Regulation of substantia nigra dopamine neurons

A substantial amount of research has focused on determining the factors that alter the activity of substantia nigra dopamine neurons. Much of this research has indicated that several mechanisms that regulate dopamine neuron activity have the capability to maintain the baseline activity of dopamine cells at a fairly constant rate. For example, the intrinsic membrane conductances present on dopamine neurons, which generate the spike activity of these cells, appear to maintain the activity of spontaneously active neurons and suppress the induction of activity in quiescent cells. In addition, dopamine cell activity can be regulated by afferent systems that appear to be capable of preventing dopamine neurons from displaying sustained variations in electrophysiological activity. Specifically, inputs from the striatum or from the subthalamic nucleus may each exert opposing influences on dopamine cell activity via direct vs. indirect afferent projection pathways. In addition, the dendritic release of dopamine may provide negative feedback; dopamine cell firing may increase the dendritic release of dopamine within the substantia nigra, providing a local feedback inhibition of dopamine neuron activity. Factors such as the intrinsic membrane properties, afferent input, and the dendritic release of dopamine all work together in a complex manner to regulate the activity level of dopamine neurons.     

Striatal extracellular dopamine levels in rats with haloperidol-induced depolarization block of substantia nigra dopamine neurons

Correlations between substantia nigra (SN) dopamine (DA) cell activity and striatal extracellular DA were examined using simultaneous extracellular single-unit recordings and in vivo microdialysis performed in drug-naive rats and in rats treated repeatedly with haloperidol (HAL). Intact rats treated with HAL for 21-28 d exhibited significantly fewer active DA cells, indicating the presence of depolarization block (DB) in these cells. However, in rats that received surgical implantation of the microdialysis probe followed by a 24 hr recovery period, HAL-induced DA cell DB was reversed, as evidenced by a number of active DA neurons that was significantly higher than that in HAL-treated intact rats and similar to that of drug-naive rats. In contrast, using a modified probe implantation procedure that did not reverse SN DA neuron DB, we found striatal DA efflux to be significantly lower than in controls and significantly correlated with the reduction in DA neuron spike activity. Furthermore, although basal striatal DA efflux was independent of SN DA cell burst-firing activity in control rats, these variables were significantly correlated in rats with HAL-induced DA cell DB. Therefore, HAL-induced DB of SN DA neurons is disrupted by implantation of a microdialysis probe into the striatum using standard procedures. However, a modified microdialysis method that allowed reinstatement of DA neuron DB revealed that the HAL-induced inactivation of SN DA neurons was associated with significantly lower extracellular DA levels in the striatum. Moreover, the residual extracellular DA maintained in the presence of DB may, in part, depend on the burst-firing pattern of the noninactivated DA neurons in the SN.     

Metabotropic glutamate receptor-mediated inhibition and excitation of substantia nigra dopamine neurons

Microiontophoretic drug application and extracellular recording techniques were used to evaluate the effects of the selective metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate(1S,3R-ACPD) on dopamine (DA) neurons in the substantia nigra zona compacta (SNZC) of chloral hydrate-anesthetized rats. 1S,3R-ACPD had a biphasic effect on the firing rate of DA cells, initially decreasing, then increasing the firing rate. 1S,3R-ACPD also increased the burst-firing activity of DA neurons. Application of the ionotropic receptor (iGluR) agonists (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartate (NMDA) increased the firing rates of neurons which had responded to 1S,3R-ACPD, indicating that mGluRs and iGluRs reside on the same neurons. The initial inhibitory period was not antagonized by systemic haloperidol or iontophoretic bicuculline, indicating a lack of DA or gamma-amino-n-butyric acid (GABA) involvement in this effect. Combined application of the AMPA antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), and the NMDA antagonist, (I)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphoric acid (CPP), at currents which antagonized AMPA and NMDA, did not antagonize either the inhibitory or excitatory effects of 1S,3R-ACPD. Application of the metabotropic antagonist (S)-4-carboxy-phenylglycine antagonized both the inhibitory and excitatory effects of 1S,3R-ACPD. These results indicate that mGluRs may play a role in the modulation of dopaminergic activity in the SNZC.     

