Excitatory amino acid pathways in the brain

Splice-dependent trans-synaptic PTPδ-IL1RAPL1 interaction regulates synapse formation and non-REM sleep

Alternative splicing regulates trans‐synaptic adhesions and synapse development, but supporting in vivo evidence is limited. PTPδ, a receptor tyrosine phosphatase adhering to multiple synaptic adhesion molecules, is associated with various neuropsychiatric disorders; however, its in vivo functions remain unclear. Here, we show that PTPδ is mainly present at excitatory presynaptic sites by endogenous PTPδ tagging. Global PTPδ deletion in mice leads to input‐specific decreases in excitatory synapse development and strength. This involves tyrosine dephosphorylation and synaptic loss of IL1RAPL1, a postsynaptic partner of PTPδ requiring the PTPδ‐meA splice insert for binding. Importantly, PTPδ‐mutant mice lacking the PTPδ‐meA insert, and thus lacking the PTPδ interaction with IL1RAPL1 but not other postsynaptic partners, recapitulate biochemical and synaptic phenotypes of global PTPδ‐mutant mice. Behaviorally, both global and meA‐specific PTPδ‐mutant mice display abnormal sleep behavior and non‐REM rhythms. Therefore, alternative splicing in PTPδ regulates excitatory synapse development and sleep by modulating a specific trans‐synaptic adhesion.

Amalaki Rasayana improved memory and neuronal metabolic activity in AbPP-PS1 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder characterized by progressive loss of memory and cognitive function. The cerebral metabolic rate of glucose oxidation has been shown to be reduced in AD. The present study evaluated efficacy of dietary Amalaki Rasayana (AR), an Ayurvedic formulation used in Indian traditional system, in AbPP-PS1 mouse model of AD in ameliorating memory and neurometabolism, and compared with donepezil, a standard FDA approved drug for AD. The memory of mice was measured using Morris Water Maze analysis. The cerebral metabolism was followed by 13C labelling of brain amino acids in tissue extracts ex vivo using 1H-[13C]-NMR spectroscopy together with a short time infusion of [1,6-13C2]glucose to mice. The intervention with Amalaki Rasayana showed improved learning and memory in AbPP-PS1 mice. The 13C labelings of GluC4, GABAC2 and GlnC4 were reduced in AbPP-PS1 mice when compared with wild-type controls. Intervention of AR increased the 13C labelling of amino acids suggesting a significant enhancement in glutamatergic and GABAergic metabolic activity in AbPP-PS1 mice similar to that observed with donepezil treatment. These data suggest that AR has potential to improve memory and cognitive function in AD.

Increased astroglial activity and reduced neuronal function across brain in AβPP-PS1 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease associated with progressive loss of cognitive function, personality, and behavior. The present study evaluates neuronal and astroglial metabolic activity, and neurotransmitter cycle fluxes in AβPP-PS1 mouse model of AD by using ¹H-[¹³C]-nuclear magnetic resonance (NMR) spectroscopy together with an infusion of either [1,6-¹³C₂]glucose or [2-¹³C]acetate. The levels of N-acetyl-aspartate (NAA) and glutamate were found to be decreased in the cerebral cortex and hippocampus in AβPP-PS1 mice, when compared with wild type controls. The cerebral metabolic rate of acetate oxidation was increased in the hippocampus and cerebral cortex of AβPP-PS1 mice suggesting enhanced astroglial activity in AD. AβPP-PS1 mice exhibit severe reduction in glutamatergic and gamma-amino butyric acid (GABA)ergic neuronal metabolic activity and neurotransmitter cycling fluxes in the hippocampus, cerebral cortex, and striatum as compared with controls. These data suggest that metabolic activity of excitatory and inhibitory neurons is compromised across brain in AβPP-PS1 mouse model of AD.

Zdhhc13-dependent Drp1 S-palmitoylation impacts brain bioenergetics, anxiety, coordination and motor skills

Protein S-palmitoylation is a reversible post-translational modification mediated by palmitoyl acyltransferase enzymes, a group of Zn²⁺-finger DHHC-domain-containing proteins (ZDHHC). Here, for the first time, we show that Zdhhc13 plays a key role in anxiety-related behaviors and motor function, as well as brain bioenergetics, in a mouse model (luc) carrying a spontaneous Zdhhc13 recessive mutation. At 3 m of age, mutant mice displayed increased sensorimotor gating, anxiety, hypoactivity, and decreased motor coordination, compared to littermate controls. Loss of Zdhhc13 in cortex and cerebellum from 3- and 24 m old hetero- and homozygous male mutant mice resulted in lower levels of Drp1 S-palmitoylation accompanied by altered mitochondrial dynamics, increased glycolysis, glutaminolysis and lactic acidosis, and neurotransmitter imbalances. Employing in vivo and in vitro models, we identified that Zdhhc13-dependent Drp1 S-palmitoylation, which acting alone or in concert, enables the normal occurrence of the fission-fusion process. In vitro and in vivo direct Zdhhc13-Drp1 protein interaction was observed, confirming Drp1 as a substrate of Zdhhc13. Abnormal fission-fusion processes result in disrupted mitochondria morphology and distribution affecting not only mitochondrial ATP output but neurotransmission and integrity of synaptic structures in the brain, setting the basis for the behavioral abnormalities described in the Zdhhc13-deficient mice.

Space-Time Dynamics of Membrane Currents Evolve to Shape Excitation, Spiking, and Inhibition in the Cortex at Small and Large Scales

In the cerebral cortex, membrane currents, i.e., action potentials and other membrane currents, express many forms of space-time dynamics. In the spontaneous asynchronous irregular state, their space-time dynamics are local non-propagating fluctuations and sparse spiking appearing at unpredictable positions. After transition to active spiking states, larger structured zones with active spiking neurons appear, propagating through the cortical network, driving it into various forms of widespread excitation, and engaging the network from microscopic scales to whole cortical areas. At each engaged cortical site, the amount of excitation in the network, after a delay, becomes matched by an equal amount of space-time fine-tuned inhibition that might be instrumental in driving the dynamics toward perception and action.

