Assessment of auditory sensory processing in a neurodevelopmental animal model of schizophrenia--gating of auditory-evoked potentials and prepulse inhibition

Advances in the study of phencyclidine-induced schizophrenia-like animal models and the underlying neural mechanisms

Schizophrenia (SZ) is a chronic, severe mental disorder with heterogeneous clinical manifestations and unknown etiology. Research on SZ has long been limited by the low reliability of and ambiguous pathogenesis in schizophrenia animal models. Phencyclidine (PCP), a noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist, rapidly induces both positive and negative symptoms of SZ as well as stable SZ-related cognitive impairment in rodents. However, the neural mechanism underlying PCP-induced SZ-like symptoms is not fully understood. Nondopaminergic pathophysiology, particularly excessive glutamate release induced by NMDAR hypofunction in the prefrontal cortex (PFC), may play a key role in the development of PCP-induced SZ-like symptoms. In this review, we summarize studies on the behavioral and metabolic effects of PCP and the cellular and circuitary targets of PCP in the PFC and hippocampus (HIP). PCP is thought to target the ventral HIP-PFC pathway more strongly than the PFC-VTA pathway and thalamocortical pathway. Systemic PCP administration might preferentially inhibit gamma-aminobutyric acid (GABA) neurons in the vHIP and in turn lead to hippocampal pyramidal cell disinhibition. Excitatory inputs from the HIP may trigger sustained, excessive and pathological PFC pyramidal neuron activation to mediate various SZ-like symptoms. In addition, astrocyte and microglial activation and oxidative stress in the cerebral cortex or hippocampus have been observed in PCP-induced models of SZ. These findings perfect the hypoglutamatergic hypothesis of schizophrenia. However, whether these effects direct the consequences of PCP administration and how about the relationships between these changes induced by PCP remain further elucidation through rigorous, causal and direct experimental evidence.

Baseline prepulse inhibition dependency of orexin A and REM sleep deprivation

RATIONALE

Prepulse inhibition (PPI) impairment reflects sensorimotor gating problems, i.e. in schizophrenia. This study aims to enlighten the role of orexinergic regulation on PPI in a psychosis-like model.

OBJECTIVES

In order to understand the impact of orexinergic innervation on PPI and how it is modulated by age and baseline PPI (bPPI), chronic orexin A (OXA) injections was carried on non-sleep-deprived and sleep-deprived rats that are grouped by their bPPI.

METHODS

bPPI measurements were carried on male Wistar rats on P45 or P90 followed by grouping into low-PPI and high-PPI rats. The rats were injected with OXA twice per day for four consecutive days starting on P49 or P94, while the control groups received saline injections. 72 h REMSD was carried on via modified multiple platform technique on P94 and either OXA or saline was injected during REMSD. PPI tests were carried out 30 min. after the last injection.

RESULTS

Our previous study with acute OXA injection after REMSD without bPPI grouping revealed that low OXA doses might improve REMSD-induced PPI impairment. Our current results present three important conclusions: (1) The effect of OXA on PPI is bPPI-dependent and age-dependent. (2) The effect of REMSD is bPPI-dependent. (3) The effect of OXA on PPI after REMSD also depends on bPPI.

CONCLUSION

Orexinergic regulation of PPI response with and without REMSD can be predicted by bPPI levels. Our findings provide potential insights into the regulation of sensorimotor gating by sleep/wakefulness systems and present potential therapeutic targets for the disorders, where PPI is disturbed.

