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

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)

Descending projections to the dorsal and ventral divisions of the cochlear nucleus in guinea pig

The origins of extrinsic projections to the guinea pig dorsal and ventral cochlear nuclei were identified by examining the retrograde transport of horseradish peroxidase conjugated to wheatgerm agglutinin following its injection into each of these divisions. Major projections originated in periolivary regions of the superior olivary complex, the contralateral cochlear nucleus and the inferior colliculus. There was no contribution from the nuclei of the lateral lemniscus to these pathways. The heaviest projection from the periolivary regions to both divisions of the cochlear nucleus arose bilaterally in the ventral nucleus of the trapezoid body. The ipsilateral lateral nucleus of the trapezoid body also projected heavily to dorsal and ventral cochlear nucleus. In addition, the ventral cochlear nucleus received a substantial projection from the dorsal aspect of the ipsilateral dorsomedial periolivary nucleus. Projections originating bilaterally in the central nucleus of the inferior colliculus terminated in the deep layers of dorsal cochlear nucleus. These projections appear to be more strongly ipsilateral and specific than those reported in the cat.

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.

Intracellular labeling of afferents to the lateral superior olive in the bat, Eptesicus fuscus

An in vitro tissue slice preparation of the bat brain stem was used to label intracellularly individual axons projecting to the lateral superior olive from two different sources: the medial nucleus of the trapezoid body (MNTB) and the anteroventral cochlear nucleus (AVCN). The tracing of individually labeled MNTB axons into the lateral superior olive reaffirms the long accepted indirect route by which information from the contralateral ear reaches the lateral superior olive. While the MNTB appears to relay input from the contralateral AVCN, information from the ipsilateral ear reaches the lateral superior olive via a direct projection from the ipsilateral AVCN. Axons from the contralateral and ipsilateral pathways have different distribution patterns upon the fusiform cells of the lateral superior olive. Axon terminals of MNTB principal cells have a perisomatic and proximal dendritic distribution pattern. Axon terminal varicosities from the ipsilateral anteroventral cochlear nucleus are distributed primarily to more distal dendrites.

Synaptic connections in the dorsal cochlear nucleus of mice, in vitro

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices that contained the root of the auditory nerve and parts of the dorsal and ventral cochlear nuclei. Probably the largest and most common cells were impaled. 2. Weak shocks to the nerve usually evoked an excitatory postsynaptic potential (EPSP) that lasted about 90 ms and whose latency was often less than 1.2 ms, indicating monosynaptic input. 3. Stronger shocks elicited a larger EPSP and a later train of inhibitory postsynaptic potentials (IPSPs). Increasing the stimulus voltage shortened the latency of the train of IPSPs and increased its efficacy so that at large stimulus strengths inhibition dominated the synaptic response. 4. To determine whether any of the neuronal circuitry which generated the synaptic responses involved the ventral cochlear nucleus, recordings were made from slices containing only the dorsal nucleus. Synaptic responses to stimulation of the pial surface of the isolated DCN resembled those driven from the nerve root. That is, weak shocks evoked long-lasting, monosynaptic EPSPs and stronger stimuli elicited a larger EPSP followed by trains of IPSPs. The DCN, therefore, contains intrinsic inhibitory interneurones. 5. The parallel fibres of the DCN course superficially, near the stimulating electrodes, whereas the axons of the auditory nerve terminate in deeper areas. Thus, the monosynaptic EPSPs evoked from the pial surface are probably generated by parallel fibres. Apparently the inhibitory interneurones are also excited by a circuit including parallel fibres. 6. The putative neurotransmitter of parallel fibres, glutamate, excited all neurones tested. 7. Cells were sensitive both to glycine and to gamma-aminobutyric acid (GABA). Only strychnine, however, not picrotoxin or bicuculline, blocked IPSPs.

