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The Axon Initial Segment: An Updated Viewpoint. J Neurosci 2018; 38:2135-2145. [PMID: 29378864 DOI: 10.1523/jneurosci.1922-17.2018] [Citation(s) in RCA: 168] [Impact Index Per Article: 28.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2017] [Revised: 01/20/2018] [Accepted: 01/25/2018] [Indexed: 12/20/2022] Open
Abstract
At the base of axons sits a unique compartment called the axon initial segment (AIS). The AIS generates and shapes the action potential before it is propagated along the axon. Neuronal excitability thus depends crucially on the AIS composition and position, and these adapt to developmental and physiological conditions. The AIS also demarcates the boundary between the somatodendritic and axonal compartments. Recent studies have brought insights into the molecular architecture of the AIS and how it regulates protein trafficking. This Viewpoints article summarizes current knowledge about the AIS and highlights future challenges in understanding this key actor of neuronal physiology.
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Sartiani L, Mannaioni G, Masi A, Novella Romanelli M, Cerbai E. The Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: from Biophysics to Pharmacology of a Unique Family of Ion Channels. Pharmacol Rev 2017; 69:354-395. [PMID: 28878030 DOI: 10.1124/pr.117.014035] [Citation(s) in RCA: 82] [Impact Index Per Article: 11.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2017] [Accepted: 07/07/2017] [Indexed: 12/22/2022] Open
Abstract
Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are important members of the voltage-gated pore loop channels family. They show unique features: they open at hyperpolarizing potential, carry a mixed Na/K current, and are regulated by cyclic nucleotides. Four different isoforms have been cloned (HCN1-4) that can assemble to form homo- or heterotetramers, characterized by different biophysical properties. These proteins are widely distributed throughout the body and involved in different physiologic processes, the most important being the generation of spontaneous electrical activity in the heart and the regulation of synaptic transmission in the brain. Their role in heart rate, neuronal pacemaking, dendritic integration, learning and memory, and visual and pain perceptions has been extensively studied; these channels have been found also in some peripheral tissues, where their functions still need to be fully elucidated. Genetic defects and altered expression of HCN channels are linked to several pathologies, which makes these proteins attractive targets for translational research; at the moment only one drug (ivabradine), which specifically blocks the hyperpolarization-activated current, is clinically available. This review discusses current knowledge about HCN channels, starting from their biophysical properties, origin, and developmental features, to (patho)physiologic role in different tissues and pharmacological modulation, ending with their present and future relevance as drug targets.
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Affiliation(s)
- Laura Sartiani
- Department of Neurosciences, Psychology, Drug Research, and Child Health, University of Florence, Firenze, Italy
| | - Guido Mannaioni
- Department of Neurosciences, Psychology, Drug Research, and Child Health, University of Florence, Firenze, Italy
| | - Alessio Masi
- Department of Neurosciences, Psychology, Drug Research, and Child Health, University of Florence, Firenze, Italy
| | - Maria Novella Romanelli
- Department of Neurosciences, Psychology, Drug Research, and Child Health, University of Florence, Firenze, Italy
| | - Elisabetta Cerbai
- Department of Neurosciences, Psychology, Drug Research, and Child Health, University of Florence, Firenze, Italy
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Rowan MJM, Christie JM. Rapid State-Dependent Alteration in K v3 Channel Availability Drives Flexible Synaptic Signaling Dependent on Somatic Subthreshold Depolarization. Cell Rep 2017; 18:2018-2029. [PMID: 28228266 DOI: 10.1016/j.celrep.2017.01.068] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2016] [Revised: 12/16/2016] [Accepted: 01/25/2017] [Indexed: 10/20/2022] Open
Abstract
In many neurons, subthreshold depolarization in the soma can transiently increase action-potential (AP)-evoked neurotransmission via analog-to-digital facilitation. The mechanisms underlying this form of short-term synaptic plasticity are unclear, in part, due to the relative inaccessibility of the axon to direct physiological interrogation. Using voltage imaging and patch-clamp recording from presynaptic boutons of cerebellar stellate interneurons, we observed that depolarizing somatic potentials readily spread into the axon, resulting in AP broadening, increased spike-evoked Ca2+ entry, and enhanced neurotransmission strength. Kv3 channels, which drive AP repolarization, rapidly inactivated upon incorporation of Kv3.4 subunits. This leads to fast susceptibility to depolarization-induced spike broadening and analog facilitation independent of Ca2+-dependent protein kinase C signaling. The spread of depolarization into the axon was attenuated by hyperpolarization-activated currents (Ih currents) in the maturing cerebellum, precluding analog facilitation. These results suggest that analog-to-digital facilitation is tempered by development or experience in stellate cells.
