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

Involvement of the posteroventral caudate-putamen in memory consolidation in the Morris water maze

Male Sprague-Dawley rats implanted with bilateral intracerebral guide cannulae were trained in the standard hidden platform version of the Morris water maze and given immediate posttraining infusions of the D2 dopamine receptor antagonist sulpiride (10.0 or 100.0 ng/side) or saline vehicle into the posteroventral caudate-putamen. Retention was tested 2 days later with a probe trial. Sulpiride-treated rats spent less time swimming near the trained platform location and more time in the periphery of the maze than controls, although their latency to reach the trained platform location was not significantly affected. The pattern of results suggests that whereas the posteroventral caudate-putamen seems to be involved in consolidation of memory in the Morris water maze, it may be involved in memory for procedural aspects of the task in a manner distinct from that of other brain regions such as the hippocampus.

5-HT1A receptors modulate the consolidation of learning in normal and cognitively impaired rats

Attempts were made to further analyze the role of 5-HT1A receptors in consolidation of learning by evaluating the role of these receptors in cognitively normal and impaired animals. The effects of post-training administration of 8-OH-DPAT and 5-HT1A receptor antagonists, WAY 100135, WAY 100635, and S-UH-301, plus the cholinergic and glutamatergic antagonists, scopolamine and dizolcipine, respectively, were determined using an autoshaping learning task. The results showed that 8-OH-DPAT increased the number of conditioned responses, whereas WAY100135, WAY100635, and S-UH-301, and the 5-HT depleter, p-chloroamphetamine (PCA), had no effect. PCA did not change the silent properties of the 5-HT1A receptor antagonists. PCA, WAY100635, and S-UH-301, but not GR127935 (a 5-HT1B/1D-receptor antagonist) or MDL100907 (a 5-HT2A receptor antagonist), reversed the effect to 8-OH-DPAT. Ketanserin (a 5-HT2A/2C receptor antagonist) and ondansetron (a 5-HT3 receptor antagonist), at a dose that increased the conditioned responses by itself, reversed the effect of 8-OH-DPAT. Moreover, 8-OH-DPAT or S-UH-301 reversed the learning deficit induced by scopolamine and dizocilpine whereas WAY100635 reversed the effect of scopolamine only. These data confirm a role for presynaptic 5-HT1A receptors during the consolidation of learning and support the hypothesis that serotonergic, cholinergic, and glutamatergic systems interact in cognitively impaired animals.

Pavlovian heart rate and jaw movement conditioning in the rabbit: effects of medial prefrontal lesions

An experiment was conducted in which jaw movements (JM) and heart rate (HR) were concomitantly assessed in rabbits during simple Pavlovian conditioning. A 2-s 1200-Hz tone was the conditioned stimulus (CS) and an intraoral 1-cc pulse of 0.5 M sucrose-water solution was the unconditioned stimulus (US). Sham and medial prefrontal (mPFC)-lesioned animals received paired CS/US training with a 70- to 75-dB CS and were compared with sham- and mPFC-lesioned animals that received explicitly unpaired CS/US presentations. The percentages of JM CRs were significantly greater in the paired than the unpaired groups, but mPFC lesions had no effect on this measure. Conditioned HR decelerations occurred only in the paired groups and then only during the first session of training. Moreover, these CS-evoked cardiac decelerations were somewhat attenuated by the mPFC lesion. CS-evoked HR accelerations, which were significantly greater in unpaired than in paired animals, occurred during the four subsequent sessions. These results suggest that a CS-evoked cardioinhibitory process, mediated by the mPFC, is engendered by Pavlovian appetitive conditioning, as has been previously demonstrated for aversive conditioning. However, during JM conditioning these inhibitory changes are quickly replaced by tachycardia, possibly related to increased nonspecific somatomotor activity, since the tachycardia was somewhat greater in the unpaired animals.

Rolipram, a type IV-specific phosphodiesterase inhibitor, facilitates the establishment of long-lasting long-term potentiation and improves memory

In an attempt to improve behavioral memory, we devised a strategy to amplify the signal-to-noise ratio of the cAMP pathway, which plays a central role in hippocampal synaptic plasticity and behavioral memory. Multiple high-frequency trains of electrical stimulation induce long-lasting long-term potentiation, a form of synaptic strengthening in hippocampus that is greater in both magnitude and persistence than the short-lasting long-term potentiation generated by a single tetanic train. Studies using pharmacological inhibitors and genetic manipulations have shown that this difference in response depends on the activity of cAMP-dependent protein kinase A. Genetic studies have also indicated that protein kinase A and one of its target transcription factors, cAMP response element binding protein, are important in memory in vivo. These findings suggested that amplification of signals through the cAMP pathway might lower the threshold for generating long-lasting long-term potentiation and increase behavioral memory. We therefore examined the biochemical, physiological, and behavioral effects in mice of partial inhibition of a hippocampal cAMP phosphodiesterase. Concentrations of a type IV-specific phosphodiesterase inhibitor, rolipram, which had no significant effect on basal cAMP concentration, increased the cAMP response of hippocampal slices to stimulation with forskolin and induced persistent long-term potentiation in CA1 after a single tetanic train. In both young and aged mice, rolipram treatment before training increased long- but not short-term retention in freezing to context, a hippocampus-dependent memory task.

