Concept Practice
Agonists and Antagonists

Chapter 39. Agonists and Antagonists

agonist
a chemical or drug that increases the effect of a neurotransmitter
antagonist
a chemical or drug that decreases the effect of a neurotransmitter
axon terminals
branching fibers at the end of an axon that contain the neurotransmitters (also called synaptic terminals or terminal buttons)
enzymes
special proteins that facilitate chemical reactions; within a synapse, enzymes break neurotransmitters into pieces, stopping neurotransmitter action
neural impulse
an electrical signal that carries a message along an axon
neuron
a single nerve cell, forming the basic unit of the nervous system
neurotransmitters
chemical messengers released by the axon terminal into the synaptic gap between neurons
receptor
special area on a neuron's membrane that can bind with neurotransmitters
reuptake
process of drawing the released neurotransmitters back into the axon terminal for reuse
synapse
the junction of the axon terminal of the sending neuron with the dendrite or cell body of the receiving neuron
synaptic gap
the tiny space between the sending neuron and the receiving neuron; also called synaptic cleft
synaptic transmission
chemical process by which the neural impulse is passed from the axon terminal of one neuron to the dendrite or cell body of another neuron
Agonists and Antagonists
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The figure demonstrates how a neurotransmitter and drugs bind to receptors to cause different effects.  When a neurotransmitter fits into the receptor, binding is successful.  An agonist is similar to a neurotransmitter in that it can bind to a receptor successfully.  An antagonist does not bind successfully because it cannot fit into the receptor, but it does block the receptor from being bound by another drug or neurotransmitter.  The receptor is like a look with a specific shape, in which only keys with certain shapes can fit and  bind successfully.
Learning Objective:

Contrast the effects of agonist drugs and antagonist drugs on neural communication at a synapse.

concept_review

Review

The figure is a diagram of an axon, dendrite, and synapse.  The axon is the part of a cell that sends messages away from the cell body to another cell.  The end of the axon resembles a bulb shape and contains vesicles.  The vesicles are called synaptic vesicles and contain neurotransmitters inside them.  These vesicles move to the end of the axon, which is called the axon terminal, where they release neurotransmitters into the synapse.  The synapse, also called the synaptic gap, is the space between the axon of the sending neuron and dendrite of the receiving neuron.  The dendrite has receptors, which resemble locked gates. The neurotransmitters can fit into the lock of these receptors to keep them closed or to open them.

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.

concept_review

Review

There are two ways drugs can work with a neurotransmitter.  An agonist increases the effect of the neurotransmitters.  An antagonist decreases the neurotransmitter effect.

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.

concept_review

Review

An antagonist does not bind to the receptor but fits into it in a way that can block other neurotransmitters.  As a result the drug as no effect.

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.

concept_review

Review

An agonist binds to the receptor in the same was a neurotransmitter.  As a result the drug has the same effect as the neurotransmitter.

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.

concept_review

Review

The figure is a diagram of an axon, dendrite, and synapse.  The axon is the part of a cell that sends messages away from the cell body to another cell.  The end of the axon resembles a bulb shape and contains vesicles.  The vesicles are called synaptic vesicles and contain neurotransmitters inside them.  These vesicles move to the end of the axon, which is called the axon terminal, where they release neurotransmitters into the synapse.  The synapse, also called the synaptic gap, is the space between the axon of the sending neuron and dendrite of the receiving neuron.  The dendrite has receptors, which resemble locked gates. The neurotransmitters can fit into the lock of these receptors to keep them closed or to open them.  Neurotransmitters in the synaptic gap can be reuptaked into the axon of the sending cell to be reused.  Agonist drugs can block this reuptake process, leaving more neurotransmitters in the synaptic gap.

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.

concept_review

Review

The figure is a diagram of an axon, dendrite, and synapse.  Neurotransmitters in the synaptic gap can be reuptaked into the axon of the sending cell to be reused.  Agonist drugs can extend this reuptake process, leaving more neurotransmitters in the synaptic gap, by interfering with the enzymes that break down the neurotransmitters for reuptake.

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.

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Practice: Agonist and Antagonist Effects

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      Quiz 1

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      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.

      Left undisturbed, neurotransmitter molecules in the synaptic gap will continue to bind, unbind, and rebind to receptor sites, continuously sending their message.

      One way of stopping the activity of neurotransmitters is to have them bind permanently to the receptors on the postsynaptic membrane.

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      Quiz 2

      A pharmaceutical company has just finished testing several new chemical compounds to see whether they have psychoactive effects. Here are the most promising new drugs.
      AgonistAntagonist

      Drug Z45 targets the neurotransmitter norepinephrine. It appears to interfere with the enzyme that breaks down the norepinephrine molecules.

      Drug H22 targets the neurotransmitter dopamine. It appears to block the dopamine receptors on the postsynaptic membrane.

      Drug T06 targets the neurotransmitter serotonin. It appears to block the reuptake of the serotonin molecules into the presynaptic membrane.

      Drug M87 targets the neurotransmitter GABA. It appears to bind to the GABA receptors on the postsynaptic neuron and to fully activate those receptors.

      end_slide
      The figure demonstrates how a neurotransmitter and drugs bind to receptors to cause different effects.  When a neurotransmitter fits into the receptor, binding is successful.  An agonist is similar to a neurotransmitter in that it can bind to a receptor successfully.  An antagonist does not bind successfully because it cannot fit into the receptor, but it does block the receptor from being bound by another drug or neurotransmitter.  The receptor is like a look with a specific shape, in which only keys with certain shapes can fit and  bind successfully.
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