Antagonistic effects of perilymphatic calcium and magnesium on the activity of single cochlear afferent neurons

Nitric oxide, endothelium-derived hyperpolarizing factor, and smooth muscle-dependent mechanisms contribute to magnesium-dependent vascular relaxation in mouse arteries

AIM

Magnesium (Mg2+ ) is a vasorelaxant. The underlying physiological mechanisms driving this vasorelaxation remain unclear. Studies were designed to test the hypothesis that multiple signaling pathways including nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) in endothelial cells as well as Ca2+ antagonization and TRPM7 channels in vascular smooth muscle cells mediate Mg2+ -dependent vessel relaxation.

METHODS

To uncover these mechanisms, force development was measured ex vivo in aorta rings from mice using isometric wire myography. Concentration responses to Mg2+ were studied in intact and endothelium-denuded aortas. Key findings were confirmed in second-order mesenteric resistance arteries perfused ex vivo using pressure myography. Effects of Mg2+ on NO formation were measured in Chinese Hamster Ovary (CHO) cells, isolated mesenteric vessels, and mouse urine.

RESULTS

Mg2+ caused a significant concentration-dependent relaxation of aorta rings. This relaxation was attenuated significantly in endothelium-denuded aortas. The endothelium-dependent portion was inhibited by NO and cGMP blockade but not by cyclooxygenase inhibition. Mg2+ stimulated local NO formation in CHO cells and isolated mesenteric vessels without changing urinary NOx levels. High extracellular Mg2+ augmented acetylcholine-induced relaxation. SKCa and IKCa channel blockers apamin and TRAM34 inhibited Mg2+ -dependent relaxation. The endothelium-independent relaxation in aorta rings was inhibited by high extracellular Ca2+ . Combined blockade of NO, SKCa , and IKCa channels significantly reduced Mg2+ -dependent dilatation in mesenteric resistance vessels.

CONCLUSIONS

In mouse conductance and resistance arteries Mg2+ -induced relaxation is contributed by endothelial NO formation, EDHF pathways, antagonism of Ca2+ in smooth muscle cells, and additional unidentified mechanisms.

Calcium-Sensing Receptor Is Functionally Expressed in the Cochlear Perilymphatic Compartment and Essential for Hearing

Maintaining Ca²⁺ homeostasis in lymphatic fluids is necessary for proper hearing. Despite its significance, the mechanisms that maintain the cochlear lymphatic Ca²⁺ concentrations within a certain range are not fully clarified. We investigated the functional expression of calcium-sensing receptor (CaSR), which plays a pivotal role in sensing extracellular Ca²⁺ concentrations for feedback regulations. Western blotting for CaSR revealed an approximately 130-kDa protein expression in cochlear tissue extracts and immunohistochemical analysis revealed its expression specifically in type I fibrocytes in the spiral ligament, fibrocytes in the supralimbal and limbal regions, the epithelium of the osseous spiral lamina, and the smooth muscle cells of the spiral modiolar arteries. Ca²⁺ imaging demonstrated that extracellular Ca²⁺ increased the levels of intracellular Ca²⁺ in CaSR-expressing fibrocytes in the spiral ligament, and that this was suppressed by the CaSR inhibitor, NPS2143. Furthermore, hearing thresholds were moderately elevated by intracochlear application of the CaSR inhibitors NPS2143 and Calhex231, across a range of frequencies (8–32 kHz). These results demonstrate the functional expression of CaSR in the cochlear perilymphatic compartment. In addition, the elevated hearing thresholds that are achieved by inhibiting CaSR suggest this is a required mechanism for normal hearing, presumably by sensing perilymphatic Ca²⁺ to stabilize Ca²⁺ concentrations within a certain range. These results provide novel insight into the mechanisms regulating Ca²⁺ homeostasis in the cochlea and provide a new perspective on cochlear physiology.

