Interneuronal synapses formed by motor neurons appear to be glutamatergic.

Acetylcholine release at motor neuron synapses has been long established; however, recent discoveries indicate that synaptic transmission by motor neurons is more complex than previously thought. Using whole-cell patch clamp, we show that spontaneous excitatory postsynaptic currents of rat motor neurons in primary ventral horn cultures are entirely glutamatergic, although the cells respond to exogenous acetylcholine. Motor neurons in cultures express the vesicular glutamate transporter VGlut2, and culturing motor neurons for weeks with glutamate receptors blocked upregulates glutamate signaling without increasing cholinergic signaling. In spinal cord slices, motor neurons showed no decrease in spontaneous excitatory synaptic potentials after blocking acetylcholine receptors. Our results suggest that motor neuron synapses formed on other neurons are largely glutamatergic in culture and the spinal cord.