Excitation-inhibition imbalance disrupts visual familiarity in amyloid and non-pathology conditions.

Neuronal hyperactivity induces memory deficits in Alzheimer's disease. However, how hyperactivity disrupts memory is unclear. Using in vivo synaptic imaging in the mouse visual cortex, we show that structural excitatory-inhibitory synapse imbalance in the apical dendrites favors hyperactivity in early amyloidosis. Consistent with this, natural images elicit neuronal hyperactivity in these mice. Compensatory changes that maintain activity homeostasis disrupt functional connectivity and increase population sparseness such that a small fraction of neurons dominates population activity. These properties reduce the selectivity of neural response to natural images and render visual recognition memory vulnerable to interference. Deprivation of non-specific visual experiences improves the neural representation and behavioral expression of visual familiarity. In contrast, in non-pathological conditions, deprivation of non-specific visual experiences induces disinhibition, increases excitability, and disrupts visual familiarity. We show that disrupted familiarity occurs when the fraction of high-responsive neurons and the persistence of neural representation of a memory-associated stimulus are not constrained.

Genomic and Transcriptomic Analyses Reveals ZNF124 as a Critical Regulator in Highly Aggressive Medulloblastomas.

Medulloblastoma (MB) is the most common malignant pediatric brain tumor, however, the mechanisms underlying tumorigenesis in different MB subgroups remain incompletely understood. Although previous studies of MB predisposition have been conducted in tertiary referral centers primarily in Caucasian cohorts, it is not unclear clear whether there exist population-specific genetic alterations in MBs. In this study, we investigated the contribution of genomic and transcriptomic alterations to the risk of malignant MB in the Chinese population (designated as the Asian cohort). We analyze the genomic and transcriptomic alterations of the Asian MB cohort by using a combination of whole-exome sequencing (WES) and RNA-deep-sequencing. In addition, we integrate publicly available data with the Asian MB cohort and identify a subset of potential MB-driving genes specifically enriched in each of the MB subgroups. We further characterize a newly identified group-3-enriched transcriptional regulator, ZNF124, and demonstrate that ZNF124 is critical for the growth of the most aggressive group-3 MB cells. Together, our analyses indicate conserved yet distinct genetic alterations and gene expression patterns of MBs between different ethnic groups. Our studies further provide an important resource for identifying potential tumor-driving factors in MBs, enhancing our understanding of the disease process for developing ethnically targeted therapies in patients with MB.

Chromatin modification and epigenetic control in functional nerve regeneration.

The repair and functional recovery of the nervous system is a highly regulated process that requires the coordination of many different components including the proper myelination of regenerated axons. Dysmyelination and remyelination failures after injury result in defective nerve conduction, impairing normal nervous system functions. There are many convergent regulatory networks and signaling mechanisms between development and regeneration. For instance, the regulatory mechanisms required for oligodendrocyte lineage progression could potentially play fundamental roles in myelin repair. In recent years, epigenetic chromatin modifications have been implicated in CNS myelination and functional nerve restoration. The pro-regenerative transcriptional program is likely silenced or repressed in adult neural cells including neurons and myelinating cells in the central and peripheral nervous systems limiting the capacity for repair after injury. In this review, we will discuss the roles of epigenetic mechanisms, including histone modifications, chromatin remodeling, and DNA methylation, in the maintenance and establishment of the myelination program during normal oligodendrocyte development and regeneration. We also discuss how these epigenetic processes impact myelination and axonal regeneration, and facilitate the improvement of current preclinical therapeutics for functional nerve regeneration in neurodegenerative disorders or after injury.

Single-Cell Transcriptomics in Medulloblastoma Reveals Tumor-Initiating Progenitors and Oncogenic Cascades during Tumorigenesis and Relapse.

Progenitor heterogeneity and identities underlying tumor initiation and relapse in medulloblastomas remain elusive. Utilizing single-cell transcriptomic analysis, we demonstrated a developmental hierarchy of progenitor pools in Sonic Hedgehog (SHH) medulloblastomas, and identified OLIG2-expressing glial progenitors as transit-amplifying cells at the tumorigenic onset. Although OLIG2+ progenitors become quiescent stem-like cells in full-blown tumors, they are highly enriched in therapy-resistant and recurrent medulloblastomas. Depletion of mitotic Olig2+ progenitors or Olig2 ablation impeded tumor initiation. Genomic profiling revealed that OLIG2 modulates chromatin landscapes and activates oncogenic networks including HIPPO-YAP/TAZ and AURORA-A/MYCN pathways. Co-targeting these oncogenic pathways induced tumor growth arrest. Together, our results indicate that glial lineage-associated OLIG2+ progenitors are tumor-initiating cells during medulloblastoma tumorigenesis and relapse, suggesting OLIG2-driven oncogenic networks as potential therapeutic targets.

