Akiko Okada

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.

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.

Direct synaptic contacts between axon terminals from the horizontal limb nucleus of the diagonal band and corticotectal neurons to the superior colliculus in the medial frontal cortex

The presubiculum (PreS) plays a role of the interface between the medial entorhinal cortex (MEA) and the anterior thalamic nuclei as well as the subiculum. We reported that the layer III cells of PreS projected to MEA, while the layer II cells concerned with the associational connection in PreS. In this study, further analysis was made using the WGA-HRP and PHA-L method. The association cells were widely distributed in the PreS including area 29e, and the majority were in the temporal half. The cells in the septal PreS projected far to the temporal PreS, while the cells in the temporal PreS terminated mostly within the temporal portion. Transversely, the association cells were located densely in the mid and distal parts (far from the subiculum), but very few in the proximal part. As to the reciprocal connection between the MEA and layer III of PreS, the following topography was observed: 1) the septal PreS was connected with the lateral MEA, and the temporal PreS with the medial MEA, and 2) the distal PreS connected with the septal MEA and the proximal PreS with the temporal MEA.