Amine accumulation in behavioural pathology

When nigro-striatal and meso-cortical neurons degenerate there is a loss of dopamine in the terminal fields and an accumulation of amines in the axons of these systems as they traverse the hypothalamus through the medial forebrain bundle. Traditional lines of thought have attributed the occurrence of motor and consummatory deficits which occur after dopamine neuron degeneration to the loss of functional dopamine neurotransmitter in the terminal fields. However, we have hypothesized that hypothalamic amine accumulation represents an area of brain tissue where processes such as neurotransmitter release, ephaptic transmission or local axon swelling may be affecting adjacent neurons and may thereby participate in the production of behavioural deficits. There is a considerable amount of evidence from studies on both peripheral and central catecholamine-containing neurons indicating that when their axons degenerate a release of functional neurotransmitter can occur. Information from neuropharmacological studies indicates that several drugs which facilitate behavioural recovery from dopamine-depleting lesions may do so by affecting amine release or receptor sensitivity near areas of accumulation rather than depleted terminal fields. We conclude that amine accumulation is a component of dopamine neuron degeneration which should be considered when assessing the role of the central catecholamine systems in the control of various behavioural and physiological processes.     

Deficits in locomotor behaviour and thermoregulation produced by intrahypothalamic dopamine injections

When nigrostriatal dopamine neurones degenerate, a loss of functional dopamine in the striatum occurs and is accompanied by increased dopamine in the degenerating axons which traverse the hypothalamus. While the behavioural deficits which occur after nigrostriatal degeneration have been attributed to the loss of functional dopamine neurotransmission, evidence produced by us suggests that the increased levels of amines in the degenerating axons may be neuroactive and participate in the production of these behavioural deficits. To test this hypothesis further, albino rats were injected bilaterally with 200 nmol of dopamine in a location just rostral to the diencephalon/mesencephalon border, where amine accumulation is commonly observed following lateral hypothalamic damage. The effect of these injections upon open field performance, thermoregulation and motor reflex control was determined 40 min after dopamine injection. In a second study, pargyline (15 mg/kg. i.p.) was administered 30 min before intracerebral dopamine to determine whether this treatment would increase the severity of motor and thermoregulatory deficits which occurred after dopamine injections alone. Deficits in locomotion, rearing and the ability to regulate body temperature were seen after the dopamine injections while motor reflex control in these animals was similar to that seen in vehicle-injected controls. The behavioural deficits displayed by pargyline pretreated, dopamine injected animals were slightly but not significantly more severe than those displayed by animals receiving dopamine injections alone. Fluorescent histochemical assessment of injection sites revealed that dopamine injection produced an increase in fluorescence or “amine accumulation” at the site of injection but this was considerably less than that seen after catecholamine degeneration. These results add further support to the hypothesis that amines which accumulate in degenerating neurones may be neuroactive and may thereby participate in the production of behavioural deficits attributed previously to the loss of functional dopamine neurotransmission.     

Catecholamine-blocking drugs injected at sites of amine accumulation reverse catecholamine degeneration associated deficits

It has been hypothesized that catecholamine (CA) accumulation in the axons of degenerating neurons may represent areas of functional neurotransmitter, and may be producing some of the consummatory and locomotory deficits which occur after central CA-depleting lesions. To test this hypothesis further, haloperidol (0.5 microliter of a 7 nM sol.), propranolol (0.5 microliter of a 175 nM sol.) or isotonic saline (0.5 microliter) were injected 1.5 h, 24 h and 48 h after the injection of 6-hydroxydopamine (6-OHDA; 2 microliter of 8 micrograms/microliters) into the lateral hypothalamus (LH) of Sprague-Dawley rats to determine if the hypothermia, motor impairment and consummatory deficits could be reversed. Although haloperidol injection significantly enhanced the hypothermia seen 1.5 h after 6-OHDA injection, open field performance and consummatory responses were significantly improved after haloperidol was injected into the LH where accumulation is known to occur. Three consecutive days of intracerebral haloperidol treatment produced a recovery of body weight regulation lasting for 6 days. Treatment with propranolol enhanced open field performance 1 day after 6-OHDA injection but failed to enhance recovery of consummatory behaviour and body weight control. These results suggest that CA released from areas of accumulation act on adjacent CA receptors to participate in the production of behavioural deficits previously attributed only to the loss of functional neurotransmitter in terminal fields in the forebrain.     

