Memory dissociation: the approach to the study of retrieval processes

Changes in the level of fatty acids in the brain of rats during memory acquisition

Memory acquisition is accompanied by many cellular and molecular processes, and it is not always clear what role they play. Fatty acids (FAs) are known to be important for cognitive functions, but the details of their involvement in memory processes remain unknown. We investigated FAs in the prefrontal cortex and hippocampus of rats trained to perform a task with food reinforcement. The learning consisted of two training sessions, each of which included 10 trials. The results showed that such training altered individual FAs in the brains. The most significant changes were in the prefrontal cortex, where an increase in the level of many FAs occurred, especially after the second training session: palmitic (16:0), stearic (18:0), docosahexaenoic (22:6, n-3), arachidonic (22:4, n-6), docosapentaenoic (22:5, n-6) acids. Changes in the fatty acid level after training in rats were detected only in the left hippocampus, where the levels of palmitic, docosapentaenoic, and docosahexaenoic acids changed. The changes in the right hippocampus were not significant. In both the prefrontal cortex and the left hippocampus, 72 h after training, all FAs returned to control levels. We believe that the main role of a reversible increase in FA levels during memory acquisition is to support and protect cellular processes involved in memory acquisition. Consolidation of memory traces, which occurs mainly in the neocortex, requires protection from external influences, to which FAs makes a significant contribution. They are able to improve neuronal plasticity, enhance local blood flow, improve mitochondrial processes, and suppress pro-inflammatory signals.

Resveratrol improved hippocampal neurogenesis following lead exposure in rats through activation of SIRT1 signaling

Lead (Pb) poses a potential environmental risk factor for cognitive dysfunction during early life and childhood. Resveratrol is considered a promising antioxidant with respect to the prevention of cognitive deficits and act as a potent SIRT1 agonist. Herein, this study aims to investigate the profile of neurogenesis markers following Pb exposure and to determine the regulatory role of resveratrol in this process. We confirmed firstly the protective effects of resveratrol against Pb-induced impairments of hippocampal neurogenesis in Male SD rats. Pb exposure early in life caused the altered expression of Ki-67, NeuN, caspase-3 and SIRT1 signaling, thereby resulting in spatial cognitive impairment of adolescent rats. As expected, resveratrol reduced cognitive damage and promoted neurogenesis in Pb-induced injury by regulation of SIRT1 pathway. Collectively, our study establishes the efficacy of resveratrol as a neuroprotective agent and provides a strong rationale for further studies on SIRT1-mediated mechanisms of neuroprotective functions.

The role of anti-inflammatory cytokines in memory processing in a healthy brain

The purpose of the work was to study the role of anti-inflammatory cytokines in memory processing in a healthy brain. Wistar rats were trained to perform a task with positive (food) reinforcement; and then the task performance was tested after intraventricular injection of IL-10 or TGF-β1. A microinjection into the brain of either of the two cytokines did not affect the performance of the task and did not have an anti-amnesic effect when the retrieval was deteriorated with scopolamine. In addition, endogenous levels of IL-10 and TGF-β1 were determine in the prefrontal cortex and in the hippocampus after one and two training sessions, consisting of 10 runs each. The level of IL-10 did not change after training both in the prefrontal cortex and in the hippocampus. Endogenous level of TGF-β1 increased in the neocortex after the first training session, the second session, and recovered to the normal level three days after training. In contrast, in the hippocampus, the level of TGF-β1 was decreased: maximally after the first training session in the right hippocampus and after the second training session in the left one. Given the role of the prefrontal cortex in memory processing, we assume that a specific increase of TGF-β1 in the prefrontal cortex may indicate involvement in memory trace consolidation.

Hydrogen sulfide attenuates isoflurane-induced neuroapoptosis and cognitive impairment in the developing rat brain

Background

Isoflurane-induced neuroapoptosis and cognitive impairment has been previously reported. Hydrogen sulfide (H₂S) has been shown to be a neuromodulator that is thought to have anti-apoptotic, anti-inflammatory, and anti-oxidative benefits. However, it is not known if H₂S is protective against anesthesia-induced apoptosis and cognitive defects.

Methods

In this study, postnatal day 7 (P7) Sprague-Dawley rats were randomly divided into four groups: control group (normal saline), H₂S group (NaHS 28 μM/kg), isoflurane group (normal saline +0.75% isoflurane) and H₂S preconditioning group (NaHS 28 μM/kg + 0.75% isoflurane). After exposure to isoflurane for 6 h, half the numbers of rats in each group were euthanized, and the hippocampus and cerebral cortex were dissected and examined for apoptosis by the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) technique and western blot. After 6 weeks, the remaining rats were subjected to a Morris water maze (MWM) test for behavioral assessment.

Results

The TUNEL assay and western blot showed that when rats were preconditioned with NaHS, neuroapoptosis decreased significantly both in hippocampus and cerebral cortex compering with the isofulrane group. The MWM showed that P7 rats administration of NaHS improved cognitive impairments induced by isoflurane.

Conclusions

The current study demonstrates that H₂S attenuates isoflurane-induced neuroapoptosis and improves cognitive impairments in the developing rat brain.

