Deficient neurogenesis in forebrain-specific presenilin-1 knockout mice is associated with reduced clearance of hippocampal memory traces

To examine the in vivo function of presenilin-1 (PS1), we selectively deleted the PS1 gene in excitatory neurons of the adult mouse forebrain. These conditional knockout mice were viable and grew normally, but they exhibited a pronounced deficiency in enrichment-induced neurogenesis in the dentate gyrus. This reduction in neurogenesis did not result in appreciable learning deficits, indicating that addition of new neurons is not required for memory formation. However, our postlearning enrichment experiments lead us to postulate that adult dentate neurogenesis may play a role in the periodic clearance of outdated hippocampal memory traces after cortical memory consolidation, thereby ensuring that the hippocampus is continuously available to process new memories. A chronic, abnormal clearance process in the hippocampus may conceivably lead to memory disorders in the mammalian brain.

Neurogenesis in the subventricular zone and rostral migratory stream of the neonatal and adult primate forebrain

Throughout life, the anterior part of the postnatal rodent subventricular zone (SVZa), surrounding the lateral ventricles, contains a prolific source of neuronal progenitor cells that retain their capacity to concurrently generate neurons and migrate along the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into interneurons. This study was designed to determine whether the SVZ and RMS of the postnatal primate also harbor a specialized population of neuronal progenitors with the capacity to divide while they migrate. In order to reveal the spatial-temporal changes in the distribution and composition of the neuronal progenitor cells in the primate SVZ and RMS, seven rhesus monkeys, ranging in age from 2 days to 8 years, were given a single injection of the cell proliferation marker bromodeoxyuridine (BrdU) 3 h before they were perfused. The phenotypic identity of the BrdU(+) cells was revealed by double labeling sagittal sections with cell type-specific markers. From birth onward the distribution of BrdU(+) cells with a neuronal phenotype is extensive and largely overlapping with that of the rodent. Similar to the rodent brain the neuronal progenitors are most numerous in neonates. The BrdU(+) neurons in the primate forebrain extend lateral and ventral to the lateral ventricle and all along the RMS. The cytoarchitectonic arrangement and appearance of the neuronal progenitor cells is quite varied in the primate compared to the rodent; in some locations the cells are aligned in parallel arrays resembling the neuronal chains of the adult rodent RMS, whereas in other positions the cells have a homogeneous "honeycomb" arrangement. The chains are progressively more pervasive in older primates. Akin to the RMS of adult rodents, in the primate SVZ and RMS the astrocytes often form long tubes enveloping the chains of neuronal progenitors. Our study demonstrates that the primate forebrain, similar to the rodent forebrain, harbors a specialized population of mitotically active neuronal progenitor cells that undergo extensive rearrangements while continuing to proliferate throughout life.

Postnatal development of cholinergic system in mouse basal forebrain: acetylcholinesterase histochemistry and choline-acetyltransferase immunoreactivity

The distribution of acetylcholinesterase histochemistry and choline-O-acetyltransferase immunohistochemistry in the basal forebrain was studied in newborn mice (P0) and until 60 days of postnatal life (P60). A weak acetylcholinesterase activity was found at P0 and P2 in the anterior and intermediate parts of the basal forebrain, and higher in the posterior region. The intensity of labeling, neuronal size and dendritic growth seems to increase progressively in all regions of basal forebrain from P4 to P10. The AChE+ cell count shows that in the anterior portion of the magnocellular basal nucleus the number of cells does not vary significantly from birth to the second month of postnatal life. However, in the intermediate and posterior portions of the nucleus the mean number of labeled cells increases significantly from birth to the end of the second week of postnatal life (P13). The choline-acetyltransferase immunoreactivity appears only detectable at the end of the first week (P6) as a slight immunoreaction, which increases progressively in intensity at P8, and at P10 seems to attain the same intensity of labeling found at P60. These results seem to indicate that the acetylcholinesterase could have a non-classic cholinergic role in the first stages of postnatal development, acting as a growth and cellular differentiation factor.