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Affiliation(s)
- Matthew J M Rowan
- Max Planck Florida Institute for Neuroscience, Jupiter, FL 33458, USA
| | - Jason M Christie
- Max Planck Florida Institute for Neuroscience, Jupiter, FL 33458, USA.
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Liu C, Xie C, Grant K, Su Z, Gao W, Liu Q, Zhou L. Patch-clamp fluorometry-based channel counting to determine HCN channel conductance. J Gen Physiol 2017; 148:65-76. [PMID: 27353446 PMCID: PMC4924933 DOI: 10.1085/jgp.201511559] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2015] [Accepted: 05/23/2016] [Indexed: 11/20/2022] Open
Abstract
Counting ion channels on cell membranes is of fundamental importance for the study of channel biophysics. Channel counting has thus far been tackled by classical approaches, such as radioactive labeling of ion channels with blockers, gating current measurements, and nonstationary noise analysis. Here, we develop a counting method based on patch-clamp fluorometry (PCF), which enables simultaneous electrical and optical recordings, and apply it to EGFP-tagged, hyperpolarization-activated and cyclic nucleotide-regulated (HCN) channels. We use a well-characterized and homologous cyclic nucleotide-gated (CNG) channel to establish the relationship between macroscopic fluorescence intensity and the total number of channels. Subsequently, based on our estimate of the total number of HCN channels, we determine the single-channel conductance of HCN1 and HCN2 to be 0.46 and 1.71 pS, respectively. Such a small conductance would present a technical challenge for traditional electrophysiology. This PCF-based technique provides an alternative method for counting particles on cell membranes, which could be applied to biophysical studies of other membrane proteins.
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Affiliation(s)
- Chang Liu
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298 School of Medicine, Nankai University, Tianjin 300071, China
| | - Changan Xie
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298
| | - Khade Grant
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298
| | - Zhuocheng Su
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298
| | - Weihua Gao
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298
| | - Qinglian Liu
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298
| | - Lei Zhou
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298
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Serotonin Decreases the Gain of Visual Responses in Awake Macaque V1. J Neurosci 2017; 37:11390-11405. [PMID: 29042433 PMCID: PMC5700422 DOI: 10.1523/jneurosci.1339-17.2017] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2017] [Revised: 09/09/2017] [Accepted: 09/12/2017] [Indexed: 11/21/2022] Open
Abstract
Serotonin, an important neuromodulator in the brain, is implicated in affective and cognitive functions. However, its role even for basic cortical processes is controversial. For example, in the mammalian primary visual cortex (V1), heterogenous serotonergic modulation has been observed in anesthetized animals. Here, we combined extracellular single-unit recordings with iontophoresis in awake animals. We examined the role of serotonin on well-defined tuning properties (orientation, spatial frequency, contrast, and size) in V1 of two male macaque monkeys. We find that in the awake macaque the modulatory effect of serotonin is surprisingly uniform: it causes a mainly multiplicative decrease of the visual responses and a slight increase in the stimulus-selective response latency. Moreover, serotonin neither systematically changes the selectivity or variability of the response, nor the interneuronal correlation unexplained by the stimulus ("noise-correlation"). The modulation by serotonin has qualitative similarities with that for a decrease in stimulus contrast, but differs quantitatively from decreasing contrast. It can be captured by a simple additive change to a threshold-linear spiking nonlinearity. Together, our results show that serotonin is well suited to control the response gain of neurons in V1 depending on the animal's behavioral or motivational context, complementing other known state-dependent gain-control mechanisms.SIGNIFICANCE STATEMENT Serotonin is an important neuromodulator in the brain and a major target for drugs used to treat psychiatric disorders. Nonetheless, surprisingly little is known about how it shapes information processing in sensory areas. Here we examined the serotonergic modulation of visual processing in the primary visual cortex of awake behaving macaque monkeys. We found that serotonin mainly decreased the gain of the visual responses, without systematically changing their selectivity, variability, or covariability. This identifies a simple computational function of serotonin for state-dependent sensory processing, depending on the animal's affective or motivational state.
