Pharmacological characterization of SAGE-718, a novel positive allosteric modulator of N-methyl-d-aspartate receptors.

BACKGROUND AND PURPOSE: Select neuroactive steroids tune neural activity by modulating excitatory and inhibitory neurotransmission, including the endogenous cholesterol metabolite 24(S)-hydroxycholesterol (24(S)-HC), which is an N-methyl-d-aspartate (NMDA) receptor positive allosteric modulator (PAM). NMDA receptor PAMs are potentially an effective pharmacotherapeutic strategy to treat conditions associated with NMDA receptor hypofunction. EXPERIMENTAL APPROACH: Using in vitro and in vivo electrophysiological recording experiments and behavioural approaches, we evaluated the effect of SAGE-718, a novel neuroactive steroid NMDA receptor PAM currently in clinical development for the treatment of cognitive impairment, on NMDA receptor function and endpoints that are altered by NMDA receptor hypoactivity and assessed its safety profile. KEY RESULTS: SAGE-718 potentiated GluN1/GluN2A-D NMDA receptors with equipotency and increased NMDA receptor excitatory postsynaptic potential (EPSP) amplitude without affecting decay kinetics in striatal medium spiny neurons. SAGE-718 increased the rate of unblock of the NMDA receptor open channel blocker ketamine on GluN1/GluN2A in vitro and accelerated the rate of return on the ketamine-evoked increase in gamma frequency band power, as measured with electroencephalogram (EEG), suggesting that PAM activity is driven by increased channel open probability. SAGE-718 ameliorated deficits due to NMDA receptor hypofunction, including social deficits induced by subchronic administration of phencyclidine, and behavioural and electrophysiological deficits from cholesterol and 24(S)-HC depletion caused by 7-dehydrocholesterol reductase inhibition. Finally, SAGE-718 did not produce epileptiform activity in a seizure model or neurodegeneration following chronic dosing. CONCLUSIONS AND IMPLICATIONS: These findings provide strong evidence that SAGE-718 is a neuroactive steroid NMDA receptor PAM with a mechanism that is well suited as a treatment for conditions associated with NMDA receptor hypofunction.

SAGE-718: A First-in-Class N-Methyl-d-Aspartate Receptor Positive Allosteric Modulator for the Potential Treatment of Cognitive Impairment.

N-Methyl-d-aspartate receptor (NMDAR) positive allosteric modulators (PAMs) have received increased interest as a powerful mechanism of action to provide relief as therapies for CNS disorders. Sage Therapeutics has previously published the discovery of endogenous neuroactive steroid 24(S)-hydroxycholesterol as an NMDAR PAM. In this article, we detail the discovery of development candidate SAGE-718 (5), a potent and high intrinsic activity NMDAR PAM with an optimized pharmacokinetic profile for oral dosing. Compound 5 has completed phase 1 single ascending dose and multiple ascending dose clinical trials and is currently undergoing phase 2 clinical trials for treatment of cognitive impairment in Huntington's disease.

Preclinical characterization of zuranolone (SAGE-217), a selective neuroactive steroid GABAA receptor positive allosteric modulator.

Zuranolone (SAGE-217) is a novel, synthetic, clinical stage neuroactive steroid GABAA receptor positive allosteric modulator designed with the pharmacokinetic properties to support oral daily dosing. In vitro, zuranolone enhanced GABAA receptor current at nine unique human recombinant receptor subtypes, including representative receptors for both synaptic (γ subunit-containing) and extrasynaptic (δ subunit-containing) configurations. At a representative synaptic subunit configuration, α1ß2γ2, zuranolone potentiated GABA currents synergistically with the benzodiazepine diazepam, consistent with the non-competitive activity and distinct binding sites of the two classes of compounds at synaptic receptors. In a brain slice preparation, zuranolone produced a sustained increase in GABA currents consistent with metabotropic trafficking of GABAA receptors to the cell surface. In vivo, zuranolone exhibited potent activity, indicating its ability to modulate GABAA receptors in the central nervous system after oral dosing by protecting against chemo-convulsant seizures in a mouse model and enhancing electroencephalogram ß-frequency power in rats. Together, these data establish zuranolone as a potent and efficacious neuroactive steroid GABAA receptor positive allosteric modulator with drug-like properties and CNS exposure in preclinical models. Recent clinical data support the therapeutic promise of neuroactive steroid GABAA receptor positive modulators for treating mood disorders; brexanolone is the first therapeutic approved specifically for the treatment of postpartum depression. Zuranolone is currently under clinical investigation for the treatment of major depressive episodes in major depressive disorder, postpartum depression, and bipolar depression.

