Nigrostriatal DA System Research Articles

Autophagy and protein aggregation as a mechanism of dopaminergic degeneration in a primary human dopaminergic neuronal model

The pathophysiology underlying the loss of dopaminergic neurons in Parkinson’s disease (PD) is unclear. A gap of knowledge in the molecular and cellular events leading to degeneration of the nigrostriatal DA system is a major barrier to the development of effective therapies for PD. 1-methyl-4-phenylpyridinium (MPP+) is used as a reliable in vitro model of PD in dopaminergic neurons; however, the molecular mechanisms that lead to cell death with this model are not fully understood. Additionally, there is a lack of translational in vitro models to fully understand progressive dopaminergic neurotoxicity. Here, we propose cultures of primary human dopaminergic neuronal precursor cells (HDNPCs) as a model to study progressive dopaminergic toxicity and neuronal damage in PD. We evaluated the concentration-response of MPP+ (0–10 mM) at 24 h, using cell viability and mitochondrial activity assays (LDH, XTT, Live/Dead staining, and MitoTracker). Based on concentration-response data, we chose two concentrations (1.0 and 2.5 mM) of MPP+ to evaluate markers of autophagy and dopaminergic status [tyrosine hydroxylase (TH)] after a 24-h exposure. Exposure to MPP+ induced cytotoxicity, reduced cell viability, and decreased mitochondrial activity. MPP+ at 1.0 and 2.5 mM also induced expression of lysosome-associated membrane protein 1 (LAMP-1) and increased the ratio of light chain 3 (LC3), LC3BII/LC3BI. The expression of TH also decreased. Furthermore, α-synuclein (α-SYN) and parkin were evaluated by immunofluorescence (IF) at 1.0 and 2.5 mM MPP+ after 24 h. A qualitative analysis revealed decreased parkin expression while α-SYN aggregation was observed in the cytoplasm and the nucleus. These data suggest that in HDNPCs MPP+ can cause cytotoxicity and neuronal damage. This damage may be mediated by autophagy, dopamine synthesis, and protein aggregation. The combination of HDNPCs and MPP+ may serve as valuable in vitro model of progressive dopaminergic neurotoxicity for research into potential treatments for PD.

Control of Reactive Oxygen Species for the Prevention of Parkinson's Disease: The Possible Application of Flavonoids.

Oxidative stress reflects an imbalance between the production of reactive oxygen species (ROS) and antioxidant defense systems, and it can be associated with the pathogenesis and progression of neurodegenerative diseases such as multiple sclerosis, stroke, and Parkinson’s disease (PD). The application of antioxidants, which can defend against oxidative stress, is able to detoxify the reactive intermediates and prevent neurodegeneration resulting from excessive ROS production. There are many reports showing that numerous flavonoids, a large group of natural phenolic compounds, can act as antioxidants and the application of flavonoids has beneficial effects in the adult brain. For instance, it is well known that the long-term consumption of the green tea-derived flavonoids catechin and epigallocatechin gallate (EGCG) can attenuate the onset of PD. Also, flavonoids such as ampelopsin and pinocembrin can inhibit mitochondrial dysfunction and neuronal death through the regulation of gene expression of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway. Additionally, it is well established that many flavonoids exhibit anti-apoptosis and anti-inflammatory effects through cellular signaling pathways, such as those involving (ERK), glycogen synthase kinase-3β (GSK-3β), and (Akt), resulting in neuroprotection. In this review article, we have described the oxidative stress involved in PD and explained the therapeutic potential of flavonoids to protect the nigrostriatal DA system, which may be useful to prevent PD.

Adenosine heteroreceptor complexes in the basal ganglia are implicated in Parkinson\u2019s disease and its treatment

