Question: I am interested in the drugs that cause addiction, because my recent research on analgesic shows that opioid family of analgesic is capable of causing dependency. I am wondering how some commonly known addictive drugs, including heroin and morphine (both are members of opiates ), are affecting our brains to cause dependency.

Answer: Dopamine plays a key role in drug dependency and addiction. It is an endogenous neurotransmitter produced at a relatively low level; however, it is involved in many essential tasks in our brain, for example, body movement control. The death of dopaminergic neurons, neurons that produce dopamine, can cause difficulties in movement, as seen in Parkinson’s disease patient.1 Dopamine is also essential in reward identification, and behavior reinforcement. The activation of dopaminergic neurons signals the presence of reward. In animal behavior study, activation of dopaminergic neurons by external stimuli would cause animal to reinforce its behavior.2

An opioid is defined as a class of chemical that binds the opioid receptor, including those naturally produced, semi-synthesized, and synthesized. Opiates is a subclass of the opioid family, mainly refering to the chemicals derived from opium. Our brain syntesizes endogenous opioids, like endorphins, that function as neurotransmitters. Morphine, heroin, and oxycodone are part of the opioid family that are semi-synthesized, and considered exogenous opioid.3

When an opioid activates its receptor, a series of signal cascade will change the opening status of ion channels on the neuron, therebyreducing its activity. In a brain region called ventral tegmental area (VTA), there are a high concentration of dopaminergic neurons and GABAergic inhibitory interneurons. Under normal circumstances, these interneurons maintain control over the excitability of the dopaminergic neurons by releasing the inhibitory neurotransmitter GABA onto dopaminergic neurons. However, when exogenous opioids are present, these excess opioids bind to the opioid receptor on GABAergic interneurons and reduce the activity of the interneurons and, in turn, reduce GABA release. This decreased inhibition causes the dopaminergic neurons to release more dopamine to downstream neurons.4

As mentioned before, dopamine is related to reward recognition and behavioral reinforcement. The excess dopamine produced during opioid intake would trigger the reinforcement of this intake action, thus forming the basis of addiction to the opiod.




  1. THE BRAIN FROM TOP TO BOTTOM. Retrieved July 5, 2016, from
  2. Redish, A. D. (2015). The mind within the brain: How we make decisions and how those decisions go wrong. United States: Oxford University Press.
  3. Buprenorphine education: Opiates and Opioids heroin and opium. Retrieved July 5, 2016, from
  4. THE BRAIN FROM TOP TO BOTTOM. Retrieved July 5, 2016, from
  5. CBI-Knowledgebase for addiction related genes. Retrieved July 5, 2016, from

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