Amino acid neurotransmitters and their pathways in the mammalian central nervous system

Pharmacologically distinct sodium-dependent L-[3H]glutamate transport processes in rat brain

The transport of L-[3H]glutamate into crude synaptosomal membrane fractions prepared from cerebellum, brainstem, hippocampus, cortex, striatum, and midbrain was characterized. In all brain regions, greater than 95% of the accumulation of radiolabel was sodium-dependent and the concentration-dependence was consistent with a single high affinity site. Dihydrokainate and L-alpha-aminoadipate were region specific inhibitors of uptake; this inhibition was consistent with a competitive mechanism. In the forebrain regions examined, dihydrokainate inhibited transport with IC50s of approx. 100 microM (range from 80 to 170 microM). Transport in cerebellum was essentially dihydrokainate-insensitive L-alpha-Aminoadipate inhibited transport in forebrain regions with IC50s of approx. 700 microM (range from 590 to 800 microM) and inhibited transport in cerebellum with an IC50 of 40 microM. The inhibition data obtained with forebrain and cerebellar tissues were consistent with nearly homogeneous (greater than 80%) populations of non-interacting sites. Inhibition data obtained with tissue prepared from brainstem were best fit to a mixture of the two sites (35-50% of the type observed in cerebellum). Other previously identified uptake inhibitors, including DL-threo-hydroxyaspartate, L-aspartate-beta-hydroxamate, beta-glutamate, and L-cysteine sulfinate were not selective for the two types of transport. These data demonstrate that there are two pharmacologically distinct sodium-dependent high affinity transport systems with heterogeneous regional distributions.

Aspartate-like immunoreactivity in vestibulospinal neurons in gerbils

In an effort to further characterize vestibulospinal pathways in the gerbil, immunocytochemistry was combined with retrograde identification of neurons. Vestibulospinal neurons were retrogradely labeled following injections of horseradish peroxidase into the cervical cord of anesthetized gerbils. Sections were reacted with nickel acetate-diaminobenzidine for horseradish peroxidase, giving a black reaction product. Sections were incubated in polyclonal antisera to aspartate, incubated in an avidin-biotin-peroxidase procedure, and reacted to give a brown reaction product. Alternatively, fluoro-gold was used as a retrograde tracer and aspartate-like immunoreactivity was demonstrated with avidin conjugated to Texas red. Cells stained with aspartate-like immunoreactivity, were located in all vestibular nuclei. Double-labeled cells were located in the medial nucleus and in the lateral vestibular nucleus where many of the large cells were double labeled.

Age-related decreases of the N-methyl-D-aspartate receptor complex in the rat cerebral cortex and hippocampus

Binding activities of central excitatory amino acid receptors were examined in Triton-treated membrane preparations of the cerebral cortex and hippocampus from brains of rats at 2, 7 and 29 months after birth. Aged rats exhibited a significant reduction of [3H]glutamate (Glu) binding displaceable by N-methyl-D-aspartate (NMDA), as well as strychnine-insensitive [3H]glycine binding in both central structures, as compared with those in young rats. Binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate (MK-801), a non-competitive NMDA antagonist used to label the activated state of ion channels linked to NMDA-sensitive receptors, also decreased with aging irrespective of the experimental conditions employed. Scatchard analysis revealed that reduction of both [3H]Glu binding and [3H]MK-801 binding were due to a significant decrease in the densities of binding sites with aging, with their affinities being unaltered. Binding of [3H]D,L-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), which is a specific agonist for quisqualate-sensitive receptors, was unchanged with aging when determined in the absence of 100 mM potassium thiocyanate (KSCN). However, AMPA binding determined in the presence of added KSCN was about 25% reduced in both brain regions of aged rats. Binding of [3H]kainate to kainate-sensitive receptors was unchanged with aging. These results suggest that glutaminergic neurotransmission mediated by NMDA-sensitive receptors may be selectively impaired with aging in the hippocampus and cerebral cortex among 3 different subclasses of excitatory amino acid receptors in the brain.

Hippocampal damage produced by tetanus toxin in rats can be prevented by lesioning CA1 pyramidal cell excitatory afferents

The neuropathological effects induced by tetanus toxin (TT) bilaterally microinjected into the hippocampus were studied in rats bearing a surgical unilateral lesion of the Schaffer collaterals. TT (1000 mouse minimum lethal doses, MLDs; n = 5 rats) produced neurodegeneration in the CA1 pyramidal cell layer in the unlesioned side of the hippocampus ten days after injection. By contrast, the injection of TT into the lesioned hippocampus produced no degeneration. In rats bilaterally treated with BSA (n = 3 rats) no neuropathological effects were observed in either hippocampi. In conclusion, our results have demonstrated that the lesion of the Schaffer collaterals may protect against the neuropathological effects induced by TT in rats.

Dual effect of glycine on isolated rat suprachiasmatic neurons

Pharmacological properties of strychnine-sensitive and -insensitive glycine receptors have been investigated in rat suprachiasmatic nucleus (SCN) neurons. Because the SCN neurons were too small for stable intracellular recordings by the glass-microelectrode technique, a conventional whole cell mode patch-clamp technique was employed on the acutely dissociated SCN neurons. Dissociated SCN neurons were morphologically heterogeneous and could be distinguished into several types. All cells responded to glycine in a concentration-dependent manner. The glycine-induced current was primarily Cl- sensitive and competitively blocked by strychnine. The SCN neurons also responded to excitatory amino acids: glutamate, quisqualate, kainate, and N-methyl-D-aspartate (NMDA). Responses to glutamate and aspartate, which are endogenous neurotransmitter candidates, were enhanced by adding glycine. Glycine especially augmented the maximum response to NMDA in a full concentration range. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) did not suppress the strychnine-sensitive glycine response but did suppress the strychnine-insensitive NMDA response in a competitive manner for glycine. The results suggest that glycine influences neural activity in the SCN as a classical inhibitory neurotransmitter and an excitatory neuromodulator.

