Lack of evidence for context-dependent cocaine-induced sensitization in humans: preliminary studies

Lower prefrontal cortical synaptic vesicle binding in cocaine use disorder: An exploratory 11 C-UCB-J positron emission tomography study in humans

Preclinical studies have revealed robust and long-lasting alterations in dendritic spines in the brain following cocaine exposure. Such alterations are hypothesized to underlie enduring maladaptive behaviours observed in cocaine use disorder (CUD). The current study explored whether synaptic density is altered in CUD. Fifteen individuals with DSM-5 CUD and 15 demographically matched healthy control (HC) subjects participated in a single 11 C-UCB-J positron emission tomography scan to assess density of synaptic vesicle glycoprotein 2A (SV2A). The volume of distribution (VT ) and the plasma-free fraction-corrected form of the total volume of distribution (VT /fP ) were analysed in the anterior cingulate cortex (ACC), dorsomedial and ventromedial prefrontal cortex (PFC), lateral and medial orbitofrontal cortex (OFC) and ventral striatum. A significant diagnostic-group-by-region interaction was observed for VT and VT /fP . Post hoc analyses revealed no differences on VT , while for VT /fP showed lower values in CUD as compared with HC subjects in the ACC (-10.9%, p = 0.02), ventromedial PFC (-9.9%, p = 0.02) and medial OFC (-9.9%, p = 0.04). Regional VT /fP values in CUD, though unrelated to measures of lifetime cocaine use, were positively correlated with the frequency of recent cocaine use (p = 0.02-0.03) and negatively correlated with cocaine abstinence (p = 0.008-0.03). These findings provide initial preliminary in vivo evidence of altered (lower) synaptic density in the PFC of humans with CUD. Cross-sectional variation in SV2A availability as a function of recent cocaine use and abstinence suggests that synaptic density may be dynamically and plastically regulated by acute cocaine, an observation that merits direct testing by studies using more definitive longitudinal designs.

Social Defeat Stress, Sex, and Addiction-Like Behaviors

Social confrontation is a form of social interaction in animals where two conspecific individuals confront each other in dispute over territory, during the formation of hierarchies, and during breeding seasons. Typically, a social confrontation involves a prevailing individual and a yielding individual. The prevailing individual often exhibits aggressive postures and launches attacks, whereas the yielding individual often adopts postures of defeat. The yielding or defeated animals experience a phenomenon known as social defeat stress, in which they show exaggerated stress as well as autonomic and endocrine responses that cause impairment of both the brain and body. In laboratory settings, one can reliably generate social defeat stress by allowing a naïve (or already defeated) animal to intrude into a home cage in which its resident has already established a territory or is nursing. This resident-intruder paradigm has been widely used in both males and females to study mechanisms in the brain that underlie the stress responses. Stress has profound effects on drug reward for cocaine, methamphetamine, alcohol, and opioids. Particularly, previous experiences with social defeat can exaggerate subsequent addiction-like behaviors. The extent of these addiction-like behaviors depends on the intensity, duration, frequency, and intermittency of the confrontation episodes. This chapter describes four types of social defeat stress: acute, repeated, intermittent, and chronic. Specifically, it focuses on social defeat stress models used in laboratories to study individual, sex, and animal strain differences in addiction-like behaviors.

The effects of oral micronized progesterone on smoked cocaine self-administration in women

There are currently no FDA-approved pharmacotherapies for cocaine abuse. Converging preclinical and clinical evidence indicates that progesterone may have potential as a treatment for cocaine-abusing women, who represent a growing portion of cocaine users. We have previously shown that oral progesterone reduced the positive subjective effects of cocaine in female cocaine users during the follicular phase of the menstrual cycle, when endogenous progesterone levels were low. To extend these findings, the present study assessed the effects of oral progesterone (150 mg BID) administered during the follicular phase on smoked cocaine self-administration in women relative to the normal follicular and luteal phases. Healthy, non-treatment seeking female cocaine smokers (N=10) underwent three 4-day inpatient stays, during: 1) a normal follicular phase; 2) a normal luteal phase; and 3) a follicular phase when oral progesterone was administered. During each stay, participants completed 4 self-administration sessions in which they first smoked a "sample" dose of cocaine (0, 12, 25 or 50 mg) and then had 5 opportunities at 14-minute intervals to self-administer that dose at a cost of $5 per dose. Expected cocaine dose effects on self-administration, subjective effects, and cardiovascular effects were observed. However, there was no effect of oral progesterone administration or menstrual cycle phase on cocaine self-administration. Thus, oral progesterone was not effective in reducing cocaine use in women under the current conditions. However, based on previous literature, further research assessing the role of oral progesterone for the treatment of cocaine dependence in women is warranted.

