Functional correlates of compensatory collateral sprouting by aminergic and cholinergic afferents in the hippocampal formation.

Selective lesions of discrete aminergic and cholinergic afferent pathways to the hippocampal formation have been studied as an experimental model of behavioral recovery after partial hippocampal deafferentation in adult rats. The noradrenergic, serotoninergic and cholinergic afferents reach the hippocampal formation along three routes: a supracallosal pathway, a subcallosal pathway (along the fimbria-fornix), and a ventral pathway (along the ansa lenticularis and the ventral amygdaloid pathway). Bi-lateral lesion of the supracallosal pathway resulted in significant reductions in biochemical markers of the hippocampal noradrenergic, serotoninergic and cholinergic innervations. Within 6 weeks following lesion these markers were found to have recovered to levels close to normal. A more substantial deafferentation, obtained by a bilateral lesion of both dorsal pathways, resulted in a more profound initial decrease in the biochemical marker levels, and a less complete recovery in the long-term animals. Concomitant with the initial phase of deafferentation, rats with lesions of one or both of the dorsal afferent pathways exhibited an equally profound behavioral deficit in acquisition of a forced choice alternation learning task in a T-maze. However, while rats subjected to the combined supracallosal-fimbria-fornix lesion remained impaired, the ones with lesions of the supracallosal pathway alone exhibited a significant recovery of the capacity to perform the maze task. The correlation between behavioral recovery and recovery of the biochemical markers in the supracallosal pathway-lesioned group was significant for the serotoninergic innervation of the hippocampus. The present results suggest that reinnervation of the partially deafferented hippocampus through compensatory collateral sprouting is a valuable new experimental model system within which to analyze mechanisms underlying functional recovery after lesions in the CNS.