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Mohamed NMM, Meredith FL, Rennie KJ. Inhibition of Ionic Currents by Fluoxetine in Vestibular Calyces in Different Epithelial Loci. Int J Mol Sci 2024; 25:8801. [PMID: 39201487 PMCID: PMC11354711 DOI: 10.3390/ijms25168801] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2024] [Revised: 08/05/2024] [Accepted: 08/07/2024] [Indexed: 09/02/2024] Open
Abstract
Previous studies have suggested a role for selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac®) in the treatment of dizziness and inner ear vestibular dysfunction. The potential mechanism of action within the vestibular system remains unclear; however, fluoxetine has been reported to block certain types of K+ channel in other systems. Here, we investigated the direct actions of fluoxetine on membrane currents in presynaptic hair cells and postsynaptic calyx afferents of the gerbil peripheral vestibular system using whole cell patch clamp recordings in crista slices. We explored differences in K+ currents in peripheral zone (PZ) and central zone (CZ) calyces of the crista and their response to fluoxetine application. Outward K+ currents in PZ calyces showed greater inactivation at depolarized membrane potentials compared to CZ calyces. The application of 100 μM fluoxetine notably reduced K+ currents in calyx terminals within both zones of the crista, and the remaining currents exhibited distinct traits. In PZ cells, fluoxetine inhibited a non-inactivating K+ current and revealed a rapidly activating and inactivating K+ current, which was sensitive to blocking by 4-aminopyridine. This was in contrast to CZ calyces, where low-voltage-activated and non-inactivating K+ currents persisted following application of 100 μM fluoxetine. Additionally, marked inhibition of transient inward Na+ currents by fluoxetine was observed in calyces from both crista zones. Different concentrations of fluoxetine were tested, and the EC50 values were found to be 40 µM and 32 µM for K+ and Na+ currents, respectively. In contrast, 100 μM fluoxetine had no impact on voltage-dependent K+ currents in mechanosensory type I and type II vestibular hair cells. In summary, micromolar concentrations of fluoxetine are expected to strongly reduce both Na+ and K+ conductance in afferent neurons of the peripheral vestibular system in vivo. This would lead to inhibition of action potential firing in vestibular sensory neurons and has therapeutic implications for disorders of balance.
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Affiliation(s)
| | | | - Katherine J. Rennie
- Department of Otolaryngology-Head & Neck Surgery, University of Colorado School of Medicine, Aurora, CO 80045, USA; (N.M.M.M.); (F.L.M.)
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Meredith FL, Vu TA, Gehrke B, Benke TA, Dondzillo A, Rennie KJ. Expression of hyperpolarization-activated current ( Ih) in zonally defined vestibular calyx terminals of the crista. J Neurophysiol 2023; 129:1468-1481. [PMID: 37198134 PMCID: PMC10259860 DOI: 10.1152/jn.00135.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2023] [Revised: 05/11/2023] [Accepted: 05/15/2023] [Indexed: 05/19/2023] Open
Abstract
Calyx terminals make afferent synapses with type I hair cells in vestibular epithelia and express diverse ionic conductances that influence action potential generation and discharge regularity in vestibular afferent neurons. Here we investigated the expression of hyperpolarization-activated current (Ih) in calyx terminals in central and peripheral zones of mature gerbil crista slices, using whole cell patch-clamp recordings. Slowly activating Ih was present in >80% calyces tested in both zones. Peak Ih and half-activation voltages were not significantly different; however, Ih activated with a faster time course in peripheral compared with central zone calyces. Calyx Ih in both zones was blocked by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD7288; 100 µM), and the resting membrane potential became more hyperpolarized. In the presence of dibutyryl-cAMP (dB-cAMP), peak Ih was increased, activation kinetics became faster, and the voltage of half-activation was more depolarized compared with control calyces. In current clamp, calyces from both zones showed three different categories of firing: spontaneous firing, phasic firing where a single action potential was evoked after a hyperpolarizing pulse, or a single evoked action potential followed by membrane potential oscillations. In the absence of Ih, the latency to peak of the action potential increased; Ih produces a small depolarizing current that facilitates firing by driving the membrane potential closer to threshold. Immunostaining showed the expression of HCN2 subunits in calyx terminals. We conclude that Ih is found in calyx terminals across the crista and could influence conventional and novel forms of synaptic transmission at the type I hair cell-calyx synapse.NEW & NOTEWORTHY Calyx afferent terminals make synapses with vestibular hair cells and express diverse conductances that impact action potential firing in vestibular primary afferents. Conventional and nonconventional synaptic transmission modes are influenced by hyperpolarization-activated current (Ih), but regional differences were previously unexplored. We show that Ih is present in both central and peripheral calyces of the mammalian crista. Ih produces a small depolarizing resting current that facilitates firing by driving the membrane potential closer to threshold.
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Affiliation(s)
- Frances L Meredith
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado, United States
| | - Tiffany A Vu
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado, United States
| | - Brandon Gehrke
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado, United States
| | - Timothy A Benke
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado, United States
- Department of Pediatrics, University of Colorado School of Medicine, Aurora, Colorado, United States
- Department of Pharmacology, University of Colorado School of Medicine, Aurora, Colorado, United States
- Department of Neurology, University of Colorado School of Medicine, Aurora, Colorado, United States
| | - Anna Dondzillo
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado, United States
| | - Katherine J Rennie
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado, United States
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Meredith FL, Rennie KJ. Dopaminergic Inhibition of Na + Currents in Vestibular Inner Ear Afferents. Front Neurosci 2021; 15:710321. [PMID: 34580582 PMCID: PMC8463658 DOI: 10.3389/fnins.2021.710321] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2021] [Accepted: 08/10/2021] [Indexed: 11/13/2022] Open
Abstract
Inner ear hair cells form synapses with afferent terminals and afferent neurons carry signals as action potentials to the central nervous system. Efferent neurons have their origins in the brainstem and some make synaptic contact with afferent dendrites beneath hair cells. Several neurotransmitters have been identified that may be released from efferent terminals to modulate afferent activity. Dopamine is a candidate efferent neurotransmitter in both the vestibular and auditory systems. Within the cochlea, activation of dopamine receptors may reduce excitotoxicity at the inner hair cell synapse via a direct effect of dopamine on afferent terminals. Here we investigated the effect of dopamine on sodium currents in acutely dissociated vestibular afferent calyces to determine if dopaminergic signaling could also modulate vestibular responses. Calyx terminals were isolated along with their accompanying type I hair cells from the cristae of gerbils (P15-33) and whole cell patch clamp recordings performed. Large transient sodium currents were present in all isolated calyces; compared to data from crista slices, resurgent Na+ currents were rare. Perfusion of dopamine (100 μM) in the extracellular solution significantly reduced peak transient Na+ currents by approximately 20% of control. A decrease in Na+ current amplitude was also seen with extracellular application of the D2 dopamine receptor agonist quinpirole, whereas the D2 receptor antagonist eticlopride largely abolished the response to dopamine. Inclusion of the phosphatase inhibitor okadaic acid in the patch electrode solution occluded the response to dopamine. The reduction in calyx sodium current in response to dopamine suggests efferent signaling through D2 dopaminergic receptors may occur via common mechanisms to decrease excitability in inner ear afferents.
