Evidence for an excitatory amino acid as the transmitter of the auditory nerve in the in vitro mouse cochlear nucleus

Effects of ketamine on response properties of neurons in the superior paraolivary nucleus of the mouse

The superior paraolivary nucleus (SPON; alternative abbreviation: SPN for the same nucleus in certain species) is a prominent brainstem structure that provides strong inhibitory input to the auditory midbrain. Previous studies established that SPON neurons encode temporal sound features with high precision. These earlier characterizations of SPON responses were recorded under the influence of ketamine, a dissociative anesthetic agent and known antagonist of N-methyl-d-aspartate glutamate (NMDA) receptors. Because NMDA alters neural responses from the auditory brainstem, single unit extracellular recordings of SPON neurons were performed in the presence and absence of ketamine. In doing so, this study represents the first in vivo examination of the SPON of the mouse. Herein, independent data sets of SPON neurons are characterized that did or did not receive ketamine, as well as neurons that were recorded both prior to and following ketamine administration. In all conditions, SPON neurons exhibited contralaterally driven spikes triggered by the offset of pure tone stimuli. Ketamine lowered both evoked and spontaneous spiking, decreased the sharpness of frequency tuning, and increased auditory thresholds and first-spike latencies. In addition, ketamine limited the range of modulation frequencies to which neurons phase-locked to sinusoidally amplitude-modulated tones.

Synaptic morphology and the influence of auditory experience

The auditory experience is crucial for the normal development and maturation of brain structure and the maintenance of the auditory pathways. The specific aims of this review are (i) to provide a brief background of the synaptic morphology of the endbulb of Held in hearing and deaf animals; (ii) to argue the importance of this large synaptic ending in linking neural activity along ascending pathways to environmental acoustic events; (iii) to describe how the re-introduction of electrical activity changes this synapse; and (iv) to examine how changes at the endbulb synapse initiate trans-synaptic changes in ascending auditory projections to the superior olivary complex, the inferior complex, and the auditory cortex.

Synaptic reorganization in the adult rat's ventral cochlear nucleus following its total sensory deafferentation

Ablation of a cochlea causes total sensory deafferentation of the cochlear nucleus in the brainstem, providing a model to investigate nervous degeneration and formation of new synaptic contacts in the adult brain. In a quantitative electron microscopical study on the plasticity of the central auditory system of the Wistar rat, we first determined what fraction of the total number of synaptic contact zones (SCZs) in the anteroventral cochlear nucleus (AVCN) is attributable to primary sensory innervation and how many synapses remain after total unilateral cochlear ablation. Second, we attempted to identify the potential for a deafferentation-dependent synaptogenesis. SCZs were ultrastructurally identified before and after deafferentation in tissue treated for ethanolic phosphotungstic acid (EPTA) staining. This was combined with pre-embedding immunocytochemistry for gephyrin identifying inhibitory SCZs, the growth-associated protein GAP-43, glutamate, and choline acetyltransferase. A stereological analysis of EPTA stained sections revealed 1.11±0.09 (S.E.M.)×10(9) SCZs per mm(3) of AVCN tissue. Within 7 days of deafferentation, this number was down by 46%. Excitatory and inhibitory synapses were differentially affected on the side of deafferentation. Excitatory synapses were quickly reduced and then began to increase in number again, necessarily being complemented from sources other than cochlear neurons, while inhibitory synapses were reduced more slowly and continuously. The result was a transient rise of the relative fraction of inhibitory synapses with a decline below original levels thereafter. Synaptogenesis was inferred by the emergence of morphologically immature SCZs that were consistently associated with GAP-43 immunoreactivity. SCZs of this type were estimated to make up a fraction of close to 30% of the total synaptic population present by ten weeks after sensory deafferentation. In conclusion, there appears to be a substantial potential for network reorganization and synaptogenesis in the auditory brainstem after loss of hearing, even in the adult brain.