Spike encoding of neurotransmitter release timing by spiral ganglion neurons of the cochlea

Mammalian cochlear spiral ganglion neurons (SGNs) encode sound with microsecond precision. Spike triggering relies upon input from a single ribbon-type active zone of a presynaptic inner hair cell (IHC). Using patch-clamp recordings of rat SGN postsynaptic boutons innervating the modiolar face of IHCs from the cochlear apex, at room temperature, we studied how spike generation contributes to spike timing relative to synaptic input. SGNs were phasic, firing a single short-latency spike for sustained currents of sufficient onset slope. Almost every EPSP elicited a spike, but latency (300-1500 μs) varied with EPSP size and kinetics. When current-clamp stimuli approximated the mean physiological EPSC (≈300 pA), several times larger than threshold current (rheobase, ≈50 pA), spikes were triggered rapidly (latency, ≈500 μs) and precisely (SD, <50 μs). This demonstrated the significance of strong synaptic input. However, increasing EPSC size beyond the physiological mean resulted in less-potent reduction of latency and jitter. Differences in EPSC charge and SGN baseline potential influenced spike timing less as EPSC onset slope and peak amplitude increased. Moreover, the effect of baseline potential on relative threshold was small due to compensatory shift of absolute threshold potential. Experimental first-spike latencies in response to a broad range of stimuli were predicted by a two-compartment exponential integrate-and-fire model, with latency prediction error of <100 μs. In conclusion, the close anatomical coupling between a strong synapse and spike generator along with the phasic firing property lock SGN spikes to IHC exocytosis timing to generate the auditory temporal code with high fidelity.

The role of BKCa channels in electrical signal encoding in the mammalian auditory periphery

Large-conductance voltage- and Ca(2+)-activated K+ channels (BKCa) are involved in shaping spiking patterns in many neurons. Less is known about their role in mammalian inner hair cells (IHCs), mechanosensory cells with unusually large BKCa currents. These currents may be involved in shaping the receptor potential, implying crucial importance for the properties of afferent auditory signals. We addressed the function of BKCa by recording sound-induced responses of afferent auditory nerve (AN) fibers from mice with a targeted deletion of the pore-forming alpha-subunit of BKCa (BKalpha(-/-)) and comparing these with voltage responses of current-clamped IHCs. BKCa-mediated currents in IHCs were selectively abolished in BKalpha(-/-), whereas cochlear physiology was essentially normal with respect to cochlear sensitivity and frequency tuning.BKalpha(-/-) AN fibers showed deteriorated precision of spike timing, measured as an increased variance of first spike latency in response to tone bursts. This impairment could be explained by a slowed voltage response in the presynaptic IHC resulting from the reduced K+ conductance in the absence of BKCa. Maximum spike rates of AN fibers were reduced nearly twofold in BKalpha(-/-), contrasting with increased voltage responses of IHCs. In addition to presynaptic changes, which may be secondary to a modest depolarization of BKalpha(-/-) IHCs, this reduction in AN rates suggests a role of BKCa in postsynaptic AN neurons, which was supported by increased refractory periods. In summary, our results indicate an essential role of IHC BKCa channels for precise timing of high-frequency cochlear signaling as well as a function of BKCa in the primary afferent neuron.

Ryanodine receptors and BK channels act as a presynaptic depressor of neurotransmission in cochlear inner hair cells

Ryanodine receptors (RyRs) are known to contribute to the regulation of free cytosolic calcium concentration. This family of intracellular calcium channels plays a significant role in calcium-induced-calcium-release (CICR), and have been implicated in calcium-dependent processes requiring exquisite spatio-temporal regulation. In order to characterize the importance of these intracellular calcium channels in cochlear physiology, we perfused the guinea pig cochlea with antagonistic concentrations of ryanodine. The distortion products of the cochlear microphonic and the compound action potential of the auditory nerve were reversibly inhibited by ryanodine (IC(50)=27.3 microm, Hill coefficient=1.9), indicating an action at the cochlear amplifier. Single auditory nerve fibre recordings showed that ryanodine slightly increased spontaneous firing rates by 22%, suggesting an excitatory effect of ryanodine. This paradoxical effect could be explained by an inhibitory action of ryanodine on presynaptic BK channels of inner hair cells (IHC). Indeed, perfusing iberiotoxin also increased the spontaneous firing activity of the auditory nerve fibres. Furthermore, whole-cell patch-clamp recordings demonstrated that ryanodine inhibits BK currents at the IHC level. Conversely, immunohistochemistry demonstrated a strong expression of RyR in IHCs and, more particularly, below the cuticular plate where membranous BK channels are highly expressed. Overall, the study demonstrated a key role for RyR and CICR in signal transduction at the IHCs. We therefore propose that coupled RyR--BK channels act to suppress the fast neurotransmission in IHCs.

