Enhanced modulation of cell-type specific neuronal responses in mouse dorsal auditory field during locomotion.

As we move through the environment we experience constantly changing sensory input that must be merged with our ongoing motor behaviors - creating dynamic interactions between our sensory and motor systems. Active behaviors such as locomotion generally increase the sensory-evoked neuronal activity in visual and somatosensory cortices, but evidence suggests that locomotion largely suppresses neuronal responses in the auditory cortex. However, whether this effect is ubiquitous across different anatomical regions of the auditory cortex is largely unknown. In mice, auditory association fields such as the dorsal auditory cortex (AuD), have been shown to have different physiological response properties, protein expression patterns, and cortical as well as subcortical connections, in comparison to primary auditory regions (A1) - suggesting there may be important functional differences. Here we examined locomotion-related modulation of neuronal activity in cortical layers ⅔ of AuD and A1 using two-photon Ca2+ imaging in head-fixed behaving mice that are able to freely run on a spherical treadmill. We determined the proportion of neurons in these two auditory regions that show enhanced and suppressed sensory-evoked responses during locomotion and quantified the depth of modulation. We found that A1 shows more suppression and AuD more enhanced responses during locomotion periods. We further revealed differences in the circuitry between these auditory regions and motor cortex, and found that AuD is more highly connected to motor cortical regions. Finally, we compared the cell-type specific locomotion-evoked modulation of responses in AuD and found that, while subpopulations of PV-expressing interneurons showed heterogeneous responses, the population in general was largely suppressed during locomotion, while excitatory population responses were generally enhanced in AuD. Therefore, neurons in primary and dorsal auditory fields have distinct response properties, with dorsal regions exhibiting enhanced activity in response to movement. This functional distinction may be important for auditory processing during navigation and acoustically guided behavior.

Diversity of Receptive Fields and Sideband Inhibition with Complex Thalamocortical and Intracortical Origin in L2/3 of Mouse Primary Auditory Cortex.

Receptive fields of primary auditory cortex (A1) neurons show excitatory neuronal frequency preference and diverse inhibitory sidebands. While the frequency preferences of excitatory neurons in local A1 areas can be heterogeneous, those of inhibitory neurons are more homogeneous. To date, the diversity and the origin of inhibitory sidebands in local neuronal populations and the relation between local cellular frequency preference and inhibitory sidebands are unknown. To reveal both excitatory and inhibitory subfields, we presented two-tone and pure tone stimuli while imaging excitatory neurons (Thy1) and two types of inhibitory neurons (parvalbumin and somatostatin) in L2/3 of mice A1. We classified neurons into six classes based on frequency response area (FRA) shapes and sideband inhibition depended both on FRA shapes and cell types. Sideband inhibition showed higher local heterogeneity than frequency tuning, suggesting that sideband inhibition originates from diverse sources of local and distant neurons. Two-tone interactions depended on neuron subclasses with excitatory neurons showing the most nonlinearity. Onset and offset neurons showed dissimilar spectral integration, suggesting differing circuits processing sound onset and offset. These results suggest that excitatory neurons integrate complex and nonuniform inhibitory input. Thalamocortical terminals also exhibited sideband inhibition, but with different properties from those of cortical neurons. Thus, some components of sideband inhibition are inherited from thalamocortical inputs and are further modified by converging intracortical circuits. The combined heterogeneity of frequency tuning and diverse sideband inhibition facilitates complex spectral shape encoding and allows for rapid and extensive plasticity.SIGNIFICANCE STATEMENT Sensory systems recognize and differentiate between different stimuli through selectivity for different features. Sideband inhibition serves as an important mechanism to sharpen stimulus selectivity, but its cortical mechanisms are not entirely resolved. We imaged pyramidal neurons and two common classes of interneurons suggested to mediate sideband inhibition (parvalbumin and somatostatin positive) in the auditory cortex and inferred their inhibitory sidebands. We observed a higher degree of variability in the inhibitory sideband than in the local frequency tuning, which cannot be predicted from the relative high homogeneity of responses by inhibitory interneurons. This suggests that cortical sideband inhibition is nonuniform and likely results from a complex interplay between existing functional inhibition in the feedforward input and cortical refinement.

Adaptation to mis-pronounced speech: evidence for a prefrontal-cortex repair mechanism.