Ultrastructural evidence of neurofilament involvement in immune-mediated motor neuron injury

Amyotrophic lateral sclerosis (ALS) is a progressive neurological disorder. A pathologic hallmark of ALS is accumulation of neurofilaments in proximal axons of affected motor neurones. As the neurofilaments involved in immune-mediated spinal cord ventral horn motor neuron degeneration and loss, we developed immune-mediated motor neuron injury animal model by inoculating Lewis rats with swine spinal cord homogenate and investigated the ultrastructural features of neurofilament accumulation using transmission electron microscopy. Our results showed that there was aberrant accumulation of neurofilaments in perikarya and processes of remaining motor neurons in recipient animals, which is similar to those observed in ALS patients. These findings suggest that immune-mediated motor neuron injury may share a common pathogenesis with ALS.     

Ultrastructural evidence of neurofilament involvement in immune-mediated motor neuron injury

Abstract

Amyotrophic lateral sclerosis (ALS) is a progressive neurological disorder. A pathologic hallmark of ALS is accumulation of neurofilaments in proximal axons of affected motor neurones. As the neurofilaments involved in immune-mediated spinal cord ventral horn motor neuron degeneration and loss, we developed immune-mediated motor neuron injury animal model by inoculating Lewis rats with swine spinal cord homogenate and investigated the ultrastructural features of neurofilament accumulation using transmission electron microscopy. Our results showed that there was aberrant accumulation of neurofilaments in perikarya and processes of remaining motor neurons in recipient animals, which is similar to those observed in ALS patients. These findings suggest that immune-mediated motor neuron injury may share a common pathogenesis with ALS.     

Neuromelanin,one of the most overlooked molecules in modern medicine,is not a spectator

The loss of pigmented neurons from the human brain has long been the hallmark of Parkinson's disease (PD). Neuromelanin (NM) in the pre-synaptic terminal of dopamine neurons is emerging as a primary player in the etiology of neurodegenerative disorders including PD. This mini-review discusses the interactions be-tween neuromelanin and different molecules in the synaptic terminal and describes how these interactions might affect neurodegenerative disorders including PD. Neuromelanin can reversibly bind and interact with amine containing neurotoxins, e.g., MPTP, to augment their actions in the terminal, eventually leading to the instability and degeneration of melanin-containing neurons due to oxidative stress and mitochon-drial dysfunction. In particular, neuromelanin appears to confer susceptibility to chemical toxicity by providing a large sink of iron-bound, heme-like structures in a pi-conjugated system, a system seemingly purposed to allow for stabilizing interactions including pi-stacking as well as ligand binding to iron. Given the progressive accumulation of NM with age corresponding with an apparent decrease in dopamine syn-thetic pathways, the immediate question of whether NM is also capable of binding dopamine, the primary functional monoamine utilized in this cell, should be raised. Despite the rather glaring implications of this finding, this idea appears not to have been adequately addressed. As such, we postulate on potential mech-anisms by which dopamine might dissociate from neuromelanin and the implications of such a reversible relationship. Intriguingly, if neuromelanin is able to sequester and release dopamine in membrane bound vesicles, this intracellular pre-synaptic mechanism could be the basis for a form of chemical memory in do-pamine neurons.     

Neurodegeneration and inflammation in Parkinson's disease

Parkinson's disease (PD) is characterized by the progressive degeneration of dopamine (DA) neurons of the substantia nigra pars compacta (SNpc) accompanied by a buildup of proteinaceous aggregates termed Lewy bodies (LB). In addition to protein aggregation and the loss of DA signaling, PD is also characterized by an active immune response. T-cell infiltration accompanies activated microglial and astrocytic accumulation in and around the SNpc. Although potentially beneficial, microglial activation is most likely responsible for furthering disease pathology and DA neuron degeneration through the release of harmful substances such as pro-inflammatory cytokines, reactive oxidative species and reactive nitrogen species. Activation of the NF-κB death pathway has been shown to occur following microglial activation related release of Cox-2, IL-1β, and Toll-like receptor activation, resulting in increased degeneration of DA neurons of the SNpc. Blockade of microglial activation can lead to DA neuron protection in animal models of PD; however, clinical application of anti-inflammatory drugs has not yielded similar benefits. Future therapeutic designs must take into account the multifactorial nature of PD, including the varied roles of the adaptive and innate immune responses.     

