Anatomical distributions of four pharmacologically distinct 3H-L-glutamate binding sites

Protein kinase C redistribution within CA3 stratum oriens during acquisition of nictitating membrane conditioning in the rabbit

This manuscript describes experiments designed to investigate protein kinase C redistribution occurring during acquisition of the rabbit nictitating membrane (NM) conditioned response (CR). The first experiment defined the acquisition phase of the NM response for our laboratory. A group of rabbits (n = 6) was given 2 days of paired NM training; a second group (n = 6) was given 2 days of unpaired NM training. The data document a variable level of responding on day 1 for rabbits given paired training (mean +/- SEM, 21 +/- 11% CRs) but show that on day 2 most rabbits reached the behavioral asymptote (five of six rabbits responding with greater than 85% CRs). Rabbits responding at the behavioral asymptote were defined as having acquired the NM conditioned response. These data were interpreted to indicate that 1 day of training initiated processes necessary for behavioral acquisition (i.e., responding at the behavioral asymptote). A quantitative film autoradiographic study of [3H]phorbol 12,13-dibutyrate binding was then used to determine the distribution of hippocampal protein kinase C in rabbits sacrificed after receiving either 1 day of paired stimuli (n = 10), 1 day of unpaired stimuli (n = 6), or no stimuli (n = 6). Autoradiograms were analyzed by measuring binding in strictly defined regions of interest and from transept profiles. A significant increase in binding of the phorbol ester was found in the CA3 stratum oriens in the paired group relative to unpaired and naive controls. No other significant differences were found.

High vulnerability of dentate granule cells to the neuropathological effects induced by intrahippocampal injection of tetanus toxin

The behavioural and neuropathological effects of tetanus toxin, injected into the dentate gyrus, were studied in rats. The monolateral injection of a single dose (1000 mouse minimum lethal doses, MLDs; n = 14 rats) of tetanus toxin produced time-dependent behavioural stimulation. Wet-dog shakes and facial stereotypy were observed 3-4 days after the injection, culminating 4-5 days after treatment, in "limbic motor seizures". Ten days after injection, histological examination revealed death of dentate granule cells in the tetanus toxin-treated side but not in the contralateral, control side (treated with neutralized toxin). This effect was observed in all rats (n = 5) receiving tetanus toxin in the dentate gyrus and no damage was reported in other sectors (e.g. CA1 and CA3 pyramidal cell layers) of the hippocampus. Quantification of the neuronal damage yielded an approximately 70% reduction (P less than 0.01) in the number of granule cells in the toxin-injected dentate gyrus, compared with the control side. This was greater than that previously reported (30% reduction) in the CA1 pyramidal cell layer of rats receiving the same dose of toxin, into the regio superior of the hippocampus. In conclusion, the present experiments have shown that the focal injection of tetanus toxin into the dentate gyrus produced behavioural excitation and selective death of dentate granule cells.

Decreased glucose utilization in discrete brain regions of rat in thioacetamide-induced hepatic encephalopathy as measured with [3H]-deoxyglucose

To evaluate the possible contribution of bioenergetic failure in the particular brain regions to the pathomechanism of hepatic encephalopathy (HE), local cerebral metabolic rate for glucose (LCMRglue) was evaluated from [3H]-deoxyglucose uptake in frontal, visual and auditory cortex, striatum, cerebellum and medulla oblongata of rats with acute HE induced with a hepatotoxin--thioacetamide (TAA). HE caused a decrease of LCMRglue in all the regions studied. The strongest decrease (about 65%) was noted in hippocampus and cerebral cortex--the two regions rich in glutamatergic neurons. The results indicate a possible link between decreased energy metabolism and impaired excitatory, glutamatergic neurotransmission--the two factors whose contribution to HE has so far been implicated separately.

The pharmacological specificity of N-methyl-D-aspartate receptors in rat cerebral cortex: correspondence between radioligand binding and electrophysiological measurements

1. The pharmacological specificity of N-methyl-D-aspartate (NMDA) receptors has been investigated in the rat cerebral cortex by use of radioligand binding and electrophysiological techniques. 2. A comparison was made between a functional assay (NMDA-induced depolarizations in a rat cortical slice preparation) and NMDA-sensitive L-[3H]-glutamate binding in the same brain region and species, to provide accurate affinity values for agonists and antagonists at the NMDA recognition site. 3. In a preparation of crude postsynaptic densities (PSD) from rat cortex, L-[3H]-glutamate bound with high affinity to an NMDA-sensitive population of sites with KD (geometric mean (-s.e.mean. + s.e. mean) = 120 (114, 126) nM, Bmax (mean +/- s.e.mean) = 11.4 +/- 0.8 pmol mg-1 protein and Hill coefficient (mean +/- s.e.mean) = 1.2 +/- 0.17 (n = 3). 4. There was a good agreement between the relative affinities in radioligand binding and electrophysiological assays for the receptor agonists NMDA, N-methyl-L-aspartate, quinolinate and trans-2,3-piperidine dicarboxylate, which are poor substrates of acidic amino acid transport systems. However, agonists which are good substrates for high affinity uptake systems (L- and D-glutamate, L- and D-aspartate, D-aspartate-beta-hydroxamate and L-glutamate-gamma-hydroxamate) were much weaker in the electrophysiological experiments. 5 Schild analysis of the antagonism of NMDA responses in the rat cortical slice by DL-3(2- carboxypiperazin-4-yl)propyl-1-phosphonate, D- and DL-2-amino-5-phosphonovalerate, D- and DL-2- amino- 7-phosphonoheptanoate, D-beta-aspartylaminomethylphosphonate, D-gamma-glutamylglycine and D-Ofaminoadipate (D-AA) indicated a competitive interaction with respective pA2 values of 6.17, 5.62, 5.24, 5.28, 5.20, 5.00, 4.43 and 3.97. 6 In the radioligand binding experiments the same antagonists inhibited only the NMDA-sensitive component of L-[3H]-glutamate binding. IC50 values showed a good correlation with the pA2 values (correlation coefficient = 0.96), with the exception of D-AA which was more potent than anticipated in the binding experiments (IC50 = 9.8 microM).7 These results confirm that NMDA-sensitive L-[3H]-glutamate binding sites represent the NMDA recognition site of the NMDA receptor and provide affinity values for both agonists and antagonists in the rat cerebral cortex, agreeing well with previous estimates in this and other tissues.

Neuronal localization of pyroglutamate aminopeptidase II in primary cultures of fetal mouse brain

Pyroglutamate aminopeptidase II is a highly specific membrane-bound ectopeptidase proposed to inactivate thyrotropin releasing hormone (TRH) in brain extracellular space. Its activity was measured in primary cell cultures of fetal brain in an attempt to define its cellular localization. Enzyme activity was detected in hypothalamic or cortical cell membrane fractions from 4- to 12-day-old cultures. When proliferation of nonneuronal cells was abolished by cytosine arabinoside treatment, pyroglutamate aminopeptidase II specific activity was increased as compared to untreated cultures, the opposite was observed for pyroglutamate amino-peptidase I activity. Treatment of cortical cells with the neurotoxic agent glutamate reduced simultaneously pyroglutamate aminopeptidase II and glutamate decarboxylase activities. Glial cell cultures expressed pyroglutamate aminopeptidase I or glutamate synthase activities but not pyroglutamate aminopeptidase II. The data suggest that pyroglutamate aminopeptidase II is predominantly localized in neuronal cells. This is consistent with a role for pyroglutamate aminopeptidase II in TRH-ergic synaptic transmission.

