Decline in age-dependent, MK801-induced injury coincides with developmental switch in parvalbumin expression: somatosensory and motor cortex

Cenobamate suppresses seizures without inducing cell death in neonatal rats

GABA modulators such as phenobarbital (PB) and sodium channel blockers such as phenytoin (PHT) have long been the mainstay of pharmacotherapy for the epilepsies. In the context of neonatal seizures, both PB and PHT display incomplete clinical efficacy. Moreover, in animal models, neonatal exposure to these medications result in neurodegeneration raising concerns about safety. Cenobamate, a more recently approved medication, displays unique pharmacology as it is both a positive allosteric modulator of GABA-A receptors, and a voltage-gated sodium channel blocker. While cenobamate is approved for adult use, its efficacy and safety profile against neonatal seizures is poorly understood. To address this gap, we assessed the efficacy and safety of cenobamate in immature rodents. Postnatal day (P)7 rat pups were pretreated with cenobamate and challenged with the chemoconvulsant pentylenetetrazole (PTZ) to screen for anti-seizure effects. In a separate experiment, P7 rats were treated with cenobamate, and brains were processed to assess induction of cell death. Cenobamate displays dose-dependent anti-seizure efficacy in neonatal rats. Unlike PHB and PHT, it does not induce neurotoxicity in P7 rats. Thus, cenobamate may be effective at treating neonatal seizures while avoiding unwanted neurotoxic side effects such as cell death.

Age-Related Changes in the Primary Motor Cortex of Newborn to Adult Domestic Pig Sus scrofa domesticus

The pig has been increasingly used as a suitable animal model in translational neuroscience. However, several features of the fast-growing, immediately motor-competent cerebral cortex of this species have been adequately described. This study analyzes the cytoarchitecture of the primary motor cortex (M1) of newborn, young and adult pigs (Sus scrofa domesticus). Moreover, we investigated the distribution of the neural cells expressing the calcium-binding proteins (CaBPs) (calretinin, CR; parvalbumin, PV) throughout M1. The primary motor cortex of newborn piglets was characterized by a dense neuronal arrangement that made the discrimination of the cell layers difficult, except for layer one. The absence of a clearly recognizable layer four, typical of the agranular cortex, was noted in young and adult pigs. The morphometric and immunohistochemical analyses revealed age-associated changes characterized by (1) thickness increase and neuronal density (number of cells/mm2 of M1) reduction during the first year of life; (2) morphological changes of CR-immunoreactive neurons in the first months of life; (3) higher density of CR- and PV-immunopositive neurons in newborns when compared to young and adult pigs. Since most of the present findings match with those of the human M1, this study strengthens the growing evidence that the brain of the pig can be used as a potentially valuable translational animal model during growth and development.

CRISPR/Cas9-engineered Gad1 elimination in rats leads to complex behavioral changes: implications for schizophrenia

GABAergic dysfunctions have been implicated in the pathogenesis of schizophrenia, especially the associated cognitive impairments. The GABA synthetic enzyme glutamate decarboxylase 67-kDa isoform (GAD67) encoded by the GAD1 gene is downregulated in the brains of patients with schizophrenia. Furthermore, a patient with schizophrenia harboring a homozygous mutation of GAD1 has recently been discovered. However, it remains unclear whether loss of function of GAD1 leads to the symptoms observed in schizophrenia, including cognitive impairment. One of the obstacles faced in experimental studies to address this issue is the perinatal lethality of Gad1 knockout (KO) mice, which precluded characterization at the adult stage. In the present study, we successfully generated Gad1 KO rats using CRISPR/Cas9 genome editing technology. Surprisingly, 33% of Gad1 KO rats survived to adulthood and could be subjected to further characterization. The GABA concentration in the Gad1 KO cerebrum was reduced to ~52% of the level in wild-type rats. Gad1 KO rats exhibited impairments in both spatial reference and working memory without affecting adult neurogenesis in the hippocampus. In addition, Gad1 KO rats showed a wide range of behavioral alterations, such as enhanced sensitivity to an NMDA receptor antagonist, hypoactivity in a novel environment, and decreased preference for social novelty. Taken together, the results suggest that Gad1 KO rats could provide a novel model covering not only cognitive deficits but also other aspects of the disorder. Furthermore, the present study teaches an important lesson: differences between species should be considered when developing animal models of human diseases.

