Independent receptors for diadenosine pentaphosphate and ATP in rat midbrain single synaptic terminals

Diadenosine pentaphosphate (ApsA) and adenosine 5'-triphosphate (ATP) stimulate a intrasynaptosomal calcium, concentration [Ca2+](i) increase via specific purinergic receptors, in; rat midbrain synaptosomes, although nothing is known: about their distribution in presynaptic terminals. A microfluorimetric technique to measure [Ca2+](i) increase using the dye FURA-2AM,, has permitted study of the presence of dinucleotide and P2X receptors in independent isolated synaptic terminals. Our results demonstrate the existence of three populations of synaptosomes: one with dinucleotide receptors (12%), another with P2X receptors (20%) and a third with both (14%). It has been possible to demonstrate that the activation of these receptors occurs only in the presence of extracellular Ca2+ and that it is also coupled with voltage-dependent Ca2+ channels. Finally 54% of the synaptosomes that responded to K+ did not present any calcium increase mediated by the nucleotides used. In summary, ATP and dinucleotides exhibit specific ionotropic receptors, that can coexist or not on the same synaptic terminal.