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Publications and source records attributed to McMahon, L. L..
Withdrawal StatementThe authors have withdrawn this manuscript because subsequent analysis revealed significant errors in the collection of the original data in several of the in vivo recordings, rendering the data unusable. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.
Fear-based disorders such as posttraumatic stress disorder (PTSD) steepen age-related cognitive decline and doubles the risk for developing Alzheimers disease. Due to the seemingly hyperactive properties of fear memories, PTSD symptoms can worsen with age. Perturbations in the synaptic circuitry supporting fear memory extinction are key neural substrates of PTSD. The basolateral amygdala (BLA) is a medial temporal lobe structure critical in the encoding, consolidation, and retrieval of fear memories. As little is known about fear extinction memory and BLA synaptic dysfunction within the context of aging and AD, the goal of this study was to investigate how fear extinction memory deficits and basal amygdaloid nucleus (BA) synaptic dysfunction differentially associate in non-pathological aging and AD in the TgF344AD (TgAD) rat model of AD. Young, middle-aged, and older-aged WT and TgAD rats were trained on a delay fear conditioning and extinction procedure prior to ex vivo extracellular field potential recording experiments in the BA. Relative to young WT rats, long-term extinction memory was impaired, and in general, associated with a hyperexcitable BA and impaired LTP in TgAD rats at all ages. In contrast, long-term extinction memory was impaired in aged WT rats and associated with impaired LTP but not BA hyperexcitability. Interestingly, the middle-aged TgAD rats showed intact short-term extinction and BA LTP, suggestive of a compensatory mechanism, whereas differential neural recruitment in older-aged WT rats may have facilitated short-term extinction. As such, associations between fear extinction memory and amygdala deficits in non-pathological aging and AD are dissociable. SignificanceAdults with fear-based disorders like post-traumatic stress disorder are at an increased risk for developing age-related cognitive decline and Alzheimers disease (AD). Moreover, negative emotional affect is an early marker of AD. The link between fear-based disorders and AD creates a disadvantage for achieving positive outcomes later in life. Central to the circuitry underlying fear disorders are medial temporal lobe structures like the basal amygdaloid nucleus (BA). However, the role of the BA in fear-based disorders exacerbated by aging and AD is not well understood. Using the TgF344AD rat model of AD, we investigated how fear extinction memory impairments and BA synaptic function are impacted by aging and AD and whether these processes differentially associate in non-pathological aging and AD.
The dentate gyrus is both a critical gatekeeper for hippocampal signal processing and one of the first regions to dysfunction in Alzheimers disease (AD). Accordingly, the appropriate balance of excitation and inhibition through the dentate is a compelling target for mechanistic investigation and therapeutic intervention of early AD. Previously, we reported increased LTP magnitude at medial perforant path-dentate granule cell (MPP-DGC) synapses in slices from TgF344-AD rats compared to Wt as early as 6 months. We next determined that the enhanced LTP magnitude was due to heightened function of {beta}-ARs leading us to ask if dentate granule cells (DGCs) also have modified passive or active membrane properties which may also contribute to hyperexcitability or aberrant hippocampal processing. Although there was no detectable difference in spine density and presynaptic release probability, dentate granule cells (DGCs) themselves might have increased electrical response to synaptic input during LTP induction, which was measured as a significant increase in charge transfer during high-frequency stimulation. In this study, we found passive membrane properties and active membrane properties are altered, leading to increased TgF344-AD DGC excitability. Specifically, TgF344-AD DGCs have an increased input resistance and decreased rheobase, decreased sag, and increased action potential (AP) spike accommodation. Importantly, we find that the voltage response increased in DGCs from TgF344-AD compared to Wt accompanied by decreased delay to fire the first AP, indicating that for the same amount of depolarizing current injection TgF344-AD DGCs membranes are more excitable. Taken together, DGCs of TgF344-AD rats are more excitable, but due to heightened accommodation, may be unable to discharge at high frequency for longer durations of time, compared to their Wt littermates.
