Balance between excitation and inhibition controls the temporal organization of neuronal avalanches.

Neuronal avalanches, measured in vitro and in vivo, exhibit a robust critical behavior. Their temporal organization hides the presence of correlations. Here we present experimental measurements of the waiting time distribution between successive avalanches in the rat cortex in vitro. This exhibits a nonmonotonic behavior not usually found in other natural processes. Numerical simulations provide evidence that this behavior is a consequence of the alternation between states of high and low activity, named up and down states, leading to a balance between excitation and inhibition controlled by a single parameter. During these periods, both the single neuron state and the network excitability level, keeping memory of past activity, are tuned by homeostatic mechanisms.

The microRNA-29a Modulates Serotonin 5-HT7 Receptor Expression and Its Effects on Hippocampal Neuronal Morphology.

miRNAs are master regulators of gene expression in diverse biological processes, including the modulation of neuronal cytoarchitecture. The identification of their physiological target genes remains one of the outstanding challenges. Recently, it has been demonstrated that the activation of serotonin receptor 7 (5-HT7R) plays a key role in regulating the neuronal structure, synaptogenesis, and synaptic plasticity during embryonic and early postnatal development of the central nervous system (CNS). In order to identify putative miRNAs targeting the 3'UTR of 5-HT7R mouse transcript, we used a computational prediction tool and detected the miR-29 family members as the only candidates. Thus, since miR-29a is more expressed than other members in the brain, we investigated its possible involvement in the regulation of neuronal morphology mediated by 5-HT7R. By luciferase assay, we show that miR-29a can act as a post-transcriptional regulator of 5-HT7R mRNA. Indeed, it downregulates 5-HT7R gene expression in cultured hippocampal neurons, while the expression of other serotonin receptors is not affected. From a functional point of view, miR-29a overexpression in hippocampal primary cultures impairs the 5HT7R-dependent neurite elongation and remodeling through the inhibition of the ERK intracellular signaling pathway. In vivo, the upregulation of miR-29a in the developing hippocampus parallels with the downregulation of 5-HT7R expression, supporting the hypothesis that this miRNA is a physiological modulator of 5-HT7R expression in the CNS.

Genetic and epigenetic control of midbrain dopaminergic neuron development.

The relatively few dopaminergic (DA) neurons in the mammalian brain regulate many important neural functions, including motor integration, neuroendocrine hormone release, cognition, emotive behaviors and reward. A number of laboratories, including ours, have contributed to unravel the mechanisms of DA phenotype induction and maturation and elucidated the role of epigenetic factors involved in specification, development and maintenance of midbrain dopaminergic functions. DA progenitors are first "committed" to give rise to DA neurons by the action of two secreted factors, Sonic hedgehog and fibroblast growth factor 8 (FGF8). Subsequently, the function of selectively activated transcription factors, Nurr1 and Ptx3, is required for the DA final determination. Further development of DA neurotransmission requires specific interactions with the developing target striatal cells, which modulate key DA functions, namely synthesis and uptake of the neurotransmitter. Committed and determined DA neurons express the key genes involved in DA neurotransmission at different times in development. In rodents, synthesis and intracellular accumulation of DA is achieved shortly after expression of Nurr1, while the onset of high affinity uptake, responsible for ending the neurotransmission, takes place after a few days. Cell contacts between the presynaptic DA neurons and target striatal neurons are apparently necessary for the fine modulation of DA function, in vivo and in vitro. Strikingly, the in situ maturation and phenotypic specialization of DA neurons grafted into the adult striatum/caudate-putamen parallels the normal development of committed fetal dopamine neurons during neurogenesis. The correct matching between the right presynaptic and postsynaptic neurons is required also for grafted DA cells.

Quantifying barcodes of dendritic spines using entropy-based metrics.

Spine motility analysis has become the mainstay for investigating synaptic plasticity but is limited in its versatility requiring complex, non automatized instrumentations. We describe an entropy-based method for determining the spatial distribution of dendritic spines that allows successful estimation of spine motility from still images. This method has the potential to extend the applicability of spine motility analysis to ex vivo preparations.

