Nonretinal projections to the medial terminal accessory optic nucleus in rabbit and rat: a retrograde and anterograde transport study

The distribution and density of the nonretinal projections to the rabbit medial terminal accessory optic nucleus (MTN) have been studied after injections of horseradish peroxidase (HRP) into the MTN in seven rabbits, and confirmation for the presence of certain of these projections has been made in the rabbit or rat by utilizing anterograde transport of tritiated leucine or leucine/proline after appropriate injections into cerebral cortical areas and brainstem nuclei. In seven cases studied by the retrograde axonal transport method, HRP-labeled neurons have been identified: (A) In four visual or preoculomotor nuclei in which available autoradiographic brain series have confirmed the presence of projections to the MTN: (1) The nucleus of the optic tract/dorsal terminal accessory optic nucleus, (2) the interstitial nucleus of the superior fasciculus (posterior fibers), (3) the periaqueductal gray (including its supraoculomotor portion), and (4) the medial division of the deep mesencephalic nucleus. (B) Within the ventral lateral geniculate nucleus, from which a projection to the MTN has been confirmed autoradiographically in the rat by other workers. (C) In brainstem nuclei and cerebral cortical areas in which available autoradiographic brain series fail to confirm the presence of afferents to the MTN: (1) The nucleus reticularis pontis, pars oralis and pars caudalis, (2) the intermediate interstitial nucleus of the medial longitudinal fasciculus, (3) the nucleus raphe pontis, and (4) five cerebral cortical areas (the area retrosplenialis granularis dorsalis, the striate area, the parietal area 3, the subicular cortex, and the regio praecentralis granularis). Finally, we report retrograde labeling which, on the basis of published connectional data, we believe to result from the spread to and uptake from axons en passant. The false-positive labeling in this category is likely to result from spread of HRP into ventral tegmental nuclei or tracts adjacent to the MTN. Thus, as a result, in the medulla and pons, labeled neurons are found in the medial, lateral, and superior vestibular nuclei, the medullary reticular formation including the nucleus reticularis gigantocellularis, the lateral reticular nucleus, the nucleus raphe magnus, the spinal nucleus of V, the nucleus gracilis/nucleus cuneatus, the dorsal and ventral divisions of the parabrachial nucleus, the central pontine gray, the nucleus K of Meessen and Olszewski, and the dorsal nucleus of the lateral lemniscus.(ABSTRACT TRUNCATED AT 400 WORDS)     

A direct hypothalamic projection to the superior salivatory nucleus neurons in the rat. A study using anterograde autoradiographic and retrograde HRP methods

The location of the superior salivatory nucleus and terminal labelings of the hypothalamic descending fibers were demonstrated in the nucleus reticularis parvocellularis using HRP and the autoradiographic techniques, respectively. When both techniques were used in the same animals, some HRP-labeled neurons were seen among the accumulations of silver grains, suggesting pericellular terminations. The present study demonstrates that the hypothalamic efferents project directly to the superior salivatory nucleus innervating salivary and lacrimal glands.     

Topographically organized projections from the nucleus subceruleus to the hypoglossal nucleus in the rat: a light and electron microscopic study with complementary axonal transport techniques.

