Differential distribution of synaptic endings containing glutamate, glycine, and GABA in the rat dorsal cochlear nucleus

Glycine is a transmitter in the human and chimpanzee cochlear nuclei

Introduction

Auditory information is relayed from the cochlea via the eighth cranial nerve to the dorsal and ventral cochlear nuclei (DCN, VCN). The organization, neurochemistry and circuitry of the cochlear nuclei (CN) have been studied in many species. It is well-established that glycine is an inhibitory transmitter in the CN of rodents and cats, with glycinergic cells in the DCN and VCN. There are, however, major differences in the laminar and cellular organization of the DCN between humans (and other primates) and rodents and cats. We therefore asked whether there might also be differences in glycinergic neurotransmission in the CN.

Methods

We studied brainstem sections from humans, chimpanzees, and cats. We used antibodies to glycine receptors (GLYR) to identify neurons receiving glycinergic input, and antibodies to the neuronal glycine transporter (GLYT2) to immunolabel glycinergic axons and terminals. We also examined archival sections immunostained for calretinin (CR) and nonphosphorylated neurofilament protein (NPNFP) to try to locate the octopus cell area (OCA), a region in the VCN that rodents has minimal glycinergic input.

Results

In humans and chimpanzees we found widespread immunolabel for glycine receptors in DCN and in the posterior (PVCN) and anterior (AVCN) divisions of the VCN. We found a parallel distribution of GLYT2-immunolabeled fibers and puncta. The data also suggest that, as in rodents, a region containing octopus cells in cats, humans and chimpanzees has little glycinergic input.

Discussion

Our results show that glycine is a major transmitter in the human and chimpanzee CN, despite the species differences in DCN organization. The sources of the glycinergic input to the CN in humans and chimpanzees are not known.

The interaction between abamectin and RDL in the carmine spider mite: a target site and resistant mechanism study

The γ-aminobutyric acid receptor (GABAR) has been identified as a target site of some commonly used insecticides. Our pervious study documented an interesting phenomenon, i.e. GABA accumulation was involved in abamectin resistance in the carmine spider mite, Tetranychus cinnabarinus. However, the mechanism of this phenomenon remains to be clarified. In this study, we investigated the interaction between abamectin and GABAR. Firstly, an artificial increase of GABA content was conducted in T. cinnabarinus and toxicity assays showed that GABA accumulation could indeed increase the tolerance of T. cinnabarinus to abamectin in vivo. Subsequently a GABAR of T. cinnabarinus, RDL2, was expressed in Xenopus oocytes and its sensitiveness to abamectin was detected. The results revealed that RDL2 showed significant responses to a series of GABA concentrations and GABA inhibited the effect of abamectin in vitro, providing direct evidence of the abamectin resistance mediated by GABA content. Our data confirmed that GABAR is the action target of abamectin and the GABA accumulation is one of the mechanisms of abamectin resistance in spider mites.

Ultrastructural and molecular features of excitatory and glutamatergic synapses. The auditory nerve synapses

Glutamatergic synapses mediate fast synaptic transmission in the central nervous system. New developments highlight the importance of the synapse structural and molecular remodeling during development, aging and in neurological disorders. This chapter summarizes key structural and molecular aspects of the presynaptic and postsynaptic components of glutamatergic synapses in the brain. In addition, this chapter describes how the structure of the postsynaptic density and ionotropic glutamate content contribute to the function of auditory nerve synapses in the lower auditory brainstem.

The Role of the Brainstem in Generating and Modulating Tinnitus

Purpose The purpose of this work is to present a perspective article summarizing ideas pertaining to the brainstem's role in generating and modulating tinnitus. It is organized in 4 sections: Part 1, the role of the brainstem as a tinnitus generator; Part 2, the role of the brainstem in modulating tinnitus; Part 3, the role of the brainstem in nonauditory comorbid conditions associated with tinnitus; and Part 4, clinical implications. In Part 1, well-established neurophysiological models are discussed providing the framework of evidence that auditory brainstem nuclei play a role in generating tinnitus. In Part 2, ideas are presented explaining modulatory effects on tinnitus related to underlying pathways originating from or projecting to brainstem auditory and nonauditory nuclei. This section addresses multiple phenomena including somatic-related, attention-mediated, and emotion-mediated changes in the tinnitus percept. In Part 3, the role of the brainstem in common nonauditory comorbidities that occur in patients with tinnitus is discussed. Part 4 presents clinical implications of these new ideas related to the brainstem's involvement in generating and modulating tinnitus. Impact Knowledge of the brainstem's involvement in generating and modulating tinnitus provides a context for health care professionals to understand the temporal relationship between tinnitus and common nonauditory comorbid conditions.

Corelease of Inhibitory Neurotransmitters in the Mouse Auditory Midbrain

The central nucleus of the inferior colliculus (ICC) of the auditory midbrain, which integrates most ascending auditory information from lower brainstem regions, receives prominent long-range inhibitory input from the ventral nucleus of the lateral lemniscus (VNLL), a region thought to be important for temporal pattern discrimination. Histological evidence suggests that neurons in the VNLL release both glycine and GABA in the ICC, but functional evidence for their corelease is lacking. We took advantage of the GlyT2-Cre mouse line (both male and female) to target expression of ChR2 to glycinergic afferents in the ICC and made whole-cell recordings in vitro while exciting glycinergic fibers with light. Using this approach, it was clear that a significant fraction of glycinergic boutons corelease GABA in the ICC. Viral injections were used to target ChR2 expression specifically to glycinergic fibers ascending from the VNLL, allowing for activation of fibers from a single source of ascending input in a way that has not been previously possible in the ICC. We then investigated aspects of the glycinergic versus GABAergic current components to probe functional consequences of corelease. Surprisingly, the time course and short-term plasticity of synaptic signaling were nearly identical for the two transmitters. We therefore conclude that the two neurotransmitters may be functionally interchangeable and that multiple receptor subtypes subserving inhibition may offer diverse mechanisms for maintaining inhibitory homeostasis.SIGNIFICANCE STATEMENT Corelease of neurotransmitters is a common feature of the brain. GABA and glycine corelease is particularly common in the spinal cord and brainstem, but its presence in the midbrain is unknown. We show corelease of GABA and glycine for the first time in the central nucleus of the inferior colliculus of the auditory midbrain. Glycine and GABA are both inhibitory neurotransmitters involved in fast synaptic transmission, so we explored differences between the currents to establish a physiological foundation for functional differences in vivo In contrast to the auditory brainstem, coreleased GABAergic and glycinergic currents in the midbrain are strikingly similar. This apparent redundancy may ensure homeostasis if one neurotransmitter system is compromised.

The number and distribution of AMPA receptor channels containing fast kinetic GluA3 and GluA4 subunits at auditory nerve synapses depend on the target cells

The neurotransmitter receptor subtype, number, density, and distribution relative to the location of transmitter release sites are key determinants of signal transmission. AMPA-type ionotropic glutamate receptors (AMPARs) containing GluA3 and GluA4 subunits are prominently expressed in subsets of neurons capable of firing action potentials at high frequencies, such as auditory relay neurons. The auditory nerve (AN) forms glutamatergic synapses on two types of relay neurons, bushy cells (BCs) and fusiform cells (FCs) of the cochlear nucleus. AN-BC and AN-FC synapses have distinct kinetics; thus, we investigated whether the number, density, and localization of GluA3 and GluA4 subunits in these synapses are differentially organized using quantitative freeze-fracture replica immunogold labeling. We identify a positive correlation between the number of AMPARs and the size of AN-BC and AN-FC synapses. Both types of AN synapses have similar numbers of AMPARs; however, the AN-BC have a higher density of AMPARs than AN-FC synapses, because the AN-BC synapses are smaller. A higher number and density of GluA3 subunits are observed at AN-BC synapses, whereas a higher number and density of GluA4 subunits are observed at AN-FC synapses. The intrasynaptic distribution of immunogold labeling revealed that AMPAR subunits, particularly GluA3, are concentrated at the center of the AN-BC synapses. The central distribution of AMPARs is absent in GluA3-knockout mice, and gold particles are evenly distributed along the postsynaptic density. GluA4 gold labeling was homogenously distributed along both synapse types. Thus, GluA3 and GluA4 subunits are distributed at AN synapses in a target-cell-dependent manner.

