Photoreceptor organization and rhythmic phagocytosis in the nile rat Arvicanthis ansorgei:: A novel diurnal rodent model for the study of cone pathophysiology

PURPOSE. To characterize rod and cone distribution, organization, and phagocytosis in the diurnal mouse- like rodent Arvicanthis ansorgei. METHODS. Retinas of adult A. ansorgei were processed for histology, electron microscopy and immunohistochemistry using rod- and mouse cone- specific antibodies. For phagocytosis studies, retinas were sampled every 3 hours under a 12- hour light - dark cycle and processed for double- label immunohistochemistry. The number of phagosomes in the retinal pigmented epithelium were quantified with a morphometric system. RESULTS. A. ansorgei retinas were composed of 33% cones and 67% rods, approximately 10 times more cones than mice and rats. Cones were arranged in two cell layers at the scleral surface, distributed uniformly across the entire retina. Cone arrestin was distributed throughout the dark- adapted cones, from outer segments to synapses, whereas short- and mid-wavelength cone opsins were restricted to outer segments. Short- wavelength cone density was mapped in wholemounted retinas, in a significantly higher number in the central region. Rhodopsin immunopositive ( rod) phagosomes showed a small peak late in the dark phase, then a large burst 1 to 2 hours after light onset, after decreasing to low baseline levels by 12 AM. Mid- wavelength cone opsin immunopositive ( cone) phagosomes were 10 times less numerous than rods, and demonstrated a broad peak 1 to 2 hours after light onset. CONCLUSIONS. The diurnal rodent A. ansorgei possesses a large number of cones, organized in a strict anatomic array. Rod and cone outer segment phagocytosis and shedding can be monitored simultaneously and show similar profiles but different amplitudes. This species may constitute a valuable novel animal model for investigating cone pathophysiology.