We studied the detailed distributions and morphology of structures immunoreactive to type B monoamine oxidase, and compared them with those stained by monoamine oxidase enzyme histo-chemistry in the brain of cats treated with or without colchicine. By means of the indirect immunohisto-chemical method in conjunction with type B monoamine oxidase monoclonal antibody, we demonstrated type B monoamine oxidase immunoreactivity in neuronal cell bodies, fibers and astroglial cells in the cat brain. As expected, the distribution of type B monoamine oxidase-immunoreactive cell bodies overlapped that of serotonin-containing ones in the lower brainstem and midbrain, as well as that of histaminergic ones in the posterior hypothalamus. We found novel cell groups containing type B monoamine oxidase in the areas described below. Intense type B monoamine oxidase-immunopositive and enzymatically active neurons, corresponding to liquor-contact ones, were discovered in the wall of the central canal of the spinomedullary junction. Weak immunoreactivity was identified in neurons of the dorsal motor nucleus of the vagus, parvocellular reticular formation and locus coeruleus complex, which have been reported to contain type A monoamine oxidase enzymatic activity. Type B monoamine oxidase-immunostaining in these structures was enhanced by treatment with colchicine. In addition, lightly immunostained cells were distinguished in the caudal portion of the hypothalamic arcuate nucleus, area of tuber cinereum, retrochiasmatic area, and rostral portion of the paraventricular thalamic nucleus after colchicine treatment. These cells also displayed monoamine oxidase activity; however, it was difficult to enzymatically characterize their nature due to its weak activity and sensitivity to inhibitors of both A and B. Distinct type B monoamine oxidase-immunoreactive fibers and terminal-like dots were abundant in the whole brain, particularly in the central gray, dorsal pontine tegmentum, interpeduncular and pontine nuclei, nucleus of the solitary tract and dorsal motor nucleus of vagus, where dense innervations of serotonergic fibers have been reported. Their immunoreactive density increased after colchicine treatment, but monoamine oxidase enzymatic reaction did not. An intense immunoreactivity could be seen in many glial cells in all parts of the brain including myelinated axon pathways. The densest accumulation of such labeled glial cells was found in the central gray, inferior olive, medial geniculate body, substantia nigra, ventral tegmental area of Tsai, retrorubral area, hypothalamus, thalamus and bed nucleus of the stria terminalis. In contrast, the striatum contained less numerous type B monoamine oxidase-immunoreactive and enzymatically active astroglial cells in comparison with the other structures.

Comments are made on (1) comparison between the two methods used, (2) variations in intensity of type B monoamine oxidase immunoreaction between cells, (3) existence of type B monoamine oxidase immunoreactivity in type A monoamine oxidase-containing cells and (4) oxidation of trace amines such as tryptamine and tyramine as well as a dopaminergic neurotoxin l-methyl-4-phenyl-l,2,3,6-tetrahydro-pyridine.