An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. I. Magnitude and time course of degeneration

Synaptic replacement in the dentate gyrus after unilateral entorhinal lesion: electron microscopic analysis of the extent of replacement of synapses by the remaining entorhinal cortex

In response to a unilateral entorhinal lesion the input from the contralateral entorhinal cortex to the dentate gyrus appears to increase. We have studied this crossed projection by electron microscopy in normal animals and in animals one year or more after a unilateral entorhinal lesion. In normal animals few degenerating boutons are found after a contralateral entorhinal lesion. However, when the contralateral lesion was made one year after an ipsilateral entorhinal lesion, degenerating boutons were readily identified. The boutons were relatively few in number, but formed an abnormally large number of synaptic contacts. These results support the previous conclusion that fibres from the contralateral entorhinal cortex form additional synapses when their ipsilateral homologues are removed. However, these new cortical synapses probably account for only a small portion of those formed in response to the lesion. Thus an anatomically homologous input does not, in this case, selectively capture most of the newly available synaptic sites.

Quantitative autoradiographic analysis of the time course of proliferation of contralateral entorhinal efferents in the dentate gyrus denervated by ipsilateral entorhinal lesions

The time course of the post-lesion proliferation of contralateral entorhinal afferents which occurs in response to ipsilateral entorhinal lesions was quanititatively analyzed with autoradiographic techniques. The extent of the crossed projection to the dentate granule cells was quantified on the basis of a contralateral/ipsilateral (C/I) ratio of grain density in the entorhinal terminal zones at 6, 8, 10, 12, 14, and over 60 days post-lesion. C/I ratios of grain density indicate little if any change in the crossed projection at 6 days post-lesion. Between 8 and 12 days post-lesion, the extent of the crossed projection increases dramatically, on the basis of the C/I ratio of grain density. C/I ratios do not increase further between 12 and 14 days post-lesion, but are higher at 60 days post-lesion. These results suggest that the crossed pathway proliferates extensively within the denervated zones between 8 and 12 days post-lesion, and may continue to proliferate at a much slower rate after 12 days post-lesion.

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. II. Effects of partial entorhinal and simultaneous multiple lesions

It has been concluded previously that the septohippocampal fibers which project to the rat dentate gyrus extend or branch in the denervated area of the molecular layer following a complete ipsilateral entorhinal lesion. The septohippocampal fibers thus appear to replace some of the perforant fibers which degenerate as a result of the lesion. The reactive fibers eventually become localized to a much smaller and more superficial area after lesions of immature rats than after lesions made in adulthood. To determine whether this difference in the response results from a selective reaction to loss of the lateral perforant path in the immature rat, various portions of the entorhinal cortex were removed at the age of 11 days, and the cholinergic septohippocampal fibers were visualized by acetylcholinesterase histochemistry. An alternative possibility, that the difference between immature and adult rats is attributable to an interaction with other reactive afferents, was tested by removing other sources of input (the contralateral entorhinal cortex, contralateral hippocampal formation or both) along with the ipsilateral entorhinal cortex at the age of 11 days and then demonstrating the septohippocampal fibers histochemically. Lesions of the lateral part of the ipsilateral entorhinal cortex (source of the lateral perforant path) at 11 days of age evoked a septohippocampal reaction along the outer edge of the molecular layer, where the lateral perforant path fibers normally terminate. This result matched that produced by a complete entorhinal lesion. Lesions of the medial entorhinal cortex evoked no obvious reaction. In contrast, the septohippocampal fibers in adult rats proliferated in the denervated area of the molecular layer after lesions of either part of the entorhinal cortex. Combining lesions of other sources of innervation to the dentate gyrus with an ipsilateral entorhinal lesion at 11 days of age did not alter the response of septohippocampal fibers, as determined histochemically. Neither did the septohippocampal fibers react to removal of commissural afferents alone. The response at any age was unaffected by prior or subsequent removal of the contralateral entorhinal cortex. These results indicate that in immature rats the septohippocampal fibers respond only to loss of the lateral perforant path, but these same fibers can later react to loss of any part of the perforant path. They are regarded as support for the hypothesis that the reactive septohippocampal fibers preferentially interact with dendritic growth cones. Our results do not support explanations based on a hypothetical attraction between septohippocampal and crossed perforant path fibers (which react in the same area) or on competition with commissural fibers (which reinnervate an adjacent area). We suggest further that proximity to the degenerating elements does not in itself determine the pattern of reinnervation after lesions of the central nervous system.