Epigenetic cues in midbrain dopaminergic neuron development.

Midbrain dopaminergic (DA) neurons subserve complex and varied neural functions in vertebrate CNS. Their progenitors give rise to DA neurons by the action of two extracellular inducers, Sonic Hedgehog and FGF8. After this first commitment, the function of selectively activated transcription factors, like the orphan steroid nuclear receptor Nurr1, is required for DA final determination. Subsequently, DA function is selectively modulated by specific interaction with the developing striatal target tissue. Committed and determined DA neurons express the key genes involved in DA neurotransmission at different times in development. Synthesis and intracellular accumulation of DA is achieved shortly after expression of Nurr1, while high affinity uptake, responsible for ending the neurotransmission, takes place after a few days. Cell contacts between the presynaptic DA neurons and target striatal neurons are apparently necessary for the fine modulation of DA function, in vivo and in vitro.

Dopamine transporter gene expression in rat mesencephalic dopaminergic neurons is increased by direct interaction with target striatal cells in vitro.

By using a semi-quantitative reverse transcriptase-PCR assay (RT-PCR) we have analyzed dopamine transporter (DAT), tyrosine hydroxylase (TH) and synaptic vesicle monoamine transporter (VMAT2) gene expression in rat mesencephalic (MES) primary cultures. Consistent with previous data obtained during rat MES ontogeny, the onset of DAT transcription in vitro is delayed in embryonic day (E)13, but not in E16, MES neurons when compared to that of TH and VMAT2. In co-culture, the addition of target striatal cells (STR) to E13 MES selectively increases DAT mRNA level in DA neurons during the first 3 days in vitro; cortical cells are ineffective. On the contrary, DAT gene does not appear up-regulated in E16 MES co-cultured with target STR cells, indicating that MES DA neurons respond to STR stimulation only at defined developmental stages. Up-regulation of DAT mRNA level by STR in E13 MES seems to require direct cell interactions since target cells do not exert their effect on DAT transcription when are separated from MES cells by a porous barrier, which only allows diffusion of soluble molecules. Thus maturation of DA neurotransmission in vitro appears to follow a developmental program which can be specifically modulated by their target STR cells.

Ontogeny of AMPA receptor gene expression in the developing rat midbrain and striatum.

AMPA receptors mediate most of the fast excitatory synaptic transmission in the mammalian CNS. Their ontogeny during embryonic (E) and postnatal (P) development is still poorly understood. We have studied the expression of the genes encoding for AMPA glutamate receptor subunits (GlurA, GlurB, GlurC and GlurD) in the rat ventral mesencephalon (MES) and striatum (STR) and in fetal midbrain primary cultures. Each receptor subunit shows unique area- and temporal-expression pattern. In MES, GluRA, GlurB and GlurC mRNA are detectable from the earliest embryonic stage studied (E13) and raise thereafter between E15 and E17, to plateau at E19 to adult values. Differently, GlurD mRNA increases throughout embryonic and postnatal development reaching its highest levels in the adult MES. The pattern of AMPA proteins corresponded to the mRNA levels for all subunits. In the STR, GlurA gene expression increases between E15 and E19, GlurB mRNA levels are sustained from the first embryonic stages analyzed (E15) until E19 and gradually decrease thereafter toward adult levels, GlurC gene expression increases gradually throughout ontogeny to reach its highest levels in the adult. STR GlurD transcripts remain at constant levels in all stages studied. In embryonic MES primary cultures, every subunit show a characteristic expression profile similar to that observed in vivo. They all decrease significantly during the second week in vitro. Thus, all the AMPA receptor subunit transcripts appear independently regulated during development, probably depending on the tissue-specific environment, which seems preserved in MES cultures.

Early upregulation of medium neurofilament gene expression in developing spinal cord of the wobbler mouse mutant.

