Amygdala and dorsal hippocampus lesions block the effects of GABAergic drugs on memory storage

The Entorhinal Cortex as a Gateway for Amygdala Influences on Memory Consolidation

The amygdala, specifically its basolateral nucleus (BLA), is a critical site integrating neuromodulatory influences on memory consolidation in other brain areas. Almost 20 years ago, we reported the first direct evidence that BLA activity is required for modulatory interventions in the entorhinal cortex (EC) to affect memory consolidation (Roesler, Roozendaal, and McGaugh, 2002). Since then, significant advances have been made in our understanding of how the EC participates in memory. For example, the characterization of grid cells specialized in processing spatial information in the medial EC (mEC) that act as major relayers of information to the hippocampus (HIP) has changed our view of memory processing by the EC; and the development of optogenetic technologies for manipulation of neuronal activity has recently enabled important new discoveries on the role of the BLA projections to the EC in memory. Here, we review the current evidence on interactions between the BLA and EC in synaptic plasticity and memory formation. The findings suggest that the EC may function as a gateway and mediator of modulatory influences from the BLA, which are then processed and relayed to the HIP. Through extensive reciprocal connections among the EC, HIP, and several cortical areas, information related to new memories is then consolidated by these multiple brain systems, through various molecular and cellular mechanisms acting in a distributed and highly concerted manner, during several hours after learning. A special note is made on the contribution by Ivan Izquierdo to our understanding of memory consolidation at the brain system level.

Inhibitory actions of general anesthetics on hippocampal CA1 neurons modified by activation of amygdala circuitry are demonstrated on the novel amygdala-hippocampal slice preparation

In order to examine effects of general anesthetics on hippocampal synaptic transmission in the absence and presence of amygdala circuitry activation, we have developed a unique amygdala-hippocampal slice preparation in which projections between amygdala and hippocampal CA1 are conserved. Stimulating electrodes were placed in radiatum stratum (Rad) to stimulate CA1, and in amygdala-hippocampal area (AH) to simulate amygdala inputs to hippocampus. Two sets of extracellular recording microelectrodes were positioned in cell bodies and dendrites of CA1 to record population spikes (PSs) and excitatory post-synaptic potentials (EPSPs), respectively. Intravenous anesthetics did not elicit consistent effects on PS and EPSP following a test stimulus on Rad. A pre-pulse of AH in addition to a test-pulse on Rad produced significant reduction of PS amplitude without a change in EPSP. Pre-treatment with tetanus-pulse on AH reversed the anesthetic-induced reduction of PS. The results suggest that inhibitory actions of general anesthetics in CA1 can be modified by activation of amygdala, suggesting that preoperative anxiety and fear could modify anesthetic actions. The modification was more prominent in the presence of intravenous anesthetics than with volatile agents.

Inhibition of GABA A receptor improved spatial memory impairment in the local model of demyelination in rat hippocampus

Cognitive impairment and memory deficit are common features in multiple Sclerosis patients. The mechanism of memory impairment in MS is unknown, but neuroimaging studies suggest that hippocampal demyelination is involved. Here, we investigate the role of GABA A receptor on spatial memory in the local model of hippocampal demyelination. Demyelination was induced in male Wistar rats by bilaterally injection of lysophosphatidylcholine (LPC) 1% into the CA1 region of the hippocampus. The treatment groups were received daily intraventricular injection of bicuculline (0.025, 0.05μg/2μl/animal) or muscimol (0.1, 0.2μg/2μl/animal) 5days after LPC injection. Morris Water Maze was used to evaluate learning and memory in rats. We used Luxol fast blue staining and qPCR to assess demyelination extention and MBP expression level respectively. Immunohistochemistry (IHC) for CD45 and H&E staining were performed to assess inflammatory cells infiltration. Behavioral study revealed that LPC injection in the hippocampus impaired learning and memory function. Animals treated with both doses of bicuculline improved spatial learning and memory function; however, muscimol treatment had no effect. Histological and MBP expression studies confirmed that demylination in LPC group was maximal. Bicuculline treatment significantly reduced demyelination extension and increased the level of MBP expression. H&E and IHC results showed that bicuculline reduced inflammatory cell infiltration in the lesion site. Bicuculline improved learning and memory and decreased demyelination extention in the LPC-induced hippocampal demyelination model. We conclude that disruption of GABAergic homeostasis in hippocampal demyelination context may be involved in memory impairment with the implications for both pathophysiology and therapy.

