Visual Evoked Potentials as an Early-Stage Biomarker in the rTg4510 Tauopathy Mouse Model.

BACKGROUND: Tauopathies such as Alzheimer's disease (AD) and frontotemporal dementia (FTD) are characterized by formation of neurofibrillary tangles consisting of hyperphosphorylated tau protein. Early pathophysiological and functional changes related to neurofibrillary tangles formation are considered to occur prior to extensive neurodegeneration. Hyperphosphorylated tau has been detected in postmortem retinas of AD and FTD patients, and the visual pathway is an easily accessible system in a clinical setting. Hence, assessment of the visual function may offer the potential to detect consequences of early tau pathology in patients. OBJECTIVE: The aim of this study was to evaluate visual function in a tauopathy mouse model in relation to tau hyperphosphorylation and neurodegeneration. METHODS: In this study we explored the association between the visual system and functional consequences of tau pathology progression using a tauopathy rTg4510 mouse model. To this end, we recorded full-field electroretinography and visual evoked potentials in anesthetized and awake states at different ages. RESULTS: While retinal function remained mostly intact within all the age groups investigated, we detected significant changes in amplitudes of visual evoked potential responses in young rTg4510 mice exhibiting early tau pathology prior to neurodegeneration. These functional alterations in the visual cortex were positively correlated with pathological tau levels. CONCLUSION: Our findings suggest that visual processing could be useful as a novel electrophysiological biomarker for early stages of tauopathy.

Early impairments of visually-driven neuronal ensemble dynamics in the rTg4510 tauopathy mouse model.

Tau protein pathology is a hallmark of many neurodegenerative diseases, including Alzheimer's Disease or frontotemporal dementia. Synaptic dysfunction and abnormal visual evoked potentials have been reported in murine models of tauopathy, but little is known about the state of the network activity on a single neuronal level prior to brain atrophy. In the present study, oscillatory rhythms and single-cell calcium activity of primary visual cortex pyramidal neuron population were investigated in basal and light evoked states in the rTg4510 tauopathy mouse model prior to neurodegeneration. We found a decrease in their responsivity and overall activity which was insensitive to GABAergic modulation. Despite an enhancement of basal state coactivation of cortical pyramidal neurons, a loss of input-output synchronicity was observed. Dysfunction of cortical pyramidal function was also reflected in a reduction of basal theta oscillations and enhanced susceptibility to a sub-convulsive dose of pentylenetetrazol in rTg4510 mice. Our results unveil impairments in visual cortical pyramidal neuron processing and define aberrant oscillations as biomarker candidates in early stages of neurodegenerative tauopathies.

No Detectable Effect on Visual Responses Using Functional MRI in a Rodent Model of α-Synuclein Expression.

Parkinson's disease (PD) is a progressive neurodegenerative disease that is typically diagnosed late in its progression. There is a need for biomarkers suitable for monitoring the disease progression at earlier stages to guide the development of novel neuroprotective therapies. One potential biomarker, α-synuclein, has been found in both the familial cases of PD, as well as the sporadic cases and is considered a key feature of PD. α-synuclein is naturally present in the retina, and it has been suggested that early symptoms of the visual system may be used as a biomarker for PD. Here, we use a viral vector to induce a unilateral expression of human wild-type α-synuclein in rats as a mechanistic model of protein aggregation in PD. We employed functional magnetic resonance imaging (fMRI) to investigate whether adeno-associated virus (AAV) mediated expression of human wild-type α-synuclein alter functional activity in the visual system. A total of 16 rats were injected with either AAV-α-synuclein (n = 7) or AAV-null (n = 9) in the substantia nigra pars compacta (SNc) of the left hemisphere. The expression of α-synuclein was validated by a motor assay and postmortem immunohistochemistry. Five months after the introduction of the AAV-vector, fMRI showed robust blood oxygen level-dependent (BOLD) responses to light stimulation in the visual systems of both control and AAV-α-synuclein animals. However, our results demonstrate that the expression of AAV-α-synuclein does not affect functional activation of the visual system. This negative finding suggests that fMRI-based read-outs of visual responses may not be a sensitive biomarker for PD.

