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The Four Dopamine Pathways: How One Neurotransmitter Controls Movement, Motivation, and Hormones · Understanding the Brain

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  Speaker 1: Today we're looking at dopamine. When a patient with schizophrenia
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  Speaker 1: develops muscle rigidity after starting an antipsychotic, what does that
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  Speaker 1: tell us about dopamine? And why might another patient taking
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  Speaker 1: the same medication develop breast discharge or menstrual irregularities? Both
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  Speaker 1: questions involve dopamine, but they involve different pathways in the brain.
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  Speaker 1: Understanding these pathways is particularly important because they connect neurobiology, psychopharmacology,
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  Speaker 1: and clinical assessment. Dopamine is a neurotransmitter involved in movement, motivation,
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  Speaker 1: reinforcement learning, and hormone regulation. Although dopamine is frequently called
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  Speaker 1: the brain's pleasure chemical, that description is quite incomplete. Dopamine
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  Speaker 1: plays an important role in wanting, anticipating reward, and learning
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  Speaker 1: which behaviors are worth repeating. So let's look at the
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  Speaker 1: four pathways. First is the mesolimbic. This pathway projects from
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  Speaker 1: the ventral tegmental area of the VTA to structures including
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  Speaker 1: the nucleus accumbens. This is where reward processing occurs. It
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  Speaker 1: also participates in motivation and reinforcement learning. Schizophrenia, increased presynaptic
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  Speaker 1: dopamine synthesis, and release within the striatum are associated with psychosis.
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  Speaker 1: The traditional dopamine hypothesis links excessive mesolimbic D2 signaling to
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  Speaker 1: positive symptoms such as hallucinations and delusions. Anti-psychotic medications help
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  Speaker 1: reduce these symptoms primarily through D2 receptor blockade or partial agonism. Second,
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  Speaker 1: we look at the mesocortical pathway. The pathway projects from
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  Speaker 1: the VTA to the prefrontal cortex. It contributes to executive functioning,
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  Speaker 1: working memory, attention, and emotional regulation. Reduced prefrontal dopaminergic functioning
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  Speaker 1: has been proposed as one contributor to negative and cognitive
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  Speaker 1: symptoms of schizophrenia. Negative symptoms include diminished emotional expression, avolition,
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  Speaker 1: and reduced speech. Remember that this explanation is a simplified model.
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  Speaker 1: Schizophrenia involves multiple neurotransmitter systems, including glutamate, and its systems
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  Speaker 1: cannot be explained by dopamine alone. Third, the nigrostriatal pathway.
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  Speaker 1: This pathway extends from the substantia nigra pars compacta to
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  Speaker 1: the dorsal striatum and is essential for motor control. When
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  Speaker 1: dopamine activity is reduced in this pathway, patients can develop
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  Speaker 1: Movement abnormalities. For example, antipsychotics that block D2 receptors may
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  Speaker 1: produce extraperiminal symptoms, including drug-induced parkinsonism, acute dystonia, and akathisia.
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  Speaker 1: Parkinson's disease also involves this pathway, but its underlying problem
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  Speaker 1: is degeneration of dopaminergic neurons rather than medication-induced receptor blockade.
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  Speaker 1: And finally, the tuperinfidibular pathway. This pathway connects hypothalamic dopamine
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  Speaker 1: neurons with the pituitary system. Dopamine normally inhibits prolactin secretion
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  Speaker 1: through D2 receptors. When an antipsychotic blocks the receptors, prolactin
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  Speaker 1: levels rise. Patients can experience galacteria, menstrual irregularities, sexual dysfunction,
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  Speaker 1: or infertility. Respiradone and paliperidone are particularly associated with elevated prolactin.
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  Speaker 1: Let's connect these pathways to medications. Haloperidol is a first-generation
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  Speaker 1: antipsychotic with strong D2 receptor antagonism. It can effectively treat
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  Speaker 1: positive psychotic symptoms, but carries substantial risk of extra-parameter symptoms. Aripazole,
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  Speaker 1: a second-generation antipsychotic, acts as a D2 partial agonist. It
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  Speaker 1: generally produces less prolactin elevation than risperidone, although akathisia remains
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  Speaker 1: an important adverse effect. Another medication worth remembering is buprenine,
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  Speaker 1: an antidepressant that inhibits norepinephrine and dopamine. It is used
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  Speaker 1: for major depressive disorder and smoking cessation, but it lowers
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  Speaker 1: the seizure threshold and is contraindicated in patients with seizure
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  Speaker 1: disorders or current or prior bulimia or anorexia. Remember the
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  Speaker 1: connection between the pathway, the symptom, and the medication. Mesolimbic,
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  Speaker 1: positive psychotic symptoms. Mesocortic, negative and cognitive symptoms. Nigrostriatal, movement abnormalities.
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  Speaker 1: Tuberin-Defibrillar Prolactin Regulation. Understanding these four will help you answer
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  Speaker 1: clinical questions involving schizophrenia, antipsychotic adverse effects.