Conditioned inhibition of amphetamine sensitization.

Repeated intermittent exposure to psychostimulants, such as amphetamine, leads to a progressive enhancement of the drug's ability to increase both behavioral and brain neurochemical responses. The expression of these enhancements, known as sensitization, can be regulated by Pavlovian conditioned stimuli. Cues that are associated with drug experience can facilitate sensitization so that it only occurs in the presence of these stimuli (context-specific sensitization). In contrast, cues that are explicitly related to the absence of drugs (conditioned inhibitors) can prevent the expression of sensitization. We hypothesized that disrupting conditioned inhibition would enable amphetamine sensitization in new contexts. Using male Sprague Dawley rats and a two-context amphetamine conditioning procedure, we found that extinguishing amphetamine experience in one environment led to the loss of conditioned inhibition in a separate context. Thus, amphetamine-induced sensitized locomotion, as well as both enhanced dopamine and glutamate neurotransmission in the nucleus accumbens, were observed in a context where the drug was never experienced before. A similar loss of contextual control of sensitization was seen after using baclofen/muscimol microinjections to transiently inhibit the medial prefrontal cortex, basolateral amygdala, or ventral subiculum of the hippocampus. In other words, compared to control infusions, these intracranial injections of GABA-receptor agonists were able to block conditioned inhibitors from preventing the expression of sensitized locomotion. Together, these findings reveal the importance of conditioned inhibitors for regulating addiction-like behavior. The results suggest that dopaminergic and glutamatergic brain circuitry controls the context-specific expression of amphetamine sensitization.

Are Cocaine-Seeking "Habits" Necessary for the Development of Addiction-Like Behavior in Rats?

Drug self-administration models of addiction typically require animals to make the same response (e.g., a lever-press or nose-poke) over and over to procure and take drugs. By their design, such procedures often produce behavior controlled by stimulus-response (S-R) habits. This has supported the notion of addiction as a "drug habit," and has led to considerable advances in our understanding of the neurobiological basis of such behavior. However, to procure such drugs as cocaine, addicts often require considerable ingenuity and flexibility in seeking behavior, which, by definition, precludes the development of habits. To better model drug-seeking behavior in addicts, we first developed a novel cocaine self-administration procedure [puzzle self-administration procedure (PSAP)] that required rats to solve a new puzzle every day to gain access to cocaine, which they then self-administered on an intermittent access (IntA) schedule. Such daily problem-solving precluded the development of S-R seeking habits. We then asked whether prolonged PSAP/IntA experience would nevertheless produce "symptoms of addiction." It did, including escalation of intake, sensitized motivation for drug, continued drug use in the face of adverse consequences, and very robust cue-induced reinstatement of drug seeking, especially in a subset of "addiction-prone" rats. Furthermore, drug-seeking behavior continued to require dopamine neurotransmission in the core of the nucleus accumbens (but not the dorsolateral striatum). We conclude that the development of S-R seeking habits is not necessary for the development of cocaine addiction-like behavior in rats.SIGNIFICANCE STATEMENT Substance-use disorders are often characterized as "habitual" behaviors aimed at obtaining and administering drugs. Although the actions involved in consuming drugs may involve a rigid repertoire of habitual behaviors, evidence suggests that addicts must be very creative and flexible when trying to procure drugs, and thus drug seeking cannot be governed by habit alone. We modeled flexible drug-seeking behavior in rats by requiring animals to solve daily puzzles to gain access to cocaine. We find that habitual drug-seeking isn't necessary for the development of addiction-like behavior, and that our procedure doesn't result in transfer of dopaminergic control from the ventral to dorsal striatum. This approach may prove useful in studying changes in neuropsychological function that promote the transition to addiction.

The sensory features of a food cue influence its ability to act as an incentive stimulus and evoke dopamine release in the nucleus accumbens core.

