Nigra Pars Compacta Dopamine Neurons Research Articles

Chronic methamphetamine-induced neurodegeneration: Differential vulnerability of ventral tegmental area and substantia nigra pars compacta dopamine neurons

Methamphetamine (meth) increases monoamine oxidase (MAO)-dependent mitochondrial stress in substantia nigra pars compacta (SNc) axons; chronic administration produces SNc degeneration that is prevented by MAO inhibition suggesting that MAO-dependent axonal mitochondrial stress is a causal factor. To test whether meth similarly increases mitochondrial stress in ventral tegmental area (VTA) axons, we used a genetically encoded redox biosensor to assess mitochondrial stress ex vivo. Meth increased MAO-dependent mitochondrial stress in both SNc and VTA axons. However, despite having the same meth-induced stress as SNc neurons, VTA neurons were resistant to chronic meth-induced degeneration indicating that meth-induced MAO-dependent mitochondrial stress in axons was necessary but not sufficient for degeneration. To determine whether L-type Ca2+ channel-dependent stress differentiates SNc and VTA axons, as reported in the soma, the L-type Ca2+ channel activator Bay K8644 was used. Opening L-type Ca2+ channels increased axonal mitochondrial stress in SNc but not VTA axons. To first determine whether mitochondrial stress was necessary for SNc degeneration, mice were treated with the mitochondrial antioxidant mitoTEMPO. Chronic meth-induced SNc degeneration was prevented by mitoTEMPO thereby confirming the necessity of mitochondrial stress. Similar to results with the antioxidant, both MAO inhibition and L-type Ca2+ channel inhibition also prevented SNc degeneration. Taken together the presented data demonstrate that both MAO- and L-type Ca2+ channel-dependent mitochondrial stress is necessary for chronic meth-induced degeneration.

Female mice are resilient to age-related decline of substantia nigra dopamine neuron firing parameters

Degeneration of substantia nigra pars compacta dopamine neurons is a central feature in the pathology of Parkinson’s disease, which is characterized by progressive loss of motor and cognitive functions. The largest risk factors for Parkinson’s disease are age and sex; most cases occur after age 60 and males have nearly twice the incidence as females. Preclinical work has scarcely considered the influence of these 2 factors to disease risk and presentation. Here, we observed a progressive decline in dopamine neuron firing activity in male C57BL/6 mice by 18 months of age, while dopamine neurons from females remained largely unaffected. This was accompanied by increased mRNA expression of PINK1 in both males and females, and PARK2 primarily in males, both of which have been linked to Parkinson’s. Since the declining cell properties were accompanied by only slight decreases in locomotion in both sexes, it is likely that these age-related impairments in males represent a vulnerability to further insults that could predispose the neurons to neurodegenerative processes such as in Parkinson’s.

Temporally restricted dopaminergic control of reward-conditioned movements.

Midbrain dopamine (DA) neurons encode both reward and movement-related events, and are implicated in disorders of reward processing as well as movement. Consequently, disentangling the contribution of DA neurons in reinforcing versus generating movements is challenging and has led to lasting controversy. We dissociated these functions by parametrically varying the timing of optogenetic manipulations in a Pavlovian conditioning task, and examining the influence on anticipatory licking prior to reward delivery. Inhibiting both ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) DA neurons in the post-reward period had a significantly greater behavioral effect than inhibition in the pre-reward period of the task. Furthermore, the contribution of DA neurons to behavior decreased linearly as a function of elapsed time after reward. Together, the results indicate a temporally restricted role of DA neurons primarily related to reinforcing stimulus-reward associations, and suggest that directly generating movements is a comparatively less important function.

