Double dissociation of the behavioural effects of R(+) 7-OH-DPAT infusions in the central and basolateral amygdala nuclei upon Pavlovian and instrumental conditioned appetitive behaviours

The Role of Genetically Distinct Central Amygdala Neurons in Appetitive and Aversive Responding Assayed with a Novel Dual Valence Operant Conditioning Paradigm

To survive, animals must meet their biological needs while simultaneously avoiding danger. However, the neurobiological basis of appetitive and aversive survival behaviors has historically been studied using separate behavioral tasks. While recent studies in mice have quantified appetitive and aversive conditioned responses simultaneously (Jikomes et al., 2016; Heinz et al., 2017), these tasks required different behavioral responses to each stimulus. As many brain regions involved in survival behavior process stimuli of opposite valence, we developed a paradigm in which mice perform the same response (nose poke) to distinct auditory cues to obtain a rewarding outcome (palatable food) or avoid an aversive outcome (mild footshoock). This design allows for both within-subject and between-subject comparisons as animals respond to appetitive and aversive cues. The central nucleus of the amygdala (CeA) is implicated in the regulation of responses to stimuli of either valence. Considering its role in threat processing (Wilensky et al., 2006; Haubensak et al., 2010) and regulation of incentive salience (Warlow and Berridge, 2021), it is important to examine the contribution of the CeA to mechanisms potentially underlying comorbid dysregulation of avoidance and reward (Sinha, 2008; Bolton et al., 2009). Using this paradigm, we tested the role of two molecularly defined CeA subtypes previously linked to consummatory and defensive behaviors. Significant strain differences in the acquisition and performance of the task were observed. Bidirectional chemogenetic manipulation of CeA somatostatin (SOM) neurons altered motivation for reward and perseveration of reward-seeking responses on avoidance trials. Manipulation of corticotropin-releasing factor neurons (CRF) had no significant effect on food reward consumption, motivation, or task performance. This paradigm will facilitate investigations into the neuronal mechanisms controlling motivated behavior across valences.

The role of genetically distinct central amygdala neurons in appetitive and aversive responding assayed with a novel dual valence operant conditioning paradigm

To survive, animals must meet their biological needs while simultaneously avoiding danger. However, the neurobiological basis of appetitive and aversive survival behaviors has historically been studied using separate behavioral tasks. While recent studies in mice have quantified appetitive and aversive conditioned responses simultaneously (Heinz et al., 2017; Jikomes et al., 2016), these tasks required different behavioral responses to each stimulus. As many brain regions involved in survival behavior process stimuli of opposite valence, we developed a paradigm in which mice perform the same response (nosepoke) to distinct auditory cues to obtain a rewarding outcome (palatable food) or avoid an aversive outcome (mild footshoock). This design allows for both within- and between-subject comparisons as animals respond to appetitive and aversive cues. The central nucleus of the amygdala (CeA) is implicated in the regulation of responses to stimuli of either valence. Considering its role in threat processing (Haubensak et al., 2010; Wilensky et al., 2006) and regulation of incentive salience (Warlow and Berridge, 2021), it is important to examine the contribution of the CeA to mechanisms potentially underlying comorbid dysregulation of avoidance and reward (Bolton et al., 2009; Sinha, 2008). Using this paradigm, we tested the role of two molecularly defined CeA subtypes previously linked to consummatory and defensive behaviors. Significant strain differences in the acquisition and performance of the task were observed. Bidirectional chemogenetic manipulation of CeA somatostatin (SOM) neurons altered motivation for reward and perseveration of reward-seeking responses on avoidance trials. Manipulation of corticotropin-releasing factor neurons (CRF) had no significant effect on food reward consumption, motivation, or task performance. This paradigm will facilitate investigations into the neuronal mechanisms controlling motivated behavior across valences.

Significance Statement

It is unclear how different neuronal populations contribute to reward- and aversion-driven behaviors within a subject. To address this question, we developed a novel behavioral paradigm in which mice obtain food and avoid footshocks via the same operant response. We then use this paradigm to test how the central amygdala coordinates appetitive and aversive behavioral responses. By testing somatostatin-IRES-Cre and CRF-IRES-Cre transgenic lines, we found significant differences between strains on task acquisition and performance. Using chemogenetics, we demonstrate that CeA SOM+ neurons regulate motivation for reward, while manipulation of CeA CRF+ neurons had no effect on task performance. Future studies investigating the interaction between positive and negative motivation circuits should benefit from the use of this dual valence paradigm.

The role of D2 dopamine receptors in oxytocin induced place preference and anxiolytic effect

Neuropeptide oxytocin (OT) is involved in the regulation of social and non-social behaviour. The central nucleus of amygdala (CeA), part of the limbic system, plays an important role in learning, memory, anxiety and reinforcing mechanisms. CeA has been shown to be rich in OT receptors in rodents. Our previous findings indicated that OT in the rat CeA has a dose dependent rewarding and anxiolytic effect. The aim of our present study was to examine in the CeA the possible interaction of OT and D2 dopamine (DA) receptor antagonist Sulpiride on reinforcement in place preference test and on anxiety in elevated plus maze test. Wistar rats were microinjected bilaterally with 10 ng OT. In different group of animals 4 μg D2 DA receptor antagonist was applied. Other animals received D2 DA receptor antagonist 15 min before 10 ng OT treatment or vehicle solution into the CeA. Rats receiving 10 ng OT spent significantly longer time in the treatment quadrant during the test session in conditioned place preference test. Prior treatment with D2 DA receptor antagonist blocked the rewarding effects of OT. Antagonist in itself did not influence the time rats spent in the treatment quadrant. In elevated plus maze test, rats receiving 10 ng OT spent significantly longer time on the open arms. Prior treatment with D2 DA receptor antagonist blocked the effects of OT. Our results show that DA system plays a role in positive reinforcing and anxiolytic effects of OT because D2 DA receptor antagonist can block these actions.

Opiate exposure state controls dopamine D3 receptor and cdk5/calcineurin signaling in the basolateral amygdala during reward and withdrawal aversion memory formation

The dopamine (DA) D3 receptor (D3R) is highly expressed in the basolateral nucleus of the amygdala (BLA), a neural region critical for processing opiate-related reward and withdrawal aversion-related memories. Functionally, D3R transmission is linked to downstream Cdk5 and calcineurin signaling, both of which regulate D3R activity states and play critical roles in memory-related synaptic plasticity. Previous evidence links D3R transmission to opiate-related memory processing, however little is known regarding how chronic opiate exposure may alter D3R-dependent memory mechanisms. Using conditioned place preference (CPP) and withdrawal aversion (conditioned place aversion; CPA) procedures in rats, combined with molecular analyses of BLA protein expression, we examined the effects of chronic opiate exposure on the functional role of intra-BLA D3R transmission during the acquisition of opiate reward or withdrawal aversion memories. Remarkably, we report that the state of opiate exposure during behavioural conditioning (opiate-naïve/non-dependent vs. chronically exposed and in withdrawal) controlled the functional role of intra-BLA D3R transmission during the acquisition of both opiate reward memories and withdrawal-aversion associative memories. Thus, whereas intra-BLA D3R blockade had no effect on opiate reward memory formation in the non-dependent state, blockade of intra-BLA D3R transmission prevented the formation of opiate reward and withdrawal aversion memory in the chronically exposed state. This switch in the functional role of D3R transmission corresponded to significant increases in Cdk5 phosphorylation and total expression levels of calcineurin, and a corresponding decrease in intra-BLA D3R expression. Inhibition of either intra-BLA Cdk5 or calcineurin reversed these effects, switching intra-BLA associative memory formation back to a D3R-independent mechanism.

