Hiroaki Igarashi

Fasting and high-fat diet alter histone deacetylase expression in the medial hypothalamus.

Increasing attention is now being given to the epigenetic regulation of animal and human behaviors including the stress response and drug addiction. Epigenetic factors also influence feeding behavior and metabolic phenotypes, such as obesity and insulin sensitivity. In response to fasting and high-fat diets, the medial hypothalamus changes the expression of neuropeptides regulating feeding, metabolism, and reproductive behaviors. Histone deacetylases (HDACs) are involved in the epigenetic control of gene expression and alter behavior in response to a variety of environmental factors. Here, we examined the expression of HDAC family members in the medial hypothalamus of mice in response to either fasting or a high-fat diet. In response to fasting, HDAC3 and −4 expression levels increased while HDAC10 and −11 levels decreased. Four weeks on a high-fat diet resulted in the increased expression of HDAC5 and −8. Moreover, fasting decreased the number of acetylated histone H3- and acetylated histone H4-positive cells in the ventrolateral subdivision of the ventromedial hypothalamus. Therefore, HDACs may be implicated in altered gene expression profiles in the medial hypothalamus under different metabolic states.

The Convergence of Axon Terminals from the Mediodorsal Thalamic Nucleus and Ventral Tegmental Area on Pyramidal Cells in Layer V of the Rat Prelimbic Cortex

We investigated the ultrastructural basis of the synaptic convergence of afferent fibres from the mediodorsal thalamic nucleus (MD) and the ventral tegmental area (VTA) on the prefrontal cortical neurons of the rat by examining the synaptic relationships between thalamocortical or tegmentocortical terminals labelled with anterograde markers [lesion‐induced degeneration or transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA—HRP)] and randomly selected unlabelled apical dendrites of layer V pyramidal cells in the prelimbic cortex. WGA—HRP‐labelled terminals from the VTA ranged in diameter from 0.7 to 2.8 μm and established synaptic contacts with large dendritic profiles, i.e. proximal segments of apical dendritic shafts and spines from layer V pyramidal cells. Symmetrical synapses, i.e. inhibitory synapses, were more often seen than asymmetrical ones. Degenerating terminals from the MD formed asymmetrical synapses on dendritic spines or occasionally on small dendritic shafts of apical dendrites from layer V pyramidal cells, which received tegmentocortical synapses, mostly within layer III. Thalamocortical synapses were more distally distributed over common apical dendrites than tegmentocortical synapses, although some of them overlapped. The numerical density of direct synaptic inputs from the MD and VTA was low. These results suggest that fibres from the VTA exert their inhibitory effects directly on pyramidal cells in layer V via synaptic junctions with apical dendrites of these pyramidal cells, and that the tegmentocortical fibres are in an ideal anatomical position to modulate the reverberatory circuits between the MD and the prelimbic cortex.

Cisplatin-induced early and delayed emesis in the pigeon.

Intravenously injected cisplatin at a dose of 4 mg kg−1 induced early and delayed emesis in all pigeons without occurrence of lethality during a 72 h observation period. The early emetic response occurred with a latency of 81.3±8.0 min (n=15) and reached a peak at 2–3 h, and decreased gradually within 8 h after injection. Then the delayed emetic response, whose peak was found between 10 to 23 h, lasted up to 48 h. The emetic response markedly declined after 48 h. Reserpine markedly reduced monoamine levels in both brain and intestine and completely abolished the early and delayed emesis. Dexamethasone markedly reduced not only the early but also the delayed emetic responses. p‐Chlorophenylalanine decreased the level of serotonin in brain and intestine without affecting noradrenaline and dopamine and partly reduced the early emetic response, but did not affect delayed emesis. Bilateral vagotomy prolonged the latency time to the onset of early emesis, and reduced the emetic responses in both the early and delayed phases. The above results suggest that the cisplatin‐induced early emesis in the pigeon is partially mediated via the vagal nerve and reserpine‐sensitive monoaminergic systems including the serotonergic system; the delayed emesis is associated with monoaminergic but not the serotonergic systems.