Pattern of expression of the tetraspanin Tspan-5 during brain development in the mouse

Here, we report the pattern of expression of the tetraspanin mTspan-5 at various developmental stages. Expression was first seen at E10 and remained until adulthood, with increased levels between P0 and P5. mTspan-5 showed high expression in the cortical areas and Purkinje cells throughout development. Moreover, transcripts were also detected in the developing heart, forelimbs, and hindlimbs.

Ontogenesis of neurons immunoreactive for nitric oxide synthase in rat forebrain and midbrain

We studied the immunohistochemical localization of neuronal nitric oxide synthase (nNOS) in the developing rat brain on embryonic days 13 (E13), 15 (E15), 17 (E17) and 19 (E19) and postnatal days 0 (P0), 7 (P7) and 14 (P14). A few neurons positive for nNOS were first detected at E15 in the hypothalamus and pons. At E17, many positive cells became detectable in the thalamus. At E19, the positive cells in these three regions were rapidly increased in number, and a few positive neurons were also observed in such regions as the cerebral cortex and striatum. Positive cells in the hypothalamus tended to locate ventrolaterally. Positive neurons, stained very intensely as in adult rats, were seen in the pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus and parafascicular nucleus. Two weeks after birth, positive neurons of larger somata with many processes were distributed widely in the cerebral cortex and hippocampus. The present study indicates that, in the forebrain and midbrain, the distribution pattern of nNOS-containing neurons is fundamentally completed by E19.

Cell proliferation in the post-natal and adult mammalian central nervous system

In the mammalian central nervous system cell proliferation is generally linked to developmental processes that are ultimated in the perinatal period. Few exceptions to this rule are known in certain regions of the mammalian brain, namely the post-natal cerebellar cortex and the adult subependymal layer. We report here the results of our studies about cell proliferation and related phenomena in these regions. Cell proliferation was visualised after bromodeoxyuridine incorporation and labeling of the proliferating cell nuclear antigen (PCNA), an endogenous protein expressed during the cell cycle. The occurrence of programmed cell death in the post-natal cerebellar cortex and the persistence of the embryonic isoform of neural cell adhesion molecule (NCAM) associated with proliferating cells in the adult subependymal layer were also investigated.

Differential palmitoylation of two mouse glutamate receptor interacting protein 1 forms with different N-terminal sequences

Glutamate receptor interacting protein (GRIP) is a member of the PDZ domain-containing protein family that is localized in the postsynaptic density area. This protein has been reported to interact specifically with the C-termini of AMPA-selective glutamate receptor channel subunits, GluRalpha2 and GluRalpha3 through its PDZ domains. To clarify the physiological functions of GRIP, we cloned mouse GRIP1, and found that there are three sites for alternative splicing and two putative translational start codons by characterizing GRIP1 cDNA clones and reverse transcription-polymerase chain reaction products. Metabolic labeling of COS-7 cells expressing two N-terminal GRIP1 proteins demonstrated that these proteins differed in their pattern of palmitoylation. These findings suggested that the molecular diversity of GRIP1 underlies the localization and functional heterogeneity of this protein.

Postnatal cell proliferation and death in a lateralized, gender-related, asymmetric nucleus

Sexual differentiation and lateralization of neurone number in a discrete forebrain nucleus (SDApc) related to masculine vocal emission, occur contemporaneously in postnatal (P0-P15) gerbils. Stereological estimates of cell proliferation and death during SDApc organization were made by BrdU labelling and pyknosis, respectively. Results confirmed that rates of apoptosis were greater in females and lateralized in males. Immunoreactive BrdU cells, located in the SDApc at P0-P6, with low levels at P15, were not numerically different between the sexes. Only at one age, P0, in males, was a left-right difference seen in BrdU-immunoreactive cell numbers. Microglia, identified by isolectin immunostaining, were numerically similar to BrdU cells. We suggest that apoptosis, rather than neurogenesis, differentiates and lateralizes SDApc organization, and proliferating cells are microglia, phagocytosing debris.

