Nucleus accumbens dopamine and discriminated approach learning: interactive effects of 6-hydroxydopamine lesions and systemic apomorphine administration.
BACKGROUND
Although dopaminergic mechanisms have been implicated in incentive motivational processes, their role in appetitive conditioning remains poorly understood.


OBJECTIVE
To investigate the effects of dopamine (DA) depleting lesions of the nucleus accumbens and the direct acting dopamine receptor agonist apomorphine on the learning of discriminated appetitive approach behavior in a Pavlovian autoshaping paradigm.


METHODS
Rats received bilateral infusions of either phosphate-buffered saline (shams) or 6-hydroxydopamine (lesions) directly into the nucleus accumbens. Ten days later, rats were trained on an autoshaping task whereby a previously neutral light stimulus was paired with food reward. Presentation of another stimulus (CS-) was never followed by reward. Over 100 pairings subjects developed a conditioned response of approaching the reward-predictive stimulus (CS+). Prior to each autoshaping session subjects were administered either saline (1 ml/kg SC) or apomorphine (30 microg/kg; 100 microg/kg), in a between-subjects design.


RESULTS
Lesioned subjects showed a delay in the acquisition of discriminated approach and were insensitive to a subsequent change in the contingency of the task whereby approaches to the CS+ now prevented food delivery. Low dose apomorphine profoundly impaired learning in both sham and lesioned subjects. Despite increasing the overall number of CS+ and CS- approaches, the higher dose of apomorphine allowed discriminated approach in sham-operated animals only.


CONCLUSIONS
These data are compatible with the hypothesis that nucleus accumbens dopamine serves to energize rather than guide conditioned approach to appetitive cues. They also support the notion that DA inputs in this region confer flexibility of approach to cues predictive of reward.
Dalley
Jeffrey W
JW
Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, UK. jwd20@cus.cam.ac.uk
Chudasama
Yogita
Y
Theobald
David E
DE
Pettifer
Candice L
CL
Fletcher
Catherine M
CM
Robbins
Trevor W
TW
D000818:Animals; D001058:Apomorphine / Q000494:pharmacology*; D004193:Discrimination Learning / Q000187:drug effects* / Q000502:physiology; D004298:Dopamine / Q000378:metabolism*; D004347:Drug Interactions / Q000502:physiology; D008297:Male; D009714:Nucleus Accumbens / Q000187:drug effects* / Q000378:metabolism; D016627:Oxidopamine / Q000494:pharmacology*; D051381:Rats
D016627:Oxidopamine; D001058:Apomorphine; D004298:Dopamine