Transcranial fluorescence imaging of auditory cortical plasticity regulated by acoustic environments in mice

Multistimuli-Responsive Chromism of Vinylene-Linked Bisflavin Based on the Aggregation and Redox Properties

Multistimuli-responsive chromism was observed for vinylene-linked bisflavin 1 a with an extended π-conjugated platform. The yellow emission of a dilute solution of 1 a in CHCl₃ (0.2 mM) observed at 298 K under UV excitation was changed to orange or red emission upon (1) an increase of concentration, (2) a decrease of temperature, and (3) variation of the solvent. This is in contrast to the almost non stimuli-responsive chromism of the N-methylated bisflavin analogue 1 b and monoflavin 2 a. Mechanistic investigation by ¹ H NMR analysis under various conditions revealed that the extended π-conjugation platform and imide moiety of 1 a generate controllability in the formation of lower- and higher-ordered aggregates, which induce variation of the emission color upon change. Bisflavin 1 a also exhibited redox-induced chromism, where the orange emission of 1 a was quenched by the addition of hydrazine under anaerobic conditions, and changed back to the original emission upon subsequent bubbling of O₂ gas.

Transcranial cortex-wide Ca2+ imaging for the functional mapping of cortical dynamics

Visualization and tracking of the information flow in the broader brain area are essential because nerve cells make a vast network in the brain. Fluorescence Ca²⁺ imaging is a simultaneous visualization of brain cell activities in a wide area. Instead of classical chemical indicators, developing various types of transgenic animals that express Ca²⁺-sensitive fluorescent proteins enables us to observe brain activities in living animals at a larger scale for a long time. Multiple kinds of literature have reported that transcranial imaging of such transgenic animals is practical for monitoring the wide-field information flow across the broad brain regions, although it has a lower spatial resolution. Notably, this technique is helpful for the initial evaluation of cortical function in disease models. This review will introduce fully intact transcranial macroscopic imaging and cortex-wide Ca²⁺ imaging as practical applications.

Auditory independent low-intensity ultrasound stimulation of mouse brain is associated with neuronal ERK phosphorylation and an increase of Tbr2 marked neuroprogenitors

Transcranial ultrasound stimulation is an emerging technique for the development of a non-invasive neuromodulation device for the treatment of various types of neurodegenerations and brain damages. However, there are very few studies that have quantified the optimal ultrasound dosage and the long-term associated effects of transcranial ultrasound treatments of brain diseases. In this study, we used a simple ex vivo hippocampal tissues stimulated by different dosages of ultrasound in combination with different chemical treatments to quantify the required energy for a measurable effect. After determining the most desirable ex vivo stimulation conditions, it was then replicated for the in vivo mouse brains. It was discovered that transcranial ultrasound promoted the increase of Tbr2-expressing neural progenitors in an ASIC1a-dependent manner. Furthermore, such effect was observable at least a week after the initial ultrasound treatments and was not abolished by auditory toxicity.

Characteristic Effects of the Cardiac Non-Neuronal Acetylcholine System Augmentation on Brain Functions

Since the discovery of non-neuronal acetylcholine in the heart, this specific system has drawn scientific interest from many research fields, including cardiology, immunology, and pharmacology. This system, acquired by cardiomyocytes independent of the parasympathetic nervous system of the autonomic nervous system, helps us to understand unsolved issues in cardiac physiology and to realize that the system may be more pivotal for cardiac homeostasis than expected. However, it has been shown that the effects of this system may not be restricted to the heart, but rather extended to cover extra-cardiac organs. To this end, this system intriguingly influences brain function, specifically potentiating blood brain barrier function. Although the results reported appear to be unusual, this novel characteristic can provide us with another research interest and therapeutic application mode for central nervous system diseases. In this review, we discuss our recent studies and raise the possibility of application of this system as an adjunctive therapeutic modality.

Postnatal development of subfields in the core region of the mouse auditory cortex

The core region of the rodent auditory cortex has two subfields: the primary auditory area (A1) and the anterior auditory field (AAF). Although the postnatal development of A1 has been studied in several mammalian species, few studies have been conducted on the postnatal development of AAF. Using a voltage-sensitive-dye-based imaging method, we examined and compared the postnatal development of AAF and A1 in mice from postnatal day 11 (P11) to P40. We focused on the postnatal development of tonotopy, the relative position between A1 and AAF, and the properties of tone-evoked responses in the subfields. Tone-evoked responses in the mouse auditory cortex were first observed at P12, and tonotopy was found in both A1 and AAF at this age. Quantification of tonotopy using the cortical magnification factor (CMF; octave difference per unit cortical distance) revealed a rapid change from P12 to P14 in both A1 and AAF, and a stable level from P14. A similar time course of postnatal development was found for the distance between the 4 kHz site in A1 and AAF, the distance between the 16 kHz site in A1 and AAF, and the angle between the frequency axis of A1 and AAF. The maximum amplitude and rise time of tone-evoked signals in both A1 and AAF showed no significant change from P12 to P40, but the latency of the responses to both the 4 kHz and 16 kHz tones decreased during this period, with a more rapid decrease in the latency to 16 kHz tones in both subfields. The duration of responses evoked by 4 kHz tones in both A1 and AAF showed no significant postnatal change, but the duration of responses to 16 kHz tones decreased exponentially in both subfields. The cortical area activated by 4 kHz tones in AAF was always larger than that in A1 at all ages (P12-P40). Our results demonstrated that A1 and AAF developed in parallel postnatally, showing a rapid maturation of tonotopy, slow maturation of response latency and response duration, and a dorsal-to-ventral order (high-frequency site to low-frequency site) of functional maturation.

Auditory cortical activity elicited by infrared laser irradiation from the outer ear in Mongolian gerbils

Infrared neural stimulation has been studied for its potential to replace an electrical stimulation of a cochlear implant. No studies, however, revealed how the technic reliably evoke auditory cortical activities. This research investigated the effects of cochlear laser stimulation from the outer ear on auditory cortex using brain imaging of activity-dependent changes in mitochondrial flavoprotein fluorescence signal. An optic fiber was inserted into the gerbil’s ear canal to stimulate the lateral side of the cochlea with an infrared laser. Laser stimulation was found to activate the identified primary auditory cortex. In addition, the temporal profile of the laser-evoked responses was comparable to that of the auditory responses. Our results indicate that infrared laser irradiation from the outer ear has the capacity to evoke, and possibly manipulate, the neural activities of the auditory cortex and may substitute for the present cochlear implants in future.

