Gamma-D-glutamylaminomethyl sulfonic acid (GAMS) distinguishes subtypes of glutamate receptor in the chick cochlear nucleus (nuc. magnocellularis)

Unitary IPSPs drive precise thalamic spiking in a circuit required for learning

Song learning in birds requires a basal ganglia-thalamo-pallial loop that contains a calyceal GABAergic synapse in the thalamus. Information processing within this circuit is critical for proper song development; however, it is unclear whether activation of the inhibitory output of the basal ganglia structure Area X can drive sustained activity in its thalamic target, the medial portion of the dorsolateral thalamic nucleus (DLM). We show that high-frequency, random activation of this GABAergic synapse can drive precisely timed firing in DLM neurons in brain slices in the absence of excitatory input. Complex IPSP trains, including spike trains recorded in vivo, drive spiking in slices with high reproducibility, even between animals. Using a simple model, we can predict much of DLM's response to natural stimulus trains. These data elucidate basic rules by which thalamic relay neurons translate IPSPs into suprathreshold output and demonstrate extrathalamic GABAergic activation of thalamus.

Group I and II metabotropic glutamate receptors are necessary for the activity-dependent regulation of ribosomes in chick auditory neurons

Elimination of eighth-nerve activity results in the death of 30% of the neurons in the chick cochlear nucleus, nucleus magnocellularis (NM). One early event in this cell death cascade is the disruption of ribosomes in NM neurons which can be observed within 1 h following deafferentation. These rapid changes in ribosomes can be visualized using Y10B, a monoclonal antibody that recognizes ribosomal RNA. Previous studies using a brain slice preparation of the avian brain stem auditory system have shown that activation of metabotropic glutamate receptors (mGluRs) is necessary for the activity-dependent maintenance of Y10B antigenicity. The purpose of the present study was to determine if group I and/or II mGluRs are necessary for this activity-dependent regulation. This was accomplished by selectively blocking group I or II receptors while unilaterally stimulating the auditory nerve in vitro. In normal media, unilateral stimulation of the auditory nerve resulted in darker Y10B immunolabeling of NM neurons on the stimulated side of the slice. The group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) and the group II antagonists LY341495 and (S)-alpha-ethylglutamic acid (EGLU) all prevented the activity-dependent difference in Y10B immunolabeling. These data suggest that both group I and II mGluRs play vital roles in the activity-dependent regulation of ribosomes in NM.

Pharmacological characterization of glutamatergic agonists and antagonists at recombinant human homomeric and heteromeric kainate receptors in vitro

An increasing body of evidence suggests that native kainate receptors form ion channels from homomeric and heteromeric combinations of five receptor subunits: GluR5, GluR6, GluR7, KA1 and KA2. We have examined the activity of agonists and antagonists at recombinant human kainate receptors expressed in HEK293 cells, using both whole-cell electrophysiological recording and 96-well plate fluo-3 based calcium microfluorimetry (FLIPR). Both homomeric (GluR5 and GluR6) and heteromeric (GluR5/6, GluR5/KA2 and GluR6/KA2) receptors were examined. Heteromeric receptor assemblies showed electrophysiological and pharmacological profiles which were distinct from homomeric channels. Several agonists, including AMPA, ATPA and (S)-5-iodowillardiine, and antagonists, including gamma-D-glutamylaminomethylsulphonic acid (GAMS) and the decahydroisoquinoline compounds LY293558, LY377770 and LY382884, were found to act at GluR5-containing channels while having no effect at GluR6 homomers. AMPA, ATPA and (S)-5-iodowillardiine did activate GluR6/KA2 heteromers, but only as partial agonists. Additionally, ATPA was shown to act as an antagonist at homomeric GluR6 receptors at high concentrations (IC50 approximately 2 mM). Kynurenic acid was also found to differentiate between GluR6 and GluR6/KA2 receptors, antagonizing glutamate at GluR6 (IC50 = 0.4 mM), while having no effect at GluR6/KA2 channels. The results of the current study provide a broad pharmacological characterization of both homomeric and heteromeric recombinant human kainate receptors, and identify which compounds are likely to be useful tools for studying these various receptor subtypes.

