Changes with aging in the levels of amino acids in rat CNS structural elements. II. Taurine and small neutral amino acids

Effects of Age on Responses of Principal Cells of the Mouse Anteroventral Cochlear Nucleus in Quiet and Noise

Older listeners often report difficulties understanding speech in noisy environments. It is important to identify where in the auditory pathway hearing-in-noise deficits arise to develop appropriate therapies. We tested how encoding of sounds is affected by masking noise at early stages of the auditory pathway by recording responses of principal cells in the anteroventral cochlear nucleus (AVCN) of aging CBA/CaJ and C57BL/6J mice in vivo. Previous work indicated that masking noise shifts the dynamic range of single auditory nerve fibers (ANFs), leading to elevated tone thresholds. We hypothesized that such threshold shifts could contribute to increased hearing-in-noise deficits with age if susceptibility to masking increased in AVCN units. We tested this by recording the responses of AVCN principal neurons to tones in the presence and absence of masking noise. Surprisingly, we found that masker-induced threshold shifts decreased with age in primary-like units and did not change in choppers. In addition, spontaneous activity decreased in primary-like and chopper units of old mice, with no change in dynamic range or tuning precision. In C57 mice, which undergo early-onset hearing loss, units showed similar changes in threshold and spontaneous rate at younger ages, suggesting they were related to hearing loss and not simply aging. These findings suggest that sound information carried by AVCN principal cells remains largely unchanged with age. Therefore, hearing-in-noise deficits may result from other changes during aging, such as distorted across-channel input from the cochlea and changes in sound coding at later stages of the auditory pathway.

Mechanisms of age-related hearing loss at the auditory nerve central synapses and postsynaptic neurons in the cochlear nucleus

Sound information is transduced from mechanical vibration to electrical signals in the cochlea, conveyed to and further processed in the brain to form auditory perception. During the process, spiral ganglion neurons (SGNs) are the key cells that connect the peripheral and central auditory systems by receiving information from hair cells in the cochlea and transmitting it to neurons of the cochlear nucleus (CN). Decades of research in the cochlea greatly improved our understanding of SGN function under normal and pathological conditions, especially about the roles of different subtypes of SGNs and their peripheral synapses. However, it remains less clear how SGN central terminals or auditory nerve (AN) synapses connect to CN neurons, and ultimately how peripheral pathology links to structural alterations and functional deficits in the central auditory nervous system. This review discusses recent progress about the morphological and physiological properties of different subtypes of AN synapses and associated postsynaptic CN neurons, their changes during aging, and the potential mechanisms underlying age-related hearing loss.

Objective evidence of temporal processing deficits in older adults

The older listener's ability to understand speech in challenging environments may be affected by impaired temporal processing. This review summarizes objective evidence of degraded temporal processing from studies that have used the auditory brainstem response, auditory steady-state response, the envelope- or frequency-following response, cortical auditory-evoked potentials, and neural tracking of continuous speech. Studies have revealed delayed latencies and reduced amplitudes/phase locking in subcortical responses in older vs. younger listeners, in contrast to enhanced amplitudes of cortical responses in older listeners. Reconstruction accuracy of responses to continuous speech (e.g., cortical envelope tracking) shows over-representation in older listeners. Hearing loss is a factor in many of these studies, even though the listeners would be considered to have clinically normal hearing thresholds. Overall, the ability to draw definitive conclusions regarding these studies is limited by the use of multiple stimulus conditions, small sample sizes, and lack of replication. Nevertheless, these objective measures suggest a need to incorporate new clinical measures to provide a more comprehensive assessment of the listener's speech understanding ability, but more work is needed to determine the most efficacious measure for clinical use.

Linking anatomical and physiological markers of auditory system degeneration with behavioral hearing assessments in a mouse (Mus musculus) model of age-related hearing loss

Age-related hearing loss is a very common sensory disability, affecting one in three older adults. Establishing a link between anatomical, physiological, and behavioral markers of presbycusis in a mouse model can improve the understanding of this disorder in humans. We measured age-related hearing loss for a variety of acoustic signals in quiet and noisy environments using an operant conditioning procedure and investigated the status of peripheral structures in CBA/CaJ mice. Mice showed the greatest degree of hearing loss in the last third of their lifespan, with higher thresholds in noisy than in quiet conditions. Changes in auditory brainstem response thresholds and waveform morphology preceded behavioral hearing loss onset. Loss of hair cells, auditory nerve fibers, and signs of stria vascularis degeneration were observed in old mice. The present work underscores the difficulty in ascribing the primary cause of age-related hearing loss to any particular type of cellular degeneration. Revealing these complex structure-function relationships is critical for establishing successful intervention strategies to restore hearing or prevent presbycusis.

Taurine Promotes Neurite Outgrowth and Synapse Development of Both Vertebrate and Invertebrate Central Neurons

Taurine is a sulfur-containing amino acid that is widely expressed throughout the human brain, heart, retina, and muscle tissues. Taurine deficiency is associated with cardiomyopathy, renal dysfunction, abnormalities of the developing nervous system, and epilepsy which suggests a role specific to excitable tissues. Like vertebrates, invertebrates maintain high levels of taurine during embryonic and larval development, which decline during aging, indicating a potential developmental role. Notwithstanding its extensive presence throughout, taurine’s precise role/s during early brain development, function, and repair remains largely unknown in both vertebrate and invertebrate. Here, we investigated whether taurine affects neurite outgrowth, synapse formation, and synaptic transmission between postnatal day 0 rat cortical neurons in vitro, whereas its synaptogenic role was tested more directly using the Lymnaea soma-soma synapse model. We provide direct evidence that when applied at physiological concentrations, taurine exerts a significant neurotrophic effect on neuritic outgrowth and thickness of neurites as well as the expression of synaptic puncta as revealed by immunostaining of presynaptic synaptophysin and postsynaptic PSD95 proteins in rat cortical neurons, indicating direct involvement in synapse development. To demonstrate taurine’s direct effects on neurons in the absence of glia and other confounding factors, we next exploited individually identified pre- and postsynaptic neurons from the mollusk Lymnaea stagnalis. We found that taurine increased both the incidence of synapse formation (percent of cells that form synapses) and the efficacy of synaptic transmission between the paired neurons. This effect was comparable, but not additive, to Lymnaea trophic factor-induced synaptogenesis. This study thus provides direct morphological and functional evidence that taurine plays an important role in neurite outgrowth, synaptogenesis, and synaptic transmission during the early stages of brain development and that this role is conserved across both vertebrate and invertebrate species.

