Denervation of the Lacrimal Gland Leads to Corneal Hypoalgesia in a Novel Rat Model of Aqueous Dry Eye Disease.

Dry eye disease (DED) represents a group of disorders related to disruption of lacrimal function; a primary feature is an altered sensory perception of corneal stimuli. Patients with DED demonstrate either increased or decreased responses to noxious corneal stimulation and sometimes experience spontaneous pain, hyperalgesia, or allodynia.1–5 Changes in corneal sensory perception in DED have been postulated to be the result of sensitization of corneal sensory fibers due to an aqueous deficit at the ocular surface. Paradoxically, many DED patients do not have dry eyes or overt loss of lacrimal function. Numerous findings support the notion that basal tear production is not a good indicator of corneal sensory dysfunction.5,6 A recent study found that DED symptoms were significantly associated with nonocular pain and depression, but were not correlated with tear film measurements.7 In the present study we used two methods to disrupt the tear reflex circuit to determine the effect on sensory responses to noxious corneal stimulation. Tear production, as well as pain, can be evoked by corneal stimulation. The reflex for tear production involves motor neurons within the superior salivatory nucleus (SSN),8 which send projections to parasympathetic cholinergic motor neurons in the pterygopalatine ganglion (PPG) that innervate the lacrimal gland and evoke tear production through stimulation of the acini within the gland (Fig. 1, dotted lines).9 In contrast, the reflex pathway involving the sensory perception of noxious corneal stimuli involves a pathway from the cornea to the trigeminal dorsal horn to neurons in the parabrachial nuclei10,11 and higher brain centers (Fig. 1, solid lines). The motor response to noxious stimulation of the cornea involves stereotypical eye wipe behaviors with the ipsilateral forelimb.12 Figure 1 Schematic of sensory and reflex pathways regulating tear production and corneal sensation in the rat. Corneal sensation and tear film status are monitored by corneal afferents that innervate the corneal epithelium, have their cell bodies in the trigeminal ... We will contrast two models, one that involves pharmacologic blockade of cholinergic receptors to reduce tear production, and a novel rat model of DED that we call saporin DED, which involves denervation of the extraorbital lacrimal gland using a saporin toxin conjugated to a monoclonal antibody that binds to the p75 neurotrophin receptor (p75NTR).13 The ribosome-inactivating saporin toxin is injected into the gland where it is taken up by nerves expressing p75NTR and transported to their cell bodies in the PPG, specifically ablating those nerves (Fig. 1).13 There are a variety of conditions in which lacrimal gland nerves are damaged by either an immune response or chemical exposure, and thus we felt that specifically damaging the nerves in the lacrimal gland might yield a scientifically interesting DED model by disrupting the lacrimal reflex pathway. To assess change in corneal nociception, we test responses to ocular stimuli that activate specific classes of nociceptors. Capsaicin activates sensory fibers that contain TRPV1, which is also responsive to noxious heat stimulation,14 and is present in corneal polymodal nociceptors.15 Menthol activates cold-sensitive fibers,16 which represent a smaller but distinct population of corneal nociceptors that have been implicated in dry eye mechanisms.17 Since responses to capsaicin and menthol are mediated by distinct molecular mechanisms, it is important to assess nociceptive responses to stimuli representing distinct modalities.