Neurochemical evidence that cocaine- and amphetamine-regulated transcript (CART) 55-102 peptide modulates the dopaminergic reward system by decreasing the dopamine release in the mouse nucleus accumbens

Blockade of the orexin 1-receptors and 'cocaine- and amphetamine-regulated transcript' in the nucleus accumbens shell alters impulse control in rats

Impulsivity is a multifaceted construct and alterations in impulsiveness are often associated with psychiatric diseases, including drug addiction and binge eating disorder. Impulse control involves several brain regions. The present study assessed the role of the orexigenic, appetite stimulating neuropeptide orexin (OX) and the anorexigenic, appetite reducing neuropeptide cocaine- and amphetamine-regulated transcript (CART) within the nucleus accumbens shell (NAcSh) in impulse control in rats. The animals were ranked for their trait impulsivity based on a screening in the 5-choice serial reaction time task (5-CSRTT). The rats' performances were analysed after bilateral infusions of the OX 1-receptor antagonist SB-334867 (SB) and CART-antibodies (CART-ABs) into the NAcSh. After SB infusions, there was no change in premature responses observed on average. Further analysis revealed a negative linear correlation between the effect of intra-NAcSh SB infusions on premature responses and trait impulsivity. The effect of SB ranged from an increase, no change to a decrease in premature responses in the individual animals with increasing trait impulsivity. Infusions of CART-ABs led to consistently enhanced impulse control with fewer irrelevant actions, independent of trait impulsivity. These data suggest that both OX, especially OX A, and CART in the NAcSh, can be considered endogenous regulators of impulsive action, dependent on underlying impulsivity in the case of OX and independent from trait impulsivity in the case of CART.

Molecular pattern of a decrease in the rewarding effect of cocaine after an escalating-dose drug regimen

BACKGROUND

Long-term cocaine exposure leads to dysregulation of the reward system and initiates processes that ultimately weaken its rewarding effects. Here, we studied the influence of an escalating-dose cocaine regimen on drug-associated appetitive behavior after a withdrawal period, along with corresponding molecular changes in plasma and the prefrontal cortex (PFC).

METHODS

We applied a 5 day escalating-dose cocaine regimen in rats. We assessed anxiety-like behavior at the beginning of the withdrawal period in the elevated plus maze (EPM) test. The reinforcement properties of cocaine were evaluated in the Conditioned Place Preference (CPP) test along with ultrasonic vocalization (USV) in the appetitive range in a drug-associated context. We assessed corticosterone, proopiomelanocortin (POMC), β-endorphin, CART 55-102 levels in plasma (by ELISA), along with mRNA levels for D2 dopaminergic receptor (D2R), κ-receptor (KOR), orexin 1 receptor (OX1R), CART 55-102, and potential markers of cocaine abuse: miRNA-124 and miRNA-137 levels in the PFC (by PCR).

RESULTS

Rats subjected to the escalating-dose cocaine binge regimen spent less time in the cocaine-paired compartment, and presented a lower number of appetitive USV episodes. These changes were accompanied by a decrease in corticosterone and CART levels, an increase in POMC and β-endorphin levels in plasma, and an increase in the mRNA for D2R and miRNA-124 levels, but a decrease in the mRNA levels for KOR, OX1R, and CART 55-102 in the PFC.

CONCLUSIONS

The presented data reflect a part of a bigger picture of a multilevel interplay between neurotransmitter systems and neuromodulators underlying processes associated with cocaine abuse.

Δ 9-Tetrahydrocannabinol During Adolescence Reprograms the Nucleus Accumbens Transcriptome, Affecting Reward Processing, Impulsivity, and Specific Aspects of Cocaine Addiction-Like Behavior in a Sex-Dependent Manner

BACKGROUND

Cannabis exposure during adolescence is associated with emotional and motivational alterations that may entail an enhanced risk of developing psychiatric disorders. In rodent models, exposure to cannabinoids during adolescence leads to increased self-administration of opiates and cocaine, however, the psychological and neural mechanisms and the sex-specificity of this phenomenon are largely unknown.