Evidence that rapid vesicle replenishment of the synaptic ribbon mediates recovery from short-term adaptation at the hair cell afferent synapse

We have employed both in vitro patch clamp recordings of hair cell synaptic vesicle fusion and in vivo single unit recording of cochlear nerve activity to study, at the same synapse, the time course, control, and physiological significance of readily releasable pool dynamics. Exocytosis of the readily releasable pool was fast, saturating in less than 50 ms, and recovery was also rapid, regaining 95% of its initial amplitude following a 200-ms period of repolarization. Longer depolarizations (greater than 250 ms) yielded a second, slower kinetic component of exocytosis. Both the second component of exocytosis and recovery of the readily releasable pool were blocked by the slow calcium buffer, EGTA. Sound-evoked afferent synaptic activity adapted and recovered with similar time courses as readily releasable pool exhaustion and recovery. Comparison of readily releasable pool amplitude, capture distances of calcium buffers, and number of vesicles tethered to the synaptic ribbon suggested that readily releasable pool dynamics reflect the depletion of release-ready vesicles tethered to the synaptic ribbon and the reloading of the ribbon with vesicles from the cytoplasm. Thus, we submit that rapid recovery of the cochlear hair cell afferent fiber synapse from short-term adaptation depends on the timely replenishment of the synaptic ribbon with vesicles from a cytoplasmic pool. This apparent rapid reloading of the synaptic ribbon with vesicles underscores important functional differences between synaptic ribbons in the auditory and visual systems.

Effects of a calcium channel blocker on spontaneous neural noise and gross action potential waveforms in the guinea pig cochlea

The effects of the L-type Ca2+ channel blocker nimodipine on the spectrum of the spontaneous neural noise (SNN) and the waveform of the gross sound-evoked compound action potential (CAP) were investigated by perilymphatic perfusion in the guinea pig cochlea. Both the SNN and the CAP were reversibly suppressed by nimodipine. The percentage reduction in SNN was dose-dependent in a manner very similar to the results obtained with the measures of CAP threshold changes. The reduction in the peak SNN caused by 10 microM nimodipine was the same as that caused by 500 microM kainic acid, which totally eliminated any neural responses. For 1 microM nimodipine there was an apparent dissociation between the SNN and CAP changes such that the SNN could be markedly suppressed with only very small changes in CAP thresholds. These results imply that spontaneous release of neurotransmitter from the inner hair cell is more sensitive to block of calcium channels than evoked release. There was no evidence for any marked shift caused by nimodipine, in the position of the main (900 Hz) spectral peak in the SNN. Comparison of the CAP waveform before and after nimodipine perfusion showed that the CAP waveforms were unchanged despite the change in sensitivity. These data do not support the notion of any significant postsynaptic site of action of nimodipine. The data hence provide further support for an exclusively presynaptic role for L-type Ca2+ channels in the regulation of both evoked and spontaneous neurotransmitter release from inner hair cells.

The synaptic physiology of cochlear hair cells

Mechanosensory hair cells of the vertebrate inner ear are so-called 'short' receptors that communicate to the central nervous system by way of chemical synapses with afferent neurons. In turn, hair cells are the targets of olivocochlear fibers that carry efferent inhibitory feedback from the brain. These synaptic activities contribute to, or modulate the hair cell's receptor potentials through the gating of associated ion channels. Thus for example, voltage-gated calcium channels open to trigger vesicle fusion and release of transmitter by entry of extracellular calcium. The inward calcium current also depolarizes the membrane and could lead to generation of 'all-or-none' action potentials. However, regenerative depolarization is prevented in most hair cells by prominent voltage-gated potassium conductances that rapidly repolarize the membrane. The magnitude and speed of these delayed potassium conductances determine the size and shape of the resulting receptor potential, and subsequent transmitter release, produced by sound. Efferent feedback is provided by the release of acetylcholine (ACh) from olivocochlear nerve fibers onto outer hair cells in the mammalian cochlea. The hair cell's ACh receptors are ligand-gated cation channels related to the nicotinic receptors of nerve and muscle. Calcium influx through the ACh receptors activates nearby calcium-gated potassium channels, resulting in hyperpolarization and inhibition of the hair cell. Calcium influx during efferent inhibition is regulated by a 'synaptic cistern' that also may act as a calcium store that is triggered by ACh under some conditions.

