8.2.6 Imbalances in Brain Chemicals Leading to Diseases

• Neurotransmitters are chemicals that transmit nerve impulses across synapses 
• Some disorders and diseases are linked to an imbalance of neurotransmitters in the brain
• Two examples are
  • Parkinson's Disease
  • Depression

Parkinson's disease

• Parkinson's disease is a brain disorder that affects the co-ordination of movement, caused by the loss of neurones in some parts of the brain
• Symptoms include
  • A tremor to the specific parts of the body
  • Slow movement
  • Stiff and inflexible muscles
  • Difficulties with balance
  • Changes to speech
• The lost neurones normally produce the neurotransmitter dopamine
  • Dopamine is involved in muscle control
• Individuals that suffer from Parkinson's disease produce insufficient amounts of dopamine due to the loss of dopamine-producing neurones
  • Less dopamine is released into the synaptic cleft meaning less is able to bind with receptors on the postsynaptic membrane 
  • Fewer sodium channels on the membrane are opened so depolarisation of the postsynaptic neurone does not occur
  • This leads to fewer action potentials which creates the symptoms such as tremors and slow movement
• Different types of drugs can be used to treat this disease
  • Dopamine agonists
    • Produce the same effect as dopamine by binding to and activating the dopamine receptors on the postsynaptic membrane
  • Dopamine precursors 
    • These are chemicals that can be converted into dopamine in the neurones
    • E.g. L-dopa
  • Enzyme inhibitors
    • Monoamine oxidase B (MAOB) inhibitors inhibit the activity of enzymes that would normally break down dopamine in the synaptic cleft, raising levels of dopamine present in the brain
• Research into other treatments for Parkinson's disease is currently ongoing, with some promising future possibilities in the areas of
  • Gene therapy
    • This would involve the addition of genes to the affected cells in the brain to either increase dopamine production or prevent the destruction of dopamine-producing cells
  • Stem cell therapy
    • Stem cells could be used to replace the lost dopamine-producing cells in the brain

Depression

• Low levels of the neurotransmitter serotonin have been linked to depression
  • Serotonin transmits nerve impulses through the areas of the brain that control mood
  • Low levels of serotonin increase episodes of depression
• Other brain chemicals linked to depression include noradrenaline and dopamine
• Some drugs that have been developed for the treatment of depression, known as antidepressants, work by increasing the levels of relevant neurotransmitters in the brain
  • SSRIs (selective serotonin reuptake inhibitors) are a class of antidepressant that prevent the uptake of serotonin at synapses; this increases the overall levels of serotonin in the brain
  • TCAs (tricyclic antidepressants) increase levels of both serotonin and noradrenaline in the brain
  • MAOB inhibitors inhibit enzymes that would otherwise break down neurotransmitters in the synaptic clefts in the brain