Timing of impulses from the central amygdala and bed nucleus of the stria terminalis to the brain stem

The role of the bed nucleus of the stria terminalis in the motivational control of instrumental action

We review recent studies assessing the role of the bed nucleus of the stria terminalis (BNST) in the motivational control of instrumental conditioning. This evidence suggests that the BNST and central nucleus of the amygdala (CeA) form a circuit that modulates the ventral tegmental area (VTA) input to the nucleus accumbens core (NAc core) to control the influence of Pavlovian cues on instrumental performance. In support of these claims, we found that activity in the oval region of BNST was increased by instrumental conditioning, as indexed by phosphorylated ERK activity (Experiment 1), but that this increase was not due to exposure to the instrumental contingency or to the instrumental outcome per se (Experiment 2). Instead, BNST activity was most significantly incremented in a test conducted when the instrumental outcome was anticipated but not delivered, suggesting a role for BNST in the motivational effects of anticipated outcomes on instrumental performance. To test this claim, we examined the effect of NMDA-induced cell body lesions of the BNST on general Pavlovian-to-instrumental transfer (Experiment 3). These lesions had no effect on instrumental performance or on conditioned responding during Pavlovian conditioning to either an excitory conditioned stimulus (CS) or a neutral CS (CS0) but significantly attenuated the excitatory effect of the Pavlovian CS on instrumental performance. These data are consistent with the claim that the BNST mediates the general excitatory influence of Pavlovian cues on instrumental performance and suggest BNST activity may be central to CeA-BNST modulation of a VTA-NAc core circuit in incentive motivation.

Amygdala-driven apnea and the chemoreceptive origin of anxiety

Although the amygdala plays an important part in the pathogenesis of anxiety and generation of exteroceptive fear, recent discoveries have challenged the directionality of this brain-behavior relationship with respect to interoceptive fear. Here we highlight several paradoxical findings including: (1) amygdala lesion patients who experience excessive fear and panic following inhalation of carbon dioxide (CO₂), (2) clinically anxious patients who have significantly smaller (rather than larger) amygdalae and a pronounced hypersensitivity toward CO₂, and (3) epilepsy patients who exhibit apnea immediately following stimulation of their amygdala yet have no awareness that their breathing has stopped. The above findings elucidate an entirely novel role for the amygdala in the induction of apnea and inhibition of CO₂-induced fear. Such a role is plausible given the strong inhibitory connections linking the central nucleus of the amygdala with respiratory and chemoreceptive centers in the brainstem. Based on this anatomical arrangement, we propose a model of Apnea-induced Anxiety (AiA) which predicts that recurring episodes of apnea are being unconsciously elicited by amygdala activation, resulting in transient spikes in CO₂ that provoke fear and anxiety, and lead to characteristic patterns of escape and avoidance behavior in patients spanning the spectrum of anxiety. If this new conception of AiA proves to be true, and activation of the amygdala can repeatedly trigger states of apnea outside of one's awareness, then it remains possible that the chronicity of anxiety disorders is being interoceptively driven by a chemoreceptive system struggling to maintain homeostasis in the midst of these breathless states.

Behavioral and brain mechanisms mediating conditioned flight behavior in rats

Environmental contexts can inform animals of potential threats, though it is currently unknown how context biases the selection of defensive behavior. Here we investigated context-dependent flight responses with a Pavlovian serial-compound stimulus (SCS) paradigm that evokes freeze-to-flight transitions. Similar to previous work in mice, we show that male and female rats display context-dependent flight-like behavior in the SCS paradigm. Flight behavior was dependent on contextual fear insofar as it was only evoked in a shock-associated context and was reduced in the conditioning context after context extinction. Flight behavior was only expressed to white noise regardless of temporal order within the compound. Nonetheless, rats that received unpaired SCS trials did not show flight-like behavior to the SCS, indicating it is associative. Finally, we show that pharmacological inactivation of two brain regions critical to the expression of contextual fear, the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST), attenuates both contextual fear and flight responses. All of these effects were similar in male and female rats. This work demonstrates that contextual fear can summate with cued and innate fear to drive a high fear state and transition from post-encounter to circa-strike defensive modes.

