Evaluation of a radioautographic neuroanatomical tracing method

Fluorescent Molecules That Help Reveal Previously Unidentified Neural Connections in Adult, Neonatal and Peripubertal Mammals

One hundred and twenty-five years ago there was a lively discussion between Hungarian and Spanish neuroscientists on the nature of neural connections. The question was whether the neurofibrils run from one neuron to the next and connect neurons as a continuous network or the fibrils form an internal skeleton in the neurons and do not leave the cell; however, there is close contact between the neurons. About 50 years later, the invention of the electron microscope solved the problem. Close contacts between individual neurons were identified and named as synapses. In the following years, the need arose to explore distant connections between neuronal structures. Tracing techniques entered neuroscience. There are three major groups of tracers: (A) non-transsynaptic tracers used to find direct connections between two neuronal structures; (B) tracers passing gap junctions; (C) transsynaptic tracers passing synapses that are suitable to explore multineuronal circuits. According to the direction of the transport mechanism, the tracer may be ante- or retrograde. In this review, we focus on the ever-increasing number of fluorescent tracers that we have also used in our studies. The advantage of the use of these molecules is that the fluorescence of the tracer can be seen in histological sections without any other processes. Genes encoding fluorescent molecules can be inserted in various neuropeptide or neurotransmitter expressing transcriptomes. This makes it possible to study the anatomy, development or functional relations of these neuronal networks in transgenic animals.

Can transsynaptic viral strategies be used to reveal functional aspects of neural circuitry?

Viruses have proved instrumental to elucidating neuronal connectivity relationships in a variety of organisms. Recent advances in genetic technologies have facilitated analysis of neurons directly connected to a defined starter population. These advances have also made viral transneuronal mapping available to the broader neuroscience community, where one-step rabies virus mapping has become routine. This method is commonly used to identify inputs onto defined cell populations, to demonstrate the quantitative proportion of inputs coming from specific brain regions, or to compare input patterns between two or more cell populations. Furthermore, the number of inputs labeled is often assumed to reflect the number of synaptic connections, and these viruses are commonly believed to label strong synapses more efficiently than weak synapses. While these maps are often interpreted to provide a quantitative estimate of the synaptic landscape onto starter cell populations, in fact very little is known about how transneuronal transmission takes place. We do not know how these viruses transmit between neurons, if they display biases in the cell types labeled, or even if transmission is synapse-specific. In this review, we discuss the experimental evidence against or in support of key concepts in viral tracing, focusing mostly on the use of one-step rabies input mapping and related methods. Does spread of these viruses occur specifically through synaptic connections, preferentially through synapses, or non-specifically? How efficient is viral transneuronal transmission, and is this efficiency equal in all cell types? And lastly, to what extent does viral labeling reflect functional connectivity?

Viral Vectors for Neural Circuit Mapping and Recent Advances in Trans-synaptic Anterograde Tracers

Viral tracers are important tools for neuroanatomical mapping and genetic payload delivery. Genetically modified viruses allow for cell-type-specific targeting and overcome many limitations of non-viral tracers. Here, we summarize the viruses that have been developed for neural circuit mapping, and we provide a primer on currently applied anterograde and retrograde viral tracers with practical guidance on experimental uses. We also discuss and highlight key technical and conceptual considerations for developing new safer and more effective anterograde trans-synaptic viral vectors for neural circuit analysis in multiple species.

Neuroanatomy goes viral!

The nervous system is complex not simply because of the enormous number of neurons it contains but by virtue of the specificity with which they are connected. Unraveling this specificity is the task of neuroanatomy. In this endeavor, neuroanatomists have traditionally exploited an impressive array of tools ranging from the Golgi method to electron microscopy. An ideal method for studying anatomy would label neurons that are interconnected, and, in addition, allow expression of foreign genes in these neurons. Fortuitously, nature has already partially developed such a method in the form of neurotropic viruses, which have evolved to deliver their genetic material between synaptically connected neurons while largely eluding glia and the immune system. While these characteristics make some of these viruses a threat to human health, simple modifications allow them to be used in controlled experimental settings, thus enabling neuroanatomists to trace multi-synaptic connections within and across brain regions. Wild-type neurotropic viruses, such as rabies and alpha-herpes virus, have already contributed greatly to our understanding of brain connectivity, and modern molecular techniques have enabled the construction of recombinant forms of these and other viruses. These newly engineered reagents are particularly useful, as they can target genetically defined populations of neurons, spread only one synapse to either inputs or outputs, and carry instructions by which the targeted neurons can be made to express exogenous proteins, such as calcium sensors or light-sensitive ion channels, that can be used to study neuronal function. In this review, we address these uniquely powerful features of the viruses already in the neuroanatomist's toolbox, as well as the aspects of their biology that currently limit their utility. Based on the latter, we consider strategies for improving viral tracing methods by reducing toxicity, improving control of transsynaptic spread, and extending the range of species that can be studied.

Knowledge engineering tools for reasoning with scientific observations and interpretations: a neural connectivity use case

Background

We address the goal of curating observations from published experiments in a generalizable form; reasoning over these observations to generate interpretations and then querying this interpreted knowledge to supply the supporting evidence. We present web-application software as part of the 'BioScholar' project (R01-GM083871) that fully instantiates this process for a well-defined domain: using tract-tracing experiments to study the neural connectivity of the rat brain.

