[Role of the lateral habenula and dopamine neurons in reward processing]

Dopamine neurons in the substantia nigra pars compacta and ventral tegmental area have been implicated in reward-related functions. These neurons are excited by reward and sensory stimuli that predict reward and inhibited by reward omission. These excitatory and inhibitory responses are thought to guide reward-seeking behaviors. Indeed, reward-seeking behaviors can be impaired by injecting dopamine antagonists into the brain areas targeted by dopamine neurons (e.g., caudate nucleus). However, it was unknown which parts of the brain provide dopamine neurons with reward-related signals necessary to induce their responses. Recent studies showed evidence that the lateral habenula, a part of the structure called the epithalamus, is a good candidate for a source of reward-related signals in dopamine neurons. The lateral habenula projects to midbrain structures such as the substantia nigra pars compacta and ventral tegmental area which contain dopamine neurons. Electrical stimulation of the lateral habenula inhibits the activity of dopamine neurons. Neurons in the lateral habenula also encode reward-related signals but in a manner opposite to that obeerved for dopamine neurons (i.e., lateral habenula neurons are inhibited by reward and sensory stimuli predicting reward and excited by reward omission). These anatomical and physiological findings suggest that the lateral habenula transmits reward-related signals to dopamine neurons by inhibiting them. Thereby, the lateral habenula could contribute to reward-seeking behaviors through its projections to the dopaminergic systems.