Central, but not basolateral, amygdala involvement in the anxiolytic-like effects of midazolam in rats in the elevated plus maze

Stimulation of lateral parabrachial (LPB) to central amygdala (CeA) pituitary adenylate cyclase-activating polypeptide (PACAP) neurons induces anxiety-like behavior and mechanical allodynia

BACKGROUND

Anxiety disorders are the most prevalent psychiatric disorders, and they are highly comorbid with chronic pain conditions. The central nucleus of the amygdala (CeA) is known not only for its role in the regulation of anxiety but also as an important site for the negative affective dimension of pain. Pituitary adenylate cyclase activating polypeptide (PACAP), a neuropeptide whose terminals are abundant in the CeA, is strongly implicated in the stress response as well as in pain processing. Here, using Cre-dependent viral vectors, we explored in greater detail the role of the PACAP projection to the CeA that originates in the lateral parabrachial nucleus (LPB).

METHODS

We first performed a circuit mapping experiment by injecting an anterograde Cre-dependent virus expressing a fluorescent reporter in the LPB of PACAP-Cre mice and observing their projections. Then, we used a chemogenetic approach (a Cre-dependent Designer Receptors Activated by Designer Drugs, DREADDs) to assess the effects of the direct stimulation of the PACAP LPB to CeA projection on general locomotor activity, anxiety-like behavior (using a defensive withdrawal test), and mechanical pain sensitivity (using the von Frey test).

RESULTS

We found that the CeA, together with other areas, is one of the major downstream projection targets of PACAP neurons originating in the lateral parabrachial nucleus (LPB). In the DREADD experiment, we then found that the selective activation of this neuronal pathway is sufficient to increase both anxiety-like behavior and mechanical pain sensitivity in mice, without affecting general locomotor activity.

CONCLUSION

In conclusion, our data suggest that the dysregulation of this circuit may contribute to a variety of anxiety disorders and chronic pain states, and that PACAP may represent an important therapeutic target for the treatment of these conditions.

Anxiety-like Behavior and GABAAR/BDZ Binding Site Response to Progesterone Withdrawal in a Stress-Vulnerable Strain, the Wistar Kyoto Rats

Stress susceptibility could play a role in developing premenstrual anxiety due to abnormalities in the hypothalamus–pituitary–adrenal (HPA) axis and impairments in the GABAA receptors’ benzodiazepine (BDZ) site. Hence, we studied the stress-vulnerable Wistar Kyoto rat strain (WKY) to evaluate progesterone withdrawal (PW) effects on anxiety, HPA axis response, and to explore indicators of GABAA functionality in the BDZ site. For five days, ovariectomized WKY rats were administered 2.0 mg/kg of progesterone. Twenty-four hours after the last administration, rats were tested in the anxiety-like burying behavior test (BBT) or elevated plus maze test (EPM), and corticosterone was determined. [3H]Flunitrazepam binding autoradiography served as the BDZ binding site index of the GABAA receptor in amygdala nuclei and hippocampus’s dentate gyrus (DG). Finally, different doses of diazepam in PW-WKY rats were tested in the BBT. PW induced anxiety-like behaviors in both BBT and EPM compared with No-PW rats. PW increased corticosterone, but was blunted when combined with PW and BBT. PW increased [3H]Flunitrazepam binding in the DG and central amygdala compared with No-PW rats. Diazepam at a low dose induced an anxiogenic-like response in PW rats, suggesting a paradoxical response to benzodiazepines. Overall, PW induced anxiety-like behavior, a blunted HPA axis response, and higher GABAAR/BZD binding site sensitivity in a stress-vulnerable rat strain. These findings demonstrate the role of stress-susceptibility in GABAAR functionality in a preclinical approximation of PMDD.