Postsynaptic nicotinic receptors on dopaminergic neurons in the substantia nigra pars compacta of the rat

Previous studies have shown that application of nicotinic agonists in the substantia nigra pars compacta increases the firing rate of dopaminergic neurons. We have used intracellular recordings to show that the response of these neurons to nicotine is postsynaptic, since it persists in the presence of low-calcium buffer containing tetrodotoxin. Burst firing in the presence of nicotine was not observed. The presence of postsynaptic nicotinic receptors was confirmed by immunohistochemical localization of the alpha4 nicotinic receptor subunit on dendrites in the substantia nigra pars compacta. The majority of tyrosine hydroxylase-immunopositive neurons in the substantia nigra pars compacta were also immunopositive for the alpha4 subunit. Immunohistochemical localization of the alpha4 and beta2 subunits in adjacent brain sections produced similar patterns of staining. Electron micrographs clearly indicated the presence of alpha4 subunit at postsynaptic densities. The predominant role of nicotinic receptors in the central nervous system has been suggested to be the presynaptic modulation of neurotransmitter release [McGehee D. S. and Role L. W. (1995) A. Rev. Physiol. 57, 521-546]. Although several postsynaptic nicotinic responses have also been reported in the literature, it is unclear as to whether the postsynaptic nicotinic receptors mediating responses to exogenously applied agonists are involved in synaptic transmission. From our electrophysiological and immunohistochemical results, we conclude that alpha4-containing nicotinic receptors are found at synapses on dopaminergic neurons. These synapses are similar to the cholinergic synapses described at these neurons, suggesting that nicotinic receptors are important in modulating the excitability of dopaminergic neurons by direct synaptic transmission.     

Place learning strategy of substantia nigra pars compacta-lesioned rats.

The substantia nigra pars compacta (SNc) and the dorsal striatum are often considered to be necessary for stimulus-response (S-R) habit learning, whereas the dorsal hippocampus is considered to be necessary for relational (declarative) memory. Spatial learning is a kind of relational learning that occurs when a rat is released from different locations (variable start) in a water maze to find a submerged platform that is kept in a constant location. However, when the rat is always released from the same starting position (constant start), it can learn to find the platform oriented by a fixed configuration of cues, that is, by S-R learning. To test the critical role of the SNc in S-R and relational learning, the authors tested adult male Wistar rats, sham-operated or with a lesion in the SNc, in these 2 versions of the water maze task. The SNc lesion was induced by bilateral intranigral infusion of 0.5 μmol 1-methyl-4-phenyl- 1,2,3,6-tetrahydropyridine. Although the SNc-lesioned rats learned the variable-start version as effectively as sham rats did, they were significantly impaired in learning the constant-start version of the task. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Single subthalamic nucleus neurons project to both the globus pallidus and substantia nigra in rat

The organization of the major efferents of the rat subthalamic nucleus (STN) was investigated using a fluorescent retrograde double-labeling technique. Red fluorescent Evans Blue was injected into the globus pallidus and blue fluorescent DAPI-Primuline was injected into the substantia nigra. After retrograde axonal transport many double-labeled neurons were seen throughout the STN. Occasionaly double-labeled cells were seen in the lateral hypothalamus just medial to the STN and in a thin lateral strip of neurons extending laterally from the STN. Evidence for a mediolateral topography in both the STN-pallidal and STN-nigral pathways was obtained. The STN contains few, if any, local interneurons. Cell counts revealed that at least 94% of, and possibly all, STN neurons send axon collaterals to both the globus pallidus and substantia nigra.     

Ultrastructure of developing substantia nigra in humans

Electron microscopy of the maturing neurons and developing and maturing synapses in the substantia nigra of 14 human embryos/foetuses of 8-24 weeks of gestation are reported. At 8 weeks, cells were immature with very little cytoplasm and cellular organelles. Contact sites of processes appeared more electron dense than the other areas. At 12 weeks, many of the cells had acquired more cytoplasm and cellular organelles and could be identified as neurons. Asymmetric synapses with clear, round synaptic vesicles also were identifiable at this age. Such synapses, first to appear in the developing substantia nigra, are reported to be formed by recurrent collateral nigro-striatal fibres. Substance P fibres from the striatum also are contributing to this type of synapse. At 15-16 weeks, not only was the number of such synapses increased, but many appeared morphologically mature. Symmetric synapses having clear round vesicles along with a few dense core vesicles also appeared at this stage, suggesting striatal input. By 24 weeks of gestation, most of the neurons had cytological features comparable to that of the mature neurons. There was an increase in the total number of synapses and the individual variety from 15 to 24 weeks of gestation. The present study indicates that synaptogenesis starts at 8 weeks and continues beyond 24 weeks of gestation.     