Phosphodiesterase type 1 inhibition alters medial prefrontal cortical activity during goal-driven behaviour and partially reverses neurophysiological deficits in the rat phencyclidine model of schizophrenia.

Positive modulation of cAMP signalling by phosphodiesterase (PDE) inhibitors has recently been explored as a potential target for the reversal of cognitive and behavioural deficits implicating the corticoaccumbal circuit. Previous studies show that PDE type 1 isoform B (PDE1B) inhibition may improve memory function in rodent models; however, the contribution of PDE1B inhibition to impulsivity, attentional and motivational functions as well as its neurophysiological effects have not been investigated. To address this, we recorded single unit activity in medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) in Lister Hooded rats treated with the PDE1B inhibitor Lu AF64386 and tested in the 5-choice serial reaction time task (5-CSRTT). We also asked whether PDE1B inhibition modulates neurophysiological deficits produced by subchronic phencyclidine (PCP) treatment, a rat pharmacological model of schizophrenia. Lu AF64386 significantly affected behavioural parameters consistent with a reduction in goal-directed behaviour, however without affecting accuracy. Additionally, it reduced mPFC neuronal activity. Pre-treatment with PCP did not affect behavioural parameters, however it significantly disrupted overall neuronal firing while increasing phasic responses to reward-predicting cues and disrupting mPFC-NAc cross-talk. The latter two effects were reversed by Lu AF64386. These findings suggest PDE1B inhibition may be beneficial in disorders implicating a dysfunction of the mPFC-NAc network.

Ongoing Electroencephalographic Rhythms Related to Exploratory Movements in Transgenic TASTPM Mice.

BACKGROUND: The European PharmaCog study (http://www.pharmacog.org) has reported a reduction in delta (1-6 Hz) electroencephalographic (EEG) power (density) during cage exploration (active condition) compared with quiet wakefulness (passive condition) in PDAPP mice (hAPP Indiana V717F mutation) modeling Alzheimer's disease (AD) amyloidosis and cognitive deficits. OBJECTIVE: Here, we tested the reproducibility of that evidence in TASTPM mice (double mutation in APP KM670/671NL and PSEN1 M146V), which develop brain amyloidosis and cognitive deficits over aging. The reliability of that evidence was examined in four research centers of the PharmaCog study. METHODS: Ongoing EEG rhythms were recorded from a frontoparietal bipolar channel in 29 TASTPM and 58 matched "wild type" C57 mice (range of age: 12-24 months). Normalized EEG power was calculated. Frequency and amplitude of individual delta and theta frequency (IDF and ITF) peaks were considered during the passive and active conditions. RESULTS: Compared with the "wild type" group, the TASTPM group showed a significantly lower reduction in IDF power during the active over the passive condition (p < 0.05). This effect was observed in 3 out of 4 EEG recording units. CONCLUSION: TASTPM mice were characterized by "poor reactivity" of delta EEG rhythms during the cage exploration in line with previous evidence in PDAPP mice. The reliability of that result across the centers was moderate, thus unveiling pros and cons of multicenter preclinical EEG trials in TASTPM mice useful for planning future studies.

Progressive Effects of Sildenafil on Visual Processing in Rats.

