Differential DNA damage in response to the neonatal and adult excitotoxic hippocampal lesion in rats

Dexamethasone induces apoptosis in the developing rat amygdala in an age-, region-, and sex-specific manner

Exposure to glucocorticoids (GCs) in early development can lead to long-term changes in brain function and behavior, although little is known about the underlying neural mechanisms. Perinatal exposure to GCs alters adult anxiety and neuroendocrine responses to stress. Therefore, we investigated the effects of either late gestational or neonatal exposure to the GC receptor agonist dexamethasone (DEX), on apoptosis within the amygdala, a region critical for emotional regulation. DEX was administered to timed-pregnant rat dams from gestational day 18 until parturition, or postnatal day 4-6. Offspring were sacrificed the day following the last DEX treatment, and tissue was processed for immunohistochemical detection of cleaved caspase-3, a marker for apoptotic cells. Prenatal DEX treatment significantly increased the number of cleaved caspase-3-positive cells in the amygdala of both sexes, largely due to increases within the medial and basomedial subregions. Postnatal DEX treatment also increased cleaved caspase-3 immunoreactivity within the amygdala, although effects reached significance only in the central nucleus of females. Overall, DEX induction of cleaved caspase-3 in the amygdala was greater following prenatal compared with postnatal treatment, yet in both instances, elevations in cleaved caspase-3 correlated with an increase in pro-apoptotic Bax mRNA expression. Dual-label immunohistochemistry of cleaved caspase-3 and the neuronal marker NeuN confirmed that virtually all cleaved caspase-3-positive cells in the amygdala were neurons, and a subset of these cells (primarily following postnatal treatment) expressed a GABAergic calcium-binding protein phenotype (calbindin or calretinin). Together these results indicate that early developmental GC exposure induces neuronal apoptosis within the amygdala in an age-, sex-, and region-dependent manner.

Tracing the development of psychosis and its prevention: what can be learned from animal models

Schizophrenia (SCZ) is a neurodevelopmental disorder manifested symptomatically after puberty whose pharmacotherapy remains unsatisfactory. In recent years, longitudinal structural neuroimaging studies have revealed that neuroanatomical aberrations occur in this disorder and in fact precede symptom onset, raising the exciting possibility that SCZ can be prevented. There is some evidence that treatment with atypical antipsychotic drugs (APDs) prior to the development of the full clinical phenotype reduces the risk of transition to psychosis, but results remain controversial. It remains unknown whether progressive structural brain aberrations can be halted. Given the diagnostic, ethical, clinical and methodological problems of pharmacological and imaging studies in patients, getting such information remains a major challenge. Animal neurodevelopmental models of SCZ are invaluable for investigating such questions because they capture the notion that the effects of early brain damage are progressive. In recent years, data derived from such models have converged on key neuropathological and behavioral deficits documented in SCZ attesting to their strong validity, and making them ideal tools for evaluating progression of pathology following in-utero insults as well as its prevention. We review here our recent studies that use longitudinal in vivo structural imaging to achieve this aim in the prenatal immune stimulation model that is based on the association of prenatal infection and increased risk for SCZ. Pregnant rats were injected on gestational day 15 with the viral mimic polyriboinosinic-polyribocytidylic acid (poly I:C) or saline. Male and female offspring were imaged and tested behaviorally on postnatal days (PNDs) 35, 46, 56, 70 and 90. In other experiments, offspring of poly I:C- and saline-treated dams received the atypical antipsychotic drugs (APDs) clozapine or risperidone in two developmental windows: PND 34-47 and PND 48-61, and underwent behavioral testing and imaging at adulthood. Prenatal poly I:C-induced interference with fetal brain development led to aberrant postnatal brain development as manifested in structural abnormalities in the hippocampus, the striatum, the prefrontal cortex and lateral ventricles (LV), as seen in SCZ. The specific trajectories were region-, age- and sex-specific, with females having delayed onset of pathology compared to males. Brain pathology was accompanied by development of behavioral abnormalities phenotypic of SCZ, attentional deficit and hypersensitivity to amphetamine, with same sex difference. Hippocampal volume loss and LV volume expansion as well as behavioral abnormalities were prevented in the offspring of poly I:C mothers who received clozapine or risperidone during the asymptomatic period of adolescence (PND 34-47). Administration at a later window, PNDs 48-61, exerted sex-, region- and drug- specific effects. Our data show that prenatal insult leads to progressive postnatal brain pathology, which gradually gives rise to "symptoms"; that treatment with atypical APDs can prevent both brain and behavioral pathology; and that the earlier the intervention, the more pathological outcomes can be prevented.

