Hippocampal hyperactivity in a rat model of Alzheimer's disease.

Neuronal network dysfunction is a hallmark of Alzheimer's disease (AD). However, the underlying pathomechanisms remain unknown. We analyzed the hippocampal micronetwork in transgenic McGill-R-Thy1-APP rats (APPtg) at the beginning of extracellular amyloid beta (Aß) deposition. We established two-photon Ca2+ -imaging in vivo in the hippocampus of rats and found hyperactivity of CA1 neurons. Patch-clamp recordings in brain slices in vitro revealed increased neuronal input resistance and prolonged action potential width in CA1 pyramidal neurons. We did neither observe changes in synaptic inhibition, nor in excitation. Our data support the view that increased intrinsic excitability of CA1 neurons may precede inhibitory dysfunction at an early stage of Aß-deposition and disease progression.

Glutamatergic synaptic integration of locomotion speed via septoentorhinal projections.

The medial septum and diagonal band of Broca (MSDB) send glutamatergic axons to medial entorhinal cortex (MEC). We found that this pathway provides speed-correlated input to several MEC cell-types in layer 2/3. The speed signal is integrated most effectively by pyramidal cells but also excites stellate cells and interneurons. Thus, the MSDB conveys speed information that can be used by MEC neurons for spatial representation of self-location.

Locomotion, Theta Oscillations, and the Speed-Correlated Firing of Hippocampal Neurons Are Controlled by a Medial Septal Glutamatergic Circuit.

Before the onset of locomotion, the hippocampus undergoes a transition into an activity-state specialized for the processing of spatially related input. This brain-state transition is associated with increased firing rates of CA1 pyramidal neurons and the occurrence of theta oscillations, which both correlate with locomotion velocity. However, the neural circuit by which locomotor activity is linked to hippocampal oscillations and neuronal firing rates is unresolved. Here we reveal a septo-hippocampal circuit mediated by glutamatergic (VGluT2(+)) neurons that is activated before locomotion onset and that controls the initiation and velocity of locomotion as well as the entrainment of theta oscillations. Moreover, via septo-hippocampal projections onto alveus/oriens interneurons, this circuit regulates feedforward inhibition of Schaffer collateral and perforant path input to CA1 pyramidal neurons in a locomotion-dependent manner. With higher locomotion speed, the increased activity of medial septal VGluT2 neurons is translated into increased axo-somatic depolarization and higher firing rates of CA1 pyramidal neurons. VIDEO ABSTRACT.

Dendritic structural degeneration is functionally linked to cellular hyperexcitability in a mouse model of Alzheimer's disease.

Dendritic structure critically determines the electrical properties of neurons and, thereby, defines the fundamental process of input-to-output conversion. The diversity of dendritic architectures enables neurons to fulfill their specialized circuit functions during cognitive processes. It is known that this dendritic integrity is impaired in patients with Alzheimer's disease and in relevant mouse models. It is unknown, however, whether this structural degeneration translates into aberrant neuronal function. Here we use in vivo whole-cell patch-clamp recordings, high-resolution STED imaging, and computational modeling of CA1 pyramidal neurons in a mouse model of Alzheimer's disease to show that structural degeneration and neuronal hyperexcitability are crucially linked. Our results demonstrate that a structure-dependent amplification of synaptic input to action potential output conversion might constitute a novel cellular pathomechanism underlying network dysfunction with potential relevance for other neurodegenerative diseases with abnormal changes of dendritic morphology.