Adolescent and adult responsiveness to the incentive value of cocaine reward in mice: role of neuronal nitric oxide synthase (nNOS) gene

Further proof on the role of accumbal nNOS in cocaine-seeking behavior in rats

BACKGROUND

Cocaine use disorder (CUD) remains a severe health problem with no effective pharmacological therapy. One of the potential pharmacological strategies for CUD pharmacotherapy includes manipulations of the brain glutamatergic (Glu) system which is particularly involved in drug withdrawal and relapse. Previous research indicated a pivotal role of ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic receptors' type 5 (mGlu5) receptors in controlling the reinstatement of cocaine. Stimulation of the above molecules results in the activation of the downstream signaling targets such as neuronal nitric oxide synthase (nNOS) and the release of nitric oxide.

METHODS

In this paper, we investigated the molecular changes in nNOS in the prefrontal cortex and nucleus accumbens following 3 and 10 days of cocaine abstinence as well as the effectiveness of nNOS blockade with the selective enzyme inhibitor N-ω-propyl-L-arginine hydrochloride (L-NPA) on cocaine seeking in male rats. The effect of L-NPA on locomotor activity in drug-naïve animals was investigated.

RESULTS

Ten-day (but not 3-day) cocaine abstinence from cocaine self-administration increased nNOS gene and protein expression in the nucleus accumbens, but not in the prefrontal cortex. L-NPA (0.5-5 mg/kg) administered peripherally did not change locomotor activity but attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue.

CONCLUSIONS

Our findings support accumbal nNOS as an important molecular player for cocaine seeking while its inhibitors could be considered as anti-cocaine pharmacological tools in male rats.

Interaction of gonadal hormones, dopaminergic system, and epigenetic regulation in the generation of sex differences in substance use disorders: A systematic review

Substance use disorder (SUD) is a chronic condition characterized by pathological drug-taking and seeking behaviors. Remarkably different between males and females, suggesting that drug addiction is a sexually differentiated disorder. The neurobiological bases of sex differences in SUD include sex-specific reward system activation, influenced by interactions between gonadal hormone level changes, dopaminergic reward circuits, and epigenetic modifications of key reward system genes. This systematic review, adhering to PICOS and PRISMA-P 2015 guidelines, highlights the sex-dependent roles of estrogens, progesterone, and testosterone in SUD. In particular, estradiol elevates and progesterone reduces dopaminergic activity in SUD females, whilst testosterone and progesterone augment SUD behavior in males. Finally, SUD is associated with a sex-specific increase in the rate of opioid and monoaminergic gene methylation. The study reveals the need for detailed research on gonadal hormone levels, dopaminergic or reward system activity, and epigenetic landscapes in both sexes for efficient SUD therapy development.

Nitric oxide and sex differences in dendritic branching and arborization

Nitric oxide (NO) is an important signaling molecule with many functions in the nervous system. Derived from the enzymatic conversion of arginine by several nitric oxide synthases (NOS), NO plays significant roles in neuronal developmental events such as the establishment of dendritic branching or arbors. A brief summary of the discovery, molecular biology, and chemistry of NO, and a description of important NO-mediated signal transduction pathways with emphasis on the role for NO in the development of dendritic branching during neurodevelopment are presented. Important sex differences in neuronal nitric oxide synthase expression during neuronal development are considered. Finally, a survey of endogenous and exogenous substances that disrupt dendritic patterning is presented with particular emphasis on how these molecules may drive NO-mediated sex differences in dendritic branching.

nNOS-mediated protein-protein interactions: promising targets for treating neurological and neuropsychiatric disorders

Neurological and neuropsychiatric disorders are one of the leading causes of disability worldwide and affect the health of billions of people. Nitric oxide (NO), a free gas with multitudinous bioactivities, is mainly produced from the oxidation of L-arginine by neuronal nitric oxide synthase (nNOS) in the brain. Inhibiting nNOS benefits a variety of neurological and neuropsychiatric disorders, including stroke, depression and anxiety disorders, post-traumatic stress disorder, Parkinson's disease, Alzheimer's disease, chronic pain, and drug addiction. Due to critical roles of nNOS in learning and memory and synaptic plasticity, direct inhibition of nNOS may cause severe side effects. Importantly, interactions of several proteins, including post-synaptic density 95 (PSD-95), carboxy-terminal PDZ ligand of nNOS (CAPON) and serotonin transporter (SERT), with the PSD/Disc-large/ZO-1 homologous (PDZ) domain of nNOS have been demonstrated to influence the subcellular distribution and activity of the enzyme in the brain. Therefore, it will be a preferable means to interfere with nNOS-mediated protein-protein interactions (PPIs), which do not lead to undesirable effects. Herein, we summarize the current literatures on nNOS-mediated PPIs involved in neurological and neuropsychiatric disorders, and the discovery of drugs targeting the PPIs, which is expected to provide potential targets for developing novel drugs and new strategy for the treatment of neurological and neuropsychiatric disorders.

African American Children's Diminished Returns of Subjective Family Socioeconomic Status on Fun Seeking

Background: Reward sensitivity (fun-seeking) is a risk factor for a wide range of high-risk behaviors. While high socioeconomic status (SES) is known to reduce reward sensitivity and associated high-risk behaviors, less is known about the differential effects of SES on reward sensitivity. It is plausible to expect weaker protective effects of family SES on reward sensitivity in racial minorities, a pattern called Minorities' Diminished Returns (MDRs). Aim: We compared Caucasian and African American (AA) children for the effects of subjective family SES on children's fun-seeking. Methods: This was a cross-sectional analysis of 7061 children from the Adolescent Brain Cognitive Development (ABCD) study. The independent variable was subjective family SES. The main outcome was children's fun-seeking measured by the behavioral approach system (BAS) and behavioral avoidance system (BIS). Age, gender, marital status, and household size were the covariates. Results: In the overall sample, high subjective family SES was associated with lower levels of fun-seeking. We also found a statistically significant interaction between race and subjective family SES on children's fun-seeking in the overall sample, suggesting that high subjective family SES is associated with a weaker effect on reducing fun-seeking among AA than Caucasian children. In race-stratified models, high subjective family SES was protective against fun-seeking of Caucasian but not AA children. Conclusion: Subjective family SES reduces the fun-seeking for Caucasian but not AA children.

Reward Responsiveness in the Adolescent Brain Cognitive Development (ABCD) Study: African Americans' Diminished Returns of Parental Education

(1) Background: Reward responsiveness (RR) is a risk factor for high-risk behaviors such as aggressive behaviors and early sexual initiation, which are all reported to be higher in African American and low socioeconomic status adolescents. At the same time, parental education is one of the main drivers of reward responsiveness among adolescents. It is still unknown if some of this racial and economic gap is attributed to weaker effects of parental education for African Americans, a pattern also called minorities' diminished returns (MDRs). (2) Aim: We compared non-Hispanic White and African American adolescents for the effects of parent education on adolescents RR, a psychological and cognitive construct that is closely associated with high-risk behaviors such as the use of drugs, alcohol, and tobacco. (3) Methods: This was a cross-sectional analysis that included 7072 adolescents from the adolescent brain cognitive development (ABCD) study. The independent variable was parent education. The main outcome as adolescents' RR measured by the behavioral inhibition system (BIS) and behavioral activation system (BAS) measure. (4) Results: In the overall sample, high parent education was associated with lower levels of RR. In the overall sample, we found a statistically significant interaction between race and parent education on adolescents' RR. The observed statistical interaction term suggested that high parent education is associated with a weaker effect on RR for African American than non-Hispanic White adolescents. In race-stratified models, high parent education was only associated with lower RR for non-Hispanic White but not African American adolescents. (5) Conclusion: Parent education reduces RR for non-Hispanic White but not African American adolescents. To minimize the racial gap in brain development and risk-taking behaviors, we need to address societal barriers that diminish the returns of parent education and resources in African American families. We need public and social policies that target structural and societal barriers, such as the unequal distribution of opportunities and resources. To meet such an aim, we need to reduce the negative effects of social stratification, segregation, racism, and discrimination in the daily lives of African American parents and families. Through an approach like this, African American families and parents can effectively mobilize their resources and utilize their human capital to secure the best possible tangible outcomes for their adolescents.

