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.