Individual neurons in the rat lateral habenular complex project mostly to the dopaminergic ventral tegmental area or to the serotonergic raphe nuclei.

The lateral habenular complex (LHb) is a bilateral epithalamic brain structure involved in the modulation of ascending monoamine systems in response to afferents from limbic regions and basal ganglia. The LHb is implicated in various biological functions, such as reward, sleep-wake cycle, feeding, pain processing, and memory formation. The modulatory role of the LHb is partially assumed by putative spontaneously active LHb neurons projecting to the dopaminergic ventral tegmental area (VTA) and to the serotonergic median (MnR) and dorsal raphe nuclei (DR). All four nuclei form a complex and coordinated network to evoke appropriate responses to reward-related stimuli. At present it is not known whether individual LHb neurons project to only one or to more than one monoaminergic nucleus. To answer this question, we made dual injections of two different retrograde tracers into the rat VTA and either DR or MnR. Tracers were visualized by immunohistochemistry. In coronal sections, the different retrogradly labeled habenular neurons were quantified and assigned to the corresponding habenular subnuclei. Our results show that 1) the distribution of neurons in the LHb projecting to the three monoamine nuclei is similar and exhibits a great overlap, 2) the vast majority of LHb projection neurons target one monoaminergic nucleus only, and 3) very few, heterogeneously distributed LHb neurons project to both dopaminergic and serotonergic nuclei. These results imply that the LHb forms both separate and interconnected circuits with each monoaminergic nucleus, permitting the LHb to modulate its output to different monoamine systems either independently or jointly.