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For normal cochlear function, outer hair cells (OHCs) require a precise control of intracellular Ca2+ levels. In the absence of regulatory elements such as proteinaceous buffers or extrusion pumps, OHCs degenerate, leading to profound hearing impairment. Influx of Ca2+ occurs both at the stereocilia tips and the basolateral membrane. In this latter compartment, two different origins for Ca2+ influx have been poorly explored: voltage-gated L-type Ca2+ channels (VGCCs) at synapses with Type II afferent neurons, and a9a10 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in mouse OHCs, we dissected Ca2+ influx individually through each of these sources, either by applying step depolarizations to activate VGCC, or stimulating MOC axons. Ca2+ ions originated in MOC synapses, but not by VGCC activation, was confined by Ca2+-ATPases most likely present in nearby synaptic cisterns. Although Ca2+ currents in OHCs are small, VGCC Ca2+ signals were comparable in size to those elicited by a9a10 receptors, and were potentiated by ryanodine receptors (RyRs). In contrast, no evidence of potentiation by RyRs was found for MOC Ca2+ signals over a wide range of presynaptic stimulation strengths. Our study shows that despite the fact that these two Ca2+ entry sites are closely positioned, they differ in their regulation by intracellular cisterns and/or organelles, suggesting the existence of well-tuned mechanisms to separate the two different OHC synaptic functions. For normal cochlear function, outer hair cells (OHCs) require a precise control of intracellular Ca2+ levels. In the absence of regulatory elements such as proteinaceous buffers or extrusion pumps, OHCs degenerate, leading to profound hearing impairment. Influx of Ca2+ occurs both at the stereocilia tips and the basolateral membrane. In this latter compartment, two different origins for Ca2+ influx have been poorly explored: voltage-gated L-type Ca2+ channels (VGCCs) at synapses with Type II afferent neurons, and a9a10 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in mouse OHCs, we dissected Ca2+ influx individually through each of these sources, either by applying step depolarizations to activate VGCC, or stimulating MOC axons. Ca2+ ions originated in MOC synapses, but not by VGCC activation, was confined by Ca2+-ATPases most likely present in nearby synaptic cisterns. Although Ca2+ currents in OHCs are small, VGCC Ca2+ signals were comparable in size to those elicited by a9a10 receptors, and were potentiated by ryanodine receptors (RyRs). In contrast, no evidence of potentiation by RyRs was found for MOC Ca2+ signals over a wide range of presynaptic stimulation strengths. Our study shows that despite the fact that these two Ca2+ entry sites are closely positioned, they differ in their regulation by intracellular cisterns and/or organelles, suggesting the existence of well-tuned mechanisms to separate the two different OHC synaptic functions. [ABSTRACT FROM AUTHOR]

The lateral line (LL) is a sensory system that allows fish and amphibians to detect water currents. LL responsiveness is modulated by efferent neurons that aid in distinguishing between external and self-generated stimuli, maintaining sensitivity to relevant cues. One component of the efferent system is cholinergic, the activation of which inhibits afferent activity. LL hair cells (HCs) share structural, functional, and molecular similarities with those of the cochlea, making them a popular model for studying human hearing and balance disorders. Because of these commonalities, one could propose that the receptor at the LL efferent synapse is a α9α10 nicotinic acetylcholine receptor (nAChR). However, the identities of the molecular players underlying ACh-mediated inhibition in the LL remain unknown. Surprisingly, through the analysis of single-cell expression studies and in situ hybridization, we describe that α9, but not the α10, subunits are enriched in zebrafish HCs. Moreover, the heterologous expression of zebrafish α9 subunits indicates that homomeric receptors are functional and exhibit robust AChgated currents blocked by a-bungarotoxin and strychnine. In addition, in vivo Ca21 imaging on mechanically stimulated zebrafish LL HCs show that ACh elicits a decrease in evoked Ca21 signals, regardless of HC polarity. This effect is blocked by both a-bungarotoxin and apamin, indicating coupling of ACh-mediated effects to small-conductance Ca21-activated potassium (SKs) channels. Our results indicate that an α9-containing (α9p) nAChR operates at the zebrafish LL efferent synapse. Moreover, the activation of α9p nAChRs most likely leads to LL HC hyperpolarization served by SK channels. [ABSTRACT FROM AUTHOR]

