CHARACTERIZATION OF ALDEHYDE DEHYDROGENASE-POSITIVE AMACRINE CELLS RESTRICTED IN DISTRIBUTION TO THE DORSAL RETINA

A class 1 aldehyde dehydrogenase (ALDH) catalyzes oxidation of retinal dehyde to retinoic acid in bovine retina. We used immunocytochemistry and in situ hybridization to localize this enzyme in adult and fetal b ovine retinas. Specific ALDH immunoreactivity was present in the cytop lasm of wide-field amacrine cells restricted in distribution to the do rsal part of the adult retina. The somata diameters ranged from simila r to 8 mu m to similar to 15 mu m, and the cells increased in density from similar to 125 cells/mm(2) near the horizontal meridian to simila r to 425 cells/mm(2) in the superior far periphery. The ALDH-positive cells had somata on both sides of the inner plexiform layer (IPL) and processes in two IPL strata. The majority of ALDH-positive cells were unreactive with antibodies against known amacrine cell enzymes and neu rotransmitters, including GABA and glycine. The ALDH-positive amacrine cells also did not react with anti-cellular retinoic acid-binding pro tein, which was present in a subset of GABA-positive amacrine cells. I n flat-mounted retinas processed by in situ hybridization, the larger ALDH-positive amacrine cells tended to be more heavily labeled. In add ition to amacrine cells, Muller cell processes in the inner retina wer e weakly immunoreactive for ALDH; however, these glial cells did not c ontain ALDH mRNA. The pattern of ALDH expression in fetal bovine retin as was documented by immunocytochemistry. No ALDH reactivity was found before 5.5 months; for the remainder of the fetal period, ALDH immuno reactivity was present in amacrine cells similar to those in adult ret ina. The ALDH-positive amacrine cells in bovine retina are novel, bein g limited in distribution to the dorsal retina and unlabeled with othe r amacrine cell-specific markers. Identification of ALDH in amacrine c ells provides additional evidence that cells of the inner retina are i nvolved in retinoid metabolism.