Startle Amplitude Research Articles

Alpha9alpha10 knockout mice show altered physiological and behavioral responses to signals in masking noise.

Medial olivocochlear (MOC) efferents modulate outer hair cell motility through specialized nicotinic acetylcholine receptors to support encoding of signals in noise. Transgenic mice lacking the alpha9 subunits of these receptors (α9KOs) have normal hearing in quiet and noise, but lack classic cochlear suppression effects and show abnormal temporal, spectral, and spatial processing. Mice deficient for both the alpha9 and alpha10 receptor subunits (α9α10KOs) may exhibit more severe MOC-related phenotypes. Like α9KOs, α9α10KOs have normal auditory brainstem response (ABR) thresholds and weak MOC reflexes. Here, we further characterized auditory function in α9α10KO mice. Wild-type (WT) and α9α10KO mice had similar ABR thresholds and acoustic startle response amplitudes in quiet and noise, and similar frequency and intensity difference sensitivity. α9α10KO mice had larger ABR Wave I amplitudes than WTs in quiet and noise. Other ABR metrics of hearing-in-noise function yielded conflicting findings regarding α9α10KO susceptibility to masking effects. α9α10KO mice also had larger startle amplitudes in tone backgrounds than WTs. Overall, α9α10KO mice had grossly normal auditory function in quiet and noise, although their larger ABR amplitudes and hyperreactive startles suggest some auditory processing abnormalities. These findings contribute to the growing literature showing mixed effects of MOC dysfunction on hearing.

Counterirritation by pain inhibits the responsiveness to aversive loud tones: the role of state anxiety and state fear triggered in the NPU paradigm

Aim of the study The application of a noxious stimulus reduces the perception and responsiveness to other pain stimuli. This inhibition can be experimentally assessed with a method called ‘counterirritation’. The question arises if counterirritation acts also on the perception and responsiveness to aversive but non-nociceptive stimuli (e.g., loud tones). Since aversive stimulation is often associated with state anxiety or state fear, we investigated in addition the modulatory effects of these emotions on counterirritation. Material and Methods 51 subjects participated in our study. We presented tones with aversive loudness (105 dB), first alone then during counterirritation (immersion of the hand in a hot water bath of 46 °C) to assess inhibition of loudness perception and responsiveness. Influences of state anxiety and state fear on counterirritation were investigated by using the Neutral-Predictable(fear)- Unpredictable(anxiety) Paradigm (NPU), which is based on classical conditioning. Loudness ratings (perception of the aversive tones) and startle reflex (defensive reaction to aversive tones) were assessed. Results Counterirritation reduced startle reflex amplitudes, but not the loudness ratings. Although state anxiety and state fear were successfully induced, counterirritation remained unaffected. Conclusions Our study showed that pain inhibits the responsiveness to aversive stimuli (loud tones). Thus, the postulate that ‘pain inhibits pain’ might be better changed to ‘pain inhibits aversiveness’. Consequently, our findings may also question the assumption of a clear pain specificity in inhibitory action as assumed by theoretical approaches like ‘conditioned pain modulation’ (CPM). Furthermore, counterirritation appeared one more time resistant to the influence of negative emotions.

Raising laying hens: housing complexity and genetic strain affect startle reflex amplitude and behavioural response to fear-inducing stimuli.

Individual variation in fearfulness can be modified during ontogeny, and high levels of fear can affect animal welfare. We asked whether early-life environmental complexity and genetic strain affect fear behaviour in young laying hens (pullets). Four replicates of brown (B) and white (W) genetic strains (breeds) of layers were each raised in four environmental treatments (housing): conventional cages (Conv) and different rearing aviaries with increasing space and complexity (Low < Mid < High). We used a startle reflex test (weeks 4 and 14) to measure startle amplitude and autonomic response (i.e. comb temperature). A combination of novel arena (NA) and novel object (NO) tests was used (week 14) to assess NA exploration and alertness, latency to approach the centre and initial NO avoidance and investigation. Housing × strain affected startle amplitude (B-Conv, B-High < B-Low, B-Mid; B > W; no housing effect in W) but not autonomic response. Fear behaviour was affected by housing (NA exploration, investigation: Conv < Low, Mid, High; NO avoidance: Conv, High < Low, Mid), strain (NA alertness: B > W, NO avoidance: W > B) and their interaction (NA centre approach: B-Conv < all other groups). We present evidence for strain-specific fear responses depending on early experience.

