Brainstem Motor Research Articles

The quantitative autoradiographic distribution of [ 3H]MK-801 binding sites in the normal human brainstem in relation to motor neuron disease

The distribution of N-methyl- d-aspartate (NMDA) receptors in the normal human brainstem has been investigated using MK-801. Specific [ 3H]MK-801 binding showed a heterogeneous distribution, the greatest density of binding sites being found in the substantia nigra, locus coeruleus, and the hypoglossal and inferior olivary nuclei. Brainstem motor nuclei subserving eye movements, which tend to be spared in motor neuron disease (MND), had significantly lower densities of binding compared to other cranial nerve motor nuclei (V, VII, X, XII) which tend to be affected. The anatomical distribution of NMDA receptors may be one factor determining selective vulnerability to excitotoxic injury.

Immunohistochemical labeling of androgen receptors in the brain of rat and monkey

Androgen receptor antibodies have recently been developed using fusion proteins containing fragments of human prostatic androgen receptor. We have used a polyclonal antibody raised in rabbits to label androgen receptors in brain sections from male and female rats and monkeys. Free-floating frozen sections were incubated in primary antibody, and processed by the peroxidase-avidin-biotin complex method using biotinylated anti-rabbit IgG. Nickel intensified diaminobenzidine was used as the chromagen, and neurons were labeled in the amygdala, hippocampus, bed nucleus of stria terminalis, septum, preoptic area, in several hypothalamic nuclei including the supraoptic and paraventricular nuclei, in several brain stem motor nuclei and in cerebral cortex. Staining was most intense in cell nuclei but also occurred in cytoplasm and in some neuronal processes. Labeling was more restricted in monkey than in rat brain. Omitting the primary antibody or pre-incubating the primary antibody with rat prostatic cytosol for control purposes demonstrated the specificity of staining.

Expression of adrenergic receptors in individual astrocytes and motor neurons isolated from the adult rat brain.

Attempts to show the distribution of adrenergic receptors (ARs) in autoradiographs of a brainstem motor nucleus following elimination of motor neurons yielded the unexpected result of an increase in beta-AR density. This increase was related to the gliosis accompanying the motor neuron degeneration. To determine the cells on which the AR subtypes were located, we dissociated cells from various regions of the adult rat brain and subsequently identified astrocytes by glial fibrillary acidic protein (GFAP) immunofluorescence. Slides containing the astrocytes were prepared for autoradiography using the nonselective beta ligand 125I-iodocyanopindolol (125ICYP) or the alpha 1 ligand 125IBE 2254 (125I-HEAT). The addition of the selective beta 1 blocker betaxolol or the beta 2 blocker ICI 118.551 to the incubation medium to displace 125ICYP binding was used to determine the binding of beta-AR subtypes. The great majority (greater than 88%) of isolated astrocytes sampled from the trigeminal motor nucleus, cerebral cortex, striatum, and cerebellum showed beta-AR binding. Astrocytes from the first three regions had similar average densities of beta-ARs, whereas the density in cerebellar astrocytes was 2- to 3-fold greater. The beta 2-AR subtype was proportionally greater than the beta 1 subtype in each region. Reactive astrocytes isolated from the trigeminal motor nucleus after degeneration of motor neurons showed a beta-AR density nearly 2-fold greater than resting astrocytes from the same region, with the beta 1 subtype showing the greater proportional increase. There was no beta-AR binding on trigeminal motor neurons. Astrocytes also showed a significant level of alpha 1-AR binding. No differences in alpha 1-AR binding were found in normal astrocytes isolated from the different regions, nor was there an increase in reactive astrocytes. In contrast, trigeminal motor neurons had an alpha 1-AR density nearly 10 times greater than astrocytes. In terms of the NE modulation of synaptic responses in motor neurons, the distribution of ARs would permit NE to act indirectly through alpha 1 and beta receptors on astrocytes and directly through alpha 1 receptors on motor neurons.