Projections from the nucleus subceruleus (nSC) to the hypoglossal nucleus (XII) were investigated with complementary retrograde and anterograde axonal transport techniques at the light and electron microscopic level in the rat. Injections of WGA-HRP into XII resulted in labeling of neurons in and around the nSC. Labeled nSC neurons were few in number (less than 4 per 40-60 microns sections) and variable in size and shape. Most labeled nSC neurons were medium-sized (mean = 16.89 microns), fusiform, triangular, or oval, with 3-4 dendrites typically oriented dorsomedially and ventrolaterally. These neurons were found throughout the rostrocaudal extent of the nSC but were most numerous medial, dorsomedial, and ventromedial to the motor trigeminal nucleus. Others were observed rostral to the motor trigeminal nucleus and ventral to the parabrachial nuclear complex. Confirmation of retrograde results was obtained following injections of tritiated amino acids or WGA-HRP into the nSC. This resulted in labeling throughout the rostrocaudal extent of XII mainly ipsilaterally. Labeled fibers descended the brainstem in the dorsolateral and, to a lesser extent, in the ventromedial component of Probst's tract. Fibers entered XII mainly rostrally along the lateral border of the nucleus. All regions of XII were recipients of nSC afferents, but the caudoventromedial quadrant contained the greatest density of terminal labeling. Electron microscopic evaluation confirmed that nSC afferents synapsed on motoneurons in XII. Axon terminals containing WGA-HRP reaction product were found contacting dendrites and somata, but primarily the former (81.3% versus 10.6%). Axodendritic terminals synapsed mainly on medium-to-small sized dendrites (less than 3 microns in diameter). The majority of labeled axodendritic terminals (90.1%) contained small, round, and clear synaptic vesicles (S-type: 20-50 nm) and were associated with an asymmetric (60.6%), symmetric (11.4%), or no (18%) postsynaptic specialization. By contrast, most axosomatic terminals contained flattened vesicles (F-type) and formed a symmetric or no postsynaptic specialization (75%). Large dense core vesicles (55-90 nm) were observed within a small proportion of all labeled axon terminals (1.3%). The results from this study demonstrate that the nSC projects to XII, preferentially targets a specific subgrouping of protrusor motoneurons, and synapses on both somata and dendrites, although mainly on the latter. The implications of these data are discussed relative to tongue control.     

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.     

Pretectal and brain stem projections of the medial terminal nucleus of the accessory optic system of the rabbit and rat as studied by anterograde and retrograde neuronal tracing methods

The projections of the medial terminal nucleus (MTN) of the accessory optic system have been studied in the rabbit and rat following injection of 3H-leucine or 3H-leucine/3H-proline into the MTN and the charting of the course and terminal distribution of the MTN efferents. The projections of the MTN, as demonstrated autoradiographically, have been confirmed in retrograde transport studies in which horseradish peroxidase (HRP) has been injected into nuclei shown in the autoradiographic series to contain fields of terminal axons. The following projections of the MTN have been identified in the rabbit and rat. The largest projection is to the ipsilateral nucleus of the optic tract and dorsal terminal nucleus (DTN) of the accessory optic system. Labeled axons course through the midbrain reticular formation and the superior fasiculus, posterior fibers of the accessory optic system, to reach the nucleus of the optic tract and the DTN in both rabbit and rat. Axons also run forward to traverse the lateral thalamus and to distribute to rostral portions of the nucleus of the optic tract in rat only. A second, large projection is to the contralateral dorsolateral portion of the nucleus parabrachialis pigmentosus of the ventral tegmental area together with an adjacent segment of the midbrain reticular formation. The patchy terminal field observed has been named the visual tegmental relay zone (VTRZ). This fiber projection courses within the posterior commissure and along its path to the VTRZ, provides terminals to the interstitial nucleus of Cajal and the nucleus of Darkschewitsch, both bilaterally. A third, large MTN projection distributes ipsilaterally to the deep mesencephalic nucleus, pars medialis, and the oral pontine reticular formation. Further, this projection also supplies input to the medial nucleus of the periaqueductal gray matter, bilaterally in the rabbit and rat, and in the rabbit also to the ipsilateral superior and lateral vestibular nuclei. A fourth projection crosses the midline and courses caudally to reach, contralaterally, the dorsolateral division of the basilar pontine complex and the above nuclei of the vestibular complex. A fifth projection of the MTN utilizes the medial longitudinal fasciiculus to reach the rostral medulla, in which its axons distribute ispilaterally to the dorsal cap, its ventrolateral outgrowth, and the beta nucleus of the inferior olivary complex. There is also a contralateral contingent of this projection that leaves the medial longitudinal fasciculus to innervate a small rostral segment of the contralateral dorsal cap.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projection from the cerebellar lateral nucleus to precerebellar nuclei in the mossy fiber pathway is glutamatergic: A study combining anterograde tracing with immunogold labeling in the rat

The pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) are sources of an excitatory projection to the cerebellar cortex via mossy fibers and a direct excitatory projection to the cerebellar nuclei. These precerebellar nuclei, in turn, receive a feedback projection from the cerebellar nuclei, which mostly originate in the lateral nucleus (LN). It has been suggested that the feedback projection from the LN partially uses γ-aminobutyric acid (GABA) as a transmitter. We tested this hypothesis by using a combination of anterograde tracing (biotinylated dextran amine injection into the LN) and postembedding GABA and glutamate immunogold histochemistry. The pattern of labeling in the PN and the NRTP was compared with that of cerebellonuclear terminals in two other target structures, the parvocellular part of the nucleus ruber (RNp) and the ventromedial and ventrolateral thalamus (VM/VL). The projection to the inferior olive (IO), which is known to be predominantly GABAergic, served as a control. A quantitative analysis of the synaptic terminals labeled by the tracer within the PN, the NRTP, and the VL/VM revealed no GABA immunoreactivity. Only one clearly labeled terminal was found in the RNp. In contrast, 72% of the terminals in the IO were clearly GABA immunoreactive, confirming the reliability of our staining protocol. Correspondingly, glutamate immunohistochemistry labeled the majority of the cerebellonuclear terminals in the PN (88%), the NRTP (90%), the RNp (93%), and the VM/VL (63%) but labeled only 5% in the IO. These data do not support a role for GABAergic inhibition either in the feedback systems from the LN to the PN and the NRTP or within the projections to the RNp and the VM/VL. J. Comp. Neurol. 381:320-334, 1997. © 1997 Wiley-Liss, Inc.     

Projection from the nucleus reuniens thalami to the hippocampal region: light and electron microscopic tracing study in the rat with the anterograde tracer Phaseolus vulgaris-leucoagglutinin

In order to study the morphological substrate of possible thalamic influence on the cells of origin and area of termination of the projection from the entorhinal cortex to the hippocampal formation, we examined the pathways, terminal distribution, and ultrastructure of the innervation of the hippocampal formation and parahippocampal region by the nucleus reuniens of the thalamus (NRT). We employed anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L). Injections of PHA-L in the NRT produce fiber and terminal labeling in the stratum lacunosum-moleculare of field CA1 of the hippocampus, the molecular layer of the subiculum, layers I and III/IV of the dorsal subdivision of the lateral entorhinal area (DLEA), and layers I and III-VI of the ventral lateral (VLEA) and medial (MEA) divisions of the entorhinal cortex. Terminal labeling is most dense in the stratum lacunosum-moleculare of field CA1, the molecular layer of the ventral part of the subiculum, MEA, and layer I of the perirhinal cortex. In layer I of the caudal part of DLEA and in MEA, terminal labeling is present in clusters. Injections in the rostral half of the NRT produce the same distribution in the hippocampal region as those in the caudal half of the NRT, although the projections from the rostral half of the NRT are much stronger. A topographical organization is present in the projections from the head of the NRT, so that the dorsal part projects predominantly to dorsal parts of field CA1 and the subiculum and to lateral parts of the entorhinal cortex, whereas the ventral part projects in greatest volume to ventral parts of field CA1 and the subiculum and to medial parts of the entorhinal cortex. The distribution of the reuniens fibers coursing in the cingulate bundle was determined by comparing cases with and without transections of this bundle. The fibers carried by the cingulate bundle exclusively innervate field CA1 of the hippocampus, the dorsal part of the subiculum, and the presubiculum and parasubiculum. They participate in the innervation of the ventral part of the subiculum and MEA. Electron microscopy was used to visualize the axon terminals of PHA-L-labeled reuniens fibers. These terminals possess spherical synaptic vesicles and form asymmetric synaptic contacts with dendritic spines or with thin shafts of spinous dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)     

A direct neuronal projection from the entopeduncular nucleus to the globus pallidus. A PHA-L anterograde tracing study in the rat

The efferent projections of the rat entopeduncular nucleus were examined by use of the anterogradely transported lectin Phaseolus vulgaris-leucoagglutinin (PHA-L). Injections of PHA-L into different parts of the entopeduncular nucleus resulted in a moderate number of labeled nerve fibers in the ipsilateral globus pallidus. The fibers displayed a heterogeneous morphology: some were of small caliber with few delicate varicosities, others were of medium caliber with several more bulbous nerve terminals. Restricted injections in the dorsal and ventral parts of the entopeduncular nucleus, respectively, showed that the dorsal part of the entopeduncular nucleus projects to the dorsal and rostral parts of the dorsal pallidum and the ventral part to the ventral and caudal parts.     

