Visual Function with Aging in Normal and Alzheimer's Disease Model Mice

While mice are nocturnal, they are not innately light aversive. If the environment is familiar, a mouse will spend a significant portion of time in a brightly lit arena. We perform extensive habituation to the testing environment under low light levels to ensure that, once we start testing manipulations to assess light aversion, we are measuring a response to light and not the neophobia small prey-like animals often experience in an unfamiliar surrounding where predators may be present.

Diabetes, hypertension, obesity all affects the retina in addition to the rest of the CNS. With Adaptive Optics and Optical Coherence Tomography, researchers are identifying patterns of photoreceptor loss and vascular function that could be used as diagnostic criteria.

An excellent reference for vascular and photoreceptor changes in diabetes can be found here:

https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2022.826643/full

An excellent reference for the relationship between retinal and other DNA vascular changes can be found here:

https://iovs.arvojournals.org/article.aspx?articleid=2707482

There are 6 different classes of ipRGCs, and they have some shared and some specific connections to at least 18 different brain regions. And yet, we still don’t understand how and where ipRGCs or melanopsin-expressing cells exert all their influence. The key will be to understanding the interplay between cell types, their projections and resultant behaviors.

An excellent reference for ipRGCs and their projections can be found at:

https://www.sciencedirect.com/science/article/pii/S0896627319306427?via%3Dihub

All behaviors can be viewed as an input-output relationship and differentiating the site of dysfunction in this relationship can be very complicated. This is particularly true in disease states in which multiple sites may be coordinately or independently affected. In animal models, this can be addressed using multiple approaches from histology to identify physical changes, to functional (electrophysiological) recordings to bypass-style experiments such as providing optogenetic or chemogenetic stimulation to test specific sites. For our current studies, we can use histological analyses to substantiate loss at the peripheral site and follow up with analyses in the brain in future studies.

This requires a sophisticated approach beyond detecting melanopsin at the protein or gene expression level. There are a few ipRGC-specific markers that are co-expressed with a high degree of correlation that can be used as a surrogate for melanopsin, for example, PACAP. Alternatively, one can use behaviors to distinguish loss of the protein versus loss of the cells. This functional differentiation can be effective as the ipRGCs receive information from specific photoreceptor pathways, integrating this extrinsic response with its own intrinsic detection of photons by melanopsin, and then relaying this information via distinct neural circuits to effect specific behavioral responses. For example, loss of melanopsin will reduce the PLR in medium light but loss of ipRGCs will result in a much stronger deficit at brighter levels of illumination.

An excellent reference for ipRGCs and their projections can be found at:

https://iovs.arvojournals.org/article.aspx?articleid=2124147

Our analyses are ongoing and thus a specific conclusion requires completion and statistical analyses. We do observe changes over that time frame.

There is a vast literature on learning and memory deficits in mouse AD models including Pavlovian and Operant Conditioning paradigms. There are many good reviews that summarize the outcomes from the past 20 years.

Yes, subjective tests can be used where the patient actively rates their level of discomfort/pain. Objective measures of light sensitivity are based on facial responses, i.e. squinting that can be measures by electromyography to measure the muscles around the eye and quantitatively measures the amplitude of response. Similarly, imaging analysis of squint is being adapted as a less invasive assay and may become a method of choice in the future.

Both greater aging and cortical/hippocampus analyses are ongoing or future studies.

It’s the AB-N38 rabbit anti-melanopsin antibody from Advanced Targeting Systems.

This is an ongoing analysis.

This is a planned study with the goal of identifying early pathologies in affected areas. As such, we first are identifying whether specific cell types (rod or cone photoreceptors, retinal ganglion cells, intrinsically photosensitive retinal ganglion cells, inner retinal neurons, etc.) are compromised in order to target potential pathological mechanistic analyses to these cells.

Studies of aging have shown that while the pupil diameter does change across decades, the relative pupillary light reflex (PLR) is conserved. This suggests that the ability to restrict light entering the eye is very important function with safeguards, and likely multiple mechanisms are in place to ensure relatively normal function. If there are PLR deficits in Alzheimer’s Disease, this would be an easy diagnostic to translate to clinical care. Whether it can be used to prognose cognitive impairment will depend on when PLR dysfunction occurs relative to other neural pathway dysfunction.

We’ve only performed a standard full field ERG on these mice. There are numerous tests that could be performed to differentiate the different components of the retina however, we are focusing on identifying histological markers of retinal changes. Other researchers have looked at both human and mouse models for some of these functions.

We have not looked at pattern ERGs or Visually Evoked Potentials (VEPs) which can both be used to assess retinal ganglion cells or the entire visual pathway, respectively. By contrast, the full field ERG, which are performing, evaluates outer retinal function (photoreceptor, inner retinal neurons). Other researchers have looked at both human and mouse models for these functions.

The analogy to a window implies a clear, easy view. This is possible in the retina where we can non-invasively assess global and local function with full-field and multi-focal electroretinography, visualize single photoreceptors with adaptive optics, and evaluate degeneration over time using optical coherence tomography. These tasks are typically more difficult, more invasive, and/or of lower resolution for the brain. There is no doubt that changes in the retina and the rest of the CNS are likely paralleled, however, the retina is simply much more amenable to examination, particularly in clinical patient care.