Deletion of alpha-synuclein decreases impulsivity in mice

Abstract

The presynaptic protein alpha-synuclein, associated with Parkinson's Disease (PD), plays a role in dopaminergic neurotransmission and is implicated in impulse control disorders (ICDs) such as drug addiction. In this study we investigated a potential causal relationship between alpha-synuclein and impulsivity, by evaluating differences in motor impulsivity in the 5-choice serial reaction time task (5-CSRTT) in strains of mice that differ in the expression of the alpha-synuclein gene. C57BL/6JOlaHsd mice differ from their C57BL/6J ancestors in possessing a chromosomal deletion resulting in the loss of two genes, snca, encoding alpha-synuclein, and mmrn1, encoding multimerin-1. C57BL/6J mice displayed higher impulsivity (more premature responding) than C57BL/6JOlaHsd mice when the pre-stimulus waiting interval was increased in the 5-CSRTT. In order to ensure that the reduced impulsivity was indeed related to snca, and not adjacent gene deletion, wild type (WT) and mice with targeted deletion of alpha-synuclein (KO) were tested in the 5-CSRTT. Similarly, WT mice were more impulsive than mice with targeted deletion of alpha-synuclein. Interrogation of our ongoing analysis of impulsivity in BXD recombinant inbred mouse lines revealed an association of impulsive responding with levels of alpha-synuclein expression in hippocampus. Expression of beta- and gamma-synuclein, members of the synuclein family that may substitute for alpha-synuclein following its deletion, revealed no differential compensations among the mouse strains. These findings suggest that alpha-synuclein may contribute to impulsivity and potentially, to ICDs which arise in some PD patients treated with dopaminergic medication.

Figure 1. Performance of C57BL/ 6J mice from Charles River (C, clear bars) and C57BL/6JOlaHsd mice from Harlan (H, dark bars) in the 5-CSRTT for the baseline (B: mean of the last three sessions on Stage 6) and the three long ITI sessions (L1, L2, L3)

The values represent the mean ± SE of accuracy of responding (a), percentage omissions (b), percentage of premature responses (c), latency to make a correct response (d), latency to collect the liquid reinforcer (e) and total perseverative responses (f). ^*P < 0.05, ^**P < 0.01, significant differences between sessions; P < 0.05, significant differences between strains (post hoc tests).

Figure 3. Distribution of total responses (correct + incorrect + premature) during the total time in which a response can be made for each trial (ITI: 10 seconds + SD: 1.8 seconds + LH: 5 seconds; total = 16.8 seconds) of the long ITI sessions (L1, L2 and L3) in the 5-CSRTT

Figures a, b and c show the performance of C57BL/6J Charles River mice (C, clear bars) vs. C57BL/6JOlaHsd Harlan mice (H, dark bars) and d, e and f show the performance of alpha-synuclein wild type (WT, clear bars) vs. knock-out mice (KO, dark bars). ITI: inter-trial interval, SD: stimulus duration, LH: Limited hold (time allowed for nose-pose).

Figure 2. Performance of alpha-synuclein wild type (WT, clear bars) and knock-out (KO, dark bars) mice in the 5-CSRTT for the baseline (B: mean of the last three sessions on Stage 6) and the three long ITI sessions performed (L1, L2, L3)

The values represent the mean ± SE of accuracy of responding (a), percentage omissions (b), percentage of premature responses (c), latency to make a correct response (d), latency to collect the liquid reinforcer (e) and total perseverative responses (f). ^*p < 0.05, ^**p < 0.01, significant differences between sessions; P < 0.05, significant differences between strains (post hoc tests).

Figure 4. Correlation of mRNA levels of alpha-synuclein in hippocampus (y axis) and percentage of premature responding in the third long ITI session (L3, x axis) for DBA2/J, C57BL/6J and BXD5, 11, 12, 21, 29, 31, 32 and 33 strains (no mRNA data available for BXD18 and BXD36 strains)

A highly significant positive correlation between level of expression of alpha-synuclein in hippocampus and percentage of premature responding was seen (r = 0.857, P = 0.00069).

Figure 5. mRNA quantification of gene expression by means of quantitative RT-PCR

Messenger RNA levels were normalized against GAPDH. Gene expression of gamma, beta and alpha-synuclein in the Prefrontal Cortex (PFC) (a) and Hippocampus (b) is shown as fold change in expression of Harlan (H) and alpha-synuclein knock-out mice (KO) vs. combined data of WT+Charles mice (±SEM).