Projections from the lateral geniculate nucleus to the hypothalamus of the Mongolian gerbil (Meriones unguiculatus): an anterograde and retrograde tracing study.

The lateral geniculate nucleus of the thalamus sends efferents to the hypothalamic suprachiasmatic nucleus, which is involved in generation and entrainment of several circadian rhythms. It seems reasonable to believe that the lateral geniculate conveys visual information about the length of the photoperiod to the circadian oscillator. In order to study in more detail the topographical relationship between the lateral geniculate and the suprachiasmatic nucleus, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde tracing with wheatgerm agglutinin coupled to horseradish peroxidase (WGA-HRP) were performed in the gerbil. After iontophoretic injections of PHA-L in the lateral geniculate, a large number of PHA-L-immunoreactive fibers and nerve terminals were observed in the ventrolateral part of the suprachiasmatic nucleus. Nerve fibers were also present in the ventromedial and dorsolateral portions, particularly in the caudal half of the nucleus. PHA-L-immunoreactive nerve fibers continued outside the borders of the suprachiasmatic nucleus to the adjacent anterior hypothalamic, the periventricular, and the subparaventricular areas. A moderate number of fibers entered the lateral hypothalamic area and the tuber cinerum via the optic tract and chiasm. Moreover, the paraventricular nucleus, the supraoptic nucleus, the medial preoptic area, the lateral preoptic area, and the supramammillary nucleus contained a few labeled fibers. In all parts of the hypothalamus receiving an input from the lateral geniculate, fine beaded immunoreactive fibers with varicosities and nerve terminals were observed, some of which were found in close apposition to hypothalamic neurons. Only after labeling of neurons in the intergeniculate leaflet of the lateral geniculate nucleus, fibers were found in the hypothalamus. This topographical organization of the geniculohypothalamic pathway was supported by retrograde tracing after injections of WGA-HRP in the suprachiasmatic area. In these experiments, retrograde labeled neurons were observed in the intergeniculate leaflet and, in agreement with the anterograde studies, most of labeling was observed in the ipsilateral side. These results confirm that the suprachiasmatic nucleus receives a substantial input from the intergeniculate leaflet of the lateral geniculate. Moreover, the present data demonstrate that the suprachiasmatic nucleus is not the only nucleus that receives a direct visual input. Thus other hypothalamic areas might be influenced by a direct rhythmic neuronal input as well.     

Organization of projections from the medial agranular cortex to the superior colliculus in the rat: a study using anterograde and retrograde tracing methods

The organization of corticotectal projections from the medial agranular cortex (AGm), which has been considered to contain rat's frontal eye field, was examined using anterograde and retrograde tracing techniques. When biotinylated dextranamine (BDA) injections were made into the rostral part of the AGm, small numbers of BDA-labeled axons were found in the rostral two-thirds of the superior colliculus (SC) while some labeled axons were seen in the caudal one-third of the SC. These labeled axons were distributed mainly in the lateral part of the stratum griseum intermediale. On the other hand, after BDA injections into the caudal part of the AGm, moderate to dense plexuses of labeled axons were found in the rostral two-thirds of the SC while some labeled axons were seen in the caudal one-third of the SC. These labeled axons were distributed in the ventromedial and dorsolateral marginal zones of the stratum griseum intermediale as well as in the stratum griseum profundum. The corticotectal projections were largely uncrossed. After combined injections of BDA into the caudal part of the AGm on one side and cholera toxin B subunit (CTb) into the paramedian pontine reticular formation on the opposite side or into the interstitial nucleus of Cajal on the same side, the overlapping distributions of BDA-labeled axons and CTb-labeled neurons were found in the ventromedial marginal zone of the stratum griseum intermediale ipsilateral to the site of BDA injection. These results suggest that the caudal part of the AGm plays a more significant role in the oculomotor function than does the rostral part of the AGm.     