The cellular and molecular basis of in vivo synaptic plasticity in rodents

Plasticity within the neuronal networks of the brain underlies the ability to learn and retain new information. The initial discovery of synaptic plasticity occurred by measuring synaptic strength in vivo, applying external stimulation and observing an increase in synaptic strength termed long-term potentiation (LTP). Many of the molecular pathways involved in LTP and other forms of synaptic plasticity were subsequently uncovered in vitro. Over the last few decades, technological advances in recording and imaging in live animals have seen many of these molecular mechanisms confirmed in vivo, including structural changes both pre- and postsynaptically, changes in synaptic strength, and changes in neuronal excitability. A well-studied aspect of neuronal plasticity is the capacity of the brain to adapt to its environment, gained by comparing the brains of deprived and experienced animals in vivo, and in direct response to sensory stimuli. Multiple in vivo studies have also strongly linked plastic changes to memory by interfering with the expression of plasticity and by manipulating memory engrams. Plasticity in vivo also occurs in the absence of any form of external stimulation, i.e., during spontaneous network activity occurring with brain development. However, there is still much to learn about how plasticity is induced during natural learning and how this is altered in neurological disorders.

Evidence of Hyperacusis in Adult Rats Following Non-traumatic Sound Exposure

Manipulations that enhance neuroplasticity may inadvertently create opportunities for maladaptation. We have previously used passive exposures to non-traumatic white noise to open windows of plasticity in the adult rat auditory cortex and induce frequency-specific functional reorganizations of the tonotopic map. However, similar reorganizations in the central auditory pathway are thought to contribute to the generation of hearing disorders such as tinnitus and hyperacusis. Here, we investigate whether noise-induced reorganizations are accompanied by electrophysiological or behavioral evidence of tinnitus or hyperacusis in adult Long-Evans rats. We used a 2-week passive exposure to moderate-intensity (70 dB SPL) broadband white noise to reopen a critical period for spectral tuning such that a second 1-week exposure to 7 kHz tone pips produced an expansion of the 7 kHz frequency region in the primary auditory cortex (A1). We demonstrate for the first time that this expansion also takes place in the ventral auditory field (VAF). Sound exposure also led to spontaneous and sound-evoked hyperactivity in the anterior auditory field (AAF). Rats were assessed for behavioral evidence of tinnitus or hyperacusis using gap and tone prepulse inhibition of the acoustic startle response. We found that sound exposure did not affect gap-prepulse inhibition. However, sound exposure led to an improvement in prepulse inhibition when the prepulse was a 7 kHz tone, showing that exposed rats had enhanced sensorimotor gating for the exposure frequency. Together, our electrophysiological and behavioral results provide evidence of hyperacusis but not tinnitus in sound-exposed animals. Our findings demonstrate that periods of prolonged noise exposure may open windows of plasticity that can also be understood as windows of vulnerability, potentially increasing the likelihood for maladaptive plasticity to take place.

Associative responses to visual shape stimuli in the mouse auditory cortex

Humans can recall various aspects of a characteristic sound as a whole when they see a visual shape stimulus that has been intimately associated with the sound. In subjects with audio-visual associative memory, auditory responses that code the associated sound may be induced in the auditory cortex in response to presentation of the associated visual shape stimulus. To test this possibility, mice were pre-exposed to a combination of an artificial sound mimicking a cat’s “meow” and a visual shape stimulus of concentric circles or stars for more than two weeks, since such passive exposure is known to be sufficient for inducing audio-visual associative memory in mice. After the exposure, we anesthetized the mice, and presented them with the associated visual shape stimulus. We found that associative responses in the auditory cortex were induced in response to the visual stimulus. The associative auditory responses were observed when complex sounds such as “meow” were used for formation of audio-visual associative memory, but not when a pure tone was used. These results suggest that associative auditory responses in the auditory cortex represent the characteristics of the complex sound stimulus as a whole.

Potentiating a non-neuronal cardiac cholinergic system reinforces the functional integrity of the blood brain barrier associated with systemic anti-inflammatory responses

We previously reported that the heart-specific choline acetyltransferase (ChAT) gene overexpressing mice (ChAT tg) show specific phenotypes including ischemic tolerance and the CNS stress tolerance. In the current study, we focused on molecular mechanisms responsible for systemic and localized anti-inflammatory phenotypes of ChAT tg. ChAT tg were resistant to systemic inflammation induced by lipopolysaccharides due to an attenuated cytokine response. In addition, ChAT tg, originally equipped with less reactive Kupffer cells, were refractory to brain cold injury, with decreased blood brain barrier (BBB) permeability and reduced inflammation. This is because ChAT tg brain endothelial cells expressed more claudin-5, and their astrocytes were less reactive, causing decreased hypertrophy. Moreover, reconstruction of the BBB integrity in vitro confirmed the consolidation of ChAT tg. ChAT tg were also resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neuronal toxicity due to lower mortality rate and neuronal loss of substantia nigra. Additionally, ChAT tg subjected to MPTP showed attenuated BBB disruption, as evident from reduced sodium fluorescein levels in the brain parenchyma. The activated central cholinergic pathway of ChAT tg lead to anti-convulsive effects like vagus nerve stimulation. However, DSP-4, a noradrenergic neuron-selective neurotoxin against the CNS including the locus ceruleus, abrogated the beneficial phenotype and vagotomy attenuated expression of claudin-5, suggesting the link between the cholinergic pathway and BBB function. Altogether, these findings indicate that ChAT tg possess an anti-inflammatory response potential, associated with upregulated claudin-5, leading to the consolidation of BBB integrity. These characteristics protect ChAT tg against systemic and localized inflammatory pathological disorders, which targets the CNS.

Acute spatial spread of NO-mediated potentiation during hindpaw ischaemia in mice

Key points

Neuropathic pain spreads spatially beyond the injured sites, and the mechanism underlying the spread has been attributed to inflammation occurring in the spinal cord.

However, the spatial spread of spinal/cortical potentiation induced by conduction block of the peripheral nerves can be observed prior to inflammation.

In the present study, we found that spreading potentiation and hypersensitivity acutely induced by unilateral hindpaw ischaemia are nitric oxide (NO)‐dependent and that NO is produced by ischaemia and quickly diffuses within the spinal cord.

We also found that NO production induced by ischaemia is not observed in the presence of an antagonist for group II metabotropic glutamate receptors (mGluRs) and that neuronal NO synthase‐positive dorsal horn neurons express group II mGluRs.

These results suggest strongly that NO‐mediated spreading potentiation in the spinal cord is one of the trigger mechanisms for neuropathic pain.

