Julie Hamaide

Brain inflammation in a chronic epilepsy model: Evolving pattern of the translocator protein during epileptogenesis.

AIMS A hallmark in the neuropathology of temporal lobe epilepsy is brain inflammation which has been suggested as both a biomarker and a new mechanistic target for treatments. The translocator protein (TSPO), due to its high upregulation under neuroinflammatory conditions and the availability of selective PET tracers, is a candidate target. An important step to exploit this target is a thorough characterisation of the spatiotemporal profile of TSPO during epileptogenesis. METHODS TSPO expression, microglial activation, astrocyte reactivity and cell loss in several brain regions were evaluated at five time points during epileptogenesis, including the chronic epilepsy phase in the kainic acid-induced status epilepticus (KASE) model (n = 52) and control Wistar Han rats (n = 33). Seizure burden was also determined in the chronic phase. Furthermore, ¹⁸F-PBR111 PET/MRI scans were acquired longitudinally in an additional four KASE animals. RESULTS TSPO expression measured with in vitro and in vivo techniques was significantly increased at each time point and peaked two weeks post-SE in the limbic system. A prominent association between TSPO expression and activated microglia (p < 0.001; r = 0.7), as well as cell loss (p < 0.001; r = -0.8) could be demonstrated. There was a significant positive correlation between spontaneous seizures and TSPO upregulation in several brain regions with increased TSPO expression. CONCLUSIONS TSPO expression was dynamically upregulated during epileptogenesis, persisted in the chronic phase and correlated with microglia activation rather than reactive astrocytes. TSPO expression was correlating with spontaneous seizures and its high expression during the latent phase might possibly suggest being an important switching point in disease ontogenesis which could be further investigated by PET imaging.

The power of using functional fMRI on small rodents to study brain pharmacology and disease

Functional magnetic resonance imaging (fMRI) is an excellent tool to study the effect of pharmacological modulations on brain function in a non-invasive and longitudinal manner. We introduce several blood oxygenation level dependent (BOLD) fMRI techniques, including resting state (rsfMRI), stimulus-evoked (st-fMRI), and pharmacological MRI (phMRI). Respectively, these techniques permit the assessment of functional connectivity during rest as well as brain activation triggered by sensory stimulation and/or a pharmacological challenge. The first part of this review describes the physiological basis of BOLD fMRI and the hemodynamic response on which the MRI contrast is based. Specific emphasis goes to possible effects of anesthesia and the animal’s physiological conditions on neural activity and the hemodynamic response. The second part of this review describes applications of the aforementioned techniques in pharmacologically induced, as well as in traumatic and transgenic disease models and illustrates how multiple fMRI methods can be applied successfully to evaluate different aspects of a specific disorder. For example, fMRI techniques can be used to pinpoint the neural substrate of a disease beyond previously defined hypothesis-driven regions-of-interest. In addition, fMRI techniques allow one to dissect how specific modifications (e.g., treatment, lesion etc.) modulate the functioning of specific brain areas (st-fMRI, phMRI) and how functional connectivity (rsfMRI) between several brain regions is affected, both in acute and extended time frames. Furthermore, fMRI techniques can be used to assess/explore the efficacy of novel treatments in depth, both in fundamental research as well as in preclinical settings. In conclusion, by describing several exemplary studies, we aim to highlight the advantages of functional MRI in exploring the acute and long-term effects of pharmacological substances and/or pathology on brain functioning along with several methodological considerations.

