Which major excitatory neurotransmitter activates NMDA receptors?

Glutamate is the major excitatory neurotransmitter in the central nervous system and plays a crucial role in synaptic plasticity, which is fundamental for learning and memory. NMDA (N-Methyl-D-Aspartate) receptors are a specific type of glutamate receptor that is essential for various brain functions, including the generation of long-term potentiation, a cellular mechanism associated with the enhancement of synaptic strength. When glutamate binds to NMDA receptors, this allows for the influx of calcium ions into the neuron, which then activates intracellular signaling pathways critical for neuronal development and activity.

Dopamine, norepinephrine, and serotonin are important neurotransmitters, but they primarily function as modulators of various physiological processes rather than serving as the primary activators of NMDA receptors. For instance, dopamine is mostly involved in reward and motivational behavior, norepinephrine functions in arousal and alertness, and serotonin is associated with mood regulation. Therefore, glutamate stands out as the correct answer due to its direct role in activating NMDA receptors and mediating excitatory neurotransmission.