Acoustic startle reflex in rhesus monkeys: a review

Quinpirole, but not muscimol, infused into the nucleus accumbens disrupts prepulse inhibition of the acoustic startle in rhesus macaques

Sensorimotor gating is the ability to suppress motor responses to irrelevant sensory inputs. This response is disrupted in a range of neuropsychiatric disorders. Prepulse inhibition (PPI) of the acoustic startle response (ASR) is a form of sensorimotor gating in which a low-intensity prepulse immediately precedes a startling stimulus, resulting in an attenuation of the startle response. PPI is conserved across species and the underlying circuitry mediating this effect has been widely studied in rodents. However, recent work from our laboratories has shown an unexpected divergence between the circuitry controlling PPI in rodents as compared to macaques. The nucleus accumbens, a component of the basal ganglia, has been identified as a key modulatory node for PPI in rodents. The role of the nucleus accumbens in modulating PPI in primates has yet to be investigated. We measured whole-body PPI of the ASR in six rhesus macaques following (1) pharmacological inhibition of the nucleus accumbens using the GABAA agonist muscimol, and (2) focal application of the dopamine D2/3 agonist quinpirole (at 3 doses). We found that quinpirole, but not muscimol, infused into the nucleus accumbens disrupts prepulse inhibition in monkeys. These results differ from those observed in rodents, where both muscimol and quinpirole disrupt prepulse inhibition.

Mechanisms of Short- and Long-Latency Sensory Suppression: Magnetoencephalography Study

Prepulse inhibition (PPI) is sensory suppression whose mechanism (i.e., whether PPI originates from specific inhibitory mechanisms) remains unclear. In this study, we applied the combination of short-latency PPI and long-latency paired pulse suppression in 17 healthy subjects using magnetoencephalography to investigate the mechanisms of sensory suppression. Repeats of a 25-ms pure tone without a blank at 800 Hz and 70 dB were used for a total duration of 1600 ms. To elicit change-related cortical responses, the sound pressure of two consecutive tones in this series at 1300 ms was increased to 80 dB (Test). For the conditioning stimuli, the sound pressure was increased to 73 dB at 1250 ms (Pre 1) and 80 dB at 700 ms (Pre 2). Six stimuli were randomly presented as follows: (1) Test alone, (2) Pre 1 alone, (3) Pre 1 + Test, (4) Pre 2 + Test, (5) Pre 2 + Pre 1, and (6) Pre 2 + Pre 1 + Test. The inhibitory effects of the conditioning stimuli were evaluated using N100m/P200m components. The results showed that both Pre 1 and Pre 2 significantly suppressed the Test response. Moreover, the inhibitory effects of Pre 1 and Pre 2 were additive. However, when both prepulses were present, Pre 2 significantly suppressed the Pre 1 response, suggesting that the Pre 1 response amplitude was not a determining factor for the degree of suppression. These results suggested that the suppression originated from a specific inhibitory circuit independent of the excitatory pathway.

A midbrain-reticulotegmental circuit underlies exaggerated startle under fear emotions

Exaggerated startle has been recognized as a core hyperarousal symptom of multiple fear-related anxiety disorders, such as post-traumatic stress disorder (PTSD) and panic disorder. However, the mechanisms driving this symptom are poorly understood. Here we reveal a neural projection from dorsal raphe nucleus (DRN) to a startle-controlling center reticulotegmental nucleus (RtTg) that mediates enhanced startle response under fear condition. Within RtTg, we identify an inhibitory microcircuit comprising GABAergic neurons in pericentral RtTg (RtTgP) and glutamatergic neurons in central RtTg (RtTgC). Inhibition of this RtTgP-RtTgC microcircuit leads to elevated startle amplitudes. Furthermore, we demonstrate that the conditioned fear-activated DRN 5-HTergic neurons send inhibitory projections to RtTgP GABAergic neurons, which in turn upregulate neuronal activities of RtTgC glutamatergic neurons. Chemogenetic activation of the DRN-RtTgP projections mimics the increased startle response under fear emotions. Moreover, conditional deletion of 5-HT_1B receptor from RtTgP GABAergic neurons largely reverses the exaggeration of startle during conditioned fear. Thus, our study establishes the disinhibitory DRN-RtTgP-RtTgC circuit as a critical mechanism underlying exaggerated startle under fear emotions, and provides 5-HT_1B receptor as a potential therapeutic target for treating hyperarousal symptom in fear-associated psychiatric disorders.

Music Listening and Homeostatic Regulation: Surviving and Flourishing in a Sonic World

This paper argues for a biological conception of music listening as an evolutionary achievement that is related to a long history of cognitive and affective-emotional functions, which are grounded in basic homeostatic regulation. Starting from the three levels of description, the acoustic description of sounds, the neurological level of processing, and the psychological correlates of neural stimulation, it conceives of listeners as open systems that are in continuous interaction with the sonic world. By monitoring and altering their current state, they can try to stay within the limits of operating set points in the pursuit of a controlled state of dynamic equilibrium, which is fueled by interoceptive and exteroceptive sources of information. Listening, in this homeostatic view, can be adaptive and goal-directed with the aim of maintaining the internal physiology and directing behavior towards conditions that make it possible to thrive by seeking out stimuli that are valued as beneficial and worthy, or by attempting to avoid those that are annoying and harmful. This calls forth the mechanisms of pleasure and reward, the distinction between pleasure and enjoyment, the twin notions of valence and arousal, the affect-related consequences of music listening, the role of affective regulation and visceral reactions to the sounds, and the distinction between adaptive and maladaptive listening.

Robust BOLD Responses to Faces But Not to Conditioned Threat: Challenging the Amygdala's Reputation in Human Fear and Extinction Learning

Most of our knowledge about human emotional memory comes from animal research. Based on this work, the amygdala is often labeled the brain's "fear center", but it is unclear to what degree neural circuitries underlying fear and extinction learning are conserved across species. Neuroimaging studies in humans yield conflicting findings, with many studies failing to show amygdala activation in response to learned threat. Such null findings are often treated as resulting from MRI-specific problems related to measuring deep brain structures. Here we test this assumption in a mega-analysis of three studies on fear acquisition (n = 98; 68 female) and extinction learning (n = 79; 53 female). The conditioning procedure involved the presentation of two pictures of faces and two pictures of houses: one of each pair was followed by an electric shock [a conditioned stimulus (CS+)], the other one was never followed by a shock (CS-), and participants were instructed to learn these contingencies. Results revealed widespread responses to the CS+ compared with the CS- in the fear network, including anterior insula, midcingulate cortex, thalamus, and bed nucleus of the stria terminalis, but not the amygdala, which actually responded stronger to the CS- Results were independent of spatial smoothing, and of individual differences in trait anxiety and conditioned pupil responses. In contrast, robust amygdala activation distinguished faces from houses, refuting the idea that a poor signal could account for the absence of effects. Moving forward, we suggest that, apart from imaging larger samples at higher resolution, alternative statistical approaches may be used to identify cross-species similarities in fear and extinction learning.SIGNIFICANCE STATEMENT The science of emotional memory provides the foundation of numerous theories on psychopathology, including stress and anxiety disorders. This field relies heavily on animal research, which suggests a central role of the amygdala in fear learning and memory. However, this finding is not strongly corroborated by neuroimaging evidence in humans, and null findings are too easily explained away by methodological limitations inherent to imaging deep brain structures. In a large nonclinical sample, we find widespread BOLD activation in response to learned fear, but not in the amygdala. A poor signal could not account for the absence of effects. While these findings do not disprove the involvement of the amygdala in human fear learning, they challenge its typical portrayals and illustrate the complexities of translational science.

