Ventral Tegmental Area (VTA)

Introduction

The Ventral Tegmental Area, also known as the VTA, is a small cluster of neurons in our midbrain right in the bottom center. The VTA plays a vital role in the regulation of our body’s reward system. It comprises a bunch of neurons positioned in the middle of the brain on the midbrain’s floor.

 It is known for containing the cell bodies of dopamine, also known as dopaminergic neurons. It performs a vital role in our reward cognition, inhibiting and regulating emotions and learning. Apart from dopamine cells, the VTA also consists of GABA and Glutamate neurons. They also have a significant role. 

The ventral tegmental area gets knowledge from different areas of the brain that tell it about the essential human needs and desires and how they are fulfilled. VTA transfers this information towards another part of the brain called the nucleus accumbens using dopamine neurons as a messenger. 

This action takes place on a specific pathway from the VTA through the nucleus accumbens onto the other parts of the brain. This pathway is called the mesolimbic pathway. This whole activity is performed in a very synchronized manner, and the dopamine cells are released concurrently with the help of GABA and Glutamate neurons present in the VTA. 

Location

The ventral tegmental is located right in the centre of our brain, right next to a brain part called substantia nigra. While the ventral tegmental area carries various types of neurons, it is widely known for carrying dopaminergic neurons. 

The Ventral tegmental area releases these neurons through the mesolimbic pathway towards other parts of the brain, completing the reward circuit. The VTA is deemed an integral part of our brain’s reward structure responsible for reinforcing desired behaviors in us by releasing dopamine, the reward hormone.

 The blue dot in the image above signifies the location of VTA in our brain.

Structure: 

VTA is made up of a cluster of neurons situated in the lower central part of the human brain, primarily responsible for transmitting dopaminergic neurons to other parts of the brain, i.e. cortical and limbic areas.

 Due to the heterogeneous cytoarchitectonic nature and lack of clear borders that segregate it from other parts, the VTA is considered an ‘Area’ in the brain rather than a ‘part’ by neurobiologists. The VTA is located right next to the substantia nigra. Initially, the neurologists faced great difficulty in distinguishing it from the substantia nigra due to the proximity and lack of boundary. 

To understand the structure and role of the ventral tegmental area, it is necessary to understand the subpopulation of neurons in this area. As per the present knowledge, the VTA consists of three types of neurons: Dopamine neurons, GABA neurons, and Glutamate neurons. 

These three types of neurons make up the Ventral Tegmental Area and all three types of neurons are regulated from here to other parts of the brain depending on the need and desire through the designated pathways. The VTA is a complex structure in our brain that contains various pathways for regulating the neurons depending on the need.

Subdivisions:

Currently, neuroscientists have subdivided the VTA into four zones or subareas. The Parabrachial pigmented area, Paranigralis nucleus area, and the Rostromedial tegmental nucleus, and the Parafasciculus retroflexus area.

 The paranigral and parabrachial areas are densely populated with a high level of dopamine neurons in terms of the types of neurons these subareas carry. In contrast, the other two parts, parafasciculus, and Rostrmedial contain a low level of dopamine. On the other hand, the paranigral and parabrachial area contains large-sized and separately stained tyrosine hydroxylase neurons.

In contrast, the parafasciculus and rostrmedial areas contain a low density of small to moderately sized tyrosine hydroxylase neurons. All these neurons are essential for our healthy functioning, and these neurons are co-released in a combination to meet and require desires. Currently, more research is being carried out to find more about the subparts of the VTA and their role in our brain activity. 

The function of VTA:

Out of many designated functions, the Ventral tegmental area plays an important function in regulating dopamine through the reward circuit. The VTA has a role in all the behavioral and mental activities in which dopamine is released in our brain. Any motivating behavior, pleasurable activity, chemical and non-chemical addiction and any behavior that contains a reward are known as dopamine-related functions. 

The release of dopamine gives a sense of achievement and pleasure to the person, and in return, the chances of repeating that behavior is reinforced to get that dopamine rush again. The VTA releases dopamine through a designated mesolimbic pathway, and the dopamine neurons travel and complete the circuit creating pleasure and good feelings.

