Osnat Ben-Shahar

Compulsive Drug‐Seeking Behavior and Relapse

Abstract: The development of addiction and vulnerability to relapse following withdrawal is proposed to be the result of neuroadaptive processes within the central nervous system that oppose the acute reinforcing actions of drugs of abuse. These changes lead to impairment in the mechanisms that mediate positive reinforcement and the emergence of affective changes such as anxiety, dysphoria, and depression during withdrawal. Considerable evidence exists implicating perturbations in DA and 5‐HT transmission in the nucleus accumbens—neurochemical systems that are activated by cocaine and ethanol self‐administration and deficient during withdrawal—as potential substrates for these affective changes. In addition, growing evidence suggests that enhanced CRF release in the central nucleus of the amygdala represents a mechanism underlying the anxiogenic and stress‐like consequences of withdrawal that are common to all drugs of abuse. A growing body of evidence also implicates dysregulation of the non‐neuroendocrine CRF stress system within the central nucleus of the amygdala as a common factor in the anxiogenic and aversive consequences of withdrawal from drugs of abuse. Moreover, a possible link may exist between long‐lasting abnormalities in CRF function in the CeA and vulnerability to relapse during protracted abstinence. Another presumably critical element contributing to the chronic relapsing nature of drug addiction is the learned responses to drug‐related stimuli. The long‐lasting efficacy of drug‐ and alcohol‐associated contextual stimuli in eliciting drug‐seeking behavior in animal models of relapse resembles the endurance of conditioned cue reactivity and cue‐induced cocaine craving in humans and confirms a significant role of learning factors in the long‐lasting addictive potential of cocaine. With cocaine, D1‐dependent neural mechanisms within the medial prefrontal cortex and basolateral amygdala may be important substrates for the motivating effects of drug‐related environmental stimuli. With ethanol, available data suggest a role for opioid receptors in the mediation of conditioned drug‐seeking behavior. Finally, conditioning factors (i.e., exposure to drug‐associated stimuli) and stress can interact to augment vulnerability to relapse. This finding emphasizes that it will be important to consider the simultaneous effects of multiple environmental triggers for relapse in the development of treatment and medication strategies.

Enduring Resistance to Extinction of Cocaine-Seeking Behavior Induced by Drug-Related Cues

The conditioning of cocaines pharmacological actions with environmental stimuli is thought to be a critical factor in long-lasting relapse risk associated with cocaine addiction. To study the significance of environmental stimuli in enduring vulnerability to relapse, the resistance to extinction of drug-seeking behavior elicited by a cocaine-related stimulus was examined. Male Wistar rats were trained to associate discriminative stimuli (SD) with the availability of intravenous cocaine (S+) vs. the availability of non-rewarding (S−) saline solution, and then placed on extinction conditions during which intravenous solutions and SD were withheld. The rats were then presented with the S+ or S− alone in 60-min reinstatement sessions conducted at 3-day intervals. To examine the long-term persistence of the motivating effects of the cocaine S+, a subgroup of rats was re-tested following an additional three months of abstinence during which time the rats remained confined to their home cages. Re-exposure to the cocaine S+ selectively elicited robust responding at the previously active lever. The efficacy and selectivity of this stimulus to elicit responding remained unaltered throughout a 34-day phase of repeated testing as well as following the additional extended abstinence period. In pharmacological tests, conducted in a separate group of rats, the dopamine (DA) D1 antagonist SCH 39166 (10 μg/kg), the D2/3 antagonist nafadotride (1 mg/kg), and the D2/3 agonist PD 128907 (0.3 mg/kg) suppressed the cue-induced response reinstatement while the D1 agonist SKF 81297 (1.0 mg/kg) produced a variable behavioral profile attenuating cue-induced responding in some rats while exacerbating this behavior in others. The results suggest that the motivating effects of cocaine-related stimuli are highly resistant to extinction. The undiminished efficacy of the cocaine S+ to induce drug-seeking behavior both with repeated testing and following long-term abstinence parallels the long-lasting nature of conditioned cue reactivity and cue-induced cocaine craving in humans, and confirms a significant role of learning factors in long-lasting vulnerability to relapse associated with cocaine addiction. Finally, the results support a role of DA neurotransmission in cue-induced cocaine-seeking behavior.