First evidence that drugs of abuse produce behavioral sensitization and cross sensitization in planarians

Behavioral sensitization in mammals, including humans, is sensitive to factors such as administration route, testing environment, and pharmacokinetic confounds, unrelated to the drugs themselves that are difficult to eliminate. Simpler animals less susceptible to these confounding influences may be advantageous substitutes for studying sensitization. We tested this hypothesis by determining whether planarians display sensitization and cross sensitization to cocaine and glutamate. Planarian hyperactivity was quantified as the number of C-like hyperkinesias during a 1-min drug exposure. Planarians exposed initially to cocaine (or glutamate) on day 1 were challenged with cocaine (or glutamate) after 2 or 6 days of abstinence. Acute cocaine or glutamate produced concentration-related hyperactivity. Cocaine or glutamate challenge after 2 and 6 days of abstinence enhanced the hyperactivity, indicating the substances produced planarian behavioral sensitization. Cross-sensitization experiments showed that cocaine produced greater hyperactivity in planarians earlier exposed to glutamate than in glutamate-naive planarians, and vice versa. Behavioral responses were pharmacologically selective because neither scopolamine nor caffeine produced planarian behavioral sensitization despite causing hyperactivity after initial administration, and acute gamma-aminobutyric acid did not cause hyperactivity. Demonstration of pharmacologically selective behavioral sensitization in planarians suggests that these flatworms represent a sensitive in-vivo model to study cocaine behavioral sensitization and to screen potential abuse-deterrent therapeutics.

The pursuit of value: sensitization or tolerance?

Two issues are raised. (1) What is the nature of the drug effect Heyman thinks confers value to drugs? (2) What is the evidence that drug use decreases the value of drugs and of conventional incentives over the long-term? There is considerable evidence for the opposite; a persistent increase in the sensitivity of neural systems that mediate drug value.

Future directions for the melioration model of addiction

For use in applied settings, the melioration model will need to incorporate changes in the shapes of local value functions over time, treat current value as a continuous function of time since previous choices, and take into account discounting of the effects of current behavior on future value. The policy implications of the model for regulating drugs are limited.

Behavioral choice theory can enhance our understanding of drug dependence and other behavioral disorders

I support the major theme of Heyman's target article that behavioral choice theory can enhance our understanding of drug dependence, but I raise concerns about the critique of the operant model of drug dependence, the underscoring of melioration to the exclusion of other theories of choice, and assertions about the unique effects of drug reinforcement.

Stimulus factors in addiction

Heyman's analysis of addiction in terms of matching to local relative value can be supplemented by stimulus-control processes. Stimulus equivalence can broaden the set of situations that occasion addictive behavior, and the situation-reinforcer correlation can enhance its persistence. The joint effects of stimulus-control and reinforcement processes may complicate treatment.

Understanding addiction: Conventional rewards and lack of control

The conflict between drug and conventional rewards leads to a paradox: Sanctions against drug use decrease access to conventional rewards and push drug users toward drug abuse, whereas increased access to the rewards of family, friends, and work may help reduce drug abuse. Lack of control is not specific to drug addiction and is unlikely to yield to a shift in bookkeeping.

Who determines the value of drug-taking behavior? Cultural considerations for a theory of behavioral choice

Heyman's analysis of addiction suggests that drug taking is irrational. The irrationality of drug taking, however, may depend on the acceptance of mainstream society's view of what is valuable. Consideration of the addict's viewpoint and cultural aspects of drug taking may be useful in trying to fathom the “rationality” of drug taking.

An economic perspective on addiction and matching

Economic models of addiction are choice-based. These models push the choice paradigm too far by modeling addiction as rational, normative behavior. Heyman's target article provides a sensible alternative to this economic approach by emphasizing that addiction is characterized by ambivalence and a perceived loss of self control. However, matching may not be a satisfactory platform on which to build this alternative model. Matching experiments do not provide evidence of ambivalence or perceived loss of self control.

Resolving the contradictions of addiction

Research findings on addiction are contradictory. According to biographical records and widely used diagnostic manuals, addicts use drugs compulsively, meaning that drug use is out of control and independent of its aversive consequences. This account is supported by studies that show significant heritabilities for alcoholism and other addictions and by laboratory experiments in which repeated administration of addictive drugs caused changes in neural substrates associated with reward. Epidemiological and experimental data, however, show that the consequences of drug consumption can significantly modify drug intake in addicts. The disease model can account for the compulsive features of addiction, but not occasions in which price and punishment reduced drug consumption in addicts. Conversely, learning models of addiction can account for the influence of price and punishment, but not compulsive drug taking. The occasion for this target article is that recent developments in behavioral choice theory resolve the apparent contradictions in the addiction literature. The basic argument includes the following four statements: First, repeated consumption of an addictive drug decreases its future value and the future value of competing activities. Second, the frequency of an activity is a function of its relative (not absolute) value. This implies that an activity that reduces the values of competing behaviors can increase in frequency even if its own value also declines. Third, a recent experiment (Heyman & Tanz 1995) shows that the effective reinforcement contingencies are relative to a frame of reference, and this frame of reference can change so as to favor optimal or suboptimal choice. Fourth, if the frame of reference is local, reinforcement contingencies will favor excessive drug use, but if the frame of reference is global, the reinforcement contingencies will favor controlled drug use. The transition from a global to a local frame of reference explains relapse and other compulsive features of addiction.