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56
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Guo Y, Su ZJ, Chen YK, Chai Z. Brain-derived neurotrophic factor/neurotrophin 3 regulate axon initial segment location and affect neuronal excitability in cultured hippocampal neurons. J Neurochem 2017; 142:260-271. [DOI: 10.1111/jnc.14050] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2016] [Revised: 04/07/2017] [Accepted: 04/18/2017] [Indexed: 12/11/2022]
Affiliation(s)
- Yu Guo
- State Key Laboratory of Membrane Biology; College of Life Sciences; Peking University; Beijing China
| | - Zi-jun Su
- State Key Laboratory of Membrane Biology; College of Life Sciences; Peking University; Beijing China
| | - Yi-kun Chen
- State Key Laboratory of Membrane Biology; College of Life Sciences; Peking University; Beijing China
| | - Zhen Chai
- State Key Laboratory of Membrane Biology; College of Life Sciences; Peking University; Beijing China
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Huguet G, Meng X, Rinzel J. Phasic Firing and Coincidence Detection by Subthreshold Negative Feedback: Divisive or Subtractive or, Better, Both. Front Comput Neurosci 2017; 11:3. [PMID: 28210218 PMCID: PMC5288357 DOI: 10.3389/fncom.2017.00003] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2016] [Accepted: 01/16/2017] [Indexed: 11/26/2022] Open
Abstract
Phasic neurons typically fire only for a fast-rising input, say at the onset of a step current, but not for steady or slow inputs, a property associated with type III excitability. Phasic neurons can show extraordinary temporal precision for phase locking and coincidence detection. Exemplars are found in the auditory brain stem where precise timing is used in sound localization. Phasicness at the cellular level arises from a dynamic, voltage-gated, negative feedback that can be recruited subthreshold, preventing the neuron from reaching spike threshold if the voltage does not rise fast enough. We consider two mechanisms for phasicness: a low threshold potassium current (subtractive mechanism) and a sodium current with subthreshold inactivation (divisive mechanism). We develop and analyze three reduced models with either divisive or subtractive mechanisms or both to gain insight into the dynamical mechanisms for the potentially high temporal precision of type III-excitable neurons. We compare their firing properties and performance for a range of stimuli. The models have characteristic non-monotonic input-output relations, firing rate vs. input intensity, for either stochastic current injection or Poisson-timed excitatory synaptic conductance trains. We assess performance according to precision of phase-locking and coincidence detection by the models' responses to repetitive packets of unitary excitatory synaptic inputs with more or less temporal coherence. We find that each mechanism contributes features but best performance is attained if both are present. The subtractive mechanism confers extraordinary precision for phase locking and coincidence detection but only within a restricted parameter range when the divisive mechanism of sodium inactivation is inoperative. The divisive mechanism guarantees robustness of phasic properties, without compromising excitability, although with somewhat less precision. Finally, we demonstrate that brief transient inhibition if properly timed can enhance the reliability of firing.