Brainstem control of orienting movements: intrinsic coordinate systems and underlying circuitry.

A fundamental issue in the understanding of how the nervous system processes information is the way in which sensory information is used to initiate and guide movements. Recent progress has been made by taking an information processing approach in which information--for example, the spatial location of an object towards which an animal will orient--is tracked through the nervous system from sensory to motor levels. In this approach, neurally encoded information is characterized in terms of its representation within a neural or intrinsic coordinate system or set of neural coding parameters. For example, the retina codes spatial location in terms of the location of activity on the retinal surface, whereas motoneurons code spatial location in terms of the pulling directions of the muscles they activate. In between these two peripheral stages, the information passes through intermediate coordinate systems. These intermediate coordinate systems can be characterized by recording or altering the activity of small groups of neurons while an animal is performing a well-defined sensorimotor task. Spatial location information is used to guide orienting movements, those movements made by the eyes, ears, head, or body which function to center an object of interest in the animal's visual field. The optic tectum and forebrain, their connections to the medial mesencephalic and rhombencephalic brainstem tegmental cell groups, and subsequent connections to brainstem motor nuclei and spinal cord are employed to control fundamental aspects of this behavior. Studies reviewed herein indicate that following the retinotopic coding of spatial location in the retina and tectum, spatial location information appears to enter a different coordinate system at tegmental levels in which spatial aspects of orienting movement are coded in terms of their discrete horizontal and vertical components. This Cartesian coordinate system is an example of an abstract neural coordinate system, in that it is a simple, low-dimensional representation of spatial location which differs greatly from both sensory and motor representations. Also, this Cartesian representation may be common to many orienting movements, yet it appears to differ from the coordinate systems controlling other movement types such as stabilization or phasic movements. This suggests an hypothesis in which coordinate systems, especially at intermediate levels of processing, may be organized according to behavioral task as opposed to being determined by the particular sensory or motor system involved in the behavior. Understanding the evolutionary heritage and computational function of abstract neural coordinate systems, and the relation between different coordinate systems and behavioral tasks may be useful in understanding general aspects of sensory information processing and motor control.

Nerve growth factor differentially modulates the expression of its receptor within the CNS.

The effect of nerve growth factor on the expression of nerve growth factor receptor in the central nervous system of newborn and adult rats was studied by means of immunohistochemistry with the monoclonal antibody 192-IgG. Both during development and in adulthood, the intracerebroventricular administration of nerve growth factor elicited a pronounced increase of nerve growth factor receptor-like immunoreactivity in the cell bodies and neural processes of the basal forebrain cholinergic nuclei, as compared to cytochrome c-treated rats (controls). A pronounced nerve growth factor-induced increase of nerve growth factor receptor-like immunoreactivity was also observed in central regions innervated by trigeminal and spinal ganglia. A moderate to a marked increase of nerve growth factor receptor-like immunoreactivity was evident in some mesencephalic visual system-related structures and thalamic nuclei expressing nerve growth factor receptor. In contrast, NGF treatment did not induce appreciable modification of nerve growth factor receptor-like immunoreactivity in cerebellar, brainstem, and spinal motor structures of newborn rats. In adult nerve growth factor-treated rats, a decrease of nerve growth factor receptor-like immunoreactivity was detected in the cerebellum, whereas no re-expression of nerve growth factor receptor-like immunoreactivity occurred in the motor structures that had expressed it in the first postnatal week. Finally, nerve growth factor was also found to enhance, in both adult and newborn rats, nerve growth factor receptor-like immunoreactivity associated with ependymal and subependymal cellular elements of the lateral and third ventricles, as well as with the leptomeninges overlying the superior colliculus and supraoptic area. The present results indicate that endogenous nerve growth factor or nerve growth factor-like molecules may play a dynamic role in a variety of cell populations of both the developing and mature mammalian central nervous system. We thus propose the nerve growth factor ability to modulate its receptor in vivo as a novel criterion to define nerve growth factor or nerve growth factor-like molecules, sensitive neuronal, and non-neuronal cells. Whereas this criterion does not intrinsically possess absolute physiological validity, its pharmacological concomitants might be relevant in view of the proposed therapeutical use of this trophic factor.

