Functional heterogeneity of the rat medial prefrontal cortex: effects of discrete subarea-specific lesions on drug-induced conditioned place preference and behavioural sensitization

Effects of adolescent alcohol exposure on oligodendrocyte lineage cells and myelination in mice: Age and subregion differences

Adolescence is an important phase for the structural and functional development of the brain. The immaturity of adolescent brain development is associated with high susceptibility to exogenous disturbances, including alcohol. In this study, the acquisition of conditioned place preference (CPP) in adolescent mice by alcohol (2 g/kg) and the parvalbumin-positive interneurons (PV+ interneurons), oligodendrocyte lineage cells (OPCs), and myelination in the medial prefrontal cortex (mPFC) were assessed. We aim to determine the age- and subregional-specificity of the effects of alcohol. Alcohol (2 g/kg) was injected intraperitoneally on even days, and saline was injected intraperitoneally on odd days. The control group received a continuous intraperitoneal injection with saline. Differences in alcohol-induced CPP acquisition were assessed, followed by immunohistochemical staining. The results showed a pronounced CPP acquisition in 4- and 5-week-old mice. In the mPFC, there were reduced PV+ interneurons and OPCs in 3-week-old mice and reduced oligodendrocyte numbers in 4-week-old mice. The 5-week-old mice showed impaired myelination and a decrease in the number of PV+ interneurons, mature oligodendrocytes, and OPCs in the mPFC. Since the alterations in 5-week-old mice are more pronounced, we further explored the mPFC-associated subregional-specificity. In the alcohol-exposed mice, the oligodendrocyte numbers were decreased in the anterior cingulate cortex (ACC), PV+ interneuron numbers were declined in the prelimbic cortex (PL), and the number of oligodendrocytes, PV+ interneurons, and OPCs was also decreased with impaired myelination in the infralimbic cortex (IL). Our data suggest that adolescent alcohol exposure notably affected the acquisition of CPP, myelin formation, and the counts of PV+ interneurons, mature oligodendrocytes, and OPCs in the mPFC in 5-week-old mice. Also, the IL subregion was the worst-affected subregion of the mPFC in alcohol-exposed 5-week-old mice. It reveals that the effects of alcohol on adolescence and its mPFC myelination show obvious age- and subregional-specificity.

Glutamate inputs from the laterodorsal tegmental nucleus to the ventral tegmental area are essential for the induction of cocaine sensitization in male mice

RATIONALE

Drug-induced potentiation of ventral tegmental area (VTA) glutamate signaling contributes critically to the induction of sensitization - an enhancement in responding to a drug following exposure which is thought to reflect neural changes underlying drug addiction. The laterodorsal tegmental nucleus (LDTg) provides one of several sources of glutamate input to the VTA.

OBJECTIVE

We used optogenetic techniques to test either the role of LDTg glutamate cells or their VTA afferents in the development of cocaine sensitization in male VGluT2::Cre mice. These were inhibited using halorhodopsin during each of five daily cocaine exposure injections. The expression of locomotor sensitization was assessed following a cocaine challenge injection 1-week later.

RESULTS

The locomotor sensitization seen in control mice was absent in male mice subjected to inhibition of LDTg-VTA glutamatergic circuitry during cocaine exposure. As sensitization of nucleus accumbens (NAcc) dopamine (DA) overflow is also induced by this drug exposure regimen, we used microdialysis to measure NAcc DA overflow on the test for sensitization. Consistent with the locomotor sensitization results, inhibition of LDTg glutamate afferents to the VTA during cocaine exposure prevented the sensitization of NAcc DA overflow observed in control mice.

CONCLUSIONS

These data identify the LDTg as the source of VTA glutamate critical for the development of cocaine sensitization in male mice. Accordingly, the LDTg may give rise to the synapses in the VTA at which glutamatergic plasticity, known to contribute to the enhancement of addictive behaviors, occurs.

Prelimbic Ensembles Mediate Cocaine Seeking After Behavioral Acquisition and Once Rats Are Well-Trained

Substance use disorder (SUD) is a chronic relapsing condition characterized by continued use of drugs despite negative consequences. SUD is thought to involve disordered learning and memory wherein drug-paired cues gain increased salience, and ultimately drive craving and relapse. These types of associations are thought to be encoded within sparsely distributed sets of neurons, called neuronal ensembles, that drive encoded behaviors through synchronous activity of the participant neurons. We have previously found that Fos-expressing neuronal ensembles within the prefrontal cortex are required for well-trained cocaine seeking. However, less is known about how quickly cortical neuronal ensembles form during the initiation of cocaine seeking behavior. Here, we seek to further elucidate the role of Fos-expressing neuronal ensembles within the prelimbic cortex (PL) after the initial acquisition of cocaine self-administration (SA), or, after 10 days of additional SA training (well-trained). We trained Fos-LacZ transgenic rats to lever press for cocaine under an FR1 schedule of reinforcement. Once rats met acquisition criteria for cocaine self-administration, we ablated Fos-expressing neuronal ensembles in the PL using the Daun02 inactivation method, either 1 or 10 days after the rats met the acquisition criteria. Targeted ablation of Fos-expressing neuronal ensembles in the PL attenuated active lever pressing both 1 day and 10 days after rats acquired cocaine self-administration. Together, this suggests that Fos-expressing neuronal ensembles rapidly form in the PL and continue to mediate maintained cocaine seeking behavior.

Prelimbic medial prefrontal cortex has bidirectional control over the expression of behavioral sensitization to 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) depending on the context of drug administration

Behavioral sensitization to MDMA is observed in the vast majority of rats if tested in the same environment in which previous MDMA exposure occurred, but not if tested in a novel, unpaired context. Previous studies have revealed a critical role for the prelimbic region of medial prefrontal cortex (PL) in the expression of sensitization to MDMA, but these studies assessed sensitization only in MDMA-paired environments. Given that PL activity can both facilitate and suppress behavior depending on context, we tested the hypothesis that PL has bidirectional control over the expression of locomotor sensitization to MDMA depending on the context of drug administration. Rats were treated with either saline or MDMA (5.0 mg/kg) twice daily for 5 days, in either their home cages (unpaired groups) or the activity monitors that were used for tests of sensitization on challenge days (paired groups). Prior to MDMA challenge injections (2.5 mg/kg; at ∼2 weeks of withdrawal), rats received bilateral PL microinjections of either lidocaine (100 μg/0.5 μl/side) or physiological saline (0.5 μl/side). Locomotor activity in response to MDMA challenge was unaffected by PL inactivation in saline pretreated rats. However, PL inactivation caused a decrease in locomotion to the challenge injection in MDMA/paired rats and an increase in locomotion in MDMA/unpaired rats. These results establish a novel role for PL in suppressing the expression of behavioral sensitization when subjects are challenged in a drug-unpaired context, adding to the literature implicating PL activity in both the expression and inhibition of other drug-related behaviors.

Fos-expressing neuronal ensembles in rat infralimbic cortex encode initial and maintained oxycodone seeking in rats

Neuronal ensembles within the infralimbic cortex (IL) and their projections to the nucleus accumbens (NAc) mediate opiate seeking in well-trained rats. However, it is unclear how early this circuitry is recruited during oxycodone self-administration. Here, we used retrograde labelling (CTb) and immunohistochemistry to identify NAc-projecting neurons in the IL that were activated during initial oxycodone seeking. Next, we sought to determine the role of IL neuronal ensembles in initial oxycodone self-administration. We used the Daun02 procedure in male and female Fos-LacZ rats to chemogenetically inactivate IL Fos-expressing neurons at different time points in oxycodone self-administration training: immediately after meeting criteria for acquisition of behaviour and following nine daily sessions with increasing schedules of reinforcement (FR1, FR2 and FR3) in which rats demonstrated stable oxycodone intake under increasing effort to self-administer. We found that Daun02 infusions attenuated oxycodone seeking at both the initial learning and well-trained time points. These results suggest that IL neuronal ensembles are formed during initial learning of oxycodone self-administration and required for the maintenance and expression of oxycodone seeking.

