PHA-L can be transported anterogradely through fibers of passage

Magnetic resonance diffusion tensor tractography of a midbrain auditory circuit in Alligator

Knowledge of brain circuitry is critical for understanding the organization, function, and evolution of central nervous systems. Most commonly, brain connections have been elucidated using histological and experimental methods that require animal sacrifice. On the other hand, magnetic resonance diffusion tensor imaging and associated tractography have emerged as a preferred method to noninvasively visualize brain white matter tracts. However, existing studies have primarily examined large, heavily myelinated fiber tracts. Whether tractography can visualize fiber bundles that contain thin and poorly myelinated axons is uncertain. To address this question, the midbrain auditory pathway to the thalamus was investigated in Alligator. This species was chosen because of its evolutionary importance as it is the reptilian group most closely related to birds and because its brain contains many thin and poorly myelinated tracts. Furthermore, this auditory pathway is well documented in other reptiles, including a related crocodilian. Histological observations and experimental determination of anterograde connections confirmed this path in Alligator. Tractography identified these tracts in Alligator and provided a 3-dimensional picture that accurately identified the neural elements of this circuit. In addition, tractography identified one possible unrecognized pathway. These results demonstrate that tractography can visualize circuits containing thin, poorly myelinated fibers. These findings open the door for future studies to examine these types of pathways in other vertebrates.

Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Differential projections from the lateral habenula to the rostromedial tegmental nucleus and ventral tegmental area in the rat

The mesopontine rostromedial tegmental nucleus (RMTg) is a mostly γ-aminobutyric acid (GABA)ergic structure believed to be a node for signaling aversive events to dopamine (DA) neurons in the ventral tegmental area (VTA). The RMTg receives glutamatergic inputs from the lateral habenula (LHb) and sends substantial GABAergic projections to the VTA, which also receives direct projections from the LHb. To further specify the topography of LHb projections to the RMTg and VTA, small focal injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were aimed at different subdivisions of the LHb. The subnuclear origin of LHb inputs to the VTA and RMTg was then confirmed by injections of the retrograde tracer cholera toxin subunit b into the VTA or RMTg. Furthermore, we compared the topographic position of retrogradely labeled neurons in the RMTg resulting from VTA injections with that of anterogradely labeled axons emerging from the LHb. As revealed by anterograde and retrograde tracing, LHb projections were organized in a strikingly topographic manner, with inputs to the RMTg mostly arising from the lateral division of the LHb (LHbL), whereas inputs to the VTA mainly emerged from the medial division of the LHb (LHbM). In the RMTg, profusely branched LHb axons were found in close register with VTA projecting neurons and were frequently apposed to the latter. Overall, our findings demonstrate that LHb inputs to the RMTg and VTA arise from different divisions of the LHb and provide direct evidence for a disynaptic pathway that links the LHbL to the VTA via the RMTg.

Development of tectal connectivity across metamorphosis in the bullfrog (Rana catesbeiana)

In the bullfrog (Rana catesbeiana), the process of metamorphosis culminates in the appearance of new visual and visuomotor behaviors reflective of the emergence of binocular vision and visually-guided prey capture behaviors as the animal transitions to life on land. Using several different neuroanatomical tracers, we examined the substrates that may underlie these behavioral changes by tracing the afferent and efferent connectivity of the midbrain optic tectum across metamorphic development. Intratectal, tectotoral, tectotegmental, tectobulbar, and tecto-thalamic tracts exhibit similar trajectories of neurobiotin fiber label across the developmental span from early larval tadpoles to adults. Developmental variability was apparent primarily in intensity and distribution of cell and puncta label in target nuclei. Combined injections of cholera toxin subunit β and Phaseolus vulgaris leucoagglutinin consistently label cell bodies, puncta, or fiber segments bilaterally in midbrain targets including the pretectal gray, laminar nucleus of the torus semicircularis, and the nucleus of the medial longitudinal fasciculus. Developmentally stable label was observed bilaterally in medullary targets including the medial vestibular nucleus, lateral vestibular nucleus, and reticular gray, and in forebrain targets including the posterior and ventromedial nuclei of the thalamus. The nucleus isthmi, cerebellum, lateral line nuclei, medial septum, ventral striatum, and medial pallium show more developmentally variable patterns of connectivity. Our results suggest that even during larval development, the optic tectum contains substrates for integration of visual with auditory, vestibular, and somatosensory cues, as well as for guidance of motivated behaviors.

The Origin of Brainstem Noradrenergic and Serotonergic Projections to the Spinal Cord Dorsal Horn in the Rat

Although it has been proposed that the locus coeruleus is the predominant, if not exclusive, brainstem origin of the noradrenergic innervation of the spinal dorsal horn, pharmacological studies argue otherwise. In this study we made localized injections of the retrograde tracer wheatgerm agglutinin conjugated to apo-horseradish peroxidase gold (WGA:apoHRP-Au), in conjunction with immunocytochemical labeling for tyrosine hydroxylase (TH) or serotonin (5-HT), to identify the brainstem source of the noradrenaline (NA) and 5-HT innervation of the dorsal horn of the rat. Our studies were concentrated in the C5 spinal segment. The pattern of labeling was only studied in animals in which the tracer injection was restricted to the dorsal horn. In these rats, TH-immunoreactive neurons in widespread regions of the brainstem, including the locus coeruleus, subcoeruleus, A5, and A7 cell groups, were found to project to the dorsal horn. In terms of absolute numbers of double-labeled cells, no one noradrenergic cell group predominated. As expected, dorsal-horn-projecting 5-HT-immunoreactive neurons were found within the 5-HT populations of the rostroventromedial medulla and caudal pons, including the nucleus raphe magnus, nucleus paragigantocellularis (PGi), and ventral portions of the nucleus gigantocellularis (Gi). The majority of retrogradely labeled 5-HT-immunoreactive cells were, however, located off the midline, in the ipsilateral PGi and ventral Gi. Finally, a large number of retrogradely labeled, non-5-HT cells were found intermingled among the 5-HT cells of this region. Our results provide evidence that the noradrenergic regulation of nociceptive transmission at the spinal cord level arises from direct spinal projections of several brainstem noradrenergic cell groups.

Anterograde axonal tract tracing

The mammalian brain contains a myriad of interconnected regions. An examination of the complex circuitry of these areas requires sensitive neuroanatomical tract tracing techniques. The anterograde tracers, Phaseolus vulgaris leucoagglutinin (PHA-L) and biotinylated dextran amines (BDA) are powerful tools that can be used to label fiber tracts that project from one particular brain region. When injected iontophoretically, PHA-L and BDA are readily taken up by neurons and transported anterogradely along their axonal tracts. Combined with immunocytochemistry for neurotransmitters, neuropeptides, and receptors, tract tracing methods may be used to elucidate the phenotype of synapses that form the microcircuitry of specific neural systems.

Organization of ventral tegmental area projections to the ventral tegmental area-nigral complex in the rat

The ventral tegmental area (VTA) is a nodal link in reward circuitry. Based on its striatal output, it has been subdivided in a caudomedial part which targets the ventromedial striatum, and a lateral part which targets the ventrolateral striatum [Ikemoto S (2007) Dopamine reward circuitry: two projection systems from the ventral midbrain to the nucleus accumbens-olfactory tubercle complex. Brain Res Rev 56:27-78]. Whether these two VTA parts are interconnected and to what extent the VTA innervates the substantia nigra compacta (SNc) and retrorubral nucleus (RR) are critical issues for understanding information processing in the basal ganglia. Here, VTA projections to the VTA-nigral complex were examined in rats, using Phaseolus vulgaris leucoagglutinin (PHA-L) as anterograde tracer. The results show that the dorsolateral VTA projects to itself, as well as to the dorsal tier of the SNc and RR, largely avoiding the caudomedial VTA. The ventrolateral VTA innervates mainly the interfascicular nucleus. The components of the caudomedial VTA (the interfascicular, paranigral and caudal linear nuclei) are connected with each other. In addition, the caudomedial VTA (especially the paranigral and caudal linear nuclei) innervates the lateral VTA, and, to a lesser degree, the SNc and RR. The caudal pole of the VTA sends robust, bilateral projections to virtually all the VTA-nigral complex, which terminate in the dorsal and ventral tiers. Modest inputs from the medial supramammillary nucleus to ventromedial parts of the VTA-nigral complex were also identified. In double-immunostained sections, PHA-L-labeled varicosities were sometimes found apposed to tyrosine hydroxylase-positive neurons in the ventral mesencephalon. Overall, the results underscore that VTA projections to the VTA-nigral complex are substantial and topically organized. In general, these projections, like the spiralated striato-nigro-striatal loops, display a medial-to-lateral organization. This anatomical arrangement conceivably permits the ventromedial striatum to influence the activity of the lateral striatum. The caudal pole of the VTA appears to be a critical site for a global recruitment of the mesotelencephalic system.

