Uptake and Release of Glycine in the Guinea Pig Cochlear Nucleus After Axotomy of Afferent or Centrifugal Fibers

Glycine may be an inhibitory transmitter in the mammalian cochlear nucleus (CN). This study attempts to determine if cochlear and/or centrifugal projections to the CN use glycine as a transmitter. The high-affinity uptake and electrically evoked release of exogenous [14C]glycine were measured in vitro in the three major subdivisions of the guinea pig CN: the anteroventral, posteroventral, and dorsal cochlear nuclei (AVCN, PVCN, and DCN, respectively). [14C]Glycine (3.4 microM) was taken up by each subdivision, reaching tissue concentrations six to seven times that in the medium. Subsequent electrical stimulation evoked a Ca2+-dependent release of [14C]glycine from each subdivision. These activities were compared in subdivisions fr0m unlesioned animals, and from animals with lesions of centrifugal or cochlear projections to the CN. Two knife-cut lesions were made to interrupt centrifugal projections to the CN lying in the right acoustic striae and trapezoid body. In one group of animals, centrifugal fibers projecting mainly to the right AVCN and PVCN were severed, which reduced [14C]glycine uptake and release by 44-53% in these subdivisions, but not in the right DCN. In another group of animals, fibers projecting mainly to the right PVCN and DCN were severed, which reduced [14C]glycine uptake and release by 33-47% in these subdivisions, but not in the right AVCN. In CN subdivisions contralateral to either lesion there was no significant change in [14C]glycine uptake or release. Neither of these lesions altered the uptake or release of D-[3H]aspartate in the right or the left CN. Ablation of the left cochlea, which presumably destroyed cochlear nerve fibers unilaterally, had no effect on [14C]glycine uptake and release. These observations suggest that centrifugal projections contribute a proportion of the glycinergic synaptic endings in the CN. In addition, some glycinergic endings probably arise from neurons intrinsic to the CN. The cochlear nerve contains very few, if any, glycinergic fibers.     

Uptake and Release of γ-Aminobutyric Acid in the Guinea Pig Cochlear Nucleus After Axotomy of Cochlear and Centrifugal Fibers

Abstract: This study attempts to determine if γ-aminobutyric acid (GABA) may be a transmitter of cochlear nerve fibers projecting from the cochlea to the cochlear nucleus, and of centrifugal fibers projecting to the cochlear nucleus via the trapezoid body and the acoustic striae of the medulla. The uptake and the electrically evoked release of exogenous [¹⁴C]GABA were measured, in vitro, in the three major subdivisions of the guinea pig cochlear nucleus: the anteroventral, posteroventral, and dorsal cochlear nuclei. These activities were compared using unlesioned animals, animals with bilateral cochlear ablations, and animals whose trapezoid body and acoustic striae were interrupted on the right side of the medulla. Subdivisions from unlesioned animals took up [¹⁴C]GABA, achieving concentrations in the tissues that were 11–19 times that in the medium. Electrical stimulation evoked a Ca²⁺-dependent release of [¹⁴C]GABA from each subdivision. Bilateral cochlear ablation, which presumably destroyed the cochlear nerve fibers, had no effect on [¹⁴C]GABA uptake and release. Section of the trapezoid body and the acoustic striae on the right side of the medulla typically severed all known connections of the right posteroventral and dorsal cochlear nuclei with the rest of the brain, but left intact many connections involved with the right anteroventral cochlear nucleus. This lesion partially depressed [¹⁴C]GABA uptake and release in the right posteroventral and dorsal cochlear nuclei, but not in the right anteroventral cochlear nucleus. These findings suggest that one or more of the centrifugal tracts projecting to the cochlear nucleus may be GABAergic, 88% or more of the cochlear nerve fibers probably are not GABAergic, and some neurons of the cochlear nucleus are probably GABAergic.     

