Synaptic connections formed by grafts of different types of cholinergic neurons in the host hippocampus

Abstract

The present experiment was performed to determine whether different types of grafted central cholinergic neurons are able to form synaptic contacts with host hippocampal neurons. Grafts from the septal-diagonal band area, which contain the neurons that normally innervate the hippocampal formation, were compared to those from the nucleus basalis magnocellularis region (NBM), the striatum, the pontomesencephalic tegmentum of the brain stem, and the spinal cord. The regions were dissected from 14- to 16-day-old rat fetuses, and the same number of viable cells (35 × 10 4) from each of the different regions was stereotaxically injected as a cell suspension into the hippocampus of rats subjected to a complete fimbria-fornix lesion, transecting the intrinsic septohippocampal pathways. At 14 to 17 weeks after transplantation, the brains were processed for choline acetyltransferase (ChAT) immunocytochemistry at the light and electron microscopic levels and acetylcholinesterase (AChE) histochemistry at the light microscopic level. There was a great variation in the number of surviving ChAT-positive cells among the different graft types. The septal grafts contained the highest number of ChAT-positive cells, and the striatal grafts showed the lowest numbers. The NBM, brain stem, and spinal cord grafts were inbetween. The differences in the number of ChAT-positive neurons between the groups matched, in general, the differences found in the magnitude of graft-derived AChE-positive fiber growth into the host hippocampal formation. At the electron microscopical level, all types of grafts were capable of forming synaptic contacts with host elements, however, with vast differences in the number of synapses found. The septal grafts produced the highest number of contacts, whereas the striatal and spinal cord grafts produced very few contacts. The ultrastructure of the cholinergic fibers from grafts obtained from the forebrain areas, i.e., septum, NBM, and striatum all appeared normal, whereas brain stem and spinal cord grafts produced different types of anomalies. The results show that grafted cholinergic neurons, that normally do not innervate the hippocampus, can send axons and form synaptic contacts in the host hippocampus. The ability to reinnervate the denervated hippocampal target appears to be shared by the embryologically closely related forebrain cholinergic neuron types, i.e., the septal, NBM, and striatal neurons. The marked differences in overall fiber ingrowth and number of synapses observed between these different types of grafts could be explained largely on the basis of differences in survivability of each grafted neuron type. By contrast, the reinnervation obtained from the grafted brain stem and spinal cord neurons were both quantitatively and qualitatively abnormal. This indicates that neuronal properties beyond the transmitter type are of importance in the formation of graft-host connectivity.