Descending Brainstem Pathways Research Articles

Interplay between subthalamic nucleus and spinal cord controls parkinsonian nociceptive disorders.

Pain is a non-motor symptom that impairs quality of life in Parkinson's patients. Pathological nociceptive hypersensitivity in patients could be due to changes in the processing of somatosensory information at the level of the basal ganglia, including the subthalamic nucleus (STN), but the underlying mechanisms are not yet defined. Here, we investigated the interaction between the STN and the dorsal horn of the spinal cord (DHSC), by first examining the nature of STN neurons that respond to peripheral nociceptive stimulation and the nature of their responses under normal and pathological conditions. Next, we studied the consequences of deep brain stimulation (DBS) of the STN on the electrical activity of DHSC neurons. Then, we investigated whether the therapeutic effect of STN-DBS would be mediated by the brainstem descending pathway involving the rostral ventromedial medulla (RVM). Finally, to better understand how the STN modulates allodynia, we used Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) expressed in the STN. The study was carried out on the 6-OHDA rodent model of Parkinson's disease, obtained by stereotactic injection of the neurotoxin into the medial forebrain bundle of rats and mice. In these animals, we used motor and nociceptive behavioral tests, in vivo electrophysiology of STN and wide dynamic range (WDR) DHSC neurons in response to peripheral stimulation, deep brain stimulation of the STN and the selective DREADD approach. Vglut2-ires-cre mice were used to specifically target and inhibit STN glutamatergic neurons. STN neurons are able to detect nociceptive stimuli, encode their intensity and generate windup-like plasticity, like WDR neurons in the DHSC. These phenomena are impaired in dopamine-depleted animals, as the intensity response is altered in both spinal and subthalamic neurons. Furthermore, As with L-Dopa, STN-DBS in rats ameliorated 6-OHDA-induced allodynia, and this effect is mediated by descending brainstem projections leading to normalization of nociceptive integration in DHSC neurons. Furthermore, this therapeutic effect was reproduced by selective inhibition of STN glutamatergic neurons in Vglut2-ires-cre mice. Our study highlights the centrality of the STN in nociceptive circuits, its interaction with the DHSC and its key involvement in pain sensation in Parkinson's disease. Furthermore, our results provide for the first-time evidence that subthalamic DBS produces analgesia by normalizing the responses of spinal WDR neurons via descending brainstem pathways. These effects are due to direct inhibition, rather than activation of glutamatergic neurons in the STN or passage fibers, as shown in the DREADDs experiment.

Bridging the gap: a reticulo-propriospinal detour bypassing an incomplete spinal cord injury.

Anatomically incomplete spinal cord injuries are often followed by considerable functional recovery in patients and animal models, largely because of processes of neuronal plasticity. In contrast to the corticospinal system, where sprouting of fibers and rearrangements of circuits in response to lesions have been well studied, structural adaptations within descending brainstem pathways and intraspinal networks are poorly investigated, despite the recognized physiological significance of these systems across species. In the present study, spontaneous neuroanatomical plasticity of severed bulbospinal systems and propriospinal neurons was investigated following unilateral C4 spinal hemisection in adult rats. Injection of retrograde tracer into the ipsilesional segments C3-C4 revealed a specific increase in the projection from the ipsilesional gigantocellular reticular nucleus in response to the injury. Substantial regenerative fiber sprouting of reticulospinal axons above the injury site was demonstrated by anterograde tracing. Regrowing reticulospinal fibers exhibited excitatory, vGLUT2-positive varicosities, indicating their synaptic integration into spinal networks. Reticulospinal fibers formed close appositions onto descending, double-midline crossing C3-C4 propriospinal neurons, which crossed the lesion site in the intact half of the spinal cord and recrossed to the denervated cervical hemicord below the injury. These propriospinal projections around the lesion were significantly enhanced after injury. Our results suggest that severed reticulospinal fibers, which are part of the phylogenetically oldest motor command system, spontaneously arborize and form contacts onto a plastic propriospinal relay, thereby bypassing the lesion. These rearrangements were accompanied by substantial locomotor recovery, implying a potential physiological relevance of the detour in restoration of motor function after spinal injury.

