Final answer:
Mescaline is not a depressant; it is a hallucinogen that alters perception and mood. Depressants like tranquilizers, opioids, and barbiturates suppress central nervous system activity by acting as GABA agonists, leading to sedation and muscle relaxation.
Step-by-step explanation:
All but one of the following are depressants: tranquilizers, opioids, mescaline, and barbiturates. The correct answer to which of these is not a depressant is mescaline. Mescaline is a hallucinogen derived from the Peyote cactus, and unlike depressants, it is known for altering perception, consciousness, and mood, rather than predominantly depressing central nervous system activity.
Depressants tend to suppress central nervous system activity, with effects that often include sedation, relief from anxiety, and muscle relaxation. They exert their effects through various mechanisms, one of which is serving as agonists of the gamma-Aminobutyric acid (GABA) neurotransmitter system, which has a quietening effect on the brain. Tranquilizers, such as benzodiazepines, and barbiturates are classic examples of depressants, as they are GABA agonists and produce a depressant effect. Opioids, although often associated with pain relief, also have depressant effects such as sedation and respiratory depression.