Changes in the modulation of retinocollicular transmission through group III mGluRs long after an increase in intraocular pressure in a rat model of glaucoma

Influences of Glaucoma on the Structure and Function of Synapses in the Visual System

Significance: Glaucoma is an age-related neurodegenerative disorder of the visual system associated with sensitivity to intraocular pressure (IOP). It is the leading irreversible cause of vision loss worldwide, and vision loss results from damage and dysfunction of the retinal output neurons known as retinal ganglion cells (RGCs). Recent Advances: Elevated IOP and optic nerve injury triggers pruning of RGC dendrites, altered morphology of excitatory inputs from presynaptic bipolar cells, and disrupted RGC synaptic function. Less is known about RGC outputs, although evidence to date indicates that glaucoma is associated with altered mitochondrial and synaptic structure and function in RGC-projection targets in the brain. These early functional changes likely contribute to vision loss and might be a window into early diagnosis and treatment. Critical Issues: Glaucoma affects different RGC populations to varying extents and along distinct time courses. The influence of glaucoma on RGC synaptic function as well as the mechanisms underlying these effects remain to be determined. Since RGCs are an especially energetically demanding population of neurons, altered intracellular axon transport of mitochondria and mitochondrial function might contribute to RGC synaptic dysfunction in the retina and brain as well as RGC vulnerability in glaucoma. Future Directions: The mechanisms underlying differential RGC vulnerability remain to be determined. Moreover, the timing and mechanisms of RGCs synaptic dysfunction and degeneration will provide valuable insight into the disease process in glaucoma. Future work will be able to capitalize on these findings to better design diagnostic and therapeutic approaches to detect disease and prevent vision loss. Antioxid. Redox Signal. 37, 842-861.

Neuroglobin Can Prevent or Reverse Glaucomatous Progression in DBA/2J Mice

Mitochondrial dysfunction is responsible for hereditary optic neuropathies. We wished to determine whether preserving mitochondrial bioenergetics could prevent optic neuropathy in a reliable model of glaucoma. DBA/2J mice exhibit elevated intraocular pressure, progressive degeneration of their retinal ganglion cells, and optic neuropathy that resembles glaucoma. We established that glaucoma in these mice is directly associated with mitochondrial dysfunction: respiratory chain activity was compromised in optic nerves 5 months before neuronal loss began, and the amounts of some mitochondrial proteins were reduced in retinas of glaucomatous mice. One of these proteins is neuroglobin, which has a neuroprotective function. Therefore, we investigated whether gene therapy aimed at restoring neuroglobin levels in the retina via ocular administration of an adeno-associated viral vector could reduce neuronal degeneration. The approach of treating 2-month-old mice impeded glaucoma development: few neurons died and respiratory chain activity and visual cortex activity were comparable to those in young, asymptomatic mice. When the treatment was performed in 8-month-old mice, the surviving neurons acquired new morphologic and functional properties, leading to the preservation of visual cortex activity and respiratory chain activity. The beneficial effects of neuroglobin in DBA/2J retinas confirm this protein to be a promising candidate for treating glaucoma.

Ocular hypertension results in retinotopic alterations in the visual cortex of adult mice

PURPOSE

Glaucoma is a group of optic neuropathies characterized by the loss of retinal ganglion cells (RGCs). Since ocular hypertension (OHT) is a main risk factor, current therapies are predominantly based on lowering eye pressure. However, a subset of treated patients continues to lose vision. More research into pathological mechanisms underlying glaucoma is therefore warranted in order to develop novel therapeutic strategies. In this study we investigated the impact of OHT from eye to brain in mice.

METHODS

Monocular hypertension (mOHT) was induced in CD-1 mice by laser photocoagulation (LP) of the perilimbal and episcleral veins. The impact on the retina and its main direct target area, the superficial superior colliculus (sSC), was examined via immunostainings for Brn3a, VGluT2 and GFAP. Alterations in neuronal activity in V1 and extrastriate areas V2L and V2M were assessed using in situ hybridization for the activity reporter gene zif268.

