The reward system is a group of neural structures responsible for incentive salience (i.e., "wanting", desire, or craving), pleasure (i.e., hedonic "liking"), and reinforcement learning (e.g., positive reinforcement). Reward is the attractive and motivational property of a stimulus that induces appetitive behavior – also known as approach behavior – and consummatory behavior. In its description of a rewarding stimulus (i.e., "a reward"), a review on reward neuroscience noted, "any stimulus, object, event, activity, or situation that has the potential to make us approach and consume it is by definition a reward." In operant conditioning, rewarding stimuli function as positive reinforcers; the converse statement is also true: positive reinforcers are rewarding.
Primary rewards are those necessary for the survival of one's self and offspring, and include homeostatic (e.g., palatable food) and reproductive (e.g., sexual contact and parental investment) rewards. Intrinsic rewards are unconditioned rewards that are attractive and motivate behavior because they are inherently pleasurable. Extrinsic rewards (e.g., money) are conditioned rewards that are attractive and motivate behavior, but are not inherently pleasurable. Extrinsic rewards derive their motivational value as a result of a learned association (i.e., conditioning) with intrinsic rewards. Extrinsic rewards may also elicit pleasure (e.g., from winning a lot of money in a lottery) after being classically conditioned with intrinsic rewards.
In neuroscience, the reward system is a collection of brain structures and neural pathways that are responsible for reward-related cognition, including reinforcement learning (e.g., positive reinforcement), incentive salience (i.e., "wanting", desire, or craving), and pleasure (i.e., hedonic "liking"). Terms that are commonly used to describe behavior related to the "wanting" component of reward include appetitive behavior, preparatory behavior, instrumental behavior, anticipatory behavior, and seeking. Terms that are commonly used to describe behavior related to the "liking" component of reward include consummatory behavior and taking behavior.
Anatomy of the reward system
Brain structures that compose the reward system are primarily within the Cortico-basal ganglia-thalamo-cortical loop; the basal ganglia portion of the loop drives activity within the reward system. Most of the pathways that connect structures within the reward system are glutamatergic interneurons, GABAergic medium spiny neurons, and dopaminergic projection neurons, although other types of projection neurons contribute (e.g., orexinergic projection neurons). The reward system includes the ventral tegmental area, ventral striatum (primarily the nucleus accumbens, but also the olfactory tubercle), dorsal striatum (i.e., caudate nucleus and putamen), substantia nigra (i.e., the pars compacta and pars reticulata), prefrontal cortex, anterior cingulate cortex, insular cortex, hippocampus, hypothalamus (particularly, the orexinergic nucleus in the lateral hypothalamus), thalamus (multiple nuclei), subthalamic nucleus, globus pallidus (both external and internal), ventral pallidum, parabrachial nucleus, amygdala, and the remainder of the extended amygdala.
Among the pathways that connect the structures in the cortico–basal ganglia–thalamic loop, the group of neurons known as the mesolimbic dopamine pathway, which connect the ventral tegmental area (VTA) to the nucleus accumbens (NAcc), along with the associated GABAergic D1-type medium spiny neurons in the nucleus accumbens shell, is a critical component of the reward system that is directly involved in the immediate perception of the motivational component of a reward (i.e., "wanting"). Most of the dopamine pathways (i.e., neurons that use the neurotransmitter dopamine to transmit a signal to other structures) that originate in the VTA are part of the reward system; in these pathways, dopamine acts on D1-like receptors or D2-like receptors to either stimulate (D1-like) or inhibit (D2-like) the production of cAMP. The GABAergic medium spiny neurons of the striatum are all components of the reward system as well. The glutamatergic projection nuclei in the subthalamic nucleus, prefrontal cortex, hippocampus, thalamus, and amygdala connect to other parts of the reward system via glutamate pathways. The medial forebrain bundle, which is a set of many neural pathways that mediate brain stimulation reward (i.e., reward derived from direct electrochemical stimulation of the lateral hypothalamus), is also a component of the reward system.
