In neuroscience, an agonist is a drug that increases the level of
a particular neurotransmitter. Agonists can work in a few different
ways; it'll be easier to understand them if you're familiar with how a neuron works (and how signals are transmitted across the synapse).
- Increasing the level of neurotransmitter. Simple,
right? If you want to increase the level of neurotransmitter in the
brain, you can just feed the person more neurotransmitter. Along
the same lines, you can give a drug that acts just like the
neurotransmitter, even though it differs chemically (for example,
nicotine stimulates acetylcholine receptors).
- Increasing the levels of neurotransmitter precursors.
In some cases, though, you can't use the approach I just mentioned. Many
neurotransmitters would be broken down in the gut if you took them orally, and most of them can't make it through the blood-brain
barrier. Therefore, it's more effective to give precursors,
which are chemicals that can be converted into neurotransmitters. L-DOPA, a common drug for Parkinson's disease, works this way.
- Facilitating the release of neurotransmitters. Within
neuron, neurotransmitters are stored inside vesicles. When a
neuron is stimulated, these vesicles bind to the end of the neuron and
release the neurotransmitters into the synapse. Some drugs can make it
easier for the vesicle Amphetamines work this way (in
part).
- Blocking enzymes that break down the
neurotransmitter. The synapse contains chemicals that break down
neurotransmitters into inactive forms. If you inhibit these
enzymes, more active neurotransmitter will be left in the
synapse. Older drugs for depression--the MAOIs--work this
way.
- Blocking autoreceptors. Autoreceptors are receptors on the presynaptic neuron (the one that originally released the
neurotransmitter). They serve as a negative feedback mechanism;
stimulation of the autoreceptors "tells" the neuron not to release any
more neurotransmitter. Thus, if you block the autoreceptors, you turn off
the feedback mechanism, and increase the level of neurotransmitter in the
brain.
- Blocking reuptake. Sometimes, neurotransmitters can
be reabsorbed into the presynaptic neuron, where they're broken down by
enzymes (which may be different than the ones I mentioned above). If you block reuptake--or if you inhibit the enzymes--you'll
have more neurotransmitter floating around in the brain. SSRIs like
Prozac and Zoloft work this way.
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