Repeated vagus nerve stimulation produces anxiolytic effects via upregulation of AMPAR function in centrolateral amygdala of male rats

Repeated vagus nerve stimulation (rVNS) exerts anxiolytic effect by activation of noradrenergic pathway. Centrolateral amygdala (CeL), a lateral subdivision of central amygdala, receives noradrenergic inputs, and its neuronal activity is positively correlated to anxiolytic effect of benzodiazepines. The activation of β-adrenergic receptors (β-ARs) could enhance glutamatergic transmission in CeL. However, it is unclear whether the neurobiological mechanism of noradrenergic system in CeL mediates the anxiolytic effect induced by rVNS. Here, we find that rVNS treatment produces an anxiolytic effect in male rats by increasing the neuronal activity of CeL. Electrophysiology recording reveals that rVNS treatment enhances the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR)-mediated excitatory neurotransmission in CeL, which is mimicked by β-ARs agonist isoproterenol or blocked by β-ARs antagonist propranolol. Moreover, chemogenetic inhibition of CeL neurons or pharmacological inhibition of β-ARs in CeL intercepts both enhanced glutamatergic neurotransmission and the anxiolytic effects by rVNS treatment. These results suggest that the amplified AMPAR trafficking in CeL via activation of β-ARs is critical for the anxiolytic effects induced by rVNS treatment.

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rVNS amplifies the noradrenergic system in CeL and results in anxiolysis.

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rVNS treatment enhances AMPAR-mediated excitatory neurotransmission CeL via β-ARs.

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Pharmacological inhibition β-ARs in CeL intercept the anxiolytic effects by rVNS.

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Exciting CeL neurons lead to an increase in inhibitory inputs into CeM neurons.

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Inhibiting CeL neurons abate inhibitory inputs into CeM and anxiolysis by rVNS.

Looking for Novelty in an "Old" Receptor: Recent Advances Toward Our Understanding of GABAARs and Their Implications in Receptor Pharmacology

Diverse populations of GABA_A receptors (GABA_ARs) throughout the brain mediate fast inhibitory transmission and are modulated by various endogenous ligands and therapeutic drugs. Deficits in GABA_AR signaling underlie the pathophysiology behind neurological and neuropsychiatric disorders such as epilepsy, anxiety, and depression. Pharmacological intervention for these disorders relies on several drug classes that target GABA_ARs, such as benzodiazepines and more recently neurosteroids. It has been widely demonstrated that subunit composition and receptor stoichiometry impact the biophysical and pharmacological properties of GABA_ARs. However, current GABA_AR-targeting drugs have limited subunit selectivity and produce their therapeutic effects concomitantly with undesired side effects. Therefore, there is still a need to develop more selective GABA_AR pharmaceuticals, as well as evaluate the potential for developing next-generation drugs that can target accessory proteins associated with native GABA_ARs. In this review, we briefly discuss the effects of benzodiazepines and neurosteroids on GABA_ARs, their use as therapeutics, and some of the pitfalls associated with their adverse side effects. We also discuss recent advances toward understanding the structure, function, and pharmacology of GABA_ARs with a focus on benzodiazepines and neurosteroids, as well as newly identified transmembrane proteins that modulate GABA_ARs.

Central amygdala circuit dynamics underlying the benzodiazepine anxiolytic effect

Benzodiazepines (BZDs) have been a standard treatment for anxiety disorders for decades, but the neuronal circuit interactions mediating their anxiolytic effect remain largely unknown. Here, we find that systemic BZDs modulate central amygdala (CEA) microcircuit activity to gate amygdala output. Combining connectome data with immediate early gene (IEG) activation maps, we identified the CEA as a primary site for diazepam (DZP) anxiolytic action. Deep brain calcium imaging revealed that brain-wide DZP interactions shifted neuronal activity in CEA microcircuits. Chemogenetic silencing showed that PKCδ⁺/SST^- neurons in the lateral CEA (CEAl) are necessary and sufficient to induce the DZP anxiolytic effect. We propose that BZDs block the relay of aversive signals through the CEA, in part by local binding to CEAl SST⁺/PKCδ^- neurons and reshaping intra-CEA circuit dynamics. This work delineates a strategy to identify biomedically relevant circuit interactions of clinical drugs and highlights the critical role for CEA circuitry in the pathophysiology of anxiety.

