Intra-amygdala infusions of scopolamine impair performance on a conditioned place preference task but not a spatial radial maze task

Functional neuroanatomy of basal forebrain projections to the basolateral amygdala: Transmitters, receptors, and neuronal subpopulations

The projections of the basal forebrain (BF) to the hippocampus and neocortex have been extensively studied and shown to be important for higher cognitive functions, including attention, learning, and memory. Much less is known about the BF projections to the basolateral nuclear complex of the amygdala (BNC), although the cholinergic innervation of this region by the BF is actually far more robust than that of cortical areas. This review will focus on light and electron microscopic tract-tracing and immunohistochemical (IHC) studies, many of which were published in the last decade, that have analyzed the relationship of BF inputs and their receptors to specific neuronal subtypes in the BNC in order to better understand the anatomical substrates of BF-BNC circuitry. The results indicate that BF inputs to the BNC mainly target the basolateral nucleus of the BNC (BL) and arise from cholinergic, GABAergic, and perhaps glutamatergic BF neurons. Cholinergic inputs mainly target dendrites and spines of pyramidal neurons (PNs) that express muscarinic receptors (MRs). MRs are also expressed by cholinergic axons, as well as cortical and thalamic axons that synapse with PN dendrites and spines. BF GABAergic axons to the BL also express MRs and mainly target BL interneurons that contain parvalbumin. It is suggested that BF-BL circuitry could be very important for generating rhythmic oscillations known to be critical for emotional learning. BF cholinergic inputs to the BNC might also contribute to memory formation by activating M1 receptors located on PN dendritic shafts and spines that also express NMDA receptors.

Post-training intra-basolateral complex of the amygdala infusions of clenbuterol enhance memory for conditioned place preference and increase ARC protein expression in dorsal hippocampal synaptic fractions

The basolateral complex of the amygdala (BLA) is critically involved in modulation of memory by stress hormones. Noradrenergic activation of the BLA enhances memory consolidation and plays a necessary role in the enhancing or impairing effects of stress hormones on memory. The BLA is not only involved in the consolidation of aversive memories but can regulate appetitive memory formation as well. Extensive evidence suggests that the BLA is a modulatory structure that influences consolidation of arousing memories through modulation of plasticity and expression of plasticity-related genes, such as the activity regulated cytoskeletal-associated (Arc/Arg 3.1) protein, in efferent brain regions. ARC is an immediate early gene whose mRNA is localized to the dendrites and is necessary for hippocampus-dependent long-term potentiation and long-term memory formation. Post-training intra-BLA infusions of the β-adrenoceptor agonist, clenbuterol, enhances memory for an aversive task and increases dorsal hippocampus ARC protein expression following training on that task. To examine whether this function of BLA noradrenergic signaling extends to the consolidation of appetitive memories, the present studies test the effect of post-training intra-BLA infusions of clenbuterol on memory for the appetitive conditioned place preference (CPP) task and for effects on ARC protein expression in hippocampal synapses. Additionally, the necessity of increased hippocampal ARC protein expression was also examined for long-term memory formation of the CPP task. Immediate post-training intra-BLA infusions of clenbuterol (4 ng/0.2 µL) significantly enhanced memory for the CPP task. This same memory enhancing treatment significantly increased ARC protein expression in dorsal, but not ventral, hippocampal synaptic fractions. Furthermore, immediate post-training intra-dorsal hippocampal infusions of Arc antisense oligodeoxynucleotides (ODNs), which reduce ARC protein expression, prevented long-term memory formation for the CPP task. These results suggest that noradrenergic activity in the BLA influences long-term memory for aversive and appetitive events in a similar manner and the role of the BLA is conserved across classes of memory. It also suggests that the influence of the BLA on hippocampal ARC protein expression and the role of hippocampal ARC protein expression are conserved across classes of emotionally arousing memories.

Instrumental and Pavlovian Mechanisms in Alcohol Use Disorder

Purpose of Review

Current theories of alcohol use disorders (AUD) highlight the importance of Pavlovian and instrumental learning processes mainly based on preclinical animal studies. Here, we summarize available evidence for alterations of those processes in human participants with AUD with a focus on habitual versus goal-directed instrumental learning, Pavlovian conditioning, and Pavlovian-to-instrumental transfer (PIT) paradigms.

Recent Findings

The balance between habitual and goal-directed control in AUD participants has been studied using outcome devaluation or sequential decision-making procedures, which have found some evidence of reduced goal-directed/model-based control, but little evidence for stronger habitual responding. The employed Pavlovian learning and PIT paradigms have shown considerable differences regarding experimental procedures, e.g., alcohol-related or conventional reinforcers or stimuli.

Summary

While studies of basic learning processes in human participants with AUD support a role of Pavlovian and instrumental learning mechanisms in the development and maintenance of drug addiction, current studies are characterized by large variability regarding methodology, sample characteristics, and results, and translation from animal paradigms to human research remains challenging. Longitudinal approaches with reliable and ecologically valid paradigms of Pavlovian and instrumental processes, including alcohol-related cues and outcomes, are warranted and should be combined with state-of-the-art imaging techniques, computational approaches, and ecological momentary assessment methods.

Acetylcholine is released in the basolateral amygdala in response to predictors of reward and enhances the learning of cue-reward contingency

The basolateral amygdala (BLA) is critical for associating initially neutral cues with appetitive and aversive stimuli and receives dense neuromodulatory acetylcholine (ACh) projections. We measured BLA ACh signaling and activity of neurons expressing CaMKIIα (a marker for glutamatergic principal cells) in mice during cue-reward learning using a fluorescent ACh sensor and calcium indicators. We found that ACh levels and nucleus basalis of Meynert (NBM) cholinergic terminal activity in the BLA (NBM-BLA) increased sharply in response to reward-related events and shifted as mice learned the cue-reward contingency. BLA CaMKIIα neuron activity followed reward retrieval and moved to the reward-predictive cue after task acquisition. Optical stimulation of cholinergic NBM-BLA terminal fibers led to a quicker acquisition of the cue-reward contingency. These results indicate BLA ACh signaling carries important information about salient events in cue-reward learning and provides a framework for understanding how ACh signaling contributes to shaping BLA responses to emotional stimuli.

