Effects of DL-alpha-aminoadipate on synaptically and chemically evoked excitation of anteroventral cochlear nucleus neurons of the cat

Quantitative distribution of choline acetyltransferase activity in rat trapezoid body

There is evidence for a function of acetylcholine in the cochlear nucleus, primarily in a feedback, modulatory effect on auditory processing. Using a microdissection and quantitative microassay approach, choline acetyltransferase activity was mapped in the trapezoid bodies of rats, in which the activity is relatively higher than in cats or hamsters. Maps of series of sections through the trapezoid body demonstrated generally higher choline acetyltransferase activity rostrally than caudally, particularly in its portion ventral to the medial part of the spinal trigeminal tract. In the lateral part of the trapezoid body, near the cochlear nucleus, activities tended to be higher in more superficial portions than in deeper portions. Calculation of choline acetyltransferase activity in the total trapezoid body cross-section of a rat with a comprehensive trapezoid body map gave a value 3-4 times that estimated for the centrifugal labyrinthine bundle, which is mostly composed of the olivocochlear bundle, in the same rat. Comparisons with other rats suggest that the ratio may not usually be this high, but it is still consistent with our previous results suggesting that the centrifugal cholinergic innervation of the rat cochlear nucleus reaching it via a trapezoid body route is much higher than that reaching it via branches from the olivocochlear bundle. The higher choline acetyltransferase activity rostrally than caudally in the trapezoid body is consistent with evidence that the centrifugal cholinergic innervation of the cochlear nucleus derives predominantly from locations at or rostral to its anterior part, in the superior olivary complex and pontomesencephalic tegmentum.

Cholinergic modulation of spontaneous activity in rat dorsal cochlear nucleus

Extracellular recordings were made from brain stem slices to test the effects of bath application of cholinergic agonists and antagonists on the firing rates of spontaneously active dorsal cochlear nucleus neurons. About 90% of neurons responded to carbachol. A higher proportion responded to muscarine than to nicotine. Muscarine elicited larger responses at lower concentrations than nicotine. Responses to either carbachol or muscarine were always blocked by atropine or scopolamine. The nicotinic antagonists d-tubocurarine, hexamethonium, and mecamylamine blocked the responses to nicotine, but did not decrease the responses to carbachol. Regularly firing neurons showed only increases of firing rate during exposure to cholinergic agonists. About half of responsive bursting neurons showed increased firing; half showed increased followed by decreased firing to 10 microM carbachol or muscarine. All phases of the responses of most bursting neurons were greatly decreased or abolished in low calcium, high magnesium medium, while responses of regular neurons were not detectably affected. Thus, cholinergic agonists appear to act directly on regularly firing neurons, while their actions on bursting neurons may require synaptic activity. The data suggest that cholinergic transmission in the dorsal cochlear nucleus is predominantly muscarinic, and that most regularly firing spontaneously active neurons have muscarinic receptors.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

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)

Glutamate neurotoxicity in rat auditory system: cochlear nuclear complex

In other systems such as the hypothalamus and hippocampus, it has been shown that cells postsynaptic with respect to glutamatergic inputs degenerate when exposed to large doses of glutamate ("glutamate neurotoxicity"). We have shown that large doses of glutamate administered intraperitoneally are toxic to spiral ganglion cells in the inner ear of the rat. In the present study, we have investigated whether similar levels of glutamate cause alterations in the neurons of the cochlear nuclei. Specifically, we have studied the morphology and size of the cochlear nuclear complex and its subdivisions as well as the size and density of cochlear nucleus neurons following administration of glutamate. The morphological evidence indicates that glutamate caused severe anatomical alteration of the cochlear nuclei. The changes were most pronounced in the anteroventral cochlear nucleus, especially in the neurons that receive terminals of the end bulbs of Held from the cochlear nerve. This could be a direct effect of glutamate in the cochlear nuclei or secondary to degeneration of cochlear nerve fibers in the inner ear.

