Determinations of catecholamine half-lives and turnover rates in discrete catecholamine nerve terminal systems of the hypothalamus, the preoptic region and the forebrain by quantitative histofluorimetry.
To determine whether amine pool sizes, half-lives and turnover rates could be measured in discrete hypothalamic, preoptic and forebrain catecholamine (CA) nerve terminal networks by quantitative histofluorimetry, the CA fluorescence disappearance was studied at different time intervals after tyrosine hydroxylase inhibition. For comparison, the depletion of DA and NA following tyrosine hydroxylase inhibition in discrete brain regions was determined by high pressure liquid chromatography (HPLC). Following tyrosine hydroxylase inhibition using alpha-methyl-DL-p-tyrosine methyl ester, an apparently monophasic decline of the CA stores was demonstrated in all brain regions analysed both histochemically and by HPLC. A multiphasic DA disappearance was measured by HPLC in the peri- and paraventricular hypothalamic area. The DA nerve terminal networks generally had shorter half-lives than the NA nerve terminal networks. The shortest half-life (99 min) of the regions demonstrating a monophasic decline of CA stores was found in the CA nerve terminal system in the medial palisade zone of the median eminence. By the use of CA standards in the histochemically prepared sections, it was possible to convert the measured CA fluorescence into absolute amounts of catecholamines expressed in nmol X g-1 of tissue wet weight. It was shown that the CA stores and the turnover rates measured by quantitative histofluorimetry were 20-30 times greater than those measured using HPLC. The difference has been related to amine dilution with amine-poor areas in the specimens analysed by HPLC. By studying the accumulation of catecholamines after monoamine oxidase inhibition, it could be demonstrated that no concentration-dependent quenching of CA fluorescence occurred.