Thalamocortical projection from the ventral posteromedial nucleus sends its collaterals to layer I of the primary somatosensory cortex in rat

Here we examined quantitatively axonal projections originating from the ventral posteromedial thalamic nucleus (VPM) to layer I of the primary somatosensory cortex (SI) by extracellular and intracellular injections of biocytin as an anterograde tracer. Following the extracellular injections, two types of VPM afferents with different arborization patterns in SI were observed. The type I extended vertically, forming dense plexus in layers IV and VI, and projected collaterals to layer I. The type II rarely branched in SI, converged in the plexus formed by the type I, and projected no collaterals to the supragranular layers. The labeled fibers in layer I derived from the first type ran parallel to the brain surface, and their mean length was 339.7 +/- 87.5 microm. Intracellular injection into VPM neurons bearing both types of afferent demonstrated the full axonal arborization in both the reticular thalamic nucleus (Rt) and SI. The total length of the axon of a neuron bearing the type I was 86,968.8 microm, and the length of axonal collaterals in layer I of SI was 433.1 microm. The total axonal length of a neuron bearing the type II was very small. The present study is the first to demonstrate substantial projections from VPM to layer I of SI, and provide quantitative data on the entire extent of the axonal arborization of thalamocortical projections from single VPM neurons.

Dopamine D5 receptor immunoreactivity is differentially distributed in GABAergic interneurons and pyramidal cells in the rat medial prefrontal cortex.

In the rodent neocortex, the dopamine D5 receptor (D5R) appears to be the predominant subtype of D1-like receptors that are generally considered to play important roles in cognitive functions subserved by the prefrontal cortex (PFC). In this study, to identify the precise localization of D5R in rat PFC, we used a receptor-specific antibody and observed the immunolabeled structures by light and confocal laser scanning microscopies. D5R immunolabeling was found in nearly all neurons, both excitatory and inhibitory neurons. Most of the excitatory neurons showing D5R immunolabeling appear to be pyramidal neurons. In these neurons, D5R immunolabeling was observed throughout somata and dendrites including dendritic spines. In neuropil, almost all of the fiber terminals, represented by synaptophysin immunopositivity, were devoid of D5R. Among inhibitory neuronal subpopulations, we examined parvalbumin-immunopositive neurons (PV neurons), because they form a major subpopulation of fast-spiking neurons. Because parvalbumin immunolabeling enables detection of somata and dendrites as well as axonal profiles, we analyzed the intracellular distribution pattern of D5R immunolabeling. As a result, we found that D5R immunolabeling was mainly in somata and proximal dendrites. The density of intradendritic D5R immunolabeling decreased toward the distal regions. Thus, the distribution pattern of D5R immunolabeling is markedly different between pyramidal neurons and PV neurons. D5R may underlie dopamine modulation of cognitive function in PFC.

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and layer III pyramidal cells in the prelimbic cortex of the rat.

A combined study of anterograde axonal degeneration and Golgi electron microscopic technique was designed to examine the distribution and density of axon terminals from the mediodorsal thalamic nucleus (MD) over layer III pyramidal cells in the prelimbic cortex of the rat. The reconstructive analysis of serial ultrathin sections of gold-toned apical and basal dendrites of layer III pyramidal cells showed that degenerating thalamocortical axon terminals from MD formed asymmetrical synaptic contacts predominantly with dendritic spines of the identified basal dendrites as well as apical dendrites. There was little difference in the numerical density of thalamocortical synapses from MD per unit length of both apical and basal dendrites.

Immunolocalization of muscarinic receptor subtypes in the reticular thalamic nucleus of rats

In this study, to identify the precise localization of the muscarinic receptor subtypes m2, m3 and m4 in the rostral part of the rat reticular thalamic nucleus (rRt), namely, the limbic sector, we used receptor-subtype-specific antibodies and characterized the immunolabeled structures by light, confocal laser scanning, and electron microscopies. The m2-immunolabeling was preferentially distributed in the distal dendrite region where cholinergic afferent fibers tend to terminate and in the peripheral region of somata, whereas the m3-immunolabeling was more preferentially distributed in a large part of somata and in proximal dendrite shafts than in the distal dendrite region. Dual-immunofluorescence experiments demonstrated that majority of rRt neurons with parvalbumin immunoreactivity contain both m2 and m3. Neither m2 nor m3 was detected in presynaptic terminals or axonal elements. No m4-immunolabeling was detected in the rostral part of the thalamus including rRt. These results show the different distributions of m2 and m3 in rRt neurons, and strongly suggest that m2 is more closely associated with cholinergic afferents than m3.