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.

The effect of a nighttime zoo event on spider monkey (Ateles geoffroyi) behavior

The relationship between zoo animals, particularly nonhuman primates, and visitors is complex and varies by species. Adding complexity to this relationship is the trend for zoos to host events outside of normal operating hours. Here, we explored whether a late-night haunted-house style event influenced the behavior of spider monkeys. We conducted behavioral observations both on event nights and nights without the event. The spider monkeys were active and outside more frequently on event nights compared to the control nights indicating that their typical nighttime behavior was altered. However, it is difficult to definitively conclude whether the behavioral changes were a result of the event being aversive or enriching. Our findings suggest that zoos should conduct behavioral observations of and collect physiological data from their animals, especially if they are sensitive to environmental changes, when implementing new events, including those occurring outside of normal operating hours to ensure high levels of animal welfare.

Test-retest reliability of prepulse inhibition paradigm using auditory evoked potentials

Prepulse inhibition (PPI) is a neurological phenomenon in which a weak initial stimulus reduces the level of responses to a subsequent stronger stimulus. Although acoustic startle reflexes are usually used for PPI examinations, recent studies have observed similar phenomena with event-related cortical potentials. In the present study, test-retest reliability of PPI measured using auditory change-related cortical responses was assessed in 35 healthy adults. Four sound stimuli were randomly presented at an even probability; Standard, Test alone, Prepulse alone, and Test + Prepulse. The Standard stimulus was a train of 25-ms tone pulses at 70 dB for 650 ms, while for Test alone and Prepulse alone, the sound pressure was increased to 80 dB at 350 ms and 73 dB at 300 ms, respectively. Measurements were performed twice with at least 7 days separation, and validity was evaluated using intra-class correlation (ICC) for latency, amplitude, and suppression rate of the P50, N100, and P200 components. The results showed high ICC values for the latency and amplitude of nearly all components, except for response to Prepulse alone (0.3-0.6). Furthermore, ICC for suppression rate was greater than 0.5 for the peak-to-peak amplitude. Good reproducibility for N100 and P200 components was obtained with this method. The present results support the PPI paradigm as a reliable tool for clinical measurements of inhibitory functions.

Assessing behavior and cognition in rodents, nonhuman primates, and humans: where are the limits of translation?

New psychopharmacological treatments are needed for affective and nonaffective psychoses, especially for the associated negative and cognitive symptoms. Earlier developments mostly failed, probably partly because of limitations in behavioral models used for validation. Now, deeper understanding of the genetics underlying disease pathogenesis and progress in genetic engineering will generate many rodent models with increased construct validity. To improve these models’ translational value, we need complementary data from nonhuman primates. We also have to improve and streamline behavioral test systems to cope with increased demand. Here, we propose a comprehensive neurocognitive test battery that should overcome the disadvantages of single tests and yield cognitive/behavioral profiles for modeling subsets of patient symptoms. Further, we delineate a concept for classifying disease-relevant cognitive endophenotypes to balance between face and construct validity and clinical diagnostics. In summary, this review discusses new concepts and the limitations and future potential of translational research on cognition in psychiatry.


Concurrent amygdalar and ventromedial prefrontal cortical responses during emotion processing: a meta-analysis of the effects of valence of emotion and passive exposure versus active regulation

Anatomically interconnected, the ventromedial prefrontal cortex (vmPFC) and amygdala interact in emotion processing. However, no meta-analyses have focused on studies that reported concurrent vmPFC and amygdala activities. With activation likelihood estimation (ALE) we examined 100 experiments that reported concurrent vmPFC and amygdala activities, and distinguished responses to positive vs. negative emotions and to passive exposure to vs. active regulation of emotions. We also investigated whole-brain experiments for other regional activities. ALE and contrast analyses identified convergent anterior and posterior vmPFC response to passive positive and negative emotions, respectively, and a subregion in between to mixed emotions. A smaller area in the posterior ventral vmPFC is specifically involved in regulation of negative emotion. Whereas bilateral amygdala was involved during emotional exposure, only the left amygdala showed convergent activities during active regulation of negative emotions. Whole-brain analysis showed convergent activity in left ventral striatum for passive exposure to positive emotions and downregulation of negative emotions, and in the posterior cingulate cortex and ventral precuneus for passive exposure to negative emotions. These findings highlight contrasting, valence-specific subregional vmPFC as well as other regional responses during passive exposure to emotions. The findings also suggest that hyperactivation of the vmPFC is associated with diminished right amygdala activities during regulation of negative emotions. Together, the findings extend the literature by specifying the roles of subregional vmPFC and amygdala activities in emotion processing.

Making translation work: Harmonizing cross-species methodology in the behavioural neuroscience of Pavlovian fear conditioning

Translational neuroscience bridges insights from specific mechanisms in rodents to complex functions in humans and is key to advance our general understanding of central nervous function. A prime example of translational research is the study of cross-species mechanisms that underlie responding to learned threats, by employing Pavlovian fear conditioning protocols in rodents and humans. Hitherto, evidence for (and critique of) these cross-species comparisons in fear conditioning research was based on theoretical viewpoints. Here, we provide a perspective to substantiate these theoretical concepts with empirical considerations of cross-species methodology. This meta-research perspective is expected to foster cross-species comparability and reproducibility to ultimately facilitate successful transfer of results from basic science into clinical applications.

Dispositional negativity, cognition, and anxiety disorders: An integrative translational neuroscience framework

When extreme, anxiety can become debilitating. Anxiety disorders, which often first emerge early in development, are common and challenging to treat, yet the underlying mechanisms have only recently begun to come into focus. Here, we review new insights into the nature and biological bases of dispositional negativity, a fundamental dimension of childhood temperament and adult personality and a prominent risk factor for the development of pediatric and adult anxiety disorders. Converging lines of epidemiological, neurobiological, and mechanistic evidence suggest that dispositional negativity increases the likelihood of psychopathology via specific neurocognitive mechanisms, including attentional biases to threat and deficits in executive control. Collectively, these observations provide an integrative translational framework for understanding the development and maintenance of anxiety disorders in adults and youth and set the stage for developing improved intervention strategies.