Effects of posttraining intrahippocampal injections of platelet-activating factor and PAF antagonists on memory

The present experiments examined the effects of posttraining intrahippocampal injections of the degradative enzyme-resistant methylcarbamyl analog of the bioactive phospholipid platelet-activating factor (mc-PAF) and the platelet-activating factor (PAF) receptor antagonists BN52021 and BN 50730 on memory in male Long-Evans rats trained in a hidden platform version of the Morris water maze. Following an eight-trial training session, rats received a unilateral intrahippocampal injection of mc-PAF (0.5, 1.0, or 2.0 microgram/0.5 microliter), lyso-PAF (1.0 microgram/0.5 microliter), the cell surface PAF receptor antagonist BN 52021 (0.25, 0.5, or 1.0 micrigram/0.5 microliter/, the intracellular PAF receptor antagonist BN 50730 (2.0, 5.0, or 10.0 microgram/0.5 microliter), or vehicle (50% DMSO in 0.9% saline; 0.5 microliter). On a retention test conducted 24 h after training, the escape latencies of rats administered mc-PAF (1.0 or 2.0 microgram) were significantly lower than those of the vehicle-injected controls, demonstrating a memory-enhancing effect of mc-PAF. Injections of lyso-PAF, a structurally similar metabolite of PAF, had no influence on memory, indicating that the memory-enhancing effect of mc-PAF is not caused by membrane perturbation by the phospholipid. The retention test escape latencies of rats administered BN 52021 (0.5 microgram) and BN 50730 (5.0 or 10 microgram) were significantly higher than those of the controls, indicating a memory impairing effect of both PAF antagonists. When mc-PAF, BN 52021, or BN 50730 was administered 2 h posttraining, no effect on retention was observed, indicating a time-dependent effect of the neuroactive substances on memory storage. The findings suggest a role for endogenous PAF in hippocampal-dependent memory processes.

Posttraining estrogen and memory modulation

The present paper provides a review of recent research carried out in this laboratory investigating the effects of posttraining peripheral and intrahippocampal injection of estradiol on memory in rats, and estradiol-acetylcholine interactions in memory modulation. Ovariectomized rats received an eight-trial training session in a hippocampal-dependent hidden platform water maze task. Immediately following training, rats received a posttraining peripheral or intrahippocampal injection of estradiol-cyclodextrin complex or vehicle. Twenty-four hours later rats were returned to the maze for a retention test session, and latency to escape was used as a measure of memory for the previous day's training. Peripheral posttraining injection of estradiol enhances memory relative to vehicle-treated rats. Injections of estradiol given 2 h posttraining has no effect on retention, indicating a time-dependent effect of estradiol on memory storage processes. A time-dependent memory enhancing effect of posttraining intrahippocampal injections of estradiol has also been observed in both male and ovariectomized female rats. The memory enhancing effect of peripheral posttraining injection of estradiol in ovariectomized rats is blocked by a subeffective dose of the acetylcholine muscarinic receptor antagonist scopolamine, suggesting that estradiol interacts with cholinergic systems in memory modulation. Concurrent peripheral posttraining injection of a subeffective dose of estradiol and a subeffective dose of the cholinergic agonist oxotremorine produces a synergistic memory enhancing effect. The findings suggest that: (1) estradiol selectively influences memory storage independent of an effect on nonmnemonic processes, (2) the hippocampus is a potential neuroanatomical site of action mediating estrogenic effects on memory, and (3) estradiol interacts with cholinergic systems in memory modulation.

Amygdala modulation of multiple memory systems: hippocampus and caudate-putamen

A series of five experiments examined the differential mnemonic roles of the hippocampus and caudate-putamen and the modulatory influence of the amygdala on hippocampal and caudate-putamen memory processes. Findings indicate that (a) posttraining intrahippocampal injections of amphetamine selectively enhance memory in a hidden platform water maze task, (b) posttraining intracaudate injections of amphetamine selectively enhance memory in a visible platform water maze task, (c) posttraining intra-amygdala injections of amphetamine enhance memory in both water maze tasks, (d) preretention intrahippocampal lidocaine injections block expression of the memory enhancing effects of posttraining intrahippocampal amphetamine injections in the hidden platform task, (e) preretention intracaudate lidocaine injections block expression of the memory enhancing effects of posttraining intracaudate amphetamine injections in the visible platform task, (f) preretention intra-amygdala lidocaine injections do not block the memory enhancing effect of posttraining intra-amygdala amphetamine injections on either task, (g) in the hidden platform task, posttraining intrahippocampal, but not intracaudate, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine, (h) in the visible platform task, posttraining intracaudate, but not intrahippocampal, lidocaine injections block the memory enhancing effects of posttraining intra-amygdala amphetamine. The findings indicate a double dissociation between the roles of the hippocampus and caudate-putamen in memory and suggest that the amygdala exerts a modulatory influence on both the hippocampal and caudate-putamen memory systems.

Estrogen enhances performance of female rats during acquisition of a radial arm maze

Estrogen can influence the expression of behaviors not associated directly with reproduction, including learning and memory. However, the effects of estrogen on learning and memory in mammals are complex, dependent on a variety of factors. The radial arm maze is a traditional experimental task that takes advantage of the natural foraging strategy of rats and provides an appropriate measure for studying the effects of estrogen on working memory in this species. In the experiments reported here, ovariectomized rats were implanted subcutaneously with 5-mm Silastic capsules containing 25% estradiol diluted with cholesterol. Control females received 5-mm Silastic capsules containing 100% cholesterol. Results of three separate experiments demonstrated that estradiol administered by Silastic implants for 30 days prior to eight-arm radial maze training, during the 24 days of maze training, or both significantly improved working memory performance compared to females treated with cholesterol alone, as indicated by improved arm choice accuracy over trials. The positive effect of estradiol exposure prior to training suggests that estrogen may induce neuronal changes that persist beyond the period of exposure with functional consequences for behavior.

