Heterogeneous distribution of functionally important amino acids in brain areas of adult and aging humans

Identification of Therapeutic Targets for Medulloblastoma by Tissue-Specific Genome-Scale Metabolic Model

Medulloblastoma (MB), occurring in the cerebellum, is the most common childhood brain tumor. Because conventional methods decline life quality and endanger children with detrimental side effects, computer models are needed to imitate the characteristics of cancer cells and uncover effective therapeutic targets with minimum toxic effects on healthy cells. In this study, metabolic changes specific to MB were captured by the genome-scale metabolic brain model integrated with transcriptome data. To determine the roles of sphingolipid metabolism in proliferation and metastasis in the cancer cell, 79 reactions were incorporated into the MB model. The pathways employed by MB without a carbon source and the link between metastasis and the Warburg effect were examined in detail. To reveal therapeutic targets for MB, biomass-coupled reactions, the essential genes/gene products, and the antimetabolites, which might deplete the use of metabolites in cells by triggering competitive inhibition, were determined. As a result, interfering with the enzymes associated with fatty acid synthesis (FAs) and the mevalonate pathway in cholesterol synthesis, suppressing cardiolipin production, and tumor-supporting sphingolipid metabolites might be effective therapeutic approaches for MB. Moreover, decreasing the activity of succinate synthesis and GABA-catalyzing enzymes concurrently might be a promising strategy for metastatic MB.

Experimental evidence of tyrosine neurotoxicity: focus on mitochondrial dysfunction

Tissue exposure to high levels of tyrosine, which is characteristic of an inborn error of metabolism named Tyrosinemia, is related to severe symptoms, including neurological alterations. The clinical manifestations and pathogenesis of tyrosine neurotoxicity can be recapitulated in experimental models in vivo and in vitro. A widely used experimental model to study brain tyrosine damage is the chronic and acute administration of this amino acid in infant rats. Other research groups and we have extensively studied the pathogenic events in the brain structures of rats exposed to high tyrosine levels. Rats administered acutely and chronically with tyrosine presented decreased and inhibition of the essential metabolism enzymes, e.g., Krebs cycle enzymes and mitochondrial respiratory complexes in the brain structures. These alterations induced by tyrosine toxicity were associated with brain oxidative stress, astrocytes, and, ultimately, cognitive impairments. Notably, in vivo data were corroborated by in vitro studies using cerebral regions homogenates incubated with tyrosine excess. Considering metabolism's importance to brain functioning, we hypothesized that mitochondrial and metabolic dysfunctions are closely related to neurological alterations induced by tyrosine neurotoxicity. Herein, we reviewed the main mechanisms associated with tyrosine neurotoxicity in experimental models, emphasizing the role of mitochondrial dysfunction.

GABAA Receptors in the Mongolian Gerbil: a PET Study Using [18F]Flumazenil to Determine Receptor Binding in Young and Old Animals

PURPOSE

Plastic changes in the central auditory system involving the GABAergic system accompany age-related hearing loss. Such processes can be investigated with positron emission tomography (PET) imaging using [¹⁸F]flumazenil ([¹⁸F]FMZ). Here, [¹⁸F]FMZ PET-based modeling approaches allow a simple and reliable quantification of GABA_A receptor binding capacity revealing regional differences and age-related changes.

PROCEDURES

Sixty-minute list-mode PET acquisitions were performed in 9 young (range 5-6 months) and 11 old (range 39-42 months) gerbils, starting simultaneously with the injection of [¹⁸F]FMZ via femoral vein. Non-displaceable binding potentials (BPnd) with pons as reference region were calculated for auditory cortex (AC), inferior colliculus (IC), medial geniculate body (MGB), somatosensory cortex (SC), and cerebellum (CB) using (i) a two-tissue compartment model (2TCM), (ii) the Logan plot with image-derived blood-input (Logan (BI)), (iii) a simplified reference tissue model (SRTM), and (iv) the Logan reference model (Logan (RT)). Statistical parametric mapping analysis (SPM) comparing young and old gerbils was performed using 3D parametric images for BPnd based on SRTM. Results were verified with in vitro autoradiography from five additional young gerbils. Model assessment included the Akaike information criterion (AIC). Hearing was evaluated using auditory brainstem responses.

RESULTS

BPnd differed significantly between models (p < 0.0005), showing the smallest mean difference between 2TCM as reference and SRTM as simplified procedure. SRTM revealed the lowest AIC values. Both volume of distribution (r² = 0.8793, p = 0.018) and BPnd (r² = 0.8216, p = 0.034) correlated with in vitro autoradiography data. A significant age-related decrease of receptor binding was observed in auditory (AC, IC, MGB) and other brain regions (SC and CB) (p < 0.0001, unpaired t test) being confirmed by SPM using pons as reference (p < 0.0001, uncorrected).

CONCLUSION

Imaging of GABA_A receptor binding capacity in gerbils using [¹⁸F]FMZ PET revealed SRTM as a simple and robust quantification method of GABA_A receptors. Comparison of BPnd in young and old gerbils demonstrated an age-related decrease of GABA_A receptor binding.

Aging and Central Auditory Disinhibition: Is It a Reflection of Homeostatic Downregulation or Metabolic Vulnerability?

