Mk801-induced caspase-3 in the postnatal brain: Inverse relationship with calcium binding proteins

Early cellular and synaptic changes in dopaminoceptive forebrain regions of juvenile mice following gestational exposure to valproate

Gestational exposure of mice to valproic acid (VPA) is one currently used experimental model for the investigation of typical failure symptoms associated with autism spectrum disorder (ASD). In the present study we hypothesized that the reduction of dopaminergic source neurons of the VTA, followed by perturbed growth of the mesotelencephalic dopamine pathway (MT), should also modify pattern formation in the dopaminoceptive target regions (particularly its mesoaccumbens/mesolimbic portion). Here, we investigated VPA-evoked cellular morphological (apoptosis-frequency detected by Caspase-3, abundance of Ca-binding proteins, CaBP), as well as synaptic proteomic (western blotting) changes, in selected dopaminoceptive subpallial, as compared to pallial, regions of mice, born to mothers treated with 500 mg/kg VPA on day 13.5 of pregnancy. We observed a surge of apoptosis on VPA treatment in nearly all investigated subpallial and pallial regions; with a non-significant trend of similar increase the nucleus accumbens (NAc) at P7, the age at which the MT pathway reduction has been reported (also supplemented by current findings). Of the CaBPs, calretinin (CR) expression was decreased in pallial regions, most prominently in retrosplenial cortex, but not in the subpallium of P7 mice. Calbindin-D 28K (CB) was selectively reduced in the caudate-putamen (CPu) of VPA exposed animals at P7 but no longer at P60, pointing to a potency of repairment. The VPA-associated overall increase in apoptosis at P7 did not correlate with the abundance and distribution of CaBPs, except in CPu, in which the marked drop of CB was negatively correlated with increased apoptosis. Abundance of parvalbumin (PV) at P60 showed no significant response to VPA treatment in any of the observed regions we did not find colocalization of apoptotic (Casp3+) cells with CaBP-immunoreactive neurons. The proteomic findings suggest reduction of tyrosine hydroxylase in the crude synaptosome fraction of NAc, but not in the CPu, without simultaneous decrease of the synaptic protein, synaptophysin, indicating selective impairment of dopaminergic synapses. The morpho-functional changes found in forebrain regions of VPA-exposed mice may signify dendritic and synaptic reorganization in dopaminergic target regions, with potential translational value to similar impairments in the pathogenesis of human ASD.

Effect of preconditioning on propofol-induced neurotoxicity during the developmental period

At therapeutic concentrations, propofol (PPF), an anesthetic agent, significantly elevates intracellular calcium concentration ([Ca2 +]i) and induces neural death during the developmental period. Preconditioning enables specialized tissues to tolerate major insults better compared with tissues that have already been exposed to sublethal insults. Here, we investigated whether the neurotoxicity induced by clinical concentrations of PPF could be alleviated by prior exposure to sublethal amounts of PPF. Cortical neurons from embryonic day (E) 17 Wistar rat fetuses were cultured in vitro, and on day in vitro (DIV) 2, the cells were preconditioned by exposure to PPF (PPF-PC) at either 100 nM or 1 μM for 24 h. For morphological observations, cells were exposed to clinical concentrations of PPF (10 μM or 100 μM) for 24 h and the survival ratio (SR) was calculated. Calcium imaging revealed significant PPF-induced [Ca2+]i elevation in cells on DIV 4 regardless of PPF-PC. Additionally, PPF-PC did not alleviate neural cell death induced by PPF under any condition. Our findings indicate that PPF-PC does not alleviate PPF-induced neurotoxicity during the developmental period.

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.