Excitatory amino acid receptors, neural membrane phospholipid metabolism and neurological disorders

Excitatory amino acids and their receptors play an important role in membrane phospholipid metabolism. Persistent stimulation of excitatory amino acid receptors by glutamate may be involved in neurodegenerative diseases and brain and spinal cord trauma. The molecular mechanism of neurodegeneration induced by excitatory amino acids is, however, not known. Excitotoxin induced calcium entry causes the stimulation of phospholipases and lipases. These enzymes act on neural membrane phospholipids and their stimulation results in accumulation of free fatty acids, diacylglycerols, eicosanoids and lipid peroxides in neurodegenerative diseases and brain and spinal cord trauma. Other enzymes such as protein kinase C and calcium-dependent proteases may also contribute to the neuronal injury. Excitotoxin-induced alteration in membrane phospholipid metabolism in neurodegenerative diseases and neural trauma can be studied in animal and cell culture models. The models can be used to study the molecular mechanisms of the neurodegenerative processes and to screen the efficacy of therapeutic drugs for neurodegenerative disease and brain and spinal cord trauma.

Kinetic analysis of interactions between kainate and AMPA: evidence for activation of a single receptor in mouse hippocampal neurons

AMPA but not kainate produces a rapidly desensitizing response in mouse hippocampal neurons. The characteristic action of these agonists appears to arise from activation of a single receptor with active and desensitized states, for which AMPA and kainate have different relative affinity. The equilibrium potency of a series of five agonists that produce rapidly desensitizing responses at non-NMDA receptors (EC50 1 microM to 4 mM) was similar to their equilibrium potency for block of kainate responses. Increasing the concentration of kainate overcame such block, but in the presence of AMPA the rate of activation of responses to kainate was slowed. Conversely, in the presence of kainate the amplitude of rapidly desensitizing responses evoked by AMPA was reduced, and the rate of onset of desensitization was slowed.

Ischemic neuronal injury in the rat hippocampus following transient forebrain ischemia: evaluation using in vivo microdialysis

Neuronal vulnerability to ischemia in the rat hippocampus was investigated by the measurement of high potassium evoked overflow of neurotransmitters using in vivo microdialysis. Changes in the extracellular level of amino acids caused by high potassium (100 mM) stimulation were measured on the 5th day after 20 min of forebrain ischemia, and the ratio of stimulated to basal levels or the peak concentration following the stimulation were correlated to neuronal activities. The responses to high potassium stimulation of glutamate and aspartate were reduced to 35-40% of the control values on the 5th day after 20 min ischemia, whereas the responses of gamma-aminobutyric acid (GABA) and taurine were not reduced on the 5th day after the ischemia. These results suggest that excitatory amino acid neurons (glutamatergic and aspartatergic) are more vulnerable than inhibitory amino acid neurons (GABAergic and taurinergic) in the hippocampus. Histologically, hippocampal CA1 pyramidal cells, which are believed to be glutamatergic or aspartatergic, demonstrated a marked neuronal necrosis on the 5th days after 20 min ischemia. Biochemical features revealed by high potassium stimulation may be an expression of 'delayed neuronal death' in the hippocampal CA1 area.

Dendritic location of neural BC1 RNA

In nerve cells, a specialized protein synthetic machinery is thought to operate in local compartments of dendrites, in particular beneath synaptic junctions, and thereby to facilitate swift adjustments of the postsynaptic protein repertoire in situ. This notion has been supported by the identification of polyribosomes and selected mRNAs in those compartments. In this study, we report the discovery of a specific RNA polymerase III transcript in dendrites. This RNA, a noncoding, 152-nucleotide-long, single-gene transcript known as BC1 RNA, is expressed almost exclusively in the nervous system. In adult rats as well as in immature rats in late developmental stages, BC1 RNA has been located in the dendrites and somata of a subset of neurons in the central and peripheral nervous system. The colocalization of BC1 RNA with dendritic mRNAs and polyribosomes may indicate a role--possibly within the functional unit of a high molecular mass ribonucleoprotein particle--in specific pre- or posttranslational processes in postsynaptic compartments of neurons.

The action of MPTP on synaptic transmission is affected by changes in Ca2+ concentrations

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes a disruption of nigrostriatal dopaminergic function which resembles parkinsonism. On the cellular level, the disruption involves a Ca2+ independent release of dopamine, depletion of nigral and striatal dopamine, and the death of dopaminergic neurons. We have previously reported that MPTP can cause a non-reversible inhibition of neostriatal synaptic transmission. In this study we investigated the effect of altering Ca2+ concentration on MPTP's actions in the mouse nigrostriatal brain slice. We report finding that the MPTP induced non-reversible decrease in N-2 amplitude did not occur if synaptic transmission had been blocked using a low Ca2(+)-high Mg2+ artificial cerebrospinal fluid (ACSF) during MPTP application. Low Ca2(+)-high Mg2+ ACSF did not however alter the decrease in slice dopamine content caused by MPTP. These data provide initial support for the hypothesis that MPTP's ability to alter functional synaptic transmission is Ca2+ dependent whereas its releasing action on dopamine is Ca2+ independent.

Neurochemical aspects of the N-methyl-D-aspartate receptor complex

The N-methyl-D-aspartic acid (NMDA)-sensitive subclass of brain excitatory amino acid receptors is supposed to be a receptor-ionophore complex consisting of at least 3 different major domains including an NMDA recognition site, glycine (Gly) recognition site and ion channel site. Biochemical labeling of the NMDA domain using [3H]L-glutamic acid (Glu) as a radioactive ligand often meets with several critical methodological pitfalls and artifacts that cause a serious misinterpretation of the results. Treatment of brain synaptic membranes with a low concentration of Triton X-100 induces a marked disclosure of [3H]Glu binding sensitive to displacement by NMDA with a concomitant removal of other several membranous constituents with relatively high affinity for the neuroactive amino acid. The NMDA site is also radiolabeled by the competitive antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid that reveals possible heterogeneity of the site. The Gly domain is sensitive to D-serine and D-alanine but insensitive to strychnine, and this domain seems to be absolutely required for an opening of the NMDA channels by agonists. The ionophore domain is radiolabeled by a non-competitive type of NMDA antagonist that is only able to bind to the open but not closed channels. The binding of these allosteric antagonists is markedly potentiated by NMDA agonists in a manner sensitive to antagonism by isosteric antagonists in brain synaptic membranes and additionally enhanced by further inclusion of Gly agonists through the Gly domain. Furthermore, physiological and biochemical responses mediated by the NMDA receptor complex are invariably potentiated by several endogenous polyamines, suggesting a novel polyamine site within the complex. At any rate, activation of the NMDA receptor complex results in a marked influx of Ca2+ as well as Na+ ions, which subsequently induces numerous intracellular metabolic alterations that could be associated with neuronal plasticity or excitotoxicity. Therefore, any isosteric and allosteric antagonists would be of great benefit for the therapy and treatment of neurodegenerative disorders with a risk of impairing the acquisition and formation process of memories.