 Apart from the dopamine neurons, the VTA also consists of the GABA neurons and Glutamate neurons. The VTA is also responsible for releasing these neurons along with the dopamine in a co-releasing format allowing these two to inhibit dopamine. GABA is considered a chief inhibitory neuron that is responsible for our inhibitions. It inhibits us from behaving or thinking, or saying incongruently. 

Clinical Significance:

Disorders of VTA:

The Ventral tegmental area has been linked with numerous developmental, neurodegenerative, and psychopathological disorders, i.e. Schizophrenia, Drug Addiction, Major Depressive Disorder, Attention deficit hyperactivity disorder, and Parkinson’s Disease, to name a few. Dopamine, being a key neurotransmitter affected in most of the aforementioned disorders, is predominantly found in the VTA. 

It is estimated that out of all the neurons present in VTA, 60 % consist of dopamine. VTA has a vital clinical significance in terms of psychiatric disorders as the imbalance of dopamine is one of the key contributors to these disorders. Since it is regulated from the VTA, the VTA has a key clinical significance when linked with the disorders.

Drug Addiction:

The ventral tegmental area and the nucleus accumbens are the two main areas where any licit or illicit drugs cause the addiction. The licit drugs consist of prescribed medicines containing dependency effects, such as anti anxiety medicines or sleeping pills. Illicit drugs are illegal drugs, i.e. cocaine, hash, alcohol, marijuana, etc.

 When an individual consumes these drugs, dopamine is released from the VTA, which passes through its pathway into the nucleus accumbens and further into other parts pleasure to the person taking that drug. The release and pleasure effect of dopamine due to the intake of the drug is so soothing and pleasurable that the person keeps on taking that drug to get that pleasure again and again.

 The repetitive release of dopamine results in a permanent change in the mesolimbic pathway. With time, our brain and body adapt our delimitation of pleasure and reward with dopamine release due to addiction. Small achievements or awards due to other things don’t give that pleasure which a person gets when he takes a drug.

 It is when a person becomes dependent and develops tolerance. The VTA fails to provide the same stimulation of dopamine; hence the dose of the drug is increased to achieve that dopamine rush. This cycle continues and the person becomes more dependent, so the dopamine starts to deplete, and the VTA fails to provide the same amount of dopamine when a drug is taken. 

Comparative anatomy & evolution:

Neuroscientists have been studying the ventral tegmental areas for decades to understand their structure and function. The studies have emphasized significant similarities in the VTA structure of most of the animals tested. Studies have been conducted mostly on rats, cats, dogs, and humans. Apart from a slight change in some areas, the VTA has been the same in almost all of them. The slightest changes were noticed in  A10 cells in the VTA. 

Talking about evolution, the role of VTA is said to have evolved with time. Research indicates that the number of dopamine neurons in VTA has increased as the species progress. Research indicates that rats and humans have different dopamine neurons, with an estimated 25000 neurons in rats, whereas 2.5 million neurons in humans. 

Chemical characteristics of VTA

It is often confusing for neuroscience experts to accurately calculate the chemical characteristics of VTA due to its loose and boundary-less structure. Still, it is estimated that the three main chemicals or neurotransmitters found in VTA are Dopamine, Glutamate, and GABA. Dopamine cells take up most of the space, with up to 60% of neurons present in VTA. 

The rest of the area is covered by the GABA with an estimate of 30% and Glutamate neurons taking the rest of the space. All these neurons play a significant role, and they are secreted through their designated pathways from the VTA to other parts of the brain. 

Role of Dopamine:

Dopamine plays a major role in our lives. It is released from the VTA to provide pleasure and good feelings. Dopamine also contributes to maintaining a healthy lifestyle. The imbalance of dopamine may have severe consequences on us. Studies show that dopamine has been linked to almost all the psychiatric disorders that exist today. In some disorders, it has a major role, while it plays a contributing role in others.

 At first, it was considered that dopamine and VTA have a significant role in drug addiction only. With time, the excess of dopamine or dopamine deficiency plays an important role in other psychiatric disorders, i.e. depression in which there is a deficiency of dopamine, Schizophrenia in which dopamine levels are abnormally high, ADHD where dopamine levels are low, resulting in low dopamine levels in low concentration. This is why VTA is crucial for proper and healthy brain function due to its significance in releasing dopamine neurons. 

