Striatal enkephalinergic differences in rats selectively bred for intrinsic running capacity.
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.


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.


CONCLUSIONS
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.
Department of Kinesiology, University of Georgia, 330 River Road, Athens, GA 30602-6554, USA. Electronic address: dmon@uga.edu.
Derek C
DC
Monroe
Psychology Department, University of Georgia, Athens, GA, USA. Electronic address: pvholmes@uga.edu.
Philip V
PV
Holmes
Department of Anesthesiology, University of Michigan, Ann Arbor, MI, USA. Electronic address: lgkoch@med.umich.edu.
Lauren G
LG
Koch
Department of Anesthesiology, University of Michigan, Ann Arbor, MI, USA. Electronic address: brittons@med.umich.edu.
Steven L
SL
Britton
Department of Kinesiology, University of Georgia, 330 River Road, Athens, GA 30602-6554, USA. Electronic address: rdishman@uga.edu.
Rodney K
RK
Dishman
D000818:Animals; D004745:Enkephalins / Q000235:genetics / Q000378:metabolism*; D005260:Female; D008297:Male; D009043:Motor Activity; D009714:Nucleus Accumbens / Q000378:metabolism; D066208:Olfactory Tubercle / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D012420:Running / Q000502:physiology; D066328:Ventral Striatum / Q000378:metabolism*
D004745:Enkephalins; D012333:RNA, Messenger
Activity wheel; Enkephalin; In situ hybridization; Nucleus accumbens; Olfactory tubercle