Retrograde peroxidase tracing of neurons combined with transmitter histochemistry

Treadmill training induced lumbar motoneuron dendritic plasticity and behavior recovery in adult rats after a thoracic contusive spinal cord injury

Spinal cord injury (SCI) is devastating, causing sensorimotor impairments and paralysis. Persisting functional limitations on physical activity negatively affect overall health in individuals with SCI. Physical training may improve motor function by affecting cellular and molecular responses of motor pathways in the central nervous system (CNS) after SCI. Although motoneurons form the final common path for motor output from the CNS, little is known concerning the effect of exercise training on spared motoneurons below the level of injury. Here we examined the effect of treadmill training on morphological, trophic, and synaptic changes in the lumbar motoneuron pool and on behavior recovery after a moderate contusive SCI inflicted at the 9th thoracic vertebral level (T9) using an Infinite Horizon (IH, 200 kDyne) impactor. We found that treadmill training significantly improved locomotor function, assessed by Basso-Beattie-Bresnahan (BBB) locomotor rating scale, and reduced foot drops, assessed by grid walking performance, as compared with non-training. Additionally, treadmill training significantly increased the total neurite length per lumbar motoneuron innervating the soleus and tibialis anterior muscles of the hindlimbs as compared to non-training. Moreover, treadmill training significantly increased the expression of a neurotrophin brain-derived neurotrophic factor (BDNF) in the lumbar motoneurons as compared to non-training. Finally, treadmill training significantly increased synaptic density, identified by synaptophysin immunoreactivity, in the lumbar motoneuron pool as compared to non-training. However, the density of serotonergic terminals in the same regions did not show a significant difference between treadmill training and non-training. Thus, our study provides a biological basis for exercise training as an effective medical practice to improve recovery after SCI. Such an effect may be mediated by synaptic plasticity, and neurotrophic modification in the spared lumbar motoneuron pool caudal to a thoracic contusive SCI.

A half century of experimental neuroanatomical tracing

Most of our current understanding of brain function and dysfunction has its firm base in what is so elegantly called the 'anatomical substrate', i.e. the anatomical, histological, and histochemical domains within the large knowledge envelope called 'neuroscience' that further includes physiological, pharmacological, neurochemical, behavioral, genetical and clinical domains. This review focuses mainly on the anatomical domain in neuroscience. To a large degree neuroanatomical tract-tracing methods have paved the way in this domain. Over the past few decades, a great number of neuroanatomical tracers have been added to the technical arsenal to fulfill almost any experimental demand. Despite this sophisticated arsenal, the decision which tracer is best suited for a given tracing experiment still represents a difficult choice. Although this review is obviously not intended to provide the last word in the tract-tracing field, we provide a survey of the available tracing methods including some of their roots. We further summarize our experience with neuroanatomical tracers, in an attempt to provide the novice user with some advice to help this person to select the most appropriate criteria to choose a tracer that best applies to a given experimental design.

Localization of substance P in neuronal pathways

The main neuronal systems containing substance P are summarized on the basis of immunohistochemical evidence. The substance P striatonigral projection is one of the most conspicuous of these. Electron microscopic studies using the peroxidase-antiperoxidase technique reveal some heterogeneity in the substance P-immunostained material in the substantia nigra. Immunoreactivity for the peptide is found in terminals establishing both symmetrical and asymmetrical synapses with substantia nigra dendrites. Substance P immunoreactivity in the substantia gelatinosa of the trigeminal nerve and in the skin of the trigeminal territory was found to be depleted after sensory denervation. Electron microscopy showed that in this area of the rat brain substance P-immunoreactive elements are largely associated with dendrites and establish asymmetrical axo-dendritic synapses. Substance P-immunoreactive terminals synapsing with presynaptic dendrites were also observed (i.e. dendrites that themselves are presynaptic to other dendrites). The origin of substance P-containing fibres in the prevertebral ganglia has been investigated in the guinea-pig by combining surgical procedures and immunohistochemistry. Only procedures which disconnected dorsal root ganglia from prevertebral ganglia depleted substance P immunofluorescence in the latter. This substance P-immunoreactive material disappeared after administration of capsaicin. Electron microscopic studies in prevertebral ganglia show that substance P-immunoreactive varicosities establish axodendritic contacts with the sympathetic neurons. These observations provide strong evidence for direct synaptic sensory-autonomic interactions in the prevertebral ganglia involving substance P-containing collaterals of peripheral sensory nerve fibres.

A sequential fluorescence method for neurotransmitter-specific retrograde tracing in the central nervous system of the rat; utilizing True Blue and immunohistochemistry in combination with computer-assisted photography

Aiming to map the distribution of spinally projecting, hypothalamic neurons containing neuronal nitric oxide synthase (nNOS), True Blue (TB) is injected into the rat spinal cord. After survival times of 7-14 days the animals are anaesthetized and perfused transcardially with a solution containing paraformaldehyde and sucrose. After dissection, the injection site is further fixed for 4-8 h, cut in a cryostat, and documented by computer-assisted digital photography. The brain region of interest is fixed for 4 h, rinsed in phosphate buffer for 48 h, sectioned, and photographically documented utilizing filter settings for visualization of TB. The brain sections are then immunohistochemically processed using a primary antibody against nNOS and a Texas Red (TR)-labelled secondary antibody and once again photographically documented, now using filter settings for visualization of TB and TR, respectively. Utilizing the Photoshop program, the TB containing cells can then be exactly aligned and the presence of TB and/or TR fluorescence in the same cell bodies are evaluated. This method for neurotransmitter-specific retrograde tracing derives its high sensitivity from the optimization of fixation/rinsing parameters, the use of appropriate fluorophores, and sequential digital microphotography.

