NMDA Receptor Blockade in the Prelimbic Cortex Activates the Mesolimbic System and Dopamine-Dependent Opiate Reward Signaling

Rationale

N-Methyl-d-aspartate (NMDA) receptors in the medial prefrontal cortex (mPFC) are involved in opiate reward processing and modulate sub-cortical dopamine (DA) activity. NMDA receptor blockade in the prelimbic (PLC) division of the mPFC strongly potentiates the rewarding behavioural properties of normally sub-reward threshold doses of opiates. However, the possible functional interactions between cortical NMDA and sub-cortical DAergic motivational neural pathways underlying these effects are not understood.

Objective

This study examines how NMDA receptor modulation in the PLC influences opiate reward processing via interactions with sub-cortical DAergic transmission. We further examined whether direct intra-PLC NMDA receptor modulation may activate DA-dependent opiate reward signaling via interactions with the ventral tegmental area (VTA).

Methods

Using an unbiased place conditioning procedure (CPP) in rats, we performed bilateral intra-PLC microinfusions of the competitive NMDA receptor antagonist, (2R)-amino-5-phosphonovaleric acid (AP-5), prior to behavioural morphine place conditioning and challenged the rewarding effects of morphine with DA receptor blockade. We next examined the effects of intra-PLC NMDA receptor blockade on the spontaneous activity patterns of presumptive VTA DA or GABAergic neurons, using single-unit, extracellular in vivo neuronal recordings.

Results

We show that intra-PLC NMDA receptor blockade strongly activates sub-cortical DA neurons within the VTA while inhibiting presumptive non-DA GABAergic neurons. Behaviourally, NMDA receptor blockade activates a DA-dependent opiate reward system, as pharmacological blockade of DA transmission blocked morphine reward only in the presence of intra-PLC NMDA receptor antagonism.

Conclusions

These findings demonstrate a cortical NMDA-mediated mechanism controlling mesolimbic DAergic modulation of opiate reward processing.     

Inactivation of the basolateral amygdala during opiate reward learning disinhibits prelimbic cortical neurons and modulates associative memory extinction

Rationale

Neurons within the basolateral amygdala (BLA) and prelimbic cortex (PLC) are involved in associative learning during morphine reward memory recall and extinction. However, the nature by which the BLA regulates PLC neuronal encoding of associative opiate reward learning is not presently understood.

Objective

The purpose of this study was to examine the functional effects of reversible inactivation of the BLA on behavioral and neuronal activity patterns in the PLC during either the acquisition or extinction phases of opiate reward memory processing.

Methods

Using a combination of in vivo neuronal population recordings in the rat PLC and pharmacological inactivation of the BLA during a place conditioning procedure, we examined the functional impact of BLA inactivation during the acquisition, recall, and extinction of opiate reward memory.

Results

Inactivation of the BLA caused an increase in the spontaneous firing and bursting activity of PLC neurons. Inactivation of the BLA during the acquisition phase of opiate reward conditioning caused a subsequent acceleration in the extinction of the previously learned opiate reward memory and behavioral aversions to morphine-paired environments. While BLA inactivation during extinction training led to a delay in extinction memory recall.

Conclusions

Our findings demonstrate a functional link between the BLA and neuronal populations in the PLC specifically during the acquisition and extinction phases of opiate reward memory and suggest that BLA input to the PLC modulates the processing of opiate-related extinction memory.     

Role of GABAA receptors in dorsal raphe nucleus in stress-induced reinstatement of morphine-conditioned place preference in rats

Rationale

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Our data indicate that stress inhibits the dorsal raphe nucleus (DRN)-5-HT system via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor and, more recently, that morphine history sensitizes DRN-5-HT neurons to GABAergic inhibitory effects of stress.

Objectives

We tested the hypothesis that DRN GABA_A receptors contribute to stress-induced reinstatement of morphine-conditioned place preference (CPP).

Methods

First, we tested if activation of GABA_A receptors in the DRN would reinstate morphine CPP. Second, we tested if blockade of GABA_A receptors in the DRN would attenuate swim stress-induced reinstatement of morphine CPP. CPP was induced by morphine (5 mg/kg) in a 4-day conditioning phase followed by a conditioning test. Upon acquiring conditioning criteria, subjects underwent 4 days of extinction training followed by an extinction test. Upon acquiring extinction criteria, animals underwent a reinstatement test. For the first experiment, the GABA_A receptor agonist muscimol (50 ng) or vehicle was injected into the DRN prior to the reinstatement test. For the second experiment, the GABA_A receptor antagonist bicuculline (75 ng) or vehicle was injected into the DRN prior to a forced swim stress, and then, animals were tested for reinstatement of CPP.