The nucleus accumbens and learning and memory

Recent research on the nucleus accumbens (NA) indicates that this brain region is involved in learning and memory processes in a way that is separable from its other well-known roles in behavior, such as motivation, reward, and locomotor activity. These findings have suggested that 1) the NA may be involved in declarative, or hippocampal formation-dependent learning and memory, and not in several other non-declarative forms of learning and memory, and 2) the NA may be selectively involved in certain stages of learning and memory. These characteristics suggest that the NA may be part of a larger striatal system which subserves acquisition and consolidation, but is not a site of long-term storage, of different forms of learning and memory.

Posttraining estradiol injections enhance memory in ovariectomized rats: cholinergic blockade and synergism

The present experiments examined acute posttraining estrogenic influences on memory in ovariectomized rats. In experiment 1 rats received a single 8-trial (30-s ITI) training session with a submerged escape platform located in the same quadrant of a circular water maze on all trials. Following trial 8, rats received a posttraining intraperitoneal injection of either an estradiol-cyclodextrin inclusion complex (0.1, 0.2, or 0.4 mg/kg) or saline. On a retention test session 24 h later, the escape latencies of rats given injections of estradiol (0.2 mg/kg) were significantly lower than those of saline-treated rats, indicating an enhancement of memory. Injections of estradiol delayed 2 h posttraining did not affect retention, demonstrating a time-dependent effect of estradiol on memory storage processes. In experiment 2a, posttraining injections of the cholinergic muscarinic receptor antagonist scopolamine (0.4 mg/kg) impaired memory in ovariectomized rats. In experiment 2b, the memory-enhancing effect of estradiol (0.2 mg/kg) was blocked by concurrent posttraining administration of a subeffective dose (0.1 mg/kg) of scopolamine, suggesting an interaction between estradiol and muscarinic cholinergic systems in memory modulation. In experiment 3a, posttraining injections of the cholinergic muscarinic receptor agonist oxotremorine (0.2 mg/kg) enhanced memory in ovariectomized rats. In experiment 3b, concurrent posttraining injection a subeffective dose of estradiol (0.1 mg/kg) and a subeffective dose of oxotremorine (0.1 mg/kg) enhanced memory, indicating a synergistic effect of estradiol and muscarinic receptor activation on memory.

Posttraining injections of MK-801 produce a time-dependent impairment of memory in two water maze tasks

The role of glutamatergic N-methyl-D-aspartate (NMDA) receptors in memory storage processes was examined using systemic posttraining injections of MK-801. Male Long-Evans rats received an eight-trial (30-s ITI) training session on a spatial or cued water maze task. In the spatial task, a submerged escape platform was located in the same quadrant of the maze on all trials. In the cued task, a visible escape platform was located in a different quadrant of the maze on each trial. Following Trial 8 in both tasks, the rats received a posttraining intraperitoneal injection of the NMDA receptor antagonist MK-801 (0.025, 0.05, 0.1, or 0.2 mg/kg) or saline. On a retention test session 24 h later, latency to mount the escape platform was used as a measure of memory. In both tasks, the retention test escape latencies of animals given MK-801 (0.05 and 0.1 mg/kg) were significantly higher than those of saline-injected controls, indicating a drug-induced impairment of memory. Injections of MK-801 (0.05 mg/kg) did not affect retention when administered 2 h posttraining in either task, indicating that the effects of MK-801 on retention are not due to an influence on non-mnemonic factors. Control experiments indicated that the memory impairing effects of MK-801 were due to an influence on memory for the type of discrimination training given (i.e., spatial or cued) and not due to an influence on a mnemonic strategy common to both tasks. The findings indicate a time-dependent role for NMDA receptor function in memory storage processes.

Strain-dependent effects of D2 dopaminergic and muscarinic-cholinergic agonists and antagonists on memory consolidation processes in mice

The interaction between muscarinic-cholinergic and dopaminergic systems in the modulation of memory storage of Y-maze discrimination (YMD) task was examined in C57BL/6 and DBA/2 strains of mice. In C57BL/6 mice, post-training systemic (i.p.) administration of the D2-agonist quinpirole facilitated retention and the D2-antagonist (-)-sulpiride impaired retention. Opposite effects were observed in DBA/2 strain. The facilitating or impairing effects of quinpirole and (-)-sulpiride were blocked by simultaneous post-training administration of muscarinic-cholinergic agonists and antagonists. The memory enhancing effects of the cholinergic agonist oxotremorine were not blocked by simultaneous administration of sulpiride in C57BL/6 mice or quinpirole in DBA/2 mice. Furthermore, the memory impairing effects of the cholinergic antagonist atropine were not blocked by simultaneous administration of quinpirole in C57BL/6 mice or sulpiride in DBA/2 mice. These findings indicate that the effects of D2-receptor agonists and antagonists on retention of YMD task are strain-dependent and mediated through muscarinic-cholinergic mechanisms.

Effects of 5-HT4 receptor agonists and antagonists in learning

In the present work, the effects of pre- or post-training (ip) injection of BIMU1 and BIMU8 (5-HT4 agonists) were figured out in the autoshaping learning task. Furthermore, the post-training effects of these agonists after treatment with SDZ 205-557 and GR 125487D (5-HT4 antagonists) or p-Chloroamphetamine (PCA) were also explored. Animals were individually trained in a lever-press response on the autoshaping task and 24 hours later were tested. The results showed that pre-training injection of BIMU1 (5 20 mg/Kg) or BIMU8 (20 mg/Kg) increased the CR; in contrast, the post-training administration of BIMU1 (10-20 mg/Kg) or BIMU8 (5 and 20 mg/Kg) decreased it. Further experiments revealed that the post-training injections of SDZ 205-557 (1.0-10.0 mg/Kg) or GR 125487D (0.39-1.56 mg/Kg) by themselves did not alter the CR. When BIMU1 or BIMU8 was administered to rats pretreated with SDZ 205-557 (10 mg/Kg) or GR 125487D (0.78 mg/Kg), the decrement induced by 5-HT4 the agonists was reversed; in contrast, the administration of PCA failed to modify the CR or the agonist-induced responses. The findings showed that the pre-training stimulation of 5-HT4 receptors enhanced the acquisition of CR, while, post-training activation of 5-HT4 receptors, impaired the consolidation of learning. The latter effect was not altered by PCA pretreatment. The data show that 5-HT4 receptors are involved in the acquisition and consolidation of learning. It seems that postsynaptic 5-HT4 receptors are involved in the latter effect.

