Maturation of the Neurokinin 1 (NK1) Receptor Immunoreactive Amacrine Cells in the Rat Retina during Postnatal Development

Kang, Tae Hoon; Chung, Jin Woong

Abstract

We examined the morphological maturation of amacrine cells expressing neurokinin 1 (NK1) receptor, whose ligand is substance P, in the rat retina, focusing on the period from postnatal day 5 (P5) when the outer plexiform layer is formed, to postnatal day 13 (P13) when the eyes open, with immunohistochemistry using a specific antiserum against NK1 receptor, and we compared maturing NK1 receptor-immunoreactive (NK1 receptor-IR) amacrine cells with adult one. In the adult retina, numerous NK1 receptor-IR amacrine cells were located in the inner part of the inner nuclear layer (INL) adjacent to the inner plexiform layer (IPL), and their processes emerging from the somata branched and stratified at 1, 2, and 5 strata of within the IPL. NK1 receptor-IR amacrine cells were already observed at P5. The cell bodies were located in the inner INL away from the IPL and their processes branched and formed two distinct bands in the IPL. Afterwards, somata of NK1 receptor-IR amacrine cells moved toward the inner part of the INL, and thus, were located in the INL adjacent to the IPL. Their processes formed three distinct bands at P10 and then, at P13, three bands occupied the same strata as those of the adult, which were posed at 1, 2, and 5 strata of the IPL. During the postnatal development, most of NK1 receptor-IR amacrine cells directly extended one or a few primary dendrites toward the IPL and formed the strata. However, some of the labeled cells located at the outermost row had horizontal processes emerging from their primary dendrites, and these horizontal processes branched and formed plexuses in the INL. The NK1 receptor-IR amacrine cells with horizontal processes were frequently observed at P7, rarely at P10, and not at P13 and in the adult. These results indicate that the NK1 receptor-IR amacrine cells of the rat retina morphologically mature by way of migration of their somata within the INL and formation of distinct processes during postnatal development, and suggest that they morphologically and functionally complete the maturation process about the time of P13.