Changes of 2-deoxyglucose uptake in the rat auditory pathway after bilateral ablation of the cochlea

Hypothalamus-hippocampus circuitry regulates impulsivity via melanin-concentrating hormone

Behavioral impulsivity is common in various psychiatric and metabolic disorders. Here we identify a hypothalamus to telencephalon neural pathway for regulating impulsivity involving communication from melanin-concentrating hormone (MCH)-expressing lateral hypothalamic neurons to the ventral hippocampus subregion (vHP). Results show that both site-specific upregulation (pharmacological or chemogenetic) and chronic downregulation (RNA interference) of MCH communication to the vHP increases impulsive responding in rats, indicating that perturbing this system in either direction elevates impulsivity. Furthermore, these effects are not secondary to either impaired timing accuracy, altered activity, or increased food motivation, consistent with a specific role for vHP MCH signaling in the regulation of impulse control. Results from additional functional connectivity and neural pathway tracing analyses implicate the nucleus accumbens as a putative downstream target of vHP MCH1 receptor-expressing neurons. Collectively, these data reveal a specific neural circuit that regulates impulsivity and provide evidence of a novel function for MCH on behavior.

Brain Plasticity Can Predict the Cochlear Implant Outcome in Adult-Onset Deafness

Sensory plasticity, which is associated with deafness, has not been as thoroughly investigated in the adult brain as it has in the developing brain. In this study, we examined the brain reorganization induced by auditory deprivation in people with adult-onset deafness and its clinical relevance by measuring glucose metabolism before cochlear implant (CI) surgery. F-18 fluorodeoxyglucose positron emission tomography (¹⁸F-FDG-PET) scans were performed in 37 postlingually deafened patients during the preoperative workup period, and in 39 normal-hearing (NH) controls. Behavioral CI outcomes were measured at 1 year after implantation using a phoneme identification test with auditory cueing only. In the deaf individuals, areas involved in the auditory pathway such as the inferior colliculus and bilateral superior temporal gyri were hypometabolic compared to the NH controls. The hypometabolism observed in the deaf auditory cortices gradually returned to levels similar to the controls as the duration of deafness increased. However, contrary to our previous findings in congenitally deaf children, this metabolic recovery failed to have a significant prognostic value for the recovery of the speech perception ability in adult CI patients. In a broad occipital area centered on the primary visual cortices, glucose metabolism was higher in the deaf patients than the controls, suggesting that the area had become visually hyperactive for sensory compensation immediately after the onset of deafness. In addition, a negative correlation between the metabolic activity and behavioral speech perception outcomes was observed in the visual association areas. In the medial frontal cortices, cortical metabolism in most patients decreased, but patients who had preserved metabolic activities showed better speech performance. These results suggest that the auditory cortex in people with adult-onset deafness is relatively resistant to cross-modal plasticity, and instead, individual traits in late-stage visual processing and cognitive control seem to be more reliable prognostic markers for adult-onset deafness.

Neural Plastic Changes in the Subcortical Auditory Neural Pathway after Single-Sided Deafness in Adult Mice: A MEMRI Study

Single-sided deafness (SSD) induces cortical neural plastic changes according to duration of deafness. However, it is still unclear how the auditory cortical changes accompany the subcortical neural changes. The present study aimed to find the neural plastic changes in the cortical and subcortical auditory system following adult-onset single-sided deafness (SSD) using Mn-enhanced magnetic resonance imaging (MEMRI). B57BL/6 mice (postnatal 8-week-old) were divided into three groups: the SSD-4-week group (postnatal 12-week-old, n = 11), the SSD-8-week group (postnatal 16-week-old, n = 11), and a normal-hearing control group (postnatal 8-week-old, n = 9). The left cochlea was ablated in the SSD groups. White Gaussian noise was delivered for 24 h before MEMRI acquisition. T1-weighted MRI data were analyzed from the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), inferior colliculus (IC), medial geniculate body (MG), and auditory cortex (AC). The differences in relative Mn2+-enhanced signal intensities (Mn2+SI) and laterality were analyzed between the groups. Four weeks after the SSD procedure, the ipsilateral side of the SSD showed significantly lower Mn2+SI in the CN than the control group. On the other hand, the contralateral side of the SSD demonstrated significantly lower Mn2+SI in the SOC, LL, and IC. These decreased Mn2+SI values were partially recovered at 8 weeks after the SSD procedure. The interaural Mn2+SI differences representing the interaural dominance were highest in CN and then became less prominently higher in the auditory neural system. The SSD-8-week group still showed interaural differences in the CN, LL, and IC. In contrast, the MG and AC did not show any significant intergroup or interaural differences in Mn2+SI. In conclusion, subcortical auditory neural activities were decreased after SSD, and the interaural differences were diluted in the higher auditory nervous system. These findings were attenuated with time. Subcortical auditory neural changes after SSD may contribute to the change in tinnitus severity and the outcomes of cochlear implantation in SSD patients.

