Modulation of fear-potentiated startle and vocalizations in juvenile rhesus monkeys by morphine, diazepam, and buspirone

Single-dose Diazepam Administration Improves Pairing Success of Unfamiliar Adult Male Rhesus Macaques (Macaca mulatta)

Social housing is one of the best forms of environmental enhancement for nonhuman primates, and current research into pair compatibility and introduction techniques focuses on improving safety and outcome. The gradual steps method (GS), which is widely used for introducing indoor-housed macaques, involves an initial phase of limited physical contact to allow animals to acclimate to one another prior to full contact. A safer, more efficacious introduction method is needed. The administration of diazepam, a sedating anxiolytic medication, is known to increase affiliative behavior in familiar, socially housed rhesus macaques. We hypothesized that administration of a single dose of diazepam prior to full contact introduction without a protected contact phase would improve the success rate of isosexual introductions of unfamiliar macaques as compared with the success rate of GS. We administered 3.2 mg/kg oral diazepam to 34 adult male rhesus macaques (Macaca mulatta) 30-45 min prior to introduction into full contact. Pairs were deemed successful after 14 consecutive days of compatible full-contact housing. Behavioral data collected during these introductions was compared with data collected on 58 adult males during social introductions using GS. Sixteen of 17 introductions (94%) employing diazepam were successful. This success rate was significantly higher than the 45% success rate of introductions using GS. We also found that a longer duration of single housing and increased age were predictive of pair failure in animals introduced using GS. Our results suggest that diazepam administration prior to full contact introductions increases the success rate of male social introductions.

Behavioral Effects of Buspirone in Juvenile Zebrafish of Two Different Genetic Backgrounds

Anxiety continues to represent a major unmet medical need. Despite the availability of numerous anxiolytic drugs, a large proportion of patients do not respond well to current pharmacotherapy, or their response diminishes with chronic drug application. To discover novel compounds and to investigate the mode of action of anxiolytic drugs, animal models have been proposed. The zebrafish is a novel animal model in this research. It is particularly appropriate, as it has evolutionarily conserved features, and drug administration can be employed in a non-invasive manner by immersing the fish into the drug solution. The first step in the analysis of anxiolytic drugs with zebrafish is to test reference compounds. Here, we investigate the effects of buspirone hydrochloride, an anxiolytic drug often employed in the human clinic. We utilize two genetically distinct populations of zebrafish, AB_SK, derived from the quasi-inbred AB strain, and WT, a genetically heterogeneous wild-type population. We placed juvenile (10–13-day, post-fertilization, old) zebrafish singly in petri dishes containing one of four buspirone concentrations (0 mg/L control, 5 mg/L, 20 mg/L or 80 mg/L) for 1 h, with each fish receiving a single exposure to one concentration, a between subject experimental design. Subsequently, we recorded the behavior of the zebrafish for 30 min using video-tracking. Buspirone decreased distance moved, number of immobility episodes and thigmotaxis, and it increased immobility duration and turn angle in a quasi-linear dose dependent but genotype independent manner. Although it is unclear whether these changes represent anxiolysis in zebrafish, the results demonstrate that behavioral analysis of juvenile zebrafish may be a sensitive and simple way to quantify the effects of human anxiolytic drugs.

Inhibition of the Deep and Intermediate Layers of the Superior Colliculus Disrupts Sensorimotor Gating in Monkeys

The deep and intermediate layers of the superior colliculus (DLSC) respond to visual, auditory, and tactile inputs and act as a multimodal sensory association area. In turn, activity in the DLSC can drive orienting and avoidance responses-such as saccades and head and body movements-across species, including in rats, cats, and non-human primates. As shown in rodents, DLSC also plays a role in regulating pre-pulse inhibition (PPI) of the acoustic startle response (ASR), a form of sensorimotor gating. DLSC lesions attenuate PPI and electrical stimulation of DLSC inhibits the startle response. While the circuitry mediating PPI is well-characterized in rodents, less is known about PPI regulation in primates. Two recent studies from our labs reported a species difference in the effects of pharmacological inhibition of the basolateral amygdala and substantia nigra pars reticulata (SNpr) on PPI between rats and macaques: in rats, inhibition of these structures decreased PPI, while in macaques, it increased PPI. Given that the SNpr sends direct inhibitory projections to DLSC, we next sought to determine if this species difference was similarly evident at the level of DLSC. Here, we transiently inactivated DLSC in four rhesus macaques by focal microinfusion of the GABAA receptor agonist muscimol. Similar to findings reported in rodents, we observed that bilateral inhibition of the DLSC in macaques significantly disrupted PPI. The impairment was specific to the PPI as the ASR itself was not affected. These results indicate that our previously reported species divergence at the level of the SNpr is not due to downstream differences at the level of the DLSC. Species differences at the level of the SNpr and basolateral amygdala emphasize the importance of studying the underlying circuitry in non-human primates, as impairment in PPI has been reported in several disorders in humans, including schizophrenia, autism, and PTSD.

Cannabidiol effects on prepulse inhibition in nonhuman primates

Prepulse inhibition (PPI) of acoustic startle reflex is a well-established behavior paradigm to measure sensorimotor gating deficits. PPI is disrupted in several neuropsychiatric disorders, including schizophrenia. PPI tests can be used to screen new drugs for treatment of such disorders. In this review, we discuss how PPI paradigm can help in screening the therapeutic effects of cannabidiol (CBD). We look into recent literature about CBD effects on PPI response in animal models, especially in nonhuman primates. CBD has been shown to modify PPI inN-methyld-aspartate receptor antagonist models for schizophrenia, both in rodents and in nonhuman primates. These results show CBD as a potential drug for the treatment of neurologic disorders that present alterations in sensorimotor system, such as schizophrenia. Moreover, the PPI paradigm seems to be a useful and relative simple paradigm to test the efficacy of CBD as a potential therapeutic drug.