Results

Intraraphe injection of muscimol reinstated morphine CPP, while intraraphe injection of bicuculline attenuated swim stress-induced reinstatement.

Conclusions

These data provide evidence that GABA_A receptor-mediated inhibition of the serotonergic DRN contributes to stress-induced reinstatement of morphine CPP.     

Involvement of insular muscarinic cholinergic receptors in morphine-induced conditioned place preference in rats

Rationale

Drug addiction represents a pathological usurpation of neural processes involved in learning and memory. Retrieval of drug-related memories can result in drug craving and relapse. Recently, the insula was identified as part of the neuronal circuit responsible for the processing of drug memory; however, its precise role remains unclear.

Objective

To investigate the involvement of insular muscarinic acetylcholine receptors (mAChRs) in the processing of drug memory.

Method

The morphine-induced conditioned place preference (CPP) was used to assess drug memory. All rats were first trained with morphine to establish the CPP. Sub-groups of these rats were used for contextual cue-induced CPP reinstatement. Other sub-groups of rats underwent extinction of the CPP, and 5 m/kg morphine was used for priming-induced CPP reinstatement. Microinjection of mAChR antagonists or agonists into the insula was performed prior to the CPP tests in order to evaluate their effect on CPP expression.

Results

Insular microinjections of the nonselective mAChR antagonist, scopolamine, and the M₁ antagonist, pirenzepine, significantly inhibited CPP expression in both contextual cue- and priming-induced CPP reinstatement; the M₁ agonist, MCN-A-343, and the M₄ antagonist, tropicamide, enhanced CPP expression. The M₄ agonist, LY2033298, inhibited CPP expression. The M₂ antagonist, methoctramine, and M₃ antagonist, 4-DAMP, had no effect on CPP expression.

Conclusion

Our results demonstrate that insular mAChRs play a role in the processing of drug memory. M₁ and M₄ mAChRs work paradoxically; M₁ activation and M₄ inhibition attenuate the expression of drug memory, while M₁ inhibition and M₄ activation augment the expression of drug memory.     

The effect of environmental factors on morphine withdrawal in C57BL/6J mice: running wheel access and group housing

Rationale

Glutamatergic projection neurons in the medial prefrontal cortex (mPFC) are hyperexcitable in cocaine-sensitized animals, resulting in increased excitatory output to addiction-associated regions such as the ventral tegmental area (VTA) and nucleus accumbens. Evidence suggests that Group I metabotropic glutamate receptor 5 (mGluR5) is necessary for cocaine sensitization, and stimulation of this receptor in the mPFC potentially alters cell excitability directly through glutamate release or indirectly through downstream signaling cascades.

Objectives and methods

Experiments in this report examined the role of mPFC mGluR5 in behavioral sensitization to cocaine. Group I mGluR agonist dihydroxyphenylglycine (DHPG) (15 nmol/side), mGluR5 antagonist 3((2-methyl-4-thiazolyl)ethynyl)pyridine (MTEP) (15 nmol/side), mGluR1 antagonist YM298198 (15 nmol/side), AMPA receptor antagonist CNQX (1 nmol/side), and/or saline were administered through cannulae implanted 1 mm above the mPFC and/or VTA in male rats. Cocaine (15 mg/kg, i.p.) was systemically administered for four consecutive days to induce sensitization and/or once on test day immediately preceding locomotor monitoring.

Results

Intra-mPFC DHPG induced an mGluR5-mediated cross-sensitization to cocaine preventable through the prior administration of an AMPA receptor antagonist in the VTA. Furthermore, mGluR5 blockade in the mPFC failed to prevent the initiation of sensitization. However, intra-mPFC injections of the mGluR5 antagonist MTEP prevented the expression of cocaine sensitization at 21, but not 7, days following daily cocaine injections suggesting a possible role for mPFC mGluR5 in the persistence of the cocaine-sensitized state.

Conclusions

These data suggest that stimulation of mGluR5s in the mPFC is sufficient to induce cocaine sensitization and is necessary for the expression of this sensitized response.     