Plasticity of auditory medullary-midbrain connectivity across metamorphic development in the bullfrog, Rana catesbeiana

On the basis of patterns of anterograde, retrograde, and bi-directional transport of tracers from both the superior olivary nucleus (SON) and the torus semicircularis (TS), we report anatomical changes in brainstem connectivity across metamorphic development in the bullfrog, Rana catesbeiana. In early and late stages of larval development (Gosner stages 25-37), anterograde or bi-directional tracers injected into the SON produce terminal/fiber label in the contralateral SON and in the ipsilateral TS. Between stages 38-41 (deaf period), only sparse or no terminal/fiber label is visible in these target nuclei. During metamorphic climax (stages 42-46), terminal/fiber label reappears in both the contralateral SON and in the ipsilateral TS, and now also in the contralateral TS. Injections of retrograde tracers into the SON fail to label cell bodies in the ipsilateral TS in deaf period animals, mirroring the previously-reported failure of retrograde transport from the TS to the ipsilateral SON during this developmental time. Bilateral cell body label emerges in the dorsal medullary nucleus and the lateral vestibular nucleus bilaterally as a result of SON transport during the late larval period, while cell body label in the contralateral TS emerges during climax. At all larval stages, injections into the SON produce anterograde and retrograde label in the medial vestibular nucleus bilaterally. These data show anatomical stability in some pathways and plasticity in others during larval development, with the most dramatic changes occurring during the deaf period and metamorphic climax. Animals in metamorphic climax show patterns of connectivity similar to that of froglets and adults, indicating the maturation during climax of central anatomical substrates for hearing in air.

Medial vestibular connections with the hypocretin (orexin) system

The mammalian medial vestibular nucleus (MVe) receives input from all vestibular endorgans and provides extensive projections to the central nervous system. Recent studies have demonstrated projections from the MVe to the circadian rhythm system. In addition, there are known projections from the MVe to regions considered to be involved in sleep and arousal. In this study, afferent and efferent subcortical connectivity of the medial vestibular nucleus of the golden hamster (Mesocricetus auratus) was evaluated using cholera toxin subunit-B (retrograde), Phaseolus vulgaris leucoagglutinin (anterograde), and pseudorabies virus (transneuronal retrograde) tract-tracing techniques. The results demonstrate MVe connections with regions mediating visuomotor and postural control, as previously observed in other mammals. The data also identify extensive projections from the MVe to regions mediating arousal and sleep-related functions, most of which receive immunohistochemically identified projections from the lateral hypothalamic hypocretin (orexin) neurons. These include the locus coeruleus, dorsal and pedunculopontine tegmental nuclei, dorsal raphe, and lateral preoptic area. The MVe itself receives a projection from hypocretin cells. CTB tracing demonstrated reciprocal connections between the MVe and most brain areas receiving MVe efferents. Virus tracing confirmed and extended the MVe afferent connections identified with CTB and additionally demonstrated transneuronal connectivity with the suprachiasmatic nucleus and the medial habenular nucleus. These anatomical data indicate that the vestibular system has access to a broad array of neural functions not typically associated with visuomotor, balance, or equilibrium, and that the MVe is likely to receive information from many of the same regions to which it projects.

Commissural propriospinal connections between the lateral aspects of laminae III-IV in the lumbar spinal cord of rats

It has been established that there is a strong functional link between sensory neural circuits on the two sides of the spinal cord. In one of our recent studies we provided a morphological confirmation of this functional phenomenon, presenting evidence for the presence of a direct commissural connection between the lateral aspects of the dorsal horn on the two sides of the lumbar spinal cord. By using a combination of neural tracing and immunocytochemical detection of neural markers like vesicular glutamate transporters, glutamic acid decarboxylase, glycine transporter, and met-enkephalin (which are characteristic of various subsets of excitatory and inhibitory neurons), we investigated here the distribution, synaptic relations, and neurochemical characteristics of the commissural axon terminals. We found that the cells of origin of commissural fibers in the lateral aspect of the dorsal horn were confined to laminae III-IV and projected to the corresponding area of the contralateral gray matter. Most of the commissural axon terminals established synaptic contacts with dendrites. Axospinous or axosomatic synaptic contacts were found in limited numbers. We demonstrated that interactions among commissural neurons also exist. More than three-fourths of the labeled axon terminals were immunostained for glutamic acid decarboxylase and/or glycine transporter, but none of them showed positive immunoreaction for met-enkephalin and vesicular glutamate transporters. The results indicate that there is a substantial reciprocal commissural synaptic interaction between the lateral aspects of laminae III-IV on the two sides of the lumbar spinal cord and that this pathway may transmit both inhibitory and excitatory signals to their postsynaptic targets.

The projections of the midbrain periaqueductal grey to the pons and medulla oblongata in rats

It is now established that stimulation of the ventrolateral midbrain periaqueductal grey (PAG) evokes inhibition of nociceptive spinal neurons, which results in analgesia and a powerful attenuation of pain behaviour. It is postulated that the PAG exerts this inhibitory effect on spinal nociceptive functions through the activation of descending serotonergic and noradrenergic pathways that arise from the rostral ventromedial medulla (RVM) and pontine noradrenergic nuclei. To investigate the neuroanatomical substrate of this functional link between the PAG and RVM, as well as the pontine noradrenergic nuclei in the rat, we labelled axons that project from the ventrolateral PAG to various regions of the pons and medulla oblongata using the anterograde tracing substance, Phaseolus vulgaris leucoagglutinin. We demonstrated that some of PAG efferents really do terminate in the RVM and pontine noradrenergic nuclei, but a substantial proportion of them project to the intermediate subdivision of the pontobulbar reticular formation. Combining the axonal tracing with serotonin- and tyrosine-hydroxylase-immunohistochemistry, we also found that, in contrast to previous results, PAG efferents make relatively few appositions with serotonin- and tyrosine-hydroxylase-immunoreactive neurons in the RVM and pontine noradrenergic nuclei; most of them terminate in nonimmunoreactive territories. The results suggest that the ventrolateral PAG may activate a complex pontobulbar neuronal assembly including neurons in the intermediate subdivision of the pontobulbar reticular formation, serotonin- and tyrosine-hydroxylase-immunoreactive and nonimmunoreactive neurons in the RVM and pontine noradrenergic nuclei. This pontobulbar neural circuitry, then, may mediate the PAG-evoked activities towards the spinal dorsal horn resulting in the inhibition of spinal nociceptive functions.

