Quantitative autoradiographic study of L-glutamate binding sites in normal and atrophic human cerebellum

Glutamate Dehydrogenase Deficiency in Cerebellar Degenerations: Clinical, Biochemical and Molecular Genetic Aspects

Glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism, is significantly reduced in patients with heterogenous neurological disorders characterized by multiple system atrophy (MSA) and predominant involvement of the cerebellum and its connections. In human brain, GDH exists in multiple isoforms differing in their isoelectric point and molecular mass. These are differentially reduced in quantity and altered in catalytic activity in patients with clinically distinct forms of MSA, thus suggesting that these GDH isoproteins are under different genetic control. Dysregulation of glutamate metabolism occurs in patients with GDH deficiency and is thought to mediate the disease’s neurodegeneration via neuroexcitotoxic mechanisms. This possibility is supported by additional data showing that glutamate binding sites are significantly decreased in cerebellar tissue obtained at autopsy from MSA patients. At the molecular biological level, several cDNAs specific for human GDH have been isolated recently and cloned. Northern blot analysis of various human tissues, including brain, has revealed the presence of multiple GDH-specific mRNAs. In addition, multiple GDH-specific genes are present in humans and these data are consistent with the possibility that the various GDH isoproteins are encoded by different genes. These advances have laid the groundwork for characterizing the human GDH genes and their products in health and disease.

Neurotransmitters, neuropeptides and calcium binding proteins in developing human cerebellum: a review

Many endogenous neurochemicals that are known to have important functions in the mature central nervous system have also been found in the developing human cerebellum. Cholinergic neurons, as revealed by immunoreactivities towards choline acetyltransferase or acetylcholinesterase, appear early at 23 weeks of gestation in the cerebellar cortex and deep nuclei. Immunoreactivities gradually increase until the first postnatal month. Enkephalin is localized in the developing cerebellum, initially in the fibers of the cortex and deep nuclei at 16-20 weeks and then also in the Purkinje cells, granule cells, basket cells and Golgi cells at 23 weeks onward. Another neuropeptide, substance P, is localized mainly in the fibers of the dentate nucleus from 9 to 24 weeks but substance P immunoreactivity declines thereafter. GABA, an inhibitory neurotransmitter of the central nervous system, starts to appear at 16 weeks in the Purkinje cells, stellate cells, basket cells, mossy fibers and neurons of deep nuclei. GABA expression is gradually upregulated toward term forming networks of GABA-positive fibers and neurons. Catecholaminergic fibers and neurons are also detected in the cortex and deep nuclei at as early as 16 weeks. Calcium binding proteins, calbindin D28K and parvalbumin, make their first appearance in the cortex and deep nuclei at 14 weeks and then their expression decreases toward term, while calretinin appears later at 21 weeks but its expression increases with fetal age. The above findings suggest that many neurotransmitters, neuropeptides and calcium binding proteins (1) appear early during development of the cerebellum; (2) have specific temporal and spatial expression patterns; (3) may have functions other than those found in the mature neural systems; and (4) may be able to interact with each other during early development.

Autoradiography of glutamate receptor binding in adult Lurcher mutant mice

The mutation Lurcher, resulting from a gain of malfunction of the delta2 glutamate receptor expressed specifically by cerebellar Purkinje cells, causes a primary total loss of these neurons of the cerebellar cortex, as well as the secondary degeneration of cerebellar granule and inferior olive neurons. The distributions of glutamate receptors sensitive to amino-methylisoxazole-propionic acid (AMPA), to kainic acid (KA), and to N-methyl-D-aspartic acid (NMDA) as well as metabotropic sites (MET1 and MET2) were examined in wild type and Lurcher mice by quantitative autoradiography. This study was undertaken to determine the gene effect on the distribution of the various glutamate receptor subtypes, as well as how the cerebellar lesion affects the glutamatergic system in other brain regions. In cerebellum, there were postsynaptic AMPA and metabotropic receptors on Purkinje cells, postsynaptic NMDA receptors on granule cells, as well as KA receptors on granule cells or on parallel fibers. Taking into account surface areas, binding to all receptor subtypes was lower in the cerebellar cortex of Lurcher mutants than in wild type mice, while in the deep cerebellar nuclei only KA receptors were diminished. In other brain regions, the alterations followed always the same pattern characterized by a decrease of NMDA and KA receptors but with an increase of AMPA sites; these reciprocal changes were seen in thalamus. neostriatum, limbic regions, and motor cerebral cortical regions. Comparisons of glutamate receptor distribution in Lurcher mutants and in human autosomal cerebellar ataxia may permit further understanding of the role of glutamate-induced toxicity on neuronal death in these heredo-degenerative diseases.