Neuroactive Steroid N-Methyl-d-aspartate Receptor Positive Allosteric Modulators: Synthesis, SAR, and Pharmacological Activity.

Neuroactive steroids (NASs) play a pivotal role in maintaining homeostasis is the CNS. We have discovered that one NAS in particular, 24(S)-hydroxycholesterol (24(S)-HC), is a positive allosteric modulator (PAM) of NMDA receptors. Using 24(S)-HC as a chemical starting point, we have identified other NASs that have good in vitro potency and efficacy. Herein, we describe the structure activity relationship and pharmacokinetic optimization of this series that ultimately led to SGE-301 (42). We demonstrate that SGE-301 enhances long-term potentiation (LTP) in rat hippocampal slices and, in a dose-dependent manner, improves cognition in a rat social recognition study.

Hypothesis-driven investigations of diverse pharmacological targets in two mouse models of autism.

Autism spectrum disorder is a neurodevelopmental syndrome diagnosed primarily by persistent deficits in social interactions and communication, unusual sensory reactivity, motor stereotypies, repetitive behaviors, and restricted interests. No FDA-approved medical treatments exist for the diagnostic symptoms of autism. Here we interrogate multiple pharmacological targets in two distinct mouse models that incorporate well-replicated autism-relevant behavioral phenotypes. Compounds that modify inhibitory or excitatory neurotransmission were selected to address hypotheses based on previously published biological abnormalities in each model. Shank3B is a genetic model of a mutation found in autism and Phelan-McDermid syndrome, in which deficits in excitatory neurotransmission and synaptic plasticity have been reported. BTBR is an inbred strain model of forms of idiopathic autism in which reduced inhibitory neurotransmission and excessive mTOR signaling have been reported. The GABA-A receptor agonist gaboxadol significantly reduced repetitive self-grooming in three independent cohorts of BTBR. The TrkB receptor agonist 7,8-DHF improved spatial learning in Shank3B mice, and reversed aspects of social deficits in BTBR. CX546, a positive allosteric modulator of the glutamatergic AMPA receptor, and d-cycloserine, a partial agonist of the glycine site on the glutamatergic NMDA receptor, did not rescue aberrant behaviors in Shank3B mice. The mTOR inhibitor rapamycin did not ameliorate social deficits or repetitive behavior in BTBR mice. Comparison of positive and negative pharmacological outcomes, on multiple phenotypes, evaluated for replicability across independent cohorts, enhances the translational value of mouse models of autism for therapeutic discovery. GABA agonists present opportunities for personalized interventions to treat components of autism spectrum disorder. Autism Res 2019, 12: 401-421 © 2019 The Authors. Autism Research published by International Society for Autism Research published by Wiley Periodicals, Inc. LAY SUMMARY: Many of the risk genes for autism impair synapses, the connections between nerve cells in the brain. A drug that reverses the synaptic effects of a mutation could offer a precision therapy. Combining pharmacological and behavioral therapies could reduce symptoms and improve the quality of life for people with autism. Here we report reductions in repetitive behavior by a GABA-A receptor agonist, gaboxadol, and improvements in social and cognitive behaviors by a TrkB receptor agonist, in mouse models of autism.

