Enhanced motivation to self-administer cocaine is predicted by self-grooming behaviour and relates to dopamine release in the rat medial prefrontal cortex and amygdala

Self-administration acquisition latency predicts locomotor sensitivity to cocaine in male rats

Individual differences in drug use emerge soon after initial exposure, and only a fraction of individuals who initiate drug use go on to develop a substance use disorder. Variability in vulnerability to establishing drug self-administration behavior is also evident in preclinical rodent models. Latent characteristics that underlie this variability and the relationship between early drug use patterns and later use remain unclear. Here, we attempt to determine whether propensity to establish cocaine self-administration is related to subsequent cocaine self-administration behavior in male Sprague-Dawley rats (n = 14). Prior to initiating training, we evaluated basal locomotor and anxiety-like behavior in a novel open field test. We then trained rats to self-administer cocaine in daily 3h cocaine (0.75mg/kg/infusion) self-administration sessions until acquisition criteria (≥30 active lever presses with ≥70% responding on the active lever in one session) was met and divided rats into Early and Late groups by median-split analysis based on their latency to meet acquisition criteria. After each rat met acquisition criteria, we gave them 10 additional daily cocaine self-administration sessions. We then conducted a progressive ratio, cocaine-induced locomotor sensitivity test, and non-reinforced cocaine seeking test after two weeks of forced abstinence. Early Learners exhibited significantly less locomotion after an acute injection of cocaine, but the groups did not differ in any other behavioral parameter examined. These results indicate that cocaine self-administration acquisition latency is not predictive of subsequent drug-taking behavior, but may be linked to physiological factors like drug sensitivity that can predispose rats to learn the operant task.

Selective estrogen receptor activation prior to global cerebral ischemia in female rats impacts microglial activation and anxiety-like behaviors without effects on CA1 neuronal injury

Estrogen receptor (ER) activation by 17-ß estradiol (E2) can attenuate neuronal injury and behavioral impairments following global cerebral ischemia (GCI) in rodents. This study sought to further examine the discrete roles of ERs through characterization of the effects of selective ER activation on post-ischemic pro-inflammatory microglial activation, hippocampal neuronal injury, and anxiety-like behaviors. Forty-six ovariectomized (OVX) adult female Wistar rats received daily s.c injections (100 μg/kg/day) of propylpyrazole triol (PPT; ERα agonist), diarylpropionitrile (DPN; ERβ agonist), G-1 (G-protein coupled ER agonist; GPER), E2 (activating all receptors), or vehicle solution (VEH) for 21 days. After final injection, rats underwent GCI via 4-vessel occlusion (n=8 per group) or sham surgery (n=6, vehicle injections). The Open Field Test (OFT), Elevated Plus Maze (EPM), and Hole Board Test (HBT) assessed anxiety-like behaviors. Microglial activation (Iba1, CD68, CD86) in the basolateral amygdala (BLA), CA1 of the hippocampus, and paraventricular nucleus of the hypothalamus (PVN) was determined 8 days post-ischemia. Compared to sham rats, Iba1 activation and CA1 neuronal injury were increased in all ischemic groups except DPN-treated rats, with PPT-treated ischemic rats also showing increased PVN Iba1-ir expression. Behaviorally, VEH ischemic rats showed slightly elevated anxiety in the EPM compared to sham counterparts, with no significant effects of agonists. While no changes were observed in the OFT, emotion regulation via grooming in the HBT was increased in G-1 rats compared to E2 rats. Our findings support selective ER activation to regulate post-ischemic microglial activation and coping strategies in the HBT, despite minimal impact on hippocampal injury.

Neural substrates for regulating self-grooming behavior in rodents

Grooming, as an evolutionarily conserved repetitive behavior, is common in various animals, including humans, and serves essential functions including, but not limited to, hygiene maintenance, thermoregulation, de-arousal, stress reduction, and social behaviors. In rodents, grooming involves a patterned and sequenced structure, known as the syntactic chain with four phases that comprise repeated stereotyped movements happening in a cephalocaudal progression style, beginning from the nose to the face, to the head, and finally ending with body licking. The context-dependent occurrence of grooming behavior indicates its adaptive significance. This review briefly summarizes the neural substrates responsible for rodent grooming behavior and explores its relevance in rodent models of neuropsychiatric disorders and neurodegenerative diseases with aberrant grooming phenotypes. We further emphasize the utility of rodent grooming as a reliable measure of repetitive behavior in neuropsychiatric models, holding promise for translational psychiatry. Herein, we mainly focus on rodent self-grooming. Allogrooming (grooming being applied on one animal by its conspecifics via licking or carefully nibbling) and heterogrooming (a form of grooming behavior directing towards another animal, which occurs in other contexts, such as maternal, sexual, aggressive, or social behaviors) are not covered due to space constraints.

Neuroinflammatory Response in Reward-Associated Psychostimulants and Opioids: A Review

Substance abuse is one of the significant problems in social and public health worldwide. Vast numbers of evidence illustrate that motivational and reinforcing impacts of addictive drugs are primarily attributed to their ability to change dopamine signaling in the reward circuit. However, the roles of classic neurotransmitters, especially dopamine and neuromodulators, monoamines, and neuropeptides, in reinforcing characteristics of abused drugs have been extensively investigated. It has recently been revealed that central immune signaling includes cascades of chemokines and proinflammatory cytokines released by neurons and glia via downstream intracellular signaling pathways that play a crucial role in mediating rewarding behavioral effects of drugs. More interestingly, inflammatory responses in the central nervous system modulate the mesolimbic dopamine signaling and glutamate-dependent currents induced by addictive drugs. This review summarized researches in the alterations of inflammatory responses accompanied by rewarding and reinforcing properties of addictive drugs, including cocaine, methamphetamine, and opioids that were evaluated by conditioned place preference and self-administration procedures as highly common behavioral tests to investigate the motivational and reinforcing impacts of addictive drugs. The neuroinflammatory responses affect the rewarding properties of psychostimulants and opioids.

