Magnocellular tuberomammillary nucleus input to the supraoptic nucleus in the rat: anatomical and in vitro electrophysiological investigations.

Anatomical and electrophysiological methods were used to investigate the existence and role of inputs from the magnocellular tuberomammillary nucleus to the supraoptic nucleus. After injecting either Fluoro-Gold or rhodamine-labeled latex microspheres into the supraoptic nucleus, consistent patterns of retrogradely labeled neurons within the tuberomammillary nucleus were observed. The results indicate that both subdivisions of the supraoptic nucleus, the tuberal and the anterior, receive input from the tuberomammillary nucleus. Injections into the tuberal supraoptic nucleus tended to label more cells in the contralateral tuberomammillary nucleus, while injections into the anterior supraoptic nucleus may label more cells on the ipsilateral side. The in vitro intracellular electrophysiological results support the anatomical findings and extend them in several ways. Some tuberomammillary neurons were found to project to the supraoptic nuclei on both sides of the brain. Intracellular Lucifer Yellow injections into tuberomammillary cells after electrophysiological recording revealed labeled axons that were traceable into the supraoptic nucleus, where apparent varicosities (possible en passant terminals) were seen. Magnocellular tuberomammillary nucleus neurons had characteristic passive and active membrane properties and morphology, similar to histaminergic neurons in this area studied by other workers. Finally, in two of the 21 cases, Lucifer Yellow injection into one neuron revealed dye-coupled pairs of tuberomammillary neurons. Previous work by others has shown that histamine excited cells in the tuberal subdivision of the supraoptic nucleus, stimulating vasopressin release, and that the tuberomammillary nucleus provides histaminergic input to the anterior portion of the supraoptic. The present findings show that the tuberomammillary nucleus supplies input to both subdivisions of the supraoptic nucleus and that this input is provided bilaterally. Taken together with previous work, these data suggest that the tuberomammillary nucleus provides histaminergic input to the supraoptic nucleus and may be involved specifically with vasopressin release.