Results

The main contribution of this work is to provide the first instantiation of a knowledge representation for experimental observations called 'Knowledge Engineering from Experimental Design' (KEfED) based on experimental variables and their interdependencies. The software has three parts: (a) the KEfED model editor - a design editor for creating KEfED models by drawing a flow diagram of an experimental protocol; (b) the KEfED data interface - a spreadsheet-like tool that permits users to enter experimental data pertaining to a specific model; (c) a 'neural connection matrix' interface that presents neural connectivity as a table of ordinal connection strengths representing the interpretations of tract-tracing data. This tool also allows the user to view experimental evidence pertaining to a specific connection. BioScholar is built in Flex 3.5. It uses Persevere (a noSQL database) as a flexible data store and PowerLoom^® (a mature First Order Logic reasoning system) to execute queries using spatial reasoning over the BAMS neuroanatomical ontology.

Conclusions

We first introduce the KEfED approach as a general approach and describe its possible role as a way of introducing structured reasoning into models of argumentation within new models of scientific publication. We then describe the design and implementation of our example application: the BioScholar software. This is presented as a possible biocuration interface and supplementary reasoning toolkit for a larger, more specialized bioinformatics system: the Brain Architecture Management System (BAMS).

Knowledge management of the neuroscientific literature: the data model and underlying strategy of the NeuroScholar system

This paper describes the underlying strategy and system's design of a knowledge management system for the neuroscientific literature called 'NeuroScholar'. The problem that the system is designed to address is to delineate fully the neural circuitry involved in a specific behaviour. The use of this system provides experimental neuroscientists with a new method of building computational models ('knowledge models') of the contents of the published literature. These models may provide input for analysis (conceptual or computational), or be used as constraint sets for conventional neural modelling work. The underlying problems inherent in this approach, the general framework for the proposed solution, the practical issues concerning usage of the system and a detailed, technical account of the system are described. The author uses a widely used software specification language (the Universal Modelling Language) to describe the design of the system and present examples from published work concerned with classical eyeblink conditioning in the rabbit.

Changing views of brain evolution

Although brain studies began in ancient Egypt, speculations on vertebrate brain evolution occurred only much later, after the publication of Darwin's Origin of Species in 1859. Subsequently, views of brain evolution have been shaped by a complex interplay of theory and technique. Darwin's theory allowed the variation in brain size and complexity to be re-interpreted within an evolutionary context, albeit an erroneous pre-Darwinian context based on scala naturae. With the development of histological techniques, research shifted to descriptions of cellular structure, cellular aggregates and their putative interconnections. In spite of these technical advances, brain evolution continued to be viewed within the context of scala naturae. Following the publication of The Comparative Anatomy of the Nervous System of Vertebrates by Ariëns Kappers, Huber, and Crosby in 1936, there followed a period of stasis, after which biological views of evolution were radically altered by the confluence of genetics, paleontology, and systematics, termed the Evolutionary Synthesis. Against this background, the development of new experimental techniques for establishing neural connections resulted in a new flowering of comparative neuroanatomy. While comparative descriptive and experimental studies of brain organization continue, the rapprochement of embryology and genetics is fueling a new renaissance that promises to increase our understanding of brain evolution and its genetic basis.

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.

Branching and/or collateral projections of spinal dorsal horn neurons

Branching and/or collateral projections of spinal dorsal horn neurons is a common phenomenon. Evidence is presented for the existence of STTm/STTl, STTc/STTi, STT/SMT, STT/SRT, SCT/DCPS, SST/DCPS, SCT/SST, STT/SHT, STeT/SHT, STeTs and other doubly or multiply projecting spinal neurons that have been anatomically and physiologically identified and named based on the locations of the cells of origin and their terminations in the brain. These newly discovered spinal projection neurons are characterized by a single cell body and branched axons and/or collaterals that project to two or more target areas in the brain. These novel populations of neurons seem to be a fuzzy set of spinal projection neurons that function as an intersection set of the corresponding single projection spinal neurons and to be at an intermediate stage phylogenetically. Identification strategies are discussed, and general concluding remarks are made in this review.

An easily constructed pipette for pressure microinjections into the brain

A simple device for making pressure microinjections into the brain, its application for delivery of a tracer substance, biotin dextran amine, and an example of the resulting axonal transport are described. The device is based on the use of a Luer Up Hamilton microliter syringe mated directly to the plastic hub of an injection needle assembly in which the metal needle has been replaced by a glass pipette. In model experiments, the injection sites were measured in brain sections of rats, which were perfused with fixative immediately after administration of the tracer, and the relationship between the injected volume and the area of the injection site was evaluated. The results showed that the device provided accurate and reproducible delivery of the fluid.

Time-related changes in the labeling pattern of motor and sensory neurons innervating the gastrocnemius muscle, as revealed by the retrograde transport of the cholera toxin B subunit

Morphological changes in the motor and sensory neurons in the lumbar spinal cord and the dorsal root ganglia were investigated at different survival times following the injection of the B subunit of cholera toxin (CTB) into the medial gastrocnemius muscle. Unconjugated CTB, visualized immunohistochemically, was found to be retrogradely transported through ventral and dorsal roots to motor neurons in the anterior horn, each lamina in the posterior horn, and ganglion cells in the dorsal root ganglia at L3-L6. The largest numbers of labeled motor neurons and ganglion cells were observed 72 h after the injection of CTB. Thereafter, labeled ganglion cells were significantly decreased in number, whereas the amount of labeled motor neurons showed a slight reduction. Motor neurons had extensive dendritic trees filled with CTB, reaching lamina VII and even the pia mater of the lateral funiculus. Labeling was also seen in the posterior horn, but the central and medial parts of laminae II and III had the most extensively labeled varicose fibers, the origin of which was the dorsal root ganglion cells. The results indicate that CTB is taken up by nerve terminals and can serve as a sensitive retrogradely transported marker for identifying neurons that innervate a specific muscle.