Time course of changes in lipid peroxidation, pre- and postsynaptic cholinergic indices, NMDA receptor binding and neuronal death in the gerbil hippocampus following transient ischemia

Brief (5 min) bilateral carotid occlusion in the gerbil produces forebrain ischemia resulting, as previously reported, in almost complete neuronal loss in the CA1 region of the hippocampus; this neuronal destruction occurs between the 4th and 7th day post-ischemia. Various hippocampal biochemical indices were measured from just after such ischemia to 21 days of recirculation, and the temporal pattern of changes compared with that of cell loss. The level of thiobarbiturate reacting substances (TBARS), a measure of lipid peroxidation, was greatly elevated at 30 min after ischemia, rapidly returned to normal levels (by 60 min), but was again elevated on days 4-14. The beginning of this second period of elevation correlated closely with the onset of neuronal loss and the very abrupt and large (to about 32%) decrease in specific N-methyl-D-aspartate (NMDA) binding sites, measured with radioactive CPP. The number of muscarinic binding sites, measured with radioactive quinuclidinyl benzilate, showed an even greater decrease (to 13%) at 21 days post-ischemia, but the decrease was delayed (starting at day 7) and much more gradual than the loss in NMDA binding. In neither case was there any change in binding affinity at any time studied. Acetylcholine (ACh) concentrations were initially greatly decreased (to about 15% at 5 min), transiently increased (to about 130% at 30 min), and then decreased again (to about 15% at 60 min), after which gradual recovery occurred and was completed by day 14. Since no inhibition of choline acetyltransferase activity was observed at any time, the reversible depression in ACh must depend upon some factor other than loss of this key synthetic enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of heat shock protein 72-like immunoreactivity in the hippocampal formation following transient global ischemia

Global ischemia was produced in adult rats by combining bilateral carotid artery occlusions with systemic hypotension for 5 or 10 minutes. Induction of the 72 kD heat shock protein (HSP72) in the hippocampus was examined immunocytochemically 18-24 hours later. Several patterns of HSP72-like immunoreactivity (HSP72LI) were observed. Five minutes of ischemia induced HSP72 in isolated columns of CA1a pyramidal neurons, or throughout CA1 pyramidal neurons and dentate hilar neurons. Ten minutes of ischemia induced marked HSP72LI in CA3 pyramidal neurons, moderate HSP72LI in dentate granule cells, and minimal HSP72LI in CA1 pyramidal, dentate hilar neurons, and hippocampal glia. Two hippocampi subjected to 10 minutes of ischemia exhibited marked HSP72LI in capillary endothelial cells but no neuronal or glial HSP72LI. It is proposed that (a) the induction of HSP72 in hippocampal sectors correlates with their vulnerability to global ischemia (CA1 greater than hilus greater than CA3 greater than dentate gyrus); (b) the induction of HSP72 in hippocampal cells correlates with their vulnerability to global ischemia in that mild ischemia induced HSP72 only in neurons, moderate ischemia in neurons and glia, and severe ischemia only in capillary endothelial cells; (c) the failure to induce HSP72 in hippocampal neurons in 2 cases of 10 min ischemia may be related to severe injury causing disruption of protein synthesis in these cells.

Effects of neonatal gamma-ray irradiation on rat hippocampus--II. Development of excitatory amino acid binding sites

In the rat, neonatal irradiation produces a destruction of dentate granule cells and prevents the development of the mossy fibre-CA3 pyramidal cell synapse. The developmental increase of high affinity kainate binding sites in the stratum lucidum was reduced on the irradiated side as compared with the control side. This suggests that a proportion of high affinity kainate binding sites is associated with mossy fibres. In contrast, the developmental profile of N-methyl-D-aspartate binding sites, which are associated with associational and commissural synapses in CA3, was not affected by irradiation. The role that afferent fibres may play in the development of pyramidal cells is discussed in connection with the modulatory effects of glutamate receptors on the development of neurons.

Permanently altered hippocampal structure, excitability, and inhibition after experimental status epilepticus in the rat: the "dormant basket cell" hypothesis and its possible relevance to temporal lobe epilepsy

The relationship between an episode of status epilepticus, the resulting hippocampal pathology, and the subsequent development of pathophysiological changes possibly relevant to human epilepsy was explored using the experimental epilepsy model of perforant path stimulation in the rat. Granule cell hyperexcitability and decreased feedforward and feedback inhibition were evident immediately after 24 hours of intermittent perforant path stimulation and persisted relatively unchanged for more than 1 year. All of the pathophysiological changes induced by perforant path stimulation were replicated in normal animals by a subconvulsive dose of bicuculline, suggesting that the permanent "epileptiform" abnormalities produced by sustained perforant path stimulation may be due to decreased GABA-mediated inhibition. Granule cell pathophysiology was seen only in animals that exhibited a loss of adjacent dentate hilar mossy cells and hilar somatostatin/neuropeptide Y-immunoreactive neurons. GABA-immunoreactive dentate basket cells survived despite the extensive loss of adjacent hilar neurons. However, parvalbumin immunoreactivity, present normally in a subpopulation of GABA-immunoreactive dentate basket cells, was absent on the stimulated side. Whether this represents decreased parvalbumin synthesis in surviving basket cells or a loss of a specific subset of inhibitory cells is unclear. Hyperexcitability and decreased paired-pulse inhibition in response to ipsilateral perforant path stimulation were also present in the CA1 pyramidal cell layer on the previously stimulated side, despite minimal damage to CA1 pyramidal cells or interneurons. The possibility that CA1 inhibitory neurons were hypofunctional or "dormant" due to a loss of excitatory input to inhibitory cells from damaged CA3 pyramidal cells was tested by stimulating the contralateral perforant path in order to activate the same CA1 basket cells via different inputs. Contralateral stimulation evoked CA1 pyramidal cell paired-pulse inhibition immediately in the previously stimulated hippocampus. Thus, we propose the "dormant basket cell" hypothesis, which implies that despite malfunction, inhibitory systems remain intact in "epileptic" tissue and are capable of functioning if appropriately activated.

Cerebellar excitatory amino acid binding sites in normal, granuloprival, and Purkinje cell-deficient mice

Using quantitative autoradiography, the cellular localization and characterization of cerebellar excitatory amino acid binding sites in normal, Purkinje cell-deficient and granuloprival (granule cell-deficient) mouse cerebella were investigated. In the molecular layer of normal mouse cerebellum, the quisqualate subtype of excitatory amino acid receptor (assayed by [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitroquinoxaline-2,3-dione binding) predominated. In the granule cell layer of the cerebellum, N-methyl-D-aspartate-sensitive L-[3H]glutamate and [3H]glycine binding sites were predominant. In the molecular layer of Purkinje cell-deficient mutant mice, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding were reduced to 24% (P less than 0.01) and 36% (P less than 0.001) of control, respectively, while quisqualate-sensitive [3H]glutamate binding sites were reduced to 54% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In the granule cell layer of these mouse cerebella, there was no change in excitatory amino acid receptor binding. In the molecular layer of granuloprival mouse cerebella, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding was increased to 205% of control (P less than 0.01), [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding was increased to 136% of control (P less than 0.02), and quisqualate-sensitive [3H]glutamate binding was increased to 152% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In areas of granule cell depletion N-methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were reduced to 68% (P less than 0.01) and 59% (P less than 0.01) of control, respectively. In the granule cell layer, binding to quisqualate receptors was not significantly different from binding in controls with any of the ligands tested. These results suggest that three different receptor assays: [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding can be used to demonstrate that quisqualate receptor specific binding sites are located on Purkinje cell dendrites in the molecular layer of cerebellum, and that these binding sites apparently up-regulate in response to granule cell ablation and Purkinje cell deafferentation.