The quantitative autoradiographic distribution of [3H]MK-801 binding sites in the normal human spinal cord

The distribution of NMDA receptors in the normal human spinal cord has been investigated using the non-competitive channel blocking agent MK-801. Specific [3H]MK-801 binding was present throughout the spinal grey matter at all segmental levels, the greatest density of binding being found in the substantia gelatinosa. Focal areas of high binding were also found in a distribution corresponding to lower motor neurones in the ventral horns. This study provides anatomical evidence that NMDA receptors are likely to be important in motor as well as sensory spinal synaptic transmission. The anatomical distribution of NMDA receptors in relation to motor neurone somata may have important implications in selective vulnerability to excitotoxic injury.

Cortical ablation reduces veratridine evoked release of endogenous glutamate from superfused substantia nigra slices

A brain slice superfusion system and HPLC coupled to fluorometric detection were used to study depolarization-evoked release of endogenous glutamate (Glu) and aspartate (Asp) from substantia nigra (SN) slices. Veratridine (15-25 microM) and K+ (50 mM) evoked both a Ca2(+)-dependent and Ca2(+)-dependent release of Glu and Asp. However, the pattern of Glu release was different from that of Asp. In addition, cortical ablation significantly reduced the evoked release of Glu but not that of Asp. There results indicate the existence in SN of functional Glu- and Asp-releasing nerve terminals. Some of the Glu-enriched nerve terminals might belong to a cortico-nigral neuronal pathway.

Evolutionary aspects of transmitter molecules, their receptors and channels

Classical transmitters are present in all phyla that have been studied; however, our detailed understanding of the process of neurotransmission in these phyla is patchy and has centred on those neurotransmitter receptor mechanisms which are amenable to study with the tools available at the time, for example, high-affinity ligands, tissues with high density of receptor protein, suitable electrophysiological recording systems. Studies also clearly show that many neurones exhibit co-localization of classical transmitters and neuropeptides. However, the physiological implications of this co-localization have yet to be elucidated in the vast majority of examples. The application of molecular biological techniques to the study of neurotransmitter receptors (to date mainly in vertebrates) is contributing to our understanding of the evolution of these proteins. Striking similarities in the structure of ligand-gated receptors have been revealed. Thus, although ligand-gated receptors differ markedly in terms of the endogenous ligands they recognize and the ion channels that they gate, the structural similarities suggest a strong evolutionary relationship. Pharmacological differences also exist between receptors that recognize the same neurotransmitter but in different phyla, and this may also be exploited to further the understanding of structure-function relationships for receptors. Thus, for instance, some invertebrate GABA receptors are similar to mammalian GABAA receptors but lack a modulatory site operated by benzodiazepines. Knowledge of the structure and subunit composition of these receptors and comparison with those that have already been elucidated for the mammalian nervous system might indicate the functional importance of certain amino acid residues or receptor subunits. These differences could also be exploited in the development of new agents to control agrochemical pests and parasites of medical importance. The study of the pharmacology of receptor proteins for neurotransmitters in invertebrates, together with the application of biochemical and molecular biological techniques to elucidate the structure of these molecules, is now gathering momentum. For certain receptors, e.g. the nicotinic receptor, we can expect to have fundamental information on the function of this receptor at the molecular level in both invertebrates and vertebrates in the near future.

Recovery of amino acid neurotransmitters from the spinal cord during posterior epidural stimulation: a preliminary study

Continuous posterior epidural spinal cord stimulation (SCS) has been an effective method for treating spasticity. The mechanisms of SCS include activation of inhibitory segmental neuronal systems and suprasegmental structures that produce inhibitory descending control. The neurochemical correlates of the mechanism of action have not been clearly defined. Microdialysis of the spinal cord extracellular space in an in vivo preparation during continuous epidural SCS was performed. The recovery of amino acid neurotransmitters, aspartate, glutamate, gamma-aminobutyric acid (GABA), glycine, and taurine from stimulated animals was compared to non-stimulated animals. Evoked potentials from the cortex and spinal cord of the stimulated animals were recorded to insure that there had been adequate epidural stimulation and normal segmental cord function. A significant increase in the concentration of glycine was seen after 90 minutes of continuous stimulation. The levels of the other amino acids were not significantly elevated. These results suggest that amelioration of spasticity with epidural SCS may involve enhanced glycine release, the major inhibitory neurotransmitter of the spinal cord.

Segmental release of amino acid neurotransmitters from transcranial stimulation

The present study used microdialysis techniques in an intact rabbit model to measure the release of amino acids within the lumbar spinal cord in response to transcranial electrical stimulation. Dialysis samples from the extracellular space were obtained over a stimulation period of 90 minutes and were examined using high pressure liquid chromatography. Neuronal excitation was verified by recording corticomotor evoked potentials (CMEPs) from the spinal cord. A significant increase in the release of glycine and taurine compared to sham animals was measured after 90 minutes of transcranial stimulation. Glutamate and aspartate release was not significantly elevated. GABA concentrations were consistently low. CMEP components repeatedly showed adequate activation of descending fiber pathways and segmental interneuron pools during dialysis sampling. Since glycine, and to a lesser extent taurine, have been shown to inhibit motor neuron activity and are closely associated with segmental interneuron pools, suprasegmental modulation of motor activity may be, in part, through these inhibitory amino acid neurotransmitters in the rabbit lumbar spinal cord.

