Axon hyperexcitability in the contralateral projection following unilateral optic nerve crush in mice

Abstract

Optic neuropathies are characterized by degeneration of retinal ganglion cell axonal projections to the brain, including acute conditions like optic nerve trauma and progressive conditions such as glaucoma. Despite different etiologies, retinal ganglion cell axon degeneration in traumatic optic neuropathy and glaucoma share common pathological signatures. We compared how early pathogenesis of optic nerve trauma and glaucoma influence axon function in the mouse optic projection. We assessed pathology by measuring anterograde axonal transport from retina to superior colliculus, current-evoked optic nerve compound action potential, and retinal ganglion cell density one week following unilateral optic nerve crush or intraocular pressure elevation. Nerve crush reduced axon transport, compound axon potential, and retinal ganglion cell density, which were unaffected by intraocular pressure elevation. Surprisingly, optic nerves contralateral to crush demonstrated 5-fold enhanced excitability in compound action potential compared to naïve nerves. Enhanced excitability in contralateral sham nerves is not due to increased accumulation of voltage-gated sodium channel 1.6, or ectopic voltage-gated sodium channel 1.2 expression within nodes of Ranvier. Our results indicate hyperexcitability is driven by intrinsic responses of αON-Sustained retinal ganglion cells. We found αON-Sustained retinal ganglion cells in contralateral, sham, eyes demonstrated increased responses to depolarizing currents compared to those from naïve eyes, while light-driven responses remained intact. Dendritic arbors of αON-Sustained retinal ganglion cells of the sham eye were like naïve, but soma area and non-phosphorylated neurofilament H increased. Current- and light-evoked responses of sham αOFF-Sustained retinal ganglion cells remained stable along with somato-dendritic morphologies. In retinas directly affected by crush, light responses of αON- and αOFF-Sustained retinal ganglion cells diminished compared to naïve cells along with decreased dendritic field area or branch points. Like light responses, αOFF-Sustained retinal ganglion cell current-evoked responses diminished, but surprisingly, αON-Sustained retinal ganglion cell responses were similar to those from naïve retinas. Optic nerve crush reduced dendritic length and area in αON-Sustained retinal ganglion cells in eyes ipsilateral to injury, while crush significantly reduced dendritic branching in αOFF-Sustained retinal ganglion cells. Interestingly, one week of intraocular pressure elevation only affected αOFF-Sustained retinal ganglion cell physiology, depolarizing resting membrane potential in cells of affected eyes and blunting current-evoked responses in cells of saline-injected eyes. Collectively, our results suggest that neither saline nor sham surgery provide a true control, chronic vs. acute optic neuropathies differentially affect retinal ganglion cells composing the ON and OFF pathways, and acute stress can have near-term effects on the contralateral projection.

Cilastatin as a Potential Anti-Inflammatory and Neuroprotective Treatment in the Management of Glaucoma

“…We already observed changes in the CONTRA eyes in our previous study [10] although this was not paired with RGC degeneration; thus, we hypothesize that it could be due to a disruption in the BRB due to OHT-induced damage, which may induce the release of inflammatory signals through the bloodstream to the CONTRA eye. Herein, we propose that signaling through the optic nerve of the OHT eye may also explain the higher microglial activation in the nasal and central areas of the CONTRA eye, as previously reported in a similar model of unilateral ocular damage [32]. Regarding the analysis per retinal layer, the CONTRA eyes only differed from the NAÏVE eyes in both plexiform layers, with these being the two retinal layers with the highest amount of microglial cells, probably having a higher capability to respond to retinal damage than the other layers.…”

“…In this regard, despite some studies reporting no affection of Brn3a in the CONTRA eye [ 29 ], some others have reported a slight decrease in the CONTRA eyes in some temporal areas [ 8 ]. These effects in the CONTRA eye have been related to the glial-induced cytokine dysregulation that peaks within this time window [ 8 ], probably due to the inflammatory signals that travel through the optic nerve of the OHT eye and reach the optic nerve of the CONTRA eye, further traveling anterogradely to the retina [ 32 ].…”

Optic nerve injury impairs intrinsic mechanisms underlying electrical activity in a resilient retinal ganglion cell

“…Uninjured animals without sham surgery served as controls for all experiments ( Bodeutsch et al ., 1999 ; Panagis et al ., 2005 ; Ananthakrishnan et al ., 2008 ; Macharadze et al. , 2009 ; Liu et al ., 2014 ; McGrady et al ., 2022 ).…”

“…The incongruence of AP firing and synaptic currents post-crush suggests that an alternative mechanism underlies AlphaONS RGC dysfunction. Indeed, ONC in one eye can cause changes to the contralateral eye, and in one case RGCs in an uninjured eye showed dysfunction of intrinsic electrical properties ( McGrady et al ., 2022 ). Reduced intrinsic excitability has also been reported in other models of neuronal injury ( Stys et al ., 1992a ; Stys et al ., 1992b ; Waxman et al ., 1992 ; Boal et al ., 2023 ).…”

Neutral sphingomyelinase inhibition promotes local and network degeneration in vitro and in vivo

“…Immunohistochemistry was performed as previous described [ 32 ]. At the time of use, mice were transcardially perfused with PBS followed by PFA (4%).…”