R-Baclofen Reverses Cognitive Deficits and Improves Social Interactions in Two Lines of 16p11.2 Deletion Mice.

Human chromosome 16p11.2 microdeletion is among the most common gene copy number variations (CNVs) known to confer risk for intellectual disability (ID) and autism spectrum disorder (ASD) and affects an estimated 3 in 10 000 people. Caused by a single copy deletion of ~27 genes, 16p11.2 microdeletion syndrome is characterized by ID, impaired language, communication and socialization skills, and ASD. Studies in animal models where a single copy of the syntenic 16p11.2 region has been deleted have revealed morphological, behavioral, and electrophysiological abnormalities. Previous studies suggested the possibility of some overlap in the mechanisms of pathophysiology in 16p11.2 microdeletion syndrome and fragile X syndrome. Improvements in fragile X phenotypes have been observed following chronic treatment with R-baclofen, a selective agonist of GABAB receptors. We were therefore motivated to investigate the effects of chronic oral R-baclofen administration in two independently generated mouse models of 16p11.2 microdeletion syndrome. In studies performed across two independent laboratories, we found that chronic activation of GABAB receptors improved performance on a series of cognitive and social tasks known to be impaired in two different 16p11.2 deletion mouse models. Our findings suggest that R-baclofen may have clinical utility for some of the core symptoms of human 16p11.2 microdeletion syndrome.

Differential roles for Akt and mTORC1 in the hypertrophy of Pten mutant neurons, a cellular model of brain overgrowth disorders.

Mutations in the PI3K/Akt/mTOR signaling pathway or in the upstream negative regulator Pten cause human brain overgrowth disorders, such as focal cortical dysplasia and megalencephaly, and are characterized by the presence of hypertrophic neurons. These disorders often have a pediatric onset and a high comorbidity with drug-resistant epilepsy; however, effective pharmacological treatments are lacking. We established forebrain excitatory neuron-specific Pten-deficient cultures as an in vitro model of brain overgrowth disorders, and investigated the effects of this Pten mutation on PI3K/Akt/mTOR signaling and neuronal growth. Mutant neurons exhibit excessive PI3K/Akt/mTOR signaling activity, enlarged somas and increased dendritic arborization. To understand the contributions of Akt and mTORC1 kinases to the hypertrophy phenotype, we evaluated the effects of short-term treatment with the Akt inhibitor MK-2206, and the mTORC1 inhibitor RAD001, which have shown safety and efficacy in human cancer clinical trials. We found that RAD001 treatment only partially reversed the morphological abnormalities of Pten mutant neurons, whereas MK-2206 treatment completely rescued the phenotype. Interestingly, neither treatment altered the size or morphology of normal neurons. Our results suggest that Akt is a major determinant of neuronal growth, and that Akt inhibition may be an effective strategy for pharmacological intervention in brain overgrowth disorders.

Translational Mouse Models of Autism: Advancing Toward Pharmacological Therapeutics.

Animal models provide preclinical tools to investigate the causal role of genetic mutations and environmental factors in the etiology of autism spectrum disorder (ASD). Knockout and humanized knock-in mice, and more recently knockout rats, have been generated for many of the de novo single gene mutations and copy number variants (CNVs) detected in ASD and comorbid neurodevelopmental disorders. Mouse models incorporating genetic and environmental manipulations have been employed for preclinical testing of hypothesis-driven pharmacological targets, to begin to develop treatments for the diagnostic and associated symptoms of autism. In this review, we summarize rodent behavioral assays relevant to the core features of autism, preclinical and clinical evaluations of pharmacological interventions, and strategies to improve the translational value of rodent models of autism.

Evaluation of the neuroactive steroid ganaxolone on social and repetitive behaviors in the BTBR mouse model of autism.

