Branching projections from subcoeruleus area neurons to medial preoptic area and cervical spinal cord revealed by double retrograde neuronal labeling

In this study we utilized a double retrograde axonal tracing technique to investigate the possible existence of collateralized axonal projections of subcoeruleus area neurons to both 'medial preoptic area (MPA) and cervical (C1-C3) spinal cord'. Following microinjections of fluorescent tracers (Fast blue (FB) and Diamidino yellow (DY) within MPA and C1-C3, substantial numbers of FB and DY single-labeled neurons as well as FB-DY double-labeled branched neurons have been found within subcoeruleus area.     

Descending phenylethanolamine-N-methyltransferase projections to the monkey spinal cord: an immunohistochemical double labeling study

In the present study, we determined that a population of spinally projecting neurons in the monkey brainstem also contained the enzyme phenylethanolamine-N-methyltransferase (PNMT). Following bilateral placements of horseradish peroxidase (HRP) in the cervical spinal cord, brainstem sections containing retrogradely labeled cells were immunohistochemically stained for PNMT. Single labeled PNMT-positive cells were found in a distinctive pattern in the dorsomedial and ventrolateral medulla. A population of double labeled cells was observed in the latter group only. This population was dispersed among other single labeled HRP and single labeled PNMT neurons. Possible functional roles of descending PNMT cells include involvement in sympathetic control of cardiovascular mechanisms and/or tonic descending inhibition of dorsal horn neurons.     

Cortical neurons with collateral projections to both the caudate nucleus and the centromedian-parafascicular thalamic complex: A fluorescent retrograde double labeling study in the cat

The retrograde fluorescent technique was used to label cortical neurons which project to both the caudate nucleus and also to the centromedian-parafascicular (CM-Pf) thalamic nuclear complex. After experimentation with many other pairs of fluorescent tracers, Evans Blue (EB) and Fast Blue (FB) were chosen as the best combination for studying the systems involved. Following injections of EB into the caudate nucleus and FB into the CM-Pf complex, doubly labeled medium-sized pyramidal neurons were present within layer V and VI of specific cortical regions. These cells were found on the inferior bank of the cruciate sulcus, in the anterior limbic area, in the cingulate and anterior sylvian gyri and within the buried cortex of the presylvian sulcus. The doubly labeled cells were relatively few in number compared to the more numerous singly labeled FB (corticothalamic) cells found in layers V and VI, and the very numerous singly labeled EB (corticostriatal) neurons, located in layers II, III, V, and VI.     

Lumbar dorsal root projections to spinocerebellar cell groups in the rat spinal cord: a double labeling study

Summary The aim of this study has been to investigate projections to spinocerebellar cell groups from lumbar dorsal root ganglia (DRGs) in the rat. The binding subunit of cholera toxin conjugated to horseradish peroxidase (B-HRP) was used to label primary afferent fibers. Spinocerebellar neurons were labeled retrogradely by Fluoro-Gold (FG). To determine the orientation of dendrites, retrogradely labeled spinocerebellar neurons were studied, following injections of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into cerebellum. FG or WGA-HRP labeled neurons were found mainly in laminae V and VII, in the lateral group of lamina IX, in Clarke's column (CC) and in the dorsal funiculus. B-HRP labeled primary afferent fibers overlapping with FG labeled cells were observed at all these locations after injections of B-HRP into different DRGs. The overlap in lamina V was found mainly medially and dorsolaterally. CC was found to receive dense projections from DRGs L1–6. In the lumbar part of CC, labeling from DRGs L4–5 overlapped and was distributed over the entire mediolateral extent of the CC, whereas labeling from DRGs L1–3 was somatotopically organized and projected to successively more dorsomedial areas. The central area of lamina VII showed moderate labeling from DRGs L3–5. The lateral group of lamina IX received only smaller amounts of labeled fibers from DRGs L3–5.     

