From the thrill of hearing an ice cream truck approaching to the spikes of
pleasure while sipping a fine wine, the neurological messenger known as
dopamine has been popularly described as the brain's "feel good" chemical
related to reward and pleasure.
A ubiquitous neurotransmitter that carries signals between brain cells,
dopamine, among its many functions, is involved in multiple aspects of
cognitive processing. The chemical messenger has been extensively studied
from the perspective of external cues, or "deterministic" signals. Instead,
University of California San Diego researchers recently set out to
investigate less understood aspects related to spontaneous impulses of
dopamine. Their results, published July 23 in the journal Current Biology,
have shown that mice can willfully manipulate these random dopamine pulses.
Rather than only occurring when presented with pleasurable, or reward-based
expectations, UC San Diego graduate student Conrad Foo led research that
found that the neocortex in mice is flooded with unpredictable impulses of
dopamine that occur approximately once per minute.
Working with colleagues at UC San Diego (Department of Physics and Section
of Neurobiology) and the Icahn School of Medicine at Mount Sinai in New
York, Foo investigated whether mice are in fact aware that these impulses --
documented in the lab through molecular and optical imaging techniques --
are actually occurring. The researchers devised a feedback scheme in which
mice on a treadmill received a reward if they showed they were able to
control the impromptu dopamine signals. Not only were mice aware of these
dopamine impulses, the data revealed, but the results confirmed that they
learned to anticipate and volitionally act upon a portion of them.
"Critically, mice learned to reliably elicit (dopamine) impulses prior to
receiving a reward," the researchers note in the paper. "These effects
reversed when the reward was removed. We posit that spontaneous dopamine
impulses may serve as a salient cognitive event in behavioral planning."
The researchers say the study opens a new dimension in the study of dopamine
and brain dynamics. They now intend to extend this research to explore if
and how unpredictable dopamine events drive foraging, which is an essential
aspect of seeking sustenance, finding a mate and as a social behavior in
colonizing new home bases.
"We further conjecture that an animal's sense of spontaneous dopamine
impulses may motivate it to search and forage in the absence of known
reward-predictive stimuli," the researchers noted.
In their efforts to control dopamine, the researchers clarified that
dopamine appears to invigorate, rather than initiate, motor behavior.
"This started as a serendipitous finding by a talented, and curious,
graduate student with intellectual support from a wonderful group of
colleagues," said study senior co-author David Kleinfeld, a professor in the
Department of Physics (Division of Physical Sciences) and Section of
Neurobiology (Division of Biological Sciences). "As an unanticipated result,
we spent many long days expanding on the original study and of course
performing control experiments to verify the claims. These led to the
current conclusions."
Reference:
Conrad Foo, Adrian Lozada, Johnatan Aljadeff, Yulong Li, Jing W. Wang, Paul
A. Slesinger, David Kleinfeld. Reinforcement learning links spontaneous
cortical dopamine impulses to reward. Current Biology, 2021; DOI:
10.1016/j.cub.2021.06.069