Amperometric measurements of cocaine cue and novel context-evoked glutamate and nitric oxide release in the nucleus accumbens core

Cue-induced reinstatement of cocaine seeking after self-administration (SA) and extinction relies on glutamate release in the nucleus accumbens core (NAcore), which activates neuronal nitric oxide synthase interneurons. Nitric oxide (NO) is required for structural plasticity in NAcore medium spiny neurons, as well as cued cocaine seeking. However, NO release in the NAcore during reinstatement has yet to be directly measured. Furthermore, the temporal relationship between glutamate release and the induction of an NO response also remains unknown. Using wireless amperometric recordings in awake behaving rats, we quantified the magnitude and temporal dynamics of novel context- and cue-induced reinstatement-evoked glutamate and NO release in the NAcore. We found that re-exposure to cocaine-conditioned stimuli following SA and extinction increased extracellular glutamate, leading to release of NO in the NAcore. In contrast, exposing drug-naïve rats to a novel context led to a lower magnitude rise in glutamate in the NAcore relative to cue-induced reinstatement. Interestingly, novel context exposure evoked a higher magnitude NO response relative to cue-induced reinstatement. Despite differences in magnitude, novel context evoked-NO release in the NAcore was also temporally delayed when compared to glutamate. These results demonstrate a dissociation between the magnitude of cocaine cue- and novel context-evoked glutamate and NO release in the NAcore, yet similarity in the temporal dynamics of their release. Together, these data contribute to a greater understanding of the relationship between glutamate and NO, two neurotransmitters implicated in encoding the valence of distinct contextual stimuli.

Age-Related Decline in Children's Reward Sensitivity: Blacks' Diminished Returns

BACKGROUND

It is important to study the correlates of reward sensitivity since it predicts high-risk behaviors. While ageing reduces children's reward sensitivity and its associated risk taking, there is more to find out about racial differences in regard to the effect of age on reward sensitivity. Minorities' Diminished Returns (MDRs) suggest that resources and assets show weaker effects on Black children than White children.

AIM

We compared White children to Black children as for the effects of age on reward sensitivity.

METHODS

This cross-sectional study included 10533 American children who participated in the baseline of the Adolescent Brain Cognitive Development (ABCD) study. The independent variable was age, while the dependent variable was reward sensitivity as captured by the behavioral approach/behavioral avoidance system (BAS-BIS). Gender, parental education, marital status, parental education, and household income were the covariates.

RESULTS

Higher age was associated with less reward sensitivity. A significant interaction was found between race and age when it comes to children's reward sensitivity. It suggested that age is associated with a smaller gain in terms of reduced reward sensitivity in Black children than White children.

CONCLUSION

Age is more likely to reduce reward sensitivity in White children than Black children. This finding is in line with MDRs, and may be due to social racism, segregation, stratification, and discrimination.

The role of neuronal nitric oxide synthase in cocaine place preference and mu opioid receptor expression in the nucleus accumbens

RATIONALE

There is evidence that central mu opioid receptors (MORs) are implicated in several aspects of cocaine addiction, and that MOR expression is elevated by cocaine in vitro and in the nucleus accumbens (NAc) when administered in vivo.

OBJECTIVE

To understand the cellular mechanisms involved in regulating MOR expression, this study explored whether neuronal nitric oxide synthase (nNOS) modulates the neurochemical and behavioral effects of acute and repeated cocaine administration.

METHODS

Male Sprague-Dawley rats received a single cocaine injection (20 mg/kg, i.p.) in combination with the selective nNOS inhibitor 7-nitroindazole (7-NI) (0, 25, or 50 mg/kg, i.p.), and the expression of MOR and nNOS messenger RNA (mRNA) and protein levels in the NAc were measured. In a separate conditioned place preference (CPP) experiment, 7-NI (0, 25, or 50 mg/kg, i.p.) was administered prior to cocaine (0 or 20 mg/kg, i.p.) conditioning sessions, and levels of MOR and nNOS mRNA and protein in the NAc were measured following CPP test.

RESULTS

Acute cocaine administration significantly enhanced nNOS and MOR mRNA and protein expression in the NAc, and this increase in MOR expression was blocked by 7-NI. Furthermore, in 7-NI pre-treated rats, cocaine-induced CPP was not statistically significant and the increase in MOR mRNA expression in the NAc in these animals was attenuated.

CONCLUSIONS

These findings suggest that nNOS modulates MOR expression following acute cocaine administration, and that cocaine CPP and associated upregulation of MOR expression involve both nNOS-dependent and independent mechanisms. Elucidation of these molecular events may identify useful therapeutic target for cocaine addiction.

Nitric oxide in the medial prefrontal cortex contributes to the acquisition of cocaine place preference and synaptic plasticity in the laterodorsal tegmental nucleus

Nitric oxide (NO), a gaseous neurotransmitter, is involved in a variety of brain functions, including drug addiction. Although previous studies have suggested that NO plays an important role in the development of cocaine addiction, the brain region(s) in which NO acts and how it contributes to cocaine addiction remain unclear. In this study, we examined these issues using a cocaine-induced conditioned place preference (CPP) paradigm and ex vivo electrophysiological recordings in rats. Specifically, we focused on the medial prefrontal cortex (mPFC) and laterodorsal tegmental nucleus (LDT), brain regions associated with cocaine CPP development and cocaine-induced plasticity. Intra-mPFC injection of the non-selective NO synthase (NOS) inhibitor L-NAME or the neuronal NOS (nNOS) selective inhibitor L-NPA during the conditioning phase disrupted cocaine CPP. Additionally, intra-mPFC injection of L-NPA prior to each cocaine injection prevented the induction of presynaptic plasticity, induced by repeated cocaine administration, in LDT cholinergic neurons. These findings indicate that NO generated in the mPFC contributes to the acquisition of cocaine CPP and the induction of neuroplasticity in LDT cholinergic neurons. Together with previous studies showing that NO induces membrane plasticity in mPFC neurons, that mPFC neurons project to the LDT, and that LDT activity is critical for the acquisition of cocaine CPP, the present findings suggest that NO-mediated neuroplasticity induced in the mPFC-LDT circuitry is critical for the development of cocaine addiction.

Cardiovascular Mitochondrial Dysfunction Induced by Cocaine: Biomarkers and Possible Beneficial Effects of Modulators of Oxidative Stress

Cocaine abuse has long been known to cause morbidity and mortality due to its cardiovascular toxic effects. The pathogenesis of the cardiovascular toxicity of cocaine use has been largely reviewed, and the most recent data indicate a fundamental role of oxidative stress in cocaine-induced cardiovascular toxicity, indicating that mitochondrial dysfunction is involved in the mechanisms of oxidative stress. The comprehension of the mechanisms involving mitochondrial dysfunction could help in selecting the most appropriate mitochondria injury biological marker, such as superoxide dismutase-2 activity and glutathionylated hemoglobin. The potential use of modulators of oxidative stress (mitoubiquinone, the short-chain quinone idebenone, and allopurinol) in the treatment of cocaine cardiotoxic effects is also suggested to promote further investigations on these potential mitochondria-targeted antioxidant strategies.

Influence of the Novelty-Seeking Endophenotype on the Rewarding Effects of Psychostimulant Drugs in Animal Models

Novelty seeking (NS), defined as a tendency to pursue novel and intense emotional sensations and experiences, is one of the most relevant individual factors predicting drug use among humans. High novelty seeking (HNS) individuals present an increased risk of drug use compared to low novelty seekers. The NS endophenotype may explain some of the differences observed among individuals exposed to drugs of abuse in adolescence. However, there is little research about the particular response of adolescents to drugs of abuse in function of this endophenotype, and the data that do exist are inconclusive. The present work reviews the literature regarding the influence of NS on psychostimulant reward, with particular focus on adolescent subjects. First, the different animal models of NS and the importance of this endophenotype in adolescence are discussed. Later, studies that have used the most common animal models of reward (self-administration, conditioned place preference paradigms) to evaluate how the NS trait influences the rewarding effects of psychostimulants are reviewed. Finally, possible explanations for the enhanced risk of developing substance dependence among HNS individuals are discussed. In conclusion, the studies referred to in this review show that the HNS trait is associated with: (1) increased initial sensitivity to the rewarding effects of psychostimulants, (2) a higher level of drug craving when the subject is exposed to the environmental cues associated with the drug, and (3) enhanced long-term vulnerability to relapse to drug consumption after prolonged abstinence.

