Cobalt-permeable non-NMDA receptors in developing chick brainstem auditory nuclei

Development of excitatory synaptic transmission to the superior paraolivary and lateral superior olivary nuclei optimizes differential decoding strategies

The superior paraolivary nucleus (SPON) is a prominent structure in the mammalian auditory brainstem with a proposed role in encoding transient broadband sounds such as vocalized utterances. Currently, the source of excitatory pathways that project to the SPON and how these inputs contribute to SPON function are poorly understood. To shed light on the nature of these inputs, we measured evoked excitatory postsynaptic currents (EPSCs) in the SPON originating from the intermediate acoustic stria and compared them with the properties of EPSCs in the lateral superior olive (LSO) originating from the ventral acoustic stria during auditory development from postnatal day 5 to 22 in mice. Before hearing onset, EPSCs in the SPON and LSO are very similar in size and kinetics. After the onset of hearing, SPON excitation is refined to extremely few (2:1) fibers, with each strengthened by an increase in release probability, yielding fast and strong EPSCs. LSO excitation is recruited from more fibers (5:1), resulting in strong EPSCs with a comparatively broader stimulus-response range after hearing onset. Evoked SPON excitation is comparatively weaker than evoked LSO excitation, likely due to a larger fraction of postsynaptic GluR2-containing Ca²⁺-impermeable AMPA receptors after hearing onset. Taken together, SPON excitation develops synaptic properties that are suited for transmitting single events with high temporal reliability and the strong, dynamic LSO excitation is compatible with high rate-level sensitivity. Thus, the excitatory input pathways to the SPON and LSO mature to support different decoding strategies of respective coarse temporal and sound intensity information at the brainstem level.

Development of glutamatergic synaptic transmission in binaural auditory neurons

Glutamatergic synaptic transmission is essential for binaural auditory processing in birds and mammals. Using whole cell voltage clamp recordings, we characterized the development of synaptic ionotropic glutamate receptor (iGluR) function from auditory neurons in the chick nucleus laminaris (NL), the first nucleus responsible for binaural processing. We show that synaptic transmission is mediated by AMPA- and N-methyl-d-aspartate (NMDA)-type glutamate receptors (AMPA-R and NMDA-R, respectively) when hearing is first emerging and dendritic morphology is being established across different sound frequency regions. Puff application of glutamate agonists at embryonic day 9 (E9) revealed that both iGluRs are functionally present prior to synapse formation (E10). Between E11 and E19, the amplitude of isolated AMPA-R currents from high-frequency (HF) neurons increased 14-fold. A significant increase in the frequency of spontaneous events is also observed. Additionally, AMPA-R currents become faster and more rectifying, suggesting developmental changes in subunit composition. These developmental changes were similar in all tonotopic regions examined. However, mid- and low-frequency neurons exhibit fewer spontaneous events and evoked AMPA-R currents are smaller, slower, and less rectifying than currents from age-matched HF neurons. The amplitude of isolated NMDA-R currents from HF neurons also increased, reaching a peak at E17 and declining sharply by E19, a trend consistent across tonotopic regions. With age, NMDA-R kinetics become significantly faster, indicating a developmental switch in receptor subunit composition. Dramatic increases in the amplitude and speed of glutamatergic synaptic transmission occurs in NL during embryonic development. These changes are first seen in HF neurons suggesting regulation by peripheral inputs and may be necessary to enhance coincidence detection of binaural auditory information.

Spatial compartmentalization of AMPA glutamate receptor subunits at the calyx of Held synapse

The mature calyx of Held ending on principal neurons of the medial nucleus of the trapezoid body (MNTB) has very specialized morphological and molecular features that make it possible to transmit auditory signals with high fidelity. In a previous work we described an increased localization of the ionotropic alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid (AMPA) glutamate receptor (GluA) subunits at postsynaptic sites of the calyx of Held-principal cell body synapses from postnatal development to adult. The aim of the present study was to investigate whether the pattern of the synaptic distribution of GluA2/3/4c and -4 in adult MNTB principal cell bodies correlated with preferential subcellular domains (stalks and swellings) of the calyx. We used a postembedding immunocytochemical method combined with specific antibodies to GluA2/3/4c and GluA4 subunits. We found that the density of GluA2/3/4c in calyceal swellings (19 +/- 1.54 particles/microm) was higher than in stalks (10.93 +/- 1.37 particles/microm); however, the differences for GluA4 were not statistically significant (swellings: 13.84 +/- 1.39 particles/microm; stalks: 10.42 +/- 1.24 particles/microm). Furthermore, GluA2/3/4c and GluA4 labeling co-localized to some extent in calyceal stalks and swellings. Taking these data together, the distribution pattern of GluA subunits in postsynaptic specializations are indicative of a spatial compartmentalization of AMPA subunits in mature calyx-principal neuron synapses that may support the temporally precise transmission required for sound localization in the auditory brainstem.

Dendritic calcium channels and their activation by synaptic signals in auditory coincidence detector neurons

The avian nucleus laminaris (NL) encodes the azimuthal location of low-frequency sound sources by detecting the coincidence of binaural signals. Accurate coincidence detection requires precise developmental regulation of the lengths of the fine, bitufted dendrites that characterize neurons in NL. Such regulation has been suggested to be driven by local, synaptically mediated, dendritic signals such as Ca(2+). We examined Ca(2+) signaling through patch clamp and ion imaging experiments in slices containing nucleus laminaris from embryonic chicks. Voltage-clamp recordings of neurons located in the NL showed the presence of large Ca(2+) currents of two types, a low voltage-activated, fast inactivating Ni(2+) sensitive channel resembling mammalian T-type channels, and a high voltage-activated, slowly inactivating Cd(2+) sensitive channel. Two-photon Ca(2+) imaging showed that both channel types were concentrated on dendrites, even at their distal tips. Single action potentials triggered synaptically or by somatic current injection immediately elevated Ca(2+) throughout the entire cell. Ca(2+) signals triggered by subthreshold synaptic activity were highly localized. Thus when electrical activity is suprathreshold, Ca(2+) channels ensure that Ca(2+) rises in all dendrites, even those that are synaptically inactive.

