Expression Of Cocaine Conditioned Place Preference Research Articles

Sex differences in hippocampal β-adrenergic receptor subtypes drive retrieval, retention, and learning of cocaine-associated memories.

Drug seeking behavior occurs in response to environmental contexts and drug-associated cues. The presence of these pervasive stimuli impedes abstinence success. β-adrenergic receptors (β-ARs) have a long-standing historical implication in driving processes associated with contextual memories, including drug-associated memories in substance use disorders. However, sex differences in the role of β-adrenergic receptors in drug memories remain unknown. Prior reports indicate a selective role for β2-ARs in retrieval and retention of contextual drug memories in males, and substantial sex differences exist in the expression of β-ARs of male and female rats. Therefore, we hypothesized that there are sex differences in selective recruitment of β-ARs during different stages of memory encoding and retrieval. The role of β-ARs in driving retrieval and learning of contextual cocaine memories was investigated using cocaine conditioned place preference (CPP) in adult male and female Sprague-Dawley rats. Rats were infused directly to the dorsal hippocampus with Propranolol (β1 and β2) or ICI-118,551 (β1) and/or Betaxolol (β2), immediately prior to testing (retrieval), or paired to each cocaine (10 mg/kp, IP) conditioning session (learning). In males, administration of either β1, β2, or combined β1 and β2-ARs before the initial CPP testing reduced the expression of a CPP compared to vehicle administration. In females, β2-ARs transiently decreased CPP memories, whereas β1 had long lasting but not immediate effects to decrease CPP memories. Additionally, β1 and combined β1 and β2-ARs had immediate and persistent effects to decrease CPP memory expression. DG Fos + neurons predicted cocaine CPP expression in males, whereas CA1 and CA3 Fos + neurons predicted cocaine CPP expression in females. There are significant sex differences in the role of dorsal hippocampus β-ARs in the encoding and expression of cocaine conditioned place preference. Furthermore, sub regions of the dorsal hippocampus appear to activate differently between male and female rats during CPP. Therefore DG, CA3, and CA1 may have separate region- and sex-specific impacts on driving drug- associated, or context-associated cues.

Sex difference in the effect of environmental enrichment on food restriction-induced persistence of cocaine conditioned place preference and mechanistic underpinnings

Psychosocial and environmental factors, including loss of natural reward, contribute to the risk of drug abuse. Reward loss has been modeled in animals by removal from social or sexual contact, transfer from enriched to impoverished housing, or restriction of food. We previously showed that food restriction increases the unconditioned rewarding effects of abused drugs and the conditioned incentive effects of drug-paired environments. Mechanistic studies provided evidence of decreased basal dopamine (DA) transmission, adaptive upregulation of signaling downstream of D1 DA receptor stimulation, synaptic upscaling and incorporation of calcium-permeable AMPA receptors (CP-AMPARs) in medium spiny neurons (MSNs) of nucleus accumbens (NAc). These findings align with the still evolving ‘reward deficiency’ hypothesis of drug abuse. The present study tested whether a compound natural reward that is known to increase DA utilization, environmental enrichment, would prevent the persistent expression of cocaine conditioned place preference (CPP) otherwise observed in food restricted rats, along with the mechanistic underpinnings. Because nearly all prior investigations of both food restriction and environmental enrichment effects on cocaine CPP were conducted in male rodents, both sexes were included in the present study. Results indicate that environmental enrichment curtailed the persistence of CPP expression, decreased signaling downstream of the D1R, and decreased the amplitude and frequency of spontaneous excitatory postsynaptic currents (EPSCs) in NAc MSNs of food restricted male, but not female, rats. The failure of environmental enrichment to significantly decrease food restriction-induced synaptic insertion of CP-AMPARs, and how this may accord with previous pharmacological findings that blockade of CP-AMPARs reverses behavioral effects of food restriction is discussed. In addition, it is speculated that estrous cycle-dependent fluctuations in DA release, receptor density and MSN excitability may obscure the effect of increased DA signaling during environmental enrichment, thereby interfering with development of the cellular and behavioral effects that enrichment produced in males.