Coordinated activity of neuron pairs in anesthetized rat dorsal cochlear nucleus

We have recorded from small groups of neurons in the dorsal cochlear nucleus of anesthetized rats in an effort to study neuronal interactions. Multi-unit recordings on each single electrode were sorted by waveform into spike trains from individual neurons using a principal components spike sorter. Pairs of such sorted spike trains were studied with cross-correlation analysis to detect excitatory and/or inhibitory interactions. In a few cases recordings were obtained from two electrodes simultaneously, thus allowing cross-correlation studies without the consequences of spike train waveform sorting. All neurons were characterized by their strongest response frequency (at a fixed sound pressure level) and peristimulus histogram responses to 55 ms tone bursts. Fifty-eight percent of the neuron pairs studied showed peaks in their cross-correlograms indicative of coordinated neural activity. Of these pairs, 86% showed peak configurations (i.e. correlograms with asymmetrically located peaks) consistent with the interpretation that one cell induced the other to discharge. The remaining correlograms contained symmetric peaks which were centrally located, possibly due to shared input to these neuron pairs. Latencies of asymmetric peaks in cross-correlograms were typically 2 ms; consequently, an intervening excitatory synapse may be involved. Similar results were obtained from at least one pair of neurons where each neuron was recorded by a separate electrode. Strongest response frequencies of each neuron pair, for which they could be determined, were within 0.17 log units. Peristimulus histograms from each neuron in these pairs revealed that it was common for adjacent cells to respond with differing time patterns under the same stimulus conditions. The variations in histogram patterns of interconnected neurons suggests some relatively complex integrative function for these circuits.

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.

Pentobarbital and ketamine alter the pattern of 2-deoxyglucose uptake in the central auditory system of the gerbil

Relative 2-deoxyglucose (2-DG) uptake was investigated during pentobarbital and/or ketamine anesthesia, when animals were either kept in silence or stimulated with wide band noise at 85 dB SPL. In the absence of anesthesia, noise stimulation produced a large increase in relative 2-DG uptake, when compared to silence, in all auditory nuclei up to and including the inferior colliculus. Much more modest noise-induced increases were seen in the medial geniculate nucleus and auditory cortex. These effects were markedly altered by anesthesia. Pentobarbital, and especially pentobarbital plus ketamine, enhanced stimulus-evoked increases in relative 2-DG uptake in lower auditory nuclei: the cochlear nuclei, superior olivary complex and ventral nucleus of the lateral lemniscus. At the same time, stimulus-evoked increases were decreased in the dorsal nucleus of the lateral lemniscus and inferior colliculus, and virtually eliminated in the medial geniculate and auditory cortex. The results of this study permit more meaningful comparison of 2-DG techniques with electrophysiological measures of central auditory activity, and illuminate the utility and limitations of each method. The data indicate that 2-DG observations from barbiturate-anesthetized preparations should be interpreted with some caution. They further suggest that the 2-DG technique is inappropriate for the study of stimulus-evoked activity in the medial geniculate and auditory cortex of barbiturate-anesthetized animals.

Evidence for the involvement of kainate receptors in synaptic transmission in the avian cochlear nucleus

Previous studies using various excitatory amino acid antagonists have shown that synaptic transmission between the auditory nerve and the cochlear nucleus of chickens (nuc. magnocellularis; NM) is mediated by non-N-methyl-D-aspartate (non-NMDA) receptors. In the present study we have attempted to define the subclass of non-NMDA receptor in the NM by examining the effects of various excitatory amino acid agonists on synaptically evoked field potentials in an in vitro preparation of the chicken brain stem. Both quisqualate and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), whose actions operationally define the quisqualate receptor class, caused variable and weak depression of evoked responses in the NM, as did L-glutamate. Kainic acid, on the other hand, completely blocked postsynaptic responses at micromolar concentrations. We conclude that kainate-preferring non-NMDA receptors play a predominant role in mediating transmission in the NM.

