Biomedical subjects

W Tokuyama

Publications and source records attributed to W Tokuyama.

11 recordsLinked to original sources

PubMed2005

Deeply located granule cells and mitral cells undergo apoptosis after transection of the central connections of the main olfactory bulb in the adult rat.

The main olfactory bulb (MOB) is the first relay station of the olfactory system: it receives afferents from sensory neurons and sends efferents to the primary olfactory cortex. The MOB also receives many centrifugal afferents from various regions. Transection of peripheral afferents to the MOB has been reported to induce cell death in granule cells. However, little is known about the effect of transection of these central connections of the MOB in adult rats. Here, we used a unilateral olfactory peduncle transection model in the adult rat to examine neuronal degeneration in the MOB. In the MOB ipsilateral to the surgery, the granule cell layer (GCL) was smaller, and the number of mitral cells was decreased compared with the contralateral MOB at 7 days after surgery. Many degenerating cells were present in both the mitral cell layer (MCL) and GCL in the ipsilateral MOB at 3 days after surgery, although there were no obvious changes in the gross morphology. We also found terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-digoxigenin nick end labeling (TUNEL)-positive cells in the MCL and GCL in the ipsilateral MOB at 3 days after surgery. The majority of the degenerating and TUNEL-positive cells were located in the deep, rather than the superficial, GCL. Immunohistochemistry for activated caspase-9 further supported the occurrence of apoptotic cell death in the mitral and deeply located granule cells. These results indicate that not only axotomized mitral cells, but also deeply located granule cells that were not directly injured, underwent apoptosis after transection of the central connections, and suggest that sensitivities to transection of the central connections differ among granule cells according to their depth in the GCL.

Animals↗

PubMed2001

Spatiotemporal dynamics of brain-derived neurotrophic factor mRNA induction in the vestibulo-olivary network during vestibular compensation.

Vestibular compensation, which is the behavioral recovery from vestibular dysfunction produced by unilateral labyrinthectomy (UL), is attributed to functional and structural reorganization of neural networks in the central vestibular system. To assess the possible contribution of brain-derived neurotrophic factor (BDNF) to this recovery process, we investigated changes in mRNA expression levels in the central vestibular system after UL. We evaluated BDNF mRNA expression levels by quantitative reverse transcription-PCR and in situ hybridization. We found that BDNF mRNA is differentially induced in the medial vestibular nucleus ipsilateral to UL and in the prepositus hypoglossi and inferior olive on the contralateral side. The BDNF mRNA induction lasted for at least 24 hr and returned to the basal expression level within 72 hr after UL. In contrast to BDNF mRNA induction, the expression of an immediate-early gene, c-fos, quickly reached the maximum level at 3 hr and decreased to the basal level within 24 hr after UL. Neither BDNF or c-fos induction was observed in sham-operated animals. The persistent induction of BDNF after UL temporally corresponded to early behavioral manifestations of vestibular compensation. We further found that trkB mRNA was expressed in the central vestibular network at high levels, although its expression levels did not change over time after UL. Because BDNF is implicated in regulating synaptic structure and function, these results provide support for the hypothesis that BDNF is involved in neuronal reorganization that allows vestibular compensation.

Adaptation, Physiological↗

PubMed2001

Differential induction of brain-derived neurotrophic factor mRNA in rat inferior olive subregions following unilateral labyrinthectomy.

