Efferent connections of the dorsal cortex of the lizard Gekko gecko studied with Phaseolus vulgaris-leucoagglutinin

The efferent connections from the dorsal cortex of the lizard Gekko gecko have been studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. It appeared that the dorsal cortex is not a homogeneous structure as far as the efferent connections are concerned. All parts of the dorsal cortex project to the septum. All parts except the most medial project to the dorsal ventricular ridge, amygdala, nucleus periventricularis hypothalami, area lateralis hypothalami, and the anterior olfactory nucleus. The most medial part, in addition to the septal projections, is connected with the medial cortex and the contralateral medial and dorsal cortices. From the rostral part additional projections could be traced to the nucleus dorsolateralis hypothalami, nucleus ventromedialis thalami, nucleus dorsolateralis thalami, striatum, pallial thickening, medial cortex, nucleus olfactorius anterior, and the main and accessory olfactory bulbs. From the caudal part additional projections exist to the nucleus dorsomedialis thalami, nucleus accumbens, and the contralateral dorsal cortex. A system of intrinsic connections exists that can be subdivided into four subsystems, each of which subserves the interconnections within four subdivisions of the cortex: 1) the superficial medial part, 2) the deep medial part, 3) the caudal lateral and caudal intermediate parts, and 4) the rostral lateral and rostral intermediate parts. Connections between these four areas are scarce. From the present results the conclusion is drawn that the dorsal cortex of the lizard Gekko gecko has many hodological aspects in common with the ventral subiculum of mammals. The present results do not support the hypothesis that the dorsal cortex is the reptilian equivalent of the mammalian neocortex.     

Morphology and projection pattern of medial and dorsal pallial neurons in the frog Discoglossus pictus and the salamander Plethodon jordani

In the frog Discoglossus pictus and the salamander Plethodon jordani, the morphology and axonal projection pattern of neurons in the medial and dorsal pallium were determined by intracellular biocytin labeling. A total of 77 pallial neurons were labeled in the frog and 58 pallial neurons in the salamander. Within the medial pallium (MP) of the frog, four types of neurons were identified on the basis of differences in their axonal projection pattern. Type I neurons have bilateral projections into telencephalic and diencephalic areas; type II neurons have bilateral projections to telencephalic areas and ipsilaterally descending projections to diencephalic regions; type III neurons have only intratelencephalic connections, and a single type IV neuron has ipsilaterally descending projections. The somata of the four types occupy four nonoverlapping zones. Neurons of the dorsal pallium (DP) project exclusively to the ipsilateral MP and to the dorsal edge of the lateral pallium. In the ventral MP of the salamander, neurons have mostly intratelencephalic projections. Neurons in the dorsal MP project bilaterally to diencephalic and telencephalic regions. Neurons in the medial DP project ipsilaterally to the MP, lateral septum, nucleus accumbens, medial amygdala, and the internal granule layer of the olfactory bulb. In five cases, fibers were found in the commissura hippocampi, but in only two cases could these fibers be followed toward the contralateral MP and septum. Neurons in the lateral DP had no contralateral projections; they projected to the ipsilateral MP and in eight cases to the ipsilateral septum as well. Based on similarities of cytoarchitecture and projection pattern in neurons of the MP and DP, it is proposed that both frogs and salamanders have an MP subdivided into a ventral and dorsal portion, and a DP subdivided into a medial and a lateral portion.     

Organization of the sensory input to the telencephalon in the fire-bellied toad, Bombina orientalis