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Affiliation(s)
- Frances L Meredith
- Department of Otolaryngology - Head & Neck Surgery, School of Medicine, University of Colorado, Aurora, CO, United States
| | - Katherine J Rennie
- Department of Otolaryngology - Head & Neck Surgery, School of Medicine, University of Colorado, Aurora, CO, United States.,Department of Physiology & Biophysics, School of Medicine, University of Colorado, Aurora, CO, United States
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Meredith FL, Rennie KJ. Persistent and resurgent Na + currents in vestibular calyx afferents. J Neurophysiol 2020; 124:510-524. [PMID: 32667253 DOI: 10.1152/jn.00124.2020] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Vestibular afferent neurons convey information from hair cells in the peripheral vestibular end organs to central nuclei. Primary vestibular afferent neurons can fire action potentials at high rates and afferent firing patterns vary with the position of nerve terminal endings in vestibular neuroepithelia. Terminals contact hair cells as small bouton or large calyx endings. To investigate the role of Na+ currents (INa) in firing mechanisms, we investigated biophysical properties of INa in calyx-bearing afferents. Whole cell patch-clamp recordings were made from calyx terminals in thin slices of gerbil crista at different postnatal ages: immature [postnatal day (P)5-P8, young (P13-P15), and mature (P30-P45)]. A large transient Na+ current (INaT) was completely blocked by 300 nM tetrodotoxin (TTX) in mature calyces. In addition, INaT was accompanied by much smaller persistent Na+ currents (INaP) and distinctive resurgent Na+ currents (INaR), which were also blocked by TTX. ATX-II, a toxin that slows Na+ channel inactivation, enhanced INaP in immature and mature calyces. 4,9-Anhydro-TTX (4,9-ah-TTX), which selectively blocks Nav1.6 channels, abolished the enhanced INa in mature, but not immature, calyces. Therefore, Nav1.6 channels mediate a component of INaT and INaP in mature calyces, but are minimally expressed at early postnatal days. INaR was expressed in less than one-third of calyces at P6-P8, but expression increased with development, and in mature cristae INaR was frequently found in peripheral calyces. INaR served to increase the availability of Na+ channels following brief membrane depolarizations. In current clamp, the rate and regularity of action potential firing decreased in mature peripheral calyces following 4,9-ah-TTX application. Therefore, Nav1.6 channels are upregulated during development, contribute to INaT, INaP, and INaR, and may regulate excitability by enabling higher mean discharge rates in a subpopulation of mature calyx afferents.NEW & NOTEWORTHY Action potential firing patterns differ between groups of afferent neurons innervating vestibular epithelia. We investigated the biophysical properties of Na+ currents in specialized vestibular calyx afferent terminals during postnatal development. Mature calyces express Na+ currents with transient, persistent, and resurgent components. Nav1.6 channels contribute to resurgent Na+ currents and may enhance firing in peripheral calyx afferents. Understanding Na+ channels that contribute to vestibular nerve responses has implications for developing new treatments for vestibular dysfunction.
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Affiliation(s)
- Frances L Meredith
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado
| | - Katherine J Rennie
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado.,Department of Physiology & Biophysics, University of Colorado School of Medicine, Aurora, Colorado
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Spaiardi P, Tavazzani E, Manca M, Russo G, Prigioni I, Biella G, Giunta R, Johnson SL, Marcotti W, Masetto S. K + Accumulation and Clearance in the Calyx Synaptic Cleft of Type I Mouse Vestibular Hair Cells. Neuroscience 2020; 426:69-86. [PMID: 31846752 PMCID: PMC6985899 DOI: 10.1016/j.neuroscience.2019.11.028] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2019] [Revised: 11/07/2019] [Accepted: 11/16/2019] [Indexed: 11/29/2022]
Abstract
Vestibular organs of Amniotes contain two types of sensory cells, named Type I and Type II hair cells. While Type II hair cells are contacted by several small bouton nerve terminals, Type I hair cells receive a giant terminal, called a calyx, which encloses their basolateral membrane almost completely. Both hair cell types release glutamate, which depolarizes the afferent terminal by binding to AMPA post-synaptic receptors. However, there is evidence that non-vesicular signal transmission also occurs at the Type I hair cell-calyx synapse, possibly involving direct depolarization of the calyx by K+ exiting the hair cell. To better investigate this aspect, we performed whole-cell patch-clamp recordings from mouse Type I hair cells or their associated calyx. We found that [K+] in the calyceal synaptic cleft is elevated at rest relative to the interstitial (extracellular) solution and can increase or decrease during hair cell depolarization or repolarization, respectively. The change in [K+] was primarily driven by GK,L, the low-voltage-activated, non-inactivating K+ conductance specifically expressed by Type I hair cells. Simple diffusion of K+ between the cleft and the extracellular compartment appeared substantially restricted by the calyx inner membrane, with the ion channels and active transporters playing a crucial role in regulating intercellular [K+]. Calyx recordings were consistent with K+ leaving the synaptic cleft through postsynaptic voltage-gated K+ channels involving KV1 and KV7 subunits. The above scenario is consistent with direct depolarization and hyperpolarization of the calyx membrane potential by intercellular K+.
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Affiliation(s)
- P Spaiardi
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia 27100, Italy
| | - E Tavazzani
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia 27100, Italy
| | - M Manca
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia 27100, Italy
| | - G Russo
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia 27100, Italy
| | - I Prigioni
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia 27100, Italy
| | - G Biella
- Department of Biology and Biotechnology, University of Pavia, Pavia 27100, Italy
| | - R Giunta
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia 27100, Italy
| | - S L Johnson
- Department of Biomedical Science, University of Sheffield, Sheffield S10 2TN, UK
| | - W Marcotti
- Department of Biomedical Science, University of Sheffield, Sheffield S10 2TN, UK
| | - S Masetto
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia 27100, Italy.