Inferior Colliculus Transcriptome After Status Epilepticus in the Genetically Audiogenic Seizure-Prone Hamster GASH/Sal

The Genetic Audiogenic Seizure Hamster from Salamanca (GASH/Sal), an animal model of reflex epilepsy, exhibits generalized tonic–clonic seizures in response to loud sound with the epileptogenic focus localized in the inferior colliculus (IC). Ictal events in seizure-prone strains cause gene deregulation in the epileptogenic focus, which can provide insights into the epileptogenic mechanisms. Thus, the present study aimed to determine the expression profile of key genes in the IC of the GASH/Sal after the status epilepticus. For such purpose, we used RNA-Seq to perform a comparative study between the IC transcriptome of GASH/Sal and that of control hamsters both subjected to loud sound stimulation. After filtering for normalization and gene selection, a total of 36 genes were declared differentially expressed from the RNA-seq analysis in the IC. A set of differentially expressed genes were validated by RT-qPCR showing significant differentially expression between GASH/Sal hamsters and Syrian control hamsters. The confirmed differentially expressed genes were classified on ontological categories associated with epileptogenic events similar to those produced by generalized tonic seizures in humans. Subsequently, based on the result of metabolomics, we found the interleukin-4 and 13-signaling, and nucleoside transport as presumably altered routes in the GASH/Sal model. This research suggests that seizures in GASH/Sal hamsters are generated by multiple molecular substrates, which activate biological processes, molecular processes, cellular components and metabolic pathways associated with epileptogenic events similar to those produced by tonic seizures in humans. Therefore, our study supports the use of the GASH/Sal as a valuable animal model for epilepsy research, toward establishing correlations with human epilepsy and searching new biomarkers of epileptogenesis.

Supplemental taurine during adolescence and early adulthood has sex-specific effects on cognition, behavior and neurotransmitter levels in C57BL/6J mice dependent on exposure window

The mammalian brain goes through final maturation during late adolescence and early adulthood with sex differences in timing. The key cellular processes, including changes in neurotransmitter receptor density and synaptic pruning, make this age uniquely vulnerable to neurotoxic insults. Teenagers and young adults are the major consumers of energy drinks, which contain high levels of taurine and caffeine. Taurine is one of the most abundant amino acids in the central nervous system, but the effects of supplemental taurine consumption during adolescence has not been well studied. We conducted an initial short-term exposure study with 0.12% taurine in drinking water and a long-term exposure dose-response study using 0.06 and 0.12% taurine in male and female C57BL/6J mice. We examined a broad range of cognitive functions and behaviors and measured neurotransmitter levels. We found no significant differences in anxiety, open field locomotor activity, or sensorimotor gating. However, we found impairments in novel object recognition and sex differences in Morris water maze. When taurine treatment stopped before behavioral experiments began, male mice had significant impairments in spatial learning and memory. In the dose-response study when taurine treatment continued throughout behavioral experiments, females had significant impairments. We also found sex differences in neurotransmitter levels with females having higher levels of glutamate, DOPAC and 5-HIAA. We conclude that both females and males are at risk from excess taurine consumption during final brain maturation.

D-Stellate Neurons of the Ventral Cochlear Nucleus Decrease in Auditory Nerve-Evoked Activity during Age-Related Hearing Loss

Age-related hearing loss (ARHL) is associated with weakened inhibition in the central auditory nervous system including the cochlear nucleus. One of the main inhibitory neurons of the cochlear nucleus is the D-stellate neuron, which provides extensive glycinergic inhibition within the local neural network. It remains unclear how physiological activities of D-stellate neurons change during ARHL and what are the underlying mechanisms. Using in vitro whole-cell patch clamp technique, we studied the intrinsic membrane properties of D-stellate neurons, the changes of their firing properties, and the underlying mechanisms in CBA/CaJ mice at the ages of 3–4 months (young), 17–19 months (middle age), and 27–33 months (aged). We found that the intrinsic membrane properties of D-stellate neurons were unchanged among these three age groups. However, these neurons showed decreased firing rate with age in response to sustained auditory nerve stimulation. Further investigation showed that auditory nerve-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced in strength with age. These findings suggest that D-stellate neurons receive weakened synaptic inputs from the auditory nerve and decreased sound driven activity with age, which are expected to reduce the overall inhibition and enhance the central gain in the cochlear nucleus during ARHL.

Top-down or bottom up: decreased stimulus salience increases responses to predictable stimuli of auditory thalamic neurons

KEY POINTS

Temporal imprecision leads to deficits in the comprehension of signals in cluttered acoustic environments, and the elderly are shown to use cognitive resources to disambiguate these signals. To mimic ageing in young rats, we delivered sound signals that are temporally degraded, which led to temporally imprecise neural codes. Instead of adaptation to repeated stimuli, with degraded signals, there was a relative increase in firing rates, similar to that seen in aged rats. We interpret this increase with repetition as a repair mechanism for strengthening the internal representations of degraded signals by the higher-order structures.

ABSTRACT

To better understand speech in challenging environments, older adults increasingly use top-down cognitive and contextual resources. The medial geniculate body (MGB) integrates ascending inputs with descending predictions to dynamically gate auditory representations based on salience and context. A previous MGB single-unit study found an increased preference for predictable sinusoidal amplitude modulated (SAM) stimuli in aged rats relative to young rats. The results suggested that the age-degraded/jittered up-stream acoustic code may engender an increased preference for predictable/repeating acoustic signals, possibly reflecting increased use of top-down resources. In the present study, we recorded from units in young-adult MGB, comparing responses to standard SAM with those evoked by less salient SAM (degraded) stimuli. We hypothesized that degrading the SAM stimulus would simulate the degraded ascending acoustic code seen in the elderly, increasing the preference for predictable stimuli. Single units were recorded from clusters of advanceable tetrodes implanted above the MGB of young-adult awake rats. Less salient SAM significantly increased the preference for predictable stimuli, especially at higher modulation frequencies. Rather than adaptation, higher modulation frequencies elicited increased numbers of spikes with each successive trial/repeat of the less salient SAM. These findings are consistent with previous findings obtained in aged rats suggesting that less salient acoustic signals engage the additional use of top-down resources, as reflected by an increased preference for repeating stimuli that enhance the representation of complex environmental/communication sounds.

Evaluation of taurine neuroprotection in aged rats with traumatic brain injury

Despite higher rates of hospitalization and mortality following traumatic brain injury (TBI) in patients over 65 years old, older patients remain underrepresented in drug development studies. Worse outcomes in older individuals compared to younger adults could be attributed to exacerbated injury mechanisms including oxidative stress, inflammation, blood-brain barrier disruption, and bioenergetic dysfunction. Accordingly, pleiotropic treatments are attractive candidates for neuroprotection. Taurine, an endogenous amino acid with antioxidant, anti-inflammatory, anti-apoptotic, osmolytic, and neuromodulator effects, is neuroprotective in adult rats with TBI. However, its effects in the aged brain have not been evaluated. We subjected aged male rats to a unilateral controlled cortical impact injury to the sensorimotor cortex, and randomized them into four treatment groups: saline or 25 mg/kg, 50 mg/kg, or 200 mg/kg i.p. taurine. Treatments were administered 20 min post-injury and daily for 7 days. We assessed sensorimotor function on post-TBI days 1-14 and tissue loss on day 14 using T2-weighted magnetic resonance imaging. Experimenters were blinded to the treatment group for the duration of the study. We did not observe neuroprotective effects of taurine on functional impairment or tissue loss in aged rats after TBI. These findings in aged rats are in contrast to previous reports of taurine neuroprotection in younger animals. Advanced age is an important variable for drug development studies in TBI, and further research is required to better understand how aging may influence mechanisms of taurine neuroprotection.