The functional organization of sensory activity in the amphibian telencephalon is poorly understood. We used an in vitro brain preparation to compare the anatomy of afferent pathways with the localization of electrically evoked sensory potentials and single neuron intracellular responses in the telencephalon of the toad Bombina orientalis. Anatomical tracing showed that the anterior thalamic nucleus innervates the anterior parts of the medial, dorsal, and lateral pallia and the rostralmost part of the pallium in addition to the subpallial amygdala/ventral pallidum region. Additional afferents to the medial telencephalon originate from the thalamic eminence. Electrical stimulation of diverse sensory nerves and brain regions generated evoked potentials with distinct characteristics in the pallium, subpallial amygdala/ventral pallidum, and dorsal striatopallidum. In the pallium, this sensory activity is generated in the anterior medial region. In the case of olfaction, evoked potentials were recorded at all sites, but displayed different characteristics across telencephalic regions. Stimulation of the anterior dorsal thalamus generated a pattern of activity comparable to olfactory evoked potentials, but it became similar to stimulation of the optic nerve or brainstem after bilateral lesion of the lateral olfactory tract, which interrupted the antidromic activation of the olfactohabenular tract. Intracellular bimodal sensory responses were obtained in the anterior pallium, medial amygdala, ventral pallidum, and dorsal striatopallidum. Our results demonstrate that the amphibian anterior pallium, medial amygdala/ventral pallidum, and dorsal striatopallidum are multimodal sensory centers. The organization of the amphibian telencephalon displays striking similarities with the brain pathways recently implicated in mammalian goal-directed behavior.     

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)     

The pallial amygdala of amniote vertebrates: evolution of the concept, evolution of the structure

Embryological studies indicate that the amygdala includes pallial structures, namely the cortical amygdala (olfactory and vomeronasal) and the basolateral complex deep to it. In squamate reptiles, the cortical amygdala includes secondary olfactory (the ventral anterior amygdala) and vomeronasal centres (the nucleus sphericus). In birds, the situation is far less clear, due to the relative underdevelopment of the chemosensory systems. The basolateral amygdala of squamate reptiles includes two ventropallial structures: the posterior dorsal ventricular ridge and the lateral amygdala. Like their mammalian counterparts, these centres give rise to glutamatergic projections to the striatal (centromedial) amygdala and the ventromedial hypothalamus. Using the same criteria, the caudal neostriatum and the ventral intermediate archistriatum may represent the ventral pallial amygdala of birds. The basal nucleus of the mammalian amygdala is a lateropallial territory. In reptiles, the lateral pallium includes the dorsolateral amygdala, which, like the mammalian basal nucleus, projects bilaterally to the striatum/accumbens and receives distinct cholinergic and dopaminergic innervations. In the avian brain, the same embryological, hodological, and histochemical criteria are met by the area temporo-parieto-occipitalis, the caudolateral neostriatum and the dorsal intermediate archistriatum. Therefore, the projections from these structures to the paleostriatum and the lobus paraolfactorius are amygdalostriatal, rather than corticostriatal connections.     

Morphology and axonal projection pattern of neurons in the telencephalon of the fire-bellied toad Bombina orientalis: an anterograde, retrograde, and intracellular biocytin labeling study

The connectivity and cytoarchitecture of telencephalic centers except dorsal and medial pallium were studied in the fire-bellied toad Bombina orientalis by anterograde and retrograde biocytin labeling and intracellular biocytin injection (total of 148 intracellularly labeled neurons or neuron clusters). Our findings suggest the following telencephalic divisions: (1) a central amygdala-bed nucleus of the stria terminalis in the caudal midventral telencephalon, connected to visceral-autonomic centers; (2) a vomeronasal amygdala in the caudolateral ventral telencephalon receiving input from the accessory olfactory bulb and projecting mainly to the preoptic region/hypothalamus; (3) an olfactory amygdala in the caudal pole of the telencephalon lateral to the vomeronasal amygdala receiving input from the main olfactory bulb and projecting to the hypothalamus; (4) a medial amygdala receiving input from the anterior dorsal thalamus and projecting to the medial pallium, septum, and hypothalamus; (5) a ventromedial column formed by a nucleus accumbens and a ventral pallidum projecting to the central amygdala, hypothalamus, and posterior tubercle; (6) a lateral column constituting the dorsal striatum proper rostrally and the dorsal pallidum caudally, and a ventrolateral column constituting the ventral striatum. We conclude that the caudal mediolateral complex consisting of the extended central, vomeronasal, and olfactory amygdala of anurans represents the ancestral condition of the amygdaloid complex. During the evolution of the mammalian telencephalon this complex was shifted medially and involuted. The mammalian basolateral amygdala apparently is an evolutionary new structure, but the medial portion of the amygdalar complex of anurans reveals similarities in input and output with this structure and may serve similar functions.     

