Ischemic brain injury is a major problem associated with stroke. It has been increasingly recognized that acid-sensing ion channels (ASICs) contribute significantly to ischemic neuronal damage, but the underlying mechanism has remained elusive. Here, we show that extracellular spermine, one of the endogenous polyamines, exacerbates ischemic neuronal injury through sensitization of ASIC1a channels to extracellular acidosis. Pharmacological blockade of ASIC1a or deletion of the ASIC1 gene greatly reduces the enhancing effect of spermine in ischemic neuronal damage both in cultures of dissociated neurons and in a mouse model of focal ischemia. Mechanistically, spermine profoundly reduces desensitization of ASIC1a by slowing down desensitization in the open state, shifting steady-state desensitization to more acidic pH, and accelerating recovery between repeated periods of acid stimulation. Spermine-mediated potentiation of ASIC1a activity is occluded by PcTX1 (psalmotoxin 1), a specific ASIC1a inhibitor binding to its extracellular domain. Functionally, the enhanced channel activity is accompanied by increased acid-induced neuronal membrane depolarization and cytoplasmic Ca2+ overload, which may partially explain the exacerbated neuronal damage caused by spermine. More importantly, blocking endogenous spermine synthesis significantly attenuates ischemic brain injury mediated by ASIC1a but not that by NMDA receptors. Thus, extracellular spermine contributes significantly to ischemic neuronal injury through enhancing ASIC1a activity. Our data suggest new neuroprotective strategies for stroke patients via inhibition of polyamine synthesis and subsequent spermine–ASIC interaction.

Acidosis is a common feature of human brain during ischemic stroke and is known to independently cause neuronal injury. However, the mechanism underlying acidosis-mediated injury of human brain remains elusive. We show that lowering the extracellular pH evoked inward currents characteristic of acid-sensing ion channels (ASICs) and increased intracellular Ca2+ in cultured human cortical neurons. ASICs in human cortical neurons show electrophysiological and pharmacological properties distinct from neurons in rodent brain. RT-PCR and Western blot detected high level of ASIC1a subunit with little or no expression of other ASIC subunits. Treatment of human cortical neurons with acidic solution induced substantial cell injury, which was attenuated by ASIC1a blockade. Thus, functional homomeric ASIC1a channels are predominantly expressed in neurons from human brain. Activation of these channels plays an important role in acidosis-mediated injury of human brain neurons.

Chronically implanted biosensors typically lose sensitivity 1-2 months after implantation, due in large part to the development of a collagen-rich capsule that prevents analytes of interest from reaching the biosensor. Corticosteroids are likely candidates for reducing collagen deposition but these compounds have many serious side effects when given over a prolonged period. One method of assessing whether or not locally released corticosteroids have a systemic effect is to measure cortisol concentrations in venous serum. We hypothesized that a very low release rate of the potent corticosteroid, dexamethasone, would lead to a localized anti-inflammatory effect without systemic effects. We found that reduction in subcutaneous granulocytes (primarily eosinophils), and to a lesser extent, reduction of macrophages served as a good local indicator of the steroid effect. When released over a 28-day period, a total dexamethasone dose of < or =0.1 mg/kg led to a consistent reduction in the number of granulocytes and macrophages found in the local vicinity of the implant without a reduction of these cells at distant tissue locations. The lack of suppression of serum cortisol with these doses confirmed that low-release rates of dexamethasone can lead to consistent local anti-inflammatory effects without distant, systemic effects. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.