Selective vasopressin-1a receptor antagonist prevents brain edema, reduces astrocytic cell swelling and GFAP, V1aR and AQP4 expression after focal traumatic brain injury

Abstract

A secondary and often lethal consequence of traumatic brain injury is cellular edema that we posit is due to astrocytic swelling caused by transmembrane water fluxes augmented by vasopressin-regulated aquaporin-4 (AQP4). We therefore tested whether vasopressin 1a receptor (V1aR) inhibition would suppress astrocyte AQP4, reduce astrocytic edema, and thereby diminish TBI-induced edematous changes. V1aR inhibition by SR49059 significantly reduced brain edema after cortical contusion injury (CCI) in rat 5 h post-injury. Injured-hemisphere brain water content (n=6 animals/group) and astrocytic area (n=3/group) were significantly higher in CCI-vehicle (80.5±0.3%; 18.0±1.4 µm2) versus sham groups (78.3±0.1%; 9.5±0.9 µm2), and SR49059 blunted CCI-induced increases in brain edema (79.0±0.2%; 9.4±0.8 µm2). CCI significantly up-regulated GFAP, V1aR and AQP4 protein levels and SR49059 suppressed injury induced up regulation (n=6/group). In CCI-vehicle, sham and CCI-SR49059 groups, GFAP was 1.58±0.04, 0.47±0.02, and 0.81±0.03, respectively; V1aR was 1.00±0.06, 0.45±0.05, and 0.46±0.09; and AQP4 was 2.03± 0.34, 0.49±0.04, and 0.92±0.22. Confocal immunohistochemistry gave analogous results. In CCI-vehicle, sham and CCI-SR49059 groups, fluorescence intensity of GFAP was 349±38, 56±5, and 244±30, respectively, V1aR was 601±71, 117.8±14, and 390±76, and AQP4 was 818±117, 158±5, and 458±55 (n=3/group). The results support that edema was predominantly cellular following CCI and documented that V1aR inhibition with SR49059 suppressed injury-induced up regulation of GFAP, V1A and AQP4, blunting edematous changes. Our findings suggest V1aR inhibitors may be potential therapeutic tools to prevent cellular swelling and provide treatment for post-traumatic brain edema.

Real-Time Monitoring of Changes in Brain Extracellular Sodium and Potassium Concentrations and Intracranial Pressure after Selective Vasopressin-1a Receptor Inhibition following Focal Traumatic Brain Injury in Rats

“…30 Overall, we showed that V1aR inhibition decreases brain AQP4 expression, brain edema, and astrocytic cell swelling, and reduces expression of V1aR. 30,40 Importantly, these studies showed a clear correlation between the time course of the traumainduced shift in intracerebral Na + concentrations and the increase in percent brain water content. 22,30,31,39,41 Our findings were consistent with previous reports demonstrating similar shifts between water and Na + content.…”

“…Nevertheless, an excess cellular Na + uptake relative to K + loss with associated counter ions could create an osmotic gradient, drive water in cells, especially astrocytes, and lead to brain edema. 30,40,49,50 For example, total tissue brain Na + as measured by flame photometry remained elevated at 5 h after CCI. 41 Interpreted in combination with the present ISE data showing a fall and recovery of [Na + ] e post CCI, the elevation of tissue Na + content previously reported 41 suggests that intracellular Na + must have been elevated post CCI and is likely to contribute to concomitant cytotoxic edema.…”

“…ICP remained elevated for 5 h in untreated CCI despite the recovery of extracellular cations, and our previous work supports ionic dysfunction, brain edema, astrocytic cell swelling, and increased AQP4 expression at the same time point. 30,40,41,55 Elevated ICP requires an increase in water and osmolytes within the brain, and one can infer that a net flux of water from vascular compartment to intracellular and/or extracellular compartments must have occurred. We posit that the transient shifts in [Na + ] e and [K + ] e that were observed may ''light the fire'' that initiates a deprivation of energy supply, cellular metabolic imbalance, increased production of osmotically active products by metabolism, neurotransmitter release, and alterations in cellular function that give rise to and sustain brain edema.…”

“…V1aR expression is significantly increased after traumatic brain injury in the neocortex, 20,21 and SR49059 application reduces brain edema after injury, 30 astrocytic swelling, and AQP4 expression. 40 Because transmembrane Na + flux is coupled to osmotic water flux, SR49059 may reduce water influx into cells after brain injury, in part by balancing the rate of osmotically active Na + and K + recovery. These actions would give ECF osmolytes and water a greater opportunity to drain by bulk flow through the ECF and CSF, and in turn, minimizing cell swelling and brain edema, and limiting the elevation in ICP.…”

Anti-edema effect of melatonin on spinal cord injury in rats

“…Astrocytes are much more prone to swelling than neurons 42 , for which GFAP is a specific marker 43 . In this study, melatonin (100 mg/kg) could obviously decrease the GFAP protein level in the spinal cord from 24 to 72 h after SCI.…”