Assessing the Cntnap2 knockout rat prepulse inhibition deficit through prepulse scaling of the baseline startle response curve

Many neurodevelopmental disorders, including autism spectrum disorder (ASD), are associated with changes in sensory processing and sensorimotor gating. The acoustic startle response and prepulse inhibition (PPI) of startle are widely used translational measures for assessing sensory processing and sensorimotor gating, respectively. The Cntnap2 knockout (KO) rat has proven to be a valid model for ASD, displaying core symptoms, including sensory processing perturbations. Here, we used a novel method to assess startle and PPI in Cntnap2 KO rats that allows for the identification of separate scaling components: startle scaling, which is a change in startle amplitude to a given sound, and sound scaling, which reflects a change in sound processing. Cntnap2 KO rats show increased startle due to both an increased overall response (startle scaling) and a left shift of the sound/response curve (sound scaling). In the presence of a prepulse, wildtype rats show a reduction of startle due to both startle scaling and sound scaling, whereas Cntnap2 KO rats show normal startle scaling, but disrupted sound scaling, resulting in the reported PPI deficit. These results validate that startle and sound scaling by a prepulse are indeed two independent processes, with only the latter being impaired in Cntnap2 KO rats. As startle scaling is likely related to motor output and sound scaling to sound processing, this novel approach reveals additional information on the possible cause of PPI disruptions in preclinical models.

Fear-Conditioning to Unpredictable Threats Reveals Sex and Strain Differences in Rat Fear-Potentiated Startle (FPS).

Fear-potentiated startle (FPS) has been widely used to study fear processing in humans and rodents. Human studies showed higher startle amplitudes and exaggerated fear reactivity to unpredictable vs. predictable threats in individuals suffering from post-traumatic stress disorder (PTSD). Although human FPS studies use both sexes, a surprisingly limited number of rodent FPS studies use females. Here we investigate the effects of signal-threat contingency, signal-threat order and threat predictability on FPS in both sexes. We use a classic fear-conditioning protocol (100% contingency of cue and shock pairings, with forward conditioning such that the cue co-terminates with the shock) and compare it to modified fear-conditioning protocols (70% contingency; backward conditioning; or cue and shock un-paired). Although there are no sex differences in the startle amplitudes when corrected for body weight, females consistently demonstrate higher shock reactivity during fear-conditioning. Both sexes and strains demonstrate comparable levels of cued, non-cued, and contextual fear in the classic FPS and FPS following fear-conditioning with 70% contingency or backward order (cue co-starts with shock). However, in the classic FPS, Sprague-Dawley females show reduced proportion between cued fear and cue-elicited vigilant state than males. Lastly, a prominent sex difference is uncovered following unpredictable fear-conditioning (cue and shock un-paired), with Wistar, but not Sprague-Dawley, females showing significantly higher startle overall during the FPS recall, regardless of trial type, and higher contextual fear than males. This striking sex difference in processing unpredictable threats in rodent FPS might help to understand the mechanisms underlying higher incidence of PTSD in women.

Sprague Dawley rats from different vendors vary in the modulation of prepulse inhibition of startle (PPI) by dopamine, acetylcholine, and glutamate drugs

RationaleRodent vendors are often utilized interchangeably, assuming that the phenotype of a given strain remains standardized between colonies. Several studies, however, have found significant behavioral and physiological differences between Sprague Dawley (SD) rats from separate vendors. Prepulse inhibition of startle (PPI), a form of sensorimotor gating in which a low-intensity leading stimulus reduces the startle response to a subsequent stimulus, may also vary by vendor. Differences in PPI between rat strains are well known, but divergence between colonies within the SD strain lacks thorough examination.ObjectivesWe explored intrastrain variation in PPI by testing SD rats from two vendors: Envigo and Charles River (CR).MethodsWe selected drugs acting on four major neurotransmitter systems that have been repeatedly shown to modulate PPI: dopamine (apomorphine; 0.5, 1.5, 3.0 mg/kg), acetylcholine (scopolamine; 0.1, 0.5, 1.0 mg/kg), glutamate (dizocilpine; 0.5, 1.5, 2.5 mg/kg), and serotonin (2,5-Dimethoxy-4-iodoamphetamine, DOI; 0.25, 0.5, 1.0 mg/kg). We determined PPI and startle amplitude for each drug in male and female Envigo and CR SD rats.ResultsSD rats from Envigo showed dose-dependent decreases in PPI after apomorphine, scopolamine, or dizocilpine administration, without significant effects on startle amplitude. SD rats from CR were less sensitive to modulation of PPI and/or more sensitive to modulation of startle amplitude, across the three drugs.ConclusionsSD rats showed vendor differences in sensitivity to pharmacological modulation of PPI and startle. We encourage researchers to sample rats from separate vendors before experimentation to identify the most suited source of subjects for their specific endpoints.