Pleasure centers 
Pleasure is a component of reward, but not all rewards are pleasurable (e.g., money does not elicit pleasure unless this response is conditioned). Stimuli that are naturally pleasurable, and therefore attractive, are known as intrinsic rewards, whereas stimuli that are attractive and motivate approach behavior, but are not inherently pleasurable, are termed extrinsic rewards. Extrinsic rewards (e.g., money) are rewarding as a result of a learned association with an intrinsic reward. In other words, extrinsic rewards function as motivational magnets that elicit "wanting", but not "liking" reactions once they have been acquired.
The hedonic hotspots or pleasure centers – i.e., brain structures that mediate pleasure or "liking" reactions from intrinsic rewards – within the reward system that have been identified as of May 2015[update] are contained in subcompartments within the nucleus accumbens shell, ventral pallidum, and parabrachial nucleus of the pons; the insular cortex and orbitofrontal cortex likely contain hedonic hotspots as well. However, opioid and endocannabinoid, but not dopamine, injections in the ventrorostral region of the nucleus accumbens is able to increase liking, while injection in other regions may actually produce aversion or just an increase in wanting, as dopamine microinjections do.
Incentive salience is the "wanting" or "desire" attribute, which includes a motivational component, that is assigned to a rewarding stimulus by substructure within the nucleus accumbens shell (NAcc shell). The degree of dopamine neurotransmission into the NAcc shell from the mesolimbic pathway is highly correlated with the magnitude of incentive salience for rewarding stimuli.
Activation of the dorsorostral region of the nucleus accumbens correlates with increases in wanting without concurrent increases in liking. However, the dopaminergic neurotransmission into the nucleus accumbens shell is not only responsible for appetitive motivational salience (i.e., incentive salience) towards rewarding stimuli, but also for aversive motivational salience which directs behavior away from undesirable stimuli. D1-type medium spiny neurons within the NAcc shell confer incentive salience for rewarding stimuli, while D2-type medium spiny neurons within the NAcc shell confer aversive motivational salience for undesirable stimuli.
Animals vs. humans
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Kent Berridge, a researcher in affective neuroscience, found that sweet (liked) and bitter (disliked) taste produced distinct orofacial expressions, and these expressions were similarly displayed by human newborns, orangutans, and rats. This was evidence that pleasure (specifically, liking) has objective features and was essentially the same in various animals. Most neuroscience studies have shown that the more dopamine released by the reward, the more effective the reward is. This is called the hedonic impact, which can be changed by the effort for the reward and the reward itself. Berridge discovered that blocking dopamine systems did not seem to change the positive reaction, measured by facial expression, to something sweet. In other words, the hedonic impact did not change based on the amount of sugar. This discounted the conventional assumption that dopamine mediates pleasure. Even with more intense dopamine alterations, the data seemed to remain constant. Berridge developed the incentive salience hypothesis to addresses the wanting aspect of rewards. It explains the compulsive use of drugs by drug addicts even when the drug no longer produces euphoria, or the cravings experienced even after the individual has been successfully withdrawn. Some addicts respond to certain stimuli involving neural changes caused by drugs. This sensitization in the brain is similar to the effect of dopamine because wanting and liking reactions occur. Human and animal brains and behaviors experience similar changes regarding reward systems because they both are so prominent.
James Olds and Peter Milner were researchers who found the reward system in 1954. They discovered, while trying to teach rats how to solve problems and run mazes, stimulation of certain regions of the brain where the stimulation was found seemed to give pleasure to the animals. They tried the same thing with humans and the results were similar.
