Neocortical ectopias are associated with attenuated neurophysiological responses to rapidly changing auditory stimuli

Auditory evoked-potential abnormalities in a mouse model of 22q11.2 Deletion Syndrome and their interactions with hearing impairment

The 22q11.2 deletion is a risk factor for multiple psychiatric disorders including schizophrenia and also increases vulnerability to middle-ear problems that can cause hearing impairment. Up to 60% of deletion carriers experience hearing impairment and ~30% develop schizophrenia in adulthood. It is not known if these risks interact. Here we used the Df1/+ mouse model of the 22q11.2 deletion to investigate how hearing impairment might interact with increased genetic vulnerability to psychiatric disease to affect brain function. We measured brain function using cortical auditory evoked potentials (AEPs), which are commonly measured non-invasively in humans. After identifying one of the simplest and best-validated methods for AEP measurement in mice from the diversity of previous approaches, we measured peripheral hearing sensitivity and cortical AEPs in Df1/+ mice and their WT littermates. We exploited large inter-individual variation in hearing ability among Df1/+ mice to distinguish effects of genetic background from effects of hearing impairment. Central auditory gain and adaptation were quantified by comparing brainstem activity and cortical AEPs and by analyzing the growth of cortical AEPs with increasing sound level or inter-tone interval duration. We found that level-dependent AEP growth was abnormally large in Df1/+ mice regardless of hearing impairment, but other AEP measures of central auditory gain and adaptation depended on both genotype and hearing phenotype. Our results demonstrate the relevance of comorbid hearing loss to auditory brain dysfunction in 22q11.2DS and also identify potential biomarkers for psychiatric disease that are robust to hearing impairment.

The Orphan GPCR Receptor, GPR88, Interacts with Nuclear Protein Partners in the Cerebral Cortex

GPR88 is an orphan G-protein-coupled receptor (GPCR) highly expressed in striatal medium spiny neurons (MSN), also found in cortical neurons at low level. In MSN, GPR88 has a canonical GPCR plasma membrane/cytoplasmic expression, whereas in cortical neurons, we previously reported an atypical intranuclear localization. Molecular size analysis suggests that GPR88, expressed in plasma membrane of MSN or in nuclear compartment of cortical neurons, corresponds to the full-length protein. By transfection of cortical neurons, we showed that GPR88 fluorescent chimeras exhibit a nuclear localization. This localization is contingent on the third intracytoplasmic loop and C-terminus domains, even though these domains do not contain any known nuclear localization signals (NLS). Using yeast two-hybrid screening with these domains, we identified the nuclear proteins ATRX, TOP2B, and BAZ2B, all involved in chromatin remodeling, as potential protein partners of GPR88. We also validated the interaction of GPR88 with these nuclear proteins by proximity ligation assay on cortical neurons in culture and coimmunoprecipitation experiments on cortical extracts from GPR88 wild-type (WT) and knockout (KO) mice. The identification of GPR88 subcellular partners may provide novel functional insights for nonclassical modes of GPCR action that could be relevant in the maturating process of neocortical neurons.

Intravital Imaging of Neocortical Heterotopia Reveals Aberrant Axonal Pathfinding and Myelination around Ectopic Neurons

Neocortical heterotopia consist of ectopic neuronal clusters that are frequently found in individuals with cognitive disability and epilepsy. However, their pathogenesis remains poorly understood due in part to a lack of tractable animal models. We have developed an inducible model of focal cortical heterotopia that enables their precise spatiotemporal control and high-resolution optical imaging in live mice. Here, we report that heterotopia are associated with striking patterns of circumferentially projecting axons and increased myelination around neuronal clusters. Despite their aberrant axonal patterns, in vivo calcium imaging revealed that heterotopic neurons remain functionally connected to other brain regions, highlighting their potential to influence global neural networks. These aberrant patterns only form when heterotopia are induced during a critical embryonic temporal window, but not in early postnatal development. Our model provides a new way to investigate heterotopia formation in vivo and reveals features suggesting the existence of developmentally modulated, neuron-derived axon guidance and myelination factors.

