Coupled oscillator model of the dopaminergic neuron of the substantia nigra

Calcium imaging using fura-2 and whole cell recording revealed the effective location of the oscillator mechanism on dopaminergic neurons of the substantia nigra, pars compacta, in slices from rats aged 15-20 days. As previously reported, dopaminergic neurons fired in a slow rhythmic single spiking pattern. The underlying membrane potential oscillation survived blockade of sodium currents with TTX and was enhanced by blockade of voltage-sensitive potassium currents with TEA. Calcium levels increased during the subthreshold depolarizing phase of the membrane potential oscillation and peaked at the onset of the hyperpolarizing phase as expected if the pacemaker potential were due to a low-threshold calcium current and the hyperpolarizing phase to calcium-dependent potassium current. Calcium oscillations were synchronous in the dendrites and soma and were greater in the dendrites than in the soma. Average calcium levels in the dendrites overshot steady-state levels and decayed over the course of seconds after the oscillation was resumed after having been halted by hyperpolarizing currents. Average calcium levels in the soma increased slowly, taking many cycles to achieve steady state. Voltage clamp with calcium imaging revealed the voltage dependence of the somatic calcium current without the artifacts of incomplete spatial voltage control. This showed that the calcium current had little or no inactivation and was half-maximal at -40 to -30 mV. The time constant of calcium removal was measured by the return of calcium to resting levels and depended on diameter. The calcium sensitivity of the calcium-dependent potassium current was estimated by plotting the slow tail current against calcium concentration during the decay of calcium to resting levels at -60 mV. A single compartment model of the dopaminergic neuron consisting of a noninactivating low-threshold calcium current, a calcium-dependent potassium current, and a small leak current reproduced most features of the membrane potential oscillations. The same currents much more accurately reproduced the calcium transients when distributed uniformly along a tapering cable in a multicompartment model. This model represented the dopaminergic neuron as a set of electrically coupled oscillators with different natural frequencies. Each frequency was determined by the surface area to volume ratio of the compartment. This model could account for additional features of the dopaminergic neurons seen in slices, such as slow adaptation of oscillation frequency and may produce irregular firing under different coupling conditions.     

GABAergic control of rat substantia nigra dopaminergic neurons: role of globus pallidus and substantia nigra pars reticulata

Dopaminergic neurons in vivo fire spontaneously in three distinct patterns or modes. It has previously been shown that the firing pattern of substantia nigra dopaminergic neurons can be differentially modulated by local application of GABA(A) and GABA(B) receptor antagonists. The GABA(A) antagonists, bicuculline or picrotoxin, greatly increase burst firing in dopaminergic neurons whereas GABA(B) antagonists cause a modest shift away from burst firing towards pacemaker-like firing. The three principal GABAergic inputs to nigral dopaminergic neurons arise from striatum, globus pallidus and from the axon collaterals of nigral pars reticulata projection neurons, each of which appear to act in vivo primarily on GABA(A) receptors (see preceding paper). In this study we attempted to determine on which afferent pathway(s) GABA(A) antagonists were acting to cause burst firing. Substantia nigra dopaminergic neurons were studied by single unit extracellular recordings in urethane anesthetized rats during pharmacologically induced inhibition and excitation of globus pallidus. Muscimol-induced inhibition of pallidal neurons produced an increase in the regularity of firing of nigral dopaminergic neurons together with a slight decrease in firing rate. Bicuculline-induced excitation of globus pallidus neurons produced a marked increase in burst firing together with a modest increase in firing rate. These changes in firing rate were in the opposite direction to what would be expected for a monosynaptic GABAergic pallidonigral input. Examination of the response of pars reticulata GABAergic neurons to similar manipulations of globus pallidus revealed that the firing rates of these neurons were much more sensitive to changes in globus pallidus neuron firing rate than dopaminergic neurons and that they responded in the opposite direction. Pallidal inhibition produced a dramatic increase in the firing rate of pars reticulata GABAergic neurons while pallidal excitation suppressed the spontaneous activity of pars reticulata GABAergic neurons. These data suggest that globus pallidus exerts significant control over the firing rate and pattern of substantia nigra dopaminergic neurons through a disynaptic pathway involving nigral pars reticulata GABAergic neurons and that at least one important way in which local application of bicuculline induces burst firing of dopaminergic neurons is by disinhibition of this tonic inhibitory input.     

