Peripheral epinephrine modulates the effects of post-training amygdala stimulation on memory

Observer's adrenal corticosterone secretion involvement in vicarious fear conditioning

Vicarious learning represents a far-reaching value for the survival of social animals. Adrenal hormones are known to affect many forms of learning, yet the roles of adrenal hormones in vicarious learning remain unexplored. This study was undertaken to assess whether observation-stimulated corticosterone (CORT) secretion may affect the magnitude of a vicarious fear conditioning. Mouse observers were individually subjected to an observational compartment next to the training compartment wherein three their cage-mate demonstrators received (1) 5 days of 15 randomly-scheduled footshocks (0.5 mA, 2 s in duration over a 30 min session) (G1); (2) a 30-min presentation of vanilla odors (G2); or (3) footshock delivery and vanilla odors in combination (G3). Demonstrator mice receiving G3 training session and their respective observer mice were found to exhibit greater training-induced and slightly greater observation-stimulated CORT secretion, greater vanilla odors-induced fear responses (FR) and conditioned place aversion (CPA), as compared with the observers vicariously learning from demonstrators receiving G1 or G2 sessions. Observers held in their home cages during demonstrators' trainings and those receiving null demonstrator (No Demonstrator) failed to exhibit vanilla odors-induced FR. Moreover, observers undergoing adrenalectomy (ADX) and G3 sessions exhibited lower vanilla odors-induced FR and CPA as compared to sham surgical (Sham) observers observing G3 sessions. Furthermore, systemic metyrapone injections (50 and 100 mg/kg) prior to daily vicarious G3 training session resulted in decreases in vanilla odors-induced FR and CPA magnitudes in observers. Finally, CORT (1 mg/kg)-pretreated G2 observers failed to display odors-induced FR escalation. These results, taken together, suggest that observation-stimulated CORT secretion is necessary for reliable establishment of vicarious fear conditioning in observer mice.

Epinephrine modulates memory of latent learning in an inhibitory avoidance task

The present study examined the memory modulatory effect of epinephrine on latent learning of an inhibitory avoidance task. Male Sprague-Dawley rats on the first day were subjected to one of three conditions (no, short or long) in pre-exposure to the task apparatus. One day or several days later, they received the typical inhibitory avoidance training with a 0.5 mA/0.5 s foot shock. Memory of the inhibitory avoidance response was tested one day after the foot-shock training. The long pre-exposure group showed better memory than the no or short pre-exposure group, and this latent memory could last for 6 days: Retention scores of the long pre-exposure group were significantly better than those of the no pre-exposure group if the shock training was given 3 or 6 days, but not 12 or 21 days, after the pre-exposure. Epinephrine injected after the pre-exposure training modulated the latent memory in a dose- and time-dependent manner: 0.01 mg/kg given shortly after the short pre-exposure enhanced the memory, but 0.5 mg/kg given shortly after the long pre-exposure impaired it. Epinephrine injected 4 h after the pre-exposure had no effect, neither did that given to rats pre-exposed to a different context. Epinephrine (0.01 mg/kg) also made the latent memory lasting longer as the rats treated with it showed significant avoidance behavior when they had the shock training at 12 or 21 days after the pre-exposure. These findings suggest that epinephrine could modulate memory formed in the latent learning.

Significance and Remembrance: The Role of Neuromodulatory Systems

It is now well known that the retention of newly-acquired information can be modulated by drugs or hormones administered shortly following training. It is generally thought that such treatments influence retention by modifying processes underlying the storage of information. The fact that susceptibility to posttraining memory modulation is seen in many species, including bees, fish, birds, and mammals, argues that some common time-dependent memory storage processes have been conserved in evolution. Recent research findings have provided strong support for the view that such susceptibility to posttraining influences provides opportunity for modulation of memory storage by endogenous neurohormonal systems. In rats and mice, posttraining administration of hormones such as epinephrine that are normally released by training experiences enhances subsequent retention. Comparable effects are found with posttraining administration of opiate receptor antagonists such as naloxone. Findings of recent experiments indicate that these treatments affect memory by influencing the release of norepinephrine within the amygdaloid complex. The endogenous regulation of memory storage appears to involve interaction of neurohormones and transmitters in activating brain systems involved in memory storage.

