Sensory Experience as a Regulator of Structural Plasticity in the Developing Whisker-to-Barrel System.

Cellular structures provide the physical foundation for the functionality of the nervous system, and their developmental trajectory can be influenced by the characteristics of the external environment that an organism interacts with. Historical and recent works have determined that sensory experiences, particularly during developmental critical periods, are crucial for information processing in the brain, which in turn profoundly influence neuronal and non-neuronal cortical structures that subsequently impact the animals' behavioral and cognitive outputs. In this review, we focus on how altering sensory experience influences normal/healthy development of the central nervous system, particularly focusing on the cerebral cortex using the rodent whisker-to-barrel system as an illustrative model. A better understanding of structural plasticity, encompassing multiple aspects such as neuronal, glial, and extra-cellular domains, provides a more integrative view allowing for a deeper appreciation of how all aspects of the brain work together as a whole.

Plasma dilution improves cognition and attenuates neuroinflammation in old mice.

Our recent study has established that young blood factors are not causal, nor necessary, for the systemic rejuvenation of mammalian tissues. Instead, a procedure referred to as neutral blood exchange (NBE) that resets signaling milieu to a pro-regenerative state through dilution of old plasma, enhanced the health and repair of the muscle and liver, and promoted better hippocampal neurogenesis in 2-year-old mice (Mehdipour et al., Aging 12:8790-8819, 2020). Here we expand the rejuvenative phenotypes of NBE, focusing on the brain. Namely, our results demonstrate that old mice perform much better in novel object and novel texture (whisker discrimination) tests after a single NBE, which is accompanied by reduced neuroinflammation (less-activated CD68+ microglia). Evidence against attenuation/dilution of peripheral senescence-associated secretory phenotype (SASP) as the main mechanism behind NBE was that the senolytic ABT 263 had limited effects on neuroinflammation and did not enhance hippocampal neurogenesis in the old mice. Interestingly, peripherally acting ABT 263 and NBE both diminished SA-ßGal signal in the old brain, demonstrating that peripheral senescence propagates to the brain, but NBE was more robustly rejuvenative than ABT 263, suggesting that rejuvenation was not simply by reducing senescence. Explaining the mechanism of the positive effects of NBE on the brain, our comparative proteomics analysis demonstrated that dilution of old blood plasma yields an increase in the determinants of brain maintenance and repair in mice and in people. These findings confirm the paradigm of rejuvenation through dilution of age-elevated systemic factors and extrapolate it to brain health and function.

Barrels XXXII meeting report: whiskers in the windy city.

The 32nd Annual Barrels meeting was hosted at the Northwestern University Feinberg School of Medicine in Chicago, Illinois on October 17th and 18th, 2019. The annual meeting brings together researchers who utilize the rodent whisker-to-barrel system as a means to understand cortical function and development. This year's meeting focussed on social behaviours, development and cerebellar functions within the barrel system and beyond.

Development and sensory experience dependent regulation of microglia in barrel cortex.

The barrel cortex is within the primary somatosensory cortex of the rodent, and processes signals from the vibrissae. Much focus has been devoted to the function of neurons, more recently, the role of glial cells in the processing of sensory input has gained increasing interest. Microglia are the principal immune cells of the nervous system that survey and regulate the cellular constituents of the dynamic nervous system. We investigated the normal and disrupted development of microglia in barrel cortex by chronically depriving sensory signals via whisker trimming for the animals' first postnatal month. Using immunohistochemistry to label microglia, we performed morphological reconstructions as well as densitometry analyses as a function of developmental age and sensory experience. Findings suggest that both developmental age and sensory experience has profound impact on microglia morphology. Following chronic sensory deprivation, microglia undergo a morphological transition from a monitoring or resting state to an altered morphological state, by exhibiting expanded cell body size and retracted processes. Sensory restoration via whisker regrowth returns these morphological alterations back to age-matched control values. Our results indicate that microglia may be recruited to participate in the modulation of neuronal structural remodeling during developmental critical periods and in response to alteration in sensory input.