大鼠黑质损毁后多巴胺能神经元形态及纹状体c-fos表达的变化

目的:观察毁损黑质后,多巴胺(DA)能神经元形态学变化和纹状体内相关神经元c-fos表达情况,探讨c-fos表达与毁损程度的关系。方法:利用6羟多巴胺(6OHDA)特异毁损大鼠黑质DA能神经元,采用阿朴吗啡(APO)诱导旋转实验观察术后1、7、14和21d行为学变化;利用HE染色、Nissl染色、免疫组织化学方法和电镜,观察各时间点黑质DA能神经元形态学变化和纹状体c-fos表达。结果:毁损侧DA能神经元逐渐减少,超微结构损伤逐渐加重;DA神经元丢失比例≥80%时,纹状体毁损侧c-fos表达上调,APO诱导的旋转实验>7r/min。结论:黑质DA能神经元丢失是毁损大鼠行为改变的病理学基础;cfos的表达与DA能神经元的毁损程度、行为改变有一定的关系。     

Regular firing of a single output neuron reduces its own inhibition through endocannabinoids in substantia nigra pars reticulata of juvenile mice

Depolarization-induced suppression of inhibition in substantia nigra pars reticulata suggests that burst-like activity but not regular firing suffices to activate presynaptic endocannabinoid CB1 receptors. To more closely determine the type of activity required, we applied gramicidin perforated patch recording under visual control to substantia nigra slices of juvenile mice. We found that evoked inhibitory postsynaptic currents (eIPSCs) were reduced in amplitude by the spontaneous firing of a neuron under study, whereas silencing this neuron enhanced inhibitory responses. Autonomous firing reduced eIPSCs to 78%±2% in a time- but not frequency-dependent manner. The phenomenon which we termed firing-induced suppression of inhibition was cannabinoid receptor subtype 1–dependent, whereas adenosine A1 receptors played only a minor role. Depletion of intracellular Ca²⁺ stores abolished the firing-induced suppression of inhibition suggesting that Ca²⁺ release from internal stores is necessary for the production of endocannabinoids during autonomous firing. We suggest that the Ca²⁺ influx during autonomous activity of pars reticulata neurons suffices to selectively dampen incoming inhibition from striatal neurons because it is amplified by ryanodine receptor-mediated Ca²⁺ release from intracellular stores.     

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.     

Dissociated high-purity dopaminergic neuron cultures from the substantia nigra and the ventral tegmental area of the postnatal rat.

