Neuronal Populations Research Articles

Hypocretin in the nucleus accumbens shell modulates social approach in female but not male California mice

The hypocretin (Hcrt) system modulates arousal and anxiety-related behaviors and has been considered as a novel treatment target for stress-related affective disorders. We examined the effects of Hcrt acting in the nucleus accumbens shell (NAcSh) and anterodorsal bed nucleus of the stria terminalis (adBNST) on social behavior in male and female California mice (Peromyscus californicus). In female but not male California mice, infusion of Hcrt1 into NAcSh decreased social approach. Weak effects of Hcrt1 on social vigilance were observed in both females and males. No behavioral effects of Hcrt1 infused into the adBNST were observed. Analyses of sequencing data from California mice and Mus musculus NAc showed that Hcrtr2 was more abundant than Hcrtr1, so we infused the selective Hcrt receptor 2 antagonist into the NAcSh, which increased social approach in females previously exposed to social defeat. A calcium imaging study in the NAcSh of females before and after stress exposure showed that neural activity increased immediately following the expression of social avoidance but not during freezing behavior. This observation is consistent with previous studies that identified populations of neurons in the NAc that drive avoidance. Intriguingly, calcium transients were not affected by stress. These data suggest that hypocretin acting in the NAcSh plays a key role in modulating stress-induced social avoidance.

Multi-omic analysis of Huntington’s disease reveals a compensatory astrocyte state

The mechanisms underlying the selective regional vulnerability to neurodegeneration in Huntington’s disease (HD) have not been fully defined. To explore the role of astrocytes in this phenomenon, we used single-nucleus and bulk RNAseq, lipidomics, HTT gene CAG repeat-length measurements, and multiplexed immunofluorescence on HD and control post-mortem brains. We identified genes that correlated with CAG repeat length, which were enriched in astrocyte genes, and lipidomic signatures that implicated poly-unsaturated fatty acids in sensitizing neurons to cell death. Because astrocytes play essential roles in lipid metabolism, we explored the heterogeneity of astrocytic states in both protoplasmic and fibrous-like (CD44+) astrocytes. Significantly, one protoplasmic astrocyte state showed high levels of metallothioneins and was correlated with the selective vulnerability of distinct striatal neuronal populations. When modeled in vitro, this state improved the viability of HD-patient-derived spiny projection neurons. Our findings uncover key roles of astrocytic states in protecting against neurodegeneration in HD.

Echoes from Sensory Entrainment in Auditory Working Memory for Pitch.

Ongoing neural oscillations reflect cycles of excitation and inhibition in local neural populations, with individual neurons being more or less likely to fire depending upon the oscillatory phase. As a result, the oscillations could determine whether or not a sound is perceived and/or whether its neural representation enters into later processing stages. While empirical support for this idea has come from sound detection studies, large gaps in knowledge still exist regarding memory for sound events. In the current study, it was investigated how sensory entrainment impacts the fidelity of working memory representations for pitch. In two separate experiments, an 8 Hz amplitude modulated (AM) entraining stimulus was presented prior to a multitone complex having an f0 between 270 and 715 Hz. This "target" sound could be presented at phases from 0 to 2π radians in relation to the previous AM. After a retention interval of 4 s (Experiment 1; n = 26) or 2 s (Experiment 2; n = 28), listeners were tasked to reproduce the target sound's pitch by moving their finger along the horizontal axis of a response pad. It was hypothesized that if entrainment modulates auditory working memory fidelity, reproductions of a target's pitch would be more accurate and precise when targets were presented in phase with the entrainment. Cosine fits of the average data for both experiments showed a significant entrainment "echo" in the accuracy of pitch matches. There was no apparent echo in the matching precision. Fitting of the individual data accuracy showed that the optimal phase was consistent across individuals, aligning near the next AM peak had the AM continued. The results show that sensory entrainment modulates auditory working memory in addition to stimulus detection, consistent with the proposal that ongoing neural oscillatory activity modulates higher-order auditory processes.

Distinct basal forebrain-originated neural circuits promote homoeostatic feeding and suppress hedonic feeding in male mice.

