Uptake of monoamines into central neurones and the blood‐brain barrier in the infant rat

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Abstract
1 . A fluorescence histochemical study was undertaken to investigate the ability of systemically administered monoamines to penetrate into the central nervous system of infant rats. 2 . It was found that after subcutaneous injections of large doses, l-noradrenaline, l-α-methyl-noradrenaline, dl-α-methyl-dopamine, l-α-methyl-dopa and 5-hydroxytryptamine could cross the blood-brain barrier of newborn to 2 week old rats and be taken up by neurones normally containing monoamine as well as by several neurones which normally contained no monoamine. 3 . 5-Hydroxytryptamine containing neurones took up 5-hydroxytryptamine as well as catecholamines and α-methyl-dopa. The pinealocytes, which contain large amounts of 5-hydroxytryptamine, were depleted of their endogenous fluorescence by α-methyl-dopa and showed no selective uptake of catecholamines. 4 . The uptake of monoamines was not antagonized by previous reserpine treatment or by inhibition of catecholamine synthesis, and occurred throughout the whole extent of the monoamine-containing neurones (cell body, axon, terminals). 5 . The monoamines were also taken up by the endothelial cells and pericytes of capillaries in the central nervous system (CNS) which are thought to constitute the physical barrier to these substances in the adult. 6 . The peripheral sympathetic nerves to pial blood vessels, the pineal and pituitary glands also took up the monoamines and α-methyl-dopa; uptake was also noted in cells of the pituitary gland, in epithelial and connective tissue elements of the meninges and choroid plexuses and in the epithelial, connective and muscular tissue of pial blood vessels. 7 . It is concluded that the blood-brain barrier to monoamines may not be fully developed in infant rats, at least for high levels of circulating monoamines; the central monoamine-containing neurones possess the ‘membrane pump’ mechanism for uptake of monoamines from the time of birth, even though they are not fully developed morphologically.