Abstract

Cortical/spinal responses to hindpaw stimulation are bilaterally potentiated by unilateral hindpaw ischaemia in mice. We tested the hypothesis that hindpaw ischaemia produces nitric oxide (NO), which diffuses in the spinal cord to induce spatially spreading potentiation. Using flavoprotein fluorescence imaging, we confirmed that the spreading potentiation in hindpaw responses was induced during ischaemia in the non‐stimulated hindpaw. This spreading potentiation was blocked by spinal application of l‐NAME, an inhibitor of NO synthase (NOS). Furthermore, no spreading potentiation was observed in neural NOS (nNOS) knockout mice. Spinal application of an NO donor was enough to induce cortical potentiation and mechanical hypersensitivity. The spatial distribution of NO during unilateral hindpaw ischaemia was visualized using 4‐amino‐5‐methylamino‐2′,7′‐difluorofluorescein (DAF‐FM). An increase in fluorescence derived from the complex of DAF‐FM with NO was observed on the ischaemic side of the spinal cord. A similar but smaller increase was also observed on the contralateral side. Somatosensory potentiation after hindpaw ischaemia is known to be inhibited by spinal application of LY354740, an agonist of group II metabotropic glutamate receptors (mGluRs). We confirmed that the spinal DAF‐FM fluorescence increases during hindpaw ischaemia were not observed in the presence of LY354740. We also confirmed that approximately half of the nNOS‐positive neurons in the superficial laminae of the dorsal horn expressed mGluR2 mRNA. These results suggest that disinhibition of mGluR2 produces NO which in turn induces a spreading potentiation in a wide area of the spinal cord. Such spreading, along with the consequent non‐specific potentiation in the spinal cord, may trigger neuropathic pain.

Neuropathic pain spreads spatially beyond the injured sites, and the mechanism underlying the spread has been attributed to inflammation occurring in the spinal cord.

However, the spatial spread of spinal/cortical potentiation induced by conduction block of the peripheral nerves can be observed prior to inflammation.

In the present study, we found that spreading potentiation and hypersensitivity acutely induced by unilateral hindpaw ischaemia are nitric oxide (NO)‐dependent and that NO is produced by ischaemia and quickly diffuses within the spinal cord.

We also found that NO production induced by ischaemia is not observed in the presence of an antagonist for group II metabotropic glutamate receptors (mGluRs) and that neuronal NO synthase‐positive dorsal horn neurons express group II mGluRs.

These results suggest strongly that NO‐mediated spreading potentiation in the spinal cord is one of the trigger mechanisms for neuropathic pain.

Developmental deprivation-induced perceptual and cortical processing deficits in awake-behaving animals

Sensory deprivation during development induces lifelong changes to central nervous system function that are associated with perceptual impairments. However, the relationship between neural and behavioral deficits is uncertain due to a lack of simultaneous measurements during task performance. Therefore, we telemetrically recorded from auditory cortex neurons in gerbils reared with developmental conductive hearing loss as they performed an auditory task in which rapid fluctuations in amplitude are detected. These data were compared to a measure of auditory brainstem temporal processing from each animal. We found that developmental HL diminished behavioral performance, but did not alter brainstem temporal processing. However, the simultaneous assessment of neural and behavioral processing revealed that perceptual deficits were associated with a degraded cortical population code that could be explained by greater trial-to-trial response variability. Our findings suggest that the perceptual limitations that attend early hearing loss are best explained by an encoding deficit in auditory cortex.

Brief Stimulus Exposure Fully Remediates Temporal Processing Deficits Induced by Early Hearing Loss

In childhood, partial hearing loss can produce prolonged deficits in speech perception and temporal processing. However, early therapeutic interventions targeting temporal processing may improve later speech-related outcomes. Gap detection is a measure of auditory temporal resolution that relies on the auditory cortex (ACx), and early auditory deprivation alters intrinsic and synaptic properties in the ACx. Thus, early deprivation should induce deficits in gap detection, which should be reflected in ACx gap sensitivity. We tested whether earplugging-induced, early transient auditory deprivation in male and female Mongolian gerbils caused correlated deficits in behavioral and cortical gap detection, and whether these could be rescued by a novel therapeutic approach: brief exposure to gaps in background noise. Two weeks after earplug removal, animals that had been earplugged from hearing onset throughout auditory critical periods displayed impaired behavioral gap detection thresholds (GDTs), but this deficit was fully reversed by three 1 h sessions of exposure to gaps in noise. In parallel, after earplugging, cortical GDTs increased because fewer cells were sensitive to short gaps, and gap exposure normalized this pattern. Furthermore, in deprived animals, both first-spike latency and first-spike latency jitter increased, while spontaneous and evoked firing rates decreased, suggesting that deprivation causes a wider range of perceptual problems than measured here. These cortical changes all returned to control levels after gap exposure. Thus, brief stimulus exposure, perhaps in a salient context such as the unfamiliar placement into a testing apparatus, rescued impaired gap detection and may have potential as a remediation tool for general auditory processing deficits.SIGNIFICANCE STATEMENT Hearing loss in early childhood leads to impairments in auditory perception and language processing that can last well beyond the restoration of hearing sensitivity. Perceptual deficits can be improved by training, or by acoustic enrichment in animal models, but both approaches involve extended time and effort. Here, we used a novel remediation technique, brief periods of auditory stimulus exposure, to fully remediate cortical and perceptual deficits in gap detection induced by early transient hearing loss. This technique also improved multiple cortical response properties. Rescue by this efficient exposure regime may have potential as a therapeutic tool to remediate general auditory processing deficits in children with perceptual challenges arising from early hearing loss.

Using Intrinsic Flavoprotein and NAD(P)H Imaging to Map Functional Circuitry in the Main Olfactory Bulb

Neurons exhibit strong coupling of electrochemical and metabolic activity. Increases in intrinsic fluorescence from either oxidized flavoproteins or reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] in the mitochondria have been used as an indicator of neuronal activity for the functional mapping of neural circuits. However, this technique has not been used to investigate the flow of olfactory information within the circuitry of the main olfactory bulb (MOB). We found that intrinsic flavoprotein fluorescence signals induced by electrical stimulation of single glomeruli displayed biphasic responses within both the glomerular (GL) and external plexiform layers (EPL) of the MOB. Pharmacological blockers of mitochondrial activity, voltage-gated Na⁺ channels, or ionotropic glutamate receptors abolished stimulus-dependent flavoprotein responses. Blockade of GABA_A receptors enhanced the amplitude and spatiotemporal spread of the flavoprotein signals, indicating an important role for inhibitory neurotransmission in shaping the spread of neural activity in the MOB. Stimulus-dependent spread of fluorescence across the GL and EPL displayed a spatial distribution consistent with that of individual glomerular microcircuits mapped by neuroanatomic tract tracing. These findings demonstrated the feasibility of intrinsic fluorescence imaging in the olfactory systems and provided a new tool to examine the functional circuitry of the MOB.

