Neonatal exposure to MK801 promotes prepulse-induced delay in startle response time in adult rats

GABAB Receptor Agonist R-Baclofen Reverses Altered Auditory Reactivity and Filtering in the Cntnap2 Knock-Out Rat

Altered sensory information processing, and auditory processing, in particular, is a common impairment in individuals with autism spectrum disorder (ASD). One prominent hypothesis for the etiology of ASD is an imbalance between neuronal excitation and inhibition. The selective GABA_B receptor agonist R-Baclofen has been shown previously to improve social deficits and repetitive behaviors in several mouse models for neurodevelopmental disorders including ASD, and its formulation Arbaclofen has been shown to ameliorate social avoidance symptoms in some individuals with ASD. The present study investigated whether R-Baclofen can remediate ASD-related altered sensory processing reliant on excitation/inhibition imbalance in the auditory brainstem. To assess a possible excitation/inhibition imbalance in the startle-mediating brainstem underlying ASD-like auditory-evoked behaviors, we detected and quantified brain amino acid levels in the nucleus reticularis pontis caudalis (PnC) of rats with a homozygous loss-of-function mutation in the ASD-linked gene Contactin-associated protein-like 2 (Cntnap2) and their wildtype (WT) littermates using Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI MS). Abnormal behavioral read-outs of brainstem auditory signaling in Cntnap2 KO rats were accompanied by increased levels of GABA, glutamate, and glutamine in the PnC. We then compared the effect of R-Baclofen on behavioral read-outs of brainstem auditory signaling in Cntnap2 KO and WT rats. Auditory reactivity, sensory filtering, and sensorimotor gating were tested in form of acoustic startle response input-output functions, short-term habituation, and prepulse inhibition before and after acute administration of R-Baclofen (0.75, 1.5, and 3 mg/kg). Systemic R-Baclofen treatment improved disruptions in sensory filtering in Cntnap2 KO rats and suppressed exaggerated auditory startle responses, in particular to moderately loud sounds. Lower ASR thresholds in Cntnap2 KO rats were increased in a dose-dependent fashion, with the two higher doses bringing thresholds close to controls, whereas shorter ASR peak latencies at the threshold were further exacerbated. Impaired prepulse inhibition increased across various acoustic prepulse conditions after administration of R-Baclofen in Cntnap2 KO rats, whereas R-Baclofen did not affect prepulse inhibition in WT rats. Our findings suggest that GABA_B receptor agonists may be useful for pharmacologically targeting multiple aspects of sensory processing disruptions involving neuronal excitation/inhibition imbalances in ASD.

Effect of Neonatal Treatment With the NMDA Receptor Antagonist, MK-801, During Different Temporal Windows of Postnatal Period in Adult Prefrontal Cortical and Hippocampal Function

The neonatal MK-801 model of schizophrenia has been developed based on the neurodevelopmental and NMDA receptor hypofunction hypotheses of schizophrenia. This animal model is generated with the use of the NMDA receptor antagonist, MK-801, during different temporal windows of postnatal life of rodents leading to behavioral defects in adulthood. However, no studies have examined the role of specific postnatal time periods in the neonatal MK-801 (nMK-801) rodent model and the resulting behavioral and neurobiological effects. Thus, the goal of this study is to systematically investigate the role of NMDA hypofunction, during specific temporal windows in postnatal life on different cognitive and social behavioral paradigms, as well as various neurobiological effects during adulthood. Both female and male mice were injected intraperitoneally (i.p.) with MK-801 during postnatal days 7-14 (p7-14) or 11-15 (p11-15). Control mice were injected with saline during the respective time period. In adulthood, mice were tested in various cognitive and social behavioral tasks. Mice nMK-801-treated on p7-14 show impaired performance in the novel object, object-to-place, and temporal order object recognition (TOR) tasks, the sociability test, and contextual fear extinction. Mice nMK-801-treated on p11-15 only affects performance in the TOR task, the social memory test, and contextual fear extinction. No differences were identified in the expression of NMDA receptor subunits, the synapsin or PSD-95 proteins, either in the prefrontal cortex (PFC) or the hippocampus (HPC), brain regions significantly affected in schizophrenia. The number of parvalbumin (PV)-expressing cells is significantly reduced in the PFC, but not in the HPC, of nMK-801-treated mice on p7-14 compared to their controls. No differences in PV-expressing cells (PFC or HPC) were identified in nMK-801-treated mice on p11-15. We further examined PFC function by recording spontaneous activity in a solution that allows up state generation. We find that the frequency of up states is significantly reduced in both nMK-801-treated mice on p7-14 and p11-15 compared to saline-treated mice. Furthermore, we find adaptations in the gamma and high gamma activity in nMK-801-treated mice. In conclusion, our results show that MK-801 treatment during specific postnatal temporal windows has differential effects on cognitive and social behaviors, as well as on underlying neurobiological substrates.