Neuroplasticity and MRI: A perfect match

Numerous studies have illustrated the benefits of physical workout and cognitive exercise on brain function and structure and, more importantly, on decelerating cognitive decline in old age and promoting functional rehabilitation following injury. Despite these behavioral observations, the exact mechanisms underlying these neuroplastic phenomena remain obscure. This gap illustrates the need for carefully designed in-depth studies using valid models and translational tools which allow to uncover the observed events up to the molecular level. We promote the use of in vivo magnetic resonance imaging (MRI) because it is a powerful translational imaging technique able to extract functional, structural, and biochemical information from the entire brain. Advanced processing techniques allow performing voxel-based analyses which are capable of detecting novel loci implicated in specific neuroplastic events beyond traditional regions-of-interest analyses. In addition, its non-invasive character sets it as currently the best global imaging tool for performing dynamic longitudinal studies on the same living subject, allowing thus exploring the effects of experience, training, treatment etc. in parallel to additional measures such as age, cognitive performance scores, hormone levels, and many others. The aim of this review is (i) to introduce how different animal models contributed to extend the knowledge on neuroplasticity in both health and disease, over different life stages and upon various experiences, and (ii) to illustrate how specific MRI techniques can be applied successfully to inform on the fundamental mechanisms underlying experience-dependent or activity-induced neuroplasticity including cognitive processes.

Exploring sex differences in the adult zebra finch brain: In vivo diffusion tensor imaging and ex vivo super-resolution track density imaging.

Abstract Zebra finches are an excellent model to study the process of vocal learning, a complex socially‐learned tool of communication that forms the basis of spoken human language. So far, structural investigation of the zebra finch brain has been performed ex vivo using invasive methods such as histology. These methods are highly specific, however, they strongly interfere with performing whole‐brain analyses and exclude longitudinal studies aimed at establishing causal correlations between neuroplastic events and specific behavioral performances. Therefore, the aim of the current study was to implement an in vivo Diffusion Tensor Imaging (DTI) protocol sensitive enough to detect structural sex differences in the adult zebra finch brain. Voxel‐wise comparison of male and female DTI parameter maps shows clear differences in several components of the song control system (i.e. Area X surroundings, the high vocal center (HVC) and the lateral magnocellular nucleus of the anterior nidopallium (LMAN)), which corroborate previous findings and are in line with the clear behavioral difference as only males sing. Furthermore, to obtain additional insights into the 3‐dimensional organization of the zebra finch brain and clarify findings obtained by the in vivo study, ex vivo DTI data of the male and female brain were acquired as well, using a recently established super‐resolution reconstruction (SRR) imaging strategy. Interestingly, the SRR‐DTI approach led to a marked reduction in acquisition time without interfering with the (spatial and angular) resolution and SNR which enabled to acquire a data set characterized by a 78 &mgr;m isotropic resolution including 90 diffusion gradient directions within 44 h of scanning time. Based on the reconstructed SRR‐DTI maps, whole brain probabilistic Track Density Imaging (TDI) was performed for the purpose of super resolved track density imaging, further pushing the resolution up to 40 &mgr;m isotropic. The DTI and TDI maps realized atlas‐quality anatomical maps that enable a clear delineation of most components of the song control and auditory systems. In conclusion, this study paves the way for longitudinal in vivo and high‐resolution ex vivo experiments aimed at disentangling neuroplastic events that characterize the critical period for vocal learning in zebra finch ontogeny. HighlightsIn vivo DTI successfully detects sex differences in the adult zebra finch brain.Ex vivo track density imaging provides an exquisite view on zebra finch anatomy.Super‐resolution DTI reduced scanning time without compromising spatial and angular resolution.

fMRI Reveals a Novel Region for Evaluating Acoustic Information for Mate Choice in a Female Songbird

Selection of sexual partners is among the most critical decisions that individuals make and is therefore strongly shaped by evolution. In social species, where communication signals can convey substantial information about the identity, state, or quality of the signaler, accurate interpretation of communication signals for mate choice is crucial. Despite the importance of social information processing, to date, relatively little is known about the neurobiological mechanisms that contribute to sexual decision making and preferences. In this study, we used a combination of whole-brain functional magnetic resonance imaging (fMRI), immediate early gene expression, and behavior tests to identify the circuits that are important for the perception and evaluation of courtship songs in a female songbird, the zebra finch (Taeniopygia guttata). Female zebra finches are sensitive to subtle differences in male song performance and strongly prefer the longer, faster, and more stereotyped courtship songs to non-courtship renditions. Using BOLD fMRI and EGR1 expression assays, we uncovered a novel region involved in auditory perceptual decision making located in a sensory integrative region of the avian central nidopallium outside the traditionally studied auditory forebrain pathways. Changes in activity in this region in response to acoustically similar but categorically divergent stimuli showed stronger parallels to behavioral responses than an auditory sensory region. These data highlight a potential role for the caudocentral nidopallium (NCC) as a novel node in the avian circuitry underlying the evaluation of acoustic signals and their use in mate choice.

