Synergistic Inhibition of Glycinergic Transmission In Vitro and In Vivo by Flavonoids and Strychnine

The inhibitory glycine receptor (GlyR) is a key mediator of synaptic signaling in spinal cord, brain stem, and higher central nervous system regions. The flavonoids quercetin and genistein have been identified previously as promising GlyR antagonists in vitro, but their detailed mechanism of action was not known. Here, inhibition of recombinant human α1 GlyRs in HEK 293 cells by genistein, quercetin, and strychnine was studied using whole-cell recording techniques. The interaction of several inhibitors applied alone or in combination was analyzed using a minimum mechanism of receptor activation and inhibition. Receptor inhibition in vivo was studied in a mouse model of strychnine toxicity. Genistein, quercetin, and strychnine were noncompetitive GlyR inhibitors. The inhibitory potency of one flavonoid (either genistein or quercetin) was not affected by simultaneous application of the other, suggesting that both flavonoids target the same site on the receptor. In combination with strychnine, flavonoid inhibition was augmented, indicating that strychnine binds to a position on the receptor physically distant from the flavonoid site. Potentiation of strychnine inhibition by flavonoids was also observed in vivo, where harmless doses of flavonoids enhanced strychnine toxicity in mice. Thus, in vitro and in vivo studies demonstrated a true synergism between flavonoids and strychnine for GlyR inhibition. The mechanism-based approach used here allows a rapid analysis of the effects of single drugs versus drug combinations.