Cholinergic Hypofunction in Presbycusis-Related Tinnitus With Cognitive Function Impairment: Emerging Hypotheses

Presbycusis (age-related hearing loss) is a potential risk factor for tinnitus and cognitive deterioration, which result in poor life quality. Presbycusis-related tinnitus with cognitive impairment is a common phenotype in the elderly population. In these individuals, the central auditory system shows similar pathophysiological alterations as those observed in Alzheimer's disease (AD), including cholinergic hypofunction, epileptiform-like network synchronization, chronic inflammation, and reduced GABAergic inhibition and neural plasticity. Observations from experimental rodent models indicate that recovery of cholinergic function can improve memory and other cognitive functions via acetylcholine-mediated GABAergic inhibition enhancement, nicotinic acetylcholine receptor (nAChR)-mediated anti-inflammation, glial activation inhibition and neurovascular protection. The loss of cholinergic innervation of various brain structures may provide a common link between tinnitus seen in presbycusis-related tinnitus and age-related cognitive impairment. We hypothesize a key component of the condition is the withdrawal of cholinergic input to a subtype of GABAergic inhibitory interneuron, neuropeptide Y (NPY) neurogliaform cells. Cholinergic denervation might not only cause the degeneration of NPY neurogliaform cells, but may also result in decreased AChR activation in GABAergic inhibitory interneurons. This, in turn, would lead to reduced GABA release and inhibitory regulation of neural networks. Reduced nAChR-mediated anti-inflammation due to the loss of nicotinic innervation might lead to the transformation of glial cells and release of inflammatory mediators, lowering the buffering of extracellular potassium and glutamate metabolism. Further research will provide evidence for the recovery of cholinergic function with the use of cholinergic input enhancement alone or in combination with other rehabilitative interventions to reestablish inhibitory regulation mechanisms of involved neural networks for presbycusis-related tinnitus with cognitive impairment.

Taurine, caffeine, and energy drinks: Reviewing the risks to the adolescent brain

Energy drinks are emerging as a major component of the beverage market with sales projected to top $60 billion globally in the next five years. Energy drinks contain a variety of ingredients, but many of the top-selling brands include high doses of caffeine and the amino acid taurine. Energy drink consumption by children has raised concerns, due to potential caffeine toxicity. An additional risk has been noted among college-aged consumers of energy drinks who appear at higher risk of over-consumption of alcohol when the two drinks are consumed together. The differential and combinatorial effects of caffeine and taurine on the developing brain are reviewed here with an emphasis on the adolescent brain, which is still maturing. Key data from animal studies are summarized to highlight both reported benefits and adverse effects reported following acute and chronic exposures. The data suggest that age is an important factor in both caffeine and taurine toxicity. Although the aged or diseased brain might benefit from taurine or caffeine supplementation, it appears that adolescents are not likely to benefit from supplementation and may, in fact, suffer ill effects from chronic ingestion of high doses. Additional work is needed though to address gaps in our understanding of how taurine affects females, since the majority of animal studies focused exclusively on male subjects.

Understanding taurine CNS activity using alternative zebrafish models

Taurine is a highly abundant "amino acid" in the brain. Despite the potential neuroactive role of taurine in vertebrates has long been recognized, the underlying molecular mechanisms related to its pleiotropic effects in the brain remain poorly understood. Due to the genetic tractability, rich behavioral repertoire, neurochemical conservation, and small size, the zebrafish (Danio rerio) has emerged as a powerful candidate for neuropsychopharmacology investigation and in vivo drug screening. Here, we summarize the main physiological roles of taurine in mammals, including neuromodulation, osmoregulation, membrane stabilization, and antioxidant action. In this context, we also highlight how zebrafish models of brain disorders may present interesting approaches to assess molecular mechanisms underlying positive effects of taurine in the brain. Finally, we outline recent advances in zebrafish drug screening that significantly improve neuropsychiatric translational researches and small molecule screens.

Responses to Predictable versus Random Temporally Complex Stimuli from Single Units in Auditory Thalamus: Impact of Aging and Anesthesia

Human aging studies suggest that an increased use of top-down knowledge-based resources would compensate for degraded upstream acoustic information to accurately identify important temporally rich signals. Sinusoidal amplitude-modulated (SAM) stimuli have been used to mimic the fast-changing temporal features in speech and species-specific vocalizations. Single units were recorded from auditory thalamus [medial geniculate body (MGB)] of young awake, aged awake, young anesthetized, and aged anesthetized rats. SAM stimuli were modulated between 2 and 1024 Hz with the modulation frequency (fm) changed randomly (RAN) across trials or sequentially (SEQ) after several repeated trials. Units were found to be RAN-preferring, SEQ-preferring, or nonselective based on total firing rate. Significant anesthesia and age effects were found. The majority (86%) of young anesthetized units preferred RAN SAM stimuli; significantly fewer young awake units (51%, p < 0.0001) preferred RAN SAM signals with 16% preferring SEQ SAM. Compared with young awake units, there was a significant increase of aged awake units preferring SEQ SAM (30%, p < 0.05). We examined RAN versus SEQ differences across fms by measuring selective fm areas under the rate modulation transfer function curve. The largest age-related differences from awake animals were found for mid-to-high fms in MGB units, with young units preferring RAN SAM while aged units showed a greater preference for SEQ-presented SAM. Together, these findings suggest that aged MGB units/animals employ increased top-down mediated stimulus context to enhance processing of "expected" temporally rich stimuli, especially at more challenging higher fms.

SIGNIFICANCE STATEMENT

Older individuals compensate for impaired ascending acoustic information by increasing use of cortical cognitive and attentional resources. The interplay between ascending and descending influences in the thalamus may serve to enhance the salience of speech signals that are degraded as they ascend to the cortex. The present findings demonstrate that medial geniculate body units from awake rats show an age-related preference for predictable modulated signals relative to randomly presented signals, especially at higher, more challenging modulation frequencies. Conversely, units from anesthetized animals, with little top-down influences, strongly preferred randomly presented modulated sequences. These results suggest a neuronal substrate for an age-related increase in experience/attentional-based influences in processing temporally complex auditory information in the auditory thalamus.

