Differential involvement of hippocampal and amygdalar NMDA receptors in contextual and aversive aspects of inhibitory avoidance memory in rats

Adult male rats bilaterally implanted with guide canullae aimed either at the dorsal hippocampus (dHIP) or the basolateral nucleus of the amygdala (BLA) were trained in a step-down inhibitory avoidance task (IA) and tested for retention 24 h after training. Immediately after training, animals were given a bilateral infusion of the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP5) (5.0 microg) into the dHIP or the BLA. Both intrahippocampal and intraamygdala infusions of AP5 blocked IA retention. Preexposure to the training box, but not to a different environment 24 h prior to training prevented the impairing effect of intrahippocampal infusion of AP5 on retention. Preexposure did not affect the retention impairment induced by intraamygdala infusion of AP5. These data suggest that hippocampal NMDA receptors might be involved in the contextual and spatial aspects, while amygdalar NMDA receptors might be involved in the aversive aspects of memory for IA.     

(+/-)-Alpha-methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor blocker, impairs retention of an inhibitory avoidance task in rats when infused into the basolateral nucleus of the amygdala

The amygdala is important for memory processes of emotionally motivated learning and the amygdala glutamatergic system may play a key role in this process. In this study we assessed the effect of the infusion of (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG), a metabotropic glutamate receptor (mGluR) antagonist, into the basolateral complex of the amygdala (BLA) on the learning and retention of an emotionally motivated task. Rats received either vehicle or three different doses of MCPG (0.2, or 1.0, or 5.0 microg/0.2 microl/side, respectively) bilaterally into the BLA, 5 min before they were trained in a continuous multiple-trial inhibitory avoidance (CMIA) task. Response latencies during the training were recorded. Retention was assessed 8 days later. MCPG in the doses given did not significantly affect the acquisition of the CMIA task. However, MCPG at a dose of 5.0 microg/0.2 microl/side impaired the long-term retention test performance. Additionally, a nociception test indicated that dose of MCPG infused into the BLA did not affect the footshock sensitivity. Our results indicate that MCPG, when infused into the BLA of rats prior to the training, impaired long-term memory of aversive training without affecting acquisition.     

Effects of intra-amygdala injections of NMDA receptor antagonists on acquisition and retention of inhibitory avoidance

These experiments examined the effects of intra-amygdala injections of NMDA receptor antagonists on the acquisition and retention of inhibitory avoidance. In Expt. I, rats received bilateral intra-amygdala injections of the NMDA antagonists D,L-AP5 (1-10 micrograms), D-AP5 (0.03-1 micrograms), CPP (0.125 or 0.375 microgram), or MK-801 (0.2 or 0.5 microgram) prior to training in a continuous multiple-trial inhibitory avoidance (CMIA) task. Acquisition of the task was not significantly affected by any of the drug injections. In contrast, all three competitive antagonists, D,L-AP5, D-AP5 and CPP, produced dose-dependent impairment of 48 h retention performance. Although the MK-801 injections did not significantly impair retention performance, the retention scores of the 0.5 microgram MK-801 group were bimodally distributed, indicating retention impairment in a subgroup of the animals given that dose. Intra-amygdala injections of 3 or 10 micrograms D,L-AP5 did not affect footshock sensitivity (Expt. II) or locomotor activity (Expt. III) and their retention-impairing effects were not due to induction of state dependency (Expt. IV). The retention-impairing effects of intra-amygdala injections of NMDA antagonists were not due to diffusion of the drugs dorsally: injections of 1 microgram D-AP5 into the striatal area directly above the amygdala impaired acquisition but not retention performance (Expt. V). The retention-impairing effects of 1 microgram D-AP5 or 0.5 microgram MK-801 were attenuated by giving additional training to the animals shortly after receiving intra-amygdala injections (Expt. VI). The implications of these findings for hypotheses concerning amygdala function in learning and memory are discussed.     

