Parvalbumin-containing nonpyramidal neurons in intracortical transplants of rat hippocampal and neocortical tissue: a light and electron microscopic immunocytochemical study

Morphological features of the entorhinal-hippocampal connection

The goal of this review in an overview of the structural elements of the entorhinal-hippocampal connection. The development of the dendrites of hippocampal neurons will be outlined in relation to afferent pathway specificity and the mature dendritic structure compared. Interneurons will be contrasted to pyramidal cells in terms of processing of physiological signals and convergence and divergence in control of hippocampal circuits. Mechanisms of axonal guidance and target recognition, target structures, the involvement of receptor distribution on hippocampal dendrites and the involvement of non-neuronal cellular elements in the establishment of specific connections will be presented. Mechanisms relevant for the maintenance of shape and morphological specializations of hippocampal dendrites will be reviewed. One of the significant contexts in which to view these structural elements is the degree of plasticity in which they participate, during development and origination of dendrites, mature synaptic plasticity and after lesions, when the cells must continue to maintain and reconstitute function, to remain part of the circuitry in the hippocampus. This review will be presented in four main sections: (1) interneurons-development, role in synchronizing influence and hippocampal network functioning; (2) principal cells in CA1, CA3 and dentate gyrus regions-their development, function in terms of synaptic integration, differentiating structure and alterations with lesions; (3) glia and glia/neuronal interactions-response to lesions and developmental guidance mechanisms; and (4) network and circuit aspects of hippocampal morphology and functioning. Finally, the interwoven role of these various elements participating in hippocampal network function will be discussed.

Embryonic cortical neurons differentiate into various types of interneurons when heterotopically transplanted into the adult rat brain

The adult cortex represents a heterogeneous mixture of different classes of pyramidal neurons and non-pyramidal interneurons. After grafting embryonic cortical anlage into the adult striatum, the present study investigated whether the development of different populations of interneurons in heterotopic cortical grafts is similar to the adult cortex. The presence of specific subpopulations of interneurons in grafts was assessed by immunocytochemistry using various antibodies against marker molecules for interneurons such as neuropeptides and calcium-binding proteins. These molecules are expressed to a different extent in specific subpopulations of cortical interneurons. Cortical primordia obtained on embryonic day 14 (E14) were stereotactically grafted into the center of the left striatum of adult recipient rats. After an 8-week differentiation period, host rats were perfusion fixed and immunocytochemistry was performed using antibodies against neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), somatostatin, parvalbumin and calbindin D-28k. Within the grafts, the number of immunopositive interneurons as well as the intensity of immunostaining for different marker molecules corresponded well with those of the adult cortex. In contrast, the expression pattern in the graft demonstrated clear differences when compared with the surrounding host striatum. The present study demonstrates, that at E14 at least some cells of the cortical anlage are primed to develop into different classes of interneurons independent of their normal environment and their regular synaptic connections. Thus, different interneuron progenitor cells survive transplantation and develop cell-specific morphological and cytochemical characteristics. Differentiation into various subpopulations of neurons may be a prerequisite for potential therapeutic approaches in humans.

Morphological alterations of hippocampal pyramidal neurons heterotopically transplanted into the somatosensory cortex of adult rats: a quantitative Golgi study

A characteristic feature of hippocampal organization is the laminated termination of extrinsic and intrinsic afferents. At present, it is not known to what extent these layer-specific fiber projections modulate the development and final shape of the dendritic arbor of hippocampal target neurons. In the present study, pieces of late embryonic (E18) rat hippocampus were transplanted heterotopically into a cavity in the somatosensory cortex of 6-8 week-old recipient rats. Here, the transplanted neurons differentiated and survived up to several months in the absence of their specific extrinsic afferents. Moreover, tracing of transplant connections with the carbocyanine dye DiI revealed only a limited projection between the transplant and the host neocortex. Golgi-impregnated transplants were used to analyze the postsynaptic structures (dendrites and spines) of hippocampal pyramidal cells quantitatively. Compared with controls, the transplanted pyramidal neurons showed a significant reduction of apical primary dendrites and basal dendritic branches, i.e. of peripheral dendritic portions that originate farther from the soma. In contrast, the number of basal primary dendrites originating directly from the perikaryon was enhanced. Spine density on the main apical dendritic shaft was significantly lower in all peripheral dendritic segments in transplanted neurons. We conclude from our results that the absence of layer-specific extrinsic afferents that normally terminate on peripheral parts of the dendritic arbor of hippocampal pyramidal neurons caused a reduction of these peripheral dendrites and spines. In contrast, the increase of dendrites and spines near the cell body might be induced by intrinsic fibers that normally terminate on these proximal dendritic portions and are known to sprout under transplant conditions.

