The monotonic dependency of prepulse inhibition of the acoustic startle reflex on the intensity of the startle-eliciting stimulus

Effects of acute and chronic methylphenidate on prepulse inhibition: A sex difference study in Wistar rats

BACKGROUND

The utilization of methylphenidate (MPH) is experiencing a notable surge within the adult population. This growth can be attributed to two key factors: its recreational and cognitive enhancement purposes, as well as the rising prevalence of ADHD diagnoses within this population. This study examined acute and chronic oral MPH effects on attention in male and female Wistar rats. To this end, we used a prepulse inhibition (PPI) task, which is widely used to assess psychoactive drug effects in both humans and rodents. This task allowed us to evaluate changes in attention by analyzing sensorimotor gating associated with stimulus selection process.

METHODS

Animals were administered a clinically relevant dose of MPH (5 mg/kg) daily for seven days. The estrous cycle phases of the female rats were measured during behavioral sessions. The PPI task was conducted 20 min after drug administration on day 1 (acute), day 7 (chronic), and 48 h post-treatment.

RESULTS

Results indicated that both acute and chronic MPH treatment impaired PPI expression in male rats, but not in female rats, regardless of their estrous cycle phase. Furthermore, a differential effect of chronic MPH treatment on the PPI task was found in male rats. Specifically, on the seventh treatment day, the PPI effect was observed when animals undertook the PPI task for the first time but was impaired in those animals in which the initial PPI session occurred under the acute influence of the drug (day 1).

CONCLUSIONS

These findings suggest that the impact of MPH on sensorimotor gating responses may vary based on sex and task experience, possibly leading to state-dependent effects in healthy individuals.

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.

Within-subjects vs between-subjects co-variation of prepulse-elicited reaction and the diminution of startle to the succeeding pulse stimulus in the prepulse inhibition paradigm

Prepulse inhibition (PPI) refers to the diminution of the startle reflex to a sudden and intense acoustic stimulus (pulse) when this startle-eliciting pulse is preceded shortly by a weaker prepulse stimulus. PPI is widely used in evaluating the effects of psychomimetic and antipsychotic drugs on sensorimotor gating, but individual differences in PPI expression have received scant attention. We have previously shown that mice and rats exhibiting stronger motor response to the prepulse also exhibit more PPI. It remains unexplored, however, if this between-subjects correlation may be similarly observed across trials from a within-subjects perspective. Here, we mapped the prepulse-elicited response to the diminution of the startle response to the succeeding pulse stimulus, trial-by-trial, across nine prepulse-pulse definitions with varying prepulse and pulse intensities. The resulting within-subjects correlation independently obtained in 113 adult C57BL6 mice revealed that trials registering a stronger prepulse reaction also recorded a larger startle response to the pulse stimulus, indicative of weaker PPI, especially when higher-intensity prepulses were paired with low-intensity pulses. The within- and between-subjects analyses have apparently yielded two contrasting relationships between the direct motor response to the prepulse and the inhibition of subsequent startle reaction induced by the same prepulse. One interpretation is that the within-subjects correlation reflects state-dependent variation, whereas the between-subjects correlation stems from trait-dependent individual variation. Finally, whether our present findings may depend on the nature of the prepulse reaction is further discussed.

Robust and replicable measurement for prepulse inhibition of the acoustic startle response

Measuring animal behavior in the context of experimental manipulation is critical for modeling, and understanding neuropsychiatric disease. Prepulse inhibition of the acoustic startle response (PPI) is a behavioral phenomenon studied extensively for this purpose, but the results of PPI studies are often inconsistent. As a result, the utility of this phenomenon remains uncertain. Here, we deconstruct the phenomenon of PPI and confirm several limitations of the methodology traditionally utilized to describe PPI, including that the underlying startle response has a non-Gaussian distribution, and that the traditional PPI metric changes with different stimuli. We then develop a novel model that reveals PPI to be a combination of the previously appreciated scaling of the startle response, as well as a scaling of sound processing. Using our model, we find no evidence for differences in PPI in a rat model of Fragile-X Syndrome (FXS) compared with wild-type controls. These results in the rat provide a reliable methodology that could be used to clarify inconsistent PPI results in mice and humans. In contrast, we find robust differences between wild-type male and female rats. Our model allows us to understand the nature of these differences, and we find that both the startle-scaling and sound-scaling components of PPI are a function of the baseline startle response. Males and females differ specifically in the startle-scaling, but not the sound-scaling, component of PPI. These findings establish a robust experimental and analytical approach that has the potential to provide a consistent biomarker of brain function.

