Abstract
The type of neurotransmitter secreted by a neuron is a product of the vesicular transporters present on its synaptic vesicle membranes and the available transmitters in the local cytosolic environment where the synaptic vesicles reside. Synaptic vesicles isolated from electroplaques of the marine ray, Torpedo californica, have served as model vesicles for cholinergic neurotransmission. Many lines of evidence support the idea that in addition to acetylcholine, additional neurotransmitters and/or neuromodulators are also released from cholinergic synapses. We identified the types of vesicular neurotransmitter transporters present at the electroplaque using immunoblot and immunofluoresence techniques with antibodies against the vesicle acetylcholine transporter (VAChT), the vesicular glutamate transporters (VGLUT1, 2, and 3), and the vesicular nucleotide transporter (VNUT). We found that VAChT, VNUT, VGLUT 1 and 2, but not 3 were present by immunoblot, and confirmed that the antibodies were specific to proteins of the axons and terminals of the electroplaque. We used a single‐vesicle imaging technique to determine whether these neurotransmitter transporters were present on the same or different populations of synaptic vesicles. We found that greater than 85% of vesicles that labeled for VAChT colabeled with VGLUT1 or VGLUT2, and approximately 70% colabeled with VNUT. Based upon confidence intervals, at least 52% of cholinergic vesicles isolated are likely to contain all four transporters. The presence of multiple types of neurotransmitter transporters – and potentially neurotransmitters – in individual synaptic vesicles raises fundamental questions about the role of cotransmitter release and neurotransmitter synergy at cholinergic synapses.
Highlights
Neurons are often classified by the small molecule neurotransmitters they release from synaptic vesicles along their axons and axon terminals
Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society
We used single-vesicle imaging to examine whether cholinergic synaptic vesicles possess additional types of neurotransmitter transporters
Summary
Neurons are often classified by the small molecule neurotransmitters they release from synaptic vesicles along their axons and axon terminals. The simple idea that each neuron secretes only one type of neurotransmitter has been thrown into doubt with examples of cholinergic, GABAergic (gamma-Aminobutyric acid) and noradrenergic neurons that appear to corelease glutamate (El Mestikawy et al 2011; Hnasko and Edwards 2012). Whether this corelease involves packaging of multiple transmitters into single synaptic vesicles is less clear.
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