Neuropeptides and astroglia in intracerebral hippocampal transplants: An immunohistochemical study in the rat

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Abstract
The rat hippocampus and fascia dentata contain neurons and terminal fields that react immunohistochemically with antisera raised against the peptides somatostatin, cholecystokinin (CCK), and enkephalin. Neurons with overlapping distribution synthesize acetylcholinesterase (AChE) as shown histochemically after systemic administration of diisopropylfluorophosphate (DFP). We used these techniques to examine the organization of peptidergic neurons and fibers and AChE‐positve neurons in transplants of late embryonic and early postnatal rat hippocampi inserted into the brain of immature and adult rats. The astroglial organization was examined using an antibody raised against the astroglial marker, glial fibrillary acidic protein (GFAP). Eighty percent of the transplants were recovered. Within the transplants the peptidergic neurons, and the AChE‐producing neurons retained their characteristic distributions. An organotypical astroglial pattern was also retained with some indication of a higher than normal density of cells and processes. The transplant neuropil displayed characteristic patterns of terminal field‐like staining with the enkephalin and CCK antibodies. An enkephalin‐like reactivity characteristic of the large mossy fiber terminals was found in the normal mossy fiber zones. A similar staining, but of smaller particle size, was found in and above the granule cell layer. It corresponded to the aberrant, supragranular mossy fiber projection, known to form after denervation of the dentate molecular layer. Also the CCK‐like reactivity of the transplant neuropil was distributed in laminar patterns characteristic of the different subfields. Deviations from the normal patterns again followed known principles for lesion‐induced aberrant nerve growth. Normalization of the enkephalin and CCK staining patterns in the dentate molecular layer was observed in transplants with intimate contact with the host dentate molecular layer or entorhinal cortex, but only in immature recipients, and not in every case. The staining was typically of the enkephalin‐reacting lateral perforant path and the CCK‐reacting medial perforant path and distributed in normal, mutually exclusive laminae in the dentate molecular layer. An invasion of the transplant dentate molecular layer by heterologous, host enkephalin‐ and CCK‐like reacting afferents in transplants located in the host basal ganglia was limited at most. Outgrowth of CCK and enkephalin reacting transplant fibers into the host brains was not observed in this study. We conclude (1) that peptidergic and AChE‐producing hippocampal and dentate neurons can survive intracerebral transplantation and that an organotypic, astroglial pattern of the tissue was retained after transplantation, (2) that the enkephalin‐ and CCK‐reacting afferents have a growth capacity similar to most other hippocampal afferents and participate in the intrinsic connective reorganizations like these afferents, and (3) that enkephalin‐ and CCK‐reacting host afferents, presumably of entorhinal origin, can innervate the transplant dentate molecular layer in a homotypic, laminar fashion, in immature recipients with close contact between the transplant and the host dentate or entorhinal area.

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