Feeding behaviour is influenced by two primary factors: homoeostatic needs driven by hunger and hedonic desires for pleasure even in the absence of hunger. While efficient homoeostatic feeding is vital for survival, excessive hedonic feeding can lead to adverse consequences such as obesity and metabolic dysregulations. However, the neurobiological mechanisms that orchestrate homoeostatic versus hedonic food consumption remain largely unknown. Here we show that GABAergic proenkephalin (Penk) neurons in the diagonal band of Broca (DBB) of male mice respond to food presentation. We further demonstrate that a subset of DBBPenk neurons that project to the paraventricular nucleus of the hypothalamus are preferentially activated upon food presentation during fasting periods and transmit a positive valence to facilitate feeding. On the other hand, a separate subset of DBBPenk neurons that project to the lateral hypothalamus are preferentially activated when detecting a high-fat high-sugar (HFHS) diet and transmit a negative valence to inhibit food consumption. Notably, when given free choice of chow and HFHS diets, mice with the whole DBBPenk population ablated exhibit reduced consumption of chow but increased intake of the HFHS diet, resulting in accelerated development of obesity and metabolic disturbances. Together, we identify a molecularly defined neural population in male mice that is crucial for the maintenance of energy balance by facilitating homoeostatic feeding while suppressing hedonic overeating.

Prenatal protein malnutrition decreases neuron numbers in the parahippocampal region but not prefrontal cortex in adult rats

ABSTRACT Objective Prenatal protein malnutrition produces anatomical and functional changes in the developing brain that persist despite immediate postnatal nutritional rehabilitation. Brain networks of prenatally malnourished animals show diminished activation of prefrontal areas and an increased activation of hippocampal regions during an attentional task [1]. While a reduction in cell number has been documented in hippocampal subfield CA1, nothing is known about changes in neuron numbers in the prefrontal or parahippocampal cortices. Methods In the present study, we used unbiased stereology to investigate the effect of prenatal protein malnutrition on the neuron numbers in the medial prefrontal cortex and the cortices of the parahippocampal region that comprise the larger functional network. Results Results show that prenatal protein malnutrition does not cause changes in the neuronal population in the medial prefrontal cortex of adult rats, indicating that the decrease in functional activation during attentional tasks is not due to a reduction in the number of neurons. Results also show that prenatal protein malnutrition is associated with a reduction in neuron numbers in specific parahippocampal subregions: the medial entorhinal cortex and presubiculum. Discussion The affected regions along with CA1 comprise a tightly interconnected circuit, suggesting that prenatal malnutrition confers a vulnerability to specific hippocampal circuits. These findings are consistent with the idea that prenatal protein malnutrition produces a reorganization of structural and functional networks, which may underlie observed alterations in attentional processes and capabilities.

Inhibitory basal ganglia nuclei differentially innervate pedunculopontine nucleus subpopulations and evoke opposite motor and valence behaviors.

The canonical basal ganglia model predicts that the substantia nigra pars reticulata (SNr) and the globus pallidus externa (GPe) will have specific effects on locomotion: the SNr inhibiting locomotion and the GPe enhancing it. In this manuscript, we use in vivo optogenetics to show that a projection-defined neural subpopulation within each structure exerts non-canonical effects on locomotion. These non-canonical subpopulations are defined by their projection to the pedunculopontine nucleus (PPN) and mediate opposing effects on reward. To understand how these structures differentially modulate the PPN, we use ex vivo whole-cell recording with optogenetics to comprehensively dissect the SNr and GPe connections to regionally- and molecularly-defined populations of PPN neurons. The SNr inhibits all PPN subtypes, but most strongly inhibits caudal glutamatergic neurons. The GPe selectively inhibits caudal glutamatergic and GABAergic neurons, avoiding both cholinergic and rostral cells. This circuit characterization reveals non-canonical basal ganglia pathways for locomotion and valence.

A therapeutic small molecule enhances γ-oscillations and improves cognition/memory in Alzheimer’s disease model mice

Brain rhythms provide the timing for recruitment of brain activity required for linking together neuronal ensembles engaged in specific tasks. The γ-oscillations (30 to 120 Hz) orchestrate neuronal circuits underlying cognitive processes and working memory. These oscillations are reduced in numerous neurological and psychiatric disorders, including early cognitive decline in Alzheimer's disease (AD). Here, we report on a potent brain-permeable small molecule, DDL-920 that increases γ-oscillations and improves cognition/memory in a mouse model of AD, thus showing promise as a class of therapeutics for AD. We employed anatomical, in vitro and in vivo electrophysiological, and behavioral methods to examine the effects of our lead therapeutic candidate small molecule. As a novel in central nervous system pharmacotherapy, our lead molecule acts as a potent, efficacious, and selective negative allosteric modulator of the γ-aminobutyric acid type A receptors most likely assembled from α1β2δ subunits. These receptors, identified through anatomical and pharmacological means, underlie the tonic inhibition of parvalbumin (PV) expressing interneurons (PV+INs) critically involved in the generation of γ-oscillations. When orally administered twice daily for 2 wk, DDL-920 restored the cognitive/memory impairments of 3- to 4-mo-old AD model mice as measured by their performance in the Barnes maze. Our approach is unique as it is meant to enhance cognitive performance and working memory in a state-dependent manner by engaging and amplifying the brain's endogenous γ-oscillations through enhancing the function of PV+INs.

