Brain Imaging Center, National Institute on Drug Abuse, Baltimore, MD 21224, USA
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Abstract |
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Introduction |
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The OFC is placed in a position to code the motivational attributes of responses to stimuli. It is a heterogeneous region that has connections with other prefrontal, limbic, sensory and premotor areas (Cavada et al., 2000; Öngür and Price, 2000
). Linked to the mesolimbic dopamine system that is critical for drug reward (Di Chiara and Imperato, 1988
; Koob and Bloom, 1988
; London et al., 1996
; Wise, 1996
), it receives inputs from association areas of each sensory modality (olfactory, gustatory, visual, auditory and somatosensory) (Pandya and Yeterian, 1990
; Morecraft et al., 1992
; Cavada et al., 2000
; Öngür and Price, 2000
). Therefore, the OFC has the information needed to integrate the sensory characteristics of a stimulus-object (Carmichael and Price, 1996
). The OFC can attach an emotional valence to the stimulus-object through its relationship with the amygdala (Barbas and De Olmos, 1990
; Morecraft et al., 1992
). Furthermore, it can evaluate these characteristics against previous experience through its connections with regions known to subserve memory [dorsolateral prefrontal cortex and mediodorsal nucleus of the thalamus, pars magnocellularis (Morecraft et al., 1992
; Ray and Price, 1993
), hippocampus and hippocampal gyrus (Cavada et al., 2000
; Öngür and Price, 2000
)]. Finally, the OFCstriatumglobus pallidusthalamusOFC loop (Alexander and Crutcher, 1990
) can mediate reinforcement of the motivational attribute of a stimulus-object.
A modular approach to the role of the OFC, although heuristic, may help tease out the various mechanisms that mediate substance abuse. Using such a strategy, individual components of aberrant behavior in substance abusers can be studied separately. One of these components is expectancy that is based on predictions of reward and attribution of probabilistic rewarding properties to the stimulus-object. Another is compulsive drive (motivational state) to use drugs, which is linked to craving. Lastly, decision making, based on the motivational attributes of the stimulus and the balance between expectation of immediate reward and long-term losses, is an important aspect of substance abuse behavior.
While considering how the OFC is involved in these three behaviors, it is important to appreciate the heterogeneous nature of this region, histologically and anatomically (Morecraft et al., 1992; Carmichael et al., 1994
). Anatomically, the OFC can be divided into anterior regions, which connect with higher association cortices (such as the dorsolateral prefrontal cortex), and posterior regions, which have selective connections with the amygdala, and entorhinal and perirhinal cortices. Therefore, anterior regions schematically would be important in decisionmaking processes, and posterior regions in the emotional and autonomic aspects of craving. Both regions could contribute to expectancy, which depends on the evaluation of the emotional valence of the stimulus. Along a transaxial plane, the medial OFC is connected with the ventral striatum and the lateral OFC is connected with the caudate nucleus, which suggests that the medial OFC would be more important to reinforcement than the lateral OFC.
This review covers findings from brain imaging studies that support important contributions of the OFC to the persistent behavioral states characteristic of addiction. To date, most functional imaging studies have been unable to distinguish accurately between the different regions of the OFC that might take part in the respective behaviors. The enhanced spatial resolution of new generation positron emission tomography (PET) scanners and improved technology of functional magnetic resonance imaging (fMRI), however, offer the promise of isolating functional activity in subregions of the OFC.
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OFC Metabolism in Polysubstance Abusers: Personality Traits, Expectancy and Motivation |
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Mean global metabolic rate for glucose (mg/100 g tissue/min) in the substance abusers (6.91 ± 0.17) did not differ significantly from the rate in the control group (7.27 ± 0.21), but the pattern of rCMRglc differed between the two groups. Visual inspection of the brain images revealed that control participants showed relatively uniform cortical glucose metabolism compared with the substance abusers, whose rCMRglc was reduced in posterior regions (Fig. 1). When metabolic rate data for each region were submitted to analysis of covariance with the global metabolic rate as a covariate, the substance abusers showed higher glucose utilization than the control group in the OFC, superior frontal gyrus, middle temporal gyrus and insula (Table 1
, Fig. 1
).