Photoreceptors are light-sensitive cells in the retina converting visual stimuli into electrochemical signals. These signals are evaluated and interpreted in the visual pathway, a process referred to as visual processing. Phosphodiesterase type 5 and 6 (PDE5 and 6) are abundant enzymes in retinal vessels and notably photoreceptors where PDE6 is exclusively present. The effects of the PDE inhibitor sildenafil on the visual system, have been studied using electroretinography and a variety of clinical visual tasks. Here we evaluate effects of sildenafil administration by electrophysiological recordings of flash visual evoked potentials (VEPs) and steady-state visual evoked potentials (SSVEPs) from key regions in the rodent visual pathway. Progressive changes were investigated in female Sprague-Dawley rats at 10 timepoints from 30 min to 28 h after peroral administration of sildenafil (50 mg/kg). Sildenafil caused a significant reduction in the amplitude of VEPs in both visual cortex and superior colliculus, and a significant delay of the VEPs as demonstrated by increased latency of several VEP peaks. Also, sildenafil-treatment significantly reduced the signal-to-noise ratio of SSVEPs. The effects of sildenafil were dependent on the wavelength condition in both assays. Our results support the observation that while PDE6 is a key player in phototransduction, near full inhibition of PDE6 is not enough to abolish the complex process of visual processing. Taken together, VEPs and SSVEPs are effective in demonstrating progressive effects of drug-induced changes in visual processing in rats and as the same paradigms may be applied in humans, representing a promising tool for translational research.

Classification of α-synuclein-induced changes in the AAV α-synuclein rat model of Parkinson's disease using electrophysiological measurements of visual processing.

Biomarkers suitable for early diagnosis and monitoring disease progression are the cornerstone of developing disease-modifying treatments for neurodegenerative diseases such as Parkinson's disease (PD). Besides motor complications, PD is also characterized by deficits in visual processing. Here, we investigate how virally-mediated overexpression of α-synuclein in the substantia nigra pars compacta impacts visual processing in a well-established rodent model of PD. After a unilateral injection of vector, human α-synuclein was detected in the striatum and superior colliculus (SC). In parallel, there was a significant delay in the latency of the transient VEPs from the affected side of the SC in late stages of the disease. Inhibition of leucine-rich repeat kinase using PFE360 failed to rescue the VEP delay and instead increased the latency of the VEP waveform. A support vector machine classifier accurately classified rats according to their `disease state' using frequency-domain data from steady-state visual evoked potentials (SSVEP). Overall, these findings indicate that the latency of the rodent VEP is sensitive to changes mediated by the increased expression of α-synuclein and especially when full overexpression is obtained, whereas the SSVEP facilitated detection of α-synuclein across reflects all stages of PD model progression.

Effects of Vortioxetine and Escitalopram on Electroencephalographic Recordings - A Randomized, Crossover Trial in Healthy Males.

The antidepressant drug vortioxetine has a multimodal action modulating neurotransmission through inhibition of the serotonin transporter and modulation of serotonin receptors. Vortioxetine has also been shown to alleviate cognitive symptoms in preclinical studies and in patients with depression. However, it is largely unclear how vortioxetine affects the brain processing in humans. The present study was conducted in 32 healthy males in a randomized, double-blinded, placebo-controlled, active comparator, four-way crossover design. Treatments were 10 and 20 mg/day vortioxetine, 15 mg/day escitalopram, and placebo, administered orally once daily for three days. Results were compared to placebo. Treatment effect was assessed by recording spontaneous electroencephalography (EEG) and 40 Hz auditory steady state responses. For the spontaneous EEG, both vortioxetine and escitalopram decreased the frequency content in the theta band (4-8 Hz) and increased power in the beta (12-32 Hz) and gamma (32-45 Hz) bands. Vortioxetine and escitalopram decreased connectivity during rest in the theta band and increased connectivity in the gamma bands. Finally, both treatments caused decreased power in the evoked gamma band in response to 40 Hz auditory stimulation. Although the global EEG changes were comparable between vortioxetine and escitalopram, subtle differences between treatment effects on the EEG in terms of effect size and regional distribution of the EEG changes were apparent. To our knowledge, the current results are the first data on how vortioxetine affects EEG in humans. The present study calls for further investigations addressing the possible electrophysiological and cognitive effects of vortioxetine.

Acute dosing of vortioxetine strengthens event-related brain activity associated with engagement of attention and cognitive functioning in rats.