Immunocytochemical evidence for retrograde axonal transport of exogenous albumin in adult rat brain stem motor neurons

Serum albumin is present in cranial nerve motor neurons in the brain. This is suggested to be due to uptake into peripheral axon terminals with subsequent retrograde axonal transport to the perikarya of the motor nuclei. Indirect evidence for uptake and retrograde axonal transport of albumin in a cranial nerve has been presented previously e.g. by demonstration of fluorescence of the albumin-bound dye Evans blue in the neurons of the hypoglossal nucleus subsequent to injection in the tongue. In this study we present direct evidence for retrograde axonal transport of albumin. Human albumin was injected into the tongue of adult rats, and immunocytochemically detected with an antibody, which selectively reacted with human albumin. In brains processed 24 hours or later after injection, a positive reaction was seen in the neurons of the hypoglossal nucleus. The immunoreaction product was localized as perinuclear granules in an otherwise unstained perikaryon probably associated with the endosomal-lysosomal system. Thus, albumin present in hypoglossal motor neurons appears to originate from uptake in peripheral axon terminals. Judging from the early appearance after injection, this transport probably occurs by a fast component of retrograde axonal transport.

Computed tomography, magnetic resonance imaging and positron emission tomography with [18F] fluorodeoxyglucose in multiple system atrophy and pure autonomic failure

We studied 45 patients who had autonomic failure with computed tomography, magnetic resonance imaging and positron emission tomography with [18F]fluorodeoxyglucose to characterize the neuroimaging features of multiple system atrophy and pure autonomic failure and determine the utility of these techniques in distinguishing multiple system atrophy from pure autonomic failure. There were 30 patients with multiple system atrophy and 15 with pure autonomic failure. In the multiple system atrophy group, eight patients had mainly cerebellar signs, seven extrapyramidal and 15 had combinations of cerebellar and extrapyramidal signs. Cerebellar atrophy on computerized tomography and magnetic resonance imaging, signal hypointensity in the posterolateral putamen on magnetic resonance imaging and a generalized reduction in glucose utilization rate with positron emission tomography with [18F]fluorodeoxyglucose, were the main findings and were seen only in the patients with multiple system atrophy. Decreased glucose utilization (hypometabolism) was most prominent in the cerebellum, brainstem, striatum and frontal and motor cortices. These results indicate clear differences, using neuroimaging studies, between multiple system atrophy and pure autonomic failure.

Early MR Abnormality Indicating Functional Recovery from Spontaneous Intracerebral Hemorrhage

Magnetic resonance (MR) imaging as an indicator of recovery from hemiparesis was evaluated in 60 patients with spontaneous intracerebral hemorrhage. T2-weighted MR images revealed early MR abnormality (EMA) of the corticospinal tract within 1 week of ictus. Most patients without EMA recovered beyond Brunnstrom's Recovery Stage 3 while only a few patients with EMA did so. Patients with EMA cannot regain motor function because EMA is almost always followed by complete tract degeneration. EMA in the brainstem and poor motor function recovery are closely correlated.

Transforming growth factor beta isoforms in the adult rat central and peripheral nervous system