Early Auditory Experience Induces Frequency-Specific, Adaptive Plasticity in the Forebrain Gaze Fields of the Barn Owl

Binaural acoustic cues such as interaural time and level differences (ITDs and ILDs) are used by many species to determine the locations of sound sources. The relationship between cue values and locations in space is frequency dependent and varies from individual to individual. In the current study, we tested the capacity of neurons in the forebrain localization pathway of the barn owl to adjust their tuning for binaural cues in a frequency-dependent manner in response to auditory experience. Auditory experience was altered by raising young owls with a passive acoustic filtering device that caused frequency-dependent changes in ITD and ILD. Extracellular recordings were made in normal and device-reared owls to characterize frequency-specific ITD and ILD tuning in the auditory archistriatum (AAr), an output structure of the forebrain localization pathway. In device-reared owls, individual sites in the AAr exhibited highly abnormal, frequency-dependent variations in ITD tuning, and across the population of sampled sites, there were frequency-dependent shifts in the representation of ITD. These changes were in a direction that compensated for the acoustic effects of the device on ITD and therefore tended to restore a normal representation of auditory space. Although ILD tuning was degraded relative to normal at many sites in the AAr of device-reared owls, the representation of frequency-specific ILDs across the population of sampled sites was shifted in the adaptive direction. These results demonstrate that early auditory experience shapes the representation of binaural cues in the forebrain localization pathway in an adaptive, frequency-dependent manner.

Development and distribution of FMRFamide-like immunoreactivity in the toad (Bufo bufo) brain

By using immunohistochemistry, we studied the development and distribution of the FMRFamide-like immunoreactive (ir) neuronal system in the toad brain during the ontogeny. In addition to this, experimental evidence was provided to show that the rostral forebrain-located FMRFamide neurons originate in the olfactory placode and then migrate into the brain along the olfactory pathway. During early development, within the brain, FMRFamide-ir perikarya first appeared in the periventricular hypothalamus. Later in development, FMRFamide-ir cells were visualized in the rostralmost forebrain simultaneously with similar ir cells in the developing olfactory mucosa. Selective ablation of the olfactory placode(s), prior to the appearance of the first FMRFamide-ir cells in the brain, resulted in the total absence of ir cells in the telencephalon (medial septum and mediobasal telencephalon) of the operated sides(s). The preoptic-suprachiasmatic-infundibular hypothalamus-located FMRFamide-ir neurons were not affected by olfactory placodectomy, arguing that they do not originate in the placode. This result points to the placode as the sole source of such neurons in the rostral forebrain.

Ontogeny of brain-derived neurotrophic factor gene expression in the forebrain of prairie and montane voles

Brain-derived neurotrophic factor (BDNF) plays an important role in normal brain development. In the present study, we examined the ontogenetic pattern of BDNF gene expression in both monogamous prairie voles (Microtus ochrogaster) and promiscuous montane voles (M. montanus); two closely related microtine rodents that differ in life strategy and social behavior. In both species, BDNF mRNA showed an early appearance and a transient expression in a regionally specific manner. In the dentate gyrus and CA3 region of the hippocampus, BDNF mRNA was found neonatally, increased gradually during development, and reached a peak at weaning, followed by a subsequent decline to the adult level. In the paraventricular nucleus of the hypothalamus, levels of BDNF mRNA persisted until weaning, followed by a significant increase to the adult levels at 3 months of age. BDNF mRNA also demonstrated a species-specific developmental pattern. In the cingulate cortex, BDNF mRNA labeling displayed a transient increase in the second and third postnatal weeks followed by a subsequent decrease to the adult level in prairie voles, but persisted throughout the course of development in montane voles. In general, montane voles achieved an adult pattern of BDNF mRNA expression earlier than did prairie voles. Together, these data indicate that BDNF may function differently in infant and adult brains, and that the two species of voles differ in the ontogenetic pattern of BDNF mRNA expression in a regional-specific manner, which may be associated with their different life strategy and brain and behavioral development.