Scanless volumetric imaging by selective access multifocal multiphoton microscopy.

Simultaneous, high-resolution imaging across a large number of synaptic and dendritic sites is critical for understanding how neurons receive and integrate signals. Yet, functional imaging that targets a large number of submicrometer-sized synaptic and dendritic locations poses significant technical challenges. We demonstrate a new parallelized approach to address such questions, increasing the signal-to-noise ratio by an order of magnitude compared to previous approaches. This selective access multifocal multiphoton microscopy uses a spatial light modulator to generate multifocal excitation in three dimensions (3D) and a Gaussian-Laguerre phase plate to simultaneously detect fluorescence from these spots throughout the volume. We test the performance of this system by simultaneously recording Ca2+ dynamics from cultured neurons at 98-118 locations distributed throughout a 3D volume. This is the first demonstration of 3D imaging in a "single shot" and permits synchronized monitoring of signal propagation across multiple different dendrites.

The genetic insulator RiboJ increases expression of insulated genes.

A primary objective of synthetic biology is the construction of genetic circuits with behaviors that can be predicted based on the properties of the constituent genetic parts from which they are built. However a significant issue in the construction of synthetic genetic circuits is a phenomenon known as context dependence in which the behavior of a given part changes depending on the choice of adjacent or nearby parts. Interactions between parts compromise the modularity of the circuit, impeding the implementation of predictable genetic constructs. To address this issue, investigators have devised genetic insulators that prevent these unintended context-dependent interactions between neighboring parts. One of the most commonly used insulators in bacterial systems is the self-cleaving ribozyme RiboJ. Despite its utility as an insulator, there has been no systematic quantitative assessment of the effect of RiboJ on the expression level of downstream genetic parts. Here, we characterized the impact of insulation with RiboJ on expression of a reporter gene driven by a promoter from a library of 24 frequently employed constitutive promoters in an Escherichia coli model system. We show that, depending on the strength of the promoter, insulation with RiboJ increased protein abundance between twofold and tenfold and increased transcript abundance by an average of twofold. This result demonstrates that genetic insulators in E. coli can impact the expression of downstream genes, information that is essential for the design of predictable genetic circuits and constructs.

Scattering reduction by structured light illumination in line-scanning temporal focusing microscopy.

Line-scanning temporal focusing microscopy (LineTFM) is capable of imaging biological samples more than 10 times faster than two-photon laser point-scanning microscopy (TPLSM), while achieving nearly the same lateral and axial spatial resolution. However, the image contrast taken by LineTFM is lower than that by TPLSM because LineTFM is severely influenced by biological tissue scattering. To reject the scattered photons, we implemented LineTFM using both structured illumination and uniform illumination combined with the HiLo post-processing algorithm, called HiLL microscopy (HiLo-Line-scanning temporal focusing microscopy). HiLL microscopy significantly reduces tissue scattering and improves image contrast. We demonstrate HiLL microscopy with in vivo brain imaging. This approach could potentially find applications in monitoring fast dynamic events and in mapping high resolution structures over a large volume.

Spine Dynamics: Are They All the Same?

Since Cajal's first drawings of Golgi stained neurons, generations of researchers have been fascinated by the small protrusions, termed spines, studding many neuronal dendrites. Most excitatory synapses in the mammalian CNS are located on dendritic spines, making spines convenient proxies for excitatory synaptic presence. When in vivo imaging revealed that dendritic spines are dynamic structures, their addition and elimination were interpreted as excitatory synapse gain and loss, respectively. Spine imaging has since become a popular assay for excitatory circuit remodeling. In this review, we re-evaluate the validity of using spine dynamics as a straightforward reflection of circuit rewiring. Recent studies tracking both spines and synaptic markers in vivo reveal that 20% of spines lack PSD-95 and are short lived. Although they account for most spine dynamics, their remodeling is unlikely to impact long-term network structure. We discuss distinct roles that spine dynamics can play in circuit remodeling depending on synaptic content.