Developmental hearing loss impedes auditory task learning and performance in gerbils

The consequences of developmental hearing loss have been reported to include both sensory and cognitive deficits. To investigate these issues in a non-human model, auditory learning and asymptotic psychometric performance were compared between normal hearing (NH) adult gerbils and those reared with conductive hearing loss (CHL). At postnatal day 10, before ear canal opening, gerbil pups underwent bilateral malleus removal to induce a permanent CHL. Both CHL and control animals were trained to approach a water spout upon presentation of a target (Go stimuli), and withhold for foils (Nogo stimuli). To assess the rate of task acquisition and asymptotic performance, animals were tested on an amplitude modulation (AM) rate discrimination task. Behavioral performance was calculated using a signal detection theory framework. Animals reared with developmental CHL displayed a slower rate of task acquisition for AM discrimination task. Slower acquisition was explained by an impaired ability to generalize to newly introduced stimuli, as compared to controls. Measurement of discrimination thresholds across consecutive testing blocks revealed that CHL animals required a greater number of testing sessions to reach asymptotic threshold values, as compared to controls. However, with sufficient training, CHL animals approached control performance. These results indicate that a sensory impediment can delay auditory learning, and increase the risk of poor performance on a temporal task.

Auditory cortical field coding long-lasting tonal offsets in mice

Although temporal information processing is important in auditory perception, the mechanisms for coding tonal offsets are unknown. We investigated cortical responses elicited at the offset of tonal stimuli using flavoprotein fluorescence imaging in mice. Off-responses were clearly observed at the offset of tonal stimuli lasting for 7 s, but not after stimuli lasting for 1 s. Off-responses to the short stimuli appeared in a similar cortical region, when conditioning tonal stimuli lasting for 5–20 s preceded the stimuli. MK-801, an inhibitor of NMDA receptors, suppressed the two types of off-responses, suggesting that disinhibition produced by NMDA receptor-dependent synaptic depression might be involved in the off-responses. The peak off-responses were localized in a small region adjacent to the primary auditory cortex, and no frequency-dependent shift of the response peaks was found. Frequency matching of preceding tonal stimuli with short test stimuli was not required for inducing off-responses to short stimuli. Two-photon calcium imaging demonstrated significantly larger neuronal off-responses to stimuli lasting for 7 s in this field, compared with off-responses to stimuli lasting for 1 s. The present results indicate the presence of an auditory cortical field responding to long-lasting tonal offsets, possibly for temporal information processing.

Non-neuronal cardiac cholinergic system influences CNS via the vagus nerve to acquire a stress-refractory propensity

We previously developed cardiac ventricle-specific choline acetyltransferase (ChAT) gene-overexpressing transgenic mice (ChAT tgm), i.e. an in vivo model of the cardiac non-neuronal acetylcholine (NNA) system or non-neuronal cardiac cholinergic system (NNCCS). By using this murine model, we determined that this system was responsible for characteristics of resistance to ischaemia, or hypoxia, via the modulation of cellular energy metabolism and angiogenesis. In line with our previous study, neuronal ChAT-immunoreactivity in the ChAT tgm brains was not altered from that in the wild-type (WT) mice brains; in contrast, the ChAT tgm hearts were the organs with the highest expression of the ChAT transgene. ChAT tgm showed specific traits in a central nervous system (CNS) phenotype, including decreased response to restraint stress, less depressive-like and anxiety-like behaviours and anti-convulsive effects, all of which may benefit the heart. These phenotypes, induced by the activation of cardiac NNCCS, were dependent on the vagus nerve, because vagus nerve stimulation (VS) in WT mice also evoked phenotypes similar to those of ChAT tgm, which display higher vagus nerve discharge frequency; in contrast, lateral vagotomy attenuated these traits in ChAT tgm to levels observed in WT mice. Furthermore, ChAT tgm induced several biomarkers of VS responsible for anti-convulsive and anti-depressive-like effects. These results suggest that the augmentation of the NNCCS transduces an effective and beneficial signal to the afferent pathway, which mimics VS. Therefore, the present study supports our hypothesis that activation of the NNCCS modifies CNS to a more stress-resistant state through vagus nerve activity.

Cerebellar Nuclei Neurons Show Only Small Excitatory Responses to Optogenetic Olivary Stimulation in Transgenic Mice: In Vivo and In Vitro Studies

To study the olivary input to the cerebellar nuclei (CN) we used optogenetic stimulation in transgenic mice expressing channelrhodopsin-2 (ChR2) in olivary neurons. We obtained in vivo extracellular Purkinje cell (PC) and CN recordings in anesthetized mice while stimulating the contralateral inferior olive (IO) with a blue laser (single pulse, 10-50 ms duration). Peri-stimulus histograms (PSTHs) were constructed to show the spike rate changes after optical stimulation. Among 29 CN neurons recorded, 15 showed a decrease in spike rate of variable strength and duration, and only 1 showed a transient spiking response. These results suggest that direct olivary input to CN neurons is usually overridden by stronger PC inhibition triggered by climbing fiber responses. To further investigate the direct input from the climbing fiber collaterals we also conducted whole cell recordings in brain slices, where we used local stimulation with blue light. Due to the expression of ChR2 in PC axons as well as the IO in our transgenic line, strong inhibitory responses could be readily triggered with optical stimulation (13 of 15 neurons). After blocking this inhibition with GABAzine, only in 5 of 13 CN neurons weak excitatory responses were revealed. Therefore our in vitro results support the in vivo findings that the excitatory input to CN neurons from climbing fiber collaterals in adult mice is masked by the inhibition under normal conditions.

Specific distribution of non-phosphorylated neurofilaments characterizing each subfield in the mouse auditory cortex

Recent imaging studies revealed the presence of functional subfields in the mouse auditory cortex. However, little is known regarding the morphological basis underlying the functional differentiation. Distribution of particular molecules is the key information that may be applicable for identifying auditory subfields in the post-mortem brain. Immunoreactive patterns using SMI-32 monoclonal antibody against non-phosphorylated neurofilament (NNF) have already been used to identify or parcellate various brain regions in various animals. In the present study, we investigated whether distribution of NNF is a reliable marker for identifying functional subfields in the mouse auditory cortex, and found that each auditory subfield has region-specific cellular and laminar patterns of immunoreactivity for NNF.

Spinal mechanisms underlying potentiation of hindpaw responses observed after transient hindpaw ischemia in mice

Transient ischemia produces postischemic tingling sensation. Ischemia also produces nerve conduction block that may modulate spinal neural circuits. In the present study, reduced mechanical thresholds for hindpaw-withdrawal reflex were found in mice after transient hindpaw ischemia, which was produced by a high pressure applied around the hindpaw for 30 min. The reduction in the threshold was blocked by spinal application of LY354740, a specific agonist of group II metabotropic glutamate receptors. Neural activities in the spinal cord and the primary somatosensory cortex (S1) were investigated using activity-dependent changes in endogenous fluorescence derived from mitochondrial flavoproteins. Ischemic treatment induced potentiation of the ipsilateral spinal and contralateral S1 responses to hindpaw stimulation. Both types of potentiation were blocked by spinal application of LY354740. The contralateral S1 responses, abolished by lesioning the ipsilateral dorsal column, reappeared after ischemic treatment, indicating that postischemic tingling sensation reflects a sensory modality shift from tactile sensation to nociception in the spinal cord. Changes in neural responses were investigated during ischemic treatment in the contralateral spinal cord and the ipsilateral S1. Potentiation already appeared during ischemic treatment for 30 min. The present findings suggest that the postischemic potentiation shares spinal mechanisms, at least in part, with neuropathic pain.