Affect-Driven Attention Biases as Animal Welfare Indicators: Review and Methods

Attention bias describes the differential allocation of attention towards one stimulus compared to others. In humans, this bias can be mediated by the observer's affective state and is implicated in the onset and maintenance of affective disorders such as anxiety. Affect-driven attention biases (ADABs) have also been identified in a few other species. Here, we review the literature on ADABs in animals and discuss their utility as welfare indicators. Despite a limited research effort, several studies have found that negative affective states modulate attention to negative (i.e., threatening) cues. ADABs influenced by positive-valence states have also been documented in animals. We discuss methods for measuring ADAB and conclude that looking time, dot-probe, and emotional spatial cueing paradigms are particularly promising. Research is needed to test them with a wider range of species, investigate attentional scope as an indicator of affect, and explore the possible causative role of attention biases in determining animal wellbeing. Finally, we argue that ADABs might not be best-utilized as indicators of general valence, but instead to reveal specific emotions, motivations, aversions, and preferences. Paying attention to the human literature could facilitate these advances.

StartReact effects in first dorsal interosseous muscle are absent in a pinch task, but present when combined with elbow flexion

Objective

To provide a neurophysiological tool for assessing sensorimotor pathways, which may differ for those involving distal muscles in simple tasks from those involving distal muscles in a kinetic chain task, or proximal muscles in both.

Methods

We compared latencies and magnitudes of motor responses in a reaction time paradigm in a proximal (biceps brachii, BB) and a distal (first dorsal interosseous, FDI) muscle following electrical stimuli used as imperative signal (IS) delivered to the index finger. These stimuli were applied during different motor tasks: simple tasks involving either one muscle, e.g. flexing the elbow for BB (FLEX), or pinching a pen for FDI (PINCH); combined tasks engaging both muscles by pinching and flexing simultaneously (PINCH-FLEX). Stimuli were of varying intensity and occasionally elicited a startle response, and a StartReact effect.

Results

In BB, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities for non-startling trials, while for trials containing startle responses, latencies were uniformly shortened and response amplitudes similarly augmented across all IS intensities in both FLEX and PINCH-FLEX. In FDI, response latencies decreased gradually and response amplitudes increased progressively with increasing IS intensities in both PINCH and PINCH-FLEX for non-startling trials, but, unlike in BB for the simple task, in PINCH for trials containing startle responses as well. In PINCH-FLEX, FDI latencies were uniformly shortened and amplitudes similarly increased across all stimulus intensities whenever startle signs were present.

Conclusions

Our results suggest the presence of different sensorimotor pathways supporting a dissociation between simple tasks that involve distal upper limb muscles (FDI in PINCH) from simple tasks involving proximal muscles (BB in FLEX), and combined tasks that engage both muscles (FDI and BB in PINCH-FLEX), all in accordance with differential importance in the control of movements by cortical and subcortical structures.

Significance

Simple assessment tools may provide useful information regarding the differential involvement of sensorimotor pathways in the control of both simple and combined tasks that engage proximal and distal muscles.

Inhibition of the substantia nigra pars reticulata produces divergent effects on sensorimotor gating in rats and monkeys

The basal ganglia are an evolutionarily old group of structures, with gross organization conserved across species. Despite this conservation, there is evidence suggesting that anatomical organization of a key output nucleus of the basal ganglia, the substantia nigra pars reticulata (SNpr), diverges across species. Nevertheless, there are relatively few comparative studies examining the impact of manipulations of SNpr across species. Here, we evaluated the role of SNpr in a highly conserved behavior: prepulse inhibition of the acoustic startle response (PPI). We performed parallel experiments in both rats and rhesus macaques using intracranial microinfusions of GABAA agonist muscimol to investigate the role of SNpr in PPI. SNpr inactivation significantly disrupted PPI in rats, congruent with prior studies; however, in macaques, SNpr inactivation resulted in facilitation of PPI. We suggest that this difference in circuit function results from a divergence in anatomical connectivity, underscoring the importance of circuit dissection studies across species.

Behavioral methods to study anxiety in rodents

Stress is a precipitating factor for anxiety-related disorders, which are among the leading forms of psychiatric illness and impairment in the modern world. Rodent-based behavioral tests and models are widely used to understand the mechanisms by which stress triggers anxiety-related behaviors and to identify new treatments for anxiety-related disorders. Although substantial progress has been made and many of the key neural circuits and molecular pathways mediating stress responsiveness have been characterized, these advances have thus far failed to translate into fundamentally new treatments that are safer and more efficacious in humans. The purpose of this article is to describe methods that have been historically used for this type of research and to highlight new approaches that align with recent conceptualizations of disease symptomatology and that may ultimately prove to be more fruitful in facilitating the development of improved therapeutics.

Probing for Neuroadaptations to Unpredictable Stressors in Addiction: Translational Methods and Emerging Evidence

Stressors clearly contribute to addiction etiology and relapse in humans, but our understanding of specific mechanisms remains limited. Rodent models of addiction offer the power, flexibility, and precision necessary to delineate the causal role and specific mechanisms through which stressors influence alcohol and other drug use. This review describes a program of research using startle potentiation to unpredictable stressors that is well positioned to translate between animal models and clinical research with humans on stress neuroadaptations in addiction. This research rests on a solid foundation provided by three separate pillars of evidence from (a) rodent behavioral neuroscience on stress neuroadaptations in addiction, (b) rodent affective neuroscience on startle potentiation, and (c) human addiction and affective science with startle potentiation. Rodent stress neuroadaptation models implicate adaptations in corticotropin-releasing factor and norepinephrine circuits within the central extended amygdala following chronic alcohol and other drug use that mediate anxious behaviors and stress-induced reinstatement among drug-dependent rodents. Basic affective neuroscience indicates that these same neural mechanisms are involved in startle potentiation to unpredictable stressors in particular (vs. predictable stressors). We believe that synthesis of these evidence bases should focus us on the role of unpredictable stressors in addiction etiology and relapse. Startle potentiation in unpredictable stressor tasks is proposed to provide an attractive and flexible test bed to encourage tight translation and reverse translation between animal models and human clinical research on stress neuroadaptations. Experimental therapeutics approaches focused on unpredictable stressors hold high promise to identify, repurpose, or refine pharmacological and psychosocial interventions for addiction.

Evidence for Startle Effects due to Externally Induced Lower Limb Movements: Implications in Neurorehabilitation

Passive limb displacement is routinely used to assess muscle tone. If we attempt to quantify muscle stiffness using mechanical devices, it is important to know whether kinematic stimuli are able to trigger startle reactions. Whether kinematic stimuli are able to elicit a startle reflex and to accelerate prepared voluntary movements (StartReact effect) has not been studied extensively to date. Eleven healthy subjects were suspended in an exoskeleton and were exposed to passive left knee flexion (KF) at three intensities, occasionally replaced by fast right KF. Upon perceiving the movement subjects were asked to perform right wrist extension (WE), assessed by extensor carpi radialis (ECR) electromyographic activity. ECR latencies were shortest in fast trials. Startle responses were present in most fast trials, yet being significantly accelerated and larger with right versus left KF, since the former occurred less frequently and thus less expectedly. Startle responses were associated with earlier and larger ECR responses (StartReact effect), with the largest effect again upon right KF. The results provide evidence that kinematic stimuli are able to elicit both startle reflexes and a StartReact effect, which depend on stimulus intensity and anticipation, as well as on the subjects' preparedness to respond.