Histological, behavioural and neurochemical evaluation of medial forebrain bundle and striatal 6-OHDA lesions as rat models of Parkinson's disease

We compared the effect of an injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle (MFB) and into the striatum on different parameters for evaluation of motor dysfunction and dopamine denervation in rats, as a function of time. A combination of behavioural, neurochemical and histological techniques was employed. Amphetamine-induced rotation is shown to provide a first rough estimation of motor impairment. Indeed, the number of rotations observed after amphetamine administration can distinguish between a partial and a near complete (>90%) denervation in the substantia nigra. However, lesion sizes of 50-80% resulted in similar rotational behaviour. Similarly, the elevated body swing test (EBST) can determine severe lesions, but is not sensitive enough in the partial model. In both models, determination of the dopamine tissue content with HPLC is a more precise measure of striatal dopamine innervation than striatal TH-immunostaining. The number of cells estimated by TH- and Nissl-staining correlated well in the striatal model, but there was a discrepancy between both measures in the MFB-lesioned animals. Therefore, additional Nissl-staining is necessary for better estimation of the size of the lesion at the level of the substantia nigra or ventral tegmental area in the severely lesioned animals. The MFB lesion model mimics end-stage Parkinson's disease. The striatal injection of 6-OHDA described here cannot be considered a progressive model, since there was no change in the number of TH-immunoreactive cells in the substantia nigra up to 8 weeks post-lesioning. However, the partial denervation renders its quite suitable for mimicking early stage Parkinson's disease, and is thus suitable for testing possible neuroprotective and neurotrophic drugs.

In vivo characterization of somatodendritic dopamine release in the substantia nigra of 6-hydroxydopamine-lesioned rats

We investigated the effect of an injection of 6-hydroxydopamine (6-OHDA) into the rat medial forebrain bundle (MFB) on the degeneration and the function of the dopaminergic cell bodies in the substantia nigra (SN) 3 and 5 weeks after lesioning. After injection of 6-OHDA into the MFB a complete loss of dopamine content was apparent in the striatum 3 weeks after lesioning. In the SN the amount of tyrosine hydroxylase-immunoreactive dopamine cells decreased gradually, with a near-complete lesion (> 90%) obtained only after 5 weeks, indicating that neurodegeneration of the nigral cells was still ongoing when total dopamine denervation of the striatum had already been achieved. Baseline dialysate and extracellular dopamine levels in the SN, as determined by in vivo microdialysis, were not altered by the lesion. A combination of compensatory changes of the remaining neurones and dopamine originating from the ventral tegmental area may maintain extracellular dopamine at near-normal levels. In both intact and lesioned rats, the somatodendritic release was about 60% tetrodotoxin (TTX) dependent. Possibly two pools contribute to the basal dopamine levels in the SN: a fast sodium channel-dependent portion and a TTX-insensitive one originating from diffusion of dopamine. Amphetamine-evoked dopamine release and release after injection of the selective dopamine reuptake blocker GBR 12909 were attenuated after a near-complete denervation of the SN (5 weeks after lesioning). So, despite a 90% dopamine cell loss in the SN 5 weeks after an MFB lesion, extracellular dopamine levels in the SN are kept at near-normal levels. However, the response to a pharmacological challenge is severely disrupted.

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.

Dose-dependent lesions of the dopaminergic nigrostriatal pathway induced by intrastriatal injection of 6-hydroxydopamine

Animal models with partial lesions of the dopaminergic nigrostriatal pathway may be useful for developing neuroprotective and neurotrophic therapies for Parkinson's disease. To develop such a model, different doses of 6-hydroxydopamine (0.0, 0.625, 1.25, 2.5 and 5.0 micrograms/microliters in 3.5 microliters of saline) were unilaterally injected into the striatum of rats. Animals that received 1.25 to 5.0 micrograms/microliters 6-hydroxydopamine displayed dose-dependent amphetamine and apomorphine-induced circling. 6-Hydroxydopamine also caused dose-dependent reductions in [3H]mazindol-labeled dopamine uptake sites in the lesioned striatum and ipsilateral substantia nigra pars compacta (up to 93% versus contralateral binding), with smaller losses in the nucleus accumbens, olfactory tubercle and ventral tegmental area. In the substantia nigra pars compacta and the ventral tegmental area, the number of Nissl-stained neurons decreases in parallel with the reduction in [3H]mazindol binding. The reduction in [3H]mazindol binding in the striatum and the nucleus accumbens, and the reduction in [3H]mazindol binding and in the number of Nissl-stained neurons in the substantia nigra pars compacta and the ventral tegmental area is stable for up to 12 weeks after the lesion. Macroscopically, forebrain coronal sections showed normal morphology, except for rats receiving 5.0 micrograms/microliters 6-hydroxydopamine in which striatal cross-sectional area was reduced, suggesting that this high dose non-specifically damages intrinsic striatal neurons. Nissl-stained sections revealed an area of neuronal loss and intense gliosis centered around the needle track, which increased in size with the dose of neurotoxin. Striatal [3H]sulpiride binding was increased by 2.5 micrograms/microliters and 5.0 micrograms/microliters 6-hydroxydopamine, suggesting up-regulation of dopamine D2 receptors. Striatal binding of [3H]CGS 21680-labeled adenosine A2a receptors, but not of [3H]SCH 23390-labeled dopamine D1 receptors, was reduced at the highest dose, suggesting preservation of the striatal intrinsic neurons with the lower doses. This study indicates that intrastriatal injection of different doses of 6-hydroxydopamine can be used to cause increasing amounts of dopamine denervation, which could model Parkinson's disease of varying degrees of severity. Injecting 3.5 microliters of 2.5 micrograms/microliters 6-hydroxydopamine appears to be particularly useful as a general model of early Parkinson's disease, since it induces a lesion characterized by robust drug-induced rotation, changes in binding consistent with approximately 70% dopamine denervation, approximately 19% dopamine D22 receptor up-regulation, negligible intrinsic striatal damage and stability for at least 12 weeks. This study outlines a technique for inducing partial lesions of the nigrostriatal dopamine pathway in rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunolabeling of retrogradely transported Fluoro-Gold: sensitivity and application to ultrastructural analysis of transmitter-specific mesolimbic circuitry