We have developed dissociated primary neuronal cultures obtained from the substantia nigra and from the ventral tegmental area of postnatal rats (two to three days old). After making brain slices, the regions of the substantia nigra and the ventral tegmental area were separately dissected. The removed fragments of brain tissue were dissociated and cultured on a glial feeder layer. Double immunocytochemical labeling for tyrosine hydroxylase and GABA on cultures grown for two to three weeks showed the presence of 42% dopaminergic and 39% GABAergic neurons in substantia nigra cultures, whereas in ventral tegmental area cultures there were 65% dopaminergic and 21% GABAergic neurons. The dopaminergic neurons were characterized by thick and straight primary processes dividing into several branches. Varicosities were found mainly on distal parts of the processes. In contrast, GABAergic neurons possessed highly branched thick and thin primary processes with intensive arborization and numerous varicosities. Co-existence of dopamine and cholecystokinin was found in about 70% of dopaminergic neurons from the substantia nigra and in about 35% of dopaminergic neurons from the ventral tegmental area. Physiological properties of these cultured dopaminergic neurons were investigated with the whole-cell version of the patch-clamp method. After each physiological experiment, immunocytochemical labeling confirmed that the cell was dopaminergic. Properties of single action potentials, with an action potential height of 92 mV and duration of 1.6 ms, were similar to those reported for dopaminergic neurons in brain slices. The neurons showed a high resting potential, and no spontaneous firing of action potentials. Constant current depolarizations elicited trains of action potentials. In the majority of cells, the train stopped firing within a few seconds, while in some cells it lasted indefinitely. When the cell was hyperpolarized, the voltage response started to decline slowly (sag), indicating the presence of hyperpolarization-activated currents (time-dependent inward rectification). These results show that by using our culture method it is possible to obtain separate dissociated cultures of the substantia nigra and the ventral tegmental area from newborn rats. Because they are rich in functional dopaminergic neurons, these cultures will be a useful tool for studying various properties of dopaminergic neurons.     

Nigrostriatal dopamine neurons are required to maintain basal levels of substance P in the rat substantia nigra

Striatal dopamine was depleted in adult rats by unilateral infusion of 6-hydroxydopamine near the dopamine neurons of the pars compacta of the substantia nigra. Following survival periods of 1, 3, 4, 6 or 12 weeks, changes in levels of the tachykinin neuropeptide, substance P, in striatonigral axon terminals were assessed by quantitative radioimmunocytochemistry. Substance P levels in the ipsilateral substantia nigra were consistently lower than levels on the control (non-lesion) side at every time point examined, reaching a maximum decline of about 30% at 3 weeks after the lesion. These data show that there is no recovery of nigral substance P content to basal levels up to 3 months post-lesion, and suggest strongly that intact striatal dopamine innervation is required for the maintenance of basal substance P levels in the terminals of striatonigral substance P neurons.     

Synaptic activation of dendritic AMPA and NMDA receptors generates transient high-frequency firing in substantia nigra dopamine neurons in vitro

Transient high-frequency activity of substantia nigra dopamine neurons is critical for striatal synaptic plasticity and associative learning. However, the mechanisms underlying this mode of activity are poorly understood because, in contrast to other rapidly firing neurons, high-frequency activity is not evoked by somatic current injection. Previous studies have suggested that activation of dendritic N-methyl-D-aspartate (NMDA) receptors and/or G-protein-coupled receptor (GPCR)-mediated reduction of action potential afterhyperpolarization and/or activation of cation channels underlie high-frequency activity. To address their relative contribution, transient high-frequency activity was evoked using local electrical stimulation (1 s, 10–100 Hz) in brain slices prepared from p15–p25 rats in the presence of GABA and D2 dopamine receptor antagonists. The frequency, pattern, and morphology of action potentials evoked under these conditions were similar to those observed in vivo. Evoked activity and reductions in action potential afterhyperpolarization were diminished greatly by application of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or NMDA receptor selective antagonists and abolished completely by co-application of AMPA and NMDA antagonists. In contrast, application of glutamatergic and cholinergic GPCR antagonists moderately enhanced evoked activity. Dendritic pressure-pulse application of glutamate evoked high-frequency activity that was similarly sensitive to antagonism of AMPA or NMDA receptors. Taken together, these data suggest that dendritic AMPA and NMDA receptor-mediated synaptic conductances are sufficient to generate transient high-frequency activity in substantia nigra dopamine neurons by rapidly but transiently overwhelming the conductances underlying action potential afterhyperpolarization and/or engaging postsynaptic voltage-dependent ion channels in a manner that overcomes the limiting effects of afterhyperpolarization.     