Neuropharmacological evaluation on four traditional herbs used as nervine tonic and commonly available as Shankhpushpi in India

Background

Shankhpushpi is an Ayurvedic drug, widely used for its actions on the central nervous system, especially to improve intellect and boost memory. Four botanicals viz. Canscora decussata Schult. (CD), Clitorea ternatea Linn. (CT), Convolvulus pluricaulis Choisy. (CP) and Evolvulus alsinoides Linn. (EA) are considered as sources of Shankhpushpi by Indian practitioners on the basis of their morphological descriptions given in ancient texts.

Objective

The present study was undertaken to evaluate the neuropharmacological effect of four herbs commonly identified as source of Shankhpushpi.

Materials and methods

Methanol extracts of all four varieties were tested and evaluated in vitro and in vivo for their neuropharmacological effects. Experiments such as protection against β-amyloid induced neurotoxicity on brain cell line (Neuro 2A), antioxidant potential, AchE (acetylcholinesterase enzyme) inhibition, and 5-LOX (lipoxygenase) enzyme inhibition were conducted for in vitro evaluation. For in vivo evaluation, scopolamine (0.3 mg/kg i.p.) induced memory retrieval using pole climbing apparatus and Morris water maze were performed in rat models.

Results

It was found that protective effects of EA and CD against β-amyloid induced neurotoxicity in Neuro 2A cells were significantly higher than CT and CP. EA proved to be superior than other varieties on the basis of antioxidant activity, AchE inhibitory and LOX inhibitory activities. The preventive activity of EA on scopolamine induced memory retrieval in pole climbing and Morris water maze task in rats was found to be higher than that of CD, CT and CP.

Conclusion

EA has remarkable neuropharmacological effect as compared to other three varieties of Shankhpushpi. This effect may be attributed due to the presence of steroids (stigmasterol and betulinic acid), coumarins (scopoletin) and flavonoids (β-carotene and chlorogenic acid). Hence it can be used as a promising lead in development and management of neuronal disorders including Alzheimer's disease.

Afferent and Efferent Connections of the Cortex-Amygdala Transition Zone in Mice

The transitional zone between the ventral part of the piriform cortex and the anterior cortical nucleus of the amygdala, named the cortex-amygdala transition zone (CxA), shows two differential features that allow its identification as a particular structure. First, it receives dense cholinergic and dopaminergic innervations as compared to the adjacent piriform cortex and amygdala, and second, it receives projections from the main and accessory olfactory bulbs. In this work we have studied the pattern of afferent and efferent projections of the CxA, which are mainly unknown, by using the retrograde tracer Fluorogold and the anterograde tracer biotinylated dextranamine. The results show that the CxA receives a relatively restricted set of intratelencephalic connections, originated mainly by the olfactory system and basal forebrain, with minor afferents from the amygdala. The only relevant extratelencephalic afference originates in the ventral tegmental area (VTA). The efferent projections of the CxA reciprocate the inputs from the piriform cortex and olfactory amygdala. In addition, the CxA projects densely to the basolateral amygdaloid nucleus and the olfactory tubercle. The extratelencephalic projections of the CxA are very scarce, and target mainly hypothalamic structures. The pattern of connections of the CxA suggests that it is indeed a transitional area between the piriform cortex and the cortical amygdala. Double labeling with choline acetyltransferase indicates that the afferent projection from the basal forebrain is the origin of its distinctive cholinergic innervation, and double labeling with dopamine transporter shows that the projection from the VTA is the source of dopaminergic innervation. These connectivity and neurochemical features, together with the fact that it receives vomeronasal in addition to olfactory information, suggest that the CxA may be involved in processing olfactory information endowed with relevant biological meaning, such as odors related to reproductive or defensive behaviors.

Input-specific contributions to valence processing in the amygdala

Reward and punishment are often thought of as opposing processes: rewards and the environmental cues that predict them elicit approach and consummatory behaviors, while punishments drive aversion and avoidance behaviors. This framework suggests that there may be segregated brain circuits for these valenced behaviors. The basolateral amygdala (BLA) is one brain region that contributes to both types of motivated behavior. Individual neurons in the BLA can favor positive over negative valence, or vice versa, but these neurons are intermingled, showing no anatomical segregation. The amygdala receives inputs from many brain areas and current theories posit that encoding of positive versus negative valence by BLA neurons is determined by the wiring of each neuron. Specifically, many projections from other brain areas that respond to positive and negative valence stimuli and predictive cues project strongly to the BLA and likely contribute to valence processing within the BLA. Here we review three of these areas, the basal forebrain, the dorsal raphe nucleus and the ventral tegmental area, and discuss how these may promote encoding of positive and negative valence within the BLA.

Impact of basal forebrain cholinergic inputs on basolateral amygdala neurons

In addition to innervating the cerebral cortex, basal forebrain cholinergic (BFc) neurons send a dense projection to the basolateral nucleus of the amygdala (BLA). In this study, we investigated the effect of near physiological acetylcholine release on BLA neurons using optogenetic tools and in vitro patch-clamp recordings. Adult transgenic mice expressing cre-recombinase under the choline acetyltransferase promoter were used to selectively transduce BFc neurons with channelrhodopsin-2 and a reporter through the injection of an adeno-associated virus. Light-induced stimulation of BFc axons produced different effects depending on the BLA cell type. In late-firing interneurons, BFc inputs elicited fast nicotinic EPSPs. In contrast, no response could be detected in fast-spiking interneurons. In principal BLA neurons, two different effects were elicited depending on their activity level. When principal BLA neurons were quiescent or made to fire at low rates by depolarizing current injection, light-induced activation of BFc axons elicited muscarinic IPSPs. In contrast, with stronger depolarizing currents, eliciting firing above ∼ 6-8 Hz, these muscarinic IPSPs lost their efficacy because stimulation of BFc inputs prolonged current-evoked afterdepolarizations. All the effects observed in principal neurons were dependent on muscarinic receptors type 1, engaging different intracellular mechanisms in a state-dependent manner. Overall, our results suggest that acetylcholine enhances the signal-to-noise ratio in principal BLA neurons. Moreover, the cholinergic engagement of afterdepolarizations may contribute to the formation of stimulus associations during fear-conditioning tasks where the timing of conditioned and unconditioned stimuli is not optimal for the induction of synaptic plasticity.