Fluorescence microscopy shows extensive filling of perikarya and distal dendrites following injections of Fluoro-Gold (FG) into their terminal fields. However, elucidation of synaptic contacts onto identified projection neurons has been limited by the lack of compatibility between electron-dense markers required for ultrastructural analysis and morphology preservation. The recent advent of antisera to FG has revealed numerous potential applications for analyzing chemically defined synaptic circuitry. To take advantage of the high sensitivity of this retrograde tracer in ultrastructural studies, we extended and detailed the original description of single immunocytochemical labeling of FG by comparing the advantages of immunodetection of an antiserum against FG using 2 distinct electron-dense markers: (1) avidin-biotin peroxidase (ABC) reacted with 3,3'-diaminobenzidine and darkened with osmium tetroxide, or (2) silver-intensified 1 nm colloidal gold particles. We subsequently examined the utility of combining these markers in single sections for detection of transmitters (e.g., gamma-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT)) in axon terminals presynaptic to retrogradely labeled neurons. Both analyses were carried out on the well-characterized mesolimbic pathway originating from perikarya in the ventral tegmental area (VTA) that project to the nucleus accumbens. Injections of FG were stereotaxically placed in the nucleus accumbens of anesthetized adult rats. From these animals, vibratome sections of aldehyde-fixed brains were examined for light-microscopic detection of FG using: (1) epi-fluorescence without immunocytochemistry, (2) immunoperoxidase, or (3) immunogold-silver. All 3 methods revealed circumscribed injections in the nucleus accumbens. Additionally, both immunocytochemical methods appeared to be as sensitive as epi-fluorescence in light-microscopic detection of retrogradely labeled perikarya and fine-caliber dendrites extending for 2-3 branch points beyond the soma. Electron microscopy showed that the FG was detectable not only in lysosomes but also throughout the cytoplasmic matrix of perikarya and dendrites using either immunoperoxidase or immunogold-silver labeling methods. In the second part of this analysis, single sections of tissue were processed for dual labeling using either immunoperoxidase or immunogold-silver for detection of FG in conjunction with the converse label for GABA or 5-HT, respectively. Regardless of the labeling combinations, the peroxidase and gold-silver reactions were readily distinguished within sections examined by light or electron microscopy. Synaptic junctions from unlabeled or from GABA or 5-HT labeled terminals were most readily identified when the targets were lightly immunoreactive for peroxidase or labeled using silver-intensified colloidal gold.(ABSTRACT TRUNCATED AT 400 WORDS)

DiI labeling combined with conventional immunocytochemical techniques for correlated light and electron microscopic studies

In order to obtain a detailed understanding of the chemical identity of callosal neurons and of their synaptic targets during development of the rat, a technique was developed combining anterograde and retrograde transport of the carbocyanine dye, DiI, previously applied in living or fixed tissue with conventional immunocytochemistry for peptides. It is reported here that photoconversion of the fluorescent DiI label to a stable diaminobenzidine reaction product is fully compatible with the application of the most widely used immunocytochemical techniques peroxidase-antiperoxidase (PAP) or avidin-biotin (ABC) on the same tissue section, for correlated light and electron microscopic studies. Advantages of this double-labeling procedure over previously described techniques which permit concurrent visualization of projection systems and chemically defined neuronal elements are discussed.

Chemically distinct rat olivocochlear neurons

We have produced a neurochemical map of the cell bodies of origin of the cochlear efferent terminals in rat by combining glutamic acid decarboxylase (GAD), choline acetyltransferase (ChAT), or calcitonin gene-related peptide (CGRP) immunocytochemistry with retrograde transport of horseradish peroxidase. The locations of cochlear efferent cell bodies are in general agreement with the medial and lateral systems described by White and Warr (J. Comp. Neurol. 219:203-214, 1983) with some minor modifications. The lateral system consists of at least two pools of chemically distinct neurons located within the lateral superior olive (LSO) ipsilateral to the injected cochlea. One pool immunostains with an antibody to GAD while the other immunostains with antibodies to ChAT and to CGRP. The medial efferent system consists of periolivary neurons that are almost exclusively large and ChAT-positive but CGRP-negative. They are located both ipsilateral and contralateral to the cochlea they project to. There are a few GAD-positive small neurons in the medioventral and rostral periolivary regions that project ipsilaterally, but these may prove tobe ectopic neurons. The ipsilateral lateroventral periolivary region (LVPO) contains some efferent neurons, all of which are ChAT-positive but CGRP-negative. Additional cochlear efferent neurons, some of which are ChAT-positive and others GAD-positive, are present within and immediately dorsal to the fiber capsule surrounding the medial limb, and to a lesser extent the lateral limb, of the ipsilateral LSO. Not all GAD-positive or ChAT-positive olivary cells project to the cochlea. We have complemented the results in the brainstem by demonstrating two immunocytochemically distinct populations of efferent terminals in the cochlea simultaneously, one CGRP-positive and the other GAD-positive. Approximately equal numbers of boutons immunoreactive for both markers are present beneath inner hair cells throughout the entire length of the cochlea. Surprisingly high numbers of GAD-positive and CGRP-positive boutons are also present on outer hair cells, with each class having its spatially and morphologically distinct features. The lack of CGRP-positive periolivary cells that are retrogradely labeled by cochlear injections of HRP suggests that the lateral olivocochlear system sends projections to outer hair cells. Our results raise questions about species differences in the organization of targets of the lateral and medial olivocochlear systems.

Iontophoretic application of unconjugated cholera toxin B subunit (CTb) combined with immunohistochemistry of neurochemical substances: a method for transmitter identification of retrogradely labeled neurons

In this report, we demonstrate that cholera-toxin B subunit (CTb) is a very sensitive retrograde tracer in the central nervous system when recognized by streptavidin-peroxidase immunohistochemistry. We further show that: (1) injection of a small volume of CTb gives rise to small sharply defined injection sites limited to the cell group of interest associated with the labeling of all the known afferent projections, (2) CTb is taken up, and anterogradely as well as retrogradely transported in damaged but not intact fibers of passage, (3) CTb can be applied iontophoretically, allowing us to study the afferents to small cell groups without any evidence of tissue necrosis in the sites and therefore without artefactual labeling due to uptake by damaged fibers of passage, (4) the use of 4% paraformaldehyde fixative ideally suited for the preservation of most neural antigens, the addition of a 48 h colchicine treatment and the development of a double immunohistochemical method allow the biochemical characterization of the cell of origin of particular pathways in the CNS, (5) CTb is also anterogradely transported with an extensive filling of axons and axon terminals and thereby opens up the possibility of identifying simultaneously the afferents as well as the efferents of the group of cells studied and finally (6) the very long conservation of the preparation, the possibility of counterstaining it and of making camera lucida drawings allow easy and precise localization of the retrogradely labeled cells.