In a fundamental discovery made in 1954, researchers James Olds and Peter Milner found that low-voltage electrical stimulation of certain regions of the brain of the rat acted as a reward in teaching the animals to run mazes and solve problems. It seemed that stimulation of those parts of the brain gave the animals pleasure, and in later work humans reported pleasurable sensations from such stimulation. When rats were tested in Skinner boxes where they could stimulate the reward system by pressing a lever, the rats pressed for hours. Research in the next two decades established that dopamine is one of the main chemicals aiding neural signaling in these regions, and dopamine was suggested to be the brain's "pleasure chemical".
Ivan Pavlov was a psychologist who used the reward system to study classical conditioning. Pavlov used the reward system by rewarding dogs with food after they had heard a bell or another stimulus. Pavlov was rewarding the dogs so that the dogs associated food, the reward, with the bell, the stimulus. Edward L. Thorndike used the reward system to study operant conditioning. He began by putting cats in a puzzle box and placing food outside of the box so that the cat wanted to escape. The cats worked to get out of the puzzle box to get to the food. Although the cats ate the food after they escaped the box, Thorndike learned that the cats attempted to escape the box without the reward of food. Thorndike used the rewards of food and freedom to stimulate the reward system of the cats. Thorndike used this to see how the cats learned to escape the box.
ΔFosB (delta FosB), a gene transcription factor, is the common factor among virtually all forms of addiction (behavioral addictions and drug addictions) that, when overexpressed in D1-type medium spiny neurons in the nucleus accumbens, induces addiction-related behavioral and neural plasticity; in particular, ΔFosB promotes self-administration, reward sensitization, and reward cross-sensitization effects among specific addictive drugs and behaviors.
Addictive drugs and addictive behaviors are rewarding and reinforcing (i.e., are addictive) due to their effects on the dopamine reward pathway. They not only prompt these brain reward systems more vigorously than natural rewards, but they also alter them. Continuous drug use and engagement in addictive activities reduces the drug-A state and causes tolerance. Whereas abruptly ceasing to engage in an addictive behaviour causes dopamine to drop below normal levels and results in the B-state of withdrawal.
- Schultz W. Neuronal reward and decision signals: from theories to data. Physiological Reviews. 2015 [archived 6 September 2015; Retrieved 24 September 2015];95(3):853–951. doi:10.1152/physrev.00023.2014. "Rewards in operant conditioning are positive reinforcers. ... Operant behavior gives a good definition for rewards. Anything that makes an individual come back for more is a positive reinforcer and therefore a reward. Although it provides a good definition, positive reinforcement is only one of several reward functions. ... Rewards are attractive. They are motivating and make us exert an effort. ... Rewards induce approach behavior, also called appetitive or preparatory behavior, and consummatory behavior. ... Thus any stimulus, object, event, activity, or situation that has the potential to make us approach and consume it is by definition a reward. ... Rewarding stimuli, objects, events, situations, and activities consist of several major components. First, rewards have basic sensory components (visual, auditory, somatosensory, gustatory, and olfactory) ... Second, rewards are salient and thus elicit attention, which are manifested as orienting responses (FIGURE 1, middle). The salience of rewards derives from three principal factors, namely, their physical intensity and impact (physical salience), their novelty and surprise (novelty/surprise salience), and their general motivational impact shared with punishers (motivational salience). A separate form not included in this scheme, incentive salience, primarily addresses dopamine function in addiction and refers only to approach behavior (as opposed to learning) ... Third, rewards have a value component that determines the positively motivating effects of rewards and is not contained in, nor explained by, the sensory and attentional components (FIGURE 1, right). This component reflects behavioral preferences and thus is subjective and only partially determined by physical parameters. Only this component constitutes what we understand as a reward. It mediates the specific behavioral reinforcing, approach generating, and emotional effects of rewards that are crucial for the organism’s survival and reproduction, whereas all other components are only supportive of these functions. ... Rewards can also be intrinsic to behavior (31, 546, 547). They contrast with extrinsic rewards that provide motivation for behavior and constitute the essence of operant behavior in laboratory tests. Intrinsic rewards are activities that are pleasurable on their own and are undertaken for their own sake, without being the means for getting extrinsic rewards. ... Intrinsic rewards are genuine rewards in their own right, as they induce learning, approach, and pleasure, like perfectioning, playing, and enjoying the piano. Although they can serve to condition higher order rewards, they are not conditioned, higher order rewards, as attaining their reward properties does not require pairing with an unconditioned reward. ... These emotions are also called liking (for pleasure) and wanting (for desire) in addiction research (471) and strongly support the learning and approach generating functions of reward."