Branched projections from the nucleus prepositus hypoglossi to the oculomotor nucleus and the cerebellum. A retrograde fluorescent double-labeling study in the cat

The retrograde transport of fluorescent substances was used in order to investigate divergent axon collaterals of neurons in the nucleus prepositus hypoglossi (Ph). Fast blue (FB) was injected into the flocculus, paraflocculus and/or the vermis, while nuclear yellow (NY) was injected into the oculomotor nucleus alone or combined with injections in the nucleus of Darkschewitsch, the interstitial nucleus of Cajal and the medial longitudinal fascicle. Within optimal survival time, separate populations of single-labeled neurons of both dyes were found in Ph in all cases. Double-labeled neurons were seen in the rostral Ph following FB injections into the flocculus and the paraflocculus and NY injections restricted to the oculomotor nucleus. The present findings demonstrate that many neurons in the rostral Ph give collateral branches to the cerebellum and to the oculomotor nucleus.     

Branching projections from mesopontine nuclei to the nucleus reticularis and related thalamic nuclei: A double labelling study in the rat

Branching projections from pedunculopontine and laterodorsal tegmental nuclei to different thalamic targets were studied by means of a double retrograde tracing technique. The results show a topographic distribution of mesopontine neurons projecting to different thalamic targets. In addition, the present data demonstrate that a small percentage (≤ 5%) of mesopontine neurons projecting to the intralaminar nuclei or to the rostral pole of the reticular nucleus innervate both these areas by means of branching axons. By contrast, a large number of mesopontine neurons projecting to the sensorimotor thalamic nuclei send axon collaterals to the caudal part of the reticular nucleus. The present findings support the hypothesis of an inhomogeneity of different sectors of the thalamic reticular nucleus. Thus, this nucleus can be differentiated into two functional areas, in accordance with their connections with functionally different cortical fields and thalamic districts. The possibility that these two areas of the thalamic reticular nucleus subserve different mechanisms during sleep phenomena is discussed. © 1993 Wiley-Liss, Inc.     

Efferent projections from the periventricular and medial parvicellular subnuclei of the hypothalamic paraventricular nucleus to circumventricular organs of the rat: a Phaseolus vulgaris-leucoagglutinin (PHA-L) tracing study.

The heterogeneous hypothalamic paraventricular nucleus (PVN) is intimately involved in the regulation of several homeostatic functions. These regulations might, at least partly, be mediated via neuronal projections from the PVN to circumventricular organs outside the blood-brain barrier. To study the efferent projections of the medial and periventricular parvicellular subnuclei of the PVN with particular emphasis on the projections to the circumventricular organs, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) was applied. Three major efferent pathways and one minor one coursed from the medial and periventricular parvicellular subnuclei to the circumventricular organs. The major fiber projections included a rostral, a lateral, and a dorsocaudal projection tract, whereas the minor projection coursed ventrally. Fibers of the rostral projection were followed to the preoptic area and along the fornix to the subfornical organ. Single fibers originating from this projection coursed further rostrally to the organum vasculosum laminae terminalis. The lateral projection equivalent to the hypothalamo-pituitary tract passed through the lateral hypothalamic area to the median eminence, and nerve terminals were observed throughout the rostrocaudal extent of this structure. A few fibers of this bundle continued into the infundibular stalk and some terminated in the posterior pituitary lobe. Few fibers of the lateral projection descended to caudal pontine levels, where they reached descending fibers of the dorsocaudal projection. The dorsocaudal projection was essentially restricted to midline structures. Along the midline, fibers were followed from the hypothalamus either dorsally through the thalamus to the dorsal part of the third ventricle or caudally alongside the ventricular wall to the mesencephalic periaqueductal grey. The density of fibers decreased along the caudal direction of the neuraxis. The dorsal part of this projection gave rise to terminals in the deep pineal gland and pineal stalk, whereas the caudal part of this projection sent terminating fibers into the area postrema. The minor ventrally directed projection could be followed through the periventricular region to the rostral part of the median eminence. The number of terminals in the circumventricular organs varied. Within the median eminence, a high density of afferents was observed in the entire rostrocaudal extent of the external zone, whereas a low density of fibers was seen in the internal zone. A medium density of afferents was observed in the organum vasculosum laminae terminalis, whereas a relative low density of nerve terminals was observed in the posterior pituitary, the deep pineal gland, the subfornical organ, and the area postrema.(ABSTRACT TRUNCATED AT 400 WORDS)