Variables affecting prepulse inhibition of the startle reflex and the response to antipsychotics in DBA/2NCrl mice

Effects of total sleep deprivation on sensorimotor gating in humans

Sensorimotor gating is a measure of pre-attentional information processing and can be measured by prepulse inhibition (PPI) of the startle reflex. Sleep deprivation has been shown to disrupt PPI in animals and humans, and has been proposed as an early phase 2 model to probe antipsychotic efficacy in heathy humans. To further investigate the reliability and efficacy of sleep deprivation to produce PPI deficits we tested the effects of total sleep deprivation (TSD) on PPI in healthy controls in a highly controlled sleep laboratory environment. Participants spent 4 days and nights in a controlled laboratory environment with their sleep monitored with polysomnography. Participants were randomly assigned to either normal sleep on all 4 nights (N=17) or 36hours of TSD on the 3^rd or 4^th night (N=40). Participants were assessed for sleepiness using the Karolinska Sleepiness Scale (KSS) and underwent a daily PPI task (interstimlulus intervals 30-2000 ms) in the evening. Both within-subject effects (TSD vs. normal sleep in TSD group alone) and between-subject effects (TSD vs. no TSD group) of TSD on PPI were assessed. TSD increased subjective sleepiness measured with the KSS, but did not significantly alter overall startle, habituation or PPI. Sleep measures including duration, rapid eye movement and slow wave sleep duration were also not associated with PPI performance. The current results show that human sensorimotor gating may not be reliably sensitive to sleep deprivation. Further research is required for TSD to be considered a dependable model of PPI disruption for drug discovery in humans.

Sensorimotor gating is disrupted by acute but not chronic systemic exposure to caffeine in mice

RATIONALE

Caffeine is a psychostimulant drug that blocks adenosine A₁ and A₂A receptors (A₁Rs and A₂ARs). However, its ability to disrupt early sensory gating as indexed by prepulse inhibition (PPI), which is consistently disrupted by other psychostimulant agents, has never been convincingly demonstrated.

OBJECTIVES

To compare the impact of caffeine on PPI expression in C57BL/6 mice by two dose-response experiments differing in terms of chronicity, regimen, and route of administration. To study separately the acute effect of selective antagonists against A₁R or A₂AR.

METHODS

Caffeine (10, 30, 100 mg/kg, intraperitoneal (i.p.)) was either administered shortly before testing or via caffeinated drinking water (0.3, 1.0, 2 g/l) in home cages over 3 weeks. Two separate dose-response studies tested the acute effect of the selective A₁R antagonist, 1,3 dipropyl-8 cyclopentyl xanthine (DPCPX), and the selective A₂AR antagonist, 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c] (SCH 58261) (0.2, 1.0, 5.0 mg/kg, i.p.). The two drugs were combined in a final experiment to identify their potential synergistic interaction.

RESULTS

While the two lower acute doses of caffeine attenuated PPI, the highest dose potentiated PPI. By contrast, chronic caffeine exposure did not affect PPI. Neither DPCPX nor SCH 58261 altered PPI, and no synergism was observed when the two drugs were combined.

CONCLUSIONS

This is the first demonstration that acute caffeine disrupts PPI, but the relative contribution of A₁R and A₂AR blockade remains unclear, and possible non-adenosinergic mechanisms cannot be ruled out. The null effect under chronic caffeine exposure might involve the development of tolerance, but the precise receptor subtypes involved also warrant further investigation.

Influence of aripiprazole, risperidone, and amisulpride on sensory and sensorimotor gating in healthy 'low and high gating' humans and relation to psychometry

Despite advances in the treatment of schizophrenia spectrum disorders with atypical antipsychotics (AAPs), there is still need for compounds with improved efficacy/side-effect ratios. Evidence from challenge studies suggests that the assessment of gating functions in humans and rodents with naturally low-gating levels might be a useful model to screen for novel compounds with antipsychotic properties. To further evaluate and extend this translational approach, three AAPs were examined. Compounds without antipsychotic properties served as negative control treatments. In a placebo-controlled, within-subject design, healthy males received either single doses of aripiprazole and risperidone (n=28), amisulpride and lorazepam (n=30), or modafinil and valproate (n=30), and placebo. Prepulse inhibiton (PPI) and P50 suppression were assessed. Clinically associated symptoms were evaluated using the SCL-90-R. Aripiprazole, risperidone, and amisulpride increased P50 suppression in low P50 gaters. Lorazepam, modafinil, and valproate did not influence P50 suppression in low gaters. Furthermore, low P50 gaters scored significantly higher on the SCL-90-R than high P50 gaters. Aripiprazole increased PPI in low PPI gaters, whereas modafinil and lorazepam attenuated PPI in both groups. Risperidone, amisulpride, and valproate did not influence PPI. P50 suppression in low gaters appears to be an antipsychotic-sensitive neurophysiologic marker. This conclusion is supported by the association of low P50 suppression and higher clinically associated scores. Furthermore, PPI might be sensitive for atypical mechanisms of antipsychotic medication. The translational model investigating differential effects of AAPs on gating in healthy subjects with naturally low gating can be beneficial for phase II/III development plans by providing additional information for critical decision making.