- Malenka RC, Nestler EJ, Hyman SE (2009). "Chapter 15: Reinforcement and Addictive Disorders". In Sydor A, Brown RY. Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. pp. 364–375. ISBN 9780071481274.
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- Yager LM, Garcia AF, Wunsch AM, Ferguson SM (August 2015). "The ins and outs of the striatum: Role in drug addiction". Neuroscience. 301: 529–541. doi:10.1016/j.neuroscience.2015.06.033. PMID 26116518.
[The striatum] receives dopaminergic inputs from the ventral tegmental area (VTA) and the substantia nigra (SNr) and glutamatergic inputs from several areas, including the cortex, hippocampus, amygdala, and thalamus (Swanson, 1982; Phillipson and Griffiths, 1985; Finch, 1996; Groenewegen et al., 1999; Britt et al., 2012). These glutamatergic inputs make contact on the heads of dendritic spines of the striatal GABAergic medium spiny projection neurons (MSNs) whereas dopaminergic inputs synapse onto the spine neck, allowing for an important and complex interaction between these two inputs in modulation of MSN activity ... It should also be noted that there is a small population of neurons in the [nucleus accumbens] NAc that coexpress both D1 and D2 receptors, though this is largely restricted to the NAc shell (Bertran- Gonzalez et al., 2008). ... Neurons in the NAc core and NAc shell subdivisions also differ functionally. The NAc core is involved in the processing of conditioned stimuli whereas the NAc shell is more important in the processing of unconditioned stimuli; Classically, these two striatal MSN populations are thought to have opposing effects on basal ganglia output. Activation of the dMSNs causes a net excitation of the thalamus resulting in a positive cortical feedback loop; thereby acting as a 'go’ signal to initiate behavior. Activation of the iMSNs, however, causes a net inhibition of thalamic activity resulting in a negative cortical feedback loop and therefore serves as a 'brake’ to inhibit behavior ... there is also mounting evidence that iMSNs play a role in motivation and addiction (Lobo and Nestler, 2011; Grueter et al., 2013). For example, optogenetic activation of NAc core and shell iMSNs suppressed the development of a cocaine CPP whereas selective ablation of NAc core and shell iMSNs ... enhanced the development and the persistence of an amphetamine CPP (Durieux et al., 2009; Lobo et al., 2010). These findings suggest that iMSNs can bidirectionally modulate drug reward. ... Together these data suggest that iMSNs normally act to restrain drug-taking behavior and recruitment of these neurons may in fact be protective against the development of compulsive drug use.
- Taylor SB, Lewis CR, Olive MF (2013). "The neurocircuitry of illicit psychostimulant addiction: acute and chronic effects in humans". Subst Abuse Rehabil. 4: 29–43. doi:10.2147/SAR.S39684. PMC . PMID 24648786.