Spontaneous Activity Defines Effective Convergence Ratios in an Inhibitory Circuit

Many neurons fire spontaneously, and the rate of this firing is subject to neuromodulation. How this firing affects functional connectivity within a neural network remains largely unexplored. Here we show that changes in spontaneous firing of cartwheel interneurons in the mouse dorsal cochlear nucleus (DCN) alter the effective convergence ratio of interneurons onto their postsynaptic targets through short-term synaptic plasticity. Spontaneous firing of cartwheel cells led to activity-dependent synaptic depression of individual cartwheel synapses. Depression was rapid and profound at stimulation frequencies between 10 and 200 Hz, suggesting the presence of high release probability (Pr) vesicles at these inhibitory synapses. Weak, transient synaptic facilitation could be induced after synapses were predepressed, indicating that low-Pr vesicles are also recruited, and may thus support steady-state transmission. A two-pool vesicle depletion model with 10-fold differences in Pr could account for the synaptic depression over a wide range of stimulus conditions. As a result of depression during high spontaneous activity, more cartwheel interneurons were required for effective inhibition. Convergence of four interneurons was sufficient to compensate for the effects of depression during physiologically expected rates of activity. By simulating synaptic release during spontaneous firing, we found that recruitment of low-Pr vesicles at the synapse plays a critical role in maintaining effective inhibition within a small population of interneurons. The interplay between spontaneous spiking, short-term synaptic plasticity, and vesicle recruitment thus determines the effective size of a convergent neural network.

SIGNIFICANCE STATEMENT

We examined the relationship between the structure of a small neural circuit and the properties of its individual synapses. Successful synaptic inhibition of a target cell firing requires a critical inhibitory synaptic strength. Synapses often become depressed during spontaneous presynaptic activity, and this increases the number of presynaptic neurons needed to mediate inhibition. We show that depression is limited by the presence of a pool of vesicles that resist depletion. Thus, the size of this vesicle pool determines the size of the circuit needed to mediate inhibition during different patterns of activity.

Cochlear ablation effects on amino acid levels in the chinchilla cochlear nucleus

Inner ear damage can lead to hearing disorders, including tinnitus, hyperacusis, and hearing loss. We measured the effects of severe inner ear damage, produced by cochlear ablation, on the levels and distributions of amino acids in the first brain center of the auditory system, the cochlear nucleus. Measurements were also made for its projection pathways and the superior olivary nuclei. Cochlear ablation produces complete degeneration of the auditory nerve, which provides a baseline for interpreting the effects of partial damage to the inner ear, such as that from ototoxic drugs or intense sound. Amino acids play a critical role in neural function, including neurotransmission, neuromodulation, cellular metabolism, and protein construction. They include major neurotransmitters of the brain - glutamate, glycine, and γ-aminobutyrate (GABA) - as well as others closely related to their metabolism and/or functions - aspartate, glutamine, and taurine. Since the effects of inner ear damage develop over time, we measured the changes in amino acid levels at various survival times after cochlear ablation. Glutamate and aspartate levels decreased by 2weeks in the ipsilateral ventral cochlear nucleus and deep layer of the dorsal cochlear nucleus, with the largest decreases in the posteroventral cochlear nucleus (PVCN): 66% for glutamate and 63% for aspartate. Aspartate levels also decreased in the lateral part of the ipsilateral trapezoid body, by as much as 50%, suggesting a transneuronal effect. GABA and glycine levels showed some bilateral decreases, especially in the PVCN. These results may represent the state of amino acid metabolism in the cochlear nucleus of humans after removal of eighth nerve tumors, which may adversely result in destruction of the auditory nerve. Measurement of chemical changes following inner ear damage may increase understanding of the pathogenesis of hearing impairments and enable improvements in their diagnosis and treatment.

Mapping and morphometric analysis of synapses and spines on fusiform cells in the dorsal cochlear nucleus

Fusiform cells are the main integrative units of the mammalian dorsal cochlear nucleus (DCN), collecting and processing inputs from auditory and other sources before transmitting information to higher levels of the auditory system. Despite much previous work describing these cells and the sources and pharmacological identity of their synaptic inputs, information on the three-dimensional organization and utltrastructure of synapses on these cells is currently very limited. This information is essential since an understanding of synaptic plasticity and remodeling and pathologies underlying disease states and hearing disorders must begin with knowledge of the normal characteristics of synapses on these cells, particularly those features that determine the strength of their influence on the various compartments of the cell. Here, we employed serial block face scanning electron microscopy (SBFSEM) followed by 3D reconstructions to map and quantitatively characterize synaptic features on DCN fusiform cells. Our results reveal a relative sparseness of synapses on the somata of fusiform cells but a dense distribution of synapses on apical and basal dendrites. Synapses on apical dendrites were smaller and more numerous than on basal dendrites. The vast majority of axosomatic terminals were found to be linked to other terminals connected by the same axon or different branches of the same axon, suggesting a high degree of divergent input to fusiform cells. The size of terminals was correlated with the number of mitochondria and with the number of active zones, which was highly correlated with the number of postsynaptic densities, suggesting that larger terminals exert more powerful influence on the cell than smaller terminals. These size differences suggest that the input to basal dendrites, most likely those from the auditory nerve, provide the most powerful sources of input to fusiform cells, while those to apical dendrites (e.g., parallel fiber) are weaker but more numerous.

Tonotopic organization of vertical cells in the dorsal cochlear nucleus of the CBA/J mouse

The systematic and topographic representation of frequency is a first principle of organization throughout the auditory system. The dorsal cochlear nucleus (DCN) receives direct tonotopic projections from the auditory nerve (AN) as well as secondary and descending projections from other sources. Among the recipients of AN input in the DCN are vertical cells (also called tuberculoventral cells), glycinergic interneurons thought to provide on- or near-best-frequency feed-forward inhibition to principal cells in the DCN and various cells in the anteroventral cochlear nucleus (AVCN). Differing lines of physiological and anatomical evidence suggest that vertical cells and their projections are organized with respect to frequency, but this has not been conclusively demonstrated in the intact mammalian brain. To address this issue, we retrogradely labeled vertical cells via physiologically targeted injections in the AVCN of the CBA/J mouse. Results from multiple cases were merged with a normalized 3D template of the cochlear nucleus (Muniak et al. [2013] J. Comp. Neurol. 521:1510-1532) to demonstrate quantitatively that the arrangement of vertical cells is tonotopic and aligned to the innervation pattern of the AN. These results suggest that vertical cells are well positioned for providing immediate, frequency-specific inhibition onto cells of the DCN and AVCN to facilitate spectral processing.

Target- and input-dependent organization of AMPA and NMDA receptors in synaptic connections of the cochlear nucleus

We examined the synaptic structure, quantity, and distribution of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)- and N-methyl-D-aspartate (NMDA)-type glutamate receptors (AMPARs and NMDARs, respectively) in rat cochlear nuclei by a highly sensitive freeze-fracture replica labeling technique. Four excitatory synapses formed by two distinct inputs, auditory nerve (AN) and parallel fibers (PF), on different cell types were analyzed. These excitatory synapse types included AN synapses on bushy cells (AN-BC synapses) and fusiform cells (AN-FC synapses) and PF synapses on FC (PF-FC synapses) and cartwheel cell spines (PF-CwC synapses). Immunogold labeling revealed differences in synaptic structure as well as AMPAR and NMDAR number and/or density in both AN and PF synapses, indicating a target-dependent organization. The immunogold receptor labeling also identified differences in the synaptic organization of FCs based on AN or PF connections, indicating an input-dependent organization in FCs. Among the four excitatory synapse types, the AN-BC synapses were the smallest and had the most densely packed intramembrane particles (IMPs), whereas the PF-CwC synapses were the largest and had sparsely packed IMPs. All four synapse types showed positive correlations between the IMP-cluster area and the AMPAR number, indicating a common intrasynapse-type relationship for glutamatergic synapses. Immunogold particles for AMPARs were distributed over the entire area of individual AN synapses; PF synapses often showed synaptic areas devoid of labeling. The gold-labeling for NMDARs occurred in a mosaic fashion, with less positive correlations between the IMP-cluster area and the NMDAR number. Our observations reveal target- and input-dependent features in the structure, number, and organization of AMPARs and NMDARs in AN and PF synapses.

