Olfactory Pallium Research Articles

The brain of the tree pangolin (Manis tricuspis). IX. The pallial telencephalon.

A cyto‐, myelo‐, and chemoarchitectonic analysis of the pallial telencephalon of the tree pangolin is provided. As certain portions of the pallial telencephalon have been described previously (olfactory pallium, hippocampal formation, and amygdaloid complex), we focus on the claustrum and endopiriform nuclear complex, the white matter and white matter interstitial cells, and the areal organization of the cerebral cortex. Our analysis indicates that the organization of the pallial telencephalon of the tree pangolin is similar to that observed in many other mammals, and specifically quite similar to the closely related carnivores. The claustrum of the tree pangolin exhibits a combination of insular and laminar architecture, while the endopiriform nuclear complex contains three nuclei, both reminiscent of observations made in other mammals. The population of white matter interstitial cells resembles that observed in other mammals, while a distinct laminated organization of the intracortical white matter was revealed with parvalbumin immunostaining. The cerebral cortex of the tree pangolin presented with indistinct laminar boundaries as well as pyramidalization of the neurons in both layers 2 and 4. All cortical regions typically found in mammals were present, with the cortical areas within these regions often corresponding to what has been reported in carnivores. Given the similarity of the organization of the pallial telencephalon of the tree pangolin to that observed in other mammals, especially carnivores, it would be reasonable to assume that the neural processing afforded the tree pangolin by these structures does not differ dramatically to that of other mammals.

The Zebrafish Amygdaloid Complex – Functional Ground Plan, Molecular Delineation, and Everted Topology

In mammals and other tetrapods, a multinuclear forebrain structure, called the amygdala, forms the neuroregulatory core essential for emotion, cognition, and social behavior. Currently, higher circuits of affective behavior in anamniote non-tetrapod vertebrates (“fishes”) are poorly understood, preventing a comprehensive understanding of amygdala evolution. Through molecular characterization and evolutionary-developmental considerations, we delineated the complex amygdala ground plan of zebrafish, whose everted telencephalon has made comparisons to the evaginated forebrains of tetrapods challenging. In this radical paradigm, thirteen telencephalic territories constitute the zebrafish amygdaloid complex and each territory is distinguished by conserved molecular properties and structure-functional relationships with other amygdaloid structures. Central to our paradigm, the study identifies the teleostean amygdaloid nucleus of the lateral olfactory tract (nLOT), an olfactory integrative structure that links dopaminergic telencephalic groups to the amygdala alongside redefining the putative zebrafish olfactory pallium (“Dp”). Molecular characteristics such as the distribution of substance P and the calcium-binding proteins parvalbumin (PV) and calretinin (CR) indicate, that the zebrafish extended centromedial (autonomic and reproductive) amygdala is predominantly located in the GABAergic and isl1-negative territory. Like in tetrapods, medial amygdaloid (MeA) nuclei are defined by the presence of substance P immunoreactive fibers and calretinin-positive neurons, whereas central amygdaloid (CeA) nuclei lack these characteristics. A detailed comparison of lhx5-driven and vGLut2a-driven GFP in transgenic reporter lines revealed ancestral topological relationships between the thalamic eminence (EmT), the medial amygdala (MeA), the nLOT, and the integrative olfactory pallium. Thus, the study explains how the zebrafish amygdala and the complexly everted telencephalon topologically relate to the corresponding structures in mammals indicating that an elaborate amygdala ground plan evolved early in vertebrates, in a common ancestor of teleosts and tetrapods.

Efferent and afferent connections of the olfactory bulb and prepiriform cortex in the pigeon (Columba livia)

Although olfaction in birds is known to be involved in a variety of behaviors, there is comparatively little detailed information on the olfactory brain. In the pigeon brain, the olfactory bulb (OB) is known to project to the prepiriform cortex (CPP), piriform cortex (CPi), and dorsolateral corticoid area (CDL), which together are called the olfactory pallium, but centrifugal pathways to the OB have not been fully explored. Fiber connections of CPi and CDL have been reported, but those of other olfactory pallial nuclei remain unknown. The present study examines the fiber connections of OB and CPP in pigeons to provide a more detailed picture of their connections using tract-tracing methods. When anterograde and retrograde tracers were injected in OB, projections to a more extensive olfactory pallium were revealed, including the anterior olfactory nucleus, CPP, densocellular part of the hyperpallium, tenia tecta, hippocampal continuation, CPi, and CDL. OB projected commissural fibers to the contralateral OB but did not receive afferents from the contralateral olfactory pallium. When tracers were injected in CPP, reciprocal ipsilateral connections with OB and nuclei of the olfactory pallium were observed, and CPP projected to the caudolateral nidopallium and the limbic system, including the hippocampal formation, septum, lateral hypothalamic nucleus, and lateral mammillary nucleus. These results show that the connections of OB have a wider distribution throughout the olfactory pallium than previously thought and that CPP provides a centrifugal projection to the OB and acts as a relay station to the limbic system.