In search of the relevant behavioral variables

Heyman s analysis of the relevant complex behavioral variables associated with addiction is evaluated in relation to identifying the appropriate variables in behavior analysis. The model of behavioral allocation and choice known as melioration, discussed by Heyman as a way to understand the complexities of addiction, is examined and contrasted with another model of matching called ratio invariance, which is offered in this commentary as another behavioral account with a significant potential for resolving the contradictions of addiction.

Positive reinforcement, the matching law and morality

Addictive behavior has never seemed rational because it persists in spite of drastic aversive consequences. This is a particular problem for models of addiction such as operant psychology which hold that behavior is controlled by its consequences. Inspite of claims to the contrary, Heymans target article illustrates how operant psychology resolves this contradiction. By using the matching law, Heyman suggests a mechanism that explains why delayed aversive events may not control behavior, and a conceptual framework in which we can understand successful therapies.

Melioration and addiction

I discuss various theoretical issues concerning maximizing, matching, and melioration. The model of addiction based on melioration has the key feature that a reduction in drag use increases both the value of drag-taking and the value of conventional activities. I found Heyman's target article stimulating. I don't feel competent to comment on drags but I do have some thoughts, both general and specific, on the theoretical issues.

Is melioration the addiction theory of choice?

Heyman makes a convincing case that a melioration choice strategy is sufficient to produce addictive behavior. But given a plethora of addiction theories, the question is whether melioration theory is superior to rivals more sophisticated than a simple disease model or operant conditioning account. Heyman offers little direct evidence that melioration actually causes the addictions we observe.

A scientific fix for the classical account of addiction

Heyman's two crucial theses are that people over-value immediate rewards, and that addictive substances “subvert the value of competing commodities.” These perennial ideas were discussed by Plato. Whereas Heyman provides scientific clarification and support for the first, the second remains problematic. I outline how this deficiency might be remedied via evolutionary considerations.

Control versus causation of addiction

Heyman explains useful ways to bring addictive drug use under environmental control. We doubt that relapse is explained by drug features such as immediate reinforcement, clouding of judgment, and so forth. Relapse may require explanation in terms of enduring sensitization of incentive neural substrates, but even if its causal assumptions are wrong, Heyman's model makes useful predictions for behavioral control.

Self-control and impulsiveness: Resolution of apparent contradictions in choice behavior

Choosing the smaller, less delayed reinforcers of drug abuse (including cessation of withdrawal) over the larger, more delayed reinforcers of a healthy life can be described as impulsiveness (the opposite of self-control). Examination of drug abuse using a self-control/impulsiveness framework can help explain many of the puzzles about this behavior, while tying drug abuse into a large research database.

Who is at risk for addiction?

Numerous individual differences determine the effectiveness of the multiple external reinforcement contingencies that control drug seeking behavior. Differences in the capacity to frame reinforcement and also the toxic effects of drugs modulate susceptibility to addiction. As a result, not all individuals are at the same degree of risk for addiction.

Imaging neurotransmitter release by drugs of abuse

Previous studies have shown that imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) radiotracers that are specific for brain dopamine receptors can be used to indirectly image the change in the levels of neurotransmitters within the brain. Most of the studies in addiction have focused on dopamine, since the dopamine neurons that project to the striatum have been shown to play a critical role in mediating addictive behavior. These imaging studies have shown that increased extracellular dopamine produced by psychostimulants can be measured with PET and SPECT. However, there are some technical issues associated with imaging changes in dopamine, and these are reviewed in this chapter. Among these are the loss of sensitivity, the time course of dopamine pulse relative to PET and SPECT imaging, and the question of affinity state of the receptor. In addition, animal studies have shown that most drugs of abuse increase extracellular dopamine in the striatum, yet not all produce a change in neurotransmitter that can be measured. As a result, imaging with a psychostimulant has become the preferred method for imaging presynaptic dopamine transmission, and this method has been used in studies of addiction. The results of these studies suggest that cocaine and alcohol addiction are associated with a loss of dopamine transmission, and a number of studies show that this loss correlates with severity of disease.

