Priming of experimental anxiety by repeated subthreshold GABA blockade in the rat amygdala

Identification of a novel perifornical-hypothalamic-area-projecting serotonergic system that inhibits innate panic and conditioned fear responses

The serotonin (5-HT) system is heavily implicated in the regulation of anxiety and trauma-related disorders such as panic disorder and post-traumatic stress disorder, respectively. However, the neural mechanisms of how serotonergic neurotransmission regulates innate panic and fear brain networks are poorly understood. Our earlier studies have identified that orexin (OX)/glutamate neurons within the perifornical hypothalamic area (PFA) play a critical role in adaptive and pathological panic and fear. While site-specific and electrophysiological studies have shown that intracranial injection and bath application of 5-HT inhibits PFA neurons via 5-HT1a receptors, they largely ignore circuit-specific neurotransmission and its physiological properties that occur in vivo. Here, we investigate the role of raphe nuclei 5-HT inputs into the PFA in panic and fear behaviors. We initially confirmed that photostimulation of glutamatergic neurons in the PFA of rats produces robust cardioexcitation and flight/aversive behaviors resembling panic-like responses. Using the retrograde tracer cholera toxin B, we determined that the PFA receives discrete innervation of serotonergic neurons clustered in the lateral wings of the dorsal (lwDRN) and in the median (MRN) raphe nuclei. Selective lesions of these serotonergic projections with saporin toxin resulted in similar panic-like responses during the suffocation-related CO2 challenge and increased freezing to fear-conditioning paradigm. Conversely, selective stimulation of serotonergic fibers in the PFA attenuated both flight/escape behaviors and cardioexcitation responses elicited by the CO2 challenge and induced conditioned place preference. The data here support the hypothesis that PFA projecting 5-HT neurons in the lwDRN/MRN represents a panic/fear-off circuit and may also play a role in reward behavior.

Dopaminergic and cholinergic modulation of the amygdala is altered in female mice with oestrogen receptor β deprivation

The amygdala is modulated by dopaminergic and cholinergic neurotransmission, and this modulation is altered in mood disorders. Therefore, this study was designed to evaluate the presence/absence of quantitative alterations in the expression of main dopaminergic and cholinergic markers in the amygdala of mice with oestrogen receptor β (ERβ) knock-out which exhibit increased anxiety, using immunohistochemistry and quantitative methods. Such alterations could either contribute to increased anxiety or be a compensatory mechanism for reducing anxiety. The results show that among dopaminergic markers, the expression of tyrosine hydroxylase (TH), dopamine transporter (DAT) and dopamine D₂-like receptor (DA₂) is significantly elevated in the amygdala of mice with ERβ deprivation when compared to matched controls, whereas the content of dopamine D₁-like receptor (DA₁) is not altered by ERβ knock-out. In the case of cholinergic markers, muscarinic acetylcholine type 1 receptor (AChR_M1) and alpha-7 nicotinic acetylcholine receptor (AChR_α7) display overexpression while the content of acetylcholinesterase (AChE) and vesicular acetylcholine transporter (VAChT) remains unchanged. In conclusion, in the amygdala of ERβ knock-out female the dopaminergic and cholinergic signalling is altered, however, to determine the exact role of ERβ in the anxiety-related behaviour further studies are required.

Neonatal inflammation via persistent TGF-β1 downregulation decreases GABAAR expression in basolateral amygdala leading to the imbalance of the local excitation-inhibition circuits and anxiety-like phenotype in adult mice

Neonatal inflammation can increase the risk of anxiety disorder in adulthood. The balance between glutamatergic excitatory and GABAergic inhibitory transmissions in the basolateral amygdala (BLA) plays a vital role in controlling anxiety state. Based on the reports that early-life inflammation had adverse effects on GABAergic system, the aim of this study was to investigate whether and how neonatal inflammation affects excitatory-inhibitory circuits in the BLA resulting in anxiety disorder. Neonatal mice received a daily subcutaneous injection of lipopolysaccharide (LPS, 50 μg/kg) or saline on postnatal days 3-5. LPS-treated mice developed anxiety behaviors accompanied by the hyperactivity of adrenal axis in adulthood. Electrophysiological study revealed the increase of postsynaptic neuronal excitability in the cortical-BLA excitatory synapses of LPS mice which could be recovered by bath-application of GABA_AR agonist suggesting the impairment of GABAergic system in LPS mice. Compared with controls, GABA_ARα2 subunit expression and density of GABA-evoked current in BLA principal neurons were reduced in LPS mice. Additionally, neonatal LPS treatment resulted in the down-regulation of transforming growth factor-beta 1 (TGF-β1) expression and PKC signaling pathway in the adult BLA. The local TGF-β1 overexpression in the BLA improved GABA_ARα2 expression via up-regulating the activity of PKC signaling, which corrected GABA_AR-mediated inhibition leading to the abolishment of anxiety-like change in adrenal axis regulation and behaviors in LPS mice. These data suggest the persistent TGF-β1deficit induces the down-regulation of GABA_ARα2 expression and subsequent disruption of the excitation-inhibition balance in the BLA circuits, which is the important mechanisms of neonatal inflammation-induced anxiety disorder.

Exposure of male mice to perfluorooctanoic acid induces anxiety-like behaviors by increasing corticotropin-releasing factor in the basolateral amygdala complex

Perfluorooctanoic acid (PFOA), a hazardous environmental pollutant, has been found to enhance hepatic synthesis of fibroblast growth factor 21 (FGF21). FGF21 can enter the brain and increase the expression of corticotropin-releasing factor (CRF) in the paraventricular nucleus (PVN). In this study, adult male mice were orally administered PFOA to evaluate how it regulates emotion. Exposure of mice to PFOA (1 mg kg-1 bw) for 10 consecutive days (PFOA-mice) caused anxiety-like behaviors and a peroxisome proliferator-activated receptor α (PPARα)-dependent increase in hepatic FGF21 synthesis. The levels of CRF expression in not only PVN but also basolateral amygdala complex (BLA) neurons of PFOA-mice were increased via FGF receptor 1 (FGF-R1) activation. However, the microinjection of FGF-R1 or CRF 1 receptor (CRF-R1) antagonist in the BLA rather than the PVN of PFOA-mice could relieve their anxiety-like behaviors. In addition, external capsule-BLA synaptic transmission in PFOA-mice was enhanced by increasing CRF-R1-mediated presynaptic glutamate release, which was corrected by the blockade of PPARα, FGF-R1 and CRF-R1 or the inhibition of PKA. Furthermore, the threshold of frequency-dependent long-term potentiation (LTP) induction was decreased in the BLA of PFOA-mice, which depended on the activation of PPARα, FGF-R1, CRF-R1, PKA and NMDA receptor (NMDAR), whereas long-term depression (LTD) induction was unchanged. Thus, the results indicate that the exposure of male mice to PFOA (1 mg kg-1 bw) enhances CRF expression in BLA neurons by increasing hepatic FGF21 synthesis, which then enhances CRF-R1-mediated presynaptic glutamate release to facilitate NMDAR-dependent BLA-LTP induction, leading to the production of anxiety-like behaviors.

Cannabinoid receptor type 1 in the bed nucleus of the stria terminalis modulates cardiovascular responses to stress via local N-methyl-D-aspartate receptor/neuronal nitric oxide synthase/soluble guanylate cyclase/protein kinase G signaling

BACKGROUND

Endocannabinoid neurotransmission in the bed nucleus of the stria terminalis is involved in the control of cardiovascular responses to stress. However, the local mechanisms involved is this regulation are not known.

AIMS

The purpose of this study was to assess an interaction of bed nucleus of the stria terminalis endocannabinoid neurotransmission with local nitrergic signaling, as well as to investigate the involvement of local N-methyl-D-aspartate glutamate receptor and nitric oxide signaling in the control of cardiovascular responses to acute restraint stress by bed nucleus of the stria terminalis endocannabinoid neurotransmission in rats.

METHODS

The first protocol evaluated the effect of intra-bed nucleus of the stria terminalis microinjection of the selective cannabinoid receptor type 1 receptor antagonist AM251 in nitrite/nitrate content in the bed nucleus of the stria terminalis following restraint stress. The other protocols evaluated the impact of local pretreatment with the selective N-methyl-D-aspartate glutamate receptor antagonist LY235959, the selective neuronal nitric oxide synthase inhibitor N^ω-propyl-L-arginine, the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, or the protein kinase G inhibitor KT5823 in restraint-evoked cardiovascular changes following bed nucleus of the stria terminalis treatment with AM251.

