neurotrophic factors that are critical for the early development and survival of trigeminal sensory neurons. On the other hand, the morphofunctional integrity of some ocular tissues like the cornea, appears to be dependent on the presence of an intact sensory innervation. Stimulation of ocular sensory pathways by noxious mechanical, chemical and thermal stimulation of cornea, conjunctiva or of other eye structures, evokes distinct types of ocular sensations. Differences in the quality of pain sensation presumably result from the magnitudes of activation of the various sub-populations of ocular nociceptive neurons by different stimulus modalities. In addition to conscious sensations, injurious stimuli evoke protective reflexes (blinking and lacrimation) aimed at protecting the eye and minimizing further ocular damage by noxious stimuli.

Ocular surface wetness is regulated by TRPM8-dependent cold thermoreceptors of the cornea

Basal tearing is crucial to maintaining ocular surface wetness. Corneal cold thermoreceptors sense small oscillations in ambient temperature and change their discharge accordingly. Deletion of the cold-transducing ion channel Transient receptor potential cation channel subfamily M member 8 (TRPM8) in mice abrogates cold responsiveness and reduces basal tearing without affecting nociceptor-mediated irritative tearing. Warming of the cornea in humans also decreases tearing rate. These findings indicate that TRPM8-dependent impulse activity in corneal cold receptors contributes to regulating basal tear flow.

Sensory experiences in humans and single‐unit activity in cats evoked by polymodal stimulation of the cornea

1 The cornea of human subjects and of anaesthetised cats was stimulated with a jet of air of controlled flow, temperature and CO2 concentration delivered by a gas aesthesiometer. 2 In humans, the intensity and magnitude of various components of the sensory experience (intensity of the sensation, degree of irritation, magnitude of burning and stinging pain, magnitude of the cold and warm components of the sensation) were measured using separate visual analog scales. In anaesthetised cats, the impulse response to the same stimuli was recorded from single mechanosensory, polymodal and cold‐sensitive corneal fibres in the ciliary nerves. 3 Intensity‐response curves for mechanical stimulation showed that all parameters of the sensation experienced by humans increased with the intensity of the stimulus. Mechanical stimuli recruited mainly phasic mechanosensory and polymodal afferents in the cat. 4 Acidic stimulation with gas mixtures of increasing CO2 concentration evoked irritation, burning and to a lesser extent stinging pain of a magnitude roughly proportional to the intensity of the stimulus in humans. CO2 primarily recruited polymodal afferents and weakly excited cold‐sensitive fibres in the cats cornea. 5 Heat stimuli evoked in humans a sensation profile similar to CO2 but accompanied by a warmth component. In the cats cornea, heat excited only polymodal fibres and silenced cold‐sensitive corneal units. 6 Cold stimuli applied to the human cornea elicited a sensation of cooling that became irritant at the lowest temperatures. Corneal cold‐sensitive fibres of the cat were activated in a manner proportional to the temperature drop, while polymodal nociceptor fibres were recruited only by the lowest temperatures. Topical menthol (0.2 mm) applied to humans evoked and later eliminated cold sensations produced by cold stimuli while the irritation sensation caused by low temperature stimuli still persisted. 7 Human subjects were able to identify masked mechanical, thermal and chemical stimuli applied to the cornea. 8 Irritation and cold sensations can therefore be evoked separately from the cornea by selective activation of mechanosensory, polymodal and cold corneal sensory afferents. Stimulation with different forms of energy usually leads to combined activation and/or inhibition of the different populations of sensory afferent fibres, evoking blended sensations that include irritation and thermal components in a variable degree.

Nerves and Sensations from the Eye Surface

Because vision plays a critical role in obtaining information from the external world, evolutionary development has provided the structures that sustain this function with special protection against injury. Thus, the cornea possesses the richest sensory innervation of the body to detect noxious stimuli. The trigeminal sensory neurons that innervate the eye vary in their chemical composition and electrophysiological properties, and can be classified according to the stimuli that activate them preferentially: mechanical forces, temperature, or irritant chemicals. Different classes of noxious stimuli (mechanical injuries, heat, extreme cold) activate to a different degree the various populations of sensory fibers of the ocular surface and evoke unpleasant sensations of distinct quality. When injured either accidentally or following ocular surgery, sensory nerve fibers of the ocular surface may form neuromas that develop abnormal activity and become the source of unpleasant sensations, such as pain, dryness, grittiness, etc. In parallel, their response to natural stimuli is diminished. The possibility of hypesthesia and dysaesthesias must be considered in the assessment of the risks of therapeutic procedures that involve damage to ocular sensory nerves.

CO2 stimulation of the cornea: a comparison between human sensation and nerve activity in polymodal nociceptive afferents of the cat.

