6 ± 5.3 mV and time to peak of 36.1 ± 16.3 ms. Consistent with previous results (Gruber and O’Donnell, 2009), ten-pulse, 50 Hz train stimulation of the PFC elicited a prolonged depolarization but rarely action potentials in VS MSNs (Figure 2A). Only 4 of 27 MSNs responded with action potential firing during the PFC train stimulation; the majority remained silent during

the PFC-evoked depolarization. buy PS-341 We evaluated MSN responses to fimbria stimulation before and following PFC burst stimulation. At a short, 50 ms latency following the final pulse in the PFC train stimulus, the amplitude of the fimbria-evoked EPSP (F2) was 1.7 ± 2.0 mV, a value significantly reduced compared to the fimbria-evoked EPSP recorded 500 ms prior to PFC stimulation (F1) (t(13) = 5.679; p < 0.0001; Figure 2A), find more without affecting time to peak. HP afferent stimulation 500 ms after the last pulse in the PFC train did not show a suppression relative to the F1 response (t(11) = 1.462; p = 0.17; Figure 2B). These data indicate that strong PFC activation similar to what is observed during instrumental behavior in awake animals transiently attenuates synaptic responses to HP afferents in VS MSNs.

Because PFC train stimulation evoked a sustained depolarization in MSNs, it is possible that the attenuation observed in F2 EPSPs resulted from the depolarization itself; the membrane potential may have neared the reversal potential of the fimbria-evoked response following the PFC stimulation. To evaluate this possibility, we assessed F1 and F2 EPSP magnitudes evoked at similar membrane potentials. We achieved these conditions either by considering F1 EPSPs evoked during spontaneous up states (eight neurons) or by injecting depolarizing current into those the recorded cells through the recording electrode (four neurons). We tailored the amount of current injected for each cell to adjust the

membrane potential to values similar to those evoked by the PFC train. When we compared F1 and F2 EPSPs recorded at similar membrane potentials, the amplitude of the F2 EPSP evoked 50 ms after the PFC train was still attenuated relative to that of the depolarized F1 EPSP (t(11) = 5.304; p < 0.0003; Figure 2C). These data suggest that depolarization-induced changes in ionic conductances are not responsible for the PFC-evoked attenuation of the F2 EPSP. Stimulating HP afferents twice within a few hundred milliseconds could suppress the second response independently of any effect of the intervening PFC stimulation. To address this possibility, we omitted the PFC train from the stimulus protocol in a subset of neurons (n = 6). In these cases, we found no difference in EPSP amplitude between the F1- and F2-evoked responses (t(5) = 0.506; p = 0.635; Figure 2D). Furthermore, a single-pulse PFC stimulus did not reduce the amplitude of the F2 EPSP evoked 50 ms after the PFC pulse (t(5) = 0.266; p = 0.80; Figure 2E).