Ageing-related changes in GABAergic inhibition in mouse auditory cortex, measured using in vitro flavoprotein autofluorescence imaging

KEY POINTS

Ageing is associated with hearing loss and changes in GABAergic signalling in the auditory system. We tested whether GABAergic signalling in an isolated forebrain preparation also showed ageing-related changes. A novel approach was used, whereby population imaging was coupled to quantitative pharmacological sensitivity. Sensitivity to GABAA blockade was inversely associated with age and cortical thickness, but hearing loss did not independently contribute to the change in GABAA ergic sensitivity. Redox states in the auditory cortex of young and aged animals were similar, suggesting that the differences in GABAA ergic sensitivity are unlikely to be due to differences in slice health. To examine ageing-related changes in the earliest stages of auditory cortical processing, population auditory cortical responses to thalamic afferent stimulation were studied in brain slices obtained from young and aged CBA/CAj mice (up to 28 months of age). Cortical responses were measured using flavoprotein autofluorescence imaging, and ageing-related changes in inhibition were assessed by measuring the sensitivity of these responses to blockade of GABAA receptors using bath-applied SR95531. The maximum auditory cortical response to afferent stimulation was not different between young and aged animals under control conditions, but responses to afferent stimulation in aged animals showed a significantly lower sensitivity to GABA blockade with SR95531. Cortical thickness, but not hearing loss, improved the prediction of all imaging variables when combined with age, particularly sensitivity to GABA blockade for the maximum response. To determine if the observed differences between slices from young and aged animals were due to differences in slice health, the redox state in the auditory cortex was assessed by measuring the FAD+/NADH ratio using fluorescence imaging. We found that this ratio is highly sensitive to known redox stressors such as H2 O2 and NaCN; however, no difference was found between young and aged animals. By using a new approach to quantitatively assess pharmacological sensitivity of population-level cortical responses to afferent stimulation, these data demonstrate that auditory cortical inhibition diminishes with ageing. Furthermore, these data establish a significant relationship between cortical thickness and GABAergic sensitivity, which had not previously been observed in an animal model of ageing.

Taurine increases hippocampal neurogenesis in aging mice

Aging is associated with increased inflammation and reduced hippocampal neurogenesis, which may in turn contribute to cognitive impairment. Taurine is a free amino acid found in numerous diets, with anti-inflammatory properties. Although abundant in the young brain, the decrease in taurine concentration with age may underlie reduced neurogenesis. Here, we assessed the effect of taurine on hippocampal neurogenesis in middle-aged mice. We found that taurine increased cell proliferation in the dentate gyrus through the activation of quiescent stem cells, resulting in increased number of stem cells and intermediate neural progenitors. Taurine had a direct effect on stem/progenitor cells proliferation, as observed in vitro, and also reduced activated microglia. Furthermore, taurine increased the survival of newborn neurons, resulting in a net increase in adult neurogenesis. Together, these results show that taurine increases several steps of adult neurogenesis and support a beneficial role of taurine on hippocampal neurogenesis in the context of brain aging.

Insult-induced adaptive plasticity of the auditory system

The brain displays a remarkable capacity for both widespread and region-specific modifications in response to environmental challenges, with adaptive processes bringing about the reweighing of connections in neural networks putatively required for optimizing performance and behavior. As an avenue for investigation, studies centered around changes in the mammalian auditory system, extending from the brainstem to the cortex, have revealed a plethora of mechanisms that operate in the context of sensory disruption after insult, be it lesion-, noise trauma, drug-, or age-related. Of particular interest in recent work are those aspects of auditory processing which, after sensory disruption, change at multiple—if not all—levels of the auditory hierarchy. These include changes in excitatory, inhibitory and neuromodulatory networks, consistent with theories of homeostatic plasticity; functional alterations in gene expression and in protein levels; as well as broader network processing effects with cognitive and behavioral implications. Nevertheless, there abounds substantial debate regarding which of these processes may only be sequelae of the original insult, and which may, in fact, be maladaptively compelling further degradation of the organism's competence to cope with its disrupted sensory context. In this review, we aim to examine how the mammalian auditory system responds in the wake of particular insults, and to disambiguate how the changes that develop might underlie a correlated class of phantom disorders, including tinnitus and hyperacusis, which putatively are brought about through maladaptive neuroplastic disruptions to auditory networks governing the spatial and temporal processing of acoustic sensory information.

Wistar rats: a forgotten model of age-related hearing loss

Age-related hearing loss (ARHL) is one of the most frequent sensory impairments in senescence and is a source of important socio-economic consequences. Understanding the pathological responses that occur in the central auditory pathway of patients who suffer from this disability is vital to improve its diagnosis and treatment. Therefore, the goal of this study was to characterize age-related modifications in auditory brainstem responses (ABR) and to determine whether these functional responses might be accompanied by an imbalance between excitation and inhibition in the cochlear nucleus of Wistar rats. To do so, ABR recordings at different frequencies and immunohistochemistry for the vesicular glutamate transporter 1 (VGLUT1) and the vesicular GABA transporter (VGAT) in the ventral cochlear nucleus (VCN) were performed in young, middle-aged and old male Wistar rats. The results demonstrate that there was a significant increase in the auditory thresholds, a significant decrease in the amplitudes and an increase in the latencies of the ABR waves as the age of the rat increased. Additionally, there were decreases in VGLUT1 and VGAT immunostaining in the VCN of older rats compared to younger rats. Therefore, the observed age-related decline in the magnitude of auditory evoked responses might be due in part to a reduction in markers of excitatory function; meanwhile, the concomitant reduction in both excitatory and inhibitory markers might reflect a common central alteration in animal models of ARLH. Together, these findings highlight the suitability of the Wistar rat as an excellent model to study ARHL.

Neuroprotective role of taurine during aging

Aging of the brain is characterized by several neurochemical modifications involving structural proteins, neurotransmitters, neuropeptides and related receptors. Alterations of neurochemical indices of synaptic function are indicators of age-related impairment of central functions, such as locomotion, memory and sensory performances. Several studies demonstrate that ionotropic GABA receptors, glutamate decarboxylase (GAD), and somatostatinergic subpopulations of GABAergic neurons are markedly decreased in experimental animal brains during aging. Additionally, levels of several neuropeptides co-expressed with GAD decrease during aging. Thus, the age-related decline in cognitive functions could be attributable, at least in part, to decrements in GABA inhibitory neurotransmission. In this study, we showed that chronic supplementation of taurine to aged mice significantly ameliorated the age-dependent decline in spatial memory acquisition and retention. We also demonstrated that concomitant with the amelioration in cognitive function, taurine caused significant alterations in the GABAergic and somatostatinergic system. These changes included (1) increased levels of the neurotransmitters GABA and glutamate, (2) increased expression of both isoforms of GAD (65 and 67) and the neuropeptide somatostatin, (3) decreased hippocampal expression of the β3 subunits of the GABAA receptor, (4) increased expression in the number of somatostatin-positive neurons, (5) increased amplitude and duration of population spikes recorded from CA1 in response to Schaefer collateral stimulation and (6) enhanced paired pulse facilitation in the hippocampus. These specific alterations of the inhibitory system caused by taurine treatment oppose those naturally occurring in the aging brain, suggesting a protective role of taurine in this process. An increased understanding of age-related neurochemical changes in the GABAergic system will be important in elucidating the underpinnings of the functional changes of aging. Taurine supplementation might help forestall the age-related decline in cognitive functions through interaction with the GABAergic system.