The neural basis of swap errors in working memory

When making decisions in a cluttered world, humans and other animals often have to hold multiple items in memory at once-such as the different items on a shopping list. Psychophysical experiments in humans and other animals have shown remembered stimuli can sometimes become confused, with participants reporting chimeric stimuli composed of features from different stimuli. In particular, subjects will often make "swap errors" where they misattribute a feature from one object as belonging to another object. While swap errors have been described behaviorally and theoretical explanations have been proposed, their neural mechanisms are unknown. Here, we elucidate these neural mechanisms by analyzing neural population recordings from monkeys performing two multistimulus working memory tasks. In these tasks, monkeys were cued to report the color of an item that either was previously shown at a corresponding location or will be shown at the corresponding location. Animals made swap errors in both tasks. In the neural data, we find evidence that the neural correlates of swap errors emerged when correctly remembered information is selected from working memory. This led to a representation of the distractor color as if it were the target color, underlying the eventual swap error. We did not find consistent evidence that swap errors arose from misinterpretation of the cue or errors during encoding or storage in working memory. These results provide evidence that swap errors emerge during selection of correctly remembered information from working memory, and highlight this selection as a crucial-yet surprisingly brittle-neural process.

Dimension Reduction of Multidimensional Structured and Unstructured Datasets through Ensemble Learning of Neural Embeddings

Dimension reduction aims to project a high‐dimensional dataset into a low‐dimensional space. It tries to preserve the topological relationships among the original data points and/or induce clusters. NetDRm, an online dimensionality reduction method based on neural ensemble learning that integrates different dimension reduction methods in a synergistic way, is introduced. NetDRm is designed for datasets of multidimensional points that can be either structured (e.g., images) or unstructured (e.g., point clouds, tabular data). It starts by training a collection of deep residual encoders that learn the embeddings induced by multiple dimension reduction methods applied to the input dataset. Subsequently, a dense neural network integrates the generated encoders by emphasizing topological preservation or cluster induction. Experiments conducted on widely used multidimensional datasets (point‐cloud manifolds, image datasets, tabular record datasets) show that the proposed method yields better results in terms of topological preservation ( curves), cluster induction (V measure), and classification accuracy than the most relevant dimension reduction methods.

Neuromodulation with transcranial direct current stimulation contributes to motor function recovery via microglia in spinal cord injury

Spinal cord injury (SCI) is damage or trauma to the spinal cord, which often results in loss of function, sensation, or mobility below the injury site. Transcranial direct current stimulation (tDCS) is a non-invasive and affordable brain stimulation technique used to modulate neuronal circuits, which changes the morphology and activity of microglia in the cerebral cortex. However, whether similar morphological changes can be observed in the spinal cord remains unclear. Therefore, we evaluated neuronal population activity in layer 5 (L5) of M1 following SCI and investigated whether changes in the activities of L5 neurons affect microglia-axon interactions using C57BL/6J mice. We discovered that L5 of the primary motor cortex (corticospinal neurons) exhibited reduced synchronized activity after SCI that correlates with microglial morphology, which was recovered using tDCS. This indicates that tDCS promotes changes in the morphological properties and recovery of microglia after SCI. Combining immunotherapy with tDCS may be effective in treating SCI.

Transfer Learning Artificial Neural Network-based Ensemble Voting of Water Quality Classification for Different Types of Farming

Transfer Learning Artificial Neural Network-based Ensemble Voting of Water Quality Classification for Different Types of Farming

Optogenetic inhibition of light-captured alcohol-taking striatal engrams facilitates extinction and suppresses reinstatement.

