Mónica Folgueira

Experimental study of the connections of the telencephalon in the rainbow trout (Oncorhynchus mykiss). II: Dorsal area and preoptic region

In this study and the accompanying article (Folgueira et al., 2004a ), the fluorescent carbocyanine dye 1,1′‐dioctadecyl 3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI) was used in fixed tissue to comprehensively analyze the connections of the different regions of the telencephalic lobes and the preoptic region of the rainbow trout. Here, we analyze the connections of the dorsal area (D; pallium) of the telencephalon, and the preoptic region, as well as the telencephalic connections of several structures in the diencephalon and brainstem of juvenile trout. The dorsal plus dorsolateral pallial zone of D (Dd+Dl‐d) receives afferents from contralateral Dd+Dl‐d, the ventral area of the telencephalon, preoptic nucleus, suprachiasmatic nucleus, medial thalamus, preglomerular complex, anterior and lateral tuberal nuclei, posterior tuberal nucleus, posterior hypothalamic lobe, superior raphe nucleus, and the rhombencephalic central gray and reticular formation, and projects to the central zone of D (Dc), medial thalamus, and some caudomedial hypothalamic regions. The medial zone of D (Dm) maintains reciprocal connections with the preglomerular complex and also receives afferents from the preoptic nucleus, suprachiasmatic nucleus, anterior tuberal nucleus, preglomerular tertiary gustatory nucleus, posterior tubercle, superior raphe nucleus, locus coeruleus, and the rhombencephalic central gray, and reticular formation. Dc receives fibers mainly from Dd+Dl‐d, preoptic nucleus, preglomerular complex, and torus semicircularis and projects to several extratelencephalic centers, including the paracommissural nucleus, optic tectum, torus semicircularis, thalamus, preglomerular complex, posterior tubercle nuclei, and inferior hypothalamic lobes. The posterior zone of D (Dp) is mainly connected with the olfactory bulbs, the ventral and supracommissural nuclei of the ventral area (subpallium), the preoptic nucleus, and the preglomerular complex and projects to wide hypothalamic and posterior tubercular regions. The preoptic nucleus projects to the olfactory bulb, to most regions of the telencephalic lobes, and to several diencephalic and brainstem structures. These results reveal complex and specialized connectional patterns in the rainbow trout dorsal telencephalon and preoptic region. Most of these connections have not been described previously in salmonids. These connections indicate that the salmonid telencephalon is involved in multisensorial processing and modulation of brain activity. J. Comp. Neurol. 480:204–233, 2004.

An experimental study of the connections of the telencephalon in the rainbow trout (Oncorhynchus mykiss). I: Olfactory bulb and ventral area

The fluorescent carbocyanine dye 1,1′‐dioctadecyl 3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI) was used in fixed tissue to comprehensively analyze the connections of the olfactory bulbs and the different regions of the ventral (V) area of the telencephalic lobes (subpallium) of the rainbow trout. With this goal, DiI was applied to the different telencephalic nuclei and zones, as well as to the olfactory nerve, the olfactory bulb, the retina, and to several structures in the diencephalon and brainstem of juvenile trout. The olfactory bulbs maintain reciprocal connections with several regions of the telencephalon [ventral nucleus of V (Vv), supracommissural nucleus (Vs), posterior zone of D (Dp), preoptic nucleus], and also project to the diencephalon (posterior tuberal nucleus, posterior hypothalamic lobe). Vv receives afferents from Vs, the dorsal nucleus of V (Vd), the preoptic nucleus, and from several nuclei in the diencephalon and brainstem (suprachiasmatic nucleus, anterior and lateral tuberal nuclei, preglomerular complex, tertiary gustatory nucleus, posterior tubercle, inferior hypothalamic lobes, thalamus, torus semicircularis, secondary gustatory nucleus, locus coeruleus, superior raphe nucleus, central gray, and reticular formation), and projects to dorsal (pallial) regions and most of the nuclei afferent to Vv. The dorsal nucleus of V (Vd) and Vs mainly project to the dorsal area. In an accompanying article (Folgueira et al., 2004 ), we present the results of application of DiI to dorsal (pallial) telencephalic regions, as well as of several experiments of tracer application to extratelencephalic regions. The results presented here, together with those of the accompanying article, reveal a complex connectional pattern of the rainbow trout ventral telencephalon, most of these connections having not been described previously in salmonids. J. Comp. Neurol. 480:180–203, 2004.