Microglial ecto-Ca2+-ATPase activity in a rat model of focal homologous blood clot embolic cerebral ischemia: an enzyme histochemical study

Post-ischemic changes in ecto-Ca(2+)-ATPase activity in microglia and the infarcted tissue were studied in a rat model of focal embolic cerebral ischemia using an enzyme histochemical method. Ecto-Ca(2+)-ATPase activity was observed in whole brains in non-operated and sham-operated control animals. In addition, this enzyme activity was determined to be localized in ramified microglia. At 30 min after ischemia, non-microglial ecto-Ca(2+)-ATPase activity in the infarcted tissue slightly decreased and continued to decrease thereafter. The ecto-Ca(2+)-ATPase activity in microglia did not appear changed at this time. The decrease of enzyme activity in the infarcted tissue made it much easier to clearly observe ecto-Ca(2+)-ATPase-positive microglia. The enzyme activity of microglia in the ischemic area began to decrease 2 or 4h after embolization and remarkably decreased, except in the perinuclear cytoplasm, apical parts of the processes, and several parts along the processes, 8h after ischemia. By 12h after onset of embolization, the enzyme activity of microglia and infarcted tissue had almost completely disappeared. Ecto-Ca(2+)-ATPase of microglia is likely to play an important role in the metabolism of extracellular nucleotides in the ischemic area immediately after the onset of embolization by means of ecto-enzymes. Thus, the findings of the present study suggest that microglia might serve to protect the infarcted tissue in the ischemic brain.

High-voltage electron microscopy of E-PTA-stained synaptic junctions in the rat frontal cortex.

The E-PTA-stained synaptic junctions in the adult rat frontal cortex were examined with high-voltage electron microscopy (HVEM). Perforated whole synaptic junctions were clearly shown in the stereo image. The E-PTA staining procedure provides a useful marker for studies of the 3-dimensional structure of synaptic junctions by means of HVEM.

SYNAPTIC CHARACTERISTICS BETWEEN CORTICAL CELLS IN THE RAT PREFRONTAL CORTEX AND AXON TERMINALS FROM THE VENTRAL TEGMENTAL AREA THAT UTILIZE DIFFERENT NEUROTRANSMITTERS

Projections from the ventral tegmental area (VTA) have been demonstrated to terminate in the prefrontal cortex (PFC) and to be dopaminergic and/or γ-aminobutyric acidergic (GABAergic), forming a neural circuit implicated in certain memory and cognitive processes. However, it has not been determined whether γ-aminobutyric acid (GABA) and dopamine (DA) are localized in certain types of axon terminals from the VTA to the PFC. To determine the synaptic characteristics made by postsynaptic prefrontal cortical structures and mesoprefrontal fibers utilizing either GABA or DA, we performed a double-labeling method for electron microscopy, in which we combined peroxidase markers for anterograde tract-tracing with postembedding immunogold labeling for tyrosine hydroxylase, DA, and GABAin rats. The anterograde tract-tracing studies showed that tegmentocortical fibers from the VTA terminated as both symmetric and asymmetric axon terminals with the predominantly symmetric synaptic type in the prelimbic cortex of the rat. Furthermore, a study using the combination of anterograde tract-tracing and postembedding immunocytochemistry indicated that tegmentocortical axon terminals forming symmetric synapses were either GABAergic or dopaminergic, whereas a small fraction of tegmentocortical terminals ending as asymmetric synapses were not immunopositive for DA or GABA. These findings indicate that the mesocortical projections to the PFC exert an inhibitory effect on the spontaneous activity of PFC cells via symmetric synapses that use DA and GABA as neurotransmitters and that these projections also have as yet unknown effects via asymmetric synapses using other neurotransmitters.