These synaptic vesicles contain neurotransmitters such as acetylcholine, neuropeptides, catecholamines, etc.
Though, adrenaline or epinephrine is a hormone released by the adrenal gland, it is also produced by a few cells in the central nervous system and acts as a neurotransmitter helping neuronal communication.
Final answer: The synaptic vesicles discharge all of the above.