Interplay of chemical neurotransmitters regulates developmental increase in electrical synapses

Abstract

Coupling of neurons by electrical synapses (gap junctions) transiently increases during embryonic and/or early postnatal development of the mammalian central nervous system and plays an important role in a number of developmental events. A previous study revealed the mechanisms that control the developmental uncoupling of neuronal gap junctions, however, developmental regulation of neuronal gap junction coupling is largely unknown and is addressed in this dissertation. The current study revealed that the developmental increase in neuronal gap junction coupling is regulated by the interplay between the activity of group II metabotropic glutamate receptors (mGluR) and GABAA receptors (GABAAR). Specifically, the experiments including dye coupling, electrotonic coupling, western blots and siRNA technology in the rat and mouse hypothalamus and cortex in vivo and in vitro demonstrated that activation of group II mGluRs augments, and inactivation prevents, the developmental increase in neuronal gap junction coupling and connexin36 (Cx36, neuronal gap junction protein) expression. In contrast, changes in GABAA receptor activity have the opposite effects. The regulation by group II mGluRs is through cyclic AMP/protein kinase A-dependent signaling, while the GABAAR-dependent regulation is via influx of Ca2+ through L-type voltage-gated Ca2+ channels and activation of protein kinase C-dependent signaling. Further, the receptor mediated up-regulation of Cx36 requires a neuron-restrictive silencer element in the Cx36 gene promoter and the down-regulation involves the 3' untranslated region of the Cx36 mRNA, as shown using real-time quantitative polymerase chain reaction and luciferase reporter activity analysis. In addition, the methyl thiazolyl tetrazolium analysis indicates that mechanism for the developmental increase in neuronal gap junction coupling directly control the death/survival mechanisms in developing neurons. Altogether, the results suggest a multi-tiered strategy for chemical synapses in developmental regulation of electrical synapses.