Vol 9, No 4 - Addictive Substances
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Overview eward is a complex construct that entails a feel- ing and an action. Components of reward include the hedonic aspects, ie, the degree to which a stimulus is asso- ciated  with  pleasure, and  the  incentive  motivational aspects, ie, the degree to which a stimulus induces an action  towards  obtaining  it.1 Typically, the  feeling  is described as “pleasurable” or “positive” and the actions comprise behavior aimed at approaching the stimulus that is associated with reward. However, importantly, both feeling and action are highly dependent on the homeostatic state of the individual.2 That is, the degree to which a stimulus elicits a reward-consistent response depends  in  turn  on  the  internal  state  of  the  subject. Therefore, to understand the neurobiology of reward, one needs to examine the neural substrates that process the feeling, and action associated with a stimulus as it relates to the internal state of the individual.As a conse- quence, treatments of disorders of reward systems need to be focused on modulating the interoceptive system and its underlying neural substrates, instead of altering the hedonic or incentive properties of the stimulus asso- ciated with the reward, or the underlying neural systems that process these associations. To this end, experiments will need to be conducted that examine how modulating the interoceptive state using C-fiber modulation will affect  reward  processing. This  review  provides  an overview of the integration of the hedonic and incentiveT r a n s l a t i o n a l  r e s e a r c h R Copyright © 2007 LLS SAS.  All rights reserved www.dialogues-cns.org Neural basis of reward and craving —a homeostatic point of view Martin P. Paulus, MD Keywords: reward; pleasure; urge, craving; interoception; insula Author affiliations: Departments of Psychiatry, University of California,   San Diego  (UCSD), California, USA; Psychiatry Service, Veterans Affairs  San  Diego Health Care System, San Diego, California, USA Address for correspondence: Martin P. Paulus, Professor in Residence, Department of  Psychiatry,  Laboratory  of  Biological  Dynamics  and  Theoretical  Medicine, University of California San Diego, 8950 Villa La Jolla Dr Suite C213, La Jolla CA 92037-0985, USA (e-mail: mpaulus@ucsd.edu) Here, it is argued that the interoceptive system, which pro- vides information about the subject’s internal state and is integrated in the insular cortex, and not the subcortical ventral striatum, is the critical neural substrate for reward- related processes. Understanding the internal state of the individual, which is processed via this system, makes it pos- sible to develop new interventions that are aimed at treat- ing reward-dysfunction disorders, ie, substance and alco- hol  dependence.  Although  the  ventral  striatum  is important for signaling the degree to which rewarding stimuli are predicted to occur, this system alone cannot account for the complex affective, cognitive, and behav- ioral phenomena that occur when individuals come into contact with potentially rewarding stimuli. On the other hand, the interoceptive system is able to make connections between all cortical, subcortical, and limbic systems to orchestrate a complex set of responses. Craving and urges are among the most notable responses, and may have important functions to preserve homeostasis. © 2007, LLS SAS Dialogues Clin Neurosci. 2007;9:379-387.