Inhibitory Synapses Are Repeatedly Assembled and Removed at Persistent Sites In Vivo.

Older concepts of a hard-wired adult brain have been overturned in recent years by in vivo imaging studies revealing synaptic remodeling, now thought to mediate rearrangements in microcircuit connectivity. Using three-color labeling and spectrally resolved two-photon microscopy, we monitor in parallel the daily structural dynamics (assembly or removal) of excitatory and inhibitory postsynaptic sites on the same neurons in mouse visual cortex in vivo. We find that dynamic inhibitory synapses often disappear and reappear again in the same location. The starkest contrast between excitatory and inhibitory synapse dynamics is on dually innervated spines, where inhibitory synapses frequently recur while excitatory synapses are stable. Monocular deprivation, a model of sensory input-dependent plasticity, shortens inhibitory synapse lifetimes and lengthens intervals to recurrence, resulting in a new dynamic state with reduced inhibitory synaptic presence. Reversible structural dynamics indicate a fundamentally new role for inhibitory synaptic remodeling--flexible, input-specific modulation of stable excitatory connections.

Experience-Dependent Structural Plasticity in the Visual System.

During development, the environment exerts a profound influence on the wiring of brain circuits. Due to the limited resolution of studies in fixed tissue, this experience-dependent structural plasticity was once thought to be restricted to a specific developmental time window. The recent introduction of two-photon microscopy for in vivo imaging has opened the door to repeated monitoring of individual neurons and the study of structural plasticity mechanisms at a very fine scale. In this review, we focus on recent work showing that synaptic structural rearrangements are a key mechanism mediating neural circuit adaptation and behavioral plasticity in the adult brain. We examine this work in the context of classic studies in the visual systems of model organisms, which have laid much of the groundwork for our understanding of activity-dependent synaptic remodeling and its role in brain plasticity.

Pharmacogenetic analysis reveals a post-developmental role for Rac GTPases in Caenorhabditis elegans GABAergic neurotransmission.

The nerve-cell cytoskeleton is essential for the regulation of intrinsic neuronal activity. For example, neuronal migration defects are associated with microtubule regulators, such as LIS1 and dynein, as well as with actin regulators, including Rac GTPases and integrins, and have been thought to underlie epileptic seizures in patients with cortical malformations. However, it is plausible that post-developmental functions of specific cytoskeletal regulators contribute to the more transient nature of aberrant neuronal activity and could be masked by developmental anomalies. Accordingly, our previous results have illuminated functional roles, distinct from developmental contributions, for Caenorhabditis elegans orthologs of LIS1 and dynein in GABAergic synaptic vesicle transport. Here, we report that C. elegans with function-altering mutations in canonical Rac GTPase-signaling-pathway members demonstrated a robust behavioral response to a GABA(A) receptor antagonist, pentylenetetrazole. Rac mutants also exhibited hypersensitivity to an acetylcholinesterase inhibitor, aldicarb, uncovering deficiencies in inhibitory neurotransmission. RNA interference targeting Rac hypomorphs revealed synergistic interactions between the dynein motor complex and some, but not all, members of Rac-signaling pathways. These genetic interactions are consistent with putative Rac-dependent regulation of actin and microtubule networks and suggest that some cytoskeletal regulators cooperate to uniquely govern neuronal synchrony through dynein-mediated GABAergic vesicle transport in C. elegans.

Serotonin release from mesencephalic raphe neurons grafted to the 5,7-dihydroxytryptamine-lesioned rat hippocampus: effects of behavioral activation and stress.