Intracellular mechanisms and behavioral changes in mouse model of attention deficit hyperactivity disorder: Importance of age-specific NMDA receptor blockade

Exposure of NMDA receptor antagonists during developmental stages leads to behavioral consequences like attention deficit hyperactivity disorder (ADHD). However, the underlying molecular mechanisms have remained poorly understood. Herein, we studied the phosphorylated Akt (pAkt) and caspase-3, the key regulators of neuronal cell survival/death, as the probable downstream targets of MK-801 often used to engender ADHD-like condition. Swiss albino mice at postnatal days (PND) 7, 14 or 21 were injected with a single dose of MK-801 and evaluated for hyperactivity (open field test) and memory deficit at adolescence (PND 30) and adult stages (PND 60). PND 7 or 14 treatment groups (but not PND 21) consistently showed hyperactivity at the adolescence stage. A significant increase in working and reference memory errors in radial arm maze was noted at the adolescence age. PND 7 group continued to display the symptoms even in adulthood. All the treatment groups showed a significant decrease in the percent alterations (Y-maze) and discrimination index (novel object recognition test) at adolescence age. A significant increase in caspase-3 expression was noted in the prefrontal cortex (PFC) and hippocampus, whereas increased pAkt was noticed only in the hippocampus, following a single injection of MK-801 at PND 7. Concurrently, PND 7 treatment group showed significantly decreased neuronal nuclei (NeuN) expression (a marker for mature neurons) in the dentate gyrus, cornu ammonis-3 and PFC, but not in cornu ammonis-1, at adolescence age. We suggest that the observed symptoms of ADHD at adolescence and adulthood stages may be linked to alteration in pAkt and caspase-3 followed MK-801 treatment at PND 7.

Neurotoxicity of low-level lead exposure: History, mechanisms of action, and behavioral effects in humans and preclinical models

Lead is a neurotoxin that produces long-term, perhaps irreversible, effects on health and well-being. This article summarizes clinical and preclinical studies that have employed a variety of research techniques to examine the neurotoxic effects of low levels of lead exposure. A historical perspective is presented, followed by an overview of studies that examined behavioral and cognitive outcomes. In addition, a short summary of potential mechanisms of action is provided with a focus on calcium-dependent processes. The current level of concern, or reference level, set by the CDC is 5 μg/dL of lead in blood and a revision to 3.5 μg/dL has been suggested. However, levels of lead below 3 μg/dL have been shown to produce diminished cognitive function and maladaptive behavior in humans and animal models. Because much of the research has focused on higher concentrations of lead, work on low concentrations is needed to better understand the neurobehavioral effects and mechanisms of action of this neurotoxic metal.