NMDA receptors in the CeA and BNST differentially regulate fear conditioning to predictable and unpredictable threats

Considerable work demonstrates that Pavlovian fear conditioning depends on N-methyl-D-aspartate (NMDA) receptor-dependent plasticity within the amygdala. In addition, the bed nucleus of the stria terminalis (BNST) has also been implicated in fear conditioning, particularly in the expression of fear to poor predictors of threat. We recently found that the expression of backward (BW) fear conditioning, in which an auditory conditioned stimulus (CS) follows a footshock unconditioned stimulus (US), requires the BNST; the expression of forward (FW) fear conditioning was not disrupted by BNST inactivation. However, whether NMDA receptors within the BNST contribute to the acquisition of fear conditioning is unknown. Moreover, the central nucleus of the amygdala (CeA), which has extensive connections with the BNST, is critically involved in FW conditioning, however whether it participates in BW conditioning has not been explored. Here we test the specific hypothesis that the CeA and the BNST mediate the acquisition of FW and BW fear conditioning, respectively. Adult female and male rats were randomly assigned to receive bilateral infusions of the NMDA receptor antagonist, D,L-2-amino-5-phosphonovalerate (APV), into the CeA or BNST prior to FW or BW fear conditioning. We found that intra-CeA APV impaired the acquisition of both FW and BW conditioning, whereas intra-BNST APV produced selective deficits in BW conditioning. Moreover, APV in the BNST significantly reduced contextual freezing, whereas CeA NMDA receptor antagonism impeded early but not long-lasting contextual fear. Collectively, these data reveal that CeA and BNST NMDA receptors have unique roles in fear conditioning.

Resting-state fMRI effective connectivity between the bed nucleus of the stria terminalis and amygdala nuclei

The bed nucleus of the stria terminalis (BNST) and the laterobasal nucleus (LB), centromedial nucleus (CM), and superficial nucleus (SF) of the amygdala form an interconnected dynamical system, whose combined activity mediates a variety of behavioral and autonomic responses in reaction to homeostatic challenges. Although previous research provided deeper insight into the structural and functional connections between these nuclei, studies investigating their resting-state functional magnetic resonance imaging (fMRI) connectivity were solely based on undirected connectivity measures. Here, we used high-quality data of 391 subjects from the Human Connectome Project to estimate the effective connectivity (EC) between the BNST, the LB, CM, and SF through spectral dynamic causal modeling, the relation of the EC estimates with age and sex as well as their stability over time. Our results reveal a time-stable asymmetric EC structure with positive EC between all amygdala nuclei, which strongly inhibited the BNST while the BNST exerted positive influence onto all amygdala nuclei. Simulation of the impulse response of the estimated system showed that this EC structure shapes partially antagonistic (out of phase) activity flow between the BNST and amygdala nuclei. Moreover, the BNST-LB and BNST-CM EC parameters were less negative in males. In conclusion, our data points toward partially separated information processing between BNST and amygdala nuclei in the resting-state.

Functional Connectivity within the Primate Extended Amygdala Is Heritable and Associated with Early-Life Anxious Temperament

Children with an extremely inhibited, anxious temperament (AT) are at increased risk for anxiety disorders and depression. Using a rhesus monkey model of early-life AT, we previously demonstrated that metabolism in the central extended amygdala (EAc), including the central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST), is associated with trait-like variation in AT. Here, we use fMRI to examine relationships between Ce-BST functional connectivity and AT in a large multigenerational family pedigree of rhesus monkeys (n = 170 females and 208 males). Results demonstrate that Ce-BST functional connectivity is heritable, accounts for a significant but modest portion of the variance in AT, and is coheritable with AT. Interestingly, Ce-BST functional connectivity and AT-related BST metabolism were not correlated and accounted for non-overlapping variance in AT. Exploratory analyses suggest that Ce-BST functional connectivity is associated with metabolism in the hypothalamus and periaqueductal gray. Together, these results suggest the importance of coordinated function within the EAc for determining individual differences in AT and metabolism in brain regions associated with its behavioral and neuroendocrine components.SIGNIFICANCE STATEMENT Anxiety disorders directly impact the lives of nearly one in five people, accounting for substantial worldwide suffering and disability. Here, we use a nonhuman primate model of anxious temperament (AT) to understand the neurobiology underlying the early-life risk to develop anxiety disorders. Leveraging the same kinds of neuroimaging measures routinely used in human studies, we demonstrate that coordinated activation between the central nucleus of the amygdala and the bed nucleus of the stria terminalis is correlated with, and coinherited with, early-life AT. Understanding how these central extended amygdala regions work together to produce extreme anxiety provides a neural target for early-life interventions with the promise of preventing lifelong disability in at-risk children.