Propriospinal afferent and efferent connections of the lateral and medial areas of the dorsal horn (laminae I-IV) in the rat lumbar spinal cord

The different subdivisions along the mediolateral extent of the superficial dorsal horn of the spinal cord are generally regarded as identical structures that execute the function of sensory information processing without any significant communication with other regions of the spinal gray matter. In contrast to this standing, here we endeavor to show that neural assemblies along the mediolateral extent of laminae I-IV cannot be regarded as identical structures. After injecting Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine into various areas of the superficial dorsal horn (laminae I-IV) at the level of the lumbar spinal cord in rats, we have demonstrated that the medial and lateral areas of the superficial dorsal horn show the following distinct features in their propriospinal afferent and efferent connections: 1) A 300- to 400-microm-long section of the medial aspects of laminae I-IV projects to and receives afferent fibers from a three segment long compartment of the spinal dorsal gray matter, whereas the same length of the lateral aspects of laminae I-IV projects to and receives afferent fibers from the entire rostrocaudal extent of the lumbar spinal cord. 2) The medial aspects of laminae I-IV project extensively to the lateral areas of the superficial dorsal horn. In contrast to this, the lateral areas of laminae I-IV, with the exception of a few fibers at the segmental level, do not project back to the medial territories. 3) There is a substantial direct commissural connection between the lateral aspects of laminae I-IV on the two sides of the lumbar spinal cord. The medial part of laminae I-IV, however, does not establish any direct connection with the gray matter on the opposite side. 4) The lateral aspects of laminae I-IV appear to be the primary source of fibers projecting to the ipsi- and contralateral ventral horns and supraspinal brain centers. Projecting fibers arise from the medial subdivision of laminae I-IV in a substantially lower number. The findings indicate that the medial and lateral areas of the superficial spinal dorsal horn of rats may play different roles in sensory information processing.

Efferent projections of the intergeniculate leaflet and the ventral lateral geniculate nucleus in the rat

The intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (VLG) are ventral thalamic derivatives within the lateral geniculate complex. In this study, IGL and VLG efferent projections were compared by using anterograde transport of Phaseolus vulgaris-leucoagglutinin and retrograde transport of FluoroGold. Projections from the IGL and VLG leave the geniculate in four pathways. A dorsal pathway innervates the thalamic lateral dorsal nucleus (VLG), the reuniens and rhomboid nuclei (VLG and IGL), and the paraventricular nucleus (IGL). A ventral pathway runs through the geniculohypothalamic tract to the suprachiasmatic nucleus and the anterior hypothalamus (IGL). A medial pathway innervates the zona incerta and dorsal hypothalamus (VLG and IGL); the lateral hypothalamus and perifornical area (VLG); and the retrochiasmatic area (RCA), dorsomedial hypothalamic nucleus, and subparaventricular zone (IGL). A caudal pathway projects medially to the posterior hypothalamic area and periaqueductal gray and caudally along the brachium of the superior colliculus to the medial pretectal area and the nucleus of the optic tract (IGL and VLG). Caudal IGL axons also terminate in the olivary pretectal nucleus, the superficial gray of the superior colliculus, and the lateral and dorsal terminal nuclei of the accessory optic system. Caudal VLG projections innervate the lateral posterior nucleus, the anterior pretectal nucleus, the intermediate and deep gray of the superior colliculus, the dorsal terminal nucleus, the midbrain lateral tegmental field, the interpeduncular nucleus, the ventral pontine reticular formation, the medial and lateral pontine gray, the parabrachial region, and the accessory inferior olive. This pattern of IGL and VLG projections is consistent with our understanding of the distinct functions of each of these ventral thalamic derivatives.

Development, neurochemical properties, and axonal projections of a population of last-order premotor interneurons in the white matter of the chick lumbosacral spinal cord

There is general agreement that last-order premotor interneurons-a set of neurons that integrate activities generated by the spinal motor apparatus, sensory information and volleys arising from higher motor centres, and transmit the integrated signals to motoneurons through monosynaptic contacts-play crucial roles in the initiation and maintenance of spinal motor activities. Here, we demonstrate the development, neurochemical properties, and axonal projections of a unique group of last-order premotor interneurons within the ventrolateral aspect of the lateral funiculus of the chick lumbosacral spinal cord. Neurons expressing immunoreactivity for neuron-specific enolase were first detected in the ventrolateral white matter at embryonic day 9 (E9). The numbers of immunoreactive neurons were significantly increased at E10-E12, while most of them were gradually concentrated in small segmentally arranged nuclei (referred to as major nuclei of Hofmann) protruding from the white matter in a necklace like fashion dorsal to the ventral roots. The major nuclei of Hofmann became more prominent at E12-E16, but substantial numbers of cells were still located within the ventrolateral white matter (referred to as minor nucleus of Hofmann). The distribution of immunoreactive neurons achieved by E16 was maintained during later developmental stages and was also characteristic of adult animals. After injection of Phaseolus vulgaris-leucoagglutinin unilaterally into the minor nucleus of Hofmann, labeled fibres were detected in the ventrolateral white matter ipsilateral to the injection site. Ascending and descending fibres were revealed throughout the entire rostro-caudal length of the lumbosacral spinal cord. Axon terminals were predominantly found within the lateral motor column and the ventral regions of lamina VII ipsilateral to the injection site. Several axon varicosities made close appositions with somata and dendrites of motoneurons, which were identified as synaptic contacts in a consecutive electron microscopic study. With the postembedding immunogold method, 21 of 97 labeled terminals investigated were immunoreactive for glycine and 2 of them showed immunoreactivity for gamma-aminobutyric acid (GABA). The axon trajectories of neurons within the minor nucleus of Hofmann suggest that some of these cells might represent a population of last-order premotor interneurons. J. Exp. Zool. 286:157-172, 2000.

Recent techniques for tracing pathways in the central nervous system of developing and adult mammals

Over the last 20 years, the choice of neural tracers has increased manyfold, and includes newly introduced anterograde tracers that allow quantitation of single-axon morphologies, and retrograde tracers that can be combined with intracellular fills for the study of dendritic arbors of neurons which have a specific projection pattern. The combination of several different tracers now permits the comparison of multiple connections in the same animal, both quantitatively and qualitatively. Moreover, the finding of new virus strains, which infect neural cells without killing them, provides a tool for studying multisynaptic connections that participate in a circuit. In this paper, the labeling characteristics, mechanism of transport and advantages/disadvantages of use are discussed for the following recently introduced neural tracers: carbocyanine dyes, fluorescent latex microspheres, fluorescent dextrans, biocytin, dextran amines, Phaseolus vulgaris leucoagglutinin, cholera toxin and viruses. We also suggest the choice of specific tracers, depending on the experimental animal, age and type of connection to be studied, and discuss quantitative methodologies.

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

The ascending serotonergic system in the hamster: comparison with projections of the dorsal and median raphe nuclei

The ascending serotonergic projections are derived largely from the midbrain median and dorsal raphe nuclei, and contribute to the regulation of many behavioral and physiological systems. Serotonergic innervation of the hamster circadian system has been shown to be substantially different from earlier results obtained with other methods and species. The present study was conducted to determine whether similar differences are observed in other brain regions. Ascending projections from the hamster dorsal or median raphe were identified using an anterograde tracer, Phaseolus vulgans leucoagglutinin, injected by iontophoresis into each nucleus. Brains were processed for tracer immunoreactivity, and drawings were made of the median raphe and dorsal raphe efferent projection patterns. The efferents were also compared to the distribution of normal serotonergic innervation of the hamster midbrain and forebrain. The results show widespread, overlapping projection patterns from both the median and dorsal raphe, with innervation generally greater from the dorsal raphe. In several brain regions, including parts of the pretectum, lateral geniculate and basal forebrain, nuclei are innervated by the dorsal, but not the median, raphe. The hypothalamic suprachiasmatic nucleus is the only site innervated exclusively by the median and not by the dorsal raphe. The pattern of normal serotonin fiber and terminal distribution is generally more robust than would be inferred from the anterograde tracer material. However, there is good qualitative similarity between the two sets of data. The oculomotor nucleus and the medial habenula are unusual to the extent that each has a moderately dense serotonin terminal plexus, although neither receives innervation from the median or dorsal raphe. In contrast, the centrolateral thalamic nucleus and lateral habenula have little serotonergic innervation, but receive substantial other neural input from the raphe nuclei. The normal serotonergic innervation of the hamster brain is similar to that in the rat, although there are exceptions. The anterograde tracing of ascending median or dorsal raphe projections reveals a high, but imperfect, degree of correspondence with the serotonin innervation data, and with data from rats derived from immunohistochemical and autoradiographic tract-tracing techniques.