Sensitization processes in drug addiction

In 1993, Robinson and Berridge published their first review that laid out the incentive sensitization theory of addiction (Robinson and Berridge 1993 Brain Res Rev 18:247). Its basic point is that repeated exposure to drugs of abuse causes hypersensitivity to drugs and drug-associated stimuli of the neural circuits mediating incentive salience, an important way in which motivational stimuli influence behavior. In laymen's terms, it states that this drug-induced hypersensitivity of motivational circuitry would mediate an increase in drug "wanting," thus being responsible for the dramatically exaggerated motivation for drugs displayed by addicts. This theory has been exceptionally influential, as evidenced by the fact that the original review paper about this theory (Robinson and Berridge 1993 Brain Res Rev 18:247) has been cited 2,277 times so far, and subsequent updates of this view (Robinson and Berridge 2000 Addiction 95(Suppl 2):S91; Robinson and Berridge 2001 Addiction 96:103; Robinson and Berridge 2003 Ann Rev Psychol 54:25) have been cited 274, 297, and 365 times, respectively, adding up to more than 3,200 citations within 15 years. The present chapter aims to delineate the merits and limitations of the incentive sensitization view of addiction, and whether incentive sensitization occurs in humans. We conclude that since incentive sensitization most prominently occurs after the first few drug exposures, it may represent an important initial step in the addiction process. During the expression of full-blown addiction, characterized by loss of control over drug intake and use of large quantities of drugs, the expression of incentive sensitization may be transiently suppressed. However, detoxification and the gradual disappearance of tolerance and withdrawal symptoms may unmask sensitization, which could then play an important role in the high risk of relapse.

Repeated dosing with oral cocaine in humans: assessment of direct effects, withdrawal, and pharmacokinetics

Cocaine withdrawal symptoms are thought to play a role in relapse; studies characterizing the symptomatology have yielded mixed findings. This study sought to examine the pharmacodynamic/pharmacokinetic profile of repeated high dose exposure to oral cocaine and characterize acute and protracted withdrawal in cocaine abusers. This study employed a repeated-dosing, single-blind design in which subjects (n = 9), resided for 40 days on a closed ward. They were maintained for two 4-day cocaine exposure periods (Days 1-4 & Days 9-12, cocaine 175 mg, p.o.; 5 hourly doses; 875 mg/day) separated by a 4-day matched placebo exposure period (Days 5-8). After these 12 days, an additional period of 28 days of placebo maintenance followed (Days 13-40). Test sessions were conducted during each phase; measures of mood, drug effects, sleep, pharmacokinetics, and prolactin were collected throughout the study. The dosing regimen produced cocaine plasma concentrations (Cmax of 680 ng/mL) two to threefold higher than typically seen in acute dose studies. Prototypic psychostimulant effects, including subjective ratings of euphoric effects (liking, high, good effects) and significant cardiopressor effects, were sustained during the active dosing periods, corresponding to the rise and fall of plasma cocaine. Withdrawal-like symptoms (i.e., disruptions of sleep, increased ratings of anxiety, irritability, crashing) were observed within 24-hr after cessation of dosing. Cocaine reduced prolactin acutely, but no sustained alterations were observed for this measure or for other signs or symptoms during the 28-day abstinence period. These findings indicate that exposure to controlled high doses of cocaine produces modest symptoms consistent with cocaine withdrawal within hours of cessation of dosing but provide no evidence of symptoms persisting beyond 24 hours.

Cardiovascular and subjective effects of repeated smoked cocaine administration in experienced cocaine users

Studies using rodents have shown that behavioral responses to a stimulant are enhanced when the stimulant is given within the same context as previous stimulant administrations; this increase in effect related to context is often referred to as sensitization. We examined the role of environmental stimuli in modulating the subjective and cardiovascular effects of cocaine in humans (1) within a daily "binge" and (2) after cocaine abstinence. Ten non-treatment seeking users of smoked cocaine were admitted to the hospital for 17 consecutive days. Participants smoked cocaine (25mg/dose) under two counterbalanced conditions: paired stimuli (same stimuli presented each session) and unpaired stimuli (varied stimuli presented each session). Under each stimulus condition, participants had cocaine test sessions for three consecutive days, no sessions for the next 3 days, then another cocaine test session on the following day, for a total of eight test days. Stimulus condition had no effect on cardiovascular or subjective effects so data were analyzed as a function of repeated cocaine administration over 2 weeks. Maximal ratings on "good drug" and "drug rating" subjective effects clusters decreased over days of repeated cocaine exposure. In contrast, baseline and peak heart rate and systolic pressure increased over days of repeated cocaine administration. Thus, repeated administration of smoked cocaine to experienced cocaine users resulted in increases in baseline blood pressure and heart rate and modest decreases in positive subjective effects. These data indicate modest tolerance rather than sensitization to the positive subjective effects of cocaine with repeated exposure.

Cocaine abuse and sensitization of striatal dopamine transmission: a critical review of the preclinical and clinical imaging literature

Much effort has been devoted in the preclinical addiction literature to understanding the phenomenon of sensitization, an enhanced dopaminergic response in the nucleus accumbens that occurs after repeated exposure to psychostimulant drugs. Although sensitization has been reported in preclinical studies, studies of sensitization in humans measuring behavioral and physiological responses have been mixed and inconclusive. However, imaging studies with positron emission tomography (PET) and single photon emission computed tomography (SPECT) using a stimulant challenge to induce dopamine (DA) release provide a unique opportunity to probe DA transmission in cocaine dependent human subjects. In contrast to the basic science literature that predicted sensitization, three independent cohorts have shown a blunted DA response, or the opposite of sensitization, in human cocaine dependent subjects. This article reviews the methodological differences between the preclinical and clinical PET studies that have investigated DA sensitization in order to address the discrepancy between the human and animal literature.