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Meredith FL, Rennie KJ. Regional and Developmental Differences in Na + Currents in Vestibular Primary Afferent Neurons. Front Cell Neurosci 2018; 12:423. [PMID: 30487736 PMCID: PMC6246661 DOI: 10.3389/fncel.2018.00423] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2018] [Accepted: 10/29/2018] [Indexed: 02/04/2023] Open
Abstract
The vestibular system relays information about head position via afferent nerve fibers to the brain in the form of action potentials. Voltage-gated Na+ channels in vestibular afferents drive the initiation and propagation of action potentials, but their expression during postnatal development and their contributions to firing in diverse mature afferent populations are unknown. Electrophysiological techniques were used to determine Na+ channel subunit types in vestibular calyx-bearing afferents at different stages of postnatal development. We used whole cell patch clamp recordings in thin slices of gerbil crista neuroepithelium to investigate Na+ channels and firing patterns in central zone (CZ) and peripheral zone (PZ) afferents. PZ afferents are exclusively dimorphic, innervating type I and type II hair cells, whereas CZ afferents can form dimorphs or calyx-only terminals which innervate type I hair cells alone. All afferents expressed tetrodotoxin (TTX)-sensitive Na+ currents, but TTX-sensitivity varied with age. During the fourth postnatal week, 200–300 nM TTX completely blocked sodium currents in PZ and CZ calyces. By contrast, in immature calyces [postnatal day (P) 5–11], a small component of peak sodium current remained in 200 nM TTX. Application of 1 μM TTX, or Jingzhaotoxin-III plus 200 nM TTX, abolished sodium current in immature calyces, suggesting the transient expression of voltage-gated sodium channel 1.5 (Nav1.5) during development. A similar TTX-insensitive current was found in early postnatal crista hair cells (P5–9) and constituted approximately one third of the total sodium current. The Nav1.6 channel blocker, 4,9-anhydrotetrodotoxin, reduced a component of sodium current in immature and mature calyces. At 100 nM 4,9-anhydrotetrodotoxin, peak sodium current was reduced on average by 20% in P5–14 calyces, by 37% in mature dimorphic PZ calyces, but by less than 15% in mature CZ calyx-only terminals. In mature PZ calyces, action potentials became shorter and broader in the presence of 4,9-anhydrotetrodotoxin implicating a role for Nav1.6 channels in firing in dimorphic afferents.
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Affiliation(s)
- Frances L Meredith
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, United States
| | - Katherine J Rennie
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, United States.,Department of Physiology & Biophysics, University of Colorado School of Medicine, Aurora, CO, United States
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Spaiardi P, Tavazzani E, Manca M, Milesi V, Russo G, Prigioni I, Marcotti W, Magistretti J, Masetto S. An allosteric gating model recapitulates the biophysical properties of I K,L expressed in mouse vestibular type I hair cells. J Physiol 2017; 595:6735-6750. [PMID: 28862328 PMCID: PMC5663832 DOI: 10.1113/jp274202] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2017] [Accepted: 08/30/2017] [Indexed: 12/11/2022] Open
Abstract
Key points Vestibular type I and type II hair cells and their afferent fibres send information to the brain regarding the position and movement of the head. The characteristic feature of type I hair cells is the expression of a low‐voltage‐activated outward rectifying K+ current, IK,L, whose biophysical properties and molecular identity are still largely unknown. In vitro, the afferent nerve calyx surrounding type I hair cells causes unstable intercellular K+ concentrations, altering the biophysical properties of IK,L. We found that in the absence of the calyx, IK,L in type I hair cells exhibited unique biophysical activation properties, which were faithfully reproduced by an allosteric channel gating scheme. These results form the basis for a molecular and pharmacological identification of IK,L.
Abstract Type I and type II hair cells are the sensory receptors of the mammalian vestibular epithelia. Type I hair cells are characterized by their basolateral membrane being enveloped in a single large afferent nerve terminal, named the calyx, and by the expression of a low‐voltage‐activated outward rectifying K+ current, IK,L. The biophysical properties and molecular profile of IK,L are still largely unknown. By using the patch‐clamp whole‐cell technique, we examined the voltage‐ and time‐dependent properties of IK,L in type I hair cells of the mouse semicircular canal. We found that the biophysical properties of IK,L were affected by an unstable K+ equilibrium potential (VeqK+). Both the outward and inward K+ currents shifted VeqK+ consistent with K+ accumulation or depletion, respectively, in the extracellular space, which we attributed to a residual calyx attached to the basolateral membrane of the hair cells. We therefore optimized the hair cell dissociation protocol in order to isolate mature type I hair cells without their calyx. In these cells, the uncontaminated IK,L showed a half‐activation at –79.6 mV and a steep voltage dependence (2.8 mV). IK,L also showed complex activation and deactivation kinetics, which we faithfully reproduced by an allosteric channel gating scheme where the channel is able to open from all (five) closed states. The ‘early’ open states substantially contribute to IK,L activation at negative voltages. This study provides the first complete description of the ‘native’ biophysical properties of IK,L in adult mouse vestibular type I hair cells. Vestibular type I and type II hair cells and their afferent fibres send information to the brain regarding the position and movement of the head. The characteristic feature of type I hair cells is the expression of a low‐voltage‐activated outward rectifying K+ current, IK,L, whose biophysical properties and molecular identity are still largely unknown. In vitro, the afferent nerve calyx surrounding type I hair cells causes unstable intercellular K+ concentrations, altering the biophysical properties of IK,L. We found that in the absence of the calyx, IK,L in type I hair cells exhibited unique biophysical activation properties, which were faithfully reproduced by an allosteric channel gating scheme. These results form the basis for a molecular and pharmacological identification of IK,L.
