Abstracts

Rationale: Following transient exposure to the group I metabotropic glutamate receptor (mGluR) agonist dihydroxyphenylglycine (S-DHPG), the CA3 region displays long-lasting network activity that is epileptiform in character and mimics interictal and ictal synchronization. Following DHPG exposure, CA3 pyramidal neurons show a change in membrane properties that include the presence of a voltage dependent cation current and loss of the currents that underlie the medium and slow afterhyperpolarizations. We evaluated spontaneously occurring post synaptic currents (PSCs) and miniature PSCs in CA3 neurons from control slices and in slices that had been transiently exposed to DHPG. Methods: Hippocampal slices were prepared from 10-15 day old Sprague-Dawley rats and slices were either incubated in control artificial cerebrospinal fluid (aCSF) or aCSF with 50 ?M S-DHPG for 60-120 minutes. Slices were transferred to a submerged chamber to visualize CA3 neurons using diffusion interference contrast infra-red techniques. CA3 neurons were recorded from using cesium methane sulfonate containing electrodes. Spontaneously occurring excitatory PSCs (EPSCs) were recorded at -70 mV and inhibitory PSCs (IPSCs) were recorded at 0 mV. Following these recordings, tetrodoxin (1 ?M) was added to the aCSF and miniature IPSCs and EPSCs were recorded. Results: Neurons from slices that were exposed to DHPG had a significant increase in the frequency (median of 1.1 vs. 5. 6 Hz, p < 0.001, n = 18 control and 17 for DHPG exposed) and amplitude (18.2 1.2 vs. 23.7 1.7 pA, p = 0.01) of sEPSCs. Spontaneously occurring IPSCs tended to be more frequent in DHPG-exposed neurons (median of 5.5 vs. 8 Hz, p = 0.06, n = 13 for control and 14 for DHPG exposed), and had a trend toward lower amplitudes in neurons from slices that had been DHPG-exposed (median of 27.9 vs. 23.2, p = 0.08). Miniature EPSCs recorded from CA3 neurons had similar amplitudes of currents (15.7 1.5 vs. 14.9 0.9 pA) but occurred less frequently in slices that had been exposed to DHPG (median 0.3 vs. 0.1, p =0.033). mIPSCs tended to occur less frequently (median of 0.95 vs. 0.6 Hz, p = 0.09) and were smaller in amplitude (19.6 1.0 vs. 15.8 0.8 pA, p = 0.007) in slices that had been exposed to DHPG. Conclusions: Our results demonstrate that DHPG exposure produced an increase in network driven EPSCs both in terms of amplitude and frequency compared to IPSCs. These changes favor abnormal synchronous activation of pyramidal neurons and epileptiform activity. At a miniature PSC level, both mEPSCs and mIPSCs showed a decrease in frequency in slices exposed to DHPG, suggesting a presynaptic alteration that depresses release. The reduction in mIPSC amplitude following DHPG exposure suggests a post synaptic change in GABA receptors. Following transient DPHP exposure, a long-lasting change in network excitability occurred that can be explained by enhanced excitatory and decreased inhibitory neuronal activity. Supported by the Department of Veterans Affairs Research and Development Service (grant to PAR).