Catecholamine synthesis regulation in hypothalamic synaptosomes.

Abstract

In order to characterize the properties of synaptosomal catecholamine formation in a predominantly noradrenergic preparation, we have studied catecholamine synthesis in rat brain hypothalamic synaptosomes. Kinetic analysis revealed an apparent Km for tyrosine of 2.5 microM and an apparent Vmax of 2.1 nmol/h/g. In the hypothalamus, norepinephrine-induced synthesis inhibition was completely reversed by preincubation with desipramine, a blocker of catecholamine uptake into noradrenergic tissue. In contrast, desipramine was relatively ineffective in the predominantly dopaminergic striatum, indicating that most of the catecholamine synthesis observed in the hypothalamic synaptosomes was taking place in noradrenergic, as opposed to dopaminergic, terminals. Synthesis was stimulated approximately 30% in the hypothalamus by elevated (55 mM) potassium. This stimulation was markedly antagonized in a calcium-free buffer and by the addition of tetraethylammonium chloride. Phenylethylamine compounds could produce either stimulation (amphetamine) or inhibition (tyramine). The most effective synthesis stimulator was dibutyryl cyclic AMP (80% stimulation at 2 mM). Lowering the pH of the incubation buffer from 7.2 to 6.2 increased the basal rate but decreased the stimulatory response to elevated potassium. These data suggest that synaptosomal preparations from the hypothalamus offer a convenient system for studying drug effects on catecholamine synthesis in noradrenergic terminals.