Posttraining intra-basolateral amygdala scopolamine impairs food- and amphetamine-induced conditioned place preferences

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

RATIONALE

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

OBJECTIVE

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

METHOD

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

RESULTS

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

CONCLUSION

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

Role of amygdala in drug memory

Drug addiction is a chronic brain disorder with the hallmark of a high rate of relapse to compulsive drug seeking and drug taking even after long-term abstinence. Addiction has been considered as an aberrant memory that has been termed "addiction memory." Drug-related memory plays a critical role in the maintenance of learned addictive behaviors and emergence of relapse. Disrupting these long-lasting memories by administering amnestic agents or other manipulations during specific phases of drug memory is a promising strategy for relapse prevention. Recent studies on the processes of drug addiction and relapse have demonstrated that the amygdala is involved in associative drug addiction learning processes. In this review, we focus on preclinical studies that used conditioned place preference and self-administration models to investigate the differential roles of the amygdala in each phase of drug-related memory, including acquisition, consolidation, retrieval, reconsolidation, and extinction. These studies indicate that the amygdala plays a critical role in both cue-associative learning and the expression of cue-induced relapse to drug-seeking behavior.

Memory modulation

Our memories are not all created equally strong: Some experiences are well remembered while others are remembered poorly, if at all. Research on memory modulation investigates the neurobiological processes and systems that contribute to such differences in the strength of our memories. Extensive evidence from both animal and human research indicates that emotionally significant experiences activate hormonal and brain systems that regulate the consolidation of newly acquired memories. These effects are integrated through noradrenergic activation of the basolateral amygdala that regulates memory consolidation via interactions with many other brain regions involved in consolidating memories of recent experiences. Modulatory systems not only influence neurobiological processes underlying the consolidation of new information, but also affect other mnemonic processes, including memory extinction, memory recall, and working memory. In contrast to their enhancing effects on consolidation, adrenal stress hormones impair memory retrieval and working memory. Such effects, as with memory consolidation, require noradrenergic activation of the basolateral amygdala and interactions with other brain regions.

Different effects of scopolamine on the retrieval of spatial memory and fear memory

Retrieval of memory is fundamental for our life as individuals. The participation of cholinergic system in memory consolidation process has been extensively studied, but there are few data concerning the function of this system in memory retrieval process. In the current study, we inject non-selective muscarinic antagonist scopolamine peripherally 20 min before training or testing to see whether cholinergic modulation has effects on the acquisition or retrieval of spatial memory by water maze task and fear memory by inhibitory avoidance task. We find that the cholinergic system is essential for the acquisition of both spatial memory and fear memory. As for the memory retrieval, the cholinergic system has a positive role in the retrieval of spatial memory, because mice injected with scopolamine 20 min before the testing in the water maze show impaired spatial memory retrieval. Whereas injection of scopolamine 20 min before the testing in the inhibitory avoidance task does not cause memory retrieval deficits. That indicates the cholinergic system is not essential for the retrieval of fear memory.

The effects of infusions of CART 55-102 into the basolateral amygdala on amphetamine-induced conditioned place preference in rats

RATIONALE

The affective aspects of D: -amphetamine (AMPH) may be mediated, in part, by cocaine- and amphetamine-regulated transcript (CART) peptides in the basolateral amygdala (BLA). The formation of context-drug associations produces either conditioned place preference (CPP) or conditioned place aversion (CPA).

OBJECTIVES

The aim of the present study was to determine whether intra-BLA infusions of CART 55-102 are either rewarding or aversive and modulate AMPH reward.

MATERIALS AND METHODS

Rats were implanted with bilateral cannulae in the BLA, were subjected to place conditioning, and were tested for CPP or CPA. Rats were conditioned with either intra-BLA infusions of artificial cerebral spinal fluid or one of three dose of CART 55-102 (1, 2, or 4 microg/side), intra-BLA infusions of a subrewarding dose of CART 55-102 (1 microg/side) plus injections of a subrewarding dose of AMPH (0.1 mg/kg, i.p.), or intra-BLA infusions of an aversive dose of CART 55-102 (4 microg/side) plus injections of a rewarding dose of AMPH (1.0 mg/kg, i.p.).

RESULTS

Intra-BLA infusions of 2 microg/side CART 55-102 produced CPP, 4 microg/side produced CPA, and 1 microg/side produced neither CPP nor CPA. Intra-BLA infusions of a subrewarding dose of CART 55-102 (1 microg/side) plus injections of a subrewarding dose of AMPH (0.1 mg/kg, i.p.) produced CPP. Intra-BLA infusions of an aversive dose of CART 55-102 (4 microg/side) plus injections of a rewarding dose of AMPH (1.0 mg/kg, i.p.) produced neither CPP nor CPA.

CONCLUSIONS

Both the affective properties of intra-BLA CART 55-102 and its ability to either facilitate or block AMPH reward are dose dependent.

