Demonstration of histaminergic neurons in horizontal cells of guinea pig retina.

The existence of L-histidine decarboxylase (HDC, EC 4.1.1.22)-like immunoreactive (HDC-I) cells in guinea pig retina was demonstrated using antiserum raised against HDC purified from fetal rat liver. The anti-HDC antiserum partially cross-reacted guinea pig L-DOPA decarboxylase (DDC, EC 4.1.1.28), so the histaminergic neurons were carefully identified. Comparison of HDC-I and DDC-like immunoreactive (DDC-I) cell types in adjacent sections revealed that HDC-I structures were found in some horizontal cells and amacrine cells, and double-staining procedures with anti-HDC antiserum and monoclonal anti-DDC antibody showed that HDC-I horizontal cells had no DDC-I structures, but all the HDC-I amacrine cells had DDC-I structures. From the results, some horizontal cells (with HDC-like immunoreactivities but without DDC-like immunoreactivities) were concluded to be histaminergic.