Auditory nerve input is not an absolute requirement for the expression, distribution and calcium permeability of AMPA receptors in the adult rat ventral cochlear nucleus

In order to understand whether glutamatergic excitatory presynaptic input is an absolute requirement for the adult regulation of postsynaptic glutamate receptors we analyzed if a period of 11 days of excitatory deprivation affects the expression, distribution and Ca(2+) permeability of AMPA receptor subunits in the ventral cochlear nucleus of the rat. Bilateral cochlear ablations were performed in 30-day-old rats. After 11 days of survival, immunohistochemistry for GluR1, GluR2/3 and GluR4 AMPA receptor subunits showed no changes in the normal pattern of distribution, with GluR2/3 and GluR4 immunoreactivity predominating, and little GluR1. No changes in the amount of these AMPA receptor subunits were found between normal and cochleotomized rats in Western blots. AMPA receptors lacking the GluR2 subunit are Ca(2+) permeable. Kainate-induced Co(2+) uptake histochemistry, which labels AMPA Ca(2+) permeable receptors, demonstrated no changes in somatic labeling intensity for Co(2+), 11 days after cochleotomy. Therefore, our data indicate that excitatory input is not an absolute requirement to maintain AMPA receptor subunit expression, distribution and functional properties such as Ca(2+) permeability in VCN neurons. Nevertheless, subtle changes in AMPA receptors through regulatory post-transductional mechanisms cannot be ruled out.

Beyond timing in the auditory brainstem: intensity coding in the avian cochlear nucleus angularis

Many of the computational principles for sound localization have emerged from the study of avian brains, especially for the construction of codes for interaural timing differences. Our understanding of the neural codes for interaural level differences, and other intensity-related, non-localization sound processing, has lagged behind. In birds, cochlear nucleus angularis (NA) is an obligatory relay for intensity processing. We present our current knowledge of the cell types found in NA, their responses to auditory stimuli, and their likely coding roles. On a cellular level, our recent experimental and modeling studies have shown that short-term synaptic plasticity in NA is a major player in the division of intensity and timing information into parallel pathways. NA projects to at least four brain stem and midbrain targets, suggesting diverse involvement in a range of different sound processing circuits. Further studies comparing processing in NA and analogous neurons in the mammalian cochlear nucleus will highlight which features are conserved and perhaps may be computationally advantageous, and which are species- or clade-specific details demonstrating either disparate environmental requirements or different solutions to similar problems.

Developmental increase in postsynaptic alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid receptor compartmentalization at the calyx of Held synapse

The pattern of expression of ionotropic glutamate receptor (GluR) subunits 1-4 in the medial nucleus of the trapezoid body (MNTB) has been reported to change during development. The aim of this study was to compare the distribution of the GluR1-4 subunits in the MNTB at postnatal day (P) 9, before high-frequency signal transmission in the auditory system has developed, with that observed in mature adult rats. GluR1-4 subunits were studied by preembedding and postembedding immunocytochemical methods. Increased levels of GluR1, 2/3, and 4 associated with development were evident only at postsynaptic sites of MNTB principal cell bodies receiving calyces of Held synapses, whereas receptor density at nonsynaptic sites was found to remain unaltered. Taken together, the expression pattern of GluR subunits and the density of immunoparticles in postsynaptic specializations are indicative of a compartmentalization of alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid (AMPA) subunits upon development. These developmental changes provide a morphological basis for establishment of the postsynaptic properties needed for high-frequency synaptic transmission of auditory signals to MNTB principal neurons.

Synaptic physiology in the cochlear nucleus angularis of the chick

Nucleus angularis (NA), one of the two cochlear nuclei in birds, is important for processing sound intensity for localization and most likely has role in sound recognition and other auditory tasks. Because the synaptic properties of auditory nerve inputs to the cochlear nuclei are fundamental to the transformation of auditory information, we studied the properties of these synapses onto NA neurons using whole cell patch-clamp recordings from auditory brain stem slices from embryonic chickens (E16-E20). We measured spontaneous excitatory postsynaptic currents (EPSCs), and evoked EPSCs and excitatory postsynaptic potentials (EPSPs) by using extracellular stimulation of the auditory nerve. These excitatory EPSCs were mediated by AMPA and N-methyl-D-aspartate (NMDA) receptors. The spontaneous EPSCs mediated by AMPA receptors had submillisecond decay kinetics (556 micros at E19), comparable with those of other auditory brain stem areas. The spontaneous EPSCs increased in amplitude and became faster with developmental age. Evoked EPSC and EPSP amplitudes were graded with stimulus intensity. The average amplitude of the EPSC evoked by minimal stimulation was twice as large as the average spontaneous EPSC amplitude (approximately 110 vs. approximately 55 pA), suggesting that single fibers make multiple contacts onto each postsynaptic NA neuron. Because of their small size, minimal EPSPs were subthreshold, and we estimate at least three to five inputs were required to reach threshold. In contrast to the fast EPSCs, EPSPs in NA had a decay time constant of approximately 12.5 ms, which was heavily influenced by the membrane time constant. Thus NA neurons spatially and temporally integrate auditory information arriving from multiple auditory nerve afferents.

