Anteroventral Cochlear Nucleus Neurons Research Articles

Modulation of Neuronal Excitability by Low- Intensity Ultrasound in Two Principal Neurons of Rat Anteroventral Cochlear Nucleus.

Ultrasonic neuromodulation has proved to be a promising new approach for direct neuromodulation or potential noninvasive deep brain stimulation technology for treating various neurological disorders. Previous studies have demonstrated that ultrasonic waves can noninvasively diffuse through the intact skull and thus precisely target specific brain regions with high spatial resolution. However, its neuromodulatory effects over different cell types of target nuclei have not been fully elucidated. In the present study, we investigated the neuronal excitability resulted from ultrasound stimulation on the two major neurons of anteroventral cochlear nucleus (AVCN) in vitro. Our results demonstrated that bushy cells (BCs) were well maintaining one action potential (AP) in response to the pairing of a sequence of depolarizing current pulses and 60-s continuous low-intensity ultrasound (LIUS), and meanwhile, stellate cells (SCs) significantly increased the firing rate. The ultrasonic waves with an acoustic pressure of 0.13 MPa were elicited by an on-chip ultrasonic stimulation system compatible with patch-clamp recording. Furthermore, LIUS significantly improved the neuronal excitability in both BCs and SCs based on their intrinsic excitability. Modulation of membrane properties among cell types was due to the LIUS-induced increase in the total inward sodium currents ( INa ) and outward potassium currents ( IKv ). LIUS significantly, at a similar rate, increased the amplitude of total inward sodium currents in both cell types. Meanwhile, LIUS induces a higher rate of the outward potassium currents in the BCs compared with SCs. Therefore, this study could provide new evidence for safe use of ultrasonic neuromodulation and its potential therapy for many auditory diseases, such as the central auditory processing disorder.

Principal Neurons in the Anteroventral Cochlear Nucleus Express Cell-Type Specific Glycine Receptor α Subunits

Signal processing in the principal neurons of the anteroventral cochlear nucleus (AVCN) is modulated by glycinergic inhibition. The kinetics of IPSCs are specific to the target neurons. It remains unclear what glycine receptor subunits are involved in generating such target-specific IPSC kinetics in AVCN principal neurons. We investigated the expression patterns of glycine receptor α (GlyRα) subunits in AVCN using immunohistochemical labeling of four isoforms of GlyRα subunits (GlyRα1–α4), and found that AVCN neurons express GlyRα1 and GlyRα4, but not GlyRα2 and GlyRα3 subunits. To further identify the cell type-specific expression patterns of GlyRα subunits, we combined whole-cell patch clamp recording with immunohistochemistry by recording from all three types of AVCN principal neurons, characterizing the synaptic properties of their glycinergic inhibition, dye-filling the neurons, and processing the slice for immunostaining of different GlyRα subunits. We found that AVCN bushy neurons express both GlyRα1 and GlyRα4 subunits that underlie their slow IPSC kinetics, whereas both T-stellate and D-stellate neurons express only GlyRα1 subunit that underlies their fast IPSC kinetics. In conclusion, AVCN principal neurons express cell-type specific GlyRα subunits that underlie their distinct IPSC kinetics, which enables glycinergic inhibition from the same source to exert target cell-specific modulation of activity to support the unique physiological function of these neurons.

Relationships between neuronal birthdates and tonotopic positions in the mouse cochlear nucleus.

Tonotopy is a key anatomical feature of the vertebrate auditory system, but little is known about the mechanisms underlying its development. Since date of birth of a neuron correlates with tonotopic position in the cochlea, we investigated if it also correlates with tonotopic position in the cochlear nucleus (CN). In the cochlea, spiral ganglion neurons are organized in a basal to apical progression along the length of the cochlea based on birthdates, with neurons in the base (responding to high-frequency sounds) born early around mouse embryonic day (E) 9.5-10.5, and those in the apex (responding to low-frequency sounds) born late around E12.5-13.5. Using a low-dose thymidine analog incorporation assay, we examine whether CN neurons are arranged in a spatial gradient according to their birthdates. Most CN neurons are born between E10.5 ānd E13.5, with a peak at E12.5. A second wave of neuron birth was observed in the dorsal cochlear nucleus (DCN) beginning on E14.5 and lasts until E18.5. Large excitatory neurons were born in the first wave, and small local circuit neurons were born in the second. No spatial gradient of cell birth was observed in the DCN. In contrast, neurons in the anteroventral cochlear nucleus (AVCN) were found to be arranged in a dorsal to ventral progression according to their birthdates, which are aligned with the tonotopic axis. Most of these AVCN neurons are endbulb-innervated bushy cells. The correlation between birthdate and tonotopic position suggests testable mechanisms for specification of tonotopic position.

