Seizures in early life suppress hippocampal dendrite growth while impairing spatial learning

Abstract

Impaired learning and memory are common in epilepsy syndromes of childhood. Clinical investigations suggest that the developing brain may be particularly vulnerable to the effects of intractable seizure disorders. Magnetic resonance imaging (MRI) studies have demonstrated reduced volumes in brain regions involved in learning and memory. The earlier the onset of an epilepsy the larger the effects seem to be on both brain anatomy and cognition. Thus, childhood epilepsy has been proposed to interfere in some unknown way with brain development. Experiments reported here explore these ideas by examining the effects of seizures in infant mice on learning and memory and on the growth of CA1 hippocampal pyramidal cell dendrites. Fifteen brief seizures were induced by flurothyl between postnatal days 7 and 11 in mice that express green fluorescent protein (GFP) in hippocampal pyramidal cells. One to 44 days later, dendritic arbors were reconstructed to measure growth. Spatial learning and memory were also assessed in a water maze. Our results show that recurrent seizures produced marked deficits in learning and memory. Seizures also dramatically slowed the growth of basilar dendrites while neurons in littermate control mice continued to add new dendritic branches and lengthen existing branches. When experiments were performed in older mice, seizures had no measureable effects on either dendrite arbor complexity or spatial learning and memory. Our results suggest that the recurring seizures of intractable childhood epilepsy contribute to associated learning and memory deficits by suppressing dendrite growth.

Introduction

Cognitive deficits are among the common neurobehavioral comorbidities of epilepsy (Berg, 2011, Elger et al., 2004, Hermann et al., 2008) and while impaired cognition is particularly prominent in the catastrophic epilepsies of childhood, it is not restricted to these syndromes. Studies of a variety of epilepsies have reported intellectual ability to be below that considered normal for age (Nolan et al., 2003, Schoenfeld et al., 1999). While, the basic mechanisms underlying these deficits have been the subject of much speculation (Brooks-Kayal, 2011, Jensen, 2011), it is clear that the nervous system of children appears to be particularly vulnerable to the effects of intractable epilepsy. Numerous studies have compared the risk of comorbidities as a function of age of onset of epilepsy (Berg et al., 2008, Bjørnaes et al., 2001, Dikmen et al., 1975, Dodrill and Matthews, 1992, Hermann et al., 2002). Results have shown that the earlier the age of onset the poorer the cognitive abilities. These observations have led some investigators to propose a neurodevelopmental origin for the comorbidities of epilepsy and that childhood seizure disorders negatively impact the normal growth and maturation of the nervous system (Hermann et al., 2008). This concept has been bolstered in recent years by imaging studies. For instance, quantitative MRI studies have reported reductions in regional brain tissue volume in adult (Lee et al., 1998, Marsh et al., 1997, Oyegbile et al., 2004, Theodore et al., 2003) and pediatric (Lawson et al., 2002, Pardoe et al., 2008, Pulsipher et al., 2009) epilepsy patients with a history of early-onset epilepsy. In concert with psychological findings, investigators have also reported greater reductions in whole brain grey-matter and white-matter volumes in patients with early-onset when compared to later-onset epilepsy (Hermann et al., 2002, Weber et al., 2007).

Potential factors that contribute to such changes in brain anatomy and human behavior are likely numerous and not easily identified or understood through clinical observations alone. Recurrent seizures themselves have long been suspected to impact brain development. However co-existing neuropathologies and/or anticonvulsant therapy are confounding factors. In this regard, experiments in animals can be informative since the effects of seizures alone can be evaluated. Indeed, experiments in several animal models of early-life seizures have shown that brief but recurrent seizures can produce deficits in learning and memory(Karnam et al., 2009a, Karnam et al., 2009b, Lee et al., 2001, Lynch et al., 2000, Sogawa et al., 2001, Stafstrom, 2002). How these deficits are produced remains unclear, but previous molecular studies have shown that the expression of postsynaptic markers for glutamatergic synapses is suppressed in a developmentally-dependent manner (Swann et al., 2007b). As animals matured the differences between them and their controls increased. Additional experiments in slice cultures showed that similar biochemical alterations take place in response to seizure-like activity (Swann et al., 2007a) but neuroanatomical experiments revealed that these effects may be best explained by a suppression of dendrite growth (Nishimura et al., 2008).

These results have led us to suspect that the induction of recurrent seizures during a critical period of dendrite maturation could retard their growth. Results from experiments reported here show that seizures can indeed suppress dendrite growth and do not produce similar effects after a critical period of marked dendrite growth.