Figure 3.

Anesthetic effects on paired pulse population spike responses recorded from CA1 neurons were concentration-dependent and exhibited differential involvement of GABA_A receptor-mediated inhibition. Low concentrations of pentobarbital (50 μM; A) produced little effect on first pulse responses, but reduced second pulse responses to approximately 20% of control. This depression of second pulse responses appeared to be mediated entirely by enhanced GABA_A-mediated inhibition, because it was completely reversed by bicuculline. Higher concentrations of pentobarbital (200 μM; B and 300 μM; C) produced increasing degrees of first pulse response depression that was only partially reversed by bicuculline. Similarly, second pulse depression produced by these concentrations was only partially reversed by the GABA_A receptor antagonist. A transient excitatory effect on first pulse responses became evident at the highest concentrations of pentobarbital studied (300 and 500 μM; C and D). Symbols represent the mean ± SD for at least five time matched experiments from separate brain slices for each concentration studied.