METHODS

We exposed male and female adolescent rats to Δ9-tetrahydrocannabinol (THC) and studied at adulthood the effects of such treatment on psychological processes related to reward, such as Pavlovian conditioned approach, Pavlovian to instrumental transfer, habit formation and waiting impulsivity. In the light of these data and given the involvement of the nucleus accumbens in the processes examined, we performed an RNASeq transcriptomic study and assessed cocaine addiction-like behavior.

RESULTS

THC exposure increased goal-tracking (in males and females) and enhanced Pavlovian to instrumental transfer (especially in males) but did not affect habit formation. THC-exposed rats exhibited subtle, state-dependent changes in premature responding in the 2-CSRTT task. RNASeq data showed gene expression alterations in a marked sex-specific manner. While no effects were found on the acquisition of cocaine self-administration or punished drug-seeking, rats exposed to THC self-administered more cocaine under a progressive ratio schedule (males), had a higher rebound upon returning to continuous access to the drug (females) and showed reduced drug-seeking after 30 days of withdrawal (females).

CONCLUSIONS

Adolescent THC affects specific aspects of reward- (and cocaine-) guided behavior and the function of a key brain region mediating these effects, in a remarkable sex-specific manner.

Cocaine- and amphetamine-regulated transcript peptide- and dopamine-containing systems interact in the ventral tegmental area of the zebra finch, Taeniopygia guttata, during dynamic changes in energy status

The mesolimbic dopamine (DA)-pathway regulates food-reward, feeding-related behaviour and energy balance. Evidence underscores the importance of feeding-related neuropeptides in modulating activity of these DA neurons. The neuropeptide, CART, a crucial regulator of energy balance, modulates DA-release, and influences the activity of ventral tegmental area (VTA) DAergic neurons in the mammalian brain. Whether CART- and DA-containing systems interact at the level of VTA to regulate energy balance, however, is poorly understood. We explored the interaction between CART- and DA-containing systems in midbrain of the zebra finch, Taeniopygia guttata, an interesting model to study dynamic changes in energy balance due to higher BMR/daytime body temperature, and rapid responsiveness of the feeding-related neuropeptides to changes in energy state. Further, its midbrain DA-neurons share similarities with those in mammals. In the midbrain, tyrosine hydroxylase-immunoreactive (TH-i) neurons were seen in the substantia nigra (SN) and VTA [anterior (VTAa), mid (VTAm) and caudal (VTAc)]; those in VTA were smaller. In the VTA, CART-immunoreactive (CART-i)-fibers densely innervated TH-i neurons, and both CART-immunoreactivity (CART-ir) and TH-immunoreactivity (TH-ir) responded to energy status-dependent changes. Compared to fed and fasted birds, refeeding dramatically enhanced TH-ir and the percentage of TH-i neurons co-expressing FOS in the VTA. Increased prepro-CART-mRNA, CART-ir and a transient appearance of CART-i neurons was observed in VTAa of fasted, but not fed birds. To test the functional interaction between CART- and DA-containing systems, ex-vivo superfused midbrain-slices were treated with CART-peptide and changes in TH-ir analysed. Compared to control tissues, CART-treatment increased TH-ir in VTA but not SN. We propose that CART is a potential regulator of VTA DA-neurons and energy balance in T. guttata.

Adolescent oxycodone exposure inhibits withdrawal-induced expression of genes associated with the dopamine transmission