Speech is a complex and ambiguous acoustic signal that varies significantly within and across speakers. Despite the processing challenge that such variability poses, humans adapt to systematic variations in pronunciation rapidly. The goal of this study is to uncover the neurobiological bases of the attunement process that enables such fluent comprehension. Twenty-four native English participants listened to words spoken by a "canonical" American speaker and two non-canonical speakers, and performed a word-picture matching task, while magnetoencephalography was recorded. Non-canonical speech was created by including systematic phonological substitutions within the word (e.g. [s] → [sh]). Activity in the auditory cortex (superior temporal gyrus) was greater in response to substituted phonemes, and, critically, this was not attenuated by exposure. By contrast, prefrontal regions showed an interaction between the presence of a substitution and the amount of exposure: activity decreased for canonical speech over time, whereas responses to non-canonical speech remained consistently elevated. Grainger causality analyses further revealed that prefrontal responses serve to modulate activity in auditory regions, suggesting the recruitment of top-down processing to decode non-canonical pronunciations. In sum, our results suggest that the behavioural deficit in processing mispronounced phonemes may be due to a disruption to the typical exchange of information between the prefrontal and auditory cortices as observed for canonical speech.

Homeostatic Control of Spontaneous Activity in the Developing Auditory System.

Neurons in the developing auditory system exhibit spontaneous bursts of activity before hearing onset. How this intrinsically generated activity influences development remains uncertain, because few mechanistic studies have been performed in vivo. We show using macroscopic calcium imaging in unanesthetized mice that neurons responsible for processing similar frequencies of sound exhibit highly synchronized activity throughout the auditory system during this critical phase of development. Spontaneous activity normally requires synaptic excitation of spiral ganglion neurons (SGNs). Unexpectedly, tonotopic spontaneous activity was preserved in a mouse model of deafness in which glutamate release from hair cells is abolished. SGNs in these mice exhibited enhanced excitability, enabling direct neuronal excitation by supporting cell-induced potassium transients. These results indicate that homeostatic mechanisms maintain spontaneous activity in the pre-hearing period, with significant implications for both circuit development and therapeutic approaches aimed at treating congenital forms of deafness arising through mutations in key sensory transduction components.

The dual pattern of corticothalamic projection of the primary auditory cortex in macaque monkey.

The distribution and terminal morphology of the corticothalamic projection originating from the primary auditory cortex (A1) were established in a macaque monkey, using the anterograde (and retrograde) tracer biotinylated dextran amine. A dense corticothalamic projection from A1 was found in the ventral (vMGB) and dorsal (dMGB) divisions of the medial geniculate body and, to a lesser extent, in the medial division (mMGB), the posterior thalamic nucleus (PO) and the suprageniculate nucleus. Most terminal boutons were small (<1 microm), except some large boutons (2-6 microm) located in PO and vMGB. The data demonstrate that the corticothalamic projection from A1 in primate consists of two types of terminals (small and giant endings) in line with previous observations in rat and cat. Retrogradely labeled thalamocortical neurons formed clusters generally overlapping the corticothalamic terminal fields.

Neuronal and astrocyte expression of nicotinic receptor subunit beta4 in the adult mouse brain.

Neuronal nicotinic acetylcholine receptor (nAChR) expression and function are customized in different brain regions through assembling receptors from closely related but genetically distinct subunits. Immunohistochemical analysis of one of these subunits, nAChRbeta4, in the mouse brain suggests an extensive and potentially diverse role for this subunit in both excitatory and inhibitory neurotransmission. Prominent immunostaining included: 1) the medial habenula, efferents composing the fasciculus retroflexus, and the interpeduncular nucleus; 2) nuclei and ascending tracts of the auditory system inclusive of the medial geniculate; 3) the sensory cortex barrel field and cell bodies of the ventral thalamic nucleus; 4) olfactory-associated structures and the piriform cortex; and 5) sensory and motor trigeminal nuclei. In the hippocampus, nAChRbeta4 staining was limited to dendrites and soma of a subset of glutamic acid dehydrogenase-positive neurons. In C57BL/6 mice, but to a lesser extent in C3H/J, CBA/J, or CF1 mice, a subpopulation of astrocytes in the hippocampal CA1 region prominently expressed nAChRbeta4 (and nAChRalpha4). Collectively, these results suggest that the unique functional and pharmacological properties exerted by nAChRbeta4 on nAChR function could modify and specialize the development of strain-specific sensory and hippocampal-related characteristics of nicotine sensitivity including the development of tolerance.