Acoustic experience alters the aged auditory system

OBJECTIVES

Presbyacusis, one of the most common ailments of the elderly, is often treated with hearing aids, which serve to reintroduce some or all of those sounds lost to peripheral hearing loss. However, little is known about the underlying changes to the ear and brain as a result of such experience with sound late in life. The present study attempts to model this process by rearing aged CBA mice in an augmented acoustic environment (AAE).

DESIGN

Aged (22-23 months) male (n = 12) and female (n = 9) CBA/CaJ mice were reared in either 6 weeks of low-level (70 dB SPL) broadband noise stimulation (AAE) or normal vivarium conditions. Changes as a function of the treatment were measured for behavior, auditory brainstem response thresholds, hair cell cochleograms, and gamma aminobutyric acid neurochemistry in the key central auditory structures of the inferior colliculus and primary auditory cortex.

RESULTS

The AAE-exposed group was associated with sex-specific changes in cochlear pathology, auditory brainstem response thresholds, and gamma aminobutyric acid neurochemistry. Males exhibited significantly better thresholds and reduced hair cell loss (relative to controls) whereas females exhibited the opposite effect. AAE was associated with increased glutamic acid decarboxylase (GAD67) levels in the inferior colliculus of both male and female mice. However, in primary auditory cortex AAE exposure was associated with increased GAD67 labeling in females and decreased GAD67 in males.

CONCLUSIONS

These findings suggest that exposing aged mice to a low-level AAE alters both peripheral and central properties of the auditory system and these changes partially interact with sex or the degree of hearing loss before AAE. Although direct application of these findings to hearing aid use or auditory training in aged humans would be premature, the results do begin to provide direct evidence for the underlying changes that might be occurring as a result of hearing aid use late in life. These results suggest the aged brain retains significantly anatomical, electrophysiological, and neurochemical plasticity.

Glycinergic synaptic transmission in the cochlear nucleus of mice with normal hearing and age-related hearing loss

The principal inhibitory neurotransmitter in the mammalian cochlear nucleus (CN) is glycine. During age-related hearing loss (AHL), glycinergic inhibition becomes weaker in CN. However, it is unclear what aspects of glycinergic transmission are responsible for weaker inhibition with AHL. We examined glycinergic transmission onto bushy cells of the anteroventral CN in normal-hearing CBA/CaJ mice and in DBA/2J mice, a strain that exhibits an early onset AHL. Glycinergic synaptic transmission was examined in brain slices of mice at 10-15 postnatal days old, 20-35 days old, and at 6-7 mo old. Spontaneous inhibitory postsynaptic current (sIPSC) event frequency and amplitude were the same among all three ages in both strains of mice. However, the amplitudes of IPSCs evoked (eIPSC) from stimulating the dorsal CN were smaller, and the failure rate was higher, with increasing age due to decreased quantal content in both mouse strains, independent of hearing status. The coefficient of variation of the eIPSC amplitude also increased with age. The decay time constant (τ) of sIPSCs and eIPSCs were constant in CBA/CaJ mice at all ages, but were significantly slower in DBA/2J mice at postnatal days 20-35, following the onset of AHL, and not at earlier or later ages. Our results suggest that glycinergic inhibition at the synapses onto bushy cells becomes weaker and less reliable with age through changes in release. However, the hearing loss in DBA/2J mice is accompanied by a transiently enhanced inhibition, which could disrupt the balance of excitation and inhibition.

Impact of sound exposure and aging on brain-derived neurotrophic factor and tyrosine kinase B receptors levels in dorsal cochlear nucleus 80 days following sound exposure

Recent studies suggested that acute sound exposure resulting in a temporary threshold shift in young adult animals within a series of maladaptive plasticity changes in central auditory structures. Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is involved in post-trauma peripheral hair cell and spiral ganglion cell survival and has been shown to modulate synaptic strength in cochlear nucleus following sound exposure. The present study evaluated levels of BDNF and its receptor (tyrosine kinase B, [TrkB]) in the dorsal cochlear nucleus (DCN) following a unilateral moderate sound exposure in young (7-8 months) and aged (28-29 months) Fischer Brown Norway (FBN) rats. Eighty days post-exposure, auditory brainstem response (ABR) thresholds for young exposed rats approached control values while aged exposed rats showed residual permanent threshold shifts (PTS) relative to aged controls. BDNF protein levels were significantly up-regulated by 9% in young exposed fusiform cells ipsilateral to the exposure. BDNF levels in aged sound-exposed fusiform cells increased 31% ipsilateral to the exposure. Protein levels of the BDNF receptor, TrkB, were also significantly increased in aged but not in young sound-exposed DCN fusiform cells. The present findings suggest a relationship between the up-regulation of BDNF/TrkB and the increase in spontaneous and driven activity previously observed for aged and sound-exposed fusiform cells. This might be due to a selective maladaptive compensatory down-regulation of glycinergic inhibition in DCN fusiform cells.

Supplementation of deprenyl attenuates age associated alterations in rat cerebellum

Aging, a multifactorial process of enormous complexity is characterised by physio-chemical and biological aspects of cellular functions. It is closely associated with changes in metabolism of various biological molecules in the system. In the present study, we have investigated the effect of deprenyl on cerebellum during ageing process in male Wistar rats with respect to the changes in levels of protein, glycoproteins and amino acids in experimental rats of three age groups (6, 12 and 18 months old). Intraperitoneal administration of liquid deprenyl (2 mg/kg body weight/day for a period of 15 days i.p., significantly P < 0.05) attenuated age-associated alterations in the levels of amino acids (taurine, aspartate, glutamate, arginine, hydroxy proline and homocysteine), protein content and glycoprotein components (hexose and hexosamine) in the rat cerebellum. The results of the present investigation indicate that the protective effect of deprenyl is probably related to its ability to strengthen the neuronal membrane by its membrane stabilizing action or to a counteraction of free radicals by its antioxidant property.

Plasticity at glycinergic synapses in dorsal cochlear nucleus of rats with behavioral evidence of tinnitus

Fifteen percent to 35% of the United States population experiences tinnitus, a subjective "ringing in the ears". Up to 10% of those afflicted report severe and disabling symptoms. Tinnitus was induced in rats using unilateral, 1 h, 17 kHz-centered octave-band noise (116 dB SPL) and assessed using a gap-startle method. The dorsal cochlear nucleus (DCN) is thought to undergo plastic changes suggestive of altered inhibitory function during tinnitus development. Exposed rats showed near pre-exposure auditory brainstem response (ABR) thresholds for clicks and all tested frequencies 16 weeks post-exposure. Sound-exposed rats showed significantly worse gap detection at 24 and 32 kHz 16 weeks following sound exposure, suggesting the development of chronic, high frequency tinnitus. Message and protein levels of alpha(1-3,) and beta glycine receptor subunits (GlyRs), and the anchoring protein, gephyrin, were measured in DCN fusiform cells 4 months following sound exposure. Rats with evidence of tinnitus showed significant GlyR alpha(1) protein decreases in the middle and high frequency regions of the DCN while alpha(1) message levels were paradoxically increased. Gephyrin levels showed significant tinnitus-related increases in sound-exposed rats suggesting intracellular receptor trafficking changes following sound exposure. Consistent with decreased alpha(1) subunit protein levels, strychnine binding studies showed significant tinnitus-related decreases in the number of GlyR binding sites, supporting tinnitus-related changes in the number and/or composition of GlyRs. Collectively, these findings suggest the development of tinnitus is likely associated with functional GlyR changes in DCN fusiform cells consistent with previously described behavioral and neurophysiologic changes. Tinnitus related GlyR changes could provide a unique receptor target for tinnitus pharmacotherapy or blockade of tinnitus initiation.

