title: Navβ4 regulates fast resurgent sodium currents and excitability in sensory neurons.
abstract: BACKGROUND
  Increased electrical activity in peripheral sensory neurons including dorsal root ganglia (DRG) and trigeminal ganglia neurons is an important mechanism underlying pain. Voltage gated sodium channels (VGSC) contribute to the excitability of sensory neurons and are essential for the upstroke of action potentials. A unique type of VGSC current, resurgent current (INaR), generates an inward current at repolarizing voltages through an alternate mechanism of inactivation referred to as open-channel block. INaRs are proposed to enable high frequency firing and increased INaRs in sensory neurons are associated with pain pathologies. While Nav1.6 has been identified as the main carrier of fast INaR, our understanding of the mechanisms that contribute to INaR generation is limited. Specifically, the open-channel blocker in sensory neurons has not been identified. Previous studies suggest Navβ4 subunit mediates INaR in central nervous system neurons. The goal of this study was to determine whether Navβ4 regulates INaR in DRG sensory neurons.
  
  
  RESULTS
  Our immunocytochemistry studies show that Navβ4 expression is highly correlated with Nav1.6 expression predominantly in medium-large diameter rat DRG neurons. Navβ4 knockdown decreased endogenous fast INaR in medium-large diameter neurons as measured with whole-cell voltage clamp. Using a reduced expression system in DRG neurons, we isolated recombinant human Nav1.6 sodium currents in rat DRG neurons and found that overexpression of Navβ4 enhanced Nav1.6 INaR generation. By contrast neither overexpression of Navβ2 nor overexpression of a Navβ4-mutant, predicted to be an inactive form of Navβ4, enhanced Nav1.6 INaR generation. DRG neurons transfected with wild-type Navβ4 exhibited increased excitability with increases in both spontaneous activity and evoked activity. Thus, Navβ4 overexpression enhanced INaR and excitability, whereas knockdown or expression of mutant Navβ4 decreased INaR generation.
  
  
  CONCLUSIONS
  INaRs are associated with inherited and acquired pain disorders. However, our ability to selectively target and study this current has been hindered due to limited understanding of how it is generated in sensory neurons. This study identified Navβ4 as an important regulator of INaR and excitability in sensory neurons. As such, Navβ4 is a potential target for the manipulation of pain sensations.
authors.affiliation: Department of Pharmacology and Toxicology, Indiana University, Indianapolis, IN, USA. cbarbosa@iupui.edu.|Department of Pharmacology and Toxicology, Stark Neurosciences Research Institute, 320 West 25th Street, NB-414F, Indianapolis, IN, 46202-2266, USA. cbarbosa@iupui.edu.
authors.forename: Cindy
authors.initials: C
authors.lastname: Barbosa
authors.affiliation: Department of Pharmacology and Toxicology, Indiana University, Indianapolis, IN, USA. zt2@iupui.edu.|Department of Pharmacology and Toxicology, Stark Neurosciences Research Institute, 320 West 25th Street, NB-414F, Indianapolis, IN, 46202-2266, USA. zt2@iupui.edu.
authors.forename: Zhi-Yong
authors.initials: ZY
authors.lastname: Tan
authors.affiliation: Department of Pharmacology and Toxicology, Indiana University, Indianapolis, IN, USA. rewang@iupui.edu.
authors.forename: Ruizhong
authors.initials: R
authors.lastname: Wang
authors.affiliation: Department of Anesthesiology, University of Cincinnati, Cincinnati, OH, USA. xiewe@ucmail.uc.edu.
authors.forename: Wenrui
authors.initials: W
authors.lastname: Xie
authors.affiliation: Department of Anesthesiology, University of Cincinnati, Cincinnati, OH, USA. strongja@ucmail.uc.edu.
authors.forename: Judith A
authors.initials: JA
authors.lastname: Strong
authors.affiliation: Medical Neuroscience Graduate Program, Indiana University School of Medicine, Indianapolis, IN, USA. patelr@iupui.edu.|Department of Pharmacology and Toxicology, Stark Neurosciences Research Institute, 320 West 25th Street, NB-414F, Indianapolis, IN, 46202-2266, USA. patelr@iupui.edu.
authors.forename: Reesha R
authors.initials: RR
authors.lastname: Patel
authors.affiliation: Department of Pharmacology and Toxicology, Indiana University, Indianapolis, IN, USA. vaskom@iupui.edu.|Department of Pharmacology and Toxicology, Stark Neurosciences Research Institute, 320 West 25th Street, NB-414F, Indianapolis, IN, 46202-2266, USA. vaskom@iupui.edu.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Vasko
authors.affiliation: Department of Anesthesiology, University of Cincinnati, Cincinnati, OH, USA. Jun-Ming.Zhang@uc.edu.
authors.forename: Jun-Ming
authors.initials: JM
authors.lastname: Zhang
authors.affiliation: Department of Pharmacology and Toxicology, Indiana University, Indianapolis, IN, USA. trcummin@iupui.edu.|Department of Pharmacology and Toxicology, Stark Neurosciences Research Institute, 320 West 25th Street, NB-414F, Indianapolis, IN, 46202-2266, USA. trcummin@iupui.edu.
authors.forename: Theodore R
authors.initials: TR
authors.lastname: Cummins
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005727:Ganglia, Spinal / Q000378:metabolism; D055785:Gene Knockdown Techniques; D006801:Humans; D015640:Ion Channel Gating; D008297:Male; D008954:Models, Biological; D008969:Molecular Sequence Data; D062557:NAV1.6 Voltage-Gated Sodium Channel / Q000378:metabolism; D017207:Rats, Sprague-Dawley; D011984:Sensory Receptor Cells / Q000378:metabolism*; D062548:Voltage-Gated Sodium Channel beta-2 Subunit; D062585:Voltage-Gated Sodium Channel beta-4 Subunit / Q000737:chemistry / Q000378:metabolism*
chemical_list: D062557:NAV1.6 Voltage-Gated Sodium Channel; C100681:SCN8A protein, human; D062548:Voltage-Gated Sodium Channel beta-2 Subunit; D062585:Voltage-Gated Sodium Channel beta-4 Subunit