Optogenetic Study of Anterior BNST and Basomedial Amygdala Projections to the Ventromedial Hypothalamus

The basomedial amygdala (BM) influences the ventromedial nucleus of the hypothalamus (VMH) through direct glutamatergic projections as well as indirectly, through the anterior part of the bed nucleus of the stria terminalis (BNSTa). However, BM and BNSTa axons end in a segregated fashion in VMH. BM projects to the core of VMH, where VMH’s projection cells are located, whereas BNSTa projects to the shell of VMH, where GABAergic cells that inhibit core neurons are concentrated. However, the consequences of this dual regulation of VMH by BM and BNSTa are unknown. To study this question, we recorded the responses of VMH’s shell and core neurons to the optogenetic activation of BM or BNSTa inputs in transgenic mice that selectively express Cre-recombinase in glutamatergic or GABAergic neurons. Glutamatergic BM inputs fired most core neurons but elicited no response in GABAergic shell neurons. Following BM infusions of AAV-EF1α-DIO-hChR2-mCherry in Vgat-ires-Cre-Ai6 mice, no anterograde labeling was observed in the VMH, suggesting that GABAergic BM neurons do not project to the VMH. In contrast, BNSTa sent mostly GABAergic projections that inhibited both shell and core neurons. However, BNSTa-evoked IPSPs had a higher amplitude in shell neurons. Since we also found that activation of GABAergic shell neurons causes an inhibition of core neurons, these results suggest that depending on the firing rate of shell neurons, BNSTa inputs could elicit a net inhibition or disinhibition of core neurons. Thus, the dual regulation of VMH by BM and BNSTa imparts flexibility to this regulator of defensive and social behaviors.

Amygdala-stimulation-induced apnea is attention and nasal-breathing dependent

OBJECTIVE

Evidence suggests that disordered breathing is critically involved in Sudden Unexpected Death in Epilepsy (SUDEP). To that end, evaluating structures that are activated by seizures and can activate brain regions that produce cardiorespiratory changes can further our understanding of the pathophysiology of SUDEP. Past preclinical studies have shown that electrical stimulation of the human amygdala induces apnea, suggesting a role for the amygdala in controlling respiration. In this study, we aimed to both confirm these findings in a larger group of patients with intractable temporal lobe epilepsy (TLE) and also further explore the anatomical and cognitive properties of this effect.

METHODS

Seven surgical TLE patients had depth electrodes implanted in the amygdala that were used to deliver electrical stimulation during functional mapping preceding resection. Real-time respiratory monitoring was performed in each patient to confirm apnea.

RESULTS

Our data confirm that amygdala stimulation reliably induces apnea (occurring in all 7 patients) and further suggest that apnea can be overcome by instructing the patient to inhale, and can be prevented entirely by breathing through the mouth before electrical stimulation. Finally, stimulation-induced apnea occurred only when stimulating the medial-most amygdalar contacts located in the central nucleus.

INTERPRETATION

These findings confirm a functional connection between the amygdala and respiratory control in humans. Moreover, they suggest specific amygdalar nuclei may be critical in mediating this effect and that attentional state is critical to apnea mediated by amygdala activation-perhaps alluding to future development of strategies for the prevention of SUDEP. Ann Neurol 2018;83:460-471.