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.

Neuronal death, not axonal degeneration, results in significant gliosis within the cochlear nucleus of adult chickens

Injury to the central nervous system initiates a series of events that leads to neuronal cell death and glial activation. Astrocytes respond to damage and disease by becoming hyperplastic and hypertrophied. This 'reactive gliosis' is also accompanied by the upregulation of the intermediate filament protein glial fibrillary acidic protein, the release of growth factors and the formation of the glial scar. However, the signaling cascades which regulate these events, and the molecular mechanisms that give rise to this diverse response, have not been fully elucidated. For example, the role played by degenerating neurons vs. degenerating axons in the activation of astrocytes remains to be determined. To investigate the influence of neuronal cell death vs. axonal degeneration on gliosis, the current study examines the astrocyte response to cochlea removal in two different breeds of adult chickens, one of which exhibits neuronal cell death within the brainstem nucleus magnocellularis (NM) following the lesion and one which does not. Our results indicate that degeneration of NM neurons leads to large increases in both glial proliferation and hypertrophy, while eighth nerve degeneration without NM cell death results in very small increases in glial proliferation.

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.

Non-NMDA glutamate receptors modulate capsaicin induced c-fos expression within trigeminal nucleus caudalis

1. We examined the effects of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzol[f]quinoxaline-7-sulpho namide (NBQX), the kainate receptor antagonists gamma-(R-)-glutamylaminomethanesulphonic acid (GAMS) and 6,7,8,9-tetrahydro-5-nitro-1H-benz[g]indole-2,3-dione-3-oxime (NS-102), and the group III metabotropic glutamate receptor (mGluR) agonist 2-amino-4-phosphono-S-butanoic acid (L-AP4) on c-fos-like immunoreactivity (c-fos LI) in trigeminal caudalis (Sp5C), lateral reticular (LRt), medullary reticular (Md) and solitary tract (Sol) nuclei, after intracisternal injection of capsaicin in urethane anaesthetized Sprague-Dawley rats. 2. Few c-fos labelled cells were observed within Sp5C in capsaicin-vehicle treated animals. The number of positive c-fos cells increased by 17 fold after intracisternal capsaicin (5 nmol) administration. 3. Pretreatment with CNQX (0.02, 0.1, 0.6, 3 and 15 mg kg-1) or NBQX (0.01, 0.1 and 1 mg kg-1), administered intraperitoneally 15 min before capsaicin, significantly reduced labelled cells within Sp5C by a maximum of 45 and 34%, respectively. The number of c-fox LI cells within LRt, Md and Sol was not affected. Pretreatment with L-AP4 (1, 3 and 10 mg kg-1) decreased the number of Sp5C c-fos LI cells by a maximum of 30%, whereas GAMS (1 and 10 mg kg-1) and NS-102 (1 and 5 mg kg-1) did not show any significant effect. 4. These results suggest that blockade of AMPA receptors, but not kainate receptors, or the activation of group III mGluRs, decrease the response of Sp5C neurons to trigeminovascular activation. Thus, in addition to NMDA receptors, mGluRs and AMPA receptors may modulate cephalic pain and may provide a potential therapeutic target for antimigraine drugs.

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.

Breed differences in deafferentation-induced neuronal cell death and shrinkage in chick cochlear nucleus

