Many drugs, both medicines and recreational drugs, work by affecting synaptic transmission. Because transmission at a synapse involves several separate steps, a drug can act at any one of them.Drugs are often described by their effect on receptors:• an agonist has a shape similar to the neurotransmitter, binds to its receptors and activates them, mimicking the neurotransmitter
• an antagonist binds to the receptors but does not activate them, blocking the neurotransmitter from bindingDrugs can also act by:• increasing or decreasing the release of neurotransmitter from the synaptic knob, for example by affecting calcium ion channels
• inhibiting the enzyme that breaks down the neurotransmitter, so it stays in the cleft longer. Organophosphate insecticides inhibit acetylcholinesterase, causing continuous stimulation of muscles
• blocking reuptake of the neurotransmitter, which also increases its concentration in the cleft. Many antidepressants block the reuptake of serotonin
• blocking ion channels in the neurone membrane so action potentials cannot be generated
• acting as a precursor, such as L-dopa, which the brain converts into dopamine in the treatment of Parkinson's diseaseIn general, drugs that increase the effect of an excitatory neurotransmitter cause more action potentials in the postsynaptic neurone, and drugs that decrease its effect cause fewer.When a drug is used repeatedly, the nervous system can adapt, for example by changing the number of receptors on the postsynaptic membrane. This can lead to tolerance, where a larger dose is needed for the same effect, and to dependence.The next lessons look at three examples: nicotine, lidocaine and cobra venom.
• an antagonist binds to the receptors but does not activate them, blocking the neurotransmitter from bindingDrugs can also act by:• increasing or decreasing the release of neurotransmitter from the synaptic knob, for example by affecting calcium ion channels
• inhibiting the enzyme that breaks down the neurotransmitter, so it stays in the cleft longer. Organophosphate insecticides inhibit acetylcholinesterase, causing continuous stimulation of muscles
• blocking reuptake of the neurotransmitter, which also increases its concentration in the cleft. Many antidepressants block the reuptake of serotonin
• blocking ion channels in the neurone membrane so action potentials cannot be generated
• acting as a precursor, such as L-dopa, which the brain converts into dopamine in the treatment of Parkinson's diseaseIn general, drugs that increase the effect of an excitatory neurotransmitter cause more action potentials in the postsynaptic neurone, and drugs that decrease its effect cause fewer.When a drug is used repeatedly, the nervous system can adapt, for example by changing the number of receptors on the postsynaptic membrane. This can lead to tolerance, where a larger dose is needed for the same effect, and to dependence.The next lessons look at three examples: nicotine, lidocaine and cobra venom.