A framework for understanding and advancing intertemporal choice research using rodent models

Intertemporal choices are common and consequential to private and public life. Thus, there is considerable interest in understanding the neural basis of intertemporal decision making. In this minireview, we briefly describe conceptual and psychological perspectives on intertemporal choice and then provide a comprehensive evaluation of the neural structures and signals that comprise the underlying cortico-limbic-striatal circuit. Even though great advances have been made, our understanding of the neurobiology of intertemporal choice is still in its infancy because of the complex and dynamic nature of this form of decision making. We close by briefly discussing recommendations for the future study of intertemporal choice research.

Subanesthetic ketamine decreases the incentive-motivational value of reward-related cues

The attribution of incentive-motivational value to reward-related cues contributes to cue-induced craving and relapse in addicted patients. Recently, it was demonstrated that subanesthetic ketamine increases motivation to quit and decreases cue-induced craving in cocaine-dependent individuals. Although the underlying mechanism of this effect is currently unknown, one possibility is that subanesthetic ketamine decreases the incentive-motivational value of reward-related cues. In the present study, we used a Pavlovian conditioned approach procedure to identify sign-trackers, rats that attribute incentive-motivational value to reward-related cues, and goal-trackers, rats that assign only predictive value to reward-related cues. This model is of interest because sign-trackers are more vulnerable to cue-induced reinstatement of drug-seeking behavior and will persist in this drug-seeking behavior despite adverse consequences. We tested the effect of subanesthetic ketamine on the expression of Pavlovian conditioned approach behavior and the conditioned reinforcing properties of a reward-related cue in sign- and goal-trackers. We found that subanesthetic ketamine decreased sign-tracking and increased goal-tracking behavior in sign-trackers, though it had no effect on conditioned reinforcement. These results suggest that subanesthetic ketamine may be a promising pharmacotherapy for addiction that acts by decreasing the incentive-motivational value of reward-related cues.

Studies on a new potential dopaminergic agent: in vitro BBB permeability, in vivo behavioural effects and molecular docking evaluation

2-Amino-N-[2-(3,4-dihydroxy-phenyl)-ethyl]-3-phenyl-propionamide (DA-PHEN) has been previously synthesized to obtain a potential prodrug capable of release dopamine (DA) into CNS. However, DA-PHEN could act per se as a dopaminergic drug. In this study, the permeability transport (Pe), obtained by parallel artificial permeability assay (PAMPA), indicated a low passive transcellular transport (Pe = 0.32 ± 0.01 × 10(-6 )cm/s). Using the Caco-2 cell system, the Papp AP-BL in absorptive direction (3.36 ± 0.02 × 10(-5 )cm/s) was significantly higher than the Papp BL-AP in secretive direction (1.75 ± 0.07 × 10(-5 )cm/s), suggesting a polarized transport. The efflux ratio (Papp AP-BL/Papp BL-AP = 0.52 ± 0.02) indicated a low affinity of DA-PHEN to efflux carriers. The forced swim test highlighted a reduction of immobility time in both pre-test and test sessions (p < 0.0001), with an exacerbation in the number of headshakes and divings in the pretest (p < 0.0001). Morris water maze strengthened the hypothesis that DA-PHEN induces adaptive responses to environmental challenges which are involved on cognitive functions (DA-PHEN versus CTR: escape latency; p < 0.001; distance swum p < 0.001, time spent on target quadrant p < 0.001), without any change in locomotor activity for the administered dose. The molecular docking revealed the interaction of DA-PHEN with the identified D1 site mapping human brain receptor.

Repeated nicotine exposure during adolescence alters reward-related learning in male and female rats

RATIONALE

Repeated nicotine exposure causes neuroadaptations in limbic cortico-striatal circuits involved in learning and motivation. Such alterations are relevant to addiction because they are suggested to mediate the ability of smoking-associated stimuli to control behavior and to enhance nicotine-seeking and -taking behaviors. Female smokers report higher cue reactivity relative to their male counter parts, yet little is known about putative gender-specific effects of adolescent nicotine exposure on reward-related learning. Prior repeated nicotine exposure in adult male rats enhances Pavlovian approach behavior and conditioned reinforcement.

OBJECTIVE

Given that smoking is typically initiated during adolescence, here we assessed the extent to which adolescent nicotine exposure impacts Pavlovian approach and conditioned reinforcement in male and female rats.

METHODS

Rats were injected with nicotine on postnatal days 31-45 prior to training on Pavlovian approach behavior starting on day 51. They were trained to associate a conditioned stimulus (CS), illumination of a magazine light, and tone, with an unconditioned stimulus (US), the delivery of water, for 10-daily sessions, and then were tested on the acquisition of responding with conditioned reinforcement.

RESULTS

Adolescent nicotine exposure selectively increased approach to the magazine during the CS in males but decreased approach to the magazine during the CS in female rats. Vehicle-exposed female rats, however, showed greater magazine approach during the CS than did male control rats. Prior nicotine exposure also enhanced conditioned reinforcement in both male and female rats.

CONCLUSIONS

Repeated exposure to nicotine during adolescence had opposite effects on Pavlovian approach behavior in male and female rats but enhanced acquisition of a new response with conditioned reinforcement. Novel information on how nicotine exposure influences reward-related learning during adolescence may increase our understanding of neurobiological mechanisms involved in the initiation of smoking behavior.

Dopaminergic reward signals selectively decrease fMRI activity in primate visual cortex

Stimulus-reward coupling without attention can induce highly specific perceptual learning effects, suggesting that reward triggers selective plasticity within visual cortex. Additionally, dopamine-releasing events-temporally surrounding stimulus-reward associations-selectively enhance memory. These forms of plasticity may be evoked by selective modulation of stimulus representations during dopamine-inducing events. However, it remains to be shown whether dopaminergic signals can selectively modulate visual cortical activity. We measured fMRI activity in monkey visual cortex during reward-only trials apart from intermixed cue-reward trials. Reward without visual stimulation selectively decreased fMRI activity within the cue representations that had been paired with reward during other trials. Behavioral tests indicated that these same uncued reward trials strengthened cue-reward associations. Furthermore, such spatially-specific activity modulations depended on prediction error, as shown by manipulations of reward magnitude, cue-reward probability, cue-reward familiarity, and dopamine signaling. This cue-selective negative reward signal offers a mechanism for selectively gating sensory cortical plasticity.

