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Mass spectrometric measurement of catecholamine turnover in rat hypothalamus after long-term L-dopa infusion.

To test the effect of L-dopa on norepinephrine turnover in rat hypothalamus, we gave steady-state, long-term (19-hour) infusions of L-dopa to conscious, restrained rats. After 19 hours, infusions were switched to identical doses of 2,5,6-trideutero-L-dopa (D3-L-dopa) and production of D3-dopamine and D3-norepinephrine was followed at intervals thereafter. Animals were sacrificed and their hypothalami were dissected and divided into left and right halves. Using gas chromatography-mass spectrometry, we measured isotopic enrichment with one side of the hypothalamus and, after the addition of deuterated internal standards, determined total concentration of dopamine and norepinephrine in the other half of the same hypothalamus. Maximal steady-state enrichment by a given dose of D3-L-dopa was determined by 19-hour infusions of D3-L-dopa alone. Results show that total dopamine production is proportional to dopa dose. Norepinephrine synthesis, on the other hand, proceeds at a constant rate that is independent of dopa dose or dopamine concentration and which is similar to the endogenous rate reported by others using tracer techniques.