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 lim bic 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 hab enula 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 com bined results of these experiments showed that the pattern of habenula r connections is very simple. Most afferents appear to originate from the lobus subhippocampalis and neighboring area, whereas the only effe rents found coursed in the fasciculus retroflexus to the neuropil of t he nucleus interpeduncularis. This neuropil comprises a commissural re gion in the rostral mesencephalon, two long bilateral areas extending in the basal mesencephalon and medulla oblongata to the trigeminal lev el, and, finally, a caudal commissural zone. The conspicuous habenular commissure contains interhemispheric fibers that appear to form occas ional contacts within the habenulae. The lamprey habenula also receive s a few immunocytochemically identified fibers (somatostatinergic, cat echolaminergic, and serotoninergic fibers) from other sources. (C) 199 4 Wiley-Liss, Inc.