Noradrenergic actions in the basolateral complex of the amygdala modulate Arc expression in hippocampal synapses and consolidation of aversive and non-aversive memory

The basolateral complex of the amygdala (BLA) plays a role in the modulation of emotional memory consolidation through its interactions with other brain regions. In rats, memory enhancing infusions of the β-adrenergic receptor agonist clenbuterol into the BLA immediately after training enhances expression of the protein product of the immediate early gene Arc in the dorsal hippocampus and memory-impairing intra-BLA treatments reduce hippocampal Arc expression. We have proposed that the BLA may modulate memory consolidation through an influence on the local translation of synaptic plasticity proteins, like Arc, in recently active synapses in efferent brain regions. To date, all work related to this hypothesis is based on aversive memory tasks such as inhibitory avoidance (IA). To determine whether BLA modulation of hippocampal Arc protein expression is specific to plasticity associated with inhibitory avoidance memory, or a common mechanism for multiple types of memory, we tested the effect of intra-BLA infusions of clenbuterol on memory and hippocampal synaptic Arc expression following IA or object recognition training. Results indicate that intra-BLA infusions of clenbuterol enhance memory for both tasks; however, Arc expression in hippocampal synaptoneurosomes was significantly elevated only in rats trained on the aversive IA task. These findings suggest that regulation of Arc expression in hippocampal synapses may depend on co-activation of arousal systems. To test this hypothesis, a "high arousal" version of the OR task was used where rats were not habituated to the testing conditions. Posttraining intra-BLA infusions of clenbuterol enhanced consolidation of the high-arousing version of the task and significantly increased Arc protein levels in dorsal hippocampus synaptic fractions. These findings suggest that the BLA modulates multiple forms of memory and affects the synaptic plasticity-associated protein Arc in synapses of the dorsal hippocampus when emotional arousal is elevated.

Learned spatiotemporal sequence recognition and prediction in primary visual cortex

Learning to recognize and predict temporal sequences is fundamental to sensory perception, and is impaired in several neuropsychiatric disorders, but little is known about where and how this occurs in the brain. We discovered that repeated presentations of a visual sequence over a course of days causes evoked response potentiation in mouse V1 that is highly specific for stimulus order and timing. Remarkably, after V1 is trained to recognize a sequence, cortical activity regenerates the full sequence even when individual stimulus elements are omitted. This novel neurophysiological report of sequence learning advances the understanding of how the brain makes “intelligent guesses” based on limited information to form visual percepts and suggests that it is possible to study the mechanistic basis of this high–level cognitive ability by studying low–level sensory systems.

The effects of the intraamygdalar melatonin injections on the anxiety like behavior and the spatial memory performance in male Wistar rats

In the present study, the effects of intraamygdalar administrations of melatonin (1 and 100μg/kg), saline and diazepam on the anxiety-like behavior and spatial memory performance in pinealectomized and sham-pinealectomized Wistar rats were investigated. The animals were tested by open field and elevated plus maze tests for anxiety-like behavior, and Morris water maze test for spatial memory. In open field, (a) diazepam was more effective in reducing the anxiety, (b) control subjects were more mobile than pinealectomized subjects and (c) 100μg/kg melatonin administrations reduced the velocity of the animals. In elevated plus maze, (a) 100μg/kg melatonin administrations increased the distance totally travelled and (b) enhanced the time spent in open arms, however, after the pinealectomy, 1μg/kg melatonin administrations decreased it and (c) control animals were less mobile than pinealectomized ones. In Morris water maze, (a) diazepam group travelled more distance than the others in control condition whereas, in pinealectomy condition high dose of melatonin and saline groups travelled more distance than the others, (b) in pinealectomy condition subjects who received 100μg/kg melatonin also travelled more distance than those who received 1μg/kg melatonin and diazepam, (c) the subjects who received 1μg/kg spent less time than those who received other treatments, and (d) in control condition subjects who received 100μg/kg melatonin were slower than those who received the other treatments. In conclusion, melatonin administration to amygdala decreased the anxiety; however, spatial memory performance of the rats was impaired by the pinealectomy and melatonin administrations.

Cholinergic innervation of pyramidal cells and parvalbumin-immunoreactive interneurons in the rat basolateral amygdala

The basolateral nucleus of the amygdala receives an extremely dense cholinergic innervation from the basal forebrain that is critical for memory consolidation. Although previous electron microscopic studies determined some of the postsynaptic targets of cholinergic afferents, the majority of postsynaptic structures were dendritic shafts whose neurons of origin were not identified. To make this determination, the present study analyzed the cholinergic innervation of the anterior subdivision of the basolateral amygdalar nucleus (BLa) of the rat using electron microscopic dual-labeling immunocytochemistry. The vesicular acetylcholine transporter (VAChT) was used as a marker for cholinergic terminals; calcium/calmodulin-dependent protein kinase II (CaMK) was used as a marker for pyramidal cells, the principal neurons of the BLa; and parvalbumin (PV) was used as a marker for the predominant interneuronal subpopulation in this nucleus. VAChT(+) terminals were visualized by using diaminobenzidine as a chromogen, whereas CAMK(+) or PV(+) neurons were visualized with Vector very intense purple (VIP) as a chromogen. Quantitative analyses revealed that the great majority of dendritic shafts receiving cholinergic inputs were CAMK(+) , indicating that they were of pyramidal cell origin. In fact, 89% of the postsynaptic targets of cholinergic terminals in the BLa were pyramidal cells, including perikarya (3%), dendritic shafts (47%), and dendritic spines (39%). PV(+) structures, including perikarya and dendrites, constituted 7% of the postsynaptic targets of cholinergic axon terminals. The cholinergic innervation of both pyramidal cells and PV(+) interneurons may constitute an anatomical substrate for the generation of oscillatory activity involved in memory consolidation by the BLa.