MRI and X-ray scanning images of the brain of 3-, 6- and 9-month-old rats with bilateral neonatal ventral hippocampus lesions

Rats with bilateral neonatal ventral hippocampus lesions (NVHL) are commonly used for modeling developmental aspects of the pathophysiology of schizophrenia. Given that functional changes become significant only after puberty, NVHL as well as sham-operated rats were analyzed at the ages of 21, 42 and 63days (i.e. as pups, adolescents and adults), using MRI to examine the damage caused by surgery over time. Morphometric evaluations were considered and lesions were classified as small, medium and large. The volume of lesions increased regularly with age, to a greater extent than increases in overall brain size. This was relatively linear, corresponding to a gradually shrinking forebrain, and these observations held true for each class of lesions considered. Following the observation that the lesion procedure elicited calcifications in the brain, the same rats were subjected to 3D X-ray scanning the day after each MRI session, allowing precise measurements of skull size to be carried out. The NVHL rats had smaller skulls; however, the dimensions of the calcifications did not grow more than the skull size over time. The mechanisms underlying the progressive anatomical changes following surgery are discussed, and we propose this in vivo follow-up method to investigate therapeutic strategies aimed at countering or limiting the post-lesion consequences of a neonatal brain damage.

Acoustic hypersensitivity in adult rats after neonatal ventral hippocampus lesions

Rats with a bilateral neonatal ventral hippocampus lesion (NVHL) are used as models of neurobiological aspects of schizophrenia. In view of their decreased number of GABAergic interneurons, we hypothesized that they would show increased reactivity to acoustic stimuli. We systematically characterized the acoustic reactivity of NVHL rats and sham operated controls. They were behaviourally observed during a loud white noise. A first cohort of 7 months' old rats was studied. Then the observations were reproduced in a second cohort of the same age after characterizing the reactivity of the same rats to dopaminergic drugs. A third cohort of rats was studied at 2, 3, 4, 5 and 6 months. In subsets of lesioned and control rats, inferior colliculus auditory evoked potentials were recorded. A significant proportion of rats (50-62%) showed aberrant audiogenic responses with explosive wild running resembling the initial phase of audiogenic seizures. This was not correlated with their well-known enhanced reactivity to dopaminergic drugs. The proportion of rats showing this strong reaction increased with rats' age. After the cessation of the noise, NVHL rats showed a long freezing period that did neither depend on the size of the lesion nor on the rats' age. The initial negative deflection of the auditory evoked potential was enhanced in the inferior colliculus of only NVHL rats that displayed wild running. Complementary anatomical investigations using X-ray scans in the living animal, and alizarin red staining of brain slices, revealed a thin layer of calcium deposit close to the medial geniculate nuclei in post-NVHL rats, raising the possibility that this may contribute to the hyper-reactivity to sounds seen in these animals. The findings of this study provide complementary information with potential relevance for the hyper-reactivity noted in patients with schizophrenia, and therefore a tool to investigate the underlying biology of this endophenotype.

Neonatal administration of N-acetyl-l-aspartyl-l-glutamate induces early neurodegeneration in hippocampus and alters behaviour in young adult rats

N-acetyl-L-aspartyl-L-glutamate (NAAG) is a dipeptide that could be considered a sequestered form of L-glutamate. As much as 25% of L-glutamate in brain may be present in the form of NAAG. NAAG is also one of the most abundant neuroactive small molecules in the CNS: it is an agonist at Group II metabotropic glutamate receptors (mGluR II) and, at higher concentrations, at the N-methyl-D-aspartate (NMDA) type of ionotropic glutamate receptors. As such, NAAG can be either neuroprotective or neurotoxic and, in fact, both characteristics have been discussed and described in the literature. In the present studies, 250 nmol NAAG was infused into each lateral cerebral ventricle of 12-day-old rat pups and, using Nissl-stained sections, neurodegeneration in the hippocampus was evaluated 24 or 96 h after the infusion. In several experiments, the neuronal death was also visualised by Fluoro-Jade B staining and studied by TUNEL technique. Some of the NAAG-treated animals were allowed to survive until 50 days post partum and subjected to behavioural (open field) tests. The administration of NAAG to 12-day-old rats resulted in extensive death of neurons particularly in the dentate gyrus of the hippocampus. The neurodegeneration was, in part, prevented by administration of an NMDA receptor antagonist MK-801 (0.1 mg/kg). The nuclear DNA-fragmentation demonstrated by TUNEL technique pointed to the presence of non-specific single-strand DNA cleavage. The NAAG-associated neonatal neuronal damage may have perturbed development of synaptic circuitry during adolescence as indicated by an altered performance of the experimental animals in the open field testing (changes in grooming activity) at postnatal day 50. The results underscore the potential neurotoxicity of NAAG in neonatal rat brain and implicate neonatally induced, NMDA receptor-mediated neuronal loss in the development of abnormal behaviour in young adult rats.