Non-N-methyl-D-aspartate receptors mediating synaptic transmission in the avian cochlear nucleus: effects of kynurenic acid, dipicolinic acid and streptomycin

We have examined the effects of a number of excitatory amino acid antagonists on transmission at the cochlear nerve-nucleus magnocellularis synapse in the chicken. Using an in vitro preparation and bath application of drugs, we studied the effects of kynurenic acid and several related substances, streptomycin and a selective N-methyl-D-aspartate receptor antagonist, DL-alpha-aminosuberate. The last compound had no effect on evoked transmission. Of the various kynurenic acid-related compounds tested, only kynurenic and dipicolinic acid selectively altered responses in nucleus magnocellularis. Quinolinic acid, a kynurenic acid analogue that is structurally akin to dipicolinic acid but which acts selectively at N-methyl-D-aspartate receptors, was without effect. The effect of kynurenic acid was solely inhibitory, completely blocking postsynaptic responses with a potency dependent on the frequency of nerve stimulation. No such frequency dependence was seen with dipicolinic acid although this compound also completely suppressed evoked responses. In addition dipicolinic acid potentiated postsynaptic responses at concentrations only slightly lower than those causing inhibition. Streptomycin inhibited responses in nucleus magnocellularis but this effect seems to result partially from the ability of the drug to inhibit presynaptic calcium influx. Our finding that selective antagonists of N-methyl-D-aspartate receptors were ineffective while antagonists of both receptor types, such as kynurenic and dipicolinic acids, inhibited evoked responses reinforces the conclusion that postsynaptic receptors mediating transmission at this synapse are of the non-N-methyl-D-aspartate type [Nemeth et al. (1983) Neurosci. Lett. 40, 39-44].(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acid pharmacology of the auditory nerve and nucleus magnocellularis of the chicken

Experiments were performed on the nucleus magnocellularis and auditory nerve in tissue slices of 19-20-day-old chick embryos. Bath-applied kainate, quisqualate and N-methyl-D-aspartate induced dose-dependent alterations in the antidromic responses of nucleus magnocellularis neurons. The sensitivity of these agonist-induced responses to 2,3-cis-piperidine dicarboxylate, glutamate diethylester and D-alpha-aminoadipate were tested, as was the sensitivity of auditory nerve transmission. The data suggest that receptors for all three agonists are present on nucleus magnocellularis neurons and that the postsynaptic receptor of the nucleus magnocellularis-auditory nerve synapse is of the kainate type. The effects of bath-applied baclofen were also studied. Baclofen blocked orthodromic responses suggesting that an excitatory amino acid is released from the presynaptic terminal.

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

Microionophoretically applied excitatory amino acids induced firing of extracellularly recorded single units in a tissue slice preparation of the mouse cochlear nucleus, and the similarly applied antagonist 2-amino-5-phosphonovalerate (2APV) was demonstrated to be a selective N-methyl-D-aspartate (NMDA) receptor antagonist. In addition, the effect of various bath-applied excitatory amino acid receptor antagonists on auditory nerve evoked field potentials was studied. Antagonists which block NMDA type receptors, blocked auditory nerve evoked potentials in a dose-dependent manner. The 50% effective concentration (EC50) for three of the antagonists used was: D-alpha-aminoadipate, 7.8 mM; 2APV, 4.2 mM; and 2,3-cis-piperidine dicarboxylate, 1.1 mM. Glutamate diethylester (5 mM) had no effect. The results suggest that NMDA, kainate and quisqualate receptors are present in the cochlear nucleus and that auditory nerve transmission in the mouse is mediated by an NMDA type receptor. This is consistent with the concept that the auditory nerve postsynaptic receptor in mammals is of the NMDA type.