Regional heterogeneity in the distribution of neurotransmitter markers in the rat hippocampus

A detailed neurochemical analysis of the distribution of markers for the most relevant neurotransmitter systems within the rat hippocampal formation has been performed. The hippocampi, obtained from unfrozen brains of male Sprague-Dawley rats were subdissected into tissue parts containing mainly CA1, CA3 or the dentate gyrus, respectively. Each part was further divided into ventral and dorsal halves. In these six hippocampal subregions the concentrations of noradrenaline, dopamine, serotonin, 3-methoxy-4-hydroxyphenylglycol, 5-hydroxyindoleacetic acid and the putative neurotransmitter amino acids glutamate, aspartate, GABA, glycine and taurine, and the levels of somatostatin and neuropeptide Y and the activities of choline acetyltransferase, acetylcholinesterase and glutamate decarboxylase were measured. A marked heterogeneity in the subregional distribution of markers for various neurotransmitter systems within the hippocampal formation was observed. Each neuronal marker was characterized by an individual pattern of distribution. Most of the markers showed a concentration-gradient, increasing from dorsal to ventral; only taurine was more abundant in the dorsal than in the ventral parts and no dorsoventral difference was seen for aspartate, glycine and neuropeptide Y. The highest molar ratios of total 3-methoxy-4-hydroxyphenylglycol to noradrenaline and 5-hydroxyindoleacetic acid to serotonin were found in the dorsal hippocampus. The levels of noradrenaline, GABA and glutamate decarboxylase activity were highest in the dentate gyrus and lowest in CA1. The concentrations of somatostatin were highest in CA1; those of serotonin were highest in CA3. Highest activities of choline acetyltransferase and acetylcholinesterase were found in the dentate gyrus; lowest activities were found in CA3. In CA3 the lowest values of glutamate, aspartate, taurine and somatostatin were also found. The heterogeneity in the distribution of individual neurochemical markers allows insights into possible functional differences of hippocampal subregions and provides a relevant basis for future neurochemical investigations in this brain area.

Distribution of the calcium-binding proteins parvalbumin and calbindin-D28k in the sensorimotor cortex of the rat

This study examined and compared the immunocytochemical distribution of the two calcium-binding proteins parvalbumin and calbindin-D28k in the primary motor and somatosensory areas of the rat neocortex. Parvalbumin-immunoreactive cells were found in all layers of the cortex except layer 1 and reached their peak density in the middle layers. The two cortical areas differed markedly in the number, cell size and morphology of immunoreactive cells. Parvalbumin-positive cells were more than twice as numerous in the somatosensory cortex compared to the motor cortex. In addition, the average size of their cell bodies was 25-30% larger in the somatosensory area. Parvalbumin cells in the motor area represented several classes of nonpyramidal cells, while the somatosensory cortex contained in addition many large cells with thick vertically oriented primary dendrites. Some of these cells resembled regular or inverted pyramidal neurons. Punctate neuropil labeling was much heavier in the upper layers of the somatosensory than in the motor cortex and was especially heavy in layer 4. Dense parvalbumin-positive perisomatic puncta surrounded large, unstained pyramidal cells in layer 5B of the motor cortex. Calbindin-D28k neuronal staining in both areas was confined to two populations. The most prominent was darkly labeled, small nonpyramidal cells confined to two bands in layers 2/3 and 5/6. There was also a lighter stained population composed of many pyramidal cells distributed throughout layers 2 and 3. In addition, the motor area contained a band of lightly stained, large pyramidal cells in layer 5B. Calbindin-D28k neuropil labeling was heaviest in layers 1 to 3. In contrast to parvalbumin, we found only minor differences in distribution, size and morphology of calbindin-D28k cell body or neuropil staining in the two cortical areas. Double-labeling immunocytochemistry showed that the large majority of immunoreactive cells contained only calbindin-D28k or parvalbumin, but a distinct population of multipolar cells in the upper layers of the somatosensory cortex contained both. The clear parcellation of parvalbumin immunoreactivity in the rat neocortex suggests that parvalbumin is preferentially associated with specific neuronal populations and terminals in the somatosensory cortex. The more general and homogeneous labeling of the upper layers of the cortex indicates that calbindin-D28k could be related to the relatively high density of calcium channels or N-methyl-D-aspartate receptors in the superficial layers of the rat cortex.

Antibodies to a C-terminal peptide of the rat brain glutamate receptor subunit, GluR-A, recognize a subpopulation of AMPA binding sites but not kainate sites

Antibodies were made to a thirteen amino acid synthetic peptide corresponding to the C-terminal portion of the glutamate (glu) receptor, GluR-A. The immunoprecipitation of kainic acid (KA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) binding sites by the anti-peptide antibodies was studied using a detergent-solubilized preparation of rat brain membranes. Under these conditions a subpopulation of AMPA binding sites was recognized by the antibodies, but no KA binding sites were recognized. Scatchard analysis of this subpopulation of AMPA binding sites yields a curvilinear plot which fits a two-site model with dissociation constants of 4.6 and 323 nM. These studies show that the glu receptor complex, GluR-A, binds AMPA but not KA and suggest that (i) the binding sites for these two ligands reside on different proteins, and (ii) the KA receptor identified physiologically is not equivalent to the KA binding sites identified with 3H-labelled KA.

Developmental study of [3H]TCP and [3H]glycine binding sites in the rat hippocampus

The postnatal development of [3H]thienylphencyclidine ([3H]TCP) sites in rat hippocampus has been studied autoradiographically and with membrane preparations. [3H]TCP binding increased progressively from birth to adulthood; this is due to a change in the maximal number of sites (Bmax) but not in the affinity (Kd). A different developmental pattern was found for strychnine-insensitive [3H]glycine binding which also increased after birth, but reached adult levels earlier than [3H]TCP binding. The ontogenesis of TCP or glycine sites also differed from that previously described for N-methyl-D-aspartate (NMDA) sites in the hippocampus. In neonatal, as in adult hippocampus, [3H]TCP binding was enhanced by NMDA or glycine and reduced by Mg2+. We suggest that TCP sites are functionally coupled to the NMDA receptor-ion channel complex in developing as in mature hippocampus, but that there are developmental changes in the receptor channel complex.

Distribution of hypoxia-induced calcium accumulation in gerbil hippocampal slice

Microfluorometry was used to examine distribution of hypoxia-induced intracellular free calcium accumulations in Mongolian gerbil hippocampal slice. Acute increase of intracellular free calcium was detected 80-170 s after the beginning of hypoxia. Large calcium accumulations were seen in the stratum radiatum, stratum lacunosum and stratum oriens of CA1 region and in the inner portions of the dentate gyrus molecular layer, and moderate accumulations were seen in the other laminae of CA1 region, other portions of dentate gyrus and the CA3 region.

Excitatory amino acid transmitters and their receptors in neural circuits of the cerebral neocortex

In 1954, L-glutamate (Glu) and L-aspartate (Asp) were first suggested as being excitatory synaptic transmitters in the cerebral cortex. Since then, evidence has mounted steadily in favor of the view that Glu and Asp are major excitatory transmitters in the neocortex. Many of the experimental studies which reported how Glu/Asp came to satisfy the criteria for transmitters in the neocortex are reviewed here, according to the methods employed. Since the question of which particular synaptic sites in cortical neural circuits Glu/Asp operate as excitatory transmitters has not previously been reviewed, particular attention is given to efferent, afferent and intrinsic neural circuits of the visual and somatosensory cortices, where circuitry is relatively clearly delineated. Recent studies using chemical assays of released amino acids, high-affinity uptake mechanisms of Glu/Asp from nerve terminals, the direct micro-iontophoretic administration of Glu/Asp antagonists, and immunocytochemical techniques have demonstrated that almost all corticofugal efferent projections employ Glu/Asp as excitatory synaptic transmitters. Evidence indicating that thalamocortical afferent projections, including geniculocortical projections and some intrinsic connections are glutamatergic, is also reviewed. Thus, the results highlighted here indicate that the main framework of neocortical circuitry is operated by Glu/Asp. Pharmacological studies indicate that synaptic receptors for Glu/Asp can be classified into a few subtypes, including N-methyl-D-aspartate (NMDA) and quisqualate/kainate (non-NMDA) types. Some evidence indicating the sites of operation of NMDA and non-NMDA receptors in neocortical circuitry is reviewed, and the distinct, functional significance of these two types of Glu/Asp receptors in information processing in the neocortex is proposed.

