Paroxysmal dyskinesias in the lethargic mouse mutant

Abstract

Lethargic mutant mice carry a mutation in the CCHB4 gene, which encodes the beta4 subunit of voltage-regulated calcium channels. These mutants have been shown to display a complex neurobehavioral phenotype that includes EEG discharges suggestive of absence epilepsy, chronic ataxia, and hypoactivity. The current studies demonstrate a fourth element of their phenotype, consisting of transient attacks of severe dyskinetic motor behavior. These attacks can be triggered by specific environmental and chemical influences, particularly those that stimulate locomotor activity. Behavioral and EEG analyses indicate that the attacks do not reflect motor epilepsy, but instead resemble a paroxysmal dyskinesia. The lethargic mutants provide additional evidence that calcium channelopathies can produce paroxysmal dyskinesias and provide a novel model for studying this unusual movement disorder.

Fig. 1.

Age-related changes in motor function.A, Rotarod. B, Cling test.C, Gross motor activity during overnight recordings.D, Gross motor activity during the first hour of testing. E, Dyskinesia scores. Results reflect average values ± SEM for 10–12 normal mice (open circles) and 8–10 lethargic mutants (filled circles) at each time point. Two-way ANOVA for the rotarod revealed significant effects for genotype (F_(1,261) = 1308;p < 0.001) and age (F_(12,261) = 2.5; p< 0.005). For the cling test, there were also significant effects for both genotype (F_(1,261) = 1435;p < 0.001) and age (F_(1,261) = 14.0; p< 0.001). For overnight activity, there was a significant effect of age (F_12,261 = 4.4;p < 0.001) but no significant effect for genotype (F_(1,261) = 4.5; p= 0.04). For the first hour of activity, there were again significant effects for genotype (F_(1,261) = 65.3;p < 0.001) and age (F_(12,261) = 3.8; p< 0.001). For dyskinesia scores, one-way ANOVA revealed significant differences among ages (F_(13,149) = 2.8; p < 0.002); post hoc Tukeyt tests indicated that the 4-week-old group was significantly different (asterisk) from all other age groups (p < 0.01).

Fig. 2.

Temporal profile of gross motor activity (A) and dyskinesia scores (B). Results reflect average values ± SEM for six normal mice (open circles) and seven lethargic mutants (filled circles) that were 12–16 weeks of age. Two-way ANOVA revealed significant differences between genotypes (F_(1,132) = 222.8;p < 0.001) and time (F_(11,132) = 18.3;p < 0.001) for gross motor activity. A similar analysis also showed significant differences between genotype (F_(1,132) = 123.5;p < 0.001) and time (F_(11,132) = 13.2;p < 0.001) for dyskinesia scores.

Fig. 3.

Dyskinetic movements during attacks in lethargic mutants. A, Exaggerated truncal flexion with forepaw clonus. B, Truncal flexion with tonic forelimb retraction. C, Listing left with tonic rear limb retraction. D, Exaggerated truncal flattening.

Fig. 4.

Environmental influences on total dyskinesia scores in lethargic mice. Results reflect average values ± SEM for 12–13 mice at 12–16 weeks of age for each condition.A, Cage size. B, Time of day.C, Stress. ANOVA revealed significant differences among the conditions (F_(8,103) = 31.5;p < 0.001). Post hoc Tukey tests provided p < 0.001 (asterisks) for each of the following comparisons: large cage scores were significantly higher than both small and medium cage scores; 9:30 p.m.scores were significantly higher than both the 7:30 p.m.and 6:00 a.m. scores; and shock or restraint stress scores were significantly higher than baseline scores.

Fig. 5.

Temporal profile of dyskinesias in lethargic mice. Mice at 12–16 weeks of age were habituated for 2 hr to the stationary treadmill and then scored every 60 sec for 20 min after starting it in motion. The treadmill was stopped at the onset of an attack. Filled circles show data for individual mice; the striped zone depicts average values for the entire group.

Fig. 6.

Relationship between gross level of activity (line graphs) and total dyskinesia scores (bar graphs) 10–15 min after administration of caffeine, amphetamine, diazepam, or haloperidol. Results reflect average values ± SEM for six to eight mice at 12–16 weeks of age per condition. ANOVA for activity scores revealed significant effects for each drug: caffeine (F_(5,30) = 5.7;p < 0.001), amphetamine (F_(5,30) = 6.3; p< 0.001), diazepam (F_(5,36) = 16.5;p < 0.001), and haloperidol (F_(5,36) = 25.6; p< 0.001). ANOVA for dyskinesia scores also revealed significant effects for each drug: caffeine (F_(5,30) = 8.0; p< 0.001), amphetamine (F_(5,30) = 17.3;p < 0.001), diazepam (F_(5,36) = 28.8; p< 0.001), and haloperidol (F_(5,36) = 17.0; p < 0.001). Across all drugs, the overall Spearman rank correlation between activity level and dyskinesia score was 0.87.

Fig. 7.

Relationship between polyspike EEG activity and total dyskinesia scores. A, Typical appearance of polyspike discharges in control and drug-treated animals, both 12–16 weeks of age. B, Polyspike discharges per hour.C, Dyskinesia scores presented as a percentage of simultaneously treated controls. Results reflect average values ± SEM for five to six mice per group. ANOVA revealed significant differences among the groups for both polyspikes (F_(2,13) = 24.7; p< 0.001) and dyskinesia scores (F_(3,16) = 37.6; p< 0.0001). Asterisks indicate significant differences from the associated control group (p < 0.001).