Transient Ca2+-permeable AMPA receptors in postnatal rat primary auditory neurons

Fast excitatory transmission in the nervous system is mostly mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors whose subunit composition governs physiological characteristics such as ligand affinity and ion conductance properties. Here, we report that AMPA receptors at inner hair cell (IHC) synapses lack the GluR2 subunit and are transiently Ca2+-permeable before hearing onset as evidenced using agonist-induced Co2+ accumulation, Western blots and GluR2 confocal microscopy in the rat cochlea. AMPA (100 microM) induced Co2+ accumulation in primary auditory neurons until postnatal day (PND) 10. This accumulation was concentration-dependent, strengthened by cyclothiazide (50 microM) and blocked by GYKI 52466 (80 microM) and Joro spider toxin (1 microM). It was unaffected by D-AP5 (50 microM), and it could not be elicited by 56 mM K+ or 1 mM NMDA + 10 microM glycine. Western blots showed that GluR1 immunoreactivity, present in homogenates of immature cochleas, had disappeared by PND12. GluR2 immunoreactivity was not detected until PND10 and GluR3 and GluR4 immunoreactivities were detected at all the ages examined. Confocal microscopy confirmed that the GluR2 immunofluorescence was not located postsynaptically to IHCs before PND10. In conclusion, AMPA receptors on maturing primary auditory neurons differ from those on adult neurons. They are probably composed of GluR1, GluR3 and GluR4 subunits and have a high Ca2+ permeability. The postsynaptic expression of GluR2 subunits may be continuously regulated by the presynaptic activity allowing for variations in the Ca2+ permeability and physiological properties of the receptor.

Modulation by topiramate of AMPA and kainate mediated calcium influx in cultured cerebral cortical, hippocampal and cerebellar neurons

The effect of the antiepileptic drug topiramate on Ca2+ uptake through (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate (AMPA) and kainate (KA) receptors was investigated in different cell culture systems consisting of neurons from the cerebral cortex, hippocampus, and cerebellum. Ca2+ influx was assayed using a fluorescent Ca2+ chelator to monitor changes in the intracellular Ca2+ concentration or cobalt staining to assess the effect of topiramate on Ca2+-permeable AMPA/KA receptors. In all types of neuronal cultures studied, AMPA and KA were found to elicit an influx of Ca2+ in a subset of the neuronal population. Topiramate, at concentrations of 30 and 100 microM, inhibited Ca2+ influx by up to 60%. Modulation of AMPA and KA-evoked Ca2+ influx may contribute to both the antiepileptic and neuroprotective properties of topiramate.

The role of AMPA receptor gating in the development of high-fidelity neurotransmission at the calyx of Held synapse

During early postnatal development of auditory synapses, the decay time course of AMPA receptor (AMPAR) EPSCs accelerates markedly, but the mechanisms underlying this process remain uncertain. Using the developing calyx of Held synapse in the mouse auditory brainstem, we have examined presynaptic and postsynaptic elements that may regulate decay kinetics of AMPAR EPSCs. We found that the decay time kinetics was voltage dependent in both immature and mature synapses, being slower at positive potentials than negative potentials. By recording evoked miniature events in extracellular Ca2+ or Sr2+, we revealed a significant decrease in decay time constants of EPSCs as maturation progresses. On the basis of internal and external polyamine block of AMPAR EPSCs and immunohistochemistry assays with subunit-specific antibodies, we demonstrated that the glutamate receptor (GluR) 2 subunit is virtually absent at all developmental ages. Antibody staining patterns suggest a gradual shift in subunit composition from GluR1- to GluR3/4-dominant phenotypes. Kinetic analyses of deactivation, desensitization, and recovery from desensitization in outside-out patches in response to ultrafast application of glutamate lend supportive evidence that such a shift in the gating phenotype likely accounts for the accelerated time course throughout development. Finally, by pharmacologically manipulating AMPAR gating and using simulated EPSCs to evoke action potentials, we demonstrated that rapid decay kinetics of AMPAR EPSCs is essential for this synapse to accommodate high-frequency firing without compromising spike amplitude. Hence, developmental alterations in the subunit composition likely dictate changes in the time course of AMPAR EPSCs and play an indispensable role in the refinement of high-fidelity neurotransmission at the calyx of Held synapse.

Catecholamine-independent transient expression of tyrosine hydroxylase in primary auditory neurons is coincident with the onset of hearing in the rat cochlea

During the last stages of neuronal maturation, tyrosine hydroxylase is transiently expressed in the absence of the other catecholamine-synthesizing enzymes. We show here that it is expressed in rat spiral ganglion neurons between postnatal days 8 and 20, with a peak of expression at postnatal day 12. These tyrosine hydroxylase-immunoreactive neurons did not display aromatic amino acid decarboxylase- or dopamine-beta-hydroxylase-immunoreactivities, ruling out the possibilities of dopamine or noradrenaline synthesis. They also did not display peripherin- or intense neurofilament 200-kDa-immunoreactivities, two indicators of type II primary auditory neurons. Tyrosine hydroxylase-immunoreactive dendrites were seen in synaptic contact with the inner hair cells and expressed the GluR2 subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors, further confirming the type I nature of the neurons transiently expressing the enzyme. The end of the tyrosine hydroxylase expression was not due to cell death because the immunoreactive neurons did not show TUNEL-labelled nuclei. Finally, all the type I neurons expressed the tyrosine hydroxylase mRNA at postnatal day 12, suggesting that the expression of the enzyme is a maturational step common to all these neurons and that the expression of the protein is not synchronized. Because the period of transient expression of tyrosine hydroxylase in type I neurons parallels the periods of maturation of evoked exocytosis in inner hair cells and of appearance and maturation of the cochlear potentials, we propose that the expression of the enzyme indicates the onset of hearing in individual type I primary auditory neurons. This enzyme expression could rely on a Ca2+ activation of its encoding gene subsequent to a sudden and massive Ca2+ entry through voltage-activated Ca2+ channels.

Primary innervation of the avian and mammalian cochlear nucleus

The auditory nerve of birds and mammals exhibits differences and similarities, but given the millions of years since the two classes diverged from a common ancestor, the similarities are much more impressive than the differences. The avian nerve is simpler than that of mammals, but share many fundamental features including principles of development, structure, and physiological properties. Moreover, the available evidence shows that the human auditory nerve follows this same general organizational plan. Equally impressive are reports that homologous genes in worms, flies, and mice exert the same heredity influences in man. The clear implication is that animal studies will produce knowledge that has a direct bearing on the human condition.