Glutamate immunoreactivity in the rat basilar pons: light and electron microscopy reveals labeled boutons and cells of origin of afferent projections

Immunohistochemical methods that employed a polyclonal antiserum directed against a glutamate-hemocyanin conjugate were utilized to examine the rat basilar pontine nuclei at both light and electron microscopic levels in order to identify putative glutamatergic neural elements. A large number of cells ranging in size from 11 to 32 microns in diameter and present in all subdivisions and at all rostrocaudal levels of the basilar pons exhibited intense glutamate immunoreactivity. Immunoreactive punctate structures, confirmed by electron microscopy to be axon terminals, were homogeneously distributed throughout the pontine neuropil, although a somewhat greater accumulation was apparent medially at mid-levels of the basilar pons and laterally at more caudal levels. Immunolabeled axons were also present throughout the pontine nuclei. In order to demonstrate possible extrinsic sources of glutamate-immunoreactive axon terminals within the pontine gray, injections of wheat germ agglutinin-horseradish peroxidase were made directly into the basilar pons. Tissue was then evaluated for the presence of retrogradely transported wheat germ agglutinin-horseradish peroxidase and the same tissue sections processed for glutamate immunocytochemistry. Following this combined protocol, neuronal somata exhibiting both wheat germ agglutinin-horseradish peroxidase and glutamate immunoperoxidase reaction products were observed within layer Vb of the cerebral cortex, zona incerta, the dentate nucleus of the cerebellum, nucleus paragigantocellularis of the medullary reticular formation, and the dorsal column nuclei. Such double-labeled cells were considered to represent glutamatergic neurons that provide axonal projections to the basilar pons. Ultrastructural studies of the pontine nuclei confirmed the presence of glutamate immunogold labeling in dendrites, neuronal somata, axons, and axon terminals. Immunoreactive boutons contained round vesicles and primarily formed asymmetric synapses at various postsynaptic loci which included glutamate-immunolabeled dendritic profiles and somata. These results suggest that glutamatergic basilar pontine neurons form one segment of a multisynaptic pathway involving glutamatergic afferents to the basilar pons, glutamatergic pontocerebellar projection neurons, and the glutamatergic granule cells of the cerebellar cortex.

Aspartate-like immunoreactivity in primary afferent neurons

There is now good evidence that amino acids act as neurotransmitters in primary afferent neurons of dorsal root ganglia. Glutamate is the primary candidate for such a role, and there are reasons to believe that release of glutamate may be accompanied by the release of other neuroactive substances. Using immunocytochemical techniques, we have tested the hypothesis that some dorsal root ganglion neurons contain elevated levels of aspartate as well as glutamate. Antisera raised against conjugates of aspartate or glutamate were used for this purpose. Blocking experiments confirmed that these antibodies were specific to their antigens in cryostat sections of dorsal root ganglia. Aspartate immunoreactivity was found in approximately 30% of neurons in cervical dorsal root ganglia. The relation between cell size and staining intensity for aspartate was examined using quantitative video microscopy; the great majority of cells immunopositive for aspartate were small (15-30 microns in diameter); about 85% of these cells stained for aspartate, although staining intensities varied over a wide range. By reacting consecutive sections with anti-aspartate and anti-glutamate it was shown that elevated levels of aspartate were found in the same cells which contained elevated levels of glutamate. By measuring the staining intensity of individual cells for both aspartate and glutamate, it was also shown that there was a positive correlation between staining intensity and, presumably, concentration of the two amino acids. The presence of high levels of aspartate in terminals located in the superficial laminae of the dorsal horn was verified by pre- and post-embedding immunocytochemistry with the electron microscope. Aspartate was demonstrated in scalloped terminals, including dark scalloped terminals believed to be associated with unmyelinated fibers of nociceptors. This evidence supports the hypothesis that aspartate as well as glutamate is present in the cell bodies and terminals of nociceptive primary afferents, and may be released by the terminals of these afferents to activate neurons in the superficial laminae of the dorsal horn.

Tectospinal neurons in hamster contain glutamate-like immunoreactivity

Immunocytochemistry with a monoclonal antibody directed against glutamate revealed numerous immunoreactive cells in the hamster's superior colliculus (SC). A large number of these neurons were located in the deep layers and many were in the stratum album intermedium (SAI). These neurons appeared similar to the large multipolar cells that have been shown to project to the cervical spinal cord in this species. The combination of retrograde tracing using either Fluoro-Gold- or FITC-labelled latex beads with immunocytochemistry for glutamate revealed that many of the immunoreactive cells did, in fact, project into the predorsal bundle and extend axons as far as the cervical spinal cord.

GABA-like immunoreactivity in a population of locust intersegmental interneurones and their inputs

Intracellular labelling of locust intersegmental interneurones with lucifer yellow or horseradish peroxidase was carried out in combination with light and electron microscope immunocytochemistry by using an antibody raised against gamma amino butyric acid (GABA). Fifteen percent (four out of 27) of intracellularly stained interneurones showed GABA-like immunoreactivity. This is in agreement with previous physiological observations that 20% of the interneurones in this population make inhibitory output connections in the metathoracic ganglion. GABA-like immunoreactivity was also found in processes presynaptic to the interneurones in the mesothoracic ganglion. The presence of such immunoreactive inputs onto the intersegmental interneurones correlates well with physiological evidence that their receptive fields are in part shaped by direct input from GABA-ergic spiking local interneurones.