Memory modulation

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

Endogenous noradrenergic activation and memory for emotional material in men and women

A plethora of evidence from the animal and human literature suggests that emotionally arousing material is often remembered better than is neutral material, and that this effect critically involves noradrenergic activation during and soon after exposure to the emotional material. A crucial prediction of this hypothesis is that endogenous adrenergic activation should relate positively and selectively to memory for emotional events in humans. Salivary alpha-amylase (sAA), a biomarker for adrenergic activity was measured in response to viewing a series of mixed emotional and neutral images to test this prediction in healthy men and women. One week after viewing these images subjects returned for a surprise free recall test. Endogenous noradrenergic activation, defined as an increase in sAA immediately after versus before slide viewing, occurred in 24 of 67 subjects. Regression analysis of the data revealed a significant positive correlation between the increase in sAA and the percentage of emotional pictures recalled. No correlation existed in the same subjects between sAA and the percentage of neutral pictures recalled. Additionally, the difference between these two correlations closely approached significance. The findings therefore demonstrate a relationship between a measure of endogenous noradrenergic activation and long-term memory performance in humans. The results support the view that adrenergic activation underlies enhanced memory for emotional material in humans, namely, that endogenous adrenergic activation in response to an emotional event should predict long-term memory for the event. The selectivity of the relationship for emotional, and not neutral, material supports the view derived from earlier research that stress activation does not necessarily enhance memory for all aspects of an emotional event; rather, that it acts disproportionately to influence memory for the more emotional aspects of an event. These findings are the first involving human subjects to indicate that the degree of endogenous noradrenergic activation in response to emotionally arousing stimuli predicts the strength of long-term memory for those stimuli.

Drug enhancement of memory consolidation: historical perspective and neurobiological implications

INTRODUCTION

Studies of drug enhancement of cognition began with Lashley's (Psychobiology 1:141-170, 1917) report that strychnine administered before daily training trials enhanced rats' maze learning. Many subsequent studies confirmed that finding and found that stimulant drugs also enhance the learning of a wide range of tasks.

DISCUSSION

A central problem in interpreting such findings is that of distinguishing the drug effects on brain processes underlying memory formation from many other possible effects of the drugs on the behavior used to assess learning. The subsequent finding that comparable learning enhancement can be obtained by posttraining drug administration provided compelling evidence that drugs can enhance memory by acting on memory consolidation processes. Such evidence stimulated the investigation of endogenous regulation of memory consolidation by arousal-released adrenal stress hormones.

CONCLUSION

Considerable evidence now indicates that such hormones regulate memory consolidation via activation of the basolateral amygdala and subsequent influences on many efferent brain regions involved in processing recent experiences. The implications of these findings for the development of cognitive enhancing drugs are discussed.

The role of the noradrenergic system in emotional memory

This contribution is an overview on the role of noradrenaline as neurotransmitter and stress hormone in emotional memory processing. The role of stress hormones in memory formation of healthy subjects can bear significance for the derailment of memory processes, for example, in post traumatic stress disorder (PTSD). Increased noradrenaline levels lead to better memory performance, whereas blocking the noradrenergic receptors with a betablocker attenuates this enhanced memory for emotional information. Noradrenaline appears to interact with cortisol in emotional memory processes, varying from encoding to consolidation and retrieval. Imaging studies show that confronting human subjects with emotional stimuli results in increased amygdala activation and that this activation is noradrenergic dependent. The role of noradrenaline in other brain areas, such as hippocampus and prefrontal cortex, is shortly summarized. Finally, the pros and cons of a therapeutic application of betablockers in the (secondary) prevention of PTSD will be discussed.

Reconsolidation of appetitive memories for both natural and drug reinforcement is dependent on {beta}-adrenergic receptors

We have investigated the neurochemical mechanisms of memory reconsolidation and, in particular, the functional requirement for intracellular mechanisms initiated by beta-adrenergic signaling. We show that propranolol, given in conjunction with a memory reactivation session, can specifically disrupt the conditioned reinforcing properties of a previously appetitively reinforced conditioned stimulus (CS), whether the stimulus had been associated with self-administered cocaine or with sucrose. These data show that memories for both drug and nondrug CS-US associations are dependent on beta-adrenergic receptor-mediated signaling for their reconsolidation, with implications for the potential development of a novel treatment for drug addiction and some forms of obesity.