Divergence and inheritance of neocortical heterotopia in inbred and genetically-engineered mice

Cortical function emerges from the intrinsic properties of neocortical neurons and their synaptic connections within and across lamina. Neurodevelopmental disorders affecting migration and lamination of the neocortex result in cognitive delay/disability and epilepsy. Molecular layer heterotopia (MLH), a dysplasia characterized by over-migration of neurons into layer I, are associated with cognitive deficits and neuronal hyperexcitability in humans and mice. The breadth of different inbred mouse strains that exhibit MLH and inheritance patterns of heterotopia remain unknown. A neuroanatomical survey of numerous different inbred mouse strains, 2 first filial generation (F1) hybrids, and one consomic strain (C57BL/6J-Chr 1^A/J/NaJ) revealed MLH only in C57BL/6 mice and the consomic strain. Heterotopia were observed in numerous genetically-engineered mouse lines on a congenic C57BL/6 background. These data indicate that heterotopia formation is a weakly penetrant trait requiring homozygosity of one or more C57BL/6 alleles outside of chromosome 1. These data are relevant toward understanding neocortical development and disorders affecting neocortical lamination.

Mind the Gap: Two Dissociable Mechanisms of Temporal Processing in the Auditory System

High temporal acuity of auditory processing underlies perception of speech and other rapidly varying sounds. A common measure of auditory temporal acuity in humans is the threshold for detection of brief gaps in noise. Gap-detection deficits, observed in developmental disorders, are considered evidence for "sluggish" auditory processing. Here we show, in a mouse model of gap-detection deficits, that auditory brain sensitivity to brief gaps in noise can be impaired even without a general loss of central auditory temporal acuity. Extracellular recordings in three different subdivisions of the auditory thalamus in anesthetized mice revealed a stimulus-specific, subdivision-specific deficit in thalamic sensitivity to brief gaps in noise in experimental animals relative to controls. Neural responses to brief gaps in noise were reduced, but responses to other rapidly changing stimuli unaffected, in lemniscal and nonlemniscal (but not polysensory) subdivisions of the medial geniculate body. Through experiments and modeling, we demonstrate that the observed deficits in thalamic sensitivity to brief gaps in noise arise from reduced neural population activity following noise offsets, but not onsets. These results reveal dissociable sound-onset-sensitive and sound-offset-sensitive channels underlying auditory temporal processing, and suggest that gap-detection deficits can arise from specific impairment of the sound-offset-sensitive channel.

SIGNIFICANCE STATEMENT

The experimental and modeling results reported here suggest a new hypothesis regarding the mechanisms of temporal processing in the auditory system. Using a mouse model of auditory temporal processing deficits, we demonstrate the existence of specific abnormalities in auditory thalamic activity following sound offsets, but not sound onsets. These results reveal dissociable sound-onset-sensitive and sound-offset-sensitive mechanisms underlying auditory processing of temporally varying sounds. Furthermore, the findings suggest that auditory temporal processing deficits, such as impairments in gap-in-noise detection, could arise from reduced brain sensitivity to sound offsets alone.

Neural Mechanisms of Language-Based Learning Impairments: Insights from Human Populations and Animal Models

The acquisition of speech perception and consequent expression of language represent fundamental aspects of human functioning. Yet roughly 7% to 8% of children who are otherwise healthy and of normal intelligence exhibit unexplained delays and impairments in acquiring these skills. Ongoing research has revealed several key features of language disability that may pro-vide more direct insight into underlying anomalous neural functioning. For example, evidence supports a strong association between basic defects in processing rapidly changing acoustic information and emergent disruptions in speech perception, as well as cascading effects on other forms of language development (including reading). Considerable neurobiological research has thus focused on developmental factors that might deleteriously influence rapid sensory processing. Additional research focuses on mechanisms of neural plasticity, including how such brains might be “retrained” for improved processing of language. These and related findings from human clinical studies, electrophysiological studies, neuroimaging studies, and animal models are reviewed.