Candidate Strategies for Development of a Rapid-Acting Antidepressant Class That Does Not Result in Neuropsychiatric Adverse Effects: Prevention of Ketamine-Induced Neuropsychiatric Adverse Reactions

Non-competitive N-methyl-D-aspartate/glutamate receptor (NMDAR) antagonism has been considered to play important roles in the pathophysiology of schizophrenia. In spite of severe neuropsychiatric adverse effects, esketamine (racemic enantiomer of ketamine) has been approved for the treatment of conventional monoaminergic antidepressant-resistant depression. Furthermore, ketamine improves anhedonia, suicidal ideation and bipolar depression, for which conventional monoaminergic antidepressants are not fully effective. Therefore, ketamine has been accepted, with rigorous restrictions, in psychiatry as a new class of antidepressant. Notably, the dosage of ketamine for antidepressive action is comparable to the dose that can generate schizophrenia-like psychotic symptoms. Furthermore, the psychotropic effects of ketamine precede the antidepressant effects. The maintenance of the antidepressive efficacy of ketamine often requires repeated administration; however, repeated ketamine intake leads to abuse and is consistently associated with long-lasting memory-associated deficits. According to the dissociative anaesthetic feature of ketamine, it exerts broad acute influences on cognition/perception. To evaluate the therapeutic validation of ketamine across clinical contexts, including its advantages and disadvantages, psychiatry should systematically assess the safety and efficacy of either short- and long-term ketamine treatments, in terms of both acute and chronic outcomes. Here, we describe the clinical evidence of NMDAR antagonists, and then the temporal mechanisms of schizophrenia-like and antidepressant-like effects of the NMDAR antagonist, ketamine. The underlying pharmacological rodent studies will also be discussed.

Brain NMDA Receptors in Schizophrenia and Depression

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP), dizocilpine (MK-801) and ketamine have long been considered a model of schizophrenia, both in animals and humans. However, ketamine has been recently approved for treatment-resistant depression, although with severe restrictions. Interestingly, the dosage in both conditions is similar, and positive symptoms of schizophrenia appear before antidepressant effects emerge. Here, we describe the temporal mechanisms implicated in schizophrenia-like and antidepressant-like effects of NMDA blockade in rats, and postulate that such effects may indicate that NMDA receptor antagonists induce similar mechanistic effects, and only the basal pre-drug state of the organism delimitates the overall outcome. Hence, blockade of NMDA receptors in depressive-like status can lead to amelioration or remission of symptoms, whereas healthy individuals develop psychotic symptoms and schizophrenia patients show an exacerbation of these symptoms after the administration of NMDA receptor antagonists.

Dose-dependent and opposite effects of orexin A on prepulse inhibition response in sleep-deprived and non-sleep-deprived rats

Orexin is a novel neurotransmitter released from lateral hypothalamus, that is a crucial modulator in sleep/wakefulness system. Recent studies also suggest its possible role in the neurodevelopmental disorders, such as schizophrenia. Our study consists of two experiments, where we investigate the effect of orexin A (OXA), one of two isoforms of orexin that can pass blood brain barrier, on the prepulse inhibition of acoustic startle reflex. The first experiment tested the effect of OXA on PPI response of non-sleep-deprived rats via intraperitoneal injection 30min before testing. Our results show that 40μg/kg OXA attenuates PPI% at 78dB and 86dB prepulse intensities. The second experiment utilized 72-h REM sleep deprivation, as a model for sleep-deprivation-induced impairment of PPI response. Here, we tested the effect of OXA on PPI% of sleep-deprived rats via intraperitoneal injection at the last 30min of sleep deprivation, testing for PPI immediately afterwards. Our results showed that (1) sleep deprivation attenuates the PPI% at 74dB, 78dB and 86dB prepulse intensities and (2) 10μg/kg OXA completely restores the impaired PPI% at 78dB only, where the highest PPI% impairment was observed. These results suggest that orexin A modulates PPI response in rats in a dose-dependent manner, oppositely for non-sleep-deprived and sleep-deprived rats, and a more detailed investigation for the etiology of this effect should follow.

