Loss of Purkinje cell-associated benzodiazepine receptors spares a high affinity subpopulation: a study with pcd mutant mice

The AGTPBP1 gene in neurobiology

The function of the Agtpbp1 gene has mainly been delineated by studying Agtpbp1pcd (pcd) mutant mice, characterized by losses in cerebellar Purkinje and granule cells along with degeneration of retinal photoreceptors, mitral cells of the olfactory bulb, thalamic neurons, and alpha-motoneurons. As a result of cerebellar degeneration, cerebellar GABA and glutamate concentrations in Agtpbp1pcd mutants decreased while monoamine concentrations increased. The salient behavioral phenotypes include cerebellar ataxia, a loss in motor coordination, and cognitive deficits. Similar neuropathogical and behavioral profiles have been described in childhood-onset human subjects with biallelic variants of AGTPBP1, including cerebellar ataxia and hypotonia.

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.

[3H]muscimol and [3H]flunitrazepam binding sites in the developing cerebellum of mice treated with methylazoxymethanol at different postnatal ages

Two models of perturbed cerebellar ontogenesis were obtained by a single administration of methylazoxymethanol (MAM), a potent antimitotic agent, to mouse pups either on the day of birth (MAM0 mice) or at postnatal day 5 (MAM5 mice). The alterations of the cerebellar GABAergic system were studied by measuring glutamic acid decarboxylase activity, [3H]muscimol binding sites, which are known to be concentrated in the GABAA receptors in the internal granular layer, and [3H]flunitrazepam binding sites, which are more abundant in the molecular layer. The primary target of the antimitotic agent are the precursors of the glutamatergic and GABAceptive granule cells. In both models GABAergic structures, as revealed by GAD activity measurements, appear to be relatively spared, and recovery of granule cell numbers occurs during development in MAM5 mice. In MAM treated mice the number of [3H]muscimol binding sites (on a per cerebellum basis) decrease as the number of granule cells decrease, although some recovery occurred in MAM5 mice, but not in MAM0 mice. In MAM5 mice, [3H]flunitrazepam binding sites (on a per cerebellum basis) were relatively unaffected, while they were decreased significantly, but to a lesser extent than [3H]muscimol binding sites, in MAM0 animals. The more significant reduction of granule cell numbers and the cytoarchitectural disruption resultant from the more precocious application of the antimitotic appear responsible for the significant alteration and lack of recovery in MAM0 mice.

Cerebellar ganglioside abnormalities in pcd mutant mice

The distribution of cerebellar gangliosides was studied in Purkinje cell degeneration (pcd/pcd) mutant mice at postnatal days 25, 30, 50, and 150. These mutants lose the majority of Purkinje cells between 18 and 50 days of age. A reactive gliosis accompanies Purkinje cell loss and a partial loss of granule cells occurs in pcd/pcd mice older than p50. Purkinje cell loss is associated with significant reductions in cerebellar weight and ganglioside concentration. This neuronal loss was also developmentally correlated with reductions of gangliosides (GT1a/LD1 and GT1b and with elevations of GD3. These results agree with previous findings in other cerebellar mutants that GT1a/LD1 and GT1b are concentrated in Purkinje cells and that GD3 is enriched in reactive glial cells. A slight, but significant, reduction in GD1a concentration occurred only in older pcd/pcd mice, consistent with previous findings in weaver and staggerer mice that GD1a is enriched in mature granule cells. The findings with pcd/pcd and other neurological mutants indicate that certain gangliosides can serve as cell-surface markers for monitoring changes in cerebellar cytoarchitecture that accompany development or disease.

