Neuronal Gap Junctions Are Required for NMDA Receptor - Mediated 1

19 20 21 2 ABSTRACT 22 23 NMDA receptors (NMDAR) play an important role in cell survival versus cell 24 death decisions during neuronal development, ischemia, trauma and epilepsy. 25 Coupling of neurons by electrical synapses (gap junctions) is high or increases in 26 neuronal networks during all these conditions. In the developing CNS, neuronal gap 27 junctions are critical for two different types of NMDAR-dependent cell death. However, 28 whether neuronal gap junctions play a role in NMDAR-dependent neuronal death in 29 the mature CNS was not known. Using Fluoro-Jade B staining, we show that a single 30 intraperitoneal administration of NMDA (100 mg/kg) to adult wild-type mice induces 31 neurodegeneration in three forebrain regions, including rostral dentate gyrus. However, 32 the NMDAR-mediated neuronal death is prevented by pharmacological blockade of 33 neuronal gap junctions (with mefloquine, 30 mg/kg) and does not occur in mice lacking 34 neuronal gap junction protein, connexin 36. Using western blots, electrophysiology, 35 calcium imaging and gas chromatography-mass spectrometry in wild-type and 36 connexin 36 knockout mice, we demonstrate that the reduced level of neuronal death 37 in knockout animals is not due to the reduced expression of NMDARs, activity of 38 NMDARs, or permeability of the blood-brain barrier to NMDA. In wild-type animals, 39 this neuronal death is not due to up-regulation of connexin 36 by NMDA. Finally, 40 pharmacological and genetic inactivation of neuronal gap junctions in mice also 41 dramatically reduce neuronal death caused by photothrombotic focal cerebral 42 ischemia. The results indicate that neuronal gap junctions are required for NMDAR-43 dependent excitotoxicity and play critical role in ischemic neuronal death.