Acute dissociation for analyses of NMDA receptor function in cortical neurons during aging

Non-invasive optogenetics with ultrasound-mediated gene delivery and red-light excitation

Optogenetics has revolutionized the capability of controlling genetically modified neurons in vitro and in vivo and has become an indispensable neuroscience tool. Using light as a probe for selective neuronal activation or inhibition and as a means to read out neural activity has dramatically enhanced our understanding of complex neural circuits. However, a common limitation of optogenetic studies to date is their invasiveness and spatiotemporal range. Direct viral injections into the brain tissue along with implantation of optical fibers and recording electrodes can disrupt the neuronal circuitry and cause significant damage. Conventional approaches are spatially limited around the site of the direct injection and insufficient in examining large networks throughout the brain. Lastly, optogenetics is currently not easily scalable to large animals or humans. Here, we demonstrate that optogenetic excitation can be achieved entirely non-invasively through the intact skull in mice. Using a needle-free combination of focused ultrasound-mediated viral delivery and extracorporeal illumination with red light, we achieved selective neuronal activation at depths up to 4 mm in the murine brain, confirmed through cFos expression and electrophysiology measurements within the treated areas. Ultrasound treatment significantly reduced freezing time during recall in fear conditioning experiments, but remote light exposure had a moderate effect on the freezing behavior of mice treated with viral vectors. The proposed method has the potential to open new avenues of studying, but also stimulating, neuronal networks, in an effort to elucidate normal or dysfunctional brain activity and treat neurological diseases. Finally, the same non-invasive methodology could be combined with gene therapy and applied to other organs, such as the eye and the heart.

The aging mouse brain: cognition, connectivity and calcium

Aging is a complex process that differentially impacts multiple cognitive, sensory, neuronal and molecular processes. Technological innovations now allow for parallel investigation of neuronal circuit function, structure and molecular composition in the brain of awake behaving adult mice. Thus, mice have become a critical tool to better understand how aging impacts the brain. However, a more granular systems-based approach, which considers the impact of age on key features relating to neural processing, is required. Here, we review evidence probing the impact of age on the mouse brain. We focus on a range of processes relating to neuronal function, including cognitive abilities, sensory systems, synaptic plasticity and calcium regulation. Across many systems, we find evidence for prominent age-related dysregulation even before 12 months of age, suggesting that emerging age-related alterations can manifest by late adulthood. However, we also find reports suggesting that some processes are remarkably resilient to aging. The evidence suggests that aging does not drive a parallel, linear dysregulation of all systems, but instead impacts some processes earlier, and more severely, than others. We propose that capturing the more fine-scale emerging features of age-related vulnerability and resilience may provide better opportunities for the rejuvenation of the aged brain.

Changes in Serine Racemase-Dependent Modulation of NMDA Receptor: Impact on Physiological and Pathological Brain Aging

The N-methyl-D-Aspartate glutamate receptors (NMDARs) are pivotal for the functional and morphological plasticity that are required in neuronal networks for efficient brain activities and notably for cognitive-related abilities. Because NMDARs are heterogeneous in subunit composition and associated with multiple functional regulatory sites, their efficacy is under the tonic influence of numerous allosteric modulations, whose dysfunction generally represents the first step generating pathological states. Among the enzymatic candidates, serine racemase (SR) has recently gathered an increasing interest considering that it tightly regulates the production of d-serine, an amino acid now viewed as the main endogenous co-agonist necessary for NMDAR activation. Nowadays, SR deregulation is associated with a wide range of neurological and psychiatric diseases including schizophrenia, amyotrophic lateral sclerosis, and depression. This review aims at compelling the most recent experimental evidences indicating that changes in SR-related modulation of NMDARs also govern opposite functional dysfunctions in physiological and pathological (Alzheimer's disease) aging that finally results in memory disabilities in both cases. It also highlights SR as a relevant alternative target for new pharmacological strategies aimed at preventing functional alterations and cognitive impairments linked to the aging process.