Functionally Distinct Connectivity of Developmentally Targeted Striosome Neurons

SUMMARYOne long-standing model of striatal function divides the striatum into compartments called striosome and matrix. While some anatomical evidence suggests that these populations represent distinct striatal pathways with differing inputs and outputs, functional investigation has been limited by the methods for identifying and manipulating these populations. Here, we utilize hs599CreER mice as a new tool for targeting striosome projection neurons and testing their functional connectivity. Extending anatomical work, we demonstrate that striosome neurons receive greater synaptic input from prelimbic cortex, whereas matrix neurons receive greater input from primary motor cortex. We also identify functional differences in how striosome and matrix neurons process excitatory input, providing the first electrophysiological method for delineating striatal output neuron subtypes. Lastly, we provide the first functional demonstration that striosome neurons are the predominant striatal output to substantia nigra pars compacta dopamine neurons. These results identify striosome and matrix as functionally distinct striatal pathways.

LRRK2 BAC transgenic rats develop progressive, L-DOPA-responsive motor impairment, and deficits in dopamine circuit function.

Mutations in leucine-rich repeat kinase 2 (LRRK2) lead to late-onset, autosomal dominant Parkinson's disease, characterized by the degeneration of dopamine neurons of the substantia nigra pars compacta, a deficit in dopamine neurotransmission and the development of motor and non-motor symptoms. The most prevalent Parkinson's disease LRRK2 mutations are located in the kinase (G2019S) and GTPase (R1441C) encoding domains of LRRK2. To better understand the sequence of events that lead to progressive neurophysiological deficits in vulnerable neurons and circuits in Parkinson's disease, we have generated LRRK2 bacterial artificial chromosome transgenic rats expressing either G2019S or R1441C mutant, or wild-type LRRK2, from the complete human LRRK2 genomic locus, including endogenous promoter and regulatory regions. Aged (18–21 months) G2019S and R1441C mutant transgenic rats exhibit L-DOPA-responsive motor dysfunction, impaired striatal dopamine release as determined by fast-scan cyclic voltammetry, and cognitive deficits. In addition, in vivo recordings of identified substantia nigra pars compacta dopamine neurons in R1441C LRRK2 transgenic rats reveal an age-dependent reduction in burst firing, which likely results in further reductions to striatal dopamine release. These alterations to dopamine circuit function occur in the absence of neurodegeneration or abnormal protein accumulation within the substantia nigra pars compacta, suggesting that nigrostriatal dopamine dysfunction precedes detectable protein aggregation and cell death in the development of Parkinson's disease. In conclusion, our longitudinal deep-phenotyping provides novel insights into how the genetic burden arising from human mutant LRRK2 manifests as early pathophysiological changes to dopamine circuit function and highlights a potential model for testing Parkinson's therapeutics.

What is the Most Promising Treatment for Parkinson‘s Disease: Genes, Cells, Growth Factors or None of the Above?

Author for correspondence: Cambridge Centre for Brain Repair & the Department of Neurology, Department of Clinical Neurosciences, University of Cambridge, Cambridge, UK Tel.: +44 1223 331160 Fax: +44 1223 331174 rab46@cam.ac.uk have received levodopa for more than 5 years, in 80% of patients treated for 10 years, and in nearly all patients with young-onset disease. In a bid to control these motor fluctuations, patients’ therapy may need to be escalated to include more intensive pharmacotherapies such as infusions of subcutaneous apomorphine and jejunal Duodopa infusions. In addition, deep brain stimulation (DBS) surgery, either of the subthalamic nucleus or the globus pallidus pars interna, can be useful for treating PD patients at this stage when the motor fluctuations and dyskinesias cannot be adequately managed with pharmacological manipulation. The 5-year follow-up of patients who have received bilateral stimulation of the subthalamic nucleus in PD clearly shows that their improvement in motor function is sustained over time. However, disability still progresses from year to year, indicating ongoing degeneration in both dopaminergic and nondopaminergic sites. Furthermore, DBS is also associated with significant cognitive and psychiatric side effects, including severe depression, impulsivity and apathy, all of which may help explain the discrepancy between changes in quality-of-life assessment and motor disability in patients having had this therapy [3]. None of the currently available therapies for PD can slow or halt the disease progression. Therefore, there is a clear need for new therapies in PD that not only are capable of providing dopamine replacement more effectively, but also serve to replace or protect the substantia nigra pars compacta dopamine neurons from further degeneration. In a bid to meet this clinical challenge, novel therapeutic approaches have evolved over the last 20 years, which include a variety of gene therapy approaches, neurotrophic factor delivery and cell replacement therapies. Three main strategies under investigation using gene transfer for targeted protein expression include: “...it has to be realized that cell therapies are not a cure for Parkinson’s disease...” What is the most promising treatment for Parkinson’s disease: genes, cells, growth factors or none of the above?