Afferent and efferent connections of the olfactory bulbs in the lizard Podarcis hispanica.

The connections of the olfactory bulbs of Podarcis hispanica were studied by tract-tracing of injected horseradish peroxidase. Restricted injections into the main olfactory bulb (MOB) resulted in bilateral terminallike labeling in the medial part of the anterior olfactory nucleus (AON) and in the rostral septum, lateral cortex, nucleus of the lateral olfactory tract, and ventrolateral amygdaloid nucleus. Bilateral retrograde labeling was found in the rostral lateral cortex and in the medial and dorsolateral AON. Ipsilaterally the dorsal cortex, nucleus of the diagonal band, lateral preoptic area, and dorsolateral amygdala showed labeled cell bodies. Retrogradely labeled cells were also found in the midbrain raphe nucleus. Results from injections into the rostral lateral cortex and lateral olfactory tract indicate that the mitral cells are the origin of the centripetal projections of the MOB. Injections in the accessory olfactory bulb (AOB) produced ipsilateral terminallike labeling of the ventral AON, bed nucleus of the accessory olfactory tract, central and ventromedial amygdaloid nuclei, medial part of the bed nucleus of the stria terminalis, and nucleus sphericus. Retrograde labeling of neurons was observed ipsilaterally in the bed nucleus of the accessory olfactory tract and stria terminalis, in the central amygdaloid nucleus, dorsal cortex, and nucleus of the diagonal band. Bilateral labeling of somata was found in the ventral AON, the nucleus sphericus (hilus), and in the mesencephalic raphe nucleus and locus coeruleus. Injections into the dorsal amygdala showed that the mitral neurons are the cells of origin of the AOB centripetal projections. Reciprocal connections are present between AOB and MOB. To our knowledge, this is the first study to address the afferent connections of the olfactory bulbs in a reptile. On the basis of the available data, a discussion is provided of the similarities and differences between the reptilian and mammalian olfactory systems, as well as of the possible functional role of the main olfactory connections in reptiles.     

Hodological characterization of the septum in anuran amphibians: II. Efferent connections

The efferent connections of the septum of the gray treefrog Hyla versicolor were studied by combining anterograde and retrograde tracing with biotin ethylendiamine (Neurobiotin). The lateral septal complex projects mainly to the medial pallium, limbic regions (e.g., amygdala and nucleus accumbens), and hypothalamic areas but also to sensory nuclei in the diencephalon and midbrain. The central septal complex strongly innervates the medial pallium, limbic, and hypothalamic areas but also specific sensory (including olfactory) regions. The medial septal complex sends major projections to all olfactory nuclei and a weaker projection to the hypothalamus. Our results indicate that all septal nuclei may modify the animal's internal state via efferents to limbic and hypothalamic areas. Via projections to the medial pallium, lateral and central septal complexes may be involved in learning processes as well. Because of their connections to specific sensory areas, all septal areas are in a position to influence sensory processing. Furthermore, our data suggest that both the postolfactory eminence and the bed nucleus of the pallial commissure are not part of the septal complex, rather, the postolfactory eminence seems to be comparable to the mammalian primary olfactory cortex, whereas the bed nucleus may be analogous to the mammalian subfornical organ.     