Lawrence and Kuypers (1968a, b) revisited: copies of the original filmed material from their classic papers in Brain

This article aims to reintroduce two classic papers on motor control published in Brain in 1968, in which Lawrence and Kuypers reported their systematic studies of the effects of lesions to the corticospinal system (Lawrence and Kuypers, 1968a), and subsequently to the descending brainstem pathways (Lawrence and Kuypers, 1968b) in the Old World macaque monkey. They showed that the capacity for independent movements of the digits was permanently lost after a complete, bilateral lesion of the corticospinal system. These studies also revealed that the brainstem pathways contribute to fundamentally different aspects of motor control, with one set of pathways (the ventromedial system) involved in the control of head, trunk and girdle movements, while the other, lateral set of fibres control movements of the extremity such as reach and grasp. There is still much to learn today from these papers. However, an important part of their scientific legacy, the films illustrating the different cases, has long been unavailable. Much of this filmed material is now made available again in video format accessible on the Brain web site, complete with supplementary notes and histological detail. This article summarizes this newly available material for these classic papers in Brain.

Sexually Dimorphic Neural Phenotypes in Golden-Collared Manakins (Manacus vitellinus)

Male golden-collared manakins (Manacus vitellinus) perform a high-speed acrobatic courtship display punctuated by loud ‘snaps’ produced by the wings. Females join males on display courts to select individuals for copulation; females follow displaying males but do not perform acrobatics or make wing snaps. Sexually dimorphic courtship displays such as those performed by manakins are the result of intense sexual selection and suggest that differences between sexes exist at neural levels as well. We examined sex differences in the volume of brain areas that might be involved in the male manakin courtship display and in the female assessment of this display. We found that males had a larger hippocampus (HP, spatial learning) and arcopallium (AP, motor and limbic areas) than females when adjusted for the size of the telencephalon (TELE) minus the target area. Females had a larger ventrolateral mesopallium (MVL) both when adjusting for the size of the remaining TELE and by direct comparison. The entopallium (E) was not sexually dimorphic. The E is part of the avian tectofugal pathway and the MVL is linked to this pathway by reciprocal connections. The MVL likely modulates visually guided behavior via descending brainstem pathways. We found no sex differences in the volume of the cerebellum or cerebellar nuclei. We speculate that the HP is important to males for cross-season site fidelity and for local spatial memory, the AP for sexually driven motor patterns that are complex in males, and that the MVL facilitates female visual processing in selecting male display traits. These results are consistent with the idea that sexual selection has acted to select sex-specific behaviors in manakins that have neural correlates in the brain.

The changing balance of brainstem–spinal cord modulation of pain processing over the first weeks of rat postnatal life

Brainstem-spinal cord connections play an essential role in adult pain processing, and the modulation of spinal pain network excitability by brainstem nuclei is known to contribute to hyperalgesia and chronic pain. Less well understood is the role of descending brainstem pathways in young animals when pain networks are more excitable and exposure to injury and stress can lead to permanent modulation of pain processing. Here we show that up to postnatal day 21 (P21) in the rat, the rostroventral medulla of the brainstem (RVM) exclusively facilitates spinal pain transmission but that after this age (P28 to adult), the influence of the RVM shifts to biphasic facilitation and inhibition. Graded electrical microstimulation of the RVM at different postnatal ages revealed a robust shift in the balance of descending control of both spinal nociceptive flexion reflex EMG activity and individual dorsal horn neuron firing properties, from excitation to inhibition, beginning after P21. The shift in polarity of descending control was also observed following excitotoxic lesions of the RVM in adult and P21 rats. In adults, RVM lesions decreased behavioural mechanical sensory reflex thresholds, whereas the same lesion in P21 rats increased thresholds. These data demonstrate, for the first time, the changing postnatal influence of the RVM in spinal nociception and highlight the central role of descending brainstem control in the maturation of pain processing.

Potential value of vestibular evoked myogenic potentials in paediatric neuropathies

Showing the interest of vestibular evoked myogenic potentials in paediatric neurological vestibulospinal pathology detection and followup. The vestibular evoked myogenic potentials testing apparatus presented is now commonly used in ENT clinics for patients from 1 month of age. Our system and protocol permits control to evoke and select the best EMG level and makes possible a comparison of data from different sides or level of stimulation or different sessions. Normal vestibular evoked myogenic potentials latencies obtained with tone bursts were remarkably stable (P: 13 +/- 0.8 ms, N: 19.6 +/- 1.6 ms). The reported case illustrates abnormal vestibular evoked myogenic potentials latencies in neuropathy. A 6 y.o. child with progressive imbalance was referred to the ENT department for vestibular functional evaluation. Abnormally long latencies in the vestibular evoked myogenic potentials and neurological examination oriented the diagnosis towards Guillain-Barre syndrome and immediate referral to a neurology department. Vestibular evoked myogenic potentials also helped to monitor the neurological recovery. The present case shows the potential value of vestibular evoked myogenic potentials in diagnosis and evaluation of descending brainstem pathways in neuropathies like Guillain-Barre syndrome in complement to neurological evaluation.

