Neurochemical Properties of Adrenergic Nerves Regenerated After 6‐hydroxydopamine

The uptake‐storage properties and synthesis of noradrenaline, and fluorescence morphology of adrenergic nerves which have been allowed to regenerate for 4 weeks after a chemical sympathectomy produced by 6‐hydroxydopamine have been investigated in mouse iris and atrium. The regenerated nerve terminals displayed a lower formaldehyde‐induced fluorescence intensity whereas the non‐terminal axons exhibited a stronger fluorescence intensity and a more beaded appearance compared with mature nerves. The endogenous noradrenaline concentration after 6‐hydroxydopamine was 30% in iris and 45% in atrium compared to control values. Recovery of [3H]noradrenaline uptake was found to be more rapid than that of endogenous noradrenaline concentration after the 6‐hydroxydopamine treatment. [3H]Noradrenaline uptake in regenerating and adult mature nerves both obeyed Michaelis‐Menten kinetics having identical Km values. There was a close correlation between [3H]noradrenaline uptake and nerve density of adrenergic nerves regenerated after 6‐hydroxydopamine. These results show that [3H]noradrenaline uptake is a better index for the number of regenerated nerve terminals than is the endogenous noradrenaline concentration. The retention of [3H]noradrenaline taken up and accumulated in vitro was about the same in regenerated and mature nerves, although a slight tendency to less effective retention was observed in the regenerated nerves. Subcellular distribution studies showed that relatively less [3H]noradrenaline was recovered in the microsomal fraction after 6‐hydroxydopamine treatment. The formation of 14C‐labelled catecholamines from [14C]DOPA was higher in regenerating nerves than indicated by the endogenous noradrenaline concentration but lower than that indicated by the [3H]noradrenaline. It is concluded that the regenerating nerves contain less endogenous noradrenaline than adult mature nerves and that the uptake mechanism develops promptly, whereas the development of the storage mechanism lags behind.