When the front fails, the rear wins. Cerebellar correlates of prefrontal dysfunction in cocaine-induced memory in male rats

Reciprocal pathways connecting the cerebellum to the prefrontal cortex provide a biological and functional substrate to modulate cognitive functions. Dysfunction of both medial prefrontal cortex (mPFC) and cerebellum underlie the phenotypes of several neuropsychiatric disorders that exhibit comorbidity with substance use disorder (SUD). In people with SUD, cue-action-reward associations appears to be particularly strong and salient, acting as powerful motivational triggers for craving and relapse. Studies of cue reactivity in human with SUD have shown cerebellar activations when drug-related cues are presented. Our preclinical research showed that cocaine-induced conditioned preference increases neural activity and upregulates perineuronal nets (PNNs) around Golgi interneurons in the posterior cerebellar cortex. In the present investigation, we aimed at evaluating cerebellar signatures of conditioned preference for cocaine when drug learning is established under mPFC impairment. We used lidocaine to temporarily inactivate in male rats either the Prelimbic (PL) or the Infralimbic (IL) cortices during cocaine-induced conditioning. The inactivation of the IL, but not the PL, encouraged the acquisition of preference for cocaine-related cues, increased posterior cerebellar cortex activity, and upregulated the expression of PNNs around Golgi interneurons. Moreover, IL impairment not only increased vGluT2- and vGAT-related activity around Golgi cells but also regulated PNNs differently on subpopulations of Golgi cells, increasing the number of neurogranin+ PNN-expressing Golgi cells. Our findings suggest that IL dysfunction may facilitate the acquisition of cocaine-induced memory and cerebellar drug-related learning hallmarks. Overall, IL perturbation during cocaine-induced Pavlovian learning increased cerebellar activity and drug effects. Importantly, cerebellum involvement requires a contingent experience with the drug, and it is not the effect of a mere inactivation of IL cortex.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

Morphine-element interactions - The influence of selected chemical elements on neural pathways associated with addiction

Addiction is a pressing social problem worldwide and opioid dependence can be considered the strongest and most difficult addiction to treat. Mesolimbic and mesocortical dopaminergic pathways play an important role in modulation of cognitive processes and decision making and, therefore, changes in dopamine metabolism are considered the central basis for the development of dependence. Disturbances caused by excesses or deficiency of certain elements have a significant impact on the functioning of the central nervous system (CNS) both in physiological conditions and in pathology and can affect the cerebral reward system and therefore, may modulate processes associated with the development of addiction. In this paper we review the mechanisms of interactions between morphine and zinc, manganese, chromium, cadmium, lead, fluoride, their impact on neural pathways associated with addiction, and on antinociception and morphine tolerance and dependence.

The Role of Dorsal Raphe Serotonin Neurons in the Balance between Reward and Aversion

BACKGROUND

Reward processing is fundamental for animals to survive and reproduce. Many studies have shown the importance of dorsal raphe nucleus (DRN) serotonin (5-HT) neurons in this process, but the strongly correlative link between the activity of DRN 5-HT neurons and rewarding/aversive potency is under debate. Our primary objective was to reveal this link using two different strategies to transduce DRN 5-HT neurons.

METHODS

For transduction of 5-HT neurons in wildtype mice, adeno-associated virus (AAV) bearing the mouse tryptophan hydroxylase 2 (TPH2) gene promoter was used. For transduction in Tph2-tTA transgenic mice, AAVs bearing the tTA-dependent TetO enhancer were used. To manipulate the activity of 5-HT neurons, optogenetic actuators (CheRiff, eArchT) were expressed by AAVs. For measurement of rewarding/aversive potency, we performed a nose-poke self-stimulation test and conditioned place preference (CPP) test.

RESULTS

We found that stimulation of DRN 5-HT neurons and their projections to the ventral tegmental area (VTA) increased the number of nose-pokes in self-stimulation test and CPP scores in both targeting methods. Concomitantly, CPP scores were decreased by inhibition of DRN 5-HT neurons and their projections to VTA.

CONCLUSION

Our findings indicate that the activity of DRN 5-HT neurons projecting to the VTA is a key modulator of balance between reward and aversion.

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.

Corticolimbic circuitry in the modulation of chronic pain and substance abuse

The transition from acute to chronic pain is accompanied by increased engagement of emotional and motivational circuits. Adaptations within this corticolimbic circuitry contribute to the cellular and behavioral maladaptations associated with chronic pain. Central regions within the corticolimbic brain include the mesolimbic dopamine system, the amygdala, and the medial prefrontal cortex. The evidence reviewed herein supports the notion that chronic pain induces significant changes within these corticolimbic regions that contribute to the chronicity and intractability of pain. In addition, pain-induced changes in corticolimbic circuitry are poised to impact motivated behavior and reward responsiveness to environmental stimuli, and may modulate the addiction liability of drugs of abuse, such as opioids.

Chromium picolinate reduces morphine-dependence in rats, while increasing brain serotonin levels

Chromium is an essential trace element with anti-diabetic and anti-depressant effect; the latter is considered related to chromium properties of increasing brain serotonin. Cr³⁺ salts were shown to improve some forced swimming-parameters and to induce rewarding effects, which are additive to those of morphine, but Cr effect on addictive processes has not been tested.

AIM

The present study aimed to assess chromium picolinate (CrPi) influence on morphine-dependence in rats.

MATHERIAL AND METHODS

We used five groups of 10 rats. Groups 1 and 2 (controls) received saline, respectively CrPi, 0.01 mg/kg/day, for 10 days. In groups 3, 4 and 5 dependence was induced with progressively-increased morphine doses (from 5 - day 1-90 mg/kg/day - day 10, s.c.). Group 3 received only morphine, while groups 4 and 5 received CrPi, i.p., 10 and respectively 5 μg/kg/day, during the 10 days of dependence induction. On day 11, groups 3, 4, and 5 were administered 90 mg/kg morphine, and, 2 h later, all rats received naloxone, 2 mg/kg s.c., to precipitate withdrawal. We compared withdrawal intensity in group 3 vs. groups 4 and 5, assessing both individual symptoms and Gellert-Holtzman global withdrawal score. Upon rats sacrifice at the end of the experiments, brain serotonin (5HT) in certain areas and serum Cr were assessed.

RESULTS

Some withdrawal signs were unequally influenced by CrPi: compulsive mastication was reduced by both CrPi doses (p < 0.05), while teeth chattering and grooming were significantly reduced only by the higher dose (p < 0.05). Withdrawal score was reduced by both CrPi doses: from 132.4 ± 9.87 - group 3 to 122.2 ± 6.47 - group 4 (p < 0.01 vs. group 3) and 124.1 ± 8.41 - group 5 (p < 0.05 vs. group 3). CrPi reduction of withdrawal is accompanied by increased brain 5 H T, mainly in the prefrontal cortex (646.3 ± 8.51 - group 3 vs. 661.5 ± 14.63 - group 4, p < 0.01 and 660.7 ± 14.01 pg/mg tissue - group 5, p < 0.05 vs. group 3). CrPi also increases brain 5 H T in non-dependent rats (prefrontal cortex: 541.6 ± 31.80, group 1 and 565.5 ± 16. 46 pg/mg tissue, group 2, p < 0.05). Administration of CrPi determined a dose-dependent increase of serum Cr.

CONCLUSIONS

Our study evidenced a slight, but significant reduction of morphine dependence in rats induced by chromium picolinate, accompanied by increased brain serotonin. This might be considered a supplementary evidence for chromium anti-depressant effect and its serotonin-mediated mechanisms.