Counterirritation by Pain Inhibits Responses to and Perception of Aversive Loud Tones.

The application of a noxious stimulus reduces the perception of other noxious stimuli, which can be assessed by an experimental method called "counterirritation." The question arises whether this type of inhibition also affects the processing of other aversive (but not nociceptive) stimuli, such as loud tones. If aversiveness or, in other words, negative emotional valence qualifies a stimulus to be affected by counterirritation, the general emotional context may also play a role in modulating counterirritation effects. We involved 63 participants in this study (M age = 38.8, SD = 10.5 years; 33 males, 30 females). We tried to counterirritate their perceptual and startle reactions to aversively loud tones (105 db) by immersing the hand into a painful hot water bath (46°C) in two emotional valence conditions (i.e., a neutral and a negative valence block in which we showed either neutral pictures or pictures of burn wounds). We assessed Inhibition by loudness ratings and startle reflex amplitudes. Counterirritation significantly reduced both loudness ratings and startle reflex amplitudes. The emotional context manipulation did not affect this clear inhibitory effect, showing that counterirritation by a noxious stimulus affects aversive sensations not induced by nociceptive stimuli. Thus, the assumption that "pain inhibits pain" should be widened to "pain inhibits the processing of aversive stimuli." This broadened understanding of counterirritation leads to a questioning of the postulate of clear pain specificity in paradigms like "conditioned pain modulation" (CPM) or "diffuse noxious inhibitory controls" (DNIC).

Different outcomes of neonatal and adult Zika virus infection on startle reflex and prepulse inhibition in mice

Zika virus (ZIKV) infection causes severe neurological consequences in both gestationally-exposed infants and adults. Sensorial gating deficits strongly correlate to the motor, sensorial and cognitive impairments observed in ZIKV-infected patients. However, no startle response or prepulse inhibition (PPI) assessment has been made in patients or animal models. In this study, we identified different outcomes according to the age of infection and sex in mice: neonatally infected animals presented an increase in PPI and delayed startle latency. However, adult-infected male mice presented lower startle amplitude, while a PPI impairment was observed 14 days after infection in both sexes. Our data further the understanding of the functional impacts of ZIKV on the developing and mature nervous system, which could help explain other behavioral and cognitive alterations caused by the virus. With this study, we support the startle reflex testing in ZIKV-exposed patients, especially infants, allowing for early detection of functional neuromotor damage and early intervention.

Age-related changes in prepulse inhibition of the startle response.

Acoustic prepulse inhibition of the startle response (PPI) is a phenomenon characterized by the reduction in the startle reflex caused by the presence of weak and brief stimulus before an intense and sudden stimulus (pulse). These phenomena can be observed in several species, but in humans it is commonly measured by the eyeblink using electromyography. PPI works as an operational measure of sensorimotor gating, which is the ability to suppress motor responses for sensory stimulus. Healthy aging is marked by several changes in neural processing, like inhibitory functioning decline. In this line, PPI measure can be a potential biomarker for changes related to the aging process. In this research we aim to investigate if PPI is reduced with aging and if this reduction would be associated with cognitive functioning of older adults. To this aim, we compared PPI levels of older adults (over 60 years old) with PPI levels of young adults (from 18 to 28 years old). With that, we found, significantly lower PPI level (F[1,25] = 7.44 p = 0.01) and lower startle amplitude startle amplitude: (U = 26.000 p = 0.001) in older adults than in young adults. However, we did not find differences in levels of habituation (T = -1.1 p = 0.28) and correlation between PPI and cognition within the sample of healthy older adults. Our results demonstrate that aging is a factor that affects PPI and that it does not seem to predict cognition, however, future studies should explore the potential of using PPI for monitoring cognitive changes associated with techniques such as cognitive training.