Socio-demographic predictors of prepulse inhibition: A prospective study in children and adolescents from Mexico City

Prepulse inhibition (PPI) is a sensorimotor gating mechanism that reduces interfering influences to the neural processing of incoming stimuli, and is associated with several neurodevelopmental disorders. To date, research on PPI and neurodevelopmental disorders has primarily been in cross-sectional, clinical settings. In this prospective, epidemiologic study, we used a data-driven prediction model to identify socio-demographic predictors of PPI in children and adolescents from Mexico City to inform future etiologic studies evaluating PPI. We conducted variable selection and validation using a modified version of the multiple imputation random lasso (MIRL) variable selection algorithm. MIRL identified six predictors of PPI at a stimulus onset asynchrony of 120 ms or 240 ms. Of those six predictors, maternal education, birthweight, and total breastfeeding months were highlighted as previously unstudied variables associated with enhanced PPI. Our findings highlight the potential value of PPI as an adjunct screening tool for identifying children at risk for neurodevelopmental disorders and underscore the relevance for validation research on this topic.

A conceptual and practical guide to the behavioural evaluation of animal models of the symptomatology and therapy of schizophrenia

Schizophrenia is a chronic debilitating brain disorder characterized by a complex set of perceptual and behavioural symptoms that severely disrupt and undermine the patient's psychological well-being and quality of life. Since the exact disease mechanisms remain essentially unknown, holistic animal models are indispensable tools for any serious investigation into the neurobiology of schizophrenia, including the search for remedies, prevention of the disease and possible biological markers. This review provides some practical advice to those confronted with the task of evaluating their animal models for relevance to schizophrenia, a task that inevitably involves behavioural tests with animals. To a novice, this challenge not only is a technical one but also entails attention to interpretative issues concerning validity and translational power. Here, we attempt to offer some guidance to help overcome these obstacles by drawing on our experience of diverse animal models of schizophrenia based on genetics, strain difference, brain lesions, pharmacological induction and early life developmental manipulations. The review pays equal emphasis to the general (theoretical) considerations of experimental design and the illustration of the problems related to critical test parameters and the data analysis of selected exemplar behavioural tests. Finally, the individual differences of behavioural expression in relevant tests observed in wild-type animals might offer an alternative approach in order to explore the mechanism of schizophrenia-related behavioural dysfunction at the molecular, cellular and structural levels, all of which are of more immediate relevance to cell and tissue research.

Baseline prepulse inhibition expression predicts the propensity of developing sensitization to the motor stimulant effects of amphetamine in C57BL/6 mice

RATIONALE

The startle reflex to a sudden intense acoustic pulse stimulus is attenuated if the pulse is shortly preceded by a weak prepulse stimulus. This represents a form of sensory gating, known as prepulse inhibition (PPI), observable across species. PPI is modulated by dopamine and readily disrupted by acute amphetamine. Prior repeated exposures to amphetamine also disrupt PPI even when the drug is not present during test, suggesting that a sensitized mesolimbic dopamine system-inducible even by a single exposure to amphetamine-might be responsible. However, this causative link has been challenged by inconsistent efficacy between different amphetamine pre-treatment regimes, which all robustly sensitize the behavioral response to amphetamine.

METHODS

Here, the presence of such a link in reverse was tested by comparing the propensity to develop amphetamine sensitization between high- and low-PPI expressing individuals identified within a homogeneous cohort of C57BL/6 mice. Comparison of dopamine content including its metabolites was performed separately in drug naïve mice by post-mortem HPLC.

RESULTS

Behavioral sensitization was substantially stronger in the low-PPI group compared with the high-PPI group, while the magnitude of their response to the first amphetamine challenge was similar. Dopamine content within the nucleus accumbens and medial prefrontal cortex was significantly higher in low-PPI relative to high-PPI mice.

CONCLUSION

Individuals with weak sensory gating characterized by low basal PPI expression may be more susceptible to the development of dopamine sensitization and therefore at greater risk of developing schizophrenia. Conversely, high baseline expression might predict a resistance to dopaminergic sensitization.

