ADORs accomplish a variety of physiological effects in diffe

ADORs accomplish a variety of physiological effects in different tissues. In neurons, ADORs regulate the release of neurotransmitters such as dopamine and glutamate (Ferré et al., 1992; Fredholm and Dunwiddie, 1988; Ginsborg and Hirst, 1972; Gonçalves et al., 2015; Quarta et al., 2004; Stella et al., 2003). In smooth muscle, ADORA1 and ADORA2b regulate proliferation and differentiation (Dubey et al., 1998b; Gerwins and Fredholm, 1995). In cardiac fibroblasts, ADORA2b inhibits collagen formation (Dubey et al., 1998a). However, ADORA2a stimulates collagen production in other tissues, such as skin and liver (Chan et al., 2006a, 2006b). Treatment with caffeine generally reduces fibrosis, for example, in the skin, liver and lung (Donejko et al., 2014; Tatler et al., 2016; Wang et al., 2014). In the eye, adenosine increases blood flow in the choroid and optic nerve head, suggesting that ADORs on vascular tlr4 promote vasodilation (Polska et al., 2003). Adenosine signaling via ADORs plays a role in the pathologic vascularization seen in retinopathy of prematurity and diabetic retinopathy (Chen et al., 2017; Vindeirinho et al., 2016). ADORA1 and ADORA2b in the corneal endothelium regulate hydration and promote deturgescence (Tan-Allen et al., 2005), and ADORA1 and ADORA2 receptors regulate fluid transport across the retinal pigment epithelium (Kawahara et al., 2005). The functions of ADORs in scleral fibroblasts have not been fully elucidated; however, knockout of ADOR2a reduces collagen fibril diameter in mouse sclera, and activation of ADORA2a in cultured human scleral fibroblasts stimulates expression of collagen type I, III, and V mRNA (Zhou et al., 2010). Though studies have investigated the localization of different adenosine receptors in discreet parts of the eye in different species, systematic studies encompassing the entire eye are lacking in species used in myopia research. A summary of previous studies reporting ADOR distribution can be seen in Table 1. One study examined the expression of all four ADORs in the entire rat eye (Kvanta et al., 1997), but species-specific differences between the rat and larger species have been noted: specifically, ADORA3 is expressed in human and guinea pig sclera, but not in rat sclera (Cui et al., 2008, 2010; Kvanta et al., 1997); and ADORA1 is expressed in human and monkey photoreceptors, but not in rat photoreceptors (Braas et al., 1987). The Rhesus monkey (Macaca mulatta) has been used extensively in myopia research (Bradley et al., 1999; Fernandes et al., 2003; Hung et al., 2018; Qiao-Grider et al., 2007, 2010; Smith, 2013; Smith and Hung, 1999; Tigges et al., 1990), but to date no study has examined the distribution of ADORs in the Rhesus eye. The retina of a related species, the Cynamolgus monkey (Macaca fascicularis) was found to contain ADORA1 in the retinal nerve fiber layer, retinal ganglion cells, the inner plexiform layer, the inner nuclear layer, and in photoreceptors (Braas et al., 1987); but no other ocular tissues were examined, and no other ADORs were investigated. This study sought to examine the distribution of the ADORs in the anterior and posterior segments of normal Rhesus monkeys using immunohistochemistry (IHC) and reverse-transcription quantitative polymerase chain reaction (RT-qPCR) to begin to understand potential mechanisms by which adenosine receptor blockers slow the progression of myopia and to identify the structures most likely to be affected by ADOR inhibition.
Materials and methods
Results
Discussion Expression of ADORs in the retinal ganglion cell layer is consistent with previous studies, though their function is not entirely understood. ADORs in nervous tissue are known to modulate neurotransmitter release. In the central nervous system, ADORA1 activation has been shown to modulate potassium and calcium currents, thereby resulting in presynaptic inhibition (Dunwiddie and Masino, 2001). Adenosine applied to purified retinal ganglion cell cultures or to intact rat retina preparations decreases glutamate receptor-induced calcium influx (Hartwick et al., 2004). Stimulation of ADORA3 may play a neuroprotective role by reducing Ca+2 overload in retinal ganglion cells (Zhang et al., 2006b).