We reasoned that since the vHPC and mPFC are required for and syn

We reasoned that since the vHPC and mPFC are required for and synchronize during anxiety (Adhikari et al., 2010b), mPFC single units with more robust anxiety-related firing patterns might be more strongly influenced by vHPC activity. Indeed, EPM scores were higher in units significantly phase-locked to vHPC theta (Rayleigh’s test p < 0.05) compared to other units (Figure 8C, mean score = 0.31 ± 0.07 and 0.17 ± 0.04, for phase-locked and other units, find more respectively, p < 0.05, n = 69 units). Importantly, this result is not due to differences in firing rates, as EPM scores and phase-locking to vHPC theta were correlated, even when phase-locking values were calculated on a subsample of 100 spikes

from each unit (r = +0.25, p < 0.03; Figure S2). These results demonstrate

that cells that receive vHPC input have stronger anxiety-related firing patterns. Consistent with previous results (Adhikari et al., 2010b), this effect was specific for the theta-frequency range, as EPM scores did not differ with phase-locking to vHPC delta- (1–4 Hz) VX-770 cell line or gamma-frequency (30–80 Hz) oscillations (data not shown). Furthermore, phase-locking of mPFC single units to dHPC theta oscillations was not related to EPM scores (Figure 8D), in agreement with lesion (Kjelstrup et al., 2002) and physiology (Adhikari et al., 2010b) studies suggesting that the dHPC is not required for normal anxiety-related behavior in the EPM. The above results suggest that mPFC single units with robust anxiety-related firing patterns are

preferentially recruited into a circuit involving the vHPC. The projection from the vHPC to the mPFC Amisulpride is unidirectional (Parent et al., 2010 and Verwer et al., 1997), and hippocampal theta-range activity has been shown to lead the mPFC (Adhikari et al., 2010a, Siapas et al., 2005 and Sigurdsson et al., 2010). We reasoned that if the vHPC input plays a role in the generation of anxiety-related firing patterns, mPFC single units that follow vHPC theta should have stronger paradigm-related firing patterns compared to units that do not. To find which cells follow hippocampal theta activity, MRL values were calculated after shifting the spike train of each mPFC single unit in time, relative to the vHPC theta-filtered LFP (see Experimental Procedures). Consistent with the known anatomy and previous results, the overall mean lag for maximal phase-locking was negative, indicating that on average, mPFC unit activity followed vHPC activity (mean lag = −13.8 ± 8.1 ms). However, units with positive lags relative to hippocampal theta were also found, similarly to previous reports (Adhikari et al., 2010b, Siapas et al., 2005 and Sigurdsson et al., 2010). Positive lag units may result from chance, or may be involved in polysynaptic modulation of hippocampal activity. Consistent with our prediction, cells that followed the vHPC had significantly higher EPM scores than other units (Figure 9D, mean score = 0.24 ± 0.047 and 0.07 ± 0.

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