Activation of the septohippocampal system differentiates anxiety from fear in startle paradigms

Response Flexibility: The Role of the Lateral Habenula

The ability to make appropriate decisions that result in an optimal outcome is critical for survival. This process involves assessing the environment as well as integrating prior knowledge about the environment with information about one’s current internal state. There are many neural structures that play critical roles in mediating these processes, but it is not yet known how such information coalesces to influence behavioral output. The lateral habenula (LHb) has often been cited as a structure critical for adaptive and flexible responding when environmental contexts and internal state changes. A challenge, however, has been understanding how LHb promotes response flexibility. In this review, we hypothesize that the LHb enables flexible responding following the integration of context memory and internal state information by signaling downstream brainstem structures known to drive hippocampal theta. In this way, animals respond more flexibly in a task situation not because the LHb selects a particular action, but rather because LHb enhances a hippocampal neural state that is often associated with greater attention, arousal, and exploration. In freely navigating animals, these are essential conditions that are needed to discover and implement appropriate alternative choices and behaviors. As a corollary to our hypothesis, we describe short- and intermediate-term functions of the LHb. Finally, we discuss the effects on the behavior of LHb dysfunction in short- and intermediate-timescales, and then suggest that new therapies may act on the LHb to alleviate the behavioral impairments following long-term LHb disruption.

Dysregulation of the Lateral Habenula in Major Depressive Disorder

Clinical and preclinical evidence implicates hyperexcitability of the lateral habenula (LHb) in the development of psychiatric disorders including major depressive disorder (MDD). This discrete epithalamic nucleus acts as a relay hub linking forebrain limbic structures with midbrain aminergic centers. Central to reward processing, learning and goal directed behavior, the LHb has emerged as a critical regulator of the behaviors that are impaired in depression. Stress-induced activation of the LHb produces depressive- and anxiety-like behaviors, anhedonia and aversion in preclinical studies. Moreover, deep brain stimulation of the LHb in humans has been shown to alleviate chronic unremitting depression in treatment resistant depression. The diverse neurochemical processes arising in the LHb that underscore the emergence and treatment of MDD are considered in this review, including recent optogenetic studies that probe the anatomical connections of the LHb.

Robust age, but limited sex, differences in mu-opioid receptors in the rat brain: relevance for reward and drug-seeking behaviors in juveniles

In the brain, the µ-opioid receptor (MOR) is involved in reward-seeking behaviors and plays a pivotal role in the mediation of opioid use disorders. Furthermore, reward-seeking behaviors and susceptibility to opioid addiction are particularly evident during the juvenile period, with a higher incidence of opioid use in males and higher sensitivity to opioids in females. Despite these age and sex differences in MOR-mediated behaviors, little is known regarding potential age and sex differences in the expression of MORs in the brain. Here, we used receptor autoradiography to compare MOR binding densities between juvenile and adult male and female rats. Age differences were found in MOR binding density in 12 out of 33 brain regions analyzed, with 11 regions showing higher MOR binding density in juveniles than in adults. These include the lateral septum, as well as sub-regions of the bed nucleus of the stria terminalis, hippocampus, and thalamus. Sex differences in MOR binding density were observed in only two brain regions, namely, the lateral septum (higher in males) and the posterior cortical nucleus of the amygdala (higher in females). Overall, these findings provide an important foundation for the generation of hypotheses regarding differential functional roles of MOR activation in juveniles versus adults. Specifically, we discuss the possibility that higher MOR binding densities in juveniles may allow for higher MOR activation, which could facilitate behaviors that are heightened during the juvenile period, such as reward and drug-seeking behaviors.

