To do or not to do: Cracking the code of motivation

Date:
August 6, 2021
Source:
The National Institutes for Quantum and Radiological Science
and Technology
Summary:
By manipulating the dopamine receptors of monkeys and studying their
behavior, scientists reveal how our brain makes us feel motivated.



FULL STORY ==========================================================================
Our motivation to put effort for achieving a goal is controlled by a
reward system wired in the brain. However, many neuropathological
conditions impair the reward system, diminishing the will to
work. Recently, scientists in Japan experimentally manipulated the reward system network of monkeys and studied their behavior. They deciphered a
few critical missing pieces of the reward system puzzle that might help
in increasing motivation.


==========================================================================
Why do we do things? What persuades us to put an effort to achieve goals, however mundane? What, for instance, drives us to search for food? Neurologically, the answer is hidden in the reward system of the brain
-- an evolutionary mechanism that controls our willingness to work
or to take a risk as the cost of achieving our goals and enjoying the
perceived rewards. In people suffering from depression, schizophrenia,
or Parkinson's disease, often the reward system of the brain is impaired, leading them to a state of diminished motivation for work or chronic
fatigue.

To find a way to overcome the debilitating behavioral blocks,
neuroscientists are investigating the "anatomy" of the reward system and determining how it evaluates the cost-benefit trade-off while deciding on whether to pursue a task. Recently, Dr. Yukiko Hori of National Institutes
for Quantum and Radiological Science and Technology, Japan, along with
her colleagues have conducted a study that has answered some of the
most critical questions on benefit- and cost-based motivation of reward systems. The findings of their study have been published in PLoS Biology.

Discussing what prompted them to undertake the study, Dr. Hori explains: "Mental responses such as 'feeling more costly and being too lazy to act,'
are often a problem in patients with mental disorders such as depression,
and the solution lies in the better understanding of what causes such responses. We wanted to look deeper into the mechanism of motivational disturbances in the brain." To do so, Dr. Hori and her team focused
on dopamine (DA), the "neurotransmitter" or the signaling molecule that
plays the central role in inducing motivation and regulation of behavior
based on cost-benefit analysis.

The effect of DA in the brain transmits via DA receptors, or molecular
anchors that bind the DA molecules and propagate the signals through
the neuronal network of the brain. However, as these receptors have
distinct roles in DA signal transduction, it was imperative to assess
their relative impacts on DA signaling. Therefore, using macaque monkeys
as models, the researchers aimed to decipher the roles of two classes
of DA receptors -- the D1-like receptor (D1R) and the D2-like receptor
(D2R) -- in developing benefit- and cost-based motivation.

In their study, the researchers first trained the animals to perform
"reward size" tasks and "work/delay tasks." These tasks allowed them to
measure how perceived reward size and required effort influenced the task-performing behavior. Dr. Takafumi Minamimoto, the corresponding
author of the study explains, "We systematically manipulated the D1R and
D2R of these monkeys by injecting them with specific receptor-binding
molecules that dampened their biological responses to DA signaling. By
positron emission tomography-based imaging of the brains of the animals,
the extent of bindings or blockades of the receptors was measured." Then,
under experimental conditions, they offered the monkeys the chance to
perform tasks to achieve rewards and noted whether the monkeys accepted
or refused to perform the tasks and how quickly they responded to the
cues related to the tasks.

Analysis of these data unearthed some intriguing insights into the neurobiological mechanism of the decision-making process. The researchers observed that decision-making based on perceived benefit and cost required
the involvement of both D1R and D2R, in both incentivizing the motivation
(the process in which the size of the rewards inspired the monkeys to
perform the tasks) and in increasing delay discounting (the tendency to
prefer immediate, smaller rewards over larger, but delayed rewards). It
also became clear that DA transmission via D1R and D2R regulates the
cost-based motivational process by distinct neurobiological processes
for benefits or "reward availability" and costs or "energy expenditure associated with the task." However, workload discounting -- the process
of discounting the value of the rewards based on the proportion of the
effort needed -- was exclusively related to D2R manipulation.

Prof. Hori emphasizes, "The complementary roles of two dopamine receptor subtypes that our study revealed, in the computation of the cost-benefit
trade- off to guide action will help us decipher the pathophysiology of psychiatric disorders." Their research brings the hope of a future when
by manipulating the inbuilt reward system and enhancing the motivation
levels, lives of many can be improved.

========================================================================== Story Source: Materials provided by The_National_Institutes_for_Quantum_and_Radiological
Science_and_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yukiko Hori, Yuji Nagai, Koki Mimura, Tetsuya Suhara, Makoto
Higuchi,
Sebastien Bouret, Takafumi Minamimoto. D1- and D2-like receptors
differentially mediate the effects of dopaminergic transmission on
cost- benefit evaluation and motivation in monkeys. PLOS Biology,
2021; 19 (7): e3001055 DOI: 10.1371/journal.pbio.3001055 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210806104359.htm

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