Estradiol-concentrating forebrain and midbrain neurons project directly to the medulla

The location and number of estradiol (E2)-concentrating neurons afferent to the dorsal medulla were determined by combining retrograde fluorescent tract tracing with steroid hormone autoradiography. Injections of Fluro-Gold were made into the medulla of 80 day old, ovariectomized, and adrenalectomized female rats. After 7 days survival to allow for retrograde transport, females were injected with [3H]estradiol; they were then perfused and their brains processed for steroid hormone autoradiography. Following a 4-12 month exposure period, autoradiograms were developed and microscopically analyzed for the presence of E2-concentrating neurons that project to the medulla. Numerous E2-concentrating neurons were identified that send axons directly to the medulla; the majority were found in the bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, central nucleus of the amygdala, and the central gray. Of the E2-concentrating neurons in the bed nucleus of the stria terminalis, 12.7% also projected to the medulla. E2-concentrating neurons that sent axons to the medulla were also identified in and ventromedial to the lateral parvicellular subdivision in the caudal half of the paraventricular nucleus of the hypothalamus (69.4%). Over one-third of the E2-concentrating neurons found in the central nucleus of the amygdala coursed to the medulla. The central gray was the only mesencephalic brain region that contained E2-concentrating neurons that projected to the medulla (41.9%). The medulla-bound E2-concentrating forebrain and midbrain neurons identified in the present study may influence autonomic tone via direct projections.

New perspectives on the functional anatomical organization of the basolateral amygdala

We have examined the functional anatomical organization of the basolateral amygdaloid nucleus (BL) in the rat and guinea pig using combined light and electron microscopic methods. Afferent and efferent connections as well as the internal organization of the BL have been studied with combined tracing, immunohistochemical, and Golgi techniques. We have found that the BL receives an intense cholinergic innervation from the ventral forebrain cholinergic system and, for the first time, described a group of intrinsic cholinergic neurons in the BL. The innervation from the primary olfactory cortex and the thalamus, as well as the GABAergic innervation of the amygdalostriatal projection neurons, is also described. Electron microscopic analyses have shown that the cholinergic system as well as the thalamic afferents primarily innervate the distal dendritic arbor of the projection neurons in the BL, whereas the GABAergic fibers are directed primarily towards their soma and proximal dendrites. Correlated light and electron microscopic studies have revealed that the projection neurons in the BL share many features with pyramidal and spiny stellate cells in the cerebral cortex. The ultrastructural characteristics of the afferent fiber systems and of the non-projection neurons in the BL are also reminiscent of the situation in the cerebral cortex. The observations reported in this study lend further support to the concept of a cortical-like organization of the BL. The anatomical observations of the BL are discussed particularly in relation to three major forebrain systems: 1. the ventral striatopallidal system, 2. the continuum formed by the centromedial amygdala, the substantia innominata and the bed nucleus of the stria terminalis, and 3. the cholinergic ventral forebrain system. The clinical implications of the results obtained in this series of experimental studies are discussed in relation to Alzheimer's disease and complex partial seizures. The cholinergic system, in particular, has attracted much interest in relation to senile dementia of Alzheimer's type (SDAT), which often seems to be characterized by disruption of the ventral forebrain cholinergic projection system. We have found that the cholinergic innervation of the BL is often significantly reduced in SDAT, but interestingly enough, the areas of the basolateral amygdala with the highest content of cholinergic markers contain the smallest numbers of senile plaques.

Brainstem projections to spinal motoneurons: an update

1. The existence of direct projections to spinal motoneurons and interneurons from the raphe pallidus and obscurus, the adjoining ventral medial reticular formation and the locus coeruleus and subcoeruleus is now well substantiated by various anatomical techniques. 2. The spinal projections from the raphe nuclei and the adjoining medial reticular formation contain serotonergic and non-serotonergic fibres. These projections also contain various peptides, several of which are contained within the serotonergic fibres. Whether still other transmitter substances (e.g. acetylcholine) are present in the various descending brainstem projections to motoneurons remains to be determined. 3. The spinal projections from the locus coeruleus and subcoeruleus are mainly noradrenergic, but there also exists a non-noradrenergic spinal projection. 4. Pharmacological, physiological and behavioural studies indicate an overall facilitatory action of noradrenaline and serotonin (including several peptides) on motoneurons. This may lead to an enhanced susceptibility for excitatory inputs from other sources. 5. The brainstem areas in question receive an important projection from several components of the limbic system. This suggests that the emotional brain can exert a powerful influence on all regions of the spinal cord and may thus control both its sensory input and motor output.