Direct hypothalamic innervation of the trigeminal motor nucleus: a retrograde tracer study

It is currently thought that the hypothalamus influences motor output through connections with premotor structures which in turn project to motor nuclei. However, hypocretinergic/orexinergic projections to different motor pools have recently been demonstrated. The present study was undertaken to examine whether hypocretinergic/orexinergic neurons are the only source of projections from the hypothalamus to the trigeminal motor nucleus in the guinea-pig. Cholera toxin subunit b was injected into the trigeminal motor nucleus in order to retrogradely label premotor neurons. Two anatomically separated populations of labeled neurons were observed in the hypothalamus: one group was distributed along the dorsal zone of the lateral hypothalamic area, the lateral portion of the dorsomedial hypothalamic nucleus and the perifornical nucleus; the other was located within the periventricular portion of the dorsomedial hypothalamic nucleus. Numerous cholera toxin subunit b+ neurons in both populations displayed glutamate-like immunoreactivity. In addition, premotor neurons containing hypocretin/orexin were distributed throughout the lateral dorsomedial hypothalamic nucleus, perifornical nucleus and lateral hypothalamic area. Other premotor neurons were immunostained for melanin concentrating hormone; these cells, which were located within the lateral hypothalamic area and the perifornical nucleus, were intermingled with glutamatergic and hypocretinergic/orexinergic neurons. Nitrergic premotor neurons were located only in the periventricular zone of the dorsomedial hypothalamic nucleus. None of the hypothalamic premotor neurons were GABAergic, cholinergic or monoaminergic. The existence of diverse neurotransmitter systems projecting from the hypothalamus to the trigeminal motor pool indicates that this diencephalic structure may influence the numerous functions that are subserved by the trigeminal motor system.     

Anatomical re-evaluation of the corticostriatal projections to the caudate nucleus: a retrograde labeling study in the cat

The distribution of cortical neurons projecting to the cat caudate nucleus (CN) was examined using retrograde labeling methods. Single injections of either horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA), or the fluorescent tracers Fast Blue (FB) or Diamidino Yellow (DY) were made into different regions of the CN. This study confirms the following previous findings. (1) Labeled neurons were observed in the frontal and parieto-temporal cortices. (2) The corticocaudate cells were mainly located in layer V, although some cells were also observed in layer III and occasionally in layers II and VI. (3) Dorsal injections into the rostral CN yielded more dorsal labeling in the cerebral cortex. However, ventral cortical areas such as the ventral part of the prelimbic (PL) cortical area and the insular cortex (sylvian anterior (SA), agranular and disgranular insular areas) presented retrograde labeling after both dorsal and ventral injections into the CN. (4) Dorsal injections into the CN labeled all subdivisions of areas 4 and 6 whereas the ventral ones labeled only the areas 4delta, 6alphabeta, 6aalpha, 6iffu. The novel findings of this study are as follows. (1) The cortical area 6betabeta and the dorsolateral prefrontal area (PfDl) were labeled in all our cases. In addition, PL, anterior limbic, SA and rostral part of cingulate (Cg) cortical areas were also labeled in most of our cases. (2) Ventral injections into the CN elicited a higher number of retrogradely labeled neurons in the ventral prefrontal area than dorsal injections. (3) A topographical relationship was found between the caudal CN and the dorsomedial prefrontal area so that dorsal injections in the caudal CN elicited retrograde labeling in the rostral PfDl, whereas ventral injections labeled the caudal PfDl. (4) A topography from dorsal to rostral and ventral to caudal was also observed between injections into the CN and PL and Cg. (5) A mediolateral topography was observed in the presylvian, cruciate and splenial sulci.     