Vestibular compensation, the neuronal process underlying behavioral recovery from vestibular dysfunction produced by unilateral labyrinthectomy, is attributed to functional reorganization of neuronal circuits in the brainstem and cerebellum. Climbing fibers originating from the inferior olive are suggested to play a crucial role in this compensatory process. To assess the possible contribution of brain-derived neurotrophic factor (BDNF) to the function of climbing fibers during vestibular compensation, we investigated the BDNF mRNA expression in the rat inferior olive after unilateral labyrinthectomy by quantitative in situ hybridization. We found several induction patterns depending on the subregions of the inferior olive 6 h after unilateral labyrinthectomy. First, in the dorsal cap and the ventrolateral outgrowth, the expression levels increased on the side contralateral to the lesion and decreased on the ipsilateral side. Second, in the beta nucleus, C subnucleus of the medial accessory olive, and A/B subnuclei of the medial accessory olive, we detected an increase in the mRNA expression level on the side contralateral to the lesion, but no changes in the expression level on the ipsilateral side. In the beta nucleus, where the strongest induction was observed, the mRNA expression level increased nearly five-fold. Third, in the dorsomedial cell column, the mRNA expression levels increased on both sides. Finally, we did not detect significant changes in mRNA expression levels in the other subregions of the inferior olive, the dorsal accessory olive, principal olive and rostral medial accessory olive. The changes in BDNF mRNA expression reverted to control levels by 72 h after the labyrinthectomy. The inferior olive subregions that showed changes in BDNF mRNA expression levels send climbing fibers to the cerebellar cortical regions that, in turn, project to the vestibular nuclei. Therefore, BDNF induced in these subregions could contribute to the functional reorganization of the olivo-cerebellar system for vestibular control.

Animals↗

PubMed2000

Expression of brain-derived neurotrophic factor, neurotrophin-3 and their receptor messenger RNAs in monkey rhinal cortex.

The primate rhinal cortex, consisting of areas 36 and 35 of the perirhinal cortex and the entorhinal cortex (area 28), plays a crucial role in perception and memory. We investigated the expression of messenger RNAs for brain-derived neurotrophic factor and neurotrophin-3, as well as those for their respective tyrosine kinase receptors, TrkB and TrkC, in the monkey rhinal cortex. Results from in situ hybridization revealed that each of these messenger RNAs was expressed in neurons with distinct laminar and areal patterns of distribution. Brain-derived neurotrophic factor messenger RNA was principally detected in layers V/ VI of area 36, and layers II/III and V of the entorhinal cortex. Some of the messenger RNA-positive cells in the deep layers of the rhinal cortex were confirmed to exhibit a pyramidal cell-like morphology. Neurotrophin-3 messenger RNA expression was confined to layers II/III of the entorhinal cortex. In contrast, trkB and trkC messenger RNAs were expressed rather homogeneously and abundantly throughout the rhinal cortex. The laminar and cellular distributions of brain-derived neurotrophic factor and neurotrophin-3 messenger RNAs indicate the predominant expression of these neurotrophins in projection neurons. These results suggest that brain-derived neurotrophic factor and neurotrophin-3 regulate neuronal connectivities of forward and backward projections from the rhinal cortex and contribute to functional reorganization underlying the formation and maintenance of long-term memory in primates.

Animals↗

PubMed2000

BDNF upregulation during declarative memory formation in monkey inferior temporal cortex.

In primates, visual long-term memory of objects is presumably stored in the inferior temporal (IT) cortex. Because brain-derived neurotrophic factor (BDNF) is involved in activity-dependent neural reorganization, we tested the hypothesis that BDNF would be upregulated in IT cortex during formation of visual pair-association memory. To eliminate genetic and cognitive variations between individual animals, we used split-brain monkeys for intra-animal comparison in PCR-based mRNA quantitation. The monkeys learned a pair-association (PA) task using one hemisphere and a control visual task using the other, to balance the amount of visual input. We found that BDNF was upregulated selectively in area 36 of IT cortex during PA learning, but not in areas involved in earlier stages of visual processing. In situ hybridization showed that BDNF-expressing cells were localized in a patchlike cluster. The results suggest that BDNF contributes to reorganization of neural circuits for visual long-term memory formation in the primate.

Actins↗

PubMed1999

Quantitative evaluation of neurotrophin and trk mRNA expression in visual and limbic areas along the occipito-temporo-hippocampal pathway in adult macaque monkeys.