Comparative Aspects of Forebrain Organization in the Ray-Finned Fishes: Touchstones or Not?

The comparative interpretation of forebrain organization of the ray-finned fishes presents a number of challenges. The telencephalon develops by an eversion process, and much of its surface is ventricular rather than pial. A topological map of the ventricular surface shows that the area ventralis (presumed subpallium) is bordered dorsally by the dorsomedial (DM) zone of the area dorsalis, which is, in turn, bordered by the olfactory-recipient dorsoposterior (DP) zone of the area dorsalis. Within the diencephalon small dorsal and ventral thalami and a large group of migrated nuclei assigned to the posterior tuberculum (but of uncertain status) are present. Both the dorsal thalamus and the migrated nuclei of the posterior tuberculum project to the telencephalon. A review of the known connections of the four major zones of the area dorsalis together with their topological positions leads to the following tentative interpretations. Zone DP is homologous to the primary olfactory cortex. There is not enough information concerning the dorsodorsal zone (DD) to speculate on its comparative relationships. Zone DM is a limbic-like area lying between the subpallium and the olfactory pallium and is possibly homologous to the pallial amygdala. The dorsolateral zone (DL) displays a pattern of connectivity with the cerebellum and the optic tectum suggesting a possible homology with non-olfactory, non-limbic pallial areas. Caveats are raised concerning the above interpretations of DM and DL. One or both of them may have evolved independently of the telencephalic zones in tetrapods and may thus represent examples of homoplasy.

An immunohistochemical study of the telencephalon of the senegal bichir (Polypterus senegalus).

The telencephalon in ray-finned fish (actinopterygians) is everted, in contrast to the evaginated telencephalic hemispheres in all other vertebrates. In the more derived ray-finned fish, the teleosts, proliferation of neurons and their migration from the ependymal zone of the pallium renders comparisons between telencephalic cell groups of the teleosts and members of other vertebrate groups extremely difficult. The telencephalon of Polypterus (a primitive living ray-finned fish), although everted, is cytoarchitecturally much simpler than that of teleosts. We have thus applied immunohistochemical techniques to the study of the telencephalon of Polypterus to help clarify the evolution of the telencephalon in teleosts and facilitate comparisons between the telencephalon in ray-finned fish and other vertebrates. Antisera against the following neuroactive substances were used: 1) serotonin (5HT), 2) tyrosine hydroxylase (TH), 3) substance P (SP), 4) leucine-enkephalin (ENK), 5) neuropeptide Y (NPY), and 6) the neurotensin-related hexapeptide LANT6. Several features of the labeling patterns obtained suggested that the dorsal and ventral subdivisions of the area ventralis are homologous as a field to the basal ganglia and septum plus other basal telencephalic regions of land vertebrates, sharks and lungfish: 1) an abundance of SP+, NPY+, and ENK+ fibers; 2) an abundance of TH+ fibers, possibly of posterior tubercle/tegmental origin; 3) the presence of an SP+ fiber bundle that appeared to descend from basal telencephalic levels and terminate in the posterior tubercle/tegmentum, which contain TH+ (possibly dopaminergic) neurons; and 4) an abundance of 5HT+ fibers, presumably of posterior tubercle/tegmental origin. It was not possible, however, to recognize distinct pallidal and striatal subdivisions within the area ventralis of Polypterus. The olfactory pallium (P1) was generally poor in most of the substances examined, except for the presence of LANT6+ fibers. The P3 pallial field was conspicuously rich in SP+ and ENK+ fibers throughout its extent, and the caudal and lateral parts of the P2 field were rich in SP+ fibers and ENK+ fibers. Since this is characteristic of the medial pallial and/or dorsomedial pallial walls of the telencephalon in lungfish, sharks, frogs, and reptiles, the P3 field and caudolateral part of the P2 field may be homologous to these portions of the telencephalon in other vertebrates. More rostromedial parts of P2 may correspond to those parts of the pallium in land vertebrates that are in receipt of specific sensory input from the thalamus, since low neuropeptide levels are characteristic of these regions.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical study of the telencephalon of the spiny dogfish, Squalus acanthias