Prescription opioid misuse is a major public health concern among children and adolescents in the United States. Opioids are the most commonly abused drugs and are the fastest growing drug problem among adolescents. In humans and animals, adolescence is a particularly sensitive period associated with an increased response to drugs of abuse. Our previous studies indicate that oxycodone exposure during adolescence increases morphine reward in adulthood. How early drug exposure mediates long-term changes in the brain and behavior is not known, but epigenetic regulation is a likely mechanism. To address this question, we exposed mice to oxycodone or saline during adolescence and examined epigenetic modifications at genes associated with dopamine activity during adulthood at early and late withdrawal, in the ventral tegmental area (VTA). We then compared these with alterations in the VTA of adult-treated mice following an equivalent duration of exposure and withdrawal to determine if the effects of oxycodone are age dependent. We observed persistence of adolescent-like gene expression following adolescent oxycodone exposure relative to age-matched saline exposed controls, although dopamine-related gene expression was transiently activated at 1 day of withdrawal. Following prolonged withdrawal enrichment of the repressive histone mark, H3K27me3, was maintained, consistent with inhibition of gene regulation following adolescent exposure. By contrast, mice exposed to oxycodone as adults showed loss of the repressive mark and increased gene expression following 28 days of withdrawal following oxycodone exposure. Together, our findings provide evidence that adolescent oxycodone exposure has long-term epigenetic consequences in VTA of the developing brain.

Extracellular Dopamine Levels in Nucleus Accumbens after Chronic Stress in Rats with Persistently High vs. Low 50-kHz Ultrasonic Vocalization Response

Fifty-kHz ultrasonic vocalizations (USVs) in response to an imitation of rough-and-tumble play ('tickling') have been associated with positive affective states and rewarding experience in the rat. This USV response can be used as a measure of inter-individual differences in positive affect. We have previously shown that rats with persistently low positive affectivity are more vulnerable to the effects of chronic variable stress (CVS). To examine whether these differential responses are associated with dopaminergic neurotransmission in the nucleus accumbens (NAc), juvenile male Wistar rats were categorized as of high or low positive affectivity (HC and LC, respectively), and after reaching adulthood, extracellular dopamine (DA) levels in the NAc shell were measured using in vivo microdialysis after three weeks of CVS. Baseline levels of DA were compared as well as the response to K⁺-induced depolarization and the effect of glial glutamate transporter EAAT2 inhibition by 4 mM l-trans-pyrrolidine-2,4-dicarboxylate (PDC). DA baseline levels were higher in control LC-rats, and stress significantly lowered the DA content in LC-rats. An interaction of stress and affectivity appeared in response to depolarization where stress increased the DA output in HC-rats whereas it decreased it in LC-rats. These results show that NAc-shell DA is differentially regulated in response to stress in animals with high and low positive affect.

A novel LC-MS/MS method for quantification of unstable endogenous 3,4-dihydroxyphenylacetaldehyde in rat brain after chemical derivatization

3,4-dihydroxyphenylacetaldehyde (DOPAL), a toxic intermediary metabolite of dopamine (DA), causes catecholaminergic neurodegeneration via covalent binding with functional proteins or other biomolecules. Accurate quantification of DOPAL is essential to investigate the etiological factors associated with DOPAL and the pathogenetic role of DOPAL in Parkinson's disease (PD). However, no validated quantitative methods are available. Quantification of DOPAL in biosample is challenging since it is a reactive endogenous aldehyde with poor ionization efficiency and chromatographic behavior in the LC-MS system. Here, a sensitive, simple, and robust UPLC-MS/MS method has been established and validated for the determination of DOPAL in rat brain tissue specimens. DOPAL was found to be unstable in biosample due to reactive aldehyde whereas it was stable in acidic condition. The analyte was stabilized by pH and temperature control during the sample preparation and derivatization. Then, a chemical derivatization method that can be readily performed in acidic conditions and at low temperature was employed using 2-hydrazino-4-(trifluoromethyl)-pyrimidine (HTP) to block the reactive aldehyde and improve the detection sensitivity (about 100-fold increase) and chromatographic retention. Bovine serum albumin was used as a surrogate matrix, which was validated by the parallelism assay and post-column infusion experiment. This method was fully validated and the lower limit of quantification (LLOQ) was 0.5 ng/mL. With the method, a significant increase of DOPAL level was found in striatum region of rats received 6-hydroxydopamine (6-OHDA) injection for 12 h, indicating DOPAL may play a pathogenic role in 6-OHDA-induced PD model.

