Abstracts

RATIONALE: At the end of this presentation, participants should be able to discuss the effects of in utero irradiation on development of neocortical inhibitory neurons in the perinatal period. In utero irradiation in rats produces offspring with diffuse cortical dysplasia and heterotopic gray matter. Previous studies of have shown a selective reduction of inhibition in dysplastic cortex from adolescent and adult irradiated rats (Roper et al., 1999; Zhu and Roper, 2000) but it was not known if this was a direct effect of the irradiation or some secondary effect. The current study was performed to determine if the deleterious effect of in utero irradiation could be detected in the perinatal period.
METHODS: Pregnant rats were exposed to 225 cGy of external [gamma]-irradiation on gestational day 17 (E17). Irradiated and control litters were studied at E21 and postnatal day 6 (P6). Brains from 5 irradiated and 5 control animals at each time point were fixed and coronally sectioned at 30 [mu]m. Sections were alternately stained for either GABA immunoreactivity (GABA-Ir) or cresyl violet (CV). Whole brain neocortical neuronal counts were estimated using the optical fractionator technique for both GABA and CV. The sampling parameters were optimized to reduce error coefficients to [lt]10%. Comparisons between treatment groups were made using the t-test. For each brain, the percentage of GABAergic neurons within the neocortex was determined as the ratio of total GABA-Ir neurons to total CV-stained neurons.
RESULTS: Irradiated brains at ages E21 and P6 have significantly fewer total neocortical neurons than age matched controls; however, both irradiated and control animals demonstrated a significant, two-fold increase in total neocortical neuron counts from E21 to P6. In control animals, total number of neocortical GABA-Ir cells significantly increased from E21 (2.9x10⁵ +/- 3x10⁴) to P6 (3.5x10⁶ +/-3.8x10⁵) and the percent of neocortical GABA-Ir cells significantly increased from 2.8 +/-0.15% to 17.5 +/-0.72%. In contrast, irradiated animals demonstrated no significant difference in total number of neocortical GABA-Ir cells between the two age groups (6.4x10⁵ +/-4.2x10⁴ and 5.5 x10⁵ +/-1.4x10⁴) while there was a clear reduction in the percentage of neocortical GABA-Ir cells from E21 to P6 of 17.1 +/-0.15% and 8.9 +/-0.10% respectively. At age P6, the irradiated rat brains had a significantly smaller fraction of neocortical GABAergic neurons (8.9%) compared with controls (17.5%).
CONCLUSIONS: This study has shown that the selective deficit of interneurons in irradiated rats comes from an arrest of the addition of new interneurons to the neocortex and a persistent capacity to add new, non-GABAergic neocortical neurons. Therefore, the selective vulnerability of inhibitory neurons to in utero irradiation is not due to increased destruction of the interneurons bur rather an impaired ability to recover and generate new interneurons after the injury. This is in contrast to non-GABAergic neurons, which continue to double their numbers during the perinatal period after in utero irradiation. These findings may have important implications for cortical development after a variety of in utero insults.
[Supported by: This research was supported by NIH grants NS11129 and NS35651.]