Ontogeny of receptor binding sites for [3H]glutamic acid and [3H]kainic acid in the rat cerebellum

Simultaneous determination of binding of a variety of radioligands related to ionotropic excitatory amino acid receptors in fetal and neonatal rat brains

Expression of ionotropic excitatory amino acid receptors was assessed by membrane binding assays using a variety of radioligands in fetal and neonatal rat brains. In fetal rat brain, receptors sensitive to N-methyl-D-aspartate (NMDA) exhibited delayed onset of expression during the last 7 days before birth as compared with those insensitive to NMDA. In addition, developmental increases in agonist-preferring sites preceded those in antagonist-preferring sites within the first 7 postnatal days in particular brain structures with respect to each domain on the NMDA receptor complex. Growth of animals led to drastic increments of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801) binding to the NMDA channel in telencephalic regions until 21 to 28 days after birth, with concomitant desensitization to inhibition by protons of [3H]MK-801 binding in cortical membranes. By contrast, three different agonists were invariably effective in more potently potentiating [3H]MK-801 binding in cortical membranes of 14- and 28-day-old rats than in those of 5-day-old rats. These results suggest that the NMDA-sensitive subclass may play more critical roles in mechanisms underlying postnatal development of rat telencephalon than do the NMDA-insensitive subclasses.

Kainate binding to the AMPA receptor in rat brain

Displacement of [3H]AMPA and [3H]CNQX by kainate was measured in membranes and solubilized fractions from rat brain. In soluble fractions, plots of [3H]AMPA and [3H]CNQX binding displaced by kainate resulted in one-site fits with Ki values in the range of 1-3 microM. In membranes, plots of [3H]AMPA binding displaced by kainate resulted in graphs which were better fit by two-site regression analysis than by a one-site fit. The Ki value for the high-affinity component of these two-site fits was 3-9 microM and the low-affinity component Ki was in the range of 70-120 microM; similar values were determined for kainate displacement of [3H]CNQX. The presence of thiocyanate ions had no effect on kainate displacement of [3H]CNQX. Since the affinity for kainate of the presumed synaptic AMPA receptor is in the range of EC50 values for kainate determined from physiological studies, these data contribute further evidence for the idea that kainate binding to synaptic AMPA receptors may be responsible for many of kainate's physiological effects.

Regional distribution of transient [3H]kainic acid-binding sites in the central nervous system of the developing mouse: an autoradiographic study

[3H]kainate-binding site distribution in mouse brain was studied by in vitro autoradiography during postnatal development. Sites, highly concentrated at early postnatal ages and undetectable at adult ages, were observed in deep cerebellar nuclei, inferior olive, pontine nuclei, inferior colliculus and stratum lacunosum moleculare of the area CA1 in the hippocampus as well as in previously described rat brain areas. It is suggested that the molecules carrying these sites play a role in the development of the regions where they are transiently expressed.

Developmental changes in the effects of drugs acting at NMDA or non-NMDA receptors on synaptic transmission in the chick cochlear nucleus (nuc. magnocellularis)

The developmental pharmacology of excitatory amino acid (EAA) receptors in the chick cochlear nucleus (nucleus magnocellularis, NM) was studied by means of bath application of drugs and recording of synaptically-evoked field potentials in brain slices taken from chicks aged embryonic day (E) 14 through hatching (E21). The abilities of various EAA agonists (N-methyl-D-aspartate [NMDA], kainic acid, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA]) to suppress postsynaptic responses by depolarization block and of EAA antagonists ((3-[RS]-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CCP], dizocilpine [MK-801], 6-nitro-7-sulfamoyl-benzo(F)quinoxaline-2,3 dione [NBQX], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6,7-dinitroquinoxaline-2,3-dione [DNQX]) to suppress these responses directly were assessed quantitatively. The results support the existence of NMDA receptors in NM and suggest that the ability of these receptors to influence synaptically-evoked responses declines dramatically during the last week of embryonic life. The results similarly suggest that the non-NMDA receptors in NM undergo changes in density and/or function during a period of development when the cochlear nucleus is undergoing a variety of morphological and functional transformations.