The organization of nucleus tegmenti pedunculopontinus neurons projecting to basal ganglia and thalamus: a retrograde fluorescent double labeling study in the rat

The organization of nucleus tegmenti pedunculopontinus (PPN) projections to the basal ganglia and thalamus was studied in the rat by using retrograde transport of fluorescent dyes. Fast blue was injected into the substantia nigra (SN) while Nuclear yellow was delivered to one of the following nuclei: globus pallidus (GP), entopeduncular nucleus, subthalamic nucleus (STN) or parafascicular nucleus of the thalamus. Retrogradely labeled cells were observed throughout the PPN without topographical arrangement. The cells labeled from the SN outnumbered those labeled from other structures. In all cases the majority of cells were single labeled and only a few cells double labeled from SN-GP or SN-STN were found. Labeled cells were either fusiform or multipolar in shape. These data suggest that distinct PPN cells project to their basal ganglia and thalamic targets without a prominent branched organization.     

Collateral projections from nucleus reuniens of thalamus to hippocampus and medial prefrontal cortex in the rat: a single and double retrograde fluorescent labeling study

The nucleus reuniens (RE) of the midline thalamus has been shown to strongly innervate structures of the limbic forebrain, prominently including the hippocampus (HF) and the medial prefrontal cortex (mPFC) and to exert pronounced excitatory effects on HF and mPFC. It was unknown, however, whether RE projections to, and hence actions on, the HF and mPFC originate from a common or largely separate groups of RE neurons. Using fluorescent retrograde tracing techniques, we examined the patterns of distribution of RE cells projecting to HF, to the mPFC or to both sites via axon collaterals. Specifically, injections of the retrograde tracers Fluorogold (FG) or Fluororuby (FR) were made in the mPFC and in various subfields of HF and patterns of single (FG or FR) or double labeled (FG + FR) cells in RE were determined. Pronounced numbers of (single) labeled neurons were present throughout RE with FG or FR injections, and although intermingled in RE, cells projecting to the mPFC were preferentially distributed along the midline or in the perireuniens nucleus (pRE), whereas those projecting to HF occupied a wide mediolateral cross sectional area of RE lying between cells projecting to the mPFC. Approximately, tenfold more labeled cells were present in RE with ventral compared to dorsal CA1 injections. Like single labeled neurons, double labeled cells were found throughout RE, but were most densely concentrated in areas of greatest overlap of FG+ and FR+ neurons or mainly in the lateral one-third of RE, medial to pRE. Depending on specific combinations of injections, double labeled cells ranged from approximately 3–9% of the labeled neurons. The nucleus reuniens has been shown to be a vital link in limbic subcortical–cortical communication and recent evidence indicates a direct RE involvement in hippocampal and medial prefrontal cortical-dependent behaviors. The present findings indicate that RE is critically positioned to influence the HF and mPFC, and their associated behaviors, via separate or collateral projections to these sites.     

The relationship between descending serotonin projections and ascending projections in the nucleus raphe magnus: a double labeling study

The nucleus raphe magnus and rostral parts of the nucleus raphe obscurus were found to have extensive efferent projections: a major ascending non-serotonergic (5-HT) projection through the median forebrain bundle, and a descending system consisting of both 5-HT and non-5-HT neurons. Differences in the localizations of their cells of origin suggest that they form two distinct efferent systems from the caudal medullary raphe nuclei.     

Bulbar neurones with axonal projections to the trigeminal motor nucleus in the cat