Brainstem projections to lumbar motoneurons in rat--I. An ultrastructural study using autoradiography and the combination of autoradiography and horseradish peroxidase histochemistry

In 11 rats the descending projections from the ventrolateral medullary medial reticular formation, the medullary raphe nuclei and the area of the nucleus coeruleus and subcoeruleus to lumbar motoneuronal cell groups were studied by means of electron microscopical autoradiography after [3H]leucine injections in the respective brainstem areas. The distribution of the transported radioactivity in the autoradiographs was determined using the circle method [Williams (1977), in Practical Methods in Electron Microscopy, Vol. 6, pp. 85-173] which showed that the vast majority of the silver grains was located over terminal profiles. In the motoneuronal cell groups six different types of terminals were distinguished. After injections in the ventrolateral medial reticular formation the majority of the silver grains was located over F-type terminal profiles while many fewer silver grains were found over S- and G-types. After injections in the raphe nuclei and the adjoining medial reticular formation approximately equal numbers of silver grains were found over F- and G-type terminals while fewer were found over S-type. A small proportion of silver grains was present over C-type terminals and only after injections in the ventrolateral medial reticular formation. After [3H]leucine injections in the area of the nucleus coeruleus and subcoeruleus the majority of silver grains were located over E- and S-type terminals whereas relatively few were located over F-type terminals. The E-type terminal, which has not been described before in the motoneuronal cell groups, is characterized by the fact that it contains relatively small vesicles and occasionally elongated or canaliculi-like structures. In the three groups of experiments approximately 40-50% of the labelled S- and F-type terminal profiles established synaptic contacts, but only approximately 10% of the labelled E- and G-type terminal profiles did so. In all cases these synaptic contacts were established mainly with proximal dendrites. In the autoradiographs some of the silver grains were concentrated into clusters. The vast majority of these clusters, consisting of six or more silver grains, were centred over terminal profiles. The differential distribution of these clusters over the different types of terminal profiles in the various experiments was roughly the same as found by means of the circle method. In two rats [3H]leucine injections in the ventrolateral medial reticular formation were combined with horseradish peroxidase injections in the ipsilateral hindleg muscles, resulting in retrograde labelling of the corresponding motoneurons as visualized by means of the tetramethyl benzidine incubation method.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoradiographic study on the distribution of vagal afferent nerve fibers in the gastroduodenal wall of the rabbit

Following unilateral supranodose vagotomy which eliminated vagal efferent fibers, [3H]leucine was injected into the ipsilateral no-dose ganglion of the rabbit, which was allowed to survive for 10 days. Autoradiographic examination of the distribution of vagal afferent fibers in the gastroduodenal wall revealed many nerve bundles of labeled afferent fibers present in the subserous plexus between the serosa and the muscle coat, where they branched and descended into the muscle coat. Some of the fibers appeared to interact with some myenteric neurons and muscle fibers in a manner suggesting that the afferent fibers may make synaptic contact with the enteric neurons and innervate or attach to the muscle fibers. Furthermore, the afferent fibers were observed in the submucosal plexus between the muscle coat and the muscularis mucosae. In the mucosa the afferent nerve branched in filaments containing one or several afferent fibers extending from the muscularis mucosae through the mucous lamina propria and to the mucous membrane composed of epithelial cells. It was speculated that the afferent fibers terminated as free endings near the mucous membrane.

Two rapid methods of counterstaining fluorescent dye tracer containing sections without reducing the fluorescence

A method is described for counterstaining neural tissue containing cells that are retrogradely labeled by fluorescent dyes or horseradish peroxidase (HRP). Specifically, protocols are detailed for the combined use of the tracers with Methylene blue for a Nissl stain or with silver methods for the detection of acetylcholine esterase. The usefulness of these techniques is evaluated in relation to cortico-cortico and thalamocortico projections. The findings indicate that the methods do not mask the labeling of the most sensitive fluorescent dyes or by HRP. Only the yellow dyes are significantly affected by the Methylene blue counterstain. Further, Fast blue labeling in neurons is not significantly diminished by the Bodian fiber stain. The effect of coverslipping sections containing fluorescent dye labeled cells also was evaluated and found to significantly extend the life of the labeling while not reducing the sensitivity. Thus the two counterstaining techniques provide excellent structural information, do not seriously affect tracer labeling and have few of the disadvantages common to other counterstaining methods.

Afferent projections to the orbicularis oculi motoneuronal cell group. An autoradiographical tracing study in the cat

The motoneurons innervating the orbicularis oculi muscle from a subgroup within the facial nucleus, called the intermediate facial subnucleus. This makes it possible to study afferents to these motoneurons by means of autoradiographical tracing techniques. Many different injections were made in the brainstem and diencephalon and the afferent projections to the intermediate facial subnucleus were studied. The results indicated that these afferents were derived from the following brainstem areas: the dorsal red nucleus and the mesencephalic tegmentum dorsal to it; the olivary pretectal nucleus and/or the nucleus of the optic tract; the dorsolateral pontine tegmentum (parabrachial nuclei and nucleus of Kölliker-Fuse) and principal trigeminal nucleus; the ventrolateral pontine tegmentum at the level of the motor trigeminal nucleus; the caudal medullary medial tegmentum; the lateral tegmentum at the level of the rostral pole of the hypoglossal nucleus and the ventral part of the trigeminal nucleus and the nucleus raphe pallidus and caudal raphe magnus including the adjoining medullary tegmentum. These latter projections probably belong to a general motoneuronal control system. The mesencephalic projections are mainly contralateral, the caudal pontine and upper medullary lateral tegmental projections are mainly ipsilateral and the caudal medullary projections are bilateral. It is suggested that the different afferent pathways subserve different functions of the orbicularis oculi motoneurons. Interneurons in the dorsolateral pontine and lateral medullary tegmentum may serve as relay for cortical and limbic influences on the orbicularis oculi musculature, while interneurons in the ventrolateral pontine and caudal medullary tegmentum may take part in the neuronal organization of the blink reflex.