Cholinergic modulation of memory in the basolateral amygdala involves activation of both m1 and m2 receptors

Muscarinic cholinergic activation is a critical component of basolateral amygdala (BLA)-mediated modulation of memory consolidation. The receptor(s) mediating this activation during consolidation have not been elucidated. This study investigated the roles of muscarinic subtype 1 (m1) and subtype 2 (m2) receptors in memory enhancement, by post-training intra-BLA infusions of the non-selective muscarinic agonist oxotremorine. Rats received intra-BLA infusions of either oxotremorine alone (10 microg in 0.2 microl per side), oxotremorine together with the selective m1 antagonist telenzipine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with the selective m2 antagonist methoctramine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with a combination of the above doses of telenzipine and methoctramine, or only vehicle, immediately after inhibitory avoidance training. Performance on a 48-hour retention test was significantly enhanced in oxotremorine-treated rats relative to vehicle-infused controls. Intra-BLA co-infusion of oxotremorine with either telenzipine (5, 17 or 50 nmol/side) or methoctramine (17 or 50 nmol/side) blocked the oxotremorine-induced enhancement. Combinations of these antagonists did not act additively to block memory enhancement by oxotremorine. These findings indicate that modulation of memory consolidation induced by cholinergic influences within the BLA requires activation of both m1 and m2 receptor synapses. Plausible mechanisms for m1- and m2-mediated influences on BLA circuitry are discussed.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Synapses on GABAergic neurons in the basolateral nucleus of the rat amygdala: double-labeling immunoelectron microscopy

Although the basolateral nucleus (BL) of the amygdala is known to contain an abundance of gamma-aminobutyric acid (GABA)ergic neurons that regulate the amygdaloid projection neurons and influence storage and consolidation of memory, it remains to be determined what type of neuronal input controls GABAergic neurons in the BL. We examined the synapses that GABAergic neurons form with GABAergic and noradrenergic neurons and terminals with unknown transmitters by double-labeling immunoelectron microscopy using anti-GABA and dopamine-beta-hydroxylase (DBH) antisera. The medium and small dendrites of the GABAergic neurons were shown to receive symmetric, inhibitory-type synapses from GABAergic axon terminals and asymmetric, excitatory-type synapses from noradrenergic axon terminals. Each segment of the GABAergic neurons from perikarya to dendritic spines received both symmetric and asymmetric synapses from unlabeled axon terminals of various forms and sizes. The incidence rates of the two types of synapses were almost identical. Our results suggest that GABAergic neurons in the BL of the rat amygdala might be affected by the excitatory influence of the noradrenergic system and the inhibitory influence of the GABAergic system. Furthermore, these neurons are also strongly influenced by both excitatory and inhibitory-type synapses from neuronal systems other than the GABAergic and noradrenergic systems.

Light and electron microscopic study of cholinergic and noradrenergic elements in the basolateral nucleus of the rat amygdala: evidence for interactions between the two systems

Pharmacological studies have suggested that the cholinergic (ACh) and noradrenergic (NA) systems in the amygdala (AM) play an important role in learning and memory storage and that the two systems interact to modulate memory storage. To obtain anatomical evidence for the interaction, the organization of the ACh and NA fibers in rat AM was investigated by immunocytochemistry for choline acetyltransferase (ChAT) and dopamine-beta-hydroxylase (DBH) in conjunction with light, confocal laser scanning, and electron microscopy (LM, CLSM, and TEM, respectively). LM showed that the ChAT immunoreactivity was densest in the basolateral nucleus (BL), whereas the DBH immunoreactivity was densest in the posterior BL. CLSM demonstrated that the ChAT-immunoreactive profiles in the BL were frequently located in juxtaposition to the DBH-immunoreactive axons. The TEM observations were as follows: The majority of the synapses formed by ChAT-immunoreactive terminals were symmetric, but DBH-immunoreactive axons formed both asymmetric and symmetric synapses. The ChAT-immunoreactive terminals usually established the symmetric synaptic contacts with the DBH-immunoreactive terminals and varicosities. The DBH-immunoreactive terminals formed the asymmetric synapses with the ChAT-immunoreactive dendrites of the intrinsic neurons within the AM. The results provide anatomical substrates for mnemonic functions of the ACh and NA systems and for the interactions between the two systems in the AM.

The amygdala: vigilance and emotion

Here we provide a review of the animal and human literature concerning the role of the amygdala in fear conditioning, considering its potential influence over autonomic and hormonal changes, motor behavior and attentional processes. A stimulus that predicts an aversive outcome will change neural transmission in the amygdala to produce the somatic, autonomic and endocrine signs of fear, as well as increased attention to that stimulus. It is now clear that the amygdala is also involved in learning about positively valenced stimuli as well as spatial and motor learning and this review strives to integrate this additional information. A review of available studies examining the human amygdala covers both lesion and electrical stimulation studies as well as the most recent functional neuroimaging studies. Where appropriate, we attempt to integrate basic information on normal amygdala function with our current understanding of psychiatric disorders, including pathological anxiety.