Variations in the stimulus salience of cocaine reward influences drug-associated contextual memory

Drugs of abuse act as reinforcers because they influence learning and memory processes resulting in long-term memory of drug reward. We have previously shown that mice conditioned by fixed daily dose of cocaine (Fix-C) or daily escalating doses of cocaine (Esc-C) resulted in short- and long-term persistence of drug memory, respectively, suggesting different mechanisms in acquisition of cocaine memory. The present study was undertaken to investigate the differential contribution of N-methyl-D-aspartate receptor (NMDAR) subunits in the formation of Fix-C and Esc-C memory in C57BL/6J mice. Training by Esc-C resulted in marked elevation in hippocampal expression of Grin2b mRNA and NR2B protein levels compared with training by Fix-C. The NR2B-containing NMDAR antagonist ifenprodil had similar attenuating effects on acquisition and reconsolidation of Fix-C and Esc-C memory. However, the NMDAR antagonist MK-801 had differential effects: (1) higher doses of MK-801 were required for post-retrieval disruption of reconsolidation of Esc-C memory than Fix-C memory; and (2) pre-retrieval MK-801 inhibited extinction of Fix-C memory but it had no effect on Esc-C memory. In addition, blockade of NMDAR downstream signaling pathways also showed differential regulation of Fix-C and Esc-C memory. Inhibition of neuronal nitric oxide synthase attenuated acquisition and disrupted reconsolidation of Fix-C but not Esc-C memory. In contrast, the mitogen-activating extracellular kinase inhibitor SL327 attenuated reconsolidation of Esc-C but not Fix-C memory. These results suggest that NMDAR downstream signaling molecules associated with consolidation and reconsolidation of cocaine-associated memory may vary upon changes in the salience of cocaine reward during conditioning.

Emergence of sex differences in the development of substance use and abuse during adolescence

Substance use and abuse begin during adolescence. Male and female adolescent humans initiate use at comparable rates, but males increase use faster. In adulthood, more men than women use and abuse addictive drugs. However, some women progress more rapidly from initiation of use to entry into treatment. In animal models, adolescent males and females consume addictive drugs similarly. However, reproductively mature females acquire self-administration faster, and in some models, escalate use more. Sex/gender differences exist in neurobiologic factors mediating both reinforcement (dopamine, opioids) and aversiveness (CRF, dynorphin), as well as intrinsic factors (personality, psychiatric co-morbidities) and extrinsic factors (history of abuse, environment especially peers and family) which influence the progression from initial use to abuse. Many of these important differences emerge during adolescence, and are moderated by sexual differentiation of the brain. Estradiol effects which enhance both dopaminergic and CRF-mediated processes contribute to the female vulnerability to substance use and abuse. Testosterone enhances impulsivity and sensation seeking in both males and females. Several protective factors in females also influence initiation and progression of substance use including hormonal changes of pregnancy as well as greater capacity for self-regulation and lower peak levels of impulsivity/sensation seeking. Same sex peers represent a risk factor more for males than females during adolescence, while romantic partners increase risk for women during this developmental epoch. In summary, biologic factors, psychiatric co-morbidities as well as personality and environment present sex/gender-specific risks as adolescents begin to initiate substance use.

Sex differences in conditioned nicotine reward are age-specific

Women constitute half of all smokers and many studies suggest that adult males and females differ in factors that maintain tobacco smoking, yet there is limited information about sex differences in nicotine reward during adolescence. Limited studies suggest that adolescent male rats self-administer more nicotine than adults, suggesting that drug administration during adolescence leads to different behavioral effects than during adulthood. In the present study, male rats developed a significant conditioned place preference (CPP) to lower doses of nicotine than females, regardless of age. In addition, adolescents were more sensitive than adults. In female rats, adolescents exhibited a CPP of greater magnitude than adult females. In males, the magnitude of the CPP did not differ as a function of age, but adolescents exhibited CPP to lower doses than adults. There also were differences in nicotinic acetylcholinergic receptor binding in nucleus accumbens and caudate putamen in response to nicotine across age and sex. These findings suggest that it is necessary to consider sex- and age-specific effects of drugs such as nicotine when developing strategies for improving smoking cessation treatments.

The strength of aversive and appetitive associations and maladaptive behaviors

Certain maladaptive behaviors are thought to be acquired through classical Pavlovian conditioning. Exaggerated fear response, which can develop through Pavlovian conditioning, is associated with acquired anxiety disorders such as post-traumatic stress disorders (PTSDs). Inflated reward-seeking behavior, which develops through Pavlovian conditioning, underlies some types of addictive behavior (e.g., addiction to drugs, food, and gambling). These maladaptive behaviors are dependent on associative learning and the development of long-term memory (LTM). In animal models, an aversive reinforcer (fear conditioning) encodes an aversive contextual and cued LTM. On the other hand, an appetitive reinforcer results in conditioned place preference (CPP) that encodes an appetitive contextual LTM. The literature on weak and strong associative learning pertaining to the development of aversive and appetitive LTM is relatively scarce; thus, this review is particularly focused on the strength of associative learning. The strength of associative learning is dependent on the valence of the reinforcer and the salience of the conditioned stimulus that ultimately sways the strength of the memory trace. Our studies suggest that labile (weak) aversive and appetitive LTM may share similar signaling pathways, whereas stable (strong) aversive and appetitive LTM is mediated through different pathways. In addition, we provide some evidence suggesting that extinction of aversive fear memory and appetitive drug memory is likely to be mediated through different signaling molecules. We put forward the importance of studies aimed to investigate the molecular mechanisms underlying the development of weak and strong memories (aversive and appetitive), which would ultimately help in the development of targeted pharmacotherapies for the management of maladaptive behaviors that arise from classical Pavlovian conditioning.

Oxycodone-induced conditioned place preference and sensitization of locomotor activity in adolescent and adult mice

Nonmedical use of the prescription opioid oxycodone has become a major public health problem in the United States, with special concern for adolescents. Although adults and adolescents have different sensitivities for drugs, little is known about the rewarding effects of oxycodone in adolescents compared to adults, even in rodent models. Here, we investigate sensitivity to oxycodone by the conditioned place preference assay of conditioned reward, and effect on the locomotor activity in adolescent (4 weeks old) and adult (10 weeks old) C57BL/6J mice. Mice of both ages were trained with multiple doses of oxycodone (0, 0.3, 1, and 3 mg/kg) and showed conditioned preference in a dose-dependent manner. The adult mice developed conditioned preference to the lowest dose tested (0.3 mg/kg), but adolescent mice did not. Dose-dependent oxycodone-induced increases in locomotor activity were observed across the conditioning session. Interestingly, adolescent mice developed greater sensitization to the locomotor-activating effects of oxycodone than adult mice. Thus differences in sensitivity to oxycodone, such as the lower initial sensitivity for conditioned preference but greater locomotor sensitization in adolescent mice, may indicate contributing factors in oxycodone abuse and later addiction in human adolescents.

Influence of abstinence and intervals between extinction trials on the expression of cocaine-conditioned place preference in adolescent rats

RATIONALE

Disruption of acquired drug-cue associations can effectively decrease relapse. The benefits of extinction training as opposed to abstinence have been reported. Timing of extinction trials is an important variable. Finding an effective extinction regimen can optimize addiction therapies.

OBJECTIVE

To determine the effects of different drug-free periods on cocaine-conditioned place preference (CPP) in rats that either did or did not receive non-reinforced exposure to drug-associated stimuli.

MATERIALS AND METHODS

Male adolescent rats were trained for cocaine-CPP (5, 10, or 15 mg/kg, i.p.) in a biased manner for 8 days and then tested following different intervals.

RESULTS

Rats treated with 15 mg/kg cocaine displayed high and equal CPP on the first test, performed 1, 4, 7, or 14 days following conditioning. Expression of CPP during the test performed 1 day after conditioning was equal in the groups conditioned with 5, 10, or 15 mg/kg cocaine. When the interval before the first test was extended to 14 days, the group treated with 5 mg/kg did not show CPP. Rats treated with the three doses and tested repeatedly at 1, 7, and 14 days did not display CPP on the third test. CPP after treatment with 10 or 15 mg/kg cocaine was already extinguished in the second test but only for an interval of 1-14 days.