The adenosine homo, iso and heteroreceptor complexes in the basal ganglia play a highly significant role in modulating the indirect and direct pathways and the striosomal projections to the nigro-striatal DA system. The major adenosine receptor complexes in the striato-pallidal GABA neurons can be the A2AR–D2R and A2AR–D2R–mGluR5 receptor complexes, in which A2AR protomers and mGluR5 protomers can allosterically interact to inhibit D2R protomer signaling. Through a reorganization of these heteroreceptor complexes upon chronic dopaminergic treatment a pathological and prolonged inhibition of D2R receptor protomer signaling can develop with motor inhibition and wearing off of the therapeutic effects of levodopa and dopamine receptor agonists. The direct pathway is enriched in D1R in and around glutamate synapses enhancing the ability of these GABA neurons to be activated and increase motor initiation. The brake on these GABA neurons is in this case exerted by A1R forming A1R–D1R heteroreceptor complexes in which they allosterically inhibit D1R signaling and thereby reduce motor initiation. Upon chronic levodopa treatment a reorganization of the D1R heteroreceptor complexes develops with the formation of putative A1R–D1R–D3 in addition to D1R–D3R complexes in which D3R enhances D1R protomer signaling and may make the A1R protomer brake less effective. Alpha-synuclein monomers–dimers are postulated to form complexes with A2AR homo and heteroprotomers in the plasma membrane enhancing alpha-synuclein aggregation and toxicity. The alpha-synuclein fibrils formed in the A2AR enriched dendritic spines of the striato-pallidal GABA neurons may reach the surrounding DA terminals via extracellular-vesicle-mediated volume transmission involving internalization of the vesicles and their cargo (alpha-synuclein fibrils) into the vulnerable DA terminals, enhancing their degeneration followed by retrograde flow of these fibrils in the DA axons to the vulnerable nigral DA nerve cells.

Subthalamic deep brain stimulation restores automatic response activation and increases susceptibility to impulsive behavior in patients with Parkinson’s disease

Repeatedly reported deficits of patients with Parkinson’s disease (PD) in selecting an appropriate action in the face of competing response alternatives has led to the conclusion of a basal ganglia (BG) involvement in response selection and impulse control. Despite capacious research, it remains elusive how BG dysfunction affects processes subserving goal-directed behavior. Even more problematically, since PD pathology transcends a BG dysfunction due to dopamine depletion in the nigrostriatal DA system (by also comprising alterations in extrastriatal dopamine availability and other neurotransmitter systems), it is not yet clear which aspects of these deficits are actually caused by BG dysfunction. To address this question, the present study investigated 13 off-medication PD patients with bilateral therapeutic subthalamic deep brain stimulation (DBS) both with and without stimulation (DBSON and DBSOFF, respectively) and 26 healthy controls. All participants performed a task that tests the relation between automatic response impulses and goal-directed action selection. Results show an improvement of automatic response activation under DBSON, increasing the susceptibility to impulsive responses, and a reduced impact of automatic response activation under DBSOFF. We argue that the BG determine the efficiency of the regulation and transmission of stimulus-driven bottom-up response activation required for efficient response selection.

Operant Self-Stimulation of Dopamine Neurons in the Substantia Nigra

We examined the contribution of the nigrostriatal DA system to instrumental learning and behavior using optogenetics in awake, behaving mice. Using Cre-inducible channelrhodopsin-2 (ChR2) in mice expressing Cre recombinase driven by the tyrosine hydroxylase promoter (Th-Cre), we tested whether selective stimulation of DA neurons in the substantia nigra pars compacta (SNC), in the absence of any natural rewards, was sufficient to promote instrumental learning in naive mice. Mice expressing ChR2 in SNC DA neurons readily learned to press a lever to receive laser stimulation, but unlike natural food rewards the lever pressing did not decline with satiation. When the number of presses required to receive a stimulation was altered, mice adjusted their rate of pressing accordingly, suggesting that the rate of stimulation was a controlled variable. Moreover, extinction, i.e. the cessation of action-contingent stimulation, and the complete reversal of the relationship between action and outcome by the imposition of an omission contingency, rapidly abolished lever pressing. Together these results suggest that selective activation of SNC DA neurons can be sufficient for acquisition and maintenance of a new instrumental action.