Reacquisition of heroin and cocaine place preference involves a memory consolidation process sensitive to systemic and intra-ventral tegmental area naloxone

To investigate the effect of naloxone on a putative memory consolidation process underlying reacquisition of heroin and cocaine conditioned place preference, four studies were conducted in male Sprague-Dawley rats using a common procedure involving: place conditioning (0.3 or 1mg/kg heroin or 20mg/kg cocaine; x4 sessions), extinction (vehiclex4 sessions), and reconditioning (0 or 1mg/kg heroin or 20mg/kg cocaine; x1 session). Systemic naloxone injections (0, 1 and 3mg/kg) or bilateral intra-ventral tegmental area (VTA) naloxone methiodide infusions (2 nmol in 0.5 microl x side) were administered at different times following reconditioning. Post-reconditioning administration of naloxone dose-dependently blocked, attenuated and had no effect on reacquisition of heroin CPP when administered immediately, 1h and 6h after reconditioning, respectively. The highest dose of naloxone also blocked reacquisition of cocaine CPP, and did not produce a conditioned place aversion in heroin-naïve and heroin pre-treated animals. Post-reconditioning infusions in the VTA, but not in adjacent structures, blocked reacquisition of heroin CPP when administered immediately, but not 6h, after reconditioning. These data suggest that reacquisition of drug-cues associations involves a memory consolidation process sensitive to manipulations of the endogenous opioid system, and indicate that opioid receptors in the VTA may be critically involved in the re-emergence of drug seeking behavior.

Overnight therapy? The role of sleep in emotional brain processing

Cognitive neuroscience continues to build meaningful connections between affective behavior and human brain function. Within the biological sciences, a similar renaissance has taken place, focusing on the role of sleep in various neurocognitive processes and, most recently, on the interaction between sleep and emotional regulation. This review surveys an array of diverse findings across basic and clinical research domains, resulting in a convergent view of sleep-dependent emotional brain processing. On the basis of the unique neurobiology of sleep, the authors outline a model describing the overnight modulation of affective neural systems and the (re)processing of recent emotional experiences, both of which appear to redress the appropriate next-day reactivity of limbic and associated autonomic networks. Furthermore, a rapid eye movement (REM) sleep hypothesis of emotional-memory processing is proposed, the implications of which may provide brain-based insights into the association between sleep abnormalities and the initiation and maintenance of mood disturbances.

The neurosteroid 3alphaDIOL modulates place preference when infused in the basolateral amygdala according to sex

Three different behavioral tasks were used to study the role of the neurosteroid 5alpha -androstane-3alpha, 17beta-diol (3alphaDIOL) in affective components of behavior when infused into the basolateral amygdala (BLA) of both sexes. Female rats were ovariectomized; half received implants containing estradiol benzoate (OVX-EB), whereas the other half received empty implants (OVX). Male rats were gonadally intact. No differences were noted in male behavior according to the conditioned place preference (CPP) test, the modified Vogel conflict test (VCT), or the elevated plus maze (EPM) upon infusion of 3alphaDIOL. In contrast, 3alphaDIOL modulated CPP and VCT performance among female rats. Therefore, the authors propose that 3alphaDIOL modulates affect through the BLA via a sex-specific mechanism.

Critical role of the cholinergic system for object-in-place associative recognition memory

Object-in-place memory, which relies on the formation of associations between an object and the place in which it was encountered, depends upon a neural circuit comprising the perirhinal (PRH) and medial prefrontal (mPFC) cortices. This study examined the contribution of muscarinic cholinergic neurotransmission within this circuit to such object-in-place associative memory. Intracerebral administration of scopolamine in the PRH or mPFC impaired memory acquisition, but not retrieval and importantly we showed that unilateral blockade of muscarinic receptors simultaneously in both regions in opposite hemispheres, significantly impaired performance. Thus, object-in-place associative memory depends upon cholinergic modulation of neurones within the PRH-PFC circuit.

Age-related dendritic hypertrophy and sexual dimorphism in rat basolateral amygdala

Little research has examined the influence of aging or sex on anatomical measures in the basolateral amygdala. We quantified spine density and dendritic material in Golgi-Cox stained tissue of the basolateral nucleus in young adult (3-5 months) and aged (20-24 months) male and female Long-Evans rats. Dendritic branching and spine density were measured in principal neurons. Age, but not sex, influenced the dendritic tree, with aged animals displaying significantly more dendritic material. Previous findings from our laboratory in the same set of subjects indicate an opposite effect of aging on dendritic material in the medial prefrontal cortex and hippocampus. We also report here a sex difference across ages in dendritic spine density, favoring males.

The food-conditioned place preference task in adolescent, adult and aged rats of both sexes

The rat basolateral amygdala shows neuroanatomical sex differences, continuing development after puberty and aging-related alterations. Implications for amygdala-dependent memory processes were explored here by testing male and female hooded rats in adolescence, adulthood and old age on the food-conditioned place preference task. While aged rats were unimpaired, adolescents failed to learn the task. This finding may be related to ongoing development of the basolateral amygdala and related memory systems during the adolescent period.