Effects of MPTP on the fine structure of neurons in substantia nigra of dogs

This study evaluates fine structural changes in neurons from pars compacta of substantia nigra in dogs 1 and 4 days after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The toxin induced a disruption and high amplitude swelling of mitochondria, and dispersion of rough endoplasmic reticulum at 4 days. Mitochondria in dendrites were less damaged than those in the soma. Swelling of myelinated axons in the nigrostriatal pathway was evident at 1 and 4 days after injection. Similar morphologic changes are produced by axotomy and inhibitors of mitochondrial function.

Mesostriatal projections in BALB/c and CBA mice: a quantitative retrograde neuroanatomical tracing study

A major portion of the midbrain dopamine (DA)-containing neurons project to the striatum and make up the mesostriatal DA system. The purpose of the present experiment was to map the location and quantify the density of mesostriatal neurons within two inbred mouse strains (BALB/c and CBA) known to possess different numbers of midbrain DA neurons. Computer-assisted reconstructions were made of both the wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) striatal injection site and the retrogradely labeled midbrain cells. There was no strain differences in the major source or topographical pattern of innervation of the striatum from the midbrain cellular regions. Even following small striatal injections, the labeled midbrain cells were found throughout most of the rostrocaudal extent of the midbrain DA nuclei; some labeled cells were found within the substantia nigra, the ventral tegmental area and the adjacent retrorubral field. Although the BALB/c strain has 20-25% more midbrain DA neurons than the CBA, given comparable striatal injection volumes, there was no significant difference in the number of HRP-filled mesostriatal neurons between the two mouse strains. These data suggest that the mesostriatal neurons give rise to comparable axonal branching within the striatum in the two mouse strains.

The iontophoretic application of Fluoro-Gold for the study of afferents to deep brain nuclei

A method is described for identifying the afferents to confined areas within the central nervous system using iontophoretic application of the fluorescent tracer, Fluoro-Gold (FG). Unlike other fluorescent tracers, it is possible to make focal iontophoretic injections through small-tipped micropipettes, and electrophysiological recordings from the injection pipette can be used to define structures prior to injections. Retrograde labeling with FG appears to be as sensitive as wheatgerm agglutinin-conjugated horseradish peroxidase visualized with tetramethylbenzidine. Furthermore, iontophoretically applied FG does not appear to be taken up and transported retrogradely by fibers of passage. Finally, retrograde transport of FG can be combined with immunofluorescence without appreciable loss of sensitivity in either label.

Combined axonal transport tracing and immunocytochemistry for mapping pathways of peptide-containing nerves in the peripheral nervous system

The various combinations of axonal transport tracing and immunocytochemistry used for mapping pathways of peptide-containing nerves, and in particular those of the peripheral nervous system, are reviewed. The advantages and disadvantages of these methods are discussed. The applications and results presented illustrate the future potential value of this approach.

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

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

Ultrastructural morphology of dopaminergic nerve terminals and synapses in the striatum of the rat using tyrosine hydroxylase immunocytochemistry: a topographical study

Structures immunoreactive for TH were examined in the rat striatum (including caudate-putamen, nucleus accumbens and globus pallidus) by electron microscopy using the indirect peroxidase-labeled antibody method. Axon profiles and nerve terminals were the only structures stained by DAB precipitates in the axoplasm. The reactive boutons frequently contained a population of large pleomorphic vesicles (40-60 nm in diameter) but their interiors remained free of reactions. The synaptic contacts formed belonged principally to the symmetric type 2 of Gray while asymmetric Gray's type 1 synapses were rarely observed. The former were mostly apposed to dendritic trunks (rarely to perikarya) and the latter to dendritic spines. In addition, numerous immunoreactive nerve terminals were often found in close contact with small structures identified as the neck of dendritic spines. The active zone of these presumed synapses was characterized by a prominent thickening of the presynaptic membrane but the post-synaptic thickening was lacking. For similar reasons, it was difficult to assert the existence of one axo-axonic synapse when a positive nerve terminal was closely apposed to another one (generally unreactive). The exact morphology of dopaminergic synapses, or even their existence, have not been firmly established owing to large discrepancies between previous reports. No synapses, or synaptic contacts of either asymmetric or symmetric type were described according to the technique used. Our work was undertaken to elucidate further this problem, and in particular, we thought that regional differences in the synaptic organization might explain the divergent data. However, regional quantitative analysis performed in this study did not show significant differences in the percentage of either kind of synapses in the various striatal regions.

Evidence that adrenaline neurons in the rostral ventrolateral medulla have a vasopressor function

Focal electrical or chemical stimulation in the rostral ventrolateral medulla of the rat and rabbit evoked large increases in arterial pressure when the stimulus sites were in the region containing a high density of adrenaline synthesizing neurons, but much smaller or no responses when the sites were outside this region. The adrenaline neurons were identified in the rat by an immunohistochemical procedure, and in the rabbit by a modification of the FAGLU catecholamine fluorescence method. By combining the fluorescence procedure with the method of retrograde transport of horseradish peroxidase, many of the adrenaline synthesizing neurons in the rabbit were shown to project to the spinal cord.

Intracellular and extracellular electrophysiology of nigral dopaminergic neurons--1. Identification and characterization

