Progesterone decreases isoflurane requirement for both balance disturbance and loss of righting reflex in male mice

It has been known that progesterone has central effects, as measured by minimum alveolar concentration in various experimental settings. Previously we showed that progesterone reduces sevoflurane requirement for the loss of righting reflex (LORR) using male mice. However, the combination of progesterone and isoflurane has not been studied. Therefore, we tested the effect of progesterone on isoflurane requirement. Male C57BL/6 mice were treated with either progesterone (75 mg/kg) + olive oil or only olive oil. Animals were studied in closed cylinder supplied with oxygen and isoflurane that rotated 4 times per min. Balance disturbance and loss of the righting reflex were counted. The data were analyzed by using a multiple independent variable logistics regression model. Concentrations for the onset of balance disturbances; the effective dose 50% (ED 50 ) and effective dose 95% (ED 95 ) of isoflurane for control group and progesterone group were 0.37% and 0.45%, 0.34% and 0.41%, respectively. Concentrations for LORR; the ED 50 and ED 95 for control group and progesterone group were 0.55% and 0.62%, 0.53% and 0.60%, respectively. Subcutaneous injection of progesterone 75 mg/kg decreased isoflurane requirement significantly for both balance disturbance ( p =0.0022) and LORR ( p =0.0218). We conclude that progesterone decreased isoflurane concentration for both balance disturbance and LORR.