CONCLUSIONS

Maintenance of CPP was evident at least 2 weeks after forced abstinence. Extinguished CPP can be obtained after a single extinction trial, performed close to original training and followed by prolonged abstinence. However, with low doses of cocaine, abstinence alone may be sufficient to disrupt drug-cue associations.

A role for the prefrontal cortex in heroin-seeking after forced abstinence by adult male rats but not adolescents

Adolescent drug abuse is hypothesized to increase the risk of drug addiction. Yet male rats that self-administer heroin as adolescents show attenuated drug-seeking after abstinence, compared with adults. Here we explore a role for neural activity in the medial prefrontal cortex (mPFC) in age-dependent heroin-seeking. Adolescent (35-day-old at start; adolescent-onset) and adult (86-day-old at start) male rats acquired lever-pressing maintained by heroin using a fixed ratio one reinforcement schedule (0.05 and 0.025 mg/kg per infusion). Following 12 days of forced abstinence, rats were tested for heroin-seeking over 1 h by measuring the number of lever presses on the active lever. Unbiased stereology was then used to estimate the number of Fos-ir(+) and Fos-ir(-) neurons in prelimbic and infralimbic mPFC. As before, adolescents and adults self-administered similar amounts of heroin, but subsequent heroin-seeking was attenuated in the younger rats. Similarly, the adolescent-onset group failed to show significant neural activation in the prelimbic or infralimbic mPFC during the heroin-seeking test, whereas the adult-onset heroin self-administration group showed two to six times more Fos-ir(+) neurons than their saline counterparts in both mPFC subregions. Finally, the overall number of neurons in the infralimbic cortex was greater in rats from the adolescent-onset groups than adults. The mPFC may thus have a key role in some age-dependent effects of heroin self-administration.

Cocaine-induced reinstatement of a conditioned place preference in developing rats: involvement of the d2 receptor

Reinstatement of conditioned place preferences have been used to investigate physiological mechanisms mediating drug-seeking behavior in adolescent and adult rodents; however, it is still unclear how psychostimulant exposure during adolescence affects neuron communication and whether these changes would elicit enhanced drug-seeking behavior later in adulthood. The present study determined whether the effects of intra-ventral tegmental area (VTA) or intra-nucleus accumbens septi (NAcc) dopamine (DA) D2 receptor antagonist infusions would block (or potentiate) cocaine-induced reinstatement of conditioned place preferences. Adolescent rats (postnatal day (PND 28–39)) were trained to express a cocaine place preference. The involvement of D2 receptors on cocaine-induced reinstatement was determined by intra-VTA or intra-NAcc infusion of the DA D2 receptor antagonist sulpiride (100 μM) during a cocaine-primed reinstatement test (10 mg/kg cocaine, i.p.). Infusion of sulpiride into the VTA but not the NAcc blocked reinstatement of conditioned place preference. These data suggest intrinsic compensatory mechanisms in the mesolimbic DA pathway mediate responsivity to cocaine-induced reinstatement of a conditioned place preference during development.

Adolescent male rats are less sensitive than adults to the anxiogenic and serotonin-releasing effects of fenfluramine

Risk taking behavior increases during adolescence, which is also a critical period for the onset of drug abuse. The central serotonergic system matures during the adolescent period, and its immaturity during early adolescence may contribute to adolescent risk taking, as deficits in central serotonergic function have been associated with impulsivity, aggression, and risk taking. We investigated serotonergic modulation of behavior and presynaptic serotonergic function in adult (67-74 days old) and adolescent (28-34 days old) male rats. Fenfluramine (2 mg/kg, i.p.) produced greater anxiogenic effects in adult rats in both the light/dark and elevated plus maze tests for anxiety-like behavior, and stimulated greater increases in extracellular serotonin in the adult medial prefrontal cortex (mPFC) (1, 2.5, and 10 mg/kg, i.p.). Local infusion of 100 mM potassium chloride into the mPFC also stimulated greater serotonin efflux in adult rats. Adult rats had higher tissue serotonin content than adolescents in the prefrontal cortex, amygdala, and hippocampus, but the rate of serotonin synthesis was similar between age groups. Serotonin transporter (SERT) immunoreactivity and SERT radioligand binding were comparable between age groups in all three brain regions. These data suggest that lower tissue serotonin stores in adolescents limit fenfluramine-stimulated serotonin release and so contribute to the lesser anxiogenic effects of fenfluramine.

The effect of phosphodiesterase inhibitors on the extinction of cocaine-induced conditioned place preference in mice

Several phosphodiesterase inhibitors (PDEis) improve cognition, suggesting that an increase in brain cAMP and cGMP facilitates learning and memory. Since extinction of drug-seeking behavior requires associative learning, consolidation and formation of new memory, the present study investigated the efficacy of three different PDEis in the extinction of cocaine-induced conditioned place preference (CPP) in B6129S mice. Mice were conditioned by escalating doses of cocaine which was resistant to extinction by free exploration. Immediately following each extinction session mice received (a) saline/vehicle, (b) rolipram (PDE4 inhibitor), (c) BAY-73-6691 (PDE9 inhibitor) or (d) papaverine (PDE10A inhibitor). Mice that received saline/vehicle during extinction training showed no reduction in CPP for >10 days. BAY-73-6691 (a) dose-dependently increased cGMP in hippocampus and amygdala, (b) significantly facilitated extinction and (c) diminished the reinstatement of cocaine CPP. Rolipram, which selectively increased brain cAMP levels, and papaverine which caused increases in both cAMP and cGMP levels, had no significant effect on the extinction of cocaine CPP. The results suggest that increase in hippocampal and amygdalar cGMP levels via blockade of PDE9 has a prominent role in the consolidation of extinction learning.

Changes in the magnitude of drug-unconditioned stimulus during conditioning modulate cocaine-induced place preference in mice

Drug reinforcement learning is relevant for the development of addiction. The present study investigated how changes in the magnitude of drug-unconditioned stimulus during associative learning modulate the acquisition and extinction of cocaine-induced conditioned place preference (CPP). B6;129S F2 mice were conditioned by three dosing schedules of cocaine: (1) ascending, (2) fixed and (3) descending daily doses. Following acquisition of CPP, extinction was induced by (1) context re-exposure, (2) reconditioning by saline and (3) reconditioning by descending doses of cocaine. The magnitude of CPP following conditioning by daily ascending doses of cocaine (2, 4, 8 and 16 mg/kg) was significantly higher than that obtained from conditioning by either a fixed daily dose (16 mg/kg × 4 days) or daily descending doses (24, 12, 6 and 3 mg/kg). Extinction following context re-exposure showed persistent CPP in the 'ascending' group compared to the other two groups. However, extinction via reconditioning by saline was equally effective in all groups. Interestingly, reconditioning by descending doses of cocaine (1) extinguished CPP and (2) resulted in partial resistance to the reinstatement of conditioned response by cocaine priming. Results underscore the significance of daily changes in cocaine dosage in the development and extinction of drug-induced conditioned response. Increase and decrease in cocaine dosage strengthens and weakens cocaine-associated memory, respectively. Moreover, extinction by 'tapering down' drug reward may be superior to extinction by saline.

Histone acetylation rescues contextual fear conditioning in nNOS KO mice and accelerates extinction of cued fear conditioning in wild type mice

Epigenetic regulation of chromatin structure is an essential molecular mechanism that contributes to the formation of synaptic plasticity and long-term memory (LTM). An important regulatory process of chromatin structure is acetylation and deacetylation of histone proteins. Inhibition of histone deacetylase (HDAC) increases acetylation of histone proteins and facilitate learning and memory. Nitric oxide (NO) signaling pathway has a role in synaptic plasticity, LTM and regulation of histone acetylation. We have previously shown that NO signaling pathway is required for contextual fear conditioning. The present study investigated the effects of systemic administration of the HDAC inhibitor sodium butyrate (NaB) on fear conditioning in neuronal nitric oxide synthase (nNOS) knockout (KO) and wild type (WT) mice. The effect of single administration of NaB on total H3 and H4 histone acetylation in hippocampus and amygdala was also investigated. A single administration of NaB prior to fear conditioning (a) rescued contextual fear conditioning of nNOS KO mice and (b) had long-term (weeks) facilitatory effect on the extinction of cued fear memory of WT mice. The facilitatory effect of NaB on extinction of cued fear memory of WT mice was confirmed in a study whereupon NaB was administered during extinction. Results suggest that (a) the rescue of contextual fear conditioning in nNOS KO mice is associated with NaB-induced increase in H3 histone acetylation and (b) the accelerated extinction of cued fear memory in WT mice is associated with NaB-induced increase in H4 histone acetylation. Hence, a single administration of HDAC inhibitor may rescue NO-dependent cognitive deficits and afford a long-term accelerating effect on extinction of fear memory of WT mice.