Tracking the fear memory engram: discrete populations of neurons within amygdala, hypothalamus, and lateral septum are specifically activated by auditory fear conditioning

Memory formation is thought to occur via enhanced synaptic connectivity between populations of neurons in the brain. However, it has been difficult to localize and identify the neurons that are directly involved in the formation of any specific memory. We have previously used fos-tau-lacZ (FTL) transgenic mice to identify discrete populations of neurons in amygdala and hypothalamus, which were specifically activated by fear conditioning to a context. Here we have examined neuronal activation due to fear conditioning to a more specific auditory cue. Discrete populations of learning-specific neurons were identified in only a small number of locations in the brain, including those previously found to be activated in amygdala and hypothalamus by context fear conditioning. These populations, each containing only a relatively small number of neurons, may be directly involved in fear learning and memory.

Effects of lorazepam and WAY-200070 in larval zebrafish light/dark choice test

Zebrafish larvae spend more time in brightly illuminated area when placed in a light/dark testing environment. Here we report that the anxiolytic drugs lorazepam and diazepam increased the time larval fish spent in the dark compartment in the light/dark test. Lorazepam did not affect the visual induced optokinetic response of larval fish. Gene expression levels of c-fos and crh were significantly increased in the hypothalamus of fish larvae underwent light/dark choice behavior, whilst lorazepam treatment alleviated the increased c-fos and crh expressions. Furthermore, we found estrogen receptor β gene expression level was increased in fish larvae underwent light/dark choice. We next examined effects of estrogen receptor modulators (estradiol, BPA, PHTPP, and WAY-200070) in the light/dark test. We identified WAY-200070, a highly selective ERβ agonist significantly altered the light/dark choice behavior of zebrafish larvae. Further investigation showed WAY-200070 treatment caused a reduction of crh expression level in the hypothalamus, suggesting activation of ERβ signaling attenuate the stress response. Interestingly, WAY-200070 treatment caused marked increase of c-fos expression in the habenula of fish larvae underwent behavior test. These results suggest WAY-200070 activation of ERβ mediated signaling may regulate anxiety related behavior in zebrafish through modulation of neuronal activity in habenula.

In a rat model of panic, corticotropin responses to dorsal periaqueductal gray stimulation depend on physical exertion

Panic disorder patients are exquisitely and specifically sensitive to hypercapnia. The demonstration that carbon dioxide provokes panic in fear-unresponsive amygdala-calcified Urbach-Wiethe patients emphasizes that panic is not fear nor does it require the activation of the amygdala. This is consonant with increasing evidence suggesting that panic is mediated caudally at midbrain's dorsal periaqueductal gray matter (DPAG). Another startling feature of the apparently spontaneous clinical panic is the counterintuitive lack of increments in corticotropin, cortisol and prolactin, generally considered 'stress hormones'. Here we show that the stress hormones are not changed during DPAG-evoked panic when escape is prevented by stimulating the rat in a small compartment. Neither did the corticotropin increase when physical exertion was statistically adjusted to the same degree as non-stimulated controls, as measured by lactate plasma levels. Conversely, neuroendocrine responses to foot-shocks were independent from muscular effort. Data are consonant with DPAG mediation of panic attacks.

Corticotropin-releasing factor modulation of forebrain GABAergic transmission has a pivotal role in the expression of anabolic steroid-induced anxiety in the female mouse

Increased anxiety is commonly observed in individuals who illicitly administer anabolic androgenic steroids (AAS). Behavioral effects of steroid abuse have become an increasing concern in adults and adolescents of both sexes. The dorsolateral bed nucleus of the stria terminalis (dlBnST) has a critical role in the expression of diffuse anxiety and is a key site of action for the anxiogenic neuromodulator, corticotropin releasing factor (CRF). Here we demonstrate that chronic, but not acute, exposure of female mice during adolescence to AAS augments anxiety-like behaviors; effects that were blocked by central infusion of the CRF receptor type 1 antagonist, antalarmin. AAS treatment selectively increased action potential (AP) firing in neurons of the central amygdala (CeA) that project to the dlBnST, increased the frequency of GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in dlBnST target neurons, and decreased both c-FOS immunoreactivity (IR) and AP frequency in these postsynaptic cells. Acute application of antalarmin abrogated the enhancement of GABAergic inhibition induced by chronic AAS exposure whereas application of CRF to brain slices of naïve mice mimicked the actions of this treatment. These results, in concert with previous data demonstrating that chronic AAS treatment results in enhanced levels of CRF mRNA in the CeA and increased CRF-IR in the dlBnST neuropil, are consistent with a mechanism in which the enhanced anxiety elicited by chronic AAS exposure involves augmented inhibitory activity of CeA afferents to the dlBnST and CRF-dependent enhancement of GABAergic inhibition in this brain region.