Endogenous opioid system modulates conditioned cocaine reward in a sex-dependent manner.

Cocaine predictive cues and contexts exert powerful control over behaviour and can incite cocaine seeking and taking. This type of conditioned behaviour is encoded within striatal circuits, and these circuits and behaviours are, in part, regulated by opioid peptides and receptors expressed in striatal medium spiny neurons. We previously showed that augmenting levels of the opioid peptide enkephalin in the striatum facilitates acquisition of cocaine conditioned place preference (CPP), while opioid receptor antagonists attenuate expression of cocaine CPP. However, whether striatal enkephalin is necessary for acquisition of cocaine CPP and maintenance during extinction remains unknown. To address this, we generated mice with a targeted deletion of enkephalin from dopamine D2-receptor expressing medium spiny neurons and tested them in a cocaine CPP paradigm. Low striatal enkephalin levels did not attenuate acquisition of CPP. However, expression of preference, assessed after acute administration of the opioid receptor antagonist naloxone, was blocked in females, regardless of genotype. When saline was paired with the cocaine context during extinction sessions, females, regardless of genotype, extinguished preference faster than males, and this was prevented by naloxone when paired with the cocaine context. We conclude that while striatal enkephalin is not necessary for acquisition, expression, or extinction of cocaine CPP, expression and extinction of cocaine preference in females is mediated by an opioid peptide other than striatal enkephalin. The unique sensitivity of females to opioid antagonists suggests sex should be a consideration when using these compounds in the treatment of cocaine use disorder.

Expression of stable and reliable preference and aversion phenotypes following place conditioning with psychostimulants.

Drug-seeking behavior occurs more readily in some individuals than others. This phenomenon is considered in studies of drug self-administration in which high drug-seeking/taking individuals can be identified. In contrast, studies of conditioned place preference (CPP) often involve a random sample of drug-naïve rodents that includes phenotypes not considered relevant to addiction. The main objective of the current studies was to determine if a priori identification of different conditioning phenotypes could improve the validity and sensitivity of CPP expression as a preclinical test for vulnerability to addiction. Analysis of cocaine place conditioning data from 443 Swiss-Webster mice revealed a trimodal distribution with peaks corresponding to means of k = 3 clusters. The cluster means occurred at high, low, or negative preference scores, the latter suggesting a phenotype acquiring conditioned place aversion (CPA). The same clusters were identified in mice conditioned with methamphetamine, MDPV, or amphetamine, and these clusters remained stable and reliable during three additional expression tests spaced at 24h. A meta-analysis of effect sizes obtained from CPP literature revealed a positively skewed distribution affected by sample size, consistent with the existence of a CPA phenotype within the populations tested. A dopamine receptor antagonist, flupentixol, blocked cocaine CPP expression in a group containing all phenotypes, but sensitivity improved markedly when CPA phenotypes were excluded from the dataset. These studies suggest that taking phenotype into consideration when designing place conditioning studies will improve their application as a preclinical tool in addiction biology and drug discovery.

The GLT-1 enhancer clavulanic acid suppresses cocaine place preference behavior and reduces GCPII activity and protein levels in the rat nucleus accumbens

