The bed nucleus of the stria terminalis is required for the expression of contextual but not auditory freezing in rats with basolateral amygdala lesions
Research highlights
► Rats with basolateral amygdala lesions acquire Pavlovian fear conditioning after overtraining. ► Excitotoxic lesions of the bed nucleus of the stria terminalis impair the expression of conditional freezing to the training context, but not an auditory conditional stimulus, after overtraining. ► Reversible inactivation of the bed nucleus of the stria terminalis with the glutamate antagonist, NBQX, impairs the expression of conditional freezing to the training context, but not an auditory conditional stimulus, after overtraining. ► The bed nucleus of the stria terminalis has a selective role in the expression of contextual fear, but not a general role in the associative processes that maintain memories of Pavlovian fear conditioning.
Introduction
Neuroscientists have long sought to understand the neural basis of learning and memory. An influential approach to this problem pioneered by Richard F. Thompson and colleagues is the model-systems approach whereby the neural circuitry underlying a simple form of learning, such as classical conditioning, is dissected (Christian & Thompson, 2003). In recent years, Pavlovian fear conditioning has proved to be an important model for studying the neural mechanisms and circuitry of emotional learning and memory (Davis, 1992, LeDoux, 2000, Maren, 2001, Maren, 2005). In the paradigm, a conditioned stimulus (CS), such as the context of the conditioning chamber or a discrete auditory cue, is paired with an unconditioned stimulus (US), such as a mild footshock. After conditioning, the CS elicits a conditioned fear response (CR), characterized by an increase in heart rate, blood pressure, release of stress hormones, and somatomotor immobility (i.e. freezing).
Years of work have now revealed that the amygdala is essential for the acquisition and expression of Pavlovian fear memories (Davis and Whalen, 2001, Fendt and Fanselow, 1999, LeDoux, 2000, Maren, 2001). Specifically, the basolateral complex of the amygdala (BLA) is believed to be the critical site of CS–US convergence underlying the acquisition and of Pavlovian fear memories. However, we have discovered that deficits in fear conditioning in rats with BLA lesions can be overcome with overtraining (Maren, 1999a, Zimmerman et al., 2007). The capacity for fear learning in rats with BLA lesions suggests other brain areas are sufficient for the acquisition and expression of conditional fear. Indeed, we have recently shown that the amygdaloid central nucleus (CEA), which also receives CS and US information, is essential for the acquisition and expression of conditional fear in rats with BLA lesions. These findings suggest that the CEA mediates the acquisition of fear in rats with BLA lesions, although this memory requires many more trials to acquire (Maren, 1999a) and is short-lived (Poulos et al., 2009).
Another brain structure that might mediate fear conditioning in the absence of the BLA is the bed nucleus of the stria terminalis (BNST). The BNST possesses similar afferent and efferent connectivity to that of the CEA (Dong et al., 2001, Walker et al., 2003). Furthermore, Sullivan and colleagues recently demonstrated a role for the BNST in the expression of conditioned fear (Sullivan et al., 2004). Specifically, they found that lesions of the BNST block the expression of contextual fear, but not fear to an auditory CS (Sullivan et al., 2004, Waddell et al., 2006). It is possible therefore that the bed nucleus of the stria terminalis (BNST), like the CEA (Zimmerman et al., 2007), may be able to compensate for the loss of the BLA following overtraining.
In support of this possibility, Poulos, Ponnusamy, Dong, and Fanselow (2010) have reported that BNST lesions or inhibition of BNST protein synthesis impairs contextual fear conditioning in rats with BLA lesions. However, it is not clear whether the BNST has a global role in mediating fear in the absence of the BLA, or if it has a selective role in the expression of contextual fear independent of the BLA. The following experiments address this possibility. Rats received bilateral excitotoxic BLA lesions prior to overtraining in an auditory fear conditioning task. They then received either post-training lesions of the BNST or pre-testing infusions of the AMPA receptor antagonist NBQX into the BNST before retention tests in which conditional freezing to the shock-associated context and auditory CS was assessed. We report that although BNST lesions or inactivation disrupt the expression of context freezing in rats with BLA lesions, they did not effect the expression of fear to the auditory CS. These results reveal that although the BNST is critical for the expression of contextual fear, it is not the essential locus of compensation for fear learning in the absence of the BLA.
