Abstract
Streams of action potentials or long depolarizations evoke a massive exocytosis of transmitters and peptides from the surface of dendrites, axons and cell bodies of different neuron types. Such mode of exocytosis is known as extrasynaptic for occurring without utilization of synaptic structures. Most transmitters and all peptides can be released extrasynaptically. Neurons may discharge their contents with relative independence from the axon, soma and dendrites. Extrasynaptic exocytosis takes fractions of a second in varicosities or minutes in the soma or dendrites, but its effects last from seconds to hours. Unlike synaptic exocytosis, which is well localized, extrasynaptic exocytosis is diffuse and affects neuronal circuits, glia and blood vessels. Molecules that are liberated may reach extrasynaptic receptors microns away. The coupling between excitation and exocytosis follows a multistep mechanism, different from that at synapses, but similar to that for the release of hormones. The steps from excitation to exocytosis have been studied step by step for the vital transmitter serotonin in leech Retzius neurons. The events leading to serotonin exocytosis occur similarly for the release of other transmitters and peptides in central and peripheral neurons. Extrasynaptic exocytosis occurs commonly onto glial cells, which react by releasing the same or other transmitters. In the last section, we discuss how illumination of the retina evokes extrasynaptic release of dopamine and ATP. Dopamine contributes to light-adaptation; ATP activates glia, which mediates an increase in blood flow and oxygenation. A proper understanding of the workings of the nervous system requires the understanding of extrasynaptic communication.
Highlights
Our view of the workings of the nervous system have been dominated by four threads of fundamental evidence: First, Cajal defined nerve circuits as networks of neurons connected in stereotyped manner, forming transmission lines for specific information processing
Serotonin that had been released extrasynaptically was discovered by Dalstrom and Fuxe in the 60s using the FalckHillarp technique, by which exposure to formaldehyde vapors transforms the monoamines serotonin, dopamine or adrenaline into fluorescent derivatives (Fuxe et al, 2007; Borroto-Escuela et al, 2015)
Similar observations made in dopaminergic neurons, plus the fact that peptides can be released far away from their receptors led to the concept of volume transmission by Fuxe and his colleagues, meaning that molecules act on receptors located distantly from the release sites (Borroto-Escuela et al, 2015)
Summary
Streams of action potentials or long depolarizations evoke a massive exocytosis of transmitters and peptides from the surface of dendrites, axons and cell bodies of different neuron types. Such mode of exocytosis is known as extrasynaptic for occurring without utilization of synaptic structures. Most transmitters and all peptides can be released extrasynaptically Neurons may discharge their contents with relative independence from the axon, soma and dendrites. The events leading to serotonin exocytosis occur for the release of other transmitters and peptides in central and peripheral neurons. Extrasynaptic exocytosis occurs commonly onto glial cells, which react by releasing the same or other transmitters.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
