Abstracts

RATIONALE: At the end of this activity participants should understand alterations in cortical circuitry that occur in an experimental model of neuronal heterotopia.
Syndromes of neuronal heterotopia have a strong association with epilepsy but the underlying mechanisms are poorly understood. We used in utero irradiation to produce heterotopic gray matter in rats to study circuit abnormalities in these displaced neurons. Previous studies from this laboratory showed a selective impairment of inhibition in normotopic, dysplastic cortex in this modelm (Zhu and Roper, 2000). This study was undertaken to determine if similar alterations in inhibitory synaptic transmission were present in heterotopic cortex in irradiated rats.
METHODS: Pregnant rats were exposed to 225 cGy of external [gamma]-irradiation on gestational day 17 (E17). Offspring were sacrificed for testing at postnatal days 21-28. 300 [mu]-thick hemispheric slices were obtained at the level of the anterior commissure and placed in a submersion-type recording chamber. Pyramidal cells in areas of heterotopic gray matter were identified using infrared, differential interference contrast microscopy and compared to layer II/III pyramidal cells from control animals. The whole cell voltage clamp technique was used to record spontaneous and miniature inhibitory (IPSCs) and excitatory (EPSCs) post-synaptic currents. EPSCs were recorded in the presence of the GABA-A receptor antagonist, picrotoxin (50 [mu]M), and IPSCs were recorded in the presence of CNQX (10 [mu]M) and AP5 (50 [mu]M). 5-minute continuous recordings were analyzed and compared between heterotopic and control pyramidal neurons. Groups values were compared using the t-test with significance defined as P[lt]0.05.
RESULTS: The average frequency of spontaneous IPSCs was significantly reduced in heterotopic pyramidal neurons (4.12 [plusminus] 0.39 Hz, n = 18) compared to control pyramidal neurons (7.44 [plusminus] 0.58 Hz, n = 18). The average frequency of miniature IPSCs was also significantly reduced in heterotopic pyramidal neurons (2.23 [plusminus] 0.35 Hz, n = 15) compared to control pyramidal neurons (3.33 [plusminus] 0.27 Hz, n = 16). There were no differences in amplitude and kinetics of spontaneous and miniature IPSCs between heterotopic and control pyramidal neurons. There were no differences in frequency or amplitude of spontaneous and miniature EPSCs between the two groups.
CONCLUSIONS: Irradiation at E17 produces a selective and lasting impairment of inhibition in heterotopic pyramidal neurons. Coupled with similar findings from normotopic, dysplastic cortex in this model, these results demonstrate a pervasive effect on inhibition throughout all putative neocortical structures. The current work further defines the scope of the profound and deleterious effect of a single, in utero injury on subsequent cortical development.
[Supported by: This work was supported by a grant from NINDS (NS35651).]