8.7 The Effects of Drugs on Nervous Transmission

• The chemicals in drugs can have a major impact on the functioning of the brain and nervous system
• Many drugs impact the nervous system by altering the events that occur at synapses
• Drugs can increase transmission of impulses at a synapse by
  • Causing more neurotransmitter to be produced in the synaptic knob 
  • Causing more neurotransmitter to be released at the presynaptic membrane
  • Imitating the effect of a neurotransmitter by binding to and activating receptors on the postsynaptic membrane
  • Preventing the breakdown of neurotransmitters by enzymes
  • Preventing the reuptake of neurotransmitters by the presynaptic cell
• Drugs can decrease transmission of impulses at a synapse by
  • Preventing production of neurotransmitter in the presynaptic knob
  • Preventing the release of neurotransmitter at the presynaptic membrane
  • Enabling neurotransmitter to gradually leak out of the presynaptic knob so there is little left when an action potential arrives
    • The neurotransmitter that leaks out of the cell is destroyed by enzymes
  • Binding to receptors on the postsynaptic membrane and so preventing neurotransmitters from binding

Drugs can influence the transmission of nerve impulses at synapses

Nicotine

• Nicotine is the addictive chemical found in tobacco
• Nicotine affects synapses in more than one way
  • It mimics acetylcholine
    • Nicotine binds to a type of acetylcholine receptor on the postsynaptic neurone known as a nicotinic receptor
    • The binding of nicotine to nicotinic receptors initiates an action potential in the postsynaptic neurone
    • After stimulation by nicotine these receptors become unresponsive to other stimulation
      • While it is normal for receptors to be briefly unresponsive to further stimulation after being activated, nicotine causes a prolonged period of unresponsiveness
  • It stimulates release of dopamine
    • Dopamine is released from the pleasure centres in the brain in response to nicotine
    • The release of dopamine is thought to reinforce rewarding behaviours, in this case smoking, increasing the likelihood that we will carry out that behaviour again
• Nicotine increases heart rate and blood pressure, as well as increasing the likelihood that an individual will continue smoking; this further impacts the circulatory system as well as increasing the risk of other health problems such as lung cancer 

Lidocaine

• Lidocaine is often used as a local anaesthetic for numbing small areas of the body 
  • E.g. it is used by dentists before dental procedures such as tooth extraction
• It can also be used to regulate the heart beat in people suffering from irregular heart rhythms
• Lidocaine works by blocking voltage gated sodium channels
  • This prevents a large influx of sodium ions in the postsynaptic neurone, therefore preventing an action potential from being generated

Cobra venom 

• Cobra venom, also known as -cobratoxin, is a type of venom produced by some species of cobra
  • Receiving a snake bite from a cobra can be fatal
  • = alpha
• -cobratoxin binds to acetylcholine receptors on the postsynaptic membrane, preventing an influx of sodium ions and therefore the generation of an action potential
• When this occurs at the synapses between motor neurones and muscle fibres, known as neuromuscular junctions, this can lead to muscle paralysis
  • Eventual paralysis of the muscles that control breathing leads to death
• Small quantities of -cobratoxin can be used as a muscle relaxant during asthma attacks

L-dopa 

• L-dopa is a drug used to treat the symptoms of Parkinson's disease
• It has a very similar structure to dopamine; a neurotransmitter present at lower levels than usual in the brains of those who suffer from Parkinson's disease
• L-dopa is transported from the blood into the brain, where it is converted into dopamine in a reaction catalysed by the enzyme dopa-decarboxylase
• The effect is to increase levels of dopamine in the brain
  • Note, dopamine cannot be given directly to those who have Parkinson's disease as it cannot cross the barrier between the blood and the brain
• Increased levels of dopamine mean that more nerve impulses are transmitted in parts of the brain that control movement, giving sufferers better control over their movement and lessening the symptoms of Parkinson's disease 

MDMA

• MDMA is a recreational drug that is also known as ecstasy
  • Its use and sale are criminal offences in most parts of the world
• MDMA effects multiple neurotransmitters, most notably serotonin
  • MDMA inhibits the reuptake of serotonin into the presynaptic neurone by binding to the specific proteins that enable serotonin reuptake, located on the presynaptic membrane; this increases the amount of serotonin present in the brain
    • Serotonin is usually reabsorbed into the presynaptic neurone to be recycled for future action potentials
  • MDMA also triggers the release of further serotonin from presynaptic neurones, further adding to the increase
• Serotonin can affect people in many ways including their mood, anxiety and sleep
• When an individual takes MDMA they may feel extreme euphoria and enhanced touch and bodily sensations