Neonatal Ethanol Disturbs the Normal Maturation of Parvalbumin Interneurons Surrounded by Subsets of Perineuronal Nets in the Cerebral Cortex: Partial Reversal by Lithium

Reduction in parvalbumin-positive (PV+) interneurons is observed in adult mice exposed to ethanol at postnatal day 7 (P7), a late gestation fetal alcohol spectrum disorder model. To evaluate whether PV+ cells are lost, or PV expression is reduced, we quantified PV+ and associated perineuronal net (PNN)+ cell densities in barrel cortex. While PNN+ cell density was not reduced by P7 ethanol, PV cell density decreased by 25% at P90 with no decrease at P14. PNN+ cells in controls were virtually all PV+, whereas more than 20% lacked PV in ethanol-treated adult animals. P7 ethanol caused immediate apoptosis in 10% of GFP+ cells in G42 mice, which express GFP in a subset of PV+ cells, and GFP+ cell density decreased by 60% at P90 without reduction at P14. The ethanol effect on PV+ cell density was attenuated by lithium treatment at P7 or at P14-28. Thus, reduced PV+ cell density may be caused by disrupted cell maturation, in addition to acute apoptosis. This effect may be regionally specific: in the dentate gyrus, P7 ethanol reduced PV+ cell density by 70% at P14 and both PV+ and PNN+ cell densities by 50% at P90, and delayed lithium did not alleviate ethanol's effect.

Short-Term Exposure to Enriched Environment in Adult Rats Restores MK-801-Induced Cognitive Deficits and GABAergic Interneuron Immunoreactivity Loss

Perinatal injections of N-methyl-D-aspartate (NMDA) receptor antagonist in rodents emulate some cognitive impairments and neurochemical alterations, such as decreased GABAergic (gamma aminobutyric acid) interneuron immunoreactivity, also found in schizophrenia. These features are pervasive, and developing neuroprotective or neurorestorative strategies is of special interest. In this work, we aimed to investigate if a short exposure to enriched environment (EE) in early adulthood (P55-P73) was an effective strategy to improve cognitive dysfunction and to restore interneuron expression in medial prefrontal cortex (mPFC) and hippocampus (HPC). For that purpose, we administered MK-801 intraperitoneally to Long Evans rats from postnatal days 10 to 20. Twenty-four hours after the last injection, MK-801 produced a transient decrease in spontaneous motor activity and exploration, but those abnormalities were absent at P24 and P55. The open field test on P73 manifested that EE reduced anxiety-like behavior. In addition, MK-801-treated rats showed cognitive impairment in novel object recognition test that was reversed by EE. We quantified different interneuron populations based on their calcium-binding protein expression (parvalbumin, calretinin, and calbindin), glutamic acid decarboxylase 67, and neuronal nuclei-positive cells by means of unbiased stereology and found that EE enhanced interneuron immunoreactivity up to normal values in MK-801-treated rats. Our results demonstrate that a timely intervention with EE is a powerful tool to reverse long-lasting changes in cognition and neurochemical markers of interneurons in an animal model of schizophrenia.