The β-lactam antibiotic ceftriaxone (CTX) is a glutamate transporter subtype 1 (GLT-1) enhancer that reduces cocaine reinforcing efficacy and relapse in rats, but pharmacokinetic liabilities limit translational utility. An attractive alternative is clavulanic acid (CLAV), a structurally related β-lactamase inhibitor and component of FDA-approved Augmentin. CLAV retains the GLT-1 enhancing effects of CTX but displays greater oral bioavailability, brain penetrability and negligible antibacterial activity. CLAV reduces morphine conditioned place preference (CPP) and ethanol consumption in rats, but knowledge about the efficacy of CLAV in preclinical models of drug addiction remains sparse. Here, we investigated effects of CLAV (10 mg/kg, IP) on the acquisition, expression, and maintenance of cocaine CPP in rats, and on two glutamate biomarkers associated with cocaine dependence, GLT-1 and glutamate carboxypeptidase II (GCPII). CLAV administered during cocaine conditioning (10 mg/kg, IP x 4 d) did not affect the development of cocaine CPP. However, a single CLAV injection, administered after the conditioning phase, reduced the expression of cocaine CPP. In rats with established cocaine preference, repeated CLAV administration facilitated extinction of cocaine CPP. In the nucleus accumbens, acute CLAV exposure reduced GCPII protein levels and activity, and a 10-d CLAV treatment regimen enhanced GLT-1 levels. These results suggest that CLAV reduces expression and maintenance of cocaine CPP but lacks effect against development of CPP. Moreover, the ability of a single injection of CLAV to reduce both GCPII activity and protein levels, as well as expression of cocaine CPP, points toward studying GCPII as a therapeutic target of CLAV.

(-)-Stepholidine blocks expression, but not development, of cocaine conditioned place preference in rats

The purpose of this study was to investigate the effects of (-)-stepholidine (SPD), a compound with dopamine D1 partial agonist and D2/D3 antagonist properties, on the development and expression of cocaine conditioned place preference (CPP). Subjects (N = 65; male Long Evans rats) were tested using a CPP procedure consisting of 3 phases: (1) a 15-min pre-exposure session where animals could explore each compartment freely, (2) eight 30-min conditioning sessions where animals were restricted to one side or the other with cocaine (10 mg/kg) or saline, respectively, on alternating days and (3) a 15-minute preference test session where animals could explore each compartment freely. To test the effects of SPD on expression of cocaine CPP, rats were administered vehicle (distilled water with 20 % DMSO), 10, 15 or 20 mg/kg SPD (intraperitoneally) 30 min prior to the test session. We found that 20 mg/kg of SPD significantly blocked the expression of cocaine CPP. To test the effects of SPD on the development of CPP, 0 (vehicle), 10, 15 or 20mg/kg SPD were administered 30 min prior to each cocaine conditioning session and vehicle before each saline conditioning session; no treatment was given prior to the test session. A preference test showed that each SPD group maintained a CPP similar to the vehicle group. These data indicate that SPD can block the expression of a cocaine CPP but has no effect on its development, suggesting that it inhibits the effects of cocaine cues on cocaine incentive motivated behavior. These results suggest that SPD may be a potential treatment for cue-driven aspects of cocaine use disorder.

Cocaine reward and memory after chemogenetic inhibition of distinct serotonin neuron subtypes in mice.

We probed serotonin neurons, those denoted by their developmental gene expression as r2Hoxa2-Pet1 (experiment 1) and Drd1a-Pet1 (experiment 2), for differential modulation of cocaine reward and memory as revealed by the expression and development of conditioned place preference (CPP) in transgenic mice. To query roles in CPP, we inhibited neurons cell autonomously in vivo by activating the transgenically expressed, synthetic DREADD receptor hM4Di (Di) with the exogenous ligand clozapine-N-oxide (CNO). To examine CPP expression, mice were conditioned using behaviorally active doses of cocaine (10.0 or 17.8 mg/kg) vs. saline followed by CPP assessment, first without neuron inhibition (post-conditioning session 1), and then with CNO-mediated neuron inhibition (post-conditioning session 2), followed by 4 more post-conditioning sessions. To examine CPP development, we administered CNO during conditioning sessions and then assayed CPP across 6 post-conditioning sessions. In r2Hoxa2-Pet1-Di mice, post-conditioning CNO administration did not impact cocaine CPP expression, but after CNO administration during conditioning, cocaine CPP (17.8 mg/kg) persisted across post-conditioning sessions compared with that in controls, suggesting a deficit in extinguishing cocaine memory. Drd1a-Pet1-Di mice, prior to CNO-Di-triggered neuronal inhibition, unexpectedly expressed heightened cocaine CPP (10.0 and 17.8 mg/kg) compared with controls, and this basal phenotype was transiently blocked by acute post-conditioning CNO administration and persistently blocked by repeated CNO administration during conditioning. Cocaine reward and memory likely map to distinct serotonergic Pet1 neuron subtypes. r2Hoxa2-Pet1 neurons normally may limit the durability of cocaine memory, without impacting initial cocaine reward magnitude. Drd1a-Pet1 neurons normally may help to promote cocaine reward.