Exercise offers anxiolytic potential: a role for stress and brain noradrenergic-galaninergic mechanisms

Although physical activity reduces anxiety in humans, the neural basis for this response is unclear. Rodent models are essential to understand the mechanisms that underlie the benefits of exercise. However, it is controversial whether exercise exerts anxiolytic-like potential in rodents. Evidence is reviewed to evaluate the effects of wheel running, an experimental mode of exercise in rodents, on behavior in tests of anxiety and on norepinephrine and galanin systems in neural circuits that regulate stress. Stress is proposed to account for mixed behavioral findings in this literature. Indeed, running promotes an adaptive response to stress and alters anxiety-like behaviors in a manner dependent on stress. Running amplifies galanin expression in noradrenergic locus coeruleus (LC) and suppresses stress-induced activity of the LC and norepinephrine output in LC-target regions. Thus, enhanced galanin-mediated suppression of brain norepinephrine in runners is supported by current literature as a mechanism that may contribute to the stress-protective effects of exercise. These data support the use of rodents to study the emotional and neurobiological consequences of exercise.

Social defeat interacts with Disc1 mutations in the mouse to affect behavior

DISC1 (Disrupted-in-schizophrenia 1) is a strong candidate susceptibility gene for psychiatric disease that was originally discovered in a family with a chromosomal translocation severing this gene. Although the family members with the translocation had an identical genetic mutation, their clinical diagnosis and presentation varied significantly. Gene-environment interactions have been proposed as a mechanism underlying the complex heritability and variable phenotype of psychiatric disorders such as major depressive disorder and schizophrenia. We hypothesized that gene-environment interactions would affect behavior in a mutant Disc1 mouse model. We examined the effect of chronic social defeat (CSD) as an environmental stressor in two lines of mice carrying different Disc1 point mutations, on behaviors relevant to psychiatric illness: locomotion in a novel open field (OF), pre-pulse inhibition (PPI) of the acoustic startle response, latent inhibition (LI), elevated plus maze (EPM), forced swim test (FST), sucrose consumption (SC), and the social interaction task for sociability and social novelty (SSN). We found that Disc1-L100P +/- and wild-type mice have similar anxiety responses to CSD, while Q31L +/- mice had a very different response. We also found evidence of significant gene-environment interactions in the OF, EPM and SSN.

Prepulse inhibition of the startle reflex and response to antipsychotic treatments in two outbred mouse strains in comparison to the inbred DBA/2 mouse

RATIONALE

Naturally low prepulse inhibition (PPI) in DBA/2 mice is increased by marketed antipsychotics and compounds acting at novel targets relevant to schizophrenia. Whether other mouse strains with naturally low PPI respond similarly and could be translational models of schizophrenia is unknown.

OBJECTIVE

Baseline levels of PPI were determined in outbred CF-1 and Black Swiss mice. CF-1 and Black Swiss mice were then compared to DBA/2 mice for their responses to typical (haloperidol) and atypical (clozapine) antipsychotics and to compounds with potential antipsychotic activity, a histamine H(3) receptor antagonist (thioperamide) and a glycine transporter-1 inhibitor (SSR504734).

RESULTS

CF-1 and Black Swiss mice had naturally low PPI, similar to the level in C57BL/6 mice, but higher than that in DBA/2 mice. Haloperidol (0.3-1 mg/kg) increased PPI in DBA/2, CF-1, and Black Swiss mice. Clozapine (3 mg/kg) increased PPI in DBA/2 and CF-1 mice, but not in Black Swiss mice. Thioperamide (10-30 mg/kg) and SSR504734 (30 mg/kg) increased PPI only in DBA/2 mice. Strain differences in PPI responsiveness were not due to differences in brain concentrations of the tested compounds.