Characterization of GABAergic neurons in hippocampal cell cultures

The morphological characteristics of GABAergic neurons and the distribution of GABAergic synaptic terminals were examined in cultures of hippocampal neurons from 4-35 days in vitro. Neurons expressing GABA immunoreactivity represented about 6% of the total number of cultured neurons at all time points. Although the morphological characteristics of GABAergic cells suggested a heterogeneous population, GABAergic cells as a class were notably different from the non-GABAergic, presumably pyramidal cells. Most GABAergic cells had more fusiform or polygonal shaped somata, non-spiny and less tapering dendrites and appeared more phase-dense than nonGABAergic cells. Quantitative analysis revealed that GABAergic cells had fewer primary dendrites, more elongated dendritic arbors, and longer dendritic segments than non-GABAergic neurons-characteristics that are similar to GABAergic cells in situ. Double immunostaining revealed that GAD65-positive varicosities were also immunopositive for synapsin I, suggesting that GAD65-positive varicosities that contacted somata and dendrites represented presynaptic specializations. Confocal microscopy revealed the proportion of the synaptic specializations on the cell soma that were GAD65-positive was greater than on the dendrites, suggesting that somata and dendrites differ in their ability to induce the formation of presynaptic specializations by GABAergic axons. These data indicate that the GABAergic cells that develop in culture exhibit distinctive morphological characteristics and participate in different synaptic interactions that nonGABA cells. Thus many of the features that distinguish GABAergic neurons in culture are reminiscent of the characteristics that distinguish GABAergic neurons in situ.

Morphological assessment of grafted rat and mouse cortical neurons: a light and electron microscopic study

The morphology of cortical neurons grafted into (or near) the rat striatum was studied by means of intracellular Lucifer yellow injections in fixed slices. Rat donor syngeneic cortical tissue (from postnatal day 1 old rats; AO strain) as well as mouse donor xenogeneic cortical tissue (prenatal day 19; C3H/HE strain) were grafted as solid pieces into 8-12 week-old rats (AO strain). Recipients of mouse xenografts were immunosuppressed with a monoclonal antibody against the interleukin-2 receptor. After perfusion and sectioning of the graft-containing areas, individual slices were incubated in the DNA stain 4.6-diamidino-2-phenylindole (DAPI) to visualize the cell nuclei. Grafts could be easily identified by a surrounding rim of astrocytes which outline the border between grafted and host tissue. Grafted cortical neurons were intracellularly filled with Lucifer yellow, DAB-photoconverted, and further processed for light and electron microscopy. In general, no cortical lamination could be observed in the grafted rat and mouse cortical tissue, but neurons were loosely packed throughout the graft. Two major cell types could be identified in all grafts investigated so far. The majority resembled those described as spiny neurons (85%), which could be further classified into pyramid-like, spiny stellate-like or fusiform spiny neurons, with somata ranging between 15 and 25 microns in diameter. The remaining 15% resembled non-spiny neurons with either a multipolar basket-like or fusiform morphology. Dendrites of spiny and non-spiny neurons, which could extend to distances up to 400 microns, were never seen to cross the astrocytic border, but some main axon and axonal collaterals of spiny neurons were found to leave the graft. On the basis of light microscopic observations no difference was found between mouse and rat grafted cortical neurons. The results of this study show that grafted cortical neurons retain some of the characteristic features of neurons in the intact adult cerebral cortex, although there appears to be a greater preponderance of spiny neurons in grafted tissue. This may reflect an immaturity of the grafted tissue or a response to the striatal environment.

Distribution of calretinin, calbindin-D28k, and parvalbumin in the rat thalamus

The localization of three calcium-binding proteins, calretinin, calbindin-D28k, and parvalbumin, in the rat thalamus was immunohistochemically examined. a) Some thalamic regions revealed cells almost exclusively containing one of the calcium-binding proteins. For example, almost only calretinin-stained cells were found in the central medial and paraventricular nuclei. Calbindin-D28k-stained cells were mostly found in the centrolateral, interanteromedial, anteromedial, and posterior nuclei. Only parvalbumin-positive cells were found in the central part of the reticular nucleus. b) Other regions expressed overlap between the distributions of two cell components composed of different calcium-binding proteins. For example, both calretinin-stained cells and calbindin-D28k-labeled cells were found in the lateroposterior, intermediodorsal, rhomboid, and reuniens nuclei. c) Other regions showed no cells stained for any of the calcium-binding proteins. For example, generally no calcium-binding protein was detected in neurons of the anterodorsal, anteroventral, ventrolateral, ventral posterolateral, ventral posteromedial, or gelatinosus nuclei, or of the central part of the mediodorsal nucleus. These three proteins serve as useful marker for localizing subpopulations of neurons within the thalamus.

Immunohistochemical localization of calretinin-, calbindin-D28k- and parvalbumin-containing cells in the hypothalamic paraventricular and supraoptic nuclei of the rat

The localization of three calcium-binding proteins, calretinin, calbindin-D28k and parvalbumin, in the hypothalamic paraventricular and supraoptic nuclei of the rat was immunohistochemically examined on adjacent sections and their distribution patterns were compared. Overlap between the distribution of calretinin-immunoreactive cells and that of calbindin-D28k-immunostained cells was found in the rostrodorsal part of the supraoptic nucleus, and the caudoventral part of this nucleus contained predominantly calbindin-D28k-stained cells. Cells of the medial and lateral magnocellular subdivisions of the paraventricular nucleus were almost devoid of the calcium-binding proteins examined. No parvalbumin-immunostained cells were observed in either nucleus. This study provides a further characterization of cell bodies in the hypothalamic paraventricular and supraoptic nuclei.