The role of inositol 1,4,5-trisphosphate 3-kinase A in regulating emotional behavior and amygdala function

Inositol 1,4,5-trisphosphate 3-kinase A (IP₃K-A) is a molecule enriched in the brain and neurons that regulates intracellular calcium levels via signaling through the inositol trisphosphate receptor. In the present study, we found that IP₃K-A expression is highly enriched in the central nucleus of the amygdala (CeA), which plays a pivotal role in the processing and expression of emotional phenotypes in mammals. Genetic abrogation of IP₃K-A altered amygdala gene expression, particularly in genes involved in key intracellular signaling pathways and genes mediating fear- and anxiety-related behaviors. In agreement with the changes in amygdala gene expression profiles, IP₃K-A knockout (KO) mice displayed more robust responses to aversive stimuli and spent less time in the open arms of the elevated plus maze, indicating high levels of innate fear and anxiety. In addition to behavioral phenotypes, decreased excitatory and inhibitory postsynaptic current and reduced c-Fos immunoreactivity in the CeA of IP₃K-A KO mice suggest that IP₃K-A has a profound influence on the basal activities of fear- and anxiety-mediating amygdala circuitry. In conclusion, our findings collectively demonstrate that IP₃K-A plays an important role in regulating affective states by modulating metabotropic receptor signaling pathways and neural activity in the amygdala.

The role of sensory modality in prepulse inhibition: An ontogenetic study

Deficits in prepulse inhibition (PPI), a measure of sensorimotor gating, are observed in neurodevelopmental and neuropsychiatric disorders. Despite the large PPI literature, the majority of studies characteristically employ tests with one interstimulus interval (ISI), of one modality, at one age. In the context of the auditory startle response (ASR), the present study examined (1) the profile for the ontogeny of PPI through adulthood in Long-Evans hooded rats with a reasonably comprehensive ISI function, (2) whether the ontogenetic profile for PPI is sensitive to modality of the prepulse stimulus, as a within-session variable, and (3) whether the maturation of PPI differs for males and females. Despite the basic effect of more pronounced PPI in adult relative to preweanling animals, each sensory modality displayed a unique ontogenetic profile for PPI, without any compelling evidence for major differences between males and females, in accordance with the known temporal course of peripheral and central maturational profiles of sensory systems in the rat. The context for assessing auditory PPI (auditory and tactile vs. auditory and visual prepulses) influenced the overall startle response, i.e., a shift in the height of the entire profile, but did not significantly impact the auditory PPI profile per se. The translational relevance of preclinical sensorimotor assessments to patients with neurodevelopmental and/or neuropsychiatric disorders depends partly on an understanding of the ontogeny of sensorimotor gating in different sensory systems, and can be strengthened with the use of a reasonably comprehensive number of ISIs to provide relatively precise and defined response functions.

Human fear acquisition deficits in relation to genetic variants of the corticotropin-releasing hormone receptor 1 and the serotonin transporter--revisited

We recently showed that a genetic polymorphism (rs878886) in the human corticotropin-releasing hormone receptor 1 (CRHR1) is associated with reduced fear-conditioned responses to a threat cue. This is a potentially important finding considering that the failure to acquire fear contingencies can leave an individual in a maladaptive state of more generalized anxiety. Consistent with that idea, the CRHR1-dependent fear acquisition deficit translated into heightened contextual anxiety when taking genetic variability within the serotonin transporter long polymorphic region (5-HTTLPR) into account. To replicate our previous findings, we conducted a replication study in 224 healthy medication-free human subjects using the exact same cue and context virtual reality fear-conditioning procedure as in study by Heitland et al. (2013). In the replication study, consistent with the original findings, CRHR1 rs878886 G-allele carriers showed reduced acquisition of cue-specific fear-conditioned responses compared with C/C homozygotes. Also, in this larger sample the cue acquisition deficit of G-allele carriers translated into heightened contextual anxiety, even independent of 5-HTT gene variation. In contrast to our earlier findings, there was an additional interaction effect of CRHR1 rs878886 and the triallelic 5-HTTLPR/rs25531 variant on cued fear acquisition. In summary, this study replicated the initially reported association of the CRHR1 rs878886 G-allele with cued fear acquisition deficits, albeit with a different pattern of results regarding the interaction with 5-HTT variation. This further supports the notion that the human corticotropin-releasing hormone plays a role in the acquisition of fears.

Rudimentary empathy in macaques' social decision-making

Primates live in highly social environments, where prosocial behaviors promote social bonds and cohesion and contribute to group members' fitness. Despite a growing interest in the biological basis of nonhuman primates' social interactions, their underlying motivations remain a matter of debate. We report that macaque monkeys take into account the welfare of their peers when making behavioral choices bringing about pleasant or unpleasant outcomes to a monkey partner. Two macaques took turns in making decisions that could impact their own welfare or their partner's. Most monkeys were inclined to refrain from delivering a mildly aversive airpuff and to grant juice rewards to their partner. Choice consistency between these two types of outcome suggests that monkeys display coherent motivations in different social interactions. Furthermore, spontaneous affilitative group interactions in the home environment were mostly consistent with the measured social decisions, thus emphasizing the impact of preexisting social bonds on decision-making. Interestingly, unique behavioral markers predicted these decisions: benevolence was associated with enhanced mutual gaze and empathic eye blinking, whereas indifference or malevolence was associated with lower or suppressed such responses. Together our results suggest that prosocial decision-making is sustained by an intrinsic motivation for social affiliation and controlled through positive and negative vicarious reinforcements.

Targeting Neuronal Networks with Combined Drug and Stimulation Paradigms Guided by Neuroimaging to Treat Brain Disorders

Improved therapy of brain disorders can be achieved by focusing on neuronal networks, utilizing combined pharmacological and stimulation paradigms guided by neuroimaging. Neuronal networks that mediate normal brain functions, such as hearing, interact with other networks, which is important but commonly neglected. Network interaction changes often underlie brain disorders, including epilepsy. "Conditional multireceptive" (CMR) brain areas (e.g., brainstem reticular formation and amygdala) are critical in mediating neuroplastic changes that facilitate network interactions. CMR neurons receive multiple inputs but exhibit extensive response variability due to milieu and behavioral state changes and are exquisitely sensitive to agents that increase or inhibit GABA-mediated inhibition. Enhanced CMR neuronal responsiveness leads to expression of emergent properties--nonlinear events--resulting from network self-organization. Determining brain disorder mechanisms requires animals that model behaviors and neuroanatomical substrates of human disorders identified by neuroimaging. However, not all sites activated during network operation are requisite for that operation. Other active sites are ancillary, because their blockade does not alter network function. Requisite network sites exhibit emergent properties that are critical targets for pharmacological and stimulation therapies. Improved treatment of brain disorders should involve combined pharmacological and stimulation therapies, guided by neuroimaging, to correct network malfunctions by targeting specific network neurons.