RESULTS

Bilateral microinjection of AM251 into the bed nucleus of the stria terminalis increased local nitric oxide release during restraint stress. Bed nucleus of the stria terminalis treatment with the cannabinoid receptor type 1 receptor antagonist also enhanced the tachycardia caused by restraint stress, but without affecting arterial pressure increase and sympathetic-mediated cutaneous vasoconstriction. The facilitation of restraint-evoked tachycardia following bed nucleus of the stria terminalis treatment with the cannabinoid receptor type 1 receptor antagonist was completely inhibited by local pretreatment with LY235959, N^ω-propyl-L-arginine, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, or KT5823.

CONCLUSIONS

Our results provide evidence that bed nucleus of the stria terminalis endocannabinoid neurotransmission inhibits local N-methyl-D-aspartate/neuronal nitric oxide synthase/soluble guanylate cyclase/protein kinase G signaling, and this mechanism is involved in the control of the cardiovascular responses to stress.

Using loss- and gain-of-function approaches to target amygdala-projecting serotonergic neurons in the dorsal raphe nucleus that enhance anxiety-related and conditioned fear behaviors

BACKGROUND

The central serotonergic system originating from the dorsal raphe nucleus (DR) plays a critical role in anxiety and trauma-related disorders such as posttraumatic stress disorder. Although many studies have investigated the role of serotonin (5-HT) within pro-fear brain regions such as the amygdala, the majority of these studies have utilized non-selective pharmacological approaches or poorly understood lesioning techniques which limit their interpretation.

AIM

Here we investigated the role of amygdala-projecting 5-HT neurons in the DR in innate anxiety and conditioned fear behaviors.

METHODS

To achieve this goal, we utilized (1) selective lesion of 5-HT neurons projecting to the amygdala with saporin toxin conjugated to anti-serotonin transporter (SERT) injected into the amygdala, and (2) optogenetic excitation of amygdala-projecting DR cell bodies with a combination of a retrogradely transported canine adenovirus-expressing Cre-recombinase injected into the amygdala and a Cre-dependent-channelrhodopsin injected into the DR.

RESULTS

While saporin treatment lesioned both local amygdalar 5-HT fibers and neurons in the DR as well as reduced conditioned fear behavior, optical activation of amygdala-projecting DR neurons enhanced anxious behavior and conditioned fear response.

CONCLUSION

Collectively, these studies support the hypothesis that amygdala-projecting 5-HT neurons in the DR represent an anxiety and fear-on network.

The Impact of Stressor Exposure and Glucocorticoids on Anxiety and Fear

Anxiety disorders and trauma- and stressor-related disorders, such as posttraumatic stress disorder (PTSD), are common and are associated with significant economic and social burdens. Although trauma and stressor exposure are recognized as a risk factors for development of anxiety disorders and trauma or stressor exposure is recognized as essential for diagnosis of PTSD, the mechanisms through which trauma and stressor exposure lead to these disorders are not well characterized. An improved understanding of the mechanisms through which trauma or stressor exposure leads to development and persistence of anxiety disorders or PTSD may result in novel therapeutic approaches for the treatment of these disorders. Here, we review the current state-of-the-art theories, with respect to mechanisms through which stressor exposure leads to acute or chronic exaggeration of avoidance or anxiety-like defensive behavioral responses and fear, endophenotypes in both anxiety disorders and trauma- and stressor-related psychiatric disorders. In this chapter, we will explore physiological responses and neural circuits involved in the development of acute and chronic exaggeration of anxiety-like defensive behavioral responses and fear states, focusing on the role of the hypothalamic-pituitary-adrenal (HPA) axis and glucocorticoid hormones.

The GAD65 knock out mouse - a model for GABAergic processes in fear- and stress-induced psychopathology

The γ-amino butyric acid (GABA) synthetic enzyme glutamic acid decarboxylase (GAD)65 is critically involved in the activity-dependent regulation of GABAergic inhibition in the central nervous system. It is also required for the maturation of the GABAergic system during adolescence, a phase that is critical for the development of several neuropsychiatric diseases. Mice bearing a null mutation of the GAD65 gene develop hyperexcitability of the amygdala and hippocampus, and a phenotype of increased anxiety and pathological fear memory reminiscent of posttraumatic stress disorder. Although genetic association of GAD65 in human has not yet been reported, these findings are in line with observations of reduced GABAergic function in these brain regions of anxiety disorder patients. The particular value of GAD65(-/-) mice thus lies in modeling the effects of reduced GABAergic function in the mature nervous system. The expression of GAD65 and a second GAD isozyme, GAD67, are differentially regulated in response to stress in limbic brain areas suggesting that by controlling GABAergic inhibition these enzymes determine the vulnerability for the development of pathological anxiety and other stress-induced phenotypes. In fact, we could recently show that GAD65 haplodeficiency, which results in delayed postnatal increase of GABA levels, provides resilience to juvenile-stress-induced anxiety to GAD65(+/-) mice thus foiling the increased fear and anxiety in homozygous GAD65(-/-) mice.

Melanocortin 4 receptor activation inhibits presynaptic N-type calcium channels in amygdaloid complex neurons

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor involved in food intake and energy expenditure regulation. MC4R activation modifies neuronal activity but the molecular mechanisms by which this regulation occurs remain unclear. Here, we tested the hypothesis that MC4R activation regulates the activity of voltage-gated calcium channels and, as a consequence, synaptic activity. We also tested whether the proposed effect occurs in the amygdala, a brain area known to mediate the anorexigenic actions of MC4R signaling. Using the patch-clamp technique, we found that the activation of MC4R with its agonist melanotan II specifically inhibited 34.5 ± 1.5% of N-type calcium currents in transiently transfected HEK293 cells. This inhibition was concentration-dependent, voltage-independent and occluded by the Gαs pathway inhibitor cholera toxin. Moreover, we found that melanotan II specifically inhibited 25.9 ± 2.0% of native N-type calcium currents and 55.4 ± 14.4% of evoked inhibitory postsynaptic currents in mouse cultured amygdala neurons. In vivo, we found that the MC4R agonist RO27-3225 increased the marker of cellular activity c-Fos in several components of the amygdala, whereas the N-type channel blocker ω conotoxin GVIA increased c-Fos expression exclusively in the central subdivision of the amygdala. Thus, MC4R specifically inhibited the presynaptic N-type channel subtype, and this inhibition may be important for the effects of melanocortin in the central subdivision of the amygdala.

The recovery of acetylcholinesterase activity and the progression of neuropathological and pathophysiological alterations in the rat basolateral amygdala after soman-induced status epilepticus: relation to anxiety-like behavior

Organophosphorus nerve agents are powerful neurotoxins that irreversibly inhibit acetylcholinesterase (AChE) activity. One of the consequences of AChE inhibition is the generation of seizures and status epilepticus (SE), which cause brain damage, resulting in long-term neurological and behavioral deficits. Increased anxiety is the most common behavioral abnormality after nerve agent exposure. This is not surprising considering that the amygdala, and the basolateral nucleus of the amygdala (BLA) in particular, plays a central role in anxiety, and this structure suffers severe damage by nerve agent-induced seizures. In the present study, we exposed male rats to the nerve agent soman, at a dose that induce SE, and determined the time course of recovery of AChE activity, along with the progression of neuropathological and pathophysiological alterations in the BLA, during a 30-day period after exposure. Measurements were taken at 24 h, 7 days, 14 days, and 30 days after exposure, and at 14 and 30 days, anxiety-like behavior was also evaluated. We found that more than 90% of AChE is inhibited at the onset of SE, and AChE inhibition remains at this level 24 h later, in the BLA, as well as in the hippocampus, piriform cortex, and prelimbic cortex, which we analyzed for comparison. AChE activity recovered by day 7 in the BLA and day 14 in the other three regions. Significant neuronal loss and neurodegeneration were present in the BLA at 24 h and throughout the 30-day period. There was no significant loss of GABAergic interneurons in the BLA at 24 h post-exposure. However, by day 7, the number of GABAergic interneurons in the BLA was reduced, and at 14 and 30 days after soman, the ratio of GABAergic interneurons to the total number of neurons was lower compared to controls. Anxiety-like behavior in the open-field and the acoustic startle response tests was increased at 14 and 30 days post-exposure. Accompanying pathophysiological alterations in the BLA - studied in in vitro brain slices - included a reduction in the amplitude of field potentials evoked by stimulation of the external capsule, along with prolongation of their time course and an increase in the paired-pulse ratio. Long-term potentiation was impaired at 24 h, 7 days, and 14 days post-exposure. The loss of GABAergic interneurons in the BLA and the decreased interneuron to total number of neurons ratio may be the primary cause of the development of anxiety after nerve agent exposure.

