The effects of acute restraint stress and dexamethasone on retrieval of long-term memory in rats: an interaction with opiate system

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1 Behavioural Brain Research 154 (2004) Research report The effects of acute restraint stress and dexamethasone on retrieval of long-term memory in rats: an interaction with opiate system Ali Rashidy-Pour a,, Hassan Sadeghi a, Abbas Ali Taherain a, Abbas Ali Vafaei a, Yaghoub Fathollahi b a Department of Physiology, School of Medicine, Semnan University of Medical Sciences, Semnan, Iran b Department of Physiology, School of Medical Sciences, Tarbiat Modarres University, Tehran, Iran Received 31 December 2003; received in revised form 8 February 2004; accepted 9 February 2004 Available online 11 March 2004 Abstract This study investigated whether application of acute restraint stress or dexamethasone, as a glucocorticoid receptor agonist, impaired retrieval of long-term memory and if pretreatment with opiate antagonist naloxone blocked their effects on memory retrieval. Young adult male rats were trained in one trial inhibitory avoidance task (1 ma, 1.5 s footshock). On retention test given 48 h after training, the latency to re-enter dark compartment of the apparatus was recorded. Thirty minutes before retention test, the rats were exposed to a 10 min of restraint stress in a Plexiglass tube or were injected with dexamethasone (1 mg/kg) with or without prior treatment of naloxone (1 or 2 mg/kg). The results showed that both acute restraint stress and dexamethasone impaired retention performance. Both doses of naloxone were effective in blocking the impairing effect of stress, but only higher dose of naloxone blocked dexamethasone-induced impairment. The applied stress increased circulating corticosterone levels as assessed immediately after the retention test, indicating that stress-induced impairment of memory retrieval is mediated, in part, by increased plasma levels of glucocorticoids. These findings further indicate that acute restraint stress and glucocorticoids impair retrieval of long-term memory, and provide evidence for the existence of an interaction between glucocortioids and opiate system on this process Elsevier B.V. All rights reserved. Keywords: Restraint stress; Dexamethasone; Memory retrieval; Naloxone; Opiate system 1. Introduction Memory is a process that consists of an encoding or acquisition phase, followed by an extended consolidation or stabilization phase. Memory is expressed during a retrieval phase, which can occur before or after consolidation. Extensive evidence indicates that stress and glucocorticoids modulate memory process in both animals and humans [1,15,21]. It is important to dissociate the effects of these treatments on various phases of memory processing: acquisition, consolidation and retrieval. In animals and humans studies, an inverted U-shape relationship has been reported between the doses of glucocorticoids administered and memory acquisition or consolidation: extreme low and high doses may disrupt acquisition or consolidation, but intermediate doses enhance memory acquisition or consolidation [3,14,15,25,28]. However, the effects of stress and Corresponding author. Tel.: ; fax: address: Rashidy-pour@sem-ums.ac.ir (A. Rashidy-Pour). glucocorticoids on retrieval process are poorly understood. The disrupting effects of stress or glucocorticoids on retention performance when animals or human subjects were tested shortly after training, while plasma levels of glucocorticoid are still elevated [6,13], led to this hypothesis that glucocorticoids not only affect memory consolidation, but also memory retrieval. The results of two recent studies supported this hypothesis. In one study, both footshock stress and acute injection of corticosterone was shown to impair retrieval of spatial information in a Morris water maze task in rats [5]. In another work, it was shown that acute administration of cortisone impairs retrieval of hippocampal-dependent free recall of previously learned words in humans [4]. Therefore, these findings indicate that, in addition to modulation of acquisition and consolidation phases, stress and glucocorticoids also affect retrieval of spatial memory in rats and declarative information in humans. It remains to be determined, however, whether comparable effects are found with different types of stress or different training tasks in animals or humans /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.bbr

