The morphological organization of monoaminergic cells and fibers in the hypothalamus of the lizards Lacerta sicula and Lacerta muralis was investigated by fluorescence histochemistry. An extensive monoaminergic system emanates from the nucleus organi paraventricularis (NOP), a circumventricular organ of the medial and posterior hypothalamus containing numerous monoaminergic perikarya. Fluorescent processes extending from these cells end as intraventricular thickenings. An extensive fiber system presumably arising from the NOP innervates the accompanying nucleus ventromedialis hypothalami (NVH) as well as the nucleus periventricularis hypothalami (NPH) and the median eminence. A monoaminergic fiber path of extrahypothalamic origin enters the preoptic/anterior hypothalamus, terminating in the nucleus paraventricularis (NP). A discrete pathway of catecholaminergic fibers courses through the hypothalamus along the ventral border of the optic tract. Levels of fluorescence intensity are highest in the spring and in castrated animals and lowest in lizards during testicular regression.
