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

Spontaneous and induced mouse mutations with cerebellar dysfunctions: behavior and neurochemistry

Grid2(Lc) (Lurcher), Grid2(ho) (hot-foot), Rora(sg) (staggerer), nr (nervous), Agtpbp1(pcd) (Purkinje cell degeneration), Reln(rl) (reeler), and Girk2(Wv) (Weaver) are spontaneous mutations with cerebellar atrophy, ataxia, and deficits in motor coordination tasks requiring balance and equilibrium. In addition to these signs, the Dst(dt) (dystonia musculorum) spinocerebellar mutant displays dystonic postures and crawling. More recently, transgenic models with human spinocerebellar ataxia mutations and alterations in calcium homeostasis have been shown to exhibit cerebellar anomalies and motor coordination deficits. We describe neurochemical characteristics of these mutants with respect to regional brain metabolism as well as amino acid and biogenic amine concentrations, uptake sites, and receptors.

Topology of ionotropic glutamate receptors in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of a G-protein inwardly rectifying K(+) channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, ionotropic glutamate receptors sensitive to N-methyl-D-aspartate (NMDA), amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA) were examined by autoradiography with [(3)H]MK-801, [(3)H]AMPA, and [(3)H]KA. These surveys were carried out in selected areas of cerebral cortex, hippocampus and related limbic regions, basal ganglia, thalamus, hypothalamus, brainstem, and cerebellum from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild-type (+/+) mice. In wv/+ and wv/wv mutants, NMDA receptor levels were lower in cortical areas, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus, and in the cerebellar granular layer. Densities of KA receptors were lower in cortical areas, hippocampus, limbic system structures, neostriatum, nucleus accumbens, thalamus and hypothalamus, superior and inferior colliculi, and cerebellar cortex of wv/wv mutants. Levels of AMPA receptors in the weaver were higher than in +/+ mice, particularly in somatosensory and piriform cortices and periaqueductal gray of wv/+, and in somatosensory cortex, CA1 field of Ammon's horn and cerebellar granular layer of wv/wv. Abnormal developmental signals, aberrant cellular responses, or a distorted balance between neurotransmitter interactions may underlie such widespread and reciprocal glutamate receptor alterations, while in the case of cerebellar cortex, NMDA receptors are lacking due to a massive disappearance of cerebellar granule cells and some loss of Purkinje neurons.

Distribution of glutamate receptors of the NMDA subtype in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of an G-protein inwardly rectifying K+ channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, glutamate receptors sensitive to N-methyl-D-aspartate (NMDA) were examined by autoradiography with [3H]MK-801 in 36 brain regions from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild type (+/+) mice. In wv/+ and wv/wv mutants labelling decreased in cortical regions, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus and in the cerebellar granular layer. The reductions in [3H]MK-801 binding were particularly specific in the cerebellar granular layer of wv/wv mutants, but an ubiquitous altered NMDA receptor topology was revealed in other brain regions. Abnormal developmental signals, or aberrant cellular responses, may underlie widespread NMDA receptor reductions, while in cerebellar cortex they could be lacking due to the massive loss of cerebellar granule cells.

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.

Induction of neuronal nitric oxide synthase by methylmercury in the cerebellum

A free radical, nitric oxide (NO), besides being a messenger molecule in the brain, becomes a neurotoxin if overproduced. We recently reported that methylmercury (MeHg) induces neuronal NO synthase (nNOS) in Purkinje cells. In the present study, we examined the distribution and the mechanism of nNOS induction by MeHg. Subcutaneous administration of MeHg chloride to mice, 10 mg/kg/day for 9 days, increased calcium-dependent NOS activity to 60% more than the controls only in the cerebellum but not in other brain regions. The Western blots showed a comparable increase in nNOS protein in the cerebellum. A N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, did not block, but rather enhanced, the increase in the nNOS activity. Another NMDA antagonist, 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), did not affect the nNOS activity. The Western blots of protein kinase C (PKC), which is an important cofactor regulating nNOS, did not change after the administration of MeHg. These results show that MeHg induces biologically active nNOS selectively in the cerebellum. The induction is independent of PKC and is not reduced by the blockade of the NMDA receptor.