Cells in the anteroventral cochlear nucleus are insensitive to L-glutamate and L-aspartate; excitatory synaptic responses are not blocked by D-alpha-aminoadipate

Auditory nerve fibers transmit signals from the cochlea to the 3 regions of the cochlear nuclear complex, the anteroventral (AVCN), posteroventral, and dorsal cochlear nucleus in the brainstem. It has been suggested that the amino acids L-aspartate and L-glutamate might serve as a neurotransmitter in auditory nerve fibers. The sensitivity of postsynaptic cells in the cochlear nuclei to these amino acids has been tested by iontophoretic techniques. One difficulty with these experiments is that responses were recorded only extracellularly. A second difficulty is that the concentrations needed to affect cells could not be determined. To avoid these difficulties a brain slice preparation was used to test the sensitivity of cells in the AVCN to bath applied L-glutamate and L-aspartate at concentrations ranging from 10(-5) to 10(-2) M. All cells that were tested in the cochlear nuclear complex were insensitive at all concentrations used; the resting potentials and the input resistances remained unchanged and the synaptic responses to electrical stimulation of the auditory nerve were not desensitized. All cells that were tested in the hippocampus, however, depolarized in the presence of 10(-4) M L-glutamate and L-aspartate. The synaptic responses to electrical stimulation of the auditory nerve were not blocked by D-alpha-aminoadipate, an amino acid which has been shown to block excitation of cells in the cochlear nuclei by auditory nerve fibers. The results are not consistent with L-glutamate and L-aspartate serving as neurotransmitters in the AVCN.

Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors

This review summarizes studies designed to label and characterize mammalian synaptic receptors for glutamate, aspartate and related acidic amino acids using in vitro ligand binding techniques. The binding properties of the 3 major ligands employed--L-[3H]glutamate, L-[3H]aspartate and [3H]kainate--are described in terms of their kinetics, the influence of ions, pharmacology, molecular nature, localization and physiological/pharmacological function. In addition, the binding characteristics are described of some new radioligands--[3H]AMPA, L-[3H]cysteine sulphinate, L-[35S]cysteate, D-[3H]aspartate, D,L-[3H]APB, D-[3H]APV and D,L-[3H]APH. Special emphasis is placed on recent findings which allow a unification of the existing binding data, and detailed comparisons are made between binding site characteristics and the known properties of the physiological/pharmacological receptors for acidic amino acids. Through these considerations, a binding site classification is suggested which differentiates 5 different sites. Four of the binding site subtypes are proposed to correspond to the individual receptor classes identified in electrophysiological experiments; thus, A1 = NMDA receptors; A2 = quisqualate receptors; A3 = kainate receptors; A4 = L-APB receptors; the fifth site is proposed to be the recognition site for a Na+-dependent acidic amino acid membrane transport process. An evaluation of investigations designed to elucidate regulatory mechanisms at acidic amino acid binding sites is made; hypotheses such as the Ca2+-activated protease hypothesis of long-term potentiation are assessed in terms of the new binding site/receptor classification scheme, and experiments are suggested which will clarify and expand this exciting area in the future.

Aspartate aminotransferase activity in fiber tracts of the rat brain

Activity of aspartate aminotransferase, an enzyme which catalyzes the interconversion of the excitatory transmitter candidates, glutamate and aspartate, has been measured in fiber tracts of rat, with an emphasis on sensory and motor systems of the brain. Most tracts had significantly higher activities than the cholinergic facial nerve root, consistent with the possibility that a component of aspartate aminotransferase activity might serve as a marker for neurons using glutamate and/or aspartate as neurotransmitter. Highest activity was in the auditory nerve root. On the other hand, a close correlation was found between aspartate aminotransferase and malate dehydrogenase activities in the fiber tracts, raising the question whether aspartate aminotransferase activity may be more closely related to energy metabolism than to transmitter metabolism.