Regions of the basal ganglia, which include the dorsal and ventral striatum, internal and external segments of the globus pallidus, subthalamic nucleus, and dopaminergic cell bodies in the substantia nigra, are highly implicated not only in fine motor control but also in [prefrontal cortex] PFC function.43 Of these regions, the [nucleus accumbens] NAc (described above) and the [dorsal striatum] DS (described below) are most frequently examined with respect to addiction. Thus, only a brief description of the modulatory role of the basal ganglia in addiction-relevant circuits will be mentioned here. The overall output of the basal ganglia is predominantly via the thalamus, which then projects back to the PFC to form cortico-striatal-thalamo-cortical (CSTC) loops. Three CSTC loops are proposed to modulate executive function, action selection, and behavioral inhibition. In the dorsolateral prefrontal circuit, the basal ganglia primarily modulate the identification and selection of goals, including rewards.44 The [orbitofrontal cortex] OFC circuit modulates decision-making and impulsivity, and the anterior cingulate circuit modulates the assessment of consequences.44 These circuits are modulated by dopaminergic inputs from the [ventral tegmental area] VTA to ultimately guide behaviors relevant to addiction, including the persistence and narrowing of the behavioral repertoire toward drug seeking, and continued drug use despite negative consequences.43–45
- Grall-Bronnec M, Sauvaget A. The use of repetitive transcranial magnetic stimulation for modulating craving and addictive behaviours: a critical literature review of efficacy, technical and methodological considerations. Neurosci. Biobehav. Rev.. 2014;47:592–613. doi:10.1016/j.neubiorev.2014.10.013. PMID 25454360. "Studies have shown that cravings are underpinned by activation of the reward and motivation circuits (McBride et al., 2006, Wang et al., 2007, Wing et al., 2012, Goldman et al., 2013, Jansen et al., 2013 and Volkow et al., 2013). According to these authors, the main neural structures involved are: the nucleus accumbens, dorsal striatum, orbitofrontal cortex, anterior cingulate cortex, dorsolateral prefrontal cortex (DLPFC), amygdala, hippocampus and insula."
- Malenka RC, Nestler EJ, Hyman SE. In: Sydor A, Brown RY, eds. Molecular Neuropharmacology: A Foundation for Clinical Neuroscience. 2nd ed. New York: McGraw-Hill Medical; 2009. ISBN 978-0-07-148127-4. "The neural substrates that underlie the perception of reward and the phenomenon of positive reinforcement are a set of interconnected forebrain structures called brain reward pathways; these include the nucleus accumbens (NAc; the major component of the ventral striatum), the basal forebrain (components of which have been termed the extended amygdala, as discussed later in this chapter), hippocampus, hypothalamus, and frontal regions of cerebral cortex. These structures receive rich dopaminergic innervation from the ventral tegmental area (VTA) of the midbrain. Addictive drugs are rewarding and reinforcing because they act in brain reward pathways to enhance either dopamine release or the effects of dopamine in the NAc or related structures, or because they produce effects similar to dopamine. ... A macrostructure postulated to integrate many of the functions of this circuit is described by some investigators as the extended amygdala. The extended amygdala is said to comprise several basal forebrain structures that share similar morphology, immunocytochemical features, and connectivity and that are well suited to mediating aspects of reward function; these include the bed nucleus of the stria terminalis, the central medial amygdala, the shell of the NAc, and the sublenticular substantia innominata." p. 365–366, 376.
- Berridge KC, Kringelbach ML. Pleasure systems in the brain. Neuron. May 2015;86(3):646–664. doi:10.1016/j.neuron.2015.02.018. PMID 25950633. "In the prefrontal cortex, recent evidence indicates that the [orbitofrontal cortex] OFC and insula cortex may each contain their own additional hot spots (D.C. Castro et al., Soc. Neurosci., abstract). In specific subregions of each area, either opioid-stimulating or orexin-stimulating microinjections appear to enhance the number of liking’’ reactions elicited by sweetness, similar to the [nucleus accumbens] NAc and [ventral pallidum] VP hot spots. Successful confirmation of hedonic hot spots in the OFC or insula would be important and possibly relevant to the orbitofrontal mid-anterior site mentioned earlier that especially tracks the subjective pleasure of foods in humans (Georgiadis et al., 2012; Kringelbach, 2005; Kringelbach et al., 2003; Small et al., 2001; Veldhuizen et al., 2010). Finally, in the brainstem, a hindbrain site near the parabrachial nucleus of dorsal pons also appears able to contribute to hedonic gains of function (Söderpalm and Berridge, 2000). A brainstem mechanism for pleasure may seem more surprising than forebrain hot spots to anyone who views the brainstem as merely reflexive, but the pontine parabrachial nucleus contributes to taste, pain, and many visceral sensations from the body and has also been suggested to play an important role in motivation (Wu et al., 2012) and in human emotion (especially related to the somatic marker hypothesis) (Damasio, 2010)."
