Sribas Chowdhury, Adamas University, Kolkata
The human brain is a fascinating organ. Holding the key to a whole lot of mysteries of human psyche and behavioral patterns, it has drawn attention of numerous researchers across the world for decades. One such research team from Bruchas Lab at University of Washington was intrigued about dopamine. It is a neurotransmitter that is known to play a key role in giving rise to our senses of achievement or excitement associated to any work or any other exciting incident. These senses, in words of neuroscientists, are termed as a “reward”. Interestingly, during the study, the researchers eventually found out that only holding dopamine responsible isn’t a fair judgement. The study revealed that in addition to dopamine, there’s yet another factor involved in the work-for-reward function of our brain.
The tale of dopamine and reward
The motivational gurus nowadays seem to be very fond of the word “dopamine-rush”. There are tons of articles and videos on how to increase dopamine levels. But most of them have rather vague information on how it actually works. So what is dopamine? As we might know, the neurons are the basic units on which our brain functions. They receive and send signals from different parts to brain and vice versa, which enables us to perceive things as they are. Neurotransmitters serve as the carriers of those signals. They are chemical messengers which pick up electrical impulses from any part of the body and transport them to their target destination.
Dopamine is one such neurotransmitter. In particular, it serves the purpose of giving us a sense of excitement, achievement, pleasure or lust. When any incident occurs that excites us or gives us pleasure, the neurons that produce dopamine are activated. These then form synapses with the help of dopamine to give us that sense of pleasure and to also create a memory associated with the event. This is known as the reward system of brain. Our expectations arise mostly due to the formation of these pleasure related memory synapses and dopamine plays the key role in it. The events in involved in this function of brain are collectively called a reward pathway or reward circuit. The most commonly accepted one is the mesolimbic reward pathway.
The mesolimbic reward pathway
The neurons that release dopamine originate from the Ventral Tegmental Area (VTA), located in the mid-brain. These are therefore, called VTA neurons. The mesolimbic pathway connects the VTA to the ventral striatum, present in forebrain. The ventral striatum consists of a structure called nucleus accumbens. The pathway describes that the nucleus accumbens serves as the interface for the VTA neurons to form synapses during any rewarding stimuli. This then facilitates the behavioral pattern related to that reward and defines the reward-related learning of motor neurons.
Dopamine, being the primary neurotransmitter associated with VTA neurons, plays an important role in the reward pathway. Due to its effect on behavioral pattern, dopamine is also believed to be one of the key factors behind behavioral disorders like drug addiction, abuse, trauma and other mental problems like schizophrenia, depression etc.
Looking beyond dopamine
The study conducted by the researchers revealed that there’s another underlying pathway behind the reward system. It was carried out on male and female mice to asses the effect of dopamine and other neurotransmitters on them under reward-related stimuli. Interestingly, the results revealed that approximately 30% of neurons in VTA region are GABA neurons. GABA (gamma-aminobutyric acid) is a type of neurotransmitter that blocks impulses in the brain.
These GABAergic neurons (neurons that use GABA are called GABAergic neurons) were found inhibit cholinergic interneurons, which are known to be primarily associated with reward related learning. Moreover these neurons seemed to project from VTA to the ventral part of nucleus accumbens, the region responsible for reward reinforcement. This gives off the notion that maybe, dopamine aren’t the primary players in the reward circuitry of the brain. A possibility arises that the reward system is a combination of antagonistic neurotransmitters to execute reward related learning in a controlled manner.
Significance and future prospects
The study was medically significant due to its deep insights into the functioning of sub regions of the brain and how those parts contribute to its efficient functioning. According to lead researcher Dr. Gowrishankar, this is just a puzzle in the piece to understand how the brain is wired as a bigger picture. The scientists are hopeful that this study will give them more insights into disorders like drug addiction, abuse, use of nicotine and other drugs, that arise due to altering reward pathways. It is also hoped that the VTA neurons will serve as the mediator towards understanding and treating mental health problems like schizophrenia, anxiety, depression in an effective manner. The study showed one way of how the differences play out in our brain. This, in future, can give us better insights into the plasticity and wiring of the brain to understand the organ with greater clarity.
Also read: A rare and aggressive form of early onset Alzheimer’s disease
Reference:
- Al-Hasani, R., Gowrishankar, R., Schmitz, G. P., Pedersen, C. E., Marcus, D. J., Shirley, S. E., Hobbs, T. E., Elerding, A. J., Renaud, S. J., Jing, M., Li, Y., Alvarez, V. A., Lemos, J. C., & Bruchas, M. R. (2021). Ventral tegmental area GABAergic inhibition of cholinergic interneurons in the ventral nucleus accumbens shell promotes reward reinforcement. Nature Neuroscience. https://doi.org/10.1038/s41593-021-00898-2
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