Neurophysiological evidence of an association between cognitive control and defensive reactivity processes in young children

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ERN, Pe, startle reflex, and parietal asymmetry were measured in young children.

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Reduced ERN was related to a larger startle and greater right parietal activity.

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Age predicted smaller startle, larger ERN, and better behavioral performance.

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Age did not moderate the association between ERN and startle.

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Age did not moderate the association between ERN and parietal asymmetry.

Interactions between cognitive control and affective processes, such as defensive reactivity, are intimately involved in healthy and unhealthy human development. However, cognitive control and defensive reactivity processes are often studied in isolation and rarely examined in early childhood. To address these gaps, we examined the relationships between multiple neurophysiological measures of cognitive control and defensive reactivity in young children. Specifically, we assessed two event-related potentials thought to index cognitive control processes – the error-related negativity (ERN) and error positivity (Pe) – measured across two tasks, and two markers of defensive reactivity processes – startle reflex and resting parietal asymmetry – in a sample of 3- to 7-year old children. Results revealed that measures of cognitive control and defensive reactivity were related such that evidence of poor cognitive control (smaller ERN) was associated with high defensive reactivity (larger startle and greater right relative to left parietal activity). The strength of associations between the ERN and measures of defensive reactivity did not vary by age, providing evidence that poor cognitive control relates to greater defensive reactivity across early childhood years.

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.

Using the threat probability task to assess anxiety and fear during uncertain and certain threat

Fear of certain threat and anxiety about uncertain threat are distinct emotions with unique behavioral, cognitive-attentional, and neuroanatomical components. Both anxiety and fear can be studied in the laboratory by measuring the potentiation of the startle reflex. The startle reflex is a defensive reflex that is potentiated when an organism is threatened and the need for defense is high. The startle reflex is assessed via electromyography (EMG) in the orbicularis oculi muscle elicited by brief, intense, bursts of acoustic white noise (i.e., "startle probes"). Startle potentiation is calculated as the increase in startle response magnitude during presentation of sets of visual threat cues that signal delivery of mild electric shock relative to sets of matched cues that signal the absence of shock (no-threat cues). In the Threat Probability Task, fear is measured via startle potentiation to high probability (100% cue-contingent shock; certain) threat cues whereas anxiety is measured via startle potentiation to low probability (20% cue-contingent shock; uncertain) threat cues. Measurement of startle potentiation during the Threat Probability Task provides an objective and easily implemented alternative to assessment of negative affect via self-report or other methods (e.g., neuroimaging) that may be inappropriate or impractical for some researchers. Startle potentiation has been studied rigorously in both animals (e.g., rodents, non-human primates) and humans which facilitates animal-to-human translational research. Startle potentiation during certain and uncertain threat provides an objective measure of negative affective and distinct emotional states (fear, anxiety) to use in research on psychopathology, substance use/abuse and broadly in affective science. As such, it has been used extensively by clinical scientists interested in psychopathology etiology and by affective scientists interested in individual differences in emotion.

Why primate models matter

Research involving nonhuman primates (NHPs) has played a vital role in many of the medical and scientific advances of the past century. NHPs are used because of their similarity to humans in physiology, neuroanatomy, reproduction, development, cognition, and social complexity-yet it is these very similarities that make the use of NHPs in biomedical research a considered decision. As primate researchers, we feel an obligation and responsibility to present the facts concerning why primates are used in various areas of biomedical research. Recent decisions in the United States, including the phasing out of chimpanzees in research by the National Institutes of Health and the pending closure of the New England Primate Research Center, illustrate to us the critical importance of conveying why continued research with primates is needed. Here, we review key areas in biomedicine where primate models have been, and continue to be, essential for advancing fundamental knowledge in biomedical and biological research.

Alcohol stress response dampening during imminent versus distal, uncertain threat

Research indicates that fear and anxiety are distinct processes with separable neurobiological substrates. Predictable versus unpredictable shock administration has been used to elicit fear versus anxiety, respectively. Recent research has demonstrated that alcohol may reduce anxiety but not fear. However, previous manipulations of predictability have varied both probability and temporal uncertainty of shock threat, leaving unresolved questions regarding which stimulus characteristics elicit anxiety and are sensitive to alcohol stress-response dampening (SRD). We developed a novel paradigm to closely parallel basic research in animals that systematically varied temporal uncertainty of threat while holding threat probability constant. Intoxicated (0.08% target blood alcohol concentration), placebo, and no-alcohol control participants viewed a series of visual threat cues. Certain cue duration (5 s) blocks were equivalent to predictable shock blocks eliciting fear in earlier research. Uncertain cue duration (5, 20, 50, or 80 s, intermixed) blocks introduced temporal uncertainty regarding impending shock to elicit anxiety. Startle potentiation relative to matched cue periods in no-shock blocks provided the primary measure of affective response. All threat cues produced robust startle potentiation. Alcohol reduced startle potentiation during the first 5 s of threat cue presentation in uncertain but not certain duration blocks. Alcohol also reduced startle potentiation at later times among longer uncertain duration cues, suggesting that alcohol SRD persisted. Trait negative emotionality and binge drinking status moderated alcohol SRD magnitude during uncertain threat. These translational findings corroborate previous reports regarding distinct substrates of fear versus anxiety and have implications for both alcoholism etiology and comorbidity with anxiety disorders.

Pain-relief learning in flies, rats, and man: basic research and applied perspectives

Memories relating to a painful, negative event are adaptive and can be stored for a lifetime to support preemptive avoidance, escape, or attack behavior. However, under unfavorable circumstances such memories can become overwhelmingly powerful. They may trigger excessively negative psychological states and uncontrollable avoidance of locations, objects, or social interactions. It is therefore obvious that any process to counteract such effects will be of value. In this context, we stress from a basic-research perspective that painful, negative events are "Janus-faced" in the sense that there are actually two aspects about them that are worth remembering: What made them happen and what made them cease. We review published findings from fruit flies, rats, and man showing that both aspects, respectively related to the onset and the offset of the negative event, induce distinct and oppositely valenced memories: Stimuli experienced before an electric shock acquire negative valence as they signal upcoming punishment, whereas stimuli experienced after an electric shock acquire positive valence because of their association with the relieving cessation of pain. We discuss how memories for such punishment- and relief-learning are organized, how this organization fits into the threat-imminence model of defensive behavior, and what perspectives these considerations offer for applied psychology in the context of trauma, panic, and nonsuicidal self-injury.

