Startle Habituation: A Tool for Assessing Information Processing Deficits in Zebrafish Model of Schizophrenia

Prepulse inhibition (PPI) and habituation of acoustic startle reflex have been extensively used to assess deficits in the sensorimotor functions of human patients and animal models of schizophrenia. These assays require expensive and sophisticated experimental setup for fine control of acoustic stimuli and sound attenuation. In this study, we investigate whether startle habituation assay based on mechanical (tap) stimuli can induce similar impairment in the habituation response in the schizophrenia model of larval zebrafish. For this purpose, a custom startle apparatus consisting of a 9 V push and pull solenoid and an Arduino Uno microcontroller was used to generate tap stimuli at desired intervals. Our results showed that tap stimuli at 1 Hz effectively evoked startle response in the control fishes which habituated after a few trials. The habituation response was significantly impaired in the MK801-induced schizophrenia model, similar to that elicited by acoustic startle stimuli in a previous study. We propose this simple and inexpensive method as an alternative tool for studying information processing and attention deficits in the pharmacological model of schizophrenia in zebrafish.

A model-based quantification of startle reflex habituation in larval zebrafish

Zebrafish is an established animal model for the reproduction and study of neurobiological pathogenesis of human neurological conditions. The ‘startle reflex’ in zebrafish larvae is an evolutionarily preserved defence response, manifesting as a quick body-bend in reaction to sudden sensory stimuli. Changes in startle reflex habituation characterise several neuropsychiatric disorders and hence represent an informative index of neurophysiological health. This study aimed at establishing a simple and reliable experimental protocol for the quantification of startle reflex response and habituation. The fish were stimulated with 20 repeated pulses of specific vibratory frequency, acoustic intensity/power, light-intensity and interstimulus-interval, in three separate studies. The cumulative distance travelled, namely the sum of the distance travelled (mm) during all 20 stimuli, was computed as a group-level description for all the experimental conditions in each study. Additionally, by the use of bootstrapping, the data was fitted to a model of habituation with a first-order exponential representing the decay of locomotor distance travelled over repeated stimulation. Our results suggest that startle habituation is a stereotypic first-order process with a decay constant ranging from 1 to 2 stimuli. Habituation memory lasts no more than 5 min, as manifested by the locomotor activity recovering to baseline levels. We further observed significant effects of vibratory frequency, acoustic intensity/power and interstimulus-interval on the amplitude, offset, decay constant and cumulative distance travelled. Instead, the intensity of the flashed light did not contribute to significant behavioural variations. The findings provide novel insights as to the influence of different stimuli parameters on the startle reflex habituation and constitute a helpful reference framework for further investigation.

CDER Experience With Juvenile Animal Studies for CNS Drugs

A survey was undertaken to evaluate juvenile animal studies conducted for drug applications reviewed by the Center for Drug Evaluation and Research between 2009 and 2014. Some conclusions about the nonclinical pediatric safety assessment based on studies performed in support of central nervous system–active compounds are presented here. A total of 44 completed studies from 32 New Drug Applications submitted to the Divisions of Psychiatry and Neurology Products were evaluated. Data on animal species and age range used, endpoints evaluated, and outcomes included in labeling were analyzed. Of the drugs evaluated, all but one had studies conducted in rats. In some cases, a second study in a nonrodent species (dog) was also conducted. Indices of growth and development and standard general toxicity parameters were included in all of the studies. Expanded neurohistopathology evaluations, bone mineral density measurements, and reproductive and neurobehavioral functional assessments were also generally carried out. A variety of neurological and neurobehavioral tests were employed. In the majority of rat studies, the potential for long-term cognitive impairment was evaluated using a complex water maze. Juvenile animal studies provided safety information considered relevant to drug use in children and that was included in labeling for 78% of the applications surveyed. The most commonly reported findings in labeling were for neurobehavioral effects, including changes in locomotor activity, auditory startle habituation, and learning and memory. Of the studies described in labeling with neurobehavioral effects, 54% found these effects to be persistent and to provide evidence of developmental neurotoxicity.