Localization of KCNC1 (Kv3.1) potassium channel subunits in the avian auditory nucleus magnocellularis and nucleus laminaris during development

The KCNC1 (previously Kv3.1) potassium channel, a delayed rectifier with a high threshold of activation, is highly expressed in the time coding nuclei of the adult chicken and barn owl auditory brainstem. The proposed role of KCNC1 currents in auditory neurons is to reduce the width of the action potential and enable neurons to transmit high frequency temporal information with little jitter. Because developmental changes in potassium currents are critical for the maturation of the shape of the action potential, we used immunohistochemical methods to examine the developmental expression of KCNC1 subunits in the avian auditory brainstem. The KCNC1 gene gives rise to two splice variants, a longer KCNC1b and a shorter KCNC1a that differ at the carboxy termini. Two antibodies were used: an antibody to the N-terminus that does not distinguish between KCNC1a and b isoforms, denoted as panKCNC1, and another antibody that specifically recognizes the C terminus of KCNC1b. A comparison of the staining patterns observed with the panKCNC1 and the KCNC1b specific antibodies suggests that KCNC1a and KCNC1b splice variants are differentially regulated during development. Although panKCNC1 immunoreactivity is observed from the earliest time examined in the chicken (E10), a subcellular redistribution of the immunoproduct was apparent over the course of development. KCNC1b specific staining has a late onset with immunostaining first appearing in the regions that map high frequencies in nucleus magnocellularis (NM) and nucleus laminaris (NL). The expression of KCNC1b protein begins around E14 in the chicken and after E21 in the barn owl, relatively late during ontogeny and at the time that synaptic connections mature morphologically and functionally.

Development of the specialized AMPA receptors of auditory neurons

At maturity, the AMPA receptors of auditory neurons exhibit very rapid desensitization kinetics and high permeability to calcium, reflecting the predominance of GluR3 flop and GluR4 flop subunits and the paucity of GluR2. We used mRNA analysis and immunoblotting to contrast the development of AMPA receptor structure in the chick cochlear nucleus [nucleus magnocellularis (NM)] with that of the slowly desensitizing and calcium-impermeable AMPA receptors of brainstem motor neurons in the nucleus of the glossopharyngeal/vagal nerves. The relative abundance of transcripts for GluRs 1-4 changes substantially in auditory (but not motor) neurons after embryonic day (E)10, with large decreases in GluR2 and increases in GluR3 and GluR4. Relative to the motor neurons, NM neurons show a higher abundance of flop isoforms of GluRs 2-4 at E10, suggesting that auditory neurons are already biased toward expression of flop isoforms before the onset of synaptic function at E11. Immunoreactivities in NM show very distinct developmental patterns from E13 onward: GluR2 declines by >90%, GluR3 increases threefold, and GluR4 remains relatively constant. Our results show that there are a series of critical points during normal development, most occurring after the onset of function, when rapid changes in receptor structure (occurring via both transcriptional and post-transcriptional control mechanisms) produce the specialized AMPA receptor functions that enable auditory neurons to accurately encode acoustic information.

Developmental profiles of glutamate receptors and synaptic transmission at a single synapse in the mouse auditory brainstem

Using whole-cell recordings from presynaptic terminals and postsynaptic principal neurons in the mouse medial nucleus of the trapezoid body (MNTB), we have characterized properties of the calyx of Held synapse during the first three postnatal weeks. We observed that evoked excitatory postsynaptic currents (EPSCs) mediated by NMDA receptors (NMDAR) increased until postnatal day 11/12 (P11/12) after which they declined to very low or undetectable levels at P16. Meanwhile, EPSCs mediated by AMPA receptors (AMPAR) showed an approximate three-fold increase in amplitude. These changes were paralleled by NMDAR and AMPAR currents evoked by exogenous NMDA and kainate to MNTB neurons except that whole-cell kainate currents remained constant after P7/8 while AMPAR-EPSCs continued to increase. We found that the decay time constant tau for NMDAR-EPSCs and AMPAR-EPSCs declined by about 30 % and 70 %, respectively. Analyses of NMDAR-EPSCs with subunit-specific pharmacological agents including ifenprodil, N,N,N',N'-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN), zinc and Mg(2+) revealed subtle developmental changes in subunit composition. As maturation progressed, this synapse displayed a reduction in the number of presynaptic spike failures and the extent of synaptic depression in response to trains of stimuli (50-300 Hz) while the recovery rate from depression accelerated. These results demonstrate profound changes in the size and kinetics of postsynaptic glutamate receptors and in the spike-firing capability of presynaptic terminals at the calyx of Held-MNTB synapse during early development. We suggest that these concurrent presynaptic and postsynaptic adaptations represent important steps for synapse consolidation and refinement and ultimately for the development of fast high-fidelity transmission at this synapse.

Early activation of Ca(2+)-permeable AMPA receptors reduces neurite outgrowth in embryonic chick retinal neurons

Calcium entry through Ca(2+)-permeable AMPA/kainate receptors may activate signaling cascades controlling neuronal development. Using the fluorescent Ca(2+)-indicator Calcium Green 1-AM we showed that the application of kainate or AMPA produced an increase of intracellular [Ca(2+)] in embryonic chick retina from day 6 (E6) onwards. This Ca(2+) increase is due to entry through AMPA-preferring receptors, because it was blocked by the AMPA receptor antagonist GYKI 52466 but not by the N-methyl-D-aspartic acid (NMDA) receptor antagonist AP5, the voltage-gated Ca(2+) channel blockers diltiazem or nifedipine, or by the substitution of Na+ for choline in the extracellular solution to prevent the depolarizing action of kainate and AMPA. In dissociated E8 retinal cultures, application of glutamate, kainate, or AMPA reduced the number of neurites arising from these cells. The effect of kainate was prevented by the AMPA/kainate receptor antagonist CNQX and by GYKI 52466 but not by AP5, indicating that the reduction in neurite outgrowth resulted from the activation of AMPA receptors. Blocking Ca(2+) influx through L-type voltage-gated Ca(2+) channels with diltiazem and nifedipine prevented the effect of 10-100 microM kainate but not that of 500 microM kainate. In addition, joro spider toxin-3, a blocker of Ca(2+)-conducting AMPA receptors, prevented the effect of all doses of kainate. Neither GABA, which is depolarizing at this age in the retina, nor the activation of metabotropic glutamate receptors with tACPD mimicked the effects of AMPA receptor activation. Calcium entry via AMPA receptor channels themselves may therefore be important in the regulation of neurite outgrowth in developing chick retinal cells.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