RATIONALE: Abnormalities in excitatory/inhibitory neurotransmission are hypothesized to contribute to autism spectrum disorder (ASD) etiology. BTBR T (+) Itpr3 (tf) /J (BTBR), an inbred mouse strain, displays social deficits and repetitive self-grooming, offering face validity to ASD diagnostic symptoms. Reduced GABAergic neurotransmission in BTBR suggests that GABAA receptor positive allosteric modulators (PAMs) could improve ASD-relevant BTBR phenotypes. The neuroactive steroid ganaxolone acts as a PAM, displaying anticonvulsant properties in rodent epilepsy models and an anxiolytic-like profile in the elevated plus-maze. OBJECTIVES: We evaluated ganaxolone in BTBR and C57BL/6J mice in standardized assays for sociability and repetitive behaviors. Open field and anxiety-related behaviors were tested as internal controls and for comparison with the existing neuroactive steroid literature. RESULTS: Ganaxolone improved aspects of social approach and reciprocal social interactions in BTBR, with no effect on repetitive self-grooming, and no detrimental effects in C57BL/6J. Ganaxolone increased overall exploratory activity in BTBR and C57BL/6J in the open field, social approach, and elevated plus-maze, introducing a confound for the interpretation of social improvements. Allopregnanolone and diazepam similarly increased total entries in the elevated plus-maze, indicating that behavioral activation may be a general property of GABAA receptor PAMs in these strains. CONCLUSIONS: Ganaxolone shows promise for improving sociability. In addition, ganaxolone, as well as other GABAA receptor PAMs, enhanced overall BTBR activity. The translational implications of specific sociability improvements and nonspecific behavioral activation by ganaxolone in the BTBR model remain to be determined. Future studies to explore whether PAMs provide a novel profile with unique benefits for ASD treatment will be worthwhile.

Antagonism of the adenosine A2A receptor attenuates akathisia-like behavior induced with MP-10 or aripiprazole in a novel non-human primate model.

Akathisia is a subset of the larger antipsychotic side effect profile known as extrapyramidal syndrome (EPS). It is associated with antipsychotic treatment and is characterized as a feeling of inner restlessness that results in a compulsion to move. There are currently no primate models available to assess drug-induced akathisia; the present research was designed to address this shortcoming. We developed a novel rating scale based on both the Barnes Akathisia Rating Scale (BARS) and the Hillside Akathisia Scale (HAS) to measure the objective, observable incidence of antipsychotic-induced akathisia-like behavior in Cebus apella non-human primates (NHPs). To induce akathisia, we administered the atypical antipsychotic aripiprazole (1 mg/kg) or the selective phosphodiesterase 10A (PDE10A) inhibitor MP-10 (1-3 mg/kg). Treatment with both compounds produced significantly greater akathisia scores on the rating scale than vehicle treatment. Characteristic behaviors observed included vocalizations, stereotypies, teeth grinding, restless limb movements, and hyperlocomotion. Adenosine A2A receptor antagonists have previously been shown to be effective in blocking antipsychotic-induced EPS in primates. The selective A2A receptor antagonist, SCH 412348 (10-30 mg/kg), effectively reduced or reversed akathisia-like behavior induced by both aripiprazole and MP-10. This work represents the first NHP measurement scale of akathisia and demonstrates that NHPs are responsive to akathisia-inducing agents. As such, it provides a useful tool for the preclinical assessment of putative antipsychotics. In addition, these results provide further evidence of the utility of A2A receptor antagonists for the treatment of antipsychotic-induced movement disorders.

Modeling fragile X syndrome in the Fmr1 knockout mouse.

