The effect of isoflurane on unmyelinated and myelinated fibres in the rat brain.

The effect of isoflurane on unmyelinated and myelinated fibres of the rat brain was investigated in vitro. The amplitude of the unmyelinated fibre potential (the prevolley of the hippocampal CA1 region) was reduced in a dose-dependent manner with increasing isoflurane concentrations (up to 5%). The conduction velocity was slightly decreased. The minimal alveolar concentration (MAC) anesthetizing 50% of the animals is 1.38%. At this concentration the presynaptic volley was reduced by 18% and the conduction velocity was decreased by about 1%. The effect on myelinated fibres (fimbria) was small and significantly different from the effect on unmyelinated fibres (P = 0.03 and 0.005 at 1 and 2% isoflurane, respectively.