ATP signaling in the integrative neural center of Aplysia californica.

ATP and its ionotropic P2X receptors are components of the most ancient signaling system. However, little is known about the distribution and function of purinergic transmission in invertebrates. Here, we cloned, expressed, and pharmacologically characterized the P2X receptors in the sea slug Aplysia californica-a prominent neuroscience model. AcP2X receptors were successfully expressed in Xenopus oocytes and displayed activation by ATP with two-phased kinetics and Na+-dependence. Pharmacologically, they were 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, the multifunctional integrative neurosecretory center. AcP2X receptors were also detected 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 the evolution of neurotransmitters.

Localization and Possible Function of P2X Receptors in Normal and Diseased Retinae.

Purines, when present in the extracellular space, can mediate fast neurotransmission in the retina and central nervous system. Over the last decade there has been emerging evidence for the expression of P2X and P2Y receptors in a range of retinal neuronal subtypes. These results have highlighted important roles for purines in modulating specific retinal circuits, including the rod pathway and amacrine cell circuits. Traditionally, synaptic release of adenosine triphosphate (ATP) involves the novel anion vesicular nucleotide transporter, VNUT, which has recently been identified in a single wide-field amacrine cell population. In addition, nontraditional, conductive mechanisms of release have also been described in the retina. In the synapse, the enzymes involved in rapid degradation of purines are present in both plexiform layers of the retina. A role for P2X receptors in retinal diseases has also emerged recently. High concentrations of ATP lead to photoreceptor loss, through mechanisms involving P2X7 receptors. In addition, activation of P2X7 receptors is associated with activation of the inflammasome, a protein complex important for the release of proinflammatory cytokines. P2X receptors, especially P2X7, are emerging as targets to combat retinal disease.

Distribution of purinergic P2X receptors in the equine digit, cervical spinal cord and dorsal root ganglia.

Purinergic pathways are considered important in pain transmission, and P2X receptors are a key part of this system which has received little attention in the horse. The aim of this study was to identify and characterise the distribution of P2X receptor subtypes in the equine digit and associated vasculature and nervous tissue, including peripheral nerves, dorsal root ganglia and cervical spinal cord, using PCR, Western blot analysis and immunohistochemistry. mRNA signal for most of the tested P2X receptor subunits (P2X1-5, 7) was detected in all sampled equine tissues, whereas P2X6 receptor subunit was predominantly expressed in the dorsal root ganglia and spinal cord. Western blot analysis validated the specificity of P2X1-3, 7 antibodies, and these were used in immunohistochemistry studies. P2X1-3, 7 receptor subunits were found in smooth muscle cells in the palmar digital artery and vein with the exception of the P2X3 subunit that was present only in the vein. However, endothelial cells in the palmar digital artery and vein were positive only for P2X2 and P2X3 receptor subunits. Neurons and nerve fibres in the peripheral and central nervous system were positive for P2X1-3 receptor subunits, whereas glial cells were positive for P2X7 and P2X1 and 2 receptor subunits. This previously unreported distribution of P2X subtypes may suggest important tissue specific roles in physiological and pathological processes.