Common and dissociable effects of oxytocin and lorazepam on the neurocircuitry of fear

Benzodiazepines (BZDs) represent the gold standard of anxiolytic pharmacotherapy; however, their clinical benefit is limited by side effects and addictive potential. Consequently, there is an urgent need to develop novel and safe anxiolytics. The peptide hormone oxytocin (OXT) exhibits anxiolytic-like properties in animals and humans, but whether OXT and BZDs share similar effects on the neural circuitry of fear is unclear. Therefore, the rationale of this ultra-high-field functional MRI (fMRI) study was to test OXT against the clinical comparator lorazepam (LZP) with regard to their neuromodulatory effects on local and network responses to fear-related stimuli. One hundred twenty-eight healthy male participants volunteered in this randomized double-blind, placebo-controlled, between-group study. Before scanning using an emotional face-matching paradigm, participants were randomly administered a single dose of OXT (24 IU), LZP (1 mg), or placebo. On the behavioral level, LZP, but not OXT, caused mild sedation, as evidenced by a 19% increase in reaction times. On the neural level, both OXT and LZP inhibited responses to fearful faces vs. neutral faces within the centromedial amygdala (cmA). In contrast, they had different effects on intra-amygdalar connectivity; OXT strengthened the coupling between the cmA and basolateral amygdala, whereas LZP increased the interplay between the cmA and superficial amygdala. Furthermore, OXT, but not LZP, enhanced the coupling between the cmA and the precuneus and dorsomedial prefrontal cortex. These data implicate inhibition of the cmA as a common denominator of anxiolytic action, with only OXT inducing large-scale connectivity changes of potential therapeutic relevance.

Effects of Repeated Treatment with Midazolam in SHR and SLA16 Rat Strains in the Triple Test

We exposed male and female rats of SHR (Spontaneously Hypertensive Rats) and SLA16 (SHR.LEW-Anxrr16) strains, in a non-drugged state, for five consecutive days to the Triple Test (experiment 1); or after repeated treatment with midazolam (MDZ), for four consecutive days. The fifth day was performed without treatment (experiment 2). The first experiment showed that males did not avoid and females increased the exploration of the open arms over the days. In experiment 2, SLA16 from both sexes approached more the open arms than SHR rats. The MDZ anxiolytic-like effect was sustained in both strains and sexes over the days. On the fifth day, SLA16 still approached more the open arms than SHR rats. Data suggest an absence of repeated-trial tolerance to MDZ anxiolytic-like effects. Testing the SHR and SLA16 strains, especially females, could be necessary for the future search for the genes and molecular pathways underlying anxiety/emotionality.

Anandamide reverses depressive-like behavior, neurochemical abnormalities and oxidative-stress parameters in streptozotocin-diabetic rats: Role of CB1 receptors

The pathophysiology associated with increased prevalence of depression in diabetics is not completely understood, although studies have pointed the endocannabinoid system as a possible target. Then, we aimed to investigate the role of this system in the pathophysiology of depression associated with diabetes. For this, diabetic (DBT) male Wistar rats were intraperitoneally treated with cannabinoid CB1 (AM251, 1mg/kg) or CB2 (AM630, 1mg/kg) receptor antagonists followed by anandamide (AEA, 0.005mg/kg) and then submitted to the forced swimming test (FST). Oxidative stress parameters, CB1 receptor expression and serotonin (5-HT) and noradrenaline levels in the hippocampus (HIP) and prefrontal cortex (PFC) were also performed. It was observed that DBT animals presented a more pronounced depressive-like behavior and increase of CB1 receptor expression in the HIP. AEA treatment induced a significant improvement in the depressive-like behavior, which was reversed by the CB1 antagonist AM251, without affecting the hyperglycemia or weight gain. AEA was also able to restore the elevated CB1 expression and also to elevate the reduced level of 5-HT in the HIP from DBT animals. In addition, AEA restored the elevated noradrenaline levels in the PFC and induced a neuroprotective effect by restoring the decreased reduced glutathione and increased lipid hydroperoxides levels along with the decreased superoxide dismutase activity observed in HIP or PFC. Together, our data suggest that in depression associated with diabetes, the endocannabinoid anandamide has a potential to induce neuroadaptative changes able to improve the depressive-like response by its action as a CB1 receptor agonist.