Sources of presumptive glutamatergic/aspartatergic afferents to the magnocellular basal forebrain in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of [3H]-D-aspartate into the magnocellular basal forebrain of the rat was compared with the distribution of neurons labeled by comparable injections of the nonspecific retrograde axonal tracer wheat germ agglutinin conjugated to horseradish peroxidase. Cells retrogradely labeled by wheat germ agglutinin-horseradish peroxidase were found in a wide range of limbic and limbic-related structures in the forebrain and brainstem. In the telencephalon, labeled neurons were seen in the orbital, medial prefrontal, and agranular insular cortical areas, the amygdaloid complex, and the hippocampal formation. Labeled cells were also seen in the olfactory cortex, the lateral septum, the ventral striatopallidal region, and the magnocellular basal forebrain itself. In the diencephalon, neurons were labeled in the midline nuclear complex of the thalamus, the lateral habenular nucleus, and the hypothalamus. In the brainstem, labeled cells were found bilaterally in the ventral midbrain, the central gray, the reticular formation, the parabrachial nuclei, the raphe nuclei, the laterodorsal tegmental nucleus, and the locus coeruleus. A significant fraction of the afferents to the magnocellular basal forebrain appear to be glutamatergic and/or aspartatergic. Only a few of the regions labeled with wheat germ agglutinin-horseradish peroxidase were not also labeled with [3H]-D-aspartate in the comparable experiments. Most prominent among the non-glutamatergic/aspartatergic projections were those from fields CA1 and CA3 of the hippocampus, the hilus of the dentate gyrus, the dorsal subiculum, the tuberomammillary nucleus, and the ventral pallidum. In addition, most of the lateral hypothalamic and brainstem projections to the magnocellular basal forebrain were not significantly labeled with [3H]-D-aspartate. In addition to these inputs, a commissural projection from the region of the contralateral nucleus of the horizontal limb of the diagonal band was confirmed with both wheat germ agglutinin-horseradish peroxidase and the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin. This projection did not label with [3H]-D-aspartate or [3H]-GABA, suggesting that it is not glutamatergic/aspartatergic or GABAergic. Furthermore, double labeling experiments with the fluorescent retrograde tracer True Blue and antibodies against choline acetyltransferase indicate that the projection is not cholinergic.

Excitatory and inhibitory transmission from dorsal root afferents to neonate rat motoneurons in vitro

Intracellular recordings were made from antidromically identified motoneurons in neonate (12-22 days) rat transverse spinal cord slices and the transmitters and receptors probably involved in initiating the excitatory (EPSP) and inhibitory (IPSP) postsynaptic potentials were investigated. Stimulation of dorsal roots elicited in motoneurons an EPSP, an IPSP, or an EPSP followed by an IPSP. EPSPs in 70% of motoneurons had a short latency (less than or equal to 1 ms) and in the remaining cells a latency longer than 1 ms. The IPSPs had a long latency (greater than or equal to 1 ms). Short- and long-latency EPSPs were enhanced by the acidic amino acid uptake inhibitor L-aspartic acid-beta-hydroxamate (AAH) and depressed by the non-selective glutamate receptor antagonists gamma-D-glutamylglycine (DGG) and kynurenic acid. Short-latency EPSPs were suppressed by the quisqualate/kainate (QA/KA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) but not by the N-methyl-D-aspartate (NMDA) receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (APV) and ketamine. Long-latency EPSPs were reduced by DNQX as well as by APV and ketamine. Superfusion of the slices with a Mg-free solution increased the EPSPs and unmasked a late, APV-sensitive component. The IPSP was reduced by the glycine antagonist strychnine as well as by APV and ketamine but resistant to DNQX. The results indicate that stimulation of dorsal roots elicited in motoneurons a monosynaptic EPSP mediated by glutamate/aspartate acting predominantly on the QA/KA subtype of glutamate receptors; an NMDA component can be unveiled in Mg-free solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Quisqualate- and NMDA-sensitive [3H]glutamate binding in primate brain

Excitatory amino acids (EAA) such as glutamate and aspartate are probably the neurotransmitters of a majority of mammalian neurons. Only a few previous studies have been concerned with the distribution of the subtypes of EAA receptor binding in the primate brain. We examined NMDA- and quisqualate-sensitive [3H]glutamate binding using quantitative autoradiography in monkey brain (Macaca fascicularis). The two types of binding were differentially distributed. NMDA-sensitive binding was most dense in dentate gyrus of hippocampus, stratum pyramidale of hippocampus, and outer layers of cerebral cortex. Quisqualate-sensitive binding was most dense in dentate gyrus of hippocampus, inner and outer layers of cerebral cortex, and molecular layer of cerebellum. In caudate nucleus and putamen, quisqualate- and NMDA-sensitive binding sites were nearly equal in density. However, in globus pallidus, substantia nigra, and subthalamic nucleus, quisqualate-sensitive binding was several-fold greater than NMDA-sensitive binding. In thalamus, [3H]glutamate binding was generally low for both subtypes of binding except for the anterior ventral, lateral dorsal, and pulvinar nuclei. In the brainstem, low levels of binding were found, and strikingly the red nucleus and pons, which are thought to receive glutamatergic projections, had approximately 1/20 the binding observed in cerebral cortex. These results demonstrate that NMDA- and quisqualate-sensitive [3H]glutamate binding are observed in all regions of primate brain, but that in some regions one subtype predominates over the other. In addition, certain areas thought to receive glutamatergic projections have low levels of both types of binding.

Prevention by the NMDA receptor antagonist, MK801 of neuronal loss produced by tetanus toxin in the rat hippocampus

1. The behavioural and neuropathological effects of tetanus toxin, microinjected directly into the hippocampus, were studied in rats. 2. A single dose (1000 minimum lethal doses, MLDs) of tetanus toxin, injected unilaterally into the hippocampus produced a time-dependent neuronal loss in the CA1 pyramidal cell layer. In comparison with the contralateral, untreated side these effects became statistically significant (P less than 0.05) 7 days (22.0 +/- 1.1% reduction) and 10 days (29.2 +/- 1.7% reduction) after the injection. No significant changes were observed 7 days after treatment with 500 MLDs whereas a reduction of 37.5 +/- 3.1% in the CA1 area cell number was produced 4 days after the injection of 2000 MLDs. 3. Behavioral stimulatory effects were also induced by tetanus toxin (1000 MLDs) within 48 h of the injection and these culminated in generalized convulsions 5-7 days later. Convulsions were observed after a shorter period of latency in rats receiving 2000 MLDs tetanus toxin whereas 500 MLDs were ineffective. 4. No behavioural and neuropathological effects were observed in rats treated with neutralized tetanus toxin (1000 MLDs), bovine serum albumin or phosphate buffer. 5. Pretreatment with MK801 (0.3 mg kg-1, i.p., given 1 h before and after the injection with tetanus toxin and then once daily for 4 or 7 days) prevented the behavioural and neuropathological effects induced by tetanus toxin (1000-2000 MLDs). In addition, such treatment fully protected the animals from the lethal effects induced by 1000 MLDs tetanus toxin. In addition, such treatment fully protected the animals from the lethal effects induced by 1000MLDs tetanus toxin. By contrast, pretreatment with diazepam (3.Omgkg-1, i.p.) using the same schedule as for MK801 did not antagonize the effects of tetanus toxin (1000-2000 MLDs). 6. In conclusion, the present experiments have demonstrated that the intrahippocampal injection of tetanus toxin produces in rats a dose- and time-dependent behavioural stimulation and neuronal loss in the CAl pyramidal cell layer which can be prevented by the non-competitive NMDA antagonist, MK801.