Age-related changes in glycine receptor subunit composition and binding in dorsal cochlear nucleus

Age-related hearing loss, presbycusis, can be thought of, in part, as a slow progressive peripheral deafferentation. Previous studies suggest that certain deficits seen in presbycusis may partially result from functional loss of the inhibitory neurotransmitter glycine in dorsal cochlear nucleus (DCN). The present study assessed age-related behavioral gap detection changes and neurochemical changes of postsynaptic glycine receptor (GlyRs) subunits and their anchoring protein gephyrin in fusiform cells of young (7-11 months) and aged (28-33 months) Fischer brown Norway (FBN) rats. Aged rats showed significantly (20-30 dB) elevated auditory brainstem-evoked response thresholds across all tested frequencies and worse gap detection ability compared to young FBN rats. In situ hybridization and quantitative immunocytochemistry were used to measure GlyR subunit message and protein levels. There were significant age-related increases in the alpha(1) subunit message with significant age-related decreases in alpha(1) subunit protein. Gephyrin message and protein showed significant increases in aged DCN fusiform cells. The pharmacologic consequences of these age-related subunit changes were assessed using [3H] strychnine binding. In support of the age-related decrease of alpha(1) subunit protein levels in DCN, there was a significant age-related decrease in the total number of GlyR binding sites with no significant change in affinity. These age-related changes may reflect an effort to reestablish a homeostatic balance between excitation and inhibition impacting on DCN fusiform cells by downregulation of inhibitory function in the face of an age-related loss of peripheral input. Age-related decrease in presynaptic glycine release results in altered subunit composition and this may correlate with loss of temporal coding of the aged fusiform cell in DCN. The previously reported role for gephyrin in retrograde intracellular receptor subunit trafficking could contribute to the alpha(1) decrease in the face of increased message.

Functional implication of taurine in aging

Age-related impairment of central functions is though to result from alterations of neurochemical indices of synaptic function. These neurochemical modifications involve structural proteins, neurotransmitters, neuropeptides and related receptors. Several studies demonstrated that GABA receptors, glutamic acid decarboxylase (GAD65&67), and different subpopulations of GABAergic neurons are markedly decreased in experimental animal brains during aging. Thus, the age-related decline in cognitive functions could be attributable, at least in part, to decrements in the function of the GABAergic inhibitory neurotransmitter system. In this study we show that chronic supplementation of taurine to aged mice significantly ameliorated the age-dependent decline in memory acquisition and retention, and caused alterations in the GABAergic system. These changes include increased levels of the neurotransmitters GABA and glutamate, increased expression of glutamic acid decarboxylase and the neuropeptide somatostatin and increased in the number of somatostatin-positive neurons. These specific alterations of the inhibitory system caused by taurine treatment oppose those naturally-occurring during aging, and suggest a protective role of taurine in this process. Increased understanding of age-related neurochemical changes in the GABAergic system will be important in elucidating the underpinnings of the functional changes of aging. Taurine might help forestall the age-related decline in cognitive functions through interaction with the GABAergic system.

Inhibitory neurotransmission, plasticity and aging in the mammalian central auditory system

Aging and acoustic trauma may result in partial peripheral deafferentation in the central auditory pathway of the mammalian brain. In accord with homeostatic plasticity, loss of sensory input results in a change in pre- and postsynaptic GABAergic and glycinergic inhibitory neurotransmission. As seen in development, age-related changes may be activity dependent. Age-related presynaptic changes in the cochlear nucleus include reduced glycine levels, while in the auditory midbrain and cortex, GABA synthesis and release are altered. Presumably, in response to age-related decreases in presynaptic release of inhibitory neurotransmitters, there are age-related postsynaptic subunit changes in the composition of the glycine (GlyR) and GABA(A) (GABA(A)R) receptors. Age-related changes in the subunit makeup of inhibitory pentameric receptor constructs result in altered pharmacological and physiological responses consistent with a net down-regulation of functional inhibition. Age-related functional changes associated with glycine neurotransmission in dorsal cochlear nucleus (DCN) include altered intensity and temporal coding by DCN projection neurons. Loss of synaptic inhibition in the superior olivary complex (SOC) and the inferior colliculus (IC) likely affect the ability of aged animals to localize sounds in their natural environment. Age-related postsynaptic GABA(A)R changes in IC and primary auditory cortex (A1) involve changes in the subunit makeup of GABA(A)Rs. In turn, these changes cause age-related changes in the pharmacology and response properties of neurons in IC and A1 circuits, which collectively may affect temporal processing and response reliability. Findings of age-related inhibitory changes within mammalian auditory circuits are similar to age and deafferentation plasticity changes observed in other sensory systems. Although few studies have examined sensory aging in the wild, these age-related changes would likely compromise an animal's ability to avoid predation or to be a successful predator in their natural environment.

Aged-related loss of temporal processing: altered responses to amplitude modulated tones in rat dorsal cochlear nucleus

Loss of temporal processing is characteristic of age-related loss of speech understanding observed in the elderly. Inhibitory glycinergic circuits provide input onto dorsal cochlear nucleus (DCN) projection neurons which likely serve to modulate excitatory responses to time-varying complex acoustic signals. The present study sought to test the hypothesis that age-related loss of inhibition would compromise the ability of output neurons to encode sinusoidally amplitude modulated (SAM) tones. Extracellular recordings were obtained from young and aged FBN rat DCN putative fusiform cells. Stimuli were SAM tones at three modulation depths (100, 50, and 20%) at 30 dB hearing level with the carrier frequency set to the unit's characteristic frequency. Discharge rate and synchrony were calculated to describe SAM responses. There were significant age-related changes in the shape and peak vector strength [best modulation frequency (BMF)] of temporal modulation transfer functions (tMTFs), with no significant age-related changes in rate modulation transfer functions (rMTFs) at BMF. Young neurons exhibited band-pass tMTFs for most SAM conditions while aged fusiform cells exhibited significantly more low-pass or double-peaked tMTFs. There were significant differences in tMTFs between buildup, pauser-buildup, and wide-chopper temporal response types. Young and aged wide-choppers displayed significantly lower vector strength values than the other two temporal DCN response types. Age-related decreases in the number of pauser-buildup response types and increases in wide-chopper types reported previously, could account, in part, for the observed loss of temporal coding of the aged fusiform cell. Age-related changes in SAM coding were similar to changes observed with receptor blockade of glycinergic inhibition onto fusiform cells and consistent with previously observed age-related loss of endogenous glycine levels and changes in normal adult glycine receptor function. DCN changes in SAM coding could, in part, underpin temporal processing deficits observed in the elderly.