The neurotrophins have been implicated in shaping and remodeling the connectivity of neural circuits. To explore the role of neurotrophins and their receptors, Trks, in cortical neural circuits of adult macaque monkeys, we determined mRNA expression levels of neurotrophins and Trk receptors in various visual and limbic areas along the occipito-temporo-hippocampal pathway by using a quantitative reverse-transcription polymerase chain reaction technique. The expression level of brain-derived neurotrophic factor (BDNF) mRNA was lowest in the primary visual cortex (V1), moderate in the temporal visual association area, and highest in the hippocampus. The expression levels of trkB mRNA isoforms, the full-length form that encodes a receptor tyrosine kinase and the truncated form that encodes a noncatalytic receptor, were also low in V1, moderate in the visual association area, and high in the entorhinal cortex. However, in contrast to their ligand BDNF, the expression levels of both trkB isoforms in the hippocampus were significantly lower than those in the entorhinal cortex. NT-3 mRNA was detectable only in the hippocampus and the entorhinal cortex, whereas both the full-length and the truncated forms of trkC mRNA were widely distributed throughout the neocortex and the limbic cortex. The expression levels of NGF and trkA mRNAs in these cortical areas were too low to determine quantitatively. The present findings suggest that, among neurotrophin/Trk signaling systems, the BDNF/TrkB-mediated signal most likely contributes to stabilization, remodeling, or both, of neural circuits in cortical areas along the occipito-temporo-hippocampal pathway in the adult macaque monkey.

Animals↗

PubMed1999

Quantification of neurotrophin-3 mRNA in the rat hippocampal subregions using the RT-PCR-based coamplification method.

Quantitative reverse transcription-polymerase chain reaction (RT-PCR) is a suitable method for determining the expression levels of rare mRNAs in small amounts of tissue. To compare the mRNA expression levels across specific brain regions, we adopted an RT-PCR method in which a target gene was coamplified with an endogenous internal standard gene in single reaction tubes. Use of the endogenous internal standard can control fluctuations in target quantification resulting from various factors, including tube-to-tube variation in amplification efficiency and variation in mRNA content among the total RNAs prepared from different tissues. In this study, we quantitatively determined the mRNA expression levels for NT-3, a member of the neurotrophin family of growth factors, in the hippocampal subregions: the entorhinal cortex, dentate gyrus and CA1. NT-3 gene was simultaneously coamplified with an endogenous internal standard gene, hypoxanthine-guanine phosphoribosyltransferase (HPRT), in the same reaction tube. Using this RT-PCR coamplification method, we detected a regional difference in the NT-3 mRNA expression levels across the hippocampal subregions. Our method can serve as a useful quantification method to investigate molecular signaling cascades in a specific cortical region.

Animals↗

PubMed1998

Highest trkB mRNA expression in the entorhinal cortex among hippocampal subregions in the adult rat: contrasting pattern with BDNF mRNA expression.

Brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, regulate synaptic functions in the hippocampus of the adult rodent. In previous studies, in situ hybridization methods have been used to evaluate regional differences in BDNF and trkB mRNA expression levels in hippocampal subregions. However, these studies have failed to reach consensus regarding the regional differences in the mRNA expression levels. In the present study, we quantitated mRNA expression levels using two different methods, ribonuclease protection assays and a quantitative reverse-transcription polymerase chain reaction technique, in four hippocampal subregions: the entorhinal cortex, dentate gyrus (DG), CA3 and CA1. These two methods yielded the same results. We found that BDNF and trkB mRNA expression levels did not covary in the four subregions. BDNF and full length trkB (trkB FL) mRNA in the entorhinal cortex and the DG show contrasting expression patterns. The expression level of BDNF mRNA was highest in the DG among the hippocampal subregions and low in the entorhinal cortex and the CA1, whereas the trkB FL mRNA expression level was highest in the entorhinal cortex, low in the DG and lowest in the CA3. These results suggest regional differences in BDNF/TrkB signaling for maintenance and modifiability of neuronal connections in the hippocampal formation.

