Cholinergic neural transplants into hippocampus restore learning ability in monkeys with fornix transections.

Monkeys with bilateral transections of the fornix were severely but selectively impaired at learning visuospatial conditional tasks presented in a Wisconsin General Test Apparatus. Bilateral transplantation of cholinergic-rich embryonic basal forebrain tissue into the hippocampus led to complete recovery from this specific learning impairment across a range of task difficulties. Administration of the direct cholinergic agonist pilocarpine to ungrafted animals immediately before testing also reduced this impairment, suggesting that the graft-associated recovery was mediated by acetylcholine release. Transection of the fornix produced a marked loss of acetylcholinesterase (AChE) staining confined to hippocampus and entorhinal cortex relative to controls. In all transplanted animals densely AChE-staining cellular masses were seen bilaterally in temporal lobe structures, with fibre outgrowth into surrounding host tissue.