Dopaminergic Neurons In Substantia Nigra Compacta Research Articles

Angiotensin-II Modulates GABAergic Neurotransmission in the Mouse Substantia Nigra.

GABAergic projections neurons of the substantia nigra reticulata (SNr), through an extensive network of dendritic arbors and axon collaterals, provide robust inhibitory input to neighboring dopaminergic neurons in the substantia nigra compacta (SNc). Angiotensin-II (Ang-II) receptor signaling increases SNc dopaminergic neuronal sensitivity to insult, thus rendering these cells susceptible to dysfunction and destruction. However, the mechanisms by which Ang-II regulates SNc dopaminergic neuronal activity are unclear. Given the complex relationship between SN dopaminergic and GABAergic neurons, we hypothesized that Ang-II could regulate SNc dopaminergic neuronal activity directly and indirectly by modulating SNr GABAergic neurotransmission. Here, using transgenic mice, slice electrophysiology, and optogenetics, we provide evidence of an AT1 receptor-mediated signaling mechanism in SNr GABAergic neurons where Ang-II suppresses electrically-evoked neuronal output by facilitating postsynaptic GABAA receptors (GABAARs) and prolonging the action potential (AP) duration. Unexpectedly, Ang-II had no discernable effects on the electrical properties of SNc dopaminergic neurons. Also, and indicating a nonlinear relationship between electrical activity and neuronal output, following phasic photoactivation of SNr GABAergic neurons, Ang-II paradoxically enhanced the feedforward inhibitory input to SNc dopaminergic neurons. In sum, our observations describe an increasingly complex and heterogeneous response of the SN to Ang-II by revealing cell-specific responses and nonlinear effects on intranigral GABAergic neurotransmission. Our data further implicate the renin-angiotensin-system (RAS) as a functionally relevant neuromodulator in the substantia nigra, thus underscoring a need for additional inquiry.

Electrical Abnormalities in Dopaminergic Neurons of the Substantia Nigra in Mice With an Aromatic L-Amino Acid Decarboxylase Deficiency.

Aromatic L-acid decarboxylase (AADC) deficiency causes severe motor disturbances in affected children. A putamen-targeted gene therapy improves the motor function of patients. The present study investigated the electrical properties of dopaminergic (DA) neurons in the substantia nigra compacta (SNc) of mice with an AADC deficiency (DdcKI). The basal firing of DA neurons, which determines DA release in the putamen, was abnormal in the DdcKI mice, including a low frequency and irregular firing pattern, because of a decrease in the after-hyperpolarization (AHP) amplitude of action potentials (APs). The frequency of spontaneous excitatory postsynaptic currents (sEPSCs) increased and that of spontaneous inhibitory PSCs (sIPSCs) decreased in the SNc DA neurons from the DdcKI mice, suggesting an elevation in glutamatergic excitatory stimuli and a reduction in GABAergic inhibitory stimuli, respectively. Altered expression patterns of genes encoding receptors and channels were also observed in the DdcKI mice. Administration of a widespread neuron-specific gene therapy to the brains of the DdcKI mice partially corrected these electric abnormalities. The overexcitability of SNc DA neurons in the presence of generalized dopamine deficiency likely underlies the occurrence of motor disturbances.

N-oleoyldopamine modulates activity of midbrain dopaminergic neurons through multiple mechanisms

N-oleoyl-dopamine (OLDA) is an amide of dopamine and oleic acid, synthesized in catecholaminergic neurons. The present study investigates OLDA targets in midbrain dopaminergic (DA) neurons. Substantia Nigra compacta (SNc) DA neurons recorded in brain slices were excited by OLDA in wild type mice. In transient receptor potential vanilloid 1 (TRPV1) knockout (KO) mice, however, SNc DA neurons displayed sustained inhibition of firing. In the presence of the dopamine type 2 receptor (D2R) antagonist sulpiride or the dopamine transporter blocker nomifensine no such inhibition was observed. Under sulpiride OLDA slightly excited SNc DA neurons, an action abolished upon combined application of the cannabinoid1 and 2 receptor antagonists AM251 and AM630. In ventral tegmental area (VTA) DA neurons from TRPV1 KO mice a transient inhibition of firing by OLDA was observed. Thus OLDA modulates the firing of nigrostriatal DA neurons through interactions with TRPV1, cannabinoid receptors and dopamine uptake. These findings suggest further development of OLDA-like tandem molecules for the treatment of movement disorders including Parkinson's disease.

Abnormal Development of Glutamatergic Synapses Afferent to Dopaminergic Neurons of the Pink1(-/-) Mouse Model of Parkinson's Disease.