The role of δ-opioid receptors in learning and memory underlying the development of addiction

Opioids are important endogenous ligands that exist in both invertebrates and vertebrates and signal by activation of opioid receptors to produce analgesia and reward or pleasure. The μ-opioid receptor is the best known of the opioid receptors and mediates the acute analgesic effects of opiates, while the δ-opioid receptor (DOR) has been less well studied and has been linked to effects that follow from chronic use of opiates such as stress, inflammation and anxiety. Recently, DORs have been shown to play an essential role in emotions and increasing evidence points to a role in learning actions and outcomes. The process of learning and memory in addiction has been proposed to involve strengthening of specific brain circuits when a drug is paired with a context or environment. The DOR is highly expressed in the hippocampus, amygdala, striatum and other basal ganglia structures known to participate in learning and memory. In this review, we will focus on the role of the DOR and its potential role in learning and memory underlying the development of addiction.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Further evidence for involvement of the dorsal hippocampus serotonergic and γ-aminobutyric acid (GABA)ergic pathways in the expression of contextual fear conditioning in rats

Intra-dorsal hippocampus (DH) injections of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a serotonin-1A (5-hydroxytryptamine (5-HT)-1A) receptor agonist, were previously shown to inhibit the expression of contextual fear when administered six hours after conditioning. However, further understanding of the consolidation and expression of aversive memories requires investigations of these and other mechanisms at distinct time points and the regions of the brain to which they are transferred. Thus, the purpose of the present study was to investigate the role of DH serotonergic and γ-aminobutyric acid (GABA)ergic mechanisms in the expression of contextual fear 24 h after conditioning, reflected by fear-potentiated startle (FPS) and freezing behavior. The recruitment of the amygdala and medial prefrontal cortex (mPFC) in these processes was also evaluated by measuring Fos protein immunoreactivity. Although intra-DH injections of 8-OH-DPAT did not produce behavioral changes, muscimol reduced both FPS and the freezing response. Fos protein immunoreactivity revealed that contextual fear promoted wide activation of the mPFC, which was significantly reduced after intra-DH infusions of muscimol. The present findings, together with previous data, indicate that in contrast to 5-HT, which appears to play a role during the early phases of contextual aversive memory consolidation, longer-lasting GABA-mediated mechanisms are recruited during the expression of contextual fear memories.

Learning-dependent plasticity of hippocampal CA1 pyramidal neuron postburst afterhyperpolarizations and increased excitability after inhibitory avoidance learning depend upon basolateral amygdala inputs

Hippocampal pyramidal neurons in vitro exhibit transient learning-dependent reductions in the amplitude and duration of calcium-dependent postburst afterhyperpolarizations (AHPs), accompanied by other increases in excitability (i.e., increased firing rate, or reduced spike-frequency accommodation) after trace eyeblink conditioning or spatial learning, with a time-course appropriate to support consolidation of the learned tasks. Both these tasks require multiple days of training for acquisition. The hippocampus also plays a role in acquisition of single trial inhibitory avoidance learning. The current study assessed AHP plasticity in this single-trial learning task using in vitro tissue slices prepared at varying intervals posttrial using intracellular current-clamp recordings. Reduced AHPs and reduced accommodation were seen in ventral CA1 pyramidal neurons within 1 h posttraining, plasticity which persisted 24 h but was extinguished >72 h posttrial. There was also a reduction in ventral CA1 AHPs and accommodation 1 h following simple exposure to the IA apparatus (a novel context) but this change was extinguished by 24 h postexposure. Reductions in AHPs and accommodation were also seen in dorsal CA1 pyramidal neurons, but were delayed until 24 h posttrial and extinguished at >72 h posttrial. Finally, transient inactivation of the basolateral complex of the amygdala (with the local anesthetics lidocaine or bupivacaine) either immediately before or immediately posttrial blocked both learning and learning-dependent changes in excitability in the hippocampus assessed 24 h posttrial. CA3 pyramidal neurons showed no reductions in AHP peak amplitude or accommodation following IA training or context exposure.

Behavioral and neural analysis of GABA in the acquisition, consolidation, reconsolidation, and extinction of fear memory

The current review systematically documents the role of gamma-amino-butyric acid (GABA) in different aspects of fear memory-acquisition and consolidation, reconsolidation, and extinction, and attempts to resolve apparent contradictions in the data in order to identify the function of GABA(A) receptors in fear memory. First, numerous studies have shown that pre- and post-training administration of drugs that facilitate GABAergic transmission disrupt the initial formation of fear memories, indicating a role for GABA(A) receptors, possibly within the amygdala and hippocampus, in the acquisition and consolidation of fear memories. Similarly, recent evidence indicates that these drugs are also detrimental to the restorage of fear memories after their reactivation. This suggests a role for GABA(A) receptors in the reconsolidation of fear memories, although the precise neural circuits are yet to be identified. Finally, research regarding the role of GABA in extinction has shown that GABAergic transmission is also disruptive to the formation of newly acquired extinction memories. We argue that contradictions to these patterns are the result of variations in (a) the location of drug infusion, (b) the dosage of the drug and/or (c) the time point of drug administration. The question of whether these GABA-induced memory deficits reflect deficits in retrieval is discussed. Overall, the evidence implies that the processes mediating memory stability consequent to initial fear learning, memory reactivation, and extinction training are dependent on a common mechanism of reduced GABAergic neurotransmission.

