Refinement of the dopaminergic system of anuran amphibians based on connectivity with habenula, basal ganglia, limbic system, pallium, and spinal cord

Abstract

Whereas our understanding of the dopaminergic system in mammals allows for a distinction between ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), no clear evidence for separate structures in anamniotes has been presented to date. To broaden the insight into the organization and regulation of neuromodulatory systems in anuran amphibians, tracing and immunohistochemical investigations were performed in the Oriental fire-bellied toad, Bombina orientalis. Topographically organized catecholaminergic "nigrostriatal," "mesolimbic," "mesocortical," and spinal cord projections arising from the posterior tubercle and mesencephalic tegmentum were identified. We compared these results with published data from lampreys, chondrichthyes, teleosts, amphibians, reptiles, birds, and mammals. Based on the pattern of organization, as well as the differential innervation by the habenular nuclei, domains gradually comparable to the mammalian paranigral VTA, ventral tier of the SNc, interfascicular nucleus of the VTA, and supramamillary/retromamillary area were identified. Additionally, we could demonstrate topographic separate populations of habenula neurons projecting via a direct excitatory or indirect GABAergic pathway onto the catecholaminergic VTA/SNc homologs and serotonergic raphe nuclei. The indirect GABAergic habenula pathway derives from neurons in the superficial mamillary area, which in terms of its connectivity and chemoarchitecture resembles the mammalian rostromedial tegmental nucleus. These results demonstrate a much more elaborate interconnection principle of the anuran dopaminergic system than previously assumed. Based on the data presented it seems that most features of the Abbreviations: Accnucleus of the ventral tegmental area; IP, interpeduncular nucleus; IR, infundibular recess; l.DlHb, left dorsolateral habenula; l.DmHb, left dorsomedial habenula; l.VHb, left ventral habenula; LHb, lateral habenula; Mam, mamillary area; med, medulla oblongata; MHb, medial habenula; NMLF, nucleus of the medial longitudinal fasciculus; NPv, nucleus of the periventricular organ; p.s., partial habenula septum; r, rostral; r.DHb, right dorsal habenula; r.VHb, right ventral habenula; RaCS, superior central raphe nucleus; RaD, dorsal raphe nucleus; RaM, nucleus raphe magnus; Rm, retromamillary area; RMTg, rostromedial tegmental nucleus; Sm, superficial mamillary area; SNc, substantia nigra pars compacta; SNr, substantia nigra pars reticulata; Str, striatum; TP, posterior tubercle; TPc, caudal part of the posterior tubercle; TPdl, dorsolateral nucleus of the posterior tubercle; TPdm, dorsomedial nucleus of the posterior tubercle; Tub, tuberomamillary nucleus; v, ventral; VH, ventral hypothalamus; VHb, ventral habenula; vMAM, ventral mamillary region; VTA, ventral tegmental area; VTApn, paranigral VTA; Zip, periventricular nucleus of the zona incerta.Wolfgang Walkowiak and Arndt von Twickel contributed equally to this work.

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

“…(4) The innervation of the supramamillary region (i.e., retromamillary area) by the LHb in rat (Herkenham & Nauta, ) and cat (Araki et al, ) should be in accordance with VHb innervation of the Rm. More detailed descriptions can be found in a follow‐up paper (Freudenmacher et al, ). Although Laberge and Smith () described the habenula to project to the posterior tubercle and ventral tegmentum, they did not regard this as evidence for the phylogenetic conservation of the habenula circuitry.…”

“…The brains were fixed overnight at 5 °C by submersion in 4% (vol/vol) paraformaldehyde and 14% (vol/vol) saturated picric acid in 0.1 M phosphate buffer (PB). Because the brains were also used for immunohistochemical studies (Freudenmacher, Schauer, Walkowiak, & von Twickel, ), glutaraldehyde was added or 1‐ethyl‐3‐(3‐diimethylamino‐propyl)‐carbodiimide hydrochloride (Sigma‐Aldrich, Cat# E7750) was used to fix the brains. The brains then were cryoprotected with 20% (wt/vol) sucrose in PB for 3–12 hr, embedded in Tissue‐Tek mounting medium (Sakura, Cat# 4583) and rapidly frozen.…”

The Dopaminergic Control of Movement-Evolutionary Considerations

“…A similar situation can be observed in birds and reptiles, although dopaminergic neurons have a more restricted distribution [ 113 , 114 ]. In amphibians, the dopaminergic neurons projecting to the striatum are mostly located in the diencephalon, although they also occupy mesencephalic domains [ 115 , 116 , 117 ]. In elasmobranchs, dopaminergic neurons projecting to basal telencephalic regions form separated populations in the diencephalon and mesencephalon [ 118 , 119 ].…”

“…However, it is important to note that further studies are needed, given that data concerning other basal vertebrates remains sparse. In amphibians, the organization of the dopaminergic innervation of the striatum shows a high degree of similarity with mammals, and as the three pathways are present, their appearance precedes the split of tetrapods [ 115 , 116 , 117 ], although when and how this occurred remain unknown.…”

“…The presence of a VTA component in lampreys should therefore not yet be excluded. In amphibians, neurons in the SNc and VTA are intermingled [ 117 ]. As a result, more studies are necessary to test whether a separate VTA-like population is also present in lampreys.…”