Visualization of the auditory pathway in rats with 18F-FDG PET activation studies based on different auditory stimuli and reference conditions including cochlea ablation

Activation studies with positron emission tomography (PET) in auditory implant users explained some of the mechanisms underlying the variability of achieved speech comprehension. Since future developments of auditory implants will include studies in rodents, we aimed to inversely translate functional PET imaging to rats. In normal hearing rats, activity in auditory and non-auditory regions was studied using ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET with 3 different acoustic conditions: sound attenuated laboratory background, continuous white noise and rippled noise. Additionally, bilateral cochlea ablated animals were scanned. 3D image data were transferred into a stereotaxic standard space and evaluated using volume of interest (VOI) analyses and statistical parametric mapping (SPM). In normal hearing rats alongside the auditory pathway consistent activations of the nucleus cochlearis (NC), olivary complex (OC) and inferior colliculus (IC) were seen comparing stimuli with background. In this respect, no increased activation could be detected in the auditory cortex (AC), which even showed deactivation with white noise stimulation. Nevertheless, higher activity in the AC in normal hearing rats was observed for all 3 auditory conditions against the cochlea ablated status. Vice versa, in ablated status activity in the olfactory nucleus (ON) was higher compared to all auditory conditions in normal hearing rats. Our results indicate that activations can be demonstrated in normal hearing animals based on ¹⁸F-FDG PET in nuclei along the central auditory pathway with different types of noise stimuli. However, in the AC missing activation with respect to the background advises the need for more rigorous background noise attenuation for non-invasive reference conditions. Finally, our data suggest cross-modal activation of the olfactory system following cochlea ablation–underlining, that ¹⁸F-FDG PET appears to be well suited to study plasticity in rat models for cochlear implantation.

Neurofilament heavy chain expression and neuroplasticity in rat auditory cortex after unilateral and bilateral deafness

Deafness induces many plastic changes in the auditory neural system. For instance, dendritic changes cause synaptic changes in neural cells. SMI-32, a monoclonal antibody reveals auditory areas and recognizes non-phosphorylated epitopes on medium- and high-molecular-weight subunits of neurofilament proteins in cortical pyramidal neuron dendrites. We investigated SMI-32-immunoreactive (-ir) protein levels in the auditory cortices of rats with induced unilateral and bilateral deafness. Adult male Sprague-Dawley rats were divided into unilateral deafness (UD), bilateral deafness (BD), and control groups. Deafness was induced by cochlear ablation. All rats were sacrificed, and the auditory cortices were harvested for real-time quantitative polymerase chain reaction (RT-qPCR) and western blot analyses at 2, 4, 6, and 12 weeks after deafness was induced. Immunohistochemical staining was performed to evaluate the location of SMI-32-ir neurons. Neurofilament heavy chain (NEFH) mRNA expression and SMI-32-ir protein levels were increased in the BD group. In particular, SMI-32-ir protein levels increased significantly 6 and 12 weeks after deafness was induced. In contrast, no significant changes in protein level were detected in the right or left auditory cortices at any time point in the UD group. NEFH mRNA level decreased at 4 weeks after deafness was induced in the UD group, but recovered thereafter. Taken together, BD induced plastic changes in the auditory cortex, whereas UD did not affect the auditory neural system sufficiently to show plastic changes, as measured by neurofilament protein level.