Patients with Alzheimers disease (AD) often have fragmentation of sleep/wake cycles and disrupted 24-h (circadian) activity. Despite this, little work has investigated the potential underlying day/night disruptions in cognition and neuronal physiology in the hippocampus. The molecular clock, an intrinsic transcription-translation feedback loop that regulates circadian behavior, may also regulate hippocampal neurophysiological activity. We hypothesized that disrupted diurnal variation in clock gene expression in the hippocampus corresponds with loss of normal day/night differences in membrane excitability, synaptic physiology, and cognition. We previously reported that the Tg-SwDI mouse model of AD has disrupted circadian locomotor rhythms and neurophysiological output of the suprachiasmatic nucleus (the primary circadian clock). Here, we report that Tg-SwDI mice failed to show day-night differences in a spatial working memory task, unlike wild-type controls that exhibited enhanced spatial working memory at night. Moreover, Tg-SwDI mice had lower levels of Per2, one of the core components of the molecular clock, at both mRNA and protein levels when compared to age-matched controls. Interestingly, we discovered neurophysiological impairments in area CA1 of the Tg-SwDI hippocampus. In controls, spontaneous inhibitory post-synaptic currents (sIPSCs) in pyramidal cells showed greater amplitude and lower inter-event interval during the day than the night. However, the normal day/night differences in sIPSCs were absent (amplitude) or reversed (inter-event interval) in pyramidal cells from Tg-SwDI mice. In control mice, current injection into CA1 pyramidal cells produced more firing during the night than during the day, but no day/night difference in excitability was observed in Tg-SwDI mice. The normal day/night difference in excitability in controls was blocked by GABA receptor inhibition. Together, these results demonstrate that the normal diurnal regulation of inhibitory transmission in the hippocampus is diminished in a mouse model of AD, leading to decreased daytime inhibition onto hippocampal CA1 pyramidal cells. Uncovering disrupted day/night differences in circadian gene regulation, hippocampal physiology, and memory in AD mouse models may provide insight into possible chronotherapeutic strategies to ameliorate Alzheimers disease symptoms or delay pathological onset.
The central noradrenergic (NA) system is critical for maintenance of attention, behavioral flexibility, spatial navigation, and learning and memory, those cognitive functions lost first in early Alzheimers disease (AD). In fact, the locus coeruleus (LC), the sole source of norepinephrine (NE) for >90% of the brain, is the first site of pathological tau accumulation in human AD with axon loss throughout forebrain, including hippocampus. The dentate gyrus (DG) is heavily innervated by LC-NA axons, where released norepinephrine (NE) acts on {beta}-adrenergic receptors (ARs) at excitatory synapses from entorhinal cortex (EC) to facilitate long-term synaptic plasticity and memory formation. These synapses dysfunction in early AD prior to cognitive impairment. In the TgF344-AD rat model, degeneration of LC-NA axons in hippocampus recapitulates human AD, providing a preclinical model to investigate synaptic and behavioral consequences. Using immunohistochemistry, Western blot analysis, and brain slice electrophysiology in 6-9 month old wild type and TgF344-AD rats, we discovered that loss of LC-NA axons co-insides with heightened {beta}-AR function at medial perforant path-dentate granule cell synapses (MPP-DCG) that is responsible for the increase in LTP magnitude at these synapses. Furthermore, novel object recognition is facilitated in TgF344-AD rats that requires {beta}-ARs, and pharmacological blockade of {beta}-ARs unmasks a deficit in extinction learning only in TgF344-AD rats, indicating a greater reliance on {beta}-ARs in both behaviors. Thus, a compensatory increase in {beta}-AR function during prodromal AD in TgF344-AD rats heightens synaptic plasticity and preserves some forms of learning and memory. Significance StatementThe locus coeruleus (LC), a brain region located in the brainstem which is responsible for attention and arousal, is damaged first by Alzheimers disease pathology. The LC sends axons to hippocampus where released norepinephrine (NE) modulates synaptic function required for learning and memory. How degeneration of LC axons and loss of NE in hippocampus in early AD impacts synaptic function and learning and memory is not well understood despite the importance of LC in cognitive function. We used a transgenic AD rat model with LC axon degeneration mimicking human AD and found that heightened function of {beta} adrenergic receptors in the dentate gyrus increased synaptic plasticity and preserved learning and memory in early stages of the disease.