Rejuvenation of brain, liver and muscle by simultaneous pharmacological modulation of two signaling determinants, that change in opposite directions with age.

We hypothesize that altered intensities of a few morphogenic pathways account for most/all the phenotypes of aging. Investigating this has revealed a novel approach to rejuvenate multiple mammalian tissues by defined pharmacology. Specifically, we pursued the simultaneous youthful in vivo calibration of two determinants: TGF-beta which activates ALK5/pSmad 2,3 and goes up with age, and oxytocin (OT) which activates MAPK and diminishes with age. The dose of Alk5 inhibitor (Alk5i) was reduced by 10-fold and the duration of treatment was shortened (to minimize overt skewing of cell-signaling pathways), yet the positive outcomes were broadened, as compared with our previous studies. Alk5i plus OT quickly and robustly enhanced neurogenesis, reduced neuro-inflammation, improved cognitive performance, and rejuvenated livers and muscle in old mice. Interestingly, the combination also diminished the numbers of cells that express the CDK inhibitor and marker of senescence p16 in vivo. Summarily, simultaneously re-normalizing two pathways that change with age in opposite ways (up vs. down) synergistically reverses multiple symptoms of aging.

Oligodendrocyte- and Neuron-Specific Nogo-A Restrict Dendritic Branching and Spine Density in the Adult Mouse Motor Cortex.

Nogo-A has been well described as a myelin-associated inhibitor of neurite outgrowth and functional neuroregeneration after central nervous system (CNS) injury. Recently, a new role of Nogo-A has been identified as a negative regulator of synaptic plasticity in the uninjured adult CNS. Nogo-A is present in neurons and oligodendrocytes. However, it is yet unclear which of these two pools regulate synaptic plasticity. To address this question we used newly generated mouse lines in which Nogo-A is specifically knocked out in (1) oligodendrocytes (oligoNogo-A KO) or (2) neurons (neuroNogo-A KO). We show that both oligodendrocyte- and neuron-specific Nogo-A KO mice have enhanced dendritic branching and spine densities in layer 2/3 cortical pyramidal neurons. These effects are compartmentalized: neuronal Nogo-A affects proximal dendrites whereas oligodendrocytic Nogo-A affects distal regions. Finally, we used two-photon laser scanning microscopy to measure the spine turnover rate of adult mouse motor cortex layer 5 cells and find that both Nogo-A KO mouse lines show enhanced spine remodeling after 4 days. Our results suggest relevant control functions of glial as well as neuronal Nogo-A for synaptic plasticity and open new possibilities for more selective and targeted plasticity enhancing strategies.

Astrocytic Contributions to Synaptic and Learning Abnormalities in a Mouse Model of Fragile X Syndrome.

BACKGROUND: Fragile X syndrome (FXS) is the most common type of mental retardation attributable to a single-gene mutation. It is caused by FMR1 gene silencing and the consequent loss of its protein product, fragile X mental retardation protein. Fmr1 global knockout (KO) mice recapitulate many behavioral and synaptic phenotypes associated with FXS. Abundant evidence suggests that astrocytes are important contributors to neurological diseases. This study investigates astrocytic contributions to the progression of synaptic abnormalities and learning impairments associated with FXS. METHODS: Taking advantage of the Cre-lox system, we generated and characterized mice in which fragile X mental retardation protein is selectively deleted or exclusively expressed in astrocytes. We performed in vivo two-photon imaging to track spine dynamics/morphology along dendrites of neurons in the motor cortex and examined associated behavioral defects. RESULTS: We found that adult astrocyte-specific Fmr1 KO mice displayed increased spine density in the motor cortex and impaired motor-skill learning. The learning defect coincided with a lack of enhanced spine dynamics in the motor cortex that normally occurs in response to motor skill acquisition. Although spine density was normal at 1 month of age in astrocyte-specific Fmr1 KO mice, new spines formed at an elevated rate. Furthermore, fragile X mental retardation protein expression in only astrocytes was insufficient to rescue most spine or behavioral defects. CONCLUSIONS: Our work suggests a joint astrocytic-neuronal contribution to FXS pathogenesis and reveals that heightened spine formation during adolescence precedes the overabundance of spines and behavioral defects found in adult Fmr1 KO mice.