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.

Results of N-methyl-D-aspartate antagonists in perinatal cerebral asphyxia therapy

Perinatal cerebral asphyxia, which results in significant neurologic and cognitive disabilities in infants and children, remains a major health problem. Potential neurologic sequelae include cerebral palsy, mental retardation, and epilepsy. Over the next few years, neuroprotective agents that prevent asphyxial neuronal injury and death are likely to be developed. These agents may also be effective in prophylaxis and treatment of chronic neurologic disorders, including epilepsy and neurodegenerative disorders, such as Huntington disease.

Abnormal distribution of phosphorylated neurofilaments in neuronal degeneration induced by kainic acid

An abnormal distribution of phosphorylated neurofilaments is present in some human neurodegenerative disorders including Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis. This study reports the changes of phosphorylated neurofilaments observed in rat spinal cord after intrathecal injection of kainic acid. This receptor agonist of excitatory amino acid produces abnormal phosphorylation of neurofilaments in the cell body and proximal neurites of degenerating neurons. These immunocytochemical modifications observed 2 and 10 days after injections are predominantly located in ventral horn neurons. This study indicates that one of the neuronal responses to excitatory amino acid toxicity is the pathological distribution of phosphorylated neurofilaments in affected neurons. Pathological findings are comparable to those observed in neurodegenerative diseases such as amyotrophic lateral sclerosis.

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.

Distribution of phosphate-activated glutaminase in the human cerebral cortex

Phosphate-activated glutaminase (PAG), which catalyses conversion of glutamine to glutamate, is a potential marker for glutamatergic, and possibly GABA, neurons in the central nervous system. A polyclonal antibody, raised in rabbits against rat brain PAG, was applied to postmortem human brain tissue to reveal the distribution of PAG in the cerebral cortex. PAG immunoreactivity was observed in pyramidal and non-pyramidal neurons but not in glial cells. In the neocortex, large to medium-sized pyramidal neurons in layers III and V were stained most intensely, while the majority of smaller pyramidal cells were labeled either lightly or moderately. Such modified pyramids as the giant Betz cells, the large pyramidal cells of Meynert, and the solitary cells of Ramón y Cajal were also stained intensely. Fusiform cells in layer VI showed moderate to intense labeling. A number of cortical non-pyramidal neurons of various sizes stained moderately to intensely. These included large basket cells which were identified by their characteristic morphology and size in primary cortical areas. Pyramidal cells in the hippocampal formation as well as basket cells of the stratum oriens stained moderately to intensely. Since pyramidal cells are believed to be glutamatergic and large basket cells GABAergic, these results suggest that PAG plays a role in generating not only transmitter glutamate, but also GABA precursor glutamate.

N-methyl-D-aspartate receptors mediate responses of rat dorsal horn neurones to hindlimb ischemia

We have investigated the role of NMDA receptors in the rat dorsal horn in mediating neuronal responses to noxious hindlimb ischemia, induced by acute occlusion of the femoral artery, as well as in the hyperalgesia evident when noxious mechanical stimuli were applied to the ischemic limb. Two specific NMDA antagonists, D-2-amino-5-phosphonovalerate (APV) and ketamine hydrochloride were applied intrathecally directly on to the spinal cord, in enflurane-anaesthetised rats. Both APV (1 microM and 100 microM) and ketamine (1 mM and 100 mM) inhibited the increase of dorsal horn neuronal firing rate induced by ischemia, but did not alter the neuronal response to noxious pinching or innocuous brushing of the receptive field. Both agents, however, abolished the hyperalgesia to noxious pinching induced by ischemia. Our results support the hypothesis that the excitatory amino acids are involved in the transmission of nociceptive information in the spinal dorsal horn, and also favour a central mechanism for hyperalgesia at the spinal level, possibly also mediated by the NMDA receptor.

Glutamate-like immunoreactivity in the peripheral vestibular system of mammals

Using a specific antibody raised against glutamate (Glu) conjugated to bovine serum albumin with glutaraldehyde, the distribution of Glu-like immunoreactivity was studied by postembedding staining in semithin sections of nonosmicated or osmicated tissue through the vestibular sensory epithelia and ganglia of different mammalian species (mouse, rat and cat). Strong immunoreactive staining was found in all ganglion neurons and their peripheral and central nerve processes as well as in the two types of sensory hair cells whereas, in contrast, supporting cells were devoid of immunoreactivity. Glu-like immunoreactivity found in vestibular fibers and ganglion neurons, is in good agreement with the proposition of glutamate as the neurotransmitter involved in vestibular nerve transmission. In sensory hair cells, glutamate, apart from its metabolic function, may play a role in synaptic transmission between the sensory cells and the vestibular afferent fibers.