The transition from controlled to compulsive drug use is associated with a loss of sensitization

Rats provided limited daily access to cocaine (1 h) maintain stable levels of drug self-administration over time while those switched to longer access (6 h or more) exhibit escalating patterns of drug intake. These results are reminiscent of human recreational and compulsive drug-taking behavior, respectively. We found that the brains of cocaine-self-administering rats were also qualitatively different in subjects having experienced 6-h (Coc6h) daily access compared to 1-h (Coc1h) access. Fourteen days after an eight-day protocol of cocaine self-administration, all subjects received one infusion of cocaine. Coc1h animals showed enhanced c-Fos reactivity in dopaminergic mesocorticolimbic brain regions and a sensitized locomotor response to IV cocaine. In contrast, both the neural and behavioral sensitization to cocaine was diminished in Coc6h animals. These data suggest that the transition to escalating patterns of drug use is associated with neuroadaptive changes that counteract those initially associated with controlled stable patterns of drug use.

Extended daily access to cocaine results in distinct alterations in Homer 1b/c and NMDA receptor subunit expression within the medial prefrontal cortex

Human cocaine addicts show altered function within the basal ganglia and the medial prefrontal cortex (mPFC) and altered glutamate function within these areas is postulated to be critical for cocaine addiction. The present project utilized a highly valid animal model of cocaine addiction, to test whether excessive use of cocaine alters glutamate function within these brain areas. Rats were trained to lever‐press for i.v. saline vehicle or cocaine (0.25 mg/infusion) over seven 1‐h daily sessions, after which, saline controls and half of cocaine self‐administering animals (brief access condition) received 10 more 1‐h daily sessions, whereas the remaining cocaine animals received 10 additional 6‐h daily sessions (extended access condition). One, 14, or 60 days after the last self‐administration session, animals were sacrificed. Tissue samples from the ventral tegmental area (VTA), nucleus accumbens (N.Acc) core and shell, and mPFC were analyzed by immunoblotting for expression of Homer1b/c, Homer2a/b, mGluR1, mGluR5, NR2a, and NR2b subunits of the NMDA receptor. Brief and extended access to cocaine failed to alter protein levels within the VTA, and produced transient and similar changes in N.Acc protein expression, which were more pronounced in the core subregion. In contrast, extended access to cocaine resulted in distinct and long lasting alterations of protein expression within the mPFC that included: increased levels of Homer1b/c at 1 day, NR2b at 14 days, and NR2a at 60 days, of withdrawal. These data support the notion that altered NMDA function within the mPFC may contribute, in part, to the transition to excessive uncontrolled consumption of cocaine. Synapse 63:598–609, 2009.

Changes in levels of D1, D2, or NMDA receptors during withdrawal from brief or extended daily access to IV cocaine

We previously reported that brief (1 h), but not extended (6 h), daily access to cocaine results in a sensitized locomotor response to cocaine and in elevated c-Fos immunoreactivity and DAT binding in the nucleus accumbens (N.Acc) core. In order to better our understanding of the neural adaptations mediating the transition from controlled drug use to addiction, the current experiments were set to further explore the neural adaptations resulting from these two access conditions. Rats received either brief daily access to saline or cocaine, or brief daily access followed by extended daily access to cocaine. Subjects were then sacrificed either 20 min, or 14 or 60 days, after the last self-administration session. Samples of the ventral tegmental area (VTA), N.Acc core and shell, dorsal striatum, and medial prefrontal cortex (mPFC) were taken for analysis of D1 ([3H]SCH-23390), D2 ([3H]Spiperone), and NMDA ([3H]MK-801) receptor binding (using the method of receptor autoradiography). At 20 min into withdrawal, D2 receptors were elevated and NMDA receptors were reduced in the mPFC of the brief access animals while D1 receptors were elevated in the N.Acc shell of the extended access animals, compared to saline controls. D2 receptors were reduced in the N.Acc shell of the brief access animals compared to saline controls after 14 days, and compared to extended access animals after 60 days of withdrawal. In summary, extended access to cocaine resulted in only transient changes in D1 receptors binding. These results suggest that the development of compulsive drug use is largely unrelated to changes in total binding of D2 or NMDA receptors.

