P2y4 Receptors Modulate Rod Pathway Signalling in the Mammalian Retina

Abstract

Purpose:: Extracellular ATP acts as a neurotransmitter in the retina through the activation of ionotropic P2X receptors and metabotropic P2Y receptors. The expression of various P2Y receptor subtypes has been demonstrated in the retina, however the specific localization of P2Y receptors and their role in retinal signalling remains ill defined. This study aims to determine the precise localisation and function of the P2Y₄ receptor subtype in the rat retina

Methods:: Using indirect immunofluorescence immunocytochemistry and pre-embedding immunoelectron microscopy on lightly fixed rat retina, we investigated the specific localisation of the P2Y₄ receptor subtype under light and dark adapted conditions and following retinal degeneration using commercially available antisera. The functional role of the receptors was determined using the twin flash paradigm of the electroretinogram (ERG) on anaesthetised (ketamine:xylazine 60:5 mg/kg i.m.), dark adapted rats. Baseline recordings were obtained prior to intravitreal injection of either uridine-triphosphate (UTP; n=5), a P2Y₄ receptor agonist or a control injection of phosphate buffered saline (n=5). Following intravitreal injection waveforms were collected at 3 minute intervals for 45 minutes and later analysed according to photoreceptoral and post-receptoral contributions.

Results:: P2Y₄ receptor expression was strongly colocalised with protein kinase Cα labelled rod bipolar cells in the rat retina. Ultrastructurally, the receptors were present pre-synaptically on rod bipolar cell axon terminals and in putative amacrine cell processes post-synaptic to cone bipolar cell terminals. In the dark adapted retina, there was a significant decrease in the number of rod bipolar cell axon terminals that were P2Y₄ receptor positive. Similarly, retinal degeneration caused an alteration in the rod bipolar cell expression of P2Y₄ receptors. Intravitreal administration of UTP resulted in an immediate and sustained decrease in the post-receptoral b-wave, with no significant change in the amplitude of the photoreceptoral a-wave.

Conclusions:: P2Y₄ receptors were strongly expressed in the inner retina, particularly in the rod pathway. Furthermore, inner retinal signalling was modulated following administration of an agonist of P2Y₄ receptors. Taken together, these results suggest a role for ATP in inner retinal signalling.

Keywords: retina • microscopy: light/fluorescence/immunohistochemistry • electroretinography: non-clinical