Something new and something blue: Responses to novelty in a rodent model of depression and epilepsy comorbidity

A bidirectional comorbidity exists between depression and epilepsy such that patients with epilepsy are at higher risk for developing depression, and vice versa. Each of these conditions individually can be complicated by behavioral effects that worsen quality of life, but less is known about these interactions within the comorbidity of depression and epilepsy. The SwLo rat has been selectively bred for depression-relevant behaviors and exhibits enhanced limbic seizure susceptibility. This study sought to characterize the effects of novelty and stress on the SwLo rodent model of this comorbidity. It was hypothesized that SwLo rats would exhibit altered responses to novelty, reflected in hyperactivity-, anxiety-, sensation seeking-, and/or compulsive behaviors, and that this would be exacerbated with stress. Compared to the SwHi rat (their depression- and epilepsy-resistant counterparts), SwLo rats showed increased entries in all areas of the Open Field Test and spent significantly more time in the light compartment of the Light-Dark Box. SwLo rats also had a significantly higher number of rearing behaviors in the inner squares of the Open Field Test, the closed arms of the Elevated Plus Maze, and both areas of the Light-Dark Box. They demonstrated increased Nestlet shredding but showed no difference in a marble burying task or in latency to consume food in a novelty suppressed feeding task. Interestingly, restraint stress showed little effect on these behaviors, despite increasing corticosterone levels. Combined, these results suggest an increase in exploratory sensation seeking and hypervigilant information-gathering behaviors in the SwLo rat that are not dependent on corticosterone levels. This shows the utility of this model for studying behavioral effects of comorbid depression and epilepsy and allows for their use in identifying underlying mechanisms or screening treatment strategies for this complex comorbidity.

Neural Mechanisms Underlying Repetitive Behaviors in Rodent Models of Autism Spectrum Disorders

Autism spectrum disorder (ASD) is comprised of several conditions characterized by alterations in social interaction, communication, and repetitive behaviors. Genetic and environmental factors contribute to the heterogeneous development of ASD behaviors. Several rodent models display ASD-like phenotypes, including repetitive behaviors. In this review article, we discuss the potential neural mechanisms involved in repetitive behaviors in rodent models of ASD and related neuropsychiatric disorders. We review signaling pathways, neural circuits, and anatomical alterations in rodent models that display robust stereotypic behaviors. Understanding the mechanisms and circuit alterations underlying repetitive behaviors in rodent models of ASD will inform translational research and provide useful insight into therapeutic strategies for the treatment of repetitive behaviors in ASD and other neuropsychiatric disorders.

Chronic alcohol disrupts hypothalamic responses to stress by modifying CRF and NMDA receptor function

The chronic inability of alcoholics to effectively cope with relapse-inducing stressors has been linked to dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and corticotropin-releasing factor (CRF) signaling. However, the cellular mechanisms responsible for this dysregulation are yet to be identified. After exposure of male Sprague Dawley rats to chronic intermittent ethanol (CIE; 5-6 g/kg orally for 35 doses over 50 days) or water, followed by 40-60 days of protracted withdrawal, we investigated CIE effects on glutamatergic synaptic transmission, stress-induced plasticity, CRF- and ethanol-induced NMDAR inhibition using electrophysiological recordings in parvocellular neurosecretory cells (PNCs) of the paraventricular nucleus. We also assessed CIE effects on hypothalamic mRNA expression of CRF-related genes using real-time polymerase chain reaction, and on HPA axis function by measuring stress-induced increases in plasma adrenocorticotropic hormone, corticosterone, and self-grooming. In control rats, ethanol-mediated inhibition of NMDARs was prevented by CRF1 receptor (CRFR1) blockade with antalarmin, while CRF/CRFR1-mediated NMDAR blockade was prevented by intracellularly-applied inhibitor of phosphatases PP1/PP2A, okadaic acid, but not the selective striatal-enriched tyrosine protein phosphatase inhibitor, TC-2153. CIE exposure increased GluN2B subunit-dependent NMDAR function of PNCs. This was associated with the loss of both ethanol- and CRF-mediated NMDAR inhibition, and loss of stress-induced short-term potentiation of glutamatergic synaptic inputs, which could be reversed by intracellular blockade of NMDARs with MK801. CIE exposure also blunted the hormonal and self-grooming behavioral responses to repeated restraint stress. These findings suggest a cellular mechanism whereby chronic alcohol dysregulates the hormonal and behavioral responses to repetitive stressors by increasing NMDAR function and decreasing CRFR1 function.

Effects of DCM Leaf Extract of Gnidia glauca (Fresen) on Locomotor Activity, Anxiety, and Exploration-Like Behaviors in High-Fat Diet-Induced Obese Rats

Obesity is the main component of metabolic syndromes involving distinct etiologies that target different underlying behavioral and physiological functions within the brain structures and neuronal circuits. An alteration in the neuronal circuitry stemming from abdominal or central obesity stimulates a cascade of changes in neurochemical signaling that directly or indirectly mediate spontaneously emitted behaviors such as locomotor activity patterns, anxiety, and exploration. Pharmacological agents available for the treatment of neurologic disorders have been associated with limited potency and intolerable adverse effects. These have necessitated the upsurge in the utilization of herbal prescriptions due to their affordability and easy accessibility and are firmly embedded within wider belief systems of many people. Gnidia glauca has been used in the management of many ailments including obesity and associated symptomatic complications. However, its upsurge in use has not been accompanied by empirical determination of these folkloric claims. The present study, therefore, is aimed at determining the modulatory effects of dichloromethane leaf extract of Gnidia glauca on locomotor activity, exploration, and anxiety-like behaviors in high-fat diet-induced obese rats in an open-field arena. Obesity was experimentally induced by feeding the rats with prepared high-fat diet and water ad libitum for 6 weeks. The in vivo antiobesity effects were determined by oral administration of G. glauca at dosage levels of 200, 250, and 300 mg/kg body weight in high-fat diet-induced obese rats from the 6^th to 12^th week. Phytochemical analysis was done using gas chromatography linked to mass spectroscopy. Results indicated that Gnidia glauca showed anxiolytic effects and significantly increased spontaneous locomotor activity and exploration-like behaviors in HFD-induced obese rats. The plant extract also contained phytocompounds that have been associated with amelioration of the main neurodegenerative mediators, viz., inflammation and oxidative stress. These findings provide “qualified leads” for the synthesis of new alternative therapeutic agents for the management of neurologic disorders. However, there is a need to conduct toxicity studies of Gnidia glauca to establish its safety profiles.