Increased melatonin may play dual roles in the striata of a 6-hydroxydopamine model of Parkinson's disease

AimsTo investigate dynamic changes and roles of melatonin (MLT) in the striata of 6-hydroxydopamine (6-OHDA)-treated rats. Main methodsA Parkinson's disease (PD) rat was established by a unilateral injection of 6-OHDA into the right substantia nigra pars compacta (SNc) and the right medial forebrain bundle (MFB) to achieve a complete lesion of the ipsilateral nigrostriatal DA system. Dialysates were collected in the lesioned striatum at different time intervals by in vivo microdialysis. In addition, both contralateral and ipsilateral striatum tissues were collected at two time intervals (10:00 and 22:00h) at 3 and 6weeks after lesioning. The levels of DA, 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the dialysates, as well as MLT in the dialysates and tissues were determined using HPLC. Key findingsThe dialysate contents of DA, DOPAC and HVA in the lesioned striatum were significantly decreased (P<0.001) in comparison with those in the controls or in the unlesioned side 3weeks after lesioning while the extracellular level of MLT in the lesioned striatum in these corresponding time intervals distinctly increased when compared with those in the controls (P<0.05). The tissue MLT contents increased in the bilateral striata in different degrees at 6weeks post-lesion (P<0.05). Moreover, increased MLT levels correlate well with rotations or DA changes in the lesioned striatum. SignificanceThese data suggest that 6-OHDA lesion manipulates the MLT secretion pattern. Increased striatal MLT level by a unilateral intracerebral injection of 6-OHDA may play dual roles in the progression of PD in rats.

Acute and long‐term response of dopamine nigrostriatal synapses to a single, low‐dose episode of 3‐nitropropionic acid‐mediated chemical hypoxia

The goal of the present investigation was to determine the persistence of striatal (DA) dopaminergic dysfunction after a mild chemically induced hypoxic event in Fisher 344 rats. To this end, we gave a single injection of the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NP; 16.5 mg/kg, i.p.) to 2-month old male F344 rats and measured various indices of striatal DA functioning and lipid peroxidation over a 3-month span. Separate groups of rats were used to measure rod walking, evoked DA release, DA content, malondialdehyde (MDA) accumulation, DA receptor binding, and tyrosine hydroxylase (TH) activity. The results showed that 3-NP exposure reduced most measures of DA functioning including motoric ability, DA release, and D(2) receptor densities for 1 to 3 months postdrug administration. Interestingly, DA content was reduced 1 week after 3-NP exposure, but rose to 147% of control values 1 month after 3-NP treatment. MDA accumulation, a measure of lipid peroxidation activity, was increased 24 h and 1 month after 3-NP treatment. 3-NP did not affect TH activity, suggesting that alterations in DA functioning were not the result of nigrostriatal terminal loss. These data demonstrate that a brief mild hypoxic episode caused by 3-NP exposure has long-term detrimental effects on the functioning of the nigrostriatal DA system.

PI3 kinase/Akt activation mediates estrogen and IGF-1 nigral DA neuronal neuroprotection against a unilateral rat model of Parkinson's disease.

Recently, using the medial forebrain bundle (MFB) 6-hydroxydopmaine (6-OHDA) lesion rat model of Parkinson's disease (PD), we have demonstrated that blockade of central IGF-1 receptors (IGF-1R) attenuated estrogen neuroprotection of substantia nigra pars compacta (SNpc) DA neurons, but exacerbated 6-OHDA lesions in IGF-1 only treated rats (Quesada and Micevych [2004]: J Neurosci Res 75:107-116). This suggested that the IGF-1 system is a central mechanism through which estrogen acts to protect the nigrostriatal DA system. Moreover, these results also suggest that IGF-1R-induced intracellular signaling pathways are involved in the estrogen mechanism that promotes neuronal survival. In vitro, two convergent intracellular signaling pathways used by estrogen and IGF-1, the mitogen-activated protein kinase (MAPK/ERK), and phosphatidyl-inositol-3-kinase/Akt (PI3K/Akt), have been demonstrated to be neuroprotective. Continuous central infusions of MAPK/ERK and PI3K/Akt inhibitors were used to test the hypothesis that one or both of these signal transduction pathways mediates estrogen and/or IGF-1 neuroprotection of SNpc DA neurons after a unilateral administration of 6-OHDA into the MFB of rats. Motor behavior tests and tyrosine hydroxylase immunoreactivity revealed that the inhibitor of the PI3K/Akt pathway (LY294002) blocked the survival effects of both estrogen and IGF-1, while an inhibitor of the MAPK/ERK signaling (PD98059) was ineffective. Western blot analyses showed that estrogen and IGF-1 treatments increased PI3K/Akt activation in the SN; however, MAPK/ERK activation was decreased in the SN. Indeed, continuous infusions of inhibitors blocked phosphorylation of PI3K/Akt and MAPK/ERK. These findings indicate that estrogen and IGF-1-mediated SNpc DA neuronal protection is dependent on PI3K/Akt signaling, but not on the MAPK/ERK pathway.

