The C57BL/10Bg sps/sps mouse: a mutant with absence-like seizures; neurochemical and behavioral correlates

Abnormalities in brain structure and biochemistry associated with mdx mice measured by in vivo MRI and high resolution localized (1)H MRS

Duchenne muscular dystrophy (DMD), an X-linked disorder caused by the lack of dystrophin, is characterized by the progressive wasting of skeletal muscles. To date, what is known about dystrophin function is derived from studies of dystrophin-deficient animals, with the most common model being the mdx mouse. Most studies on patients with DMD and in mdx mice have focused on skeletal muscle and the development of therapies to reverse, or at least slow, the severe muscle wasting and progressive degeneration. However, dystrophin is also expressed in the CNS. Both mdx mice and patients with DMD can have cognitive and behavioral changes, but studies in the dystrophic brain are limited. We examined the brain structure and metabolites of mature wild type (WT) and mdx mice using magnetic resonance imaging and spectroscopy (MRI/MRS). Both structural and metabolic alterations were observed in the mdx brain. Enlarged lateral ventricles were detected in mdx mice when compared to WT. Diffusion tensor imaging (DTI) revealed elevations in diffusion diffusivities in the prefrontal cortex and a reduction of fractional anisotropy in the hippocampus. Metabolic changes included elevations in phosphocholine and glutathione, and a reduction in γ-aminobutyric acid in the hippocampus. In addition, an elevation in taurine was observed in the prefrontal cortex. Such findings indicate a regional structural change, altered cellular antioxidant defenses, a dysfunction of GABAergic neurotransmission, and a perturbed osmoregulation in the brain lacking dystrophin.

entla, a novel epileptic and ataxic Cacna2d2 mutant of the mouse

entla (ent) is a novel recessive phenotype of mice. The underlying mutation was mapped to chromosome 9 (60.1 centimorgans) and identified as an allele of the Cacna2d2 gene encoding the alpha2delta-2 subunit of voltage-gated calcium channels. The Cacna2d2entla allele harbors a 38-kb duplication comprising the 117 nucleotides of exon 3. The predicted duplication of 39 amino acid residues near the subunit's N terminus results in the expression of a full-length, membrane-associated protein. Western blot data were consistent with correct cleavage of the alpha2delta-2entla precursor into alpha2entla and delta2 proteins but indicated loss of the disulfide linkage between the two proteins. ent/ent mice develop ataxia by postnatal day 13-15, followed by paroxysmal dyskinesia a few days later. Two distinct types of cortical and hippocampal epileptic activity at 2 and 4 Hz were recorded, indicative of absence epilepsy. Homozygotes display reduced size and weight, increased mortality before weaning, and female infertility. No overt neuroanatomical abnormalities were detected. Ca2+ current densities recorded from acutely dissociated Purkinje cells of homozygous entla animals were reduced by 50% compared with wild type. Ligand binding assays using the antiepileptic drug [3H]gabapentin, a specific ligand of the alpha2delta-1 and alpha2delta-2 subunits, revealed a >60% reduced maximum binding to cerebellar membranes of ent/ent compared with unaffected littermates. entla is allelic to ducky and ducky2J, representing the third murine Cacna2d2 allele identified and so far the only one encoding an untruncated protein that is incorporated into membranes.

High affinity [3H]glutamate uptake systems in normal and audiogenic seizure-susceptible mice

Two high affinity sodium-dependent and PDC-sensitive glutamate (GLU) uptake systems are present in a whole brain synaptosomal preparation from adult C57BL/10 SPS/SPS normal mice. System 1 has an apparent Km of 3.65 microM while that of System 2 is 46.8 microM. Glutamate uptake in the normal mice increases gradually during development, displaying a striking peak at postnatal day 15, and decreases rapidly between PN 16 and PN 20 until it reaches adult levels. The developmental pattern of GLU uptake System 1 and System 2 in audiogenic seizure susceptible mice is similar to that described in normal mice. However, there are differences between GLU uptake system 1 and 2 during ontogenesis: (1) System 1 could not be detected until PN 15 while being markedly diminished in adulthood; and (2) GLU uptake by System 2 is increased in adults. In addition, the Umax for System 2 is significantly greater than that of normal mice at PN 2 and PN 15.

GABAergic neurotransmission in the C57BL/10 sps/sps mouse mutant: a model of absence seizures

The C57BL/10 sps/sps mouse mutant displays generalized absence seizure-like behavior. In these mice, glutamic acid decarboxylase activity is reduced in the cortex and hippocampus. Tritiated flunitrazepam binding (3H-flu) is reduced in these areas, as well as in midbrain, cerebellum, and pons-medulla. Quantitative [3H]-flunitrazepam binding autoradiography confirms these observations. GABA uptake by synaptosomes from sps/sps mice is also reduced in all the areas studied. Potassium-stimulated, Ca(2+)-dependent release of radioactivity from synaptosomes preloaded with [14C]-GABA is reduced in the hippocampus, increased in midbrain and pons-medulla, but remains unaltered in the cortex. These results suggest region-specific alterations in GABAergic neurotransmission that may be responsible for the absence-like seizures in C57BL/10 sps/sps mice.