Date Published: September 30, 2011
Publisher: Informa Healthcare
Author(s): Melinda Paizs, Massimo Tortarolo, Caterina Bendotti, Jozsef I Engelhardt, László Siklós.
We tested the efficacy of treatment with talampanel in a mutant SOD1 mouse model of ALS by measuring intracellular calcium levels and loss of spinal motor neurons. We intended to mimic the clinical study; hence, treatment was started when the clinical symptoms were already present. The data were compared with the results of similar treatment started at a presymptomatic stage. Transgenic and wild-type mice were treated either with talampanel or with vehicle, starting in pre-symptomatic or symptomatic stages. The density of motor neurons was determined by the physical disector, and their intracellular calcium level was assayed electron microscopically. Results showed that motor neurons in the SOD1 mice exhibited an elevated calcium level, which could be reduced, but not restored, with talampanel only when the treatment was started presymptomatically. Treatment in either presymptomatic or symptomatic stages failed to rescue the motor neurons. We conclude that talampanel reduces motoneuronal calcium in a mouse model of ALS, but its efficacy declines as the disease progresses, suggesting that medication initiation in the earlier stages of the disease might be more effective.
Amyotrophic lateral sclerosis (ALS) is a multifactorial (1), multisystem (2), non-cell autonomous disease (3). Several pathomechanisms of the degeneration of motor neurons (MNs) have been identified, which contribute to the progression of the motor dysfunction during the disease. Excitotoxicity has attracted considerable attention since an abnormal glutamate metabolism was documented in pathogenesis-oriented clinical studies (4), and was confirmed in in vitro experiments and in animal models of ALS (5). Moreover, riluzole, the only drug to date to display a potential to increase patient survival, has an anti-excitotoxic effect (6). Since glutamate-induced excitotoxicity to MNs is mediated through calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (7), a protective effect was expected from treatment with AMPA receptor antagonists. Indeed, different competitive (8) or non-competitive (9,10) AMPA receptor inhibitors have been reported to prolong the survival of transgenic (Tg) mice with mutant Cu/Zn superoxide dismutase (SOD1) when administered before the onset of motor impairment. Using the non-competitive AMPA receptor inhibitor, we modestly reduced the loss of MNs in the lumbar spinal cord of SOD1-G93A mice, while the morphology of the remaining MNs, and especially that of the den-drites was well preserved by the treatment (9). As AMPA receptor-mediated excitotoxicity on MNs in in vitro paradigms is thought to be mediated by an excessive calcium influx particularly at the level of the dendrites, we hypothesized that the beneficial effect of AMPA receptor inhibitors could be a result of a reduced permeability to calcium. The drug used in our previous study belongs in the class of active non-competitive antagonists (or negative allosteric modulators) of the AMPA subtype of glutamatergic excitatory amino acid receptor inhibitors similar to talampanel (8-methyl-7H-1,3-dioxolo(2,3)benzodiazepine) (10).
In parallel with the morphological signs of degeneration, a higher number of EDDs, reflecting an increased level of calcium, was seen in the cytoplasm of the surviving MNs of the mutant SOD 1 Tg animals compared with wild-type ones (Figure 1). The treatment with talampanel reduced the elevated calcium level to a significant extent, but only in the group aged 12 weeks (Figure 2A, B). At this age, a 62% increase (5.57 ±0.37% vs. 3.45 ±0.24%; Figure 2A) in the volume density of the EDDs was noted in the vehicle-treated Tg animals relative to the similarly treated wild-type mice (p = 0.0002). This elevated calcium level was reduced by a significant 22% by talampanel treatment (to 4.33 ± 0.39%; p = 0.01) (Figure 2A), although this level was still significantly higher than that for the wild-type group (3.02 ± 0.21%; p = 0.009) (Figure 2A). At 19 weeks of age, the MNs from the vehicle-treated mutant SOD1 Tg mice displayed similar increases in the volume density of the EDDs to those from the 12-week-old animals relative to the wild-type mice (5.13 ± 0.70% vs. 3.15 ± 0.39%; p = 0.014) (Figure 2B), but a comparison with the corresponding talampanel-treated mutant SOD1 Tg mouse group demonstrated that talampanel was non-effective (4.88 ±0.13%; p = 0.721) (Figure 2B).
Since no significant effect of talampanel on the number of MNs could be demonstrated in either of the mouse strains in the present study, we pooled the numerical density data for the mutant SOD1 Tg and wild-type strains in the two age groups. The comparison with non-Tg littermates revealed an overall cell loss of 25% at 12 weeks of age in the mutant SOD1 Tg animals, before any motor deficit was detectable. At 19 weeks of age, the mutant SOD1 Tg mice exhibited a decline of about 50% in their grip strength (15), which may be compared with a 35% cell loss obtained in our experiments, since even at a late symptomatic stage the reported loss of MNs does not exceed 50% (16). Otherwise, our finding that talampanel itself had no effect on the number of spinal MNs in the wild-type mice suggests that the drug (at the applied dose) may safely be used in these animals.