title: Cortical Interneuron Subtypes Vary in Their Axonal Action Potential Properties.
abstract: The role of interneurons in cortical microcircuits is strongly influenced by their passive and active electrical properties. Although different types of interneurons exhibit unique electrophysiological properties recorded at the soma, it is not yet clear whether these differences are also manifested in other neuronal compartments. To address this question, we have used voltage-sensitive dye to image the propagation of action potentials into the fine collaterals of axons and dendrites in two of the largest cortical interneuron subtypes in the mouse: fast-spiking interneurons, which are typically basket or chandelier neurons; and somatostatin containing interneurons, which are typically regular spiking Martinotti cells. We found that fast-spiking and somatostatin-expressing interneurons differed in their electrophysiological characteristics along their entire dendrosomatoaxonal extent. The action potentials generated in the somata and axons, including axon collaterals, of somatostatin-expressing interneurons are significantly broader than those generated in the same compartments of fast-spiking inhibitory interneurons. In addition, action potentials back-propagated into the dendrites of somatostatin-expressing interneurons much more readily than fast-spiking interneurons. Pharmacological investigations suggested that axonal action potential repolarization in both cell types depends critically upon Kv1 channels, whereas the axonal and somatic action potentials of somatostatin-expressing interneurons also depend on BK Ca(2+)-activated K(+) channels. These results indicate that the two broad classes of interneurons studied here have expressly different subcellular physiological properties, allowing them to perform unique computational roles in cortical circuit operations.
  
  
  CONCLUSIONS
  Neurons in the cerebral cortex are of two major types: excitatory and inhibitory. The proper balance of excitation and inhibition in the brain is critical for its operation. Neurons contain three main compartments: dendritic, somatic, and axonal. How the neurons receive information, process it, and pass on new information depends upon how these three compartments operate. While it has long been assumed that axons are simply for conducting information from the cell body to the synapses, here we demonstrate that the axons of different types of interneurons, the inhibitory cells, possess differing electrophysiological properties. This result implies that differing types of interneurons perform different tasks in the cortex, not only through their anatomical connections, but also through how their axons operate.
authors.affiliation: Department of Neurobiology, Kavli Institute for Neuroscience, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510.
authors.forename: Amanda E
authors.identifier: http://orcid.org/0000-0003-0874-6897
authors.initials: AE
authors.lastname: Casale
authors.affiliation: Department of Neurobiology, Kavli Institute for Neuroscience, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510.
authors.forename: Amanda J
authors.identifier: http://orcid.org/0000-0002-4895-0502
authors.initials: AJ
authors.lastname: Foust
authors.affiliation: Department of Neurobiology, Kavli Institute for Neuroscience, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510.
authors.forename: Thierry
authors.identifier: http://orcid.org/0000-0003-0662-1716
authors.initials: T
authors.lastname: Bal
authors.affiliation: Department of Neurobiology, Kavli Institute for Neuroscience, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510 david.mccormick@yale.edu.
authors.forename: David A
authors.identifier: http://orcid.org/0000-0002-9803-8335
authors.initials: DA
authors.lastname: McCormick
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D001369:Axons / Q000502:physiology*; D005260:Female; D007395:Interneurons / Q000502:physiology*; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D013003:Somatosensory Cortex / Q000166:cytology* / Q000502:physiology*
keywords: SOM; action potential; cortex; d-current; fast spiking; interneuron