Early Postnatal Rat Brain Research Articles

Dexmedetomidine alleviates neurobehavioral impairments and myelination deficits following lipopolysaccharide exposure in early postnatal rats

AimsWhite matter injury (WMI) is the main form of brain injury in preterm neonate survivors, and perinatal inflammation is implicated in the pathogenesis of WMI. It has been demonstrated that dexmedetomidine, an anesthetic adjuvant, possesses neuroprotective effects in both preclinical and clinical trials. The present study was conducted to explore whether dexmedetomidine could protect against neurobehavioral impairments and myelination deficits caused by lipopolysaccharide (LPS) exposure in the early postnatal rat brain. Main methodsLPS (2 mg/kg) was intraperitoneally (i.p.) injected in Sprague–Dawley rat pups on postnatal day 2 (P2). Dexmedetomidine (25 μg/kg) or vehicle was given i.p. immediately after LPS injection. STAT3 and p-STAT3 expression were detected by western blot in rat brain 24 h after drug administration. Immunostaining for GFAP to was performed to evaluate astrocytic response at 24 h post-LPS and P14. Neurobehavioral tests (the righting reflex, negative geotaxis, and wire hanging maneuver tests) were performed from P5 to P10. Histological analysis of myelin content was accessed by immunohistochemistry for CNPase and MBP at P14. Key findingsOur results showed that treatment with dexmedetomidine significantly ameliorated LPS-induced neurobehavioral abnormalities and myelin damage, which is accompanied by suppression of STAT3 activation and reactive astrogliosis. SignificanceDexmedetomidine can alleviate neurobehavioral impairments and myelination deficits after LPS exposure in early postnatal rats, probably by mitigating STAT3-mediated reactive astrogliosis. Our results suggest that dexmedetomidine might be a promising agent to treat brain injury in neonates.

Acute peripheral immune activation alters cytokine expression and glial activation in the early postnatal rat brain

BackgroundNeuroinflammation can modulate brain development; however, the influence of an acute peripheral immune challenge on neuroinflammatory responses in the early postnatal brain is not well characterized. To address this gap in knowledge, we evaluated the peripheral and central nervous system (CNS) immune responses to a mixed immune challenge in early postnatal rats of varying strains and sex.MethodsOn postnatal day 10 (P10), male and female Lewis and Brown Norway rats were injected intramuscularly with either a mix of bacterial and viral components in adjuvant, adjuvant-only, or saline. Immune responses were evaluated at 2 and 5 days post-challenge. Cytokine and chemokine levels were evaluated in serum and in multiple brain regions using a Luminex multiplex assay. Multi-factor ANOVAs were used to compare analyte levels across treatment groups within strain, sex, and day of sample collection. Numbers and activation status of astrocytes and microglia were also analyzed in the cortex and hippocampus by quantifying immunoreactivity for GFAP, IBA-1, and CD68 in fixed brain slices. Immunohistochemical data were analyzed using a mixed-model regression analysis.ResultsAcute peripheral immune challenge differentially altered cytokine and chemokine levels in the serum versus the brain. Within the brain, the cytokine and chemokine response varied between strains, sexes, and days post-challenge. Main findings included differences in T helper (Th) type cytokine responses in various brain regions, particularly the cortex, with respect to IL-4, IL-10, and IL-17 levels. Additionally, peripheral immune challenge altered GFAP and IBA-1 immunoreactivity in the brain in a strain- and sex-dependent manner.ConclusionsThese findings indicate that genetic background and sex influence the CNS response to an acute peripheral immune challenge during early postnatal development. Additionally, these data reinforce that the developmental time point during which the challenge occurs has a distinct effect on the activation of CNS-resident cells.

