Efferent Axonal Projections of the Habenular Complex in the Fire-Bellied Toad <b><i>Bombina orientalis</i></b>

Abstract

The habenular complex and its associated axonal pathways are often thought of as phylogenetically conserved features of the brain among vertebrates despite the fact that detailed studies of this brain region are limited to a few species. Here, the gross morphology and axonal projection pattern of the habenular complex of an anuran amphibian, the fire-bellied toad Bombina orientalis, was studied to allow comparison with the situation in other vertebrates. Axonal pathways were traced using biocytin applications in dissected brain preparations. The results show that the rostral part of the left dorsal nucleus is enlarged in this species, while the rostral ventral nucleus and caudal parts do not show left-right size differences. Biocytin applications revealed widespread axonal projections of the habenular complex to the posterior tuberculum/dorsal hypothalamic region, ventral tegmentum, interpeduncular nucleus (IPN), and raphe median. Additionally, axons targeting the lateral hypothalamus originated from the ventral habenular nuclei. The results also suggest an asymmetrical pattern of projection to the IPN in the rostral part of the habenular complex, where the left habenula preferentially targeted the dorsal IPN while the right habenula preferentially targeted the ventral IPN. The caudal habenular nuclei showed no asymmetry of projections as both sides targeted the ventral IPN. Comparison of the habenular complex axonal connectivity across vertebrates argues against strong phylogenetic conservation of the axonal projection patterns of different habenular nuclei.

Regional chemoarchitecture of the brain of lungfishes based on calbindin D‐28K and calretinin immunohistochemistry

“…In contrast, in birds and mammals habenular asymmetry was not described as regards the distribution of these proteins (Braun, ; Celio, ; Résibois & Rogers, ). Moreover, in amphibians habenular asymmetry has been observed in urodeles, but not all species of anurans share this feature (Guglielmotti, Cristino, Sada, & Bentivoglio, ; Morona & González, ), although they might indeed possess asymmetry in their connectivity (Laberge & Smith, ). It is noteworthy that as in basal actinopterygian fishes (Graña et al, ; López, Perlado, Morona, Northcutt, & González, ; Lozano, Morona, & González, 2016), CB is found in cholinergic cells in the dorsal habenula of lungfishes (López et al, ) and both markers stain the fasciculus retroflexus, which marks the boundary between p2 and p1.…”

The habenula as an evolutionary conserved link between basal ganglia, limbic, and sensory systems—A phylogenetic comparison based on anuran amphibians

“…It was therefore argued that the context, in which the habenula circuitry is recruited might have changed (Stephenson‐Jones, Floros, et al, ). Habenular efferents terminate predominantly in diencephalic, mesencephalic, and rhombencephalic areas, with an additional forebrain projection in tetrapods [lampreys: Yáñez & Anadón, , Stephenson‐Jones, Floros, et al, ; chondrichthyes: Giuliani et al, ; teleosts: Villani et al, , Yáñez & Anadón, , Amo et al, ; amphibians: Kemali et al, , Laberge & Smith, , present study; reptiles: Distel & Ebbesson, , Díaz & Puelles, ; mammals: Cragg, , Akagi & Powell, , Smaha & Kaelber, , Herkenham & Nauta, , Araki, McGeer, & Kimura, ]. In the course of vertebrate evolution, the primordial habenula circuitry seemingly underwent various evolutionary adaptations and the complexity of the afferent and efferent connectivity increased in the tetrapod line.…”

“…In the following, arguments are provided which support a homology of the anuran DHb and VHb to mammalian MHb and LHb, respectively. This contradicts the assessment of Laberge and Smith (), who suggested that the conservation of the habenula circuitry among vertebrates is overestimated. Figure displays the habenula circuitry in key positions during evolution.…”

“…However, the extent of the phylogenetic conservation of the habenula circuitry among vertebrates is strongly debated (cf. Amo et al, ; Kemali, Guglielmotti, & Gioffre, ; Laberge & Smith, ; Stephenson‐Jones, Floros, et al, ). The primary function of the habenula is hypothesized to suppress motor activity under adverse conditions (Hikosaka, ).…”

“…In the present study, an emphasis was placed on the VHb, because its connectivity is supposedly the least preserved among vertebrates (cf. Amo et al, ; Laberge & Smith, ). Consequently, here, the hypothesis was tested that the habenula of amphibians has network structures that can be regarded to reflect the ancestral state of the tetrapod habenula.…”