A genetic deletion of the serotonin transporter differentially influences the behavioural effects of MDMA

Background: While (±)-3,4-methylenedioxymethamphetamine (MDMA) primarily induces serotonin release, it also affects dopamine and noradrenaline transmission. It is, however, unclear what role each of these neurotransmitters play in the behavioural profile of MDMA. Methods: In this study we used the drug discrimination (DD) and the acoustic startle (ASR) paradigms to examine the behaviour of rats with and without a genetic deletion of the serotonin transporter SERT (SERT−/− and SERT+/+ rats). In DD, rats were trained to respond on different levers following an injection of 1.5 mg/kg MDMA, or saline. After acquisition, they were given a challenge dose of 0.5 mg/kg amphetamine (AMPH). In the ASR paradigm, SERT+/+ and SERT−/− rats were given 0, 5 or 10 mg/kg MDMA. Results: In DD, significantly fewer SERT−/− rats acquired MDMA discrimination. When the acquirers were challenged with AMPH, SERT+/+ showed partial, while SERT−/− rats showed full generalisation to MDMA. In the ASR paradigm, MDMA significantly reduced prepulse inhibition and startle habituation in SERT+/+ rats, while having no effect in SERT−/− rats. Conclusion: Together these data suggest that in wildtype rats the interoceptive cues of MDMA are primarily mediated by serotonin and to a lesser extent by dopamine and noradrenaline, while the effects in the startle paradigm are almost exclusively mediated via serotonin. Together, these data contribute to our understanding of the complex pharmacodynamics of MDMA.

Startle Habituation Depends on Selective Attention in Schizophrenia Patients and Cannabis Users

IntroductionSchizophrenia is associated with cannabis use and deficits in selective attention.Objectives/aimsThis study investigated these relationships using habituation of the startle reflex in schizophrenia patients relative to cannabis users during selective attentional tasks.MethodsParticipants included 12 healthy controls (CON), 16 healthy cannabis users in the last 12 months (THC), and eight schizophrenia patients (SCZ). Auditory startle reflex was recorded from orbicularis oculi muscle while participants were attending to (Attend Task) or ignoring (Ignore Task) 100 dB startling pulses. Startle habituation was measured as the absolute reduction in startle magnitude on block 2 (last nine trials) vs. block 1 (first nine trials) on each attentional task and in each group.ResultsAll three groups were matched on demographics, alcohol, and caffeine consumption. ANCOVA with two within-subject factors (attention and habituation with 2 levels each), one between-subject factor (group with 3 levels), and one covariate (nicotine use which was higher in SCZ vs. CON or THC) showed a significant startle habituation with moderate to large effect sizes in all three groups on the Ignore Task (Cohen's d = 0.67 in CON, d = 0.59 in THC, d = 0.90 in SCZ) but not on the Attend Task. Attentional modulation of the startle reflex occurred only in CON (d = 1.33) and THC (d = 1.17), but not in SCZ.ConclusionAuditory startle habituation depends on selective attention but is not affected by schizophrenia or cannabis use. Deficient attentional modulation of the startle reflex suggests that a more severe deficit in selective attention occurs in schizophrenia relative to cannabis use.Disclosure of interestThe authors have not supplied their declaration of competing interest.

Startle Habituation and Midfrontal Theta Activity in Parkinson Disease

The ability to adapt to aversive stimuli is critical for mental health. Here, we investigate the relationship between habituation to startling stimuli and startle-related activity in medial frontal cortex as measured by EEG in both healthy control participants and patients with Parkinson disease (PD). We report three findings. First, patients with PD exhibited normal initial startle responses but reduced startle habituation relative to demographically matched controls. Second, control participants had midfrontal EEG theta activity in response to startling stimuli, and this activity was attenuated in patients with PD. Finally, startle-related midfrontal theta activity was correlated with the rate of startle habituation. These data indicate that impaired startle habituation in PD is a result of attenuated midfrontal cognitive control signals. Our findings could provide insight into the frontal regulation of startle habituation.