2 194 A. Rashidy-Pour et al. / Behavioural Brain Research 154 (2004) Extensive evidence indicates that opiate drugs modulate memory process. When injected after training in a variety of tasks, opiate receptor agonist and antagonists impair and enhance memory consolidation, respectively [16]. Opiate drugs also influence on memory retrieval process. Peripheral administration of naloxone or ICI (mu and delta opiate receptor antagonists, respectively) prior to testing has been found to enhance memory retrieval in rats. On the other hand, activation of kappa opiate receptor by dynorphin enhanced memory retrieval [10]. In addition, intra-cerebral administration of a mu opiate receptor agonist impairs memory retrieval in mice [30]. It has been shown that injection of different doses of morphine before retention test, impaired memory retrieval dose-dependently and naloxone antagonized the impairing effects of morphine on memory retrieval in mice [24], while in another study, similar high dose of morphine has been observed to facilitate memory retrieval in mice and the effect was completely blocked by naloxone pre-treatment [27]. These findings considered together with other results [2] indicate that the opiate system modulates memory retrieval process. The ability of opiate agonist or antagonist to modulate the effects of some kinds of stress provides evidence that opiate system may play a mediation role during stress [17]. Although the effects of some types of stress including immobilization or restraint stress on some behaviors such as analgesia appear to be opiate mediated [12,28,31,32], it is not known whether the effects of stress on memory retrieval involve interaction with opiate system. The aims of the present study were to examine: (1) the effects of acute restraint stress and dexamethasone, as a synthetic glucocorticoid agonist, on long-term memory retrieval in an inhibitory avoidance task, a rodent model to investigate emotionally relevant memory, and (2) whether pretreatment with naloxone, as an opiate antagonist, can modulate the influences of acute restraint stress or dexamethasone on memory retrieval. 2. Materials and methods 2.1. Animals Adult male Wistar rats ( g) were used in this study. They were housed five per cage in a room with natural light cycle and constant temperature (24 ± 2 C). Food and water were available ad libitum. All experiments were performed between 10:00 and 13:00 h during the light cycle Drugs Dexamethasone (Synopharm, Italy) was dissolved in 100% ethanol and subsequently diluted in 0.9% saline to reach its final concentration (1 mg/kg). The final concentration of ethanol was 2%. Dexamethasone was injected subcutaneously at a volume of 1 ml/kg. Control animals received the vehicle only (a saline solution containing 2% ethanol). Naloxone hydrochloride (Sigma) as an opiate receptor blocker was dissolved in saline and was injected i.p. The drug doses were mainly derived from pilot studies, and a survey of reports on these drugs [21,33] 2.3. Inhibitory avoidance training Apparatus The apparatus used for inhibitory training consisted of a trough-shape alley (90 cm long; 20 cm deep; 20 cm wide at the top, 6.5 cm wide at the bottom) divided by a sliding door into two compartments [19,20]. The illuminated compartment (30 cm long) was made from transparent plastic and was connected through an 8 8 guillotine door to the dark compartment (60 cm long). The walls and ceiling of the dark compartment were black and opaque and the floor of the dark compartment was constructed of stainless steel rods (3 mm in diameter, 10 mm apart) through which footshock could be delivered from a constant current source Training All experimental groups were first habituated to the apparatus. The rat was placed in the illuminated compartment and 5 s later, the guillotine door was raised. Upon entering the dark compartment, the door was closed and the rat was taken from the dark compartment into the home cage. The acquisition trial was done 30 min later during which the guillotine door was closed and a 50 Hz, 1 ma constant current shock was applied for 1.5 s immediately after the rat had entered the dark compartment. The rat was removed from the dark compartment about 5 s after receiving the shock and returned to his home cage Retrieval test Avoidance memory of all rats was assessed by a 10 min extinction trail 48 h later, during which shock was never delivered. The rat was placed in the illuminated chamber, as in the training session, and the latency of entering the dark chamber (step-through latency) during a 10 min period was recorded. Longer latencies were interpreted as indicating better memory retention Restraint stress procedure The rats were gently immobilized for 10 min in a Plexiglass tube (20 cm length, 6.5 cm diameter) 30 min before retrieval test Radioimmunoassay for corticosterone For plasma corticosterone determination, immediately after retention test in experiment 1, the control and stressed rats were decapitated and trunk blood was collected in tubes with heparin and centrifuged (3000 g, 20 min) and the plasma was stored at 20 C until the corticosterone assay.

