Anne-Noël Samaha

Continuous, but not Intermittent, Antipsychotic Drug Delivery Intensifies the Pursuit of Reward Cues

Chronic exposure to antipsychotic medications can persistently change brain dopamine systems. Most studies on the functional significance of these neural changes have focused on motor behavior and few have addressed how long-term antipsychotic treatment might influence dopamine-mediated reward function. We asked, therefore, whether a clinically relevant antipsychotic treatment regimen would alter the incentive motivational properties of a reward cue. We assessed the ability of a Pavlovian-conditioned stimulus to function as a conditioned reward, as well as to elicit approach behavior in rats treated with haloperidol, either continuously (achieved via subcutaneous osmotic minipump) or intermittently (achieved via daily subcutaneous injections). Continuous, but not intermittent, treatment enhanced the ability of amphetamine to potentiate the conditioned reinforcing effects of a cue associated with water. This effect was not related to differences in the ability to attribute predictive value to a conditioned stimulus (as measured by conditioned approach behavior), but was potentially linked to the development of behavioral supersensitivity to amphetamine and to augmented amphetamine-induced immediate early-gene expression (c-fos and Nur77) in dorsal striatopallidal and striatonigral cells. By enhancing the ability of reward cues to control behavior and by intensifying dopamine-mediated striatopallidal and striatonigral cell activity, standard (ie, continuous) antipsychotic treatment regimens might exacerbate drug-seeking and drug-taking behavior in schizophrenia. Achieving regular but transiently high antipsychotic levels in the brain (as modeled in the intermittent condition) might be a viable option to prevent these changes. This possibility should be explored in the clinic.

Prior Haloperidol, but not Olanzapine, Exposure Augments the Pursuit of Reward Cues: Implications for Substance Abuse in Schizophrenia

Drug abuse and addiction are excessively common in schizophrenia. Chronic antipsychotic treatment might contribute to this comorbidity by inducing supersensitivity within the brains dopamine system. Dopamine supersensitivity can enhance the incentive motivational properties of reward cues, and reward cues contribute to the maintenance and severity of drug addiction. We have shown previously that rats withdrawn from continuous haloperidol (HAL) treatment (via subcutaneous minipump) develop dopamine supersensitivity and pursue reward cues more vigorously than HAL-naive rats following an amphetamine (AMPH) challenge. Atypical antipsychotic drugs are thought to be less likely than typicals to produce dopamine supersensitivity. Thus, we compared the effects of HAL and the atypical antipsychotic olanzapine (OLZ) on the pursuit of reward cues. Rats were trained to associate a light-tone cue with water then treated with HAL or OLZ. Following antipsychotic withdrawal, we assessed AMPH-induced enhancement of lever pressing for the cue. Withdrawal from HAL, but not from OLZ, enhanced this effect. HAL, but not OLZ, also enhanced AMPH-induced psychomotor activation and c-fos mRNA expression in the caudate-putamen. Thus, prior HAL, but not OLZ, enhanced conditioned reward following an AMPH challenge, and this was potentially linked to enhanced behavioral sensitivity to AMPH and AMPH-induced engagement of the caudate-putamen. These findings suggest that HAL, but not an atypical like OLZ, modifies reward circuitry in ways that increase responsiveness to reward cues. Because enhanced responsiveness to reward cues can promote drug-seeking behavior, it should be investigated whether atypical antipsychotics might be a preferential option in schizophrenic patients at risk for drug abuse or addiction.

Can antipsychotic treatment contribute to drug addiction in schizophrenia

Individuals with schizophrenia are at very high risk for drug abuse and addiction. Patients with a coexisting drug problem fare worse than patients who do not use drugs, and are also more difficult to treat. Current hypotheses cannot adequately account for why patients with schizophrenia so often have a co-morbid drug problem. I present here a complementary hypothesis based on evidence showing that chronic exposure to antipsychotic medications can induce supersensitivity within the brains dopamine systems, and that this in turn can enhance the rewarding and incentive motivational effects of drugs and reward cues. At the neurobiological level, these effects of antipsychotics are potentially linked to antipsychotic-induced increases in the striatal levels of dopamine D2 receptors and D2 receptors in a high-affinity state for dopamine, particularly at postsynaptic sites. Antipsychotic-induced dopamine supersensitivity and enhanced reward function are not inevitable consequences of prolonged antipsychotic treatment. At least two parameters appear to promote these effects; the use of antipsychotics of the typical class, and continuous rather than intermittent antipsychotic exposure, such that silencing of dopaminergic neurotransmission via D2/3 receptors is unremitting. Thus, by inducing forms of neural plasticity that facilitate the ability of drugs and reward cues to gain control over behaviour, some currently used treatment strategies with typical antipsychotics might contribute to compulsive drug seeking and drug taking behaviours in vulnerable schizophrenia patients.