Calbindin-1 Expression in the Hippocampus following Neonatal Hypoxia-Ischemia and Therapeutic Hypothermia and Deficits in Spatial Memory

Hippocampal injury following neonatal hypoxia-ischemia (HI) leads to memory impairments despite therapeutic hypothermia (TH). In the hippocampus, the expression of calbindin-1 (Calb1), a Ca2+-buffering protein, increases during postnatal development and decreases with aging and neurodegenerative disorders. Since persistent Ca2+ dysregulation after HI may lead to ongoing injury, persistent changes in hippocampal expression of Calb1 may contribute to memory impairments after neonatal HI. We hypothesized that, despite TH, neonatal HI persistently decreases Calb1 expression in the hippocampus, a change associated with memory deficits in the mouse. We induced cerebral HI in C57BL6 mice at postnatal day 10 (P10) with right carotid ligation and 45 min of hypoxia (FiO2 = 0.08), followed by normothermia (36°C, NT) or TH (31°C) for 4 h with anesthesia-shams as controls. Nissl staining and glial fibrillary acidic protein (GFAP) immunohistochemistry (IHC) were used to grade brain injury and astrogliosis at P11, P18, and P40 prior to the assessment of Calb1 expression by IHC. The subset of mice followed to P40 also performed a memory behavior task (Y-maze) at P22-P26. Nonparametric statistics stratified by sex were applied. In both anterior and posterior coronal brain sections, hippocampal Calb1 expression doubled between P11 and P40 due to an increase in the cornus ammonis (CA) field (Kruskal-Wallis [KW] p < 0.001) and not the dentate gyrus (DG). Neonatal HI produced delayed (P18) and late (P40) deficits in the expression of Calb1 exclusively in the CA field (KW p = 0.02) in posterior brain sections. TH did not attenuate Calb1 deficits after HI. Thirty days after HI injury (at P40), GFAP scores in the hippocampus (p < 0.001, r = -0.47) and CA field (p < 0.001, r = -0.39) of posterior brain sections inversely correlated with their respective Calb1 expression. Both sexes demonstrated deficits in Y-maze testing, including approximately 40% lower spontaneous alterations performance and twice as much total impairment compared to sham mice (KW p < 0.001), but it was only in females that these deficits correlated with the Calb1 expression in the hippocampal CA field (p < 0.05) of the posterior sections. Hippocampal atrophy after neonatal HI also correlated with worse deficits in Y-maze testing, but it did not predict Calb1 deficits. Neonatal HI produces a long-lasting Calb1 deficit in the hippocampal CA field during development, which is not mitigated by TH. Late Calb1 deficit after HI may be the result of persistent astrogliosis and can lead to memory impairment, particularly in female mice.

Modifications of inhibitory transmission onto pyramidal neurons by postnatal exposure to MK-801: Effects of enriched environment

Early enriched environment (EE) prevents several deficits associated with postnatal MK-801 [N-Methyl-d-Aspartate (NMDA) receptor antagonist] treatment such as cognitive and locomotor deficits. We sought physiological correlates to such changes by looking at inhibitory synaptic inputs onto pyramidal cells in a prefrontal cortex slice preparation. Pharmacologically isolated γ-amino-butyric acid _A (GABA_A) receptor-mediated currents were measured using whole-cell patch clamp recordings. Wistar rats were raised in standard or EE from birth up to the time of experiments and were injected with saline or MK-801 (1mg/kg) on postnatal days (P) 6-10. We recorded miniature inhibitory post-synaptic currents (mIPSCs) of pyramidal cells in layer II/III of prefrontal cortex and measured their frequency, amplitude and kinetics. In control animals, the amplitude and frequency of mIPSCs increased strikingly during development from P21 to P28. MK-801 accelerated the development of mIPSCs frequency but caused a significant decrease in the amplitude of mIPSCs on P28 suggesting a significant reduction of inhibition onto pyramidal cells. EE per se led to a significant increase in both frequency and amplitude of mIPSCs, but its application to MK-801-treated rats resulted in moderate rescue of GABAergic transmission on P28. We conclude that postnatal MK-801 leads to reduced inhibitory transmission onto pyramidal cells of prefrontal cortex at adolescence which may underlie behavioural and morphological differences detected in vivo in rats. EE presentation from birth rather prevents GABAergic alterations associated with postnatal MK-801 treatment at adolescence.