Stimulation of dopamine release from cultured rat mesencephalic cells by naturally occurring excitatory amino acids: involvement of both N-methyl-D-aspartate (NMDA) and non-NMDA receptor subtypes

In rat mesencephalic cell cultures, L-glutamate at concentrations ranging from 100 microM to 1 mM stimulated release of [3H]dopamine that was attenuated by the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 6,7-dinitroquinoxalinedione, but not by the selective NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; 10 microM) and 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (300 microM). Even at 1 mM glutamate, this release was Ca2+ dependent. These observations suggest that the release was mediated by a non-NMDA receptor. Only release stimulated by a lower concentration (10 microM) of glutamate was inhibited by MK-801 (10 microM), indicating that glutamate at this concentration activates the NMDA receptor. By contrast, L-aspartate at concentrations of 10 microM to 1 mM evoked [3H]dopamine release that was completely inhibited by MK-801 (10 microM) and was also Ca2+ dependent (tested at 1 and 10 mM aspartate). Thus, effects of aspartate involved activation of the NMDA receptor. Sulfur-containing amino acids (L-homocysteate, L-homocysteine sulfinate, L-cysteate, L-cysteine sulfinate) also evoked [3H]dopamine release. Release evoked by submillimolar concentrations of these amino acids was attenuated by MK-801 (10 microM), indicating involvement of the NMDA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinal cord ischemia-induced elevation of amino acids: extracellular measurement with microdialysis

Excitatory amino acids have been implicated in the production of calcium mediated neuronal death following central nervous system ischemia. We have used microdialysis to investigate changes in the extracellular concentrations of amino acids in the spinal cord after aortic occlusion in the rabbit. Glutamate, aspartate, glutamine, asparagine, glycine, taurine, valine, and leucine were measured in the microdialysis perfusate by high pressure liquid chromatography. The concentrations of glutamate, glycine, and taurine were significantly higher during ischemia and reperfusion than controls. Delayed elevations in the concentrations of asparagine and valine were also detected. The elevation of glutamate is consistent with the hypothesis that excitotoxins may mediate neuronal damage in the ischemic spinal cord. Increased extracellular concentrations of asparagine and valine may reflect preferential use of amino acids for energy metabolism under ischemic conditions. The significance of increased concentrations of inhibitory amino acid neurotransmitters is unclear.

Effect of nimodipine on ischemia-induced brain edema and mortality in a novel transient middle cerebral artery occlusion model

A novel transient middle cerebral artery (MCA) occlusion model in the rat was used to evaluate the effect of nimodipine on brain edema and mortality. Nimodipine (30 micrograms/kg) administered immediately after 3 hr of transient unilateral MCA occlusion attenuated significantly the post-ischemic increase of tissue water content and partly attenuated 45Ca accumulation in the parieto-temporal cortex ipsilateral to the left MCA occlusion 3 hr after reperfusion. Nimodipine decreased the mortality rate at 6 and 9 hr after recirculation, although the survival rate at 24 hr after recirculation was not different from the control group. These results suggest that nimodipine has beneficial effects in the early phase of the reperfusion period.

Immunocytochemical characterization of supporting cells in the enteric nervous system in Hirschsprung's disease

The enteric nervous system (ENS) is composed of two distinct neural components, extrinsic and intrinsic, and its supporting cells uniquely possess some characteristics of both central nervous system (CNS) astrocytes and peripheral nervous system (PNS) Schwann cells. To provide further insight into the neural defects in Hirschsprung's disease, the supporting cells in biopsied normal gut, ganglionic, and aganglionic segments from six cases of Hirschsprung's disease were investigated immunocytochemically for localization of three neuroglial markers, glial fibrillary acidic protein (GFAP), S-100 protein, and glutamine synthetase (GS), by the avidin-biotin-horseradish peroxidase complex method applied to free-floating thick cryostat sections. In normal control gut and ganglionic segments of Hirschsprung's colon, all of the GFAP, S-100, and GS were expressed strongly by the supporting cells of the myenteric and submucosal plexuses, interconnecting nerve fiber bundles of the plexuses, and fine nerve strands in the muscular layer. The nerve bundles of the subserosa merging into the muscular layer were also immunoreactive for GFAP and S-100, but negative or only faintly positive for GS. On the other hand, aberrantly proliferated nerve bundles in the aganglionic segment of the Hirschsprung's colon were accompanied by supporting cells strongly positive for GFAP and S-100, but negative or faintly positive for GS. These results indicate that the supporting cells of the enteric neurons proper, enteric glia, express GFAP, S-100, and GS, whereas the supporting cells of the extrinsic components, which accompany PNS axons, are negative or very weakly positive for GS. Thus, GS immunocytochemistry may delineate intrinsic and extrinsic neural components in the ENS, and may provide an important clue for differential diagnosis of Hirschsprung's disease.

Localization of GABAA-receptor alpha 1 subunit mRNA-containing neurons in the lower brainstem of the rat

The localization of gamma-aminobutyric acid-A (GABAA) receptors (GABAA-R) in the lower brainstem of the rat was examined by means of in situ hybridization histochemistry using an oligonucleotide probe to the sequence of the alpha 1 subunit (GABAA-R alpha 1). Strongly labeled neurons were found in the cranial motor nuclei, the dorsal motor nucleus of the vagus, reticular formation (large neurons), lateral vestibular nucleus, dorsal nucleus of the lateral lemniscus, central nucleus of the inferior colliculus, intermediate and white layers of the superior colliculus, red nucleus and substantia nigra. In addition, moderately labeled cells were abundant in the nucleus of the solitary tract, medial and inferior vestibular nuclei, parabrachial area, dorsal and ventral tegmental nuclei of Gudden, central gray matter, ventral nucleus of the lateral lemniscus, and reticular formation (small neurons). This study has therefore revealed some of the target neurons of GABA-containing fibers in the lower brainstem.

Very rapid and simple assay of taurine in the brain within two minutes by high-performance liquid chromatography with electrochemical detection

We describe a very rapid and simple method for the assay of taurine (Tau) in the brain tissue, using high-performance liquid chromatography (HPLC) with electrochemical detection in combination with precolumn o-phthalaldehyde (OPA) derivatization. The present method permits Tau assay within 2 min in one chromatographic run. Recovery for Tau was 107.4 +/- 1.3% (SD, n = 10). Within-run coefficients of variation were +/- 1.6% (n = 15). The limit of quantitative detection of the method was 0.1 pmol for Tau. The present method has been applied to the measurement of Tau levels in several discrete brain areas of the mouse.