During a critical developmental period, cochlear inner hair cells (IHCs) exhibit sensory-independent activity, featuring action potentials in which Ca2+ ions play a fundamental role in driving both spiking and glutamate release onto synapses with afferent auditory neurons. This spontaneous activity is controlled by a cholinergic input to the IHC, activating a specialized nicotinic receptor with high Ca2+ permeability, and coupled to the activation of hyperpolarizing SK channels. The mechanisms underlying distinct excitatory and inhibitory Ca2+ roles within a small, compact IHC are unknown. Making use of Ca2+ imaging, afferent auditory bouton recordings, and electron microscopy, the present work shows that unusually high intracellular Ca2+ buffering and "subsynaptic" cisterns provide efficient compartmentalization and tight control of cholinergic Ca2+ signals. Thus, synaptic efferent Ca2+ spillover and cross-talk are prevented, and the cholinergic input preserves its inhibitory signature to ensure normal development of the auditory system. [ABSTRACT FROM AUTHOR]

Hearing relies on rapid, temporally precise, and sustained neurotransmitter release at the ribbon synapses of sensory cells, the inner hair cells (IHCs). This process requires otoferlin, a six C2-domain, Ca2+-binding transmembrane protein of synaptic vesicles. To decipher the role of otoferlin in the synaptic vesicle cycle, we produced knock-in mice (Otof Ala515,Ala517/Ala515,Ala517) with lower Ca2+-binding affinity of the C²C domain. The IHC ribbon synapse structure, synaptic Ca²+ currents, and otoferlin distribution were unaffected in these mutant mice, but auditory brainstem response wave-I amplitude was reduced. Lower Ca2+ sensitivity and delay of the fast and sustained components of synaptic exocytosis were revealed by membrane capacitance measurement upon modulations of intracellular Ca2+ concentration, by varying Ca2+ influx through voltage-gated Ca2+- channels or Ca2+ uncaging. Otoferlin thus functions as a Ca2+ sensor, setting the rates of primed vesicle fusion with the presynaptic plasma membrane and synaptic vesicle pool replenishment in the IHC active zone. [ABSTRACT FROM AUTHOR]

The auditory system processes time and intensity through separate brainstem pathways to derive spatial location as well as other salient features of sound. The independent coding of time and intensity begins in the cochlea, where afferent neurons can fire action potentials at constant phase throughout a wide range of stimulus intensities. We have investigated time and intensity coding by simultaneous presynaptic and postsynaptic recording at the hair cell-afferent synapse from rats. Trains of depolarizing steps to die hair cell were used to elicit postsynaptic currents that occurred at constant phase for a range of membrane potentials over which release probability varied significandy. To probe the underlying mechanisms, release was examined using single steps to various command voltages. As expected for vesicular release, first synaptic events occurred earlier as presynaptic calcium influx grew larger. However, synaptic depression produced smaller responses with longer first latencies. Thus, during repetitive hair cell stimulation, as the hair cell is more strongly depolarized, increased calcium channel gating hurries transmitter release, but the resulting vesicular depletion produces a compensatory slowing. Quantitative simulation of ribbon function shows that these two factors varied reciprocally with hair cell depolarization (stimulus intensity) to produce constant synaptic phase. Finally, we propose that the observed rapid vesicle replenishment would help maintain the vesicle pool, which in turn would equilibrate with the stimulus intensity (and therefore the number of open Ca2+ channels), so that for trains of different levels the average phase will be conserved. [ABSTRACT FROM AUTHOR]

Inner hair cells (IHCs) in the mammalian cochlea are able to continuously release neurotransmitter in the presence of constant stimuli. Nonetheless, strong synaptic depression is observed over the first few milliseconds of stimulation. This process most likely underlies adaptation in the auditory nerve. In the present study we demonstrate that under certain conditions of stimulation, facilitation can occur at the IHC ribbon synapse. Using simultaneous whole-cell, voltage-clamp recordings from IHCs and afferent fiber endings in excised postnatal rat cochleae, we stimulated IHCs with 2 ms long test depolarizations from a holding potential of −89 mV. Synaptic currents in afferent fibers occurred with high failure rates of ∼50%. However, when a pre-depolarization to values of −55 to −49 mV was implemented before the test pulse, success rates of the synaptic response increased to 100%, the strength of the synaptic response increased ∼2.8-fold, and synaptic latency was reduced by ∼50%. When calcium influx was minimized during pre-depolarization, none of these effects were found, suggesting that calcium influx during pre-depolarizations is required for synaptic conditioning. Similarly, in response to paired-pulse protocols, short term facilitation occurred. The response to the second stimulus increased up to ∼5-fold, and its latency was reduced by up to 35% compared to the response to the first stimulus. We propose that at the IHC resting membrane potential, the ribbon synapse operates in a constantly facilitated mode caused by Ca2+ influx, optimizing the size and timing of the postsynaptic response in auditory nerve fibers. [ABSTRACT FROM AUTHOR]

