Addiction is a disease that results from a combination of behavioral, environmental and biological factors. In Dr. Stephan Steidl’s article, “Opioid-induced rewards, locomotion and dopamine activation: A proposed model for control by mesopontine and rostromedial tegmental neurons”, he and his colleagues assessed the relationship between the dopamine rewards system and environmental cues in relation to motivated behavior.
In Dr. Steidl’s experiments, he utilized optogenetic activation in rats, to observe dopamine uptake in response to glutamate and acetylcholine. In each experiment, the rodent subject was given the ability to self-administer the neurotransmitter. For example, in the glutamate studies, rats were given the ability to self-administer glutamate simply by entering one of two chambers: a light (stimulation) chamber or a non-stimulation chamber. When rats entered the light chamber, a blue light would be sent to stimulate the ventral tegmental area (VTA). Seeing as though stimulation could only be disengaged by the rat leaving the chamber, the amount of stimulation each rat received was contingent upon independent volition.
At the conclusion of the experiments, it was found that the rats in the glutamate studies spent less time in the stimulation chamber, but returned more frequently then they had in the acetylcholine experiments. Steidl explains how this finding may be attributed to acetylcholine having a slower dopamine uptake, but a longer dopamine activation period; whereas glutamate may have a faster dopamine uptake, but a shorter dopamine activation period. Thus, the rats in the glutamate experiments were motivated to return to the stimulation chamber more frequently to get the dopamine stimulation. The same may be true for those struggling with opioid addiction.
A recent study conducted by the University of Minnesota Medical School, revealed that dopamine neurons assign extrinsic values to environmental cues. It is through these values, that dopamine is able to motivate our actions and behaviors. Through the use of a Pavlovian model, researchers were able to assess whether a simple cue (such as turning on a light) before the activation of dopamine neurons would motivate behavior. Two types of dopamine neurons were observed: those found within the substantia nigra (SN) and those within the ventral tegmental area (VTA). Researchers discovered that the different responses elicited, were dependent upon the location of the neuron activated (either within the SN or the VTA). Those neurons located within the SN evoked a more rapid and energetic response, to which they described as “get up and go”. Whereas those cues that activated the neurons in the VTA produced a response analogous to “where do I go?”. Seeing as though the activation of VTA neurons has been linked to addiction, the results of the experiment demonstrate that the brain can assign a motivational value to certain environmental cues; ultimately leading to the release of dopamine. In other words, an opioid addict may be motivated to relapse (or increase their drug use) when environmental cues (such as a bar, or a needle) are presented and cause an increase in dopamine release.
The research outlined in Dr. Steidl’s talk, as well as the research conducted by the University of Minnesota, demonstrate how important the interplay between environmental cues and the dopamine reward system is. However, if proven dysfunctional, these environmental cues can have maladaptive consequences -one of which being sensitization. In the case of opioid addiction, the constant administration (or injection) of opioids, such as heroin, results in behavioral sensitization. Steidl describes behavioral sensitization as the long-term enhancement ability of a stimulus to activate dopamine neurotransmitters and elicit appetitive behaviors (cravings). Dr. Steidl’s research on rats with pre exposure to amphetamines, demonstrates this enhanced dopamine response. In his study, the rats with a history of amphetamine use worked harder to obtain an I.V. drug, and displayed enhanced dopamine reactivity and dependence; all of which are anticipated with sensitization.
Although the future of the opioid epidemic looks bleak, by utilizing the information presented in these two studies, researchers can better understand the process of addiction. Researchers at the University of Minnesota, hope that future studies will help to decipher the difference between healthy motivation and dysfunctional motivation -giving a solution to the drawn-out crisis of addiction.
Works cited:
University of Minnesota Medical School(2018, August 3). How Cues Drive Our Behavior. NeuroscienceNews. Retrieved August 3, 2018 from http://neurosciencenews.com/behavior-cues-dopamine-9654/
Addiction as a Disease. (2017, April 14). Retrieved October 18, 2018, from https://www.centeronaddiction.org/what-addiction/addiction-disease
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