A pharmacological analysis of serotonergic receptors: effects of their activation of blockade in learning

1. The authors have tested several 5-HT selective agonists and antagonists (5-HT1A/1B, 5-HT2A/2B/2C, 5-HT3 or 5-HT4), an uptake inhibitor and 5-HT depletors in the autoshaping learning task. 2. The present work deals with the receptors whose stimulation increases or decreases learning. 3. Impaired consolidation of learning was observed after the presynaptic activation of 5-HT1B, 5-HT3 or 5-HT4 or the blockade of postsynaptic 5-HT2C/2B receptors. 4. In contrast, an improvement occurred after the presynaptic activation of 5-HT1A, 5-HT2C, and the blockade of presynaptic 5-HT2A, 5-HT2C and 5-HT3 receptors. 5. The blockade of postsynaptic 5-HT1A, 5-HT1B, 5-HT3 or 5-HT4 receptors and 5-HT inhibition of synthesis and its depletion did no alter learning by themselves. 6. The present data suggest that multiple pre- and postsynaptic serotonergic receptors are involved in the consolidation of learning. 7. Stimulation of most 5-HT receptors increases learning, however, some of 5-HT subtypes seem to limit the data storage. 8. Furthermore, the role of 5-HT receptors in learning seem to require an interaction with glutamatergic, GABAergic and cholinergic neurotransmission systems.

Interaction of neuromodulatory systems in modulating memory storage

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

A rat model of distractibility: effects of drugs modifying dopaminergic, noradrenergic and GABAergic neurotransmission

A procedure for analyzing effects of drugs on distractibility is proposed. Rats are trained to traverse a straight runway with a sucrose solution as reinforcement. Once the response has been acquired, an additional runway ending in an empty box is connected. The time spent investigating this additional runway is the measure of distractibility. Amphetamine, 1 mg/kg i.p., increased distractibility. In rats that were never reinforced, amphetamine at a dose of 1 mg/kg reduced the time spent in the additional runway. This shows that the effects of amphetamine in the reinforced animals cannot be interpreted as enhanced exploration. Furthermore, the benzodiazepines diazepam (2 and 4 mg/kg, i.p.) and chlordiazepoxide (2.5, 5 and 10 mg/kg, i.p.), known to enhance exploration of novel environments, did not affect the time spent in the additional runway in sucrose-reinforced animals. It was concluded that the procedure indeed is a model of distractibility. The dopamine antagonist cis(Z)-flupenthixol, at a dose of 0.25 mg/kg, i.p., blocked the effects of amphetamine, 1 mg/kg. Flupenthixol itself, in doses of 0.25 and 0.5 mg/kg, did not affect the time spent in the additional runway. This suggests that enhanced dopaminergic activity indeed is responsible for the effects. This proposal is further supported by experiments showing that the noradrenaline precursor dihydroxyphenylserine (10 mg/kg + carbidopa, 50 mg/kg, both i.p.) and the noradrenergic neurotoxin DSP4 (50 mg/kg, i.p.) had no effect on distractibility. Moreover, amfonelic acid, a dopamine releaser with slight or no effect on noradrenergic neurotransmission, had effects very similar to those of amphetamine when given in doses of 0.25 and 0.5 mg/kg, i.p. A lower dose, 0.125 mg/ kg, was ineffective. Taken together, these data suggest that enhanced dopaminergic neurotransmission increases distractibility in the rat. However, both amphetamine and amfonelic acid may stimulate serotonin release. Until serotonergic drugs have been tested, a contribution of this transmitter cannot be ruled out. The distraction procedure may constitute an animal model of some kinds of disordered information processing.

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.

Acquisition of a morris water maze task is impaired during early but not late withdrawal from morphine

Behavioral changes in male Sprague-Dawley rats during early and late withdrawal from morphine were investigated. Morphine-treated subjects (M) were implanted (SC) with osmotic pumps containing 2.0 ml of a 159 mg/ml morphine sulfate solution while control subjects (C) received sham implants. Implants were removed after 7 days. M subjects exhibited a significant decrease in body weight during withdrawal that recovered by 21 days after pump removal. Beginning 1 or 21 days following pump removal, subjects were tested for 8 days in a Morris water maze (MWM) task. M subjects trained in the MWM during early withdrawal exhibited significantly longer escape latencies than C subjects. However, during sequential probe trials, the same subjects exhibited a significant preference for the target quadrant of the maze and executed accurate searches for the escape platform. Though these subjects failed to locate the platform as efficiently as controls during training trials, they learned the location of the escape platform. M rats trained during late withdrawal exhibited no deficits in any measure of MWM performance relative to C subjects. The data suggest that a variety of processes involved in the acquisition and performance of the MWM task are differentially affected during early withdrawal from morphine.