Origin and function of short-latency inputs to the neural substrates underlying the acoustic startle reflex

The acoustic startle reflex (ASR) is a survival mechanism of alarm, which rapidly alerts the organism to a sudden loud auditory stimulus. In rats, the primary ASR circuit encompasses three serially connected structures: cochlear root neurons (CRNs), neurons in the caudal pontine reticular nucleus (PnC), and motoneurons in the medulla and spinal cord. It is well-established that both CRNs and PnC neurons receive short-latency auditory inputs to mediate the ASR. Here, we investigated the anatomical origin and functional role of these inputs using a multidisciplinary approach that combines morphological, electrophysiological and behavioral techniques. Anterograde tracer injections into the cochlea suggest that CRNs somata and dendrites receive inputs depending, respectively, on their basal or apical cochlear origin. Confocal colocalization experiments demonstrated that these cochlear inputs are immunopositive for the vesicular glutamate transporter 1 (VGLUT1). Using extracellular recordings in vivo followed by subsequent tracer injections, we investigated the response of PnC neurons after contra-, ipsi-, and bilateral acoustic stimulation and identified the source of their auditory afferents. Our results showed that the binaural firing rate of PnC neurons was higher than the monaural, exhibiting higher spike discharges with contralateral than ipsilateral acoustic stimulations. Our histological analysis confirmed the CRNs as the principal source of short-latency acoustic inputs, and indicated that other areas of the cochlear nucleus complex are not likely to innervate PnC. Behaviorally, we observed a strong reduction of ASR amplitude in monaural earplugged rats that corresponds with the binaural summation process shown in our electrophysiological findings. Our study contributes to understand better the role of neuronal mechanisms in auditory alerting behaviors and provides strong evidence that the CRNs-PnC pathway mediates fast neurotransmission and binaural summation of the ASR.

Superficial stellate cells of the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) integrates auditory and multisensory signals at the earliest levels of auditory processing. Proposed roles for this region include sound localization in the vertical plane, head orientation to sounds of interest, and suppression of sensitivity to expected sounds. Auditory and non-auditory information streams to the DCN are refined by a remarkably complex array of inhibitory and excitatory interneurons, and the role of each cell type is gaining increasing attention. One inhibitory neuron that has been poorly appreciated to date is the superficial stellate cell. Here we review previous studies and describe new results that reveal the surprisingly rich interactions that this tiny interneuron has with its neighbors, interactions which enable it to respond to both multisensory and auditory afferents.

Chemical synaptic transmission onto superficial stellate cells of the mouse dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is a cerebellum-like auditory brain stem region whose functions include sound localization and multisensory integration. Although previous in vivo studies have shown that glycinergic and GABAergic inhibition regulate the activity of several DCN cell types in response to sensory stimuli, data regarding the synaptic inputs onto DCN inhibitory interneurons remain limited. Using acute DCN slices from mice, we examined the properties of excitatory and inhibitory synapses onto the superficial stellate cell, a poorly understood cell type that provides inhibition to DCN output neurons (fusiform cells) as well as to local inhibitory interneurons (cartwheel cells). Excitatory synapses onto stellate cells activated both NMDA receptors and fast-gating, Ca(2+)-permeable AMPA receptors. Inhibition onto superficial stellate cells was mediated by glycine and GABAA receptors with different temporal kinetics. Paired recordings revealed that superficial stellate cells make reciprocal synapses and autapses, with a connection probability of ∼ 18-20%. Unexpectedly, superficial stellate cells co-released both glycine and GABA, suggesting that co-transmission may play a role in fine-tuning the duration of inhibitory transmission.

Regulation of interneuron excitability by gap junction coupling with principal cells

Electrical coupling of inhibitory interneurons can synchronize activity across multiple neurons, thereby enhancing the reliability of inhibition onto principal cell targets. It is unclear whether downstream activity in principal cells controls the excitability of such inhibitory networks. Using paired patch-clamp recordings, we show that excitatory projection neurons (fusiform cells) and inhibitory stellate interneurons of the dorsal cochlear nucleus form an electrically coupled network through gap junctions containing connexin36 (Cxc36, also called Gjd2). Remarkably, stellate cells were more strongly coupled to fusiform cells than to other stellate cells. This heterologous coupling was functionally asymmetric, biasing electrical transmission from the principal cell to the interneuron. Optogenetically activated populations of fusiform cells reliably enhanced interneuron excitability and generated GABAergic inhibition onto the postsynaptic targets of stellate cells, whereas deep afterhyperpolarizations following fusiform cell spike trains potently inhibited stellate cells over several hundred milliseconds. Thus, the excitability of an interneuron network is bidirectionally controlled by distinct epochs of activity in principal cells.

Noise-induced hearing loss is correlated with alterations in the expression of GABAB receptors and PKC gamma in the murine cochlear nucleus complex

Noise overexposure may induce permanent noise-induced hearing loss (NIHL). The cochlear nucleus complex (CNC) is the entry point for sensory information in the central auditory system. Impairments in gamma-aminobutyric acid (GABA)—mediated synaptic transmission in the CNC have been implicated in the pathogenesis of auditory disorders. However, the role of protein kinase C (PKC) signaling pathway in GABAergic inhibition in the CNC in NIHL remains elusive. Thus, we investigated the alterations of glutamic acid decarboxylase 67 (GAD67, the chemical marker for GABA-containing neurons), PKC γ subunit (PKCγ) and GABA_B receptor (GABA_BR) expression in the CNC using transgenic GAD67-green fluorescent protein (GFP) knock-in mice, BALB/c mice and C57 mice. Immunohistochemical results indicate that the GFP-labeled GABAergic neurons were distributed in the molecular layer (ML) and fusiform cell layer (FCL) of the dorsal cochlear nucleus (DCN). We found that 69.91% of the GFP-positive neurons in the DCN were immunopositive for both PKCγ and GABA_BR1. The GAD67-positive terminals made contacts with PKCγ/GABA_BR1 colocalized neurons. Then we measured the changes of auditory thresholds in mice after noise exposure for 2 weeks, and detected the GAD67, PKCγ, and GABA_BR expression at mRNA and protein levels in the CNC. With noise over-exposure, there was a reduction in GABA_BR accompanied by an increase in PKCγ expression, but no significant change in GAD67 expression. In summary, our results demonstrate that alterations in the expression of PKCγ and GABA_BRs may be involved in impairments in GABAergic inhibition within the CNC and the development of NIHL.

Rapid, activity-independent turnover of vesicular transmitter content at a mixed glycine/GABA synapse

The release of neurotransmitter via the fusion of transmitter-filled, presynaptic vesicles is the primary means by which neurons relay information. However, little is known regarding the molecular mechanisms that supply neurotransmitter destined for vesicle filling, the endogenous transmitter concentrations inside presynaptic nerve terminals, or the dynamics of vesicle refilling after exocytosis. We addressed these issues by recording from synaptically coupled pairs of glycine/GABA coreleasing interneurons (cartwheel cells) of the mouse dorsal cochlear nucleus. We find that the plasma membrane transporter GlyT2 and the intracellular enzyme glutamate decarboxylase supply the majority of glycine and GABA, respectively. Pharmacological block of GlyT2 or glutamate decarboxylase led to rapid and complete rundown of transmission, whereas increasing GABA synthesis via intracellular glutamate uncaging dramatically potentiated GABA release within 1 min. These effects were surprisingly independent of exocytosis, indicating that prefilled vesicles re-equilibrated upon acute changes in cytosolic transmitter. Titration of cytosolic transmitter with postsynaptic responses indicated that endogenous, nonvesicular glycine/GABA levels in nerve terminals are 5-7 mm, and that vesicular transport mechanisms are not saturated under basal conditions. Thus, cytosolic transmitter levels dynamically set the strength of inhibitory synapses in a release-independent manner.

Ultrastructure of spines and associated terminals on brainstem neurons controlling auditory input

Spines are unique cellular appendages that isolate synaptic input to neurons and play a role in synaptic plasticity. Using the electron microscope, we studied spines and their associated synaptic terminals on three groups of brainstem neurons: tensor tympani motoneurons, stapedius motoneurons, and medial olivocochlear neurons, all of which exert reflexive control of processes in the auditory periphery. These spines are generally simple in shape; they are infrequent and found on the somata as well as the dendrites. Spines do not differ in volume among the three groups of neurons. In all cases, the spines are associated with a synaptic terminal that engulfs the spine rather than abuts its head. The positions of the synapses are variable, and some are found at a distance from the spine, suggesting that the isolation of synaptic input is of diminished importance for these spines. Each group of neurons receives three common types of synaptic terminals. The type of terminal associated with spines of the motoneurons contains pleomorphic vesicles, whereas the type associated with spines of olivocochlear neurons contains large round vesicles. Thus, spine-associated terminals in the motoneurons appear to be associated with inhibitory processes but in olivocochlear neurons they are associated with excitatory processes.