Removal of functional presynaptic input can result in a variety of changes in postsynaptic neurons in the central nervous system, including altered metabolism, changes in neuronal cell size, and even death of the postsynaptic cell. Age-dependent neuronal cell death and shrinkage has been documented in second order auditory neurons in the chick brainstem (nucleus magnocellularis, NM) following cochlea removal (Born and Rubel, 1985. J. Comp. Neurol. 231, 435-445). Here we examined whether the extent of neuronal cell death and shrinkage is also breed-dependent. We performed unilateral cochlea removal on both hatchling and adult birds of either a broiler breed (Arbor Acres Cross) or egg layer breed (Hy-Line, H and N) and killed birds one week later. Changes in neuronal cell number and cross sectional area were determined from Nissl-stained sections. We observed 25% neuronal cell loss and a 15-20% decrease in neuronal cross sectional area after cochlea removal in either broiler or egg layer hatchling birds. In adult birds, however, neuronal cell loss is breed-dependent. Adult egg layer birds lose an average of 37% of NM neurons after cochlea removal, while adult broiler birds show no cell loss. In both breeds of adult birds, cochlea removal results in a 20% decrease in neuronal cross sectional area. These results suggest that analysis of differences between breeds as well as ages of birds will prove fruitful in determining how afferent input controls neuronal survival and metabolism.

Activation of metabotropic glutamate receptors is necessary for transneuronal regulation of ribosomes in chick auditory neurons

Elimination of auditory nerve activity results in atrophy and death of nucleus magnocellularis (NM) neurons in the chick. One early event in the degeneration of NM neurons is a disruption of their ribosomes. This experiment examines the role of metabotropic glutamate receptors in afferent regulation of ribosomes. The auditory nerve on one side of a chick brainstem slice was stimulated in vitro. Rapid stimulation-dependent changes in ribosomes were visualized by immunolabeling using an antibody, called Y10B, that recognizes ribosomal RNA. In normal media, NM neurons on the stimulated side of the slice show greater Y10B labeling than the unstimulated NM neurons on the opposite side of the same slice. The role of metabotropic glutamate receptors was evaluated by unilaterally stimulating the auditory nerve in media containing the metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4-carboxyphenyl-glycine (MCPG). Addition of MCPG to the bath did not block EPSPs produced by stimulating the auditory nerve. However, MCPG did prevent the stimulation-dependent regulation of ribosomes in NM neurons (as indexed by Y10B labeling). These data suggest that glutamate may play a trophic role in the young auditory system through activation of metabotropic glutamate receptors.

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.

Differences in expression of GABAA receptor subunits, but not benzodiazepine binding, in the chick brainstem auditory system

Neurons in nucleus magnocellularis (NM) and nucleus laminaris (NL) of the chick brainstem auditory system show an unusual physiological response to GABA. Examination of these nuclei using in situ hybridization for GABAA receptor subunits showed a differential expression of the gamma 2 and alpha 1 subunits. The gamma 2 subunit was found in both NM and NL, but the alpha 1 subunit was found in NL only. Like NL, other areas of the tissue section that showed labeling with the gamma 2 probe, such as the medial vestibular nucleus (VeM) and granule cells of the cerebellum (CB), also labeled with the alpha 1 probe. Thus, given that NM labeled with the gamma 2 probe, the absence of the alpha 1 subunit was unusual in this tissue. This difference in subunit composition suggests that there may also be a difference in GABA receptor function in NM compared to these neighboring areas. One feature of the GABAA receptor believed to be related to the presence of gamma 2 and alpha 1 subunits is specific pharmacological properties of the benzodiazepine modulatory site. It has been proposed that the alpha 1 subunit is necessary for producing a GABAA receptor with a benzodiazepine site that has Type I binding characteristics. The present experiments challenge this hypothesis. Based on the differential presence of the alpha 1 subunit, one would expect that GABA receptors in NM would show different benzodiazepine binding properties than NL, VeM, and CB. However, displacement of 3H-flunitrazepam binding using CL 218,872, which differentiates between the Type I and Type II receptors, showed no difference between these areas. Additionally, the relatively high affinity for CL 218,872 suggests that even NM contains Type I receptors.

