Colchicine inhibits GABA(A) receptors independently of microtubule depolymerization

Colchicine is a microtubule depolymerizing agent used extensively in the study of cytoskeleton-dependent cell functions. In studying the possible functional interaction between the GABA(A) receptor and the cytoskeleton, we found that colchicine inhibits GABA(A) receptor function by mechanisms independent of microtubule depolymerization. Human GABA(A) receptor alpha1beta2gamma2L subunits were co-expressed in Xenopus oocytes and the effects of colchicine on GABA(A) receptor function was assessed using the two-electrode voltage-clamp technique. Co-application of GABA (10 microM) with colchicine (100 microM) resulted in a 59.9% inhibition of GABA-gated chloride currents. This effect was instantaneous in onset with no pre-incubation required and reversed within seconds. Other depolymerizing agents, such as nocodazole (20 microM) and vinblastine (200 microM), did not affect GABA(A) receptor function using the same co-application protocol used with colchicine. The polymerizing agent taxol (10-50 microM) did not affect colchicine inhibition of the GABA responses and did not itself alter GABA-gated chloride currents. The inhibitory effect of colchicine was present under conditions in which the oocyte microtubules had been depolymerized by cold temperature. These results indicate that colchicine inhibits the GABA(A) receptor via mechanisms unrelated to microtubule depolymerization. To further examine the inhibitory effect of colchicine on the GABA response, GABA (10-3000 microM) concentration-response curves were performed in the absence or presence of various concentrations of colchicine (30-300 microM). In the presence of colchicine, the GABA concentration-response curve was shifted to the right in a parallel fashion. A Schild plot of this data yielded a linear slope indicating that colchicine acts as a competitive antagonist at the GABA binding site. We conclude that colchicine is a competitive antagonist at the GABA(A) receptor and that studies using colchicine to examine the functional interaction between GABA(A) receptors and microtubules should be interpreted with caution.