Injections of 6-hydroxydopamine in the substantia nigra of the rat brain: morphological and biochemical effects

Comparison between Tail Suspension Swing Test and Standard Rotation Test in Revealing Early Motor Behavioral Changes and Neurodegeneration in 6-OHDA Hemiparkinsonian Rats

The unilateral 6-hydroxydopamine (6-OHDA) model of Parkinson’s disease (PD) is one of the most commonly used in rodents. The anatomical, metabolic, and behavioral changes that occur after severe and stable 6-OHDA lesions have been extensively studied. Here, we investigated whether early motor behavioral deficits can be observed in the first week after the injection of 6-OHDA into the right substantia nigra pars compacta (SNc), and if they were indicative of the severity of the dopaminergic (DAergic) lesion in the SNc and the striatum at different time-points (day 1, 3, 5, 7, 14, 21). With this aim, we used our newly modified tail suspension swing test (TSST), the standard rotation test (RT), and immunohistochemical staining for tyrosine hydroxylase (TH). The TSST, but not the standard RT, revealed a spontaneous motor bias for the 6-OHDA-lesioned rats from the day 1 post-surgery. Both tests detected the motor asymmetry induced by (single and repeated) apomorphine (APO) challenges that correlated, in the first week, with the DAergic neuronal degeneration. The described TSST is fast and easy to perform, and in the drug-free condition is useful for the functional assessment of early motor asymmetry appearing after the 6-OHDA-lesion in the SNc, without the confounding effect of APO challenges.

Intranigral Grafts of Fetal Ventral Mesencephalic Tissue in Adult 6-Hydroxydopamine-Lesioned Rats can Induce Behavioral Recovery

Intrastriatal grafts of fetal ventral mesencephalon in rats with unilateral 6-hydroxydopamine lesions can reduce and even reverse rotational behavior in response to direct and indirect dopamine agonists. These grafts can ameliorate deficits on simple spontaneous behaviors, but do not improve complex behaviors that require the skilled integration of the use of both paws. We report here that rats with grafts into the DA-depleted substantia nigra, that receive cyclosporine A, can experience recovery on spontaneous behaviors that mimic those observed in Parkinson's disease. Specific cyclosporine A treatment conditions can differentially affect whether intranigral grafts normalize paw use during initiation or termination of a movement sequence. These findings may have important implications for the treatment of Parkinson's disease.

Low cumulative manganese exposure affects striatal GABA but not dopamine

The introduction of the anti-knock methylcyclopentadienyl manganese (Mn) tricarbonyl (MMT) in gasoline has raised concerns about the potential for manganese neurotoxicity. Because subpopulations such as the elderly in the early stages of neurodegenerative disease may be at increased risk for manganese toxicity, a pre-Parkinsonism rat model was used to evaluate whether sub-chronic manganese exposure can aggravate the neurochemical and behavioral dysfunctions characteristic of Parkinsonism. Sub-threshold levels of dopamine depletion of 3.5, 53 and 68% were generated via intrastriatal unilateral 6-hydroxydopamine (6-OHDA) doses. A sub-chronic dosing regimen of low cumulative manganese exposure (4.8 mg Mn/kg body weight, 3 i.p. injections per week x 5 weeks) was started 4 weeks after 6-OHDA treatments. Neurochemical and neuromotor (functional observational battery (FOB)) measures were evaluated. Manganese produced significant (P < 0.05) reductions of 30-60% in motor function. This effect was exacerbated in the presence of a pre-Parkinsonism condition [Neurotox. Teratol. 22 (2000) 851]. Manganese did not affect striatal dopamine, but resulted in significant increases in striatal y-aminobutyric acid (GABA) of 16 and 22% (P < 0.01) in both striati and a borderline non-significant 4% increase in frontal cortex (P = 0.076). Manganese treatment produced increased aspartate (P < 0.01) in the manganese and 6-OHDA treated striatum. In light of previous studies predominantly showing dopamine depletion with elevated manganese exposures, the significant effects of manganese on striatal GABA but not on striatal dopamine at the low cumulative exposure administered here suggest a progression in manganese toxicity with increasing cumulative dose, whereby GABA levels are adversely affected before striatal dopamine levels. Because these neurochemical disruptions were accompanied by motor dysfunction that was exacerbated in the presence of a pre-Parkinsonism condition, an increased environmental burden of manganese may have deleterious effects on populations with sub-threshold neurodegeneration in the basal ganglia (e.g. pre-Parkinsonism).