Glucocorticoid enhancement of memory consolidation in the rat is blocked by muscarinic receptor antagonism in the basolateral amygdala

Glucocorticoid-induced memory enhancement is known to depend on beta-adrenoceptor activation in the basolateral amygdala (BLA). Additionally, inactivation of muscarinic cholinergic receptors in the rat amygdala blocks memory enhancement induced by concurrent beta-adrenergic activation. Together, these findings suggest that glucocorticoid-induced modulation of memory consolidation requires cholinergic as well as adrenergic activation in the BLA. Two experiments investigated this issue. The first experiment examined whether blockade of muscarinic cholinergic receptors in the BLA with atropine alters the memory-enhancing effects of the systemically administered glucocorticoid dexamethasone. Dexamethasone (0.3, 1.0 or 3.0 mg/kg, s.c.) administered to rats immediately after inhibitory avoidance training produced dose-dependent enhancement of 48-h retention. Concurrent bilateral infusions of the muscarinic cholinergic antagonist atropine (0.5 microg in 0.2 microL per side) into the BLA blocked the memory enhancement. The second experiment investigated whether the BLA is a locus of interaction between glucocorticoid and muscarinic activation. The specific glucocorticoid receptor (GR or type II) agonist RU 28362 (1.0, 3.0 or 10 ng) was infused into the BLA either alone or together with atropine immediately after training. The GR agonist produced dose-dependent memory enhancement and atropine blocked the memory enhancement. These findings indicate that muscarinic cholinergic activation within the BLA is critical for enabling glucocorticoid enhancement of memory consolidation and that enhancement of memory induced by GR activation in the BLA requires cholinergic activation within the BLA.

Role of norepinephrine in mediating stress hormone regulation of long-term memory storage: a critical involvement of the amygdala

There is extensive evidence indicating that the noradrenergic system of the amygdala, particularly the basolateral nucleus of the amygdala (BLA), is involved in memory consolidation. Infusions of norepinephrine or beta-adrenoceptor agonists into the BLA enhance memory for inhibitory avoidance as well as water maze training. Other findings show that alpha 1-adrenoceptor activation also enhances memory for inhibitory avoidance training through an interaction with beta-adrenergic mechanisms. The central hypothesis guiding the research reviewed in this chapter is that stress hormones released during emotionally arousing experiences activate noradrenergic mechanisms in the BLA, resulting in enhanced memory for those events. Findings from experiments using rats have shown that the memory-modulatory effects of the adrenocortical stress hormones epinephrine and glucocorticoids are mediated by influences involving activation of beta-adrenoceptors in the BLA. In addition, both behavioral and microdialysis studies have shown that the noradrenergic system of the BLA also mediates the influences of other neuromodulatory systems such as opioid peptidergic and GABAergic systems on memory storage. Other findings indicate that this stress hormone-induced activation of noradrenergic mechanisms in the BLA regulates explicit/declarative memory storage in other brain regions.

Involvement of catecholamines in recall of the conditioned NK cell response

The primary goal of the study was to identify the types of catecholamines and the associated receptors which might be involved in the recall of the conditioned NK cell response. Specific catecholamine receptor antagonists were selected to block the conditioned NK cell response at the recall step. The regional contents of dopamine (DA), norepinephrine (NE), and epinephrine were determined in the brain of the conditioned animals by using the high performance liquid chromatography with electrochemical detection (HPLC/ED). Results showed that pre-disruption of the central alpha1-, alpha2-, beta1-, beta2-, D1-, or D2-receptors at the conditioned recall stage, interrupted the conditioned enhancement in NK cell activity. The NE contents at the cerebellum, and DA contents at the striatum and hippocampus, were significantly higher in the brain of the conditioned animals when compared to that of the control animals. These information indicated the possible roles of the central noradrenergic and dopaminergic systems in regulating the recall of the conditioned NK cell response.

Mechanisms of emotional arousal and lasting declarative memory

Neuroscience is witnessing growing interest in understanding brain mechanisms of memory formation for emotionally arousing events, a development closely related to renewed interest in the concept of memory consolidation. Extensive research in animals implicates stress hormones and the amygdaloid complex as key, interacting modulators of memory consolidation for emotional events. Considerable evidence suggests that the amygdala is not a site of long-term explicit or declarative memory storage, but serves to influence memory-storage processes in other brain regions, such as the hippocampus, striatum and neocortex. Human-subject studies confirm the prediction of animal work that the amygdala is involved with the formation of enhanced declarative memory for emotionally arousing events.