Sirtuin 3 regulates Foxo3a-mediated antioxidant pathway in microglia

Microglia are the prime cellular sources of reactive oxygen species (ROS) in the central nervous system (CNS). Chronic activation of microglia has been linked to aging-associated neurodegenerative diseases such as Alzheimer’s disease (AD) and Parkinson’s disease (PD) since they produce excessive amounts of ROS for a prolonged duration leading to oxidative stress. The present study was aimed at investigating the expression and role of Sirtuin 3 (Sirt3), a protein deacetylase which is implicated in regulating cellular ROS levels. It has been shown that Sirt3 reduces cellular ROS levels by deacetylating forkhead box O 3a (Foxo3a), a transcription factor which transactivates antioxidant genes, catalase (Cat) and manganese superoxide dismutase (mnSod).In the present study, Sirt3 immunoreactivity was localized in the ameboid microglial cells distributed in the corpus callosum (CC) of the early postnatal rat brain and diminished in the ramified microglial cells in the CC of the adult rat brain. A marked induction of Sirt3 expression was seen in lipopolysaccharide (LPS)-activated microglia in vivo and in vitro as well as in adult rat brains subjected to traumatic brain injury (TBI). Knockdown of Sirt3 in microglia led to an increase in the cellular and mitochondrial ROS and decrease in the expression of antioxidant, mnSod which is indicative of the function of Sirt3 in ROS regulation in microglia. Conversely, Sirt3 overexpression led to increase in the expression of antioxidants Cat and mnSod. Further, increase in the expression and nuclear translocation of Foxo3a was observed following Sirt3 overexpression, suggesting that Sirt3 regulates ROS by inducing the expression of antioxidants via activation of Foxo3a. The above results point to an antioxidant defense mechanism presented by Sirt3 through the activation of Foxo3a, in microglia.

The expression of non-clustered protocadherins in adult rat hippocampal formation and the connecting brain regions

Non-clustered protocadherins (PCDHs) are calcium-dependent adhesion molecules which have attracted attention for their possible roles in the neuronal circuit formation during development and their implications in the neurological disorders such as autism and mental retardation. Previously, we found that a subset of the non-clustered PCDHs exhibited circuit-dependent expression patterns in thalamo-cortical connections in early postnatal rat brain, but such patterns disappeared in adulthood. In this study, we identified that the non-clustered PCDHs showed differential expression patterns along the septotemporal axis in the subregions of adult hippocampus and dentate gyrus with topographical preferences. The expressions of PCDH1, PCDH9, PCDH10 and PCDH20 showed septal preferences, whereas the expressions of PCDH8, PCDH11, PCDH17 and PCDH19 showed temporal preferences, suggesting that they play roles in the formation/maintenance of intrahippocampal circuits. PCDHs also exhibited the region-specific expression patterns in the areas connected to hippocampal formation such as entorhinal cortex, lateral septum, and basolateral amygdaloid complex. Furthermore, the expression levels of three PCDHs (PCDH8, PCDH19 and PCDH20) were regulated by the electroconvulsive shock stimulation of the brain in the adult hippocampus and dentate gyrus. These results suggest that non-clustered PCDHs are involved in the maintenance and plasticity of adult hippocampal circuitry.

Amphetamine-Induced Changes in Dopamine Receptors in Early Postnatal Rat Brain

Amphetamines are among the most widely abused drugs. The user population includes a large proportion of women of child-bearing age. The early ontogeny of the axons in the neocortex and other neural structures positions them to influence the development and connectivity of non-aminergic dendrites and axons in these structures. A cascade of abnormalities in neural circuitry may result from the effects of amphetamines on the dopaminergic system. An attempt has been made to investigate the possible changes in the dopaminergic system in neonatal rats (a human third trimester equivalent model) following chronic D-amphetamine exposure. Neonatal rats were administered 5–15 mg/kg D-amphetamine subcutaneously daily from postnatal day 4 to day 10. Several parameters related to the dopaminergic system were measured. The results showed that tyrosine hydroxylase enzyme levels were significantly decreased in the prefrontal cortex, dorsal striatum and nucleus accumbens. Dopamine D1 receptor (DRD1) levels increased in the dorsal striatum whereas dopamine D2 receptor (DRD2) levels significantly decreased in both the prefrontal cortex and the dorsal striatum but significantly increased in the nucleus accumbens. In order to investigate whether these changes occurred at the transcriptional level, DRD1 and DRD2 mRNAs were detected. The results showed that DRD1 mRNA levels were significantly increased in the dorsal striatum whereas DRD2 mRNA levels were significantly increased in all three brain regions. These results indicate that early D-amphetamine exposure altered the dopaminergic system in the developing rat brain. This change may lead to abnormal perinatal stimulation that may yield long-term consequences.

