Membrane and synaptic actions of halothane on rat hippocampal pyramidal neurons and inhibitory interneurons

A relatively small number of inhibitory interneurons can control the excita bility and synchronization of large numbers of pyramidal neurons in hippoca mpus and other cortical regions. Thus, anesthetic modulation of interneuron s could play an important role during anesthesia. The aim of this study was to investigate effects of a general anesthetic, halothane, on membrane and synaptic properties of rat hippocampal interneurons. GABA receptor-mediate d IPSCs were recorded with whole-cell patch-clamp techniques in visually id entified CA1 pyramidal cells and interneurons located at the border of stra tum lacunosum-moleculare and stratum radiatum. Halothane (0.35 mM congruent to 1.2 vol%) depressed evoked IPSC amplitudes recorded from both pyramidal cells and inhibitory interneurons. Also, halothane considerably prolonged the decay time constant of evoked IPSCs in pyramidal cells and interneurons . The frequencies of miniature IPSCs were increased by halothane (two- to t hreefold) in both types of neuron. On the other hand, halothane effects on resting membrane potentials were variable but minimal in both types of neur ons. In current-clamp recordings, halothane depressed EPSP amplitudes and i ncreased IPSP amplitudes recorded from both types of neurons. In addition, halothane increased the failure rate of synaptically evoked action potentia ls. Taken together, these data provide evidence that halothane increases GA BA(A) receptor-mediated synaptic inhibition between synaptically connected interneurons and depresses excitatory transmission, similar to effects obse rved in pyramidal neurons.