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

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

Effect of the GABAergic System on Memory Formation and State-Dependent Learning Induced by Morphine in Rats

In the present study, the effects of intraperitoneal injections of GABA(A) receptor agonist and antagonist on memory formation and morphine state-dependent learning were investigated in rats. Pre-training administration of morphine (1-15 mg/kg) in a step-down passive avoidance task induced state-dependent learning with impaired memory retrieval on the test day. The impairment of memory was restored after the pre-test administration of the same dose of morphine. The pre-test administration of the GABA(A) receptor agonist, muscimol (0.01, 0.05 and 0.1 mg/kg), significantly decreased state-dependent retrieval induced by pre-test morphine (5 mg/kg). The state-dependency effect of morphine (1 mg/kg) was significantly potentiated by the pre-test administration of the GABA(A) receptor antagonist, bicuculline (0.125, 0.25 and 0.5 mg/kg). Furthermore, the pre-training injection of muscimol (0.01 mg/kg) impaired memory retrieval which was restored by pre-test morphine (1, 3 and 5 mg/kg) administration. However, the pre-training administration of bicuculline did not affect retention by itself. In addition, amnesia induced by pre-training morphine (5 mg/kg) was significantly reversed in rats which had received pre-test injections of muscimol (0.01, 0.05 and 0.1 mg/kg). Pre-test injections of bicuculline (0.125, 0.25 and 0.5 mg/kg) significantly decreased morphine-induced amnesia. It is concluded that the GABA(A) receptor mechanisms may be involved in the memory formation and it is postulated that these receptors may play an important role in morphine state-dependent learning.

Intra-amygdala muscimol injections impair freezing and place avoidance in aversive contextual conditioning

Rats were trained by shocking them in a closed compartment. When subsequently tested in the same closed compartment with no shock, normal rats showed an increased tendency to freeze. They also showed an increased tendency to actively avoid the compartment when given access to an adjacent neutral compartment for the first time. Amygdala inactivation with bilateral muscimol injections before training attenuated freezing and eliminated avoidance during the test. Rats trained in a normal state and given intra-amygdala muscimol injections before the test did not freeze or avoid the shock-paired compartment. This pattern of effects suggests that amygdala inactivation during training impaired acquisition of a conditioned response (CR) due either to inactivation of a neural substrate essential for its storage or to elimination of a memory modulation effect that facilitates its storage in some other brain region(s). The elimination of both freezing and active avoidance by amygdala inactivation during testing suggests that neither of these behaviors is the CR. The possibility that the CR is a set of internal responses that produces both freezing and avoidance as well as other behavioral effects is discussed.

Sialyltransferase ST8Sia-II Assembles a Subset of Polysialic Acid That Directs Hippocampal Axonal Targeting and Promotes Fear Behavior

Polysialic acid (PSA) is a post-translational protein modification that is widely expressed among neural cell types during development. Found predominantly on the neural cell adhesion molecule (NCAM), PSA becomes restricted to regions of neurogenesis and neuroplasticity in the adult. In the mammalian genome, two polysialyltransferases termed ST8Sia-II and ST8Sia-IV have been hypothesized to be responsible for the production of PSA in vivo. Approaches to discover PSA function have involved the application of endoneuraminidase-N to remove PSA and genetic manipulations in the mouse to deplete either NCAM or ST8Sia-IV. Here we report the production and characterization of mice deficient in the ST8Sia-II polysialyltransferase. We observed alterations in brain PSA expression unlike those observed in mice lacking ST8Sia-IV. This included a PSA deficit in regions of neurogenesis but without changes in the frequency of mitotic neural progenitor cells. In further contrast with ST8Sia-IV deficiency, loss of ST8Sia-II did not impair hippocampal synaptic plasticity but instead resulted in the misguidance of infrapyramidal mossy fibers and the formation of ectopic synapses in the hippocampus. Consistent with studies of animal models bearing these morphological changes, ST8Sia-II-deficient mice exhibited higher exploratory drive and reduced behavioral responses to Pavlovian fear conditioning. PSA produced by the ST8Sia-II polysialyltransferase modifies memory and behavior processes that are distinct from the neural roles reported for ST8Sia-IV. This genetic partitioning of PSA formation engenders discrete neurological processes and reveals that this post-translational modification forms the predominant basis for the multiple functions attributed to the NCAM glycoprotein.

The amygdala modulates the consolidation of memories of emotionally arousing experiences

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

Amygdala inactivation blocks expression of conditioned memory modulation and the promotion of avoidance and freezing

Rats were exposed to shock-paired cues immediately after training on an appetitive preference task. Elevated levels of freezing in and active avoidance of the shock-paired compartment were observed, and memory for the appetitive task was improved when tested 24 hr later. Intra-amygdala muscimol injected before the posttraining exposure eliminated freezing, avoidance, and memory modulation. The blockade of both freezing and active avoidance, which involve competing behavioral tendencies, makes it unlikely that the amygdala itself generates either behavior. The elimination of conditioned memory modulation suggests that conditioned neurohormonal responses were blocked. These conditioned internal responses may comprise the intervening variable of "conditioned fear" and may promote observable behaviors, the form of which is determined by the environment in which they occur.

Role of the basolateral amygdala in memory consolidation

Memories of emotionally arousing events tend to be more vivid and to persist longer than do memories of neutral or trivial events. Moreover, memories of emotionally influenced information may endure after a single experience. Recent findings strongly suggest that the influence of emotional arousal on memory consolidation is mediated by the release of adrenal stress hormones (epinephrine and glucocorticoids) and neurotransmitters that converge in modulating the noradrenergic system within the amygdala. Considerable evidence also indicates that amygdala activation influences memory by regulating consolidation in other brain regions. The findings suggest further that this memory-modulatory system may be involved in the formation of traumatic memories and posttraumatic stress disorder in human subjects.