The distribution of transforming growth factor-beta isoforms 1, 2 and 3 and transforming growth factor-beta 2 and 3 mRNAs in adult rat central and peripheral nervous system was examined using Northern blotting and isoform specific antibodies for immunocytochemistry. Transforming growth factor-beta 2 and 3 mRNA were present in all brain areas including cerebral cortex, hippocampus, striatum, cerebellum and brainstem. In sciatic nerve, transforming growth factor-beta 3 mRNA was highly expressed, but transforming growth factor-beta 2 mRNA was not detectable. Transforming growth factor-beta 1-like immunoreactivity was confined to meninges and choroid plexus in the brain and connective tissue in peripheral ganglia and nerves. Transforming growth factor-beta 2 and 3 immunore-activity entirely overlapped and, in general, were found in large multipolar neurons. Highest densities of immunoreactive neuronal perikarya were present in spinal cord and brainstem motor nuclei, hypothalamus, amygdaloid complex, hippocampus and cerebral cortical layers II, III and V. Most thalamic nuclei, superior colliculi, periaqueductal gray and striatum were almost devoid of transforming growth factor-beta 2- and 3-immunoreactive neurons. Fibrous astrocytes in white matter areas were intensely immunostained. Most dorsal root ganglionic neurons, their satellite cells and Schwann cells in peripheral nerves were also labeled. Transforming growth factor-beta 2- and 3-immunoreactive neurons were localized in brain regions that have been shown to contain neurons synthesizing and/or storing basic fibroblast growth factor suggesting possible opposing or synergistic effects of these peptide growth factors. However, the precise functions of local synthesis and storage of the transforming growth factor-beta isoforms in the nervous system are as yet unknown.

Colchicine enhances mRNAs encoding the precursor of calcitonin gene-related peptide in brainstem motoneurons

Hybridization signals indicating mRNAs encoding the precursor of calcitonin gene-related peptide (CGRP) and CGRP immunoreactivity were detected on parallel sections containing brainstem motor nuclei using in situ hybridization histochemistry and immunohistochemistry. In untreated and saline-injected rats the motoneurons in the hypoglossal, facial motor nuclei and in the ambiguus nucleus showed weak to moderate hybridization signals. In these motoneurons CGRP immunoreactivity was restricted to the Nissl bodies of the perikarya. Twenty-four and 42 hours after intracerebroventricular colchicine injection the intensity of both the hybridization signal and the immunoreaction product increased. The distribution of CGRP immunoreactivity changed from discrete perikaryal localization to diffuse reaction in the perikarya and along the proximal dendritic tree. Motoneurons in the rest of the brainstem motor nuclei (VIth, Vth, IVth and IIIrd) of untreated and saline-injected rats showed neither hybridization signal nor CGRP immunoreactivity. After intracerebroventricular injection of colchicine these motoneurons showed both hybridization signal and CGRP immunoreactivity. In all nuclei the size of motoneurons decreased and their Nissl structure changed to an amorphous basophilic mass following colchicine treatment.

Generation patterns of immunocytochemically identified cholinergic neurons in rat brainstem

Combined [ 3H]thymidine autoradiographic and choline acetyltransferase (ChAT)-immunocytochemical techniques were used to answer questions concerning the generation of specific classes and subclasses of cholinergic neurons in rat brainstem. First, the generation of rostrally and caudally located neurons of the same class (i.e. somatic efferent oculomotor and hypoglossal nuclei, respectively) were compared. Results indicated that, although embryonic day 11 (E11) was the peak birthday for both nuclei, hypoglossal neurons were generated significantly earlier than oculomotor neurons, indicating a caudorostral generation gradient for brainstem somatic motor nuclei. Second, the generation patterns of 3 different subclasses of motor neurons at the same brainstem level were compared; namely those of the somatic efferent hypoglossal nucleus (XII), the general visceral efferent dorsal nucleus of the vagus (X), and the predominantly special visceral efferent nucleus ambiguus. All 3 subclasses of cholinergic cells had the same peak day (E11) and overall period of generation (E11–12). However, statistical analyses indicated a precocious generation of nucleus ambiguus, but no developmental differences between N. XII and N. X. It is suggested that nucleus ambiguus is formed earlier than N. XII and N. X, due to its more ventral location within a ventrodorsal neurogenetic gradient. Third, the generation patterns of different classes of large cholinergic neurons were examined. Specifically, the birthdays of cholinergic non-motor projection neurons of the pedunculopontine-laterodorsal tegmental nuclei (PPT-LDT) were contrasted to those of the cholinergic brainstem motor neurons. The peak birthdays of both rostrally and caudally located motor neurons were two days earlier than those of the PPT-LDT neurons. Thus, large cholinergic cells projecting to peripheral targets are born significantly earlier than those projecting within the CNS, even though the former are located more rostrally on the caudorostral neurogenetic gradient. This represents an apparent exception to the emerging rule that cholinergic neurons obey the general gradients of neurogenesis manifest in the regions of the central nervous system where they reside.