Summary The location of bulbar neurones with axons projecting to the ipsi- and contralateral trigeminal motor nucleus were investigated in cats anaesthetized with sodium pentobarbital. Wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected in amounts of 5–24 nl. A volume-calibrated microelectrode was used for recording of evoked potentials and pressure injection of WGA-HRP. The injection site was guided by the position where a maximal antidromic response was evoked by electrical stimulation of the masseteric nerve. The survival time was 19–22 h. In preparations with the depot located in the masseteric subnucleus retrogradely stained neurones were found bilaterally in the borderzone of the trigeminal motor nucleus. Dense populations of stained neurones were observed ipsi- and contralaterally in the dorsal division of the main sensory trigeminal nucleus and the subnucleus- of the oral nucleus of the spinal trigeminal tract. Clusters of WGA-HRP-neurones were observed bilaterally in the lateral tegmental field at the level of the subnucleus- of the oral nucleus of the spinal trigeminal tract, bilaterally dorsal to the facial nucleus and contralaterally adjacent to the hypoglossal nucleus. No stained neurones were found in the gigantocellular reticular nucleus. A group of stained neurones was located in the marginal nucleus of brachium conjunctivum and some were found in the raphé nuclei near obex. Cell profiles were of two types: medium-sized neurones with a triangular profile and 30–40 m diameter, and fusiform neurones 10×50–70 m. Convergence of descending cortical and trigeminal afferent inputs on interneurones located in the lateral borderzone of the trigeminal motor nucleus, i.e. the intertrigeminal area, is reported in the preceding paper.List of Abbreviations BCM Marginal nucleus of the brachium conjunctivum - CAE Nucleus caeruleus - CI Inferior central nucleus - Cu Cuneate nucleus - Cux External cuneate nucleus - DMV Dorsal motor nucleus of the vagus - FTG Gigantocellular tegmental field - FTL Lateral tegmental field - FTP Paralemniscal tegmental field - Gr Gracile nucleus - Mb Medial borderzone of NVmt - NintV Intertrigeminal area - NsV Supratrigeminal nucleus (area) - NVmes Mesencephalic trigeminal nucleus - NVmt Trigeminal motor nucleus - NVsnpr Main sensory trigeminal nucleus - NVsnpr-d Main sensory trigeminal nucleus, dorsal division - NVsnpr-v Main sensory trigeminal nucleus, ventral division - NVspc Caudal nucleus of the spinal trigeminal tract - NVspo- Subnucleus- of the oral nucleus of the spinal trigeminal tract - NVspo- Subnucleus- of the oral nucleus of the spinal trigeminal tract - NVspo- Subnucleus- of the oral nucleus of the spinal trigeminal tract - V Spinal trigeminal tract - NVII Facial nucleus - VII Facial nerve - NXII Hypoglossal nucleus - XII Hypoglossal nerve - Ols Superior olive - Rb Rostral borderzone of NVmt - Vb Ventral borderzone of NVmt - VIN Inferior vestibular nucleus - VSL Superior vestibular nucleus, lateral division     

An anatomical study of brainstem projections to the trigeminal motor nucleus of lampreys.

This study was undertaken to identify and describe populations of brainstem neurons that project to the area of the nucleus motorius nervi trigemini in lampreys as a first step in the study of neurons that control feeding behavior in this species. To identify these neurons, the retrograde tracer cobalt-lysine was injected into the nucleus motorius nervi trigemini on one side of the in vitro isolated brainstem preparation of seven spawning adult lampreys (Petromyzon marinus). Transport times ranged from 42 to 48 h. Retrogradely labeled neurons were found within the rostral spinal cord, the rhombencephalon, the mesencephalon and the caudal diencephalon. This study concentrates on the labeled neurons in the rhombencephalon, since the essential circuits for mastication and swallowing are confined to this region in higher vertebrates. Within the rhombencephalon, labeled cells were in the nucleus sensibilis nervi trigemini on both sides. A densely packed column of labeled neurons was found medial to the nucleus motorius nervi trigemini on the ipsilateral side, extending further rostrally in the isthmic region. Continuous columns of labeled cells were observed in the lateral reticular formation on each side in the basal plate ventral to rhombencephalic cranial motor nuclei. They extended from the rostral trigeminal region down into the rostral spinal cord. A comparison with data from cats and rats shows that the distribution of neurons that project to the nucleus motorius nervi trigemini is very similar in mammals and in agnathes. We conclude that the organization of the motor command network of the trigeminal system is well preserved throughout phylogeny and that the in vitro isolated brainstem of lampreys should be a useful model for the study of vertebrate feeding behavior.     