Cue-induced reinstatement of alcohol-seeking behavior is associated with increased ERK1/2 phosphorylation in specific limbic brain regions: blockade by the mGluR5 antagonist MPEP

Relapse to alcohol use after periods of abstinence is a hallmark behavioral pathology of alcoholism and a major clinical problem. Emerging evidence indicates that metabotropic glutamate receptor 5 (mGluR5) antagonists attenuate relapse to alcohol-seeking behavior but the molecular mechanisms of this potential therapeutic effect remain unexplored. The extracellular signal-regulated kinase (ERK1/2) pathway is downstream of mGluR5 and has been implicated in addiction. We sought to determine if cue-induced reinstatement of alcohol-seeking behavior, and its reduction by an mGluR5 antagonist, is associated with changes in ERK1/2 activation in reward-related limbic brain regions. Selectively-bred alcohol-preferring (P) rats were trained to lever press on a concurrent schedule of alcohol (15% v/v) vs. water reinforcement. Following 9 days of extinction, rats were given an additional extinction trial or injected with the mGluR5 antagonist MPEP (0, 1, 3, or 10mg/kg) and tested for cue-induced reinstatement. Brains were removed 90-min later from the rats in the extinction and MPEP (0 or 10mg/kg) conditions for analysis of p-ERK1/2, total ERK1/2, and p-ERK5 immunoreactivity (IR). Cue-induced reinstatement of alcohol-seeking behavior was associated with a three to five-fold increase in p-ERK1/2 IR in the basolateral amygdala and nucleus accumbens shell. MPEP administration blocked both the relapse-like behavior and increase in p-ERK1/2 IR. p-ERK1/2 IR in the central amygdala and NAcb core was dissociated with the relapse-like behavior and the pharmacological effect of mGluR5 blockade. No changes in total ERK or p-ERK5 were observed. These results suggest that exposure to cues previously associated with alcohol self-administration is sufficient to produce concomitant increases in relapse-like behavior and ERK1/2 activation in specific limbic brain regions. Pharmacological compounds, such as mGluR5 antagonists, that reduce cue-induced ERK1/2 activation may be useful for treatment of relapse in alcoholics that is triggered by exposure to environmental events.

Antagonism of muscarinic M1 receptors by dicyclomine inhibits the consolidation of morphine-associated contextual memory

M1 muscarinic receptor has been shown to be involved in cognitive functions of the brain. Conditioned place preference (CPP) paradigm involves memory for the association between environmental stimuli and the rewarding properties produced by a treatment. Using a balanced CPP design, we studied the possible involvement of M1 muscarinic receptors on the acquisition, expression and consolidation of morphine place conditioning in male mice. Subcutaneous administration of morphine sulphate-induced CPP in a dose-dependent manner. Using a 6-day schedule of conditioning, it was found that dicyclomine, an M1 muscarinic antagonist, significantly reduced the time spent by mice in the morphine compartment when given immediately, but not 6h, after each conditioning session (consolidation). It had no effect when administered 30 min before each conditioning session during CPP training period (acquisition) or 30 min before testing for place preference in the absence of morphine (expression). It is concluded that M1 muscarinic receptors may play a time-dependent role in the consolidation of reward-related memory of morphine.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Long-term memory of cocaine-associated context: disruption and reinstatement

Long-term memory of cocaine-associated context was established by conditioned place preference learning. After 1 week, exposure to context in the absence of cocaine (memory retrieval) was paired with one of the following treatments: saline, scopolamine (muscarinic acetylcholine receptor antagonist), dizocilpine (MK-801; noncompetitive N-methyl-D-aspartate antagonist) or D-cycloserine (partial N-methyl-D-aspartate agonist). In subsequent conditioned place preference tests, place preference was suppressed in the drug-treated groups but not saline-treated groups. Results suggest that the amnesic agents, scopolamine and MK-801, disrupted reconsolidation of cocaine-associated contextual memory. In contrast, the mnemonic agent D-cycloserine might have facilitated extinction learning during context exposure in the absence of cocaine. Challenge administration of cocaine reinstated place preference in all groups except the MK-801 group, suggesting that suppression of conditioned response may or may not suppress memory evoked by drug-context reexposure.

Behavioral effects of morphine and cocaine in M1 muscarinic acetylcholine receptor-deficient mice

RATIONALE

Muscarinic acetylcholine receptors (M1-M5) modulate the activity of the central nervous system and an array of physiological functions. Recent evidence has also implicated muscarinic receptors in behavioral effects of drugs of abuse such as morphine and cocaine. However, the genetic similarity between muscarinic receptors and the coexpression of multiple subtypes in most cells has impeded the development of selective antagonists and the determination of the role of each muscarinic receptor subtype in morphine's and cocaine's behavioral effects.

OBJECTIVE

The present studies employ mice deficient in the M1 receptor subtype (M1 KO) to assess morphine antinociception (2.5, 5.0, 10, or 20 mg/kg) and the conditioned rewarding effects of morphine and cocaine (2.5, 5.0, or 10 mg/kg).

METHODS

M1 KO and their wild-type (WT) littermates were tested using a 56 degrees C hotplate assay and a conditioned place preference procedure. Parallel studies using the M1 receptor antagonist, pirenzepine, were also conducted in the background strain C57BL6 mice.