Vessicular glutamate transporters 1 and 2 are differentially associated with auditory nerve and spinal trigeminal inputs to the cochlear nucleus

Projections of glutamatergic somatosensory and auditory fibers to the cochlear nucleus (CN) are mostly nonoverlapping: projections from the spinal trigeminal nucleus (Sp5) terminate primarily in the granule cell domains (GCD) of CN, whereas type I auditory nerve fibers (ANFs) project to the magnocellular areas of the VCN (VCNm) and deep layers of Dorsal CN (DCN). Vesicular glutamate transporters (VGLUTs), which selectively package glutamate into synaptic vesicles, have different isoforms associated with distinct subtypes of excitatory glutamatergic neurons. Here we examined the distributions of VGLUT1 and VGLU2 expression in the CN and their colocalization with Sp5 and ANF terminals following injections of anterograde tracers into Sp5 and the cochlea in the guinea pig. The CN regions that showed the most intense expression of VGLUT1 and VGLUT2 were largely nonoverlapping and were consistent with ANF and Sp5 projections, respectively: VGLUT1 was highly expressed in VCNm and the molecular layer of the DCN, whereas VGLUT2 was expressed predominantly in the GCD. Half (47% +/- 3%) of the Sp5 mossy fiber endings colabeled with VGLUT2, but few (2.5% +/- 1%) colabeled with VGLUT1. In contrast, ANFs colabeled predominantly with VGLUT1. The pathway-specific expression of VGLUT isoforms in the CN may be associated with the intrinsic synaptic properties that are unique to each sensory pathway.

Activity-dependent plasticity in the adult auditory brainstem

Over the past few years we have studied the plasticity of the adult auditory brainstem in the rat following unilateral changes to the pattern of sensory activation, either by intracochlear electrical stimulation or by deafening. We discovered that modifications to afferent activity induced changes in the molecular composition and cellular morphology throughout the auditory brainstem, including its major centers: the cochlear nucleus complex, the superior olivary complex, and the inferior colliculus. The time window studied ranged from 2 h to over 1 year following induction of changes to afferent activity. The molecular markers employed include the NMDA receptor subunit type 1, the cAMP response element binding protein (CREB), the immediate early gene products c-Fos, c-Jun and Egr-1, the growth and plasticity-associated protein GAP-43 and its mRNA, the calcium binding protein calbindin, the cell adhesion molecule integrin-alpha(1), the microtubule-associated protein MAP-1b, and the neurofilament light chain (NF-L). As a consequence of the specific electrical stimulation of the auditory afferents or the loss of hearing, a cascade of events is triggered that apparently modifies the integrative action and computational abilities of the central auditory system. An attempt is made to relate the diverse phenomena observed to a common molecular signaling network that is suspected to bridge sensory experience to changes in the structure and function of the brain. Eventually, a thorough understanding of these events will be essential for the specific diagnosis of patients, optimal timing for implantation, and suitable parameters for running of a cochlear implant or an auditory brainstem implant in humans. In this report an overview of the results obtained in the past years in our lab is presented, flanked by an introduction into the history of plasticity research and a model proposed for intracellular signal cascades related to activity-dependent plasticity.

Intracellular responses of the rat anteroventral cochlear nucleus to intracochlear electrical stimulation

The anteroventral cochlear nucleus (AVCN) is the first central processing site for acoustic information. The influence and extent of convergent auditory nerve input to AVCN neurons was investigated using brief (<0.2 ms) intracochlear electrical activation of spiral ganglion cells. In 40 neurons recorded in vivo, the major intracellular response to stimulation was an excitatory postsynaptic potential (EPSP) with short latency (approximately 1 ms) and fast rise time (<1 ms). Graduated EPSP amplitude increases were also seen with increasing stimulation strength resulting in spike generation. Hyperpolarization followed excitation in most neurons, its extent distinguished three response types: Type I showed no hyperpolarization; Type II and Type III displayed short (<10 ms) and long (>19 ms) duration hyperpolarization, respectively. Hyperpolarization was attributed to an inhibitory postsynaptic potential (IPSP) in addition to spike after hyperpolarization. Neurobiotin filling identified Type I and II neurons as stellate and Type III as bushy cells. These results suggests that AVCN neurons receive direct, possibly convergent, excitatory input from auditory nerves emanating from spiral ganglion cells with hyperpolarization resulting from polysynaptic inhibitory input.