Highly fluorescent flavins: rational molecular design for quenching protection based on repulsive and attractive control of molecular alignment

Unprecedented intense fluorescent emission was observed for a variety of flavin compounds bearing a perpendicular cyclic imide moiety at the C7 position of an isoalloxazine platform. A series of alloxan-substituted flavins was prepared selectively by reduction of the corresponding N-aryl-2-nitro-5-alkoxyanilines with zinc dust and subsequent reaction with alloxan monohydrate in the presence of boric acid. Analogues bearing oxazolidine-2,4-dione functionality were obtained on methylation of the alloxan-substituted flavins with methyl iodide and subsequent rearrangement in the presence of an inorganic base. The flavin compounds exhibit intense white-green fluorescent emission in the solution state under UV excitation at 298 K, with emission efficiencies Φ298 K greater than 0.55 in CH3 CN, which are higher than the values for all reported flavin compounds under similar conditions. The highest Φ298 K value of 0.70 was obtained in CH3 CN for isoalloxazine bearing C7-alloxan and N10-2,6-diisopropylphenyl groups. The temperature dependence of the emission intensities indicates that the pronounced emission properties at 298 K are attributable to the highly heat resistant properties towards emission decay with increasing temperature. Mechanistic studies, including X-ray diffraction analysis, revealed that the good emission properties and high heat resistance of the alloxan-substituted flavins are due to a synergetic effect of the associative nature of the C7-alloxan unit and the repulsive nature of the perpendicular bulky substituents at the C7 and N10 positions.

Cell-specific activity-dependent fractionation of layer 2/3→5B excitatory signaling in mouse auditory cortex

Auditory cortex (AC) layer 5B (L5B) contains both corticocollicular neurons, a type of pyramidal-tract neuron projecting to the inferior colliculus, and corticocallosal neurons, a type of intratelencephalic neuron projecting to contralateral AC. Although it is known that these neuronal types have distinct roles in auditory processing and different response properties to sound, the synaptic and intrinsic mechanisms shaping their input-output functions remain less understood. Here, we recorded in brain slices of mouse AC from retrogradely labeled corticocollicular and neighboring corticocallosal neurons in L5B. Corticocollicular neurons had, on average, lower input resistance, greater hyperpolarization-activated current (Ih), depolarized resting membrane potential, faster action potentials, initial spike doublets, and less spike-frequency adaptation. In paired recordings between single L2/3 and labeled L5B neurons, the probabilities of connection, amplitude, latency, rise time, and decay time constant of the unitary EPSC were not different for L2/3→corticocollicular and L2/3→corticocallosal connections. However, short trains of unitary EPSCs showed no synaptic depression in L2/3→corticocollicular connections, but substantial depression in L2/3→corticocallosal connections. Synaptic potentials in L2/3→corticocollicular connections decayed faster and showed less temporal summation, consistent with increased Ih in corticocollicular neurons, whereas synaptic potentials in L2/3→corticocallosal connections showed more temporal summation. Extracellular L2/3 stimulation at two different rates resulted in spiking in L5B neurons; for corticocallosal neurons the spike rate was frequency dependent, but for corticocollicular neurons it was not. Together, these findings identify cell-specific intrinsic and synaptic mechanisms that divide intracortical synaptic excitation from L2/3 to L5B into two functionally distinct pathways with different input-output functions.

Delineation of a frequency-organized region isolated from the mouse primary auditory cortex

The primary auditory cortex (AI) is the representative recipient of information from the ears in the mammalian cortex. However, the delineation of the AI is still controversial in a mouse. Recently, it was reported, using optical imaging, that two distinct areas of the AI, located ventrally and dorsally, are activated by high-frequency tones, whereas only one area is activated by low-frequency tones. Here, we show that the dorsal high-frequency area is an independent region that is separated from the rest of the AI. We could visualize the two distinct high-frequency areas using flavoprotein fluorescence imaging, as reported previously. SMI-32 immunolabeling revealed that the dorsal region had a different cytoarchitectural pattern from the rest of the AI. Specifically, the ratio of SMI-32-positive pyramidal neurons to nonpyramidal neurons was larger in the dorsal high-frequency area than the rest of the AI. We named this new region the dorsomedial field (DM). Retrograde tracing showed that neurons projecting to the DM were localized in the rostral part of the ventral division of the medial geniculate body with a distinct frequency organization, where few neurons projected to the AI. Furthermore, the responses of the DM to ultrasonic courtship songs presented by males were significantly greater in females than in males; in contrast, there was no sex difference in response to artificial pure tones. Our findings offer a basic outline on the processing of ultrasonic vocal information on the basis of the precisely subdivided, multiple frequency-organized auditory cortex map in mice.

Spinal autofluorescent flavoprotein imaging in a rat model of nerve injury-induced pain and the effect of spinal cord stimulation

Nerve injury may cause neuropathic pain, which involves hyperexcitability of spinal dorsal horn neurons. The mechanisms of action of spinal cord stimulation (SCS), an established treatment for intractable neuropathic pain, are only partially understood. We used Autofluorescent Flavoprotein Imaging (AFI) to study changes in spinal dorsal horn metabolic activity. In the Seltzer model of nerve-injury induced pain, hypersensitivity was confirmed using the von Frey and hotplate test. 14 Days after nerve-injury, rats were anesthetized, a bipolar electrode was placed around the affected sciatic nerve and the spinal cord was exposed by a laminectomy at T13. AFI recordings were obtained in neuropathic rats and a control group of naïve rats following 10 seconds of electrical stimulation of the sciatic nerve at C-fiber strength, or following non-noxious palpation. Neuropathic rats were then treated with 30 minutes of SCS or sham stimulation and AFI recordings were obtained for up to 60 minutes after cessation of SCS/sham. Although AFI responses to noxious electrical stimulation were similar in neuropathic and naïve rats, only neuropathic rats demonstrated an AFI-response to palpation. Secondly, an immediate, short-lasting, but strong reduction in AFI intensity and area of excitation occurred following SCS, but not following sham stimulation. Our data confirm that AFI can be used to directly visualize changes in spinal metabolic activity following nerve injury and they imply that SCS acts through rapid modulation of nociceptive processing at the spinal level.

