Expression of neuropeptides and their receptors in the developing retina of mammals

The present review examines various aspects of the developmental expression of neuropeptides and of their receptors in mammalian retinas, emphasizing their possible roles in retinal maturation. Different peptidergic systems have been investigated with some detail during retinal development, including substance P (SP), somatostatin (SRIF), vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase-activating polypeptide (PACAP), neuropeptide Y (NPY), opioid peptides and corticotrophin-releasing factor (CRF). Overall, the developmental expression of most peptides is characterized by early appearance, transient features and achievement of the mature pattern at the time of eye opening. Concerning possible developmental actions of neuropeptides, recent studies imply a role of SP in the modulation of cholinergic neurotransmission in early postnatal rabbit retinas, when cholinergic cells participate in the retinal spontaneous waves of activity. In addition, the presence of transient SRIF expressing ganglion cells and recent observations in SRIF receptor knock-out mice indicate variegated roles of this peptide in the development of the retina and of retinofugal projections. Furthermore, VIP and PACAP exert protective and growth-promoting actions that may sustain retinal neurons during their development, and opioid peptides may control cell proliferation in the developing retina. Finally, a peak in the expression of certain peptides, including VIP, NPY and CRF, is present around the time of eye opening, when the retina begins the analysis of structured visual information, suggesting important roles of these peptides during this delicate phase of retinal development. In summary, although the physiological actions of peptides during retinal development are far from being clarified, the data reviewed herein indicate promising perspectives in this field of study.