Presynaptic and postsynaptic actions of halothane at glutamatergic synapses in the mouse hippocampus

Whole‐cell patch‐clamp recordings in adult mouse hippocampal slices were used to test the mechanism by which the volatile anesthetic halothane inhibits glutamate receptor‐mediated synaptic transmission. Non‐ N ‐methyl‐ D ‐aspartate (nonNMDA) and NMDA receptor‐mediated currents in CA1 pyramidal cells were pharmacologically isolated by bath application of D , L ‐2‐amino‐5‐phosphonovaleric acid (APV; 100 μ M ) or 6‐cyano‐7‐nitro‐quinoxaline‐2,3‐dione (CNQX; 5 μ M ), respectively. Halothane blocked both nonNMDA and NMDA receptor‐mediated excitatory postsynaptic currents (EPSCs) to a similar extent (IC 50 values of 0.66 and 0.57 m M , respectively). Partial blockade of the EPSCs by lowering the extracellular concentration of calcium ([Ca 2+ ] o ), but not by application of CNQX (1 μ M ), was accompanied by an increase in paired‐pulse facilitation (PPF). Halothane‐induced blockade of the EPSCs also was associated with an increase in PPF. The effects of halothane on α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA) and NMDA receptor‐mediated currents induced by agonist iontophoresis, were compared. AMPA‐induced currents were blocked with an IC 50 of 1.7 m M . NMDA‐induced currents were significantly less sensitive to halothane (IC 50 of 5.9 m M ). The effect of halothane on iontophoretic AMPA dose‐response curves was tested. Halothane suppressed the maximal response to AMPA without affecting its EC 50 , suggesting a noncompetitive mechanism of inhibition. All effects of halothane were reversible upon termination of the exposure to the drug. These data suggest that halothane blocks central glutamatergic synaptic transmission by presynaptically inhibiting glutamate release and postsynaptically blocking the AMPA subtype of glutamate receptors.British Journal of Pharmacology (1998) 124 , 1607–1614; doi: 10.1038/sj.bjp.0701996