3 A. Rashidy-Pour et al. / Behavioural Brain Research 154 (2004) Corticosterone was measured using RIA kits (Immunodiagnostic Systems, Boldon, UK). The sensitivity of the assay was 0.39 ng/ml Locomotor activity measurement Locomotor activity of animal was measured using an automated activity monitor system (TSE inframot, TSE, Bad Homburg, Germany). The system uses so called passive infrared sensors. These sensors register the activity of animal by sensing the body head image, i.e. infrared radiation, and its spatial displacement over time. The sensor unit (contained infrared sensors) was placed on the metal grid cover of a home cage. Locomotor activity of each rat was measured for five 2-min intervals. Only one animal was placed in each activity chamber per measurement time Statistical analysis The inhibitory avoidance retention data were analyzed with a two-way analysis of variance (ANOVA) with dexamethasone or stress condition and naloxone treatment both as between-subjects variables. Locomotor activity was analyzed using two-way ANOVA with group as the independent measure and time as the repeated measure. Two-way ANOVA followed by Turkey s test for multiple comparisons. Corticosterone levels were analyzed by Student t-test. Values of P<0.05 were considered significant Experiments Experiment 1 In first experiment the effect of acute restraint stress on retrieval of long-term memory was examined in presence or absence of naloxone (NAL). Rats were randomly divided into six groups (n = 12 in each group) as follows: control (no stress, NS), saline (SAL) + stress (S), NAL (1 mg/kg) + NS, NAL (2 mg/kg) + NS, NAL (1 mg/kg) + S and NAL (2 mg/kg) + S. The rats were stressed 30 min prior to retention test. NAL was injected 5 min before application of stress. Immediately after retention test, blood samples were taken from control and stressed rats and plasma corticosterone was measured according to procedures explained in Section Experiment 2 In this experiment the effect of dexamethasone (DEX, 1 mg/kg) on retrieval of long-term memory was examined in presence or absence of NAL. Rats were randomly divided into six groups (n = in each group) and given two injections: the first injection was given 35 min prior to retention test and the second injection was given 5 min after the first injection. The six pairs of injections were SAL + vehicle (VEH), SAL + DEX, NAL (1 mg/kg) + VEH, NAL (2 mg/kg)+veh, NAL (1 mg/kg)+dex, NAL (2 mg/kg)+ DEX Experiment 3 In this experiment the effects of acute stress (S) or DEX (1 mg/kg) on motor activity were examined in presence or absence of NAL (2 mg/kg). Rats were randomly divided into six groups (n = 7 in each group) as follows: control, SAL + S, DEX, NAL + S, NAL + DEX and NAL. Thirty minutes after treatment, the motor activity was measured according to procedures explained in Section Results 3.1. Experiment 1 The results of experiment 1 are shown in Fig. 1. A two-way ANOVA on retention latencies data revealed significant effects of acute restraint stress (F 1,70 = 5.95; P = 0.01), no effect of naloxone (F 2,70 = 2.14; P = 0.12), and a significant interaction (F 2,70 = 5.05; P = 0.008). Post hoc analysis showed that the step-through latencies of animals in SAL + S group were significantly shorter than those of animals in groups of NS (P <0.01), NAL (1 mg/kg) + S(P<0.01) and NAL (2 mg/kg) + S(P< 0.01). The step through latencies of animals in the NS group were not different from those of animals in groups of NAL (1 mg/kg) + NS, NAL (2 mg/kg) + NS, NAL (1 mg/kg) + S or NAL (2 mg/kg) + S. These results show that both doses of naloxone per se did not affect memory retrieval but attenuated the impairing effect of stress on memory retrieval. Fig. 2 showed plasma corticosterone levels in control and stressed animals. As expected, exposure to a 10 min restraint stress significantly (P <0.01) increased plasma corticosterone concentrations in stressed group in comparison with control group Experiment 2 The results of experiment 2 are shown in Fig. 3. A two-way ANOVA on retention latencies data revealed Fig. 1. Step-through latencies (mean ± S.E.M.) for a 48-h inhibitory avoidance test. Effects of acute restraint stress on retrieval of long-term memory in presence or absence of naloxone. P<0.01 as compared with control group. P<0.01 as compared with saline + stress group. SAL, saline; S, stress; NAL, naloxone; (n = 12 for each group).