Pituitary adenylate cyclase-activating polypeptide (PACAP) in the central nucleus of the amygdala induces anxiety via melanocortin receptors

RATIONALE

Anxiety disorders are the most common mental disorders in the USA. Characterized by feelings of uncontrollable apprehension, they are accompanied by physical, affective, and behavioral symptoms. The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor PAC1 (PAC1R) are highly expressed in the central nucleus of the amygdala (CeA), and they have gained growing attention for their proposed role in mediating the body's response to stress.

OBJECTIVES

The aim of this study was to evaluate the anxiogenic effects of PACAP in the CeA and its effects on the hypothalamic-pituitary-adrenal (HPA) axis. Furthermore, the mechanism of action of PACAP in the CeA was investigated.

METHODS

PACAP was microinfused into the CeA of rats, and its effects in the elevated plus maze (EPM), the defensive withdrawal tests, and plasma corticosterone levels were evaluated. The ability of the melanocortin receptor antagonist SHU9119 to block PACAP effect in the EPM was assessed.

RESULTS

Intra-CeA PACAP exerted a dose-dependent anxiogenic effect and activated the HPA axis. In contrast, PACAP microinfused into the basolateral nucleus of the amygdala (BlA) had no effect. Finally, the anxiogenic effect of intra-CeA PACAP was prevented by SHU9119.

CONCLUSIONS

These data prove an anxiogenic role for the PACAP system of the CeA and reveal that the melanocortin receptor 4 (MC4R) system of CeA mediates these effects. Our data provide insights into this neuropeptide system as a mechanism for modulating the behavioral and endocrine response to stress and suggest that dysregulations of this system may contribute to the pathophysiology of anxiety-related disorders.

Ethanol and corticotropin releasing factor receptor modulation of central amygdala neurocircuitry: An update and future directions

The central amygdala is a critical brain region for many aspects of alcohol dependence. Much of the work examining the mechanisms by which the central amygdala mediates the development of alcohol dependence has focused on the interaction of acute and chronic ethanol with central amygdala corticotropin releasing factor signaling. This work has led to a great deal of success in furthering the general understanding of central amygdala neurocircuitry and its role in alcohol dependence. Much of this work has primarily focused on the hypothesis that ethanol utilizes endogenous corticotropin releasing factor signaling to upregulate inhibitory GABAergic transmission in the central amygdala. Work that is more recent suggests that corticotropin releasing factor also plays an important role in mediating anxiety-like behaviors via the enhancement of central amygdala glutamatergic transmission, implying that ethanol/corticotropin releasing factor interactions may modulate excitatory neurotransmission in this brain region. In addition, a number of studies utilizing optogenetic strategies or transgenic mouse lines have begun to examine specific central amygdala neurocircuit dynamics and neuronal subpopulations to better understand overall central amygdala neurocircuitry and the role of neuronal subtypes in mediating anxiety-like behaviors. This review will provide a brief update on this literature and describe some potential future directions that may be important for the development of better treatments for alcohol addiction.

Dorsal periaqueductal gray post-stimulation freezing is counteracted by neurokinin-1 receptor antagonism in the central nucleus of the amygdala in rats