Taurine protects against bilirubin-induced hyperexcitation in rat anteroventral cochlear nucleus neurons

No effective medication for hyperbilirubinemia yet exists. Taurine is believed to exert a neuroprotective action. The aim of the present study was to determine whether taurine protected neurons of the rat anteroventral cochlear nucleus (AVCN) against bilirubin-induced neuronal hyperexcitation. AVCN neurons were isolated from 13 to 15-day-old Sprague–Dawley rats. The effects of bilirubin on the spontaneous excitatory postsynaptic currents (sEPSCs) and action potential currents were compared with those exerted by bilirubin and taurine together. Bilirubin dramatically increased the frequencies of sEPSCs and action potential currents, but not sEPSC amplitude. Taurine suppressed the enhanced frequency of action potentials induced by bilirubin, in a dose-dependent manner. In addition, taurine decreased the amplitude of voltage-dependent calcium channel currents that were enhanced upon addition of bilirubin. We explored the mechanism of the protective effects exerted by taurine using GABAA and glycine receptor antagonists, bicuculline and strychnine, respectively. Addition of bicuculline and strychnine eliminated the protective effects of taurine. Neither bilirubin nor taurine affected the sensitivity of the glutamate receptor. Our findings thus indicate that taurine protected AVCN neurons against bilirubin-induced neuronal hyperexcitation by activating the GABAA and glycine receptors and inhibiting calcium flow through voltage-gated channels. Thus, taurine may be effective in treatment of neonatal hyperbilirubinemia.

Trauma‐specific insults to the cochlear nucleus in the rat

The effect of acoustic overstimulation on the neuronal number of the cochlear nucleus (CN) was investigated by using unbiased stereological methods in rats. We found that, after 9 weeks of recovery, neurons in the anteroventral cochlear nucleus (AVCN) degenerated, whereas those in the posteroventral and dorsal cochlear nuclei (PVCN and DCN) were preserved. The noise trauma induced near complete loss of the outer hair cells throughout the cochlea, and the inner hair cells were preserved only in the more apical regions. This pattern of selective loss of AVCN neurons in this study was different from trauma induced by auditory deafferentation by mechanical compression of auditory neurons. In contrast to noise trauma, mechanical compression caused loss of neurons in the PVCN and DCN. After 5 weeks of recovery from mechanical compression, there was no loss of inner or outer hair cells. These findings indicate that auditory deprivation, induced by different experimental manipulations, can have strikingly different consequences for the central auditory system. We hypothesized that AVCN neuronal death was induced by excitotoxic mechanisms via AMPA-type glutamate receptors and that excitatory neuronal circuits developed after acoustic overstimulation protected the PVCN and DCN against neuronal death. The results of the present study demonstrate that hearing loss from different etiologies will cause different patterns of neuronal degeneration in the CN. These findings are important for enhancing the performance of cochlear implants and auditory brainstem implants, because diverse types of hearing loss can selectively affect neuronal degeneration of the CN.

Responses of Auditory Nerve and Anteroventral Cochlear Nucleus Fibers to Broadband and Narrowband Noise: Implications for the Sensitivity to Interaural Delays

The quality of temporal coding of sound waveforms in the monaural afferents that converge on binaural neurons in the brainstem limits the sensitivity to temporal differences at the two ears. The anteroventral cochlear nucleus (AVCN) houses the cells that project to the binaural nuclei, which are known to have enhanced temporal coding of low-frequency sounds relative to auditory nerve (AN) fibers. We applied a coincidence analysis within the framework of detection theory to investigate the extent to which AVCN processing affects interaural time delay (ITD) sensitivity. Using monaural spike trains to a 1-s broadband or narrowband noise token, we emulated the binaural task of ITD discrimination and calculated just noticeable differences (jnds). The ITD jnds derived from AVCN neurons were lower than those derived from AN fibers, showing that the enhanced temporal coding in the AVCN improves binaural sensitivity to ITDs. AVCN processing also increased the dynamic range of ITD sensitivity and changed the shape of the frequency dependence of ITD sensitivity. Bandwidth dependence of ITD jnds from AN as well as AVCN fibers agreed with psychophysical data. These findings demonstrate that monaural preprocessing in the AVCN improves the temporal code in a way that is beneficial for binaural processing and may be crucial in achieving the exquisite sensitivity to ITDs observed in binaural pathways.