Cortico-cortical communication dynamics

In principle, cortico-cortical communication dynamics is simple: neurons in one cortical area communicate by sending action potentials that release glutamate and excite their target neurons in other cortical areas. In practice, knowledge about cortico-cortical communication dynamics is minute. One reason is that no current technique can capture the fast spatio-temporal cortico-cortical evolution of action potential transmission and membrane conductances with sufficient spatial resolution. A combination of optogenetics and monosynaptic tracing with virus can reveal the spatio-temporal cortico-cortical dynamics of specific neurons and their targets, but does not reveal how the dynamics evolves under natural conditions. Spontaneous ongoing action potentials also spread across cortical areas and are difficult to separate from structured evoked and intrinsic brain activity such as thinking. At a certain state of evolution, the dynamics may engage larger populations of neurons to drive the brain to decisions, percepts and behaviors. For example, successfully evolving dynamics to sensory transients can appear at the mesoscopic scale revealing how the transient is perceived. As a consequence of these methodological and conceptual difficulties, studies in this field comprise a wide range of computational models, large-scale measurements (e.g., by MEG, EEG), and a combination of invasive measurements in animal experiments. Further obstacles and challenges of studying cortico-cortical communication dynamics are outlined in this critical review.

Binge-like eating attenuates nisoxetine feeding suppression, stress activation, and brain norepinephrine activity

Stress is often associated with binge eating. A critical component of the control of stress is the central norepinephrine system. We investigated how dietary-induced binge eating alters central norepinephrine and related behaviors. Young male Sprague Dawley rats received calorie deprivation (24 h) and /or intermittent sweetened fat (vegetable shortening with sucrose; 30 min) twice a week for 10 weeks. The groups were Restrict Binge (calorie deprivation/sweetened fat), Binge (sweetened fat), Restrict (calorie deprivation), and Naive (no calorie deprivation/no sweetened fat). Dietary-induced binge eating was demonstrated by Restrict Binge and Binge, which showed an escalation in 30-min intake over time. Feeding suppression following nisoxetine (3 mg/kg; IP), a selective norepinephrine reuptake inhibitor, was not evident in Restrict Binge (Restrict Binge: 107±13, Binge: 52±9, Restrict: 80±8, Naive: 59±13% of saline injection at 1 h). In subsequent experiments with Restrict Binge and Naive, Restrict Binge had reduced corticosterone (Restrict Binge: 266±25; Naive: 494±36 ng/ml) and less feeding suppression (Restrict Binge: 81±12, Naive: 50±11% of non-restraint intake at 30 min) following restraint stress (1 h). Dietary-induced binge eating in Restrict Binge was not altered by a dorsal noradrenergic bundle lesion caused by N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4), but frontal cortex norepinephrine was positively correlated with the average 30-min intake post-lesion (0.69; p<0.01). In a separate set of animals, single-unit in vivo electrophysiological recording of locus coeruleus–norepinephrine neural activity demonstrated reduced sensory-evoked response as a consequence of the Restrict Binge schedule (Restrict Binge: 8.1±0.67, Naive: 11.9±1.09 Hz). These results, which suggest that a consequence of dietary-induced binge eating is to attenuate the responsiveness of the brain norepinephrine system, will further our understanding of how highly palatable foods dampen the stress neuraxis.

Glutamatergic and GABAergic TCA cycle and neurotransmitter cycling fluxes in different regions of mouse brain

The (13)C nuclear magnetic resonance (NMR) studies together with the infusion of (13)C-labeled substrates in rats and humans have provided important insight into brain energy metabolism. In the present study, we have extended a three-compartment metabolic model in mouse to investigate glutamatergic and GABAergic tricarboxylic acid (TCA) cycle and neurotransmitter cycle fluxes across different regions of the brain. The (13)C turnover of amino acids from [1,6-(13)C2]glucose was monitored ex vivo using (1)H-[(13)C]-NMR spectroscopy. The astroglial glutamate pool size, one of the important parameters of the model, was estimated by a short infusion of [2-(13)C]acetate. The ratio Vcyc/VTCA was calculated from the steady-state acetate experiment. The (13)C turnover curves of [4-(13)C]/[3-(13)C]glutamate, [4-(13)C]glutamine, [2-(13)C]/[3-(13)C]GABA, and [3-(13)C]aspartate from [1,6-(13)C2]glucose were analyzed using a three-compartment metabolic model to estimate the rates of the TCA cycle and neurotransmitter cycle associated with glutamatergic and GABAergic neurons. The glutamatergic TCA cycle rate was found to be highest in the cerebral cortex (0.91 ± 0.05 μmol/g per minute) and least in the hippocampal region (0.64 ± 0.07 μmol/g per minute) of the mouse brain. In contrast, the GABAergic TCA cycle flux was found to be highest in the thalamus-hypothalamus (0.28 ± 0.01 μmol/g per minute) and least in the cerebral cortex (0.24 ± 0.02 μmol/g per minute). These findings indicate that the energetics of excitatory and inhibitory function is distinct across the mouse brain.

Glutamatergic and GABAergic metabolism in mouse brain under chronic nicotine exposure: implications for addiction

Background and Purpose

The effects of nicotine on cerebral metabolism and its influence on smoking behavior is poorly understood. An understanding of the chronic effects of nicotine on excitatory and inhibitory metabolic demand, and corresponding neurotransmission may provide clues for designing strategies for the optimal smoking cessation intervention. The objective of the current study was to investigate neuronal and astroglial metabolism in mice exposed to nicotine (0.5 and 2.0 mg/kg, sc) three times in a day for 4 weeks.

Experimental Approach/Principal Findings

Metabolic measurements were carried out by co-infusing [U-¹³C₆]glucose and [2-¹³C]acetate, and monitoring ¹³C labeling of amino acids in brain tissue extract using ¹H-[¹³C] and ¹³C-[¹H]-NMR spectroscopy. Concentration of ¹³C-labeled glutamate-C4 was increased significantly from glucose and acetate with chronic nicotine treatment indicating an increase in glucose oxidation by glutamatergic neurons in all brain regions and glutamate-glutamine neurotransmitter cycle in cortical and subcortical regions. However, chronic nicotine treatment led to increased labeling of GABA-C2 from glucose only in the cortical region. Further, increased labeling of glutamine-C4 from [2-¹³C]acetate is suggestive of increased astroglial activity in subcortical and cerebellum regions of brain with chronic nicotine treatment.

Conclusions and Significance

Chronic nicotine exposure enhanced excitatory activity in the majority of brain regions while inhibitory and astroglial functions were enhanced only in selected brain regions.