Heat shock genes and the heat shock response in zebrafish embryos

Heat shock genes exhibit complex patterns of spatial and temporal regulation during embryonic development in a wide range of organisms. Our laboratory has initiated an analysis of heat shock protein gene expression in the zebrafish, a model system that is now utilized extensively for the examination of early embryonic development of vertebrates. We have cloned members of the zebrafish hsp47, hsp70, and hsp90 gene families and shown them to be closely related to their counterparts in higher vertebrates. Whole mount in situ hybridization and Northern blot analyses have revealed that these genes are regulated in distinct spatial, temporal, and stress-specific manners. Furthermore, the tissue-specific expression patterns of the hsp47 and hsp90 alpha genes correlate closely with the expression of genes encoding known chaperone targets of Hsp47 and Hsp90 in other systems. The data raise a number of interesting questions regarding the function and regulation of these heat shock genes in zebrafish embryos during normal development and following exposure to environmental stress.     

Effects of substantia nigra and caudate nucleus lesions on avoidance learning in rats.

Compared to 21 operated and 14 nonoperated controls, 36 male Sprague-Dawley albino rats with small bilateral lesions in the anteroventral caudate nucleus or the rostral substantia nigra were significantly impaired in the acquisition of 1-way active avoidance, passive avoidance requiring the inhibition of the previously acquired 1-way response, and shuttle-box avoidance. Ss with nigral lesions took significantly more trials to criterion than Ss with caudate lesions on 1-way avoidance. Results are considered in terms of the intimate anatomical and neurochemical relationships between these structures, and a circuit of structures involved in avoidance learning is suggested. (34 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of YM872 on atrophy of substantia nigra reticulata after focal ischemia in rats

Middle cerebral artery (MCA) occlusion causes atrophy in the ipsilateral substantia nigra reticulata (SNR). The effects of glutamate AMPA receptor antagonism on SNR atrophy, which is supposed to inhibit excitatory inputs from the subthalamic nucleus to the SNR, was investigated in rats with permanent MCA occlusions. Histological examination revealed marked atrophy two weeks after MCA occlusion in the saline-treated control group. However, constant i.v. infusion of YM872, a selective AMPA receptor antagonist, for 2 weeks significantly reduced SNR atrophy; neurological deficits also decreased. These results suggest that the AMPA receptor may be involved in the pathogenesis of SNR atrophy during the subacute phase of focal cerebral ischemia.     

Association between the low threshold calcium spike and activation of NMDA receptors in guinea-pig substantia nigra pars compacta neurons

The aim of this study was to examine the interaction between N-methyl-D-aspartate (NMDA) receptor activation and the low threshold calcium spike (LTS) of phasically firing neurons in the rostral part of the substantia nigra pars compacta (SNpc) in mid-brain slices. Bath perfusion of 10 microM NMDA gradually increased the LTS area and the effect reached a maximum after 6 min of perfusion. This enhancement of the LTS by NMDA was blocked both by a competitive and non-competitive NMDA receptor antagonist, 50 microM D-AP5 and 10 microM MK801, respectively, demonstrating that this effect of NMDA was mediated through NMDA receptors. Prolonged exposure to increasing concentrations of NMDA (0.1-100 microM) progressively decreased the LTS area. The higher doses led to an irreversible marked depolarization and decrease of the membrane resistance. These results suggest that the LTS of SNpc neurons can trigger a NMDA receptor-dependent response which may have physiological and pathological roles.     