Prenatal exposure to lamotrigine: effects on postnatal development and behaviour in rat offspring

Use of antiepileptic drugs (AEDs) in pregnancy warrants various side effects and also deleterious effects on fetal development. The present study was carried out to assess the effects of prenatal exposure to lamotrigine (LTG) on postnatal development and behavioural alterations of offspring. Adult male and female Sprague Dawley rats weighing 150-180 g b. wt. were allowed to copulate and pregnancy was confirmed by vaginal cytology. Pregnant rats were treated with LTG (11.5, 23, and 46 mg/kg, p.o) from gestational day 3 (GND 3) and this treatment continued till postnatal day 11 (PND 11). Offspring were separated from their dam on day 21 following parturition. LTG, at 46 mg/kg, p.o, produced severe clinical signs of toxicity leading to death of dam between GND 15 and 17. LTG, at 11.5 and 23 mg/kg, p.o, showed significant alterations in offspring's incisors eruption and vaginal opening when compared to age matched controls. LTG (23 mg/kg, p.o) exposed female offspring expressed hyperactive behaviour and decreased GABA-A receptor expression when compared to control rats. These results reveal that prenatal exposure to LTG may impart differential postnatal behavioural alterations between male and female rats which paves way for further investigations.

Chronic mild stress-induced changes of risk assessment behaviors in mice are prevented by chronic treatment with fluoxetine but not diazepam

As an important part of risk-related defensive behavior and central element of anxiety, risk assessment in rodents is particularly sensitive to psychosocial stress and may consequently influence the following decision-making and behavioral output. In this study, using a mouse-test battery, we evaluated the possible impacts of chronic mild stress (CMS) on risk assessment behaviors and action selections. For non-stressed control animals, a close relationship between risk assessment and choice behavior was observed in EPM and LDT. For stressed animals, however, 5 weeks of CMS exposure not only increased risk assessment behaviors, but also abolished the correlations between risk assessment and action selection. Pharmacological intervention with GABA-A receptor modulator diazepam (0.25-4 mg/kg) blocked the alterations of conventional spatiotemporal behaviors in response to CMS, but had no effect on the CMS-induced risk assessment behavioral changes. In contrast, 4-weeks of chronic treatment with fluoxetine (4-20mg/kg), a selective serotonin reuptake inhibitor, not only ameliorated the CMS-affected risk-assessment behaviors, but also restored the CMS-impaired correlations between risk assessment and decision making-related action selection. The present findings may shed new light on the better understanding of emotional reactivity and decision making under stressful situations. These results also indicate a differential pharmacological sensitivity in CMS-affected emotional response and risk-assessment behaviors.

Differential activation of neuronal cell types in the basolateral amygdala by corticotropin releasing factor

Enhanced corticotropin releasing factor (CRF) release in the basolateral amygdala (BLA) is strongly associated with the generation of behavioral stress responses through activation of the CRF-R1 receptor subtype. Stress and anxiety-like behavior are modulated in part by the balance of peptide actions such as excitatory CRF and inhibitory neuropeptide Y (NPY) receptor activation in the BLA. While the actions of CRF are clear, little is known about the cell type influenced by CRF receptor stimulation. These studies were designed to identify the cell types within the BLA activated by intra-BLA administration of CRF using multi-label immunohistochemistry for cFos and markers for pyramidal (CaMKII-immunopositive) and interneuronal [glutamic acid decarboxylase (GAD65)] cell populations. Administration of CRF into the BLA produced a dose-dependent increase in the expression of cFos-ir. Intra-BLA injection of CRF induced significant increases in cFos-ir in the CaMKII-ir population. Although increases in cFos-ir in GAD65-ir cells were observed, this did not reach statistical significance perhaps in part due to the decreased numbers of GAD65-ir cells within the BLA after CRF treatment. These findings demonstrate that CRF, when released into the BLA, activates projection neurons and that the activity of GABAergic interneurons is also altered by CRF treatment. Decreases in the number of GAD65-ir neurons could reflect either increased or decreased activity of these cells and future studies will more directly address these possibilities. The expression of cFos is associated with longer term regulation of gene expression which may be involved in the profound long term effects of neuropeptides, such as CRF, on the activity and plasticity of BLA pyramidal neurons.

Focal uncaging of GABA reveals a temporally defined role for GABAergic inhibition during appetitive associative olfactory conditioning in honeybees

Throughout the animal kingdom, the inhibitory neurotransmitter γ-aminobutyric acid (GABA) is a key modulator of physiological processes including learning. With respect to associative learning, the exact time in which GABA interferes with the molecular events of learning has not yet been clearly defined. To address this issue, we used two different approaches to activate GABA receptors during appetitive olfactory conditioning in the honeybee. Injection of GABA-A receptor agonist muscimol 20 min before but not 20 min after associative conditioning affects memory performance. These memory deficits were attenuated by additional training sessions. Muscimol has no effect on sensory perception, odor generalization, and nonassociative learning, indicating a specific role of GABA during associative conditioning. We used photolytic uncaging of GABA to identify the GABA-sensitive time window during the short pairing of the conditioned stimulus (CS) and the unconditioned stimulus (US) that lasts only seconds. Either uncaging of GABA in the antennal lobes or the mushroom bodies during the CS presentation of the CS-US pairing impairs memory formation, while uncaging GABA during the US phase has no effect on memory. Uncaging GABA during the CS presentation in memory retrieval also has no effect. Thus, in honeybee appetitive olfactory learning GABA specifically interferes with the integration of CS and US during associative conditioning and exerts a modulatory role in memory formation depending on the training strength.

Low dose prenatal ethanol exposure induces anxiety-like behaviour and alters dendritic morphology in the basolateral amygdala of rat offspring

Prenatal exposure to high levels of alcohol is strongly associated with poor cognitive outcomes particularly in relation to learning and memory. It is also becoming more evident that anxiety disorders and anxiety-like behaviour can be associated with prenatal alcohol exposure. This study used a rat model to determine if prenatal exposure to a relatively small amount of alcohol would result in anxiety-like behaviour and to determine if this was associated with morphological changes in the basolateral amygdala. Pregnant Sprague Dawley rats were fed a liquid diet containing either no alcohol (Control) or 6% (vol/vol) ethanol (EtOH) throughout gestation. Male and Female offspring underwent behavioural testing at 8 months (Adult) or 15 months (Aged) of age. Rats were perfusion fixed and brains were collected at the end of behavioural testing for morphological analysis of pyramidal neuron number and dendritic morphology within the basolateral amygdala. EtOH exposed offspring displayed anxiety-like behaviour in the elevated plus maze, holeboard and emergence tests. Although sexually dimorphic behaviour was apparent, sex did not impact anxiety-like behaviour induced by prenatal alcohol exposure. This increase in anxiety – like behaviour could not be attributed to a change in pyramidal cell number within the BLA but rather was associated with an increase in dendritic spines along the apical dendrite which is indicative of an increase in synaptic connectivity and activity within these neurons. This study is the first to link increases in anxiety like behaviour to structural changes within the basolateral amygdala in a model of prenatal ethanol exposure. In addition, this study has shown that exposure to even a relatively small amount of alcohol during development leads to long term alterations in anxiety-like behaviour.