The speed of cocaine delivery determines the subsequent motivation to self-administer the drug.

The rapid delivery of drugs of abuse to the brain is associated with an increased likelihood and severity of addiction. Here we evaluated the hypothesis that rapidly delivered cocaine facilitates the addiction process by promoting the development of enhanced motivation for the drug. Rats lever-pressed for cocaine delivered intravenously over 5 or 90 s under fixed ratio (FR) during 6-h sessions. The motivation for cocaine was subsequently assessed using a progressive ratio (PR) schedule, where each successive drug injection cost an exponentially greater number of lever presses, until the cessation of responding. Throughout all self-administration sessions, all rats could only take one injection every 90 s. The 5-s groups self-administered more drug than the 90-s groups across the FR sessions. Under PR, animals that had chronically self-administered rapidly delivered cocaine took more cocaine across a range of doses and regardless of whether the drug was delivered over 5 or 90 s during PR testing. The speed of delivery also determined the long-term neurobiological impact of cocaine. Fourteen days following cocaine withdrawal, caudate-putamen D2 levels were decreased only in the 90-s rats, and quinpirole-mediated Gαi/o-protein activation was increased to a greater extent in the 90- vs 5-s rats. Thus, rapid delivery promotes the pursuit of cocaine in the face of rising costs and alters cocaine-induced changes in striatal D2 receptor number and function. As such, rapidly delivered cocaine might facilitate addiction because it more readily alters brain motivation circuits in ways that contribute to the compulsive pursuit of the drug.

Intermittent intake of rapid cocaine injections promotes robust psychomotor sensitization, increased incentive motivation for the drug and mGlu2/3 receptor dysregulation

&NA; The choice between smoking, injecting or swallowing a drug influences the risk of addiction, as this determines both how much drug gets into the brain and how fast. Most animal studies on addiction focus on how much drug it takes to produce pathological drug use. How fast drugs get to the brain is generally ignored. A few studies have examined the influence of the speed of drug onset, but speed varied along with cumulative intake. Here we held average cumulative intake constant and determined whether variation in the speed of cocaine onset alone predicts outcome. Two groups of rats self‐administered intravenous cocaine (0.25 mg/kg/injection) during daily sessions. Cocaine was available intermittently during each session. This produces the spikes and troughs in brain levels of cocaine thought to model how addicts take the drug. To vary the speed of cocaine onset, each injection was delivered over 5 s to one group, and over 90 s to the other. Average cumulative cocaine intake was the same in the two groups. However, rapid injections promoted robust psychomotor sensitization and potentiated incentive motivation for cocaine (0.063–0.25 mg/kg/injection). This addiction‐relevant phenotype was accompanied by enhanced functional activity of metabotropic glutamate group II receptors (mGluR2/3s) in the prelimbic cortex and nucleus accumbens. Pharmacological activation of mGluR2/3s with LY379268 also preferentially decreased the motivation to take cocaine in rats previously exposed to rapid drug injections. Thus, varying the speed of drug onset can be used to parse the neurobiology of addiction from that of mere drug taking. HighlightsCocaine is more addictive when it enters the brain rapidly.Rats were given intermittent access to rapid or slower i.v. cocaine injections.Rapid injections evoked locomotor sensitization and greater motivation for cocaine.Rapid injections led to mGluR2/3 dysregulation in prelimbic cortex and accumbens.Activating mGluR2/3s reduced the motivation for cocaine in rapid injection‐rats.