Direct comparison of projections from the central amygdaloid region and nucleus accumbens shell

Certain neurochemical and connectional characteristics common to extended amygdala and the nucleus accumbens shell suggest that the two represent a single functional-anatomical continuum. If this is so, it follows that the outputs of the two structures should be substantially similar. To address this, projections from the caudomedial shell and central nucleus of the amygdala, a key extended amygdala structure, were demonstrated in Sprague-Dawley rats with different anterograde axonal tracers processed separately to exhibit distinguishable brown and blue-black precipitates. The caudomedial shell projection is strong in the ventral pallidum and along the medial forebrain bundle through the lateral preopticohypothalamic continuum into the ventral tegmental area, distal to which it thins abruptly. The central nucleus projects strongly to the bed nucleus of the stria terminalis and the sublenticular extended amygdala, but substantially to the lateral hypothalamus only at levels behind the rostral part of the entopeduncular nucleus. Innervation of the ventral tegmental area by the central amygdala is minimal, but the lateral one-third of the substantia nigra, pars compacta and an adjacent lateral part of the retrorubral field receive substantial central amygdala input. Central amygdaloid projections are robust in caudal brainstem sites, such as the reticular formation, parabrachial nucleus, nucleus of the solitary tract and dorsal vagal complex, all of which receive little input from the accumbens. The substantial differences in the output systems of the caudomedial shell of accumbens and central amygdala suggest that the two represent distinct functional-anatomical systems.

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.

Heterogeneous projections of the cat posteroventral cochlear nucleus

The anterograde tracer Phaseolus vulgaris-leucoagglutinin was used to identify the projections of the posteroventral cochlear nucleus in cats. After labeling predominately cells of the core and multipolar regions, varicose fibers were observed in a variety of auditory nuclei. Ipsilaterally, most varicose fibers were located in periolivary regions situated lateral to the medial superior olive of the superior olivary complex. Contralaterally, the majority of labeled fibers were located in the ventral nucleus of the trapezoid body and the ventral nucleus of the lateral lemniscus. Labeled varicose fibers were also observed in regions not commonly identified as receiving input from the posteroventral cochlear nucleus. These regions included bilaterally the principal nuclei of the superior olivary complex, some periolivary regions, and the sagulum, as well as the ipsilateral intermediate and dorsal nucleus of the lateral lemniscus, inferior colliculus, and lateral pontine nucleus. Both similarities and differences were observed in the projections of the core and multipolar regions. With the exception of calyceal-type endings in the contralateral ventral nucleus of the lateral lemniscus, the varicose fibers in all regions, including the contralateral medial nucleus of the trapezoid body, were beaded, en passant type terminal varicosities.

Postnatal development of amygdaloid projections to the prefrontal cortex in the rat studied with retrograde and anterograde tracers

The prefrontal cortex (PFC) and the amygdala are involved in a number of common functions, such as emotional and social behavior, stress, visceral functions, ingestive behavior, self-stimulation, and certain aspects of learning and memory. The amygdala massively projects to the PFC and may play a role in the developmental plasticity reported for several of these functions. We have studied the normal postnatal development of the amygdaloid projections to the rat prefrontal cortex by using the retrogradely transported fluorescent dye fast blue and the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Shortly after birth some fibers were observed in the frontal pole of the rat brain. These fibers were scattered throughout all prefrontal cortical areas. The majority of the amygdaloid cells contributing to this pattern at that stage of development were located in the anterior and ventral basolateral nuclei, whereas a minority were located in the posterior basolateral nucleus. The transition from a diffuse fiber distribution to a characteristic bilaminar pattern occurred around postnatal day 12 in the lateral and rostral medial PFC. The PHA-L injections confirmed the existence of a topographical organization of the amygdalo-prefrontocortical projections. Our observations suggest that the development of amygdala innervation of the PFC parallels the emergence of PFC cytoarchitectural organization.

Amygdala input to medial prefrontal cortex (mPFC) in the rat: a light and electron microscope study

This paper describes the termination pattern and synaptic connectivity of the pathway from the basolateral nucleus of the amygdala (BLA) to the medial prefrontal cortex (mPFC; areas 25, 32, and 24b) of the rat. Discrete injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) were made in the BLA and detailed light microscopical observations made of the distribution of PHA-L labelled fibres and boutons in the mPFC. Labelled fibres were distributed in two tiers: predominantly within deep layer 1/layer 2 and also in layers 5/6. Fibre plexi in layers 2 and 5 were highly varicose. Electron microscopical examination of 120 labelled boutons in area 32 (60 in layer 2 and 60 in layer 5) indicated that 116 (97%) established asymmetrical synaptic contacts with dendritic spines and 4 (3%) were in synaptic contact with small dendritic shafts. No significant differences in target structures were found between layers 2 and 5. The results indicate that BLA input to mPFC in the rat predominantly innervates spine bearing dendrites in layers 2 and 5. This suggests that the neuronal operations of these processes are influenced by direct feedforward excitation from the BLA.

Differential synaptic innervation of striatofugal neurones projecting to the internal or external segments of the globus pallidus by thalamic afferents in the squirrel monkey

It is well established that the centromedian nucleus (CM) is the major source of thalamic afferents to the sensorimotor territory of the striatum in monkeys. However, the projection sites of striatal neurons contacted by thalamic afferents still remain to be determined. We therefore carried out an anatomical study aimed at elucidating the hodology of striatal neurones that receive input from the CM in squirrel monkeys. Our approach was to combine the anterograde transport of Phaseolus vulgaris-leucoagglutinin (PHA-L) or biocytin from the CM with the retrograde transport of biotinylated dextran-amine (bio-dex) or PHA-L from the internal (GPi) or external (GPe) segments of the globus pallidus. Following CM injections, rich plexuses of anterogradely labelled, thin varicose fibres aggregated in the form of bands that were confined to the postcommissural region of the putamen. On the other hand, injections into the GPe or GPi led to profuse retrograde labelling of a multitude of medium-sized spiny neurones. In cases where the injections involved the caudoventral two-thirds of the GPe or GPi, the retrogradely labelled striatopallidal cells and the anterogradely labelled thalamostriatal fibres occurred in the sensorimotor territory of the putamen. After injections into either pallidal segments, clusters of retrogradely labelled cells were in register with bands of anterogradely labelled thalamic fibres. However, electron microscopic analysis of striatal regions containing both anterogradely labelled thalamic afferents and retrogradely labelled cells revealed that terminals from the CM frequently form asymmetric synapses with dendritic shafts and spines of striato-GPi cells but rarely with those of striato-GPe cells. In conclusion, our findings demonstrate that thalamic afferents from the CM innervate preferentially striatopallidal neurones projecting to the GPi in monkeys. These results indicate that the striatopallidal neurones contributing to the "direct" and "indirect" output pathways are differentially innervated by thalamic afferents in primates.

Efferent projections of the retrorubral nucleus to the substantia nigra and ventral tegmental area in cats as shown by anterograde tracing

The aim of the present study was to determine whether the retrorubral nucleus projects to the dopaminergic nuclei in the ventral midbrain of the cat. For this purpose, injections of biotinylated dextran-amine or Phaseolus vulgaris-leucoagglutinin were placed into the retrorubral nucleus under stereotaxic guidance. The tracers were visualized by means of (immuno) histochemical procedures. In addition, tyrosine hydroxylase immunohistochemistry was used to evaluate the location of the injection sites and the distribution of the anterogradely labeled fibers. Both tracers reveal the same topography of labeled fibers in the ventral mesencephalon. Labeled fibers with varicosities were found ipsilaterally in the substantia nigra pars compacta, the substantia nigra pars lateralis, the ventral tegmental area and, contralaterally, in the substantia nigra pars compacta, the ventral tegmental area, and the retrorubral nucleus. A considerable number of labeled axons with varicosities were observed to be wrapped around the dendrites and perikarya of tyrosine hydroxylase-positive neurons in these areas. The present results are discussed in view of the possible role of the A8 dopaminergic cell group in the coordination of A9 nigrostriatal and A10 mesolimbic systems, as well as in the progressive pathology seen in patients suffering from Parkinson's disease.

