Trial Inhibitory Avoidance Research Articles

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

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.

Participation of hippocampal nicotinic receptors in acquisition, consolidation and retrieval of memory for one trial inhibitory avoidance in rats

One-trial step-down inhibitory avoidance in rats involves the activation of two separate memory types, a short-term system (STM) that lasts 3–6 h, and a long-term system (LTM) that takes 3–6 h to be formed and lasts for many days or even months. Here we investigate the effect of nicotinic receptor (nAChR) ligands infused bilaterally in the hippocampus on STM and LTM formation and on LTM retrieval of this task. Rats were implanted with chronic cannulae in the CA1 region of the dorsal hippocampus, trained using a 0.5 mA foot shock, and tested twice, first 1.5 h after training to measure STM, and again at 24 h to measure LTM. The drugs used were the nAChR antagonists, mecamylamine (1, 3 and 10 μg/side) and dihydro-β-erythroidine (DHβE; 2, 6 and 18 μg/side) and the agonist, nicotine (0.6, 1 and 3 μg/side). They were given either 15 min before training, immediately after training or 15 min prior to LTM retrieval. Mecamylamine and DHβE impaired and nicotine enhanced STM, LTM and retrieval similarly. The results indicate that nAChRs in CA1 participate in the regulation of both STM and LTM formation, and on the retrieval of LTM.

ATP-sensitive potassium channels mediate the effects of a peripheral injection of glucose on memory storage in an inhibitory avoidance task

A peripheral injection of glucose has been found to improve memory in rats, but the underlying mechanisms are unknown. One possible mechanism by which peripheral glucose might act on memory storage is by regulating ATP-sensitive potassium (K-ATP) channels. The present study investigated the effects of K-ATP channel modulators (minoxidil, a K-ATP channel opener and glibenclamide, a K-ATP channel closer) on memory enhancement induced by peripheral injection of glucose in an inhibitory avoidance task. Rats were trained in a one trial inhibitory avoidance task (50 Hz, 1 mA, 3 s footshock), and minoxidil (12.5, 25 or 50 mg/kg) or glibenclamide (2.5, 5, 10 or 20 mg/kg) was injected interaperitoneally (I.P) 30 min before training. Immediately after training the rats were injected with glucose (100 mg/kg, I.P). Retention was tested 2 days later. The results indicate that systemic post-training injection of glucose improved memory storage and this effect was attenuated and enhanced by pretreatment of minoxidil and glibenclamide, respectively. The drug minoxidil or glibenclamide alone did not significantly affect memory storage. This shows that K-ATP channels mediate the memory improving effects of systemic glucose.

Infusions of physostigmine into the hippocampus or the entorhinal cortex attenuate avoidance retention deficits produced by intra-septal infusions of the GABA agonist muscimol

Septal γ-aminobutyric acid (GABA) receptor activation is known to disrupt memory formation, although the mechanisms underlying this impairment remain unclear. The present study explored the possibility that high levels of septal GABA receptor activity might impair memory by down-regulating acetylcholine (ACh) function in archicortex and entorhinal cortex. To test this possibility, rats were trained on an avoidance task 15 min after receiving intra-septal infusions of vehicle or muscimol (5 nmol/0.5 μl) combined with unilateral intra-hippocampal (10 μl/1 μl) or intra-entorhinal cortex (1.875 μg/0.25 μl) infusions of vehicle or the acetylcholinesterase inhibitor physostigmine. We demonstrate that these infusions do not alter acquisition performance on a continuous multiple trial inhibitory avoidance task. However, intra-septal infusions of muscimol dramatically impair retention performance 48 h later. More importantly, infusions of physostigmine into the hippocampus or the entorhinal cortex, at doses that do not influence acquisition or retention performance when infused alone, attenuate the impairing effects of the muscimol infusions on retention. We suggest that high levels of septal GABA receptor activity might impair memory by down-regulating ACh levels in the hippocampal region, and that such memory impairments can be ameliorated by increasing ACh levels in the hippocampus or entorhinal cortex.