Cerebellar benzodiazepine receptors: cellular localization and consequences of neurological mutations in mice

The distribution of cerebellar [3H]flunitrazepam binding sites was studied autoradiographically in Purkinje cell degeneration (pcd/pcd), weaver (wv/wv), staggerer (sg/sg) and reeler (rl/rl) mutant mice. In the normal 78-day-old C57BL/6J mouse cerebellum, the highest concentration of [3H]flunitrazepam binding sites was observed over the molecular layer. Intermediate grain density was present over the Purkinje cell layer and intermediate to high density over the deep cerebellar nuclei. Low labeling was observed over the granule cell layer. Negligible concentrations of binding sites were seen in the white matter. In 45-49-day-old Purkinje cell degeneration mutants, where essentially all Purkinje cells have disappeared by day 45, there was a small decrease in grain density over the cerebellar cortex. Concomitantly, a substantial increase in grain density was observed over the deep cerebellar nuclei of the pcd/pcd mutants when compared to littermate controls. A significant increase in [3H]flunitrazepam labeling was observed over the cerebellar cortex of 81-86-day-old wv/wv mutants; this was most pronounced in the vermis where the granule cell loss was greatest. Over the hemispheres, where fewer granule cells degenerate, a lower density of binding sites was seen. Grain density over the wv/wv deep cerebellar nuclei was comparable to that of littermates. Substantially lower [3H]flunitrazepam labeling was detected over the cerebellar cortex of 25-27-day-old sg/sg mutants in which the number of granule, Purkinje and Golgi cells is greatly reduced; the labeling over the deep nuclei, however, was significantly increased. In 27-29-day-old rl/rl mutant cerebella, where all classes of cells are malpositioned, labeling density over all areas of the cerebellar cortex, including the Purkinje cell masses, was increased. Our autoradiographic data suggest that a proportion of cerebellar cortical benzodiazepine receptors are associated with Purkinje cells; we propose that the remainder of the receptors are localized on Golgi cells, while granule cells are devoid of receptors. In the deep cerebellar nuclei, the observed increase in benzodiazepine receptors in the pcd/pcd and sg/sg mutants may be a manifestation of denervation supersensitivity subsequent to the loss of innervation by Purkinje cell axon terminals. The finding of a high receptor density in the Purkinje cell masses of the rl/rl mutant, where Purkinje cells are devoid of afferent basket cell input, suggests that benzodiazepine receptors are expressed and maintained in the absence of a full complement of GABAergic afferents.

Cerebellar benzodiazepine receptor distribution: an autoradiographic study of the normal C57BL/6J and Purkinje cell degeneration mutant mouse

The distribution of [3H]flunitrazepam binding sites in the cerebella of normal mice and Purkinje cell degeneration mutant mice was studied by light microscopic autoradiography. In the cerebellar cortex of normal mice, a high density of [3H]flunitrazepam binding was observed over the molecular layer, an intermediate density over the Purkinje cell layer and a low density over the granule cell layer; the white matter was devoid of labeling. The deep cerebellar nuclei were labeled to an intermediate density. In the 54-day-old Purkinje cell degeneration mutant cerebellum, which is depleted of Purkinje cells, a 36% reduction in labeling density of the cerebellar cortex was observed. The density was reduced by approximately equal amounts in both the molecular and granule cell layers; labeling in the deep cerebellar nuclei was, however, substantially increased.

Noradrenergic innervation of the cerebellar cortex in normal and in Purkinje cell degeneration mutant mice: evidence for long term survival following loss of the two major cerebellar cortical neuronal populations

