Neurons communicate with each other, for the most part, via a tiny gap called a synapse. Simply put, when a nerve impulse reaches the end of a nerve cell, neurotransmitter molecules are released into the synapse from synaptic vesicles - little storage sacs in which they are temporarily housed. Neurotransmitters such as dopamine move, or diffuse, across the synapse, exciting or inhibiting the post-synaptic nerve cell. Most neuroscientists are of the view that a specific amount of neurotransmitter is found in any one vesicle, that the acidity within that vesicle is constant, and that the amount of neurotransmitter released by a neuron is fixed.
However, recent research with the fruit fly and mouse turns this theory on its head. Researchers demonstrated that dopamine neurons can alter the amount of dopamine released depending on their level of activity. After dopamine is released, and prior to the vesicles becoming empty, transport channels in the cell membrane open permitting an influx of negatively-charged glutamate ions into the neuron, increasing acidity within the vesicle. Changes in acidity appear to allow synaptic vesicles to modify their levels of a particular neurotransmitter in response to the demands on the neuron. The conventional model which holds that synaptic vesicles have fixed levels of a single neurotransmitter is here called into question, the neurons studied containing both dopamine and glutamate. One obvious next step is to explore whether imbalances in such chemicals at the vesicular level might be linked to diseases such as schizophrenia.
Read the abstract here.