Excitation of nociceptors by low pH has been proposed as a cause of pain following tissue injury. Here we have studied the effect of pH reductions caused by application of CO2 pulses to the cornea on the activity of corneal afferent nerves of the cat and on the magnitude of pain sensations in humans. Single‐unit activity was recorded from corneal afferent fibres in anaesthetized cats. The corneal receptive field of A‐delta or C polymodal nociceptive units was exposed for 30 s to a gas mixture with different concentrations of CO2 in air (0, 35, 50, 65, 80 and 98.5%). Responses to CO2 were evoked at a mean threshold concentration of 40 ± 3% CO2. They consisted of a discharge of impulses that decayed gradually to a tonic level. In 15% of the units the initial burst was absent. The CO2 concentration and firing frequency data could be fitted to a power function with an exponent of 1.12. Pulses of CO2 were also applied to the cornea of 16 human volunteers. Sensations experienced were measured by means of a visual analogue scale and a verbal descriptor scale. Flow was adjusted below the mechanical stimulation threshold (2.8 ± 0.5 mg). When mixtures containing 10‐90% CO2 in 5% steps were applied as 3 s pulses, threshold sensation, described as a mild stinging pain, was evoked at 33.5 ± 4.0% CO2. This sensation became overtly painful with higher CO2 concentrations (47.5 ± 3.6% CO2). For the same subject the sensory threshold was remarkably constant, though it changed with longer exposure times. The relationship between CO2 concentration and magnitude of pain could be adjusted to a power function with a power exponent of 1.12. Curves of CO2 concentration versus neural discharges in the cat and versus psychophysical sensation in humans were very similar. These results show that corneal polymodal nociceptors respond to protons, and encode changes in CO2 concentration presumably reflecting pH changes. The same stimulus evokes corneal pain sensations in humans, thus opening the possibility of using CO2 as an effective stimulus for corneal aesthesiometry.

Tear fluid hyperosmolality increases nerve impulse activity of cold thermoreceptor endings of the cornea

Summary Hyperosmolality equivalent to that seen in evaporative dry eye primarily increases background activity of corneal cold thermoreceptors, maybe underlying discomfort sensations developed during dry eye condition. ABSTRACT Dry eye disease (DED) is a multifactorial disorder affecting the composition and volume of tears. DED causes ocular surface dryness, cooling, and hyperosmolality, leading ultimately to corneal epithelium damage and reduced visual performance. Ocular discomfort is the main clinical symptom in DED. However, the peripheral neural source of such unpleasant sensations is still unclear. We analyzed in excised, superfused mouse eyes, the effect of NaCl‐induced hyperosmolality (325–1005 mOsm·kg−1) on corneal cold thermoreceptor and polymodal nociceptor nerve terminal impulse (NTI) activity. Osmolality elevations at basal corneal temperature (33.6°C) linearly increased the ongoing NTI frequency of cold thermoreceptors, at a mean rate of 0.34 imp·s−1/10 mOsm. This frequency increase became significant with osmolality values greater than 340 mOsm. Comparison of cold thermoreceptor activity increase induced by a dynamic temperature reduction of 1.8°C under iso‐ and hyperosmolal (360‐mOsm) conditions provided evidence that more than 50% of the increased firing response was attributable to hyperosmolality. Comparatively, activation of corneal polymodal nociceptor endings by hyperosmolal solutions started with values of 600 mOsm and greater. Sensitization of polymodal nociceptors by continuous perfusion with an “inflammatory soup” (bradykinin, histamine, prostaglandin E2 [PGE2], serotonin, and adenosine triphosphate [ATP]) did not enhance their activation by hyperosmolal solutions. High osmolality also altered the firing pattern and shape of cold and polymodal NTIs, possibly reflecting disturbances in local membrane currents. Results strongly suggest that tear osmolality elevations in the range observed in DED predominantly excite cold thermoreceptors, supporting the hypothesis that dryness sensations experienced by these patients are due, at least in part, to an augmented activity of corneal cold thermoreceptors.

Quantification and immunocytochemical characteristics of trigeminal ganglion neurons projecting to the cornea: effect of corneal wounding.

The number and immunocytochemical characteristics of trigeminal ganglion neurons providing sensory innervation to the cornea were studied in the mouse. Corneal neurons were retrogradely labelled with fluorogold placed on the cornea after removal of the epithelium with n‐heptanol. Corneal neurons were counted, sized and characterized immunocytochemically with antisera against substance P (SP), calcitonin gene‐related peptide (CGRP), calbindin, calretinin, and with a monoclonal antibody (RT97) against neurofilament proteins. A total of 258 corneal neurons were counted, most of them located in the ophthalmic division of the trigeminal ganglion. They represent only a small fraction (1.3%) of the population of trigeminal ganglion neurons. More than 70% of corneal neurons were classified as ‘small dark’ according to their cell body area and the absence of immunoreactivity to RT97. A low percentage of corneal neurons, usually large in size, contained calcium binding proteins. Fifty‐eight percent of the corneal neurons were immunoreactive to CGRP, and 20% to SP Corneal wounding with NaOH, which affects stromal nerve trunk, did not modify the total number of corneal neurons or their neuropeptide content. However, this increased the total number of calbindin‐positive and decreased the RT97‐positive neurons. Thus, unlike in other nociceptive neurons, peripheral axotomy did not modify the SP/CGRP content of corneal neurons.

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.