Two series of experiments were done in the rat to investigate whether
neurons in arcuate nucleus of the hypothalamus (Are) containing neurot
ensin (NT)-like immunoreactivity projected to subfornical organ (SFO).
In the first series, the anterograde tract-tracer Phaseolus vulgaris
leucoagglutinin (PHA-L) was microiontophoresed into the region of Are
that contains NT neurons. After a 9-12 day survival period the animals
were sacrificed and forebrain sections that contained SFO were proces
sed for combined PHA-L and NT immunoreactivity. In the second series o
f experiments, unilateral or bilateral electrolytic lesions of Are wer
e made and after a 10-17 day survival period SFO was examined to deter
mine the relative contribution of NT Are neurons to NT immunoreactivit
y within SFO. PHA-L labelled fibers with terminal-like boutons were fo
und in SFO primarily on the side ipsilateral to the site of injection.
A small number of the PHA-L labelled fibers in the lateral aspect of
SFO was also immunoreactive to NT. Unilateral lesions reduced, whereas
bilateral lesions of Are eliminated most of the NT-like: immunoreacti
vity within SFO. These data demonstrate the existence of a direct path
way from Are to SFO that contains the putative neurotransmitter NT. Th
ese results suggest that this pathway may function in the modulation o
f neural and/or humoral events related to cardiovascular regulation an
d body fluid homeostasis by influencing the activity of SFO neurons.