Alcohol use disorder (AUD) is a complex condition, and it remains unclear which specific neuronal substrates mediate alcohol-seeking and -taking behaviors. Engram cells and their related ensembles, which encode learning and memory, may play a role in this process. We aimed to assess the precise neural substrates underlying alcohol-seeking and -taking behaviors and determine how they may affect one another. Using FLiCRE (Fast Light and Calcium-Regulated Expression; a newly developed technique which permits the trapping of acutely activated neuronal ensembles) and operant self-administration (OSA), we tagged striatal neurons activated during alcohol-taking behaviors. We used FLiCRE to express an inhibitory halorhodopsin in alcohol-taking neurons, permitting loss-of-function manipulations. We found that the inhibition of OSA-tagged alcohol-taking neurons decreased both alcohol-seeking and -taking behaviors in future OSA trials. In addition, optogenetic inhibition of these OSA-tagged alcohol-taking neurons during extinction training facilitated the extinction of alcohol-seeking behaviors. Furthermore, inhibition of these OSA-tagged alcohol-taking neurons suppressed the reinstatement of alcohol-seeking behaviors, but, interestingly, it did not significantly suppress alcohol-taking behaviors during reinstatement. Our findings suggest that alcohol-taking neurons are crucial for future alcohol-seeking behaviors during extinction and reinstatement. These results may help in the development of new therapeutic approaches to enhance extinction and suppress relapse in individuals with AUD.

Y chromosome genes interplay with interval timing in regulating mating duration of male Drosophila melanogaster

We explore the understudied role of Y chromosome genes in modulating mating behaviors and interval timing in male fruit flies. Our findings reveal a significant impact of these genes on mating duration, a critical aspect of sexual selection and reproductive success. Through the use of XO males lacking a Y chromosome and RNA interference (RNAi) techniques to knockdown specific Y chromosome genes, we demonstrate that the Y chromosome and its genes WDY and CCY are essential for the generation of Longer-Mating-Duration (LMD) and Shorter-Mating-Duration (SMD) behaviors. Notably, the neuronal knockdown of Ppr-Y, a gene highly expressed in both neuronal and glial cells, leads to profound disruptions in courtship and mating behaviors without affecting fertility. Utilizing the fly SCope scRNA-seq data platform, we identified that several Y chromosome genes, including kl-3, kl-5, WDY, and PRY, are preferentially expressed in fru-positive neurons, suggesting a role in male-specific neuronal populations. Our work not only advances the understanding of the Y chromosome's contribution to complex mating behaviors but also sets the stage for future investigations into the molecular mechanisms underlying sexual dimorphism and reproductive strategies in Drosophila.

Lineage-tracing reveals an expanded population of NPY neurons in the inferior colliculus.

Growing evidence suggests that neuropeptide signaling shapes auditory computations. We previously showed that neuropeptide Y (NPY) is expressed in the inferior colliculus (IC) by a population of GABAergic stellate neurons and that NPY regulates the strength of local excitatory circuits in the IC. NPY neurons were initially characterized using the NPY-hrGFP mouse, in which humanized renilla green fluorescent protein (hrGFP) expression indicates NPY expression at the time of assay, i.e., an expression-tracking approach. However, studies in other brain regions have shown that NPY expression can vary based on several factors, suggesting that the NPY-hrGFP mouse might miss NPY neurons not expressing NPY on the experiment date. Here, we hypothesized that neurons with the ability to express NPY represent a larger population of IC GABAergic neurons than previously reported. To test this hypothesis, we used a lineage-tracing approach to irreversibly tag neurons that expressed NPY at any point prior to the experiment date. We then compared the physiological and anatomical features of neurons labeled with this lineage-tracing approach to our prior data set, revealing a larger population of NPY neurons than previously found. In addition, we used optogenetics to test the local connectivity of NPY neurons and found that NPY neurons provide inhibitory synaptic input to other neurons in the ipsilateral IC. Together, our data expand the definition of NPY neurons in the IC, suggest that NPY expression might be dynamically regulated in the IC, and provide functional evidence that NPY neurons form local inhibitory circuits in the IC.NEW & NOTEWORTHY Across brain regions, neuropeptide Y (NPY) expression is dynamic and influenced by extrinsic and intrinsic factors. We previously showed that NPY is expressed by a class of inhibitory neurons in the auditory midbrain. Here, we find that this neuron class also includes neurons that previously expressed NPY, suggesting that NPY expression is dynamically regulated in the auditory midbrain. We also provide functional evidence that NPY neurons contribute to local inhibitory circuits in the auditory midbrain.

Chronic Stress Alters Synaptic Inhibition/Excitation Balance of Pyramidal Neurons But Not PV Interneurons in the Infralimbic and Prelimbic Cortices of C57BL/6J Mice.