Inhibition of the Nociceptive R2 Blink Reflex after Supraorbital or Index Finger Stimulation is Normal in Migraine Without Aura Between Attacks

In order to explore possible interictal brainstem dysfunctions in migraine, we have studied the R2 component of the nociceptive specific blink reflex (nBR) after conditioning by supraorbital or index finger stimuli in 14 untreated migraine without aura patients (MO) between attacks and in 15 healthy volunteers. We determined the R2 recovery curve at increasing inter-stimulus intervals between 50 and 600 ms. The nBR was conditioned by a paired supraorbital stimulus and, in another session, by an ipsilateral electrical shock delivered to the index finger. The R2 nBR recovery curves were normal in MO patients for both the supraorbital and peripheral conditioning. These results do not favour persistent interictal sensitization in the spinal trigeminal sensory system. They also suggest that the control exerted by descending brainstem pathways on medullary R2 interneurones is normal in migraine between attacks.

Arm Function after Stroke: From Physiology to Recovery

There are varying degrees of spontaneous improvement in arm paresis over the first 6 months after stroke. The degree of improvement at 6 months is best predicted by the motor deficit at 1 month despite standard rehabilitative interventions in the ensuing 5 months. Animal studies indicate that the loss of fine motor control, especially individuation of the digits, is due to interruption of monosynaptic corticomotoneuronal connections. Spasticity occurs because of loss of cortical modulatory control on descending brain stem pathways and spinal segmental circuits but is not a major cause of motor dysfunction. Quantitative studies of reaching movements in patients suggest that arm paresis consists of higher-order motor planning and sensorimotor integration deficits that cannot be attributed to weakness or presence of synergies. Cortical stimulation experiments in animals and functional imaging studies in humans indicate that motor learning and recovery after stroke share common brain reorganization mechanisms. Rehabilitation techniques enhance learning-related changes after stroke and contribute to recovery. Future research will benefit from using quantitative methods to characterize the motor impairment after stroke and by applying concepts in motor learning to devise more physiologically based rehabilitation techniques.

Ascending sensory, but not other long-tract axons, regenerate into the connective tissue matrix that forms at the site of a spinal cord injury in mice.

Mice exhibit a unique wound healing response following spinal cord injury in which the lesion site fills in with a connective tissue matrix. Previous studies have revealed that axons grow into this matrix, but the source of the axons remained unknown. The present study assesses whether any of these axons were the result of long tract regeneration. C57Bl/6 mice received crush injuries and were allowed to survive for 6 weeks to 7 months. Biotinylated dextran amine (BDA) was injected into the somato-motor cortex to trace descending corticospinal tract (CST) axons, into the midbrain to label descending brainstem pathways including the rubrospinal and reticulospinal tracts, or into the L5 dorsal root ganglion to trace ascending projections of first-order sensory neurons. Spinal cords from other mice were prepared for immunocytochemistry using antibodies against neurofilament protein (NF), 5-HT to reveal descending serotonergic axons, calcitonin gene-related protein (CGRP) to reveal ascending sensory axons, and chondroitin sulfate proteoglycan (CSPG) to assess the distribution of molecules that are inhibitory to axon growth. NF immunostaining revealed axons in the connective tissue matrix at the lesion site, confirming previous studies that used protargol staining. CST axons did not enter the connective tissue matrix, but did sprout extensively in segments adjacent to the injury site. Rubrospinal and reticulospinal tract axons also did not grow into the lesion site. 5-HT-positive axons extended to the edge of the lesion, and a few axons followed astrocyte processes into the margins of the lesion site. In contrast to the other pathways, BDA-labeled ascending sensory axons did extend into and arborized extensively within the connective tissue matrix, although the subgroup of ascending axons that are positive for CGRP did not. These results indicate that the connective tissue matrix is permissive for regeneration of some classes of ascending sensory axons but not for other axonal systems.

Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. I. Walking on a level surface.

