Gating of fear in prelimbic cortex by hippocampal and amygdala inputs

Abstract

The prefrontal cortex (PFC) regulates emotional responses, but it is unclear how PFC integrates diverse inputs to select the appropriate response. We therefore evaluated the contribution of basolateral amygdala (BLA) and ventral hippocampus (vHPC) inputs to fear signaling in the prelimbic (PL) cortex, a PFC region critical for the expression of conditioned fear. In conditioned rats trained to press for food, BLA inactivation decreased the activity of projection cells in PL, and reduced PL conditioned tone responses. In contrast, vHPC inactivation decreased activity of interneurons in PL and increased PL conditioned tone responses. Consistent with hippocampal gating of fear after extinction, vHPC inactivation increased fear and PL pyramidal activity in extinguished, but not in conditioned, rats. These results suggest a prefrontal circuit whereby hippocampus gates amygdala-based fear. Thus, deficient hippocampal inhibition of PFC may underlie emotional disorders, especially in light of reduced hippocampal volume observed in depression and PTSD.

Figure 1. Basolateral amygdala and ventral hippocampus differentially modulate the spontaneous activity of prelimbic pyramidal neurons and interneurons after conditioning

(A) Conditioned rats previously trained to press a lever to obtain food were implanted with unit recording electrodes in the prelimbic (PL) prefrontal cortex and locally infused with muscimol (MUS) in the basolateral amygdala (BLA) or ventral hippocampus (vHPC). Coronal drawings show the location of recording electrodes (left) and cannula injector tips (right). Inset shows example extracellular waveforms. (B & C) Histograms show ten minutes of spontaneous PL firing rate of representative neurons before (grey) and after (orange) MUS infusion (orange arrow) into either the BLA (B) or vHPC (C) (bin width = 1 min). Delay between pre- and post-infusion recordings was between 2 h and several days. Insets illustrate the proportion of PL cells that significantly increased rate, decreased rate, or showed no significant changes. (D) Classification of PL neurons into putative pyramidal excitatory cells (blue) and interneurons (green) based on waveform duration and firing rate. Dendrogram inset shows the unsupervised cluster analysis that identified two main clusters. (E) Averaged cross-correlation analysis for 10 pairs of simultaneously recorded PL interneurons and pyramidal cells, illustrating inhibitory interactions between interneurons (reference cell, firing at 0) and pyramidal cells (binned spike count normalized to overall firing rate) (bin width = 20 ms). Dashed horizontal line indicates mean; dotted lines indicate 95% confidence intervals. (F) BLA inactivation significantly decreased activity of PL pyramidal neurons (blue bars) without affecting activity of PL interneurons (green bars). (G) vHPC inactivation significantly decreased the activity of PL interneurons without affecting activity of pyramidal neurons. In this and subsequent figures, error bars illustrate s.e.m.; *p<0.05.

Figure 2. Conditioned tone responses in PL neurons were decreased by BLA inactivation, but increased by vHPC inactivation

(A) Upper: Peri-event time histogram of a representative PL tone-responsive neuron before (grey) and after (orange) BLA inactivation (bin width = 3 s), showing reduction in tone response with no change in spontaneous activity (Hz; pre-tone). Data represent the average of two conditioned tones before/after input inactivation. Lower: Group data showing that BLA inactivation significantly reduced tone responsiveness of PL neurons, as indicated by z-scores (P<0.001). Data represent averaged conditioned tone presentations over repeated days. Shaded areas represent s.e.m. Inset shows decreased PL tone responses were due in part to decreased pyramidal cell activity. (B) Upper: Peri-event time histogram of a representative PL neuron that increased its tone response following inactivation of vHPC. Lower: Group data showing that vHPC inactivation significantly increased tone responsiveness of PL neurons, as indicated by z-scores (P<0.05). Inset shows that increased PL tone responses were due to increased pyramidal cell activity. (C) Line plots tracking the tone responses (TRs) of individual PL neurons classified as significantly tone responsive (Z> 2.58; P<0.01; dotted line) before and/or after inactivation. Pie charts illustrate the percentage of PL neurons that lost TR (green), became TR (purple), or remained TR (white) after inactivation. The majority of PL neurons lost their tone response (green) after BLA inactivation. (D) In contrast to BLA, inactivation of vHPC caused the majority of PL neurons to maintain their tone response (white) or become tone responsive (purple). *p<0.05; **p<0.01. See also Figure 2S.

Figure 3. Examples of interaction between vHPC and BLA inputs within single PL neurons from conditioned rats

(A) Experimental design: PL neuron activity without brain inactivation(s) (top), PL neuron activity with vHPC inactivation (middle) and PL neuron activity with vHPC + BLA inactivations (bottom) (B) Peri-event time histograms showing the tone responses of two PL neurons (left and right) recorded prior to any manipulation (top, grey), after vHPC inactivation (middle; orange), and after vHPC + BLA inactivation (bottom; red). Note that these two PL neurons were not originally TR, but became TR after vHPC inactivation. Downward pointing arrows represent succession of events. Subsequent inactivation of BLA eliminated the latent tone responses. (C) Mean z-scores (10 bins) show that vHPC inactivation increased tone responses significantly, while subsequent BLA inactivation of the same cells decreased tone responses significantly. Tukey post hoc before inactivation (grey) vs. after vHPC inactivation (orange), and vHPC inactivation vs. BLA inactivation (red): *p<0.05; ***p<0.001.

Figure 4. Inactivation of vHPC induced moderate fear and increased PL activity after, but not before, extinction

(A) Rats trained to press a bar to obtain food were locally infused with saline (SAL) or muscimol (MUS) to temporarily inactivate the vHPC. Coronal drawing shows the location of cannula injector tips in vHPC. Press rates prior to the onset of the first tone were used as a measure of fear to the context, and rates during the tone as a measure of fear to the tone. (B) Inactivation of vHPC in naïve rats did not affect pressing during pre-tone or tone periods. (C) Inactivation of vHPC in conditioned rats increased pre-tone and tone pressing (decreased fear), but had no effect on spontaneous firing rate of PL pyramidal cells. (D) In contrast, inactivation of vHPC in extinguished rats significantly reduced pre-tone and tone pressing (increased fear) and increased the firing rate of PL pyramidal cells. Press rates following vHPC inactivation resembled pre-extinction rates in the conditioned group (see 4C); however, freezing was not increased (inset). Diagrams in the bottom of (C) and (D) suggest the underlying circuit recruited under different conditions. Following conditioning, heightened BLA activity (driven in part by vHPC input) controls PL activity. Following extinction, reduced excitatory drive emanating from BLA is balanced by increased inhibition of PL by vHPC. *p<0.05; **p<0.01. See also Figure 3S.