Studies of the antidepressant vortioxetine have demonstrated beneficial effects on cognitive dysfunction associated with depression. To elucidate how vortioxetine modulates neuronal activity during cognitive processing we investigated the effects of vortioxetine (3 and 10mg/kg) in rats performing an auditory oddball (deviant target) task. We investigated neuronal activity in target vs non-target tone responses in vehicle-treated animals using electroencephalographic (EEG) recordings. Furthermore, we characterized task performance and EEG changes in target tone responses of vortioxetine vs controls. Quantification of event-related potentials (ERPs) was supplemented by analyses of spectral power and inter-trial phase-locking. The assessed brain regions included prelimbic cortex, the hippocampus, and thalamus. As compared to correct rejection of non-target tones, correct target tone responses elicited increased EEG power in all regions. Additionally, neuronal synchronization was increased in vehicle-treated rats during both early and late ERP responses to target tones. This indicates a significant consistency of local phases across trials during high attentional load. During early sensory processing, vortioxetine increased both thalamic and frontal synchronized gamma band activity and EEG power in all brain regions measured. Finally, vortioxetine increased the amplitude of late hippocampal P3-like ERPs, the rodent correlate of the human P300 ERP. These findings suggest differential effects of vortioxetine during early sensory registration and late endogenous processing of auditory discrimination. Strengthened P3-like ERP response may relate to the pro-cognitive profile of vortioxetine in rodents. Further investigations are warranted to explore the mechanism by which vortioxetine increases network synchronization during attentive and cognitive processing.

Adapted wavelet transform improves time-frequency representations: a study of auditory elicited P300-like event-related potentials in rats.

OBJECTIVE: Active auditory oddball paradigms are simple tone discrimination tasks used to study the P300 deflection of event-related potentials (ERPs). These ERPs may be quantified by time-frequency analysis. As auditory stimuli cause early high frequency and late low frequency ERP oscillations, the continuous wavelet transform (CWT) is often chosen for decomposition due to its multi-resolution properties. However, as the conventional CWT traditionally applies only one mother wavelet to represent the entire spectrum, the time-frequency resolution is not optimal across all scales. To account for this, we developed and validated a novel method specifically refined to analyse P300-like ERPs in rats. APPROACH: An adapted CWT (aCWT) was implemented to preserve high time-frequency resolution across all scales by commissioning of multiple wavelets operating at different scales. First, decomposition of simulated ERPs was illustrated using the classical CWT and the aCWT. Next, the two methods were applied to EEG recordings obtained from prefrontal cortex in rats performing a two-tone auditory discrimination task. MAIN RESULTS: While only early ERP frequency changes between responses to target and non-target tones were detected by the CWT, both early and late changes were successfully described with strong accuracy by the aCWT in rat ERPs. Increased frontal gamma power and phase synchrony was observed particularly within theta and gamma frequency bands during deviant tones. SIGNIFICANCE: The study suggests superior performance of the aCWT over the CWT in terms of detailed quantification of time-frequency properties of ERPs. Our methodological investigation indicates that accurate and complete assessment of time-frequency components of short-time neural signals is feasible with the novel analysis approach which may be advantageous for characterisation of several types of evoked potentials in particularly rodents.

On-going electroencephalographic rhythms related to cortical arousal in wild-type mice: the effect of aging.