Optogenetics is a widely used tool for studying neural circuits. However, non-invasive methods for light delivery in the brain are needed to avoid physical damage typically caused by intracranial insertion of light guides. An innovative strategy could employ X-ray activation of radioluminescent particles (RLPs) to emit localized light. We previously reported that RLPs composed of cerium doped lutetium oxyorthosilicate (LSO:Ce), an inorganic scintillator that emits blue light, are biocompatible with neuronal function and synaptic transmission. However, little is known about the consequences of acute X-ray exposure on synaptic function and long-term plasticity. Furthermore, modulation of neuronal or synaptic function by X-ray induced radioluminescence from RLPs has not yet been demonstrated. Here we show that 30 minutes of X-ray exposure at a rate of 0.042 Gy/second caused no change in the strength of basal glutamatergic transmission during extracellular dendritic field recordings in mouse hippocampal slices. Additionally, long-term potentiation (LTP), a robust measure of synaptic integrity, was able to be induced after X-ray exposure and expressed at a magnitude not different from control conditions (absence of X-rays). This is important as synaptic plasticity is critical to learning and memory. Next, we used molecular and electrophysiological approaches to determine if X-ray dependent radioluminescence emitted from RLPs can activate light sensitive proteins. We found that X-ray stimulation of RLPs elevated cAMP levels in HEK293T cells expressing OptoXR, a chimeric opsin receptor that combines the extracellular light-sensitive domain of channelrhodopsin-2 (ChR2) with an intracellular second messenger signaling cascade. This demonstrates that X-ray radioluminescence from LSO:Ce particles can activate OptoXR. Next, we tested whether X-ray activation of the RLPs can enhance synaptic activity in whole-cell recordings from hippocampal neurons expressing ChR2, both in cell culture and acute hippocampal slices. Importantly, X-ray radioluminescence caused an increase in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in both systems, indicating activation of ChR2 and excitation of neurons. Together, our results show that X-ray activation of LSO:Ce particles can heighten cellular and synaptic function. The combination of LSO:Ce inorganic scintillators and X-rays is therefore a viable method for optogenetics as an alternative to more invasive light delivery methods.
Genome-wide association studies identified the BIN1 locus as a leading modulator of genetic risk in Alzheimers disease (AD). One limitation in understanding BIN1s contribution to AD is its unknown function in the brain. AD-associated BIN1 variants are generally noncoding and likely change expression. Here, we determined the effects of increasing expression of the major neuronal isoform of human BIN1 in cultured hippocampal neurons. Higher BIN1 induced network hyperexcitability on multielectrode arrays, increased frequency of synaptic transmission, and elevated calcium transients, indicating that increasing BIN1 drives greater neuronal activity. In exploring the mechanism of these effects on neuronal physiology, we found that BIN1 interacted with L-type voltage-gated calcium channels (LVGCCs) and that BIN1-LVGCCs interactions were modulated by Tau in vitro and in vivo. Finally, Tau reduction prevented BIN1-induced network hyperexcitability. These data shed light on BIN1s neuronal function and suggest that it may contribute to Tau-dependent hyperexcitability in AD.
O-GlcNAcylation, a post-translational modification involving O-linkage of {beta}-N-acetylglucosamine to Ser/Thr residues on target proteins, is increasingly recognized as a critical regulator of brain function in health and disease. Enzymes that catalyze O-GlcNAcylation are found at both presynaptic and postsynaptic sites, and O-GlcNAcylated proteins localize to synaptosomes. An acute increase in O-GlcNAcylation induces long-term depression (LTD) of excitatory transmission at hippocampal CA3-CA1 synapses, and depresses hyperexcitable circuits in vitro and in vivo. Yet, no study has investigated how O-GlcNAcylation modulates the efficacy of inhibitory neurotransmission. Here we show an acute increase in O-GlcNAc dampens GABAergic currents onto principal cells in rodent hippocampus likely through a postsynaptic mechanism, and has a variable effect on the excitation/inhibition balance. The overall effect of increased O-GlcNAc is reduced synaptically-driven spike probability via synaptic depression and decreased intrinsic excitability. Our results position O-GlcNAcylation as a novel regulator of the overall excitation/inhibition balance and neuronal output.
