Studying ACh In Rats
January 24, 2013

Researchers Train Rats To Respond To Visual Cues

Alan McStravick for — Your Universe Online

Ivan Pavlov, a Russian physiologist, was instrumental in the formation of the school of study of conditioned response. His work, spanning some 40 years, has pervaded the public consciousness ever since. We are all, in one way or another, unwitting participants in his research. For instance, who among us doesn´t jump up to grab our mobile phone when it dings at us, signaling an incoming text message or e-mail?

Researchers at Johns Hopkins University School of Medicine and the Massachusetts Institute of Technology have released findings of a recent study in the current issue of the journal Neuron. Their research, supported by grants from the National Institute of Mental Health, the National Institute on Drug Abuse, the National Eye Institute, the National Institute of Child Health and Human Development and Johns Hopkins University, shows how their work with rat subjects allowed them to pinpoint the positive and negative reward centers in the brain.

The research team created tiny, light-emitting goggles with which their rat subjects were outfitted. The scientists claim they have learned the brain´s initial vision processing center not only works to relay visual stimuli, but that it can also “learn” time intervals, creating specifically timed expectations of future rewards among their test subjects. They state this research helps to focus on both learning and memory-making. The study could lead to an explanation for why people with Alzheimer´s disease often have difficulty recalling recent events.

Marshall Hussain Shuler, PhD, assistant professor of neuroscience at the Institute for Basic Biomedical Sciences at the Johns Hopkins University School of Medicine states the conclusions of their study suggest connections within nerve cell networks in the vision-processing center are able to be improved by the neurochemical acetylcholine (ACh), a brain chemical that is, it is believed, secreted after a reward is received. The light-emitting goggles allowed the team to observe nerve cell networks that were able to be specifically targeted. The flashes of light, which were sent through the goggles, showed that the targeted network was the only area affected by ACh. This is important to note because this shows those nerve networks were able to associate the visual cue with the upcoming reward. The researchers see a parallel between the rat and human for this study because the brain structures of mammals are typically highly conserved.

"We've discovered that nerve cells in this part of the brain, the primary visual cortex, seem to be able to develop molecular memories, helping us understand how animals learn to predict rewarding outcomes," says Hussain Shuler.

As we progress through our lives, it is important our brains learn to remember which cues precede a positive or a negative event. This allows us to maximize our survival traits. We can, through this neural memory, alter our behavior to increase rewards received and to decrease negative consequences to our actions. The Hopkins-MIT study worked to establish clarity about how the brain links visual information to more complex information about both timing and reward.

Until now, according to Hussain Shuler, the popular theory assumed this connection was made in areas of the brain devoted to “high-level” processing, like the frontal cortex. The frontal cortex, it is known, is important for both learning and memory. The prevailing theory stated the primary visual cortex was only useful in receiving information from the eyes and then collecting that data to “re-piece” the visual world together before presenting it to the decision making parts of the brain.

As mentioned above, the method the researchers used in this study involved specially designed goggles that allowed the researchers to select whether a flash of light was presented to either the left or right eye of their rat subjects. Within the testing chamber was a water spout meant to act as the reward for the rats. As they would approach the spout, the researchers would present a brief visual cue to one eye of the animal.

When the rat received a visual cue to the left eye, they would need only lick the spout a few times before water was distributed. On the other hand, if light was presented to the right eye, the rat would have to lick many more times before water came. It required only a few daily conditioning sessions before the rats would learn the timing intervals for when water would be released. If, after receiving their visual cue, the water did not come in the expected amount of time, the rats would simply abandon their effort and move away from the water spout.

The team was able to determine, after monitoring the pattern of electrical signals given off by individual nerve cells in the rat brains, that the signals´ “spikes” weren´t solely reflecting the visual cue, alone. In fact, the signals seemed to relay the time of expected reward delivery through an altered spiking pattern. Additionally, they observed how many of the nerve cells would report one or the other visual cue-reward interval, but not both.

To better explain this idea, cells which were stimulated by a flash of light to the left eye would return to its baseline level after only a short delay that appeared synced to the timing of the water reward. Alternately, a cue to the right eye showed a correlation with a longer delay that was also in sync with the lengthened reward timing. The researchers claim this finding shows the return to the nerve cells resting state was the brain´s way of setting up a “timed expectation.”

The researchers next needed to see if they could follow the ACh delivery to the vision-processing center from the basal forebrain. The basal forebrain, it is known, plays an important role in learning. What the researchers needed to do was implement a testing method that could remove the nerve cells from the equation. To do this, they paired a neurotoxin with a sort of “homing device” that specifically targeted the ACh-releasing neurons coming from the basal forebrain. Using the already conditioned rats, they then repeated their experiments, utilizing some rats that had received the neurotoxin and others that had not. What they learned was nerve cell signals continued to relay the old time intervals. This suggested, according to the research team, ACh and the basal forebrain were not necessary in the expression of previously learned time information.

The next experiment involved the same rats and was performed to find if ACh was necessary for nerve cells to learn new time delays. This experiment involved switching the visual cues so the left eye receiving a light flash would result in a longer delay before the water reward was presented, and a flash of light to the right eye meant a shorter delay. Rats, for whom the vision-processing nerve cells were still able to receive ACh, were able to adapt to the signal and timing switch. However, those for whom the delivery of ACh was inhibited continued to relay the old associations. The team claims this suggests ACh is necessary to make new associations but not for the expression of old ones.

"When a reward is received, ACh is sent throughout the brain and reinforces only those nerve cell connections that were recently active,” according to Hussain Shuler. “The process of conditioning continues to strengthen these nerve connections, giving rise to a timed expectation of reward in the brain."

Previous studies, according to Hussain Shuler, have shown Alzheimer´s patients typically have lower levels of ACh and, therefore, have trouble forming new memories. While the use of pharmaceuticals can work to elevate ACh, the alleviation of the symptoms is minimal, at best. “Our research explains that limitation. Therapeutically, we predict that the problem isn´t just low levels of ACh — the timing of ACh delivery is key.”

The work represented in this study is, for sufferers and family members of Alzheimer´s patients, an important step forward in understanding and eliminating the disease known as “the long goodbye.”

Other authors of the report include Emma Roach of the Johns Hopkins University School of Medicine and Alexander Chubykin and Mark Bear of the Massachusetts Institute of Technology.