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Affiliation(s)
- Gemma Huguet
- Departament de Matemàtiques, Universitat Politècnica de Catalunya Barcelona, Spain
| | - Xiangying Meng
- Biology Department, University of Maryland College Park, MD, USA
| | - John Rinzel
- Center for Neural Science, New York UniversityNew York, NY, USA; Courant Institute of Mathematical Sciences, New York UniversityNew York, NY, USA
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58
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Tapia M, Dominguez A, Zhang W, Del Puerto A, Ciorraga M, Benitez MJ, Guaza C, Garrido JJ. Cannabinoid Receptors Modulate Neuronal Morphology and AnkyrinG Density at the Axon Initial Segment. Front Cell Neurosci 2017; 11:5. [PMID: 28179879 PMCID: PMC5263140 DOI: 10.3389/fncel.2017.00005] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2016] [Accepted: 01/10/2017] [Indexed: 12/27/2022] Open
Abstract
Neuronal polarization underlies the ability of neurons to integrate and transmit information. This process begins early in development with axon outgrowth, followed by dendritic growth and subsequent maturation. In between these two steps, the axon initial segment (AIS), a subcellular domain crucial for generating action potentials (APs) and maintaining the morphological and functional polarization, starts to develop. However, the cellular/molecular mechanisms and receptors involved in AIS initial development and maturation are mostly unknown. In this study, we have focused on the role of the type-1 cannabinoid receptor (CB1R), a highly abundant G-protein coupled receptor (GPCR) in the nervous system largely involved in different phases of neuronal development and differentiation. Although CB1R activity modulation has been related to changes in axons or dendrites, its possible role as a modulator of AIS development has not been yet explored. Here we analyzed the potential role of CB1R on neuronal morphology and AIS development using pharmacological and RNA interference approaches in cultured hippocampal neurons. CB1R inhibition, at a very early developmental stage, has no effect on axonal growth, yet CB1R activation can promote it. By contrast, subsequent dendritic growth is impaired by CB1R inhibition, which also reduces ankyrinG density at the AIS. Moreover, our data show a significant correlation between early dendritic growth and ankyrinG density. However, CB1R inhibition in later developmental stages after dendrites are formed only reduces ankyrinG accumulation at the AIS. In conclusion, our data suggest that neuronal CB1R basal activity plays a role in initial development of dendrites and indirectly in AIS proteins accumulation. Based on the lack of CB1R expression at the AIS, we hypothesize that CB1R mediated modulation of dendritic arbor size during early development indirectly determines the accumulation of ankyrinG and AIS development. Further studies will be necessary to determine which CB1R-dependent mechanisms can coordinate these two domains, and what may be the impact of these early developmental changes once neurons mature and are embedded in a functional brain network.
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Affiliation(s)
- Mónica Tapia
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC) Madrid, Spain
| | - Ana Dominguez
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC) Madrid, Spain
| | - Wei Zhang
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC) Madrid, Spain
| | - Ana Del Puerto
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC) Madrid, Spain
| | - María Ciorraga
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC) Madrid, Spain
| | - María José Benitez
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC)Madrid, Spain; Department of Quimica Fisica Aplicada, Universidad Autónoma de MadridMadrid, Spain
| | - Carmen Guaza
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC) Madrid, Spain
| | - Juan José Garrido
- Instituto Cajal, Consejo Superior de Investigaciones Científicas (CSIC) Madrid, Spain
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Crofton EJ, Nenov MN, Zhang Y, Scala F, Page SA, McCue DL, Li D, Hommel JD, Laezza F, Green TA. Glycogen synthase kinase 3 beta alters anxiety-, depression-, and addiction-related behaviors and neuronal activity in the nucleus accumbens shell. Neuropharmacology 2017; 117:49-60. [PMID: 28126496 DOI: 10.1016/j.neuropharm.2017.01.020] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2015] [Revised: 01/15/2017] [Accepted: 01/22/2017] [Indexed: 11/24/2022]
Abstract
Psychiatric disorders such as anxiety, depression and addiction are often comorbid brain pathologies thought to share common mechanistic biology. As part of the cortico-limbic circuit, the nucleus accumbens shell (NAcSh) plays a fundamental role in integrating information in the circuit, such that modulation of NAcSh circuitry alters anxiety, depression, and addiction-related behaviors. Intracellular kinase cascades in the NAcSh have proven important mediators of behavior. To investigate glycogen-synthase kinase 3 (GSK3) beta signaling in the NAcSh in vivo we knocked down GSK3beta expression with a novel adeno-associated viral vector (AAV2) and assessed changes in anxiety- and depression-like behavior and cocaine self-administration in GSK3beta knockdown rats. GSK3beta knockdown reduced anxiety-like behavior while increasing depression-like behavior and cocaine self-administration. Correlative electrophysiological recordings in acute brain slices were used to assess the effect of AAV-shGSK3beta on spontaneous firing and intrinsic excitability of tonically active interneurons (TANs), cells required for input and output signal integration in the NAcSh and for processing reward-related behaviors. Loose-patch recordings showed that TANs transduced by AAV-shGSK3beta exhibited reduction in tonic firing and increased spike half width. When assessed by whole-cell patch clamp recordings these changes were mirrored by reduction in action potential firing and accompanied by decreased hyperpolarization-induced depolarizing sag potentials, increased action potential current threshold, and decreased maximum rise time. These results suggest that silencing of GSK3beta in the NAcSh increases depression- and addiction-related behavior, possibly by decreasing intrinsic excitability of TANs. However, this study does not rule out contributions from other neuronal sub-types.
