Pharmacologically induced calcium oscillations protect neurons from increases in cytosolic calcium after trauma.

Increases in cytosolic calcium ([Ca2+](i)) following mechanical injury are often considered a major contributing factor to the cellular sequelae in traumatic brain injury (TBI). However, very little is known on how developmental changes may affect the calcium signaling in mechanically injured neurons. One key feature in the developing brain that may directly impact its sensitivity to stretch is the reduced inhibition which results in spontaneous [Ca2+](i) oscillations. In this study, we examined the mechanism of stretch-induced [Ca2+](i) transients in 18-days in vitro (DIV) neurons exhibiting bicuculline-induced [Ca2+](i) oscillations. We used an in vitro model of mechanical trauma to apply a defined uniaxial strain to cultured cortical neurons and used increases in [Ca2+](i) as a measure of the neuronal response to the stretch insult. We found that stretch-induced increases in [Ca2+](i) in 18-DIV neurons were inhibited by pretreatment with either the NMDA receptor antagonist, APV [D(-)-2-Amino-5-phosphonopentanoic acid], or by depolymerizing the actin cytoskeleton prior to stretch. Blocking synaptic NMDA receptors prior to stretch significantly attenuated most of the [Ca2+](i) transient. In comparison, cultures with pharmacologically induced [Ca2+](i) oscillations showed a substantially reduced [Ca2+](i) peak after stretch. We provide evidence showing that a contributing factor to this mechanical desensitization from induced [Ca2+](i) oscillations is the PKC-mediated uncoupling of NMDA receptors (NMDARs) from spectrin, an actin-associated protein, thereby rendering neurons insensitive to stretch. These results provide novel insights into how the [Ca2+](i) response to stretch is initiated, and how reduced inhibition - a feature of the developing brain - may affect the sensitivity of the immature brain to trauma.