Autoradiographic characterization of [3H]L-glutamate binding sites in developing mouse cerebellar cortex

Postnatal changes of [3H]L-glutamate binding sites in mouse cerebellum were studied by in vitro autoradiography. These sites were already present at birth, their density globally increased until postnatal day 25, and at all ages it was higher when Cl- and Ca2+ were present in the incubation buffer. At birth, these binding sites were diffused through the whole cerebellar mass, but became distinctly concentrated in the molecular and the internal granular layers by postnatal day 10. From this age on, binding site sensitivity to ions and glutamate analogues takes a different course in each layer. The external granular layer and the white matter never displayed significant amounts of binding. In the molecular layer the Cl-/Ca2+ effect increased during ontogeny until, in adults, the ion-dependent binding was threefold higher than the ion-independent binding. Quisqualate-sensitive sites accounted for 80% of the total binding sites already at postnatal day 15, while displacement by alpha-amino-3-hydroxy-methyl-4-isoxazolepropionic and ibotenic acids attained the maximum (68%) at postnatal day 60. N-Methyl-D-aspartate displaced glutamate binding (50%) only in the presence of Cl- and Ca2+. Starting from postnatal day 15, binding site density in the molecular layer of lobules VIb and VII of the vermis was lower than in other lobules. In the internal granular layer, the Cl-/Ca2+ effect observed in young animals decreased during development. These transient binding sites were sensitive to quisqualic and ibotenic acid. In adults, the majority of glutamate binding sites were ion-independent and mainly sensitive to D,L-amino-5-phospho-valeric acid and N-methyl-D-aspartate. Throughout development and in both layers, sites displaced by kainate were present at low density and sites displaced by D,L-2-amino-4-phosphonobutyric acid were not detected. The localized postnatal changes of the [3H]L-glutamate binding sites were correlated with the events occurring during growth and maturation of cerebellar structures. The increase of the Cl-/Ca(2+)-dependent binding in the molecular layer is simultaneous with the growth of Purkinje cell dendrites and of parallel fibres and with the formation of the synapses between them. This suggests that these binding sites are localized in these synapses. The changing pattern of sensitivity to different agonists during development might correspond to the maturation of these synapses. The low density of [3H]L-glutamate binding in the molecular layer of lobules VIb and VII probably indicates the presence of specific nerve projections to these areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in messenger RNAs coding for neurotransmitter receptors and voltage-operated channels in the developing rat cerebral cortex

The ontogenetic development of poly(A)+ mRNAs coding for receptors to several neurotransmitters (kainate, glutamate, acetylcholine, and serotonin) and voltage-operated channels (sodium and calcium) was studied by isolating total poly(A)+ mRNA from the brains of rats at various developmental stages and injecting it into Xenopus oocytes. The oocytes translated the foreign mRNA and incorporated functional receptor/ion channel complexes into the cell membrane. Thus, recording of induced membrane currents in voltage-clamped oocytes gave a measure of the relative amounts of the different messengers. Responses induced by kainate, glutamate, acetylcholine, and serotonin all increased with age and reached a maximum in oocytes injected with mRNA from adult cortex. Messenger RNAs for the earliest ages examined, Embryonic Days 15 and 18, expressed little or no response to kainate, glutamate, or acetylcholine, while 50-70% of the adult response was reached by Postnatal Day 10. In contrast, the serotonin-induced response was already comparatively large (16% of the adult level) in oocytes injected with mRNA from Embryonic Day 15 brain and increased postnatally to adult levels. The expression of voltage-dependent sodium and calcium channels was small in oocytes injected with mRNA from embryonic animals and increased postnatally to reach a maximum in oocytes injected with mRNA from adult animals.

Developmentally regulated changes of glutamate binding sites in mouse deep cerebellar nuclei

The expression of L-[3H]glutamate binding sites of different ionic and pharmacological sensitivities was studied in mouse deep cerebellar nuclei during early postnatal development by means of in vitro autoradiography. Ca2+/Cl(-)-dependent, quisqualate/AMPA/ibotenate-sensitive, and APB-insensitive binding sites are present at high density in the deep cerebellar nuclei of young animals, but greatly decrease between the 10th and 25th postnatal day and remain low in the adult. The density of Ca2+/Cl(-)-independent binding sites remains low and constant during the whole of postnatal development. The possible involvement of the Ca2+/Cl(-)-dependent binding sites in brain development is discussed.