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Affiliation(s)
- Elizabeth J Crofton
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - Miroslav N Nenov
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - Yafang Zhang
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - Federico Scala
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA; Biophysics Graduate Program, Institute of Human Physiology, Universita Cattolica, Rome, Italy
| | - Sean A Page
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - David L McCue
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - Dingge Li
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - Jonathan D Hommel
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - Fernanda Laezza
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA
| | - Thomas A Green
- Center for Addiction Research, The University of Texas Medical Branch, Galveston, TX, USA; Mitchell Center for Neurodegenerative Diseases, The University of Texas Medical Branch, Galveston, TX, USA; Department of Pharmacology and Toxicology, The University of Texas Medical Branch, Galveston, TX, USA.
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60
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Felix RA, Elde CJ, Nevue AA, Portfors CV. Serotonin modulates response properties of neurons in the dorsal cochlear nucleus of the mouse. Hear Res 2016; 344:13-23. [PMID: 27838373 DOI: 10.1016/j.heares.2016.10.017] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/08/2016] [Revised: 10/10/2016] [Accepted: 10/26/2016] [Indexed: 01/15/2023]
Abstract
The neurochemical serotonin (5-hydroxytryptamine, 5-HT) is involved in a variety of behavioral functions including arousal, reward, and attention, and has a role in several complex disorders of the brain. In the auditory system, 5-HT fibers innervate a number of subcortical nuclei, yet the modulatory role of 5-HT in nearly all of these areas remains poorly understood. In this study, we examined spiking activity of neurons in the dorsal cochlear nucleus (DCN) following iontophoretic application of 5-HT. The DCN is an early site in the auditory pathway that receives dense 5-HT fiber input from the raphe nuclei and has been implicated in the generation of auditory disorders marked by neuronal hyperexcitability. Recordings from the DCN in awake mice demonstrated that iontophoretic application of 5-HT had heterogeneous effects on spiking rate, spike timing, and evoked spiking threshold. We found that 56% of neurons exhibited increases in spiking rate during 5-HT delivery, while 22% had decreases in rate and the remaining neurons had no change. These changes were similar for spontaneous and evoked spiking and were typically accompanied by changes in spike timing. Spiking increases were associated with lower first spike latencies and jitter, while decreases in spiking generally had opposing effects on spike timing. Cases in which 5-HT application resulted in increased spiking also exhibited lower thresholds compared to the control condition, while cases of decreased spiking had no threshold change. We also found that the 5-HT2 receptor subtype likely has a role in mediating increased excitability. Our results demonstrate that 5-HT can modulate activity in the DCN of awake animals and that it primarily acts to increase neuronal excitability, in contrast to other auditory regions where it largely has a suppressive role. Modulation of DCN function by 5-HT has implications for auditory processing in both normal hearing and disordered states.
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Affiliation(s)
- Richard A Felix
- School of Biological Sciences and Integrative Physiology and Neuroscience, Washington State University, Vancouver, WA, USA.
| | - Cameron J Elde
- School of Biological Sciences and Integrative Physiology and Neuroscience, Washington State University, Vancouver, WA, USA
| | - Alexander A Nevue
- School of Biological Sciences and Integrative Physiology and Neuroscience, Washington State University, Vancouver, WA, USA
| | - Christine V Portfors
- School of Biological Sciences and Integrative Physiology and Neuroscience, Washington State University, Vancouver, WA, USA
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Yamada R, Kuba H. Structural and Functional Plasticity at the Axon Initial Segment. Front Cell Neurosci 2016; 10:250. [PMID: 27826229 PMCID: PMC5078684 DOI: 10.3389/fncel.2016.00250] [Citation(s) in RCA: 86] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2016] [Accepted: 10/12/2016] [Indexed: 11/13/2022] Open
Abstract
The axon initial segment (AIS) is positioned between the axonal and somato-dendritic compartments and plays a pivotal role in triggering action potentials (APs) and determining neuronal output. It is now widely accepted that structural properties of the AIS, such as length and/or location relative to the soma, change in an activity-dependent manner. This structural plasticity of the AIS is known to be crucial for homeostatic control of neuronal excitability. However, it is obvious that the impact of the AIS on neuronal excitability is critically dependent on the biophysical properties of the AIS, which are primarily determined by the composition and characteristics of ion channels in this domain. Moreover, these properties can be altered via phosphorylation and/or redistribution of the channels. Recently, studies in auditory neurons showed that alterations in the composition of voltage-gated K+ (Kv) channels at the AIS coincide with elongation of the AIS, thereby enhancing the neuronal excitability, suggesting that the interaction between structural and functional plasticities of the AIS is important in the control of neuronal excitability. In this review, we will summarize the current knowledge regarding structural and functional alterations of the AIS and discuss how they interact and contribute to regulating the neuronal output.