Prenatal ethanol exposure attenuates GABAergic inhibition in basolateral amygdala leading to neuronal hyperexcitability and anxiety-like behavior of adult rat offspring

Prenatal exposure to a relatively high-dose ethanol (EtOH) caused anxiety-like behavior of adult male rat offspring. Previous studies have demonstrated that GABA system in the basolateral amygdala complex (BLA) is involved in the pathogensis of anxiety-related disorders. The role of GABAergic system in the BLA was investigated in anxiety-like behavior evoked by prenatal EtOH exposure. The infusion of midazolam (MDZ), a positive modulator of GABA(A) receptor, into the BLA prevented anxiety-like behavior in EtOH-offspring without affecting the corresponding behavior of control offspring. The data suggest that anxiety-like behavior could be causally related to increased neuronal excitability attributable to depressed GABAergic inhibition in the BLA. To test this hypothesis, evoked potential was studied using brain slices from EtOH-offspring. Potential evoked in the BLA by single stimuli applied to external capsule showed multispike responses, indicative of GABAergic disinhibition. These multiple responses were no longer evident after the perfusion with MDZ. In the slices from EtOH-offspring, paired-pulse inhibition (GABA(A)-dependent) was suppressed. Also, in EtOH-offspring, long-term potentiation (LTP) was induced by a single train of high frequency stimulation, which did not induce LTP in control rats. Moreover, MDZ pretreatment prevented the facilitating effect of EtOH on LTP induction. The data provide the functional evidence that prenatal EtOH exposure attenuates GABAergic inhibition in the BLA resulting in neuronal hyperexcitability and anxiety-like behavior of adult rat offspring.

SK2 potassium channel overexpression in basolateral amygdala reduces anxiety, stress-induced corticosterone secretion and dendritic arborization

The basolateral amygdala is critical for generation of anxiety. In addition, exposure to both stress and glucocorticoids induces anxiety. Demonstrated ability of the amygdala to change in response to stress and glucocorticoids could thus be important therapeutic target for anxiety management. Several studies have reported a relationship between anxiety and dendritic arborization of the amygdaloid neurons. In this study we employed a gene therapeutic approach to reduce anxiety and dendritic arborization of the amygdala neurons. Specifically, we overexpressed SK2 potassium channel in the basolateral amygdala using a herpes simplex viral system. Our choice of therapeutic cargo was guided by the indications that activation of the amygdala might underlie anxiety and that SK2 could reduce neuronal activation by exerting inhibitory influence on action potentials. We report that SK2 overexpression reduced anxiety and stress-induced corticosterone secretion at a systemic level. SK2 overexpression also reduced dendritic arborization of the amygdala neurons. Hence, SK2 is a potential gene therapy candidate molecule that can be used against stress-related neuropsychiatric disorders such as anxiety.

Amygdalar GABAergic-rich neural grafts attenuate anxiety-like behavior in rats

Transplantation experiments have shown that neurologic deficits may be reversed by engrafting fresh tissue or engineered cells within dysfunctional neural circuitry. In experimental and clinical settings, this approach has provided insights into the pathology and treatment of neurologic diseases, primarily movement disorders. The present experiments were designed to investigate whether a similar strategy is feasible as a method to investigate, and perhaps repair, circuitry integral to emotional disorders. We focused on the amygdala, a macrostructure known to be involved in the expression of anxiety- and fear-related behaviors. GABAergic cell-rich suspensions were prepared from E17 rat lateral ganglionic eminence and engrafted bilaterally into the lateral and basolateral amygdaloid nuclei of young adult rats. After 6 weeks, increased numbers of GABAergic neurons were identified in the vicinity of the graft sites, and electron microscopy provided evidence for functional integration of transplanted cells. Rats with these grafts spent more time in the open arms of the elevated-plus maze, consistent with an anxioloytic-like phenotype. These rats were also less sensitive to the unconditioned anxiogenic effects of light on the acoustic startle response, although fear-potentiated startle was not affected, suggesting that the grafts produced an attenuation of unlearned fear but did not affect acquisition of conditioned fear. Our results raise the possibility that distinct components of emotion can be modulated by strategic neural engraftment.

COPD as a lung disease with systemic consequences--clinical impact, mechanisms, and potential for early intervention

The natural course of chronic obstructive pulmonary disease (COPD) is complicated by the development of systemic consequences and co-morbidities. These may be major features in the clinical presentation of COPD, prompting increasing interest. Systemic consequences may be defined as non-pulmonary manifestations of COPD with an immediate cause-and-effect relationship, whereas co-morbidities are diseases associated with COPD. The major systemic consequences/co-morbidities now recognized are: deconditioning, exercise intolerance, skeletal muscle dysfunction, osteoporosis, metabolic impact, anxiety and depression, cardiovascular disease, and mortality. The mechanisms by which these develop are unclear. Probably many factors are involved. Two appear of paramount importance: systemic inflammation, which presents in some patients with stable disease and virtually all patients during exacerbations, and inactivity, which may be a key link to most COPD-related co-morbidities. Further studies are required to determine the role of inflammatory cells/mediators involved in systemic inflammatory processes in causing co-morbidities; the link between activity and co-morbidities; and how COPD therapy may affect activity. Both key mechanisms appear to be influenced significantly by COPD exacerbations. Importantly, although the prevalence of systemic consequences increases with increasing severity of airflow obstruction, both systemic consequences and co-morbidities are already present in the Global Initiative for Chronic Obstructive Lung Disease Stage II. This supports the concept of early intervention in chronic obstructive pulmonary disease. Although at present early intervention studies in COPD are lacking, circumstantial evidence suggests that current treatments may influence events leading to the systemic consequences and co-morbidities, and thus may affect the clinical manifestations of the disease.

Behavioural assays to model cognitive and affective dimensions of depression and anxiety in rats

Animal models have been used extensively to investigate neuropsychiatric disorders, such as depression, and their treatment. However, the aetiology and pathophysiology of many such disorders are largely unknown, which makes validation of animal models particularly challenging. Furthermore, many diagnostic symptoms are difficult to define, operationalize and quantify, especially in experimental animals such as rats. Thus, rather than attempting to model complex human syndromes such as depression in their entirety, it can be more productive to define and model components of the illness that may account for clusters of co-varying symptoms, and that may share common underlying neurobiological mechanisms. In preclinical investigations of the neural regulatory mechanisms linking stress to depression and anxiety disorders, as well as the mechanisms by which chronic treatment with antidepressant drugs may exert their beneficial effects in these conditions, we have employed a number of behavioural tests in rats to model specific cognitive and anxiety-like components of depression and anxiety disorders. In the present study, we review the procedures for conducting four such behavioural assays: the attentional set-shifting test, the elevated-plus maze, the social interaction test and the shock-probe defensive burying test. The purpose is to serve as a guide to the utility and limitations of these tools, and as an aid in optimising their use and productivity.

Lasting changes in social behavior and amygdala function following traumatic experience induced by a single series of foot-shocks

Neuronal plasticity within the amygdala mediates many behavioral effects of traumatic experience, and this brain region also controls various aspects of social behavior. However, the specific involvement of the amygdala in trauma-induced social deficits has never been systematically investigated. We exposed rats to a single series of electric foot-shocks--a frequently used model of trauma--and studied their behavior in the social avoidance and psychosocial stimulation tests (non-contact versions of the social interaction test) at different time intervals. Social interaction-induced neuronal activation patterns were studied in the prefrontal cortex (orbitofrontal and medial), amygdala (central, medial, and basolateral), dorsal raphe and locus coeruleus. Shock exposure markedly inhibited social behavior in both tests. The effect lasted at least 4 weeks, and amplified over time. As shown by c-Fos immunocytochemistry, social interactions activated all the investigated brain areas. Traumatic experience exacerbated this activation in the central and basolateral amygdala, but not in other regions. The tight correlation between the social deficit and amygdala activation patterns suggest that the two phenomena were associated. A real-time PCR study showed that CRF mRNA expression in the amygdala was temporarily reduced 14, but not 1 and 28 days after shock exposure. In contrast, amygdalar NK1 receptor mRNA expression increased throughout. Thus, the trauma-induced social deficits appear to be associated with, and possibly caused by, plastic changes in fear-related amygdala subdivisions.

Measurement of panic-like responses following intravenous infusion of sodium lactate in panic-prone rats

This unit describes a putative animal model for panic disorder. The basic premise is that pharmacological disruption of critical brain regions implicated in the circuitry of anxiety will lead to a condition similar to that of the human disorder. A clinically relevant test, the sodium lactate challenge, is utilized to assess parallels between the human condition and this rat model.