(±)-α-Methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor blocker, impairs retention of an inhibitory avoidance task in rats when infused into the basolateral nucleus of the amygdala

The amygdala is important for memory processes of emotionally motivated learning and the amygdala glutamatergic system may play a key role in this process. In this study we assessed the effect of the infusion of (±)-α-methyl-4-carboxyphenylglycine (MCPG), a metabotropic glutamate receptor (mGluR) antagonist, into the basolateral complex of the amygdala (BLA) on the learning and retention of an emotionally motivated task. Rats received either vehicle or three different doses of MCPG (0.2, or 1.0, or 5.0 μg/0.2μl/side, respectively) bilaterally into the BLA, 5 min before they were trained in a continuous multiple-trial inhibitory avoidance (CMIA) task. Response latencies during the training were recorded. Retention was assessed 8 days later. MCPG in the doses given did not significantly affect the acquisition of the CMIA task. However, MCPG at a dose of 5.0 μg/0.2 μl/side impaired the long-term retention test performance. Additionally, a nociception test indicated that dose of MCPG infused into the BLA did not affect the footshock sensitivity. Our results indicate that MCPG, when infused into the BLA of rats prior to the training, impaired long-term memory of aversive training without affecting acquisition.     

Post-training intrabasolateral amygdala infusions of dopamine modulate consolidation of inhibitory avoidance memory: involvement of noradrenergic and cholinergic systems

There is extensive evidence that several neurotransmitter systems within the basolateral amygdala (BLA) influence memory consolidation. The present study investigated the influence of dopamine (DA) in the BLA on the consolidation of memory for inhibitory avoidance (IA) training. Male Sprague-Dawley rats (approximately 300 g) were trained on a step-through IA task and, 48 h later, tested for retention as indexed by their latencies to enter the shock compartment on the test day. Drugs were infused into the BLA or central amygdala nucleus (CEA) immediately or 3 h after training via bilateral cannulae. DA infused into the BLA immediately after training enhanced retention, whereas DA infused into the BLA 3 h after training or into the CEA did not affect retention. Infusions of the dopaminergic antagonist cis-Flupenthixol together with DA blocked the DA-induced memory enhancement. Immediate post-training intra-BLA infusions of the D1 receptor antagonist SCH 23390 or the D2 receptor antagonist sulpiride impaired retention. beta-adrenergic or muscarinic cholinergic receptor antagonists coinfused into the BLA with DA blocked the memory enhancing effects of DA. These findings indicate that dopaminergic activation within the BLA modulates memory consolidation and that the modulation involves activation of both D1 and D2 receptors and concurrent activation of beta-adrenergic and cholinergic influences within the BLA.     

Impairment of object recognition memory by rapamycin inhibition of mTOR in the amygdala or hippocampus around the time of learning or reactivation

The role of the basolateral complex of the amygdala (BLA) in recognition memory remains poorly understood. The mammalian target of rapamycin (mTOR) in the BLA and other brain areas has been implicated in synaptic plasticity and memory. We have recently shown that mTOR signaling in both the BLA and the dorsal hippocampus (DH) is required for formation and reconsolidation of inhibitory avoidance, a fear-motivated memory task. Here we examined the effects of infusions of the mTOR inhibitor rapamycin into the BLA before or after either training or reactivation on retention of novel object recognition (NOR) memory in rats, and compared the effects with those obtained using intra-DH infusions. Male Wistar rats received bilateral infusions of vehicle or rapamycin into the BLA or DH before or after NOR training or reactivation. Rapamycin impaired NOR retention tested 24 h after training when given either before or immediately after training into the BLA or DH. Rapamycin also impaired retention measured 24 h after reactivation when infused before reactivation into the BLA or DH, or immediately after reactivation into the BLA, but not when given 6 h after reactivation into either the BLA or DH. The results suggest that mTOR signaling in the BLA and DH is involved in NOR memory formation and stabilization.     

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 μg/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 μl) into the NTS or amygdala and peripheral injections of saline or BBS (5 μg/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.     