Transplants of fetal midbrain raphe neurons into the adult brain have been shown to promote recovery of complex behavioral deficits in several experimental models, but the mechanisms underlying these effects are only partially understood. In the present study, we have used a well-characterized model system to ascertain whether midbrain raphe graft can display behaviorally relevant changes in transmitter release and/or metabolism. Fetal mesencephalic raphe neurons were grafted unilaterally into the hippocampus previously deprived of its innate serotonergic innervation by intraventricular injections of 5,7-dihydroxytryptamine. The contralateral hippocampus remained as a nongrafted, lesioned control. Microdialysis probes were implanted in the hippocampus 5-7 months postgrafting. Under baseline conditions, extracellular levels of serotonin were similar to normal in the grafted hippocampi, but undetectable on the contralateral, nongrafted side. Levels of the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were markedly higher than normal in the grafted hippocampi, but dramatically reduced on the contralateral nongrafted side. Handling stimulation (gentle stroking of a rat's fur and tail for 15 min) induced a 64% increase in serotonin output in the intact rats and a small but significant 12% increase in the grafted animals. Non-noxious tail-pinch (15 min) enhanced serotonin release by 86% in the intact rats and 28% in the grafted ones. Extracellular 5-HIAA levels remained unchanged during both handling and tail-pinch in both the intact and the grafted rats. Forced immobilization of the rats for 15 min induced a transient 124% increase in extracellular serotonin levels in the intact rats and a significant 19% increase in the grafted animals, whereas swimming in temperate water (25-30 degrees C; 15 min) induced no detectable changes in serotonin output in any of the groups. 5-HIAA levels remained unchanged during forced immobilization, but were significantly reduced during the swimming session in both the intact (-38%) and grafted (-15%) animals. The present results indicate that median raphe grafts can become functionally integrated in the denervated host hippocampus and respond by altered indole output when the animal is exposed to different types of environmental challenges.

Transmitter release from transplants of fetal ventral mesencephalon or locus coeruleus in the rat frontal cortex and nucleus accumbens: effects of pharmacological and behaviorally activating stimuli.

The present study was performed in order to establish whether dopamine (DA) release from behaviorally functional intracerebral DA transplants is dependent on changes in neuronal impulse flow, and is under control of the host brain. Rats were subjected to combined intraventricular and ventral tegmental injections of 6-hydroxydopamine (6-OHDA) in order to obtain a severe bilateral lesion of the ascending mesocorticolimbic DA projections. Cell suspension grafts of fetal ventral mesencephalic neurons were thereafter implanted into the medial frontal cortex (MFC) and the nucleus accumbens (NAc). Since the neurotoxin injections removed also the ascending noradrenergic systems, fetal locus coeruleus neurons were added to the graft suspension in one group of animals. Age-matched lesion-only and normal animals served as controls. The lesion-induced alterations in spontaneous, amphetamine- and apomorphine-induced locomotor activity and in a skilled paw reaching task were evaluated before transplantation, and at 3 and 6 months post-grafting. Microdialysis probes were finally implanted in the MFC and NAc in order to monitor extracellular DA and noradrenaline (NA) levels (i) during administration of pharmacological agents which augment or depress catecholamine release in the intact brain; (ii) during exposure of the rats to stressful manipulations (handling and immobilization) or appetitive stimuli (eating) known to enhance cortical and limbic DA or NA release in intact animals. The lesion-induced reduction in amphetamine-induced locomotor activity was reversed in all grafted animals, which also showed a higher than normal spontaneous overnight activity. Daytime spontaneous locomotor activity (which was reduced in the lesion-only rats) as well as apomorphine-induced hyperactivity was reversed by the grafts of DA neurons only. By contrast, the lesion-induced impairment in skilled forelimb use was not alleviated by the grafts. The grafted DA neurons restored normal steady-state DA overflow in the NAc, whereas they enhanced cortical DA overflow to significantly higher than normal levels. Restoration of both cortical and striatal NA overflow was observed in the group that received mixed DA and NA grafts, whereas animals that received DA grafts only did not differ from the lesioned controls. The changes in extracellular DA and NA levels measured in the grafted MFC and NAc under potassium depolarization (100 mM KCl), inhibition of terminal catecholamine reuptake (10 microM nomifensine), and sodium channel blockade (1 microM TTX) indicated that graft-derived DA or NA release had normal neuronal properties, and was dependent on an intact axonal impulse flow.(ABSTRACT TRUNCATED AT 400 WORDS)

Seizure suppression in kindling epilepsy by intracerebral implants of GABA- but not by noradrenaline-releasing polymer matrices.