Electrical stimulation of the dorsal periaqueductal gray (dPAG) in rats generates defensive responses that are characterized by freezing and escape behaviors, followed by post-stimulation freezing that resembles symptoms of panic attacks. dPAG post-stimulation freezing involves the processing of ascending aversive information to prosencephalic centers, including the amygdala, which allows the animal to evaluate the consequences of stressful situations. The basolateral nucleus of the amygdala (BLA) is thought to act as a filter for innate and learned aversive information that is transmitted to higher structures. The central (CeA) and medial (MeA) nuclei of the amygdala constitute an output for the expression of fear reactions through projections to limbic and brainstem regions. Neurokinin (NK) receptors are abundant in the CeA, MeA, and BLA, but their role in the expression of defensive responses and processing of aversive information that is evoked by electrical stimulation of the dPAG is still unclear. In the present study, we examined the role of NK1 receptors in these amygdala nuclei in the expression of defensive responses induced by electrical stimulation of the dPAG in rats and fear memory of this aversive stimulation. Rats were implanted with an electrode into the dPAG for electrical stimulation and one cannula in the CeA, MeA, or BLA for injections of vehicle (phosphate-buffered saline) or the NK1 receptor antagonist spantide (SPA; 100 pmol/0.2 μl). Injections of SPA into the CeA but not BLA or MeA reduced the duration of post-stimulation freezing evoked by electrical stimulation of the dPAG, without changing the aversive thresholds of freezing or escape. Twenty-four hours later, exploratory behavior was evaluated in the elevated plus maze test (EPM) in the CeA group of rats. Electrical stimulation of the dPAG rats that received vehicle exhibited higher aversion to the open arms of the EPM than sham rats that did not receive any dPAG stimulation. SPA injections into the CeA prevented the proaversive effects of electrical stimulation of the dPAG assessed in the EPM 24 h later. The present results suggest that neurokininergic modulation via NK1 receptors in the CeA but not BLA or MeA is involved in the processing of aversive information derived from dPAG stimulation. The long-lasting consequences of electrical stimulation of the dPAG may be prevented by NK1 receptor antagonism in the CeA.

Activation of 5-HT₁A receptors in the medial subdivision of the central nucleus of the amygdala produces anxiolytic effects in a rat model of Parkinson's disease

Although the medial subdivision of the central nucleus of the amygdala (CeM) and serotonin-1A (5-HT1A) receptors are involved in the regulation of anxiety, their roles in Parkinson's disease (PD)-associated anxiety are still unknown. Here we assessed the importance of CeM 5-HT1A receptors for anxiety in rats with unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle (MFB). The lesion induced anxiety-like behaviors, increased the firing rate and burst-firing pattern of CeM γ-aminobutyric acid (GABA) neurons, as well as decreased dopamine (DA) levels in the striatum, medial prefrontal cortex (mPFC), amygdala and ventral part of hippocampus (vHip). Intra-CeM injection of the selective 5-HT1A receptor agonist 8-OH-DPAT produced anxiolytic effects in the lesioned rats, and decreased the firing rate of CeM GABAergic neurons in two groups of rats. Compared to sham-operated rats, the duration of the inhibitory effect on the firing rate of GABAergic neurons was shortened in the lesioned rats. The injection increased DA levels in the mPFC and amygdala in two groups of rats and the vHip in the lesioned rats, and increased 5-HT level in the lesioned rats, whereas it decreased NA levels in the mPFC in two groups of rats and the vHip in the lesioned rats. Moreover, the mean density of 5-HT1A receptor and GABA double-labeled neurons in the CeM was reduced after the lesioning. These results suggest that activation of CeM 5-HT1A receptor produces anxiolytic effects in the 6-OHDA-lesioned rats, which involves decreased firing rate of the GABAergic neurons, and changed monoamine levels in the limbic and limbic-related brain regions.

Benzodiazepine site agonists differentially alter acetylcholine release in rat amygdala

BACKGROUND

Agonist binding at the benzodiazepine site of γ-aminobutric acid type A receptors diminishes anxiety and insomnia by actions in the amygdala. The neurochemical effects of benzodiazepine site agonists remain incompletely understood. Cholinergic neurotransmission modulates amygdala function, and this study tested the hypothesis that benzodiazepine site agonists alter acetylcholine (ACh) release in the amygdala.

METHODS

Microdialysis and high-performance liquid chromatography quantified ACh release in the amygdala of Sprague-Dawley rats (n = 33). ACh was measured before and after IV administration (3 mg/kg) of midazolam or eszopiclone, with and without anesthesia. ACh in isoflurane-anesthetized rats during dialysis with Ringer's solution (control) was compared with ACh release during dialysis with Ringer's solution containing (100 μM) midazolam, diazepam, eszopiclone, or zolpidem.

