Abstracts

In many types of brain injury, an initial excitotoxic event is the triggering point for a series of events that culminate in a persistent pathological condition. For example, in acquired epilepsies, an initial seizure episode or injurious event may lead to long-term functional abnormalities. Commonly, this type of initial insult involves the loss of neurons. Numerous studies have focused on the details of intracellular signaling pathways that lead to neuronal cell death; these studies pursue a goal of developing novel interventions that may promote cell survival and improve clinical outcome. However, it is the surviving neurons that are responsible for resultant functional pathology [ndash] e.g., seizures. It is therefore important to determine whether surviving neurons maintain physiological normalcy. We characterized an [italic]in vitro[/italic] model for the longitudinal study of neuronal function following excitotoxic injury. Mouse organotypic hippocampal slice cultures were exposed to physiologically relevant glutamate levels (50 [mu]M glutamate with 10% glycine) to mimic excitotoxic insult. Using intracellular electrophysiological recordings, we evaluated changes in the intrinsic and synaptic properties of surviving CA1 and CA3 pyramidal neurons. Recordings were also obtained from cultures exposed to GABA receptor-specific agonists or antagonists. We found few differences in the passive properties of surviving glutamate-challenged neurons (compared to cells in age-matched, sister control cultures). Spontaneous activity was monitored and quantified to characterize the consequences of excitotoxicity on the overall activity levels. Neurons that survived an excitotoxic challenge demonstrated significant increases in the number, amplitude and duration of excitatory and inhibitory synaptic events. We found significant changes in synaptic patterns evoked by stimulation of afferent inputs. Approximately 20% of the CA1 and CA3 neurons in glutamate-treated cultures demonstrated significant reductions in early GABA[sub]A[/sub]-mediated inhibition. However, the most dramatic effect was on late inhibition; approximately 90% of the CA1 and CA3 neurons showed dramatic reductions (and in some cases a complete loss) of late GABA[sub]B[/sub]-mediated hyperpolarization. These studies identify significant alterations in the synaptic input to neurons that have been exposed to an excitotoxic challenge. Our findings implicate functional circuitry changes [ndash] particularly, the loss of GABA[sub]B[/sub]-mediated inhibition -- as a significant factor in the development of long-term excitability following excitotoxic insult. (Supported by an American Epilepsy Society predoctoral fellowship (LSP) and the University of California Brain Injury Research program.)