Neuronal mechanisms mediating drug-induced cognition enhancement: cognitive activity as a necessary intervening variable

The conceptual foundations of a research aimed at the determination of potential neuronal, neuropharmacological, and behavioral/cognitive mechanisms mediating drug-induced cognition enhancement are discussed. The available evidence justifies a focus on attentional processes as a target for drug-induced cognition enhancement. Neuropharmacological mechanisms that may mediate drug-induced enhancement of attentional functions are proposed to interact necessarily with attention-associated neuronal activity. The elements of a transsynaptic approach to increase the excitability of basal forebrain cholinergic neurons and hence, attentional functions are discussed. Experimental tests of this hypothesis require the demonstration of interactions between cognition-induced increases in the activity of cortical cholinergic afferents and the effects of putative cognition enhancers. The available data illustrate that the effects of benzodiazepine receptor (BZR) agonists and inverse agonists on cortical acetylcholine (ACh) efflux interact with the state of activity in this system. The feasibility, potential heuristic power, and the experimental and conceptual problems of studies attempting to simultaneously assess drug effects on behavioral/cognitive abilities, ACh efflux, and neuronal activity have been revealed by an experiment intended to correlate performance in a task measuring sustained attention with medial prefrontal ACh efflux and medial prefrontal single-unit activity. The rational development of a psychopharmacology of cognition enhancers requires a union among behavioral/cognitive pharmacology, neuropharmacological and electrophysiological approaches.

Acetylcholine: a neurotransmitter for learning and memory?

The cholinergic hypothesis claims that the decline in cognitive functions in dementia is predominantly related to a decrease in cholinergic neurotransmission. This hypothesis has led to great interest in the putative involvement of the cholinergic neurotransmission in learning and memory processes. This review aims to assess the data of studies in which the role of acetylcholine (ACh) in cognitive functions was investigated. For this purpose, studies from three different fields of research, namely: (1) behavioral pharmacology (effects of drugs on behavior); (2) behavioral neuroscience (effects of brain lesions on behavior); and (3) dementia, are discussed separately. The experimental tools that have been used in pharmacological studies may appear to be inadequate to enable conclusions to be drawn about the involvement of ACh in learning and memory processes. Especially, the use of scopolamine as a pharmacological tool is criticized. In the field of behavioral neuroscience a highly specific cholinergic toxin has been developed. It appears that the greater and more specific the cholinergic damage, the fewer effects can be observed at the behavioral level. The correlation between the decrease in cholinergic markers and the cognitive decline in dementia may not be as clearcut as has been assumed. The involvement of other neurotransmitter systems in cognitive functions is briefly discussed. Taking into account the results of the different fields of research, the notion that ACh plays a pivotal role in learning and memory processes seems to be overstated. Even when the role of other neurotransmitter systems in learning and memory is taken into consideration, it is unlikely that ACh has a specific role in these processes. On basis of the available data, ACh seems to be more specifically involved in attentional processes than in learning and memory processes.

Effect of fluoxetine on learning and memory involves multiple 5-HT systems

Diverse evidence suggests that 5-HT uptake blockers enhance learning and memory. However, there is no information about the mechanisms of action involved in such effects. The aim of the present work was to investigate the nature of the receptors involved in the effects of fluoxetine on learning. Therefore, a dose-response curve of posttraining injection (intraperitoneal) of fluoxetine was carried out in an associative learning task (auto-shaping). Fluoxetine or the vehicle was injected 10 min after 5-HT antagonists: (+/-)-pindolol, (+/-)-propanolol, NAN-190, ketanserin, ritanserin, mesulergine, MDL 72222, or SDZ 205-557. Presynaptic activity was eliminated by means of chloroamphetamine pretreatment. Scopolamine (an anticholinergic) and dizocilpine (a noncompetitive NMDA receptor antagonist) were also used. Results showed that fluoxetine enhanced learning of the conditioned response (CR) in a dose-dependent fashion. All 5-HT antagonists had no effects by themselves but inhibited the effects of fluoxetine at different degrees. Decrement of CR produced by scopolamine was reversed by fluoxetine. Dizocilpine did not affect CR but prevented the effects of fluoxetine. The present findings suggest that the actions of fluoxetine on learning are due to an interaction with multiple receptors of postsynaptic nature.

Effects of amylin on appetite regulation and memory

Amylin has been demonstrated to decrease food intake in mice and rats. Amylin is effective when delivered both peripherally and directly into the central nervous system. Amylin's effect on food intake is not aversive. Amylin may produce its effect on food intake by modulating nitric oxide synthesis. Calcitonin gene related peptide also decreases food intake after peripheral and central administration. In addition, amylin has been demonstrated to modulate memory at both peripheral and central sites.

Hippocampal corticosteroid receptor mRNA expression and spatial learning in the aged Wistar rat

The expression of mineralocorticoid (MR) and glucocorticoid receptor (GR) mRNA in the hippocampus of aged (18-24 months) and young (7 months) Wistar rats in relation to spatial learning in the water maze was investigated. All aged rats showed impaired performance of the task to locate a hidden platform compared to young controls, but the Wistar strain as a whole showed inferior performance to a group of young Lister hooded controls (added for the purposes of comparison) with even young Wistar rats apparently failing to learn platform location. The aged Wistar rats spent nearly 70% of their time near the side walls of the pool throughout the 5 days of training, while the young Wistar controls showed a gradual reduction in the percentage time spent there. MR and GR mRNA expression were unaltered in any hippocampal subregion in aged rats compared to young controls. Plasma corticosterone levels were also unchanged in aged rats. These results show that alterations in hippocampal corticosteroid receptor gene expression are not an inevitable consequence of aging in the rat, but that aging may be associated with strategy changes in the performance of a spatial learning task that are independent of glucocorticoid hypersecretion or alterations in hippocampal corticosteroid receptor mRNA expression.

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

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

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.