Baclofen reduces tone-evoked activity of cochlear nucleus neurons

Recent evidence suggests that an excitant amino acid may be a neurotransmitter at acoustic nerve synapses in cochlear nucleus (CN). Release of excitant amino acids is reportedly reduced by baclofen, a lipophilic GABA-mimetic used to treat the spasticity of multiple sclerosis and spinal injury. Microiontophoresis of (-)baclofen suppressed spontaneous and tone-evoked activity in CN neurons. GABA inhibited the responses of most neurons responsive to (-)baclofen. However, iontophoresis of these two substances onto the same CN neuron resulted in dramatic differences in time course to maximum effect and to recovery. Onset and offset of (-)baclofen-induced firing reduction were gradual at all doses (currents), but even the highest doses rarely caused total suppression of firing. Inhibition of firing by GABA was abrupt, and total suppression was frequently observed over the range of doses used. GABA desensitization (fading) commonly occurred while the (-)baclofen response never faded. The same CN neurons were also suppressed by D-alpha-aminoadipate, which blocks certain excitatory amino acid receptors, while the GABA antagonist bicuculline had no effect on the (-)baclofen response. These findings support the hypothesis that an excitant amino acid may be a transmitter at acoustic nerve synapses in CN.

Immunocytochemical localization of glutaminase-like immunoreactivity in the auditory nerve

The immunocytochemical localization of glutaminase, which we have proposed as a marker for excitatory amino acid neurotransmitters was determined in the guinea pig auditory nerve. Glutaminase-like immunoreactivity was seen in auditory nerve terminals in the cochlear nucleus and in the cell bodies of the auditory nerve in the cochlea. This staining was seen in type I and not type II spiral ganglion cells. Glutaminase-like immunoreactivity was also observed in granule cells in the cochlear nucleus.

Uptake and release of D-aspartate in the guinea pig cochlear nucleus

This study attempted to determine if L-glutamate (L-Glu) and/or L-aspartate (L-Asp) might be the transmitters of neurons that provide synaptic endings to the cochlear nucleus of the medulla. The uptake and release of D-[3H]aspartate (D-Asp), a putative marker for L-Glu and L-Asp, were measured in the guinea pig cochlear nucleus before and after destruction of the cochlear afferents by cochlear ablation. The cochlear nucleus was dissected into the anteroventral (AVCN), posteroventral (PVCN), and dorsal (DCN) cochlear nuclei. Subdivisions from unlesioned animals took up D-Asp, achieving concentrations in the tissues that were 13-20 times that in the medium. Subsequently, electrical stimulation evoked a Ca2+-dependent release of part of the D-Asp from each subdivision. Disarticulation of the middle ear ossicles, which attenuates acoustic stimulation, produced a modest inhibition of D-Asp release in each subdivision, but did not alter the uptake of D-Asp. Cochlear ablation strongly depressed both the uptake and the release of D-Asp in each subdivision, presumably as a result of destruction of the cochlear nerve endings in the cochlear nucleus. Nevertheless, after lesions, there was a preservation of the uptake and release of D-Asp in the DCN relative to the AVCN and PVCN. These residual activities in the DCN may be mediated by the axonal endings of the granule cells of the cochlear nucleus. The present findings support the hypothesis that the granule cells of the cochlear nucleus, as well as the cochlear nerve fibers, use L-Glu and/or L-Asp as transmitters.

Pharmacologic evidence for synaptic transmission mediated by non-N-methyl-D-aspartate receptors in the avian cochlear nucleus

The hypothesis that synaptic transmission between the auditory nerve and the cochlear nucleus is mediated by an excitatory amino acid acting through N-methyl-D-aspartate (NMDA) receptors was examined in an in vitro preparation of the chicken brainstem. The ability of various bath-applied excitatory amino acid receptor antagonists to inhibit synaptically-evoked responses was assessed by recording field potentials from nucleus magnocellularis (NM) following electrical stimulation of the cochlear nerve. Antagonists that selectively block responses mediated by NMDA receptors, such as D-alpha-aminoadipate and 2-amino-5-phosphonovalerate, were without effect on evoked transmission in NM. In contrast, antagonists that additionally act on non-NMDA receptors, such as cis-2,3-piperidine dicarboxylate and gamma-D-glutamylglycine, reversibly suppressed transmission. The results indicate that (1) transmission in the chicken auditory system is mediated by non-NMDA receptors, and (2) a substance(s) chemically akin to aspartate and glutamate may be the transmitter used by the auditory nerve in NM.