Evidence for corticotropin-releasing hormone projections from Barrington's nucleus to the periaqueductal gray and dorsal motor nucleus of the vagus in the rat

The present study used anterograde and retrograde tract tracing and immunohistochemistry to determine the efferent projections of corticotropin-releasing hormone (CRH) neurons of Barrington's nucleus in the rat. Injections of Phaseolus vulgaris-leucoagglutinin into Barrington's nucleus resulted in anterograde labeling in the dorsal motor nucleus of the vagus, periaqueductal gray, medial thalamic nuclei, lateral hypothalamus, paraventricular nucleus of the hypothalamus, lateral preoptic area, and lateral septum. The retrograde tract tracer, fluorogold, injected into the lumbosacral spinal cord labeled many, but not all, CRH-immunoreactive neurons in Barrington's nucleus. Moreover, some Barrington's neurons that were retrogradely labeled from the spinal cord were not CRH-immunoreactive. Several CRH-immunoreactive Barrington's neurons were retrogradely labeled by fluorogold injections into the periaqueductal gray, and these were located predominantly in the dorsal part of the nucleus. Additionally, some CRH-immunoreactive Barrington's neurons were retrogradely labeled from fluorogold injections into the dorsal motor nucleus of the vagus. In contrast, fluorogold injections into the lateral hypothalamus, lateral preoptic area, or lateral septum did not result in double labeling of CRH-immunoreactive neurons in Barrington's nucleus. These results suggest that many, but not all, CRH-containing neurons of Barrington's nucleus project to the lumbosacral spinal cord. In addition to their previously documented projections to the spinal cord, these neurons may be a source of CRH in the periaqueductal gray and dorsal motor nucleus of the vagus. CRH projections of Barrington's nucleus may play a role in behavioral or autonomic aspects of stress responses, in addition to their proposed role in micturition.

Projections from the cochlear nucleus to the superior paraolivary nucleus in guinea pigs

Axonal tracing techniques were used to study the projection from the cochlear nucleus to the superior paraolivary nucleus in guinea pigs. Different tracers were used to identify the cell types that give rise to the projections, the morphology of their axons, and the cell types that they contact in the superior paraolivary nucleus. Injections of Fluoro-Gold or peroxidase-labeled-WGA and HRP into the superior paraolivary nucleus labeled multipolar cells and octopus cells bilaterally in the ventral cochlear nucleus, mainly on the contralateral side. Injections of PHAL into the ventral cochlear nucleus labeled two types of axons in the superior paraolivary nucleus. Thin axons branch infrequently and give rise primarily to small, en passant boutons. Thick axons have larger boutons, many of which are terminal boutons that arise from short collaterals. Thin axons appear to originate from multipolar cells, whereas thick axons probably originate from octopus cells. Both types are found bilaterally after an injection into the ventral cochlear nucleus on one side. Individual thick or thin axons may contact multiple cell types in the superior paraolivary nucleus. Individual cells in the superior paraolivary nucleus can receive convergent input from both thick and thin axons. Combined anterograde and retrograde transport of different fluorescent tracers was used to identify the projections of the cells in the superior paraolivary nucleus that receive inputs from the ventral cochlear nucleus. Cells in the superior paraolivary nucleus that projected to the ipsilateral cochlear nucleus or to the ipsilateral inferior colliculus appeared to be contacted by axons that were labeled by anterograde transport from the contralateral ventral cochlear nucleus. Thus the projections to the superior paraolivary nucleus are in a position to affect the activity in both ascending and descending auditory pathways.

Anatomical evidence for inputs to ventrolateral medullary catecholaminergic neurons from the midbrain periaqueductal gray of the rat

Previous studies have shown that the midbrain periaqueductal gray (PAG) projects to the ventrolateral medulla (VLM). Here, we studied PAG projections to the area of A1/C1 neurons in the VLM in the rat using phaseolus vulgaris leucoagglutinin (PHA-L) anterograde tracing combined with immunocytochemistry for tyrosine hydroxylase (TH) or phenylethanolamine N-methyl transferase (PNMT). Following PAG injections, PHA-L labeled fibers and terminals were intermingled among TH-immunoreactive (TH-ir) neurons in the VLM. High-power light microscopic examination revealed that some of the PHA-L labeled varicose fibers and boutons were in close contiguity with TH-ir elements. Such apparent appositions appeared more frequently on TH-ir elements in the A1 area than on TH-ir or PNMT-ir neurons in the C1 area. These results indicate that some PAG inputs to the VLM may directly innervate A1/C1 neurons.

Topographical organization of the rat suprachiasmatic-paraventricular projection

The suprachiasmatic nucleus (SCN) is a dominant pacemaker involved in the generation of circadian rhythms in mammals. Surprisingly, the expression of the many rhythms appears to be mediated via a limited efferent projection system of the pacemaker, of which the largest pathway terminates in the subparaventricular area and in the paraventricular nucleus of the hypothalamus. In order to investigate a possible topographical organization of this major outflow pathway of the SCN, microiontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) or the retrograde tracer cholera toxin subunit B (ChB) were centered in distinct subparts of the SCN (PHA-L) or in the subparaventricular area-paraventricular nucleus (ChB), respectively. PHA-L injections involving the entire SCN revealed not only a major projection to the subparaventricular area, but also one directed towards the medial and dorsal parvicellular subnuclei of the paraventricular nucleus. As opposed to injections involving the entire nucleus, injections of PHA-L centered in the dorsomedial subdivision of the SCN resulted in a relatively larger number of PHA-L-immunoreactive fibers in the parvicellular subdivisions of the PVN, whereas the terminal field in the subparaventricular area was less substantial. A topography of the SCN efferent output system was also revealed by the retrograde tracing with ChB. Injections of ChB in the dorsal part of the paraventricular hypothalamic nucleus, not involving the underlying subparaventricular area, gave rise to a population of retrogradely labeled cells in the dorsomedial part of the SCN. In contrast, ChB injections in the subparaventricular area resulted in labeling of neurons clustered in a more ventrolateral aspect of the SCN. The present data provide evidence for a topography in the major efferent projection system from the SCN, implying that different subparts of the rat SCN, presumably containing partly different potential neurotransmitter substances, may regulate different circadian rhythms.

Evidence that cholera toxin B subunit (CTb) can be avidly taken up and transported by fibers of passage

It has been reported that cholera toxin B subunit (CTb) is a sensitive neuronal tracer with unique features. However, the possible uptake of CTb by non-terminal fibers passing through the injection site has not been examined thoroughly. In the present study, small iontophoretic injections (current = +2 microA) of CTb were made in the olivocerebellar pathway in the rat ventrolateral medulla. A large number of retrogradely labeled neurons were seen in the contralateral inferior olive. In addition, prominent anterogradely labeled climbing fibers/terminals were found in the cerebellum ipsilateral to the injection site. This study, in contrast to previous report(s), indicates that CTb can be taken up avidly by fibers of passage and transported both anterogradely and retrogradely.