Intravenous anesthetic-induced calcium dysregulation and neurotoxic shift with age during development in primary cultured neurons

Anesthetic-induced neurotoxicity in the developing brain is a concern. This neurotoxicity is closely related to anesthetic exposure time, dose, and developmental stages. Using calcium imaging and morphological examinations in vitro, we sought to determine whether intravenous anesthetic-induced direct neurotoxicity varies according to different stages of the days in vitro (DIV) of neurons in primary culture. Cortical neurons from E17 Wistar rats were prepared. On DIV 3, 7, and 13, cells were exposed to the intravenous anesthetics thiopental sodium (TPS), midazolam (MDZ), or propofol (PPF), to investigate direct neurotoxicity using morphological experiments. Furthermore, using calcium imaging, the anesthetic-induced intracellular calcium concentration ([Ca²⁺]i) elevation was monitored in cells on DIV 4, 8, and 13. All anesthetics elicited significant [Ca²⁺]i increases on DIV 4. While TPS (100 μM) and MDZ (10 μM) did not alter neuronal death, PPF (10 μM and 100 μM) decreased the survival ratio (SR) significantly. On DIV 8, TPS and MDZ did not elicit [Ca²⁺]i elevation or SR decrease, while PPF still induced [Ca²⁺]i elevation (both at 10 μM and 100 μM) and significant SR decrease at 100 μM (0.76 ± 0.03; P < 0.05), but not at 10 μM (0.91 ± 0.03). Such anesthetic-induced [Ca²⁺]i elevation and SR decrease were not observed on DIV 13-14 for any of the anesthetic drugs. Our study indicates that more caution may be exercised when using PPF compared to TPS or MDZ during development.

Anaesthesia in early childhood - is the development of the immature brain in danger?

Experimental studies performed on immature animal brains had demonstrated a neurotoxic effect following sedation and general anaesthetics administration. The same magnitude of neurotoxicity has been suggested but not been proven to neonates, infants and small children who have undergone anaesthesia. There is a justified and increasing inquiry regarding the administration of general anaesthesia to paediatric patients.

Pten Inhibitor-bpV Ameliorates Early Postnatal Propofol Exposure-Induced Memory Deficit and Impairment of Hippocampal LTP

Early postnatal propofol administration has potential detrimental effects on hippocampal synaptic development and memory. Therapeutic method is still lack due to unknown mechanisms. In this study, a 7-day propofol protocol was applied to model anesthesia in neonatal mice. Phosphatase and tensin homolog deleted on chromosome ten (Pten) inhibitor bisperoxovanadium (bpV) was pre-applied before propofol to study its potential protection. After propofol application, Pten level increased while phospho-AKT (p-AKT) (Ser473) decreased in dorsal hippocampus. Interestingly, i.p. injection of Pten inhibitor reversed the decrease of p-AKT. Two months after administration, basal synaptic transmission, hippocampal long-term potentiation (LTP) and long-term memory were reduced in propofol-administrated mice. By contrast, i.p. injection of Pten inhibitor at a dose of 0.2 mg/kg/day before propofol reversed the detrimental effects due to propofol application. Consistently, bpV injection also reversed propofol application-induced decrease of synaptic plasticity-related proteins, including p-CamKIIα, p-PKA and postsynaptic density protein 95. Taken together, our results demonstrate that bpV injection could reverse early propofol exposure-induced decrease of memory and hippocampal LTP. bpV might be a potential therapeutic for memory impairment after early propofol postnatal application.

Hydrogen-rich water protects against ischemic brain injury in rats by regulating calcium buffering proteins

