Synaptic plasticity studies and their applicability in mouse models of neurodegenerative diseases

Effects of PI3K/AKT/mTOR pathway regulation of HIF-1α on Lanthanum-induced neurotoxicity in rats

This study examined the effects of the AKT/mTOR/HIF-1α signaling pathway on learning and memory in offspring rats induced by lanthanum from neuroethology and molecular biology perspectives. 32 pregnant adult Wistar rats were divided into four groups randomly: control group (NC), 0.25%, 0.5% and 1.0% LaCl3 groups (n = 8). All rats were poisoned through free drinking from day 0 of pregnancy to postnatal day 21 (suckling period). All offspring rats were poisoned through free drinking from delactation to postnatal day 48. Offspring rats aged 49-days-old were used as sampling objects to construct an LaCl3 poisoning model of offspring rats. Changes in hippocampal neurons, apoptosis of hippocampal neurons, learning and memory abilities of LaCl3-poisoned animals were measured by Nissl staining, TUNEL method and the shuttle box test, respectively. Expressions of PI3K, AKT, and mTOR, HIF-1α, and VEGF in the hippocampus were tested by qPCR and Western blot. Distributions of PI3K and p-AKT in hippocampal neurons were observed through the immunohistochemical method. With increasing LaCl3 dose, lightning strike time and active avoidance incubation period of offspring rats in the different LaCl3 groups were significantly prolonged. The Nissl body positive neurons of hippocampal neurons gradually declined while apoptosis in cells increased. The expressions of both mRNA (PI3K, AKT, mTOR, HIF-1α, VEGF) and proteins (PI3K, p-AKT, p-mTOR, HIF-1α, VEGF) in the hippocampus of the LaCl3 groups were significantly lower than those of NC group (p < 0.05). LaCl3 poisoning can induce developmental injuries in hippocampal neurons and can increase cell apoptosis. As a result, learning and memory abilities of offspring rats, as well as the expressions of PI3K/AKT/mTOR, are decreased, thus inhibiting activation of HIF-1α and influencing the expression of the downstream VEGF gene.

Learning as a Functional State of the Brain: Studies in Wild-Type and Transgenic Animals

Contemporary neuroscientists are paying increasing attention to subcellular, molecular, and electrophysiological mechanisms underlying learning and memory processes. Recent studies have examined the development of transgenic mice affected at different stages of the learning process, or have emulated in animals various human pathological conditions involving cognition and motor learning. However, a parallel effort is needed to develop stimulating and recording techniques suitable for use in behaving mice in order to understand activity-dependent synaptic changes taking place during the very moment of the learning process. The in vivo models should incorporate information collected from different molecular and in vitro approaches. Long-term potentiation (LTP) has been proposed as the neural mechanism underlying synaptic plasticity, and NMDA receptors have been proposed as the molecular substrate of LTP. It now seems necessary to study the relationship of both LTP and NMDA receptors to functional changes in synaptic efficiency taking place during actual learning in selected cerebral cortical structures. Here, we review data collected in our laboratory during the past 10 years on the involvement of different hippocampal synapses in the acquisition of the classically conditioned eyelid responses in behaving mice. Overall the results indicate a specific contribution of each cortical synapse to the acquisition and storage of new motor and cognitive abilities. Available data show that LTP, evoked by high-frequency stimulation of Schaffer collaterals, disturbs both the acquisition of conditioned eyelid responses and the physiological changes that occur at hippocampal synapses during learning. Moreover, the administration of NMDA-receptor antagonists is able not only to prevent LTP induction in vivo, but also to hinder both the formation of conditioned eyelid responses and functional changes in the strength of the CA3-CA1 synapse. Nevertheless, many other neurotransmitter receptors, intracellular mediators, and transcription factors are also involved in learning and memory processes. In summary, it can be proposed that learning and memory in behaving mammals are the result of the activation of complex and distributed functional states involving many different cerebral cortical synapses, with the participation also of various neurotransmitter systems.

Functional basis of associative learning and its relationships with long-term potentiation evoked in the involved neural circuits: Lessons from studies in behaving mammals

While contemporary neuroscience is paying increasing attention to subcellular and molecular events and other intracellular phenomena underlying the acquisition, storage, and retrieval of newly acquired motor and cognitive abilities, parallel attention should be paid to the study of the electrophysiological phenomena taking place at selected cortical and subcortical neuronal and synaptic sites during the precise moment of learning acquisition, extinction, and recall. These in vivo approaches to the study of learning and memory processes will allow the proper integration of the important information collected from in vitro and delayed molecular studies. Here, we summarize studies in behaving mammals carried out in our laboratory during the past ten years on the relationships between experimentally evoked long-term potentiation (LTP) and activity-dependent changes in synaptic strength taking place in hippocampal, prefrontal and related cortical and subcortical circuits during the acquisition of classical eyeblink conditioning or operant learning tasks. These studies suggest that different hippocampal synapses are selectively modified in strength during the acquisition of classical, but not instrumental, learning tasks. In contrast, selected prefrontal and striatum synapses are more directly modified by operant conditioning. These studies also show that besides N-methyl-D-aspartate (NMDA) receptors, many other neurotransmitter, intracellular mediating, and transcription factors participate in these two types of associative learning. Although experimentally evoked LTP seems to prevent the acquisition of classical eyeblink conditioning when induced at selected hippocampal synapses, it proved to be ineffective in preventing the acquisition of operant conditioned tasks when induced at numerous hippocampal, prefrontal, and striatal sites. The differential roles of these cortical structures during these two types of associative learning are discussed, and a diagrammatic representation of their respective functions is presented.