Distribution of spinal cord projections from the medullary subnucleus reticularis dorsalis and the adjacent cuneate nucleus: a Phaseolus vulgaris-leucoagglutinin study in the rat

The distribution and organization of descending spinal projections from the dorsal part of the caudal medulla were studied in the rat following injections of Phaseolus vulgaris-leucoagglutinin into small areas of the subnucleus reticularis dorsalis (SRD) and the adjacent cuneate nucleus (Cu). The caudal aspect of the Cu projected only to the dorsal horn of the ipsilateral cervical cord via the dorsal funiculus. These projections were mainly to laminae I, IV, and V. More ventrally located reticular structures projected to the full length of the cord. Fibers originating from the SRD travelled through the ipsilateral dorsolateral funiculus and terminated within the deep dorsal horn and upper layers of the ventral horn, mainly in laminae V-VII. Fibers originating from subnucleus reticularis ventralis (SRV) travelled ipsilaterally through the lateral and ventrolateral funiculi and bilaterally through the ventromedial funiculus. These fibers terminated within the ventral horn. The density of labeling within the gray matter varied at different levels of the cord was as follows: cervical > sacral > thoracic > lumbar. The reciprocal connections between the caudal medulla and the spinal cord suggest that the former is an important link in feedback loops that regulate spinal outflow.

The pallidostriatal projection in the rat: a recurrent inhibitory loop?

The pallidostriatal projection in the rat was investigated employing the PHA-L tracing technique. Following inotophoretic injections into the lateral aspect of the globus pallidus external segment, the ipsilateral striatum showed patches of dense anterograde labeling separated by areas containing sparse anterograde labeling and isolated retrogradely labeled neurons. The densely labeled patches did not correspond to any known compartments of the striatum. The retrogradely labeled neurons consistently showed similar distribution and morphological features reminiscent of striatal type II projection neurons. As all projection neurons of the striatum and all pallidal neurons are GABAergic, the complementary pattern of anterogradely and retrogradely labeled profiles from the globus pallidus suggest a possible mechanism whereby a horizontal inhibition may be exerted on groups of striatal neurons via the striato-pallido-striatal pathway.

Distribution of the postsynaptic dorsal column projection in the cuneate nucleus of monkeys

Cells in the spinal cord that are postsynaptic to primary afferent fibers project to the dorsal column nuclei in the postsynaptic dorsal column pathway. The projection of cells in the cervical spinal cord of monkeys to the cuneate nucleus has been reported to avoid pars rotunda of that nucleus, the part that contains the somatotopic representation of the ipsilateral hand. We used the sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin to reexamine this projection. We made multiple iontophoretic injections into the cervical enlargements of three monkeys (two Macaca fascicularis and one Macaca mulatta). Control injections were made in the contralateral dorsal columns of one of these and in the dorsal roots of a fourth animal (M. fascicularis) to test for transport by fibers of passage. After 28-39 days, the animals were deeply anesthetized and perfused, and the tissue was processed for immunohistochemical detection of the label. In all cases (excluding control injections), labeled fibers and varicosities were distributed widely in the ipsilateral cuneate and external cuneate nuclei, including pars rotunda. The dorsal column nuclei ipsilateral to control injections contained no label or only very few poorly labeled fibers, indicating that labeling through fibers of passage did not contribute importantly to the results. This study indicates that the postsynaptic projection to the cuneate nucleus is widespread and includes pars rotunda. Such projections may contribute to transmission of information originating in nociceptors through the dorsal column-medial lemniscal system to the ventrobasal thalamus.

Pedunculopontine nucleus in the squirrel monkey: projections to the basal ganglia as revealed by anterograde tract-tracing methods

The efferent projections of the pedunculopontine nucleus (PPN) to the basal ganglia have been studied in the squirrel monkey (Saimiri sciureus) with [3H]leucine and Phaseolus vulgaris-leucoagglutinin (PHA-L) as anterograde tracers. Following unilateral injections of [3H]leucine or PHA-L in the central portion of the PPN, numerous autoradiographic linear profiles or PHA-L-labeled fibers ascend to the forebrain, both ipsilaterally and contralaterally. These fibers form a compact bundle that courses in the central portion of the mesopontine tegmentum. At rostral mesencephalic levels, this bundle splits into ventromedial and dorsolateral fascicles that arborize in basal ganglia and thalamic nuclei, respectively. The substantia nigra and the subthalamic nucleus are by far the most densely innervated structures of the basal ganglia. In these two nuclei, labeled fibers arborize profusely ipsilaterally and less abundantly contralaterally. The labeled fibers in the substantia nigra are thin and varicose and arborize almost exclusively in the pars compacta, where they closely surround the soma and proximal dendrites of dopaminergic neurons. In the subthalamic nucleus, labeled fibers are also thin and appear to contact more than one neuron along their course. Numerous labeled fibers also occur in the pallidal complex, where they arborize most profusely in the internal segment. Several thick, labeled fibers oriented dorsolaterally in the pallidal complex give rise to thinner fibers that closely surround the soma and proximal dendrites of pallidal neurons. Some labeled fibers are also scattered in the striatum. These fibers abound in the peripallidal and ventral portions of the putamen, are more sparsely distributed in the remaining portion of the putamen as well as in the caudate nucleus, and are virtually absent in the ventral striatum. These results reveal that the PPN gives rise to a massive and highly ordered innervation of the basal ganglia in the squirrel monkey. This nucleus may thus act as an important relay in the basal ganglia circuitry in primates.

Commissural connections of the cat periaqueductal gray matter studied with anterograde and retrograde tract-tracing techniques

The commissural connections of the periaqueductal gray matter were investigated by light and electron microscopy by using the anterograde tracer Phaseolus vulgaris leucoagglutinin and the retrograde tracer horseradish peroxidase. In the first group of seven animals (1-7), single injections of Phaseolus vulgaris leucoagglutinin were performed iontophoretically (4.5 microA for 30 min) into various subdivisions of the periaqueductal gray matter. On light microscopic examination, injection sites were characterized by several immunolabeled neurons of different sizes and morphology, with the cytoplasm, nucleus and neuronal processes intensely stained. Many labeled fibers turned from injection sites toward all contralateral periaqueductal gray matter subdivisions, but anterograde labeling was densest in the regions homotopic to those injected. Commissural fibers bore along their course many en passant boutons of different sizes and morphology, and gave off spine-like processes, at the end of which one terminal bouton was observed. Labeled fibers branched into numerous collaterals which ended in a terminal array of 10-20 en passant and en grappe boutons. At the electron microscopic level, commissural axons were observed in close proximity to the cytoplasmic membranes of cells. Axon terminals formed symmetric or asymmetric synapses mainly on dendritic shafts of neurons and rarely on vesicle-containing profiles. Horseradish peroxidase experiments were carried out in four cats (1-4). The tracer was injected iontophoretically into different regions of the periaqueductal gray matter of three cats (1-3). Retrogradely labeled neurons giving rise to commissural connections had a morphology similar to that of polygonal, triangular and fusiform cells described in previous Golgi studies. The perikaryal cross-sectional area of commissural neurons was smaller than that of neurons projecting outside the periaqueductal gray matter (mean value of commissural neurons 149.77 microns 2 vs 261.19 microns 2 for projecting neurons), which were retrogradely labeled by pressure-injecting horseradish peroxidase into several targets of periaqueductal gray matter (4). Moreover, since the distribution of sizes of the two populations of the periaqueductal gray matter overlapped in the range of 90-300 microns 2, a considerable number of projecting neurons were as small as commissural neurons. The present results suggest that commissural fibers could reciprocally connect zones of the periaqueductal gray matter with similar functions, and originate from small and medium-sized neurons, some of which are also projecting neurons.