Upregulation of CCL7 and CCL2 in reward system mediated through dopamine D1 receptor signaling underlies methamphetamine-induced place preference in mice

We previously showed that the CC-chemokine ligand 2 (CCL2)-CC-chemokine receptor 2 (CCR2) system is responsible for conditioned place preference (CPP) by methamphetamine (Meth). In this study, we investigated the roles for other chemokines mediating Meth-induced CPP and the upstream factors upregulating chemokines in mice. We found that CCL7 mRNA level was upregulated in the prefrontal cortex (PFC) after Meth administration (3mg/kg, subcutaneous), and increased CCL7 immunoreactivity was localized to the PFC NeuN-positive neurons. Meth-induced CPP was blocked by the dopamine D1 receptor antagonist SCH 23390 but not by the D2 receptor antagonists raclopride or haloperidol. The D1 receptor agonist SKF 81297 alone elicited CPP, suggesting a critical role of D1 receptor signaling in Meth-induced reward. Consistent with these results, the Meth-induced upregulation of CCL7 and CCL2 were attenuated by SCH 23390, and a single administration of SKF 81297 upregulated mRNA expression levels of CCL7 and CCL2 in the PFC. Furthermore, Meth-induced CPP was prevented by INCB 3284, a selective antagonist of CCR2, a receptor that binds both CCL7 and CCL2. Collectively, we identified two CC-chemokines (i.e., CCL7 and CCL2) as key regulatory factors in Meth-induced reward. Pharmacological inhibitors of these chemokines may warrant development as novel therapeutics for ameliorating Meth addiction.

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

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

Cocaine and Caffeine Effects on the Conditioned Place Preference Test: Concomitant Changes on Early Genes within the Mouse Prefrontal Cortex and Nucleus Accumbens

Caffeine is the world's most popular psychostimulant and is frequently used as an active adulterant in many illicit drugs including cocaine. Previous studies have shown that caffeine can potentiate the stimulant effects of cocaine and cocaine-induced drug seeking behavior. However, little is known about the effects of this drug combination on reward-related learning, a key process in the maintenance of addiction and vulnerability to relapse. The goal of the present study was thus to determine caffeine and cocaine combined effects on the Conditioned Place Preference (CPP) test and to determine potential differential mRNA expression in the Nucleus Accumbens (NAc) and medial prefrontal cortex (mPFC) of immediate-early genes (IEGs) as well as dopamine and adenosine receptor subunits. Mice were treated with caffeine (5 mg/kg, CAF), cocaine (10 mg/kg, COC), or their combination (caffeine 5 mg/kg + cocaine 10 mg/kg, CAF-COC) and trained in the CPP test or treated with repeated injections inside the home cage. NAc and mPFC tissues were dissected immediately after the CPP test, after a single conditioning session or following psychostimulant injection in the home cage for mRNA expression analysis. CAF-COC induced a marked change of preference to the drug conditioned side of the CPP and a significant increase in locomotion compared to COC. Gene expression analysis after CPP test revealed specific up-regulation in the CAF-COC group of Drd1a, cFos, and FosB in the NAc, and cFos, Egr1, and Npas4 in the mPFC. Importantly, none of these changes were observed when animals received same treatments in their home cage. With a single conditioning session, we found similar effects in both CAF and CAF-COC groups: increased Drd1a and decreased cFos in the NAc, and increased expression of Drd1a and Drd2, in the mPFC. Interestingly, we found that cFos and Npas4 gene expression were increased only in the mPFC of the CAF-COC. Our study provides evidence that caffeine acting as an adulterant could potentiate reward-associated memories elicited by cocaine. This is associated with specific changes in IEGs expression that were observed almost exclusively in mice that received the combination of both psychostimulants in the context of CPP memory encoding and retrieval. Our results highlight the potential relevance of caffeine in the maintenance of cocaine addiction which might be mediated by modifying neural plasticity mechanisms that strengthen learning of the association between drug and environment.

Distinct roles for the deacetylase domain of HDAC3 in the hippocampus and medial prefrontal cortex in the formation and extinction of memory

Histone deacetylases (HDACs) are chromatin modifying enzymes that have been implicated as powerful negative regulators of memory processes. HDAC3has been shown to play a pivotal role in long-term memory for object location as well as the extinction of cocaine-associated memory, but it is unclear whether this function depends on the deacetylase domain of HDAC3. Here, we tested whether the deacetylase domain of HDAC3has a role in object location memory formation as well as the formation and extinction of cocaine-associated memories. Using a deacetylase-dead point mutant of HDAC3, we found that selectively blocking HDAC3 deacetylase activity in the dorsal hippocampus enhanced long-term memory for object location, but had no effect on the formation of cocaine-associated memory. When this same point mutant virus of HDAC3 was infused into the prelimbic cortex, it failed to affect cocaine-associated memory formation. With regards to extinction, impairing the HDAC3 deacetylase domain in the infralimbic cortex had no effect on extinction, but a facilitated extinction effect was observed when the point mutant virus was delivered to the dorsal hippocampus. These results suggest that the deacetylase domain of HDAC3 plays a selective role in specific brain regions underlying long-term memory formation of object location as well as cocaine-associated memory formation and extinction.

Reduction of Cocaine-Induced Locomotor Effects by Enriched Environment Is Associated with Cell-Specific Accumulation of ΔFosB in Striatal and Cortical Subregions

BACKGROUND

Early exposure to enriched environments has been shown to decrease the locomotor effects induced by repeated injections of cocaine and modify basal and cocaine-induced total protein levels of the transcription factor ΔFosB in the whole striatum of mice. In this study, we aimed at characterizing whether the profile of ΔFosB accumulation induced by enriched environments and cocaine would be similar or different in terms of brain areas and cell type.

METHODS

We used mice expressing the eGFP protein in D1 receptor positive (D1R(+)) neurons to determine whether Δ FosB induced by enriched environment or cocaine injections (5×15 mg/kg) would occur in selective subpopulations of neurons in several subregions of the striatum and prefrontal cortex.

RESULTS

We found that: (1) exposure to enriched environment reduces cocaine-induced locomotor activation, confirming our previous findings; (2) exposure to enriched environment by itself increases the accumulation of Δ FosB mostly in D1R(-) cells in the shell part of the nucleus accumbens and dorsal striatum, whereas in the nucleus accumbens core, Δ FosB accumulates in both D1R(+) and D1R(-) neurons; (3) in standard environment mice, cocaine induces accumulation of Δ FosB selectively in D1R(+) cells in the nucleus accumbens, dorsal striatum, and infralimbic cortex; and (4) the effects of enriched environments and cocaine on accumulation of Δ FosB were reciprocally blocked by their combination.

CONCLUSIONS

Altogether, these results suggest that the enriched environment-induced reduction in behavioral effects of cocaine might result from 2 distinct effects on ΔFosB in striatal medium-sized spiny neurons belonging to the direct and indirect pathways.

The Extracellular Matrix Protein Brevican Limits Time-Dependent Enhancement of Cocaine Conditioned Place Preference

Cocaine-associated environmental cues sustain relapse vulnerability by reactivating long-lasting memories of cocaine reward. During periods of abstinence, responding to cocaine cues can time-dependently intensify a phenomenon referred to as 'incubation of cocaine craving'. Here, we investigated the role of the extracellular matrix protein brevican in recent (1 day after training) and remote (3 weeks after training) expression of cocaine conditioned place preference (CPP). Wild-type and Brevican heterozygous knock-out mice, which express brevican at ~50% of wild-type levels, received three cocaine-context pairings using a relatively low dose of cocaine (5 mg/kg). In a drug-free CPP test, heterozygous mice showed enhanced preference for the cocaine-associated context at the remote time point compared with the recent time point. This progressive increase was not observed in wild-type mice and it did not generalize to contextual-fear memory. Virally mediated overexpression of brevican levels in the hippocampus, but not medial prefrontal cortex, of heterozygous mice prevented the progressive increase in cocaine CPP, but only when overexpression was induced before conditioning. Post-conditioning overexpression of brevican did not affect remote cocaine CPP, suggesting that brevican limited the increase in remote CPP by altering neuro-adaptive mechanisms during cocaine conditioning. We provide causal evidence that hippocampal brevican levels control time-dependent enhancement of cocaine CPP during abstinence, pointing to a novel substrate that regulates incubation of responding to cocaine-associated cues.