Afferent modulation of dopamine neuron firing differentially regulates tonic and phasic dopamine transmission

The mesolimbic dopamine system is centrally involved in reward and goal-directed behavior, and it has been implicated in multiple psychiatric disorders. Understanding the mechanism by which dopamine participates in these activities requires comprehension of the dynamics of dopamine release. Here we report dissociable regulation of dopamine neuron discharge by two separate afferent systems in rats; inhibition of pallidal afferents selectively increased the population activity of dopamine neurons, whereas activation of pedunculopontine inputs increased burst firing. Only the increase in population activity increased ventral striatal dopamine efflux. After blockade of dopamine reuptake, however, enhanced bursting increased dopamine efflux three times more than did enhanced population activity. These results provide insight into multiple regulatory systems that modulate dopamine system function: burst firing induces massive synaptic dopamine release, which is rapidly removed by reuptake before escaping the synaptic cleft, whereas increased population activity modulates tonic extrasynaptic dopamine levels that are less influenced by reuptake.     

中脑黑质多巴胺神经元对适当剂量鱼藤酮毒性损伤的特殊敏感性

目的:探索中脑黑质多巴胺神经元对适当剂量鱼藤酮毒性损伤是否具有特殊敏感性。方法:采用颈背部皮下注射鱼藤酮的方法建立大鼠中脑黑质多巴胺神经元损伤模型,进行中脑黑质和纹状体酪氨酸羟化酶(TH)免疫组织化学染色加尼氏染色;同时通过HE染色及尼氏染色方法分别观察心、肝、脾、肾等重要胸腹腔脏器及海马、顶叶皮质的形态学变化。结果:中脑黑质TH免疫染色和尼氏染色结果显示组间中脑黑质致密区以外部位尼氏小体数目无差异;大脑顶叶皮质和海马尼氏染色及胸腹腔重要脏器HE染色结果表明各组大鼠均未出现相应部位损伤。结论:低剂量颈部皮下注射鱼藤酮能选择性诱导中脑黑质多巴胺神经元损伤,说明中脑黑质多巴胺神经元对鱼藤酮具有高度敏感性。     

Differential sensitivity of presumed dopaminergic and non-dopaminergic neurones in rat substantia nigra to electrophoretically applied substance P

Substance P (SP) was administered by microelectrophoresis to physiologically identified substantia nigra neurones in halothane anaesthetized rats. Dopaminergic neurones of the substantia nigra compacta (SNC) were markedly less sensitive to SP than non-dopaminergic cells of the substantia nigra reticulata (SNR) when the peptide was administered with the same micropipettes. Identified substantia nigra neurones were also differentially sensitive to other putative transmitters including dopamine, 5-HT and acetylcholine. No interactions could be demonstrated between SP and the responses to other transmitters.     

Possible roles of kainate receptors on GABAergic nerve terminals projecting to rat substantia nigra dopaminergic neurons

GABAergic afferent inputs are thought to play an important role in the control of the firing pattern of substantia nigra pars compacta (SNc) dopaminergic neurons. We report here the actions of presynaptic kainite (KA) receptors in GABAergic transmission of rat SNc dopaminergic neurons. In mechanically dissociated rat SNc dopaminergic neurons attached with native presynaptic nerve terminals, GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded by use of conventional whole cell patch recording mode. In the voltage-clamp condition, KA (3 microM) significantly increased GABAergic mIPSC frequency without affecting the current amplitude. This facilitatory effect of KA was not affected in the presence of 20 microM GYKI52466, a selective AMPA receptor antagonist, but was completely inhibited in the presence of 20 microM CNQX, an AMPA/KA receptor antagonist. Presynaptic KA receptors on GABAergic terminals were mainly permeable to Na+ but impermeable to Ca2+ because KA-induced facilitation of mIPSC frequency was completely suppressed in either Na+-free or Ca2+-free external solutions, and in the presence of 200 microM Cd2+, a general voltage-dependent Ca2+ channel blocker. In the slice preparation, KA increased GABAergic spontaneous mIPSC frequency, but significantly suppressed evoked IPSC (eIPSC) amplitude. However, this inhibitory action on eIPSCs was reversed by 10 microM CGP55845, a selective GABAB receptor antagonist, implicating the possible involvement of GABAB autoreceptors in KA-induced modulation of GABAergic transmission. Thus presynaptic KA receptors on GABAergic nerve terminals synapsing onto SNc neurons may play functional roles contributing the fine control of neuronal excitability and firing pattern of SNc.     