Resting state electroencephalographic (EEG) rhythms reflect the fluctuation of cortical arousal and vigilance in a typical clinical setting, namely the EEG recording for few minutes with eyes closed (i.e., passive condition) and eyes open (i.e., active condition). Can this procedure be back-translated to C57 (wild type) mice for aging studies? On-going EEG rhythms were recorded from a frontoparietal bipolar channel in 85 (19 females) C57 mice. Male mice were subdivided into 3 groups: 25 young (4.5-6 months), 18 middle-aged (12-15 months), and 23 old (20-24 months) mice to test the effect of aging. EEG power density was compared between short periods (about 5 minutes) of awake quiet behavior (passive) and dynamic exploration of the cage (active). Compared with the passive condition, the active condition induced decreased EEG power at 1-2 Hz and increased EEG power at 6-10 Hz in the group of 85 mice. Concerning the aging effects, the passive condition showed higher EEG power at 1-2 Hz in the old group than that in the others. Furthermore, the active condition exhibited a maximum EEG power at 6-8 Hz in the former group and 8-10 Hz in the latter. In the present conditions, delta and theta EEG rhythms reflected changes in cortical arousal and vigilance in freely behaving C57 mice across aging. These changes resemble the so-called slowing of resting state EEG rhythms observed in humans across physiological and pathological aging. The present EEG procedures may be used to enhance preclinical phases of drug discovery in mice for understanding the neurophysiological effects of new compounds against brain aging.

Hippocampal P3-like auditory event-related potentials are disrupted in a rat model of cholinergic degeneration in Alzheimer's disease: reversal by donepezil treatment.

P300 (P3) event-related potentials (ERPs) have been suggested to be an endogenous marker of cognitive function and auditory oddball paradigms are frequently used to evaluate P3 ERPs in clinical settings. Deficits in P3 amplitude and latency reflect some of the neurological dysfunctions related to several psychiatric and neurological diseases, e.g., Alzheimer's disease (AD). However, only a very limited number of rodent studies have addressed the back-translational validity of the P3-like ERPs as suitable markers of cognition. Thus, the potential of rodent P3-like ERPs to predict pro-cognitive effects in humans remains to be fully validated. The current study characterizes P3-like ERPs in the 192-IgG-SAP (SAP) rat model of the cholinergic degeneration associated with AD. Following training in a combined auditory oddball and lever-press setup, rats were subjected to bilateral intracerebroventricular infusion of 1.25 µg SAP or PBS (sham lesion) and recording electrodes were implanted in hippocampal CA1. Relative to sham-lesioned rats, SAP-lesioned rats had significantly reduced amplitude of P3-like ERPs. P3 amplitude was significantly increased in SAP-treated rats following pre-treatment with 1 mg/kg donepezil. Infusion of SAP reduced the hippocampal choline acetyltransferase activity by 75%. Behaviorally defined cognitive performance was comparable between treatment groups. The present study suggests that AD-like deficits in P3-like ERPs may be mimicked by the basal forebrain cholinergic degeneration induced by SAP. SAP-lesioned rats may constitute a suitable model to test the efficacy of pro-cognitive substances in an applied experimental setup.

Impaired hippocampal acetylcholine release parallels spatial memory deficits in Tg2576 mice subjected to basal forebrain cholinergic degeneration.

The Alzheimer's disease (AD) mouse model Tg2576 overexpresses an AD associated mutant variant of human APP and accumulates amyloid beta (Aß) in an age-dependent manner. Using the selective cholinergic immunotoxin mu p75-saporin (SAP), we induced a partial basal forebrain cholinergic degeneration (BFCD) in 3 months old male Tg2576 mice to co-express cholinergic degeneration with Aß overexpression as these characteristics constitutes key hallmarks of AD. At 9 months, SAP lesioned Tg2576 mice were cognitively impaired in two spatial paradigms addressing working memory and mid to long-term memory. Conversely, there was no deterioration of cognitive functioning in sham lesioned Tg2576 mice or wild type littermates (wt) receiving the immunotoxin. At 10 months of age, release of acetylcholine (ACh) was addressed by microdialysis in conscious mice. Scopolamine-induced increases in hippocampal ACh efflux was significantly reduced in SAP lesioned Tg2576 mice compared to sham lesioned Tg2576 mice. Intriguingly, there was no significant difference in ACh efflux between wt treatment groups. Following SAP treatment, choline acetyltransferase activity was reduced in the hippocampus and frontal cortex and the reduction was comparable between groups. Our results suggest that partial BFCD acts collectively with increased levels of Aß to induce cognitive decline and to compromise cholinergic release. Tg2576 mice with BFCD may constitute a new and suitable AD mouse model to study the interrelations between cholinergic deficits and amsyloid deposition.

