Compartmentation of catecholamines in rat brain: Effects of agonists and antagonists

Abstract

The subsynaptosomal distributions of dopamine (DA) in striatum and of norepinephrine (NE) in hypothalamus and cerebral cortex were examined. Isolated nerve-endings from each region were osmotically disrupted and subfractionated into a soluble cytoplasmic fraction (end supernatant, Se) and a synaptic vesicle fraction (P2V). DA and NE were measured in the crude homogenate and in subcellular fractions by a radioenzymatic assay. Levels of NE and DA were 3--5 times higher in the nerve-ending cytoplasm than in the synaptic vesicles, suggesting that catecholamines within the nerve-endings are predominantly in soluble form. Amphetamine increased DA levels in the tissue homogenate and in the nerve-ending cytoplasm but not in synaptic vesicles. Pargyline and gamma-butyrolactone (GBL) increased DA levels in all fractions with the greatest increase occurring in the cytoplasmic fraction. Both 6-hydroxydopamine (6-OHDA) and alpha-methyltyrosine (AMT) caused uniform DA decreases in all fractions. Hypothalamic levels of NE in the two nerve-ending compartments were also reduced to a similar extent after AMT. Reserpine produced uniform depletions of striatal DA in both nerve-ending fractions while the rate of DA repletion was more rapid in the vesicular compartment. Levels of hypothalamic NE were also uniformly depleted by reserpine at the times examined. The cytoplasmic storage compartment is discussed in terms of a possible anatomical correlate such as the smooth endoplasmic reticulum.