Allopregnanolone induces state-dependent fear via the bed nucleus of the stria terminalis

Abstract

Gonadal steroids and their metabolites have been shown to be important modulators of emotional behavior. Allopregnanolone (ALLO), for example, is a metabolite of progesterone that has been linked to anxiety-related disorders such as posttraumatic stress disorder. In rodents, it has been shown to reduce anxiety in a number of behavioral paradigms including Pavlovian fear conditioning. We have recently found that expression of conditioned contextual (but not auditory) freezing in rats can be suppressed by infusion of ALLO into the bed nucleus of the stria terminalis (BNST). To further explore the nature of this effect, we infused ALLO into the BNST of male rats prior to both conditioning and testing. We found that suppression of contextual fear occurred when the hormone was present during either conditioning or testing but not during both procedures, suggesting that ALLO acts in a state-dependent manner within the BNST. A shift in interoceptive context during testing for animals conditioned under ALLO provided further support for this mechanism of hormonal action on contextual fear. Interestingly, infusions of ALLO into the basolateral amygdala produced a state-independent suppression of both conditioned contextual and auditory freezing. Altogether, these results suggest that ALLO can influence the acquisition and expression of fear memories by both state-dependent and state-independent mechanisms.

Keywords: Basolateral amygdala; Contextual fear; Fear conditioning; Neurosteroid.

Fig. 1

Effects of intra-BNST ALLO infusions on the acquisition and expression of contextual and cued fear. A) Representative thionin-stained coronal section showing cannula placements. B) Schematic coronal sections showing infusion sites for vehicle (VEH) or allopregnanolone (ALLO). Experiment 1 sites are indicated by circles and Experiment 3 sites are indicated by triangles. Coronal brain section images are adapted from Swanson (2003). C) Mean percentage of freezing (±SEM) during the conditioning session (data are shown for a 3-min pre-trial period followed by five tone-shock pairings). Freezing was quantified before the first trial (baseline, BL) and during the 1-min period after each trial. D) Mean percentage of freezing (±SEM) averaged across the 10-min context test. The drug states for each group are indicated by a label on each bar indicating the drug infused before conditioning followed by the drug infused before testing (V for VEH, A for ALLO). The V/V group differed from the V/A (p < 0.01) and A/V (p < 0.05) groups. The A/A group differed from the V/A (p < 0.05) and A/V (p < 0.01) groups. E) Mean percentage of freezing (±SEM) averaged across each 1-min period after four tone presentations.

Fig. 2

Effects of intra-BLA ALLO infusions on the acquisition and expression of contextual and cued fear. A) Representative thionin-stained coronal section showing cannula placements. B) Schematic coronal sections showing infusion sites for vehicle (VEH) or allopregnanolone (ALLO). Experiment 2 sites are indicated by circles. Coronal brain section images are adapted from Swanson (2003). C) Mean percentage of freezing (±SEM) during the conditioning session (data are shown for a 3-min pre-trial period followed by five tone-shock pairings). Freezing was quantified before the first trial (baseline, BL) and during the 1-min period after each trial. D) Mean percentage of freezing (±SEM) averaged across the 10-min context test. The drug states for each group are indicated by a label on each bar indicating the drug infused before conditioning followed by the drug infused before testing (V for VEH, A for ALLO). The V/V group differed from the A/A (p < 0.01), V/A (p < 0.0001), and A/V (p < 0.05) groups. E) Mean percentage of freezing (±SEM) averaged across each 1-min period after four tone presentations. The V/V group differed from the A/A (p < 0.01), V/A (p < 0.0001), and A/V (p < 0.01) groups.

Fig. 3

Contribution of intra-BNST ALLO to interoceptive and exteroceptive states. A) Schematic diagram showing the design for Experiment 3. For conditioning, all animals were in the same interoceptive (infused with allopregnanolone; ALLO) and exteroceptive (Context A) states. For context testing, animals were assigned to one of four groups with varying degrees of shift in interoceptive and exteroceptive states: 1) the None group had no shifts, 2) the Context group had a shift in exteroceptive state (infused with ALLO, exposed to Context B), 3) the Drug group had a shift in interoceptive state (infused with vehicle, exposed to Context A), and 4) the Context & drug group had shifts in both states (infused with vehicle, exposed to Context B). B) Mean percentage of freezing (±SEM) averaged across the 10-min context test. The None group differed from the Drug (p < 0.05) and Context & drug groups (p < 0.01).