Habenula and the asymmetric development of the vertebrate brain
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REVIEW ARTICLE
Habenula and the asymmetric development of the vertebrate brain Hidenori Aizawa
Received: 4 September 2012 / Accepted: 4 October 2012 / Published online: 20 October 2012 Ó Japanese Association of Anatomists 2012
Abstract Habenula is a relay nucleus connecting the forebrain with the brain stem and plays a pivotal role in cognitive behaviors by regulating serotonergic and dopaminergic activities. The mammalian habenula is divided into the medial and lateral habenulae, each of which consists of a heterogeneous population of neurons. Recent comparative analyses of zebrafish and rodent habenulae have provided molecular insights into the developmental mechanism of the habenula. Hodological and gene expression analyses revealed that these two habenular pathways are conserved phylogenetically between fish and mammals. The anatomical information make the zebrafish and rodent model animals amenable to the genetic analysis of the development and physiological role of the vertebrate habenula. Intriguingly, habenula has also attracted interest as a model for brain asymmetry, since many vertebrates show left–right differences in habenular size and neural circuitry. Left–right asymmetry is a common feature of the central nervous system in vertebrates. Despite its prevalence and functional importance, few studies have addressed the molecular mechanism for generation of the asymmetric brain structure, probably due to the absence of genetically accessible model animals showing obvious asymmetry. The results from recent studies on zebrafish habenula suggest that development of habenular asymmetry is mediated by differential regulation of the neurogenetic period for generating specific neuronal subtypes. Since the orientation and size ratio of the medial and lateral habenulae differs across species, evolution of those
H. Aizawa (&) Department of Molecular Neuroscience, Medical Research Institute, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan e-mail: [email protected]
subregions within the habenula may also reflect changes in neurogenesis duration for each habenular subdivision according to the evolutionary process. Keywords Asymmetry Habenula Lateralization Monoamines Zebrafish
Habenula as a center for regulation of the monoaminergic activities in the central nervous system Habenula is a nucleus in the epithalamus connecting the basal forebrain with the midbrain nuclei that contains abundant serotonergic and dopaminergic neurons (Fig. 1a). This nucleus was previously implicated in regulation of the activity of these monoaminergic neurons, since destruction of the habenula led to increased metabolism of serotonin and dopamine in the central nervous system (Nishikawa et al. 1986; Nishikawa and Scatton 1985), and electrical stimulation resulted in a decreased firing rate in serotonergic and dopaminergic neurons (Wang and Aghajanian 1977; Christoph et al. 1986). Neurotracing studies in the late 1970s revealed that the habenula consists of the medial and lateral hab
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