Forebrain PENK and PDYN gene expression levels in three inbred strains of mice and their relationship to genotype-dependent morphine reward sensitivity

Rationale

Vulnerability to drug abuse disorders is determined not only by environmental but also by genetic factors. A body of evidence suggests that endogenous opioid peptide systems may influence rewarding effects of addictive substances, and thus, their individual expression levels may contribute to drug abuse liability.

Objectives

The aim of our study was to assess whether basal genotype-dependent brain expression of opioid propeptides genes can influence sensitivity to morphine reward.

Methods

Experiments were performed on inbred mouse strains C57BL/6J, DBA/2J, and SWR/J, which differ markedly in responses to morphine administration: DBA/2J and SWR/J show low and C57BL/6J high sensitivity to opioid reward. Proenkephalin (PENK) and prodynorphin (PDYN) gene expression was measured by in situ hybridization in brain regions implicated in addiction. The influence of the κ opioid receptor antagonist nor-binaltorphimine (nor-BNI), which attenuates effects of endogenous PDYN-derived peptides, on rewarding actions of morphine was studied using the conditioned place preference (CPP) paradigm.

Results

DBA/2J and SWR/J mice showed higher levels of PDYN and lower levels of PENK messenger RNA in the nucleus accumbens than the C57BL/6J strain. Pretreatment with nor-BNI enhanced morphine-induced CPP in the opioid-insensitive DBA/2J and SWR/J strains.

Conclusions

Our results demonstrate that inter-strain differences in PENK and PDYN genes expression in the nucleus accumbens parallel sensitivity of the selected mouse strains to rewarding effects of morphine. They suggest that high expression of PDYN may protect against drug abuse by limiting drug-produced reward, which may be due to dynorphin-mediated modulation of dopamine release in the nucleus accumbens.     

Interactions between VTA orexin and glutamate in cue-induced reinstatement of cocaine seeking in rats

Rationale

Glutamate and orexin/hypocretin systems are involved in Pavlovian cue-triggered drug seeking.

Objectives

Here, we asked whether orexin and glutamate interact within ventral tegmental area (VTA) to promote reinstatement of extinguished cocaine seeking in a rat self-administration paradigm.

Methods/results

We first found that bilateral VTA microinjections of the orexin 1 receptor (OX1R) antagonist SB-334867 (SB) or a cocktail of the AMPA and NMDA glutamate receptor antagonists CNQX/AP-5 reduced reinstatement of cocaine seeking elicited by cues. In contrast, neither of these microinjections nor systemic SB reduced cocaine-primed reinstatement. Additionally, unilateral VTA OX1R blockade combined with contralateral VTA glutamate blockade attenuated cue-induced reinstatement, indicating that VTA orexin and glutamate are simultaneously necessary for cue-induced reinstatement. We further probed the receptor specificity of glutamate actions in VTA, finding that CNQX, but not AP-5, dose-dependently attenuated cue-induced reinstatement, indicating that AMPA but not NMDA receptor transmission is required for this type of cocaine seeking. Given the necessary roles of both OX1 and AMPA receptors in VTA for cue-induced cocaine seeking, we hypothesized that these signaling pathways interact during this behavior. We found that PEPA, a positive allosteric modulator of AMPA receptors, completely reversed the SB-induced attenuation of reinstatement behavior. Intra-VTA PEPA alone did not alter cue-induced reinstatement, indicating that potentiating AMPA activity with this drug specifically compensates for OX1R blockade, rather than simply inducing or enhancing reinstatement itself.

Conclusions

These findings show that cue-induced, but not cocaine-primed, reinstatement of cocaine seeking is dependent upon orexin and AMPA receptor interactions in VTA.     

Involvement of nucleus accumbens AMPA receptor trafficking in augmentation of D- amphetamine reward in food-restricted rats

Rationale

Chronic food restriction (FR) increases behavioral responsiveness to drugs of abuse and associated environments. Pre- and postsynaptic neuroadaptations have been identified in the mesoaccumbens dopamine pathway of FR subjects but the mechanistic basis of increased drug reward magnitude remains unclear.

Objectives

Effects of FR on basal and d-amphetamine-induced trafficking of AMPA receptor subunits to the nucleus accumbens (NAc) postsynaptic density (PSD) were examined, and AMPA receptor involvement in augmentation of d-amphetamine reward was tested.