Effects of buprenorphine on responses to social stimuli in healthy adults

In addition to its classical role in mediating responses to pain, the opioid system is strongly implicated in the regulation of social behavior. In young laboratory animals, low doses of opioid analgesic drugs reduce responses to isolation distress and increase play behavior. However, little is known about how opioid drugs affect responses to social stimuli in humans. Here we examined the effects of buprenorphine, a mu-opioid partial agonist and kappa-antagonist, on three dimensions of social processing: (i) responses to simulated social rejection, (ii) attention to emotional facial expressions, and (iii) emotional responses to images with and without social content. Healthy adults (N=36) attended two sessions during which they received either placebo or 0.2mg sublingual buprenorphine in randomized order, under double-blind conditions. Ninety minutes after drug administration, they completed three behavioral tasks: (i) a virtual ball-toss game in which they were first included and then excluded by the other players; (ii) an attention task in which they were shown pairs of faces (one emotional and one neutral), while the direction of their gazes was recorded using electrooculography, and (iii) a picture-viewing task, in which they rated standardized images with and without social content. During the ball-toss game, buprenorphine decreased perceived social rejection. During the attention task, the drug reduced initial attention to fearful facial expressions, without influencing attention to angry, happy, or sad faces. Finally, during the picture-viewing task, buprenorphine increased ratings of positivity of images with social content without affecting ratings of nonsocial images. These results suggest that even at low doses, opioid analgesic drugs reduce responses to some types of negative social stimuli while enhancing positive responses to social stimuli. This provides further support for the role of the opioid system in mediating responses to social rejection and social reward.

Sensorimotor gating impairments induced by MK-801 treatment may be reduced by tolerance effect and by familiarization in monkeys

Dizocilpine (MK-801) is a non-competitive NMDA antagonist that induces schizophreniclike effects. It is therefore widely used in experimental models of schizophrenia including prepulse inhibition (PPI) impairments in rodents. Nevertheless, MK-801 has never been tested in monkeys on a PPI paradigm. In order to evaluate MK-801 effects on monkeys’ PPI, we tested eight capuchin monkeys (Sapajus spp.) using three different doses of MK-801 (0.01; 0.02; 0.03 mg/kg). Results show PPI impairment in acute administration of the highest dose (0.03 mg/kg). PPI impairment induced by MK-801 was reversed by re-exposure to the PPI test throughout treatment trials, in contrast with rodent studies. These results indicate that tolerance effect and familiarization with PPI test may reduce the sensorimotor gating deficits induced by MK-801 in monkeys, suggesting a drug-training interaction.

Opioid-dependent regulation of high and low fear responses in two inbred mouse strains

The molecular mechanisms underlying the susceptibility or resilience to trauma-related disorders remain incompletely understood. Opioids modulate emotional learning, but the roles of specific receptors are unclear. Here, we aimed to analyze the contribution of the opioid system to fear responses in two inbred mouse strains exhibiting distinct behavioral phenotypes. SWR/J and C57BL/6J mice were subjected to five consecutive electric footshocks (1mA each), and the contextual freezing time was measured. Stress-induced alterations in gene expression were analyzed in the amygdala and the hippocampus. In both strains, the fear response was modulated using pharmacological tools. SWR/J mice did not develop conditioned fear but exhibited increased transcriptional expression of Pdyn and Penk in the amygdala region. Blocking opioid receptors prior to the footshocks using naltrexone (2 mg/kg) or naltrindole (5 mg/kg) increased the freezing responses in these animals. The C57BL/6J strain displayed high conditioned fear, although no alteration in the mRNA abundance of genes encoding opioid precursors was observed. Double-injection of morphine (20 mg/kg) following stress and upon context re-exposure prevented the enhancement of freezing. Moreover, selective delta and kappa agonists caused a reduction in conditioned fear responses. To summarize, the increased expression of the Pdyn and Penk genes corresponded to reduced intensity of fear responses. Blockade of the endogenous opioid system restored freezing behavior in stress-resistant animals. The pharmacological stimulation of the kappa and delta opioid receptors in stress-susceptible individuals may alleviate fear. Thus, subtype-selective opioid receptor agonists may protect against the development of trauma-related disorders.

Assessing anxiety in nonhuman primates

Anxiety can be broadly described as a psychological state in which normally innocuous environmental stimuli trigger negative emotional expectations. Human anxiety disorders are multidimensional and may be organic or acquired, situational or pervasive. The broad ranging nature of the anxiety phenotype speaks to the need for models that identify its various components and root causes to develop effective clinical treatments. The cross-species comparative approach to modeling anxiety disorders in animals aims to understand mechanisms that both contribute to and modulate anxiety. Nonhuman primate models provide an important bridge from nonprimate model systems because of the complexity of nonhuman primates' biobehavioral capacities and their commonalities with human emotion. The broad goal of this review is to provide an overview of various procedures available to study anxiety in the nonhuman primate, with a focus on the behavioral aspects of anxiety. Commonly used methods covered in this review include assessing animals in their home environment or in response to an ethologically relevant threat, associative conditioning and startle response tests, and cognitive bias tests. We also discuss how these procedures can help veterinarians and researchers care for captive nonhuman primates.

Whole-body prepulse inhibition protocol to test sensorymotor gating mechanisms in monkeys

Prepulse inhibition (PPI) is the decrease of startle reflex amplitude when a slight stimulus is previously generated. This paradigm may provide valuable information about sensorimotor gating functionality. Here we aimed at determining the inhibited and uninhibited startle response of capuchin monkeys (Sapajus spp.), and to evaluate the role of the superior colliculus in PPI. Capuchin monkeys were tested in a whole-body protocol, to determine the best startle amplitude and interstimuli interval. Additionally we tested two subjects with bilateral superior colliculus damage in this protocol. Results show that 115 dB auditory pulse has induced the best startle response. In contrast to reports in other species, no habituation to the auditory stimuli was observed here in capuchins. Also, startle reflex inhibition was optimal after 120 msec interstimuli interval. Finally, there was a downward tendency of percentage inhibition in superior colliculus-lesioned monkeys. Our data provides the possibility of further studies with whole-body protocol in capuchin monkeys and reinforces the importance of the superior colliculus in PPI.

