Monoclonal antibodies reveal molecular differences between terminal fields in the rat dentate gyrus

Monoclonal Antibodies to Late-differentiating Epitopes Identify Mossy Fibre Terminals Innervating Normal and Transplanted Hippocampal CA3 Pyramidal Cells

We have derived two monoclonal antibodies, MF-1 and MF-2, which both recognize the same 58-kD antigen. Light and electron microscopic immunocytochemistry showed that this antigen is highly expressed in the large mossy fibre terminals innervating the proximal portion of the apical dendrites of pyramidal neurons in hippocampal field CA3. Staining was seen in the adult hippocampus in rats and mice, and in a post mortem human sample. Comparison with the Timm stain showed that the antibodies recognize mossy fibres from all parts of the adult dendate gyrus except for the tip of the infrapyramidal blade (the latest part of the dentate gyrus to develop). The MF antigen is expressed by mature terminals, and is not detected immunohistochemically in developing hippocampal mossy terminals until the end of the first postnatal week (i.e. later than the Timm-positive material). It was also found in host mossy fibre terminals innervating embryonic CA3 pyramids transplanted into adult hosts, but not in areas of the graft containing transplanted CA1 pyramids. These results indicate that this previously undescribed, late-developing antigen provides a useful specific marker for the mossy fibre projection in both the normal hippocampus and in situations of experimentally manipulated connectivity.

Sema3C and netrin-1 differentially affect axon growth in the hippocampal formation

The interaction between outgrowing neurons and their targets is a central element in the development of the afferent and efferent connections of the hippocampal system. This requires that axonal growth cones recognize specific guidance cues in the appropriate target area. At present, little is known about the mechanisms that determine the lamina-specific termination of hippocampal afferents. In order to understand the role of different guidance factors, we analyzed the effects of Sema3C and Netrin-1 on explants from the entorhinal cortex, dentate gyrus, cornu ammonis regions CA1 and CA3 and medial septum in a collagen coculture assay. Our observations suggest that both semaphorins and netrin play important roles in the neuron-target interactions in the hippocampal system. Sema3C is involved in the control of the ingrowth of the septohippocampal projection. We also show that netrin-1 is involved in attracting commissural neurons from dentate gyrus/hilus and CA3 to their target area in the contralateral hippocampus.

Target- and maturation-specific membrane-associated molecules determine the ingrowth of entorhinal fibers into the hippocampus

In this study the role of membrane-associated molecules involved in entorhinohippocampal pathfinding was examined. First outgrowth preferences of entorhinal neurites were analyzed on membrane carpets obtained from their proper target area, the hippocampus, and compared to preferences on control membranes from brain regions which do not receive afferent connections from the entorhinal cortex. On a substrate consisting of alternating lanes of hippocampal and control membranes, entorhinal neurites exhibited a strong tendency to grow on lanes of hippocampal membrane. These tissue-specific outgrowth preferences were maintained even on membrane preparations from adult brain tissue devoid of myelin. To determine the possible maturation dependence of these membranes, we examined guidance preferences of entorhinal neurites on hippocampal membranes of different developmental stages ranging from embryonic to postnatal and adult. Given a choice between alternating lanes of embryonic (E15-E16) and neonatal (P0-P1) hippocampal membranes, entorhinal neurites preferred to extend on neonatal membranes. No outgrowth preferences were observed on membranes obtained between E19 and P10. From P10 onward there was a reoccurrence of a preference for postnatal membrane lanes when neurites were presented with a choice between P15, P30, and adult membranes (>P60). This choice behavior of entorhinal neurites temporally correlates with the ingrowth of the perforant path into the hippocampus and with the stabilization of this brain area in vivo. Experiments in which postnatal and adult hippocampal membranes were heat inactivated or treated to remove molecules sensitive to phosphatidylinositol-specific phospholipase C demonstrated that entorhinal fiber preferences were controlled in this assay by attractive guidance cues and were independent of phosphatidylinositol-sensitive linked molecules. Moreover, entorhinal neurites displayed a positive discrimination for membrane-associated guidance cues of their target field, thus preferring to grow on membranes from the molecular layer of the dentate gyrus compared with CA3 or hilus membranes. Heat-inactivation experiments indicated that preferential growth of entorhinal axons is due to a specific attractivity of the molecular layer substrate. The data presented demonstrate that outgrowth of entorhinal fibers on hippocampal membranes is target and maturation dependent.