Inhibition of neuronal nitric oxide synthase prevents alterations in medial prefrontal cortex excitability induced by repeated cocaine administration

RATIONALE

The medial prefrontal cortex (mPFC), a forebrain region that regulates cognitive function and reward-motivated behaviors, has been implicated in the neuropathological mechanisms of drug addiction and withdrawal. In cocaine-abstinent human addicts, neuronal activity of the mPFC is increased in response to cocaine re-exposure or drug-associated cues. Additionally, repeated cocaine exposure alters the membrane properties and ion channel function of mPFC pyramidal neurons in drug-withdrawn rats, leading to an increased firing in response to excitatory stimuli. Nitric oxide (NO), a diffusible neuromodulator of neuronal excitability, may play a role in initiating and maintaining behavioral effects of psychostimulants. However, the role of NO in the mechanisms by which cocaine affects membrane excitability is not well clarified.

OBJECTIVES

In this study, we attempted to determine whether inhibition of neuronal nitric oxide synthase (nNOS) altered the changes induced by repeated cocaine exposure and withdrawal.

METHODS

Visualized whole-cell current clamp recordings in brain slices containing the mPFC of rats administered (once per day for 5 days) with either vehicle (10% Cremophor EL in saline 0.9%), cocaine (15 mg/kg, i.p.), or cocaine and the nNOS inhibitor 7-NI (50 mg/kg, i.p.) were employed.

RESULTS

We found that nNOS inhibition prevented cocaine sensitization and the increased membrane excitability of pyramidal cells, evidenced by an increased number of evoked spikes and reductions in inward rectification observed after short-term withdrawal from cocaine.

CONCLUSIONS

These findings suggest that NO plays an important role in chronic cocaine-induced deregulation of the mPFC activity that may contribute to the development of behavioral sensitization and cocaine withdrawal.

Juvenile and adult rats differ in cocaine reward and expression of zif268 in the forebrain

Adolescents are more likely to experiment with and become addicted to drugs of abuse. A number of studies indicate that the developmental forebrain may be responsible for making adolescents vulnerable to the addictive properties of such drugs. The aim of this study was to first compare behavioral responses to novelty and cocaine between juvenile and adult rats and then compare levels of the immediate-early gene zif268 activation in several forebrain areas via in situ hybridization. We found that juveniles demonstrated higher locomotion scores and required a higher dose of cocaine than adults to establish a conditioned place preference. Additionally, at this higher dose, juvenile rats exhibited higher levels of zif268 mRNA in the prefrontal cortex compared with adults. A developmental effect for increased zif268 mRNA was also observed in the striatum and nucleus accumbens, but there was no interaction with the cocaine dose. These findings hold interesting implications for the study of the molecular mechanisms underlying juvenile drug addiction.

Early methylphenidate exposure enhances cocaine self-administration but not cocaine-induced conditioned place preference in young adult rats

RATIONALE

Previous studies in rodents show that early exposure to methylphenidate alters later responsiveness to drugs of abuse. An interesting feature of these studies is that early methylphenidate treatment decreases the rewarding value of cocaine when measured by conditioned place preference (CPP), but the same treatment increases cocaine self-administration.

OBJECTIVE

The goal of the present study was to examine the effects of early methylphenidate exposure on cocaine-induced responding using both reward paradigms.

METHODS

Rats were treated with methylphenidate (0, 2, or 5 mg/kg) from postnatal days (PDs) 11 to 20, and then cocaine-induced CPP or cocaine self-administration was measured in separate groups of rats in adulthood. The CPP procedure included 8 days of acquisition training, 8 days of extinction training, and a reinstatement test. Rats were conditioned with 0, 10, or 20 mg/kg cocaine. Reinstatement was assessed after a priming dose of cocaine (10 mg/kg). For the self-administration experiment, a jugular catheter was implanted and rats were trained to press a lever reinforced with cocaine (0.25 or 0.75 mg/kg/infusion) on a fixed ratio (FR) one schedule. Rats were gradually moved from an FR1 to an FR10 schedule and, after criterion was reached, rats were placed on a progressive ratio schedule for 5 days.

RESULTS

Cocaine produced robust rewarding effects as determined by both the CPP and self-administration experiments; however, early methylphenidate exposure only enhanced the reinforcing effects of cocaine on the self-administration paradigm. Interestingly, this methylphenidate enhancement was only seen in male rats.

CONCLUSIONS

These data suggest that in males, methylphenidate enhances the reinforcing value of cocaine, but not cocaine-associated cues.

Long-term memory of visually cued fear conditioning: roles of the neuronal nitric oxide synthase gene and cyclic AMP response element-binding protein

Nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in late-phase long-term potentiation (LTP) and long-term memory (LTM) formation. Our recent studies implicated NO signaling in contextual and auditory cued fear conditioning. The present study investigated the role of NO signaling in visually cued fear conditioning. First, visually cued fear conditioning was investigated in wild-type (WT) and nNOS knockout (KO) mice. Second, the effects of pharmacological modulators of NO signaling on the acquisition of visually cued fear conditioning were investigated. Third, plasma levels of corticosterone were measured to determine a relationship between physiological and behavioral responses to fear conditioning. Fourth, levels of extracellular signal-related kinase (ERK1/2) and cyclic AMP response element binding protein (CREB) phosphorylation, downstream of NO signaling, were determined in the amygdala as potential correlates of fear learning. Mice underwent single or multiple (4) spaced trainings that consisted of a visual cue (blinking light) paired with footshock. WT mice acquired cued and contextual LTM following single and multiple trainings. nNOS KO mice acquired neither cued nor contextual LTM following a single training; however, multiple trainings improved contextual but not cued LTM. The selective nNOS inhibitor S-methyl-thiocitrulline (SMTC) impaired cued and contextual LTM in WT mice. The NO donor molsidomine recovered contextual LTM but had no effect on cued LTM in nNOS KO mice. Re-exposure to the visual cue 24 h posttraining elicited freezing response and a marked increase in plasma corticosterone levels in WT but not nNOS KO mice. The expression of CREB phosphorylation (Ser-133) was significantly higher in naive nNOS KO mice than in WT counterparts, and pharmacological modulators of NO had significant effects on levels of CREB phosphorylation and expression. These findings suggest that visual cue-dependent LTM is impaired in nNOS KO mice, and aberrant modulation of CREB in the absence of the nNOS gene may hinder cued and contextual LTM formation.

Stress-induced reinstatement of amphetamine-conditioned place preference and changes in tyrosine hydroxylase in the nucleus accumbens in adolescent rats

Drug abuse among humans often begins during adolescence. Exposure to psychostimulants during this age period may have long-term consequences which can render the organism more susceptible to drug abuse and relapse later in life. It has been demonstrated that exposure to stress can promote relapse to drug use even after long periods of withdrawal. The reinstatement of conditioned place preference (CPP) is a useful animal model for studying relapse. In humans and animals, changes in tyrosine hydroxylase (TH) have been related to drug addiction. Our study examined whether amphetamine-induced CPP during adolescence could be reinstated by exposure to stress 1 (adolescence) and 30 (adulthood) days after the extinction test. We also investigated TH levels following the reinstatement of CPP. Our results showed that amphetamine-induced CPP during adolescence can be reinstated by stress exposure 1day (P42, end of adolescence) but not 30days after extinction (P71, adulthood). Moreover the reinstatement of AMPH-induced CPP by stress exposure occurred in the presence of decreased TH in the nucleus accumbens. In conclusion, our data add new evidence that neuroadaptations on TH may mediate relapse to drug-seeking behavior induced by stress within adolescence.

