Abstracts

The mechanisms responsible for clearance of glutamate from the synaptic cleft at excitatory synapses are not well understood. Although a family of high-affinity glutamate transporters has been characterized in both neuronal and astroglial cells it is not clear whether these transporters subserve a homoeostatic function to keep overall levels of glutamate below neurotoxic levels, or whether they play a more dynamic role in shaping the glutamate concentration transient that occurs following a release event. The importance of these molecules is underscored by the finding that knock out of the EAAT2 (Glt-1) gene, the predominant CNS glutamate transporter, leads status epilepticus and perinatal mortality. Pharmacologic manipulation of transporter function is hampered because of relatively low affinity of uptake blockers, and their cross-reactivity with glutamate receptors. We have taken the novel approach of overexpressing the EAAT2 glutamate transporter under the control of a glial specific promoter GFAP in a transgenic mouse. The transgene was detected by the presence of a GFP tag, and functional activity was evidenced by a 3-5 fold increase in 3H D-Aspartate uptake in crude cortical and hippocampal synaptosome preparations. To investigate the consequences of enhanced glutamate transporter activity on synaptic physiology we attempted to elicit long-term potentiation (LTP) in hippocampal area CA1. We used a 100 HZ high frequency stimulation protocol since we reasoned that transporter activity was likely to be most important under conditions where glutamate release was high. We found that LTP was significantly blunted in EAAT2 overexpressing mice compared to wild-type (wt) litter mates. Approximately 40% of transgenic animals showed some LTP (>20%) compared to 90% of wt animals. These findings were confirmed in a second independent line of transgenic animals. These data suggest that increased glutamate clearance can impair synaptic plasticity at hippocampal synapses. Supported by NIMH (MLS).