Final answer:
Psychoactive drugs exert their effects by acting as either agonists or antagonists to neurotransmitters in the brain, enhancing or inhibiting their activities respectively, which influences mood, perception, and behavior.
Step-by-step explanation:
The mechanism by which psychoactive drugs elicit their responses is through their actions on brain chemistry, affecting neurotransmitter systems. These drugs tend to have specific actions as either agonists or antagonists. Agonists enhance the activity of neurotransmitters by increasing their synthesis, reducing their reuptake, or mimicking their action at receptors. Antagonists, on the other hand, decrease neurotransmitter activity by blocking the action at the receptors or decreasing their synthesis.
For example, depressants act predominantly as agonists to the neurotransmitter GABA, leading to an inhibitory effect overall, reducing arousal or stimulation in various areas of the brain. Antagonists may block receptors to inhibit the action of certain neurotransmitters, thus altering mood, perception, and behavior.