Lack of feedback inhibition on rat basolateral amygdala following stress or withdrawal from sedative-hypnotic drugs

Previous research has demonstrated that suppression of inhibition in projection neurons of the basolateral complex of the amygdala (BLA) represents an essential mechanism underlying the emergence of negative emotional responses, including exaggerated fear and anxiety. The present work evaluates inhibitory postsynaptic potentials (IPSPs) in pyramidal projection neurons of the BLA in rats subjected to either diazepam or ethanol withdrawal or uncontrollable stress. These are experimental paradigms conducive to a negative emotional state. In slices containing the BLA, IPSPs were studied using whole-cell patch clamp. In control animals, a small IPSP was evoked by sub-threshold stimulation of the external capsule. When an action potential (AP) was evoked by supra-threshold stimuli, IPSPs were considerably larger; these IPSPs were sensitive to blockade of GABA(A) receptors by picrotoxin. However, IPSPs were clearly reduced in diazepam- or ethanol-withdrawn and in stressed rats. Firing of an AP by a depolarizing pulse applied through the patch pipette consistently evoked an inhibitory postsynaptic current (IPSC) in the pyramidal neurons of control animals from all three experimental models; these IPSCs were mediated by GABA(A) receptor activation and were blocked after suppression of glutamatergic transmission. In contrast, no IPSCs were observed in slices from diazepam- or ethanol-withdrawn or stressed animals, although the depolarizing pulse regularly evoked an AP in pyramidal neurons. It is concluded that, in withdrawn or stressed rats, GABAergic disinhibition occurs due to attenuation or suppression of feedback inhibition.

Angiotensin-II is a putative neurotransmitter in lactate-induced panic-like responses in rats with disruption of GABAergic inhibition in the dorsomedial hypothalamus

Intravenous sodium lactate infusions or the noradrenergic agent yohimbine reliably induce panic attacks in humans with panic disorder but not in healthy controls. However, the exact mechanism of lactate eliciting a panic attack is still unknown. In rats with chronic disruption of GABA-mediated inhibition in the dorsomedial hypothalamus (DMH), achieved by chronic microinfusion of the glutamic acid decarboxylase inhibitor L-allylglycine, sodium lactate infusions or yohimbine elicits panic-like responses (i.e., anxiety, tachycardia, hypertension, and tachypnea). In the present study, previous injections of the angiotensin-II (A-II) type 1 receptor antagonist losartan and the nonspecific A-II receptor antagonist saralasin into the DMH of "panic-prone" rats blocked the anxiety-like and physiological components of lactate-induced panic-like responses. In addition, direct injections of A-II into the DMH of these panic-prone rats also elicited panic-like responses that were blocked by pretreatment with saralasin. Microinjections of saralasin into the DMH did not block the panic-like responses elicited by intravenous infusions of the noradrenergic agent yohimbine or by direct injections of NMDA into the DMH. The presence of the A-II type 1 receptors in the region of the DMH was demonstrated using immunohistochemistry. Thus, these results implicate A-II pathways and the A-II receptors in the hypothalamus as putative substrates for sodium lactate-induced panic-like responses in vulnerable subjects.

Exposure to high- and low-light conditions in an open-field test of anxiety increases c-Fos expression in specific subdivisions of the rat basolateral amygdaloid complex

Anxiety states and anxiety-related behaviors appear to be regulated by a distributed and highly interconnected system of forebrain structures including the basolateral amygdaloid complex (basolateral amygdala). Despite a wealth of research examining the role of the basolateral amygdala in anxiety-related behaviors and anxiety states, the specific subdivisions of the basolateral amygdala that are involved in responses to anxiogenic stimuli have not been examined. In this study, we investigated the effects of exposure to a novel open-field environment, with either low- or high-levels of illumination, on expression of the protein product of the immediate-early gene c-Fos in subdivisions of the rat basolateral amygdala. The subdivisions studied included the lateral, ventrolateral and ventromedial parts of the lateral amygdaloid nucleus, the anterior, posterior and ventral parts of the basolateral amygdaloid nucleus and the anterior and posterior part of the basomedial amygdaloid nucleus. Small increases in the number of c-Fos-immunoreactive cells were observed in several, but not all, of the subdivisions of the basolateral amygdala studied following exposure of rats to either the high- or low-light conditions, compared to home cage or handled control groups. Open-field exposure in both the high- and low-light conditions resulted in a marked increase in c-Fos expression in the anterior part of the basolateral amygdaloid nucleus compared to either home cage or handled control groups. These findings point toward anatomical and functional heterogeneity within the basolateral amygdaloid complex and an important role of the anterior part of the basolateral amygdaloid nucleus in the neural mechanisms underlying physiological or behavioral responses to this anxiety-related stimulus.

Panic-prone state induced in rats with GABA dysfunction in the dorsomedial hypothalamus is mediated by NMDA receptors

Rats with chronic inhibition of GABA synthesis and consequently enhanced glutamatergic excitation in the dorsomedial hypothalamus (DMH) develop panic-like responses, defined as tachycardia, tachypnea, hypertension, and increased anxiety as measured by a social interaction (SI) test, after intravenous sodium lactate infusions, a phenomenon similar to patients with panic disorder. Therefore, the present studies tested the role of the postsynaptic NMDA and AMPA type glutamatergic receptors in the lactate-induced panic-like responses in these rats. Rats were fit with femoral arterial and venous catheters and Alzet pumps [filled with the GABA synthesis inhibitor L-allylglycine (L-AG; 3.5 nmol/0.5 microl per hour) or its inactive isomer D-AG] into the DMH. After 4-5 d of recovery only those rats with L-AG pumps exhibited panic-like responses to lactate infusions. Using double immunocytochemistry, we found that rats exhibiting panic-like responses (e.g., L-AG plus lactate) had increased c-Fos immunoreactivity in DMH neurons expressing the NMDA receptor 1 (NR1) subunit, but not those expressing the glutamate receptor 2 and 3 subunits of the AMPA receptors. To confirm this pharmacologically, we tested another group of rats implanted with l-AG pumps with intravenous lactate infusions preceded by injections of either NMDA [aminophosphonopentanoic acid (AP-5) or (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate (MK-801)] or non-NMDA [CNQX or 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodazepin-5-yl)-benzenamine dihydrochloride (GYKI52466)] antagonists into the DMH. Injections of NMDA, but not non-NMDA, antagonists into the DMH resulted in dose-dependent blockade of the tachycardia, tachypnea, hypertension, and SI responses after lactate infusions. These results suggest that NMDA, and not non-NMDA, type glutamate receptors regulate lactate-induced panic-like responses in rats with GABA dysfunction in the DMH.

The role of neuropeptide Y in the amygdala on corticotropin-releasing factor receptor-mediated behavioral stress responses in the rat

Neuropeptide Y (NPY) is one of the most abundant peptides in the brain and has been shown to be a critical regulator of emotionality, most notably for its effect in decreasing anxiety-like behaviors. The stress response in both humans and animals has been shown to involve a cascade of biological events initiated by corticotropin releasing factor (CRF), another centrally acting peptide. Interestingly, NPY and CRF are present in similar brain regions mediating stress responses and may act in an opposing fashion. The basolateral nucleus of the amygdala (BLA) is a distinct division of the amygdala and contains CRF receptors and the highest concentration of NPY neurons. The current study investigates the behavioral effects in rodents when NPY is injected directly into the BLA prior to the pharmacological stressor, urocortin I (Ucn; a CRF receptor agonist) or the emotional stressor, restraint. The animals that underwent restraint were evaluated in the social interaction (SI) test, while those injected with Ucn into the BLA were assessed in the two floor choice test, a modified version of the conditioned-place avoidance paradigm. The results showed that injections of NPY into the BLA prior to Ucn significantly blocked the development of the avoidance behavior in the two floor choice test and the decrease in SI time that is usually seen following restraint stress. These results provide further support that an interaction between NPY and CRF within the BLA may be critical for maintaining a normal homeostatic emotional state.