Age-related changes in the response properties of cartwheel cells in rat dorsal cochlear nucleus

The fusiform cell and deep layers of the dorsal cochlear nucleus (DCN) show neurotransmitter and functional age-related changes suggestive of a downregulation of inhibitory efficacy onto DCN output neurons. Inhibitory circuits implicated in these changes include vertical and D-multipolar cells. Cartwheel cells comprise a large additional population of DCN inhibitory neurons. Cartwheel cells receive excitatory inputs from granule cell parallel fibers and provide a source of glycinergic inhibitory input onto apical dendrites of DCN fusiform cells. The present study compared the response properties from young and aged units meeting cartwheel-cell criteria in anesthetized rats. Single unit recordings from aged cartwheel cells revealed significantly higher thresholds, increased spontaneous activity and significantly altered rate-level functions characterized by hyperexcitability at higher intensities. Aged cartwheel cells showed a significant reduction in off-set suppression. Collectively, these findings suggest a loss of tonic and perhaps response inhibition onto aged DCN cartwheel neurons. These changes likely reflect a compensatory downregulation of synaptic inhibition in response to a loss of excitatory drive from auditory and non-auditory excitatory inputs via granule cells. The impact of increased excitability of cartwheel cells on DCN output neurons is likely to be complex, influenced by loss of glycinergic release and/or subunit receptor changes which would only partially off-set age-related loss of inhibition onto the somata and basal dendrites of fusiform cells.

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.

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.

Age-related retinal degeneration in animal models of aging: possible involvement of taurine deficiency and oxidative stress

There is strong evidence that the retina degenerates with age. Electroretinogram deficits and photoreceptor cell death and structural abnormalities have been observed in both animal and human studies of aging. The mechanism behind this phenomenon is a very interesting area for scientific and medical study. Current data support the link between retinal degeneration and increased oxidative stress. Taurine is a free amino acid found in high millimolar concentrations in the retina, and age-related deficiency in retinal levels of taurine may contribute to the retinal degeneration associated with age. Taurine acts as an antioxidant and taurine replenishment is known to alleviate oxidative stress in the retina. Thus taurine supplementation may be useful in the treatment of age-related retinal dysfunction.

Selective neuroinhibitory effects of taurine in slices of rat main olfactory bulb

Taurine is abundant in the main olfactory bulb, exceeding glutamate and GABA in concentration. In whole-cell patch-clamp recordings in rat olfactory bulb slices, taurine inhibited principal neurons, mitral and tufted cells. In these cells, taurine decreased the input resistance and caused a shift of the membrane potential toward the chloride equilibrium potential. The taurine actions were sustained under the blockade of transmitter release and were reversible and dose-dependent. At a concentration of 5 mM, typically used in this study, taurine showed 90% of its maximal effect. GABA(A) antagonists, bicuculline and picrotoxin, blocked the taurine actions, whereas the glycine receptor antagonist strychnine and GABA(B) antagonists, CGP 55845A and CGP 35348, were ineffective. These findings are consistent with taurine directly activating GABA(A) receptors and inducing chloride conductance. Taurine had no effect on periglomerular and granule interneurons. The subunit composition of GABA(A) receptors in these cells, differing from those in mitral and tufted cells, may account for taurine insensitivity of the interneurons. Taurine suppressed olfactory nerve-evoked monosynaptic responses of mitral and tufted cells while chloride conductance was blocked. This action was mimicked by the GABA(B) agonist baclofen and abolished by CGP 55845A; CGP 35348, which primarily blocks postsynaptic GABA(B) receptors, was ineffective. The taurine effect most likely was due to GABA(B) receptor-mediated inhibition of presynaptic glutamate release. Neither taurine nor baclofen affected responses of periglomerular cells. The lack of a baclofen effect implies that functional GABA(B) receptors are absent from olfactory nerve terminals that contact periglomerular cells. These results indicate that taurine decreases the excitability of mitral and tufted cells and their responses to olfactory nerve stimulation without influencing periglomerular and granule cells. Selective effects of taurine in the olfactory bulb may represent a physiologic mechanism that is involved in the inhibitory shaping of the activation pattern of principal neurons.

Age-related synaptic changes in the anteroventral cochlear nucleus of Fischer-344 rats

Previous studies have demonstrated age-related decreases in the transmitters glycine and glutamate in the cochlear nucleus (CN) of the Fischer-344 (F344) rat, along with declining levels of binding for glycine receptors. The purpose of this study was to evaluate structural correlates to the transmitter and receptor losses that accompany aging in the anteroventral CN (AVCN). Thin sections were obtained from the middle-frequency area of the right AVCNs from five 3-month-, four 19-month-, and five 28-month-old F344 rats. Montages were constructed from electron micrographs taken of several sites in each AVCN section. The presynaptic terminals were classified by vesicle type and postsynaptic target, and their perimeters and synaptic lengths were traced using morphometry software. The calibers of all dendritic profiles were also measured, and cell counts were performed on semi-thin sections. The data were compared among the three age groups using analysis of variance followed by Tukey's Honestly Significant Difference for pairwise comparisons. There were significant age-related decreases in the size of terminals contacting small-caliber (<2 microm) dendrites. Dendrites of this size comprised the largest percentage of dendrites in the AVCN. On these targets, round and pleomorphic-vesicle terminals were reduced in volume by nearly 44% and 24%, respectively, in 28-month olds when compared to the 3-month olds. On the other hand, the densities and numbers of synaptic terminals and dendritic profiles did not differ among age groups, and no neuronal losses were evident in the older animals. Also, there were no detectable changes in synaptic area among groups. The decrease in terminal size may be related to age-associated reductions in neurotransmitter levels previously described in the F344 CN. The observations presented here contrast with those previously described in the inferior colliculus (IC), in which there were significant age-related losses of synaptic terminals and dendrites, but no change in the size of synaptic terminals. The lack of synaptic and dendritic losses suggests that the structural connectivity of the rat AVCN remains relatively intact during aging, which is interesting in light of the synaptic and dendritic changes evident in the IC, a major target of its projections.

Endogenous glutamate-taurine interaction in striatum and nucleus accumbens of the freely moving rat: studies during the normal process of aging

Using microdialysis, the effects of endogenous glutamate on extracellular concentrations of taurine, in striatum and nucleus accumbens of the awake rat during the normal process of aging were investigated. The glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) was perfused through the microdialysis probe to increase the endogenous concentration of glutamate. Young (2-4 months), middle aged (12-14 months), aged (27-32 months) and very aged (37 months) male Wistar rats were used. PDC (1, 2 and 4 mM) produced a dose-related increase of extracellular concentrations of glutamate and taurine in striatum and nucleus accumbens in all groups of age. Increases of glutamate, but not of taurine, decreased during aging in striatum. In nucleus accumbens there were no age-related changes in the increases of glutamate and taurine induced by PDC. In all age groups, increases of extracellular taurine were significantly correlated with increases of extracellular glutamate. However, when these taurine-glutamate correlations for all groups of age were compared, no statistical differences were found. These results show first that a decrease in the increases of glutamate produced by the glutamate reuptake inhibitor in striatum, but not in nucleus accumbens, is produced by age; second that glutamate-taurine interaction in striatum and nucleus accumbens does not change during the normal process of aging.