Bilateral subcortical heterotopia with partial callosal agenesis in a mouse mutant

Cognition and behavior depend on the precise placement and interconnection of complex ensembles of neurons in cerebral cortex. Mutations that disrupt migration of immature neurons from the ventricular zone to the cortical plate have provided major insight into mechanisms of brain development and disease. We have discovered a new and highly penetrant spontaneous mutation that leads to large nodular bilateral subcortical heterotopias with partial callosal agenesis. The mutant phenotype was first detected in a colony of fully inbred BXD29 mice already known to harbor a mutation in Tlr4. Neurons confined to the heterotopias are mainly born in midgestation to late gestation and would normally have migrated into layers 2-4 of overlying neocortex. Callosal cross-sectional area and fiber number are reduced up to 50% compared with coisogenic wildtype BXD29 substrain controls. Mutants have a pronounced and highly selective defect in rapid auditory processing. The segregation pattern of the mutant phenotype is most consistent with a two-locus autosomal recessive model, and selective genotyping definitively rules out the Tlr4 mutation as a cause. The discovery of a novel mutation with strong pleiotropic anatomical and behavioral effects provides an important new resource for dissecting molecular mechanisms and functional consequences of errors of neuronal migration.

Neocortical molecular layer heterotopia in substrains of C57BL/6 and C57BL/10 mice

Abnormal development of the neocortex is often associated with cognitive deficits and epilepsy. Rodent models are widely used to study normal and abnormal cortical development and have revealed the roles of many important genetic and environmental factors. Interestingly, several inbred mouse strains commonly used in behavioral, anatomical, and/or physiological studies display neocortical malformations including C57BL/6J mice, which are among the most widely utilized mice. In the present report we describe the prevalence and cytoarchitecture of molecular-layer heterotopia in C57BL/6J mice and related strains obtained from three commercial vendors as well as mice bred in academic vivaria from founders obtained commercially. In particular, we found that the prevalence of molecular-layer heterotopia vaired according to the sex as well as the vendor-of-origin of the mouse. These data are relevant to the use of this strain as a mouse-model in the study of brain-behavior relationships.

Layer I neocortical ectopia: cellular organization and local cortical circuitry

Focal cortical dysplasia (FCD) are associated with neurological disorders and cognitive impairments in humans. Molecular layer ectopia, clusters of misplaced cells in layer I of the neocortex, have been identified in patients with developmental dyslexia and psychomotor retardation. Mouse models of this developmental disorder display behavioral impairments and increased seizure susceptibility. Although there is a correlation between cortical malformations and neurological dysfunction, little is known about the morphological and physiological properties of cells within cortical malformations. In the present study we used electrophysiological and immunocytochemical analyses to examine the distribution of neuronal and non-neuronal cell types within and surrounding layer I neocortical ectopia in NXSMD/EiJ mice. We show that cells within ectopia have membrane properties of both pyramidal and a variety of non-pyramidal cell types, including fast-spiking cells. Immunocytochemical analysis for different interneuronal subtypes demonstrates that ectopia contain nonpyramidal cells immunoreactive for calbindin-D28K (CALB), parvalbumin (PARV), and calretinin (CR). Ectopia also contains astrocytes, positive for glial fibrillary acidic protein (GFAP) and oligodendrocyte precursor cells positive for NG2 proteoglycan (NG2). Lastly, we provide electrophysiological and morphological evidence to demonstrate that cells within ectopia receive input from cells within layers I, upper and deeper II/III, and V and provide outputs to cells within deep layer II/III and layer V, but not layers I and upper II/III. These results indicate that ectopia contain cells of different lineages with diverse morphological and physiological properties, and appear to cause disruptions in local cortical circuitry.

Progress towards a cellular neurobiology of reading disability

Reading Disability (RD) is a significant impairment in reading accuracy, speed and/or comprehension despite adequate intelligence and educational opportunity. RD affects 5-12% of readers, has a well-established genetic risk, and is of unknown neurobiological cause or causes. In this review we discuss recent findings that revealed neuroanatomic anomalies in RD, studies that identified 3 candidate genes (KIAA0319, DYX1C1, and DCDC2), and compelling evidence that potentially link the function of candidate genes to the neuroanatomic anomalies. A hypothesis has emerged in which impaired neuronal migration is a cellular neurobiological antecedent to RD. We critically evaluate the evidence for this hypothesis, highlight missing evidence, and outline future research efforts that will be required to develop a more complete cellular neurobiology of RD.