Ontogeny of binding sites for [3H] kainic acid in chick and rat cerebellar membranes: a comparative study

We have analyzed the developmental properties of kainate receptors in cerebellar membranes prepared from chick and rat, two vertebrate species with contrasting patterns of functional maturation. Single populations of binding sites have been characterized in the avian and rodent membranes with apparent dissociation constants (Kd) in the 210-280 nM and 40-55 nM ranges, respectively; the number of binding sites (Bmax) increases with age in both species, reaching a maximum of 187 pmol/mg in the case of 10-day chicks vs. 1.28 pmol/mg in 75-day rats. The ontogenetic profiles of kainate receptors in chick and rat cerebella are in consonance with the patent differences in motor development at birth.

Excitatory amino acid receptors on sustained retinal ganglion cells in the kitten during the critical period of development

Effects of iontophoretically applied excitatory amino acid analogues, kainate, quisqualate and N-methyl-D-aspartate (NMDA) and their receptor antagonists on the sustained class of retinal ganglion cells were studied in the optically intact eye of pentobarbitone-anaesthetized kittens (7-9 weeks of age). These results were compared with the effects obtained in adult cats. All 3 excitatory amino acid agonists had excitatory actions on the majority of On- and Off-sustained ganglion cells in the kitten but at higher current levels than those required for adult cells, suggesting all 3 types of receptors of weaker sensitivity are present on the kitten cells. Whilst the relative potency of kainate, quisqualate and NMDA was 15:3:1 in the adult cells, it was 5:2:1 in the kitten cells. As for other neurones in the CNS, an increase in the potency of kainate receptors and a decrease in that of NMDA receptors appear, therefore, to characterize the postnatal development of the excitatory amino acid receptors on the retinal ganglion cells. In accordance with the agonist results, a broadband receptor antagonist, kynurenate, powerfully antagonised responses of kitten cells as well as those of adult cells. The pure NMDA receptor antagonist, 3((+-)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), however, only suppressed spontaneous firing of kitten cells. Furthermore, in kitten cells, the visually-driven firing was depressed while the level of firing was raised by these excitatory amino acid analogous, and a long period of inhibition of firing followed the agonist-induced excitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Subcellular localization of a putative kainate receptor in Bergmann glial cells using a monoclonal antibody in the chick and fish cerebellar cortex

A monoclonal antibody, IX-50, that was raised against a kainate binding protein (Mr = 49,000) from chicken cerebellum, was used in light and electron microscopic immunocytochemical studies to localize putative kainate receptors. Pre- and postembedding immunoperoxidase and immunogold methods were used in the cerebellar cortices of one to 26-day old chickens and adult rainbow trout. Immunoreactivity was detected only in association with Golgi epithelial/Bergmann glial cells. Intracellular immunoreactivity was present in the granular and agranular endoplasmic reticulum, Golgi apparatus and in lysosomes, representing the sites of synthesis, glycosylation and degradation of the protein. In the fish the granular endoplasmic reticulum was not immunoreactive. Extracellular immunoreactivity was associated with the plasma membrane. In the fish it was established that the epitope is on the outer surface of the membrane. The protein seems to be uniformly distributed along the membrane including the somata, the radial stem processes and the leafy lamellae surrounding Purkinje cell dendrites. Areas of the glial membrane in contact with other glial cells were also immunopositive. High-resolution light microscopy demonstrated all the Bergmann glial plasma membrane in the cortex, providing a "negative" image of Purkinje cell dendrites. It is apparent that Bergmann glial processes selectively outline the dendrites of the Purkinje cells by surrounding the parallel fibre terminal/Purkinje cell spine synaptic complexes. The parallel fiber terminals were highly immunoreactive for glutamate, as shown by an immunogold procedure. The association of Bergmann glial processes, carrying the Mr = 49,000 kainate binding protein, with the Purkinje cell dendrites and spine synapses could provide a basis for neuronal signalling to the Bergmann glia, possibly by glutamate.

Divergent ontogeny of sigma and phencyclidine binding sites in the rat brain

The postnatal developmental patterns of sigma (sigma) and phencyclidine (PCP) binding sites were compared in the rat brain. The results show diametrically different ontogenic patterns for the sites. While both the affinity and the density of sigma sites remain constant throughout the developmental period tested (postnatal day 1 to 1 year), the density of PCP binding sites increases from the time of birth, reaching the adult level by postnatal day 14. The differences in developmental patterns provide evidence for distinctive properties of cerebral sigma and PCP binding sites.

