PURPOSE: Orexin-A, -B play a crucial role in arousal and feeding by activating two G-protein-coupled receptors: OX1R and OX2R. Orexins and orexin receptors are expressed in retinal neurons. Indeed, orexins have been found to modulate the activity of amacrine and ganglion cells in the inner retina. However, the function of these peptides in the outer retina remains unknown. In this work, we investigated whether and how orexin-B modulates synaptic transmission of rod-dominant ON type bipolar cells (RBCs)in the outer retina of rat.
METHODS: The in vivo scotopic flash electroretinogram (ERG) recordings were conducted to monitor the effect of orexin-B on RBC activity. Whole cell patch-clamp recording techniques in both retinal slices and dissociated RBCs were used to investigate the effect of orexin-B on synaptic transmission of RBCs.
RESULTS: Intravitreal injection of orexin-B increased the amplitudes of the scotopic ERG b-wave, a reflection of RBC activity. Consistently, patch clamp recordings in rat retinal slices further showed that orexin-B potentiated the responses of RBCs to simulated light flashes, which could avoid presynaptic and lateral interference from photoreceptors and horizontal cells. Effects of orexin-B on GABAergic synaptic transmission were then examined. Presynaptically, orexin-B suppressed GABA receptor-mediated inhibitory postsynaptic currents (IPSCs) of RBCs in retinal slices. Postsynaptically, as whole-cell recordings and calcium imaging from isolated RBCs showed, orexin-B suppressed GABAC receptor-, but not GABAA receptor-, mediated currents, which was blocked by the OX1R and OX2R antagonists. The orexin-B-induced inhibition of GABAC currents was likely mediated by a Gi/o/PC-PLC/Ca2+-independent PKC signaling pathway, as such inhibition was absent when each step of the above-pathway was blocked with GDP-β-S/pertussis toxin (for Gi/o), D609 (for PLC), bisindolylmaleimide IV (for PKC)/rottlerin (for PKCδ), respectively.
CONCLUSIONS: The orexin-B-induced potentiation of RBC activity may improve visual acuity and contrast sensitivity of the animal during the dark period (wake phase).