The beta-lactam antibiotics, penicillin-G and cefoselis have different mechanisms and sites of action at GABA(A) receptors.

The action of the beta-lactam antibiotics, penicillin-G (PCG) and cefoselis (CFSL) on GABA(A) receptors (GABA(A)-R) was investigated using the two-electrode voltage clamp technique and Xenopus oocyte expressed murine GABA(A)-R. Murine GABA(A)-Rs were expressed in Xenopus oocytes by injecting cRNA that encoded for each subunit (alpha1, beta2, and gamma2) and the effects of PCG and CFSL on the alpha1beta2gamma2s subunit receptors were examined using two-electrode voltage clamp. Using the alpha1beta2gamma2s GABA(A)-R, PCG and CFSL inhibited GABA-induced currents in a concentration-dependent manner, with IC(50)s of 557.1+/-125.4 and 185.0+/-26.6 microM, respectively. The inhibitory action of PCG on GABA-induced currents was non-competitive whereas that of CFSL was competitive. Mutation of tyrosine to phenylalanine at position 256 in the beta2 subunit (beta2(Y256F)), which is reported to abolish the inhibitory effect of picrotoxin, drastically reduced the potency of PCG (IC(50)=28.4+/-1.42 mM) for the alpha1beta2(Y256F)gamma2s receptor without changing the IC(50) of CFSL (189+/-26.6 microM). These electrophysiological data indicate that PCG and CFSL inhibit GABA(A)-R in a different manner, with PCG acting non-competitively and CFSL competitively. The mutational study indicates that PCG might act on an identical or nearby site to that of picrotoxin in the channel pore of the GABA(A)-R.