Projections of the olfactory bulb and nervus terminalis in the silver lamprey

The connections of the olfactory bulb were traced using horseradish peroxidase. A homologue of the medial olfactory tract in gnathostomes projects to the ipsilateral septal nucleus, preoptic area and, possibly, the rostral striatum. A homologue of the lateral olfactory tract projects to the ipsilateral lateral pallium, dorsal pallium and, possibly, the medial pallium, as well as to the posterior diencephalon. A component of the lateral olfactory tract decussates in the habenular and posterior commissures and distributes to the contralateral hemisphere and caudal diencephalon. A dorsal component of secondary olfactory fibers terminates, ipsilaterally, in a dorsomedially situated neuropil that has previously been interpreted as a single glomerulus of the olfactory bulb or as an accessory olfactory bulb, as well as in the contralateral olfactory bulb after decussation in the dorsal commissure. Afferents to the olfactory bulb arise from the ipsilateral dorsal pallium, lateral pallium, a cell-poor region adjacent to the preoptic area, and the midbrain tegmentum. The extent of the secondary olfactory projections in silver lampreys could be interpreted to support the phylogenetic hypothesis that all regions of the telencephalon received secondary olfactory projections in the earliest vertebrates, but this interpretation is not unequivocal, due to questions concerning the pallial homologues in lampreys and gnathostomes. Application of horseradish peroxidase to the olfactory epithelium revealed projections to the striatum, preoptic area, hypothalamus and posterior tuberculum that are comparable to projections of the nervus terminalis in other vertebrates.     

Efferent projections of the dorsal ventricular ridge and the striatum in the Tegu lizard. Tupinambis nigropunctatus.

A H3 proline-leucine mixture was injected into the dorsal ventricular ridge (DVR) and striatum of the Tegu lizard in order to determine their efferent projections. The brains were processed according to standard radioautographic technique, and counterstained with cresyl violet. DVR projections were generally restricted to the telencephalon, while striatal projections were limited to diencephalic and mesencephalic structures. Thus the anterior DVR projects ipsilaterally to nuclei sphericus and lateralis amygdalae, striatum (ipsilateral and contralateral) ventromedial nucleus of the hypothalamus, nucleus accumbens, anterior olfactory nucleus, nucleus of the lateral olfactory tract and lateral pallium. Posterior DVR projections enter ipsilateral anterior olfactory nucleus, lateral and interstitial amygdalar nuclei, olfactory tubercle and bulb, nucleus of the lateral olfactory tract and a zone surrounding the ventromedial hypothalamic nucleus. Labeled axons from striatal injections pass caudally in the lateral forebrain bundle to enter (via dorsal peduncle) nuclei dorsomedialis, medialis posterior, entopeduncularis anterior, and a zone surrounding nucleus rotundus. Others join the ventral peduncle of LFB and enter ventromedial nucleus (thalami), while the remaining fibers continue caudally in the ventral peduncle to the mesencephalic prerubral field, central gray, substantia nigra, nucleus intercollicularis, reticular formation and pretectal nucleus posterodorsalis. These results are discussed in relation to the changing notions regarding terminology, classification and functions of dorsl ventricular ridge and striatum.     

Afferent and efferent connections of the bullfrog medial pallium.

Horseradish peroxidase or tritiated proline was unilaterally injected into the medial pallium in bullfrogs in order to determine the sources of afferent projections to the medial pallium and the targets of pallial efferent projections. Some cells in all telencephalic centers, except the corpus striatum and the pars lateralis of the amygdala, project to the ipsilateral medial pallium. The medial pallium receives projections from fewer centers in the contralateral hemisphere, which include the medial septal nucleus, the pars medialis of the amygdala, the bed nucleus of the pallial commissure and the medial pallium. The raphe nucleus and the anterior thalamic nuclei appear to be the only sources of afferents to the medial pallium from outside the telencephalon. Efferents of the medial pallium are far more extensive than reported in earlier studies. The medial pallium projects ipsilaterally to all telencephalic nuclei, with the exception of a large part of the corpus striatum, and contralaterally to the medial septal nucleus, the olfactory tubercle, amygdala, medial pallium and bed nucleus of the pallial commissure. Extensive efferent projections also terminate in preoptic and hypothalamic regions, as well as in most thalamic relay nuclei, the pretectum and, possibly, the optic tectum. Similarities to the medial pallium in other tetrapods and to that in mammals suggest that the medial pallium in anurans is homologous to the subicular and CA fields and, possibly, the dentate gyrus in mammals. However, the extensive projections of the medial pallium to the dorsal thalamus and pretectum in anurans may be primitive features of the medial pallium retained in anurans, or uniquely derived features in anurans.     