Multiscale optical Ca2+ imaging of tonal organization in mouse auditory cortex

Spatial patterns of functional organization, resolved by microelectrode mapping, comprise a core principle of sensory cortices. In auditory cortex, however, recent two-photon Ca2+ imaging challenges this precept, as the traditional tonotopic arrangement appears weakly organized at the level of individual neurons. To resolve this fundamental ambiguity about the organization of auditory cortex, we developed multiscale optical Ca2+ imaging of unanesthetized GCaMP transgenic mice. Single-neuron activity monitored by two-photon imaging was precisely registered to large-scale cortical maps provided by transcranial widefield imaging. Neurons in the primary field responded well to tones; neighboring neurons were appreciably cotuned, and preferred frequencies adhered tightly to a tonotopic axis. By contrast, nearby secondary-field neurons exhibited heterogeneous tuning. The multiscale imaging approach also readily localized vocalization regions and neurons. Altogether, these findings cohere electrode and two-photon perspectives, resolve new features of auditory cortex, and offer a promising approach generalizable to any cortical area.

Frequency transformation in the auditory lemniscal thalamocortical system

The auditory lemniscal thalamocortical (TC) pathway conveys information from the ventral division of the medial geniculate body to the primary auditory cortex (A1). Although their general topographic organization has been well characterized, functional transformations at the lemniscal TC synapse still remain incompletely codified, largely due to the need for integration of functional anatomical results with the variability observed with various animal models and experimental techniques. In this review, we discuss these issues with classical approaches, such as in vivo extracellular recordings and tracer injections to physiologically identified areas in A1, and then compare these studies with modern approaches, such as in vivo two-photon calcium imaging, in vivo whole-cell recordings, optogenetic methods, and in vitro methods using slice preparations. A surprising finding from a comparison of classical and modern approaches is the similar degree of convergence from thalamic neurons to single A1 neurons and clusters of A1 neurons, although, thalamic convergence to single A1 neurons is more restricted from areas within putative thalamic frequency lamina. These comparisons suggest that frequency convergence from thalamic input to A1 is functionally limited. Finally, we consider synaptic organization of TC projections and future directions for research.

Learning, neural plasticity and sensitive periods: implications for language acquisition, music training and transfer across the lifespan

Sensitive periods in human development have often been proposed to explain age-related differences in the attainment of a number of skills, such as a second language (L2) and musical expertise. It is difficult to reconcile the negative consequence this traditional view entails for learning after a sensitive period with our current understanding of the brain's ability for experience-dependent plasticity across the lifespan. What is needed is a better understanding of the mechanisms underlying auditory learning and plasticity at different points in development. Drawing on research in language development and music training, this review examines not only what we learn and when we learn it, but also how learning occurs at different ages. First, we discuss differences in the mechanism of learning and plasticity during and after a sensitive period by examining how language exposure versus training forms language-specific phonetic representations in infants and adult L2 learners, respectively. Second, we examine the impact of musical training that begins at different ages on behavioral and neural indices of auditory and motor processing as well as sensorimotor integration. Third, we examine the extent to which childhood training in one auditory domain can enhance processing in another domain via the transfer of learning between shared neuro-cognitive systems. Specifically, we review evidence for a potential bi-directional transfer of skills between music and language by examining how speaking a tonal language may enhance music processing and, conversely, how early music training can enhance language processing. We conclude with a discussion of the role of attention in auditory learning for learning during and after sensitive periods and outline avenues of future research.

Auditory cortical areas activated by slow frequency-modulated sounds in mice

Species-specific vocalizations in mice have frequency-modulated (FM) components slower than the lower limit of FM direction selectivity in the core region of the mouse auditory cortex. To identify cortical areas selective to slow frequency modulation, we investigated tonal responses in the mouse auditory cortex using transcranial flavoprotein fluorescence imaging. For differentiating responses to frequency modulation from those to stimuli at constant frequencies, we focused on transient fluorescence changes after direction reversal of temporally repeated and superimposed FM sweeps. We found that the ultrasonic field (UF) in the belt cortical region selectively responded to the direction reversal. The dorsoposterior field (DP) also responded weakly to the reversal. Regarding the responses in UF, no apparent tonotopic map was found, and the right UF responses were significantly larger in amplitude than the left UF responses. The half-max latency in responses to FM sweeps was shorter in UF compared with that in the primary auditory cortex (A1) or anterior auditory field (AAF). Tracer injection experiments in the functionally identified UF and DP confirmed that these two areas receive afferent inputs from the dorsal part of the medial geniculate nucleus (MG). Calcium imaging of UF neurons stained with fura-2 were performed using a two-photon microscope, and the presence of UF neurons that were selective to both direction and direction reversal of slow frequency modulation was demonstrated. These results strongly suggest a role for UF, and possibly DP, as cortical areas specialized for processing slow frequency modulation in mice.

Improved detection of electrical activity with a voltage probe based on a voltage-sensing phosphatase

  One of the most awaited techniques in modern physiology is the sensitive detection of spatiotemporal electrical activity in a complex network of excitable cells. The use of genetically encoded voltage probes has been expected to enable such analysis. However, in spite of recent progress, existing probes still suffer from low signal amplitude and/or kinetics too slow to detect fast electrical activity. Here, we have developed an improved voltage probe named Mermaid2, which is based on the voltage-sensor domain of the voltage-sensing phosphatase from Ciona intestinalis and Förster energy transfer between a pair of fluorescent proteins. In mammalian cells, Mermaid2 permits ratiometric readouts of fractional changes of more than 50% over a physiologically relevant voltage range with fast kinetics, and it was used to follow a train of action potentials at frequencies of up to 150 Hz. Mermaid2 was also able to detect single action potentials and subthreshold voltage responses in hippocampal neurons in vitro, in addition to cortical electrical activity evoked by sound stimuli in single trials in living mice.

Deep cerebellar nuclei play an important role in two-tone discrimination on delay eyeblink conditioning in C57BL/6 mice

Previous studies have shown that deep cerebellar nuclei (DCN)-lesioned mice develop conditioned responses (CR) on delay eyeblink conditioning when a salient tone conditioned stimulus (CS) is used, which suggests that the cerebellum potentially plays a role in more complicated cognitive functions. In the present study, we examined the role of DCN in tone frequency discrimination in the delay eyeblink-conditioning paradigm. In the first experiment, DCN-lesioned and sham-operated mice were subjected to standard simple eyeblink conditioning under low-frequency tone CS (LCS: 1 kHz, 80 dB) or high-frequency tone CS (HCS: 10 kHz, 70 dB) conditions. DCN-lesioned mice developed CR in both CS conditions as well as sham-operated mice. In the second experiment, DCN-lesioned and sham-operated mice were subjected to two-tone discrimination tasks, with LCS+ (or HCS+) paired with unconditioned stimulus (US), and HCS− (or LCS−) without US. CR% in sham-operated mice increased in LCS+ (or HCS+) trials, regardless of tone frequency of CS, but not in HCS− (or LCS−) trials. The results indicate that sham-operated mice can discriminate between LCS+ and HCS− (or HCS+ and LCS−). In contrast, DCN-lesioned mice showed high CR% in not only LCS+ (or HCS+) trials but also HCS− (or LCS−) trials. The results indicate that DCN lesions impair the discrimination between tone frequency in eyeblink conditioning. Our results suggest that the cerebellum plays a pivotal role in the discrimination of tone frequency.