Gamma-aminobutyric acid (GABA)-releasing polymer matrices were implanted bilaterally, immediately dorsal to the substantia nigra, in rats previously kindled in the amygdala. Two days after implantation, rats with GABA-releasing matrices exhibited only focal limbic seizures in response to electrical stimulation, whereas animals with control matrices devoid of GABA had generalized convulsions. GABA release from the polymer matrices was high during the first days after implantation, as demonstrated both in vitro and, using microdialysis, in vivo. The anticonvulsant effect was no longer observed at 7 and 14 days at which time GABA release was found to be low. In a parallel experiment, polymer matrices containing noradrenaline (NA) were implanted bilaterally into the hippocampus of rats with extensive forebrain NA depletion induced by an intraventricular 6-hydroxydopamine injection. No effect on the development of hippocampal kindling was observed, despite extracellular NA levels exceeding those of rats with intrahippocampal locus coeruleus grafts that have previously been shown to retard kindling rate. The results indicate that GABA-releasing implants located in the substantia nigra region can suppress seizure generalization in epilepsy, even in the absence of synapse formation and integration with the host brain. In contrast, the failure of NA-releasing polymer matrices to retard the development of seizures in NA-depleted rats suggests that such an effect can only be exerted by grafts acting through a well-regulated, synaptic release of NA.

Characterization of in vivo noradrenaline release from superior cervical ganglia or fetal locus coeruleus transplanted to the subcortically deafferented hippocampus in the rat.

Solid grafts of autologous superior cervical ganglia (SCG) or fetal locus coeruleus (LC) were implanted unilaterally into a fimbria-fornix lesion cavity adjacent to the hippocampal formation after a 6-hydroxydopamine lesion of the intrinsic noradrenergic system. Twelve to 15 months after transplantation, one microdialysis probe was implanted in the dorsal hippocampus ipsilateral to the graft, and extracellular levels of noradrenaline (NA) were monitored during the application of pharmacological or behavioral stimuli. Age-matched intact and lesion-only animals served as controls. Morphological examination of the grafts was performed on sections processed for dopamine-beta-hydroxylase (DBH) immunohistochemistry. In the lesion-only controls, the hippocampus was totally devoid of DBH-immunoreactive fibers and hippocampal levels of NA were generally undetectable. Although both SCG and LC grafts gave rise to an extensive DBH-immunoreactive fiber ingrowth in the ipsilateral hippocampus, baseline NA release was strikingly different in the two graft groups, being markedly lower than normal in the SCG-grafted rats (3.5 +/- 0.1 fmol/30 microliters) and significantly higher than normal in the LC-grafted rats (44.5 +/- 12.3 fmol/30 microliters). The response to potassium-induced depolarization (100 mM KCl in the perfusion fluid), neuronal uptake blockade (5 microM desipramine), and sodium-channel blockade (1 microM TTX) was similar to normal in both graft groups. Exposure of the animals to mild (handling) or severe (immobilization) stressful stimuli significantly enhanced NA release in the intact controls, whereas no clear-cut effect could be detected in either graft group. Electrical stimulation of the medial septum, applied in an attempt to activate possible afferents to the grafts from the host septum, did not enhance NA release in any of the groups. The results show that grafts of both central and peripheral noradrenergic neurons can provide a source of steady-state NA release in the denervated hippocampus, but that the spontaneous activity of the grafted ganglionic neurons is very low compared to that of the LC neurons, probably due to the absence of a functional preganglionic input to the grafted SCG neurons. Although extracellular NA recovered from both the SCG- and the LC-grafted hippocampi is likely to derive from impulse-dependent neuronal release, it was largely unaffected by physiological stimuli applied to the host.

Extracellular gamma-aminobutyric acid levels in the rat caudate-putamen: monitoring the neuronal and glial contribution by intracerebral microdialysis.

