No abstract

In previous studies, fibers demonstrating somatostatin-like immunoreactivity were observed in the outer half of the molecular layer of the dentate gyrus in the rat and monkey. They occupy the same region as those of the perforant pathway that originates in the entorhinal cortex. Numerous somatostatin immunoreactive neuronal cell bodies were also observed in the hilar region, though stained axonal profiles could not be followed from these cells into the molecular layer. In the present study, several experimental procedures were employed to determine the origin of the somatostatin-positive fibers in the molecular layer. Transection of the perforant path fibers resulted in such characteristic changes as shrinkage of the molecular layer and sprouting of AChE-positive fibers. There was no apparent decrease, however, in the density of somatostatin-positive fibers. In fact, since the stained fibers occupied a narrower band in the shrunken molecular layer, their density appeared greater. Injections of kainic acid into the hilar region produced a lesion of hilar neurons, including those positive for somatostatin. In the region of cell loss, there was a marked reduction of somatostatin-immunoreactive fibers in the ipsilateral molecular layer, with no detectable changes in the homotopic contralateral molecular layer. The distribution of AChE fibers, which presumably have an extrinsic origin, was not altered by the treatment. In a final series of experiments, the retrograde tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected into the hilar region and sections were prepared for the simultaneous demonstration of the tracer and of somatostatin-like immunoreactivity. Somatostatin-positive neurons demonstrating WGA-HRP reaction product were observed primarily in the ipsilateral hilar region, but a few double-labeled cells were also seen in the same area of the contralateral side. These studies indicate that a population of intrinsic neurons located in the polymorphic layer of the dentate gyrus projects to the outer half of the ipsilateral molecular layer. A similar, but very much smaller, projection also extends to the contralateral dentate gyrus. Taken together, these projections appear to account for much of the somatostatin-like immunoreactivity in the molecular layer of the dentate gyrus.

In order to study the distribution of acetylcholinesterase (AChE) in the primate hippocampal formation, we have stained serial sections through the brains of nine macaque monkeys for AChE by two variants of the Koelle acetylthiocholine method. We have found a distinctive pattern of staining in the hippocampal formation which varies in intensity both from region to region, and along rostrocaudal and radial gradients within each region. In the dentate gyrus, there is intense staining of the inner one-third of the molecular layer with much lighter staining in the rest of the molecular layer except for a moderately stained band at its outer edge. In the caudal half of the dentate gyrus, the inner portion of the molecular layer is less intensely stained though there is a distinctly denser band of staining just above, and partly within, the superficial margin of the granule cell layer. The granule cells are unstained but there are AChE-positive fibers which run through the granule cell layer to the molecular layer. The hilar region of the dentate gyrus has a narrow band of heavy staining (which corresponds to an acellular layer in Nissl-stained sections) just subjacent to the granule cell layer; the remainder of the hilus, where most of the hilar cells reside, is less intensely stained and at caudal levels is almost entirely unstained. In the regio inferior of the hippocampus, there is intense staining of the stratum oriens which extends into the pyramidal cell layer; the stratum radiatum and the stratum lacunosum- moleculare are also stained and here the staining pattern shows some degree of stratification. By contrast, most of the alveus, the pyramidal cell somata, and the layer of mossy fibers (stratum lucidum) are unstained. The border region between regio inferior and regio superior of the hippocampus (field CA2 of Lorente de No, '34) is especially heavily stained. This contrasts markedly with regio superior, which is more lightly stained than regio inferior. Stratum oriens and stratum radiatum of regio superior have a more evenly distributed pattern of staining, though the intensity of staining increases sharply at the border with the subiculum. Stratum lacunosum- moleculare is only lightly stained throughout much of the transverse extent of regio superior but there is also a conspicuous and constant patch of heavier staining at the border with the subiculum.(ABSTRACT TRUNCATED AT 400 WORDS)

The distribution of somatostatin-like immunoreactivity was studied in the hippocampal formation of the Old World (Macaca fascicularis) and New World (Saimiri sciureus) monkeys. Series of coronal sections were processed by the unlabeled second antiserum method using primary antisera which recognize somatostatin-28 (S309) or somatostatin-28(1-12) (S320). Neuronal cell bodies were more readily stained with antiserum S309 and were observed throughout the hippocampal formation. The most prominent accumulations of stained neurons occur in the hilar region of the dentate gyrus, in strata oriens and pyramidale of regio inferior of the hippocampus, and in the deep layers of the entorhinal cortex. Both antisera demonstrated extensive fiber systems which varied in density regionally in the hippocampal formation. Stained fibers were most prominent in the outer two-thirds of the molecular layer of the dentate gyrus, in stratum lacunosum-moleculare of the hippocampus, in layer I of the presubiculum and in layers I, III, and V of the entorhinal cortex.

We have examined the interactions between axons regenerating from dorsal root ganglia (DRGs) derived from newborn rats and oligodendrocytes cultured by three different techniques. Cultures examined after 2 days have a profuse outgrowth of axons from the DRGs, forming a dense mat on the culture surface. However, the axons avoid growing on oligodendrocytes; axons are seen all around these cells, but do not grow over them. We have also performed time-lapse video studies of the interactions between axonal growth cones and oligodendrocytes. Axons grow normally until their growth cone comes into direct contact with an oligodendrocyte, following which the growth cone remains motile for 30–60 min, but without making any progress over the cell. The growth cone then suddenly collapses, and the axon retracts, leaving a thin strand in contact with the cell. After this a new growth cone is usually elaborated and the process repeated. Oligodendrocytes are therefore inhibitory to axonal growth, and this may partially explain the failure of axons to regenerate in the mammalian central nervous system.