Telencephalic connections in the pacific hagfish (Eptatretus stouti), with special reference to the thalamopallial system

The pallium of hagfishes (myxinoids) is unique: It consists of a superficial “cortical” mantle of gray matter which is subdivided into several layers and fields, but it is not clear whether or how these subdivisions can be compared to those of other craniates, i.e., lampreys and gnathostomes. The pallium of hagfishes receives extensive secondary olfactory projections (Wicht and Northcutt [1993] J. Comp. Neurol. 337:529–542), but there are no experimental data on its nonolfactory connections. We therefore investigated the pallial and dorsal thalamic connections of the Pacific hagfish. Injections of tracers into the pallium labeled many cells bilaterally in the olfactory bulbs. Other pallial afferents arise from the contralateral pallium, the dorsal thalamic nuclei, the preoptic region, and the posterior tubercular nuclei. Descending pallial efferents reach the preoptic region, the dorsal thalamus, and the mesencephalic tectum but not the motor or premotor centers of the brainstem. Injections of tracers into the dorsal thalamus confirmed the presence of reciprocal thalamopallial connections. In addition, these injections revealed that there is no “preferred” pallial target for the ascending thalamic fibers; instead, ascending thalamic and secondary olfactory projections overlap throughout the pallium. The mesencephalic tectum and tegmentum, which receive afferents from a variety of sensory sources, are interconnected with the dorsal thalamus; thus, ascending nonolfactory sensory information may reach myxinoid pallia via a tectal-thalamic-telencephalic route. A comparative analysis of pallial organization reveals that the subdivisions of the pallium in gnathostomes (i.e., medial, dorsal, and lateral pallia) cannot be recognized with certainty in hagfishes. J. Comp. Neurol. 395:245–260, 1998. © 1998 Wiley-Liss, Inc.     

Dorsomedial telencephalon of lungfishes: A pallial or subpallial structure? criteria based on histology,connectivity, and histochemistry

The dorsomedial telencephalon of lepidosirenid lungfishes has been interpreted in two divergent ways: earlier investigators regarded it as a subpallial (septal) structure; more recently, it has been reinterpreted as the medial pallium (hippocampus). To resolve this question, we identified parameters that are conclusive in their association with either the medial pallium or the septum in anamniotes. The present study examines the position of ependymal thickenings and the distribution of acetylcholinesterase (AchE) in the cerebral hemispheres of the African lungfish Protopterus, the Australian lungfish Neoceratodus, and the amphibian species Xenopus and Ambystoma. In addition, projections from the hypothalamus (paraventricular organ) to the telencephalon are investigated in Protopterus. Ependymal specializations are located dorsally and ventrally in the lateral ventricles of amphibians, but laterally and medially in lungfishes. In Protopterus, the paraventricular organ projects to the medial telencephalic hemisphere, but not to the dorsal roof. High levels of AchE are present in restricted neuropil regions of the medial hemisphere and in the ventral and ventrolateral telencephalon, but they are lacking in the dorsal roof. Intensely AchE-stained neuronal cell bodies are located in the ventral telencephalon (rostrally) and the dorsomedial telencephalon (at mid-level). In Neoceratodus, AchE staining is pronounced in the septal area, but absent in the pallium. The terminal nerve proper lacks AchE stain in Protopterus; nerve fibres of the preoptic nerve are AchE-positive in both lungfish species. In Xenopus, AchE staining of fibers and terminals is restricted to the subpallium (medial septum, tuberculum olfactorium, striatum, nucleus accumbens, and medial amygdala); cell bodies are AchE positive in parts of the subpallium and rostral pallium. Comparison of cytological, histochemical, and "connectional" parameters substantiates the interpretation that the dorsomedial telencephalon of lungfishes represents a subpallial, but not a "medial pallial" structure. The dorsomedial part of the lepidosirenid telencephalon corresponds to the septum in the most plesiomorphic living lungfish, Neoceratodus forsteri, but it differs considerably from the dorsomedial telencephalon (medial pallium) in amphibians.     