Pharmacological modulators of nitric oxide signaling and contextual fear conditioning in mice

RATIONALE

Nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) is a retrograde neuronal messenger that participates in synaptic plasticity, including late-phase long-term potentiation (LTP) and long-term memory (LTM) formation. Our recent studies have shown that nNOS knockout (KO) mice have a severe deficit in contextual fear conditioning compared to wild type (WT) counterparts (Kelley et al. 2009).

OBJECTIVES

Given the role of the nNOS gene in fear conditioning, we investigated whether systemic administration of modulators of NO signaling affect the formation of contextual and cued fear memories and the effects of these modulators on cyclic 3'5'-guanosine monophosphate (cGMP) levels in the hippocampus and amygdala.

METHODS

The preferential nNOS inhibitor S-methyl-L-thiocitrulline (SMTC; 10-200 mg/kg) was administered (IP) to WT mice, and the NO donor molsidomine (10 mg/kg) was administered (IP) to nNOS KO mice either 30 min pretraining or immediately posttraining.

RESULTS

Pretraining SMTC administration to WT mice impaired both short- and long-term memories of contextual (36% inhibition) but not cued fear conditioning. Pretraining molsidomine administration to nNOS KO mice improved their deficit in short- and long-term memories of contextual fear conditioning (46% increase). Posttraining drug administration had no effect on WT and nNOS KO mice. The systemic administration of SMTC dose-dependently decreased cGMP concentrations down to 25% of control, while molsidomine increased cGMP concentration (three- and five-fold) in amygdala and hippocampus, respectively.

CONCLUSIONS

These findings suggest that neuronal NO and its downstream second messenger cGMP are important for acquisition and subsequent consolidation of LTM of contextual fear conditioning.

Discrimination between cocaine-associated context and cue in a modified conditioned place preference paradigm: role of the nNOS gene in cue conditioning

The conditioned place preference (CPP) paradigm entails appetitive learning and is utilized to investigate the motivational effects of drug and natural reward in rodents. However, a typical CPP design does not allow dissociation between cue- and context-dependent appetitive learning. In humans, context and cues that had been associated with drug reward can elicit conditioned response and drug craving. Therefore, we investigated (a) methods by which to discriminate between cue- and context-dependent appetitive learning, and (b) the role of the neuronal nitric oxide synthase (nNOS) gene in appetitive learning. Wild-type (WT) and nNOS knockout (KO) mice were trained by cocaine (20 mg/kg) in a discrete context paired with a light cue (a compound context-cue stimulus). In test 1, approach behaviour to either the training context or to the cue in a novel context was determined. WT mice showed robust preference for both cocaine-associated context and cue. nNOS KO mice acquired approach behaviour for the cocaine-associated context but not cue. This finding suggests that the nNOS gene is required for cue-dependent appetitive learning. On the following day (test 2), mice were tested for approach behaviour to the compound context-cue stimulus. Context but not cue exposure in test 1 reduced approach behaviour to the compound context-cue stimulus in test 2, suggesting that repeated context but not cue exposures diminished the conditioned response. Hence, this modified CPP paradigm is useful for the investigation of approach behaviour for both drug-associated context and cue, and allows further investigation of mechanisms underlying cue- and context-dependent appetitive learning.

Social experiences affect reinstatement of cocaine-induced place preference in mice

RATIONALE

Drug addiction is a multifactorial disorder resulting from an interaction between genetic and environmental factors, and negative and positive environmental conditions may increase or reduce, respectively, vulnerability to drug addiction.

OBJECTIVES

The influence of different social experiences on the acquisition, extinction, and reinstatement of a cocaine-induced conditioned place preference (CPP) was evaluated.

METHODS

In experiment 1, adolescent and adult male OF1 mice housed under four different conditions (grouped, isolated, crowded, and cohabitating with a female) were conditioned with 50, 12.5, or 3.125 mg/kg of cocaine. All mice underwent extinction sessions until the CPP was extinguished. The effects of cocaine priming injections on the reinstatement of CPP were then evaluated. In experiment 2, the effect of different social experiences on the maintenance and reinstatement of cocaine-CPP in adult mice was studied.

RESULTS

Although housing conditions did not affect the acquisition of cocaine-CPP, it did modify reinstatement after extinction. Adolescent mice living in crowded conditions or cohabitating with a female did not present reinstatement after cocaine priming. Similarly, neither isolated adult mice nor adults cohabitating with a female presented reinstatement. In grouped adult mice, isolation after acquisition of the CPP and social defeat before reinstatement increased the vulnerability to reinstatement induced by cocaine priming. Conversely, both exposure to females and a brief social interaction undermined cocaine-induced reinstatement.

CONCLUSIONS

Social experiences modify vulnerability to reinstatement, acting as prevention or risk factors in the development of drug addiction.

Are adolescents more vulnerable to drug addiction than adults? Evidence from animal models

BACKGROUND AND RATIONALE

Epidemiological evidence suggests that people who begin experimenting with drugs of abuse during early adolescence are more likely to develop substance use disorders (SUDs), but this correlation does not guarantee causation. Animal models, in which age of onset can be tightly controlled, offer a platform for testing causality. Many animal models address drug effects that might promote or discourage drug intake and drug-induced neuroplasticity.

METHODS

We have reviewed the preclinical literature to investigate whether adolescent rodents are differentially sensitive to rewarding, reinforcing, aversive, locomotor, and withdrawal-induced effects of drugs of abuse.

RESULTS AND CONCLUSIONS

The rodent model literature consistently suggests that the balance of rewarding and aversive effects of drugs of abuse is tipped toward reward in adolescence. However, increased reward does not consistently lead to increased voluntary intake: age effects on voluntary intake are drug and method specific. On the other hand, adolescents are consistently less sensitive to withdrawal effects, which could protect against compulsive drug seeking. Studies examining neuronal function have revealed several age-related effects but have yet to link these effects to vulnerability to SUDs. Taken together, the findings suggest factors which may promote recreational drug use in adolescents, but evidence relating to pathological drug-seeking behavior is lacking. A call is made for future studies to address this gap using behavioral models of pathological drug seeking and for neurobiologic studies to more directly link age effects to SUD vulnerability.

Role of the nNOS gene in ethanol-induced conditioned place preference in mice

Nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in synaptic plasticity, and evidence suggests its role in a range of effects produced by alcohol in the central nervous system. The aim of the current study was to investigate the role of the nNOS gene in the development of ethanol-induced conditioned place preference (CPP) in mice. The CPP paradigm is designed to investigate the reinforcing properties of drugs of abuse and the development of maladaptive behaviors, such as conditioned response to drug-associated stimuli, after repeated drug exposure. Adult male and female wild type (WT) and nNOS knockout (KO) mice on a mixed B6;129S genetic background were trained by a morning saline session and afternoon ethanol (1, 2, and 3 g/kg; intraperitoneally) session for 4 days. Place preference in a drug-free state was recorded on the following day. Results show that WT males and females developed robust CPP, whereas nNOS KO mice did not (with the exception of female nNOS KO mice conditioned by 2 g/kg ethanol). The differential response of WT and nNOS KO mice was not due to genotypic differences in motor behavior. To investigate if the absence of the nNOS gene causes specific impairment in processing the motivational effect of ethanol or an overall impairment in associative learning, WT and nNOS KO mice were trained by LiCl (150 mg/kg) which causes conditioned place aversion (CPA). Results show that both WT and nNOS KO mice developed significant CPA. The findings that the absence of the nNOS gene impaired ethanol-induced CPP but not LiCl-induced CPA suggest that NO signaling has a specific role in processing the motivational effect of ethanol. Hence, inhibition of nNOS may attenuate the development of maladaptive behaviors associated with alcohol exposure.