Injections of urocortin 1 into the basolateral amygdala induce anxiety-like behavior and c-Fos expression in brainstem serotonergic neurons

The amygdala plays a key role in emotional processing and anxiety-related physiological and behavioral responses. Previous studies have shown that injections of the anxiety-related neuropeptide corticotropin-releasing factor or the related neuropeptide urocortin 1 into the region of the basolateral amygdaloid nucleus induce anxiety-like behavior in several behavioral paradigms. Brainstem serotonergic systems in the dorsal raphe nucleus and median raphe nucleus may be part of a distributed neural system that, together with the basolateral amygdala, regulates acute and chronic anxiety states. We therefore investigated the effect of an acute bilateral injection of urocortin 1 into the basolateral amygdala on behavior in the social interaction test and on c-Fos expression within serotonergic neurons in the dorsal raphe nucleus and median raphe nucleus. Male rats were implanted with bilateral cannulae directed at the region of the basolateral amygdala; 72 h after surgery, rats were injected with urocortin 1 (50 fmol/100 nl) or vehicle (100 nl of 1% bovine serum albumin in distilled water). Thirty minutes after injection, a subgroup of rats from each experimental group was exposed to the social interaction test; remaining animals were left in the home cage. Two hours after injection rats were perfused with paraformaldehyde and brains were removed and processed for immunohistochemistry. Acute injection of urocortin 1 had anxiogenic effects in the social interaction test, reducing total interaction time without affecting locomotor activity or exploratory behavior. These behavioral effects were associated with increases in c-Fos expression within brainstem serotonergic neurons. In home cage rats and rats exposed to the social interaction test, urocortin 1 treatment increased the number of c-Fos-immunoreactive serotonergic neurons within subdivisions of both the dorsal raphe nucleus and median raphe nucleus. These results are consistent with the hypothesis that the basolateral amygdala and serotonergic neurons within the midbrain raphe complex are part of an integrated neural system modulating anxiety state.

Cortico-amygdala circuits: role in the conditioned stress response

The amygdala plays a crucial role in the orchestration and modulation of the organism response to aversive, stressful events. This response could be conceived as the result of two interdependent components. The first is represented by sets of visceral and motor responses aimed at helping the organism to cope with the present event. The second is the acquisition and modulation of memories relative to the stressful stimulus and its context. This latter component contributes to the instatement of conditioned stress responses that are essential to the capability of the organism to predict future exposures to similar stimuli in order to avoid them or counteract them effectively. In the amygdala, these two components become fully integrated. Massive networks link the amygdala to the hypothalamus, midbrain and brainstem. These networks convey visceral, humoral and nociceptive information to the amygdala and mediate its effects on the hypothalamic-pituitary-adrenal axis as well on autonomic and motor centers. On the other hand, interactions between the amygdala and interconnected cortical networks play a crucial role in acquisition, consolidation and extinction of learning relative to the stressful stimulus. Within the scope of this review, current evidence relative to the interaction between the amygdala and cortical networks will be considered in relationship to the integration of the conditioned response to stress.

Retigabine: chemical synthesis to clinical application

Retigabine [D23129; N-(2-amino-4-(4-fluorobenzylamino)-phenyl)carbamic acid ethyl ester] is an antiepileptic drug with a recently described novel mechanism of action that involves opening of neuronal K(V)7.2-7.5 (formerly KCNQ2-5) voltage-activated K(+) channels. These channels (primarily K(V)7.2/7.3) enable generation of the M-current, a subthreshold K(+) current that serves to stabilize the membrane potential and control neuronal excitability. In this regard, retigabine has been shown to have a broad-spectrum of activity in animal models of electrically-induced (amygdala-kindling, maximal electroshock) and chemically-induced (pentylenetetrazole, picrotoxin, NMDA) epileptic seizures. These encouraging results suggest that retigabine may also prove useful in the treatment of other diseases associated with neuronal hyperexcitability. Neuropathic pain conditions are characterized by pathological changes in sensory pathways, which favor action potential generation and enhanced pain transmission. Although sometimes difficult to treat with conventional analgesics, antiepileptics can relieve some symptoms of neuropathic pain. A number of recent studies have reported that retigabine can relieve pain-like behaviors (hyperalgesia and allodynia) in animal models of neuropathic pain. Neuronal activation within several key structures within the CNS can also be observed in various animal models of anxiety. Moreover, amygdala-kindled rats, which have a lowered threshold for neuronal activation, also display enhanced anxiety-like responses. Retigabine dose-dependently reduces unconditioned anxiety-like behaviors when assessed in the mouse marble burying test and zero maze. Early clinical studies have indicated that retigabine is rapidly absorbed and distributed, and is resistant to first pass metabolism. Tolerability is good in humans when titrated up to its therapeutic dose range (600-1200 mg/day). No tolerance, dependence or withdrawal potential has been reported, although adverse effects can include mild dizziness, headache, nausea and somnolence. Thus, retigabine may prove to be useful in the treatment of a diverse range of disease states in which neuronal hyperexcitability is a common underlying factor.

Stress and central Urocortin increase anxiety-like behavior in the social interaction test via the CRF1 receptor

Corticotropin releasing factor (CRF) and Urocortin are important neurotransmitters in the regulation of physiological and behavioral responses to stress. Centrally administered CRF or Urocortin produces anxiety-like responses in numerous animal models of anxiety disorders. Previous studies in our lab have shown that Urocortin infused into the basolateral nucleus of the amygdala produces anxiety-like responses in the social interaction test. Subsequently, in the current study we prepared a specific CRF1 receptor antagonist (N-Cyclopropylmethyl-2,5-dimethyl-N-propyl-N'-(2,4,6-trichloro-phenyl)-pyrimidine-4,6-diamine, NBI3b1996) to examine in this paradigm. This CRF1 receptor antagonist inhibited the ex vivo binding of 125I-sauvagine to rat cerebellum with an ED50 of 6 mg/kg, i.p. NBI3b1996 produced a dose-dependent antagonism of Urocortin-induced anxiety-like behavior in Social Interaction test with an ED50 of 6 mg/kg, i.p. The compound had no effect on baseline social interaction. In addition, the CRF1 receptor antagonist prevented the stress-induced decrease in social interaction. These results provide further support for the CRF1 receptor in anxiety-like behavior and suggest this pathway is quiescent in unstressed animals.

Inactivation of the medial prefrontal cortex with the GABAA receptor agonist muscimol increases open-arm activity in the elevated plus-maze and attenuates shock-probe burying in rats

This study examined the effects of infusions of a direct GABA(A) receptor agonist, muscimol, into the medial prefrontal cortex (MPFC), on fear behavior measured in the elevated plus-maze and shock-probe burying tests. Bilateral infusions of either a 0.175 or 4 nmol/0.5 microl dose of muscimol increased the percentage of entries and time spent in the open arms, and attenuated shock-probe burying. These findings indicate that intra-MPFC infusions of muscimol induce anxiolysis, and suggest that the direct stimulation of MPFC GABA(A) receptors attenuates fear-related behavior.

Corticotrophin releasing factor-induced synaptic plasticity in the amygdala translates stress into emotional disorders

The amygdala is involved in the associative processes for both appetitive and aversive emotions, and its function is modulated by stress hormones. The neuropeptide corticotrophin releasing factor (CRF) is released during stress and has been linked to many stress-related behavioral, autonomic, and endocrine responses. In the present study, nonanxiety-inducing doses of a potent CRF type 1 and 2 receptor agonist, urocortin (Ucn), was infused locally into the basolateral amygdala (BLA) of rats. After 5 daily injections of Ucn, the animals developed anxiety-like responses in behavioral tests. Intravenous administration of the anxiogenic agent sodium lactate elicited robust increases in blood pressure, respiratory rate, and heart rate. Furthermore, in the absence of any additional Ucn treatment, these behavioral and autonomic responses persisted for >30 d. Whole-cell patch-clamp recordings from BLA neurons of these hyper-reactive animals revealed a pronounced reduction in both spontaneous and stimulation-evoked IPSPs, leading to a hyperexcitability of the BLA network. This Ucn-induced plasticity appears to be dependent on NMDA receptor and subsequent calcium-calmodulin-dependent protein kinase II (CaMKII) activation, because it is blocked by pretreatment with NMDA receptor antagonists and by coadministration of CaMKII inhibitors. Our results show for the first time a stress peptide-induced behavioral syndrome that can be correlated with cellular mechanisms of neural plasticity, a novel mechanism that may explain the etiological role of stress in several chronic psychiatric and medical disorders.

The amygdala, panic disorder, and cardiovascular responses

The amygdala is implicated in a number of emotional responses including conditioned fear and anxiety, and it appears to regulate the behavioral and autonomic responses associated with such emotional responses. The basolateral nucleus of the amygdala (BLA) is under tonic GABAergic inhibition, and acutely blocking this inhibition results in increased anxiety-like behavior, conditioned avoidance, and sympathetically mediated cardiovascular activation. By contrast, activation of the BLA with the stress-related neuropeptide corticotropin-releasing factor results in anxiety-like behavior, but not cardiovascular activation. Furthermore, repeated activation of this region with subthreshold GABA blockade or corticotropin-releasing factor-mediated excitation (priming) results in a chronic anxiety-like state, with susceptibility to panic-like arousal following intravenous lactate infusions. The chronic anxiety state appears to result from a loss of basal inhibitory drive in the BLA as a result of NMDA-dependent synaptic plasticity involving cyclic AMP and calcium calmodulin kinase II (CAM KII)-mediated changes. The lactate-induced panic-like response appears to involve angiotensin-II mediated activation of the BLA. These results suggest that the BLA has a significant role in regulating anxiety, autonomic responses, and the development of anxiety disorders.