Ndrg4 enhances NGF-induced ERK activation uncoupled with Elk-1 activation

Ndrg4 is expressed predominantly in the early postnatal rat brain and may be related to neural cell differentiation. PC12 cell lines stably expressing increased levels of Ndrg4 protein display enhanced NGF-induced phosphorylation of MEK and ERK. In contrast, the Ndrg4-C2-overexpressed PC12 cell lines showed attenuated NGF-promoted phosphorylation of Elk-1, which is a nuclear target of ERK. A reporter assay also indicated that Ndrg4-C2 suppresses Elk-1-mediated transcriptional activation and SRE reporter expression. The suppressive effect of Ndrg4-C2 on NGF-induced activation of Elk-1 was abolished by colchicine but not by cytochalasin D, suggesting that microtubules are involved in the reduced activation of Elk-1 by Ndrg4. Ndrg4 may play a role in supporting the activation of ERK and its target proteins needed for neuronal differentiation and in reducing the activation of Elk-1 implicated in cell growth.

Stimulation of Endocannabinoid Formation in Brain Slice Cultures through Activation of Group I Metabotropic Glutamate Receptors

Activation of group I metabotropic glutamate (mGlu) receptors drives the endocannabinoid system to cause both short- and long-term changes of synaptic strength in the striatum, hippocampus, and other brain areas. Although there is strong electrophysiological evidence for a role of endocannabinoid release in mGlu receptor-dependent plasticity, the identity of the endocannabinoid transmitter mediating this phenomenon remains undefined. In this study, we show that activation of group I mGlu receptors triggers the biosynthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG), but not anandamide, in primary cultures of corticostriatal and hippocampal slices prepared from early postnatal rat brain. Pharmacological studies suggest that 2-AG biosynthesis is initiated by activation of mGlu5 receptors, is catalyzed by phospholipase C (PLC) and 1,2-diacylglycerol lipase (DGL) activities, and is dependent on intracellular Ca2+ ions. Realtime polymerase chain reaction and immunostaining analyses indicate that DGL-beta is the predominant DGL isoform expressed in corticostriatal and hippocampal slices and that this enzyme is highly expressed in striatal neurons, where it is colocalized with PLC-beta1. The results suggest that 2-AG is a primary endocannabinoid mediator of mGlu receptor-dependent neuronal plasticity.

Inhibition of neurite outgrowth by reduced level of NDRG4 protein in antisense transfected PC12 cells

NDRG4, a member of the new NDRG gene family, was originally cloned as a gene that was expressed predominantly in the early postnatal rat brain. To determine whether the NDRG4 protein contributes to differentiation of neural cells, the effect of lowering the cellular NDRG4 protein level on the nerve growth factor (NGF)-induced neurite formations and transcription factor activations in PC12 cells was examined. An antisense construct of rat NDRG4 cDNA was made and transfected to PC12 cells, which constitutively express a basal level of the NDRG4 protein. Of the stably transfected antisense cell clones that expressed exogenous NDRG4 antisense RNA, six clones showed reduced levels of the NDRG4 protein, but unexpectedly two clones showed quite higher levels of NDRG4 protein than the control cells. The clones having decreased levels of the NDRG4 protein extended shorter neurites than control cells in response to NGF or dibutyryl cAMP. In contrast, the NDRG4 protein-highly expressing clones did not show suppressed neurite outgrowth induced by NGF. NGF-mediated activation of the transcription factor AP-1 was found to be suppressed in the NDRG4 protein-diminished clone and enhanced in the NDRG4 protein-upregulated clone as compared with those in the control cells. These results suggest that NDRG4 plays a role in neurite outgrowth and has an influence on an NGF-stimulated AP-1 activation by an undefined mechanism in PC12 cells.

Molecular Analysis of a Novel Protein, BDM1, Upregulated in Early Postnatal Rat Brain Development

Molecular Analysis of a Novel Protein, BDM1, Upregulated in Early Postnatal Rat Brain Development

Expression and coupling to polyphosphoinositide hydrolysis of group I metabotropic glutamate receptors in early postnatal and adult rat brain.