Abstract: We studied the functional activation of different polymorphic variants of the human dopamine D4 receptors by the three major central monoamines, dopamine, noradrenaline and serotonin. Dopamine D4 receptors carrying two (D4.2), four (D4.4) or seven (D4.7) repeats within the third intracellular domain were co-expressed with G protein-regulated inwardly rectifying potassium channels (GIRK1) in frog oocytes. All the dopamine D4 receptor variants coupled to oocyte Gi/o proteins and modulated co-expressed GIRK1 channels. Monoamine-induced responses were detected as increases in voltage-clamp recorded GIRK1 currents. Dopamine, noradrenaline as well as serotonin stimulated dopamine D4 receptors. Dose-response analysis showed that dopamine and noradrenaline are full agonists whereas serotonin acted as partial agonist. Dopamine was 5-fold more potent on D4.2 and D4.7 (EC50 =1 nM) than on D4.4 (EC50 =5 nM) suggesting that the actions of dopamine and therapeutic drugs on dopamine D4 receptors might vary among individuals depending on their repertoire of expressed alleles. In contrast, noradrenaline and serotonin did not discriminate among dopamine D4 receptor variants (EC50 NA =50 nM, EC50 5-HT =1.5 μM). All monoamine effects were blocked by the specific dopaminergic D4 antagonist (S)-(−)-4-[4-[2-(Isochroman-1-yl)ethyl]piperazin-1-yl]benzenesulfonamide (PNU101387). Sequence analyses of dopamine D4 receptors and related monoamine receptors revealed that dopamine D4 receptors have most aminoacidic residues necessary for binding of dopamine, noradrenaline and serotonin. Our data indicate that dopamine D4 receptors can be pharmacologically stimulated by any the three major central monoamines. [Copyright &y& Elsevier]

1: The mechanisms of action of antagonists of the ?-aminobutyric acid C (GABAC) receptor picrotoxin, quercetin and pregnanolone were studied. 2: Ionic currents (chloride), mediated through human homomeric GABA?1 receptors expressed in Xenopus oocytes, were recorded by two-electrode voltage clamp. 3: Dose-response (D-R) curves and kinetic measurements of GABA?1 currents were carried out in the presence or absence of antagonists. Use-dependent actions were also evaluated. 4: Picrotoxin, quercetin and pregnanolone exerted noncompetitive actions. 5: IC50 values measured at the EC50 for GABA (1?µM) were as follows: picrotoxin 0.6±0.1?µM (Hill coefficient n=1.0±0.2); quercetin 4.4±0.4?µM (n=1.5±0.2); pregnanolone 2.1±0.5?µM (n=0.8±0.1). 6: These antagonists produced changes only in the slope of the linear current-voltage relationships, which was indicative of voltage-independent effects. 7: The effect of picrotoxin on GABA?1 currents was use-dependent, strongly relied on agonist concentration and showed a slow onset and offset. The mechanism was compatible with an allosteric inhibition and receptor activation was a prerequisite for antagonism. 8: The effect of quercetin was use-independent, showed relatively fast onset and offset, and resulted in a slowed time course of the GABA-evoked currents. 9: The effect of pregnanolone was use-independent, presented fast onset and a very slow washout, and did not affect current activation. 10: All the antagonists accelerated the time course of deactivation of the GABA?1 currents.British Journal of Pharmacology (2004) 141, 717-727. doi:10.1038/sj.bjp.0705657 [ABSTRACT FROM AUTHOR]

The modulation of ionotropic γ-aminobutyric acid (GABA) receptors (GABA-gated Cl− channels) by a group of natural and synthetic flavonoids was studied in electrophysiological experiments. Quercetin, apigenin, morine, chrysin and flavone inhibited ionic currents mediated by α1β1γ2s GABAA and ρ1 GABAC receptors expressed in Xenopus laevis oocytes in the micromolar range. α1β1γ2s GABAA and ρ1 GABAC receptors differ largely in their sensitivity to benzodiazepines, but they were similarly modulated by different flavonoids. Quercetin produced comparable actions on currents mediated by α4β2 neuronal nicotinic acetylcholine, serotonin 5-HT3A and glutamate AMPA/kainate receptors. Sedative and anxiolytic flavonoids, like chrysin or apigenin, failed to potentiate but antagonized α1β1γ2s GABAA receptors. Effects of apigenin and quercetin on α1β1γ2s GABAA receptors were insensitive to the benzodiazepine antagonist flumazenil. Results indicate that mechanism/s underlying the modulation of ionotropic GABA receptors by some flavonoids differs from that described for classic benzodiazepine modulation. [Copyright &y& Elsevier]

