Developmental Brain Research最新文献

Fetal or neonatal exposure to chlorpyrifos (CPF) or related organophosphate pesticides leads to abnormalities of brain cell development, synaptic function, and behavior. Recent studies in rats indicate profound effects on serotonin (5HT) systems that originate during CPF exposure and that are still present at 2 months posttreatment in the young adult. To determine if these changes are permanent, we administered 1 mg/kg of CPF daily to neonatal rats on postnatal days 1–4, a regimen devoid of systemic toxicity, and examined 5HT synaptic markers at 5 months of age: radioligand binding to 5HT_1A and 5HT₂ receptors and to the 5HT transporter. There were global elevations in all three synaptic proteins, with pronounced sex selectivity (effects on males > females) and a regional hierarchy of effects, viz. striatum > midbrain ≈ brainstem > cerebral cortex. Because there is a normal sex disparity for 5HT synaptic proteins, with females having higher values than males, the increase caused by CPF exposure in males completely eliminated this difference. Our findings at 5 months of age replicate those seen in young adulthood and strongly suggest that the effects of neonatal CPF exposure on 5HT systems are permanent.

Previously, an RNA interference (RNAi) knockdown of GABA_B(1) subunit in adult Drosophila was used for behavioral studies. Here we report on developmental deficits caused by embryonic Drosophila GABA_B(1) RNAi and drug antagonism. Injecting embryos with CGP54626 (a GABA_B receptor antagonist) reduced hatching and caused lethality. Similar effects were produced by injecting embryos with GABA_B(1) double-stranded RNA (RNAi). The surviving GABA_B(1) RNAi larvae were significantly smaller than controls and showed a peculiar phenotype; their tracheae were folded. Our results suggest that GABA_B receptors are required for normal development and that the Drosophila model could be used to investigate the participating molecular mechanisms.

Methanesulfonyl fluoride (MSF) is a CNS-selective acetylcholinesterase (AChE) inhibitor, currently being developed and tested for the treatment of symptoms of Alzheimer's disease [D.E. Moss, P. Berlanga, M.M. Hagan, H. Sandoval, and C. Ishida, Methanesulfonyl fluoride (MSF): a double-blind, placebo-controlled study of safety and efficacy in the treatment of senile dementia of the Alzheimer type, Alzheimer Dis. Assoc. Disord., 13 (1999) 20–25] [43]. We have previously confirmed that a single in utero exposure to MSF at clinically appropriate doses inhibits AChE activity in fetal rat brain by 20%, and when administered throughout gestation, MSF achieves a 40% level of inhibition. Here, we show that rats chronically exposed in utero to MSF display marked sex-specific differences in morphological development of the cerebral cortical layers compared with controls at 7 days of age. Forebrain size and cortical thickness were increased in females and decreased in males. An analysis of gene expression in neonate brain on the day of birth revealed sex-specific differential expression of over 25 genes, including choline acetyltransferase (ChAT), which were affected by prenatal MSF exposure. Many of these genes are associated with sexual differentiation and brain development, while others are involved in more generalized cellular and metabolic processes. The changes observed in cortical morphology and gene expression suggest a critical developmental role for AChE in the fetal nervous system, most likely through its effect on cholinergic neurotransmission.

Transcription/translation feedback loops consisting of multiple clock genes are thought to be essential for circadian oscillations at cellular, tissue and organismal levels. We examined the developmental expressions of three clock genes (Bmal1, Cry1 and Per1) in the Syrian hamster to probe the oscillatory properties of the suprachiasmatic nucleus (SCN) over the first 4 days after the completion of SCN neurogenesis. Samples were taken at the dam's circadian times 6, 12, and 18 daily over 4 days in constant dim light and processed for in situ hybridization using ³⁵S-labeled RNA probes. Collection times were based on the phases of Bmal1 and Per1 rhythms in adult SCN and on an observed difference in Per1 mRNA at CT6 and 18 on postnatal day 2. For the developmental study, sections from each brain were processed in parallel for the three genes. Bmal1 was prominently expressed in the fetal SCN while Per1 and Cry1 were only weakly expressed. Transcripts of all three genes showed higher abundance just after birth. At subsequent ages, Bmal1 showed a significant decrease, while Per1 continued to be greater than prenatal levels. Significant variation was detected across circadian times for Cry1, but no circadian variation was detected for Per1 and Bmal1. Molecular oscillations equivalent to those observed in adults were not present in the fetal SCN despite evidence for an entrainable pacemaker at that time. An absence of robust oscillations during early SCN development may in part explain the strong phase-setting effects of pharmacological agents on the fetal/neonatal clock.

