Selective Vulnerability of Hippocampal Vs. Cortical Neurons to Mechanically Induced Increases in Plasma Membrane Permeability

Traumatic brain injuries (TBI) are characterized by a high rate impact to the head and often result in functional deficits or death. Experimental models of TBI have revealed that the severity of impact directly correlates to the amount of functional and histological damage. However, selective regions of the hippocampus are severely injured despite their distance from the initial insult to the cortex. Hippocampal neurons may be intrinsically more vulnerable to mechanical insult than cortical neurons due to increased NMDA receptor densities and lower energy capacities, as evidenced after experimental ischemia. We hypothesized that the neuronal response to mechanical insult depended on both the applied strain parameters and the specific neuronal subtype. This theory was evaluated using an in vitro model of TBI where either primary rat cortical or hippocampal neurons seeded onto silicone substrates were subjected to graded levels of mechanical stretch. We found a rate- and magnitude-dependent plasma membrane permeability increase that may be the initiating mechanism that translates mechanical stretch to cellular dysfunction. Various sized fluorescent molecules were added to the bathing media either immediately before injury or 1, 2, 5, or 10 minutes after injury and removed one minute later. Severe stretch (10s^-1, 0.30) resulted in significant uptake of all tested molecules (ranging between 0.5 to 8.9 nm) with up to 60% of cells positively stained. The neurons remained permeable to carboxyfluorescein up to five minutes after severe stretch but were only permeable to larger molecules (≥10kDa) immediately after stretch. Independent of the initial permeability change, however, hippocampal neurons were selectively vulnerable to mechanical stretch compared to cortical neurons. Injury of hippocampal neurons resulted in higher intracellular free calcium concentration [Ca²⁺]_i increases, lactate dehydrogenase (LDH) release, and cell death. Stretch-induced [ATP]_i deficits were apparent by 60 min after injury in cortical neurons but recovered by 24 hrs. Significant deficits in [ATP]_i were not observed in hippocampal neurons until 24 hrs after injury. As the hippocampus is the primary region responsible for cognitive deficits associated with TBI, understanding why this region is selectively damaged could lead to the development of better mechanical tolerances and more effective pharmaceutical agents

Year	Entry
1999	Thurman DJ, Alverson C, Dunn KA, Guerrero J, Sniezek JE. Traumatic brain injury in the United States: a public health perspective. J Head Trauma Rehabil. December 1999;14(6):602-615.
1994	Pettus EH, Christman CW, Giebel ML, Povlishock JT. Traumatically induced altered membrane permeability: its relationship to traumatically induced reactive axonal change. J Neurotrauma. October 1994;11(5):507-522.
2001	Geddes DM, Cargill RS II. An in vitro model of neural trauma: device characterization and calcium response to mechanical stretch. J Biomech Eng. June 2001;123(3):247-255.
1993	Galbraith JA, Thibault LE, Matteson DR. Mechanical and electrical responses of the squid giant axon to simple elongation. J Biomech Eng. February 1993;115(1):13-22.
1996	Cargill RS II, Thibault LE. Acute alterations in [Ca²⁺]_i in NG108-15 cells subjected to high strain rate deformation and chemical hypoxia: an in vitro model for neural. J Neurotrauma. July 1996;13(7):395-407.
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Year

Entry

1999

Thurman DJ, Alverson C, Dunn KA, Guerrero J, Sniezek JE. Traumatic brain injury in the United States: a public health perspective. J Head Trauma Rehabil. December 1999;14(6):602-615.

1994

Pettus EH, Christman CW, Giebel ML, Povlishock JT. Traumatically induced altered membrane permeability: its relationship to traumatically induced reactive axonal change. J Neurotrauma. October 1994;11(5):507-522.

2001

Geddes DM, Cargill RS II. An in vitro model of neural trauma: device characterization and calcium response to mechanical stretch. J Biomech Eng. June 2001;123(3):247-255.

1993

Galbraith JA, Thibault LE, Matteson DR. Mechanical and electrical responses of the squid giant axon to simple elongation. J Biomech Eng. February 1993;115(1):13-22.

1996

Cargill RS II, Thibault LE. Acute alterations in [Ca²⁺]_i in NG108-15 cells subjected to high strain rate deformation and chemical hypoxia: an in vitro model for neural. J Neurotrauma. July 1996;13(7):395-407.