Activation of GABAergic Neurons in the Nucleus Accumbens Mediates the Expression of Cocaine-Associated Memory.

Cocaine-associated environmental cues elicit craving and relapse to cocaine use by recalling the rewarding memory of cocaine. However, the neuronal mechanisms underlying the expression of cocaine-associated memory are not fully understood. Here, we investigated the possible contribution of γ-aminobutyrate (GABA)ergic neurons in the nucleus accumbens (NAc), a key brain region associated with the rewarding and reinforcing effects of cocaine, to the expression of cocaine-associated memory using the conditioned place preference (CPP) paradigm combined with designer receptors exclusively activated by designer drugs (DREADD) technology. The inhibitory DREADD hM4Di was selectively expressed in NAc GABAergic neurons of vesicular GABA transporter-Cre (vGAT-Cre) mice by infusing adeno-associated virus (AAV) vectors. Ex vivo electrophysiological recordings revealed that bath application of clozapine-N-oxide (CNO) significantly hyperpolarized membrane potentials and reduced the number of spikes induced by depolarizing current injections in hM4Di-positive NAc neurons. Additionally, systemic CNO injections into cocaine-conditioned mice 30 min before posttest session significantly reduced CPP scores compared to saline-injected mice. These results indicate that chemogenetic inhibition of NAc GABAergic neurons attenuated the expression of cocaine CPP, suggesting that NAc GABAergic neuronal activation is required for the environmental context-induced expression of cocaine-associated memory.

A neurotensin analog blocks cocaine-conditioned place preference and reinstatement.

Neurotensin (NT) is a neuropeptide that acts as a neurotransmitter and neuromodulator in the central nervous system. Several studies suggest a therapeutic role for NT analogs in nicotine and other psychostimulant addictions. We studied the effects of the nonselective NT receptor agonist NT69L, which has equal affinity for the two major NT receptors, NTS1 and NTS2, on the expression of cocaine-conditioned place preference (cocaine-CPP) and reinstatement after extinction. Robust cocaine-CPP was obtained after 5 days of conditioning. Extinction was induced using eight repeated daily injections of saline. Reinstatement was prompted by priming with one injection of cocaine (12 mg/kg intraperitoneally). On the test day, NT69L (1 mg/kg intraperitoneally) was administered 30 min before assessing cocaine-CPP. Extinction led to the loss of cocaine-CPP. One injection of cocaine (12 mg/kg intraperitoneally) for cocaine priming reinstated cocaine-CPP. NT69L blocked cocaine-CPP reinstatement in cocaine-primed animals. In addition, NT69L blocked cocaine-CPP reinstatement when administered before priming with cocaine. Thus, the NT agonist NT69L blocked both cocaine-CPP and reinstatement to cocaine preference. NT69L may exert this action by modulating the mesocorticolimbic dopamine and glutamatergic pathways involved in addiction and relapse processes. Therefore, NT agonists may represent a novel therapy for the treatment of addiction to cocaine and possibly to other psychostimulants.

Nociceptin receptor activation does not alter acquisition, expression, extinction and reinstatement of conditioned cocaine preference in mice

Growing evidence indicates that targeting nociceptin receptor (NOP) signaling may have therapeutic efficacy in treating alcohol and opioid addiction. However, little is known about the therapeutic value of selective NOP agonists for the treatment of cocaine dependence. Recently, we identified a highly selective, brain-penetrant NOP small molecule agonist (SR-8993), and using this compound, we previously showed that nociceptin receptor activation attenuated consolidation of fear-related memories. Here, we sought to determine whether SR-8993 also affects the rewarding properties of cocaine. Using a conditioned place preference (CPP) procedure, we show that SR-8993 (3 or 10mg/kg) failed to disrupt acquisition or expression of cocaine CPP (7.5 or 15mg/kg) in C57BL/6 mice. Additionally, SR-8993 did not affect rate of extinction or reinstatement (yohimbine- and cocaine-induced) of cocaine CPP. These studies indicate that selective activation of NOP may not be sufficient in reducing behavioral responses to cocaine.