Parkinsonian motor deficits are reflected by proportional A9/A10 dopamine neuron degeneration in the rat

In a model of Parkinson's disease (PD), amphetamine, a dopamine (DA)-releasing drug, fails to induce ipsilateral drug rotations in a proportion of rats with complete unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle and DA neurons of the substantia nigra. To investigate this phenomenon, individual 6-OHDA lesions (measured by tyrosine hydroxylase immunohistochemistry) in the substantia nigra pars compacta (A9), ventral tegmental area (A10), and striatum were examined in conjunction with outcomes of four behavioral tests. The behavioral tests were skilled paw reaching, a head-turning test, and apomorphine (0.05 mg/kg) and amphetamine (4 mg/kg) drug-induced rotations. Four weeks postlesion, ipsilateral side bias measured by the head-turning test correlated strongly with extent of A9 DA neuronal lesion. Additional A10 neuronal DA lesions did not substantially improve the model fit, indicating that the head-turning bias was primarily A9 dependent. In contrast, total head-turning activity increased monotonically with lesions of A10 striatal DA fibers. Skilled paw-reaching accuracy decreased with increased lesion of both A9 and A10 DA neuronal systems. Associating amphetamine-induced rotations with extent of A9 DA lesion generated a second-order polynomial model, y = -11.1x + 0.20 x(2) + 208.7 (R(2) = 0.73), with an overall F ratio (df = 2,21) of 28.4 (P < 0.0001). This model predicts that an A9 DA lesion of about 50% is required to induce an ipsilateral turning bias, after which rotations increase with the degree of A9 DA neuronal lesion. No further change in rotational behavior was seen until an additional A10 DA lesion reached 60%, after which the rotational response decreased. This analysis provides tests that differentiate between A9 DA degeneration and combined A9/A10 lesions in animal models and in addition allows predictive testing of PD therapeutic intervention at a preclinical level.

Forced limb-use effects on the behavioral and neurochemical effects of 6-hydroxydopamine

Rats with unilateral depletion of striatal dopamine (DA) show marked preferential use of the ipsilateral forelimb. Previous studies have shown that implementation of motor therapy after stroke improves functional outcome (Taub et al., 1999). Thus, we have examined the impact of forced use of the impaired forelimb during or soon after unilateral exposure to the DA neurotoxin 6-hydroxydopamine (6-OHDA). In one group of animals, the nonimpaired forelimb was immobilized using a cast, which forced exclusive use of the impaired limb for the first 7 d after infusion. The animals that received a cast displayed no detectable impairment or asymmetry of limb use, could use the contralateral (impaired) forelimb independently for vertical and lateral weight shifting, and showed no contralateral turning to apomorphine. The behavioral effects were maintained throughout the 60 d of observation. In addition to the behavioral sparing, these animals showed remarkable sparing of striatal DA, its metabolites, and the expression of the vesicular monoamine transporter, suggesting a decrease in the extent of DA neuron degeneration. Behavioral and neurochemical sparing appeared to be complete when the 7 d period of immobilization was initiated immediately after 6-OHDA infusion, only partial sparing was evident when immobilization was initiated 3 d postoperatively, and no sparing was detected when immobilization was initiated 7 d after 6-OHDA treatment. These results suggest that physical therapy may be beneficial in Parkinson's disease.

Akinesia and postural abnormality after unilateral dopamine depletion

Following unilateral nigrostriatal dopamine depletion rats engage in drug-induced and spontaneous turning. These asymmetric behaviors are associated with a hemispheric asymmetry in the dopamine function. Several tests of 'spontaneous' behavior have revealed other abnormalities, primarily in posture and akinesia. A test is described here which quantifies postural abnormalities, as well as difficulties in initiating and terminating movement. This test may be suitable for evaluating aspects of dopamine-mediated behaviors not addressed by drug tests and may prove valuable in evaluating the potential of new treatment strategies for Parkinson's disease.