Stimulation of GABA release from retinal horizontal cells by potassium and acidic amino acid agonists

The release of [3H]GABA from horizontal cells of goldfish retina was studied by biochemical analysis of perfused isolated retina. Retinas were incubated for 15 min in 0.72 microM [3H]GABA, rinsed for 30 min and then perfused with 1 min pulses of increasing concentrations of K+ and acidic amino acid agonists under a variety of conditions. Radioactivity in the perfusate was determined by liquid scintillation spectroscopy. The main findings are: (1) virtually all of the [3H]GABA released by L-glutamate (L-Glu) and L-aspartate (L-Asp) and 50% of the K+-evoked release, is calcium independent; (2) K+-evoked [3H]GABA release is only 10% of that released by L-Glu; (3) threshold [3H]GABA release occurs with 320 microM L-Glu, 1175 microM L-Asp, 4 microM quisqualic acid (QA), 4 microM kainic acid (KA) and 53 microM N-methyl-DL-aspartate (NMDLA); (4) the quisqualate antagonist glutamic acid diethyl ester (GDEE), has no specific inhibitory action on any of the agonists, whereas D-alpha-aminoadipic acid (D alpha AA), an NMDA antagonist, potently inhibits the action of NMDLA and L-Asp; (5) the presence of Mg2+, even at 1 mM, totally inhibits NMDLA and also inhibits the action of L-Glu and L-Asp below 1 mM; (6) D-Asp potentiates the action of L-Glu by 0.6-0.8 log units and completely inhibits the action of L-Asp; (7) L-Asp at a ratio of 3:1 potentiates the effect of L-Glu. From these and other results one concludes that: (a) [3H]GABA release from H1 cells is calcium independent and depends on factors other than passive depolarization, probably sodium; (2) the likely transmitter of red cones is L-Glu acting on quisqualate or kainate receptors, and (3) L-Asp acts predominantly on NMDA receptors and may provide a modulatory role in the outer retina by potentiating the action of L-Glu.

Effects of large brain stem lesions on the cholinergic system in the rat cochlear nucleus

Large lesions were made medial to one cochlear nucleus in rats, in order to cut virtually centrifugal pathways to it. To estimate the contribution of these centrifugal pathways to cholinergic synapses in the cochlear nucleus, choline acetyltransferase and acetylcholinesterase activities were mapped, by quantitative histochemical procedures, in lesion and control side cochlear nuclei. Choline acetyltransferase activities were reduced by 85-90% in most regions of the lesion side cochlear nucleus and by 65-75% in granular regions. Acetylcholinesterase activities were reduced by 50% or less in the same regions. The choline acetyltransferase results are consistent with a conclusion that by far most cholinergic synapses in the rat cochlear nucleus derive from centrifugal pathways. Additionally, the effects of the lesions on enzyme activities in the lateral superior olivary nucleus and ventral nucleus of the trapezoid body, and in the facial, motor trigeminal, and spinal trigeminal nuclei were examined. In the lesion side facial nucleus, 60% and 40% decreases in choline acetyltransferase and acetylcholinesterase activities, respectively, were apparently consequences of facial root transection. Lesion-control enzyme activity differences in the other nuclei were much smaller.

Single unit analysis of the posteroventral cochlear nucleus of the decerebrate cat