RESULTS

The results demonstrate that M1 KO mice display a greater antinociceptive effect of morphine in the hotplate assay; however, the effects of morphine as well as cocaine were attenuated in the conditioned place preference procedure. Comparable results were obtained with the pharmacological antagonism of the M1 receptor by pirenzepine.

CONCLUSIONS

These results suggest a modulatory role of the M1 muscarinic receptor in opioid antinociception and conditioned drug reward, and demonstrate the utility of M1 receptor knockout models for the determination of the role of the M1 subtype in the behavioral effects of morphine and cocaine.

Post-training reversible inactivation of the rat's basolateral amygdala interferes with hippocampus-dependent place avoidance memory in a time-dependent manner

In this study, tetrodotoxin (TTX) inactivation was employed to evaluate the involvement of the rat's basolateral amygdala (BLA) in hippocampus-dependent spatial memory using a place avoidance learning task. Rats were trained in single 30 min session to avoid a 60 degrees segment of the stable circular (80-cm diameter) arena, entering which was punished by a mild shock. Bilateral injections of TTX or saline were made either immediately, 1 or 2h after training. Retention was tested 24h later in a 30 min extinction session. Retention was impaired when both BLA inactivated immediately or 1h after training, but not 2h after training. These data indicate that activity in the BLA, at least 60 min after training, is necessary for the post-training processing of a hippocampus-dependent place avoidance memory.

Endogenous histamine inhibits the development of morphine-induced conditioned place preference

AIM

The conditioned place preference (CPP) paradigm was used to investigate the effects of endogenous histamine on the processes leading to morphine-induced reward-seeking behavior in Sprague-Dawley rats.

METHODS

The model of CPP was used to assess the rewarding effect of morphine. The levels of histamine, glutamate, gamma-aminobutyric acid (GABA), dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) in rat brains were measured with high-performance liquid chromatography. Immunohistochemistry technique was used to observe the morphological changes of neurons.

RESULTS

Intraperitoneal injection of morphine (2, 5 or 10 mg/kg) induced the development of CPP in a dose-dependent manner. In addition, morphine administrations (10 mg/kg) decreased the histamine content and reduced the number and size of histaminergic neurons in the tubero-mammillary nucleus (TM), as well as markedly increasing the DOPAC/DA ratios in the ventral tegmental area (VTA) and nucleus accumbens (NAc). Intraperitoneal injection of histidine (50, 100 or 200 mg/kg) dose-dependently inhibited the development of morphine-induced CPP. Bilateral lesions of the TM, which decreased the histamine levels in the VTA and NAc, potentiated the development of CPP induced by morphine (1 mg/kg, a dose that produced no appreciable effect when given alone) and increased the DOPAC/DA ratios in the VTA and NAc, but did not change the glutamate or GABA levels in these nuclei. Histidine reversed the effects of the TM lesions.

CONCLUSION

These results indicate that endogenous histamine plays a role in inhibiting the development of morphine-induced reward-seeking behavior, and the inhibition may involve the modulation of dopaminergic activity.

Involvement of the rostral anterior cingulate cortex in consolidation of inhibitory avoidance memory: interaction with the basolateral amygdala

Previous findings suggest that the rostral anterior cingulate cortex (rACC) is involved in memory for emotionally arousing training. There is also extensive evidence that the basolateral amygdala (BLA) modulates the consolidation of emotional arousing training experiences via interactions with other brain regions. The present experiments examined the effects of posttraining intra-rACC infusions of the cholinergic agonist oxotremorine (OXO) on inhibitory avoidance (IA) retention and investigated whether the BLA and rACC interact in enabling OXO effects on memory. In the first experiment, male Sprague-Dawley rats were implanted with bilateral cannulae above the rACC and given immediate posttraining OXO infusions. OXO (0.5 or 3 ng) induced significant enhancement of retention performance on a 48-h test. In the second experiment, unilateral posttraining OXO infusions (0.5, 3.0 or 10 ng) enhanced retention when infused into rACC, but not caudal ACC, consistent with previous evidence that ACC is composed of functionally distinct regions. A third experiment investigated the effects of posttraining intra-rACC OXO infusions (0.5 or 10 ng) in rats with bilateral sham or NMDA-induced lesions of the BLA. The BLA lesions did not impair IA retention, but blocked the enhancement induced by posttraining intra-rACC OXO infusions. Lastly, unilateral NMDA lesions of rACC blocked the enhancement of IA retention induced by posttraining ipsilateral OXO infusions into the BLA. These findings support the hypothesis that the rACC is involved in modulating the storage of emotional events and provide additional evidence that the BLA modulates memory consolidation through interactions with efferent brain regions, including the cortex.