KCNQ5 reaches synaptic endings in the auditory brainstem at hearing onset and targeting maintenance is activity‐dependent

Kv7.5/KCNQ5, a voltage-dependent potassium channel that generates a subthreshold K+ current (also called M-current), is localized in excitatory endings of auditory brainstem nuclei in the adult rat. Here, we focus on how specific targeting develops from birth to adulthood in the rat. We first analyzed by immunocytochemistry the distribution of KCNQ5 during postnatal development of neurons in the anteroventral cochlear nucleus (AVCN) and their targets in the medial nucleus of the trapezoid body (MNTB). From postnatal days (P) 0 to 12, KCNQ5 immunoreactivity was restricted to cell bodies, whereas from P13 onward a shift in labeling pattern was seen, with KCNQ5 immunoreactivity becoming confined to synaptic endings in both the AVCN and MNTB. The developmental synaptic targeting was also accompanied by a downregulation of KCNQ5 transcripts in the cochlear nucleus from P13 onward, as seen with quantitative reverse transcriptase polymerase chain reaction. We further tested whether auditory nerve activity at hearing onset (approximately P12) regulates synaptic targeting of the channel. Cochleae were removed at P10, before hearing onset. In the MNTB, 3 days after cochlear ablation, at P13, KCNQ5 immunoreactivity was seen in calyces of Held, as in normal age-matched controls. However, immunolabeling virtually disappeared from MNTB calyces 40 days after cochlear ablation but reappeared in the somata of neurons in AVCN. These findings suggest that synaptic targeting of KCNQ5 in brainstem auditory neurons occurs around the time of hearing onset, regardless of auditory nerve activity. However, long-term synaptic localization after hearing onset depends on peripheral input.

Effects of a high-frequency augmented acoustic environment on parvalbumin immunolabeling in the anteroventral cochlear nucleus of DBA/2J and C57BL/6J mice

Neurons in the anteroventral cochlear nucleus (AVCN) of DBA/2J (D2) and C57BL/6J (B6) mice were immunohistochemically labeled for the calcium binding protein parvalbumin (PV). Prior to this, mice were treated for 12 h nightly with a “high-frequency” augmented acoustic environment (HAAE: repetitive bursts of a 70 dB sound pressure level, half-octave noise band centered at 20 kHz). This was done during the period that hearing loss occurs: pre-weaning to 55 days in D2 mice and weaning to 9 months in B6 mice. After HAAE treatment in D2 mice, high-frequency hearing loss was ameliorated and fewer PV-labeled neurons were found in the AVCN compared to untreated controls. HAAE treatment in B6 mice exacerbated high-frequency hearing loss, yet the number of PV-labeled AVCN neurons in treated mice did not differ significantly from that of control mice. The findings suggest that HAAE treatment provides relief from physiological stress caused by deprivation of auditory input from the impaired cochlea.

Deafferentation-Induced Activation of NFAT (Nuclear Factor of Activated T-Cells) in Cochlear Nucleus Neurons during a Developmental Critical Period: A Role for NFATc4-Dependent Apoptosis in the CNS

During the development and maturation of sensory neurons, afferent activity is required for normal maintenance. There exists a developmental window of time when auditory neurons, including neurons of the anteroventral cochlear nucleus (AVCN), depend on afferent input for survival. This period of time is often referred to as a critical period. The cellular and molecular mechanisms that underlie AVCN neuron susceptibility to deafferentation-induced death remain unknown. Here, we show that only during this critical period deafferentation of mouse AVCN neurons by in vivo cochlea removal results in rapid nuclear translocation and activation of the transcription factor NFATc4 (nuclear factor of activated T-cells isoform 4). NFAT activation is abolished by in vivo treatment with the calcineurin inhibitor FK506 and the specific NFAT-inhibitor 11R-VIVIT. Inhibition of NFAT significantly attenuates deafferentation-induced apoptosis of AVCN neurons and abolishes NFAT-mediated expression of FasL, an initiator of apoptotic pathways, in the cochlear nucleus. These data suggest that NFAT-mediated gene expression plays a role in deafferentation-induced apoptosis of cochlear nucleus neurons during a developmental critical period.

