Abstracts

Rationale: It is a widely known fact that homeostatic plasticity plays an important role in stabilizing neuronal networks in response to changes in network activity. Disruptions of this mechanism can lead the network to debilitating states such as epilepsy. It is still unclear whether this plasticity is due to presynaptic or postsynaptic changes, and the mechanisms that drive these changes are largely unknown. Our team investigated the presence of homeostatic plasticity in a neocortical neuronal network using organotypic slice cultures, which allow for the study of the response of the network to isolation from the rest of the brain while maintaining intact local connectivity. Methods: Organotypic neocortical slice cultures were obtained from P5-6 mice. Slice cultures were maintained in culture media containing Earle’s balanced salt solution, Eagle’s basal medium, horse serum, D-glucose, L-glutamine and Gentamycin. Whole-cell patch clamp recordings were performed in a visual electrophysiology setup during 7-20 days in vitro. Forty-eight hours prior to recording, the experimental group was treated with 1uM TTX. For miniature excitatory post-synaptic potentials (mEPSP) recordings, synaptic activity was isolated by applying 1uM TTX in the recording solution. Results: The group treated with 1uM TTX exhibited paroxysmal depolarizing shifts (PDS) in all slices (n=10) whereas the untreated group demonstrated predominantly normal upstates with only 5.77% of the slices produced PDS (n=52). The difference in upstate amplitude in the control group and PDS amplitude in the TTX treated group was significantly different (control group; 17.51 mV versus 45.58mV). Additionally, the frequency of PDS in the TTX treated group (0.073 Hz) was almost three times that of the control group (0.026 Hz). To investigate the specifics of synaptic modulation we measured mEPSP in both control and TTX treated organotypic neocortical slice cultures. There was a slight increase in the amplitude of mEPSPs in the TTX treated group (-19.27 mV, n=5) compared to the control group (-17.59 mV, n=11). The frequency of mEPSPs was markedly increased in the TTX treated group (1.70 Hz) compared to the control group (0.94 Hz). Conclusions: Our preliminary data suggest that homeostatic plasticity induced the control network to produce seizure like activity (PDS). The synaptic changes could be due to both pre and post synaptic modifications but more pronounced presynaptic changes. Presynaptic modulation could be due to a higher release probability of vesicles or an increase in functional synapses. This work is supported by Dr.Ralph and Marian Falk Medical Research Trust