Single-nucleus genomics in outbred rats with divergent cocaine addiction-like behaviors reveals changes in amygdala GABAergic inhibition

The amygdala processes positive and negative valence and contributes to addiction, but the cell-type-specific gene regulatory programs involved are unknown. We generated an atlas of single-nucleus gene expression and chromatin accessibility in the amygdala of outbred rats with high and low cocaine addiction-like behaviors following prolonged abstinence. Differentially expressed genes between the high and low groups were enriched for energy metabolism across cell types. Rats with high addiction index (AI) showed increased relapse-like behaviors and GABAergic transmission in the amygdala. Both phenotypes were reversed by pharmacological inhibition of the glyoxalase 1 enzyme, which metabolizes methylglyoxal-a GABAA receptor agonist produced by glycolysis. Differences in chromatin accessibility between high and low AI rats implicated pioneer transcription factors in the basic helix-loop-helix, FOX, SOX and activator protein 1 families. We observed opposite regulation of chromatin accessibility across many cell types. Most notably, excitatory neurons had greater accessibility in high AI rats and inhibitory neurons had greater accessibility in low AI rats.

α7 nicotinic acetylcholine receptors in the medial prefrontal cortex control rewarding but not aversive memory expression in a dopamine-sensitive manner

Emotional learning involves the association between sensory cues and rewarding or aversive stimuli, and this stored information can be recalled during memory retrieval. In this process, the medial prefrontal cortex (mPFC) plays an essential role. We have previously shown that the antagonism of α7 nicotinic acetylcholine receptors (nAChRs) by methyllycaconitine (MLA) in the mPFC blocked cue-induced cocaine memory retrieval. However, little is known about the involvement of prefrontal α7 nAChRs in the retrieval of aversive memories. Here, by using pharmacology and different behavioral tasks, we found that MLA did not affect aversive memory retrieval, indicating a differential effect of cholinergic prefrontal control of appetitive and aversive memories. Despite being shown that acetylcholine modulates dopamine release in the mPFC, it remains unknown if those modulatory systems act together to control reward-based behavior. We examined that question and found that dopamine type 1 receptor (D1R) activation prevented MLA-induced blockade of cocaine CPP retrieval. Our results suggest that α7 nAChRs and D1R signaling interact in the mPFC to modulate cocaine-associated memory retrieval.

The endocannabinoid system and drug-associated contextual memories

Drug abuse and addiction can be initiated and reinstated by contextual stimuli previously paired with the drug use. The influence exerted by the context on drug-seeking behaviour can be modelled in experimental animals with place-conditioning protocols. Here, we review the effects of cannabinoids in place conditioning and the therapeutic potential of the endocannabinoid system for interfering with drug-related memories. The phytocannabinoid Δ9-tetrahydrocannabinol (THC) tends to induce conditioned place preference (CPP) at low doses and conditioned place aversion at high doses; cannabidiol is devoid of any effect, yet it inhibits CPP induced by some drugs. Synthetic CB1 receptor agonists tend to recapitulate the biphasic profile observed with THC, whereas selective antagonists/inverse agonists inhibit CPP induced by cocaine, nicotine, alcohol and opioids. However, their therapeutic use is limited by potential psychiatric side effects. The CB2 receptor has also attracted attention, because selective CB2 receptor agonists inhibit cocaine-induced CPP. Inhibitors of endocannabinoid membrane transport and hydrolysis yield mixed results. In targeting the endocannabinoid system for developing new treatments for drug addiction, future research should focus on 'neutral' CB1 receptor antagonists and CB2 receptor agonists. Such compounds may offer a well-tolerated pharmacological profile and curb addiction by preventing drug-seeking triggered by conditioned contextual cues.

Distinct Role of Dopamine in the PFC and NAc During Exposure to Cocaine-Associated Cues

BACKGROUND

Dopamine neurotransmission plays a critical role in reward in drug abuse and drug addiction. However, the role of dopamine in the recognition of drug-associated environmental stimuli, retrieval of drug-associated memory, and drug-seeking behaviors is not fully understood.

METHODS

Roles of dopamine neurotransmission in the prefrontal cortex (PFC) and nucleus accumbens (NAc) in the cocaine-conditioned place preference (CPP) paradigm were evaluated using in vivo microdialysis.