Transneuronal regulation of ribosomes after blockade of ionotropic excitatory amino acid receptors

Elimination of auditory nerve activity results in death and atrophy of neurons in the cochlear nucleus, nucleus magnocellularis (NM), of the chick. One early event believed to lead to cell death and atrophy is the disruption of ribosomes in the NM neuron. A useful assay for visualizing these ribosomal changes is immunolabeling with the antibody Y10B, which recognizes ribosomal RNA. Activity-dependent changes in Y10B labeling have been observed both in vivo, after unilateral cochlea removal and in vitro after unilateral auditory nerve stimulation. Although it is clear that activity is crucial for maintaining ribosomal integrity, the identity of the important transynaptic signal(s) is not known. It is possible that this trophic signal is glutamate, the neurotransmitter release from the auditory nerve. The present study investigates the role of ionotropic glutamate receptors in the activity-dependent regulation of ribosomes, as measured by the Y10B immunoreactivity. Brain slices containing the auditory nerve and NM on both sides were obtained from hatchling chicks. The auditory nerve on one side of the slice was stimulated for 1 h. The slice was then processed for Y10B immunoreactivity. As expected, greater Y10B immunolabeling was observed on the stimulated side of the slice. Unexpectedly, however, this immunolabeling difference was still observed after blocking NMDA receptors (50 microM DL-APV), non-NMDA receptors (20 microM CNQX), or blocking both ionotropic receptor subtypes (APV and CNQX). This was true even though CNQX eliminated driven postsynaptic potentials. These data suggest that ionotropic glutamate receptors are not necessary for the activity-dependent regulation of ribosomes in NM neurons.

The mechanism by which NBQX enhances NMDA currents in retinal ganglion cells

When the quinoxaline NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline), a KA/AMPA antagonist, is bath applied to the tiger salamander retina, a paradoxical action is evident in the light-evoked synaptic responses of ganglion cells: NBQX enhances excitatory synaptic currents at light onset observed under whole-cell voltage-clamp conditions in a perfused retinal slice preparation. This observation was surprising because synaptic inputs into ganglion cells that are mediated by KA/AMPA receptors are entirely blocked by NBQX. Thus, the NBQX-enhanced current is entirely mediated by NMDA receptors. The purpose of this study was to determine the mechanism(s) by which blocking KA/AMPA receptors appears to enhance NMDA currents. Using hyperosmotic sucrose stimulation to activate neurotransmitter release from the inner retina, we observed that NBQX augmented the sucrose-evoked response, suggesting that at least a component of this enhancement may reside in the inner retina. NBQX does not enhance NMDA currents activated by bath applied NMDA, demonstrating that the NBQX-induced enhancement does not result from modulation of NMDA receptors. Voltage-clamp studies, carried out at the appropriate holding potential, indicate that NBQX enhances glutamatergic transmission and reduces inhibitory inputs onto ganglion cells. In the presence of strychnine and picrotoxin, the NBQX-induced enhancement of NMDA currents is eliminated, suggesting that NBQX facilitates the expression of NMDA currents by a selective and partial reduction of inhibitory mechanisms. Additional studies suggest that part of the NMDA enhancement by NBQX is evident at the postsynaptic level, but a presynaptic component probably also participates, perhaps at the level of bipolar cell terminals. One way to account for this observation is to assume that a subpopulation of inhibitory amacrine cells requires KA/AMPA receptors exclusively for their synaptic activation: previous studies of sustained amacrine cells support this interpretation. Thus the NBQX-induced enhancement phenomenon may reflect a network-selective distribution of NMDA and KA/AMPA receptors among third-order neurons.

Activity-related features of synapse morphology: a study of endbulbs of held

The myelinated fibers of the auditory nerve can be divided into two separate populations on the basis of sensitivity to sound, average levels of spike activity, and central branching patterns. The synaptic endings of these populations were investigated for the presence of structural specializations that might correlate with levels of neural activity. We applied intracellular recording and staining methods in cats to analyze directly the relationship between spike activity and the structure of synapses using endbulbs of Held, the large synaptic endings in the anteroventral cochlear nucleus. Endbulbs from fibers having low or high levels of activity were examined and compared using light and electron microscopic methods. All endbulbs exhibited relatively large but incomplete coverage by one-to-several lamellae of glial processes. Endbulbs of high activity fibers were large and contained larger mitochondria than endbulbs of low activity fibers. Furthermore, the synapses of high activity endbulbs were on average smaller but more numerous, possessed greater numbers of associated synaptic vesicles, and exhibited greater curvature of their postsynaptic densities. These structural features are hypothesized to reflect specializations that optimize synaptic transmission.