Kainate-activated cobalt uptake in the primary gustatory nucleus in goldfish: visualization of the morphology and distribution of cells expressing AMPA/kainate receptors in the vagal lobe

Gustatory afferent fibers of the vagus nerve that innervate taste buds of the oropharynx of the goldfish, Carassius auratus, project to the vagal lobe, which is a laminated gustatory nucleus in the dorsal medulla. As in the mammalian gustatory system, responses by second-order cells in the goldfish medulla are mediated by N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic glutamate receptors. We utilized a cobalt uptake technique to label vagal lobe neurons that possess cobalt-permeable ionotropic glutamate receptors. Vagal lobe slices were bathed in kainate (40 microM) or glutamate (0.5 or 1 mM) in the presence of CoCl(2), which can pass into cells through the ligand-gated cation channels of non-NMDA receptors made up of certain subunit combinations. Cobalt-filled cells and dendrites were observed in slices that were activated by kainate or glutamate, but not in control slices that were bathed in CoCl(2) alone, nor in slices that were bathed with the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (10 microM) in addition to an agonist. Likewise, simple depolarization of the cells with KCl failed to induce cobalt loading. Cobalt-filled round unipolar cells, elongate or globular bipolar cells, and multipolar cells with elongate or polygonal perikarya were distributed throughout the cell layers in the sensory zone of the vagal lobe. Numerous labeled neurons had dendrites spanning layers IV and VI, the two principal layers of primary afferent input. Apical and basal dendrites often extended radially through neighboring laminae, but many cells also extended dendrites tangential to the lamination of the sensory zone. In the motor layer, cell bodies and proximal dendrites of small, multipolar neurons, and large motoneurons were regularly loaded with cobalt.

The AMPA receptors of auditory neurons

The ionotropic glutamate receptor (GluR) subtype known as the AMPA receptor, which mediates rapid excitatory synaptic transmission in many regions of the nervous system, is composed of four different protein subunits, termed GluRs 1-4. The functional properties of each AMPA receptor are determined by the relative levels of GluRs 1-4 and by post-transcriptional modifications of these proteins through mRNA editing and alternative exon splicing. The present paper reviews the published evidence for (1) localization of mRNAs and immunoreactivity for GluRs 1-4 in the cochlea and subcortical central nervous system auditory pathways of mammals and birds, and (2) involvement of AMPA receptors in synaptic transmission in the auditory system. Recent biochemical and electrophysiological evidence concerning the specialized properties of AMPA receptors on brainstem auditory neurons is also reviewed, along with data concerning how these properties emerge during normal development.

Differential distribution of glutamate receptors in the cochlear nuclei

Glutamate receptors are the major excitatory neurotransmitter receptors of the mammalian central nervous system, and include AMPA, kainate, delta, NMDA, and metabotropic types. In the cochlear nucleus (CN), the AMPA receptor subunits GluR2-4 are found in major kinds of neurons, while GluR1 subunit distribution is more restricted. GluR2 is low in the anteroventral CN, suggesting that many AMPA receptors here are calcium-permeable. Delta receptors are most prevalent in cartwheel cells in the dorsal CN. Of the NMDA receptors, NR1 is widespread while the NR2 subunits show more restricted distributions. Of the metabotropic glutamate receptors, mGluR1alpha is most prevalent in the dorsal CN, and mGluR2 is concentrated in Golgi cells and unipolar brush cells. AMPA receptors in endbulb synapses in the anteroventral CN are mainly GluR3+4 complexes: probably an adaptation for rapid auditory neurotransmission. Glutamate receptors are differentially distributed in synapses of fusiform cells of the dorsal CN; GluR4 and mGluR1alpha are present only at basal dendrite synapses (auditory nerve), while other glutamate receptors occupy both apical and basal synapses. Analysis of cytoplasmic distribution suggests that a selective targeting mechanism may restrict movement of GluR4 and mGluR1alpha to basal dendrites, although other targeting mechanisms may be present.

Development of the auditory brainstem of birds: comparison between barn owls and chickens

Birds have proved to be extremely useful models for the study of hearing function. In particular, chickens and barn owls have been widely used by a number of researchers to study diverse aspects of auditory function. These studies have benefited from the advantages offered by each of these two species, including differences of auditory specialization. Direct comparisons between chickens and barn owls become complicated when the degree of auditory specialization and their modes of development are brought into consideration. In this article we review the available literature on the development of the auditory brainstem of chickens and barn owls in the context of such differences. In addition, we present a time line constructed on the basis of common stages of structural differentiation, rather than chronological time. We suggest that such a time line should be considered when discussing comparative data between these two species. Such an approach should facilitate the interpretation of similarities and differences observed in the developmental processes of the auditory system of chickens and barn owls.

Long-term specification of AMPA receptor properties after synapse formation

AMPA receptors expressed at auditory nerve synapses in the mammalian and avian cochlear nuclei display exceptionally rapid channel gating, an adaptation well suited for acoustic processing. We examined whether cellular interactions during development might determine the subunit composition of these receptors. After synapse formation in the avian nucleus magnocellularis (nMag) in vivo, the rate of receptor desensitization increased threefold, sensitivity to channel block by polyamines increased, and sensitivity to cyclothiazide, an inhibitor of desensitization, increased, indicating a reduction in glutamate receptor subunit 2 and of flip splice variants. This phenotypic switch was prevented, but not reversed, by isolating nMag neurons in a cell-culture environment. We propose that the switch in receptor kinetics is an outcome of cellular interactions during a critical period that result in the long-term determination of receptor phenotype.