Modulation of learning and memory by natural polyamines

Spermine and spermidine are natural polyamines that are produced mainly via decarboxylation of l-ornithine and the sequential transfer of aminopropyl groups from S-adenosylmethionine to putrescine by spermidine synthase and spermine synthase. Spermine and spermidine interact with intracellular and extracellular acidic residues of different nature, including nucleic acids, phospholipids, acidic proteins, carboxyl- and sulfate-containing polysaccharides. Therefore, multiple actions have been suggested for these polycations, including modulation of the activity of ionic channels, protein synthesis, protein kinases, and cell proliferation/death, within others. In this review we summarize these neurochemical/neurophysiological/morphological findings, particularly those that have been implicated in the improving and deleterious effects of spermine and spermidine on learning and memory of naïve animals in shock-motivated and nonshock-motivated tasks, from a historical perspective. The interaction with the opioid system, the facilitation and disruption of morphine-induced reward and the effect of polyamines and putative polyamine antagonists on animal models of cognitive diseases, such as Alzheimer's, Huntington, acute neuroinflammation and brain trauma are also reviewed and discussed. The increased production of polyamines in Alzheimer's disease and the biphasic nature of the effects of polyamines on memory and on the NMDA receptor are also considered. In light of the current literature on polyamines, which include the description of an inborn error of the metabolism characterized by mild-to moderate mental retardation and polyamine metabolism alterations in suicide completers, we can anticipate that polyamine targets may be important for the development of novel strategies and approaches for understanding the etiopathogenesis of important central disorders and their pharmacological treatment.

Arcaine and MK-801 make recall state-dependent in rats

RATIONALE

The polyamines putrescine, spermidine, and spermine are a group of aliphatic amines that may act as physiological modulators of the N-methyl-D-aspartate (NMDA) receptor, a glutamate receptor implicated in memory formation and consolidation. Arcaine is a competitive antagonist of the polyamine binding site at the NMDA receptor, the post-training administration of which impairs memory of various tasks.

OBJECTIVES

In this study, we investigated whether the administration of arcaine and MK-801 alters the memory of the step-down inhibitory avoidance task, and whether the effects of these NMDA antagonists involve state-dependency mechanisms, in adult male Wistar rats.

RESULTS

The administration of arcaine (30 mg/kg, i.p.) or MK-801 (0.03 mg/kg, i.p.) immediately after training impaired inhibitory avoidance performance at testing. Arcaine- and MK-801-induced performance impairment was reversed by the administration of arcaine (30 mg/kg, i.p.) and MK-801 (0.03 mg/kg, i.p.), respectively, 30 min before testing. Response transfer also occurred if arcaine substituted MK-801 at testing, and vice-versa.

CONCLUSION

These results suggest that arcaine and MK-801 induce state-dependent recall and that, probably due to their ability to decrease NMDA receptor function, one drug can substitute for the other at testing, demonstrating a cross-state dependency between arcaine and MK-801.

Atropine-induced, state-dependent learning for spatial information, but not for visual cues

This study investigates state-dependent learning employing atropine. The reaction of rats to (1) the presentation of novel stimuli, (2) habituation to intermittent presentations of the same stimulus at the same local, (3) spatial change at the site of stimulus presentation, and (4) a visual stimulus change, was investigated in the straight alleyway test, controlling for the possible development of behavioral and/or pharmacological tolerance. Our findings reveal that rats habituated to stimulus presentation at a specific location, when under an atropine effect, do react to stimulus presentation at another location, or to a different stimulus, when under an atropine effect, indicating that this drug does not interfere with the acquisition of spatial or visual information. Differently, however, rats habituated to stimulus presentation at a specific location in the absence of an atropine effect are unable to react to spatial change when under the atropine effect, but do react to a visual stimulus change. This suggests that atropine interferes either with the retrieval of previously acquired spatial information or with the comparison of previously acquired spatial information with current information, but does not interfere with visual recognition. These findings reveal that atropine interferes with the use of spatial information acquired in the absence of a drug effect.

Are the effects of the antidepressants amitriptyline, maprotiline, and fluoxetine on inhibitory avoidance state-dependent?

State-dependent learning (SDL) is a phenomenon in which the retrieval of newly acquired information is possible if the subject is in the same physiological state as during the encoding phase. SDL makes it possible to separate the effects of drugs per se on learning from the effects due to changes in drug state during the task. The present work was designed to investigate whether the antidepressants amitriptyline (30 mg/kg), maprotiline (25 mg/kg), and fluoxetine (15 mg/kg) produce SDL of the inhibitory avoidance conditioning in male and female CD1 mice. In three separate experiments, independent groups were used for each pharmacological treatment and for each sex using a 2 x 2 experimental design. The results do not show SDL in any of the drugs. In the case of amitriptilyline, the data can be attributed to a memorization deficit, while the maprotiline results are interpreted as simultaneously influenced by memorization deficit and performance facilitation due to motor impairment. Fluoxetine treatment did not produce any deteriorating effect on the conditioning. Drugs had some different effects on the performance of males and females, males showing a slightly higher deterioration than females with administration of amitriptyline and maprotiline. This study shows that these antidepressants affect the acquisition/consolidation but not the retrieval process in the inhibitory avoidance learning.