GlycineB Receptor Antagonists and Partial Agonists Prevent Memory Deficits in Inhibitory Avoidance Learning

Activation of N-methyl-d-aspartate (NMDA) receptors has been hypothesized to mediate certain forms of learning and memory. This hypothesis is based on the ability of competitive and uncompetitive NMDA receptor antagonists to disrupt learning. We investigated the effects of glycine site antagonists and partial agonists on deficits of acquisition (learning) and consolidation (memory) in a single trial inhibitory avoidance learning paradigm. Posttraining administration of either hypoxia (exposure to 7% oxygen) or the convulsant drug pentylenetetrazole (PTZ) (45 mg/kg) to mice impaired consolidation without producing neuronal cell death. Pretreatment with the competitive glycine antagonist 7-chlorokynurenic acid (7KYN) and the glycine partial agonists 1-aminocyclopropanecarboxylic acid (ACPC) and (+)HA-966 prevented memory deficits induced by hypoxia and PTZ, but did not affect scopolamine-induced learning impairment. In addition, ACPC prevented consolidation deficits evoked by a nonexcitotoxic concentration of l-trans-pyrrolidine-2, 4-dicarboxylate, a competitive inhibitor of glutamate transport that increases extracellular levels of glutamate. Moreover, (+)HA-966, 7KYN, and ACPC facilitated both acquisition and consolidation of inhibitory avoidance training, an effect that was dose-dependent and reversed by glycine. These results indicate that memory deficits induced by both hypoxia and PTZ involve NMDA receptor activation. Furthermore, the present findings demonstrate that glycine site antagonists and partial agonists prevent memory deficits of inhibitory avoidance learning by affecting consolidation, but not acquisition processes.

Experience-dependent decrease in synaptically localized Fra-1

The Fos family of transcription factors has been repeatedly shown to participate in the long-term neural responses associated with a variety of physiological stimuli, including activity-dependent plastic processes. Quite recently, several transcription factors have been found in synaptic regions, localized in dendrites and presynaptic terminals. Here we show that the transcription factor Fos-related antigen-1 (Fra-1) was detected in synaptosomes (Syn) and synaptic plasma membrane (SPM) fractions from the rat cerebral cortex and hippocampus as a single band migrating with M r 42–43 kDa. The 55-kDa c-Fos protein was also detected in syn and SPM fractions. Conversely, the inducible 62–65-kDa c-Fos is present in nuclear fractions from metrazole-treated animals (positive control), but not in Syn or SPM fractions. Furthermore, no Fra-2, Fos B or c-Jun immunoreactivities were detected in these same synaptic regions. DNA-mobility shift assays showed the presence of specific AP-1 binding activity in synaptic protein extracts. Immunoelectronmicroscopic analysis of cortical and hippocampal tissues revealed that Fra-1 and Fos-like immunoreactivities are localized in association with presynaptic plasma membranes. One trial inhibitory avoidance training, a hippocampal-dependent task, is associated with a time-dependent decrease (−31%) in Fra-1, but not in 55-kDa c-Fos, levels in hippocampal SPM fractions. In hippocampal homogenates, we do not detect significant changes in Fra-1 immunoreactivity, suggesting that this behavioural experience is probably accompanied by a subcellular redistribution of Fra-1 protein. These results suggest that Fra-1 may participate in the communication between synapse and the nucleus and in experience-dependent hippocampal plasticity.

Rapid and transient learning-associated increase in NMDA NR1 subunit in the rat hippocampus.