Light enhances learned fear

The ability to learn, remember, and respond to emotional events is a powerful survival strategy. However, dysregulated behavioral and physiological responses to these memories are maladaptive. To fully understand learned fear and the pathologies that arise during response malfunction we must reveal the environmental variables that influence learned fear responses. Light, a ubiquitous environmental feature, modulates cognition and anxiety. We hypothesized that light modulates responses to learned fear. Using tone-cued fear conditioning, we found that light enhances behavioral responses to learned fear in C57BL/6J mice. Mice in light freeze more in response to a conditioned cue than mice in darkness. The absence of significant freezing during a 2-wk habituation period and during intertrial intervals indicated that light specifically modulates freezing to the learned acoustic cue rather than the context of the experimental chamber. Repeating our assay in two photoreceptor mutant models, Pde6b(rd1/rd1) and Opn4(-/-) mice, revealed that light-dependent enhancement of conditioned fear is driven primarily by the rods and/or cones. By repeating our protocol with an altered lighting regimen, we found that lighting conditions acutely modulate responses when altered between conditioning and testing. This is manifested either as an enhancement of freezing when light is added during testing or as a depression of freezing when light is removed during testing. Acute enhancement, but not depression, requires both rod/cone- and melanopsin-dependent photoreception. Our results demonstrate a modulation by light of behavioral responses to learned fear.

Calcitonin gene-related peptide in the bed nucleus of the stria terminalis produces an anxiety-like pattern of behavior and increases neural activation in anxiety-related structures

Calcitonin gene-related peptide (CGRP) evokes anxiety-like responses when infused into the lateral ventricle of rats. Because the bed nucleus of the stria terminalis (BNST) lies immediately adjacent to the lateral ventricle, is rich in CGRP receptors, and has itself been implicated in anxiety, we evaluated the hypothesis that these effects are attributable to stimulation of CGRP receptors within the BNST itself. Bilateral intra-BNST, but not dorsal, CGRP infusions (0, 200, 400, 800 ng/side) enhanced startle amplitude in a dose-dependent manner, and produced an anxiety-like response on the elevated plus maze. Intra-BNST infusion of the CGRP antagonist, αCGRP(8-37), blocked the effect of CGRP on startle, and also blocked startle potentiation produced by exposure to trimethylthiazoline (a component of fox feces that induces anxiety-like behavior in rats). Intra-BNST, but not dorsal, CGRP infusions also increased c-Fos immunoreactivity in a number of anxiety-related brain areas (central nucleus of the amygdala, locus ceruleus, ventrolateral septal nucleus, paraventricular hypothalamic nucleus, lateral hypothalamus, lateral parabrachial nucleus, dorsal raphe nucleus, and nucleus accumbens shell), all of which receive direct projections from the BNST. Together, the results indicate that the activation of BNST CGRP receptors is both necessary and sufficient for some anxiety responses and that these effects may be mediated by activation of a wider network of BNST efferent structures. If so, inhibition of CGRP receptors may be a clinically useful strategy for anxiety reduction.

Pavlovian fear conditioning activates a common pattern of neurons in the lateral amygdala of individual brains

Understanding the physical encoding of a memory (the engram) is a fundamental question in neuroscience. Although it has been established that the lateral amygdala is a key site for encoding associative fear memory, it is currently unclear whether the spatial distribution of neurons encoding a given memory is random or stable. Here we used spatial principal components analysis to quantify the topography of activated neurons, in a select region of the lateral amygdala, from rat brains encoding a Pavlovian conditioned fear memory. Our results demonstrate a stable, spatially patterned organization of amygdala neurons are activated during the formation of a Pavlovian conditioned fear memory. We suggest that this stable neuronal assembly constitutes a spatial dimension of the engram.