To examine the function of descending brain stem pathways in the control of locomotion, we have characterized the discharge patterns of identified vestibulo- and reticulospinal neurons (VSNs and RSNs, respectively) recorded from the lateral vestibular nucleus (LVN) and the medullary reticular formation (MRF), during treadmill walking. Data during locomotion were obtained for 44 VSNs and for 63 RSNs. The discharge frequency of most VSNs (42/44) was phasically modulated in phase with the locomotor rhythm and the averaged peak discharge frequency ranged from 41 to 165 Hz (mean = 92.8 Hz). We identified three classes of VSNs based on their discharge pattern. Type A, or double peak, VSNs (20/44 neurons, 46%) showed two peaks and two troughs of activity in each step cycle. One of the peaks was time-locked to the activity of extensor muscles in the ipsilateral hindlimb while the other occurred anti-phase to this period of activity. Type B, or single pause, neurons (13/44 neurons, 30%) were characterized by a tonic or irregular discharge that was interrupted by a single pronounced and brief period of decreased activity that occurred just before the onset of swing in the ipsilateral hindlimb; some type B VSNs also exhibited a brief pulse of activity just preceding this decrease. Type C, or single peak, neurons (9/44 neurons, 23%) exhibited a single period of increased activity that, in most cells, was time-locked to the burst of activity of either extensor or flexor muscles of a single limb. The population of RSNs that we recorded included neurons that showed phasic activity related to the activity of flexor or extensor muscles [electromyographically (EMG) related, 26/63, 41%], those that were phasically active but whose activity was not time-locked to the activity of any of the recorded muscles (13/63, 21%) and those that were completely unrelated to locomotion (24/63, 38%). Most of the EMG-related RSNs showed one (15/26) or two (11/26) clear phasic bursts of activity that were temporally related to either flexor or extensor muscles. The discharge pattern of double-burst RSNs covaried with ipsilateral and contralateral flexor muscles. Peak averaged discharge activity in these EMG-related RSNs ranged from 4 to 98 Hz (mean = 35.2 Hz). We discuss the possibility that most VSNs regulate the overall activity of extensor muscles in the four limbs while RSNs provide a more specific signal that has the flexibility to modulate the activity of groups of flexor and extensor muscles, in either a single or in multiple limbs.

Abnormal muscle coactivation patterns during isometric torque generation at the elbow and shoulder in hemiparetic subjects.

To study abnormal spatial patterns of muscle activation in hemiparetic stroke, we compared EMG activity in paretic and contralateral elbow and shoulder muscles of 10 hemiparetic subjects during 1.5-s voluntary isometric contractions, against five to eight different loads. Isometric forces were generated in eight directions, referenced to a plane orthogonal to the long axis of the forearm, and were recorded by a three degrees of freedom load cell, mounted at the wrist. Surface and intramuscular EMGs of six elbow and six shoulder muscles were recorded from both impaired and contralateral upper extremities of each subject. The spatial characteristics of EMG activation of individual muscles were summarized using two measures. The first, called the 'net resultant EMG vector' is a new measure which calculated the vector sum of EMG magnitudes for each of the eight directions, and the second, index of EMG focus, is a measure of the range of EMG activation recorded for each load level. Use of these measures permitted us to describe spatial EMG characteristics quantitatively, which has not been done previously. We observed consistent and statistically significant shifts in the resultant EMG vector directions in the impaired limb, especially in shoulder and other proximal muscles. Significant increases in the angular range of EMG activity were also identified and were most evident at the elbow. Correlation analysis techniques were used to assess the degree of coactivation of different muscle pairs. There were consistent EMG coactivation patterns observed across all subjects (both normal and hemiparetic). However, in spasticparetic limbs, additional novel coactivational relationships were also recorded, especially between elbow flexors/shoulder abductors and elbow extensors/shoulder adductors. These novel coactivation patterns represent a reduction in the number of possible muscle combinations, or in the number of possible 'synergies' in the paretic limb of the stroke subject. This reduction in number of 'synergies' could result from a loss of descending command options; from an increased reliance on residual, descending brainstem pathways (such as the reticulospinal and vestibulospinal projections); from changes in spinal interneuronal excitability; or from a combination of several of these factors. The relative merits of these hypotheses are addressed.