Several lines of evidence indicate that glutamate NMDA receptors are critically involved in long-term potentiation (LTP) and in certain forms of learning. It was previously demonstrated that memory formation of an inhibitory avoidance task in chick is specifically associated with an increase in the density of NMDA receptor in selected brain regions. Here we report on the effect of a one trial inhibitory avoidance training in rats, a hippocampal-dependent learning task, on the levels of different subunits of the glutamate NMDA receptor in synaptic plasma membranes (SPM) isolated from the hippocampus. Training rats on a one trial inhibitory avoidance task results in a rapid, transient and selective increase (+33%, p < 0.05) in NMDA NRI subunit expression in hippocampal SPM of rats sacrificed 30 min posttraining. No changes were observed at 0 or 120 min after training or in shocked animals in comparison to naive control rats. In addition, no training-associated increase in the levels of NMDA NR2A and NR2B or AMPA GluR 2/3 subunits was observed at any timepoint tested. In conclusion, the present findings support the hypothesis that alterations in expression of synaptic NMDA NR1 subunits in the hippocampus are specifically associated with memory formation of an inhibitory avoidance task and strongly suggest that hippocampal NMDA receptors are crucially involved in the neural mechanisms underlying certain forms of learning.

Glucocorticoid interactions with the β-adrenoceptor-cyclic AMO system in the basolateral amygdala in regulating memory consolidation

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

Task-dependent effects of the antidepressant/antipanic drug phenelzine on memory.

Phenelzine (PLZ) is a non-selective monoamine oxidase (MAO) inhibitor commonly used to treat depression and panic disorder. In addition to increasing levels of biogenic amines in the brain, PLZ elevates brain levels of the amino acid gamma-aminobutyric acid (GABA; Baker et al. 1991; present study). Given the extensive evidence implicating biogenic amines and GABA in mnemonic processes, PLZ may affect learning and memory. To investigate this possibility, male Sprague-Dawley rats were given PLZ sulfate (15 or 30 mg/kg, based on free base weight) 2 h prior to training in a continuous multiple trial inhibitory avoidance (CMIA) and spatial water maze task. Retention was assessed 48 h later. The results indicated that PLZ enhanced CMIA and impaired water maze retention performance. Compared to control rats, rats given PLZ took significantly longer to re-enter the shock compartment and swam longer distances before reaching the escape platform on the retention tests. These effects of PLZ did not appear to be the result of PLZ-induced changes in acquisition or retrieval processes, activity levels, or footshock sensitivity. Combined, these findings indicate that PLZ influences memory in a task-dependent manner. These differential effects of PLZ may be the result of contrasting influences of GABA and biogenic amines on memory.

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.

Learning-specific, time-dependent increases in hippocampal Ca2+/calmodulin-dependent protein kinase II activity and AMPA GluR1 subunit immunoreactivity.

Ca2+/calmodulin-dependent protein kinase II (CAMK II) and one of its target, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), glutamate receptors have been shown to participate in both long-term potentiation (LTP) in the hippocampus, and in spatial, as well as in a variety, of learning paradigms. Recently, we were able to demonstrate that the intrahippocampal infusion of a specific inhibitor of CAMK II (KN62) provoked full retrograde amnesia of an inhibitory avoidance learning in rats when given immediately, but not 120 or 240 min, after training. Furthermore, this task is accompanied by a rapid, selective and reversible increase in hippocampal [3H] AMPA receptor binding. Here we report the effect of this aversively motivated learning task on CAMK II activity, and AMPA GluR1 subunit phosphorylation and immunoreactivity in the hippocampus. One trial inhibitory avoidance training is associated with a learning-specific, time-dependent increase (25-78%) in both total and Ca2+-independent activities of CAMK II in the hippocampus of rats killed immediately (0 min), but not 120 min, after training. In addition, immunoblotting experiments showed an increment in the amount of the alpha-subunit of CAMK II at 0, 30 and 120 min after training. An increase in the in vitro phosphorylation of alpha- and beta-subunits of CAMK II was also observed in hippocampal synaptosomal membranes (SPM) of trained rats killed immediately and 30 min post-training. In addition, inhibitory avoidance is accompanied by a 20% increase in GluR1 phosphorylation and a 33% increase in GluR1 immunoreactivity 120 min after training. No significant changes were observed in shocked animals. Phosphorylation of hippocampal SPM from naive control animals in conditions suitable for CAMK II activation resulted in a large increase in the density of [3H] AMPA binding (+ 100%). Taken together, these findings confirm and extend previous data suggesting that CAMK II and AMPA glutamate receptors in the hippocampus participate in the early phase of memory formation of an inhibitory avoidance learning.

