Trigeminal brainstem modulation of persistent orbicularis oculi muscle activity in a rat model of dry eye

Abstract

Altered corneal reflex activity is a common feature of dry eye disease (DE). Trigeminal sensory nerves supply the ocular surface and terminate at the trigeminal interpolaris/caudalis (ViVc) transition and spinomedullary (VcC1) regions. Although both regions contribute to corneal reflexes, their role under dry eye conditions is not well defined. This study assessed the influence of local inhibitory and excitatory amino acid neurotransmission at the ViVc transition and VcC1 regions on hypertonic saline (HS) evoked orbicularis oculi muscle activity (OOemg) in urethane-anesthetized male rats after exorbital gland removal (DE). HS increased the magnitude of long duration OOemg activity (OOemgL, >200 ms) in DE compared to sham rats, while short duration OOemg activity (OOemgS, <200 ms) was similar for both groups. Inhibition of the ViVc transition by muscimol, a GABAA receptor agonist, greatly reduced HS-evoked OOemgL activity in DE rats, whereas injections at the VcC1 region had only minor effects in both groups. Blockade of GABAA receptors by bicuculline methiodide at the ViVc transition or VcC1 region increased HS-evoked OOemgL activity in DE rats. Blockade of N-methyl-D-aspartate (NMDA) receptors at either region reduced HS-evoked OOemgL activity in DE and sham rats. GABAαβ3 receptor density was reduced at the ViVc transition, while NMDA receptor density was increased at both regions in DE rats. Loss of GABAergic inhibition at the ViVc transition coupled with enhanced NMDA excitatory amino acid neurotransmission at the ViVc transition and the VcC1 region likely contribute to altered corneal reflexes under dry eye conditions.

Title: P2x7 Receptor Activation and Estrogen Status Drive Neuroinflammatory Mechanisms in a Rat Model for Dry Eye

“…Several methods have been used to estimate chronic ocular hyperalgesia in animals: spontaneous and evoked eyeblink rates, palpebral opening and forelimb eye wiping to ocular surface stimulation ( de Castro et al, 1998 ; Bates et al, 2010 ; Yorek et al, 2016 ; Fakih et al, 2019 ; Mecum et al, 2020 ). Recently, we reported that HS-evoked OOemg activity was a reliable surrogate for ocular hyperalgesia in anesthetized male rats ( Rahman et al, 2017 ). HS-evoked long duration, squint-like OOemg activity that was enhanced in DED rats, corresponded well to cornea-evoked eye wiping behavior and involved TRPV1 activation, results that were consistent with the notion of HS-evoked OOemg as a nociceptive behavior ( Bereiter et al, 2018 ).…”

“…Rats were allowed to breathe spontaneously. The rat was placed in a stereotaxic frame and Teflon-coated copper wires (0.12 mm diameter) were implanted by a 26-gauge needle near the center of the upper and lower OO muscles, proximal to the lid margins, and grounded by a wire inserted in the neck muscle and at least 1 h elapsed before recording began ( Rahman et al, 2017 ).…”

“…Ocular surface stimulation consisted of instillation of normal saline (0.15M NaCl) and hypertonic saline (HS) in concentrations of 1 and 2.5 M NaCl. Previously we determined that HS-evoked total OOemg activity consisted mainly (>90% of total) of a period of long duration activity (>200 ms) and a minor contribution from short duration (<200 ms) and that total OOemg activity was a valid measure of ocular hyperalgesia in a rat model for DED ( Rahman et al, 2017 ; Bereiter et al, 2018 ).…”

“…The data were presented as mean ± SEM and the significant level set at p < 0.05, n = 5-6 rats per group. Sample size was based on results from previous studies ( Rahman et al, 2017 ; Bereiter et al, 2018 ), which we calculated would provide 80% power at p < 0.05. Three female rats were excluded from further analysis due to low blood pressure at the time of recording.…”

Sex, blinking, and dry eye

“…Rats were anesthetized (ketamine, 90 mg/kg and xylazine, 10 mg/kg), and the subcutaneous exorbital lacrimal gland, which is located just anterior and slightly below the ear (Lorber 1993;Walcott et al 2005) ipsilateral to the OOemg electrodes, was removed. Exorbital gland removal created an~50% reduction in tearing in the ipsilateral eye (Bereiter et al 2018;Katagiri et al 2015;Meng et al 2015;Rahman et al 2015bRahman et al , 2017. This mild dry eye did not produce corneal ulcerations because rats have two additional glands that contribute to the aqueous portion of the tear film (Williams 2002).…”