Section snippets
Subjects
The subjects were 66 male Long-Evans rats (200–224 g; Blue Spruce) obtained from a commercial supplier (Harlan Sprague Dawley, Indianapolis, IN). After arrival, the animals were individually housed in clear plastic cages hanging from a standard stainless-steel rack. The vivarium lights were on a 14:10 light:dark cycle (lights on at 7:00 am) and the rats had free access to food and tap water. After housing, the rats were handled (15–20 s each) for 5 days to acclimate them to the experimenter. All
Histology
Based on the histological results, 10 of 66 rats were excluded. Rats were excluded if their lesions were larger than intended, misplaced, largely unilateral, or produced substantial damage in the CEA. This yielded the following group sizes: BLA–BNST (n = 13), BLA–SH (n = 14), SH–BNST (n = 11), and SH–SH (n = 18). The extent of the amygdala and BNST damage for rats included in the analyses are depicted in Fig. 1. As can be seen damage was generally confined to the targeted structure and was estimated to
Discussion
The results of the present study indicate that BNST lesions or inactivation selectively impairs the expression of contextual fear after overtraining. This impairment was not observed to an auditory CS, and was manifest in both intact rats and rats with BLA lesions. Hence, the effect of BNST lesions on conditional freezing was not due to a performance deficit. These results are consistent with previous studies that have demonstrated that lesions of the BNST selectively disrupt contextual fear
Conclusions
Rats with basolateral amygdala lesions acquire both contextual and auditory fear after overtraining. The expression of overtrained contextual fear, but not tone fear, requires the bed nucleus of the stria terminalis. These results reveal that the bed nucleus of the stria terminalis has a selective role in the expression of context fear, rather than a general role in the associative processes mediating fear memory in the absence of the basolateral amygdala. This latter role appears to be
Acknowledgment
The research reported in this manuscript was supported by a grant from the National Institutes of Health (R01MH73655).
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NMDA receptors in the CeA and BNST differentially regulate fear conditioning to predictable and unpredictable threats
2020, Neurobiology of Learning and MemoryCitation Excerpt :Specifically, this work suggests that although the amygdala is critical for phasic fear responses to predictable threat cues, the bed nucleus of the stria terminalis (BNST) mediates sustained fear states evoked by uncertain threat (Alvarez, Chen, Bodurka, Kaplan, & Grillon, 2011; Davis, Walker, Miles, & Grillon, 2010; Walker & Davis, 2008; Walker, Miles, & Davis, 2009). Although initial studies suggested a role for the BNST in contextual (but not cued) fear (Davis & Walker, 2014; Hott et al., 2012, 2017; LeDoux, Iwata, Cicchetti, & Reis, 1988; Poulos, Ponnusamy, Dong, & Fanselow, 2010; Resstel et al., 2008; Sink et al., 2013; Sullivan et al., 2004; Zimmerman & Maren, 2011), more recent work suggests the role of the BNST in fear conditioning may be more nuanced than previously appreciated (Goode, Ressler, Acca, Miles, & Maren, 2019; Goode, Acca, & Maren, 2020; Hammack, Todd, Kocho-Schellenberg, & Bouton, 2015; Waddell, Morris, & Bouton, 2006). In a recent study from our laboratory, we found that reversible inactivation of BNST impaired the expression of fear to discrete CSs that poorly signaled when shock would occur [e.g., backward (BW) or temporally randomized]; in contrast, this manipulation had no effect on the expression of fear to forward (FW) CSs that reliably predict shock onset (Goode et al., 2019).
Threat imminence dictates the role of the bed nucleus of the stria terminalis in contextual fear
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Chronic stress induces cell type-selective transcriptomic and electrophysiological changes in the bed nucleus of the stria terminalis
2019, NeuropharmacologyCitation Excerpt :Consistent with previous observations, the 7-day CSS paradigm also caused a long-lasting increase in anxiety-like behavior as measured by an elevated ASR 6 days after the last day of shock stress (Hazra et al., 2012). The BNST is known to be critical for context fear conditioning and an important modulator of the startle reflex (Gewirtz et al., 1998; Zimmerman and Maren, 2011; Davis and Walker, 2014; Hammack et al., 2015). Significantly, the chronic stress-induced increase in anxiety-like behavior and mRNA changes appear to require a period of “incubation” (Gewirtz et al., 1998; Bangasser et al., 2005; Lezak et al., 2014) suggesting that a tipping point may need to be reached in the properties of discrete ovBNST neurons after which long-term behavioral changes are observed.