Adenosine A2A receptors, dopamine D2 receptors and their interactions in Parkinson's disease

Future therapies in Parkinson's disease may substantially build on the existence of intra-membrane receptor-receptor interactions in DA receptor containing heteromeric receptor complexes. The A(2A)/D(2) heteromer is of substantial interest in view of its specific location in cortico-striatal glutamate terminals and in striato-pallidal GABA neurons. Antagonistic A(2A)/D(2) receptor interactions in this heteromer demonstrated at the cellular level, and at the level of the striato-pallidal GABA neuron and at the network level made it possible to suggest A(2A) antagonists as anti-parkinsonian drugs. The major mechanism is an enhancement of D(2) signaling leading to attenuation of hypokinesia, tremor, and rigidity in models of Parkinson's disease with inspiring results in two clinical trials. Other interactions are antagonism at the level of the adenylyl cyclase; heterologous sensitization at the A(2A) activated adenylyl cyclase by persistent D(2) activation and a compensatory up-regulation of A(2A) receptors in response to intermittent Levodopa treatment. An increased dominance of A(2A) homomers over D(2) homomers and A(2A)/D(2) heteromers after intermittent Levodopa treatment may therefore contribute to development of Levodopa induced dyskinesias and to the wearing off of the therapeutic actions of Levodopa giving additional therapeutic roles of A(2A) antagonists. Their neuroprotective actions may involve an increase in the retrograde trophic signaling in the nigro-striatal DA system.

Influence of noradrenaline denervation on MPTP-induced deficits in mice

C57/BL6 mice were administered either DSP4 (50 mg/kg, s.c., 30 min after injection of zimeldine, 20 mg/kg, s.c.) or vehicle (saline) at 63 days of age. Three weeks later, one group (n = 10) of DSP4-treated and one group of vehicle-treated mice were administered MPTP (2 x 40 mg/kg, s.c., 24 hours between injections; the High dose groups), one group (n = 10) of DSP4-treated and one group of vehicle-treated mice were administered MPTP (2 x 20 mg/kg, s.c., 24 hours between injections; the Low dose groups), and one group (n = 10) of DSP4-treated and one group of vehicle-treated mice were administered vehicle. Three weeks later, all six groups were tested in motor activity test chambers, followed by injections of L-Dopa (20 mg/kg, s.c.), and then tested over a further 360 min in the activity test chambers. It was found that pretreatment with the selective NA neurotoxin, DSP4, deteriorated markedly the dose-dependent motor activity deficits observed in the vehicle pretreated MPTP treated mice. These 'ultra-deficits' in the spontaneous motor behaviour of MPTP-treated mice were observed over all three parameters: locomotion, rearing and total activity, and were restricted to the 1(st) and 2(nd) 20-min periods. Administration of L-Dopa (20 mg/kg) following the 60-min testing of spontaneous behaviour restored the motor activity of Vehicle + MPTP treated mice (neither the Vehicle + MPTP-Low nor the Vehicle + MPTP-High groups differed from the Vehicle-Vehicle group, here) but failed to do so in the DSP4 pretreated mice. Here, a dose-dependent deficit of L-Dopa-induced motor activity (over all three parameters) was obtained thereby offering further evidence of an 'ultra-deficit' of function due to previous denervation of the NA terminals. The present findings support the notion that severe damage to the locus coeruleus noradrenergic system, through systemic DSP4, disrupts the facilitatory influence on the nigrostriatal DA system, and interferes with the ability of the nigrostriatal pathway to compensate for or recover from marked injury, MPTP treatment.