KCNQ4 potassium channel subunit deletion leads to exaggerated acoustic startle reflex in mice.

The potassium voltage-gated channel subfamily Q member 4 (KCNQ4) subunit forms channels responsible for M-current, a muscarine-sensitive potassium current regulating neuronal excitability. In contrast to other KCNQ subunits, its expression is restricted to the cochlear outer hair cells, the auditory brainstem and other brainstem nuclei in a great overlap with structures involved in startle reflex. We aimed to show whether startle reflexis affected by the loss of KCNQ4 subunit and whether these alterations are similar to the ones caused by brainstem hyperexcitability. Young adult KCNQ4 knockout mice and wild-type littermates, as well as mice expressing hM3D chemogenetic actuator in the pontine caudal nucleus and neurons innervating it were used for testing acoustic startle. The acoustic startle reflex was significantly increased in knockout mice compared with wild-type littermates. When mice expressing human M3 muscarinic (hM3D) in nuclei related to startle reflex were tested, a similar increase of the first acoustic startle amplitude and a strong habituation of the further responses was demonstrated. We found that the acoustic startle reflex is exaggerated and minimal habituation occurs in KCNQ4 knockout animals. These changes are distinct from the effects of the hyperexcitability of nuclei involved in startle. One can conclude that the exaggerated startle reflex found with the KCNQ4 subunit deletion is the consequence of both the cochlear damage and the changes in neuronal excitability of startle networks.

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-HT1B 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-HT1B receptor as a potential therapeutic target for treating hyperarousal symptom in fear-associated psychiatric disorders.

Stress resilience-associated behaviors following predator scent stress are accompanied by upregulated nucleus accumbens mGlu5 transcription in female Sprague Dawley rats

Despite the higher prevalence of post-traumatic stress disorder (PTSD) in women, the majority of preclinical research has been conducted utilizing male subjects. We have found that male rats exposed to the predator scent 2,4,5-trimethyl-3-thiazoline (TMT) show heterogenous long-term anxiety-like behavior and conditioned fear to the TMT environment. Stress-Resilient males exhibit increased mGlu5 mRNA expression in the basolateral amygdala (BLA) and prefrontal cortex (PFC). Here we sought to determine whether the same behavioral and genetic responses would be observed in female rats exposed to TMT. Female Sprague-Dawley rats were exposed to TMT for ten minutes, while Controls were exposed to an unscented environment. Anxiety and anhedonia were assessed 7–14 days later with elevated plus maze (EPM), acoustic startle response, light-dark box, and sucrose preference test (SPT). TMT-exposed females spent less time in the EPM open arms, exhibited greater startle amplitude, and reduced sucrose intake compared to Controls. Median split analyses conducted on EPM and SPT data yielded stress-Susceptible and -Resilient phenotypes that displayed behavior in the light-dark box consistent with EPM and SPT behavior. Susceptible females displayed greater BLA mGlu5 mRNA expression than Resilient and Control rats and did not show conditioned fear, in contrast to previous results in males. Resilient females displayed greater mGlu5 mRNA expression in the nucleus accumbens. These data indicate that the predator scent stress model of PTSD produces distinct stress-Susceptible and Resilient phenotypes in female rats that are associated with changes in mGlu5 mRNA expression in several brain regions.

A Protocol for the Induction of Posttraumatic Stress-Disorder (PTSD)-like Behavior in Mice.

