Neostigmine (prostigmin) is used to increase accumulation in nerve synapses

The answer to this question would be: acetylcholine 

Neostigmine a drug that will inhibit the acetylcholine esterase which was an enzyme that works to break down the acetylcholine. In the end, this drug would cause an increase in acetylcholine. The drug is used for treating miastenia gravis, a disease where the acetylcholine destroyed.