fMRI study of the role of glutamate NMDA receptor in the olfactory processing in monkeys

Longitudinal evaluation of the functional connectivity changes in the secondary somatosensory cortex (S2) of the monkey brain during acute stroke

Background

Somatosensory deficits are frequently seen in acute stroke patients and may recover over time and affect functional outcome. However, the underlying mechanism of function recovery remains poorly understood. In the present study, progressive function alteration of the secondary somatosensory cortex (S2) and its relationship with regional perfusion and neurological outcome were examined using a monkey model of stroke.

Methods and materials

Rhesus monkeys (n = 4) were induced with permanent middle cerebral artery occlusion (pMCAo). Resting-state functional MRI, dynamic susceptibility contrast perfusion MRI, diffusion-weighted, T₁ and T₂ weighted images were collected before surgery and at 4-6, 48, and 96 h post stroke on a 3T scanner. Progressive changes of relative functional connectivity (FC), cerebral blood flow (CBF), and CBF/Tmax (Time to Maximum) of affected S2 regions were evaluated. Neurological deficits were assessed using the Spetzler approach.

Results

Ischemic lesion was evidently seen in the MCA territory including S2 in each monkey. Relative FC of injured S2 regions decreased substantially following stroke. Spetzler scores dropped substantially at 24 h post stroke but slightly recovered from Day 2 to Day 4. Relative FC progressively increased from 6 to 48 and 96 h post stroke and correlated significantly with relative CBFand CBF/Tmax changes.

Conclusion

The present study revealed the progressive alteration of function connectivity in S2 during acute stroke. The preliminary results suggested the function recovery might start couple days post occlusion and collateral circulation might play a key role in the recovery of somatosensory function after stroke insult. The relative function connectivity in S2 may provide additional information for prediction of functional outcome in stroke patients.

The current status and trend of the functional magnetic resonance combined with stimulation in animals

As a non-radiative, non-invasive imaging technique, functional magnetic resonance imaging (fMRI) has excellent effects on studying the activation of blood oxygen levels and functional connectivity of the brain in human and animal models. Compared with resting-state fMRI, fMRI combined with stimulation could be used to assess the activation of specific brain regions and the connectivity of specific pathways and achieve better signal capture with a clear purpose and more significant results. Various fMRI methods and specific stimulation paradigms have been proposed to investigate brain activation in a specific state, such as electrical, mechanical, visual, olfactory, and direct brain stimulation. In this review, the studies on animal brain activation using fMRI combined with different stimulation methods were retrieved. The instruments, experimental parameters, anesthesia, and animal models in different stimulation conditions were summarized. The findings would provide a reference for studies on estimating specific brain activation using fMRI combined with stimulation.

Effects of Anesthesia on Cerebral Blood Flow and Functional Connectivity of Nonhuman Primates

Nonhuman primates (NHPs) are the closest living relatives of humans and play a critical and unique role in neuroscience research and pharmaceutical development. General anesthesia is usually required in neuroimaging studies of NHPs to keep the animal from stress and motion. However, the adverse effects of anesthesia on cerebral physiology and neural activity are pronounced and can compromise the data collection and interpretation. Functional connectivity is frequently examined using resting-state functional MRI (rsfMRI) to assess the functional abnormality in the animal brain under anesthesia. The fMRI signal can be dramatically suppressed by most anesthetics in a dose-dependent manner. In addition, rsfMRI studies may be further compromised by inter-subject variations when the sample size is small (as seen in most neuroscience studies of NHPs). Therefore, proper use of anesthesia is strongly demanded to ensure steady and consistent physiology maintained during rsfMRI data collection of each subject. The aim of this review is to summarize typical anesthesia used in rsfMRI scans of NHPs and the effects of anesthetics on cerebral physiology and functional connectivity. Moreover, the protocols with optimal rsfMRI data acquisition and anesthesia procedures for functional connectivity study of macaque monkeys are introduced.

Nav1.7 target modulation and efficacy can be measured in nonhuman primate assays

Humans with loss-of-function mutations in the Na_v1.7 channel gene (SCN9A) show profound insensitivity to pain, whereas those with gain-of-function mutations can have inherited pain syndromes. Therefore, inhibition of the Na_v1.7 channel with a small molecule has been considered a promising approach for the treatment of various human pain conditions. To date, clinical studies conducted using selective Na_v1.7 inhibitors have not provided analgesic efficacy sufficient to warrant further investment. Clinical studies to date used multiples of in vitro IC₅₀ values derived from electrophysiological studies to calculate anticipated human doses. To increase the chance of clinical success, we developed rhesus macaque models of action potential propagation, nociception, and olfaction, to measure Na_v1.7 target modulation in vivo. The potent and selective Na_v1.7 inhibitors SSCI-1 and SSCI-2 dose-dependently blocked C-fiber nociceptor conduction in microneurography studies and inhibited withdrawal responses to noxious heat in rhesus monkeys. Pharmacological Na_v1.7 inhibition also reduced odor-induced activation of the olfactory bulb (OB), measured by functional magnetic resonance imaging (fMRI) studies consistent with the anosmia reported in Na_v1.7 loss-of-function patients. These data demonstrate that it is possible to measure Na_v1.7 target modulation in rhesus macaques and determine the plasma concentration required to produce a predetermined level of inhibition. The calculated plasma concentration for preclinical efficacy could be used to guide human efficacious exposure estimates. Given the translatable nature of the assays used, it is anticipated that they can be also used in phase 1 clinical studies to measure target modulation and aid in the interpretation of phase 1 clinical data.