Reversible neurotoxicity of kanamycin on dorsal cochlear nucleus

The time course of aminoglycoside neurotoxic effect on cochlear nucleus is still obscure. We examined dynamic pathological changes of dorsal cochlear nucleus (DCN) and investigated whether apoptosis or autophagy was upregulated in the neurotoxic course of kanamycin on DCN after kanamycin treatment. Rats were treated with kanamycin sulfate/kg/day at a dose of 500mg by subcutaneous injection for 10 days. Dynamic pathological changes, neuron density and neuron apoptosis of the DCN were examined at 1, 7, 14, 28, 56, 70 and 140 days after kanamycin treatment. The expressions of JNK1, DAPK2, Bcl-2, p-Bcl-2, Caspase-3, LC3B and Beclin-1 were also detected. Under transmission electron microscopy, the mitochondrial swelling and focal vacuoles as well as endoplasmic reticulum dilation were progressively aggravated from 1 day to 14 days, and gradually recovered from 28 days to 140 days. Meanwhile, both autophagosomes and autolysosomes were increased from 1 day to 56 days. Only few neurons were positive to the TUNEL staining. Moreover, neither the expressions of caspase-3 and DAPK2 nor neurons density of DCN changed significantly. LC3-II was drastically increased at 7 days. Beclin-1 was upgraded at 1 and 7 days. P-Bcl-2 increased at 1, 7, 14 and 28 days. JNK1 increased at 7 days, and Bcl-2 was downgraded at 140 days. LC3-B positive neurons were increased at 1, 7 and 14 days. These data demonstrated that the neurons damage of the DCN caused by kanamycin was reversible and autophagy was upregulated in the neurotoxic course of kanamycin on DCN through JNK1-mediated phosphorylation of Bcl-2 pathway.

Tensor tympani motoneurons receive mostly excitatory synaptic inputs

The tensor tympani is a middle ear muscle that contracts in two different situations: in response to sound or during voluntary movements. To gain insight into the inputs and neural regulation of the tensor tympani, we examined the ultrastructure of synaptic terminals on labeled tensor tympani motoneurons (TTMNs) using transmission electron microscopy. Our sample of six TTMNs received 79 synaptic terminals that formed 126 synpases. Two types of synapses are associated with round vesicles and form asymmetric junctions (excitatory morphology). One of these types has vesicles that are large and round (Lg Rnd) and the other has vesicles that are smaller and round (Sm Rnd) and also contains at least one dense core vesicle. A third synapse type has inhibitory morphology because it forms symmetric synapses with pleomorphic vesicles (Pleo). These synaptic terminals can be associated with TTMN spines. Two other types of synapse are found on TTMNs but they are uncommon. Synaptic terminals of all types form multiple synapses but those from a single terminal are always the same type. Terminals with Lg Rnd vesicles formed the most synpases per terminal (avg. 2.73). Together, the synaptic terminals with Lg Rnd and Sm Rnd vesicles account for 62% of the terminals on TTMNs, and they likely represent the pathways driving the contractions in response to sound or during voluntary movements. Having a high proportion of excitatory inputs, the TTMN innervation is like that of stapedius motoneurons but proportionately different from other types of motoneurons.

Anatomical characterization of a rabbit cerebellar eyeblink premotor pathway using pseudorabies and identification of a local modulatory network in anterior interpositus

Rabbit eyeblink conditioning is a well characterized model of associative learning. To identify specific neurons that are part of the eyeblink premotor pathway, a retrograde transsynaptic tracer (pseudorabies virus) was injected into the orbicularis oculi muscle. Four time points (3, 4, 4.5, and 5 d) were selected to identify sequential segments of the pathway and a map of labeled structures was generated. At 3 d, labeled first-order motor neurons were found in dorsolateral facial nucleus ipsilaterally. At 4 d, second-order premotor neurons were found in reticular nuclei, and sensory trigeminal, auditory, vestibular, and motor structures, including contralateral red nucleus. At 4.5 d, labeled third-order premotor neurons were found in the pons, midbrain, and cerebellum, including dorsolateral anterior interpositus nucleus and rostral fastigial nucleus. At 5 d, labeling revealed higher-order premotor structures. Labeled fourth-order Purkinje cells were found in ipsilateral cerebellar cortex in cerebellar lobule HVI and in lobule I. The former has been implicated in eyeblink conditioning and the latter in vestibular control. Labeled neurons in anterior interpositus were studied, using neurotransmitter immunoreactivity to classify individual cell types and delineate their interconnectivity. Labeled third-order premotor neurons were immunoreactive for glutamate and corresponded to large excitatory projection neurons. Labeled fourth-order premotor interneurons were immunoreactive for GABA (30%), glycine (18%), or both GABA and glycine (52%) and form a functional network within anterior interpositus involved in modulation of motor commands. These results identify a complete eyeblink premotor pathway, deep cerebellar interconnectivity, and specific neurons responsible for the generation of eyeblink responses.

Intrinsic and synaptic properties of vertical cells of the mouse dorsal cochlear nucleus

Multiple classes of inhibitory interneurons shape the activity of principal neurons of the dorsal cochlear nucleus (DCN), a primary target of auditory nerve fibers in the mammalian brain stem. Feedforward inhibition mediated by glycinergic vertical cells (also termed tuberculoventral or corn cells) is thought to contribute importantly to the sound-evoked response properties of principal neurons, but the cellular and synaptic properties that determine how vertical cells function are unclear. We used transgenic mice in which glycinergic neurons express green fluorescent protein (GFP) to target vertical cells for whole cell patch-clamp recordings in acute slices of DCN. We found that vertical cells express diverse intrinsic spiking properties and could fire action potentials at high, sustained spiking rates. Using paired recordings, we directly examined synapses made by vertical cells onto fusiform cells, a primary DCN principal cell type. Vertical cell synapses produced unexpectedly small-amplitude unitary currents in fusiform cells, and additional experiments indicated that multiple vertical cells must be simultaneously active to inhibit fusiform cell spike output. Paired recordings also revealed that a major source of inhibition to vertical cells comes from other vertical cells.

Selective degeneration of synapses in the dorsal cochlear nucleus of chinchilla following acoustic trauma and effects of antioxidant treatment

The purpose of this study was to reveal synaptic plasticity within the dorsal cochlear nucleus (DCN) as a result of noise trauma and to determine whether effective antioxidant protection to the cochlea can also impact plasticity changes in the DCN. Expression of synapse activity markers (synaptophysin and precerebellin) and ultrastructure of synapses were examined in the DCN of chinchilla 10 days after a 105 dB SPL octave-band noise (centered at 4 kHz, 6 h) exposure. One group of chinchilla was treated with a combination of antioxidants (4-hydroxy phenyl N-tert-butylnitrone, N-acetyl-l-cysteine and acetyl-l-carnitine) beginning 4 h after noise exposure. Down-regulated synaptophysin and precerebellin expression, as well as selective degeneration of nerve terminals surrounding cartwheel cells and their primary dendrites were found in the fusiform soma layer in the middle region of the DCN of the noise exposure group. Antioxidant treatment significantly reduced synaptic plasticity changes surrounding cartwheel cells. Results of this study provide further evidence of acoustic trauma-induced neural plasticity in the DCN and suggest that loss of input to cartwheel cells may be an important factor contributing to the emergence of hyperactivity in the DCN after noise exposure. Results further suggest that early antioxidant treatment for acoustic trauma not only rescues cochlear hair cells, but also has impact on central auditory structures.

Combined LTP and LTD of modulatory inputs controls neuronal processing of primary sensory inputs

A hallmark of brain organization is the integration of primary and modulatory pathways by principal neurons. However, the pathway interactions that shape primary input processing remain unknown. We investigated this problem in mouse dorsal cochlear nucleus (DCN) where principal cells integrate primary, auditory nerve input with modulatory, parallel fiber input. Using a combined experimental and computational approach, we show that combined LTP and LTD of parallel fiber inputs to DCN principal cells and interneurons, respectively, broaden the time window within which synaptic inputs summate. Enhanced summation depolarizes the resting membrane potential and thus lowers the response threshold to auditory nerve inputs. Combined LTP and LTD, by preserving the variance of membrane potential fluctuations and the membrane time constant, fixes response gain and spike latency as threshold is lowered. Our data reveal a novel mechanism mediating adaptive and concomitant homeostatic regulation of distinct features of neuronal processing of sensory inputs.

Mechanisms underlying input-specific expression of endocannabinoid-mediated synaptic plasticity in the dorsal cochlear nucleus

A hallmark of brain organization is the integration of primary and modulatory pathways by principal neurons. Primary sensory inputs are usually not plastic, while modulatory inputs converging to the same principal neuron can be plastic. However, the mechanisms determining this input-specific expression of synaptic plasticity remain unknown. We investigated this problem in the dorsal cochlear nucleus (DCN), where principal cells integrate primary auditory nerve input with plastic, parallel fiber input. Our previous DCN studies have shown that parallel fiber inputs exhibit short- and long-term plasticities mediated by endocannabinoid signaling. Here we show that auditory nerve inputs to principal cells do not show short- or long-term endocannabinoid-mediated synaptic plasticity. Electrophysiological and electron microscopy studies indicate that input specificity arises from selective expression of presynaptic cannabinoid (CB1) receptors in parallel fiber terminals, but not in auditory nerve terminals. However, pairing of parallel fiber activity with auditory nerve activity elicits plasticity in parallel fiber inputs, thus suggesting a role for synaptic plasticity in multisensory integration.

