Assessment of motor function, sensory motor gating and recognition memory in a novel BACHD transgenic rat model for huntington disease

Abstract

Rationale: Huntington disease (HD) is frequently first diagnosed by the appearance of motor symptoms; the diagnosis is subsequently confirmed by the presence of expanded CAG repeats (> 35) in the HUNTINGTIN (HTT) gene. A BACHD rat model for HD carrying the human full length mutated HTT with 97 CAG-CAA repeats has been established recently. Behavioral phenotyping of BACHD rats will help to determine the validity of this model and its potential use in preclinical drug discovery studies.

Objectives: The present study seeks to characterize the progressive emergence of motor, sensorimotor and cognitive deficits in BACHD rats.

Materials and methods: Wild type and transgenic rats were tested from 1 till 12 months of age. Motor tests were selected to measure spontaneous locomotor activity (open field) and gait coordination. Sensorimotor gating was assessed in acoustic startle response paradigms and recognition memory was evaluated in an object recognition test.

Results: Transgenic rats showed hyperactivity at 1 month and hypoactivity starting at 4 months of age. Motor coordination imbalance in a Rotarod test was present at 2 months and gait abnormalities were seen in a Catwalk test at 12 months. Subtle sensorimotor changes were observed, whereas object recognition was unimpaired in BACHD rats up to 12 months of age.

Conclusion: The current BACHD rat model recapitulates certain symptoms from HD patients, especially the marked motor deficits. A subtle neuropsychological phenotype was found and further studies are needed to fully address the sensorimotor phenotype and the potential use of BACHD rats for drug discovery purposes.

Figure 1. Phenotype.

Results are expressed as Mean ± SEM. (a) Body weight. There is no significant difference between TG and WT control rats. (b) Locomotor activity. Compared to WT, TG rats have a higher total activity at one month followed by a lower activity starting at 4 months of age. (c-d) Rotarod. Presented are the latency to fall off the rod during (c) constant speed (12 r.p.m) and (d) accelerating speed (4 - 40 r.p.m). A significant difference between groups was present already at 2 months of age [constant speed (2 months: t= 3.373, P< 0.001; 3 months: t= 3.53, P< 0.01; 4 months: t= 4.798; P< 0.001; 6 months: t=5.433; p< 0.001 and 9 months: t= 2.742, P< 0.05); accelerating speed (1 month: t= 1.066, P> 0. 1; 2 months: t= 4.172, P< 0.001; 3 months: t= 3.549, P< 0.01; 4 months: t= 6.493, p< 0.001; 6 months: t= 4.263, P< 0.001 and 9 months: t= 3.015, P< 0.01)]. Asterisks indicate significant differences between WT and TG rats (*p < 0.05; **p < 0.01 and ***p < 0.001).

Figure 2. Catwalk gait analysis in 12 months old BACHD rats.

Results are expressed as Mean ± SEM. There was no significant difference between TG and WT control rats in walking speed (data not shown). (a) Static parameters: Stride length. Compared to WT, TG rats had a significant shorter stride length for both front and hind paws during walking. (b-c) Dynamic parameters: presented are the Stand (c) and the Swing (d). TG rats had a significant shorter time of stand (or Stance) for front and hind paws and a shorter time in front swing than WT rats. Asterisks indicate significant differences between WT and TG rats (*p < 0.05 and **p < 0.01).

Figure 3. Object Recognition Test (ORT).

Results are expressed as Mean ± SEM. Three WT rats of the 4 months old group and one WT rat from the 12 months old group were excluded from the analysis as they did not explore the objects (a) Exploration time (e1) and (e2) during T1 and T2 respectively. TG rats showed a significantly higher exploration time during e2 at 12 months of age. (b) Recognition testing during T2: 4 months and 12 months old BACHD rats had a significantly higher exploration time to the novel object than the familiar object. (c) The discrimination index (d2) for both groups are above zero and were significant at each age. Asterisks indicate statistical significance (*p < 0.05; **p < 0.01 and ***p < 0.001).

Figure 4. Startle testing.

Results are expressed as Mean ± SEM. (a) Prepulse inhibition. A 2 way-ANOVA revealed a GENOTYPE difference in 9 months old BACHD rats especially at PP 6 and PP 12; however any significant differences were found in 1, 4 and 12 months old rats. (b) Startle habituation amplitude in 6 months old rats. Each trial consisted of 10 blocks of 120 dB startle stimuli. WT and TG rats presented a normal startle habituation. (c) Startle threshold. Amplitude to varying startling stimulus intensities in 9 months old rats. WT and TG response amplitude increased with higher stimulus intensities. No GENOTYPE effect was observed. However, a significant difference was detected at 120 dB. Asterisks indicate statistical significance in BACHD rats (*p < 0.05).