Isoflurane Bidirectionally Modulates the Paired-Pulse Responses in the Rat Hippocampal CA1 Field In Vivo

Abstract

We studied the effects of isoflurane on hippocampal synaptic transmission and paired-pulse plasticity, under in vivo intact interneuron circuitry. Using rats chronically implanted with electrodes, excitatory postsynaptic potential (EPSP) and population spike amplitude (PSA) were measured in the hippocampal CA1 field by stimulating Schaffer collaterals. The lungs of the rats were mechanically ventilated with 0.25-1.5 minimum alveolar anesthetic concentration (MAC) isoflurane. A control value was obtained in the absence of isoflurane. Isoflurane depressed EPSP responses and enhanced synaptic efficacy. PSA was not depressed except under high concentrations, presumably reflecting a well-balanced combination with the decreased EPSP and enhanced synaptic efficacy. Low concentrations of isoflurane (0.25 and 0.5 MAC) increased paired-pulse facilitation (PPF), whereas a high concentration of isoflurane (1.5 MAC) prolonged the paired-pulse depression. Isoflurane appeared to affect multiple sites of CA1 synapses: 1) the depression of presynaptic glutamatergic transmission as shown by depressed EPSP and increased PPF; 2) the depression of pyramidal neurons as shown by prolonged PPF and depressed PSA under high concentration; and 3) the depression of interneurons as shown by the greater synaptic efficacy. The degree of each of these inhibitory effects seemed to vary at different concentrations, and the overall direction of the synaptic properties may depend on the balances between these inhibitory effects in vivo.