Immune lesions of central noradrenergic neurons produced by antibodies to dopamine-β-hydroxylase

(1) Intraventricular injection of antibodies to dopamine-beta-hydroxylase (DBH) caused degeneration of central noradrenergic nerve terminals in rats and guinea-pigs. In rats it was necessary to infuse exogenous complement in the form of guinea-pig serum together with the anti-DBH, whereas in guinea-pigs the anti-DBH was effective on its own. Control animals were infused with equivalent amounts of non-immune serum and complement and showed no signs of degeneration other than in the region of the needle tract. (2) There was a loss of varicosities in most terminal fields of the noradrenergic projections and swollen distorted axons were seen in both ascending and descending noradrenergic pathways. Noradrenergic cell bodies in the locus coeruleus and subcoeruleus appeared unaffected. No histochemical changes were observed in dopaminergic neurons. (3) The ultrastructural changes in degenerating axons that were first identifided by fluorescence histochemistry included swelling, vacuolation, accumulation of dense cored vesicles, lysosome-like bodies and smooth membranous sacs. The surrounding neuropil appeared normal. (4) There was a significant depletion of noradrenaline in all regions of the rat brain ranging from 20% in the hypothalamus to 80% in the neocortex. Dopamine concentrations were unaffected. (5) These observations provide a new approach to the production of selective lesions in specific neurotransmitter pathways that could be extended to non-adrenergic neurones. They may also be useful as a model for the study of autoimmune diseases of the nervous system.     

Sensory neuron degeneration in familial Kugelberg-Welander disease

A 53 year old man developed symptoms of motor neuron disease in childhood. There was a family history of a similar disorder and it was felt to represent a form of Kugelberg-Welander disease. In addition to the motor deficits, sensory abnormalities in his legs were documented during life. Autopsy revealed anterior horn cell loss throughout the length of the spinal cord, with preservation of the phrenic nucleus. The lumbar dorsal root ganglia showed active degeneration of sensory neurons, with nuclear changes exceeding cytoplasmic ones. The fasciculus gracilis showed Wallerian degeneration. The findings provide direct evidence that sensory neurons can degenerate in some forms of motor neuron disease, and that the "demyelination" or "degeneration" of posterior columns sometimes seen in the various forms of motor neuron disease may actually be secondary to cell body disease in the dorsal root ganglia.     

Limb use and complex ultrasonic vocalization in a rat model of Parkinson's disease: deficit-targeted training

Recent evidence in animal models of Parkinson's disease (PD) suggests that exercise and other forms of motor enhancement can be beneficial when applied during the degeneration of dopamine neurons. Behaviours that depend on adequate levels of striatal dopamine may provide particularly favourable targets for therapeutic motor interventions. Task-specific motor enrichment procedures have been used to improve functional and neural outcomes following unilateral infusions of 6-hydroxydopamine (6-OHDA) into the nigrostriatal pathway in rats. In contrast, forced non-use procedures can exaggerate the degree of degeneration. Limb-use akinesia and ultrasonic vocalization in the 50-kHz range may be useful behavioural indices of nigrostriatal integrity and may model common deficits found in PD. These deficits in movement initiation and fine sensorimotor control are potential targets for early training interventions.     

Persistence of Neuromuscular Junctions after Axotomy in Mice with Slow Wallerian Degeneration (C57BL/Wlds)

The present study was undertaken to examine the fate of neuromuscular junctions in C57BL/Wld^s mice (formerly known as OLA mice) after nerve injury. When a peripheral nerve is injured, the distal axons normally degenerate within 1-3 days. For motor axons, an early event is deterioration of motor nerve terminals at neuromuscular junctions. Previously, the vulnerability of motor terminals has been attributed either to a 'signal'originating at the site of nerve injury and transported rapidly to the terminals or to their continual requirement for essential maintenance factors synthesized in the motor neuron cell body and supplied to the terminals by fast axonal transport. Mice of the Wld^s strain have normal axoplasmic transport but show an abnormally slow rate of axon and myelin degeneration. Structure and function are retained in the axons of distal nerve stumps for several days or even weeks after nerve injury in these mice. The results of the present study show that Wld^s neuromuscular junctions are also preserved and continue to release neurotransmitter and recycle synaptic vesicle membrane for at least 3 days and in some cases up to 2 weeks after nerve injury. Varying the site of the nerve lesion delayed degeneration by -1-2 days per centimetre of distal nerve remaining. These findings suggest that the mechanisms of nerve terminal degeneration after injury are more complex than can be accounted for simply by the failure of motor neuron cell bodies to supply their terminals with essential maintenance factors. Rather, the data support the view that nerve section normally activates cellular components or processes already present, but latent, in motor nerve endings, and that in Wld^s mice either the trigger or the cellular response is abnormal.     