Cues Paired with either Rapid or Slower Self- Administered Cocaine Injections Acquire Similar Conditioned Rewarding Properties

The faster drugs of abuse reach the brain, the more addictive they can be. It is not known why this is. Environmental stimuli associated with drugs can promote the development and persistence of addiction by invigorating and precipitating drug-seeking behaviour. We determined, therefore, whether cues associated with the self-administration of rapidly delivered cocaine (injected intravenously over 5 versus 90 seconds) would acquire greater conditioned rewarding properties, as assessed by the performance of an operant response reinforced solely by the cues. Rats nose-poked for intravenous cocaine infusions delivered either over 5 or 90 seconds. Discrete visual cues accompanied each infusion. The rats could then press a lever to obtain the cues—now a conditioned reward—or an inactive lever. Rats in both the 5- and 90-second groups pressed more on the active versus inactive lever following extensive (24 sessions) but not following limited (3 sessions) self-administration training. There were no group differences in this behaviour. Following withdrawal from cocaine self-administration, lever discrimination progressively abated in both groups and was lost by withdrawal day 30. However, the rewarding properties of the cues were not “forgotten” because on withdrawal days 32–33, amphetamine selectively enhanced active-lever pressing, and did so to a similar extent in both groups. Thus, cues paired with rapid or slower cocaine delivery acquire similar conditioned rewarding properties. We conclude, therefore, that the rapid delivery of cocaine to the brain promotes addiction by mechanisms that might not involve a greater ability of drug cues to control behaviour.

Antipsychotic treatment leading to dopamine supersensitivity persistently alters nucleus accumbens function

Chronic exposure to some antipsychotic medications can induce supersensitivity to dopamine receptor stimulation. This is linked to a worsening of clinical outcome and to antipsychotic treatment failure. Here we investigated the role of striatal subregions [nucleus accumbens (NAc) and caudate-putamen (CPu)] in the expression of antipsychotic-induced dopamine supersensitivity. We treated rats with haloperidol (HAL) or olanzapine (OLZ), using regimens that achieve clinically relevant kinetics of striatal D2 receptor occupancy. Under these conditions, HAL produces dopamine supersensitivity whereas OLZ does not. We then assessed behaviors evoked by the dopamine agonist amphetamine (AMPH). We either injected AMPH into the striatum or inhibited striatal function with microinjections of GABA receptor agonists prior to injecting AMPH systemically. HAL-treated rats were dopamine supersensitive, as indicated by sensitization to systemic AMPH-induced potentiation of both locomotor activity and operant responding for a conditioned reward (CR). Intra-CPu injections of AMPH had no effect on these behaviors, in any group. Intra-NAc injections of AMPH enhanced operant responding for CR in OLZ-treated and control rats, but not in HAL-treated rats. In HAL-treated rats, inhibition of the NAc also failed to disrupt systemic AMPH-induced potentiation of operant responding for CR. Furthermore, while intra-NAc AMPH enhanced locomotion in both HAL-treated and control animals, inhibition of the NAc disrupted systemic AMPH-induced locomotion only in control rats. Thus, antipsychotic-induced dopamine supersensitivity persistently disrupts NAc function, such that some behaviors that normally depend upon NAc dopamine no longer do so. This has implications for understanding dysfunctions in dopamine-mediated behaviors in patients undergoing chronic antipsychotic treatment.

5-HT2 receptors modulate the expression of antipsychotic-induced dopamine supersensitivity