Impairments in fear conditioning in mice lacking the nNOS gene

The fear conditioning paradigm is used to investigate the roles of various genes, neurotransmitters, and substrates in the formation of fear learning related to contextual and auditory cues. In the brain, nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) functions as a retrograde neuronal messenger that facilitates synaptic plasticity, including the late phase of long-term potentiation (LTP) and formation of long-term memory (LTM). Evidence has implicated NO signaling in synaptic plasticity and LTM formation following fear conditioning, yet little is known about the role of the nNOS gene in fear learning. Using knockout (KO) mice with targeted mutation of the nNOS gene and their wild-type (WT) counterparts, the role of NO signaling in fear conditioning was investigated. Plasma levels of the stress hormone corticosterone were measured to determine the relationship between physiological and behavioral response to fear conditioning. Contextual fear learning was severely impaired in male and female nNOS KO mice compared with WT counterparts; cued fear learning was slightly impaired in nNOS KO mice. Sex-dependent differences in both contextual and cued fear learning were not observed in either genotype. Deficits in contextual fear learning in nNOS KO mice were partially overcome by multiple trainings. A relationship between increase in plasma corticosterone levels following footshock administration and the magnitude of contextual, but not cued freezing was also observed. Results suggest that the nNOS gene contributes more to optimal contextual fear learning than to cued fear learning, and therefore, inhibition of the nNOS enzyme may ameliorate context-dependent fear response.

The neuronal nitric oxide synthase (nNOS) gene contributes to the regulation of tyrosine hydroxylase (TH) by cocaine

Recently, we demonstrated that intact nitric oxide (NO) signaling is essential for the development of cocaine behavioral sensitization in adulthood [M.A. Balda, K.L. Anderson, Y. Itzhak, Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice, Psychopharmacology (Berl) 200 (2008) 509-519]. Given the requirement of dopamine (DA) transmission in cocaine-induced behavioral sensitization and the interactions between NO and DA systems, the present study investigated the role of the neuronal nitric oxide synthase (nNOS) gene and the effect of cocaine on the expression of tyrosine hydroxylase (TH)-immunoreactive (-ir) neurons. Adult (postnatal day 80) wild type (WT) and nNOS knockout (KO) mice received saline or a sensitizing regimen of cocaine (20mg/kg) for 5 days. After 24h, TH immunoreactivity was assessed in the ventral tegmental area (VTA) and the dorsal striatum (dST) using stereology and Western blotting, respectively. We report that (a) nNOS KO mice express lower levels of TH-ir neurons in the VTA compared to WT counterparts, (b) cocaine administration to WT mice significantly increased striatal TH expression, and (c) the same cocaine administration to nNOS KO mice significantly decreased striatal TH expression. Thus, the nitrergic system may contribute to cocaine-induced behavioral sensitization by regulating dopaminergic neurotransmission.

Ovarian hormones modulate 'compulsive' lever-pressing in female rats

Life events related to the female hormonal cycle may trigger the onset of obsessive-compulsive disorder (OCD) or exacerbate symptoms in women already suffering from it. These observations suggest a possible role for ovarian hormones in the course of this disorder. Yet, the mechanisms that may subserve the modulatory effect of ovarian hormones are currently unknown. The aim of the present study was therefore to test the role of ovarian hormones in the signal attenuation rat model of OCD. Experiment 1 compared the behavior of pre-pubertal and adult male and female rats in the model, and found no age and sex differences in compulsive responding. Experiment 2 found that compulsive responding fluctuates along the estrous cycle, being highest during late diestrous and lowest during estrous. Acute administration of estradiol to pre-pubertal female rats was found to attenuate compulsive behavior (Experiment 3), and withdrawal from chronic administration of estradiol was shown to increase this behavior (Experiment 4). These findings extend the use of the signal attenuation model of OCD to female rats, and by demonstrating that the model is sensitive to the levels of ovarian hormones, provide the basis for using the model to study the role of ovarian hormones in OCD. In addition, the present findings support the hypothesis that the increased risk of onset and exacerbation of OCD in women post-partum may be a result of the decrease in the level of estradiol, which was elevated during pregnancy.

Development and persistence of long-lasting behavioral sensitization to cocaine in female mice: role of the nNOS gene

Our recent studies have shown that the neuronal nitric oxide synthase (nNOS) gene is required for the development and persistence of psychomotor sensitization to cocaine in adult but not adolescent male mice (Balda, M.A., Anderson, K.L., Itzhak, Y., 2008. Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice. Psychopharmacology (Berlin) 200, 509-519.). The aim of the present study was to investigate the contribution of the nNOS gene to cocaine-induced behavioral sensitization in adolescent and adult female mice. Adolescent and adult wild type (WT) and nNOS knockout (KO) mice received saline or cocaine (20 mg/kg) for 5 days and then were challenged with cocaine (20 mg/kg) after a drug-free period of either 10, 30, or 90 days. Context-dependent sensitization was determined by measuring saline-induced locomotor activity in the previously cocaine-paired environment. Results show that adolescent females of both genotypes, like their adult counterparts, developed long-lasting behavioral sensitization to cocaine (a three-month period), suggesting high vulnerability of females to cocaine regardless of age. An effect of genotype was observed in the initiation of sensitization, e.g., delayed onset in the absence of the nNOS gene. The only age-dependent difference observed was that adult, but not adolescent mice developed context-dependent sensitization. The present study suggests that long-term expression of cocaine-induced behavioral sensitization in females (adolescent and adult) is nNOS-independent, unlike our previous findings in adult males.

Sensitivity to cocaine conditioned reward depends on sex and age

Human and animal laboratory studies show that females and males respond differently to drugs and that drug administration during adolescence leads to different behavioral effects than during adulthood. Adult female rats are more sensitive to the behavioral effects of cocaine than adult males, but it is not known if the same effect of sex exists during adolescence. In the present study, sensitivity to the conditioned reward of cocaine was evaluated using a conditioned place preference (CPP) paradigm where adolescent (PND 34) and adult (PND 66) male and female rats were trained and tested for the development of CPP to multiple doses of cocaine. Female rats developed CPP at lower doses than males, regardless of age. In addition, adolescent male and female rats established a CPP at lower doses of cocaine than adult male and female rats, respectively. Thus, both age and sex altered cocaine conditioned reward with the order of sensitivity being adolescent females > adult females > adolescent males > adult males. These data show that adolescents are more sensitive to the conditioned rewarding properties of cocaine than adults and that females respond to lower doses of cocaine compared to males regardless of age.

Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice

RATIONALE

Previous studies have suggested the involvement of neuronal nitric oxide synthase (nNOS) in the development of sensitization to psychostimulants. Ontogeny-dependent differences in the response to psychostimulants have been reported.

OBJECTIVE

The objectives were to investigate (a) the short- and long-term consequences of adolescent and adult cocaine exposure on behavioral sensitization and (b) the role of the nNOS gene in behavioral sensitization in adolescent and adult mice.

MATERIALS AND METHODS

Adolescent and adult wild type (WT) and nNOS knockout (KO) mice received saline or cocaine (20 mg/kg) for 5 days and then were challenged with cocaine (20 mg/kg) after a drug-free period of 10 or 30 days. Locomotor activity was recorded by infrared beam interruptions. nNOS immunoreactive (ir) neurons in the dorsal and ventral striatum were quantified 24 h after repeated administration of cocaine to adolescent and adult WT mice.

RESULTS

Repeated administration of cocaine to either WT or nNOS KO mice during adolescence resulted in locomotor sensitization, which persisted into adulthood. WT but not KO adult mice developed long-term sensitization to cocaine. Repeated cocaine administration resulted in a 96% increase in the expression of nNOS-ir neurons in the dorsal striatum of adult but not adolescent WT mice.

CONCLUSIONS

The nNOS gene is essential for the induction of behavioral sensitization to cocaine in adulthood but not in adolescence. The increased expression of nNOS-ir neurons in the dorsal striatum may underlie the induction of behavioral sensitization in adulthood. Thus, the NO-signaling pathway has an ontogeny-dependent role in the neuroplasticity underlying cocaine behavioral sensitization.

Chronic social stress in adolescence influenced both amphetamine conditioned place preference and locomotor sensitization

We previously reported that chronic social stress (SS) in adolescence, but not in adulthood, increased the locomotor-activating effects of nicotine in females, and not males, when tested in adulthood. However, SS rats had decreased locomotor response to nicotine when tested in adolescence. Here, we investigated age-related changes in the effects of SS on both conditioned place preference (CPP) and locomotor sensitization to amphetamine. In the CPP experiment, SS females tested in adolescence had increased preference for the 1.0 mg/kg dose of amphetamine, whereas SS rats of both sexes showed a decrease in CPP for the 0.5 mg/kg dose when tested as adults. Irrespective of time of testing, SS males and females had enhanced locomotor sensitization compared to controls. Thus, adolescent SS produced both immediate and enduring effects on behavioral responses to amphetamine, likely by altering the development of the mesocorticolimbic dopamine system, which holds implications for vulnerability to addiction.

