C2.2.12 Effects of exogenous chemicals on synaptic transmission (HL) Notes

Exogenous Chemicals Alter Synaptic Transmission by Blocking or Enhancing Signals

1. Synaptic transmission can be influenced by a variety of exogenous chemicals, such as pesticides, drugs, and other substances.
2. These chemicals can either block or enhance the transmission of signals across synapses, affecting the normal functioning of the nervous system.

Neonicotinoids Disrupt Insect Synapses by Prolonged Receptor Activation

How Neonicotinoids Work

1. Neonicotinoids are synthetic compounds resembling nicotine.
2. They target cholinergic synapses, which use acetylcholine as a neurotransmitter.
3. Neonicotinoids bind to nicotinic acetylcholine receptors, much like acetylcholine (the natural neurotransmitter), but they do not get broken down by acetylcholinesterase, leading to prolonged receptor activation.

Why Neonicotinoids Are Selective

1. Neonicotinoids are more toxic to insects than mammals because:
   1. Insects have a higher proportion of cholinergic synapses.
   2. Neonicotinoids bind more strongly to insect receptors than mammalian ones.

Result

1. The prolonged activation leads to an interruption in normal synaptic signaling, which can paralyze and kill insects.
2. However, the impact on the nervous system of insects can cause death, and exposure to humans and other animals may also lead to harmful effects, particularly in large amounts.

Real-World Implications

1. Neonicotinoids are widely used in agriculture, but their effects on non-target species, such as bees, have sparked controversy.
2. The disruption of synaptic transmission in bees can lead to colony collapse, affecting pollination and ecosystems.

Cocaine Blocks Dopamine Reuptake

How Cocaine Affects Synapses

1. Cocaine targets synapses that use dopamine, a neurotransmitter involved in reward and pleasure pathways.
2. Cocaine works by blocking the reuptake of dopamine at the synapse.
3. Normally, dopamine is released into the synapse and then reabsorbed by the presynaptic neuron through dopamine transporters.
4. Cocaine inhibits these transporters, resulting in prolonged dopamine activity in the synapse.

Why Cocaine Is Addictive

1. The prolonged presence of dopamine in the synapse intensifies the reward signal.
2. Over time, the brain adapts by reducing dopamine receptors, making it harder to experience pleasure without the drug.
3. This cycle of dependency is what makes cocaine highly addictive.

Health Implications

Cocaine’s effects extend beyond euphoria. It can cause:

1. Increased heart rate and blood pressure.
2. Risk of heart attack or stroke.
3. Long-term changes in brain chemistry, leading to addiction and mental health issues.

Other Exogenous Chemicals Affecting Synaptic Transmission

1. Amphetamines

1. Mechanism: Amphetamines increase the release of dopamine and norepinephrine and inhibit their reuptake.
2. Effect: This increases sympathetic nervous activity, leading to increased alertness, euphoria, and stimulated heart rate. Overuse can lead to addiction and long-term neurological damage.

2. Morphine

1. Mechanism: Morphine binds to opioid receptors on the postsynaptic neurons, mimicking the action of natural pain-relieving molecules (endorphins).
2. Effect: It inhibits synaptic transmission, reducing pain signals and producing a feeling of well-being and relaxation. Chronic use can lead to tolerance and dependence.

3. Nicotine

1. Mechanism: Nicotine mimics the action of acetylcholine, binding to nicotinic acetylcholine receptors on the postsynaptic neuron. This leads to an increase in dopamine release in the brain’s reward pathways.
2. Effect: The stimulating effect of nicotine results in increased heart rate, alertness, and a sense of pleasure. However, long-term use can lead to addiction and adverse health effects.

4. Ethanol (Alcohol)

1. Mechanism: Ethanol affects neurotransmission by enhancing the action of gamma-aminobutyric acid (GABA), the brain's primary inhibitory neurotransmitter, and inhibiting glutamate, the primary excitatory neurotransmitter.
2. Effect: This leads to sedation, impaired motor function, and a depressant effect on the central nervous system. Chronic use can lead to neurodegeneration and addiction.