P2Y receptors in the cornea and sclera
Metabotropic receptors for nucleotides in the cornea have been identified by means of in situ hybridisation techniques. P2Y2 receptors are predominantly expressed in the cornea, sclera, goblet cells and meibomian glands of monkeys and rabbits . In our experiments, we describe P2Y2 receptors and the presence of P2Y1, P2Y4 and P2Y6. The differences between our results and those described in the literature can be due to the animal model used. In the rabbit corneal epithelium the possible existence of P2Y4 and P2Y6 receptors cannot be discarded. Experiments studying the effect of nucleotides on corneal wound healing demonstrate that, apart from the involvement of a P2Y2 receptor in the re-epithelialisation process, the participation of P2Y4/P2Y6 receptors needs to be taken into consideration .
In the endothelium, the existence of receptors for ATP has been demonstrated. These receptors are of the P2Y subtype, although there are not a detailed description of the particular subtype . We may suggest, according to our results, that P2Y2 and P2Y4 are good candidates to fulfill the results described by Srinivas, who clearly identified different responses, therefore suggesting the presence of more than one P2Y receptor subtype.
P2Y4 receptors have been also identified by in situ hybridisation techniques in the conjunctiva, and a role on the CI− movement on conjunctival epithelial cells has been suggested. Hosoya et al.  indicate that P2Y2 and P2Y4 receptors control the ion flux across the conjunctiva. This ion flux can be measured by activating P2Y2 receptors with UTP. This receptor stimulation produces a clear movement of CI− ions and the concomitant fluid transport .
Conjunctival goblet cells can modulate the release of mucins by means of P2Y2 receptors, as suggested by Dartt  and Murakami and co-workers .
P2Y receptors in the iris ciliary body
There is not much information about the presence of P2 receptors in the iris. We have observed the presence of P2Y1, P2Y2 and mainly P2Y4. Previous functional studies demonstrated the presence of P2Y receptors, although nothing indicated the receptor subtype. In these studies, Fuder and co-workers  demonstrated the effect of ATP on the iris, and how this effect is mediated by a P2Y-like receptor . P2Y receptors present in the iris may regulate the pupil size by either controlling the muscle contraction/relaxation or by modulating the sympathetic nervous system which controls the iris physiology .
Concerning the ciliary body, the presence of P2Y receptors has been demonstrated in pigmented and non-pigmented epithelial cells by measuring IP3 generation and cytosolic Ca2+ levels . Other experiments performed by Shaindullah and Wilson  also demonstrate that the effects of UTP and ATP are equipotent and that the receptors they activate may belong to the P2Y2 purinoceptor subtype. In our case, we have been able to identify different P2Y receptors; nevertheless, we were able to see a differential distribution of the receptors in the ciliary processes. P2Y1, P2Y2 and P2Y4 showed immunoreactivity in the ciliary non-pigmented and pigmented epithelium. The existence of P2Y2 receptors in non-pigmented and pigmented epithelial cells has been described by in situ hybridisation techniques . This P2Y2 receptors and also P2Y1 have been described in the rabbit ciliary body epithelium by means of functional studies that reveal the metabotropic nature of these receptors . These receptors, when activated, can increase cytosolic Ca2+ levels plus PGE2 and cAMP. This combination produces the activation of chloride channels which reduce the aqueous humour formation and a reduction in IOP . The P2Y6 receptor appears only in the stroma of the ciliary processes. This fact could be due to the existence of a fenestrated blood vessel network in this area. Since P2Y6 receptors have been described in the endothelium of blood vessels , it could be the case that our results are showing positive immunoreactivity to the P2Y6 receptor present in the vessels that can modify the blood flow in this area.
Concerning the trabecular meshwork, our results, indicating the presence of P2Y1 P2Y2 and P2Y4 in the rat slices, cope quite well with the results described recently by Crosson and co-workers . They also described the presence of P2Y11 in a human trabecular meshwork cells line . We were not able to see a positive immunoreactivity against this receptor in the rat eye slices.
P2Y receptors in the retina
The distribution in the retina concerning the P2Y receptor is complicated, mainly due to the great cellular heterogeneity. The presence of P2Y2 receptors has been demonstrated in the retinal pigmented epithelium (RPE) in situ hybridisation , and functional studies . These studies fit well with our results, which indicate strong immunoreactivity to the P2Y2 receptor. The role of this P2Y2 receptor seems to be very interesting from the therapeutic point of view The presence of this receptors on the apical membrane of RPE cells is very important for the re-absorption of fluids present in the inter-retinal space . This fact has invited us to think about the possibility of using selective P2Y2 agonists for the treatment of pathologies such as the retinal detachment.
In the neural retina, and comparing our P2Y2 results with others previously published, we were unable to see labelling in other retinal areas apart from the photoreceptors. This is in clear contrast with the results of Cowlen et al. , who described the presence of P2Y2 mRNA in the inner nuclear layer and ganglion cell layer.
The other P2Y receptor which showed imunoreactivity in the neural retina was the P2Y6 receptor, which was present at the synaptic location (the plexiform layers) as indicated by the co-localisation with synaptophysin. Only P2X1 receptors have been described as associated to the plexiforms layers in the retina; these include P2X1 [26–29].
Vinmentin antibody is a useful tool to identify Müller cells in the retina . In this sense, we have been able to identify P2Y4 and P2Y6 co-localisation with vimentin in retinal slices, indicating the presence of these two purinoceptors subtypes in Müller cells. Reports about the presence of P2Y receptors in glial cells have been done [31, 32]. The existence of a P2Y receptors in Müller cells produces the typical increase in the cytosolic Ca2+ levels . Recently, Bringmann and co-workers  have demonstrated how P2Y receptors can modify physiological processes such as K+ turnover in Müller cells, which is a crucial aspect in the neurophysiology of the retina.
Very recently, Fries and co-workers  described the expression of P2Y receptors in the rat retina. The use of in situ hybridization techniques demonstrated the presence of P2Y1, 4 and 6 in the inner layers of the retina. We have similar results with the P2Y4 and P2Y6 receptor antibodies; nevertheless, we do not get labelling with the P2Y2. Also, and in clear contrast with the results presented by these authors, we do not observe labelling with the P2Y1 and P2Y2 in the ganglion cell layer but we do find the P2Y4 receptor. This group also describes immunoreactivity to the P2Y1 and P2Y4 in the inner plexiform layer. In our case, the only receptor located in this retinal area is the P2Y6. The differences among our results and the ones described by Fries et al., can be, in part, due to the strain and age of the animals (they used Brown Norway, we used Wistar; they used adults, we used young animals). Changes in the results can also be due to the methodologies used by both groups, which differ slightly.
In summary, P2Y receptors are widely distributed in the eye. Some of the receptors were identified as P2Y although they have not been fully characterised to know which of the P2Y subtype is present in each area. We hope that this work will help researchers to apply better pharmacological tools when investigating these receptors in the different ocular structures.