Sensorimotor gating and D2 receptor signalling: evidence from a molecular genetic approach

Converging evidence from pharmacological investigations, genetic association studies and schizophrenia research indicates an important influence of the dopamine system on sensorimotor gating as measured by prepulse inhibition (PPI) of the acoustic startle response. In particular, D2 receptor agonists have been shown to disrupt PPI in humans and rodents. In the present study, we investigated the associations of two functional DRD2 related single nucleotide polymorphisms (rs4648317 and rs1800497, the latter also known as DRD2/ANKK1 Taq1A) with PPI in two independent healthy human samples (overall n=197; Munich n=101; London n=96). Taq1A is a prominent marker of striatal D2 receptor signalling and was therefore hypothesized to impact on PPI. In line with our hypothesis, we report here reduced PPI levels in individuals with higher striatal D2 receptor signalling as indicated by the Taq1A genotype. Meta-analysis across both samples confirmed this finding. In contrast, an association between rs4648317 and PPI found in the Munich sample could not be confirmed in the London sample. Overall, the present study helps to bridge the gap between pharmacological manipulations of PPI and molecular genetics of the dopaminergic system.

Reversal of scopolamine-induced disruption of prepulse inhibition by clozapine in mice

Prepulse inhibition (PPI) of the acoustic startle reflex refers to the reduction of the startle response to an intense acoustic pulse stimulus when it is shortly preceded by a weak non-startling prepulse stimulus and provides a cross-species measure of sensory-motor gating. PPI is typically impaired in schizophrenia patients, and a similar impairment can be induced in rats by systemic scopolamine, a muscarinic cholinergic receptor antagonist that can evoke a range of cognitive and psychotic symptoms in healthy humans that are commonly referred to as the "anti-muscarinic syndrome" resembling some clinical features of schizophrenia. Scopolamine-induced PPI disruption has therefore been proposed as an anti-muscarinic animal model of schizophrenia, but parallel investigations in the mouse remain scant and the outcomes are mixed and often confounded by an elevation of startle reactivity. Here, we distinguished the PPI-disruptive and the confounding startle-enhancing effects of scopolamine (1 and 10mg/kg, i.p.) in C57BL/6 wild-type mice by showing that the latter partly stemmed from a shift in spontaneous baseline reactivity. With appropriate correction for between-group differences in startle reactivity, we went on to confirm that the PPI-disruptive effect of scopolamine could be nullified by clozapine pre-treatment (1.5mg/kg, i.p.) in a dose-dependent manner. This is the first demonstration that scopolamine-induced PPI disruption is sensitive to atypical antipsychotic drugs. In concert with previous data showing its sensitivity to haloperidol the present finding supports the predictive validity of the anti-muscarinic PPI disruption model for both typical and atypical antipsychotic drug action.

Are DBA/2 mice associated with schizophrenia-like endophenotypes? A behavioural contrast with C57BL/6 mice

RATIONALE

Due to its intrinsic deficiency in prepulse inhibition (PPI), the inbred DBA/2 mouse strain has been considered as an animal model for evaluating antipsychotic drugs. However, the PPI impairment observed in DBA/2 mice relative to the common C57BL/6 strain is confounded by a concomitant reduction in baseline startle reactivity. In this study, we examined the robustness of the PPI deficit when this confound is fully taken into account.

MATERIALS AND METHODS

Male DBA/2 and C57BL/6 mice were compared in a PPI experiment using multiple pulse stimulus intensities, allowing the possible matching of startle reactivity prior to examination of PPI. The known PPI-enhancing effect of the antipsychotic, clozapine, was then evaluated in half of the animals, whilst the other half was subjected to two additional schizophrenia-relevant behavioural tests: latent inhibition (LI) and locomotor reaction to the psychostimulants-amphetamine and phencyclidine.

RESULTS

PPI deficiency in DBA/2 relative to C57BL/6 mice was essentially independent of the strain difference in baseline startle reactivity. Yet, there was no evidence that DBA/2 mice were superior in detecting the PPI-facilitating effect of clozapine when startle difference was balanced. Compared with C57BL/6 mice, DBA/2 mice also showed impaired LI and a different temporal profile in their responses to amphetamine and phencyclidine.

CONCLUSION

Relative to the C57BL/6 strain, DBA/2 mice displayed multiple behavioural traits relevant to schizophrenia psycho- and physiopathology, indicative of both dopaminergic and glutamatergic/N-methyl-D: -aspartic acid receptor dysfunctions. Further examination of their underlying neurobiological differences is therefore warranted in order to enhance the power of this specific inter-strain comparison as a model of schizophrenia.