Intracellular recordings were obtained from directly identified rat nigral dopamine cells in vivo. This identification was based on an increase in glyoxylic acid-induced catecholamine fluorescence in the impaled dopamine neurons. One of three compounds was injected intracellularly into each cell to produce the heightened fluorescence: (1) L-DOPA, to increase the intracellular dopamine content by precursor loading; (2) tetrahydrobiopterin, a cofactor for tyrosine hydroxylase, to increase intracellular dopamine concentration through activation of the rate-limiting enzyme for dopamine synthesis and (3) colchicine, to arrest intraneuronal transport and thus allow the build-up of dopamine synthesizing enzymes and dopamine in the soma. In addition, dopamine cells were antidromically activated from the caudate nucleus and collision with a directly elicited action potential was demonstrated. Identified dopamine neurons were shown to possess an input resistance of 31.2 +/- 7.4 M omega (means +/- SD) and a time constant of 12.1 +/- 3.2 ms. The action potentials were of long duration (2.75 +/- 0.5 ms) with a marked break between the initial segment and the somatodendritic spike components. The initial segment was the only component commonly elicited during antidromic activation. Spontaneously occurring action potentials were usually preceded by a slow, pacemaker-like depolarization. Burst firing by summation of depolarizing afterpotentials was observed to occur spontaneously, but could not be triggered by short depolarizing current pulses. Intravenously administered apomorphine demonstrated the same inhibitory effect on cell firing that was previously reported to occur when recording extracellularly from identified dopaminergic neurons. The determination of the electrophysiological characteristics of a population of cells directly identified as containing a specific neurotransmitter (in this case, dopamine) may allow one to construct better models of a system's functioning. Thus, the high input resistance and long time constant of dopamine-containing cells, combined with their burst/pause firing mode, may be important functionally with respect to a possible modulatory effect of dopamine in postsynaptic target areas.

Motoneurons innervating the cremaster muscle of the rat are characteristically densely innervated by serotoninergic fibers as revealed by combined immunohistochemistry and retrograde fluorescence DAPI-labelling

The lumbar spinal cord of the rat was studied by combined retrograde fluorescent labelling with 4',6-diamidino-2-phenylindole-2HCl (DAPI) and immunoperoxidase procedure using serotonin antiserum. A peculiar small neuronal group endowed more densely than other anterior horn neurons with serotonin-like immunoreactive fibers was recognized in the anterior column of lumbar segments L1-L2. At the same time, this small nucleus was shown to contain the motoneurons innervating the cremaster muscle by means of retrograde labelling with DAPI. It is tentatively suggested that the bulbospinal descending serotonin system is particularly intimately connected with the function of the cremaster muscle.

The effect of forebrain lesions in the neonatal rat: survival of midbrain dopaminergic neurons and the crossed nigrostriatal projection

Lesions were placed in the striatum and the olfactory tubercle of 1-day-old rat pups. Control and experimental animals were raised to adulthood. Efferent projections of mesencephalic neurons were examined by injecting the retrograde tracers horseradish peroxidase or Fast Blue into the undamaged striata of some experimental animals. The survival of the mesencephalic dopaminergic neurons was monitored by using immunocytochemical localization of tyrosine hydroxylase. Small lesions in the caudate-putamen had no appreciable effect on the survival of tyrosine hydroxylase-containing neurons in the mesencephalon, but the density of dopaminergic terminals adjacent to the lesion increased in the remaining caudate-putamen. Striatal lesions that involved an estimated area of more than one-third resulted in loss of dopamine neurons of the substantia nigra compacta. Rostral lesions in the striatum affected mostly rostrally positioned neurons in the substantia nigra. Dorsal lesions of the caudate-putamen resulted in disappearance of dorsal A9 neurons. Reduction of the A10 and A8 dopamine neuron groups occurred if the neonatal lesions involved the olfactory tubercle and nucleus accumbens. Some tyrosine hydroxylase-containing neurons persisted even after the largest lesions. These dopaminergic neurons formed a crossed nigrostriatal pathway which was confirmed by retrogradely transported tracers. The density of this crossed projection in the adult appeared unaffected by the neonatal lesion. We concluded that dopaminergic neurons form topographically ordered projections with their targets in the newborn rat. Rearrangement of these fibers appeared limited, but compensatory increase of axon terminal density was evident in partially lesioned target areas.

Enkephalin-containing sympathetic preganglionic neurons projecting to the inferior mesenteric ganglion: evidence from combined retrograde tracing and immunohistochemistry

The origin of fibers containing enkephalin immunoreactivity in the inferior mesenteric ganglion of the guinea-pig was studied by combining retrograde axonal tracing and indirect immunofluorescence techniques. Fast Blue was applied into the inferior mesenteric ganglion. Three days later colchicine was administered into the subarachnoid space in order to increase the peptide content of the spinal cord cell bodies. The drug was injected through a catheter which was inserted into the cisterna magna and moved to the appropriate spinal cord levels. After the colchicine injection the animals were perfused with formalin and the L2-L3 spinal cord segments were dissected. Cryostat sections of the spinal cord were analyzed in a fluorescence microscope and subsequently processed for indirect immunohistochemistry using antiserum against enkephalin. Several sympathetic preganglionic neurons containing both Fast Blue and enkephalin-like immunoreactivity were seem mainly in the intermediolateral cell column of the cord. The observations strongly support the view that at least some of the enkephalin-containing fibers in the inferior mesenteric ganglion originate in the sympathetic preganglionic nuclei of the spinal cord. These findings are discussed in view of recent physiological studies which have shown that enkephalin may have a presynaptic inhibitory action on preganglionic neurons as well as on substance P containing primary afferent neurons in the inferior mesenteric ganglion.

The projections of the ventral tegmental area and adjacent regions: a combined fluorescent retrograde tracer and immunofluorescence study in the rat

The organization of projection neurons in the ventral tegmental area (VTA), and in adjacent parts of the raphe nuclei medial to it (the central and rostral linear, and interfascicular nuclei), the mammillary body (the supramammillary region and the tuberomammillary nucleus), and the substantia nigra have been examined in the rat with Kuypers' retrograde double labeling method, and with a combined retrograde labeling (with true blue)-immunohistochemical method for the demonstration of dopaminergic neurons. First, the distribution, within the VTA and adjacent regions, of dopaminergic and non-dopaminergic cells that project to terminal fields in the telencephalon (nucleus accumbens, lateral septum, pre- and supragenual fields of the anterior limbic cortex, amygdala, dorsal hippocampus, and entorhinal area), in the diencephalon (lateral habenula), and in the brainstem (locus coeruleus, and parabrachial nucleus) was determined. Then, 15 different combinations of injections of the tracers bisbenzimide and true blue into different terminal fields were made to determine whether individual cells in the region of the VTA send collaterals to more than one site. Taken together, the results indicate that essentially separate groups of cells in the VTA and adjacent regions of the raphe project to each terminal field. In addition, each group can be further divided into dopaminergic and non-dopaminergic components, although the proportion of dopaminergic cells in each group can vary from over 80% (e.g., to the nucleus accumbens) to less than 1% (to the lateral habenula and to the locus coeruleus). In addition, it was found that the supramammillary region, which contains a dense extension of the A10 cell group in its medial part, and the tuberomammillary nucleus, project to, or through, most of the regions injected with retrograde tracers. Virtually all of the projections from the VTA and adjacent regions are partially crossed, the percentage of cells on the uninjected side ranging from over 40% (e.g., for locus coeruleus injections) to only about 2% (e.g., for amygdalar injections). Most of the groups of projection neurons in the region of the VTA are considerably intermixed with the exception of those that project to the lateral septum, to the lateral habenula, and to the hippocampal formation, which are concentrated in ventral and medial parts of the VTA, and in the raphe nuclei medial to the VTA. It was concluded that in the ventral part of the midbrain, essentially separate groups of aminergic and non-aminergic neurons in both the reticular formation (VTA) and in the adjacent nuclei of the raphe project bilaterally to a variety of similar terminal fields in the telencephalon, diencephalon, and brainstem. Further work at the single cell level is needed to determine whether these cell groups are differentially innervated by known inputs to the VTA and adjacent regions, most of which appear to descend through the medial forebrain bundle from sites in the limbic system and hypothalamus.

