New frontiers in the study of memory mechanisms

Hippocampal cholinergic receptors and the mTOR participation in fear-motivated inhibitory avoidance extinction memory

Evidence has demonstrated the hippocampal cholinergic system and the mammalian target of rapamycin (mTOR) participation during the memory formation of aversive events. This study assessed the role of these systems in the hippocampus for the extinction memory process by submitting male Wistar rats to fear-motivated step-down inhibitory avoidance (IA). The post-extinction session administration of the nicotinic and muscarinic cholinergic receptor antagonists, mecamylamine and scopolamine, respectively, both at doses of 2 µg/µl/side, and rapamycin, an mTOR inhibitor (0.02 µg/µl/side), into the CA1 region of the dorsal hippocampus, impaired the IA extinction memory. Furthermore, the nicotinic and muscarinic cholinergic receptor agonists, nicotine and muscarine, respectively, had a dose-dependent effect on the IA extinction memory when administered intra-CA1, immediately after the extinction session. Nicotine (0.6 µg/µl/side) and muscarine (0.02 µg/µl/side), respectively, had no effect, while the higher doses (6 and 2 µg/µl/side, respectively) impaired the IA extinction memory. Interestingly, the co-administration of muscarine at the lower dose blocked the impairment that was induced by rapamycin. This effect was not observed when nicotine at the lower dose was co-administered. These results have demonstrated the participation of the cholinergic receptors and mTOR in the hippocampus for IA extinction, and that the cholinergic agonists had a dose-dependent effect on the IA extinction memory. This study provides insights related to the behavioural aspects and the neurobiological properties underlying the early stage of fear-motivated IA extinction memory consolidation and suggests that there is hippocampal muscarinic receptor participation independent of mTOR in this memory process.

Does the enriched environment alter memory capacity in malnourished rats by modulating BDNF expression?

Environmental factors interfere in the neural plasticity processes. Among these, malnutrition in the early stages of life stands out as one of the main non-genetic factors that can interfere in the morphofunctional development of the nervous system. Furthermore, sensory stimulation from enriched environments (EE) also interferes with neural development. These two factors can modify areas related to memory and learning as the hippocampus, through mechanisms related to the gene expression of brain-derived neurotrophic factor (BDNF). The BDNF may interfere in synaptic plasticity processes, such as memory. In addition, these changes in early life may affect the functioning of the hippocampus during adulthood through mechanisms mediated by BDNF. Therefore, this study aims to conduct a literature review on the effects of early malnutrition on memory and the relationship between the underlying mechanisms of EE, BDNF gene expression, and memory. In addition, there are studies that demonstrate the effect of EE reversal on exposure to changes in the functioning of hippocampal malnutrition in adult rats that were prematurely malnourished. Thereby, evidence from the scientific literature suggests that the mechanisms of synaptic plasticity in the hippocampus of adult animals are influenced by malnutrition and EE, and these alterations may involve the participation of BDNF as a key regulator in memory processes in the adult animal hippocampus.

Forgetting Unwanted Memories: Active Forgetting and Implications for the Development of Psychological Disorders

Intrusive memories are a common feature of many psychopathologies, and suppression-induced forgetting of unwanted memories appears as a critical ability to preserve mental health. In recent years, biological and cognitive studies converged in revealing that forgetting is due to active processes. Recent neurobiological studies provide evidence on the active role of main neurotransmitter systems in forgetting, suggesting that the brain actively works to suppress retrieval of unwanted memories. On the cognitive side, there is evidence that voluntary and involuntary processes (here termed "intentional" and "incidental" forgetting, respectively) contribute to active forgetting. In intentional forgetting, an inhibitory control mechanism suppresses awareness of unwanted memories at encoding or retrieval. In incidental forgetting, retrieval practice of some memories involuntarily suppresses the retrieval of other related memories. In this review we describe recent findings on deficits in active forgetting observed in psychopathologies, like post-traumatic stress disorder, depression, schizophrenia, and obsessive-compulsive disorder. Moreover, we report studies in which the role of neurotransmitter systems, known to be involved in the pathogenesis of mental disorders, has been investigated in active forgetting paradigms. The possibility that biological and cognitive mechanisms of active forgetting could be considered as hallmarks of the early onset of psychopathologies is also discussed.

