Untangling a Cholinergic Pathway from Wakefulness to Memory

Consolidation of memory traces in cultured cortical networks requires low cholinergic tone, synchronized activity and high network excitability

In systems consolidation, encoded memories are replayed by the hippocampus during slow-wave sleep (SWS), and permanently stored in the neocortex. Declarative memory consolidation is believed to benefit from the oscillatory rhythms and low cholinergic tone observed in this sleep stage, but underlying mechanisms remain unclear. To clarify the role of cholinergic modulation and synchronized activity in memory consolidation, we applied repeated electrical stimulation in mature cultures of dissociated rat cortical neurons with high or low cholinergic tone, mimicking the cue replay observed during systems consolidation under distinct cholinergic concentrations. In the absence of cholinergic input, these cultures display activity patterns hallmarked by network bursts, synchronized events reminiscent of the low frequency oscillations observed during SWS. They display stable activity and connectivity, which mutually interact and achieve an equilibrium. Electrical stimulation reforms the equilibrium to include the stimulus response, a phenomenon interpreted as memory trace formation. Without cholinergic input, activity was burst-dominated. First application of a stimulus induced significant connectivity changes, while subsequent repetition no longer affected connectivity. Presenting a second stimulus at a different electrode had the same effect, whereas returning to the initial stimuli did not induce further connectivity alterations, indicating that the second stimulus did not erase the 'memory trace' of the first. Distinctively, cultures with high cholinergic tone displayed reduced network excitability and dispersed firing, and electrical stimulation did not induce significant connectivity changes. We conclude that low cholinergic tone facilitates memory formation and consolidation, possibly through enhanced network excitability. Network bursts or SWS oscillations may merely reflect high network excitability.

Low acetylcholine during early sleep is important for motor memory consolidation

The synaptic homeostasis theory of sleep proposes that low neurotransmitter activity in sleep optimizes memory consolidation. We tested this theory by asking whether increasing acetylcholine levels during early sleep would weaken motor memory consolidation. We trained separate groups of adult mice on the rotarod walking task and the single pellet reaching task, and after training, administered physostigmine, an acetylcholinesterase inhibitor, to increase cholinergic tone in subsequent sleep. Post-sleep testing showed that physostigmine impaired motor skill acquisition of both tasks. Home-cage video monitoring and electrophysiology revealed that physostigmine disrupted sleep structure, delayed non-rapid-eye-movement sleep onset, and reduced slow-wave power in the hippocampus and cortex. Additional experiments showed that: (1) the impaired performance associated with physostigmine was not due to its effects on sleep structure, as 1 h of sleep deprivation after training did not impair rotarod performance, (2) a reduction in cholinergic tone by inactivation of cholinergic neurons during early sleep did not affect rotarod performance, and (3) stimulating or blocking muscarinic and nicotinic acetylcholine receptors did not impair rotarod performance. Taken together, the experiments suggest that the increased slow wave activity and inactivation of both muscarinic and nicotinic receptors during early sleep due to reduced acetylcholine contribute to motor memory consolidation.

Anti-Oxidative Stress Activity Is Essential for Amanita caesarea Mediated Neuroprotection on Glutamate-Induced Apoptotic HT22 Cells and an Alzheimer's Disease Mouse Model

Amanita caesarea, an edible mushroom found mainly in Asia and southern Europe, has been reported to show good antioxidative activities. In the present study, the neuroprotective effects of A. caesarea aqueous extract (AC) were determined in an l-glutamic acid (l-Glu) induced HT22 cell apoptosis model, and in a d-galactose (d-gal) and AlCl₃-developed experimental Alzheimer’s disease (AD) mouse model. In 25 mM of l-Glu-damaged HT22 cells, a 3-h pretreatment with AC strongly improved cell viability, reduced the proportion of apoptotic cells, restored mitochondrial function, inhibited the over-production of intracellular reactive oxygen species (ROS) and Ca²⁺, and suppressed the high expression levels of cleaved-caspase-3, calpain 1, apoptosis-inducing factor (AIF) and Bax. Compared with HT22 exposed only to l-Glu cells, AC enhanced the phosphorylation activities of protein kinase B (Akt) and the mammalian target of rapamycin (mTOR), and suppressed the phosphorylation activities of phosphatase and tensin homolog deleted on chromosome ten (PTEN). In the experimental AD mouse, 28-day AC administration at doses of 250, 500, and 1000 mg/kg/day strongly enhanced vertical movements and locomotor activities, increased the endurance time in the rotarod test, and decreased the escape latency time in the Morris water maze test. AC also alleviated the deposition of amyloid beta (Aβ) in the brain and improved the central cholinergic system function, as indicated by an increase acetylcholine (Ach) and choline acetyltransferase (ChAT) concentrations and a reduction in acetylcholine esterase (AchE) levels. Moreover, AC reduced ROS levels and enhanced superoxide dismutase (SOD) levels in the brain of experimental AD mice. Taken together, our data provide experimental evidence that A. caesarea may serve as potential food for treating or preventing neurodegenerative diseases.