Neuropharmacological modulation of the P3-like event-related potential in a rat two-tone auditory discrimination task with modafinil and NS9283, a positive allosteric modulator of α4ß2 nAChRs.

The P300 (P3) event-related potential (ERP) is a neurophysiological signal believed to reflect cognitive processing of salient cues, and is thus used as a measure of attention and working memory. Additionally, P3 amplitude and latency is altered in neurological diseases and can be pharmacologically modulated. As P3-like ERPs can be recorded in rodents, it may serve as a potential translational biomarker of value for drug discovery. Here we investigated whether a positive allosteric modulator of α4ß2 nicotinic acetylcholine receptors, NS9283, and the psychostimulant modafinil could modulate P3-like ERPs in healthy adult rats performing an auditory oddball discrimination task. ERPs were recorded with electroencephalography electrodes implanted into mediodorsal (MD) thalamus, medial prefrontal cortex, hippocampus and auditory cortex (AC). P3-like ERPs were detected in all brain regions, displaying larger amplitudes in target trials compared to non-target trials. Administration of modafinil (64 mg/kg) decreased P3-like ERP latency in MD thalamus and AC, whereas NS9283 augmented P3-like ERP amplitude in MD thalamus at 0.3 mg/kg and in AC at 3.0 mg/kg. Additionally, N1 pre-attention peak amplitude in MD thalamus was increased with 0.3 mg/kg NS9283. Neither of the compounds enhanced task performance. Rather, modafinil lowered correct rejections in non-target trials. In summary, our findings reveal pharmacological modulation of the rat P3-like ERP in cortical and subcortical regions by modafinil and NS9283. These findings encourage further exploration of the rat P3-like ERP in order to promote the understanding of its meaning within cognition, as well as its applicability as a translatable biomarker in drug development.

Cholinergic degeneration is associated with increased plaque deposition and cognitive impairment in APPswe/PS1dE9 mice.

Cholinergic dysfunction and deposition of plaques containing amyloid ß-peptides (Aß) are two of the characteristics of Alzheimer's disease. Here, we combine APPswe/PS1dE9 (APP/PS1) mice with the cholinergic immunotoxin mu p75-saporin (SAP) to integrate partial basal forebrain cholinergic degeneration and the neuropathology of APP/PS1 mice. By 6 months of age, APP/PS1 mice and wild type littermates (Wt) received intracerebroventricular injection of 0.6 µg SAP (lesion) or PBS (sham). Two months following surgery, APP/PS1 mice treated with SAP were significantly impaired compared to sham treated APP/PS1 mice in a behavioural paradigm addressing working memory. Conversely, the performance of Wt mice was unaffected by SAP treatment. Choline acetyltransferase activity was reduced in the hippocampus and frontal cortex following SAP treatment. The selective effect of a mild SAP lesion in APP/PS1 mice was not due to a more extensive cholinergic degeneration since the reduction in choline acetyltransferase activity was similar following SAP treatment in APP/PS1 mice and Wt. Interestingly, plaque load was significantly increased in SAP treated APP/PS1 mice relative to sham lesioned APP/PS1 mice. Additionally, APP/PS1 mice treated with SAP showed a tendency towards an increased level of soluble and insoluble Aß1-40 and Aß1-42 measured in brain tissue homogenate. Our results suggest that the combination of cholinergic degeneration and Aß overexpression in the APP/PS1 mouse model results in cognitive decline and accelerated plaque burden. SAP treated APP/PS1 mice might thus constitute an improved model of Alzheimer's disease-like neuropathology and cognitive deficits compared to the conventional APP/PS1 model without selective removal of basal forebrain cholinergic neurons.