Dissociation of β1- and β2-adrenergic receptor subtypes in the retrieval of cocaine-associated memory

Drug seeking is maintained by encounters with drug-associated cues, and disrupting retrieval of these drug–cue associations would reduce the risk of relapse. Retrieval of cocaine-associated memories is dependent on β-adrenergic receptor (β-AR) activation, and blockade of these receptors induces a persistent retrieval deficit. Whether retrieval of cocaine-associated memory is mediated by a specific β-AR subtype, however, remains unclear. Using a cocaine conditioned place preference (CPP) procedure, we examined whether retrieval of a cocaine CPP memory is mediated collectively by β1- and β2-ARs, or by one of these β-AR subtypes alone. We show that co-blockade of β1- and β2-ARs abolished CPP expression on that and subsequent drug-free CPP tests, resulting in a long-lasting retrieval deficit that prevented subsequent cocaine-induced reinstatement. To dissociate the necessity of either β1- or β2-ARs alone, we administered subtype-specific antagonists prior to retrieval. Administration of a β1-AR antagonist before the initial CPP trial dose-dependently reduced expression of a CPP on that and subsequent drug-free trials as compared to vehicle administration. In contrast, administration of a β2-AR antagonist had no effect on initial CPP expression, although the highest dose reduced subsequent CPP expression. Importantly, either β1- or β2-AR blockade prior to an initial retrieval trial prevented subsequent cocaine-induced reinstatement. Our findings indicate that the β1-AR subtype mediates retrieval of a cocaine CPP, and that acutely blocking either β1- or β2-ARs can prevent subsequent cocaine-induced reinstatement. Thus, β-AR antagonists, particularly β1-ARs antagonists, could serve as adjuncts for addiction therapies to prevent retrieval of drug-associated memories and provide protection against relapse.

Critical role of cholinergic transmission from the laterodorsal tegmental nucleus to the ventral tegmental area in cocaine-induced place preference

Conditioned place preference (CPP) is widely used to investigate the rewarding properties of cocaine. Various brain regions and neurotransmitters are involved in developing cocaine CPP. However, the contribution of cholinergic transmission in the ventral tegmental area (VTA) to cocaine CPP remains largely unexplored. Here, we examined the role of cholinergic input arising from the laterodorsal tegmental nucleus (LDT) to the VTA in the acquisition and expression of cocaine CPP in rats. Intra-LDT injection of carbachol, which hyperpolarizes LDT neurons, and of NMDA and AMPA receptor antagonists before cocaine conditioning blocked and attenuated cocaine CPP, respectively, indicating the necessity of LDT activity for acquiring the CPP. Additionally, intra-VTA injection of scopolamine or mecamylamine before cocaine conditioning also attenuated cocaine CPP, demonstrating the contribution of cholinergic transmission via muscarinic and nicotinic acetylcholine receptors in CPP acquisition. Furthermore, intra-VTA injection of scopolamine or mecamylamine immediately before the test attenuated cocaine CPP, indicating that cholinergic signaling is also associated with the expression of CPP. These results suggest that cholinergic transmission from the LDT to the VTA is critically involved in both acquiring and retrieving cocaine-associated memories in cocaine CPP.