Endogenously generated spontaneous spiking activities recorded from postnatal spiral ganglion neurons in vitro

Spontaneous spiking activities in the nervous system play an important role in the neuronal development and the coding of sensory information. Such firings could be initiated by transmitter leaked from the first-order sensory receptors or as a result of the internal operation of voltage-dependent ion channels intrinsic to the neuron. We recorded endogenously-generated spontaneous action potentials (APs) from postnatal spiral ganglion (SG) neurons of mouse in vitro. SG neurons in cultures displayed statistically stable spontaneous firings with no obvious bursting, rhythmic spiking and long silent gaps for as long as the recording configuration could be maintained. Average firing rates ranged from less than 1 to over 10 spikes/s, with most cells fired around 4 spikes/s. Interpulse interval histograms were remarkably similar to those recorded in vivo from the auditory nerve, with characteristics of a Poisson-like distribution. Resting membrane potential greatly altered the AP width and the rate of spontaneous firings. Spontaneous firing rates were also found to be controlled by the availability of the Shaw-like potassium channels. In contrast, matured SG neurons did not display any spontaneous APs, probably due to a large increase in the expression of the whole-cell potassium currents in comparison to their postnatal counterparts. This study provided the first direct evidence that postnatal SG neurons were capable of generating spontaneous APs independent of inputs from hair cells. Intracellular mechanisms for generating the spontaneous random spikes and the possible roles of such spontaneous activities in the postnatal development of SG neurons are discussed.

Role of L-type Ca(2+) channels in transmitter release from mammalian inner hair cells I. Gross sound-evoked potentials

Intracochlear perfusion and gross potential recording of sound-evoked neural and hair cell responses were used to study the site of action of the L-type Ca(2+) channel blocker nimodipine in the guinea pig inner ear. In agreement with previous work nimodipine (1-10 microM) caused changes in both the compound auditory nerve action potential (CAP) and the DC component of the hair cell receptor potential (summating potential, or SP) in normal cochleae. For 20-kHz stimulation, the effect of nimodipine on the CAP threshold was markedly greater than the effect on the threshold of the negative SP. This latter result was consistent with a dominant action of nimodipine at the final output stage of cochlear transduction: either the release of transmitter from inner hair cells (IHCs) or the postsynaptic spike generation process. In animals in which the outer hair cells (OHCs) had been destroyed by prior administration of kanamycin, nimodipine still caused a large change in the 20-kHz CAP threshold, but even less change was observed in the negative SP threshold than in normal cochleae. When any neural contamination of the SP recording in kanamycin-treated animals was removed by prior intracochlear perfusion with TTX, nimodipine caused no significant change in SP threshold. Some features of the data also suggest a separate involvement of nimodipine-sensitive channels in OHC function. Perfusion of the cochlea with solutions containing Ni(2+) (100 microM) caused no measurable change in either CAP or SP. These results are consistent with, but do not prove, the notion that L-type channels are directly involved in controlling transmitter release from the IHCs and that T-type Ca(2+) channels are not involved at any stage of cochlear transduction.

Medial efferent effects on auditory-nerve responses to tail-frequency tones. I. Rate reduction

One way medial efferents are thought to inhibit responses of auditory-nerve fibers (ANFs) is by reducing the gain of the cochlear amplifier thereby reducing motion of the basilar membrane. If this is the only mechanism of medial efferent inhibition, then medial efferents would not be expected to inhibit responses where the cochlear amplifier has little effect, i.e., at sound frequencies in the tails of tuning curves. Inhibition at tail frequencies was tested for by obtaining randomized rate-level functions from cat ANFs with high characteristic frequencies (CF > or = 5 kHz), stimulated with tones two or more octaves below CF. It was found that electrical stimulation of medial efferents can indeed inhibit ANF responses to tail-frequency tones. The amplitude of efferent inhibition depended on both sound level (largest near to threshold) and frequency (largest two to three octaves below CF). On average, inhibition of high-CF ANFs responding to 1 kHz tones was around 5 dB. Although an efferent reduction of basilar-membrane motion cannot be ruled out as the mechanism producing the inhibition of ANF responses to tail frequency tones, it seems more likely that efferents produce this effect by changing the micromechanics of the cochlear partition.

Exponential onset and recovery of temporary threshold shift after loud sound: evidence for long-term inactivation of mechano-electrical transduction channels

The onset and recovery of temporary threshold shift (TTS) in one human subject (the author) has been studied during and after pure-tone overstimulation lasting between minutes and days. Under the conditions of these experiments the time courses appeared reproducible, and thresholds always recovered to normal within 3 days. The onset and recovery followed a multiple-exponential time course, with the time constants for the onset being 6.5 and 800 min, and the recovery time constants being 30, 240 and 800 min. The observed time courses were consistent with data previously reported in humans, and with the view that the threshold elevation was due to an inactivation and reactivation of the stretch-activated channels at the apex of the outer hair cells of the cochlea. The time constants of the multi-exponential onset and recovery do not appear to depend on the duration of the overstimulation, but the exponential coefficients do. A simple kinetic model of the onset and recovery is described (for more detail see Patuzzi (1998)). It is suggested that the rapid recovery in the first 5 min after exposure is due to a short-lived disruption of the synapses between the inner hair cells and the primary afferent neurones. Intermittent exposures were found to produce much less TTS than continuous tones, and this reduction was found to be inconsistent with the Equal Energy Hypothesis, in that the TTS produced by intermittent tones was much less than predicted using the Equal Energy model, and the recovery time course was also different from that expected from a shorter exposure to a continuous tone of equal energy.