Ultrastructure, synaptic organization, and molecular components of bushy cell networks in the anteroventral cochlear nucleus of the rhesus monkey

Bushy cells (BCs) process auditory information in the ventral cochlear nucleus (VCN). Yet, most neuroanatomical findings come from studies in cats and rodents, and the ultrastructural morphological features of BCs in humans and higher nonhuman primates are unknown. In this study, we combined histological, immunocytochemical, and ultrastructural methods to examine the morphology and synaptic organization of BCs in the rhesus monkey VCN. We observed that BCs were organized in a complex neural network that appears to interconnect the cells. The fine structure of BC somata and dendrites, as well as their synaptic inputs, are similar to those in other mammals. We found that BCs received numerous endbulb-like VGLUT1- and VGLUT2-immunopositive endings. In addition, they expressed glutamate AMPA (GluR2/3 and GluR4), NMDA (NR1), delta1/2 receptor subunits, and the α1 subunit of the glycine receptor. These receptor types and subunits mediate fast excitatory synaptic transmission from the cochlea and inhibitory neurotransmission from noncochlear inputs. Parvalbumin immunostaining and semithin sections showed that BC dendrites are oriented toward neighboring BC somas to form neuronal clusters. Within the cluster, the incoming inputs established multiple, divergent synaptic contacts. Thus, BCs were connected by specialized dendrosomatic and somasomatic membrane junctions. Our results indicate that the cytoarchitectural organization of BCs is well conserved between primates and other mammalian species.

Vesicular glutamate transporters: spatio-temporal plasticity following hearing loss

An immunocytochemical comparison of vGluT1 and vGluT3 in the cochlear nucleus (CN) of deafened versus normal hearing rats showed the first example of vGluT3 immunostaining in the dorsal and ventral CN and revealed temporal and spatial changes in vGluT1 localization in the CN after cochlear injury. In normal hearing rats vGluT1 immunostaining was restricted to terminals on CN neurons while vGluT3 immunolabeled the somata of the neurons. This changed in the ventral cochlear nucleus (VCN) 3 days following deafness, where vGluT1 immunostaining was no longer seen in large auditory nerve terminals but was instead found in somata of VCN neurons. In the dorsal cochlear nucleus (DCN), while vGluT1 labeling of terminals decreased, there was no labeling of neuronal somata. Therefore, loss of peripheral excitatory input results in co-localization of vGluT1 and vGluT3 in VCN neuronal somata. Postsynaptic glutamatergic neurons can use retrograde signaling to control their presynaptic inputs and these results suggest vGluTs could play a role in regulating retrograde signaling in the CN under different conditions of excitatory input. Changes in vGluT gene expression in CN neurons were found 3 weeks following deafness using qRT-PCR with significant increases in vGluT1 gene expression in both ventral and dorsal CN while vGluT3 gene expression decreased in VCN but increased in DCN.

Age-related increase in PKC gamma expression in the cochlear nucleus of hearing impaired C57BL/6J and BALB/c mice

Age-dependent alteration in cellular signaling is implicated in the onset of age-related hearing loss (presbycusis). The gamma subtype of protein kinase C (PKCγ) is a PKC isoenzyme exclusively expressed in central nervous system but its potential role in the presbycusis remains unclear. Using two presbycusis-like animal models (C57BL/6J strain and BALB/c strain), the auditory thresholds were assessed by auditory brainstem response (ABR) in young (2-month-old), adult (8-month-old) and old (24-month-old) groups, and the localization and expression of PKCγ in the cochlear nucleus (CN) was examined by immunohistochemistry, Western blotting and Real-Time PCR. The results showed that PKCγ immmunoreactive (-ir) neurons were mainly concentrated in the molecular layer and fusiform layer of the dorsal CN (DCN) and their number was increased significantly with aging (p<0.05). Moreover, compared with 2-month-old mice, PKCγ expression in the CN at both protein and mRNA levels was significantly increased in the 8-month-old mice and 24-month-old mice (p<0.05). Thus our findings demonstrate a potential link between the increased PKCγ expression and the age-related hearing loss in these mice, suggesting novel strategies for the prevention and therapy of age-associated auditory disorders.

Molecular layer inhibitory interneurons provide feedforward and lateral inhibition in the dorsal cochlear nucleus

In the outer layers of the dorsal cochlear nucleus, a cerebellum-like structure in the auditory brain stem, multimodal sensory inputs drive parallel fibers to excite both principal (fusiform) cells and inhibitory cartwheel cells. Cartwheel cells, in turn, inhibit fusiform cells and other cartwheel cells. At the microcircuit level, it is unknown how these circuit components interact to modulate the activity of fusiform cells and thereby shape the processing of auditory information. Using a variety of approaches in mouse brain stem slices, we investigated the synaptic connectivity and synaptic strength among parallel fibers, cartwheel cells, and fusiform cells. In paired recordings of spontaneous and evoked activity, we found little overlap in parallel fiber input to neighboring neurons, and activation of multiple parallel fibers was required to evoke or alter action potential firing in cartwheel and fusiform cells. Thus neighboring neurons likely respond best to distinct subsets of sensory inputs. In contrast, there was significant overlap in inhibitory input to neighboring neurons. In recordings from synaptically coupled pairs, cartwheel cells had a high probability of synapsing onto nearby fusiform cells or other nearby cartwheel cells. Moreover, single cartwheel cells strongly inhibited spontaneous firing in single fusiform cells. These synaptic relationships suggest that the set of parallel fibers activated by a particular sensory stimulus determines whether cartwheel cells provide feedforward or lateral inhibition to their postsynaptic targets.

Effects of sodium salicylate on spontaneous and evoked spike rate in the dorsal cochlear nucleus

Spontaneous hyperactivity in the dorsal cochlear nucleus (DCN), particularly in fusiform cells, has been proposed as a neural generator of tinnitus. To determine if sodium salicylate, a reliable tinnitus inducer, could evoke hyperactivity in the DCN, we measured the spontaneous and depolarization-evoked spike rate in fusiform and cartwheel cells during salicylate superfusion. Five minute treatment with 1.4 mM salicylate suppressed spontaneous and evoked firing in fusiform cells; this decrease partially recovered after salicylate washout. Less suppression and greater recovery occurred with 3 min treatment using 1.4 mM salicylate. In contrast, salicylate had no effect on the spontaneous or evoked firing of cartwheel cells indicating that salicylate's suppressive effects are specific to fusiform cells. To determine if salicylate's suppressive effects were a consequence of increased synaptic inhibition, spontaneous inhibitory postsynaptic currents (IPSC) were measured during salicylate treatment. Salicylate unexpectedly reduced IPSC thereby ruling out increased inhibition as a mechanism to explain the depressed firing rates in fusiform cells. The salicylate-induced suppression of fusiform spike rate apparently arises from unidentified changes in the cell's intrinsic excitability.

NMDA receptors activated by subventricular zone astrocytic glutamate are critical for neuroblast survival prior to entering a synaptic network

Even before integrating into existing circuitry, adult-born neurons express receptors for neurotransmitters, but the intercellular mechanisms and their impact on neurogenesis remain largely unexplored. Here, we show that neuroblasts born in the postnatal subventricular zone (SVZ) acquire NMDA receptors (NMDARs) during their migration to the olfactory bulb. Along their route, neuroblasts are ensheathed by astrocyte-like cells expressing vesicular glutamate release machinery. Increasing calcium in these specialized astrocytes induced NMDAR activity in neuroblasts, and blocking astrocytic vesicular release eliminated spontaneous NMDAR activity. Single-cell knockout of NMDARs using neonatal electroporation resulted in neuroblast apoptosis at the time of NMDAR acquisition. This cumulated in a 40% loss of neuroblasts along their migratory route, demonstrating that NMDAR acquisition is critical for neuroblast survival prior to entering a synaptic network. In addition, our findings suggest an unexpected mechanism wherein SVZ astrocytes use glutamate signaling through NMDARs to control the number of adult-born neurons reaching their final destination.

