Role for neuronal contact site and protein trafficking in determining the strength of electrical transmission

Abstract

All cells, including neurons, must communicate to ensure proper functioning of both organs and the whole animal. Electrical coupling is a fast and reliable means of neuron-to-neuron communication that occurs via inter-cellular channels known as gap junctions. In the sea snail, Aplysia californica, electrical transmission between bag cell neurons promotes firing synchrony during a prolonged afterdischarge, which results in the neurohemal release of egg-laying hormone and deposition of fertilized eggs. In vivo, gap junctions are present between both bag cell neuron axons and somata. Here, I duplicated these configurations between cultured bag cell neurons. In vitro, junctional conductance was always modestly voltage-dependent, but significantly higher for soma-soma vs. axon-axon pairs (~3 vs. ~1 nS). Similarly, both soma-soma coupling coefficient (~0.6 vs. ~0.35) and electrotonic potential (~15 vs. ~ 5 mV) were significantly larger, although pre- or postsynaptic input resistance (300-350 MΩ) did not differ between groups. Furthermore, although the action potential threshold was not different between arrangements, the propensity for presynaptic input to evoke a postsynaptic spike was greater in soma-soma coupling (40% vs. never). Thus, the strength of bag cell neuron electrical transmission depends on contact configuration, meaning synchronization may be facilitated by electrical synapse location. It is well established that gap junctions are formed by docking of two hemichannels, known as innexons, from neighbouring neurons; these innexons have a high turnover rate, exhibiting half-lives of mere hours. I examined trafficking of innexin subunits in cultured soma-soma paired bag cell neurons using either brefeldin A, an endoplasmic reticulum/Golgi transport disruptor (reduces insertion), or lactacystin, a proteasome inhibitor (slows degradation). While any change with lactacystin was not significant, brefeldin A caused a significant decrease in junctional conductance (from ~3 to ~1.5 nS), coupling coefficient (from ~0.4 to ~0.2), and the electrotonic potential (from ~7 to ~ 3 mV), but no change in presynaptic action potential height (both ~60 mV), suggesting a trafficking-specific effect. Insertion of innexons likely contributes to bag cell neuron electrical coupling and the firing synchrony needed for successful reproduction. Collectively, my results suggest that gap junction location and trafficking have the potential to profoundly influence neuroendocrine output.