In the past two weeks the Brain Update was focused on the prefrontal cortex, which has been characterized as a brain area that integrates information and is sensitive to the emotional intensity of events. This week we turn our attention to an area of the brain that receives information from the prefrontal cortex and is pivotal in translating that information into action: the dorsomedial striatum.
The striatum’s function is to influence motor behavior. When you decide (prefrontal cortex) that it’s time for your midmorning coffee jolt, you must convert that decision into moving out of your chair, reaching for your wallet, making your way to the Coffee Procurement Facility, and extracting the stack of money from your wallet in exchange for the magical elixir. Meanwhile, you smell the aroma of the coffee, and the baked goods that go so well with it. You hear the familiar voices of the baristas and your fellow addicts, and you experience the initial rush of that first sip. All of these sensory experiences are processed by various areas in the cortex, and then are sent to the striatum. If the decision to get coffee is followed by the positive outcome of the coffee’s effect, then the context surrounding this process gets linked to the enjoyable experience, and tomorrow your decision will lead you to the same place to get more coffee. With enough experience, the role of the prefrontal cortex in the decision-making process is minimized, and the striatum takes over. Habit formed.
You can understand why neuroscientists are interested in how the striatum converts an idea into action. It would be possible to help someone who has a serious addiction, to something like cocaine or heroin or nicotine, if the neural basis of compulsive behavior was understood. The authors of this week’s featured article also point out that even a minor malfunction in this system can lead to disorganized behavior, like that observed in obsessive-compulsive disorder or schizophrenia.
|This image is from a very cool study in which the researchers traced the path of new neurons in the hippocampus.|
In this week’s Journal of Neuroscience, Ferguson, Phillips, Roth, Wess, and Neumaier have published their research entitled Direct-Pathway Striatal Neurons Regulate the Retention of Decision-Making Strategies. Remember the awesome viral-vector optogenetics technique from last time? These authors used a very similar approach, but instead of selectively activating a set of neurons with a light, they used a drug that would only affect the neurons with a particular receptor protein inserted into them. So, the dorsomedial striatum was injected with the viral vector, which was taken up only by cells that make a specific neurotransmitter and are known to be connected to an output of the striatum, the so-called “direct” pathway. They injected two types of receptors, one that would stimulate the cells and one that would inhibit them. Once the receptors were expressed on this small population of cells, the drug that activates the receptor, but would have no other effect in the rat, could be injected. With Ferguson and colleagues’ technique, they could target a specific population of cells and thus show which part of the striatum’s complex circuitry was responsible for changing behavior.
The researchers examined the effect of this technique on decision making in rats. They trained the rats to press two levers for food. Pressing one lever gave the rat a single food pellet. Not bad, if you’re a hungry rat. Pressing the other lever, however, led to delivery of four food pellets: even better if you’re a hungry rat. Thus, one lever was deemed “high-reward” and the other “low-reward” and the rats demonstrated a clear preference for the high-reward option.
Whether the neurons were stimulated or inhibited during the training, the rats were still able to make the right choice. But, if they were asked a week later to do the same task, the ones that had their cells inhibited had trouble learning the difference between high and low reward levers, and they didn’t show a strong preference for the high-reward. Conversely, the rats that had their cells stimulated a week earlier learned the difference more efficiently and showed a greater preference for the high-reward option. One of the most interesting challenges in understanding addiction is to describe the brain changes that occur when a person develops a habit and permanently alters their need for a drug. Even after years of abstinence, an addict will report that they crave the drug, especially if they find themselves in a drug-related situation, like the coffee shop or a bar. Ferguson and her colleagues suggest that the neurons they studied in the dorsomedial striatum are involved in this long-term change in the brain’s response.
|Lace-weight cashmere (above) or classic shetland wool?|
So which of the following would you choose?
If you have prior experience with cashmere and you enjoy making lace, you might opt for the pretty green yarn, it being your “high-reward” option. But what if you are currently on a Fair Isle knitting kick? It depends on your experience.
|I expected to be cured of the fair isle compulsion for awhile once this was done.|
Were you especially pleased with your latest Fair Isle project? Are you in the same context (season, maybe) as when you had such an enjoyable experience making steeks? Any positive past knitting experience might have changed the sensitivity of your dorsomedial striatum, which would influence the choice you make in beginning your next project.
|I find the combination of colors so much fun to arrange.|
|But, this lacy confection is appealing, too.|