Risperidone pretreatment prevents elevated locomotor activity following neonatal hippocampal lesions

Long-standing behavioral abnormalities emerge after puberty in rats following neonatal hippocampal lesion, providing a developmental model of abnormal rat behavior that may have predictive validity in identifying compounds effective in treating symptoms of schizophrenia. We sought to test the predictive validity of the neonatal hippocampal lesion model in identifying preventive treatment for first-episode psychosis. We determined the effect of risperidone, recently studied for prevention of first-episode psychosis, on the development of elevated locomotor activity following neonatal hippocampal lesions. Rat pups received hippocampal or sham lesions on postnatal day 7, followed by treatment with risperidone or vehicle from postnatal days 35 to 56. Locomotor activity in response to novelty, amphetamine, and nocturnal locomotion were determined on postnatal day 57. Low-dose risperidone (45 microg/kg) pretreatment prevented elevated locomotor activity in some, but not all, of the behavioral tasks following neonatal hippocampal lesions. In contrast, higher risperidone pretreatment was less effective in preventing elevated locomotor activity following neonatal hippocampal lesions. Because low risperidone dosages were also found to be effective in preventing first-episode psychosis in human studies, these data support the predictive validity of the hippocampal lesion model in identifying medications for prevention of first-episode psychosis. Additionally, these data support the use of low-dose risperidone in psychosis prevention, and suggest the possibility that higher risperidone doses could be less effective in this application.

Organotypic Hippocampal Slice Cultures: A Model System to Study Basic Cellular and Molecular Mechanisms of Neuronal Cell Death, Neuroprotection, and Synaptic Plasticity

The hippocampus has become one of the most extensively studied areas of the mammalian brain, and its proper function is of utmost importance, particularly for learning and memory. The hippocampus is the most susceptible brain region for damage, and its impaired function has been documented in many human brain diseases, e.g. hypoxia, ischemia, and epilepsy regardless of the age of the affected patients. In addition to experimental in vivo models of these disorders, the investigation of basic anatomical, physiological, and molecular aspects requires an adequate experimental in vitro model, which should meet the requirements for well-preserved representation of various cell types, and functional information processing properties in the hippocampus. In this review, the characteristics of organotypic hippocampal slice cultures (OHCs) together with the main differences between the in vivo and in vitro preparations are first briefly outlined. Thereafter, the use of OHCs in studies focusing on neuron cell death and synaptic plasticity is discussed.

Effects of ventral hippocampal lesion on thermal and mechanical nociception in neonates and adult rats

The proper maturation of the hippocampus is essential for the development of different behaviours, including memory, pain responses and avoidance. The mechanisms involved in the neurodevelopment of nociception have also been implicated in several neuropsychiatric disorders. The neonatal lesion of the ventral hippocampus (VH) in rats, an animal model of schizophrenia, can be utilized to study the developmental neurobiology of animal behaviour. We examined the nociceptive responses in this animal model at different stages of development. Rat pups were lesioned at postnatal day 7 by injecting ibotenic acid into the VH bilaterally, and then tested for thermal and mechanical nociception at the age of 35, 65 and 180 days. The nociceptive tests used were the hot plate (HP), paw pressure (PP) and tail flick (TF) tests. Another group of adult rats had the same lesion in the VH and then underwent the same tests at 28, 56 and 168 days post-lesions. When compared with sham controls, the rats with neonatal VH lesion showed decreased latency for the HP and PP tests only after puberty. The TF test showed significant increase in latency for both groups at age 65 and 180 days. The adult rats with VH lesion showed no major changes over all periods of testing. These results suggest that early lesion of VH can alter the development of the neural mechanisms involved in the processing of thermal and mechanical nociception.

Neonatal lesions in the amygdala or ventral hippocampus disrupt prepulse inhibition of the acoustic startle response; implications for an animal model of neurodevelopmental disorders like schizophrenia

Prepulse inhibition of the acoustic startle response is a behavioural tool applied to assess sensorimotor gating processes in humans and rats. Schizophrenic patients show deficits in prepulse inhibition of the acoustic startle response. The animal model of neurodevelopmental disorders such as schizophrenia, as purported in earlier reports and the present study, is based on the assumption that damage to brain structures early in life (on day 7) disrupts brain maturation of structures connected to the damaged areas, measurable by behavioural changes, whereas similar damage later in life (on day 21) does not result in these behavioural changes. Locomotor activity, the acoustic startle response and its prepulse inhibition were investigated in adult rats lesioned in the amygdala or ventral hippocampus on day 7 or 21 of life. The acoustic startle response was increased in animals lesioned in the amygdala on day 7 or 21 of life, but not in animals lesioned in the ventral hippocampus. Prepulse inhibition was impaired and locomotor activity enhanced in animals lesioned in the amygdala or ventral hippocampus on day 7, but not in animals lesioned in these structures on day 21 of life. The results on the acoustic startle response are suggestive of amygdaloid influences on modulation of the acoustic startle response. The effects of early postnatal lesions on prepulse inhibition and locomotor activity are in support of the animal model of neurodevelopmental disorders like schizophrenia.