- Richard JM, Castro DC, Difeliceantonio AG, Robinson MJ, Berridge KC. Mapping brain circuits of reward and motivation: in the footsteps of Ann Kelley. Neurosci. Biobehav. Rev.. November 2013;37(9 Pt A):1919–1931. doi:10.1016/j.neubiorev.2012.12.008. PMID 23261404. "
Figure 3: Neural circuits underlying motivated 'wanting' and hedonic 'liking'."
- Dumitriu D, Laplant Q, Grossman YS, Dias C, Janssen WG, Russo SJ, Morrison JH, Nestler EJ. Subregional, dendritic compartment, and spine subtype specificity in cocaine regulation of dendritic spines in the nucleus accumbens. J. Neurosci.. 2012;32(20):6957–66. doi:10.1523/JNEUROSCI.5718-11.2012. PMID 22593064. "The enduring spine density change in core but not shell fits well with the established idea that the shell is preferentially involved in the development of addiction, while the core mediates the long-term execution of learned addiction-related behaviors (Ito et al., 2004; Di Chiara, 2002; Meredith et al., 2008). Consistent with the idea of [nucleus accumbens] NAc core being the locus of long-lasting drug-induced neuroplasticity, several studies have shown that electrophysiological changes in core persist longer than their shell counterparts. ... Furthermore, data presented here support the idea that NAc shell is preferentially involved in immediate drug reward, while the core might play a more explicit role in longer-term aspects of addiction."
- Trantham-Davidson H, Neely LC, Lavin A, Seamans JK. Mechanisms underlying differential D1 versus D2 dopamine receptor regulation of inhibition in prefrontal cortex. The Journal of Neuroscience. 2004;24(47):10652–10659. doi:10.1523/jneurosci.3179-04.2004.
- You ZB, Chen YQ, Wise RA. Dopamine and glutamate release in the nucleus accumbens and ventral tegmental area of rat following lateral hypothalamic self-stimulation. Neuroscience. 2001;107(4):629–639. doi:10.1016/s0306-4522(01)00379-7. PMID 11720786.
- Berridge KC, Kringelbach ML. Neuroscience of affect: brain mechanisms of pleasure and displeasure. Current Opinion in Neurobiology. 1 June 2013;23(3):294–303. doi:10.1016/j.conb.2013.01.017. PMID 23375169. "rather opioid stimulation has the special capacity to enhance 'liking’ only if the stimulation occurs within an anatomical hotspot whereas dopamine never does anywhere dopamine, probably the most popular brain neurotransmitter candidate for pleasure two decades ago, turns out not to cause pleasure or 'liking’ at all. Rather dopamine more selectively mediates a motiva- tional process of incentive salience, which is a mechanism for 'wanting’ rewards but not for 'liking’ them"
- Kringelbach ML, Berridge KC. The Joyful Mind. Scientific American. 2012 [Retrieved 17 January 2017]:44-45. "So it makes sense that the real pleasure centers in the brain—those directly responsible for generating pleasurable sensations—turn out to lie within some of the structures previously identified as part of the reward circuit. One of these so-called hedonic hotspots lies in a subregion of the nucleus accumbens called the medial shell. A second is found within the ventral pallidum, a deep-seated structure near the base of the forebrain that receives most of its signals from the nucleus accumbens. ...