Apoptosis in substantia nigra following developmental hypoxic-ischemic injury

We have previously observed that either hypoxic-ischemic or excitotoxic striatal injury during development is associated with a reduction in the adult number of dopaminergic neurons in the substantia nigra. This decrease occurs in the presence of preserved striatal dopaminergic markers and in the absence of direct nigral injury. We have also observed that natural cell death, with the morphology of apoptosis, occurs in the substantia nigra, and that there is an induced apoptotic cell death event following early striatal excitotoxic injury. We now report that forebrain hypoxic-ischemic injury is also associated with an induced cell death event in the substantia nigra, with both morphological and histochemical features of apoptosis. Induced apoptotic cell death occurs in immunohistochemically defined dopaminergic neurons. While the mechanisms for this induced cell death are not yet known, in the case of the pars compacta it may be related to the loss of normal striatal target-derived developmental support. Since dopaminergic neurons are postmitotic at the time of the injury, we conclude that this induced cell death is responsible for the diminished adult number of dopaminergic neurons. We also conclude that hypoxic-ischemic injury to the developing brain in general causes not only direct, necrotic injury to vulnerable regions, but also induced apoptotic death at remote sites. The significance of this finding is that apoptosis is a distinct death mechanism, with unique regulatory pathways, which can potentially be modified by approaches different from those which might influence cell death in regions of direct injury. In view of the present finding that apoptosis can occur in the setting of hypoxic-ischemic injury, and our previous work demonstrating its occurrence following excitotoxic injury, it seems likely that it may occur following other forms of injury to the immature brain in which excitotoxic injury plays a role, such as seizures, head trauma and hypoglycemia.     

Electrophysiological studies of rat substantia nigra neurons in an in vitro slice preparation after middle cerebral artery occlusion

We studied sequential changes in electrophysiological profiles of the ipsilateral substantia nigra neurons in an in vitro slice preparation obtained from the middle cerebral artery-occluded rats. Histological examination revealed marked atrophy and neurodegeneration in the ipsilateral substantia nigra pars reticulata at 14 days after middle cerebral artery occlusion. Compared with the control group, there was no significant change in electrical membrane properties and synaptic responses of substantia nigra pars reticulata neurons examined at one to two weeks after middle cerebral artery occlusion. On the other hand, there was a significant increase in the input resistance and spontaneous firing rate of substantia nigra pars compacta neurons at 13-16 days after middle cerebral artery occlusion. Furthermore, inhibitory postsynaptic potentials evoked by stimulation of the subthalamus in substantia nigra pars compacta neurons was suppressed at five to eight days after middle cerebral artery occlusion. At the same time excitatory postsynaptic potentials evoked by the subthalamic stimulation was increased. Bath application of bicuculline methiodide (50 microM), a GABA(A) receptor antagonist, significantly increased the firing rate of substantia nigra pars compacta neurons from intact rats. These results strongly suggest that changes in electrophysiological responses observed in substantia nigra pars compacta neurons is caused by degeneration of GABAergic afferents from the substantia nigra pars reticulata following middle cerebral artery occlusion. While previous studies indirectly suggested that hyperexcitation due to deafferentation from the neostriatum may be a major underlying mechanism in delayed degeneration of substantia nigra pars reticulata neurons after middle cerebral artery occlusion, the present electrophysiological experiments provide evidence of hyperexcitation in substantia nigra pars compacta neurons but not in pars reticulata neurons at the chronic phase of striatal infarction.     

(-)-Stepholidine acts as a D1 partial agonist on firing activity of substantia nigra pars reticulata neurons in 6-hydroxydopamine-lesioned rats

This cross-sectional survey was conducted in 20 randomly selected streets in Moradbad city in North India to determine the association of magnesium and antioxidant vitamins with risk of ageing. There were 595 subjects (314 males, 281 females) between 50-84 years of age inclusive. The overall prevalence of hypo-magnesemia was 11.8 per cent (n = 60) with a prevalence of 13.2 per cent (n = 33) in males and 10.6 per cent (n = 27) in females. The prevalence of hypomagnesemia showed significant declining trend in the concentration of serum magnesium, vitamin C, vitamin E and beta-carotene and a rising trend in lipid peroxides and diene conjugates with increase in age from 50-59 years to 70-84 years in both men and women. Multivariate logistic regression analysis showed that serum magnesium, vitamin C, vitamin E and beta-carotene were significant risk factors of ageing in both men and women. The findings suggest that some urban populations of India can benefit by consuming higher dietary magnesium, potassium and antioxidant vitamins for prevention of ageing.     

