Drugs are substances that have the ability to affect how neurotransmitters work. Therefore, by definition, they can either enhance the normal action of a neurotransmitter, thereby increasing its effects, or they can interfere with the normal action of a neurotransmitter, and decrease its effects.
A drug that enhances the effectiveness of a neurotransmitter is an agonist.
A drug that decreases the effectiveness of a neurotransmitter is an antagonist.
There are several ways that drugs can function as agonists, and several ways they can function as antagonists. Generally speaking, drugs influence the effects of neurotransmitters by affecting one of the 5 steps of synaptic transmission. Remember, those are: synthesis, transport, release, interaction with the postsynaptic receptor, and inactivation of the neurotransmitter.
WAYS THAT DRUGS CAN ACT AS AGONISTS:
1. Increase release of the neurotransmitter
If more neurotransmitter is in the synapse because more has been released, this will help the neurotransmitter bind every possible postsynaptic receptor. This will enhance the effect of the neurotransmitter. Examples of drugs that increase release of neurotransmitter are black widow spider venom (Ach), MDMA (5-HT), and amphetamine (DA & NE).
***Note: Just because a drug is an agonist and therefore has a positive effect on the action of a neurotransmitter does NOT mean that the drug has a positive effect on the physiology or the behavior of the organism. Black widow spider venom is a poison, and therefore bad for the bitten organism, but it exerts those bad effects by having a positive, enhancing effect on Ach.
2. Mimicking the neurotransmitter at the postsynaptic receptor
A drug can pretend to be a neurotransmitter by binding that neurotransmitter's receptors on a postsynaptic cell and opening ion channels just like the neurotransmitter would. This will help ensure that every possible postsynaptic recpetor has been activated. Examples of drugs that act by mimicking the neurotransmitter at postsynaptic receptors are LSD (5-HT) and nicotine (Ach).
3. Inhibiting enzymatic breakdown
Some drugs increase the effectiveness of a neurotransmitter by preventing it from being degraded. This will ensure that more neurotransmitter is around in the synapse to activate the postsynaptic cell. Some drugs only inhibit enzymatic degradation a little bit, like MAO inhibitors (used as anti-depressants because they slow the breakdown of DA & NE by MAO) or physostigmine, which inhibits the breakdown of Ach by AchE. However, some drugs, like nerve gas, are so effective and irreversibly inhibit the action of an enzyme, that they disrupt neurotransmission so badly that the organism dies.
4. Inhibit reuptake of neurotransmitter Some drugs enhance the effects of a neurotransmitter by preventing it from being taken back up by the presynaptic cell. Again, this ensures that as much neurotransmitter as possible can activate postsynaptic receptors. Examples of drugs that inhibit reuptake include cocaine (DA) and Prozac (5-HT).
WAYS THAT DRUGS CAN ACT AS ANTAGONISTS
1. Block synthesis of neurotransmitter
If no neurotransmitter is synthesized, then none can be released and contact the postsynaptic cell. This greatly inhibits the effects of a neurotransmitter. I did not give you an example of a drug that blocks synthesis of a neurotransmitter.
2. Block vesiculation
Even though a neurotransmitter might be synthesized successfully, if it is not packaged into vesicles, it cannot be released. Again, if no neurotransmitter is released, no neurotransmitter can contact the postsynaptic cell. An example of a drug that is an antagonist because it prevents vesiculation is reserpine (DA & NE).
3. Block release of a neurotransmitter
Again, if a neurotransmitter cannot be released, it cannot contact the post-synaptic receptors. An example of a drug that blocks release of a neurotransmitter is botulinum toxin (Ach).
4. Block post-synaptic receptors
A drug can block the action of a neurotransmitter by plugging up that neurotransmitter's postsynaptic receptors. Drugs that act as antagonists this way do not themselves have any effect at the postsynaptic receptors. Instead, all they do is prevent the neurotransmitter from getting to its postsynaptic receptors and having an effect there. Examples of drugs that work this way are scopolamine (Ach), anti-psychotics (DA) and PCP (glutamate).
***NOTE: Just because a drug inhibits the action of a neurotransmitter does not mean it has an adverse effect on behavior. It might, like PCP, or it might be beneficial. For example, anti-psychotic drugs have an inhibitory effect on dopaminergic transmission, but that translates into a beneficial effect on the behavior of schizophrenics, since it decreases their hallucinations and delusions.