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. I. Time course after complete unilateral entorhinal lesion at various ages

The entorhinal cortex of rats was removed at various times during development, and the reaction of the cholinergic septohippocampal input to the dentate gyrus was examined by use of acetylcholinesterase histochemistry. When the ipsilateral entorhinal cortex is completely removed, the outer 70-75% of the molecular layer of the dentate gyrus is almost completely denervated. After such a lesion at 5 to 33 days of age, the acetylcholinesterase staining initially intensified throughout the denervated area, indicating that the septohippocampal fibers branched or elongated. This reaction could be detected within one day after a lesion at 11 days of age and within three or five days after lesions at earlier or later times. Whereas the initial response of the septohippocampal fibers was independent of the age at which the lesion was made, their final localization depended on the developmental state of the animal. After lesions at the age of 5 or 11 days, the reactive septohippocampal fibers became restricted to the outer one-sixth to one-third of the molecular layer within two days after appearance of their initial reaction. A similar concentration of reactive fibers was demonstrable after lesions at 16, 18 or 21 days of age, but some reaction persisted in the middle third of the molecular layer. Finally, after lesions at 26 or 33 days of age the proliferating cholinergic fibers ultimately were uniformly distributed throughout the outer 60% of the molecular layer. These results suggest that septohippocampal fibers initially extend or sprout throughout the denervated area to replace the lost perforant path fibers. However, the reactive fiber population becomes restricted to the outer edge of the molecular layer if the entorhinal lesion is made before the period of cholinergic synaptogenesis and concentrates in this same zone if it is made while cholinergic synapses are forming. We suggest that either the proliferative reaction continues in the outer part of the molecular layer and subsides in other parts of the denervated area or septohippocampal fibers move outward through the molecular layer to assume a more superficial location. After entorhinal lesions at 16 days of age or later the pale-staining zone (containing fibers that originate in hippocampus regio inferior) immediately deep to the denervated area widened. If the lesion was made earlier, this zone never developed at most septotemporal levels of the dentate gyrus. These results are probably related to the extension of regio inferior fibers into the denervated area.

The synaptic spinule in the dendritic spine: electron microscopic study of the hippocampal dentate gyrus

The present report calls attention to a component of certain synaptic junctions which has received little attention since its description in 1962 by Westrum and Blackstad. This component, which we term the synaptic spinule, is found in dendritic spine synapses in rat telencephalon (e.g., hippocampus, caudate nucleus, temporal and frontal cortex). Its major feature is an invagination of the presynaptic terminal by the presynaptic membrane, with the postsynaptic membrane protruding into this invagination. The synaptic spinule occurs in close association with the post-synaptic density, often occurring between breaks in this specialization. Serial sections reveal that when a synaptic spinule is present, a spine apparatus is observed near the dendritic protrusion. Coated vesicles are sometimes observed associated with the synaptic spinule; in all such instances they are associated with the presynaptic membrane. We studied the distribution of synaptic spinules in the dentate gyrus of the hippocampus. In contrast to its presence in the molecular layer, the synaptic spinule has not been observed in terminals of the subgranular layer of the dentate hilus. It is speculated that the synaptic spinule may play a role in exchange of material at dendritic spine synaptic junctions.

An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. II. Reappearance of morphologically normal synaptic contacts

Intact synapses in the denervated area of the rat dentate gyrus are reduced to 14% of those normally present 2-4 days following a unilateral entorhinal lesion. By 160-240 days after lesion, the former entorhinal terminal zone is repopulated with new synapses. In all, there is more than a 5-fold increase in the density of intact synapses in the denervated zone between 2 and 240 days post-lesion, and the denervated zone of the molecular layer is restored to 80% of control values. The synapses are Gray type I and are formed on simple and complex spines which closely resemble those normally present. A few boutons have an abnormally large number of synaptic junctions. Reinnervation seems to progress at differential rates. Synapses are rapidly regained up to 30 days after operation, but thereafter the reacquisition of synaptic connections is much slower. Reinnervation is more rapid in the portion of the denervated zone nearest the granule cells, where the maximal densities are attained within 30 days. The time course of reinnervation differed from that of degeneration. A portion of the new synapses in the reinnervated molecular layer appear to arise by the assembly of new synaptic junctions. Over time, the number of post-synaptic contact sites along a given length of dendritic surface recovers, suggesting the formation of new synaptic sites. Our data indicate that granule cells retain a capacity even into adulthood to manufacture, position and assemble postsynaptic components of a synapse and, in concert with reactive afferents, form normal-appearing synapses.