Pharmacological characterization of SAGE-718, a novel positive allosteric modulator of N-methyl-d-aspartate receptors.

BACKGROUND AND PURPOSE: Select neuroactive steroids tune neural activity by modulating excitatory and inhibitory neurotransmission, including the endogenous cholesterol metabolite 24(S)-hydroxycholesterol (24(S)-HC), which is an N-methyl-d-aspartate (NMDA) receptor positive allosteric modulator (PAM). NMDA receptor PAMs are potentially an effective pharmacotherapeutic strategy to treat conditions associated with NMDA receptor hypofunction. EXPERIMENTAL APPROACH: Using in vitro and in vivo electrophysiological recording experiments and behavioural approaches, we evaluated the effect of SAGE-718, a novel neuroactive steroid NMDA receptor PAM currently in clinical development for the treatment of cognitive impairment, on NMDA receptor function and endpoints that are altered by NMDA receptor hypoactivity and assessed its safety profile. KEY RESULTS: SAGE-718 potentiated GluN1/GluN2A-D NMDA receptors with equipotency and increased NMDA receptor excitatory postsynaptic potential (EPSP) amplitude without affecting decay kinetics in striatal medium spiny neurons. SAGE-718 increased the rate of unblock of the NMDA receptor open channel blocker ketamine on GluN1/GluN2A in vitro and accelerated the rate of return on the ketamine-evoked increase in gamma frequency band power, as measured with electroencephalogram (EEG), suggesting that PAM activity is driven by increased channel open probability. SAGE-718 ameliorated deficits due to NMDA receptor hypofunction, including social deficits induced by subchronic administration of phencyclidine, and behavioural and electrophysiological deficits from cholesterol and 24(S)-HC depletion caused by 7-dehydrocholesterol reductase inhibition. Finally, SAGE-718 did not produce epileptiform activity in a seizure model or neurodegeneration following chronic dosing. CONCLUSIONS AND IMPLICATIONS: These findings provide strong evidence that SAGE-718 is a neuroactive steroid NMDA receptor PAM with a mechanism that is well suited as a treatment for conditions associated with NMDA receptor hypofunction.

Evoked potentials as a translatable biomarker to track functional remyelination.

Enhancing remyelination is a key therapeutic strategy for demyelinating diseases such as multiple sclerosis. To achieve this goal, a central challenge is being able to quantitatively and longitudinally track functional remyelination, especially with translatable biomarkers that can be performed in both preclinical models and in the clinic. We developed the methodology to stably measure multi-modal sensory evoked potentials from the skull surface over the course of months in individual mice and applied it to a genetic mouse model of oligodendrocyte ablation and demyelination. We found that auditory and somatosensory evoked potential latencies reliably increased over time during the early phase of the model and recovered spontaneously and almost completely during a later phase. Histological examination supported the interpretation that the evoked potential latency changes dynamically reflect changes in CNS myelination. Specifically, we found reduction of myelination in corresponding brain regions at the time that sensory evoked potentials were maximally impacted. Importantly, we also found that myelination levels recovered when evoked potential latencies recovered. Other changes known to associate with demyelination were also observed at the time of delayed evoked potentials, including the emergence of white matter vacuoles and increased markers for activated microglia and macrophages; these changes also fully reversed by the time that evoked potentials recovered. Our results support the hypothesis that skull-surface recorded evoked potential latencies can dynamically track CNS myelination changes. The methods developed here allow for longitudinally tracking functional myelination changes in vivo in preclinical rodent models with a quantitative biomarker that can also be applied clinically and will facilitate translational development of CNS remyelinating therapies.

Antisense oligonucleotides extend survival and reverse decrement in muscle response in ALS models.

Mutations in superoxide dismutase 1 (SOD1) are responsible for 20% of familial ALS. Given the gain of toxic function in this dominantly inherited disease, lowering SOD1 mRNA and protein is predicted to provide therapeutic benefit. An early generation antisense oligonucleotide (ASO) targeting SOD1 was identified and tested in a phase I human clinical trial, based on modest protection in animal models of SOD1 ALS. Although the clinical trial provided encouraging safety data, the drug was not advanced because there was progress in designing other, more potent ASOs for CNS application. We have developed next-generation SOD1 ASOs that more potently reduce SOD1 mRNA and protein and extend survival by more than 50 days in SOD1G93A rats and by almost 40 days in SOD1G93A mice. We demonstrated that the initial loss of compound muscle action potential in SOD1G93A mice is reversed after a single dose of SOD1 ASO. Furthermore, increases in serum phospho-neurofilament heavy chain levels, a promising biomarker for ALS, are stopped by SOD1 ASO therapy. These results define a highly potent, new SOD1 ASO ready for human clinical trial and suggest that at least some components of muscle response can be reversed by therapy.

Membrane potential dynamics of populations of cortical neurons during auditory streaming.

How a mixture of acoustic sources is perceptually organized into discrete auditory objects remains unclear. One current hypothesis postulates that perceptual segregation of different sources is related to the spatiotemporal separation of cortical responses induced by each acoustic source or stream. In the present study, the dynamics of subthreshold membrane potential activity were measured across the entire tonotopic axis of the rodent primary auditory cortex during the auditory streaming paradigm using voltage-sensitive dye imaging. Consistent with the proposed hypothesis, we observed enhanced spatiotemporal segregation of cortical responses to alternating tone sequences as their frequency separation or presentation rate was increased, both manipulations known to promote stream segregation. However, across most streaming paradigm conditions tested, a substantial cortical region maintaining a response to both tones coexisted with more peripheral cortical regions responding more selectively to one of them. We propose that these coexisting subthreshold representation types could provide neural substrates to support the flexible switching between the integrated and segregated streaming percepts.