Differences in the dopaminergic reward system in rats that passively and actively behave in the Porsolt test

The aim of the study was to assess appetitive responses and central dopaminergic neurotransmission in passive and active rats divided according to their immobility time in the Porsolt swim test and exposed to restraint stress. Passive rats had more episodes of appetitive 50-kHz ultrasonic vocalization (USV) during rat encounter after social isolation and spent significantly more time in the amphetamine-associated context in conditioned place preference test, compared to active rats. Restraint stress decreased sucrose preference, but increased appetitive vocalization and reinforced the conditioned place preference only in passive animals that was associated with increased dopamine concentration in the amygdala. Restraint stress increased also the level of Cocaine- and Amphetamine Regulated Transcript (CART) peptide, a neuromodulator linked to dopamine neurotransmission, in the central nucleus of amygdala, while decreasing it the nucleus accumbens shell in passive rats. In the parvocellular region of paraventricular nucleus of the hypothalamus passive animals had a higher expression of CART compared to passive restraint rats and active control rats. The obtained results show that active and passive rats in the Porsolt test differ significantly in response to appetitive stimuli, which can be additionally changed under stress conditions. The underlying mechanisms are probably associated with differences in dopaminergic activity and CART signaling in reward system.

Cocaine- and amphetamine-regulated transcript peptide in the nucleus accumbens shell inhibits cocaine-induced locomotor sensitization to transient over-expression of α-Ca2+ /calmodulin-dependent protein kinase II

Cocaine- and amphetamine-regulated transcript (CART) peptide is a widely distributed neurotransmitter that attenuates cocaine-induced locomotor activity when injected into the nucleus accumbens (NAc). Our previous work first confirmed that the inhibitory mechanism of the CART peptide on cocaine-induced locomotor activity is related to a reduction in cocaine-enhanced phosphorylated Ca²⁺ /calmodulin-dependent protein kinaseIIα (pCaMKIIα) and the enhancement of cocaine-induced D3R function. This study investigated whether CART peptide inhibited cocaine-induced locomotor activity via inhibition of interactions between pCaMKIIα and the D3 dopamine receptor (D3R). We demonstrated that lentivirus-mediated gene transfer transiently increased pCaMKIIα expression, which peaked at 10 days after microinjection into the rat NAc shell, and induced a significant increase in Ca²⁺ influx along with greater behavioral sensitivity in the open field test after intraperitoneal injections of cocaine (15 mg/kg). However, western blot analysis and coimmunoprecipitation demonstrated that CART peptide treatment in lentivirus-transfected CaMKIIα-over-expressing NAc rat tissues or cells prior to cocaine administration inhibited the cocaine-induced Ca²⁺ influx and attenuated the cocaine-increased pCaMKIIα expression in lentivirus-transfected CaMKIIα-over-expressing cells. CART peptide decreased the cocaine-enhanced phosphorylated cAMP response element binding protein (pCREB) expression via inhibition of the pCaMKIIα-D3R interaction, which may account for the prolonged locomotor sensitization induced by repeated cocaine treatment in lentivirus-transfected CaMKIIα-over-expressing cells. These results provide strong evidence for the inhibitory modulation of CART peptide in cocaine-induced locomotor sensitization. Cover Image for this issue: doi: 10.1111/jnc.14187.

Epigenetic regulation of motivated behaviors by histone deacetylase inhibitors

Growing evidence has begun to elucidate the contribution of epigenetic mechanisms in the modulation and maintenance of gene expression and behavior. Histone acetylation is one such epigenetic mechanism, which has been shown to profoundly alter gene expression and behaviors. In this review, we begin with an overview of the major epigenetic mechanisms including histones acetylation. We next focus on recent evidence about the influence of environmental stimuli on various motivated behaviors through histone acetylation and highlight how histone deacetylase inhibitors can correct some of the pathologies linked to motivated behaviors including substance abuse, feeding and social attachments. Particularly, we emphasize that the effects of histone deacetylase inhibitors on motivated behaviors are time and context-dependent.