Wisconsin, Minnesota professors use rats to quantify link between chronic alcohol exposure, pathways related to Alzheimer's

Chronic excessive alcohol use is connected to an increase in the risk of cognitive decline and dementia, but how exactly does this occur? 

A group of researchers from the University of Minnesota and the University of Wisconsin carried out an experiment designed to shed light on the mechanisms involved. The study, sponsored by the Mayo Clinic, appears in the American Chemical Society journal ACS Omega, Dec. 6.

The authors note that "the percentage of older adult drinkers has had a greater escalation in recent years, and older adults often consume alcohol in a hazardous binge drinking problem."  

Therefore, they specifically designed their study to probe how chronic intermittent ethanol (alcohol) exposure affects the cognitive behavior of young rats versus aged rats.

The rodent experiments

The researchers used two groups of rodents to investigate the different molecular pathways that underlie the cognitive decline associated with Alzheimer's disease. These included eight young adult rats (8 weeks old) and eight retired breeder rats (70 weeks old).

First, the experimenters trained all the rats in a 6-foot-diameter water maze, having them swim to find a hidden underwater platform in a fixed position that allowed them to escape. After successful training in escaping the maze, the rats were divided into two groups, one that were subjected to an alcohol gavage every other day for 20 days, and another that received a water gavage every other day for 20 days.

To test their spatial memory, the experimenters then removed the underwater platform and allowed each rat to swim for 1 minute. An underwater camera was used to measure how far they got to the escape part of the maze. Here the young rats and the aged rats performed with no significant differences between each group. Also, the spatial memory of rats in each group that had the chronic intermittent ethanol exposure was not altered.

But when the experiment was changed to look at behavioral flexibility, the researchers found a differences. In these experiments, the submerged escape platform was moved 180 degrees. After two trials of training with the new setup, the rats were put through six trials. The young adult rats showed no impact of the chronic intermittent ethanol exposure, but the aged rats with chronic intermittent ethanol exposure had "significantly impaired behavioral flexibility performance."

The authors state: "Behavioral flexibility is a measure of cognition where subjects have to learn a new response strategy in the face of previously learned stimuli and therefore requires a measure of cognitive flexibility. The current results demonstrate a selective impairment in behavioral flexibility in aged animals undergoing CIEE [chronic intermittent ethanol exposure]."

Proteomic differences

The third part of the research investigated the proteome, the entire group of proteins that is part of a cell, tissue, or organism.

After the first sets of experiments were completed, the rats were sacrificed and their brains were dissected to look at changes in the hippocampus and perform protein analysis.

Here the researchers found notable changes in proteins and phosphosites in the aged rats with chronic intermittent ethanol exposure. In particular, the Prkcd (protein kinase C delta) protein was increased (upregulated) in this group, compared with the water-treated aged rats. This upregulation did not occur in the young adult rats who were treated with chronic intermittent ethanol exposure.

The authors note that Prkcd levels are implicated in the brain tissue of people with Alzheimer's disease, compared with people who don't have the disease.

Several other proteomic pathways were also found to be altered by chronic intermittent ethanol exposure in the aged rats, including the insulin receptor signaling pathway and insulin resistance, which are both associated with Alzheimer's disease.

Conclusions

The researchers summarize their results: “Our bioinformatic analysis revealed notable changes in pathways involved in neurotransmission regulation, synaptic plasticity, neuronal apoptosis, and insulin receptor signaling. In conclusion, our behavioral and proteomic results identified several candidate proteins and pathways potentially associated with alcohol-induced cognitive decline in aged adults.”

The authors note that future research is needed to validate the pathways that they identified with chronic intermittent ethanol exposure and to "further explore the molecular impact" of this exposure in aged animals. They also cited two caveats: their research was limited to male rats, and the brains dissected after sacrifice might not reflect immediate changes in the living animals.

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Ada Man-Choi Ho et al. "Chronic Intermittent Ethanol Exposure Alters Behavioral Flexibility in Aged Rats Compared to Adult Rats and Modifies Protein and Protein Pathways Related to Alzheimer’s Disease." ACS Omega, Dec. 6, 2022.

DOI: 10.1021/acsomega.2c04528