Inhibitory avoidance learning alters the amygdala calcium/calmodulin-dependent protein kinase II activity in rats

This study investigated the role of amygdala CaM-kinase II (calcium/calmodulin-dependent protein kinase II) in affective learning and memory. In Experiment I, two groups of rats were trained on a one-trial step through inhibitory avoidance learning task. The experimental group received a high intensity foot shock contingent upon the stepping-through behavior, whereas the control group received a series of non-contingent low intensity foot shock during training. The experimental rats showed significantly higher retention scores than the control rats. Correspondingly, rats in the experimental group showed significantly higher Ca2+-independent activity of CaM-kinase II than the controls. Intra-amygdala injection of a specific CaM-kinase II inhibitor, KN-62, before the training trial disrupted affective learning. In comparison with the vehicle-injected controls, pretraining injection of KN-62 impaired the acquisition of affective specific learning. These results, taken together, indicated that the activation of amygdala CaM-kinase II in the amygdala is associated with the affective learning behavior, and may be one of the neural mechanisms underlying formation of affective memory.

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.

Modulation of memory storage

For several decades, the concept of modulation of memory storage has significantly influenced research investigating neurobiological memory mechanisms. New evidence provides additional support for the view that stress hormones released during emotionally arousing situations modulate memory processes. Recent experiments have investigated the role of sympathetic adrenomedullary hormones in emotional memory in humans, as well as the role of adrenocortical hormones, primarily in animal studies. Further, it is becoming increasingly clear that the sympathetic adrenomedullary and the pituitary adrenocortical systems interact to modulate memory storage. Other new evidence emphasizes the role of peripheral influences to the brain on emotional memory, as well as the critical contribution of the amygdaloid complex in modulation of memory by emotional arousal.

Noradrenergic and cholinergic interactions in the amygdala and the modulation of memory storage

Numerous studies have reported that, in rats, memory can be affected by manipulations of the amygdala noradrenergic system. Typically, low doses of norepinephrine facilitate while higher doses impair memory storage. Muscarinic cholinergic agonists facilitate, while antagonists impair memory storage. Recent evidence from studies using systemic injection of drugs, indicates that these two systems interact in modulating memory storage. The experiments reported here examined interactions between the amygdala noradrenergic and muscarinic cholinergic systems. The results indicate that activation of muscarinic cholinergic mechanisms in the amygdala enhances retention, and that such activation mediates the facilitatory effects of systemically administered oxotremorine. beta-Noradrenergic agonists appear to exert their effects in the amygdala by activating the release of acetylcholine.

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.

Effects of vagotomy on Leu-enkephalin-induced changes in memory storage processes

When given before or after training, Leu-enkephalin impairs later retention of learning. To replicate these findings, Experiment 1 determined if Leu-enkephalin impairs retention when administered after inhibitory avoidance training. Posttraining injection of 100.0 micrograms/kg of Leu-enkephalin impaired retention relative to saline controls or those receiving a lower dose of this peptide. Since Leu-enkephalin does not cross freely from the blood stream into the brain, this peptide may exert its influence on mnemonic processes by activating peripheral receptors that transmit neural messages to the brain via the vagus nerve. In Experiment 2, 100.0 micrograms/kg Leu-enkephalin impaired retention in unoperated and sham-operated animals although vagotomized animals did not differ significantly from these groups or saline controls. These results suggest that subdiaphragmatic vagotomy may not be sufficient to disrupt the mnemonic effects of Leu-enkephalin. Alternatively, the vagus nerve may be one pathway but not the only pathway by which the activation of peripheral systems may influence the memory storage process.

Evidence that cholecystokinin-enhanced retention is mediated by changes in opioid activity in the amygdala

Mice, partially trained to avoid footshock in a T-maze, showed enhanced retention relative to vehicle-injected mice when treated peripherally with arecoline, D-amphetamine, cholecystokinin octapeptide (CCK-8), epinephrine or naloxone. Both intra-amygdaloid and intraventricular injections of beta-endorphin resulted in amnesia. D-amphetamine and arecoline blocked the amnestic effect of beta-endorphin administered into the amygdala but it required higher doses for CCK-8, epinephrine and naloxone to block the amnestic effect of beta-endorphin. The effects of CCK-8, epinephrine and naloxone showed a differential ability to block amnesia induced by beta-endorphin intraventricularly with epinephrine and naloxone preventing amnesia but CCK-8 not improving retention. This data suggests that the memory enhancement produced by peripherally administered CCK-8 involves the amygdala and that both CCK-8 and epinephrine interact with opioid amnestic mechanisms within the amygdala to alter memory processing.