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Affiliation(s)
- Rei Yamada
- Department of Cell Physiology, Graduate School of Medicine, Nagoya University Nagoya, Japan
| | - Hiroshi Kuba
- Department of Cell Physiology, Graduate School of Medicine, Nagoya University Nagoya, Japan
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Griggs RB, Yermakov LM, Susuki K. Formation and disruption of functional domains in myelinated CNS axons. Neurosci Res 2016; 116:77-87. [PMID: 27717670 DOI: 10.1016/j.neures.2016.09.010] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2016] [Revised: 09/19/2016] [Accepted: 09/23/2016] [Indexed: 12/15/2022]
Abstract
Communication in the central nervous system (CNS) occurs through initiation and propagation of action potentials at excitable domains along axons. Action potentials generated at the axon initial segment (AIS) are regenerated at nodes of Ranvier through the process of saltatory conduction. Proper formation and maintenance of the molecular structure at the AIS and nodes are required for sustaining conduction fidelity. In myelinated CNS axons, paranodal junctions between the axolemma and myelinating oligodendrocytes delineate nodes of Ranvier and regulate the distribution and localization of specialized functional elements, such as voltage-gated sodium channels and mitochondria. Disruption of excitable domains and altered distribution of functional elements in CNS axons is associated with demyelinating diseases such as multiple sclerosis, and is likely a mechanism common to other neurological disorders. This review will provide a brief overview of the molecular structure of the AIS and nodes of Ranvier, as well as the distribution of mitochondria in myelinated axons. In addition, this review highlights important structural and functional changes within myelinated CNS axons that are associated with neurological dysfunction.
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Affiliation(s)
- Ryan B Griggs
- Department of Neuroscience, Cell Biology, and Physiology, Boonshoft School of Medicine, Wright State University, Dayton, OH, United States
| | - Leonid M Yermakov
- Department of Neuroscience, Cell Biology, and Physiology, Boonshoft School of Medicine, Wright State University, Dayton, OH, United States
| | - Keiichiro Susuki
- Department of Neuroscience, Cell Biology, and Physiology, Boonshoft School of Medicine, Wright State University, Dayton, OH, United States.
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Boychuk JA, Farrell JS, Palmer LA, Singleton AC, Pittman QJ, Teskey GC. HCN channels segregate stimulation-evoked movement responses in neocortex and allow for coordinated forelimb movements in rodents. J Physiol 2016; 595:247-263. [PMID: 27568501 DOI: 10.1113/jp273068] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2016] [Accepted: 08/17/2016] [Indexed: 11/08/2022] Open
Abstract
KEY POINTS The present study tested whether HCN channels contribute to the organization of motor cortex and to skilled motor behaviour during a forelimb reaching task. Experimental reductions in HCN channel signalling increase the representation of complex multiple forelimb movements in motor cortex as assessed by intracortical microstimulation. Global HCN1KO mice exhibit reduced reaching accuracy and atypical movements during a single-pellet reaching task relative to wild-type controls. Acute pharmacological inhibition of HCN channels in forelimb motor cortex decreases reaching accuracy and increases atypical movements during forelimb reaching. ABSTRACT The mechanisms by which distinct movements of a forelimb are generated from the same area of motor cortex have remained elusive. Here we examined a role for HCN channels, given their ability to alter synaptic integration, in the expression of forelimb movement responses during intracortical microstimulation (ICMS) and movements of the forelimb on a skilled reaching task. We used short-duration high-resolution ICMS to evoke forelimb movements following pharmacological (ZD7288), experimental (electrically induced cortical seizures) or genetic approaches that we confirmed with whole-cell patch clamp to substantially reduce Ih current. We observed significant increases in the number of multiple movement responses evoked at single sites in motor maps to all three experimental manipulations in rats or mice. Global HCN1 knockout mice were less successful and exhibited atypical movements on a skilled-motor learning task relative to wild-type controls. Furthermore, in reaching-proficient rats, reaching accuracy was reduced and forelimb movements were altered during infusion of ZD7288 within motor cortex. Thus, HCN channels play a critical role in the separation of overlapping movement responses and allow for successful reaching behaviours. These data provide a novel mechanism for the encoding of multiple movement responses within shared networks of motor cortex. This mechanism supports a viewpoint of primary motor cortex as a site of dynamic integration for behavioural output.
