Positive allosteric modulation by ultraviolet irradiation on GABAA, but not GABAC, receptors expressed in Xenopus oocytes

Recombinant rat GABAA (α1β2, α1β2γ2, β2γ2) and human GABAC (ρ1) receptors were expressed in Xenopus oocytes to examine the effect of ultraviolet (UV) light on receptor function. GABA-induced currents in individual oocytes expressing GABA receptors were tested by two-electrode voltage clamp before, and immediately after, 312 nm UV irradiation. UV irradiation significantly potentiated 10 μm GABA-induced currents in α1β2γ2 GABA receptors. The modulation was irradiation dose dependent, with a maximum potentiation of more than 3-fold. The potentiation was partially reversible and decayed exponentially with a time constant of 8.2 ± 1.2 min toward a steady-state level which was still significantly elevated (2.7 ± 0.3-fold) compared to the control level. The effect of UV irradiation on GABAA receptors varied with receptor subunit composition. UV irradiation decreased the EC50 of the α1β2, α1β2γ2 and β2γ2 GABAA receptors, but exhibited no significant effect on the ρ1 GABAC receptor. UV irradiation also significantly increased the maximum current 2-fold in α1β2 GABAA receptors with little effect on the maximum of α1β2γ2 (1.1-fold) or β2γ2 (1.1-fold) GABAA receptors. The effect of UV irradiation on GABAA receptors did not overlap the effect of the GABA receptor- allosteric modulator, diazepam. The UV effect on GABAA receptors was not prevented by the treatment of the oocytes before and during UV irradiation with one of the following free-radical scavengers: 40 mmd-mannitol, 40 mm imidazole or 40 mm sodium azide. In addition, the effect was not mimicked by the free-radical generator, H2O2. Potential significance and mechanism(s) of the UV effect on GABA receptors are discussed. γ-Aminobutyric acid (GABA)-gated chloride channels mediate fast inhibitory synaptic transmission, thereby controlling neuronal excitability. These chloride channels can be classified into GABAA and GABAC receptors according to their pharmacological properties. GABAA receptors are allosterically modulated by benzodiazepines, barbiturates and neurosteroids, and antagonized by bicuculline, whereas GABAC receptors are insensitive to these compounds (Woodward et al. 1992; Feigenspan et al. 1993; Qian & Dowling, 1993; Macdonald & Olsen, 1994; Johnston, 1996; Lukasiewicz, 1996; Ueno et al. 1997). These two types of GABA receptors also differ in their physiological properties and distribution. For example, GABAA receptors have fast activation and deactivation kinetics and show significant desensitization during prolonged agonist exposure, whereas GABAC receptors have slow kinetics without significant desensitization (Cutting et al. 1991, Amin & Weiss, 1994). While GABAA receptors are widely distributed in the central nervous system and retina, GABAC receptors are mainly localized in the retina (Enz et al. 1995, 1996; Yeh et al. 1996; Albrecht et al. 1997; Wegelius et al. 1998). Molecular cloning has revealed several GABA-gated ion channel subunits and their isoforms α1–6, β1–4, γ1–3, ρ1–3, δ, e, π and χ (Barnard et al. 1987; Schofield et al. 1987; Khrestchatisky et al. 1989; Olsen & Tobin, 1990; Cutting et al. 1991; Garret et al. 1997; Hedblom & Kirkness, 1997; Whiting et al. 1997). The exogenous expression of GABA receptors simplifies studies by avoiding the complication of heterogeneous populations of GABA receptor subtypes in the same cell. Such studies have revealed that the distinct physiological and pharmacological properties of GABAA and GABAC receptors are due to the differences in their subunit compositions. Recombinant αβγ GABA receptors have pharmacological and physiological properties similar to native GABAA receptors (Levitan et al. 1988; Pritchett et al. 1989; Malherbe et al. 1990; Sigel et al. 1990; Verdoorn et al. 1990), whereas exogenously expressed ρ1 homomeric GABA receptors have properties similar to native GABAC receptors (Cutting et al. 1991). The distinct pharmacological properties and spatial distribution for receptors with different subunit composition is the basis for developing subunit-specific therapeutic approaches for the treatment of insomnia, epilepsy and other neurological disorders. In this study, we observed that UV irradiation has differential effects on recombinant GABAA and GABAC receptors. α1β2, β2γ2 and α1β2γ2 GABA receptors can be potentiated by UV irradiation whereas ρ1 GABA receptors are insensitive to UV light. The potentiation of GABAA receptors by UV irradiation does not overlap with the effect of the GABAA receptor-allosteric modulator, diazepam. The elucidation of the molecular mechanism could facilitate the identification of new structural element(s) crucial for GABAA receptor function. It could also help us to understand the pathophysiology of UV-induced retina damage, and facilitate the development of strategies to prevent or reverse the damage.