Selective reduction of cerebral cortex GABA neurons in a late gestation model of fetal alcohol spectrum disorder

Fetal alcohol spectrum disorders (FASD) are associated with cognitive and behavioral deficits, and decreased volume of the whole brain and cerebral cortex. Rodent models have shown that early postnatal treatments, which mimic ethanol toxicity in the third trimester of human pregnancy, acutely induce widespread apoptotic neuronal degeneration and permanent behavioral deficits. However, the lasting cellular and anatomical effects of early ethanol treatments are still incompletely understood. This study examined changes in neocortex volume, thickness, and cellular organization that persist in adult mice after postnatal day 7 (P7) ethanol treatment. Post mortem brain volumes, measured by both MRI within the skull and by fluid displacement of isolated brains, were reduced 10-13% by ethanol treatment. The cerebral cortex showed a similar reduction (12%) caused mainly by lower surface area (9%). In spite of these large changes, several features of cortical organization showed little evidence of change, including cortical thickness, overall neuron size, and laminar organization. Estimates of total neuron number showed a trend level reduction of about 8%, due mainly to reduced cortical volume but unchanged neuron density. However, counts of calretinin (CR) and parvalbumin (PV) subtypes of GABAergic neurons showed a striking >30% reduction of neuron number. Similar ethanol effects were found in male and female mice, and in C57BL/6By and BALB/cJ mouse strains. Our findings indicate that the cortex has substantial capacity to develop normal cytoarchitectonic organization after early postnatal ethanol toxicity, but there is a selective and persistent reduction of GABA cells that may contribute to the lasting cognitive and behavioral deficits in FASD.

Behavioral and neurochemical consequences of cortical oxidative stress on parvalbumin-interneuron maturation in rodent models of schizophrenia

Oxidative stress, in response to the activation of the superoxide-producing enzyme Nox2, has been implicated in the schizophrenia-like behavioral dysfunction that develops in animals that were subject to either neonatal NMDA receptor-antagonist treatment or social isolation. In both of these animal models of schizophrenia, an environmental insult occurring during the period of active maturation of the fast-spiking parvalbumin-positive (PV+) interneuronal circuit leads to a diminished expression of parvalbumin in GABA-inhibitory neurons when animals reach adulthood. The loss of PV+ interneurons in animal models had been tentatively attributed to the death of these neurons. However, present results show that for the perinatal NMDA-R antagonist model these interneurons are still alive when animals are 5-6 weeks of age even though they have lost their phenotype and no longer express parvalbumin. Alterations in parvalbumin expression and sensory-evoked gamma-oscillatory activity, regulated by PV+ interneurons, are consistently observed in schizophrenia. We propose that cortical networks consisting of faulty PV+ interneurons interacting with pyramidal neurons may be responsible for the aberrant oscillatory activity observed in schizophrenia. Thus, oxidative stress during the maturation window for PV+ interneurons by alteration of normal brain development, leads to the emergence of schizophrenia-like behavioral dysfunctions when subjects reach early adulthood.

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.

A thalamo-hippocampal-ventral tegmental area loop may produce the positive feedback that underlies the psychotic break in schizophrenia

The N-methyl-D-aspartate receptor (NMDAR) hypofunction model of schizophrenia is based on the ability of NMDAR antagonists to produce many symptoms of the disease. Recent work in rats shows that NMDAR antagonist works synergistically with dopamine to produce delta frequency bursting in the thalamus. This finding, together with other results in the literature, suggests a mechanism for the sudden onset of schizophrenia. Among the thalamic nuclei most activated by NMDAR antagonist is the nucleus reuniens. This nucleus excites the cornu ammonis area 1 (CA1) region of the hippocampus. Experiments indicate that such activation can lead to excitation of dopaminergic cells of the ventral tegmental area by a polysynaptic pathway. The resulting elevation of dopamine in the thalamus will enhance thalamic bursting, thereby creating a loop with the potential for positive feedback. We show through computer simulations that in individuals with susceptibility to schizophrenia (e.g., because of partially compromised NMDAR function), an event that stimulates the dopamine system, such as stress, can cause the system to reach the threshold for thalamic bursting. When this occurs, positive feedback in the loop will cause all components to become highly active and to remain active after the triggering stimulus is removed. This is a physiologically specific hypothesis for the sudden and lasting transition that underlies the psychotic break in schizophrenia. Furthermore, the model provides an explanation for the observed selective activation of the CA1 hippocampal region in schizophrenia. The model also predicts an increase of basal activity in the dopamine system and thalamus; the relevant evidence is reviewed.

