Abstract
In the mammalian CNS, excessive release of glutamate and overactivation of glutamate receptors are responsible for the secondary (delayed) neuronal death following neuronal injury, including ischemia, traumatic brain injury (TBI), and epilepsy. The coupling of neurons by gap junctions (electrical synapses) increases during neuronal injury. We report here that the ischemic increase in neuronal gap junction coupling is regulated by glutamate via group II metabotropic glutamate receptors (mGluRs). Specifically, using electrotonic coupling, Western blots, and siRNA in the mouse somatosensory cortex in vivo and in vitro, we demonstrate that activation of group II mGluRs increases background levels of neuronal gap junction coupling and expression of connexin 36 (Cx36) (neuronal gap junction protein), and inactivation of group II mGluRs prevents the ischemia-mediated increases in the coupling and Cx36 expression. We also show that the regulation is via cAMP/PKA (cAMP-dependent protein kinase)-dependent signaling and posttranscriptional control of Cx36 expression and that other glutamate receptors are not involved in these regulatory mechanisms. Furthermore, using the analysis of neuronal death, we show that inactivation of group II mGluRs or genetic elimination of Cx36 both dramatically reduce ischemia-mediated neuronal death in vitro and in vivo. Similar results are obtained using in vitro models of TBI and epilepsy. Our results indicate that neuronal gap junction coupling is a critical component of glutamate-dependent neuronal death. They also suggest that causal link among group II mGluR function, neuronal gap junction coupling, and neuronal death has a universal character and operates in different types of neuronal injuries.