Autoradiographic characterization of the non-N-methyl-D-aspartate binding sites in human cerebellum using the antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione

Using quantitative autoradiography, we have characterized the binding properties of the non-N-methyl-D-aspartate (NMDA) glutamate receptor antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in adult human cerebellum. Saturation experiments revealed [3H]CNQX binding to a single class of sites with similar affinity in the molecular and granule cell layer (Kd = 89.0 +/- 6.4 and 83.3 +/- 9.9 nM, respectively). The maximum number of [3H]CNQX binding sites was much higher in the molecular compared to the granule cell layer (Bmax = 16.2 +/- 1.1 and 2.8 +/- 0.5 pmol/mg protein, respectively). Inhibition experiments were performed in order to examine the pharmacological profile of [3H]CNQX binding in the molecular layer. [3H]CNQX labeled sites with high affinity for both non-NMDA agonists, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate. Dose-response curves for inhibition of [3H]CNQX by AMPA and kainate were biphasic. The potency of AMPA for displacement of [3H]CNQX binding (Ki: 2.8 +/- 0.8 nM and 12.5 +/- 0.8 microM) was 4- to 6-fold greater than the corresponding potency of kainate (Ki: 18.1 +/- 5.7 nM and 48.7 +/- 9.3 microM). In conclusion, the pharmacological analysis of [3H]CNQX binding in the human cerebellar molecular layer reflects the existence of multiple binding sites of the non-NMDA receptor that have different affinities for both AMPA and kainate.

Excitatory amino acid AMPA receptor mRNA localization in several regions of normal and neurological disease affected human brain. An in situ hybridization histochemistry study

In situ hybridization histochemistry was used to localize the mRNAs coding for four alpha-aminoisoxazole propionic acid-sensitive glutamate receptor subunits in human brain (age range 51-95 years, postmortem delay 4.5-10 h). High levels of the B receptor subunit mRNA were present in all the studied regions, followed by the A-subunit and the C-subunit. Only very low levels of the D-subunit mRNA were detected. In hippocampus, the mRNA coding for the B-subunits of the glutamate receptor was observed in granule cells of dentate gyrus and in the pyramidal cells of Ammon's horn. In cortex, the highest levels of glutamate receptor subunit mRNAs were found in layer I and layers III-IV of entorhinal and temporal cortex, although significant levels were also observed in the other cell layers. A differential distribution was seen in cerebellum where the A-subunit mRNA is expressed mainly by Purkinje cells, while the B-subunit mRNA is present in the internal granule cell layer. These results correlate well with previous data from autoradiographic studies on the localization of excitatory amino acid binding sites in human brain and pinpoint the cells where these receptors are synthesized. In situ hybridization in the hippocampus of patients affected by Alzheimer's disease (age range 77-82 years, postmortem delay 19-25.5 h) revealed a decrease on the content of the mRNAs coding for these excitatory amino acid receptors, while an increase was detected in surgically dissected epileptic human hippocampi. These results corroborate and extend the previous data from in vitro autoradiography and suggest alteration of the excitatory amino acid disfunction during these neurodegenerative processes.

Autoradiographic comparison of the distribution of [3H]MK801 and [3H]CNQX in the human cerebellum during development and aging

The autoradiographic distribution of N-methyl-D-aspartate (NMDA) and D,L-a-amino-3-hydroxyl-5-methyl-4-isoxazoleproprionic acid/quisqualate (AMPA/QUIS) receptors was determined in cerebellum obtained at autopsy from 37 human individuals, aged from 24 weeks gestation to 95 years. [3H]MK801 was used to label the NMDA receptor and [3H]CNQX to label the AMPA/QUIS receptor. AMPA/QUIS receptors were concentrated in the cerebellar molecular layer, and NMDA receptors in the granular layer. Significant (3- to 4-fold) increases in binding were seen for both ligands from the fetal to neonatal periods in the molecular layer (CNQX) and in both molecular and granular layers (MK801). MK801 binding in the molecular layer continued to increase with age up to the tenth decade and together with binding in the granular layer, increased 2-fold between 10-40 years. The Purkinje cell layer was negative for MK801 binding until the 6-7th decade when it became positive. [3H]CNQX binding in the molecular layer increased significantly with age between the fetal period and the tenth decade, whereas in the granular layer binding increased from neonate to 40 years, but then decreased significantly from 60 years to the tenth decade. Lamination of the molecular and granular layers was absent during the fetal period and appeared with both ligands during the neonatal period. These marked differences in age-related expression of ligand binding sites in the granular layer during development and aging are of potential significance in relation both to selective vulnerability to ischemia, and synaptic plasticity and remodelling related to neuronal loss in senescence.

Glutamate-like immunoreactivity in chick cerebellum and optic tectum

Glutamate was coupled via glutaraldehyde to bovine serum albumin. The conjugate was used for raising specific anti-glutamate antibodies. The purified antibody was used for immunostaining of chick cerebellum and optic tectum. Staining was intense in the molecular layer and in cell bodies of the granule cell layer. In the optic tectum a diffuse staining was detected in the superficial layers of stratum griseum fibrosum superficiale and in cell bodies especially in the layers a and e. Large cell bodies located in the stratum griseum centrale were also stained.