As the severity and scope of the
nation’s opioid crisis worsens, addition research becomes even more salient.
Most addictive drugs, including opioids, activate the reward system in the
human brain. The reward system deals in a neurotransmitter called dopamine.
 |
|
The brain's
reward system. (n.d.). Retrieved October 01, 2017,
from
https://bigpictureeducation.com/brains-reward-system
|
The
image shows a simplified pathway of dopamine in the reward center. The number 1
indicates the Ventral Tegmental Area (VTA). The VTA is a population of neurons
in the midbrain that sends dopamine on to Nucleus Accumbens (NAc), indicated by
the number 2. The dopamine pathway continues to the prefrontal cortex, a region
that is involved in many cognitive functions, such as decision-making. According to Dr. Steidl, a researcher here at
Loyola University Chicago who studies the reward pathway, “the dopamine system
can pick up on environmental cues that signal a potential reward” (Steidl,
2017).
Dr. Steidl recently gave a talk to
the Loyola neuroscience community, in which he described his studies on the
role of conditioned place preference in appetitively motivated behaviors, such
as drug addiction. A conditioned place preference is “a modified version of
Pavlovian conditioning” (Steidl, 2017) that results in a preference for a
location, even without a rewarding stimulus. Researchers at the Medical
University of South Carolina (MUSC) are also studying the role that a drug
user’s surroundings play in motivation to self-administer drugs. While both
research designs involve the administration of a reward to rodents via a
retractable lever-press apparatus, the two groups of scientists are using the
subfields of genetics to understand different aspects of addition. The findings
of both research teams bring us closer to an understanding of the role of
environment in addiction, and thus closer to predicting and potentially
preventing these “persistent and powerful triggers for relapse” (MUSC, 2017).
Dr. Steidl is using optogenetics to
understand the contribution of specific neural structures to the dopamine
reward system. Optogenetics refers to genes that encode light-sensitive
proteins. In Dr. Steidl’s work, the genes code for light-gated ion channels,
meaning his team can induce the activity of dopamine-releasing neurons in the
areas of interest. Dr. Steidl is interested in an area thought to input to the
VTA, thus preceding the known interactions of the dopamine system. This area is called the laterodorsal
tegmental nucleus (LDT-g) and is located in the brainstem. Dr. Steidl’s
optogenetics experiments demonstrate that excitation of the LDT-g elicits
rewarding effects, creating conditioned place preferences.
Down at the Medical University of
South Carolina, a team of researchers led by professor Christopher W. Cowan is
examining addiction in terms of epigenetics. Epigenetics is the subfield of
genetics that considers when certain genes are “turned on” and thus their
protein products are produced. Dr. Cowan’s goal is “to discover the brain
mechanisms responsible for the rewarding effects of the drug and the motivation
to seek it even after long periods of abstinence” (MUSC, 2017), specifically
through considering which genes are turned on as an addictive behavior
develops. This research led to an epigenetic enzyme called histone deacetylase
5 (HDAC5), which slows the rodent brain’s formation of associations between
cocaine and simple cues in the environment, such as light and sound. Cowan’s
team thought it had promise for preventing cues that would trigger a return to
drugs. In fact, it did prevent relapse after a one-week abstinence period for rodents.
It is important to note that while HDAC5 did prevent relapse, it did not
prevent from developing addictive behaviors in the first place. Another
significant gene identified is one called NPAS4, an early-onset gene that helps
form associations between cocaine and environmental cues. Cowan and his team
are hopeful that their research in this field can addiction mechanisms with
relevance “to multiple substance use disorders” (MUSC, 2017). In understanding
the mechanisms by which appetitevely motivated behaviors form and persist, we
can then develop methods to alleviate addiction and relapse.
Works Cited
(Image) The
brain's reward system. (n.d.). Retrieved October 01, 2017, from
https://bigpictureeducation.com/brains-reward-system
Medical
University of South Carolina (2017, September 27). Epigenetics of Addiction:
Untangling Relapse and Addiction in the Brain. NeuroscienceNews. Retrieved
September 27, 2017 from http://neurosciencenews.com/epigenetics-relapse-addiction-7597/
Steidl, S. (2017). Speech presented at Neuroscience Seminar,
Loyola University Chicago.
Steidl, S., Wang,
H., Ordonez, M., Zhang, S., & Morales, M. (2016). Optogenetic excitation in
the ventral tegmental area of glutamatergic or cholinergic inputs from the
laterodorsal tegmental area drives reward. European Journal of
Neuroscience, 45(4), 559-571. doi:10.1111/ejn.13436