Potentiation of inputs from the posterolateral amygdala to the dentate gyrus and resistance to stress ulcers formation in rats

Physical restraint was found to increase the activity of a number of multiple units in the lateral amygdala of rats. High-frequency electrical stimulation of units in the posterolateral amygdala increased the amplitudes of granule cell potentials in the dentate gyrus. This bilateral long-term potentiation (LTP) of inputs from posterior areas of the lateral amygdala also attenuated the severity of stress ulcers produced by physical restraint. This effect was reversed by intraventricular injections of the selective N-methyl-D-aspartate receptor blocker, aminophosphonovaleric acid. LTP in this pathway also reduced "struggling" behaviour during restraint. The data were interpreted to indicate that LTP in this temporal lobe pathway increased the coping ability because of faster habituation to stressors.

MK-801 attenuates capillary bed compression and hypoperfusion following incomplete focal cerebral ischemia

The effects of the N-methyl-D-aspartate (NMDA) antagonist MK-801 on capillary beds and CBF following 1 h of transient incomplete focal cerebral ischemia were studied by examining 133Xe CBF, capillary diameter, and area of perfused vasculature. Capillary diameter increased from a control of 5.24 +/- 0.37 to 8.62 +/- 0.57 microns (p less than 0.001) and area of perfused vasculature from 20,943 +/- 1,151 to 30,442 +/- 1,691 microns2/x 10 magnification field (p less than 0.001) with MK-801 1.0 mg/kg administered 30 min prior to ischemia. After flow restoration in control animals, there was a relative hypoperfusion with eventual normalization of CBF over 60 min. Alternatively, in MK-801 1.0 mg/kg animals, there was rapid normalization of CBF upon flow restoration without the postischemic hypoperfusion observed in controls. On histological analysis, there was consistently less neuronal edema in MK-801-treated animals. These results support the hypothesis that hypoperfusion following incomplete focal cerebral ischemia may be due in part to NMDA-mediated cellular edema with subsequent extravascular capillary bed compression.

Protection against ischemia-induced neuronal damage by the alpha 2-adrenoceptor antagonist idazoxan: influence of time of administration and possible mechanisms of action

The protective effect of the alpha 2-receptor antagonist idazoxan against neuronal damage in the neocortex and in the hippocampal CA1 region was studied in rats exposed to 10 min of incomplete forebrain ischemia. When administered i.v. immediately after ischemia (0.1 mg/kg) and subsequently for 6 h (10 micrograms/kg/min), idazoxan significantly reduced neuronal damage in the hippocampus (from 84 to 26%) and in the vulnerable parts of the neocortex (from 15 to 1%). The bolus dose alone provided no significant protection. When idazoxan administration was delayed for 30 min, no significant protection was noticed in the neocortex, and the effect in the hippocampus was ambiguous. A transient elevation of plasma corticosterone levels was induced during ischemia. Idazoxan administration for 2 h did not affect postischemic changes in corticosterone levels compared with saline infusion. Idazoxan (10(-7)-10(-4) M) did not influence the in vitro binding to glutamate receptors in brain slices. Thus, the protective effect of idazoxan cannot be explained by suppression of the plasma corticosteroid levels or via an antagonistic effect on glutamate receptors. Idazoxan apparently protects neurons when given during the first hours of postischemic reperfusion, while histopathological necrosis of neurons becomes visible 48-72 h after ischemia. Detrimental processes causing delayed neuronal death occur in the early postischemic phase and can be influenced by adrenoceptor ligands. Idazoxan may protect by several mechanisms but probably exerts its protective postischemic effect mainly through an increased noradrenergic neuronal activity and an elevation of extracellular noradrenaline (NA) levels in the brain. The favorable effects of NA may either be due to inhibition of excitotoxic neurotransmission or activation of survival-promoting and trophic processes.

Conantokin-G: a novel peptide antagonist to the N-methyl-D-aspartic acid (NMDA) receptor

Conantokin-G is a 17 amino acid peptide isolated from the venom of the fish-eating snail Conus geographus which produces hyperactivity when injected into the brains of adult mice. We show that this peptide is a selective N-methyl-D-aspartate (NMDA) antagonist based on its ability to block NMDA-induced elevation of cGMP in rat cerebellar slices in vitro (IC50 = 171 nM), but not kainic acid-induced elevations. This inhibition could not be overcome by increasing the NMDA concentration, indicating non-competitive inhibition. Conantokin-G displayed no affinity for binding sites for thienylcyclohexylpiperidine, various glutamate subclasses or those for several other neurotransmitters/neuromodulators. This peptide, however, enhanced [3H]glycine binding to rat forebrain membranes but not to spinal cord membranes. The activity profile of the peptide in various assays indicates that it is a novel type of non-competitive NMDA antagonist.

[3H]kainate localization in goldfish brain: receptor autoradiography and membrane binding

The anatomical distribution of specific [3H]kainate binding in goldfish brain was investigated by membrane binding and autoradiographical techniques. Saturation binding of the radioligand was determined in 8 anatomically defined regions and demonstrated a single class of high affinity sites with Kd values ranging from 290 to 650 nM. Kainate receptor densities, however, varied significantly. The cerebellum contained the highest concentration of binding sites (964 pmol/mg prot.), while the optic tectum had the lowest (96 pmol/mg prot.). Binding site distributions determined by autoradiographic studies demonstrated the same regional variation and allowed more specific localization of the binding sites. Within the cerebellum, the molecular layers of the corpus, valvula and lobus caudalis displayed a uniform and highly intense image while the granule cell layers (except for the medial granule cell mass of the lobus caudalis) did not. Other areas of intense binding were the posterior tubercle of the diencephalon, inferior lobes of the hypothalamus and layers 1 and 2 of the optic tectum (deep to the periventricular granule cells).

Brain glucose and energy metabolism during normal aging

The mature, healthy, non-starved mammalian brain uses glucose only as a source of energy in the form of ATP, which is necessary for several metabolic processes, such as the maintenance of cellular homeostasis via ion homeostasis, maintenance of the integrity of cellular compartments, and intracellular transportation processes for the formation of several neurotransmitters, neurotransmission itself and a few anabolic reactions. Glucose breakdown contributes to the formation of the neurotransmitters: acetylcholine, glutamate, aspartate, gamma-aminobutyrate, and glycine. Normal cerebral aging is associated with an incipient perturbation in both cerebral glucose and related metabolism, that determines an energy deficit and thus an imbalance in cell homeostasis after the 7th or 8th decade of human life, indicating a threshold phenomenon. This is evidenced by morphological/morphobiological abnormalities comprising neuronal loss and structural changes. These events are thought to cause a marked reduction in the biological plasticity of the brain, which may be severely involved after additional stress situations such as ischemia, hypoxia or hypoglycemia. The age-related increasing perturbation of neuronal homeostasis may represent a stress situation capable of inducing heat shock proteins effecting gene activity. Thus, several age-related metabolic abnormalities at the cellular level, starting with a deficient neuronal glucose and energy metabolism, can be regarded as risk factors for neuronal damage and death, and hence reduced mental capacity.

Molecular cloning and functional expression of glutamate receptor subunit genes

Three closely related genes, GluR1, GluR2, and GluR3, encode receptor subunits for the excitatory neurotransmitter glutamate. The proteins encoded by the individual genes form homomeric ion channels in Xenopus oocytes that are sensitive to glutamatergic agonists such as kainate and quisqualate but not to N-methyl-D-aspartate, indicating that binding sites for kainate and quisqualate exist on single receptor polypeptides. In addition, kainate-evoked conductances are potentiated in oocytes expressing two or more of the cloned receptor subunits. Electrophysiological responses obtained with certain subunit combinations show agonist profiles and current-voltage relations that are similar to those obtained in vivo. Finally, in situ hybridization histochemistry reveals that these genes are transcribed in shared neuroanatomical loci. Thus, as with gamma-aminobutyric acid, glycine, and nicotinic acetylcholine receptors, native kainate-quisqualate-sensitive glutamate receptors form a family of heteromeric proteins.

