Intra‐Retrosplenial Cortical Transplantation of Cholinergic Neurons Derived from the Septal Nucleus of Fetal Rats: Pattern of Innervation

The retrosplenial cortex (RSC) receives extensive cholinergic afferents from the medial septal nucleus /3/. Our earlier studies have demonstrated that this cholinergic input traverses the fornix pathway in addition to the cingulum pathway and innervates the RSC in a highly laminated pattern. In the present report we have studied whether the pattern of cholinergic reinnervation by intra-retrosplenial grafts of septal tissue mimics that of intrinsic cholinergic inputs. Two microliters of cell suspension containing 100,000 viable cells obtained from the septal nucleus of fetal rats 15-17 days in gestation were injected into RSC of male adult Sprague-Dawley rats (250-275 g body weight) bilaterally at 4 different sites. The two anterior sites for transplantation were4.8 mm posterior to the bregma, 0.8 mm lateral to the midline, and 1.6 mm ventral to the cortical surface. The two posterior sites were: 5.8 mm posterior to the bregma, 1.0 mm lateral to the midline, and 2.5 mm ventral to the cortical surface. Bilateral transection of the cingulum bundle and the fornix by aspiration were performed immediately after the injection. Five months after surgery animals with fornix lesions (FX), and those with lesions and grafts (TN) were perfused and processed for choline acetyltransferase (CHAT) immunohistochemistry along with normal controls (NC). We found that the RSC in NC animals received extensive eholinergic afferents which exhibited a pattern of highest density in layers I and IV, moderate density in layers V/VI, and lowest in layers II and III. In FX animals this pattern of cholinergic innervation was dramatically disrupted as revealed by a nearly complete loss of ChAT-positive nerve terminals. These results confirm those reported by other studies. Eckenstein et al. /1/, using the same technique, demonstrated a high density in layers I and IV, and none in layers II and III. Lysakowski et al./2/ reported similar findings. They showed extensive ChAT stain in layer VI, moderate in layers VI, V, and IV, and very faint in layers II and Ill. Our previous studies, using a Fluoro-Gold retrograde fluorescent axonal tracing technique, revealed that the RSC receives, extensive afferents from the medial septal nucleus, and the vertical and horizontal diagonal bands. With AChE histochemistry we previously found that animals with fornix lesions displayed a nearly complete loss of AChE stain in the RSC. Our high affinity choline uptake studies also demonstrated dramatic decreases in high affinity choline uptake in the RSC after fornix lesions. In the TN group we observed clusters of large ChAT positive cell bodies with extensive ChAT positive nerve fibers radiating from the clusters and into the immediate vicinity of the host cortex. The RSC of these animals demonstrated heavy cholinergic innervation with the highest density in layers I and IV, moderate in layers V/VI, and light in layers II and III, and resembled the pattern of the NC animals. The density of the cholinergic reinnervation by the fetal septal grafts in these fornix-lesioned animals was also comparable to that of the normal animal. We therefore conclude that the retrosplenial cortex exhibits a unique laminar pattern of cholinergic innervation which can be significantly depleted by transection of the septohippocampal and cingulate pathways. Intraretrosplenial cortical transplantation of cholinergic rich neurons derived from the septal