Injection of oxotremorine in nucleus accumbens shell reduces cocaine but not food self-administration in rats

Mesencephalic dopamine neurons form synapses with acetylcholine (ACh)-containing interneurons in the nucleus accumbens (NAcc). Although their involvement in drug reward has not been systematically investigated, these large aspiny interneurons may serve an important integrative function. We previously found that repeated activation of nicotinic cholinergic receptors enhanced cocaine intake in rats but the role of muscarinic receptors in drug reward is less clear. Here we examined the impact of local changes in muscarinic receptor activation within the NAcc on cocaine and food self-administration in rats trained on a progressive ratio (PR) schedule of reinforcement. Animals were given a minimum of 9 continuous days of drug access before testing in order to establish a stable breaking point (BP) for intravenous cocaine infusions (0.75 mg/kg/infusion). Rats in the food group acquired stable responding on the PR schedule within 7 days. On the test day, rats were bilaterally infused in the NAcc with the muscarinic receptor agonist oxotremorine methiodide (OXO: 0.1, 0.3 or 1 nmol/side), OXO plus the M(1) selective antagonist pirenzepine (PIRENZ; 0.3 nmol/side) or aCSF 15 min before cocaine or food access. OXO dose dependently reduced BP values for cocaine reinforcement (-17%, -44% [p<0.05] and -91% [p<0.0001] for 0.1, 0.3 and 1.0 nmol, respectively) and these reductions dissipated by the following session. Pretreatment with PIRENZ blocked the BP-reducing effect of 0.3 nmol OXO. Notably, OXO (0.1, 0.3 and 1.0 nmol/side) injection in the NAcc did not affect BP for food reward. The results suggest that muscarinic ACh receptors in the caudomedial NAcc may play a role in mediating the behavior reinforcing effects of cocaine.

The role of the basolateral amygdala in stimulus-reward memory and extinction memory consolidation and in subsequent conditioned cued reinstatement of cocaine seeking

The consolidation of cue-cocaine associations and extinction learning (i.e. cue-no cocaine associations) into long-term memory probably regulates the long-lasting control of conditioned stimuli (CS) over cocaine-seeking behaviour, and the basolateral amygdala (BLA) may play a role in this phenomenon. To test this hypothesis, rats previously trained to self-administer cocaine underwent a single classical conditioning (CC) session, during which they received passive pairings of cocaine infusions and a novel light + tone stimulus complex. After additional self-administration sessions in the absence of CS presentation and subsequent extinction training sessions, the ability of the CS to reinstate cocaine-seeking on five test days was assessed. Rats received intra-BLA microinfusions of vehicle or the Na+-channel blocker tetrodotoxin (TTX) immediately after CC (consolidation of CS-cocaine associations) or immediately after reinstatement testing (consolidation of extinction learning). TTX administered immediately after CC attenuated subsequent CS-induced reinstatement. In contrast, TTX administered after the first reinstatement test impaired the extinction of cocaine-seeking behaviour during a second reinstatement test by disrupting extinction memory. Overall, these findings suggest that Na+ channel-mediated mechanisms within the BLA mediate the consolidation of both cocaine-stimulus association and extinction learning, two processes that have opposite effects on subsequent cue-induced cocaine-seeking behaviour.

Dorsal hippocampus inhibition disrupts acquisition and expression, but not consolidation, of cocaine conditioned place preference

Cocaine abusers may experience drug craving upon exposure to environmental contexts where cocaine was experienced. The dorsal hippocampus (DHC) is important for contextual conditioning, therefore the authors examined the specific role of the DHC in cocaine conditioned place preference (CPP). Muscimol was used to temporarily inhibit the DHC and was infused before conditioning sessions or tests for CPP to investigate acquisition and expression of cocaine CPP, respectively. To investigate consolidation, rats received intra-DHC muscimol either immediately or 6 hr after conditioning sessions. Inhibition of DHC, but not the overlying cortex, disrupted acquisition and expression of cocaine CPP. It is interesting to note that there was no effect of post-conditioning DHC inhibition. The findings suggest that the DHC is important for both acquisition and recall, but not consolidation, of context-cocaine associations.

Protein synthesis inhibition in the basolateral amygdala following retrieval does not impair expression of morphine-associated conditioned place preference

Conditioned place preference is an animal model used to evaluate the affective properties of natural rewards and drugs of abuse. This animal model is a kind of classical conditioning that depends on learning and memory. The basolateral amygdala (BLA) plays an important role in the consolidation and extinction of memory for this task. However, there is a lack of evidence demonstrating protein synthesis dependent reconsolidation following retrieval in conditioned animals. In other words, is it possible to observe morphine-associated place preference if recall of this preference is disrupted? Accordingly, we investigated this hypothesis by BLA infusion of protein synthesis inhibitor, anisomycin, immediately after retrieval (test) in conditioned place preference paradigm. In the first experiment, the conditioned animals were exposed to the two sides of the apparatus for 15 min in a drug-free state during retrieval. In the second experiment, the animals received an injection of morphine (7.5 mg/kg, i.p.) and immediately after, they were exposed to the two sides of the apparatus for 15 min. Finally in the third experiment, after habituation and training in the conditioned place preference task, the animals received an injection of the unconditioned stimulus (morphine, i.p.; 7.5 mg/kg) followed by confinement for 10 min in the morphine-paired compartment (conditioned stimulus) during memory retrieval. For the three experiments the animals were subsequently exposed in a free-drug state to the two sides of the apparatus for the retest. Our results show that the protein synthesis inhibition in all of these experimental designs had no effect on conditioned place preference memory under conditions that would initiate reconsolidation, suggesting that if reconsolidation of a conditioned place preference task exists it is not mediated by protein synthesis in basolateral amygdala. The effect of anisomycin on consolidation of contextual fear conditioning was also investigated as a positive control to assure that the negative results were not due to methodological problems. Using the same dose of anisomycin (62.5 microg/1 microl) in morphine-associated place preference procedures, we have found that this anisomycin dose blocks the consolidation of contextual fear memory, ruling out the possibility that these negative results can be attributed to methodological problem of some sort.