Locus coeruleus (LC) provides the sole source of noradrenergic (NA) innervation to hippocampus, and it undergoes significant degeneration early in Alzheimers disease (AD). Norepinephrine (NE) modulates synaptic transmission and plasticity at hippocampal synapses which likely contributes to hippocampus dependent learning and memory. We previously reported that pharmacological activation of 1 adrenergic receptors (1ARs) induces long-term depression (LTD) at CA3-CA1 synapses. Here we investigated whether accumulation of endogenous NE via pharmacological blockade of NE transporters (NET) and the NE degradative enzyme, monoamine oxidase (MAO), can induce 1AR LTD, as these inhibitors are used clinically. Further, we sought to determine how degeneration of hippocampal NA innervation, as occurs in AD, impacts 1AR function and 1AR LTD. Bath application of NET and MAO inhibitors in slices from control rats reliably induced 1AR LTD when {beta} adrenergic receptors were inhibited. To induce degeneration of LC-NA innervation, rats were treated with the specific NA neurotoxin DSP-4 and recordings performed 1-3 weeks later when NA axon degeneration had stabilized. Even with 85% loss of hippocampal NA innervation, 1AR LTD was successfully induced using either the 1AR agonist phenylephrine or the combined NET and MAO inhibitors, and importantly, the LTD magnitude was not different from saline-treated control. These data suggest that despite significant decreases in NA input to hippocampus, the mechanisms necessary for the induction of 1AR LTD remain functional. Furthermore, we posit that 1AR activation could be a viable therapeutic target for pharmacological intervention in AD and other diseases involving malfunctions of NA neurotransmission.
Optogenetics is widely used in neuroscience to control neural circuits. However, non-invasive methods for light delivery in brain are needed to avoid physical damage caused by current methods. One potential strategy could employ x-ray activation of radioluminescent particles (RPLs), enabling localized light generation within the brain. RPLs composed of inorganic scintillators can emit light at various wavelengths depending upon composition. Cerium doped lutetium oxyorthosilicate (LSO:Ce), an inorganic scintillator that emits blue light in response to x-ray or UV stimulation, could potentially be used to control neural circuits through activation of channelrhodopsin-2 (ChR2), a light-gated cation channel. Whether inorganic scintillators themselves negatively impact neuronal processes and synaptic function is unknown, and was investigated here using cellular, molecular, and electrophysiological approaches. As proof of principle, we applied UV stimulation to 4 m LSO:Ce particles during whole-cell recording of CA1 pyramidal cells in acutely prepared hippocampal slices from mice that expressed ChR2 in glutamatergic neurons. We observed an increase in frequency and amplitude of spontaneous excitatory postsynaptic currents (EPSCs), indicating UV activation of ChR2 and excitation of neurons. Importantly, we found that LSO:Ce particles have no effect on survival of primary mouse cortical neurons, even after 24 hours of exposure. In extracellular dendritic field potential recordings, we observed no change in strength of basal glutamatergic transmission up to 3 hours of exposure to LSO:Ce microparticles. However, there was a slight decrease in the frequency of spontaneous EPSCs in whole-cell voltage-clamp recordings from CA1 pyramidal cells, with no change in current amplitudes. No changes in the amplitude or frequency of spontaneous inhibitory postsynaptic currents (IPSCs) were observed. Finally, long term potentiation (LTP), a synaptic modification believed to underlie learning and memory and a robust measure of synaptic integrity, was successfully induced, although the magnitude was slightly reduced. Together, these results show LSO:Ce particles are biocompatible even though there are modest effects on baseline synaptic function and long-term synaptic plasticity. Importantly, we show that light emitted from LSO:Ce particles is able to activate ChR2 and modify synaptic function. Therefore, LSO:Ce inorganic scintillators are potentially viable for use as a new light delivery system for optogenetics.