Acetylcholine and attention

Historically, ACh has been implicated in learning and short-term memory functions. However, more recent studies have provided support for a role of cortical ACh in attentional effort, orienting and the detection of behavioral significant stimuli. The current review article summarizes studies in animals and humans which have investigated the role of ACh in attention and cognition. An attempt has been made to differentiate between brain regions involved in attentional processes versus those important for other cognitive functions. To this purpose, various experimental methods and interventions were used. Animal behavioral studies have injected the selective immunotoxin IgG-saporin to induce specific cholinergic lesions, employed electrochemical techniques such as microdialysis, or have administered cholinergic compounds into discrete parts of the brain. Human studies that give some indication on the link between central cholinergic signaling and cognition are obviously confined to less invasive, imaging methods such as fMRI. The brain areas that are deemed most important for intact attentional processing in both animals and humans appear to be the (pre)frontal, parietal and somatosensory (especially visual) regions, where ACh plays a vital role in the top-down control of attentional orienting and stimulus discrimination. In contrast, cholinergic signaling in the septohippocampal system is suggested to be involved in memory processes. Thus, it appears that the role of ACh in cognition is different per brain region and between nicotinic versus muscarinic receptor subtypes.

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.

Effects of scopolamine and ketamine on reconsolidation of morphine conditioned place preference in rats

Persistent memory associated with addictive drugs contributes to the relapse of drug abuse. The current study was conducted to examine the effects of scopolamine and ketamine on reconsolidation of morphine-induced conditioned place preference (CPP). In experiment 1, after morphine CPP was acquired, rats were injected with ketamine (60 mg/kg, intraperitoneally) and scopolamine (2 mg/kg, intraperitoneally), respectively, after reexposure to an earlier morphine-paired context or in their home cages. The CPP was reassessed 24 and 48 h after reexposure. An additional group of rats received saline following reexposure to the earlier morphine-paired context. In experiment 2, two groups of rats were only given saline during the CPP training and subsequent administration of ketamine or scopolamine during the reexposure. In experiment 1, rats failed to exhibit morphine CPP when ketamine and scopolamine were administered only after reexposure to a morphine-paired context. CPP was not abolished by ketamine or scopolamine administration in the animals' home cages. Also, the animals receiving only saline injections showed strong morphine CPP 24 h after a short exposure to the morphine-paired context. In experiment 2, ketamine or scopolamine treatment alone did not induce CPP or aversion. Administration of scopolamine and ketamine, after reexposure to a drug-paired context, resulted in the disruption of morphine CPP, suggesting the potential effects of scopolamine and ketamine in disrupting memory associated with environmental cues and addictive drugs.

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

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.

Intra-hippocampal inhibition of protein kinase AII attenuates morphine-induced conditioned place preference

Morphine and other drugs of abuse modulate protein kinase A (PKA) signaling within the mesolimbic reward pathway. Using a balanced conditioned place preference (CPP) paradigm, we studied the possible involvement of protein kinase AII (PKA II) on the acquisition, expression and consolidation of morphine place conditioning in male Wistar rats. Subcutaneous administration of various doses of morphine sulfate (1-9 mg/kg) induced CPP in a dose-dependent manner. H-89, a selective PKA II inhibitor, was administered into CA1 region of the hippocampus at 1, 2.5 and 5 microM/rat. Using a 3-day schedule of conditioning, it was found that the H-89 did not produce a significant place preference or place aversion. H-89 (1, 2.5 and 5 microM/rat) significantly reduced the time spent by rats in the morphine compartment when given immediately after each conditioning session (consolidation), whereas it had no effect when administered before morphine during the conditioning phase (acquisition) or before testing for place preference in the absence of morphine (expression). It is concluded that the PKA II may play an active role in the consolidation of reward-related memory of morphine in CA1 region of the hippocampus.

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.

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.

Role of the cholinergic system in the rat basolateral amygdala on morphine-induced conditioned place preference

The effects of intra-basolateral amygdala (intra-BLA) injections of physostigmine, atropine, nicotine and/or mecamylamine on morphine-induced conditioned place preference (CPP) in rats was investigated by using an unbiased 3-day schedule of place conditioning design. Animals that received 3 daily injections of morphine (0.5-10 mg/kg) subcutaneously (s.c.) or saline (1.0 ml/kg, s.c.) showed a significant preference for compartment paired with morphine. The maximum response was observed with 7.5 mg/kg of the opioid. Administration of the anticholinesterase drug, physostigmine (1, 3 and 5 microg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant CPP. Injections of antimuscarinic receptor agent, atropine (1, 4 and 7 microg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. The injections of nicotine (0.75, 1 and 2 microg/rat) potentiated the morphine (0.5 mg/kg)-induced place preference, while the nicotinic receptor antagonist, mecamylamine (1, 3 and 6 microg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. Furthermore, administration of atropine (7 microg/rat) but not mecamylamine (6 microg/rat) reduced the response induced by different doses of physostigmine plus morphine. Moreover, mecamylamine (6 microg/rat) but not atropine (7 microg/rat) reduced the response induced by different doses of nicotine plus morphine. It is concluded that the muscarinic and nicotinic receptor mechanisms in the BLA may be involved in the acquisition of morphine-induced place preference.