Hydrogen-rich water (HRW) has anti-oxidant activities, and it exerts neuroprotective effects during ischemia-reperfusion brain injury. Parvalbumin and hippocalcin are two calcium buffering proteins, which are involved in neuronal differentiation, maturation and apoptosis. The aim of this study was to investigate whether HRW could moderate parvalbumin and hippocalcin expression during ischemic brain injury and glutamate toxicity-induced neuronal cell death. Focal brain ischemia was induced in male Sprague-Dawley rats by middle cerebral artery occlusion (MCAO). Rats were treated with H2O or HRW (6 ml/kg per rat) before and after MCAO, and cerebral cortical tissues were collected 1, 7 and 14 days after MCAO. Based on our results, HRW treatment was able to reduce brain infarct volume and improve neurological function following ischemic brain injury. In addition, HRW prevented the ischemia-induced reduction of parvalbumin and hippocalcin levels in vivo and also reduced the glutamate toxicity-induced death of neurons, including the dose-dependent reduction of glutamate toxicity-associated proteins in vitro. Moreover, HRW attenuated the glutamate toxicity-induced elevate in intracellular Ca(2+) levels. All these results suggest that HRW could protect against ischemic brain injury and that the maintenance of parvalbumin and hippocalcin levels by HRW during ischemic brain injury might contribute to the neuroprotective effects against neuron damage.

Developmental neurotoxicity and anticonvulsant drugs: a possible link

In utero exposure to antiepileptic drugs (AEDs) may affect neurodevelopment causing postnatal cognitive and behavioral alterations. Phenytoin and phenobarbital may lead to motor and learning dysfunctions in the pre-exposed children. These disorders may reflect the interference of these AEDs with the development of hippocampal and cerebellar neurons, as suggested by animal studies. Exposure to valproic acid may result in inhibition of neural stem cell proliferation and/or immature neuron migration in the cerebral cortex with consequent increased risk of neurodevelopmental impairment, such as autistic spectrum disorders. A central issue in the prevention of AED-mediated developmental effects is the identification of drugs that should be avoided in women of child-bearing potential and during pregnancy. The aim of this review is to explore the possible link between AEDs and neurodevelopmental dysfunctions both in human and in animal studies. The possible mechanisms underlying this association are also discussed.

Mechanistic insights into neurotoxicity induced by anesthetics in the developing brain

Compelling evidence has shown that exposure to anesthetics used in the clinic can cause neurodegeneration in the mammalian developing brain, but the basis of this is not clear. Neurotoxicity induced by exposure to anesthestics in early life involves neuroapoptosis and impairment of neurodevelopmental processes such as neurogenesis, synaptogenesis and immature glial development. These effects may subsequently contribute to behavior abnormalities in later life. In this paper, we reviewed the possible mechanisms of anesthetic-induced neurotoxicity based on new in vitro and in vivo findings. Also, we discussed ways to protect against anesthetic-induced neurotoxicity and their implications for exploring cellular and molecular mechanisms of neuroprotection. These findings help in improving our understanding of developmental neurotoxicology and in avoiding adverse neurological outcomes in anesthesia practice.

The absence of the calcium-buffering protein calbindin is associated with faster age-related decline in hippocampal metabolism

Although reductions in the expression of the calcium-buffering proteins calbindin D-28K (CB) and parvalbumin (PV) have been observed in the aging brain, it is unknown whether these changes contribute to age-related hippocampal dysfunction. To address this issue, we measured basal hippocampal metabolism and hippocampal structure across the lifespan of C57BL/6J, calbindin D-28k knockout (CBKO) and parvalbumin knockout (PVKO) mice. Basal metabolism was estimated using steady state relative cerebral blood volume (rCBV), which is a variant of fMRI that provides the highest spatial resolution, optimal for the analysis of individual subregions of the hippocampal formation. We found that like primates, normal aging in C57BL/6J mice is characterized by an age-dependent decline in rCBV-estimated dentate gyrus (DG) metabolism. Although abnormal hippocampal fMRI signals were observed in CBKO and PVKO mice, only CBKO mice showed accelerated age-dependent decline of rCBV-estimated metabolism in the DG. We also found age-independent structural changes in CBKO mice, which included an enlarged hippocampus and neocortex as well as global brain hypertrophy. These metabolic and structural changes in CBKO mice correlated with a deficit in hippocampus-dependent learning in the active place avoidance task. Our results suggest that the decrease in CB that occurs during normal aging is involved in age-related hippocampal metabolic decline. Our findings also illustrate the value of using multiple MRI techniques in transgenic mice to investigate mechanisms involved in the functional and structural changes that occur during aging.

