Citation

Background: The habenula is part of the brain reward system and modulates many biological behaviours. It is connected with the hypothalamus and several brain stem nuclei and serves as a relay between reward system and homoeostatic systems. Ten subnuclei can be distinguished in the lateral habenular complex (LHb) by light microscopical criteria. Methods: Neuronal tract tracing studies aimed to clearify the distribution of habenular efferents to the dorsal raphe nucleus (DR) considering their subnuclear organisation. These data were compaired with results from studies of diencephalic afferents to the LHb. Results: Axons from the lateral (LHbL) and the medial (LHbM) part of the LHb terminate in different areas of the DR. These areas differ in neuronal morphology and content of neuromarkers. Not all subnuclei of LHb similary contribute to the efferents to the DR. Most projection neurons reside in the central, the parvocellular and the anterior subnucleus of the LHbM. Their axons terminate in individual areas of the DR. Many projections originate in the marginal subnucleus of the LHbM, less in the superior or basal subnucleus and very few in the oval subnucleus. The parvocellular and the marginal subnuleus oft he LHbL only send scarce projections to the DR. Conclusions: The results of this study as well as those of the hypothalamic afferents prove that the projections of the LHb show a subnuclear pattern. The subnuclei of the LHb possess different sensitivities to dopaminergic stimulation as demonstrated in recent studies. Hence a functional importance of those subnuclei is likely. In summary the combined analysis of the present hodological data considering the subnuclear organisation and specific distribution of neurotransmitters and neuromodulators may provide a more specific view on neuronal loops, their possible functions and interactions. Such data will help in the development of novel drugs for anxiolysis or treatment of depression and dependence.