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M.M. Ward et al.

Abbracchio et al. 2009). Both P2X and P2Y receptors are believed to play a role in neuronal and glial cell transmission and neurodegenerative diseases.

44.2 Mechanisms of ATP Release and Degradation

44.2.1 ATP Release

Under physiological conditions, ATP is released through a variety of mechanisms where it acts as a glioand neurotransmitter. Like other conventional neurotransmitters, ATP is stored and released from secretory vesicles in the presynaptic terminal of neurons. It is thought to act as a co-transmitter in many cells, where it is released with both inhibitory (GABA) and excitatory (glutamate) neurotransmitters and neuropeptides (Sperlágh et al. 1998; Jo and Schlichter 1999; Jo and Role 2002). Until recently, the mechanism of ATP transport into secretory vesicles was unknown. However, a newly identified vesicular transporter, from the same family as the vesicular glutamate transporters (VGluTs), was shown to accumulate ATP, ADP and GTP (Sawada et al. 2008). Immunohistochemical analysis showed that the vesicular nucleotide transporter (VNUT) was expressed in a population of astrocytes however the localization of VNUT in other systems is yet to be thoroughly explored. In addition to exocytosis, ATP may also be released by ATP-binding cassette transporters, gap junction hemichannels and through P2X7Rs (Fields and Burnstock 2006; Suadicani et al. 2006).

There is evidence for the storage and release of nucleotides from retinal cells. Cholinergic amacrine cells from the rabbit retina release ATP in conjunction with acetylcholine in response to light (Neal and Cunningham 1994) and cultured cholinergic-like amacrine cells from chick retina release ATP, likely via an exocytotic mechanism (Santos et al. 1999). In an eye-cup preparation of rat retina, light stimulation lead to an increase in extracellular ATP levels, purported to be released from amacrine and/or ganglion cells (Newman 2005). Non-neuronal cells are also a source of ATP in the retina. Extracellular ATP-mediated calcium waves are a feature of retinal astrocytes and Müller cells (Newman and Zahs 1997; Newman 2001) and ATP is also released from the retinal pigment epithelium (Mitchell 2001).

44.2.2 Degradation of ATP

Common to all neurotransmitters is the requirement for a mechanism for cessation of neurotransmission. In the following section we will discuss the methods of extracellular ATP degradation.

In the retina, deactivation of neurotransmission typically occurs through the re-uptake and recycling of neurotransmitter. Other mechanisms to terminate neurotransmission include receptor desensitization or enzymatic degradation of the signaling molecule in the extracellular space. In the case of extracellular nucleotide signaling, a family of surface bound ecto-enzymes, known as ectonucleotidases, hydrolyse and thereby inactivate, synaptically released ATP (Zimmermann 1996,