Influence of Inhibitory Inputs on Rate and Timing of Responses in the Anteroventral Cochlear Nucleus

Anatomical and physiological studies have shown that anteroventral cochlear nucleus (AVCN) neurons receive glycinergic and GABAergic inhibitory inputs. In this study, changes in the temporal responses of AVCN neurons to pure tones and complex sounds after blocking inhibition were analyzed. Blocking inhibition influenced the temporal responses of each type of AVCN neuron. Choppers showed more chopping peaks and shortened chopping cycles after blocking inhibition. Sustained and slowly adapting choppers showed increased regularity throughout the response duration after blocking inhibition, whereas most transient choppers showed increased regularity in the early part of the response. Diverse changes in temporal response patterns were observed in neurons with primary-like and unusual responses, with several neurons showing a large decrease in the first-spike latency after blocking inhibition. This result disagreed with previous findings that onset responses are less affected than sustained responses by manipulating inhibition. Although blocking inhibition had a greater effect on spontaneous activity than that on tone-evoked activity, the change in spontaneous activity was less significant because of larger variability. In addition, for relatively high level masker noises, blocking inhibition had similar effects on responses to noise-alone and noise-plus-tone stimuli, in contrast with previous studies with low-level background noise. In general, inhibition had an enhancing effect on temporal contrast only for responses to amplitude-modulated tones, for which envelope synchrony was enhanced. Results of this study contribute new information about the characteristics, functional roles, and possible sources of inhibitory inputs received by AVCN neurons.

Effects of exposing C57BL/6J mice to high- and low-frequency augmented acoustic environments: Auditory brainstem response thresholds, cytocochleograms, anterior cochlear nucleus morphology and the role of gonadal hormones

Gonadectomized and intact adult C57BL/6J (B6) mice of both sexes were exposed for 12 h nightly to an augmented acoustic environment (AAE): repetitive bursts of a 70 dB SPL noise band. The high-frequency AAE (HAAE) was a half-octave band centered at 20 kHz; the low-frequency AAE (LAAE) was a 2–8 kHz band. The effects of sex, gonadectomy, and AAE treatment on genetic progressive hearing loss (a trait of B6 mice) were evaluated by obtaining auditory brainstem response (ABR) thresholds at ages 3-, 6-, and 9-months. At 9-months of age, hair cell counts (cytocochleograms) were obtained, and morphometric measures of the anteroventral cochlear nucleus (AVCN) were obtained. LAAE treatment caused elevation in ABR thresholds (8–24 kHz), with the highest thresholds occurring in intact females. LAAE treatment caused some loss of outer hair cells in the basal half of the cochlea (in addition to losses normally occurring in B6 mice), with intact females losing more cells than intact males. The loss of AVCN neurons and shrinkage of tissue volume that typically occur in 9-month-old B6 mice was lessened by LAAE treatment in intact (but not gonadectomized) male mice, whereas the degenerative changes were exacerbated in intact (but not gonadectomized) females. These LAAE effects were prominent in, but not restricted to, the tonotopic low-frequency (ventral) AVCN. HAAE treatment resulted in some loss of neurons in the high-frequency (dorsal) AVCN. In general, LAAE treatment plus male gonadal hormones (intact males) had an ameliorative effect whereas HAAE or LAAE treatment plus ovarian hormones (intact females) had a negative effect on age-related changes in the B6 auditory system.

