Purinergic neuron-glia interactions in sensory systems

The purine adenosine 5'-triphosphate (ATP) and its breakdown products, ADP and adenosine, act as intercellular messenger molecules throughout the nervous system. While ATP contributes to fast synaptic transmission via activation of ionotropic P2X receptors as well as neuromodulation via metabotropic P2Y receptors, ADP and adenosine only stimulate P2Y and P1 receptors, respectively, thereby adjusting neuronal performance. Often glial cells are recipient as well as source for extracellular ATP. Hence, purinergic neuron-glia signalling contributes bidirectionally to information processing in the nervous system, including sensory organs and brain areas computing sensory information. In this review, we summarize recent data of purinergic neuron-glia communication in two sensory systems, the visual and the olfactory systems. In both retina and olfactory bulb, ATP is released by neurons and evokes Ca(2+) transients in glial cells, viz. Müller cells, astrocytes and olfactory ensheathing cells. Glial Ca(2+) signalling, in turn, affects homeostasis of the nervous tissue such as volume regulation and control of blood flow. In addition, 'gliotransmitter' release upon Ca(2+) signalling--evoked by purinoceptor activation--modulates neuronal activity, thus contributing to the processing of sensory information.