Laminar boundaries persist in the hippocampal dentate molecular layer of the mutant Shaking Rat Kawasaki despite aberrant granule cell migration

The present report provides the first detailed description of the hippocampus in the Shaking Rat Kawasaki (SRK) mutant by using a panel of antibody markers to delineate its laminar organization. The mutant was characterised at postnatal day 21 by severe malformations of both neuronal position and orientation, the most striking of which was the presence of a rounded central granule cell mass in the dentate gyrus rather than the normal V-shaped granule cell layer. Despite this finding, the SRK dentate gyrus not only retained a cell-sparse molecular layer (thinner but similar in gross appearance to that of control littermates), but the sharp laminar boundary between its inner and outer parts was as clearly marked by IM1 and OM4 antibody staining as it was in the normal dentate gyrus. These immunocytochemical data suggest that the entorhinal terminal field of the dentate gyrus may be relatively normal in the mutant, despite entorhinal afferents appearing to take an abnormal trajectory after they fail to cross the hippocampal fissure. Laminar malformations included disruption of the SRK pyramidal cell layer, with spreading of the CA3 mossy fibre projection to an ectopic infrapyramidal position, radial displacement of CA1 pyramids, and transposition of a hitherto unremarked longitudinal fibre bundle immunoreactive for calretinin from its normal position in the stratum lacunosum-moleculare of field CA2 to an alvear position in SRK. The SRK malformations were very like but not identical to those seen in the mouse reeler mutant, suggesting similar underlying developmental mechanisms.

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.

Lamina-specific synaptic connections of hippocampal neurons in vitro

By using slice cultures as a model, we demonstrate here that different target selectivities exist among the various afferent fibers to the hippocampus. As in intact animals, septohippocampal cholinergic fibers, provided by a slice culture of septum, innervate a co-cultured slice of hippocampus diffusely, that is, without forming distinct layers of termination. As in vivo, the septal cholinergic fibers establish synapses with a variety of target cells. Conversely, fibers from an entorhinal slice co-cultured to a hippocampal slice display their normal laminar specificity. They preferentially terminate in the outer molecular layer of the fascia dentata, thereby selectively contacting peripheral dendrites of the granule cells. This preferential termination on peripheral dendritic segments is remarkable, since these fibers do not have to compete with commissural fibers, hypothalamic fibers, and septal afferents for dendritic space under these culture conditions. Moreover, in triplet cultures in which first two hippocampal slices were co-cultured and then, with a delay of 5 days, an entorhinal slice was added, the fibers from the entorhinal slice and those from the hippocampal culture terminated in their appropriate layers in the hippocampal target culture. However, in this approach the normal sequence of ingrowth of these two afferents was reversed. In normal ontogenetic development, entorhinal afferents arrive in the hippocampus before the commissural fibers. The results show that there are different degrees of target selectivity of hippocampal afferents and that the characteristic lamination of certain afferent fibers in the hippocampus is not determined by their sequential ingrowth during development.

Rapid decline in the ability of entorhinal axons to innervate the dentate gyrus with increasing time in organotypic co-culture

We have used the species-specific monoclonal antibodies OM1 and OM4 to identify the histiotypic pattern of projection from late embryonic rat entorhinal explants to the outer molecular layer of the dentate gyrus in organotypic cultures of 6-day postnatal mouse hippocampal slices. The presence of this entorhinal projection was detectable with the rat-specific OM1 and OM4 markers after 3-7 days in co-culture, and confirmed by use of the later-forming rat neuron-specific marker THy-1.1, which appeared during the second week. Hippocampal slices confronted with control explants of superior colliculus for 4 weeks in culture showed only sparse, non-specific growth of axons with no histiotypic pattern in the dentate gyrus. In order to assess whether the formation of specific entorhino-dentate projections in vitro is age-dependent, embryonic rat entorhinal cortical explants were cultured alone for periods of 1-5 weeks before cutting across the halo of axons radiating into the collagen matrix and presenting each with 6-day-old mouse hippocampal slices as targets to innervate. After allowing a 2 week period for fibre growth to take place, the density of immunostained axonal outgrowth was scored on a five-point scale for each weekly interval. The amount of new axon growth when the cuts were made after 1 week was slightly reduced compared to undamaged control cultures. However, outgrowth was greatly diminished when the cuts were made after 2 or 3 weeks, and essentially abolished if the interval was extended to > or = 4 weeks. Thus we demonstrate that, although hippocampal slices can survive in organotypic co-culture with entorhinal explants and maintain previously formed connections, the explants show an age-related failure in the ability to form new connections. Such a system provides a possible in vitro model for study of the factors influencing the failure of regeneration in the adult central nervous system.