The sensory properties of a reward-paired cue (a conditioned stimulus; CS) may impact the motivational value attributed to the cue, and in turn influence the form of the conditioned response (CR) that develops. A cue with multiple sensory qualities, such as a moving lever-CS, may activate numerous neural pathways that process auditory and visual information, resulting in CRs that vary both within and between individuals. For example, CRs include approach to the lever-CS itself (rats that "sign-track"; ST), approach to the location of reward delivery (rats that "goal-track"; GT), or an "intermediate" combination of these behaviors. We found that the multimodal sensory features of the lever-CS were important to the development and expression of sign-tracking. When the lever-CS was covered, and thus could only be heard moving, STs not only continued to approach the lever location but also started to approach the food cup during the CS period. While still predictive of reward, the auditory component of the lever-CS was a much weaker conditioned reinforcer than the visible lever-CS. This plasticity in behavioral responding observed in STs closely resembled behaviors normally seen in rats classified as "intermediates." Furthermore, the ability of both the lever-CS and the reward-delivery to evoke dopamine release in the nucleus accumbens was also altered by covering the lever-dopamine signaling in STs resembled neurotransmission observed in rats that normally only GT. These data suggest that while the visible lever-CS was attractive, wanted, and had incentive value for STs, when presented in isolation, the auditory component of the cue was simply predictive of reward, lacking incentive salience. Therefore, the specific sensory features of cues may differentially contribute to responding and ensure behavioral flexibility.

Individual variation in incentive salience attribution and accumbens dopamine transporter expression and function.

Cues (conditioned stimuli; CSs) associated with rewards can come to motivate behavior, but there is considerable individual variation in their ability to do so. For example, a lever-CS that predicts food reward becomes attractive and wanted, and elicits reward-seeking behavior, to a greater extent in some rats ('sign-trackers'; STs) than others ('goal-trackers'; GTs). Variation in dopamine (DA) neurotransmission in the nucleus accumbens (NAc) core is thought to contribute to such individual variation. Given that the DA transporter (DAT) exerts powerful regulation over DA signaling, we characterized the expression and function of the DAT in the accumbens of STs and GTs. STs showed greater DAT surface expression in ventral striatal synaptosomes than GTs, and ex vivo fast-scan cyclic voltammetry recordings of electrically evoked DA release confirmed enhanced DAT function in STs, as indicated by faster DA uptake, specifically in the NAc core. Consistent with this, systemic amphetamine (AMPH) produced greater inhibition of DA uptake in STs than in GTs. Furthermore, injection of AMPH directly into the NAc core enhanced lever-directed approach in STs, presumably by amplifying the incentive value of the CS, but had no effect on goal-tracking behavior. On the other hand, there were no differences between STs and GTs in electrically-evoked DA release in slices, or in total ventral striatal DA content. We conclude that greater DAT surface expression may facilitate the attribution of incentive salience to discrete reward cues. Investigating this variability in animal sub-populations may help explain why some people abuse drugs while others do not.

Rapid dopamine transmission within the nucleus accumbens: dramatic difference between morphine and oxycodone delivery.

While most drugs of abuse increase dopamine neurotransmission, rapid neurochemical measurements show that different drugs evoke distinct dopamine release patterns within the nucleus accumbens. Rapid changes in dopamine concentration following psychostimulant administration have been well studied; however, such changes have never been examined following opioid delivery. Here, we provide novel measures of rapid dopamine release following intravenous infusion of two opioids, morphine and oxycodone, in drug-naïve rats using fast-scan cyclic voltammetry and rapid (1 min) microdialysis coupled with high-performance liquid chromatography - tandem mass spectrometry (HPLC-MS). In addition to measuring rapid dopamine transmission, microdialysis HPLC-MS measures changes in GABA, glutamate, monoamines, monoamine metabolites and several other neurotransmitters. Although both opioids increased dopamine release in the nucleus accumbens, their patterns of drug-evoked dopamine transmission differed dramatically. Oxycodone evoked a robust and stable increase in dopamine concentration and a robust increase in the frequency and amplitude of phasic dopamine release events. Conversely, morphine evoked a brief (~ 1 min) increase in dopamine that was coincident with a surge in GABA concentration and then both transmitters returned to baseline levels. Thus, by providing rapid measures of neurotransmission, this study reveals previously unknown differences in opioid-induced neurotransmitter signaling. Investigating these differences may be essential for understanding how these two drugs of abuse could differentially usurp motivational circuitry and powerfully influence behavior.

Sign-tracking and goal-tracking in humans: Utilising eye-tracking in clinical and non-clinical populations.

