Contrast the effects of agonist drugs and antagonist drugs on neural communication at a synapse.
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1. Agonists and antagonists are chemicals that influence synaptic transmission. Normally, a message is passed from one neuron to the next at a synapse. Neurotransmitters released from the sending neuron drift across the synaptic gap and bind to receptors on the receiving neuron, triggering a neural impulse in the receiving neuron.
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2. An agonist drug works with the neurotransmitter, enhancing its effect and making it more likely that the message will be passed. An antagonist drug works against the neurotransmitter, reducing its effect and making it less likely to trigger a neural impulse in the receiving neuron.
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3. Most antagonist drugs directly block the neurotransmitter. The shape of the antagonist molecule is just similar enough to the neurotransmitter that it can fit partway into the receptors, but not similar enough to stimulate the receptors. Because this prevents the actual neurotransmitter from binding to the receptors, it produces the same effect as decreasing the number of neurotransmitters in the synaptic gap.
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4. Some agonist drugs mimic the neurotransmitter. The shape of the agonist molecule is similar enough to the neurotransmitter that it can bind directly to the receptors and stimulate them, producing the same effect as releasing additional neurotransmitters into the synaptic gap.
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5. Other agonist drugs block the reuptake of the neurotransmitter into the axon terminal. This enhances the effect of the neurotransmitter by prolonging the time that neurotransmitters remain in the synaptic gap.
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6. Finally, some agonist drugs slow down the removal of the neurotransmitter from the synaptic gap by interfering with the enzymes that break the neurotransmitter molecules into smaller pieces that can be reabsorbed into the axon terminal. This increases the effect of the neurotransmitter by lengthening the time that the neurotransmitters remain in the synaptic gap.
Practice: Agonist and Antagonist Effects
Play the animation to watch the effect of the agonist drug Nicotine and the antagonist drug Haldol on neurotransmitter function.
Quiz 1
For each statement, select one of the buttons to indicate whether the statement is True or False. When responses have been chosen for all the statements, select the CHECK ANSWER button.
| True | False | |
|---|---|---|
Reuptake is the process of removing neurotransmitter molecules from the synaptic gap by absorbing them back into the axon terminal so that they can be released when the next neural impulse arrives. |
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Left undisturbed, neurotransmitter molecules in the synaptic gap will continue to bind, unbind, and rebind to receptor sites, continuously sending their message. |
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One way of stopping the activity of neurotransmitters is to have them bind permanently to the receptors on the postsynaptic membrane. |
Quiz 2
For each scenario, select one of the buttons to indicate whether the drug would be considered as having an Agonist or Antagonist effect of the specified neurotransmitter. When responses have been chosen for all the scenarios, select the CHECK ANSWER button.
| Agonist | Antagonist | |
|---|---|---|
Drug Z45 targets the neurotransmitter norepinephrine. It appears to interfere with the enzyme that breaks down the norepinephrine molecules. |
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Drug H22 targets the neurotransmitter dopamine. It appears to block the dopamine receptors on the postsynaptic membrane. |
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Drug T06 targets the neurotransmitter serotonin. It appears to block the reuptake of the serotonin molecules into the presynaptic membrane. |
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Drug M87 targets the neurotransmitter GABA. It appears to bind to the GABA receptors on the postsynaptic neuron and to fully activate those receptors. |