The neostriatal mosaic. I. Compartmental organization of projections from the striatum to the substantia nigra in the rat

Combined neuroanatomical techniques were used to examine the organization of the striatal projection to the substantia nigra in the rat. Both double anterograde axonal tracing methods (Phaseolus vulgaris leuco-agglutinin (PHA-L) and 3H-amino acid tract tracing) and double fluorescent retrograde axonal transport tracing methods were used to examine the relationship among striatal neurons projecting to separate areas of the substantia nigra. Additionally, the distributions of retrogradely labeled striatonigral projection neurons were charted relative to the neurochemically distinct striatal "patch" compartment, identified by substance P- or leu-enkephalin-like immunoreactivity, and the complementary "matrix" compartment, identified by somatostatin-like immunoreactive fibers. These studies show two distinct types of organization in the striatonigral projections. One type is topographic in that the mediolateral relationships among these striatal efferent neurons are roughly maintained by their termination patterns in the substantia nigra, while the dorsoventral relationships are inverted. Projections from any part of the striatum, however, are distributed throughout the rostrocaudal axis of the substantia nigra. Despite their general topographic organization, the variable and dispersed nature of such projections from individual striatal loci results in partial overlap of afferent fields from separate striatal areas. The second type of organization is nontopographic and provides a different system for convergence of inputs from separated striatal areas that is superimposed on the rough topographic system. In this other projection system the mediolateral and dorsoventral relationships typical of the topographically ordered system are not maintained and are sometimes reversed. For example, PHA-L injected into the dorsal striatum labels a topographic (inverted relationship) projection to the ventral substantia nigra pars reticulata but also a smaller and separate projection to the dorsal pars reticulata and adjacent pars compacta. Retrograde tracer deposits in the pars compacta label neurons in the ventral striatum (the inverted relationship) but also clusters of neurons in the dorsal striatum. These clusters are in the neurochemically defined patch compartment whereas neurons in the matrix are labeled by injections into the pars reticulata. The dendrites of both retrogradely filled patch and matrix neurons are confined to the compartment containing their cell bodies, suggesting a restriction that would functionally segregate extrinsic striatal afferents shown in other studies to be confined to either patches or matrix.(ABSTRACT TRUNCATED AT 400 WORDS)

Transneuronal transport of wheatgerm agglutinin conjugated with horseradish peroxidase in the somatosensory system of the rat: a light- and electron-microscopic study

Experiments were performed in order to investigate, at the light- and electron-microscopic levels, the transneuronal transport of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA:HRP) in the somatosensory system of rats. In five adult albino rats, various amounts of WGA:HRP at different concentrations were bilaterally injected in the dorsal column nuclei (DCN). In one additional animal, WGA:HRP was injected in one side, whereas free HRP was injected in the contralateral DCN. In another five rats, WGA:HRP was injected in the first somatosensory cortex (SI). The postinjection survival time of the animals ranged from 30 to 48 hr. The histochemical visualization of the enzyme was performed using either paraphenylenediamine-pyrocatechol (PPD-PC) or tetraethylbenzidine (TMB) as chromogens on adjacent horizontal serial sections. All the reacted samples were studied at the light-microscopic level, and sections from four animals were processed for the ultrastructural investigation. After DCN injections, a massive anterograde labeling was always observed in nucleus ventralis posterolateralis (VPL) of the thalamus, where also labeled neurons and glial cells were detected at both the light- and the electron-microscopic levels. Labeled neurons and terminals in the lateral border of nucleus reticularis (Re) of the thalamus were also observed after either DCN or SI injection of WGA:HRP. Our results show that WGA:HRP is effectively transported not only anterogradely and retrogradely through the somatosensory system of the rat, but also transneuronally. The transneuronal transfer of the tracer seems to be mainly related to the postlabeling survival time of the animal, and it does not occur when free HRP is injected. In controlled experimental conditions, WGA:HRP can therefore be useful for tracing secondary projections. Moreover, in the present report, the existence of a mediolateral arrangement of the projections of the somatosensory-related area of Re toward VPL is directly demonstrated. As for the histochemical procedure employed, differences in sensitivity are shown between PPD-PC and TMB, although the same general pattern of labeling is present with both chromogens.

A method for specific transmitter identification of retrogradely labeled neurons: immunofluorescence combined with fluorescence tracing

In the present article a method is described which allows the delineation of the projections of a single neuron as well as the identification of one or more of its chemical components. The technique is a combination of retrograde tracing and fluorescent dyes based on the work of Kuypers and collaborators and indirect immunofluorescence histochemistry as originally described by Coons and collaborators. The crucial parameters including the selection of the dyes, the injection technique and tissue processing as well as the appropriate immunohistochemical fluorescent markers and filter combinations are discussed. The method of choice involves the use of the retrogradely transported dyes Fast Blue, True Blue or Propidium Iodide, and in addition, for double labeling experiments, Diamidino Yellow or Primuline. They are combined with FITC (Propidium Iodide) or TRITC (Fast Blue, True Blue, Diamidino Yellow, Primuline) as immunofluorescence markers.