We investigated the expression and coupling to the phospholipase C signal transduction pathway of metabotropic glutamate receptor (mGluR) subtypes by Western blot analysis and agonist-stimulated inositol monophosphate formation in several brain regions of postnatal day 9 (P9) and adult rats. In the cerebral cortex, hippocampus, corpus striatum, olfactory bulb, cerebellum and hypothalamus, the expression level of mGluR5 was greater at P9 than in adulthood. The mGluR5 signal was very low or absent in the adult cerebellum and hypothalamus. The expression of mGluR1a was slightly greater at P9 in the hypothalamus, hippocampus and olfactory bulb, whereas it substantially increased with age in the cerebellum, and did not change in the cerebral cortex and corpus striatum. mGluR1b and -1c were nearly undetectable by Western blot analysis. The expression level of mGluR5, but not that of mGluR1a, was significantly correlated with the extent of phosphoinositide hydrolysis stimulated by mGluR agonists in slices prepared from these brain regions. The mGluR antagonist cyclopropan[b]chromen-1a-carboxylic acid ethylester (CPCCOEt), potently antagonized responses mediated by mGluR1, but much less potently those mediated by mGluR5a in recombinant cells. CPCCOEt, at a concentration which efficiently blocks mGluR1 responses, did not substantially affect the polyphosphoinositide response in hippocampal or cerebellar slices from newborn animals, and antagonized only a minor component of the polyphosphoinositide response in adult hippocampal slices. CPCCOEt, however, prevented the small stimulation of polyphosphoinositide hydrolysis by mGluR agonists in adult cerebellar slices. We conclude that (i) the efficient mGluR-mediated polyphosphoinositide hydrolysis in 9-day-old rats is mediated by mGluR5; (ii) the increased expression of mGluR1 in the adult cerebellum does not substitute for the decline of mGluR5 expression in the ability to mediate polyphosphoinositide hydrolysis; and therefore (iii) mGluR1a might couple less efficiently than mGluR5 to polyphosphoinositide hydrolysis.

Characterization of a 30-kDa Protein Expressed in Prenatal and Early Postnatal Rat Brain Cytosol

Characterization of a 30-kDa Protein Expressed in Prenatal and Early Postnatal Rat Brain Cytosol

Effects of Donor Age and Brain-Derived Neurotrophic Factor on the Survival of Dopaminergic Neurons and Axonal Growth in Postnatal Rat Nigrostriatal Cocultures

Early postnatal rat brain tissue can be grown for several weeks as organotypic slice cultures by the roller-tube method. We have here used this method to study the effects of donor age and brain-derived neurotrophic factor (BDNF) on the survival and growth of tyrosine hydroxylase immunoreactive (TH-i), dopaminergic (DA) neurons during the postnatal period when their nerve fibers normally innervate the striatal target. Tissue slices of ventral mesencephalon (VM) and striatum were prepared from newborn and 7-day-old rats and cocultured for 3–3.5 weeks with different combinations of the two donor ages. After immunocytochemical staining the number of TH-i, ventral mesencephalic neurons were counted, and the growth of TH-i fibers into the striatal part of the cocultures was evaluated. Co-cultures, with both VM and striatal slices prepared from newborn rats, contained a significantly higher number of TH-i neurons and displayed a significantly increased innervation of the striatal slices compared with other combinations of donor ages. Addition of BDNF resulted in both an increased survival of TH-i neurons and an increased growth of TH-i fibers into the cocultured striatal slices. Significant neurotrophic effect of BDNF did, however, require young donor age of both VM and striatal slices. It is suggested that BDNF induces more cells, possibly progenitor cells, to express TH immunoreactivity. Alternatively BDNF may suppress apoptotic cell death documented by others to occur in the postnatal rat substantia nigra pars compacta. Irrespective of the mechanisms, survival of more TH-i neurons was related to an increased innervation of the striatal slices by TH-i nerve fibers. The observed effects of BDNF on both survival and fiber growth of TH-i neurons indicate a potential role of BDNF for treatment of Parkinson's disease or grafts of immature DA neurons transplanted to patients with Parkinson's disease. A significant trophic effect of BDNF did, however, seem to depend on young developmental age of both striatum and VM. Parallel treatment with striatal neurotrophic factors may therefore be a necessary prerequisite to a trophic effect of BDNF under clinical conditions.

Fatty acid composition of late embryonic and early postnatal rat brain

The fatty acid (FA) composition and distribution in a variety of phospholipids (PL) and neutral lipids (NL) at two discrete stages during the embryonic rat brain development were investigated. Over 96% of the FA were acylated into fetal brain PL at embryonic day 17 after the peak of neuronal proliferation and at embryonic day 20, one day prior to delivery. Phosphatidylcholine constituted approximately 60% of the total PL pool, phosphatidylethanolamine (PE) 30%, phosphatidylserine (PS) 6%, and phosphatidylinositol (PI) 4%. The diacylglycerols and triacylglycerols constituted 1-2% of the fetal brain lipids. alpha-Linolenic acid (18:3n-3) and linoleic acid (18:2n-6) were found in very low amounts in all fetal brain PL and NL. The percentage of the n-6 polyunsaturated FA, consisting of arachidonic acid (AA), 22:4n-6 and 22:5n-6, remained unchanged in all the fractions, except in PI, in which the proportion of AA increased. The concentration of docosahexaenoic acid (DHA) increased with age in all the fractions, with the bulk of accumulation accounted for by its increase in PE and, to a lesser extent, in PS. This finding suggests a "DHA accretion spurt" during the last three days of pregnancy.