Pharmacological aspects of excitatory synaptic transmission to second-order vestibular neurons in the frog

Synaptic excitation of second-order vestibular neurons is mediated by two principal afferents: vestibular afferents projecting into the brain via the VIIIth cranial nerve and commissural afferents from the contralateral vestibular nuclear complex. The shape of the excitatory postsynaptic potentials (EPSPs) generated by selectively activating these two inputs differs qualitatively, such that ipsilateral VIIIth nerve afferents generate a faster-rising EPSP than do the commissural afferents. We have investigated the synaptic pharmacology of these two inputs in the isolated, intact medulla of the frog in order to determine the nature of the transmitter substances released by the afferents and the nature of the subsynaptic receptors with which these transmitters interact. Electrical stimulation of the ipsilateral VIIIth cranial nerve evokes in the region of the vestibular nuclear complex a field potential that exhibits a presynaptic (afferent volley) and a postsynaptic (slow negativity) component. Bath application of glutamate receptor antagonists, such as kynurenic acid (KENYA), blocks the postsynaptic component of this field potential in a dose-dependent manner, without affecting the presynaptic volley, suggesting that the VIIIth nerve afferent releases glutamate and/or similar substances as its neurotransmitter. A comparison of the actions of various glutamate receptor antagonists to block this postsynaptic negativity gives a rank order of effectiveness such that KENYA greater than gamma-D-glutamylglycine (gamma DGG) = gamma-D-glutamylaminomethylsulfonic acid (GAMS) greater than gamma-D-glutamyltaurine (gamma DGT) much greater than gamma-D-glutamylaminomethylphosphonic acid (GAMP) greater than D-2-amino-5-phosphonovaleric acid (D-APV) greater than D,L-APV greater than D-2-amino-7-phosphonoheptanoic acid (APH). This rank order of effectiveness suggests that the VIIIth nerve transmitter activates second-order neurons through kainate (KA)/quisqualate (QUIS) synaptic receptors. Intracellular studies support these conclusions. Chemically mediated EPSPs evoked from ipsilateral VIIIth nerve stimulation are completely blocked by high concentrations of KENYA (greater than or equal to 1 mM). Occasionally an extremely short-latency, probably electrically mediated, component to these EPSPs persists in the presence of KENYA. The slower-rising EPSPs evoked from contralateral VIIIth nerve or contralateral vestibular nuclear complex stimulation are also completely blocked by KENYA, suggesting that the transmitter released by the commissural afferents is also glutamate and/or related compounds.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoradiographic localization of receptors in the cochlear nucleus of the mouse

Light microscopic autoradiography of bound radiolabeled ligands was used to describe the distribution of six receptor types in the dorsal and ventral mouse cochlear nuclei: Glycine receptor ([3H]strychnine); GABA receptor ([3H]muscimol); benzodiazepine receptor ([3H]flunitrazepam); adenosine receptor ([3H]cyclohexyladenosine); muscarinic ACh receptor ([3H]quinuclidinyl benzilate); histamine receptor ([3H]mepyramine). The most intense [3H]strychnine labeling was observed in the deep region of the dorsal cochlear nucleus (DCN), with slightly lower levels in the molecular and pyramidal layers. Highest density of [3H]muscimol binding sites was observed in the granule cell layer of the posterior ventral nucleus (PVCN) and in the pyramidal layer of the DCN. Diffuse [3H]flunitrazepam labeling was distributed over all laminar regions of the DCN; the highest grain density was observed over the granule cell layer of the PVCN. Intense [3H]cyclohexyladenosine labeling was seen over the molecular layer, possibly extending into the pyramidal layer, of the DCN. The granule cell layer of the PVCN was also densely labeled. High concentrations of [3H]quinuclidinyl benzilate sites were seen in the molecular layer, possibly extending into the pyramidal layer, of the DCN. A thin band of high grain density was also visible over the granule cell layer of the PVCN. Moderate, diffuse [3H]mepyramine labeling was visible throughout the DCN, with slightly higher grain density over the molecular, and possibly the pyramidal layers, than over the deep region of the DCN.

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.