In a preceding study, we showed that in adult pink1−/− mice, a monogenic animal model of Parkinson’s disease (PD), striatal neurons display aberrant electrical activities that precede the onset of overt clinical manifestations. Here, we tested the hypothesis that the maturation of dopaminergic (DA) neurons of the pink1−/− substantia nigra compacta (SNc) follows, from early stages on, a different developmental trajectory from age-matched wild type (wt) SNc DA neurons. We used immature (postnatal days P2–P10) and young adult (P30–P90) midbrain slices of pink1−/− mice expressing the green fluorescent protein in tyrosine hydroxylase (TH)-positive neurons. We report that the developmental sequence of N-Methyl-D-aspartic acid (NMDA) spontaneous excitatory postsynaptic currents (sEPSCs) is altered in pink1−/− SNc DA neurons, starting from shortly after birth. They lack the transient episode of high NMDA receptor-mediated neuronal activity characteristic of the immature stage of wt SNc DA neurons. The maturation of the membrane resistance of pink1−/− SNc DA neurons is also altered. Collectively, these observations suggest that electrical manifestations occurring shortly after birth in SNc DA neurons might lead to dysfunction in dopamine release and constitute an early pathogenic mechanism of PD.

Neuroprotective subthalamotomy in Parkinson's disease. The role of magnetic resonance-guided focused ultrasound in early surgery

Subthalamic nucleus hyperactivity in Parkinson's disease may be a very early phenomenon. Its start is not well known, and it may occur during the pre-symptomatic disease stage. Glutamatergic hyperactivity may be neurotoxic over the substantia nigra compacta dopaminergic neurons. If this occurred, the excitatory neurotransmitter, glutamate, should affect the neurons that maintain a high turnover as a compensatory mechanism. Would a subthalamic nucleus lesion decrease this hyperactivity and thus be considered as a neuroprotective mechanism for dopaminergic neurons? The authors hypothesise about the possibility to perform surgery on a subthalamic nucleus lesion at a very early stage in order to avoid the neurotoxic glutamatergic effect over the dopaminergic neurons, and therefore be considered as a neuroprotective surgery able to alter the progress of the disease during early motor symptoms. In this regard, magnetic resonance-guided focused ultrasound techniques open a new window in the stereotactic armamentarium.

Differential distribution of hypoxia-inducible factor 1-beta (ARNT or ARNT2) in mouse substantia nigra and ventral tegmental area

Hypoxia has been proposed as a mechanism underlying gene–environment interactions in the neurodevelopmental model of schizophrenia, and hypoxia-inducible factor 1 (HIF-1) could mediate the interactions. In the current study, we analyzed the HIF-1 beta subunit, as formed by aryl hydrocarbon receptor nuclear translocator (ARNT) or ARNT2, in the mouse substantia nigra (SN) and the ventral tegmental area (VTA). We performed immunohistochemical studies of ARNT and ARNT2 in the adult mouse brain, and colocalization analyses, with specific emphasis on dopaminergic cells, i.e. tyrosine hydroxylase (TH) immunoreactive cells. Bioinformatic analyses identified shared protein partners for ARNT and ARNT2. ARNT immunoreactivity showed widespread neuronal labeling without overt regional specificity. We observed co-localization of ARNT and TH in the SN compacta and VTA. Nuclei strongly labeled for ARNT2 were observed in the SN reticulata, while only weak immunoreactivity for ARNT2 was found in TH-immunoreactive neurons in SN compacta and VTA. Stereological analysis showed that ARNT was preferentially expressed in dopaminergic neurons in SN compacta and VTA. Nuclei strongly labeled for ARNT2 were present in neocortex and CA1 of hippocampus. Differential expression of ARNT and ARNT2 in dopaminergic neurons may relate to the vulnerability of distinct dopaminergic projections to hypoxia and to functional vulnerability in schizophrenia and other neuropsychiatric disorders.

Manganese mixture inhalation is a reliable Parkinson disease model in rats

Manganese (Mn) is an essential trace metal. Regardless of its essentiality, it has been reported that the overexposure causes neurotoxicity manifested as extrapyramidal symptoms similar to those observed in Parkinson disease (PD). Recently, our group reported that mice that inhaled for 5 months the mixture of manganese chloride (MnCl2) and manganese acetate Mn(OAc)3 developed movement abnormalities, significant loss of substantia nigra compacta (SNc) dopaminergic neurons, dopamine depletion and improved behavior with l-DOPA treatment. However, this model has only been characterized in mice. In order to have a well-supported and generalizable model in rodents, we used male Wistar rats that inhaled a mixture of 0.04M MnCl2 and 0.02M Mn(OAc)3, 1h three times a week for 6 months. Before Mn exposure, animals were trained to perform motor tests (Beam-walking and Single-pellet reaching tasks) and were evaluated each week after the exposure. The mixture of MnCl2/Mn(OAc)3 caused alterations in the motor tests, 75.95% loss of SNc dopaminergic neurons, and no cell alterations in Globus Pallidus or striatum. With these results we conclude that the inhalation of the mixture of Mn compounds is a useful model in rodents for the study of PD.