Materials and methods

FR and ad libitum fed (AL) rats were injected with d-amphetamine (2.5 mg/kg, i.p.) or vehicle. Brains were harvested and subcellular fractionation and Western analyses were used to assess AMPA receptor abundance in NAc homogenate and PSD fractions. A follow-up experiment used a curve-shift protocol of intracranial self-stimulation to assess the effect of 1-naphthylacetyl spermine (1-NASPM), a blocker of Ca²⁺-permeable AMPA receptors, on rewarding effects of d-amphetamine microinjected in NAc shell.

Results

FR increased GluA1 in the PSD, and d-amphetamine increased p-Ser845-GluA1, GluA1, GluA2, but not GluA3, with a greater effect in FR than AL rats. d-amphetamine lowered reward thresholds, with greater effects in FR than AL rats, and 1-NASPM selectively reversed the enhancing effect of FR.

Conclusions

Results suggest that FR leads to increased synaptic incorporation of GluA1 homomers to potentiate rewarding effects of appetitive stimuli and, as a maladaptive byproduct, d-amphetamine. The d-amphetamine-induced increase in synaptic p-Ser845-GluA1, GluA1, and GluA2 may contribute to the rewarding effect of d-amphetamine, but may also be a mechanism of synaptic strengthening and behavior modification.     

Opposing roles of prelimbic and infralimbic dopamine in conditioned cue and place preference

Rationale

Increasing evidence points to the prelimbic (PL) and infralimbic (IL) cortices of the medial prefrontal cortex (mPFC) and their dopaminergic innervations subserving opposing roles in the regulation of instrumental behavior. However, it is at present unclear if they hold similar roles in the regulation of Pavlovian learning.

Objective

The present study investigated the role of the dopaminergic innervations of the PL and IL in the modulation of Pavlovian appetitive cue and place conditioning, previously shown to be dependent on the basolateral amygdala and hippocampus, respectively.

Methods

Rats received preconditioning microinfusions of d-amphetamine, cis-flupenthixol, or vehicle solution directly into the PL or IL and were trained to simultaneously acquire conditioned cue and place preference in a radial maze.

Results

Preconditioning blockade of dopamine neurotransmission in the PL and amphetamine microinfusions in the IL had the same effect of attenuating place conditioning. In contrast, place conditioning remained intact following preconditioning amphetamine microinfusions in the PL and dopamine receptor blockade in the IL. Instead, conditioned cue preference was attenuated following IL dopamine receptor blockade.

Conclusion

These data indicate that PL dopaminergic mechanisms are critical for the acquisition of appetitive place learning, while IL dopamine may oppose the influence of PL dopamine upon hippocampal-dependent learning. Furthermore, they implicate a functional reciprocity between mPFC and associated subregions of the nucleus accumbens in the regulation of limbic information processing.     

Differential effects on natural reward processing in rats after repeated heroin

Rationale

Heroin users report reward deficits as well as reward enhancements (to drug stimuli). To better understand the causal relation between chronic heroin and alterations in natural reward processing, we used experimental techniques in animal models.

Methods

Separate groups of rats were trained in several food reward paradigms: conditioned place preference (CPP), food-reinforced lever pressing under a progressive ratio schedule of reinforcement, free feeding, and lever pressing with conditioned reinforcement. After training, the rats were subjected to 10 daily heroin (2 mg/kg) or saline vehicle injections and tested at 3, 15, and 30 days post-treatment.

Results

Repeated heroin treatment abolished the CPP and significantly reduced break points for food reward at 3, 15, and 30 days post-treatment. Repeated heroin did not affect free feeding. Finally, repeated heroin significantly enhanced responding for a food-based conditioned reinforcer.

Conclusions

Repeated heroin decreases the attractiveness of food-associated cues and reduces motivation to work for natural reward. However, it appears to enhance natural conditioned reward processes that involve the acquisition of novel responding. Thus, repeated heroin appears to produce differential effects on natural reward processing depending on the nature of the reward-directed behavior.     

Decreased prefrontal cortex dopamine activity following adolescent social defeat in male rats: role of dopamine D2 receptors

Rationale

Adverse social experience in adolescence causes reduced medial prefrontal cortex (mPFC) dopamine (DA) and associated behavioral deficits in early adulthood.

Objective

This study aims to determine whether mPFC DA hypofunction following social stress is specific to adolescent experience and if this results from stress-induced DA D₂ receptor activation.