Experimental study of the connections of the gustatory system in the rainbow trout, Oncorhynchus mykiss.

Salmonids are a group of teleosts with a nonspecialized gustatory system. With the aim of describing the gustatory connections in a member of this group, we carried out tract‐tracing experiments using the lipophilic carbocyanine dye 1,1′‐dioctadecyl 3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI) in fixed brains of the rainbow trout (Oncorhynchus mykiss). The neural tracer was applied to the primary viscerosensory column, secondary gustatory visceral nucleus (SGN), torus lateralis (TL), and tertiary gustatory nucleus (TGN), the dorsal part of the ventral area of the telencephalon (dorsal‐Vv), and the medial area of the dorsal telencephalon (Dm). The primary viscerosensory column projects mainly to the SGN. DiI application to the SGN showed a bilateral and reciprocal connection with the TGN and a rostral portion of the nucleus of the lateral hypothalamic recess. The application of DiI in the dorsal‐Vv and Dm at levels rostral to the anterior commissure led to labeling of a restricted group of diencephalic neurons in the TGN and sending dendrites to the TL. DiI application to the TL/TGN anterogradely labeled fibers that coursed in the medial forebrain bundle innervating the precommissural portion of the dorsal‐Vv and Dm. Caudally, this type of application led to labeling of fibers in the viscerosensory column and perikarya in the SGN. Tract‐tracing results showed direct projections from the diencephalic and rhombencephalic gustatory nuclei to the telencephalon. There was a direct and reciprocal connection between the SGN and the ventral telencephalon. The results showed that the gustatory connections of the trout are similar to those of teleosts, with highly specialized gustatory centers as in cyprinids and ictalurids, and to that observed in the percomorph tilapia, thus demonstrating a basic organization that is shared by most teleosts. J. Comp. Neurol. 465:604–619, 2003.

Morphogenesis underlying the development of the everted teleost telencephalon

BackgroundAlthough the mechanisms underlying brain patterning and regionalization are very much conserved, the morphology of different brain regions is extraordinarily variable across vertebrate phylogeny. This is especially manifest in the telencephalon, where the most dramatic variation is seen between ray-finned fish, which have an everted telencephalon, and all other vertebrates, which have an evaginated telencephalon. The mechanisms that generate these distinct morphologies are not well understood.ResultsHere we study the morphogenesis of the zebrafish telencephalon from 12 hours post fertilization (hpf) to 5 days post fertilization (dpf) by analyzing forebrain ventricle formation, evolving patterns of gene and transgene expression, neuronal organization, and fate mapping. Our results highlight two key events in telencephalon morphogenesis. First, the formation of a deep ventricular recess between telencephalon and diencephalon, the anterior intraencephalic sulcus (AIS), effectively creates a posterior ventricular wall to the telencephalic lobes. This process displaces the most posterior neuroepithelial territory of the telencephalon laterally. Second, as telencephalic growth and neurogenesis proceed between days 2 and 5 of development, the pallial region of the posterior ventricular wall of the telencephalon bulges into the dorsal aspect of the AIS. This brings the ventricular zone (VZ) into close apposition with the roof of the AIS to generate a narrow ventricular space and the thin tela choroidea (tc). As the pallial VZ expands, the tc also expands over the upper surface of the telencephalon. During this period, the major axis of growth and extension of the pallial VZ is along the anteroposterior axis. This second step effectively generates an everted telencephalon by 5 dpf.ConclusionOur description of telencephalic morphogenesis challenges the conventional model that eversion is simply due to a laterally directed outfolding of the telencephalic neuroepithelium. This may have significant bearing on understanding the eventual organization of the adult fish telencephalon.