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Affiliation(s)
- Paolo Spaiardi
- Department of Brain and Behavioural Sciences, University of Pavia, Pavia, 27100, Italy
| | - Elisa Tavazzani
- Department of Brain and Behavioural Sciences, University of Pavia, Pavia, 27100, Italy
| | - Marco Manca
- Department of Brain and Behavioural Sciences, University of Pavia, Pavia, 27100, Italy
| | - Veronica Milesi
- Instituto de Estudios Inmunológios y Fisiopatológicos (IIFP) - CONICET, Universidad Nacional de La Plata, La Plata, 1900, Argentina
| | - Giancarlo Russo
- Department of Brain and Behavioural Sciences, University of Pavia, Pavia, 27100, Italy
| | - Ivo Prigioni
- Department of Brain and Behavioural Sciences, University of Pavia, Pavia, 27100, Italy
| | - Walter Marcotti
- Department of Biomedical Science, University of Sheffield, Sheffield, S10 2TN, UK
| | - Jacopo Magistretti
- Department of Biology and Biotechnology, University of Pavia, Pavia, 27100, Italy
| | - Sergio Masetto
- Department of Brain and Behavioural Sciences, University of Pavia, Pavia, 27100, Italy
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Kirk ME, Meredith FL, Benke TA, Rennie KJ. AMPA receptor-mediated rapid EPSCs in vestibular calyx afferents. J Neurophysiol 2017; 117:2312-2323. [PMID: 28298303 DOI: 10.1152/jn.00394.2016] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2016] [Revised: 03/10/2017] [Accepted: 03/10/2017] [Indexed: 01/21/2023] Open
Abstract
In the vestibular periphery neurotransmission between hair cells and primary afferent nerves occurs via specialized ribbon synapses. Type I vestibular hair cells (HCIs) make synaptic contacts with calyx terminals, which enclose most of the HCI basolateral surface. To probe synaptic transmission, whole cell patch-clamp recordings were made from calyx afferent terminals isolated together with their mature HCIs from gerbil crista. Neurotransmitter release was measured as excitatory postsynaptic currents (EPSCs) in voltage clamp. Spontaneous EPSCs were classified as simple or complex. Simple events exhibited a rapid rise time and a fast monoexponential decay (time constant < 1 ms). The remaining events, constituting ~40% of EPSCs, showed more complex characteristics. Extracellular Sr2+ greatly increased EPSC frequency, and EPSCs were blocked by the AMPA receptor blocker NBQX. The role of presynaptic Ca2+ channels was assessed by application of the L-type Ca2+ channel blocker nifedipine (20 µM), which reduced EPSC frequency. In contrast, the L-type Ca2+ channel opener BAY K 8644 increased EPSC frequency. Cyclothiazide increased the decay time constant of averaged simple EPSCs by approximately twofold. The low-affinity AMPA receptor antagonist γ-d-glutamylglycine (2 mM) reduced the proportion of simple EPSCs relative to complex events, indicating glutamate accumulation in the restricted cleft between HCI and calyx. In crista slices EPSC frequency was greater in central compared with peripheral calyces, which may be due to greater numbers of presynaptic ribbons in central hair cells. Our data support a role for L-type Ca2+ channels in spontaneous release and demonstrate regional variations in AMPA-mediated quantal transmission at the calyx synapse.NEW & NOTEWORTHY In vestibular calyx terminals of mature cristae we find that the majority of excitatory postsynaptic currents (EPSCs) are rapid monophasic events mediated by AMPA receptors. Spontaneous EPSCs are reduced by an L-type Ca2+ channel blocker and notably enhanced in extracellular Sr2+ EPSC frequency is greater in central areas of the crista compared with peripheral areas and may be associated with more numerous presynaptic ribbons in central hair cells.
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Affiliation(s)
- Matthew E Kirk
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado
| | - Frances L Meredith
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado
| | - Timothy A Benke
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado.,Departments of Pediatrics, Neurology, and Pharmacology, University of Colorado School of Medicine, Aurora, Colorado
| | - Katherine J Rennie
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado; .,Department of Physiology and Biophysics, University of Colorado School of Medicine, Aurora, Colorado; and
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9
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Contini D, Price SD, Art JJ. Accumulation of K + in the synaptic cleft modulates activity by influencing both vestibular hair cell and calyx afferent in the turtle. J Physiol 2016; 595:777-803. [PMID: 27633787 DOI: 10.1113/jp273060] [Citation(s) in RCA: 50] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2016] [Accepted: 09/11/2016] [Indexed: 12/16/2022] Open
Abstract
KEY POINTS In the synaptic cleft between type I hair cells and calyceal afferents, K+ ions accumulate as a function of activity, dynamically altering the driving force and permeation through ion channels facing the synaptic cleft. High-fidelity synaptic transmission is possible due to large conductances that minimize hair cell and afferent time constants in the presence of significant membrane capacitance. Elevated potassium maintains hair cells near a potential where transduction currents are sufficient to depolarize them to voltages necessary for calcium influx and synaptic vesicle fusion. Elevated potassium depolarizes the postsynaptic afferent by altering ion permeation through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and contributes to depolarizing the afferent to potentials where a single EPSP (quantum) can generate an action potential. With increased stimulation, hair cell depolarization increases the frequency of quanta released, elevates [K+ ]cleft and depolarizes the afferent to potentials at which smaller and smaller EPSPs would be sufficient to trigger APs. ABSTRACT Fast neurotransmitters act in conjunction with slower modulatory effectors that accumulate in restricted synaptic spaces found at giant synapses such as the calyceal endings in the auditory and vestibular systems. Here, we used dual patch-clamp recordings from turtle vestibular hair cells and their afferent neurons to show that potassium ions accumulating in the synaptic cleft modulated membrane potentials and extended the range of information transfer. High-fidelity synaptic transmission was possible due to large conductances that minimized hair cell and afferent time constants in the presence of significant membrane capacitance. Increased potassium concentration in the cleft maintained the hair cell near potentials that promoted the influx of calcium necessary for synaptic vesicle fusion. The elevated potassium concentration also depolarized the postsynaptic neuron by altering ion permeation through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. This depolarization enabled the afferent to reliably generate action potentials evoked by single AMPA-dependent EPSPs. Depolarization of the postsynaptic afferent could also elevate potassium in the synaptic cleft, and would depolarize other hair cells enveloped by the same neuritic process increasing the fidelity of neurotransmission at those synapses as well. Collectively, these data demonstrate that neuronal activity gives rise to potassium accumulation, and suggest that potassium ion action on HCN channels can modulate neurotransmission, preserving the fidelity of high-speed synaptic transmission by dynamically shifting the resting potentials of both presynaptic and postsynaptic cells.
