Dopaminergic and behavioural changes in a loss-of-imprinting model of Cdkn1c

Abstract

The imprinted gene Cdkn1c is expressed exclusively from the maternally inherited allele as a consequences of epigenetic regulation. Cdkn1c exemplifies many of the functional characteristics of imprinted genes, playing a role in foetal growth and placental development. However, Cdkn1c also plays an important role in the brain, being key to the appropriate proliferation and differentiation of midbrain dopaminergic neurons. Using a transgenic model (Cdkn1c^BACx1 ) with a twofold elevation in Cdkn1c expression that mimics loss-of-imprinting, we show that increased expression of Cdkn1c in the brain gives rise to neurobiological and behavioural changes indicative of a functionally altered dopaminergic system. Cdkn1c^BACX1 mice displayed altered expression of dopamine system-related genes, increased tyrosine hydroxylase (Th) staining and increased tissue content of dopamine in the striatum. In addition, Cdkn1c^BACx1 animals were hypersensitive to amphetamine as showed by c-fos expression in the nucleus accumbens. Cdkn1c^BACX1 mice had significant changes in behaviours that are dependent on the mesolimbic dopaminergic system. Specifically, increased motivation for palatable food stuffs, as indexed on a progressive ratio task. In addition, Cdkn1c^BACX1 mice displayed enhanced social dominance. These data show, for the first time, the consequence of elevated Cdkn1c expression on dopamine-related behaviours highlighting the importance of correct dosage of this imprinted gene in the brain. This work has significant relevance for deepening our understanding of the epigenetic factors that can shape neurobiology and behaviour.

Keywords: Dopamine; epigenetics; imprinted genes; p57kip2; reward; social dominance.

Figure 1

Elevated expression of Cdkn1c results in an altered dopaminergic state in the striatum in Cdkn1c ^BACx1 animals. (a) Th immuno‐reactivity in the striatum (b) Adult Th expression normalized to WT (c) Dat expression normalized to WT. Whole tissue dopamine concentration in DS (d) and VS (e). Average NeuN^+ve cell count in striatum and cortex for Cdkn1c ^BACx1. Dorsal striatum (DS) ventral striatum (VS) Data shown ±SEM. *P < 0.05, **P < 0.01.

Figure 2

Cdkn1c ^BACx1 animals display increased neural reactivity to an intraperitoneal injection of amphetamine associated with increased mesolimbic input cell number. Cfos expression in ventral striatum following a saline of amphetamine was significantly increased only in Cdkn1c ^BACx1 animals. Data shown is mean fold change ± SEM. *P < 0.05 main effect of DRUG. Cdkn1c ^BACx1 had significantly more Th‐positive cells than WT in VTA, but not SNc. Data shown is mean cell number per section ± SEM. *P < 0.05.

Figure 3

Cdkn1c ^BACx1 animals show increased motivation to obtain a sucrose reward compared with WT. (a) Average number of trials completed during CRF trials. When minimal work (i.e. a single nose poke) was required to obtain a reward there was no difference in number of trails completed between Cdkn1c ^BACx1 animals and their WT littermates. (b) When the number of nose‐pokes required to obtain a reward increased within a session, Cdkn1c ^BACx1 made more nose‐pokes that WT before stopping (BP). (c) Duration of time between successive nose‐pokes was shorter in Cdkn1c ^BACx1 animals. Average inter nose‐pokes interval. (d) Consequently, average time to complete a trial was shorter in Cdkn1c ^BACx1 animals. Data shown ±SEM. **P < 0.01, ***P < 0.001.

Figure 4

Cdkn1c ^BACx1 maintained elevated BP when presented with a less palatable sucrose concentration or a calorie‐free sweetener. Cdkn1c ^BACx1 animals maintained a higher average BP compared with WT when sucrose concentration was reduced from 8% to 2%. Cdkn1c ^BACx1 animals maintained a higher BP in an FR2 schedule than WT animals when working for the calorie‐free sweetener, saccharin. Data shown ±SEM. **P < 0.01.

Figure 5

Cdkn1c ^BACx1 males are more dominant in a tube test. (a) Cdkn1c ^BACx1 animals won significantly more encounters against unfamiliar animals in the tubes test than WT. There was no difference in proportion of encounters won between Cdkn1c ^BACLacZ and WT animals.