If you wanted to study the kinetics of neurotransmitter release at neuronal synapses, how would you do it? There has been a traditional answer to this question in the research community. Stick a recording electrode in the post-synaptic neuron, stimulate the pre-synaptic one with another electrode, and record the electrical response of the post-synaptic cell. However, indirect approaches like this limit our ability to detect what is really going on in pre-synaptic active zones. An innovative electrophysiological technique can address this problem. It’s known as amperometry, and this is how it works. Say you want to study the release of dopamine from neurons. Place a carbon fiber electrode in solution just over the pre-synaptic active zone. Then, generate a voltage of +700mV in the electrode. This is a high enough voltage to oxidize dopamine molecules that come into contact with the electrode, and thus generate a current in the electrode. The magnitude of this current is therefore proportional to the amount of dopamine released. This allows for direct recording of the amount of dopamine released from synapses over time in response to stimulation. Amperometry has enabled a team of scientists at Columbia University to shed light on the nature of the synaptic release of dopamine.
If you are already familiar with the basics of synaptic transmission, you know that depolarization of pre-synaptic cells causes the release of neurotransmitter via fusion of synaptic vesicles with the pre-synaptic membrane. However, this fusion isn’t always complete. It is thought that often, especially with vesicles that contain small molecule neurotransmitters such as dopamine, vesicles may partially fuse with the membrane and then separate and close up again. Sometimes vesicles may repeat this partial fusion and dissociation, known as “kiss and run exocytosis”, several times in quick succession. Using amperometry, the researchers as Columbia were able to provide convincing evidence that this is the primary means of exocytosis in dopaminergic neurons. Using neurons cultured in vitro, they recorded dopamine release in response to depolarization. Their recordings showed two distinct current patterns. Sometimes, single spikes of current were recorded, termed simple events by the researchers. Other times, they recorded repeated spikes of current, with the initial spike being comparable in amplitude to the single spike event, but the subsequent spikes always decreasing in amplitude over time. These were labeled complex events. The researchers interpreted these recordings as evidence of kiss and run exocytosis. The single spikes represented instances of single transient fusion of vesicles with the membrane. The compound spike patterns were thought to represent repeated fusion of the same vesicle with the membrane. The continual decrease in current would correspond to the slowly depleting dopamine stores in the vesicles as it continually fused and resealed, releasing more of its neurotransmitter with each fusion event.
This study provides solid evidence of the prevalence of kiss and run exocytosis in dopaminergic neurons. The major limitation of these results is that these recordings were performed using cultured neurons in vitro. It will be interesting to see if these same results hold true for neurons in vivo. However, this study still lays the groundwork for uncovering the intricate details of vesicle fusion in neurons. Soon we may see if other types of neurons also use this pattern of release and to what extent, as well as what functional advantage it may convey to these neurons.
Posted by Sean McDougall (2)