Withdrawal from escalated cocaine self-administration impairs reversal learning by disrupting the effects of negative feedback on reward exploitation: a behavioral and computational analysis

Addiction is regarded as a disorder of inflexible choice with behavior dominated by immediate positive rewards over longer-term negative outcomes. However, the psychological mechanisms underlying the effects of self-administered drugs on behavioral flexibility are not well understood. To investigate whether drug exposure causes asymmetric effects on positive and negative outcomes we used a reversal learning procedure to assess how reward contingencies are utilized to guide behavior in rats previously exposed to intravenous cocaine self-administration (SA). Twenty-four rats were screened for anxiety in an open field prior to acquisition of cocaine SA over six daily sessions with subsequent long-access cocaine SA for 7 days. Control rats (n = 24) were trained to lever-press for food under a yoked schedule of reinforcement. Higher rates of cocaine SA were predicted by increased anxiety and preceded impaired reversal learning, expressed by a decrease in lose-shift as opposed to win-stay probability. A model-free reinforcement learning algorithm revealed that rats with high, but not low cocaine escalation failed to exploit previous reward learning and were more likely to repeat the same response as the previous trial. Eight-day withdrawal from high cocaine escalation was associated, respectively, with increased and decreased dopamine receptor D2 (DRD2) and serotonin receptor 2C (HTR2C) expression in the ventral striatum compared with controls. Dopamine receptor D1 (DRD1) expression was also significantly reduced in the orbitofrontal cortex of high cocaine-escalating rats. These findings indicate that withdrawal from escalated cocaine SA disrupts how negative feedback is used to guide goal-directed behavior for natural reinforcers and that trait anxiety may be a latent variable underlying this interaction.

Pharmacological evaluation of the hydro-alcoholic extract of Campomanesia phaea fruits in rats

Campomanesia phaea (Myrtaceae), popularly known as cambuci, is one of several species of plants producing comestible fruits, largely used in human nutrition. Despite its consumption and economic potential, limited scientific research is available on the Campomanesia, especially those related to its therapeutic benefits. It is reported by traditional medicine the use of the plant in the treatment of different disorders, such as cardiovascular and nervous system disturbances. So, the aim of this study was to carry out the pharmacological evaluation of the hydro-alcoholic extract (HAE) of Campomanesia fruits in rats by screening consisting of tests: a) neuropharmacological observation, b) test on the cardiovascular system. The HAE, prepared from the extraction of fruits with water/ethanol, was concentrated and freeze-dried. Behavioral responses in rats were investigated in open field test and the cardiovascular actions were investigated by a register of indirect blood pressure and the register of spontaneous beating rate right atrium. The results revealed that HAE induced grooming, hypotension and bradycardia. So, this study identified an action on the central nervous system, represented by grooming, and a cardiovascular activity of Campomanesia. The hypotension, attributed in part to bradycardia, was not related to a cholinergic effect, discarding a possible cholinomimetic action of the plant that could justify both cardiovascular and central actions.

Stress resilience evidenced by grooming behaviour and dopamine levels in male mice selected for high and low immobility using the tail suspension test

Grooming behaviour has different functions on many species during development and can be observed and affected during periods of stress. By selecting male mice with high (HI) and low (LI) immobility traits in the tail suspension test, a screening for antidepressant drugs, we investigate how these phenotypes associated with grooming behaviour may be influenced by the effects of repeated restraint stress. For this we used the sucrose preference test and the splash test in a novel and a familiar cage performed before and after exposure to 2 days of restraint stress. Animals were submitted to an additional day of restraint stress before the hypothalamus, prefrontal cortex and midbrain extraction for dopamine activity analysis. Corticosterone analysis was made in three distinct moments: without stress (prior first restraint session), immediately after the last restrain, and 1 hr after the last restrain episode. Compared to LI group, HI animals exhibited an increased frequency and decreased time of grooming in the familiar cage. In the novel cage, stress increased frequency and time of grooming of HI animals compared to LI. Corticosterone levels were increased in HI animals after 3 days of stress. Lower hypothalamic dopaminergic activity without stress and decreased hypothalamic dopaminergic activity immediately after stress in HI group were observed. The HI group displayed decreased prefrontal cortex dopaminergic activity and increased activity in the mesolimbic area. We proposed that through the influence of stress the two phenotypes manifested as a resilient (LI) and a not resilient (HI) trait in response to restraint stress.

Modelling Differential Vulnerability to Substance Use Disorder in Rodents: Neurobiological Mechanisms

Despite the prevalence of drug use within society, only a subset of individuals actively taking addictive drugs lose control over their intake and develop compulsive drug-seeking and intake that typifies substance use disorder (SUD). Although research in this field continues to be an important and dynamic discipline, the specific neuroadaptations that drive compulsive behaviour in humans addicted to drugs and the neurobiological mechanisms that underlie an individual's innate susceptibility to SUD remain surprisingly poorly understood. Nonetheless, it is clear from research within the clinical domain that some behavioural traits are recurrently co-expressed in individuals with SUD, thereby inviting the hypothesis that certain behavioural endophenotypes may be predictive, or at least act in some way, to modify an individual's probability for developing this disorder. The analysis of such endophenotypes and their catalytic relationship to the expression of addiction-related behaviours has been greatly augmented by experimental approaches in rodents that attempt to capture diagnostically relevant aspects of this progressive brain disorder. This work has evolved from an early focus on aberrant drug reinforcement mechanisms to a now much richer account of the putatively impaired cognitive control processes that ultimately determine individual trajectories to compulsive drug-related behaviours. In this chapter we discuss the utility of experimental approaches in rodents designed to elucidate the neurobiological and genetic underpinnings of so-called risk traits and how these innate vulnerabilities collectively contribute to the pathogenesis of SUD.

Escalated cocaine "binges" in rats: enduring effects of social defeat stress or intra-VTA CRF

RATIONALE

Exposure to intermittent social defeat stress elicits corticotropin releasing factor (CRF) release into the VTA and induces long-term modulation of mesocorticolimbic dopamine activity in rats. These adaptations are associated with an intense cocaine-taking phenotype, which is prevented by CRF receptor antagonists.

OBJECTIVE

The present studies examine whether infusion of CRF into the VTA is sufficient to escalate cocaine-taking behavior, in the absence of social defeat experience. Additionally, we aimed to characterize changes in cocaine valuation that may promote binge-like cocaine intake.

METHODS

Male Long-Evans rats were microinjected into the VTA with CRF (50 or 500 ng/side), vehicle, or subjected to social defeat stress, intermittently over 10 days. Animals were then trained to self-administer IV cocaine (FR5). Economic demand for cocaine was evaluated using a within-session behavioral-economics threshold procedure, which was followed by a 24-h extended access "binge."

RESULTS

Rats that experienced social defeat or received intra-VTA CRF microinfusions (50 ng) both took significantly more cocaine than controls over the 24-h binge but showed distinct patterns of intake. Behavioral economic analysis revealed that individual demand for cocaine strongly predicts binge-like consumption, and demand elasticity (i.e. α) is augmented by intra-VTA CRF, but not by social defeat. The effects of CRF on cocaine-taking were also prevented by intra-VTA pretreatment with CP376395, but not Astressin-2B.

