 |
 |
Date: Januari 2000 Source: University Medical Center St. Radboud, Nijmegen, The Netherlands. Systemic administration of the propargylamine CGP 3466B prevents behavioral and immunocytochemical deficits in rats with 6-hydroxydopamine-induced lesions in the substantia nigra. G. Andringa, RV. van Oosten, W. Unger, JC. Stoof, TGM. Hafmans, J. Veening and AR. Cools CGP 3466B has been reported to potently prevent neuronal apoptosis in vitro (Kragten et al., 1998). Moreover, in vivo studies have suggested that this compound reduces the degenerative processes in animal models of amyotrophic lateral sclerosis (Kato et al., 1996) and ischemia (Paterson et al., 1998b). The present study is the first to show that CGP 3466B is able to prevent both the behavioral and morphological consequences of degeneration of nigrostriatal dopaminergic neurons induced by 6-OHDA, in a prominent rodent model of Parkinson’s disease. Effects of 6-OHDA The bilateral intranigral administration of 3.0 ug 6-OHDA induced both behavioral and morphological abnormalities. First, the 6-OHDA treatment induced motor deficits: it enhanced the retraction time of the fore- and hindlimbs and elicited abnormal stepping. These deficits clearly reflect the extensively reported sensorimotor deficits that are induced by damage to the dopaminergic nigrostriatral tract and decreased dopaminergic input of the striatum (Barneoud et al., 1995). Second, 6-OHDA induced learning deficits: in the Morris water maze, it increased the escape latency time to find the platform at trial 4 to 6, but not at the later trials, showing that the 6-OHDA treatment delayed the acquisition of this task. The fact that 6-OHDA did not prevent the acquisition together with the finding that it did not alter the swimming speed of the rats reveals that the noted delay is not simply due to motor deficits. As shown earlier (Cools, 1980), the striatal dopamine allows the organism to select arbitrarily the best strategy under stressful conditions. Therefore, the 6-OHDA treatment may have hampered this function of the striatum, resulting in the noted delay of the acquisition in the Morris water maze task. Finally, and in concord with many studies (Hefti et al., 1980), 6-OHDA induced morphological deficits: it reduced the TH positive area in the SNc, which reflects the extent of the damage to the dopaminergic neurons in this area. (Wolf et al., 1989). While the classical unilateral lesion procedures often induce widespread degeneration of all midbrain dopaminergic areas at one side of the brain (Ungerstedt et al., 1974; Zigmond, Stricker 1989; Zigmond, Stricker, 1989; Schwarting, Huston, 1997), the bilateral degeneration produced by the updated procedure affected the TH activity only in the SNc. In addition, unlike the classical models (Ungerstedt, 1971; Hodge, Butcher, 1980; Whishaw et al., 1997), this updated model did not induce severely incapacitating behavioral deficits: it did not alter grooming, rearing and walking behavior of the rats. Moreover, it did not significantly reduce the body weight. In sum, our data confirm that a moderate dose of 6-OHDA locally and bilaterally administered into the SNc induces a relatively specific destruction of the nigrostriatal dopaminergic system, which is accompanied by moderate but characteristic behavioral disturbances (Schwarting, Huston, 1997). For these reasons, the present model appeared suitable for studying the effects of compounds like CGP 3466B. Effects of CGP 3466B CGP 3466B was able to prevent the display of all the deficits seen in our 6-OHDA-treated rats. Remarkably, however, the dose-range required to prevent the symptoms varied per deficit studied: impaired stepping was prevented by 0.0014 and 0.014 mg/kg CGP 3466B; impairments in the paw test were prevented by 0.014 to 0.14 mg/kg; impaired acquisition in the Morris water maze was prevented by 1.4 mg/kg; and, finally, impaired TH-ir was prevented by 0.0014 and 0.014 mg/kg CGP 3466B. Thus, it prevented the display of behavioral and morphological deficits that normally occur in rats with nigral dopamine depletion. These data clearly show that CGP 3466B that is highly effective in rescuing cells in in vitro studies is also extremely potent in preventing dopaminergic degeneration in vivo. However, at the highest dose of CGP 3466B (1.4 mg/kg), the protective effects against 6-OHDA in the motor tests as well as those seen in the quantitative TH evaluation were lost. Moreover, this dose increased the HRT, generated abnormal stepping and reduced the amount of TH-ir in sham-treated rats qualitatively similar to those seen after local administration of 6-OHDA. This suggests a biphasic dose-response relationship of the compound, with an optimum around 0.014 – 0.14 mg/kg s.c. Whether this biphasic action is inherent to the mechanism of action or reflects independent processes occurring at different dose levels is impossible to say on the basis of the present experiments. It is very unlikely that the effects of CGP 3466B were symptomatic, since the effects of CGP 3466B were evaluated at least 14 days after the last administration of the compound. Moreover, unlike most propargylamines (Gerlach et al., 1992), CGP 3466B does not increase synaptic dopamine via an inhibitory effect on MAO-B (Waldmeier, personal communication). It is also not conceivable that CGP 3466B interfered with the toxicity or uptake of 6-OHDA, since it was administered two hours after the injection of 6-OHDA. Therefore, it is suggested that CGP 3466B mediates its protective effects via a direct interference with the degenerative process initiated by 6-OHDA. Although the precise action of CGP 3466B remains elusive, (Kragten et al., 1998) have reported that CGP 3466B binds to glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Besides its role as enzyme in the glycolysis, GAPDH has been found to play a role in apoptosis (Sunaga et al., 1995). Indeed, CGP 3466B prevents neuronal apoptosis induced by GAPDH over-expression in vitro (Kragten et al., 1998). These data strongly suggest that GAPDH is involved in the mechanism by which CGP 3466B rescues cells from apoptosis. In conclusion, CGP 3466B was able to prevent all the 6-OHDA-induced deficits, though at different doses. The data suggest that, at appropriate doses, CGP 3466B may be able to prevent the progression of dopaminergic degeneration in patients with PD, especially in the early stage of the disease.