Wiring and Molecular Features of Prefrontal Ensembles Representing Distinct Experiences

A major challenge in understanding the cellular diversity of the brain has been linking activity during behavior with standard cellular typology. For example, it has not been possible to determine whether principal neurons in prefrontal cortex active during distinct experiences represent separable cell types, and it is not known whether these differentially active cells exert distinct causal influences on behavior. Here, we develop quantitative hydrogel-based technologies to connect activity in cells reporting on behavioral experience with measures for both brain-wide wiring and molecular phenotype. We find that positive and negative-valence experiences in prefrontal cortex are represented by cell populations that differ in their causal impact on behavior, long-range wiring, and gene expression profiles, with the major discriminant being expression of the adaptation-linked gene NPAS4. These findings illuminate cellular logic of prefrontal cortex information processing and natural adaptive behavior and may point the way to cell-type-specific understanding and treatment of disease-associated states.

Molecular and neuronal plasticity mechanisms in the amygdala-prefrontal cortical circuit: implications for opiate addiction memory formation

The persistence of associative memories linked to the rewarding properties of drugs of abuse is a core underlying feature of the addiction process. Opiate class drugs in particular, possess potent euphorigenic effects which, when linked to environmental cues, can produce drug-related "trigger" memories that may persist for lengthy periods of time, even during abstinence, in both humans, and other animals. Furthermore, the transitional switch from the drug-naïve, non-dependent state to states of dependence and withdrawal, represents a critical boundary between distinct neuronal and molecular substrates associated with opiate-reward memory formation. Identifying the functional molecular and neuronal mechanisms related to the acquisition, consolidation, recall, and extinction phases of opiate-related reward memories is critical for understanding, and potentially reversing, addiction-related memory plasticity characteristic of compulsive drug-seeking behaviors. The mammalian prefrontal cortex (PFC) and basolateral nucleus of the amygdala (BLA) share important functional and anatomical connections that are involved importantly in the processing of associative memories linked to drug reward. In addition, both regions share interconnections with the mesolimbic pathway's ventral tegmental area (VTA) and nucleus accumbens (NAc) and can modulate dopamine (DA) transmission and neuronal activity associated with drug-related DAergic signaling dynamics. In this review, we will summarize research from both human and animal modeling studies highlighting the importance of neuronal and molecular plasticity mechanisms within this circuitry during critical phases of opiate addiction-related learning and memory processing. Specifically, we will focus on two molecular signaling pathways known to be involved in both drug-related neuroadaptations and in memory-related plasticity mechanisms; the extracellular-signal-regulated kinase system (ERK) and the Ca(2+)/calmodulin-dependent protein kinases (CaMK). Evidence will be reviewed that points to the importance of critical molecular memory switches within the mammalian brain that might mediate the neuropathological adaptations resulting from chronic opiate exposure, dependence, and withdrawal.

A Role for p38 Mitogen-activated Protein Kinase-mediated Threonine 30-dependent Norepinephrine Transporter Regulation in Cocaine Sensitization and Conditioned Place Preference

The noradrenergic and p38 mitogen-activated protein kinase (p38 MAPK) systems are implicated in cocaine-elicited behaviors. Previously, we demonstrated a role for p38 MAPK-mediated norepinephrine transporter (NET) Thr(30) phosphorylation in cocaine-induced NET up-regulation (Mannangatti, P., Arapulisamy, O., Shippenberg, T. S., Ramamoorthy, S., and Jayanthi, L. D. (2011) J. Biol. Chem. 286, 20239-20250). The present study explored the functional interaction between p38 MAPK-mediated NET regulation and cocaine-induced behaviors. In vitro cocaine treatment of mouse prefrontal cortex synaptosomes resulted in enhanced NET function, surface expression, and phosphorylation. Pretreatment with PD169316, a p38 MAPK inhibitor, completely blocked cocaine-mediated NET up-regulation and phosphorylation. In mice, in vivo administration of p38 MAPK inhibitor SB203580 completely blocked cocaine-induced NET up-regulation and p38 MAPK activation in the prefrontal cortex and nucleus accumbens. When tested for cocaine-induced locomotor sensitization and conditioned place preference (CPP), mice receiving SB203580 on cocaine challenge day or on postconditioning test day exhibited significantly reduced cocaine sensitization and CPP. A transactivator of transcription (TAT) peptide strategy was utilized to test the involvement of the NET-Thr(30) motif. In vitro treatment of synaptosomes with TAT-NET-Thr(30) (wild-type peptide) completely blocked cocaine-mediated NET up-regulation and phosphorylation. In vivo administration of TAT-NET-Thr(30) peptide but not TAT-NET-T30A (mutant peptide) completely blocked cocaine-mediated NET up-regulation and phosphorylation. In the cocaine CPP paradigm, mice receiving TAT-NET-Thr(30) but not TAT-NET-T30A on postconditioning test day exhibited significantly reduced cocaine CPP. Following extinction, TAT-NET-Thr(30) when given prior to cocaine challenge significantly reduced reinstatement of cocaine CPP. These results demonstrate that the direct inhibition of p38 MAPK or the manipulation of NET-Thr(30) motif/phosphorylation via a TAT peptide strategy prevents cocaine-induced NET up-regulation, locomotor sensitization, and CPP, suggesting a role for Thr(30)-linked NET regulation in cocaine-elicited behaviors.

Involving the cerebellum in cocaine-induced memory: pattern of cFos expression in mice trained to acquire conditioned preference for cocaine

Because of its primary role in drug-seeking, consumption and addictive behaviour, there is a growing interest in identifying the neural circuits and molecular mechanisms underlying the formation, maintenance and retrieval of drug-related memories. Human studies, which focused on neuronal systems that store and control drug-conditioned memories, have found cerebellar activations during the retrieval of drug-associated cue memory. However, at the pre-clinical level, almost no attention has been paid to a possible role of the cerebellum in drug-related memories. In the present study, we ought to fill this gap by aiming to investigate the pattern of neuronal activation (as revealed by cFos expression) in different regions of the prefrontal cortex and cerebellum of mice trained to develop conditioned preference for an olfactory stimulus (CS+) paired with cocaine. Our results indicate that CS+ preference was directly associated with cFos expression in cells at the apical region of the granule cell layer of the cerebellar vermis; this relationship being more prominent in some specific lobules. Conversely, cFos+ immunostaining in other cerebellar regions seems to be unrelated to CS+ preference but to other aspects of the conditioning procedure. At the prefrontal cortex, cFos expression seemed to be related to cocaine administration rather than to its ability to establish conditioned preference. The present results suggest that as it has been observed in some clinical studies, the cerebellum might be an important and largely overlooked part of the neural circuits involved in generating, maintaining and/or retrieving drug memories.

Medial prefrontal cortex inactivation attenuates the diurnal rhythm in amphetamine reward

Psychostimulant reward, as assessed via the conditioned place preference (CPP) paradigm, exhibits a daily rhythm with peaks in the late dark and early light periods, and a nadir near the light-to-dark transition. While this diurnal rhythm is correlated with neural activity in several corticolimbic structures, the brain regions mediating this behavioral rhythm remain unknown. Here, we examine the role of the ventral medial prefrontal cortex (mPFC). The effects of excitotoxic mPFC lesions on daily rhythms in amphetamine CPP were examined at previously observed peak (zeitgeber time [ZT] 23) and nadir times (ZT11). mPFC lesions encompassing the prelimbic and infralimbic subregions increased the CPP for amphetamine at the nadir time, thereby eliminating the daily rhythm in amphetamine reward. To examine the effects of transient mPFC inactivation, rats received intra-mPFC infusions of GABA receptor agonists during the acquisition or expression phases of CPP testing. Inactivation of the ventral mPFC at either of these phases also eliminated the daily rhythm in amphetamine-induced CPP via an increase in drug-paired chamber dwell time at the baseline nadir. Together, these results indicate that the ventral mPFC plays a critical role in mediating the diurnal rhythm in amphetamine CPP during both the acquisition and expression of learned reward-context associations. Moreover, as the loss of rhythmicity occurs via an increase at the nadir point, these results suggest that excitatory output from the ventral mPFC normally inhibits context-elicited reward seeking prior to the light-to-dark transition.