Effects of neonatal amygdala lesions on fear learning, conditioned inhibition, and extinction in adult macaques

Fear conditioning studies have demonstrated the critical role played by the amygdala in emotion processing. Although all lesion studies until now investigated the effect of adult-onset damage on fear conditioning, the current study assessed fear-learning abilities, as measured by fear-potentiated startle, in adult monkeys that had received neonatal neurotoxic amygdala damage or sham-operations. After fear acquisition, their abilities to learn and use a safety cue to modulate their fear to the conditioned cue, and, finally, to extinguish their response to the fear conditioned cue were measured with the AX+/BX- Paradigm. Neonatal amygdala damage retarded, but did not completely abolish, the acquisition of a learned fear. After acquisition of the fear signal, four of the six animals with neonatal amygdala lesions discriminated between the fear and safety cues and were also able to use the safety signal to reduce the potentiated-startle response and to extinguish the fear response when the air-blast was absent. In conclusion, the present results support the critical contribution of the amygdala during the early phases of fear conditioning that leads to quick, robust responses to potentially threatening stimuli, a highly adaptive process across all species and likely to be present in early infancy. The neonatal amygdala lesions also indicated the presence of amygdala-independent alternate pathways that are capable to support fear learning in the absence of a functional amygdala. This parallel processing of fear responses within these alternate pathways was also sufficient to support the ability to flexibly modulate the magnitude of the fear responses.

A novel AX+/BX- paradigm to assess fear learning and safety-signal processing with repeated-measure designs

One of the core symptoms of anxiety disorders, such as post-traumatic stress disorder, is the failure to overcome feelings of danger despite being in a safe environment. This deficit likely stems from an inability to fully process safety signals, which are cues in the environment that enable healthy individuals to over-ride fear in aversive situations. Studies examining safety signal learning in rodents, humans, and non-human primates currently rely on between-groups designs. Because repeated-measure designs reduce the number of subjects required, and facilitate a broader range of safety signal studies, the current project sought to develop a repeated-measures safety-signal learning paradigm in non-human primates. Twelve healthy rhesus macaques of both sexes received three rounds of auditory fear-potentiated startle training and testing using an AX+/BX- design with all visual cues. Cue AX was paired with an aversive blast of air, whereas the same X cue in compound with another B cue (BX) signaled the absence of an air blast. Hence, cue B served as a safety signal. Once animals consistently discriminated between the aversive (AX+) and safe (BX-) cues, measured by greater startle amplitude in the presence of AX vs. BX, they were tested for conditioned inhibition by eliciting startle in the presence of a novel ambiguous combined cue (AB). Similar to previous AX+/BX- studies, healthy animals rapidly learned to discriminate between the AX+ and BX- cues as well as demonstrate conditioned inhibition in the presence of the combined AB cue (i.e. lower startle amplitude in the presence of AB vs. AX). Additionally, animals performed consistently across three rounds of testing using three new cues each time. The results validate this novel method that will serve as a useful tool for better understanding the mechanisms for the regulation of fear and anxiety.

The brain orexin system and almorexant in fear-conditioned startle reactions in the rat

RATIONALE

The rat fear-potentiated startle (FPS) paradigm is a translational model of conditioned fear involving central amygdala pathways of the brain. Hypothalamic orexin neurons have input-output projections to the amygdala; they modulate vigilance and stress-related responses.

OBJECTIVE

To investigate whether the transient pharmacological blockade of orexin receptors moderates the conditioned fear response.

METHODS

F344 rats received acute oral treatment with the dual orexin receptor antagonist almorexant (30-300 mg/kg) or with one of the clinically effective anxiolytics diazepam (1-10 mg/kg), buspirone (10-100 mg/kg), fluoxetine (3-30 mg/kg), and sertraline (10-100 mg/kg). Drug effects on startle responses were assessed in both fear- and non-fear-conditioned rats; on forepaw grip and horizontal wire motor performance, and on elevated plus maze (EPM) behavior.

RESULTS

Diazepam and almorexant both dose-dependently decreased FPS in the presence of the fear-conditioned stimulus (CS; light) more prominently than background startle in absence of the CS (dark). Diazepam induced myorelaxation and reduced startle responses in control non-fear-conditioned rats. Almorexant had no myorelaxant effects and left startle responses under light in non-fear-conditioned rats intact. On the EPM, diazepam showed anxiolytic-like effects, almorexant not. Buspirone demonstrated anxiolytic-like effects on FPS by simultaneously reducing CS-related startle and increasing no-CS-background startle. Fluoxetine did not affect FPS, whereas sertraline showed anxiogenic-like effects.

CONCLUSIONS

Almorexant reduced FPS, but did not affect EPM behavior. Almorexant's overall pattern of effects on FPS was comparable to but less pronounced than that of the anxiolytic benzodiazepine diazepam. The endogenous orexin system actively contributes to fear-conditioned startle reactions in the rat.