Mechanisms of seizure suppression during rapid-eye-movement (REM) sleep in cats

REM sleep is the most antiepileptic state in the sleep-wake cycle for human generalized epilepsy, yet the neural mechanism is unknown. This study verified the antiepileptic properties of REM sleep in feline generalized epilepsy and also isolated the responsible factors. Conclusions are based on 20 cats evaluated for generalized EEG and motor seizure susceptibility before and after dissociation of specific REM sleep components. Bilateral electrolytic lesions of the medial-lateral pontine tegmentum created a syndrome of REM sleep without atonia. Systemic atropine created a syndrome of REM sleep without thalamocortical EEG desynchronization. Identical results were obtained in two seizure models, systemic penicillin epilepsy and electroconvulsive shock. (1) Normal REM sleep retarded the spread of EEG seizure discharges and had even more potent anticonvulsant effects. (2) Selective loss of ‘sleep paralysis’ (skeletal muscle atonia) during REM abolished REM sleep protection against myoclonus and convulsions without affecting generalized EEG paroxysms. (3) Conversely, selective loss of thalamocortical EEG desynchronization abolished REM sleep protection against generalized EEG seizures without affecting clinical motor accompaniment. These results suggest that the descending brainstem pathways which mediate lower motor neuron inhibition also protect against generalized motor seizures during REM sleep. Protection against spread of EEG paroxysms is governed by a separate mechanism, presumably the ascending brainstem pathways mediating intense thalamocortical EEG desynchronization during REM sleep.

Neurons containing calcitonin gene-related peptide in the parabrachial nucleus project to the central nucleus of the amygdala.

The source, distribution, and morphology of axons displaying calcitonin gene-related peptide (CGRP) immunoreactivity in the central amygdaloid nucleus of the adult rat were investigated with immunohistochemical techniques, both alone and in combination with retrograde transport of fluorescent tracers. An extremely dense plexus of CGRP-immunoreactive axons is differentially concentrated within the lateral capsular and lateral central subdivisions of the central nucleus, and much lighter concentrations of labeled fibers are present in the rostral part of the medial subdivision. No immunoreactive neurons were observed in the central nucleus in any of the experimental animals. The immunoreactive axons characteristically form prominent pericellular terminal arborizations surrounding unlabeled neurons. The number of cells receiving this dense input increases at caudal levels of the central nucleus. Retrograde label of central nucleus neurons by dye transport from injections into the pontine parabrachial nucleus and the nucleus of the tractus solitarius combined with CGRP immunohistochemistry established that many neurons in the central nucleus which receive dense pericellular innervation from CGRP-immunoreactive axons are projecting caudally to the parabrachial nucleus or, to a lesser extent, to the nucleus tractus solitarii. Central amygdaloid injections of rhodamine-labeled microspheres or fluorogold followed by immunohistochemical localization of cellular CGRP immunoreactivity revealed that the central amygdaloid CGRP fiber plexus originates bilaterally from the parabrachial nucleus. These multipolar CGRP-containing neurons are preferentially concentrated in the external medial and external lateral subnuclei, in the ventral aspect of the parabrachial nucleus. These results relating central amygdaloid CGRP to ascending and descending brainstem pathways, taken together with the extreme density of the fiber plexus, strongly suggest the relevance of the CGRP input to central nucleus function in cardiovascular and other autonomic regulation.

Retrograde double labeling of neurons: the combined use of horseradish peroxidase and diamidino yellow dihydrochloride (DY·2HCl) compared with true blue and DY·2HCl in rat descending brainstem pathways

Three series of double-labeling experiments were carried out in a study of the collateralization of brainstem nuclei which project to the spinal cord in the rat. The fluorescent tracer Diamidino Yellow Dihydrochloride (DY·2HCl) was injected in one half of the spinal gray and white matter at T7-T8 or T13-L1. Subsequently, either True Blue (TB), wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) or free HRP were injected ipsilaterally in the gray matter at C5–C8. The distributions of single and double retrogradely laabeled neurons were studied in the following cell groups: red nucleus, interstitial nucleus of Cajal, ventrolateral pontine tegmentum, nuclei coeruleus and subcoeruleus and nuclei raphe magnus and raphe pallidus including the adjoining ventral reticular formation. The numbers of TB- or HRP-labeled neurons present in those nuclei were counted and the percentages of double-labeled neurons were calculated when TB or HRP had been used in combination with DY·2HCl. The results indicate: (1) The HRP-TMB reaction product and the DY·2HCl fluorescence can be visualized simultaneously in retrogradely single- or double-labeled neurons. (2) The distributions of single- and double-labeled neurons in the various brainstem nuclei were entirely comparable when using TB with DY·2HCl or HRP with DY·2HCl. (3) The percentages of double-labeled neurons obtained with HRP and DY·2HCl were consistent over a series of cases, and were comparable to those obtained with TB and DY·2HCl in several structures. However, in the red nucleus slightly lower percentages of double-labeled neurons were obtained using HRP and DY·2HCl as compared with the percentages obtained using TB and DY·2HCl.

