P2X7 receptors in rat brain: presence in synaptic terminals and granule cells.

ATP stimulates [Ca2+]i increases in midbrain synaptosomes via specific ionotropic receptors (P2X receptors). Previous studies have demonstrated the implication of P2X3 subunits in these responses, but additional P2X subunits must be involved. In the present study, ATP and BzATP proved to be able to induce intrasynaptosomal calcium transients in the midbrain synaptosomes, their effects being potentiated when assayed in a Mg2+-free medium. Indeed, BzATP was shown to be more potent than ATP, and their effects could be inhibited by PPADS and KN-62, but not by suramin. This activity profile is consistent with the presence of functional P2X7 receptors in the midbrain terminals. The existence of presynaptic responses to selective P2X7 agonists could be confirmed by means of a microfluorimetric technique allowing [Ca2+]i measurements in single synaptic terminals. Additionally, the P2X7 receptor protein could be identified in the midbrain synaptosomes and in axodendritic prolongations of cerebellar granule cells by immunochemical staining.