Event-related oscillations in mice: effects of stimulus characteristics

Event-related oscillations (EROs) are rhythmic changes that are evoked by sensory and/or cognitive processes that influence the dynamics of the EEG. EROs are estimated by a decomposition of the EEG signal into phase and magnitude information for a range of frequencies and then changes in those frequencies are characterized over a millisecond time scale with respect to task events. EROs have been demonstrated to be sensitive measures of both normal and abnormal cognitive functioning in humans but have not been fully described in mice. The results of these studies demonstrate that EROs can be generated in cortical sites in mice in the delta, theta, alpha/beta frequency ranges in response to auditory stimuli. Oscillations in the 7.5-40 Hz frequencies were significantly affected in the 0-50 ms time range in response to differences in tone frequency. Whereas, changes in tone loudness produced changes in oscillations in the 7.5-40 Hz frequencies in the 350-800 ms range. No significant changes in EROs were found to differences in tone probability. These studies suggest that EROs are an electrophysiological assay sensitive to tone characteristics and as such may be suitable for the exploration of the effects of genetic or neuropharmacological manipulations on neurosensory processing in mice.

Cytoarchitecture and transcriptional profiles of neocortical malformations in inbred mice

Malformations of neocortical development are associated with cognitive dysfunction and increased susceptibility to epileptogenesis. Rodent models are widely used to study neocortical malformations and have revealed important genetic and environmental mechanisms that contribute to neocortical development. Interestingly, several inbred mice strains commonly used in behavioral, anatomical, and/or physiological studies display neocortical malformations. In the present report we examine the cytoarchitecture and myeloarchitecture of the neocortex of 11 inbred mouse strains and identified malformations of cortical development, including molecular layer heterotopia, in all but one strain. We used in silico methods to confirm our observations and determined the transcriptional profiles of cells found within heterotopia. These data indicate cellular and transcriptional diversity present in cells in malformations. Furthermore, the presence of dysplasia in nearly every inbred strain examined suggests that malformations of neocortical development are a common feature in the neocortex of inbred mice.

Postnatal analysis of the effect of embryonic knockdown and overexpression of candidate dyslexia susceptibility gene homolog Dcdc2 in the rat

Embryonic knockdown of candidate dyslexia susceptibility gene (CDSG) homologs in cerebral cortical progenitor cells in the rat results in acute disturbances of neocortical migration. In the current report we investigated the effects of embryonic knockdown and overexpression of the homolog of DCDC2, one of the CDSGs, on the postnatal organization of the cerebral cortex. Using a within-litter design, we transfected cells in rat embryo neocortical ventricular zone around embryonic day (E) 15 with either 1) small hairpin RNA (shRNA) vectors targeting Dcdc2, 2) a DCDC2 overexpression construct, 3) Dcdc2 shRNA along with DCDC2 overexpression construct, 4) an overexpression construct composed of the C terminal domain of DCDC2, or 5) an overexpression construct composed of the DCX terminal domain of DCDC2. RNAi of Dcdc2 resulted in pockets of heterotopic neurons in the periventricular region. Approximately 25% of the transfected brains had hippocampal pyramidal cell migration anomalies. Dcdc2 shRNA-transfected neurons migrated in a bimodal pattern, with approximately 7% of the neurons migrating a short distance from the ventricular zone, and another 30% migrating past their expected lamina. Rats transfected with Dcdc2 shRNA along with the DCDC2 overexpression construct rescued the periventricular heterotopia phenotype, but did not affect the percentage of transfected neurons that migrate past their expected laminar location. There were no malformations associated with any of the overexpression constructs, nor was there a significant laminar disruption of migration. These results support the claim that knockdown of Dcdc2 expression results in neuronal migration disorders similar to those seen in the brains of dyslexics.

