Photodynamic effects of steroid-conjugated fluorophores on GABA_A receptors

We have shown that fluorescent, 7-nitro-2,1,3-benzoxadiazol-4-yl amino (NBD)-conjugated neurosteroid analogs photopotentiate GABA_A receptor function. These compounds seem to photosensitize a modification of receptor function, resulting in long-lived increases in responses to exogenous or synaptic GABA. Here we extend this work to examine the effectiveness of different fluorophore positions, conjugations, steroid structures, and fluorophores. Our results are generally in agreement with the idea that steroids with activity at GABA_A receptors are the most potent photopotentiators. In particular, we find that an unnatural enantiomer of an effective photopotentiating steroid is relatively weak, excluding the idea that membrane solubility alone, which is identical for enantiomer pairs, is solely responsible for potent photopotentiation. Furthermore, there is a significant correlation between baseline GABA_A receptor activity and photopotentiation. Curiously, both sulfated steroids, which bind a presumed external neurosteroid antagonist site, and hydroxysteroids, which bind an independent site, are effective. We also find that a rhodamine dye conjugated to a 5β-reduced 3α-hydroxy steroid is a particularly potent and effective photopotentiator, with minimal baseline receptor activity up to 10 μM. Steroid conjugated fluorescein and Alexa Fluor 546 also supported photopotentiation, although the Alexa Fluor conjugate was weaker and required 10-fold higher concentration to achieve similar potentiation to the best NBD and rhodamine conjugates. Filling cells with steroid-conjugated or free fluorophores via whole-cell patch pipette did not support photopotentiation. FM1-43, another membrane-targeted, structurally unrelated fluorophore, also produced photopotentiation at micromolar concentrations. We conclude that further optimization of fluorophore and carrier could produce an effective, selective, light-sensitive GABA_A receptor modulator.