Localization of glycine receptors in the human forebrain, brainstem, and cervical spinal cord: an immunohistochemical review

Inhibitory neurotransmitter receptors for glycine (GlyR) are heteropentameric chloride ion channels that are comprised of four functional subunits, alpha1–3 and beta and that facilitate fast-response, inhibitory neurotransmission in the mammalian brain and spinal cord. We have investigated the distribution of GlyRs in the human forebrain, brainstem, and cervical spinal cord using immunohistochemistry at light and confocal laser scanning microscopy levels. This review will summarize the present knowledge on the GlyR distribution in the human brain using our established immunohistochemical techniques. The results of our immunohistochemical labeling studies demonstrated GlyR immunoreactivity (IR) throughout the human basal ganglia, substantia nigra, various pontine regions, rostral medulla oblongata and the cervical spinal cord present an intense and abundant punctate IR along the membranes of the neuronal soma and dendrites. This work is part of a systematic study of inhibitory neurotransmitter receptor distribution in the human CNS, and provides a basis for additional detailed physiological and pharmacological studies on the inter-relationship of GlyR, GABA_AR and gephyrin in the human brain. This basic mapping exercise, we believe, will provide important baselines for the testing of future pharmacotherapies and drug regimes that modulate neuroinhibitory systems. These findings provide new information for understanding the complexity of glycinergic functions in the human brain, which will translate into the contribution of inhibitory mechanisms in paroxysmal disorders and neurodegenerative diseases such as Epilepsy, Huntington's and Parkinson's Disease and Motor Neuron Disease.

A bushy cell network in the rat ventral cochlear nucleus

Geometry of the dendritic tree and synaptic organization of afferent inputs are essential factors in determining how synaptic input is integrated by neurons. This information remains elusive for one of the first brainstem neurons involved in processing of the primary auditory signal from the ear, the bushy cells (BCs) of the ventral cochlear nucleus (VCN). Here, we labeled the BC dendritic trees with retrograde tracing techniques to analyze their geometry and synaptic organization after immunofluorescence for excitatory and inhibitory synaptic markers, electron microscopy, morphometry, double tract-tracing methods, and 3D reconstructions. Our study revealed that BC dendrites provide space for a large number of compartmentalized excitatory and inhibitory synaptic interactions. The dendritic inputs on BCs are of cochlear and noncochlear origin, and their proportion and distribution are dependent on the branching pattern and orientation of the dendritic tree in the VCN. Three-dimensional reconstructions showed that BC dendrites branch and cluster with those of other BCs in the core of the VCN. Within the cluster, incoming synaptic inputs establish divergent multiple-contact synapses (dyads and triads) between BCs. Furthermore, neuron-neuron connections including puncta adherentia, sarcoplasmic junctions, and gap junctions are common between BCs, which suggests that these neurons are electrically coupled. Overall, our study demonstrates the existence of a BC network in the rat VCN. This network may establish the neuroanatomical basis for acoustic information processing by individual BCs as well as for enhanced synchronization of the output signal of the VCN.

Cochlear nucleus neurons redistribute synaptic AMPA and glycine receptors in response to monaural conductive hearing loss

Neurons restore their function in response to external or internal perturbations and maintain neuronal or network stability through a homeostatic scaling mechanism. Homeostatic responses at synapses along the auditory system would be important for adaptation to normal and abnormal fluctuations in the sensory environment. We investigated at the electron microscopic level and after postembedding immunogold labeling whether projection neurons in the cochlear nucleus responded to modifications of auditory nerve activity. After unilaterally reducing the level of auditory inputs by approximately 20 dB by monaural earplugging, auditory nerve synapses on bushy cells somata and basal dendrites of fusiform cells of the ventral and dorsal cochlear nucleus, respectively, upregulated GluR3 AMPA receptor subunit, while inhibitory synapses decreased the expression of GlyRalpha1 subunit. These changes in expression levels were fully reversible once the earplug was removed, indicating that activity affects the trafficking of receptors at synapses. Excitatory synapses on apical dendrites of fusiform cells (parallel fibers) with different synaptic AMPA receptor subunit composition, were not affected by sound attenuation, as the expression levels of AMPA receptor subunits were the same as in normal hearing littermates. GlyRalpha1 subunit expression at inhibitory synapses on apical dendrites of fusiform cells was also found unaffected. Furthermore, fusiform and bushy cells of the contralateral side to the earplugging upregulated the GluR3 subunit at auditory nerve synapses. These results show that cochlear nucleus neurons innervated by the auditory nerve, are able to respond to small changes in sound levels by redistributing specific AMPA and glycine receptor subunits.

Two distinct types of inhibition mediated by cartwheel cells in the dorsal cochlear nucleus

Individual neurons have been shown to exhibit target cell-specific synaptic function in several brain areas. The time course of the postsynaptic conductances (PSCs) strongly influences the dynamics of local neural networks. Cartwheel cells (CWCs) are the most numerous inhibitory interneurons in the dorsal cochlear nucleus (DCN). They are excited by parallel fiber synapses, which carry polysensory information, and in turn inhibit other CWCs and the main projection neurons of the DCN, pyramidal cells (PCs). CWCs have been implicated in "context-dependent" inhibition, producing either depolarizing (other CWCs) or hyperpolarizing (PCs) post synaptic potentials. In the present study, we used paired whole cell recordings to examine target-dependent inhibition from CWCs in neonatal rat DCN slices. We found that CWC inhibitory postsynaptic potentials (IPSPs) onto PCs are large (1.3 mV) and brief (half-width = 11.8 ms), whereas CWC IPSPs onto other CWCs are small (0.2 mV) and slow (half-width = 36.8 ms). Evoked IPSPs between CWCs exhibit paired-pulse facilitation, while CWC IPSPs onto PCs exhibit paired-pulse depression. Perforated-patch recordings showed that spontaneous IPSPs in CWCs are hyperpolarizing at rest with a mean estimated reversal potential of -67 mV. Spontaneous IPSCs were smaller and lasted longer in CWCs than in PCs, suggesting that the kinetics of the receptors are different in the two cell types. These results reveal that CWCs play a dual role in the DCN. The CWC-CWC network interactions are slow and sensitive to the average rate of CWC firing, whereas the CWC-PC network is fast and sensitive to transient changes in CWC firing.

Neuronal subtype identity in the rat auditory brainstem as defined by molecular profile and axonal projection

The nuclei of the auditory brainstem harbor a diversity of neuronal cell types and are interconnected by excitatory as well as inhibitory ascending, descending, and commissural pathways. Classically, neurons have been characterized by size and shape of their cell body and by the geometry of their dendrites. Our study is based on the use of axonal tracers in combination with immunocytochemistry to identify and distinguish neuronal subtypes by their molecular signature in dorsal and ventral cochlear nucleus, lateral superior olive, medial superior olive, medial nucleus of the trapezoid body, and inferior colliculus of the adult rat. The presumed neurotransmitters glutamate, glycine, and GABA were used alongside the calcium-binding proteins parvalbumin, calretinin, and calbindin-D28k as molecular markers. Our data provide distinct extensions to previous characterizations of neuronal subtypes and reveal regularities and differences across auditory brainstem nuclei that are discussed for their functional implications.

Distinct functional and anatomical architecture of the endocannabinoid system in the auditory brainstem

Endocannabinoids (ECs) act as retrograde messengers that enable postsynaptic cells to regulate the strength of their synaptic inputs. Here, by using physiological and histological techniques, we showed that, unlike in other parts of the brain, excitatory inputs are more sensitive than inhibitory inputs to EC signaling in the dorsal cochlear nucleus (DCN), an auditory brainstem nucleus. The principal cells of the DCN, fusiform cells, integrate acoustic signals through nonplastic synapses located in the deep layer with multimodal sensory signals carried by plastic parallel fibers in the molecular layer. Parallel fibers contact fusiform cells and inhibitory interneurons, the cartwheel cells, which in turn inhibit fusiform cells. Postsynaptic depolarization or pairing of postsynaptic potentials (PSPs) with action potentials (APs) induced EC-mediated modulation of excitatory inputs but did not affect inhibitory inputs. Quantitative electron microscopical studies showed that glutamatergic terminals express more cannabinoid 1 receptors (CB1Rs) than glycinergic terminals. Fusiform and cartwheel cells express diacylglycerol lipase alpha and beta (DGLalpha/beta), the two enzymes involved in the generation of the EC, 2-arachidonoyl-glycerol (2-AG). DGLalpha and DGLbeta are found in the spines of cartwheel but not fusiform cells indicating that the synthesis of ECs is more distant from parallel fiber synapses in fusiform than cartwheel cells. The differential localization and density of DGLalpha/beta and CB1Rs leads to cell- and input-specific EC signaling that favors activity-dependent EC-mediated suppression at synapses between parallel fibers and cartwheel cell spines, thus leading to reduced feedforward inhibition in fusiform cells. We propose that EC signaling is a major modulator of the balance of excitation and inhibition in auditory circuits.

