Expression of histamine receptors in the gerbil retinal neurons

Purpose The presence of histamine receptors ( HR s) in mammalian retinae has been reported by several laboratories. In order to confirm the presence of histaminergic pathway in the retina, we made experiments using physiological and immunohistochemical analyses. Methods All experiments were performed using the gerbil ( Meriones unguiculatus ). The activity of the HR was measured by fura‐2 based calcium‐imaging technique and by whole‐cell patch‐clamp technique from slice preparation of the retina. Localizations of the subtypes of HR s, H1 receptor (H1R), H2 receptor (H2R) and H3 receptor (H3R), were examined by avidin‐biotin‐peroxidase complex immunocytochemical staining or immunofluorescence in retinae from 1 to 350 postnatal days. All animal experiments are approved by the ethics committee of Fujita Health University. Results Physiology : A bath application of 100 µM histamine increased the intracellular calcium concentration in some retinal ganglion cells ( RGC s). Under voltage‐clamp condition, histamine increased the amplitude of the outward or inward current in some RGC s. Immunohistochemistry : We found that H1R, H2R and H3R expressed on RGC s. H1R expresses through the retinal maturation. On the other hand, the expressions of H2R and H3R became maximum from 14 to 21 postnatal days. Since the gerbil opens the eyes at 3 weeks old, it is considered that the H2R and H3R play some specific roles at the formation of the early visual system. Histidine decarboxylase, which produces histamine from histidine, also expressed in RGC s, and moreover, each of HR s and histidine decarboxylase were co‐localized at the same RGC s. Conclusions Our findings suggest that RGC s interact with each other via histamine, and that histamine is one of the important neurotransmitters and/or neuromodulators in the visual information processings of the mammalian retina.