Localization of forebrain neurons which project directly to the medulla and spinal cord of the rat by retrograde tracing with wheat germ agglutinin

Wheat germ agglutinin (WGA) in a slow-release polyacrylamide gel pellet was implanted in the medulla or spinal cord of the rat. Large numbers of retrogradely labeled cells were visualized by immunocytochemical procedures in specific nuclei of the forebrain mainly ipsilateral to the implant site following implants as far caudal as the sacral segments of the spinal cord. Total average number of labeled forebrain cells (three brains per category; 100 micron per 150 micron of brain tissue were examined microscopically): medulla, 2,115; cervical, 1,878; lumbar, 1,017; sacral, 385. After WGA-gel implants in the medulla or cervical cord the majority of retrogradely labeled neurons were seen in the lateral hypothalamic area, the zona incerta, and in subdivisions of the paraventricular nucleus. A continuum of labeled cells extended from the caudal part of the paraventricular nucleus into the posterior hypothalamus and into the central gray of the midbrain. Labeled cells were also seen in the medial basal hypothalamus and the rostral part of the bed nucleus of the stria terminalis. A few labeled cells were observed in the medial and lateral preoptic areas, the rostral part of the paraventricular nucleus, and in the arcuate nucleus. Following WGA-gel implants in the lumbar or sacral cord many retrogradely labeled cells were observed mainly in the paraventricular nucleus, the lateral hypothalamus, zona incerta, medial basal hypothalamus, and posterior hypothalamic area. The continuum of labeled cells described above was also seen following these implants. Our data indicate that the lateral hypothalamus and zona incerta, as well as specific parts of the paraventricular nucleus, are major loci of neurons which project directly to the medulla and spinal cord of the rat. The consistency with which labeled cells were localized across all brains examined within categories of implant sites and the large numbers of labeled cells counted within these areas appeared to verify the sensitivity of our retrograde tracing method. Therefore, we interpret the paucity or absence of labeled cells in particular brain regions to indicate that cells of these regions do not project to the medulla or spinal cord.

Seeing with the visual cortex

A short analysis of the input-output organization of the primary visual cortical areas in the cat and monkey is followed by a description of the salient microelectrophysiological properties of retino-geniculo-cortical system neurons. It is concluded that a strict hierarchical model of cortical processing of visual information is no longer tenable.

The relationship of dorsal root afferents to motoneuron somata and dendrites in the adult bullfrog: a light and electron microscopic study using horseradish peroxidase

The relationship of lumbar dorsal root afferents to lateral motor column motoneurons was studied using anterograde injury filling of dorsal roots and retrograde injury filling of ventral roots with horseradish peroxidase. At the light microscopic level, horseradish peroxidase labelled dorsal root axons were observed to separate into a medial division of large diameter axons which enter the dorsal funiculus and a lateral division of small diameter axons which form a compact bundle in the dorsolateral funiculus which may be homologous to the mammalian tract of Lissauer. Within the spinal gray, primary afferents terminate in two distinct regions. The more ventral of these terminal fields, which receives collaterals of primary afferent axons in the dorsal funiculus, overlaps the dendritic arborizations of the lateral motor column motoneurons. Some axons leave the ventral terminal field to enter the dorsal lateral motor column. Here they terminate on the primary dendrites and somata of lateral motor column motoneurons. At the electron microscopic level, labelled primary afferent terminals were seen to synapse upon lateral motor column motoneuron dendrites as well as upon the somata of dorsally positioned lateral motor column motoneurons. These terminals contain small spherical vesicles and occasional dense-cored vesicles. The synaptic specializations are characterized by a small amount of postsynaptic material. The lateral motor column may be divided into dorsal and ventral portions on the basis of the primary afferent distribution and this is in accord with functional, physiological and developmental data.

Dental sensory receptors

Teeth are innervated by unmyelinated sympathetic axons, and by unmyelinated and small myelinated sensory axons. Some sensory axons in teeth are terminal branches of larger parent axons, so that conduction from teeth to CNS in trigeminal nerves includes C-fiber, A-delta, and A-beta velocities. Sensory dental axons contain acetylcholine or substance P-like immunoreactivity. The sympathetic axons contain noradrenalin. Other neuropeptides may also be present, such as vasoactive intestinal peptide and serotonin. Dental axons of mature teeth of many species (man, monkey, cat, rodents, fish) are essentially the same, but continuously erupting teeth have smaller and fewer axons. Free sensory nerve endings in mature teeth are found in the peripheral plexus of Raschkow, the odontoblastic layer, the predentin, and the dentin. Free nerve endings are most numerous in those regions near the tip of the pulp horn, where more than 40% of the dentinal tubules can be innervated. Many dentinal tubules contain more than one free nerve ending. Intradentinal axons can extend as far as 0.2 mm into dentin but usually end less than 0.1 mm from the pulp. Some sensory endings also occur along pulpal blood vessels. In continuously erupting teeth nerve endings do not enter the dentin but remain within the pulp. Nerve endings in dentin are labeled by axonal transport. They are therefore as viable and active as the nerve endings in pulp. The axoplasm of the free nerve endings contains organelles typical of other somatosensory receptors. These organelles are most common in the successive beaded regions along the free nerve endings and include mitochondria, clear and dense-core vesicles, multivesicular bodies, profiles of smooth endoplasmic reticulum, and relatively few microtubules and neurofilaments. The beads can vary in size from about 0.2 to 2.0 microns and can have varying amounts of receptor organelles. The interbead axonal regions are thin and contain mainly microtubules and neurofilaments. Nerve endings are associated with companion cells after they leave the coronal nerve bundles; these companion cells include Schwann cells, fibroblasts, and odontoblasts. There is no good evidence of gap junctions or synapses between nerve endings and odontoblasts. Instead, the two cell types form appositions that have a 20-40 nm extracellular cleft and parallel apposed plasmalemmas but no unusual membrane-associated material. No special organelles occur in the odontoblastic cytoplasm at these sites.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of the striate-recipient zone of the cats lateralis posterior-pulvinar complex and its relations with the geniculostriate system