BACKGROUND: In Pavlovian conditioning, learned behaviour varies according to the perceived value of environmental cues. For goal-trackers (GT), the cue merely predicts a reward, whilst for sign-trackers (ST), the cue holds incentive value. The sign-tracking/goal-tracking model is well-validated in animals, but translational work is lacking. Despite the model's relevance to several conditions, including attention deficit hyperactivity disorder (ADHD), we are unaware of any studies that have examined the model in clinical populations. METHODS: The current study used an eye-tracking Pavlovian conditioning paradigm to identify ST and GT in non-clinical (N = 54) and ADHD (N = 57) participants. Eye movements were recorded whilst performing the task. Dwell time was measured for two areas of interest: sign (i.e., cue) and goal (i.e., reward), and an eye-gaze index (EGI) was computed based on the dwell time sign-to-goal ratio. Higher EGI values indicate sign-tracking behaviour. ST and GT were determined using median and tertiary split approaches in both samples. RESULTS: Despite greater propensity for sign-tracking in those with ADHD, there was no significant difference between groups. The oculomotor conditioned response was reward-specific (CS+) and present, at least partly, from the start of the task indicating dispositional and learned components. There were no differences in externalising behaviours between ST and GT for either sample. CONCLUSIONS: Sign-tracking is associated with CS+ trials only. There may be both dispositional and learned components to sign-tracking, potentially more common in those with ADHD. This holds translational potential for understanding individual differences in reward-learning.

Voluntary Self-Exclusion and Contingency Management for the Treatment of Problematic and Harmful Gambling in the UK: An Exploratory Study.

Research into self-directed methods for reducing problematic and harmful gambling is still in its infancy. One strategy that individuals use to prevent gambling involves voluntary self-exclusion (VSE) programs. For example, VSE programs can make it challenging to access betting sites or enable banks to block gambling-related transactions. Although individual VSEs can be helpful when used alone, it is unclear whether their efficacy is enhanced when combined. Furthermore, it is unknown how VSE compliance can be improved. We propose that contingency management (CM), an evidence-based strategy to incentivise abstinence, could encourage continued VSE use, promoting long-lasting recovery from problematic or harmful gambling. Here, we conducted exploratory analyses on VSE use and CM for gambling in two populations (members of the UK general population recruited and students). Participants responded favourably regarding combined VSE use. They felt that providing vouchers exchangeable for goods/services could incentivise gambling abstinence during VSE. However, some were concerned about people potentially "gaming" the system. Participants believed supplementing VSE and CM with social support could encourage abstinence. These attitudes, and recent research on treatment providers' opinions on CM for gambling, suggest that experimental evidence should be sought to determine the efficacy of combined VSE use and CM for gambling.

The Effects of Different Exercise Approaches on Attention Deficit Hyperactivity Disorder in Adults: A Randomised Controlled Trial.

Attention deficit hyperactivity disorder (ADHD) results in significant functional impairment. Current treatments, particularly for adults, are limited. Previous research indicates that exercise may offer an alternative approach to managing ADHD, but research into different types of exercise and adult populations is limited. The aim of this study was to examine the effects of acute exercise (aerobic cycling vs mind-body yoga exercises) on symptoms of ADHD in adults. Adults with ADHD (N = 82) and controls (N = 77) were randomly allocated to 10 min of aerobic (cycling) or mind-body (Hatha yoga) exercise. Immediately before and after exercise, participants completed the Test of Variables of Attention task, Delay Discounting Task, and Iowa Gambling Task to measure attention and impulsivity. Actigraphy measured movement frequency and intensity. Both groups showed improved temporal impulsivity post-exercise, with cycling beneficial to all, whilst yoga only benefited those with ADHD. There were no effects of exercise on attention, cognitive or motor impulsivity, or movement in those with ADHD. Exercise reduced attention and increased movement in controls. Exercise can improve temporal impulsivity in adult ADHD but did not improve other symptoms and worsened some aspects of performance in controls. Exercise interventions should be further investigated.

Gambling disorder in the UK: key research priorities and the urgent need for independent research funding.