Effects of nimodipine on noise-induced hearing loss

The effects of nimodipine, a calcium channel blocker, on noise-induced hearing loss were examined in gerbils. Animals were implanted subcutaneously with a timed-release pellet containing either nimodipine (approximately 10 mg/kg/day) or placebo and exposed to either 102 or 107 dBA noise. Serum levels were tested in two subjects and were in the range known to protect humans from cerebral artery vasospasm and ischemia-related neurologic deficits. Nimodipine and control groups had similar amounts of noise-induced (a) permanent threshold shift; (b) reductions in distortion product otoacoustic emissions; (c) reductions in tuning and suppression of the compound action potential; and (d) loss of outer hair cells. The results suggest that nimodipine, at a dose which results in clinically relevant serum levels, does not provide protection from the effects of moderately intense noise exposures.

Cochlear basal and apical differences reflected in the effects of cooling on responses of single auditory nerve fibers

Responses of single auditory nerve fibers in the Mongolian gerbil were examined before and during rapid, moderate cooling of the cochlea. Reducing cochlear temperature from 35-39 degrees C to 29-32 degrees C led to stable, reversible changes in spontaneous firing rates (SRs), and responses to tonebursts, as characterized by frequency tuning curves and rate-versus-intensity curves. The nature and extent of effects of cooling were strongly linked to characteristic frequency (CF). Rate thresholds at the CF were increased by 0-15 dB for fibers with CFs below 8 kHz, and by 10-30 dB for higher CFs. Although SRs were generally reduced, the percent reduction in SR was striking CF dependent. For fibers with CFs below 4 kHz, the reduction did not exceed 50% of the initial SR. For higher CFs, the reduction was always greater than 50%. The effects of cooling on intensity curve shape differed qualitatively for fibers with CFs below and above 3-4 kHz. The slope of the curve was reduced by an average of 50% for lower CFs, but on average was unchanged for higher CFs. Cooling-related increases in CF threshold probably reflect impairment of active mechanical processes. The CF dependence of these increases suggests either that active mechanical processes are more impaired in the cochlear base relative to the apex, or that they play a more crucial role in determining sensitivity in the base. The CF-dependent changes in SR and in the shape of rate-intensity curves caused by cooling correspond to an enhancement of basal/apical differences seen at normal temperatures. They are best explained by longitudinal gradients in the properties of the inner hair cells and their afferent synapses. Basal and apical differences in the distribution of SRs and in supra-threshold response properties suggest that stimulus coding strategies differ between low and high frequency regions of the cochlea.

Immunohistochemical localization of S-100 protein in auditory and vestibular end organs of the mouse and hamster

The distribution of S-100-like immunoreactivity in mouse and hamster auditory and vestibular end organs was determined by the use of immunohistochemistry. Within the organ of Corti, the cytoplasm of cells of Deiter and Hensen were strongly immunoreactive. Inner hair cells and the peripheral processes and cell bodies of the spiral ganglion were weakly immunoreactive for S-100, whereas the supranuclear regions of outer hair cells and cells underlying the basilar membrane were unstained. Immunoreactivity was observed near the base of outer hair cells. In the lateral wall of the cochlea, cellular components of the spiral ligament and a subpopulation of epithelial cells in the stria vascularis, identified as predominantly basal cells, were immunoreactive. For the saccule, utricle, and semicircular canals, S-100 immunoreactivity was observed in vestibular hair cells, types I and II, and the nerve calyces surrounding type I hair cells as well as in nerve fibers underlying the sensory epithelium. Weak S-100-like immunoreactivity was associated with vestibular nerve fibers and cell bodies in the vestibular ganglion. The localization of S-100-like immunoreactivity to the sensory cells and nerve fibers of the peripheral auditory and vestibular end organs is consistent with a functional role for S-100 proteins at these sites.

Immunohistochemical localization of S-100 protein in the saccule of the rainbow trout (Salmo gairdnerii R.)

