The effects of general anesthesia on passive and active membrane properties of trigeminal root ganglion neurons of decerebrate guinea pigs have been determined using frequency-domain analyses of small-amplitude perturbations of membrane voltage. Quantification of the effects was accomplished by fitting the complex impedance locus diagrams computed from the neuronal responses with a membrane model based on linearized Hodgkin-Huxley-like equations. 2. Endotracheal administrations of isoflurane (2-3% for periods of 30-180 s), the most extensively studied of five general anesthetics, did not elicit large changes in membrane potential or in electrical properties in 26 of the 38 neurons. In this relatively unresponsive group, application of isoflurane in higher concentrations (3-4%) tended to evoke small but significant changes (less than 20%) in membrane properties without altering membrane potential by greater than 5 mV. These changes consisted of increases in the effective input capacitance and input conductance. 3. The impedance magnitude functions were reduced in amplitude consistently in 12 of the 38 neurons during induction of general anesthesia with isoflurane (2-4%) or, in several cases, with halothane (2%). Such applications evoked depolarizations of 8-32 mV, which also were observed in several instances of anesthesia with enflurane and cyclopropane. Quantification of these effects on electrical properties by curve fitting with the linearized Hodgkin-Huxley model revealed increases in the effective input capacitance, in the time-invariant resting conductance, Gr, and in the voltage- and time-dependent conductance, GL. Sometimes, an initial decrease preceded the increase of Gr, and the relaxation time constant associated with GL usually was reduced by the anesthetic agent in the 12 neurons. 4. In 10 neurons, membrane resonance behavior (which was apparent as a large hump at low frequencies of the impedance magnitude functions) was reduced in amplitude, as well as broadened in bandwidth, when peak changes in membrane properties were evident, i.e., during surgical or deep anesthesia. These actions of isoflurane or halothane were correlated to a reduction in spike electrogenesis and they may account for the reduced tendency of neurons to fire repetitive action potentials during anesthesia with isoflurane or halothane.(ABSTRACT TRUNCATED AT 400 WORDS)