Presynaptic dopamine modulation by stimulant self-administration

The mesolimbic dopamine system is an essential participant in the initiation and modulation of various forms of goal-directed behavior, including drug reinforcement and addiction processes. Dopamine neurotransmission is increased by acute administration of all drugs of abuse, including the stimulants cocaine and amphetamine. Chronic exposure to these drugs via voluntary self-administration provides a model of stimulant abuse that is useful in evaluating potential behavioral and neurochemical adaptations that occur during addiction. This review describes commonly used methodologies to measure dopamine and baseline parameters of presynaptic dopamine regulation, including exocytotic release and reuptake through the dopamine transporter in the nucleus accumbens core, as well as dramatic adaptations in dopamine neurotransmission and drug sensitivity that occur with acute non-contingent and chronic, contingent self-administration of cocaine and amphetamine.

The many paths to fear

Fear is an emotion that has powerful effects on behaviour and physiology across animal species. It is accepted that the amygdala has a central role in processing fear. However, it is less widely appreciated that distinct amygdala outputs and downstream circuits are involved in different types of fear. Data show that fear of painful stimuli, predators and aggressive members of the same species are processed in independent neural circuits that involve the amygdala and downstream hypothalamic and brainstem circuits. Here, we discuss data supporting multiple fear pathways and the implications of this distributed system for understanding and treating fear.

Inadequate early social experience increases the incentive salience of reward-related cues in adulthood

The mechanisms by which childhood abuse and/or neglect become risk factors for the development of drug addiction, problem gambling, and other disorders of behavioral inhibition are unknown. The loss of behavioral inhibition is often triggered by reward-related cues that acquire incentive salience. This study examined whether inadequate early-life social experience in rats affects the incentive salience of reward-related cues. Rats were deprived of early-life social experience with the mother and litter through artificial-rearing (AR). A group of AR rats (AR+STM) received additional tactile stimulation that mimicked maternal licking, a critical component of rat maternal care. Control rats were maternally reared (MR). The incentive salience attributed to a food cue was measured in adult rats using a conditioned approach task, where a conditional stimulus (CS; lever) was paired with food delivery, and in a conditional reinforcement task. The dependent measures were approach towards the CS (sign-tracking) versus approach towards the place of food delivery (goal-tracking) and instrumental responding for the CS. AR rats made significantly more sign-tracking responses than MR rats. AR rats also made more instrumental responses when reinforced with the CS. AR+STM rats' responses were intermediate to MR and AR rats. Thus, inadequate early-life social experience enhanced the incentive salience of a reward-related cue in adulthood. Replacement of maternal licking partially reversed this effect. These results highlight a potential link between early-life social adversity and susceptibility to disorders of behavioral inhibition.

Stimulation of dopamine D2/D3 but not D1 receptors in the central amygdala decreases cocaine-seeking behavior

Alterations in dopamine output within the various subnuclei of the amygdala have previously been implicated in cocaine reinforcement, as well as cocaine-seeking behavior. To elucidate the potential for increased stimulation of D1- and D2-like receptors (D1Rs and D2Rs, respectively) specifically in the central nucleus of the amygdala (CeA) to modulate cue- and cocaine-elicited reinstatement of cocaine-seeking behavior, we infused either the D1R agonist, SKF-38393 (0-4.0 microg/side) or the D2R agonist, 7-OH-DPAT (0-4.0 microg/side) into the CeA immediately prior to tests for cue and cocaine-primed reinstatement. We also examined the effects of 7-OH-DPAT on cocaine self-administration as a positive behavioral control. 7-OH-DPAT decreased cue-and cocaine-primed reinstatement, and reduced the number of cocaine infusions obtained during self-administration; SKF-38393 produced no discernable effects. The results suggest that enhanced stimulation of D2Rs, but not D1Rs, in the CeA is sufficient to inhibit expression of the incentive motivational effects of cocaine priming and cocaine-paired cues. Together with previous findings that D1R blockade attenuates reinstatement of cocaine-seeking behavior, the results suggest that D1R stimulation may be necessary, but not sufficient, to modulate the incentive motivational effects of cues and cocaine priming.

Role of the mesoamygdaloid dopamine projection in emotional learning

RATIONALE

Amygdala dopamine is crucially involved in the acquisition of Pavlovian associations, as measured via conditioned approach to the location of the unconditioned stimulus (US). However, learning begins before skeletomotor output, so this study assessed whether amygdala dopamine is also involved in earlier 'emotional' learning.

OBJECTIVES

A variant of the conditioned reinforcement (CR) procedure was validated where training was restricted to curtail the development of selective conditioned approach to the US location, and effects of amygdala dopamine manipulations before training or later CR testing assessed.

METHODS

Experiment 1a presented a light paired (CS+ group) or unpaired (CS- group) with a US. There were 1, 2 or 10 sessions, 4 trials per session. Then, the US was removed, and two novel levers presented. One lever (CR+) presented the light, and lever pressing was recorded. Experiment 1b also included a tone stimulus. Experiment 2 applied intra-amygdala R(+) 7-OH-DPAT (10 nmol/1.0 microl/side) before two training sessions (Experiment 2a) or a CR session (Experiment 2b).

RESULTS

For Experiments 1a and 1b, the CS+ group preferred the CR+ lever across all sessions. Conditioned alcove approach during 1 or 2 training sessions or associated CR tests was low and nonspecific. In Experiment 2a, R(+) 7-OH-DPAT before training greatly diminished lever pressing during a subsequent CR test, preferentially on the CR+ lever. For Experiment 2b, R(+) 7-OH-DPAT infusions before the CR test also reduced lever pressing.

CONCLUSIONS

Manipulations of amygdala dopamine impact the earliest stage of learning in which emotional reactions may be most prevalent.

The psychological and neurochemical mechanisms of drug memory reconsolidation: implications for the treatment of addiction

Memory reconsolidation is the process by which memories, destabilised at retrieval, require restabilisation to persist in the brain. It has been demonstrated that even old, well-established memories require reconsolidation following retrieval; therefore, memory reconsolidation could potentially be exploited to disrupt, or even erase, aberrant memories that underlie psychiatric disorders, thereby providing a novel therapeutic target. Drug addiction is one such disorder; it is both chronic and relapsing, and one prominent risk factor for a relapse episode is the presentation of environmental cues that have previously been associated with drugs of abuse. This 'cue-induced relapse' can be accounted for in psychological terms by reinforcing memories of the pavlovian association between the cue and the drug, which can thus influence behaviour through at least three psychologically and neurobiologically dissociable mechanisms: conditioned reinforcement, conditioned approach and conditioned motivation. As each of these psychological processes could contribute to the resumption of drug-seeking following abstinence, it is important to develop treatments that can reduce drug-seeking re-established via influences on each or all of these pavlovian processes, in order to minimise the risk of a subsequent relapse. Investigation of the memory reconsolidation mechanisms of the memories underlying conditioned reinforcement, conditioned approach and conditioned motivation indicate that they depend upon different neurochemical systems, including the glutamatergic and adrenergic systems within limbic corticostriatal circuitry. We also discuss here the subsequent translation to the clinic of this preclinical work.