Six principles of visual cortical dynamics

A fundamental goal in vision science is to determine how many neurons in how many areas are required to compute a coherent interpretation of the visual scene. Here I propose six principles of cortical dynamics of visual processing in the first 150 ms following the appearance of a visual stimulus. Fast synaptic communication between neurons depends on the driving neurons and the biophysical history and driving forces of the target neurons. Under these constraints, the retina communicates changes in the field of view driving large populations of neurons in visual areas into a dynamic sequence of feed-forward communication and integration of the inward current of the change signal into the dendrites of higher order area neurons (30-70 ms). Simultaneously an even larger number of neurons within each area receiving feed-forward input are pre-excited to sub-threshold levels. The higher order area neurons communicate the results of their computations as feedback adding inward current to the excited and pre-excited neurons in lower areas. This feedback reconciles computational differences between higher and lower areas (75-120 ms). This brings the lower area neurons into a new dynamic regime characterized by reduced driving forces and sparse firing reflecting the visual areas interpretation of the current scene (140 ms). The population membrane potentials and net-inward/outward currents and firing are well behaved at the mesoscopic scale, such that the decoding in retinotopic cortical space shows the visual areas' interpretation of the current scene. These dynamics have plausible biophysical explanations. The principles are theoretical, predictive, supported by recent experiments and easily lend themselves to experimental tests or computational modeling.

SAT1, A Glutamine Transporter, is Preferentially Expressed in GABAergic Neurons

Subsets of GABAergic neurons are able to maintain high frequency discharge patterns, which requires efficient replenishment of the releasable pool of GABA. Although glutamine is considered a preferred precursor of GABA, the identity of transporters involved in glutamine uptake by GABAergic neurons remains elusive. Molecular analyses revealed that SAT1 (Slc38a1) features system A characteristics with a preferential affinity for glutamine, and that SAT1 mRNA expression is associated with GABAergic neurons. By generating specific antibodies against SAT1 we show that this glutamine carrier is particularly enriched in GABAergic neurons. Cellular SAT1 distribution resembles that of GAD67, an essential GABA synthesis enzyme, suggesting that SAT1 can be involved in translocating glutamine into GABAergic neurons to facilitate inhibitory neurotransmitter generation.

System A transporter SAT2 mediates replenishment of dendritic glutamate pools controlling retrograde signaling by glutamate

Glutamate mediates several modes of neurotransmission in the central nervous system including recently discovered retrograde signaling from neuronal dendrites. We have previously identified the system N transporter SN1 as being responsible for glutamine efflux from astroglia and proposed a system A transporter (SAT) in subsequent transport of glutamine into neurons for neurotransmitter regeneration. Here, we demonstrate that SAT2 expression is primarily confined to glutamatergic neurons in many brain regions with SAT2 being predominantly targeted to the somatodendritic compartments in these neurons. SAT2 containing dendrites accumulate high levels of glutamine. Upon electrical stimulation in vivo and depolarization in vitro, glutamine is readily converted to glutamate in activated dendritic subsegments, suggesting that glutamine sustains release of the excitatory neurotransmitter via exocytosis from dendrites. The system A inhibitor MeAIB (alpha-methylamino-iso-butyric acid) reduces neuronal uptake of glutamine with concomitant reduction in intracellular glutamate concentrations, indicating that SAT2-mediated glutamine uptake can be a prerequisite for the formation of glutamate. Furthermore, MeAIB inhibited retrograde signaling from pyramidal cells in layer 2/3 of the neocortex by suppressing inhibitory inputs from fast-spiking interneurons. In summary, we demonstrate that SAT2 maintains a key metabolic glutamine/glutamate balance underpinning retrograde signaling by dendritic release of the neurotransmitter glutamate.

Hypoglutamatergic activity in the STOP knockout mouse: A potential model for chronic untreated schizophrenia

In mice, the deletion of the STOP protein leads to hyperdopaminergia and major behavioral disorders that are alleviated by neuroleptics, representing a potential model of schizophrenia. The reduction of the glutamatergic synaptic vesicle pool in the hippocampus could reflect a disturbance in glutamatergic neurotransmission in this model. Here we examined potential disturbances in energy metabolism and interactions between neurons and glia in 15-week-old STOP KO, wild-type, and heterozygous mice. Animals received [1-(13)C]glucose and [1,2-(13)C]acetate, the preferential substrates of neurons and astrocytes, respectively. Extracts from the whole forebrain and midbrain were analyzed by HPLC, (13)C and (1)H NMR spectroscopy. Amounts and labeling of most metabolites were unchanged. However, glutamine concentration and amount of [4,5-(13)C]glutamine derived from [1,2-(13)C]acetate significantly decreased by 17% and 18%, respectively, in STOP KO compared with wild-type mice. The amount of [4-(13)C]glutamate was decreased in STOP KO and heterozygous compared with wild-type mice. gamma-Aminobutyric acid labeling was not influenced by the genotype. Because STOP-deficient mice have a lower synaptic vesicle density, less glutamate is released to the synaptic cleft, leading to decreased stimulation of the postsynaptic glutamate receptors, reflecting increased glutamine metabolism only in the vicinity of the postsynapse of STOP KO mice.

Ultrastructural features of neurons and synaptic contacts in the posterodorsal medial amygdala of adult male rats

The aim of the present study was to describe the ultrastructure of neurons (from eight animals) and to analyse the synaptic terminal distribution (from two animals) in the posterodorsal subnucleus of the medial amygdala (MePD) of adult male rats. Using transmission electron microscopy, it was possible to identify many spiny and aspiny dendrites, unmyelinated axonal bundles, single axonal processes, a few myelinated axons, blood vessels and glial processes in the neuropil. Axodendritic synapses were the most frequently observed (67.5%), appearing to be of either the inhibitory or the excitatory types. The presynaptic region contained round or flattened vesicles that occurred either singly or with dense-cored vesicles (DCVs). The dendrites often received many synapses on a single shaft, and axon terminals displayed synaptic contacts with one or more postsynaptic structures. Dendritic spines showed different morphologies and the synapses on them (23.1%) formed a single and apparently excitatory synaptic contact with round, electron-lucid vesicles alone or, less frequently, with DCVs. Inhibitory and excitatory axosomatic synapses (8.2%) and excitatory axoaxonic synapses (1.2%) were also identified. The present report provides new findings relevant to the study of the MePD cellular organization and could be combined with other morphological data in order to reveal the functional activity of this area in male rats.

Selective amino acids changes in the medial and lateral preoptic area in the formalin test in rats

A combination of microdialysis in freely moving rats and capillary zone electrophoresis coupled to laser induced fluorescence detection was used to measure extracellular concentrations of amino acid neurotransmitters in different hypothalamic areas during noxious stimulation. Arginine, glutamate and aspartate were monitored every 30 s before and after a s.c. injection of formalin (5%, 50 microl) or saline (0.9%) in the right hind paw. In the medial and lateral preoptic area, calcium and nerve impulse dependent increases of arginine, glutamate and aspartate were observed during the first 2 min after formalin injection. However, amino acid changes were not detected in the lateral hypothalamus or in the ventromedial nucleus when compared with pre-injection levels or with the levels from animals injected with saline in the hind paw. Flinching behavior was also scored during the first 10 min following the formalin or saline injection. Flinching frequency was maximum at minute 2 after formalin injection, whereas saline injection did not elicited any flinching behavior. These results show that nociceptive stimulation induces rapid and differential amino acids changes in discrete areas of the hypothalamus that can be associated with pain-related behavior.