Transient expression of acetylcholinesterase messenger RNA and enzyme activity in developing rat thalamus studied by quantitative histochemistry and in situ hybridization.

The molecular basis for transient expression of acetylcholinesterase in noncholinergic regions of the early postnatal rat brain was studied by in situ hybridization histochemistry. A 33P-labelled 63-mer DNA oligonucleotide was used to probe acetylcholinesterase messenger RNA in the brains of rat pups at one, two, six, nine, 12, 16 and 21 days of age (birth = day 0). Cryostat brain-sections were hybridized with probe and exposed to X-ray film or emulsion coatings. Acetylcholinesterase messenger RNA was quantitated by counting silver grains and by measuring X-ray film density with video imaging and computer-based densitometry. Adjacent sections were stained histochemically for acetylcholinesterase activity, also quantitated by video densitometry. Overall there was a significant correlation between apparent levels of acetylcholinesterase activity and acetylcholinesterase messenger RNA. Increases in message tended to accompany the surges of acetylcholinesterase activity that marked the maturation of thalamocortical sensory relay pathways. Acetylcholinesterase expression in the youngest rats was generally sparse but it increased markedly during the first postnatal week, especially in the sensory relay nuclei of the thalamus. Levels of message and enzyme activity in the medial and dorsolateral geniculate and the ventral posteromedial and ventral posterolateral nuclei rose to a peak, typically about day 9. Beyond this time there was a gradual decline. By day 21 the staining and in situ hybridization patterns resembled those in adult brains, whose thalamic relay nuclei are impoverished in acetylcholinesterase activity and messenger RNA. Thus, acetylcholinesterase expression is strongly modulated in certain thalamic systems as they undergo neural morphogenesis.

Neurons express ciliary neurotrophic factor mRNA in the early postnatal and adult rat brain

The regional and subcellular localization in the central nervous system (CNS) of postnatal day 5, day 15, and adult rats of ciliary neurotrophic factor (CNTF) mRNA was examined by in situ hybridization with biotinylated riboprobes. Probe specificity was determined by Northern blot analysis of poly(A)+ RNA extracted from adult rat brain using digoxigenin labeled riboprobes and chemiluminescent detection. Both a 4 kb and a 1.2 kb transcript were detected in the cortex and brainstem. In situ hybridization revealed that CNTF mRNA was widely distributed in neurons and glia throughout the CNS at each of the developmental time points. The density of the neuronal hybridization signal was found to be greater in neuronal nuclei than in their cytoplasm. In the nucleus of most neurons, CNTF mRNA distribution was concentrated in a perinucleolar fashion. Alternate sections from the same animals, which were incubated with a specific polyclonal antibody against a CNTF peptide fragment, revealed that both neurons and glia in postnatal day 5, day 15, and adult rat brain were immunoreactive for CNTF.

Exposure of neonatal rats to alcohol by vapor inhalation demonstrates specificity of microcephaly and Purkinje cell loss but not astrogliosis.

The artificial rearing model (AR) of fetal alcohol syndrome (FAS) has been shown to induce several major pathologies in the early postnatal rat brain development: microcephaly, selective neuronal cell loss, and activation of astroglia in the neocortex. The purpose of this study was to determine whether these pathologies were specific to the action of alcohol or, in contrast, could result from confounds attributed to this model of FAS. For this purpose, the pathological effects of AR were compared with those of a vapor inhalation (VI) model of FAS. Our studies showed that the microcephaly that developed after exposure to periodic blood alcohol levels (BALs) of 300-350 mg% during postnatal days 4-9 could be achieved by both AR and IV models of FAS, and thus is independent of the method of alcohol administration. In contrast, the gliosis measured by glial fibrillary acidic protein (GFAP) mRNA levels in cortex, as well as by immunohistochemical staining for GFAP, was found only in the AR-FAS model, but not in the VI model. However, the lack of gliosis in VI was apparently not due to a less intrusive intervention of alcohol, because VI exposure resulted in a reduction in Purkinje cell number comparable with that found after AR or intragastric intubation of alcohol. Based on these observations, we conclude that the activation of gliosis observed after AR is not a specific effect of alcohol, but rather is caused by an interaction of alcohol with as yet unidentified factors present in AR.