Fragile X Syndrome (FXS) is a commonly inherited form of intellectual disability and one of the leading genetic causes for autism spectrum disorder. Clinical symptoms of FXS can include impaired cognition, anxiety, hyperactivity, social phobia, and repetitive behaviors. FXS is caused by a CGG repeat mutation which expands a region on the X chromosome containing the FMR1 gene. In FXS, a full mutation (> 200 repeats) leads to hypermethylation of FMR1, an epigenetic mechanism that effectively silences FMR1 gene expression and reduces levels of the FMR1 gene product, fragile X mental retardation protein (FMRP). FMRP is an RNA-binding protein that is important for the regulation of protein expression. In an effort to further understand how loss of FMR1 and FMRP contribute to FXS symptomology, several FXS animal models have been created. The most well characterized rodent model is the Fmr1 knockout (KO) mouse, which lacks FMRP protein due to a disruption in its Fmr1 gene. Here, we review the behavioral phenotyping of the Fmr1 KO mouse to date, and discuss the clinical relevance of this mouse model to the human FXS condition. While much remains to be learned about FXS, the Fmr1 KO mouse is a valuable tool for understanding the repercussions of functional loss of FMRP and assessing the efficacy of pharmacological compounds in ameliorating the molecular and behavioral phenotypes relevant to FXS.

Dab1 is required for synaptic plasticity and associative learning.

Disabled-1 (Dab1) is an adaptor protein that is an obligate effector of the Reelin signaling pathway, and is critical for neuronal migration and dendrite outgrowth during development. Components of the Reelin pathway are highly expressed during development, but also continue to be expressed in the adult brain. Here we investigated in detail the expression pattern of Dab1 in the postnatal and adult forebrain, and determined that it is expressed in excitatory as well as inhibitory neurons. Dab1 was found to be localized in different cellular compartments, including the soma, dendrites, presynaptic and postsynaptic structures. Mice that are deficient in Dab1, Reelin, or the Reelin receptors ApoER2 and VLDLR exhibit severely perturbed brain cytoarchitecture, limiting the utility of these mice for investigating the role of this signaling pathway in the adult brain. In this study, we developed an adult forebrain-specific and excitatory neuron-specific conditional knock-out mouse line, and demonstrated that Dab1 is a critical regulator of synaptic function and hippocampal-dependent associative and spatial learning. These dramatic abnormalities were accompanied by a reduction in dendritic spine size, and defects in basal and plasticity-induced Akt and ERK1/2 signaling. Deletion of Dab1 led to no obvious changes in neuronal positioning, dendrite morphology, spine density, or synaptic composition. Collectively, these data conclusively demonstrate an important role for Reelin-Dab1 signaling in the adult forebrain, and underscore the importance of this pathway in learning and memory.

Development and characterization of NEX- Pten, a novel forebrain excitatory neuron-specific knockout mouse.

The phosphatase and tensin homolog located on chromosome 10 (PTEN) suppresses the activity of the phosphoinositide-3-kinase/Akt/mammalian target of rapamycin (mTOR) pathway, a signaling cascade critically involved in the regulation of cell proliferation and growth. Human patients carrying germ line PTEN mutations have an increased predisposition to tumors, and also display a variety of neurological symptoms and increased risk of epilepsy and autism, implicating PTEN in neuronal development and function. Consistently, loss of Pten in mouse neural cells results in ataxia, seizures, cognitive abnormalities, increased soma size and synaptic abnormalities. To better understand how Pten regulates the excitability of principal forebrain neurons, a factor that is likely to be altered in cognitive disorders, epilepsy and autism, we generated a novel conditional knockout mouse line (NEX-Pten) in which Cre, under the control of the NEX promoter, drives the deletion of Pten specifically in early postmitotic, excitatory neurons of the developing forebrain. Homozygous mutant mice exhibited a massive enlargement of the forebrain, and died shortly after birth due to excessive mTOR activation. Analysis of the neonatal cerebral cortex further identified molecular defects resulting from Pten deletion that likely affect several aspects of neuronal development and excitability.

Targeting mTOR as a novel therapeutic strategy for traumatic CNS injuries.