Excitotoxic basolateral amygdala lesions potentiate the memory impairment effect of muscimol injected into the medial septal area

In rats, the septo-hippocampal system is important for memory encoding. Previous reports indicate that muscimol, a specific GABAergic agonist induces learning and memory deficits when infused into the medial septal area. The basolateral nucleus of the amygdala (BLA) modulates memory encoding in other brain areas, including the hippocampus. To explore the interactions between the septo-hippocampal system and amygdala in memory, we studied the effects of intra-medial septal infusions of muscimol in rats with BLA lesions. Animals received sham surgery or excitotoxic BLA lesions and were given infusions of either vehicle or muscimol (5 nmol) into the medial septal area 5 min prior to training sessions in inhibitory avoidance and water maze tasks. In the inhibitory avoidance task, muscimol-induced memory impairment was potentiated by BLA amygdala lesions. Additionally, in the water maze task, BLA-lesioned rats given muscimol infusions into the medial septal also showed memory impairment. These findings indicate that the MSA interacts with the BLA in the processing of memory storage.     

NMDA receptor antagonism in the basolateral but not central amygdala blocks the extinction of Pavlovian fear conditioning in rats

Glutamate receptors in the basolateral complex of the amygdala (BLA) are essential for the acquisition, expression and extinction of Pavlovian fear conditioning in rats. Recent work has revealed that glutamate receptors in the central nucleus of the amygdala (CEA) are also involved in the acquisition of conditional fear, but it is not known whether they play a role in fear extinction. Here we examine this issue by infusing glutamate receptor antagonists into the BLA or CEA prior to the extinction of fear to an auditory conditioned stimulus (CS) in rats. Infusion of the α‐amino‐3‐hydroxyl‐5‐methyl‐4‐isoxazole‐propionate (AMPA) receptor antagonist, 2,3‐dihydroxy‐6‐nitro‐7‐sulfamoyl‐benzo[f]quinoxaline‐2,3‐dione (NBQX), into either the CEA or BLA impaired the expression of conditioned freezing to the auditory CS, but did not impair the formation of a long‐term extinction memory to that CS. In contrast, infusion of the N‐methyl‐d ‐aspartate (NMDA) receptor antagonist, d,l ‐2‐amino‐5‐phosphonopentanoic acid (APV), into the amygdala, spared the expression of fear to the CS during extinction training, but impaired the acquisition of a long‐term extinction memory. Importantly, only APV infusions into the BLA impaired extinction memory. These results reveal that AMPA and NMDA receptors within the amygdala make dissociable contributions to the expression and extinction of conditioned fear, respectively. Moreover, they indicate that NMDA receptor‐dependent processes involved in extinction learning are localized to the BLA. Together with previous work, these results reveal that NMDA receptors in the CEA have a selective role acquisition of fear memory.     

Inhibitory avoidance learning altered ensemble activity of amygdaloid neurons in rats

In this study, we examined single-unit activity in the amygdala before and after a rat had acquired an inhibitory avoidance task. Long-Evans rats with microwires chronically implanted into the central nucleus (CeA) or basolateral complex (BLC) of the amygdala were acclimatized to the apparatus of a step-through inhibitory avoidance task for three sessions. On the fourth session, rats in the experimental group received an inescapable footshock (3 mA, 1 s) as they stepped from the lit side into the dark side of the task apparatus, whereas rats in the control group received the same amount of shock on a different apparatus. All rats were tested for retention in the task apparatus 1 day after shock training. The experimental rats showed better retention than the controls as they stayed longer in the lit side. Ensemble unit activities were recorded in the amygdala nuclei from the indwelling wire bundles during the acclimation and test sessions. The data collected from well-isolated amygdala units showed that neuronal discharge habituated from the first to the third acclimation session. In the test session, the experimental group, but not the control group, showed elevated firing rates in the CeA or BLC neurons located on either side of the brain. These findings provide the first piece of evidence showing that learning of an inhibitory avoidance task leads to an increase in amygdala neuronal discharges during a retention test.     

