The Polyamine Inhibitor Alpha-Difluoromethylornithine Modulates Hippocampus-Dependent Function after Single and Combined Injuries

Exposure to uncontrolled irradiation in a radiologic terrorism scenario, a natural disaster or a nuclear battlefield, will likely be concomitantly superimposed on other types of injury, such as trauma. In the central nervous system, radiation combined injury (RCI) involving irradiation and traumatic brain injury may have a multifaceted character. This may entail cellular and molecular changes that are associated with cognitive performance, including changes in neurogenesis and the expression of the plasticity-related immediate early gene Arc. Because traumatic stimuli initiate a characteristic early increase in polyamine metabolism, we hypothesized that treatment with the polyamine inhibitor alpha-difluoromethylornithine (DFMO) would reduce the adverse effects of single or combined injury on hippocampus structure and function. Hippocampal dependent cognitive impairments were quantified with the Morris water maze and showed that DFMO effectively reversed cognitive impairments after all injuries, particularly traumatic brain injury. Similar results were seen with respect to the expression of Arc protein, but not neurogenesis. Given that polyamines have been found to modulate inflammatory responses in the brain we also assessed the numbers of total and newly born activated microglia, and found reduced numbers of newly born cells. While the mechanisms responsible for the improvement in cognition after DFMO treatment are not yet clear, the present study provides new and compelling data regarding the potential use of DFMO as a potential countermeasure against the adverse effects of single or combined injury

Blockade of TRPM7 Channel Activity and Cell Death by Inhibitors of 5-Lipoxygenase

TRPM7 is a ubiquitous divalent-selective ion channel with its own kinase domain. Recent studies have shown that suppression of TRPM7 protein expression by RNA interference increases resistance to ischemia-induced neuronal cell death in vivo and in vitro, making the channel a potentially attractive pharmacological target for molecular intervention. Here, we report the identification of the 5-lipoxygenase inhibitors, NDGA, AA861, and MK886, as potent blockers of the TRPM7 channel. Using a cell-based assay, application of these compounds prevented cell rounding caused by overexpression of TRPM7 in HEK-293 cells, whereas inhibitors of 12-lipoxygenase and 15-lipoxygenase did not prevent the change in cell morphology. Application of the 5-lipoxygenase inhibitors blocked heterologously expressed TRPM7 whole-cell currents without affecting the protein's expression level or its cell surface concentration. All three inhibitors were also effective in blocking the native TRPM7 current in HEK-293 cells. However, two other 5-lipoxygenase specific inhibitors, 5,6-dehydro-arachidonic acid and zileuton, were ineffective in suppressing TRPM7 channel activity. Targeted knockdown of 5-lipoxygenase did not reduce TRPM7 whole-cell currents. In addition, application of 5-hydroperoxyeicosatetraenoic acid (5-HPETE), the product of 5-lipoxygenase, or 5-HPETE's downstream metabolites, leukotriene B4 and leukotriene D4, did not stimulate TRPM7 channel activity. These data suggested that NDGA, AA861, and MK886 reduced the TRPM7 channel activity independent of their effect on 5-lipoxygenase activity. Application of AA861 and NDGA reduced cell death for cells overexpressing TRPM7 cultured in low extracellular divalent cations. Moreover, treatment of HEK-293 cells with AA861 increased cell resistance to apoptotic stimuli to a level similar to that obtained for cells in which TRPM7 was knocked down by RNA interference. In conclusion, NDGA, AA861, and MK886 are potent blockers of the TRPM7 channel capable of attenuating TRPM7's function during cell stress, making them effective tools for the biophysical characterization and suppression of TRPM7 channel conductance in vivo

Experimental traumatic brain injury

Traumatic brain injury, a leading cause of death and disability, is a result of an outside force causing mechanical disruption of brain tissue and delayed pathogenic events which collectively exacerbate the injury. These pathogenic injury processes are poorly understood and accordingly no effective neuroprotective treatment is available so far. Experimental models are essential for further clarification of the highly complex pathology of traumatic brain injury towards the development of novel treatments. Among the rodent models of traumatic brain injury the most commonly used are the weight-drop, the fluid percussion, and the cortical contusion injury models. As the entire spectrum of events that might occur in traumatic brain injury cannot be covered by one single rodent model, the design and choice of a specific model represents a major challenge for neuroscientists. This review summarizes and evaluates the strengths and weaknesses of the currently available rodent models for traumatic brain injury

Astrocytes convert network excitation to tonic inhibition of neurons

<p>Abstract</p> <p>Background</p> <p>Glutamate and γ-aminobutyric acid (GABA) transporters play important roles in balancing excitatory and inhibitory signals in the brain. Increasing evidence suggest that they may act concertedly to regulate extracellular levels of the neurotransmitters.</p> <p>Results</p> <p>Here we present evidence that glutamate uptake-induced release of GABA from astrocytes has a direct impact on the excitability of pyramidal neurons in the hippocampus. We demonstrate that GABA, synthesized from the polyamine putrescine, is released from astrocytes by the reverse action of glial GABA transporter (GAT) subtypes GAT-2 or GAT-3. GABA release can be prevented by blocking glutamate uptake with the non-transportable inhibitor DHK, confirming that it is the glutamate transporter activity that triggers the reversal of GABA transporters, conceivably by elevating the intracellular Na⁺concentration in astrocytes. The released GABA significantly contributes to the tonic inhibition of neurons in a network activity-dependent manner. Blockade of the Glu/GABA exchange mechanism increases the duration of seizure-like events in the low-[Mg²⁺] <it>in vitro </it>model of epilepsy. Under <it>in vivo </it>conditions the increased GABA release modulates the power of gamma range oscillation in the CA1 region, suggesting that the Glu/GABA exchange mechanism is also functioning in the intact hippocampus under physiological conditions.</p> <p>Conclusions</p> <p>The results suggest the existence of a novel molecular mechanism by which astrocytes transform glutamat<it>ergic </it>excitation into GABA<it>ergic </it>inhibition providing an adjustable, <it>in situ </it>negative feedback on the excitability of neurons.</p