Effects of sciatic nerve transection on glucose uptake in the presence and absence of lactate in the frog dorsal root ganglia and spinal cord

Frogs have been used as an alternative model to study pain mechanisms because the simplicity of their nervous tissue and the phylogenetic aspect of this question. One of these models is the sciatic nerve transection (SNT), which mimics the clinical symptoms of “phantom limb”, a condition that arises in humans after amputation or transverse spinal lesions. In mammals, the SNT increases glucose metabolism in the central nervous system, and the lactate generated appears to serve as an energy source for nerve cells. An answerable question is whether there is elevated glucose uptake in the dorsal root ganglia (DRG) after peripheral axotomy. As glucose is the major energy substrate for frog nervous tissue, and these animals accumulate lactic acid under some conditions, bullfrogs Lithobates catesbeianus were used to demonstrate the effect of SNT on DRG and spinal cord 1-[14C] 2-deoxy-D-glucose (14C-2-DG) uptake in the presence and absence of lactate. We also investigated the effect of this condition on the formation of 14CO2 from 14C-glucose and 14C-L-lactate, and plasmatic glucose and lactate levels. The 3-O-[14C] methyl-D-glucose (14C-3-OMG) uptake was used to demonstrate the steady-state tissue/medium glucose distribution ratio under these conditions. Three days after SNT, 14C-2-DG uptake increased, but 14C-3-OMG uptake remained steady. The increase in 14C-2-DG uptake was lower when lactate was added to the incubation medium. No change was found in glucose and lactate oxidation after SNT, but lactate and glucose levels in the blood were reduced. Thus, our results showed that SNT increased the glucose metabolism in the frog DRG and spinal cord. The effect of lactate on this uptake suggests that glucose is used in glycolytic pathways after SNT.

Glucose metabolism in the primary auditory cortex of postlingually deaf patients: an FDG-PET study

BACKGROUND/PURPOSE

Previous FDG-PET studies have indicated neuroplasticity in the adult auditory cortex in cases of postlingual deafness. In the mature brain, auditory deprivation decreased neuronal activity in primary auditory and auditory-related cortices. In order to reevaluate these issues, we used statistical analytic software, namely a three-dimensional stereotaxic region of interest template (3DSRT), in addition to statistical parametric mapping (SPM; Institute of Neurology, University College of London, UK).

MATERIALS AND METHODS

(18)F-FDG brain PET scans were performed on 7 postlingually deaf patients and 10 healthy volunteers. Significant increases and decreases of regional cerebral glucose metabolism in the patient group were estimated by comparing their PET images with those of healthy volunteers using SPM analysis and 3DSRT.

RESULTS

SPM revealed that the glucose metabolism of the deaf patients was lower in the right superior temporal gyrus, both middle temporal gyri, left inferior temporal gyrus, right inferior lobulus parietalis, right posterior cingulate gyrus, and left insular cortex than that of the control subjects. 3DSRT data also revealed significantly decreased glucose metabolism in both primary auditory cortices of the postlingually deaf patients.

CONCLUSION

SPM and 3DSRT analyses indicated that glucose metabolism decreased in the primary auditory cortex of the postlingually deaf patients. The previous results of PET studies were confirmed, and our method involving 3DSRT has proved to be useful.

Noise-induced hearing loss (NIHL) as a target of oxidative stress-mediated damage: cochlear and cortical responses after an increase in antioxidant defense

This study addresses the relationship between cochlear oxidative damage and auditory cortical injury in a rat model of repeated noise exposure. To test the effect of increased antioxidant defenses, a water-soluble coenzyme Q10 analog (Qter) was used. We analyzed auditory function, cochlear oxidative stress, morphological alterations in auditory cortices and cochlear structures, and levels of coenzymes Q9 and Q10 (CoQ9 and CoQ10, respectively) as indicators of endogenous antioxidant capability. We report three main results. First, hearing loss and damage in hair cells and spiral ganglion was determined by noise-induced oxidative stress. Second, the acoustic trauma altered dendritic morphology and decreased spine number of II-III and V-VI layer pyramidal neurons of auditory cortices. Third, the systemic administration of the water-soluble CoQ10 analog reduced oxidative-induced cochlear damage, hearing loss, and cortical dendritic injury. Furthermore, cochlear levels of CoQ9 and CoQ10 content increased. These findings indicate that antioxidant treatment restores auditory cortical neuronal morphology and hearing function by reducing the noise-induced redox imbalance in the cochlea and the deafferentation effects upstream the acoustic pathway.

