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. 2014 Jul 14:1572:11-7.
doi: 10.1016/j.brainres.2014.05.014. Epub 2014 May 17.

Striatal enkephalinergic differences in rats selectively bred for intrinsic running capacity

Derek C Monroe  1 , Philip V Holmes  2 , Lauren G Koch  3 , Steven L Britton  4 , Rodney K Dishman  5
Affiliations

Striatal enkephalinergic differences in rats selectively bred for intrinsic running capacity

Derek C Monroe et al. Brain Res. .

Abstract

Rats selectively bred for high- and low-capacity for running on a treadmill (HCR; LCR) also differ in wheel-running behavior, but whether wheel-running can be explained by intrinsic or adaptive brain mechanisms is not as yet understood. It is established that motivation of locomotory behavior is driven by dopaminergic transmission in mesolimbic and mesostriatal systems. However, whether voluntary wheel running is associated with enkephalinergic activity in the ventral striatum is not known.

Materials and methods: 40 male (20 HCR and 20 LCR) and 40 female (20 HCR and 20 LCR) rats were randomly assigned to 3 weeks of activity wheel exposure or sedentary conditions without wheel access. After 3 weeks of activity-wheel running, rats were decapitated and brains were extracted. Coronal sections were analyzed utilizing in situ hybridization histochemistry for enkephalin (ENK) mRNA in the ventral striatum.

Results: HCR rats expressed less ENK than LCR rats in the nucleus accumbens among females (p<0.01) and in the olfactory tubercle among both females (p<0.05) and males (p<0.05). There was no effect of wheel running on ENK mRNA expression.

Conclusion: Line differences in ENK expression in the olfactory tubercle, and possibly the nucleus accumbens, partly explain divergent wheel-running behavior. The lower striatal ENK in the HCR line is consistent with enhanced dopaminergic tone, which may explain the increased motivation for wheel running observed in the HCR line.

Keywords: Activity wheel; Enkephalin; In situ hybridization; Nucleus accumbens; Olfactory tubercle.

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Figures

Fig. 1
Fig. 1
Mean daily running distances (± SEM) on the activity-wheel over 3 weeks. High-capacity running (HCR) rats ran more on average than low-capacity running (LCR) rats. There was an interaction effect between lines over 3 weeks in females; the effect was independent of line in males.
Fig. 2
Fig. 2
(a) Average enkephalin (ENK) mRNA expression (± SEM) in the nucleus accumbens septi (NAS). High capacity running (HCR) rats expressed less ENK than low capacity running (LCR) rats; OT=olfactory tubercle, NAS= nucleus accumbens septi. (b) Average enkephalin (ENK) mRNA expression (± SEM) in the dorsal striatum. There were no significant line differences in ENK expression.
Fig. 2
Fig. 2
(a) Average enkephalin (ENK) mRNA expression (± SEM) in the nucleus accumbens septi (NAS). High capacity running (HCR) rats expressed less ENK than low capacity running (LCR) rats; OT=olfactory tubercle, NAS= nucleus accumbens septi. (b) Average enkephalin (ENK) mRNA expression (± SEM) in the dorsal striatum. There were no significant line differences in ENK expression.
Fig. 3
Fig. 3
Mean daily running distance and ENK mRNA expression among female rats in the nucleus accumbens septi (NAS; panel A) and the olfactory tubercle (OT; panel B). Correlations between running distance and ENK mRNA in the NAS and the OT were mainly explained by lower mean mRNA and higher mean running distance in HCR females compared to LCR females.
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