Selective changes in AMPA receptors in rabbit cerebellum following classical conditioning of the eyelid-nictitating membrane response

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors are critically involved in several forms of synaptic plasticity proposed to be neural substrates for learning and memory, e.g., long-term potentiation and long-term depression (LTD). The present study was designed to determine changes in cerebellar AMPA receptors following classical conditioning of the eyeblink-nictitating membrane response (NMR) in the rabbit. Quantitative autoradiography was used to assess changes in ligand binding properties of cerebellar AMPA receptors following NMR conditioning elicited by pairing electrical stimulation of the pontine nuclei with an airpuff to the eye. [3H]AMPA and [3H]-6-cyano-7-nitroquinoxaline-2,3-dion (CNQX) binding were determined following preincubation of frozen-thawed brain tissue sections at 0 or 35 degreesC. With 0 degreesC preincubation, no significant differences in [3H]AMPA binding to cerebellar AMPA receptors were seen between any of the experimental groups tested. In contrast, preincubation at 35 degreesC revealed significant decreases in [3H]AMPA binding to the trained side of the cerebellar cortex resulting from paired presentations of the conditioned and the unconditioned stimuli, while unpaired presentations of the stimuli resulted in no significant effect. With 35 degreesC preincubation, there were no significant differences in [3H]CNQX binding between any of the experimental groups and no significant differences in [3H]AMPA binding in the untrained side of the cerebellum. These results indicate that NMR conditioning is associated with a selective modification of AMPA-receptor properties in brain structures involved in the storage of the associative memory. Furthermore, they support the hypothesis that cerebellar LTD, resulting from decreased synaptic efficacy at parallel fiber-Purkinje cell synapses mediated by a change in AMPA-receptor properties, is a form of synaptic plasticity that supports this type of learning.

Distribution of metabotropic glutamate receptor type 1a in Purkinje cell dendritic spines is independent of the presence of presynaptic parallel fibers

The metabotropic glutamate receptor type 1a (mGluR1a) is expressed at a high level in the molecular layer of the cerebellar cortex, where it is localized mostly in dendritic spines of Purkinje cells, innervated by parallel fibers. Treatment with methylazoxymethanol (MAM) of mouse pups at postnatal days (PND) 0 + 1 or 5 + 6 results in the partial loss of granule cells, the extent of which depends on the age of the animal at the time of injection. As a consequence of hypogranularity, the number of parallel fibers is decreased to such an amount that many of the postsynaptic Purkinje cell dendritic spines are devoid of axonal input, and only a limited number of spines participate in the formation of parallel fiber synapses, or, infrequently, in heterologous or heterotopic synapses with other presynaptic partners. At PND 30, 50% of the spines in the cerebella of mice treated with MAM at PND 0 + 1 was not contacted by any presynaptic element, compared to 5% in controls or 15% in the cerebella of mice treated with MAM at PND 5 + 6. The localization of mGluR1a was visualized by immunocytochemistry on ultrathin sections: approximately 80% of all Purkinje cell dendritic spines were immunopositive in controls and in both groups of MAM-treated mice, indicating that mGluR1a was present in Purkinje dendritic spines even when the corresponding synaptic input was absent. This observation indicates that the expression and subcellular distribution of mGluR1a are inherent, genetically determined properties of Purkinje cells.

Cerebellar Grafts Partially Reverse Amino Acid Receptor Changes Observed in the Cerebellum of Mice with Hereditary Ataxia: Quantitative Autoradiographic Studies

We used quantitative autoradiography of [3H]CNQX (200 nM), [3H]muscimol (13 nM), and [3H]flunitrazepam (10 nM) binding to study the distribution of non-NMDA and GABAA receptors in the cerebellum of pcd mutant mice with unilateral cerebellar grafts. Nonspecific binding was determined by incubation with 1 mM Glu, 200 μM GABA, or 1 μM clonazepam, respectively. Saturation parameters were defined in wild-type and mutant cerebella. In mutants, non-NMDA receptors were reduced by 38% in the molecular layer and by 47% in the granule cell layer. The reduction of non-NMDA receptors in the pcd cerebellar cortex supports their localization on Purkinje cells. [3H] CNQX binding sites were visualized at higher density in grafts that had migrated to the cerebellar cortex of the hosts (4.1 and 11.0 pmol/mg protein, respectively, at 23 and 37 days after grafting) than in grafts arrested intraparen-chymally (2.6 and 6.2 pmol/mg protein, respectively, at 23 and 37 days after grafting). The pattern of expression of non-NMDA receptors in cortical vs. parenchymal grafts suggests a possible regulation of their levels by transacting elements from host parallel fibers. GABAA binding levels in the grafts for both ligands used were similar to normal molecular layer. Binding was increased in the deep cerebellar nuclei of pcd mutants: the increase in [3H]muscimol binding over normal was 215% and the increase in [3H]flunitrazepam binding was 89%. Such increases in the pcd deep cerebellar nuclei may reflect a denervation-induced supersensitivity subsequent to the loss of Purkinje axon terminal innervation. In the deep nuclei of pcd mutants with unilateral cerebellar grafts, [3H]muscimol binding was 31% lower in the grafted side than in the contralateral nongrafted side at 37 days after transplantation; [3H]fluni-trazepam binding was also lower in the grafted side by 15% compared to the nongrafted side. Such changes in GABAA receptors suggest a significant, albeit partial, normalizing trend of cerebellar grafts on the state of postsynaptic supersensitive receptors in the host cerebellar nuclei.