Role of the bed nucleus of the stria terminalis in aversive learning and memory

Surviving threats in the environment requires brain circuits for detecting (or anticipating) danger and for coordinating appropriate defensive responses (e.g., increased cardiac output, stress hormone release, and freezing behavior). The bed nucleus of the stria terminalis (BNST) is a critical interface between the "affective forebrain"-including the amygdala, ventral hippocampus, and medial prefrontal cortex-and the hypothalamic and brainstem areas that have been implicated in neuroendocrine, autonomic, and behavioral responses to actual or anticipated threats. However, the precise contribution of the BNST to defensive behavior is unclear, both in terms of the antecedent stimuli that mobilize BNST activity and the consequent defensive reactions. For example, it is well known that the BNST is essential for contextual fear conditioning, but dispensable for fear conditioning to discrete conditioned stimuli (CSs), at least as indexed by freezing behavior. However, recent evidence suggests that there are circumstances in which contextual freezing may persist independent of the BNST. Furthermore, the BNST is involved in the reinstatement (or relapse) of conditioned freezing to extinguished discrete CSs. As such, there are critical gaps in understanding how the BNST contributes to fundamental processes involved in Pavlovian fear conditioning. Here, we attempt to provide an integrative account of BNST function in fear conditioning. We discuss distinctions between unconditioned stress and conditioned fear and the role of BNST circuits in organizing behaviors associated with these states. We propose that the BNST mediates conditioned defensive responses-not based on the modality or duration of the antecedent threat or the duration of the behavioral response to the threat-but rather as consequence the ability of an antecedent stimulus to predict when an aversive outcome will occur (i.e., its temporal predictability). We argue that the BNST is not uniquely mobilized by sustained threats or uniquely involved in organizing sustained fear responses. In contrast, we argue that the BNST is involved in organizing fear responses to stimuli that poorly predict when danger will occur, no matter the duration, modality, or complexity of those stimuli. The concepts discussed in this review are critical to understanding the contribution of the human BNST to fear and anxiety disorders.

Heightened extended amygdala metabolism following threat characterizes the early phenotypic risk to develop anxiety-related psychopathology

Children with an anxious temperament are prone to heightened shyness and behavioral inhibition (BI). When chronic and extreme, this anxious, inhibited phenotype is an important early-life risk factor for the development of anxiety disorders, depression and co-morbid substance abuse. Individuals with extreme anxious temperament often show persistent distress in the absence of immediate threat and this contextually inappropriate anxiety predicts future symptom development. Despite its clear clinical relevance, the neural circuitry governing the maladaptive persistence of anxiety remains unclear. Here, we used a well-established nonhuman primate model of childhood temperament and high-resolution ¹⁸fluorodeoxyglucose positron emission tomography (FDG-PET) imaging to understand the neural systems governing persistent anxiety and to clarify their relevance to early-life phenotypic risk. We focused on BI, a core component of anxious temperament, because it affords the moment-by-moment temporal resolution needed to assess contextually appropriate and inappropriate anxiety. From a pool of 109 peri-adolescent rhesus monkeys, we formed groups characterized by high or low levels of BI, as indexed by freezing in response to an unfamiliar human intruder's profile. The high-BI group showed consistently elevated signs of anxiety and wariness across >2 years of assessments. At the time of brain imaging, 1.5 years after initial phenotyping, the high-BI group showed persistently elevated freezing during a 30-min 'recovery' period following an encounter with the intruder-more than an order of magnitude greater than the low-BI group-and this was associated with increased metabolism in the bed nucleus of the stria terminalis, a key component of the central extended amygdala. These observations provide a neurobiological framework for understanding the early phenotypic risk to develop anxiety-related psychopathology, for accelerating the development of improved interventions, and for understanding the origins of childhood temperament.

