Final answer:
Acetylcholine is the neurotransmitter used at the synapse between pre-ganglionic and post-ganglionic neurons in both the sympathetic and parasympathetic nervous systems.
Step-by-step explanation:
The neurotransmitter used at the synapse between pre-ganglionic and post-ganglionic neurons, in both the sympathetic and parasympathetic nervous systems, is acetylcholine (ACh). This is the case for both preganglionic parasympathetic and preganglionic sympathetic neurons. However, it's important to note that postganglionic sympathetic neurons primarily release norepinephrine, while postganglionic parasympathetic neurons release ACh. Drugs that affect both divisions of the autonomic system would thus potentially interact with both nicotinic and muscarinic receptors. The release of acetylcholine at the synapse between these neurons leads to the transmission of signals and regulation of various physiological processes.