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.

Effects of neonatal MK-801 treatment on p70S6K-S6/eIF4B signal pathways and protein translation in the frontal cortex of the developing rat brain

Systemic injections of MK-801, a selective NMDAR antagonist, into neonatal rats induces long-term neurochemical and behavioural changes. It has been suggested that these changes form the neurodevelopmental basis for schizophrenia-like behaviour in rats. In this study, 7-d-old rats were treated with MK-801, and their frontal cortices were examined to investigate the effects on p70S6K-S6 signal pathway and on protein translation, which play crucial roles in the neurodevelopmental process. MK-801, in doses of 0.5 and 1.0 mg/kg, induced a decrease in phosphorylation of p70S6K and its substrates, S6 and eIF4B, in the first 8 h, and no change at 24 and 48 h. These effects were more prominent after two injections of MK-801 than one. Decreased S6 phosphorylation by MK-801 was evident in the prefrontal, cingulate, and insular cortex. In two representative upstream p70S6K-S6 pathways related to ERK1/2 and Akt, changes in ERK1/2-p90RSK phosphorylation were accompanied by changes of p70S6K-S6. Although two MK-801 injections induced a dose-dependent decrease in phosphorylation of Akt and mTOR at 4 and 8 h, a single injection did not produce a significant effect. Protein synthesis rate, measured by [3H]leucine incorporation in frontal cortical tissue, was reduced until 24 h after two MK-801 (1.0 mg/kg) injections. In summary, this study found that neonatal MK-801 treatment induced dysregulation in the p70S6K-S6/eIF4B pathway and protein translation in the frontal cortex of the developing rat brain, which may suggest an important role of protein translation machinery in the MK-801 neurodevelopmental animal model of schizophrenia.

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.

Spermatocyte apoptosis, which involves both intrinsic and extrinsic pathways, explains the sterility of Graomys griseoflavus x Graomys centralis male hybrids

Spermatogenic impairment and the apoptotic pathways involved in establishing sterility of male hybrids obtained from crossing Graomys griseoflavus females with Graomys centralis males were studied. Testes from G. centralis, G. griseoflavus and hybrids were compared at different ages. Terminal transferase-mediated dUTP nick-end labelling assay (TUNEL), Fas, Bax and cytochrome c labelling were used for apoptosis evaluation, and calbindin D(28k) staining as an anti-apoptotic molecule. In 1-month-old animals, spermatocytes were positive for all apoptotic markers, but moderate TUNEL (+) spermatocyte frequency was only found in G. centralis. At subsequent ages, the apoptotic markers were downregulated in testes from parental cytotypes, but not in hybrid testes. TUNEL (+) spermatocytes were present at 78% and 44% per tubule cross-section in 2- and 3-month-old hybrid animals, respectively. Pachytene spermatocyte death in adult hybrids occurs via apoptosis, as revealed by high caspase-3 expression. Calbindin was highly expressed in spermatocytes of adult hybrids, in which massive cell death occurs via apoptosis. Calbindin co-localisation with TUNEL or Fas, Bax and cytochrome c was very limited, suggesting an inverse regulation of calbindin and apoptotic markers. Hybrid sterility is due to breakdown of spermatogenesis at the pachytene spermatocyte stage. Both extrinsic and intrinsic pathways are involved in apoptosis of spermatocytes, which are the most sensitive cell type to apoptotic stimuli.

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.

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.

