Technique for producing directionally specific brain lesions with radio frequency current

Functional cerebral activity during regeneration from entorhinal lesions in the rat

The consequences of an unilateral electrolytic entorhinal lesion on the functional activity in all major anatomically defined brain regions were evaluated in the rat. The 14C-2 deoxyglucose method served as a tool to quantify alterations of local cerebral glucose utilization (LCGU) ipsilateral and contralateral to the lesion at 4 days, 2 weeks, or 3 months after stereotaxic surgery. Apart from a few minor increases in the contralateral hemisphere, the predominant pattern consisted of reductions in the range of 10-40% in the ipsilateral hemisphere. Ipsilaterally, in extrahippocampal areas, LCGU had regained control levels at 2 weeks postlesion in contrast to hippocampal regions, where reductions were more pronounced than in other brain areas and partially persisted for up to 3 months. Interestingly, the termination zones of entorhinal fibers in the dentate gyrus did not regain control levels within 3 months. We conclude from the data that functional recovery of denervated primary target areas does not occur within 3 months after entorhinal lesions and that altered functional activity may be found beyond the primary target areas predominantly during the acute recovery period after the lesion. The data suggest that sprouting fibers do not reestablish a fully functional neuronal network during the recovery period.

A disconnection analysis of hippocampal function

A disconnection analysis determined the extent to which the fornix, hippocampus and entorhinal cortex are components of the same functional system in tasks that require working memory. Preoperatively, rats were trained to perform accurately on a radial arm maze. Then various combinations of unilateral and bilateral lesions were placed in the fornix and entorhinal cortex, either with or without a transection of the hippocampal commissures. When the lesions left intact at least one pathway through the hippocampus interconnecting the fornix and entorhinal cortex, rats performed normally. Either an uncrossed pathway (following a unilateral lesion of the fornix, transection of the hippocampal commissures, and an ipsilateral lesion of the entorhinal cortex) or a crossed pathway (following a unilateral lesion of the fornix and a contralateral lesion of the entorhinal cortex, leaving the hippocampal commissures intact) was sufficient. When the lesions produced a complete bilateral disconnection of the fornix and entorhinal cortex, rats performed poorly. The results indicate that the hippocampal system provides a functional connection between the subcortical structures associated with the fornix and the neocortical structures associated with the entorhinal cortex, and that without this connection normal processing of working memory can not occur.

Hippocampal connections and spatial discrimination

Rats were tested in a new spatial discrimination procedure which measured working memory. Following preoperative testing, lesions were placed to disrupt each of the major extrinsic fiber connections of the hippocampal formation. Destruction of the entorhinal area, body of the fimbria-fornix anterior to hippocampus, septum, or postcommisural fornix produced a severe and consistent impairment in performance. Analysis of error patterns indicated that when animals with limbic lesions made errors, they were likely to make these errors in the same sequence as the original choices. These data support the hypothesis that the hippocampus has an important role in the processing of information about spatial location, and that normal performance on this task requires an intact hippocampal circuitry.

Hippocampal influence on hyperreactivity induced by septal lesions

Rats were tested for their responses to 4 stimuli in order to measure hyperreactivity. Animals with septal lesions emitted the expected hyperreactivity. Lesions of the postcommisural fornix, precommissural fornix, or anterior hippocampus, administered 16 days prior to a septal lesion, blocked the expected hyperreactivity. Lesions localized to the medial or the lateral fibers of the fornix decreased the magnitude the duration of the expected hyperreactivity following septal lesions but did not block it. Lesions of the posterior hippocampus-entorhinal cortex had no reliable effect on the expected hyperreactivity after septal lesions. These data indicate that the appearance of hyperreactivity following a septal lesion depends upon a circuit involving the septum, precommissural fornix, anterior hippocampus, postcommissural fornix, and hypothalamus, and document an important functional relationship between the septum, anterior hippocampus, and hypothalamus.