Contrasting molecular composition and channel properties of AMPA receptors on chick auditory and brainstem motor neurons

1. Neurons in the brainstem auditory pathway exhibit a number of specializations for transmitting signals reliably at high rates, notably synaptic AMPA receptors with very rapid kinetics. Previous work has not revealed a common structural pattern shared by the AMPA receptors of auditory neurons that could account for their distinct functional properties. 2. We have used whole-cell patch-clamp recordings, mRNA analysis, immunofluorescence, Western blots and agonist-evoked cobalt uptake to compare AMPA receptors on the first-, second- and third-order neurons in the chick ascending auditory pathway with those on brainstem motor neurons of the glossopharyngeal/vagal nucleus, which have been shown to have very slow desensitization kinetics. 3. The results indicate that the AMPA receptors of the cochlear ganglion, nucleus magnocellularis and nucleus laminaris share a number of structural and functional properties that distinguish them from the AMPA receptors of brainstem motor neurons, namely a lower relative abundance of glutamate receptor (GluR)2 transcript and much lower levels of GluR2 immunoreactivity, higher relative levels of GluR3 flop and GluR4 flop, lower relative abundance of the C-terminal splice variants GluR4c and 4d, less R/G editing of GluR2 and 3, greater permeability to calcium, predominantly inwardly rectifying I-V relationships, and greater susceptibility to block by Joro spider toxin. 4. We conclude that the AMPA receptors of auditory neurons acquire rapid kinetics from their high content of GluR3 flop and GluR4 flop subunits and their high permeability to Ca2+ from selective post-transcriptional suppression of GluR2 expression.

Development of calretinin immunoreactivity in the brainstem auditory nuclei of the barn owl (Tyto alba)

The early development of calretinin immunoreactivity (CR-IR) was described in the auditory nuclei of the brainstem of the barn owl. CR-IR was first observed in the auditory hindbrain at embryonic day (E17) and a day later (E18) in the inferior colliculus. In each of the auditory nuclei studied, CR-IR did not develop homogeneously, but began in the regions that map high best frequencies in the adult barn owl. In the hindbrain, CR-IR was first observed in the rostromedial regions of the cochlear nucleus magnocellularis and the nucleus laminaris, and in the dorsal regions of the nucleus angularis and in the nucleus of the lateral lemniscus. In the inferior colliculus, CR-IR began in the ventral region of the central core. The edge of these gradients moved along the future tonotopic axes during the development of all nuclei studied, until adult patterns of CR-IR were achieved about a week after hatching.

Synaptic mechanisms for coding timing in auditory neurons

Neurons in the cochlear ganglion and auditory brain stem nuclei preserve the relative timing of action potentials passed through sequential synaptic levels. To accomplish this task, these neurons have unique morphological and biophysical specializations in axons, dendrites, and nerve terminals. At the membrane level, these adaptations include low-threshold, voltage-gated potassium channels and unusually rapid-acting transmitter-gated channels, which govern how quickly and reliably action potential threshold is reached during a synaptic response. Some nerve terminals are remarkably large and release large amounts of excitatory neurotransmitter. The high output of transmitter at these terminals can lead to synaptic depression, which may itself be regulated by presynaptic transmitter receptors. The way in which these different cellular mechanisms are employed varies in different cell types and circuits and reflects refinements suited to different aspects of acoustic processing.

Glutamate receptor phenotypes in the auditory brainstem and mid-brain of the developing rat

Glutamate receptors mediate most excitatory synaptic transmission in the adult vertebrate brain, but their activation in developing neurons also influences developmental processes. However, little is known about the developmental regulation of the subunits composing these receptors. Here we have studied age-dependent changes in the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits in the cochlear nucleus complex (CN), the superior olivary complex (SOC), the nuclei of the lateral lemniscus, and the inferior colliculus of the developing rat. In the lateral superior olive, the medial nucleus of the trapezoid body, and the ventral nucleus of the lateral lemniscus, the distribution of AMPA receptor subunits changed drastically with age. While GluR1 and GluR2 subunits were highly expressed in the first 2 postnatal weeks, GluR4 staining was detectable only thereafter. GluR1 and GluR2 immunoreactivities rapidly decreased during the third postnatal week, with the GluR1 subunits disappearing from most neurons. In contrast, the adult pattern of the distribution of AMPA receptor subunits emerged gradually in most of the other auditory nuclei. Thus, progressive as well as regressive events characterized AMPA receptor development in some nuclei, while a monotonically maturation was seen in other regions. In contrast, the staining patterns of NMDA receptor subunits remained stable or only decreased during the same period. Although our data are not consistent with a generalized pattern of AMPA receptor development, the abundance of GluR1 subunits is a distinctive feature of early AMPA receptors. As similar AMPA receptors are present during plasticity periods throughout the brain, neurons undergoing synaptic and structural remodelling might have a particular need for these receptors.

Development of calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in cultured neocortical neurons visualized by cobalt staining