The distribution of S-100-like immunoreactivity in the trout saccule (a presumed organ of hearing in fish) has been determined by means of immunohistochemistry. Within the sensory epithelium of the saccular macula, hair cells and myelinated saccular nerve fibers were found to be immunoreactive. Hair-cell immunoreactivity was relatively uniform throughout the macula except at the extreme periphery (rostral, caudal, ventral and dorsal), where staining was either decreased or absent. The immunoreactivity associated with myelinated nerve fibers was greatest at the peripheral edges of the nerve processes, a position corresponding to the location of Schwann cells. However, the nerve processes themselves (within and subjacent to the sensory epithelium), as well as cell bodies within the saccular nerve, were also immunoreactive. Thus, the immunoreactivity of the saccular nerve observed above the basal lamina can be attributed to the saccular nerve processes as well as to nerve-associated Schwann cells. Overall, the immunoreactivity displayed by hair cells was less intense than that associated with myelinated saccular nerve, as evidenced by a disappearance of signal in hair cells first, upon serial dilution of antibody. No S-100-like immunoreactivity was observed in supporting cells within the sensory epithelium or in epithelial cells in non-sensory regions. A concentration of S-100-like immunoreactivity in hair cells and saccular nerve is suggestive of the presence of S-100 calcium-binding protein-mediated activities in these cell types.

Otoacoustic emissions and the categorization of cochlear and retro-cochlear lesions

There are only some cochlear and retro-cochlear lesions that could be detected using a simple measurement of otoacoustic emissions. A simple description of the various lesions that lead to hearing loss and disruption of emissions is presented, a basis for categorizing cochlear and retro-cochlear lesions is discussed, and the types of clinical symptoms likely to be associated with each type of lesion are outlined.

Additivity of threshold elevations produced by disruption of outer hair cell function

We present a simple model describing the additivity of hearing loss in the mammalian cochlea produced by disruption of the outer hair cell transduction processes. The validity of this model has been tested experimentally in the guinea-pig by inducing threshold elevations using two simultaneous cochlear manipulations, including acoustic overstimulation, two-tone suppression, low-frequency acoustic biasing of the cochlear partition and electrical stimulation of the medial olivo-cochlear system of efferent fibres. The results of these experiments suggest that the model presented is an adequate description, within the measurement error of our experiments, of the hearing losses produced.

Spontaneous synaptic potentials from afferent terminals in the guinea pig cochlea

Records of spontaneous activity from units likely to be radial afferents were analyzed to find the origin of spontaneous action potentials in single auditory nerve fibers. Single synaptic events (excitatory postsynaptic potentials or EPSPs) nearly all triggered action potentials (spikes). An abrupt increase in slope during the rising phase of the EPSP often signalled the initiation of an action potential. Synaptic potentials that did not trigger spikes occurred frequently during the refractory period. These events sometimes appeared to be composed of subunits. Refractoriness appears to be the primary reason these EPSPs were ineffective. Distributions of the onset slopes of postsynaptic potentials were highly skewed. Skewing was not a consequence of refractoriness, but most likely because the amplitude distribution of spontaneous potentials is not gaussian.

Magnitude of the negative summating potential varies with perilymph calcium levels

Previous results demonstrated that nimodipine, an L-type of Ca2+ channel antagonist, abolished the negative summating potential (SP) recorded from anesthetized guinea pigs (Bobbin et al., 1990), suggesting that Ca2+ is involved in generation of the negative SP. Therefore we examined the effect of changing concentrations of perilymph Ca2+ on this cochlear potential. Perilymph spaces of guinea pig cochleae were perfused with artificial perilymph solutions containing zero mM Ca2+, zero mM Ca2+ with 2 mM EGTA, 30 mM Mg2+ and increasing levels of Ca2+ (2, 4, 8, 16 mM) at a rate of 2.5 microliters/min for 10 min. Immediately after each period of perfusion the compound action potential of the auditory nerve (CAP), cochlear microphonics (CM) and the negative SP evoked by 10 kHz tone bursts of varying intensities were recorded from a wire inserted in the basal turn scala vestibuli. Decreasing the level of Ca2+ decreased the magnitude of the negative SP, whereas increasing the level of Ca2+ progressively increased the magnitude of the negative SP. Mg2+ (30 mM) suppressed the CAP to the same extent as zero mM Ca2+ with 2 mM EGTA, but only slightly increased the magnitude of the negative SP. These results support the hypothesis that Ca2+ and L-type Ca2+ channels are involved in the function of the hair cells and the generation of the negative SP. Mg2+ appears to be a selective antagonist of the Ca2+ channel involved in transmitter release.