Fidelity of complex spike-mediated synaptic transmission between inhibitory interneurons

Complex spikes are high-frequency bursts of Na+ spikes, often riding on a slower Ca2+-dependent waveform. Although complex spikes may propagate into axons, given their unusual shape it is not clear how reliably these bursts reach nerve terminals, whether their spikes are efficiently transmitted as a cluster of postsynaptic responses, or what function is served by such a concentrated postsynaptic signal. We examined these questions by recording from synaptically coupled pairs of cartwheel cells, neurons which fire complex spikes and form an inhibitory network in the dorsal cochlear nucleus. Complex spikes in the presynaptic soma were reliably propagated to nerve terminals and elicited powerful, temporally precise postsynaptic responses. Single presynaptic neurons could prevent their postsynaptic partner from firing complex but not simple spikes, dramatically reducing dendritic Ca2+ signals in the postsynaptic neuron. We suggest that rapid transmission of complex spikes may control the susceptibility of neighboring neurons to Ca2+-dependent plasticity.

Glycinergic transmission shaped by the corelease of GABA in a mammalian auditory synapse

The firing pattern of neurons is shaped by the convergence of excitation and inhibition, each with finely tuned magnitude and duration. In an auditory brainstem nucleus, glycinergic inhibition features fast decay kinetics, the mechanism of which is unknown. By applying glycine to native or recombinant glycine receptors, we show that response decay times are accelerated by addition of GABA, a weak partial agonist of glycine receptors. Systematic variation in agonist exposure time revealed that fast synaptic time course may be achieved with submillisecond exposures to mixtures of glycine and GABA at physiological concentrations. Accordingly, presynaptic terminals generally contained both transmitters, and depleting terminals of GABA slowed glycinergic synaptic currents. Thus, coreleased GABA accelerates glycinergic transmission by acting directly on glycine receptors, narrowing the time window for effective inhibition. Packaging both weak and strong agonists in vesicles may be a general means by which presynaptic neurons regulate the duration of postsynaptic responses.

Aged-related loss of temporal processing: altered responses to amplitude modulated tones in rat dorsal cochlear nucleus

Loss of temporal processing is characteristic of age-related loss of speech understanding observed in the elderly. Inhibitory glycinergic circuits provide input onto dorsal cochlear nucleus (DCN) projection neurons which likely serve to modulate excitatory responses to time-varying complex acoustic signals. The present study sought to test the hypothesis that age-related loss of inhibition would compromise the ability of output neurons to encode sinusoidally amplitude modulated (SAM) tones. Extracellular recordings were obtained from young and aged FBN rat DCN putative fusiform cells. Stimuli were SAM tones at three modulation depths (100, 50, and 20%) at 30 dB hearing level with the carrier frequency set to the unit's characteristic frequency. Discharge rate and synchrony were calculated to describe SAM responses. There were significant age-related changes in the shape and peak vector strength [best modulation frequency (BMF)] of temporal modulation transfer functions (tMTFs), with no significant age-related changes in rate modulation transfer functions (rMTFs) at BMF. Young neurons exhibited band-pass tMTFs for most SAM conditions while aged fusiform cells exhibited significantly more low-pass or double-peaked tMTFs. There were significant differences in tMTFs between buildup, pauser-buildup, and wide-chopper temporal response types. Young and aged wide-choppers displayed significantly lower vector strength values than the other two temporal DCN response types. Age-related decreases in the number of pauser-buildup response types and increases in wide-chopper types reported previously, could account, in part, for the observed loss of temporal coding of the aged fusiform cell. Age-related changes in SAM coding were similar to changes observed with receptor blockade of glycinergic inhibition onto fusiform cells and consistent with previously observed age-related loss of endogenous glycine levels and changes in normal adult glycine receptor function. DCN changes in SAM coding could, in part, underpin temporal processing deficits observed in the elderly.

Neurochemistry of the afferents to the rat cochlear root nucleus: possible synaptic modulation of the acoustic startle

Afferents to the primary startle circuit are essential for the elicitation and modulation of the acoustic startle reflex (ASR). In the rat, cochlear root neurons (CRNs) comprise the first component of the acoustic startle circuit and play a crucial role in mediating the ASR. Nevertheless, the neurochemical pattern of their afferents remains unclear. To determine the distribution of excitatory and inhibitory inputs, we used confocal microscopy to analyze the immunostaining for vesicular glutamate and GABA transporter proteins (VGLUT1 and VGAT) on retrogradely labeled CRNs. We also used reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry to detect and localize specific neurotransmitter receptor subunits in the cochlear root. Our results show differential distributions of VGLUT1- and VGAT-immunoreactive endings around cell bodies and dendrites. The RT-PCR data showed a positive band for several ionotropic glutamate receptor subunits, M1-M5 muscarinic receptor subtypes, the glycine receptor alpha1 subunit (GlyRalpha1), GABAA, GABAB, and subunits of alpha2 and beta-noradrenergic receptors. By immunohistochemistry, we confirmed that CRN cell bodies exhibit positive immunoreaction for the glutamate receptor (GluR) 3 and NR1 GluR subunits. Cell bodies and dendrites were also positive for M2 and M4, and GlyRalpha1. Other subunits, such as GluR1 and GluR4 of the AMPA GluRs, were observed in glial cells neighboring unlabeled CRN cell bodies. We further confirmed the existence of noradrenergic afferents onto CRNs from the locus coeruleus by combining tyrosine hydroxylase immunohistochemistry and tract-tracing experiments. Our results provide valuable information toward understanding how CRNs might integrate excitatory and inhibitory inputs, and hence how they could elicit and modulate the ASR.

Revealing the molecular layer of the primate dorsal cochlear nucleus

In nonprimate mammals, the dorsal cochlear nucleus (DCN) is thought to play a role in the orientation of the head toward sounds of interest by integrating acoustic and somatosensory information. Humans and higher primates might not use this system because of reported phylogenetic changes in DCN cytoarchitecture [Moskowitz N (1969) Comparative aspects of some features of the central auditory system of primates. Ann N Y Acad Sci 167:357-369; Moore JK, Osen KK (1979) The cochlear nuclei in man. Am J Anat 154:393-418; Moore JK (1980) The primate cochlear nuclei: loss of lamination as a phylogenetic process. J Comp Neurol 193:609-629]. In this study, we re-evaluated this question from a comparative perspective and examined the rhesus monkey (cercopithecoid primate) using more sensitive probes and higher resolution imaging methods. We used electron microscopy to identify parallel fibers and their synapses, and molecular markers to determine that primates exhibit the main components of excitatory neurotransmission as other mammals. We observed that characteristics of the monkey molecular layer resembled what has been reported for nonprimates: (1) immunohistochemistry revealed many unmyelinated, thin axons and en passant glutamatergic synapses on dendritic spines; (2) immunohistochemistry for phosphodiesterase (PDE10A) showed the nuclei of granule cells distributed in the external molecular layer and the deep layers in the DCN; (3) antibodies for the inositol trisphosphate receptor (IP3r) and calbindin immunostained cartwheel cells; (4) postembedding immunogold labeling revealed synaptic expression of AMPA and delta glutamate receptor subunits on spines in parallel fiber endings; and (5) parallel fibers use vesicular glutamate transporter 1 (VGLUT1) to package glutamate into the synaptic vesicles and to mediate glutamate transport. These observations are consistent with the argument that the rhesus monkey DCN has neuronal features similar to those of other nonprimate mammals.

Presynaptic GABA(B) receptors modulate synaptic facilitation and depression at distinct synapses in fusiform cells of mouse dorsal cochlear nucleus

The mammalian dorsal cochlear nucleus (DCN) is considered to contribute to the localization of the sound sources. Fusiform cells (FCs), principal projection neurons in the DCN, integrate two excitatory inputs from auditory nerve fibers (ANFs) and parallel fibers (PFs). Although an immunohistochemical study suggested presence of GABA(B) receptors at excitatory presynaptic terminals in the DCN, it has not been elucidated how GABA(B) receptors modulate the synaptic transmission to FCs. Here, we examined effects of baclofen on the transmission in vitro. Baclofen reduced both PF-EPSC and ANF-EPSC by reducing transmitter releases, and it enhanced the facilitation in PF-FC synapses and prevented the depression in ANF-FC synapses. The enhancement and prevention were prominent during high-frequency (50Hz) synaptic input, suggesting the activation of presynaptic GABA(B) receptors may optimize both PF-FC and ANF-FC synapses for high-frequency transmission. Postsynaptic GABA(B) receptors activated GIRK current and would further modulate the activity of FCs.