Capturing the aversive state of cephalic pain preclinically

OBJECTIVE

Preclinical evaluation of headache by behavioral assessment of reward from pain relief.

METHODS

Inflammatory mediators (IMs) or control solution were applied to the rat dura mater to elicit a presumed state of cephalic pain. Hind paw incision was used in separate groups of animals to model noncephalic postsurgical pain. Drugs were given systemically or microinjected within the rostral ventromedial medulla (RVM), nucleus accumbens (NAc), or rostral anterior cingulate cortex (rACC). Peripheral nerve block was produced at the level of the popliteal fossa, and behavior was assessed using evoked sensory stimuli or conditioned place preference (CPP). Immunohistochemistry and brain microdialysis measurements were performed.

RESULTS

Dural IMs produced long-lasting generalized cutaneous allodynia. RVM lidocaine produced CPP, increased NAc c-Fos, and dopamine release selectively in rats receiving dural IMs; CPP was blocked by intra-NAc α-flupenthixol, a dopaminergic antagonist. Intravenous α-calcitonin gene-related peptide (αCGRP)(8-37) produced CPP and elicited NAc dopamine release selectively in rats treated with dural IMs. Prior lesion of the rACC or treatment with systemic sumatriptan or αCGRP(8-37) abolished RVM lidocaine-induced CPP in IM-treated rats. Sumatriptan treatment blocked NAc dopamine release in IM-treated rats receiving RVM lidocaine. Systemic sumatriptan did not alter pain relief-induced CPP in rats with incisional injury.

INTERPRETATION

Cephalic pain was unmasked in rats by assessment of motivated behavior to seek relief. Relief of pain activates the dopaminergic reward pathway to elicit negative reinforcement of behavior. Medications clinically effective for migraine headache selectively elicit relief of ongoing cephalic, but not postsurgical, noncephalic pain. These studies provide a platform for exploring migraine pathophysiology and for the discovery of new headache therapies.

Manganese-enhanced magnetic resonance imaging for mapping of whole brain activity patterns associated with the intake of snack food in ad libitum fed rats

Non-homeostatic hyperphagia, which is a major contributor to obesity-related hyperalimentation, is associated with the diet’s molecular composition influencing, for example, the energy content. Thus, specific food items such as snack food may induce food intake independent from the state of satiety. To elucidate mechanisms how snack food may induce non-homeostatic food intake, it was tested if manganese-enhanced magnetic resonance imaging (MEMRI) was suitable for mapping the whole brain activity related to standard and snack food intake under normal behavioral situation. Application of the MnCl₂ solution by osmotic pumps ensured that food intake was not significantly affected by the treatment. After z-score normalization and a non-affine three-dimensional registration to a rat brain atlas, significantly different grey values of 80 predefined brain structures were recorded in ad libitum fed rats after the intake of potato chips compared to standard chow at the group level. Ten of these areas had previously been connected to food intake, in particular to hyperphagia (e.g. dorsomedial hypothalamus or the anterior paraventricular thalamic nucleus) or to the satiety system (e.g. arcuate hypothalamic nucleus or solitary tract); 27 areas were related to reward/addiction including the core and shell of the nucleus accumbens, the ventral pallidum and the ventral striatum (caudate and putamen). Eleven areas associated to sleep displayed significantly reduced Mn²⁺-accumulation and six areas related to locomotor activity showed significantly increased Mn²⁺-accumulation after the intake of potato chips. The latter changes were associated with an observed significantly higher locomotor activity. Osmotic pump-assisted MEMRI proved to be a promising technique for functional mapping of whole brain activity patterns associated to nutritional intake under normal behavior.

Brain regions associated with the acquisition of conditioned place preference for cocaine vs. social interaction

Positive social interaction could play an essential role in switching the preference of the substance dependent individual away from drug related activities. We have previously shown that conditioned place preference (CPP) for cocaine at the dose of 15 mg/kg and CPP for four 15-min episodes of social interaction were equally strong when rats were concurrently conditioned for place preference by pairing cocaine with one compartment and social interaction with the other. The aim of the present study was to investigate the differential activation of brain regions related to the reward circuitry after acquisition/expression of cocaine CPP or social interaction CPP. Our findings indicate that cocaine CPP and social interaction CPP activated almost the same brain regions. However, the granular insular cortex and the dorsal part of the agranular insular cortex were more activated after cocaine CPP, whereas the prelimbic cortex and the core subregion of the nucleus accumbens were more activated after social interaction CPP. These results suggest that the insular cortex appears to be potently activated after drug conditioning learning while activation of the prelimbic cortex-nucleus accumbens core projection seems to be preferentially involved in the conditioning to non-drug stimuli such as social interaction.

Glutamate mechanisms underlying opiate memories

As the major excitatory neurotransmitter in the brain, glutamate plays an undisputable integral role in opiate addiction. This relates, in part, to the fact that addiction is a disorder of learning and memory, and glutamate is required for most types of memory formation. As opiate addiction develops, the addict becomes conditioned to engage in addictive behaviors, and these behaviors can be triggered by opiate-associated cues during abstinence, resulting in relapse. Some medications for opiate addiction exert their therapeutic effects at glutamate receptors, especially the NMDA receptor. Understanding the neural circuits controlling opiate addiction, and the locus of glutamate's actions within these circuits, will help guide the development of targeted pharmacotherapeutics for relapse.

The atypical antidepressant mirtazapine attenuates expression of morphine-induced place preference and motor sensitization

Opioid abuse and dependence remains prevalent despite having multiple FDA-approved medications to help maintain abstinence. Mirtazapine is an atypical antidepressant receiving attention for substance abuse pharmacotherapy, and its action includes alterations in monoaminergic transmission. As monoamines are indirectly altered by opioids, the current investigation assessed the ability of mirtazapine to ameliorate morphine-induced behaviors. Conditioned place preference (CPP) is a behavioral assay wherein a rewarding drug is paired with a distinct environmental context resulting in reward-related salience of cues through learning-related neuronal plasticity. A second behavioral assay involved motor sensitization (MSn), wherein repeated administration results in an enhanced motoric response to an acute challenge, also reflecting neuronal plasticity. Attenuation of CPP and/or MSn provides two behavioral measures to suggest therapeutic potential for addiction therapy, and the present study evaluated the effectiveness of mirtazapine to reduce both behaviors. To do so, morphine-induced CPP was established using an eight day conditioning paradigm, and expression of CPP was tested on day 10 following a 24h or 30min mirtazapine pretreatment. To determine if mirtazapine altered the expression of MSn, on day 11, rats received a pretreatment of mirtazapine, followed 30min later by a challenge injection of morphine. Pretreatment with mirtazapine 24h prior to the CPP test had no effect on CPP expression. In contrast, a 30min pretreatment of mirtazapine attenuated the expression of both CPP and MSn. Collectively, these results indicate that mirtazapine may help to maintain abstinence in opioid dependent patients.