CONCLUSIONS

Repeated infusion of CRF into the VTA persistently alters cocaine valuation and intensifies binge-like drug intake in a CRF-R1-dependent manner. Conversely, the persistent pattern of cocaine bingeing induced by social defeat stress may suggest impaired inhibitory control, independent of reward valuation.

Exposure of adolescent mice to 3,4-methylenedioxypyrovalerone increases the psychostimulant, rewarding and reinforcing effects of cocaine in adulthood

BACKGROUND AND PURPOSE

3,4-Methylenedioxypyrovalerone (MDPV) is a synthetic cathinone with powerful psychostimulant effects. It selectively inhibits the dopamine transporter (DAT) and is 10-50-fold more potent as a DAT blocker than cocaine, suggesting a high abuse liability. The main objective of the present study was to assess the consequences of an early (adolescence) MDPV exposure on the psychostimulant, rewarding and reinforcing effects induced by cocaine in adult mice.

EXPERIMENTAL APPROACH

Twenty-one days after MDPV pretreatment (1.5 mg·kg^-1 , s.c., twice daily for 7 days), adult mice were tested with cocaine, using locomotor activity, conditioned place preference and self-administration (SA) paradigms. In parallel, dopamine D₂ receptor density and the expression of c-Fos and ΔFosB in the striatum were determined.

KEY RESULTS

MDPV treatment enhanced the psychostimulant and conditioning effects of cocaine. Acquisition of cocaine SA was unchanged in mice pretreated with MDPV, whereas the breaking point achieved under a progressive ratio programme and reinstatement after extinction were higher in this group of mice. MDPV decreased D₂ receptor density but increased ΔFosB expression three-fold. As expected, acute cocaine increased c-Fos expression, but MDPV pretreatment negatively influenced its expression. ΔFosB accumulation declined during MDPV withdrawal, although it remained elevated in adult mice when tested for cocaine effects.

CONCLUSION AND IMPLICATIONS

MDPV exposure during adolescence induced long-lasting adaptive changes related to enhanced responsiveness to cocaine in the adult mice that seems to lead to a higher vulnerability to cocaine abuse. This particular behaviour correlated with increased expression of ΔFosB.

Preweaning iron deficiency increases non-contingent responding during cocaine self-administration in rats

Iron deficiency (ID) is the most prevalent single-nutrient deficiency worldwide. There is evidence that ID early in development (preweaning in rat) causes irreversible neurologic, behavioral, and motor development deficits. Many of these effects have been attributed to damage to dopamine systems, including ID-induced changes in transporter and receptor numbers in the striatum and nucleus accumbens. These mesolimbic dopaminergic neurons are, in part, responsible for mediating reward and thus play a key role in addiction. However, there has been relatively little investigation into the behavioral effects of ID on drug addiction. In 2002, we found that rats made ID from weaning (postnatal day 21) and throughout the experiment acquired cocaine self-administration significantly more slowly than controls and failed to increase responding when the dose of the drug was decreased. In the present study, we assessed addiction for self-administered cocaine in rats with a history of preweaning ID only during postnatal days 4 through 21, and iron replete thereafter. The results showed that while ID did not affect the number of cocaine infusions or the overall addiction-like behavior score, ID rats scored higher on a measure of continued responding for drug than did iron replete controls. This increase in responding, however, was less goal-directed as ID rats also responded more quickly to the non-rewarded manipulandum than did control rats. Thus, while ID early in infancy did not significantly increase addiction-like behaviors for cocaine in this small study, the pattern of data suggests a possible underlying learning or performance impairment. Future studies will be needed to elucidate the exact neuro-behavioral deficits that lead to the increase in indiscriminate responding for drug in rats with a history of perinatal ID.

Sex differences in corticotropin releasing factor-evoked behavior and activated networks

Hypersecretion of corticotropin releasing factor (CRF) is linked to the pathophysiology of major depression and post-traumatic stress disorder, disorders that are more common in women than men. Notably, preclinical studies have identified sex differences in CRF receptors that can increase neuronal sensitivity to CRF in female compared to male rodents. These cellular sex differences suggest that CRF may regulate brain circuits and behavior differently in males and females. To test this idea, we first evaluated whether there were sex differences in anxiety-related behaviors induced by the central infusion of CRF. High doses of CRF increased self-grooming more in female than in male rats, and the magnitude of this effect in females was greater when they were in the proestrous phase of their estrous cycle (higher ovarian hormones) compared to the diestrous phase (lower ovarian hormones), which suggests that ovarian hormones potentiate this anxiogenic effect of CRF. Brain regions associated with CRF-evoked self-grooming were identified by correlating a marker of neuronal activation, cFOS, with time spent grooming. In the infralimbic region, which is implicated in regulating anxiety, the correlation for CRF-induced neuronal activation and grooming was positive in proestrous females, but negative for males and diestrous females, indicating that ovarian hormones altered this relationship between neuronal activation and behavior. Because CRF regulates a number of regions that work together to coordinate different aspects of responding to stress, we then examined more broadly whether CRF-activated functional connectivity networks differed between males and cycling females. Interestingly, hormonal status altered correlations for CRF-induced neuronal activation between a variety of brain regions, but the most striking differences were found when comparing proestrous females to males, particularly when comparing neuronal activation between prefrontal cortical and other forebrain regions. These results suggest that ovarian hormones alter the way brain regions work together in response to CRF, which could drive different strategies for coping with stress in males versus females. These sex differences in stress responses could also help explain female vulnerability to psychiatric disorders characterized by CRF hypersecretion.

Aberrant self-grooming as early marker of motor dysfunction in a rat model of Huntington's disease

In the study of neurodegenerative diseases, rodent models provide experimentally accessible systems to study multiple pathogenetic aspects. The identification of early and robust behavioural changes is crucial to monitoring disease progression and testing potential therapeutic strategies in animals. Consistent experimental data support the translational value of rodent self-grooming as index of disturbed motor functions and perseverative behaviour patterns in different rodent models of brain disorders. Huntington's disease (HD) is a progressive neurodegenerative disorder, characterized by severe degeneration of basal ganglia, cognitive and psychiatric impairments and motor abnormalities. In the rat species, intrastriatal injection of the excitotoxin quinolinic acid (QA) mimics some of the neuroanatomical and behavioural changes found in HD, including the loss of GABAergic neurons and the appearance of motor and cognitive deficits. We show here that striatal damage induced by unilateral QA injection in dorsal striatum of rats triggers aberrant grooming behaviour as early as three weeks post-lesion in absence of other motor impairments: specifically, both quantitative (frequency and duration) and qualitative (the sequential pattern of movements) features of self-grooming behaviour were significantly altered in QA-lesioned rats placed in either the elevated plus-maze and the open-field. The consistent abnormalities in self-grooming recorded in two different experimental contexts support the use of this behavioural marker in rodent models of striatal damage such as HD, to assess the potential effects of drug and cell replacement therapy in the early stage of disease.