Combining neural and behavioral indicators in the assessment of internalizing psychopathology in children and adolescents

Anxiety and mood disorders are among the most prevalent mental health problems affecting our youth. We propose that assessment and treatment efforts in this area can benefit from a focus on developmentally sensitive neurobehavioral trait constructs, that is, individual difference constructs with direct referents in both neurobiology and behavior across the lifespan. This approach dovetails with the National Institute of Mental Health's Research Domain Criteria initiative, which aims to improve classification and treatment of psychopathology by delineating dimensions of functioning that transcend measurement domains and traditional diagnostic categories. We highlight two neurobehavioral dimensions with clear relevance for understanding internalizing problems at differing ages: (a) defensive reactivity and (b) cognitive control. Individual differences in defensive reactivity are posited to reflect variations in sensitivity of the brain's negative valence systems, whereas differences in cognitive control are theorized to reflect variations in neural systems dedicated to regulating behavior and affect. Focusing on these target constructs, we illustrate a psychoneurometric approach to assessment of internalizing psychopathology entailing use of neural, self-report, and behavioral indicators. We address the feasibility of the psychoneurometric approach for clinical application and present results from a pilot study demonstrating expected associations for neural, parent-report, and behavioral measures of defensive reactivity and cognitive control with internalizing symptoms in preschoolers. Together, our conceptual and empirical analyses highlight the promise of multimethod, dimensional assessment of internalizing psychopathology in the lab and in the clinic.

Effect of Suppression, Reappraisal, and Acceptance of Emotional Pictures on Acoustic Eye-Blink Startle Magnitude

To examine the effects of different emotion regulation strategies on acoustic eye-blink startle, 65 participants viewed positive, neutral, and negative pictures and were instructed to suppress, reappraise, or accept their emotional responses to these pictures using a within-group experimental design with separate blocks of pictures for each strategy. Instructions to suppress the emotional response led to an attenuation of the eye-blink startle magnitude, in comparison with instructions to reappraise or accept. Reappraisal and acceptance instructions did not differ from one another in their effect on startle. These results are discussed within the context of the existing empirical literature on emotion regulation.

Lead exposure and fear-potentiated startle in the VA Normative Aging Study: a pilot study of a novel physiological approach to investigating neurotoxicant effects

BACKGROUND

Physiologically-based indicators of neural plasticity in humans could provide mechanistic insights into toxicant actions on learning in the brain, and perhaps prove more objective and sensitive measures of such effects than other methods.

OBJECTIVES

We explored the association between lead exposure and classical conditioning of the acoustic startle reflex (ASR)-a simple form of associative learning in the brain-in a population of elderly men. Fifty-one men from the VA Normative Aging Study with cumulative bone lead exposure measurements made with K-X-Ray-Fluorescence participated in a fear-conditioning protocol.

RESULTS

The mean age of the men was 75.5years (standard deviation [sd]=5.9) and mean patella lead concentration was 22.7μg/g bone (sd=15.9). Baseline ASR eyeblink response decreased with age, but was not associated with subsequent conditioning. Among 37 men with valid responses at the end of the protocol, higher patella lead was associated with decreased awareness of the conditioning contingency (declarative learning; adjusted odds ratio [OR] per 20μg/g patella lead=0.91, 95% confidence interval [CI]: 0.84, 0.99, p=0.03). Eyeblink conditioning (non-declarative learning) was 0.44sd less (95% CI: -0.91, 0.02; p=0.06) per 20μg/g patella lead after adjustment. Each result was stronger when correcting for the interval between lead measurement and startle testing (awareness: OR=0.88, 95% CI: 0.78, 0.99, p=0.04; conditioning: -0.79sd less, 95% CI: -1.56, 0.03, p=0.04).

CONCLUSIONS

This initial exploration suggests that lead exposure interferes with specific neural mechanisms of learning and offers the possibility that the ASR may provide a new approach to physiologically explore the effects of neurotoxicant exposures on neural mechanisms of learning in humans with a paradigm that is directly comparable to animal models.

The brain basis of emotion: a meta-analytic review

Researchers have wondered how the brain creates emotions since the early days of psychological science. With a surge of studies in affective neuroscience in recent decades, scientists are poised to answer this question. In this target article, we present a meta-analytic summary of the neuroimaging literature on human emotion. We compare the locationist approach (i.e., the hypothesis that discrete emotion categories consistently and specifically correspond to distinct brain regions) with the psychological constructionist approach (i.e., the hypothesis that discrete emotion categories are constructed of more general brain networks not specific to those categories) to better understand the brain basis of emotion. We review both locationist and psychological constructionist hypotheses of brain-emotion correspondence and report meta-analytic findings bearing on these hypotheses. Overall, we found little evidence that discrete emotion categories can be consistently and specifically localized to distinct brain regions. Instead, we found evidence that is consistent with a psychological constructionist approach to the mind: A set of interacting brain regions commonly involved in basic psychological operations of both an emotional and non-emotional nature are active during emotion experience and perception across a range of discrete emotion categories.

The anxiety spectrum and the reflex physiology of defense: from circumscribed fear to broad distress

Guided by the diagnostic nosology, anxiety patients are expected to show defensive hyperarousal during affective challenge, irrespective of the principal phenotype. In the current study, patients representing the whole spectrum of anxiety disorders (i.e., specific phobia, social phobia, panic disorder with or without agoraphobia, obsessive-compulsive disorder, generalized anxiety disorder (GAD), posttraumatic stress disorder(PTSD)), and healthy community control participants, completed an imagery-based fear elicitation paradigm paralleling conventional intervention techniques. Participants imagined threatening and neutral narratives as physiological responses were recorded. Clear evidence emerged for exaggerated reactivity to clinically relevant imagery--most pronounced in startle reflex responding. However, defensive propensity varied across principal anxiety disorders. Disorders characterized by focal fear and impairment (e.g., specific phobia) showed robust fear potentiation. Conversely, for disorders of long-enduring, pervasive apprehension and avoidance with broad anxiety and depression comorbidity (e.g., PTSD secondary to cumulative trauma, GAD), startle responses were paradoxically diminished to all aversive contents. Patients whose expressed symptom profiles were intermediate between focal fearfulness and broad anxious-misery in both severity and chronicity exhibited a still heightened but more generalized physiological propensity to respond defensively. Importantly, this defensive physiological gradient--the inverse of self-reported distress--was evident not only between but also within disorders. These results highlight that fear circuitry could be dysregulated in chronic, pervasive anxiety, and preliminary functional neuroimaging findings suggest that deficient amygdala recruitment could underlie attenuated reflex responding. In summary, adaptive defensive engagement during imagery may be compromised by long-term dysphoria and stress-a phenomenon with implications for prognosis and treatment planning.

Can a one-hour session of exposure treatment modulate startle response and reduce spider fears?

Preliminary evidence suggests that 3 hours of behavior therapy can reduce fear responses to phobic stimuli. Most of this research, however, has relied on self-reports and clinician assessments, and failed to include a comparison group. To extend this literature, with 32 adults with spider phobia, we investigated the effects of a single hour of in vivo exposure on subjective and electrophysiological aspects of fear; comparisons were made to a wait-list control group. Pre- and post-assessments included phobia-relevant questionnaires and startle reflex responses to spider, negative, neutral and positive stimuli. Compared to the control group, our one-hour treatment reduced self-reported and physiological responses to spider stimuli. These data provide preliminary support for the ability of affective startle modulation to be changed by very brief exposure therapy.