Cross-modal and compensatory plasticity in adult deafened cats: a longitudinal PET study

Although much is known about the cerebral neural plasticity that occurs after deafness, it is unclear how much time is required for its development or what other cortical changes may consequently occur. This study provides a longitudinal assessment of cerebral cortical neural plasticity, as manifested in adult deafened cats. A total of 5 male cats were subjected to whole cortex analysis of glucose metabolic activity via 2-deoxy-2[(18)F] fluoro-D-glucose (FDG) micro-positron emission tomography (PET). The imaging was performed at the baseline state of normal hearing and then at 4, 9, 24, and 33 months after the induction of deafness. We compared glucose metabolism between the normal hearing state and each deafened state by using voxel-based statistical analysis (P<0.005). Significant changes were observed in the primary auditory (A1) and primary visual (V1) cortices. A bilateral metabolic decrease was observed in A1 areas and in temporal auditory fields, the extent of which was significantly increased at Month 9. Then it was declined at Month 24. And finally it was disappeared by Month 33. Auditory cortical plasticity subsequent to deafness was thus demonstrated. Furthermore, a significant metabolic upsurge occurred in bilateral occipital areas at Month 33. This increase, involving bilateral occipital and thalamic areas of V1, suggests compensatory hyperactivity of the visual cortex after deafness.

Feasibility of template-guided attenuation correction in cat brain PET imaging

PURPOSE

Attenuation correction (AC) is important in quantitative positron emission tomography (PET) imaging of medium-sized animals such as the cat. However, additional time for transmission (TX) scanning and tracer uptake is required in PET studies with animal-dedicated PET scanners because post-injection TX scanning is not available in these systems. The aim of this study was to validate a template-guided AC (TGAC) method that does not require TX PET data for AC in cat 2-deoxy-2-[F-(18)fluoro-D-glucose (FDG) brain PET imaging.

METHODS

PET scans were acquired using a microPET Focus 120 scanner. TX data were obtained using a (68)Ge point source before the injection of FDG. To generate the attention map (mu-map) template for the TGAC, a target image of emission (EM) PET was selected, and spatial normalization parameters of individual EM data onto the target were reapplied to the corresponding mu-maps. The inverse transformations of the mu-map template into the individual spaces were performed, and the transformed template was forward projected to generate the AC factor. The TGAC method was compared with measured AC (MAC) and calculated AC (CAC) methods using region of interest (ROI) and SPM analyses.

RESULTS

The ROI analysis showed that the activity of the TGAC EM PET images strongly correlated with those of the MAC data (y = 0.98x + 0.01, R(2) = 0.96). In addition, no significant difference was observed in the SPM analysis. By contrast, the CAC showed a significantly higher uptake in the deep gray regions compared to the MAC (corrected P < 0.05). The ROI correlation with MAC was worse than with the TGAC (R(2) = 0.84). In SPM analysis for the voxel-wise group comparisons between before and after the induction of deafness, only the TGAC showed equivalent results with the MAC.

CONCLUSIONS

The TGAC was reliable in cat FDG brain PET studies in terms of compatibility with the MAC method. The TGAC might be a useful option for increasing study throughput and decreasing the probability of subject movement. In addition, it might reduce the possible biological effects of long-term anesthesia on the cat brain in investigations using animal-dedicated PET scanners.

[Paediatric cochlear implantation in the critical period of the auditory pathway, our experience]

INTRODUCTION

Numerous experimental and clinical studies have suggested a critical or sensitive period in which the auditory pathway develops its greatest potential in terms of plasticity and learning. Early cochlear implantation performed in prelingual deaf children in this period provides a better prognosis for language acquisition. The aim of this study is to show the importance of cochlear implantation before this critical period ends.

