We study the mechanisms that permit rapid and sustained synaptic transmission in the mouse brain, predominantly using the calyx of Held as a model synapse. This giant glutamatergic synapse in the auditory brainstem has a number of experimental advantages that permit us to trace the fundamental mechanisms that underlie chemical neurotransmission. We apply a variety of genetic and viral transduction techniques to disrupt presynaptic function at the calyx through transgenic mouse models, and expression in neuronal populations using adeno-associated virus (AAV). We use whole cell electrophysiology to record activity from the presynaptic or postsynaptic compartments (and sometimes both!) We complement these recordings with the use of use organic and genetically-encoded probes for functional imaging of essential messengers (Ca2+ ATP, and others).