Abstracts

The glial water channel aquaporin-4 (AQP4) has been hypothesized to modulate water and potassium fluxes associated with neuronal activity, and alterations in AQP4 expression have been found in human temporal lobe epilepsy specimens. However, the role of AQP4 in neural signal transduction [italic]in vivo [/italic]has not yet been characterized. In this study, we examine the[italic] [/italic]seizure phenotype of AQP4^-/- mice using hippocampal stimulation and recording, and characterize extracellular K⁺ kinetics. We also examine the expression of AQP4 in the hippocampus for the first time., Male AQP4^-/- mice (n=8) and wild-type littermates (n=10) were implanted with bipolar electrodes into the right dorsal hippocampus. Following postoperative recovery, electrical stimulations (60 Hz, 1-sec train, 1-msec biphasic pulses) were given to determine electrographic seizure threshold and duration. [K⁺][sub]o[/sub] was measured following direct cortical stimulation using double-barreled K⁺-sensitive microelectrodes. To determine the expression of AQP4 in WT hippocampus, an enhanced immunohistochemical method was developed using AQP4^-/- sections as controls., AQP4^-/- mice were found to have dramatically prolonged stimulation-evoked seizures following hippocampal stimulation compared to wild-type controls (33 [plusmn] 2 sec [italic]vs.[/italic] 13 [plusmn] 2 sec). To assess the effect of AQP4 on[italic] [/italic]potassium kinetics, we used [italic]in vivo [/italic]recording with potassium-sensitive microelectrodes following direct cortical stimulation. The rise time to peak [K⁺][sub]o[/sub] (t[sub]1/2[/sub], 2.3 [plusmn] 0.5 sec) as well as the recovery to baseline [K⁺][sub]o[/sub] (t[sub]1/2[/sub], 15.6 [plusmn] 1.5 sec) were slowed in AQP4^-/- compared to WT mice (t[sub]1/2[/sub], 0.5 [plusmn] 0.1 sec and 6.6 [plusmn] 0.7 sec, respectively). Immunohistochemical analysis revealed a remarkable developmental and laminar-specific expression of AQP4 in the hippocampus, with highest expression in the CA1 stratum lacunosum-moleculare., These results establish a novel role for glial AQP4 in excitability and K⁺ regulation [italic]in vivo[/italic], and are consistent with the idea that AQP4 and its molecular partners ([italic]e.g.[/italic] Kir4.1) comprise a multifunctional [apos]unit[apos] responsible for clearance of K⁺ and/or H[sub]2[/sub]O following neural activity. The strikingly high expression of AQP4 in the CA1 SLM identifies this region as a potential site of astrocytic K⁺ and H[sub]2[/sub]O regulation. Further understanding of the glial modulation of ion and water homeostasis may lead to novel targets for anticonvulsant drug development., (Supported by NIH grants NS050173, EY13574, DK35124, DK43840, HL59198, DK72517, HL73856, a Van Wagenen Fellowship of the American Association of Neurological Surgeons, and a University of Bonn BONFOR grant.)