Optogenetic activation of GABAergic neurons in the nucleus accumbens decreases the activity of the ventral pallidum and the expression of cocaine-context-associated memory

GABAergic medium-sized spiny neurons (MSNs) in the nucleus accumbens (NAc) differentially express D1 and D2 dopamine receptors. Both D2- and D1-MSNs in the NAc form projections into the ventral pallidum, whereas only D1-MSNs directly project into midbrain neurons. They are critical in rewarding and aversive learning, and understanding the function of these NAc efferents and the alteration of their targeted brain regions in responding to a reward-associated context is important. In this study, we activated the GABAergic neurons in the NAc of mice expressing channelrhodopsin-2 under the control of the vesicular GABA transporter promoter by an optogenetic approach, and examined its effects on the expression of cocaine-context-associated memory. In vivo optogenetic activation of the NAc GABAergic neurons inhibited the expression of cocaine-conditioned place preference (CPP). When tested 24 h later, these mice exhibited normal cocaine-induced CPP, indicating that the inhibitory effect on the expression of CPP was transient and reversible. Activation of the NAc GABAergic neurons also attenuated the learning of cocaine-induced reinforcement, as indicated by the results of behavioural sensitization. To explore how the cocaine-context-associated information was processed and integrated, we assessed the activity of NAc MSN-targeted brain nuclei and found that the activation of NAc GABAergic neurons during CPP expression resulted in a decrease of c-Fos+ cells in the ventral palladium. Our data suggested that the NAc GABAergic efferents inhibit the ventral palladium activity and negatively regulate the expression of motivational effects induced by cocaine-context-associated cues.

Tonic Inhibition of Accumbal Spiny Neurons by Extrasynaptic α4βδ GABAAReceptors Modulates the Actions of Psychostimulants

Within the nucleus accumbens (NAc), synaptic GABAA receptors (GABAARs) mediate phasic inhibition of medium spiny neurons (MSNs) and influence behavioral responses to cocaine. We demonstrate that both dopamine D1- and D2-receptor-expressing MSNs (D-MSNs) additionally harbor extrasynaptic GABAARs incorporating α4, β, and δ subunits that mediate tonic inhibition, thereby influencing neuronal excitability. Both the selective δ-GABAAR agonist THIP and DS2, a selective positive allosteric modulator, greatly increased the tonic current of all MSNs from wild-type (WT), but not from δ(-/-) or α4(-/-) mice. Coupling dopamine and tonic inhibition, the acute activation of D1 receptors (by a selective agonist or indirectly by amphetamine) greatly enhanced tonic inhibition in D1-MSNs but not D2-MSNs. In contrast, prolonged D2 receptor activation modestly reduced the tonic conductance of D2-MSNs. Behaviorally, WT and constitutive α4(-/-) mice did not differ in their expression of cocaine-conditioned place preference (CPP). Importantly, however, mice with the α4 deletion specific to D1-expressing neurons (α4(D1-/-)) showed increased CPP. Furthermore, THIP administered systemically or directly into the NAc of WT, but not α4(-/-) or α4(D1-/-) mice, blocked cocaine enhancement of CPP. In comparison, α4(D2-/-) mice exhibited normal CPP, but no cocaine enhancement. In conclusion, dopamine modulation of GABAergic tonic inhibition of D1- and D2-MSNs provides an intrinsic mechanism to differentially affect their excitability in response to psychostimulants and thereby influence their ability to potentiate conditioned reward. Therefore, α4βδ GABAARs may represent a viable target for the development of novel therapeutics to better understand and influence addictive behaviors.

CaMKII Activity in the Ventral Tegmental Area Gates Cocaine-Induced Synaptic Plasticity in the Nucleus Accumbens

