European Journal of Neuroscience

Changes in pupil size at constant light levels reflect the activity of neuromodulatory brainstem centers that control global brain state. These endogenously driven pupil dynamics can be synchronized with cognitive acts. For example, the pupil dilates during the spontaneous switches of perception of a constant sensory input in bistable perceptual illusions. It is unknown whether this pupil dilation only indicates the occurrence of perceptual switches, or also their content. Here, we measured pupil diameter in human subjects reporting the subjective disappearance and re‐appearance of a physically constant visual target surrounded by a moving pattern (‘motion‐induced blindness’ illusion). We show that the pupil dilates during the perceptual switches in the illusion and a stimulus‐evoked ‘replay’ of that illusion. Critically, the switch‐related pupil dilation encodes perceptual content, with larger amplitude for disappearance than re‐appearance. This difference in pupil response amplitude enables prediction of the type of report (disappearance vs. re‐appearance) on individual switches (receiver‐operating characteristic: 61%). The amplitude difference is independent of the relative durations of target‐visible and target‐invisible intervals and subjects' overt behavioral report of the perceptual switches. Further, we show that pupil dilation during the replay also scales with the level of surprise about the timing of switches, but there is no evidence for an interaction between the effects of surprise and perceptual content on the pupil response. Taken together, our results suggest that pupil‐linked brain systems track both the content of, and surprise about, perceptual events.

We investigated the role of desensitization of α‐amino‐3‐hydroxy‐5‐methyl‐isoxazole‐4‐propionate (AMPA) receptors on the neurotoxicity and on the [Ca²⁺]_i changes induced by kainate or by AMPA in cultured rat hippocampal neurons. The neuronal viability was evaluated either by the 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) assay, or by analysis of cell morphology. Short‐term exposure of the neurons to kainate or AMPA (30 min) was not toxic, but the exposure for 24 h to the excitotoxic drugs caused a concentration‐dependent neurotoxic effect which was prevented by LY 303070, a noncompetitive AMPA receptor antagonist. In the presence of cyclothiazide (CTZ), kainate or AMPA was toxic (30 min exposure), or the toxic effect was significantly enhanced (24 h exposure), but in this case LY 303070 did not completely protect the cells against kainate‐induced toxicity. The alterations in the [Ca²⁺]_i caused by kainate or AMPA showed a great cell‐to‐cell variability. LY 303070 completely or partially inhibited the responses stimulated by kainate. CTZ differentially affected the responses evoked by kainate or AMPA. In the majority of hippocampal neurons, CTZ did not potentiate, or only slightly potentiated, the kainate‐stimulated responses but in 11% of neurons there was a great potentiation. In AMPA‐stimulated neurons, the responses were slightly or greatly potentiated in the majority of neurons, but not in all of them. The results show that AMPA and kainate may be toxic, depending on the time of exposure and on the blockade of the desensitization of the AMPA receptors. Overall, our results clearly show that there exist different populations of hippocampal neurons with different sensitivities to kainate, AMPA, CTZ and LY 303070. Moreover, the effects of CTZ on both [Ca²⁺]_i alterations and neurotoxicity are not fully correlated.

Huntington's disease (HD) is a neurodegenerative disorder with complex symptoms dominated by progressive motor dysfunction. Skeletal muscle atrophy is common in HD patients. Because the HD mutation is expressed in skeletal muscle as well as brain, we wondered whether the muscle changes arise from primary pathology. We used R6/2 transgenic mice for our studies. Unlike denervation atrophy, skeletal muscle atrophy in R6/2 mice occurs uniformly. Paradoxically however, skeletal muscles show age‐dependent denervation‐like abnormalities, including supersensitivity to acetylcholine, decreased sensitivity to µ‐conotoxin, and anode‐break action potentials. Morphological abnormalities of neuromuscular junctions are also present, particularly in older R6/2 mice. Severely affected R6/2 mice show a progressive increase in the number of motor endplates that fail to respond to nerve stimulation. Surprisingly, there was no constitutive sprouting of motor neurons in R6/2 muscles, even in severely atrophic muscles that showed other denervation‐like characteristics. In fact, there was an age‐dependent loss of regenerative capacity of motor neurons in R6/2 mice. Because muscle fibers appear to be released from the activity‐dependent cues that regulate membrane properties and muscle size, and motor axons and nerve terminals become impaired in their capacity to release neurotransmitter and to respond to stimuli that normally evoke sprouting and adaptive reinnervation, we speculate that in these mice there is a progressive dissociation of trophic signalling between motor neurons and skeletal muscle. However, irrespective of the cause, the abnormalities at neuromuscular junctions we report here are likely to contribute to the pathological phenotype in R6/2 mice, particularly in late stages of the disease.