Serotonin 5-HT1A receptors might control the output of cortical glutamatergic neurons in rat cingulate cortex

The present study was designed to investigate the distribution of serotonin 5-HT1A receptor protein (5-HT1A-immunoreactivity) and its localization within cortical pyramidal neurons of the rat cingulate cortex. This experimental direction was inspired by recent data showing the role of 5-HT1A receptors in the pathology of schizophrenia, and in the mechanism of action of novel antipsychotic drugs as well as by the importance of the cingulate cortex in regulation of cognitive functions. It was found that 5-HT1A-immunoreactivity was densely distributed in neuronal eyelash-like elements, and their size, shape and spatial orientation may suggest concentration of 5-HT1A-immunopositive material in the proximal fragments of axons of cortical neurons. Moreover, it was observed that these 5-HT1A-immunopositive fragments were present predominately on proximal fragments of axons of pyramidal neurons, which was evidenced by double labeling experiments using glutamate and non-phosphorylated neurofilament H as markers of the cortical pyramidal cells. The 5-HT1A receptor immunoreactivity was localized distally to the inhibitory GABAergic terminals of chandelier and basket cells surrounding the pyramidal cell bodies and occasionally surrounding short initial segment of axonal hillock of pyramidal neurons. These anatomical data indicate that 5-HT1A receptors might control the excitability and propagation of information transmitted by the pyramidal cells. Moreover, our results indicate that drugs operating via 5-HT1A receptors in the cingulate cortex might control from this level the release of glutamate in the subcortical structures. Finally, the 5-HT1A receptors present in the cingulate cortex, as demonstrated in the present study, may constitute an important target for drugs used to repair dysfunction of glutamate neurotransmission, which is observed for example in schizophrenia.

Regional cerebral blood flow correlations of somatosensory areas 3a, 3b, 1, and 2 in humans during rest: a PET and cytoarchitectural study

The concept of functional connectivity relies on the assumption that cortical areas that are directly anatomically connected will show correlations in regional blood flow (rCBF) or regional metabolism. We studied correlations of rCBF of cytoarchitectural areas 3a, 3b, 1, and 2 in the brains of 37 subjects scanned with PET during a rest condition. The cytoarchitectural areas, delineated from 10 postmortem brains with statistical methods, were transformed into the same standard anatomical format as the resting PET images. In areas 3a, 3b, and 1, somatotopically corresponding regions were intercorrelated. Area 2 was correlated with the dorsal pre-motor area. These results were in accordance with the somatosensory connectivity in macaque monkeys. In contrast, we also found correlations between areas 3b and 1 with area 4a, and SMA, and among the left and right hand sector of areas 3a, 3b, and 1. Furthermore, there were no correlations between areas 3b, 1, and 2 with SII or other areas in the parietal operculum, nor of other areas known to be directly connected with areas 3a, 3b, 1, and 2 in macaques. This indicates that rCBF correlations between cortical areas during the rest state only partly reflect their connectivity and that this approach lacks sensitivity and is prone to reveal spurious or indirect connectivity.

The GAD-given Right of Dentate Gyrus Granule Cells to Become GABAergic

Janus, the ancient Roman God of Gates and Doors had two faces: one looked into the past, and the other, into the future. Do neurons possess a Janus face when it comes to neurotransmitters, or a given neuron is to be forever solely γ-aminobutyric acid (GABA) ergic, glutamatergic, dopaminergic, peptidergic, or YOURPREFERREDTRANSMITTERergic? The answer is that the terminals of many neurons are homes to even more than two neurotransmitters. All this in spite of the “one neuron–one transmitter” usual misinterpretation of Sir Henry Hallett Dale's postulate, originally meant to indicate that a metabolic process taking place in the cell body can influence all processes of the same neuron. A large variety of neurons in the CNS, many of them GABAergic, produce and release chemicals that satisfy some of the criteria used to define neurotransmitters. The usual scenario for a dual-transmitter terminal is that the fast-acting transmitter such as GABA or glutamate is stored in regular synaptic vesicles, whereas a neuropeptide is stored in dense core vesicles ( 1 ). The vesicular zinc found in many glutamatergic terminals also may be considered to be a second neurotransmitter, based on its vesicular packaging with the aid of a specific vesicular transporter, and its postsynaptic actions through high-affinity binding sites and permeation through certain channels ( 2 ). Whenever a “fast” and a “slow” neurotransmitter are present in the same presynaptic terminal, it is customary to assume that their release can be differentially regulated ( 1 ). There is little convincing experimental support for this phenomenon in the mammalian CNS. The coexistence of two “fast” neurotransmitters in the same terminal is less frequent, but not unheard of. In neonatal sympathetic neurons cocultured with cardiac myocytes, norepinephrine and acetylcholine coexist and have opposite actions on the cardiac muscle cells ( 3 ). Very recently we learned that brain-derived neurotrophic factor acting at the low-affinity neurotrophin receptor p75NTR, perhaps as part of a programmed developmental switch, can convert the phenotype of the sympathetic neuron from noradrenergic to cholinergic ( 4 ). Other examples of two fast neurotransmitters released from the same neuron include GABA and glycine in interneurons of the spinal cord ( 5 ) and glutamate and dopamine in ventral midbrain dopamine neurons ( 6 ). Of all CNS neurons, the granule cells of the dentate gyrus appear to be the champions of neurotransmitter colocalization: glutamate, enkephalin, dynorphin, zinc, and finally GABA ( 2 , 7 – 9 ). With this many transmitters in a single neuron, there are probably different ways in which they can be released. Dynorphin and other opioid peptides can be released directly from the dendrites to inhibit excitatory transmission ( 8 ). A similar mechanism may take place for GABA, as described in cortical GABAergic neurons ( 10 ).