Unilateral reversible inactivations of the nucleus tractus solitarius and amygdala attenuate the effects of bombesin on memory storage

This study examined the role of the nucleus tractus solitarius (NTS) and amygdala in mediating the effects of bombesin (BBS) on retention of inhibitory avoidance training by reversibly inactivating these regions with lidocaine immediately following training. In Expt. 1, peripheral injection of different doses of BBS (1, 2.5, 5, 10 or 20 microg/kg) to unoperated rats immediately after training in one trial inhibitory avoidance task (1 mA, 1.5 s footshock) produced a dose-dependent response on retention test scores which was given two days after training. In Expts. 2 and 3, rats were surgically implanted unilaterally with cannula tip placed above the NTS or amygdala, were trained in task and then received unilateral injections of saline or lidocaine (2%, 0.5 microl) into the NTS or amygdala and peripheral injections of saline or BBS (5 microg/kg). The results showed that the memory enhancing effect of BBS was attenuated by unilateral inactivation of the NTS or amygdala. These findings indicate that the NTS and amygdala are involved in mediating the memory modulating effects of peripheral BBS on memory storage.

Estrogen replacement attenuates effects of scopolamine and lorazepam on memory acquisition and retention

A multiple-trial passive avoidance paradigm was used to examine and compare the ability for estrogen replacement to attenuate learning and memory deficits produced by the muscarinic antagonist scopolamine and the benzodiazepine lorazepam. The multiple-trial paradigm was used in order to distinguish effects on acquisition from effects on retention. Estrogen replacement significantly attenuated a scopolamine-induced deficit on passive avoidance acquisition, but not retention. The ability for estrogen to attenuate the effect of scopolamine on acquisition was observed only when the analysis was limited to animals with serum levels of estradiol <200 pg/ml, suggesting that higher levels of estradiol were ineffective. This observation is consistent with at least one recent study showing dose-related effects of estrogen on ChAT-like immunoreactivity in the basal forebrain and supports the hypothesis that effects of estrogen on basal forebrain cholinergic neurons can help to reduce cognitive deficits associated with cholinergic impairment. Estrogen replacement was also observed to protect against a lorazepam-induced impairment on passive avoidance retention. This effect was observed specifically in animals that received estrogen prior to and during training and was not due to any effect of estrogen on serum levels of lorazepam following acute lorazepam administration. Collectively, these data demonstrate the ability for estrogen replacement to attenuate specific pharmacologically induced impairments in learning and retention and provide additional clues as to potential mechanisms by which estrogen replacement may help to reduce cognitive deficits associated with aging and Alzheimer's disease in postmenopausal women.

Importance of the intracellular domain of NR2 subunits for NMDA receptor function in vivo

NMDA receptors, a class of glutamate-gated cation channels with high Ca2+ conductance, mediate fast transmission and plasticity of central excitatory synapses. We show here that gene-targeted mice expressing NMDA receptors without the large intracellular C-terminal domain of any one of three NR2 subunits phenotypically resemble mice made deficient in that particular subunit. Mice expressing the NR2B subunit in a C-terminally truncated form (NR2B(deltaC/deltaC) mice) die perinatally. NR2A(deltaC/deltaC) mice are viable but exhibit impaired synaptic plasticity and contextual memory. These and NR2C(deltaC/deltaC) mice display deficits in motor coordination. C-terminal truncation of NR2 subunits does not interfere with the formation of gateable receptor channels that can be synaptically activated. Thus, the phenotypes of our mutants appear to reflect defective intracellular signaling.

Opposite effects on two different measures of retention following unilateral inactivation of the amygdala

We previously reported finding differential effects of unilateral infusion of lidocaine into the right and left amygdalae prior to a retention test 24 h after one-trial inhibitory avoidance training. The present study was undertaken to determine whether the degree of training in an escape task influences the effects of unilateral inactivation of the amygdala with lidocaine prior to the retention test. Male Sprague-Dawley rats, implanted with bilateral cannulae aimed at the amygdaloid complex received 0 (no shock), 2, or 10 training trials in a footshock-motivated escape task (0.4 mA). Forty-eight h later, the rats received bilateral microinfusions into the amygdalae of either buffer, or lidocaine into one amygdala and buffer in the other, 5 min before they were tested for retention of the escape training using a continuous multiple trial inhibitory avoidance (CMIA) procedure (0.4 mA). Forty-eight h after CMIA training the animals were retested, this time without influence of drugs. As indicated by initial response latencies on the first retention test, unilateral inactivation of either the right or the left amygdala severely impaired retention performance for the escape training regardless of the number of prior escape training trials received. However, unilateral inactivation of either amygdala had no effect on acquisition of CMIA. Furthermore, unilateral inactivation of either amygdala during CMIA training did not affect retention tested 48 h later. These results suggest that the differential involvement of the right and left amygdalae may be limited to certain training conditions. Furthermore, these findings demonstrate opposite effects on 2 different measures of retention indicating that the method of testing retention affects the interpretation of the memory impairment induced by infusion of lidocaine into the amygdala.