Early cortical damage in rat somatosensory cortex alters acoustic feature representation in primary auditory cortex

Early postnatal freeze-lesions to the cortical plate result in malformations resembling human microgyria. Microgyria in primary somatosensory cortex (S1) of rats are associated with a reduced behavioral detection of rapid auditory transitions and the loss of large cells in the thalamic nucleus projecting to primary auditory cortex (A1). Detection of slow transitions in sound is intact in animals with S1 microgyria, suggesting dissociation between responding to slow versus rapid transitions and a possible dissociation between levels of auditory processing affected. We hypothesized that neuronal responses in primary auditory cortex (A1) would be differentially reduced for rapid sound repetitions but not for slow sound sequences in animals with S1 microgyria. We assessed layer IV cortical responses in primary auditory cortex (A1) to single pure-tones and periodic noise bursts (PNB) in rats with and without S1 microgyria. We found that responses to both types of acoustic stimuli were reduced in magnitude in animals with microgyria. Furthermore, spectral resolution was degraded in animals with microgyria. The cortical selectivity and temporal precision were then measured with conventional methods for PNB and tone-stimuli, but no significant changes were observed between microgyric and control animals. Surprisingly, the observed spike rate reduction was similar for rapid and slow temporal modulations of PNB stimuli. These results suggest that acoustic processing in A1 is indeed altered with early perturbations of neighboring cortex. However, the type of deficit does not affect the temporal dynamics of the cortical output. Instead, acoustic processing is altered via a systematic reduction in the driven spike rate output and spectral integration resolution in A1. This study suggests a novel form of plasticity, whereas early postnatal lesions of one sensory cortex can have a functional impact on processing in neighboring sensory cortex.

Brain abnormalities in language disorders and in autism

It has been speculated that autism and specific language impairment share common underlying neural substrates because of the overlap in language impairment issues and evidence suggesting parallels in other domains and implying a possible shared genetic risk. Anatomically the two sets of disorders have generally been studied using different methodologies, but when identical methodologies have been used substantial similarities have been noted. Functionally there is a growing body of literature suggesting sensory perception abnormalities that have parallels in both conditions and that may be upstream of language abnormalities. Finding upstream mechanisms that impact language and non-language abnormalities in autism and specific language impairment would impact the orientation taken by translational attempts to use science to design treatments.

Development of structure and function in the infant brain: implications for cognition, language and social behaviour

Recent advances in cognitive neuroscience have allowed us to begin investigating the development of both structure and function in the infant brain. However, despite the rapid evolution of technology, surprisingly few studies have examined the intersection between brain and behaviour over the first years of life. Even fewer have done so in the context of a particular research question. This paper aims to provide an overview of four domains that have been studied using techniques amenable to elucidating the brain/behaviour interface: language, face processing, object permanence, and joint attention, with particular emphasis on studies focusing on early development. The importance of the unique role of development and the interplay between structure and function is stressed throughout. It is hoped that this review will serve as a catalyst for further thinking about the substantial gaps in our understanding of the relationship between brain and behaviour across development. Further, our aim is to provide ideas about candidate brain areas that are likely to be implicated in particular behaviours or cognitive domains.

Developmental disorders of vision

This review of developmental disorders of vision focuses on only a few of the many disorders that disrupt visual development. Given the enormity of the human visual system in the primate brain and complexity of visual development, however, there are likely hundreds or thousands of types of disorders affecting high-level vision. The rapid progress seen in developmental dyslexia and WMS demonstrates the possibilities and difficulties inherent in researching such disorders, and the authors hope that similar progress will be made for congenital prosopagnosia and other disorders in the near future.

Long latency event-related potentials in mice: effects of stimulus characteristics and strain

The P3, or P300 component of the event-related potentials (ERPs) is a positive going waveform that can be averaged from the EEG approximately 250-500 ms following the presentation of task or context 'relevant' stimuli. This potential has been demonstrated to be a sensitive measure of both normal and abnormal cognitive functioning. P300 models have been developed in monkeys, cats and rats. The aim of the present study was to develop an auditory ERP paradigm suitable for use in mice that resembled those used in humans and other animal models. The results of the studies showed that late positive potentials in the 200-400 ms range could be generated in cortical sites in response to auditory stimuli. Additionally, like passive ERPs recorded in humans, mouse ERPs were sensitive to changes in stimulus characteristics. An earlier negative component designated the N1 was found to be sensitive to tone frequency and loudness but not to stimulus probability, whereas the mouse P300 component was sensitive to probability but not to tone frequency or loudness. C57BL/6 mice, a strain known to have a strong alcohol preference, were found to have significantly lower P300 amplitudes when compared to the DBA/2 strain. These findings also parallel human studies, and studies of selected lines of rats, demonstrating that decrements in P300 amplitude can be associated with a genetic vulnerability to alcoholism/alcohol preference. These studies further suggest that ERPs are an electrophysiological assay suitable for the exploration of the effects of genetic manipulations on neurosensory processing in mice.