Serotonin1A receptor deletion does not interact with maternal separation-induced increases in startle reactivity and prepulse inhibition deficits

RATIONALE

Early life stress is a risk factor for the development of psychopathology in later life. Consequences of adverse life events, however, may depend on the genetic makeup of an individual. Reduced serotonin(1A) receptor function may predispose to the development of anxiety disorders.

OBJECTIVE

Determine susceptibility of serotonin(1A) receptor knockout (1AKO) mice on different background strains to the effects of maternal separation (MS) by assessing startle plasticity in adulthood.

METHODS

1AKO mice on a 129S6 and a Swiss Webster (SW) background were used. MS groups were separated daily from their mother for 180 min/day from postnatal days 2 to 14. Control groups underwent normal animal facility rearing. In adulthood, effects on acoustic startle response, habituation, prepulse inhibition (PPI), and foot shock sensitization were determined.

RESULTS

MS increased startle reactivity and reduced PPI in 129S6 mice. These effects of MS were independent of genotype. MS had no effect on the other readouts. In SW mice, MS had no consistent effect on startle reactivity and did not alter startle plasticity in wild type or in 1AKO mice. 1AKO mice did not differ from wild-type mice in startle plasticity.

CONCLUSION

Serotonin(1A) receptor deletion does not enhance vulnerability to the effects of MS on startle plasticity. The life-long increase in startle reactivity and PPI deficit induced by MS are strain-dependent. Further, the use of startle reactivity and plasticity may have added value in translational studies relating to early life stress.

Cocaine-induced changes in affective state modulate the light-enhanced startle response

In order to evaluate the influence of changes in affective state on light-enhanced startle, the effects of positive affect, induced by acute cocaine administration, and the effect of negative affect, induced by spontaneous cocaine withdrawal-induced anxiety, were studied. Acute cocaine administration decreased LES, whereas withdrawal from chronic cocaine administration exacerbated LES 24h after withdrawal, an effect indicative of increased anxiety. This exacerbated LES was reduced, but not back to normal, 4 days after withdrawal. The finding that both cocaine-induced positive and negative affect can be detected in LES, suggests that this may be a valuable tool in studying affect regulation in rodents.

Fear-potentiated startle, but not light-enhanced startle, is enhanced by anxiogenic drugs

RATIONALE AND OBJECTIVES

The light-enhanced startle paradigm (LES) is suggested to model anxiety, because of the non-specific cue and the long-term effect. In contrast, the fear-potentiated startle (FPS) is suggested to model conditioned fear. However, the pharmacological profiles of these two paradigms are very similar. The present study investigated the effects of putative anxiogenic drugs on LES and FPS and aimed at determining the sensitivity of LES for anxiogenic drugs and to potentially showing a pharmacological differentiation between these two paradigms.

METHODS

Male Wistar rats received each dose of the alpha(2)-adrenoceptor antagonist yohimbine (0.25-1.0mg/kg), the 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP, 0.5-2.0mg/kg) or the GABA(A) inverse receptor agonist pentylenetetrazole (PTZ, 3-30mg/kg) and were subsequently tested in either LES or FPS.

RESULTS

None of the drugs enhanced LES, whereas mCPP increased percentage FPS and yohimbine increased absolute FPS values. Furthermore, yohimbine increased baseline startle amplitude in the LES, while mCPP suppressed baseline startle in both the LES and FPS and PTZ suppressed baseline startle in the FPS.

CONCLUSIONS

In contrast to findings in the FPS paradigm, none of the drugs were able to exacerbate the LES response. Thus, a clear pharmacological differentiation was found between LES and FPS.