Corticothalamic cells in layers 5 and 6 of primary and secondary sensory cortex express GAP-43 mRNA in the adult rat.

The expression of a presynaptic phosphoprotein, growth-associated protein (GAP)-43, is associated with synaptogenesis during development and synaptic remodeling in the adult. This study examined GAP-43 mRNA expression and distribution in primary and secondary areas of visual, auditory, and somatosensory cortex of the adult rat, by in situ hybridization with a digoxigenin-coupled mRNA probe, focusing particularly on the corticothalamic cells in layers 5 and 6. In the six cortical areas studied, GAP-43 mRNA was expressed predominantly in layers 5 and 6 and was greater in secondary than primary areas. There were densely labeled cells in layers 5 and 6 of all areas, which showed a restricted sublaminar distribution in primary areas and more even distribution in secondary areas. Combining retrograde transport of rhodamine beads with in situ hybridization in visual and auditory cortex showed that corticothalamic cells in layers 5 and 6 express GAP-43 mRNA. There are more of these GAP-43 mRNA positive corticothalamic cells in layer 5 of secondary areas than in primary areas. The evidence suggests that in the adult rat, plasticity related to GAP-43 is present in primary and secondary sensory cortex and more so in secondary areas.

Histochemical identification of cortical areas in the auditory region of the human brain.

Despite numerous studies stretching over the last 100 years there is still no general agreement on the number of auditory areas in the human cortex or even how to define them by histological methods. Full definition of these areas will require a combination of functional and histological methods but, by using six complementary histological methods, of which most have been used in the monkey, we provide a clearer description of these areas. The primary auditory area was located on the posteromedial two-thirds of the first transverse temporal (Heschl's) gyrus and was distinguished by a dense band of cytochrome oxidase activity in layer IV and the base of layer III, as well as a relatively thick, pale layer V and VI. Layers V and VI together made up 40% of the cortical thickness. Acetylcholinesterase (AChE)-containing pyramidal cells were sparsely distributed within the primary auditory area. The anterolateral third of Heschl's gyrus did not have a clear band of high cytochrome oxidase activity but contained a moderately high density of AChE-containing pyramidal cells and thus appeared to be part of the auditory belt. Within Heschl's sulcus there was a third area, which had a band of high cytochrome oxidase activity and bands of high parvalbumin immunoreactivity and AChE activity in layer IV. This area appeared to be part of the auditory core. Thus the use of staining methods for cytochrome oxidase, AChE and parvalbumin provided additional information which allowed a clearer definition of auditory areas than Nissl or myelin staining alone. Our results suggest that there are two core areas surrounded by at least six belt areas in the human auditory region.

Classical conditioning modifies cytochrome oxidase activity in the auditory system.

The effects of excitatory classical conditioning on cytochrome oxidase activity in the central auditory system were investigated using quantitative histochemistry. Rats in the conditioned group were trained with consistent pairings of a compound conditional stimulus (a tone and a light) with a mild footshock, to elicit conditioned suppression of drinking. Rats in the pseudorandom group were exposed to pseudorandom presentations of the same tone, light and shock stimuli without consistent pairings. Untrained rats in a naive group did not receive presentations of the experimental stimuli. The findings demonstrated that auditory fear conditioning modifies the metabolic neuronal responses of the auditory system, supporting the hypothesis that sensory neurons are responsive to behavioural stimulus properties acquired by learning. There was a clear distinction between thalamocortical and lower divisions of the auditory system based on the differences in metabolic activity evoked by classical conditioning, which lead to an overt learned behavioural response versus pseudorandom stimulus presentations, which lead to behavioural habituation. Increases in cytochrome oxidase activity indicated that tone processing is enhanced during associative conditioning at upper auditory structures (medial geniculate nucleus and secondary auditory cortices). In contrast, metabolic activation of lower auditory structures (cochlear nuclei and inferior colliculus) in response to the pseudorandom presentation of the experimental stimuli suggest that these areas may be activated during habituation to tone stimuli. Together these findings show that mapping the metabolic activity of cytochrome oxidase with quantitative histochemistry can be successfully used to map regional long-lasting effects of learning on brain systems.