Antipsychotic treatment can produce supersensitivity to dopamine receptor stimulation. This compromises the efficacy of ongoing treatment and increases the risk of relapse to psychosis upon treatment cessation. Serotonin 5-HT2 receptors modulate dopamine function and thereby influence dopamine-dependent responses. Here we evaluated the hypothesis that 5-HT2 receptors modulate the behavioural expression of antipsychotic-induced dopamine supersensitivity. To this end, we first treated rats with the antipsychotic haloperidol using a clinically relevant treatment regimen. We then assessed the effects of a 5-HT2 receptor antagonist (ritanserin; 0.01 and 0.1mg/kg) and of a 5-HT2A receptor antagonist (MDL100,907; 0.025-0.1mg/kg) on amphetamine-induced psychomotor activity. Antipsychotic-treated rats showed increased amphetamine-induced locomotion relative to antipsychotic-naïve rats, indicating a dopamine supersensitive state. At the highest dose tested (0.1mg/kg for both antagonists), both ritanserin and MDL100,907 suppressed amphetamine-induced locomotion in antipsychotic-treated rats, while having no effect on this behaviour in control rats. In parallel, antipsychotic treatment decreased 5-HT2A receptor density in the prelimbic cortex and nucleus accumbens core and increased 5-HT2A receptor density in the caudate-putamen. Thus, activation of either 5-HT2 receptors or of 5-HT2A receptors selectively is required for the full expression of antipsychotic-induced dopamine supersensitivity. In addition, antipsychotic-induced dopamine supersensitivity enhances the ability of 5-HT2/5-HT2A receptors to modulate dopamine-dependent behaviours. These effects are potentially linked to changes in 5-HT2A receptor density in the prefrontal cortex and the striatum. These observations raise the possibility that blockade of 5-HT2A receptors might overcome some of the behavioural manifestations of antipsychotic-induced dopamine supersensitivity.

High and escalating levels of cocaine intake are dissociable from subsequent incentive motivation for the drug in rats

RationaleTaking high and increasing amounts of cocaine is thought to be necessary for the development of addiction. Consequently, a widely used animal model of drug self-administration involves giving animals continuous drug access during long sessions (LgA), as this produces high and escalating levels of intake. However, human cocaine addicts likely use the drug with an intermittent rather than continuous pattern, producing spiking brain cocaine levels.ObjectivesUsing an intermittent-access (IntA) cocaine self-administration procedure in rats, we studied the relationship between escalation of cocaine intake and later incentive motivation for the drug, as measured by responding under a progressive ratio schedule of cocaine reinforcement.ResultsFirst, under IntA, rats escalated their cocaine use both within and between sessions. However, escalation did not predict later incentive motivation for the drug. Second, incentive motivation for cocaine was similar in IntA-rats limited to low- and non-escalating levels of drug intake (IntA-Lim) and in IntA-rats that took high and escalating levels of drug. Finally, IntA-Lim rats took much less cocaine than rats given continuous drug access during each self-administration session (LgA-rats). However, IntA-Lim rats later responded more for cocaine under a progressive ratio schedule of reinforcement.ConclusionsTaking large and escalating quantities of cocaine does not appear necessary to increase incentive motivation for the drug. Taking cocaine in an intermittent pattern—even in small amounts—is more effective in producing this addiction-relevant change. Thus, beyond the amount of drug taken, the temporal kinetics of drug use predict change in drug use over time.

Varying the rate of intravenous cocaine infusion influences the temporal dynamics of both drug and dopamine concentrations in the striatum

The faster drugs of abuse reach the brain, the greater is the risk of addiction. Even small differences in the rate of drug delivery can influence outcome. Infusing cocaine intravenously over 5 vs. 90–100 s promotes sensitization to the psychomotor and incentive motivational effects of the drug and preferentially recruits mesocorticolimbic regions. It remains unclear whether these effects are due to differences in how fast and/or how much drug reaches the brain. Here, we predicted that varying the rate of intravenous cocaine infusion between 5 and 90 s produces different rates of rise of brain drug concentrations, while producing similar peak concentrations. Freely moving male Wistar rats received acute intravenous cocaine infusions (2.0 mg/kg/infusion) over 5, 45 and 90 s. We measured cocaine concentrations in the dorsal striatum using rapid‐sampling microdialysis (1 sample/min) and high‐performance liquid chromatography‐tandem mass spectrometry. We also measured extracellular concentrations of dopamine and other neurochemicals. Regardless of infusion rate, acute cocaine did not change concentrations of non‐dopaminergic neurochemicals. Infusion rate did not significantly influence peak concentrations of cocaine or dopamine, but concentrations increased faster following 5‐s infusions. We also assessed psychomotor activity as a function of cocaine infusion rate. Infusion rate did not significantly influence total locomotion, but locomotion increased earlier following 5‐s infusions. Thus, small differences in the rate of cocaine delivery influence both the rate of rise of drug and dopamine concentrations, and psychomotor activity. A faster rate of rise of drug and dopamine concentrations might be an important issue in making rapidly delivered cocaine more addictive.