Involvement of amygdala dopamine and nucleus accumbens NMDA receptors in ethanol-seeking behavior in mice

Although progress has been made identifying neural mechanisms underlying ethanol's primary reinforcing effects, few studies have examined the mechanisms mediating ethanol-induced conditioned effects. A recent lesion study suggests that expression of ethanol-conditioned behaviors depends upon an intact amygdala and nucleus accumbens core. However, specific mechanisms within these nuclei are unknown. In the present experiments, we used site-specific microinfusions of dopamine and NMDA receptor antagonists to examine the roles of accumbens and amygdala in the expression of ethanol conditioned place preference (CPP) in mice. In experiments 1 and 2, a D1/D2/D3 receptor antagonist (flupenthixol) was infused into accumbens or amygdala before testing, whereas experiment 3 used pretest infusions of an NMDA antagonist (AP-5) to examine the role of intra-accumbens NMDA receptors. Dopamine antagonism of accumbens was without effect, but intra-amygdala infusions of flupenthixol blocked CPP expression. Moreover, this effect was dependent upon dopamine antagonism within the basolateral nucleus but not the central nucleus of the amygdala. Antagonism of NMDA receptors in accumbens also blocked CPP expression. The present findings suggest that expression of the ethanol-conditioned response depends upon amygdala dopamine and accumbens NMDA receptors. These are the first studies in any species to show a role for amygdala dopamine receptors and the first studies in mice to implicate accumbens NMDA receptors in ethanol-induced conditioned effects.

Afferent connections to the rostrolateral part of the periaqueductal gray: a critical region influencing the motivation drive to hunt and forage

Previous studies have shown that a particular site in the periaqueductal gray (PAG), the rostrolateral PAG, influences the motivation drive to forage or hunt. To have a deeper understanding on the putative paths involved in the decision-making process between foraging, hunting, and other behavioral responses, in the present investigation, we carried out a systematic analysis of the neural inputs to the rostrolateral PAG (rlPAG), using Fluorogold as a retrograde tracer. According to the present findings, the rlPAG appears to be importantly driven by medial prefrontal cortical areas involved in controlling attention-related and decision-making processes. Moreover, the rlPAG also receives a wealth of information from different amygdalar, hypothalamic, and brainstem sites related to feeding, drinking, or hunting behavioral responses. Therefore, this unique combination of afferent connections puts the rlPAG in a privileged position to influence the motivation drive to choose whether hunting and foraging would be the most appropriate adaptive responses.

Blockade of the acquisition, but not expression, of associative learning by pre-session intra-amygdala R(+) 7-OH-DPAT

RATIONALE

Two issues were addressed regarding the effects of amygdala dopamine manipulations on associative learning: first, an apparent contradiction between the effects of post- vs. pre-session dopaminergic manipulations and second, the ability of dopaminergic infusions to affect association formation vs. its expression following extended training.

OBJECTIVES

The ability of pre-session infusions of a dopamine receptor agonist (R(+) 7-OH-DPAT) to inhibit acquisition of a conditioned approach response was examined and compared with the same manipulation following overtraining. Further experiments extended these findings.

MATERIALS AND METHODS

Experiment 1 infused pre-session intra-amygdala R(+) 7-OH-DPAT (0, 0.1, 1 nmol) during conditioned approach acquisition. Experiment 2 applied pre-session intra-amygdala R(+) 7-OH-DPAT (0, 0.01, 0.1, 1 nmol) during expression of the same response, once well learned. Experiment 3 required the inhibition of a conditioned approach response following unconditioned stimulus (US) removal. Experiment 4 examined the ability of animals with prior drug experience to acquire a conditioned response to a novel stimulus.

RESULTS

Experiments 1-3 showed that pre-session amygdala R(+) 7-OH-DPAT impaired acquisition of either excitatory or inhibitory conditioned responding, but was ineffective following overtraining. Drug-induced impairments in acquisition of a specific conditioned stimulus (CS)-US relationship continued well beyond the cessation of drug treatment, but were found not to transfer to an alternate CS in Experiment 4.

CONCLUSIONS

Pre-session dopamine receptor activation within the amygdala may impair the acquisition, but not expression, of CS-US associations. Enhanced learning reported earlier following post-session dopamine receptor activation may occur indirectly through reduced interference with the consolidation of recent learning.

Individual differences in amphetamine self-administration: the role of the central nucleus of the amygdala

Rats categorized as high responder (HR), based on their activity in an inescapable novel environment, self-administer more amphetamine than low responder (LR) rats. The current study examined if the central nucleus of the amygdala (ACe) contributes to the elevated response for amphetamine in HR rats. Male Sprague-Dawley rats were classified as HR and LR rats based on their activity in inescapable novelty and novelty place preference, and then were trained to self-administer amphetamine (0.1 mg/kg/infusion). Once stable responding was achieved, rats received microinfusions of the GABA(A) agonist muscimol (0.5 microg/0.5 microl) or phosphate-buffered saline into the ACe immediately before self-administration of amphetamine (0.1, 0.03, 0.01, or 0.001 mg/kg/infusion) or saline. An additional group of rats was trained to lever press for sucrose rather than amphetamine. Based on the inescapable novelty test, HR rats self-administered more amphetamine than LR rats at the 0.03 and 0.01 mg/kg/infusion unit doses; there were no significant individual differences in amphetamine self-administration based on the novelty place preference test. Inactivation of the ACe with muscimol decreased self-administration at the 0.03 and 0.01 mg/kg/infusion unit doses in HR rats, but had no effect on LR rats. ACe inactivation had no reliable effect on inactive lever responding and appeared to be region specific based on anatomical controls. In addition, while inactivation of the ACe decreased responding for sucrose, inactivation did not differentially affect HR and LR rats. These results suggest that the ACe contributes to the elevated rate of amphetamine self-administration in HR rats.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Blockade of nicotinic acetylcholine or dopamine D1-like receptors in the central nucleus of the amygdala or the bed nucleus of the stria terminalis does not precipitate nicotine withdrawal in nicotine-dependent rats

