A healthy moist ocular surface is essential for proper visual functioning. It requires cooperation of an integrated system called the Lacrimal Functional Unit, which consists of the ocular surface tissues, secretory glands, and the nervous system, that together produce and maintain a normal tear film. A disturbance in any group of these components leads to dry eye. The corneal sensory afferents are one of these components. Their activation is considered to be critical to basal tear production, and thus their dysfunction could cause a chronic lack of tears, resulting in dry eye. However, despite a great amount of work done to characterize the corneal afferents, their roles in the lacrimation reflex or dry eye disease still remain to be determined. Recently, we identified a special type of cold-sensitive corneal nerve that is activated by a variety of ocular stimulations thought to be critical for tear production: drying of the cornea, gentle cooling, tear evaporation, and hyperosmolar tears. Evidence indicates that dry eye disease is in part a dysfunction of the mechanisms that detect ocular surface conditions such as cooling of the cornea and the hyperosmolar tears. Thus, we hypothesize that activation of this special type of corneal afferent will induce tear production and their dysfunction contributes to dry eye disease. If so, a selective destruction of these cold-sensitive corneal afferents should lead to dry eye. Recent evidence indicates that members of the nerve membrane receptors (TRPM8, TRPV1, TRPA1) reside in this special type of corneal afferent involved in tear production;that TRPM8 receptors mediate the cooling response;and TRPV1/TRPA1 receptors mediate the hyperosmolar response. We hypothesize that activation of these receptors together constitutes the responses of these cold-sensitive corneal afferents to drying of the cornea and leads to tearing. Thus, genetically engineered mutant mice specifically lacking these receptors should produce very few tears and exhibit dry eye. We will also show that these mutant mice display other signs of dry eye diseases such as inflammation of the ocular surface, which would damage the integrity of the ocular surface tissues, producing the abnormal morphology of the corneal and the conjunctival epithelia. Finally, we will demonstrate immunohistochemically that some well-known models of dry eye disease lack these receptors, implying that this fact is one reason that they have dry eye. If the aims of these studies are achieved, the proposed research will lead to a better understanding of the cellular and molecular mechanisms underlying the activation of the lacrimation reflex and dry eye disease, and may provide new targets for pharmacological treatments for dry eye diseases.
Dry eye disease afflicts millions of Americans annually and is the most frequent complaint from patients visiting eye care clinics. It also could lead to serious visual disturbances including blindness. We propose to study the corneal afferent mechanisms underlying the tear production reflex and to attempt to establish its relationship to dry eye disease. The results from this study may lead to a new approach to treatments for dry eye disease.
|Hirata, Harumitsu; Rosenblatt, Mark I (2014) Hyperosmolar tears enhance cooling sensitivity of the corneal nerves in rats: possible neural basis for cold-induced dry eye pain. Invest Ophthalmol Vis Sci 55:5821-33|
|Hirata, Harumitsu; Oshinsky, Michael L (2012) Ocular dryness excites two classes of corneal afferent neurons implicated in basal tearing in rats: involvement of transient receptor potential channels. J Neurophysiol 107:1199-209|