Aging-related changes have been identified at virtually every level of the central auditory system. One of the most common findings across these nuclei is a loss of synaptic inhibition with aging, which has been proposed to be at the heart of several aging-related changes in auditory cognition, including diminished speech perception in complex environments and the presence of tinnitus. Some authors have speculated that downregulation of synaptic inhibition is a consequence of peripheral deafferentation and therefore is a homeostatic mechanism to restore excitatory/inhibitory balance. As such, disinhibition would represent a form of maladaptive plasticity. However, clinical data suggest that deafferentation-related disinhibition tends to occur primarily in the aged brain. Therefore, aging-related disinhibition may, in part, be related to the high metabolic demands of inhibitory neurons relative to their excitatory counterparts. These findings suggest that both deafferentation-related maladaptive plastic changes and aging-related metabolic factors combine to produce changes in central auditory function. Here, we explore the arguments that downregulation of inhibition may be due to homeostatic responses to diminished afferent input vs. metabolic vulnerability of inhibitory neurons in the aged brain. Understanding the relative importance of these mechanisms will be critical for the development of treatments for the underlying causes of aging-related central disinhibition.

Morphological and neurochemical changes in GABAergic neurons of the aging human inferior colliculus

It is well known that quality of hearing decreases with increasing age due to changes in the peripheral or central auditory pathway. Along with the decrease in the number of neurons the neurotransmitter profile is also affected in the various parts of the auditory system. Particularly, changes in the inhibitory neurons in the inferior colliculus (IC) are known to affect quality of hearing with aging. To date, there is no information about the status of the inhibitory neurotransmitter GABA in the human IC during aging. We have collected and processed inferior colliculi of persons aged 11-97 years at the time of death for morphometry and immunohistochemical expression of glutamic acid decarboxylase (GAD67) and parvalbumin. We used unbiased stereology to estimate the number of cresyl-violet and immunostained neurons. Quantitative real-time PCR was used to measure the relative expression of the GAD67 mRNA. We found that the number of total, GABAergic and PV-positive neurons significantly decreased with increasing age (p < 0.05). The proportion of GAD67-ir neurons to total number of neurons was also negatively associated with increasing age (p = 0.004), but there was no change observed in the proportion of PV-ir neurons relative to GABAergic neurons (p = 0.25). Further, the fold change in the levels of GAD67 mRNA was negatively correlated to age (p = 0.024). We conclude that the poorer quality of hearing with increasing age may be due to decreased expression of inhibitory neurotransmitters and the decline in the number of inhibitory neurons in the IC.

Effects of hypotaurine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis in immature mice

To understand the action and mechanism of hypotaurine, an immediate precursor of taurine, on orofacial nociceptive processing, we examined the direct effects and receptor types involved in hypotaurine-induced responses using the whole-cell patch clamp technique in the substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis (Vc) of immature mice. Under the condition of high-chloride pipette solution, hypotaurine elicited inward currents or upward deflections of membrane potential, which increased in a concentration-dependent manner (30-3000 μM) with the EC₅₀ of 663.8 and 337.6 μM, respectively. The responses to 300 µM hypotaurine were reproducible and recovered upon washout. The 300 µM hypotaurine-induced currents were maintained in the presence of TTX, CNQX, and AP5, indicating direct postsynaptic action of hypotaurine on SG neurons. Responses to both low (300 µM) and high (1 or 3 mM) concentrations of hypotaurine were completely and reversibly blocked by the glycine receptor antagonist strychnine (2 µM), but unaffected by the GABA_A receptor antagonist gabazine (3 µM) which blocks synaptic GABA_A receptors at low concentration. Furthermore, responses to 300 µM hypotaurine and a maximal concentration of glycine (3 mM) were not additive, indicating that hypotaurine and glycine act on the same receptor. Hypotaurine-induced currents were partially antagonized by picrotoxin (50 µM) which blocks homomeric glycine receptors and by bicuculline (10 µM) which is an antagonist of α2 subunit-containing glycine receptors. These results suggest that hypotaurine-induced responses were mediated by glycine receptor activation in the SG neurons and hypotaurine might be used as an effective therapeutics for orofacial pain.

Reconstructed Metabolic Network Models Predict Flux-Level Metabolic Reprogramming in Glioblastoma

Developments in genome scale metabolic modeling techniques and omics technologies have enabled the reconstruction of context-specific metabolic models. In this study, glioblastoma multiforme (GBM), one of the most common and aggressive malignant brain tumors, is investigated by mapping GBM gene expression data on the growth-implemented brain specific genome-scale metabolic network, and GBM-specific models are generated. The models are used to calculate metabolic flux distributions in the tumor cells. Metabolic phenotypes predicted by the GBM-specific metabolic models reconstructed in this work reflect the general metabolic reprogramming of GBM, reported both in in-vitro and in-vivo experiments. The computed flux profiles quantitatively predict that major sources of the acetyl-CoA and oxaloacetic acid pool used in TCA cycle are pyruvate dehydrogenase from glycolysis and anaplerotic flux from glutaminolysis, respectively. Also, our results, in accordance with recent studies, predict a contribution of oxidative phosphorylation to ATP pool via a slightly active TCA cycle in addition to the major contributor aerobic glycolysis. We verified our results by using different computational methods that incorporate transcriptome data with genome-scale models and by using different transcriptome datasets. Correct predictions of flux distributions in glycolysis, glutaminolysis, TCA cycle and lipid precursor metabolism validate the reconstructed models for further use in future to simulate more specific metabolic patterns for GBM.