Approximately 70% of tobacco smokers wish to quit, but attempts are often unsuccessful partly due to the aversive nicotine withdrawal syndrome. We investigated the possible involvement of nicotinic and dopaminergic signalling in the central nucleus of the amygdala (CeA) and dorsolateral bed nucleus of the stria terminalis (dlBNST) in the anhedonic depression-like effect of precipitated nicotine withdrawal in rats. Nicotine-dependent rats exhibit elevations in intracranial self-stimulation (ICSS) thresholds compared to control rats after cessation of chronic nicotine administration (spontaneous withdrawal) or systemic or intra-ventral tegmental area (VTA), but not intra-nucleus accumbens (NAcc), administration of nicotinic acetylcholine receptor (nAchR) antagonists while exposed to nicotine (precipitated withdrawal). We examined whether intracerebral administration of the nAChR antagonist dihydro-beta-erythroidine (DHbetaE; 0.6-20 microg total bilateral dose) or the dopamine D1-like receptor antagonist SCH 23390 (2-16 microg total bilateral dose) into the CeA and dlBNST results in withdrawal-like threshold elevations in nicotine-treated rats. Nicotinic acetylcholine and D1-like receptor blockade in the CeA or the dlBNST did not induce differential threshold elevations in nicotine- and saline-treated rats. Further, the highest SCH 23390 dose (16 microg bilateral dose) injected into the dlBNST, but not the CeA, elevated thresholds similarly in both saline- and nicotine-treated rats, suggesting that dopaminergic signalling in the dlBNST may regulate brain reward function under baseline conditions. These results suggest that nACh and D1-like signalling in the CeA and the dlBNST does not develop neuroadaptations with the development of nicotine dependence that may be involved in the depression-like aspects of nicotine withdrawal.

Evolution of the amygdala: new insights from studies in amphibians

The histology of amphibian brains gives an impression of relative simplicity when compared with that of reptiles or mammals. The amphibian telencephalon is small and contains comparatively few and large neurons, which in most parts constitute a dense periventricular cellular layer. However, the view emerging from the last decade is that the brains of all tetrapods, including amphibians, share a general bauplan resulting from common ancestry and the need to perform similar vital functions. To what extent this common organization also applies to higher brain functions is unknown due to a limited knowledge of the neurobiology of early vertebrates. The amygdala is widely recognized as a brain center critical for basic forms of emotional learning (e.g., fear conditioning) and its structure in amphibians could suggest how this capacity evolved. A functional systems approach is used here to synthesize the results of our anatomical investigations of the amphibian amygdala. It is proposed that the connectivity of the amphibian telencephalon portends a capacity for multi-modal association in a limbic system largely similar to that of amniote vertebrates. One remarkable exception is the presence of new sensory-associative regions of the amygdala in amniotes: the posterior dorsal ventricular ridge plus lateral nuclei in reptiles and the basolateral complex in mammals. These presumably homologous regions apparently are capable of modulating the phylogenetically older central amygdala and allow more complex forms of emotional learning.

Persistent changes in motivation to self-administer cocaine following modulation of cyclic AMP-dependent protein kinase A (PKA) activity in the nucleus accumbens

Drug-induced neuroadaptations within the nucleus accumbens, including activation of cAMP-dependent protein kinase A (PKA), may contribute to the synaptic plasticity and behavioural changes that underlie drug addiction. As a direct test of this hypothesis, we examined the effects in rats of PKA activation (Sp-cAMPS infusions of 10 and 20 nmol/side) and inhibition (Rp-cAMPS infusions of 10 and 20 nmol/side) in the nucleus accumbens on motivation to obtain cocaine as measured by responding under the progressive-ratio schedule. Bilateral infusions of Sp-cAMPS (20 nmol/side) resulted in an increase in progressive-ratio responding for cocaine and this effect persisted for several days. In contrast, Rp-cAMPS (20 nmol/side) produced persistent decreases in progressive-ratio responding for cocaine beginning on the day of administration and lasting for several days. These data suggest that alternations in PKA activity within the nucleus accumbens as a consequence of repeated cocaine exposure may contribute to addiction by producing persistent increases in motivation to obtain cocaine.

Dopamine D1 or D2 receptor antagonism within the basolateral amygdala differentially alters the acquisition of cocaine-cue associations necessary for cue-induced reinstatement of cocaine-seeking

The basolateral amygdala complex has been implicated in the formation and utilization of cocaine-cue associations in rat models of cue-induced reinstatement to cocaine-seeking behavior. We have previously demonstrated the importance of dopamine inputs to the basolateral amygdala complex in the reinstatement of cocaine-seeking behavior following chronic cocaine self-administration. Here we show that selective blockade of amygdalar dopamine D1 and D2 receptors during acquisition of cocaine-cue associations has distinctive effects on subsequent conditioned-cued cocaine-seeking behavior. Male, Sprague-Dawley rats were first trained to self-administer i.v. cocaine on a fixed ratio 1 schedule for 5 days. Subjects then received bilateral, intra-basolateral amygdala complex infusions of a dopamine D1 receptor antagonist (SCH23390, 0.25-2.0 microg/side; experiment 1), a dopamine D2 receptor antagonist (raclopride, 0.625-5.0 microg/side; experiment 2), or vehicle just prior to a single classical conditioning session, during which a light+tone cue was discretely paired with passive infusions of cocaine in the absence of lever responding. Following five additional days of cocaine self-administration and 7-10 days of extinction training, animals underwent multiple tests for cue-induced reinstatement. SCH23390 (2.0 microg/side), administered at the time of cocaine-cue association only, produced an attenuation of reinstatement to cue-induced cocaine-seeking behavior. In contrast, low doses of raclopride potentiated, while a higher dose of raclopride attenuated cue-induced reinstatement. These results demonstrate unique contributions of D1 vs. D2 receptors in mediating dopamine inputs within the basolateral amygdala complex during the formation of cocaine-stimulus associations that are critical for cue-induced reinstatement.

Functional mapping of the prosencephalic systems involved in organizing predatory behavior in rats

The study of the neural basis of predatory behavior has been largely neglected over the recent years. Using an ethologically based approach, we presently delineate the prosencephalic systems mobilized during predation by examining Fos immunoreactivity in rats performing insect hunting. These results were further compared with those obtained from animals killed after the early nocturnal surge of food ingestion. First, predatory behavior was associated with a distinct Fos up-regulation in the ventrolateral caudoputamen at intermediate rostro-caudal levels, suggesting a possible candidate to organize the stereotyped sequence of actions seen during insect hunting. Insect predation also presented conspicuous mobilization of a neural network formed by a distinct amygdalar circuit (i.e. the postpiriform-transition area, the anterior part of cortical nucleus, anterior part of basomedial nucleus, posterior part of basolateral nucleus, and medial part of central nucleus) and affiliated sites in the bed nuclei of the stria terminalis (i.e. the rhomboid nucleus) and in the hypothalamus (i.e. the parasubthalamic nucleus). Accordingly, this network is likely to encode prey-related motivational values, such as prey's odor and taste, and to influence autonomic and motor control accompanying predatory eating. Notably, regular food intake was also associated with a relatively weak Fos up-regulation in this network. However, during regular surge of food intake, we observed a much larger mobilization in hypothalamic sites related to the homeostatic control of eating, namely, the arcuate nucleus and autonomic parts of the paraventricular nucleus. Overall, the present findings suggest potential neural systems involved in integrating prey-related motivational values and in organizing the stereotyped sequences of action seen during predation. Moreover, the comparison with regular food intake contrasts putative neural mechanisms controlling predatory related eating vs. regular food intake.

