Reduced connexin43 expression correlates with c‐Src activation,proliferation, and glucose uptake in reactive astrocytes after an excitotoxic insult

In diverse brain pathologies, astrocytes become reactive and undergo profound phenotypic changes. Connexin43 (Cx43), the main gap junction channel‐forming protein in astrocytes, is one of the proteins modified in reactive astrocytes. Downregulation of Cx43 in cultured astrocytes activates c‐Src, promotes proliferation, and increases the rate of glucose uptake; however, so far there have been no studies examining whether this cascade of events takes place in reactive astrocytes. In this work, we analyzed this pathway after a cortical lesion induced by a kainic acid injection. As previously described, astrocytes reacted to the lesion with an increase in glial fibrillary acidic protein and a decrease in Cx43 expression. Some of these reactive astrocytes proliferated, as estimated by bromodeoxyuridine incorporation and cyclins D1 and D3 upregulation. In addition, the expression of the glucose transporter GLUT‐3 and the enzyme responsible for glucose phosphorylation, Type II hexokinase (Hx‐2), were induced in reactive astrocytes, suggesting an increased glucose uptake. Previous in vitro studies reported that c‐Src is the link between Cx43 and glucose uptake and proliferation in astrocytes. Here, we found that c‐Src activity increased in the lesioned area. c‐Src activation and Cx43 downregulation preceded the peak of Hx‐2 and cyclin D3 expression, suggesting that c‐Src could mediate the effect of Cx43 on glucose uptake and proliferation in reactive astrocytes after an excitotoxic insult. Interestingly, we identify c‐Src, GLUT‐3, and Hx‐2 in the signaling mechanisms involved in the reaction of astroglia to injury. Altogether these data contribute to identify new therapeutical targets to enhance astrocyte neuroprotective activities. © 2012 Wiley Periodicals, Inc.     

Cholinergic excitation in mouse primary vs. associative cortex: region‐specific magnitude and receptor balance

Cholinergic stimulation of the cerebral cortex is essential for tasks requiring attention; however, there is still some debate over which cortical regions are required for such tasks. There is extensive cholinergic innervation of both primary and associative cortices, and transient release of acetylcholine (ACh) is detected in deep layers of the relevant primary and/or associative cortex, depending on the nature of the attention task. Here, we investigated the electrophysiological effects of ACh in layer VI, the deepest layer, of the primary somatosensory cortex, the primary motor cortex, and the associative medial prefrontal cortex. Layer VI pyramidal neurons are a major source of top‐down modulation of attention, and we found that the strength and homogeneity of their direct cholinergic excitation was region‐specific. On average, neurons in the primary cortical regions showed weaker responses to ACh, mediated by a balance of contributions from both nicotinic and muscarinic ACh receptors. Conversely, neurons in the associative medial prefrontal cortex showed significantly stronger excitation by ACh, mediated predominantly by nicotinic receptors. The greatest diversity of responses to ACh was found in the primary somatosensory cortex, with only a subset of neurons showing nicotinic excitation. In a mouse model with attention deficits only under demanding conditions, cholinergic excitation was preserved in primary cortical regions but not in the associative medial prefrontal cortex. These findings demonstrate that the effect of ACh is not uniform throughout the cortex, and suggest that its ability to enhance attention performance may involve different cellular mechanisms across cortical regions.     

Axospinous synaptic subtype‐specific differences in structure,size, ionotropic receptor expression,and connectivity in apical dendritic regions of rat hippocampal CA1 pyramidal neurons

The morphology of axospinous synapses and their parent spines varies widely. Additionally, many of these synapses are contacted by multiple synapse boutons (MSBs) and show substantial variability in receptor expression. The two major axospinous synaptic subtypes are perforated and nonperforated, but there are several subcategories within these two classes. The present study used serial section electron microscopy to determine whether perforated and nonperforated synaptic subtypes differed with regard to their distribution, size, receptor expression, and connectivity to MSBs in three apical dendritic regions of rat hippocampal area CA1: the proximal and distal thirds of stratum radiatum, and the stratum lacunosum‐moleculare. All synaptic subtypes were present throughout the apical dendritic regions, but there were several subclass‐specific differences. First, segmented, completely partitioned synapses changed in number, proportion, and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA) receptor expression with distance from the soma beyond that found within other perforated synaptic subtypes. Second, atypically large, nonperforated synapses showed N‐methyl‐D ‐aspartate (NMDA) receptor immunoreactivity identical to that of perforated synapses, levels of AMPA receptor expression intermediate to that of nonperforated and perforated synapses, and perforated synapse‐like changes in structure with distance from the soma. Finally, MSB connectivity was highest in the proximal stratum radiatum, but only for those MSBs composed of nonperforated synapses. The immunogold data suggest that most MSBs would not generate simultaneous depolarizations in multiple neurons or spines, however, because the vast majority of MSBs are comprised of two synapses with abnormally low levels of receptor expression, or involve one synapse with a high level of receptor expression and another with only a low level. J. Comp. Neurol. 512:399–418, 2009. © 2008 Wiley‐Liss, Inc.