Alcohol stress response dampening: selective reduction of anxiety in the face of uncertain threat

Problematic alcohol use and stress response dampening (SRD) are intimately interconnected. Recent evidence suggests that alcohol produces selective SRD during uncertain but not certain threat. We systematically varied shock probability in a novel task assessing alcohol SRD during low probable/uncertain threat, while holding temporal precision of threat constant. Intoxicated (0.08% target blood alcohol concentration) and placebo participants completed a cued shock threat task in which probability of shock administration at the offset of brief visual cues varied parametrically. High probability (100%) shock cues represented certain threat as used in earlier research, while lower probability (20% and 60%) shock cues provided novel uncertain threat conditions. Startle potentiation during cues and inter-trial intervals (ITIs) served as the measure of affective response. General linear model analysis indicated that alcohol SRD magnitude increased monotonically as threat uncertainty increased. Alcohol SRD was significantly greater during 20% and 60% shock threat relative to 100% shock threat. Alcohol also significantly reduced startle potentiation during distal threat in shock-free ITIs. Alcohol SRD magnitude during distal/uncertain threat was meaningfully moderated by individual differences in negative affectivity and weekly alcohol consumption. This work advances understanding of which properties of uncertainty are relevant to anxiety and anxiolytic effects of alcohol.

Decreased startle modulation during anticipation in the postpartum period in comparison to late pregnancy

Knowledge about healthy women’s psychophysiological adaptations during the large neuroendocrine changes of pregnancy and childbirth is essential in order to understand why these events have the potential to disrupt mental health in vulnerable individuals. This study aimed to compare startle response modulation, an objective psychophysiological measure demonstrated to be influenced by anxiety and depression, longitudinally across late pregnancy and the postpartum period. The acoustic startle response modulation was assessed during anticipation of affective images and during image viewing in 31 healthy women during gestational weeks 36–39 and again at 4 to 6 weeks postpartum. No startle modulation by affective images was observed at either time point. Significant modulation during anticipation stimuli was found at pregnancy assessment but was reduced in the postpartum period. The women rated the unpleasant images more negative and more arousing and the pleasant images more positive at the postpartum assessment. Self-reported anxiety and depressive symptoms did not change between assessments. The observed postpartum decrease in modulation of startle by anticipation suggests a relatively deactivated defense system in the postpartum period.

Alcohol dose effects on stress response to cued threat vary by threat intensity

RATIONALE

Clarification of alcohol's effect on stress response during threat is critical to understand motivation for alcohol use and related alcohol-use disorders. Evaluation of stress response dampening (SRD) effects of alcohol has been limited by nonsystematic use of varied experimental methods and measures.

OBJECTIVES

This experiment parametrically varied alcohol dose and shock threat intensity among social drinkers to examine their effects on startle potentiation, a physiological measure of the affective component of the stress response.

METHODS

Ninety-six participants were assigned to one of four beverage groups: placebo and target blood alcohol concentration (BAC) groups of 0.04%, 0.075%, and 0.11%. Participants viewed colored cues presented in shock and no-shock blocks. Distinct colored cues predicted imminent low, moderate, or high intensity electric shock administration. Startle potentiation during shock threat relative to no-shock cues indexed affective response.

RESULTS

High threat increased startle potentiation relative to moderate/low intensity threat. Startle potentiation decreased as BAC increased. Threat intensity moderated this BAC effect with the strongest BAC effect observed during high threat. Analysis of individual difference moderators revealed reduced effect of BAC among heavier, more problematic drinkers.

CONCLUSIONS

Clear alcohol SRD effects were observed. These SRD effects were greatest at higher BACs and during more potent threat. Failure to account for these factors may partially explain inconsistent findings in past laboratory SRD research. Furthermore, they suggest greater reinforcement from alcohol at higher doses and among individuals with greater stress. Moderation of SRD effects by alcohol consumption and problems point to possible important risk factors.

Somatic and neuroendocrine responses to standard and biologically salient acoustic startle stimuli in monkeys

The startle response, a simple defensive response to a sudden stimulus signaling proximal threat, has been well studied in rodents and humans, but has been rarely examined in monkeys. The first goal of the present studies was to develop a minimally immobilizing startle measurement paradigm and validate its usefulness by testing two core features of the startle response (habituation and graded responsivity) in squirrel monkey subjects. Two different types of startle stimuli were used: standard broad-band noise bursts, and species-specific alarm vocalizations ("yaps") which are elicited in response to threat in both wild and captive animals. The second goal of the present studies was to test whether yaps produce enhanced startle responsivity due to their increased biological salience compared to simple, non-biologically relevant noise bursts. The third goal of the present studies was to evaluate the hypothalamic-pituitary-adrenal (HPA) axis response to startle stimuli, as little is known about the stress-activating role of startle stimuli in any species. These experiments determined that the whole-body startle response in relatively unrestrained squirrel monkeys habituates across repeated stimulus presentations and is proportional to stimulus intensity. In addition, differential habituation was observed across biologically salient vs. standard acoustic startle stimuli. Responses to "yaps" were larger initially but attenuated more rapidly over trials. Responses to "yaps" were also larger in the early subepochs of the response window but then achieved a lower level than responses to noise bursts in the later subepochs. Finally, adrenocorticotropic hormone and cortisol concentrations were significantly elevated above baseline after startle stimuli presentation, though monkeys did not exhibit differential HPA axis responses to the two types of startle stimuli. The development of monkey startle methodology may further enhance the utility of this paradigm in translational studies of human stress-related psychiatric disorders.

RDX binds to the GABA(A) receptor-convulsant site and blocks GABA(A) receptor-mediated currents in the amygdala: a mechanism for RDX-induced seizures

BACKGROUND

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a high-energy, trinitrated cyclic compound that has been used worldwide since World War II as an explosive in both military and civilian applications. RDX can be released in the environment by way of waste streams generated during the manufacture, use, and disposal of RDX-containing munitions and can leach into groundwater from unexploded munitions found on training ranges. For > 60 years, it has been known that exposure to high doses of RDX causes generalized seizures, but the mechanism has remained unknown.

OBJECTIVE

We investigated the mechanism by which RDX induces seizures.

METHODS AND RESULTS

By screening the affinity of RDX for a number of neurotransmitter receptors, we found that RDX binds exclusively to the picrotoxin convulsant site of the γ-aminobutyric acid type A (GABA(A)) ionophore. Whole-cell in vitro recordings in the rat basolateral amygdala (BLA) showed that RDX reduces the frequency and amplitude of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic currents and the amplitude of GABA-evoked postsynaptic currents. In extracellular field recordings from the BLA, RDX induced prolonged, seizure-like neuronal discharges.

CONCLUSIONS

These results suggest that binding to the GABA(A) receptor convulsant site is the primary mechanism of seizure induction by RDX and that reduction of GABAergic inhibitory transmission in the amygdala is involved in the generation of RDX-induced seizures. Knowledge of the molecular site and the mechanism of RDX action with respect to seizure induction can guide therapeutic strategies, allow more accurate development of safe thresholds for exposures, and help prevent the development of new explosives or other munitions that could pose similar health risks.