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

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

B-50/GAP-43 phosphorylation and PKC activity are increased in rat hippocampal synaptosomal membranes after an inhibitory avoidance training.

Several lines of evidence indicate that protein kinase C (PKC) is involved in long-term potentiation (LTP) and in certain forms of learning. Recently, we found a learning-specific, time-dependent increase in [3H]phorbol dibutyrate binding to membrane-associated PKC in the hippocampus of rats subjected to an inhibitory avoidance task. Here we confirm and extend this observation, describing that a one trial inhibitory avoidance learning was associated with rapid and specific increases in B-50/GAP-43 phosphorylation in vitro and in PKC activity in hippocampal synaptosomal membranes. The increased phosphorylation of B-50/GAP-43, was seen at 30 min (+35% relative to naive or shocked control groups), but not at 10 or 60 min after training. This learning-associated increase in the phosphorylation of B-50/GAP-43 is mainly due to an increase in the activity of PKC. This is based on three different sets of data: 1) PKC activity increased by 24% in hippocampal synaptosomal membranes of rats sacrificed 30 min after training; 2) B-50/GAP-43 immunoblots revealed no changes in the amount of this protein among the different experimental groups; 3) phosphorylation assays, performed in the presence of bovine purified PKC or in the presence of the selective PKC inhibitor CGP 41231, exhibited no differences in B-50/GAP-43 phosphorylation between naive and trained animals. In conclusion, these results support the contention that hippocampal PKC participates in the early neural events of memory formation of an aversively-motivated learning task.

Footshock facilitates the expression of aversively motivated memory in rats given post-training amygdala basolateral complex lesions

We previously reported that increased training in an escape task partially attenuates the memory impairment produced by large amygdala lesions induced 1 week following training. The present study examined the effect of amount of preoperative training on the retention of rats with lesions restricted to the amygdala basolateral complex. Rats received 1 or 10 training trials in a footshock-motivated escape task and 1 week later sham lesions or neurotoxic lesions of the basolateral complex. Four days after recovery from the surgery they were tested for inhibitory avoidance retention and then 2 days later given continuous multiple trial inhibitory avoidance training (CMIA) in the same apparatus. The basolateral complex lesions significantly decreased the retention latencies of rats given 1 or 10 trials. However, following administration of footshock on the CMIA task, the performance of the lesioned rats reflected the degree of preoperative escape training. The basolateral complex lesions also increased open field locomotor activity, an effect that may have contributed to the shorter retention latencies in lesioned animals. These findings indicate that an intact amygdala basolateral complex is not critical for the retention of the escape training.

Infusion of the GABAergic antagonist bicuculline into the medial septal area does not block the impairing effects of systemically administered midazolam on inhibitory avoidance retention

This experiment investigated the effect of intraseptal administration of the GABAergic antagonist bicuculline methiodide on benzodiazepine-induced amnesia. Male Sprague-Dawley rats were implanted with cannula aimed at the medial septal area and allowed to recover for 1 week. Ten minutes prior to training in a continuous multiple trial inhibitory avoidance task, buffer solution or bicuculline methiodide (56 or 100 pmol/0.5 microliter) was injected into the medial septal area. This infusion was immediately followed by systemic (ip) administration of saline or midazolam (1.5 or 3.0 mg/kg). In comparison with saline controls, animals given the higher dose of midazolam (3.0 mg/kg), required more trials to reach acquisition criterion (remaining in the starting chamber for 100 s). This midazolam-induced acquisition deficit was blocked by an intraseptal infusion of bicuculline methiodide (100 pmol). On a 48-h retention test the performance of animals given either dose of midazolam was significantly impaired relative to vehicle controls. Furthermore, although intraseptal infusion of bicuculline methiodide prior to systemic injection of midazolam blocked the midazolam-induced acquisition impairment, bicuculline did not block the midazolam-induced retention impairment. These results suggest that although the medial septal area may be involved in midazolam-induced acquisition deficits, this area is not a critical site of action for benzodiazepine-induced effects on inhibitory avoidance retention.