Addictive drugs such as cocaine induce synaptic plasticity in discrete regions of the reward circuit. The aim of the present study is to investigate whether cocaine-evoked synaptic plasticity in the ventral tegmental area (VTA) and nucleus accumbens (NAc) is causally linked. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a central regulator of long-term synaptic plasticity, learning, and drug addiction. We examined whether blocking CaMKII activity in the VTA affected cocaine conditioned place preference (CPP) and cocaine-evoked synaptic plasticity in its target brain region, the NAc. TatCN21 is a CaMKII inhibitory peptide that blocks both stimulated and autonomous CaMKII activity with high selectivity. We report that intra-VTA microinjections of tatCN21 before cocaine conditioning blocked the acquisition of cocaine CPP, whereas intra-VTA microinjections of tatCN21 before saline conditioning did not significantly affect cocaine CPP, suggesting that the CaMKII inhibitor blocks cocaine CPP through selective disruption of cocaine-cue-associated learning. Intra-VTA tatCN21 before cocaine conditioning blocked cocaine-evoked depression of excitatory synaptic transmission in the shell of the NAc slices ex vivo. In contrast, intra-VTA microinjection of tatCN21 just before the CPP test did not affect the expression of cocaine CPP and cocaine-induced synaptic plasticity in the NAc shell. These results suggest that CaMKII activity in the VTA governs cocaine-evoked synaptic plasticity in the NAc during the time window of cocaine conditioning.

Inactivation of the paraventricular thalamus abolishes the expression of cocaine conditioned place preference in rats

BackgroundThe paraventricular thalamus (PVT) is rapidly becoming recognized as part of the addiction circuitry. In addition to its strong anatomical connection to most of the brain regions underlying addiction, such as the nucleus accumbens, prefrontal cortex, amygdala, and hippocampus, the PVT has recently been shown to contribute to cocaine sensitization and reinstatement. In the present study, we examined the role of the PVT in the expression of cocaine conditioned place preference (CPP). MethodsWe tested the impact of PVT inactivation by baclofen/muscimol (bac-mus) microinjection on the expression of cocaine-induced CPP in rats. Rats were implanted with guide cannulae into the PVT. Bac-mus (GABAB–GABAA agonists) or saline was injected into the PVT prior to CPP testing. ResultsInactivation of the PVT by bac-mus prevented the expression of CPP, while placements outside the PVT did not affect CPP. Intra-PVT injections of bac-mus did not affect locomotor activity during the session. ConclusionsIn the present study, we contribute to the growing body of research supporting a role for the PVT in addiction by demonstrating that the PVT is necessary for the expression of cocaine CPP.

Inhibition of hippocampal β-adrenergic receptors impairs retrieval but not reconsolidation of cocaine-associated memory and prevents subsequent reinstatement.

Retrieval of drug-associated memories is critical for maintaining addictive behaviors, as presentation of drug-associated cues can elicit drug seeking and relapse. Recently, we and others have demonstrated that β-adrenergic receptor (β-AR) activation is necessary for retrieval using both rat and human memory models. Importantly, blocking retrieval with β-AR antagonists persistently impairs retrieval and provides protection against subsequent reinstatement. However, the neural locus at which β-ARs are required for maintaining retrieval and subsequent reinstatement is unclear. Here, we investigated the necessity of dorsal hippocampus (dHipp) β-ARs for drug-associated memory retrieval. Using a cocaine conditioned place preference (CPP) model, we demonstrate that local dHipp β-AR blockade before a CPP test prevents CPP expression shortly and long after treatment, indicating that dHipp β-AR blockade induces a memory retrieval disruption. Furthermore, this retrieval disruption provides long-lasting protection against cocaine-induced reinstatement. The effects of β-AR blockade were dependent on memory reactivation and were not attributable to reconsolidation disruption as blockade of β-ARs immediately after a CPP test had little effect on subsequent CPP expression. Thus, cocaine-associated memory retrieval is mediated by β-AR activity within the dHipp, and disruption of this activity could prevent cue-induced drug seeking and relapse long after treatment.