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Affiliation(s)
- Donatella Contini
- Department of Anatomy and Cell Biology, College of Medicine, University of Illinois at Chicago, Chicago, IL, 60612, USA
| | - Steven D Price
- Department of Anatomy and Cell Biology, College of Medicine, University of Illinois at Chicago, Chicago, IL, 60612, USA
| | - Jonathan J Art
- Department of Anatomy and Cell Biology, College of Medicine, University of Illinois at Chicago, Chicago, IL, 60612, USA
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The response of guinea pig primary utricular and saccular irregular neurons to bone-conducted vibration (BCV) and air-conducted sound (ACS). Hear Res 2015; 331:131-43. [PMID: 26626360 DOI: 10.1016/j.heares.2015.10.019] [Citation(s) in RCA: 87] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/19/2015] [Revised: 10/23/2015] [Accepted: 10/29/2015] [Indexed: 01/11/2023]
Abstract
UNLABELLED This study sought to characterize the response of mammalian primary otolithic neurons to sound and vibration by measuring the resting discharge rates, thresholds for increases in firing rate and supra-threshold sensitivity functions of guinea pig single primary utricular and saccular afferents. Neurons with irregular resting discharge were activated in response to bone conducted vibration (BCV) and air conducted sound (ACS) for frequencies between 100 Hz and 3000 Hz. The location of neurons was verified by labelling with neurobiotin. Many afferents from both maculae have very low or zero resting discharge, with saccular afferents having on average, higher resting rates than utricular afferents. Most irregular utricular and saccular afferents can be evoked by both BCV and ACS. For BCV stimulation: utricular and saccular neurons show similar low thresholds for increased firing rate (around 0.02 g on average) for frequencies from 100 Hz to 750 Hz. There is a steep increase in rate change threshold for BCV frequencies above 750 Hz. The suprathreshold sensitivity functions for BCV were similar for both utricular and saccular neurons, with, at low frequencies, very steep increases in firing rate as intensity increased. For ACS stimulation: utricular and saccular neurons can be activated by high intensity stimuli for frequencies from 250 Hz to 3000 Hz with similar flattened U-shaped tuning curves with lowest thresholds for frequencies around 1000-2000 Hz. The average ACS thresholds for saccular afferents across these frequencies is about 15-20 dB lower than for utricular neurons. The suprathreshold sensitivity functions for ACS were similar for both utricular and saccular neurons. Both utricular and saccular afferents showed phase-locking to BCV and ACS, extending up to frequencies of at least around 1500 Hz for BCV and 3000 Hz for ACS. Phase-locking at low frequencies (e.g. 100 Hz) imposes a limit on the neural firing rate evoked by the stimulus since the neurons usually fire one spike per cycle of the stimulus. CONCLUSION These results are in accord with the hypothesis put forward by Young et al. (1977) that each individual cycle of the waveform, either BCV or ACS, is the effective stimulus to the receptor hair cells on either macula. We suggest that each cycle of the BCV or ACS stimulus causes fluid displacement which deflects the short, stiff, hair bundles of type I receptors at the striola and so triggers the phase-locked neural response of primary otolithic afferents.
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Meredith FL, Kirk ME, Rennie KJ. Kv1 channels and neural processing in vestibular calyx afferents. Front Syst Neurosci 2015; 9:85. [PMID: 26082693 PMCID: PMC4451359 DOI: 10.3389/fnsys.2015.00085] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2015] [Accepted: 05/18/2015] [Indexed: 11/13/2022] Open
Abstract
Potassium-selective ion channels are important for accurate transmission of signals from auditory and vestibular sensory end organs to their targets in the central nervous system. During different gravity conditions, astronauts experience altered input signals from the peripheral vestibular system resulting in sensorimotor dysfunction. Adaptation to altered sensory input occurs, but it is not explicitly known whether this involves synaptic modifications within the vestibular epithelia. Future investigations of such potential plasticity require a better understanding of the electrophysiological mechanisms underlying the known heterogeneity of afferent discharge under normal conditions. This study advances this understanding by examining the role of the Kv1 potassium channel family in mediating action potentials in specialized vestibular afferent calyx endings in the gerbil crista and utricle. Pharmacological agents selective for different sub-types of Kv1 channels were tested on membrane responses in whole cell recordings in the crista. Kv1 channels sensitive to α-dendrotoxin and dendrotoxin-K were found to prevail in the central regions, whereas K(+) channels sensitive to margatoxin, which blocks Kv1.3 and 1.6 channels, were more prominent in peripheral regions. Margatoxin-sensitive currents showed voltage-dependent inactivation. Dendrotoxin-sensitive currents showed no inactivation and dampened excitability in calyces in central neuroepithelial regions. The differential distribution of Kv1 potassium channels in vestibular afferents supports their importance in accurately relaying gravitational and head movement signals through specialized lines to the central nervous system. Pharmacological modulation of specific groups of K(+) channels could help alleviate vestibular dysfunction on earth and in space.
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Affiliation(s)
- Frances L Meredith
- Department of Otolaryngology, University of Colorado School of Medicine Aurora, Colorado, USA
| | - Matthew E Kirk
- Department of Otolaryngology, University of Colorado School of Medicine Aurora, Colorado, USA
| | - Katherine J Rennie
- Department of Otolaryngology, University of Colorado School of Medicine Aurora, Colorado, USA ; Department of Physiology and Biophysics, University of Colorado School of Medicine Aurora, Colorado, USA
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Meredith FL, Rennie KJ. Zonal variations in K+ currents in vestibular crista calyx terminals. J Neurophysiol 2014; 113:264-76. [PMID: 25343781 DOI: 10.1152/jn.00399.2014] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
We developed a rodent crista slice to investigate regional variations in electrophysiological properties of vestibular afferent terminals. Thin transverse slices of the gerbil crista ampullaris were made and electrical properties of calyx terminals in central zones (CZ) and peripheral zones (PZ) compared with whole cell patch clamp. Spontaneous action potential firing was observed in 25% of current-clamp recordings and was either regular or irregular in both zones. Firing was abolished when extracellular choline replaced Na(+) but persisted when hair cell mechanotransduction channels or calyx AMPA receptors were blocked. This suggests that ion channels intrinsic to the calyx can generate spontaneous firing. In response to depolarizing voltage steps, outward K(+) currents were observed at potentials above -60 mV. K(+) currents in PZ calyces showed significantly more inactivation than currents in CZ calyces. Underlying K(+) channel populations contributing to these differences were investigated. The KCNQ channel blocker XE991 dihydrochloride blocked a slowly activating, sustained outward current in both PZ and CZ calyces, indicating the presence of KCNQ channels. Mean reduction was greatest in PZ calyces. XE991 also reduced action potential firing frequency in CZ and PZ calyces and broadened mean action potential width. The K(+) channel blocker 4-aminopyridine (10-50 μM) blocked rapidly activating, moderately inactivating currents that were more prevalent in PZ calyces. α-Dendrotoxin, a selective blocker of KV1 channels, reduced outward currents in CZ calyces but not in PZ calyces. Regional variations in K(+) conductances may contribute to different firing responses in calyx afferents.