Purkinje cell degeneration mutant mice were examined during the course of Purkinje cell death (26 and 35 days old) and at 3, 5, 9 and 12 months of age. Glyoxylic acid fluorescence histochemistry for catecholamines was used to investigate possible alterations or reorganization of the noradrenergic fibers from the coeruleo-cerebellar system in response to the degeneration of two major cell types in the cerebellar cortex, of which one, the Purkinje cell, is reported to be the major target neuron. In control mice, noradrenergic fibers traveled in linear and tortuous profiles through the granule cell layer, formed pericellular arrays alongside Purkinje cell somata, and branched profusely into both radially oriented and longitudinally oriented chains. The density of noradrenergic varicosities diminished in the molecular layer, there was with age. In the mutants, concomitant with the progressive shrinkage of the molecular layer, there was a progressive increase in the density of noradrenergic varicosities. This was most conspicuous at 9 and 12 months of age, at which time the molecular layer has been depleted not only of Purkinje cell dendrites, but also of parallel fibers. Noradrenergic fibers in these zones formed dense parallel bundles of varicose profiles whose density reached 621.3 +/- 122.8% (mean +/- SD, n = 4) at 9-12 months of age, compared with age-matched controls. Neurochemical measurement of norepinephrine content in whole cerebellum of the Purkinje cell degeneration mutants revealed no change compared with age-matched controls. We conclude that noradrenergic innervation persists in the cerebellar cortex despite the death of Purkinje cells and most of the granule cells. Although we found an increased density of varicosities in the molecular layer of mutant mice, progressing with age, we believe that this can be explained on the basis of the resultant geometry of the altered cerebellar cortex. It appears that the health of the environment surrounding the noradrenergic fibers in cerebellar cortex has little influence on their anatomical integrity.

Residual benzodiazepine (BZ) binding in the cortex of pcd mutant cerebella and qualitative BZ binding in the deep cerebellar nuclei of control and mutant mice: an autoradiographic study

In mutant mice 'Purkinje cell degeneration' (pcd), there is an almost complete degeneration of Purkinje cells followed subsequently by a partial degeneration of granule cells. Recent neurochemical studies have revealed a 50% decrease in benzodiazepine (BZ) receptors in 45-day-old pcd mutants after degeneration of the Purkinje cells. At 300 days there is an 80% decrease in BZ receptors concomitant with granule cell losses. To determine the histological localization of these receptor changes this autoradiographic analysis was conducted. An in vitro autoradiographic technique was used to explore [3H]flunitrazepam binding. BZ receptors were found to be more concentrated in the molecular than the granular layer of mutant and control cerebellar cortices. There was, nonetheless, no statistically significant difference in grain counts between control and mutant mice in any layer. Substantial atrophy of cerebellar structures, particularly of the molecular layer, occurred in the mutant mice. It began even before 45 days of age but was extreme at 300 days. When the appropriate mathematical correction factor was introduced for the layer atrophy there was a 60% decrease in grain count in 45-day-old mutants in the molecular layer and a 84% decrease in 300-day-old mutants compared to controls. The initial decrease in total BZ receptors in the 45-day-old mutant animals is associated with a selective loss of Purkinje cells. The amount of receptor binding which persists in the 300-day-old mutants in the molecular layer would appear to reflect binding in the remaining parallel fibers from granule cells which remain.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding studies in the Lurcher mutant suggest an uneven distribution of putative benzodiazepine receptor subclasses in the mouse cerebellum

Binding studies using [3H]flunitrazepam in the cerebellum of normal and Lurcher mutant mice indicate that about 90% of the total benzodiazepine receptor population reside on the Purkinje and granule cells. Moreover, the specific binding of the BZ1 receptor subclass specific ligand [3H]propyl-beta-carboline-3-carboxylate suggests that neuronal elements of the mouse cerebellum other than the Purkinje and granule cells contain about 18% of the BZ1 receptors but only about 7% of the BZ2 receptors of the mouse cerebellum.

Properties of [3H]diazepam binding sites on cultured murine glia and neurons

[3H]Diazepam binding was assayed in situ on living cultures of fetal mouse cerebral cortex or glia in an attempt to further characterize the high and low affinity binding sites. Mixed neuronal-glial cultures were found to have a high (Kd approximately equal to 10 nM) as well as a low (Kd approximately equal to 240 nM) affinity binding site. Glial cultures also had a similarly high affinity site (Kd 13 nM). In both types of cultures, the high affinity site was Ro 5-4864 sensitive and clonazepam resistant. Since Ro 5-4864 has particular affinity for non-neuronal elements and clonazepam for neuronal elements, the data suggest that the high affinity binding site may be localized to glial elements and the low affinity site primarily neuronal.