Retrograde neuronal labeling of cells of origin of descending brainstem pathways in rat using SITS as a retrograde tracer

Recently, the retrograde tracer SITS has been introduced, a cytoplasmic label, which seems to be taken up by terminals only 15. In order to test the transport of SITS from terminals of brainstem pathways to the spinal cord SITS was injected in different rats at different spinal levels. It could be concluded that SITS is taken up primarily from axon terminals.

Brain Stem Projections to Spinal MotoneuronalCell Groups in Rat Studied by means of Electron Microscopy Autoradiography

Publisher Summary This chapter discusses brain stem projections to spinal motoneuronal cell groups in rat studied by means of electron microscopy autoradiography. Histofluorescent studies since the early sixties have shown that serotonergic and noradrenergic brain stem cell groups in different animals distribute descending fibers to the spinal grey matter throughout the spinal cord. In cat, the descending brain stem pathways have been traced into the spinal cord by means of this anterograde amino acid transport technique. Several present findings suggest that the large and medium-sized terminals of the reticulospinal fibers in the lumbar motoneuronal cell groups contain flattened vesicles. The findings in a rat with an injection in the area of the nucleus coeruleus and subcoeruleus suggest that the large to medium sized terminals of the fibers from this brain stem area contained spherical vesicles. To facilitate the correlation between the light microscopic and the electron microscopic autoradiographic findings both the semithin and the ultrathin sections of each pyramid were studied.

The Anatomy of Brain Stem Pathways to the Spinal Cord in Cat. A Labeled Amino Acid Tracing Study

This chapter discusses the anatomy of brain stem pathways to the spinal cord in cat. In the series of experiments described in the chapter, [3H] leucine injections were made in the brain stem and the spinal distribution of the labeled descending brain stem pathways was studied autoradiographically. The pontine reticulospinal tract, as well as the vestibulospinal tracts, distribute to the ventromedial parts of the intermediate zone mainly ipsilaterally. Anterograde fiber degeneration findings obtained since the early decades of this century by means of several techniques consistently showed that the descending brain stem pathways to the spinal cord terminate mainly in the spinal intermediate zone—for example, in Rexed's laminae V–VIII. However, to facilitate the identification of the various descending pathways, the brain stem distribution of the cells of origin of these pathways in the cat has been reinvestigated by means of the horseradish peroxidase (HRP) technique by using the procedure of Mesulam.

Funicular Trajectories of Descending Brain Stem Pathways in a Lizard(Varanus exanthematicus)

This chapter discusses funicular trajectories of descending brain stem pathways in a lizard. Horseradish peroxidase (HRP) slow-release gels have been applied to various parts of the lateral and ventral funiculi of the cervical enlargement in the lizard Varunus exanthematicus to obtain more conclusive evidence on funicular trajectories of various descending pathways. In an earlier study, the preparation of the HRP slow-release gel was slightly modified to obtain a slower release of the enzyme and a less fragile dried gel, which would be easier to handle and to apply. Following the implantation of HRP slow-release gel in the dorsal part of the lateral funiculus, many labeled neurons were observed throughout the brain stem. No labeled neurons were found in the hypothalamus. A few labeled cells were present in the ipsilateral interstitial nucleus of the fasciculus longitudinalis medialis (flm). At the isthmic level, labeled cells were observed in the locus coeruleus and in two reticular cell groups.

Fluorescent retrograde neuronal labeling in rat by means of substances binding specifically to adeninethymine rich DNA

Six fluorescent substances, binding specifically to adenine-thymine rich DNA, were injected in rat caudate-putamen. This resulted in retrograde axonal transport and fluorescent retrograde labeling of neurons in centermedian parafascicular nucleus, substantia nigra and dorsal raphe. Two of these substances, i.e. ‘True Blue’ and ‘Granular Blue’, give a very striking blue fluorescent retrograde neuronal labeling. Mid-thoracic spinal injections of these two substances in rat also resulted in a pronounced retrograde fluorescent labeling of neurons of the descending brainstem pathways and of neurons in the sensori-motor cortex.