Repeated administration of dopaminergic agents in the dorsal hippocampus and morphine-induced place preference

The aim of the present experiments was to investigate whether repeated intra-hippocampal CA1 (intra-CA1) administration of dopaminergic agents can affect morphine-induced conditioned place preference (CPP). Effects of repeated intra-CA1 injections of dopamine (DA) receptor agonists and antagonists on morphine-induced CPP in rats were investigated using an unbiased 3-day schedule of place conditioning. Animals receiving once-daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner: the maximum response was observed with 3 mg/kg morphine. Three days' intra-CA1 injections of apomorphine (0.25-1 microg/rat) followed by 5 days free of the drug, significantly decreased morphine CPP (1 and 3 mg/kg, s.c.). Moreover, pre-treatment with the highest dose of apomorphine (1 microg/rat) altered the effect of morphine to an aversive response. The morphine (1 and 3 mg/kg) CPP was also significantly decreased in animals that previously received three intra-CA1 injections of SKF 38393 (2-9 microg/rat), quinpirole (1-3 microg/rat) or sulpiride (1-3 microg/rat), and significantly increased in animals that had previously received three intra-CA1 injections of SCH 23390 (0.02 microg/rat). The 3-day pre-treatment with apomorphine, SKF 38393 or quinpirole reduced locomotor activity in the test session, while SCH 23390 and sulpiride did not have any influence on locomotor activity. It is concluded that repeated injections of DA receptor agents in the dorsal hippocampus, followed by 5 days free of the drugs, can affect morphine reward.

Coordination of multiple memory systems

On the basis of lesions of different brain areas, several neural systems appear to be important for processing information regarding different types of learning and memory. This paper examines the development of pharmacological and neurochemical approaches to multiple memory systems from past studies of modulation of memory formation. The findings suggest that peripheral neuroendocrine mechanisms that regulate memory processing may target their actions toward those neural systems most engaged in the processing of learning and memory. In addition, measurements of acetylcholine release in different memory systems reveals extensive interactions between memory systems, some cooperative and some competitive. These results imply that many neural systems, often characterized as relatively independent, may in fact interact extensively, blurring the dependencies of different memory tasks on specific neural systems.

NMDA systems in the amygdala and piriform cortex and nicotinic effects on memory function

Both nicotinic cholinergic and NMDA glutaminergic systems are important for memory function. Nicotine has been found repeatedly to significantly improve working memory performance in the radial-arm maze. The NMDA antagonist dizocilpine has been found to impair working memory performance. There is neuropharmacological evidence that these two systems are functionally related. Nicotine is potent at releasing many transmitters including glutamate. The current study was conducted to examine the interaction of nicotinic and NMDA systems within the amygdala with regard to working and reference memory. Rats were trained on a working/reference procedure on a 16-arm radial maze. After acquisition, local infusion cannulae were implanted bilaterally into the amygdala and piriform cortex using stereotaxic techniques. Then 20 min prior to running the rats on the radial-arm maze, they were injected subcutaneously with (-) nicotine ditartrate at doses of 0 and 0.4 mg/kg. Following this, the rats received local infusions of (+) dizocilpine maleate (MK-801) at doses of 0, 2, 6 and 18 microg per side into the lateral amygdala or piriform cortex 10 min prior to running on the radial-arm maze. Each of the eight nicotine and dizocilpine combinations was administered to each rat in a counterbalanced order. After completion of the drug sessions the rats were sacrificed, and using histological methods the cannulae placements were verified. Acute amygdalar infusions of the NMDA glutamate receptor antagonist dizocilpine induced dose-related working and reference memory deficits in the radial-arm maze. Systemic nicotine was not seen to reverse these effects. Dizocilpine infusions into the adjacent piriform cortex did not impair memory function, supporting the specificity of dizocilpine effects in the amygdala. Latency effects were seen with both drugs in both areas. Latencies were decreased with both systemic nicotine and dizocilpine in both the lateral amygdala and the piriform cortex. This study demonstrated the importance of NMDA glutamate systems in the amygdala for appetitively-motivated spatial memory performance.

Involuntary, unreinforced (pure) spatial learning is impaired by fimbria-fornix but not by dorsal hippocampus lesions

Pure spatial learning occurs when rats acquire information about an environment while exploring it in the absence of reinforcers. We previously reported that voluntary, unreinforced exploration of a radial maze retards subsequent reinforced conditioned cue preference (CCP) learning in the same maze. In the present experiment, we examined the effects of involuntary, unreinforced pre-exposure to a radial maze. During pre-exposure, rats were moved by an experimenter between the ends of two arms of a radial maze five times in 30 min. This form of pre-exposure retarded CCP learning, whereas rats that were not pre-exposed and rats that were pre-exposed to a maze in a different room displayed normal CCP learning. These findings suggest that some information specific to the maze environment was acquired during involuntary unreinforced pre-exposure to it. In experiment 2, the retardation of reinforced CCP learning by involuntary unreinforced pre-exposure was eliminated by fimbria-fornix lesions made before pre-exposure but was unaffected by fimbria-fornix lesions made after pre-exposure but before training. Large neurotoxic lesions of the dorsal hippocampus made before pre-exposure had no effect on the retardation of CCP learning, but the rats with these lesions were impaired on a standard test of reinforced spatial learning in a water maze. The lesion effects in experiment 2 are similar to those previously reported for voluntary exploration and suggest that pure spatial learning may occur during both voluntary exploration of and involuntary exposure to an environment in the absence of reinforcers. Pure spatial learning can apparently occur with exposure to two different locations within an environment, but the rats do not have to move between the locations voluntarily. An intact fimbria-fornix is required for acquisition but not expression of this form of learning. The hippocampus is not involved in this form of learning.

Muscarinic receptor antagonism in the basolateral amygdala blocks acquisition of cocaine-stimulus association in a model of relapse to cocaine-seeking behavior in rats

Recent evidence has demonstrated a critical role for the basolateral amygdala complex in the reinstatement of extinguished drug-seeking behavior produced by drug-paired cues. In the current study, we utilized a model of the acquisition and expression of cocaine-stimulus associative pairing in order to study the role of cholinergic input to the basolateral amygdala in mediating conditioned-cued reinstatement. Male, Sprague-Dawley rats were first trained daily to self-administer i.v. cocaine on a fixed ratio 1 schedule of reinforcement. The muscarinic acetylcholine receptor antagonist, scopolamine, was directly infused into the basolateral amygdala prior to: a) classically conditioned pairing of a tone+light stimulus with cocaine infusions (acquisition), or b) testing of conditioned-cued reinstatement following a period of withdrawal from cocaine and extinction of cocaine-paired lever responding. Infusion of scopolamine just prior to the classical conditioning trial produced a dose-dependent disruption of cocaine-seeking behavior maintained by cocaine-paired cues during the reinstatement test. In contrast, infusion of scopolamine prior to the reinstatement test had no effect on conditioned-cued reinstatement of cocaine-seeking behavior. These results indicate a crucial role for cholinergic innervation of muscarinic acetylcholine receptors in the basolateral amygdala during the formation, but not the expression, of stimulus-reward associations that mediate cue-induced cocaine-seeking behavior.