Uptake and Release of Glycine in the Guinea Pig Cochlear Nucleus

This study attempts to determine if the cochlear nucleus (CN) contains glycinergic synaptic endings. The uptake and release of exogenous radiolabeled glycine were measured in vitro in the three major subdivisions of the guinea pig CN: anteroventral, posteroventral, and dorsal. A kinetic analysis of [3H]glycine uptake revealed the presence in each CN subdivision of a high- and a low-affinity uptake mechanism. The high-affinity mechanism had a Km of 25.2-30.5 microM and a Vmax of 3.8-4.8 nmol/10 mg of cell water/5 min, whereas the low-affinity mechanism had a Km of 633-718 microM and a Vmax of 26.6-37.1 nmol/10 mg of cell water/5 min. At steady state, the high-affinity mechanism accumulated 10 microM [3H]glycine from the medium, achieving tissue concentrations that were 13-24 times that in the medium. The high-affinity uptake was dependent on the temperature and on the concentrations of NaCl and glucose in the incubation medium. It exhibited a high degree of substrate specificity, as determined by the effects of structural analogues of glycine on the uptake of [3H]glycine. Each CN subdivision also contained two mechanisms mediating [14C]glycine release. One was activated by depolarizing electrical stimuli, produced a rapid transient release of [14C]glycine, and was dependent on the presence of extracellular Ca2+. The other was continuous, producing a slow spontaneous efflux of [14C]glycine. Released glycine could be removed primarily by uptake, because during release measurements, the amount of [14C]glycine detected in the medium decreased when glycine uptake activity was optimized. The electrically evoked, Ca2+-dependent release and the high-affinity uptake of glycine may mediate the synaptic release and inactivation of glycine, respectively. These findings, therefore, support the presence of glycinergic synaptic endings in each CN subdivision.     

The Effects of l-Glutamate and trans-(±)-1-Amino-1,3-Cyclopentanedicarboxylate on Phosphoinositide Hydrolysis Can Be Pharmacologically Differentiated

Abstract: The excitatory amino acid analogues l -glutamate ( l -Glu), l -aspartate ( l -Asp), d -Asp, and trans -(±)-1-amino-1,3-cyclopentanedicarboxylate ( trans -ACPD) stimulate the hydrolysis of phosphoinositides (PI). In the present studies, the effects of noncompetitive and competitive inhibitors on PI hydrolysis stimulated by excitatory amino acid analogues were examined. When agonist and inhibitor were added simultaneously to hippocampal tissue, the noncompetitive inhibitor l -2-amino-3-phosphonopropionate ( l -AP3) did not block the effects of l -Glu, l -Asp, or d -Asp at concentrations that block the effects of trans -ACPD by more than 80%. When tissue was pre-incubated with l -AP3, the effects of l -Glu, l -Asp, or d -Asp were blocked (IC₅₀ values between 65 and 210 µ M ). Unlike l -AP3, l -aspartate-β-hydroxamate ( l -AβHA) inhibited PI hydrolysis stimulated by trans -ACPD, l -Glu, l -Asp, or d -Asp when agonist and inhibitor were added simultaneously in hippocampus; its effects were not time-dependent. In cerebellum, both l -AP3 and l -AβHA had agonist activity. Inhibition by the recently identified competitive inhibitor (+)-α-methyl-4-carboxyphenylglycine [(+)-MCPG] of PI hydrolysis was also examined. (+)-MCPG blocked PI hydrolysis stimulated by trans -ACPD, l -Asp, or d -Asp in both hippocampus and cerebellum (IC₅₀ values between 220 and 1,700 µ M ). The effects of (+)-MCPG were consistent with a competitive mechanism of action. (+)-MCPG (up to 3 m M ) blocked PI hydrolysis stimulated by l -Glu by less than 25% in both hippocampus and cerebellum.     

Morphology of neurons cultured from subdivisions of the mouse cochlear nucleus

This study was designed to characterize the dendritic organization of cochlear nucleus (CN) cells grown in primary cell culture and to assess differences among cultures grown from different regions of CN. Cultures were prepared from postnatal mice and processed using microtubule-associated protein 2 (MAP2) or gamma-aminobutyric acid (GABA) immunohistochemistry. CN neurons were successfully cultured from preparations grown from either the anteroventral subdivision of the nucleus (AVCN), the posterior region [posteroventral (PVCN) and dorsal (DCN) subnuclei], or the whole CN, although the cultured neurons did not exhibit complex dendritic patterns characteristic of CN neurons in vivo. Neurons cultured from the entire nucleus exhibited an increased rate of survival compared to those cultured from either the anterior or posterior regions, although similar types of cells were observed in all preparations. The majority of cultured CN neurons were GABA-positive and had soma areas that were similar to the areas of immature GABAergic neurons measured in CN sections. Small cells (soma areas or=120 microm(2)) were also present in significant numbers. Overall, CN cultures consisted of a heterogeneous population of neurons that had less elaborate dendritic organizations than cells of corresponding size that have been described in adult animals in vivo.     