Altered Auditory Processing, Filtering, and Reactivity in the Cntnap2 Knock-Out Rat Model for Neurodevelopmental Disorders

Sensory processing, and auditory processing in particular, is altered in individuals with neurodevelopmental disorders such as autism spectrum disorders (ASDs). The typical maturation of the auditory system is perturbed in these individuals during early development, which may underlie altered auditory reactivity that persists in later life. Of the many genes that regulate the auditory system development, loss-of-function mutations in the CNTNAP2 gene are strongly associated with language processing deficits and ASD. Therefore, using a novel Cntnap2 knock-out rat model, we tested the impact of Cntnap2 loss on auditory processing, filtering, and reactivity throughout development and young adulthood in male and female animals. Although hearing thresholds were not altered in Cntnap2 knock-out animals, we found a reduction in response amplitudes and a delay in response latency of the auditory brainstem response (ABR) in juvenile Cntnap2 knock-out rats compared with age-matched controls. Amplitudes and latency of the ABR largely normalized by adulthood, indicating a delayed maturation of auditory processing pathways in Cntnap2 knock-out rats. Despite the reduced ABR amplitudes, adolescent Cntnap2 knock-out animals displayed increased startle reactivity accompanied by disruptions in sensory filtering and sensorimotor gating across various conditions, most of which persisted in adulthood. All of these observations show striking parallels to disruptions reported in ASD. Our results also imply that developmental disruptions of sensory signal processing are associated with persistent changes in neural circuitries responsible for implicit auditory evoked behavior, emphasizing the need for interventions that target sensory processing disruptions early during development in ASD.SIGNIFICANCE STATEMENT This is the first study of brainstem auditory processing in a novel knock-out rat model with very high construct and face validity for autism spectrum disorders. Electrophysiological and behavioral measures of implicit auditory-evoked responses were systematically taken across developmental stages. Auditory processing, filtering, and reactivity disruptions show striking similarities to observations in autism. We also show for the first time that, whereas auditory brainstem responses normalize by adulthood, disruptions in brainstem-mediated auditory-evoked behavior persist. This indicates that early developmental perturbations in sensory processing can cause permanent maladaptive changes in circuitries responsible for auditory reactivity, underlining the importance for interventions early during development aiming at normalizing sensory processing.

Antioxidant Treatment with N-acetyl Cysteine Prevents the Development of Cognitive and Social Behavioral Deficits that Result from Perinatal Ketamine Treatment

Alterations of the normal redox state can be found in all stages of schizophrenia, suggesting a key role for oxidative stress in the etiology and maintenance of the disease. Pharmacological blockade of N-methyl-D-aspartic acid (NMDA) receptors can disrupt natural antioxidant defense systems and induce schizophrenia-like behaviors in animals and healthy human subjects. Perinatal administration of the NMDA receptor (NMDAR) antagonist ketamine produces persistent behavioral deficits in adult mice which mimic a range of positive, negative, and cognitive symptoms that characterize schizophrenia. Here we tested whether antioxidant treatment with the glutathione (GSH) precursor N-acetyl-cysteine (NAC) can prevent the development of these behavioral deficits. On postnatal days (PND) 7, 9 and 11, we treated mice with subanesthetic doses (30 mg/kg) of ketamine or saline. Two groups (either ketamine or saline treated) also received NAC throughout development. In adult animals (PND 70-120) we then assessed behavioral alterations in a battery of cognitive and psychomotor tasks. Ketamine-treated animals showed deficits in a task of cognitive flexibility, abnormal patterns of spontaneous alternation, deficits in novel-object recognition, as well as social interaction. Developmental ketamine treatment also induced behavioral stereotypy in response to an acute amphetamine challenge, and it impaired sensorimotor gating, measured as reduced prepulse inhibition (PPI) of the startle response. All of these behavioral abnormalities were either prevented or strongly ameliorated by NAC co-treatment. These results suggest that oxidative stress is a major factor for the development of the ketamine-induced behavioral dysfunctions, and that restoring oxidative balance during the prodromal stage of schizophrenia might be able to ameliorate the development of several major symptoms of the disease.

