Subnuclear organization of the rat habenular complexes

The habenular complexes represent phylogenetically constant structures in t he diencephalon of all vertebrates. Available evidence suggests that this a rea is engaged in a variety of important biological functions, such as repr oductive behaviors, central pain processing nutrition, sleep-wake cycles, s tress responses, and learning. Based on Nissl-stained sections, one medial nucleus and two lateral nuclei (divisions) have been widely accepted in the rat. Cytochemical, hodologic, and functional studies suggest a considerabl y more complex subnuclear structure. To improve our knowledge of the precis e structural composition of the habenular complexes, we have systematically investigated their fine ultrastructure in the rat. Based on the detailed a nalysis of complete series of large, semithin sections supplemented with el ectron photomicrographs of selected fields, clear criteria for the delineat ion of five distinct subnuclei of the medial and ten subnuclei of the later al habenular complexes were elaborated for the first time. All 15 subnuclei were reconstructed, and their dimensions were determined. A medial and lat eral stria medullaris were described. Different roots of the fasciculus ret roflexus were differentiated within the medial and lateral habenular comple xes. The topographical relationships with respect to the adjacent habenular areas as well as to the neighboring thalamic nuclei were identified and de monstrated. The new understanding of the subnuclear organization of the hab enular complexes certainly will facilitate further functional investigation s. Whether the newly identified subnuclei finally will be recognized as fun ctionally distinct awaits ongoing immunocytochemical, hodologic, and functi onal studies. (C) 1999 Wiley-Liss, Inc.