Extracellular level of basolateral amygdalar dopamine responding to reversal of appetitive-conditioned discrimination in young and old rats

Young and old rats, aged 3 and 24 months old, respectively, were conditioned to press a lever under lamp-on conditions for reward acquisition and lamp-off for no reward using a variable interval reinforcement schedule that averaged 15 s; i.e., the minimal requirement was 4 responses/min. Over a 30-day period, young and old groups showed increased response to lamp-on from 22 to 35/min and from 10 to 23/min, respectively, and shortened response to lamp-off after initial training. Response to lamp-on as a percentage of total response to lamp-on and -off (the discrimination ratio) was over 80%. For the next 30 days, reversal learning was imposed to reinforce discrimination of the lamp-off state. Young rats showed a steadily increasing discrimination ratio from 40% to 70%, and old rats from 30% to 60%. In comparison with the initial training, young rats showed a total response increase from 50% to 60%, while old rats showed an approximately 5% decrease without loss of reward-obtaining efficiency. In vivo microdialysis during reversal revealed that young rats had higher dopamine transmission in the basolateral amygdala than old rats. The dopamine level was positively correlated with the number of responses to state of reward in young rats and negatively with the numbers of both NRF and RF responses to lamp-on and -off states in old rats. These results suggest that in reversal discrimination, basolateral amygdalar dopamine efflux correlates with the manner of age-related conditioned response rather than the ability to learn.

Direct interactions between the basolateral amygdala and nucleus accumbens core underlie cocaine-seeking behavior by rats

An insidious feature of drug craving and drug seeking in humans is that it can be induced and maintained by conditioned stimuli after a prolonged drug-free period. Understanding the neural basis of this control over addictive behavior may aid in the development of treatments targeting drug seeking and thereby be beneficial in preventing drug use. In the present study, we used a well established animal model to investigate the functional importance of amygdala-nucleus accumbens interactions in cocaine seeking under the control of drug-associated conditioned reinforcers. To probe the direct neuroanatomical relationship between these structures within a functional corticostriatal loop, we used a neuropharmacological disconnection procedure. Thus, infusing a dopamine receptor antagonist unilaterally into the basolateral amygdala (which had no effect on its own) and an AMPA-kainate (KA) receptor antagonist into the contralateral nucleus accumbens core (which also had no effect on its own) greatly reduced cocaine seeking. We also show that bilateral infusions of the DA receptor antagonist into the amygdala, but not nucleus accumbens, or of the AMPA-KA receptor antagonist in the nucleus accumbens, but not the amygdala, also greatly reduced cocaine seeking. The results of this study demonstrate an amygdala-nucleus accumbens system that critically underlies stimulus-controlled cocaine seeking and indicate possible neurochemical targets for relapse-prevention medication.

The role of signaling molecules in reward-related incentive learning

Reward-related incentive learning involves the acquisition by neutral stimuli of an enhanced ability to elicit approach and other responses. Previous studies have shown that both dopamine (DA) and glutamate (Glu) play critical roles in this type of learning. Signaling molecules are intracellular messengers that participate in the influence of transmitter-receptor events on intracellular function including transcription in the nucleus. In recent years studies have begun to implicate signaling molecules in incentive learning. Thus, inhibition of cyclic adenosine monophosphate-dependent protein kinase (PKA) in the nucleus accumbens (NAc), that is activated by DA acting at D1-like receptors, blocks the acquisition of conditioned approach responses, lever pressing for food, conditioned place preference (CPP) based on NAc injections of amphetamine or cocaine, and conditioned activity based on NAc injections of amphetamine. Similar effects have been observed with PKA inhibition in the basolateral amygdala or medial prefrontal cortex. If animals were trained prior to testing with PKA inhibitors in NAc, no effect was seen suggesting that PKA is more important for acquisition than expression of incentive learning. Inhibition of calcium-dependent protein kinase or mitogen-activated protein kinases in NAc similarly has been shown to block the acquisition of incentive learning. Results support a model of DA-Glu synaptic interactions that form the basis of incentive learning.

Facilitation of appetitive pavlovian conditioning by d-amphetamine in the shell, but not the core, of the nucleus accumbens

The effects of postsession d-amphetamine within subregions of the ventral and dorsal striatum on appetitive Pavlovian learning were assessed. Rats acquired a conditioned approach response on presentation of a stimulus predictive of 10% sucrose solution (unconditioned stimulus [US]), but not during equally frequent presentations of a stimulus uncorrelated with the US. In Experiment 1, postsession d-amphetamine infusions enhanced acquisition of conditioned responding, with no effect on control measures. In Experiment 2, rats received postsession d-amphetamine in the accumbens shell or core. Shell infusions facilitated conditioning; core infusions did not. In Experiment 3, dorsomedial striatal infusions of d-amphetamine also were ineffective. In sum, dopaminergic activation within the shell, but not the core, of the nucleus accumbens facilitates the acquisition of a Pavlovian association.

A role for the periaqueductal grey in opioidergic inhibition of maternal behaviour

Opiates are known to be involved in the regulation of various events surrounding parturition and lactation, such as maternal behaviour in rats. The onset of this behaviour has been closely linked to opiate action in the medial pre-optic area, where administration of morphine disrupts maternal behaviour during lactation. By combining the use of Fos protein immunohistochemical detection and pharmacological manipulations, in the present paper we show that the periaqueductal grey (PAG) is another region critically involved in the opioidergic blockade of maternal behaviour. According to our observations, a critical level of morphine-induced activation of the rostral lateral PAG appears to be required to inhibit maternal behaviour in lactating rats. This hypothesis was further confirmed in experiments showing that morphine's inhibitory effect on maternal responsiveness was blocked by unilateral naloxone injection into the rostral PAG, but not into nearby regions of the mesencephalic reticular nucleus. Therefore, only a partial inhibition of the opiate's effect on the rostral PAG was needed to block the inhibitory effect of morphine on maternal behaviour. Further studies are needed to ascertain whether the rostral lateral PAG plays a role in the natural onset of maternal behaviour, playing a complementary role to the medial pre-optic area, or merely inhibits maternal behaviour in response to this specific pharmacological challenge. Conversely, the present findings may well reflect a more general role of the PAG, seemingly providing an important piece of information for proposing a hitherto unexplored concept of the PAG as an important centre for the selection of adaptive behavioural responses.

Cocaine-seeking behavior after extended cocaine-free periods in rats: role of conditioned stimuli

RATIONALE

Cocaine abstinence symptoms and conditioned stimuli (CSs) previously associated with cocaine administration are postulated to contribute to relapse to drug taking in humans.

OBJECTIVE

The present study assessed the role of both non-contingent CS presentation and experimenter-imposed extended cocaine-free periods on cocaine-seeking behavior in rats.