Deficits in Ventromedial Prefrontal Cortex Group 1 Metabotropic Glutamate Receptor Function Mediate Resistance to Extinction during Protracted Withdrawal from an Extensive History of Cocaine Self-Administration

Anomalies in prefrontal cortex (PFC) function are posited to underpin difficulties in learning to suppress drug-seeking behavior during abstinence. Because group 1 metabotropic glutamate receptors (mGluRs) regulate drug-related learning, we assayed the consequences of extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) on the PFC expression of group 1 mGluRs and the relevance of observed changes for cocaine seeking. After protracted withdrawal, cocaine-experienced animals exhibited a time-dependent intensification of cue-induced cocaine-seeking behavior and an impaired extinction of this behavior. These behavioral phenomena were associated with a time-dependent reduction in mGluR1/5 expression within ventromedial PFC (vmPFC) of cocaine-experienced animals exposed to extinction testing but not in untested ones. Interestingly, pharmacological manipulations of vmPFC mGluR1/5 produced no immediate effects on cue-induced cocaine-seeking behavior but produced residual effects on a subsequent test for cocaine seeking. At 3 d withdrawal, cocaine-experienced rats infused intra-vmPFC with mGluR1/5 antagonists, either before or after an initial test for cocaine seeking, persisted in their cocaine seeking akin to cocaine-experienced rats in protracted withdrawal. Conversely, cocaine-experienced rats infused with an mGluR1/5 agonist before the initial test for cocaine-seeking at 30 d withdrawal exhibited a facilitation of extinction learning. These data indicate that cue-elicited deficits in vmPFC group 1 mGluR function mediate resistance to extinction during protracted withdrawal from a history of extensive cocaine self-administration and pose pharmacological stimulation of these receptors as a potential approach to facilitate learned suppression of drug-seeking behavior that may aid drug abstinence.

Heightened drug-seeking motivation following extended daily access to self-administered cocaine.

Rats allowed extended daily access (6 h) to cocaine, consume high doses of the drug and escalate their cocaine intake over days, resembling the pattern of cocaine use seen in human addicts. The current study was designed to test whether such animals would also demonstrate the heightened motivation to seek cocaine seen in human addicts. Rats were trained to lever press for i.v. cocaine (0.25 mg/infusion) over a 5-day period of 1 h sessions. Subjects were then assigned to either a brief-access (1 h/day) or an extended-access condition for an additional 10 days. Control rats lever pressed for i.v. saline. Following the final self-administration session animals were tested for their motivation to receive cocaine in an operant runway apparatus. Extended-access animals exhibited significantly higher motivation for cocaine in the runway (where they received 1.0 mg/kg cocaine i.v. upon goal-box entry) as was evident by faster run times and less ambivalence about entering the goal box (i.e. retreat behavior) than either brief-access or control subjects. Brief and extended-access animals, tested in the Elevated Plus Maze, exhibited comparable and significant increases in anxiety following a single 1.0 mg/kg i.v. injection of cocaine, as compared to saline control animals that were challenged with i.v. saline infusion. Together, these data suggest that extended access to cocaine results in an especially high motivation for the drug that is not accounted for by reductions in the anxiogenic properties of cocaine.