The developmental expression of calcium (Ca2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors in cultured neocortical neurons was evaluated by using cobalt uptake, a histochemical method that identifies cells expressing Ca2+-permeable, non-N-methyl-D-aspartate (non-NMDA) receptors. At a concentration of 500 microM, AMPA was found to stimulate cobalt uptake only late in development, resulting in staining of 2.7%+/-0.3% of the neurons maintained in culture for 12 days in vitro (DIV). When AMPA receptor desensitization was blocked with 50 microM cyclothiazide, the developmental profile of cobalt uptake mediated by 25 microM AMPA changed dramatically. The cobalt staining now appeared in young cultures (5 DIV), and the percentage of stained cells increased from 3.4%+/-0.2% at 5 DIV to 21.7%+/-1.6% at 12 DIV. The effect of 200 microM kainate was similar to that seen with 25 microM AMPA plus 50 microM cyclothiazide, resulting in 17.7%+/-0.3% stained neurons at 12 DIV. The cobalt uptake was specific to AMPA and kainate receptors because NMDA receptors and voltage-gated calcium channels were found not to mediate any cobalt staining. In addition, 10 microM 6-nitro-7-sulphamoylbenzo-[f]-quinoxaline-2,3-dione (NBQX) was able to prevent all staining at 5 and 8 DIV and most of the staining at 12 DIV, indicating that the non-NMDA ionotropic glutamate receptors are involved in cobalt uptake into the neurons. The AMPA receptor-selective antagonist GYKI 53655 was used to differentiate between cobalt influx through AMPA- or kainate-preferring receptors. After pretreatment with concanavalin A (con A), an inhibitor of kainate receptor desensitization, cobalt uptake was assessed after stimulation by 200 microM kainate in the presence of 25 microM GYKI 53655. No cobalt staining was observed under these conditions, indicating that most if not all of the cobalt influx induced by kainate was mediated through AMPA receptor channels.

Activity-dependent regulation of [Ca2+]i in avian cochlear nucleus neurons: roles of protein kinases A and C and relation to cell death

Neurons of the cochlear nucleus, nucleus magnocellularis (NM), of young chicks require excitatory afferent input from the eighth nerve for maintenance and survival. One of the earliest changes seen in NM neurons after deafferentation is an increase in intracellular calcium concentration ([Ca2+]i). This increase in [Ca2+]i is due to loss of activation of metabotropic glutamate receptors (mGluR) that activate second-messenger cascades involved in [Ca2+]i regulation. Because mGluRs are known to act via the phospholipase C and adenylate cyclase signal transduction pathways, the goal of this study was to determine the roles of protein kinases A (PKA) and C (PKC) activities in the regulation of NM neuron [Ca2+]i by eighth nerve stimulation. Additionally, we sought to determine the relationship between increased [Ca2+]i and cell death as measured by propidium iodide incorporation. [Ca2+]i of individual NM neurons in brain stem slices was monitored using fura-2 ratiometric fluorescence imaging. NM field potentials were monitored in experiments in which the eighth nerve was stimulated. Five hertz orthodromic stimulation maintained NM neuron [Ca2+]i at approximately 110 nM for 180 min. In the absence of stimulation, NM neuron [Ca2+]i increased steadily to a mean of 265 nM by 120 min. This increase was attenuated by superfusion of PKC activators phorbol-12,13-myristate acetate (100 nM) or dioctanoylglycerol (50 microM) and by activators of PKA: 1 mM 8-bromoadenosine-3',5'-cyclophosphate sodium (8-Br-cAMP), 50 microM forskolin or 100 microM Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine. Inhibition of PKA (100 microM Rp-cAMPS) or PKC (50 nM bisindolymaleimide or 10 microM U73122) during continuous orthodromic stimulation resulted in an increase in NM neuron [Ca2+]i that exceeded 170 and 180 nM, respectively, by 120 min. Nonspecific kinase inhibition with 1 microM staurosporine during stimulation resulted in an [Ca2+]i increase that was greater in magnitude than that seen with either PKA or PKC inhibition alone, equal to that seen in the absence of stimulation, but much smaller than that seen with inhibition of mGluRs. In addition, manipulations that resulted in a [Ca2+]i increase >/=250 nM resulted in an increase in number and percentage of propidium iodide-labeled NM neurons. These results suggest that eighth nerve activity maintains [Ca2+]i of NM neurons at physiological levels in part via mGluR-mediated activation of PKA and PKC and that increases in [Ca2+]i due to activity deprivation or interruption of the PKA and PKC [Ca2+]i regulatory mechanisms are predictive of subsequent cell death.

Development of AMPA-selective glutamate receptors in the auditory brainstem of the barn owl

AMPA receptor specific antibodies were used to study the distribution and development of glutamate receptor subtypes (GluR1-4) in nucleus magnocellularis, angularis, laminaris, and the superior olive of the barn owl. Each nucleus in the adult barn owl expresses characteristic levels of AMPA receptor subtypes, and all are enriched in the subunits associated with rapid desensitization (GluR2 and 4). In the auditory hindbrain of the barn owl, the levels of expression of all AMPA receptors were very low at the time of hatching. In all nuclei, the level of GluR1 immunoreactivity was low to undetectable at all ages studied. In the cochlear nuclei, angularis and magnocellularis, levels of GluR2/3 and GluR4 immunoreactivity increased over the first 2 weeks after hatching, coinciding with the morphological maturation of auditory nerve terminals in NM. In the nucleus laminaris and in the superior olive, GluR2/3 and GluR4 immunoreactivity reached adult-like patterns by 3 weeks after hatching. Thus, adult-like patterns of immunoreactivity appeared at least 1 month before the end of the sensitive period in all nuclei studied.

Kinetic analysis of glycine receptor currents in ventral cochlear nucleus

Glycine plays an important role as an inhibitory neurotransmitter in the ventral cochlear nucleus. However, little is known about the kinetic behavior of glycine receptors. The present study examines the kinetics of the native inhibitory glycine receptors in neurons of the ventral cochlear nucleus, using outside-out patches from acutely dissociated cells and a fast flow system. Steps into 1 mM glycine revealed fast phases of desensitization with time constants of 13 and 129 ms, that together produced a 40% reduction in current from the peak response. Slower desensitization phases also were observed. After removal of glycine, currents deactivated with two time constants of 15 and 68 ms, and these rates were independent of the glycine concentration between 0.2 and 1 mM. Recovery from desensitization was slow relative to desensitization itself. These results demonstrate that glycine receptors can exhibit faster rates of desensitization and deactivation than previously reported.

