Panic results in unique molecular and network changes in the amygdala that facilitate fear responses

Recurrent panic attacks (PAs) are a common feature of panic disorder (PD) and post-traumatic stress disorder (PTSD). Several distinct brain regions are involved in the regulation of panic responses, such as perifornical hypothalamus (PeF), periaqueductal gray, amygdala and frontal cortex. We have previously shown that inhibition of GABA synthesis in the PeF produces panic-vulnerable rats. Here, we investigate the mechanisms by which a panic-vulnerable state could lead to persistent fear. We first show that optogenetic activation of glutamatergic terminals from the PeF to the basolateral amygdala (BLA) enhanced the acquisition, delayed the extinction and induced the persistence of fear responses 3 weeks later, confirming a functional PeF-amygdala pathway involved in fear learning. Similar to optogenetic activation of PeF, panic-prone rats also exhibited delayed extinction. Next, we demonstrate that panic-prone rats had altered inhibitory and enhanced excitatory synaptic transmission of the principal neurons, and reduced protein levels of metabotropic glutamate type 2 receptor (mGluR2) in the BLA. Application of an mGluR2-positive allosteric modulator (PAM) reduced glutamate neurotransmission in the BLA slices from panic-prone rats. Treating panic-prone rats with mGluR2 PAM blocked sodium lactate (NaLac)-induced panic responses and normalized fear extinction deficits. Finally, in a subset of patients with comorbid PD, treatment with mGluR2 PAM resulted in complete remission of panic symptoms. These data demonstrate that a panic-prone state leads to specific reduction in mGluR2 function within the amygdala network and facilitates fear, and mGluR2 PAMs could be a targeted treatment for panic symptoms in PD and PTSD patients.

Guanfacine treatment improves ADHD phenotypes of impulsivity and hyperactivity in a neurofibromatosis type 1 mouse model

Background

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder with a mutation in one copy of the neurofibromin gene (NF1^+/−). Even though approximately 40–60% of children with NF1 meet the criteria for attention deficit hyperactivity disorder (ADHD), very few preclinical studies, if any, have investigated alterations in impulsivity and risk-taking behavior. Mice with deletion of a single NF1 gene (Nf1^+/−) recapitulate many of the phenotypes of NF1 patients.

Methods

We compared wild-type (WT) and Nf1^+/− mouse strains to investigate differences in impulsivity and hyperactivity using the delay discounting task (DDT), cliff avoidance reaction (CAR) test, and open field. We also investigated whether treatment with the clinically effective alpha-2A adrenergic receptor agonist, guanfacine (0.3 mg/kg, i.p.), would reverse deficits observed in behavioral inhibition.

Results

Nf1^+/− mice chose a higher percentage of smaller rewards when both 10- and 20-s delays were administered compared to WT mice, suggesting Nf1^+/− mice are more impulsive. When treated with guanfacine (0.3 mg/kg, i.p.), Nf1^+/− mice exhibited decreased impulsive choice by waiting for the larger, delayed reward. Nf1^+/− mice also exhibited deficits in behavioral inhibition compared to WT mice in the CAR test by repetitively entering the outer edge of the platform where they risk falling. Treatment with guanfacine ameliorated these deficits. In addition, Nf1^+/− mice exhibited hyperactivity as increased distance was traveled compared to WT controls in the open field. This hyperactivity in Nf1^+/− mice was reduced with guanfacine pre-treatment.

Conclusions

Overall, our study confirms that Nf1^+/− mice exhibit deficits in behavioral inhibition in multiple contexts, a key feature of ADHD, and can be used as a model system to identify alterations in neural circuitry associated with symptoms of ADHD in children with NF1.

Development × environment interactions control tph2 mRNA expression

Adverse early life experience is thought to increase an individual's susceptibility to mental health disorders, including anxiety and affective disorders, later in life. Our previous studies have shown that post-weaning social isolation of female rats during a critical period of development sensitizes an anxiety-related serotonergic dorsal raphe nucleus (DR) system in adulthood. Therefore, we investigated how post-weaning social isolation, in combination with a challenge with the anxiogenic drug, N-methyl-beta-carboline-3-carboxamide (FG-7142; a partial inverse agonist at the benzodiazepine allosteric site on the GABAA receptor), affects home cage behavior and serotonergic gene expression in the DR of female rats using in situ hybridization histochemistry. Juvenile female rats were reared in isolation or groups of three for a 3-week period from weaning (postnatal day (PD) 21 to mid-adolescence (PD42)), after which all rats were group-reared for an additional 16 days until adulthood. Among vehicle-treated rats, isolation-reared rats had decreased rodent tryptophan hydroxylase 2 (tph2) mRNA expression in ventral and ventrolateral subdivisions of the DR, a pattern observed previously in a rat model of panic disorder. Isolation-reared rats, but not group-reared rats, responded to FG-7142 with increased duration of vigilance and arousal behaviors. In addition, FG-7142 decreased tph2 expression, measured 4h following treatment, in multiple subregions of the DR of group-reared rats but had no effect in isolation-reared rats. No treatment effects were observed on 5-HT1A receptor or serotonin transporter gene expression. These data suggest that adolescent social isolation alters tph2 expression in specific subregions of the DR and alters the effects of stress-related stimuli on behavior and serotonergic systems.