The adult central nervous system (CNS) has a remarkable ability to repair itself. However, severe brain and spinal cord injuries (SCIs) cause lasting disability and there are only a few therapies that can prevent or restore function in such cases. In this review, we provide an overview of traumatic CNS injuries and discuss several emerging pharmacological options that have shown promise in preclinical and early clinical studies. We highlight therapies that modulate mammalian target of rapamycin (mTOR) signaling, a pathway that is well known for its roles in cell growth, metabolism and cancer. Interestingly, this pathway is also gaining newfound attention for its role in CNS repair and regeneration.

Characterization of the selective mGluR1 antagonist, JNJ16259685, in rodent models of movement and coordination.

Metabotropic glutamate receptor 1 (mGluR1) antagonists interfere with learning and memory; however, their role in motor function is not well elucidated despite their abundance in brain areas implicated in the control of movement. Here, the effects of mGluR1 antagonism on movement, coordination, and motor learning were investigated. JNJ16259685, a selective mGluR1 antagonist (negative allosteric modulator), was tested in assays of motor skill, and motor learning in rats and mice. JNJ16259685 produced very minimal effects on locomotor activity and posture up to a dose of 30 mg/kg. Motor skill was unaffected for well-learned tasks (up to 30 mg/kg) in rats, but impaired in mice. Both rats and mice rats were profoundly impaired (0.3 mg/kg) in the acquisition of a novel motor skill (rotarod). These results implicate the mGluR1 receptor in the acquisition of novel motor skills. JNJ16259685 dramatically reduced rearing behavior, exploration of a novel environment and lever pressing for a food reward (rat: 0.3 mg/kg; mouse: 1 mg/kg). JNJ16259685 (30 mg/kg) had no effect on reflexive startle responses to loud auditory stimuli or foot shock in mice. Previous groups have proposed that mGluR1 antagonists induce a general reduction in motivation. The effects seen here to reduce exploration and reward are consistent with that hypothesis. Pharmacological inhibition of the mGluR1 receptor has a modest effect on motor function but blocks motor learning and may reduce motivation to perform simple behaviors.

Preladenant, a selective A(2A) receptor antagonist, is active in primate models of movement disorders.

Parkinson's Disease (PD) and Extrapyramidal Syndrome (EPS) are movement disorders that result from degeneration of the dopaminergic input to the striatum and chronic inhibition of striatal dopamine D(2) receptors by antipsychotics, respectively. Adenosine A(2A) receptors are selectively localized in the basal ganglia, primarily in the striatopallidal ("indirect") pathway, where they appear to operate in concert with D(2) receptors and have been suggested to drive striatopallidal output balance. In cases of dopaminergic hypofunction, A(2A) receptor activation contributes to the overdrive of the indirect pathway. A(2A) receptor antagonists, therefore, have the potential to restore this inhibitor imbalance. Consequently, A(2A) receptor antagonists have therapeutic potential in diseases of dopaminergic hypofunction such as PD and EPS. Targeting the A(2A) receptor may also be a way to avoid the issues associated with direct dopamine agonists. Recently, preladenant was identified as a potent and highly selective A(2A) receptor antagonist, and has produced a significant improvement in motor function in rodent models of PD. Here we investigate the effects of preladenant in two primate movement disorder models. In MPTP-treated cynomolgus monkeys, preladenant (1 or 3 mg/kg; PO) improved motor ability and did not evoke any dopaminergic-mediated dyskinetic or motor complications. In Cebus apella monkeys with a history of chronic haloperidol treatment, preladenant (0.3-3.0 mg/kg; PO) delayed the onset of EPS symptoms evoked by an acute haloperidol challenge. Collectively, these data support the use of preladenant for the treatment of PD and antipsychotic-induced movement disorders.

Automated evaluation of sensitivity to foot shock in mice: inbred strain differences and pharmacological validation.