A Guide to Single-Cell Transcriptomics in Adult Rodent Brain: The Medium Spiny Neuron Transcriptome Revisited

Recent advances in single-cell technologies are paving the way to a comprehensive understanding of the cellular complexity in the brain. Protocols for single-cell transcriptomics combine a variety of sophisticated methods for the purpose of isolating the heavily interconnected and heterogeneous neuronal cell types in a relatively intact and healthy state. The emphasis of single-cell transcriptome studies has thus far been on comparing library generation and sequencing techniques that enable measurement of the minute amounts of starting material from a single cell. However, in order for data to be comparable, standardized cell isolation techniques are essential. Here, we analyzed and simplified methods for the different steps critically involved in single-cell isolation from brain. These include enzymatic digestion, tissue trituration, improved methods for efficient fluorescence-activated cell sorting in samples containing high degree of debris from the neuropil, and finally, highly region-specific cellular labeling compatible with use of stereotaxic coordinates. The methods are exemplified using medium spiny neurons (MSN) from dorsomedial striatum, a cell type that is clinically relevant for disorders of the basal ganglia, including psychiatric and neurodegenerative diseases. We present single-cell RNA sequencing (scRNA-Seq) data from D1 and D2 dopamine receptor expressing MSN subtypes. We illustrate the need for single-cell resolution by comparing to available population-based gene expression data of striatal MSN subtypes. Our findings contribute toward standardizing important steps of single-cell isolation from adult brain tissue to increase comparability of data. Furthermore, our data redefine the transcriptome of MSNs at unprecedented resolution by confirming established marker genes, resolving inconsistencies from previous gene expression studies, and identifying novel subtype-specific marker genes in this important cell type.

High and low nightly running behavior associates with nucleus accumbens N-Methyl-d-aspartate receptor (NMDAR) NR1 subunit expression and NMDAR functional differences

The extent to which N-Methyl-d-aspartate (NMDA) receptors facilitate the motivation to voluntarily wheel-run in rodents has yet to be determined. In so, we utilized female Wistar rats selectively bred to voluntarily run high (HVR) and low (LVR) nightly distances in order to examine if endogenous differences in nucleus accumbens (NAc) NMDA receptor expression and function underlies the propensity for high or low motivation to voluntarily wheel-run. 12-14 week old HVR and LVR females were used to examine: 1.) NAc mRNA and protein expression of NMDA subunits NR1, NR2A and NR2B; 2.) NMDA current responses in isolated medium spiny neurons (MSNs) and 3.) NMDA-evoked dopamine release in an ex vivo preparation of NAc punches. Expectedly, there was a large divergence in nightly running distance and time between HVR and LVR rats. We saw a significantly higher mRNA and protein expression of NR1 in HVR compared to LVR rats, while seeing no difference in the expression of NR2A or NR2B. There was a greater current response to a 500 ms application of 300 μM of NMDA in medium-spiny neurons isolated from the NAc HVR compared to LVR animals. On average, NMDA-evoked punches (50 μM of NMDA for 10 min) taken from HVR rats retained ∼54% of the dopamine content compared to their bilateral non-evoked sides, while evoked punches from LVR animals showed no statistical decrease in dopamine content compared to their non-evoked sides. Collectively, these data suggest a potential link between NAc NR1 subunit expression as well as NMDA function and the predisposition for nightly voluntary running behavior in rats. In light of the epidemic rise in physical inactivity, these findings have the potential to explain a neuro-molecular mechanism that regulates the motivation to be physically active.