4 196 A. Rashidy-Pour et al. / Behavioural Brain Research 154 (2004) Fig. 2. Effects of acute restraint stress on plasma corticosterone levels (mean±s.e.m.) as assessed immediately after retention test in experiment 1. P<0.05 as compared with control group. significant effects of dexamethasone (F 1,66 = 6.21; P = 0.01), but with no effect of naloxone (F 2,66 = 1.5; P = 0.22), and a significant interaction (F 2,66 = 3.24; P = 0.04). Post hoc comparisons showed that step-through latencies of animals receiving SAL + DEX were significantly shorter than those of groups receiving SAL + VEH (P < 0.05) or NAL (2 mg/kg) + DEX (P <0.05), but not NAL (1 mg/kg) + DEX. The step-through latencies of animals receiving SAL + VEH were not significantly longer than those of NAL (1 mg/kg) + VEH, NAL (2 mg/kg) + VEH or NAL (2 mg/kg) + DEX. These results show that both doses of naloxone per se did not affect memory retrieval but higher dose (2 mg/kg) of it attenuated the impairing effect of restraint stress on memory retrieval. Fig. 3. Step-through latencies (mean ± S.E.M.) for a 48-h inhibitory avoidance test. Effects of dexamethasone on retrieval of long-term memory in presence or absence of naloxone. P < 0.05 as compared with vehicle-saline group. P < 0.05 as compared with the saline + dexamethasone group. SAL, saline; VEH, vehicle; DEX, dexamethasone; NAL, naloxone (n = for each group) Experiment 3 The locomotor activity results are shown in Fig. 4. There were no significant differences between groups in total activity recorded for 10 min period (F 5,36 = 0.66; P = 0.654). A two-way ANOVA (group time) of locomotor activity at each two min interval of the 10-min test showed lack of significant effect of groups (F 5,36 = 1.84; P = 0.128), a significant effect of time (F 4,144 = ; P<0.0001), Fig. 4. Effects of acute restraint stress or dexamethasone in presence or absence of naloxone on locomotor activity. (A) mean (±S.E.M.) total locomotor activity for a 10-min period; (B) mean (±S.E.M.) locomotor activity over the 10-min test (2-min interval). NAL, naloxone; SAL, saline; DEX, dexamethasone; S, stress (n = 7 for each group).

