Focal epileptiform activity in the prefrontal cortex is associated with long-term attention and sociability deficits
Introduction
Almost 40% of children with epilepsy have comorbid behavioral deficits (Brunquell et al., 2002, Holmes, 2009) including ADHD, autism, anxiety and OCD, as well as general learning and memory impairment (Brooks-Kayal, 2010, Brunquell et al., 2002, Cabaleiro, 1969, Dunn et al., 2003, Holmes, 2009, Kaufmann et al., 2009, Levin and Duchowny, 1991). Many children with these co-morbidities have frequent overt seizures and it is hypothesized that these seizures are partially responsible for adverse outcomes. The association between frequent generalized early life seizures and hippocampal-dependent cognition has been well studied in rodent models and support the view that epileptic discharges disrupt normal development of hippocampal networks (Chang et al., 2005, Huang et al., 1999, Huang et al., 2002, Liu et al., 1994). However, conclusions drawn from these models may be confounded by the effects of general brain injury due to hypoxia/ischemia and metabolic imbalance that occur at the time of the generalized seizure (McCabe et al., 2001). Furthermore these models often focus on the hippocampus, thereby neglecting the neocortical brain regions associated with many of the most common behavioral deficits seen in children with epilepsy. From a clinical perspective, these models also do not adequately model a large group of children with epilepsy who have infrequent seizures despite frequent focal neocortical interictal spikes (IIS) associated with serious cognitive and behavioral impairments, e.g. children with Landau–Kleffner Syndrome or Continuous Spike-wave in Sleep (Gordon, 1990, Gordon, 1997, Praline et al., 2003). Therefore, in order to understand these effects it is essential to develop an animal model that recapitulates the human condition. As no such model currently exists, we set out to develop a model that recapitulates the phenotype of frequent neocortical epileptiform activity through focal alteration of the inhibitory/excitatory balance during childhood in order to test if this is sufficient to generate later life network abnormalities and behavioral deficits.
We chose microinjections of bicuculline, a competitive GABAA antagonist, into the prefrontal cortex to create a model of inhibitory/excitatory network imbalance leading to focal IIS activity in adolescent rats. This model is clinically relevant as a large body of evidence implicates GABAergic system dysfunction in children with autism and ADHD that present with or without focal IIS on the EEG (Edden et al., 2012, Fatemi et al., 2009, Mori et al., 2012, Samaco et al., 2005). The PFC was chosen based on the preponderance of PFC-specific deficits in children with epilepsy, particularly deficits in executive functioning and social behaviors.
We hypothesize that transient GABA signaling disruption and resultant frequent focal IIS during development will result in long-term neural network disruptions and impairments in PFC-dependent behavior. We show that focal IIS during adolescence is associated with long-lasting alterations in short-term plasticity (STP) in the PFC, a type of plasticity that is thought to underlie information processing and synaptic filtering important for cognition (Jääskeläinen et al., 2011, Rotman et al., 2011). Using delayed-non-match to sample, sociability, motivation and open arena tasks, we also show selective behavioral deficits in measures of attention and social behavior in animals with frequent focal epileptiform discharges.
Section snippets
Animals
All experiments were performed in accordance with the guidelines set down by the National Institutes of Health and the Geisel School of Medicine at Dartmouth for the humane treatment of animals (see Fig. 1A for experimental design). The animal protocol was approved by the Institutional Animal Care and Use Committee of Dartmouth College.
Surgery
Male Sprague–Dawley rat pups were anesthetized at p17–p19 with isoflurane (2–3% in oxygen) and implanted with custom-made cannula/EEG electrodes (Plastics One,
Developmental IIS model
We first wanted to determine whether injection of bicuculline results in IIS in the absence of seizures. Immediately after each bicuculline injection, local EEG was recorded for 30 min. This 30 min window was chosen based on preliminary experiments indicating that if IIS was to develop into ictal EEG activity, this would occur within 10–20 min post-injection. Animals were visually monitored in real time and video recorded for signs of seizures during this time. Saline-injected animals never
Discussion
We show for the first time that prefrontal IIS during development are associated with both long-lasting alterations in short-term synaptic plasticity and deficits attention and sociability in rodents. This effect is independent of the presence or absence of seizures observed during a 30-minute post-injection recording session, demonstrating that transient GABAergic signaling disruption and resultant IIS activity in the developing brain can lead to deleterious behavioral and network-level
Acknowledgments/Conflicts of interest
This work was supported by 1R01NS073083 (awarded to GLH), Emmory R. Shapses Research Fund (GLH), R01NS075249 (RCS) and R01NS076763 (PPLS). RCS is funded by Great Ormond Street Children's Charity. EI is funded by State Foundation of Fundamental Research of Ukraine F46.2/001.
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