Spatiotemporal coordination of slow-wave ongoing activity across auditory cortical areas.

Natural acoustic stimuli contain slow temporal fluctuations, and the modulation of ongoing slow-wave activity by bottom-up and top-down factors plays essential roles in auditory cortical processing. However, the spatiotemporal pattern of intrinsic slow-wave activity across the auditory cortical modality is unknown. Using in vivo voltage-sensitive dye imaging in anesthetized guinea pigs, we measured spectral tuning to acoustic stimuli across several core and belt auditory cortical areas, and then recorded spontaneous activity across this defined network. We found that phase coherence in spontaneous slow-wave (delta-theta band) activity was highest between regions of core and belt areas that had similar frequency tuning, even if they were distant. Further, core and belt regions with high phase coherence were phase shifted. Interestingly, phase shifts observed during spontaneous activity paralleled latency differences for evoked activity. Our findings suggest that the circuits underlying this intrinsic source of slow-wave activity support coordinated changes in excitability between functionally matched but distributed regions of the auditory cortical network.

Tinnitus-related neural activity: theories of generation, propagation, and centralization.

The neuroscience of tinnitus represents an ideal model to explore central issues in brain functioning such as the formation of auditory percepts, in addition to opening up new treatment avenues for the condition in the long-term. The present review discusses the origin and nature of tinnitus-related neural activity. First, we review evidence for the hypothesis that tinnitus is caused by the central nervous system changes induced by sensory deprivation, even when hearing loss is not visible in the audiogram. Second, we suggest that changes in neural activity in individual central structures may not be sufficient to underlie the tinnitus percept. Instead, we propose that tinnitus may arise from functional alterations at multiple levels which promote abnormal propagation of neural activity throughout the network involved in auditory perception. In this context, functional coupling within and between central auditory structures may be especially important to consider. Investigating how sensory deprivation affects functional coupling between areas, which might be reflected in changes in temporal coherence of intrinsic ongoing activity patterns, may give critical insights into the mechanisms of tinnitus.

Stimulus-specific adaptation in auditory cortex is an NMDA-independent process distinct from the sensory novelty encoded by the mismatch negativity.

The significance of the mismatch negativity (MMN), an event-related potential measured in humans which indexes novelty in the auditory environment, has motivated a search for a cellular correlate of this process. A leading candidate is stimulus-specific adaptation (SSA) in auditory cortex units, which shares several characteristics with the MMN. Whether auditory cortex responses encode sensory novelty, a defining property of the MMN, however, has not been resolved. To evaluate this key issue, we used several variations of the auditory oddball paradigm from the human literature and examined psychophysical and pharmacological properties of multiunit activity in the auditory cortex of awake rodents. We found converging evidence dissociating SSA from sensory novelty and the MMN. First, during an oddball paradigm with frequency deviants, neuronal responses showed clear SSA but failed to encode novelty in a manner analogous to the human MMN. Second, oddball paradigms using intensity or duration deviants revealed a pattern of unit responses that showed sensory adaptation, but again without any measurable novelty correlates aligning to the human MMN. Finally NMDA antagonists, which are known to disrupt the MMN, suppressed the magnitude of multiunit responses in a nonspecific manner, leaving the process of SSA intact. Together, our results suggest that auditory novelty detection as indexed by the MMN is dissociable from SSA at the level of activity encoded by auditory cortex neurons. Further, the NMDA sensitivity reported for the MMN, which models the disruption of MMN observed in schizophrenia, may occur at a mechanistic locus outside of SSA.

Alteration of visual input results in a coordinated reorganization of multiple visual cortex maps.

In the adult visual cortex, multiple feature maps exist and have characteristic spatial relationships with one another. The relationships can be reproduced by "dimension-reduction" computational models, suggesting that the principles of continuity and coverage may underlie cortical map organization. However, the mechanisms responsible for establishing these relationships are unknown. We explored whether removing one feature map during development causes a coordinated reorganization of the remaining maps or whether the remaining maps are unaffected. We removed the ocular dominance map by monocular enucleation in newborn ferrets, so that single eye stimulation drove the cortex in a more spatially uniform manner in adult monocular animals compared with normal animals. Maps of orientation, spatial frequency, and retinotopy formed in monocular ferrets, but their structures and spatial relationships differed from those in normal ferrets. The wavelength of the orientation map increased, so that the average orientation gradient across the cortex decreased. The decrease in the orientation gradient in monocular animals was most prominent in the high gradient regions of the spatial frequency map, indicating a coordinated reorganization between these two maps. In monocular animals, the orthogonal relationship between the orientation and spatial frequency maps was preserved, and the orthogonal relationship between the orientation and retinotopic maps became more pronounced. These results were consistent with detailed predictions of a dimension-reduction model of cortical organization. Thus, the number of feature maps in a cortical area influences the relationships between them, and inputs to the cortex have a significant role in generating these relationships.

The coordinated mapping of visual space and response features in visual cortex.

Whether general principles can explain the layouts of cortical maps remains unresolved. In primary visual cortex of ferret, the relationships between the maps of visual space and response features are predicted by a "dimension-reduction" model. The representation of visual space is anisotropic, with the elevation and azimuth axes having different magnification. This anisotropy is reflected in the orientation, ocular dominance, and spatial frequency domains, which are elongated such that their directions of rapid change, or high-gradient axes, are orthogonal to the high-gradient axis of the visual map. The feature maps are also strongly interdependent-their high-gradient regions avoid one another and intersect orthogonally where essential, so that overlap is minimized. Our results demonstrate a clear influence of the visual map on each feature map. In turn, the local representation of visual space is smooth, as predicted when many features are mapped within a cortical area.