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.

Medioventral medulla-induced locomotion

Previous anatomical studies demonstrated the presence of descending projections from the physiologically identified mesencephalic locomotor region (MLR) to the medioventral medulla (MED) in the cat. The present experiments were designed to determine if a similar low threshold locomotion-inducing area is present in the rat medulla. In addition, the nature of the neurochemical control of this area of the brain was explored using localized injections of neurochemical agents in the decerebrate rat during locomotion on a treadmill. A region virtually identical to that reported in the cat was found to lead to controlled locomotion on a treadmill following stimulation at low amplitude currents (less than or equal to 60 microA). Injections of cholinergic agonists into the MED of the rat induced locomotion which could be blocked by injections of cholinergic antagonists. In addition, injections of GABA antagonists were found to induce stepping which could be blocked by injections of GABA or GABA agonists. Substance P (SP) also was found to induce walking following injection into the MED of the rat. Injections of an excitatory amino acid agonist (NMDA) also were found to induce locomotion in the rat. These effects were blocked by injections of an excitatory amino acid antagonist (APV). Since these results had not been reported for the cat MED, a short series of experiments revealed that the MED in the cat also responded to NMDA.

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.

Age-related alteration in excitatory amino acid neurotransmission in rat brain

The excitatory amino acids as neurotransmitters in the neocortex, hippocampus, striatum, thalamus, amygdala, nucleus basalis of Meynert and cerebellum from rats aged 4 months, 12 months and 24 months have been examined by measuring sodium-dependent high affinity uptake of D-[3H]-aspartate into preparations containing synaptosomes. Calcium-dependent K(+)-stimulated release of endogenous glutamate from the nucleus basalis was also measured. The hippocampus and cerebellum failed to show significant age-related changes in uptake of D-[3H]-aspartate. D-[3H]-aspartate uptake decreased significantly in the neocortex (29%), striatum (29%), nucleus basalis (26%), amygdala (19%) and thalamus (16%) in the middle-aged rats as compared to the young rats, but the changes were not progressive with age. The release of glutamate from the nucleus basalis was unaltered during the aging process.

The effects of dizocilpine maleate (MK-801), an antagonist of the N-methyl-D-aspartate receptor, on neurologic recovery and histopathology following complete cerebral ischemia in primates

The present study was designed to determine if the noncompetitive excitatory amino acid antagonist, dizocilpine maleate, when administered after a 17 min period of complete cerebral ischemia in primates, would improve postischemic neurologic function and hippocampal histopathologic outcome when compared to placebo-treated animals. Ten pigtail monkeys were anesthetized and subjected to complete cerebral ischemia using an established neck tourniquet model. Five minutes postischemia, five monkeys received dizocilpine 300 micrograms/kg i.v. over 5 min, followed by an infusion of 150 micrograms/kg/h for 10 h. This produced plasma levels of the drug in excess of 30 ng/ml for the duration of the infusion. An additional five monkeys were treated with an identical volume of saline placebo. All monkeys received intensive care for the initial 24 to 48 h postischemia. At 96 h postischemia, there was no significant difference in neurologic function between the two groups (p = 0.53, with the placebo group having the numerically better outcome). There also was no significant difference between hippocampal histopathology scores between dizocilpine and placebo-treated monkeys. The authors conclude that dizocilpine is not an efficacious therapy in the treatment of neurologic injury that occurs following complete cerebral ischemia in this primate model.

Electrical stimulation-evoked release of endogenous aspartate from rat medulla oblongata slices. Effects of inhibitors of aspartate aminotransferase and GABA transaminase

The effects of aminooxyacetic acid (AOAA), an aspartate aminotransferase (AAT) inhibitor, L-canaline, an ornithine aminotransferase inhibitor, and gamma-acetylenic GABA and gabaculine, both gamma-aminobutyric acid transaminase (GABA-T) inhibitors, on the release of aspartate from slices of rat medulla oblongata and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2(+)-dependent stimulus-evoked release of endogenous aspartate. AOAA (10(-4) and 10(-3) M) decreased the evoked release of aspartate in the medulla oblongata but not in the hippocampus. In addition, AOAA produced a decrease in the spontaneous efflux and tissue content of aspartate in the medulla oblongata. L-Canaline (5 x 10(-5) M), gamma-acetylenic GABA (10(-4) M) and gabaculine (10(-5) M) did not affect the evoked release of aspartate in the medulla oblongata, while these agents produced a decrease in spontaneous efflux and tissue content of aspartate. These findings suggest that AAT participates in the synthesis of transmitter aspartate in the medulla oblongata of the rat. It appears that there are the pools of transmitter aspartate and non-transmitter aspartate in the rat medulla oblongata.

Endogenous aspartate release in the rat hippocampus is inhibited by M2 'cardiac' muscarinic receptors

The release of endogenous aspartic acid elicited by depolarization of rat hippocampus synaptosomes with 15 mM KCl was totally calcium-dependent. Acetylcholine (ACh) added to the superfusion medium inhibited the K(+)-evoked release of aspartate in a concentration-dependent manner. The effect of ACh was mimicked by oxotremorine and carbachol. It was insensitive to the nicotinic receptor antagonist mecamylamine but blocked by the non-selective muscarinic receptor antagonist atropine. Further pharmacological characterization of the muscarinic receptor involved showed that the ACh effects was insensitive to the M1 selective muscarinic receptor antagonists pirenzepine and dicyclomine. However, the inhibition by ACh of aspartate release was counteracted by 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido-[2-3-b][1,4]benzodiazepine-6-one (AF-DX 116), a selective M2 'cardiac' receptor antagonist. The calcium dependence of the release of aspartate and its regulation through presynaptic receptors are suggestive of a transmitter role for this excitatory amino acid. Moreover, the similarities between the present results and those previously obtained with glutamate are compatible with the idea that aspartate and glutamate are co-released in the rat hippocampus.

