L-DOPA Suppressed Scratching Behavior and Modified Brain Monoamine/ Amino Acid Concentrations in Mice

The effects of L-DOPA (0, 25, and 50 mg/kg body weight) administration on peripheral pruritogen-induced scratching behavior and on brain monoamine and amino acid metabolism were investigated in mice. Intraperitoneal injection of L-DOPA significantly suppressed the number of scratches caused by subcutaneous injection of compound 48/80 (C48/80) in a dose-dependent manner. In the brain stem, C48/80 increased the norepinephrine turnover rate, but the change in the frequency of scratching was not paralleled by the changes in monoamine levels or by the monoamine turnover rates in the brain. On the other hand, most amino acids in the brain stem showed the highest levels in mice treated with C48/80 without L-DOPA, and these values were clearly decreased by L-DOPA administration. These changes in the concentrations of amino acids may be involved in the regulation of the frequency of scratching induced by L-DOPA. In particular, treatment with L-DOPA and C48/80 markedly decreased the concentrations of GABA, L-glutamine, Lglutamic acid, and glycine in the brain stem. In conclusion, L-DOPA administration may attenuate the frequency of the scratching through changing amino acid metabolism in the brain.