A light and electron microscopic study of taurine-like immunoreactivity in the main olfactory bulb of frogs

The distribution of taurine in the frog olfactory bulb was studied using light and electron microscopic immunohistochemical techniques. At the light microscopic level, taurine-like immunoreactivity (taurine-LI) was found in (i) fibers coursing from the olfactory nerve layer to the glomerular layer, (ii) cell bodies and processes primarily located in the caudal part of the granule cell layer (GCL), and (iii) puncta outlining unstained somata of mitral cells and cells in the GCL. In consecutive sections processed for taurine or GABA, numerous cells of the caudal GCL displayed taurine-LI and GABA-like immunoreactivity (GABA-LI). A bimodal distribution of the cross-sectional cell area for GABA-LI cells implied their morphological diversity, and the peak for larger GABA-LI cells coincided with the maximum for taurine-LI cells. At the electron microscopic level, single immunogold labeling showed that GABA-LI, but not taurine-LI, is present in granule cells, whereas both taurine-LI and GABA-LI were localized in a 'non-granule' type of cell. The double labeling procedure demonstrated coexistence of taurine-LI and GABA-LI in neurons of a 'non-granule' type. These cells had some ultrastructural features typical of short axon cells in the GCL of the mammalian olfactory bulb and were tentatively considered as short axon-like cells. Results suggest that, in the frog olfactory bulb, taurine is contained in primary olfactory afferents and short axon-like cells of the GCL co-localizing GABA and taurine.

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.

Effects of dietary taurine supplementation or deprivation in aged male Fischer 344 rats

Taurine is a sulfur amino acid that is present in high concentration in mammalian tissues and previously has been reported to decline in a number of tissues with advancing age. The aims of the present study were to examine: (1) the effects of dietary taurine supplementation; (2) the effects of taurine-free diets; (3) the ability of aged rats to conserve urinary taurine; and (4) the consequences of these dietary manipulations on some biochemical parameters. Male F344 rats (n = 30/group) 18 months of age were placed on control diets, diets supplemented with 1.5% taurine in the drinking water, or a taurine-free diet for 10 months. An adult control group (12 months old at the end of the study) on normal diets was included for comparison purposes. Significant (P < 0.05) age-related declines in taurine content were observed in the spleen, kidney, eye, cerebellum and serum. Taurine supplementation corrected these deficits in tissue content in aged rats and in many cases increased taurine content above that of adult controls. Urinary excretion of taurine was significantly (P < 0.05) reduced in aged rats indicating an increased need to conserve taurine. Taurine-deficient diets did not further exacerbate the age-related decline in tissue taurine content, suggesting biosynthetic adaptations to the lack of dietary taurine. Dietary taurine supplementation blunted age-related declines in serum IGF-1 and increases in serum creatinine and blood urinary nitrogen (BUN). These studies suggest that advanced aging results in a taurine-deficient state that can be corrected by dietary supplementation.

Age-related changes in [3H]strychnine binding in the vestibular nuclei of rats

Glycine plays an important role as a neurotransmitter in the four vestibular nuclei (VN). The objective of this study was to determine if the levels of glycine-receptor binding in the VN change as a function of age. Quantitative receptor autoradiography was performed on brainstem sections from three age groups (3, 18 and 26 months) of Fischer 344 rats to assess binding in the VN. Glycine receptors were localized using [3H]strychnine binding. Strychnine binding declined monotonically with increasing age, such that the level of strychnine binding in each of the VN in the 28-month-old animals was approximately one-half that in the 3-month-olds. The age-related decrease in levels of strychnine binding suggest altered glycinergic function in the VN, which may in turn contribute to disturbances in equilibrium observed in the elderly.

Procedure for the sample preparation and handling for the determination of amino acids, monoamines and metabolites from microdissected brain regions of the rat

A method is described for the analysis of amino acids, monoamines and metabolites by high-performance liquid chromatography with electrochemical detection (HPLC-ED) from individual brain areas. The chromatographic separations were achieved using microbore columns. For amino acids we used a 100x1 mm I.D. C8, 5 microm column. A binary mobile phases was used: mobile phase A consisted of 0.1 M sodium acetate buffer (pH 6.8)-methanol-dimethylacetamide (69:24:7, v/v) and mobile phase B consisted of sodium acetate buffer (pH 6.8)-methanol-dimethylacetamide (15:45:40, v/v). The flow-rate was maintained at 150 microl/min. For monoamines and metabolites we used a 150X1 mm I.D. C18 5 microm reversed-phase column. The mobile phase consisted of 25 mM monobasic sodium phosphate, 50 mM sodium citrate, 27 microM disodium EDTA, 10 mM diethylamine, 2.2 mM octane sulfonic acid and 10 mM sodium chloride with 3% methanol and 2.2% dimethylacetamide. The potential was +700 mV versus Ag/AgCl reference electrode for both the amino acids and the biogenic amines and metabolites. Ten rat brain regions, including various cortical areas, the cerebellum, hippocampus, substantia nigra, red nucleus and locus coeruleus were microdissected or micropunched from frozen 300-microm tissue slices. Tissue samples were homogenized in 50 or 100 microl of 0.05 M perchloric acid. The precise handling and processing of the tissue samples and tissue homogenates are described in detail, since care must be exercised in processing such small volumes while preventing sample degradation. An aliquot of the sample was derivatized to form the tert.-butylthiol derivatives of the amino acids and gamma-aminobutyric acid. A second aliquot of the same sample was used for monamine and metabolite analyses. The results indicate that the procedure is ideal for processing and analyzing small tissue samples.

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.

Age-related glycine receptor subunit changes in the cochlear nucleus of Fischer-344 rats

Previous studies have shown that levels of binding for the strychnine-sensitive glycine receptor in the cochlear nucleus (CN) of Fischer (F344) rats decrease with age. Given the major role glycine plays in normal CN function, changes in glycine-receptor activity may contribute to central presbycusis. To further evaluate the impact of age on glycine receptors, in situ hybridization was used to assess, in three age groups of F344 rats, changes in levels of gene expression for four of its subunits. When compared with the 3-month-old rats, expression of mRNAs for alpha1 and beta subunits in the anteroventral CN decreased significantly in the 18- and 27-month-old age groups, while mRNA expression for the alpha2 subunit increased. If protein expressions are similar, these subunit changes may alter the function of glycine receptors, thereby affecting binding to its ligands.

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.