Dopamine-degrading activity of monoamine oxidase is not detected by histochemistry in neurons of the substantia nigra pars compacta of the rat

Monoamine oxidase (MAO) activity was examined in neurons of the substantia nigra pars compacta (SNC) of the rat using a histochemical method, and compared to MAO activity in neurons of the locus coeruleus (LC) and dorsal raphe nucleus (DR). Using dopamine as a substrate, dopamine-degrading MAO activity was not detected in any SNC neurons, although LC and DR neurons were intensely stained for this activity. We further examined MAO activity in these neurons using other substrates, including serotonin (an MAO type A preferential substrate), beta-phenylethylamine (an MAO type B preferential substrate), and tyramine (a substrate common to both MAO types A and B). As for dopamine, no SNC neurons were stained for MAO activity using any of these other substrates. In contrast, LC neurons were intensely stained when either serotonin or tyramine was used, and DR neurons were darkly stained when either beta-phenylethylamine or tyramine was used. The lack of evidence of MAO activity in the SNC is surprising given that there are densely packed tyrosine hydroxylase (TH)-immunoreactive neurons in the SNC (i.e., dopaminergic neurons). By comparison, in the LC and DR the distribution patterns of the MAO-stained neurons were similar to those of TH-immunolabeled neurons (i.e., noradrenergic neurons) and serotonin-immunoreactive neurons, respectively. Our results suggest that dopamine-degrading MAO activity and MAO types A and B activities in SNC dopamine neurons are very low compared to MAO activity in LC noradrenaline neurons and in DR serotonin neurons.     

Neuromelanin of the human substantia nigra: a mixed-type melanin

Model melanins, synthesized with different cysteinyldopamine/dopamine ratios in the incubates, were oxidized with KMnO4 and the resulting compounds were analyzed by HPLC. The ratios between a phaeomelanin-derived compound, thiazole-4,5-dicarboxylic acid (TDCA), and a compound derived from eumelanin, pyrrole-2,3,5-tricarboxylic acid (PTCA), reflected the composition of the model melanins. The neuromelanin of the human substantia nigra was isolated, and the pigment, as well as intact brain tissue from human substantia nigra was oxidized with KMnO4 and the TDCA/PTCA ratios were determined. Analysis of the isolated neuromelanin showed it to contain 2.3% sulfur and 8.1% nitrogen. The sulfur content indicates the pigment is a mixed-type melanin, and the TDCA/PTCA ratio indicates that it consists of units derived from benzothiazines and from indoles in about equal amounts.     

Striatal dopamine-glutamate interactions reflected in substantia nigra reticulata firing

To gain insight into the role of striatal dopamine in basal ganglia functioning, dopaminergic drugs alone and in combination with the glutamate receptor agonist kainic acid were infused in the lateral striatum via a microdialysis probe, while single-unit recordings of substantia nigra reticulata neurons were made in chloral hydrate-anaesthetized rats. Striatal infusion of dopaminergic drugs did not significantly affect the firing rate of substantia nigra reticulata neurons, which was related to the low activity of striatal cells under basal conditions, illustrated by the lack of effect of striatal infusion of TTX on substantia nigra reticulata activity. Under glutamate-stimulated conditions, striatal infusion of d-amphetamine potentiated the inhibition of substantia nigra reticulata neurons induced by striatal kainic acid. Thus, under stimulated but not basal conditions, the modulatory role of dopamine in the striatum could be demonstrated. Dopamine potentiated the inhibitory effect of striatal kainic acid on the firing rate of the basal ganglia output neurons.     

Dual control of heat shock response: involvement of a constitutive heat shock element-binding factor

Heat shock factor (HSF) has been implicated as the key regulatory protein in the heat shock response. Our studies on the response of rodent cells to heat shock or sodium arsenite indicate that a high level of HSF-DNA-binding activity, by itself, is not sufficient for the induction of hsp70 mRNA synthesis; furthermore, a high level of HSF binding is also not necessary for this induction. Analysis of the binding of protein factors to the heat shock element (HSE) in extracts of stressed rodent cells indicates that the regulation of heat shock response involves the heat-inducible HSF and a constitutive HSE-binding factor. Our results also suggest that overexpression of human hsp70 may decrease the level of heat-induced HSF-HSE-binding activity in rat cells.