MASS-SPECTROMETRIC MEASUREMENT OF CATECHOLAMINE TURNOVER IN RAT HYPOTHALAMUS AFTER LONG-TERM L-DOPA INFUSION

Abstract

To test the effect of L-dopa or norepinephrine turnover in rat hypothalamus, steady-state, long-term (19 h) infusions of L-dopa were given to conscious, restrained rats. After 19 h, 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, isotopic enrichment was measured with one side of the hypothalamus and, after the addition of deuterated internal standards, the total concentration of dopamine and norepinephrine in the other half of the same hypothalamus was determined. Maximal steady-state enrichment by a given dose of D3-L-dopa was determined by 19-h infusions of D3-L-dopa alone. Total dopamine production is proportional to dopa dose. Norepinephrine synthesis proceeds at a constant rate that is independent dopa dose or dopamine concentration and which is similar to the endogenous rate reported by others using tracer techniques.