Abstracts

Rationale: Stroke in the developing brain is an important cause of chronic neurological morbidity. Long-term manifestations include cognitive dysfunction, motor deficits and epilepsy in 75% of children who survive their stroke, thus representing a major public health issue. To investigate the dynamics of new-cell integration into functional hippocampal circuits after neonatal stroke, long-term cognitive impairments and post-stroke epilepsy, we utilized unilateral carotid ligation alone to produce infarcts and acute ischemic seizures in postnatal day 12 CD1 mice. Methods: Ligated mice of both genders that had post-stroke acute seizures were introduced into the chronic study. New-cells, proliferating in the neurogenic niche of the subgranular zone in the dentate gyrus (DG,), were labeled with bromodeoxyuridine at ~1week (P17-21) after the ischemic insult by once daily I.P. injections of BrdU (each 50mg/kg). New-cell integration into functional circuits of stroke-injured hippocampi was ascertained by early immediate gene Arc (activity-regulated cytoskeleton associated protein) induction using a novel spatial exploration test 7 months after the stroke. Motor and cognitive tests were conducted at 3 months of age with Rotorod, T-maze alternation and open-field testing. Temporally equidistant intervals of chronic and continuous video-monitoring over a 6-month post-stroke period (i.e., 1 week per month “24/7”) was done to investigate for the occurrence of spontaneous recurrent behavioral seizures. Post-stroke circuit re-organization was investigated by staining for atypical mossy fiber sprouting in the inner molecular layer of the DG. Results: Experience-dependent Arc expression in the granule-cell-layer was impaired bilaterally in stroke-injured mice (p=0.02) compared to controls. Integration of the new-neurons into the functional circuit in the ligation-injured mice was not significantly different from controls ipsilaterally due to a large variability and impaired contralaterally, however not significantly (p=0.1). Open-field testing revealed significant impairment of spatial cognition in ligation-injured mice with not only failure to habituate within the testing cycle on day 1 but also on the subsequent day of testing. Low frequency spontaneous behavioral seizures (0.22±0.02 seizures/day) graded as class 2-4 on the Racine scale were detected in 62% (n/n=5/8) of the monitored mice that was not associated with mossy fiber sprouting in the inner molecular layer of the DG. Behaviorally, there was also an associated temporal increase in the frequency of clockwise running around the cage periphery in 3/8 (37%) mice at 3 months of age that progressed to “tail chasing” at 6 months of age. Conclusions: Impaired Arc induction, both in the ipsilateral injured and contralateral uninjured DGs, associated with poor performance in the spatial cognition tests provide valuable insights into the impaired hippocampal functionality in a new mouse model of neonatal stroke.