Time-Dependent Molecular Memory in Single Voltage-Gated Sodium Channel Tapan K. Nayak Æ S. K. Sikdar Received: 12 February 2007 / Accepted: 18 June 2007 / Published online: 1 September 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Excitability in neurons is associated with firing of action potentials and requires the opening of voltage- gated sodium channels with membrane depolarization. Sustained membrane depolarization, as seen in pathophys- iological conditions like epilepsy, can have profound implications on the biophysical properties of voltage-gated ion channels. Therefore, we sought to characterize the effect of sustained membrane depolarization on single voltage- gated Na + channels. Single-channel activity was recorded in the cell-attached patch-clamp mode from the rNa v 1.2a channels expressed in CHO cells. Classical statistical anal- ysis revealed complex nonlinear changes in channel dwell times and unitary conductance of single Na + channels as a function of conditioning membrane depolarization. Signal processing tools like weighted wavelet Z (WWZ) and dis- crete Fourier transform analyses attributed a ‘‘pseudo- oscillatory’’ nature to the observed nonlinear variation in the kinetic parameters. Modeling studies using the hidden Markov model (HMM) illustrated significant changes in kinetic states and underlying state transition rate constants upon conditioning depolarization. Our results suggest that sustained membrane depolarization induces novel nonlinear properties in voltage-gated Na + channels. Prolonged mem- brane depolarization also induced a ‘‘molecular memory’’ phenomenon, characterized by clusters of dwell time events and strong autocorrelation in the dwell time series similar to that reported recently for single enzyme molecules. The persistence of such molecular memory was found to be dependent on the duration of depolarization. Voltage-gated Na + channel with the observed time-dependent nonlinear properties and the molecular memory phenomenon may determine the functional state of the channel and, in turn, the excitability of a neuron. Keywords Voltage-gated sodium channel  Cell-attached patch-clamp  Conditioning depolarization  Pseudoperiodic oscillation  Autocorrelation  Molecular memory Introduction Spike timing, delay and information coding in the nervous system largely depend on the availability of voltage-gated Na + conductance and the ‘‘conformational preparedness’’ of the channel at the molecular level. The availability of Na + conductance is the outcome of a complex interplay between activation, inactivation and recovery from inactivation processes of Na + channel (Mickus, Jung & Spruston, 1999; Ong, Tomaselli & Balser, 2000; Goldin, 2003). The dynamics of voltage-gated Na + channel inactivation and recovery thereof are complex nonlinear functions of the amplitude and duration of depolarization (Toib, Lyakhov & Marom, 1998; Majumdar, Foster & Sikdar, 2004). Neuronal membranes can remain depolarized (above threshold) for varying and extended time durations in conditions such as epileptic seizures (Rutecki & Yang, 1998), brain ischemia (Xu, 1995), pain (Wu et al., 2005) and during spontaneous network activity observed in working memory within local circuits (McCormick et al., 2003). Epileptic seizures are known to bring about acquired changes in somatic ion channels (Chen et al., 2001; Su et al., 2002). This raises an intriguing question involving the possible role of sustained depolarization in inducing dynamic conformational changes in voltage-gated Na + channel molecules. Such changes in T. K. Nayak  S. K. Sikdar (&) Molecular Biophysics Unit, Indian Institute of Science, Bangalore-12, India e-mail: sks@mbu.iisc.ernet.in 123 J Membrane Biol (2007) 219:19–36 DOI 10.1007/s00232-007-9058-4