Identification of serotonin and non-serotonin-containing neurons of the mid-brain raphe projecting to the entorhinal area and the hippocampal formation. A combined immunohistochemical and fluorescent retrograde tracing study in the rat brain

We have studied the localization of serotonin- and non-serotonin-containing cell bodies in the midbrain raphe nuclei that project to the entorhinal area and the hippocampal formation in the rat brain, using the technique of combined retrograde fluorescent tracing and immunohistochemistry on the same tissue section. The branching properties of these neurons were studied by retrograde double labelling using two fluorochromes which emit fluorescence with different spectral characteristics. After injections of granular blue or propidium iodide into the medial entorhinal area, retrogradely-labelled cells were found situated bilaterally in the caudal half of the dorsal raphe nucleus, the medial part of the median raphe and throughout the rostrocaudal extension of the nucleus reticularis tegmentipontis. Injections placed successively more laterally in the entorhinal area labelled progressively less cells contralaterally in the dorsal raphe and the reticular tegmental nucleus of the pons. After fluorochrome injections into the dorsal part of the hippocampal formation, retrogradely-labelled cells were found in the caudal part of the dorsal raphe, in the peripheral part of the median raphe and to a minor extent in the medial part of this nucleus, but not in the nucleus reticularis tegmentipontis. The experiments with double retrograde fluorescent tracing showed that the raphe nuclei do not send bilateral projections to the entorhinal area in spite of the fact that many of these cells are located contralateral to the injected hemisphere in single labelling experiments. Injections of the fluorochromes into the entorhinal area and hippocampal formation showed that at least 10% of the raphe cells project to both areas simultaneously. Analysis of sections incubated with antiserum to serotonin showed that a majority of the retrogradely-labelled versus serotonin-immunoreactive cells was found to vary within different parts of the individual raphe nuclei: the ventromedial part of the dorsal, the medial part of the median and the nucleus reticularis tegmentipontis being the highest. The findings indicate that both serotonin- and non-serotonin-containing neurons in the raphe innervate the hippocampal region, that these projections may be crossed but not bilateral, and that the same neuron in the raphe may influence the neural activity in the entorhinal area and the hippocampus simultaneously.

Postnatal ontogeny of evoked neuronal responses in the substantia nigra of the cat

Single unit extracellular responses evoked by striatal, cortical, and somatosensory stimulation were recorded in substantia nigra (SN) neurons of kittens (1-70 days of postnatal age) and adult cats. Neuronal responses to stimulation of each site were obtained throughout postnatal development. However, 4 major developmental shifts in the response of neurons were found: (1) the responsiveness of neurons to orthodromic activation increased with age; (2) the signs of the initial responses of neurons to orthodromic activation increased with age; (2) the signs of the initial responses of neurons to caudate and cortical stimulation changed with age (excitatory and inhibitory responses in kittens vs almost entirely inhibitory responses in adults); (3) the the response latencies of neurons decreased with age; (4) nigral neurons that were activated antidromically by stimulation of nigrostriatal axons showed age-related decreases in refractory periods and increases in conduction velocities.

Substance P-containing primary sensory neurons projecting to the inferior mesenteric ganglion: evidence from combined retrograde tracing and immunohistochemistry

The origin of substance P-containing fibres in the inferior mesenteric ganglion of the guinea-pig was studied by combining retrograde tracing and indirect immunofluorescence techniques. 'True Blue' or propidium iodide was injected into the inferior mesenteric ganglion. Four days later the animals were perfused with formalin and the dorsal root ganglia L2 and L3 were dissected out. Sections of the spinal ganglia were cut on a cryostat and analysed in a fluorescence microscope with appropriate filter combinations and subsequently processed for indirect immunofluorescence using antiserum to substance P. Several ganglion cells containing both dye (True Blue or propidium iodide) and substance P-like immunoreactivity were observed, strongly supporting the view that at least some of the substance P fibres in the inferior mesenteric ganglion are of sensory nature. Taken together with physiological results from other workers, the present findings suggest that substance P can be released from peripheral, sensory nerve endings, inducing changes in the membrane potential of sympathetic neurons.

Two types of dopaminergic nerve terminals in the rat neostriatum. An ultrastructural study

In the neostriatum 7.7% of all boutons absorbed alpha-methylnoradrenaline, thus staining their synaptic vesicles profiles granular. These presumably monoaminergic synapses were divided into two types according to ultrastructural features. Type A was composed of a relatively large axon terminal and a dendritic spine with postsynaptic membrane thickening. The axon terminal of type B, that is a bouton en passant, was smaller than the postsynaptic profile, while the synaptic contact was symmetrical. Following nigral coagulation, both types of synapse degenerated. It is concluded that both types of synapse are dopaminergic.