Review: Parkinson's disease: from synaptic loss to connectome dysfunction

Parkinson's disease (PD) is a common neurodegenerative disorder with prominent loss of nigro‐striatal dopaminergic neurons. The resultant dopamine (DA) deficiency underlies the onset of typical motor symptoms (MS). Nonetheless, individuals affected by PD usually show a plethora of nonmotor symptoms (NMS), part of which may precede the onset of motor signs. Besides DA neuron degeneration, a key neuropathological alteration in the PD brain is Lewy pathology. This is characterized by abnormal intraneuronal (Lewy bodies) and intraneuritic (Lewy neurites) deposits of fibrillary aggregates mainly composed of α‐synuclein. Lewy pathology has been hypothesized to progress in a stereotypical pattern over the course of PD and α‐synuclein mutations and multiplications have been found to cause monogenic forms of the disease, thus raising the question as to whether this protein is pathogenic in this disorder. Findings showing that the majority of α‐synuclein aggregates in PD are located at presynapses and this underlies the onset of synaptic and axonal degeneration, coupled to the fact that functional connectivity changes correlate with disease progression, strengthen this idea. Indeed, by altering the proper action of key molecules involved in the control of neurotransmitter release and re‐cycling as well as synaptic and structural plasticity, α‐synuclein deposition may crucially impair axonal trafficking, resulting in a series of noxious events, whose pressure may inevitably degenerate into neuronal damage and death. Here, we provide a timely overview of the molecular features of synaptic loss in PD and disclose their possible translation into clinical symptoms through functional disconnection.     

Clenbuterol retards loss of motor function in motor neuron degeneration mice

Motor neuron degeneration (mnd) mice exhibit lysosomal accumulation of lipofuscin-like material that is associated with progressive loss of motor function and strength. Motor dysfunction scores at 8.5-9 months of age were highly correlated with the occurrence of abnormal spinal motor neurons with eccentric nuclei, although the total numbers of motor neurons were not significantly reduced. Nuclear eccentricity is a characteristic of the axon reaction that results from injury and subsequent compensatory axonal sprouting indicating axonal/synaptic dysfunction in mnd motor neurons. Treatment with clenbuterol, a beta(2)-adrenoceptor agonist that can enhance regeneration of motor neuron axons, opposed the development of motor deficits in parallel with a reduced proportion of motor neurons with eccentric nuclei consistent with improved synaptic function. Clenbuterol also opposed decreases in grip strength and muscle mass suggesting beta(2)-agonist treatment as a potential therapeutic modality for lipofuscinoses.     

Changes in the connections of the main olfactory bulb after mitral cell selective neurodegeneration

The connections of the main olfactory bulb (OB) of the mouse were studied with iontophoretic injections of biotinylated dextran amine. To sort efferences from mitral cells and tufted cells, the Purkinje cell degeneration (PCD) mouse was used. This mutant animal undergoes a specific neurodegeneration of mitral cells, whereas tufted cells do not degenerate. The unilateral tracer injections used were small and confined largely to the OB of both PCD and control mice at P120. Seven days after tracer injection, the efferences from the OB and the centrifugal afferences from secondary olfactory structures to it were studied. Although there is a large overlap of their target fields, mitral cell axons innervated more caudal regions of the olfactory cortex than tufted cell axons, thus providing definitive evidence of the differential projections of olfactory output neurons. Additionally, an important increase in retrogradely-labeled neurons was detected in the ipsilateral anterior olfactory nucleus of the mutant animals. This was not observed in any other secondary olfactory structure, suggesting a strengthening of the centrifugal input to the OB from that central area after mitral cell loss. Moreover, we recorded a complete loss of bilaterality in the olfactory connections of the PCD mice due to degeneration of the anterior commissure. These results point to an important reorganization of this essential olfactory circuit between the anterior olfactory nucleus and the OB, and hint at a transsynaptic level of plasticity not considered previously in literature.     

