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A histochemical investigation into the regional distribution of monoamine oxidase in the brain-stem of rabbit and cat with atlas of related concentrations Halsey, Nancy M.

Abstract

In the last decade, considerable interest has been focused on the role of biogenic amines and their function in the central nervous system. Certain of these, norepinephrine and serotonin, have been suggested as neurotransmitters, and evidence has accumulated that rise in the levels of these amines results in behavioural change. At the same time, it was found that the enzyme, monoamine oxidase (MAO), utilized these compounds as substrates, and that inhibition of MAO resulted in elevated levels of the catecholamines and serotonin. This knowledge has led to considerable investigation of the areas of brain that might be affected by this inhibition, but beyond the preliminary report of Shimizu et al. (1959) little had been done to determine the histochemical localization of MAO by methods of proven specificity. A study of the brain-stem has therefore been attempted, to determine the major sites of MAO activity. Rabbit and cat brain-stem have been used, and the histochemical method of Glenner, Burtner and Brown (1959) using nitro-blue tetrazolium. Fresh frozen tissue was cut on the cryostat and sections incubated with this solution, a positive result producing a purple-blue formazan precipitate. For identification and correlation of brain-stem nuclei, adjacent sections were cut and stained with toluidine blue. Controls were run in vitro and in vivo with known MAO inhibitors, as well as by incubation without substrate, application of heat and alteration of pH. Finally, an atlas has been prepared identifying the sites of MAO activity and suggesting a functional relationship based on these studies. Results indicate that within the brain, the brain-stem itself contains the highest proportion of MAO, which is concentrated within the following regions - choroid plexus, pineal gland, hypothalamus, pituitary, interpeduncular nucleus, habenulo-pedencular tract, dorsal tegmental nucleus, locus coeruleus, area postrema and cranial nerve nuclei, especially the distal portion of the trigeminal nucleus and the dorsal vagal nuclei. The thalamus, inferior colliculi and major fibre tracts were all conspicuously low in MAO. With the exception of the cells of the mesencephalic nucleus of nerve V, activity did not occur within the body of the neuron, but was present in the neuropil of the neocortex and all other positive areas of brain-stem. Certain peculiarities of distribution were noted for the glandular areas of the pineal, pituitary and choroid plexus. In the anterior pituitary and choroid plexus, MAO was found in intracellular granules, but within the pineal and posterior pituitary the activity appeared to lie in a matrix between cells.

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