Cooperation between memory systems: acetylcholine release in the amygdala correlates positively with performance on a hippocampus-dependent task

The present experiment examined the relationship between release of acetylcholine (ACh) in the amygdala and performance on a hippocampus-dependent spatial working memory task. Using in vivo microdialysis, the authors measured ACh release in rats during testing on a spontaneous alternation task. Amygdala ACh release was positively correlated with performance on the hippocampus-dependent task. These findings suggest that activation of the amygdala promotes processing in other neural systems important for learning and memory.

Nicotinic mechanisms of memory: effects of acute local DHbetaE and MLA infusions in the basolateral amygdala

Nicotine has been shown to improve working memory. The neural mechanisms underlying this effect are still being determined. The ventral hippocampus is critical for nicotinic effects on memory. Local ventral hippocampal infusions of either the nicotinic alpha7 nicotinic receptor antagonist methyllycaconitine (MLA) or the alpha4beta2 nicotinic receptor antagonist dihydro-beta-erythroidine (DHbetaE) caused working memory impairments, but no additive effects were seen. Other areas, such as the amygdala, also likely play important roles in nicotinic effects on memory. Amygdalar lesions cause memory impairment and there is a dense concentration of nicotinic receptors in the basolateral amygdala. The current study used local basolateral amygdalar infusions of the nicotinic antagonists MLA and DHbetaE to determine the involvement of alpha7 and alpha4beta2 nicotinic receptors in spatial working and reference memory. Rats (n=8) were trained in the 16-arm radial maze and were implanted with bilateral infusion cannulae into the basolateral amygdala. Acute infusions of MLA (6.75 micro g/side, P<0.0005) or DHbetaE (3.38 micro g/side, P<0.025) caused significant working memory impairments. When given together MLA and DHbetaE did not produce an additive effect. In fact, the 6.75 micro g/kg dose of DHbetaE produced a significant (P<0.0005) attenuation of the MLA-induced working memory impairment. Significant effects were not seen with reference memory or response latency. Nicotinic systems in the basolateral amygdala, as in the ventral hippocampus, are important for spatial working memory. In both the basolateral amygdala and the ventral hippocampus, MLA and DHbetaE individually caused working memory impairments. The lowest effective dose of DHbetaE was lower in the basolateral amygdala than in the ventral hippocampus. In both the basolateral amygdala and the ventral hippocampus, combined MLA and DHbetaE treatment did not produce additive working memory deficits. Unlike in the ventral hippocampus, the addition of DHbetaE to MLA in the basolateral amygdala significantly reduced the MLA-induced working memory deficit.

Involvement of the dorsomedial striatum in behavioral flexibility: role of muscarinic cholinergic receptors

The present experiments determined whether temporary inactivation or blockade of muscarinic cholinergic receptors in the dorsomedial striatum affects acquisition or reversal learning of a response discrimination. Testing occurred in a modified cross-maze across two consecutive sessions. In the acquisition phase, a rat learned to make a turn to the left or to the right for 10 consecutive correct choices. In the reversal learning phase, a rat learned to turn in the opposite direction as required during acquisition for 10 consecutive correct choices. Experiment 1 investigated the effects of the local anesthetic, 2% bupivacaine, infused into the dorsomedial striatum on acquisition and reversal learning. Experiment 2 examined the effects of the muscarinic cholinergic antagonist, scopolamine injected into the dorsomedial striatum on acquisition and reversal learning. Bupivacaine infusions did not impair acquisition, but did impair reversal learning of the response discrimination. Analysis of the errors indicated that the deficit was not due to perseveration of the previously learned strategy, but to an inability to learn the new strategy. Bilateral injections of scopolamine, 1 or 8 microg/side, did not affect acquisition. Infusions of scopolamine at 8 microg, but not 1 microg, produced a reversal learning deficit. The scopolamine-induced deficit resulted from an inability to learn the new strategy. The results suggest that the dorsomedial striatum is important for behavioral flexibility and that activation of muscarinic cholinergic receptors in this region may facilitate the learning of situationally adaptive response patterns.

Influence of nitric oxide on morphine-induced conditioned place preference in the rat central amygdala

Effects of intra-central amygdala injections of L-arginine, a nitric oxide (NO) precursor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a NO synthase (NOS) inhibitor, on morphine-induced conditioned place preference in rats were investigated by using an unbiased 3-day schedule of place conditioning design. Animals receiving once daily injections of morphine (0.5-7.5 mg/kg, subcutaneously, s.c.) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner. The maximum response was observed with 5.0 mg/kg of the opioid. Co-administration of morphine (5.0 mg/kg) with L-arginine (0.3, 1.0 and 3.0 microg/rat), but not with L-NAME (0.3, 1.0 and 3.0 microg/rat), during the acquisition of morphine-induced conditioned place preference increased morphine-induced conditioned place preference. The response to L-arginine was blocked by L-NAME preadministration. L-arginine and L-NAME by themselves did not induce conditioned place preference. When L-arginine or L-NAME at 0.3-3.0 microg/rat was administered 1 min before conditioned place preference testing, L-arginine but not L-NAME caused an increase in the expression of morphine-induced conditioned place preference, the effect that was blocked by L-NAME preadministration. A dose of L-arginine (0.3 microg/rat), but not L-NAME, during expression of morphine-induced conditioned place preference produced an increase in locomotion compared with that in the control group. It may be concluded that an increase in the NO levels in the central amygdala may have an effect on the acquisition and expression of morphine-induced conditioned place preference.