Afferent and efferent connections of the dorsolateral corticoid area and a comparison with connections of the temporo-parieto-occipital area in the pigeon (Columba livia)

The dorsolateral corticoid area (CDL) in the pigeon telencephalon is a thin, superficial part of the caudal pallium adjoining the medially situated hippocampal formation. To determine the connectivity of CDL, and to distinguish CDL from the rostrally adjacent temporo-parieto-occipital area (TPO), injections of neural tracers were made into the caudal superficial pallium at various rostrocaudal levels. The results showed that injections caudal to A 6.75 (Karten and Hodos [1967] Baltimore: Johns Hopkins University Press) gave rise to reciprocal connections with subdivisions of the hippocampal formation, TPO, piriform cortex, posterior pallial amygdala, caudoventral nidopallium, densocellular part of the hyperpallium, lateral hyperpallium, frontolateral nidopallium, and lateral intermediate nidopallium. Of these, the hippocampal formation showed very strong connectivity with CDL, and projection fibers from CDL clearly separated the dorsomedial region of the hippocampal formation into lateral and medial portions. CDL projected directly to the olfactory bulb, but did not receive projections from it. In the diencephalon, CDL received efferents from a dorsal region of the medial part of the anterior dorsolateral nucleus of the thalamus, subrotundal nucleus, and internal paramedian nucleus of the thalamus. These findings suggest that CDL in the pigeon belongs to the limbic pallium and that in some respects it may be comparable to the mammalian cingulate cortex. In contrast, injections of tracers into the pallial surface at or rostral to A 7.00 showed marked differences in the pattern of both anterograde and retrograde labeling from that resulting from injections caudal to A 6.50, thereby indicating the approximate level of transition from CDL to TPO.     

Differential projections of the main and accessory olfactory bulb in the frog.

The central projections of the main olfactory bulb and the accessory olfactory bulb of the adult leopard frog (Rana pipiens) were reexamined, by using a horseradish peroxidase anterograde tracing method that fills axons with a continuous deposit of reaction product. The fine morphology preserved by this method allowed the terminal fields of the projection tracts to be delineated reliably, and for the first time. Herrick's amygdala has been newly subdivided into cortical and medial nuclei on the basis of cytoarchitecture, dendritic morphology, and the differential projections of the main and accessory olfactory tracts. The main olfactory bulb projects through the medial and lateral olfactory tracts to the postolfactory eminence, the rostral end of the medial cortex, the rostral end of the medial septal nucleus, the cortical amygdaloid nucleus, the nucleus of the hemispheric sulcus, and both the dorsal and ventral divisions of the lateral cortex, including its retrobulbar fringe. The lateral olfactory tract overlaps the dorsal edge of the striatal plate along the ventral border of the lateral cortex, but it is not certain whether any striatal cells are postsynaptic to the tract fibers. The lateral cortex is the largest of these territories, and receives the terminals of the main olfactory projection throughout its extent. It extends from the olfactory bulb to the posterior pole, and from the striatum to the summit of the hemisphere, where it borders the dorsal cortex. The medial and lateral olfactory tracts combine in the region of the amygdala to form a part of the stria medullaris thalami. These fibers cross in the habenular commissure and terminate in the contralateral cortical amygdaloid nucleus and periamygdaloid part of the lateral cortex. Cells projecting to the main olfactory bulb are found in the diagonal band and adjacent cell groups, but there is no evidence of an interbulbar projection arising from either the olfactory bulb proper or a putative anterior olfactory nucleus. The accessory olfactory bulb projects through the accessory olfactory tract to the medial and cortical amygdaloid nuclei. A fascicle of the tract crosses in the anterior commissure to terminate in the contralateral amygdala. While the main and accessory olfactory projections may converge in the cortical amygdaloid nucleus, the medial amygdaloid nucleus is connected exclusively with the accessory olfactory bulb.     