CONCLUSIONS

CF-1 mice with naturally low PPI may be useful for testing typical and atypical antipsychotics while Black Swiss mice only responded to a typical antipsychotic. DBA/2 mice remain the only strain with naturally low PPI that responds to marketed antipsychotics, as well as to compounds with novel mechanisms of action. Thus, DBA/2 mice may be the strain of choice for screening novel chemical entities for their ability to increase PPI.

The effects of d-amphetamine, methylphenidate, sydnocarb, and caffeine on prepulse inhibition of the startle reflex in DBA/2 mice

RATIONALE

Dopamine (DA) agonists decrease prepulse inhibition (PPI) and are widely used in translational models for the sensorimotor gating deficits in schizophrenia. Reductions in PPI induced by DA agonists are routinely reversed by antipsychotics in these translational models. Nevertheless, under conditions of low-baseline PPI, DA agonists may increase PPI in humans and experimental animals. DBA/2 mice have naturally low-baseline PPI, which as in the drug-induced translational models, is increased by antipsychotics.

OBJECTIVE

Determine whether DBA/2 mice respond like other models of low-baseline PPI by evaluating the effect of psychostimulants (caffeine, 30-100 mg/kg IP) and the indirect DA agonists d-amphetamine (0.3-10 mg/kg IP), methylphenidate (10-100 mg/kg IP), and sydnocarb (10-30 mg/kg IP), a selective DA transporter inhibitor on PPI. Furthermore, baseline PPI in DBA/2 mice was increased by noise exposure and the effect of d-amphetamine was assessed.

RESULTS

PPI was increased at one dose for each of the psychostimulants when baseline PPI was low in naïve DBA/2 mice. Effective doses were 3 mg/kg of d-amphetamine, 30 mg/kg of methylphenidate, 30 mg/kg of sydnocarb, and 100 mg/kg of caffeine. Higher doses of d-amphetamine (10 mg/kg) and methylphenidate (100 mg/kg IP) decreased PPI. When the baseline PPI was increased by noise exposure, 10 mg/kg of d-amphetamine only reduced PPI.

CONCLUSION

Lower doses of psychostimulants increased PPI in naïve DBA/2 mice in a manner consistent with their naturally low-baseline PPI, and higher doses decreased PPI, consistent with effects observed in most mouse strains.

Glycine transporter (GlyT1) inhibitors with reduced residence time increase prepulse inhibition without inducing hyperlocomotion in DBA/2 mice

Inhibition of the glycine transporter type 1 (GlyT1) leading to potentiation of the glycine site (GlyB) on the N-methyl-d-aspartate (NMDA) receptor has been proposed as a novel therapeutic approach for schizophrenia. However, sarcosine-based GlyT1 inhibitors produce undesirable side effects including compulsive walking and respiratory distress. The influence of specific biochemical properties of GlyT1 inhibitors, such as mode of inhibition and residence time, on adverse effects is unknown. Two GlyT1 inhibitors that contain a sarcosine moiety, sarcosine and ALX-5407, and two compounds that do not contain a sarcosine moiety, Roche-7 and Merck (S)-13h, were evaluated for their potency, mode of inhibition, and target residence times in vitro, and modulation of prepulse inhibition (PPI) and locomotor activity in vivo. (S)-13h and sarcosine were competitive inhibitors while ALX-5407 and Roche-7 demonstrated mixed noncompetitive inhibition. Potency of GlyT1 inhibition (ALX-5407>(S)-13h>Roche-7≫sarcosine) did not correlate with residence time on GlyT1 (sarcosine=Roche-7≪(S)-13h<ALX-5407). ALX-5407 and (S)-13h induced compulsive walking, termed obstinate progression (OP), at doses that increased PPI in DBA/2 mice, demonstrating that OP was not a function of mode of inhibition or inhibitor chemotype. Sarcosine and Roche-7 increased PPI without inducing OP, suggesting that compounds with decreased GlyT1 residence time were efficacious without adverse effects. Direct activation of the GlyB site by d-serine did not produce OP. However, OP induced by (S)-13h was blocked by strychnine, a glycine receptor (GlyA) antagonist, suggesting that OP induced by GlyT1 inhibition was mediated by GlyA. Thus, GlyT1 inhibitors with short residence times demonstrated efficacy without mechanism-based adverse effects.

Mood stabilizers increase prepulse inhibition in DBA/2NCrl mice

RATIONALE

Lithium and several antiepileptic drugs have mood-stabilizing effects in bipolar disorder and schizophrenia. Both disorders are characterized by deficits in prepulse inhibition (PPI) of the acoustic startle response.