Experience-dependent regulation of tissue-type plasminogen activator in the mouse barrel cortex.

It has been suggested that tissue-type plasminogen activator (tPA), a serine protease, plays a key role in regulating the extracellular matrix core proteins, thereby impacting the structural plasticity in the cerebral cortex. Much is known about its role in regulating plasticity in the visual cortex. However, its permissive role has not been demonstrated to generalize to other cerebral cortical areas. By utilizing a combination of immunofluorescent histochemistry and confocal microscopy, we demonstrate that endogenous tPA is indeed present in the somatosensory cortex, and its expression is experience-dependent. Chronic sensory deprivation induced by whisker trimming from birth for one month leads to increased tPA immunoreactivity in all layers of the barrel cortex. Furthermore, tPA immunoreactivity remains high even after sensation has been restored to the mystacial pad (by allowing whiskers to grow back to full length for one month). Our results suggest that tPA levels in the cerebral cortex are regulated by sensory experience, and play a key role in regulating structural remodeling in the cerebral cortex.

The impact of development and sensory deprivation on dendritic protrusions in the mouse barrel cortex.

Dendritic protrusions (spines and filopodia) are structural indicators of synapses that have been linked to neuronal learning and memory through their morphological alterations induced by development and experienced-dependent activities. Although previous studies have demonstrated that depriving sensory experience leads to structural changes in neocortical organization, the more subtle effects on dendritic protrusions remain unclear, mostly due to focus on only one specific cell type and/or age of manipulation. Here, we show that sensory deprivation induced by whisker trimming influences the dendritic protrusions of basilar dendrites located in thalamocortical recipient lamina (IV and VI) of the mouse barrel cortex in a layer-specific manner. Following 1 month of whisker trimming after birth, the density of dendritic protrusions increased in layer IV, but decreased in layer VI. Whisker regrowth for 1 month returned protrusion densities to comparable level of age-matched controls in layer VI, but not in layer IV. In adults, chronic sensory deprivation led to an increase in protrusion densities in layer IV, but not in layer VI. In addition, chronic pharmacological blockade of N-methyl-d-aspartate receptors (NMDARs) increased protrusion density in both layers IV and VI, which returned to the control level after 1 month of drug withdrawal. Our data reveal that different cortical layers respond to chronic sensory deprivation in different ways, with more pronounced effects during developmental critical periods than adulthood. We also show that chronically blocking NMDARs activity during developmental critical period also influences the protrusion density and morphology in the cerebral cortex.

Spatiotemporal dynamics of dendritic spines in the living brain.

Dendritic spines are ubiquitous postsynaptic sites of most excitatory synapses in the mammalian brain, and thus may serve as structural indicators of functional synapses. Recent works have suggested that neuronal coding of memories may be associated with rapid alterations in spine formation and elimination. Technological advances have enabled researchers to study spine dynamics in vivo during development as well as under various physiological and pathological conditions. We believe that better understanding of the spatiotemporal patterns of spine dynamics will help elucidate the principles of experience-dependent circuit modification and information processing in the living brain.

An ocean full of BARRELS: Barrels XXVI meeting report.

The 26th annual Barrels meeting was convened on the campus of the University of California San Diego, not far from the shores of the Pacific Ocean. The meeting focused on three main themes: the structure and function of the thalamic reticular nucleus, the neurovasculature system and its role in brain metabolism, and the origins and functions of cortical GABAergic interneurons. In addition to the major themes, there were short talks, a data blitz, and a poster session which highlighted the diversity and quality of the research ongoing in the rodent whisker-to-barrel system.

Fetal maxillary and mandibular length in normal pregnancies from 11 weeks' to 13(+6) weeks' gestation: a Taiwanese study.