Post-traumatic stress disorder (PTSD) is a debilitating psychiatric condition that precipitates in ~10% of individuals exposed to a traumatic event (TE). Symptoms include recurrent and intrusive thoughts, sleep disturbance, hypervigilance, exaggerated startle, and reckless or destructive behavior. Given the complex and heterogeneous nature of the disease, animal models for PTSD-like symptomatology are of increasing interest to the field of PTSD research. Because resilience to PTSD-like symptomatology is an important epidemiologic aspect of PTSD, animal models that resolve vulnerable and resilient animals are of particular value. Due to the complex nature of the PTSD phenotype and the potential overlaps between PTSD-like behavior and behaviors associated with other stress-induced psychopathologies such as anxiety or depression, animal models that utilize multiple readouts for PTSD-like behavior are also of increasing value. We utilize a paradigm developed by Lebow et al. 2012 for the induction and identification of PTSD-like symptomatology in mice. This paradigm utilizes inescapable electric foot shock, administered in two decontextualized sessions over two consecutive days. Stressed mice perform four behavioral tests - dark/light transfer, marble burying, acoustic startle, and home cage activity - to generate five behavioral readouts of PTSD-like behavior: % risk assessment (%RA), % marbles buried (%MB), % prepulse inhibition (%PPI), latency to peak startle amplitude (LPSA), and % light phase activity (%LPA). PTSD-like symptomatology is characterized by decreased %RA, increased %MB, decreased %PPI, decreased LPSA, and increased %LPA. The 20% of animals displaying the most PTSD-like behavior in each test are awarded a certain number of points depending on the test, and animals scoring sufficient points are designated as PTSD-like, while animals scoring no points are designated PTSD-resilient. This paradigm identifies PTSD-like behavior in ~15% of animals, a rate comparable to that observed in humans. This protocol represents a robust and reproducible paradigm for the induction of PTSD-like behavior in mice.

Startle Latency as a Potential Marker for Amygdala-Mediated Hyperarousal

BackgroundFear-related disorders are characterized by hyperexcitability in reflexive circuits and maladaptive associative learning mechanisms. The startle reflex is suited to investigate both processes, either by probing it under baseline conditions or by deriving it in fear conditioning studies. In anxiety research, the amplitude of the fear-potentiated startle has been shown to be influenced by amygdalar circuits and has typically been the readout of interest. In schizophrenia research, prolonged startle peak latency under neutral conditions is an established readout, thought to reflect impaired processing speed. We therefore explored whether startle latency is an informative readout for human anxiety research. MethodsWe investigated potential similarities and differences of startle peak latency and amplitude derived from a classical fear conditioning task in a sample of 206 participants with varying severity levels of anxiety disorders and healthy control subjects. We first reduced startle response to stable components and regressed individual amygdala gray matter volumes onto the resulting startle measures. We then probed time, stimulus, and group effects of startle latency. ResultsWe showed that startle latency and startle amplitude were 2 largely uncorrelated measures; startle latency, but not amplitude, showed a sex-specific association with gray matter volume of the amygdala; startle latencies showed a fear-dependent task modulation; and patients with fear-related disorders displayed shorter startle latencies throughout the fear learning task. ConclusionsThese data provide support for the notion that probing startle latencies under threat may engage amygdala-modulated threat processing, making them a complementary marker for human anxiety research.

STREStrogen: The Dichotomous Role of Estrous Cycle on Anxiety‐like Behavior is Dependent upon Stress History

Stress‐induced psychiatric disorders are some of the most debilitating conditions worldwide and occur more often in women than men. Increased risk coincides with the reproductive years when circulating estrogen (E) levels peak, alluding to a critical role of ovarian hormones in susceptibility. Human stress studies support this association as E is linked to negative mood and anxiety during social stressors. However, preclinical research often reports opposing findings of protective effects of E in non‐stressed subjects, indicating reduced anxiety‐like behavioral responses. Yet, rodent studies often neglect to acknowledge the latent effects of stress on E‐induced behavioral differences, like the negative impact of a traumatic experience that perpetuates future interactions. Thus, this divergence between clinical and preclinical findings limits the capability of identifying adequate pharmacological targets for use in naturally cycling females. Therefore, we investigated if prior exposure to stress in intact adult female rats sensitized behavioral outcomes during anxiogenic tasks and whether this was contingent upon the current phase of estrous. Rats were exposed for 15 mins/day over 5 consecutive days to either control handling (CON) or witness stress (WS), a modified social defeat paradigm whereby a female observes social defeat from within a protected compartment of the aggressive resident’s cage. Forty‐eight hours after the 5th and final CON/WS exposure, rats underwent elevated plus maze (EPM) testing, and then acoustic startle response (ASR) testing the next day. Vaginal lavage was obtained immediately following EPM and ASR to confirm estrous phase. Analyses were conducted in a 2 x 2 fashion: stress exposure x estrous phase, in which metestrus/diestrus (M/D) signified putatively low levels of E, and proestrus/estrus (P/E) signified high levels. Results revealed a prominent stress exposure x estrous phase interaction for percentage of time spent in the closed and open arms of EPM. First, CON rats demonstrated reduced avoidance of the open arms when tested during P/E vs M/D. This is in line with studies reporting an anxiolytic effect of E. However, our findings in WS rats supports the notion that E is a critical component of sensitizing anxiety‐like responses in females with a history of stress. Specifically, WS rats demonstrated greater avoidance of open arms in the EPM during P/E vs M/D. Importantly, we also found that not all tests are impacted by cycle stage. Females with a history of stress demonstrated a sensitized startle amplitude compared with CON rats, independent of estrous phase, when tested for magnitude of the innate startle response. Further investigations into E receptor expression and subsequent signaling mechanisms in stress‐sensitive brain regions are necessary to determine how a history of stress modifies behavioral outcomes in the presence of high circulating E. These data can then elucidate novel therapeutic targets important for decreasing the high incidence of psychological disorders in this susceptible population.