Increased anxiety-like behaviors in rats experiencing chronic inflammatory pain

For many patients, chronic pain is often accompanied, and sometimes amplified, by co-morbidities such as anxiety and depression. Although it represents important challenges, the establishment of appropriate preclinical behavioral models contributes to drug development for treating chronic inflammatory pain and associated psychopathologies. In this study, we investigated whether rats experiencing persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA) developed anxiety-like behaviors, and whether clinically used analgesic and anxiolytic drugs were able to reverse CFA-induced anxiety-related phenotypes. These behaviors were evaluated over 28 days in both CFA- and saline-treated groups with a variety of behavioral tests. CFA-induced mechanical allodynia resulted in increased anxiety-like behaviors as evidenced by: (1) a significant decrease in percentage of time spent and number of entries in open arms of the elevated-plus maze (EPM), (2) a decrease in number of central squares visited in the open field (OF), and (3) a reduction in active social interactions in the social interaction test (SI). The number of entries in closed arms in the EPM and the distance traveled in the OF used as indicators of locomotor performance did not differ between treatments. Our results also reveal that in CFA-treated rats, acute administration of morphine (3mg/kg, s.c.) abolished tactile allodynia and anxiety-like behaviors, whereas acute administration of diazepam (1mg/kg, s.c) solely reversed anxiety-like behaviors. Therefore, pharmacological treatment of anxiety-like behaviors induced by chronic inflammatory pain can be objectively evaluated using multiple behavioral tests. Such a model could help identify/validate alternative potential targets that influence pain and cognitive dimensions of anxiety.

SSRIs and conditioned fear

Among drugs that act on serotonergic neurotransmission, selective serotonin (5-HT) reuptake inhibitors (SSRIs) are now the gold standard for the treatment of anxiety disorders. The precise mechanisms of the anxiolytic actions of SSRIs are unclear. We reviewed the literature related to the effects of SSRIs and the neurochemical changes of 5-HT in conditioned fear. Acute SSRIs and 5-HT(1A) receptor agonists reduced the acquisition and expression of contextual conditioned fear. Chronic SSRI administration enhanced anxiolytic-like effects. Microinjection studies revealed the amygdala as the target brain region of both classes of serotonergic drugs, and the hippocampus as the target of 5-HT(1A) receptor agonists. These findings highlight the contribution of post-synaptic 5-HT receptors, especially 5-HT(1A) receptors, to the anxiolytic-like effects of serotonergic drugs. These results support the new 5-HT hypothesis of fear/anxiety: the facilitation of 5-HT neurotransmission ameliorates fear/anxiety. Furthermore, these behavioral data provide a new explanation of neurochemical adaptations to contextual conditioned fear: increased 5-HT transmission seems to decrease, not increase, fear.

Tools for translational neuroscience: PTSD is associated with heightened fear responses using acoustic startle but not skin conductance measures

BACKGROUND

Posttraumatic stress disorder (PTSD) patients show heightened fear responses to trauma reminders and an inability to inhibit fear in the presence of safety reminders. Brain imaging studies suggest that this is in part due to amygdala over-reactivity as well as deficient top-down cortical inhibition of the amygdala. Consistent with these findings, previous studies, using fear-potentiated startle (FPS), have shown exaggerated startle and deficits in fear inhibition in PTSD participants. However, many PTSD studies using the skin conductance response (SCR) report no group differences in fear acquisition.

METHOD

The study included 41 participants with PTSD and 70 without PTSD. The fear conditioning session included a reinforced conditioned stimulus (CS+, danger cue) paired with an aversive airblast, and a nonreinforced conditioned stimulus (CS-, safety cue). Acoustic startle responses and SCR were acquired during the presentation of each CS.

RESULTS

The results showed that fear conditioned responses were captured in both the FPS and SCR measures. Furthermore, PTSD participants had higher FPS to the danger cue and safety cue compared to trauma controls. However, SCR did not differ between groups. Finally, we found that FPS to the danger cue predicted re-experiencing symptoms, whereas FPS to the safety cue predicted hyper-arousal symptoms. However, SCR did not contribute to PTSD symptom variance.

CONCLUSIONS

Replicating earlier studies, we showed increased FPS in PTSD participants. However, although SCR was a good measure of differential conditioning, it did not differentiate between PTSD groups. These data suggest that FPS may be a useful tool for translational research.

Pharmacokinetics and behavioral effects of liposomal hydromorphone suitable for perioperative use in rhesus macaques

INTRODUCTION

This study aims to evaluate the pharmacokinetic, behavioral, and motor effects of a liposomal preparation of hydromorphone hydrochloride (LE-hydro) in rhesus monkeys. We administered either 2 mg/kg of LE-hydro (n = 8) subcutaneous (s.c.) or 0.1 mg/kg of standard pharmaceutical hydromorphone HCl (hydro) preparation either intravenous (i.v.; n = 4) or s.c. (n = 5).

MATERIALS AND METHODS

Serial blood samples were drawn after injection and analyzed for serum hydro concentration by liquid chromatography/mass spectrometry. Following s.c. injection of 0.1 mg/kg hydro or 2 mg/kg LE-hydro, behavioral evaluations were conducted in groups of rhesus monkeys (n = 10/group) in the presence of a compatible stimulus animal and motor skills were also evaluated (n = 10/group). The motor skills test consisted of removing a food reward (carrot ring) from either a straight peg (simple task) or a curved peg (difficult task).

RESULTS

LE-hydro (MRT(0-INF) = 105.9 h) demonstrated extended-release pharmacokinetics compared to hydro when administered by either i.v. (MRT(0-INF) =1.1 h) or s.c. (MRT(0-INF) =1.3 h) routes. Hydro did not affect motor performance of the simpler task, but the monkeys' performance deteriorated on the more difficult task at 0.5 and 1 h after injection. LE-hydro had no effect on motor skills in either the simpler or more difficult task.

CONCLUSIONS

The results of these studies indicate that LE-hydro has a pharmacokinetic and behavioral side effects profile consistent with an analgesic that could be tested for surgical use in animals. Our studies also expand the use of rhesus monkeys as a translational behavioral pharmacodynamics model for testing extended-release opioid medication.

