METABOLISM OF EXOGENOUS NORADRENALINE IN SLICES OF HYPOTHALAMUS AND CAUDATE NUCLEUS OF THE BRUSH‐TAIL POSSUM, TRICHOSURUS VULPECULA

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Abstract
In the possum, Trichosurus vulpecula, the major catecholamines in the hypothalamus and caudate nucleus are noradrenaline and dopamine, respectively, suggesting that catecholaminergic transmission in these regions is similar to that in eutherian mammalian brain. Comparisons of accumulation and metabolism of 3H noradrenaline by tissue slices of the two regions indicated that in the caudate nucleus there was a seven-fold greater accumulation of unchanged noradrenaline but a lower rate of deamination to the principal metabolite, dihydroxyphenylethylene glycol (DOPEG). Cocaine inhibited DOPEG formation in the hypothalamus but nut in the caudate nucleus. To explain these results, it is suggested that in the hypothalamus noradrenaline is taken up by noradrenergic nerve terminals where it undergoes extensive deamination. However, in the caudate nucleus it is taken up by, but not deaminated within, the dopaminergic nerve terminals. In the hypothalamus the presence of a corticosteroid-sensitive O-methylating system extraneuronal to noradrenergic neurones was indicated by the inhibitory effect of hydrocortisone on the O-methylation of isoprenaline. The results do not point to any major differences between the metabolism of noradrenaline in metatherian (marsupial) and eutherian brain.

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