Acute stress modulates hippocampal to entorhinal cortex communication

Feed-forward inhibition is vital in the transfer and processing of synaptic information within the hippocampal-entorhinal loop by controlling the strength and direction of excitation flow between different neuronal populations and individual neurons. While the cellular targets in the hippocampus that receive excitatory inputs from the entorhinal cortex have been well studied, and the role of feedforward inhibitory neurons has been attributed to neurogliafom cells, the cortical interneurons providing feed-forward control over receiving layer V in the entorhinal cortex remain unknown. We used sharp-wave ripple oscillations as a natural excitatory stimulus of the entorhinal cortex, driven by the hippocampus, to study the function of synaptic interactions between neurons in the deep layers of the entorhinal cortex. We discovered that CB1R-expressing interneurons in the deep layers of the entorhinal cortex constitute the major relay station that translates hippocampal excitation into efficient inhibition of cortical pyramidal cells. The impact of inhibition provided by these interneurons is under strong endocannabinoid control and can be drastically reduced either by enhanced activity of postsynaptic targets or by stress-induced elevation of cannabinoids.

Enhancing myelin renewal reverses cognitive dysfunction in a murine model of Alzheimer's disease

Severe cognitive decline is a hallmark of Alzheimer's disease (AD). In addition to gray matter loss, significant white matter pathology has been identified in AD patients. Here, we characterized the dynamics of myelin generation and loss in the APP/PS1 mouse model of AD. Unexpectedly, we observed a dramatic increase in the rate of new myelin formation in APP/PS1 mice, reminiscent of the robust oligodendroglial response to demyelination. Despite this increase, overall levels of myelination are decreased in the cortex and hippocampus of APP/PS1 mice and postmortem AD tissue. Genetically or pharmacologically enhancing myelin renewal, by oligodendroglial deletion of the muscarinic M1 receptor or systemic administration of the pro-myelinating drug clemastine, improved the performance of APP/PS1 mice in memory-related tasks and increased hippocampal sharp wave ripples. Taken together, these results demonstrate the potential of enhancing myelination as a therapeutic strategy to alleviate AD-related cognitive impairment.

Activity-dependent plasticity of mouse hippocampal assemblies in vitro

Memory formation is associated with the generation of transiently stable neuronal assemblies. In hippocampal networks, such groups of functionally coupled neurons express highly ordered spatiotemporal activity patterns which are coordinated by local network oscillations. One of these patterns, sharp wave-ripple complexes (SPW-R), repetitively activates previously established groups of memory-encoding neurons, thereby supporting memory consolidation. This function implies that repetition of specific SPW-R induces plastic changes which render the underlying neuronal assemblies more stable. We modeled this repetitive activation in an in vitro model of SPW-R in mouse hippocampal slices. Weak electrical stimulation upstream of the CA3-CA1 networks reliably induced SPW-R of stereotypic waveform, thus representing re-activation of similar neuronal activity patterns. Frequent repetition of these patterns (100 times) reduced the variance of both, evoked and spontaneous SPW-R waveforms, indicating stabilization of pre-existing assemblies. These effects were most pronounced in the CA1 subfield and depended on the timing of stimulation relative to spontaneous SPW-R. Additionally, plasticity of SPW-R was blocked by application of a NMDA receptor antagonist, suggesting a role for associative synaptic plasticity in this process. Thus, repetitive activation of specific patterns of SPW-R causes stabilization of memory-related networks.