Animals↗

PubMed1997

Temporal and spatial dissociation of expression patterns between Zif268 and c-Fos in rat inferior olive during vestibular compensation.

We examined the expression of the inducible transcription factors Zif268 and c-Fos in the inferior olive during the behavioural recovery following unilateral labyrinthectomy known as vestibular compensation. These transcription factors were differentially induced in the two subnuclei of the inferior olive. In the beta nucleus, c-Fos induction was restricted to the side contralateral to the lesion whereas Zif268 was induced on both the ipsilateral and contralateral side. In the dorsal cap, both proteins were expressed mostly on the contralateral side. In addition to this spatial difference, the expression of Zif268 attenuated faster than that of c-Fos. The present data suggest a functional difference between these two subnuclei of the inferior olive at the level of gene expression in vestibular compensation.

Animals↗

PubMed1997

Layer-specific differential regulation of transcription factors Zif268 and Jun-D in visual cortex V1 and V2 of macaque monkeys.

To investigate intracellular mechanisms of cortical layer-specific gene regulation, we quantitatively examined the expression of two transcription factors, Zif268 and JunD, and compared their expression levels in each layer of the primary visual cortex (VI) and visual area 2 (V2) of macaque monkeys (Macaca fuscata). The brain sections were immunohistochemically stained for determination of the percentage of Zif268- or JunD-expressing neurons in the total neuronal population. We found area- and layer-specific expression of these transcription factors; Zif268 tended to be expressed at high levels in layers on the parvocellular pathway in V1, whereas JunD did not show such an expression pattern. In V1, many Zif268-immunopositive neurons were observed in layers II/III, IVC beta and VI. The percentage of Zif268-immunopositive neurons was highest in layer IVC beta and lowest in layer IVC alpha. JunD-immunopositive neurons were fewest in layer IVC beta and most abundant in layer VI. In V2, the level of expression of Zif268 was almost the same as that of JunD in layer II/II. However, layer IV of V2 tended to contain more Zif268-immunopositive neurons than JunD-immunopositive neurons, whereas layer VI contained more JunD-immunopositive neurons than Zif268-immunopositive neurons. Although it has been reported that the same extracellular signals induce both Zif268 and JunD, the present results indicate that the expression of these transcription factors is differentially controlled in each layer of the primate visual cortical areas. Furthermore, the present results suggest that these transcription factors contribute to area- and layer-specific gene regulation by mediating transmission of extracellular signals to the nucleus via different intracellular signalling pathways.

Animals↗

PubMed1996

Feedback signal from medial temporal lobe mediates visual associative mnemonic codes of inferotemporal neurons.

Functional roles of the cortical backward signal in long-term memory formation were studied in monkeys performing a visual pair-association task. Before learning of the task, the anterior commissure of the monkeys was transected, disconnecting the anterior temporal cortex of each hemisphere. After training with 12 pairs of pictures, we injected a grid of ibotenic acid unilaterally into the entorhinal and perirhinal cortex that provide massive backward projections ipsilaterally to the inferotemporal cortex. According to the histological examination, the lesions covered medial and lateral banks of the rhinal sulcus completely and most the entorhinal and perirhinal cortex. After the injections, the monkeys fixated the cue stimulus normally, relearned the preoperatively learned set (set-A) and learned a new set (set-B) of paired associates. Then single units were recorded from the same area as that for the prelesion control. We found that (i) in spite of the lesion, the sampled neurons responded strongly and selectively to both the set-A and set-B patterns, and that (ii) the paired associates elicited significantly correlated responses in the control neurons but not in the cells tested after the lesion either for set-A or set-B stimuli. We conclude that the ability of inferotemporal neurons to represent association between picture pairs was lost after disruption of backward neural signals from the limbic cortex to the inferotemporal neurons, while the ability of the neurons to respond to a particular visual stimulus was left intact.

Analysis of Variance↗