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

The acoustic startle reflex in rats can be inhibited if a prepulse stimulus is presented just before the startle stimulus (prepulse inhibition; PPI). When postnatal day 7 (P7) rats are exposed to agents that block the NMDA receptor (NMDAR), robust apoptosis is observed within hours and is thought to be followed at later ages by a significant loss of PPI. To understand these observations further, we exposed rat pups to vehicle or the NMDAR antagonist MK801 (1 mg/kg) at P6, P8, and P10. We then examined animals for PPI at P28 and P56. Compared to vehicle controls, we found no evidence for PPI deficits in the MK801-treated group, although we did observe prepulse-induced delay in response time at P56 (but not at P28). In a parallel study, we also performed histological analysis of brain sections for evidence of the pro-apoptotic marker activated caspase-3, 8 h after vehicle or MK801 injection into P6 animals. We found that there was a robust increase in this marker of cell death in the inferior colliculus of MK801 compared to vehicle-treated animals. Thus, transient blockade of the NMDAR during the postnatal period not only promotes early apoptosis in a brain region critical for acoustic processing but also leads to auditory deficits at a later age, suggesting that injury-induced loss of collicular neurons leads to network reorganization in the auditory system that is progressive in nature.

Early postnatal depletion of NMDA receptor development affects behaviour and NMDA receptor expression until later adulthood in rats--a possible model for schizophrenia

There is increasing evidence that a dysfunction of the N-methyl-d-aspartate (NMDA) receptor system plays a key role in the pathophysiology of schizophrenia. Non-competitive NMDA-antagonists induce schizophrenia-like symptoms and cognitive impairment in healthy humans as well as rodents. As receptor dysfunction precedes clinical disorder manifestation, the present study investigated whether transient perinatal NMDA antagonism constitutes a suitable long-term animal model for schizophrenia. Male Wistar rats were treated from postnatal days 6-21 with the NMDA receptor antagonist MK-801, and then subjected to behavioural analysis up to an age of 180d. Alterations in cortical NMDA receptor expression and lymphocyte cAMP-response-element-binding-protein (CREB) were assessed. In comparison to controls, MK-801-treated animals showed differences in behaviour up to an age of 180d. Western blot analysis revealed that transient perinatal application of MK-801 caused a persistent increase in cortical NMDA-R1 protein in combination with a persistent disturbance of CREB phosphorylation, a downstream target of NMDA signalling. This animal model demonstrates that early postnatal NMDA receptor blockade leads to schizophrenia-like symptoms with persistent behavioural and neurochemical disturbances throughout life. Therefore, it might provide a basis for further understanding of the disease and evaluation of new therapeutic strategies.

Deficits in adult prefrontal cortex neurons and behavior following early post-natal NMDA antagonist treatment

The prefrontal cortex (PFC) is associated with higher cognitive functions including attention and working memory and has been implicated in the regulation of impulsivity as well as the pathology of complex mental illnesses. N-methyl D-aspartate (NMDA) antagonist treatment with dizocilpine induces cell death which is greatest in the frontal cortex on post-natal day seven (P7), however the long-term structural and behavioral effects of this treatment are unknown. This study investigates both the acute neurotoxicity of P7 dizocilpine and the persistent effects of this treatment on pyramidal cells and parvalbumin interneurons in the adult PFC, a brain region involved in the regulation of impulsivity. Dizocilpine treatment on P7 increased cleaved caspase-3 immunoreactivity (IR) in the PFC on P8. In adult mice (P82), P7 dizocilpine treatment resulted in 50% fewer parvalbumin-positive interneurons (p<0.01) and 42% fewer layer V pyramidal neurons (p<0.01) in the PFC. Double immunohistochemistry revealed cleaved caspase-3 IR in both GAD67 IR interneurons and GAD67 (-) neurons. Following dizocilpine treatment at P7, adults showed reduced time in the center of the open field suggesting increased anxiety-like behavior. These findings indicate that early brain insults affecting glutamatergic neurotransmission lead to persistent brain pathology that could contribute to impulsivity and cognitive dysfunction.