Electrophysiological evidence for expression of glycine receptors in freshly isolated neurons from nucleus accumbens

In the course of studying N-methyl-D-aspartate (NMDA) receptors of the nucleus accumbens (NAcc), we found that 20% of freshly isolated medium spiny neurons, as well as all interneurons, responded in an unexpected way to long (5-s) coapplication of NMDA and glycine, the coagonist of NMDA receptors. Whereas the reversal potential of the peak NMDA current of this subset of neurons was still around 0 mV, the desensitizing current became outward at hyperpolarized potentials around -30 mV. A Cl(-)-free solution shifted the equilibrium potentials of the desensitized currents to around 0 mV. This outward current was not blocked by a Ca(2+)-free, Ba(2+)-containing solution, suggesting that the anionic conductance was not activated by Ca(2+) influx through NMDA receptor channels. Interestingly, glycine alone also evoked a current with a similar hyperpolarized reversal potential in this subset of neurons. The glycine current reversed around -50 mV, rectified outwardly, and inactivated strongly. Its desensitization was best fitted with a double exponential. Only the slow desensitization showed clear voltage dependence. The glycine current was not blocked by 200 microM picrotoxin and 10 microM zinc, was weakly antagonized by 1 microM strychnine, and was not enhanced by 1 microM zinc. In addition, 1 mM taurine, but not GABA, inactivated glycine currents, and 1 mM glycine occluded 10 mM taurine-mediated currents. These data indicate that a subset of nucleus accumbens neurons expresses glycine receptors and that either glycine or taurine could be an endogenous agonist for these receptors.

The GAD-given Right of Dentate Gyrus Granule Cells to Become GABAergic

JANUS, THE ANCIENT ROMAN GOD OF GATES AND DOORS HAD TWO FACES: one looked into the past, and the other, into the future. Do neurons possess a Janus face when it comes to neurotransmitters, or a given neuron is to be forever solely gamma-aminobutyric acid (GABA) ergic, glutamatergic, dopaminergic, peptidergic, or YOURPREFERREDTRANSMITTERergic? The answer is that the terminals of many neurons are homes to even more than two neurotransmitters. All this in spite of the "one neuron-one transmitter" usual misinterpretation of Sir Henry Hallett Dale's postulate, originally meant to indicate that a metabolic process taking place in the cell body can influence all processes of the same neuron. A large variety of neurons in the CNS, many of them GABAergic, produce and release chemicals that satisfy some of the criteria used to define neurotransmitters. The usual scenario for a dual-transmitter terminal is that the fast-acting transmitter such as GABA or glutamate is stored in regular synaptic vesicles, whereas a neuropeptide is stored in dense core vesicles (1). The vesicular zinc found in many glutamatergic terminals also may be considered to be a second neurotransmitter, based on its vesicular packaging with the aid of a specific vesicular transporter, and its postsynaptic actions through high-affinity binding sites and permeation through certain channels (2). Whenever a "fast" and a "slow" neurotransmitter are present in the same presynaptic terminal, it is customary to assume that their release can be differentially regulated (1). There is little convincing experimental support for this phenomenon in the mammalian CNS. The coexistence of two "fast" neurotransmitters in the same terminal is less frequent, but not unheard of. In neonatal sympathetic neurons cocultured with cardiac myocytes, norepinephrine and acetylcholine coexist and have opposite actions on the cardiac muscle cells (3). Very recently we learned that brain-derived neurotrophic factor acting at the low-affinity neurotrophin receptor p75(NTR), perhaps as part of a programmed developmental switch, can convert the phenotype of the sympathetic neuron from noradrenergic to cholinergic (4). Other examples of two fast neurotransmitters released from the same neuron include GABA and glycine in interneurons of the spinal cord (5) and glutamate and dopamine in ventral midbrain dopamine neurons (6). Of all CNS neurons, the granule cells of the dentate gyrus appear to be the champions of neurotransmitter colocalization: glutamate, enkephalin, dynorphin, zinc, and finally GABA (2)(7)(8)(9). With this many transmitters in a single neuron, there are probably different ways in which they can be released. Dynorphin and other opioid peptides can be released directly from the dendrites to inhibit excitatory transmission (8). A similar mechanism may take place for GABA, as described in cortical GABAergic neurons (10).

Glutamate-like Immunoreactivity in the Pigeon Optic Tectum and Effects of Retinal Ablation

The pattern of glutamate-like immunoreactivity was investigated in the pigeon optic tectum. The most impressive aspect of the labelling pattern was an accumulation of immunoreactive terminal-like elements restricted to those superficial tectal layers that correspond to the termination zone of the retinal afferents. These immunoreactive puncta occurred frequently in small clusters. At the level of electron microscopy, many of the labelled nerve endings showed the characteristics of retinal terminals. Moreover, following unilateral retinal ablation a drastic loss of immunoreactive terminal-like puncta was observed in the retinorecipient layers of the tectum contralateral to the lesion. The remaining glutamate-immunoreactive terminal-like elements had the light and electron microscopic features typical of the afferents from the nucleus isthmi, pars parvocellularis (lpc). The relation between the latter result and the transmitter specificity of the afferents from this subtectal nucleus is unclear at present. On the other hand, the light and electron microscopic labelling patterns and the effect of retinal ablation suggest that afferents from retina and from lpc are the only major sources for glutamate-immunoreactive terminals in the pigeon optic tectum. Furthermore, the results are well in line with previous data indicating glutamate as neurotransmitter at least in part of the retinal afferents to the pigeon optic tectum.

Physiological characteristics of the projection pathway from the medial preoptic to the nucleus raphe magnus of the rat and its modulation by the periaqueductal gray

Anatomical studies have shown a strong projection from the medial preoptic nucleus of the hypothalamus (MPO) to both the periaqueductal gray (PAG) and nucleus raphe magnus (NRM). In this study, we examined the physiological characteristics of MPO to NRM connections and examined how blockade of neuronal transmission and of the glutamatergic system within the PAG modifies this pathway. In deeply anesthetized rats, recordings were made from NRM neurons that were identified by their response to peripheral mechanical stimulation and designated as "E", "I", or "N" if they were excited, inhibited, or not activated by noxious stimulation. In addition, cells were identified as spinally projecting if they could be antidromically activated by stimulation of the dorsolateral funiculus at the thoracic level. The responses of 204 NRM neurons to electrical and 87 cells to both chemical and electrical stimulation of MPO were recorded. The response of NRM neurons to MPO stimulation was highly dependent on the sensory class of these cells. Chemical stimulation of MPO inhibited 50% (16/32) and excited 16% (5/32) of the I-cells. In contrast, 23% (9/39) of the E-cells were inhibited and 49% (19/39) were excited by chemical stimulation of MPO. Electrical stimulation at intensities below 80 microA at 100Hz had similar effects on the two classes of cells; 62% (24/39) of the E-cells and 31% (10/32) of the I cells were excited, and 31% (12/39) of the E-cells and 59% (19/32) of the I-cells were inhibited. The excitatory response to chemical stimulation lasted for an average of 136.8+/-73.2s and inhibitory response lasted for an average of 143.8+/-102.1s. Electrical stimulation of MPO at 1Hz excited 27%, inhibited 3%, and had no effect on 70% of NRM cells. The mean latency to peak excitation was 9.6+/-6.6ms. Antidromic activation of MPO neurons by NRM stimulation showed an average latency of 6.3+/-3.4ms. Blocking the glutamatergic transmission within the PAG (by injecting kynurenic acid (KYN) into the PAG) blocked the inhibitory response of 40% (6/15) of the I-cells and inhibitory response of 43% (3/7) of the E-cells. The excitatory response of 27% (3/11) of the I-cells and the excitatory response of 14% (1/7) of the E-cells were blocked by kynurenic injection into the PAG. It is concluded that: (1) in response to chemical stimulation of MPO, the number of I-cells that were inhibited was more than three times the number of I-cells that were excited; in contrast, the number of E-cells that were excited was more than twice the number of E-cells that were inhibited. (2) The interaction between MPO and NRM can be modulated by blockade of the neuronal transmission or blockade of the glutamatergic system in the PAG. (3) Simultaneous activity of many synapses is required for activation of the MPO-NRM pathway. (4) MPO to NRM interaction is mediated by fibers with a conduction velocity of less than 1m/s.