Development of dynorphin-like immunoreactive auditory nerve terminals in the chick

The novel discovery that auditory nerve terminals in the chick cochlear nucleus magnocellularis (NM) are immunoreactive for the opioid peptide dynorphin (DYN) was recently reported [3]. The present study examines the development of DYN-immunoreactivity (DYN-I) in auditory nerve terminals in NM from embryos through young post-hatch chicks. No DYN-I was observed in NM at embryonic day 13 (E13). DYN-I first appeared at E16 as short flat structures partially surrounding NM cell bodies. Around post-hatch day 1 (P1), these structures had a more rounded, chalice-type of morphology reminiscent of the specialized auditory nerve terminals found in birds, the end-bulbs of Held. At P6, most NM neurons were circumscribed by a prominent DYN-I calyceal-type of ending. By P13, fewer NM cells were ringed by this DYN-I and by the third post-hatch week, there was very little DYN-I in NM. There were no obvious differences in the density of DYN-I terminals across either the rostrocaudal length or the mediolateral width of NM at any age examined. These results suggest that during a restricted time of development, end-bulbs of Held in the chick NM contain DYN.

Glutamate-induced Co2+ uptake in rat auditory brainstem neurons reveals developmental changes in Ca2+ permeability of glutamate receptors

Ca2+ influx through glutamate receptors (GluRs) is thought to play a crucial part in developmental processes and neuronal plasticity. Here we have examined the spatiotemporal distribution of Ca2+-permeable GluRs in auditory brainstem neurons of the rat from birth to adulthood, using the cobalt-staining technique of Pruss and collaborators. In slices of young adult rats, 1 mM glutamate evoked intense cobalt uptake in subsets of neurons in the ventral cochlear nuclei, the medial nucleus of the trapezoid body, the lateral and the medial superior olive, and the lateral lemniscal nuclei. Neurones in the central nucleus of the inferior colliculus, and thalamic auditory nuclei appear to express few, if any, Ca2+-permeable GluRs. Thus, in adults, Ca2+-permeable GluRs are present in neurons of almost all main relay stations of the auditory brainstem. During development, cobalt-stained cells first appeared at about hearing onset (at postnatal day 12 [P12]). At P16, staining levels were highest and the pattern of distribution was already adult-like. The staining intensity slightly declined during the fourth postnatal week. In contrast, Ca2+-permeable receptors were detected in the external cortex of the inferior colliculus as early as P4. Our results show that auditory neurons, characterized by a high temporal precision in neuronal activity, display Ca2+-permeable GluRs. Because Ca2+ permeability appears at about the onset of hearing and is highest during the following 2 weeks, Ca2+ influx through GluRs is likely to be implicated in remodelling processes occurring during this ontogenetic period.

Endbulb synapses in the anteroventral cochlear nucleus express a specific subset of AMPA-type glutamate receptor subunits

The anteroventral cochlear nucleus (AVCN) acts as the first relay center in the conduction of auditory information from the ear to the brain, and it probably performs a crucial role in sound localization. Auditory nerve input to the principal neurons of the AVCN, the spherical bushy cells, appears to be mediated by an excitatory amino acid such as glutamate, which acts at a specialized, large synaptic ending called an endbulb of Held. Presumably, endbulb synapses contain some specific combination of glutamate receptors to facilitate rapid neurotransmission of auditory signals. AMPA glutamate receptor composition at the endbulb synapses was examined with both light and electron microscope immunocytochemistry. Electron microscope localization of AMPA receptors was examined with two techniques, preembedding immunoperoxidase and postembedding immunogold, which provide maximum sensitivity and greatest accuracy, respectively. Dense and frequent labeling was seen with the AMPA receptor subunit antibodies GluR2/3 and GluR4, which were colocalized at the endbulb synapses. In contrast, immunolabeling with antibody to GluR2 was low. These data indicate that the major glutamate receptor at this synapse is an AMPA receptor made up mainly of GluR3 and GluR4 subunits. Receptors composed of these subunits display properties, such as calcium permeability and rapid desensitization, that facilitate their specialized functions in auditory information processing.

Cellular mechanisms for preservation of timing in central auditory pathways

The faithful preservation of acoustic timing information, as signals are passed from one synaptic level to another, requires a convergence of morphological, biophysical, and biochemical specializations in auditory neurons. Recent studies have focused on the adaptive membrane properties of neurons in the auditory brainstem. These include analyses of neurotransmitter receptors and voltage-gated channels, as well as the mechanisms of transmitter release and its modulation. The molecular composition of the relevant proteins are now being demonstrated, including the glutamate receptor Dflop (GluR-Dflop) subunit of AMPA receptors and members of the Kv1 and Kv3 families of potassium channels.

Prevention of normally occurring and deafferentation-induced neuronal death in chick brainstem auditory neurons by periodic blockade of AMPA/kainate receptors

The role of glutamate receptors in regulating programmed neuronal death and deafferentation-induced neuronal death in the brainstem auditory nuclei was studied by in ovo drug administration to chick embryos. The nucleus laminaris (NL) undergoes programmed developmental cell death of 19% between embryonic day 9 (E9) and E17. The AMPA/kainate receptor antagonist CNQX, when administered at doses of 200-300 microg/d from E8 to E15, prevented programmed neuronal death in NL through at least posthatching day 8, without producing anatomical or behavioral abnormalities. 3-((RS)-2-Carboxypiperazin-4-yl)-propyl-1-phos-phonic acid, an antagonist of NMDA receptors, had no effect on normal cell death in the NL. CNQX, given from E8 to E15 or only from E8 to E10, also blocked the 33% neuronal loss in the nucleus magnocellularis (NM) that follows surgical destruction of the otocyst on E3, a procedure that deafferents NM neurons by preventing formation of the cochlear nerve. Treatment either with CNQX or the more highly selective NBQX from E8 to E10, before the onset of synaptic transmission in NM and NL, was also effective in preventing normal neuronal death in NL. Analysis of the effects of CNQX or NBQX on spontaneous embryonic motility at E10 showed that the doses effective in preventing neuronal death suppressed motility for <8 hr. We conclude that periodic blockade of AMPA/kainate receptors can protect CNS neurons against subsequent programmed cell death or deafferentation-induced death.