Geschatte leestijd: 4 minutenResearchers have found a nerve cell in the brain that acts as a conductor over other cells that control habits. An insight that can aid in combating bad habits and addictions.
Bad Habits
We humans are creatures of habit. That’s often a good thing. Habits, automatism, routines, they allow us to perform many actions on autopilot. Something that saves a lot of mental capacity and thus energy. However, there are also many examples where we get trapped in certain bad habits. Think of dietary habits where rational considerations often prove no match for emotional needs.
It is therefore interesting to know exactly what happens in the brain when the little devil on your left shoulder speaks louder in your ear than the angel on the right shoulder.
Behavioral scientists describe habits as “stimulus-response” behavior. To develop a new habit, the help of a deep brain region called the (dorsolateral) striatum is needed. The outgoing neurons, which form 95% of the nerve cells in that area, respond differently to incoming signals. In mice that have learned a habit, at least. For example, by rewarding them with sugar when they press a switch. However, it was unclear how the altered functioning of these nerve cells in learning a habit comes about.
“Bad habits neuron”
Neuroscientists at Duke University have made an important discovery in this context. They published their findings this month in eLife [1]. They have discovered a type of neuron (nerve cell) that acts as a kind of on-off switch for (performing) habits. Developing habits activates this important nerve cell, while suppressing this neuron with medication turns out to be enough to unlearn a bad habit. In the case of the research, the learned habit of mice in their lab to get sugar by pressing a switch.
The studied nerve cell itself occurs in very low amounts compared to other nerve cells. For example, compared to the more common nerve cells known to control behavior. Through a web of connections, the “habit cell” (ed.) can exert its influence via these other nerve cells.
The researchers had already discovered last year that the development of habits leaves its mark on this area of the brain [2]. However, they saw that it was not the 95% outgoing nerve cells that provided the explanation, but a rare type of cell known as the fast-spiking interneuron [FSI]. This cell seems to work like a conductor, causing these changes in the other outgoing cells.
For that research, they let mice develop the habit by letting them press a switch where they received sugar. The extent to which a habit was learned was determined by measuring how long the mice continued to press the switch after they had already eaten and no more sugar followed as a reward. “Habit mice” continued to press the switch for a long time, others did not.
They compared the striatum in the brains of the mice that had developed a strong habit with the mice in which that had not happened. Here they saw that both the process that leads to action in these neurons (the ‘on’ button), and the process that inhibits action (the ‘off’ button) were stronger in the ‘sugar mice’. However, they also saw that the sequence of activating these processes changed, causing the ‘on’ button to be activated first and then the ‘off’ button. That may seem quite logical in this metaphorical layman’s language, but let’s just say I’m simplifying it here. What is logical for a light switch can be more complex in a nerve cell.
In the most recent study, the researchers wanted to learn more about the functioning of the FSI. The cell belongs to a class of neurons responsible for transmitting messages between other types of nerve cells in a specific area of the brain. Although they form only one percent of the nerve cells in the striatum, they have long, branch-like protrusions with which they are connected to the other neurons (which activate the on and off buttons). This raised the suspicion that there might be a single conductor behind this operation. Among other things because the FSI turned out to be more active in the mice that had learned the habit.
This idea seemed confirmed when they subsequently switched off the ‘conductor cell’, the FSI (using chemogenetics). The outgoing cells in the sugar mice then worked again as they did before the learned habit. This broke the habit.
“Prone to addiction”
This research concerns the learned bad habit of scoring sugar. An example that is easily translated into practice for humans. Witness only the fact that while writing this piece, I keep thinking about freshly baked bread with Nutella.
When we talk about “addiction-prone,” we often refer to the ease with which new habits are learned and the difficulty with which bad habits are unlearned. As these studies, like previous studies, show, developing a habit itself also makes one more sensitive to developing a habit. Addiction strengthens the susceptibility to addiction. A downward spiral, or upward, depending on the balance between positive and less constructive habits.
Insight into the workings of the brain offers opportunities to break this spiral.
Some harmful behaviors like compulsion and addiction in humans might involve corruption of the normally adaptive habit-learning mechanisms. Understanding the neurological mechanisms underlying our habits may inspire new ways to treat these conditions.
I firmly believe that to develop new therapies to help people, we need to understand how the brain normally works, and then compare it to what the ‘broken’ brain looks like.
Nicole Calakos, Duke University
References
- Justin K O’Hare, Haofang Li, Namsoo Kim, Erin Gaidis, Kristen Ade, Jeff Beck, Henry Yin, Nicole Calakos. Striatal fast-spiking interneurons selectively modulate circuit output and are required for habitual behavior. eLife, 2017; 6 DOI: 10.7554/eLife.26231
- O’Hare JK, Ade KK, Sukharnikova T, Van Hooser SD, Palmeri ML, Yin HH, Calakos N. Pathway-Specific Striatal Substrates for Habitual Behavior. Neuron. 2016 Feb 3;89(3):472-9. doi: 10.1016/j.neuron.2015.12.032. Epub 2016 Jan 21. PubMed PMID: 26804995; PubMed Central PMCID: PMC4887103.