RESULTS

In mice that had acquired cocaine CPP, dopamine levels in the PFC, but not in the NAc, increased in response to cocaine-associated cues when mice were placed in the cocaine chamber of an apparatus with 2 separated chambers. The induction of the dopamine response and the development of cocaine CPP were mediated through activation of glutamate NMDA (N-methyl-D-aspartate)/AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor signaling in the PFC during conditioning. Activation of dopamine D1 or D2 receptor signaling in the PFC was required for cocaine-induced locomotion, but not for the induction of the dopamine response or the development of cocaine CPP. Interestingly, dopamine levels in the NAc increased in response to cocaine-associated cues when mice were placed at the center of an apparatus with 2 connected chambers, which requires motivated exploration associated with cocaine reward.

CONCLUSIONS

Dopamine neurotransmission in the PFC is activated by the exposure to the cocaine-associated cues, whereas dopamine neurotransmission in the NAc is activated in a process of motivated exploration of cues associated with cocaine reward. Furthermore, the glutamate signaling cascade in the PFC is suggested to be a potential therapeutic target to prevent the progression of drug addiction.

Medial prefrontal cortical control of reward- and aversion-based behavioral output: Bottom-up modulation

How does the brain guide our actions? This is a complex issue, where the medial prefrontal cortex (mPFC) plays a crucial role. The mPFC is essential for cognitive flexibility and decision making. These functions are related to reward- and aversion-based learning, which ultimately drive behavior. Though, cortical projections and modulatory systems that may regulate those processes in the mPFC are less understood. How does the mPFC regulate approach-avoidance behavior in the case of conflicting aversive and appetitive stimuli? This is likely dependent on the bottom-up neuromodulation of the mPFC projection neurons. In this review, we integrate behavioral-, pharmacological-, and viral-based circuit manipulation data showing the involvement of mPFC dopaminergic, noradrenergic, cholinergic, and serotoninergic inputs in reward and aversion processing. Given that an incorrect balance of reward and aversion value could be a key problem in mental diseases such as substance use disorders, we discuss outstanding questions for future research on the role of mPFC modulation in reward and aversion.

Prefrontal cortex nicotinic receptor inhibition by methyllycaconitine impaired cocaine-associated memory acquisition and retrieval

Cocaine administration has been shown to induce plastic changes in the medial prefrontal cortex (mPFC), which could represent a mechanism by which cocaine facilitates the association between cocaine rewarding effects with contextual cues. Nicotinic acetylcholine receptors (nAChRs) in the mPFC have critical roles in cognitive function including attention and memory and are key players in plasticity processes. However, whether nAChRs in the mPFC are required for the acquisition and maintenance of cocaine-associated memories is still unknown. To assess this question, we used the conditioning place preference (CPP) model to study the effect of intra-mPFC infusion of methyllycaconitine, a selective antagonist of α7 nAChRs, on the acquisition, consolidation and expression of cocaine-associated memory in adult rats. Our findings reveal that mPFC α7 nAChRs activation is necessary for the acquisition and retrieval, but not consolidation, of cocaine induced CPP. Moreover, cocaine-induced sensitization during CPP conditioning sessions was abolished by methyllycaconitine infusion in the mPFC. Together, these results identify mPFC α7 nAChRs as critical players involved in both acquiring and retrieving cocaine-associated memories. Considering that drug seeking often depends on the association between drug-paired cues and the rewarding effects of the drug, α7 nAChRs in the mPFC could be considered as potential targets for the prevention or treatment of cocaine use disorder.

