Synaptic vesicles help relay messages

Every time we move, feel emotions, think, or remember, the nerve cells, or neurons, in our body transmit messages to one another via chemical signals called neurotransmitters. Within neurons are tiny organelles called synaptic vesicles that sequester neurotransmitters and release them when needed into the synapse, or space between nerve cells, where the chemical signal is transmitted to other neutrons.

It is known that synaptic vesicles release their neurotransmitters in two different "modes"--one when the neuron is stimulated and actively relaying a message and the other through spontaneous release when the neuron is at rest, or inactive. Until now, it was believed the same synaptic vesicles were responsible for releasing neurotransmitters in both modes. However, research by scientists at the University of Texas Southwestern Medical Center, Dallas, suggests that two distinct types of synaptic vesicles are responsible for the different modes of neurotransmitter release.

"These findings question one of the core tenets of synaptic function and reveal significant complexity in organization of synaptic vesicles within individual synapses," reports Ege Kavalali, assistant professor in the Center for Basic Neuroscience and of physiology.

Neurotransmitters regulate many different aspects of mood, cognition, and behavior, such as emotional state, reactions to stress, pain, and the physical drives of sleep, appetite, and sexuality. A better understanding of such fundamental mechanisms of neurotransmitter release will aid researchers in their investigations of psychiatric and neurological disorders such as mental retardation, autism, depression, and epilepsy, all of which have been linked to abnormal neurotransmitter function.

"The functional differences of these two sets of vesicles may be the result of differences in the protein and/or lipid composition of the vesicles," Kavalali surmises. "Higher-resolution analysis is needed to test whether the two sets of vesicles are indeed distinct."

Kavalali indicates that, if the two types of vesicles have different molecular compositions, as the findings suggest, those differences may make it possible to regulate spontaneous and activity-dependent neurotransmitter release independently. Spontaneous release is thought to play a role in the development of the neural circuitry in the brain and body, while activity-dependent release is responsible for functions such as learning and memory.

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