The Phaseolus vulgaris-leucoagglutinin tracing technique for the study of neuronal connections

A taxonomy of the brain's white matter: twenty-one major tracts for the 21st century

The functional and computational properties of brain areas are determined, in large part, by their connectivity profiles. Advances in neuroimaging and network neuroscience allow us to characterize the human brain noninvasively, but a comprehensive understanding of the human brain demands an account of the anatomy of brain connections. Long-range anatomical connections are instantiated by white matter, which itself is organized into tracts. These tracts are often disrupted by central nervous system disorders, and they can be targeted by neuromodulatory interventions, such as deep brain stimulation. Here, we characterized the connections, morphology, traversal, and functions of the major white matter tracts in the brain. There are major discrepancies across different accounts of white matter tract anatomy, hindering our attempts to accurately map the connectivity of the human brain. However, we are often able to clarify the source(s) of these discrepancies through careful consideration of both histological tract-tracing and diffusion-weighted tractography studies. In combination, the advantages and disadvantages of each method permit novel insights into brain connectivity. Ultimately, our synthesis provides an essential reference for neuroscientists and clinicians interested in brain connectivity and anatomy, allowing for the study of the association of white matter's properties with behavior, development, and disorders.

Dietary Plant Lectins Appear to Be Transported from the Gut to Gain Access to and Alter Dopaminergic Neurons of Caenorhabditis elegans, a Potential Etiology of Parkinson's Disease

Lectins from dietary plants have been shown to enhance drug absorption in the gastrointestinal tract of rats, be transported trans-synaptically as shown by tracing of axonal and dendritic paths, and enhance gene delivery. Other carbohydrate-binding protein toxins are known to traverse the gut intact in dogs. Post-feeding rhodamine- or TRITC-tagged dietary lectins, the lectins were tracked from gut to dopaminergic neurons (DAergic-N) in transgenic Caenorhabditis elegans (C. elegans) [egIs1(Pdat-1:GFP)] where the mutant has the green fluorescent protein (GFP) gene fused to a dopamine transport protein gene labeling DAergic-N. The lectins were supplemented along with the food organism Escherichia coli (OP50). Among nine tested rhodamine/TRITC-tagged lectins, four, including Phaseolus vulgaris erythroagglutinin (PHA-E), Bandeiraea simplicifolia (BS-I), Dolichos biflorus agglutinin (DBA), and Arachis hypogaea agglutinin (PNA), appeared to be transported from gut to the GFP-DAergic-N. Griffonia Simplicifolia and PHA-E, reduced the number of GFP-DAergic-N, suggesting a toxic activity. PHA-E, BS-I, Pisum sativum (PSA), and Triticum vulgaris agglutinin (Succinylated) reduced fluorescent intensity of GFP-DAergic-N. PHA-E, PSA, Concanavalin A, and Triticum vulgaris agglutinin decreased the size of GFP-DAergic-N, while BS-I increased neuron size. These observations suggest that dietary plant lectins are transported to and affect DAergic-N in C. elegans, which support Braak and Hawkes' hypothesis, suggesting one alternate potential dietary etiology of Parkinson's disease (PD). A recent Danish study showed that vagotomy resulted in 40% lower incidence of PD over 20 years. Differences in inherited sugar structures of gut and neuronal cell surfaces may make some individuals more susceptible in this conceptual disease etiology model.

Controlling feeding behavior by chemical or gene-directed targeting in the brain: what's so spatial about our methods?

Intracranial chemical injection (ICI) methods have been used to identify the locations in the brain where feeding behavior can be controlled acutely. Scientists conducting ICI studies often document their injection site locations, thereby leaving kernels of valuable location data for others to use to further characterize feeding control circuits. Unfortunately, this rich dataset has not yet been formally contextualized with other published neuroanatomical data. In particular, axonal tracing studies have delineated several neural circuits originating in the same areas where ICI injection feeding-control sites have been documented, but it remains unclear whether these circuits participate in feeding control. Comparing injection sites with other types of location data would require careful anatomical registration between the datasets. Here, a conceptual framework is presented for how such anatomical registration efforts can be performed. For example, by using a simple atlas alignment tool, a hypothalamic locus sensitive to the orexigenic effects of neuropeptide Y (NPY) can be aligned accurately with the locations of neurons labeled by anterograde tracers or those known to express NPY receptors or feeding-related peptides. This approach can also be applied to those intracranial "gene-directed" injection (IGI) methods (e.g., site-specific recombinase methods, RNA expression or interference, optogenetics, and pharmacosynthetics) that involve viral injections to targeted neuronal populations. Spatial alignment efforts can be accelerated if location data from ICI/IGI methods are mapped to stereotaxic brain atlases to allow powerful neuroinformatics tools to overlay different types of data in the same reference space. Atlas-based mapping will be critical for community-based sharing of location data for feeding control circuits, and will accelerate our understanding of structure-function relationships in the brain for mammalian models of obesity and metabolic disorders.