Intracerebral microdialysis with high pressure liquid chromatography (HPLC) coupled to electrochemical detection was employed to characterize gamma-aminobutyric acid (GABA) release and the effects induced by a preceding neuron-depleting ibotenic acid (IBO) lesion in the rat caudate-putamen (CPu). Extracellular GABA overflow was monitored in the intact and excitotoxically lesioned CPu, either 7-10 days (acute) or more than 3 months post-lesioning (chronic), using loop type dialysis probes perfused at a rate of 2 microliters/min. In the intact CPuu, basal GABA levels were 0.97 pmol/30 microliters of dialysate in the awake animals and 0.76 pmol/30 microliters under halothane anaesthesia. In both the acute and chronic IBO lesioned CPu the extracellular GABA levels were reduced by 80% and 67%, respectively, under halothane anaesthesia. KCl added to the perfusion fluid at a concentration of 100 mM resulted in dramatic increases in GABA overflow from baseline levels in the intact CPu (60- to 70-fold), which were almost totally abolished (> 95%) in the excitotoxically lesioned CPu. Veratridine administered at 75 microM, produced a 45-fold increase in GABA overflow in the intact CPu, but failed to produce any effect in the lesioned CPu. The addition of nipecotic acid (0.5 mM), a GABA uptake blocker, increased basal extracellular GABA levels 6-15-fold in the intact CPu, while GABA overflow in either the acute or chronic lesioned CPu was not significantly altered. Although Ca(2+)-free conditions (with 20 mM Mg2+ added) or tetrodotoxin (TTX, 1 microM) did not alter the basal GABA overflow in the intact CPU under halothane anaesthesia, the omission of Ca2+ resulted in a 47% reduction in basal extracellular GABA levels in awake, freely moving animals. Nipecotic acid-induced GABA overflow was reduced by 22% under Ca(2+)-free conditions, and by 33% in the presence of 1 microM TTX. Moreover, KCl-evoked GABA overflow was reduced by 86% in Ca(2+)-free conditions and by 40% when administered in the presence of 1 microM TTX. These results indicate that the extracellular GABA levels recorded by intracerebral microdialysis in the CPu are derived predominantly from neuronal sources. Under baseline resting conditions only a small fraction (up to 20-30%) of the neuronal release was Ca(2+)-dependent and TTX-sensitive (i.e. possessing the characteristics of impulse-dependent vesicular release).(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of GABA release from intrastriatal striatal transplants: dependence on host-derived afferents.

Extracellular levels of GABA, derived from cell suspension transplants of embryonic day 14-15 rat striatal primordia implanted into the previously excitotoxically lesioned striatum, were measured using intracerebral microdialysis in halothane-anaesthetized rats. GABA overflow was monitored using loop type dialysis probes implanted into grafted, age-matched ibotenic acid-lesioned and intact striata, under baseline conditions and after different pharmacological manipulations. Basal and evoked GABA release, which was reduced by 58 and 96%, respectively, in the excitotoxin-lesioned striatum, was restored by the striatal grafts to levels close to or above those observed in normal striata. The graft-derived release of GABA was most likely of neuronal origin, since the K(+)-evoked (100 mM) GABA overflow was reduced by almost 80% when Ca++ was replaced by 20 mM Mg++ in the perfusion medium, and blockade of GABA uptake by nipecotic acid (0.5 mM), induced a greater than six-fold increase in GABA overflow. However, perfusion of the graft with 1 microM tetrodotoxin in combination with K+ (100 mM) resulted in little if any reduction in the K(+)-evoked overflow. Histological analysis demonstrated a dense tyrosine hydroxylase-positive fibre network in the grafts, which was removed after a 6-hydroxydopamine lesion of the ipsilateral nigrostriatal pathway. The dopamine denervating lesion resulted in an increased K(+)-evoked GABA overflow both in the intact (+76%) and the grafted striata (+181%), suggesting that the tonic dopaminergic inhibitory control of GABA release, seen in the intact striatum, is also present in the grafted striata. The glutamate analogue, kainic acid (1 mM added to the perfusion fluid), evoked a 60-74% increase in GABA overflow both in intact striata (with or without dopaminergic denervation) and in the striatal grafts. This effect seemed to be dependent on an intact corticostriatal projection, since knife-cut transections of the frontal cortex at the level of the forceps minor, abolished the response in both the intact and grafted striata. These results demonstrate that grafts of fetal striatal tissue implanted into the excitotoxically lesioned striatum restore striatal GABA overflow in a neuron-dependent manner, close to or above that seen in the normal intact striatum. Furthermore, the graft-derived GABA release appears to be under normal regulatory control from the host dopaminergic and glutamatergic systems. Since the GABAergic striatal output system is critical for the expression of striatum-related behaviours, it is proposed that the graft-induced behavioural recovery in the striatal lesion model, at least in part, may depend on the restoration of striatal GABAergic neurotransmission.

Regional differences in the regulation of dopamine and noradrenaline release in medial frontal cortex, nucleus accumbens and caudate-putamen: a microdialysis study in the rat.