Tissue correction for GABA-edited MRS: Considerations of voxel composition, tissue segmentation, and tissue relaxations

PURPOSE

To develop a tissue correction for GABA-edited magnetic resonance spectroscopy (MRS) that appropriately addresses differences in voxel gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) fractions.

MATERIALS AND METHODS

Simulations compared the performance of tissue correction approaches. Corrections were then applied to in vivo data from 16 healthy volunteers, acquired at 3T. GM, WM, and CSF fractions were determined from T1 -weighted images. Corrections for CSF content, GM/WM GABA content, and water relaxation of the three compartments are combined into a single, fully corrected measurement.

RESULTS

Simulations show that CSF correction increases the dependence of GABA measurements on GM/WM fraction, by an amount equal to the fraction of CSF. Furthermore, GM correction substantially (and nonlinearly) increases the dependence of GABA measurements on GM/WM fraction, for example, by a factor of over four when the voxel GM tissue fraction is 50%. At this tissue fraction, GABA is overestimated by a factor of 1.5. For the in vivo data, correcting for voxel composition increased measured GABA values (P < 0.001 for all regions), but did not reduce intersubject variance (P > 0.5 for all regions). Corrected GABA values differ significantly based on the segmentation procedure used (P < 0.0001) and tissue parameter assumptions made (P < 0.0001).

CONCLUSION

We introduce a comprehensive tissue correction factor that adjusts GABA measurements to correct for different voxel compositions of GM, WM, and CSF.

Two-voxel spectroscopy with dynamic B0 shimming and flip angle adjustment at 7 T in the human motor cortex

The aim of this study was to acquire high-quality in vivo (1) H spectra concurrently from two voxels at ultra-high field (7 T) without specialized hardware. To this end, an acquisition scheme was developed in which first-order shims and flip angles are dynamically updated to acquire spectra from both of the brain's motor cortices in an alternating fashion. To validate this acquisition scheme, separate, static, single-voxel acquisitions were also performed for comparison. Six subjects were examined using semi-LASER spectroscopy at 7 T. Barium titanate pads were used to increase the extent of the effective transmit field (B1 (+) ). Spectra were obtained from the hand area of both motor cortices for both acquisition schemes. LCModel was used to determine neurochemical profiles in order to examine variations between acquisition schemes and volumes of interest. The dynamic two-voxel acquisition protocol produced water linewidths (full width at half-maximum between 11.6 and 12.8 Hz) and signal-to-noise ratios similar to those from static single-voxel measurements. The concentrations of 13 individual and 3 combined metabolites with Cramér-Rao lower bounds below 30% were reliably detected for both acquisition schemes, and agreed well with previous postmortem assay and spectroscopy studies. The results show that high spectral quality from two voxels can be acquired concurrently without specialized hardware. This practical technique can be applied to many neuroscience applications.

Metabolome-wide association study of phenylalanine in plasma of common marmosets

Little systematic knowledge exists concerning the impacts of cumulative lifelong exposure, termed the exposome, on requirements for nutrients. Phenylalanine (Phe) is an essential dietary amino acid with an aromatic ring structure similar to endogenous metabolites, dietary compounds and environmental agents. Excess plasma Phe in genetic disease or nutritional deficiency of Phe has adverse health consequences. In principle, structurally similar chemicals interfering with Phe utilization could alter Phe requirement at an individual level. As a strategy to identify components of the exposome that could interfere with Phe utilization, we tested for metabolites correlating with Phe concentration in plasma of a non-human primate species, common marmosets (Callithrix jacchus). The results of tests for more than 5,000 chemical features detected by high-resolution metabolomics showed 17 positive correlations with Phe metabolites and other amino acids. Positive and negative correlations were also observed for 33 other chemicals, which included matches to endogenous metabolites and dietary, microbial and environmental chemicals in database searches. Chemical similarity analysis showed many of the matches had high structural similarity to Phe. Together, the results show that chemicals in marmoset plasma could impact Phe utilization. Such chemicals could contribute to early lifecycle developmental disorders when neurological development is vulnerable to Phe levels.

Is GABA neurotransmission enhanced in auditory thalamus relative to inferior colliculus?

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central auditory system. Sensory thalamic structures show high levels of non-desensitizing extrasynaptic GABAA receptors (GABAARs) and a reduction in the redundancy of coded information. The present study compared the inhibitory potency of GABA acting at GABAARs between the inferior colliculus (IC) and the medial geniculate body (MGB) using quantitative in vivo, in vitro, and ex vivo experimental approaches. In vivo single unit studies compared the ability of half maximal inhibitory concentrations of GABA to inhibit sound-evoked temporal responses, and found that GABA was two to three times (P < 0.01) more potent at suppressing MGB single unit responses than IC unit responses. In vitro whole cell patch-clamp slice recordings were used to demonstrate that gaboxadol, a δ-subunit selective GABAAR agonist, was significantly more potent at evoking tonic inhibitory currents from MGB neurons than IC neurons (P < 0.01). These electrophysiological findings were supported by an in vitro receptor binding assay which used the picrotoxin analog [(3)H]TBOB to assess binding in the GABAAR chloride channel. MGB GABAARs had significantly greater total open chloride channel capacity relative to GABAARs in IC (P < 0.05) as shown by increased total [(3)H]TBOB binding. Finally, a comparative ex vivo measurement compared endogenous GABA levels and suggested a trend towards higher GABA concentrations in MGB than in IC. Collectively, these studies suggest that, per unit GABA, high affinity extrasynaptic and synaptic GABAARs confer a significant inhibitory GABAAR advantage to MGB neurons relative to IC neurons. This increased GABA sensitivity likely underpins the vital filtering role of auditory thalamus.

