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Purpose: Developmental delay and cognitive impairment are common comorbidities in people with epilepsy associated with malformations of cortical development (MCDs). We studied cognition and behavior in an animal model of diffuse cortical dysplasia (CD), in utero irradiation, using a battery of behavioral tests for neuromuscular and cognitive function.
Methods: Fetal rats were exposed to 2.25 Gy external radiation on embryonic day 17 (E17). At 1 month of age they were tested using an open field task, a grip strength task, a grid walk task, inhibitory avoidance, an object recognition task, and the Morris water maze task.
Key Findings: Rats with CD showed reduced nonlocomotor activity in the open field task and impaired motor coordination for grid walking but normal grip strength. They showed a reduced tendency to recognize novel objects and reduced retention in an inhibitory avoidance task. Water maze testing showed that learning and memory were impaired in irradiated rats for both cue discrimination and spatially oriented tasks. These results demonstrate significant deficits in cortex- and hippocampus-dependent cognitive functions associated with the diffuse abnormalities of cortical and hippocampal development that have been documented in this model.
Significance: This study documents multimodal cognitive deficits associated with CD and can serve as the foundation for future investigations into the mechanisms of and possible therapeutic interventions for this problem.
Comorbidities associated with epilepsy are common and may have an impact on quality of life that equal to or greater than that of the seizures themselves (Jacobs et al., 2009; Brooks-Kayal, 2011). Accordingly, the National Institute of Neurological Disorders and Stroke (NINDS) has identified the study of comorbidities as a major goal of current epilepsy research efforts (http://www.ninds.nih.gov/research/epilepsyweb/2007_benchmarks.htm). Cortical dysplasia (CD) describes a range of malformations of cortical development and is a common cause of epilepsy in both children and adults. Developmental delay and cognitive impairment is a common comorbidity in CD, and there is some evidence that the severity of the impairment correlates with the extent of the cortical malformation (Barkovich & Kjos, 1992; Leventer et al., 1999). This study was undertaken as a first step to better understand how CD relates to impaired behavioral function in an animal model. We used in utero irradiation on embryonic day 17 (E17) to produce diffuse CD and examined the affected offspring using a battery of behavioral tests that monitor different aspects of brain function.
The effects of in utero irradiation on postnatal brain structure and behavior are dependent on both the timing and the dose of irradiation (Hicks & D’Amato, 1963; Brizzee, 1967; Norton & Kimler, 1987; Schull et al., 1990; Hossain et al., 2005; Kisková & Šmajda, 2006). Maximal effects on cortical development occur with irradiation between E15 and E18 (Cowan & Geller, 1960). Fetal irradiation can produce a wide range of structural defects including microcephaly, diffuse CD, neuronal heterotopia, ectopic pyramidal cells in the hippocampus, and agenesis of the corpus callosum (Riggs et al., 1956; Cowan & Geller, 1960; Roper et al., 1995). Irradiation can also cause reduced numbers of neurons in the cortex, hippocampus, and cerebellum (Brizzee et al., 1982; Norton & Kimler, 1987; Roper et al., 1995; Hossain et al., 2005; Schmitz et al., 2005; Zhou & Roper, 2010a), reduced numbers of dendrites and disorganized dendritic arborization (Brizzee et al., 1982), and defective neuronal connections (Rakic, 1988; Naylor et al., 2008).
CD is a common cause of medically intractable epilepsy (Taylor et al., 1971; Palmini et al., 1991), accounting for >50% of intractable epilepsy in children and up to 20% in adults (Kuzniecky & Barkovich, 1996). We have studied an animal model of CD by exposing fetal rats to 2.0–2.5 Gy γ-radiations on E17 (Roper et al., 1995, 1997), when cerebral cortex development is maximally sensitive to irradiation (Hicks & D’Amato, 1980). Postnatally, irradiated rats show spontaneous seizures in vivo (Kondo et al., 2001; Kellinghaus et al., 2004) and enhanced epileptiform activity in vitro (Roper et al., 1997). Previous electrophysiologic studies have demonstrated that an imbalance of synaptic input favors inhibition in interneurons in dysplastic cortex (Xiang et al., 2006; Zhou et al., 2009; Zhou & Roper, 2010b), but favors excitation in pyramidal neurons in dysplastic cortex (Zhu & Roper, 2000) and heterotopic gray matter (Chen & Roper, 2003). These findings complemented immunohistochemical studies that showed a selective reduction in the density of inhibitory interneurons (Roper et al., 1999; Zhou & Roper, 2010a,b) and γ-aminobutyric acid (GABA)ergic presynaptic terminals in this model (Zhou & Roper, 2010a). Impaired cortical inhibition and the subsequent increased excitation in these animals may be an important mechanism of epileptogenesis. But these widespread alterations in cortical and hippocampal circuitry may also have profound effects on other aspects of behavior and cognition.
Although previous studies have investigated behaviors in irradiated rats that were exposed to 0.3–1.5 Gy between E11.5 and E17.5 (Werboff et al., 1962; Norton, 1986; Norton & Kimler, 1987; Schull et al., 1990; Baskar & Devi, 1996, 2000; Devi et al., 1999; Kisková & Šmajda, 2006), the number of publications on behavioral effects of prenatal irradiation are still limited (Kisková & Šmajda, 2006). Not all time points and doses of irradiation have been examined, and the ages at testing are not always consistent. More importantly, although the behavioral effects of irradiation with relatively low doses have been investigated (Norton & Kimler, 1987), the behavioral effects of irradiation at higher doses, which has been demonstrated to produce CD with dyslamination (Roper et al., 1995, 1997), have not been reported. The present study was designed to investigate the behavioral effects of irradiation with 2.25 Gy on E17 on motor function, learning, and memory using a battery of behavioral tests. We found that rats with CD show impairment across multiple domains including motor coordination and both limbic-based and cortically based learning and memory.