Gambling in the modern era is pervasive owing to the variety of gambling opportunities available, including those that use technology (eg, online applications on smartphones). Although many people gamble recreationally without undue negative effects, a sizeable subset of individuals develop disordered gambling, which is associated with marked functional impairment including other mental health problems, relationship problems, bankruptcy, suicidality, and criminality. The National UK Research Network for Behavioural Addictions (NUK-BA) was established to promote understanding of, research into, and treatments for behavioural addictions including gambling disorder, which is the only formally recognised behavioural addiction. In this Health Policy paper, we outline the status of research and treatment for disordered gambling in the UK (including funding issues) and key research that should be conducted to establish the magnitude of the problem, vulnerability and resilience factors, the underlying neurobiology, long-term consequences, and treatment opportunities. In particular, we emphasise the need to: (1) conduct independent longitudinal research into the prevalence of disordered gambling (including gambling disorder and at-risk gambling), and gambling harms, including in vulnerable and minoritised groups; (2) select and refine the most suitable pragmatic measurement tools; (3) identify predictors (eg, vulnerability and resilience markers) of disordered gambling in people who gamble recreationally, including in vulnerable and minoritised groups; (4) conduct randomised controlled trials on psychological interventions and pharmacotherapy for gambling disorder; (5) improve understanding of the neurobiological basis of gambling disorder, including impulsivity and compulsivity, genetics, and biomarkers; and (6) develop clinical guidelines based on the best contemporary research evidence to guide effective clinical interventions. We also highlight the need to consider what can be learnt from approaches towards mitigating gambling-related harm in other countries.

Transient overexpression of alpha-Ca2+/calmodulin-dependent protein kinase II in the nucleus accumbens shell enhances behavioral responding to amphetamine.

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is known to contribute to the expression of psychostimulant sensitization by regulating dopamine (DA) overflow from DA neuron terminals in the nucleus accumbens (NAcc). The present experiments explored the contribution of CaMKII in NAcc neurons postsynaptic to these terminals where it is known to participate in a number of signaling pathways that regulate responding to psychostimulant drugs. Exposure to amphetamine transiently increased alphaCaMKII levels in the shell but not the core of the NAcc. Thus, HSV (herpes simplex viral) vectors were used to transiently overexpress alphaCaMKII in NAcc neurons in drug-naive rats, and behavioral responding to amphetamine was assessed. Transiently overexpressing alphaCaMKII in the NAcc shell led to long-lasting enhancement of amphetamine-induced locomotion and self-administration manifested when alphaCaMKII levels were elevated and persisting long after they had returned to baseline. Enhanced locomotion was not observed after infection in the NAcc core or sites adjacent to the NAcc. Transient elevation of NAcc shell alphaCaMKII levels also enhanced locomotor responding to NAcc AMPA and increased phosphorylation levels of GluR1 (Ser831), a CaMKII site, both soon and long after infection. Similar increases in pGluR1 (Ser831) were observed both soon and long after exposure to amphetamine. These results indicate that the transient increase in alphaCaMKII observed in neurons of the NAcc shell after viral-mediated gene transfer and likely exposure to amphetamine leads to neuroadaptations in AMPA receptor signaling in this site that may contribute to the long-lasting maintenance of behavioral and incentive sensitization by psychostimulant drugs like amphetamine.

Rapid induction of dopamine sensitization in the nucleus accumbens shell induced by a single injection of cocaine.

Repeated intermittent exposure to cocaine results in the neurochemical sensitization of dopamine (DA) transmission within the nucleus accumbens (NAc). Indeed, the excitability of DA neurons in the ventral tegmental area (VTA) is enhanced within hours of initial psychostimulant exposure. However, it is not known if this is accompanied by a comparably rapid change in the ability of cocaine to increase extracellular DA concentrations in the ventral striatum. To address this question we used fast-scan cyclic voltammetry (FSCV) in awake-behaving rats to measure DA responses in the NAc shell following an initial intravenous cocaine injection, and then again 2-h later. Both injections quickly elevated DA levels in the NAc shell, but the second cocaine infusion produced a greater effect than the first, indicating sensitization. This suggests that a single injection of cocaine induces sensitization-related plasticity very rapidly within the mesolimbic DA system.

Drug-Paired Contextual Stimuli Increase Dendritic Spine Dynamics in Select Nucleus Accumbens Neurons.