Central serotonergic projections to the nucleus tractus solitarii: evidence from a double labeling study in the rat

Projections from several brainstem serotonergic nuclei to the nucleus tractus solitarii were investigated in the rat. Experiments were performed using a double labeling method combining retrograde radioautographic tracing and serotonin immunohistochemistry. After injection of the radioactive tracer ([3H] wheat germ agglutinin) into the lateral nucleus tractus solitarii, nerve cell bodies exhibiting both radioautographic labeling and immunostaining were detected in all the serotonergic nuclei investigated, namely the nucleus raphe magnus, the ventromedial paragigantocellular nucleus, the nuclei raphe pontis, medianus and dorsalis, the medial lemniscus and the reticulotegmental nucleus of the pons. Most of the double labeled perikarya observed were in the nucleus raphe magnus, the adjacent part of the paragigantocellular nucleus and the nucleus raphe dorsalis. Nerve cell bodies retrogradely labeled but devoid of immunostaining were also observed, together with the double labeled perikarya, within serotonergic nuclei. These results provide direct evidence that brainstem serotonergic neurons contribute to the innervation of the nucleus tractus solitarii. They indicate that the nucleus raphe magnus and the nucleus raphe dorsalis constitute two major sources of central serotonergic projections to the nucleus tractus solitarii.     

Pallidal inputs to thalamocortical neurons projecting to the supplementary motor area: an anterograde and retrograde double labeling study in the macaque monkey

Summary The relationship between thalamocortical neurons projecting to the supplementary motor area (SMA) and pallidothalamic projection fibers was examined with an anterograde and retrograde double labeling technique in macaque monkeys (Macaca fuscata). In each monkey, Fast Blue (FB) was injected into the handarm area of the SMA after mapping the somatotopy using intracortical microstimulation, and horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) was injected into the ipsilateral internal segment of the globus pallidus (GPi). As a result, numerous projection neurons labeled with FB were distributed in pallidal terminal areas labeled with WGA-HRP in the ventral nuclear group of the thalamus. The present findings indicate that the SMA receives strong indirect projections from the GPi via the thalamus.     

The morphology of neurons in trigeminal nucleus oralis projecting to the medullary dorsal horn (trigeminal nucleus caudalis): a retrograde horseradish peroxidase and Golgi study

This study demonstrates that trigeminal nucleus oralis, the most rostral subdivision of the spinal trigeminal nucleus, contains four morphologically distinct types of small neurons which project to the medullary dorsal horn (trigeminal nucleus caudalis) via descending intratrigeminal pathways. Using the retrograde transport of horseradish peroxidase following injections in the medullary dorsal horn, labeled small neurons with cell bodies ranging from 8-15 microns in diameter are found principally in the ventrolateral portion of the trigeminal nucleus oralis. Most neurons are labeled ipsilaterally throughout the entire rostrocaudal extent of the ventrolateral portion of the trigeminal nucleus oralis, but a few cells are also labeled contralaterally. From this aspect of the present study it can be concluded that a specific portion of the trigeminal nucleus oralis, i.e. the ventrolateral part, contains numerous small neurons which send descending projections to the medullary dorsal horn that could affect synaptic activity there. Utilizing both the methods of Golgi and retrograde horseradish peroxidase labeling four distinct types of small descending medullary dorsal horn projection neurons can be distinguished in the ventrolateral portion of the trigeminal nucleus oralis on the basis of their morphology and the distribution of their axons and dendrites. All four neuronal cell types are present throughout the entire rostrocaudal extent of the trigeminal nucleus oralis. Type I neurons are the most frequently labeled descending medullary dorsal horn projection neurons. They are concentrated in the medial 500-550 microns of the ventrolateral portion of the trigeminal nucleus oralis and display dendritic trees which occupy spherical domains approaching 300 microns in diameter. The unmyelinated axons of many of these cells arise either directly from the cell body or a primary dendrite and give rise to a single collateral within 50 microns of their site of origin. This collateral generates a fine axonal plexus within a portion of the dendritic arbor of the parent cell while the parent axon, without branching further, travels a short distance in the ventrolateral portion of the trigeminal nucleus oralis and enters a deep axon bundle. Type II neurons are the second most frequently labeled descending medullary dorsal horn projection neuron. They generate medial and lateral dendritic arbors which together span nearly the entire medial 500-550 microns of the ventrolateral portion of the trigeminal nucleus oralis. An unmyelinated axon emerges from the cell body and within 10-30 microns of its origin gives rise to two collaterals.(ABSTRACT TRUNCATED AT 400 WORDS)     