A unique role for Fyn in CNS myelination

We analyzed the role of Fyn tyrosine kinase in CNS myelination by using fyn(-/-) null mutant mice, which express no Fyn protein. We found a severe myelin deficit in forebrain at all ages from 14 d to 1 year. The deficit was maximal at 1 month of age and was similar regardless of mouse strain background or whether it was determined by bulk isolation of myelin or by quantitation of myelin basic protein. To determine the cellular basis of the myelin deficit, we counted oligodendrocytes in tissue sections of mice expressing oligodendrocyte-targeted beta-galactosidase, and we used light and electron microscopy to examine the number and morphology of myelinated fibers and size of myelinated CNS structures. All of these parameters were reduced in fyn(-/-) mice. Unexpectedly, there were regional differences in the myelin deficit; in contrast to forebrain, fyn(-/-) cervical spinal cord exhibited no reduction in myelin content, number of oligodendrocytes, or number of myelinated fibers, nor was myelination delayed developmentally. We found that oligodendrocytes express Src, but there was no significant reduction of myelin content in null mutants lacking the Fyn-related kinases Src, Yes, or Lyn. Finally, we investigated the molecular features of Fyn that are required for myelination and found that a single amino acid substitution, which abolishes the tyrosine kinase activity of Fyn, resulted in a myelin deficit as great as that observed in the complete absence of Fyn protein. These results demonstrate that Fyn plays a unique role in myelination, one that requires its kinase activity.

Does senile impairment of cholinergic system in rats concern only disturbances in cholinergic phenotype or the progressive degeneration of neuronal cell bodies?

The trophic effect of continuous intraventricular infusion of nerve growth factor (NGF) on morphology of the basal forebrain (BF) cholinergic neurons was tested in 4- and 28-month-old male Wistar rats. All studies were conducted using behaviorally uncharacterized animals from the same breeding colony. Immunohistochemical procedure for choline acetyltransferase (ChAT) and p75NTR receptor has been applied to identify cholinergic cells in the structures of basal forebrain (BF). Using a quantitative image analyzer, morphometric and densitometric parameters of ChAT- and p75NTR-positive cells were measured immediately after cessation of NGF infusion. In 28-month-old non-treated rats the number of intensively ChAT-positive cells in all forebrain structures was reduced by 50-70% as compared with young animals. The remaining ChAT-positive cells appeared shrunken and the neuropil staining was NTR markedly reduced. In contrast, the same neurons when stained for p75 were numerous and distinctly visible with perfect morphology. Analysis of Nissl stained sections also showed that 28-month-old rats did not display significant losses of neuronal cell bodies. NGF restored the number of intensely stained ChAT-positive cells to about 90% of that for young controls and caused a significant increase in size of those cells in 28-month-old rats as compared with the control, age-matched group. NGF did not influence the morphology of p75NTR-positive neurons, which were well labeled, irrespective of treatment and age of the rats. In 4-month-old rats, NGF infusion decreased the intensity of both ChAT and p75NTR immunostaining. These data provide some evidence for preservation of BF cholinergic neurons from atrophy during aging and indicate that senile impairment of the cholinergic system in rats concerns decrease in ChAT-protein expression rather than an acute degeneration of neuronal cell bodies. Treatment with NGF resulted in restoration of cholinergic phenotype in the BF neurons of aged rats. However, the present study also rises issue of possible detrimental effects of NGF in young normal animals.