Water spray-induced grooming is negatively correlated with depressive behavior in the forced swimming test in rats

Rodents show grooming, a typical self-care behavior, under stress and non-stress conditions. Previous studies revealed that grooming under stress conditions such as the open-field test (OFT) or the elevated plus-maze test (EPM) is associated with anxiety, but the roles of grooming under non-stress conditions are not well understood. Here, we examined spray-induced grooming as a model of grooming under a non-stress condition to investigate the relationship between this grooming and depression-like behavior in the forced swim test (FST) and tail suspension test, and we compared spray-induced grooming with OFT- and EPM-induced grooming. The main finding was that the duration of spray-induced grooming, but not that of OFT/EPM-induced grooming, was negatively correlated with the duration of immobility in the FST, an index of depression-like behavior. The results suggest that spray-induced grooming is functionally different from the grooming in the OFT and EPM and is related to reduction of depressive behavior.

Neurobiology of rodent self-grooming and its value for translational neuroscience

Self-grooming is a complex innate behaviour with an evolutionarily conserved sequencing pattern and is one of the most frequently performed behavioural activities in rodents. In this Review, we discuss the neurobiology of rodent self-grooming, and we highlight studies of rodent models of neuropsychiatric disorders--including models of autism spectrum disorder and obsessive compulsive disorder--that have assessed self-grooming phenotypes. We suggest that rodent self-grooming may be a useful measure of repetitive behaviour in such models, and therefore of value to translational psychiatry. Assessment of rodent self-grooming may also be useful for understanding the neural circuits that are involved in complex sequential patterns of action.

Differential involvement of anxiety and novelty preference levels on oral ethanol consumption in rats

RATIONALE

Drug addiction is defined as a recurring cycle of intoxication, abstinence and relapse. The behavioural trait of novelty seeking is frequently observed in alcohol abusers. Moreover, converging evidence indicates that anxious individuals are also predisposed to alcohol abuse.

OBJECTIVES

We have analyzed the respective implication of those two behavioural factors on vulnerability to ethanol intake on rats in situations designed to reflect drug intoxication and relapse phases in humans.

METHODS

In a general population of Wistar rats, animals were tested in both the light/dark box and the novelty preference tests. Ethanol consumption was measured in a two-bottle free-choice procedure across three successive procedures. Animals were first exposed to increasing concentrations of ethanol (2, 4, 6, 8, 10, 12 % for 8 days at each concentration). Then, the concentration of the solution was diminished from 12 to 6 %. Finally, all rats were re-exposed to 6 % ethanol after 12 days of ethanol deprivation.

RESULTS

Novelty preference predicted the amount of ethanol consumed across all phases. In contrast, anxiety was associated with a quicker recovery of ethanol consumption after the concentration drop and a greater increase in ethanol consumption after deprivation.

CONCLUSIONS

Novelty seeking and anxiety are both but differentially implicated in predisposition to ethanol abuse. Whereas novelty seeking is related to the amount of ethanol consumed, anxiety is associated to higher ethanol consumption when ethanol concentration is decreased or after ethanol deprivation.

High locomotor reactivity to novelty is associated with an increased propensity to choose saccharin over cocaine: new insights into the vulnerability to addiction

Drug addiction is associated with a relative devaluation of natural or socially-valued reinforcers that are unable to divert addicts from seeking and consuming the drug. Before protracted drug exposure, most rats prefer natural rewards, such as saccharin, over cocaine. However, a subpopulation of animals prefer cocaine over natural rewards and are thought to be vulnerable to addiction. Specific behavioral traits have been associated with different dimensions of drug addiction. For example, anxiety predicts loss of control over drug intake whereas sensation seeking and sign-tracking are markers of a greater sensitivity to the rewarding properties of the drug. However, how these behavioral traits predict the disinterest for natural reinforcers remains unknown. In a population of rats, we identified sensation seekers (HR) on the basis of elevated novelty-induced locomotor reactivity, high anxious rats (HA) based on the propensity to avoid open arms in an elevated-plus maze and sign-trackers (ST) that are prone to approach, and interaction with, reward-associated stimuli. Rats were then tested on their preference for saccharin over cocaine in a discrete-trial choice procedure. We show that HR rats display a greater preference for saccharin over cocaine compared with ST and HA whereas the motivation for the drug was comparable between the three groups. The present data suggest that high locomotor reactivity to novelty, or sensation seeking, by predisposing to an increased choice toward non-drug rewards at early stages of drug use history, may prevent the establishment of chronic cocaine use.

Individual differences in cocaine addiction: maladaptive behavioural traits

Cocaine use leads to addiction in only a subset of individuals. Understanding the mechanisms underlying these individual differences in the transition from cocaine use to cocaine abuse is important to develop treatment strategies. There is agreement that specific behavioural traits increase the risk for addiction. As such, both high impulsivity and high anxiety have been reported to predict (compulsive) cocaine self-administration behaviour. Here, we set out a new view explaining how these two behavioural traits may affect addictive behaviour. According to psychological and psychiatric evolutionary views, organisms flourish well when they fit (match) their environment by trait and genotype. However, under non-fit conditions, the need to compensate the failure to deal with this environment increases, and, as a consequence, the functional use of rewarding drugs like cocaine may also increase. It suggests that neither impulsivity nor anxiety are bad per se, but that the increased risk to develop cocaine addiction is dependent on whether behavioural traits are adaptive or maladaptive in the environment to which the animals are exposed. This 'behavioural (mal)adaptation view' on individual differences in vulnerability to cocaine addiction may help to improve therapies for addiction.