Regional difference in GABA levels between medial prefrontal and occipital cortices

PURPOSE

To avoid the confounding effects of variations in tissue composition this study measured regional GABA differences using two voxels with the same tissue composition.

MATERIALS AND METHODS

Eighteen healthy adult volunteers were scanned using a 3 Tesla GE clinical scanner with a J-coupling based editing sequence. Spectroscopy voxels were placed in the medial prefrontal (MPFC) and occipital cortex (OCC) with essentially the same gray and white matter fractions.

RESULTS

A 16% (P = 0.0001) significantly higher GABA to creatine ratio was found in the OCC (0.1103 ± 0.0050) compared with the MPFC (0.0953 ± 0.0041). When normalized to tissue water, GABA concentrations in the OCC were 14% higher than in the MPFC.

CONCLUSION

A difference in GABA concentration was found between the OCC and MPFC voxels in healthy subjects which is attributable to differences other than tissue composition.

Age-related changes in the expression of NMDA, serotonin, and GAD in the central auditory system of the rat

CONCLUSIONS

We suggest that age-induced changes of serotonin, N-methyl-d-aspartate receptor (NMDAR), and glutamate decarboxylase (GAD) expression in rats are dependent on the specific location in the central auditory system (CAS).

OBJECTIVES

Despite the importance of understanding changes in neurotransmitters during presbycusis, only a few studies have assessed age-associated changes in neurotransmitter at each level of the CAS. We therefore evaluated effects of aging on neurotransmission in the CAS of rats.

METHODS

The concentrations of serotonin, NMDAR, and GAD were assayed immunohistochemically in the cochlear nucleus (CN), superior olivary nucleus (SON), inferior colliculus (IC), medial geniculate body (MGB), and auditory cortex (AC) of Sprague-Dawley rats, aged 2 weeks (n = 20) or 24 months (n = 20).

RESULTS

The total number of neuronal cell bodies of the CAS did not differ significantly at each level between young and aged rats (p > 0.05). Serotonin expression was increased with age in the IC and MGB, but decreased in the CN, SON, and AC (p < 0.05). NMDAR was significantly higher in the CN, MGB, and AC of aged compared with young rats, but was significantly decreased over time in the SOC and IC (p < 0.05). GAD67 was increased with age in the MGB and AC and decreased in the CN and SON (p < 0.05), but was not changed in the IC.

In vivo magnetic resonance spectroscopy measurement of gray-matter and white-matter gamma-aminobutyric acid concentration in sensorimotor cortex using a motion-controlled MEGA point-resolved spectroscopy sequence

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the brain. Understanding the GABA concentration, in vivo, is important to understand normal brain function. Using MEGA point-resolved spectroscopy sequence with interleaved water scans to detect subject motion, GABA level of sensorimotor cortex was measured using a voxel identified from a functional magnetic resonance imaging scan. The GABA level in a 20×20×20-mm(3) voxel consisting of 37%±7% gray matter, 52%±12% white matter and 11%±8% cerebrospinal fluid in the sensorimotor region was measured to be 1.43±0.48 mM. In addition, using linear regression analysis, GABA concentrations within gray and white matter were calculated to be 2.87±0.61 and 0.33±0.11 mM, respectively.

Glycine receptors involved in acamprosate's modulation of accumbal dopamine levels: an in vivo microdialysis study

BACKGROUND

Glycine receptors (GlyRs) in the nucleus accumbens (nAc) and nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) have been suggested to be involved in the positive reinforcing and dopamine elevating effects of ethanol. Recent studies have also shown that ethanol high-preferring rats substantially decrease their ethanol intake when treated with a glycine transporter 1 inhibitor (ORG 25935). Acamprosate, a drug used for relapse prevention in treatment of alcohol dependence, has also been demonstrated to elevate extracellular dopamine levels in the nAc. However, the underlying mechanism of action of acamprosate is not fully understood. Here we investigated whether acamprosate interferes with a neuronal circuitry that previously has been demonstrated to be involved in the dopamine elevating effects of ethanol and taurine.

METHODS

In vivo microdialysis in freely moving rats was used to assess accumbal dopamine levels before and during local (nAc) or systemic administration of acamprosate.

RESULTS

Perfusion of 0.5 mM acamprosate in the nAc significantly increased dopamine levels. Pretreatment either with 10 microM strychnine in the nAc or 100 microM mecamylamine in the VTA, completely antagonized the acamprosate-induced elevation of accumbal dopamine levels. Also, systemic acamprosate administration elevated accumbal dopamine output, an effect that was abolished by local (nAc) pretreatment with 10 microM strychnine.

CONCLUSIONS

These results suggest that both systemic and local application of acamprosate elevate extracellular dopamine levels in the nAc by activating accumbal GlyRs, and, secondarily, tegmental nAChRs.

