Neurochemical and Electroretinographic Effects of Neurosteroids in the Golden Hamster Retina

Abstract

Abstract: : Purpose: Neurosteroids (NEs) directly regulate the activity of GABA_A receptors, by enhancing or inhibiting their response. Although GABA is the main inhibitory neurotransmitter in vertebrate retina, the effect of NEs on retinal GABAergic activity has not been previously examined in the golden hamster. In this context, the aim of the present work was to study neurochemical and electroretinographic effects of NEs in the hamster retina. In addition, the presence of cytochrome P450scc, one of the key steps in steroidogenesis, was studied in the hamster retina. Methods: Pregnenolone sulphate (PS) and alotetrahidrodesoxicorticosterone (THDOC) were used as negative and positive GABAergic modulators respectively. Chloride uptake was assessed in a synaptoneurosomal retinal fraction by using ³⁶Cl^-. GABA uptake and release was assessed using ³H-GABA as a radioligand. Electroretinographic activity was examined in both eyes of each animal under scotopic conditions with a gold electrode, one our after the injection of PS or THDOC. The presence of cytochrome P450scc was assessed by western blotting and immunocytochemistry Results: PS inhibited and THDOC increased retinal ³⁶Cl^- influx induced by GABA. At presynaptic level, PS increased, whereas THDOC inhibited high potassium (50 mM) induced ³H-GABA release in the golden hamster retina. The effect of PS on this parameter was Ca²⁺-dependent, since in its absence, the increase in GABA release was not evident. On the other hand, we demonstrated that PS increased and THDOC inhibited ³H-GABA specific uptake. The injection of THDOC (5 mg/kg, i.p.) did not change hamster scotopic electroretinographic activity, while PS, at the same dose, decreased the b- wave amplitude. Using a specific antibody, an immunoreactive protein with a molecular weight close to that of the adrenal hamster cytochrome P450scc was detected in the hamster retina. The immunocytochemical study revealed the presence of this enzyme in retinal ganglion cells as well as at the inner nuclear layer. Conclusions: In summary, these results suggest that in the retina, NEs significantly modulate both the pre- and postsynaptic GABAergic activity. The presence cytochrome P450scc could suggest that the retina is an active site of neurosteroids biosynthesis.

Keywords: retina • inhibitory neurotransmitters • neurotransmitters/neurotransmitter systems