Immunocytochemical evidence for functional integration of newborn hippocampal neurons after stroke

It is well established that enriched environment and rehabilitative training increase the number of newborn cells in the hippocampus after focal ischemia. Likewise these therapies improve the functional outcome after stroke as demonstrated in behavioral tests like the Morris water maze. But previous findings in various models of epilepsy have shown a failing integration of newborn neurons representing a form of maladaptive plasticity. Here, we investigated whether newborn cells generated after cortical infarcts integrate into functional hippocampal circuits using immediate early gene expression (c-fos, Zif268) after different behavioral stimuli. Male adult C57BL/6J mice received photochemically induced cortical infarcts in the sensorimotor forelimb cortex. Proliferating cells were labelled by applications of 5-Bromo-2-deoxyuridine (50mg/kg, i.p., twice daily) over 13 days after surgery. Immediately after the infarct the animals were transferred to an enriched environment (EE) or a cage with a running wheel (RW) for 6 weeks whereas mice housed in standard cages (ST) served as controls. In the 8th week mice of all experimental groups were divided and either trained in the water maze (WM) as a cognitive task or on the ladder beam (LB) as a sensorimotor task. 150 minutes after the last training session the animals were sacrificed for further processing. Immunocytochemistry with antibodies against BrdU and the neuronal marker NeuN revealed a significant increase of dentate neurogenesis in the EE or RW group compared to ST controls. In all subgroups a small number of newborn neurons expressing the immediate early gene c-fos were detected. The sensorimotor ladder beam task activated significantly more newborn neurons compared to the cognitive stimulation in the water maze. Our results thereby provide evidence that newborn neurons generated after stroke are integrated into functional hippocampal circuits. The increased activation of newborn hippocampal neurons after sensorimotor stimulation indicates that these cells might be involved in motor learning in the lesioned brain.

Supported by the Ernst-Abbe-Stiftung Jena.