Posttraining infusion of lidocaine into the amygdala basolateral complex impairs retention of inhibitory avoidance training

The present experiment examined the role of the central nucleus and basolateral complex in the retention of inhibitory avoidance training by reversibly inactivating these regions with lidocaine immediately following training. Male Sprague-Dawley rats were surgically implanted bilaterally with cannulae aimed at the central nucleus or the basolateral complex. One week later, they received one trial inhibitory avoidance training (0.45 mA; 1 s), followed immediately by infusions of lidocaine hydrochloride or buffer (10 μg/0.25 μl). Retention was tested 2 days after training. Immediate posttraining infusions of lidocaine into the central nucleus did not affect retention performance; in contrast, immediate posttraining infusions of lidocaine into the basolateral complex significantly impaired retention performance. In addition, the effect of posttraining infusions of lidocaine into the basolateral complex was time-dependent: infusions administered 6 h after training also impaired memory, but infusions administered 24 h after training had no effect. Immediate posttraining infusions of lidocaine also impaired the retention performance of rats trained with a more intense footshock (0.75 mA). However, at the higher footshock intensity, administration of lidocaine 6 h after training had no effect on retention performance. The time- and footshock-dependent retrograde impairment of memory produced by posttraining reversible inactivation of the basolateral complex suggests that this region of the amygdala is involved in the consolidation of memory for inhibitory avoidance training.

Bicuculline administered into the amygdala blocks benzodiazepine-induced amnesia

This experiment investigated the effect of intra-amygdala administration of the GABAergic antagonist bicuculline methiodide on benzodiazepine-induced amnesia. Male Sprague-Dawley rats were implanted bilaterally with cannulae aimed at the amygdala and allowed to recover for 1 week. Ten minutes before training in a continuous multiple trial inhibitory avoidance task a buffer solution or bicuculline methiodide (56 pmol/0.5 microliters) was injected bilaterally into the amygdala and this injection was immediately followed by a systemic injection of saline or midazolam (1.0 mg/kg). In comparison with saline controls, midazolam-treated animals required more trials to reach the acquisition criterion of remaining in the starting chamber for 100 s. The midazolam effect on acquisition was not attenuated by intra-amygdala infusion of bicuculline methiodide, suggesting that the midazolam-induced changes in acquisition behavior do not involve the amygdaloid GABAergic system. On a 48-h retention test the performance of the midazolam-treated animals was significantly poorer than that of the controls. However, the retention performance of animals given intra-amygdala injections of bicuculline methiodide prior to the systemic injection of midazolam was comparable to that of the saline controls. These results suggest that the amygdaloid GABAergic system mediates the impairing effects of midazolam on retention of inhibitory avoidance training.

The locus coeruleus: Its possible role in memory consolidation

Three experiments were done looking at the effects on memory of discrete electrolytic lesions in the locus coeruleus of mice. In Experiment 1 mice received electrolytic lesions of the locus coeruleus immediately following training on the 1 trial inhibitory avoidance step-through task. Retention of this response, measured 48 hr later was normal, suggesting that locus coeruleus lesions per se do not interfere with the performance of this rather simple response. In Experiments 2 and 3 mice were treated exactly as in Experiment 1 except that a transcorneal ECS was administered 40 hr or 7 days after initial training and locus coeruleus lesions. Mice with locus coeruleus lesions were amnesic following a 40 hr delayed ECS treatment when tested either 8 hr or 24 hr after ECS. Mice with locus coeruleus lesions were not amnesic following a 7 day delayed ECS. These data are interpreted to suggest that the locus coeruleus is normally involved in the temporal delineation of the susceptibility period of newly formed memory. A malfunction of the locus coeruleus can result in a profound extension of the normal susceptibility period of newly formed memory to ECS produced amnesia.