Neuroscience
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The activity and/or expression of the mitogen-activated protein kinases c-Jun N-terminal kinase 1, p38 and extracellular signal-regulated kinases 1/2, as well as their substrates, the transcription factors c-Jun and activating transcription factor-2, were examined following systemic application of kainate in the cortex and hippocampus of the adult rat brain. The protein expression levels of all three mitogen-activated protein kinases remained constant during the observation period. Unexpectedly, c-Jun N-terminal kinase 1 was the only mitogen-activated protein kinase activated in this model of excitotoxicity, its activity raised from between 1 and 3 h moderate basal to maximal levels between 6 and 12 h. ⋯ A second set of supershift experiments demonstrated that c-Jun, but not activating transcription factor 2, bound to activator protein-1 sites in the promoter of substance P and collagenase genes, but not of the cyclo-oxygenase-2 gene. Our results demonstrate that activation of c-Jun N-terminal kinase 1, phosphorylation of c-Jun and selective occupation of the c-jun promoter by activating transcription factor-2 or c-Jun are part of the neuronal response following excitotoxicity that is considered as the mechanism for neuronal apoptosis in vivo. Some of these findings differ substantially from in vitro experiments and underline the necessity to analyse the neuronal stress pathways in the adult brain.
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Corticospinal neurons were identified in primary cultures of cortical neurons established from rats that had been injected with a fluorescent tracer to retrogradely label the corticospinal tract. We measured neurite outgrowth from corticospinal neurons after they had been co-cultured with astrocytes derived from either the cerebral cortex (homotopic region) or spinal cord (target region) of postnatal rats. The axon length of corticospinal neurons was increased when they were cultured on astroglial monolayers compared to a control monolayer (fibroblasts). ⋯ If the corticospinal neurons were co-cultured without physical contact with the astrocytes, axonal and dendritic outgrowth were not stimulated when compared to the fibroblast control. The data indicate that dendritic growth from corticospinal neurons is preferentially promoted by astrocytes from the cerebral cortex, whereas axonal growth is not influenced by the anatomical origin of the astrocytes. The impact of these findings on our understanding of the role of astrocytes in the development and regeneration of the corticospinal tract is discussed.
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Cortistatin is a 14-residue putative neuropeptide with strong structural similarity to somatostatin. Even if it shares several biological properties with somatostatin, the effects of cortistatin on cortical electrical activity and sleep are opposite to those elicited by somatostatin. We recently demonstrated that somatostatin could modulate glutamate sensitivity, either positively through activation of the sstl receptor subtype, or negatively through activation of the sst2 receptor subtype in hypothalamic neurons in culture which express almost exclusively these two sst subtypes. ⋯ We first determined that the affinities of cortistatin and somatostatin were similar on cloned rat sstl and sst2 receptor subtypes in transfected cells and hypothalamic neurons membranes. We then found that cortistatin, like somatostatin, depresses the glutamate response but, unlike somatostatin, never potentiates glutamate sensitivity in hypothalamic neurons. The observed effect of cortistatin is strongly suggestive of an activation of the somatostatin sst2 receptor subtype in hypothalamic neurons in culture.
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An exogenous glutamate injection into the hypothermic hippocampal CA1 during 5-min ischemia produced the same extent of extracellular glutamate levels as observed in the normothermic CA1 during 5-min ischemia; however, neuronal death was not induced in the hypothermic CA1. Glutamate is released excessively into the extracellular space during ischemia, and is thought to induce brain injury by its neurotoxicity. It has been reported that the massive glutamate release is reduced by mild hypothermia, and it has been proposed that the reduction of ischemia-induced glutamate release exerts the neuroprotective effect on postischemic neuronal death. ⋯ An injection with 1 mM L-glutamate into the hypothermic CA1 during 5-min ischemia produced a similar extent of increased glutamate (17-fold increase) to that observed in the normothermic CA1 during 5-min ischemia (16-fold increase). However, neuronal death was not induced in the hypothermic CA1. This result indicates that the neuroprotective effect of mild hypothermia cannot be explained in terms of a reduction of glutamate release during ischemia.
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Inflammatory processes may play a critical role in the degeneration of basal forebrain cholinergic cells that underlies some of the cognitive impairments associated with Alzheimer's disease. In the present study, the proinflammagen lipopolysaccharide, from the cell wall of Gram-negative bacteria, was used to produce inflammation within the basal forebrain of rats. The effects of acute, high-dose injections of lipopolysaccharide (2, 20 or 40 microg) upon basal forebrain chemistry and neuronal integrity were compared with the effects of chronic, low-dose lipopolysaccharide infusions (0.18, 0.25, 1.8 or 5.0 microg/h) for either 14, 37, 74 or 112 days. ⋯ Regional levels of five different neuropeptides were unchanged by acute, high-dose lipopolysaccharide injections. Chronic lipopolysaccharide infusions produced (i) a time-dependent, but not dose-dependent, decrease in cortical choline acetyltransferase activity that paralleled a decline in the number of choline acetyltransferase- and p75-immunoreactive cells within the basal forebrain, and (ii) a dense distribution of reactive astrocytes and microglia within the basal forebrain. Chronic neuroinflammation might underlie the genesis of some neuropathological changes associated with normal ageing or Alzheimer's disease.