Differential involvement of the hippocampus, anterior cingulate cortex, and basolateral amygdala in memory for context and footshock

Extensive evidence from contextual fear conditioning experiments suggests that the hippocampus is involved in processing memory for contextual information. Evidence also suggests that the rostral anterior cingulate cortex (rACC) may be selectively involved in memory for nociceptive stimulation. In contrast, many findings indicate that the basolateral amygdala (BLA) is more broadly involved in modulating the consolidation of different kinds of information. To investigate further the differential involvement of these brain regions in memory consolidation, the present experiments used a modified inhibitory avoidance training procedure that took place on 2 sequential days to separate context training from footshock training. Male Sprague-Dawley rats were implanted with unilateral cannulae aimed at the (i) hippocampus, (ii) rACC, or (iii) BLA, and given infusions of the muscarinic cholinergic agonist oxotremorine (OXO) immediately after either context training (day 1) or footshock training in that context (day 2). OXO enhanced retention when infused into the hippocampus after context, but not footshock, training. Conversely, OXO infusions enhanced memory when administered into the rACC immediately after footshock, but not context, training. Lastly, intra-BLA OXO infusions enhanced retention when administered after either context or footshock training. These findings are consistent with evidence that the hippocampus and rACC play selective roles in memory for specific components of training experiences. Additionally, they provide further evidence that the BLA is more liberally involved in modulating memory consolidation for various aspects of emotionally arousing experiences.

Neural substrates of cocaine-cue associations that trigger relapse

Learned associations that occur during the process of repeated drug use in addiction can later manifest as trigger factors in relapse to renewed drug-seeking and drug-taking behavior. The process of conditioned-cued relapse of drug-seeking behavior has been successfully modeled in animals using the reinstatement procedure, in which chronic drug self-administration can be extinguished or withheld, and then reinstated using conditioned stimuli previously paired with the drug. Our laboratory has extensively studied the neural circuitry underlying conditioned-cued drug-seeking during the expression of reinstatement. In order to study the learning process of drug-cue pairings, we further developed a procedure whereby discrete cocaine-cue pairings can be conducted in a single pavlovian training session in animals previously trained to self-administer cocaine. Presentation of these cues during later reinstatement trials produces robust responding over extinction levels at levels similar to those seen when animals experience the cues on a daily basis. In a series of experiments, we have shown that reversible pharmacological inactivation of the basolateral complex of the amygdala just prior to acquisition of cocaine-cue associations blocks the ability of cocaine-paired stimuli to elicit conditioned-cued reinstatement. This learning process is mediated in part by muscarinic acetylcholine and dopaminergic inputs to the basolateral complex of the amygdala, as intra-amygdala infusion of selective receptor antagonists at the time of acquisition significantly affects reinstatement. We have also recently found that disruption of neural activity within the basolateral complex of the amygdala at the time of consolidation (just after cocaine-cue pairings) will disrupt reinstatement. Taken together, these results reveal the importance of the amygdala in the acquisition, consolidation, and expression of drug-stimulus learning that drives relapse to drug-seeking behavior.

Complementary roles for the amygdala and hippocampus during different phases of appetitive information processing

Evidence collected from rodent models of memory storage suggests that rapid forms of learning engage the involvement of multiple brain regions each of which may participate in a different component of information processing. The present study used temporary inactivation of the amygdala and hippocampus during different phases of information processing on a one-trial appetitive-conditioning task to examine how these two regions might participate in the storage of appetitive memories. Male Long Evans rats were chronically implanted into the amygdala or dorsal hippocampus and food deprived. Rats were trained on a radial maze conditioned cue preference task where training occurred in one 40-min session and testing took place 24 h later. The amygdala or hippocampus was inactivated separately with muscimol (50 ng/microl) injected immediately before or after training, or immediately before testing. Saline-injected rats displayed a conditioned preference by spending more time in the arm that previously contained food than in the arm that did not contain food. Muscimol injected into the amygdala before training or testing blocked the conditioned preference. Muscimol injected into the hippocampus immediately after training blocked the conditioned preference. These results suggest that the processing of memories may require multiple contributions from separate brain systems for at least short-term (24 h) storage. The resulting output from each system may converge on a similar downstream target to influence behavior.

