Glutamatergic axons from the lateral habenula mainly terminate on GABAergic neurons of the ventral midbrain

The concept of cortical-subcortical loops emphasizes the importance of the basal ganglia for motor, psychomotor, and emotional cortical functions. These loops are bidirectionally controlled by the midbrain dopaminergic system, predominantly but not exclusively at the level of the striatum including the accumbens nucleus. Successful behaviors increase the activities of the mesostriatal (arising in the complex part of the substantia nigra) and mesolimbic (arising in the ventral tegmental area, VTA) neurons, thereby reinforcing the corresponding actions. In contrast, unsuccessful behaviors result in an increased activation of the lateral habenular complex (LHb), thereby decreasing the activities of mesolimbic neurons. Correspondingly, electrical stimulation of the LHb effectively blocks neuronal activity in the VTA. Whether this block is due to an inhibitory projection from the LHb to the VTA, or whether axons from excitatory LHb neurons target inhibitory neurons within the VTA, is presently not known. Here we show, using in situ hybridization and immunocytochemical double labeling at the light and electron microscopic level, that GABAergic neurons are scarce in the LHb and that glutamatergic axons from the LHb mostly target GABAergic neurons in the VTA and the mesopontine rostromedial tegmental nucleus (RMTg), also known as tail of the VTA (tVTA). These data explain the inhibitory effect of LHb activation on the VTA. In addition, however, a small number of LHb terminals in the VTA actually contacts dopaminergic neurons. The biological importance of these terminals requires further investigation.