[3H]kainic acid binding sites in chick cerebellar membranes

1. A total particulate fraction of chick cerebellar membranes, obtained by a simple method, has been found to specifically bind [3H]kainic acid. Non-neuronal tissue, like chick liver, does not show any appreciable specific binding under the same experimental conditions. 2. Specific [3H]kainic acid binding to chick cerebellar membranes increases linearly with tissue concentration, reaches the binding equilibrium almost instantaneously and is pH and temperature dependent. 3. Specifically bound [3H]kainic acid is displaced by suitable concentrations of unlabelled kainic acid, L-glutamic acid and other excitatory amino acid analogues, both agonist and antagonist. This pharmacological pattern agrees with the general pharmacological properties of kainic acid receptors. 4. Saturation kinetic studies of kainic acid binding sites show one single binding mode with an apparent dissociation constant KD = 278 nM and a maximum number of binding sites of 187 pmoles/mg of protein. 5. In view of the mentioned data and the high amount of receptor sites found in chick cerebellar membranes, as compared with related values in rat cerebellum, we suggest that these receptors play a different physiological role or that they have a different cellular localization in chick and rat cerebellum.

Maturational changes in retinal excitatory amino acid receptors

The appearance, kinetics and pharmacological properties of receptors for n-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate (KA), L-glutamate (Glu) and L-aspartate (Asp) was investigated using 3H-ligand binding during the development of chick embryo retina. Receptors for AMPA are maximally concentrated at embryonic day 7 (ED 7) and decline 50% in subsequent days; L-Glu receptors are low until ED 11, and the same is true for Asp and NMDA receptors which increase at ED 14 and 18 respectively. All receptors studied underwent an increase in pharmacological specificity, whereas only AMPA-receptors showed an important change in affinity during ontogeny. Results demonstrate that receptors for excitatory amino acids in the retina suffer maturational changes and suggest that while NMDA and aspartate could interact with the same receptor, AMPA and glutamate seem to bind to different sites.

L-aspartate and L-glutamate binding sites in developing normal and 'nervous' mutant mouse cerebellum

This study concerns the ontogeny and the cellular localization of L-aspartate and L-glutamate binding sites in normal and 'nervous' mutant mouse cerebellar membranes. The binding kinetics revealed for L-aspartate a single binding system (Kd = 750 nM) and for L-glutamate also a single binding component of higher affinity (Kd = 344 nM). The pharmacological study, using various amino acid analogues, revealed a differential specificity for the binding sites of the two amino acids. The developmental study showed that the binding sites of both amino acids appear mainly during the second and third week of life, a period when parallel and climbing fiber synaptogenesis occurs, but they follow a slightly different developmental pattern. The study using 'nervous', mutant mouse cerebellum showed an age-dependent decrease of L-aspartate and L-glutamate binding, which coincides in time with the Purkinje cell degeneration in this mutant, indicating a cellular localization of these binding sites on the Purkinje cell membranes. These results suggest that L-aspartate and L-glutamate binding sites may be respectively associated with the postsynaptic target of climbing and parallel fibers on the Purkinje cell dendrites. However, the decrease of specific binding in 'nervous' mutant mouse cerebellum was about 50% for L-aspartate and 60% for L-glutamate, implying that a significant number of L-aspartate and L-glutamate binding sites are located on cerebellar elements other than the Purkinje cell membranes.

Maternal ethanol consumption: binding of L-glutamate to synaptic membranes from whole brain, cortices, and cerebella of offspring

We examined the influence of chronic maternal ethanol consumption on the Na+- and Ca2+-independent binding of L-glutamate to synaptic plasma membranes from whole brain as well as from cortices and cerebella of developing offspring. The maximum specific binding (Bmax) of L-glutamate to the Na+- and Ca2+-independent binding sites in synaptic plasma membranes of brain peaked at 17 days of age in the offspring of both control and ethanol-fed rats, although at that age there were significantly fewer binding sites in the brains of the offspring of ethanol-fed rats. The regional localization of this deficit is not now known. However, it appears that one major glutamatergic region (the cortex) does not reflect the transient deficiency of L-glutamate sites in brain. In fact, the concentration of L-glutamate binding sites in cortical synaptic plasma membranes was significantly increased in the 20-day-old offspring of ethanol-fed rats. In contrast to the cortex, binding to cerebellar synaptic plasma membranes was comparable in 20-day-old offspring of control and ethanol-fed rats. Despite transient alterations in the concentrations of L-glutamate binding sites in brain and synaptic plasma membranes, the affinity of the sites for L-glutamate (Kd) was consistently normal in the 14- to 26-day-old offspring of ethanol-fed rats.

