Tachykinin (TK) peptides act on retinal neurons through neurokinin (NK) receptors. We examined the expression of neurokinin-1 (NK1; the substance P receptor), NK3 [the neurokinin B (NKB) receptor], and TK peptides in developing rat retinas. NK1 immunolabeling was found in newborn retinas in rare amacrine cells and in putative ganglion cells. At postnatal day 2 (PND 2), NK1 immunostaining was reduced greatly among ganglion cells, and it appeared in many amacrine cells and in fibers in the inner plexiform layer (IPL), with the highest density in laminae 1, 3, and 5. A similar pattern was found at PND 7. At PND 12, interplexiform NK1-immunoreactive (-IR) cells were detected, and NK1-IR fibers in the IPL were concentrated in lamina 2, similar to what was seen in adults. NK3 was expressed mainly by OFF-cone bipolar cells, and the developmental pattern of NK3 was compared with that of cone bipolar cells that were labeled with antibodies to recoverin. Immature recoverin-IR cone bipolar cells were seen at PND 2. NK3 immunolabeling was detected first in the outer plexiform layer and in sparse bipolar cell somata at PND 10, when recoverin-IR cone bipolar cells are nearly mature. By PND 15, both the NK3 immunostaining pattern and the recoverin immunostaining pattern were similar to the patterns seen in adults. TK immunoreactivity was present at PND 0 in amacrine cells and displaced amacrine cells. By PND 10, the morphologic maturation of TK-IR cells was complete. These findings indicate that, in early postnatal retinas, substance P may act on NK1 receptors, whereas NKB/NK3 interactions are unlikely, suggesting that there are different levels of importance for different TK peptides in the developing retina.
Copyright 2000 Wiley-Liss, Inc.