Abstract
Purpose: :
The experiments were designed to test the hypothesis that dopaminergic amacrine cells are among the targets of histamine released from retinopetal axons in mouse retina.
Methods: :
Dopaminergic amacrine cells were isolated from transgenic mice (C57BL/TH::RFP), expressing red fluorescent protein and plated onto glass coverslips. Cells were loaded for 45 minutes in darkness at 22° C with Fura 2 acetoxymethyl ester (1 µM), and then the coverslips were mounted on the stage of an inverted microscope with epifluorescent illumination. The cells were superfused with HEPES-buffered saline solutions containing histamine, histamine receptor 1 (HR1) antagonists or agonists in a moist oxygen atmosphere at 20 °C. Free intracellular calcium was determined from the ratio of the fluorescence signals excited at the wavelengths 360 and 380 nm. Mean calcium levels were analyzed using paired t-tests.
Results: :
Superfusion of histamine leads to an increase in mean [Ca++]i of the dopaminergic amacrine cells. The effect was dose-dependent and maximal at 5 µM. Superfusion of 5µM histamine produced an increase in mean [Ca++]i in 79% of the cells analyzed; the increase in mean [Ca++]i was 65%, on average with this dose. The HR1 antagonist pyrilamine blocked the effects of 5 µM histamine when applied at 50 µM. The selective HR1 agonist 2-(3-trifluoromethylphenyl) histamine dihydrogenmaleate applied at 5 µM significantly increased mean [Ca++]i.
Conclusions: :
The results indicate that the effects of histamine on the dopaminergic cells are mediated by HR1, as predicted in a recent anatomical study of mouse retina.
Keywords: dopamine • amacrine cells • calcium