Abstracts

Rationale: Previous research in our lab has indicated that following a single episode of early life seizures (ELS), induced using kainic acid (2 mg/kg) on post-natal day (PND) 7, rats display increased LTD (long term depression), as well as cognitive deficits at PND 60 . These changes occur in the absence of pronounced hippocampal injury (cell loss and synaptic reorganization) and are consistent with molecular alterations at the sub-cellular or synaptic level. We speculate that increased LTD (in conjunction with decreased LTP) may be responsible for the observed learning abnormalities. LTD induction can be mediated by two distinct mechanisms, NMDA receptor (NR) dependent and metabotropic glutamate receptor (mGluR) dependent. The NR dependent and mGluR dependent forms are differentiated by the effectiveness of different chemical and electrical LTD inducing stimulation paradigms. We hypothesize that alterations in LTD following ELS are the result of changes in mGluR dependent LTD. Therefore mGluR dependent LTD induction paradigms were employed to test this hypothesis. We further hypothesize that the expression levels of sub-synaptic machinery associated with LTD will reflect their role as potential mediators of the changes following ELS. Methods: LTD induction paradigms designed to isolate mGluR dependent LTD were utilized. Specifically, LTD was induced using 900 paired-pulse stimuli at 1Hz with 50-millisecond interpulse interval in the presence of D-APV (50 ?M). LTD was also induced using DHPG (100 ?M) (50 ?M), an mGluR agonist. Potential mediators of the observed changes in mGluR dependent LTD were assessed using semi-quantitative western blot expression assays. Results: A single episode of neonatal seizures resulted in enhanced mGluR mediated LTD. Field excitatory postsynaptic potentials (fEPSPs) were measured from the stratum radiatum of the CA1 region of adult hippocampal slices in response to stimulation of the Schaffer collateral-commisural pathway. ELS animals displayed enhanced LTD after induction with 900 50-millisecond paired pulses at 1Hz, as well as following DHPG wash-in (p < 0.05). Expression of activated p90S6K, ?CamKII, and ?CamKII was significantly (p < 0.05) increased. Expression of activated GSK3? and Akt were not altered (p < 0.05). Expression of total p90S6K, ?CamKII, ?CamKII, FMRP, GSK3?, Akt, and STEP were not significantly altered (p < 0.05). Conclusions: ELS results in long-term signaling changes resulting in enhanced mGluR mediated LTD compared to age matched controls. The enhanced LTD is apparent regardless of the induction paradigm (900 50-millisecond paired pulses at 1Hz, or DHPG induced). Signaling proteins associated with mGluR LTD were also altered in a consistent fashion, suggesting a mechanisms for the observed changes in LTD, as well as possible targets for therapeutic intervention. We speculate that this altered mGluR dependent LTD may underlie some of the learning abnormalities detected following ELS.