Dopamine (DA) and noradrenaline (NA) extracellular levels have been measured by microdialysis in the medial frontal cortex (MFC), nucleus accumbens (NAc) and caudate-putamen (CP) under baseline conditions in awake and halothane-anaesthetized rats, and after application of three types of stimuli which are likely to activate the brainstem catecholaminergic systems: mild stressors (handling and tail pinch), rewarded behavior (eating palatable food without prior food deprivation) and electrical stimulation of the lateral habenular nucleus. Changes were studied with and without uptake blockade (10 microM nomifensine in the perfusion fluid). The influence of calcium concentration (1.2 or 2.3 mM in the perfusion fluid) on DA and NA overflow was tested in some cases. Handling and tail pinch stimulated both DA and NA overflow in MFC, and enhanced NA overflow in NAc. By contrast, these mildly stressful stimuli had only marginal effects on DA overflow in NAc and no effects on either DA or NA overflow in CP. Eating behavior was accompanied by increased DA and NA overflow in MFC but had no effect in NAc. These regional differences were similar also when the manipulations were applied under uptake blockade, which indicates that the more pronounced changes seen in MFC did not simply reflect a more sparse innervation (i.e. lower density of uptake sites) in the MFC compared to the more densely innervated NAc and CP areas. Stimulation of the lateral habenula induced a 2-3-fold increase in NA overflow in both MFC, NAc and CP but had no consistent effect on DA overflow in any region. The effect on NA release was abolished by a transection of the ipsilateral fasciculus retroflexus (which carries the efferent output of the lateral habenula). The results show that the forebrain DA and NA projections to cortical and striatal targets are differentially regulated during ongoing behavior, that the mesocortical and mesostriatal DA systems respond quite differently to stressful and rewarding stimuli; and that the NA projection to MFC (like the dopaminergic one) is more responsive to stressful and rewarding stimuli than the ones innervating the striatum (NAc and CP). The results support the view that environmental stimuli evoking emotional arousal (whether aversive or non-aversive) are accompanied by increased DA and NA release above all in the MFC and only to a minor extent in limbic and striatal areas.

Dopaminergic transplants normalize amphetamine- and apomorphine-induced Fos expression in the 6-hydroxydopamine-lesioned striatum.

Dopamine receptor-mediated Fos protein expression in the striatum has been used to monitor dopamine receptor activation at the cellular level after dopaminergic denervation and reinnervation by fetal nigral transplants. The pattern of striatal Fos expression after systemic administration of either the dopamine receptor agonist, apomorphine, or the dopamine-releasing agent, amphetamine, was studied in rats which had received cell suspension grafts of fetal ventral mesencephalic neurons into the striatum after a complete 6-hydroxydopamine lesion of mesostriatal dopaminergic projection. Grafted animals, and normal and lesioned controls were killed 2 h after administration of either D-amphetamine (5 mg/kg, i.p.) or apomorphine (0.25 mg/kg, s.c.). Fos protein was detected immunohistochemically, and the density of Fos-immunoreactive cell nuclei was measured in 12 selected areas of caudate-putamen, nucleus accumbens and globus pallidus by computerized image analysis. Consistent with previous studies, amphetamine induced high Fos expression in the medial and dorsal parts of the intact caudate-putamen and significantly lower expression in the denervated caudate-putamen. A significant difference between lesioned and intact striata was present also in globus pallidus, but not in nucleus accumbens. In grafted rats, amphetamine-induced Fos activation was restored to normal or supranormal levels in the anterior and central caudate-putamen (i.e. close to the graft deposits), whereas in the tail of caudate-putamen Fos expression was significantly lower than normal. The side-to-side difference in globus pallidus seen in lesioned rats was no longer present in the grafted animals. Apomorphine led to high Fos activation throughout the dopamine-depleted caudate-putamen, whereas only very few immunopositive cells were observed in the intact caudate-putamen. Also in globus pallidus and nucleus accumbens, a significantly higher number of Fos-immunoreactive cells was detected on the denervated side. In the grafted rats, apomorphine-induced Fos activation was similar to normal in all striatal areas sampled, as well as in the globus pallidus. The graft-induced effect extended over a considerably larger area than that covered by the graft-derived tyrosine hydroxylase-immunoreactive innervation. These findings indicate that fetal ventral mesencephalic transplants normalize dopamine receptor-mediated function in the 6-hydroxydopamine-lesioned caudate-putamen and nucleus accumbens, as well as in a primary target of the striatal output neurons, the globus pallidus. The results support the idea that dopamine released from the grafted neurons, both under baseline conditions and after amphetamine administration, exerts functional effects over a larger volume of the striatum than that reached by the graft-derived fibers.