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Emotional state, related to expectancy of drug reward, may have also been reflected in the higher rCMRglc in the OFC of the substance abusers. As they took part in a double-blind procedure in which the injection could have been placebo or cocaine, they likely experienced a negative emotional reaction (e.g. disappointment) when they realized that they had received placebo. In contrast, control participants would not have been as likely to have this emotional response. This interpretation is supported by our findings from a recent study (Grant et al., 1996) in which normalized rCMRglc was negatively correlated with self-reported mood in a number of OFC regions when neutral (not drug-related) visual cues were presented.
The elevated rCMRglc in other cortical areas (insula, temporal lobe and superior frontal gyrus) in addition to OFC in the substance abusers suggests that the response to the placebo injection included activation of a circuit related to a component of motivation. Classic work of Mesulam and Mufson in the old world monkey has demonstrated connections between the OFC and insula, which also receives projections from the temporal pole and other paralimbic areas (Mesulam and Mufson, 1982ac
). On the basis of this and other work, these authors suggest that the paralimbic brain, particularly its insulo-orbitotemporopolar component, provides integration between extrapersonal stimuli and the internal milieu (Mesulam and Mufson, 1982a
,c
). In this light, elevated rCMRglc in the OFC and insula (and also in temporal and superior frontal areas) is consistent with an interpretive response to placebo administration in the context of expectancy of drug reward. Such integration can be critical to the motivational state or drive to use drugs.
Although the PET study situation was markedly different from the typical conditions of drug self-administration, the substance abusers may have shown conditioned drug effects, as they had previously received cocaine under test conditions that were in some ways similar to the PET session. Thus, the higher rCMRglc as compared with control values may reflect activation in response to conditioned, drug-related cues (i.e. an injection given by an investigator who administered cocaine previously in a test situation). Nonetheless, substance abusers also exhibit higher normalized rCMRglc under conditions where there are no drug-related cues or expectancy of drug administration. A supplementary analysis of the data collected in our study of cue-elicited cocaine craving (Grant et al., 1996) compared rCMRglc in seven OFC brain regions of substance abusers and controls only in the session where a non-drug (arts and crafts) stimulus complex was presented. Drug abusers exhibited a significantly higher normalized rCMRglc than controls in the medial prefrontal cortex.
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Role of the OFC and its Connections in Drug Craving |
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Our research group studied cue-elicited cocaine craving in cocaine abusers (n = 13) and control participants (n = 5), who were presented with a stimulus complex, including a cocainerelated videotape and paraphernalia in one session and a videotape and objects related to arts and crafts in another session (Grant et al., 1996). We also measured rCMRglc by the FDG method and PET. Exposure to cocaine-related cues produced self-reports of craving and activation in the medial OFC and five other composite cortical brain regions in cocaine abusers: dorsolateral prefrontal, peristriate, temporal/parietal, retrosplenial and temporal (see Fig. 2
). Furthermore, self-reports of craving were positively correlated with metabolic increases in the dorsolateral prefrontal cortex, medial temporal lobe (amygdala) and cerebellum. These effects were not seen in non-drug-abusing control participants. The activation pattern seen in our study, involving the OFC and several areas to which it is linked anatomically, is consistent with an integrative function of OFC, involving brain areas that participate in sensory processing (peristriate cortex) and episodic memory (dorsolateral prefrontal cortex and temporal areas), as well as emotional coding (amygdala).
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The issue of the anatomical substrates of cue-induced craving was also addressed with fMRI (Maas et al., 1998). Audiovisual stimuli containing alternating intervals of drug-related and neutral scenes were presented to six men with histories of crack cocaine use and six control participants. Activation was detected in the anterior cingulate and left dorsolateral prefrontal cortex in the cocaine users. In addition, self-reported levels of craving were correlated with activation in these regions. No data were presented for the OFC or the amygdala, which are difficult to evaluate with fMRI because of the susceptibility to artifacts in regions near the sinuses induced by the air-tissue interface.
A pattern of activation that was generally consistent with the studies reviewed above was observed when drug craving was elicited by script-driven imagery. In an FDGPET study, Wang et al. conducted interactive interviews in 13 active cocaine abusers about either neutral themes or cocaine-themes during the FDG uptake period (Wang et al., 1999). Cocaine-related paraphernalia were also presented to the subject during the cocaine-theme session. Increases in both absolute and normalized rCMRglc were seen in the medial OFC and left insular cortex, and increased normalized glucose metabolism was seen in the cerebellum. Since no control subjects were studied, it is possible that differences in the physical properties of the stimuli in the neutral versus cocaine-cues session could have contributed to some portion of the observed changes in cerebral metabolism. Self-reported cocaine craving was correlated with absolute and normalized rCMRglc in the right insular region.