Early Social Isolation Stress and Perinatal NMDA Receptor Antagonist Treatment Induce Changes in the Structure and Neurochemistry of Inhibitory Neurons of the Adult Amygdala and Prefrontal Cortex

The exposure to aversive experiences during early life influences brain development and leads to altered behavior. Moreover, the combination of these experiences with subtle alterations in neurodevelopment may contribute to the emergence of psychiatric disorders, such as schizophrenia. Recent hypotheses suggest that imbalances between excitatory and inhibitory (E/I) neurotransmission, especially in the prefrontal cortex and the amygdala, may underlie their etiopathology. In order to understand better the neurobiological bases of these alterations, we studied the impact of altered neurodevelopment and chronic early-life stress on these two brain regions. Transgenic mice displaying fluorescent excitatory and inhibitory neurons, received a single injection of MK801 (NMDAR antagonist) or vehicle solution at postnatal day 7 and/or were socially isolated from the age of weaning until adulthood (3 months old). We found that anxiety-related behavior, brain volume, neuronal structure, and the expression of molecules related to plasticity and E/I neurotransmission in adult mice were importantly affected by early-life stress. Interestingly, many of these effects were potentiated when the stress paradigm was applied to mice perinatally injected with MK801 ("double-hit" model). These results clearly show the impact of early-life stress on the adult brain, especially on the structure and plasticity of inhibitory networks, and highlight the double-hit model as a valuable tool to study the contribution of early-life stress in the emergence of neurodevelopmental psychiatric disorders, such as schizophrenia.

Gender and estrous cycle influences on behavioral and neurochemical alterations in adult rats neonatally administered ketamine

Neonatal N-methyl-D-aspartate (NMDA) receptor blockade in rodents triggers schizophrenia (SCZ)-like alterations during adult life. SCZ is influenced by gender in age of onset, premorbid functioning, and course. Estrogen, the hormone potentially driving the gender differences in SCZ, is known to present neuroprotective effects such as regulate oxidative pathways and the expression of brain-derived neurotrophic factor (BDNF). Thus, the aim of this study was to verify if differences in gender and/or estrous cycle phase during adulthood would influence the development of behavioral and neurochemical alterations in animals neonatally administered ketamine. The results showed that ketamine-treated male (KT-male) and female-in-diestrus (KTF-diestrus, the low estrogen phase) presented significant deficits in prepulse inhibition of the startle reflex and spatial working memory, two behavioral SCZ endophenotypes. On the contrary, female ketamine-treated rats during proestrus (KTF-proestrus, the high estradiol phase) had no behavioral alterations. This correlated with an oxidative imbalance in the hippocampus (HC) of both male and KTF-diestrus female rats, that is, decreased levels of GSH and increased levels of lipid peroxidation and nitrite. Similarly, BDNF was decreased in the KTF-diestrus rats while no alterations were observed in KTF-proestrus and male animals. The changes in the HC were in contrast to those in the prefrontal cortex in which only increased levels of nitrite in all groups studied were observed. Thus, there is a gender difference in the adult rat HC in response to ketamine neonatal administration, which is based on the estrous cycle. This is discussed in relation to neuropsychiatric conditions and in particular SCZ.

Sex-specific restoration of MK-801-induced sensorimotor gating deficit by environmental enrichment

Despite ample evidence of N-methyl-D-aspartate (NMDA) receptor dysfunction in schizophrenia, no study has addressed the effects of enriched environment (EE) on sensorimotor gating deficits induced by postnatal NMDA receptor blockade. We evaluated the effect of EE on sensorimotor gating (measured by prepulse inhibition, PPI), or on sensorimotor gating deficit induced by the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) in both sexes of Wistar rats. Rats were injected with MK-801 (1 mg/kg) on postnatal days (P) 6-10. EE was provided from birth up to the time of experiments on P28-30 or P58-60. PPI data were collected at three prepulse intensities and then averaged to yield global PPI. MK-801 treatment reduced PPI significantly in both sexes. While EE per se had no significant effect on PPI, it restored MK-801-induced PPI deficit only in male rats. An extended period of EE did not influence PPI deficit in female rats. Our results indicate that postnatal exposure to MK-801 may exert long-lasting effects on neuronal circuits underlying sensorimotor gating. Sex-specific modulation of such effects by EE suggests sexually dimorphic mechanisms are involved.