Auditory evoked BOLD responses in awake compared to lightly anaesthetized zebra finches

Functional magnetic resonance imaging (fMRI) is increasingly used in cognitive neuroscience and has become a valuable tool in the study of auditory processing in zebra finches, a well-established model of learned vocal communication. Due to its sensitivity to head motion, most fMRI studies in animals are performed in anaesthetized conditions, which might significantly impact neural activity evoked by stimuli and cognitive tasks. In this study, we (1) demonstrate the feasibility of fMRI in awake zebra finches and (2) explore how light anaesthesia regimes affect auditory-evoked BOLD responses to biologically relevant songs. After an acclimation procedure, we show that fMRI can be successfully performed during wakefulness, enabling the detection of reproducible BOLD responses to sound. Additionally, two light anaesthesia protocols were tested (isoflurane and a combination of medetomidine and isoflurane), of which isoflurane alone appeared to be the most promising given the high success rate, non-invasive induction, and quick recovery. By comparing auditory evoked BOLD responses in awake versus lightly anaesthetized conditions, we observed overall effects of anaesthetics on cerebrovascular reactivity as reflected in the extent of positive and negative BOLD responses. Further, our results indicate that light anaesthesia has limited effects on selective BOLD responses to natural versus synthetic sounds.

Song Processing in the Zebra Finch Auditory Forebrain Reflects Asymmetric Sensitivity to Temporal and Spectral Structure

Despite being commonly referenced throughout neuroscientific research on songbirds, reports of hemispheric specialization in the processing of song remain controversial. The notion of such asymmetries in songbirds is further complicated by evidence that both cerebral hemispheres in humans may be specialized for different aspects of speech perception. Some studies suggest that the auditory neural substrates in the left and right hemispheres of humans process temporal and spectral elements within speech sounds, respectively. To determine whether songbirds process their conspecific songs in such a complementary, bilateral manner, we performed functional magnetic resonance imaging (fMRI) on 15 isoflurane anesthetized adult male zebra finches (Taeniopygia guttata) while presenting them with (1) non-manipulated, (2) spectrally-filtered (reduced spectral structure), and (3) temporally-filtered (reduced temporal structure) conspecific song. Our results revealed sensitivity of both primary (Field L) and secondary (caudomedial nidopallium, NCM) auditory regions to changes in spectral and temporal structure of song. On the one hand, temporally-filtered song elicited a bilateral decrease in neural responses compared to the other stimulus types. On the other hand, spectrally filtered song elicited significantly greater responses in left Field L and NCM than temporally filtered or non-manipulated song while concurrently reducing the response relative to non-manipulated song in the right auditory forebrain. The latter hemispheric difference in sensitivity to manipulations of spectral structure in song, suggests that there is an asymmetry in spectral and temporal domain processing in the zebra finch auditory forebrain bearing some resemblance to what has been observed in human auditory cortex.

Neuroplasticity in the cerebello-thalamo-basal ganglia pathway: A longitudinal in vivo MRI study in male songbirds

ABSTRACT Similar to human speech, bird song is controlled by several pathways including a cortico‐basal ganglia‐thalamo‐cortical (C‐BG‐T‐C) loop. Neurotoxic disengagement of the basal ganglia component, i.e. Area X, induces long‐term changes in song performance, while most of the lesioned area regenerates within the first months. Importantly however, the timing and spatial extent of structural neuroplastic events potentially affecting other constituents of the C‐BG‐T‐C loop is not clear. We designed a longitudinal MRI study where changes in brain structure were evaluated relative to the time after neurotoxic lesioning or to vocal performance. By acquiring both Diffusion Tensor Imaging and 3‐dimensional anatomical scans, we were able to track alterations in respectively intrinsic tissue properties and local volume. Voxel‐based statistical analyses revealed structural remodeling remote to the lesion, i.e. in the thalamus and, surprisingly, the cerebellum, both peaking within the first two months after lesioning Area X. Voxel‐wise correlations between song performance and MRI parameters uncovered intriguing brain‐behavior relationships in several brain areas pertaining to the C‐BG‐T‐C loop supervising vocal motor control. Our results clearly point to structural neuroplasticity in the cerebellum induced by basal ganglia (striatal) damage and might point to the existence of a human‐like cerebello‐thalamic‐basal ganglia pathway capable of modifying vocal motor output. HIGHLIGHTSStriatal lesioning suggests human‐like cerebellar input in song control pathway.Song correlates with DTI metrics of cortico‐basal ganglia thalamo‐cortical loop.Striatal lesioning induces long‐term decrease in song motif duration.Lesion‐induced microstructural remodeling peaks within first the 2 months.