Bicuculline administered into the amygdala after training blocks benzodiazepine-induced amnesia

Male Sprague-Dawley rats were injected (i.p.) with either midazolam (MDZ, 2.0 mg/kg) or vehicle (1.0 ml/kg) 10 min before they were trained on a multiple-trial inhibitory avoidance task. Immediately following the training, bicuculline methiodide (BMI; 2.0, 5.6, 56.0 or 197.0 pmol/0.5 microl) or vehicle (0.5 microl) was infused bilaterally into the amygdala. On a 48 h retention test the performance of the MDZ-treated animals was significantly poorer than that of controls. The retention of MDZ-treated animals given intra-amygdala injections of the lowest dose of BMI (2.0 pmol) was comparable to that of controls, whereas higher doses of BMI impaired retention. The present results are consistent with other findings indicating that the amygdala mediates the amnestic effects of benzodiazepines on aversive learning. Furthermore, these data suggest that benzodiazepines impair memory by disrupting post-training processes underlying memory consolidation.

Interaction of neuromodulatory systems in modulating memory storage

An implicit assumption guiding many studies of neurochemical systems involved in learning and memory in animal subjects is that animal and human memory systems use the same or similar mechanisms. Because acquisition and retention performance can be influenced by many processes other than information storage, special effort is required to distinguish influences on memory processes from other factors influencing performance. This article reviews the findings of recent studies investigating the effects, on memory, of drugs affecting adrenergic, opioid peptidergic, GABAergic and cholinergic systems. The review focuses primarily on studies using posttraining treatments and tests for retention given no sooner than a day after the training. Extensive evidence suggests that such drugs interact within the amygdaloid complex and that projections from the amygdala influence memory storage in other brain regions. The assumption that comparable processes occur in animal and human subjects is supported by evidence that, in human subjects, emotionally influenced memory is blocked by a beta-adrenergic blocker and by lesions of the amygdaloid complex.

Involvement of the amygdala in memory storage: interaction with other brain systems

There is extensive evidence that the amygdala is involved in affectively influenced memory. The central hypothesis guiding the research reviewed in this paper is that emotional arousal activates the amygdala and that such activation results in the modulation of memory storage occurring in other brain regions. Several lines of evidence support this view. First, the effects of stress-related hormones (epinephrine and glucocorticoids) are mediated by influences involving the amygdala. In rats, lesions of the amygdala and the stria terminalis block the effects of posttraining administration of epinephrine and glucocorticoids on memory. Furthermore, memory is enhanced by posttraining intraamygdala infusions of drugs that activate beta-adrenergic and glucocorticoid receptors. Additionally, infusion of beta-adrenergic blockers into the amygdala blocks the memory-modulating effects of epinephrine and glucocorticoids, as well as those of drugs affecting opiate and GABAergic systems. Second, an intact amygdala is not required for expression of retention. Inactivation of the amygdala prior to retention testing (by posttraining lesions or drug infusions) does not block retention performance. Third, findings of studies using human subjects are consistent with those of animal experiments. beta-Blockers and amygdala lesions attenuate the effects of emotional arousal on memory. Additionally, 3-week recall of emotional material is highly correlated with positronemission tomography activation (cerebral glucose metabolism) of the right amygdala during encoding. These findings provide strong evidence supporting the hypothesis that the amygdala is involved in modulating long-term memory storage.

The amygdala modulates memory for changes in reward magnitude: involvement of the amygdaloid GABAergic system

Rats with bilateral amygdala cannulae were trained to run a straight alley for a large or small food reward. Muscimol (MUS), a GABAA agonist, or bicuculline methiodide (BMI), a GABAA antagonist, were infused into the amygdala immediately after a reward shift. In Experiment 1, rats in the large-reward group were shifted to the small reward and immediately after received an infusion of vehicle or MUS bilaterally into the amygdala. The runway latencies of the shifted vehicle animals were increased in comparison with those of the unshifted controls. In contrast, the shifted MUS (0.0001 nM, but not 0.01 nM) animals displayed latencies that were comparable to those of unshifted animals by the second postshift day. These findings suggest that muscimol dose-dependently attenuated the memory of the reward reduction. In Experiment 2 rats were trained as before except they received bilateral infusions of BMI into the amygdala immediately after reward reduction. The shifted BMI (0.1 nM, but not 0.3 nM) animals displayed increased runway latencies, in comparison with those of shifted vehicle animals, by the second postshift day. These findings suggest that BMI dose-dependently enhanced memory of the reward reduction. In Experiment 3, animals were trained as before except they first experienced a reward increase before receiving post-training injections of vehicle, MUS (0.001 nM) or BMI (0.1 nM). On the next day the reward was reduced. Despite reward reduction, shifted BMI animals persisted displaying low latencies for more trials than did shifted MUS animals. These findings suggest that BMI enhanced memory for the reward increase. More generally, the findings suggest that the amygdala and its GABAergic system is involved in memory consolidation for both positive and negative affective experiences.