Memory-enhancing effects of post-training dipivefrin and epinephrine: involvement of peripheral and central adrenergic receptors

These experiments examined the effects, in mice, of post-training i.p. injections of dipivefrin (DPE), a lipophilic prodrug of epinephrine, and epinephrine (EPI) on 48-h retention assessed in inhibitory avoidance and Y-maze discrimination tasks. DPE, in doses of 0.3-10 micrograms/kg significantly facilitated retention: the effects were approximately 10-fold more potent than those of EPI obtained with similar experimental conditions. The alpha-adrenergic antagonists prazosin (alpha 1; 3.0 mg/kg; i.p.), yohimbine (alpha 2; 3.0 mg/kg; i.p.) and phentolamine (alpha 1 and alpha 2; 3.0 mg/kg; i.p.) did not block the enhancement of retention induced by either DPE (10.0 micrograms/kg; i.p.) or EPI (0.1 mg/kg; i.p.). However, the beta-adrenergic antagonist propranolol (2.0 mg/kg; i.p.) attenuated the effects of both DPE and EPI. Sotalol (2.0 mg/kg; i.p.), a peripherally-acting beta-adrenergic antagonist, attenuated the effects of EPI but not those of DPE. These findings suggest the DPE-induced enhancement of memory involves central beta- but not alpha-adrenergic mechanisms while EPI's effects are initiated by activation of peripheral beta-adrenergic systems.

Effects of nucleus basolateralis amygdalae neurotoxic lesions on aversive conditioning in the rat

After bilateral stereotaxic administration of ibotenic acid on the n. basolateralis amygdalae, male adult rats were tested in the light-dark box apparatus to measure the time-course of the acquisition and retention of passive and active avoidance responses. The results show that after the lesions both passive avoidance and active avoidance acquisition were impaired. Passive avoidance responses were retained quite well, while active avoidance responses disappeared quickly. Conditioned freezing was almost completely absent. Thus it appears that the n. basolateralis plays a facilitatory role in all the conditioned responses which were investigated.

Involvement of the amygdaloid complex in neuromodulatory influences on memory storage

Neuromodulatory systems activated by training experiences appear to play a role in influencing memory storage processes. The research summarized in this paper examined the effects, on memory, of posttraining administration of treatments affecting adrenergic, opioid peptidergic and GABAergic systems. When administered after training, drugs affecting these systems all produce dose- and time-dependent effects on memory storage. The drug effects on memory are blocked by lesions of the amygdaloid complex as well as lesions of the stria terminalis, a major amygdala pathway. The effects of drugs affecting these neuromodulatory systems are also blocked by injections of beta-adrenergic antagonists administered to the amygdaloid complex. Thus, the findings suggest that the neuromodulatory systems affect memory storage through influences involving the activation of beta-adrenergic receptors within the amygdala. These findings are consistent with the view that the amygdala is involved in regulating the storage of memory in other brain regions.

Interaction between catecholaminergic and opioid systems in an active avoidance task

Male NMRI mice were given intravenous injections of the noradrenergic neurotoxin DSP4 or the vehicle 24 to 72 h prior behavioral testing. Animals were given 2 days of training on a one-way active avoidance task. Naloxone was given in one of three doses prior to training on Day 1 and Day 2 or prior to training on Day 1 only (saline was given prior to training on Day 2). There was a dose-dependent impairment of acquisition by naloxone in the vehicle-pretreated groups; 10 mg/kg naloxone produced a significant impairment of acquisition. Naloxone also modulated retention (Day 2) performance of the active avoidance task. For vehicle-pretreated mice, 1 mg/kg naloxone facilitated and 10 mg/kg naloxone-impaired performance on Day 2. DSP4 alone produced an impairment of acquisition of this task but had no effect on retention; Day 2 scores were slightly higher in the DSP4-pretreated group than in the vehicle-pretreated group. Naloxone produced somewhat different effects in DSP4-pretreated animals than in vehicle-pretreated animals. Naloxone (1 mg/kg) ameliorated the DSP4-induced impairment of acquisition; 10 mg/kg naloxone did not significantly alter the acquisition performance of this group. For the DSP4-pretreated mice that received naloxone before training on both days, the dose-response characteristics for retention scores were similar to those of vehicle-pretreated mice; 1 mg/kg naloxone was the facilitatory dose. However, for DSP4-treated mice that received naloxone before training on Day 1 only, there was a shift to the right in the effective facilitatory dose of naloxone. For these animals, 10 mg/kg naloxone but not 1 mg/kg naloxone significantly enhanced retention performance. We discuss these results in the context of a possible state-dependent modulation by naloxone in the DSP4-treated animals.