Subthalamic high frequency stimulation induced rotations are differentially mediated by D1 and D2 receptors

High frequency stimulation (HFS) of the subthalamic nucleus (STN) has clinically emerged as a promising approach in the treatment of Parkinson’s disease, epilepsy, dystonia as well as compulsive and possibly other mood disorders. The underlying mechanisms are incompletely understood, but are definitely related to high frequency and likely to involve the dopamine (DA)-system. To further test this hypothesis the present study investigated the modulation of STN-HFS-induced circling by systemic and intracerebral injection of drugs acting on DA receptors in naive freely moving rats. Within this experimental setup, unilateral STN-HFS alone induced intensity-dependent circling. Systemic injections of selective D1- (SCH-23390) and D2-((−)-sulpiride) antagonists as well as the mixed D1 and D2 agonist apomorphine dose-dependently reduced STN-HFS-induced rotational behavior. Intracerebral microinjections of (−)-sulpiride but not SCH-23390 decreased circling when injected intrastriatally and increased the number of rotations when injected intranigrally (pars reticulata (SNr)). These data reveal that STN-HFS-induced contralateral circling is differentially modulated by D1 and D2 receptors. While D2 receptor-mediated effects involve the dorso-/ventrolateral striatum and the SNr, D1 receptors probably exert their actions via brain areas outside the striatum and SNr. These findings suggest the nigrostriatal DA-system to be specifically involved in the mediation of STN-HFS-induced motor effects.

Chapter 23 Vulnerability of monoaminergic neurons in the brainstem of the zitter rat in oxidative stress

In the monograph of Santiago Ramon y Cajal, he provided a detailed description about the morphological changes in degeneration and regeneration of peripheral and central nervous systems following lesions. He discussed factors that may promote or inhibit axonal growth after peripheral and/or central nerve injury. Cajal with a brilliant insight anticipated the existence of several factors acting on degeneration and regeneration. Free radicals have been proposed to be one of such factors. These highly reactive oxygen species-derived free radicals play a pathogenetic role in neurological disorders, including ischemia, trauma, Alzheimer's disease and Parkinson's disease (PD). In this review we will discuss the similarities and differences between the morphological changes under oxidant stress and Cajal's drawings of degeneration and regeneration following the central injury. The monoaminergic neuron systems in the brainstem appear vulnerable to these free radicals, which have also been implicated in the selective degeneration of the nigrostriatal DA system. We analyzed the degeneration of fibers and the neuronal cell death of brainstem monoaminergic neuron systems in a mutant rat, which has abnormal metabolism of oxygen species in the brain. The degeneration of DA cell bodies and fibers was characterized by swollen varicosities and clustered fibers.

Neurophysiology of Parkinson's disease: from basic research to clinical correlates

The aim of the present review is to discuss recent acquisitions on the neurophysiological effects of the DA denervation of the striatum, obtained in vitro from single neurone electrophysiological recordings. Nigrostriatal DA system degeneration does not significantly alter the intrinsic properties of striatal cells, but triggers adaptive responses in the striatum, involving glutamatergic neurotransmission. By pairing intracellular recordings from 6-OHDA-denervated striatal projection neurones and the synaptic stimulation of glutamatergic corticostriatal pathway, in vitro experiments contributed to elucidate the intimate alterations that follow striatal DA deafferentation. This experimental approach also allowed to better understand the possible cellular bases of both beneficial and side effects of the pharmacological compounds commonly employed in the treatment of PD.

Early behavioral changes after nigro-striatal system damage can serve as predictors of striatal dopamine depletion

1. 1. Rats which had received a unilateral injection of 6-OHDA into the substantia nigra were assigned to four lesion groups according to the degree of DA depletion in the neostriatum. In these animals, behavioral changes in the open-field were investigated during the first post-operative week. Overall, this analysis showed that the animals could adequately be characterized by behavior on day 1 and day 7 after lesion. 2. 2. On the first day after lesion, the groups with the severest DA depletions ( > 80% and 55–80%) showed an ipsilateral asymmetry in turning. After one week, these groups showed a tendency to recover from this deficit; however, the group with the most strongest lesions ( > 80%) was still asymmetric. 3. 3. In scanning behavior, in contrast to turning, all the lesion groups displayed an initial ipsilateral asymmetry. On day 7 after lesion, only the group with DA depletions of > 80% still had an ipsilateral asymmetry. Locomotor activity and rearing were initially reduced after lesion, and showed a tendency to recover, especially in the group with the most severe DA depletions ( > 80%). There were no differences between groups neither on day 1 nor on day 7 by grooming, but this behavior increased in all the lesion groups with time. 4. 4. The correlational analyses yielded a positive relationship between the asymmetry in turning and neostriatal DA depletion. Locomotor activity and rearing on day 1 were both negatively correlated with DA depletion. The present results show that a number of behavioral parameters obtained in the open-field are affected by unilateral lesions of the nigro-striatal DA system. The degree of deficit, its time course and relation to lesion size differs among the various behavioral measures. Some of these early behavioral changes after unilateral nigrostriatal lesion are related to DA depletion and should therefore be useful to predict lesion size. 5. 5. Together, these data suggest that the study of such behavioral changes can provide an important tool, to investigate the compensatory mechanisms underlying striatal DA depletion and to understand preclinical states of the Parkinson's disease.