Amygdala lesions block the effect of cocaine on memory in mice

Post-training cocaine intraperitoneal administration enhanced memory consolidation in unoperated and sham-lesioned mice tested in a one-trial inhibitory avoidance task. Bilateral electrolytical amygdala lesions blocked this effect. The results are interpreted in terms of involvement of neurotransmitter, and in particular dopaminergic, systems in the effects of cocaine on memory.

The memory-modulatory effects of glucocorticoids depend on an intact stria terminalis

This study examined the effects of stria terminalis (ST) lesions on glucocorticoid-induced modulation of memory formation for inhibitory avoidance training and spatial learning in a water maze. Systemic (s.c.) posttraining injections of the glucocorticoid receptor agonist dexamethasone (0.3 or 1.0 mg/kg) enhanced memory for inhibitory avoidance training in rats with sham ST lesions. Removal of the adrenal glands (adrenalectomy; ADX) significantly impaired spatial memory in a water maze, and immediate posttraining injections of dexamethasone (0.3 mg/kg) attenuated the memory impairment. Bilateral lesions of the ST did not significantly affect retention of these two tasks. However, ST lesions did block the effects of short-term ADX and dexamethasone administration on memory for both tasks. These results are similar to those of previous experiments examining the effects of lesions of the basolateral nucleus of the amygdala on the glucocorticoid-induced modulation of memory for both tasks. These findings suggest that the integrity of the ST, which connects the amygdala with other brain structures, is essential for the modulating effects of glucocorticoids on memory storage.

Spatial learning in rats exposed to acute ethanol intoxication on gestational day 8

Pregnant Wistar rats were treated on gestational day 8 (GD 8) with two IP injections of either ethanol (2.9 g/kg in 24% v/v saline solution) or saline. Offspring were tested in the water-maze task at 45 or 90 days of age. The escape latencies of rats trained with a submerged escape platform at a fixed location were similar between control and experimental rats. Analyses of responses on a probe trial carried out 10 days after the training period, revealed that 90-day-old females prenatally exposed to alcohol were less likely to swim in the target region. No differences were observed in this free-swim trial in 45- and 90-day-old male, and 45-day-old female animals. Binding studies of low-affinity GABAA sites in the hippocampus showed an increase in affinity of [3H]GABAA for their binding sites in 90-day-old female offspring prenatally intoxicated with ethanol. Our results demonstrate that acute intoxication with ethanol on GD 8 did not modify acquisition but impaired the retention of spatial learning only in adult female rats. It is possible that the impaired retention will be consequence of higher GABAA receptor affinity.

Short- and long-term effects of neonatal diazepam exposure on local cerebral glucose utilization in the rat

The short- and long-term consequences of a neonatal exposure to diazepam (DZP) on the postnatal changes in local cerebral metabolic rates for glucose (LCMRglcs) were studied by the quantitative autoradiographic [14C]2-deoxyglucose method in a total number of 66 brain structures of freely moving rats. Rat pups received a daily subcutaneous injection of 10 mg/kg DZP, of the dissolution vehicle or of saline from postnatal day 2 (P2) to 21 (P21). The animals were studied at 4 ages, P10, P14, P21 and P60. DZP induced a decrease in LCMRglcs which was restricted to 13 areas at P10, mainly sensory and limbic regions. At P14, the treatment had significant metabolic effects on 48 structures belonging to all functional systems. By P21, 23 brain areas were still affected by the treatment, mainly sensory, limbic and motor areas. At P60, i.e. at about 40 days after the end of drug exposure, LCMRglcs still decreased in 14 brain regions which were mainly sensory and limbic structures. The structures most sensitive to both short- and long-term consequences of the anticonvulsant treatment are mammillary body, limbic cortices and sensory regions. The dissolution vehicle increased LCMRglcs in a few brain regions at P14 and P60, whereas it decreased metabolic levels in 5 brain regions at P21. The results of the present study show that the brain appears to be particularly vulnerable to the treatment at P14, period of active brain growth, whereas by P21, the drug is actively metabolized and a tolerance to the treatment may occur. The long-term effects of the treatment are in good accordance with the well-known effects of DZP on anxiety, sedation and memory. The structures most sensitive to early neonatal DZP exposure are the mammillary body, limbic cortices and sensory regions that all contain a high density of benzodiazepine binding sites.

Interaction of GABAergic and beta-noradrenergic drugs in the regulation of memory storage

These experiments examined the interaction of drugs affecting noradrenergic and GABAergic systems, administered post-training, in influencing retention of an inhibitory avoidance response. Male CD1 mice (23-28 g) were trained in an inhibitory avoidance task, given immediate post-training ip injections of saline or GABAergic and adrenergic drugs administered either alone or concurrently. Retention was tested 48 h later. In agreement with extensive previous evidence, the GABAergic antagonist bicuculline (0.3, 1.0, or 3.0 mg/kg) produced dose-dependent (inverted-U) enhancement of retention and the GABAergic agonist muscimol (1.0 mg/kg) impaired retention. The retention-enhancing effects of bicuculline were blocked by concurrent administration of the beta-nor-adrenoceptor antagonist propranolol (2.0 mg/kg). Also in agreement with previous evidence, the beta-adrenoceptor agonist clenbuterol (0.030, 0.100, or 0.300 mg/kg, ip) produced dose-dependent (inverted-U) enhancement of retention. Clenbuterol also blocked the retention-impairing effects of muscimol (1.0 mg/kg). In addition, propranolol (2.0 mg/kg) potentiated the retention impairing effects of muscimol (1.0 or 3.0 mg/kg, ip). These findings support the view that GABAergic systems modulate memory through an interaction with beta-noradrenergic mechanisms.