Olfactory bulb projections in the bichir, Polypterus

The central projections of the olfactory bulb were studied in Polypterus using the Nauta and Fink-Heimer techniques. Two major target areas were identified in the subpallium: the lateral subpallial nucleus and the dorsal and ventral entopeduncular nuclei. The connections are predominantly, if not exclusively, ipsilateral. In the pallium a massive ipsilateral projection to the superficial third of the medial pallium was demonstrated while the remainder of the pallium was found to be free of degeneration. Thus it appears that the pallium of Polypterus is not uniform throughout, as has been suggested in the literature. This contention is also supported by an analysis of the pallial cytoarchitecture. Because the pallium of Polypterus is everted, rather than inverted and evaginated, the topographically medial pallium is in topological correspondence with the lateral pallium of tetrapods. On the basis of this topological correspondence and the similarity of afference from the olfactory bulb, it is argued that the “medial” pallium of Polypterus is homologous to the pyriform pallium of tetrapods. The findings of this study are compared to those of similar studies in teleosts, which also have an everted pallium. An apparent conflict appears and suggestions for resolving it are offered.     

Organization of the thalamostriatal projections in the rat, with special emphasis on the ventral striatum

The organization of the thalamic projections to the ventral striatum in the rat was studied by placing injections of the retrograde tracer cholera toxin subunit B in the ventral striatum and small deposits of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in individual midline and intralaminar thalamic nuclei. In order to provide a complete map of the midline and intralaminar thalamostriatal projections, PHA-L injections were also made in those parts of the intralaminar nuclei that project to the dorsal striatum. The relationship of thalamic afferent fibres with the compartmental organization of the ventral striatum was assessed by combining PHA-L tracing and enkephalin immunohistochemistry. The various midline and intralaminar thalamic nuclei project to longitudinally oriented striatal sectors. The paraventricular thalamic nucleus sends most of its fibres to medial parts of the nucleus accumbens and the olfactory tubercle, whereas smaller contingents of fibres terminate in the lateral part of the nucleus accumbens and the most ventral, medial, and caudal parts of the caudate-putamen complex. The projections of the parataenial nucleus are directed towards central and ventral parts of the nucleus accumbens and intermediate mediolateral parts of the olfactory tubercle. The intermediodorsal nucleus projects to lateral parts of the nucleus accumbens and the olfactory tubercle and to ventral parts of the caudate-putamen. The projection of the rhomboid nucleus is restricted to the rostrolateral extreme of the striatum. A diffuse projection to the ventral striatum arises from neurons ventral and caudal to the nucleus reuniens rather than from cells inside the nucleus. Fibres from the central medial nucleus terminate centrally and dorsolaterally in the rostral part of the nucleus accumbens and medially in the caudate-putamen. Successively more lateral positions in the caudate-putamen are occupied by fibres from the paracentral and central lateral nuclei, respectively. The lateral part of the parafascicular nucleus projects to the most lateral part of the caudate-putamen, whereas projections from the medial part of this nucleus terminate in the medial part of the caudate-putamen and in the dorsolateral part of the nucleus accumbens. Furthermore, a rostral to caudal gradient in a midline or intralaminar nucleus corresponds to a dorsal to ventral and rostral to caudal gradient in the striatum. In the ventral striatum, thalamic afferent fibres in the "shell" region of the nucleus accumbens avoid areas of high cell density and weak enkephalin immunoreactivity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Connections of the lateral and medial divisions of the goldfish telencephalic pallium

