Afferent and efferent connections of the habenula in the larval sea lamprey (Petromyzon marinus L.): an experimental study.

The habenula is an integrative center between the striatum and the limbic and motor systems. With the aim of achieving further understanding of the evolution of this structure in vertebrates, we carried out an experimental study of the afferent and efferent connections of the habenula of larval sea lamprey. Experimental procedures included in vivo and in vitro transport after injections of horseradish peroxidase (HRP) into the habenula, telencephalon, pineal organ, dorsal thalamus, and posterior tubercle as well as carbocyanine dye tracing (DiI). The combined results of these experiments showed that the pattern of habenular connections is very simple. Most afferents appear to originate from the lobus subhippocampalis and neighboring area, whereas the only efferents found coursed in the fasciculus retroflexus to the neuropil of the nucleus interpeduncularis. This neuropil comprises a commissural region in the rostral mesencephalon, two long bilateral areas extending in the basal mesencephalon and medulla oblongata to the trigeminal level, and, finally, a caudal commissural zone. The conspicuous habenular commissure contains interhemispheric fibers that appear to form occasional contacts within the habenulae. The lamprey habenula also receives a few immunocytochemically identified fibers (somatostatinergic, catecholaminergic, and serotoninergic fibers) from other sources.