RESULTS

Transient mOHT resulted in diffuse and sectorial RGC degeneration. In the sSC contralateral to the OHT eye, a decrease in VGluT2 immunopositive synaptic connections was detected one week post LP, which appeared to be retinotopically linked to the sectorial RGC degeneration patterns. In parallel, hypoactivity was discerned in contralateral retinotopic projection zones in V1 and V2. Despite complete cortical reactivation 4 weeks post LP, in the sSC no evidence for recovery of RGC synapse density was found and also the concomitant inflammation was not completely resolved. Nevertheless, sSC neurons appeared healthy upon histological inspection and subsequent analysis of cell density revealed no differences between the ipsi- and contralateral sSC.

CONCLUSION

In addition to RGC death, OHT induces loss of synaptic connections and neuronal activity in the visual pathway and is accompanied by an extensive immune response. Our findings stress the importance of looking beyond the eye and including the whole visual system in glaucoma research.

Influence of the hypothalamic arcuate nucleus on intraocular pressure and the role of opioid peptides

BACKGROUND

An opioid peptide neuron/humoral feedback regulation might be involved in changes of intraocular pressure (IOP). The aims of this study are to investigate the effects of arcuate nucleus (ARC) and opioid peptides on intraocular pressure (IOP).

METHODS

Fifty-four healthy purebred New Zealand white rabbits (108 eyes) were randomly divided into 4 groups, including control group, electrical stimulation group, [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) group, and [D-Pen 2, D-Pen5]- enkephalin (DPDPE) group. Bilateral IOP was measured after unilateral electrical stimulation of the ARC or unilateral microinjection into the ARC of the selective μ-opioid receptor agonist DAMGO or the selective δ opioid receptor agonist DPDPE, both alone and after pre-administration of either the non-selective opioid receptor antagonist naloxone or saline.

RESULTS

Both electrical stimulation in ARC and micro-injection either or opioid receptor agonists, DAMGO or DPDPE, respectively, caused a significant bilateral reduction in IOP (P<0.05) which was more pronounced in the ipsilateral than in the contralateral eye. Pretreatment with naloxone prevented some, but not all IOP reductions.

CONCLUSION

The ARC takes part in the negative regulation of IOP, an action that may involve opioid neurons.

Electroretinogram and visual-evoked potential assessment of retinal and central visual function in a rat ocular hypertension model of glaucoma

PURPOSE/AIM

The aim of the study was to investigate the long-term functional changes that may occur in the retina and visual cortex in a rat ocular hypertension (OHT) model of glaucoma, used in our lab for treatment studies, using electroretinogram (ERG) and visual-evoked potential (VEP) cortical recordings in order to test the hypothesis that experimental glaucoma has differential retinal and central effects.

MATERIALS AND METHODS

Experimental glaucoma was induced unilaterally in Dark Agouti rats using hypertonic saline injection into the episcleral veins. After 3, 8, 16 and 26 weeks, ERGs and VEPs were recorded under scotopic conditions using brief full-field white flashes (10 μcd s m(-2) to 10.4 cd s m(-2)) and under photopic conditions using a rod-adapting background and white light flashes (0.13-10.4 cd s m(-2)).

RESULTS

At 16 and 26 weeks after OHT induction, there was a significant reduction in the amplitudes of the a- (50% and 30% of unoperated eye values, respectively) and b-waves (55% and 40%, respectively) of the scotopic ERG and the b-waves of the photopic ERG (55% and 45%, respectively) in the glaucomatous eyes. However, no significant changes in the VEPs simultaneously recorded over the visual cortex were seen at any of the time points.

CONCLUSIONS

The reductions in ERG amplitudes suggest that this model of glaucoma not only causes retinal ganglion cell (RGC) degeneration but also degeneration of the outer retinal cells, and this was confirmed by histology showing a reduction in the outer retinal layers in the glaucomatous eyes. Cortical VEPs did not show detrimental effects suggesting that the retinal damage in this model was not extensive enough to be detected with the VEP methods used or that there could be central compensation in this model of glaucoma.