Transient expression of synaptic zinc during development of uncrossed retinogeniculate projections

The transition metal zinc is an essential dietary constituent that is believed to serve an important intercellular signaling role at certain excitatory synapses in the central nervous system. In the present study, we used histochemical techniques to investigate the distribution of synaptic zinc during postnatal development of retinogeniculate projections in rats. From postnatal day (P) 1 until P-21, the pattern of zinc histochemical staining in the dorsal lateral geniculate nucleus (LGNd) precisely matched the distribution of axon terminals from the ipsilateral eye that were labeled by anterograde transport of horseradish peroxidase. Regions of the LGNd that contained only crossed axons were devoid of zinc staining. Abnormalities in the distribution of uncrossed retinogeniculate projections in albino versus pigmented rats were paralleled by identical variations in localization of synaptic zinc. Unilateral enucleation on P-10 was followed within 5 days by loss of zinc staining in the LGNd ipsilateral to the removed eye without affecting staining in the contralateral nucleus. Finally, the ability to detect zinc histochemically in the LGNd ceased at approximately P-24. These findings provide evidence that zinc is sequestered within synaptic boutons of a subpopulation of retinal ganglion cells whose axons terminate on the ipsilateral side of the brain. The duration of zinc staining overlaps with the major period of axonal remodeling in the LGNd, suggesting that synaptically released zinc may play a role in postnatal refinement of retinogeniculate projections.

Glutamate-like immunoreactivity during hair cell recovery after gentamicin exposure in the chinchilla vestibular sensory periphery

OBJECTIVE

Determine the expression of glutamate by immunohistochemistry in normal and recovering vestibular hair cells in the chinchilla crista ampullaris after gentamicin ototoxicity.

STUDY DESIGN

In five groups of three animals each, ototoxicity was produced by placing gentamicin (50 microg)-impregnated Gelfoam pellets within the perilymphatic space of the superior semicircular canal. Animals were sacrificed at 1, 2, 4, 8, and 16 weeks after treatment. A group of normal (n=3) animals was also processed.

METHODS

For the detection of glutamate the inner ears of these animals were dissected, and the horizontal cristae ampullaris embedded in plastic. Two-micron-thick tissue sections were obtained and incubated with monoclonal antibodies against glutamate. The immunoreaction was detected using the avidinbiotinylated-complex technique and diaminobenzidine was the chromogen.

RESULTS

Normal sensory epithelia demonstrated type I and type II hair cells with moderate glutamate-like immunoreactivity. Supporting cells demonstrated no glutamate-like immunoreactivity. Afferent nerve fibers and calyxes surrounding type I hair cells demonstrated strong glutamate-like immunoreactivity. At 1 and 2 weeks after treatment the few type II hair cells surviving ototoxic treatment (15%-18%) contained moderate glutamate-like immunoreactivity, supporting cells showed no immunoreactivity, and nerve terminals and fibers displayed strong immunoreactivity. At 4 and 8 weeks after treatment, recovered hair cells (80%) had greater glutamate-like immunoreactivity when compared with normal hair cells, supporting cells displayed no glutamate-like immunoreactivity, and afferent fibers contained strong glutamate-like immunoreactivity. At 16 weeks, glutamate-like immunoreactivity in hair cells returned to normal level.

CONCLUSION

Glutamate may be used as an indicator of hair cell differentiation and as an index of the molecular recovery of hair cells after ototoxicity.

Effects of protein kinase C modulation on NMDA receptor mediated regulation of neurotransmitter enzyme and c-fos protein in cultured neurons

The role of protein kinase C (PKC) in N-methyl-D-aspartate (NMDA) receptor-mediated biochemical differentiation and c-fos protein expression was investigated in cultured cerebellar granule neurons. The biochemical differentiation of glutamatergic granule cells was studied in terms of the specific activity of phosphate-activated glutaminase, an enzyme treatment in the synthesis of the putative neurotransmitter pool of glutamate. When the partially depolarized cells were treated with NMDA for the last 1 to 3 days (between 2 and 5 days in vitro), it elevated the specific activity of glutaminase. In contrast, NMDA had little effect on the activity of aspartate aminotransferase or of lactate dehydrogenase. Treatment of 10-day old granule neurons with NMDA also resulted in a marked increase in the immunocytochemically measured expression of c-fos protein. The increases in both the activity of glutaminase and the steady state level of c-fos protein were specific to the activation of NMDA receptors, as they were completely blocked by D,L-2-amino-5-phosphonovaleric acid. The specific stimulation of NMDA receptors in PKC-depleted granule neurons or in the presence of reasonably specific PKC inhibitors also produced significant elevation in the activity of glutaminase and the expression of c-fos protein. These increases were similar in magnitude to those observed in the granule neurons of the respective control groups. Our findings demonstrate that PKC is not directly involved in the NMDA receptor-mediated signal transduction processes associated with biochemical differentiation and c-fos induction in cerebellar granule neurons.