Stereological analysis of regional brain volumes and neuron numbers in rats displaying a spontaneous hydrocephalic condition

Stereological methods were employed to investigate a novel spontaneously occurring brain mutation in an inbred colony of Wistar rats. These mutants displayed changes (enlarged cerebral ventricles and malformed hippocampi) similar to those seen in H-Tx hydrocephalic rats. Mutant and control rats were studied at three postnatal ages: 4, 7, and 13 weeks. Brain weight in the mutant animals was significantly (P < 0.05) increased when compared to age-matched controls. Using systematic random sampling and the Cavalieri principle we estimated the volumes of various brain compartments, including the cerebral ventricles, forebrain, and cerebral cortex. We found that ventricular volume (P < 0.001) and forebrain volume (P < 0.05) were significantly increased in mutant rats when compared to control rats. Total numbers of nucleoli, estimated using the physical fractionator, were obtained for neurons in the cerebral cortex and granule cells in the dentate gyrus. Numbers were not altered significantly in mutant rats. Nor were mean soma volumes as estimated from total volumes and numbers. The changes in brain and ventricle volumes provide quantitative evidence that these animals display a hydrocephalic condition. This condition appears not to compromise cell number or mean soma size in the brain regions examined.

Migration pathways and behavior of glial progenitors in the postnatal forebrain

The postnatal subventricular zone (SVZ) gives rise to many of the glial cells in the forebrain. We investigated migration pathways and dynamics of motility of progenitors from the neonatal rat forebrain SVZ by labeling progenitors in vivo with a retrovirus encoding green fluorescent protein (GFP) and then visualizing the dynamics of their movements by time-lapse fluorescence microscopy in slice preparations. Cells within the dorso-lateral SVZ moved in an apparently undirected fashion, but migrated in a directed manner after emigration into white matter and cortex, displaying both radial and tangential migration. Cells in the striatal-SVZ, a region of SVZ along the lateral wall of the ventricle, migrated parallel to the ventricular surface, and entered the striatum, where they migrated both perpendicular and parallel to the ventricular surface. Sometimes, cells in all these regions reversed their migration back toward the SVZ. Migration involved either elongation of the leading process followed by a quick translocation of the nucleus or a synchronous advancement of the nucleus and the leading process. Two distinct patterns of cellular changes were observed at orthogonal turning: one involves the cessation of cell body movement and the formation of a new leading process, and the other involves continuous cell body movement and bending of the leading process. The dynamic behavior of progenitors may reflect local tissue architecture and contribute to the widespread distribution of glia.

Genetic control of regional identity in the developing vertebrate forebrain

In the past we isolated and characterized a number of vertebrate homeobox genes expressed in the developing brain. In particular, Emx1 and Emx2 are expressed in the developing forebrain of mouse embryos, in a region including the presumptive cerebral cortex. In the developing cerebral cortex, Emx1 is expressed in most neuroblasts and neurons at all stages of development, whereas Exm2 expression is restricted to proliferating neuroblasts of the so-called ventricular zone and to Cajal-Retzius cells, but is undetectable in most postmitotic cortical neurons. It is conceivable to hypothesize that Emx2 plays a role in the control of proliferation of cortical neuroblasts and in the regulation of their subsequent migration. This latter process has been recently analysed in some detail in null mutant mice. The expression of these and other genes has also been analysed in the developing brain of different species of vertebrates. Homologies between forebrain subdivisions have been proposed based on the conservation and divergence of gene expression patterns.

Late postnatal reorganization of GABA(A) receptor signalling in native GnRH neurons

The molecular and cellular characteristics of the gonadotropin-releasing hormone (GnRH) neurons have been difficult to ascertain due to their scattered distribution within the basal forebrain. Using morphological criteria coupled with single cell RT-PCR postidentification, we have developed a method for investigating native GnRH neurons in the mouse brain and used it to examine the development of GABA(A) receptor signalling in this phenotype. Following the harvesting of the cytoplasmic contents of individual GnRH neurons, single cell multiplex RT-PCR experiments demonstrated that GABAA receptor alpha1-5, beta1-3 and gamma2 & 3 subunit transcripts were expressed by both neonatal (postnatal day 5) and juvenile (day 15-20) GnRH neurons in a heterogeneous manner. Following puberty, this profile was reduced to a predominant alpha1, alpha5, beta1, gamma2 subunit complement in rostral preoptic area GnRH neurons of the adult female. Whole-cell patch-clamp recordings revealed little difference between juvenile and adult GnRH neurons in their resting membrane potential and spontaneous firing rates. All GnRH neurons were found to be subjected to a tetrodotoxin-insensitive, tonic GABAergic barrage signalling through the GABA(A) receptor. However, marked heterogeneity in the sensitivity of individual juvenile GnRH neurons to GABA was revealed and, in parallel with the change in subunit mRNA profile, this was dramatically reduced in the reproductively competent adult GnRH neurons. These findings provide the first electrical and molecular characterization of the GnRH phenotype and demonstrate a novel pattern of late postnatal reorganization of native GABA(A) receptor gene expression and signalling in the GnRH neuronal population.

