Abstracts

Rationale: Two major subtypes of GABAergic interneuron have been documented in the CNS which either express parvalbumin (PV) or somatostatin (SS). The molecules may not only serve as neuronal markers but also may play important roles in cellular function. It is not clear how the expression of these two molecules is regulated during normal development and in cortical dysplasia (CD). We have examined their developmental expression in normal rats and in an animal model of CD. Methods: Pregnant rats at E17 were irradiated and offspring, aged P1 to P45, were studied. Animals were perfused transcardially with 4% paraformaldehye. The brains were left overnight in the same fixative at 4 oC and cryoprotected in 30% sucrose in PBS. Coronal sections of somatosensory cortex and sagittal hemispheric sections (30 μm) were cut using a cryostat. Sections were stained with monoclonal PV (1:5000) and SS antibodies (1:1000) and fluorescently conjugated secondary antibodies (1:500). Results: In normal rats, the onset of PV expression occurred at P10, beginning in the hippocampus and layers V and VI of neocortex. At P15, PV-positive neurons were seen in all cortical layers. The density and the brightness of immunostaining increased with age. At maturity, PV neurons were evenly distributed in all cortical layers. In CD, the onset of PV expression was delayed by one to two days. At all ages, the density of PV neurons was significantly reduced in severely affected areas in CD. PV neurons were also distributed in uneven clusters throughout the cortex, with some areas showing a complete absence of these cells. In heterotopic gray matter, PV neurons were present in a relatively high density. In contrast to PV, SS staining appeared much earlier, first seen at P3 or P4 in controls and at P5 in CD. In controls at early ages (< P10), SS-positive neurons were most prominent in layers V and VI; but at ages >P20, the majority were located in layers II to IV. In CD, the density of SS neurons was also decreased at all ages. SS-positive neurons were also seen in heterotopic gray matter. Conclusions: Expression of PV and SS in cortical interneurons is developmentally regulated. The onset of SS expression occurs much earlier than that of PV. The onset of both SS and PV is delayed in experimental CD. The density of both PV- and SS-positive neurons is reduced at all ages in irradiated rats. Distribution of PV interneurons is very uneven throughout the dysplastic cortex and heterotopic gray matter has a relatively high density of these cells compared to the overlying dysplastic cortex. This suggests that in utero irradiation may produce a disruption of interneuronal migration such that PV interneurons are more likely to enter heterotopic cortex. These findings further elucidate abnormalities in the GABAergic system in the in utero irradiation model of CD.