Materials and methods

Male rats exposed to repeated social defeat during adolescence or adulthood had mPFC DA activity sampled 17 days later. Separate experiments used freely moving microdialysis to measure mPFC DA release in response to adolescent defeat exposure. At P40, 49 and 56 mPFC DA turnover was assessed to identify when DA activity decreased in relation to the adolescent defeat experience. Finally, nondefeated adolescent rats received repeated intra-mPFC infusions of the D₂ receptor agonist quinpirole, while another adolescent group received intra-mPFC infusions of the D₂ antagonist amisulpride before defeat exposure.

Results

Long-term decreases or increases in mPFC DA turnover were observed following adolescent or adult defeat, respectively. Adolescent defeat exposure elicits sustained increases in mPFC DA release, and DA turnover remains elevated beyond the stress experience before declining to levels below normal at P56. Activation of mPFC D₂ receptors in nondefeated adolescents decreases DA activity in a similar manner to that caused by adolescent defeat, while defeat-induced reductions in mPFC DA activity are prevented by D₂ receptor blockade.

Conclusions

Both the developing and mature PFC DA systems are vulnerable to social stress, but only adolescent defeat causes DA hypofunction. This appears to result in part from stress-induced activation of mPFC D₂ autoreceptors.     

Reduced emotional signs of opiate withdrawal in rats selectively bred for low (LoS) versus high (HiS) saccharin intake

Rationale

Rats bred for high (HiS) and low (LoS) saccharin intake exhibit divergent behavioral responses to multiple drugs of abuse, with HiS rats displaying greater vulnerability to drug taking. Previous research indicates that this effect may be due to increased sensitivity to reward in HiS rats and to the aversive effects of acute drug administration in LoS rats.

Objective

The current study investigated whether HiS and LoS rats also exhibit different behavioral signs of withdrawal following one or repeated opiate exposures.

Methods

Emotional signs of opiate withdrawal were assessed with potentiation of the acoustic startle reflex and conditioned place aversion (CPA) in male and female HiS and LoS rats. Startle was measured before and 4 h after a 10-mg/kg injection of morphine on days 1, 2, and 7 of opiate exposure. CPA was induced with a 2-day, naloxone-precipitated conditioning paradigm. Somatic signs of withdrawal and weight loss were also measured.

Results

Male and female LoS rats exhibited lower startle potentiation than HiS rats on the seventh day of morphine exposure. LoS male rats also failed to develop a CPA to morphine withdrawal. No differences in physical withdrawal signs were observed between HiS and LoS rats, but males of both lines had more physical signs of withdrawal than females.

Conclusions

These results suggest that LoS rats are less vulnerable to the negative emotional effects of morphine withdrawal than HiS rats. A less severe withdrawal syndrome may contribute to decreased levels of drug taking in the LoS line.     

Effects of the beta-lactam antibiotic ceftriaxone on nicotine withdrawal and nicotine-induced reinstatement of preference in mice

Rationale

Several studies suggest that repeated nicotine administration causes alterations in glutaminergic transmission that may play an important role in developing and maintaining nicotine addiction. Chronic nicotine administration in rats decreases the expression of the glutamate transporter-1 (GLT-1) and cysteine–glutamate exchanger (system x_C?) in the nucleus accumbens. We hypothesized that ceftriaxone, a GLT-1 and system x_C? activator, would decrease murine behavioral aspects of nicotine dependence.

Objective

This study aimed to investigate the effect of repeated ceftriaxone administration on the behavioral effects of nicotine using mouse models of conditioned reward and withdrawal.

Method

Using male ICR mice, the ability of repeated ceftriaxone injections to modulate the development and reinstatement of a nicotine-conditioned place preference (CPP) was evaluated. Additionally, nicotine withdrawal-associated signs were assessed. These included both physical (somatic signs and hyperalgesia) and affective (anxiety-related behaviors) withdrawal signs in mice. Finally, the effects of ceftriaxone on nicotine-induced antinociception and hypothermia after acute nicotine injection were measured.

Result

Ceftriaxone had no effect on the development of nicotine preference but significantly attenuated nicotine-induced reinstatement of CPP. Furthermore, ceftriaxone reversed all nicotine withdrawal signs measured in mice.

Conclusion

Altogether, these findings show that a β-lactam antibiotic reduces nicotine withdrawal and nicotine-seeking behavior. Our results suggest that the documented efficacy of ceftriaxone against cocaine and morphine dependence-related behaviors effects extends to nicotine.     