Birth dating of midbrain dopamine neurons identifies A9 enriched tissue for transplantation into Parkinsonian mice

Clinical trials have provided proof of principle that new dopamine neurons isolated from the developing ventral midbrain and transplanted into the denervated striatum can functionally integrate and alleviate symptoms in Parkinson's disease patients. However, extensive variability across patients has been observed, ranging from long-term motor improvement to the absence of symptomatic relief and development of dyskinesias. Heterogeneity of the donor tissue is likely to be a contributing factor in the variable outcomes. Dissections of ventral midbrain used for transplantation will variously contain progenitors for different dopamine neuron subtypes as well as different neurotransmitter phenotypes. The overall impact of the resulting graft will be determined by the functional contribution from these different cell types. The A9 substantia nigra pars compacta dopamine neurons, for example, are known to be particularly important for motor recovery in animal models. Serotonergic neurons, on the other hand, have been implicated in unwanted dyskinesias. Currently little knowledge exists on how variables such as donor age, which have not been controlled for in clinical trials, will impact on the final neuronal composition of fetal grafts. Here we performed a birth dating study to identify the time-course of neurogenesis within the various ventral midbrain dopamine subpopulations in an effort to identify A9-enriched donor tissue for transplantation. The results show that A9 neurons precede the birth of A10 ventral tegmental area dopamine neurons. Subsequent grafting of younger ventral midbrain donor tissue revealed significantly larger grafts containing more mitotic dopamine neuroblasts compared to older donor grafts. These grafts were enriched with A9 neurons and showed significantly greater innervation of the target dorso-lateral striatum and DA release. Younger donor grafts also contained significantly less serotonergic neurons. These findings demonstrate the importance of standardized methods to improve cell therapy for Parkinson's disease and have significant implications for the generation and selectivity of dopamine neurons from stem cell based sources.

Single-Cell Redox Imaging Demonstrates a Distinctive Response of Dopaminergic Neurons to Oxidative Insults

The study of the intracellular oxido-reductive (redox) state is of extreme relevance to the dopamine (DA) neurons of the substantia nigra pars compacta. These cells possess a distinct physiology intrinsically associated with elevated reactive oxygen species production, and they selectively degenerate in Parkinson's disease under oxidative stress conditions. To test the hypothesis that these cells display a unique redox response to mild, physiologically relevant oxidative insults when compared with other neuronal populations, we sought to develop a novel method for quantitatively assessing mild variations in intracellular redox state. We have developed a new imaging strategy to study redox variations in single cells, which is sensitive enough to detect changes within the physiological range. We studied DA neurons' physiological redox response in biological systems of increasing complexity--from primary cultures to zebrafish larvae, to mammalian brains-and identified a redox response that is distinctive for substantia nigra pars compacta DA neurons. We studied simultaneously, and in the same cells, redox state and signaling activation and found that these phenomena are synchronized. The redox histochemistry method we have developed allows for sensitive quantification of intracellular redox state in situ. As this method is compatible with traditional immunohistochemical techniques, it can be applied to diverse settings to investigate, in theory, any cell type of interest. Although the technique we have developed is of general interest, these findings provide insights into the biology of DA neurons in health and disease and may have implications for therapeutic intervention.