Inhibition of the amygdala: key to pathological states?

The amygdala plays an important role in associating sensory stimuli with aversive or appetitive outcomes. Conditioning procedures potentiate inputs to the amygdala, which facilitate emotional responses via subcortical and cortical outputs. Powerful inhibitory circuits exist that control expression of conditioned responses in the amygdala, including inhibition from prefrontal cortex. Deficient inhibitory tone in the amygdala could lead to overexpression of conditioned responses, producing pathological states such as anxiety disorders and drug-seeking behavior. Support for this comes from several lines of animal research: (1) GABA antagonist-induced priming of anxiety states, (2) extinction of conditioned fear, (3) modulation of inhibitory avoidance memory, and (4) cue-induced reinstatement of drug seeking. Cue-induced craving in humans is associated with feelings of fear and autonomic arousal, suggesting a link between fear and addiction in the amygdala. Future therapies aimed at increasing inhibitory tone in the amygdala, either locally or via the prefrontal cortex, may prevent anxiety disorders and addiction relapse. Novel neuropeptides, which can either excite or inhibit specific components of anxiety responses, offer promise in this regard.

Neuropeptide Y receptor subtypes in the basolateral nucleus of the amygdala modulate anxiogenic responses in rats

The behavioral effects induced by intra-amygdala stimulation of the neuropeptide Y (NPY) Y(2) and the NPY Y(5) receptor subtypes were assessed in the social interaction (SI) test. Microinjections of NPY(3-36), an NPY Y(2) preferring agonist, into the basolateral nucleus of the amygdala (BLA) produced bi-directional dose-response curve. At low doses NPY(3-36) has an anxiogenic effect while at higher doses it produced an anxiolytic effect. Pretreatment with the NPY Y(5) receptor antagonist Novartis 1(1 nmol), an analog of CGP71683A synthesized by Eli Lilly and Company, IN, blocked the anxiolytic effects of NPY(3-36) (80 pmol), while pretreatment with BIBO 3304 (200 pmol), a Y(1) antagonist, had no effect, suggesting that the Y(5), but not the Y(1) receptor was involved in the anxiolytic behavior produced following intra-amygdalar NPY(3-36) administration. In addition, the Y(5) antagonist had no behavioral effect when given alone at 1.0 nmol. These findings support the hypothesis that amygdalar Y(2) receptors may play a role in mediating anxiogenic effects, while Y(5) receptors may be involved in the anxiolytic behaviors of NPY.

Gabaergic modulation of the stress response in frontal cortex and amygdala

GABAergic neurotransmission is thought to play an important role in the modulation of the central response to stress. In the present study we evaluate the influence of a brief restraint exposure on GABA-stimulated chloride influx in diverse brain areas presumed to have a major role in the mediation of emotional behaviors following aversive stimulation. A reduced chloride uptake after stress exposure was only observed in frontal cortex and amygdala. Moreover, rats subjected to such stressor performed an anxiogenic behavior when exposed later to the elevated plus-maze. A comparable behavior in the elevated plus-maze was observed between animals that were allowed to chew during the restraint experience and those without any stressful manipulation, suggesting that chewing served as an efficient coping behavioral strategy during such threatening situations. In order to explore if chewing during the restraint experience could suppress the reduction in GABA-stimulated chloride uptake induced by this stressor, rats were allowed or not to chew during restraint and in both cases GABA-stimulated chloride influx was assayed in frontal cortex and amygdala. The finding of this experiment showed that restrained rats that have the possibility to chew exhibited a similar GABA-stimulated chloride uptake in cortical tissue to that shown by control, unstressed rats. Moreover, chewing in response to restraint attenuated the reduction of GABA-stimulated chloride uptake in amygdala, supporting the notion that chewing is an effective coping response to restraint. These experiments suggest that a reduced GABAergic inhibitory control in these areas could be implicated in the emotional sequelae generated by this uncontrollable stressor and that the suppression of this reduction seems to be associated with the occurrence of coping behavioral response to such fear-inducing stimulus.

Amygdala priming results in conditioned place avoidance

Priming involves daily stimulation of the basolateral nucleus of the amygdala (BLA) for 5 days using a dose of the GABA(A) receptor antagonist, bicuculline methiodide (BMI), that is subthreshold to generate anxiogenic-like responses. The coordinated physiological and behavioral response of the primed rat is similar to the symptoms of human panic disorder and has been used as a model to study panic attacks. If the priming procedure is indeed similar to human panic disorder, then the context in which priming occurs should become associated with aversive conditioning and avoidance as seen in secondary agoraphobia following panic attacks in humans. Therefore, the purpose of this study was to further characterize the behavioral response of priming using the conditioned place avoidance (CPA) task that utilizes distinct tactile cues of a grid floor (Grid+) or hole floor (Grid-). Male Wistar rats (275-300 g) were implanted bilaterally with guide cannulae positioned 1 mm above the BLA. Grid+ animals were placed in the conditioning chamber containing grid floors immediately after a 6-pmol (in 250 nl) BMI injection into the BLA and on hole floors following a sham (250 nl vehicle) injection. Grid animals were placed in the chamber containing hole floors after the BMI injection and on grid floors following the sham injection. Animals were placed in the chamber for 20 min following each injection and injections were separated by 4 h. After 5 days of this treatment, the animals were primed. Two days later, during avoidance testing, each animal was placed in the chamber containing both floors for 30 min. Priming with daily 6-pmol BMI injections into the BLA results in CPA or an aversion to the floor paired with the BMI injection. These results suggest that priming may result in phobic-like responses, similar to the avoidance behavior exhibited by panic disorder patients.

Increased anxiety and synaptic plasticity in estrogen receptor beta -deficient mice

Estrogens are powerful modulators of neuronal physiology and in humans may affect a broad range of functions, including reproductive, emotional, and cognitive behaviors. We studied the contribution of estrogen receptors (ERs) in modulation of emotional processes and analyzed the effects of deleting ERalpha or ERbeta in mice. Behavior consistent with increased anxiety was observed principally in ERbeta mutant females and was associated with a reduced threshold for the induction of synaptic plasticity in the basolateral amygdala. Local increase of 5-hydroxytryptamine 1a receptor expression in medial amygdala may contribute to these changes. Our data show that, particularly in females, there is an important role for ERbeta-mediated estrogen signaling in the processing of emotional behavior.

Subunit composition of strychnine-sensitive glycine receptors expressed by adult rat basolateral amygdala neurons

In neonatal rats, strychnine-sensitive glycine receptors are widely expressed in the spinal cord, brainstem and forebrain. During development, these 'neonatal' receptors are replaced by an adult isoform, the expression of which becomes restricted primarily to brain stem and spinal cord. Unlike most forebrain regions, functional strychnine-sensitive glycine receptors appear to persist within adult rat amygdala. However, the subunit composition of glycine receptors expressed by amygdala neurons and its relationship to the adult isoform in brain stem/spinal cord has not been defined precisely. In this report, we have utilized RT-PCR and single-cell RT-PCR to demonstrate that the 'neonatal' alpha2-subunit mRNA persists in adult rat amygdala neurons and is the predominant alpha-subunit. We further demonstrate that native amygdala glycine receptors are relatively insensitive to the receptor antagonist picrotoxin, suggesting that alpha2- and beta-subunits may be present together in the same multisubunit complex. We further demonstrate that alpha2- and beta-subunits cloned from adult rat amygdala can form functional channels when expressed in a heterologous system. Together, these studies highlight both the unique characteristics of strychnine-sensitive glycine receptors in the adult rat amygdala as well as the possibility that alpha2/beta channels may represent the adult forebrain isoform of the strychnine-sensitive glycine receptor.