Impaired spatial representation in CA1 after lesion of direct input from entorhinal cortex

Place-specific firing in the hippocampus is determined by path integration-based spatial representations in the grid-cell network of the medial entorhinal cortex. Output from this network is conveyed directly to CA1 of the hippocampus by projections from principal neurons in layer III, but also indirectly by axons from layer II to the dentate gyrus and CA3. The direct pathway is sufficient for spatial firing in CA1, but it is not known whether similar firing can also be supported by the input from CA3. To test this possibility, we made selective lesions in layer III of medial entorhinal cortex by local infusion of the neurotoxin gamma-acetylenic GABA. Firing fields in CA1 became larger and more dispersed after cell loss in layer III, whereas CA3 cells, which receive layer II input, still had sharp firing fields. Thus, the direct projection is necessary for precise spatial firing in the CA1 place cell population.

In vivo tracing of major rat brain pathways using manganese-enhanced magnetic resonance imaging and three-dimensional digital atlasing

The magnetic resonance imaging (MRI)-detectable T1 contrast agent manganese (Mn2+) has recently been introduced as a neural tracer in rodents, birds, and monkeys. We have tested to what extent this in vivo method is useful for three-dimensional (3-D) survey of connectivity patterns in the rat somatosensory system. A commonly available 3 T human clinical MRI scanner was used to trace neural pathways following focal injection of manganese chloride (MnCl2) in the somatosensory cortex. Six to 10 h after MnCl2 injection, we found significant signal enhancement in major projection systems, including corticocortical, corticostriatal, corticothalamic, corticotectal, corticopontine, and corticospinal pathways. To facilitate the assignment of anatomic localization to the observed Mn2+ signal enhancement, we registered the MRI data with a 3-D digital reconstruction of a stereotaxic rat brain atlas. Across-animal comparison using the digital model allowed demonstration of a corticothalamic 3-D topographic organization in agreement with previously published two-dimensional topographic schemes based on classical neural tracing data. We conclude that anterograde MnCl2/MRI tracing allows rapid analysis of topographic organization across multiple brain regions. The method allows a higher data throughput for 3-D studies of large-scale brain connectivity than conventional methods based on tissue sectioning.

Neuroanatomical tracing at high resolution

Most techniques used for the study of the fiber connectivity in the central nervous system produce results which are visualized in the conventional light microscope or fluorescence microscope. Although in some cases this may be sufficient, often proof is necessary that fibers which enter a particular brain area indeed terminate here. Alternatively, it may be necessary to determine whether the axon terminals of traced fibers form synapses with specific processes of specific neurons. With the latter neurons all cellular elements are meant which can be labeled in some way. Evidence of synaptic connectivity necessitates visualization at a higher level of resolution, that is at the electron-microscopic level. In this contribution to the Special Issue we discuss several methods currently available to visualize individual tracers, and methods developed to visualize two different markers, that is one marker attached to a fiber or an axon terminal, and the second marker attached to a presumed pre- or postsynaptic neuronal element.

Neonatal lesion of the rat's frontal cortex and subsequent transplantation of embryonic frontal cortex: evidence of appropriate synaptic integration of the graft neurons within the host thalamo-fronto-striate circuit

Previous observations in intact rats have indicated that axons from the ventrolateral thalamic nucleus (VL) establish direct axo-somatic or axo-dendritic contacts onto frontal cortical neurons projecting to the striatum. The embryonic frontal cortex was grafted into the damaged frontal cortex of newborn rats to study the capacity of homotopic transplants to restore the thalamo-fronto-striate pathway. Several months later, grafted neurons projecting to the striatum were identified by injecting a retrograde neurotracer (subunit b of the cholera toxin) into the ipsilateral caudate putamen. In the same animal, axons and terminations from the VL were labeled within the transplant with an anterograde neurotracer (Phaseolus vulgaris leuco-agglutinin) injected into the ipsilateral VL. The findings show that VL axons establish direct synaptic contacts onto grafted neurons projecting to the striatum. Although the synaptic contacts were scarce in the transplants, their organization was similar to that observed in intact rats. The contacts were axo-somatic or axo-dendritic. Our observations for the first time indicate that synaptic contacts are formed in cortical grafts and that fetal frontal cortex is susceptible to develop appropriate synaptic integration within the host thalamo-fronto-striate system.