Elevated endogenous GABA concentration attenuates glutamate-glutamine cycling between neurons and astroglia

In this study, the relationship between endogenous brain GABA concentration and glutamate-glutamine cycling flux (V (cyc)) was investigated using in vivo (1)H and (1)H{(13)C} magnetic resonance spectroscopy techniques. Graded elevations of brain GABA levels were induced in rat brain after administration of the highly specific GABA-transaminase inhibitor vigabatrin (gamma-vinyl-GABA). The glial-specific substrate [2-(13)C]acetate and (1)H{(13)C} magnetic resonance spectroscopy were used to measure V (cyc) at different GABA levels. Significantly reduced V (cyc) was found in rats pretreated with vigabatrin. The reduction in group mean V (cyc) over the range of GABA concentrations investigated in this study (1.0 +/- 0.3-5.1 +/- 0.5 micromol/g) was found to be nonlinear: Delta V (cyc)/V (cyc) = [GABA (micromol/g)](-0.35 )- 1.0 (r (2) = 0.98). The results demonstrate that V (cyc) is modulated by endogenous GABA levels, and that glutamatergic and GABAergic interactions can be studied in vivo using noninvasive magnetic resonance spectroscopy techniques.

In vivo detection of gray and white matter differences in GABA concentration in the human brain

A novel selective multiple quantum filtering-based chemical shift imaging method was developed for acquiring GABA images in the human brain at 3 T. This method allows a concomitant acquisition of an interleaved total creatine image with the same spatial resolution. Using T(1)-based image segmentation and a nonlinear least square regression analysis of GABA-to-total creatine concentration ratios in frontal and parietal lobes of healthy adult volunteers as a function of the tissue gray matter fraction, the mean GABA concentration in gray and white matter was determined to be 1.30+/-0.36 micromol/g and 0.16+/-0.16 micromol/g (mean+/-SD, n=13), respectively. It is expected that this method will become a useful tool for studying GABAergic function in the human brain in vivo.

Taurine interaction with neurotransmitter receptors in the CNS: an update

Taurine appears to have multiple functions in the brain participating both in volume regulation and neurotransmission. In the latter context it may exert its actions by serving as an agonist at receptors of the GABAergic and glycinergic neurotransmitter systems. Its interaction with GABAA and GABAB receptors as well as with glycine receptors is reviewed and the physiological relevance of such interactions is evaluated. The question as to whether local extracellular concentrations of taurine are likely to reach the threshold level for the pertinent receptor populations cannot presently be answered satisfactorily. Hence more sophisticated analytical methods are warranted in order to obtain a definite answer to this important question.

Age-related changes in the inhibitory response properties of dorsal cochlear nucleus output neurons: role of inhibitory inputs

Age-related hearing loss frequently results in a loss in the ability to discriminate speech signals, especially in noise. This is attributable, in part, to a loss in temporal resolving power and ability to adjust dynamic range. Circuits in the adult dorsal cochlear nucleus (DCN) have been shown to preserve signal in background noise. Fusiform cells, major DCN output neurons, receive focused glycinergic inputs from tonotopically aligned vertical cells that also project to the ventral cochlear nucleus. Glycine-mediated inhibition onto fusiform cells results in decreased tone-evoked activity as intensity is increased at frequencies adjacent to characteristic frequency (CF). DCN output is thus shaped by glycinergic inhibition, which can be readily assessed in recordings from fusiform cells. Previous DCN studies suggest an age-related loss of markers for glycinergic neurotransmission. The present study postulated that response properties of aged fusiform cells would show a loss of inhibition, resembling conditions observed with glycine receptor blockade. The functional impact of aging was examined by comparing response properties from units meeting fusiform-cell criteria in young and aged rats. Fusiform cells in aged animals displayed significantly higher maximum discharge rates to CF tones than those recorded from young-adult animals. Fusiform cells of aged rats displayed significantly fewer nonmonotonic CF rate-level functions and an age-related change in temporal response properties. These findings are consistent with an age-related loss of glycinergic input, likely from vertical cells, and with findings from other sensory aging studies suggesting a selective age-related decrement in inhibitory amino acid neurotransmitter function.

Endogenous neuro-protectants in ammonia toxicity in the central nervous system: facts and hypotheses

The paper overviews experimental evidence suggestive of the engagement of three endogenous metabolites: taurine, kynurenic acid, and glutathione (GSH) in the protection of central nervous system (CNS) cells against ammonia toxicity. Intrastriatal administration of taurine via microdialysis probe attenuates ammonia-induced accumulation of extracellular cyclic guanosine monophosphate (cGMP) resulting from over-activation of the N-methyl-D: -aspartate/nitric oxide (NMDA/NO) pathway, and this effect involves agonistic effect of taurine on the GABA-A and glycine receptors. Taurine also counteracts generation of free radicals, increased release of dopamine, and its metabolism to dihydroxyphenylacetic acid (DOPAC). Taurine reduces ammonia-induced increase of cell volume (edema) in cerebrocortical slices by a mechanism involving GABA-A receptors. Massive release of radiolabeled or endogenous taurine from CNS tissues by ammonia in vivo and in vitro is thought to promote its neuroprotective action, by making the amino acid available for interaction with cell membranes and/or by driving excess water out of the CNS cells (astrocytes) that underwent ammonia-induced swelling. Ammonia in vivo and in vitro affects in variable ways the synthesis of kynurenic acid (KYNA). Since KYNA is an endogenous NMDA receptor antagonist with a high affinity towards its glycine site, changes in its content may counter over-activation or depression of glutaminergic transmission observed at the different stages of hyperammonemia. GSH is a major antioxidant in the CNS whose synthesis is partly compartmented between neurons and astrocytes: astrocytic GSH is a source of precursors for the synthesis of neuronal GSH. Ammonia in vitro stimulates GSH synthesis in cultured astrocytes, which may compensate for increased GSH consumption (decreased GSH/GSSG ratio) in neurons.