Afferent and efferent connections of the cerebellum of a salmonid, the rainbow trout (Oncorhynchus mykiss): A tract-tracing study

The connections of the cerebellum of the rainbow trout were studied by experimental methods. The pretectal paracommissural nucleus has reciprocal connections with the cerebellum. Three additional pretectal nuclei project to both the corpus and valvula cerebelli, and seem to receive cerebellar afferents. A large number of cells of the lateral nucleus of the valvula project to wide regions of the cerebellum, including the valvula, the corpus, the granular eminences, and the caudal lobe, whereas the contralateral inferior olive and scattered reticular cells project only to the corpus and valvula cerebelli. Afferents to the corpus were also observed from the ventral tegmental nucleus, the “paraisthmic nucleus,” the perilemniscal nucleus, the central gray, and the octavolateral area. Valvular afferents were also observed from the torus semicircularis and the midbrain tegmental areas. In most cases of cerebellar application, labeled fibers were seen in the thalamus, the pretectum, the torus longitudinalis and torus semicircularis, the nucleus of the medial longitudinal fascicle, and midbrain and rhombencephalic reticular areas. From the corpus cerebelli some fibers also project to the posterior tubercle and the hypothalamus. Moreover, the granular eminences project to the cerebellar crest. DiI application to most of the areas showing labeled fibers after cerebellar tracer application led to the labeling of characteristic eurydendroid cells, mainly in the valvula cerebelli and the caudal lobe. A few putative eurydendroid cells were labeled from the octavolateralis regions. These results in a teleost with a generalized brain indicate several differences with respect to the cerebellar connections reported in other teleost fishes that have specialized brains. J. Comp. Neurol. 497:542–565, 2006.

The nucleus subglomerulosus of the trout hypothalamus is a link between chemosensory and visual systems: a DiI study

We have studied the connections of the nucleus subglomerulosus of the trout posterior tubercle. Main afferents to the nucleus subglomerulosus come from the dorsal telencephalon and the visceral (gustatory) secondary nucleus, while it projects to the optic tectum. In the light of the connections observed, the nucleus subglomerulosus of trout (and probably in other teleosts) appears to be involved in the modulation of sensory-motor tectal processing by olfactory and visceral information.

Experimental study of the connections of the preglomerular nuclei and corpus mamillare in the rainbow trout, Oncorhynchus mykiss

The preglomerular complex of trout consists of the anterior (aPGN) and medial (mPGN) preglomerular nuclei and the corpus mamillare (CM). In order to improve knowledge on this complex, we applied a lipophilic neuronal tracer (DiI) to the three nuclei. These nuclei received afferents from the medial part of the dorsal telencephalic area (Dm), the ventral part of the ventral telencephalic area (Vv), the preoptic nucleus, the periventricular layer of the rostral optic tectum and the central posterior thalamic nucleus. The aPGN also received numerous toral projections and, sent efferents to the anterior tuberal nucleus. In addition, both the aPGN and the mPGN nuclei gave rise to efferents to the dorsal region of the dorsal telencephalic area (Dd), whereas the medial preglomerular nucleus and the CM sent fibers to the torus lateralis and the diffuse nucleus, as confirmed by reciprocal labeling. A small mPGN/CM subgroup projected to the optic tectum. These results suggest close functional inter-relationship between the trout preglomerular complex and two telencephalic regions (Dm and Vv). In addition, all nuclei of the complex receive preoptic, tectal and dorsal thalamic afferents, whereas the aPGN and mPGN are related with acoustic-lateral ascending pathways, and the mPGN and CM with the central region of the dorsal telencephalic area and visceral/gustatory pathways.

Megalencephalic Leukoencephalopathy with subcortical Cysts protein 1 regulates glial surface localization of GLIALCAM from fish to humans