Transient intraneuronal Aβ rather than extracellular plaque pathology correlates with neuron loss in the frontal cortex of APP/PS1KI mice

The accumulation of beta-amyloid (Aβ) plaques and neurofibrillary tangles consisting of hyperphosphorylated tau protein are pathological features of Alzheimer’s disease (AD) commonly modeled in mice using known human familial mutations; however, the loss of neurons also found to occur in AD is rarely observed in such models. The mechanism of neuron degeneration remains unclear but is of great interest as it is very likely an important factor for the onset of adverse memory deficits occurring in individuals with AD. The role of Aβ in the neuronal degeneration is a matter of controversial debates. In the present study we investigated the impact of extracellular plaque Aβ versus intraneuronal Aβ on neuronal cell death. The thalamus and the frontal cortex of the APP/PS1KI mouse model were chosen for stereological quantification representing regions with plaques only (thalamus) or plaques as well as intraneuronal Aβ (frontal cortex). A loss of neurons was found in the frontal cortex at the age of 6 months coinciding with the decrease of intraneuronal immunoreactivity, suggesting that the neurons with early intraneuronal Aβ accumulation were lost. Strikingly, no neuron loss was observed in the thalamus despite the development of abundant plaque pathology with levels comparable to the frontal cortex. This study suggests that plaques have no effect on neuron death whereas accumulation of intraneuronal Aβ may be an early transient pathological event leading to neuron loss in AD. O. Wirths and T. A. Bayer have equally contributed to this work.     

Is methamphetamine abuse a risk factor in parkinsonism?

Parkinsons disease (PD) is a neurodegenerative disorder with increased incidence in individuals beyond 50 years of age. The etiology of PD is currently not known, but it appears that environmental factors may play an important role. The molecular basis of PD is the nearly complete loss of the neurotransmitter dopamine (DA) in the basal ganglia (caudate/putamen). The decrease in dopamine levels is the result of degeneration of dopamine-containing neurons in the substantia nigra. This biochemical deficit in the nigrostriatal pathway leads to the emergence of motor impairments typical of PD. Methamphetamine (METH) is a psychostimulant drug with increasing use in certain segments of the population in the United States and worldwide. In experimental animal models and human studies, METH administration has been shown to decrease markers of dopaminergic neuron terminal integrity in the basal ganglia. A long-standing question has been whether the reductions in dopaminergic markers induced by METH constitute degenerative changes or reflect drug-induced modulation. Resolving this question is important because the irreversible loss of dopaminergic function may increase the likelihood of Parkinsonism with advancing age.     

A possible cellular mechanism of neuronal loss in the dorsal root ganglia of Dystonia musculorum (dt) mice

Dystonia musculorum (dt) is a mutant mouse with hereditary sensory neuropathy. A defective bullous pemphigoid antigen 1 (BPAG1) gene is responsible for this mutation. In the present study, we examined the distribution of neuronal intermediate filament proteins in the central and peripheral processes of the dorsal root ganglia (DRG) in adult dt mice using different approaches. We found that not only BPAG1, but also alpha-internexin was absent in the DRG neurons in adult dt mice. To study the relationship between the absence of alpha-internexin and the progressive neuronal loss in the DRG of dt mice, we further cultured DRG neurons from embryonic dt mutants. Immunocytochemical assay of cultured DRG neurons from dt embryos revealed that alpha-internexin was aggregated in the proximal region of axons and juxtanuclear region of the cytoplasma, yet the other intermediate filament proteins were widely distributed in all processes. The active caspase-3 activity was observed in the dt neuron with massive accumulation of alpha-internexin. From our observations, we suggest that the interaction between BPAG1 and alpha-internexin may be one of the key factors involved in neuronal degeneration, and abnormal accumulation of alpha-internexin may impair the axonal transport and subsequently turns on the cascade of neuronal apoptosis in dt mice.     

Restoration of ascending noradrenergic projections by residual locus coeruleus neurons: compensatory response to neurotoxin-induced cell death in the adult rat brain.

