Early-Stage Ocular Hypertension Alters Retinal Ganglion Cell Synaptic Transmission in the Visual Thalamus

Abstract

27Axonopathy is a hallmark of many neurodegenerative diseases including glaucoma, where elevated 28 intraocular pressure (ocular hypertension, OHT) stresses retinal ganglion cell (RGC) axons as they exit 29 the eye and form the optic nerve. OHT causes early changes in the optic nerve such as axon atrophy, 30 transport inhibition, and gliosis. Importantly, many of these changes appear to occur prior to irreversible 31 neuronal loss, making them promising points for early diagnosis of glaucoma. It is unknown whether 32OHT has similarly early effects on the function of RGC output to the brain. To test this possibility, we 33 elevated eye pressure in mice by anterior chamber injection of polystyrene microbeads. 5 weeks post-34injection, bead-injected eyes showed a modest RGC loss in the peripheral retina, as evidenced by RBPMS 35 antibody staining. Additionally, we observed reduced dendritic complexity and lower spontaneous spike 36 rate of On-RGCs, targeted for patch clamp recording and dye filling using a Opn4-cre reporter mouse 37 line. To determine the influence of OHT on retinal projections to the brain, we expressed 38Channelrhodopsin-2 (ChR2) in melanopsin-expressing retinal ganglion cells by crossing the Opn4-cre 39 mouse line with a ChR2-reporter mouse line and recorded post-synaptic responses in thalamocortical 40relay neurons in the dorsal lateral geniculate nucleus (dLGN) of the thalamus evoked by stimulation with 41 460 nm light. The use of a Opn4-cre reporter system allowed for expression of ChR2 in a narrow subset 42of RGCs responsible for image-forming vision in mice. Five weeks following OHT induction, paired 43 pulse and high-frequency stimulus train experiments revealed that presynaptic vesicle release probability 44at retinogeniculate synapses was elevated. Additionally, miniature synaptic current frequency was slightly 45 reduced in brain slices from OHT mice and proximal dendrites of post-synaptic dLGN relay neurons, 46assessed using a Sholl analysis, showed a reduced complexity. Strikingly, these changes occurred prior to 47 major loss of RGCs labeled with the Opn4-Cre mouse, as indicated by immunofluorescence staining of 48ChR2-expressing retinal neurons. Thus, OHT leads to pre-and post-synaptic functional and structural 49 changes at retinogeniculate synapses. Along with RGC dendritic remodeling and optic nerve transport 50 changes, these retinogeniculate synaptic changes are among the earliest signs of glaucoma. 51 52

Structural and functional plasticity in the dorsolateral geniculate nucleus of mice following bilateral enucleation

“…We previously demonstrated that during a modest and sustained ocular hypertension, a key risk factor for glaucoma, vesicle release probability in the retinogeniculate synapse was increased (Bhandari et al, 2019). Although the extent of injury to the optic nerve is more severe in the present study, our results here present an interesting contrast to results from ocular hypertension.…”

“…Additionally, we have shown that in ocular hypertension, dLGN TC neuron dendritic complexity is reduced, especially in regions proximal to the TC neuron somata, where they receive retinal inputs (Bhandari et al, 2019). Findings from the current experiments indicate that similarly, there is a reduction in dendritic complexity proximal to the TC neuron somata, and these changes are prominent and significant by 14 days post-enucleation.…”

“…In previous study from our lab, we demonstrated that ocular hypertension (OHT) brings about a significant increase in Pr in mice (Bhandari et al, 2019). Therefore, our next goal was to examine whether similar changes occur in presynaptic vesicle release probability (Pr) in our model of severe injury to the optic nerve.…”

“…Optic nerve degeneration in glaucoma has been reported to alter the dendritic structure of dLGN relay neurons in primate and rodents (Gupta et al, 2007;Liu et al, 2014;Bhandari et al, 2019).…”

“…The developmental plasticity of the dLGN is well-established and involves pruning and strengthening of synaptic inputs from retinal ganglion cells, refinement of dLGN relay neuron dendritic branching, and maturation of intrinsic excitability (Chen and Regehr, 2000; Hooks and Chen, 2006, 2008; Bickford et al, 2010; Hong and Chen, 2011; Krahe et al, 2012; El-Danaf et al, 2015; Thompson et al, 2016; Litvina and Chen, 2017a; Guido, 2018). A handful of studies have also documented plasticity in the dLGN in adult mice, including in monocular deprivation (Jaepel et al, 2017; Rose and Bonhoeffer, 2018), monocular enucleation (Gonzalez et al, 2005), a mouse model of inflammatory demyelination (Araújo et al, 2017), and ocular hypertension (Bhandari et al, 2019; Van Hook et al, 2021). These studies have shown that alterations in visual function or optic nerve health lead to changes in inputs to the dLGN from retina and the cortex as well as changes in the structure and function of dLGN relay neurons (Yücel et al, 2001; Hayakawa and Kawasaki, 2010; Krahe and Guido, 2011; Ly et al, 2011; Krahe et al, 2012; You et al, 2012; El-Danaf et al, 2015; Pang et al, 2015; Araújo et al, 2017; Bhandari et al, 2019).…”