Contribution of PKMζ-dependent and independent amplification to components of experimental neuropathic pain

Injuries can induce adaptations in pain processing that result in amplification of signaling. One mechanism may be analogous to long-term potentiation and involve the atypical protein kinase C, PKMζ. The possible contribution of PKMζ-dependent and independent amplification mechanisms to experimental neuropathic pain was explored in rats with spinal nerve ligation (SNL) injury. SNL increased p-PKMζ in the rostral anterior cingulate cortex (rACC), a site that mediates, in part, the unpleasant aspects of pain. Inhibition of PKMζ within the rACC by a single administration of ζ-pseudosubstrate inhibitory peptide (ZIP) reversed SNL-induced aversiveness within 24 hours, whereas N-methyl-d-aspartate receptor blockade with MK-801 had no effects. The SNL-induced aversive state (reflecting "spontaneous" pain), was re-established in a time-dependent manner, with full recovery observed 7 days post-ZIP administration. Neither rACC ZIP nor MK-801 altered evoked responses. In contrast, spinal ZIP or MK-801, but not scrambled peptide, transiently reversed evoked hypersensitivity, but had no effect on nerve injury-induced spontaneous pain. PKMζ phosphorylation was not altered by SNL in the spinal dorsal horn. These data suggest that amplification mechanisms contribute to different aspects of neuropathic pain at different levels of the neuraxis. Thus, PKMζ-dependent amplification contributes to nerve injury-induced aversiveness within the rACC. Moreover, unlike mechanisms maintaining memory, the consequences of PKMζ inhibition within the rACC are not permanent in neuropathic pain, possibly reflecting the re-establishment of amplification mechanisms by ongoing activity of injured nerves. In the spinal cord, however, both PKMζ-dependent and independent mechanisms contribute to amplification of evoked responses, but apparently not spontaneous pain.

Reversal of cocaine-conditioned place preference through methyl supplementation in mice: altering global DNA methylation in the prefrontal cortex

Analysis of global methylation in cells has revealed correlations between overall DNA methylation status and some biological states. Recent studies suggest that epigenetic regulation through DNA methylation could be responsible for neuroadaptations induced by addictive drugs. However, there is no investigation to determine global DNA methylation status following repeated exposure to addictive drugs. Using mice conditioned place preference (CPP) procedure, we measured global DNA methylation level in the nucleus accumbens (NAc) and the prefrontal cortex (PFC) associated with drug rewarding effects. We found that cocaine-, but not morphine- or food-CPP training decreased global DNA methylation in the PFC. Chronic treatment with methionine, a methyl donor, for 25 consecutive days prior to and during CPP training inhibited the establishment of cocaine, but not morphine or food CPP. We also found that both mRNA and protein level of DNMT (DNA methytransferase) 3b in the PFC were downregulated following the establishment of cocaine CPP, and the downregulation could be reversed by repeated administration of methionine. Our study indicates a crucial role of global PFC DNA hypomethylation in the rewarding effects of cocaine. Reversal of global DNA hypomethylation could significantly attenuate the rewarding effects induced by cocaine. Our results suggest that methionine may have become a potential therapeutic target to treat cocaine addiction.

Prefrontal cell firing in male rats during approach towards sexually receptive female: interactions with cocaine

The medial prefrontal cortex (mPFC) plays a role in anticipation of rewards and goal orientation, properties that are influenced by cocaine administration. Single-unit firing was measured in the mPFC of seven male rats during the expression of approach responses toward a sexually receptive female. Nose-poking in male rats was used as a measure of approach behavior during the following periods: a baseline, first exposure to a female, a second baseline 2 h later and a second exposure to female 10 min after cocaine (15 mg kg⁻¹ i.p.). Two types of excitatory responses were identified. First, a subset of cells (23%) showed increased firing activity during nose-poke behavior upon presentation of the female, but not before. Another subset of cells (12%) showed increased firing in the presence of the female only after cocaine was administered. The present results provide preliminary evidence for neurons in the mPFC that are involved in sexually motivated approach behavior and that are modulated by cocaine.

Basolateral amygdala-driven augmentation of medial prefrontal cortex GABAergic neurotransmission in response to environmental stimuli associated with cocaine administration

Basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) interactions have been implicated in cue-elicited craving and drug seeking. However, the neurochemical mechanisms underlying drug/environment associations are ill-defined. We used in vivo microdialysis and pharmacological inactivation techniques to identify alterations in mPFC glutamate (GLU) and gamma-aminobutyric acid (GABA) transmission in response to cues previously associated with experimenter-administered cocaine (COC) and the BLA contribution to these effects. Rats received alternate day injections of COC and saline (SAL) paired with a distinct environment for 6 days. Behavioral, neurochemical and immunohistochemical studies were conducted, in drug-free animals, 24 h after the last conditioning session. Animals exposed to a COC-paired environment demonstrated an augmented locomotor activity (LMA) relative to those exposed to the SAL-paired environment. mPFC GABA neurotransmission in the COC-paired environment was significantly increased, whereas GLU overflow was unaltered. Dual labeling of cFos and glutamic acid decarboxylase 67 immunoreactivity in mPFC neurons revealed significantly greater colocalization of these proteins following exposure to the COC-associated environment (CAE) relative to pseudo-conditioned rats or rats exposed to the SAL-associated environment indicating that the conditioned neurochemical response to the COC-paired environment is associated with activation of intrinsic mPFC GABA neurons. BLA inactivation prevented the increase in LMA and the augmentation of mPFC GABA transmission produced by cue exposure. Intra-mPFC application of the AMPA/KA receptor antagonist, NBQX, produced similar effects. These findings indicate that exposure to a CAE increases mPFC GABA transmission by enhancing excitatory drive from the BLA and activation of AMPA/KA receptors on mPFC GABA neurons.

Deconstructing craving: dissociable cortical control of cue reactivity in nicotine addiction

BACKGROUND

Cue reactivity, the ability of cues associated with addictive substances to induce seeking and withdrawal, is a major contributor to addiction. Although human imaging studies show that cigarette-associated cues simultaneously activate the insula and the orbitofrontal cortex and evoke craving, how these activities functionally contribute to distinct elements of cue reactivity remains unclear. Moreover, it remains unclear whether the simultaneous activation of these cortical regions reflects coordinated functional connectivity or parallel processing.

METHODS

We selectively lesioned the insula or orbitofrontal cortex with the excitotoxin ibotenic acid in mice, and their approach to nicotine-associated cues (n = 6-13/group) and avoidance of withdrawal-associated cues (n = 5-12/group) were separately examined in place conditioning paradigms. We additionally tested the role of these two cortical structures in approach to food-associated cues (n = 6-7/group) and avoidance of lithium chloride-associated cues (n = 6-7/group).

RESULTS

Our data show a double dissociation in which excitotoxic lesions of the insula and orbitofrontal cortex selectively disrupted nicotine-induced cue approach and withdrawal-induced cue avoidance, respectively. These effects were not entirely generalized to approach to food-associated cues or avoidance of lithium chloride-associated cues.

CONCLUSIONS

Our data provide functional evidence that cue reactivity seen in addiction includes unique neuroanatomically dissociable elements and suggest that the simultaneous activation of these two cortical regions in response to smoking-related cues does not necessarily indicate functional connectivity.

Enhanced cocaine-conditioned place preference and associated brain regional levels of BDNF, p-ERK1/2 and p-Ser845-GluA1 in food-restricted rats

Previously, a learning-free measure was used to demonstrate that chronic food restriction (FR) increases the reward magnitude of a wide range of abused drugs. Moreover, a variety of striatal neuroadaptations were detected in FR subjects, some of which are known to be involved in synaptic plasticity but have been ruled out as modulators of acute drug reward magnitude. Little is known about effects of FR on drug-conditioned place preference (CPP) and brain regional mechanisms that may enhance CPP in FR subjects. The purpose of the present study was to compare the expression and persistence of a conditioned place preference (CPP) induced by a relatively low dose of cocaine (7.0mg/kg, i.p.) in ad libitum fed (AL) and FR rats and take several brain regional biochemical measures following the first CPP conditioning session to probe candidate mechanisms that may underlie the more robust CPP observed in FR subjects. Behaviorally, AL subjects displayed a CPP upon initial testing which extinguished rapidly over the course of subsequent test sessions while CPP in FR subjects persisted. Despite previous reports of elevated BDNF protein in forebrain regions of FR rats, the FR protocol used in the present study did not alter BDNF levels in dorsal hippocampus, nucleus accumbens or medial prefrontal cortex. On the other hand, FR rats, whether injected with cocaine or vehicle, displayed elevated p-ERK1/2 and p-Ser845-GluA1 in dorsal hippocampus. FR rats also displayed elevated p-ERK1/2 in medial prefrontal cortex and elevated p-ERK1 in nucleus accumbens, with further increases produced by cocaine. The one effect observed exclusively in cocaine-treated FR rats was increased p-Ser845-GluA1 in nucleus accumbens. These findings suggest a number of avenues for continuing investigation with potential translational significance.