5 A. Rashidy-Pour et al. / Behavioural Brain Research 154 (2004) and no significant interaction between the effects of groups and time (F 20,144 = 0.77; P = 0.745). Post hoc analysis indicated that in all groups locomotor activity was increased during the first two min interval (P <0.05 for all comparisons). These results indicate that there were no differential effects of restraint stress or dexamethasone in presence or absence of naloxone on locomotor activity. 4. Discussion The findings of the present experiments are that acute restraint stress or dexamethasone injection shortly before retention test impaired long-term memory retrieval in an inhibitory avoidance task, and their effects were blocked by an opiate receptor antagonist naloxone. The findings of experiment 3 revealed that acute restraint stress or dexamethasone with or without prior treatment of naloxone did not affect locomotor activity and thus suggest that the impairing effects of stress or dexamethasone on retention performance were not due to a nonspecific influence on animal behavior. Stress activates the hypothalamus-pituitary-adrenal axis and causes release of glucocorticoids. It seems that this sequence of events contributes to stress-induced behavioral responses [13]. It was found that both exposure to restraint stress and dexamethasone injection impaired memory retrieval and also the applied stress significantly increased the levels of corticosterone in stressed animals as measured at the time of testing. Thus, these findings considered together with other results [23] indicate that the actions of stress on memory retrieval are mediated by glucocorticoids. Furthermore, the impairing effects of these treatments were not equal. There was an approximately 65% decrease in memory following restraint stress (Fig. 1), but only a 25% reduction after dexamethasone (Fig. 3). It is well known that dexamethasone and other exogenous glucocorticoids can suppress corticosterone release via inhibition of hypothalamus-pituitary-adrenal axis, and thus reduce levels of corticosterone in plasma. Therefore, this may be a mechanism for a weaker memory impairment induced by dexamethasone than restraint stress. Numerous studies in the past have dealt with the role of glucocorticoids on processes of memory acquisition and consolidation in both animals and human [1,15,21], but the literature concerning the actions of glucocorticoids on memory retrieval remains much less. Only a few recent studies addressed the actions of glucocorticoids on memory retrieval [4,5]. We have found that both acute restraint stress and dexamethasone disrupted memory retrieval in an inhibitory avoidance task. Our results are in accordance with studies showing impairing effects of corticosterone on retrieval of long term declarative memory in humans [4] and footshock stress and corticosterone on spatial memory retrieval in Morris water maze in rats [5]. Based on these findings, it seems that besides the well-known effects of stress and glucocorticoids on processes of memory acquisition and consolidation, they also affect memory retrieval process. Importantly, glucocorticoids have opposing actions on memory consolidation and retrieval processes [22]. For example, same dose of dexamethasone used in this work, which impaired memory retrieval, was shown to improve memory consolidation in aversive and appetitive tasks [26,33]. The reasons for these differential actions are not clear and remain to be determined in future researches. The disrupting effects of both dexamethasone and stress-induced glucocorticoids on memory retrieval in the present work, was presumably mediated by an activation of brain glucocorticoid receptors. In agreement with this hypothesis, a very recent study has demonstrated that intra-hippocampal injection of a glucocorticoid receptor agonist has effects on memory retrieval similar to those produced by systemic administrations [23]. In this study, both exposure to acute restraint stress and dexamethasone 30 min before retention test, have been found to disrupt memory retrieval. These rapid actions may therefore involve activation of membrane glucocorticoid receptors that are independent from the classical glucocorticoid receptors. Several studies have characterized structure and function of