Glutamate-immunoreactive climbing fibres in the cerebellar cortex of the rat

The climbing fibre system, one of the two main excitatory inputs to the cerebellar cortex, is anatomically and physiologically well characterized, while the nature of its neurotransmitter is still a matter of debate. We wished to determine whether glutamate-immunoreactive profiles with the morphological characteristics of climbing fibres could be found in the rat cerebellar cortex. For this purpose, a monoclonal 'anti-glutamate' antibody has been used in combination with a sensitive postembedding immunoperoxidase method on semi-thin sections or in combination with a postembedding immunogold method on ultrathin sections. At the light microscopic level, climbing fibres appeared as strongly stained fibrous profiles, chains of interconnected varicosities or heavily labelled dots of various sizes, often in close apposition to principal Purkinje cell dendrites. At the electron microscopic level, certain labelled varicosities or more elongated profiles resembling climbing fibre terminals were in synaptic contact with dendritic spines of Purkinje cells. Quantitative analysis of gold particle densities showed that such elements were about three to four times more heavily labelled than their postsynaptic partners. The results obtained in this study demonstrate that at least a subset of climbing fibres and their terminals contain relatively high levels of glutamate-like immunoreactivity and provide additional evidence for a role of glutamate as transmitter in these cerebellar afferents.

Regulation of [3H]norepinephrine release by N-methyl-D-aspartate receptors in minislices from the dentate gyrus and the CA1-CA3 area of the rat hippocampus

It has been reported previously that N-methyl-D-aspartic acid induces a significant release of [3H]norepinephrine preaccumulated in slices from the hippocampus. In the present study, we investigated whether there are regional differences in the hippocampus regarding this N-methyl-D-aspartate effect. In the absence of Mg2+, N-methyl-D-aspartate (10-200 microM) induced the release of [3H]norepinephrine from superfused minislices containing the dentate gyrus area or the CA1-CA3 region of the hippocampus. Such N-methyl-D-aspartate effects on [3H]norepinephrine release were significantly higher in the dentate gyrus than in the CA1-CA3 area. The N-methyl-D-aspartate effects in both hippocampal areas were also reduced significantly by D-2-amino-5-phosphonovaleric acid (50 microM), an antagonist of the N-methyl-D-aspartate receptor, and by tetrodotoxin, a blocker of the voltage-dependent Na+ channels. The extent of this reduction was the same in the dentate gyrus and the CA1-CA3 area. Further experiments, conducted in the presence of Mg2+, demonstrated that N-methyl-D-aspartic acid increased K(+)-induced release of [3H]norepinephrine from dentate gyrus minislices but not from the CA1-CA3 area. The results are consistent with the existence of a higher density and/or different subtypes of N-methyl-D-aspartate receptors modulating [3H]norepinephrine release in the dentate gyrus as compared with the CA1-CA3 hippocampal area.

Extracellular amino acid levels in hippocampus during pilocarpine-induced seizures

Extracellular levels of aspartate, glutamate and glutamine were monitored by microdialysis in the dorsal hippocampus of freely moving rats following the administration of a convulsant dose of pilocarpine (400 mg/kg, i.p.). Rats were either pretreated with the glutamate uptake inhibitor, 1-trans-pyrrolidine-2,4-dicarboxylic acid (PDC, 1 mM in the perfusion medium, -25 min), or received pilocarpine directly. All rats injected with pilocarpine (with or without PDC pretreatment) developed limbic seizures (latency 15.4 +/- 2.4 min). Without PDC pretreatment there were no significant changes in extracellular levels of aspartate, glutamate and glutamine following pilocarpine administration until the onset of limbic seizures when glutamine levels fell by 35%. Following PDC pretreatment there were large and sustained increases in extracellular hippocampal aspartate (250%) and glutamate (55%) levels, but no significant change in the glutamine level. When pilocarpine was administered to this group of rats, there were further selective, significant, transient increases in the extracellular levels of aspartate (31%) and glutamate (18%) which preceded the onset of seizures. Aspartate and glutamate levels were not significantly increased (relative to PDC controls) during seizures. The conditions for pilocarpine-induced increases in aspartate and glutamate release were established in parallel groups of anaesthetised rats where pilocarpine was administered via a microdialysis probe in the dorsal hippocampus. Following the infusion of 10 mM pilocarpine there were large and rapid increases in the levels of aspartate (143%) and glutamate (179%), which were completely abolished by the absence of calcium in the perfusion medium, or by the presence of atropine (20 mM) or tetrodotoxin (1 microM).

Amygdala kindling-induced seizures selectively impair spatial memory. 1. Behavioral characteristics and effects on hippocampal neuronal protein kinase C isoforms

Protein kinase C (PKC) comprises a family of kinases consisting of nine subspecies that are differentially distributed in the central nervous system. This implies distinct functions. Its involvement is suggested in cellular and molecular mechanisms by which the hippocampus exerts influence on information processing. In this study, it was questioned whether abnormal activity in the neuronal substrate, particularly the hippocampal formation, induced by amygdala kindling indeed impairs spatial memory performance and correlated alpha, beta I/II, and gamma PKC subspecies expression. Rats were trained in a spatial discrimination task (SDT) and simultaneously kindled in the amygdala to induce abnormal, epileptiform activity. Control rats were only trained in the holeboard, a "free choice" maze, in which working (WM) and reference memory (RM) were simultaneously examined. Halfway through and at the end of the experiments the influence of kindling and SDT training on the immunoreactivity for PKC subspecies alpha, beta I/II, and gamma was evaluated in the hippocampal formation. Kindling resulted in a gradual increase in afterdischarge duration and motor seizure (MS) severity. Repeated SDT training ultimately resulted in an asymptotic level of WM and RM performance. As soon as generalized MSs developed, kindled rats failed to improve RM, whereas WM was not influenced. Compared to untrained rats, in trained controls PKC gamma but not PKC alpha beta I/II immunoreactivity was elevated in CA1 pyramidal and dentate gyrus granular cells. Generalized but not partial MSs abolished these alterations in PKC gamma immunoreactivity. The present data indicate that repeated training in a SDT affects the expression of PKC subspecies gamma but not of alpha or beta in the rat hippocampus. Generalized epileptiform activity impair both acquisition of new spatial RM information and PKC gamma expression. It is argued that PKC gamma plays a role in cellular mechanisms through which pathological brain activity impairs certain aspects of spatial memory.