The dopaminergic input to the frontal cortex has an important role in motor and cognitive functions. These effects are mediated by dopamine receptors both of type D1 and of type D2, although the neural circuits involved are not completely understood. We used in situ hybridization to determine the cellular localization of D1 and D2 receptor mRNAs in the rat frontal cortex. Retrograde tracing was used in the same animals to identify the main cortical efferent populations. Fluorogold was injected into the different cortical targets of the frontal cortex and sections were hybridized with D1 and D2 ³⁵S‐labelled cRNA probes. D1 and D2 mRNA‐containing neurons were present in all the cortical areas investigated, with greater expression in the medial prefrontal, insular and cingulate cortexes and lower expression in the motor and parietal cortexes. Neurons containing D1 mRNA were most abundant in layer Vlb; they were also present in layers Vla and V of all cortical layers and in layer II of the medial prefrontal, cingulate and insular areas. Double labelling with fluorogold demonstrated that D1 mRNA was present in corticocortical, corticothalamic and corticostriatal neurons. Neurons containing D2 mRNA were essentially restricted to layer V, but only in corticostriatal and corticocortical neurons. Neither D1 nor D2 mRNA was found in corticospinal or corticopontine neurons. The present results demonstrate that D1 and D2 receptor genes are expressed in efferent cortical populations, with higher expression for D1. In spite of an overlap in some cortical layers, the expression of D1 and D2 receptor genes is specific for different categories of pyramidal neurons.

The mechanisms by which animals adapt to an ever‐changing environment have long fascinated scientists. Different forces, conveying information regarding various aspects of the internal and external environment, interact with each other to modulate behavioral arousal. These forces can act in concert or, at times, in opposite directions. These signals eventually converge and are integrated to influence a common arousal pathway which, depending on all the information received from the environment, supports the activation of the most appropriate behavioral response. In this review we propose that the ventromedial hypothalamic nucleus (VMN) is part of the circuitry that controls food anticipation. It is the first nucleus activated when there is a change in the time of food availability, silencing of VMN ghrelin receptors decreases food‐anticipatory activity (FAA) and, although lesions of the VMN do not abolish FAA, parts of the response are often altered. In proposing this model it is not our intention to exclude parallel, redundant and possibly interacting pathways that may ultimately communicate with, or work in concert with, the proposed network, but rather to describe the neuroanatomical requirements for this circuit and to illustrate how the VMN is strategically placed and connected to mediate this complex behavioral adaptation.

Glutamine synthetase (GS) supposedly inactivates the excitatory neurotransmitter glutamate. By using immunocytochemistry for GS, we recently demonstrated a layer‐specific, perisynaptic distribution of GS‐immunoreactive astrocytes and their processes in perfusion‐fixed rat hippocampi. Highest levels of immunoreactivity were found in well defined termination zones of glutamatergic hippocampal afferents. In the present study we analysed the developmental aspect of this neuron – glia interaction by using hippocampal slice cultures lacking all extrinsic afferents. Under these conditions, no layer‐specific distribution of astrocytic GS immunoreactivity could be demonstrated. This suggests that the laminated distribution of GS immunoreactivity is formed in parallel with the segregated termination of hippocampal afferents. Thus, there is no predetermined pattern of GS‐containing astrocytes playing a role in the segregation of extrinsic fibres. The ultrastructural localization of GS immunoreactivity in fine astrocytic processes around asymmetric, probably glutamatergic excitatory spine synapses confirms earlier in situ findings, which suggests that this arrangement is a global phenomenon of glutamatergic systems.

Microglial cells with their characteristic ramified morphology are exclusively found in healthy CNS tissue, whereas various pathologies are associated with the occurrence of amoeboid, macrophage‐like cells. It is still a matter of discussion whether amoeboid cells are blood‐derived macrophages, or whether a characteristic change in morphology, reflecting activation of previously ramified microglia, takes place. Cells in dissociated microglia culture obtained from healthy rat brains, inevitably developing this amoeboid morphology, were labelled with a fluorescent dye and transferred onto organotypic hippocampal slice cultures. Prelabelled cells with amoeboid morphology invaded these slice cultures and had, after 9 days in vitro, gradually transformed into highly ramified cells. Our findings strengthen the hypothesis that the observed amoeboid and ramified cells belong to a single population of microglia, appearing with different morphologies depending on the presence of stimuli provided by the CNS microenvironment. Microglial cells obviously appear in different shapes and can switch from immunologically resting to activated modes and vice versa.