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Affiliation(s)
- Frances L Meredith
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado; and
| | - Katherine J Rennie
- Department of Otolaryngology, University of Colorado School of Medicine, Aurora, Colorado; and Department of Physiology and Biophysics, University of Colorado School of Medicine, Aurora, Colorado
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Lim R, Drury HR, Camp AJ, Tadros MA, Callister RJ, Brichta AM. Preliminary characterization of voltage-activated whole-cell currents in developing human vestibular hair cells and calyx afferent terminals. J Assoc Res Otolaryngol 2014; 15:755-66. [PMID: 24942706 PMCID: PMC4164689 DOI: 10.1007/s10162-014-0471-y] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2014] [Accepted: 05/28/2014] [Indexed: 11/28/2022] Open
Abstract
We present preliminary functional data from human vestibular hair cells and primary afferent calyx terminals during fetal development. Whole-cell recordings were obtained from hair cells or calyx terminals in semi-intact cristae prepared from human fetuses aged between 11 and 18 weeks gestation (WG). During early fetal development (11–14 WG), hair cells expressed whole-cell conductances that were qualitatively similar but quantitatively smaller than those observed previously in mature rodent type II hair cells. As development progressed (15–18 WG), peak outward conductances increased in putative type II hair cells but did not reach amplitudes observed in adult human hair cells. Type I hair cells express a specific low-voltage activating conductance, GK,L. A similar current was first observed at 15 WG but remained relatively small, even at 18 WG. The presence of a “collapsing” tail current indicates a maturing type I hair cell phenotype and suggests the presence of a surrounding calyx afferent terminal. We were also able to record from calyx afferent terminals in 15–18 WG cristae. In voltage clamp, these terminals exhibited fast inactivating inward as well as slower outward conductances, and in current clamp, discharged a single action potential during depolarizing steps. Together, these data suggest the major functional characteristics of type I and type II hair cells and calyx terminals are present by 18 WG. Our study also describes a new preparation for the functional investigation of key events that occur during maturation of human vestibular organs.
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Affiliation(s)
- Rebecca Lim
- The School of Biomedical Sciences and Pharmacy, Faculty of Health and Medicine, Hunter Medical Research Institute, The University of Newcastle, Callaghan, NSW, 2308, Australia,
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Oak MH, Yi E. Voltage-gated K(+) channels contributing to temporal precision at the inner hair cell-auditory afferent nerve fiber synapses in the mammalian cochlea. Arch Pharm Res 2014; 37:821-33. [PMID: 24925343 DOI: 10.1007/s12272-014-0411-8] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2014] [Accepted: 05/09/2014] [Indexed: 12/16/2022]
Abstract
To perform auditory tasks such as sound localization in the space, auditory neurons in the brain must distinguish sub-millisecond temporal differences in signals from two ears. Such high temporal resolution is possible when each neuron in the ascending auditory pathway fires brief action potential at very accurate timing. Various pre- and postsynaptic machineries ensuring such high temporal precision of auditory synaptic transmission have been identified. Of particular, in this review, the role of K(+) channels in shortening the duration of synaptic potentials will be discussed. First, the contribution of K(+) channels to AP firing of general auditory neurons will be discussed. Then, the focus will be moved to the inner hair cell (IHC)-auditory afferent nerve fiber (ANF) synapses, the first synapses of ascending auditory pathway. Molecular and immunohistological techniques have revealed various K(+) channels in the cell bodies and their processes of ANFs. Since the development of patch-clamp recordings from the ANF dendrites in 2002, it became possible to monitor the IHC-ANF synaptic transmission in greater detail. As revealed in brain auditory synapses, several different K(+) channels appear to participate in reducing the duration of synaptic potentials at the IHC-ANF synapses. In addition, K(+) channels at the ANF dendrites might act as potential targets of efferent feedback from the brain. The hypothesis is that, upon loud sound exposure, efferent neurotransmitters released onto the ANF dendrites activate certain K(+) channels and prevent excitotoxicity of ANFs. Therefore, K(+) channels of the ANF dendrites might provide potential sites of pharmacological actions to prevent noise-induced hearing loss.
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Affiliation(s)
- Min-Ho Oak
- College of Pharmacy and Natural Medicine Research Institute, Mokpo National University, 1666 Yeongsan-ro, Cheonggye-Myeon, Muan, Jeonnam, 534-729, Republic of Korea
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Horwitz GC, Risner-Janiczek JR, Holt JR. Mechanotransduction and hyperpolarization-activated currents contribute to spontaneous activity in mouse vestibular ganglion neurons. J Gen Physiol 2014; 143:481-97. [PMID: 24638995 PMCID: PMC3971655 DOI: 10.1085/jgp.201311126] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2013] [Accepted: 02/12/2014] [Indexed: 11/20/2022] Open
Abstract
The hyperpolarization-activated, cyclic nucleotide-sensitive current, Ih, is present in vestibular hair cells and vestibular ganglion neurons, and is required for normal balance function. We sought to identify the molecular correlates and functional relevance of Ih in vestibular ganglion neurons. Ih is carried by channels consisting of homo- or heteromeric assemblies of four protein subunits from the Hcn gene family. The relative expression of Hcn1-4 mRNA was examined using a quantitative reverse transcription PCR (RT-PCR) screen. Hcn2 was the most highly expressed subunit in vestibular neuron cell bodies. Immunolocalization of HCN2 revealed robust expression in cell bodies of all vestibular ganglion neurons. To characterize Ih in vestibular neuron cell bodies and at hair cell-afferent synapses, we developed an intact, ex vivo preparation. We found robust physiological expression of Ih in 89% of cell bodies and 100% of calyx terminals. Ih was significantly larger in calyx terminals than in cell bodies; however, other biophysical characteristics were similar. Ih was absent in calyces lacking Hcn1 and Hcn2, but small Ih was still present in cell bodies, which suggests expression of an additional subunit, perhaps Hcn4. To determine the contributions of hair cell mechanotransduction and Ih to the firing patterns of calyx terminals, we recorded action potentials in current-clamp mode. Mechanotransduction currents were modulated by hair bundle defection and application of calcium chelators to disrupt tip links. Ih activity was modulated using ZD7288 and cAMP. We found that both hair cell transduction and Ih contribute to the rate and regularity of spontaneous action potentials in the vestibular afferent neurons. We propose that modulation of Ih in vestibular ganglion neurons may provide a mechanism for modulation of spontaneous activity in the vestibular periphery.