Differential modulation of spontaneous and evoked neurotransmitter release from hair cells: some novel hypotheses

It has been generally accepted that even in the absence of mechanical stimulation of the transductional elements, a resting depolarizing current exists which is ultimately responsible for the spontaneous release of neurotransmitter. Movement of the transductional elements modulates this resting current and thereby the evoked release of neurotransmitter occurs. Recent data from our laboratory and others have led us to question whether the relationship between spontaneous and evoked neurotransmitter release is as simple as stated. Indeed, a variety of experimental manipulations appear to influence the two modes of release differently. Examination of our results and the results of others has led us to four hypotheses: 1. the two modes of neurotransmitter release are processed differently by the hair cells; 2. cyclic AMP is involved in spontaneous but not evoked neurotransmitter release; 3. there is a positive feedback step involving an excitatory amino acid and its receptor on the hair cell in evoked neurotransmitter release and; 4. different pools of calcium are involved according to the mode of release. Accordingly, there may be several biochemical steps between the transductional movement of the stereocilia at the apex of the hair cells and the ultimate release of the neurotransmitter at the base of these cells. Some of these biochemical steps are different depending on whether the mode of release is spontaneous or evoked. These biochemical steps may amplify or at least interact with the biophysical processes previously described in the hair cells.

Electrophysiological evidence for the presence of NMDA receptors in the guinea pig cochlea

An excitatory amino acid, possibly L-glutamate, which probably acts as a neurotransmitter at the inner hair cell-afferent fiber synapses in the cochlea. In the present study, we have used an electrophysiological approach to investigate at this level the presence of a major type of excitatory amino acid receptor, namely the glutamatergic receptor for which N-methyl-D-aspartate is a selective agonist. Our results show that, when N-methyl-D-aspartate and the antagonist 2-amino-5-phosphonovalerate are perfused through the perilymphatic scalae, they induced, by different mechanisms, a significant reduction of the amplitude of the compound action potential and an increase of the N1 latency, both predominant at high intensity tone burst stimulations. No significant difference was found in the presence or absence of Mg2+ in the artificial perilymph used as a vehicle. A further slight N-methyl-D-aspartate-induced decrease of the amplitude of the compound action potential, although non significant, was observed when the Mg2(+)-free perilymph contained 100 or 1000 microM glycine. In all the experimental conditions, no effect was observed on the cochlear microphonic potential. This observation is consistent with an action of N-methyl-D-aspartate and 2-amino-5-phosphonovalerate at receptors located on the auditory nerve dendrites contacting the inner hair cells. In conclusion, our results suggest the presence of N-methyl-D-aspartate receptors in the cochlea.

Effects of L-glutamate on auditory afferent activity in view of its proposed excitatory transmitter role in the mammalian cochlea

This report describes the effects of L-glutamate (Glu) introduced into the perilymph of scala tympani on the spontaneous and tone-evoked activity of guinea pig single primary auditory afferents. Concentrations below 2 mmol/l were in general ineffective, while a concentration of 5 mmol/l caused a marked decrease of the neural activity. At 2 mmol/l, roughly 60% of the Glu-perfusions were effective and produced a variety of changes. The most prominent effect was a reduction of the tone-evoked activity without a change in spontaneous rate. Indeed, in some cells, the tone-evoked activity could be almost totally abolished without affecting the spontaneous activity. More rarely observed was a moderate, generally transient increase of the spontaneous activity which was occasionally followed by a decrease in both tone-evoked and spontaneous firing rate. The increase in firing rate was always small relative to the maximum discharge rate evoked by tone stimuli. Desensitization of the Glu-evoked response without an obvious change in the spontaneous activity was also found. In a few cells Glu caused a reduction of the discharge rate below the spontaneous firing rate during loud tone presentation. Higher Glu doses generally caused a reduction of spontaneous and tone-evoked activity without any sign of a preceding increase. Thus, the effects of Glu in the mammalian cochlea appear to be complex and on balance seem inconsistent with the effects predicted for an excitatory transmitter. The findings argue against the hypothesis that Glu is the afferent transmitter released by inner hair-cells. However, the results do not exclude an involvement of Glu as a neuromodulator or co-transmitter.