METHODS

We conducted an observational, longitudinal, retrospective study of 57 children suffering profound prelingual bilateral sensorineural hearing loss who had received Advanced Bionics implants at our ENT department between June, 1998, and November, 2006. Data on their audiometric thresholds, the disyllabic word test adapted to children, open-set sentences recognition test and the Nottingham scale were analyzed.

RESULTS

The analysis of audiometric thresholds showed no differences between children receiving the implants at different ages. However, statistically significant differences (p<0.05) were found in speech tests between groups of children receiving the implants before and after 4 years of age.

CONCLUSIONS

Our results are in line with other publications showing differences in auditory performance when comparing children with early implants versus children receiving the implants at a later age. We found the greatest differences at 4 years of age. Nevertheless, these findings should not exclude children over this age from implantation.

An animal model of central auditory pathway imaging in the rat brain by high resolution small animal positron emission tomography

CONCLUSIONS

Our study demonstrated that high resolution animal positron emission tomography (PET) can non-invasively assess the change in glucose metabolism of the central auditory pathway including the inferior colliculus and auditory cortex in the rat.

OBJECTIVES

The traditional in vitro approach with immunohistochemical staining or autoradiography to assess chronological changes or topographic arrangement of central auditory pathway required sacrificing a large number of animals. Inter-individual difference is also a major concern. Therefore, development of an in vivo animal model using PET imaging would be a rational method to overcome these shortcomings.

MATERIALS AND METHODS

Small animal PET scan using (18)F-fluorodeoxyglucose (FDG) as a functional marker was performed in rats. Each animal was serially scanned before and after unilateral cochlear ablation, with and without acoustic stimulation. The images were analyzed by the region of interest (ROI) method. Ratios of radioactivity at the inferior colliculus and auditory cortex and a referenced cerebral cortex between bilateral hemispheres were measured.

RESULTS

These scans demonstrated several brain structures including the inferior colliculus (IC) and cortex (B). Moreover, unilateral cochlear ablation decreased the radioactivity at contralateral IC and auditory cortex (C) areas. Differences may reach 33% in IC and 27% in C, and average radioactivity ratios were 1.24+/-0.08 and 1.18+/-0.07 in selected adjacent sections, respectively.

Assessment of cerebral glucose metabolism in cat deafness model: strategies for improving the voxel-based statistical analysis for animal PET studies

PURPOSE

The aim of this study was to establish the procedures for 3D voxel-based statistical analysis of 2-deoxy-2-[(18)F]fluoro-D-glucose-positron emission tomography (FDG-PET) images of a cat's brain obtained using a small animal-dedicated PET system and to assess the utility of this approach in investigating the cerebral glucose metabolism in an animal model of cortical deafness.

PROCEDURES

This study compared several different strategies for the spatial processing of PET data acquired twice from eight cats before and after inducing deafness in terms of the comparability of the statistical analysis results to the established pattern of the cerebral glucose metabolic changes in the deaf animals.

RESULTS

The accuracy of the spatial preprocessing procedures and the statistical significance of the comparison were improved by removing the background activities outside the brain regions. The use of the spatial normalization parameters obtained from the mean image of the realigned data set for individual data also helped improve the statistical significance of the paired t testing. It was also found that an adjustment of the registration options was also important for increasing the precision of the realignment.

CONCLUSIONS

A method for voxel-based analysis of the PET data of a cat's brain was optimized. The results demonstrated the high localization accuracy and specificity of this method, which is expected to be useful for examining the brain PET data of medium-sized animals such as cats.

Gene expression and plasticity in the rat auditory cortex after bilateral cochlear ablation

CONCLUSION

The plastic changes in the auditory cortex after bilateral cochlear ablation are related to the immediate early genes as well as the neural plasticity-related genes. In addition, cross-modal plasticity may play an important role in the early changes in the auditory cortex after bilateral cochlear ablation.

OBJECTIVES

The purpose of this study was to identify candidate genes involved in the normal development of primary auditory cortex during the critical period as well as those genes specifically modulated under conditions of sensory deafferentation by bilateral cochlear ablation.