The binding properties and regional ontogeny for [3H]glutamic acid Na+-independent and [3H]kainic acid binding sites in chick brain

The binding kinetics, pharmacological properties and regional ontogeny of L-[3H]glutamic acid Na+-independent and [3H]kainic acid binding sites were studied in preparations of chick brain. One binding component was found for L-[3H]glutamic acid with a Kd value of 176 x 10(9) M. For [3H]kainic acid two binding components were found in the hemispheres, optic lobes and brain stem, one with high affinity and a Kd value of 12.5 x 10(9) M and one with low affinity and a Kd value of 260 x 10(9) M. In cerebellum only one binding site was detected for [3H]kainic acid with a Kd value of 144 x 10(9) M. The ontogeny of L-[3H]glutamic acid and [3H]kainic acid binding sites was studied using membrane preparations (48,000 g pellet) of hemispheres, optic lobes, brain stem and cerebellum. Binding of L-[3H]glutamic acid was already significant in all brain regions by embryonic day 11 but major increases in total receptor number per brain region or per mg of protein were apparent by embryonic day 19 and especially after hatching. Cerebral hemispheres, optic lobes and brain stem showed few [3H]kainic acid binding sites by day 13 in ovo. An increase follows which, in hemispheres and optic lobes, continues at the same rate during the first two weeks after hatching. In cerebellum, by contrast, the kainic acid binding site is almost undetectable until embryonic day 15. The appearance of these binding sites in cerebellum takes place during the restricted period between days 15 in ovo and 5 post-hatching. This pattern of development of [3H]kainic acid binding sites almost parallels the developmental patterns of the molecular layer of chick cerebellum and it is consistent with the results of our autoradiographic study showing that the great majority of kainic acid binding sites are localized in the molecular layer.

Maturation of kainic acid seizure-brain damage syndrome in the rat. III. Postnatal development of kainic acid binding sites in the limbic system

The progressive appearance of [3H]kainic acid binding sites with age has been studied in membrane suspensions prepared from various regions of the rat limbic system, and by autoradiography. Binding sites with fast dissociation rate appeared earlier than binding sites with slow dissociation rate. Scatchard analysis demonstrated apparent receptor heterogeneity for both subclasses. High affinity components were detected in the hippocampus as early as 10 days after birth, but in the amygdala + piriform lobe were found only towards the end of the third week, when animals also respond to parenteral kainic acid, for the first time, with limbic seizures accompanied by metabolic activation of the amygdala. Slice autoradiography revealed distinct labelling of the hippocampal CA3 region by postnatal day 10. A comparison with the ontogenesis of the kainic acid-induced seizure-brain damage syndrome suggests a role of high affinity receptors as mediators of metabolic nerve cell activation by kainic acid. However, this receptor interaction per se does not result in neuronal damage to the vulnerable region of the Ammon's horn, which will only occur at an age when also the amygdala is activated by the neurotoxin.

Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors

This review summarizes studies designed to label and characterize mammalian synaptic receptors for glutamate, aspartate and related acidic amino acids using in vitro ligand binding techniques. The binding properties of the 3 major ligands employed--L-[3H]glutamate, L-[3H]aspartate and [3H]kainate--are described in terms of their kinetics, the influence of ions, pharmacology, molecular nature, localization and physiological/pharmacological function. In addition, the binding characteristics are described of some new radioligands--[3H]AMPA, L-[3H]cysteine sulphinate, L-[35S]cysteate, D-[3H]aspartate, D,L-[3H]APB, D-[3H]APV and D,L-[3H]APH. Special emphasis is placed on recent findings which allow a unification of the existing binding data, and detailed comparisons are made between binding site characteristics and the known properties of the physiological/pharmacological receptors for acidic amino acids. Through these considerations, a binding site classification is suggested which differentiates 5 different sites. Four of the binding site subtypes are proposed to correspond to the individual receptor classes identified in electrophysiological experiments; thus, A1 = NMDA receptors; A2 = quisqualate receptors; A3 = kainate receptors; A4 = L-APB receptors; the fifth site is proposed to be the recognition site for a Na+-dependent acidic amino acid membrane transport process. An evaluation of investigations designed to elucidate regulatory mechanisms at acidic amino acid binding sites is made; hypotheses such as the Ca2+-activated protease hypothesis of long-term potentiation are assessed in terms of the new binding site/receptor classification scheme, and experiments are suggested which will clarify and expand this exciting area in the future.