Effects of intracranial infusions of chlordiazepoxide on spatial learning in the Morris water maze. I. Neuroanatomical specificity

The present investigation sought to determine the neuroanatomical locus through which the amnesic and anxiolytic effects of the benzodiazepine agonist chlordiazepoxide are mediated. Rats were infused with either chlordiazepoxide (60 nmol/microliters) or artificial CSF (1 microliter) into either the frontal cortex, nucleus basalis magnocellularis/substantia innominata, amygdala, medial septum, hippocampus, or cerebellum and run in the open field to assess anxiety as thigmotaxia and in the Morris water maze to assess spatial learning. Other rats were given chlordiazepoxide (5 mg/kg) or saline (1 ml/kg) systemically and run in the open field and water maze. When chlordiazepoxide was administered systemically, rats showed significantly less thigmotaxia, but not overall activity, than controls in the open field, and were deficit in spatial learning, but not cue learning or swim speed, in the water maze. Intracranial infusions revealed a neuroanatomical specificity for the amnesic and anxiolytic actions of chlordiazepoxide. Infusions of chlordiazepoxide into the amygdala, but none of the other structures, reduced thigmotaxia without affecting overall activity levels whereas infusions into the medial septum, but none of the other structures, prevented spatial learning, but not cue learning, and reduced swim speed in the water maze. Together, these finding suggest that the medial septum and the amygdala mediate the amnesic and anxiolytic actions of chlordiazepoxide, respectively. Moreover, these results provide direct evidence that the amnesic and anxiolytic actions of chlordiazepoxide are independent.

Localization in the amygdala of the amnestic action of diazepam on emotional memory

It is well known that systemically administered benzodiazepines (BZDs) induce anterograde amnesia in a variety of learning tasks. BZs effects are mediated through the GABAA complex by enhancing GABA-induced synaptic inhibition. As the GABAergic system in the amygdaloid complex (AC) is a site of action for the anxiolytic effects of BZs, such findings suggest that BZs may also influence memory through the amygdala. The present report summarizes a recent series of experiments designed to examine this implication. In a first experiment rats received either sham or bilateral AC lesion using N-methyl-D-aspartic acid (NMDA). One week later, animals were trained on an inhibitory avoidance task and tested 48 h later. Diazepam (DZP; 1.0 and 2.0 mg/kg, i.p.) or vehicle was injected 30 min prior to acquisition. The results demonstrate that DZP-induced retention deficits was blocked in rats with AC lesions. In a second experiment, in an attempt to localize the site of BZDs amnestic action in the AC, we tested the effects of DZP in rats with bilateral ibotenic acid-induced lesions of central (CE), lateral (LAT) or basolateral (BL) amygdala nuclei. The results shown that retention was impaired in animals with CE and LAT lesions but not in animals with BL lesions. In a third experiment we tested the effects of DZP microinjections in different nuclei of the AC on retention performance of rats trained in an avoidance task. The results demonstrate that DZP microinjection prior training in the BL/LAT, but not CE nuclei produce anterograde amnesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuromodulatory systems and memory storage: role of the amygdala

This article reviews findings of research examining the interaction of peripheral adrenergic systems with cholinergic, opioid peptidergic and GABAergic systems in modulating memory storage. It is well established that retention is enhanced by posttraining systemic or intra-amygdala injections of adrenergic agonists, opiate antagonists and GABAergic antagonists. These influences appear to be mediated by activation of NE receptors within the amygdala, as intra-amygdala injections of beta-adrenergic antagonists block the memory-modulating effects of hormones and drugs affecting these systems. Furthermore, these influences also appear to involve, at a subsequent step, activation of a cholinergic system: atropine blocks the memory-enhancing effects of adrenergic agonists and opiate and GABAergic antagonists and oxotremorine attenuate the memory-impairing effects of opiate agonists and GABAergic agonists. These findings suggest that the amygdala integrates the memory-modulating effects of neuromodulatory systems activated by learning experiences.