MATERIALS AND METHODS

We produced a bilaterally deaf rat model and used DNA microarray technology to analyze differential gene expression in the primary auditory cortex of bilateral cochlear ablated and sham-operated age-matched control rats. Gene expression in the auditory cortex was compared at 2, 4, and 12 weeks after surgery. For selected genes, the changes in gene expression were confirmed by real-time polymerase chain reaction (PCR).

RESULTS

In the cochlear ablation groups, the expression of immediate early genes (Egr1, 2, 3, 4, c-fos, etc.) and neural plasticity-related genes (Arc, Syngr1, Bdnf, etc.) was decreased at 2 weeks and increased at 4 weeks. The expression of neurotransmission-related genes (Gabra5, Chrnb3, Chrne, etc.) was decreased at 12 weeks.

Cortical Activity at Rest Predicts Cochlear Implantation Outcome

The functional status of central neural pathways, in particular their susceptibility to plasticity and functional reorganization, may influence speech performance of deaf cochlear implant users. In this paper, we sought to determine how brain metabolic activity measured before implantation relates to cochlear implantation outcome, that is, speech perception. In 22 prelingually deaf children between 1 and 11 years, we correlated preoperative glucose metabolism as measured by F-18 fluorodeoxyglucose positron emission tomography with individual speech perception performance assessed 3 years after implantation, while factoring out the confounding effect of age at implantation. Whereas age at implantation was positively correlated with increased activity in the right superior temporal gyrus, speech scores were selectively associated with enhanced metabolic activity in the left prefrontal cortex and decreased metabolic activity in right Heschl's gyrus and in the posterior superior temporal sulcus. These results reinforce the notion that implantation should be performed as early as possible to prevent cross-modal takeover of auditory regions and suggest that rehabilitation strategies may be more efficient if they capitalize on general cognitive functions instead of only targeting specialized circuits dedicated to auditory and audiovisual pattern recognition.

Voxel-based statistical analysis of cerebral glucose metabolism in the rat cortical deafness model by 3D reconstruction of brain from autoradiographic images

PURPOSE

Animal models of cortical deafness are essential for investigation of the cerebral glucose metabolism in congenital or prelingual deafness. Autoradiographic imaging is mainly used to assess the cerebral glucose metabolism in rodents. In this study, procedures for the 3D voxel-based statistical analysis of autoradiographic data were established to enable investigations of the within-modal and cross-modal plasticity through entire areas of the brain of sensory-deprived animals without lumping together heterogeneous subregions within each brain structure into a large region of interest.

METHODS

Thirteen 2-[1-(14)C]-deoxy-D: -glucose autoradiographic images were acquired from six deaf and seven age-matched normal rats (age 6-10 weeks). The deafness was induced by surgical ablation. For the 3D voxel-based statistical analysis, brain slices were extracted semiautomatically from the autoradiographic images, which contained the coronal sections of the brain, and were stacked into 3D volume data. Using principal axes matching and mutual information maximization algorithms, the adjacent coronal sections were co-registered using a rigid body transformation, and all sections were realigned to the first section. A study-specific template was composed and the realigned images were spatially normalized onto the template. Following count normalization, voxel-wise t tests were performed to reveal the areas with significant differences in cerebral glucose metabolism between the deaf and the control rats.

RESULTS

Continuous and clear edges were detected in each image after registration between the coronal sections, and the internal and external landmarks extracted from the spatially normalized images were well matched, demonstrating the reliability of the spatial processing procedures. Voxel-wise t tests showed that the glucose metabolism in the bilateral auditory cortices of the deaf rats was significantly (P<0.001) lower than that in the controls. There was no significantly reduced metabolism in any other area, and no area showed a significant increase in metabolism in the deaf rats with the same threshold, demonstrating the high localization accuracy and specificity of the method developed in this study.

CONCLUSION

This study established new procedures for the 3D reconstruction and voxel-based analysis of autoradiographic data which will be useful for examining the cerebral glucose metabolism in a rat cortical deafness model.