Final answer:
Curare is the drug that prevents over-stimulation by PNS neurons, while caffeine stimulates the CNS.
Step-by-step explanation:
The drug that prevents over-stimulation by the peripheral nervous system (PNS) neurons is curare.
Caffeine, on the other hand, is a central nervous system (CNS) stimulant that stimulates certain portions of the autonomic nervous system, resulting in a temporary increase in alertness and arousal.
Amphetamine and tricyclic antidepressants have different mechanisms of action and are not specifically used to prevent over-stimulation by PNS neurons.