Neurotensin attenuates the quinpirole-induced inhibition of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area

In the present study we describe the excitatory effects of the bioactive peptide neurotensin on the electrical activity of dopamine neurons (simultaneously recorded) in the substantia nigra pars compacta and the ventral tegmental area. The neurotensin fragment (8-13) induced comparable increases in firing rate of the substantia nigra and ventral tegmental area dopamine neurons (EC50 values 30 and 45 nM, respectively). The neurotensin receptor antagonist SR142948A antagonized the excitatory effects of neurotensin fragment (8-13) (pA2 values 8.4 and 8.2, respectively). Furthermore, it was found that a low concentration of neurotensin fragment (8-13) (1 nM) attenuated the inhibition of the firing rate by the selective dopamine D2 receptor agonist quinpirole in both neuron types (e.g., the effect of 0.01 microM quinpirole was reduced by approximately 60% in the presence of 1 nM neurotensin fragment [8-13]). Antagonism of this neurotensin fragment (8-13) effect by SR142948A confirms that neurotensin receptors can reduce the effect of dopamine D2 receptors at the single-cell level. These results are discussed in the light of possible roles for neurotensin in neurological disorders such as Parkinson's disease and schizophrenia.     

A presynaptic action of dopamine on globus pallidus afferents to substantia nigra in the rat

Dopamine applied iontophoretically onto substantia nigra and adjacent reticular formation units, changed the pattern of response evoked in these cells by stimulation of the globus pallidus. This change was reversible and was sensitive to iontophoretically applied dopamine. It is proposed that this effect is brought about by an interaction of dopamine with dopamine receptors located on afferent fibres coming from the globus pallidus.     

Neurotensin attenuates the quinpirole-induced inhibition of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area

In the present study we describe the excitatory effects of the bioactive peptide neurotensin on the electrical activity of dopamine neurons (simultaneously recorded) in the substantia nigra pars compacta and the ventral tegmental area. The neurotensin fragment (8–13) induced comparable increases in firing rate of the substantia nigra and ventral tegmental area dopamine neurons (EC₅₀ values 30 and 45 nM, respectively). The neurotensin receptor antagonist SR142948A antagonized the excitatory effects of neurotensin fragment (8–13) (pA₂ values 8.4 and 8.2, respectively). Furthermore, it was found that a low concentration of neurotensin fragment (8–13) (1 nM) attenuated the inhibition of the firing rate by the selective dopamine D₂ receptor agonist quinpirole in both neuron types (e.g., the effect of 0.01 μM quinpirole was reduced by ≈60% in the presence of 1 nM neurotensin fragment [8–13]). Antagonism of this neurotensin fragment (8–13) effect by SR142948A confirms that neurotensin receptors can reduce the effect of dopamine D₂ receptors at the single-cell level.These results are discussed in the light of possible roles for neurotensin in neurological disorders such as Parkinson's disease and schizophrenia.     

鱼藤酮损伤大鼠黑质至行为学及黑质多巴胺能神经元损伤

目的 探讨不同剂量鱼藤酮损伤大鼠黑质后行为学和酪氨酸羟化酶免疫活性细胞的变化.方法 采用大鼠黑质立体定位注射不同剂量鱼藤酮的方法,观察大鼠行为学变化,应用免疫组化染色检测黑质酪氨酸羟化酶(TH)免疫活性细胞的变化.结果 给鱼藤酮21d后,2.5μg/μl组大鼠行为测试记分2-4分为主,占91.3%,TH免疫活性细胞百分...