Lesions of the dorsomedial striatum disrupt prepulse inhibition

Prepulse inhibition (PPI) of startle is an experimentally tractable measure of sensorimotor gating that can be readily evaluated in mice, rats, monkeys, and humans. PPI is the inhibitory effect of a low-intensity stimulus, the prepulse, on the startle response to a subsequent high-intensity stimulus. PPI has garnered great interest as a marker of clinically relevant information processing abnormalities, because it is impaired in such neuropsychiatric conditions as schizophrenia, Tourette syndrome, and obsessive compulsive disorder (OCD). Pathology of the basal ganglia has been described in all three of these disorders, and it is therefore of great interest to determine the role of the basal ganglia in PPI. Previous work in rats described a PPI deficit after excitotoxic ventral striatal lesions and a more subtle attenuation after caudodorsal lesion, but no effect of other large lateral dorsal lesions. However, previous studies have not specifically investigated the role of the dorsomedial striatum in PPI. We investigated this issue using excitotoxic lesions in mice. We describe a marked reduction in PPI, at a variety of prepulse intensities, after bilateral lesions of dorsomedial striatum. There was no effect of lesion on baseline startle or habituation. In contrast, comparably sized lesions of the central dorsal striatum had no effect on PPI. These results reveal a role for the dorsomedial striatum in prepulse inhibition, which may have relevance for the abnormalities observed in this region in such disorders as Tourette syndrome and OCD.

Enduring sensorimotor gating abnormalities following predator exposure or corticotropin-releasing factor in rats: a model for PTSD-like information-processing deficits?

A deficit in prepulse inhibition (PPI) can be one of the clinically observed features of post-traumatic stress disorder (PTSD) that is seen long after the acute traumatic episode has terminated. Thus, reduced PPI may represent an enduring psychophysiological marker of this illness in some patients. PPI is an operational measure of sensorimotor gating and refers to the phenomenon in which a weak stimulus presented immediately before an intense startling stimulus inhibits the magnitude of the subsequent startle response. The effects of stress on PPI have been relatively understudied, and in particular, there is very little information on PPI effects of ethologically relevant psychological stressors. We aimed to develop a paradigm for evaluating stress-induced sensorimotor gating abnormalities by comparing the effects of a purely psychological stressor (predator exposure) to those of a nociceptive physical stressor (footshock) on PPI and baseline startle responses in rats over an extended period of time following stressor presentation. Male Sprague-Dawley rats were exposed (within a protective cage) to ferrets for 5 min or left in their homecage and then tested for PPI immediately, 24 h, 48 h, and 9 days after the exposure. The effects of footshock were evaluated in a separate set of rats. The effects seen with stressor presentation were compared to those elicited by corticotropin-releasing factor (CRF; 0.5 and 3 μg/6 μl, intracerebroventricularly). Finally, the effects of these stressors and CRF administration on plasma corticosterone were measured. PPI was disrupted 24 h after ferret exposure; in contrast, footshock failed to affect PPI at any time. CRF mimicked the predator stress profile, with the lowdose producing a PPI deficit 24 h after infusion. Interestingly, the high dose also produced a PPI deficit 24 h after infusion, but with this dose, the PPI deficit was evident even 9d later. Plasma corticosterone levels were elevated acutely (before PPI deficits emerged) by both stressors and CRF, but returned to normal control levels 24 h later, when PPI deficits were present. Thus, predator exposure produces a delayed disruption of PPI, and stimulation of CRF receptors recapitulates these effects. Contemporaneous HPA axis activation is neither necessary nor sufficient for these PPI deficits. These results indicate that predator exposure, perhaps acting through CRF, may model the delayed-onset and persistent sensorimotor gating abnormalities that have been observed clinically in PTSD, and that further studies using this model may shed insight on the mechanisms of information-processing deficits in this disorder. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Nicotine withdrawal increases threat-induced anxiety but not fear: neuroadaptation in human addiction

BACKGROUND

Stress response neuroadaptation has been repeatedly implicated in animal addiction models for many drugs, including nicotine. Programmatic laboratory research that examines the stress response of nicotine-deprived humans is necessary to confirm that stress neuroadaptations observed in animal models generalize to humans.

METHODS

Two experiments tested the prediction that nicotine deprivation selectively increases startle response associated with anxiety during unpredictable threat but not fear during imminent, predictable threat. Dependent smokers (n = 117) were randomly assigned to 24-hour nicotine-deprived or nondeprived groups and participated in one of two experiments wherein electric shock was administered either unpredictably (noncontingent shock; Experiment 1) or predictably (cue-contingent shock; Experiment 2).

RESULTS

Nicotine deprivation increased overall startle response in Experiment 1, which involved unpredictable administration of shock. Age of first cigarette and years of daily smoking were significant moderators of this deprivation effect. Self-reported withdrawal symptoms also predicted startle response during unpredictable shock. In contrast, nicotine deprivation did not alter overall or fear-potentiated startle in Experiment 2, which involved predictable administration of shock.

CONCLUSIONS

These results provide evidence that startle response during unpredictable threat may be a biomarker of stress neuroadaptations among smokers in nicotine withdrawal. Contrast of results across unpredictable versus predictable shock experiments provides preliminary evidence that these stress neuroadaptations manifest selectively as anxiety during unpredictable threat rather than in every stressful context. Individual differences in unpredictable threat startle response associated with withdrawal symptoms, age of first cigarette, and years daily smoking link this laboratory biomarker to clinically relevant indexes of addiction risk and relapse.

Comparison of salicylate- and quinine-induced tinnitus in rats: development, time course, and evaluation of audiologic correlates

BACKGROUND

Salicylate and quinine have been shown to reliably induce short-term tinnitus when administered at high doses. The present study compared salicylate and quinine-induced tinnitus in rats using the gap prepulse inhibition of acoustic startle (GPIAS).

METHODS

Twenty-four rats were divided into 2 groups; the first group (n = 12) was injected with salicylate (300 mg kg d), whereas the second (n = 12) was treated with quinine orally at a dose of 200 mg kg d. Animals were treated daily for 4 consecutive days. All rats were tested for tinnitus and hearing loss before and 2, 24, 48, 72, and 96 hours after the first drug administration. Tinnitus was assessed using GPIAS; hearing function was measured with distortion product otoacoustic emissions (DPOAEs) and auditory brainstem response.

RESULTS

Salicylate treatment induced transient tinnitus with a pitch near 16 kHz starting 2 hours posttreatment, persisting over the 4-day treatment period and disappearing 24 hours later. Animals in the quinine group showed GPIAS changes at a higher pitch (20 kHz); however, changes were more variable among animals, and the mean data were not statistically significant. Hearing function varied across treatments. In the salicylate group, high-level DPOAEs were slightly affected; most changes occurred 2 hours posttreatment. Low-level DPOAEs were affected at all frequencies with a progressive dose-dependent effect. In the quinine group, only high-level DPOAEs were affected, mainly at 16 kHz.

CONCLUSION

The present study highlights the similarities and differences in the frequency and the time course of tinnitus and hypoacusis induced by salicylate and quinine. Transient tinnitus was reliably induced pharmacologically with salicylate, whereas hearing loss remained subclinical with only minor changes in DPOAEs.

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.

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.