Final answer:
Psychoactive drugs can either be agonists, which increase neurotransmitter activity, or antagonists, which decrease it. Agonists mimic or enhance neurotransmitter action, while antagonists block or impede it.
Step-by-step explanation:
Psychoactive drugs influence brain chemistry and are categorized as either agonists or antagonists based on how they affect neurotransmitter systems. Agonists are compounds that enhance the activity of neurotransmitters. They can mimic neurotransmitters at receptor sites, increase their synthesis, or prevent their reuptake, thereby increasing neurotransmitter levels in the synapses. This increased activity can help correct imbalances, as seen with dopamine agonists used in Parkinson's disease treatment.
On the contrary, antagonists decrease neurotransmitter activity by blocking neurotransmitter production or by preventing neurotransmitters from binding to their receptors. This reduction is crucial in situations where there is an excess of neurotransmitters or an oversensitivity to them.