Finally, preliminary results have shown rCBF activation with PET in the basal ganglia, insula, cerebellum and left parahippocampal gyrus in heroin-dependent individuals who listened to personalized audiotapes (Weinstein et al., 1998). Subjects listened to six of their own audiotaped neutral scripts and six audiotaped craving scripts at random, each script lasting for 1.5 min, with a rest of 8 min between scripts (120 min total). The main regions of activation were the medial prefrontal gyrus and the anterior cingulate. Activation in the anterior cingulate was positively correlated with self-reports of craving.
Craving induced by drug administration has been studied with both PET and fMRI (Breiter et al., 1997; Volkow et al., 1999
). When cocaine was administered to cocaine-dependent subjects in an fMRI study, craving was correlated with early and sustained activation of regions that included the nucleus accumbens, left amygdala and right parahippocampal gyrus, and some regions of the lateral prefrontal cortex (Breiter et al., 1997
). Sustained negative signal change in the amygdala was correlated with ratings of craving. Cocaine-induced perceptions of rush were associated with short-term activation of the ventral tegmentum, pons, basal forebrain, caudate, cingulate and most regions of the lateral prefrontal cortex. In a FDGPET study, craving for cocaine in cocaine abusers was elicited by injection of methylphenidate, which has a pharmacological mechanism of action similar to that of cocaine (Volkow, 1999). Cocaine craving was correlated with increased normalized rCMRglc in the posterior portion of the OFC and the striatum of the right hemisphere. Although the report on the fMRI study indicated no signal changes in the OFC, it is unclear whether this absence reflected susceptibility artifacts that precluded imaging of this region, as in the study by Maas et al. (Maas et al., 1998
). Even though these findings suggest that activation of the OFC is common to both cueand drug-elicited craving, it is important to note that there are differences in the circuitry affected when craving is induced by cocaine itself or by cocaine-related cues. Drug-related cues activated the amygdala and medial temporal lobe, simulating the direct effects of cocaine. However, other regions, such as the nucleus accumbens, which presented no significant change in cue activation studies, showed direct pharmacological effects of cocaine.
In addition to these findings, related to experimentally induced craving, findings from brain imaging studies where spontaneous craving was noted also showed involvement of the OFC. For example, in recently abstinent cocaine abusers, rCMRglc in the OFC and dorsoprefrontal area were correlated with retrospective self-reports of cocaine craving during the week that preceded the PET scan (Volkow et al., 1991). In contrast we observed a negative correlation of cocaine-craving with rCMRglc in the OFC in additional analysis of data from the study by Grant et al. (Grant et al., 1996
). Here, increased levels of self-reported cocaine craving during presentation of neutral stimuli was related to lower levels rCMRglc in the right anterior OFC. One possible reason for the differences in direction of the correlations is that in the study by Grant et al., subjects had been abstinent from cocaine for only 48 h, whereas the subjects studied by Volkow were abstinent for up to 1 week. Another possibility may be related to the greater level of cocaine use in the subjects studied by Volkow.
These data suggest that the OFC is involved in the induction of craving, at least for cocaine; however, methodological factors complicate the issue. Craving induced by cocaine itself, though perceived by users as the drive to acquire and use more drug, may be anatomically distinct from craving that is triggered by stimuli conditioned through repetitive drug use. Whereas humans may not be able to discriminate between these states, the observations that different neurobiological substrates are associated with these states may have profound implications for therapeutic interventions.
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The OFC and Decision Making in Drug Abusers |
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As a first step toward determining whether drug abusers have a functional deficit involving the OFC, we have utilized a task that was specifically designed to evaluate human patients with lesions of the ventromedial portion of the OFC (VmOFC) Bechara et al., 2000). Patients with these lesions demonstrate generally irresponsible behaviors, including poor judgement in business and personal decisions. The Gambling Task was developed to capture these traits in a laboratory setting by requiring subjects to make choices based on the long-term consequences of complex reward/punishment contingencies. The task is performed as a card game in which the participant selects cards from four decks that differ with respect to the payoff for each card selection and the frequency and severity of penalties, indicated by certain cards. It has face validity for drug abuse since poor performance results from making choices that yield large short-term rewards even when such choices eventually lead to net losses. Indeed, one of the DSM IV criteria for substance abuse disorder states (on p. 181) that substance abuse is continued despite knowledge of having a recurrent physical or psychological problem that is likely to have been caused or exacerbated by the substance (American Psychiatric Association, 1994). Similarly, the World Health Organization (1992) uses the criterion for disorders due to psychoactive substance use of persisting . . . despite harmful consequences' (p. 321). Although the Gambling Task models some aspects of drug abuse behavior, the balance between reward and negative consequences differs somewhat from the real life situation of the drug abuser. One difference is the fact that the certainty of reward is greater when considering self-administration of a drug than drawing a card in the model task.
