The Reward Pathway:
Why do you get addicted to things? How do addictions even occur? The answer to the first question is fairly straightforward: the reward pathway. How, on the other hand, well that describes a whole process that happens in your brain and we’re talking about a series of events that lasts milliseconds long…but it’s consequences last a lifetime.
If we are going to talk about the reward pathway, it’s only right to familiarise ourselves with dopamine. Now dopamine, it’s a neurohormone. Putting it in more simple terms, it’s a specialised chemical -released by the hypothalamus- that stimulates the nervous system. In today’s popular culture and media, dopamine is portrayed as the feel good “pleasure” hormone and rightly so. However, current opinions in modern pharmacology instead argue that dopamine creates the feelings of “motivational salience”- the term used to describe the cognitive process which motivates an organism towards or away from a particular object, perceived event, outcome, thing, place whatever. Bottom line is; it’s a very important chemical in our body. And for that very reason, the study of dopamine is extraordinarily beneficial in understanding human psychology. However, that is a separate conversation altogether, one that we will discuss in the not too distant future. For now, all you need to remember is that dopamine motivates certain actions in people and that will be explained further when we talk more about the reward pathway.
In the 1950s, 2 scientists, Olds and Milner implanted electrodes into the brains of rats and allowed the rats to press a lever to receive an electrical stimulation to a specific area of the brain. The two, realised that there was one lever that the rats would repeatedly press to get the burst of electricity and therefore stimulation. They found that the region known as the septal area, was highly sensitive and received the most stimulation.It is said that one of the rats in the study in fact pressed the lever an estimated 7500 times in just 12 hours! And of course the rat would only press the lever 7500 times if it was motivated to do so. Whether that motivation came from any enjoyment or pleasure, the fact is that these experiments told us for the very first time that certain areas of the brain are dedicated for reward pathways. It paved the way for further studies to map out the “reward areas” for the brain. Through these studies, they found out that these reward centres were found along the medial forebrain bundle. What’s more is that through these studies they also discovered that dopamine neurons are activated in this reward pathway, and when a dopamine antagonist is administered to the rats, the rats would soon stop the lever pressing. From these investigations two things were established: the dedicated reward centres of the brain and the neurotransmitter responsible for motivation.
So, we have discussed the history relating to the reward system and some scientific experiments that occurred. The question still remains though: What is the mechanism for the reward pathway? Well, as already mentioned the reward systems of the brain are a group of structures that are activated whenever we engage in something we deem rewarding. Eating, relationships, sex, addicted drugs all are things which trigger the reward pathway. And when the reward pathway is triggered, it’s the primary dopamine pathways of the brain that are stimulated to release an increased amount of dopamine, the neurotransmitter commonly associated with reward and pleasure. Dopamine is mainly produced in a part of the brain known as the ventral tegmental area (or VTA for short) and once produced here is transported to different areas of the brain via 4 pathways: mesolimbic, mesocortical, nigrostriatal and the tuberoinfundibular pathways.
The mesolimbic pathway is one of the major components of the dopamine pathway. It occurs within the ventral tegmental area (VTA). The VTA is a dopamine rich group of neurons in the central nervous system. This is situated in the midbrain. The mesolimbic pathway stimulates dopaminergic action potentials to another area of the brain called the nucleus accumbens (NAc). This is where the feelings of reward and pleasure are released. When a person encounters rewarding or pleasuring stimuli (such as certain foods, sex and drugs), dopamine is released from the VTA to the NAc, creating positive feelings hence reinforcing that behaviour.
Stimulating the neuron accumbens pathway is important for our daily activities, although over-stimulation of the NAc can lead to cravings and want to feel that stimulation again. These substances directly increase dopamine releasement, causing the sensation of intense euphoria.
Similar to the mesolimbic pathway, dopamine originates at the ventral tegmental area (VTA). Action potentials then travel from the VTA to the areas in the prefrontal cortex. (PFA). The PFA area has huge roles in cognition, decision making and the working memory itself.
This helps to retain the memory of the activities that stimulate dopaminergic receptors in the brain, allowing the person to ‘remember’ the things they crave.
This is involved in an activity called motor planning. This is where voluntary movement is influenced through basal ganglia. These dopamine projections start in the substantia nigra and go off to the caudate and putamen, which are parts of the basal ganglia. It actually contains 80% of dopamine in the brain.
The nigrostriatal pathway stimulates purposeful movement. When there is a reduced amount of neurons in this pathway, this leads to impaired motor movement.
The final dopamine pathway is called the tuberoinfundibular system. The dopaminergic neurons originate in the arcuate and the periventricular cell bodies of the hypothalamus. They travel down to the infundibular region of the hypothalamus, particularly a part called the median eminence. In this pathway dopamine is released into the portal circulation, which connects the neurons to the pituitary glands. Once dopamine is released by the pituitary glands, a protein called prolactin is inhibited. This protein has an important role in metabolism, sexual satisfaction and the immune system.
When these D2 receptors are blocked, dopamine’s inhibitory effect is prevented, causing a high level of prolactin in the blood. This may affect menstrual cycles, libido, fertility, bone health, or stimulate galactorrhea (milky discharge from nipples unrelated to breastfeeding).
Our actions rewire our brain. It’s no stretch of the imagination to make that statement. The things we do, the places we go and the company we keep all shape and mould us as individuals; for better or worse. Actions become habits and habits can soon become addictions if left unchecked. And all this occurs because of a tiny process in our brain known as the reward pathway.