The subcortical afferents to caudate nucleus and putamen in primate: A fluorescence retrograde double labeling study

The cellular origin and degree of collateralization of the subcortical afferents to the caudate nucleus and the putamen in squirrel monkeys (Saimiri sciureus) were studied using the following combinations of fluorescent retrograde tracers: Evans blue and DAPI-Primuline, Fast blue and Nuclear yellow, True blue and Nuclear yellow. After the injections, cells containing the tracer delivered in caudate nucleus (caudate-labeled cells) and others labeled with the complementary tracer injected in putamen (putamen-labeled cells) occur in large number in intralaminar nuclei, substantia nigra pars compacta, midbrain raphe nuclei and central midbrain tegmentum. In addition, a small to moderate number of putamen-labeled cells is found in external pallidum, pulvinar and laterodorsal thalamic nuclei, and basolateral amygdaloid nucleus, whereas some caudate and putamen-labeled cells are scattered in ventral tegmental area and locus coeruleus. However, very few double-labeled cells are present in all these structures. In rostral intralaminar nuclei, the labeled cells are not confined to the known cytoarchitectonic boundaries of the nuclei but impinge slightly upon ventrolateral and mediodorsal nuclei. At this level, the caudate-labeled cells lie more dorsally and medially relative to putamen-labeled cells, but a high degree of intermingling exists and some double-labeled cells occur particularly in nucleus centralis lateralis. In caudal intralaminar nuclei, caudate-labeled cells are strictly confined to parafascicular nucleus and putamen-labeled cells present only in centre median, without any overlap between the two neuronal populations. In substantia nigra pars compacta, clusters of caudate-labeled cells are closely intermingled with clusters of putamen-labeled cells according to a complex mosaic-like pattern that varies along the rostrocaudal extent of the structure. Overall, however, caudate-labeled cells predominate rostrodorsally and putamen-labeled cells are more abundant caudoventrally in substantia nigra pars compacta, with only a few double-labeled cells. Some caudate and putamen-labeled cells are also scattered in contralateral substantia nigra pars compacta. In dorsal raphe nucleus, putamen-labeled cells tend to occupy a more lateral position relative to caudate-labeled cells, with again very few double-labeled neurons. The caudate and putamen-labeled cells are less numerous and more closely intermingled in nucleus centralis superior. Numerous striatal afferent cells are also found bilaterally in the peribrachial region of midbrain tegmentum, comprising the pedunculopontine nucleus area. There, the putamen-labeled cells are slightly more numerous than the caudate-labeled cells with less than 10% of these neurons being double-labeled.

Our findings suggest that the subcortical afferents to caudate and to putamen in primates arise largely from different neurons in thalamus and midbrain. These two types of striatal afferent neurons are distributed according to various patterns that are much more complex than could have been inferred from current knowledge of the topographical organization of striatal afferents.     

Divergent projections of catecholamine neurons of the locus coeruleus as revealed by fluorescent retrograde double labeling technique

Divergent projections of catecholamine (CA) neurons of the locus coeruleus have been studied by fluorescent retrograde double labeling in conjunction with monoamine histofluorescence technique. The present results indicate that the coerulo-cortical CA system is composed of two types of neurons. A predominant type possesses few divergent axons innervating a restricted region, while the other type projects widely to various areas of the cerebral cortex. The existence of divergent axonal projections of single CA neurons in the locus coeruleus to the cerebellum and the spinal cord, to the frontal cortex and the cerebellum, is also demonstrated.     