Descending pathways mediating cardiovascular response from dorsomedial hypothalamic nucleus

Physiological and anatomic methods were used to determine whether neurons in the rostral ventrolateral medulla (RVLM), nucleus tractus solitarius (NTS), or hypothalamic paraventricular nucleus (PVN) mediate the cardiovascular response evoked from the dorsomedial hypothalamic nucleus (DMH), which is believed to play a key role in mediating responses to stress. In urethane-anesthetized rats, activation of neurons in the DMH by microinjection of bicuculline resulted in a large increase in arterial pressure, heart rate, and renal sympathetic nerve activity. The pressor and sympathoexcitatory responses, but not the tachycardic response, were greatly reduced after bilateral muscimol injections into the RVLM even when baseline arterial pressure was maintained at a constant level. These responses were not reduced by muscimol injections into the PVN or NTS. Retrograde tracing experiments identified many neurons in the DMH that projected directly to the RVLM. The results indicate that the vasomotor and cardiac components of the response evoked from the DMH are mediated by pathways that are dependent and independent, respectively, of neurons in the RVLM.

Does the kindling model of epilepsy contribute to our understanding of multiple chemical sensitivity?

Multiple chemical sensitivity (MCS) is a phenomenon whereby individuals report an increased sensitivity to low levels of chemicals in the environment. Kindling is a model of synaptic plasticity whereby repeated low-level electrical stimulation to a number of brain sites leads to permanent increases in seizure susceptibility. Stimulation that is initially subthreshold for subclinical seizure provocation comes, over time, to elicit full-blown motor seizures. Kindling can also be induced by chemical stimulation, and repeated exposures to some pesticides have been shown to induce signs of behavioral seizure, facilitate subsequent electrical kindling, and induce subclinical electrographic signs of hyperexcitability in the amygdala. Many of the symptoms of MCS suggest that CNS limbic pathways involved in anxiety are altered in individuals reporting MCS. Limbic structures are among the most susceptible to kindling-induced seizures, and persistent cognitive and emotional sequelae have been associated with temporal lobe epilepsy (TLE) in humans and kindling in animals. Thus, a number of parallels exist between kindling and MCS phenomena, leading to initial speculations that MCS may occur via a kindling-like mechanism. However, kindling requires the activation of electrographic seizure discharge and has thus been primarily examined as a model for TLE. Events leading to the initial evocation of a subclinical electrographic seizure have been much less well studied. It is perhaps these events that may serve as a more appropriate model for the enhanced chemical responsiveness characteristic of MCS. Alternatively, kindling may be useful as a tool to selectively increase sensitivity in subcomponents of the neural fear circuit to address questions relating the role of anxiety in the development and expression of MCS.

Astressin, a corticotropin releasing factor antagonist, reverses the anxiogenic effects of urocortin when administered into the basolateral amygdala

Previous work in our laboratory has shown that Urocortin (Ucn), a peptide related to corticotropin releasing factor (CRF), injected into the basolateral nucleus of the amygdala (BLA) in male Wistar rats would result in an anxiogenic response as measured in the social interaction (SI) test. In addition, it was found that repeated injections of subthreshold doses of Ucn would 'prime' the animal's response. This 'priming' effect induces a sensitivity to sodium lactate infusions that results in a panic-like reaction. Currently, we examined the effects of the CRF(1) and CRF(2) antagonist Astressin (Asn) on the anxiety-like responses produced during 'priming' as well as during a sodium lactate infusion into 'primed' rats. The results showed that Asn (60 pmoles) was able to reverse the anxiogenic effects seen during acute administration of Ucn, but was only able to partially antagonize the same response following 'priming' with Ucn. Furthermore, Asn administered either alone or prior to a sodium lactate infusion had no effect on the primed rat's behavior. Autoradiographic studies, in Ucn primed and sham-primed animals indicated no significant changes in [(125)I]-Sauvagine binding to CRF(1) and CRF(2) receptors in several brain regions. Thus, a 60 pmole dose of Asn blocks the effects of an acute injection of Ucn (100 pmoles), while only partially blocking the behavioral effects after repeated injections of subthreshold doses of Ucn (6 pmoles) are given. Furthermore, Asn has no effect on anxiogenic responses due to sodium lactate infusions in 'primed' rats

Amygdalo-entorhinal inputs to the hippocampal formation in relation to schizophrenia

This chapter reviews recent postmortem studies of schizophrenic brain and discusses the potential role of the amygdala in the induction of hippocampal abnormalities in this disorder. Based on available evidence, sectors CA4, CA3, and CA2, but not CA1, show preferential changes in schizophrenic subjects, although the most pronounced changes have been found in CA3 and CA2. It seems likely that the amygdala would contribute in some way to the induction of abnormalities along the trisynaptic pathway via its direct input to sectors CA3 and CA2, as well as an indirect one that involves the entorhinal cortex and its perforant path projection to the area dentata. The postmortem findings reported to date have been integrated into a working model in which decreases of inhibitory GABAergic modulation are invoked to explain the observation from a recent PET scan study (Heckers et al., 1999) that baseline metabolic activity in the hippocampus of schizophrenics is increased. In addition, however, the apparent inability of schizophrenics to increase metabolic activity in the hippocampus when challenged with a memory retrieval task may reflect a disturbance of disinhibitory modulation postulated herein to occur in sector CA3, a key relay point along the trisynaptic pathway. Overall, it seems plausible that an increase of excitatory activity entering the hippocampus from the basolateral complex via both direct and indirect pathways may make a significant contribution to the pathophysiology of schizophrenia.

Postnatal development of a GABA deficit and disturbance of neural functions in mice lacking GAD65

The 65-kDa isoform of glutamic acid decarboxylase (GAD65) is believed to play an essential role for GABA synthesis in the central nervous system. Using mice with targeted disruption of the GAD65 gene (GAD65(-/-) mice) we investigated the contribution of GAD65 to GABA synthesis in different brain areas during postnatal development and in adulthood. In the amygdala, hypothalamus and parietal cortex of GAD65(+/+) mice an increase of GABA levels was observed during postnatal development, most prominently between the first and second month after birth. This increase appeared to be dependent on GAD65, as it was delayed by 2 months in GAD65(+/-) mice and was not observed in GAD65(-/-) mice. Likely as a consequence of their GABA deficit, adult GAD65(-/-) mice showed a largely abnormal neural activity with frequent paroxysmal discharges and spontaneous seizures. They furthermore displayed increased anxiety-like behaviour in a light/dark avoidance test and reduced intermale aggression, as well as a reduced forced-swimming-induced immobility indicative of an antidepressant-like behavioural change. Adult GAD65(+/-) mice did not show behavioural disturbances except for a reduced aggressive behaviour that was comparable to that in GAD65(-/-) mice. We conclude that GAD65-mediated GABA synthesis may be crucially involved in control of emotional behaviour and indispensable for a tonic inhibition that prevents the development of hyperexcitability in the maturating central nervous system. Aggressive, and possibly other social behaviour may be especially prone to regulation through GAD65-mediated GABA synthesis.

Sodium lactate elicits anxiety in rats after repeated GABA receptor blockade in the basolateral amygdala

Repeated administration of the GABA(A) receptor antagonist bicuculline methiodide into the basolateral nucleus of the amygdala at doses subthreshold to eliciting a full response will eventually produce long-term 'priming', such that heart rate, blood pressure as well as anxiety are increased at the lower doses. The present study was conducted in order to determine if the long-term priming of anxiety within the basolateral nucleus is producing a condition similar to that seen in human panic disorder by testing the response elicited by i.v. lactate infusions, since lactate infusions induce a panic attack in patients with panic disorder. Male Wistar rats were fitted with femoral arterial and venous catheters and chronic microinjection cannulae into the basolateral nucleus. Repeated daily injections of a subthreshold dose of bicuculline methiodide into the basolateral nucleus for 4-5 days elicited a primed response, while the same procedure with artificial cerebrospinal fluid vehicle (a-CSF; sham-primed) had no effect. Following priming, rats received both sodium lactate infusions (0.5 N, 10 ml/kg) or 0.9% saline in a random order separated by 48 h. Heart rate and blood pressure were monitored throughout the infusion and the animals were immediately placed in the social interaction test to assess their anxiety response. Only primed and not sham-primed rats responded to a lactate infusion with significant increases in heart rate, blood pressure and experimental anxiety. Thus, rats which are primed with chronic subthreshold GABA receptor blockade in the basolateral nucleus develop a sensitivity to sodium lactate, similar to human panic disorder patients.