The p57(Kip2) cyclin kinase inhibitor is expressed by a restricted set of amacrine cells in the rodent retina

Amacrine cells in the vertebrate retina can be grouped according to morphol ogy into distinct types, which are organized into characteristic mosaics. E ach type is believed to perform a unique role in visual signal processing. Neurotransmitters and calcium binding proteins have served as important mar kers for amacrine cell populations, yet many types remain to be characteriz ed at the molecular level. We have found that a cyclin kinase inhibitor, p5 7(Kip2), is expressed in a restricted group of amacrine cells in the inner nuclear layer (INL) and ganglion cell layer (GCL) of the rodent retina. Who le-mount antibody staining revealed that the p57(Kip2) amacrine cells are e venly distributed across the retina with a density of 1654 +/- 63 cells/mm( 2) in the INL and 994 +/- 26 cells/mm2 in the GCL. These amacrine cells acc umulate the major inhibitory neurotransmitter gamma -aminobutyric acid (GAB A) but do not accumulate high levels of glycine. In addition, p57(Kip2) imm unoreactivity does not colocalize with any of the previously identified ama crine cell markers including calbindin, calretinin, parvalbumin, choline ac etyltransferase, and tyrosine hydroxylase. To determine whether the p57(Kip 2) population of amacrine cells is organized into a regular or a random mos aic, nearest neighbor analysis was performed for both the INL and GCL popul ations. Results from this analysis demonstrated that the p57(Kip2)-immunore active amacrine cells are randomly organized and therefore they are likely to constitute two or more distinct populations. This new molecular marker w ill serve as a useful tool for future studies on the development and functi on of amacrine cells in the vertebrate retina. J. Comp. Neurol. 429: 601-61 4, 2001. (C) 2001 Wiley-Liss, Inc.