Bulbar raphe neurones with projections to the trigeminal nucleus caudalis and the lumbar cord in the rat: A fluorescence double-labelling study

Retrograde transport of the fluorescent dyes Evans blue and 4,6-diamidino-2-phenyl indole (DAPI) has been used to study projections from the medullary raphe nuclei to the trigeminal nucleus caudalis and to the dorsolateral quadrant of the thoracolumbar cord in the rat. The majority of projecting neurones were found in n. raphe magnus (NRM) and its ventrolateral extensions over the pyramids and inferior olive. Double labelling experiments showed that 94% of raphe-trigeminal neurones sent a collateral branch to the nucleus in the contralateral brainstem. Similarly, 92% of raphe-trigeminal neurones branched to supply the thoracolumbar cord. It is suggested that the widespread nature of the analgesia produced by electrical stimulation in NRM in conscious animals may be due to activation of a population of raphe neurones which inhibit the responses of neurones in the trigeminal nucleus and dorsal horn via an extensive system of collateral projections to these structures.     

Branching neurons in the cervical spinal cord: a retrograde fluorescent double-labeling study in the rat

Summary Branching neurons giving rise to ascending and descending collaterals were studied in the cervical spinal cord of the rat. After unilateral injection of two retrograde fluorescent tracers, i.e. DY.2HCl at T2 or more caudal levels and TB at C1 or more rostral levels, many DY-TB double-labeled neurons were found in C3 to C8. These neurons were located bilaterally throughout the spinal grey matter, as well as in the lateral spinal nucleus (LSN). However, no double-labeled neurons could be detected in the laminae I and II on either side. The double-labeled neurons must represent branching neurons giving rise to a collateral ascending to the rostral injection-site or above, and another collateral descending to the caudal injection-site or below. The descending collaterals were found to extend to various spinal levels, including the lumbosacral cord. However, most of them terminated at shorter distances from their parent cell bodies; thus 20% of the C3–C8 neurons projecting to C1 or above had a descending collateral reaching T2, 8% had a collateral reaching T9, and 3% a collateral reaching L2/L3. The ascending collaterals of the majority of the branching neurons passed into the most caudal part of the medulla oblongata, and about half of these collaterals reached the level of the rostral part of the inferior olive. In regard to the neurons located in the segments C5–C8, about 13% of those projecting to T2 or below distribute an ascending collateral restricted to C2–C4, while 29% of those had an ascending collateral to C1 or above.     

Collateral projections of trigeminal ganglion neurons to both the principal sensory trigeminal and the spinal trigeminal nuclei in the rat

Employing a combination of fluorescent retro grade double labelling and immunofluorescence histo chemistry for substance P (SP) and calcitonin gene-relat ed peptide (CGRP), we examined collateral projections from single neurons in the trigeminal ganglion (TG) of the rat to both the principal sensory trigeminal nucleus (Vp) and the oral, interpolar or caudal subnuclei of the spinal trigeminal nucleus (Vo, Vi or Vc). In the rats that were unilaterally injected with fast blue (FB) into the Vp and with diamidino yellow (DY) into the Vo, Vi or Vc, neurons labelled with FB and/or DY were observed in the TG ipsilateral to the injections. Of the labelled TG neurons, about 2% were double labelled with both trac ers in the rats that were injected with FB into the Vp and with DY into the Vo or Vi, and about 10% were double labelled in the rats that were injected with FB into the Vp and with DY into the Vc. The results indicate that TG neurons sending their axons to the Vp project, by way of axon collaterals, to the Vc more frequently than to the Vo or Vi.Some of the TG neurons double labelled with FB and DY exhibited SP-or CGRP-like immunoreactivity (LI): Of the TG neurons that were double labelled with FB injected into the Vp and with DY injected into the Vo, Vi or Vc, about 38%, 49% and 42%, respectively, displayed SP-LI, and about 54%, 58% and 59%, respectively, showed CGRP-LI. Some of the SP-or CGRP-LI TG neurons that were double labelled with FB and DY were assumed to mediate pain signals to both the Vp and the spinal trigeminal nucleus (Vo, Vi and/or Vc) by way of axon collaterals.Yun-Qing Li is on leave from the Department of Anatomy, The Fourth Military Medical University, Xian, People's Republic of China