Neurokinin-2 receptor antagonism in medial septum influences temporal-order memory for objects and forebrain cholinergic activity

In the mammalian brain the neurokinin NK(2) receptors are predominantly located in the hippocampus, thalamus, septum and frontal cortex. It has been shown that administration of the NK(2) receptor agonist, neurokinin A (NKA), into the medial septum of rats increases extracellular levels of acetylcholine (ACh) in the hippocampus and that NK(2) receptor antagonism blocks this increase. Therefore, given the prominent role of hippocampal ACh in information processing, we hypothesized that NK(2) receptor antagonism in the medial septum would negatively affect learning and memory via its influence on the cholinergic neurons of the basal forebrain. We investigated the action of local application of the peptidic NK(2) receptor antagonist, Bz-Ala-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH (1, 10 and 100pmol), into the medial septum on object memory for temporal order and spatial location using an object novelty paradigm. By means of in vivo microdialysis and HPLC analyses, we also examined the influence of NK(2) receptor antagonism in the medial septum on ACh in major cholinergic projection areas of the basal forebrain, namely, hippocampus, frontal cortex and amygdala.

RESULTS

Injection of vehicle alone into the medial septum impaired memory for temporal order and spatial location of objects. Application of 1pmol of the NK(2) receptor antagonist partially reversed this deficit by reinstating memory for temporal order. Injection of 10pmol of the NK(2) receptor antagonist into the medial septum decreased levels of ACh in the hippocampus (at 30min post-injection), and frontal cortex (at 30 and 80min post-injection) in comparison to vehicle. However, this apparent decrease was the result of the blockade of a saline-induced increase in ACh levels.

Disrupted startle modulation in animal models for affective disorders

Affective startle modulation is used to study emotional reactivity in humans, and blunted affective startle modulation has been reported in depressed patients. To determine whether blunted affective startle modulation is also a common feature in animal models for affective disorders, light-enhanced startle was studied in three models: inescapable foot shock (IFS), repeated restraint stress (RRS) and olfactory bulbectomy (OBX). In addition, prepulse inhibition was studied in these models. Light-enhanced startle was blunted following IFS and OBX and RRS decreased overall startle responding. Prepulse inhibition, however, was unaffected. These findings indicate that induction models for affective disorders may be associated with long term effects on affective startle modulation. The lack of changes in sensory motor gating suggests that these changes can be ascribed to alterations in emotional reactivity. In conclusion, our results indicate that the blunted affective startle modulation seen in animal models for affective disorders may be used to examine the mechanisms underlying altered emotional reactivity.

Medial amygdala lesions differentially influence stress responsivity and sensorimotor gating in rats

BACKGROUND

The amygdala is involved in the coordination of stress but is also an important gatekeeper involved in the regulation of vigilance. The amygdala is structurally complex, consisting of several nuclei with specific functions in the affective response to environmental stimuli. There are indications that the medial amygdaloid nucleus may be a pivotal player in acute responses to emotional environmental stimuli.

METHODS

The present study therefore aimed to study the effects of bilateral electrolytic lesions of the medial amygdala on unconditioned anxiety-related behavior as well as a sensorimotor gating parameter (prepulse inhibition, PPI) in rats. Anxiety-related behavior was assessed with the use of stress-induced hyperthermia (SIH), light-enhanced startle (LES) and open field behavior.

RESULTS

Bilateral electrolytic lesions of the medial amygdala decreased the SIH response and anxiety-related open field behavior. In contrast, lesioned animals displayed augmented LES and disrupted PPI. No changes in basal locomotor activity, body temperature and acoustic startle were found between lesioned and sham animals.

CONCLUSIONS

The present study suggests that the medial amygdala is an important player in response to acute environmental stimuli. Decreased unconditioned psychological stress responses were found, whereas LES was enhanced and sensorimotor processing was disrupted. However, considering the existing data on basolateral amygdala involvement in PPI and bed nucleus of the stria terminalis involvement in LES, local infusion studies into the MeA should be performed to further substantiate these findings.