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

The present study investigated the role of cholinergic muscarinic receptor function within the basolateral amygdala memory in the consolidation of conditioned place preference (CPP) memory. Adult male Long-Evans rats were confined to treatment- or nontreatment-paired compartments for 30 min on 4 alternating days. After training, rats received intrabasolateral amygdala infusions of scopolamine (2.5 microg or 5.0 microg/0.5 microl) or saline. The rats were then given a 20-min test session, and the time spent in each of the compartments was recorded. Immediate posttraining (but not delayed 2 hr) scopolamine (5.0 microg) blocked acquisition of food- and amphetamine-induced CPPs. The findings indicate a time-dependent role for basolateral amygdala muscarinic receptors in memory consolidation underlying CPPs for natural and drug rewards.

Effects of lesions of various brain areas on drug priming or footshock-induced reactivation of extinguished conditioned place preference

We have previously shown with a model of morphine-induced conditioned place preference (CPP) that a brief exposure to footshock stress or a priming dose of morphine could reactivate drug-seeking behavior after a long drug-free period. The present study was designed to examine the possible role of certain brain areas in such a reactivation. After the rats were successfully trained with morphine (4 mg/kg, i.p.) through a CPP paradigm (10 sessions of daily pairing of morphine with one of the two compartments), different parts of nucleus accumbens (NAc), ventral tegmental area (VTA), and central (Ce) or lateral (La) nucleus of amygdala were lesioned with a DC current passing through the respective location. After a 9-day abstinence period, random intermittent footshock (DC square wave, 0.5 mA, 0.5 s width, off time 10-70 s) or drug priming (morphine 0.25 mg/kg, s.c.) reactivated the place preference in sham lesion rats. However, the effect of drug priming could be completely abolished by lesions placed either at VTA, or the majority or shell part, but not the core of NAc. On the other hand, the effect of footshock stressor could be eliminated by a lesion placed at Ce but not La. These results suggest that, while both drug priming and footshock stress are effective in reactivating drug-seeking behavior, they might work through different neurochemical mechanisms and anatomical pathways.

Phthalic acid amygdalopetal lesion of the nucleus basalis magnocellularis induces reversible memory deficits in rats

The basolateral amygdala (BLA) is extensively implicated in emotional learning and memory. The current study investigated the contribution of cholinergic afferents to the BLA from the nucleus basalis magnocellularis in influencing aversive learning and memory. Sprague-Dawley rats were given permanent unilateral phthalic acid (300 ng) lesions of the nucleus basalis magnocellularis and were chronically implanted with cannulas aimed at the ipsilateral BLA. Lesioned rats showed a pronounced inhibitory avoidance task retention deficit that was attenuated by acute posttraining infusions of the muscarinic cholinergic agonist oxotremorine (4 ng) or the indirect agonist physostigmine (1 microg) into the BLA. Continuous multiple-trial inhibitory avoidance training and testing revealed that lesioned rats have a mild acquisition deficit, requiring approximately 1 additional shock to reach the criterion, and a pronounced consolidation deficit as indicated by a shorter latency to enter the shock compartment on the retention test. Because lesioned rats did not differ from sham-operated controls in performance on a spatial water maze task or in shock sensitivity, it is not likely that the memory impairments produced by the phthalic acid lesions are due to any general sensory or motor deficits. These findings suggest that the dense cholinergic projection from the nucleus basalis magnocellularis to the BLA is involved in both the acquisition and the consolidation of the aversive inhibitory avoidance task.

Spatial working memory and the brainstem cholinergic innervation to the anterior thalamus

The anteroventral thalamic nucleus (AV) has a role in spatial memory, but the influence of the prominent brainstem cholinergic projection to this region is unknown. Here, spatial memory in a 12-arm radial maze was examined after 0.15 microl bilateral AV infusions of scopolamine. In part one, rats visited six arms singly (the phase 1 arms) and, after a 10 min delay, were allowed free choice to both phase 1 arms and the remaining six baited arms (phase 2 arms). Scopolamine (10 microg) administered during the delay increased errors to both phase 1 and phase 2 arms, whereas PBS infusions increased phase 1 arm errors only. The PBS effect was the result of inserting the internal cannulas alone and not the infusion. The same dose of scopolamine (10 microg) infused before maze testing (part two: no phase 1 arms, no delay) also impaired spatial memory over and above the effects of both PBS and no-infusion, which did not differ markedly. Part two also showed that choice latency and choice strategies were unaffected by PBS and scopolamine. Cannulation and infusion procedures in both parts one and two did not produce any negative carryover effects across multiple control (no internal cannula) sessions, and a trypan blue manipulation indicated that infusions were restricted to the AV region. This study provides the first direct evidence that the brainstem cholinergic innervation to the limbic thalamus influences learning and memory, which may have important implications for human neurological conditions such as alcohol-related disorders and schizophrenia.

Conditioned memory modulation, freezing, and avoidance as measures of amygdala-mediated conditioned fear

Three conditioned aversive responses were used to infer the existence of an unobservable central state of "conditioned fear," and the roles of certain amygdala subregions in producing these responses were investigated. Rats received tone-shock pairings in one compartment of a shuttle box and no tones or shocks in the other, distinctive, compartment. They were then trained to find food in one arm of a Y-maze. After the final training trial they were exposed to different sets of stimuli in the shuttle box with no shock. Twenty-four hours later rats that had received immediate posttraining exposure to the conditioned stimuli (in the shock-paired compartment) made significantly more correct responses on the Y-maze than rats that had been exposed to the neutral stimuli (in the no-shock compartment) or rats that had received delayed posttraining exposure to the conditioned stimuli. This constitutes a demonstration of posttraining memory modulation by conditioned aversive stimuli. Freezing increased during posttraining exposure to the conditioned stimuli compared to the neutral stimuli. When subsequently allowed to move freely between the two compartments, the rats in all groups also showed significant conditioned avoidance of the compartment containing the conditioned stimuli. In a second experiment the effects of lesions confined to specific parts of the amygdala on the three conditioned responses (memory modulation, freezing, avoidance) were tested. Lesions of the central nucleus impaired all three conditioned responses; lesions of the medial nucleus impaired conditioned modulation and avoidance. These lesions had no effect on freezing during the training trials. Lesions of the lateral and basolateral nuclei attenuated freezing during both training and testing. The findings suggest that the central and medial nuclei of the amygdala may be important parts of neural circuits mediating conditioned responses that constitute conditioned aversive states, but that conditioned freezing may be mediated independently.

