Characterization of histamine projections and their potential cellular targets in the mouse retina.

The vertebrate retina receives histaminergic input from the brain via retinopetal axons that originate from perikarya in the posterior hypothalamus. In the nervous system, histamine acts on three G-protein-coupled receptors, histamine receptor (HR) 1, HR2 and HR3. In order to look for potential cellular targets of histamine in the mouse retina, we have examined the retina for the expression of histamine and the presence of these three receptors. Consistent with studies of retina from other vertebrates, histamine was only found in retinopetal axons, which coursed extensively through the ganglion cell and inner plexiform layers. mRNA for all three receptors was expressed in the mouse retina, and immunohistochemical studies further localized HR1 and HR2. HR1 immunoreactivity was observed on dopaminergic amacrine cells, calretinin-positive ganglion cells and axon bundles in the ganglion cell layer. Furthermore, a distinct group of processes in the inner plexiform layer was labeled, which most likely represents the processes of cholinergic amacrine cells. HR2 immunoreactivity was observed on the processes and cell bodies of the primary glial cells of the mammalian retina, the Müller cells. This distribution of histamine and its receptors is consistent with a brain-derived source of histamine acting on diverse populations of cells in the retina, including both neurons and glia.