Neonatal MK-801 treatment differentially alters the effect of adolescent or adult MK-801 challenge on locomotion and PPI in male and female rats

Schizophrenia is a neurodevelopmental disorder and is typically "triggered" by subsequent insults in life. The N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801) induces locomotor hyperactivity and prepulse inhibition (PPI) deficits, which can mimic the schizophrenia phenotype. In this experiment, we assessed whether neonatal exposure to MK-801 (postnatal days 5-14) could induce sensitization to both hyperactivity and PPI deficit caused by later-life acute MK-801 treatment during adolescence or adulthood. Our results showed that the hyperactivity induced by an acute MK-801 challenge was enhanced in male and female rats after neonatal MK-801 treatment. Notably, in the PPI test, adult female rats neonatally exposed to MK-801 exhibited a significantly greater reduction in PPI in response to acute MK-801 administration, whereas male rats receiving neonatal MK-801 treatment expressed attenuated PPI disruption in adulthood. Our data indicate that a combination of neonatal and later-life NMDA receptor blockades could induce sensitization in the locomotor activity of both sexes in adolescence and adulthood. In addition, a sex difference was observed in the effects of this treatment regime on PPI.

A two-hit model: behavioural investigation of the effect of combined neonatal MK-801 administration and isolation rearing in the rat

This study combined two neurodevelopmental manipulations, neonatal MK-801 treatment and isolation rearing, to produce a 'two-hit' model and determine whether two hits induce a more robust behavioural phenotype of an animal model of aspects of schizophrenia compared with individual manipulations alone. The effect of clozapine was also assessed. Male Sprague-Dawley rats received 0.2 mg/kg MK-801 or saline intraperitoneally (i.p.) once daily on postnatal days (PNDs) 7-10 and were assigned to group or isolation rearing at weaning (PND 21). From PND 77, they received a vehicle or 5 mg/kg clozapine (i.p.) treatment regimen and were subjected to three prepulse inhibition (PPI) tests, a locomotor activity assessment and a novel object recognition task. MK-801-treated rats reared in isolation displayed robust PPI disruptions which were consistently manifested in all three tests. PPI deficits were also detected in saline-treated rats reared in isolation but not in all tests. Only the two-hit rats demonstrated hyperlocomotion and impaired object recognition memory. Clozapine restored PPI anomalies in the two-hit rats. The two-hit model showed greater psychotic-like effects than either neonatal MK-801 or isolation rearing alone. The preliminary predictive validity shown with clozapine suggests this model may be useful for predicting the efficacy of putative antipsychotics.

Strategies to defeat ketamine-induced neonatal brain injury

Studies using animal models have shown that general anesthetics such as ketamine trigger widespread and robust apoptosis in the infant rodent brain. Recent clinical evidence suggests that the use of general anesthetics on young children (at ages equivalent to those used in rodent studies) can promote learning deficits as they mature. Thus, there is a growing need to develop strategies to prevent this injury. In this study, we describe a number of independent approaches to address therapeutic intervention. Postnatal day 7 (P7) rats were injected with vehicle (sterile PBS) or the NMDAR antagonist ketamine (20 mg/kg). After 8 h, we prepared brains for immunohistochemical detection of the pro-apoptotic enzyme activated caspase-3 (AC3). Focusing on the somatosensory cortex, AC3-positive cells were then counted in a non-biased stereological manner. We found AC3 levels were markedly increased in ketamine-treated animals. In one study, microarray analysis of the somatosensory cortex from ketamine-treated P7 pups revealed that expression of activity dependent neuroprotective protein (ADNP) was enhanced. Thus, we injected P7 animals with the ADNP peptide fragment NAPVSIPQ (NAP) 15 min before ketamine administration and found we could dose-dependently reverse the injury. In separate studies, pretreatment of P6 animals with 20 mg/kg vitamin D(3) or a nontoxic dose of ketamine (5 mg/kg) also prevented ketamine-induced apoptosis at P7. In contrast, pretreatment of P7 animals with aspirin (30 mg/kg) 15 min before ketamine administration actually increased AC3 counts in some regions. These data show that a number of unique approaches can be taken to address anesthesia-induced neurotoxicity in the infant brain, thus providing MDs with a variety of alternative strategies that enhance therapeutic flexibility.