Development of Spontaneous Miniature EPSCs in Mouse AVCN Neurons During a Critical Period of Afferent-Dependent Neuron Survival

During a critical period prior to hearing onset, cochlea ablation leads to massive neuronal death in the mouse anteroventral cochlear nucleus (AVCN), where cell survival is believed to depend on glutamatergic input. We investigated the development of spontaneous miniature excitatory postsynaptic currents (mEPSCs) in AVCN neurons using whole cell patch-clamp techniques during [postnatal day 7 (P7)] and after (P14, P21) this critical period. We also examined the effects of unilateral cochlea ablation on mEPSC development. The two main AVCN neuron types, bushy and stellate cells, were distinguished electrophysiologically. Bushy cell mEPSCs became more frequent and faster between P7 and P14/P21 but with little change in amplitude. Dendritic filtering of mEPSCs was not detected as indicated by the lack of correlation between 10 and 90% rise times and decay time constants. Seven days after cochlea ablation at P7 or P14, mEPSCs in surviving bushy cells were similar to controls, except that rise and decay times were positively correlated (R = 0.31 and 0.14 for surgery at P7 and P14, respectively). Consistent with this evidence for a shift of synaptic activity from the somata to the dendrites, SV2 staining (a synaptic vesicle marker) forms a ring around somata of control but not experimental bushy cells. In contrast, mEPSCs of stellate cells showed few significant changes over these ages with or without cochlea ablation. Taken together, mEPSCs in mouse AVCN bushy cells show dramatic developmental changes across this critical period, and cochlea ablation may lead to the emergence of excitatory synaptic inputs impinging on bushy cell dendrites.

Hyperpolarization‐activated currents are differentially expressed in mice brainstem auditory nuclei

The hyperpolarization-activated cation current (I(h)) may influence precise auditory processing by modulating resting membrane potential and cell excitability. We used electrophysiology and immunohistochemistry to investigate the properties of I(h) in three auditory brainstem nuclei in mice: the anteroventral cochlear nucleus (AVCN), the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO). I(h) amplitude varied considerably between these cell types, with the order of magnitude LSO > AVCN > MNTB. Kinetically, I(h) is faster in LSO neurons, and more active at rest, compared with AVCN and MNTB cells. The half-activation voltage is -10 mV more hyperpolarized for AVCN and MNTB cells compared with LSO neurons. HCN1 immunoreactivity strongly labelled AVCN and LSO neurons, while HCN2 staining was more diffuse in all nuclei. The HCN4 subunit displayed robust membrane staining in AVCN and MNTB cells but weak labelling of the LSO. We used a dynamic clamp, after blocking I(h), to reinsert I(h) to the different cell types. Our results indicate that the native I(h) for each cell type influences the resting membrane potential and can delay the generation of action potentials in response to injected current. Native I(h) increases rebound depolarizations following hyperpolarizations in all cell types, and increases the likelihood of rebound action potentials (particularly in multiple-firing LSO neurons). This systematic comparison shows that I(h) characteristics vary considerably between different brainstem nuclei, and that these differences significantly affect the response properties of cells within these nuclei.

Temporal Measures and Neural Strategies for Detection of Tones in Noise Based on Responses in Anteroventral Cochlear Nucleus

To examine possible neural strategies for the detection of tones in broadband noise, single-neuron extracellular recordings were obtained from the anteroventral cochlear nucleus (AVCN) in anesthetized gerbils. Detection thresholds determined by average discharge rate and several temporal metrics were compared with previously reported psychophysical detection thresholds in cats (Costalupes 1985). Because of their limited dynamic range, the average discharge rates of single neurons failed to predict psychophysical detection thresholds for relatively high-level noise at all measured characteristic frequencies (CFs). However, temporal responses changed significantly when a tone was added to a noise, even for neurons with flat masked rate-level functions. Three specific temporal analyses were applied to neural responses to tones in noise. First, temporal reliability, a measure of discharge time consistency across stimulus repetitions, decreased with increasing tone level for most AVCN neurons at all measured CFs. Second, synchronization to the tone frequency, a measure of phase-locking to the tone, increased with tone level for low-CF neurons. Third, rapid fluctuations in the poststimulus time histograms (PSTHs) decreased with tone level for a number of neurons at all CFs. For each of the three temporal measures, some neurons had detection thresholds at or below psychophysical thresholds. A physiological model of a higher-stage auditory neuron that received simple excitatory and inhibitory inputs from AVCN neurons was able to extract the PSTH fluctuation information in a form of decreased rate with tone level.

