In popular media, neurotransmitters are often viewed as on-off switches for basic human emotions, but how true is this? Neurotransmitters are chemicals that regulate brain activity by facilitating connection between neurons. Dopamine, for example, is considered to be a switch for feelings of pleasure and happiness, but the reality is slightly more complex.
It is established that dopamine is a neurotransmitter with an important role in the modulation of learning and motivation, motor function and cognition. It also acts as a hormone and is essential to maintaining homeostasis.
Dopamine plays a modulatory role in several distinct brain pathways. Its activity is very flexible due to synaptic plasticity and variable cell excitability. Dopaminergic neurons have been identified in several midbrain areas, including the ‘substantia nigra’ – loss of neurons in this area is associated with the neurodegenerative disorder Parkinson’s Disease. Another dopaminergic pathway is the mesolimbic pathway, which connects the ventral tegmental area of the midbrain with the ventral striatum of the basal ganglia located in the forebrain.
The mesolimbic pathway is part of a reward mechanism that enables long-term learning and motivation in both humans and other mammals. That is, this system is responsible for associative learning based on rewards. It consists of dopaminergic neurons, and activity here requires a rewarding stimulus. In this context, a stimulus is a change in the internal or external environment that drives a behaviour – for example, appetite. ‘Reward’ is the property of a stimulus that makes it desirable. When this reward component is added to a stimulus, it encourages the associated behaviour – in this case, eating – in pursuit of a desirable outcome – gaining energy. Because we are consistently exposed to rewards of all kinds (e.g. homeostatic or reproductive), the brain creates an estimate of the value of rewards, in order to regulate the motivation to achieve them. Every time this pathway is activated, the future reward estimate is updated. These continuous updates are the basis of learning.
A possible instance of how motivation is related to dopamine lies in observations from mild cases of Parkinson’s. In Parkinson’s Disease, neurodegeneration (loss of neurons) is localized in the substantia nigra, and this compromises the level of dopamine activity, leading to a decrease in motivation. Some scientists suspect that the decrease in movement in early Parkinson’s is not a question of motor function, but a deficit in motivation at a cellular level. The reward circuits are not powering an incentive for cells to spend more energy on normal, faster movements. As a result, the muscular tissue cannot respond properly and this leads to slower movements.
This mesolimbic pathway is fundamentally tied with the sensation of pleasure. It regulates incentive salience, the cognitive process that encourages advantageous behaviours by making them desirable. When that behaviour is executed the sensation of pleasure is experienced. This is where dopamine’s association with happiness originates. This process is also involved with addiction: drugs become perceived as rewarding stimuli,because of their pleasurable effects, so there is an increase in motivation to consume them.
Although some pathways have been theoretically established, it has been difficult to define the actual intensity of dopamine effects. Recently, scientists at MIT determined how dopamine release in the striatum affects responses locally and globally, through molecular imaging and fMRI. Li and Jasanoff observed that higher dopamine levels increase the activity time frame of neurons, meaning that higher stimuli lead to longer responses. It was also observed that the response to dopamine activity was localized in the motor cortex and the insular cortex. This confirmed that although dopaminergic neurons are located in a few small areas in the midbrain, the dopamine released can affect multiple, further regions of the brain. This study gives a basis for further research regarding dopamine activity and its location in the brain.
The ever-changing pattern of dopamine activity might be a good reason why dopamine-related research is such a great challenge for neuroscientists. It is still early to pinpoint exactly how dopamine relates to happiness, but it does seem to have a role in its pursuit: dopamine regulates reward pathways, and fulfilling the desires generated by these gives a sensation of pleasure. The pathways governing emotions are complex and although we are coming closer to a basic understanding, there is still a lot to learn. To obtain a fuller picture, there needs to be a more thorough exploration of dopaminergic pathways, and a clearer bio-chemical definition of happiness.
Written by Maria-Mihaela Avram and edited by Ailie McWhinnie.