The fast synaptic responses that we looked at in the previous chapter are mediated by ion channels that open when they bind to the neurotransmitter active at the synapse. Thus transmission at the vertebrate neuromuscular junction or electric organ is mediated by acetylcholine-gated channels, excitatory transmission at spinal motoneurons is mediated by glutamate-gated channels, and inhibitory transmission there is mediated by glycine-gated or GABA-gated channels. These channels are also receptors since they are each specifically activated by their own particular neurotransmitter.

Structure of the nicotinic acetylcholine receptor

There are two main types of acetylcholine receptor. The type we have met previously, which mediates fast synaptic transmission at vertebrate neuromuscular junctions and electric organs, is known as the nicotinic acetylcholine receptor, or nAChR for short. The second type, known as the muscarinic acetylcholine receptor, is involved in some slow synaptic transmission processes and contains no ion channel. Nicotinic receptors are activated by nicotine and blocked by curare, whereas muscarinic receptors are activated by muscarine and blocked by atropine. We will look at muscarinic receptors in chapter 9.

The richest known source of nAChRs is the electric organ of the electric ray Torpedo, 1 kg of which contains over 100 mg of the receptor protein. Lower concentrations are present in the electric organ of Electrophorus, the electric eel, and in vertebrate muscle fibres. Acetylcholine receptors have been isolated from all these sources, but the Torpedo electric organ remains the biochemists' favourite.