Use of peroxidase substrate Vector VIP for multiple staining in light microscopy

The study of the distribution of a fiber input to a particular brain area and the visualization of the anatomical relationships of that input with both projection- and interneurons, requires a triple-staining that allows the unequivocal distinction of each of the three components in one and the same histological section. In this regard, we investigated the properties of a recently introduced peroxidase chromogen, VIP (V-VIP; Vector Labs) in combination with two traditional substrates, standard diaminobenzidine (DAB, brown precipitate) and nickel-enhanced DAB (DAB-Ni, black). In rats, the anterograde tracer biotinylated dextran amine (BDA) and the retrograde tracer fluorogold (FG) were injected in the perirhinal cortex and hippocampus, respectively. Transported BDA was detected with an avidin-biotin-peroxidase complex, whereas the transported FG was detected via a PAP method. Tracing with BDA and FG was combined with parvalbumin- or calbindin-immunocytochemistry. We compared various combinations and staining sequences. The best results were obtained with a staining sequence comprising first the BDA stain with DAB-Ni as chromogen, second the FG protocol with the chromogen DAB and finally, parvalbumin- or calbinding-immunocytochemistry using the chromogen V-VIP. The order with which the chromogens were applied appeared to be critical. Partial or even total loss of V-VIP reaction product has been observed after standard dehydration in ethanol. As an alternative, a quick dehydration procedure in toluene yields much better staining. Colour separation is excellent and the sensitivity is high. This procedure may also be used for detection of any other combination of three different labels, taking the usual care to avoid cross-reactivity between antibodies.

Lateral nucleus of the rat amygdala is reciprocally connected with basal and accessory basal nuclei: a light and electron microscopic study

Information flow within the intra-amygdaloid circuitry has been generally believed to be unidirectional rather than reciprocal, in which case sensory inputs entering the amygdala via the lateral nucleus would not be modulated by inputs from other amygdaloid regions. In the present study we extend our earlier findings which indicated that the lateral nucleus of the rat amygdala is reciprocally connected with the basal and accessory basal nuclei. The type of synaptic contacts made by these connections is also characterized at the ultrastructural level. An anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into the basal (n=22) or accessory basal nuclei (n=12) of the rat amygdala. The results demonstrate that the ventrolateral division of the lateral nucleus receives projections from the basal nucleus, while the medial division receives projections from the accessory basal nucleus. Electron microscopic analyses revealed that axons projecting from the basal nucleus formed both asymmetric and symmetric contacts within the ventrolateral division of the lateral nucleus, whereas axons projecting from the accessory basal nucleus to the medial division of the lateral nucleus formed only asymmetric synapses with their targets. These findings suggest that the lateral nucleus receives both inhibitory and excitatory intra-amygdaloid projections and indicate that information flow within the amygdala is not unidirectional as previously thought. The results of this study provide evidence that the early phase of sensory processing within the amygdala is already modified by inputs from other amygdaloid nuclei.

Ventral striatopallidothalamic projection: IV. Relative involvements of neurochemically distinct subterritories in the ventral pallidum and adjacent parts of the rostroventral forebrain