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

MK801-induced activation of caspase-3 is developmentally regulated, peaking at postnatal day (P) 7 and decreasing with increasing postnatal age thereafter. Further, at P7, cells displaying activation of caspase-3 lack expression of calcium binding proteins (CaBPs). To further explore this relationship, we investigated postnatal expression of calbindin (CB), calretinin (CR) and parvalbumin (PV) in two brain regions susceptible to MK801-induced injury, the somatosensory cortex (S1) and layer II/III of motor cortex (M1/M2). Expression of CB and especially PV was low to absent prior to P7 but substantially increased from P7 through to P21 and adulthood. In contrast, CR expression was more variable at early developmental ages, stabilized to lower levels after P7 and showed a marked decline by P21. The results suggest that not only does calcium buffering capacity increase developmentally but also acquisition of enhanced buffering may be one mechanism by which neurons survive agent-induced alterations in calcium homeostasis.

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.

First egg protein with a neurotoxic effect on mice

While many invertebrates sequester toxic compounds to endow eggs with chemical defences, here we show, for the first time to our knowledge, the identification of a neurotoxin of proteinaceous nature localized inside an egg. Egg extracts from the freshwater apple snail Pomacea canaliculata displayed a neurotoxic effect in mice upon intraperitoneal injection (i.p.) (LD50, 96h 2.3mg/kg). Egg protein and total lipids were analysed separately and the only fraction displaying a highly toxic effect (LD50, 96h 0.25mg/kg, i.p.) was further purified to homogeneity as an oligomeric glyco-lipoprotein of 400kDa and two subunits biochemically and immunologically indistinguishable from the previously described perivitellin PV2. The neurotoxin was heat sensitive and there was evidence of circulating antibody response to sublethal i.p. doses on mice. Clinical signs, histopathological and immunocytochemical studies revealed damage mostly in mice spinal cord. Experiments showed chromatolysis and a decreased response to calbindin D-28K associated with a significant increase of TUNEL-positive cells in the dorsal horn neurons. These results suggest that calcium buffering and apoptosis may play a role in the neurological disorders induced by the toxin in mammalian central nervous system. This is the first report of a mollusc neurotoxin genetically encoded outside the cone-snail species.

Effects of disrupting calcium homeostasis on neuronal maturation: early inhibition and later recovery

It has become increasingly clear that agents that disrupt calcium homeostasis may also be toxic to developing neurons. Using isolated primary neurons, we sought to understand the neurotoxicity of agents such as MK801 (which blocks ligand-gated calcium entry), BAPTA (which chelates intracellular calcium), and thapsigargin (TG; which inhibits the endoplasmic reticulum Ca(2+)-ATPase pump). Thus, E18 rat cortical neurons were grown for 1 day in vitro (DIV) and then exposed to vehicle (0.1% DMSO), MK801 (0.01-20 microM), BAPTA (0.1-20 microM), or TG (0.001-1 microM) for 24 h. We found that all three agents could profoundly influence early neuronal maturation (growth cone expansion, neurite length, neurite complexity), with the order of potency being MK801 < BAPTA < TG. We next asked if cultures exposed to these agents were able to re-establish their developmental program once the agent was removed. When we examined network maturity at 4 and 7 DIV, the order of recovery was MK801 > BAPTA > TG. Thus, mechanistically distinct ways of disrupting calcium homeostasis differentially influenced both short-term and long-term neuronal maturation. These observations suggest that agents that act by altering intracellular calcium and are used in obstetrics or neonatology may be quite harmful to the still-developing human brain.

Effects of lamotrigine alone and in combination with MK-801, phenobarbital, or phenytoin on cell death in the neonatal rat brain

