Expression of sigma receptor 1 mRNA and protein in rat retina

Sigma receptor (σR), known as a unique nonopiate, nonphencyclidine brain receptor, can bind diverse classes of psychotropic drugs, neurosteroids and other synthetic compounds, such as (+)pentazocine, etc. Two types of σRs have been identified: σR1 and σR2. In this work, we examined the expression of σR1 in rat retina by reverse transcription-polymerase chain reactive (RT-PCR) analysis and immunofluorescence double labeling. RT-PCR analysis showed that σR1 mRNA was present in rat retina. Furthermore, labeling for σR1 was diffusely distributed in the outer and inner plexiform layers. The σR1-immunoreactivity (IR) was also observed in many cells in the inner nuclear layer and the ganglion cell layer. In the outer retina σR1 was expressed in all horizontal cells labeled by calbindin. In contrast, no σR1-IR was detected in several subtypes of bipolar cells, including rod-dominant ON-type bipolar cells, types 2, 3, 5 and 8 bipolar cells, labeled by protein kinase C (PKC), recoverin and hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4) respectively. In the inner retina, most of GABAergic amacrine cells, including dopaminergic and cholinergic ones, stained by tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) respectively, expressed σR1. Some glycinergic amacrine cells were also labeled by σR1, but glycinergic AII amacrine cells were not labeled. In addition, σR1-IR was seen in almost all somata of the ganglion cells retrogradely labeled by fluorogold. These results suggest that σR1 may have neuromodulatory and neuroprotective roles in the retina.