H2O2 augments cytosolic calcium in nucleus tractus solitarii neurons via multiple voltage-gated calcium channels

Reactive oxygen species (ROS) play a profound role in cardiorespiratory function under normal physiological conditions and disease states. ROS can influence neuronal activity by altering various ion channels and transporters. Within the nucleus tractus solitarii (nTS), a vital brainstem area for cardiorespiratory control, hydrogen peroxide (H₂O₂) induces sustained hyperexcitability following an initial depression of neuronal activity. The mechanism(s) associated with the delayed hyperexcitability are unknown. Here we evaluate the effect(s) of H₂O₂ on cytosolic Ca²⁺ (via fura-2 imaging) and voltage-dependent calcium currents in dissociated rat nTS neurons. H₂O₂ perfusion (200 µM; 1 min) induced a delayed, slow, and moderate increase (~27%) in intracellular Ca²⁺ concentration ([Ca²⁺]_i). The H₂O₂-mediated increase in [Ca²⁺]_i prevailed during thapsigargin, excluding the endoplasmic reticulum as a Ca²⁺ source. The effect, however, was abolished by removal of extracellular Ca²⁺ or the addition of cadmium to the bath solution, suggesting voltage-gated Ca²⁺ channels (VGCCs) as targets for H₂O₂ modulation. Recording of the total voltage-dependent Ca²⁺ current confirmed H₂O₂ enhanced Ca²⁺ entry. Blocking VGCC L, N, and P/Q subtypes decreased the number of cells and their calcium currents that respond to H₂O₂ The number of responder cells to H₂O₂ also decreased in the presence of dithiothreitol, suggesting the actions of H₂O₂ were dependent on sulfhydryl oxidation. In summary, here, we have shown that H₂O₂ increases [Ca²⁺]_i and its Ca²⁺ currents, which is dependent on multiple VGCCs likely by oxidation of sulfhydryl groups. These processes presumably contribute to the previously observed delayed hyperexcitability of nTS neurons in in vitro brainstem slices.

NMDA Receptor Function During Senescence: Implication on Cognitive Performance

N-methyl-D-aspartate (NMDA) receptors, a family of L-glutamate receptors, play an important role in learning and memory, and are critical for spatial memory. These receptors are tetrameric ion channels composed of a family of related subunits. One of the hallmarks of the aging human population is a decline in cognitive function; studies in the past couple of years have demonstrated deterioration in NMDA receptor subunit expression and function with advancing age. However, a direct relationship between impaired memory function and a decline in NMDA receptors is still ambiguous. Recent studies indicate a link between an age-associated NMDA receptor hypofunction and memory impairment and provide evidence that age-associated enhanced oxidative stress might be contributing to the alterations associated with senescence. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between age-associated impaired cognitive faculties and NMDA receptor hypofunction. The current review intends to present an overview of the research findings regarding changes in expression of various NMDA receptor subunits and deficits in NMDA receptor function during senescence and its implication in age-associated impaired hippocampal-dependent memory function.

Memory in aged mice is rescued by enhanced expression of the GluN2B subunit of the NMDA receptor

The GluN2B subunit of the N-methyl-d-aspartate (NMDA) receptor shows age-related declines in expression across the frontal cortex and hippocampus. This decline is strongly correlated to age-related memory declines. This study was designed to determine if increasing GluN2B subunit expression in the frontal lobe or hippocampus would improve memory in aged mice. Mice were injected bilaterally with either the GluN2B vector, containing cDNA specific for the GluN2B subunit and enhanced green fluorescent protein (eGFP); a control vector or vehicle. Spatial memory, cognitive flexibility, and associative memory were assessed using the Morris water maze. Aged mice, with increased GluN2B subunit expression, exhibited improved long-term spatial memory, comparable to young mice. However, memory was rescued on different days in the Morris water maze; early for hippocampal GluN2B subunit enrichment and later for the frontal lobe. A higher concentration of the GluN2B antagonist, Ro 25-6981, was required to impair long-term spatial memory in aged mice with enhanced GluN2B expression, as compared to aged controls, suggesting there was an increase in the number of GluN2B-containing NMDA receptors. In addition, hippocampal slices from aged mice with increased GluN2B subunit expression exhibited enhanced NMDA receptor-mediated excitatory post-synaptic potentials (EPSP). Treatment with Ro 25-6981 showed that a greater proportion of the NMDA receptor-mediated EPSP was due to the GluN2B subunit in these animals, as compared to aged controls. These results suggest that increasing the production of the GluN2B subunit in aged animals enhances memory and synaptic transmission. Therapies that enhance GluN2B subunit expression within the aged brain may be useful for ameliorating age-related memory declines.