Prolonged daily exposure to IV cocaine results in tolerance to its stimulant effects

We have previously shown that 1 h, but not 6 h, of daily access to i.v. cocaine induces a sensitized response to i.v. cocaine challenge after 14 days of withdrawal. Here we tried to replicate these results using an i.p. cocaine challenge and adding a group of animals that had 1 h daily access to cocaine, but maintained levels of administration comparable to that of saline animals (i.e. a Coc group). Since addiction-associated neuroadaptations are particularly long lasting, we also tested the response to cocaine challenge after a longer withdrawal period of 60 days. Rats had daily access to i.v. self-administered saline or cocaine for 1 h (Coc1h), or to cocaine for 6 h (Coc6h) over 8 days. Subsequently, after 14 days of withdrawal only Coc animals showed a sensitized locomotor response to cocaine challenge administered i.p. After 60 days of withdrawal, i.p. cocaine failed to produce a sensitized response in Coc1h animals and produced a tolerant response in Coc6h animals. The present data support the notion that 6 h of daily access to cocaine leads to different neuroadaptations than those resulting from 1 h of daily access to the drug. In addition, these data further demonstrate a dissociation between sensitization and addiction to cocaine.

Imbalances in Prefrontal Cortex CC-Homer1 versus CC-Homer2 Expression Promote Cocaine Preference

Homer postsynaptic scaffolding proteins regulate forebrain glutamate transmission and thus, are likely molecular candidates mediating hypofrontality in addiction. Protracted withdrawal from cocaine experience increases the relative expression of Homer2 versus Homer1 isoforms within medial prefrontal cortex (mPFC). Thus, this study used virus-mediated gene transfer strategies to investigate the functional relevance of an imbalance in mPFC Homer1/2 expression as it relates to various measures of sensorimotor, cognitive, emotional and motivational processing, as well as accompanying alterations in extracellular glutamate in C57BL/6J mice. mPFC Homer2b overexpression elevated basal glutamate content and blunted cocaine-induced glutamate release within the mPFC, whereas Homer2b knockdown produced the opposite effects. Despite altering mPFC glutamate, Homer2b knockdown failed to influence cocaine-elicited conditioned place preferences, nor did it produce consistent effects on any other behavioral measures. In contrast, elevating the relative expression of Homer2b versus Homer1 within mPFC, by overexpressing Homer2b or knocking down Homer1c, shifted the dose-response function for cocaine-conditioned reward to the left, without affecting cocaine locomotion or sensitization. Intriguingly, both these transgenic manipulations produced glutamate anomalies within the nucleus accumbens (NAC) of cocaine-naive animals that are reminiscent of those observed in cocaine experienced animals, including reduced basal extracellular glutamate content, reduced Homer1/2 and glutamate receptor expression, and augmented cocaine-elicited glutamate release. Together, these data provide novel evidence in support of opposing roles for constitutively expressed Homer1 and Homer2 isoforms in regulating mPFC glutamate transmission in vivo and support the hypothesis that cocaine-elicited increases in the relative amount of mPFC Homer2 versus Homer1 signaling produces abnormalities in NAC glutamate transmission that enhance vulnerability to cocaine reward.

Estrogen abolishes latent inhibition in ovariectomized female rats

Estrogen is frequently prescribed as a method of birth control and as hormone replacement therapy for post-menopausal women with varied effects on cognition. Here the effects of estrogen on attention were examined using the latent inhibition (LI) behavioral paradigm. Ovariectomized (OVX) female rats were given either estrogen benzoate (EB, 10 or 100 microg/ml/kg; SC) or sesame oil vehicle. Males and OVX females receiving vehicle displayed normal LI. In contrast, LI was abolished in OVX females receiving EB. The lack of LI in OVX females receiving EB was a result of low suppression ratios, reflecting strong conditioning between the tone and the shock in these subjects even if they were pre-exposed to the tone. Thus, estrogen impaired the ability of OVX females to ignore irrelevant stimuli. Since different cognitive tasks vary in their required ability to ignore irrelevant stimuli, these results may account for some of the variations in the current literature on estrogen and cognition.