On the other hand, intense euphoria is harder to come by than everyday pleasures. The reason may be that strong enhancement of pleasure—like the chemically induced pleasure bump we produced in lab animals—seems to require activation of the entire network at once. Defection of any single component dampens the high.
Whether the pleasure circuit—and in particular, the ventral pallidum—works the same way in humans is unclear."
- Berridge KC. From prediction error to incentive salience: mesolimbic computation of reward motivation. Eur. J. Neurosci.. April 2012;35(7):1124–1143. doi:10.1111/j.1460-9568.2012.07990.x. PMID 22487042. "Here I discuss how mesocorticolimbic mechanisms generate the motivation component of incentive salience. Incentive salience takes Pavlovian learning and memory as one input and as an equally important input takes neurobiological state factors (e.g. drug states, appetite states, satiety states) that can vary independently of learning. Neurobiological state changes can produce unlearned fluctuations or even reversals in the ability of a previously learned reward cue to trigger motivation. Such fluctuations in cue-triggered motivation can dramatically depart from all previously learned values about the associated reward outcome. ... Associative learning and prediction are important contributors to motivation for rewards. Learning gives incentive value to arbitrary cues such as a Pavlovian conditioned stimulus (CS) that is associated with a reward (unconditioned stimulus or UCS). Learned cues for reward are often potent triggers of desires. For example, learned cues can trigger normal appetites in everyone, and can sometimes trigger compulsive urges and relapse in addicts.
Cue-triggered 'wanting’ for the UCS
A brief CS encounter (or brief UCS encounter) often primes a pulse of elevated motivation to obtain and consume more reward UCS. This is a signature feature of incentive salience.
Cue as attractive motivational magnets
When a Pavlovian CS+ is attributed with incentive salience it not only triggers 'wanting’ for its UCS, but often the cue itself becomes highly attractive – even to an irrational degree. This cue attraction is another signature feature of incentive salience ... Two recognizable features of incentive salience are often visible that can be used in neuroscience experiments: (i) UCS-directed 'wanting’ – CS-triggered pulses of intensified 'wanting’ for the UCS reward; and (ii) CS-directed 'wanting’ – motivated attraction to the Pavlovian cue, which makes the arbitrary CS stimulus into a motivational magnet."
- Malenka RC, Nestler EJ, Hyman SE. In: Sydor A, Brown RY, eds. Molecular Neuropharmacology: A Foundation for Clinical Neuroscience. 2nd ed. New York: McGraw-Hill Medical; 2009. ISBN 978-0-07-148127-4. "VTA DA neurons play a critical role in motivation, reward-related behavior (Chapter 15), attention, and multiple forms of memory. This organization of the DA system, wide projection from a limited number of cell bodies, permits coordinated responses to potent new rewards. Thus, acting in diverse terminal fields, dopamine confers motivational salience ("wanting") on the reward itself or associated cues (nucleus accumbens shell region), updates the value placed on different goals in light of this new experience (orbital prefrontal cortex), helps consolidate multiple forms of memory (amygdala and hippocampus), and encodes new motor programs that will facilitate obtaining this reward in the future (nucleus accumbens core region and dorsal striatum). In this example, dopamine modulates the processing of sensorimotor information in diverse neural circuits to maximize the ability of the organism to obtain future rewards. ...
The brain reward circuitry that is targeted by addictive drugs normally mediates the pleasure and strengthening of behaviors associated with natural reinforcers, such as food, water, and sexual contact. Dopamine neurons in the VTA are activated by food and water, and dopamine release in the NAc is stimulated by the presence of natural reinforcers, such as food, water, or a sexual partner. ...
The NAc and VTA are central components of the circuitry underlying reward and memory of reward. As previously mentioned, the activity of dopaminergic neurons in the VTA appears to be linked to reward prediction. The NAc is involved in learning associated with reinforcement and the modulation of motoric responses to stimuli that satisfy internal homeostatic needs. The shell of the NAc appears to be particularly important to initial drug actions within reward circuitry; addictive drugs appear to have a greater effect on dopamine release in the shell than in the core of the NAc." p. 147–148, 367, 376.
