Astrocyte-neuron signaling, a.k.a. gliotransmission, can modulate synaptic transmission/plasticity at
tripartite synapses. Among the processes regulated by gliotransmission are sleep-regulation, respiration, and
learning/memory. Despite these roles of gliotransmission in such fundamental life processes, its mechanism is
not understood. Elucidating this mechanism should provide insights into basic brain processes, and suggest
interventions when they go awry. Two early studies of astrocyte-neuron signaling explored the hypothesis that
astrocytic glutamate release acts on neuronal glutamate receptors, but they led to different conclusions
regarding the mechanism. One study concluded that glutamate is not the messenger but instead suggested
that gap junctions might mediate astrocyte-neuron signaling. The other study concluded that the signaling is
mediated by Ca2+-dependent glutamate release from astrocytes, subsequently shown to occur by regulated
exocytosis of glutamate-containing vesicles. Virtually nothing is known about the subcellular
distribution/localization of astrocyte release sites. It is not clear if they are localized uniquely to the tripartite
synaptic regions of astrocytes or more broadly. There is much debate about the relative roles of exocytosis vs.
gap junction-mediated communication as critical for astrocyte-neuron signals. Our preliminary data point to a
novel, unifying hypothesis that these two mechanisms are, in fact, mechanistically linked.