Post-transcriptional regulation of zenk expression associated with zebra finch vocal development

In the male zebra finch, highly variable juvenile song and stereotyped adult song induce mRNA expression of the immediate early gene zenk in telencephalon. However, the functional consequences of this behavior-driven gene expression remain unknown. Here we characterize the developmental expression of zenk mRNA and protein in two forebrain song regions (HVC, the higher vocal center, and RA, the robust nucleus of the archistriatum). In HVC, singing results in similar percentages of cells producing zenk mRNA and zenk protein at different stages of vocal development. Similarly, song behavior at all stages of vocal development induces a comparable percentage of RA cells expressing zenk mRNA. However, the percentage of RA zenk immunoreactive cells is low during early vocal learning, increasing only as the vocal pattern matures. Early induction of a stereotyped vocal pattern in juvenile birds is associated with increased zenk immunoreactivity in RA, indicating that it is the form of the behavior (and not the age of the bird) that correlates with changes in zenk immunoreactivity. Together, our findings reveal a previously unrecognized relationship between behavioral development and post-transcriptional gene regulation.

Expression of "suppressor of cytokine signalling" (SOCS) genes in the developing and adult mouse nervous system

Growth factor and cytokine signalling in the developing nervous system has multiple effects, ranging from cell differentiation and cell survival to modulation of cell phenotype. Molecules that can regulate growth factor signalling pathways will therefore be of importance in determining the cellular response to factor stimulation. Members of a recently described gene family, the suppressor of cytokine signalling (SOCS) family, can regulate signalling events downstream of predominantly cytokine stimulation and may have important roles in the nervous system. We have examined the temporal and spatial expression of SOCS-1, SOCS-2, and SOCS-3 in the developing and adult nervous system by use of Northern analysis and in situ hybridisation. All three genes were expressed in the brain, with maximal expression from embryonic day 14 to postnatal day 8 and declining thereafter, with SOCS-2 being the most highly expressed. In situ hybridisation analysis showed that SOCS-1 and SOCS-3 had a low and widespread pattern of expression, whereas SOCS-2 expression was higher and tightly regulated. Its expression pattern indicated that SOCS-2 was expressed exclusively in neurons and that it was switched on developmentally at the time of neuronal differentiation.

SEK1/MKK4, c-Jun and NFKappaB are differentially activated in forebrain neurons during postnatal development and injury in both control and p75NGFR-deficient mice

The common neurotrophin receptor (p75NGFR) can signal in vitro through activation of the c-Jun N-terminal kinase (JNK) pathway and nuclear translocation of NFKappaB. Activation of JNK and its substrate c-Jun can lead to apoptosis. We investigated these activities in vivo by comparing immunoreactivity for phosphorylated(p) SEK-1 (or MKK4, which activates JNK), c-Jun (ser63, ser73) and nuclear translocation of NFKappaB-p50 in tissue sections through the forebrain of control and p75NGFR-deficient mice. During postnatal development, SEK1p-immunoreactivity was detectable in p75NGFR-positive cholinergic neurons and p75NGFR-negative neurons throughout the forebrain in control mice. During development, few cells contained c-Junp, although many neurons contained c-Jun. No obvious c-Jun immunostaining was present in the adult forebrain. At any age, NFKappaB-p50 immunoreactivity was seen in nuclei of most cells throughout the forebrain. Following fimbria fornix transection in adult mice, few basal forebrain neurons contained SEK1p while many axotomized choline acetyltransferase (ChAT)-positive neurons contained c-Junp and nuclear NFKappaB-p50. The immunostaining patterns of SEK1p, c-Junp and NFKB during development and following injury were largely similar in p75NGFR-deficient mice. During development, cells throughout the forebrain had TdT-mediated dUTP-biotin nick end labelling (TUNEL)-labelling (a potential marker for apoptosis), however, their presence was not predicted by number of neurons stained for SEK1p or c-Junp. These results suggest that the expected activation of the JNK pathway by p75NGFR, as well as the expected relationship between SEK1 and downstream activation of c-Jun do not occur in the mammalian forebrain. Also, these results suggest that this activation does not necessarily lead to cell death.