Homozygous wobbler mouse mutants develop a progressive paralysis due to spinal motoneuron degeneration. To understand the molecular aspect underlying the genetic defect we have studied the embryonic (from E13) and postnatal expression of the three neurofilament and choline acetyltransferase genes in each member from several wild-type (wt) and wobbler (wr) progenies. There are no variations among wt littermates at all ages studied. In contrast, analyses of neurofilament mRNA reveals a 3-4-fold increase of medium neurofilament (NFM) mRNA in wobbler mice (wr/wr). The pattern of increased NFM mRNA during development, prior to the appearance of the wobbler phenotype, among littermates (from heterozygous carriers) conforms to a mendelian inheritance of a single gene defect 1:2:1 (wr/wr:wr/+:+/+). Light and heavy neurofilament mRNA levels are also increased later in development exclusively in those individuals with high NFM mRNA values indicating that increase of the latter is associated with increase of the light and heavy subunit expression. Also NF proteins are increased. Expression of choline acetyltransferase gene is instead always comparable to normal control. Our study provides novel insights into the nature of the wobbler defect, strengthening the hypothesis that neurofilament accumulation plays a pivotal role in the etiopathogenesis of motoneuron degeneration.

Target cells modulate dopamine transporter gene expression during brain development.

We have analysed the expression of the dopamine transporter (DAT) gene and compared it with that of tyrosine hydroxylase, neuronal GABA transporter and synaptic vesicle monoamine transporter genes during pre- and post-natal development of rat mesencephalic dopaminergic (DA) neurones. Our results show that DAT transcripts are not detectable until embryonic day (E) 15, whilst those of the other genes analysed are already present at E12. In vitro, the level of DAT gene transcription in mesencephalic E13 DA neurones is increased in coculture with target striatal cells. Thus striatal targets cells regulate, at the transcriptional level, a key step of dopaminergic neurotransmission during DA neurone development.

Multiplex semi-quantitative reverse transcriptase-polymerase chain reaction of low abundance neuronal mRNAs.

The sequential use of reverse transcriptase and the polymerase chain reaction (RT-PCR) has provided molecular biology research with an exquisitely sensitive and fast technique for studying gene expression. This method is particularly useful to study transcripts in the nervous system, which are on average present at low levels and the amount of tissue or cells to be analyzed is often limited. Here, we describe a RT-PCR assay which allows the simultaneous detection and semi-quantitation of several transcripts (multiplex). Multiple PCR primer pairs are used to detect different target transcripts in a single reaction, together with a pair of primers able to amplify the hypoxantine-phosphoribosyl-transferase (HPRT), a gene constitutively expressed at low levels throughout the nervous system. HPRT levels remain constant also during neurogenesis and it is thus apt to be used in developmental neurobiology. This internal standard is the mRNA of reference to evaluate sample variation in RT and PCR reactions and to monitor the degradation and recovery of RNAs. Normalization with respect to HPRT cDNA allows to estimate the relative abundance of each target mRNA.

Synthesis of rat brain DNA during acquisition of an appetitive task.

We have examined the incorporation of [3H-methyl]thymidine into DNA extracted from several brain regions of rats learning a reverse handedness task, of control rats allowed to use their preferred paw, and of control rats left in their home cages. In learning animals, decrements in percent incorporation were observed in the visual cortex, remaining brain, hippocampus and entorhinal cortex. In the latter two regions less marked decreases were present in the active control group. No variation occurred in the sensory-motor cortex. In learning rats the specific radioactivity of neuronal DNA was markedly decreased in the hippocampus and remaining brain. In the former region, a less marked decrease was present in active control rats. In subcellular fractionation studies it was observed that decreases in DNA specific radioactivity prevailed in the mitochondrial fraction isolated from the hippocampus and visual cortex of learning rats. Brain radioactive DNA was widely distributed among fractions differing in their degree of repetitiveness. Its pattern of distribution did not coincide with that of bulk DNA and differed significantly among behavioural groups. The results suggest a non random origin of newly-synthesized brain DNA and its involvement in learning.

The serotonin receptor 7 promotes neurite outgrowth via ERK and Cdk5 signaling pathways.