Connectivity between the central nucleus of the amygdala and the bed nucleus of the stria terminalis in the non-human primate: neuronal tract tracing and developmental neuroimaging studies

The lateral division of the bed nucleus of the stria terminalis (BSTL) and central nucleus of the amygdala (Ce) form the two poles of the 'central extended amygdala', a theorized subcortical macrostructure important in threat-related processing. Our previous work in nonhuman primates, and humans, demonstrating strong resting fMRI connectivity between the Ce and BSTL regions, provides evidence for the integrated activity of these structures. To further understand the anatomical substrates that underlie this coordinated function, and to investigate the integrity of the central extended amygdala early in life, we examined the intrinsic connectivity between the Ce and BSTL in non-human primates using ex vivo neuronal tract tracing, and in vivo diffusion-weighted imaging and resting fMRI techniques. The tracing studies revealed that BSTL receives strong input from Ce; however, the reciprocal pathway is less robust, implying that the primate Ce is a major modulator of BSTL function. The sublenticular extended amygdala (SLEAc) is strongly and reciprocally connected to both Ce and BSTL, potentially allowing the SLEAc to modulate information flow between the two structures. Longitudinal early-life structural imaging in a separate cohort of monkeys revealed that extended amygdala white matter pathways are in place as early as 3 weeks of age. Interestingly, resting functional connectivity between Ce and BSTL regions increases in coherence from 3 to 7 weeks of age. Taken together, these findings demonstrate a time period during which information flow between Ce and BSTL undergoes postnatal developmental changes likely via direct Ce → BSTL and/or Ce ↔ SLEAc ↔ BSTL projections.

Dopamine D2 receptors gate generalization of conditioned threat responses through mTORC1 signaling in the extended amygdala

Overgeneralization of conditioned threat responses is a robust clinical marker of anxiety disorders. In overgeneralization, responses that are appropriate to threat-predicting cues are evoked by perceptually similar safety-predicting cues. Inappropriate learning of conditioned threat responses may thus form an etiological basis for anxiety disorders. The role of dopamine (DA) in memory encoding is well established. Indeed by signaling salience and valence, DA is thought to facilitate discriminative learning between stimuli representing safety or threat. However, the neuroanatomical and biochemical substrates through which DA modulates overgeneralization of threat responses remain poorly understood. Here we report that the modulation of DA D2 receptor (D2R) signaling bidirectionally regulates the consolidation of fear responses. While the blockade of D2R induces generalized threat responses, its stimulation facilitates discriminative learning between stimuli representing safety or threat. Moreover, we show that controlled threat generalization requires the coordinated activation of D2R in the bed nucleus of the stria terminalis and the central amygdala. Finally, we identify the mTORC1 cascade activation as an important molecular event by which D2R mediates its effects. These data reveal that D2R signaling in the extended amygdala constitutes an important checkpoint through which DA participates in the control of threat processing and the emergence of overgeneralized threat responses.

Tools for resolving functional activity and connectivity within intact neural circuits

Mammalian neural circuits are sophisticated biological systems that choreograph behavioral processes vital for survival. While the inherent complexity of discrete neural circuits has proven difficult to decipher, many parallel methodological developments promise to help delineate the function and connectivity of molecularly defined neural circuits. Here, we review recent technological advances designed to precisely monitor and manipulate neural circuit activity. We propose a holistic, multifaceted approach for unraveling how behavioral states are manifested through the cooperative interactions between discrete neurocircuit elements.

Paternal deprivation affects the functional maturation of corticotropin-releasing hormone (CRH)- and calbindin-D28k-expressing neurons in the bed nucleus of the stria terminalis (BNST) of the biparental Octodon degus