Conditioned cues and the expression of stimulant sensitization in animals and humans

Repeated intermittent exposure to psychostimulants can lead to long-lasting sensitization of the drugs' behavioral and biochemical effects. Such findings have figured importantly in recent theories of drug addiction proposing that sensitized nucleus accumbens (NAcc) dopamine (DA) overflow in particular acts in concert with other alterations in the neurochemistry of this nucleus to promote drug seeking and self-administration. Yet, experiments in rodents, non-human primates and humans have not always detected behavioral or biochemical sensitization following drug exposure, bringing into doubt the utility of this model. In an effort to reconcile apparent discrepancies in the literature, this review assesses conditions that might affect the expression of sensitization during testing. Specifically, the role played by conditioned cues is reviewed. A number of reports strongly support a potent and critical role for conditioned stimuli in the expression of sensitization. Findings suggest that stimuli associated either with the presence or absence of drug can respectively facilitate or inhibit sensitized responding. It is concluded that the presence or absence of such stimuli during testing for sensitization in animal and human studies could significantly affect the results obtained. It is necessary to consider this possibility especially when interpreting the results of studies that fail to observe sensitized responding.

From vice to virtue: insights from sensitization in the nonhuman primate

Repeated, intermittent administration of psychomotor stimulants, or D1 agonists in dopamine-deficient states, induces behavioral sensitization, characterized by an enhanced response to a subsequent acute low dose challenge, which may be manifested in form of altered behavior or cognitive function. Amphetamine sensitization in the nonhuman primate encompasses profound and enduring changes to similar neuronal and neurochemical substrates that occur in rodents. The process of sensitization in the monkey also results in a long-lasting depression in baseline behavioral responding, as well as emergence of hallucinatory-like behaviors reminiscent of human psychosis in response to an acute challenge. Nonhuman primates show a reduction in spine density and dendritic length in prefrontal neurons and a marked reduction in basal dopamine turnover in both prefrontal cortex and striatum. A major hallmark of amphetamine sensitization in both nonhuman primates and rodents is the manifestation of deficits in executive function and working memory which rely upon the integrity of prefrontal cortex and thereby, may yield significant insights into the cognitive dysfunction associated with addiction. Together with evidence from human and rodent studies, it can be concluded that repeated exposure to psychomotor stimulants can lead to a corruption of neuroadaptive systems in the brain by an extraordinary influence on synaptic plasticity, learning, and memory. Actively harnessing this same process by repeated, intermittent D1 agonist administration may be the key to improved working memory and decision making in addiction and other dopamine dysfunctional states, such as schizophrenia.

Conditioned and sensitized responses to stimulant drugs in humans

In animal models considerable evidence suggests that increased motivation to seek and ingest drugs of abuse are related to conditioned and sensitized activations of the mesolimbic dopamine (DA) system. Direct evidence for these phenomena in humans, though, is sparse. However, recent studies support the following. First, the acute administration of drugs of abuse across pharmacological classes increases extracellular DA levels within the human ventral striatum. Second, individual differences in the magnitude of this response correlate with rewarding effects of the drugs and the personality trait of novelty seeking. Third, transiently diminishing DA transmission in humans decreases drug craving, the propensity to preferentially respond to reward-paired stimuli, and the ability to sustain responding for future drug reward. Finally, very recent studies suggest that repeated exposure to stimulant drugs, either on the street or in the laboratory, can lead to conditioned and sensitized behavioral responses and DA release. In contrast to these findings, though, in individuals with a long history of substance abuse, drug-induced DA release is decreased. This diminished DA release could reflect two different phenomena. First, it is possible that drug withdrawal related decrements in DA cell function persist longer than previously suspected. Second, drug-paired stimuli may gain marked conditioned control over the release of DA and the expression of sensitization leading to reduced DA release when drug-related cues are absent. Based on these observations a two-factor hypothesis of the role of DA in drug abuse is proposed. In the presence of drug cues, conditioned and sensitized DA release would occur leading to focused drug-seeking behavior. In comparison, in the absence of drug-related stimuli DA function would be reduced, diminishing the ability of individuals to sustain goal-directed behavior and long-term objectives. This conditioned control of the expression of sensitized DA release could aggravate susceptibility to relapse, narrow the range of interests and perturb decision-making, accounting for a wide range of addiction related phenomena.

Cocaine sensitization and dopamine mediation of cue effects in rodents, monkeys, and humans: areas of agreement, disagreement, and implications for addiction

BACKGROUND

Sensitization of mesocorticolimbic dopamine projections has been a valuable model of neurobiological adaptation to chronic exposure to cocaine and other psychostimulants.