The neonatal rat brain is vulnerable to neuronal apoptosis induced by antiepileptic drugs (AEDs), especially when given in combination. This study evaluated lamotrigine alone or in combination with phenobarbital, phenytoin, or the glutamate antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801) for a proapoptotic action in the developing rat brain. Cell death was assessed in brain regions (striatum, thalamus, and cortical areas) of rat pups (postnatal day 8) by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay, 24 h after acute drug treatment. Lamotrigine alone did not increase neuronal apoptosis when given in doses up to 50 mg/kg; a significant increase in cell death occurred after 100 mg/kg. Combination of 20 mg/kg lamotrigine with 0.5 mg/kg MK-801 or 75 mg/kg phenobarbital resulted in a significant increase in TUNEL-positive cells, compared with MK-801 or phenobarbital treatment alone. A similar enhancement of phenytoin-induced cell death occurred after 30 mg/kg lamotrigine. In contrast, 20 mg/kg lamotrigine significantly attenuated phenytoin-induced cell death. Lamotrigine at 10 mg/kg was without effect on apoptosis induced by phenytoin. Although the functional and clinical implications of AED-induced developmental neuronal apoptosis remain to be elucidated, our finding that lamotrigine alone is devoid of this effect makes this drug attractive as monotherapy for the treatment of women during pregnancy, and for preterm or neonatal infants. However, because AEDs are often introduced as add-on medication, careful selection of drug combinations and doses may be required to avoid developmental neurotoxicity when lamotrigine is used in polytherapy.

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

Age-dependent, MK801-induced, activated caspase-3 expression in the postnatal brain is generally not observed in neurons expressing calcium-binding proteins (CaBPs), suggesting that apoptosis and calcium buffering are inversely related. In regions such as the cingulate and retrosplenial cortex, injury peaks at postnatal Day 7 (P7) and rapidly diminishes thereafter, whereas expression of calbindin (CB) and calretinin (CR) was relatively low from P0 to P7 and steadily increased from P7 to P14. At ages thereafter, CB and CR expression either remained stable then declined or rapidly declined. Parvalbumin (PV) was generally low-absent prior to P7 but expression dramatically increased from P10 onwards, peaking at P21. These studies suggest calcium entry (through N-methyl-D-aspartate receptor (NMDARs)) and buffering (by CaBPs) are integral to normal CNS maturation. Because schizophrenia is associated with glutamate hypo-function, developmental injury, and aberrant CaBP expression, our data indicate that this postnatal brain injury model may offer important insights into the nature of this disorder.

Loss of calcium and increased apoptosis within the same neuron

Loss of neuronal calcium is associated with later apoptotic injury but observing reduced calcium and increased apoptosis in the same cell would provide more definitive proof of this apparent correlation. Thus, following exposure to vehicle or the calcium chelator, BAPTA (1-20 microM), primary cortical neurons were labeled with Calcium Green-1 which was then cross-linked with EDAC, prior to immuno-staining for various proteins. We found that BAPTA-induced changes in calcium were highly correlated with changes in expression of activated caspase-3 as well as the calcium binding proteins calbindin, calretinin, and parvalbumin. Additionally, in brain slices from P7 neonatal rats, BAPTA induced significant loss of calcium in a brain region we have previously shown to express only moderate levels of calcium binding proteins as well as display robust apoptosis following calcium entry blockade. In contrast, BAPTA had little influence on calcium levels in a brain region we have previously shown to express robust calcium binding proteins as well as display far less apoptosis following calcium entry blockade. These data suggest that the ability of developing neurons to buffer changes in calcium may be critical to their long-term survival.

Widespread Neonatal Brain Damage following Calcium Channel Blockade

An abundance of evidence exists that shows calcium channel blockade promotes injury in cultured neurons. However, few studies have addressed the in vivo toxicity of such agents. We now show that the L-type calcium channel antagonist nimodipine promotes widespread and robust injury throughout the neonatal rat brain, in an age-dependent manner. Using both isolated neuronal as well as brain slice approaches, we address mechanisms behind such injury. These expanded studies show a consistent pattern of injury using a variety of agents that lower intracellular calcium. Collectively, these observations indicate that postnatal brain development represents a transitional period for still developing neurons, from being highly sensitive to reductions in intracellular calcium to being less vulnerable to such changes. These observations directly relate to current therapeutic strategies targeting neonatal brain injury.