DA uptake sites, D1 and D2 receptors, D2 and preproenkephalin mRNAs and Fos immunoreactivity in rat striatal subregions after partial dopaminergic degeneration.

Stereotaxic injection of a limited amount of 6-hydroxydopamine in the lateral part of the rat substantia nigra induces a partial degeneration of the nigrostriatal dopaminergic system. This animal model in which the destruction of the dopaminergic nigral cell population reaches approximately 50% could be considered as a preclinical Parkinson's model. Autoradiography of dopaminergic uptake sites performed with a specific marker ([3H]GBR 12935) allowed the precise determination of dopaminergic denervated and non-denervated areas in the striatum 1 month after partial lesion of the substantia nigra pars compacta. In both striatal areas, dopaminergic D1 and D2 receptor densities and dopaminergic D2 and preproenkephalin mRNAs levels were measured by autoradiography and in situ hybridization coupled to an image analysis system. Our results show that in the denervated striatal subregion, none of the dopaminergic targets were modified, contrary to the observations made after complete lesion of the nigrostriatal DA system at the same post-lesion delay. However, striatal Fos activation induced by amphetamine (5 mg/kg i.p., 2 h before killing) revealed that the number of Fos-positive cells detected in the denervated striatal subregion was lower than that observed in the non-denervated one. These data argue in favour of the existence of compensatory mechanisms different from the up-regulation of DA receptor densities, thereby allowing the maintenance of striatal dopaminergic transmission. Such mechanisms could contribute to the delay of the appearance of neurological symptoms (which are reported to be clinically apparent only when depletion of striatal dopamine levels reaches near 80%) in Parkinsonian patients.

The intrastriatal 6-hydroxydopamine model of hemiparkinsonism: quantitative receptor autoradiographic evidence of correlation between circling behavior and presynaptic as well as postsynatic nigrostriatal markers in the rat

Unilateral injections of 6-hydroxydopamine into the striatum resulted in almost immediate ipsilateral amphetamine (AMPH)- and delayed contralateral apomorphine (APO)-induced circling behavior in rats. APO-induced rotation correlated positively with that caused by AMPH. In these animals, there was al almost complete disappearance of dopamine uptake sites as well as increases in DA D 2 receptors in specific subdivision of the ipsilateral caudate-putamen CPu). Both the rate of AMPH- and APO-induced rotation correlated with the percentage of DA terminal loss in the total aspect and in various quadrants of the striatum. In contrast, AMPH- and APO-induced rotation correlated with the percentage increase in striatal D 2 receptors only in the dorsolateral (DL) aspect of the CPu. These results indicate that both AMPH- and APO-induced rotation can be sued to determine the extent of DA terminal loss in the rat basal ganglia. The positive correlation of circling behavior to only changes in DA D 2 receptors observed in the DL striatal subdivision provides further evidence for the heterogeneity of the basal ganglia. This model of hemiparkinsonism in the rat which uses a distant intrastriatal approach to the destruction of nigral DA cell bodies may be a more appropriate model to study the regenerative properties of the nigrostriatal DA system. This approach could also be used to more specifically localize peptidergic receptors on midbrain dopamine cell bodies

Persistent augmented dopamine release after acute cocaine requires dopamine receptor activation.

Pretreatment of rats with a single injection of cocaine produces a persistent augmentation of amphetamine-induced [3H]dopamine [( 3H]DA) release measured using the in vitro striatal slice preparation. Cocaine has several actions in the nigrostriatal DA system: it blocks DA uptake and thereby indirectly stimulates DA receptors and it also acts as a local anesthetic. We investigated which of these actions is responsible for the augmented amphetamine-stimulated [3H]DA release by determining whether pretreatment with drugs sharing one or more of these actions also augmented release. Release was increased in striatal slices one week after a single injection of either mazindol, a DA uptake blocker and indirect DA receptor agonist, or apomorphine, a direct-acting receptor agonist, whereas the local anesthetic lidocaine had no effect. The prerequisite of DA receptor stimulation was confirmed by pretreatment prior to the cocaine injection with either a nonselective, a D-1 selective or a D-2 selective DA receptor antagonist. Each of these blocked the long-lasting augmentation of release. From these experiments, we conclude that cocaine indirectly activates both D-1 and D-2 DA receptors to produce the persistent augmentation of striatal amphetamine-stimulated [3H]DA release.