Interaction of cholinergic-dopaminergic systems in the regulation of memory storage in aversively motivated learning tasks

These experiments examined the interaction between muscarinic cholinergic and dopaminergic systems in the modulation of memory storage. Male CD1 mice (25-30 g) were trained in an inhibitory avoidance (IA) and a Y-maze discrimination (YMD) task. The first experiment examined the dose-response effects, on retention, of agonists and antagonists specific for either D1- or D2-receptors. Immediately posttraining mice were given i.p. injections of saline, the D1-receptor agonists SKF 38393 (3.0, 10.0 or 30.0 mg/kg) or SKF 77434 (3.0, 10.0 or 30.0 mg/kg), the D1-receptor antagonist SCH 23390 (0.03, 0.1, or 1.0 mg/kg), the D2-receptor agonist quinpirole (0.3, 1.0 or 3.0 mg/kg) or the D2-receptor antagonist sulpiride (3.0, 10.0, 30.0 or 100.0 mg/kg). Retention was tested 48 h later. The drugs affecting D1-receptors did not affect retention. In contrast, in both tasks quinpirole enhanced retention and sulpiride impaired retention. In the IA task, quinpirole (3.0 mg/kg) blocked the retention impairing effects of the muscarinic cholinergic antagonist atropine (10.0 mg/kg), and sulpiride (3.0, 10.0, 30.0 or 100.0 mg/kg) significantly attenuated the memory enhancing effects of the muscarinic cholinergic agonist oxotremorine (35.0 or 70.0 micrograms/kg). D1-receptor agents did not modify the effects of either atropine or oxotremorine on retention of the IA response. These findings suggest that the effects of cholinergic muscarinic agents on retention of the IA response are mediated by influences involving D2-dopaminergic mechanisms. In the YMD task, atropine (10.0 mg/kg) blocked the memory-enhancing effects of quinpirole (3.0 mg/kg) and oxotremorine (35.0 or 70.0 micrograms/kg) attenuated the memory impairing effect of sulpiride (3.0, 10.0, 30.0 or 100.0 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Minaprine facilitates acquisition and retrieval of an active avoidance response in the rat

The nootropic activity of 3-(2-morpholino-ethylamino)-4-methyl-6-phenyl-pyridazine dihydrochloride (minaprine) has been investigated in intact male, adult Long Evans rats by means of an active avoidance paradigm. In the light-dark box apparatus, the rat had to learn the active avoidance response of going out of the normally preferred dark chamber to avoid electric foot-shocks. These were administered during one trial per day for 3 consecutive days (acquisition period). After a 72-h interval, rats underwent, for 3 consecutive days, one trial per day in which punishments were omitted (retrieval period). In the first experiment, rats were injected IP with minaprine (5, 10, and 25 mg/kg b.w.) 30 min before each trial of both periods. Rats injected with the two lower dosages showed better responding during the retrieval period than controls (saline). On the contrary, the highest dosage impaired active avoidance during both periods. In Experiment 2, minaprine (10 mg/kg b.w.) was administered either only during the acquisition or only during the retrieval period. In both instances, active avoidance was equally enhanced, if compared to controls (saline), only during the retrieval period. The results are discussed on the basis of the known facilitating activity on cholinergic systems of this compound. It is concluded that minaprine acts positively both on acquisition and retrieval of mnemonic traces.

Memory enhancement by post-training peripheral administration of low doses of dopamine agonists: possible autoreceptor effect

These experiments examined the effect of post-training injections of low doses of dopamine (DA) agonists on the acquisition of two 8-arm radial maze tasks. On a winstay simultaneous discrimination task, a light cue signaled the location of food in four randomly selected arms on each trial, and animals were required to visit each of the lit arms twice within a trial. Animals received one food trial per day and were injected immediately after training on Day 5. The direct DA receptor agonist, apomorphine (0.05 mg/kg), and the direct D2-DA receptor agonists, LY 177555 (quinpirole: 0.05, 0.1 mg/kg) and B-HT 920 (0.05 mg/kg), all improved acquisition of winstay radial maze behavior relative to saline-injected controls. On a win-shift task, rats were allowed to obtain food from four randomly selected maze arms, followed by a delay period in which they were removed from the maze. Animals were returned to the maze for a retention test in which only those arms that had not been visited prior to the delay contained food. After training on shorter delays, a delay of 18 h was imposed between the first four and second four choices, and DA agonists were injected immediately after the first four choices. Apomorphine, LY 171555, and B-HT 920 (all at 0.05 mg/kg), all improved win-shift retention relative to saline-injected controls. On both tasks, delaying the injections for 2 h post-training eliminated the memory-improving effects of all drugs. The results indicate that post-training administration of DA agonists at doses that may preferentially stimulate autoreceptors improves memory.

Benzodiazepines in the brain. Their origin and possible biological roles

Great progress has been made in the last 5 yr in demonstrating the presence of benzodiazepines (BDZs) in mammalian tissues, in beginning studies on the origin of these natural compounds, and in elucidating their possible biological roles. Many unanswered questions remain regarding the sources and biosynthetic pathways responsible for the presence of BDZs in brain and their different physiological and/or biochemical actions. This essay will focus on recent findings supporting that: (1) BDZs are of natural origin; (2) mammalian brain contains BDZs in concentrations ranging between 5 x 10(-10)-10(-8) M; (3) dietary source of BDZs might be a plausible explanation for their occurrence in animal tissues, including man; (4) the formation of BDZ-like molecules in brain is a possibility, experimentally supported; (5) BDZ-like molecules including diazepam and N-desmethyldiazepam are elevated in hepatic encephalopathy; and (6) natural BDZs in the brain are involved in the modulation of memory processes. Future studies using the full range of biochemical, physiological, behavioral, and molecular biological techniques available to the neuroscientist will hopefully continue to yield exciting and new information concerning the biological roles that BDZs might play in the normal and pathological functioning of the brain.