Sequential Role of Hippocampus and Amygdala, Entorhinal Cortex and Parietal Cortex in Formation and Retrieval of Memory for Inhibitory Avoidance in Rats

The hippocampus and amygdala, the entorhinal cortex and the parietal cortex participate, in that sequence, both in the formation and in the expression of memory for a step-down inhibitory avoidance task in rats. Bilateral infusion of AP5 or muscimol caused retrograde amnesia when given O min after training into both hippocampus and amygdala, when given or 180 min after training into the entorhinal cortex, or when given 180 min after training into the parietal cortex. Therefore, memory formation requires the sequential and integrated activity of all these areas mediated by glutamate NMDA receptors in each case. Pre-test administration of CNQX 1 day after training into hippocampus and amygdala, 1 or 31 days after training in entorhinal cortex, or 1, 31 or 60 days after training in the parietal cortex temporarily blocked retention test performance. Therefore, 1 day after training, all these brain structures are necessary for retrieval; 1 month later, the hippocampus and amygdala are no longer necessary for retrieval but the entorhinal and parietal cortex still are; and 60 days after training only the parietal cortex is needed. In all cases the mechanisms of retrieval require intact glutamate AMPA receptors.     

Glucocorticoids interact with the basolateral amygdala beta-adrenoceptor--cAMP/cAMP/PKA system in influencing memory consolidation

Infusion of a beta-adrenoceptor antagonist into the basolateral nucleus of the amygdala (BLA) blocks memory enhancement induced by systemic or intra-BLA administration of a glucocorticoid receptor (GR) agonist. As there is evidence that glucocorticoids interact with the noradrenergic signalling pathway in activating adenosine 3prime prime or minute,5prime prime or minute-cyclic monophosphate (cAMP), the present experiments examined whether glucocorticoids influence the beta-adrenoceptor--cAMP system in the BLA in modulating memory consolidation. Male, Sprague--Dawley rats received bilateral infusions of atenolol (a beta-adrenoceptor antagonist), prazosin (an alpha1-adrenoceptor antagonist) or Rp-cAMPS (a protein kinase A inhibitor) into the BLA 10 min before inhibitory avoidance training and immediate post-training intra-BLA infusions of the GR agonist, RU 28362. Atenolol and Rp-cAMPS, but not prazosin, blocked 48-h retention enhancement induced by RU 28362. A second series of experiments investigated whether a GR antagonist alters the effect of noradrenergic activation in the BLA on memory consolidation. Bilateral intra-BLA infusions of the GR antagonist, RU 38486, administered 10 min before inhibitory avoidance training completely blocked retention enhancement induced by alpha1-adrenoceptor activation and attenuated the dose--response effects of post-training intra-BLA infusions of clenbuterol (a beta-adrenoceptor agonist). However, the GR antagonist did not alter retention enhancement induced by post-training intra-BLA infusions of 8-Br-cAMP (a synthetic cAMP analogue). These findings suggest that glucocorticoids influence the efficacy of noradrenergic stimulation in the BLA on memory consolidation via an interaction with the beta-adrenoceptor--cAMP cascade, at a locus between the membrane-bound beta-adrenoceptor and the intracellular cAMP formation site.     

Bombesin/gastrin-releasing peptide receptors in the basolateral amygdala regulate memory consolidation

Several receptor and intracellular signalling systems in the basolateral amygdala (BLA) regulate memory formation. In the present study, we show that bombesin/gastrin-releasing peptide (GRP) receptors in the BLA are involved in the consolidation of affectively motivated memory. Adult male rats were trained in a single-trial step-down inhibitory avoidance task and tested for retention 24 h later. Post-training systemic injection of the bombesin/GRP receptor antagonist (D-Tpi6, Leu13 psi[CH2NH]-Leu14) bombesin (6-14) (RC-3095) impaired memory retention. In rats implanted under thionembutal anaesthesia with guide cannulae aimed at the BLA, post-training bilateral infusion of RC-3095 into the BLA dose-dependently impaired retention. Pre-training unilateral muscimol inactivation of the BLA blocked the memory-impairing effect of post-training systemic administration of RC-3095. The results suggest that bombesin/GRP receptors in the BLA are involved in the consolidation of aversive memory, and the BLA mediates the memory-impairing effect of systemic bombesin/GRP receptor blockade.     