Lesions producing REM sleep without atonia disinhibit the acoustic startle reflex without affecting prepulse inhibition

This study determined whether the brainstem motor inhibition system that mediates muscle atonia during rapid eye movement (REM) sleep is involved in the elicitation and prepulse inhibition of the acoustic startle reflex. Electrolytic or neurotoxic (glutamate) lesions were made in the dorsolateral pontine tegmentum or the medial medulla, respectively, to produce the syndrome of REM sleep without atonia. Startle responses were released during REM sleep following the lesions. However, the amount of startle suppression produced by auditory prepulse after the lesion did not differ from that seen in intact controls. We conclude that REM sleep suppression of the acoustic startle responses is mediated by the system responsible for tonic motor inhibition, but auditory prepulse inhibition of the acoustic startle is not.

Corticofugal projections to the motor nuclei of the brainstem and spinal cord in humans

We studied corticofugal projections to the motoneurons with Nauta-Gygax's technique in a patient with cerebral infarction of both hemispheres. Motoneurons in the brainstem motor nuclei and spinal anterior horns seem to receive direct cortical projections, except for the oculomotor and abducens nuclei and Onuf's nucleus in the sacral cord.

Administration of testosterone attenuates neuronal loss following axotomy in the brain-stem motor nuclei of female rats

This study was undertaken to elucidate whether (1) administration of testosterone to female rats attenuates axotomy-induced neuronal loss; (2) the efficacy of testosterone treatment is related to the age of animals, the dosage given, and the time and duration of the treatment; (3) neurons which project or terminate aberrantly can survive; and (4) the trophic actions of testosterone on neuronal survival and axonal outgrowth are operated under the same mechanisms. The hypoglossal and facial nerves were transected unilaterally at 3 and 6 weeks of age. In order to establish the dose-response curve, testosterone propionate (TP) at doses of 0.5, 1.0, 2.0, or 5.0 mg was injected subcutaneously twice weekly during the first 4 postaxotomy weeks, and once weekly thereafter for an additional 6 weeks. Neuronal numbers in the hypoglossal and facial motor nuclei were counted 10-12 weeks after axotomy in serial paraffin sections stained with cresyl violet. To determine the time course of TP effect, neuronal numbers were counted at 1, 4, 12, and 20 weeks after axotomy. In addition, neuronal loss 12 weeks after axotomy in rats treated with TP for the first 3 postaxotomy weeks only was compared with that in rats withheld TP treatment until the 5th postaxotomy week. To determine axonal projections and terminations of the surviving neurons, HRP retrograde tracing technique was used. Results indicated that TP treatment significantly attenuated neuronal loss in prepubertal and young adult female rats in a dose- and time-dependent manner. Only doses which elevated serum testosterone to levels comparable to or surpassing normal male levels were effective.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical analysis of the dopamine system in the brain and spinal cord of the European eel, Anguilla anguilla.