Regulation of dopaminergic neuron firing by heterogeneous dopamine autoreceptors in the substantia nigra pars compacta

Dopamine (DA) receptors generate many cellular signals and play various roles in locomotion, motivation, hormone production, and drug abuse. According to the location and expression types of the receptors in the brain, DA signals act in either stimulatory or inhibitory manners. Although DA autoreceptors in the substantia nigra pars compacta are known to regulate firing activity, the exact expression patterns and roles of DA autoreceptor types on the firing activity are highly debated. Therefore, we performed individual correlation studies between firing activity and receptor expression patterns using acutely isolated rat substantia nigra pars compacta DA neurons. When we performed single-cell RT-PCR experiments, D(1), D(2)S, D(2)L, D(3), and D(5) receptor mRNA were heterogeneously expressed in the order of D(2)L > D(2)S > D(3) > D(5) > D(1). Stimulation of D(2) receptors with quinpirole suppressed spontaneous firing similarly among all neurons expressing mRNA solely for D(2)S, D(2)L, or D(3) receptors. However, quinpirole most strongly suppressed spontaneous firing in the neurons expressing mRNA for both D(2) and D(3) receptors. These data suggest that D(2) S, D(2)L, and D(3) receptors are able to equally suppress firing activity, but that D(2) and D(3) receptors synergistically suppress firing. This diversity in DA autoreceptors could explain the various actions of DA in the brain.

The Origins of Oxidant Stress in Parkinson's Disease and Therapeutic Strategies

Parkinson's disease (PD) is a major world-wide health problem afflicting millions of the aged population. Factors that act on most or all cell types (pan-cellular factors), particularly genetic mutations and environmental toxins, have dominated public discussions of disease etiology. Although there is compelling evidence supporting an association between disease risk and these factors, the pattern of neuronal pathology and cell loss is difficult to explain without cell-specific factors. This article focuses on recent studies showing that the neurons at greatest risk in PD-substantia nigra pars compacta dopamine neurons-have a distinctive physiological phenotype that could contribute to their vulnerability. The opening of L-type calcium channels during autonomous pacemaking results in sustained calcium entry into the cytoplasm of substantia nigra pars compacta dopamine neurons, resulting in elevated mitochondrial oxidant stress and susceptibility to toxins used to create animal models of PD. This cell-specific stress could increase the negative consequences of pan-cellular factors that broadly challenge either mitochondrial or proteostatic competence. The availability of well-tolerated, orally deliverable antagonists for L-type calcium channels points to a novel neuroprotective strategy that could complement current attempts to boost mitochondrial function in the early stages of the disease.

Pharmacological Modulation of the Gating Properties of Small Conductance Ca2+-Activated K+ Channels Alters the Firing Pattern of Dopamine Neurons In Vivo

Dopamine (DA) neurons are autonomous pacemakers that occasionally fire bursts of action potentials, discharge patterns thought to reflect tonic and phasic DA signaling, respectively. Pacemaker activity depends on the concerted and cyclic interplay between intrinsic ion channels with small conductance Ca(2+)-activated K(+) (SK) channels playing an important role. Bursting activity is synaptically initiated but neither the transmitters nor the specific ion conductances involved have been definitively identified. Physiological and pharmacological regulation of SK channel Ca(2+) sensitivity has recently been demonstrated and could represent a powerful means of modulating the expression of tonic/phasic signaling in DA neurons in vivo. To test this premise, we characterized the effects of intravenous administration of the novel positive and negative SK channel modulators NS309 and NS8593, respectively, on the spontaneous activity of substantia nigra pars compacta DA neurons in anesthetized C57BL/6 mice. NS309, dose-dependently decreased DA cell firing rate, increased the proportion of regular firing cells, and eventually stopped spontaneous firing. By contrast, systemic administration of the negative SK channel modulator NS8593 increased firing rate and shifted the pattern toward increased irregularity/bursting; an effect similar to local application of the pore blocking peptide apamin. The altered firing patterns resulting from inhibiting SK currents persisted independently of changes in firing rates induced by administration of DA autoreceptor agonists/antagonists. We conclude that pharmacological modulation of SK channel Ca(2+)-sensitivity represents a powerful mechanism for switching DA neuron firing activity between tonic and phasic signaling modalities in vivo.