In our study, polydrug abusers were compared with a group of subjects who did not use illicit drugs of abuse (Grant et al., 2000). The scores on the GT were nearly 50% lower in drug abusers (n = 30) than in controls (n = 24), but not as low as those of patients with VmOFC lesions (Bechara et al., 1994
). In contrast to GT results, there was no difference in performance between drug abusers and controls on the Wisconsin Card Sorting Task, which is sensitive to lesions of the dorsolateral prefrontal cortex but not the OFC (Stuss et al., 1998
). These results suggest the working hypothesis that there is a functional abnormality, as opposed to a frank lesion, specifically in the ventromedial prefrontal cortex of drug abusers, but not necessarily in the frontal lobe as a whole. Further, they are consistent with the notion that the OFC is linked to the ability to balance short-term gains against longer-term consequences in the process of decision making.
Other studies have also shown that drug abusers are likely to make maladaptive decisions when faced with short-term versus long-term outcomes, especially under conditions that involve risk and uncertainty. In one of these, opioid-dependent participants placed less value on delayed monetary rewards than control subjects (Madden et al., 1997), underscoring the concept that drug abusers are overly biased towards the immediate value of rewards and less able to evaluate the consequences of their actions in the future. In another study, amphetamine abusers, opiate abusers, non-drug-using comparison subjects and patients with VmOFC lesions were tested on a decision-making task similar to the Gambling Task (Rogers et al., 1999
). Amphetamine users and lesion patients performed more poorly than subjects in the other groups, although the amphetamine users did not do as badly as the lesioned patients. Furthermore, there was a negative correlation between task performance and years of stimulant use. Interestingly, experimentally induced depletion of tryptophan, the precursor of the neurotransmitter serotonin, in non-drug-using comparison subjects led to impaired task performance. These results suggest that specific classes of drugs of abuse may have a differential impact on the processes underlying decision-making abilities, and the impact may be influenced by the drug's effect on serotonergic, and perhaps other aminergic, neurotransmission.
Our research group has begun a series of experiments to investigate whether impaired performance on the Gambling Task in drug abusers is directly due to dysfunction of the VmOFC. The aim of the first set of studies was to demonstrate that the VmOFC is involved in Gambling Task performance in neurologically intact subjects. We therefore conducted PET assays of rCMRglc during performance of the Gambling Task. Preliminary results from seven subjects indicate that the level of performance on the Gambling Task is positively correlated with activation of the VmOFC (Grant et al., 1999) (see Fig. 3
). These results suggest that successful Gambling Task performance requires anatomical integrity as well as adequate metabolic activity of the VmOFC. Our results are convergent with those of previous neuroimaging studies showing activation of the ventromedial prefrontal cortex in subjects who had to guess about task-related reward contingencies (Elliott et al., 1997
) or in subjects who experienced violations in expectancies (Nobre et al., 1999
; Elliot et al., 2000
).
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Conclusion |
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The initial impression from the work reviewed supports the view that the OFC, through its anatomical connections with sensory and limbic systems, serves as a critical node in the processing of environmental and internal cues to generate feeling states that influence the motivation to seek and selfadminister a drug. Even if this inference ultimately were proven to be true, however, several important tasks would still lay before us. We would need to identify the neural networks that code for the different behavioral components of substance abuse, such as drug expectancy, craving, and impaired decision making. It then would be important to characterize the role of the OFC in these networks as primary or secondary; and to evaluate the contribution of possible dysfunction of the OFC to the deviant behaviors of substance abuse (e.g. necessary but insufficient, sufficient but not necessary, or insufficient and not necessary). A final step would be identification of the neurochemical messengers that operate in the networks. Even before these tasks are tackled, however, it is important to consider the findings reviewed above in the design of therapeutic approaches for substance abuse. This instruction is particularly relevant to the implementation of a cognitive therapy that would require executive functioning that requires integrity of the OFC and its connections.
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Notes |
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References |
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