Discrete neurochemical coding of distinguishable motivational processes: insights from nucleus accumbens control of feeding.
OBJECTIVE
The idea that nucleus accumbens (Acb) dopamine transmission contributes to the neural mediation of reward, at least in a general sense, has achieved wide acceptance. Nevertheless, debate remains over the precise nature of dopamine's role in reward and even over the nature of reward itself. In the present article, evidence is reviewed from studies of food intake, feeding microstructure, instrumental responding for food reinforcement, and dopamine efflux associated with feeding, which suggests that reward processing in the Acb is best understood as an interaction among distinct processes coded by discrete neurotransmitter systems.


RESULTS
In agreement with several theories of Acb dopamine function, it is proposed here that allocation of motor effort in seeking food or food-associated conditioned stimuli can be dissociated from computations relevant to the hedonic evaluation of food during the consummatory act. The former appears to depend upon Acb dopamine transmission and the latter upon striatal opioid peptide release. Moreover, dopamine transmission may play a role in 'stamping in' associations between motor acts and goal attainment and perhaps also neural representations corresponding to rewarding outcomes. Finally, evidence is reviewed that amino acid transmission specifically in the Acb shell acts as a central 'circuit breaker' to flexibly enable or terminate the consummatory act, via descending connections to hypothalamic feeding control systems.


CONCLUSIONS
The heuristic framework outlined above may help explain why dopamine-compromising manipulations that strongly diminish instrumental goal-seeking behaviors leave consummatory activity relatively unaffected.
Baldo
Brian A
BA
Department of Psychiatry, University of Wisconsin-Madison School of Medicine and Public Health, 6001 Research Park Blvd., Madison, WI, 53719, USA. babaldo@wisc.edu
Kelley
Ann E
AE
D000818:Animals; D001069:Appetite Regulation / Q000187:drug effects; D001479:Basal Ganglia / Q000378:metabolism; D003216:Conditioning, Operant; D003259:Consummatory Behavior; D004298:Dopamine / Q000378:metabolism*; D015259:Dopamine Agents / Q000494:pharmacology; D018846:Excitatory Amino Acids / Q000378:metabolism; D005247:Feeding Behavior / Q000187:drug effects; D049673:History, 20th Century; D006801:Humans; D007031:Hypothalamus / Q000378:metabolism; D009042:Motivation; D009043:Motor Activity; D009448:Neurochemistry / Q000266:history; D018377:Neurotransmitter Agents / Q000378:metabolism*; D009714:Nucleus Accumbens / Q000187:drug effects / Q000378:metabolism*; D018847:Opioid Peptides / Q000378:metabolism; D012054:Reinforcement, Psychology; D012201:Reward
D015259:Dopamine Agents; D018846:Excitatory Amino Acids; D018377:Neurotransmitter Agents; D018847:Opioid Peptides; D004298:Dopamine