Retrograde and anterograde tract-tracing studies were carried out to determine whether the capacity of the nucleus accumbens to influence the thalamic mediodorsal nucleus via ventral striatopallidothalamic connections disproportionately favors the shell over the core subterritory. After injections of Fluoro-Gold into the mediodorsal thalamic nucleus, retrogradely labeled neurons were detected in sections also processed for calbindin-D 28-kD and neurotensin immunoreactivities to facilitate identification of subterritories in the ventral pallidum. Fluoro-Gold-labeled cells were counted in series of sections cut through the ventral pallidum, rostral globus pallidus, nucleus of the vertical limb of the diagonal band, preoptic region, lateral hypothalamus, and the sublenticular gray region, including parts of the extended amygdala. Data were expressed as cells/unit area and as percentages of all labeled forebrain cells. Mediodorsal nucleus-projecting rostroventral forebrain neurons were most numerous in the ventromedial part of the subcommissural ventral pallidum and pallidal parts of the olfactory tubercle. Few were observed in the dorsolateral part of the subcommissural ventral pallidum. In addition, following injections into the ventral pallidum, anterogradely transported biotinylated dextran amine was evaluated in sections processed for calbindin or tyrosine hydroxylase immunoreactivities. Injection into the ventromedial part of the subcommissural ventral pallidum resulted in robust anterograde labeling of the medial segment of the mediodorsal nucleus and ventral tegmental area and weak labeling of the substantia nigra and subthalamic nucleus. Conversely, after injection into the dorsolateral part of the subcommissural ventral pallidum, anterograde labeling was weak in the mediodorsal nucleus and ventral tegmental area, but robust in the substantia nigra and subthalamic nucleus. The results are consistent with a predominant accumbens shell influence on the mediodorsal nucleus and with cortico-ventral striatopallidal-thalamocortical pathways that begin and end in different parts of the frontal lobe.

Efferent connections of the lateral cortex of the lizard Gekko gecko: evidence for separate origins of medial and lateral pathways from the lateral cortex to the hypothalamus

The lateral cortex of the lizard Gekko gecko is composed of three parts: a dorsal and ventral part located rostrally and a posterior part located caudally. In order to obtain detailed information about the efferent connections of these lateral cortex subdivisions, iontophoretic injections of the anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran were made in the various parts. The main projection from the dorsal part terminates in the caudal part of the medial cortex. Other cortical projections were noted to the ipsi- and contralateral lateral cortex, the large-celled part of the medial cortex, and the dorsal cortex. Additional fibers were found bilaterally in the anterior olfactory nucleus and the external amygdaloid nucleus. The ventral part of the lateral cortex projects mainly to the ipsilateral, posterior part of the dorsal ventricular ridge and the external amygdaloid nucleus. Minor contralateral projections to these nuclei were also found. Other projections were observed to travel to the caudal part of the medial cortex, to the nucleus sphericus, and bilaterally to the lateral cortex and the anterior olfactory nucleus. The posterior part of the lateral cortex has similar efferent connections as the dorsal part and should be regarded as the caudal continuation of the dorsal part. Because previous studies have shown that the medial cortex and the amygdaloid complex project to different hypothalamic areas, we conclude that the dorsal and ventral parts of the lateral cortex transmit olfactory information to separate hypothalamic areas that are probably involved with different types of behavior.

Differential labeling of converging afferent pathways using biotinylated dextran amine and cholera toxin subunit B

We report a new technique for 2-tracer anterograde labeling that permits unequivocal identification of the differentially labeled projections in the same section. One pathway is labeled with biotinylated dextran amine and is visualized as a black to dark gray diaminobenzidine (DAB)-cobalt precipitate by an avidin-biotinylated peroxidase reaction. The other pathway is labeled with cholera toxin subunit B and is visualized as a reddish-brown reaction product using DAB without cobalt as the substrate for peroxidase immunohistochemistry. To maintain serial order, sections can be processed mounted on slides without any loss of sensitivity for either tracer.

Multiple anterograde tracing, combining Phaseolus vulgaris leucoagglutinin with rhodamine- and biotin-conjugated dextran amine

The simultaneous use of different neuroanatomical anterograde tracers provides a potentially powerful method to study the convergence of afferent systems in a particular brain area. However, a simple routine procedure to apply multiple anterograde tracers in conjunction with their simultaneous visualization is still missing. We report an easy and straightforward application of three sensitive anterograde tracers: Phaseolus vulgaris leucoagglutinin (PHA-L), rhodamine-conjugated dextran amine (RDA) and biotin-conjugated dextran amine (BDA). These tracers can be visualized simultaneously and permanently through a triple-staining procedure with nickel-enhanced diaminobenzidine (DAB-Ni), DAB and 1-naphthol/Azur B as chromogens. Our test model comprised the projections from the nucleus reuniens thalami and entorhinal cortex. Both projection systems show a high degree of overlap in their terminal fields in the hippocampus. Two tracers were injected in the left and right entorhinal cortex, respectively; a third tracer was injected in the nucleus reuniens. This combination of injections provided a good opportunity to compare the three tracers in one and the same animal. PHA-L, RDA and BDA, injected in either of the injection sites, turned out to be equally sensitive and revealed the morphology of the involved projection systems in great detail. The triple-staining protocol yielded an excellent, simultaneous detectability of the three tracers with a remarkably low background level. Thus, the combination of the anterograde tracers PHA-L, RDA and BDA, in conjunction with the triple-staining procedure, offers a very attractive approach for neuroanatomical research.