The distribution of dopamine-containing perikarya and fibres in the central nervous system of the eel, Anguilla anguilla, was determined by using a specific dopamine antiserum. Telencephalic dopamine-immunoreactive somata are located in the external cell layer of the olfactory bulb and throughout the rostrocaudal extent of the subpallium; immunoreactive fibres are located primarily in the bulb and in ventral and lateral portions of the hemispheres. Diencephalic dopamine-immunoreactive neurons are associated with the ventricles in the preoptic area and hypothalamus and in the posterior tubercle. Many of the neurons in the hypothalamus are liquor-contacting. Very few immunoreactive neurons are located in the mesencephalon, and no dopamine-containing cells are found in regions that can be homologized with the ventral tegmental area and substantia nigra of amniotes. There is a rich innervation of the medial octavolateralis nucleus and certain layers of the torus semicircularis and of the tectum. Dopamine-containing neurons are located in the vagal lobe, by the vagal motor nucleus and in the area postrema, which provides a rich dopaminergic innervation of the brainstem motor column and of the reticular formation. Immunoreactive liquor-contacting neurons line the central canal and another type of labelled neuron lies dorsally in the spinal cord.

Reward or reinforcement: What's the difference?

The histories of the terms "reward" and "reinforcement" are reviewed to show the difference in their origins. Reward refers to the fact that certain environmental stimuli have the property of eliciting approach responses. Evidence suggests that the ventral striatum (nucleus accumbens area) is central to the mediation of this behavior. Reinforcement refers to the tendency of certain stimuli to strengthen learned stimulus-response tendencies. The dorsolateral striatum appears to be central to the mediation of this behavior. Neuroanatomical and neurochemical data are adduced suggesting that reward may be mediated by a neural circuit including the neostriatal patch system, together with the hippocampus, limbic system (amygdala, prefrontal cortex) and ventral pallidum. The evidence also suggests that reinforcement, in the form of dopamine release in the striatal matrix, acts to promote the consolidation of sensori-motor associations. Thus, the matrix may mediate stimulus-response memory as part of a circuit including the cerebral cortex, substantia nigra pars reticulata and its projections to thalamic and brainstem motor areas.

Mechanics and Neuromorphology of Feeding in Amphibians

Salamanders and frogs exhibit strong similarities in their habitats, prey preferences and foraging strategies. Despite these similarities, they differ profoundly in the structure and function of the adult feeding apparatus, and in the participation of cranial and spinal nerves and nuclei in the control of feeding behavior. In frogs, the hyolingual skeleton plays no role in tongue projection. Only the tongue is projected out of the mouth under the control of the nn. trigeminus (tongue protraction) and hypoglossus (tongue protraction and retraction). In all salamanders, feeding is based on forward movement of the hyolingual apparatus, which is folded into a slender, far-reaching projectile in the most highly derived salamanders. Tongue protraction is controlled by nn. glossopharyngeus and vagus, and tongue retraction is controlled by the first and second spinal nerves. All muscles related to feeding are activated sequentially in frogs and simultaneously in salamanders. While the descending pathways to the brainstem and cervical spinal motor nuclei are similar in frogs and salamanders, differences in muscle function and activation pattern imply that the premotor reticular formation is also different. In frog tadpoles and salamander larvae, the hyobranchial apparatus plays different roles in feeding and breathing. This has led to differential loss, acquisition and change of function of musculoskeletal elements during the ontogeny and phylogeny of the adult feeding apparatus in frogs and salamanders. These differences in the musculoskeletal periphery imply that a reorganization at or above the level of the reticular formation is likely to have occurred in frogs, but not in salamanders.

Neural systems for early independent ingestion: regional metabolic changes during ingestive responding and dehydration.

The ingestive behavior of young rodents, studied independently of the mother and suckling, provides a system for the developmental analysis of the neurobiology of feeding and drinking. In these experiments regional neural metabolic activity, as assessed by semiquantitative deoxyglucose (DG) autoradiography, was related to dehydration and ingestive behavior in 6-day-old rat pups. During simple ingestive responding, changes in relative DG uptake, representative of changes in neural metabolic activity, occurred primarily in hindbrain sensory and motor nuclei. Producing cellular dehydration resulted in activity changes primarily in the basal forebrain. When pups were dehydrated and allowed to ingest during the DG-uptake period, activity changes were seen in both the hindbrain and forebrain areas that responded to ingestion or dehydration alone as well as in regions that were not affected by either manipulation alone. These latter changes, which result from an interaction of behavioral and physiological stimuli, call attention to areas that may subserve motivational aspects of behavior. Extracellular dehydration was found to produce fewer and different forebrain responses in neural metabolic activity. During ingestion, the only effects of extracellular dehydration that overlapped with those of cellular dehydration appeared in circumventricular hypothalamic regions and brain stem motor nuclei. Thus, there appeared to be only a limited final common pathway for these two types of dehydration-induced drinking. Taken together, these findings in infant rats depict distributed neural systems subserving ingestion and responding to state change. They provide a starting point early in ontogeny for the developmental analysis of neural substrates of ingestive systems.