Direct projections from the entopeduncular nucleus to the lower brainstem in the rat

The present study reports the existence of projection fibers from the entopeduncular nucleus to the superior colliculus and lateral parts of the pontobulbar tegmental regions (so-called lateral tegmental field) in the rat, suggesting that the entopeduncular nucleus may control eye-head and orofacial movements via these projection fibers. The anterograde axonal tracing with Phaseolus vulgaris-leucoagglutinin has revealed that the entopedunculotectal fibers terminate, bilaterally, with an ipsilateral predominance, in the deep layers of the superior colliculus through its rostral one-third level and that the entopedunculotegmental fibers terminate, bilaterally, with an ipsilateral predominance, in the parabrachial area, reticular formation surrounding the trigeminal motor nucleus, and parvicellular, dorsal, and ventral reticular nuclei. The cells of origin of the entopedunculotectal and entopedunculotegmental projections have been identified by retrograde axonal tracing with Fluoro-Gold and cholera toxin B subunit. The entopedunculotectal or entopedunculotegmental fibers originate, respectively, from the dorsal or ventral part of the entopeduncular nucleus. Additionally, a series of fluorescent retrograde double-labeling experiments with Fast Blue and Diamidino Yellow have indicated that single entopeduncular nucleus neurons projecting to the superior colliculus or lateral tegmental field often send their axon collaterals to the lateral habenular nucleus. The entopedunculotectal fibers are assumed to control head movements, which may be provoked via the tectospinal fibers, and further to participate in eye movements as the nigrotectal fibers that have been known to arise from the substantia nigra pars reticulata to end in the deep layers of the superior colliculus primarily through its caudal two-thirds level. The entopedunculotegmental fibers are presumed to be involved in control of orofacial movements, because the sites of termination of the entopedunculotegmental fibers correspond well with the reported areas of distribution of premotor interneurons for the trigeminal motor, facial, and hypoglossal nuclei.

Cholecystokinin corticostriatal pathway in the rat: evidence for bilateral origin from medial prefrontal cortical areas

The anterograde tracer Phaseolus vulgaris-leucoagglutinin was used to examine the organization of the projections to the striatum from medial prefrontal and frontal cortical areas in the rat with reference to their relation to cholecystokinin-like immunoreactivity in the striatum. Medial prefrontal cortical areas projected bilaterally, with an ipsilateral predominance, to the striatum. Most of the positive fibres were found in medial and ventral areas of the caudate-putamen and in the nucleus accumbens. Labelled fibres formed distinct patch-like arrangements throughout the dorsomedial striatum, whereas more ventrally the fibres were densely packed and spread to lateral areas. Almost no fibres were found in the dorsolateral aspects of the caudate-putamen. Cholecystokinin-like immunoreactivity in the striatum was diffusely distributed in the medial aspects, in fine punctate elements as well as in patches of fibres. Overlapping of corticostriatal clusters of fibres, from medial prefrontal cortex, with cholecystokinin-immunoreactive patches was found at all rostrocaudal levels studied, but predominantly in rostral areas. The overlap was present both in the ipsilateral and the contralateral side. Often the cluster of corticostriatal fibres was completely and precisely overlaid by a cholecystokinin-immunoreactive patch. At more caudal planes the overlap was only partial and in some instances cholecystokinin-positive patches "avoided" zones of dense corticostriatal fibre terminations. Frontal cortex injections of tracer gave rise to a network of fibres in the lateral aspects of the striatum, sparing the medial areas. No overlap with cholecystokinin-immunoreactive patches was found in these cases. These results suggest that a large number of cholecystokinin-containing striatal fibres originate in medial prefrontal cortical areas.

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.

Spinal cord and trigeminal projections to the pontine parabrachial region in the rat as demonstrated with Phaseolus vulgaris leucoagglutinin

In order to determine the regions within the parabrachial nucleus that receive synaptic input from nociceptive regions of the spinal cord and medulla in the rat, we analyzed the "Golgi-like" labeling produced by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) from discrete iontophoretic injections confined to either the superficial dorsal horn of the lumbar spinal cord or to the superficial dorsal horn of the trigeminal nucleus at the level of the obex. Labeled fibers from both the spinal cord and the medulla ascended through the ventral lateral pons and coursed with the ventral spinocerebellar tract toward the parabrachial nuclei. Spinal cord injections led to labeling of fine caliber fibers and en passant and terminal enlargements in the rostral part of the contralateral lateral parabrachial nucleus (PBL), mostly in the central lateral and dorsal lateral subnuclei. Medullary injections revealed fiber and enlargement labeling primarily in the ipsilateral caudal PBL, mostly in the central lateral, external lateral, and medial subnuclei. Injections in both regions resulted in labeled terminations in the Kölliker-Fuse nucleus. These results indicate that the nociceptive regions of the spinal cord and medulla terminate in regions of the parabrachial nucleus that have been associated with autonomic functions because of their interconnections with the hypothalamus, brainstem cardiovascular and respiratory control centers, and the amygdala.

Several neuronal and axonal types form long intrinsic connections in the cat primary auditory cortical field (AI)

Intrinsic connections in the cat primary auditory field (AI) as revealed by injections of Phaseolus vulgaris leucoagglutinin (PHA-L) or biocytin, had an anisotropic and patchy distribution. Neurons, labelled retrogradely with PHA-L were concentrated along a dorsoventral stripe through the injection site and rostral to it; the spread of rostrally located neurons was greater after injections into regions of low rather than high characteristic frequencies. The intensity of retrograde labelling varied from weak and granular to very strong and Golgi-like. Out of 313 Golgi like retrogradely labelled neurons 79.6% were pyramidal, 17.2% multipolar, 2.6% bipolar, and 0.6% bitufted; 13.4% were putatively inhibitory, i.e. aspiny or sparsely spiny multipolar, or bitufted. Individual anterogradely labelled intrinsic axons were reconstructed for distances of 2 to 7 mm. Five main types were distinguished on the basis of the branching pattern and the location of synaptic specialisations. Type 1 axons travelled horizontally within layers II to VI and sent collaterals at regular intervals; boutons were only present in the terminal arborizations of these collaterals. Type 2 axons also travelled horizontally within layers II to VI and had rather short and thin collateral branches; boutons or spine-like protrusions occurred in most parts of the axon. Type 3 axons travelled obliquely through the cortex and formed a single terminal arborization, the only site where boutons were found. Type 4 axons travelled for some distance in layer I; they formed a heterogeneous group as to their collaterals and synaptic specializations. Type 5 axons travelled at the interface between layer VI and the white matter; boutons en passant, spine-like protrusions, and thin short branches with boutons en passant were frequent all along their trajectory. Thus, only some axonal types sustain the patchy pattern of intrinsic connectivity, whereas others are involved in a more diffuse connectivity.

Interlaminar connections of the superior colliculus in the tree shrew. I. The superficial gray layer

One of the most persistent problems in the study of the superior colliculus is the relationship between its superficial and deep layers. The superficial tier of layers is considered to be visuosensory in function, whereas the deep tier is multisensory and has premotor functions. This fundamental distinction is the primary basis for the view that a visually triggered shift in the direction of gaze depends on the transfer of information from sensory cells in the superficial tier to premotor cells in the deep tier. The goal of the present experiments was to examine the interlaminar projections of the superficial gray layer in the tree shrew Tupaia belangeri. We used biocytin as the marker for tracing the pathways. The tree shrew was chosen because its large and distinctly laminated superior colliculus facilitates the task of examining connections between the layers. Biocytin was used because of its sensitivity and because it allowed us to place very small injections restricted entirely to the superficial gray layer. The results demonstrated that a prominent pathway originates in the superficial gray layer and terminates in stratum opticum. In comparison, the projection from the superficial gray layer to the layers beneath stratum opticum is extremely sparse. The pathway from the superficial gray layer to stratum opticum has a columnar distribution, extending about 100 microns rostrally and caudally from the center of the injection site. There were no signs of more remote intracollicular connections, nor of patches or bands of terminals. The biocytin injection sites also labeled pathways to nuclei as distant from the superior colliculus as the diencephalon, including the dorsal and ventral lateral geniculate bodies, and the pulvinar. The results suggest that stratum opticum may serve as a link between the superficial gray layer and the deeper layers.

