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Arginine (ARG) was injected (0.8 g/kg, i.p.) into rats and levels of ARG were determined in plasma and four brain areas in the morning and afternoon. In control rats, brain values for ARG and some amino compounds are lower in the afternoon than in the morning. After ARG administration, ARG levels increase about 10-fold in the plasma and 2- to 3-fold in the brain areas. Brain ARG levels follow plasma levels. Elevated ARG levels affect a number of related amino compounds both in the plasma and all brain areas most notably ornithine, phosphoserine, glycine, GABA and ammonia. An increase of citrulline after ARG administration suggests the possibility of ARG-stimulated nitric oxide formation in the midbrain. Thus, ARG shows a daily rhythm in the plasma and brain and its administration increases ARG brain levels which seem to follow plasma levels. In addition, ARG alters a number of other amino compounds most notably GABA, glycine, ornithine and ammonia, indicating that some pharmacological effects seen after ARG administration might be caused by elevated levels of ARG and/or changes in other amino compounds.

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