Induction of major histocompatibility complex class II antigen on amoeboid microglial cells in early postnatal rats following intraperitoneal injections of lipopolysaccharide or interferon-γ

The induction of major histocompatibility complex (MHC) class II antigen on amoeboid microglia cells (AMC) by lipopolysaccharide (LPS) and interferon-γ (INF-γ) in early postnatal rat brain was studied by immunohistochemistry. In newborn rats given successive intraperitoneal injections of LPS or INF-γ and killed at the age of 7 days, MHC class II antigen expressing AMC were consistently present in specific areas throughout the entire brain, notably in the subcortical white matter and circumventricular region. It is concluded from this study that the induction of MHC class II antigen on AMC by LPS or INF-γ is a widespread phenomenon in the developing brain. Since MHC class II antigen is essential for the initiation of immune response, it is suggested that besides their phagocytic nature, AMC may also be involved in immunological processes in the developing brain.

Immunocytochemical demonstration of developmental distribution of muscarinic acetylcholine receptors in rat parietal cortex

The present investigation reveals many cortical neurons immunopositive for M35, the monoclonal antibody raised against purified muscarinic acetylcholine receptor (mAChR) proteins, in the early postnatal rat brain. The ontogeny of mAChR expression, exemplified on the parietal neocortex, was studied in a series of rat pups from postnatal days (PD) 1, 3, 7, 14 and 21. Immunoprecipitation in the parietal somatosensory cortex was manifest in the population of pyramidal neurons during postnatal development. In particular during the early postnatal ages, until 2 weeks after birth, M35 immunoreactivity (M35-ir) was present in all neuronal compartments, indicating transportation of mAChR protein in axonal and dendritic processes as observed in light and electron microscopic analysis. The immunoprecipitation in the apical dendrites yielded dense labeling in layer 1 where the distal processes of the pyramidal dendrites branched extensively forming a plexus that intermingled with horizontal fibers in this superficial layer. At PD21, immunolabeling in layer 1 and in axons of pyramidal cells was reduced compared to earlier ages suggesting a transient expression of mAChRs in these neuronal structures. The development of M35-ir in the cortex appeared to antedate that of its cholinergic afferentation as indicated by AChE histochemical study.

Four repeat MAP2 isoforms in human and rat brain

In mammalian brain, variations in the primary structure of the characterized microtubule-associated protein 2 (MAP2) isoforms have only been observed in their projection domains whose length determines the spacing between neighboring microtubules. We now report that, as with MAP4 and tau, MAP2 isoforms containing four (4R) instead of three (3R) tandem repeats in their microtubule binding domains do exist in human and rat brain. The additional sequence, inserted between the first and second repeat of the 3R-MAP2 messages, appears on mRNAs encoding both high and low molecular weight (Hwt and Lwt) rat MAP2 variants. In contrast to the corresponding 3R-messages, 4R-Hwt MAP2 mRNA concentrations decrease during early postnatal rat brain development, while the amount of 4R-Lwt MAP2 messages remains constant. In general, 4R-/3R-MAP2 mRNA ratios appear to be low with the highest levels of 4R-messages found in the cerebellum.

Expression of major histocompatibility complex class II antigen on amoeboid microglial cells in early postnatal rat brain following intraperitoneal injections of lipopolysaccharide.

In rats given two single intraperitoneal injections of lipopolysaccharide (LPS) at 1 and 4 days of age and killed at 7 days of age, 11.5--12% of amoeboid microglial cells (AMC) in the supraventricular corpus callosum were induced to express major histocompatibility complex (MHC) class II antigen, as detected with monoclonal antibody OX-6. The MHC class II antigen induced was colocalized with MHC class I antigen and type 3 complement receptors on the same cells. The expression of MHC class II antigen on the plasma membrane of AMC was confirmed in immunoelectron microscopy. Although OX-6-positive AMC often assumed a perivascular position, the majority of them, however, were far removed from the blood vessels. The cytoplasmic processes of the perivascular OX-6-positive AMC appeared to rest directly on the vascular lamina, and in some section profiles they were in contact with a large surface area of the outer wall of small blood vessels. It is concluded from this study that although MHC class II antigen is not constitutively present on AMC, it is, however, inducible under stimulation with LPS. It is, therefore, suggested that the OX-6-positive AMC, especially the perivascular AMC, may have the potentiality to function as antigen-presenting cells in the developing brain when challenged by LPS.