Should I freeze or should I go? The ventral subiculum→BNST neurons yield the right-of-way

The ventral subiculum (vSUB), the main output of the ventral hippocampus, encodes environmental cues for context processing. Lesion or inactivation of vSUB after (but not before) fear-conditioning impairs contextual fear memory (Maren, 1999; Biedenkapp and Rudy, 2009), whereas other studies also implicate the vSUB in the exploratory behavior (Burns et al., 1996) and spatial learning (Floresco et al., 1997). In the current issue of Neuroscience, Urien et al., 2022, take a closer look at a subset of vSUB neurons, which project to the bed nucleus of the stria terminalis (BNST) (Canteras and Swanson, 1992; Cullinan et al., 1993; Radley and Sawchenko, 2011), a limbic brain region necessary for the expression of contextual fear (Sullivan et al., 2004; Urien et al., 2021). First, they demonstrate that vSUB neurons projecting to the BNST (vSUB→BNST) are distinct from vSUB neurons projecting to the basolateral nucleus of the amygdala, another important hub in the contextual fear expression (Fanselow and LeDoux, 1999; Ciocchi et al., 2010). Likewise, Wee and MacAskill, 2020, also showed that vSUB neurons send separate projections to various targets in the forebrain. Urien et al., next investigated if activity of the vSUB→BNST neurons is modulated by context fear expression. They found reduced activity in the vSUB→BNST pathway (indexed by cFos expression) in both male and female rats in aversive context, such as a chamber, in which the rats have been previously fear-conditioned. This reduction was evident when compared to control (never conditioned) rats, which showed significant activation of the vSUB→BNST neurons (and the entire vSUB, especially the posterior part) after they explored a neutral familiar context (no shock group). Although this might suggest that the vSUB→BNST neurons are tonically active and become inhibited by the aversive context, the authors excluded this possibility by indexing cFos of the vSUB→BNST neurons in naïve, home-cage controls. The latter rats were not subjected to any behavioral manipulations and showed the lowest overall activity in the vSUB and the vSUB→BNST neurons. As the vSUB has been implicated in exploration and spatial learning (Burns et al., 1996; Shankaranarayana Rao et al., 2001; Torromino et al., 2019), active exploration of the familiar neutral environment might activate the vSUB→BNST neurons, as observed in the no shock control group. These results are consistent with studies showing that high frequency stimulation of the vSUB neurons in vivo leads to long-lasting increases in active exploration of open arms of the elevated plus maze (EPM) and more time spent in the bright compartment of light-dark box. Notably, these anxiolytic effects of the vSUB stimulation were blocked by intra-BNST infusion of NMDA receptor antagonist (Glangetas et al., 2017), suggesting that indeed the vSUB→BNST pathway promotes active exploration of environments, which are not associated with a learned threat. This effect might be specific to the activity the of the vSUB→BNST neurons, as rats with general lesion (Kjelstrup et al., 2002) or inactivation (Trent and Menard, 2010) of the ventral hippocampus spent more time exploring open arms of the EPM, an opposite role to the one attributed the vSUB→BNST neurons.

More importantly, the reduced activity in the vSUB→BNST pathway in the aversive context found by Urien et al., would suggest that once rats learn that a particular environment is aversive, the vSUB→BNST pathway becomes inhibited and so is the free exploration. In other words, when the environment is no longer safe, this free exploration is traded for a defensive behavior, such as freezing, and the vSUB→BNST pathway needs to be inhibited for the freezing behavior to occur. As an alternative explanation, hippocampus-dependent spatial exploration and activity might compete with freezing behavior per se, independently of fear learning (Teitelbaum and Milner, 1963; Ohki, 1982; Good and Honey, 1997). For example, dorsal hippocampus lesions produce deficit in the contextual fear expression when the measured outcome is freezing (Kim et al., 1993; Phillips and LeDoux, 1994; Maren et al., 1997) but do not produce such deficits, when measured with potentiation of the startle response (McNish et al., 1997). However, others argued that the observed deficits in freezing during contextual fear recall cannot be exclusively attributed to motor disruption (Maren et al., 1997).

To elucidate the mechanisms of the vSUB-BNST pathway inhibition in the aversive context, Urien et al., demonstrated that activity of GABA interneurons in the vSUB was not altered by the expression of contextual fear. Although they found a relatively small, albeit significant, sex difference in the number of GABA interneurons in vSUB (with males showing greater number), no significant effects of sex on freezing to the aversive contexts or the number of activated vSUB→BNST neurons were found in the study. Future studies might shift focus toward elucidating cellular substrates of the vSUB→BNST pathway in the BNST and fear-related behaviors. As the vSUB projection is excitatory (Glangetas et al., 2017), hypothetically, if active vSUB→BNST neurons preferentially target and excite BNST inhibitory interneurons during conditions of safe exploration of neutral, familiar environment, this would inhibit BNST output neurons projecting to brainstem effector centers (such as periaqueductal gray, PAG, Hao et al., 2019), overall reducing defensive behaviors, such as freezing. In the aversive contexts however, when the vSUB→BNST neurons are inhibited, the BNST interneurons innervated by vSUB remain inactive, and therefore the BNST→PAG output neurons would enable freezing behavior during contextual fear expression. In summary, the current study by Urien et al., sheds light on how the vSUB→BNST neurons might modulate different forms of defensive and exploratory behaviors and shift activity balance between the free exploration of a neutral environment and expression of defensive behaviors such as freezing when this environment becomes aversive.