Effects of 6-hydroxydopamine on the uptake and storage of noradrenaline in sympathetic adrenergic neurons

Abstract

The time-course of 6-hydroxydopamine induced disappearance of endogenous noradrenaline in mouse heart and iris, its reappearance and changes in uptake-storage properties of the adrenergic nerves for noradrenaline have been investigated using biochemical, isotope and histochemical techniques. There was a very rapid depletion of endogenous noradrenaline. Thus 1 hr after the injection of 6-hydroxydopamine (20 mg/kg i.v.) only 20% of the endogenous noradrenaline was left in mouse heart; after 8 hr, only 6%. Subcellular distribution studies showed that soon after injection 6-hydroxydopamine caused a change in the intraneuronal distribution of previously taken up 3H-noradrenaline and that there was a preferential release from the microsomal fraction, which contains the amine storage granules. The ability of the adrenergic nerves to take up and accumulate exogenously administered noradrenaline was also lost after 1 hr, probably due to severe damage of the uptake-storage mechanisms. Previous electronmicroscopic investigations have revealed that 6-hydroxydopamine causes a selective destruction of the adrenergic nerve terminals. This effect seems to be dependent on a critical concentration of 6-hydroxydopamine acting upon the adrenergic nerves. There was a gradual restoration of the adrenergic ground-plexus and noradrenaline content with parallel recovery of the uptake capacity for noradrenaline, which for heart was complete after about 8 wk. This was probably due to regeneration of the adrenergic nerves. A low dose of 6-hydroxydopamine (1 mg/kg i.v.) resulted in a small temporary drop of the noradrenaline level and also of the noradrenaline uptake capacity. Almost complete recovery of the transmitter content was observed in a few days, whereas the noradrenaline uptake returned to normal values after a few hours. Studies with 3H-6-hydroxydopamine showed that the adrenergic nerves take up this amine, since the uptake was considerably reduced by sympathetic denervation. Desmethylimipramine of 0°C, which efficiently block the ‘membrane pump’ of the adrenergic nerves, also strongly inhibited the uptake. The ability of the microsomal fraction to take up 3H-6-hydroxydopamine was markedly blocked by reserpine, but a granular uptake, at least via the Mg 2+-ATP dependent mechanism, did not seem to be a prerequisite for the degeneration effects.