Discovery of Arylsulfonamide Nav1.7 Inhibitors: IVIVC, MPO Methods, and Optimization of Selectivity Profile

The voltage-gated sodium channel Nav1.7 continues to be a high-profile target for the treatment of various pain afflictions due to its strong human genetic validation. While isoform selective molecules have been discovered and advanced into the clinic, to date, this target has yet to bear fruit in the form of marketed therapeutics for the treatment of pain. Lead optimization efforts over the past decade have focused on selectivity over Nav1.5 due to its link to cardiac side effects as well as the translation of preclinical efficacy to man. Inhibition of Nav1.6 was recently reported to yield potential respiratory side effects preclinically, and this finding necessitated a modified target selectivity profile. Herein, we report the continued optimization of a novel series of arylsulfonamide Nav1.7 inhibitors to afford improved selectivity over Nav1.6 while maintaining rodent oral bioavailability through the use of a novel multiparameter optimization (MPO) paradigm. We also report in vitro-in vivo correlations from Nav1.7 electrophysiology protocols to preclinical models of efficacy to assist in projecting clinical doses. These efforts produced inhibitors such as compound 19 with potency against Nav1.7, selectivity over Nav1.5 and Nav1.6, and efficacy in behavioral models of pain in rodents as well as inhibition of rhesus olfactory response indicative of target modulation.

Brain Activity of Benzoate, a D-Amino Acid Oxidase Inhibitor, in Patients With Mild Cognitive Impairment in a Randomized, Double-Blind, Placebo Controlled Clinical Trial

BACKGROUND

Current anti-dementia drugs cannot benefit mild cognitive impairment (MCI). Sodium benzoate (a D-amino acid oxidase [DAO] inhibitor) has been found to improve the cognitive function of patients with early-phase Alzheimer's disease (mild Alzheimer's disease or MCI). However, its effect on brain function remains unknown. This study aimed to evaluate the influence of benzoate on functional magnetic resonance imaging in patients with amnestic MCI.

METHODS

This was a 24-week, randomized, double-blind, placebo-controlled trial that enrolled 21 patients with amnestic MCI and allocated them randomly to either of 2 treatment groups: (1) benzoate group (250-1500 mg/d), or (2) placebo group. We assessed the patients' working memory, verbal learning and memory, and resting-state functional magnetic resonance imaging and regional homogeneity (ReHo) maps at baseline and endpoint.

RESULTS

Resting-state ReHo decreased in right orbitofrontal cortex after benzoate treatment but did not change after placebo. Moreover, after benzoate treatment, the change in working memory was positively correlated with the change in ReHo in right precentral gyrus and right middle occipital gyrus; and the change in verbal learning and memory was positively correlated with the change in ReHo in left precuneus. In contrast, after placebo treatment, the change in working memory or in verbal learning and memory was not correlated with the change in ReHo in any brain region.

CONCLUSION

The current study is the first to our knowledge to demonstrate that a DAO inhibitor, sodium benzoate herein, can alter brain activity as well as cognitive functions in individuals with MCI. The preliminary finding lends supports for DAO inhibition as a novel approach for early dementing processes.

Odor habituation can modulate very early olfactory event-related potential

Odor habituation is a phenomenon that after repeated exposure to an odor, is characterized by decreased responses to it. The central nervous system is involved in odor habituation. To study odor habituation in humans, measurement of event-related potentials (ERPs) has been widely used in the olfactory system and other sensory systems, because of their high temporal resolution. Most previous odor habituation studies have measured the olfactory ERPs of (200-800) ms. However, several studies have shown that the odor signal is processed in the central nervous system earlier than at 200 ms. For these reasons, we studied whether when odors were habituated, olfactory ERP within 200 ms of odors could change. To this end, we performed an odor habituation behavior test and electroencephalogram experiments. In the behavior test, under habituation conditions, odor intensity was significantly decreased. We found significant differences in the negative and positive potentials within 200 ms across the conditions, which correlated significantly with the results of the behavior test. We also observed that ERP latency depended on the conditions. Our study suggests that odor habituation can involve the olfactory ERP of odors within 200 ms in the brain.