ATP and its ionotropic P2X receptors are components of the most ancient signaling systems. However, little is known about the distribution and function of purinergic transmission in invertebrates. Here, we cloned, expressed, and pharmacologically characterized P2X receptors in the sea slug Aplysia californica – the prominent neuroscience model. acP2X receptors were successfully expressed in Xenopus oocytes and were displayed activation by ATP with two-phased kinetics and Na+-dependence. Pharmacologically, they were quite different from other P2X receptors. The ATP analog, Bz-ATP, was a less effective agonist than ATP, and PPADS was a more potent inhibitor of the acP2X receptors than the suramin. acP2X were uniquely expressed within the cerebral F-cluster, which contains multiple secretory peptides (e.g., insulins, interleukins, and potential toxins), ecdysone-type receptors, and a district subset of ion channels. We view F-cluster as the multifunctional integrative center, remarkably different from other neurosecretory cells. acP2X receptors were also found in the chemosensory structures and the early cleavage stages. Therefore, in molluscs, rapid ATP-dependent signaling can be implicated both in development and diverse homeostatic functions. Furthermore, this study illuminates novel cellular and systemic features of P2X-type ligand-gated ion channels for deciphering evolution of neurotransmitters.
