I have my semesters tomorrow and everytime I read the Dopamine Hypothesis in relation to Schizophrenia it’s going over my head.
In: Biology
It may get a little complicated at times but I’ll try my best to make it ELI5.
So before entering into the whole Dopamine Hypothesis, let me tell you about two of the pathways in our brain. The **Mesolimbic Pathway** and the **Mesocortical Pathway** . You can think of these as roads that connect two different areas of the brain with each other.
**Mesolimbic Pathway** connects the mid brain with basal ganglia(specifically the ventral striatum of basal ganglia but for simplicity let’s just say Basal Ganglia).
**Mesocortical Pathway** again connects the mid brain(ventral tegmental area of the mid brain) to the prefrontal cortex.
Both of these pathways are Dopaminergic pathways. This means they utilize Dopamine to send information from one point of the pathway to another.
The Mesolimbic Pathway is of greater importance in this theory. It is also called the reward pathway because when dopamine travels through this pathway we get a sense of reward. We call it the *incentive salience* ie. desire for rewarding stimuli.
Now, Schizophrenia can have two types of symptoms right? Postive symptoms (hallucinations, etc) and Negative symptoms (apathy, alogia, etc).
Now we come back to the Dopaminergic Hypothesis of Schizophrenia. What this hypothesis specifically says is that the positive symptoms of Schizophrenia arise due to excessive Dopamine activity in the **Mesolimbic Pathway**. Note that I have said excessive *dopamine activity* and not *excessive dopamine*.
Why is this distinction between *excessive dopamine activity* and *excess dopamine* important?
This is important because the positive symptoms of Schizophrenia cannot be attributed to only overabundance of Dopamine in the Mesolimbic Pathway. Specifically, the overactivation of D2 receptors is attributed in causation of the positive symptoms.
D2 receptor is just one of the many receptors that Dopamine binds to to show its effects. The are five types of it ie. D1, D2, D3, D4 and D5. They can be found all over the body including different areas of brain, heart and kidneys.
Why is D2 receptor specifically mentioned? Because it just so happens that D2 is the most abundant receptor in the Mesolimbic Pathway. So any effect shown by Dopamine in this pathway is most likely through D2.
But what we have to understand about Dopaminergic theory is that it is a **theory** and not the pathogenesis of Schizophrenia. It is postulated on the basis that if we give drugs that are D2 antagonists, then the positive symptoms of Schizophrenia are relieved. So this probably means that the symptoms were caused by excessive Dopamine activity so when D2 antagonists were given, the activity decreased and the symptoms decreased.
*TLDR* : The Dopamine Hypothesis is a theory which says that the positive symptoms of Schizophrenia are due to excessive Dopaminergic activity in the brain. It is based on the fact that if Dopamine antagonists are given, the positive symptoms are relieved.
Hope this didn’t go over your head because of it being too long.
The dopamine hypothesis suggests that dopamine dysfunction plays a significant role in schizophrenia. Most antipsychotics (medications that are used to treat schizophrenia) reduce the amount of dopamine activity. The dopamine hypothesis does not say that schizophrenia is entirely caused by too much dopamine activity, but that it is a significantly contributing factor.
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