Flufenamic acid modulates multiple currents in gonadotropin-releasing hormone neurons

Reproduction in mammals is dependent upon the appropriate neurosecretion of gonadotropin-releasing hormone (GnRH), yet the endogenous generation of activity underlying GnRH secretion remains poorly understood. We have demonstrated that the depolarizing afterpotential (DAP), which modulates bursting activity, is reduced in isolated GnRH neurons from aged animals. Calcium-activated non-specific cation (CAN) channels contribute to the DAP in other vertebrate neurosecretory cells. We used the CAN channel blocker flufenamic acid (FFA) to examine the contribution of CAN channels to the DAP in GnRH neurons during aging. Recordings were performed on isolated fluorescent GnRH neurons from young, middle-aged and aged female mice. Flufenamic acid inhibited spontaneous activity, but significantly increased the DAP in neurons from young and middle-aged animals. Apamin did not significantly potentiate the DAP, but did reduce the effects of FFA, suggesting that the increased DAP is partially due to blockade of apamin-sensitive SK channels. Flufenamic acid increased the current underlying the DAP (I(ADP)) and decreased the preceding fast outward current (I(OUT)) at all ages. These current responses were not affected by apamin, but TEA evoked similar changes. Thus, a potassium current, likely mediated through BK channels, contributes to the fast AHP and appears to offset the DAP; this current is sensitive to FFA, but insensitive to age. The effect of FFA on the DAP, but not I(ADP), is diminished in aged animals, possibly reflecting an age-related modulation of the apamin-sensitive SK channel. Future studies will examine the expression of SK channels during the aging process in GnRH neurons.

Susceptibility to Calcium Dysregulation during Brain Aging

Calcium (Ca(2+)) is a highly versatile intracellular signaling molecule that is essential for regulating a variety of cellular and physiological processes ranging from fertilization to programmed cell death. Research has provided ample evidence that brain aging is associated with altered Ca(2+) homeostasis. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during senescence. The current review takes a broader perspective, assessing age-related changes in Ca(2+) sources, Ca(2+) sequestration, and Ca(2+) binding proteins throughout the nervous system. The nature of altered Ca(2+) homeostasis is cell specific and may represent a deficit or a compensatory mechanism, producing complex patterns of impaired cellular function. Incorporating the knowledge of the complexity of age-related alterations in Ca(2+) homeostasis will positively shape the development of highly effective therapeutics to treat brain disorders.

Age affects spontaneous activity and depolarizing afterpotentials in isolated gonadotropin-releasing hormone neurons

Neuronal activity underlying the pulsatile secretion of GnRH remains poorly understood, as does the endogenous generation of such activity. It is clear that changes at the level of the hypothalamus are taking place during reproductive aging, yet virtually nothing is known about GnRH neuronal physiology in aging and postreproductive animals. In these studies, we performed cell-attached and whole-cell recordings in GnRH-enhanced green fluorescent protein neurons dissociated from young (3 months), middle-aged (10 months), and old (15-18 months) female mice. All mice were ovariectomized; half were estradiol replaced. Neurons from all ages fired spontaneously, most in a short-burst pattern that is characteristic of GnRH neuronal firing. Membrane characteristics were not affected by age. However, firing frequency was significantly reduced in neurons from old animals, as was spike patterning. The amplitude of the depolarizing afterpotential, evoked by a 200-msec current pulse, was significantly smaller in aged animals. In addition, inward whole-cell currents were reduced in estradiol-treated animals, although they were not significantly affected by age. Because depolarizing afterpotentials have been shown to contribute to prolonged discharges of activity after a very brief excitatory input, a decreased depolarizing afterpotential could lead to attenuated pulses in older animals. In addition, decreases in frequency and pattern generation could lead to improper information coding. Therefore, changes in the GnRH neuron during aging could lead to dysregulated activity, potentially resulting in the attenuated LH pulses observed in the transition to reproductive senescence.