The extrageniculate visual thalamus of the cat is divisible into several major subdivisions but only one receives dense fiber projections from the striate cortex. In the present study, modern axon transport techniques and acetylcholinesterase histochemistry were used to examine the internal organization of this striate-recipient zone and some of its afferent and efferent connections. A detailed study of the corticothalamic fiber projections of the striate cortex clarified the topographic organization and boundaries of the striate-recipient zone. The nature and course of "projection lines" within the zone were defined and the subdivision was shown to correspond closely to a region of relatively weak acetylcholinesterase staining. Corticothalamic projections from two regions of the extrastriate visual cortex, area 19 and the medial division of the Clare-Bishop complex, converge with those from area 17 in the striate-recipient zone, but these extrastriate areas have more widespread projections to the extrageniculate thalamus than does the striate cortex. A weak subcortical projection to the striate-recipient zone was demonstrated, apparently originating in the superior colliculus. Retrograde tracing experiments indicated that the corticothalamic inputs of the striate-recipient zone are precisely reciprocal by thalamocortical projections. Extrageniculate thalamic projections to area 17 arise exclusively from this thalamic subdivision and are highly topographically ordered. The striate-recipient zone projects massively and apparently retinotopically to area 19 and to the medial division of the Clarc-Bishop area, as well as area 21(a), but these extrastriate areas receive additional afferents from other subdivisions of the extrageniculate thalamus. These findings appear to rule out a "non-specific" functional role for the striate-recipient zone. In its topographic organization, its reciprocal connections with areas of the visual cortex, and its sheer volume, the zone seems comparable to the dorsal lateral geniculate nucleus and may be fairly considered a satellite of the geniculocortical system. Certain of the zone's organizational and connectional features may be clues to its functional role and its possible homologues in other mammalian forms.

Brainstem projections to the facial nucleus of the opossum. A study using axonal transport techniques

The horseradish peroxidase and autoradiographic techniques have been used to determine the origin and intranuclear termination of brainstem axons projecting to the facial nucleus of the opossum and to define networks which could be utilized in some oral-facial behaviors. Two regions of the midbrain have dense projections to the facial nucleus. One region is the ventral periaqueductal gray and adjacent interstitial nucleus of the medial longitudinal fasciculus which project bilaterally to those areas of the facial nucleus supplying auricular and cervical musculature. A second is the paralemniscal zone of the caudolateral midbrain which innervates the same areas of the contralateral facial nucleus. The red nucleus and/or the adjacent tegmentum send a less dense projection to those regions of the contralateral facial nucleus which innervate buccolabial and zygomatic muscles. The dorsolateral pons (the parabrachial complex, the nucleus locus coeruleus, pars alpha, and the nucleus sensorius n. trigemini, pars dorsalis) projects densely to those areas of the ipsilateral facial nucleus which innervate buccolabial and zygomatic musculature. In contrast, the nucleus reticularis pontis, pars ventralis, projects bilaterally to parts of the facial nucleus supplying auricular and cervical muscles. There was evidence of some rostral to caudal organization in the latter projection. Neurons in medial parts of the lateral reticular formation project bilaterally to the facial nucleus. Those within the nucleus reticularis parvocellularis and the rostral nucleus reticularis medullae oblongatae ventralis innervate areas supplying buccolabial and zygomatic muscles. Neurons in the nucleus reticularis medullae oblongatae ventralis located caudal to the obex favor regions of the facial nuclei which supply auricular and cervical muscles. Neurons in the nucleus reticularis medullae oblongatae dorsalis and lamina V of the medullary and spinal dorsal horns project ipsilaterally to the facial nucleus in a manner suggesting that information from specific cutaneous areas reaches neurons supplying the muscles deep to them. The brainstem-facial connections are discussed in relation to the functionally diverse roles served by the facial nucleus in oral-facial behavior.

Collaterals of rubrospinal neurons to the cerebellum in rat. A retrograde fluorescent double labeling study

In a previous study the collateralization of the rubrospinal tract in the spinal cord of rat, cat and monkey was studied by means of the fluorescent retrograde double labeling technique. In the present study the existence of rubrospinal collaterals to the cerebellar interpositus nucleus (NI) has been studied using the same technique. In rat 'True Blue' (TB) was injected in the cerebellar NI and 'Nuclear Yellow' (NY) was injected ipsilaterally in white and gray matter of C5-C8 spinal segments. In some cases a new fluorescent retrograde tracer was used instead of NY, i.e. 'Diamidino Yellow' (DY), which produces retrograde labeling similar to NY but which migrates only very slowly out of the retrogradely labeled neurons. In these experiments only very few single TB-labeled rubrocerebellar neurons occurred, but many (+/- 90%) of the TB-fluorescent rubrocerebellar neurons were TB-NY or TB-DY double-labeled from the spinal cord. At least 37% of the NY and DY-fluorescent rubrospinal neurons were NY-TB and DY-TB double-labeled from the cerebellum. These findings indicate that, in rat, almost all rubrocerebellar fibers represent collaterals of rubrospinal neurons, and that at least 37% of the rubrospinal neurons give rise to such cerebellar collaterals.