Frequency tuning and response latencies at three levels in the brainstem of the echolocating bat,Eptesicus fuscus

To determine the level at which certain response characteristics originate, we compared monaural auditory responses of neurons in ventral cochlear nucleus, nuclei of lateral lemniscus and inferior colliculus. Characteristics examined were sharpness of frequency tuning, latency variability for individual neurons and range of latencies across neurons.Exceptionally broad tuning curves were found in the nuclei of the lateral lemniscus, while exceptionally narrow tuning curves were found in the inferior colliculus. Neither specialized tuning characteristic was found in the ventral cochlear nuclei.All neurons in the columnar division of the ventral nucleus of the lateral lemniscus maintained low variability of latency over a broad range of stimulus conditions. Some neurons in the cochlear nucleus (12%) and some in the inferior colliculus (15%) had low variability in latency but only at best frequency.Range of latencies across neurons was small in the ventral cochlear nucleus (1.3–5.7 ms), intermediate in the nuclei of the lateral lemniscus (1.7–19.8 ms) and greatest in the inferior colliculus (2.9–42.0 ms).We conclude that, in the nuclei of the lateral lemniscus and in the inferior colliculus, unique tuning and timing properties are built up from ascending inputs.Abbreviations AVCN anteroventral cochlear nucleus - BF best frequency - CV coefficient of variation - DCN dorsal cochlear nucleus - FM frequency modulation - IC inferior colliculus - NLL nuclei of lateral lemniscus - PSTH post stimulus time histogram - PVCN posteroventral cochlear nucleus - SD standard deviation - SPL sound pressure level - VCN ventral cochlear nuclei - VNLLc ventral nucleus of the lateral lemniscus, columnar division     

Uptake and Release of d-Aspartate, GABA, and Glycine in Guinea Pig Brainstem Auditory Nuclei

Abstract: This study attempts to determine if the medial (MSO) and lateral superior olive (LSO), medial nucleus of the trapezoid body (MNTB), ventral nucleus of the lateral lemniscus (VNLL), and central nucleus of the inferior colliculus (ICc) contain glutamatergic synaptic endings. Micropunch and microdissection procedures provided fresh samples of these auditory nuclei for the measurement of the high-affinity uptake and electrically evoked release of exogenous d -[³H]ASP. The study also determined if the LSO and MSO contain glycinergic synaptic endings by measuring uptake and release of [¹⁴C]-Gly in these nuclei, and whether the MNTB, VNLL, and ICc contain GABAergic endings by assessing the uptake and release of [¹⁴C]GABA in these structures. Several strategies optimized the evoked Ca²⁺-dependent release of the labeled amino acids. These included the enhancement of high-affinity uptake during loading of the markers into the tissues, inhibition of uptake during the subsequent measurement of release, and use of an electrical stimulus current that evoked maximal Ca²⁺-dependent release. Each of these nuclei manifested the high-affinity uptake and the evoked Ca²⁺-dependent release of d -[³H]Asp, suggesting the presence of synaptic endings that may use Glu or Asp as a transmitter. Similar findings suggest the presence of glycinergic synaptic endings in the LSO and MSO, and of GABAergic synaptic endings in the MNTB, VNLL, and ICc.     

Effects of High-Potassium-Induced Depolarization on Amino Acid Chemistry of the Dorsal Cochlear Nucleus in Rat Brain Slices

High K⁺ was used to depolarize glia and neurons in order to study the effects on amino acid release from and concentrations within the dorsal cochlear nucleus (DCN) of brain slices. The release of glutamate, -aminobutyrate (GABA) and glycine increased significantly during exposure to 50 mM K⁺, while glutamine and serine release decreased significantly during and/or after exposure, respectively. After 10 min of exposure to 50 mM K⁺, glutamine concentrations increased in all three layers of DCN slices, to more than 5 times the values in unexposed slices. In the presence of a glutamate uptake blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), glutamine concentrations in all layers did not increase as much during 50 mM K⁺. Similar but smaller changes occurred for serine. Mean ATP concentrations were lower in 50 mM K⁺-exposed slices compared to control. The results suggest that depolarization, such as during increased neural activity, can greatly affect amino acid metabolism in the cochlear nucleus.     