Single unit recordings were obtained in the cochlear nuclear complex of the unanesthetized, decerebrate cat. Sixty-six of 282 units were localized to the posteroventral cochlear nucleus, 17 from the multipolar cell area and 49 from the octopus cell area. Spontaneous rates ranged from less than 1 to 75 spikes per second in the multipolar cell area and from less than 1 to 135 spikes per second in the octopus cell area. Poststimulus time histograms revealed four response types, at the best frequency, in the posteroventral cochlear nucleus. These responses were: (1) primary-like (maximum response shortly after the stimulus onset, followed by a reduction in activity to a steady state); (2) chopper (similar to primary-like but with multiple peaks in the first 10-15 milliseconds); (3) onset-ex (onset response followed by a low level of excitation); and (4) onset-in (onset response followed by inhibition). The onset-in responses represented the first observations of inhibition, at best frequency, for onset units in the mammalian cochlear nuclear complex. Analysis of interspike interval distributions showed that both spontaneous and driven activity consisted of irregular intervals for all four response types. Activity-intensity functions for primary-like, chopper and onset-ex units showed monotonic increases with increases in stimulus intensity. Activity-intensity functions for onset-in units were non-monotonic. Latency-intensity functions for primary-like, chopper and onset-ex units exhibited monotonic decreases with increases in intensity. Latency-intensity functions for onset-in units were non-monotonic. In contrast to primary-like, chopper and onset-ex units, onset-in units do not retain the intensity and temporal information coded in the eighth nerve, as least for stimuli above 2 kilohertz. It is hypothesized that a depolarization block, caused by the massive eighth nerve input to octopus cells, is responsible for the inhibition observed from onset-in units.

Aspartate aminotransferase immunoreactivity in cochlea of guinea pig

The distribution of aspartate aminotransferase-like immunoreactivity in the cochlea of the guinea pig was studied at the light microscopy level. Indirect immunofluorescence histochemistry using antisera against cytoplasmic aspartate aminotransferase prepared from pig heart was applied to surface preparations of the organ of Corti and cryostat sections of the cochlea. In the modiolus, immunofluorescence was localized to spiral ganglion cells and myelinated fibers of the auditory nerve and intraganglionic spiral bundles. In the organ of Corti, immunofluorescence was seen in upper tunnel crossing fibers and at the base of outer hair cells, following a distribution similar to that of the efferent innervation of the outer hair cells. Weak immunofluorescence was seen in the inner spiral bundle and tunnel spiral bundle, but was not present in all preparations. Immunofluorescence was not seen in inner hair cells, nor at the base of inner hair cells, and may have been absent from outer hair cells. It is concluded that spiral ganglion cells and myelinated auditory nerve axons contain aspartate aminotransferase-like immunoreactivity such immunoreactivity has previously been determined in auditory nerve endings inthe cochlear nucleus. Olivocochlear neurons that innervate outer hair cells also contain such immunoreactivity while other cochlear efferents contain little or none.

Effects of kainic acid injection and cortical lesion on ornithine and aspartate aminotransferases in rat striatum

The effects of cortical lesions and intrastriatal kainic acid injections on various striatal enzyme activities were investigated. Ornithine aminotransferase decreased concomitantly with glutamate uptake in decorticated and chronic kainic acid-treated rats. It was also decreased in acute kainic acid-lesioned striatum where glutamate uptake was unaffected. Aspartate aminotransferase, however, decreased only after acute kainic acid treatment. Results for glutamate uptake, glutamate decarboxylase, and choline acetyltransferase were in agreement with previous findings. These results suggest that ornithine may act as a precursor for glutamate in nerve terminals, although the nonspecific localization does not allow ornithine aminotransferase to be a convenient biochemical marker. The decrease in aspartate aminotransferase is thought to be due to the widespread cell degeneration after acute kainic acid. Aspartate aminotransferase activities were also found to be reduced in the frontal cortex, caudate nucleus and putamen of Huntington's disease brains.

Immunocytochemical localization of aspartate aminotransferase immunoreactivity in cochlear nucleus of the guinea pig