Sex differences in rapid auditory processing deficits in ectopic BXSB/MpJ mice

Prior research with rodent models, performed predominantly in males, has demonstrated a significant association between focal neocortical malformations (e.g. ectopias and microgyria) and rate-specific auditory processing deficits. In the current study and consistent with prior findings, we report that ectopic male BXSB/MpJ mice exhibit impairments in detecting a two-tone oddball stimulus at short but not long inter-stimulus interval durations when compared to non-ectopic male littermates. However, ectopic female littermates showed no rapid auditory processing deficit when compared to non-ectopic females on this same task. Current results add growing support to: (1) an association between focal cortical malformations and impaired auditory processing in males; and (2) the existence of sex differences in the behavioral consequences of focal cortical malformations.

Auditory cortical responses to speech-like stimuli in dyslexic adults

Auditory cortical processing of speech-like sounds was studied in 9 dyslexic and 11 normal-reading adults. Noise/square-wave sequences, mimicking transitions from a fricative consonant to a vowel, were presented binaurally once every 1.1 sec and the cortical responses were recorded with a whole-scalp neuromagnetometer. The auditory cortices of both hemispheres were less reactive to acoustical changes in dyslexics than in controls, as was evident from the weaker responses to the noise/square-wave transitions. The results demonstrate that dyslexic adults are deficient in processing acoustic changes presented in rapid succession within tens to hundreds of milliseconds. The observed differences could be related to insufficient triggering of automatic auditory attention, resulting, for instance, from a general deficiency of the magnocellular system.

Layer I ectopias and increased excitability in murine neocortex

Cortical dysplasias are associated with both epilepsy and cognitive impairments in humans. Similarly, several animal models of cortical dysplasia show that dysplasia causes increased seizure susceptibility and behavioral deficits in vivo and increased levels of excitability in vitro. As most current animal models involve either global disruptions in cortical architecture or the induction of lesions, it is not yet clear whether small spontaneous neocortical malformations are also associated with increased excitability or seizure susceptibility. Small groups of displaced neurons in layer I of the neocortex, ectopias, have been identified in patients with cognitive impairments, and similar malformations occur sporadically in some inbred lines of mice where they are associated with behavioral and sensory-processing deficits. In a previous study, we characterized the physiology of cells within neocortical ectopias, in one of the inbred lines (NXSM-D/Ei) and showed that the presence of multiple ectopias is associated with the generation of spontaneous epileptiform activity in slices. In this study, we use extracellular recordings from brain slices to show that even single-layer I ectopias are associated with higher excitability. Specifically, slices that contain single ectopias display epileptiform activity at significantly lower concentrations of the GABA(A) receptor antagonist bicuculline than do slices without ectopias (either from opposite hemispheres or animals without ectopias). Moreover, because removal of ectopias from slices does not restore normal excitability, enhanced excitability is not generated within the ectopia. Finally, we show that in vivo, mice with ectopias are more sensitive to the convulsant pentylenetetrazole than are mice without ectopias. Together these results suggest that alterations in cortical hemispheres containing focal layer I malformations increase cortical excitability and that even moderately small spontaneous cortical dysplasias are associated with increased excitability in vitro and in vivo.