Effects of nitric oxide release in an area of the chick forebrain which is essential for early learning

Extracellular recording techniques were used to study the effects of the nitric oxide releasing agents diethylamine-NO (DEA-NO) and S-nitroso-N-acetyl-penicillamine (SNAP) on synaptic transmission in the intermediate and medial part of the hyperstriatum ventrale (IMHV), a part of the domestic chick forebrain that is essential for some forms of early learning. The field response evoked by local electrical stimulation was recorded in the IMHV in an in vitro slice preparation. DEA-NO (100-200 mgr) significantly depressed the field response in a concentration dependent and reversible manner. However, the depression produced by perfusion with 400 mgr DEA-NO, was not reversed following washout of the drug. With 400 mgr DEA-NO, NO reaches a maximum concentration of 10 mgr at 2 min of perfusion, and then declines slowly. SNAP (400 mgr) produced an effect similar to 400 mgr DEA-NO. Neither the immediate nor the longer-term depressive effect of NO is mediated by activation of guanylyl cyclase because in the presence of both low and high doses of ODQ, a potent and selective inhibitor of NO-stimulated guanylyl cyclase, NO produced the same depression of the field response. There is evidence however that the IMHV possesses c-GMP responsive elements since direct perfusion of 8-Br-cGMP (1 mM) produced a long-term but not an immediate depression. The long-term depression produced by 400 mgr DEA-NO was eliminated in the presence of either a selective adenosine A(1) receptor antagonist or an ADP-ribosyltransferase inhibitor. It was also possible to prevent the long-term effect in the presence of tetraethyl ammonium a K(+)-channel blocker. These results suggest that the NO may be acting presynaptically in a synergistic fashion with the adenosine A(1) receptor to depress transmitter release.

Transcription factor activator protein-2 is required for continued luteinizing hormone-releasing hormone expression in the forebrain of developing mice

LHRH is the neuropeptide responsible for reproductive function. Prenatally, LHRH expression begins when neurons are in the olfactory pit and continues as these cells migrate into the brain. Thus, LHRH neurons maintain neuropeptide expression through very distinct environments. The regulatory interactions that control onset and continued expression of the LHRH phenotype are unknown. To begin to address this question primary LHRH neurons were removed from nasal explants at different ages. A complementary DNA (cDNA) subtraction screen was performed comparing a 3.5-days in vitro LHRH neuron [approximately embryonic day 15 (E15) in vivo] to two 10.5-days in vitro LHRH neurons (approximately postnatal day 1 in vivo). The transcription factor activator protein-2 (AP-2alpha) was differentially expressed and was present in the developmentally younger LHRH neuron. In vivo analysis revealed that LHRH neurons expressed AP-2 as they migrated across the cribriform plate and into the forebrain beginning on E13.5, but that coexpression of LHRH and AP-2 was no longer detected in postnatal day 1 animals. This suggested a regulatory role for AP-2 in LHRH neurons. Analysis of animals lacking AP-2alpha revealed a dramatic decrease in forebrain LHRH neurons between E13.5 and E14.5, correlating with normal onset of AP-2 expression in LHRH neurons as they entered the central nervous system. Nasal cells robustly expressing LHRH were still present on E 14.5. The continued presence of forebrain LHRH cells is proposed based on a second marker, galanin, and lack of increased apoptotic/necrotic cells in this region. A decrease in LHRH messenger RNA in forebrain neurons indicates regulation of LHRH occurred at the transcriptional or posttranscriptional level in mutant animals. These results indicate a developmentally restricted involvement of the transcription factor AP-2 in LHRH expression once the LHRH neurons have migrated into the forebrain, but before establishment of an adult-like distribution.

