Neuromuscular NMDA Receptors Modulate Developmental Synapse Elimination.

In normal adult muscle, each muscle fiber is innervated by a single axon, but at birth, fibers are multiply innervated. Elimination of excess connections requires neural activity; because the neuromuscular junction (NMJ) is a cholinergic synapse, acetylcholine has been assumed to be the critical mediator of activity. However, glutamate receptors are also expressed at the NMJ. We found that axon removal in mice is slowed by pharmacological and molecular manipulations that decrease signaling through neuromuscular NMDA receptors, whereas application of exogenous NMDA at the NMJ accelerates synapse elimination and increases muscle calcium levels during the first 2 postnatal weeks. Therefore, neuromuscular NMDA receptors play previously unsuspected roles in neuromuscular activity and elimination of excess synaptic input during development.