NBQX, an improved non-NMDA antagonist studied in retinal ganglion cells

The quinoxaline derivative, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline (NBQX), significantly reduced the currents evoked by exogenous application of quisqualate (QQ), kainate (KA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) when applied to ganglion cells, using whole-cell recording in a slice preparation of the tiger salamander retina. A comparison between NBQX and CNQX indicates that NBQX is more effective in blocking AMPA receptors. Also, at up to 10 microM, NBQX has no effect on NMDA-induced currents. Thus at this concentration, NBQX shows no affinity for the glycine binding site of NMDA receptors. For this reason, NBQX is preferred over CNQX for a more effective and selective antagonism toward non-NMDA receptors.

Activity-dependent regulation of a ribosomal RNA epitope in the chick cochlear nucleus

Elimination of auditory nerve activity results in rapid metabolic changes, cell atrophy, and cell death in nucleus magnocellularis (NM), the cochlear nucleus of the chick. The transneuronal signals involved in the activity-dependent regulation of NM neurons are not well understood. One of the most rapid transneuronal effects is alteration in protein synthesis by NM neurons. Previous studies using an in vitro preparation of the brain stem auditory system suggested that up-regulation of protein synthesis in NM neurons requires the action of some trophic substance released by active auditory nerve fibers. Here, similar results were obtained when measuring changes in immunoreactivity using a monoclonal antibody (Y10B) that recognizes ribosomal RNA. This immunolabeling assay has advantages over the global protein synthesis assay in that it is not sensitive to possible changes in specific activity of the precursor pool or possible differences in the uptake of the labeled amino acids. Unilateral stimulation of the auditory nerve for 1 h resulted in greater immunolabeling of NM neurons on the stimulated side of the slice. This is consistent with previous in vivo results after unilateral deafferentation. Blockade of synaptic transmission by maintaining the slice in a low-Ca2+/high Mg2+ medium prevented the stimulation-induced difference in immunolabeling. Electrical stimulation of the postsynaptic NM neurons alone (antidromic stimulation, via electrical stimulation of NM neuron axons) did not result in greater immunolabeling. Rather, antidromically stimulated neurons tended to show lighter labeling. Thus, the transneuronal regulation of ribosomes in NM neurons appears to require some substance released from the active auditory nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid increase in mitochondrial volume in nucleus magnocellularis neurons following cochlea removal

Second-order auditory neurons in nucleus magnocellularis (NM) of the chick brainstem undergo a series of rapid metabolic changes following unilateral cochlea removal, culminating in the death of 25% of NM neurons. Within hours of cochlea removal, ipsilateral NM neurons show marked increases in histochemical staining for the mitochondrial enzymes succinate dehydrogenase and cytochrome oxidase. We investigated corresponding ultrastructural changes in NM neurons by preparing animals undergoing unilateral cochlea removal for transmission electron microscopy. We quantified changes in NM mitochondrial volume by stereological methods and qualitatively compared mitochondrial morphology between NM neurons destined to survive and those destined to die after cochlea removal. Within hours of cochlea removal, ipsilateral NM neurons show striking increases in mitochondrial volume (84% at 6 hours and 236% at 12 hours after cochlea removal compared to unoperated, control animals). At 2 week survival times, ipsilateral NM neurons contain fewer mitochondria than contralateral neurons. Surprisingly, anesthesia alone causes short-term increases in NM mitochondrial volume. Animals anesthetized with pentobarbital and ketamine and sacrificed 6 or 12 hours later showed a 45% increase in mitochondrial volume compared to previously unanesthetized animals. NM neurons destined to die within days of cochlea removal can be identified within several hours after deafferentation by the appearance of their ribosomes. We observed qualitative differences in mitochondrial morphology in dying neurons. Mitochondria in neurons destined to die consistently showed mitochondrial swelling and vacuolization indicative of metabolic dysfunction. Similar mitochondrial changes have been reported when mitochondria take up excess calcium. Ultrastructural changes in NM after cochlea removal display features of both programmed and pathological cell death, in which increased intracellular calcium is thought to play a role.

