Upon release, acetylcholine is metabolized into choline and acetate by acetylcholinesterase, and other nonspecific esterases. Acetylcholine release can be excitatory or inhibitory depending on the type of tissue and the nature of the receptor with which it interacts. Cholinergic receptors can be divided into two types, muscarinic and nicotinic, based on the pharmacological action of various agonists and antagonists. Muscarinic receptors originally were distinguished from nicotinic receptors by the selectivity of the agonists muscarine and nicotine respectively. Muscarinic receptors will be discussed in detail later, while nicotinic receptors will be discussed in the next section.
Nicotinic responses are of fast onset, short duration and excitatory in nature.
The pharmacology of nicotinic receptors has been studied in great detail and our understanding of how ion channel-coupled neurotransmitter receptors work is based largely on the study of this class of proteins. Nicotinic receptors are found in a variety of tissues, including the autonomic nervous system, the neuromuscular junction and the brain in vertebrates. They also are found in high quantities in the electric organs of various electric eels and rays. Agonists such as acetylcholine, carbamylcholine and nicotine produce the physiological responses associated with nicotinic cholinergic activation. Acetylcholine produces an influx of sodium through a ligand-gated ion channel.