Intrinsic GABAergic neurons in the rat central extended amygdala

The present study examined the distribution, morphology, and connections of gamma-aminobutyric acid-immunoreactive (GABA-IR) neurons in the three principal components of the central extended amygdala: the central amygdaloid nucleus, the bed nucleus of the stria terminalis (BNST) and the sublenticular substantia innominata. In the central nucleus, large numbers of GABA-IR neurons were identified in the lateral, lateral capsular, and ventral subdivisions, though in the medial subdivision, GABA-IR neurons were only present at very caudal levels. Combined immunocytochemistry-Golgi impregnation revealed that GABA-IR neurons in the lateral central nucleus were medium-sized spiny neurons that were morphologically similar to GABAergic neurons in the striatum. Injections of horseradish peroxidase into the bed nucleus of the stria terminalis labeled a major proportion of the GABA-IR neurons in the central nucleus. In the bed nucleus, the majority of GABA-IR neurons were located in the anterolateral subdivision, ventral part of the posterolateral subdivision and the parastrial subdivision. GABA-IR neurons in the anterolateral bed nucleus were of the typical medium-sized spiny type. Injections of horseradish peroxidase into the central nucleus labeled a few GABA-IR neurons in the posterior part of the anterolateral bed nucleus. GABA-IR neurons were identified in the sublenticular substantia innominata and medial shell of the nucleus accumbens and contributed to the continuum of GABA-IR extending from the central nucleus to the bed nucleus. Injections of horseradish peroxidase (HRP) into the central nucleus, but not the BNST, labeled a few GABA-IR neurons in the substantia innominata. The data point to GABA-IR neurons being a characteristic feature of the central extended amygdala and that GABA-IR neurons participate in the long intrinsic connections linking the major components of this structure. Since lesions of the stria terminalis and basolateral amygdaloid nucleus failed to deplete GABA-IR terminals in the central nucleus, the role of GABA in local and short intrinsic connections in the central extended amygdala is discussed. Further, physiological findings implicating the intrinsic GABAergic system of the central extended amygdala in the tonic inhibition of brainstem efferents are reviewed.

Neocortical, hippocampal and septal parvalbumin- and somatostatin-containing neurons in young and aged rats: correlation with passive avoidance and water maze performance

Aged (26-month-old) rats were impaired compared with young (three-month-old) rats in passive avoidance and water maze tasks. In order to study whether changes in inhibitory circuits are involved in these age-related cognitive impairments, the number of two different subpopulations of GABAergic neurons, i.e. somatostatin- and parvalbumin-containing neurons, were counted in the hippocampal formation, septum and neocortex. We found that the number of parvalbumin-containing neurons was decreased in the entorhinal, somatosensory and motor cortex as well as in the medial septum and vertical limb of the diagonal band of Broca, but not in the hippocampus of aged rats. Somatostatin-containing neurons were affected in the somatosensory and motor cortex, and in the dorsolateral septum, but not in the hippocampus or in the entorhinal cortex. The decreased number of parvalbumin-containing neurons in the entorhinal cortex of the aged rats correlated with their performance deficits in passive avoidance and spatial learning. We propose that impaired functioning of the entorhinal cortex parvalbumin-containing inhibitory neurons may, to some extent, be responsible for the learning and memory defects found in aged rats.

Basolateral amygdala lesions block diazepam-induced anterograde amnesia in an inhibitory avoidance task

This experiment examined the effects of diazepam (DZP) on acquisition and retention of an inhibitory avoidance response by rats with excitotoxic-induced lesions of central (CE), lateral (LAT), or basolateral (BL) amygdala nuclei. Sham-operated and lesioned rats received i.p. injections of DZP (2.0 mg per kg of body weight) 30 min before training in a continuous multiple-trial inhibitory avoidance task. Retention was tested 48 h later. Acquisition was not impaired by the lesions or the DZP. Retention was impaired in animals with CE and LAT lesions in comparison with sham-operated controls. DZP impaired retention in the sham-operated controls as well as CE- and LAT-lesioned animals but did not affect retention in animals with BL lesions. These findings indicate that the DZP-induced anterograde amnesia for inhibitory avoidance training is mediated through influences involving the BL amygdala nucleus.

Amygdala lesions block the amnestic effects of diazepam

This experiment examined the effects of pre-training systemic injections of the benzodiazepine (BZ) diazepam (DZP) on learning and retention of an inhibitory avoidance response in rats with bilateral lesions of the amygdaloid complex (AC) induced by intra-amygdala injections of the excitotoxin N-methyl-D-aspartic acid (NMDA). Unoperated, sham-operated and AC-lesioned rats received i.p. injections of DZP (1.0 or 2.0 mg/kg) or vehicle 30 min prior to training in a continuous multiple-trial inhibitory avoidance task. Retention was tested 48 h later. The acquisition and retention of the AC-lesioned rats were impaired, relative to that of the unoperated and sham controls. In the unoperated and sham controls, DZP impaired retention but did not affect acquisition. In contrast, in animals with AC lesions, DZP did not affect either acquisition or retention. These findings suggest that the amnestic effects of DZP are mediated, at least in part, through influences involving the AC.

Involvement of the amygdala in the memory-enhancing effects of clenbuterol

Male Sprague Dawley rats with stria terminalis (ST) or sham lesions were trained in an inhibitory avoidance task, injected immediately post-training with clenbuterol hydrochloride (0.03 mg/kg; IP), and tested for retention 48 h later. Clenbuterol enhanced retention of the sham-lesioned animals but did not affect retention of the ST-lesioned animals. Clenbuterol injected intraamygdally immediately after training also enhanced retention of the inhibitory avoidance task. These results agree with other evidence suggesting the participation of beta-noradrenergic mechanisms in the amygdala in the modulation of memory storage.