The hypothalamus receives major projections from the tuberomammillary nucleus in rat

Projections from the tuberomammillary nucleus (TM) to widespread regions within the hypothalamus were evaluated using a combined immunohistochemical-retrograde fluorescent tracing procedure. Injections of Fluoro-gold into the anterior hypothalamus labelled TM neurons identified by their immunoreactivity for the enzyme adenosine deaminase (ADA). Small injections of Fluoro-gold into the posterior hypothalamus also led to the labelling of TM neurons. The numbers of ADA-immunoreactive axonal varicosities were 5 times greater in the hypothalamus than in most other brain regions. The results indicate that the hypothalamus represents a major projection area of the TM.