ANAESTHETICS DEPRESS THE SENSITIVITY OF CORTICAL NEURONES TO l‐GLUTAMATE

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Abstract
The effects of general anaesthetics on the responses of neurones to iontophoretically applied l‐glutamate have been examined in slices of the guinea‐pig olfactory cortex in vitro. Concentrations of pentobarbitone, ether, methoxyflurane, trichloroethylene and alphaxalone that are known to depress synaptic transmission in the prepiriform cortex also depressed the sensitivity of prepiriform neurones to l‐glutamate. Halothane, in concentrations that depress synaptic transmission (< 1%) did not alter the sensitivity of neurones to glutamate. Higher concentrations (> 1%) produced a dose‐related depression of the glutamate sensitivity of neurones. All four volatile anaesthetics tested caused some cells to alter their glutamate‐evoked firing pattern to one in which the spike discharges were more closely grouped. Pentobarbitone and alphaxalone had no such effect. If the sensitivity of the neurones to the endogenous excitatory transmitter is affected by anaesthetics in the same way as the glutamate‐sensitivity, these results suggest that halothane depresses synaptic transmission by decreasing the amount of transmitter released from the nerve terminals, whereas the other anaesthetics depress the sensitivity of the post‐synaptic membrane to the released transmitter.