Age-related loss of the GABA synthetic enzyme glutamic acid decarboxylase in rat primary auditory cortex

Age-related changes within the auditory brainstem typically include alterations in inhibitory neurotransmission and coding mediated by GABA and glycinergic circuits. As part of an effort to evaluate the impact of aging on neurotransmission in the higher auditory centers, the present study examined age-related changes in the GABA synthetic enzyme, glutamic acid decarboxylase (GAD), in rat primary auditory cortex (AI), which contains a vast network of intrinsic and extrinsic GABAergic circuits throughout its layers. Message levels of the two GAD isoforms found in brain, GAD(65) and GAD(67), and GAD(67) protein levels were compared in young adult, middle-aged and aged rats using in situ hybridization and quantitative immunocytochemistry, respectively. For comparison, age-related GAD changes were also assessed in the parietal cortex and hippocampus. Significant age-related decreases in GAD(65&67) messages were observed in AI layers II-VI of aged rats relative to their young adult cohorts. The largest changes were identified in layer II (GAD(65): -26.6% and GAD(67): -40.1%). GAD(67) protein expression decreased significantly in parallel with mRNA decreases in all layers of AI. Adjacent regions of parietal cortex showed no significant GAD(67) protein changes among the age groups, except in layer IV. As previously described, GAD(67) message and protein levels in selected hippocampal regions were significantly reduced in aged rats. Age-related GAD reductions likely reflect decreases in both metabolic and pre-synaptic GABA levels suggesting a plastic down-regulation of normal adult inhibitory GABA neurotransmission. Consistent with the present findings, functional studies in primate visual cortex and preliminary studies in AI find coding changes suggestive of altered inhibitory processing in aged animals. An age-related loss of normal adult GABA neurotransmission in AI would likely alter temporal coding properties and could contribute to the loss in speech understanding observed in the elderly.

The role of inhibitory amino acidergic neurotransmission in hepatic encephalopathy: a critical overview

Gamma-Aminobutyric acid (GABA) is the main inhibitory amino acid in the central nervous system (CNS). Experiments with animal models of HE, and with brain slices or cultured CNS cells treated with ammonia, have documented changes in GABA distribution and transport, and modulation of the responses of both the GABA(A)-benzodiazepine receptor complex and GABA(B) receptors. Although many of the data point to an enhancement of GABAergic transmission probably contributing to HE, the evidence is not unequivocal. The major weaknesses of the GABA theory are (1) in a vast majority of HE models, there were no alterations of GABA content in the brain tissue and/or extracellular space, indicating that exposure of neurons to GABA may not have been altered, (2) changes in the affinity and capacity of GABA receptor binding were either absent or qualitatively different in HE models of comparable severity and duration, and (3) no sound changes in the GABAergic system parameters were noted in clinical cases of HE. Taurine (Tau) is an amino acid that is thought to mimic GABA function because of its agonistic properties towards GABA(A) receptors, and to contribute to neuroprotection and osmoregulation. These effects require Tau redistribution between the different cell compartments and the extracellular space. Acute treatment with ammonia evokes massive release of radiolabeled or endogenous Tau from CNS tissues in vivo and in vitro, and the underlying mechanism of Tau release differs from the release evoked by depolarizing conditions or hypoosmotic treatment. Subacute or chronic HE, and also long-term treatment of cultured CNS cells in vitro with ammonia, increase spontaneous Tau "leakage" from the tissue. This is accompanied by a decreased potassium- or hypoosmolarity-induced release of Tau and often by cell swelling, indicating impaired osmoregulation. In in vivo models of HE, Tau leakage is manifested by its increased accumulation in the extrasynaptic space, which may promote inhibitory neurotransmission and/or cell membrane protection. In chronic HE in humans, decreased Tau content in CNS is thought to be one of the causes of cerebral edema. However, understanding of the impact of the changes in Tau content and transport on the pathogenic mechanisms of HE is hampered by the lack of clear-cut evidence regarding the various roles of Tau in the normal CNS.

Presbyacusis and the auditory brainstem response

Age-related hearing loss (ARHL or presbyacusis) is an increasingly common form of sensorineural hearing loss (SNHL) as a result of changing demographics, and the auditory brainstem response (ABR) is a common experimental and clinical tool in audiology and neurology. Some of the changes that occur in the aging auditory system may significantly influence the interpretation of the ABR in comparison to the ABRs of younger adults. The approach of this review will be to integrate physiological and histopathological data from human and animal studies to provide a better understanding of the array of age-related changes in the ABR and to determine how age-related changes in the auditory system may influence how the ABR should be interpreted in presbyacusis. Data will be described in terms of thresholds, latencies, and amplitudes, as well as more complex auditory functions such as masking and temporal processing. Included in the review of data will be an attempt to differentiate between age-related effects that may strictly be due to threshold elevation from those that may be due to the aging process.

