Compensation in the neural circuitry of fear conditioning awakens learning circuits in the bed nuclei of the stria terminalis

Abstract

The basolateral amygdala (BLA) is thought to be essential for fear learning. However, extensive training can overcome the loss of conditional fear evident following lesions and inactivation of the BLA. Such results suggest the existence of a primary BLA-dependent and a compensatory BLA-independent neural circuit. We tested the hypothesis that the bed nuclei of the stria terminalis (BST) provides this compensatory plasticity. Using extensive context-fear conditioning, we demonstrate that combined BLA and BST lesions prevented fear acquisition and expression. Additionally, protein synthesis in the BST was critical only for consolidation of BLA-independent but not BLA-dependent fear. Moreover, fear acquired after BLA lesions resulted in greater activation of BST regions that receive hippocampal efferents. These results suggest that the BST is capable of functioning as a compensatory site in the acquisition and consolidation of context-fear memories. Unlocking such neural compensation holds promise for understanding situations when brain damage impairs normal function or failure to regulate compensatory sites leads to anxiety disorders.

Fig. 1.

Histological analysis of BLA or BST lesions for exp. 1. Reconstruction of the minimal (dark gray) and maximal (light gray) excitotoxic lesion in the region of the BLA (AP 2.6–3.3 mm) and BST (AP 0.0–1.0 mm). Coronal section images taken and adapted from Swanson (52). The largest extent of any given BLA lesion included LA, BLAa, BLAp, and BMAp; the smallest lesion only included damage to the LA. The largest extent of BST lesion included subdivisions pr, rh, al, tr, am, mg, v, and if, and the smallest lesion was limited to pr, mg, and if.

Fig. 2.

Behavioral measurement. (A) Behavioral measurement of fear learning in sham, BLA, BST, or combined BLA+BST lesion animals. Acquisition of conditional fear as indexed by percent-time observed freezing during an immediate postshock period. Mean (±SEM) percentage of observations spent freezing during a 76-footshock contextual-fear conditioning session. (B) Context-fear memory in sham, BLA, BST, or combined BLA+BST lesioned animals. Total mean (±SEM) percentage of observations spent freezing during an 8-min test session for conditional fear.

Fig. 3.

Histological analysis of BLA lesion and BST cannula placements for exp. 2. Reconstruction of the minimal (dark gray) and maximal (light gray) excitotoxic lesion in the region of the BLA (AP 2.6–3.3 mm) and cannula tip placement within the BST (square). Coronal section images taken and adapted from Swanson (52). The largest extent of any given BLA lesion included LA, BLAa, BLAp, and BMAp; the smallest lesion only included damage to the LA.

Fig. 4.

Behavioral measurement of context-fear memory in rats with combined: (i) Sham lesion and BST ACSF infusion (black), (ii) sham lesion and BST ANISO infusion (light gray), (iii) BLA lesion and BST ACSF infusion (white), (iv) BLA lesion and BST ANISO infusion (medium gray), and (v) retraining (76 trials in a novel context) in previously trained and tested BLA lesion and BST ANISO-infused animals (medium gray with description). Total mean (±SEM) percentage of observations spent freezing during an 8-min test session for conditional fear.

Fig. 5.

Retrograde labeling of neuronal tracer Fluoro-gold. (Right) The putative neuroanatomical circuits underlying fear conditioning. (Left) Retrograde neuronal tracer Fluoro-Gold was infused into the BST, as highlighted in the image in the red box. A representative infusion resulted in retrograde labeling of neurons in the mPFC, CEA, and ventral (HPF). A rat with the BLA intact showed substantial retrograde labeling in the BLA, whereas a rat with a BLA lesion showed substantially less labeling within the BLA complex.

Fig. 6.

Representative images of C-Fos–immunoreactive neurons within the BST in animals with lesions of the BLA, following an 8-min test of contextual fear. The outlined area includes BST and its subdivisions: rh, am, al, dm, mg, and v. The subdivisions in solid white text (am, dm, mg, and v) represent the anteromedial group of BST subdivisions quantified in exp. 3. (A) BLA-lesioned and not overtrained; (B) BLA-lesioned and overtrained.

Fig. 7.

Mean number of C-fos-positive cells counted in posterior ventral anteromedial region of the BST following the test of a context-fear memory (**P < 0.001).