Parvalbumin is expressed in a reciprocal circuit linking the medial geniculate body and auditory neocortex in the rabbit.

Recent studies of the rabbit auditory forebrain have shown that antibodies directed against the calcium-binding protein parvalbumin (PV) specifically demarcate auditory neocortex and the ventral division of the medial geniculate body (MGV). The auditory cortex is characterized by two PV- immunoreactive bands: dense terminal-like labeling within layer III/IV and a prominent band of PV+ somata in the upper half of layer VI. In some cases, there are distinct patches of PV immunoreactivity within layers III/IV of auditory cortex that appear similar to the patchy termination of thalamocortical axons labeled by the injection of anterograde tracers into MGV. The presence of PV+ patches in III/IV, PV+ somata in layer VI, and the high density of PV+ neurons and terminals in the MGV suggest the existence of a reciprocal PV+ circuit linking primary auditory cortex (AI) and the MGV. In the present study, double-labeling experiments in adult rabbits were carried out to provide evidence for this circuit. Focal injections of the tracers biocytin or biotinylated dextran amine (BDA) into the MGV labeled thalamocortical afferent patches within layer III/IV and retrogradely labeled corticothalamic neurons in layer VIa of the ipsilateral auditory cortex. Adjacent sections stained with antibodies against PV revealed terminal-like PV-immunoreactive patches in III/IV and PV+ somata in VIa that were in register with those labeled by BDA injections into the MGV. Serial section reconstruction of BDA-labeled corticothalamic neurons in VIa revealed pyramidal cells with tangentially oriented basal dendrites and sparsely branched apical dendrites that ascended to layer I. Fluorescent double-labeling studies demonstrated that a subpopulation of corticothalamic neurons also express PV. PV-negative corticothalamic neurons were also found. Discrete injections of BDA into auditory cortex labeled bands of neurons in the ipsilateral MGV, whose orientation paralleled the fibrodendritic laminae characteristic of this subdivision. Retrograde double-labeling experiments showed that most MGV relay neurons also express PV. Small numbers of PV-negative relay neurons were also found. These studies provide evidence for the existence of multiple, chemically coded pathways linking primary auditory cortex and the MGV.

Impaired mismatch negativity (MMN) generation in schizophrenia as a function of stimulus deviance, probability, and interstimulus/interdeviant interval.

Schizophrenia is a severe mental disorder associated with disturbances in perception and cognition. Event-related potentials (ERP) provide a mechanism for evaluating potential mechanisms underlying neurophysiological dysfunction in schizophrenia. Mismatch negativity (MMN) is a short-duration auditory cognitive ERP component that indexes operation of the auditory sensory ('echoic') memory system. Prior studies have demonstrated impaired MMN generation in schizophrenia along with deficits in auditory sensory memory performance. MMN is elicited in an auditory oddball paradigm in which a sequence of repetitive standard tones is interrupted infrequently by a physically deviant ('oddball') stimulus. The present study evaluates MMN generation as a function of deviant stimulus probability, interstimulus interval, interdeviant interval and the degree of pitch separation between the standard and deviant stimuli. The major findings of the present study are first, that MMN amplitude is decreased in schizophrenia across a broad range of stimulus conditions, and second, that the degree of deficit in schizophrenia is largest under conditions when MMN is normally largest. The pattern of deficit observed in schizophrenia differs from the pattern observed in other conditions associated with MMN dysfunction, including Alzheimer's disease, stroke, and alcohol intoxication.

Fos-like immunoreactivity in central auditory neurons of the mouse.

