6.2.9 Drug Effects on Synapses

How Drugs Affect Synapses

1. Mimicking Neurotransmitters (Agonists):

• Some drugs mimic the shape of neurotransmitters and bind to receptors on the postsynaptic membrane.
• This stimulates receptors in the same way as the natural neurotransmitter.
• Example: Nicotine mimics acetylcholine and binds to cholinergic receptors.

2. Blocking Receptors (Antagonists):

• Drugs can block neurotransmitter receptors, preventing neurotransmitters from binding.
• This inhibits the generation of an action potential in the postsynaptic neurone.
• Example: Curare blocks acetylcholine receptors at neuromuscular junctions, leading to paralysis.

3. Inhibiting Neurotransmitter Breakdown:

• Some drugs inhibit the enzymes that break down neurotransmitters in the synaptic cleft.
• This prolongs the action of the neurotransmitter, increasing stimulation of the postsynaptic neurone.
• Example: Nerve gases inhibit acetylcholinesterase, causing continuous muscle contraction.

4. Enhancing Neurotransmitter Release:

• Drugs can increase the release of neurotransmitters from the presynaptic neurone.
• This increases stimulation of the postsynaptic membrane.
• Example: Amphetamines increase the release of dopamine, enhancing alertness and focus.

5. Inhibiting Neurotransmitter Release:

• Some drugs prevent neurotransmitter release from the presynaptic neurone, reducing stimulation.
• Example: Botulinum toxin (botox) inhibits acetylcholine release, preventing muscle contraction.

6. Blocking Reuptake of Neurotransmitters:

• Drugs can block the reuptake of neurotransmitters into the presynaptic neurone.
• This prolongs their action in the synaptic cleft.
• Example: SSRIs (Selective Serotonin Reuptake Inhibitors) block the reuptake of serotonin, treating depression.

Key Examples of Drug Effects

Drug	Effect	Mechanism
Nicotine	Mimics acetylcholine, stimulates cholinergic receptors	Agonist
Curare	Blocks acetylcholine receptors, causing paralysis	Antagonist
Nerve gas	Prevents acetylcholine breakdown, causing continuous muscle contraction	Enzyme inhibition
Amphetamines	Increases dopamine release, enhancing alertness	Neurotransmitter release enhancer
SSRIs	Increases serotonin levels by blocking reuptake	Reuptake inhibition