Autoradiographical analysis of excitatory amino acid binding sites in rat hippocampus during the development of hippocampal kindling

Binding sites for excitatory amino acids have been determined by autoradiographical procedures in the rat hippocampus and striatum during hippocampal kindling. The binding sites measured were the N-methyl-D-aspartate (NMDA)-sensitive sites for L-[3H]glutamate and [3H]MK-801 sites (transmitter recognition site and ion channel of the NMDA receptor, respectively), [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) sites (quisqualate receptor), [3H]kainate sites (kainate receptor) and NMDA-insensitive sites for L-[3H]glutamate. In general, little change was apparent in the hippocampus or striatum for any of these binding sites when assessed 48 h after attaining stages 1/2, 3 or 5 of kindling. These results suggest that hippocampal kindling does not bring about a change in the excitatory amino acid receptor binding sites examined, and that the appearance of an NMDA receptor-mediated component to synaptic responses in the hippocampus produced by kindling, cannot be explained on this basis.

Functional antagonists at the NMDA receptor complex exhibit antidepressant actions

Inescapable, but not escapable, stress inhibits the induction of Long Term Potentiation (LTP) in the CA1 region of hippocampus, a process that is dependent upon activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. Since inescapable stress also produces a syndrome of behavioral depression sensitive to clinically effective antidepressants, we examined the actions of functional antagonists at the NMDA receptor complex in animal models commonly used to evaluate potential antidepressants. A competitive NMDA antagonist (2-amino-7-phosphonoheptanoic acid [AP-7]), a non-competitive NMDA antagonist (Dizolcipine [MK-801]), and a partial agonist at strychnine-insensitive glycine receptors (1-aminocylopropanecarboxylic acid [ACPC]) mimicked the effects of clinically effective antidepressants in these models. These findings indicate that the NMDA receptor complex may be involved in the behavioral deficits induced by inescapable stress, and that substances capable of reducing neurotransmission at the NMDA receptor complex may represent a new class of antidepressants. Based on these findings, the hypothesis that pathways subserved by the NMDA subtype of glutamate receptors are involved in the pathophysiology of affective disorders may have heuristic value.

Effect of septal kindling on glutamate binding and calcium/calmodulin-dependent phosphorylation in a postsynaptic density fraction isolated from rat cerebral cortex

Postsynaptic density (PSD) fractions were isolated from the cerebral cortices of control and kindled rats and assayed for glutamate and gamma-aminobutyric acid-binding capacities and for the Ca2+/calmodulin-dependent protein kinase. Glutamate binding was found to be increased by approximately 50% in the PSDs isolated from kindled rats as compared to controls; this increase was almost completely from an increase in Bmax; Kd decreased only slightly. Studies with inhibitors indicate that the receptors involved were of the N-methyl-D-aspartate and quisqualate types. PSDs isolated from control and kindled rats did not differ in gamma-aminobutyric acid or flunitrazepam binding. The in vitro autophosphorylation of the Ca2+/calmodulin-dependent protein kinase was depressed by 45-76% in PSDs isolated from kindled rats as compared to controls, with little change in amount of the kinase. Therefore, we infer that (i) the kindled state is associated with an increase in glutamate activation of postsynaptic sites, allowing Ca2+ to enter dendritic spines, (ii) a change has occurred in activity of the protein kinase, which is the major cerebral cortex PSD protein, and (iii) perhaps major alterations in the PSD are a concomitant to the long-lasting nature of the kindled state.

Effect of APV and ketamine on epileptiform activity in the CA1 and CA3 regions of the hippocampus

Intracellular recordings were obtained from hippocampal CA1 and CA3 pyramidal neurons maintained in vitro. The ability of the NMDA receptor antagonists ketamine and APV to suppress picrotoxin-induced epileptiform burst activity was examined. Activity was recorded either after a single orthodromic stimulation, which gave rise to paroxysmal depolarization shift (PDS), or during a 500 msec train of 50 Hz stimulation, which produced a sustained depolarization. In the CA1 and CA3 areas, both the PDS and sustained depolarization were reduced by APV (20 microM) and ketamine (100 microM). APV reduced the area under the PDS by 24 +/- 3% and 32 +/- 4% in CA1 and CA3 neurons, respectively. The corresponding reductions in the sustained depolarization were 10 +/- 2% and 22 +/- 4%. Ketamine reduced the PDS by 43 +/- 4% and 31 +/- 4% in CA1 and CA3 and decreased the sustained depolarization by 21 +/- 3% and 12 +/- 3%. In all cases, NMDA receptor antagonists had a significantly greater effect on the PDS than the sustained depolarization. These results indicate that, although not essential for generation of paroxysmal activity, NMDA receptors make significant contributions to epileptiform activity in both CA1 and CA3 regions of the hippocampus.

Spider toxin (JSTX-3) inhibits the convulsions induced by glutamate agonists

The effect of a spider toxin (JSTX-3)--a specific blocker of glutamate receptors--on the behavior of mice was studied using an intracerebroventricular (i.c.v.) injection technique. At higher doses (more than 12 nmol/brain), JSTX-3 increased motor activities and induced characteristic symptoms. JSTX-3 at a dose of 4.7 nmol/brain which per se did not produce any abnormal behavior, specifically antagonized quisqualate-induced convulsions but not NMDA- or kainate-induced convulsions. These results indicate that JSTX-3 is a selective antagonist of the quisqualate receptors in the mammalian central nervous system.

gamma-Aminobutyric acid (GABA) enhances glutamate cytotoxicity in a cerebellar cell line

A temperature-sensitive rat cerebellar cell line SC9 has been used to study the role of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in glutamate cytotoxicity. GABA increases glutamate toxicity in a dose-dependent fashion, but NMDA and kainic acid were not toxic in the presence or absence of GABA. The specificity of this cytotoxicity was further indicated by the NMDA-selective antagonist 2-amino-7-phosphonoheptanoic acid (APV), which does not block glutamate effect. These observations, as well as binding experiments with 3H-glutamate, suggest that glutamate cytotoxicity in these cells depends on quisqualate-selective uptake sites of the amino acid. The study may open therefore a novel pathway for understanding the cytotoxic effect of excitatory amino acids in brain structures that are enriched with GABA and glutamate uptake sites.

Ultrastructural description of glutamate-, aspartate-, taurine-, and glycine-like immunoreactive terminals from five rat brain regions

The ultrastructural localization of putative excitatory (glutamate, aspartate) and inhibitory (taurine, glycine) amino acid neurotransmitters is described in several selected rat brain regions. In general, axon terminal profiles immunoreactive for excitatory amino acids formed asymmetric synapses with non-immunoreactive small diameter dendritic profiles or dendritic spines. In the cerebellum, both mossy fiber terminals and parallel fiber terminals were immunoreactive for glutamate and aspartate. In the hippocampus, mossy fiber terminals within the stratum lucidum of the CA3 region were immunoreactive for glutamate. Localization of glutamate and aspartate to cerebellar parallel and mossy fibers, as well as the identification of glutamate in hippocampal mossy fibers, is consistent with the excitatory nature of these fibers as described in previous physiological studies. Glutamate-like immunoreactive terminals were also identified in subnucleus caudalis of the spinal trigeminal nucleus and in the dorsal horn of the spinal cord. Immunoreactive axon terminals for two putative inhibitory neurotransmitters, glycine and taurine, displayed a greater number of morphological variations in synaptic structure. In the cerebellum, taurine-like immunoreactivity was present in both basket cell axon terminals which formed symmetric synapses with Purkinje cell neurons, and in a few mossy fiber terminals which formed asymmetric synapses with dendritic spines. In the area dentata of the hippocampus, taurine-like immunoreactive profiles formed asymmetric synapses with dendritic elements. Glycine-like immunoreactive terminals formed symmetric synapses with cell perikarya in both the ventral horn of the spinal cord and in the cochlear nuclei, and on axon terminals in the spinal trigeminal and cochlear nuclei. In contrast, some glycine-like immunoreactive terminals formed asymmetric synapses with distal dendritic profiles in the spinal cord and spinal trigeminal nucleus. The localization of taurine to cerebellar basket cell axons and glycine to axon terminals that synapse on ventral horn motor neuron perikarya is consistent with the hypothesis that these amino acids are functioning as inhibitory neurotransmitters at these synapses. Taurine localization to cerebellar mossy fibers and to fibers in the molecular layer of the dentate gyrus may be more consistent with a proposed neuromodulator role of taurine.