Post-training intra-amygdala amphetamine injections given during acquisition of a stimulus–response (S–R) habit task enhance the expression of stimulus–reward learning: Further evidence for incidental amygdala learning

The effect of post-training intra-amygdala amphetamine injections was examined on the acquisition and expression of a visual discrimination task. Rats were trained to enter four lit arms for food (stimulus-response) and avoid unlit arms on an eight-arm radial maze visual discrimination task. Post-training intra-amygdala amphetamine injections (10 microg) were given for 4 consecutive days during the mid-point of training (days 20-23). The number of lit arm entries was used as a measure of stimulus-response habit learning 24 h after each injection. Twenty-four hours after the last injection, a transfer test was run to assess the effect of the same post-training manipulation. This transfer test assessed the amount of time spent in the lit arms and was used as a measure of stimulus-reward learning. Compared to saline-injected rats, rats that received post-training amphetamine spent more time in lit as opposed to dark arms during the transfer test. This occurred in the absence of an increase in the number of correct arm entries during visual discrimination training. This suggests that post-training amphetamine strengthened a stimulus-reward association that did not immediately affect behavioral output. This association may reflect a mnemonic representation stored in an ensemble of amygdala neurons.

Central muscarinic blockade interferes with retrieval and reacquisition of active allothetic place avoidance despite spatial pretraining

Animal navigation to hidden goals (place navigation) ranks among the most intensively studied types of behaviour because it requires brain representations of environments in the form of cognitive maps, demonstrated to depend on hippocampal function. Intact function of muscarinic receptors in the brain was originally assumed to be crucial for place navigation, however, recent studies using non-spatial pretraining demonstrated that animals with central blockade of muscarinic acetylcholine receptors can also learn and retrieve spatial memory engrams. In the present study we addressed whether navigation in the active allothetic place avoidance (AAPA) task, which requires animals to separate spatial stimuli into coherent representations and navigate according to the representation relevant for the task, is dependent on intact muscarinic receptors in the brain. We studied the effect of three doses of scopolamine (0.5, 1.0 and 2.0mg/kg) administered 20 min prior to training on the retention of the AAPA and re-acquisition of the AAPA in a new environment. The dose of 2.0mg/kg was found to impair both AAPA retention and re-acquisition of the AAPA in a new environment, whereas the 1.0mg/kg dose only impaired the reinforced retention of AAPA. It is concluded that, unlike navigation in classic paradigms, efficient orientation in the AAPA task is critically dependent on muscarinic receptors in the brain.

Inhibition of acetylcholine-induced activation of extracellular regulated protein kinase prevents the encoding of an inhibitory avoidance response in the rat

It has been demonstrated that the forebrain cholinergic system and the extracellular regulated kinase signal transduction pathway are involved in the mechanisms of learning, encoding, and storage of information. We investigated the involvement of the cholinergic and glutamatergic systems projecting to the medial prefrontal cortex and ventral hippocampus and of the extracellular regulated kinase signal transduction pathway in the acquisition and recall of the step-down inhibitory avoidance response in the rat, a relatively simple behavioral test acquired in a one-trial session. To this aim we studied by microdialysis the release of acetylcholine and glutamate, and by immunohistochemistry the activation of extracellular regulated kinase during acquisition, encoding and recall of the behavior. Cholinergic, but not glutamatergic, neurons projecting to the medial prefrontal cortex and ventral hippocampus were activated during acquisition of the task, as shown by increase in cortical and hippocampal acetylcholine release. Released acetylcholine in turn activated extracellular regulated kinase in neurons located in the target structures, since the muscarinic receptor antagonist scopolamine blocked extracellular regulated kinase activation. Both increased acetylcholine release and extracellular regulated kinase activation were necessary for memory formation, as administration of scopolamine and of extracellular regulated kinase inhibitors was followed by blockade of extracellular regulated kinase activation and amnesia. Our data indicate that a critical function of the learning-associated increase in acetylcholine release is to promote the activation of the extracellular regulated kinase signal transduction pathway and help understanding the role of these systems in the encoding of an inhibitory avoidance memory.

Facilitation of memory for extinction of drug-induced conditioned reward: role of amygdala and acetylcholine

These experiments examined the effects of posttrial peripheral and intra-amygdala injections of the cholinergic muscarinic receptor agonist oxotremorine on memory consolidation underlying extinction of amphetamine conditioned place preference (CPP) behavior. Male Long-Evans rats were initially trained and tested for an amphetamine (2 mg/kg) CPP. Rats were subsequently given limited extinction training, followed by immediate posttrial peripheral or intrabasolateral amygdala injections of oxotremorine. A second CPP test was then administered, and the amount of time spent in the previously amphetamine-paired and saline-paired apparatus compartments was recorded. Peripheral (0.07 or 0.01 mg/kg) or intra-amygdala (10 etag/0.5 microL) postextinction trial injections of oxotremorine facilitated CPP extinction. Oxotremorine injections that were delayed 2 h posttrial training did not enhance CPP extinction, indicating a time-dependent effect of the drug on memory consolidation processes. The findings indicate that memory consolidation for extinction of approach behavior to environmental stimuli previously paired with drug reward can be facilitated by posttrial peripheral or intrabasolateral amygdala administration of a cholinergic agonist.

