Prefrontal infralimbic cortex mediates competition between excitation and inhibition of body movements during pavlovian fear conditioning

Abstract

The infralimbic subregion of the prefrontal cortex (IL) is broadly involved in behavioral flexibility, risk assessment, and outcome reinforcement. In aversive conditioning tasks, the IL has been implicated in fear extinction and in mediating transitions between Pavlovian and instrumental responses. Here we examine the role of the IL in mediating transitions between two competing Pavlovian fear responses, conditioned motor inhibition (CMI) and conditioned motor excitation (CME). Rats were trained to fear an auditory conditioned stimulus (CS) by pairing it with periorbital shock to one eyelid (the unconditioned stimulus [US]). Trained animals exhibited CMI responses (movement suppression) to the CS when they had not recently encountered the US (>24 hr), but, after recent encounters with the US (<5 min), the CS evoked CME responses (turning in circles away from anticipated shock). Animals then received bilateral infusions of muscimol or picrotoxin to inactivate or hyperactivate the IL, respectively. Neither drug reliably affected CMI responses, but there was a bidirectional effect on CME responses; inactivation of the IL attenuated CME responses, whereas hyperactivation potentiated CME responses. These results provide evidence that activation of the IL may promote behavioral strategies that involve mobilizing the body and suppress strategies that involve immobilizing the body. © 2016 Wiley Periodicals, Inc.

Keywords: RGD ID:2308852; defensive; fear conditioning; fear expression; flight; freezing.

Figure 1. Histological reconstruction of intracranial infusion sites

A, Representative example of IL cannulation using fluorescent muscimol (red) against DAPI counterstain (blue) to visualize cannula tip location. B, Reconstructed cannula tip placements in mPFC (n=30 per hemisphere) at +2.5 mm anterior to bregma are overlaid on a coronal template from the atlas of Paxinos and Watson (1997); symbols indicate rats infused with MUS (●) or PTX (□). IL=infralimbic cortex; PL=prelimbic cortex; AC=Cingulate cortex.

Figure 2. Movement suppression and turning bias in drug-free animals

A, Left graph shows trial-by-trial averages of CS movement suppression ratio (R_CS) from pre- (PreSh) and post-shock (PostSh) trials for all rats (n=30) on the drug free day prior to bilateral MUS or PTX infusion; bar graphs at right show mean R_CS (black) during PreSh and PostSh trials, as well as mean US movement suppression ratio (R_US; white) during the CX period of PreSh versus PostSh trials. B, Left, same as ‘A’ except that mean turning velocity is plotted on the ordinate (positive and negative velocities indicate turning away from or toward the trained eyelid, respectively); bar graphs at right show mean turning bias during CX and CS periods of PreSh and PostSh trials. In all graphs, symbols indicate significance levels for Newman-Keuls posthoc comparisons; symbols in ‘A’ denote comparisons to .50, while symbols in ‘B’ denote comparisons of CX versus CS in trial graphs at left, or of PreSh versus PostSh in bar graphs at right (***= p<.001, **=p<.01, *=p<.05, ns=not significant).

Figure 3. Effects of MUS and PTX on CMI versus CME responses

In all graphs, DF data is plotted by open symbols and drug infusion data is plotted by filled symbols. A, The CS significantly suppressed movement (R_CS <.5) before and after MUS infusions, but not after PTX infusions; asterisks denote significance levels for z-test comparisons against R_CS =.5. B, Defensive turning bias during the CS period of pre- and post-shock trials following MUS versus PTX infusions; asterisks denote significance levels for z-test comparisons against zero turning bias. C, US-evoked reflex responses following MUS and PTX; carat symbols denote significance levels for paired Newman-Keuls tests comparing movement speed during CX versus US for post-shock trials (^^^p<.001). D, Shock-induced movement suppression (R_US) scores following infusions of MUS and PTX; asterisks denote significance levels for z-test comparisons against R_US =.5. Z-score significance levels in graphs A, B, and D are denoted by ***p<.001, **p<.01, *p<.05.

Figure 4. Effects of MUS and PTX on CS-evoked turning during post-shock trials

Each bar shows the mean and standard error of the within-subject change in turning bias after drug infusion, compared to the drug-free session on the previous day; **p<.01, †p=.06. Data for PTX includes n=16 subjects (outliers included).