DISCUSSIONS

In addition to providing an explanation of exaggerated responses to drugs that might explain their increased ability to serve as reinforcers, sensitization has also been incorporated into influential theories of how drug associated cues can acquire increased salience and incentive motivation. However, almost all of the work exploring behavioral and neurochemical sensitization has been conducted in rodents. Importantly, the relatively small amount of work conducted in human and nonhuman primates differs from the rodent work in some important regards. This review will examine areas of convergence and divergence between the rodent and primate literature on sensitization and the ability of drug associated environmental cues to elicit dopamine release. The implications of this comparison for expanding addiction research beyond dopaminergic mechanisms in the striatum/nucleus accumbens will be considered.

Differential neurochemical and behavioral adaptation to cocaine after response contingent and noncontingent exposure in the rat

RATIONALE

In naive rats, passive administration of drugs of abuse preferentially increases extracellular dopamine (DA) in the nucleus accumbens (NAc) shell as compared to the core. Repeated exposure to the same drugs results in behavioral and biochemical sensitization characterized by stereotyped activity and reduction of the shell/core DA response ratio.

OBJECTIVES

The aim of this work is to study the neurochemical and behavioral effects of response-contingent vs response-noncontingent drug administration in rats, who were bilaterally implanted with chronic intracerebral guide cannulae and trained to self-administer cocaine by nose poking in daily 1-h sessions for 3 weeks (5 days/week). Nose poking in the active hole by master rats resulted in intravenous injection of cocaine (0.25 mg/kg) in master rats and in rats yoked to them. Dialysate DA was monitored before, during, and for 30 min after cocaine availability on alternate days by inserting the probe into the NAc shell and core. Stereotyped and non-stereotyped behavior was recorded during the sessions.

RESULTS

In master rats, dialysate DA increased preferentially in the NAc shell during cocaine self-administration throughout the 3 weeks of cocaine exposure. In yoked rats, DA increased preferentially in the shell but to a lesser extent than in master rats. With continued exposure to cocaine, the shell/core ratio of DA changes decreased progressively and, on the third week, was reversed so that DA increased more in the core than in the shell. Yoked rats showed a progressive and faster increase in stereotyped behaviors than master rats.

CONCLUSIONS

Response-noncontingent cocaine administration is particularly prone, compared to response-contingent administration, to induce behavioral and biochemical sensitization.

Absence of behavioral sensitization in healthy human subjects following repeated exposure to ketamine

RATIONALE

Sensitization to the effects of N-methyl-D-aspartate (NMDA) glutamate receptor antagonists is robust in animals. However, the applicability of this model to humans is unclear because it currently rests on highly confounded retrospective studies of individuals who experienced protracted psychoses following repeated binges with NMDA receptor antagonists.

OBJECTIVES

The purpose of the current study was to determine whether there was evidence of sensitization to the behavioral effects of ketamine in healthy human subjects with repeated exposure to this drug.

METHODS

Data were studied from 295 healthy human subjects who participated in one or more of 11 separate studies that involved ketamine administration over 14 years. Positive and negative symptoms (Brief Psychiatric Rating Scale: BPRS), perceptual alterations (Clinician-Administered Dissociative States Scale: CADSS), and "high" and "anxiety" states (Visual Analog Scale: VAS) that were measured in all studies were included as outcome measures.

RESULTS

After including the number of previous exposures, number of previous studies, and time since first exposure as variables, repeated exposure to ketamine did not result in increased behavioral responses, suggestive of behavioral sensitization.

CONCLUSIONS

The current data do not provide evidence that repeated exposure to ketamine, albeit limited, is associated with sensitization to the behavioral effects of ketamine.

Biological markers of cocaine addiction: implications for medications development

The search for effective medications for cocaine addiction has been elusive. The failure to find such medications so far could be due to poor understanding of the underlying biology both in the premorbid condition and following the disease state of chronic cocaine use. Population heterogeneity could be a major factor in response to medications. In an attempt to highlight the issue of biomarkers we reviewed physiological, neuroendocrine and neuroimaging studies to identify specific biological changes/markers that could be used to characterize subgroups among chronic cocaine users. Merging the biology within medications studies of cocaine abusers could prove useful for targeting specific pharmacological agents to subgroups of patients, prediction of response to medication and relapse to use.