Immature rabbit hippocampus is damaged by systemic but not intraventricular kainic acid

Intraventricular injection of kainic acid produced a characteristic necrosis in the hippocampal CA3 region of adult rabbits. Neurons in immature rabbit hippocampus were relatively insensitive to the intraventricular neurotoxin, even though these injections caused seizures. In contrast, the immature hippocampus was highly sensitive to kainate delivered systemically. Systemic kainate preferentially killed neurons in the CA1 region, not the CA3 region, of immature animals. Kainate injections also caused cellular damage in brain regions other than hippocampus. In immature rabbits, the pattern of extra-hippocampal damage was similar regardless of the route of kainate delivery. This pattern of extra-hippocampal damage was distinctly different than that seen in adults following intraventricular kainate. These data suggest that kainate-induced damage in the CA3 and CA1 regions of the hippocampus are mediated by different mechanisms. Furthermore, the dissimilar patterns of extra-hippocampal damage in immature and adult rabbits suggest that mature and immature animals have differential sensitivity to kainate, different types of kainate binding sites, and/or different distributions of kainate receptors.

Synaptic junctions isolated from cerebellum and forebrain: comparisons of morphological and molecular properties

Synaptic junction (SJ) fractions have been isolated from rat cerebellum which are similar to forebrain SJs on the basis of morphology and enrichment of synaptic structures. The polypeptide compositions of SJ fractions were analyzed by one- and two-dimensional SDS-gel electrophoresis and stained by the silver technique. Equivalent numbers of proteins that possess similar relative mobilities (Mr), isoelectric points and staining intensities were present in cerebellum and forebrain synaptic fractions. A few prominent differences were observed between cerebellum and forebrain synaptic fractions; cerebellum SJs contained a 240 K protein that was not detected in the forebrain and the 52,000 K, major PSDp protein was present in forebrain SJs in amounts that are approximately 5-fold greater than in cerebellum SJ fractions. The identity of the cerebellum mPSDp was verified by electrophoretic mobility, peptide fingerprinting and [125I]calmodulin binding. Differences between various synaptic fractions in mannose containing glycoproteins were examined by the binding of [125I]concanavalin A (Con A) to gels. On the basis of apparent molecular weights, the glycoproteins in forebrain and cerebellum SPMs were very similar. In contrast, however, the prominent glycoproteins that reside in the postsynaptic junctional membrane of forebrain SJs were undetectable in SJ fractions isolated from cerebellum. SJ fractions from cerebellum contained their own distinct group of Con A binding glycoproteins. SPM and SJ fractions from forebrain and cerebellum were examined for receptors for excitatory (aspartate, glutamate and kainic acid) and inhibitory (GABA) neurotransmitters and the benzodiazepine analog flunitrazepam. On the basis of relative receptor contents, SJ fractions isolated from either brain region were qualitatively similar and bound significant amounts of excitatory and inhibitory transmitters. These findings support the notion that SJs from cerebellum contain a distinct class of synaptic elements that are in large part derived from asymmetric, type I synapses.

Quantitative autoradiography of L-[3H]glutamate binding to rat brain

A technique has been developed to investigate sodium-independent L-[3H]glutamate binding in rat brain sections using quantitative autoradiography and tritium-sensitive film. Binding is rapid (reaching equilibrium in 5 min) and reversible (having a t 1/2 of dissociation of 0.38 min). Glutamate apparently bound to a single population of sites with a Kd of about 1.0 microM. The pharmacology of this binding site is similar to that observed in homogenate studies. There is marked regional variation in the amount of glutamate bound. Of the areas analyzed in detail, the density of sites is greatest in stratum moleculare of hippocampus, followed by striatum and cortex.

N-acetylaspartylglutamate: an endogenous peptide with high affinity for a brain "glutamate" receptor

A brain peptide with high affinity (420 nM) and marked specificity for brain receptor sites labeled with L-[3H]glutamate has been identified. Amino acid analysis and mass spectroscopy indicate that the peptide is N-acetylaspartylglutamate. The peptide exhibits potent convulsant properties when injected into the rat hippocampus, similar to those produced by the glutamate receptor agonist, quisqualic acid. These findings raise the question whether endogenous brain peptides enriched in acidic amino acids may serve as excitatory neurotransmitters.