Cell groups in the lower brain stem of the rabbit projecting to the spinal cord, with special reference to catecholamine-containing neurons

Two groups of experiments were carried out in rabbits. In the first groups, the distribution of cell bodies within the pons and medulla projecting ipsilaterally and contralaterally to the thoracic or lumbar spinal cord was studied using the horseradish peroxidase (HRP)/tetramethylbenzidine (TMB) procedure. In the second group, both a previously described double-labeling technique and a new modification of it were used to determine the location of catecholamine (CA)-fluorescent pontomedullary cells projecting to the spinal cord. The results demonstrate that the catecholamine (probably norepinephrine)-containing neurons which innervate the thoracic spinal cord are confirmed almost exclusively to the pons where they were found within the A5, A7 and subcoeruleus groups, as well as the ventral portion of the principal part of the locus coeruleus and the more caudal locus coeruleus, including the A4 cell group. Within the medulla oblongata no doubly labeled A2 cells were observed and the few double labeled A1 cells which were observed were confined to the rostral portion of this group. A dense group of HRP-positive but non-fluorescent cells was found rostral to the A1 area in the ventrolateral reticular formation. These cells, which correspond in position to PNMT-containing cells in the rat, appear to project to both thoracic and lumbar segments of the spinal cord. In contrast, spinally projecting neurons within the nucleus tractus solitarius originated from different subnuclei according to their segmental destination. New information about the organization of medial reticulospinal and vestibulospinal pathways was also obtained.

Neurotransmitter-specific projection neurons revealed by combining PAP immunohistochemistry with retrograde transport of HRP

The use of PAP immunohistochemistry in combination with HRP retrograde transport is described, allowing the transmitter characterization of identified projection neurons. To assess the validity of this procedure the dorsal raphe nucleus has been studied. It has been possible in single sections to stain for 5-HT immunoreactivity cells which have been retrogradely labeled following injection of HRP into the striatum. The presence of such neurons and their distribution in the dorsal raphe demonstrated with this dual staining technique agrees very well with previous results obtained from separately performed retrograde labeling and histochemical or immunohistochemical staining. The procedure described has some advantages over other traditional and recently described methods and in addition should be applicable to electron microscopic studies.

Monosynaptic input from the nucleus accumbens--ventral striatum region to retrogradely labelled nigrostriatal neurones

After placement of lesions (either electrolytic or by injection of kainic acid) in an area including the nucleus accumbens and part of the ventral striatum in the rat, the ipsilateral substantia nigra was studied in the electron microscope. Degenerating axons and nerve terminals were found mainly in the zona reticulata and in the ventral layer of the zona compacta. Degenerating synaptic boutons were found in contact with cell bodies (symmetric synapses) and dendrites (mainly symmetric, but a few asymmetric). The postsynaptic target of some of the afferent fibres from the accumbens-ventral striatum was established by demonstrating degenerating synaptic boutons of the above types in contact with nigrostriatal neurones which had been identified by the retrograde transport of horseradish peroxidase (HRP) from the main body of the striatum. Some of the HRP-labelled cells were also impregnated by the Golgi stain and degenerating boutons were found in contact with their distal dendrites. We also observed two types of HRP-containing boutons (presumably labelled anterogradely) in the subsantia nigra after injection of HRP into the main body of the striatum : type 1 boutons contained large spherical vesicles, and formed symmetrical synapses mainly on dendritic shafts in the zona reticulata and in one case the dendrite was from a nigrostriatal neurone; type 2 boutons had pleomorphic and flattened vesicles and formed symmetrical synapses with perikarya and proximal dendrites, especially in the zona compacta. The latter type of HRP-labelled bouton was frequently found in synaptic contact with the cell bodies of nigrostriatal neurones and the same neurones sometimes also received degenerating boutons originating from neurones in the nucleus accumbens-ventral striatum. It is concluded that part of the striato-nigro-striatal circuit includes a monosynaptic link between neurones in the ventral striatum-accumbens and some nigrostriatal neurones. The possible convergence of input from different regions of the striatum on to single nigrostriatal neurones is also suggested.

Non-dopaminergic nigrostriatal pathway

The nigrostriatal projection was studied with a retrograde tracing method (Evans blue, EB) combined with a technique for dopamine histofluorescence. The study, realized in control rats and in animals with 6-hydroxydopamine-induced lesions of the dopaminergic pathway, yielded the following results. (1) In 3 control rats injected with 0.1 microliter of a 10% solution of EB in the center of the caudate-putamen 1 mm anterior to the globus pallidus, 96% of all substantia nigra neurons retrogradely labelled with the dye contained dopamine fluorescence. The remaining ones (average 350 per brain) were devoid of dopamine fluorescence and predominantly found in the posterior 75% of the substantia nigra. These last cells were confined to the upper-half of the pars reticulata. (2) In a series of 6 animals, the cytotoxic agent 6-hydroxydopamine was injected in various locations in the vicinity of either the substantia nigra or the nigrostriatal tract 12--15 days prior to the injections of 0.1 microliter of EB in the same striatal locations as in the controls. Despite a reduction of up to 85% in the number of dopaminergic cell bodies, the substantia nigra of these rats contained the same average number of EB-labelled neurons devoid of dopamine fluorescence. (3) Eight rats received smaller injections (0.1 microliter) of EB in various striatal sites and in two further cases such injections were placed in the globus pallidus to determine more accurately the anatomical location of the dopamine-negative nigral neurons retrogradely labelled with the dye. Following the striatal injections, these cells were found mostly in the upper-half of the pars reticulata and were arranged in longitudinal oriented clusters whose mediolateral location depended on the striatal injection site. Following the pallidal injections, retrogradely labelled neurons devoid of dopamine fluorescence were found in greater numbers and were located in all areas of the pars reticulata. The possibility of retrograde labelling of some nigrothalamic neurons was not entirely ruled out in these two cases. (4) Finally 6 rats received 0.1 microliter injections of EB in various parts of the parietal cortex. In these cases the substantia nigra did not contain any EB-positive dopamine-negative neurons. These results are interpreted as evidence in support of the existence of a topographically organized non-dopaminergic nigrostriatal projection.

Serotonergic and peptidergic projections to the spinal cord demonstrated by a combined retrograde HRP histochemical and immunocytochemical staining method

A simple technique is described for simultaneously visualizing the cells of origin of descending brain stem-spinal systems and immunocytochemically defined neurotransmitters. The combination of horseradish peroxidase (HRP) histochemistry, enhanced by the use of CoCl2, with the unlabeled antibody, peroxidase-antiperoxidase (PAP) immunocytochemical staining method, revealed double labeled cells containing black punctate HRP granules within a homogeneous brown cytoplasm stained positively for serotonin (5-HT), substance P or enkephalin-like immunoreactivity. The advantages of this method over other double labeling techniques are discussed.