Dorsal root projections in the clawed toad (Xenopus laevis) as demonstrated by anterograde labeling with horseradish peroxidase

Horseradish peroxidase was applied to the proximal stumps of severed cervical, thoracic and lumbar dorsal roots in the clawed toad, Xenopus laevis, in order to study the course, distribution and site of termination of dorsal root fibers in the spinal cord and brain stem. The anterograde transport of horseradish peroxidase as applied in the present study proved to be a useful and reliable technique. Results show that on entering the spinal cord, dorsal root fibers segregate into a medially placed component entering the dorsal funiculus and a more laterally situated bundle in the dorsal part of the lateral funiculus. As regards its position the latter bundle presumably represents the anuran homologue of the mammalian tract of Lissauer. Moreover, a small intermediate bundle of fibers directly enters the spinal gray matter. The labeled fibers entering the dorsal funiculus and the tract of Lissauer ascend and descend in the spinal cord, displaying a longitudinal arrangement resembling that of higher vertebrates. In the spinal gray, dorsal root fibers terminate in the dorsal, central and lateral fields of Ebbesson, with the last field being a major terminus for dorsal root fibers originating in the limb-innervating segments. No dorsal root fibers were found to project to the motoneuron fields. A dorsal column nucleus, which is divisible into medial and lateral compartments, is present in the obex region and extends from the level of the second spinal nerve to that of the entrance of the vagus and glossopharyngeal nerves. Dorsal root fibers from the lumbar and all thoracic segments project to the medial compartment of the dorsal column nucleus, whereas those of the cervical enlargement project to the lateral compartment. Although the anuran dorsal column nucleus appears to be less differentiated than that of higher vertebrates, its medial and lateral compartments can be considered to be the forerunners of the mammalian nucleus gracilis and nucleus cuneatus, respectively.

Light and electron microscopic autoradiographic study of the dorsal root projections to the cat dorsal horn

The distribution of terminals arising from dorsal root primary afferents was examined in the lumbar spinal cord of cats using light- and electron-microscopic autoradiography. Tritiated proline or leucine was injected into either the L6 or L7 dorsal root ganglion. The light-microscopic spinal cord distribution of radioactivity in the ganglia was independent of the type of amino acid used. Likewise, the length of the survival time after injection had no effect. The projections to the substantia gelatinosa and the marginal zone were consistently densest. However, the topography of the dorsal horn distribution, relative to the segment of entry, varied significantly especially in the gelatinosa, depending upon the ganglia injected. Those to the substantia gelatinosa were largely limited to the segment of entry; those to the marginal zone and nucleus proprius extended many segments beyond the level of entry. At all levels the projection was exclusively ipsilateral to the side of injection. At the electron-microscopic level the distribution of radioactivity was determined in each of the three easily recognizable areas of the dorsal horn: the marginal zone, the substantia gelatinosa and the nucleus proprius. In each dorsal horn area the total terminal population was divided into four basic categories. Each of these areas was found to contain a characteristic distribution of these four terminal categories. The difference between areas arose, primarily, as a consequence of the dorsal to ventral decreasing frequency gradient of two types of terminal: those containing large, dense-cored vesicles and the increasing gradient of those containing flattened vesicles. The terminals with small pleomorphic vesicles and those with large round vesicles were frequently encountered in all three areas without a detectable frequency gradient. Similarly the primary afferent terminal population that is the subset of the total terminal population labelled after dorsal root ganglion injection, was also characteristic of the area, and each area was dominated by a different terminal-type. In the marginal zone the terminals containing large dense-cored vesicles dominated. In the substantia gelatinosa the terminals with pleomorphic vesicles (which included the so-called 'C' type terminals) dominated. And the terminals containing large round vesicles dominated the primary afferent population in the nucleus proprius. The terminals containing flattened vesicles were never found to be specifically labeled in any of the areas examined.

A method of dark-field and simultaneous light--dark-field illumination for photomacrography of autoradiographic preparations

A technique is described for dark-field and light--dark-field illumination of autoradiographic preparations for photomacrogra. A right-angle prism is employed as a substrate through which illumination is introduced to the specimen. The technique permits uniform illumination of large fields of view, and also minimizes the image-degrading effects of chromatic dispersion and excessive noncoherent scatter which are often associated with conventional dark-field illumination.

The tectopontine projection the the rat with comments on visual pathways to the basilar pons

The projection from the superior and inferior colliculi to the basilar pons in the rat was studied with the technique of orthograde transport of labeled amino acids and autoradiography. Injections restricted to the medial or lateral regions of the superior colliculus gave rise to grain labeling representing terminal fields over the ipsilateral peduncular, dorsolateral, and ventrolateral regions of the caudal basilar pons and over the dorsomedial area of the contralateral nucleus reticularis tegmenti pontis (NRTP). The pontine projection from the superior colliculus to the lateral basilar pons is topographically organized; the medial superior colliculus projects primarily to the peduncular region, whereas the lateral superior colliculus terminates chiefly in ventrolateral pontine areas. A projection from the superior colliculus to the contralateral dorsomedial pontine and medial peduncular pontine regions, a previously undescribed finding, has also been shown. Descending fibers from the inferior colliculus do not appear to terminate extensively within the basilar pons but rather course adjacent to pontine cells of the dorsolateral region in the caudal pons. Pretectal nuclei project ipsilaterally to medial and lateral nuclei in the rostral and middle basilar pons, respectively. A rostrocaudal topography exists in the tectopontine projection; the pretectum projects to rostromiddle basilar pons, the superior colliculus to more caudal pontine regions, and the inferior colliculus (although sparsely) to further caudal areas. The pontine projection pattern from the colliculi and pretectum differs from the pontine afferents from the visual cortices. The findings of this study, when compared to our results from previous investigations on the pontocerebellar projection system, suggest that the tectal inputs to certain lateral cerebellar lobules are relayed primarily through NRTP rather than the basilar pons. The collicular projection to midvermal lobules of the cerebellum appear to be mediated in part by both NRTP and lateral pontine nuclei.