Inverted-U relationship between the level of peripheral corticosterone and the magnitude of hippocampal primed burst potentiation

Studies have shown that peripheral levels of corticosterone correlate with the magnitudes of two well-described physiological models of memory, long-term potentiation (LTP) and primed burst (PB) potentiation. In the present experiments, the authors investigated the effects of experimenter-controlled manipulations of the levels of corticosterone on the magnitude of hippocampal PB potentiation in urethane-anesthetized rats. Primed burst potentiation is a long-lasting (at least 30 minutes) increase in the amplitude of the CA1 population spike and EPSP slope in response to physiologically patterned stimulation of the hippocampal commissure. The levels of serum corticosterone were controlled by implanting corticosterone pellets in adrenalectomized rats (ADX/PELLET). In the first experiment, a significant negative linear correlation between elevated (stress) levels of serum corticosterone (greater than 20 micrograms/dL) and the magnitude of PB potentiation in ADX/PELLET subjects (r = 0.60, P < .05) was found. In the second experiment, the shape of the corticosterone-PB potentiation function was different at low and intermediate levels of corticosterone than it was at high levels of corticosterone: There was a positive correlation at low levels (0-10 micrograms/dL), a peak response at intermediate levels (11-20 micrograms/dL), and a negative correlation at high levels (21-93 micrograms/dL) of corticosterone. Thus, the overall relationship between corticosterone and PB potentiation is an inverted-U function. These findings provide strong support for the hypothesis that corticosterone exerts a concentration-dependent biphasic influence on the expression of hippocampal plasticity.

Antagonism of GABAergic transmission within the septum disrupts working/episodic memory in the rat

Male Sprague-Dawley rats, trained to perform a standard or delayed-non-match-to-sample radial arm maze task, were implanted with a single cannula aimed at the medial septal nucleus. A within-subjects design was utilized to examine the effects of intraseptal administration of the GABAergic antagonist bicuculline on performance of these tasks. Bicuculline (0-0.5 microgram/0.5 microliter) infusion produced dose-dependent impairments when administered prior to performance of a standard radial arm maze task. Post-training infusion of bicuculline (0.-0.25 microgram/0.5 microliter) also induced dose-dependent impairments in the delayed version (4 h) of the task. Further testing indicated that post-training administration of a low dose of bicuculline (0.05 microgram) in the delayed version of the task induced a deficit at a 4-h, but not a 1-h, retention interval. The latter indicates that the impairment varied as a function of bicuculline dose and increasing task difficulty (longer retention intervals). Previous observations indicated that post-training administration of the GABAergic agonist muscimol and the antagonist bicuculline could induce deficits in the performance of the delayed task. The present findings demonstrate that intraseptal bicuculline treatment can disrupt ongoing radial maze performance, as well as the maintenance and/or retrieval of memories necessary for performance of the delayed version of the task. These findings suggest that either activation or blockade of intraseptal GABA receptors is sufficient to disrupt working/episodic memory processes. The role of septum and septohippocampal pathway in working/episodic memory is discussed.

Diazepam impairs retention of spatial information without affecting retrieval or cue learning

We further investigated the effect of diazepam on the processing of spatial information in a water maze task. Diazepam significantly impaired the retention of spatial information in a group of rats trained to locate a hidden platform. In a free swim trial carried out after training, diazepam-treated rats showed no bias to the target quadrant. There was no effect of diazepam on retrieval of spatial information in well-trained rats, and diazepam was devoid of any effect on cue learning in the water maze. However, diazepam blocked latent place learning during cue training in the water maze. Our results indicate that the GABA-BZD receptor modulates spatial information processing and that diazepam specifically impairs the retention of spatial information without affecting retrieval or cue learning.

D-1 receptor involvement in reward-related learning

A comparison of the effects of apomorphine, amphetamine and dopamine (DA) receptor subtype-specific agonists in responding for conditioned reward, self-administration and place conditioning paradigms provides insights into the possible involvement of D-1 and D-2 receptors in reward-related learning. Amphetamine and the D-2 agonists bromocriptine and quinpirole enhanced responding for conditioned reward, were self-administered and produced place preferences. Apomorphine impaired responding for conditioned reward by enhancing responding on two levers, was self-administered and produced a place preference. The D-1 agonist SKF 38393 impaired responding for condition reward, did not support self-administration and produced a place preference. The failure of SKF 38393 to support self-administration may have been related to effects of this drug, for example, peripheral aversive effects or a slow onset of action, unrelated to its action at the D-1 receptor. It was suggested that a D-1 agonist might be expected to be self-administered from the point of view of the hypothesis that it is the action at D-1 receptors of DA released in association with reward that produces reward-related learning. This hypothesis was supported by the remaining data. Thus, apomorphine and SKF 38393 may have masked the DA signal associated with reward in the conditioned reward paradigm leading to a loss of control of responding by the conditioned rewarding stimulus. In self-administration, apomorphine would have its onset of action after the performance of the response which is followed immediately by a conditioned reward. The conditioned reward may effectively maintain control of behaviour by the lever and related stimuli while the drug may maintain the effectiveness of the conditioned reward. In place conditioning, there is no specific environmental stimulus that must come to control responding; therefore, apomorphine and SKF 38393 may have been seen to produce place preferences in spite of their relatively tonic action at D-1 receptors. Finally, the finding that the D-1 antagonists SCH 23390 or SCH 39166 blocked the effects of reward in these paradigms was taken as further evidence that the D-1 receptor may be critically involved in the learning produced by rewarding stimuli.