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Affiliation(s)
- Geoffrey C. Horwitz
- Department of Otolaryngology and F.M. Kirby Neurobiology Center, Boston Children’s Hospital and Harvard Medical School, Boston, MA 02115
- Department of Neuroscience and Otolaryngology, University of Virginia School of Medicine, Charlottesville, VA 22908
| | - Jessica R. Risner-Janiczek
- Department of Neuroscience and Otolaryngology, University of Virginia School of Medicine, Charlottesville, VA 22908
| | - Jeffrey R. Holt
- Department of Otolaryngology and F.M. Kirby Neurobiology Center, Boston Children’s Hospital and Harvard Medical School, Boston, MA 02115
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Goldberg JM, Holt JC. Discharge regularity in the turtle posterior crista: comparisons between experiment and theory. J Neurophysiol 2013; 110:2830-48. [PMID: 24004525 DOI: 10.1152/jn.00195.2013] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Intra-axonal recordings were made from bouton fibers near their termination in the turtle posterior crista. Spike discharge, miniature excitatory postsynaptic potentials (mEPSPs), and afterhyperpolarizations (AHPs) were monitored during resting activity in both regularly and irregularly discharging units. Quantal size (qsize) and quantal rate (qrate) were estimated by shot-noise theory. Theoretically, the ratio, σV/(dμV/dt), between synaptic noise (σV) and the slope of the mean voltage trajectory (dμV/dt) near threshold crossing should determine discharge regularity. AHPs are deeper and more prolonged in regular units; as a result, dμV/dt is larger, the more regular the discharge. The qsize is larger and qrate smaller in irregular units; these oppositely directed trends lead to little variation in σV with discharge regularity. Of the two variables, dμV/dt is much more influential than the nearly constant σV in determining regularity. Sinusoidal canal-duct indentations at 0.3 Hz led to modulations in spike discharge and synaptic voltage. Gain, the ratio between the amplitudes of the two modulations, and phase leads re indentation of both modulations are larger in irregular units. Gain variations parallel the sensitivity of the postsynaptic spike encoder, the set of conductances that converts synaptic input into spike discharge. Phase variations reflect both synaptic inputs to the encoder and postsynaptic processes. Experimental data were interpreted using a stochastic integrate-and-fire model. Advantages of an irregular discharge include an enhanced encoder gain and the prevention of nonlinear phase locking. Regular and irregular units are more efficient, respectively, in the encoding of low- and high-frequency head rotations, respectively.
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Affiliation(s)
- Jay M Goldberg
- Department of Pharmacological and Physiological Sciences, University of Chicago, Chicago, Illinois
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Cervantes B, Vega R, Limón A, Soto E. Identity, expression and functional role of the sodium-activated potassium current in vestibular ganglion afferent neurons. Neuroscience 2013; 240:163-75. [PMID: 23466807 DOI: 10.1016/j.neuroscience.2013.02.052] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2012] [Revised: 02/19/2013] [Accepted: 02/20/2013] [Indexed: 10/27/2022]
Abstract
Vestibular afferent neurons (VANs) transmit information from the vestibular end organs to the central nuclei. This information is encoded within the firing pattern of these cells and is heavily influenced by the K⁺ conductances expressed by vestibular neurons. In the present study, we describe the presence of a previously unidentified Na⁺-activated K⁺ conductance (KNa) in these cells. We observed that the blocking of Na⁺ channels by tetrodotoxin (TTX) or the substitution of choline for Na⁺ in the extracellular solution during voltage clamp pulses resulted in the reduction of a sustained outward current that was dependent on the Na⁺ current. Furthermore, increases in the intracellular concentration of Na⁺ that were made by blocking the Na⁺/K⁺ ATPase with ouabain increased the amplitude of the outward current, and reduction of the intracellular Cl⁻ concentration reduced the TTX-sensitive outward current. The substitution of Li⁺ for Na⁺ in the extracellular solution significantly reduced the amplitude of the outward current in voltage clamp pulses and decreased the afterhyperpolarization (AHP) of the action potentials in current clamp experiments. These electrophysiological results are consistent with the presence of mRNA transcripts for the KNa subunits Slick and Slack in the vestibular ganglia and in the sensory epithelium, which were detected using reverse-transcription polymerase chain reaction (RT-PCR). These results are also consistent with the immunolabeling of Slick and Slack protein in isolated vestibular neurons, in the vestibular ganglion and in the vestibular sensory epithelium. These results indicate that KNa channels are expressed in VANs and in their terminals. Furthermore, these data indicate that these channels may contribute to the firing pattern of vestibular neurons.
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Affiliation(s)
- B Cervantes
- Instituto de Fisiología, Universidad Autónoma de Puebla, 14 Sur 6301, Puebla C.P. 72570, Pue., Mexico
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Mann SE, Johnson M, Meredith FL, Rennie KJ. Inhibition of K+ Currents in Type I Vestibular Hair Cells by Gentamicin and Neomycin. ACTA ACUST UNITED AC 2013; 18:317-26. [DOI: 10.1159/000354056] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2013] [Accepted: 06/24/2013] [Indexed: 11/19/2022]
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Almanza A, Luis E, Mercado F, Vega R, Soto E. Molecular identity, ontogeny, and cAMP modulation of the hyperpolarization-activated current in vestibular ganglion neurons. J Neurophysiol 2012; 108:2264-75. [PMID: 22832570 DOI: 10.1152/jn.00337.2012] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Properties, developmental regulation, and cAMP modulation of the hyperpolarization-activated current (I(h)) were investigated by the whole cell patch-clamp technique in vestibular ganglion neurons of the rat at two postnatal stages (P7-10 and P25-28). In addition, by RT-PCR and immunohistochemistry the identity and distribution of hyperpolarization-activated and cyclic nucleotide-gated channel (HCN) isoforms that generate I(h) were investigated. I(h) current density was larger in P25-28 than P7-10 rats, increasing 410% for small cells (<30 pF) and 200% for larger cells (>30 pF). The half-maximum activation voltage (V(1/2)) of I(h) was -102 mV in P7-10 rats and in P25-28 rats shifted 7 mV toward positive voltages. At both ages, intracellular cAMP increased I(h) current density, decreased its activation time constant (τ), and resulted in a rightward shift of V(1/2) by 9 mV. Perfusion of 8-BrcAMP increased I(h) amplitude and speed up its activation kinetics. I(h) was blocked by Cs(+), zatebradine, and ZD7288. As expected, these drugs also reduced the voltage sag caused with hyperpolarizing pulses and prevented the postpulse action potential generation without changes in the resting potential. RT-PCR analysis showed that HCN1 and HCN2 subunits were predominantly amplified in vestibular ganglia and end organs and HCN3 and HCN4 to a lesser extent. Immunohistochemistry showed that the four HCN subunits were differentially expressed (HCN1 > HCN2 > HCN3 ≥ HCN4) in ganglion slices and in cultured neurons at both P7-10 and P25-28 stages. Developmental changes shifted V(1/2) of I(h) closer to the resting membrane potential, increasing its functional role. Modulation of I(h) by cAMP-mediated signaling pathway constitutes a potentially relevant control mechanism for the modulation of afferent neuron discharge.
