SELECTIVE ANESTHETIC INHIBITION OF BRAIN NITRIC-OXIDE SYNTHASE

Background: It has been postulated that nitric oxide (NO) is a neurotr ansmitter involved in consciousness, analgesia, and anesthesia. Haloth ane has been shown to attenuate NO-mediated cyclic guanosine monophosp hate accumulation in neurons, and a variety of anesthetic agents atten uate endothelium-mediated vasodilation, suggesting an interaction of a nesthetic agents and the NO-cyclic guanosine monophosphate pathway. Ho wever, the exact site of anesthetic inhibitory action in this multiste p pathway is unclear. The current study examines effects of volatile a nd intravenous anesthetic agents on the enzyme nitric oxide synthase ( NOS) in brain. Methods: NOS activity was determined by in vitro conver sion of [C-14]arginine to [C-14]citrulline. Wistar rats were decapitat ed and cerebellum quickly harvested and homogenized. Brain extracts we re then examined for NOS activity in the absence and presence of the v olatile anesthetics halothane and isoflurane, and the intravenous agen ts fentanyl, midazolam, ketamine, and pentobarbital. Dose-response cur ves of NOS activity versus anesthetic concentration were constructed. Effects of anesthetics on NOS activity were evaluated by analysis of v ariance. Results: Control activities were 57.5 +/- 4.5 pmol.mg protein (-1).min(-1) in the volatile anesthetic experiments and 51.5 +/- 6.5 p mol.mg protein(-1).min(-1) in the intravenous anesthetic experiments. NOS activity was not affected by ketamine (less than or equal to 1 X 1 0(-4) M), pentobarbital (less than or equal to 5 X 10(-5) M), fentanyl (less than or equal to 1 X 10(-5) M), and midazolam (less than or equ al to 1 X 10(-5) M). Halothane decreased NOS activity to 36.7 +/- 2.5 (64% of control, P < 0.01 from control), 23.8 +/- 4.3 (41%, P < 0.01 f rom control and < 0.05 from 0.5% halothane), 25.2 +/- 3.8 (44%, P < 0. 01 from control and < 0.05 from 0.5% halothane), and 19.7 +/- 2.8 (34% , P < 0.01 from control and < 0.05 from 0.5% halothane) pmol.mg protei n(-1).min(-1) at 0.5, 1.0, 2.0, and 3.0% vapor. Isoflurane decreased N OS activity to 48.9 +/- 6.1 (85% of control), 46.0 +/- 3.2 (80%, P < 0 .05 from control), 40.3 +/- 5.1 (70%, P < 0.05 from control), and 34.2 +/- 4.0 (60%, P < 0.05 from control and 0.5% and 1.0% isoflurane) pmo l.mg protein(-1).min(-1) at 0.5, 1.0, 1.5, 2.0% vapor, respectively. C onclusions: Volatile anesthetics inhibit brain NOS activity in an in v itro system, but the intravenous agents examined have no effect at cli nically relevant concentrations. This inhibition suggests a protein-an esthetic interaction between halothane, isoflurane, and NOS. In contra st, intravenous agents appear to have no direct effect on NOS activity . Whether intravenous agents alter signal transduction or regulatory p athways that activate NOS is unknown.