There is substantial evidence supporting the role of aspartate or glutamate as the neurotransmitter of the auditory nerve. The concentration of aspartate aminotransferase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1), an enzyme associated with the metabolism of these amino acids, is high in axons and terminals of the auditory nerve. Antibodies were raised against aspartate aminotransferase and used in immunocytochemical studies to determine its localization in the cochlear nucleus of the guinea pig. Indirect immunofluorescence techniques were used for light microscopic localization of aspartate aminotransferase-like immunoreactivity in normal guinea pigs and guinea pigs with auditory nerve lesions. Fluorescent rings of aspartate aminotransferase-like immunoreactivity were seen around spherical cells in the anteroventral cochlear nucleus. In animals with auditory nerve lesions, rings were no longer seen in the ipsilateral cochlear nucleus. Immunoreactivity was also seen on cells in the posteroventral cochlear nucleus and in auditory nerve fibers. Ultrastructural studies were done in the rostral anteroventral cochlear nucleus, using the peroxidase-antiperoxidase technique. Aspartate aminotransferase-like immunoreactivity was seen at axosomatic synapses on large spherical cells in terminals with the morphological characteristics of auditory nerve terminals. Other classes of terminals on the soma of large spherical cells showed no immunoreactivity. It was concluded that aspartate aminotransferase-like immunoreactivity is present in axons and terminals of the auditory nerve. These findings indicate that aspartate aminotransferase-like immunoreactivity may serve as a marker at terminals where aspartate or glutamate is a neurotransmitter.

Glutamate and aspartate: alteration of thresholds and response patterns of auditory neurons

Iontophoretic application of the excitant amino acids glutamate and aspartate onto neurons in the chinchilla cochlear nucleus results in a lowering of the threshold of response to auditory stimuli. Neurons that display 'on'-type phasic responses to toneburst stimuli may become tonic, sustained responders with iontophoretic application of glutamate or aspartate. The ability of either glutamate or aspartate to effect changes in thresholds and response patterns of cochlear nucleus neurons is further evidence that one of these amino acids may be the afferent transmitter of the auditory nerve. The effects seen with these excitant amino acids may also provide insight into the underlying synaptic events involved in the generation of a particular response pattern.

Neurotransmitters in the cochlea and the cochlear nucleus

The inhibitory efferent transmitter in the cochlear is most likely acetylcholine. The afferent transmitter (between hair cells and primary afferent fibres) is not known. There is some evidence for glutamate (or aspartate) but the high concentrations necessary to activate the afferents when these amino-acids are applied intracochlearly may indicate that their effects is unspecific. A number of other transmitter candidates can be safely ruled out at these synapses. In the cochlear nucleus of transmitter between primary afferents and secondary cells is probably glutamate (or aspartate).

Selective synaptic antagonism by atropine and alpha-aminoadipate of pulvinar and cortical afferents to the suprasylvian visual area (Clare-Bishop area)

The synaptic excitations of cells of the Clare-Bishop cortical region produced by electrical stimulation of the pulvinar and ipsilateral cortex, have been shown to be differentially antagonized by iontophoretically applied antagonists. Atropine attenuated the responses evoked by pulvinar stimulation without having an appreciable effect against either iontophoretically applied aspartate or cortically evoked responses. alpha-Aminoadipate antagonized aspartate elicited excitations and those obtained with cortical stimulation while leaving unaffected acetylcholine and pulvinar evoked responses. The results are supportive of the view that acetylcholine and aspartate, or a similar excitatory amino acid, act as synaptic transmitters of some afferents from the pulvinar and ipsilateral cerebral cortex, respectively.

Effects of neuronal activity on kainic acid neurotoxicity in the ventral cochlear nucleus

Kainic acid was injected into the brain stem of adult guinea pigs, and the animals were either placed in a sound reducing-chamber or stimulated with 90 dB noise. The pattern and rate of kainic acid-induced degeneration in the anteroventral cochlear nucleus (AVCN) of sound-deprived animals was similar to that in animals exposed to ambient noise [2]. The amount of degeneration was greatly increased in animals stimulated with 90 dB noise. Therefore, although decreased activity in primary auditory fibers does not protect neurons in the AVCN from kainate-induced neurotoxicity, increased auditory stimulation augments the effects of kainic acid in the cochlear nucleus.

Neurochemistry of the auditory system

Two areas of auditory biochemistry are reviewed: the identification and characterization of neurotransmitters in the auditory system and the biochemical approach to the study of genetic hearing disorders. Studies to identify neurotransmitters at major auditory synapses are outlined. Evidence supporting glutamate or aspartate as the neurotransmitter for the auditory nerve is presented. The application of biochemistry to the study of genetic hearing disorders is discussed.