Imbalance of ionic conductances contributes to diverse symptoms of demyelination

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 107; no. 48; pp. 20602 - 20609
• Main Authors: Coggan, Jay S., Prescott, Steven A., Bartol, Thomas M., Sejnowski, Terrence J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 30-11-2010; National Acad Sciences
• Series: Inaugural Article
• Subjects: Action potential; Action Potentials - physiology; Axons; Axons - physiology; Biological Sciences; Demyelinating diseases; Demyelinating Diseases - pathology; Demyelinating Diseases - physiopathology; Demyelination; Drugs; Excitability; Firing pattern; Humans; Ion Channels - metabolism; Membranes; Modeling; Models, Neurological; Multiple sclerosis; Myelin; Nerve conduction; Neurons; Neurotransmission; Potassium; Potassium channels; Saddle points; Simulation; Sodium; Sodium conductance; Symptoms; Synchronization; Three dimensional modeling; Trajectories; Two dimensional modeling
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1013798107

Fast axonal conduction of action potentials in mammals relies on myelin insulation. Demyelination can cause slowed, blocked, desynchronized, or paradoxically excessive spiking that underlies the symptoms observed in demyelination diseases. The diversity and timing of such symptoms are poorly understood, often intermittent, and uncorrelated with disease progress. We modeled the effects of demyelination (and secondary remodeling) on intrinsic axonal excitability using Hodgkin—Huxley and reduced Morris—Lecar models. Simulations and analysis suggested a simple explanation for the breadth of symptoms and revealed that the ratio of sodium to leak conductance, g Na /g L , acted as a four-way switch controlling excitability patterns that included spike failure, single spike transmission, afterdischarge, and spontaneous spiking. Failure occurred when this ratio fell below a threshold value. Afterdischarge occurred at g Na /g L just below the threshold for spontaneous spiking and required a slow inward current that allowed for two stable attractor states, one corresponding to quiescence and the other to repetitive spiking. A neuron prone to afterdischarge could function normally unless it was switched to its "pathological" attractor state; thus, although the underlying pathology may develop slowly by continuous changes in membrane conductances, a discontinuous change in axonal excitability can occur and lead to paroxysmal symptoms. We conclude that tonic and paroxysmal positive symptoms as well as negative symptoms may be a consequence of varying degrees of imbalance between g Na and g L after demyelination. The KCNK family of g L potassium channels may be an important target for new drugs to treat the symptoms of demyelination.