OBJECTIVES: This study aims to establish the normal range of maxillary and mandibular lengths within the Taiwanese population at 11(+0) weeks to 13(+6) weeks of gestation in normal singleton pregnancy as a reference value for prenatal ultrasonographic examinations. MATERIALS AND METHODS: We examined nuchal translucency in 269 normal singleton pregnancies, with the gestational age ranging from 11 weeks to 13(+6) weeks in this study. Fetal biometric measurements, with an emphasis on maxillary and mandibular lengths, were obtained from the patients during consecutive routine prenatal ultrasonographic examinations. RESULTS: Maxillary and mandibular lengths were recorded successfully in 191 patients and 179 patients, respectively. The mean maternal age was 31 years (range 19-45 years), with a corresponding gestational age of 12 + 4 weeks (range, 11(+0)-13(+6) weeks). A first-degree correlation was found to exist between the gestational age and maxillary length (r = 0.596; p < 0.0001; y = 1.491 × GA - 10.523) as well as mandibular length (r = 0.465; p < 0.0001; y = 1.050 × GA - 6.50). CONCLUSION: Normative data for ultrasonographic measurements of maxillary and mandibular lengths within the Taiwanese population were presented. Our data can serve as a reference value in congenital anomaly screening during prenatal examination.

Study motor skill learning by single-pellet reaching tasks in mice.

Reaching for and retrieving objects require precise and coordinated motor movements in the forelimb. When mice are repeatedly trained to grasp and retrieve food rewards positioned at a specific location, their motor performance (defined as accuracy and speed) improves progressively over time, and plateaus after persistent training. Once such reaching skill is mastered, its further maintenance does not require constant practice. Here we introduce a single-pellet reaching task to study the acquisition and maintenance of skilled forelimb movements in mice. In this video, we first describe the behaviors of mice that are commonly encountered in this learning and memory paradigm, and then discuss how to categorize these behaviors and quantify the observed results. Combined with mouse genetics, this paradigm can be utilized as a behavioral platform to explore the anatomical underpinnings, physiological properties, and molecular mechanisms of learning and memory.

Intrinsic properties of and thalamocortical inputs onto identified corticothalamic-VPM neurons.

Corticothalamic (CT) feedback plays an important role in regulating the sensory information that the cortex receives. Within the somatosensory cortex layer VI originates the feedback to the ventral posterior medial (VPM) nucleus of the thalamus, which in turn receives sensory information from the contralateral whiskers. We examined the physiology and morphology of CT neurons in rat somatosensory cortex, focusing on the physiological characteristics of the monosynaptic inputs that they receive from the thalamus. To identify CT neurons, rhodamine microspheres were injected into VPM and allowed to retrogradely transport to the soma of CT neurons. Thalamocortical slices were prepared at least 3 days post injection. Whole-cell recordings from labeled CT cells in layer VI demonstrated that they are regular spiking neurons and exhibit little spike frequency adaption. Two anatomical classes were identified based on their apical dendrites that either terminated by layer V (compact cells) or layer IV (elaborate cells). Thalamic inputs onto identified CT-VPM neurons demonstrated paired pulse depression over a wide frequency range (2-20 Hz). Stimulus trains also resulted in significant synaptic depression above 10 Hz. Our results suggest that thalamic inputs differentially impact CT-VPM neurons in layer VI. This characteristic may allow them to differentiate a wide range of stimulation frequencies which in turn further tune the feedback signals to the thalamus.

Alterations of dendritic protrusions over the first postnatal year of a mouse: an analysis in layer VI of the barrel cortex.

Dendritic spines are small protrusions that serve as the principal recipients of excitatory inputs onto cortical pyramidal cells. Alterations in spine and filopodia density and morphology correlate with both developmental maturity and changes in synaptic strength. In order to better understand the developmental profile of dendritic protrusion (dendritic spines + filopodia) morphology and density over the animal's first postnatal year, we used the Golgi staining technique to label neurons and their dendritic protrusions in mice. We focused on quantifying the density per length of dendrite and categorizing the morphology of dendritic protrusions of layer VI pyramidal neurons residing in barrel cortex using the computer assisted reconstruction program Neurolucida. We classified dendritic protrusion densities at seven developmental time points: postnatal day (PND) 15, 30, 60, 90, 180, 270, and 360. Our findings suggest that the dendritic protrusions in layer VI barrel cortex pyramidal neurons are not static, and their density as well as relative morphological distribution change over time. We observed a significant increase in mushroom spines and a decrease in filopodia as the animals matured. Further analyses show that as the animal mature there was a reduction in pyramidal cell dendritic lengths overall, as well as a decrease in overall protrusion densities. The ratio of apical to basilar density decreased as well. Characterizing the profile of cortical layer VI dendritic protrusions within the first postnatal year will enable us to better understand the relationship between the overall developmental maturation profile and dendritic spine functioning.

