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44 The Role of Purinergic Receptors in Retinal Function and Disease

387

2000). Conversion of ATP to nucleotide diphosphates, and ultimately adenosine occurs in a step-wise manner involving a number of ecto-enzymes (ATP ADP AMP adenosine).

Using an enzyme histochemical method, Puthussery et al. (2006) and Puthussery and Fletcher (2007) demonstrated ecto-ATPase activity (required for the first step of hydrolysis of ATP-ADP) was present in synaptic layers in the retina, providing a mechanism for the degradation of extracellular ATP. The presence of ecto-enzymes in the retina is further supported by functional studies that have shown endogenous production of adenosine following ATP release from glial cells, which can be prevented with the application of an ecto-enzyme inhibitor (Newman, 2003).

44.3 Purinergic Signaling in the Retina

44.3.1 Purinergic Modulation of Neuronal Signaling

There is an increasing body of work studying purinergic receptor expression in the retina. Many P2X and P2Y receptor subtypes have been localized to a diverse number of retinal cell classes including neurons and glia (Table 44.1), supporting a role for purinergic modulation of visual processing. Activation of P2X7Rs in the outer retina increases the photoreceptorally derived a-wave of the electroretinogram (Puthussery et al. 2006), whilst application of UTP, a P2YR agonist decreases the post-receptoral rod and cone b-waves of the electroretinogram (Ward et al. 2008); suggesting that purines modulate ‘through’ pathway transmission in the retina. OFFcholinergic cells are also responsive to ATP through activation of P2X2Rs, whilst blockade of P2 receptors with the antagonist PPADS altered firing rates of ON and OFF ganglion cells (Kaneda et al. 2008). Taken together, this data supports a role for ATP in neural modulation in ON and OFF pathways in the retina.

Table 44.1 Localization of purinergic receptors in the retina

P2 receptor

Cellular localization

References

 

 

 

P2Y1

AC, GC, MC (rat, human,

Fries et al. (2004, 2005)

 

porcine), RPE

 

P2Y2

RPE, MC

Fries et al. (2004, 2005)

P2Y4

RBC, MC, AC

Fries et al. (2004, 2005) and Ward et al. (2008)

P2Y6

MC, RPE

Fries et al. (2004, 2005)

P2X2

AC, GC

Taschenberger et al. (1999), Kaneda et al. (2004),

 

 

Puthussery and Fletcher (2006) and Kaneda et al.

 

 

(2008)

P2X3

AC, GC

Wheeler-Schilling et al. (2001) and Puthussery and

 

 

Fletcher (2007)

P2X7

PR, HC, AC, GC, MC

Pannicke et al. (2000), Innocenti et al. (2004),

 

(human), MG

Puthussery and Fletcher (2004) and Puthussery

 

 

et al. (2006)