Biotinylated dextran amine and fluorescent carbocyanine dye (DiI) were used to examine connections of the lateral (Dl) and medial (Dm) divisions of the goldfish pallium. Besides numerous intrinsic telencephalic connections to Dl and Dm, major ascending projections to these pallial divisions arise in the preglomerular complex of the posterior tuberculum, rather than in the dorsal thalamus. The rostral subnucleus of the lateral preglomerular nucleus receives auditory input via the medial pretoral nucleus, lateral line input via the ventrolateral toral nucleus, and visual input via the optic tectum, and it projects to both Dl and Dm. The anterior preglomerular nucleus and caudal subnucleus of the lateral preglomerular nucleus receive auditory input via the central toral nucleus and project to Dm. This pallial division also receives chemosensory information via the medial preglomerular nucleus. The central posterior (CP) nucleus, which receives both auditory and visual inputs, also projects to Dm and is the only dorsal thalamic nucleus projecting to the pallium. Thus, both Dl and Dm clearly receive multisensory inputs. Major projections of CP and projections of all other dorsal thalamic nuclei are to the subpallium, however. Descending projections of Dl are primarily to the preoptic area and the caudal hypothalamus, whereas descending projections of Dm are more extensive and particularly heavy to the anterior tuber and nucleus diffusus of the hypothalamus. The topography and connections of Dl are remarkably similar to those of the hippocampus of tetrapods, whereas the topography and connections of Dm are similar to those of the amygdala.     

The connections of the anterior pallium in Pleurodeles waltl and Triturus carnifex: an HRP study.

In order to provide cues about the evolution of the telencephalon in tetrapods, the connections of the anterior pallium were studied in two adult Urodeles, Pleurodeles waltl and Triturus carnifex, by means of the HRP-tracing method. The staining of HRP-immunopositive cell bodies indicates that the pallial regions studied receive afferent projections from the main olfactory bulb and are reciprocally interconnected by intrapallial associative fiber systems. In the ventral hemispheric wall, HRP-immunoreactive perikarya are observed in the pars medialis of the amygdala and in the rostral and caudal striatum. Triturus exhibits a more complex pattern of pallial afferents, including interhemispheric connections and thalamic ascending projections that were not discovered in Pleurodeles. HRP-immunopositive fibers are observed in the dorsal and medial walls of the telencephalon, from the rostral part to the foraminal level. In Triturus, the dorsal fibers extend to the caudal part of the hemisphere. Another group of labelled fibers extends, throughout the lateral and ventral walls, to the most caudal part of the telencephalon, and, through the stria medullaris and the habenular commissure, crosses over to the controlateral hemisphere. These results allow us to specify the basic pattern of the pallial connections in Urodeles and to compare them with data previously obtained in other Amphibians.     

Lateral and medial amygdala of anuran amphibians and their relation to olfactory and vomeronasal information

The amygdala of anurans is currently considered as a complex of nuclei that share many features with their counterparts in amniotes. In the present study, the subdivisions of the amygdala that are directly related to olfactory and vomeronasal information, were investigated in the anurans Rana perezi and Xenopus laevis. In particular, the connectivity of the main and accessory olfactory bulbs and their related amygdaloid nuclei was studied by means of in vivo and in vitro tract-tracing with dextran amines. The projections observed from the main olfactory bulb clearly innervate the newly redefined lateral amygdala within the ventral pallium and, to a lesser extent, the rostral portion of the medial amygdala. Injections into the accessory olfactory bulb exclusively revealed projections to the medial amygdala. Tracer applications into the lateral and medial nuclei revealed abundant intra-amygdaloid connections. The dual flow of olfactory and vomeronasal projections throughout the telencephalon was not strictly segregated since the lateral pallium and the lateral amygdala, both receiving olfactory information, were found to project to the medial amygdala (the only target of vomeronasal information), which in turn projects to the lateral amygdala. Additionally, both the lateral and the medial amygdala strongly project to the hypothalamus through the anuran equivalent of the stria terminalis. The main hodological features found in the present study suggest that forerunners of the olfactory and vomeronasal amygdaloid nuclei can be distinguished in anurans. This supports the notion that all tetrapods share a common pattern of organization of the amygdaloid complex, which links environmental (olfactory/vomeronasal) information and the behavioural response of the animal.