Coronins belong to the fundamental WD40‐repeat proteins. They are mainly found at the submembraneous area, they bind F‐actin in vitro, and most of the seven mammalian coronins have unclear roles. Coronin 3 is abundantly expressed in the adult CNS. All murine brain areas express coronin 3 during embryogenesis and the first postnatal stages. Expression in grey matter decreases postnatally, except for hippocampal pyramidal and dentate gyrus neurons, and cerebellar Purkinje cells, while levels in white matter increase in the course of myelination. Consistently, coronin 3 is abundant in differentiating neuro‐2a and PC‐12 cells and in primary oligodendrocytes. Treatment with PKC activator PMA reduced coronin 3 protein levels. To address its functions, neuro‐2a and PC‐12 cells were transfected with GFP‐tagged coronin 3 versions. Full‐length coronin 3 among other areas localized to outgrowing neurites, whereas truncated proteins efficiently suppressed neurite formation. Our results favour a role for coronin 3 in neuron morphogenesis and possibly migration.

Glutamate uptake into nerve cells and astrocytes via high‐affinity transporters controls the extracellular glutamate concentration in the brain, with major implications for physiological excitatory neurotransmission and the prevention of excitotoxicity. We report here that three recently cloned rat glutamate transporter subtypes, viz. EAAC1 (neuronal), GLT1 and GLAST (glial), possess a redox‐sensing property, undergoing opposite functional changes in response to oxidation or reduction of reactive sulphydryls present in their structure. In particular, thiol oxidation with 5,5′‐dithio‐bis(2–nitrobenzoic) acid (DTNB) and disulphide reduction with dithiothreitol (DTT) result, respectively, in reduced and increased uptake capacity by a preparation of partially purified brain transporters as well as by the three recombinant proteins reconstituted into liposomes. In this model system, EAAC1, GLT1 and GLAST react similarly to DTT/DTNB exposures despite their different contents of cysteines, suggesting that only the conserved residues might be involved in redox modulation. Redox sensitivity is a property of the glutamate transporters also when present in their native cell environment. Thus, by using cultured cortical astrocytes and the whole‐cell patch‐clamp technique we were able to observe dynamic increase and decrease of the glutamate uptake current in response to application of DTT and DTNB in sequence. Moreover, in the same paradigm, DDT‐reversible current inhibition was observed with hydrogen peroxide instead of DTNB, indicating that the SH‐based redox modulatory site is targeted by endogenous oxidants and might constitute an important physiological or pathophysiological regulatory mechanism of glutamate uptake in vivo

MicroRNA (miRNA) là một loại phân tử RNA nhỏ không mã hóa mới được công nhận, tham gia vào việc kiểm soát phát triển của biểu hiện gen. Chúng tôi đã nghiên cứu sự điều tiết của một bộ miRNA thần kinh có biểu hiện cao trong quá trình phát triển não ở chuột. Kiểm soát tạm thời là một đặc điểm của điều tiết miRNA ở C. elegans và Drosophila, và cũng nổi bật trong não của phôi. Chúng tôi đã quan sát thấy sự khác biệt đáng kể về thời gian bắt đầu và cường độ kích thích giữa các miRNA cá thể. So sánh sự biểu hiện trong các nền văn hóa tế bào thần kinh phôi và tế bào sao, chúng tôi tìm thấy độ cụ thể về dòng tế bào đối với từng miRNA trong nghiên cứu của chúng tôi. Hai miRNA có biểu hiện cao nhất ở não người lớn được biểu hiện ưa thích trong các tế bào thần kinh (mir‐124, mir‐128). Ngược lại, mir‐23, một miRNA trước đây đã được chứng minh liên quan đến sự xác định tế bào thần kinh, chỉ giới hạn ở tế bào sao. mir‐26 và mir‐29 được biểu hiện mạnh hơn ở tế bào sao so với tế bào thần kinh, trong khi những cái khác phân bố tương đối đồng đều (mir‐9, mir‐125). Độ cụ thể về dòng tế bào đã được khám phá thêm bằng cách sử dụng các cấu trúc báo cáo cho hai miRNA có mối quan tâm đặc biệt (mir‐125 và mir‐128). Sự ức chế do miRNA gây ra cho cả hai báo cáo viên đã được quan sát thấy sau khi chuyển gen vào tế bào thần kinh nhưng không ở tế bào sao. MiRNA đã được kích thích mạnh mẽ trong quá trình phân hóa thần kinh của tế bào gốc phôi, cho thấy tính hợp lệ của mô hình tế bào gốc để nghiên cứu sự điều chỉnh miRNA trong phát triển thần kinh.