OBJECTIVES

Using the DBA/2 model of naturally low PPI, which is reliably increased by antipsychotics, five mood stabilizers in clinical use were tested to determine whether they would also increase PPI in this model. All drugs were administered intraperitoneally (i.p.) 30 min before testing.

RESULTS

Lithium chloride (30 mg/kg), topiramate (100 and 300 mg/kg), carbamazepine (30, 60, and 100 mg/kg), valproic acid (178 and 316 mg/kg), and lamotrigine (3, 10, and 30 mg/kg) increased percent PPI. The antiepileptic drugs carbamazepine, valproic acid, and lamotrigine at high doses also decreased no-stimulus amplitudes and increased startle amplitudes. At high doses of carbamazepine, valproic acid, and lamotrigine, increases in percent PPI were independent of the increases in startle amplitude.

CONCLUSIONS

The demonstrated efficacy of five mood stabilizers in the DBA/2 model of naturally low PPI points to the translational value of this model in predicting therapeutic activity in schizophrenia and bipolar disorder of compounds with diverse mechanisms of action.

Realistic expectations of prepulse inhibition in translational models for schizophrenia research

INTRODUCTION

Under specific conditions, a weak lead stimulus, or "prepulse", can inhibit the startling effects of a subsequent intense abrupt stimulus. This startle-inhibiting effect of the prepulse, termed "prepulse inhibition" (PPI), is widely used in translational models to understand the biology of brainbased inhibitory mechanisms and their deficiency in neuropsychiatric disorders. In 1981, four published reports with "prepulse inhibition" as an index term were listed on Medline; over the past 5 years, new published Medline reports with "prepulse inhibition" as an index term have appeared at a rate exceeding once every 2.7 days (n=678). Most of these reports focus on the use of PPI in translational models of impaired sensorimotor gating in schizophrenia. This rapid expansion and broad application of PPI as a tool for understanding schizophrenia has, at times, outpaced critical thinking and falsifiable hypotheses about the relative strengths vs. limitations of this measure.

OBJECTIVES

This review enumerates the realistic expectations for PPI in translational models for schizophrenia research, and provides cautionary notes for the future applications of this important research tool.

CONCLUSION

In humans, PPI is not "diagnostic"; levels of PPI do not predict clinical course, specific symptoms, or individual medication responses. In preclinical studies, PPI is valuable for evaluating models or model organisms relevant to schizophrenia, "mapping" neural substrates of deficient PPI in schizophrenia, and advancing the discovery and development of novel therapeutics. Across species, PPI is a reliable, robust quantitative phenotype that is useful for probing the neurobiology and genetics of gating deficits in schizophrenia.

Olanzapine improves deficient sensory inhibition in DBA/2 mice

Most schizophrenia patients do not inhibit their P50 auditory evoked potential to the second of duplicate auditory stimuli, reflecting a failure to inhibit responses to irrelevant sensory input. Typical antipsychotic drugs do not improve this deficit while some atypical antipsychotics do. A previous study using an animal model, deficient P20-N40 (which corresponds to the human P50) inhibitory processing in DBA/2 mice found that sensory inhibition was improved by clozapine, the prototypical atypical antipsychotic, but not by haloperidol, a typical antipsychotic. The improvement after clozapine was mediated by alpha7 nicotinic receptors. The present study addresses whether another atypical antipsychotic, olanzapine, will also improve sensory inhibition deficits in the mouse model. In vivo electrophysiological recordings of the P20-N40 auditory evoked potential in anesthetized DBA/2 mice, which spontaneously exhibit a schizophrenia-like inhibitory processing deficit, were obtained after olanzapine alone (0.01, 0.033, 0.1, 0.33 mg/kg, IP) and the efficacious dose of olanzapine (0.033 mg/kg, IP) in combination with either the alpha7 nicotinic receptor antagonist alpha-bungarotoxin or the alpha4beta2 nicotinic receptor antagonist di-hydro-beta-erythroidine. All doses of olanzapine produced improved P20-N40 inhibitory processing in DBA/2 mice. The normalization observed after the 0.033 mg/kg dose of olanzapine was due to a selective decrease in response to the second auditory stimulus indicating an increase in inhibitory processing. This improvement was blocked by pre-administration of alpha-bungarotoxin but not di-hydro-beta-erythroidine. Like clozapine, olanzapine acts via alpha7 nicotinic receptors to elicit improved inhibitory processing of auditory stimuli.