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.

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

Because kainic acid (KA) is more potent than other excitatory amino acids (EAAs) in affecting synaptic transmission in the cochlear nucleus, previous reports have concluded that primary afferent neurotransmission to the cochlear nucleus in birds and mammals is mediated by KA-preferring non-N-methyl-D-aspartate (non-NMDA) EAA receptors. Since this conclusion is at odds with a number of studies suggesting that rapid excitatory neurotransmission in the CNS is mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring non-NMDA receptors, we re-examined the pharmacology of synaptic transmission between the cochlear nerve and nucleus magnocellularis (NM) in chickens, using bath application of drugs and recording of field potentials evoked in NM by electrical stimulation of the cochlear nerve in vitro. A series of EAA agonists produced complete, concentration-dependent and reversible suppression of postsynaptic responses: the order of potency was domoic acid (DO) greater than KA greater than AMPA much greater than quisqualic acid much greater than L-glutamic acid (Glu). Three quinoxalinedione antagonists of non-6-nitro-7-sulphamobenzo[f]quinoxaline-2,3-dione NMDA receptors also produced complete, concentration-dependent and reversible suppression of postsynaptic responses in NM without affecting the presynaptic action potential; the half-maximal inhibitory concentrations (IC50's) were 2.7 +/- 0.4 microM for 6-nitro-7-sulphamobenzo[f]quinoxaline-2,3-dione (NBQX), 5.3 +/- 0.1 microM for 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and 10.6 +/- 1.2 microM for 6,7-dinitroquinoxaline-2,3-dione (DNQX).(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory nerve neurotransmitter acts on a kainate receptor: evidence from intracellular recordings in brain slices from mice

Intracellular recordings from neurons in brain slice preparations of the mouse ventral cochlear nucleus (VCN) were used to examine the actions of excitatory amino acid agonists and antagonists. Synaptic responses to electrical stimulation of the auditory nerve root were partially blocked by kynurenic acid, an antagonist that is specific for glutamate receptors. The antagonists specific for N-methyl-D-aspartate (NMDA), DL-2-amino-5-phosphonovalerate (APV) and Mg2+, did not affect the response, arguing against a role for NMDA receptors at the VIIIth nerve synapse. To test postsynaptic sensitivity to excitatory amino acid agonists, responses to bath applications were measured in VCN neurons while synaptic transmission was blocked by the removal of Ca2+ from the bath or by the addition of tetrodotoxin. Neurons in the VCN were 500-1000 times more sensitive to kainate than to glutamate or aspartate. In the absence of Mg2+, they were also sensitive to NMDA. The responses to kainate and glutamate were increased by the removal of calcium from the bath. These results imply that VCN neurons have both kainate and NMDA receptors and that synaptic transmission between auditory nerve fibers and neurons in the cochlear nuclear complex could be mediated by a substance related to kainate.

Development of aspartate aminotransferase and glutaminase immunoreactivity in the rat auditory nerve

Aspartate aminotransferase and glutaminase immunoreactive labeling of the auditory nerve has previously been reported. In the present study, the development of these immunoreactivities was examined in the auditory nerve of the rat, at ages ranging from 17 days gestation to four postnatal weeks. Cells and processes were examined in the cochlea, and fibers and terminals in the cochlear nucleus. In the cochlea, immunoreactive labeling with antisera to both enzymes was first seen at 20 gestational days, in spiral ganglion cells. It was not until two postnatal weeks, however, that this immunoreactive labeling was first seen in primary afferent terminals around spherical cells in the anteroventral cochlear nucleus. This correlates with the establishment of mature synaptic connections and function.