Behavioral screening for cognition enhancers: from indiscriminate to valid testing: Part I

Preclinical efforts to detect and characterize potential cognition enhancers appear to have been dominated by a strategy of demonstrating a wide variety of apparently beneficial behavioral effects with little attention given to the specific psychological mechanisms underlying behavioral enhancement. In particular, the question of whether or not behavioral facilitation is based on relevant mnemonic mechanisms and is independent of the stimulus properties and/or the motivational and attentional components of a task is not often considered. As a result, an overwhelming number of compounds have failed to produce the clinical effects predicted for them on the basis of preclinical research. The available data suggest that a more successful approach requires deductive research strategies rather than the indiscriminate accumulation of apparently beneficial effects in a variety of behavioral tasks and animal models. The first step towards such an approach is a systematic and rigorous evaluation of the different aspects of validity for the models most frequently used in preclinical research. It is concluded that a combination of good construct validity and good face validity represents a necessary condition for screening tests with predictive validity, and that the most popular paradigms fail to fulfil these criteria. Future screening programs for cognition enhancers will probably be characterized by a depreciation of "fast and dirty tests" in favor of approaches focussing on the validity of the effects of potential cognition enhancers.

GABAA receptor modulation of memory: the role of endogenous benzodiazepines

GABAA receptors are known to downregulate memory consolidation processes: picrotoxin and bicuculline enhance memory, and benzodiazepines and muscimol depress it. The discovery of naturally occurring benzodiazepines in the brain prompted a recent investigation of whether these compounds could act as physiological regulators of the GABAA receptors involved in memory modulation. Different forms of learning cause a rapid reduction of benzodiazepine-like immunoreactivity in septum, amygdala and hippocampus; microinjection of the benzodiazepine antagonist flumazenil into these regions, at the time that consolidation is taking place, enhances memory. Ivan Izquierdo and Jorge Medina suggest that these and other findings indicate that benzodiazepines released in the septum, amygdala and hippocampus do indeed physiologically downregulate memory storage processes; moreover, benzodiazepine release could be modulated by the anxiety and/or stress associated with each type of learning.

Localized intracaudate dopamine D2 receptor activation during the post-training period improves memory for visual or olfactory conditioned emotional responses in rats

Rats with cannulas aimed at the posteroventral (PV) or ventrolateral (VL) areas of the caudate nucleus were trained on a conditioned emotional response (CER) task. Post-training microinjections of the indirect catecholamine agonist, d-amphetamine (5 micrograms), or of the dopamine D2 receptor agonist, LY171555 (1 microgram), into the PV area improved retention of a CER with a visual CS, but had no effect on a CER with an olfactory CS. Post-training injections of the same two drugs into the VL area improved retention of a CER with an olfactory CS, but had no effect on a CER with a visual CS. Post-training injections of the dopamine D1 receptor agonist, SKF38393 (0.5, 1.0, 2.0 micrograms), into either site had no effects on either CER. These findings suggest that different areas of the caudate nucleus mediate acquisition of CERs with different CSs, possibly implicating the topographically organized corticostriatal innervation in the acquisition of certain types of memories in the caudate nucleus. The findings also suggest that dopamine D2 receptors in the caudate nucleus are involved in the acquisition of these CERs.

Memory facilitation by post-training intraperitoneal, intracerebroventricular and intra-amygdala injection of Ro 5-4864

Post-training i.p. (2.0 or 5.0 mg/kg), i.c.v. (2.5 micrograms/rat), or intra-amygdala (1.6-40 ng/amygdala) administration of Ro 5-4864 causes memory facilitation of step-down inhibitory avoidance in rats. The effect is expressed as an increased latency to step down in a retention test carried out 24 h after training. Ro 5-4864 is a blocker of the Cl(-)-channel associated with GABAA receptors, at a site sensitive to the antagonist, PK11195, and different from that sensitive to picrotoxin. PK11195, given i.c.v. (2.5 micrograms/rat) or into the amygdala (8 ng/amygdala), antagonized the effect of Ro 5-4864. Intra-amygdala picrotoxin administration (80 ng/amygdala) also caused retrograde memory facilitation, but its effect was not antagonized by PK11195. At a higher dose (40 ng/amygdala), PK11195 had an amnestic effect of its own, which suggests that it might be acting against an endogenous ligand of receptor to Ro 5-4864 in the Cl(-)-channel. These findings support the hypothesis that there is a GABAA mechanism in the amygdala normally involved in the modulation of the post-training memory processing of aversive learnings.

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

Diazepam impairs place learning in the Morris water maze

The effect of diazepam (0.3, 1.0, and 3.0 mg/kg) on the acquisition and retention of place learning was evaluated. The analysis of escape latencies indicates that 1.0 and 3.0 mg/kg diazepam significantly impaired the retention of spatial information. When a free swim trial was carried out only control animals showed spatial bias to the target quadrant. The absence of spatial bias in the group that received 0.3 mg/kg suggests that the amnesic effect of diazepam can be seen at doses similar to or even lower than the anxiolytic ones, and that the GABA/benzodiazepine receptor complex is highly sensitive to the cognitive impairment induced by diazepam in spatial tasks.

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.

Post-training down-regulation of memory consolidation by a GABA-A mechanism in the amygdala modulated by endogenous benzodiazepines

In rats, amygdala benzodiazepine-like immunoreactivity decreases by 29% immediately after the animals step down from the platform of an inhibitory avoidance apparatus and decreases by a further 45% immediately after they receive a training footshock. The decrease is attributable to a release of diazepam or diazepam-like molecules. The immediate post-training intraamygdala injection of the central benzodiazepine antagonist flumazenil (10 nmole/amygdala) causes memory facilitation, and that of the GABA-A agonist muscimol (0.005 to 0.5 nmole) causes retrograde amnesia. Pretraining ip flumazenil administration (2.0 and 5.0 mg/kg) attenuates the effect of post-training muscimol by a factor of at least 100. The higher dose of pretraining flumazenil also causes memory facilitation. The data suggest that post-training consolidation is down-regulated by a GABA-A mechanism in the amygdala modulated by endogenous benzodiazepines released during training and at the time of consolidation.