Evidence for Glutamatergic Projections from the Cochlear Nucleus to the Superior Olive and the Ventral Nucleus of the Lateral Lemniscus

Abstract: This study attempts to determine if projections ascending from the guinea pig cochlear nucleus (CN) could be glutamatergic and/or aspartatergic. Multiple radio frequency lesions were made to ablate the right CN. The ablation was verified histologically. To identify the principal targets of CN efferents, silver impregnation methods were used to localize the preterminal degeneration of fibers in transverse sections of the brainstem 5 and 7 days after CN ablation. CN efferents projected heavily to the lateral superior olive (LSO) ipsilaterally, the medial superior olive (MSO) bilaterally, and contralaterally to the medial (MNTB) and ventral (VNTB) nuclei of the trapezoid body, the ventral (VNLL) and intermediate nuclei of the lateral lemniscus and the central nucleus of the inferior colliculus (ICc). There were smaller projections to the lateral nucleus of the trapezoid body ipsilaterally, the dorsal and dorsomedial periolivary nuclei bilaterally, and the dorsal nucleus of the lateral lemniscus contralaterally. There were sparse projections to the VNLL and ICc ipsilaterally and the CN contralaterally, and a very sparse projection to the contralateral LSO. To determine if CN efferents were glutamatergic and/or aspartatergic, the fresh brainstem was sectioned transversely and samples of the LSO, MSO, MNTB, VNLL, and ICc were taken to measure the electrically evoked release and the uptake of d -[³H]Asp and [¹⁴C]Gly or [¹⁴C]GABA 3–5 days after the CN ablation. The release studies suggest that only certain of the histologically identified projections ascending from the CN may be glutamatergic and/or aspartatergic. CN ablation depressed d -[³H]Asp release in the MSO bilaterally and in the contralateral MNTB and VNLL, suggesting that the CN efferents to these nuclei may use glutamate or aspartate as a transmitter. It was unclear whether a marginal depression of d -[³H]Asp release in the ipsilateral LSO reflected the presence of glutamatergic CN projections to this nucleus. d -[³H]Asp release in the ICc was unaffected, suggesting that CN efferents to this nucleus may not be glutamatergic. There were no deficits in d -[³H]Asp uptake. [¹⁴C]Gly release from the LSO and MSO was unchanged. [¹⁴C]Gly uptake was unchanged in the MSO and depressed only in the contralateral LSO, possibly reflecting subnormal uptake activity in endings contributed by contralateral MNTB cells that had lost their CN efferents. [¹⁴C]GABA uptake in the MNTB, VNLL, and ICc was unchanged. [¹⁴C]GABA release was unchanged in the VNLL and ICc. [¹⁴C]GABA release was depressed only in the contralateral MNTB, possibly reflecting the loss of a small complement of GABAergic CN efferents and the reaction of GABAergic projections from the contralateral VNTB to their loss of CN efferents.     

Increase in Chloride-Dependent l-Glutamate Transport Activity in Synaptic Membrane After In Vitro Ischemic Treatment

Abstract: The effect of energy failure on Cl⁻-dependent l -glutamate ( l -Glu) transport was examined with an in vitro preparation. Rat brain slices were incubated in low oxygen and glucose-deprived medium (in vitro ischemia), and a synaptic membrane fraction was prepared from the slices. Cl⁻-dependent l -[³H]Glu uptake into vesicles increased about twofold after 20 min of in vitro ischemia. The increased l -[³H]Glu uptake was inhibited by l -Glu, dl -2-amino-4-phosphonobutyrate, l -homocysteic acid, l -cystine, 4,4'-diisothiocyano-2,2'-disulfonic stilbene, and removal of Cl⁻. Uptakes of Na⁺-dependent l -[³H]-Glu, [³H]GABA, and [³H]taurine were not changed by the in vitro ischemia. In vitro ischemia increased the V _max value without affecting the K _m value. The increased l -[³H]Glu uptake by in vitro ischemia was reduced by subsequent incubation in a normoxic glucose-containing solution. ATP content in brain slices decreased to <10% of control values by in vitro ischemia for 10 min. The decrease in ATP content was restored by subsequent incubation in normoxic glucose-containing solution. Treatment with veratrine, 2,4-dinitrophenol, carbonyl cyanide m -chlorophenylhydrazone, and NaCN in normoxic conditions increased l -[³H]Glu uptake with a concomitant decrease in ATP content in slices. These results suggest that Cl⁻-dependent l -Glu transport activity in synaptic membranes increases in ischemia- or hypoxia-induced brain energy failures.     