Nicotine Enhances Object Recognition Memory via Stimulating α4β2 and α7 Nicotinic Acetylcholine Receptors in the Medial Prefrontal Cortex of Mice

Nicotine has been known to enhance recognition memory in various species. However, the brain region where nicotine acts and exerts its effect remains unclear. Since the medial prefrontal cortex (mPFC) is associated with memory, we examined the role of the mPFC in nicotine-induced enhancement of recognition memory using the novel object recognition test in male C57BL/6J mice. Systemic nicotine administration 10 min before training session significantly enhanced object recognition memory in test session that was performed 24 h after the training. Intra-mPFC infusion of mecamylamine, a non-selective nicotinic acetylcholine receptor (nAChR) antagonist, 5 min before nicotine administration blocked the effect of nicotine. Additionally, intra-mPFC infusion of dihydro-β-erythroidine, a selective α4β2 nAChR antagonist, or methyllycaconitine, a selective α7 nAChR antagonist, significantly suppressed the nicotine-induced object recognition memory enhancement. Finally, intra-mPFC infusion of nicotine 1 min before the training session augmented object recognition memory in a dose-dependent manner. These findings suggest that mPFC α4β2 and α7 nAChRs mediate the nicotine-induced object recognition memory enhancement.

Role of 5-HT1A receptor-mediated serotonergic transmission in the medial prefrontal cortex in acute restraint stress-induced augmentation of rewarding memory of cocaine in mice

Stress enhances cocaine craving. We recently reported that acute restraint stress increases cocaine conditioned place preference (CPP) in mice; however, the underlying mechanisms remain unclear. This study aimed to examine the role of serotonergic transmission in the medial prefrontal cortex (mPFC) in cocaine CPP enhancement by acute restraint stress, which increases extracellular serotonin (5-HT) levels in the mPFC. Intra-mPFC infusion of the selective serotonin reuptake inhibitor (S)-citalopram prior to the test session significantly increased the cocaine CPP score under non-stressed conditions. This is indicative of the substantial role of increased mPFC 5-HT levels in cocaine CPP enhancement. Moreover, intra-mPFC and systemic administration of the 5-HT_1A receptor antagonist WAY100635 immediately before restraint stress exposure significantly attenuated stress-induced cocaine CPP enhancement. Our findings suggest that enhanced serotonergic transmission via 5-HT_1A receptors in the mPFC is involved in acute stress-induced augmentation of rewarding memory of cocaine; moreover, the 5-HT_1A receptor could be a therapeutic target for stress-induced cocaine craving.

Noradrenaline enhances the excitatory effects of dopamine on medial prefrontal cortex pyramidal neurons in rats

Aim

Our previous studies showed that exposure to acute restraint stress enhanced cocaine‐induced conditioned place preference (cocaine‐CPP) and suggested the possibility that co‐activation of adrenergic transmission boosts the increase in medial prefrontal cortex (mPFC) neuronal activity by the activation of dopaminergic transmission. To examine this possibility, the effects of the co‐treatment with dopamine (DA) and noradrenaline (NA) on mPFC neurons were compared with those of treatment with DA alone using whole‐cell patch‐clamp recordings.

Methods

The effects of DA alone and a mixture of DA and NA on the membrane potentials and spontaneous excitatory postsynaptic currents (sEPSCs) were examined by electrophysiological recordings of mPFC pyramidal neurons in brain slices of male Sprague Dawley rats. Extracellular DA and NA levels in the mPFC during and after restraint stress exposure were also examined by in vivo microdialysis.

Results

Dopamine significantly produced depolarizing effects on mPFC neurons and tended to increase sEPSC frequency. Co‐administration of NA with DA produced stronger depolarizing effects and significantly increased sEPSC frequency. The findings suggest that the additional depolarizing effect of NA on DA‐responsive neurons, rather than the excitation of DA‐nonresponsive neurons by NA, contributes to the stronger effect of co‐treatment of NA with DA.

Conclusion

The present study suggests that NA released by restraint stress exposure cooperates with DA to stimulate DA‐responsive neurons in the mPFC, thereby causing the stress‐induced enhancement of cocaine‐CPP.

Our previous studies showed that exposure to acute restraint stress enhanced cocaine‐induced conditioned place preference (cocaine‐CPP). The present study revealed that noradrenaline enhanced the excitatory effects of dopamine on medial prefrontal cortex (mPFC) pyramidal neurons in rats. Noradrenaline released by restraint stress exposure may cooperate with dopamine to stimulate dopamine‐responsive neurons in the mPFC, thereby causing the stress‐induced enhancement of cocaine‐CPP.