We have investigated the changes in arrangement of the SSEA-1 immunoreactive chiasmatic neurons in the mouse ventral diencephalon from embryonic day (E) 9 to the end of gestation. A regionally specific staining of SSEA-1 was first detected in the ventricular layer of the caudal diencephalon at E10 and later at E11 on the cells in the subventricular layer. At E12, these cells formed the characteristic V-shaped configuration caudal to the optic axons in the chiasm. At E13–E15, this neuronal array changes gradually to a configuration that facilitates contact with the optic axons only at the midline and the initial segment of the optic tract. Colocalization studies showed that CD44 was localized strongly on the neurons in the central but not lateral domains of the array, suggesting existence of heterogeneity in these neurons in terms of surface antigen presentation. This difference between the central and lateral domains raises the possibility that the chiasmatic neurons may regulate the patterning of axon orders at the midline and the optic tract through presentation of distinct combination of guidance cues at these strategic positions in the optic pathway. Furthermore, exogenous Lewis-x/SSEA-1 inhibited neurite outgrowth from the E14 retinal explants; this inhibition was observed in neurites from both ventral temporal and dorsal nasal retina. These findings suggest an action of this surface carbohydrate on the control of axon growth and guidance in the mouse optic pathway.

NMDA antagonists are of potential therapeutic benefit for several conditions. However, their ability to produce neurotoxicity and psychosis has hampered their clinical use. A better understanding of these side effects and the mechanism underlying them could result in their safer use and in improving our understanding of psychotic illnesses. By disinhibiting certain multisynaptic circuits, moderate doses of NMDA antagonists produce reversible neurotoxicity in the retrosplenial cortex in rats older than 1 month. Higher doses of these same agents result in the death of neurons in the retrosplenial cortex and several other brain regions. It is unknown whether susceptibility to this irreversible neurodegeneration has a similar age dependency profile. We, therefore, examined the sensitivity of rats of various ages (PND20-60) to the irreversible neurodegenerative effect of the selective NMDA antagonist, MK-801. Quantification of the severity of neurodegeneration with stereology revealed that the retrosplenial cortex, induseum griseum, and dentate gyrus had decreasing amounts of damage with decreasing age and onset of sensitivity around PND30. The piriform cortex also displayed a decreased amount of degeneration in younger age groups. However, a low level of degeneration continued to occur in the posterior piriform cortex in the PND20-25 animals. The stage of degeneration appeared to be more advanced, suggesting that these neurons were dying by a different mechanism. We conclude that for most neuronal populations, susceptibility to the irreversible and reversible neurodegenerative effects of NMDA antagonists has a similar age dependency profile, consistent with the proposal that the same disinhibitory mechanism underlies both neurotoxicities.

White matter damage (WMD) is an important cause of disability including cerebral palsy in preterm, low birth-weight infants. Maternal infection is now recognized as one of the risk factors for WMD. Previously we reported that intrauterine inoculation of Escherichia coli to pregnant rats resulted in WMD in offspring and interleukin-10 (IL-10) was protective against this damage. The objective of this study was to elucidate the mechanism involved in the protective effect of IL-10 against neonatal WMD. We found that E. coli treatment in dams resulted in significant apoptosis in periventricular white matter of rat pups on postnatal day 0 (P0). On P8, a remarkable increase in ED-1 immunostaining (indicating either microglial activation or macrophage infiltration) was detected in brains of pups in the E. coli-treated group. Astrogliosis was also noticed in brain white matter of pups in the E. coli-treated group. In addition to the strong activation of microglia and astrocytes, oligodendrocytes (OLs) were significantly reduced in periventricular areas in the brains of pups from the E. coli-treated group. Later, on P15, hypomyelination was also noticed in rat brains from the E. coli-treated group, using myelin basic protein (MBP) immunostaining. Treatment with IL-10 after E. coli inoculation significantly reduced TUNEL staining and caspase-3 activation, and partially restored the impaired immunostaining markers for immature and mature OLs, such as CNPase, O4, adenomatous polyposis coli (APC) and MBP. These results indicate that the protective effect of IL-10 against brain WMD is linked with suppression of microglial activation/macrophage infiltration, as shown by significantly reduced ED-1+ cells in the white matter.

The development of the serotonergic (5HT) and dopaminergic (DA) systems may contribute to the onset of psychotic disorders during late adolescence and early adulthood. Previous studies in our laboratory have suggested that these systems may compete for functional territory on neurons during development, as lesions of the serotonergic system at postnatal day 5 (P5) result in an increase in the density of dopaminergic fibers in rat medial prefrontal cortex (mPFC). In the present study, the dopaminergic system of P5 rats was lesioned with intracisternal injections of 6-hydroxydopamine (6-OHDA). Quantification of serotonin-immunoreactivity (5HT-IR) in mPFC at adulthood (P70) revealed a significant decrease in fiber density within layers II and III of the Cg3 subdivision of mPFC in lesioned rats compared to sham controls. We propose that the decrease in serotonergic fibers in mPFC in response to a neonatal depletion of dopamine may be due to the loss of a trophic effect of this system on 5HT neurons and/or fibers during development. Taken together with previous findings, our data suggest that there may be an “inverse trophic” relationship between the cortical DA and 5HT systems whereby dopamine facilitates the ingrowth of 5HT fibers, while serotonin suppresses the ingrowth of DA fibers. We present a model based on neurotrophic interactions at the cortical and brainstem levels that could potentially explain these unexpected results.