The amplitude-modulation following response in young and aged human subjects

The amplitude-modulation following response (AMFR) is a steady-state auditory response which may be an objective measure of intensity discrimination. Aged subjects with normal hearing have poorer intensity discrimination for low-frequency tones measured behaviorally, which would predict poorer AMFRs for low-frequency carriers. Experiment 1 was designed to assess age-related differences in AMFR characteristics. Response amplitudes were not significantly different among the young and aged groups for either carrier frequency (520 or 4000 Hz) or modulation depth (0--100%). Response phase did not vary systematically among groups. These results suggest that the AMFR may not be directly comparable to behavioral measures of intensity discrimination in aged subjects with normal hearing. To assess the contribution of high-frequency hearing loss on the AMFR in aged subjects, Experiment 2 compared AMFR amplitudes in aged subjects and in young subjects under the condition of high-pass masking. The amplitude of the AMFR was reduced at 520 Hz for both aged subjects and masked young subjects compared to unmasked young subjects, suggesting that reduced amplitudes in aged subjects with high-frequency hearing loss were associated with threshold elevations. Furthermore, the results suggest that the base of the cochlea contributes to the AMFR for low carrier frequencies.

Age-related changes in GABA(A) receptor subunit composition and function in rat auditory system

A decline in the ability to discriminate speech from noise due to age-related hearing loss (presbycusis) may reflect impaired auditory information processing within the central nervous system. Presbycusis may result, in part, from functional loss of the inhibitory neurotransmitter GABA. The present study assessed age-related changes of the GABA(A) receptor in the inferior colliculus of young-adult, middle-aged, and aged rats related to: (i) receptor subunit composition and (ii) receptor function. Western blotting was used to measure protein levels of selected GABA(A) receptor subunits in preparations obtained from the inferior colliculus of Fischer 344 and Fischer 344/Brown-Norway F1 hybrid rats. In both strains, the aged group exhibited significant increases in gamma1 subunit protein and a decrease in alpha1 subunit protein. To examine the functional consequence of this putative age-related subunit change, we measured the ability of exogenous GABA to flux/translocate chloride ions into microsac preparations derived from Fischer 344 inferior colliculus. GABA-mediated chloride influx was significantly increased in samples prepared from the inferior colliculus of aged animals. Together with previous studies, these results strongly suggest an age-related change in GABA(A) receptor composition. These changes may reflect a compensatory up-regulation of inhibitory function in the face of significant loss of presynaptic GABA release. These findings provide one example of plastic neurotransmitter receptor changes which can occur during the ageing process.

Alteration of protease levels in different brain areas of suicide victims

Numerous recent studies found that proteases play a major role in brain function. In addition to their role in protein turnover, they have modulatory functions and an important role in apoptosis, pathological changes, and other mechanisms. To explore possible differences in brain protein metabolism of suicide victims, we examined the activity of two proteases, cathepsin D and calpain (I and II combined), in eleven discrete areas of postmortem brain tissue of 21 victims of suicide and of 31 age- and sex-matched control subjects without a history of psychiatric or neurological disease. The levels of functionally important amino acids in five of these areas were also measured. Cathepsin D activity was found to be lower in two of eleven regions of brains of suicide victims, the parahippocampal cortex and the medial hypothalamus, by 26% and 27%, respectively. Calpain activity was lower in two different areas tested, 29% in the medulla oblongata and 26% in the lateral prefrontal cortex, and was 18% higher in the midbrain. There were no significant differences in the other areas (globus pallidus, hippocampus, amygdala, caudate nucleus, ventral tegmental area, and nucleus accumbens). Protease distribution was regionally heterogeneous--the levels in the globus pallidus were low, and in the hippocampus high, with about a two-fold difference. The length of the postmortem period for obtaining tissue, the storage time of the frozen tissue, and the age of the subject had no apparent influence on the results obtained. Although there was a tendency toward higher levels of aspartate and glycine in brain areas from suicide victims, the difference was not significant. The variations among individual brains were greater in amino acid levels than in protease levels. The findings indicate the possible role of protein metabolism in depressive or suicidal behavior.

Alterations of GABAA receptor subunit mRNA levels in the aging Fischer 344 rat inferior colliculus

The inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) is critically involved in shaping neuronal responses to simple and complex acoustic stimuli in the central auditory structure, the inferior colliculus (IC). Studies in rat and human suggest that age-related changes in markers for GABA neurotransmission occur in the IC. In particular, these changes include findings indicative of an age-related increase in the efficacy/potency of GABA to inhibit ligand binding at the GABAA receptor picrotoxin site in the Fischer (F344) rat IC. Such changes in GABAA receptor modulation suggest the potential for an alteration in GABAA receptor subunit composition in the old rat IC. To test this idea, the present study used in situ hybridization to quantify age-related changes in GABAA receptor subunit mRNA levels in the three major subdivisions of the IC in the F344 rat: dorsal cortex (DCIC), external cortex (ECIC), and the central nucleus (CIC). In support of earlier findings of an age-related change in GABAA receptor modulation, the present study found: (1) GABAA receptor subunit mRNA levels were significantly altered in the IC of old rats, and (2) age-related changes in subunit levels appeared to be regionally selective and subunit specific. A highly significant increase in the level of the gamma 1 subunit mRNA was observed with little change in the levels of the alpha 1, beta 2, and gamma 2 subunit mRNAs. A nonstatistically significant increase in alpha 2 subunit mRNA was also observed. This observed increase in alpha 2 subunit mRNA could be important since previous expression studies have shown that the alpha 2 and gamma 1 subunits coassemble and are incorporated into GABAA receptors which appear to be more sensitive to GABA. If the observed changes in subunit mRNA levels with age correlate well with enhanced GABAA receptor function in the IC of old rats, this, in turn, may represent a compensatory mechanism in response to presynaptic GABAergic changes.