Opiate Exposure State Controls a D2-CaMKIIα-Dependent Memory Switch in the Amygdala-Prefrontal Cortical Circuit

The mammalian basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) comprise a functionally interconnected circuit that is critical for processing opiate-related associative memories. In the opiate-naïve state, reward memory formation in the BLA involves a functional link between dopamine (DA) D1 receptor (D1R) and extracellular signal-related kinase 1/2 (ERK1/2) signaling substrates, but switches to a DA D2 (D2R)/Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα)-dependent memory substrate following chronic opiate exposure and spontaneous withdrawal. Using conditioned place preference (CPP) in rats paired with molecular analyses, we examined the role of intra-mPFC CaMKII, ERK and DAergic activity during the formation of opiate associative memories, and how opiate exposure state may regulate the functions of these molecular memory pathways. We report that the role of CaMKIIα signaling is functionally reversed within the BLA-mPFC pathway depending on opiate exposure state. Thus, in the opiate-naïve state, intra-mPFC but not intra-BLA blockade of CaMKII signaling prevents formation of opiate reward memory. However, following chronic opiate exposure and spontaneous withdrawal, the role of CaMKII signaling in the BLA-mPFC is functionally reversed. This behavioral memory switch corresponds to a selective increase in the expression of D2R and CaMKIIα, but not other calcium/calmodulin-related molecules, nor D1R expression levels within the mPFC.     

Involvement of nucleus accumbens dopamine D1 receptors in ethanol drinking, ethanol-induced conditioned place preference, and ethanol-induced psychomotor sensitization in mice

Rationale

Dopamine D1 receptor (D1R) signaling has been associated to ethanol consumption and reward in laboratory animals.

Objectives

Here, we hypothesize that this receptor, which is located within the nucleus accumbens (NAc) neurons, modulates alcohol reward mechanisms.

Methods

To test this hypothesis, we measured alcohol consumption and ethanol-induced psychomotor sensitization and conditioned place preference (CPP) in mice that received bilateral microinjections of small interference RNA (siRNA)-expressing lentiviral vectors (LV-siD1R) producing D1R knock-down. The other group received control (LV-Mock) viral vectors into the NAc.

Results

There were no differences in the total fluid consumed and also no differences in the amount of ethanol consumed between groups prior to surgery. However, after surgery, the LV-siD1R group consumed less ethanol than the control group. This difference was not associated to taste neophobia. In addition, results have shown that down-regulation of endogenous D1R using viral-mediated siRNA in the NAc significantly decreased ethanol-induced behavioral sensitization as well as acquisition, but not expression, of ethanol-induced place preference.

Conclusions

We conclude that decreased D1R expression into the NAc led to reduced ethanol rewarding properties, thereby leading to lower voluntary ethanol consumption. Together, these findings demonstrate that the D1 receptor pathway within the NAc controls ethanol reward and intake.     

Impairing effect of amphetamine and concomitant ionotropic glutamate receptors blockade in the ventral striatum on spatial learning in mice

Rationale

Accumulating evidence supports the involvement of the ventral striatum (VS) in spatial information processing. The multiple cortical glutamatergic and mesolimbic dopaminergic (DAergic) afferences on the same neurons in the ventral striatum provide the neuroanatomical substrate for glutamate and dopamine functional interaction. However, there is little evidence in the literature on how this interaction affects the ability to encode spatial information.

Objective

First, we evaluated the effect of intra-VS bilateral infusion of different doses of amphetamine (0.3, 0.75, and 1.5 μg/side) on the ability to detect spatial novelty in mice. Next, we examined the impact produced on the same abilities by intra-VS infusion of ineffective doses of amphetamine (0.3 μg/side) in association with N-methyl-d-aspartate (NMDA) (3.125 ng/side) or α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) (0.25 ng/side) receptor antagonist.

Results

The results show that infusion of amphetamine impairs detection of spatial novelty, affecting also exploratory activity and marginally the detection of nonspatial novelty. In contrast, an association of subthreshold doses of amphetamine with NMDA or AMPA receptor antagonists exerted a selective effect on reactivity to a spatial change.

Conclusions

These findings demonstrate that enhanced DAergic activity in the VS enhances glutamate receptor antagonist-induced impairment in learning and memory.     

