Abstract
Purpose :
Orexin-A and orexin-B have been implicated in arousal and feeding behaviors by activating two G-protein-coupled receptors: OX1R and OX2R. While the expression of orexins and orexin receptors is immunohistochemically revealed in retinal neurons (Liu et al., 2011), the function of these peptides in the retina is largely unknown. Here we investigated whether and how orexin-B modulates GABAergic synaptic transmission on rod-dominant ON type bipolar cells (RBCs) in the rat retina.
Methods :
The in vivo scotopic flash electroretinogram (ERG) recordings were conducted to monitor the effect of orexin-B on ERG b-wave that mainly represents RBC depolarization. Whole cell patch-clamp recording techniques in both retinal slices and freshly dissociated RBCs were used to investigate the effect of orexin-B on GABAergic synaptic transmission on RBCs.
Results :
Intravitreal injection of orexin-B produced a marked increase in the amplitudes of scotopic ERG b-waves, suggesting orexin-B modulates RBC activity in vivo. We recorded GABA receptor (GABAR)-mediated inhibitory postsynaptic currents (IPSCs) from RBCs in rat retinal slices to test whether orexin-B modulates GABAergic feedback from amacrine cells (ACs) to RBCs in the inner plexiform layer (IPL). We found that orexin-B significantly suppressed GABAR-mediated IPSCs of RBCs. We further investigated the effect of orexin-B on GABAR-mediated currents (GABA currents) in rat isolated RBCs. Orexin-B suppressed GABACR-, but not GABAAR-mediated currents of RBCs and the effect was partially eliminated by an OX1R antagonist or an OX2R antagonist. The orexin-B effect was abolished by preincubation with a Gi/o inhibitor and phosphatidylcholine (PC)-phospholipase (PLC) inhibitor. A protein kinase C (PKC) activator suppressed the GABA currents, whereas a PKC inhibitor abolished the orexin-B-induced suppression. Moreover, orexin-B did not change [Ca2+]i of RBCs and the orexin-B effect persisted in intracellular or extracellular Ca2+-free solution. Collectively, orexin-B suppressed the GABA currents of RBCs through a distinct Gi/o/PC-PLC/Ca2+-independent PKC signaling pathway, following OX1R and OX2R activation.
Conclusions :
Orexin-B suppressed GABA currents of RBCs and GABAergic inhibitory feedback from ACs onto RBCs, and the suppression may cause increased activity of RBCs, as evidenced by the fact that orexin-B potentiated the scotopic ERG b-wave.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.