Hyperreactivity and Impaired Habituation of Startle Amplitude During Unpleasant Pictures in Borderline but Not Schizotypal Personality Disorder: Quantifying Emotion Dysregulation

BackgroundBorderline personality disorder (BPD) is characterized by greater intensity of reactions to unpleasant emotional cues and a slower-than-normal return of these responses to baseline. Habituation is defined as decreased response to repeated stimulation. Affect-modulated startle (AMS), a translational psychophysiological approach, is mediated by the amygdala and used to study emotion processing in both humans and animals. This is the first study to examine the specificity of habituation anomalies in BPD during passive emotional and neutral picture processing. MethodsA total of 90 participants were studied: patients with BPD (n = 35), patients with schizotypal personality disorder (n = 26; included as a psychopathological comparison group), and healthy control subjects (n = 29). Participants received rigorous clinical assessments, and patients were unmedicated. AMS was examined during a series of intermixed unpleasant, neutral, and pleasant pictures. ResultsCompared with the other groups, patients with BPD showed greater overall AMS during unpleasant pictures and prolonged habituation of startle amplitude during unpleasant pictures from early to later trials. The groups did not differ in AMS during neutral or pleasant pictures or self-reported picture valence. Among the patients with BPD, prolonged habituation to unpleasant pictures was associated with greater symptom severity and suicidal/self-harming behavior. ConclusionsThese findings 1) indicate that abnormal processing of and habituation to unpleasant pictures is observed in BPD but not schizotypal personality disorder, suggesting that these deficits are not simply characteristics of personality disorders in general; 2) are consistent with studies showing deficient amygdala habituation to unpleasant pictures in BPD; and 3) have significant implications for clinical assessment and treatment of BPD, e.g., alternative therapies for BPD such as gradual exposure to unpleasant emotional stimuli or amygdala neurofeedback may aid habituation deficits.

Commonly-used rodent tests of anxiety-like behavior lack predictive validity for human sex differences

Women are more likely to develop an anxiety disorder than men. Yet, preclinical models of anxiety were largely developed in male rodents, with poorly understood predictive validity for sex differences. Here, we investigate whether commonly-used anxiety-like behavior tests, elevated plus maze (EPM) and open field (OF), represent the human sex difference in adult Sprague-Dawley rats. When interpreted by EPM or OF, female rats displayed less anxiety-like behavior compared to males, as they spent twice as much time in the open arms of the EPM or the center of the OF compared to males. However, they also displayed vastly different levels of locomotor activity, possibly confounding interpretation of these locomotion-dependent tests. To exclude locomotion from the assessment, the acoustic startle response (ASR) test was used. When interpreted by the ASR test, females displayed more anxiety-like behavior compared to males, as indicated by a nearly two-fold higher startle amplitude. The observed sex differences were not driven by gonadal steroids. Overall, all but one of the tests fail to mirror the sex difference in anxiety reported in humans. Our findings suggest that the ASR might be a better fit in modelling female anxiety-like behavior.

Reduced Serum Levels of Soluble Interleukin-15 Receptor α in Schizophrenia and Its Relationship to the Excited Phenotype.