METHODS

A fixed interval (FI) second-order schedule of intravenous cocaine (0.5 mg/infusion) reinforcement of the type FI 15 min (fixed ratio 8:S) was used.

RESULTS

Non-contingent CS presentation before exposure to a cocaine binge had no effect on responding under the second-order schedule of reinforcement for cocaine after 23 h of no access to cocaine. By contrast, six non-contingent presentations of the CS during a 1-min period before the test session increased the number of responses in both no-binge (daily 2-h sessions, five infusions) and binge (two 12-h overnight sessions; maximum 48 infusions) exposed rats on day 7 of the cocaine-free period compared to no-binge- and binge-exposed rats that were not presented with the CSs. On day 30 of the cocaine-free period, only binge-exposed rats presented with the CSs exhibited a tendency for increased level of responding.

CONCLUSIONS

The results indicated that non-contingent CS presentation had no effect after 23 h of no access to cocaine, increased drug-seeking behavior on day 7 of the cocaine-free period independent of binge exposure, and a strong tendency to increase drug-seeking behavior only in binge-exposed rats, on day 30 of the cocaine-free period, illustrating the interactive effects of conditioned stimuli with the extended cocaine-free period.

Immunohistochemical assessment of mesotelencephalic dopamine activity during the acquisition and expression of Pavlovian versus instrumental behaviours

Dopaminergic activity during Pavlovian or instrumental learning in key target regions of the mesotelencephalic dopamine system was investigated immunohistochemically using antibodies raised against glutaraldehyde-conjugated dopamine. Experiment 1 examined dopamine immunoreactivity during acquisition of a Pavlovian conditioned-approach response. Observations were taken at three stages of learning: initial, intermediate and asymptotic; each with a conditioned stimulus+ (CS+) group for whom visual or auditory stimuli immediately preceded an unconditioned stimulus (sucrose), and a conditioned stimulus- (CS-) group for whom stimuli and the unconditioned stimulus were unpaired. Animals learned to approach the alcove during CS+ presentations, whilst approach behaviour of the CS- group remained low. In general, target regions exhibiting a dopaminergic reaction responded maximally during the intermediate stage of acquisition, and were less responsive initially, and not responsive at all at asymptote. Specifically, the pattern of dopaminergic response was: shell more than core of the nucleus accumbens; prefrontal cortex, central and basolateral nuclei of the amygdala also significantly responsive. Mediodorsal and laterodorsal striatal regions were reactive only very early in training. Experiment 2 examined dopaminergic reaction following acquisition of a novel conditioned instrumental response. The conditioned response+ (CR+) group responded at a much higher rate on the lever for which unconditioned stimulus-associated stimuli were presented, than on the control lever. The conditioned response- (CR-) group responded at a low rate on both levers. In contrast with experiment 1, the most responsive regions were the core of the nucleus accumbens, medial prefrontal cortex and basolateral area of the amygdala. Thus, the acquisition, but not expression of Pavlovian associations activated dopamine within several key target regions of the mesotelencephalic dopamine system, and preferentially within the shell rather than core of the nucleus accumbens. By contrast, acquisition of a novel instrumental response preferentially activated the core of the nucleus accumbens, and basolateral area of the amygdala. These data carry significant implications for the potential role of these regions in learning and memory.

An immunohistochemical examination of the effects of sensitisation on the mesotelencephalic dopaminergic response to d-amphetamine

The dopaminergic response to d-amphetamine with or without prior repeated experience with the drug was investigated immunohistochemically in key target regions of the mesotelencephalic dopamine system using antibodies raised against glutaraldehyde-conjugated dopamine. This methodology permitted the unambiguous determination of dopaminergic activity within specific subregions of structures implicated in the behavioural effects of psychomotor stimulants drugs, and in the expression of behavioural sensitisation. Experiment 1 examined dopamine immunoreactivity in central or basolateral amygdala, shell or core of the nucleus accumbens, medial and lateral caudate-putamen and medial prefrontal cortex following the administration of various doses of d-amphetamine to drug-naïve rats. Whilst dose-related increases in dopaminergic activity were detected in all regions examined, a regional heterogeneity was clearly evident. For example, d-amphetamine enhanced dopaminergic activity preferentially within the shell subregion of the nucleus accumbens both with respect to the core subregion and to other striatal and non-striatal areas. Experiment 2 examined changes in dopaminergic activity following the administration of a low dose of d-amphetamine to d-amphetamine-sensitised rats and saline-pretreated control animals. Regional heterogeneity both between and within terminal areas was again detected. Thus, there was evidence of a preferential increase in dopaminergic activity within the shell of the nucleus accumbens of sensitised rats. Moreover, sensitisation to d-amphetamine increased the dopaminergic response to acute administration of d-amphetamine within all striatal and non-striatal areas examined. Comparison of this effect across subterritories of the areas under investigation revealed that in sensitised rats, acute administration of d-amphetamine elevated dopaminergic activity within the shell of the nucleus accumbens to a greater extent than within the core. These data therefore indicate that systemic administration of d-amphetamine is associated with regionally heterogeneous changes in dopaminergic activity within terminal regions of the mesotelencephalic dopamine system in both sensitised and unsensitised rats. Moreover, the present methodology permitted resolution of these changes at an anatomical level beyond that of conventional approaches.

Attenuation of cue-controlled cocaine-seeking by a selective D3 dopamine receptor antagonist SB-277011-A

Conditioned stimuli (CS) previously paired with drugs of abuse can elicit cravings in humans, relapse to drug use, and can also reinforce drug-seeking behavior in both humans and animals, events that are believed to be subserved in part by activation of the mesolimbic dopamine system. Converging anatomical, pharmacological, and behavioral evidence implicates dopamine D(3) receptors in the mechanisms underlying cue-controlled behaviors. The purpose of the present study was therefore to investigate the effects on cocaine-seeking behavior of a novel D(3) receptor antagonist, SB-277011-A, which is 100-fold more selective for D(3) over D(2) dopamine receptors. We have established previously that second-order schedules of reinforcement provide an animal model of cue-controlled drug-seeking both prior to and after cocaine has been self-administered. SB-277011-A dose-dependently decreased cocaine-seeking maintained by a cocaine-associated conditioned reinforcer in both the first, drug-free interval and also following self-administration of cocaine. At higher doses, SB-277011-A also increased the latency to receive the first CS presentation and cocaine infusion, thereby decreasing the number of cocaine infusions self-administered under the second-order schedule of reinforcement. SB-277011-A had no effect on cocaine intake under an FR-1 schedule of reinforcement, or on responding for sucrose under a second-order schedule of reinforcement, at any dose tested. These results therefore suggest that D(3) dopamine receptors may be critically involved in cue-controlled drug-seeking behavior independently of any interaction with the reinforcing effects of cocaine itself, and may therefore provide a therapeutic target in the treatment of relapse to cocaine use induced by CSs.