Enhancement of behavioral sensitization, anxiety-like behavior, and hippocampal and frontal cortical CREB levels following cocaine abstinence in mice exposed to cocaine during adolescence

Adolescence has been linked to greater risk-taking and novelty-seeking behavior and a higher prevalence of drug abuse and risk of relapse. Decreases in cyclic adenosine monophosphate response element binding protein (CREB) and phosphorylated CREB (pCREB) have been reported after repeated cocaine administration in animal models. We compared the behavioral effects of cocaine and abstinence in adolescent and adult mice and investigated possible age-related differences in CREB and pCREB levels. Adolescent and adult male Swiss mice received one daily injection of saline or cocaine (10 mg/kg, i.p.) for 8 days. On day 9, the mice received a saline injection to evaluate possible environmental conditioning. After 9 days of withdrawal, the mice were tested in the elevated plus maze to evaluate anxiety-like behavior. Twelve days after the last saline/cocaine injection, the mice received a challenge injection of either cocaine or saline, and locomotor activity was assessed. One hour after the last injection, the brains were extracted, and CREB and pCREB levels were evaluated using Western blot in the prefrontal cortex (PFC) and hippocampus. The cocaine-pretreated mice during adolescence exhibited a greater magnitude of the expression of behavioral sensitization and greater cocaine withdrawal-induced anxiety-like behavior compared with the control group. Significant increases in CREB levels in the PFC and hippocampus and pCREB in the hippocampus were observed in cocaine-abstinent animals compared with the animals treated with cocaine in adulthood. Interestingly, significant negative correlations were observed between cocaine sensitization and CREB levels in both regions. These results suggest that the behavioral and neurochemical consequences of psychoactive substances in a still-developing nervous system can be more severe than in an already mature nervous system.

Grooming analysis algorithm: use in the relationship between sleep deprivation and anxiety-like behavior

Increased anxiety is a classic effect of sleep deprivation. However, results regarding sleep deprivation-induced anxiety-like behavior are contradictory in rodent models. The grooming analysis algorithm is a method developed to examine anxiety-like behavior and stress in rodents, based on grooming characteristics and microstructure. This study evaluated the applicability of the grooming analysis algorithm to distinguish sleep-deprived and control rats in comparison to traditional grooming analysis. Forty-six animals were distributed into three groups: control (n=22), paradoxical sleep-deprived (96 h, n=10) and total sleep deprived (6 h, n=14). Immediately after the sleep deprivation protocol, grooming was evaluated using both the grooming analysis algorithm and traditional measures (grooming latency, frequency and duration). Results showed that both paradoxical sleep-deprived and total sleep-deprived groups displayed grooming in a fragmented framework when compared to control animals. Variables from the grooming analysis algorithm were successful in distinguishing sleep-deprived and normal sleep animals regarding anxiety-like behavior. The grooming analysis algorithm and traditional measures were strongly correlated. In conclusion, the grooming analysis algorithm is a reliable method to assess the relationship between anxiety-like behavior and sleep deprivation.

The stereotypy-inducing and OCD-like effects of chronic 'binge' cocaine are modulated by distinct subtypes of nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

High rates of cigarette smoking occur in cocaine-dependent individuals, reflecting an involvement of nicotinic acetylcholine receptors (nAChRs) in cocaine-elicited behaviour. This study was designed to assess the contribution of different nAChR subtypes to the behavioural and neurochemical effects of chronic cocaine treatment.

EXPERIMENTAL APPROACH

Cocaine (15 mg·kg(-1) , i.p.) was administered to male C57BL/6J mice in a chronic 'binge' paradigm, with and without the coadministration of the α7 preferring nAChR antagonist methyllycaconitine (MLA; 5 mg·kg(-1) , i.p.) or the β2* nAChR antagonist dihydro-β-erythroidine (DHβE; 2 mg·kg(-1) , i.p.). Quantitative autoradiography was used to examine the effect of cocaine exposure on α7 and α4β2* nAChRs, and on the high-affinity choline transporter.

KEY RESULTS

MLA+cocaine administration induced an intense self-grooming behaviour, indicating a likely role for α7 nAChRs in modulating this anxiogenic, compulsive-like effect of cocaine. In the major island of Calleja, a key area of action for neuroleptics, MLA+cocaine reduced choline transporter binding compared with cocaine (with or without DHβE) administration. DHβE treatment prevented the induction of stereotypy sensitisation to cocaine but prolonged locomotor sensitisation, implicating heteromeric β2* nAChRs in the neuroadaptations mediating cocaine-induced behavioural sensitisation. 'Binge' cocaine treatment region-specifically increased α4β2* nAChR binding in the midbrain dopaminergic regions: ventral tegmental area and substantia nigra pars compacta.

CONCLUSIONS AND IMPLICATIONS

We have shown a differential, subtype-selective, contribution of nAChRs to the behavioural and neurochemical sequelae of chronic cocaine administration. These data support the clinical utility of targeting specific nAChR subtypes for the alleviation of cocaine-abuse symptomatology.

Behavioral, pharmacological, and immunological abnormalities after streptococcal exposure: a novel rat model of Sydenham chorea and related neuropsychiatric disorders

Group A streptococcal (GAS) infections and autoimmunity are associated with the onset of a spectrum of neuropsychiatric disorders in children, with the prototypical disorder being Sydenham chorea (SC). Our aim was to develop an animal model that resembled the behavioral, pharmacological, and immunological abnormalities of SC and other streptococcal-related neuropsychiatric disorders. Male Lewis rats exposed to GAS antigen exhibited motor symptoms (impaired food manipulation and beam walking) and compulsive behavior (increased induced-grooming). These symptoms were alleviated by the D2 blocker haloperidol and the selective serotonin reuptake inhibitor paroxetine, respectively, drugs that are used to treat motor symptoms and compulsions in streptococcal-related neuropsychiatric disorders. Streptococcal exposure resulted in antibody deposition in the striatum, thalamus, and frontal cortex, and concomitant alterations in dopamine and glutamate levels in cortex and basal ganglia, consistent with the known pathophysiology of SC and related neuropsychiatric disorders. Autoantibodies (IgG) of GAS rats reacted with tubulin and caused elevated calcium/calmodulin-dependent protein kinase II signaling in SK-N-SH neuronal cells, as previously found with sera from SC and related neuropsychiatric disorders. Our new animal model translates directly to human disease and led us to discover autoantibodies targeted against dopamine D1 and D2 receptors in the rat model as well as in SC and other streptococcal-related neuropsychiatric disorders.

High anxiety is a predisposing endophenotype for loss of control over cocaine, but not heroin, self-administration in rats

RATIONALE

Although high anxiety is commonly associated with drug addiction, its causal role in this disorder is unclear.

OBJECTIVES

In light of strong evidence for dissociable neural mechanisms underlying heroin and cocaine addiction, the present study investigated whether high anxiety predicts the propensity of rats to lose control over intravenous cocaine or heroin self-administration.