The medial prefrontal cortex (mPFC) plays a pivotal role in regulating working memory, executive function, and self-regulatory behaviors. Dysfunction in the mPFC circuits is a characteristic feature of several neuropsychiatric disorders including schizophrenia, depression, and post-traumatic stress disorder. Chronic stress (CS) is widely recognized as a major triggering factor for the onset of these disorders. Although evidence suggests synaptic dysfunction in mPFC circuits following CS exposure, it remains unclear how different neuronal populations in the infralimbic (IL) and prelimbic (PL) cortices are affected in terms of synaptic inhibition/excitation balance (I/E ratio). Here, using neuroproteomic analysis and whole-cell patch-clamp recordings in pyramidal neurons (PNs) and parvalbumin (PV) interneurons within the PL and IL cortices, we examined the synaptic changes after 21 d of chronic unpredictable stress, in male mice. Our results reveal distinct impacts of CS on PL and IL PNs, resulting in an increased I/E ratio in both subregions but through different mechanisms: CS increases inhibitory synaptic drive in the PL while decreasing excitatory synaptic drive in the IL. Notably, the I/E ratio and excitatory and inhibitory synaptic drive of PV interneurons remained unaffected in both PL and IL circuits following CS exposure. These findings offer novel mechanistic insights into the influence of CS on mPFC circuits and support the hypothesis of stress-induced mPFC hypofunction.

Bidirectional fear modulation by discrete anterior insular circuits in male mice.

The brain's ability to appraise threats and execute appropriate defensive responses is essential for survival in a dynamic environment. Humans studies have implicated the anterior insular cortex (aIC) in subjective fear regulation and its abnormal activity in fear/anxiety disorders. However, the complex aIC connectivity patterns involved in regulating fear remain under investigated. To address this, we recorded single units in the aIC of freely moving male mice that had previously undergone auditory fear conditioning, assessed the effect of optogenetically activating specific aIC output structures in fear, and examined the organization of aIC neurons projecting to the specific structures with retrograde tracing. Single-unit recordings revealed that a balanced number of aIC pyramidal neurons' activity either positively or negatively correlated with a conditioned tone-induced freezing (fear) response. Optogenetic manipulations of aIC pyramidal neuronal activity during conditioned tone presentation altered the expression of conditioned freezing. Neural tracing showed that non-overlapping populations of aIC neurons project to the amygdala or the medial thalamus, and the pathway bidirectionally modulated conditioned fear. Specifically, optogenetic stimulation of the aIC-amygdala pathway increased conditioned freezing, while optogenetic stimulation of the aIC-medial thalamus pathway decreased it. Our findings suggest that the balance of freezing-excited and freezing-inhibited neuronal activity in the aIC and the distinct efferent circuits interact collectively to modulate fear behavior.

Neuropathic pain has sex-specific effects on oxycodone-seeking and non-drug-seeking ensemble neurons in the dorsomedial prefrontal cortex of mice.

Approximately 50 million Americans suffer from chronic pain, and nearly a quarter of chronic pain patients have reported misusing opioid prescriptions. Repeated drug seeking is associated with reactivation of an ensemble of neurons sparsely scattered throughout the dorsomedial prefrontal cortex (dmPFC). Prior research has demonstrated that chronic pain increases intrinsic excitability of dmPFC neurons, which may increase the likelihood of reactivation during drug seeking. We tested the hypothesis that chronic pain would increase oxycodone-seeking behaviour and that the pain state would differentially increase intrinsic excitability in dmPFC drug-seeking ensemble neurons. TetTag mice self-administered intravenous oxycodone. After 7days of forced abstinence, a drug-seeking session was performed, and the ensemble was tagged. Mice received spared nerve injury (SNI) to induce chronic pain during the period between the first and second seeking session. Following the second seeking session, we performed electrophysiology on individual neurons within the dmPFC to assess intrinsic excitability of the drug-seeking ensemble and non-ensemble neurons. SNI had no impact on sucrose seeking or intrinsic excitability of dmPFC neurons from these mice. In females, SNI increased oxycodone seeking and intrinsic excitability of non-ensemble neurons. In males, SNI had no impact on oxycodone seeking or neuron excitability. Data from females are consistent with clinical reports that chronic pain can promote drug craving and relapse and support the hypothesis that chronic pain itself may lead to neuroadaptations which promote opioid seeking.

Single-cell spatial transcriptomics reveals molecular patterns of selective neuronal vulnerability to α-synuclein pathology in a transgenic mouse model of Lewy body disease.