Neurobiological mechanisms of the reinstatement of drug-conditioned place preference

Drug addiction is a chronic disorder characterized by a high rate of relapse following detoxification. There are two main versions of the reinstatement model that are employed to study relapse to drug abuse; one based on the operant self-administration procedure, and the other on the classical conditioned place preference procedure. In the last seven years, the use of the latter version has become more widespread, and the results obtained complement those obtained in self-administration studies. It has been observed that the conditioned place preference induced by opioids, psychostimulants, nicotine, ethanol and other drugs of abuse can be extinguished and reinstated by drug priming or exposure to stressful events. Herein, the neuroanatomical and neurochemical basis of drug priming- and stress-induced reinstatement of morphine and cocaine, together with the molecular correlates of reinstatement behavior, are reviewed. Differences between the conditioned place preference and self-administration studies are also discussed. Evidence suggests that data of reinstatement with the CPP are to be viewed with caution until more extensive analysis of operant procedures has been performed, and that further research will undoubtedly improve our understanding of the neurobiological mechanisms of relapse to drug seeking.

Ethanol-induced behavioral sensitization in adolescent and adult mice: role of the nNOS gene

BACKGROUND

In the brain, nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in synaptic plasticity. Recent evidence suggests the role of NO in a variety of effects produced by alcohol in the central nervous system. The current study investigated the role of the nNOS gene in the development of behavioral sensitization to ethanol in adolescent and adult mice.

METHODS

Adolescent and adult wild type (WT; B6;129SF2) and nNOS knockout (KO; B6;129S4-Nos1) mice of both sexes received saline or ethanol (1.5 g/kg; intraperitoneally) for 5 consecutive days, and locomotor activity was recorded daily. The locomotor response to challenge ethanol and saline injections was investigated at various time points following withdrawal from ethanol.

RESULTS

Adolescent WT but not nNOS KO mice developed a long-lasting sensitized response to ethanol as well as context-dependent hyperlocomotion (in response to saline) from adolescence through adulthood; sex-dependent differences were not observed. Compared to adolescent WT mice, adult WT males developed a short-term sensitized response to ethanol and context-dependent hyperlocomotion; adult WT females showed only short-term context-dependent hyperlocomotion. Adult nNOS KO males (like their adolescent counterparts) did not develop behavioral sensitization; no significant differences between adult nNOS KO and WT females were observed. Blood ethanol concentrations did not show genotype- or sex-dependent differences.

CONCLUSIONS

(1) The nNOS gene is required for the development of behavioral sensitization to ethanol in adolescent male and female mice. (2) Adolescent exposure to ethanol results in long-lasting behavioral sensitization through adulthood, while adult exposure to ethanol results in a shorter behavioral sensitization. (3) Sex-dependent differences are observed when ethanol exposure begins in adulthood but not in adolescence. (4) Ethanol-induced behavioral sensitization in adulthood is nNOS-dependent in males but not in females. Taken together, results suggest genotype-, ontogeny-, and sex-dependent differences in the development of behavioral sensitization to ethanol.

Postnatal manganese exposure alters dopamine transporter function in adult rats: Potential impact on nonassociative and associative processes

In the present study, we examined whether exposing rats to a high-dose regimen of manganese chloride (Mn) during the postnatal period would depress presynaptic dopamine functioning and alter nonassociative and associative behaviors. To this end, rats were given oral supplements of Mn (750 microg/day) on postnatal days (PD) 1-21. On PD 90, dopamine transporter (DAT) immunoreactivity and [3H]dopamine uptake were assayed in the striatum and nucleus accumbens, while in vivo microdialysis was used to measure dopamine efflux in the same brain regions. The effects of postnatal Mn exposure on nigrostriatal functioning were evaluated by assessing rotorod performance and amphetamine-induced stereotypy in adulthood. In terms of associative processes, both cocaine-induced conditioned place preference (CPP) and sucrose-reinforced operant responding were examined. Results showed that postnatal Mn exposure caused persistent declines in DAT protein expression and [3H]dopamine uptake in the striatum and nucleus accumbens, as well as long-term reductions in striatal dopamine efflux. Rotorod performance did not differ according to exposure condition, however Mn-exposed rats did exhibit substantially more amphetamine-induced stereotypy than vehicle controls. Mn exposure did not alter performance on any aspect of the CPP task (preference, extinction, or reinstatement testing), nor did Mn affect progressive ratio responding (a measure of motivation). Interestingly, acquisition of a fixed ratio task was impaired in Mn-exposed rats, suggesting a deficit in procedural learning. In sum, these results indicate that postnatal Mn exposure causes persistent declines in various indices of presynaptic dopaminergic functioning. Mn-induced alterations in striatal functioning may have long-term impact on associative and nonassociative behavior.

Delayed extinction and stronger reinstatement of cocaine conditioned place preference in adolescent rats, compared to adults

Adolescence is a transitional period during development that is associated with a greater likelihood of addiction to drugs than any other age. One possibility for this observation is that learned associations between the rewarding experience of drugs and drug-related cues may produce greater motivational salience, and thus are more difficult to extinguish. Using an unbiased place-conditioning paradigm with two doses of cocaine (10 mg/kg or 20 mg/kg), the authors show here that adolescents require 75 +/- 17% more extinction trials than adults to extinguish cocaine place-preferences. Furthermore, once extinguished, adolescents display a greater preference for a previously cocaine-paired environment upon drug-primed reinstatement compared with adults. These results suggest that adolescent vulnerability to addiction involves robust memories for drug-associated cues that are difficult to extinguish. Therefore, drug-addicted adolescents may have a higher risk of relapse than adults, leading to greater prevalence of addiction in this population.

Transient D1 dopamine receptor expression on prefrontal cortex projection neurons: relationship to enhanced motivational salience of drug cues in adolescence

Adolescence is a transitional period during development that is associated with a greater likelihood of addiction to drugs than any other age. In the prefrontal cortex (PFC), D(1) dopamine receptors mediate motivational salience attribution, which plays a role in addiction. Here, we investigated the relationship of age-related D(1) dopamine receptor expression in the PFC with the maturation of cocaine place conditioning. Confocal microscopy revealed that retrogradely traced cortical output neurons to the nucleus accumbens express higher levels of D(1) receptors during adolescence compared with younger and older ages. D(1) expression does not change on GABAergic interneurons across age. Adolescent differences in D(1) expression occur independently of cortical-accumbens connectivity, which proliferates through adulthood. Behaviorally, adolescent rats are more sensitive to cocaine place conditioning than younger and older rats. However, microinjections of the D(1) antagonist SCH23390 into the PFC blocked adolescent place preferences, whereas microinjections of D(1) agonists dose-dependently increased preferences for cocaine-associated environments previously not preferred by juveniles. These results suggest that the heightened expression of D(1) receptors on cortical-accumbens projections may help explain increased sensitivity to environmental events and addictive behaviors during adolescence, whereas the paucity of D(1)-expressing projections may reduce risk in juveniles.

Female and male rats in late adolescence differ from adults in amphetamine-induced locomotor activity, but not in conditioned place preference for amphetamine

Rodent models display differences in drug-induced behaviour between prepubertal/young adolescents and adults that parallel developmental differences in people; however, little is known as to when the transition to 'adultlike' behaviour occurs. We investigated the differences in locomotor and reward responses to amphetamines in male and female rats in late adolescence and compared them with corresponding adult responses. Long-Evans rats were tested for locomotor activity and conditioned place preference (CPP) for amphetamine (0.25, 0.5 or 1.0 mg/kg), beginning at 45 or 69 days of age. Adolescent female rats moved less to the first injection of amphetamine compared with adult female rats irrespective of dose, whereas adolescent male rats did not differ from adults. Adolescent female rats significantly increased locomotor activity in response to subsequent injections of amphetamine at all three doses, whereas such sensitization was only found at the highest dose for adult female and male rats. No effect of repeated injections at any dose was observed in adolescent male rats. No age differences were observed in CPP, but female rats showed greater CPP during the dioestrous than during the oestrous phase of the cycle. These data suggest that differences in neural systems underlying some behavioural responses to amphetamine continue to mature postpubertally into late adolescence in a sex-specific manner.