Gamma-vinyl GABA, an irreversible inhibitor of GABA transaminase, alters the acquisition and expression of cocaine-induced sensitization in male rats

We examined the effect of (+/-)-gamma-vinyl GABA (GVG, Vigabatrin), an irreversible inhibitor of the enzyme GABA transaminase, on the acquisition and expression of cocaine-induced sensitization in albino male Sprague-Dawley rats. Animals received a single injection of 1 ml/kg i.p. of 0.9% saline or 15 mg/kg i.p. of (-)-cocaine and locomotor activity was assessed using automated locomotor cages and stereotyped behaviors were scored using a 4-point rating scale (Day 1). Subsequently, animals were given 15 mg/kg i.p. of cocaine every 48 h in their home cage for 1 week (Days 3, 5, and 7) and then given no treatment for 1 week. A challenge injection of 15 mg/kg i.p. of cocaine, but not vehicle, produced a significant increase in locomotor activity and stereotyped behaviors on Day 15 compared to animals that received cocaine on Day 1. Administration of 75 mg/kg i.p. of GVG 2.5 h before the cocaine injections did not significantly alter the acquisition of cocaine-induced locomotor sensitization. However, 150 mg/kg i.p. of GVG significantly attenuated the acquisition of cocaine-induced locomotor sensitization. Administration of 150 mg/kg i.p. of GVG 2.5 h before the cocaine challenge injection on Day 15 significantly attenuated the expression of cocaine-induced locomotor sensitization. Acquisition and expression of cocaine-induced sensitization of stereotypy was also significantly attenuated by 150 mg/kg i.p. of GVG. Since sensitization may be one of the factors involved in relapse to drug use, the present results, in combination with previous findings that GVG blocks the rewarding and incentive motivating effects of cocaine, suggest that GVG might prove useful in the treatment of cocaine addiction.

Behavioral sensitization in humans

Behavioral sensitization is the process whereby repeated, intermittent stimulant administration produces a progressively greater and enduring behavioral response. For over two decades, behavioral sensitization has been reliably demonstrated in a number of different animal species and has been proposed as a model for the development of stimulant dependence. However, the application of this model to humans is limited since there have been relatively few studies of sensitization in human subjects. Nonetheless, results from these studies suggest that, similar to animal studies, enhanced behavioral responses do occur following repeated stimulant administration that resemble sensitization. Further research is necessary to examine characteristics of sensitization in humans, including the neurobiological systems involved.

Sensitization to the cardiovascular but not subject-rated effects of oral cocaine in humans

BACKGROUND

Despite a substantial nonhuman literature in the area, few studies have experimentally evaluated the effects of repeated stimulant administration in human participants. This study examined the effects of repeated cocaine in individuals with histories of abuse and dependence.

METHODS

Twenty-two individuals with recent histories of cocaine abuse received under double-blind conditions, in pseudorandom order, four administrations of oral cocaine (150 mg [n = 14] or 200 mg [n = 8]) and two administrations of placebo. All administrations were given on separate days. Cardiovascular measures were collected and included heart rate, systolic pressure, diastolic pressure, mean arterial pressure, and pressure rate product. Subject-rated effects were assessed using the Addiction Research Center Inventory (ARCI) and a 15-item drug-effect questionnaire.

RESULTS

There were significant differences between placebo days and cocaine days in both cardiovascular and subject-rated effects. Moreover, three of five cardiovascular measures analyzed revealed a significant main effect for day of cocaine administration. A planned follow-up contrast revealed a significant increasing linear trend for each of these variables across days. No significant effects were found for day of administration for the subject-rated items.

CONCLUSIONS

These results indicate that, under controlled laboratory conditions, repeated oral cocaine administration may result in sensitization to the cardiovascular effects, but not subject-rated effects.

Patterns of waking EEG spectral power in chemically intolerant individuals during repeated chemical exposures

Previous studies indicate that low level chemical intolerance (CI) is a symptom of several different controversial conditions with neuropsychiatric features, e.g., chronic fatigue syndrome, fibromyalgia, multiple chemical sensitivity, and "Persian Gulf Syndrome". Prior studies suggest that limbic and/or mesolimbic sensitization may contribute to development of CI. The purpose of this report was to document the waking electroencephalographic (EEG) patterns of individuals with CI during chemical exposures presented over repeated sessions. Three groups of adult subjects who were recruited from the community participated in the study: self-reported CI who had made associated lifestyle changes due to their intolerance (CI/ LSC), self-reported CI who had not made such changes (CI), and normal controls without self-reported CI. Subjects underwent two sessions involving one-minute EEG recordings during exposures to low level chemical odors (a probe for limbic activation). The CI, but not the CI/ LSC, subjects had increased absolute delta power after the chemical exposures during the second, but not the first, session. The findings support the neural sensitization hypothesis for intolerance to low levels of environmental chemicals in vulnerable individuals. As in human studies of stimulant drug sensitization, those with the strongest past history with sensitizing agents may not show-term sensitization to low level exposures in the laboratory.

Progressive behavioral response to repeated d-amphetamine challenge: further evidence for sensitization in humans

BACKGROUND

Behavioral sensitization is the process whereby intermittent stimulant exposure produces a time-dependent, enduring, and progressive behavioral response. Although animal models of sensitization are well established, the phenomenon has been relatively little studied in humans. In a previous study, we reported enhanced responses following a second as compared to a first amphetamine dose in eye-blink rate and ratings of increased motor activity/energy, increased speech, and elevated mood in normal human volunteers. This current study extends those findings in a new sample of normal volunteers.