The paucity of experimental data and the differences in telencephalic organization between sharks and other jawed vertebrates have complicated telencephalic comparisons. The distribution of neuropeptides has been extremely useful in recognizing and comparing major subdivisions of the telencephalon among vertebrates. Immunohistochemical techniques were therefore used to study the distribution of substance P (SP), leucin-enkephalin (LENK), and serotonin (5HT), as well as tyrosine hydroxylase (TH), an indicator of catecholamines, in the telencephalon of the spiny dogfish. The distribution of SP and LENK provides a clear distinction between pallial and subpallial portions of the telencephalon. Two regions of the ventrolateral telencephalon, area superficialis basalis and area periventricularis ventrolateralis, exhibit histochemical similarities to the pallidal and striatal subdivisions, respectively, of the basal ganglia in amniotes. Lower densities of LENK+ and SP+ perikarya and fibers occur in the medial pallium and the pars centralis of the dorsal pallium. Similar histochemical traits characterize the sensory thalamorecipient telencephalic structures in amniotes. The lateral pallium in dogfishes is distinguished by the presence of large numbers of TH+ neurons with radially oriented processes. The presence of these distinctive cells also in the medial wall of the rostral telencephalon suggests that the lateral pallium has a medial extension that is situated ventral to the medial pallium. Neurons containing TH were widely distributed in the telencephalon of spiny dogfish and were particularly abundant in the dorsal pallium, olfactory pallium, and area superficialis basalis. It is currently unclear whether these TH+ telencephalic neurons are, in fact, catecholaminergic or merely contain a TH-like substance unrelated to catecholamine synthesis.

An immunohistochemical study of the telencephalon of the african lungfish, Protopterus annectens

The telencephalon of the African lungfish, Protopterus annectens, was studied by immunohistochemical techniques in order to identify the major subdivisions of the telencephalon and determine the possible homologues of these subdivisions, if any, in other vertebrates. The distributions of four different neuropeptides (substance P, leucine-enkephalin, avian pancreatic polypeptide, and LANT6), a neurotransmitter (serotonin), and a neurotransmitter-related enzyme that is involved in catecholamine synthesis (tyrosine hydroxylase) were examined. The resultant labeling patterns indicated that the telencephalon of lungfish consists of three major subdivisions--a rostrally and dorsally situated olfactory bulb, a dorsally situated pallial region located caudal to the olfactory bulbs, and a ventrally situated subpallial regions. The dorsal and lateral pallial regions, which both receive secondary olfactory input, are somewhat distinct from one another cytoarchitectonically, but their immunohistochemical labeling characteristics did not differ. Thus, the lateral pallium and the dorsal pallium together appear to constitute an olfactory pallium in lungfishes. The medial pallium was found to consist of three immunohistochemically distinct subdivisions--a dorsal cell group, an intermediate cell group, and a ventral cell group. These medial pallial fields extend throughout the entire rostrocaudal extent of the medial wall of the telencephalon. Although one or more of these medial pallial cell groups may be homologous to specific portions of the medial pallium in land vertebrates, no specific similarities were observed to support any proposed one-to-one correspondences. The possibility that one or more of the medial pallial cell groups of lungfishes correspond to cell groups located in the dorsal pallium of land vertebrates could not be excluded. The subpallium is divided into lateral, medial, and caudal subdivisions. The lateral subdivision appears to be homologous to the basal ganglia of land vertebrates since it contains neuropeptide/neurotransmitter-specific neuronal populations that are characteristic of the striatal and pallidal portions of the basal ganglia of amniotes. The medial subdivision of the subpallium shows the topographic and immunohistochemical characteristics of the septal region and the nucleus accumbens region of the amniote telencephalon. The caudal subpallium does not show any distinctive immunohistochemical labeling characteristics and its possible homologue in land vertebrates is unclear.(ABSTRACT TRUNCATED AT 400 WORDS)