Pharmacological modulation of anxiety-like phenotypes in adult zebrafish behavioral models

Zebrafish (Danio rerio) are becoming increasingly popular in neurobehavioral research. Here, we summarize recent data on behavioral responses of adult zebrafish to a wide spectrum of putative anxiolytic and anxiogenic agents. Using the novel tank test as a sensitive and efficient behavioral assay, zebrafish anxiety-like behavior can be bi-directionally modulated by drugs affecting the gamma-aminobutyric acid, monoaminergic, cholinergic, glutamatergic and opioidergic systems. Complementing human and rodent data, zebrafish drug-evoked phenotypes obtained in this test support this species as a useful model for neurobehavioral and psychopharmacological research.

Phasic and sustained fear are pharmacologically dissociable in rats

Previous findings suggest differences in the neuroanatomical substrates of short- (seconds) vs longer-duration (minutes) fear responses. We now report that phasic and sustained fear can also be differentiated pharmacologically, based on their response to several treatments that either are or are not clinically effective anxiolytics. For these experiments, short- or long-duration clicker stimuli were paired with footshock. Acoustic startle amplitude was later measured in the absence of the clicker, or within seconds (phasic fear) or minutes (sustained fear) of its onset. Before testing, rats received a single injection of vehicle, the benzodiazepine chlordiazepoxide, the 5HT(1A) agonist and dopamine D2 antagonist buspirone, the selective serotonin reuptake inhibitor fluoxetine, or a 3-week treatment with either vehicle or fluoxetine. Chlordiazepoxide blocked sustained, but not phasic startle increases. Acute buspirone, which is not anxiolytic in human beings, did not affect sustained startle increases, but did disrupt phasic increases. Chronic fluoxetine blocked sustained startle increases and unreliably reduced phasic increases; acute fluoxetine affected neither. The results indicate that phasic and sustained fear responses can be pharmacologically dissociated, further validating this distinction, and suggest that sustained startle increases may be especially useful as anxiety models and anxiolytic screens.

Anxiolytic-like effects of the neurokinin 1 receptor antagonist GR-205171 in the elevated plus maze and contextual fear-potentiated startle model of anxiety in gerbils

Gerbils show a neurokinin (NK)1 receptor pharmacological profile, which is similar to that observed in humans, and thus have become a commonly used species to test efficacy of NK1 receptor antagonists. The aim of this study was to determine whether systemic administration of the NK1 receptor antagonist GR-205171 produced anxiolytic-like effects in the elevated plus maze and in a novel contextual conditioned fear test using fear-potentiated startle (FPS). On the elevated plus maze, treatment with GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing produced anxiolytic-like effects in an increasing dose-response manner as measured by the percentage of open arm time and percentage of open arm entries. For contextual fear conditioning, gerbils were given 10 unsignaled footshocks (0.6 mA) at a 2-min variable interstimulus interval in a distinctive training context. Twenty-four hours after training, gerbils received treatment of GR-205171 at 0, 0.3, 1.0, and 5.0 mg/kg doses, 30 min before testing in which startle was elicited in the same context in which they were trained. Contextual FPS was defined as an increase in startle over pretraining baseline values. All drug dose levels (0.3, 1.0, and 5.0 mg/kg) significantly attenuated contextual FPS when compared with the vehicle control group. A control group, which received testing in a different context, showed little FPS. These findings support other evidence for anxiolytic activity of NK1 receptor antagonists and provide a novel conditioned fear test that may be an appropriate procedure to test other NK1 antagonists for preclinical anxiolytic activity in gerbils.

Oxytocin reduces background anxiety in a fear-potentiated startle paradigm

Oxytocin reportedly decreases anxious feelings in humans and may therefore have therapeutic value for anxiety disorders, such as post-traumatic stress disorder (PTSD). As PTSD patients have exaggerated startle responses, a fear-potentiated startle paradigm in rats may have face validity as an animal model to examine the efficacy of oxytocin in treating these symptoms. Oxytocin (0, 0.01, 0.1, or 1.0 μg, subcutaneously) was given either 30 min before fear conditioning, immediately after fear conditioning, or 30 min before fear-potentiated startle testing to assess its effects on acquisition, consolidation, and expression of conditioned fear, respectively. Startle both in the presence and absence of the fear-conditioned light was significantly diminished by oxytocin when administered at acquisition, consolidation, or expression. There was no specific effect of oxytocin on light fear-potentiated startle. In an additional experiment, oxytocin had no effects on acoustic startle without previous fear conditioning. Further, in a context-conditioned test, previous light-shock fear conditioning did not increase acoustic startle during testing when the fear-conditioned light was not presented. The data suggest that oxytocin did not diminish cue-specific conditioned nor contextually conditioned fear, but reduced background anxiety. This suggests that oxytocin has unique effects of decreasing background anxiety without affecting learning and memory of a specific traumatic event. Oxytocin may have antianxiety properties that are particularly germane to the hyper-vigilance and exaggerated startle typically seen in PTSD patients.

Testosterone may increase selective attention to threat in young male macaques

Animal studies indicate that sex hormones have widespread effects on the brain, cognition and emotion, but findings in humans are inconsistent. Well-controlled studies in nonhuman primates are crucial to resolve these discrepancies. In this study, we examined the effects of testosterone (T) on emotion in male rhesus monkeys. Six young adult males were tested on two emotional tasks during three hormonal conditions in a crossover design: when intact at baseline and when pharmacologically hypogonadal with add-back of T or placebo. The emotional tasks were the Approach-Avoidance task, which tested behavioral responses to three categories of objects (familiar, novel, and negative) and a Social Playback task which tested behavioral responses to scenes of unfamiliar conspecifics engaged in three types of social activities (neutral, positive, or negative). Following a 4-week baseline period, monkeys were treated with Depot Lupron, 200μg/kg before being randomly assigned to one of two treatment groups: Depot Lupron+Testosterone Enanthate (TE, 20mg/kg) or Depot Lupron+oil vehicle. In each treatment group, monkeys received one injection of Lupron and one injection of TE or one injection of Lupron and one injection of oil at the onset of a 4-week testing period, before crossing over to the alternate treatment for an additional 4weeks of testing. TE treatment had no effect on behavioral measures in the Approach-Avoidance task. For the Social Playback task, however, TE significantly increased watching time of video clips which depicted fights between unfamiliar conspecifics. The enhancing effect of T on watching time for negative social scenes is consistent with human data suggesting that T decreases aversion or facilitates approach to threatening social stimuli. Further studies are needed to understand the mechanisms by which T may mediate responsiveness to social threat in male primates.