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Affiliation(s)
- Angélica Almanza
- Instituto de Fisiología, Universidad Autónoma de Puebla, Puebla, Mexico
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Meredith FL, Benke TA, Rennie KJ. Hyperpolarization-activated current (I(h)) in vestibular calyx terminals: characterization and role in shaping postsynaptic events. J Assoc Res Otolaryngol 2012; 13:745-58. [PMID: 22825486 DOI: 10.1007/s10162-012-0342-3] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2012] [Accepted: 06/25/2012] [Indexed: 01/11/2023] Open
Abstract
Calyx afferent terminals engulf the basolateral region of type I vestibular hair cells, and synaptic transmission across the vestibular type I hair cell/calyx is not well understood. Calyces express several ionic conductances, which may shape postsynaptic potentials. These include previously described tetrodotoxin-sensitive inward Na(+) currents, voltage-dependent outward K(+) currents and a K(Ca) current. Here, we characterize an inwardly rectifying conductance in gerbil semicircular canal calyx terminals (postnatal days 3-45), sensitive to voltage and to cyclic nucleotides. Using whole-cell patch clamp, we recorded from isolated calyx terminals still attached to their type I hair cells. A slowly activating, noninactivating current (I(h)) was seen with hyperpolarizing voltage steps negative to the resting potential. External Cs(+) (1-5 mM) and ZD7288 (100 μM) blocked the inward current by 97 and 83 %, respectively, confirming that I(h) was carried by hyperpolarization-activated, cyclic nucleotide gated channels. Mean half-activation voltage of I(h) was -123 mV, which shifted to -114 mV in the presence of cAMP. Activation of I(h) was well described with a third order exponential fit to the current (mean time constant of activation, τ, was 190 ms at -139 mV). Activation speeded up significantly (τ=136 and 127 ms, respectively) when intracellular cAMP and cGMP were present, suggesting that in vivo I(h) could be subject to efferent modulation via cyclic nucleotide-dependent mechanisms. In current clamp, hyperpolarizing current steps produced a time-dependent depolarizing sag followed by either a rebound afterdepolarization or an action potential. Spontaneous excitatory postsynaptic potentials (EPSPs) became larger and wider when I(h) was blocked with ZD7288. In a three-dimensional mathematical model of the calyx terminal based on Hodgkin-Huxley type ionic conductances, removal of I(h) similarly increased the EPSP, whereas cAMP slightly decreased simulated EPSP size and width.
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Affiliation(s)
- Frances L Meredith
- Neuroscience Program, University of Colorado School of Medicine, Aurora, CO 80045, USA.
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Liu P, Jo S, Bean BP. Modulation of neuronal sodium channels by the sea anemone peptide BDS-I. J Neurophysiol 2012; 107:3155-67. [PMID: 22442564 DOI: 10.1152/jn.00785.2011] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Blood-depressing substance I (BDS-I), a 43 amino-acid peptide from sea anemone venom, is used as a specific inhibitor of Kv3-family potassium channels. We found that BDS-I acts with even higher potency to modulate specific types of voltage-dependent sodium channels. In rat dorsal root ganglion (DRG) neurons, 3 μM BDS-I strongly enhanced tetrodotoxin (TTX)-sensitive sodium current but weakly inhibited TTX-resistant sodium current. In rat superior cervical ganglion (SCG) neurons, which express only TTX-sensitive sodium current, BDS-I enhanced current elicited by small depolarizations and slowed decay of currents at all voltages (EC(50) ∼ 300 nM). BDS-I acted with exceptionally high potency and efficacy on cloned human Nav1.7 channels, slowing inactivation by 6-fold, with an EC(50) of approximately 3 nM. BDS-I also slowed inactivation of sodium currents in N1E-115 neuroblastoma cells (mainly from Nav1.3 channels), with an EC(50) ∼ 600 nM. In hippocampal CA3 pyramidal neurons (mouse) and cerebellar Purkinje neurons (mouse and rat), BDS-I had only small effects on current decay (slowing inactivation by 20-50%), suggesting relatively weak sensitivity of Nav1.1 and Nav1.6 channels. The biggest effect of BDS-I in central neurons was to enhance resurgent current in Purkinje neurons, an effect reflected in enhancement of sodium current during the repolarization phase of Purkinje neuron action potentials. Overall, these results show that BDS-I acts to modulate sodium channel gating in a manner similar to previously known neurotoxin receptor site 3 anemone toxins but with different isoform sensitivity. Most notably, BDS-I acts with very high potency on human Nav1.7 channels.
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Affiliation(s)
- Pin Liu
- Department of Neurobiology, Harvard Medical School, Boston, MA, USA
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Meredith FL, Li GQ, Rennie KJ. Postnatal expression of an apamin-sensitive k(ca) current in vestibular calyx terminals. J Membr Biol 2011; 244:81-91. [PMID: 22057903 DOI: 10.1007/s00232-011-9400-8] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2011] [Accepted: 10/15/2011] [Indexed: 11/25/2022]
Abstract
Afferent innervation patterns in the vestibular periphery are complex, and vestibular afferents show a large variation in their regularity of firing. Calyx fibers terminate on type I vestibular hair cells and have firing characteristics distinct from the bouton fibers that innervate type II hair cells. Whole-cell patch clamp was used to investigate ionic currents that could influence firing patterns in calyx terminals. Underlying K(Ca) conductances have been described in vestibular ganglion cells, but their presence in afferent terminals has not been investigated previously. Apamin, a selective blocker of SK-type calcium-activated K(+) channels, was tested on calyx afferent terminals isolated from gerbil semicircular canals during postnatal days 1-50. Lowering extracellular calcium or application of apamin (20-500 nM) reduced slowly activating outward currents in voltage clamp. Apamin also reduced the action potential afterhyperpolarization (AHP) in whole-cell current clamp, but only after the first two postnatal weeks. K(+) channel expression increased during the first postnatal month, and SK channels were found to contribute to the AHP, which may in turn influence discharge regularity in calyx vestibular afferents.
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Affiliation(s)
- Frances L Meredith
- Department of Otolaryngology, University of Colorado at Anschutz Medical Campus, Aurora, CO 80045, USA
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