While the critical role of maternal care on the development of brain and behavior of the offspring has been extensively studied, our knowledge about the importance of paternal care for brain development of his offspring is still comparatively scarce. The aim of this study in the biparental caviomorph rodent Octodon degus was to analyze the impact of paternal care on the development of corticotropin-releasing hormone (CRH)-expressing neurons in the bed nucleus of the stria terminalis (BNST) and hypothalamic paraventricular nucleus (PVN). Both brain areas are key players in neuronal circuits that regulate hypothalamic–pituitary–adrenal axis (HPA) activity. At the age of postnatal day (PND) 21, we found that paternal deprivation resulted in a decreased density of CRH-containing neurons in the medial, but not in the lateral BNST, whereas no changes were observed in the PVN. These deprivation-induced changes were still prominent in adulthood. At PND 21, the density of Ca-binding protein calbindin D28K (CaBP-D28K)-expressing neurons was specifically increased in the medial, but not lateral BNST of father-deprived animals. In contrast, adult father-deprived animals show significantly decreased density of CaBP-D28K-expressing neurons in the lateral, but not medial BNST. Taken together, these results may have important implications for our understanding of the experience-driven development of neural circuits that regulate HPA activity mediating acute responses to stress and chronic anxiety.

Distinct extended amygdala circuits for divergent motivational states

The comorbidity of anxiety and dysfunctional reward processing in illnesses such as addiction¹ and depression² suggests that common neural circuitry contributes to these disparate neuropsychiatric symptoms. The extended amygdala, including the bed nucleus of the stria terminalis (BNST), modulates fear and anxiety^3,4, but also projects to the ventral tegmental area (VTA) ^5,6, a region implicated in reward and aversion^7–13, thus providing a candidate neural substrate for integrating diverse emotional states. However, the precise functional connectivity between distinct BNST projection neurons and their postsynaptic targets in the VTA, as well as the role of this circuit in controlling motivational states have not been described. Here, we recorded and manipulated the activity of genetically and neurochemically identified VTA-projecting BNST neurons in freely behaving mice. Collectively, aversive stimuli exposure produced heterogeneous firing patterns in VTA-projecting BNST neurons. In contrast, in vivo optically-identified glutamatergic projection neurons displayed a net enhancement of activity to aversive stimuli, whereas the firing rate of identified GABAergic projection neurons was suppressed. Channelrhodopsin-2 (ChR2) assisted circuit mapping revealed that both BNST glutamatergic and GABAergic projections preferentially innervate postsynaptic non-dopaminergic VTA neurons, thus providing a mechanistic framework for in vivo circuit perturbations. In vivo photostimulation of BNST glutamatergic projections resulted in aversive and anxiogenic behavioral phenotypes. In contrast, activation of BNST GABAergic projections produced rewarding and anxiolytic phenotypes, which were also recapitulated by direct inhibition of VTA GABAergic neurons. These data demonstrate that functionally opposing BNST to VTA circuits regulate rewarding and aversive motivational states and may serve as a critical circuit node for bidirectionally normalizing maladaptive behaviors.

Morphology, PKCδ expression, and synaptic responsiveness of different types of rat central lateral amygdala neurons

Recent findings implicate the central lateral amygdala (CeL) in conditioned fear. Indeed, CeL contains neurons exhibiting positive (CeL-On) or negative (CeL-Off) responses to fear-inducing conditioned stimuli (CSs). In mice, these cells differ in their expression of protein kinase Cδ (PKCδ) and physiological properties. CeL-Off cells are PKCδ(+) and late firing (LF), whereas CeL-On cells are PKCδ(-) and express a regular-spiking (RS) or low-threshold bursting (LTB) phenotype. However, the scarcity of LF cells in rats raises questions about the correspondence between the organization of CeL in mice and rats. Therefore, we studied the PKCδ expression, morphological properties, synaptic responsiveness, and fear conditioning-induced plasticity of rat CeL neurons. No PKCδ(+) LF cells were encountered, but ≈20-25% of RS and LTB neurons were PKCδ(+). Compared with RS neurons, a higher proportion of LTB cells projected to central medial amygdala (CeM) and they had fewer primary dendritic branches, yet the amplitude of excitatory postsynaptic potentials (EPSPs) evoked by lateral amygdala (LA) stimulation was similar in RS and LTB cells. In contrast, LA-evoked inhibitory postsynaptic potentials (IPSPs) had a higher amplitude in LTB than RS neurons. Finally, fear conditioning did not induce plasticity at LA inputs to RS or LTB neurons. These findings point to major species differences in the organization of CeL. Since rat LTB cells are subjected to stronger feedforward inhibition, they are more likely to exhibit inhibitory CS responses than RS cells. This is expected to cause a disinhibition of CeM fear output neurons and therefore an increase in fear expression.