Sensory processing in autism spectrum disorders and Fragile X syndrome-From the clinic to animal models

Brains are constantly flooded with sensory information that needs to be filtered at the pre-attentional level and integrated into endogenous activity in order to allow for detection of salient information and an appropriate behavioral response. People with Autism Spectrum Disorder (ASD) or Fragile X Syndrome (FXS) are often over- or under-reactive to stimulation, leading to a wide range of behavioral symptoms. This altered sensitivity may be caused by disrupted sensory processing, signal integration and/or gating, and is often being neglected. Here, we review translational experimental approaches that are used to investigate sensory processing in humans with ASD and FXS, and in relevant rodent models. This includes electroencephalographic measurement of event related potentials, neural oscillations and mismatch negativity, as well as habituation and pre-pulse inhibition of startle. We outline robust evidence of disrupted sensory processing in individuals with ASD and FXS, and in respective animal models, focusing on the auditory sensory domain. Animal models provide an excellent opportunity to examine common mechanisms of sensory pathophysiology in order to develop therapeutics.

Effects of Neonatal Dexamethasone Exposure on Adult Neuropsychiatric Traits in Rats

The effects of early life stress in utero or in neonates has long-term consequences on hypothalamic-pituitary-adrenal (HPA) stress axis function and neurodevelopment. These effects extend into adulthood and may underpin a variety of mental illnesses and be related to various developmental and cognitive changes. We examined the potential role of neonatal HPA axis activation on adult psychopathology and dopamine sensitivity in the mature rat using neonatal exposure to the synthetic glucocorticoid receptor agonist and stress hormone, dexamethasone. We utilized a comprehensive battery of assessments for behaviour, brain function and gene expression to determine if elevated early life HPA activation is associated with adult-onset neuropsychiatric traits. Dexamethasone exposure increased startle reactivity under all conditions tested, but decreased sensitivity of sensorimotor gating to dopaminergic disruption–contrasting with what is observed in several neuropsychiatric diseases. Under certain conditions there also appeared to be mild long-term changes in stress and anxiety-related behaviours with neonatal dexamethasone exposure. Electrophysiology revealed that there were no consistent neuropsychiatric abnormalities in auditory processing or resting state brain function with dexamethasone exposure. However, neonatal dexamethasone altered auditory cortex glucocorticoid activation, and auditory cortex synchronization. Our results indicate that neonatal HPA axis activation by dexamethasone alters several aspects of adult brain function and behaviour and may induce long-term changes in emotional stress-reactivity. However, neonatal dexamethasone exposure is not specifically related to any particular neuropsychiatric disease.

Memantine Improves Attentional Processes in Fragile X-Associated Tremor/Ataxia Syndrome: Electrophysiological Evidence from a Randomized Controlled Trial

Progressive cognitive deficits are common in patients with fragile X-associated tremor/ataxia syndrome (FXTAS), with no targeted treatment yet established. In this substudy of the first randomized controlled trial for FXTAS, we examined the effects of NMDA antagonist memantine on attention and working memory. Data were analyzed for patients (24 in each arm) who completed both the primary memantine trial and two EEG recordings (at baseline and follow-up) using an auditory “oddball” task. Results demonstrated significantly improved attention/working memory performance after one year only for the memantine group. The event-related potential P2 amplitude elicited by non-targets was significantly enhanced in the treated group, indicating memantine-associated improvement in attentional processes at the stimulus identification/discrimination level. P2 amplitude increase was positively correlated with improvement on the behavioral measure of attention/working memory during target detection. Analysis also revealed that memantine treatment normalized the P2 habituation effect at the follow-up visit. These findings indicate that memantine may benefit attentional processes that represent fundamental components of executive function/dysfunction, thought to comprise the core cognitive deficit in FXTAS. The results provide evidence of target engagement of memantine, as well as therapeutically relevant information that could further the development of specific cognitive or disease-modifying therapies for FXTAS.