Maintenance of pharmacologically-immature glutamate receptors by aberrant synapses in the chick cochlear nucleus

Surgical destruction of the otocyst in chick embryos prevents formation of the *** ear, abolishes normal cochlear input to the cochlear nucleus (nucleus magnocellularis, NM) and results in axons from the contralateral NM forming (in addition to their normal bilateral endings in nucleus laminaris, NL) a novel and functional aberrant projection to the deafferented NM. We studied the pharmacology of synaptic transmission at aberrant synapses in an in vitro preparation of the brainstem in chick embryos and hatchlings. Transmission at the aberrant synapses (as with cochlear nerve synapses in NM and NM synapses in NL) is blocked by the quinoxalinedione antagonists CNQX and NBQX, confirming the presence of excitatory amino acid receptors of the non-NMDA subtype. At cochlear nerve synapses in NM, the antagonist potency of NBQX normally decreases rapidly after embryonic day (E)18 (IC50 = 0.69 +/- 0.06 microM, mean +/- S.E.M.), reaching an asymptotic value by E21 (IC50 = 2.7 +/- 0.4 microM) that is maintained at least through posthatching day (P)14 (IC50 = 3.6 +/- 0.3 microM). In the case of the aberrant endings, the potency of NBQX remained (from E21 [IC50 = 0.6 +/- 0.1 microM] through at least P14[IC50 = 0.5 +/- 0.1 microM]) at levels that are statistically indistinguishable from the E18 value for normal cochlear nerve synapses.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered distribution of synaptic densities at aberrant synapses in the chick cochlear nucleus

The role of axon-target cell interactions in shaping the prevalence and distribution of synaptic densities was studied in an experimentally-induced aberrant functional projection from the chick cochlear nucleus (nuc. magnocellularis, NM) to the contralateral NM. Contact with an abnormal target appears to induce in the aberrant axons a pattern of presynaptic densities resembling that in normal cochlear nerve endings in NM. NM axon terminals induced similar numbers of postsynaptic densities (PSDs) per unit length of membrane apposition in both their normal and abnormal targets but the longer membrane apposition in the highly invaginated aberrant terminal in NM results in a significantly greater amount of postsynaptic density per ending. These auditory neurons thus appear able to adjust a variety of features to permit assembly and maintenance of a novel functional synapse.

Developmental changes in the effects of drugs acting at NMDA or non-NMDA receptors on synaptic transmission in the chick cochlear nucleus (nuc. magnocellularis)

The developmental pharmacology of excitatory amino acid (EAA) receptors in the chick cochlear nucleus (nucleus magnocellularis, NM) was studied by means of bath application of drugs and recording of synaptically-evoked field potentials in brain slices taken from chicks aged embryonic day (E) 14 through hatching (E21). The abilities of various EAA agonists (N-methyl-D-aspartate [NMDA], kainic acid, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA]) to suppress postsynaptic responses by depolarization block and of EAA antagonists ((3-[RS]-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CCP], dizocilpine [MK-801], 6-nitro-7-sulfamoyl-benzo(F)quinoxaline-2,3 dione [NBQX], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6,7-dinitroquinoxaline-2,3-dione [DNQX]) to suppress these responses directly were assessed quantitatively. The results support the existence of NMDA receptors in NM and suggest that the ability of these receptors to influence synaptically-evoked responses declines dramatically during the last week of embryonic life. The results similarly suggest that the non-NMDA receptors in NM undergo changes in density and/or function during a period of development when the cochlear nucleus is undergoing a variety of morphological and functional transformations.