Competition between memory systems: acetylcholine release in the hippocampus correlates negatively with good performance on an amygdala-dependent task

Lesions of the amygdala impair acquisition of a food conditioned place preference (CPP) task. In contrast, lesions of the fornix facilitate acquisition on this task, showing that an intact hippocampal system can interfere with learning an amygdala-dependent task. Our recent findings indicate that acetylcholine (ACh) release in the hippocampus increases while rats perform a hippocampus-dependent spontaneous alternation task. To the extent that ACh output in the hippocampus reflects activation of that brain area in learning and memory, the results obtained with fornix lesions suggest that ACh release in the hippocampus might be negatively correlated with learning on a CPP task. Using in vivo microdialysis, release of ACh was measured in the hippocampus while rats learned and were tested on an amygdala-dependent CPP task and a hippocampus-dependent spontaneous alternation task. Release of ACh in the hippocampus increased when rats were tested on either task. The magnitude of the increase in release of hippocampal ACh was negatively correlated with good performance on the amygdala-dependent CPP task. These findings suggest that ACh release may reflect activation and participation of the hippocampus in learning and memory, but in a manner that can be detrimental to performance on a task dependent on another brain area.

The amygdala mediates memory consolidation for an amphetamine conditioned place preference

Drug-induced conditioned place preference (CPP) behavior requires memory for an association between environmental cues and the affective state produced by the drug treatment. The present study investigated whether memory consolidation underlying an amphetamine CPP could be modulated by post-training intra-amygdala infusion of the local anesthetic drug bupivacaine. On 4 alternating days adult male Long-Evans rats received peripheral injections of amphetamine (2.0 mg/kg) or saline vehicle prior to confinement for 30 min to one of two compartments of a place preference apparatus, followed by post-training intra-amygdala infusions of bupivacaine (0.75% solution/1.0 microl) or saline. On day 5 the rats were given a drug-free 20-min test session, and the amount of time spent in each of the pairing compartments of the apparatus was recorded. On the test day, rats receiving post-training intra-amygdala saline injections displayed an amphetamine conditioned place preference. Post-training intra-amygdala infusions of bupivacaine blocked amphetamine CPP. Intra-amygdala infusions of bupivacaine that were delayed 1 h post-training did not block amphetamine CPP, indicating a time-dependent effect of the treatment on memory storage processes. Pre-training or pre-retention test intra-amygdala infusions of bupivacaine also blocked acquisition and expression of an amphetamine CPP, respectively. The findings indicate that the mechanism(s) by which amphetamine elicits conditioned approach responses to environmental cues can be manipulated post-training, and suggest a role for the amygdala in acquisition, consolidation, and expression of amphetamine CPP behavior.

Lesions in the central nucleus of the amygdala: discriminative avoidance learning, discriminative approach learning, and cingulothalamic training-induced neuronal activity

The amygdala is critically involved in discriminative avoidance learning. Large lesions of the amygdala block discriminative avoidance learning and abolish cingulothalamic training-induced neuronal activity. These results indicated that amygdalar processing is critical for cingulothalamic plasticity. The larger lesions did not allow differentiation of the specific functioning of various amygdalar nuclei. Anatomical analysis showed that damage in the central (CE) nucleus of the amygdala was correlated with the severity of the behavioral deficit. The present study was carried out to determine whether smaller lesions, centered in the CE nucleus, would impair discriminative avoidance learning and block cingulothalamic plasticity. In addition, the possible role of the CE nucleus in appetitively motivated discriminative approach learning was examined for the first time. New Zealand White rabbits with CE nuclear lesions were first trained in the discriminative approach task. After attaining asymptotic performance, discriminative avoidance training sessions were alternated with continuing approach training sessions, one session each day. The rabbits with lesions were severely impaired in avoidance learning but showed no impairment of approach learning. Surprisingly, the attenuating effects of the lesions on cingulothalamic training-induced neuronal activity were more prevalent during approach learning than during avoidance learning. These results indicated that avoidance learning can be impaired by lesions centered in the CE nucleus that leave cingulothalamic plasticity largely intact and that the CE nucleus is involved in extra-cingulothalamic learning processes.

Basolateral amygdala noradrenergic influence enables enhancement of memory consolidation induced by hippocampal glucocorticoid receptor activation

Previously, we reported that bilateral excitotoxic lesions of the basolateral nucleus of the amygdala (BLA) block the enhancing effects of posttraining systemic or intrahippocampal glucocorticoid administration on memory for inhibitory avoidance training. The present study further examined the basis of this permissive influence of the BLA on hippocampal memory functioning. Immediate posttraining unilateral infusions of the specific glucocorticoid receptor agonist RU 28362 (11beta,17beta-dihydroxy-6, 21-dimethyl-17alpha-pregna-4,6-trien-20-yn-3-one; 3.0, 10.0, or 30.0 ng in 0.5 microliter) administered into the dorsal hippocampus of male Sprague-Dawley rats induced dose-dependent enhancement of 48-h inhibitory avoidance retention. Infusions of the beta-adrenoceptor antagonist atenolol (0.5 microgram in 0.2 microliter) into the ipsilateral, but not the contralateral, BLA 10 min prior to training blocked the hippocampal glucocorticoid effects on memory consolidation. Infusions of the muscarinic cholinergic antagonist atropine (0.5 microgram in 0.2 microliter) into either the ipsilateral or contralateral BLA before training did not block the hippocampal glucocorticoid effects. These findings provide further evidence that beta-adrenergic activity in the BLA is essential in enabling glucocorticoid-induced modulation of memory consolidation and are consistent with the hypothesis that the BLA regulates the strength of memory consolidation in other brain structures. The ipsilateral nature of the BLA-hippocampus interaction indicates that BLA influences on hippocampal memory processes are mediated through neural pathways rather than by influences by means of the activation of peripheral stress responses.