Given that cells with high firing rate are often interneurons, we divided the

check details PL neurons into putative pyramidal cells and interneurons by carrying out an unsupervised cluster analysis of the cells based on their firing rates and spike widths (Letzkus et al., 2011; see Experimental Procedures for details). This procedure yielded two main clusters (Figure 1D). Consistent with previous reports, one cluster contained a majority of neurons with low firing rate (<15 Hz) and broad spike waveform (>225 μs; putative excitatory pyramidal neurons), while the other contained neurons with high firing rates (>15 Hz) and narrow spike waveforms (<225 μs; putative inhibitory interneurons). Of 194 PL neurons, 174 (89.7%) were classified as putative pyramidal neurons, and 20 (10.3%) were classified as putative interneurons. Similar proportions of interneurons in the rat prefrontal cortex have been previously reported (Homayoun and Moghaddam, 2007). In support of our classification, we observed significant inhibitory interactions between putative inhibitory neurons and putative pyramidal cells, as evidenced by cross-correlation analyses MEK pathway (n = 10 of 88 pairs of neurons; Figure 1E). Classification of PL neurons into pyramidal cells and interneurons revealed dissociable effects of BLA and vHPC inputs. We limited our analysis to PL cells that exhibited significant changes in firing rate following input inactivation (p < 0.05, n = 108/194).

This additional restriction eliminated the few putative pyramidal cells that fell below the firing rate/waveform length criterion. Inactivation of BLA significantly decreased the firing rate of pyramidal neurons (n = 52; Wilcoxon test: Z = 2.57, p = 0.010), without affecting interneuron activity (n = 6; Wilcoxon test: Z = 1.36, p = 0.17; Figure 1F), suggesting that BLA input to PL is largely excitatory in conditioned

rats pressing for food. In contrast, inactivation of vHPC significantly decreased the firing rate of interneurons tuclazepam (n = 7; Wilcoxon test: Z = 2.36, p = 0.01), without affecting pyramidal cell activity (n = 43; Wilcoxon test: Z = 1.48, p = 0.13; Figure 1G). This suggests that vHPC inputs are capable of triggering feed-forward inhibition of PL neurons by exciting local interneurons. This difference between BLA and vHPC inputs could not be predicted from prior anatomical ( Carr and Sesack, 1996; Gabbott et al., 2002, 2006; Hoover and Vertes, 2007; McDonald, 1991) or physiological ( Dégenètais et al., 2003; Floresco and Tse, 2007; Laviolette et al., 2005; Tierney et al., 2004) studies. We next assessed the effect of input inactivation on tone responses of PL neurons, tested after conditioning. Conditioned tone tests occurred from 2 hr to several days postconditioning (see Experimental Procedures for details). Unilateral inactivation of either BLA or vHPC did not alter tone evoked freezing in conditioned rats (see Figure S1 available online).