Gene therapy in Parkinson's disease: rationale and current status.

Parkinson's disease is the second most common age-related neurodegenerative disorder, typified by the progressive loss of substantia nigra pars compacta dopamine neurons and the consequent decrease in the neurotransmitter dopamine. Patients exhibit a range of clinical symptoms, with the most common affecting motor function and including resting tremor, rigidity, akinesia, bradykinesia and postural instability. Current pharmacological interventions are palliative and largely aimed at increasing dopamine levels through increased production and/or inhibition of metabolism of this key neurotransmitter. The gold standard for treatment of both familial and sporadic Parkinson's disease is the peripheral administration of the dopamine precursor, levodopa. However, many patients gradually develop levodopa-induced dyskinesias and motor fluctuations. In addition, dopamine enhancement therapies are most useful when a portion of the nigrostriatal pathway is intact. Consequently, as the number of substantia nigra dopamine neurons and striatal projections decrease, these treatments become less efficacious. Current translational research is focused on the development of novel disease-modifying therapies, including those utilizing gene therapeutic approaches. Herein we present an overview of current gene therapy clinical trials for Parkinson's disease. Employing either recombinant adeno-associated virus type 2 (rAAV2) or lentivirus vectors, these clinical trials are focused on three overarching approaches: augmentation of dopamine levels via increased neurotransmitter production; modulation of the neuronal phenotype; and neuroprotection. The first two therapies discussed in this article focus on increasing dopamine production via direct delivery of genes involved in neurotransmitter synthesis (amino acid decarboxylase, tyrosine hydroxylase and GTP [guanosine triphosphate] cyclohydrolase 1). In an attempt to bypass the degenerating nigrostriatal pathway, a third clinical trial utilizes rAAV2 to deliver glutamic acid decarboxylase to the subthalamic nucleus, converting a subset of excitatory neurons to GABA-producing cells. In contrast, the final clinical trial is aimed at protecting the degenerating nigrostriatum by striatal delivery of rAAV2 harbouring the neuroprotective gene, neurturin. Based on preclinical studies, this gene therapeutic approach is posited to slow disease progression by enhancing neuronal survival. In addition, we discuss the outcome of each clinical trial and discuss the potential rationale for the marginal yet incremental clinical advancements that have thus far been realized for Parkinson's disease gene therapy.

Bar pressing for food: differential consequences of lesions to the anterior versus posterior pedunculopontine

The pedunculopontine tegmental nucleus (PPTg) is in a key position to participate in operant reinforcement via its connections with the corticostriatal architecture and the medial reticular formation. Indeed, previous work has demonstrated that rats bearing lesions of the whole PPTg are impaired when learning to make two bar presses for amphetamine reinforcement. Anterior and posterior portions of the PPTg make different anatomical connections, including preferential projections by the anterior PPTg to substantia nigra pars compacta dopamine neurons and by the posterior PPTg to ventral tegmental area dopamine neurons. We wanted to assess the effects of anterior and posterior PPTg ibotenate lesions on rats learning simple and more complex schedules of natural reinforcement. We trained rats with lesions to the anterior PPTg (n = 11) and the posterior PPTg (n = 5) [and appropriate controls (n = 15)] to bar press for food on a variety of fixed-ratio and variable-ratio reinforcement schedules and then during extinction. We found that posterior PPTg-lesioned rats bar pressed at lower rates, were slower to learn to bar press, and often had deficits characteristic of impaired learning and/or motivation. In contrast, anterior PPTg-lesioned rats learned to bar press for reinforcement at normal rates. However, they made errors of perseveration and anticipation throughout many schedules, and pressed at a higher rate than controls during extinction, deficits best characterized as reflecting disorganized response control. Together, these data suggest that the anterior PPTg and posterior PPTg (and their related circuits) contribute differently to reinforcement learning, incentive motivation, and response control, processes that are considered to malfunction in drug addiction.