- Berridge KC, Kringelbach ML. Neuroscience of affect: brain mechanisms of pleasure and displeasure. Current Opinion in Neurobiology. 1 June 2013;23(3):294–303. doi:10.1016/j.conb.2013.01.017. PMID 23375169.
- Salamone JD, Correa M. The mysterious motivational functions of mesolimbic dopamine. Neuron. 8 November 2012;76(3):470–485. doi:10.1016/j.neuron.2012.10.021. PMID 23141060.
- Calipari ES, Bagot RC, Purushothaman I, Davidson TJ, Yorgason JT, Peña CJ, Walker DM, Pirpinias ST, Guise KG, Ramakrishnan C, Deisseroth K, Nestler EJ. In vivo imaging identifies temporal signature of D1 and D2 medium spiny neurons in cocaine reward. Proc. Natl. Acad. Sci. U.S.A.. March 2016;113(10):2726–2731. doi:10.1073/pnas.1521238113. PMID 26831103. "Previous work has demonstrated that optogenetically stimulating D1 [medium spiny neurons] MSNs promotes reward, whereas stimulating D2 MSNs produces aversion. ... Studies using in vivo pharmacological approaches have demonstrated differential roles of NAc D1 and D2 receptors in drug conditioning by use of selective receptor agonists or antagonists, further supporting a role for both dopamine and D1 and D2 MSN subtypes in associative learning (27). Whereas this work has focused on the VTA-to-NAc dopamine circuit, tracking postsynaptic responses in NAc MSNs is particularly important because they integrate information not only from VTA dopamine neurons but also from numerous glutamatergic projections (28, 29). From a network perspective, D1 and D2 MSNs receive inputs from several regions known to encode and store information about context or context–drug associations such as the prefrontal cortex, basolateral amygdala, and hippocampus (30). ... Our data highlight the important role played by D1 MSNs in NAc core in establishing context–reward associations and in controlling the strength of these associations after cocaine exposure. ... Here we show that regulation of associative learning, and its dysregulation by cocaine, is driven primarily through alterations in D1 MSNs in NAc core, which both impair the extinction of previously learned associations and enhance reinstatement following abstinence."
- Baliki MN, Mansour A, Baria AT, Huang L, Berger SE, Fields HL, Apkarian AV. Parceling human accumbens into putative core and shell dissociates encoding of values for reward and pain. J. Neurosci.. October 2013;33(41):16383–16393. doi:10.1523/JNEUROSCI.1731-13.2013. PMID 24107968. "Recent evidence indicates that inactivation of D2 receptors, in the indirect striatopallidal pathway in rodents, is necessary for both acquisition and expression of aversive behavior, and direct pathway D1 receptor activation controls reward-based learning (Hikida et al., 2010; Hikida et al., 2013). It seems we can conclude that direct and indirect pathways of the NAc, via D1 and D2 receptors, subserve distinct anticipation and valuation roles in the shell and core of NAc, which is consistent with observations regarding spatial segregation and diversity of responses of midbrain dopaminergic neurons for rewarding and aversive conditions, some encoding motivational value, others motivational salience, each connected with distinct brain networks and having distinct roles in motivational control (Bromberg-Martin et al., 2010; Cohen et al., 2012; Lammel et al., 2013). ... Thus, the previous results, coupled with the current observations, imply that the NAc pshell response reflects a prediction/anticipation or salience signal, and the NAc pcore response is a valuation response (reward predictive signal) that signals the negative reinforcement value of cessation of pain (i.e., anticipated analgesia)."
- Berridge KC, Kringelbach ML. Affective neuroscience of pleasure: reward in humans and animals. Psychopharmacology. 2008;199(3):457–480. doi:10.1007/s00213-008-1099-6. PMID 18311558. PMC 3004012.
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