The potassium channel KCNQ5/Kv7.5 is localized in synaptic endings of auditory brainstem nuclei of the rat

KCNQ, also called Kv7, is a family of voltage-dependent potassium channels with important roles in excitability regulation. Of its five known subunits, KCNQ5/Kv7.5 is extensively expressed in the central nervous system and it contributes to the generation of M-currents. The distribution of KCNQ5 was analyzed in auditory nuclei of the rat brainstem by high-resolution immunocytochemistry. Double labeling with anti-KCNQ5 antibodies and anti-synaptophysin or anti-syntaxin, which mark synaptic endings, or anti-microtubule-associated protein 2 (MAP2) antibodies, which mark dendrites, were used to analyze the subcellular distribution of KCNQ5 in neurons in the cochlear nucleus, superior olivary complex, nuclei of the lateral lemniscus, and inferior colliculus. An abundance of KCNQ5 labeling in punctate structures throughout auditory brainstem nuclei along with colocalization with such synaptic markers suggests that a preferred localization of KCNQ5 is in synaptic endings in these auditory nuclei. Punctate KCNQ5 immunoreactivity virtually disappeared from the cochlear nucleus after cochlea removal, which strongly supports localization of this channel in excitatory endings of the auditory nerve. Actually, neither glycinergic endings, labeled with an anti-glycine transporter 2 (GlyT2) antibody, nor gamma-aminobutyric acid (GABA)ergic endings, labeled with an anti-glutamic acid decarboxylase (GAD65) antibody, contained KCNQ5 immunoreactivity, suggesting that KCNQ5 is mostly in excitatory endings throughout the auditory brainstem. Overlap of KCNQ5 and MAP2 labeling indicates that KCNQ5 is also targeted to dendritic compartments. These findings predict pre- and postsynaptic roles for KCNQ5 in excitability regulation in auditory brainstem nuclei, at the level of glutamatergic excitatory endings and in dendrites.

Regional variations of cytochrome oxidase activity in the central auditory system of Relnrl-Orl (reeler) mutant mice

Despite preserved cell differentiation, the Reln(rl-Orl) phenotype comprises laminar abnormalities of cell position in auditory cortex and dorsal cochlear nucleus. The metabolic consequences of the cell ectopias were determined by estimating cytochrome oxidase (CO) activity, a marker of neuronal activity. CO activity increased in the granular cell layer of dorsal cochlear nucleus, trapezoid body nucleus, intermediate lateral lemniscus, central and external inferior colliculus, and pyramidal cell layer of primary auditory cortex. On the contrary, CO activity decreased in the superficial molecular layer of dorsal cochlear nucleus as well as in the medioventral periolivary nucleus. These metabolic variations are discussed in terms of their possible relation to morphologic anomalies observed in the mutant.

Coactivation of pre- and postsynaptic signaling mechanisms determines cell-specific spike-timing-dependent plasticity

Synapses may undergo long-term increases or decreases in synaptic strength dependent on critical differences in the timing between pre-and postsynaptic activity. Such spike-timing-dependent plasticity (STDP) follows rules that govern how patterns of neural activity induce changes in synaptic strength. Synaptic plasticity in the dorsal cochlear nucleus (DCN) follows Hebbian and anti-Hebbian patterns in a cell-specific manner. Here we show that these opposing responses to synaptic activity result from differential expression of two signaling pathways. Ca2+/calmodulin-dependent protein kinase II (CaMKII) signaling underlies Hebbian postsynaptic LTP in principal cells. By contrast, in interneurons, a temporally precise anti-Hebbian synaptic spike-timing rule results from the combined effects of postsynaptic CaMKII-dependent LTP and endocannabinoid-dependent presynaptic LTD. Cell specificity in the circuit arises from selective targeting of presynaptic CB1 receptors in different axonal terminals. Hence, pre- and postsynaptic sites of expression determine both the sign and timing requirements of long-term plasticity in interneurons.

Coactivation of pre- and postsynaptic signaling mechanisms determines cell-specific spike-timing-dependent plasticity

Synapses may undergo long-term increases or decreases in synaptic strength dependent on critical differences in the timing between pre-and postsynaptic activity. Such spike-timing-dependent plasticity (STDP) follows rules that govern how patterns of neural activity induce changes in synaptic strength. Synaptic plasticity in the dorsal cochlear nucleus (DCN) follows Hebbian and anti-Hebbian patterns in a cell-specific manner. Here we show that these opposing responses to synaptic activity result from differential expression of two signaling pathways. Ca2+/calmodulin-dependent protein kinase II (CaMKII) signaling underlies Hebbian postsynaptic LTP in principal cells. By contrast, in interneurons, a temporally precise anti-Hebbian synaptic spike-timing rule results from the combined effects of postsynaptic CaMKII-dependent LTP and endocannabinoid-dependent presynaptic LTD. Cell specificity in the circuit arises from selective targeting of presynaptic CB1 receptors in different axonal terminals. Hence, pre- and postsynaptic sites of expression determine both the sign and timing requirements of long-term plasticity in interneurons.

Glycine receptors in the striatum, globus pallidus, and substantia nigra of the human brain: An immunohistochemical study

Glycine receptors (GlyRs) are heteropentameric chloride ion channels that facilitate fast-response, inhibitory neurotransmission in the mammalian spinal cord and brain. GlyRs have four functional subunits, alpha1-3 and beta, which likely exist in heteromeric alphabeta combinations. Mutations in GlyR alpha1 and beta subunits are well known for their involvement in hyperekplexia, a paroxysmal motor disorder. In this study we present the first detailed immunohistochemical investigation at the regional, cellular, and subcellular levels of GlyRs in the human basal ganglia. The results show that GlyRs are present at the regional level in low concentrations in the striatum and globus pallidus and are present in the highest concentrations in the substantia nigra. At the cellular level, GlyRs are present only in discrete populations of neurons immunoreactive for choline acetyltransferase (ChAT), parvalbumin, and calretinin in the human striatum, on a subpopulation of parvalbumin- and calretinin-positive neurons in the globus pallidus, and in the substantia nigra GlyRs are present on approximately three-fourths of all pars compacta and one-third of all pars reticulata neurons. They also form a distinct band of immunoreactive neurons in the intermedullary layers of the globus pallidus. At the subcellular level in the substantia nigra pars reticulata (SNr), GlyRs show a localized distribution on the soma and dendrites that partially complements but does not overlap with the distribution of gamma-aminobutyric acid (GABA)A receptors. Our results demonstrate the precise cellular and subcellular localization of GlyRs in the human basal ganglia and suggest that glycinergic receptors may play an important complementary role to other inhibitory receptors in modulating cholinergic, dopaminergic, and GABAergic neuronal pathways in the basal ganglia.

Synaptic and nonsynaptic localization of GABAA receptors containing the alpha5 subunit in the rat brain

The alpha5 subunit of the GABA(A) receptors (GABA(A)Rs) has a restricted expression in the brain. Maximum expression of this subunit occurs in the hippocampus, cerebral cortex, and olfactory bulb. Hippocampal pyramidal cells show high expression of alpha5 subunit-containing GABA(A)Rs (alpha5-GABA(A)Rs) both in culture and in the intact brain. A large pool of alpha5-GABA(A)Rs is extrasynaptic and it has been proposed to be involved in the tonic GABAergic inhibition of the hippocampus. Nevertheless, there are no studies on the localization of the alpha5-GABA(A)Rs at the electron microscope (EM) level. By using both immunofluorescence of cultured hippocampal pyramidal cells and EM postembedding immunogold of the intact hippocampus we show that, in addition to the extrasynaptic pool, there is a pool of alpha5-GABA(A)Rs that concentrates at the GABAergic synapses in dendrites of hippocampal pyramidal cells. The results suggest that the synaptic alpha5-GABA(A)Rs might play a role in the phasic GABAergic inhibition of pyramidal neurons in hippocampus and cerebral cortex.

Redistribution of synaptic AMPA receptors at glutamatergic synapses in the dorsal cochlear nucleus as an early response to cochlear ablation in rats

This study investigated whether unilateral deafferentation of the presynaptic neuron is key in the control of morphology and the subunit composition and expression of AMPA type glutamate receptors (GluRs) in neurons of the dorsal cochlear nucleus (DCN). Data showed that there are morphological changes at the postsynaptic sites which precede presynaptic changes at the auditory nerve (AN) synaptic ending in response to peripheral damage, in particular that the postsynaptic densities (PSD) of the AN on fusiform cells (FC) are thicker after denervation. Moreover, GluR2, GluR3 and GluR4 AMPA receptor subunits were redistributed, not only at the synapse of FCs receiving direct contact with the AN, but also at the glutamatergic synapse of the parallel fibers on FC and on cartwheel cells (CwC) which are indirectly innervated by the AN. Interestingly, the same synapses in the DCN contralateral to the lesion and with a normal AN synaptic input also redistributed AMPA receptor subunits at the synapse in respond to deafferentation. In these synapses, there was an increase of immunogold labeling for GluR2/3 subunits but not for GluR2 at 2 days after deafferentation.