One of the unifying pathological hallmarks of Parkinson's disease (PD) and dementia with Lewy bodies (DLB) is the presence of misfolded, aggregated, and often phosphorylated forms of the protein α-synuclein in neurons. α-Synuclein pathology appears in select populations of neurons throughout various cortical and subcortical regions, and little is currently known about why some neurons develop pathology while others are spared. Here, we utilized subcellular-resolution imaging-based spatial transcriptomics (IST) in a transgenic mouse model that overexpresses wild-type human α-synuclein (α-syn-tg) to evaluate patterns of selective neuronal vulnerability to α-synuclein pathology. By performing post-IST immunofluorescence for α-synuclein phosphorylated at Ser129 (pSyn), we identified cell types in the cortex and hippocampus that were vulnerable or resistant to developing pSyn pathology. Next, we investigated the transcriptional underpinnings of the observed selective vulnerability using a set of custom probes to detect genes involved in α-synuclein processing and toxicity. We identified expression of the kinase:substrate pair Plk2 , which phosphorylates α-synuclein at Ser129, and human SNCA ( hSNCA ), as underlying the selective vulnerability to pSyn pathology. Finally, we performed differential gene expression analysis, comparing non-transgenic cells to pSyn - and pSyn + α-syn-tg cells to reveal gene expression changes downstream of hSNCA overexpression and pSyn pathology, which included pSyn-dependent alterations in mitochondrial and endolysosomal genes. This study provides a comprehensive use case of IST, yielding new biological insights into the formation of α-synuclein pathology and its downstream effects in a PD/DLB mouse model.

Corticotropin-Releasing Factor Release From a Unique Subpopulation of Accumbal Neurons Constrains Action-Outcome Acquisition in Reward Learning

BackgroundThe nucleus accumbens (NAc) mediates reward learning and motivation. Despite an abundance of neuropeptides, peptidergic neurotransmission from the NAc has not been integrated into current models of reward learning. The existence of a sparse population of neurons containing corticotropin-releasing factor (CRF) has been previously documented. Here, we provide a comprehensive analysis of their identity and functional role in shaping reward learning. MethodsOur multidisciplinary approach included fluorescent in situ hybridization (n = ≥3 mice), tract tracing (n = 5 mice), ex vivo electrophysiology (n = ≥30 cells), in vivo calcium imaging with fiber photometry (n = ≥4 mice), and use of viral strategies in transgenic lines to selectively delete CRF peptide from NAc neurons (n = ≥4 mice). Behaviors used were instrumental learning, sucrose preference, and spontaneous exploration in an open field. ResultsWe showed that the vast majority of NAc CRF-containing neurons are spiny projection neurons (SPNs) comprising dopamine D1-, D2-, or D1/D2-containing SPNs that primarily project and connect to the ventral pallidum and to a lesser extent the ventral midbrain. As a population, they display mature and immature SPN firing properties. We demonstrated that NAc CRF-containing neurons track reward outcomes during operant reward learning and that CRF release from these neurons acts to constrain initial acquisition of action-outcome learning and at the same time facilitates flexibility in the face of changing contingencies. ConclusionsCRF release from this sparse population of SPNs is critical for reward learning under normal conditions.

Laminar pattern of sensory-evoked dynamic high-frequency oscillatory activity in the macaque auditory cortex.

High-frequency (>60Hz) neuroelectric signals likely have functional roles distinct from low-frequency (<30Hz) signals. While high-gamma activity (>60Hz) does not simply equate to neuronal spiking, they are highly correlated, having similar information encoding. High-gamma activity is typically considered broadband and poorly phase-locked to sensory stimuli and thus is typically analyzed after transformations into absolute amplitude or spectral power. However, those analyses discard signal polarity, compromising the interpretation of neuroelectric events that are essentially dipolar. In the spectrotemporal profiles of field potentials in auditory cortex, we show high-frequency spectral peaks not phase-locked to sound onset, which follow the broadband peak of phase-locked onset responses. Isolating the signal components comprising the high-frequency peaks reveals narrow-band high-frequency oscillatory events, whose instantaneous frequency changes rapidly from >150 to 60Hz, which may underlie broadband high-frequency spectral peaks in previous reports. The laminar amplitude distributions of the isolated activity had two peak positions, while the laminar phase patterns showed a counterphase relationship between those peaks, indicating the formation of dipoles. Our findings suggest that nonphase-locked HGA arises in part from oscillatory or recurring activity of supragranular-layer neuronal ensembles in auditory cortex.