There is clinical and experimental evidence that monoamine neurons respond to lesions with a wide range of compensatory adaptations aimed at preserving their functional integrity. Neurotoxin-induced lesions are followed by increased synthesis and release of transmitter from residual monoamine fibers and by axonal sprouting. However, the fate of lesioned neurons after long survival periods remains largely unknown. Whether regenerative sprouting may contribute significantly to recovery of function following lesions which induce cell loss has been questioned. We have previously analyzed the response of locus coeruleus (LC) neurons to systemic administration of the noradrenergic (NE) neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to adult rats. This drug causes ablation of nearly all LC axon terminals within 2 weeks after administration, followed by a profound loss of LC cell bodies 6 months later. The present study was conducted to determine the fate of surviving LC neurons and to characterize their potential for regenerative sprouting during a 16 month period after DSP-4 treatment. The time-course and extent of LC neuron degeneration were analyzed quantitatively in Nissl-stained sections, and the regenerative response of residual neurons was characterized by dopamine-beta-hydroxylase immunohistochemistry. The results document that LC neurons degenerate gradually after DSP-4 treatment, cell loss reaching on average 57% after 1 year. LC neurons which survive the lesion exhibit a vigorous regenerative response, even in those animals in which cell loss exceeds 60-70%. This regenerative process leads progressively to restoration of the NE innervation pattern in the forebrain, with some regions becoming markedly hyperinnervated. In stark contrast to the forebrain, very little reinnervation takes place in the brainstem, cerebellum and spinal cord. These findings suggest that regenerative sprouting of residual neurons is an important compensatory mechanism by which the LC may regain much of its functional integrity in the presence of extensive cell loss. Furthermore, regeneration of LC axons after DSP-4 treatment is region-specific, suggesting that the pattern of reinnervation is controlled by target areas. Elucidation of the factors underlying recovery of LC neurons after DSP-4 treatment may provide insights into the compensatory mechanisms of central neurons after injury and in disease states.     

Reversible changes in the activities and amounts of tyrosine hydroxylase in dopamine neurons of the substantia nigra in response to axonal injury as studied by immunochemical and immunocytochemical methods

The effects of unilateral electrolytic lesions of axons of nigrostriatal dopamine neurons of the rat brain on the activity and amount of the neurotransmitter synthesizing enzyme tyrosine hydroxylase (TH) in the axon terminals in the striatum and in the parent cell bodies in the substantia nigra (A9 group) were examined. Lesions in posterolateral hypothalamus damaging axons close to cell bodies resulted, by 72 h, in a permanent anterograde reduction of TH activity to 20% of control in the ipsilateral striatum. By immunochemical titration and immunocytochemical localization with a specific antibody to TH, the reduction was demonstrated as due to loss of enzyme protein. In the reactive A9 cell bodies TH activity was increased to 175% of control by 24 h followed by a gradual and permanent fall to 40% of control by day 14. The retrograde reduction of TH was due to a loss of enzyme protein in turn reflecting retrograde cell death. Striatal lesions of moderate size resulted in a fall in the activity and amount of TH in A9 substantia nigra cell bodies by day 7 to 60–65% of control, with full recovery by day 21. We conclude that the retrograde reaction in central dopamine neurons is dependent upon the proximity of the lesion to the parent cell body: proximal lesions result in retrograde cell death; distal lesions result in a reversible retrograde reduction in the amount and activity of TH. The findings confirm previous evidence that: (a) a reduced accumulation of neurotransmitter synthesizing enzymes is a biochemical concomitant of the retrograde reaction in central neurons; (b) that intrinsic neurons of the CNS may undergo a retrograde response which is reversible. The reduction of TH may indicate a reordering of protein biosynthesis favoring production of protein required for axonal regeneration at the expense of those involved in neurotransmission.     

Basal ganglia dopamine loss due to defect in purine recycling

Several rare inherited disorders have provided valuable experiments of nature highlighting specific biological processes of particular importance to the survival or function of midbrain dopamine neurons. In both humans and mice, deficiency of hypoxanthine-guanine phosphoribosyl transferase (HPRT) is associated with profound loss of striatal dopamine, with relative preservation of other neurotransmitters. In the current studies of knockout mice, no morphological signs of abnormal development or degeneration were found in an exhaustive battery that included stereological and morphometric measures of midbrain dopamine neurons, electron microscopic studies of striatal axons and terminals, and stains for degeneration or gliosis. A novel culture model involving HPRT-deficient dopaminergic neurons also exhibited significant loss of dopamine without a morphological correlate. These results suggest that dopamine loss in HPRT deficiency has a biochemical rather than anatomical basis and imply that purine recycling to be a biochemical process of particular importance to the function of dopaminergic neurons.