Effects Of Modulation Of Central Nitric Oxide Signaling Pathway On Exercise Performance In Exhausted Rats

Objective: At present, there have been few reports about whether nitric oxide (NO) in central nervous system is involved in the regulation of exercise performance and fatigue-induced stress. Furhtermore, because there are difference among exercise patterns, sampling regions and sampling time, assay methods in published reports, the results about central NO in exercise-induce fatigue were inconsistent. To picture the relationship between central NO and exercise-induced fatigue, studies were performed to assess (1) the effects of exhaustive exercise on central NO level and expression of neuronal nitric oxide synthase (nNOS) in brain, (2) the effects of modulation of central NO pathway on rat's exercise performance and NO's role in modulation of fatigue stress through intracerebroventricular (i.c.v.) microinjection of L-arginine (L-Arg), a NO precursor, and L-nitro-arginine methyl ester (L-NAME), a NOS inhibitor.Methods: Rats were randomly assigned to five groups: control group, exhausted group, Saline/exhausted group, L-Arg/exhausted group, and L-NAME/exhausted group. After recovery from surgery that a cannula was implanted into the lateral cerebral ventricle of the male SD rats, rats were injected with normal Saline, L-Arg or L-NAME for four consecutive days. Then, a one-time exhaustive treadmill exercise (on the speed of 18 m/min, an inclination of 5°) was performed. The time of exercise till exhaustion was recorded and total workload was calculated, which represented the running performance. The levels of nitrate/nitrite (NOx) in plasma, hypothalamus and hippocampus after exhaustive treadmill exercise were assayed by a colorimetric technique. Expression of nNOS in paraventricular nucleus (PVN) and hippocampal formation (hippocampus and dentate gyrus) was measured four hours after exhaustive exercise.Results: (1) Compared with those in the control group, plasma NOx level and usea nitrogen concentration were significantly inscreased in the exhausted group (P < 0.01). Results from an open-field test showed that the locomotor activity at pre-surgery, pre-injection and pre-exercise had no significant differences among Saline/exhausted group, L-Arg/exhausted group and L-NAME/exhausted group.(2) The hypothalamic NOx level, the number and area of nNOS immunoreactive-postive neurons in PVN decreased significantly in the exhausted group (P < 0.001, P < 0.05, P < 0.01 respectively). The number, area and gray degree value of nNOS immunoreactive-postive neurons in hippocampal CA1,CA2,CA3 and dentate gyrus (DG) of rats in the exhausted group were higher than those in the control group (P < 0.05), but not the NOx levels.(3) Injection (i.c.v.) of L-Arg or L-NAME had significant effects on exercise performance. Compared with those in the Saline/exhausted group, the time of exercise till exhaustion and total workload increased by 151.8% and 150.08% respectively in the L-Arg/exhausted group (P < 0.05), and decreased by 70.22% and 68.90% respectively in the L-NAME /exhausted group (P < 0.01).(4) Injection (i.c.v.) of L-Arg or L-NAME had effects on hypothalamic NOx level and nNOS postive neurons in PVN after exhaustive exercise. Compared with those in the Saline/exhausted group, hypothalamic NOx level and the number and area of nNOS postive neurons in PVN were increased in the L-Arg/exhausted group (P < 0.001, P < 0.001, P < 0.05 respectively). Injection of L-NAME resulted in marked decreases of hypothalamic NOx level (P < 0.05) and the number and area of nNOS postive neurons in PVN (P < 0.05, P < 0.001) as compared with Saline injection.(5) Injection (i.c.v.) of L-Arg or L-NAME had significant effects on NOx levels and expression of nNOS postive neurons in hippocampal formation following exercise-induced fatigue. The number and area of nNOS postive neurons in the L-Arg/exhausted group were higer than those in the Saline/exhausted group (P < 0.05), but there was no significant change of NOx level in hippocampus. Injection of L-NAME decreased the number and area of nNOS positive neurons (P < 0.05) and the NOx level in hippocampal formation (P < 0.001). Conclusion: Modulation of central NO signaling pathway by i.c.v injection of L-Arg or L-NAME can affect rat's exercise performance in exhaustive exercise. The down-regulation of NO/nNOS in hypothalamus and hippocampal formation may contribute the low exercise performance and fatigue-induced stress. Hypothalamus may be a key brain region in regulation of exercise performance for central NO/nNOS. L-Arg supplementation may improve exercise performance and delay the emergence of exercise-induced fatigue through L-Arg-NO signaling pathway.