A method for tracing biochemically defined pathways in the central nervous system using combined fluorescence retrograde transport and immunohistochemical techniques

A simple method for the simultaneous localization of an antigen and a retrogradely transported fluorescent dye within single neurons is described. The method is based upon: (1) the efficiency of retrograde neuronal labeling with the fluorescent marker 'true blue'; (2) the near-quantitative persistence of retrogradely transported true blue localizations after subsequent processing of the tissue for immunohistochemistry; and (3) the ability to distinguish clearly between true blue- and immunohistochemically-stained cells by simply using appropriate excitation wavelengths for each. First we examined the characteristics of two fluorescent tracers which are effectively transported over long distances in the rat. The results confirm that true blue and bisbenzimide are transported from terminal fields to parent cell bodies and that both tracers are taken up and transported by damaged fibers and by undamaged fibers-of-passage. No evidence for transneuronal transport of either dye in the anterograde or in the retrograde direction was found. Next, using the projection of the paraventricular nucleus of the hypothalamus (PVH) to the spinal cord as a test system, it was found by direct cell counts that a considerably greater percentage of cells in a specific subdivision of the nucleus was labeled following true blue injections into the spinal cord (88%) than was labeled after comparable injections of bisbenzimide (58%), or horseradish peroxidase (HRP) (24%), or after HRP-polyacrylamide gel implants (39%). A comparison of cell counts of true blue-labeled cells in normal material and in series of adjacent sections that were processed for immunohistochemistry suggested that only about 5% of the true blue-labeled cells are no longer detectable following the immunohistochemical procedures employed. Finally, by coupling the fluorescent retrograde tracing method with an indirect immunofluorescence technique, we have been able to localize reproducibly both retrogradely transported true blue and an antigen in individual neurons. The perfusion and staining method employed provided adequate staining of cell bodies that cross-reacted with antisera to one or another of the 9 peptides and enzymes tested. The results indicate that true blue is a versatile and highly sensitive marker for retrograde tracing studies that can be used alone, or in conjunction with respect to their biochemistry as well as their efferent connections.

The distribution and orientation of serotonin fibers in the entorhinal and other retrohippocampal areas. An immunohistochemical study with anti-serotonin antibodies in the rats brain

The serotonin (5-hydroxytryptamine; 5-HT) innervation of the retrohippocampal region (subiculum, pre- and parasubiculum, area 29e, medial and lateral entorhinal area) in the rat brain has been examined with antibodies against 5-HT used in combination with fluorescence histochemistry. Analysis of consecutive sections cut in the coronal, sagittal, and horizontal planes revealed a widespread distribution of 5-HT immunoreactive fibers throughout the retrohippocampal region. This innervation was with regard to the morphological characteristics of the 5-HT fibers, their density and their spatial orientation. On the basis of morphological criteria, four different types of 5-HT positive processes were distinguished: (a) fine, convoluted fibers with small (approximately 0.5-0.8 micron), round and evenly spaced varicosities: (b) fine fibers with small elongated, irregularly distributed varicosities; (c) thick, possibly myelinated fibers, and (d) a terminal plexus with large (5-10 micron), irregularly spaced varicosities. Analysis of the laminar distribution of the 5-HT fibers showed that whereas all layers contain 5-HT positive fibers, the molecular layer was the most densely innervated. The 5-HT fibers were found to be oriented both parallel and transverse to the longitudinal axis of medial and lateral entorhinal area. This grid-like arrangement was less pronounced in the presubiculum. Although the 5-HT innervation of the retrohippocampal region was found to be dominated by a widespread and apparently diffuse pattern, several areas contained dense clusters of preterminal 5-HT processes: area 29e, dorsal presubiculum (layer II), lateral entorhinal area (layer III and ventral layer II) and the transitional zone of the ventral entorhinal area. The 5-HT fibers were found to enter the retrohippocampal region primarily by three different routes; from the ventral and dorsal aspects and from the piriform and lateral neocortex (via the perirhinal area). Most of the fibers enter the region by the ventral route and these were found to ascend in all layers but predominantly in layer I. The location of the 5-HT cells giving rise to the innervation of the entorhinal area was studied by combining retrograde transport of fluorescent tracers with immunohistochemistry on the same tissue section. Both ipsi- and contralaterally located cells in the dorsal and median raphe nuclei were found to project to the entorhinal area. Most, but not all, of these retrogradely labeled cell bodies also contain 5-HT immunoreactivity.

Importance of new catecholamine pathways in control of blood pressure

The first comprehensive maps of central catecholamine pathways referred to both noradrenaline and dopamine neurons. They described the catecholamine neurons as having their cell bodies in the brainstem with spinal axons descending mainly from two medullary cell groups (A1 and A2) and with ascending axons arising mainly from more rostral groups (A3-A13). More recent work utilising immunohistochemistry has established the presence of adrenaline neurons in the brain, in two medullary cell groups (C1 and C2). While these were originally thought to lie within the rostral part of the A1 and A2 cell groups, work from this laboratory has now established that the adrenaline neurons are topographically distinct from the A1 and A2 cells, do not fluoresce with standard methods, and are collected into three groups, the third group (C3) lying in the midline of the rostral medulla. Work in this laboratory using a combination of histochemical fluorescence and retrograde transport of horseradish peroxidase has demonstrated that existence of a descending dopaminergic projection from the hypothalamus to the spinal cord, indicating that descending pathways can arise well above the medulla. Recent studies on the A1 neurons have established the presence of projections from the A1 cells to the median eminence of the hypothalamus and towards the nucleus tractus solitarius in the medulla. Other experiments have demonstrated that most of the descending catecholamine axons do not arise from the A1 and A2 cell groups in the caudal medulla, but higher up in the brainstem especially from the A5 and A7 cell groups. These new descriptions of central catecholamine neurons will necessarily modify the interpretation of many experiments on the central regulation of arterial pressure.