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.     

Relaxin receptor activation in the basolateral amygdala impairs memory consolidation

The peptide-hormone relaxin has well-established actions in male and female reproductive tracts, and has functional effects in circumventricular regions of brain involved in neurohormonal secretion. In the current study, we initially mapped the distribution of mRNA encoding the relaxin receptor--leucine-rich repeat-containing G-protein-coupled receptor 7 (LGR7)- and [33P]-human relaxin-binding sites in extra-hypothalamic sites of male Sprague-Dawley rats. The basolateral amygdala (BLA) expressed high levels of LGR7 mRNA and relaxin-binding sites and, although relaxin peptide was not detected in the BLA, several brain regions that send projections to the BLA were found to contain relaxin-expressing neurons. As it is well established that the BLA is involved in regulating the consolidation of memory for emotionally arousing experiences, we investigated whether activation of LGR7 in the BLA modulated memory consolidation for aversively motivated inhibitory avoidance training. Bilateral infusions of human relaxin (10-200 ng in 0.2 microL) into the BLA immediately after inhibitory avoidance training impaired 48-h retention performance in a dose-dependent manner. Delayed infusions of relaxin into the BLA 3 h after training were ineffective, indicating that the retention impairment was due to influences on memory consolidation. Post-training infusions of relaxin into the adjacent central amygdala, which is devoid of LGR7, did not impair retention. These findings suggest a novel function for endogenous relaxin-LGR7 signalling in rat brain involving regulation of memory consolidation.     

Norepinephrine attenuation of amnesia produced by diethyldithiocarbamate

Rats given 680 mg/kh diethyldithiocarbamate, approximately one half hour before training in an inhibitory avoidance task, had impaired retention performance when tested one week after training. Intracerebroventricular or subcutaneous injections of norepinephrine administered shortly after training attenuated the disruptive effects of DDC on retention performance. The effect depended upon the footshock intensity used during training. NE(0.01 microgram) administered centrally attenuated the DDC induced retention deficit when animals were trained with a high (2 mA) but not a low footshock (0.5 mA). The effect of peripherally administered NE also varied with intensity of footshock. The lowest dose of subcutaneously administered NE (5 microgram/kg) was effective in attenuating DDC induced retention deficits only when animals were trained with higher footshock. Higher doses of NE (50 microgram/kg, 500 microgram/kg) were more effective when animals were trained with lower footshock.     

Basolateral Amygdala Lesions Block the Memory-enhancing Effect of Glucocorticoid Administration in the Dorsal Hippocampus of Rats

These experiments examined the effects of bilateral amygdala nuclei lesions on modulation of memory storage induced by bilateral intrahippocampal microinfusions of glucocorticoids in male Sprague-Dawley rats. Post-training infusions of the glucocorticoid receptor (type II) agonist RU 28362 (3.0 or 10.0 ng) enhanced inhibitory avoidance retention, and infusions of the glucocorticoid receptor antagonist RU 38486 (3.0 or 10.0 ng) administered shortly before training in a water maze spatial task did not affect acquisition, but impaired retention. In both tasks, neurochemically induced lesions of the basolateral but not of the central amygdala blocked the memory-modulatory effects of the intrahippocampal infusions of the drugs affecting glucocorticoid receptors. Lesions of the central amygdala alone impaired inhibitory avoidance retention, but basolateral amygdala lesions alone did not affect acquisition or retention in either task. These findings are consistent with previous evidence indicating that lesions of the basolateral amygdala block the memory-modulatory effects of systemically administered glucocorticoids, and provide further evidence that the basolateral amygdala is a critical area involved in regulating glucocorticoid effects in other brain regions involved in memory storage.     

