Final answer:
The receptor subtype that is not metabotropic is the nicotinic cholinergic receptor, which is instead an ionotropic receptor acting as a channel for ions to quickly change a cell's membrane potential.
Step-by-step explanation:
All of the following receptor sub-types are metabotropic, except nicotinic cholinergic receptors. A metabotropic receptor, such as the muscarinic cholinergic, alpha adrenergic, and beta adrenergic receptors, are part of a complex system that causes metabolic changes in the cell when a neurotransmitter binds to it. Unlike these, the nicotinic cholinergic receptor is an ionotropic receptor, which means it acts as a ion channel that opens directly upon the neurotransmitter's binding.
Ionotropic receptors like the nicotinic cholinergic receptor work by quickly allowing ions to enter or exit the cell, thereby changing the cell's membrane potential and causing a rapid response. Metabotropic receptors, on the other hand, activate a second messenger pathway which can result in a range of cellular responses including changes in gene transcription and metabolic alterations. The NMDA receptor mentioned is also ionotropic, primarily allowing calcium ions to enter the cell, and is not in the same category as the aforementioned metabotropic receptors.