Type B monoamine-oxidase-containing cells and fibers in the cat hypothalamus demonstrated by an improved enzyme histochemical method

The present study, using a diaminobenzidine (DAB)-coupled peroxidation method, examined the distribution and morphological characteristics of neuronal structures containing type B monoamine oxidase (MAO-B) in the cat hypothalamus. Large and intensely stained, distinctive MAO-B-positive cells, multipolar and with long dendritic arbors, were principally distributed in the ventral hypothalamus extending from A7 to A12.5 of the Horsley-Clarke plane. These cells were located caudally in the ventral surface of the brain including the tuberomamillary nucleus (TM) and the region surrounding the mamillary nuclei. Rostrally, they were aggregated in the area surrounding the fornix, particularly in the lateral perifornical region, and dispersed in the anterior mamillary nucleus, lateral hypothalamic area (HLA), and the medial tip of the entopeduncular nucleus. The most rostral positive cell group was identified in a narrow space between the optic tract and the entopeduncular nucleus at the A12.5 level. In addition to these large cells, the present study disclosed the presence of "small" to "very small" MAO-B-positive cells in the area surrounding the mamillary recess and the lateral part of the caudal arcuate nucleus. Distinct MAO-B-stained fibers were identified in all regions of the hypothalamus. A large number of thick labeled fibers were observed in the ventral hypothalamus including the TM and premamillary nucleus and posterior and lateral hypothalamic areas. A dense network of MAO-B-positive terminal-like fibers was observed in the dorsomedial nucleus where very small labeled cells were scattered. Many intensely stained thick and straight fibers were seen running ventrolaterally in the anterior part of the HLA and in the narrow space between the entopeduncular nucleus and optic tract. In the area of the tuber cinereum and the ventral part of the HLA, there were many positive fibers cut transversely, possibly projecting to the more anterior parts of the brain such as the diagonal band of Broca or septal nuclei.