Age‐related decline in striatal dopamine content and motor performance occurs in the absence of nigral cell loss in a genetic mouse model of Parkinson's disease

Dopamine cytotoxicity is thought to contribute towards the selective loss of substantia nigra pars compacta dopamine neurons and disease progression in Parkinson's disease. However, the long-term toxicity of dopamine in vivo has not previously been established. The vesicular monoamine transporter 2 (VMAT2) sequesters monoamines into synaptic vesicles, a process that, in addition to being important in normal transmission, may also act to keep intracellular levels of monoamine neurotransmitters below potentially toxic thresholds. The homozygous VMAT2-hypomorphic mouse has an insertion in the VMAT2 gene (Slc18a2). Consequently, VMAT2-deficient mice (VD(-/-)) have an approximately 95% reduction in VMAT2 expression and an equivalent level of dopamine depletion in the striatum which results in moderate motor impairment. Here, we show that L-DOPA induces locomotor hyperactivity in VD(-/-) mice and reverses the deficit in motor coordination and balance as tested with the rotarod. We report that evidence for cytosolic accumulation of dopamine in substantia nigra neurons in these mice is two-fold: firstly, there is reduced phosphorylation of tyrosine hydroxylase at the residue associated with catechol feedback inhibition; and, secondly, there are increased rates of dopamine turnover at 6, 12 and 24 months of age. These animals exhibit a progressive decline in striatal monoamine levels and rotarod performance with increasing age. However, despite these data, there was no loss of nigral dopamine neurons as estimated by quantification of tyrosine hydroxylase-immunoreactive cells in the substantia nigra pars compacta of old VD(-/-) mice (24-month-old), implying that these age-dependent manifestations may be due to senescence alone.

Intracranial delivery of proteins and peptides as a therapy for neurodegenerative diseases.

Parkinson's disease is characterized by a progressive degeneration of the substantia nigra pars compacta dopamine neurons that innervate the striatum. Unlike current treatments for PD, GDNF administration could potentially slow or halt the continued degeneration of nigral dopaminergic neurons. GDNF does not cross the blood-brain barrier and needs to be administered directly into the brain. Due to the progressive nature of PD, sustained delivery of trophic factors may be necessary for optimal, long-term neuronal effects. Novel methods for sustained delivery of GDNF into the nigrostriatal pathway are currently being studied in non-human primates, including computer-controlled infusion pumps. Using this approach, we have demonstrated that chronic infusions of nominally 7.5 or 22.5 microg/day GDNF into the lateral ventricle, the putamen or the substantia nigra, using programmable pumps, promotes restoration of the nigrostriatal dopaminergic system and significantly improves motor functions in MPTP-lesioned rhesus monkeys with neural deficits modeling the terminal stages of PD and in aged rhesus monkeys modeling the early stages of PD. Based on the promising studies of the chronic effects of GDNF in non-human primate models of PD, a study was recently conducted in England on five advanced PD patients. Chronic GDNF infusion into the dorsal putamen, via programmable pumps, resulted in improved motor function in all patients and limited side effects were observed. However, while the data from this intraparenchymal clinical trial in humans look encouraging, extensive blinded efficacy trials will need to be conducted before it can be determined if chronic treatment with GDNF or other trophic molecules will prove useful in treating patients with PD.

