Electrophysiology and LTP

Animals were anesthetized using a brief isoflurance exposure, then killed by decapitation. Three hundred micrometer thick coronal hippocampal slices were prepared as previously described,^12,13 placed in an interface chamber and continuously perfused with oxygenated artificial cerebral spinal fluid (aCSF) while they equilibrated for at least 30 min. A bipolar stimulating electrode placed in stratum radiatum was used to stimulate the Schaffer collateral pathway. An aCSF-filled glass microelectrode with resistance approximately 2–3 MΩ placed in the stratum radiatum region of CA1 was used to record the resulting field-potentials (fEPSPs). Data were acquired using Clampex (Molecular Devices, Sunnyvale, CA, USA), and analyzed using Clampfit (Molecular Devices). Stimulations occurred every 20 s and three traces were averaged to yield a single data point per minute. A minimum of a 10-min baseline was recorded for each experiment, recordings continued for at least 1 h after long-term potentiation (LTP) induction. Initial fEPSP slopes were normalized against the average of the baseline traces. Theta burst stimulation was 40 ms in duration, 100-Hz bursts delivered at 5 Hz for 3 s (15 bursts of 4 pulses per burst, for a total of 60 pulses). In all LTP figures, representative sample sweeps are shown for each experiment. The black sweeps represent the average of 5 baseline sweeps, and the red sweeps represent traces recorded after LTP induction. The amplitude of the presynaptic fiber volley was measured in all experiments. Slices for which the amplitude of the fiber volley varied by more than 10% were not used in the analysis. For rescue experiments, D-serine (Sigma-Aldrich, St Louis, MO, USA) was dissolved in aCSF at a saturating dose^14,15 of 100 μM and bath applied during the entire recording period for saline- and cocaine-sired slices.