GABA & Glutamate Neurotransmitters:

The ventral tegmental area also plays a major role in controlling and releasing glutamate and GABA neurotransmitters. These neurotransmitters are co-released with dopamine and have their fair amount of share in maintaining the healthy functioning of our brain. GABA is primarily responsible for maintaining the inhibition of an individual. It inhibits our emotions and feelings and makes sure that we act congruently. Studies show that up to 30% of the VTA consist of GABA neurons. Glutamate is also another important neurotransmitter in the VTA. Although present in almost every part of the brain, it is vital to regulate cell activity in the VTA, specifically related to the release of dopamine. These two neurons also make sure to reinforce the behaviors that are beneficial for us.

Summary:

  • The ventral tegmental area in the brain, also known as the VTA, is a small cluster of neurons present in our midbrain, and it has a vital role in our reward system.
  • The VTA contains the cell bodies of dopaminergic neurons, GABA neurons, and glutamate neurons primarily.
  • It performs a vital role in our reward cognition, inhibiting and regulating emotions and learning.
  • The VTA is considered to be an integral part of our brain’s reward structure. It helps in the reinforcement of the desired behaviors by releasing dopamine, the reward hormone.
  • To understand the structure and role of the ventral tegmental area, it is vital to acknowledge the subpopulation of neurons in this area. Current studies state that the VTA consists of three types of neurons: Dopamine neurons, GABA neurons, and Glutamate neurons.
  • The VTA is considered a complex structure in our brain that contains various pathways for regulating the neurons depending on the need.
  • The Ventral tegmental area plays an important function in releasing the dopamine chemicals in our brain through the reward circuit.
  • Any motivating behavior, pleasurable activity, chemical and non-chemical addiction and any behavior that contains a reward are known as dopamine-related functions.
  • The other two prominent neurotransmitters present in the VTA are the GABA neurons and Glutamate neurons. The VTA is also responsible for releasing these neurons along with the dopamine in a co-releasing format allowing these two to inhibit dopamine.
  • Dopamine cells take up most of the space in the VTA, with up to 60% of neurons present are dopamine neurons. The rest of the area is covered by the GABA and Glutamate neurons.
  • Studies show that up to 30% of the VTA consist of GABA neurons.
  • Glutamate is also another important neurotransmitter in the VTA.
  • Apart from the control and release of dopamine, The ventral tegmental area also plays an important role in regulating glutamate and GABA neurotransmitters.
  • GABA is primarily responsible for maintaining the inhibition of an individual.
  • The Ventral tegmental area has been widely linked with numerous developmental, neurodegenerative, and psychopathological disorders i.e. Schizophrenia, Drug Addiction, Major Depressive Disorder, Attention deficit hyperactivity disorder, and Parkinson’s Disease.
  • The ventral tegmental area and the nucleus accumbens are the two main areas where any licit or illicit drug acts cause the addiction.
  • When an individual consumes a drug, dopamine is released from the VTA, passing through its pathway into the nucleus accumbens and further into other parts, releasing pleasure. The release and effect of dopamine due to the drug intake are so soothing and pleasurable that the person keeps on taking that drug to get that pleasure again and again. The repetitive release of dopamine results in a permanent change in the mesolimbic pathway through the dopamine travels upon its release.
  • This cycle continues, and the person becomes more dependent, so the dopamine starts to deplete, and the VTA fails to provide the same amount of dopamine when a drug is taken.
  • Neuroscientists have been studying the ventral tegmental areas for decades to understand their structure and function. Apart from a slight change in some areas, the VTA has been the same in almost all mammals.
  • The role of VTA is said to have evolved with time. Research indicates that the number of dopamine neurons in VTA has increased as the species progress. 
  • Studies show that dopamine has been linked to almost all psychiatric disorders. Excess dopamine or the deficiency of dopamine both play an important role in other psychiatric disorders, i.e. depression, Schizophrenia, ADHD, and other psychiatric disorders.

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