Hyperpolarization‐activated (Ih) currents in auditory brainstem neurons of normal and congenitally deaf mice

We have investigated the membrane properties of brainstem auditory neurons in a mouse model of congenital deafness (dn/dn). Whole-cell recordings were made from visualized neurons in slices of the medial nucleus of the trapezoid body (MNTB) and anteroventral cochlear nucleus (AVCN). We have recently demonstrated that MNTB neurons in deaf mice are more excitable than in normal mice, due in part to a reduced expression of low-threshold potassium currents. In this study, we have examined the contribution of hyperpolarization-activated (Ih) channels to the membrane properties of MNTB and AVCN neurons. Our results show that Ih is larger in MNTB neurons from deaf mice than in normal mice. In contrast, no significant differences were found in Ih or excitability between AVCN bushy cells from dn/dn and normal mice. Experimental evidence and neuronal modelling suggests that, in the MNTB of normal mice, a small contribution of Ih helps to reduce temporal summation of synaptic potentials. A larger Ih in neurons from deaf mice has a much greater effect in reducing temporal summation of synaptic potentials, counteracting to some extent the greater excitability of these cells. Our results provide further insight into the role of activity during development in regulating the membrane and firing properties of central neurons.

Ameliorative Effects of Exposing DBA/2J Mice to an Augmented Acoustic Environment on Histological Changes in the Cochlea and Anteroventral Cochlear Nucleus

DBA/2J (D2) mice, which exhibit very early progressive sensorineural hearing loss, were treated nightly with an augmented acoustic environment (AAE) initiated before the onset of hearing, and consisting of repetitive bursts of a 70-dB sound pressure level (SPL), 4-25 kHz noise band. At 55 days of age, AAE-treated mice exhibited less elevation of auditory brainstem response thresholds, fewer missing hair cells, and greatly reduced loss of anteroventral cochlear nucleus (AVCN) volume and neuron number compared to untreated control mice. It was hypothesized that the central neuroprotective effect was associated with increased afferent input to AVCN neurons evoked by the AAE as well as a healthier cochlea.

Cochlear ablation in mice lacking SHP-1 results in an extended period of cell death of anteroventral cochlear nucleus neurons

Cochlear ablation results in the death of anteroventral cochlear nucleus (AVCN) neurons from birth to approximately postnatal day 14 (P14) in the murine brainstem. It is not known whether microglial activation contributes to AVCN neuronal death following deafferentation. In order to determine whether microglial activation helps to define the period of neuronal susceptibility within AVCN, we performed unilateral cochlear ablation on mice lacking the protein tyrosine phosphatase SHP-1 ( me/ me). These mice have been shown to have an exaggerated microglial response following ischemic injury. In the present study, the glial and neuronal response to deafferentation within AVCN was examined in wild-type and me/ me mice at P5, P14, and P21. Lack of SHP-1 results in robust microglial but not astrocyte activation within the ablated P14 me/ me AVCN. These mice also exhibit approximately 28% neuronal death at P14, a time when normal wild-type littermate controls show little cell death. Glial activation and neuronal loss at P5 and P21 were similar between the two phenotypes, suggesting a role of activated microglia in inducing neuronal death beyond P14 but not P21. These results indicate that activated microglia may participate in determining whether neurons in AVCN live or die following deafferentation.

Glycinergic mIPSCs in mouse and rat brainstem auditory nuclei: modulation by ruthenium red and the role of calcium stores.

Spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in central neurons are usually highly variable in amplitude due to many factors such as intrinsic postsynaptic channel fluctuations at each release site, site-to-site variability between release sites, electrotonic attenuation due to variable dendritic locations of synapses, and the possibility of synchronous multivesicular release. A detailed knowledge of these factors is essential for the interpretation of mIPSC amplitude distributions and mean quantal size. We have studied glycinergic mIPSCs in two auditory brainstem nuclei, the rat anteroventral cochlear nucleus (AVCN) and the mouse medial nucleus of the trapezoid body (MNTB). Our previous results have demonstrated the location of glycinergic synapses on these neurons to be somatic, thus avoiding electrotonic complications. Spontaneous glycinergic mIPSCs were recorded from AVCN and MNTB neurons in brainstem slices, in the presence of TTX to block action potentials, and 6-cyano-7-nitroquinoxaline-2, 3-dione, (+/-)-2-amino-5-phosphonopentanoic acid and bicuculline to block glutamatergic and GABAergic synaptic currents. Ruthenium red (RuR), which was used to increase the frequency of mIPSCs, significantly changed the shape of most (90 %) mIPSC amplitude distributions by increasing the proportion of large-amplitude mIPSCs. The possibility was investigated (following previous evidence at GABAergic synapses) that large-amplitude glycinergic mIPSCs are due to synchronous multivesicular release initiated by presynaptic calcium sparks from ryanodine-sensitive calcium stores. Interval analysis of mIPSCs indicated that the number of potentially undetected (asynchrony < 0.5 ms) multivesicular mIPSCs was low in comparison with the number of large-amplitude mIPSCs. Ryanodine, thapsigargin and calcium-free perfusate did not reduce the frequency of large-amplitude mIPSCs (> 150 pA), arguing against a significant role for presynaptic calcium stores. Our results support previous evidence suggesting that RuR increases miniature postsynaptic current (mSC) frequency by a mechanism that does not involve presynaptic calcium stores. Our results also indicate that at glycinergic synapses in the AVCN and MNTB, site-to-site variability in mIPSC amplitude, rather than multivesicular release, is a major factor underlying the large range of amplitudes of glycinergic mIPSCs.