Assessing foot shock sensitivity in rodents can be useful in identifying analgesic or hyperalgesic drugs, and phenotyping inbred or genetically altered mice. Furthermore, as foot shock is an integral part of several rodent behavioral models, sensitivity should also be assessed to accurately interpret behavioral measures from these models. To eliminate variability and increase the efficiency of manually scored shock sensitivity paradigms, we utilized a startle reflex system to automatically quantify responses to varying levels of foot shock. Eight inbred mouse strains were tested for reactivity to foot shock in this system, as well as inherent startle response activity to loud noise bursts. Strain rank order for shock reactivity differed from that for acoustic startle, suggesting that pathways activated in response to each differed. Analgesic doses of morphine and acetaminophen specifically reduced foot shock responses without affecting motor reflexive responses to loud noises in each strain tested. We also tested diazepam and scopolamine, which are often used to disrupt behavior in shock-related paradigms to illustrate the usefulness of this assay. Overall, these results demonstrate that our automated method is a quick and simple way to accurately assess potential foot shock sensitivity differences owing to strain, genotype or drug treatments.

Age-progressing cognitive impairments and neuropathology in transgenic CRND8 mice.

Patients with Alzheimer's disease suffer from progressive cognitive impairments and show distinct post-mortem neuropathology, including beta-amyloid plaques. Transgenic (Tg) CRND8 mice carry a mutated human amyloid precursor protein gene and show age-related increases in beta-amyloid production and plaque deposition. It was previously reported that during the early stages of plaque deposition, Tg CRND8 mice demonstrated Morris maze impairments. However, it is unknown if Tg mice would be impaired at an earlier age prior to plaque deposition or more impaired at a later age with more extensive plaque deposition. In the current study, we describe Tg CRND8 age-progressing beta-amyloid neuropathology and cognitive abilities in greater detail. At all ages, Tg mice showed normal short-term memory in the Y-maze. Pre-plaque Tg and age-matched Non-Tg mice did not differ in learning the spatial Morris water maze. However, both early and late plaque Tg mice showed impairments during acquisition. In addition, although early plaque Tg mice performed well in the probe trial, late plaque Tg mice demonstrated impaired probe trial performance. Therefore compared to their Non-Tg littermates, Tg CRND8 mice demonstrate cognitive impairments that progressed with age and seemed to coincide with the onset of beta-amyloid plaque deposition.

Increased auditory startle response and reduced prepulse inhibition of startle in transgenic mice expressing a double mutant form of amyloid precursor protein.

Prepulse inhibition (PPI), a form of sensorimotor gating, occurs when an auditory startle response is markedly inhibited by a preceding sub-threshold stimulus (prepulse). Deficits in PPI have been demonstrated in patients with certain psychiatric disorders, such as schizophrenia, and in laboratory animals following specific pharmacological manipulations. Patients with Alzheimer's disease (AD) have not been tested in PPI, but have been shown to have abnormal sensory gating in another paradigm. Transgenic (Tg) CRND8 mice, which model Alzheimer's disease, carry the Swedish and Indiana familial Alzheimer's disease mutations of the human amyloid precursor protein gene and show age-related increases in beta-amyloid (Abeta) production, as well as plaque deposition. The present experiment investigated auditory startle threshold and PPI in TgCRND8 mice at various ages. In two longitudinal studies, PPI was examined in male TgCRND8 mice and non-transgenic (non-Tg) controls at 6-8 weeks of age (pre-plaque), and every 2 weeks thereafter until all mice were at least 16 weeks old (post-plaque). In a cross-sectional study, three different age sets of nai;ve TgCRND8 and non-Tg mice were tested: 10-12, 12-14, and 15-17 weeks old. In all three studies, TgCRND8 mice consistently and robustly demonstrated an enhanced response to a range of auditory startle stimuli compared to non-Tg mice. In addition, the TgCRND8 mice exhibited modest reductions in PPI, compared to non-Tg controls. These PPI deficits were present at pre- and post-plaque time points and did not appear to intensify with age; thus, they do not seem to correlate with the known neuropathology of TgCRND8 mice.