Activation Of The Sigma Receptor 1 Modulates NMDA Receptors In Rat Retinal Ganglion Cells

The sigma receptor (σR), once considered as a subtype of the opioid receptor, is now descrided as a distinct pharmacological entity and represents unique nonopiate, nonphencyclidine binding sites in mammalian nervous systems. Two types ofσR have been identified:σR type 1 (σR1) and type 2 (σR2). aRl is widely expressed in the central nervous system which has quite high expression levels in olfactory bulb and hypothalamus, but less in cortex and hippocampus. aRl is invovled in modulating voltage-regulated and ligand-gated ion channels, neuronal firing and neurotransmitter release. Moreover,σRl participates in several physical and pathological processes, including neuroprotection, learning and memory, antipsychosis, drug abuse. In the retina, many studies have demonstrated the efficacy of aRl ligands in the protection of retinal neurons, however, little is known about transmission and modulation of visual signals byσR1 in this tissue. Ganglion cells (GCs) are the third-order neurons, which convey signal from bipolar cells and amacrine cells to visual cortex. Both ionotropic and metabotropic glutamate receptors are expressed on GCs, in which NMDA receptors are located extrasynaptically and could be activated only by the concomitant release of many vesicles. In other words, NMDA may play physiological roles in a special mode. Besides, NMDA receptors are closely related to glutamate-caused excitotoxicity in the retina. Excessive stimulation of NMDA receptors result in the degeneration of retinal neurons in glaucoma, retinal ischemia or diabetic retinopathy. In this work, using immunohistochemistry, western blot analysis and patch clamp recording in the preparation of rat retinal slice, we report for the first time that aRl suppressed NMDA receptor-mediated current responses and light-evoked responses on different types of GCs, by modulating G protein signal transduction. The results reveal a new aspect of neuromodulation byσR1 in the retina and also provide hints to understand functional roles ofσR1 in the CNS.In the present work, by immunohistochemistry, we showed that aRl was strongly expressed on some amacrine cells and GCs in the rat retina. Labeling for aRl was also observed in both outer and inner plexiform layers. Using whole-cell recording made from retinal slice preparations, we first characterized NMDA receptor-mediated currents on GCs. A robust inward current was induced from GCs with local puff of 1 mM NMDA to the dendrites or with puff of 120 mM KC1 onto the somata of associated bipolar cells in the absence of Mg2+. The current could be completely blocked by D-AP5, a specific NMDA receptor antagonist. We further show that the aRl agonist SKF10047 (SKF) or PRE-084 suppressed NMDA-induced currents in a dose-dependent manner from both ON and OFF type GCs and the effect could be reversed by the aRl antagonist BD1047 or Haloperidol. Internal infusion of G protein inhibitor GDP-β-S and Gi/o activator Mastoparan greatly reduced the suppression effect of SKF on NMDA currents. Internal application of the phosphatidylinostiol (PI)-specific phospholipase C (PLC) inhibitor U73122, but not the phosphatidylcholine (PC)-PLC inhibitor D609, could block the SKF-induced suppression. Moreover, in intracellular Ca2+-free solution, buffered with 10 mM BAPTA, application of SKF failed to suppress the currents, while L-type Ca2+ channels blocker nimodipine had no effect on SKF effect. Furthermore, application of the ryanodine receptor modulators ryanodine or caffeine, SKF persisted to suppress the NMDA responses. In contrast, internal application of IP3 receptor antagonist heparin or xestospongins-C could suppress the NMDA responses, and the SKF caused suppression was no longer observed.Application of the protein kinase C inhibitor Bis IV and Go6976 could also abolish the SKF effect. However, cAMP, PKA inhibitor Rp-cAMP, cGMP and PKG inhibitor KT5823 failed to block the SKF effect. Therefore, conclusion can be drawn that PI-PLC by the SKF induced modulation on G protein, but not PC-PLC, may be responsible for the SKF-caused NMDA induced current suppression via PKC pathway. Finally, we characterized NMDA receptor-mediated light evoked excitatory postsynaptic currents (eEPSC), and showed thatσR1 agonist SKF suppressed the eEPSC at different holding potential in ON, OFF and ON-OFF GCs. Bath application of SKF had no effect on AMPA receptor-mediated miniature EPSC, suggesting that activation ofσR1 does not affect the glutamate release of bipolar cells. These results suggest that theσR1-mediated neuroprotective effects on GCs may be, at least in part, due to the suppression of NMDA receptors caused by activation ofσR1 via G protein-PLC-PKC signaling pathway.