Altered cerebral blood flow and neurocognitive correlates in adolescent cannabis users

Rationale

In drug-dependent individuals, the primary excessive motivation is for drugs. Studies also indicate altered interest for “natural” rewarding activities associated with motivational disorders that may be relevant to drug dependence. However, to date, the impact of drug dependence and withdrawal upon motivation for “natural” rewards remains unclear.

Methods and objectives

In the present study, we use a food-driven operant behavior paradigm to assess the impact of opiate intake and withdrawal upon the motivational properties of highly palatable food (HPF) in mice.

Results

Our findings indicate that early (8-h) opiate withdrawal does not affect either the motivational or the discriminative properties of HPF intake. However, starting 32?h after the last morphine injection, opiate withdrawal increases operant behavior aimed at obtaining HPF. The increased HPF-driven behavior lasts at least 12?days following opiate withdrawal, indicating long-lasting effects upon motivation. Using a paradigm of reward contingency reversal, we also address the impact of opiate withdrawal upon cognitive functions. Our results indicate that opiate withdrawal does not affect the ability to learn a new operant rule to obtain HPF. Indeed, opiate withdrawal ameliorates the acquisition of the new HPF-driven operant task, most probably due to the persistent and long-lasting increased motivation. Finally, analysis of ambulatory activity and body weight (BW) changes reveal that motivational and cognitive effects are totally independent of caloric and/or motor effects of opiate dosing and withdrawal.

Conclusions

These results clearly demonstrate that excessive opiate intake and withdrawal produces dramatic and long-lasting motivational disorders relevant to drug dependence.     

Delay Discounting of Oral Morphine and Sweetened Juice Rewards in Dependent and Non-Dependent Rats

Rationale

Opioid-dependent humans are reported to show accelerated delay discounting of opioid rewards when compared to monetary rewards. It has been suggested that this may reflect a difference in discounting of consumable and non-consumable goods not specific to dependent individuals. Here, we evaluate the discounting of similar morphine and non-morphine oral rewards in dependent and non-dependent rats

Methods

We first tested the analgesic and rewarding effects of our morphine solution. In a second experiment, we assigned rats randomly to either dependent or non-dependent groups that, 30 min after daily testing, received 30 mg/kg subcutaneous dose of morphine, or saline, respectively. Delay discounting of drug-free reward was examined prior to initiation of the dosing regimen. We tested discounting of the morphine reward in half the rats and retested the discounting of the drug-free reward in the other half. All tests were run 22.5 h after the daily maintenance dose.

Results

Rats preferred the morphine cocktail to the drug-free solution and consumed enough to induce significant analgesia. The control quinine solution did not produce these effects. Dependent rats discounted morphine rewards more rapidly than before dependence and when compared to discounting drug-free rewards. In non-dependent rats both reward types were discounted similarly.

Conclusions

These results show that morphine dependence increases impulsiveness specifically towards a drug reward while morphine experience without dependence does not.     

Effects of acute withdrawal on ethanol-induced conditioned place preference in DBA/2J mice

Rationale

Reexposure to ethanol during acute withdrawal might facilitate the transition to alcoholism by enhancing the rewarding effect of ethanol.

Objective

The conditioned place preference (CPP) procedure was used to test whether ethanol reward is enhanced during acute withdrawal.

Methods

DBA/2J mice were exposed to an unbiased one-compartment CPP procedure. Ethanol (0.75, 1.0, or 1.5 g/kg IP) was paired with a distinctive floor cue (CS+), whereas saline was paired with a different floor cue (CS?). The withdrawal (W) group received CS+ trials during acute withdrawal produced by a large dose of ethanol (4 g/kg) given 8 h before each trial. The no-withdrawal (NW) group did not experience acute withdrawal during conditioning trials but was matched for acute withdrawal experience. Floor preference was tested in the absence of ethanol or acute withdrawal.

Results

All groups eventually showed a dose-dependent preference for the ethanol-paired cue, but development of CPP was generally more rapid and stable in the W groups than in the NW groups. Acute withdrawal suppressed the normal activating effect of ethanol during CS+ trials, but there were no group differences in test activity.

Conclusions

Acute withdrawal enhanced ethanol’s rewarding effect as indexed by CPP. Since this effect depended on ethanol exposure during acute withdrawal, the enhancement of ethanol reward was likely mediated by the alleviation of acute withdrawal, i.e., negative reinforcement. Enhancement of ethanol reward during acute withdrawal may be a key component in the shift from episodic to chronic ethanol consumption that characterizes alcoholism.