Predatory hunting and exposure to a live predator induce opposite patterns of Fos immunoreactivity in the PAG

Considering the periaqueductal gray's (PAG) general roles in mediating motivational responses, in the present study, we compared the Fos expression pattern in the PAG induced by innate behaviors underlain by opposite motivational drivers, in rats, namely, insect predation and defensive behavior evoked by the confrontation with a live predator (a cat). Exposure to the predator was associated with a striking Fos expression in the PAG, where, at rostral levels, an intense Fos expression was found largely distributed in the dorsomedial and dorsolateral regions, whereas, at caudal levels, Fos-labeled cells tended to be mostly found in the lateral and ventrolateral columns, as well as in the dorsal raphe nucleus. Quite the opposite, insect predation was associated with increased Fos expression predominantly in the rostral two thirds of the lateral PAG, where the majority of the Fos-immunoreactive cells were found at the oculomotor nucleus levels. Remarkably, both exposure to the cat and insect predation upregulated Fos expression in the supraoculomotor region and the laterodorsal tegmental nucleus. Overall, the present results clearly suggest that the PAG activation pattern appears to reflect, at least partly, the animal's motivational status. It is well established that the PAG is critical for the expression of defensive responses, and, considering the present findings, it will be important to investigate how the PAG contributes to the expression of the predatory behavior, as well.

Effects of excitotoxic lesions of the central or basolateral nucleus of the amygdala on naloxone-precipitated withdrawal-induced conditioned place aversion in morphine-dependent rats

We examined the effects of discrete, bilateral excitotoxic lesions of the central or basolateral nucleus of the amygdala on naloxone-precipitated withdrawal-induced conditioned place aversion in morphine-dependent rats. Lesions of the central nucleus significantly attenuated the conditioned place aversion, while lesions of the basolateral nucleus had little effect. These results suggest that the central nucleus of the amygdala, rather than the basolateral nucleus, plays a crucial role in the negative affective component of morphine abstinence.

Nucleus accumbens shell and core dopamine: differential role in behavior and addiction

Drug addiction can be conceptualized as a disturbance of behavior motivated by drug-conditioned incentives. This abnormality has been explained by Incentive-Sensitization and Allostatic-Counteradaptive theories as the result of non-associative mechanisms acting at the stage of the expression of incentive motivation and responding for drug reinforcement. Each one of these theories, however, does not account per se for two basic properties of the motivational disturbance of drug addiction: (1). focussing on drug- at the expenses of non-drug-incentives; (2). virtual irreversibility. To account for the above aspects we have proposed an associative learning hypothesis. According to this hypothesis the basic disturbance of drug addiction takes place at the stage of acquisition of motivation and in particular of Pavlovian incentive learning. Drugs share with non-drug rewards the property of stimulating dopamine (DA) transmission in the nucleus accumbens shell but this effect does not undergo habituation upon repeated drug exposure, as instead is the case of non-drug rewards. Repetitive, non-decremental stimulation of DA transmission by drugs in the nucleus accumbens septi (NAc) shell abnormally strengthens stimulus-drug associations. Thus, stimuli contingent upon drug reward acquire powerful incentive properties after a relatively limited number of predictive associations with the drug and become particularly resistant to extinction. Non-contingent occurrence of drug-conditioned incentive cues or contexts strongly facilitates and eventually reinstates drug self-administration. Repeated drug exposure also induces a process of sensitization of drug-induced stimulation of DA transmission in the NAc core. The precise significance of this adaptive change for the mechanism of drug addiction is unclear given the complexity and uncertainties surrounding the role of NAc core DA in responding but might be more directly related to instrumental performance.

Appetitive conditioning-induced plasticity is expressed during paradoxical sleep in the medial geniculate, but not in the lateral amygdala

This study examined whether neurons in the medial division of the medial geniculate (MGm) and the dorsal part of the lateral amygdala (LAd) express learning-induced plasticity in paradoxical sleep (PS) after appetitive conditioning, as they do in PS after fear conditioning. Rats received tone-food pairings in 3 sessions. After each session, the tone was presented at a nonawakening intensity during PS. Multiunit activity was simultaneously recorded in MGm and LAd. During waking, increases in tone-evoked discharges developed in MGm and LAd; however, as training continued, they lessened in LAd, but not in MGm. During PS, conditioned tone responses were expressed in MGm, but not in LAd. Thus, these results demonstrate dissociation of MGm and LAd plasticity. Moreover, compared with fear conditioning results, they suggest that expression of amygdalar plasticity in PS depends on the emotional salience of the stimulus.

Blockade of sensitisation-induced facilitation of appetitive conditioning by post-session intra-amygdala nafadotride

Prior D-amphetamine experience has been reported to enhance appetitive Pavlovian conditioning. The present study assessed the involvement of the mesoamygdaloid dopamine projection in this effect. Bilateral post-session intra-amygdala infusions of the D3 dopamine receptor antagonist, L-nafadotride, or vehicle were given during acquisition of a Pavlovian association in sensitised and unsensitised rats. During these sessions, subjects received presentations of a stimulus (CS(+)) paired with 10% sucrose availability. A second stimulus (CS(-)) was also presented but never paired with sucrose. Sensitised animals infused post-session with vehicle acquired a Pavlovian conditioned approach response during CS(+) presentations more rapidly than controls, as we have shown previously. However, post-session intra-amygdala L-nafadotride selectively retarded conditioned responding to the CS(+) in both groups of animals, abolishing the difference between sensitised and unsensitised rats. These results, therefore, extend the evidence for the involvement of the mesoamygdaloid dopamine projection in Pavlovian conditioning, and the facilitation of associative learning following sensitisation.

Isolation rearing-induced facilitation of Pavlovian learning: abolition by postsession intra-amygdala nafadotride

It has been shown previously in this laboratory that rats reared in social isolation acquire a Pavlovian-conditioned approach task much more rapidly than their respective controls. This study assessed the involvement specifically of the mesoamygdaloid dopamine pathway in this facilitated learning of isolates. Thus, animals were required to associate arbitrary stimuli with a pulsed light stimulus (unconditioned stimulus, US). The US, while without biological significance, was nevertheless capable of eliciting an intrinsic and sustained alerting response. Procedures ensured that the arbitrary stimuli (tone or clicker) did not elicit a response in the first instance, and were presented either paired (CS+) or unpaired (CS-) with the US. Isolates and socially reared controls received intra-amygdala infusions of the D3 dopamine receptor antagonist, L-nafadotride, or vehicle immediately following the end of each training session. The conditioned response increased over sessions in both groups of vehicle-infused rats during presentations of the CS+ stimulus, but not CS-, and isolates acquired this association more rapidly than controls. However, acquisition of this association was abolished by postsession intra-amygdala L-nafadotride. Responding to the US was largely unaffected by drug or rearing conditions. Hence, these data provide strong evidence for the specific involvement of the mesoamygdaloid dopamine projection in the facilitation of associative learning by isolation rearing.