Behavioral asymmetries and recovery in rats with different degrees of unilateral striatal dopamine depletion

A detailed behavioral analysis during the first postoperative week was performed in rats which had sustained various degrees of unilateral neostriatal dopamine (DA) lesions by administration of the neurotoxin 6-hydroxydopamine into the substantia nigra. These animals were assigned to different groups according to their residual DA levels in the damaged neostriatum (as percentage of the intact side). On the first day after toxin injection into the substantia nigra, turning asymmetries (tight turns) toward the side of the lesion were observed in animals with a mean residual DA level of 32% or less. Out of these, the strongest asymmetries were observed in animals with a mean residual DA of 3%. After one week, the asymmetry in tight turns had totally recovered except in those groups with mean residual DA levels of 17% or less. Partial recovery was found in animals with mean residual DA of 9 and 17%, whereas no indication for recovery was found in animals with the most severe lesions (mean residual DA 3%). Measurement of thigmotactic scanning also revealed an asymmetry for the side of the lesion on the first post-operative day. This asymmetry was observed over a wider range of DA lesion than that observed in turning, namely up to a mean residual DA level of 78%. Furthermore, recovery to symmetry was observed in all lesion-groups except in those with more severe lesions (mean residual DA 17% or less). In contrast to turning, the strongest asymmetries were not displayed by the animals with the most severe lesions. Furthermore, locomotor activity was affected by the lesion, since on the first postoperative day locomotion was reduced in animals with mean residual DA of 39% or less. On day 7, this lesion-dependent deficit had recovered to control levels. Finally, the analysis of net turns allowed the prediction of lesion size in animals with residual DA levels of less than 15%. These results are discussed with respect to mechanisms of recovery, the role of lesion size, and the value of different behavioral measures to predict the degree of DAergic lesion.

Behavioural, pharmacological and biochemical effects of acute and chronic administration of ketamine in the rat

The effects of N-methyl-D-aspartate (NMDA) antagonist ketamine given acutely or chronically were investigated on dopamine-related motor functions. Acute administration (15, 22.5, 30 mg/kg, i.p.) reversed the catalepsy induced by a dopamine (DA) antagonist (haloperidol, 0.25 mg/kg, i.p.) in the rat. When given orally and chronically (15 mg/kg per day) during at least 60 days, no alteration of spontaneous motor behaviour was observed, but the responsiveness to a DA agonist (apomorphine, 0.125 or 0.25 mg/kg s.c.) and to haloperidol was enhanced, suggesting an hypersensitivity of the DA receptors following the chronic blockade of NMDA receptors. However, following prolonged administration of ketamine there were no alteration of DA levels and turnover. Taken together these results suggest that the mechanisms involved in this DA receptor hypersensitivity should be postsynaptic.

Relationships between indices of behavioral asymmetries and neurochemical changes following mesencephalic 6-hydroxydopamine injections

Behavioral and neurochemical changes were investigated in rats that had received one of 3 doses of 6-hydroxydopamine (6-OHDA), injected unilaterally into the ventral mesencephalon. The behavioral analysis comprised that of tight turns (diameter less than 30 cm), wide turns (diameter greater than 55 cm), and locomotor activity. 6-OHDA-injected animals were assigned to 3 different groups according to their degree of asymmetry in tight turns, both in spontaneous behavior and after the dopamine receptor agonist apomorphine (0.05 mg/kg). Thus, 6-OHDA-injected animals showed either (i) no spontaneous ipsiversive asymmetry (group 1), or (ii) an ipsiversive asymmetry, from which they did not recover during the three postoperative weeks of testing and which could not be reversed by apomorphine (group 2), or (iii) an even stronger ipsiversive asymmetry from which they did not recover but which could be reversed by apomorphine (group 3). The analysis of wide turns, which might reflect exploratory behavior of the environment, namely thigmotactic scanning, provided further information, as it indicated an asymmetry even in group 1; however, in contrast to the other groups a contraversive asymmetry was observed. Neurochemically, the three experimental groups were clearly different from each other with respect to the degree of neostriatal dopamine depletion, and the increase in dopamine metabolism in the damaged hemisphere as indicated by increased metabolite/transmitter ratios. The ipsiversive asymmetry in tight turns was negatively correlated with dopamine levels in the damaged neostriatum and positively correlated with the increase in metabolism. Furthermore, indications of changes in neostriatal serotonin activity were found. These results are discussed with respect to the necessity of differential measures of behavioral asymmetry, the role of dopaminergic mechanisms of 6-OHDA-induced deficits and mechanisms of recovery. The asymmetries in tight versus wide turns are suggested to reflect the preponderance of a motor deficit in the former case versus that of sensory neglect in the latter. Thus, the analysis of tight versus wide turns may provide distinctive and sensitive indices related to different functional deficits in animal models of hemiparkinsonism.