Effects of time of feeding on psychostimulant reward, conditioned place preference, metabolic hormone levels, and nucleus accumbens biochemical measures in food-restricted rats

Chronic food restriction (FR) increases rewarding effects of abused drugs and persistence of a cocaine-conditioned place preference (CPP). When there is a single daily meal, circadian rhythms are correspondingly entrained, and pre- and postprandial periods are accompanied by different circulating levels of metabolic hormones that modulate brain dopamine function. The present study assessed whether rewarding effects of d-amphetamine, cocaine, and persistence of cocaine-CPP differ between FR subjects tested in the pre- and postprandial periods. Rats were stereotaxically implanted with intracerebral microinjection cannulae and an electrode in lateral hypothalamus. Rewarding effects of d-amphetamine and cocaine were assessed using electrical self-stimulation in rats tested 1-4 or 18-21h after the daily meal. Nonimplanted subjects acquired a cocaine-CPP while ad libitum fed and then were switched to FR and tested for CPP at these same times. Rewarding effects of intranucleus accumbens (NAc) d-amphetamine, intraventricular cocaine, and persistence of cocaine-CPP did not differ between rats tested 18-21 h food-deprived, when ghrelin and insulin levels were at peak and nadir, respectively, and those tested 1-4h after feeding. Rats that expressed a persistent CPP had elevated levels of p-ERK1, GluA1, and p-Ser845-GluA1 in NAc core, and the latter correlated with CPP expression. Psychostimulant reward and persistence of CPP in FR rats are unaffected by time of testing relative to the daily meal. Further, NAc biochemical responses previously associated with enhanced drug responsiveness in FR rats are associated with persistent CPP expression.

Prkcz null mice show normal learning and memory

Protein kinase M ζ (PKMζ) is a constitutively active form of atypical PKC that is exclusively expressed in the brain and implicated in the maintenance of long-term memory1–9. Most studies that support a role for PKMζ in memory maintenance have used pharmacological PKMζ inhibitors such as the myristoylated zeta inhibitory peptide (ZIP) or chelerythrine. Here, we used a genetic approach and targeted exon 9 of the Prkcz gene to generate mice that lack both protein kinase C ζ (PKCζ) and PKMζ (Prkcz−/− mice). Prkcz−/− mice showed normal behavior in a cage environment and in baseline tests of motor function and sensory perception, but displayed reduced anxiety-like behavior. Surprisingly, they did not show deficits in learning or memory in tests of cued fear conditioning, novel object recognition, object location recognition, conditioned place preference (CPP) for cocaine, or motor learning, when compared with wild-type littermates. ZIP injection into the nucleus accumbens (NAc) reduced expression of cocaine CPP in Prkcz−/− mice. In vitro, ZIP and scrambled ZIP inhibited PKMζ, PKCι and PKCζ with similar Ki values. Chelerythrine was a weak inhibitor of PKMζ (Ki = 76 µM). Our findings show that absence of PKMζ does not impair learning and memory in mice, and that ZIP can erase reward memory even when PKMζ is not present.

Regulation of the ventral tegmental area by the bed nucleus of the stria terminalis is required for expression of cocaine preference

Lateral hypothalamus (LH) orexin neurons are essential for the expression of a cocaine place preference. However, the afferents that regulate the activity of these orexin neurons during reward behaviors are not completely understood. Using tract tracing combined with Fos staining, we examined LH afferents for Fos induction during cocaine preference in rats. We found that the ventral bed nucleus of the stria terminalis (vBNST) was a major input to the LH orexin cell field that was significantly Fos-activated during cocaine conditioned place preference (CPP). Inactivation of the vBNST with baclofen plus muscimol blocked expression of cocaine CPP. Surprisingly, such inactivation of the vBNST also increased Fos induction in LH orexin neurons; as activity in these cells is normally associated with increased preference, this result indicates that a vBNST-orexin connection is unlikely to be responsible for CPP that is dependent on vBNST activity. Because previous studies have revealed that vBNST regulates dopamine cells in the ventral tegmental area (VTA), which is known to be involved in CPP and other reward functions, we tested whether vBNST afferents to the VTA are necessary for cocaine CPP. We found that disconnection of the vBNST and VTA (using local microinjections of baclofen plus muscimol unilaterally into the vBNST and contralateral VTA) significantly attenuated expression of cocaine preference. However, blocking ionotropic glutamatergic afferents to the VTA from the vBNST did not significantly reduce cocaine preference. These results indicate that a non-glutamatergic vBNST-VTA projection is involved in expression of cocaine preference.