a membrane corticosteroid receptor [7,8,17]. The results on rapid electrophysiological and behavioral actions of glucocortioids provide new evidence that these membrane receptors are responsible for mediating non-classical (non-genomic) actions of glucocorticoids [9,11]. Further studies are necessary to determine whether membrane receptors for glucocortioids are responsible for mediating the disrupting effects of glucocortioids on memory retrieval process. Naloxone, as an opiate antagonist, did not affect memory retrieval alone, but rather blocked the impairing effects of both acute stress and dexamethasone. This finding indicates involvement of naloxone sensitive pathway in mediating the influences of acute stress or dexamethasone on memory retrieval. Previous studies have shown that the opiate drugs affect memory retrieval, but their effects are complex and depend on receptor subtypes [2,24,27,30]. There are two possible explanations for an interaction between restraint stress or dexamethasone and naloxone on memory retrieval. First, it is possible that restraint stress or dexamethasone activates the endogenous opiate system and which mediates their influences on memory retrieval. Although, there are some evidences indicating that the effects of restraint stress on some behaviors such as analgesia is mediated, at least in part, by the endogenous opiate system [12,29], to our knowledge there are no reports in literature concerning such a mediation on memory retrieval. Another explanation for these results might be that naloxone interacts with stress-induced glucocorticoids or dexamethasone in plasma membrane of target neurons in brain. As noted above, retrieval deficit was found as soon as 30 min after dexamethasone injection or restraint stress. This is probably too fast to explain genomic action of glucocortioids via intracellular receptors and it may reflect an involvement of membrane

6 198 A. Rashidy-Pour et al. / Behavioural Brain Research 154 (2004) receptors. In light of this idea, a membrane receptor for glucocorticoids in brain of the roughskin newt (Taricha granulosa) as an amphibian model has been identified recently [7], and this receptor appears to be a G-protein coupled receptor [18]. This receptor mediates the rapid effects of acute stress or corticosterone on reproductive behaviors and neural activity of the animal [9,11]. Ligand binding assays with radiolabeled corticosterone revealed that this receptor binds corticosterone and cortisol, but also recognizes specific opiate ligands such as dynorphine and naloxone. These opiate ligands interact directly by competing for the same binding site [8]. Therefore, opiate antagonists that bind to membrane receptor for glucocorticoids should block the behavioral effects of glucocorticoids. This idea is supported by the present findings which indicate that naloxone blocks the deteriorating effects of both acute restraint stress and dexamethasone on memory retrieval process. In conclusion, although the underlying mechanisms remain to be determined, the present results provide evidence for the existence of an interaction between glucocorticoids and opiate system on long-term memory retrieval. Acknowledgements We thank Dr. A.R. Bandegi for performing corticosterone assay. This work was supported by a grant from Research Office of Management and Planning Organization of Iran to Dr. A. Rashidy-Pour and Dr. Y. Fathollahi. References [1] Belanoff JK, Gross K, Yagar A, Schatzberg AF. Corticosteroids and cognition. J Psychiat Res 2001;35: [2] Bruins Slot LA, Colpaert FC. Opiate states of memory: receptor mechanisms. J Neurosci 1999;19: [3] Conrad CD, Galea LAM, Kuroda Y, McEwen BS. Chronic stress impairs rat spatial memory on the Y-maze, and this effect is blocked by tianeptine pretreatment. Behav Neurosci 1996;110: [4] de Quervain DJ-F, Roozendaal B, Nistsch RM, McGaugh JL, Hock C. Acute cortisone administration impairs retrieval of long term declarative memory in humans. Nature Neurosci 2000;3: [5] de Quervain DJ-F, Roozendaal B, McGaugh JL. Stress and glucocorticoids impair retrieval of long term spatial memory. Nature 1998;394: [6] Diamond DM, Park CR, Heman KL, Rose GM. Exposing rats to a predator impairs spatial working memory in the radial water maze. Hippocampus 1999;9: [7] Evans