The effect of gonadectomy on prepulse inhibition and fear-potentiated startle in adolescent rhesus macaques

Sex steroids, such as testosterone, can regulate brain development, cognition and modify psychiatric conditions. However, the role of adolescent testosterone in the emergence of cognitive deficits relevant to psychiatric illness has not been directly studied in primates. We examined whether removing testosterone during adolescence in rhesus macaques would affect prepulse inhibition (PPI) and fear-potentiated startle (FPS), which are translational tests of cognition affected in psychiatric disorders. Prepubertal macaques (30 months old) were castrated (n=6) or sham operated (n=6), and PPI and (FPS) were tested before the onset of puberty (34 months old) and after the pubertal surge in sex hormones 16 months later (50 months old). As expected there were no differences between the gonadectomized and intact groups' level of startle amplitude, PPI or (FPS) before puberty. After puberty, the intact group displayed substantially less PPI than the gonadectomized group, consistent with evidence that PPI is attenuated by endogenous increases in sex hormones. At the end of the study, testosterone among the intact monkeys was also correlated with tyrosine hydroxylase levels in the putamen, suggesting the attenuation of PPI by gonadal sex hormones may be influenced by subcortical dopamine. Thus, puberty involves significant increases in sex hormones, which in turn may modulate subcortical dopamine synthesis and affect cognitive functions impaired in psychiatric illnesses such as schizophrenia.

Increased auditory startle reflex in children with functional abdominal pain

OBJECTIVE

To test the hypothesis that children with abdominal pain-related functional gastrointestinal disorders have a general hypersensitivity for sensory stimuli.

STUDY DESIGN

Auditory startle reflexes were assessed in 20 children classified according to Rome III classifications of abdominal pain-related functional gastrointestinal disorders (13 irritable bowel syndrome [IBS], 7 functional abdominal pain syndrome; mean age, 12.4 years; 15 girls) and 23 control subjects (14 girls; mean age, 12.3 years) using a case-control design. The activity of 6 left-sided muscles and the sympathetic skin response were obtained by an electromyogram. We presented sudden loud noises to the subjects through headphones.

RESULTS

Both the combined response of 6 muscles and the blink response proved to be significantly increased in patients with abdominal pain compared with control subjects. A significant increase of the sympathetic skin response was not found. Comorbid anxiety disorders (8 patients with abdominal pain) or Rome III subclassification did not significantly affect these results.

CONCLUSIONS

This study demonstrates an objective hyperresponsivity to nongastrointestinal stimuli. Children with abdominal pain-related functional gastrointestinal disorders may have a generalized hypersensitivity of the central nervous system.

Behavioral correlates of anxiety

The tripartite model of anxiety includes three response domains: cognitive (most often identified by self report), behavioral, and physiological. Each is suggested to bring a separate element of response characteristics and, in some cases, potentially independent underlying mechanisms to the construct of anxiety. In this chapter, commonly used behavioral correlates of anxiety in human research, including startle reflex, attentional bias, and avoidance tasks, as well as future tasks using virtual reality technology will be discussed. The focus will be in evaluating their translational utility supported by (1) convergent validity with other measures of anxiety traits or anxiety disorders, (2) their use in identifying neural and genetic mechanisms of anxiety, and (3) ability to predict treatment efficacy.

Phasic vs sustained fear in rats and humans: role of the extended amygdala in fear vs anxiety

Data will be reviewed using the acoustic startle reflex in rats and humans based on our attempts to operationally define fear vs anxiety. Although the symptoms of fear and anxiety are very similar, they also differ. Fear is a generally adaptive state of apprehension that begins rapidly and dissipates quickly once the threat is removed (phasic fear). Anxiety is elicited by less specific and less predictable threats, or by those that are physically or psychologically more distant. Thus, anxiety is a more long-lasting state of apprehension (sustained fear). Rodent studies suggest that phasic fear is mediated by the amygdala, which sends outputs to the hypothalamus and brainstem to produce symptoms of fear. Sustained fear is also mediated by the amygdala, which releases corticotropin-releasing factor, a stress hormone that acts on receptors in the bed nucleus of the stria terminalis (BNST), a part of the so-called 'extended amygdala.' The amygdala and BNST send outputs to the same hypothalamic and brainstem targets to produce phasic and sustained fear, respectively. In rats, sustained fear is more sensitive to anxiolytic drugs. In humans, symptoms of clinical anxiety are better detected in sustained rather than phasic fear paradigms.

Pharmacokinetics and behavioral effects of an extended-release, liposome-encapsulated preparation of oxymorphone in rhesus macaques

The objectives of the study were to determine the pharmacokinetics of oxymorphone (oxy) and of ammonium sulfate-loaded, liposome-encapsulated oxymorphone (LE-ASG oxy) and to evaluate the behavioral effects of both opioid preparations by using ethographic evaluation specific to rhesus monkeys. Rhesus monkeys (n = 8) were injected with 2.0 mg/kg LE-ASG oxy s.c.. Blood samples were collected at serial time points up to 144 h in six monkeys and up to 456 h in two monkeys. Separate groups of monkeys were injected with 0.1 mg/kg oxy s.c. (n = 4) or i.v. (n = 5). Blood samples were collected at serial time points up to 24 h after injection. Pharmacokinetic parameters were calculated by using commercially available software. Behavior was recorded in a different group of 10 monkeys administered LE-ASG oxy (2.0 mg/kg s.c.) or oxy (0.1 mg/kg s.c.) on separate occasions. Behavioral evaluations were made at serial time points while monkeys were in an extended cage with a compatible stimulus animal. Oxymorphone was rapidly eliminated from the serum in the oxy group. Measurable drug was present in serum for up to 4 h after oxy was administered subcutaneously or intravenously. LE-ASG oxy was present in serum in measurable concentrations for more than 2 weeks. Neither oxy nor LE-ASG oxy produced observable sedation. LE-ASG oxy decreased some environmentally directed behaviors, but this drug formulation increased watchfulness, decreased self-directed and elimination behaviors, increased nonspecific social contact, and decreased threat behaviors. LE-ASG oxy persisted for an extended period in rhesus monkey serum and produced behavioral changes consistent with this opioid.