Consequences of early life MK-801 administration: long-term behavioural effects and relevance to schizophrenia research

Animal models contribute significantly to advancing the understanding of schizophrenia neurobiology, in addition to being an important tool for the screening of antipsychotic potential of new compounds. However, the entire spectrum or all the symptoms manifested in schizophrenia cannot be straightforwardly reproduced in animals due to the complexity of the disorder, difference in mental capacities and behaviours, and the ability to quantify or measure the changes. Blockade of the NMDA receptor by the use of MK-801, a non-competitive NMDA receptor antagonist, during the early postnatal period has been proposed to be an experimental model which induces behavioural changes that mimic several aspects of the disorder. The long term behavioural profile arising from this early life manipulation is reviewed herein, with a specific focus on behaviours relevant to a schizophrenia-like condition. Some of the reported neurochemical changes are also compiled. Although this method may be suitable to model some aspects of schizophrenia in rodents, there are unmet areas which need to be addressed, notably the characterisation of its predictive value.

Low-level lead exposure triggers neuronal apoptosis in the developing mouse brain

While the toxic effects of lead have been recognized for millennia, it has remained a significant public health concern due to its continued use and toxicological potential. Of particular interest is the increased susceptibility of young children to the toxic effects of lead. Although the exact mechanism(s) for lead toxicity is currently not well understood, research has established that it can be a potent NMDA antagonist. Previous research has established that exposure to NMDA antagonists during the brain growth spurt period (first 2 weeks of life in mice) can produce apoptotic neurodegeneration throughout the brain. Based on this information, the ability of lead exposure (two injections of 350 mg/kg lead 4h apart) to produce apoptosis in the neonatal mouse brain was assessed histologically 8-24h after treatment using activated caspase-3 immunohistochemistry, De Olmos silver technique, Nissl staining, and electron microscopy. Lead exposure produced significant neurodegeneration in the caudate/putamen, hippocampus, subiculum, and superficial and deep cortical layers of the frontal cortical regions. Further ultrastructural examination revealed cellular profiles consistent with apoptotic cell death. Statistical results showed that lead exposure significantly increased apoptotic neurodegeneration above that seen in normal controls in animals treated at postnatal day 7, but not on day 14. The results of this study may provide a basis for further elucidation of mechanisms through which the immature nervous system may be particularly susceptible to lead exposure.

Is age-dependent, ketamine-induced apoptosis in the rat somatosensory cortex influenced by temperature?

General anesthetics have long been thought to be relatively safe but recent clinical studies have revealed that exposure of very young children (4 years or less) to agents that act by blocking the N-methyl-D-aspartate receptor (NMDAR) can lead to cognitive deficits as they mature. In rodent and non-human primate studies, blockade of this receptor during the perinatal period leads to a number of molecular, cellular and behavioral pathologies. Despite the overwhelming evidence from such studies, doubt remains as to their clinical relevance. A key issue is whether the primary injury (apoptotic cell death) is specific to receptor blockade or due to non-specific, patho-physiological changes. Principal to this argument is that loss of core body temperature following NMDAR blockade could explain why injury is observed hours later. We therefore examined the neurotoxicity of the general anesthetic ketamine in P7, P14 and P21 rats while monitoring core body temperature. We found that, at P7, ketamine induced the pro-apoptotic enzyme activated caspase-3 in a dose-dependent manner. As expected, injury was greatly diminished by P14 and absent by P21. However, contrary to expectations, we found that core body temperature was not a factor in determining injury. Our data imply that injury is directly related to receptor blockade and is unlikely to be overcome by artificially changing core body temperature.