Fos-like immunoreactivity was used to study sound-induced activation of neurons in the auditory brainstem. Immunoreactivity was assayed with a polyclonal antibody to Fos. In response to 6-kHz tone bursts, the pattern of staining was a band of immunoreactive neurons positioned at the tonotopically appropriate position within the cochlear nucleus and the inferior colliculus. The band was narrow at low sound pressure levels but wider along the tonotopic axis at higher sound levels. In response to noise bursts, the pattern was broader and often extended throughout the auditory nuclei. Often within this broad pattern were "sub-bands" of immunostained neurons, interspersed with bands of unstained neurons. With increasing sound pressure levels above 35-55 dB, the number of Fos-like immunoreactive neurons increased for the cochlear nucleus, superior olivary complex, and inferior colliculus. In the cochlear nucleus and inferior colliculus, the stained cells were small, and hence their activity would be difficult to sample in electrophysiological studies. In the medial nucleus of the trapezoid body, the stained neurons had larger somata and other characteristics of principal cells. Anesthesia with Nembutal or Avertin, but not with ketamine or urethane, decreased the number of Fos-like immunoreactive neurons in the cochlear nucleus. The different anesthetics produced more variable results in the inferior colliculus. In anesthetized, monaurally stimulated animals, the presence of staining in the contralateral cochlear nucleus indicates that some Fos-like immunoreactivity may be mediated by descending or commissural systems. These observations indicate that Fos assays are useful for studying the pattern of neuronal activation in the auditory system and may also be useful in studying the descending auditory pathways.

Fos-like immunoreactivity elicited by sound stimulation in the auditory neurons of the big brown bat Eptesicus fuscus.

C-fos immunocytochemistry was used as a rapid and sensitive technique for identification of sound activated neurons in the cerebral cortex, the cerebellum and subcortical nuclei of the big brown bat, Eptesicus fuscus. When bats were stimulated with sounds under the both-ears opened conditions, Fos-like immunoreactive neurons were bilaterally and symmetrically distributed in all subcortical auditory nuclei, the auditory cortex, the superior colliculus, the pontine nuclei and the cerebellar deep nuclei. Interestingly, when bats were stimulated with sounds under the monaurally plugged conditions, a larger (31-74% more) number of Fos-like immunoreactive neurons were observed. They were predominantly distributed in all contralateral auditory nuclei from the level of the nucleus of the lateral lemniscus down and in all ipsilateral auditory nuclei from the level of inferior colliculus up as well as in the contralateral superior colliculus, pontine nuclei and cerebellar deep nuclei. Implications of these observations in relation to known mammalian auditory pathways and electrophysiological studies are discussed.

Topography of excitatory bandwidth in cat primary auditory cortex: single-neuron versus multiple-neuron recordings.

1. The spatial distribution of the sharpness of tuning of single neurons along the dorsoventral extent of primary auditory cortex (AI) was studied. A sharpness of tuning gradient was initially obtained with multiple-unit recordings, and in combination with the cochleotopic organization, served as a frame of reference for the locations of single neurons. The frequency selectivity or "integrated excitatory bandwidth" of multiple units varied systematically along the dorsoventral extent of AI. The most sharply tuned unit clusters were found at the approximate center of the dorsoventral extent. A gradual broadening of the integrated excitatory bandwidth in both dorsal and ventral directions was consistently seen. 2. The multiple-unit measures of the bandwidth 10 (BW10) and 40 dB (BW40) above minimum threshold, pooled across several animals and expressed in octaves, were similar to those described within individual cases in cats. As in the individual animals, the bandwidth maps were V shaped with minima located at the approximate center of the dorsal-ventral extent of AI. The location of the minimum in the multiple-unit bandwidth map (i.e., the most sharply tuned area) was used as a reference point to pool single-neuron data across animals. 3. For single neurons, the dorsal half of the BW40 distribution showed a gradient paralleling that found for multiple units. For both single and multiple units, the average excitatory bandwidth increased at a rate of approximately 0.27 octaves/mm from the center of AI toward the dorsal fringe. Differing from the dorsal half of AI, the ventral half of AI showed no clear BW40 gradient for single units along its dorsoventral extent. At 40 dB above minimum threshold, most ventral neurons encountered were sharply tuned. By contrast, the multiple-unit BW40 showed a gradient similar to the dorsal half with 0.23 octaves/mm increasing from the center toward the ventral border of AI. 4. For single neurons, BW10 showed no clear systematic spatial distribution in AI. Neither the dorsal nor the ventral gradient was significantly different from zero slope, although the dorsal half showed a trend toward increasing BW10s. Contrasting single neurons, both dorsal and ventral halves of AI showed BW10 slopes for multiple units confirming a V-shaped map of the integrated excitatory bandwidth within the dorsoventral extent of AI. 5. On the basis of the distribution of the integrated (multiple-unit) excitatory bandwidth, AI was parceled into three regions: the dorsal gradient, the ventral gradient, and the central, narrowly tuned area.(ABSTRACT TRUNCATED AT 400 WORDS)