Evidence for a Glutamatergic Pathway from the Guinea Pig Auditory Cortex to the Inferior Colliculus

Abstract: We attempt to provide evidence that the projection from the guinea pig auditory cortex (AC) to the inferior colliculus (IC) may contain glutamatergic or GABAergic fibers. Seven days after unilateral AC aspiration, histological studies indicated almost complete AC destruction and preterminal degeneration of fibers and terminal fields in the dorsal cortex (DCIC), external cortex (ECIC), and central nucleus (CNIC) of the IC ipsilateral to the ablated AC. Contralaterally, degeneration appeared in the DCIC. AC ablation depressed the electrically evoked Ca²⁺-dependent release of d -[³H]aspartate ( d -[³H]Asp) in the ipsilateral DCIC, ECIC, and CNIC, and d -[³H]Asp uptake in the CNIC. Together with other evidence that the corticocollicular pathway is excitatory, these findings suggest that this projection may contain glufamatergic and/or aspartatergic (Glu/Asp-ergic) fibers. Glutamic acid decarboxylase immunoreactivity was not apparent in presumed pyramidal cells of layer V of the AC retrogradely labeled with biotinylated dextran injected into the ipsilateral IC. Thus, corticocollicular neurons probably do not synthesize GABA and may not be GABAergic. However, AC ablation depressed [¹⁴C]GABA release from the ipsilateral DCIC and ECIC, and [¹⁴C]GABA uptake in the DCIC. These findings are consistent with the atrophy or down-regulation of some subcortical neurons that mediate GABAergic transmission in the IC.     

Ethanol-Induced Inhibition of [3H]Thienylcyclohexylpiperidine (TCP) Binding to NMDA Receptors in Brain Synaptic Membranes and to a Purified Protein Complex

Abstract: N -Methyl- d -asparate receptors (NMDARs) are a major target of ethanol effects in the nervous system. Haloperidol-insensitive, but dizocilpine (MK-801)-sensitive, binding of N -[1-(2-[³H]thienyl)cyclohexyl]piperidine ([³H]TCP) to synaptic membranes has the characteristics of ligand interaction with the ion channel of NMDARs. In the present studies, ethanol produced a concentration-dependent decrease in the maximal activation of [³H]TCP binding to synaptic membranes by NMDA and Gly, but a moderate change in the activation by l -Glu when l -Glu was present at concentrations < 100 µ M . However, ethanol (100 m M ) inhibited completely the activation of [³H]TCP binding produced by high concentrations of l -Glu (200–400 µ M ). It also inhibited strongly the activation of [³H]TCP binding by spermidine or spermidine plus Gly. In a purified complex of proteins that has l -Glu-, Gly-, and [³H]TCP-binding sites, ethanol (100 m M ) decreased significantly the maximal activation of [³H]TCP binding produced by either l -Glu or Gly. Activation constants ( K _act) for l -Glu and Gly acting on the purified complex were 12 and 28 µ M, respectively. Ethanol had no significant effect on the K _act of l -Glu but caused an increase in the K _act of Gly. These studies have identified at least one protein complex in neuronal membranes whose response to both l -Glu and Gly is inhibited by ethanol. These findings may explain some of the effects of acute and chronic ethanol treatment on the function and expression of the subunits of this complex in brain neurons.     

Remodeling of neuronal membranes as an early response to deafferentation. A freeze-fracture study

The early effects of deafferentation on the postsynaptic membrane beneath the end bulb of Held in the anteroventral cochlear nucleus (AVCN) were studied with the freeze-fracture technique. Three distinct responses were seen on the external membrane leaflet after cochlear ablation. Within 12 h the number of nonaggregate particles increased 147% by the addition of new particles to the membrane. The increase in number of nonaggregate particles continued until 4 days after cochlear ablation. The other responses occurred later, after degenerative changes were present in the end bulb. Between 1 and 2 days after cochlear ablation, the number of perisynaptic aggregates surrounding the postsynaptic active zone decreased to 10% of normal numbers. By 4 days, all perisynaptic aggregates had disappeared from the membrane. Coated vesicles may be involved in removing these aggregates. Between 1 and 3 days, the number of junctional aggregates decreased, but the size of the aggregates increased, apparently as a result of coalescence of nearby junctional aggregates. The total number of particles in junctional aggregates in the membrane was not altered during the first 6 days after cochlear ablation. The three separate responses suggest the existence of at least three different types of intramembranous particles on the external leaflet of the principal cell membrane, with each type dependent upon different cues for its maintenance in the membrane.     

