Although astrocytes have been considered to be supportive, rather than
transmissive, in the adult nervous system, recent studies have challe
nged this assumption by demonstrating that astrocytes possess function
al neurotransmitter receptors. Astrocytes are now shown to directly mo
dulate the free cytosolic calcium, and hence transmission characterist
ics, of neighboring neurons. When a focal electric field potential was
applied to single astrocytes in mixed cultures of rat forebrain astro
cytes and neurons, a prompt elevation of calcium occurred in the targe
t cell. This in turn triggered a wave of calcium increase, which propa
gated from astrocyte to astrocyte. Neurons resting on these astrocytes
responded with large increases in their concentration of cytosolic ca
lcium. The gap junction blocker octanol attenuated the neuronal respon
se, which suggests that the astrocytic-neuronal signaling is mediated
through intercellular connections rather than synaptically. This neuro
nal response to local astrocytic stimulation may mediate local interce
llular communication within the brain.