Neuroteratology and Animal Modeling of Brain Disorders

Over the past 60 years, a large number of selective neurotoxins were discovered and developed, making it possible to animal-model a broad range of human neuropsychiatric and neurodevelopmental disorders. In this paper, we highlight those neurotoxins that are most commonly used as neuroteratologic agents, to either produce lifelong destruction of neurons of a particular phenotype, or a group of neurons linked by a specific class of transporter proteins (i.e., dopamine transporter) or body of receptors for a specific neurotransmitter (i.e., NMDA class of glutamate receptors). Actions of a range of neurotoxins are described: 6-hydroxydopamine (6-OHDA), 6-hydroxydopa, DSP-4, MPTP, methamphetamine, IgG-saporin, domoate, NMDA receptor antagonists, and valproate. Their neuroteratologic features are outlined, as well as those of nerve growth factor, epidermal growth factor, and that of stress. The value of each of these neurotoxins in animal modeling of human neurologic, neurodegenerative, and neuropsychiatric disorders is discussed in terms of the respective value as well as limitations of the derived animal model. Neuroteratologic agents have proven to be of immense importance for understanding how associated neural systems in human neural disorders may be better targeted by new therapeutic agents.

Comparing Pharmacological Modulation of Sensory Gating in Healthy Humans and Rats: The Effects of Reboxetine and Haloperidol

Sensory gating is the brain's ability to filter out irrelevant information before it reaches high levels of conscious processing. In the current study we aimed to investigate the involvement of the noradrenergic and dopaminergic neurotransmitter systems in sensory gating. Furthermore, we investigated cross-species reliability by comparing effects in both healthy humans and rats, while keeping all experimental conditions as similar as possible between the species. The design of the human experiment (n=21) was a double-blind, placebo-controlled, cross-over study where sensory gating was assessed following a dose of either reboxetine (8 mg), haloperidol (2 mg), their combination or placebo at four separate visits. Similarly in the animal experiment sensory gating was assessed in rats, (n=22) following a dose of reboxetine (2 mg/kg), haloperidol (0.08 mg/kg), their combination or placebo. The sensory gating paradigms in both experiments were identical. In humans, we found significantly reduced P50 suppression following separate administration of reboxetine or haloperidol, while their combined administration did not reach statistical significance compared with placebo. In the rats, we found a similar significant reduction of sensory gating (N40) following treatment with haloperidol and the combination of haloperidol and reboxetine, but not with separate reboxetine treatment, compared with placebo. Our study indicates that even when experimental conditions are kept as similar as possible, direct human to rat cross-species translation of pharmacological effects on sensory gating is challenging, which calls for more focussed research in this important translational area.

Reverse translation of clinical electrophysiological biomarkers in behaving rodents under acute and chronic NMDA receptor antagonism

Electroencephalogram (EEG) stands out as a highly translational tool for psychiatric research, yet rodent and human EEG are not typically obtained in the same way. In this study we developed a tool to record skull EEG in awake-behaving rats in a similar manner to how human EEG are obtained and then used this technique to test whether acute NMDA receptor antagonism alters rodent EEG signals in a similar manner as in humans. Acute MK-801 treatment elevated gamma power and reduced beta band power, which closely mirrored EEG data from healthy volunteers receiving acute ketamine. To explore the mechanisms behind these oscillatory changes, we examined the effects of GABA-A receptor blockade, finding that picrotoxin (PTX) recapitulated the decrease in sound-evoked beta oscillations observed with acute MK-801, but did not produce changes in gamma band power. Chronic treatment with either PTX or MK-801 did not affect frequency-specific oscillatory activity when tested 24 h after the last drug injection, but decreased total broadband oscillatory power. Overall, this study validated a novel platform for recording rodent EEG and demonstrated similar oscillatory changes after acute NMDA receptor antagonism in both humans and rodents, suggesting that skull EEG may be a powerful tool for further translational studies.