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Affiliation(s)
- Jeffery A Boychuk
- Department of Physiology, University of Kentucky, Lexington, Kentucky, USA.,Epilepsy Center, University of Kentucky, Lexington, Kentucky, USA.,Department of Cell Biology and Anatomy, University of Calgary, Calgary, Alberta, Canada.,Department of Neuroscience, University of Calgary, Calgary, Alberta, Canada.,Department of Physiology and Pharmacology, University of Calgary, Calgary, Alberta, Canada
| | - Jordan S Farrell
- Department of Cell Biology and Anatomy, University of Calgary, Calgary, Alberta, Canada.,Department of Neuroscience, University of Calgary, Calgary, Alberta, Canada
| | - Laura A Palmer
- Department of Cell Biology and Anatomy, University of Calgary, Calgary, Alberta, Canada.,Department of Neuroscience, University of Calgary, Calgary, Alberta, Canada
| | - Anna C Singleton
- Department of Neuroscience, University of Calgary, Calgary, Alberta, Canada
| | - Quentin J Pittman
- Department of Neuroscience, University of Calgary, Calgary, Alberta, Canada.,Department of Physiology and Pharmacology, University of Calgary, Calgary, Alberta, Canada.,Hotchkiss Brain Institute, Calgary, Alberta, Canada
| | - G Campbell Teskey
- Department of Cell Biology and Anatomy, University of Calgary, Calgary, Alberta, Canada.,Department of Neuroscience, University of Calgary, Calgary, Alberta, Canada.,Department of Physiology and Pharmacology, University of Calgary, Calgary, Alberta, Canada.,Hotchkiss Brain Institute, Calgary, Alberta, Canada
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64
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How Azobenzene Photoswitches Restore Visual Responses to the Blind Retina. Neuron 2016; 92:100-113. [PMID: 27667006 DOI: 10.1016/j.neuron.2016.08.038] [Citation(s) in RCA: 52] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2015] [Revised: 07/02/2016] [Accepted: 08/25/2016] [Indexed: 01/13/2023]
Abstract
Azobenzene photoswitches confer light sensitivity onto retinal ganglion cells (RGCs) in blind mice, making these compounds promising candidates as vision-restoring drugs in humans with degenerative blindness. Remarkably, photosensitization manifests only in animals with photoreceptor degeneration and is absent from those with intact rods and cones. Here we show that P2X receptors mediate the entry of photoswitches into RGCs, where they associate with voltage-gated ion channels, enabling light to control action-potential firing. All charged photoswitch compounds require permeation through P2X receptors, whose gene expression is upregulated in the blind retina. Photoswitches and membrane-impermeant fluorescent dyes likewise penetrate through P2X receptors to label a subset of RGCs in the degenerated retina. Electrophysiological recordings and mapping of fluorescently labeled RGC dendritic projections together indicate that photosensitization is highly selective for OFF-RGCs. Hence, P2X receptors are a natural conduit allowing cell-type-selective and degeneration-specific delivery of photoswitches to restore visual function in blinding disease.