Glutamate and glycine induce a negative wave on hippocampal field response through NMDA receptors

In rats under urethane anesthesia, iontophoresis of large amounts (30-300 nA) of glutamate in the hippocampus induced a negative wave on the field potential evoked by stimulation of fimbria/commissura or perforant pathway. The amplitudes of the negative waves ranged between 0.2 and 9.8 mV and their mean duration was 341 +/- 12 ms. This activity was antagonized by iontophoresis of N-methyl-D-aspartate (NMDA) antagonists: Mg2+ (80-100 nA), ketamine (50-150 nA), MK-801 (50-150 nA) and by systemic ketamine (5 mg/kg, i.v.) administration. Iontophoresis of N-methyl-DL-aspartate (NMDLA) (20-40 nA) and glycine (25-100 nA) also elicited a negative wave which was blocked by NMDA antagonists. The negative waves were induced in all hippocampal layers except the dentate hilus by glutamate, NMDLA and glycine. Pyramidal regions were found to be as sensitive as dendritic layers; the mean amplitudes of glutamate-induced negative waves on the field response were 4.1 +/- 0.6 and 4.2 +/- 0.5 mV for CA1 stratum pyramidale and radiatum, respectively. These data suggest that large amounts of glutamate activate NMDA receptor/ion channels causing appearance of a long-lasting negative wave on the hippocampal field response. The data also demonstrate that glycine leads to a significant participation of NMDA receptors during glutamatergic transmission which is largely mediated through non-NMDA receptors.

The adenosine antagonist 8-cyclopentyltheophylline reduces the depression of hippocampal neuronal responses during hypoxia

Exposure of rat hippocampal slices to hypoxic conditions for 15 min produced a rapid, profound, but completely reversible depression of evoked synaptic potentials. The specific A1 adenosine receptor antagonist 8-cyclopentyltheophylline (8-CPT) significantly reduced hypoxia-induced synaptic depression in a concentration-dependent manner. It is concluded that adenosine, which is neuroprotective when exogenously applied during severe hypoxia because of its ability to depress synaptic transmission, may have an important and exploitable endogenous role in the protection of sensitive neurons.

The distribution of chromogranin A-like immunoreactivity in the human hippocampus coincides with the pattern of resistance to epilepsy-induced neuronal damage

The distribution of chromogranin A-like immunoreactivity in the hippocampus of adult humans who were free of neurological disease was examined by immunohistochemical methods. Immunoreactivity was restricted to the cytoplasm of certain neuronal populations, most notably the mossy fibers of denate granule cells (and a subset of their perikarya), and the perikarya of pyramidal cells of the cornu Ammonis 2 (CA2) sector. Additionally, staining was observed in neurons in the stratum oriens, a population of neurons at the periphery of the CA4 sector, scattered, probably short-axon perikarya in the CA1 sector, and fibers in the perforant path and the molecular layer of the dentate gyrus. Pyramidal neurons in the CA1 and CA3 sectors were not immunoreactive. The two prominently immunoreactive neuronal populations, CA2 pyramids and dentate granule cells, are those spared in human and experimental epileptic brain damage, whereas CA1 and CA3 pyramids, lacking chromogranin, are characteristically destroyed in this condition. The known activities of chromogranin in the periphery as a calcium-binding protein and as a precursor of active peptides (autocrine inhibitory modulators) suggest that its distribution in the hippocampus may help to explain the observed pattern of resistance to epileptic brain damage.

Excitatory amino acid receptors in the brain: membrane binding and receptor autoradiographic approaches

In last month's article in this series, Lodge and Johnson discussed the contribution of noncompetitive excitatory amino acid antagonists to understanding of these receptors. In this third article, Anne Young and Graham Fagg describe how radioligand binding experiments have helped to fuel the recent burst of progress in understanding excitatory amino acid receptors in the brain. New and selective radioligands have facilitated mapping the distributions of the major excitatory receptor subtypes in normal and diseased brain, examining allosteric interactions within the NMDA receptor, searching for novel therapeutic agents and determining drug mechanisms, and making first steps along the path to defining receptor structure at the molecular level.

Brain damage in a new hemorrhagic shock model in the rat using long-term recovery

A new shock model in the rat using hemorrhagic hypotension for production of brain damage is described. Hemorrhagic shock was induced by lowering arterial blood pressure with bleeding. The MABP was maintained at approximately 25 mm Hg, accompanied by isoelectric EEG, and then shed blood was retransfused. At 1 week of recovery, morphological and 45Ca autoradiographic changes were examined. No brain damage was observed in rats after 1 min of isoelectric EEG. Mild neuronal damage in the hippocampal CA1 subfield was seen in some animals after 2 min of isoelectric EEG. Severe and consistent neuronal loss in the hippocampal CA1 subfield was recognized after 3 min of isoelectric EEG. Additional damage was also seen in the dentate hilus and the thalamus in some animals. This model can be used to study the pathophysiology of postshock brain damage and to assess new therapies following shock.

Autoradiographic characterization and localization of quisqualate binding sites in rat brain using the antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione: comparison with (R,S)-[3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites

Using quantitative autoradiography, we have investigated the binding sites for the potent competitive non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist [3H]6-cyano-7-nitro-quinoxaline-2,3-dione ([3H]-CNQX) in rat brain sections. [3H]CNQX binding was regionally distributed, with the highest levels of binding present in hippocampus in the stratum radiatum of CA1, stratum lucidum of CA3, and molecular layer of dentate gyrus. Scatchard analysis of [3H]CNQX binding in the cerebellar molecular layer revealed an apparent single binding site with a KD = 67 +/- 9.0 nM and Bmax = 3.56 +/- 0.34 pmol/mg protein. In displacement studies, quisqualate, L-glutamate, and kainate also appeared to bind to a single class of sites. However, (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) displacement of [3H]CNQX binding revealed two binding sites in the cerebellar molecular layer. Binding of [3H]AMPA to quisqualate receptors in the presence of potassium thiocyanate produced curvilinear Scatchard plots. The curves could be resolved into two binding sites with KD1 = 9.0 +/- 3.5 nM, Bmax = 0.15 +/- 0.05 pmol/mg protein, KD2 = 278 +/- 50 nM, and Bmax = 1.54 +/- 0.20 pmol/mg protein. The heterogeneous anatomical distribution of [3H]CNQX binding sites correlated to the binding of L-[3H]glutamate to quisqualate receptors and to sites labeled with [3H]AMPA. These results suggest that the non-NMDA glutamate receptor antagonist [3H]CNQX binds with equal affinity to two states of quisqualate receptors which have different affinities for the agonist [3H]AMPA.