The anterograde neuroanatomical tracer biotinylated dextran-amine: comparison with the tracer Phaseolus vulgaris-leucoagglutinin in preparations for electron microscopy

We investigated the properties of biotinylated dextran-amine (BDA) as a neuroanatomical tracer at the electron microscopic level and we compared the results with those obtained previously with another tracer, the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L). BDA was injected into various brain areas of rats. Following survival and fixation, vibratome sections were cut, subjected to a freeze-thaw treatment, and incubated overnight with avidin-biotin complex (ABC). Following reaction with diaminobenzidine (DAB)-hydrogen peroxide, the sections were processed for electron microscopy. In the electron microscope we observed that the reaction product occurred in the cytoplasm of cell bodies and in the matrices of dendrites, axons and axon terminals following ABC histochemistry of BDA-containing brain sections. The ultrastructural details of BDA-labelled neurones were generally better preserved than in PHA-L-labelled material, whereas at the same time penetration of the reagent into the sections was complete (incomplete in sections of PHA-L material). We conclude that the use of BDA as a neuroanatomical tracer in electron microscopy is a good substitute for PHA-L. The detection method for transported BDA is much faster and less complicated than that for PHA-L, while the results are better; that is, there is improved penetration coinciding with good preservation of ultrastructure. Keeping the limitations of BDA as a neuroanatomical tracer in mind, e.g., retrograde transport into local collaterals of axons that intermingle with anterogradely labelled axons, BDA seems well suited as a neuroanatomical tracer for electron microscopy.

Medial cortex of the lizard Gekko gecko: a hodological study with emphasis on regional specialization

There is increasing evidence that the archicortex in mammals and reptiles is not a homogeneous structure. However, little is known about the regional specialization of this cortical area in reptiles. Therefore, the efferent connections of the medial cortex of the lizard Gekko gecko were studied with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The small-celled part of the medial cortex (Cxms) projects to various parts of the septum in a topological way: the rostral part projects to the anterior septal nucleus, whereas the caudal part projects to the lateral septal nucleus and the nucleus septi impar. In addition, Cxms projects to the large-celled part of the medial cortex (Cxml). Axons that originate from the dorsal part of Cxms terminate at the proximal parts of the apical and basal dendrites of the neurons of Cxml caudal to the injection site. In contrast, fibers originating from the ventral part terminate on more distal parts of the dendrites of neurons of Cxml rostral to the injection site. Other projections from Cxms to the dorsal cortex (Cxd) and the external amygdaloid nucleus were found. The Cxml projects bilaterally to Cxms. These projections terminate in the superficial and deep plexiform layers. In addition, projections to the cell plate of Unger, Cxd, and to the lateral septal nucleus were found. It appears, on the basis of the efferent connections, that Cxms can be divided into a rostral and caudal part, while hodological differences also exist between the dorsal and ventral parts of Cxms. The results of the present study do not suggest a subdivision of Cxml. The regional variations of the medial cortex in the lizard Gekko gecko differ from the regional variations described in other reptilian species.

Restricted origin and distribution of projections from the lateral to the medial septal complex in rat and guinea pig

The origin and distribution of projections from the lateral to the medial septal complex were studied at the light- and electron-microscopical level in the rat and the guinea pig, with the use of sensitive anterograde tracing techniques. Injections in the lateral septal complex resulted in only weak to moderate labeling in the medial septal nucleus. In contrast, injections in all but the ventral subdivision of the lateral septal complex labeled restricted terminal arborizations in the angular portions of the vertical diagonal band. Analyses at the electron-microscopical level indicated that these fibers form both symmetrical and asymmetrical synapses with dendrites and somata, but not with axons. These findings are discussed in the light of a presumed functional circuit from the hippocampal formation, via the lateral septal complex, to cells in the medial septal complex that originate projections to the hippocampal formation.