Volumetric development of the zebra finch brain throughout the first 200 days of post-hatch life traced by in vivo MRI

&NA; The first months of life are characterized by massive neuroplastic processes that parallel the acquisition of skills and abilities vital for proper functioning in later life. Likewise, juvenile songbirds learn the song sung by their tutor during the first months after hatching. To date, most studies targeting brain development in songbirds exclusively focus on the song control and auditory pathways. To gain a comprehensive insight into structural developmental plasticity of the entire zebra finch brain throughout the different subphases of song learning, we designed a longitudinal study in a group of male (16) and female (19) zebra finches. We collected T2‐weighted 3‐dimensional anatomical scans at six developmental milestones throughout the process of song learning, i.e. 20, 30, 40, 65, 90 and 120 days post hatching (dph), and one additional time point well after song crystallization, i.e. 200 dph. We observed that the total brain volume initially increases, peaks around 30–40 dph and decreases towards the end of the study. Further, we performed brain‐wide voxel‐based volumetric analyses to create spatio‐temporal maps indicating when specific brain areas increase or decrease in volume, relative to the subphases of song learning. These maps informed (1) that most areas implicated in song control change early, i.e. between 20 and 65 dph, and are embedded in large clusters that cover major subdivisions of the zebra finch brain, (2) that volume changes between consecutive subphases of vocal learning appear highly similar in males and females, and (3) that only more rostrally situated brain regions change in volume towards later ages. Lastly, besides detecting sex differences in local tissue volume that align with previous studies, we uncovered two additional brain loci that are larger in male compared to female zebra finches. These volume differences co‐localize with areas related to the song control and auditory pathways and can therefore be associated to the behavioral difference as only male zebra finches sing. In sum, our data point to clear heterochronous patterns of brain development similar to brain development in mammalian species and this work can serve as a reference for future neurodevelopmental imaging studies in zebra finches.

A three-dimensional digital neurological atlas of the mustached bat (Pteronotus parnellii)

&NA; Substantial knowledge of auditory processing within mammalian nervous systems emerged from neurophysiological studies of the mustached bat (Pteronotus parnellii). This highly social and vocal species retrieves precise information about the velocity and range of its targets through echolocation. Such high acoustic processing demands were likely the evolutionary pressures driving the over‐development at peripheral (cochlea), metencephalic (cochlear nucleus), mesencephalic (inferior colliculus), diencephalic (medial geniculate body of the thalamus), and telencephalic (auditory cortex) auditory processing levels in this species. Auditory researchers stand to benefit from a three dimensional brain atlas of this species, due to its considerable contribution to auditory neuroscience. Our MRI‐based atlas was generated from 2 sets of image data of an ex‐vivo male mustached bats brain: a detailed 3D‐T2‐weighted‐RARE scan [(59 × 63 x 85) &mgr;m3] and track density images based on super resolution diffusion tensor images [(78) &mgr;m3] reconstructed from a set of low resolution diffusion weighted images using Super‐Resolution‐Reconstruction (SRR). By surface‐rendering these delineations and extrapolating from cortical landmarks and data from previous studies, we generated overlays that estimate the locations of classic functional subregions within mustached bat auditory cortex. This atlas is freely available from our website and can simplify future electrophysiological, microinjection, and neuroimaging studies in this and related species.