We are focusing on the habenula, a nucleus located in the dorsal diencephalon controlling monoaminergic neurons. In mammals, the habenula is divided into two nuclei: medial nucleus and lateral nucleus (Figure 5). The medial nucleus receives the external environmental information and associated emotional information from the hippocampus and amygdala via the septum and stria terminalis, respectively (Figure 5). Left and right medial habenula send their projection to the interpeduncular nucleus (IPN) to control indirectly monoaminergic neurons such as dopaminergic neurons in the ventral tegmental area (VTA) or serotonergic neurons in Raphé. On the contrary, the lateral habenula receives information about behavioral programs from the cortico-basal ganglia circuit via ventral pallidum (Figure 5). The lateral habenula directly projects to the VTA or Raphé to control monoaminergic neurons.
From the characteristics of this neural circuit including habenula, we have a working hypothesis that the habenula may calculate the difference between the external information with some value information and the internal information of behavioral programs with emotional motivation so that animals can adapt their behavior upon the environmental change. In deed, several studies including that from Hikosaka laboratory (Matsumoto and Hikosaka, 2007, Nature 447) have reported that neurons in the lateral habenula increase their activity to suppress dopaminergic neurons when the expected reward was omitted. Nevertheless, functions of the medial habenula and the role of the interaction between the medial and lateral habenula remain unknown.
We are using zebrafish, mice and rats to unveil the role of the habenula in emotional behaviors.