Glycogen synthase kinase (GSK)-3beta levels and activity in a neurodevelopmental rat model of schizophrenia

We have previously reported reduced GSK-3beta protein levels and GSK-3 total (alpha + beta isoforms) activity in postmortem frontal cortex of schizophrenic patients. We now studied whether GSK-3beta is altered in the frontal cortex of rats with the neonatal excitotoxic hippocampal lesion used as a model of schizophrenia. Rats were infused with ibotenic acid (or artificial CSF in controls) bilaterally into the ventral hippocampus (VH) at postnatal day 7, then killed at postnatal day 35 (pre-puberty) or 56 (post-puberty). GSK-3beta protein levels were reduced in the frontal cortex of the lesioned rats as compared to sham animals; post-hoc comparisons revealed that the reduction was statistically significant at a pre-pubertal age. Total GSK-3 (alpha + beta) activity was not different between lesioned and sham rats at any age. These results demonstrate that reduced frontal cortical GSK-3beta levels may occur as a result of neonatal hippocampal damage and suggest that this animal model may be utilized to study the mechanism of GSK-3 reduction in schizophrenia, a disorder in which postmortem changes in GSK-3 were found.

Glutamate inhibition of NMDA-induced hydroxyl radical release: an ontogenic study in rat

Hydroxyl radicals (.OH) are frequently associated with glutamate excitotoxicity and may be critical in the occurrence of perinatal brain damage. We thus investigated the mechanisms regulating the glutamate-induced release of toxic.OH during development, using microdialysis and salicylate as an.OH trap. Glutamate inhibited.OH release until post-natal day 14, but stimulated this release from day 21 onwards. DHPG [(RS)-3,5-dihydroxyphenylglycine], a group-I metabotropic glutamate receptor agonist, similarly reduced the.OH release at day 14, but was ineffective afterwards. DHPG also completely blunted the tremendous NMDA-induced.OH release at day 14 but not at day 21. Glutamate itself therefore tonically inhibited a possible free radical release through NMDA channel activation during early development.

BDNF mRNA expression in rat hippocampus and prefrontal cortex: effects of neonatal ventral hippocampal damage and antipsychotic drugs

Brain-derived neurotrophic factor (BDNF) plays an important role in development, synapse remodelling and responses to stress and injury. Its abnormal expression has been implicated in schizophrenia, a neuropsychiatric disorder in which abnormal neural development of the hippocampus and prefrontal cortex has been postulated. To clarify the effects of antipsychotic drugs used in the therapy of schizophrenia on BDNF mRNA, we studied its expression in rats treated with clozapine and haloperidol and in rats with neonatal lesions of the ventral hippocampus, used as an animal model of schizophrenia. Both antipsychotic drugs reduced BDNF expression in the hippocampus of control rats, but did not significantly lower its expression in the prefrontal cortex. The neonatal hippocampal lesion itself suppressed BDNF mRNA expression in the dentate gyrus and tended to reduce its expression in the prefrontal cortex. These results indicate that, unlike antidepressants, antipsychotics down-regulate BDNF mRNA, and suggest that their therapeutic properties are not mediated by stimulation of this neurotrophin. To the extent that the lesioned rat models some pathophysiological aspects of schizophrenia, our data suggest that a neurodevelopmental insult might suppress expression of the neurotrophin in certain brain regions.

Lesion size and amphetamine hyperlocomotion after neonatal ventral hippocampal lesions: more is less

Neonatal hippocampal lesions in rats produce behavioral and neurochemical abnormalities post-puberty that are used in animal models for developmentally linked pathology in schizophrenia. In one model, adult rats exhibit enhanced sensitivity to the locomotor-activating effects of amphetamine, if they had sustained excitotoxic lesions of the ventral hippocampus on post-natal day 7. The hippocampal elements responsible for these lesion-induced developmental changes have not been fully characterized. The present study assessed the locomotor-activating effects of amphetamine in adult rats that on day 7 had sustained either sham or ibotenic acid lesions of the ventral hippocampus alone ("standard lesions"), or the ventral hippocampus plus surrounding portions of entorhinal cortex and dorsal hippocampus ("large lesions"). "Standard lesions" produced the expected "supersensitive" locomotor response to amphetamine, while "large lesions" did not. No differences between these lesion groups were observed in baseline levels of locomotor activity or habituation. These data suggest that models of enhanced behavioral sensitivity to dopamine agonists after neonatal hippocampal lesions require functionality in the entorhinal cortex and/or dorsal hippocampus. It is possible that the behavioral abnormalities in the "neonatal hippocampal lesion model" reflect, at least in part, aberrant function within spared elements of the hippocampal complex.