A functional role for the sympathohippocampal pathway in the recovery of function after medial septal damage in rats

Abstract

Several investigators have established that a proliferation of sympathetic fibers into the hippocampus occurs following lesions to the medial septal (MS) area. These fibers are postganglionic fibers from the superior sympathetic ganglion (SCG). Until recently, no behavioral function had been identified for the ingrowth of sympathetic fibers. However, recent evidence suggests a functional role for the ingrowth. When the MS area is lesioned, a behavioral reactivity is produced. The time course of recovery which follows, corresponds to the time course of the establishment of the ingrowth. When animals with the ingrowth receive a ganglionectomy, there is a return of the reactivity. The present study had four specific aims: (1) To reaffirm earlier results, an substantiate a behavioral role for the sympathetic ingrowth. Therefore, rather than a ganglion removal, ligations of pre- or postganglionic fibers were done; (2) To determine if the return of reactivity seen after sympathetic interruption is due to the loss of neural modulation of reactivity, or to a change in light aversion. Light aversion was measured as the time animals spent in a light versus a dark chamber; (3) To establish if the contribution of the ingrowth is through direct neural modulation, or to the mere presence of functional axon terminals. Thus, comparisons were made between animals with pre- and postganglionic ligations; (4) To determine if the ingrowth is necessary for recovery. Determination of the last specific aim was attempted by freezing pre- or postganglionic fibers to allow regenerative replacement of the fibers. If replacement of functional connections reestablished the recovery, it would suggest that the ingrowth is necessary for recovery. The results related to each specific aim suggest that: (1) there is a behaviorally related function for the ingrowth, (2) the changes in behavior are not related to changes in light aversion, (3) the ingrowth related recovery requires a complete "spino-sympatho-hippocampal" pathway, and (4) the possible reestablishment of functional ingrowth did possibly contribute to the reestablishment of recovery.