Repeated exposure to amphetamine leads to both associative conditioning and nonassociative sensitization. Here we assessed the contribution of neuronal ensembles in the nucleus accumbens (NAcc) to these behaviors. Animals exposed to amphetamine IP or in the ventral tegmental area (VTA) showed a sensitized locomotor response when challenged with amphetamine weeks later. Both exposure routes also increased ΔFosB levels in the NAcc. Further characterization of these ΔFosB+ neurons, however, revealed that amphetamine had no effect on dendritic spine density or size, indicating that these neurons do not undergo changes in dendritic spine morphology that accompany the expression of nonassociative sensitization. Additional experiments determined how neurons in the NAcc contribute to the expression of associative conditioning. A discrimination learning procedure was used to expose rats to IP or VTA amphetamine either Paired or Unpaired with an open field. As expected, compared with Controls, Paired rats administered IP amphetamine subsequently showed a conditioned locomotor response when challenged with saline in the open field, an effect accompanied by an increase in c-Fos+ neurons in the medial NAcc. Further characterization of these c-Fos+ cells revealed that Paired rats showed an increase in the density of dendritic spines and the frequency of medium-sized spines in the NAcc. In contrast, Paired rats previously exposed to VTA amphetamine showed neither conditioned locomotion nor conditioned c-Fos+ expression. Together, these results suggest a role for c-Fos+ neurons in the medial NAcc and rapid changes in the morphology of their dendritic spines in the expression of conditioning evoked by amphetamine-paired contextual stimuli.

Rats that sign-track are resistant to Pavlovian but not instrumental extinction.

Individuals vary in the extent to which they attribute incentive salience to a discrete cue (conditioned stimulus; CS) that predicts reward delivery (unconditioned stimulus; US), which results in some individuals approaching and interacting with the CS (sign-trackers; STs) more than others (goal-trackers; GTs). Here we asked how periods of non-reinforcement influence conditioned responding in STs vs. GTs, in both Pavlovian and instrumental tasks. After classifying rats as STs or GTs by pairing a retractable lever (the CS) with the delivery of a food pellet (US), we introduced periods of non-reinforcement, first by simply withholding the US (i.e., extinction training; experiment 1), then by signaling alternating periods of reward (R) and non-reward (NR) within the same session (experiments 2 and 3). We also examined how alternating R and NR periods influenced instrumental responding for food (experiment 4). STs and GTs did not differ in their ability to discriminate between R and NR periods in the instrumental task. However, in Pavlovian settings STs and GTs responded to periods of non-reward very differently. Relative to STs, GTs very rapidly modified their behavior in response to periods of non-reward, showing much faster extinction and better and faster discrimination between R and NR conditions. These results highlight differences between Pavlovian and instrumental extinction learning, and suggest that if a Pavlovian CS is strongly attributed with incentive salience, as in STs, it may continue to bias attention toward it, and to facilitate persistent and relatively inflexible responding, even when it is no longer followed by reward.

Stimuli associated with the presence or absence of amphetamine regulate cytoskeletal signaling and behavior.

Drug-paired stimuli rapidly enlarge dendritic spines in the nucleus accumbens (NAcc). While increases in spine size and shape are supported by rearrangement of the actin cytoskeleton and facilitate the synaptic expression of AMPA-type glutamate receptors, it remains unclear whether drug-related stimuli can influence signaling pathways known to regulate these changes in spine morphology. These pathways were studied in rats trained on a discrimination learning paradigm using subcellular fractionation and protein immunoblotting to isolate proteins within dendritic spine compartments in the NAcc shell. An open field chamber was repeatedly associated with amphetamine in one group (Paired) and explicitly unpaired with amphetamine in another (Unpaired). Rats in a third group were exposed to the open field but never administered amphetamine (Control). When administered saline and returned to the open field one week later, Paired rats as expected displayed a conditioned locomotor response relative to rats in the other two groups. NAcc shell tissues were harvested immediately after this 30-minute test. Re-exposing Paired rats to the drug-paired excitatory context significantly decreased p-GluA2(S880), an effect consistent with reduced internalization of this subunit and increased spine proliferation in these rats. In contrast, re-exposing Unpaired rats to the drug-unpaired context, capable of inhibiting conditioned responding in these animals, significantly decreased levels of both actin binding protein Arp2/3 and p-cofilin, consistent with spine volatility, shrinkage, and inhibition of spine proliferation in these rats. These findings show that contextual stimuli previously associated with either the presence or absence of amphetamine differentially regulate cytoskeletal signaling pathways in the NAcc.