Impaired prepulse inhibition and prepulse-elicited reactivity but intact reflex circuit excitability in unmedicated schizophrenia patients: a comparison with healthy subjects and medicated schizophrenia patients

Deficient sensorimotor gating as indexed by prepulse inhibition (PPI) of the startle response has been reported repeatedly in patients suffering from schizophrenia. According to the widely accepted "protective hypothesis," PPI reflects the protection of ongoing information processing against interference by other stimuli. Alternatively, it has been proposed that PPI might be regulated by startle reflex circuit excitability. In the present study, we evaluated these 2 conceptually divergent approaches underlying the regulation of PPI. To this end, we assessed sensorimotor gating as indexed by PPI, the reactivity to the prepulse-alone stimulus indexed as prepulse-elicited reactivity (PPER), and acoustic blink reflex excitability in terms of paired pulse suppression (PPS) within a single recording session in 13 unmedicated and 24 medicated (11 first break) schizophrenia patients in comparison to 43 healthy control subjects. The results showed that PPI was significantly reduced in unmedicated, but not in medicated schizophrenia patients. Furthermore, unmedicated patients could be distinguished from the medicated patients and control subjects in terms of PPER. In contrast to PPI, PPS did not differ between patients and control subjects. These findings are in line with the "protective hypothesis" of PPI and indicate that reduced sensorimotor gating in schizophrenia patients might be based on a reduced perception and/or processing of the prepulse stimulus. The extent to which PPER may or may not be causally associated with sensorimotor gating in schizophrenia has to be further investigated in human and animal studies.

Prepulse inhibition of the startle reflex depends on the catechol O-methyltransferase Val158Met gene polymorphism

BACKGROUND

Recent evidence suggests that dopamine (DA) agonist-induced disruption of prepulse inhibition (PPI) depends on basal PPI values, in a manner that suggests an inverted U-shaped relationship between PPI and prefrontal DA levels. This is the first study to examine possible genetic determinants of PPI and the catechol O-methyltransferase (COMT) Val158Met polymorphism, the main catabolic pathway of released DA in the prefrontal cortex (PFC).

METHOD

PPI was measured in 93 healthy males presented with 75-dB and 85-dB prepulses at 60-ms and 120-ms prepulse-pulse intervals. Subjects were grouped according to their COMT status into a Val/Val, a Val/Met and a Met/Met group.

RESULTS

ANOVAs showed that at all prepulse and interval conditions, Val/Val individuals had the lowest PPI, Met/Met the highest, and Val/Met were intermediate.

CONCLUSIONS

These results suggest that PPI is regulated by DA neurotransmission in the PFC and its levels depend on the COMT Val158Met gene polymorphism. These findings enhance the value of the PPI paradigm in examining individual variability of early information processing in healthy subjects and psychiatric disorders associated with changes in PFC DA activity and attentional deficits such as schizophrenia.

Defizite der sensomotorischen Filterleistung bei psychiatrischen Erkrankungen

Die Präpuls-Inhibition (PPI) des akustischen Schreckreflexes gilt als operationales Maß für einen teils vorbewußten attentionalen Filterprozeß, der auch als sensomotorisches Gating bezeichnet wird. Die PPI wird durch ein cortico-striato-pallido-pontines (CSPP) Netzwerk reguliert, welches frontale und mediotemporale Hirnareale, das ventrale Striatum, das ventrale Pallidum und pontine Bereiche des Hirnstamms mit einbezieht. Verschiedene psychiatrische und neurologische Erkrankungen zeigen beeinträchtigte Gating-Prozesse, doch insbesondere die konsistenten Befunde eines PPI-Defizits in der Schizophrenie haben dazu beigetragen, daß die Schizophrenie heute auch als Filterstörung verstanden wird. Die PPI hat sich mittlerweile als translationales Modell für gestörte Filterprozesse in der Schizophrenie etabliert, da sie bei verschiedenen Versuchstieren abgeleitet werden kann und pharmakologisch manipulierbar ist. Darüber hinaus wurde die PPI als vielversprechender Endophänotyp, d. h. als Gen-naher biologischer Marker, der Schizophrenie vorgeschlagen. Man erhofft sich von der Identifizierung solcher Endophänotypen eine verbesserte Entschlüsselung der krankheitsmitverursachenden Gene im Vergleich zu bislang nicht zielführenden genetischen Assoziationsstudien mit den komplexeren Krankheitsphänotypen. Des Weiteren wird die Korrektur künstlich erzeugter PPI-Defizite bei Versuchstieren als Modell für antipsychotische Wirksamkeit neu entwickelter Substanzen zur Behandlung der Schizophrenie genutzt. Der vorliegende Artikel soll einen Überblick über die Anwendungsmöglichkeiten und Grenzen des PPI-Paradigmas in der klinischen und grundlagenorientierten psychologischen und psychiatrischen Forschung geben.