Animal models of anxiety based on classical conditioning: the conditioned emotional response (CER) and the fear-potentiated startle effect

Stimuli consistently paired with shock become capable of suppressing ongoing operant or consummatory behavior (the conditioned emotional response--CER) or elevating the amplitude of the startle reflex (fear-potentiated startle). These changes are used to infer a central state of fear which involves the central nucleus of the amygdala and its efferent projections to the brainstem. The present paper reviews how psychoactive drugs affect these measures. Both the CER and fear-potentiated startle are reduced by benzodiazepines, barbiturates and opiates. Advantages and disadvantages of these animal tests of anxiety are discussed.

Impairment of active avoidance by the noradrenergic neurotoxin, DSP4: attenuation by post-training epinephrine

Male Sprague-Dawley rats were treated with the selective noradrenergic neurotoxin, DSP4 prior to behavioral assessment. They were trained in an inhibitory avoidance and a one-way active avoidance task and given post-training treatment with epinephrine (EPI, 0.1 mg/kg, SC) or physiological saline. Performance on these tests was assessed at time points after treatment with DSP4 when (1) both central NE and sympathetic catecholamines were depleted and when (2) sympathetic catecholamines had returned to control levels and central NE remained depleted. Activity was also assessed at two time points after DSP4 treatment. DSP4 treatment had no effect upon inhibitory avoidance retention but impaired one-way active avoidance shuttle performance at both time points following DSP4 treatment. There was a transient depression of spontaneous activity which may indicate a deficit of behavioral initiation during the early phase after DSP4 treatment when the sympathetic catecholamine levels were depleted. This finding suggests that the peripheral sympathetic system may support some aspect of behavioral initiation. Epinephrine (0.1 mg/kg SC) administered after active avoidance training ameliorated the active avoidance retention performance deficit seen 4 days after DSP4 treatment. Post-training EPI did not significantly affect active avoidance retention performance when animals were trained and tested after peripheral sympathetic recovery (approximately 2 weeks after DSP4 treatment).(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociating learning and performance: drug and hormone enhancement of memory storage

This paper reviews selected studies examining the enhancing effects of drugs and hormones on learning and memory. Many strategies have been used in an effort to dissociate drug effects on learning from drug effects on other processes affecting the performance of responses. These strategies include the use of tasks with various motivational and response requirements, the use of studies explicitly examining drug influences on performance, the use of posttraining drug administration and the use of various forms of latent learning tasks. It seems clear from these studies that the dissociation of learning and performance effects of drugs cannot rest on one task or one experiment. Overall, the evidence summarized in this paper provides strong support for the conclusion that drugs can and do enhance retention and that the effects are due to influences on memory storage rather than to other factors that influence performance.

Modulation of memory by post-training epinephrine: involvement of cholinergic mechanisms

Extensive evidence indicates that memory storage can be modulated by peripheral epinephrine as well as by drugs affecting the muscarinic cholinergic system. Low doses of epinephrine (Epi) facilitate memory while high doses induce amnesia. Retention is enhanced by post-training administration of cholinergic muscarinic agonists and impaired by antagonists. The present experiments examined the interaction of peripheral Epi and cholinergic drugs in memory modulation. Male CFW mice (60 days old) were trained on an inhibitory avoidance response (IA) or a Y-maze discrimination response (YMD), injected (IP) immediately post-training and tested 24 h later. In the Y-maze task, retention was assessed by discrimination reversal training. Findings obtained in the two tasks were comparable. Epi (IA: 3.0 micrograms/kg; YMD: 30.0 micrograms/kg) potentiated the memory-enhancing effect of low doses of oxotremorine (Otm) (IA: 16; YMD: 5.0 micrograms/kg) and physostigmine (Phy) (6.8 and 22.0 micrograms/kg for both IA and YMD). The memory-facilitating effect of Otm (50.0 micrograms/kg) was not blocked by an amnestic dose of Epi (IAT: 0.3 mg/kg; YMD: 1.0 mg/kg). Retention was not affected when these amnestic doses of Epi were administered together with a memory-facilitating dose of Phy (68.0 micrograms/kg). In contrast, atropine (IA: 10.0 mg/kg; YMD: 3.0 mg/kg) completely blocked the facilitatory effect of Epi (IA: 10.0 micrograms/kg; YMD: 300 micrograms/kg). These findings indicate that, when administered in low doses, Epi interacts with Otm and Phy. However, cholinergic drugs can block both the memory-enhancing and the memory-impairing effects of Epi. The findings suggest that Epi may modulate memory through cholinergic systems.