Propriospinal input to thoracolumbar sympathetic nuclei from cervical and lumbar lamina I neurons in the cat and the monkey

The possibility that specific thermoreceptive and nociceptive influences on sympathetic outflow are conveyed directly to spinal sympathetic regions by lamina I neurons was investigated anatomically with the immunofluorescent PHA-L technique in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-II in the cervical (C6-8) or lumbar (L6-7) enlargement. Bilateral (symmetric) terminal arborizations were observed (with an ipsilateral predominance) in the intermediolateral, intermediomedial, and intervening regions of the thoracolumbar intermediate zone. In serial horizontal sections, patches of terminal labeling appeared at regular longitudinal intervals in the intermediolateral region. Longitudinally coursing fibers that had multiple varicosities and gave off small terminal branches were observed in the intermediolateral and the intermediomedial regions. Mediolateral strips of labeling that extended from labeling in the intermediolateral region to labeling in the intermediomedial region occurred at fairly regular longitudinal intervals. Because the longitudinal distribution of these terminations corresponds very well with the characteristic (ladder-like) longitudinal pattern of organization of the neuropil of the thoracolumbar sympathetic nuclei, i.e., the principal part of the intermediolateral cell column, the central autonomic n., and the intervening n. intercalatus, it is inferred that these lamina I terminations occur within these nuclei. After cervical injections, the labeling was most dense in the upper thoracic T2-4 spinal cord segments in both the cat and the monkey; labeling was also present in the T10-12 segments. After lumbar injections, labeling in the cat was located in the L4 segment; labeling in the monkey was present in the T4-6 and T10-12 segments. The labeling obtained was much more dense in the monkey than in the cat. These observations reveal a spinal lamina I projection that could provide a direct pathway for the somatosympathetic reflex effects of thermal and noxious stimuli. Considered together with reports that lamina I and the sympathetic nuclei both receive descending input from certain key autonomic regions, this result emphasizes the importance of lamina I for homeostasis, in addition to its probable roles in behavioral arousal, affect, and sensation. These observations thus support the proposed concept that lamina I processes and distributes in a functionally specific manner the sensory input relevant to the physiological status of the tissues and organs of the entire organism.

GABA-immunoreactive basal forebrain afferents innervate GABA-immunoreactive non-pyramidal cells in the cerebral cortex of the lizard Podarcis hispanica

The basal forebrain projection to the cerebral cortex was studied in the lizard Podarcis hispanica by anterograde transport of Phaseolus vulgaris leucoagglutinin. After injections of the lectin into the septal-basal forebrain area, Phaseolus vulgaris leucoagglutinin-labelled fibres were mainly detected in the outer plexiform layer of the medial cortex and in the inner plexiform layer of the dorsal and dorsomedial cortices. Ultrathin sections from these areas were obtained and processed for postembedding immunogold staining for GABA. Most of the Phaseolus vulgaris leucoagglutinin-labelled boutons in the dorsal and dorsomedial cortical areas were GABA immunoreactive and all the double-labelled boutons established symmetric synaptic contacts on cell bodies and dendrites that were also found to be GABA immunoreactive in all cases. In contrast, Phaseolus vulgaris leucoagglutinin-labelled varicosities in the outer plexiform layer of the medial cortex made asymmetric synaptic contacts on GABA-immunonegative profiles and they were themselves negative for GABA. In double-labelled sections, GABA-, calbindin D28k- and neuropeptide Y-immunoreactive neurons were found to be innervated by multiple Phaseolus vulgaris leucoagglutinin-labelled varicosities in the dorsal and dorsomedial cortical areas, whereas in the medial cortex Phaseolus vulgaris leucoagglutinin-labelled fibres were not observed in contact with any subpopulation of GABAergic cells. The results demonstrate that in lizards the septal-basal forebrain projection to the cortex has a GABAergic component, which selectively terminates on GABAergic non-pyramidal cells including the neuropeptide Y- and the calbindin D28k-containing subpopulations. This synaptic organization is remarkably similar to that in mammals, and suggests that the mechanisms of control of the cortical activity by the basal forebrain have been highly preserved during phylogeny.

The termination pattern and postsynaptic targets of rubrospinal fibers in the rat spinal cord: a light and electron microscopic study

The spinal course, termination pattern, and postsynaptic targets of the rubrospinal tract, which is known to contribute to the initiation and execution of movements, were studied in the rat at the light and electron microscopic levels by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in combination with calbindin-D28k (CaBP), gamma-aminobutyric acid (GABA), and glycine immunocytochemistry. After injections of PHA-L unilaterally into the red nucleus, labelled fibers and terminals were detected at cervical, thoracic, and lumbar segments of the spinal cord. Most of the descending fibers were located in the dorsolateral funiculus contralateral to the injection site, but axons descending ipsilaterally were also revealed. Rubrospinal axon terminals were predominantly found in laminae V-VI and in the dorsal part of lamina VII at all levels and on both sides of the spinal cord, but stained collaterals were also seen in the ventrolateral aspect of Clark's column and in the ventral regions of lamina VII on both sides. The proportion of axonal varicosities revealed on the ipsilateral side varied at different segments and represented 10-28% of the total number of labelled boutons. Most of the labelled boutons were engaged in synaptic contacts with dendrites. Of the 137 rubrospinal boutons investigated, only 2 were found to establish axosomatic synaptic junctions in the lumbar spinal cord contralateral to the PHA-L injection. With the postembedding immunogold method, 80.8% of dendrites establishing synaptic contacts with rubrospinal terminals did not show immunoreactivity for either GABA or glycine, whereas 19.2% of them were immunoreactive for both amino acids. Rubrospinal axons made multiple contacts with CaBP-immunoreactive neurons in laminae V-VI. Synaptic contacts between rubrospinal terminals and CaBP-immunoreactive dendrites were identified at the electron microscopic level, and all CaBP-containing postsynaptic dendrites investigated were negative for both GABA and glycine. The results suggest that rubrospinal terminals establish synaptic contacts with both excitatory and inhibitory interneurons in the rat spinal cord, and a population of excitatory interneurons receiving monosynaptic rubrospinal input is located in laminae V-VI.

Convergence and segregation of septal and median raphe inputs onto different subsets of hippocampal inhibitory interneurons

The convergence and segregation of medial septal and median raphe afferents in the innervation of different subpopulations of GABAergic interneurons was investigated in the rat hippocampal formation. Following local injections of 5,7-dihydroxytryptamine into the median raphe nucleus destroying all serotonergic neurons, iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHAL) into the medial septum resulted in anterograde labelling of axons in the hippocampus. The labelled varicose fibres made multiple contacts with calbindin D28K-, parvalbumin-, and cholecystokinin-immunoreactive interneurons. These results disproved the possibility that PHAL-labelled afferents innervating hippocampal interneurons following septal PHAL injections would have been raphe axons passing through the injection site. In the second set of experiments a double anterograde tracing technique (PHAL from the septum and biotinylated-PHAL from the median raphe) and a triple or double immunostaining procedure was used to determine the types of interneurons (calbindin D28K-, parvalbumin-, or cholecystokinin-immunoreactive) innervated by one, or the other, or both pathways. The results showed that parvalbumin-containing neurons were innervated by septal afferents but avoided by raphe axons, whereas calbindin D28K-containing cells, and to a smaller extent cholecystokinin-containing cells served as targets for both pathways. In some cases the same individual calbindin D28K- or cholecystokinin-containing neurons received multiple contacts from afferents of both septal and raphe origin. Thus, our results indicate that different subcortical nuclei modulate largely different inhibitory circuits in the hippocampal formation. However, considering the occasional convergence of the two subcortical nuclei not only onto the same type, but even onto the same individual calbindin D28K-containing interneurons, we propose that a particular inhibitory function, most probably feed-forward inhibition in the distal dendritic region, is under the control of both pathways.

Notes on a light and electron microscopic double-labeling method combining anterograde tracing with Phaseolus vulgaris leucoagglutinin and retrograde tracing with cholera toxin subunit B

Investigations of monosynaptic connections in the central nervous system have been hindered by the lack of compatible markers that can be used at both light and electron microscopic levels. In attempts to determine synaptic contacts between fibers originating in the substantia nigra and neurons projecting to the spinal cord, we have developed a double immunolabeling technique using anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde transport of unconjugated cholera toxin subunit B (CTB). In this report, we describe technical modifications which consistently produced superior labeling together with adequate ultrastructural preservation of the tissue and discuss the advantages of the two tracers.