Distribution of the mRNAs encoding torsinA and torsinB in the normal adult human brain

To gain insight into the neural pathways involved in the pathogenesis of DYT1 dystonia, we have mapped the cellular expression of the mRNA encoding torsinA and the closely related family member, torsinB, in normal adult human brain. Here, we report an intense expression of torsinA mRNA in the substantia nigra pars compacta dopamine neurons, the locus ceruleus, the cerebellar dentate nucleus, Purkinje cells, the basis pontis, numerous thalamic nuclei, the pedunculopontine nucleus, the oculomotor nucleus, the hippocampal formation, and the frontal cortex. Within the caudateputamen, the cellular expression of torsinA mRNA was heterogeneous; a moderate signal was found overlying large cholinergic neurons, and most striatal neurons exhibited only a very weak signal. A moderate signal was detected in numerous midbrain and hindbrain nuclei. A weak cellular signal was detected in neurons of the globus pallidus and subthalamic nucleus. In marked contrast to torsinA, no specific mRNA signal was detected for torsinB. That torsinA mRNA is enriched in several basal ganglia nuclei, including the dopamine neurons in the substantia nigra, is intriguing since it suggests that DYT1 dystonia may be associated with a dysfunction in dopamine transmission.

Distribution of the mRNAs encoding torsinA and torsinB in the normal adult human brain

To gain insight into the neural pathways involved in the pathogenesis of DYT1 dystonia, we have mapped the cellular expression of the mRNA encoding torsinA and the closely related family member, torsinB, in normal adult human brain. Here, we report an intense expression of torsinA mRNA in the substantia nigra pars compacta dopamine neurons, the locus ceruleus, the cerebellar dentate nucleus, Purkinje cells, the basis pontis, numerous thalamic nuclei, the pedunculopontine nucleus, the oculomotor nucleus, the hippocampal formation, and the frontal cortex. Within the caudate-putamen, the cellular expression of torsinA mRNA was heterogeneous; a moderate signal was found overlying large cholinergic neurons, and most striatal neurons exhibited only a very weak signal. A moderate signal was detected in numerous midbrain and hindbrain nuclei. A weak cellular signal was detected in neurons of the globus pallidus and subthalamic nucleus. In marked contrast to torsinA, no specific mRNA signal was detected for torsinB. That torsinA mRNA is enriched in several basal ganglia nuclei, including the dopamine neurons in the substantia nigra, is intriguing since it suggests that DYT1 dystonia may be associated with a dysfunction in dopamine transmission.

Oscillation of interspike interval length in substantia nigra dopamine neurons: effects of nicotine and the dopaminergic D2 agonist LY 163502 on electrophysiological activity.

The rates and patterns of discharge activity exhibited by 16 spontaneously active substantia nigra pars compacta dopamine neurons were studied in halothane-anesthetized rats using three types of quantitative measures: 1) mean discharge rates, 2) population characteristics of interspike interval samples, and 3) interspike interval time-series measures which were used to examine patterns in the ordering of interspike intervals. The mean discharge rate of these 16 cells was 2.9 +/- 0.3 spikes/sec, and each cell was classified as bursting (25% of the cells) or non-bursting (75%). The distribution of interspike intervals of non-bursting neurons were more normally distributed. Time-series analyses (raw time-series plots, return maps, and phase portraits) revealed a substantial oscillatory tendency in the magnitudes of consecutive interspike intervals in these neurons under baseline conditions: Successive interspike intervals tended to alternate between short and long durations, although short bursts often occurred. Under baseline conditions, these cells exhibited both multispike bursts and consecutive long intervals less frequently than would have been predicted by chance ordering of the interspike intervals. These results imply that there are mechanisms acting to reduce the probability of these types of events. Locally infused nicotine enhanced discharge rates in these neurons. Burst firing increased in four neurons, while five neurons did not show any change in burst firing. LY 163502 induced significant decreases in both discharge rate and bursting activity in all cells tested. The variation coefficient, skew, and kurtosis of the interspike interval distributions were not consistently altered by either drug. The local infusion of either nicotine or LY 163502 decreased the oscillatory phenomenon seen in the baseline condition. Neither the nicotine or LY 163502 time-series data exhibited a larger proportion of long-short and short-long pairs (relative to the median interval) than would be expected by chance. It is hypothesized that these neurons have intrinsic mechanisms, made manifest under anesthesia, which induce oscillations in interspike interval length. The oscillatory effect of these mechanisms can be overridden by tonic increases in either excitatory or inhibitory tone.