Novelty causes time-dependent retrograde amnesia for one-trial avoidance in rats through NMDA receptor- and CaMKII-dependent mechanisms in the hippocampus

Exposure to a novel environment (an open field) for 2 min, 1 h after one-trial inhibitory avoidance training, hindered memory of the avoidance task measured 24 h later. The effect was seen regardless of the intensity of the avoidance training footshock. The effect was not seen if the exposure to novelty was carried out 5 min before, or 6 h after, the avoidance training, or if the animals did not perceive the open field as new and react accordingly. The amnesic effect of the novelty presented 1 h after avoidance training was blocked by the intrahippocampal infusion of D-2-amino-5-phosphono-pentanoic acid (AP5, 25 nmoles per side) or 1-(N, O-bis-[5-isoquinolinylsulphonyl]-N-methyl-L-tyrosyl)-4- phenylpiperazine (KN62, 100 micromoles per side) but not by that of C32H25N3O6 (KT5720, 90 micromoles per side) given 5 min before the novelty. In the open field there was habituation, measured by the decrease in exploration between the first and second minute. AP5 and KN62 impaired this habituation, but not KT5720. Exploration of the open field was similar in the groups exposed to the avoidance task 5 min later, or 1 h or 6 h before. Therefore, there was no reciprocity between the effect of the two tasks: novelty was amnesic for the one-trial avoidance task, but the opposite was not true. The amnesic effect of novelty appears to rely on N-methyl-D-aspartate (NMDA) receptor- and calcium/calmodulin-dependent protein kinase II (CaMKII)-dependent, but not on PKA-dependent, aspects of its habituation learning.     

Posterior hypothalamic deafferentation abolishes the amnestic effect of electroconvulsive shock in rats

The amnestic effect of immediate post-training transcorneal electroconvulsive shock ECS (15.0 mA, 60 Hz, 2 sec) on step-down inhibitory avoidance learning (0.5 mA, 60 Hz training footshock) was studied in intact rats and in rats submitted to bilateral surgical transection of the dorsal fornix, to anterior or posterior hypothalamic deafferentation, and in sham-operated animals. Animals were tested for retention 24 hr after training. The amnestic effect of ECS was observed in all groups except in the one with the posterior hypothalamic lesion. Fornix-lesioned animals showed a moderate retention deficit which was considerably worsened by the ECS treatment. The results indicate that the amnestic effect of ECS requires integrity of posterior hypothalamic pathways. One possibility is that the amnestic effect of ECS may be mediated by posterior afferent fibers to the hypothalamus acting on hypothalamic opioid systems such as have been previously proposed to play a role in ECS-induced amnesia.     

Facilitating influence of stress on the consolidation of fear memory induced by a weak training: Reversal by midazolam pretreatment

It is well known that an emotionally arousing experience usually results in a robust and persistent memory trace. The present study explored the potential mechanisms involved in the influence of stress on the consolidation of a contextual fear memory in animals subjected to a weak fear training protocol, and whether pretreatment with intra-basolateral amygdala or systemic administration of midazolam (MDZ) prevents the potential stress-induced influence on fear memory formation. A previous restraint session facilitated fear retention, this effect was not due to a sensitized effect of restraint on the footshock experience. MDZ, both systemically or intra-basolateral amygdala infusion prior to the restraint, attenuated the stress-induced promoting influence on fear memory formation. In addition, stress exposure activated the ERK1/2 pathway in basolateral amygdala (BLA) after the weak training procedure but not after the immediate footshock protocol. Similar to our behavioral findings, MDZ attenuated stress-induced elevation of phospho-ERK2 (p-ERK2) in BLA following the acquisition session. Given that the activation of ERK1/2 pathway is essential for associative learning, we propose that stress-induced facilitation of p-ERK2 in BLA is an important mechanism for the promoting influence of stress on the consolidation of contextual fear memory.