How the brain makes, and breaks, a habit

Neuroscience study identifies brain chemicals, neural pathway involved in switching between habitual behavior, deliberate decision-making

May 26, 2016

Science Daily/University of California - San Diego

Not all habits are bad. Some are even necessary. It's a good thing, for example, that we can find our way home on "autopilot" or wash our hands without having to ponder every step. But inability to switch from acting habitually to acting in a deliberate way can underlie addiction and obsessive compulsive disorders.

Working with a mouse model, an international team of researchers demonstrates what happens in the brain for habits to control behavior.

The study is published in Neuron and was led by Christina Gremel, assistant professor of psychology at the University of California San Diego, who began the work as a postdoctoral researcher at the National Institute on Alcohol Abuse and Alcoholism of the National Institutes of Health. Senior authors on the study are Rui Costa, of the Champalimaud Centre for the Unknown in Lisbon, and David Lovinger of the NIAAA/NIH.

The study provides the strongest evidence to date, Gremel said, that the brain's circuits for habitual and goal-directed action compete for control -- in the orbitofrontal cortex, a decision-making area of the brain -- and that neurochemicals called endocannabinoids allow for habit to take over, by acting as a sort of brake on the goal-directed circuit.

Endocannabinoids are a class of chemicals produced naturally by humans and other animals. Receptors for endocannabinoids are found throughout the body and brain, and the endocannabinoid system is implicated in a variety of physiological processes -- including appetite, pain sensation, mood and memory. It is also the system that mediates the psychoactive effects of cannabis.

Earlier work by Gremel and Costa had shown that the orbitofrontal cortex, or OFC, is an important brain area for relaying information on goal-directed action. They found that by increasing the output of neurons in the OFC with a technique called optogenetics -- precisely turning neurons on and off with flashes of light -- they increased goal-directed actions. In contrast, when they decreased activity in the same area with a chemical approach, they disrupted goal-directed actions and the mice relied on habit instead.

"Habit takes over when the OFC is quieted," Gremel said.

In the current study, since endocannabinoids are known to reduce the activity of neurons in general, the researchers hypothesized that endocannabinoids may be quieting or reducing activity in the OFC and, with it, the ability to shift to goal-directed action. They focused particularly on neurons projecting from the OFC into the dorsomedial striatum.

They trained mice to perform the same lever-pressing action for the same food reward but in two different environments that differentially bias the development of goal-directed versus habitual actions. Like humans who don't suffer from neuropsychiatric disorders, healthy mice will readily shift between performing the same action using a goal-directed versus habitual action strategy. To stick with the earlier example of getting home, we can switch the homing autopilot off and shift to goal-directed behavior when we need to get to a new or different location.

To test their hypothesis on the role played by endocannabinoids, the researchers then deleted a particular endocannabinoid receptor, called cannabinoid type 1, or CB1, in the OFC-to-striatum pathway. Mice missing these receptors did not form habits -- showing the critical role played by the neurochemicals as well as that particular pathway.

"We need a balance between habitual and goal-directed actions. For everyday function, we need to be able to make routine actions quickly and efficiently, and habits serve this purpose," Gremel said. "However, we also encounter changing circumstances, and need the capacity to 'break habits' and perform a goal-directed action based on updated information. When we can't, there can be devastating consequences."

The findings may suggest, the authors say, a new therapeutic target for people suffering from OCD or addictions: To stop overreliance on habit and restore the ability to shift from habit to goal-directed action, it may be helpful to treat the brain's endocannabinoid system and so reduce habitual control over behavior. Treatment could be pharmaceutical or might involve behavioral therapy. Further research is needed.

https://www.sciencedaily.com/releases/2016/05/160526185419.htm

Go for a run or eat chocolate: A choice dictated by the cannabinoid receptors

March 19, 2019

INSERM (Institut national de la santé et de la recherche médicale)

Physical inactivity is a common factor in lifestyle diseases -- and one that is often linked to the excessive consumption of fatty and/or sugary foods. The opposite scenario of excessive physical activity at the expense of caloric intake can also be harmful, as cases of anorexia nervosa illustrate. These data therefore point to the crucial need to research the neurobiological processes that control the respective motivations for exercise and food intake. A study by Inserm and CNRS researchers published on March 7, 2019 in JCI Insight reveals that the cannabinoid type 1 (CB1) receptors play an essential role in the choice between running and eating chocolatey food.

The authors of this paper had previously reported that the cannabinoid type-1 (CB1) receptors, present on several types of neurons, play a key role in performance during physical activity in mice. A conclusion based on the performances achieved by animals with free access to an exercise wheel -- a model in which it was not possible to distinguish the mechanism involved (motivation, pleasure...). Given that the motivation for a reward can only be estimated by measuring the efforts that the individual -- whether human or animal -- is prepared to make to get that reward, the researchers devised a model in which each access to the wheel was conditional on a prior effort. This involved the animal repeatedly introducing its snout into a recipient, an essential prerequisite for unlocking the wheel. After a training period during which the level of effort required to unlock the wheel remained the same, the mice were confronted with a test in which the effort required was gradually increased. When exposed to this test, the mice lacking CB1 receptors showed an 80 % deficit in the maximum effort they were prepared to make to unlock the wheel, and without a decrease in performance during their access to it. This finding indicates that the CB1 receptors play a major role in controlling motivation for exercise. The use of other genetically-modified mice also enabled the researchers to demonstrate that these CB1 receptors controlling motivation for exercise are located on GABAergic neurons.

The researchers then examined whether the CB1 receptors in the GABAergic neurons control the motivation for another reward: chocolatey food (like humans, mice love it even when they are otherwise well-fed). While the CB1 receptors also play a role in motivation for food -- albeit to a lesser extent than in motivation for exercise -- the CB1 receptors located on the GABAergic neurons are not implicated in the motivation for eating chocolatey food.

In our daily life, we are faced with an ongoing choice between various rewards. A fact which has encouraged the researchers to develop a model in which following a learning period the mice had the choice -- in return for the efforts described above -- between exercise and chocolatey food. The motivation for exercise was greater than that for chocolatey food, with the exception of the mice lacking CB1 -- whether generally or just on GABAergic neurons -- whose preference was for the food.

In addition to these findings indicating that the cannabinoid receptor is essential for the motivation for exercise, this study opens up avenues for researching the neurobiological mechanisms behind pathological increases in this motivation. One illustration is provided by anorexia nervosa which often combines the decreased motivation to eat with an increased motivation to exercise.

https://www.sciencedaily.com/releases/2019/03/190319121721.htm