Acoustic overexposure increases the expression of VGLUT-2 mediated projections from the lateral vestibular nucleus to the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is a first relay of the central auditory system as well as a site for integration of multimodal information. Vesicular glutamate transporters VGLUT-1 and VGLUT-2 selectively package glutamate into synaptic vesicles and are found to have different patterns of organization in the DCN. Whereas auditory nerve fibers predominantly co-label with VGLUT-1, somatosensory inputs predominantly co-label with VGLUT-2. Here, we used retrograde and anterograde transport of fluorescent conjugated dextran amine (DA) to demonstrate that the lateral vestibular nucleus (LVN) exhibits ipsilateral projections to both fusiform and deep layers of the rat DCN. Stimulating the LVN induced glutamatergic synaptic currents in fusiform cells and granule cell interneurones. We combined the dextran amine neuronal tracing method with immunohistochemistry and showed that labeled projections from the LVN are co-labeled with VGLUT-2 by contrast to VGLUT-1. Wistar rats were exposed to a loud single tone (15 kHz, 110 dB SPL) for 6 hours. Five days after acoustic overexposure, the level of expression of VGLUT-1 in the DCN was decreased whereas the level of expression of VGLUT-2 in the DCN was increased including terminals originating from the LVN. VGLUT-2 mediated projections from the LVN to the DCN are likely to play a role in the head position in response to sound. Amplification of VGLUT-2 expression after acoustic overexposure could be a compensatory mechanism from vestibular inputs in response to hearing loss and to a decrease of VGLUT-1 expression from auditory nerve fibers.     

Protein markers in the microcyst of the posteroventral cochlear nucleus of the gerbil

Microcysts are most evident in the posteroventral and anteroventral cochlear nuclei (PVCN and AVCN) of the Mongolian gerbil. The origin and contents of the microcyst are not elucidated at present. The present study investigated the possible inclusions in the microcyst by employing immunocytochemical labeling to localize the existence of various protein markers. Thirty and 100 microm thick sections were used to substitute and reconstruct between 6 and 20 paraffin serial sections, respectively. In 30-microm-thick slice sections, immunoreactivity of glial fibrillary acidic protein (GFAP-IR), mitochondria inner membrane (MCA-In-IR), S-100 (S-100-IR), serotonin (5-HT-IR), myelin proteolipid protein (PLP-IR) and substance P (SP-IR) abutted on the perimeter of the microcyst. The immunolabeled SP-positive cells were adjacent to the evagination of the microcyst. In 100-microm-thick slice sections, immunoreactivity of nitric oxide synthase (NOS-IR) and somatostatin (SOM-IR) mainly precipitated as flocculent structures in the small to medium-sized microcysts. 5-HT-IR also precipitated as an elongated flocculent stalk adjacent to the large microcyst or randomly distributed in the neuropil. The findings suggest that GFAP, MCA-In, S-100, 5-HT, PLP, SP, NOS and SOM may be involved in modulating the physiological functions and maintaining micro-environmental homeostasis of the microcyst in the cochlear nucleus of the gerbil.     

Effect of Electroconvulsive Shock on the Uptake and Release of Noradrenaline and 5-Hydroxytryptamine in Rat Brain Slices

Abstract: The uptake and release of [³H]noradrenaline and [³H]-5-hydroxytryptamine (5-HT) were studied in cerebral cortex slices from rats 30 min and 24 h after a single electroconvulsive shock (ECS) and 24 h after a series of five shocks given over 10 days. Both the K _m and V _max for 5-HT uptake were lower than controls 24 h after a single ECS, whereas after 5 ECS spread over 10 days both parameters remained depressed, though only the fall in V_max was significant. Noradrenaline uptake was not altered after a single ECS, but the V_max and K _m were elevated following chronic ECS treatment. Neither ECS treatment schedule had any effect on the potassium-stimulated release of either transmitter. It is possible that the changes in monoamine uptake seen following ECS are an adaptive response to alterations in the synaptic cleft concentration of these transmitters.     