Diminished cortical inhibition in an aging mouse model of chronic tinnitus

Flavoprotein autofluorescence imaging was used to examine auditory cortical synaptic responses in aged animals with behavioral evidence of tinnitus and hearing loss. Mice were exposed to noise trauma at 1-3 months of age and were assessed for behavioral evidence of tinnitus and hearing loss immediately after the noise trauma and again at ~24-30 months of age. Within 2 months of the final behavioral assessment, auditory cortical synaptic transmission was examined in brain slices using electrical stimulation of putative thalamocortical afferents, and flavoprotein autofluorescence imaging was used to measure cortical activation. Noise-exposed animals showed a 68% increase in amplitude of cortical activation compared with controls (p = 0.008), and these animals showed a diminished sensitivity to GABA(A)ergic blockade (p = 0.008, using bath-applied 200 nm SR 95531 [6-Imino-3-(4-methoxyphenyl)-1(6H)-p yridazinebutanoic acid hydrobromide]). The strength of cortical activation was significantly correlated to the degree of tinnitus behavior, assessed via a loss of gap detection in a startle paradigm. The decrease in GABA(A) sensitivity was greater in the regions of the cortex farther away from the stimulation site, potentially reflecting a greater sensitivity of corticocortical versus thalamocortical projections to the effects of noise trauma. Finally, there was no relationship between auditory cortical activation and activation of the somatosensory cortex in the same slices, suggesting that the increases in auditory cortical activation were not attributable to a generalized hyperexcitable state in noise-exposed animals. These data suggest that noise trauma can cause long-lasting changes in the auditory cortical physiology and may provide specific targets to ameliorate the effects of chronic tinnitus.

Significance of horizontal propagation of synchronized activities in human epileptic neocortex investigated by optical imaging and immunohistological study

To characterize the physiological condition of human epileptic neocortex, we employed flavoprotein fluorescence imaging (FFI), an optical imaging method which detects intrinsic signals accompanying neural activation, and immunohistologically studied human cortical specimens. The experimented materials were cortical tissues surrounding various intracerebral lesions obtained from 5 patients with epilepsy (epileptic patients: EPs) and 5 without epilepsy (non-epileptic patients: NEPs). These tissues were immersed in oxygenated artificial cerebrospinal fluid immediately after removal in the operating room. Signal changes of FFI in the cortical layers subjected to electrical stimulation were observed under bicuculline methiodide perfusion. Immunohistological staining for parvalbumin (PV), calbindin, and calretinin were performed on the same specimens to evaluate expressions of calcium-binding protein positive cells. The FFI study showed the characteristic cortical propagation pattern of elicited activities horizontally along the cortical layers in EPs but not in NEPs. The propagated area with more than 0.5% signal changes was significantly larger in EPs than in NEPs (p=0.008). Only the expression of PV positive neurons was significantly lower in EPs than in NEPs (p=0.006). The propagated area on FFI and the decrease in PV positive neurons correlated significantly (R=-0.78, p=0.04). The present study visualized the unique horizontal propagation of signal changes on FFI and demonstrated a correlation of this propagation with immunohistological decreases in PV positive neurons in human epileptic cortex. Further investigations may elucidate the mechanism of hyper-excitability and hyper-synchronization in epileptic cortical tissue itself.

Auditory map reorganization and pitch discrimination in adult rats chronically exposed to low-level ambient noise

Behavioral adaption to a changing environment is critical for an animal's survival. How well the brain can modify its functional properties based on experience essentially defines the limits of behavioral adaptation. In adult animals the extent to which experience shapes brain function has not been fully explored. Moreover, the perceptual consequences of experience-induced changes in the brains of adults remain unknown. Here we show that the tonotopic map in the primary auditory cortex of adult rats living with low-level ambient noise underwent a dramatic reorganization. Behaviorally, chronic noise-exposure impaired fine, but not coarse pitch discrimination. When tested in a noisy environment, the noise-exposed rats performed as well as in a quiet environment whereas the control rats performed poorly. This suggests that noise-exposed animals had adapted to living in a noisy environment. Behavioral pattern analyses revealed that stress or distraction engendered by the noisy background could not account for the poor performance of the control rats in a noisy environment. A reorganized auditory map may therefore have served as the neural substrate for the consistent performance of the noise-exposed rats in a noisy environment.

Restoration of contralateral representation in the mouse somatosensory cortex after crossing nerve transfer

Avulsion of spinal nerve roots in the brachial plexus (BP) can be repaired by crossing nerve transfer via a nerve graft to connect injured nerve ends to the BP contralateral to the lesioned side. Sensory recovery in these patients suggests that the contralateral primary somatosensory cortex (S1) is activated by afferent inputs that bypassed to the contralateral BP. To confirm this hypothesis, the present study visualized cortical activity after crossing nerve transfer in mice through the use of transcranial flavoprotein fluorescence imaging. In naïve mice, vibratory stimuli applied to the forepaw elicited localized fluorescence responses in the S1 contralateral to the stimulated side, with almost no activity in the ipsilateral S1. Four weeks after crossing nerve transfer, forepaw stimulation in the injured and repaired side resulted in cortical responses only in the S1 ipsilateral to the stimulated side. At eight weeks after crossing nerve transfer, forepaw stimulation resulted in S1 cortical responses of both hemispheres. These cortical responses were abolished by cutting the nerve graft used for repair. Exposure of the ipsilateral S1 to blue laser light suppressed cortical responses in the ipsilateral S1, as well as in the contralateral S1, suggesting that ipsilateral responses propagated to the contralateral S1 via cortico-cortical pathways. Direct high-frequency stimulation of the ipsilateral S1 in combination with forepaw stimulation acutely induced S1 bilateral cortical representation of the forepaw area in naïve mice. Cortical responses in the contralateral S1 after crossing nerve transfer were reduced in cortex-restricted heterotypic GluN1 (NMDAR1) knockout mice. Functional bilateral cortical representation was not clearly observed in genetically manipulated mice with impaired cortico-cortical pathways between S1 of both hemispheres. Taken together, these findings strongly suggest that activity-dependent potentiation of cortico-cortical pathways has a critical role for sensory recovery in patients after crossing nerve transfer.