PDE and cognitive processing: beyond the memory domain

Phosphodiesterase inhibitors (PDE-Is) enhance cAMP and/or cGMP signaling via reducing the degradation of these cyclic nucleotides. Both cAMP and cGMP signaling are essential for a variety of cellular functions and exert their effects both pre- and post-synaptically. Either of these second messengers relays and amplifies incoming signals at receptors on the cell surface making them important elements in signal transduction cascades and essential in cellular signaling in a variety of cell functions including neurotransmitter release and neuroprotection. Consequently, these processes can be influenced by PDE-Is as they increase cAMP and/or cGMP concentrations. PDE-Is have been considered as possible therapeutic agents to treat impaired memory function linked to several brain disorders, including depression, schizophrenia and Alzheimer's disease (AD). This review will, however, focus on the possible role of phosphodiesterases (PDEs) in cognitive decline beyond the memory domain. Here we will discuss the involvement of PDEs on three related domains: attention, information filtering (sensory- and sensorimotor gating) and response inhibition (drug-induced hyperlocomotion). Currently, these are emerging cognitive domains in the field of PDE research. Here we discuss experimental studies and the potential beneficial effects of PDE-I drugs on these cognitive domains, as effects of PDE-Is on these domains could potentially influence effects on memory performance. Overall, PDE4 seems to be the most promising target for all domains discussed in this review.

Repeated potentiation of the metabotropic glutamate receptor 5 and the alpha 7 nicotinic acetylcholine receptor modulates behavioural and GABAergic deficits induced by early postnatal phencyclidine (PCP) treatment

The underlying mechanism of the GABAergic deficits observed in schizophrenia has been proposed to involve NMDA receptor hypofunction. An emerging treatment strategy therefore aims at enhancing GABAergic signalling by increasing the excitatory transmission onto interneurons. We wanted to determine whether behavioural and GABAergic functional deficits induced by the NMDA receptor channel blocker, phencyclidine (PCP), could be reversed by repeated administration of two drugs known to enhance GABAergic transmission: the positive allosteric modulator (PAM) of the metabotropic glutamate receptor 5 (mGluR5), ADX47273, and the partial agonist of the α7 nicotinic acetylcholine receptor (α7 nAChR), SSR180711. Adolescent rats (4-5 weeks) subjected to PCP treatment during the second postnatal week displayed a consistent deficit in prepulse inhibition (PPI), which was reversed by a one-week treatment with ADX47273 or SSR180711. We examined GABAergic transmission by whole cell patch-clamp recordings of miniature inhibitory postsynaptic currents (mIPSC) in pyramidal neurons in layer II/III of prefrontal cortex (PFC) and by activation of extrasynaptic δ-containing GABAA receptors by THIP. Following PCP treatment, pyramidal neurons displayed a reduced mIPSC frequency and up-regulation of extrasynaptic THIP-induced current. ADX47273 treatment restored this up-regulation of THIP-induced current. Reduced receptor function seems to be the underlying cause of the reported changes, since repeated treatment with ADX47273 and SSR180711 decreased the induction of spontaneous inhibitory current caused by acute and direct agonism of mGluR5s and α7 nAChRs in slices. These results show that repeated administration of ADX47273 or SSR180711 reverses certain behavioural and functional deficits induced by PCP, likely through down-regulation or desensitisation of mGluR5s and α7 nAChRs, respectively.

The PDE5 inhibitor vardenafil does not affect auditory sensory gating in rats and humans

RATIONALE

Sensory gating is an adaptive mechanism of the brain to prevent overstimulation. Patients suffering from clinical disorders such as Alzheimer's disease or schizophrenia exhibit a deficit in gating, which indicates not only an impairment in basic information processing that might contribute to the cognitive problems seen in these patients. Phosphodiesterase type 5 inhibitors (PDE5-Is) have been shown to improve cognition in rodents in various behavioural tasks and might consequently be an interesting target for cognition enhancement. However, the effects of PDE5-Is on sensory gating are not known yet.

OBJECTIVES

This work aims to study the effects of PDE5 inhibition on auditory sensory gating in rats and humans.

METHODS

In the rat study, vehicle or 0.3-3 mg/kg of the PDE5-I vardenafil was given orally 30 min before testing and electrode locations were the vertex, hippocampus and the striatum. The human subjects received placebo, 10-20 mg vardenafil 85 min before testing and sensory gating was measured at the cortex (Fz, Fcz and Cz) electrodes.