Developmental profiles of growth-associated protein (Gap43), Ngfb, Bndf and Ntf4 mRNA levels in the rat forebrain after exposure to 60 Hz magnetic fields

Fischer 344 rats were exposed to 60 Hz magnetic fields (EMFs) during gestation and lactation. Rats received continuous exposure to 2-, 200- or 1000-microT magnetic fields for 18.5 h per day, 7 days a week, or sham exposure (sham controls). During postnatal development, on postnatal days 1, 3, 6, 9, 15 and 20, forebrain tissue from male pups was examined for alterations in mRNA level for developmentally regulated central nervous system-specific proteins. Alterations in these factors during critical periods of development could result in alterations in the final neural network. Gap43 (growth-associated protein 43) mRNA was measured by Northern hybridization as a developmental indicator of axonal growth during the development of the neuron. Between postnatal days 1 and 9, detectable levels of Gap43 mRNA displayed a similar pattern across all sham control and exposure groups. In addition to Gap43, mRNA levels for the nervous system-specific growth factors ciliary neurotrophic factor (Cntf), brain-derived neurotrophic factor (Bdnf), beta nerve growth factor (Ngfb), neurotrophin-3 (Ntf3), and neurotrophin-4 (Ntf4) were examined by RNase protection assay. While there is public concern for developmental neurotoxicity associated with exposure to EMFs, these data, generated from animals exposed to 2-, 200- or 1000-microT magnetic fields during both gestational and lactational periods of development, suggest that under these conditions no significant alterations in these critical factors for brain development occur.

Characterization of the subventricular zone of the adult human brain: evidence for the involvement of Bcl-2

The subventricular zone (SVZ) is an embryonic remnant that persists and remains mitotically active throughout adulthood. The rodent SVZ harbors neuronal precursors, principally in its anterior part, and generates neuroblasts that migrate tangentially into the olfactory bulb, thus forming the so-called rostral migratory stream. This study aimed at characterizing the SVZ in the human brain. Antibodies raised against the widely used SVZ molecular markers nestin, glial fibrillary acidic protein, beta-tubulin-III and polysialylated neural cell adhesion molecule, have allowed us to characterize in detail a zone similar to the rodent SVZ in humans. Virtually all portions of the lateral ventricle, as well as the ventral (hypothalamic) sector of the third ventricle, displayed immunoreactivity for most of the molecular markers. The midline region of the septum (septal recess) and the ventral portion of the SVZ displayed a particularly intense immunostaining for all SVZ markers. These two regions may represent zones of adult neurogenesis that are unique to primates. Furthermore, the anti-apoptotic protein Bcl-2 was found to be actively synthesized and co-expressed with all the other markers throughout the entire SVZ. This study reveals that a well-developed SVZ exists in the adult human brain and suggests that Bcl-2 might play an important role in the functional organization of such a system.

Slow NMDA-EPSCs at synapses critical for song development are not required for song learning in zebra finches

Birdsong, like human speech, is learned via auditory experience during a developmentally restricted sensitive period. Within projection neurons of two avian forebrain nuclei, NMDA receptor-mediated EPSCs (NMDA-EPSCs) become fast during song development, a transition posited to limit learning. To discover whether slow NMDA-EPSCs at these synapses are required for learning, we delayed song learning beyond its normal endpoint, post-hatch day (PHD) 65, by raising zebra finches in isolation from song tutors. At PHD45, before learning, isolation delayed NMDA-EPSC maturation, but only transiently. By PHD65, NMDA-EPSCs in isolates were fast and adult-like, yet isolates presented with tutors readily learned song. Thus song learning did not require slow NMDA-EPSCs at synapses critical for song development.