Augmentation by glycine and blockade by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) of responses to excitatory amino acids in slices of rat neocortex

Responses of neocortical pyramidal cells to excitatory amino acids were recorded intracellularly. Agonists and antagonists were applied electrophoretically from a separate multibarrel pipette and care taken to ensure that the pipette was positioned to evoke optimal responses to N-methyl-D-aspartate (NMDA), or homocysteic acid, before control responses were recorded. Responses to NMDA, but not those to alpha-amino-3-hydroxy-5-methyl-4-isoxazdepropionic acid (AMPA) or quisqualate, were enhanced when glycine was co-applied. Responses to AMPA, quisqualate and NMDA were reduced by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) applied either electrophoretically, or in the bathing medium, with responses to quisqualate being the least and those to AMPA being the most sensitive to CNQX. The blockade of NMDA responses by CNQX was selectively reversed by additional glycine confirming that CNQX blocks NMDA receptor-channel complexes at the glycine, rather than at the NMDA site. Under control conditions, responses to glutamate resembled responses to quisqualate, and were relatively insensitive to CNQX, 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid and 2-amino-5-phosphonovalerate, while responses to homocysteic acid resembled responses to NMDA and were blocked by these antagonists. This suggested that homocysteic acid acted at NMDA receptors, while glutamate acted primarily at non-NMDA receptors. However, responses to both glutamate and homocysteic acid were augmented by additional glycine when these transmitter candidates were applied close to a "hot spot" for NMDA receptor activation. The glycine enhancement of responses to glutamate was sensitive to NMDA antagonists, indicating that glutamate can activate NMDA receptors in an intact preparation if glycine levels are high enough.

Anticonvulsant effects of dihydropyridine Ca2+ antagonists in electrocortical shock seizures

The dihydropyridine calcium antagonists nimodipine (NMD), PN200-110, and nicardipine were compared with phenytoin (PHT) as potential anticonvulsants in electrocortical shock (ECS)-induced seizures in the white New Zealand rabbit. Before treatment, seizure duration ranged from 43.8 +/- 5.1 to 49.6 +/- 5.2 s with an ECS stimulus of 10-V, 100-Hz, 0.1-ms pulses for 5 s. Each drug was administered into the right internal intracarotid artery 2 min before the ECS. A cumulative nimodipine dose of 440 micrograms/kg decreased seizure discharge to 6.6 +/- 5.0 s (p less than 0.001), whereas a total dose of 1.0 mg/kg PN200-110 was required to achieve a similar effect. Nicardipine was ineffective. A cumulative dose of 7 mg/kg phenytoin was required to suppress seizure discharge. These results indicate that Ca2+ channels modulated by dihydropyridines play a facilitating role in ECS-induced seizures. We propose that the anticonvulsant effects of nimodipine and PN200-110 are due to inhibition of neuronal calcium L-channels. Dihydropyridine Ca2+ antagonists that penetrate the blood-brain barrier (BBB) and bind to neuronal tissue may emerge as a novel class of anticonvulsants.

Structure-activity relationships in the development of excitatory amino acid receptor agonists and competitive antagonists

Development of new selective ligands for excitatory amino acid receptors has been fundamental in supporting this rapidly developing field. Some of the most important ligands have come from the laboratories of Jeff Watkins, Povl Krogsgaard-Larsen and Tage Honoré, who collaborate in this double-length review to describe the chemical features and SARs of agonists and antagonists, particularly those features associated with subtype selectivity.

MK-801 pretreatment enhances N-methyl-D-aspartate-mediated brain injury and increases brain N-methyl-D-aspartate recognition site binding in rats

Direct intracerebral administration of N-methyl-D-aspartate typically produces focal brain injury. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-immi ne maleate (MK-801), a non-competitive N-methyl-D-aspartate antagonist, can protect against N-methyl-D-aspartate-mediated brain injury when administered shortly before or after an intracerebral injection of N-methyl-D-aspartate. However, in this study we report that in perinatal rats if MK-801 (1 mg/kg) is administered intraperitoneally 24 h prior to a unilateral intrastriatal N-methyl-D-aspartate injection, N-methyl-D-aspartate-mediated brain injury is paradoxically enhanced. The severity of resulting brain injury is 15-25% greater in groups that received MK-801 in comparison with saline-treated controls (P less than 0.001, linear regression analysis). In contrast, the severity of brain injury resulting from intrastriatal injection of the glutamate agonist quisqualate is not altered by a similar 24 h MK-801 pretreatment. Furthermore, the enhanced toxicity of N-methyl-D-aspartate produced by a 24 h pretreatment with MK-801 is completely blocked if a second dose of MK-801 is administered 15 min after the intrastriatal injection of N-methyl-D-aspartate. To determine if MK-801 produced alterations in glutamate receptor pharmacology co-incident with the enhanced toxicity of N-methyl-D-aspartate, in vitro quantitative autoradiography for excitatory amino acid receptor subtypes was performed with [3H]glutamate and [3H]N-1-(2-thienyl)cyclohexyl-3,4-piperidine in seven-day-old rats killed 2 or 24 h after MK-801 (1 mg/kg) administration. A 2 h MK-801 pretreatment produced a 30-50% increase in [3H]glutamate binding at N-methyl-D-aspartate preferring recognition sites in all four brain regions examined (areas CA1 and CA3 of the hippocampus, corpus striatum, cingulate cortex) in comparison with saline-treated controls (P less than 0.05, ANOVA). [3H]N-1-(2-Thienyl)cyclohexyl-3,4-piperidine binding to the phencyclidine site associated with the N-methyl-D-aspartate receptor was reduced by 60-80% in all brain regions examined (P less than 0.001). Quisqualate-sensitive [3H]glutamate binding was not altered by a 2 h MK-801 pretreatment. In animals that received a 24 h MK-801 pretreatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Systemic administration of MK-801 protects against N-methyl-D-aspartate- and quisqualate-mediated neurotoxicity in perinatal rats

MK-801, a non-competitive antagonist of N-methyl-D-aspartate-type glutamate receptors, was tested for its ability to antagonize excitotoxic actions of N-methyl-D-aspartate or quisqualic acid injected into the brains of seven-day-old rats. Stereotaxic injection of N-methyl-D-aspartate (25 nmol/0.5 microliters) or quisqualic acid (100 nmol/1.0 microliter) into the corpus striatum under ether anesthesia consistently produced severe unilateral neuronal necrosis in the basal ganglia, dorsal hippocampus and overlying neocortex. The distribution of the damage corresponded to the topography of glutamate receptors in the vulnerable regions demonstrated by previous autoradiographic studies. N-Methyl-D-aspartate produced severe, confluent neuronal destruction while quisqualic acid typically caused more selective neuronal necrosis. Intraperitoneal administration of MK-801 (0.1-1.0 mg/kg) 30 min before N-methyl-D-aspartate injection had a prominent dose-dependent neuroprotective effects as assessed morphometrically by comparison of bilateral striatal, hippocampal and cerebral hemisphere cross-sectional areas five days later. A 1 mg/kg dose of MK-801 given as pre-treatment completely protected the infant brain. The same dose of MK-801 was also completely protective when administered 30 or 40 min after N-methyl-D-aspartate and afforded partial protection when given 2 h later. MK-801 pre-treatment also prevented the electrically confirmed behavioral seizures induced by N-methyl-D-aspartate. The drug significantly reduced striatal but not hippocampal or neocortical injury when given as two doses (1 mg/kg) 30 min prior to and immediately following quisqualic acid injection. The data indicate that systemic administration of MK-801 can prevent N-methyl-D-aspartate-induced neuronal injury in perinatal rat brain even when administered after the initial insult. MK-801 also partially antagonized quisqualic acid-mediated neurotoxicity, suggesting that quisqualic acid-induced toxicity is, in part, mediated through N-methyl-D-aspartate receptor activation. The sensitivity of the developing brain to the toxicity of N-methyl-D-aspartate provides a sensitive and reproducible in vivo model for exploring these issues and for screening prospective neuroprotective drugs that act at the N-methyl-D-aspartate receptor-channel complex.