The patch-clamp technique was used to study whether cocultivation of neurons and astrocytes modulates the expression of calcium channel activity in astrocytes. Whole-cell patch-clamp recordings from rat brain astrocytes cocultured with rat embryonic neurons revealed two types of voltagedependent inward currents carried by Ca2l and blocked by either Cd2+ or Co2" that otherwise were not detected in purified astrocytes. This expression of calcium channel activity in astrocytes was neuron dependent and was not observed when astrocytes were cocultured with purified oligodendrocytes.Cell-cell interaction in the nervous system has long been considered a major determinant of cell growth and differentiation. Evidence that glial cells play an essential role in neuronal function has supported the concept of neuron-glia interaction as an important element in understanding the dynamics of the nervous system. Glial cells are no longer considered as only buffers of extracellular K+ and neurotransmitters, but also as part of an active molecular interaction with neighboring neurons (1, 2), which ranges from cell growth and differentiation (1, 2) to exchange of molecules and nutrients (1-4).Coculture systems have provided a useful tool to study neuron-glia interactions (2, 5-10). We have used the patchclamp technique to study whether the interactions between neurons and astrocytes established in coculture modulate calcium channel activity in astrocytes. Neuronal modulation of the expression of calcium channel activity would have a profound significance on astrocyte physiology because of the widespread role of calcium in excitability and intracellular signal transduction (11,12).In culture, purified type 1 astrocytes display only K+ outward currents (13). However, calcium currents can be induced when astrocytes are treated with substances that increase the intracellular levels of cAMP (14,15). In this study, we report that in purified astrocytes, calcium current activity can be expressed by cocultivating neurons and astrocytes. It is also shown that this effect is specifically dependent on neuron-astrocyte interactions, since calcium currents are not detected from astrocytes cocultured with oligodendrocytes. MATERIAL AND METHODS