During development, inner hair cells (IHCs) in the mammalian cochlea are unresponsive to acoustic stimuli but instead exhibit spontaneous activity. During this same period, neurons originating from the medial olivocochlear complex (MOC) transiently innervate IHCs, regulating their firing pattern which is crucial for the correct development of the auditory pathway. Although the MOC-IHC is a cholinergic synapse, previous evidence indicates the widespread presence of gamma-aminobutyric acid (GABA) signaling markers, including presynaptic GABA B receptors (GABA B R). In this study, we explore the source of GABA by optogenetically activating either cholinergic or GABAergic fibers. The optogenetic stimulation of MOC terminals from GAD;ChR2-eYFP and ChAT;ChR2-eYFP mice evoked synaptic currents in IHCs that were blocked by α-bungarotoxin. This suggests that GABAergic fibers release ACh and activate α9α10 nicotinic acetylcholine receptors (nAChRs). Additionally, MOC cholinergic fibers release not only ACh but also GABA, as the effect of GABA on ACh response amplitude was prevented by applying the GABA B -R blocker (CGP 36216). Using optical neurotransmitter detection and calcium imaging techniques, we examined the extent of GABAergic modulation at the single synapse level. Our findings suggest heterogeneity in GABA modulation, as only 15 out of 31 recorded synaptic sites were modulated by applying the GABA B R specific antagonist, CGP (100-200 μM). In conclusion, we provide compelling evidence that GABA and ACh are co-released from at least a subset of MOC terminals. In this circuit, GABA functions as a negative feedback mechanism, locally regulating the extent of cholinergic inhibition at certain efferent-IHC synapses during an immature stage., Competing Interests: The authors declare no competing financial interests.

For normal cochlear function, outer hair cells (OHCs) require a precise control of intracellular Ca 2+ levels. In the absence of regulatory elements such as proteinaceous buffers or extrusion pumps, OHCs degenerate, leading to profound hearing impairment. Influx of Ca 2+ occurs both at the stereocilia tips and the basolateral membrane. In this latter compartment, two different origins for Ca 2+ influx have been poorly explored: voltage-gated L-type Ca 2+ channels (VGCCs) at synapses with Type II afferent neurons, and α9α10 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in mouse OHCs, we dissected Ca 2+ influx individually through each of these sources, either by applying step depolarizations to activate VGCC, or stimulating MOC axons. Ca 2+ ions originated in MOC synapses, but not by VGCC activation, was confined by Ca 2+ -ATPases most likely present in nearby synaptic cisterns. Although Ca 2+ currents in OHCs are small, VGCC Ca 2+ signals were comparable in size to those elicited by α9α10 receptors, and were potentiated by ryanodine receptors (RyRs). In contrast, no evidence of potentiation by RyRs was found for MOC Ca 2+ signals over a wide range of presynaptic stimulation strengths. Our study shows that despite the fact that these two Ca 2+ entry sites are closely positioned, they differ in their regulation by intracellular cisterns and/or organelles, suggesting the existence of well-tuned mechanisms to separate the two different OHC synaptic functions. SIGNIFICANCE STATEMENT Outer hair cells (OHCs) are sensory cells in the inner ear operating under very special constraints. Acoustic stimulation leads to fast changes both in membrane potential and in the intracellular concentration of metabolites such as Ca 2+ Tight mechanisms for Ca 2+ control in OHCs have been reported. Interestingly, Ca 2+ is crucial for two important synaptic processes: inhibition by efferent cholinergic neurons, and glutamate release onto Type II afferent fibers. In the current study we functionally imaged Ca 2+ at these two different synapses, showing close positioning within the basolateral compartment of OHCs. In addition, we show differential regulation of these two Ca 2+ sources by synaptic cisterns and/or organelles, which could result crucial for functional segregation during normal hearing., (Copyright © 2021 the authors.)