Enduring effects of adolescent ketamine exposure on cocaine- and sucrose-induced reward in male and female C57BL/6 mice

Ketamine has shown promising antidepressant efficacy for adolescent treatment-resistant depression. However, the potential enduring consequences of ketamine exposure have not been thoroughly evaluated. Thus, we examined if juvenile ketamine treatment results in long-lasting changes for the rewarding properties of sucrose and cocaine in adulthood, across three separate experiments. In Experiment 1, adolescent male and female C57BL/6 mice received ketamine (20 mg/kg) for 15 consecutive days (Postnatal Day [PD] 35-49). Twenty-one days later (PD70; adulthood) we examined their behavioral responsivity to sucrose (1%) on a two-bottle choice design, or cocaine (0, 5, 10 mg/kg) using the conditioned place preference (CPP) test. We found that juvenile ketamine-pretreatment increased preference for sucrose and environments paired with cocaine in male, but not female, adult mice. This long-term outcome was not observed when male and female mice received ketamine as adults (PD70-84) and tested for sucrose and cocaine preference 21-days later (Experiment 2). Similarly, in Experiment 3, no long-lasting differences in these measures were observed when adolescent male mice were exposed to concomitant ketamine and social stressors (PD35-44), namely the social defeat or vicarious defeat stress paradigms-procedures that mediated a depression-related phenotype (along with a ketamine antidepressant-like response). Collectively, we demonstrate that in the absence of physical or psychological stress, adolescent ketamine exposure increases later life preference for the rewarding properties of sucrose and cocaine in a sex- and age-specific manner. As such, this preclinical work provides awareness for the potential long-term behavioral consequences associated with juvenile ketamine exposure.

Effect of Topiramate on Morphine-induced Conditioned Place Preference (CPP) in Rats: Role of ERK and CREB Proteins in Hippocampus and Cerebral Cortex

In this study, the effect of topiramate, as an antiepileptic drug, was evaluated on morphine craving in rats. The conditioned place preference (CPP) test was used for this purpose. Repeated administration of morphine (10 mg/kg, i.p. for 4 days) induced significant CPP. Administration of topiramate (50 and 100 mg/kg, i.p. for 4 days) with each morphine administration decreased the acquisition of morphine-induced CPP. At the next step, the levels of extracellular signal-regulated kinase (ERK), p-ERK, cAMP responsive element binding (CREB), and p-CREB proteins were evaluated in hippocampus and cerebral cortex using western blot analysis. Following the repeated administration of morphine, the level of p-ERK protein markedly enhanced in both tissues, while topiramate could significantly reduce the phosphorylation of ERK in these brain regions. Additionally, the level of CREB and p-CREB proteins did not change in different groups. Memantine as a positive control reduced the acquisition of morphine-induced CPP. Also, memantine significantly decreased the level of p-ERK protein in hippocampus and cerebral cortex. These results demonstrated that topiramate can attenuate the acquisition of morphine-induced CPP in rats. This effect in part can be mediated through down regulation of p-ERK protein in hippocampus and cerebral cortex.

Glutamatergic neurons in the medial prefrontal cortex mediate the formation and retrieval of cocaine-associated memories in mice

In drug addiction, environmental stimuli previously associated with cocaine use readily elicit cocaine-associated memories, which persist long after abstinence and trigger cocaine craving and consumption. Although previous studies suggest that the medial prefrontal cortex (mPFC) is involved in the expression of cocaine-addictive behaviors, it remains unclear whether excitatory and inhibitory neurons in the mPFC are causally related to the formation and retrieval of cocaine-associated memories. To address this issue, we used the designer receptors exclusively activated by designer drugs (DREADD) technology combined with a cocaine-induced conditioned place preference (CPP) paradigm. We suppressed mPFC neuronal activity in a cell-type- and timing-dependent manner. C57BL/6J wild-type mice received bilateral intra-mPFC infusion of an adeno-associated virus (AAV) expressing inhibitory DREADD (hM4Di) under the control of CaMKII promotor to selectively suppress mPFC pyramidal neurons. GAD67-Cre mice received bilateral intra-mPFC infusion of a Cre-dependent AAV expressing hM4Di to specifically silence GABAergic neurons. Chemogenetic suppression of mPFC pyramidal neurons significantly attenuated both the acquisition and expression of cocaine CPP, while suppression of mPFC GABAergic neurons affected neither the acquisition nor expression of cocaine CPP. Moreover, chemogenetic inhibition of mPFC glutamatergic neurons did not affect the acquisition and expression of lithium chloride-induced conditioned place aversion. These results suggest that the activation of glutamatergic, but not GABAergic, neurons in the mPFC mediates both the formation and retrieval of cocaine-associated memories.