Locating chronically implanted subdural electrodes using surface reconstruction.

OBJECTIVE: To determine the accuracy of locating subdural electrodes by means of 3-D surface rendering of CT scans. METHODS: Open source software has been developed and posted on the web which segments the electrodes into 3-D surfaces and allows their 3-D locations to be exported to other EEG analysis programs. The accuracy of the technique was determined by studying 410 subdural electrodes implanted in four epilepsy patients. Accuracy was determined by comparing the locations from the rendering analysis to the locations of the same electrodes determined by conventional analysis of their appearance on individual CT slices. RESULTS: The average accuracy of a study of 410 electrodes imaged in four patients repeated two times by three observers was 0.91 (+/- 0.41) mm, with a maximum error of 3.3 mm, about half of the diameter of an electrode. CONCLUSIONS: The location of subdural electrodes can easily and quickly be determined within high-resolution CT scans through the use of 3-D rendering. SIGNIFICANCE: This relatively fast and easy method for determining the location of subdural electrodes should facilitate their use in both clinical and research investigations.

Locomotor conditioning by amphetamine requires cyclin-dependent kinase 5 signaling in the nucleus accumbens.

Intermittent systemic exposure to psychostimulants leads to several forms of long-lasting behavioral plasticity including nonassociative sensitization and associative conditioning. In the nucleus accumbens (NAcc), the protein serine/threonine kinase cyclin-dependent kinase 5 (Cdk5) and its phosphorylation target, the guanine-nucleotide exchange factor kalirin-7 (Kal7), may contribute to the neuroadaptations underlying the formation of conditioned associations. Pharmacological inhibition of Cdk5 in the NAcc prevents the increases in dendritic spine density normally observed in this site following repeated cocaine. Mice lacking the Kal7 gene display similar effects. As increases in spine density may relate to the formation of associative memories and both Cdk5 and Kal7 regulate the generation of spines following repeated drug exposure, we hypothesized that either inhibiting Cdk5 or preventing its phosphorylation of Kal7 in the NAcc may prevent the induction of drug conditioning. In the present experiments, blockade in rats of NAcc Cdk5 activity with roscovitine (40 nmol/0.5 µl/side) prior to each of 4 injections of amphetamine (1.5 mg/kg; i.p.) prevented the accrual of contextual locomotor conditioning but spared the induction of locomotor sensitization as revealed on tests conducted one week later. Similarly, transient viral expression in the NAcc exclusively during amphetamine exposure of a threonine-alanine mutant form of Kal7 [mKal7(T1590A)] that is not phosphorylated by Cdk5 also prevented the accrual of contextual conditioning and spared the induction of sensitization. These results indicate that signaling via Cdk5 and Kal7 in the NAcc is necessary for the formation of context-drug associations, potentially through the modulation of dendritic spine dynamics in this site.

Amphetamine-induced changes in dendritic morphology in rat forebrain correspond to associative drug conditioning rather than nonassociative drug sensitization.

BACKGROUND: Systemic exposure to amphetamine (AMPH) leads to a number of long-lasting neuroadaptations including changes in dendritic morphology in rat forebrain. It remains unknown whether these changes relate to associative drug conditioning or to nonassociative drug sensitization, two forms of plasticity produced by systemic exposure to AMPH. METHODS: We compared the behavioral, neuronal, and morphologic consequences of exposing rats to intraperitoneal (IP) AMPH to those of exposure to AMPH applied to the ventral tegmental area (VTA), infusions that sensitize AMPH-induced locomotion and nucleus accumbens (NAcc) DA overflow but do not produce drug conditioning. RESULTS: Both IP and VTA AMPH exposure sensitized locomotion and NAcc DA overflow, but only IP AMPH exposure produced conditioned locomotion. Importantly, whereas IP AMPH exposure increased spine density and dendritic length and branching in the NAcc, exposure to VTA AMPH produced the opposite effects. A similar differentiation of effects was observed in cortical areas. CONCLUSIONS: Together these findings suggest that the morphological changes seen following IP AMPH exposure reflect associative drug conditioning rather than nonassociative drug sensitization. The decreases observed in the NAcc of VTA AMPH exposed rats may reflect the inability of these infusions to support conditioning.