Selective retention of MPP + within the monoaminergic systems of the primate brain following MPTP administration: An in vivo autoradiographic study

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) selectively destroys dopaminergic neurons of the substantia nigra pars compacta in humans and other primates, producing a parkinsonian condition. MPTP is metabolized to the toxin 1-methyl-4-phenylpyridine (MPP +) which is taken up by dopamine terminals. The subsequent events culminating in cell death in the substantia nigra pars compacta are not understood. To examine these events we first produced a chronic hemiparkinsonian condition in monkeys by administering a toxic dose of MPTP via the right carotid artery. One year later, these monkeys were given a trace dose of [ 14C]MPTP intravenously and allowed to survive 1, 3, or 10 days. In two acute conditions, monkeys were either given the radiolabeled trace dose intravenously immediately following the toxic intracarotid dose, or were given a single toxic intracarotid radiolabeled dose, and allowed to survive 1, 3, or 10 days. We show by histology and autoradiography that the chronic hemiparkinsonian condition is characterized by selective unilateral loss of nigrostriatal dopamine neurons and absence of MPP + retention in the caudate-putamen. In the acute conditions, MPP + is accumulated and selectively retained in high concentrations in the caudate-putamen bilaterally and throughout the nigrostriatal pathway only on the side receiving the toxic dose. In the substantia nigra pars compacta, MPP + is accumulated in very low concentrations in the dopamine cell bodies and is not selectively retained there. At 10 days survival, the caudate-putamen on the side receiving the toxic dose loses its ability to retain MPP +. The apparent degeneration of the dopamine axon terminals in the caudate-putamen and the development of Parkinson-like behavioral signs seen at 10 days survival were observed to precede the loss of cell bodies in the substantia nigra, which appeared normal by the criteria of Nissl staining and neuromelanin content at all time points in the acute conditions. Other areas of dense MPP + retention in all cases include noradrenergic and serotonergic cell groups and noradrenergic pathways. MPP + in the locus coeruleus and other caudal catecholaminergic cell groups is apparently retrogradely transported there after uptake in terminal regions, and although it is retained in high concentrations, no cell loss occurs. These findings suggest that experimentally induced Parkinsonism results from molecular events initiated in the neostriatum and selectively elaborated in the nigrostriatal pathway, ultimately resulting in the death of substantia nigra pars compacta dopamine neurons. They do not support a significant role for neuromelanin binding in the toxicity of MPP +.

Neurophysiological investigation of effects of the D-1 agonist SKF 38393 on tonic activity of substantia nigra dopamine neurons.

The effects of the D-1 agonist SKF 38393 on tonic activity of rat substantia nigra pars compacta dopamine neurons were studied using extracellular, single-unit recording techniques. Unlike nonselective D-1/D-2 dopamine agonists or the D-2 agonist quinpirole, SKF 38393 did not inhibit dopamine neuronal activity when applied iontophoretically or when administered intravenously in doses up to 20 mg/kg to chloral hydrate-anesthetized rats. Moreover, pretreatment with SKF 38393 did not alter the inhibitory response of these neurons to apomorphine or the D-2 agonist quinpirole. However, in locally anesthetized, gallamine-treated, artificially respired rats, dopamine cell activity was significantly altered by i.v. administration of SKF 38393; firing rate increases and decreases were observed. Administration of the inactive enantiomer of SKF 38393, S-SKF 38393, did not induce similar changes in parallel experiments. These results support the idea that unlike D-2 autoreceptor stimulation, D-1 receptor stimulation does not exert a direct local effect on dopamine neurons in the substantia nigra pars compacta and suggest that D-1 receptor stimulation at sites postsynaptic to the dopamine cells may indirectly affect the activity of some dopamine neurons through long-loop feedback mechanisms.