Tinnitus,Unipolar Brush Cells,and Cerebellar Glutamatergic Function in an Animal Model

Unipolar brush cells (UBCs) are excitatory interneurons found in the dorsal cochlear nucleus (DCN) and the granule cell layer of cerebellar cortex, being particularly evident in the paraflocculus (PFL) and flocculus (FL). UBCs receive glutamatergic inputs and make glutamatergic synapses with granule cells and other UBCs. It has been hypothesized that UBCs comprise local networks of tunable feed-forward amplifiers. In the DCN they might also participate in feed-back amplification of signals from higher auditory centers. Recently it has been shown that UBCs, in the vestibulocerebellum and DCN of adult rats, express doublecortin (DCX), previously considered a marker of newborn and migrating neurons. In an animal model, both the DCN, and more recently the PFL, have been implicated in contributing to the sensation of acoustic-exposure-induced tinnitus. These studies support the working hypothesis that tinnitus emerges after loss of peripheral sensitivity because inhibitory processes homeostatically down regulate, and excitatory processes up regulate. Here we report the results of two sequential experiments that examine the potential role of DCN and cerebellar UBCs in tinnitus, and the contribution of glutamatergic transmission in the PFL. In Experiment 1 it was shown that adult rats with psychophysical evidence of tinnitus induced by a single unilateral high-level noise exposure, had elevated DCX in the DCN and ventral PFL. In Experiment 2 it was shown that micro-quantities of glutamatergic antagonists, delivered directly to the PFL, reversibly reduced chronically established tinnitus, while similarly applied glutamatergic agonists induced tinnitus-like behavior in non-tinnitus controls. These results are consistent with the hypothesis that UBC up regulation and enhanced glutamatergic transmission in the cerebellum contribute to the pathophysiology of tinnitus.     

The cochlear nuclei in man.

The human cochlear nuclei are composed of a ventral and a dorsal nucleus which are similar, though not identical, in their cytoarchitecture to those of other mammals. The ventral cochlear nucleus (VCN) consists of a rostral area of spherical cells, a central area of multipolar and globular cells, a posterior area of octopus cells, and laterodorsal cap of small neurons. The interareal boundaries are less distinct in man than in the cat. The central region of multipolar cells and the cap area of small cells constitute the bulk of the human VCN. The spherical, globular, and octopus cells appear relatively less numerous in man than in other mammals. The dorsal cochlear nucleus (DCN) in man is relatively large, but lacks the typical stratification seen in other mammals, with only vestiges of the granular and molecular layers remaining. Virtually the entire DCN consists of an area of cochlear fiber neuropil containing pyramidal cells, small neurons, and occasional giant cells. The pyramidal cells have lost their typical radial orientation and lie scattered within the cochlear neuropil. Thus the entire human DCN may be equivalent to layers 2 and 3 of this nucleus in other mammals. In spite of the relatively large DCN, the acoustic striae appear small. This is in contrast to the large trapezoid body leaving the VCN. Intrinsic and descending fiber pathways to the cochlear nuclei are not clearly defined and may be less prominent in man than in the cat.     

THE EFFECT OF DELTA-AMINOLAEVULINIC ACID ON THE UPTAKE AND EFFLUX OF AMINO ACID NEUROTRANSMITTERS IN RAT BRAIN SYNAPTOSOMES

Abstract— δ-Aminolaevulinic acid (δ-ALA) is an omega amino acid structurally similar to γ-aminobutyric acid (GABA) and l -glutamate. We have examined the effect of δ-ALA on the uptake and efflux of radiolabelled GABA and l -glutamate in rat cortical synaptosomes and report: (1) low concentrations of δ-ALA reduced the potassium-stimulated release of [³H]GABA from the synaptosome preparation. This effect was reversed by the GABA receptor antagonist bicuculline. We postulate that GABA release is modulated by a feedback mechanism on presynaptic GABA receptors, and that δ-ALA has agonist activity at these receptors. (2) δ-ALA at high concentrations (0.75-5.0 m m ) stimulated the efflux of l -[¹⁴C]glutamate from preloaded synaptosomes. (3) δ-ALA had no effect on potassium-stimulated release of l -glutamate. (4) Uptake of labelled l -glutamate was inhibited by δ-ALA in a noncompetitive fashion. (5) Synaptosomes did not accumulate [¹⁴C]δ-ALA in the range 0.5-50 δ m . These results are discussed in relation to the control of GABA release from nerve endings, and the role of δ-ALA in the neuropsychiatric manifestations of the acute porphyric attack.