Serotonergic neurotransmission is mediated by at least 14 subtypes of 5-HT receptors. Among these, the CNS serotonin receptor 7 (5-HTR7) is involved in diverse physiological processes. Here we show that treatment of murine striatal and cortical neuronal cultures with 5-HTR7 agonists (8-OH-DPAT and LP-211) significantly enhances neurite outgrowth. This effect is abolished by the selective 5-HTR7 antagonist SB-269970, by the ERK inhibitor U0126, by the cyclin-dependent kinase 5 (Cdk5) inhibitor roscovitine, as well as by cycloheximide, an inhibitor of protein synthesis. These data indicate that 5-HTR7 activation stimulates extensive neurite elongation in CNS primary cultures, subserved by ERK and Cdk5 activation, and de novo protein synthesis. Two-dimensional (2D) gel electrophoresis coupled to Western blot analyses reveals both qualitative and quantitative expression changes in selected cytoskeletal proteins, following treatment of striatal primary cultures with LP-211. In particular, the 34 kDa isoform of MAP1B is selectively expressed in stimulated cultures, consistent with a role of this protein in tubulin polymerization and neurite elongation. In summary, our results show that agonist-dependent activation of the endogenous 5-HTR7 in CNS neuronal primary cultures stimulates ERK- and Cdk5-dependent neurite outgrowth, sustained by modifications of cytoskeletal proteins. These data support the hypothesis that the 5-HTR7 might play a crucial role in shaping neuronal morphology and behaviorally relevant neuronal networks, paving the way to new approaches able to modulate CNS connectivity.

Methylphenidate to adolescent rats drives enduring changes of accumbal Htr7 expression: implications for impulsive behavior and neuronal morphology.

Methylphenidate (MPH) administration to adolescent rodents produces persistent region-specific changes in brain reward circuits and alterations of reward-based behavior. We show that these modifications include a marked increment of serotonin (5-hydroxy-tryptamine) receptor type 7 (Htr7) expression and synaptic contacts, mainly in the nucleus accumbens, and a reduction of basal behavioral impulsivity. We show that neural and behavioral consequences are functionally related: administration of a selective Htr7 antagonist fully counteracts the MPH-reduced impulsive behavior and enhances impulsivity when administered alone in naive rats. Agonist-induced activation of endogenous Htr7 significantly increases neurite length in striatal neuron primary cultures, thus suggesting plastic remodeling of neuronal morphology. The mixed Htr (1a/7) agonist, 8-OH-DPAT, reduces impulsive behavior in adolescent rats and in naive adults, whose impulsivity is enhanced by the Htr7 antagonist. In summary, behavioral pharmacology experiments show that Htr7 mediates self-control behavior, and brain primary cultures experiments indicate that this receptor may be involved in the underlying neural plasticity, through changes in neuronal cytoarchitecture.

Chronic cocaine administration modulates the expression of transcription factors involved in midbrain dopaminergic neuron function.

Chronic cocaine use leads to pronounced alterations in neuronal functions in brain circuits associated with reward. In the present study, we examined in the rat midbrain the effects of acute, subchronic (5 days) and chronic cocaine treatments (14 days) on the gene expression of transcription factors involved in the development and maintenance of dopaminergic neurons. We show that chronic, but not acute or subchronic, cocaine administration downregulates Nurr1 and Pitx3 transcripts whereas En1 transcripts are upregulated. Conversely, Lmx1b and En2 transcripts are not affected by the drug treatment, indicating that the modulation of the midbrain transcription factors analyzed is highly selective. Interestingly, modification of the gene expression for these transcription factors persists in midbrain as long as two weeks after the last drug administration, suggesting that it may account for some of the enduring alterations in midbrain dopaminergic circuits associated with chronic cocaine use.

Epigenetic factors and midbrain dopaminergic neurone development.

In the mammalian brain dopamine systems play a central role in the control of movement, hormone release, emotional balance and reward. Alteration of dopaminergic neurotransmission is involved in Parkinson's disease and other movement disorders, as well as in some psychotic syndromes. This review summarises recent findings, which shed some light on signals and cellular interactions involved in the specification and maturation of the dopaminergic function during neurogenesis. In particular we will focus on three major issues: (1) the differentiation of dopaminergic neurones triggered by direct contact with the midbrain floor plate cells through the action of sonic hedgehog; (2) the neurotrophic factors acting on dopaminergic neurones; and (3) the role of target striatal cells on the survival and the axonal growth of developing or grafted dopaminergic neurones.