X-linked recessive bulbospinal neuronopathy. A clinicopathological study.

A clinicopathological study on X-linked recessive bulbospinal neuronopathy was undertaken on 9 cases, with morphological observations on 3 autopsied cases and sural nerve biopsies from 6 patients. Both lower motor and primary sensory neurons were involved. Lower motor neurons were markedly depleted through all spinal segments and in brainstem motor nuclei except for the third, fourth and sixth cranial nerves. Primary sensory neurons were less severely affected. A quantitative study of primary sensory axons at several levels in the peripheral nervous system suggested that a distally accentuated axonopathy was the salient pathological process. Segmental demyelination and remyelination clustered on individual fibres, and g ratios (axon diameter: total fibre diameter) in the sural nerve showed an increased scatter in some cases. Evidence of regeneration was inconspicuous. Unmyelinated fibres were well preserved throughout all the nerves examined. Neurons in the Onufrowicz nuclei, in the intermediolateral columns and in Clarke's columns of the spinal cord were generally well preserved. These observations indicate that a lower motor and primary sensory neuronopathy is a major neurological manifestation in this disease.

Immunohistochemical analysis of the neurotoxic effects of DSP-4 identifies two populations of noradrenergic axon terminals

N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) is a potent and highly selective neurotoxin which induces degeneration of noradrenergic axons. The effects of DSP-4 vary considerably in different brain regions: the drug produces nearly complete depletion of noradrenaline in neocortex, hippocampus, cerebellum and spinal cord, but only partial depletion in hypothalamus and brainstem. In this study we have employed an immunohistochemical method to assess the neurotoxic effects of DSP-4 on the structural integrity of central noradrenergic neurons in the rat, and to identify those noradrenergic axons that remain in the central nervous system 2–4 weeks after DSP-4 treatment. The staining results identified noradrenergic axon terminals as the principal site of action of DSP-4; noradrenergic cell bodies and preterminal axons were not noticeably affected. DSP-4 produced an almost all or none neurotoxic effect on noradrenergic axon terminals in different brain regions. Nearly all noradrenergic axon terminals were destroyed in the neocortex, hippocampus, olfactory bulb, thalamus, tectum, cerebellum and spinal cord dorsal horn. In contrast, most noradrenergic axons were unaffected in the basal forebrain, hypothalamus, reticular formation, brainstem motor nuclei and spinal cord ventral horn. These remaining noradrenergic axon terminals differed morphologically from sensitive axons by their thickness, size and spacing of their varicosities and their dense arborizations within terminal fields. The distribution of noradrenergic axons susceptible to DSP-4 correlates very closely with the distribution of locus coeruleus axons and possibly all regions in which noradrenergic terminals are unaffected by DSP-4 receive their major noradrenergic input from non-locus coeruleus neurons. This study provides the first direct evidence that DSP-4 destroys noradrenergic axon terminals from the locus coeruleus, but not those from non-locus coeruleus neurons. This profound differential sensitivity of noradrenergic axons to DSP-4 is matched by distinct differences in their morphology and their topographic projections. The results support the view that locus coeruleus and non-locus coeruleus noradrenergic neurons constitute two separate subsystems, which differ not only in their projections but also with respect to the pharmacological properties of their axon terminals.