Specific binding of [3H]L-glutamate to cerebellar membranes: evidence for recognition site heterogeneity

The kinetics of interaction of excitatory amino acid analogues and reputed antagonists with a specific binding site for [3H]L-glutamate, having a Kd of 600 nM, were examined in washed cerebellar membranes incubated at 37 degrees C. Displacement curves were analyzed by an iterative computer program for a non-cooperative two-site competitive inhibition model. L-Glutamate, D-glutamate and D-aspartate exhibited simple, mass action kinetics with Hill coefficients near unity and Kis of 1.1, 9.3 and 23.3 microM, respectively. Quisqualate, ibotenate and cyclopentylglutamate had Hill coefficients less than 0.85 and bound to an high affinity component with KHS of 0.4, 0.8 and 1.7 microM, respectively. Neither N-methyl-D-aspartate nor derivatives of kainate, with the exception of a-keto kainate, had KiS less than 0.1 mM. Linear analogues of glutamate with reputed antagonistic properties all exhibited shallow displacement curves with Hill coefficients less than 0.6 and KHS varying from 0.5 to 6.6 microM. Notably, 2-amino-6-phosphonocaproic acid had negligible affinity for the site in contrast to the valeric and pimelic phosphono analogues. The results indicate that [3H]L-glutamate labels a single class of sites that can be resolved into subpopulations by agonists and antagonists and provide additional evidence of excitatory amino acid receptor heterogeneity.

Kainic acid receptors and neurotoxicity in adult and immature rat cerebellar slices

The neurotoxic actions of kainate were examined in incubated slices of adult and immature rat cerebellum using light- and electron-microscopy. In the adult, Purkinje cells and inhibitory interneurones became selectively necrotic at concentrations between 5 micro M and 20 micro M. At 30 micro M, granule cells also became affected. In the immature cerebellum, at an age (8 days after birth) when the parallel fibres (thought to use glutamate as transmitter) are largely yet to be developed, selective toxicity was still evident but Purkinje cells and inhibitory interneurones were about 10-fold, and granule cells about 30-fold, less sensitive to kainate than in the adult. Kainate and other excitotoxins also increased cyclic GMP levels in cerebellar slices, apparently through the activation of excitatory amino acid receptors. In the adult tissue, the dose-cyclic GMP response curve to kainate was biphasic suggesting the presence of two components. The lower concentrations of kainate eliciting the first component mirrored those inducing selective necrosis of Purkinje cells and inhibitory interneurones while the second component correlated with necrosis of granule cells. Similar correlations applied to the immature cerebellum, but here kainate neurotoxicity appeared to be associated with the activation of receptor types different from those evident in the adult. It is suggested that kainate receptors, whose activation is associated with both neurotoxic damage and elevation of cyclic GMP levels, are located on all cell types in the adult cerebellum, with Purkinje cells and inhibitory interneurones displaying a higher sensitivity to kainate than granule cells. The lower sensitivity of immature cerebellum to the neurotoxic effect of kainate is probably due to lower levels of kainate receptors.

Characterization of [L-3H]aspartate binding to chick retinal subcellular fractions

Binding of [L-3H]aspartate to synaptic receptors was examined in membranes from whole chick retina and subcellular fractions enriched with photoreceptor terminals (P1) or terminals from the inner plexiform layer (P2), Na+-independent, stereospecific, high affinity binding was concentrated in the P1 fraction (Kb = 40 nM). P2 fraction also showed a high affinity binding system (KB = 11.8 nM) with lower capacity than in the P1 fraction. Comparative studies with [L-3H]-aspartate, [L-3H]-glutamate and [H3]-kainate showed that L-aspartate and L-glutamate are the most potent inhibitors of the binding of the three ligands. Aspartate and glutamate binding were effectively displaced by N-methyl-DL-aspartate and alpha-amino adipate, whereas only [3H]-glutamate binding was significantly inhibited by glutamate-diethyl-ester. Kainic acid exhibited negligible affinity for aspartate and glutamate binding sites. Results indicate the presence of different receptors for glutamate and aspartate in both plexiform layers of the retina.