Regional brain catecholamines and memory: effects of footshock, amygdala implantation, and stimulation

Previous findings have revealed a correlation between post-training release of whole brain norepinephrine (NE) and later retention performance. The present experiment examined changes after a training footshock in NE levels, as well as the levels of the major central NE metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), dopamine (DA), and epinephrine (EPI) in eight brain regions. Brain levels of these amines and the metabolite were assessed 10 min after training in a one-trial inhibitory (passive) avoidance task. The results indicate that NE levels decreased significantly in neocortex, neostriatum, hypothalamus, frontal pole, septum, and brainstem, but not in hippocampus or thalamus. The decreases in NE levels were generally accompanied by increases in MHPG; the MHPG/NE ratio increased significantly in all areas in which decreases in NE were observed. DA levels decreased in neostriatum and increased in neocortex and brainstem. Epinephrine levels decreased only in the brainstem sample. Thus, the effects of training on NE are widespread, probably reflecting the release of the amine in most brain regions. Such findings are consistent with the view that posttraining release of brain NE may modulate the storage of new information in many brain regions. One especially potent treatment for modulating memory storage is electrical stimulation of the amygdala. Therefore, we also examined the effects of amygdala implantation and stimulation on brain catecholamine levels to determine whether such changes might be correlated with the effects of amygdala stimulation on memory. The results indicate that electrode implantation into the amygdala results in pervasive changes in NE levels in most brain regions tested. Against this modified baseline, the results of training and electrical stimulation were region specific and very difficult to interpret. The major conclusion which can be derived from this portion of the experiment is that the amygdala damage produced by electrode implantation produces a brain which is substantially different from that of intact animals.

Epinephrine-induced memory facilitation: attenuation by adrenoceptor antagonists

The present study examined the relative importance of alpha- and beta-adrenoceptors in the memory modulatory effects of epinephrine. Posttraining epinephrine administration enhanced retention performance of a one-trial inhibitory avoidance response. Further pretraining injections of a variety of adrenoceptor antagonists, including selective alpha 1-, alpha 2-, beta 1- and/or beta 2-adrenoceptor antagonists, attenuated the retention enhancing effects of posttraining epinephrine. These results suggest that alpha- and beta-adrenoceptors of both subtypes are involved in the memory-modulating effects of epinephrine.

Epinephrine modulates long-term retention of an aversively motivated discrimination

These experiments examined the effects of post-training epinephrine (Epi) on retention of an aversively motivated discrimination task. Male CFW mice were trained to escape from footshock by entering one of two alleys of a Y-maze. On a 24-h retention test (six trials) the correct alley was reversed. The findings of Experiment 1 indicate that errors on the discrimination reversal varied directly with number of trials (criterion of 0, 3, or 6 successive correct choices) on the original training. These findings indicate that errors on discrimination reversal training provide a sensitive index of retention of the original training. In Experiment 2, mice were trained to a criterion of three successive correct choices and were given post-training injections of saline or Epi (0.1, 0.3, or 1.0 mg/kg ip). On a 24-h discrimination reversal test mice given the low doses of Epi made more errors than did saline controls while mice given the high dose made fewer errors. In Experiment 3, mice trained as in Exp 2 received post-training saline or Epi (0.3 or 1.0 mg/kg) and were tested for retention either 1 week or 1 month later. At each retention interval, performance was comparable to that found with a 24-h retention interval. The findings provide additional evidence that post-training Epi produces long-lasting dose-dependent modulating effects on memory storage.

Modulating effects of posttraining epinephrine on memory: involvement of the amygdala noradrenergic system

These experiments examined the effects, on retention, of posttraining intra-amygdala administration of norepinephrine (NE), and propranolol. Rats were trained on a one-trial step-through inhibitory avoidance task and tested for retention 24 h later. Injections were administered bilaterally (1.0 microliter/injection) through chronically-implanted cannulae. Low doses of NE (0.1 or 0.3 microgram) administered shortly after training enhanced retention while higher doses (1.0 or 5.0 micrograms) were ineffective. Retention was not affected by NE administered 3 h after training. The effect of intra-amygdala NE on retention is blocked by simultaneous administration of propranolol (0.2 microgram). This finding suggests that the memory-enhancing effect of NE may be mediated by beta-receptors. Posttraining intra-amygdala NE also attenuated the retention deficit produced by adrenal demedullation. Further, intra-amygdala injections of propranolol (0.2 microgram) blocked the enhancing effect, on retention, of posttraining s.c. injections of epinephrine. These findings suggest that activation of noradrenergic receptors in the amygdala may be involved in memory processing and may play a role in the memory-modulating effect of peripheral epinephrine.