International A Level Biology / Coordination in animals and plants
Brain chemicals: dopamine, serotonin and new drugs revision notes
Edexcel International A Level Biology (YBI11), chapter 24: Coordination in animals and plants, specification point 8.16. Free lesson.
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Dopamine and serotonin affect impulses in brain pathways, so changes in their levels can alter movement and mood. Understanding these chemicals and the genes linked to their targets helps explain current treatments and how more personalised drugs can be developed.
How dopamine loss causes Parkinson’s symptoms
In Parkinson’s disease, dopamine-producing neurones in the substantia nigra, part of the brain stem, die over time. Their axons run to the striatum, a movement control area. With fewer neurones, less dopamine is released at synapses there, so fewer dopamine receptors are activated. This means fewer impulses pass along pathways controlling movement, and the motor area has less control over muscle contraction. Symptoms include tremor (shaking), stiff muscles, slow movement and poor balance. Symptoms usually appear only after many dopamine-producing neurones have died, so the disease…
How L-DOPA reduces symptoms
Dopamine itself cannot cross the blood-brain barrier, the lining of brain capillaries that keeps most substances out. L-DOPA is the molecule neurones use to make dopamine, and it can cross this barrier. It replaces some of the dopamine that the dead neurones would have produced, allowing more impulses in movement pathways and better muscle control. L-DOPA reduces symptoms, but it does not cure Parkinson’s because the neurones continue to die.
How other Parkinson’s drugs work
Other drugs act at different stages of the problem. An inhibitor stops an enzyme from working. A dopamine agonist acts like dopamine by binding to and activating dopamine receptors. Carbidopa inhibits the enzyme that converts L-DOPA into dopamine outside the brain, so more L-DOPA reaches the brain. A dopamine agonist binds to dopamine receptors on the postsynaptic membrane and activates them, doing dopamine’s job. An MAO-B inhibitor stops the enzyme that breaks dopamine down, so dopamine lasts longer at synapses.
How SSRIs affect serotonin
Depression is linked with low serotonin levels and fewer impulses in pathways that affect mood. Normally, serotonin is released from the presynaptic neurone, diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Transporter proteins then take it back into the presynaptic neurone. This is called reuptake. An SSRI (selective serotonin reuptake inhibitor) blocks the transporter proteins, so serotonin is not taken back up. Serotonin stays in the cleft for longer. More binds to receptors, so more impulses pass along pathways that affect mood and…
How the genome can lead to new and personalised drugs
Understanding brain chemistry helps researchers identify a chemical that is out of balance and the receptor, transporter or enzyme involved. The Human Genome Project sequenced human DNA, allowing researchers to find and study genes for these proteins. A drug can then be designed to fit a target protein. Different alleles can affect how a person responds to a drug. For example, differences in a receptor or in a liver enzyme that breaks down a drug can change its effects. In personalised medicine, a patient’s genotype is tested before choosing a suitable drug and dose. The aim is fewer…