Electrical Synapse and Their Functional Interactions with Chemical Synapse

NeuSci014 : Synaptic Transmission  

Journal : Electrical Synapse and their Functional Interactions with Chemical Synapse

Communication between neurons is required for brain function and the quality of such communication enables hardwired neural networks to act in a dynamic fashion. Functional interactions between neurons occur at anatomically identifiable cellular regions called synapses. There are two main modalities of synaptic transmission which are chemical and electrical synapse. At chemical synapse, the information is transferred through the release of neurotransmitter from one neuron and detection of the neurotransmitter by an adjacent cell whereas in electrical synapse, electrical transmission between neurons is mediated by gap junctions, which are clusters of intercellular channels that directly connect the interiors of two adjacent cells enabling the bidirectional passage of electrical currents and small molecules

Gap junction channels are formed by the docking of two hexameric connexin ‘hemichannels’ (also called ‘connexons’) that is, one from each adjacent cell. Strikingly, although they assemble into almost identical structures, connexons in invertebrates and vertebrates are formed by two different multigene families of proteins namely, connexins and a second family including innexins and pannexins. In vetebrates neuron, connexin 36 (CX36) is the most abundant. Gap junctions are highly dynamic, complex structures and that their channels are actively turned over.

In all stage of brain development, there is an interaction between electrical and chemical synapse. This interaction is required for brain to develop and function normally as it is important in the formation of neural circuit and maintaining the synaptic strength. This synaptic strength is maintain by constitutive trafficking and regulated trafficking of ligand gated ion channel to and from the plasma membrane together with the modification of gap junction conductance. This interaction can be seen in glutamatergic synapse. At glutamergic synapse, ionotropic glutamate receptors are trafficked in and out of synapse. This trafficking involved interaction between glutamate receptor carboxy termini and various cytosolic proteins and scaffolding proteins, which are mostly located at the postsynaptic density (PSD). PSD95 and CaMKII are both abundant component in PSD.

Like their chemical counterparts, electrical synapses are highly modifiable by the action of neuromodulators such as dopamine and are capable of activity-dependent plasticity. Modification of gap junction in electrical synapse required the molecular machinery to mediate the turnover of gap junction channel. Gap junction considered to be a part of multiprotein complex. In electrical synapse, there is an electron dense area that called as a semi dense cytoplasmic matrix. This area is same like PSD area in chemical synapse.

 In semi dense cytoplasmic matrix, new connexon are trafficked as unpaired hemichannels to the membrane in vesicles, where they are inserted at the periphery of the gap junction plaque and dock with hemichannels in the apposing membrane. They are internalized as small clusters of entire channels into either of the coupled cells from regions near the centre of the plaque. Various proteins make up the ‘semi-dense cytoplasmic matrix’, which acts as a scaffold for these channels. Zonula occludens protein 1 (ZO1) is a structural component of this scaffold and it have similar role in neuronal gap junctions to that of PSD95 at glutamatergic synapses where it conserved regions of the C terminus of CX36 mediate its interactions with ZO1 and are required for the insertion of new gap junction proteins into electrical synapses.