Monoamine Oxidase in the Hypothalamo-hypophysial Region of the Brown Smooth Dogfish, Triakis scyllia

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Distribution of monoamine oxidase (MAO) was histochemically determined in the hypothalamo-hypophysial region of the dogfish, Triakis scyllia. MAO positive neurons, which seemed to be monoaminergic, were observed in the nucleus medius hypothalamicus (NMH) and the nucleus tuberis (NT). NMH was mainly innervated by the MAO-positive fibers arising from the tractus strio-thalamicus et hypothalamicus, and NT was innervated by those from the commissure pre-infundibulaire. The median eminence was innervated by MAO-positive fibers from both NMH and NT, and these were more abundant in the posterior region. These fibers tended to accumulate around the blood capillaries of the primary plexus. The possibility of the involvement of a monoaminergic mechanism in controlling the secretion of adenohypophysial hormones is discussed. In the neurointermediate lobe, MAO-positive fibers were often observed between the cells of this lobe. The cylindrical cells around the sinusoid-like capillaries showed rather moderate MAO activity. The cells in the cellular cord of each lobule showed very weak activity. MAO positive cells were also observed in the pars distalis.