Abstracts

Rationale: Hypoxia is the leading cause of perinatal seizures, which affects approximately 1-3% of infants. In postnatal day (P)10 rats, hypoxic seizures do not lead to neuronal death, but result in increased seizure susceptibility, spontaneous seizures, and neurobehavioral deficits in later life. We recently reported that P10 seizures result in acute (within mins-hrs) increases in sEPSC amplitude and frequency in s. radiatum of hippocampal area CA1 and decreases in sIPSC frequency. Following P10 seizures, rats exhibit long-term excitability in hippocampal CA1 as well as in vivo spontaneous seizures. P10 is a time window of intense dendritic and synaptic development. Dendritic regions of CA1 pyramidal neurons differ in synaptic input, dendritic spine density, and ion channel distribution, and in the adult, proximal primary apical dendrites receive predominantly inhibitory input, whereas oblique dendrites receive primarily excitatory input. We thus hypothesized that primary and secondary segments of apical dendrites may be differentially affected by early life seizures. Methods: We analyzed dendritic spine density in rats perfused at 1hr (P10), 48 hrs (P12), or 1 week (P17) after hypoxia-induced seizures at P10. We used confocal microscopy to image near the first branch point (which included segments 30-150 μm from soma) of primary and oblique apical dendrites of DiI-labeled CA1 neurons. We counted spines in 3D, including spines projecting into the z-plane. Results: In slices from normally developing control rats, we observed a developmental trend towards decreased spine density on the proximal segment of the primary apical dendrite from P10-P17 (mean P17=1.7; p=0.07). While slices from rats post P10 seizures were not different from controls at 1 and 48 hrs following seizures (mean spines/μm: control P10=2.2, P12=2.4; hypoxic P10=2.6,P12= 2.5; p=0.58), we observed that mean spine density was significantly higher than those of controls (mean=2.3, p=0.03) and also did not decrease from the 1 and 48 hr time points following seizures. In contrast, there was no change in oblique dendrite spine density after seizures at any of these time points (P10 hyp mean=1.2, p=0.36; P17 hyp mean=1.9, p=0.44) compared to controls (mean=P10, 1.3; P17, 1.9; p=0.06). Conclusions: Our control measurements indicate that normal spine density development differs in primary and oblique apical dendrites. These data also suggest that seizures in the first two postnatal weeks, at peak synaptogenesis, result in abnormal spine development only in the primary dendrite. These data suggest that early life seizures may result in a developmental delay specific to inhibitory CA1 networks. Future studies will be required to determine how long term seizure susceptibility is related to elevated spine number as well as to dysregulation of developmental receptor expression and ratio of inhibitory:excitatory inputs onto these dendritic regions. Supported by NIH RO1-NS31718, DP1-OD003347, T32-NS007473