Sensitized activation of Fos and brain-derived neurotrophic factor in the medial prefrontal cortex and ventral tegmental area accompanies behavioral sensitization to amphetamine

Behavioral sensitization, or augmented locomotor response to successive drug exposures, results from neuroadaptive changes contributing to addiction. Both the medial prefrontal cortex (mPFC) and ventral tegmental area (VTA) influence behavioral sensitization and display increased immediate-early gene and BDNF expression after psychostimulant administration. Here we investigate whether mPFC neurons innervating the VTA exhibit altered Fos or BDNF expression during long-term sensitization to amphetamine. Male Sprague-Dawley rats underwent unilateral intra-VTA infusion of the retrograde tracer Fluorogold (FG), followed by 5 daily injections of either amphetamine (2.5 mg/kg, i.p.) or saline vehicle. Four weeks later, rats were challenged with amphetamine (1.0 mg/kg, i.p.) or saline (1.0 mL/kg, i.p.). Repeated amphetamine treatment produced locomotor sensitization upon drug challenge. Two hours later, rats were euthanized, and mPFC sections were double-immunolabeled for either Fos-FG or Fos-BDNF. Tissue from the VTA was also double-immunolabeled for Fos-BDNF. Amphetamine challenge increased Fos and BDNF expression in the mPFC regardless of prior drug experience, and further augmented mPFC BDNF expression in sensitized rats. Similarly, more Fos-FG and Fos-BDNF double-labeling was observed in the mPFC of sensitized rats compared to drug-naïve rats after amphetamine challenge. Repeated amphetamine treatment also increased VTA BDNF, while both acute and repeated amphetamine treatment increased Fos and Fos-BDNF co-labeling, an effect enhanced in sensitized rats. These findings point to a role of cortico-tegmental BDNF in long-term amphetamine sensitization.

Acquisition, extinction, and recall of opiate reward memory are signaled by dynamic neuronal activity patterns in the prefrontal cortex

The medial prefrontal cortex (mPFC) comprises an important component in the neural circuitry underlying drug-related associative learning and memory processing. Neuronal activation within mPFC circuits is correlated with the recall of opiate-related drug-taking experiences in both humans and other animals. Using an unbiased associative place conditioning procedure, we recorded mPFC neuronal populations during the acquisition, recall, and extinction phases of morphine-related associative learning and memory. Our analyses revealed that mPFC neurons show increased activity both in terms of tonic and phasic activity patterns during the acquisition phase of opiate reward-related memory and demonstrate stimulus-locked associative activity changes in real time, during the recall of opiate reward memories. Interestingly, mPFC neuronal populations demonstrated divergent patterns of bursting activity during the acquisition versus recall phases of newly acquired opiate reward memory, versus the extinction of these memories, with strongly increased bursting during the recall of an extinction memory and no associative bursting during the recall of a newly acquired opiate reward memory. Our results demonstrate that neurons within the mPFC are involved in both the acquisition, recall, and extinction of opiate-related reward memories, showing unique patterns of tonic and phasic activity patterns during these separate components of the opiate-related reward learning and memory recall.

Lesion of the rostral anterior cingulate cortex eliminates the aversiveness of spontaneous neuropathic pain following partial or complete axotomy

Neuropathic pain is often "spontaneous" or "stimulus-independent." Such pain may result from spontaneous discharge in primary afferent nociceptors in injured peripheral nerves. However, whether axotomized primary afferent nociceptors give rise to pain is unclear. The rostral anterior cingulate cortex (rACC) mediates the negative affective component of inflammatory pain. Whether the rACC integrates the aversive component of chronic spontaneous pain arising from nerve injury is not known. Here, we used the principle of negative reinforcement to show that axotomy produces an aversive state reflecting spontaneous pain driven from injured nerves. Additionally, we investigated whether the rACC contributes to the aversiveness of nerve injury-induced spontaneous pain. Partial or complete hind paw denervation was produced by sciatic or sciatic/saphenous axotomy, respectively. Conditioned place preference resulting from presumed pain relief was observed following spinal clonidine in animals with sciatic axotomy but not in sham-operated controls. Similarly, lidocaine administration into the rostral ventromedial medulla (RVM) produced place preference selectively in animals with sciatic/saphenous axotomy. In rats with spinal nerve ligation (SNL) injury, lesion of the rACC blocked the reward elicited by RVM lidocaine but did not alter acute stimulus-evoked hypersensitivity. Lesion of the rACC did not block cocaine-induced reward, indicating that rACC blockade did not impair memory encoding or retrieval but did impair spontaneous aversiveness. These data indicate that spontaneous pain arising from injured nerve fibers produces a tonic aversive state that is mediated by the rACC. Identification of the circuits mediating aversiveness of chronic pain should facilitate the development of improved therapies.

Mediodorsal nucleus of the thalamus is critical for the expression of memory of methamphetamine-produced conditioned place preference in rats

Methamphetamine (MA) is a powerful and highly addictive psychostimulant. However, the neural substrate mediating MA-induced conditioned effects, an essential part of addiction, remain unclear. The present study investigated the involvement of the anterior cingulate cortex (ACC), the lateral nucleus of amygdala (LNA), and the mediodorsal nucleus of the thalamus (MD) in MA-conditioned place preference (CPP). Rats underwent bilateral radio-frequency lesions of the ACC, LNA, or MD followed by MA CPP training. Lesions of the MD, but not the ACC or LNA, disrupted MA CPP learning. To clarify the role of the MD on the different stages of the MA CPP memory process, bilateral microinfusions of lidocaine into the MD were performed 5 min prior to each conditioning trial, immediately after the conditioning trial, or 5 min before the testing phase. Pretesting, but not pre- or post-conditioning, infusions of lidocaine into the MD impaired MA CPP. Furthermore, a clear preference for the previously conditioned MA paired cues was expressed when the rats were tested again 24 h after infusions of lidocaine. These results are interpreted as indicating that the MD is specifically involved in the memory retrieval process of MA associated memory which suggests the MD could have an important role in relapse in individuals suffering from MA addiction.

Blockade of opioid receptors in anterior cingulate cortex disrupts ethanol-seeking behavior in mice

The anterior cingulate cortex (ACC) and opioid receptors have been suggested to play a role in attributing incentive motivational properties to drug-related cues. We examined whether blockade of ACC opioid receptors would reduce cue-induced ethanol-seeking behavior in mice. We show that intra-ACC opioid receptor blockade disrupted expression of an ethanol-induced conditioned place preference, suggesting that endogenous opioid modulation in the ACC may be critical for maintaining the cue's conditioned rewarding effects.

Role of dopamine D1 receptors in the activation of nucleus accumbens extracellular signal-regulated kinase (ERK) by cocaine-paired contextual cues

Exposure to drug-paired cues can trigger addicts to relapse into drug seeking. Although the molecular mechanisms underlying cue-elicited cocaine seeking are incompletely understood, the protein kinase extracellular signal-regulated kinase (ERK) is known to have an important role. Psychostimulants and their associated cues can activate ERK in medium spiny neurons of the nucleus accumbens core (AcbC). These medium spiny neurons can be classified according to their projections (to ventral pallidum and/or substantia nigra) and by their mRNA expression. The present experiments were designed to determine which distinct set of AcbC projection neurons expresses phosphorylated ERK (pERK) in response to cocaine-paired contextual cues. Combined use of the retrograde label Flurogold with immunohistochemical staining of pERK was used to show that the AcbC pERK accompanying preference for cocaine-paired contexts occurs in both the accumbens (Acb)-nigral and Acb-pallidal projections. The gene expression characteristics of the neurons expressing pERK in response to cocaine-paired cues was further investigated using combined in situ hybridization and immunocytochemistry to show that AcbC pERK+ cells correspond to D1, but not preproenkephalin, mRNA+ cells. Furthermore, intra-AcbC infusion of the D1-antagonist SCH23390 attenuated cue-induced AcbC pERK expression. In aggregate, these results indicate that (i) the D1-expressing AcbC neurons evidence long-term plasticity related to drug-cue memories and (ii) local dopamine D1 receptors are necessary for the expression of cocaine-paired cue-induced pERK in these AcbC neurons.