Transneuronal changes in dendrites of GABAergic parvalbumin-containing neurons of the rat fascia dentata following entorhinal lesion

The perforant path fibers from the entorhinal cortex form synapses with both granule cells and GABAergic, parvalbumin-containing (PARV) nongranule cells. The authors recently reported a persistent reduction of PARV-positive dendrites in the termination zones of entorhinal fibers in the hippocampus proper and fascia dentata after lesion of the entorhinal cortex. In the present study the authors analyzed the effects of de-entorhination on the ultrastructure of postsynaptic PARV-positive dendrites in the molecular layer of the fascia dentata. PARV immunocytochemistry was performed 2, 8, 55, and 360 days after an ipsilateral entorhinal lesion and, for comparison, 10 days after an ipsilateral fimbria-fornix transection that disconnects the hippocampus from its septal and commissural afferents. Two days after entorhinal lesion, the authors observed swelling of the tissue close to the hippocampal fissure. Adjacent distal dendritic tips of PARV-positive dentate neurons appeared bloated and reduced in number. Reduction of PARV-positive dendrites in the former perforant path termination zone persisted 55 days after entorhinal lesion and could still observed after postlesional survival times for 1 year. Degenerating axon terminals were still present 55 days following lesion and PARV-positive dendrites exhibited abnormal invaginations. Fimbria transection did not result in similar dendritic changes in PARV-positive neurons. The results indicate a long-lasting process of reorganization in the molecular layer of the fascia dentata following entorhinal lesion and persisting changes in the morphology of PARV-immunoreactive dendrites. Entorhinal fibers seem to play a specific role for the maintenance of these dendrites, since similar changes did not occur following removal of septal and commissural fibers.

The OM series of terminal field-specific monoclonal antibodies demonstrate reinnervation of the adult rat dentate gyrus by embryonic entorhinal transplants

Monoclonal antibodies OM-1 to OM-4 and IM-1 [Woodhams et al. (1991) Neuroscience 46, 57-69] have complementary immunostaining patterns in the molecular (dendritic) layer of the adult rat dentate gyrus, with OM-1 to OM-4 selectively recognizing the outer (distal) two-thirds (i.e. the entorhinal afferent zone), and IM-1 the inner (proximal) one-third (i.e. the hippocampal commissural/associational zone). Immunoblotting suggests that OM-1 recognizes a single glycoprotein antigen of mol. wt around 93,000, and OM-2, OM-3, and OM-4 all recognize a second glycoprotein antigen of mol. wt around 36,000. At four weeks after removal of the ipsilateral entorhinal cortex the background OM immunostaining of the entorhinal afferent zone is abolished and replaced by a network of densely stained granules, which we interpret as degenerating entorhinal afferent axons. At the same time, the proximal, IM immunoreactive zone expands by about 10 microns in width (while the distal deafferented zone shrinks by about 80 microns). Attempts were made to restore the OM immunoreactivity of the distal zone by grafting either small pieces or cell suspensions of embryonic day 18 entorhinal cortex directly into the dentate molecular layer of entorhinally deafferented adult hosts. About half (14/26) of the animals with successfully positioned grafts showed restoration of OM-2 to OM-4 immunostaining throughout the entire width of the outer two-thirds (entorhinal afferent zone) of the dentate molecular layer. Strikingly, however, in adjacent serial sections the restoration of OM-1 immunoreactivity was restricted to the "middle" molecular layer, i.e. the most proximal part of the distal (entorhinal) two-thirds of the dentate molecular layer. In no case did the OM-1 immunoreactivity extend to the outer margin of the molecular layer. This did not appear to be associated with incompleteness of the removal of the host entorhinal projection, since it occurred in grafted cases where the hippocampus had been completely isolated from the entorhinal area. The simplest explanation of the observed pattern of OM loss and restitution is that the epitopes are located on the entorhinodentate axons, but it is not clear whether the antigens recognized by OM-1 and OM-2 to OM-4 are expressed in different parts of the same group of axons, or in different subsets of entorhinodentate axons. Nor is it clear why the pattern of OM-1 is only restored to the "middle" molecular layer, while that of OM-2 to OM-4 is restored to the entire outer two-thirds.(ABSTRACT TRUNCATED AT 400 WORDS)