Ventral Hippocampal Input to Infralimbic Cortex Is Necessary for the Therapeutic-Like Effects of Extinction in Stressed Rats

Abstract

Background: Posttraumatic stress disorder is characterized by deficits in cognitive flexibility related to dysfunction of the medial prefrontal cortex (mPFC). Exposure therapy can effectively reverse these deficits. Fear extinction in rodents bears similarity to exposure therapy. Extinction reverses chronic stress-induced deficits in cognitive flexibility on the attentional set-shifting test (AST), an mPFC-mediated process. This therapeutic effect requires activity of pyramidal neurons and brain derived neurotrophic factor (BDNF) signaling in infralimbic cortex (IL). However, the circuit mechanisms governing BDNF-mediated plasticity initiated by extinction in IL are unknown. The ventral hippocampus (vHipp) plays a role in regulating IL activity during extinction, and plasticity in vHipp is necessary for extinction memory consolidation. Therefore, we investigated the role of vHipp input to IL in the effects of extinction in reversing stress-induced cognitive deficits.

Methods: vHipp input to IL was silenced using a Gi-Designer Receptors Exclusively Activated by Designer Drugs (DREADD) via local infusion of clozapine-N-oxide (CNO) into IL before extinction. A day later, rats were tested on AST. In a separate experiment, we tested whether vHipp input to the IL induces BDNF signaling to exert therapeutic effects. We activated the vHipp using a Gq-DREADD, and injected an anti-BDNF neutralizing antibody into IL. Rats were tested on the AST 24 hours later.

Results: Silencing the vHipp input to IL prevented the beneficial effects of extinction in reversing stress-induced cognitive deficits. Activating vHipp input to IL in the absence of extinction was sufficient to reverse stress-induced deficits in set-shifting. The beneficial effects were blocked by local infusion of a neutralizing anti-BDNF antibody into IL.

Conclusions: vHipp-driven BDNF signaling in IL is critical for extinction to counteract the deleterious cognitive effects of chronic stress.

Keywords: Chronic stress; fear extinction; infralimbic cortex; ventral hippocampus.

Figure 1.

Chemogenetically silencing ventral hippocampal (vHipp) terminals in the infralimbic cortex(IL) decreases TrkB phosphorylation in the IL after extinction. (A) Inhibiting vHipp terminals in the IL attenuated the phosphorylation of TrkB in the IL after extinction (*P < .05). Bars represent SEM. (B) Top: Histological representation of a guide cannula track targeting IL cortex, indicated by the asterisk marking the clozapine-N-oxide injection site located 1 mm beyond the cannula tip (diagram from Paxinos and Watson, 2007). Bottom left: Expression of virus in cell bodies in the vHipp, indicated by the EGFP reporter. Bottom right: Expression of virus in ventral hippocampal terminals in the IL, indicated by the EGFP reporter. Abbreviations: Cg1, cingulate cortex, area 1; IL, infralimbic cortex; PrL, prelimbic cortex. Scale bar = 50 μm.

Figure 2.

Ventral hippocampal input to the infralimbic cortex (IL) is necessary for the effects of extinction after chronic stress. (A) Experimental timelines for behavioral experiments in male (14-day CUS) and female rats (21-day CUS). (B) Fear conditioning did not differ between groups (i.e., before stress exposure). Insets show male and female data separately (n = 3–6 per group for both sexes). (C) Freezing during fear extinction also did not differ between groups. Insets show male and female data separately (n = 3–6 per group for both sexes). Freezing behavior for some rats could not be scored due to technical issues (the rats managed to hide behind the mat used to form the circular enclosure while freezing during some of the tone presentations). (D) Chemogenetically silencing ventral hippocampal terminals in the IL during extinction blocked the therapeutic effects of extinction. Chronic stress induced a cognitive deficit in the extradimensional set shifting task (*P < .001). This effect was reversed by extinction (⁺P < .001). Inhibiting ventral hippocampal input in the IL during extinction blocked the effects of extinction (^#P < .001) (n = 10–13 per group). Data disaggregated by sex are shown separately in the insets (males top, n = 3–8 per group; females bottom, n = 3–6 per group). Bars represent SEM.

Figure 3.

Chemogenetic activation of ventral hippocampal terminals in the infralimbic cortex (IL) reverses stress-induced deficits in set shifting, and this effect is dependent on Brain-Derived Neurotrophic Factor(BDNF) signaling. (A) Experimental timelines for male (14-day CUS) and female rats (21-day CUS). (B) Chemogenetic activation of ventral hippocampal terminals in the IL is sufficient to reverse stress induced deficits in set-shifting performance. Stress induced a deficit in set shifting (*P < .05) that was reversed by chemogenetic activation of ventral hippocampal terminals in the IL (⁺P < .05). Blocking BDNF with a neutralizing antibody in the IL prevented the beneficial effects of chemogenetic activation (^#P < .05) (n = 7–11 per group). Data disaggregated by sex are shown separately in the insets (males top, (n = 3–6 per group); females bottom, (n = 3–8 per group). Bars represent SEM.