In this study we combined ERG recordings with measurements of visuomotor sensitivity and immunohistochemistry to determine how treatment with a carbohydrate-rich diet affects retinal function and structure in a mouse model of chronic hypoglycemia. We found that treatment with a carbohydrate-rich diet raised blood glucose levels and improved some aspects of retinal function, as measured with the ERG, but that it did not rescue synaptic connections in the OPL within the 1-month duration of our study. These results have important implications for our understanding of the connection between metabolism and retinal function and structure.
mice exhibit late-onset retinal degeneration and loss of retinal function.18
The extent of cell death is modest compared with the substantial losses in retinal function. In this study, we show that Gcgr−/−
mice experience a disruption of the synaptic contacts between rods and second-order cells that parallel the early signs of reorganization observed in other forms of retinal degeneration. Such changes in connectivity, also known as remodeling, are a common event in degenerations caused by defects in the sensory retina and is generally associated with photoreceptor death (see reviews by Marc et al.27
and Jones and Marc28
). Cone death,29,30
and lack of rods during development36
are associated with cellular remodeling and ectopic synaptogenesis. Cellular remodeling can also result from aging,37,38
and detachment and reattachment of the RPE.40
Maintenance of established synapses in the central nervous system depends both on the presence of neurotrophic factors and the level of synaptic activity.41
Our results suggest that in Gcgr−/−
retinas, synapses located at the dendritic tips and farthest from the bipolar cell body are particularly vulnerable to metabolic stress. Although the cause of selective loss (as opposed to uniform loss) of synapses is unclear, it is possible that a hierarchy or spatial gradient exists, whereby synapses located in the periphery receive fewer metabolic resources than those positioned closer to the perinuclear housekeeping machinery. By this scenario, apical synapses may be at increased risk during the chronic hypoglycemia and metabolic deprivation characteristic of Gcgr−/−
mice. Synaptic loss has also been linked to chronic stress42,43
and related pathologic conditions such as Cushing's disease.44
mice have altered stress responses,17
we cannot rule out that stress consequent to chronic hypoglycemia is detrimental to the long-term stability of apical rod-to-bipolar cell synapses.
The physical loss of synapses cannot account for the loss of ERG sensitivity in Gcgr−/−
mice. Indeed, the 20% reduction in the number of rod-to-bipolar cell contacts that we measured is probably insufficient to explain the substantial (~10- to 100-fold) loss in ERG b-wave sensitivity (assuming that every synapse contributes equally to the generation of the postsynaptic signal in rod bipolar cells45
). Further support for this notion follows from the observation that treatment with a carbohydrate-rich diet promotes a strong recovery of the ERGs without a corresponding recovery in the number of synaptic contacts. On the other hand, the permanent 20% reduction in the number of synapses between rods and bipolar cells may explain, to a good extent, the failure of the ERG b-wave to recover completely, which, as shown in and , is approximately 30% below normal.
A major result of this study was the demonstration that long-term treatment with a carbohydrate-rich diet can rescue retinal sensitivity in Gcgr−/−
mice. Because glucose is essential for retinal function,46–48
the expectation is that acute restoration of the amount of available glucose in Gcgr−/−
mice should lead to an immediate reestablishment of retinal function. However, when we previously tested this hypothesis, we found that acute increments in blood glucose levels by ingestion of dextrose did not improve retinal function in these mice, suggesting that metabolically challenged retinal cells lose their function irreversibly.18
In this study, we administered a carbohydrate-rich diet to elevate blood glucose levels for an extended period. Blood glucose levels recovered almost completely after 1 month of treatment. Surprisingly, the treatment also rescued retinal function. The sluggish reversal of the loss of retinal function suggests that retinal cells can sense their energy status, perhaps by way of the AMP-kinase, a regulator of metabolic energy balance,49–51
and can adjust the allocation of energy resources between metabolically demanding tasks involved in signaling the absorption of photons47,48,52,53
and processes that promote cell survival during metabolic stress conditions.
In summary, short-term reductions in glucose availability cause dramatic yet readily reversible alterations in retinal and visual function.4,10,11
However, chronic hypoglycemia also leads to loss of neurons18
and synaptic remodeling. In mice, the improvement of retinal function is possible after 1 month of treatment with a carbohydrate-rich diet; however, the same treatment did not rescue synaptic connectivity. Future studies should aim at determining end points for the recovery of retinal function.