Comparative study on the effects of cigarette smoke exposure, ethanol consumption and association: Behavioral parameters, apoptosis, glial fibrillary acid protein and S100β immunoreactivity in different regions of the rat hippocampus

Exposure to cigarette smoke and ethanol are proposed to trigger neurotoxicity, apoptosis, and to impair neuronal signaling. However, it is little known how the combination of both might trigger astrogliosis and the morphological changes capable of affecting a differential susceptibility of hippocampal regions to these licit drugs. The present study investigated the chronic effects of exposure to cigarette smoke and/or ethanol on behavioral parameters, apoptosis, and alteration in immunoreactivity of glial fibrillary acid protein (GFAP) and S100β in the CA1, CA3, and dentate gyrus (DG) of the rat hippocampus. Adult male Wistar rats (n = 32) were divided into four groups: vehicle (VE, glucose 3% in water, 10 mL/kg), cigarette smoke (TOB, total 12 cigarettes per day), ethanol (ethanol, 2 g/kg), and cigarette smoke plus ethanol (TOB plus ethanol, total 12 cigarettes per day plus ethanol 2 g/kg) for 54 days. The groups were submitted to tail-flick, open-field, and inhibitory avoidance tasks. The results showed that ethanol per se worsened the short-term memory. The association between TOB and ethanol increased the immunoreactivity of cleaved caspase-3 in the CA3 and DG regions. The TOB plus ethanol group showed a lower immunoreactivity to GFAP in all regions of the hippocampus. In addition, ethanol and TOB per se also reduced the immunoreactivity for GFAP in the DG. Ethanol increased S100β immunoreactivity only in the DG. In conclusion, this study showed that only ethanol worsened short-term memory, and the DG became more susceptible to changes in the markers investigated. This evidence suggests that DG is more sensitive to neurotoxicity induced by cigarette smoke and ethanol.

Memory corticalization triggered by REM sleep: mechanisms of cellular and systems consolidation

Once viewed as a passive physiological state, sleep is a heterogeneous and complex sequence of brain states with essential effects on synaptic plasticity and neuronal functioning. Rapid-eye-movement (REM) sleep has been shown to promote calcium-dependent plasticity in principal neurons of the cerebral cortex, both during memory consolidation in adults and during post-natal development. This article reviews the plasticity mechanisms triggered by REM sleep, with a focus on the emerging role of kinases and immediate-early genes for the progressive corticalization of hippocampus-dependent memories. The body of evidence suggests that memory corticalization triggered by REM sleep is a systemic phenomenon with cellular and molecular causes.

Protein kinase-based neural signaling pathways for ginsenosides: a retrospective review

Ginsenosides are the main active components of ginseng, which have been reported to target brain tissues and produce multiple neuroprotective effects. Ginsenosides have been shown to improve learning ability and memory in normal aged animals, and in an animal model of memory impairment. However, its underlying pharmacological mechanisms are very complicated, especially with regard to its effects on the activation of protein kinases in neurons. Previous reports have shown that some protein kinases may be affected by ginsenosides, including protein kinase C, calcium/calmodulin-dependent protein kinase II, c-Jun-N terminal kinase, and protein tyrosine kinase. In this paper, protein kinases that may underlie the mechanisms of ginsenosides will be discussed.

Common mechanisms of Alzheimer's disease and ischemic stroke: the role of protein kinase C in the progression of age-related neurodegeneration

Ischemic stroke and Alzheimer's disease (AD), despite being distinct disease entities, share numerous pathophysiological mechanisms such as those mediated by inflammation, immune exhaustion, and neurovascular unit compromise. An important shared mechanistic link is acute and chronic changes in protein kinase C (PKC) activity. PKC isoforms have widespread functions important for memory, blood-brain barrier maintenance, and injury repair that change as the body ages. Disease states accelerate PKC functional modifications. Mutated forms of PKC can contribute to neurodegeneration and cognitive decline. In some cases the PKC isoforms are still functional but are not successfully translocated to appropriate locations within the cell. The deficits in proper PKC translocation worsen stroke outcome and amyloid-β toxicity. Cross talk between the innate immune system and PKC pathways contribute to the vascular status within the aging brain. Unfortunately, comorbidities such as diabetes, obesity, and hypertension disrupt normal communication between the two systems. The focus of this review is to highlight what is known about PKC function, how isoforms of PKC change with age, and what additional alterations are consequences of stroke and AD. The goal is to highlight future therapeutic targets that can be applied to both the treatment and prevention of neurologic disease. Although the pathology of ischemic stroke and AD are different, the similarity in PKC responses warrants further investigation, especially as PKC-dependent events may serve as an important connection linking age-related brain injury.

The role of progesterone in memory: an overview of three decades

Memory comprises acquisition, consolidation and retrieval of information. Many substances can influence these different phases. It is well demonstrated that sex hormones, mainly estrogen, impact cognitive function. More recently, progesterone has also been documented as playing an important role in cognition, since it influences brain regions involved in memory. Currently, many women are under hormone treatment, which contain progesterone to decrease the risk of development of endometrial cancer. This affords the opportunity to study the real effects of this hormonal replacement on cognition. There are many contradictory results regarding the role of progesterone in memory. Therefore, the aim of this review was to synthesize these studies using the new perspective of the influence of hormone replacement on cognition in women.