Effects of divalent cations on the frequency of spontaneous action potentials from the lateral line organ of Xenopus laevis

The effect of superfusion of the internal surface of the skin of Xenopus laevis with saline containing Co2+, Ca2+, Mg2+, or Ba2+, on the frequency of spontaneous action potentials of the lateral line nerve, was studied to investigate the role of extracellular Ca2+ in spontaneous neural activity. Addition of divalent cations to frog saline, either singly or as a mixture of two different ions, produced concentration-dependent suppression of spontaneous rate. The rank order of potency for suppression by each ion, perfused alone, was Co2+ greater than Ca2+ greater than Mg2+ greater than Ba2+. Suppression by combinations of Mg2+ and Co2+, or of Ca2+ and Co2+, was approximated by the sum of the suppressive effects of each cation. Ca2+ was more suppressive than Mg2+ when each of these ions was paired with the same amount of Co2+, while Ca2+ was approximately as suppressive as Co2+ when similarly paired with Mg2+. One interpretation of the suppression by Ca2+ invokes the hypothesis that divalent cations suppress spontaneous activity by charge screening of voltage-sensitive Na+ channels on afferent dendrites and that release of neurotransmitter by the influx of extracellular Ca2+ through voltage-sensitive Ca2+ channels of hair cells may not be the sole mechanism for generation of spontaneous activity in the lateral line. These results quantify the relative suppressive potency of common divalent cations in the lateral line, and serve as a caveat to investigators who interpret a blockade of action potentials by high concentrations of Co2+ or Mg2+ as sufficient evidence for dependence of neurotransmission upon extracellular Ca2+, particularly in acousticolateralis systems.

Outer hair cell receptor current and sensorineural hearing loss

It is argued in this paper that many nonlinear phenomena in audition and many types of sensorineural hearing loss can be explained by a disruption of the mechano-electrical transduction process at the apex of the outer hair cells. This is done using experimental data and a simple model of the active role of outer hair cells in cochlear mechanics based on our previous experiments with acoustic trauma. The causes of sensorineural loss addressed include acoustic trauma, aminoglycoside ototoxicity, intoxication with loop diuretics, hypoxia and Meniere's disease. The nonlinear phenomena discussed include loudness compression, two-tone suppression and modulation of cochlear sensitivity by very low-frequency tones. In every case considered the reduction in neural sensitivity was related to the reduction in outer hair cell receptor current in a quantitatively similar way. We conclude that the link is causal.

Effect of spider venom on cochlear nerve activity consistent with glutamatergic transmission at hair cell-afferent dendrite synapse

Venom from the spider Argiope trifasciata, a highly specific blocker of the ionic channels associated with invertebrate glutamatergic receptors, was perfused through scala tympani of the basal turn of the pig cochlea. Its effect on spontaneous and driven activity of single afferent neurons was studied. 0.1 U/ml spider venom altered the maximum driven activity without an effect on spontaneous activity. 1 U/ml spider venom suppressed both spontaneous and driven activity. These findings are consistent with the hypothesis that L-glutamate is the neurotransmitter of the synapse between inner hair cells and primary auditory afferent neurons. The results also suggest that the differences in spontaneous activity between neurons may originate in variations in neurotransmitter release.

Actions of excitatory amino acid acid agonists and antagonists on the primary afferents of the vestibular system of the axolotl (Ambystoma mexicanum)

In order to determine the nature of the transmitter in the synapse between hair cells and primary afferent fibers, both resting and evoked spike activity of vestibular system afferents were recorded. Excitatory amino acid agonists and antagonists were applied by micro perfusion. Excitatory amino acid agonists consistently increased the firing rate of these afferents. The rank order in potencies of the agonists tested was: kainate greater than or equal to quisqualate greater than D-aspartate greater than or equal to L-glutamate greater than or equal to L-aspartate greater than N-methyl D-aspartate. Blockade of synaptic transmission with high-Mg2+ and low-Ca2+ solutions did not seem to affect the responses to the excitatory amino acid agonists indicating their postsynaptic action. Excitatory amino acid antagonists inhibit both resting and physiologically evoked activity. The rank order of inhibitory potency was: kynurenate greater than L-glutamate diethyl ester greater than D,L-2-amino-4-phosphono-butyrate greater than D-alpha-amino adipate greater than D,L-2-amino-5-phosphonovalerate. These findings suggest that an amino acid-related compound may be the transmitter at this synapse. The relative potencies of agonists and antagonists tested provide evidence that the transmitter released from the hair cells' basal pole in the axolotl vestibular system interacts with postsynaptic kainic/quisqualic type receptors.