Orexin-A differentially modulates AMPA-preferring responses of ganglion cells and amacrine cells in rat retina

- The work demonstrates physiological functions of orexins in retina for the first time.
- Orexin-A suppresses AMPA currents of ganglion cells (GCs) via OX1R activation.
- Orexin-A potentiates AMPA currents of amacrine cells (ACs) via OX2R activation.
- Gi/o/PI-PLC/IP3/Ca2+/PKC signaling pathway mediates the orexin-A effect on GCs.
- Gi/o/PC-PLC/novel PKC signaling pathway mediates the orexin-A effect on ACs.

By activating their receptors (OX1R and OX2R) orexin-A/B regulate wake/sleeping states, feeding behaviors, but the function of these peptides in the retina remains unknown. Using patch-clamp recordings and calcium imaging in rat isolated retinal cells, we demonstrated that orexin-A suppressed α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-preferring receptor-mediated currents (AMPA-preferring currents) in ganglion cells (GCs) through OX1R, but potentiated those in amacrine cells (ACs) through OX2R. Consistently, in rat retinal slices orexin-A suppressed light-evoked AMPA-preferring receptor-mediated excitatory postsynaptic currents in GCs, but potentiated those in ACs. Intracellular dialysis of GDP-β-S or preincubation with the Gi/o inhibitor pertussis toxin (PTX) abolished both the effects. Either cAMP/the protein kinase A (PKA) inhibitor Rp-cAMP or cGMP/the PKG blocker KT5823 failed to alter the orexin-A effects. Whilst both of them involved activation of protein kinase C (PKC), the effects on GCs and ACs were respectively eliminated by the phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor and phosphatidylcholine (PC)-PLC inhibitor. Moreover, in GCs orexin-A increased [Ca2+]i and the orexin-A effect was blocked by intracellular Ca2+-free solution and by inositol 1,4,5-trisphosphate (IP3) receptor antagonists. In contrast, orexin-A did not change [Ca2+]i in ACs and the orexin-A effect remained in intracellular or extracellular Ca2+-free solution. We conclude that a distinct Gi/o/PI-PLC/IP3/Ca2+-dependent PKC signaling pathway, following the activation of OX1R, is likely responsible for the orexin-A effect on GCs, whereas a Gi/o/PC-PLC/Ca2+-independent PKC signaling pathway, following the activation of OX2R, mediates the orexin-A effect on ACs. These two actions of orexin-A, while working in concert, provide a characteristic way for modulating information processing in the inner retina.