DNA turnover in rat cerebral cortex.

After the intracranial injection of [methyl-3H]thymidine the specific activity of rat cortical DNA increases rapidly, reaching a maximum at about 5 h. More than half of the radioactive DNA disappears from the tissue in the following few hours. During the same period of time the concentration of radioactive DNA in liver remains essentially constant. Minor variations occur in both organs after 41 h. An apparent rapid turnover of DNA is also present in a fraction of purified neuronal perikarya prepared from the cerebral cortex.

Ribosomal RNAs synthesized by isolated squid nerves and ganglia differ from native ribosomal RNAs.

The large rRNA of the squid comprises two chains that may be dissociated by heating at 65 degrees C. A single chain constitutes the small rRNA. Surprisingly, the RNAs synthesized by dissected squid fin nerves and stellate nerves and ganglia differed in size from native rRNAs and did not manifest thermal instability. Nonetheless, they resembled native rRNAs in relative abundance, subcellular distribution, lack of poly(A), and metabolic stability. In addition, newly synthesized RNA was localized in nerve and glial cells, as shown by autoradiographic analysis, and was assembled into 80S ribosomes, which supported the synthesis of neuron-specific neurofilament proteins. Following incubation of nerves and ganglia for >10 h, native rRNAs started to disappear, while two major newly synthesized RNAs progressively accumulated. As a result, after 20 h, native rRNAs were substituted by the two novel RNAs. With use of 32P-cDNA synthesized from the latter RNAs as a probe, the novel RNAs demonstrated a considerable degree of homology with native rRNA in northern analysis. Taken together, the data suggest that in dissected squid nerves and ganglia, the synthesis of native rRNAs is gradually terminated while two novel rRNAs are being synthesized, presumably as a correlate of reactive gliosis and/or neuronal degeneration/regeneration.

Regionalized neurofilament accumulation and motoneuron degeneration are linked phenotypes in wobbler neuromuscular disease.

Abnormal neurofilament aggregates are pathological hall-mark of most neurodegenerative diseases, although their pathogenic role remains unclear. Increased expression of medium neurofilament (NFM) is an early molecular marker of wobbler mouse, an animal model of motoneuron disease. In the wr/wr, a vacuolar neuronal degeneration (VND) starts at 15 days postnatally, selectively in cervical spinal cord and brain stem motoneurons. Here we show that nfm gene hyperexpression is restricted to the aforementioned motoneurons and is specific for wr mutation. NF proteins accumulate in wr/wr before VND. wr/+ mice, which are asymptomatic, show intermediate NF accumulation between wr/wr and +/+ littermates, suggesting a gene dosage dependence of the wobbler pathology. Altogether our data indicate that NF hyperexpression and regionalized motoneuron degeneration are linked to the wr mutation, although with a still unknown relationship to the mutant gene activity.

Dystrophin localization and gene expression in the developing nervous system of the chick.

The presence and distribution of dystrophin was studied in selected areas of the chick embryo nervous system and in primary cultures. Dystrophin was examined at the protein level by immunocytochemistry and at the transcriptional level by a semiquantitative reverse transcriptase-polymerase chain reaction analysis. Immunofluorescence staining shows that dystrophin is present early during embryogenesis in dorsal root ganglia, spinal cord, and ciliary ganglia and colocalizes with neurofilament subunits. Cultured dorsal root ganglion, spinal cord, and ciliary ganglion neurons show immunoreactivity for dystrophin, both in cell bodies and along fibers. Dystrophin mRNA level in ciliary and dorsal root ganglia is higher than in spinal cord throughout development and shows a tissue-specific pattern of expression. In primary cultures of dorsal root ganglia and ciliary ganglia, dystrophin mRNA level increases with time in vitro. However, in spinal cord cultures, dystrophin mRNA drastically decreases with time in vitro, but it is significantly increased when embryonic muscle extract is added to the cultures. Our results show that dystrophin is present in neurons from different areas of embryonic chick nervous system and that its mRNA level is developmentally regulated both in vivo and in vitro.