P2X7 receptors have been suggested to be located both on neurons and astrocytes of the central and peripheral nervous systems. In the present Ca(2+)-imaging and patch-clamp study, we reinvestigated these findings on mixed neuronal-astrocytic cell cultures prepared from embryonic or newborn rat hippocampi. We found in a Mg(2+)-free bath medium that the prototypic P2X7 receptor agonist dibenzoyl-adenosine triphosphate (Bz-ATP) increased the intracellular Ca(2+) concentration ([Ca(2+)]i) both in the neuronal cell bodies and in their axo-dendritic processes only to a very minor extent. However, Bz-ATP produced marked [Ca(2+)]i transients in the neuronal processes, when they grew above a glial carpet, which was uniformly sensitive to Bz-ATP. These glial signals might be misinterpreted as neuronal responses because of the poor focal discrimination by a fluorescent microscope. Most astrocytes had a polygonal shape without clearly circumscribable boundaries, but a subgroup of them had neuron-like appearance. The cellular processes of this astrocytic subgroup, just as their cell somata and their polygonal counterparts, appeared to possess a high density of functional P2X7 receptors. In contrast to astrocytes, in a low Ca(2+)/no Mg(2+)-containing bath medium, hippocampal neurons failed to respond to Bz-ATP with membrane currents. In addition, neither the amplitude nor the frequency of spontaneous excitatory postsynaptic currents, representing the quantal release of glutamate, was modified by Bz-ATP. We conclude that cultured hippocampal neurons, in contrast to astrocytes, possess P2X7 receptors, if at all, only at a low density.