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. 2022 Feb 10;15:818782. doi: 10.3389/fnbeh.2021.818782

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Copyright © 2022 Ogawa and Parhar.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Schematics showing the habenular pathways in mammals and teleosts. (A) The habenula consists of two major subnuclei, medial habenula (MHb) and lateral habenula (LHb) in mammals. LHb can be further divided into medial (LHbM) and lateral (LHbL) parts. MHb primarily projects to serotonergic center: the dorsal and median raphe nuclei (DRN and MRN), and dopaminergic center: substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) via the interpeduncular nucleus (IPN). The MHb also sends projections to other brain regions such as the periaqueductal gray (PAG), nucleus incertus (NI), and locus coeruleus (LC). LHb projects serotonergic raphe nuclei and midbrain dopaminergic center. However, LHbL act on serotonergic and dopaminergic centers via the rostromedial tegmental nucleus (RMTg, which is also known as tail of the VTA). (B) In teleosts, the habenula structure consists of two subnuclei: dorsal habenula (dHb) and ventral habenula (vHb), which are structurally and biochemically homologous to mammalian MHb and LHb, respectively. dHb and their pathways are structurally asymmetric, hence they can be considered subnuclei: lateral (left) and medial (right) dHb (dHbL and dHbM). dHbL and dHbM project to the different regions of IPNs: dorsal (dIPN) and ventral (vIPN) part of the IPN, respectively. dIPN further innervates to the griseum centrale (GC, a homolog of the mammalian PAG), while vIPN is connected to the MRN. vHb sends projection to the ventro-anterior corner of the MRN, which may further extend toward the serotonergic DRN. Modified from Ogawa et al. (2021).