The Role of Histamine in the Retina: Studies on the Hdc Knockout Mouse

The role of histamine in the retina is not well understood, despite it regulating a number of functions within the brain, including sleep, feeding, energy balance, and anxiety. In this study we characterized the structure and function of the retina in mice that lacked expression of the rate limiting enzyme in the formation of histamine, histidine decarboxylase (Hdc(−/−) mouse). Using laser capture microdissection, Hdc mRNA expression was assessed in the inner and outer nuclear layers of adult C57Bl6J wildtype (WT) and Hdc(−/−)-retinae. In adult WT and Hdc(−/−)-mice, retinal fundi were imaged, retinal structure was assessed using immunocytochemistry and function was probed by electroretinography. Blood flow velocity was assessed by quantifying temporal changes in the dynamic fluorescein angiography in arterioles and venules. In WT retinae, Hdc gene expression was detected in the outer nuclear layer, but not the inner nuclear layer, while the lack of Hdc expression was confirmed in the Hdc(−/−) retina. Preliminary examination of the fundus and retinal structure of the widely used Hdc(−/−)mouse strain revealed discrete lesions across the retina that corresponded to areas of photoreceptor abnormality reminiscent of the rd8 (Crb1) mutation. This was confirmed after genotyping and the strain designated Hdc(rd8/rd8). In order to determine the effect of the lack of Hdc-alone on the retina, Hdc(−/−) mice free of the Crb1 mutation were bred. Retinal fundi appeared normal in these animals and there was no difference in retinal structure, macrogliosis, nor any change in microglial characteristics in Hdc(−/−) compared to wildtype retinae. In addition, retinal function and retinal blood flow dynamics showed no alterations in the Hdc(−/−) retina. Overall, these results suggest that histamine plays little role in modulating retinal structure and function.