RESULTS

In unanesthetized rats, ACh in the amygdala was decreased by IV midazolam (-51.1%; P = 0.0029; 95% confidence interval [CI], -73.0% to -29.2%) and eszopiclone (-39.6%; P = 0.0222; 95% CI, -69.8% to -9.3%). In anesthetized rats, ACh in the amygdala was decreased by IV administration of midazolam (-46.2%; P = 0.0041; 95% CI, -67.9% to -24.5%) and eszopiclone (-34.0%; P = 0.0009; 95% CI, -44.7% to -23.3%), and increased by amygdala delivery of diazepam (43.2%; P = 0.0434; 95% CI, 2.1% to 84.3%) and eszopiclone (222.2%; P = 0.0159; 95% CI, 68.5% to 375.8%).

CONCLUSIONS

ACh release in the amygdala was decreased by IV delivery of midazolam and eszopiclone. Dialysis delivery directly into the amygdala caused either increased (eszopiclone and diazepam) or likely no significant change (midazolam and zolpidem) in ACh release. These contrasting effects of delivery route on ACh release support the interpretation that systemically administered midazolam and eszopiclone decrease ACh release in the amygdala by acting on neuronal systems outside the amygdala.

Distributed circuits underlying anxiety

Anxiety is of paramount importance for animals, as it allows assessment of the environment while minimizing exposure to potential threats. Furthermore, anxiety disorders are highly prevalent. Consequently, the neural circuitry underlying anxiety has been a topic of great interest. In this mini review, we will discuss current views on anxiety circuits. We will focus on rodent anxiety paradigms, but we will also consider results from human neuroimaging and clinical studies. We briefly review studies demonstrating the central role that the amygdala and the bed nucleus of the stria terminals (BNST) play in modulating anxiety and present evidence showing how the bed nucleus uses different output pathways to influence specific features of anxiolysis. Lastly, we propose that several brain regions, such as the medial prefrontal cortex (mPFC) and the ventral hippocampus (vHPC), act in a coordinated fashion with the amygdala and BNST, forming a distributed network of interconnected structures that control anxiety both in rodents and humans.

Participation of NK1 receptors of the amygdala on the processing of different types of fear

The amygdala, medial hypothalamus, dorsal periaqueductal gray (dPAG), superior and inferior colliculus together constitutes the encephalic aversion system which has been considered the main neural substrate for the integration of unconditioned aversive behavioral states. Within the amygdala the basolateral nucleus (BLA) is thought to act as a filter for innate and learned aversive information to higher structures, whereas the central nucleus (CeA) is considered the main output for the expression of fear reactions through projections to limbic and brainstem regions. Although neurokinin (NK) receptors are abundant in the amygdala, their role in the processing and expression of fear is yet unclear. In this study, we examined the role of SP/NK1 receptor system of the CeA and BLA on the expression of defensive responses elaborated by Wistar rats submitted to elevated plus maze (EPM) and to electrical stimulation (ES) of the dPAG. For EPM test, cannulae were implanted in the CeA and BLA for injections of substance P (SP - 10 and 100pmol/0.2μL) and spantide (SPA - 10, 100 and 500pmol/0.2μL). For ES of dPAG, aversive thresholds for freezing and escape responses as well as post-stimulation freezing (PSF) were measured in rats treated with PBS and SPA (100pmol/0.2μL) in CeA. Injections of SP into the CeA, but not the BLA, produced anxiogenic-like effects in the EPM test. SPA injected into the CeA had no effect on the exploratory behavior of rats submitted to the EPM but blocked the effects of SP. The duration of dPAG-PSF was also reduced significantly following injection of SPA in CeA but had no effect on thresholds for freezing and escape responses. The EPM gives the animal a control over its environment i.e. the option to choose or not to enter into the open arm and dPAG-PSF is thought to reflect a period when the animal evaluates the significance of dPAG-evoked aversion once the unconditioned responses of freezing and escape were elicited. The data indicate that SP may be involved in mediating responses of the animal in only certain types of aversive behavior and suggests a differential participation of the NK1 receptors in the processing of distinct types of fear in the amygdala.