Evidence for coordinated functional activity within the extended amygdala of non-human and human primates

Neuroanatomists posit that the central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST) comprise two major nodes of a macrostructural forebrain entity termed the extended amygdala. The extended amygdala is thought to play a critical role in adaptive motivational behavior and is implicated in the pathophysiology of maladaptive fear and anxiety. Resting functional connectivity of the Ce was examined in 107 young anesthetized rhesus monkeys and 105 young humans using standard resting-state functional magnetic resonance imaging (fMRI) methods to assess temporal correlations across the brain. The data expand the neuroanatomical concept of the extended amygdala by finding, in both species, highly significant functional coupling between the Ce and the BST. These results support the use of in vivo functional imaging methods in nonhuman and human primates to probe the functional anatomy of major brain networks such as the extended amygdala.

Common and distinct neural inputs to the medial central nucleus of the amygdala and anterior ventrolateral bed nucleus of stria terminalis in rats

The central nucleus of the amygdala (CEA) and lateral bed nucleus of stria terminalis (BST) are highly interconnected limbic forebrain regions that share similar connectivity with other brain regions that coordinate behavioral and physiological responses to internal and environmental stressors. Their similar connectivity is frequently referred to when describing the CEA and lateral BST together as a unified "central extended amygdala". However, the CEA and BST reportedly play distinct roles in behavioral and physiological responses associated with fear, anxiety, and social defeat, presumably due to differences in connectivity. To identify common and unique sources of input to the CEA and lateral BST, we performed dual retrograde tracing. Fluorogold and cholera toxin β were iontophoresed into the medial CEA (CEAm) and the anterior ventrolateral BST (BSTvl) of adult male rats. The anatomical distribution of tracer-labeled neurons was mapped throughout the brain. Regions with overlapping populations of CEAm- and BSTvl-projecting neurons were further examined for the presence of double-labeled neurons. Although most regions with input to the mCEA also projected to the BSTvl, and vice versa, cortical and sensory system-related regions projected more robustly to the CEAm, while motor system-related regions primarily innervated the BSTvl. The incidence of double-labeled neurons with collateralized axonal inputs to the CEAm and BSTvl was relatively small (~2 to 13%) and varied across regions, suggesting regional differences in the degree of coordinated CEAm and BSTvl input. The demonstrated similarities and differences in inputs to CEAm and BSTvl provide new anatomical insights into the functional organization of these limbic forebrain regions.

The bed nucleus of the stria terminalis mediates inter-individual variations in anxiety and fear

While learning to fear stimuli that predict danger promotes survival, the inability to inhibit fear to inappropriate cues leads to a pernicious cycle of avoidance behaviors. Previous studies have revealed large inter-individual variations in fear responding with clinically anxious humans exhibiting a tendency to generalize learned fear to safe stimuli or situations. To shed light on the origin of these inter-individual variations, we subjected rats to a differential auditory fear conditioning paradigm in which one conditioned auditory stimulus (CS+) was paired to footshocks whereas a second (CS-) was not. We compared the behavior of rats that received pretraining excitotoxic lesions of the bed nucleus of the stria terminalis (BNST) to that of sham rats. Sham rats exhibit a continuum of anxious/fearful behaviors. At one end of the continuum were rats that displayed a poor ability to discriminate between the CS+ and CS-, high contextual freezing, and an anxiety-like trait in the elevated plus maze (EPM). At the other end were rats that display less fear generalization to the CS-, lower freezing to context, and a nonanxious trait in the EPM. Although BNST-lesioned rats acquired similarly high levels of conditioned fear to the CS+, they froze less than sham rats to the CS-. In fact, BNST-lesioned rats behaved like sham rats with high discriminative abilities in that they exhibited low contextual fear and a nonanxious phenotype in the EPM. Overall, this suggests that inter-individual variations in fear generalization and anxiety phenotype are determined by BNST influences on the amygdala and/or its targets.