METHODS

Sixty-four rats were assessed for anxiety in the elevated plus-maze, prior to extended access to intravenous cocaine or heroin self-administration.

RESULTS

High-anxious rats, identified in the lower quartile of the population, showed a greater escalation of cocaine, but not heroin, self-administration compared with low-anxious rats selected in the upper quartile of the population. Anxiety scores were also positively correlated with the extent of escalation of cocaine self-administration.

CONCLUSIONS

The present data suggest that high anxiety predisposes rats to lose control over cocaine-but not heroin-intake. High anxiety may therefore be a vulnerability trait for the escalation of stimulant but not opiate self-administration.

Differences in rat dorsal striatal NMDA and AMPA receptors following acute and repeated cocaine-induced locomotor activation

Sprague-Dawley rats can be classified as low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor activity induced by an acute low dose of cocaine. Upon repeated cocaine exposure, LCRs display greater locomotor sensitization, reward, and reinforcement than HCRs. Altered glutamate receptor expression in the brain reward pathway has been linked to locomotor sensitization and addiction. To determine if such changes contribute to the differential development of locomotor sensitization, we examined protein levels of total, phosphorylated, and cell surface glutamate N-methyl D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors (Rs) following acute or repeated cocaine (10 mg/kg, i.p.) in LCRs, HCRs and saline controls. Three areas involved in the development and expression of locomotor sensitization were investigated: the ventral tegmental area (VTA), nucleus accumbens (NAc) and dorsal striatum (dSTR). Our results revealed differences only in the dSTR, where we found that after acute cocaine, GluN2B(Tyr-1472) phosphorylation was significantly greater in LCRs, compared to HCRs and controls. Additionally in dSTR, after repeated cocaine, we observed significant increases in total GluA1, phosphorylated GluA1(Ser-845), and cell surface GluA1 in all cocaine-treated animals vs. controls. The acute cocaine-induced increases in NMDARs in dSTR of LCRs may help to explain the more ready development of locomotor sensitization and susceptibility to addiction-like behaviors in rats that initially exhibit little or no cocaine-induced activation, whereas the AMPAR increases after repeated cocaine may relate to recruitment of more dorsal striatal circuits and maintenance of the marked cocaine-induced locomotor activation observed in all of the rats.

Mice lacking neuropeptide Y show increased sensitivity to cocaine

There is increasing data implicating neuropeptide Y (NPY) in the neurobiology of addiction. This study explored the possible role of NPY in cocaine-induced behavior using NPY knockout mice. The transgenic mice showed a hypersensitive response to cocaine in three animal models of cocaine addiction. Whether this is due to an observed compensatory increase in striatal dopamine transporter binding or an anxiogenic phenotype of the transgenic mice remains to be determined.

D1 and D2 dopaminergic systems in the rat basolateral amygdala are involved in anxiogenic-like effects induced by histamine

Involvement of the dopamine receptors in the basolateral amygdala (BLA) in the effects of histamine on anxiety-like behaviors of the elevated plus maze in male Wistar rats was investigated. The results showed that bilateral intra-BLA injections of histamine (2.5, 5 and 7.5 µg/rat) induced an anxiogenic-like effect, revealed by decreases in percentage of open arm time (%OAT) and open arm entries (%OAE). Intra-BLA administration of dopamine D1 receptor agonist, SKF38393 (0.25 µg/rat), and dopamine D2 receptor agonist, quinpirole (0.03 and 0.05 µg/rat), decreased %OAT but not %OAE. Conversely, intra-BLA administration of dopamine D1 receptor antagonist, SCH23390 (0.5 and 1 µg/rat), and dopamine D2 receptor antagonist, sulpiride (0.3 and 0.5 µg/rat), increased %OAT and %OAE, suggesting an anxiolytic-like effect for both drugs. Interestingly, co-administration of a silent dose of SCH23390 or sulpiride prevented anxiogenic-like effects of SKF38393 and quinpirole, respectively. Conjoint administration of a sub-effective dose of SKF38393 (0.125 µg/rat) or quinpirole (0.01 µg/rat) along with lower doses of histamine (1 and 2.5 µg/rat) induced anxiolytic-like effects. On the other hand, intra-BLA pretreatment with a silent dose of SCH23390 (0.25 µg/rat) or sulpiride (0.1 µg/rat) prevented the anxiogenic-like effect of higher doses of histamine (5 and 7.5 µg/rat). No significant change was observed in total closed arm entries, as an index for motor activity of the animals. It can be concluded that the dopamine D1 and D2 receptors in the BLA may be involved in the anxiogenic-like effects induced by histamine.

Acquisition of cocaine self-administration in male Sprague-Dawley rats: effects of cocaine dose but not initial locomotor response to cocaine

RATIONALE

We have previously described a model in which adult outbred male Sprague-Dawley rats are classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on acute cocaine-induced open-field activation. This model revealed important individual differences in cocaine's effects, including that LCRs exhibited greater responding than HCRs on a progressive ratio schedule of cocaine reinforcement. However, no LCR/HCR differences in acquisition of cocaine self-administration (0.25 mg/kg/12 s infusion) were observed under these conditions.

OBJECTIVES

To determine if LCRs and HCRs differ in the effectiveness of cocaine to function as a reinforcer under a broader range of conditions, the present study assessed the acquisition of cocaine self-administration (fixed ratio 1 schedule of reinforcement) as a function of i.v. cocaine dose (0.1875, 0.375, 0.5, 1, or 1.5 mg/kg/6 s infusion).

RESULTS

LCRs and HCRs did not differ significantly on any measure of acquisition examined, including the day to meet acquisition criterion, percent acquired, and cocaine intake. The effect of dose on percent acquired and rate of acquisition peaked at the 1-mg/kg/infusion dose of cocaine. In contrast, the effect of dose on cocaine intake was linear, with the highest rate of intake occurring at the 1.5-mg/kg/infusion dose of cocaine.

CONCLUSIONS

LCRs and HCRs do not appear to differ in their acquisition of cocaine-reinforced operant responding across a range of cocaine doses, including conditions that lead to high levels of cocaine intake.

