Chronic treatment with nitric oxide synthase (NOS) inhibitor profoundly reduces cerebellar NOS activity and cyclic guanosine monophosphate but does not modify minimum alveolar anesthetic concentration.

We previously found that acute administration of a nitric oxide synthase (NOS) inhibitor (N omega-nitro-L-arginine methyl ester [L-NAME]) does not reduce the minimum alveolar anesthetic concentration (MAC) of halothane in rats. However, a recent study has suggested that brain NOS activity could not be inhibited by more than approximately 50% by acute administration of L-NAME. To investigate the effect of marked inhibition of NOS activity on the MAC of halothane, we measured cerebellar NOS activity, cerebellar cyclic guanosine monophosphate (cGMP) levels, and halothane MAC in rats chronically treated with L-NAME and compared the results to those of the saline-treated control group. Although the cerebellar NOS activity and cGMP levels were significantly decreased (14% and 2.7% of control, respectively) by L-NAME, the value of the halothane MAC was not significantly affected. These results suggest that the anesthetic action of halothane, as measured by its MAC in rats, is not related to NOS activity or cGMP levels in the brain.