Our previous studies documented that interleukin-15 receptor α (IL-15Rα) knockout (KO) mice exhibited hyperactivity, memory impairment, and desperate behavior, which are core features of schizophrenia and depression. Due to the overlapping symptomology and pathogenesis observed for schizophrenia and depression, the present study attempted to determine whether IL-15Rα was associated with the risk of schizophrenia or depression. One hundred fifty-six participants, including 63 schizophrenia patients, 29 depressive patients, and 64 age-matched healthy controls, were enrolled in the study. We investigated the circulating levels of soluble IL-15Rα and analyzed potential links between the IL-15Rα levels and clinical symptoms present in schizophrenia or depressive patients. We observed reduced serum IL-15Rα levels in schizophrenia patients, but not depressive patients compared with controls. Moreover, a significant negative association was observed between the circulating IL-15Rα levels and excited phenotypes in the schizophrenia patients. The IL-15Rα KO mice displayed pronounced pre-pulse inhibition impairment, which was a typical symptom of schizophrenia. Interestingly, the IL-15Rα KO mice exhibited a remarkable elevation in the startle amplitude in the startle reflex test compared to wild type mice. These results demonstrated that serum levels of soluble IL-15Rα were reduced in schizophrenia and highlighted the relationship of IL-15Rα and the excited phenotype in schizophrenia patients and mice.

Prepulse inhibition deficits in inbred and outbred rats and between-strain differences in startle habituation do not depend on startle reactivity levels

The acoustic startle response and prepulse inhibition (PPI) of startle are measures related to information processing, which is impaired in schizophrenia. Some studies have provided inconclusive patterns of association between both measures in rodents. We assessed the influence of baseline startle response on PPI in large samples of Roman high-(RHA) and low-avoidance (RLA) rat strains and in genetically heterogeneous stock (HS) rats. Results show that RHAs exhibit a PPI deficit compared to RLA rats, which is present regardless of the startle response levels. HS rats were stratified in two sub-samples according to their high or low PPI (HS-highPPI or HS-lowPPI, respectively) scores, and then they were grouped by their differential baseline startle amplitude (high reactivity –HR- or low reactivity –LR-) within each sub-sample. Differences between high- and low-PPI-stratified HS rats remained regardless of their high or low startle amplitude scores. Thus, the impairments in %PPI found in both RHA and HS-LowPPI rats are present irrespective of the relatively high or low levels of startle amplitude in pulse-alone trials. Another objective of the present study was to evaluate whether habituation to the startling stimulus (i.e., pulse) depends on the initial baseline startle response. RLA rats habituated to the startling stimulus more effectively than RHAs regardless of their baseline startle responses. Conversely, there were no differences in startle habituation in the HS rats grouped by their extreme scores of baseline startle. Altogether, these findings suggest a deficit in information processing in RHA rats, which along with evidence indicating that this strain displays other attentional/cognitive impairments, strengthens the validity of the RHA strain as a putative model of schizophrenia-relevant features.

Time- and frequency-dependent changes in acoustic startle reflex amplitude following cyclodextrin-induced outer and inner cell loss

The acoustic startle reflex (ASR) amplitude can be enhanced or suppressed by noise-induced hearing loss or age-related hearing loss; however, little is known about how the ASR changes when ototoxic drugs destroy outer hair cells (OHCs) and inner hair cells (IHCs). High doses of 2-hydroxypropyl-beta-cyclodextrin (HPβCD), a cholesterol-lowering drug used to treat Niemann-Pick Type disease type C1, initially destroy OHCs and then the IHCs 6–8 weeks later. Adult rats were treated with doses of HPβCD designed to produce a diversity of hair cell lesions and hearing losses. When HPβCD destroyed OHCs and IHCs in the extreme base of the cochlea and caused minimal high-frequency hearing loss, the ASR amplitudes were enhanced at 4-, 8- and 16 kHz. Enhanced ASR occurred during the first few weeks post-treatment when only OHCs were missing; little change in the ASR occurred 6–8-WK post-treatment. If HPβCD destroyed most OHCs and many IHCs in the basal half of the cochlea, high-frequency thresholds increased ∼50 dB, and ASR amplitudes were reduced ∼50% at 4-, 8- and 16-kHz. The ASR amplitude reduction occurred in the first few weeks post-treatment when the OHCs were degenerating. The ASR was largely abolished when most of the OHCs were missing over the basal two-thirds of the cochlea and a 40–50 dB hearing loss was present at most frequencies. These results indicate that high-doses of HPβCD generally lead to a decline in ASR amplitude as OHCs degenerate; however, ASR amplitudes were enhanced in a few cases when hair cell loss was confined to the extreme base of the cochlea.