The lateral line (LL) is a sensory system that allows fish and amphibians to detect water currents. LL responsiveness is modulated by efferent neurons that aid in distinguishing between external and self-generated stimuli, maintaining sensitivity to relevant cues. One component of the efferent system is cholinergic, the activation of which inhibits afferent activity. LL hair cells (HCs) share structural, functional, and molecular similarities with those of the cochlea, making them a popular model for studying human hearing and balance disorders. Because of these commonalities, one could propose that the receptor at the LL efferent synapse is a α9α10 nicotinic acetylcholine receptor (nAChR). However, the identities of the molecular players underlying ACh-mediated inhibition in the LL remain unknown. Surprisingly, through the analysis of single-cell expression studies and in situ hybridization, we describe that α9, but not the α10, subunits are enriched in zebrafish HCs. Moreover, the heterologous expression of zebrafish α9 subunits indicates that homomeric receptors are functional and exhibit robust ACh-gated currents blocked by α-bungarotoxin and strychnine. In addition, in vivo Ca 2+ imaging on mechanically stimulated zebrafish LL HCs show that ACh elicits a decrease in evoked Ca 2+ signals, regardless of HC polarity. This effect is blocked by both α-bungarotoxin and apamin, indicating coupling of ACh-mediated effects to small-conductance Ca 2+ -activated potassium (SKs) channels. Our results indicate that an α9-containing (α9*) nAChR operates at the zebrafish LL efferent synapse. Moreover, the activation of α9* nAChRs most likely leads to LL HC hyperpolarization served by SK channels. SIGNIFICANCE STATEMENT The fish lateral line (LL) mechanosensory system shares structural, functional, and molecular similarities with those of the mammalian cochlea. Thus, it has become an accessible model for studying human hearing and balance disorders. However, the molecular players serving efferent control of LL hair cell (HC) activity have not been identified. Here we demonstrate that, different from the hearing organ of vertebrate species, a nicotinic acetylcholine receptor composed only of α9 subunits operates at the LL efferent synapse. Activation of α9-containing receptors leads to LL HC hyperpolarization because of the opening of small-conductance Ca 2+ -activated potassium channels. These results will further aid in the interpretation of data obtained from LL HCs as a model for cochlear HCs., (Copyright © 2021 the authors.)

Hearing relies on rapid, temporally precise, and sustained neurotransmitter release at the ribbon synapses of sensory cells, the inner hair cells (IHCs). This process requires otoferlin, a six C 2 -domain, Ca 2+ -binding transmembrane protein of synaptic vesicles. To decipher the role of otoferlin in the synaptic vesicle cycle, we produced knock-in mice ( Otof Ala515,Ala517/Ala515,Ala517 ) with lower Ca 2+ -binding affinity of the C 2 C domain. The IHC ribbon synapse structure, synaptic Ca 2+ currents, and otoferlin distribution were unaffected in these mutant mice, but auditory brainstem response wave-I amplitude was reduced. Lower Ca 2+ sensitivity and delay of the fast and sustained components of synaptic exocytosis were revealed by membrane capacitance measurement upon modulations of intracellular Ca 2+ concentration, by varying Ca 2+ influx through voltage-gated Ca 2+ -channels or Ca 2+ uncaging. Otoferlin thus functions as a Ca 2+ sensor, setting the rates of primed vesicle fusion with the presynaptic plasma membrane and synaptic vesicle pool replenishment in the IHC active zone.

Whole-cell patch clamping is a widely applied method to record currents across the entire membrane of a cell. This protocol describes application of this method to record currents from the sensory inner hair cells in the intact auditory sensory epithelium, the organ of Corti, isolated from rats or mice. This protocol particularly outlines the basic equipment required, provides instructions for the preparation of solutions and small equipment items, and methodology for recording voltage-activated and evoked synaptic currents from the inner hair cells.

1. The mechanisms of action of antagonists of the gamma-aminobutyric acid C (GABA(C)) receptor picrotoxin, quercetin and pregnanolone were studied. 2. Ionic currents (chloride), mediated through human homomeric GABA rho(1) receptors expressed in Xenopus oocytes, were recorded by two-electrode voltage clamp. 3. Dose-response (D-R) curves and kinetic measurements of GABA rho(1) currents were carried out in the presence or absence of antagonists. Use-dependent actions were also evaluated. 4. Picrotoxin, quercetin and pregnanolone exerted noncompetitive actions. 5. IC(50) values measured at the EC(50) for GABA (1 microM) were as follows: picrotoxin 0.6+/-0.1 microM (Hill coefficient n=1.0+/-0.2); quercetin 4.4+/-0.4 microM (n=1.5+/-0.2); pregnanolone 2.1+/-0.5 microM (n=0.8+/-0.1). 6. These antagonists produced changes only in the slope of the linear current-voltage relationships, which was indicative of voltage-independent effects. 7. The effect of picrotoxin on GABA rho(1) currents was use-dependent, strongly relied on agonist concentration and showed a slow onset and offset. The mechanism was compatible with an allosteric inhibition and receptor activation was a prerequisite for antagonism. 8. The effect of quercetin was use-independent, showed relatively fast onset and offset, and resulted in a slowed time course of the GABA-evoked currents. 9. The effect of pregnanolone was use-independent, presented fast onset and a very slow washout, and did not affect current activation. 10. All the antagonists accelerated the time course of deactivation of the GABA rho(1) currents.