Phaclofen-insensitive presynaptic inhibitory action of (+/-)-baclofen in neonatal rat motoneurones in vitro

1. Intracellular recordings were made from antidromically identified motoneurones in transverse spinal cord slices from neonatal (12-16 day) rats. 2. Superfusion of (+/-)-baclofen (0.5-50 microM) reduced the excitatory postsynaptic potentials (e.p.s.ps) and inhibitory postsynaptic potentials (i.p.s.ps) evoked by dorsal root or dorsal root entry zone stimulation in a concentration-dependent manner; the calculated EC50 was 2.4 microM. Baclofen in comparable concentrations also reversibly eliminated spontaneously occurring e.p.s.ps and i.p.s.ps. 3. (-)-Baclofen was more effective as compared to baclofen in reducing the synaptic responses, whereas (+)-baclofen at concentrations as high as 50 microM was ineffective. 4. Baclofen (less than 5 microM) attenuated the synaptic responses without causing a significant change of passive membrane properties and depolarizations induced by exogenously applied glutamate. In addition to synaptic depression, baclofen (greater than 5 microM) caused a hyperpolarization associated with decreased membrane resistance in some of the motoneurones; the glutamate responses were also attenuated. 5. Baclofen reversibly depressed the spike after-hyperpolarization of the motoneurones. 6. GABA (1-10 mM) depressed synaptic transmission and depolarized or hyperpolarized motoneurones. While potentiated by the uptake inhibitor nipecotic acid, the synaptic depressant effect of GABA was not antagonized by bicuculline. 7. The synaptic depressant effect of baclofen was neither blocked by GABAA antagonists bicuculline and picrotoxin (10-50 microM) nor by the GABAB antagonist phaclofen (0.1-1 mM). 8. It is suggested that baclofen depresses excitatory and inhibitory transmission in rat motoneurones by primarily a presynaptic mechanism in reducing the liberation of chemical transmitters from nerve endings via a phaclofen-insensitive GABAB receptor.

Immunohistochemical demonstration of glutamate dehydrogenase in the postnatally developing rat hippocampal formation and cerebellar cortex: comparison to activity staining

Distribution patterns of activity and immunohistochemical staining for glutamate dehydrogenase were compared during the postnatal development of rat hippocampal formation and cerebellar cortex. On postnatal day 5, dendritic layers of the hippocampal formation showed only a very weak enzyme activity. Similarly, when studied at the same age, the external granule cell layer and Purkinje cells of the cerebellar cortex exhibited a very faint and moderate staining, respectively. With advancing age, in both brain regions a marked postnatal increase in glutamate dehydrogenase activity occurred in neuropil area as glutamatergic structures matured. However, compared to activity staining, both brain regions of early postnatal stages showed a relatively high level of glutamate dehydrogenase-like immunoreactivity. In this case, the immunohistochemical staining of hippocampal dendritic layers and of the molecular layer of the cerebellar cortex was rather diffuse, being not very similar to parameters of the maturation of the respective glutamatergic structures. In contrast to the activity staining for the enzyme, the immunohistochemical labelling in adult rats revealed a selective predominance of immunoreactivity in astroglial cells from postnatal day 5 onwards. The Bergmann glia in the cerebellar cortex exhibited the strongest intensity of immunoreactivity. Generally, the patterns of immunoreactivity were found to depend on the fixation procedure adopted. Concluding from our results, glutamate dehydrogenase is demonstrable in glial and in neuronal cell elements as well. Therefore, it is recommended that activity staining and the immunohistochemical procedure be combined to study qualitative and quantitative aspects of glutamate dehydrogenase in nervous tissues.

Quantitative analysis of gamma-aminobutyric acid (GABA) receptors of Purkinje cell layer from rat cerebellar slices

1. Extracellular recordings were made from Purkinje cell layer of rat cerebellar slices. Cell activity was sensitive to both magnesium and manganese ions. 2. Glutamic and aspartic acids both excited cell activity while gamma-aminobutyric acid (GABA), muscimol, taurine, beta-alanine and delta-aminolaevulinic acid all inhibited activity. 3. The sensitivity to GABA varied with depth while no such effect was observed with muscimol. 4. Bicuculline methiodide, picrotoxin and pitrazepin blocked the action of muscimol with pA2 values of 5.92, 5.97 and 5.71 respectively. 5. The benzodiazepines flurazepam and RU 32007 both potentiated the GABA inhibition and this potentiation was blocked by Ro 15-1788, a benzodiazepine antagonist.

Physiological and pathophysiological roles of excitatory amino acids during central nervous system development

Recent studies suggest that excitatory amino acids (EAAs) have a wide variety of physiological and pathophysiological roles during central nervous system (CNS) development. In addition to participating in neuronal signal transduction, EAAs also exert trophic influences affecting neuronal survival, growth and differentiation during restricted developmental periods. EAAs also participate in the development and maintenance of neuronal circuitry and regulate several forms of activity-dependent synaptic plasticity such as LTP and segregation of converging retinal inputs to tectum and visual cortex. Pre- and post-synaptic markers of EAA pathways in brain undergo marked ontogenic changes. These markers are commonly overexpressed during development; periods of overproduction often coincide with times when synaptic plasticity is great and when appropriate neuronal connections are consolidated. The electrophysiological and biochemical properties of EAA receptors also undergo marked ontogenic changes. In addition to these physiological roles of EAAs, overactivation of EAA receptors may initiate a cascade of cellular events which produce neuronal injury and death. There is a unique developmental profile of susceptibility of the brain to excitotoxic injury mediated by activation of each of the EAA receptor subtypes. Overactivation of EAA receptors is implicated in the pathophysiology of brain injury in several clinical disorders to which the developing brain is susceptible, including hypoxia-ischemia, epilepsy, physical trauma and some rare genetic abnormalities of amino acid metabolism. Potential therapeutic approaches may be rationally devised based on recent information about the developmental regulation of EAA receptors and their involvement in the pathogenesis of these disorders.