Plasticity of postsynaptic, but not presynaptic, GABA \u3c inf\u3e B receptors inSSADH deficient mice

Succinic semialdehyde dehydrogenase (SSADH) deficiency is an autosomal-recessively inherited disorder of γ-aminobutyrate (GABA) catabolism characterized by ataxia and epilepsy. Since SSADH is responsible for GABA break-down downstream of GABA transaminase, patients manifest high extracellular levels of GABA, as well as the GABAB receptor (GABABR) agonist γ-hydroxybutyrate (GHB). SSADH knockout (KO) mice display absence seizures, which progress into lethal tonic-clonic seizures at around 3weeks of age. It is hypothesized that desensitization of GABABRs plays an important role in the disease, although detailed studies of pre- and postsynaptic GABABRs are not available. We performed patch-clamp recordings from layer 2/3 pyramidal neurons in neocortical brain slices of wild-type (WT) and SSADH KO mice. Electrical stimulation of GABAergic fibers during wash in of the GABABR agonist baclofen revealed no difference in presynaptic GABABR mediated inhibition of GABA release between WT and SSADH KO mice. In contrast, a significant decrease in postsynaptic baclofen-induced potassium currents was seen in SSADH KO mice. This reduction was unlikely to be caused by accumulation of potassium, GABA or GHB in the brain slices, or an altered expression of regulators of G-protein signaling (RGS) proteins. Finally, adenosine-induced potassium currents were also reduced in SSADH KO mice, which could suggest heterologous desensitization of the G-protein dependent effectors, leading to a reduction in G-protein coupled inwardly rectifying potassium (GIRK) channel responses. Our findings indicate that high GABA and GHB levels desensitize postsynaptic, but not certain presynaptic, GABABRs, promoting a decrease in GIRK channel function. These changes could contribute to the development of seizures in SSADH KO mice and potentially also in affected patients. © 2010 Elsevier Inc

Stroke infarct volume estimation in fixed tissue: Comparison of diffusion kurtosis imaging to diffusion weighted imaging and histology in a rodent MCAO model

<div><p>Diffusion kurtosis imaging (DKI) is a new promising MRI technique with microstructural sensitivity superior to conventional diffusion tensor (DTI) based methods. In stroke, considerable mismatch exists between the infarct lesion outline obtained from the two methods, kurtosis and diffusion tensor derived metrics. We aim to investigate if this mismatch can be examined in fixed tissue. Our investigation is based on estimates of mean diffusivity (MD) and mean (of the) kurtosis tensor (MKT) obtained using recent fast DKI methods requiring only 19 images. At 24 hours post stroke, rat brains were fixed and prepared. The infarct was clearly visible in both MD and MKT maps. The MKT lesion volume was roughly 31% larger than the MD lesion volume. Subsequent histological analysis (hematoxylin) revealed similar lesion volumes to MD. Our study shows that structural components underlying the MD/MKT mismatch can be investigated in fixed tissue and therefore allows a more direct comparison between lesion volumes from MRI and histology. Additionally, the larger MKT infarct lesion indicates that MKT do provide increased sensitivity to microstructural changes in the lesion area compared to MD.</p></div

Whole brain volumes.

<p>The whole brain volumes measured by MD and hematoxylin stain. (n = 7). Data are represented as a scatter plot. MD: Mean diffusivity.</p

Infarct volumes.

<p>The infarct volumes visualized as the correlation between A; MD and Hematoxylin, B; MKT and Hematoxylin and C; MKT and MD (n = 7). MD: Mean diffusivity. MK: Mean Kurtosis Tensor.</p

Example of difference in visualization.

<p>Infarcted area depicted by A) MD B) MKT C) Anatomical RARE image and D) Hematoxylin stain to illustrate the difference in resolution and contrast. MD: Mean diffusivity. MKT: Mean Kurtosis Tensor.</p

Example of the H- and MAP2-stained post-hoc brains.

<p>Infarcted area on an additional post-hoc brain depicted by A) Hematoxylin stain B) MAP2-stain.</p

The intra- and inter-variability.

<p>The intra- and inter-variability obtained from all three infarct visualization methods. MD: Mean diffusivity. MK: Mean Kurtosis Tensor.</p