[Analyses of cocaine rewarding memories by AAV vector-induced introduction of DREADD system]

The development and persistence of drug addiction are associated with the activation and adaptation of the brain reward circuitry, which consists of dopaminergic projection from the ventral tegmental area to the nucleus accumbens (NAc) and the medial prefrontal cortex (mPFC). In cocaine addiction, cocaine-induced activation and neuroplasticity in the brain reward circuitry may contribute to the acquisition and expression of rewarding memory of cocaine, which is critical for the reinstatement of cocaine seeking. However, it remains unclear which neuronal types causally contribute to the retrieval of cocaine-associated rewarding memory. To address this issue, we used DREADD (Designer Receptors Exclusively Activated by Designer Drugs) technology. To selectively suppress mPFC excitatory neurons, we infused an adeno-associated virus (AAV5 or AAV-DJ) vector expressing hM4Di, an inhibitory DREADD, under the control of CaMKII promotor into the mPFC of wildtype mice. To selectively suppress GABAergic neurons, we infused a Cre-dependent AAV (AAV5 or AAV-DJ) vector expressing hM4Di into the mPFC of GAD67-Cre mice or the NAc of vGAT-Cre mice. We found that, in cocaine conditioned place preference paradigm, the activity of mPFC pyramidal and NAc GABAergic neurons is causally related to the retrieval of cocaine-associated memory. The findings suggest that the mPFC-NAc circuit can be a potential therapeutic target for the drug addiction.

Acute Cocaine Reduces Excitatory Synaptic Transmission in Pyramidal Neurons of the Mouse Medial Prefrontal Cortex

The medial prefrontal cortex (mPFC) plays critical roles in the development of cocaine addiction. Numerous studies have reported about the effects of cocaine on neuronal and synaptic activities in the nucleus accumbens and ventral tegmental area, which are brain regions associated with cocaine addiction; however, a limited number of studies have reported the effect of cocaine on mPFC neuronal activity. In this study, using whole-cell patch-clamp recordings in brain slices, we present that under the condition where synaptic transmission is enhanced by increasing extracellular K⁺ concentration, cocaine significantly reduced the frequency but not amplitude of spontaneous excitatory postsynaptic currents. These findings suggest that cocaine exposure could be a trigger to induce hypofrontality, which is related to the compulsive craving for cocaine use.

Stress augments the rewarding memory of cocaine via the activation of brainstem-reward circuitry

Effects of stress on the reward system are well established in the literature. Although previous studies have revealed that stress can reinstate extinguished addictive behaviors related to cocaine, the effects of stress on the rewarding memory of cocaine are not fully understood. Here, we provide evidence that stress potentiates the expression of rewarding memory of cocaine via the activation of brainstem-reward circuitry using a cocaine-induced conditioned place preference (CPP) paradigm combined with restraint stress in rats. The rats exposed to 30-minute restraint stress immediately before posttest exhibited significantly larger CPP scores compared with non-stressed rats. Intra-laterodorsal tegmental nucleus (LDT) microinjection of a β or α2 adrenoceptor antagonist attenuated the stress-induced enhancement of cocaine CPP. Consistent with this observation, intra-LDT microinjection of a β or α2 adrenoceptor agonist before posttest increased cocaine CPP. Additionally, intra-ventral tegmental area (VTA) microinjection of antagonists for the muscarinic acetylcholine, nicotinic acetylcholine or glutamate receptors attenuated the stress-induced enhancement of cocaine CPP. Finally, intra-medial prefrontal cortex (mPFC) microinjection of a D1 receptor antagonist also reduced the stress-induced enhancement of cocaine CPP. These findings suggest a mechanism wherein the LDT is activated by noradrenergic input from the locus coeruleus, leading to the activation of VTA dopamine neurons via both cholinergic and glutamatergic transmission and the subsequent excitation of the mPFC to enhance the memory of cocaine-induced reward value.

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.