The amygdala modulates the consolidation of memories of emotionally arousing experiences

Converging findings of animal and human studies provide compelling evidence that the amygdala is critically involved in enabling us to acquire and retain lasting memories of emotional experiences. This review focuses primarily on the findings of research investigating the role of the amygdala in modulating the consolidation of long-term memories. Considerable evidence from animal studies investigating the effects of posttraining systemic or intra-amygdala infusions of hormones and drugs, as well as selective lesions of specific amygdala nuclei, indicates that (a) the amygdala mediates the memory-modulating effects of adrenal stress hormones and several classes of neurotransmitters; (b) the effects are selectively mediated by the basolateral complex of the amygdala (BLA); (c) the influences involve interactions of several neuromodulatory systems within the BLA that converge in influencing noradrenergic and muscarinic cholinergic activation; (d) the BLA modulates memory consolidation via efferents to other brain regions, including the caudate nucleus, nucleus accumbens, and cortex; and (e) the BLA modulates the consolidation of memory of many different kinds of information. The findings of human brain imaging studies are consistent with those of animal studies in suggesting that activation of the amygdala influences the consolidation of long-term memory; the degree of activation of the amygdala by emotional arousal during encoding of emotionally arousing material (either pleasant or unpleasant) correlates highly with subsequent recall. The activation of neuromodulatory systems affecting the BLA and its projections to other brain regions involved in processing different kinds of information plays a key role in enabling emotionally significant experiences to be well remembered.

p-Chloroamphetamine blocks physostigmine-induced memory enhancement in rats with unilateral nucleus basalis lesions

The present experiment examined whether p-chloroamphetamine (PCA), a serotonergic releasing/depleting agent, would block the memory-enhancing effect of physostigmine in rats with N-methyl-D-aspartic acid (NMDA)-induced unilateral lesions of the nucleus basalis of Meynert (uni-nbM). Six groups of subjects with uni-nbM lesions in addition to an isolated sham-operated control group were included. Subjects were trained and tested 72 h later on a one-trial passive avoidance task. Thirty minutes before training, rats with uni-nbM lesions were injected with either 1.0 or 5.0 mg/kg PCA or saline. Immediately after training, approximately half the subjects in each group were injected with either saline or 0.06 mg/kg physostigmine. Animals in the sham group received saline injections. Saline-injected animals with uni-nbM lesions performed poorly at test, a deficit that was reversed with physostigmine. Pretraining injections of PCA blocked physostigmine's memory-enhancing effect, although motor impairment during training may have contributed to decrements in test performance in animals injected with 5.0 mg/kg. Subjects were killed about 10 days later and their frontal cortices examined for choline acetyltransferase (ChAT). Results from the neurochemical analysis revealed that the lesion decreased ChAT levels and that the injection of 1.0 mg/kg PCA exaggerated this lesion-induced depletion. Implications for the interaction between acetylcholine and serotonin are discussed.

Post-training intra-basolateral amygdala infusions of norepinephrine enhance consolidation of memory for contextual fear conditioning

Post-training infusions of drugs, including noradrenergic agonists and antagonists, into the basolateral amygdala (BLA) influence the consolidation of memory for training in several tasks, including inhibitory avoidance. There is, however, conflicting evidence concerning whether post-training intra-BLA drug infusions modulate the consolidation of contextual fear conditioning (CFC). In the present study, norepinephrine (NE) was infused bilaterally into the BLA of male Sprague Dawley rats immediately after training on two CFC tasks: a Y-maze and a straight alley. Post-training intra-BLA infusions enhanced memory of CFC training in the Y-maze, as assessed by percentage of time spent freezing and shock arm entrance latencies. Post-training intra-BLA infusions of NE enhanced 48 hr retention of CFC training in the straight alley, as assessed by shock compartment entrance latencies and the number of shocks required to learn to avoid entering the shock compartment. These findings indicate that the consolidation of memory for CFC, like that for inhibitory avoidance training, is influenced by post-training neuromodulatory influences within the BLA. Thus, the findings provide additional evidence consistent with the hypothesis that the BLA has a general role in modulating memory consolidation.

Systemic or intra-amygdala injections of glucose facilitate memory consolidation for extinction of drug-induced conditioned reward

The conditioned place preference (CPP) task has been used extensively to investigate the neurobiological bases of drug-induced reward. The initial expression of a CPP involves memory for an association between environmental stimuli and the affective state produced by a rewarding treatment. The present experiments examined the hypothesis that post-trial administration of glucose can facilitate memory consolidation processes underlying the extinction of drug-induced CPP behaviour. Adult male Long-Evans rats acquired an amphetamine CPP, and subsequently received extinction training. Immediately following extinction training, separate groups of rats received peripheral (100 mg/kg, 500 mg/kg, or 2 g/kg) or intra-amygdala (basolateral nucleus; 1.5 micro g/0.5 micro L or 10 micro g/0.5 micro L) injections of glucose or vehicle. Peripheral (100 mg/kg and 2 g/kg) and intra-amygdala (1.5 and 10 micro g) glucose injections facilitated the extinction of amphetamine CPP behaviour relative to vehicle-injected controls. Postextinction trial peripheral or intra amygdala glucose injections that were delayed 2 h had no effect. The findings indicate that: (i) extinction of approach behaviour to drug-associated cues involves the formation of new memories that undergo a time-dependent consolidation process; and (ii), systemic or intra-amygdala administration of a known memory-enhancing agent facilitates extinction of drug-induced CPP behaviour.