Detection of virtual pitch up to 5kHz by mice

Natural sounds consist of a component at the fundamental frequency (f0) and its overtones. Pitch is perceived at f0, even when spectral energy at f0 is missing. This missing f0, or 'virtual pitch', is thought to be detected in the auditory cortex and related cortical areas, but the precise neural mechanisms are unknown. One possibility is that virtual pitch can be retrieved from the periodicity of sound waveforms. However, this mechanism requires the temporal accuracy in periodicity detection, and so far the detection of virtual pitch has only been demonstrated at frequencies lower than 1kHz. We investigated the ability of mice to detect virtual pitch up to 5kHz using a two-step sound discrimination test. In the first step of this test, mice were trained to discriminate between tone bursts at 2.5 and 5kHz. In the second step, we tested the ability of mice to discriminate between virtual pitches at 2.5kHz and at 5kHz. It was demonstrated that the performance of mice to discriminate between virtual pitches at 2.5 and 5kHz was significantly affected by previous discrimination learning between tone bursts, indicating that mice can detect virtual pitch up to 5kHz.

Autofluorescent flavoprotein imaging of spinal nociceptive activity

Pain arises from activation of peripheral nociceptors, and strong noxious stimuli may cause an increase in spinal excitability called central sensitization, which is likely involved in many pathological pain states. So far, it has not been achieved to simultaneously visualize in vivo both the temporal and spatial aspects of spinal activity, including central sensitization. Using autofluorescent flavoprotein imaging (AFI), an optical technique suitable for mapping activity in nervous tissue, we demonstrate a close temporal and spatial correlation of electrically evoked nociceptive input with the spinal AFI signal, representing spinal neuronal activity. The AFI signal increases linearly with stimulation intensity. Furthermore, we found that the AFI signal was much larger in intensity and size when the same electrical stimulation was applied after the induction of central sensitization by a subcutaneous capsaicin injection. Finally, innocuous palpation of the hindpaw did not evoke an AFI response in naive animals, but after capsaicin injection a strong response was obtained. This is the first report demonstrating simultaneously the temporal and spatial propagation of spinal nociceptive activity in vivo.

Effect of transient forebrain ischemia on flavoprotein autofluorescence and the somatosensory evoked potential in the rat

In order to evaluate the effect of cerebral ischemia on the flavoprotein fluorescence (FPF), we compared the changes in the FPF and somatosensory evoked potential (SEP) during transient cerebral ischemia in the rat. We measured the FPF and SEP simultaneously via a cranial window made over the right sensorimotor cortex during the left median nerve stimulation in F344 rats. We compared change in FPF and SEP during cerebral ischemia for 60 min. The rCBF were rapidly recovered after reperfusion. However, the recovery rates of the FPF were significantly faster than those of the SEP after reperfusion. These findings indicate that activity-dependent changes of the FPF do not necessarily correlate with the electrical activity after transient cerebral ischemia.

Pulsed noise experience disrupts complex sound representations

Cortical sound representations are adapted to the acoustic environment. Early exposure to exponential frequency-modulated (FM) sweeps results in more neurons selective to the experienced sounds. Here we examined the influence of pulsed noise experience on the development of sound representations in the primary auditory cortex (AI) of the rat. In naïve animals, FM sweep direction selectivity depends on the characteristic frequency (CF) of the neuron--low CF neurons tend to select for upward sweeps and high CF neurons for downward sweeps. Such a CF dependence was not observed in animals that had received weeklong exposure to pulsed noise in periods from postnatal day 8 (P8) to P15 or from P24 to P39. In addition, AI tonotopicity, tuning bandwidth, intensity threshold, tone-responsiveness, and sweep response magnitude were differentially affected by the noise experience depending on the exposure time windows. These results are consistent with previous findings of feature-dependent multiple sensitive periods. The different effects induced here by pulsed noise and previously by FM sweeps further indicate that plasticity in cortical complex sound representations is specific to the sensory input.

Transcranial imaging of somatotopic map plasticity after tail cut in mice

Peripheral afferent denervation induces reorganization of somatotopic maps in the primary somatosensory cortex (S1). In the present study, we investigated somatotopic map plasticity after tail cut. Neonatal mice at postnatal days (P) 2-3 and adult mice at eight weeks of age were anesthetized with ether, and approximately two thirds of the tail was cut from the tip. Both groups of mice were anesthetized with urethane (1.7g/kg, i.p.) at 10weeks of age, and transcranial flavoprotein fluorescence imaging was performed in the S1. Neural activities in the S1 were elicited by vibratory stimulation applied to the contralateral hindpaw or the tail in control mice. The cortical areas activated by hindpaw, tail base, and tail tip stimuli were placed in this order according to the medial and posterior direction. In mice with tail cut, the tail base area moved to the more medial and posterior area corresponding to the tail tip in control mice. The shift of the tail base area was observed in both neonatal and adult tail cut mice, indicating the absence of a critical period before eight weeks. Medial and posterior shift of the tail base area with regard to the bregma was confirmed in tail cut mice. These data suggest that transcranial flavoprotein fluorescence imaging is a useful technique for investigating somatosensory map plasticity in mice.

Tuning up the developing auditory CNS

Although the auditory system has limited information processing resources, the acoustic environment is infinitely variable. To properly encode the natural environment, the developing central auditory system becomes somewhat specialized through experience-dependent adaptive mechanisms that operate during a sensitive time window. Recent studies have demonstrated that cellular and synaptic plasticity occurs throughout the central auditory pathway. Acoustic-rearing experiments can lead to an over-representation of the exposed sound frequency, and this is associated with specific changes in frequency discrimination. These forms of cellular plasticity are manifest in brain regions, such as midbrain and cortex, which interact through feed-forward and feedback pathways. Hearing loss leads to a profound re-weighting of excitatory and inhibitory synaptic gain throughout the auditory CNS, and this is associated with an over-excitability that is observed in vivo. Further behavioral and computational analyses may provide insights into how theses cellular and systems plasticity effects underlie the development of cognitive functions such as speech perception.

Confocal reflectance and two-photon microscopy studies of a songbird skull for preparation of transcranial imaging

We present experiments and analyses of confocal reflectance and two-photon microscopy studies of zebra finch skull samples. The thin and hollow structure of these birds' skulls is quite translucent, which can allow in vivo transcranial two-photon imaging for brain activation monitoring. However, the skull structure is also quite complex, with high refractive index changes on a macroscopic scale. These studies aim at exploring the geometrical and scattering properties of these skull samples with the use of several confocal microscopy contrasts. Moreover, the study of the axial reflectance exponential decay is used to estimate the scattering coefficients of the bone. Finally, two-photon imaging experiments of a fluorescent object located beneath the skull are carried out. It reveals that two-photon fluorescence can be collected through the skull with a strong signal. It also reveals that the spatial resolution loss is quite high and cannot be fully explained by the bulk scattering properties of the bone, but also by the presence of the high refractive index inhomogeneity of this pneumatic skull structure. Even if the optical properties of the skull are different during in vivo experiments, these preliminary studies are aimed at preparing and optimizing transcranial brain activation monitoring experiments on songbirds.