Regulation of neurotransmitter enzyme by quisqualate subtype glutamate receptors in cultured cerebellar and hippocampal neurons

The possible involvement of ionotropic and metabotropic quisqualate (QA) receptors in neuronal plasticity was studied in cultured glutamatergic cerebellar or hippocampal cells in terms of the specific activity of phosphate-activated glutaminase, an enzyme important in the synthesis of the putative neurotransmitter pool of glutamate. When cerebellar or hippocampal neurons were treated with QA, it elevated the specific activity of glutaminase in a dose-dependent manner. The half-maximal effect was obtained at about 0.1 microM, the maximum increase was at about 1 microM, but levels higher than 10 microM QA produced progressive reduction in glutaminase activity. In contrast, QA had little effects on the activities of lactate dehydrogenase and aspartate aminotransferase and the amount of protein, indicating that the increase in glutaminase was relatively specific. The QA-mediated increase in glutaminase was mimicked by the ionotropic QA receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; EC50, about 0.5 microM), but not by the metabotropic QA receptor agonist trans-(+-)-1-amino-cyclopentyl-1,3,dicarboxylate (t-ACPD; up to 0.5 mM). The specific ionotropic QA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) inhibited QA- and AMPA-mediated increases in glutaminase activity in a dose-dependent manner, whereas other glutamate receptor antagonists, D,L-2-amino-5-phosphonovalerate, gamma-D-glutamyl aminomethyl sulphonic acid and gamma-D-glutamyl diethyl ester were ineffective. The elevation of neurotransmitter enzyme was Ca(2+)-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of perforant path lesion on pattern of glutamate-like immunoreactivity in rat dentate gyrus

To investigate the relation between perforant path and the pattern of glutamate-like immunoreactivity in its target regions in the rat hippocampal formation, unilateral lesions of various size and location were placed to interrupt certain contingents of these afferent fibers. Postembedding immunohistochemistry at the level of light microscopy yielded the same pattern of immunoreactivity in the hippocampal formation contralateral to the lesion as in untreated animals. On the ipsilateral side, however, extensive transections of the perforant path led to a drastic loss of glutamate-immunoreactive terminal-like elements in the outer part of the dentate molecular layer. More restricted lesions induced a loss of punctate glutamate-like immunoreactivity in narrower bands within this zone. The width and the location of the affected bands appeared to depend on the extent of the transections and their topographical relation to the perforant path fiber system. These results and those obtained using a postembedding immunogold method at the level of electron microscopy strongly indicate that perforant path terminals in the dentate molecular layer of the rat contain high levels of glutamate and, thus, provide further support for an already well-documented role of this excitatory amino acid as neurotransmitter in this system.

Lesions of excitatory pathways reduce hippocampal cell death after transient forebrain ischemia in the gerbil

Transient forebrain ischemia produces a spatially and temporally selective pattern of neuronal degeneration in the hippocampal formation of the Mongolian gerbil. Ischemic neuronal death has been suggested to depend on the activation of excitatory hippocampal pathways that project to the vulnerable neurons. This idea was tested by examining the effect of a unilateral entorhinal cortical lesion or a unilateral knife cut lesion of intrahippocampal pathways on the neuropathology produced by 5 min of complete forebrain ischemia. A prior lesion of either the ipsilateral entorhinal cortex or the mossy fiber and Schaffer collateral-commissural pathways partially prevented the destruction of CA1b pyramidal cells in most animals. It did not, however, reduce the extent of ischemic neuronal death in any other hippocampal subfield. Within area CA1b, an entorhinal lesion protected an average of 23% of the pyramidal cells and a transection of both mossy and Schaffer collateral-commissural fibers protected an average of 36.5%. CA1b pyramidal cells saved from ischemia-induced degeneration appeared clearly abnormal when stained with cresyl violet or by silver impregnation. It is suggested that lesions of excitatory pathways attenuate ischemic damage to area CA1b by directly or indirectly reducing the level of synaptic excitation onto the vulnerable neurons. However, only a relatively small percentage of hippocampal neurons can be protected by these lesions in the gerbil ischemia model and there is reason to believe that the neurons protected in this manner may not be electrophysiologically competent. Synaptic excitation therefore appears to play an important, but not an essential, role in this model of ischemic brain damage.

Glutamate-like immunoreactivity revealed in rat olfactory bulb, hippocampus and cerebellum by monoclonal antibody and sensitive staining method

Although there is good evidence favoring L-glutamate as a major excitatory amino acid transmitter, relatively little is known about the distribution of nerve terminals using this substance. A method visualizing glutamate-like immunoreactivity at the light microscopic level by means of a monoclonal antibody, mAb 2D7, is described. --The antigen used for immunization was a glutaraldehyde-linked glutamate-BSA conjugate, and hybridomas were differentially screened by ELISA for production of antibodies recognizing glutamate- but not aspartate-BSA. The crossreactivity of 'anti-glutamate' mAb 2D7 as estimated in absorption tests was low even with conjugates closely related to glutamate-BSA.--Semithin sections from rapidly perfusion-fixed, plastic-embedded rat brain tissues were etched and stained by a combination of the peroxidase-antiperoxidase method and silver enhancement of the diaminobenzidine reaction product. Only this amongst several other immunohistochemical methods tried produced labeling patterns which showed terminal-like elements in brain regions such as olfactory bulb, hippocampus and cerebellum, and which were mostly consistent with already available information on systems using glutamate as neurotransmitter. Particularly striking was the staining of elements reminiscent of mossy fiber terminals in hippocampus and cerebellum as well as of cerebellar parallel fiber terminals.

2-Amino-7-phosphonoheptanoic acid inhibits insulin-induced convulsions and striatal aspartate accumulation in rats with frontal cortical ablation

Pretreatment of rats with the excitatory amino acid antagonist 2-amino-7-phosphonoheptanoic acid (2-APH; 0.5 mmol/kg, i.p.) protected against insulin-induced clonic seizures. Complete protection was observed in 38% of the rats and partial protection in an additional 50%. Lesioning of the corticostriatal pathway by frontal cortical ablation caused decreases in the striatal levels of aspartate (-28%) and glutamate (-18%), an increase in striatal glutamine level (45%), and decreased high-affinity uptake of D-[3H]aspartate (-27%) in the lesioned dorsal neostriatum. Insulin-induced hypoglycemia caused a predicted sharp increase in aspartate level (165%) and decreased glutamate (-20%) and glutamine (-38%) levels in the intact striatum. Pretreatment of rats with 2-APH significantly reversed the insulin-induced changes in striatal aspartate, glutamate, and glutamine levels, especially in the intact hemisphere. In normoglycemic control rats, the "metabolic," i.e., concentration in the lesioned hemisphere, aspartate pool constituted 72% and the "synaptic," i.e., the concentration difference between the intact and lesioned hemispheres, 28% of the total striatal aspartate pool. 2-APH had no effect on the level of "metabolic" aspartate in the striata of normoglycemic rats but caused an almost complete suppression of "synaptic" aspartate. Following insulin-induced hypoglycemia, the "metabolic" aspartate pool doubled, whereas the "synaptic" aspartate pool increased 3.5-fold in the absence of 2-APH. The insulin-induced rise in "synaptic" aspartate level was almost completely blocked by 2-APH (a 5% rise instead of a 3.5-fold rise).(ABSTRACT TRUNCATED AT 250 WORDS)