Anesthetics Discriminate Between Tonic and Phasic γ-Aminobutyric Acid Receptors on Hippocampal CA1 Neurons

BACKGROUND: Anesthesia is produced by a depression of neuronal signaling in the central nervous system (CNS); however, the mechanism(s) of action underlying this depression remain unclear. Recent studies have indicated that anesthetics can enhance inhibition of CNS neurons by increasing current flow through tonic gamma-aminobutyric acid (GABA(A)) receptor gated chloride channels in their membranes. Enhanced tonic inhibition would contribute to CNS depression produced by anesthetics, but it remains to be determined to what extent anesthetic actions at these receptors contribute to CNS depression. In the present study, we compared and contrasted the involvement of tonic versus synaptic GABA(A) receptors in the functional depression of CNS neurons produced by isoflurane and thiopental. METHODS: In rat hippocampal slices, whole cell patch clamp recordings were used to study anesthetic effects on CA1 neuron intrinsic excitability, and population spike recordings were used to investigate effects on synaptically evoked discharge. These responses were chosen to test whether anesthetic effects on GABA receptors alter single neuron discharge and/or circuit level synaptic functioning. Phasic (synaptic) GABA receptors were selectively blocked using the GABA(A) antagonist gabazine and tonic responses were blocked using the chloride channel blocker picrotoxin. RESULTS: Clinically relevant and equi-effective concentrations of thiopental and isoflurane depressed CA1 neuron synaptically evoked discharge. This depression was partially reversed by blocking synaptic GABA(A) receptors with gabazine (20 mu M). The thiopental-induced depression was reversed by approximately 60%, but the isoflurane-induced depression was reversed by only approximately 20%. Blocking tonic GABA(A) receptors with the addition of 100 mu M picrotoxin produced an additional 40% reversal of the thiopental-induced depression, but no additional reversal was seen for isoflurane-depressed responses. In response to direct DC current injection, CA1 neuron discharge was depressed by thiopental and membrane conductance was increased. Both of these effects were reversed by picrotoxin, but not by gabazine. Isoflurane, in contrast, neither depressed current-evoked discharge, nor altered the membrane conductance of CA1 neurons. CONCLUSIONS: These results indicate that general anesthetics discriminate between synaptic and tonic GABA(A) receptors. Effects on both phasic and tonic receptors combined to depress circuit responses produced by thiopental, whereas only effects on synaptic GABA receptors appeared to play an important role for isoflurane. Together with the other known sites of action for these two anesthetics, our results support a multisite, agent-specific mechanism for anesthetic actions.