mGluR5 knockout mice display increased dendritic spine densities.

Alterations in dendritic spine densities and morphologies have been correlated with the abnormal functioning of the synapse. Specifically the metabotropic glutamate receptor 5 (mGluR5) has been implicated in dendrogenesis and spineogenesis, since its activation triggers various signaling cascades that have been demonstrated to play roles in synaptic maturation and plasticity. Here we used the Golgi impregnation technique to analyze the dendritic spines of mGluR5(-/-) knockout mice in comparison to their heterozygote mGluR5(+/-) littermates. mGluR5(-/-) mice had elevated spine densities irrespective of spine type or location along their dendritic trees in comparison to mGluR5(+/-) animals. Such anatomical changes may underlie the hyperexcitability observed in mGluR5 total knockout mice.

Ductus venosus Doppler velocimetry in normal pregnancies from 11 to 13 + 6 weeks' gestation - a Taiwanese study.

BACKGROUND: To investigate flow in the ductus venosus at 11-13 + 6 weeks of gestation in women with normal pregnancies in the Taiwanese population. METHODS: Two hundred and fifty-two normal singleton pregnancies with gestational ages ranging from 11 to 13 + 6 weeks were examined in this study. The pulsatility index for veins (PIV), resistance index (RI), peak velocity during ventricular systole (S-wave), and peak velocity during ventricular diastole (D-wave) were recorded from the ductus venosus. RESULTS: We analyzed 252 participants who all fulfilled the inclusion and exclusion criteria of our study. The mean maternal age was 31 (range 19-45 years), with a corresponding gestational age of 12 + 4 weeks (range 11-13 + 6). No significant change was found in the vascular indices as gestational age increased for the S-wave (S-wave = 1.4214 (GA) + 17.448, r = 0.09, P = 0.154), PIV (PIV = -0.0358 (GA) + 1.4143, r = -0.05, P = 0.378) and RI (RI = -0.035 (GA) + 1.1478, r = -0.064, P = 0.468). In contrast, the D-wave behaved differently from the other variables. There was a significant increase (r = 0.155, P = 0.013) in the D-wave with gestational age (D-wave = 1.4896 (GA) - 7.1547). CONCLUSION: D-wave velocity in the ductus venosus increased with gestational age. S-wave peak velocity showed an increasing trend and PIV showed a decreasing trend with gestational age, but they did not reach statistical significance.

Sensory deprivation differentially impacts the dendritic development of pyramidal versus non-pyramidal neurons in layer 6 of mouse barrel cortex.

Early postnatal sensory experience can have profound impacts on the structure and function of cortical circuits affecting behavior. Using the mouse whisker-to-barrel system we chronically deprived animals of normal sensory experience by bilaterally trimming their whiskers every other day from birth for the first postnatal month. Brain tissue was then processed for Golgi staining and neurons in layer 6 of barrel cortex were reconstructed in three dimensions. Dendritic and somatic parameters were compared between sensory-deprived and normal sensory experience groups. Results demonstrated that layer 6 non-pyramidal neurons in the chronically deprived group showed an expansion of their dendritic arbors. The pyramidal cells responded to sensory deprivation with increased somatic size and basilar dendritic arborization but overall decreased apical dendritic parameters. In sum, sensory deprivation impacted on the neuronal architecture of pyramidal and non-pyramidal neurons in layer 6, which may provide a substrate for observed physiological and behavioral changes resulting from whisker trimming.