Stress is characterized by the deployment of response systems to promote adaptation in the face of threats. Among these, the neuroendocrine hypothalamic-pituitary-adrenal (HPA) axis has received considerable attention due to the potent acute and chronic effects of its glucocorticoid end-products, including cortisol in humans and corticosterone (CORT) in rodents. Stress also simultaneously elicits conserved behavioral responses that may be key to understanding how animals and humans cope with ongoing threats. Both neuroendocrine and behavioral responses to psychological stress are thought to originate from, and are modulated by, complex neurocircuitry residing within the limbic forebrain. However, to date these responses have largely been studied in functional and neuroanatomical isolation. The experiments here described are intended to shed light on the circuitry underlying the dual modulation of behavioral and HPA output.
Chapter 2 investigates a pathway from the prelimbic subfield (PL) of the medial prefrontal cortex (mPFC) to the anteroventral bed nuclei of the stria terminalis (avBST). Using an optogenetic approach, we found that this pathway simultaneously suppresses both immobility behavior and HPA output during an acute psychological stressor (tail suspension, TS). We go on to show that this pathway also suppresses behavioral passivity in the shock probe defensive burying test (SPDB), a test of coping behavior. Furthermore, endogenous activity in this pathway, as measured by Fos immunoreactivity in avBST–projecting PL neurons, was negatively correlated with passive coping behavior in the SPDB. Follow-up experiments found that PL axonal terminals within avBST were glutamatergic and photoexcitation of these terminals produced excitatory post-synaptic potentials in avBST neurons. Next, a downstream pathway from avBST to the ventrolateral periaqueductal gray (vlPAG) was investigated as a candidate mediator of the observed effects on passive coping. Photoinhibition of avBST terminals in vlPAG recapitulated the effects of PL–avBST photoinhibition. Finally, avBST terminals within vlPAG were found to be GABAergic, consistent with a role for avBST inputs in inhibiting passive coping-related activity in this region.
Chapter 3 expands on the role of avBST and its output pathways in modulating behavioral and neuroendocrine stress responses. Photoinhibition of avBST cell bodies during TS produced a marked increase in both immobility and HPA output while photoexcitation was sufficient to suppress the neuroendocrine stress axis. Follow-up studies found that the HPA-modulatory effects of avBST cell body manipulations were likely mediated by direct avBST inputs to the paraventricular nucleus of the hypothalamus (PVH). We found that avBST terminals within PVH terminated in close proximity to putatively neurosecretory corticotropin releasing factor (CRF)-immunoreactive neurons. Photoinhibition of avBST terminals in PVH during TS produced elevations in HPA output that were comparable to those observed follow avBST cell body inhibition. Finally, photoinhibition of avBST terminals in vlPAG was associated with increased immobility during both TS and acute exposure to the forced swim test, consistent with a role for this pathway in suppressing passive behaviors across a variety of behavioral tests.
Chapter 4 studies parallel pathways from mPFC to distinct cell columns within the periaqueductal gray (PAG). The PAG is a highly conserved region of the midbrain that surrounds the cerebral aqueduct and has been implicated in the regulation of defensive behaviors. Prior work suggests that ventrolateral aspects of the structure promote passive defensive behaviors (e.g., freezing and immobility), whereas activation of the dorsal (d) cell column produces active behavior (threat confrontation or flight). In these experiments, we again utilized the SPDB; rats are exposed to an electrified probe mounted on their cage wall, whereby after receiving electric shock, they display both active (probe burying with cage bedding) and passive (immobility) coping behavior. Consistent with previous reports, we found that rostral mPFC provided dense innervation of ventrolateral PAG, whereas caudal mPFC provided innervation of dorsal PAG. Using an optogenetic approach we found that photoinhibition enhanced, and photoexcitation of the rostral mPFC–ventrolateral PAG pathway diminished passive coping during the SPDB, but active coping behavior was unaffected. Next, we investigated the contributions of the caudal mPFC–dorsal PAG pathway during the SPDB. Here, pathway photoexcitation enhanced probe burying behavior, the primary measure of active coping, while other behaviors remained unaffected. This result suggested that activation of a single pathway was sufficient to drive active coping. Finally, we tested the effects of caudal mPFC–dorsal PAG pathway photoexcitation under conditions where active coping behavior is prohibited, by removal of the cage bedding to prevent rats from the ability to bury the shock probe. In control animals acutely deprived of bedding during the SPDB, we observed increased immobility behavior and ultrasonic vocalizations, as well as autonomic and HPA output, while each of these were decreased in bedding-deprived animals that received caudal mPFC–dPAG pathway photoexcitation. This final series of experiments implicate separate prefrontal-PAG pathways in either the suppression of passive, or promotion of active coping behavior. They further suggest that the caudal mPFC-dorsal PAG pathway provides a neural basis linking active coping with stress-buffering effects, marked by decreases in displacement behavior and neuroendocrine activation.
These results show that separate pathways from the medial prefrontal cortex to the bed nuclei of the stria terminalis and periaqueductal gray are simultaneously and differentially able modulate passive and active coping in response to aversive stimuli in rats. The prefrontal–avBST pathway coordinates the inhibition of something akin to a “passive response set” – i.e., by gating passive coping behavior and restraining neuroendocrine activation. Complementary, parallel prefrontal–periaqueductal gray pathways are able to independently support either the suppression of passive, or promotion of active coping behavior. The discussion will consider the naturalistic contexts accounting for how activity in mPFC may provide for the cooperative engagement of an active behavioral response set, and how differentially engaging these pathways may promote distinct adaptative strategies as based upon changing environmental conditions. Finally, we will consider how these data offer a neural basis linking active coping with stress-buffering effects, and how perturbations in these circuits may lead to chronic stress-related dysfunction of multiple systems and inform disease susceptibility in humans.