The efferent projections of the rat lateral habenular nucleus revealed by the PHA-L anterograde tracing method.

The efferent connections of the rat lateral habenular nucleus (LHb) were demonstrated using anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA-L). Following PHA-L injections into the LHb, neuronal somata located in the lateral two thirds of the LHb were labeled with PHA-L. Individual axonal fibers and terminal specializations were clearly visible. This permitted detailed mapping of both efferent fiber pathways and terminal distributions. Previous reports on fiber pathways were substantially confirmed and several new findings were revealed. (1) Major rostrally oriented fibers enter the medial forebrain bundle via 3 routes which initially branch from the fasciculus retroflexus: the mediodorsal thalamic nucleus and ventromedial thalamic nucleus; the zona incerta and fields of Forel; and the ventral tegmental area of Tsai. (2) A major decussation to the contralateral thalamic nuclei occurs in the central medial thalamic nucleus. (3) Caudally directed fibers follow two courses: one to the deep mesencephalic nucleus, central grey, dorsal raphe nucleus and the deep layers of the superior colliculus; and the other to the median raphe nucleus, oral pontine reticular formation and raphe pontis nucleus. The present results offer more detailed information concerning the dorsal diencephalic system.