Agonists drugs bind to and trigger receptors in the human body. For example, a dopamine agonist binds to dopamine receptors and produces effects are very similar, or the same as, increasing dopamine levels.
Antagonists bind to receptors and prevent them from being triggered. This results in the influence of a receptor being severely reduced or eliminated. Antagonists don't actually reduce the level of a chemical, but the net effect is the same.
Partial Antagonists bind to receptors and reduce the signals they send. But they do not block the signal altogether like antagonists can. Their effect is somewhere between the effect of an agonist drug and an antagonist drug.
Inverse Agonists bind to receptor sites in the same manner as an agonist. But they produce the opposite effect, when compared to what an agonist would.
Neutral Antagonists do not do anything when there is no agonist or inverse agonist present. But they can block the activity of either, when an agonist or inverse agonist is present.
Competitive Antagonists will bind to receptors but not activate them. When an agonist is present, competitive antagonists will compete for receptor binding sites. In sufficient quantity competitive antagonists will displace an agonist.
Competitive antagonists are utilized to prevent drugs from having an effect, and to reverse the effects of substances that have been ingested. For example, naloxone is a competitive antagonist employed to reverse overdose caused by opioids like heroin.
Selective Agonists activate certain types of a specific receptor and produce results that are similar to increasing the level of the chemical associated with it.
Selective Antagonists keep certain types of a specific receptor from being activated and produce results that are similar to decreasing the level of the chemical associated with it.
Many neurotransmitters have several different types of receptors. Representative of this is GABA-A and GABA-B receptors which will both produce different effects when activated.
A drug that is a selective GABA-A receptor agonist will only bind to and trigger GABA-A receptors.
A drug that is a selective GABA-A receptor antagonist will only bind to and block GABA-A receptors.
Reuptake Inhibitors slow down the rate at which chemicals are broken down. A serotonin reuptake inhibitor slows down the rate at which serotonin is broken down. This results in increased levels of serotonin available to the body.
Releasing Agents increase the quantities of chemicals that are released in the body. A histamine releasing agent increases the rate that histamine is released. This results in larger quantities of histamine being available to the body.
Precursors are the chemicals utilized to produce (synthesize) other compounds. Citric acid is converted to alpha-ketoglutaric acid, which is then converted to glutamate.
So citric acid and alpha-ketoglutaric acid are both glutamate precursors. Alpha-ketoglutaric acid is considered a direct precursor because it is the last chemical to be converted.