Topographical distribution of corticotropin-releasing factor type 2 receptor-like immunoreactivity in the rat dorsal raphe nucleus: co-localization with tryptophan hydroxylase

Corticotropin-releasing factor (CRF) and CRF-related neuropeptides are involved in the regulation of stress-related physiology and behavior. Members of the CRF family of neuropeptides bind to two known receptors, the CRF type 1 (CRF₁) receptor, and the CRF type 2 (CRF₂) receptor. Although the distribution of CRF₂ receptor mRNA expression has been extensively studied, the distribution of CRF₂ receptor protein has not been characterized. An area of the brain known to contain high levels of CRF₂ receptor mRNA expression and CRF₂ receptor binding is the dorsal raphe nucleus (DR). In the present study we investigated in detail the distribution of CRF₂ receptor immunoreactivity throughout the rostrocaudal extent of the DR. CRF₂ receptor-immunoreactive perikarya were observed throughout the DR, with the highest number and density in the mid-rostrocaudal DR. Dual immunofluorescence revealed that CRF₂ receptor immunoreactivity was frequently co-localized with tryptophan hydroxylase, a marker of serotonergic neurons. This study provides evidence that CRF₂ receptor protein is expressed in the DR, and that CRF₂ receptors are expressed in topographically organized subpopulations of cells in the DR, including serotonergic neurons. Furthermore, these data are consistent with the hypothesis that CRF₂ receptors play an important role in the regulation of stress-related physiology and behavior through actions on serotonergic and non-serotonergic neurons within the DR.

Investigation of a central nucleus of the amygdala/dorsal raphe nucleus serotonergic circuit implicated in fear-potentiated startle

Serotonergic systems are thought to play an important role in control of motor activity and emotional states. We used a fear-potentiated startle paradigm to investigate the effects of a motor-eliciting stimulus in the presence or absence of induction of an acute fear state on serotonergic neurons in the dorsal raphe nucleus (DR) and cells in subdivisions of the central amygdaloid nucleus (CE), a structure that plays an important role in fear responses, using induction of the protein product of the immediate-early gene, c-Fos. In Experiment 1 we investigated the effects of fear conditioning training, by training rats to associate a light cue (conditioned stimulus, CS; 1000 lx, 2 s) with foot shock (0.5 s, 0.5 mA) in a single session. In Experiment 2 rats were given two training sessions identical to Experiment 1 on days 1 and 2, then tested in one of four conditions on day 3: (1) placement in the training context without exposure to either the CS or acoustic startle (AS), (2) exposure to 10 trials of the 2 s CS, (3) exposure to 40 110 dB AS trials, or (4) exposure to 40 110 dB AS trials with 10 of the trials preceded by and co-terminating with the CS. All treatments were conducted during a 20 min session. Fear conditioning training, by itself, increased c-Fos expression in multiple subdivisions of the CE and throughout the DR. In contrast, fear-potentiated startle selectively increased c-Fos expression in the medial subdivision of the CE and in serotonergic neurons in the dorsal part of the dorsal raphe nucleus (DRD). These data are consistent with previous studies demonstrating that fear-related stimuli selectively activate DRD serotonergic neurons. Further studies of this mesolimbocortical serotonergic system could have important implications for understanding mechanisms underlying vulnerability to stress-related psychiatric disorders, including anxiety and affective disorders.

Early life social isolation alters corticotropin-releasing factor responses in adult rats

Stress induced by early life social isolation leads to long-lasting alterations in stress responses and serotonergic activity. Corticotropin-releasing factor (CRF) is a neurotransmitter that mediates stress responses and alters serotonergic activity. We tested the hypothesis that the stress of early life isolation enhances responses to CRF in adulthood by determining the effect of CRF infusions into the dorsal raphe nucleus (dRN) on 5-HT release in the nucleus accumbens (NAc) of adult rats using in vivo microdialysis. Juvenile male rats were either isolated or housed in groups of three for a 3-week period beginning on postnatal day 21 after which, all rats were group-reared for an additional 2 weeks. Following the isolation/re-socialization procedure, infusion of 100 ng CRF into the dRN decreased 5-HT release in the NAc of group-reared rats. This treatment did not significantly affect 5-HT release in the NAc of isolation-reared animals. In contrast, infusion of 500 ng CRF into the dRN transiently increased 5-HT release in the NAc of both group-reared and isolated animals with isolated animals showing a more prolonged serotonergic response. Western blot and immunofluorescent staining for CRF receptors in the dRN showed that CRF(2) receptor levels were increased in the dRN of isolation-reared animals when compared with group-reared rats. Taken together, the results suggest that isolation during the early part of development causes alterations in both CRF receptor levels and CRF-mediated serotonergic activity. These effects may underlie the increased sensitivity to stress observed in isolates.