Interaction of Excitation and Inhibition in Anteroventral Cochlear Nucleus Neurons That Receive Large Endbulb Synaptic Endings

Spherical bushy cells (SBCs) of the anteroventral cochlear nucleus (AVCN) receive their main excitatory input from auditory nerve fibers (ANFs) through large synapses, endbulbs of Held. These cells are also the target of inhibitory inputs whose function is not well understood. The present study examines the role of inhibition in the encoding of low-frequency sounds in the gerbil's AVCN. The presynaptic action potentials of endbulb terminals and postsynaptic action potentials of SBCs were monitored simultaneously in extracellular single-unit recordings in vivo. An input-output analysis of presynaptic and postsynaptic activity was performed for both spontaneous and acoustically driven activity. Two-tone stimulation and neuropharmacological experiments allowed the effects of neuronal inhibition and cochlear suppression on SBC activity to be distinguished. Ninety-one percent of SBCs showed significant neuronal inhibition. Inhibitory sidebands enclosed the high- or low-frequency, or both, sides of the excitatory areas of these units; this was reflected as a presynaptic to postsynaptic increase in frequency selectivity of up to one octave. Inhibition also affected the level-dependent responses at the characteristic frequency. Although in all units the presynaptic recordings showed monotonic rate-level functions, this was the case in only half of the postsynaptic recordings. In the other half of SBCs, postsynaptic inhibitory areas overlapped the excitatory areas, resulting in nonmonotonic rate-level functions. The results demonstrate that the sound-evoked spike activity of SBCs reflects the integration of acoustically driven excitatory and inhibitory input. The inhibition specifically affects the processing of the spectral, temporal, and intensity cues of acoustic signals.

Bcl-2 Overexpression eliminates deprivation-induced cell death of brainstem auditory neurons.

Deprivation of afferent input in young animals results in transneuronal degeneration of postsynaptic sensory neurons in a variety of species and sensory pathways. Transneuronal degeneration is generally not seen in adult animals. The cellular and molecular basis for this dramatic developmental change in susceptibility is not understood. One possibility is that genes involved in the apoptotic process are involved in determining cell death or survival after afferent deprivation. To further investigate this possibility, we performed unilateral cochlear ablation on wild-type and bcl-2-overexpressing mice at a variety of ages. In postnatal day 5 (P5) or P8 wild-type mice, cochlea removal resulted in a 54% or 31% neuronal loss in the anteroventral cochlear nucleus (AVCN), respectively. When the same manipulation is performed on a P30 mouse, no loss of AVCN neurons occurs. This confirmed a rather abrupt change in the sensitivity to disruption of afferent input, a critical period. However, in littermates expressing bcl-2 under a neuron-specific enolase promoter, no significant loss of AVCN neurons was observed at any age after unilateral cochlear ablation. Furthermore, wild-type mice demonstrate rapid expression of activated caspase-3 in AVCN neurons within hours of deafferentation, whereas bcl-2-overexpressing mice do not. This suggests that bcl-2 can influence cell survival after removal of afferent input during the critical period and is consistent with the hypothesis that caspase-3 is one effector of cell death under these circumstances. These data are the first to indicate that known apoptotic mediators can play a role in central neuronal plasticity in models of afferent deprivation.