Arrhythmic Firing in Dopamine Neurons of Rat Substantia Nigra Evoked by Activation of Subthalamic Neurons

Intracellular recordings were made from dopaminergic neurons of the rat substantia nigra compacta (SNc) in in vitro slice preparations to study the synaptic influence from the subthalamic nucleus (STh). After microstimulation of STh, monosynaptic excitatory postsynaptic potentials (EPSPs) were produced in dopaminergic neurons. STh-induced EPSPs were composed of 6-cyano-7-nitroquinoxalene-2,3-dione- and 2-amino-5-phosphonovaleric acid-sensitive components. Subthreshold EPSPs evoked by STh stimulation could differentially trigger pacemaker-like slow depolarization (PLSD) and low-threshold Ca2+ spike (LTS) depending on the level of baseline membrane potentials. When a subthreshold EPSP was evoked by STh stimulation during rhythmic firing, the STh-induced EPSP could shift or elevate PLSD to a more depolarized level, resulting in generation of a spike at an earlier arrhythmic timing to restart the rhythmic firing. The interspike interval after the arrhythmic spike remained almost unchanged. In contrast, when a suprathreshold EPSP for evoking spikes was produced by STh stimulation during rhythmic firing, the STh-induced spike was just interposed between two spontaneous spikes the interspike interval of which was almost the same as those seen during the preceding rhythmic firing. This ectopically induced spike did not disturb or reset rhythmic firing. It was concluded that SNc dopaminergic neurons receive monosynaptic glutamatergic inputs from STh, and subthreshold and suprathreshold EPSPs evoked by STh stimulations can induce two types of arrhythmic firing in SNc dopaminergic neurons, similar to arrhythmic occurrences of the QRS complex seen in the electrocardiogram of the atrial and ventricular arrhythmias, respectively. The former arrhythmic firing may play a crucial role in desynchronization of dopaminergic neurons.

Calcium spike underlying rhythmic firing in dopaminergic neurons of the rat substantia nigra

In order to study a possible mechanism for rhythmic firing of dopaminergic (DA) neurons, intracellular recordings were obtained from 56 rhythmically firing DA neurons in the rat substantia nigra compacta (SNc), using in vitro slice preparations. In the presence of TTX, spontaneous oscillation of the membrane potential was induced in SNc DA neurons when the membrane potential was depolarized more positive from −60 to −40 mV. Each oscillation wave was characterized by a pacemaker-like slow depolarization (PLSD) followed by a relatively prompt repolarization. As the DC depolarization was increased from −60 to −40 mV, the oscillation frequency increased from 0.5 to 5 Hz, but the amplitude of the wave decreased. Of 17 neurons tested in the presence of TTX, the maximum amplitudes of the oscillation varied from 10–15 mV in 8 neurons and were less than 5 mV in 9 neurons. In those 9 neurons, an application of TEA greatly enhanced (up to 15 mV) the amplitude of oscillation. The oscillation ceased when the membrane was hyperpolarized more negative than −60 mV. At the membrane potential more negative than −60 mV in the presence of TTX an injection of a depolarizing current pulse could evoke PLSD which was an all-or-nothing regenerative spike potential. The rate of rise of the PLSD changed depending on the intensity of injected current pulses but their amplitude remained constant. Its time-to-peak was slow (up to 1400 ms), while the decay time was relatively brief (< 500 ms). The threshold membrane potential for evoking PLSD was −53.7 ± 3.2 mV ( n = 10). This was higher than the previously reputed threshold for low threshold Ca 2+ spike (LTS) (< −60 mV) and lower than that for high threshold Ca 2+ spike (HTS) (> −35 mV) in SNc DA neurons. Even at a holding potential of −45 mV, a depolarizing current pulse could trigger PLSD while LTS was completely inactivated. Cd 2+ (0.4 mM) abolished the oscillation and PLSD without marked effects on the LTS ( n = 6). A low Ca 2+ and high Mg 2+ Ringer's solution also abolished the oscillation and PLSD ( n = 4). An intracellular injection of EGTA markedly prolonged the decay time course of PLSD characterized by a slow and a relatively fast falling phase ( n = 5). This would suggest an involvement of Ca 2+-dependent K + conductance and/or Ca 2+ dependent inactivation of Ca 2+ conductance during repolarization. Characteristic features of the slow rising course of PLSD indicate a slowly inactivating property of the Ca 2+ conductance which may be responsible for this depolarization. Of 56 DA neurons which displayed oscillation or rhythmic firing, PLSD was observed in all of the neurons, while LTS was observed in only 24 neurons. In conclusion, PLSD had characteristics of a regenerative spike potential which is different in the threshold for activation and inactivation, the time-course, and pharmacology previously reported for LTS or HTS. These data suggest that a possible heterogenous Ca 2+ spike (PLSD) may be responsible for rhythmic oscillation of membrane potential in SNc DA neurons.