Activation of the sigma receptor 1 modulates AMPA receptor-mediated light-evoked excitatory postsynaptic currents in rat retinal ganglion cells

Sigma receptor (σR), a unique receptor family, is classified into three subtypes: σR1, σR2 and σR3. It was previously shown that σR1 activation induced by 1 μM SKF10047 (SKF) suppressed N-methyl-d-aspartate (NMDA) receptor-mediated responses of rat retinal ganglion cells (GCs) and the suppression was mediated by a distinct Ca2+-dependent phospholipase C (PLC)-protein kinase C (PKC) pathway. In the present work, using whole-cell patch-clamp techniques in rat retinal slice preparations, we further demonstrate that SKF of higher dosage (50 μM) significantly suppressed AMPA receptor (AMPAR)-mediated light-evoked excitatory postsynaptic currents (L-EPSCs) of retinal ON-type GCs (ON GCs), and the effect was reversed by the σR1 antagonist BD1047, suggesting the involvement of σR1. The SKF (50 μM) effect was unlikely due to a change in glutamate release from bipolar cells, as suggested by the unaltered paired-pulse ratio (PPR) of AMPAR-mediated EPSCs of ON GCs. SKF (50 μM) did not change L-EPSCs of ON GCs when the G protein inhibitor GDP-β-S or the protein kinase G (PKG) inhibitor KT5823 was intracellularly infused. Calcium imaging further revealed that SKF (50 μM) did not change intracellular calcium concentration in GCs and persisted to suppress L-EPSCs when intracellular calcium was chelated by BAPTA. The SKF (50 μM) effect was intact when protein kinase A (PKA) and phosphatidylinostiol (PI)-PLC signaling pathways were both blocked. We conclude that the SKF (50 μM) effect is Ca2+-independent, PKG-dependent, but not involving PKA, PI-PLC pathways.