Hypoglossal motor neurons display a reduced calcium increase after axotomy in mice with upregulated parvalbumin

Motor neurons that exhibit differences in vulnerability to degeneration have been identified in motor neuron disease and in its animal models. The oculomotor and hypoglossal neurons are regarded as the prototypes of the resistant and susceptible cell types, respectively. Because an increase in the level of intracellular calcium has been proposed as a feature amplifying degenerative processes, we earlier studied the calcium increase in these motor neurons after axotomy in Balb/c mice and demonstrated a correlation between the susceptibility to degeneration and the intracellular calcium increase, with an inverse relation with the calcium buffering capacity, characterized by the parvalbumin or calbindin-D(28k) content. Because the differential susceptibility of the cells might also be attributed to their different cellular environments, in the present experiments, with the aim of verifying directly that a higher calcium buffering capacity is indeed responsible for the enhanced resistance, motor neurons were studied in their original milieu in mice with a genetically increased parvalbumin level. The changes in intracellular calcium level of the hypoglossal and oculomotor neurons after axotomy were studied electron microscopically at a 21-day interval after axotomy, during which time no significant calcium increase was detected in the hypoglossal motor neurons, the response being similar to that of the oculomotor neurons. The hypoglossal motor neurons of the parental mice, used as positive controls, exhibited a transient, significant elevation of calcium. These data provide more direct evidence of the protective role of parvalbumin against the degeneration mediated by a calcium increase in the acute injury of motor neurons.

MK801-induced activated caspase-3 exhibits selective co-localization with GAD67

Blockade of the N-methyl-d-aspartate receptor (NMDAR) in postnatal day 7 (P7) rats can promote rapid and robust induction of the pro-apoptotic marker activated caspase-3 (AC3) and loss of the GABAergic marker GAD67 at P56. Thus, we hypothesized that NMDAR blockade-induced AC3 occurs in GAD67 positive cells at P7. To test this idea, we injected P7 rat pups with vehicle or MK801 and after 8h (peak of AC3 induction) we examined brain sections for both AC3 and GAD67. Compared to vehicle, MK801 profoundly induced AC3 in all brain regions examined but co-expression of GAD67 in the same cells was not observed. However, in brain regions where punctate (synaptic) GAD67 was abundant (for example, layer IV of the somatosensory cortex), AC3 was robust. These data suggest that whereas somatic expression of AC3 and GAD67 may be non-overlapping, areas that exhibit punctate GAD67 (and are high in synaptic turnover) may be more vulnerable to MK801 exposure.

NMDAR blockade-induced neonatal brain injury: Reversal by the calcium channel agonist BayK 8644

We have previously shown that P7 rat pups injected with the N-methyl-d-aspartate receptor (NMDAR) blocker MK801 displayed robust apoptotic injury within hours after injection. Further studies from our lab suggest that loss of calcium cannot be compensated for when vulnerable neurons lack calcium buffering capabilities. Thus, to elevate calcium in these neurons prior to MK801 exposure, we injected P7 rats with the calcium channel agonist BayK 8644. Whereas BayK 8644 did not induce apoptosis by itself, it was found to block MK801-induced injury in a dose-dependent manner. Reversal of MK801 toxicity was complete in the caudate-putamen, partial in the somatosensory cortex but was not observed in the retrosplenial cortex. These results suggest that postnatal brain injury resulting from agents that block the NMDAR, which include commonly used anesthetics as well as drugs of abuse, may be prevented in vulnerable neurons by compensatory increases in calcium prior to exposure to these antagonists.