METHODS

Eleven normal human volunteers were administered three single oral doses of d-amphetamine (0.25 mg/kg) at 48-hour intervals, alternating with matched placebo in a randomized, double-blind trial. Hourly behavioral ratings included eye-blink rate, symptoms (elevated mood, increased speech, increased motor activity/energy), and subjective drug effects.

RESULTS

Eye-blink rate and increased motor activity/energy ratings progressively increased following each challenge with the third amphetamine dose response significantly greater than all other conditions 4 hours postadministration. Similar, although less pronounced, responses were observed for elevated mood and subjective drug effect.

CONCLUSIONS

These results provide further evidence for sensitization of some amphetamine-induced behaviors in human subjects.

Addiction as choice? Yes. As melioration? Maybe, maybe not

Melioration is not superior and may be inferior to other extensions of behavioral choice theory to addiction studies. Progress in the addiction field will be accelerated by the demise of the disease model rather than by attempting to resolve the contradictions it has created.

Maximization should sometimes lead to abstinence

Heyman's model, paradoxically, predicts that whereas a maximizing approach to drug choice will prevent escalation of drug use it will never yield complete abstinence. We suggest an alternative model that overcomes this difficulty by focusing on changes in drug tolerance. A small modification allows maximization to predict either abstinence or moderation (e.g., social drinking).

Which behavioral consequences matter? The importance of frame of reference in explaining addiction

The target article emphasizes the relationship between a matching law-based theory of addiction and the disease model of addiction. In contrast, this response emphasizes the relationship between the matching law theory and other behavioral approaches to addiction. The basic difference, I argue, is that the matching law specifies that choice is governed by local reinforcement rates. In contrast, economics says that overall reinforcement rate controls choice, and for other approaches there are other measures or no clear prediction at all. The response also differs from the target article in that there is more emphasis on the finding that stimulus conditions determine whether choice is under local or overall reinforcement rate control.

The contradiction unresolved

Extreme sensations – thirst, pain – can focus attention on local consequences at the expense of the overall, perhaps for good evolutionary reasons. Maybe the same phenomenon evolves from prolonged use of addictive substances. The matching law explains mistaken choice, not how a person who has confronted personal catastrophe manages to ignore it in making a locally induced choice.

Addiction is not as puzzling as it seems

Heyman's target article seeks to resolve the apparent paradox surrounding the issue of control in relation to addictive behaviour. The present commentary argues that addiction is in fact less paradoxical and more easily understandable than Heyman supposes. A fully satisfactory model of addiction, however, requires a more multifaceted approach than that provided by the type of behavioural choice theory favoured by Heyman.

The Janus faces of addiction

Heyman proposes that external stimuli can promote a switch from a local to a global frame of reference for evaluating the consequences of behavior and that such a change might be critical to breaking the grip of drag addiction. Could incentive stimuli promote a switch in the opposite direction and thus contribute to relapse in the recovered addict?

Relationship between melioration and the controlling variables

Knowledge of specific instances of melioration does not imply knowledge of the variables controlling behavior. Drug self-administration has been successfully analyzed in terms of standard variables such as the immediacy and magnitude of the drug reinforcer. An appeal to melioration may not be necessary.

The behavioral economics of addiction: A comprehensive alternative

Heyman's target article makes a strong case for a behavioral approach to addiction, yet some important assumptions require justification, and promising behavioral alternatives to the author's melioration approach should be considered. In particular, the behavioral economic approach to addiction appears well developed and comprehensive. How does the melioration approach complement or improve on a behavioral economic account?

The ameliorating addict: An illusion reviewed

It is difficult to accommodate compulsive drug use that destroys regulation by consequences in a unitary framework of melioration or maximization, especially as both are strategies of behavioral regulation. The “contradiction” of addictive behavior despite aversive consequences is an illusory issue because consequences that do not decrease addictive behavior cannot be considered punishing.

Addiction requires philosophical explanation, not mere redescription

Heyman's model explains the irrationality of addictive behavior, but it does not satisfactorily answer the question of whether this behavior is voluntary because it does not address the issue of the choice of preference functions. Furthermore, although Heyman disconfirms the disease model of addiction, this does not resolve the issue of whether addiction should be classified as a mental illness.

Matching and melioration as accounts of reinforcement and drug addiction

Heyman's view that addiction can be viewed as a natural outcome predictable by melioration and the matching law is provocative. Remaining to be explained more fully, however, are exactly how his view is an improvement on other reinforcement-based accounts. Included in these elaborations should be an account of how different “bookkeeping schemes” are developed and controlled and what new approaches to treatment and prevention of drug addiction are indicated.

How do people choose between local and global bookkeeping?

The matching law accounts for both addictive behavior and the usefulness of a person's evaluating choices in overall categories. To explain why overall bookkeeping, once learned, does not easily win out over local bookkeeping, another implication of matching is needed: intertemporal bargaining. The role of melioration, though probably important for new addiction is separate.