Aspartate, glutamate, glutamine, glycine and gamma-aminobutyric acid in human bioptic neocortical areas: comparison to autoptic tissue

Amino acid concentrations were determined by high performance liquid chromatography in distinct areas of human neocortex of autoptic and bioptic origin. The concentrations in autoptic tissue were similar in all cortical areas which may be explained by postmortem proteolysis, abolishing regional differences seen in bioptic tissue. Aspartate, glutamate, glycine and gamma-aminobutyric acid concentrations were lower, but glutamine levels were higher, in biopsied than in autopsied tissue. Glycine and gamma-aminobutyric acid concentrations increased with the age of biopsied patients. The differences seen suggest that only amino acid concentrations determined in bioptic tissue may yield a reliable data base for the interpretation of pathological alterations in neocortical biopsies of patients with brain diseases.

Auditory evoked potentials in aged gerbils: responses elicited by noises separated by a silent gap

The compound action potential (CAP) and the auditory brainstem response (ABR; waves ii and iv) were recorded in young (4-8 month) and aged (33-37 month) gerbils using a paradigm similar to that used in some psychophysical studies of gap detection (a pair of identical low-pass noises separated by a silent gap). Response amplitudes were analyzed in terms of absolute amplitudes and the 'amplitude ratio' (the amplitude of the response to the second noise of a pair divided by that to the first). Response latencies were analyzed in terms of the absolute latencies as well as the 'latency shift' (the latency of the response to the second noise minus that to the first). Response amplitudes were much smaller in the aged subjects for both the first and second stimuli of a pair. There were minimal changes in amplitude ratios across age for both the CAP and ABR. Absolute latencies were similar between groups for the first stimulus of a pair, but latencies to wave iv were much longer for the aged subjects when the gap was short. Thus, the latency shift for the aged group was much longer for wave iv in the aged compared to the young group, but were similar between groups for the CAP or wave ii of the ABR. The results suggest that there may be changes in coding of temporal information in the auditory brainstem of aged gerbils which are not a direct result of abnormal temporal processing in the auditory periphery.

Responses of young and aged Fischer 344 rat inferior colliculus neurons to binaural tonal stimuli

The inferior colliculus (IC) is one nucleus of the central auditory system which displays age-related changes. Inputs to the IC use primarily the amino acid neurotransmitters glutamate and gamma-aminobutryic acid (GABA). Neurochemical and anatomical studies of the Fischer 344 (F344) rat IC have shown decreases in GABA and GABA receptor levels (see Caspary et al., 1995 for review). GABA neurotransmission affects binaural response properties in the IC (Faingold et al., 1991a, b; Vater et al., 1992a; Park and Pollak, 1993, 1994). We hypothesized that aged F344 rats would show alterations in binaural IC neuronal response properties due to an imbalance in the relative levels of inhibition and excitation. Extracellular recordings from 189 single units localized to the IC of anesthetized aged (24 month) F344 rats were compared to those obtained from 221 IC units in young adult (3 month) animals. Quantitative analyses were performed to determine the distribution of ipsilateral and binaural rate/intensity functions (RIFs) in the central nucleus of the IC and external cortex of the IC units. The majority of IC units in both young and aged F344 rats were not responsive to monaural ipsilateral characteristic frequency tone bursts. Although there was some shift in the distribution of binaural RIF shapes with age, it was not statistically significant. The shift included a reduction in the percentage of units classified as E/I (excited by contralateral stimulation/ipsilaterally inhibited during binaural stimulation), but an increase with age in the percentage of units classified as E/f (excited by contralateral stimulation/ further facilitated by the addition of low intensity ipsilateral stimulation, but inhibited by higher intensity ipsilateral stimulation). Despite the role of inhibitory neurotransmission in binaural processing in the IC, age-related neurochemical deficits in the IC do not appear to result in a major deficit in the processing of simple binaural stimuli in F344 rats.

Effect of food deprivation on glutathione and amino acid levels in brain and liver of young and aged rats

The effect of short-term food deprivation on glutathione (GSH) and amino acid levels in brain regions of young and aged rats was compared with changes observed in liver. Animals aged 3 months and 24 months were deprived of food for 48 h. GSH and amino acid levels from cerebral cortex, cerebellum, pons medulla, and liver were assayed and compared with levels in animals of the same age fed normal diets. In liver in both young and old rats, GSH levels fell 30%, from 13 mumol/g tissue to 8.7 mumol/g tissue. Significant changes were observed in other amino acids, including an increase of 30-50% in methionine, glycine, and glutamine, and a decrease of 30-50% in alanine in liver of both young and aged rats, and a 4-fold increase in taurine in young. In brain, little change was observed upon food deprivation. No decrease was observed in GSH, and only small changes were observed in other amino acids. In the aged animal aspartate, glutamate, and alanine levels were slightly lower; tyrosine in cerebellum was reduced by 30%, and both glycine and tyrosine in the pons medulla were reduced by 20-30%. In the brain areas examined, levels of GSH ranged from 1-2 mumol/g in young and 0.8-1.4 mumol/g in old; with levels in pons medulla being lower than those in cerebral cortex. In brain, in contrast to liver, levels were scarcely affected by short-term food deprivation.(ABSTRACT TRUNCATED AT 250 WORDS)