Acute effects of a parkinsonism-inducing neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on mouse body temperature

The parkinsonism-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) given in single systemic doses (i.p.) to mice produced marked hyperthermia, and subsequent long-lasting hypothermia. Administration of MPTP or its oxidized product, 1-methyl-4-phenylpyridinium ion, MPP+, via i.c.v. resulted in only hypothermia. In contrast, i.p. MPP+ administration resulted in only hyperthermia. The MPTP-induced hyperthermia (i.p.) was blocked by quaternary derivatives of anti-cholinergic agents, atropine and scopolamine, but not by the tertiary-derivative of atropine. Duration of this hyperthermic effect was potentiated by neostigmine. Pretreatment with 1-deprenyl did not prevent hypothermia, but nomifensine partially or clorgyline completely prevented the effect without preventing MPTP-induced hyperthermia. The thermic effects by MPTP, unlike its neurotoxicity for the nigrostriatal DA system, may not require metabolism to MPP+. These results indicate that peripheral cholinergic functions are responsible for the MPTP-induced hyperthermia, whereas its hypothermic effect may be centrally mediated via dysregulation of the various neuron systems.

Firing properties of substantia nigra dopaminergic neurons in freely moving rats

Single unit recordings were obtained from putative dopaminergic neurons in the substantia nigra of awake, freely moving rats. The cells exhibited waveforms, range of firing rates and types of firing patterns identical to those of identified DA neurons of anesthetized or paralyzed rats. Two firing patterns were observed: single spike activity and a bursting mode with spikes of progressively diminished amplitude and increased duration within each burst. The degree of burst firing varied considerably among the cells and individual cells sometimes switched from one pattern of firing (e.g. predominantly single spike) to another (e.g. bursting), although the determinants of these transitions are, at this time, unclear. Putative DA neurons were inhibited by i.v. apomorphine and excited by i.v. haloperidol. Haloperidol also reversed the apomorphine-induced inhibition of firing. Inhibitions and excitations were associated with a reduction and elevation, respectively, in burst firing. The effects of the two drugs were identical to their effects in immobilized rats. In several cases, a putative DA neuron was observed to fire all of its spikes in near coincidence with at least one other cell with identical electrophysiological characteristics. This form of interaction (i.e. presumed electrical coupling) between DA cells is only rarely observed in anesthetized or paralyzed rats and may play a significant role in the normal functioning of the nigrostriatal DA system.

Effect of Δ9-tetrahydrocannabinol (THC) on central dopaminergic function in circling behavior of rats

Neural mechanisms of catalepsy and aggressive behavior induced by THC were investigated in relation to brain amines in rats. In rats pretreated with 6-OHDA (intracerebroventicle), THC induced characteristic irritable aggression. We studied, therefore, the effect of THC on the central dopaminergic (DA-ergic) system using the circling behavior of rats treated with 6-OHDA (8 μg/2 μl) in the unilateral substantia nigra. After unilateral lesion of the nigro-striatal DA system, methamphetamine (MAP), a DA releaser, strongly induced a ipsilateral circling toward the lesion side in a dose-dependent fashion; whereas apomorphine,a DA receptor agonist, oppositely exhibited contralateral circling. Using rats in which induction of circling by these drugs was reproducible, the effect of THC (2, 5, 10 mg/kg i.p.) was observed on circling behavior for 30 min. THC, in a nonataxic dose (5 ng/kg) induced ipsilateral circling, and this potency was approximately 1/3 that of MAP (2 mg/kg). In addition, haloperidol (HPD: 0.1 mg/kg i.p.) markedly inhibited the effect of MAP (2 mg/kg s.c.), but moderately inhibited that of THC (5 mg/kg i.p.). These results suggest that THC has the property of activating the DAergic neurons.