Neuroanatomical and functional alterations resulting from early postnatal cerebellar insults in rodents

This review examines neuroanatomical and functional alterations in rodents resulting from postnatal insults during cerebellar development. Treatments such as irradiation and methylazoxymethanol (MAM) administration produced near birth (< postnatal day 8 for irradiation treatment and < postnatal day 4 for MAM administration) result in more severe cerebellar damage than do similar treatments administered several days after birth. Prominent among the more severe alterations are foliation abnormalities, misalignment of Purkinje cells and continued multiple innervation of climbing fibers; few or none of these occur as a result of later treatments (> postnatal day 8 for irradiation treatment and > postnatal day 4 for MAM treatment). The functional alterations also differ: insults produced near birth result in hypoactivity, ataxia, tremor and accompanying learning deficits, whereas those produced later result in hyperactivity and few learning deficits. This hyperactivity may have relevance to human disorders. Brief discussions of cerebellar and functional alterations (e.g., hyperactivity) resulting from neonatal infection with the Borna disease virus and induction of hypo- and hyperthyroidism during the preweaning period are also presented.

CGP 39653 binding in the chick CNS after NMDA receptor antagonist treatment

To analyze the affect of blocking neuronal activity on NMDA receptor levels during development, we have injected chick embryos with the competitive NMDA receptor antagonist NPC 12626 from E17 to E19. Brains from drug-treated (n = 7) and control (n = 6) embryos (E20) were processed for receptor autoradiography using the NMDA competitive antagonist [3H]CGP 39653. NPC 12626 treatment caused a significant 24 to 46% increase in [3H]CGP 39653 binding in both the forebrain and cerebellar cortex. The results support the hypothesis that NMDA receptor levels are regulated by activity-dependent mechanisms.

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.

Autoradiographic localization of [3H]-L-glutamate binding sites in a model of cerebellar granule cell ectopia generated by methylazoxymethanol treatment

The distribution of [3H]glutamate binding sites was studied in a model of altered cerebellar development obtained by injecting methylazoxymethanol (MAM) in 5-day-old mice. In these mice, at the 25th postnatal day, cerebella were smaller than normal, stratification was normal except for the presence in some lobes of a thin ectopic granule cell layer in the middle of the molecular layer, the proportion of the distribution of [3H]glutamate binding sites between molecular and internal granule cell layers was maintained but site density of both quisqualate- and NMDA-sensitive types was increased in the two layers. In the molecular layer, this increase was uniform in spite of the presence of the ectopic cell layer. In the internal granular layer, the increase of quisqualate-sensitive and NMDA-sensitive [3H]glutamate binding sites is topographically segregated and the first corresponds to areas of lesser cellular density. These results show that MAM treatment induces persistent alterations of the cerebellar glutamatergic system, which consist of receptor over-expression, possibly due to deficit of innervation, reactive gliosis and immaturity of surviving granule cells.