RESULTS

Significant gating was only found for the N1 component in rats while all three peaks P1, N1 and P2 showed gating in humans, i.e. the response to the second sound click was decreased as compared with the first for these deflections. Administration of vardenafil did neither have an effect on sensory gating in rats nor in humans.

CONCLUSIONS

These findings imply that positive effects of PDE5 inhibition on cognition are not mediated by more early phases of information processing.

Neonatal administration of phencyclidine decreases the number of putative inhibitory interneurons and increases neural excitability to auditory paired clicks in the hippocampal CA3 region of freely moving adult mice

Animals exposed to phencyclidine (PCP) during the neonatal period have fewer GABAergic interneurons in the corticolimbic area, including the hippocampus, and exhibit abnormal behaviors after attaining maturation that correspond with schizophrenic symptoms. Since a lack of inhibitory interneurons in the hippocampus has also been reported in postmortem studies of patients with schizophrenia, the deficit may induce abnormal activity of hippocampal neurons that underlies pathological states in schizophrenia. However, it remains unclear how PCP treatment during the neonatal period affects the discharge activity of hippocampal neurons in adulthood. In the current study, single unit responses of hippocampal CA3 neurons to paired auditory clicks were recorded in freely moving mice repeatedly injected with PCP or saline during the neonatal period. The recorded neurons were classified into two subpopulations, narrow-spike neurons and broad-spike neurons, based on the spike width. The spontaneous discharge rate was higher in the narrow-spike neurons than in the broad-spike neurons, indicating that the narrow-spike neurons correspond with hippocampal inhibitory neurons. The proportion of narrow-spike neurons was significantly smaller in neonatally PCP-treated mice than in saline-treated mice. The broad-spike neurons that exhibited a response magnitude to the second click as large as that to the first click (E/E-type response) showed longer response duration to the paired clicks in PCP-treated mice than in the saline-treated mice. Further, the number of neurons with E/E-type response was higher in the PCP-treated mice than in the saline-treated mice. Finally, the attenuation of an auditory-evoked potential component, N40, to the second click (sensory gating) was blunted in the PCP-treated mice when compared with that in the saline-treated mice. These results suggest that the neonatal administration of PCP induced a deficit of inhibitory interneurons and altered discharge activity of neurons in the hippocampal CA3 region to the paired clicks, thereby inducing the deficit in sensory gating.

Glutamatergic transmission in schizophrenia: from basic research to clinical practice

PURPOSE OF REVIEW

The past 20 years have seen the glutamatergic hypothesis go from theory to phase III trials of novel mechanism antipsychotics.

RECENT FINDINGS

We review the recent literature on glutamatergic theory, covering assessment and genetic studies, as well as drug development in animals and humans.

SUMMARY

Although evidence continues to accumulate in support of glutamate hypotheses, further research continues to be required and interactions with other key systems need to be explored.

Zebrafish behavioural assays of translational relevance for the study of psychiatric disease

Understanding the pathogenesis of the complex behavioural disorders that constitute psychiatric disease is a major challenge for biomedical research. Assays in rodents have contributed significantly to our understanding of the neural basis of behavioural disorders and continue to be one of the main focuses for the development of novel therapeutics. Now, owing to their genetic tractability and optical transparency (allowing in vivo imaging of circuit function) and the rapid expansion of genetic tools, zebrafish are becoming increasingly popular for behavioural genetic research. The increased development of behavioural assays in zebrafish raises the possibility of exploiting the advantages of this system to identify molecular mechanisms contributing to behavioural phenotypes associated with psychiatric disorders as well as potential therapeutics. This mini-review describes behavioural paradigms in zebrafish that can be used to address endophenotypes associated with psychiatric disease. The content reflects the interests of the author and covers tests of cognitive functions, response choice and inhibition, social interaction and executive function.