Electrical synapses interconnecting axons in the optic nerve head–a novel model of optic nerve function

Abstract

AbstractPurposeAxons in the optic nerve are arranged in bundles and conducting action potential with resistance related to their membrane. Here, we aim to characterize the function of newly identified gap junctions coupling optic nerve axons with each other’s.MethodsProximal and distal rat optic nerve stumps were processed for immunostainings, electron microscopy (EM), RT‐PCR and western blots (WB). Additional 20 animals were deeply anesthetized, and stainless‐steel screw electrodes were drilled in skulls in standard locations to record fVEPs. Recordings were performed in untouched animals, after retrobulbar injection of saline (negative control) or meclofenamic acid solution (gap junctions’ blocker). Human paraffin cross‐sections of eyeballs for immunostainings were obtained from the Human Eye Biobank for Research, University of Toronto, Canada.ResultsImmunostaining of rat and human samples revealed presence of Cx45 and 36 in axonal membranes, colocalizing with β3‐tubulin, but not with GFAP. In WB, Cx36 content in optic nerve was approximately halved when compared with retina (0.58 ± 0.005 ‐ proximal stump; 0.44 ± 0.02 ‐ distal stump), Cx45 showed higher levels (0.68 ± 0.0003 ‐ proximal stump; 0.9 ± 0.07 ‐ distal stump). Additionally, we detected transcript of Cx45 and Cx36 in RT‐PCR analysis of optic nerve homogenates. In immunogold‐EM of optic nerve sections, we found electric synapses (formed mostly by Cx45) that are directly coupling neighboring axons. In fVEPs, blocking of gap junctions’ with meclofenamic acid resulted in prolongation of the latency of P1 wave up to 140% after 5 min from retrobulbar injection of this blocker (p < 0.001) and 170% after 30 min (p < 0.001).ConclusionsOptic nerve axons do not form absolutely independent conductive channels. They are directly coupled by gap junctions formed in majority by neuronal Cx45. Coupling of axons allows to reduce axonal membrane resistance and accelerates transduction of visual signal.