Pharmacology, distribution, cellular localization, and development of GABAB binding in rodent cerebellum

Quantitative receptor autoradiography using [3H]GABA under selective conditions was used to characterize the pharmacology, distribution, cellular localization, and development of GABAB binding sites in rodent cerebellum. Pharmacologic analysis of [3H]GABA binding showed that drugs active at GABAB receptors displaced [3H]GABA with the following order of potency: 3-aminopropylphosphonous acid > CGP 35348 = 2-hydroxysaclofen > phaclofen. GTP-gamma-S and GDP-beta-S also diminished potently [3H]GABA binding in a dose-dependent manner. The pattern of [3H]GABA binding to GABAB binding sites was systematically mapped throughout the rat cerebellum. GABAB binding was greatest in the molecular layer and a pattern of parasagittal zonation was observed in the molecular layer of lobules VII-X in adult rats. The cellular localization of GABAB binding was investigated using lesion techniques. Neither methyl azoxymethanol lesions of cerebellar granule cells nor 3-acetylpyridine lesions of climbing fibers resulted in a decrease in [3H]GABA binding. Homozygote stumbler mutant mice, deficient in Purkinje cell dendrites, had a significant decrease in [3H]GABA binding in the molecular layer. These results suggest that the majority of cerebellar molecular layer GABAB binding sites detected by [3H]GABA autoradiography are located on Purkinje cell dendrites. Examination of [3H]GABA binding to GABAB binding sites during development revealed that binding in the molecular layer peaks between postnatal day 14 and postnatal day 28 and then decreases to adult levels. Transient expression of high levels of GABAB binding was observed in the deep cerebellar nuclei, peaking at postnatal day 3 and decreasing to adult levels by postnatal day 21. Our investigation of GABAB pharmacology yielded data in agreement with previously reported results. We have described a parasagittal pattern of GABAB binding in the cerebellar molecular layer and assigned the majority of cerebellar GABAB binding sites to Purkinje cell dendrites. Finally, development studies reveal transient peaks in GABAB binding in the cerebellar molecular layer and deep cerebellar nuclei.

Phospholipase A2 modulates different subtypes of excitatory amino acid receptors: autoradiographic evidence

Exogenous phospholipases have been used extensively as tools to study the role of membrane lipids in receptor mechanisms. We used in vitro quantitative autoradiography to evaluate the effect of phospholipase A2 (PLA2) on N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in rat brain. PLA2 pretreatment induced a significant increase in alpha-[3H]amino-3-hydroxy-5-methyl-isoxazole-4-propionate ([3H]AMPA) binding in the stratum radiatum of the CA1 region of the hippocampus and in the stratum moleculare of the cerebellum. No modification of [3H]AMPA binding was found in the stratum pyramidale of the hippocampus at different ligand concentrations. [3H]-Glutamate binding to the metabotropic glutamate receptor and the non-NMDA-, non-kainate-, non-quisqualate-sensitive [3H]glutamate binding site were also increased by PLA2 pretreatment. [3H]Kainate binding and NMDA-sensitive [3H]glutamate binding were minimally affected by the enzyme pretreatment. The PLA2 effect was reversed by EGTA, the PLA2 inhibitor p-bromophenacyl bromide, and prolonged pretreatment with heat. Bovine serum albumin (1%) prevented the increase in metabotropic binding by PLA2. Arachidonic acid failed to mimic the PLA2 effect on metabotropic binding. These results indicate that PLA2 can selectively modulate certain subtypes of excitatory amino acid receptors. This effect is due to the enzymatic activity but is probably not correlated with the formation of arachidonic acid metabolites. Independent of their possible physiological implications, our results provide the first autoradiographic evidence that an enzymatic treatment can selectively affect the binding properties of excitatory amino acid receptors in different regions of the CNS.

Excitatory and inhibitory amino acid neurotransmitter binding sites in the cerebellar cortex of the pigeon (Columba livia)

We used receptor autoradiography to determine the distribution of excitatory and inhibitory amino acid neurotransmitter binding sites in the cerebellar cortex of the pigeon (Columba livia). alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid, kainate and metabotropic binding sites had highest levels in the molecular layer. N-methyl-D-aspartate binding sites, assayed with both [3H]glutamate under selective conditions and with [3H]glycine binding to the associated strychnine-insensitive glycine site, had highest levels in the granule cell layer. There was little specific binding of the non-competitive N-methyl-D-aspartate antagonist, [3H]MK-801. The level of gamma-aminobutyric acid (GABA)-A binding sites was higher than GABA-B binding sites in both molecular and granule cell layers with the highest level of GABA-A sites in the granule cell layer. The highest level of GABA-B binding sites was in the molecular layer. [3H]Flunitrazepam binding levels were approximately the same in both molecular and granule cell layers. With the exception of kainate binding sites, the distribution of binding sites was identical to that seen in the cerebellar cortex of mammals. Our results support the concept that the chemoarchitecture of the cerebellar cortex has been conserved in the course of vertebrate evolution.