The topographical organization of neurons in the dorsal hypothalamic area that project to the spinal cord or to the nucleus raphé pallidus in the rat.

The present study was undertaken using retrograde labeling techniques to clarify whether the neurons in the dorsal hypothalamic area (DHA) that project to the spinal cord are the same as those that project to the nucleus raphé pallidus (NRP). Following an injection of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) in the NRP many labeled small neurons (6-13 X 9-22 microns) with an oval shape were found in the ventromedial part of the DHA. At the level of the dorsomedial hypothalamic nucleus, they formed a distinct and compact cell cluster. Labeled neurons, which were large in size (9-22 X 11-36 microns) with oval and triangular shapes, were found mainly in the dorsolateral part of the DHA after injections of horseradish peroxidase (HRP) in the spinal cord. In a double-labeling experiment, Fast Blue or True Blue, and Nuclear Yellow were injected in the NRP and in the spinal cord, respectively. A large number of blue-fluorescent neurons were located mostly in the ventromedial part of the DHA, while yellow-fluorescent ones were found in the dorsolateral part of the DHA. However, no double-labeled neurons were found in the DHA. These results show that the neurons of the DHA projecting to the NRP are essentially different from those which project to the spinal cord.