Implication of dopaminergic projection from the ventral tegmental area to the anterior cingulate cortex in μ-opioid-induced place preference

Despite the importance of prefrontal cortical dopamine in modulating reward, little is known about the implication of the specific subregion of prefrontal cortex in opioid reward. We investigated the role of neurons projecting from the ventral tegmental area (VTA) to the anterior cingulate cortex (ACG) in opioid reward. Microinjection of the retrograde tracer fluorogold (FG) into the ACG revealed several retrogradely labelled cells in the VTA. The FG-positive reactions were noted in both tyrosine hydroxylase (TH)-positive and -negative VTA neurons. The released levels of dopamine and its major metabolites in the ACG were increased by either the electrical stimulation of VTA neurons or microinjection of a selective μ-opioid receptor (MOR) agonist, (D-Ala²,N-MePhe⁴,Gly-ol⁵) enkephalin (DAMGO), into the VTA. MOR-like immunoreactivity was seen in both TH-positive and -negative VTA neurons projecting to the ACG. The conditioned place preference induced by intra-VTA injection of DAMGO was significantly attenuated by chemical lesion of dopaminergic terminals in the ACG. The depletion of dopamine in the ACG induced early extinction of μ-opioid-induced place preference. The levels of phosphorylated DARPP32 (Thr34) and phosphorylated CREB (Ser133) were increased in the ACG of rats that had maintained the morphine-induced place preference, whereas the increases of these levels induced by morphine were blocked by pre-treatment of a selective dopamine D1 receptor antagonist SCH23390. These findings suggest that VTA-ACG transmission may play a crucial role in the acquisition and maintenance of μ-opioid-induced place preference. The activation of DARPP32 and CREB through dopamine D1 receptors in the ACG could be implicated in the maintenance of μ-opioid-induced place preference.

Stimulation of medial prefrontal cortex serotonin 2C (5-HT(2C)) receptors attenuates cocaine-seeking behavior

Serotonin 2C receptor (5-HT(2C)R) agonists administered systemically attenuate both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior. To further elucidate the function of these receptors in addiction-like processes, this study examined the effects of microinfusing the 5-HT(2C)R agonist MK212 (0, 10, 30, 100 ng/side/0.2 microl) into the medial prefrontal cortex (mPFC) on cocaine self-administration and reinstatement of extinguished cocaine-seeking behavior. Male Sprague-Dawley rats were trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues. Once responding stabilized, rats received MK212 microinfusions before tests for maintenance of cocaine self-administration. Next, extinction training to reduce cocaine-seeking behavior, defined as responses performed without cocaine reinforcement available, occurred until low extinction baselines were achieved. Rats then received MK212 microinfusions before tests for reinstatement of extinguished cocaine-seeking behavior elicited by cocaine-priming injections (10 mg/kg, i.p.) or response-contingent presentations of the cocaine-associated cues; operant responses during cocaine-primed reinstatement tests produced no consequences. MK212 microinfusions into the prelimbic and infralimbic, but not anterior cingulate, regions of the mPFC dose-dependently attenuated both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior, but did not reliably affect cocaine self-administration. A subsequent experiment showed that the effects of MK212 (100 ng/side/0.2 microl) on reinstatement of extinguished cocaine-seeking behavior were blocked by co-administration of the 5-HT(2C)R antagonist SB242084 (200 ng/side/0.2 microl). MK212 administered alone into the mPFC as a drug prime produced no discernable effects on cocaine-seeking behavior. These findings suggest that stimulation of 5-HT(2C)Rs in the mPFC attenuates the incentive motivational effects produced by sampling cocaine or exposure to drug-paired cues.

Lesions of the medial prefrontal cortex cause maladaptive sexual behavior in male rats

BACKGROUND

An inability to inhibit behaviors once they become maladaptive is a component of several psychiatric illnesses, and the medial prefrontal cortex (mPFC) was identified as a potential mediator of behavioral inhibition. The current study tested if the mPFC is involved in inhibition of sexual behavior when associated with aversive outcomes.

METHODS

Using male rats, effects of lesions of the infralimbic and prelimbic areas of the mPFC on expression of sexual behavior and ability to inhibit mating were tested using a paradigm of copulation-contingent aversion.

RESULTS

Medial prefrontal cortex lesions did not alter expression of sexual behavior. In contrast, mPFC lesions completely blocked the acquisition of sex-aversion conditioning and lesioned animals continued to mate, in contrast to the robust behavioral inhibition toward copulation in mPFC intact male animals, resulting in only 22% of intact male animals continuing to mate. However, rats with mPFC lesions were capable of forming a conditioned place preference to sexual reward and conditioned place aversion for lithium chloride, suggesting that these lesions did not alter associative learning or sensitivity for lithium chloride.

CONCLUSIONS

The current study indicates that animals with mPFC lesions are likely capable of forming the associations with aversive outcomes of their behavior but lack the ability to suppress seeking of sexual reward in the face of aversive consequences. These data may contribute to a better understanding of a common pathology underlying impulse control disorders, as compulsive sexual behavior has a high prevalence of comorbidity with psychiatric disorders and Parkinson's disease.

Effect of 5-aza-2-deoxycytidine microinjecting into hippocampus and prelimbic cortex on acquisition and retrieval of cocaine-induced place preference in C57BL/6 mice

The long lasting addiction-related abnormal memory is one of the most important foundations for relapse. DNA methylation may be a possible mechanism for persistence of such memory. Here we injected the DNA methyltransferases (DNMTs) inhibitor, 5-aza-2-deoxycytidine (5-aza) into hippocampus CA1 area and prelimbic cortex during the stages of acquisition and expression of cocaine-induced place preference in C57BL/6 mice. Results showed that in CA1 DNA methylation inhibitors could restrain acquisition but had no impact on expression of the cocaine-induced conditioned place preference (CPP). On the contrary, in prelimbic cortex, 5-aza had no effect on acquisition but blocked expression. Our results indicated that DNA methylation in hippocampus is required for learning; while DNA methylation in prelimbic cortex is necessary for memory retrieval. The present finding is consistent with the role of the hippocampus as a structure contributing to cocaine-induced memory acquisition, and prelimbic cortex, a part of prefrontal cortex as an area responsible for cocaine-induced memory retrieval. In conclusion, DNA methylation does play an important role in drug-induced learning and memory although the detailed effect still calls for further research.

Role of the bed nucleus of the stria terminalis in the control of ventral tegmental area dopamine neurons

Projections from neurons of the bed nucleus of the stria terminalis (BST) to the ventral tegmental area (VTA) are crucial to behaviors related to reward and motivation. Over the past few years, we have undertaken a series of studies to understand: 1) how excitatory inputs regulate in vivo excitable properties of BST neurons, and 2) how BST inputs in turn modulate neuronal activity of dopamine neurons in VTA. Using in vivo extracellular recording techniques in anesthetized rats and tract-tracing approaches, we have demonstrated that inputs from the infralimbic cortex and the ventral subiculum exert a strong excitatory influence on BST neurons projecting to the VTA. Thus, the BST is uniquely positioned to receive emotional and learning-associated informations and to integrate these into the reward/motivation circuitry. We will discuss how changes in the activity of BST neurons projecting to the VTA could participate in the development or exacerbation of psychiatric conditions such as drug addiction.