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a leukodystrophy characterized by myelin vacuolization and caused by mutations in MLC1 or GLIALCAM. Patients with recessive mutations in either MLC1 or GLIALCAM show the same clinical phenotype. It has been shown that GLIALCAM is necessary for the correct targeting of MLC1 to the membrane at cell junctions, but its own localization was independent of MLC1 in vitro. However, recent studies in Mlc1(-/-) mice have shown that GlialCAM is mislocalized in glial cells. In order to investigate whether the relationship between Mlc1 and GlialCAM is species-specific, we first identified MLC-related genes in zebrafish and generated an mlc1(-/-) zebrafish. We have characterized mlc1(-/-) zebrafish both functionally and histologically and compared the phenotype with that of the Mlc1(-/-) mice. In mlc1(-/-) zebrafish, as in Mlc1(-/-) mice, Glialcam is mislocalized. Re-examination of a brain biopsy from an MLC patient indicates that GLIALCAM is also mislocalized in Bergmann glia in the cerebellum. In vitro, impaired localization of GlialCAM was observed in astrocyte cultures from Mlc1(-/-) mouse only in the presence of elevated potassium levels, which mimics neuronal activity. In summary, here we demonstrate an evolutionary conserved role for MLC1 in regulating glial surface levels of GLIALCAM, and this interrelationship explains why patients with mutations in either gene (MLC1 or GLIALCAM) share the same clinical phenotype.

Afferent Connectivity of the Zebrafish Habenulae

The habenulae are bilateral nuclei located in the dorsal diencephalon that are conserved across vertebrates. Here we describe the main afferents to the habenulae in larval and adult zebrafish. We observe afferents from the subpallium, nucleus rostrolateralis, posterior tuberculum, posterior hypothalamic lobe, median raphe; we also see asymmetric afferents from olfactory bulb to the right habenula, and from the parapineal to the left habenula. In addition, we find afferents from a ventrolateral telencephalic nucleus that neurochemical and hodological data identify as the ventral entopeduncular nucleus (vENT), confirming and extending observations of Amo et al. (2014). Fate map and marker studies suggest that vENT originates from the diencephalic prethalamic eminence and extends into the lateral telencephalon from 48 to 120 hour post-fertilization (hpf). No afferents to the habenula were observed from the dorsal entopeduncular nucleus (dENT). Consequently, we confirm that the vENT (and not the dENT) should be considered as the entopeduncular nucleus “proper” in zebrafish. Furthermore, comparison with data in other vertebrates suggests that the vENT is a conserved basal ganglia nucleus, being homologous to the entopeduncular nucleus of mammals (internal segment of the globus pallidus of primates) by both embryonic origin and projections, as previously suggested by Amo et al. (2014).

Organization of the torus longitudinalis in the rainbow trout (Oncorhynchus mykiss): An immunohistochemical study of the GABAergic system and a DiI tract‐tracing study

The torus longitudinalis (TL) is a tectum‐associated structure of actinopterygian fishes. The organization of the TL of rainbow trout was studied with Nissl staining, Golgi methods, immunocytochemistry with antibodies to γ‐aminobutyric acid (GABA), glutamic acid decarboxylase (GAD), and the GABAA receptor subunits δ and β2/β3, and with tract tracing methods. Two types of neuron were characterized: medium‐sized GABAergic neurons and small GABA‐negative granule cells. GABAA receptor subunit δ‐like immunoreactivity delineated two different TL regions, ventrolateral and central. Small GABAergic cells were also observed in marginal and periventricular strata of the optic tectum. These results indicate the presence of local GABAergic inhibitory circuits in the TL system. For tract‐tracing, a lipophilic dye (DiI) was applied to the TL and to presumed toropetal nuclei or toral targets. Toropetal neurons were observed in the optic tectum, in pretectal (central, intermediate, and paracommissural) nuclei, in the subvalvular nucleus, and associated with the pretectocerebellar tract. Torofugal fibers were numerous in the stratum marginale of the optic tectum. Toropetal pretectal nuclei also project to the cerebellum, and a few TL cells project to the cerebellar corpus. The pyramidal cells of the trout tectum were also studied by Golgi methods and local DiI labeling. The connections of trout TL revealed here were more similar to those recently reported in carp and holocentrids (Ito et al. [ 2003 ] J. Comp. Neurol. 457:202–211; Xue et al. [ 2003 ] J. Comp. Neurol. 462:194–212), than to those reported in earlier studies. However, important differences in organization of toropetal nuclei were noted between salmonids and these other teleosts. J. Comp. Neurol. 503:348–370, 2007.