The nonhuman primate amygdala is necessary for the acquisition but not the retention of fear-potentiated startle

BACKGROUND

In a previous study, we found that rhesus monkeys prepared with bilateral lesions of the amygdala failed to acquire fear-potentiated startle to a visual cue. However, a second group of monkeys, which received the lesion after training, successfully demonstrated fear-potentiated startle learned prior to the lesion.

METHODS

In the current experiment, the eight monkeys used in the second part of the original study, four of which had bilateral amygdala lesions and the four control animals, were trained using an auditory cue and tested in the fear-potentiated startle paradigm. This test was performed to determine whether they could acquire fear-potentiated startle to a new cue.

RESULTS

Monkeys with essentially complete damage to the amygdala (based on histological analysis) that had retained and expressed fear-potentiated startle to a visual cue learned before the lesion failed to acquire fear-potentiated startle to an auditory cue when training occurred after the lesion.

CONCLUSIONS

The results suggest that while the nonhuman primate amygdala is essential for the initial acquisition of fear conditioning, it does not appear to be necessary for the memory and expression of conditioned fear. These findings are discussed in relation to a network of connections between the amygdala and the orbitofrontal cortex that may subserve different component processes of fear conditioning.

Activation of nociceptin opioid peptide (NOP) receptor impairs contextual fear learning in mice through glutamatergic mechanisms

The present study investigated whether the selective nociceptin opioid peptide (NOP) receptor agonist, Ro64-6198, impairs acquisition of fear conditioning through glutamatergic mechanisms. Systemic administration of Ro64-6198 (0.3 and 1mg/kg) or the non-competitive NMDA receptor antagonist, MK-801 (0.03 and 0.1mg/kg) prior to conditioning severely impaired contextual but not cued fear learning in C57BL/6N mice. When administered together at sub-effective doses, Ro64-6198 (0.5mg/kg) and MK-801 (0.05mg/kg), synergistically impaired contextual fear learning, but left cued fear learning intact. We next used the immediate shock deficit paradigm (ISD) to examine the effects of Ro64-6198 and MK-801 on contextual memory formation in the absence of the foot-shock. As expected, naive mice that were shocked briefly after being placed in the training chamber displayed no contextual fear conditioning. This learning deficit was elevated by prior exposure of mice to the training context. Furthermore, administration of Ro64-6198 and MK-801, either separately at amnesic doses (1mg/kg and 0.1mg/kg, respectively) or concomitantly at sub-effective doses (0.5mg/kg and 0.05mg/kg, respectively) significantly reduced the facilitating effects of context preexposure. These findings demonstrate the existence of functional antagonism between NOP and NMDA receptors that predominantly contributes to modulation of conditioned fear learning which involves spatial-processing demands.

Animal models of anxiety and anxiolytic drug action

Animal models of anxiety attempt to represent some aspect of the etiology, symptomatology, or treatment of human anxiety disorders, in order to facilitate their scientific study. Within this context, animal models of anxiolytic drug action can be viewed as treatment models relevant to the pharmacological control of human anxiety. A major purpose of these models is to identify novel anxiolytic compounds and to study the mechanisms whereby these compounds produce their anxiolytic effects. After a critical analysis of "face," "construct," and "predictive" validity, the biological context in which animal models of anxiety are to be evaluated is specified. We then review the models in terms of their general pharmacological profiles, with particular attention to their sensitivity to 5-HTIA agonists and antidepressant compounds. Although there are important exceptions, most of these models are sensitive to one or perhaps two classes of anxiolytic compounds, limiting their pharmacological generality somewhat, but allowing in depth analysis of individual mechanisms of anxiolytic drug action (e.g., GABAA agonism). We end with a discussion of possible sources of variability between models in response to 5-HTIA agonists and antidepressant drugs.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

The neurotensin-1 receptor agonist PD149163 blocks fear-potentiated startle

Preliminary evidence suggests that the neuropeptide, neurotensin (NT) may regulate fear/anxiety circuits. We investigated the effects of PD149163, a NT1 receptor agonist, on fear-potentiated startle (FPS). Sprague Dawley rats were trained to associate a white light with a mild foot shock. In one experiment, animals were treated with either subcutaneous vehicle or PD149163 (0.01, 0.1 or 1.0 mg/kg) 24 h after training. Twenty minutes later their acoustic startle response in the presence or absence of the white light was tested. In a second experiment, saline and 1.0 mg/kg PD149163 were tested using a separate group of rats. In the first experiment, PD149163 produced a non-significant decrease in baseline acoustic startle at all three doses. As expected, saline-treated rats exhibited significant FPS. An ANOVA of percentage FPS revealed no significant effect of treatment group overall but the high dose group did not display FPS strongly suggesting an FPS effect at this dose. This finding was confirmed in the second experiment where the high dose of PD149163 reduced percent FPS relative to saline (P < 0.05). These data suggest that systemically administered NT1 agonists modulate the neural circuitry that regulates fear and anxiety to produce dose-dependent anxiolytic-like effects on FPS.