Lack of vasopressin does not prevent the behavioural and endocrine changes induced by chronic unpredictable stress

Vasopressin (VP) plays an important role in hypothalamo-pituitary-adrenal (HPA) axis regulation and in stress-related disorders. Our previous studies confirmed the role of VP in acute situations, where VP-deficient Brattleboro rats had less depression-like behaviour compared to animals that express VP. In this study, we test the hypothesis that VP-deficient rats are more resistant to the development of chronic HPA axis hyperactivity and depression-like symptoms after chronic unpredictable stress (CUS). Male VP-deficient Brattleboro rats were compared to their heterozygous littermates (controls). CUS consisted of different mild stimuli for 5 weeks. Elevated plus maze and forced swim test were used for behavioural characterization, while organs and blood for HPA axis parameters were collected at the end of the experiment. In controls, CUS resulted in the development of chronic stress state characterized by typical somatic (body weight reduction, thymus involution) and endocrine changes (resting plasma ACTH and corticosterone elevation and POMC mRNA elevation in anterior lobe of the pituitary). Floating time in the forced swim test was enhanced together with reduced open arm entries on elevated plus maze and a reduction in daily food intake. Unexpectedly, the lack of VP did not alter the effect of CUS on the somatic and behavioural measures, but only prevented CUS-induced corticosterone changes. In conclusion, lifelong VP-deficiency has a positive effect on corticosterone elevation following CUS but does not affect the behavioural consequences of CUS. It is likely that the interplay of several related factors, rather than an alteration in a single neuropeptide, modulates behaviour and disease pathogenesis.

Individual differences in cocaine-induced locomotor activity of male Sprague-Dawley rats are not explained by plasma corticosterone levels

Humans differ in their initial response to, and subsequent abuse of, addictive drugs like cocaine. Rodents also exhibit marked individual differences in responsiveness to cocaine. Previously, we classified male Sprague-Dawley rats as either low or high cocaine responders (LCRs or HCRs, respectively), based on their acute low-dose cocaine-induced locomotor activity, and found that with repeated drug exposure LCRs exhibit greater cocaine locomotor sensitization, reward and reinforcement than HCRs. Differential cocaine-induced increases in striatal dopamine help to explain the LCR/HCR phenotypes. Differential levels of stress and/or anxiety could also contribute but have not been explored. Here we measured open-field activity and plasma corticosterone levels both pre- and post-cocaine treatment in LCRs, HCRs, and saline-treated controls. The three groups did not differ in baseline locomotor activity or corticosterone levels. Importantly, LCR/HCR differences in corticosterone levels were also not observed following acute cocaine (10mg/kg, i.p.), when cocaine induced approximately 3.5-fold greater locomotor activity in HCRs than LCRs. Additionally, there were no LCR/HCR differences in plasma corticosterone levels following 5 days of once-daily cocaine, during which time LCRs developed locomotor sensitization such that their cocaine-induced locomotor activity no longer differed from that of HCRs. Likewise, there were no group activity differences in any of four concentric zones within the open-field chamber. In summary, neither plasma corticosterone levels nor thigmotaxis-type anxiety appears to be a factor that contributes to the observed cocaine-induced LCR/HCR behavioral differences.

Effect of cocaine sensitization prior to pregnancy on maternal care and aggression in the rat

RATIONALE

Although many studies have investigated the effects of gestational and lactational cocaine use on maternal behavior, few studies have examined the effects of prior adult cocaine use on maternal behavior.

METHODS

In the current study, intraperitoneal cocaine or saline was administered to adult female rats for 10 days, treatments were stopped for 4 days, and the females were then mated. Litter size and weight were recorded on days 2 and 16 of lactation, and maternal care, maternal aggression, and light/dark box activity were assessed on days 2, 9, and 16 of lactation.

RESULTS

The cocaine-treated females exhibited behavioral sensitization on treatment day 10. Mothers previously treated with cocaine retrieved pups more quickly, spent more time caring for the pups (increased pup retrieval, pup grooming, and crouching over the pups by the mother), and were more aggressive towards a male intruder compared to saline-treated control animals on day 2 of lactation. These effects were specific to maternal behavior, as other behaviors were mostly unaffected by prior cocaine treatment. Data from light/dark box tests indicate that cocaine did not affect activity in this test. There were few significant behavioral effects of prior cocaine treatment on lactation days 9 and 16.

CONCLUSIONS

Prior cocaine use may have lasting effects on offspring care and maternal aggression, specifically during early lactation. The present data support the hypothesis that maternal behavior is increased by prior cocaine sensitization, possibly due to cross-sensitization between cocaine and the natural reward of maternal behavior.

Environmental enrichment decreases the rewarding but not the activating effects of heroin

RATIONALE

Environmental conditions during adolescence, a critical period of brain maturation, can have important consequences on subsequent vulnerability to drugs of abuse. We have recently found that the behavioral effects of cocaine as well as its ability to increase expression of zif-268 are reduced in mice reared in enriched environments (EE).

OBJECTIVES

The present experiments examined whether environmental enrichment has protective influences on the effects of heroin, a drug of addiction whose mechanism of action differs from that of cocaine.

MATERIALS AND METHODS

Mice were housed either in standard environments (SE) or in EE from weaning to adulthood before any drug exposure. EE were constituted by big housing cages and contained constantly a running wheel and a small house and four to five toys that were changed once a week with new toys of different shapes and colors. We assessed the influence of EE on the ability of heroin to (1) induce conditioned place preferences, (2) induce behavioral sensitization, (3) increase dopamine levels in the nucleus accumbens (NAc), and (4) increase expression of the immediate early gene zif-268 in the striatum.

RESULTS

Conditioned place preference but not behavioral sensitization was reduced in EE mice compared to SE mice. Heroin induced similar increases in dopamine levels and in the expression of zif-268 in the NAc of EE and SE mice.

CONCLUSIONS

The rewarding effects of heroin are blunted by EE and appear to be, at least in part, independent from activation of the mesolimbic system.

Neurokinin3 receptor modulation of the behavioral and neurochemical effects of cocaine in rats and monkeys

Neurokinin3 (NK3) receptors and their endogenous ligands (e.g. the neuropeptide substance P and its C-terminal fragment) have been implicated in psychomotor activity and reinforcement mechanisms. We review here recent findings on the involvement of NK3 receptors in the behavioral and neurochemical effects of cocaine. Although NK3 receptors can modulate dopamine (DA) activity in the brain, recent results suggest that this modulation does not occur during spontaneous behavioral activity. However, NK3 receptors play a role in the regulation of cocaine-induced DA responses in the nucleus accumbens core and shell subregions. NK3 receptor agonism as well as antagonism potentiate cocaine's effects on nucleus accumbens DA subregions specifically, and modulate the acute behavioral effects of cocaine in rats and non-human primates (Callithrix penicillata). However, conditioned place preference studies in rats have, so far, failed to provide evidence for an involvement of NK3 receptors in the reinforcing effects of cocaine.