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65
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Nevue AA, Felix RA, Portfors CV. Dopaminergic projections of the subparafascicular thalamic nucleus to the auditory brainstem. Hear Res 2016; 341:202-209. [PMID: 27620513 DOI: 10.1016/j.heares.2016.09.001] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/18/2016] [Revised: 09/01/2016] [Accepted: 09/07/2016] [Indexed: 11/18/2022]
Abstract
Neuromodulators can alter the response properties of sensory neurons, including those in the auditory system. Dopamine, which plays a major role in reward and movement, has been shown to alter neural responses in the auditory brainstem and midbrain. Recently we identified the subparafascicular thalamic nucleus (SPF), part of the A11 dopaminergic cell group, as the source of dopamine to the inferior colliculus (IC). The superior olivary complex (SOC) is also a likely target of dopaminergic projections from the SPF because it receives projections from the SPF and contains fibers and terminals immunoreactive for tyrosine hydroxylase, the rate limiting enzyme in dopamine synthesis. However, it is unknown if the projections from the SPF to SOC are dopaminergic, and if single neurons in the SPF project to both the IC and SOC. Using anterograde tracing combined with fluorescent immunohistochemistry, we found that the SPF sends dopaminergic projections to the superior paraolivary nucleus and the medial nucleus of the trapezoid body, but not the lateral superior olive. We confirmed these projections using a retrograde tracer. By making dual retrograde deposits in the IC and SOC, we found that individual dopaminergic cells innervate both the IC and SOC. These results suggest dopaminergic innervation, likely released in a context dependent manner, occurs at multiple levels of the auditory pathway.
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Affiliation(s)
- Alexander A Nevue
- School of Biological Sciences, Washington State University Vancouver, 14204 NE Salmon Creek Ave, Vancouver, WA 98686, USA.
| | - Richard A Felix
- Integrative Physiology and Neuroscience, Washington State University Vancouver, 14204 NE Salmon Creek Ave, Vancouver, WA 98686, USA.
| | - Christine V Portfors
- School of Biological Sciences, Washington State University Vancouver, 14204 NE Salmon Creek Ave, Vancouver, WA 98686, USA.
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66
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Goyer D, Kurth S, Gillet C, Keine C, Rübsamen R, Kuenzel T. Slow Cholinergic Modulation of Spike Probability in Ultra-Fast Time-Coding Sensory Neurons. eNeuro 2016; 3:ENEURO.0186-16.2016. [PMID: 27699207 PMCID: PMC5035776 DOI: 10.1523/eneuro.0186-16.2016] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2016] [Revised: 07/22/2016] [Accepted: 07/23/2016] [Indexed: 11/23/2022] Open
Abstract
Sensory processing in the lower auditory pathway is generally considered to be rigid and thus less subject to modulation than central processing. However, in addition to the powerful bottom-up excitation by auditory nerve fibers, the ventral cochlear nucleus also receives efferent cholinergic innervation from both auditory and nonauditory top-down sources. We thus tested the influence of cholinergic modulation on highly precise time-coding neurons in the cochlear nucleus of the Mongolian gerbil. By combining electrophysiological recordings with pharmacological application in vitro and in vivo, we found 55-72% of spherical bushy cells (SBCs) to be depolarized by carbachol on two time scales, ranging from hundreds of milliseconds to minutes. These effects were mediated by nicotinic and muscarinic acetylcholine receptors, respectively. Pharmacological block of muscarinic receptors hyperpolarized the resting membrane potential, suggesting a novel mechanism of setting the resting membrane potential for SBC. The cholinergic depolarization led to an increase of spike probability in SBCs without compromising the temporal precision of the SBC output in vitro. In vivo, iontophoretic application of carbachol resulted in an increase in spontaneous SBC activity. The inclusion of cholinergic modulation in an SBC model predicted an expansion of the dynamic range of sound responses and increased temporal acuity. Our results thus suggest of a top-down modulatory system mediated by acetylcholine which influences temporally precise information processing in the lower auditory pathway.
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Affiliation(s)
- David Goyer
- Institute for Biology II, Department of Zoology/Animal Physiology, RWTH Aachen University, D-52074 Aachen, Germany
| | - Stefanie Kurth
- Institute for Biology II, Department of Zoology/Animal Physiology, RWTH Aachen University, D-52074 Aachen, Germany
| | - Charlène Gillet
- Institute for Biology II, Department of Zoology/Animal Physiology, RWTH Aachen University, D-52074 Aachen, Germany
| | - Christian Keine
- Institute of Biology, Faculty of Biosciences, Pharmacy and Psychology, University of Leipzig, D-04103 Leipzig, Germany
| | - Rudolf Rübsamen
- Institute of Biology, Faculty of Biosciences, Pharmacy and Psychology, University of Leipzig, D-04103 Leipzig, Germany
| | - Thomas Kuenzel
- Institute for Biology II, Department of Zoology/Animal Physiology, RWTH Aachen University, D-52074 Aachen, Germany
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