Diabetic retinopathy is one of the leading causes of blindness in diabetic patients. Müller cells are the primary glial cells of the retina and they are pivotal for the normal functioning of the retina. Histamine is involved in the blood-retinal barrier (BRB) breakdown that is associated with the pathogenesis of diabetic retinopathy, and recent evidence has specifically implicated an important role of histamine H4 receptor in vascular permeability. Photobiomodulation (PBM) refers to low-intensity infra-red light therapy which has shown promise in retinopathies. We have demonstrated, for the first time, the presence of the histamine H4 receptor on murine Müller cells, particularly on the end feet of these cells. The principal aims of this investigation were to confirm the presence and characterize the Histamine H4 receptor on human Müller cells and to determine whether H4 receptor antagonists or PBM near-infrared (NIR) wavelength 1068 nm has any therapeutic potential against diabetic retinopathy. Immunofluorescence (IF) analysis showed the presence of Histamine H4 receptor in the human Müller cells of the retina (Mio-1 cells), and this expression was upregulated in a high (4500 mg/L), versus low (3151 mg/L), glucose environment. IF analysis has also hinted at the presence of a modest nascent histamine pool in the retina through the presence of Histidine decarboxylase (HDC). Further pharmacological characterization of the receptor included the release of calcium from intracellular pools upon receptor activation with selective H4 receptor agonists, VUF8430 and ST-1006 of concentrations ranging from 10 nM – 100 µM. On testing the protective properties of H4 receptor agonist, VUF8430 (10 µM) and antagonist, JNJ7777120 (10 µM) against H2O2 induced (400 µM) oxidative and glyoxal-induced (2 mM) inflammatory damage in human Müller cells, they both showed no significant protection. However, pilot data showed that VUF8430 exacerbated inflammatory response. Single PBM treatment at 10 mW/sq cm2 power density against oxidative stress showed modest protection, but double PBM treatment of the cells both pre- and post-oxidative stress displayed complete glioprotection, and an anti-inflammatory effect upon TNF-alpha gliosecretion. In conclusion, this is the first demonstration that histamine H4 receptors are functionally expressed on human Müller cells, which indicates the therapeutic potential of neutral H4 receptor antagonists in diabetic retinopathy. The study also provides evidence for a new non-invasive therapeutic approach (antioxidant and anti-inflammatory) for diabetic retinopathy based on a PBM NIR1068 nm protocol
