Extracting rate constants for nmda receptor gating from one-channel current recordings

Like all neurotransmitter-gated channels, in response to agonist binding, ionotropic glutamate receptors produce electrical signals whose amplitudes and durations reflect intramolecular transitions between nonconducting (closed) and conducting (open) receptor conformations. Thus, delineating the reaction mechanism of synaptic channels represents an important step in understanding how information is transferred and processed in the nervous system. The recorded single-channel signal captures in real-time multiple series of discrete current amplitudes, whose complex duration distributions contain valuable information about the underlying kinetic mechanism but in most cases are difficult to decipher. For NMDA receptors, we identified conditions in which the receptor populates only two conductance levels, corresponding to closed and open channels, and we developed procedures that can organize the entire succession of closed and open durations into a comprehensive, reproducible, and testable reaction mechanism. In this chapter, we describe how to select, process, and idealize current traces recorded from patches containing one NMDA receptor, and how to build increasingly more accurate kinetic models that include transitions from the sub-millisecond to the hundreds of minutes time scales. The resulting schemes can be tested by comparing model simulations and experimental recordings elicited with similar stimulation patterns. The principles and methodology outlined here can be adapted and extended to other ion channels to gather deeper insight into the order and rates of intramolecular movements that produce stimulus-elicited electrical signals in the central nervous system.