Acoustic startle reflex in rhesus monkeys: a review

Modulation of the acoustic startle response is a simple and objective indicator of emotionality and attention in rodents and humans. This finding has proven extremely valuable for the analysis of neural systems associated with fear and anxiety. Until recently, there have been few efforts to develop acoustic startle measurement in non-human primates. Here we review recent work in which whole body acoustic startle amplitude has been measured in rhesus monkeys. Initial studies revealed that the amplitude of whole body startle in monkeys, as in rodents and humans, is directly proportional to acoustic stimulus intensity and gradually habituates with repeated exposures. Presentation of a weak acoustic stimulus 25-5,000 msec before a startle stimulus reduces startle amplitude by 40-50% depending on inter-stimulus interval length (prepulse inhibition). We have also measured significant fear-potentiated startle in the presence of a visual stimulus after pairing it with an inescapable pulse of pressurized air (fear-potentiated startle). This effect was reduced by diazepam and morphine, but not by buspirone. Ibotenic acid-induced lesions of the amygdala prevented the acquisition of fear-potentiated startle but, remarkably, did not prevent the expression of fear-potentiated startle when fear conditioning was carried out prior to the lesion. Finally, we have developed an objective measure of fear inhibition in monkeys using a novel conditioned inhibition procedure identical to one used in rats and humans. Our data demonstrate that acoustic startle in non-human primates successfully bridges rodent and human research. The opportunity now emerges to link concepts developed in rodents to the more complex neuroanatomical and cognitive processes common to monkeys and humans.

Predatory stress as an experimental strategy to measure fear and anxiety-related behaviors in non-human primates

Natural defense-inducing stimuli are being increasingly exploited as a means to investigate the neural mechanisms underlying normal and pathological anxiety, as well as for the screening of new compounds with potential therapeutic use in human anxiety disorders. Such an approach, frequently used in rodents, has recently been employed in the Marmoset Predator Confrontation Test (MPCT). In this method, marmoset monkeys are individually confronted with a taxidermized predator (wild oncilla cat) in a previously habituated maze environment, while several easily discernable fear/anxiety-related behaviors are measured. Confrontation with the cat stimulus significantly altered ongoing behaviors, each habituating distinctively during repeated exposures; e.g. complete rapid habituation (alarm call), complete slow habituation (exploration, vigilance) or only partial habituation (proximity avoidance). Pharmacological validating studies with diazepam and buspirone induced a significant dose-dependent reversal of the fear-induced proximic avoidance and scratching/scent-marking behaviors, while exploration (smell/lick the maze, leg stand) was found to increase. The neuropeptide substance P and the selective 5-HT1A receptor antagonist WAY100635 resulted in a similar anxiolytic-like profile. The response pattern observed was not influenced by social isolation, handling/manual restraint, novel environment exposure or habituation to the stimulus or its location. Persistent defensive behavior and response pattern to diazepam was observed when naive versus MPCT-experienced marmosets were tested following a recent predatory stress. Taken together, the results indicate that the MPCT is a valuable experimental procedure to measure fear and anxiety-related behaviors in nonhuman primates.

Neurochemistry of the afferents to the rat cochlear root nucleus: possible synaptic modulation of the acoustic startle

Afferents to the primary startle circuit are essential for the elicitation and modulation of the acoustic startle reflex (ASR). In the rat, cochlear root neurons (CRNs) comprise the first component of the acoustic startle circuit and play a crucial role in mediating the ASR. Nevertheless, the neurochemical pattern of their afferents remains unclear. To determine the distribution of excitatory and inhibitory inputs, we used confocal microscopy to analyze the immunostaining for vesicular glutamate and GABA transporter proteins (VGLUT1 and VGAT) on retrogradely labeled CRNs. We also used reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry to detect and localize specific neurotransmitter receptor subunits in the cochlear root. Our results show differential distributions of VGLUT1- and VGAT-immunoreactive endings around cell bodies and dendrites. The RT-PCR data showed a positive band for several ionotropic glutamate receptor subunits, M1-M5 muscarinic receptor subtypes, the glycine receptor alpha1 subunit (GlyRalpha1), GABAA, GABAB, and subunits of alpha2 and beta-noradrenergic receptors. By immunohistochemistry, we confirmed that CRN cell bodies exhibit positive immunoreaction for the glutamate receptor (GluR) 3 and NR1 GluR subunits. Cell bodies and dendrites were also positive for M2 and M4, and GlyRalpha1. Other subunits, such as GluR1 and GluR4 of the AMPA GluRs, were observed in glial cells neighboring unlabeled CRN cell bodies. We further confirmed the existence of noradrenergic afferents onto CRNs from the locus coeruleus by combining tyrosine hydroxylase immunohistochemistry and tract-tracing experiments. Our results provide valuable information toward understanding how CRNs might integrate excitatory and inhibitory inputs, and hence how they could elicit and modulate the ASR.

Role of the primate amygdala in fear-potentiated startle: effects of chronic lesions in the rhesus monkey

In experiment 1, we assessed the role of the primate amygdala and hippocampus in the acquisition of learned fear measured with fear-potentiated startle. Three groups of six rhesus monkeys were prepared with bilateral ibotenic acid lesions of the amygdaloid complex and the hippocampus or were sham operated. Selective ibotenic acid lesions of the amygdala, but not the hippocampus, blocked the acquisition of fear-potentiated startle. In experiment 2, we assessed the role of the primate amygdala in the expression of fear-potentiated startle. Surprisingly, animals that sustained amygdala damage after they successfully learned fear-potentiated startle expressed normal fear-potentiated startle, despite a complete amygdala lesion based on magnetic resonance imaging assessments. These results suggest that although the amygdala is necessary for the initial acquisition of fear-potentiated startle, it is not necessary for the retention and expression of fear-potentiated startle. These findings are discussed in relation to the role of the amygdala in emotional learning and in cross-species comparisons of emotional behavior.