The importance of rapid auditory processing abilities to early language development: evidence from converging methodologies

The ability to process two or more rapidly presented, successive, auditory stimuli is believed to underlie successful language acquisition. Likewise, deficits in rapid auditory processing of both verbal and nonverbal stimuli are characteristic of individuals with developmental language disorders such as Specific Language Impairment. Auditory processing abilities are well developed in infancy, and thus such deficits should be detectable in infants. In the studies presented here, converging methodologies are used to examine such abilities in infants with and without a family history of language disorder. Behavioral measures, including assessments of infant information processing, and an EEG/event-related potential (ERP) paradigm are used concurrently. Results suggest that rapid auditory processing skills differ as a function of family history and are predictive of later language outcome. Further, these paradigms may prove to be sensitive tools for identifying children with poor processing skills in infancy and thus at a higher risk for developing a language disorder.

Impaired processing of rapid stimulus sequences in dyslexia

Apart from their reading difficulties, dyslexic subjects often suffer from a variety of subtle sensory and motor deficits. Whether these deficits have a causal relationship to the reading disorder, form additional risk factors, or are totally independent of the reading problem, is under vivid debate. In this article, we review the evidence and suggest that 'sluggish attentional shifting' (SAS) can account for the impaired processing of rapid stimulus sequences in dyslexia. Within this novel framework attention-related prolongation of input chunks is decisive for many small deficits found in dyslexic subjects.

Effects of ectopias and their cortical location on several measures of learning in BXSB mice

About half of BXSB/MpJ-Yaa mice have ectopias, which are misplaced clusters of neurons located in layer I of cortex. This study replicated several previous findings showing that there are learning differences between mice with ectopias and those without. In addition, we had sufficient numbers of ectopic mice to investigate if ectopics learned differently depending on the cortical location of the ectopia(s). Mice with at least one ectopia located in prefrontal cortex were initially impaired in learning the Morris maze, as well as relearning the Lashley maze when it was inverted, but learned better in the radial-arm maze when compared to ectopic mice with ectopias located in nonprefrontal regions of cortex. Mice with at least one ectopia in motor cortex learned the Lashley maze better than mice with ectopias located outside motor cortex. In sum, the cortical location of the ectopia(s) affected learning performance in certain tasks within the ectopic group, but regardless of the cortical location of the ectopia(s), ectopics still learned differently than nonectopics in several tasks.

Impaired detection of variable duration embedded tones in ectopic NZB/BINJ mice

Utilizing rodent models, prior research has demonstrated a significant association between focal neocortical malformations (i.e. induced microgyria, molecular layer ectopias), which are histologically similar to those observed in human dyslexic brains, and rate-specific auditory processing deficits as seen in language impaired populations. In the current study, we found that ectopic NZB/BINJ mice exhibit significant impairments in detecting a variable duration 5.6 kHz tone embedded in a 10.5 kHz continuous background, using both acoustic reflex modification and auditory event-related potentials (AERP). The current results add further support to the association between focal cortical malformations and impaired auditory processing, and the notion that these auditory effects may occur regardless of the cortical location of the anomaly.

Electrophysiological and morphological characterization of neurons within neocortical ectopias

Focal developmental abnormalities in neocortex, including ectopic collections of neurons in layer I (ectopias), have been associated with behavioral and neurological deficits. In this study, we used infrared differential interference contrast microscopy and whole cell patch-clamp to complete the first characterization of neurons within and surrounding neocortical ectopias. Current-clamp recordings revealed that neurons within ectopias display multiple types of action potential firing patterns, and biocytin labeling indicated that approximately 20% of the cells in neocortical ectopias can be classified as nonpyramidal cells and the rest as atypically oriented pyramidal cells. All cells had spontaneous excitatory (glutamatergic) and inhibitory (GABAergic) postsynaptic currents. Exhibitory postsynaptic currents consisted of both N-methyl-D-aspartate (NMDA) receptor-mediated and AMPA/kainate (A/K) receptor-mediated currents. The NMDA receptor-mediated component had decay time constants of 15.35 +/- 2.2 (SE) ms, while the A/K component had faster decay kinetics of 7.6 +/- 1.7 ms at -20 mV. GABA(A) receptor-mediated synaptic currents in ectopic cells reversed at potentials near the Cl- equilibrium potential and had decay kinetics of 16.65 +/- 1.3 ms at 0 mV. Furthermore we show that cells within ectopias receive direct excitatory and inhibitory input from adjacent normatopic cortex and can display a form of epileptiform activity.