Presence of diverse functional P2X receptors in rat cerebellar synaptic terminals

Abstract

Studies in individual synaptic terminals have demonstrated the presence of diverse functional P2X receptors in rat cerebellum. No immunolabelling for P2X 1, P2X 4, P2X 5 and P2X 6, and scarce presence of P2X 2 were found at the cerebellar synaptic terminals. P2X 3 immunolabelling was present in 28% of isolated synaptosomes. At these synaptic terminals, nucleotides as ATP or α,β-meATP induced Ca 2+ transients in the presence of extracellular Ca 2+, showing homologous and heterologous receptor desensitization in 60% of cases. Ip 5I 10 nM did not block responses to α,β-meATP, but inhibition occurred when antagonist concentrations were equal or higher than 100 nM. These data agree with the presence of abundant P2X 3 homomeric receptors. P2X 7 immunolabelling was present in 60% of terminals and P2X 7 receptor hallmarks in Ca 2+ responses have been found. BzATP was more potent than ATP and responses were potentiated when assayed in Mg 2+-free medium. EC 50 values were, respectively, 39.4 ± 0.4 and 0.3 ± 0.1 μM for ATP in the presence or absence of Mg 2+. Maximal values of synaptosomal calcium transients, in the presence or absence of Mg 2+, were, respectively, 91.6 ± 11.9 and 132.9 ± 12.9 nM for ATP; and 104.3 ± 9.4 and 169.7 ± 17.1 nM for BzATP. In addition, Zn 2+ inhibited ATP responses in the absence of Mg 2+ and the P2X 7 specific antagonist Brilliant Blue G completely blocked these responses in one half of synaptosomes. This study reports the presence of functional P2X 3 and P2X 7 receptors at synaptic sites, which provides complexity and regulatory possibilities to the cerebellar neurotransmission.