SJ, Murray TF, Moore FJ. Partial purification and biochemical characterization of a membrane glucocorticoid receptor from an amphibian brain. J Steroid Biochem Mol Biol 2000;72: [8] Evans SJ, Searcy BT, Moore FJ. A subset of kappa opioid ligands bind to the membrane glucocorticoid receptor in an amphibian brain. Endocrinology 2000;141: [9] Hua SY, Chen YZ. Membrane receptor mediated electrophysiological effects of glucocorticoid on mammalian neurons. Endocrinology 1989;124: [10] Ilyutchenok RY, Dubrovina NI. Memory retrieval enhancement by kappa opioid agonist and mu, delta antagonists. Pharmacol Biochem Behav 1995;52: [11] Joles M, de Kloet ER. Effects of glucocorticoids and norepinephrine on the excitability in the hippocampus. Science 1989;245: [12] Kavaliers M, Yang H-YT. IgG from antiserum against endogenous mammalian FMRF-NH 2 related peptides augments morphine and stress induced analgesia in mice. Peptides 1989;10: [13] Kirschbaum C, Wolf OT, May M, Wippich W, Hellhammer DH. Stress- and treatment-induced elevations of cortisol levels associated with impaired declarative memory in healthy adults. Life Sci 1996;58: [14] Luine V, Martinez C, Villegas M, Magarinos AM, McEwen BS. Restraint stress reversibly enhances spatial memory performance. Physiol Behav 1996;59: [15] Lupien SJ, McEwen BS. The acute effects of corticosteroids on cognition: integration of animal and human model studies. Brain Res Rev 1997;24:1 27. [16] McGaugh JL. Involvement of hormonal and neuromodulatory systems in the regulation of memory storage. Ann Rev Neurosci 1989;12: [17] Olson GA, Olson RD, Kastin AJ. Endogenous opiates: Peptides 1990;11: [18] Orchinik M, Murray TF, Franklin PH, Morre FL. Guanyl nucleotides modulate binding to steroid receptors in neuronal membranes. Proc Natl Acad Sci: USA 1992;89: [19] Rashidy-Pour A, Motaghde-Larijani Z, Bures J. Reversible inactivation of medial septal area impairs consolidation but not retrieval of passive avoidance learning in rats. Behav Brain Res 1996;72: [20] Rashidy-Pour A, Razvani ME. Unilateral inactivation of the nucleus solitarius and amygdala attenuate the effects of bombesin on memory storage. Brain Res 1988;814: [21] Roozendaal B. Glucocorticoids and regulation of memory consolidation. Psychoendocrinology 2000;25: [22] Roozendaal B. Stress and memory: opposing effects of glucocorticoids on memory consolidation and memory retrieval. Neurobiol Learn Mem 2002;78: [23] Roozendaal B, Griffith QK, Buranday J, de Quervain DJ-F, McGaugh JL. The hippocampus mediates glucocorticoids induced impairment of spatial memory retrieval: dependence on the basolateral amygdala. Proc Natl Acad Sci: USA 2003;100: [24] Saha N, Datta H, Sharma PL. Effects of morphine on memory: interactions with naloxone, propranolol and haloperidol. Pharmacology 1991;42:10 4. [25] Sandi C, Loscertales M, Guaza C. Experience dependent facilitating effect of corticosterone on spatial memory formation in water maze. Eur J Neurosci 1997;9: [26] Setlow B, Roozendaal B, McGaugh JL. Involvement of a basolateral amygdala complex-nucleus accumbens pathway in glucocorticoidinduced modulation of memory consolidation. Eur J Neuosci 2000;12: [27] Shiigi Y, Takahashi M, Kaneto H. Facilitation of memory retrieval by pretest morphine mediated by mu but not delta and kappa opioid receptors. Psychopharmacology (Berlin) 1990;102: [28] Shors TJ, Weiss C, Thompson RF. Stress-induced facilitation of classical conditioning. Science 1992;257: [29] Stevens CW, Pezulla PD. Endogenous opioid system down regulation during hibernation in amphibians. Brain Res 1989;494: [30] Ukai M, Kobayashi T, Mori K, Shinkai N, Sasaki Y, Kameyama T. Attenuation of memory with Tyr-d-Arg-Phe-beta-Ala-NH2, a novel dermorphin analog with high affinity for mu-opoid receptors. Eur J Pharmacol 1995;287: [31] Van Der Kolk BA, Greenberg MS, Orr SP, Pitman RK. Endogenous opoioids, stress induced analgesia, and posttraumatic stress disorder. Psychopharmacol Bull 1989;25: [32] Wong CL. The involvement of -endorphin in swin-induced antinociception in female mice. Arch Int Pharmacodyn Ther 1989;300:22 8. [33] Zorawski M, Killcross S. Posttraining glucocorticoid receptor agonist enhances memory in appetitive and aversive Pavlovian discrete cue conditioning paradigms. Neurobiol Learn Mem 2002;78: