Microglia monitor and protect neuronal function through specialized somatic purinergic junctions

Microglia are the main immune cells in the brain and have roles in brain homeostasis and neurological diseases. Mechanisms underlying microglia-neuron communication remain elusive. Here, we identified an interaction site between neuronal cell bodies and microglial processes in mouse and human brain. Somatic microglia-neuron junctions have a specialized nanoarchitecture optimized for purinergic signaling. Activity of neuronal mitochondria was linked with microglial junction formation, which was induced rapidly in response to neuronal activation and blocked by inhibition of P2Y12 receptors. Brain injury-induced changes at somatic junctions triggered P2Y12 receptor-dependent microglial neuroprotection, regulating neuronal calcium load and functional connectivity. Thus, microglial processes at these junctions could potentially monitor and protect neuronal functions.

Preferential origin and layer destination of GAD65-GFP cortical interneurons

To identify the origin and track the migratory pathway of specific subpopulations of GABAergic interneurons, we studied tangential migration in a recently developed GAD65-GFP transgenic mouse strain. First, we used immunohistochemical methods to characterize the expression of specific neurochemical markers in the GAD65-GFP neurons. Then, organotypic cultures were used in combination with birth-dating studies to determine the time of generation, place of origin and migratory route of these cells. From E14 to E15, the highest density of GAD65-GFP cells was seen in the lower intermediate zone; however, at later stages more GAD65-GFP cells were observed in the subventricular zone. Migratory GAD65-GFP cells express GAD65, but not calretinin or reelin. Surprisingly, only 4% were calbindin immunopositive. At P21, GAD65-GFP cells were found predominantly in layers II-III and expressed calretinin and neuropeptide Y. Remarkably, almost all cholecystokinin-positive but very few parvalbumin-positive neurons expressed GFP. In vitro studies demonstrated that the caudal ganglionic eminence gives rise to a large proportion of GAD65-GFP interneurons and in vivo birth-dating experiments showed that GAD65-GFP interneurons in supragranular layers are born at late embryonic development. Taken together these results support the idea that the destination layer of GABAergic interneurons is closely linked to their place of origin and time of generation.

The zinc finger transcription factor Sp8 regulates the generation and diversity of olfactory bulb interneurons

The molecular mechanisms that regulate the production and diversity of olfactory bulb interneurons remain poorly understood. With the exception of the GABAergic/dopaminergic subtype in the glomerular layer, no information exists concerning the generation of the other subtypes. Here we show that the recently identified zinc finger transcription factor Sp8 is expressed in neurogenic regions, which give rise to olfactory bulb interneurons at embryonic and postnatal time points and remains expressed in the calretinin-expressing and GABAergic/nondopaminergic interneurons of the glomerular layer. Conditional inactivation of Sp8 in the embryonic ventral telencephalon reveals a requirement for the normal generation of these interneuron subtypes. Sp8 conditional mutants exhibit an increase in cell death within the lateral ganglionic eminence and rostral migratory stream. Moreover, mutant neuroblasts/interneurons are misspecified and display abnormal migration patterns in the olfactory bulb, indicating that Sp8 contributes to olfactory bulb interneuron diversity by regulating the survival, migration, and molecular specification of neuroblasts/interneurons.

Stringent specificity in the construction of a GABAergic presynaptic inhibitory circuit

GABAergic interneurons are key elements in neural coding, but the mechanisms that assemble inhibitory circuits remain unclear. In the spinal cord, the transfer of sensory signals to motor neurons is filtered by GABAergic interneurons that act presynaptically to inhibit sensory transmitter release and postsynaptically to inhibit motor neuron excitability. We show here that the connectivity and synaptic differentiation of GABAergic interneurons that mediate presynaptic inhibition is directed by their sensory targets. In the absence of sensory terminals these GABAergic neurons shun other available targets, fail to undergo presynaptic differentiation, and withdraw axons from the ventral spinal cord. A sensory-specific source of brain derived neurotrophic factor induces synaptic expression of the GABA synthetic enzyme GAD65--a defining biochemical feature of this set of interneurons. The organization of a GABAergic circuit that mediates presynaptic inhibition in the mammalian CNS is therefore controlled by a stringent program of sensory recognition and signaling.

Electrical coupling among irregular-spiking GABAergic interneurons expressing cannabinoid receptors

Anatomical studies have shown that the G-protein-coupled cannabinoid receptor-1 (CB1) is selectively expressed in a subset of GABAergic interneurons. It has been proposed that these cells regulate rhythmic activity and play a key role mediating the cognitive actions of marijuana and endogenous cannabinoids. However, the physiology, anatomy, and synaptic connectivity of neocortical CB1-expressing interneurons remain poorly studied. We identified a population of CB1-expressing interneurons in layer II/III in mouse neocortical slices. These cells were multipolar or bitufted, had a widely extending axon, and exhibited a characteristic pattern of irregular spiking (IS) in response to current injection. CB1-expressing-IS (CB1-IS) cells were inhibitory, establishing GABAA receptor-mediated synapses onto pyramidal cells and other CB1-IS cells. Recently, electrical coupling among other classes of cortical interneurons has been shown to contribute to the generation of rhythmic synchronous activity in the neocortex. We therefore tested whether CB1-IS interneurons are interconnected via electrical synapses using paired recordings. We found that 90% (19 of 21 pairs) of simultaneously recorded pairs of CB1-IS cells were electrically coupled. The average coupling coefficient was 6%. Signaling through electrical synapses promoted coordinated firing among CB1-IS cells. Together, our results identify a population of electrically coupled CB1-IS GABAergic interneurons in the neocortex that share a unique morphology and a characteristic pattern of irregular spiking in response to current injection. The synaptic interactions of these cells may play an important role mediating the cognitive actions of cannabinoids and regulating coherent neocortical activity.

P boutons in lamina IX of the rodent spinal cord express high levels of glutamic acid decarboxylase-65 and originate from cells in deep medial dorsal horn

Presynaptic inhibition of primary muscle spindle (group Ia) afferent terminals in motor nuclei of the spinal cord plays an important role in regulating motor output and is produced by a population of GABAergic axon terminals known as P boutons. Despite extensive investigation, the cells that mediate this control have not yet been identified. In this work, we use immunocytochemistry with confocal microscopy and EM to demonstrate that P boutons can be distinguished from other GABAergic terminals in the ventral horn of rat and mouse spinal cord by their high level of the glutamic acid decarboxylase (GAD) 65 isoform of GAD. By carrying out retrograde labeling from lamina IX in mice that express green fluorescent protein under the control of the GAD65 promoter, we provide evidence that the cells of origin of the P boutons are located in the medial part of laminae V and VI. Our results suggest that P boutons represent the major output of these cells within the ventral horn and are consistent with the view that presynaptic inhibition of proprioceptive afferents is mediated by specific populations of interneurons. They also provide a means of identifying P boutons that will be important in studies of the organization of presynaptic control of Ia afferents.

Endocannabinoid-mediated long-term depression of afferent excitatory synapses in hippocampal pyramidal cells and GABAergic interneurons

Although endocannabinoids have emerged as essential retrograde messengers in several forms of synaptic plasticity, it remains controversial whether they mediate long-term depression (LTD) of glutamatergic synapses onto excitatory and inhibitory neurons in the hippocampus. Here, we show that parvalbumin- and somatostatin/metabotropic glutamate receptor 1(a) (mGlu(1a))-positive GABAergic interneurons express diacylglycerol lipase-α (DGL-α), a synthesizing enzyme of the endocannabinoid 2-arachidonoylglycerol (2-AG), albeit at lower levels than principal cells. Moreover, this lipase accumulates postsynaptically around afferent excitatory synapses in all three cell types. To address the role of retrograde 2-AG signaling in LTD, we investigated two forms: (1) produced by postsynaptic spiking paired with subsequent presynaptic stimulation or (2) induced by group I mGlu activation by (S)-3,5-dihydroxyphenylglycine (DHPG). Neither form of LTD was evoked in the presence of the mGlu(5) antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine], the DGL inhibitor THL [N-formyl-l-leucine (1S)-1-[[(2S,3S)-3-hexyl-4-oxo-2-oxetanyl]methyl]dodecyl ester], or the intracellularly applied Ca(2+) chelator BAPTA in CA1 pyramidal cells, fast-spiking interneurons (representing parvalbumin-containing cells) and interneurons projecting to stratum lacunosum-moleculare (representing somatostatin/mGlu(1a)-expressing interneurons). Both forms of LTD were completely absent in CB(1) cannabinoid receptor knock-out mice, whereas pharmacological blockade of CB(1) led to inconsistent results. Notably, in accordance with their lower DGL-α level, a higher stimulation frequency or higher DHPG concentration was required for LTD induction in interneurons compared with pyramidal cells. These findings demonstrate that hippocampal principal cells and interneurons produce endocannabinoids to mediate LTD in a qualitatively similar, but quantitatively different manner. The shifted induction threshold implies that endocannabinoid-LTD contributes to cortical information processing during distinct network activity patterns in a cell type-specific manner.

Lateral hypothalamic GAD65 neurons are spontaneously firing and distinct from orexin- and melanin-concentrating hormone neurons

GABAergic neurons are vital for brain function. Their neurochemical and electrical features have been classically characterized in the cortex, but in the lateral hypothalamic area (LHA), such knowledge is lacking, despite the emerging roles of LHA GABAergic cells in feeding and sleep. We used GAD65-GFP transgenic mice, developed for studies of cortical GABAergic cells, to determine fundamental properties of LHA GAD65 neurons, and compare them to 'classical' GABAergic cell types of the cortex, and to previously described classes of LHA cells. Whole-cell patch-clamp recordings in acute brain slices revealed that, unlike cortical GABAergic interneurons, LHA GAD65 neurons were intrinsically depolarized and fired action potentials spontaneously. Similar to cortical GABAergic cells, LHA GAD65 cells fell into four major subtypes based on evoked firing: fast spiking, late spiking, low threshold spiking and regular spiking. Three-dimensional reconstructions of biocytin-filled neurons, performed after the patch-clamp analysis, did not reveal striking morphological differences between these electrophysiological subtypes. Peptide transmitters expressed in known classes of LHA projection neurons, namely melanin-concentrating hormone (MCH) and hypocretin/orexin (hcrt/orx), were not detected in LHA GAD65 cells. Approximately 40% of LHA GAD65 cells were directly inhibited by physiological increases in extracellular glucose concentration. Glucose inhibition was most prevalent in the fast spiking subpopulation, although some glucose-responsive neurons were found in each electrophysiological subpopulation. These results suggest that LHA GAD65 neurons are electrically different from 'classical' GABAergic neurons of the cortex, are neurochemically distinct from LHA hcrt/orx and MCH cells, but partly resemble hcrt/orx cells in their glucose responses.

NMDA receptors increase the size of GABAergic terminals and enhance GABA release

In developing cerebellar interneurons, NMDA increases spontaneous GABA release by activating presynaptic NMDA receptors. We investigated the role of these receptors on differentiating basket/stellate cells in cerebellar cultures grown under conditions allowing functional synaptic transmission. Presynaptic GABAergic boutons were visualized either by GAD65 immunostaining or by using cells derived from GAD65-enhanced green fluorescent protein (eGFP) transgenic mice, in which cerebellar basket/stellate cells express eGFP. After the first week in culture, whole-cell recordings from granule cells reveal that acute application of NMDA increases miniature IPSC (mIPSC) frequency. Interestingly, after 2 weeks, the mIPSC frequency increases compared with the first week but is not modulated by NMDA. Furthermore, in cultures chronically treated with NMDA for 1 week, the size of the GABAergic boutons increases. This growth is paralleled by increased mIPSC frequency and the loss of NMDA sensitivity. Direct patch-clamp recording from these presynaptic terminals reveals single NMDA-activated channels, showing multiple conductance levels, and electronic propagation from the somatodendritic compartment. Our results demonstrate that NMDA receptors alter GABAergic synapses in developing cerebellar cultures by increasing the size of the terminal and spontaneous GABA release. These findings parallel changes in inhibitory synaptic efficacy seen in vivo in developing GABAergic interneurons of the molecular layer of the cerebellum.

Cannabinoid sensitivity and synaptic properties of 2 GABAergic networks in the neocortex

Distinct networks of gamma-aminobutyric acidergic interneurons connected by electrical synapses can promote different patterns of activity in the neocortex. Cannabinoids affect memory and cognition by powerfully modulating a subset of inhibitory synapses; however, very little is known about the synaptic properties of the cannabinoid receptor-expressing neurons (CB(1)-positive irregular spiking [CB(1)-IS]) in the neocortex. Using paired recordings in neocortical slices, we 1st report here that synapses of CB(1)-IS cells, but not synapses of fast-spiking (FS) cells, are suppressed by release of endocannabinoids from pyramidal neurons. CB(1)-IS synapses were characterized by a very high failure rate that contrasted with the high reliability of FS synapses. Furthermore, CB(1)-IS cells received excitatory inputs less frequently compared with FS cells and made significantly less frequent inhibitory contacts onto local pyramids. These distinct synaptic properties together with their characteristic irregular firing suggest that CB(1)-IS cells play different role in neocortical function than that of FS cells. Thus, whereas the synaptic properties of FS cells can allow them to impose high-frequency rhythmic oscillatory activity, those of CB(1)-IS cells may lead to disruption of fast rhythmic oscillations. Our results suggest that activity-dependent release of cannabinoids, by blocking CB(1)-IS synapses, may alter the role of inhibition in neocortical circuits.

Properties of external plexiform layer interneurons in mouse olfactory bulb slices

In the external plexiform layer (EPL) of the main olfactory bulb, apical dendrites of inhibitory granule cells form large numbers of synapses with mitral and tufted (M/T) cells, which regulate the spread of activity along the M/T cell dendrites. The EPL also contains intrinsic interneurons, the functions of which are unknown. In the present study, recordings were obtained from cell bodies in the EPL of mouse olfactory bulb slices. Biocytin-filling confirmed that the recorded cells included interneurons, tufted cells, and astrocytes. The interneurons had fine, varicose dendrites, and those located superficially bridged the EPL space below several adjacent glomeruli. Interneuron activity was characterized by high frequency spontaneous excitatory postsynaptic potential/currents that were blocked by the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and largely eliminated by the voltage-sensitive Na+ channel blocker, tetrodotoxin. Interneuron activity differed markedly from that of tufted cells, which usually exhibited spontaneous action potential bursts. The interneurons produced few action potentials spontaneously, but often produced them in response to depolarization and/or olfactory nerve (ON) stimulation. The responses to depolarization resembled responses of late- and fast-spiking interneurons found in other cortical regions. The latency and variability of the ON-evoked responses were indicative of polysynaptic input. Interneurons expressing green fluorescent protein under control of the mouse glutamic acid decarboxylase 65 promoter exhibited identical properties, providing evidence that the EPL interneurons are GABAergic. Together, these results suggest that EPL interneurons are excited by M/T cells via AMPA/kainate receptors and may in turn inhibit M/T cells within spatial domains that are topographically related to several adjacent glomeruli.

Transmitter-phenotypes of commissural interneurons in the lumbar spinal cord of newborn mice

Commissural interneurons (CINs) are a necessary component of central pattern generators (CPGs) for locomotion because they mediate the coordination of left and right muscle activity. The projection patterns and relative locations of different classes of CINs in the ventromedial part of the rodent lumbar cord have been described (Eide et al. [1999] J Comp Neurol 403:332-345; Stokke et al. [2002] J Comp Neurol 446:349-359; Nissen et al. [2005] J Comp Neurol 483:30-47). However, the distribution and relative prevalence of different CIN neurotransmitter phenotypes in the ventral region of the mammalian spinal cord where the locomotor CPG is localized is unknown. In this study we describe the relative proportions and anatomical locations of putative inhibitory and excitatory CINs in the lumbar spinal cord of newborn mice. To directly visualize potential neurotransmitter phenotypes we combined retrograde labeling of CINs with in situ hybridization against the glycine transporter, GlyT2, or the vesicular glutamate transporter, vGluT2, in wildtype mice and in transgenic mice expressing eGFP driven by the promoters of glutamic acid decarboxylase (GAD) 65, GAD67, or GlyT2. Our study shows that putative glycinergic, GABAergic, and glutamatergic CINs are expressed in almost equal numbers, with a small proportion of CINs coexpressing GlyT2 and GAD67::eGFP, indicating a putative combined glycinergic/GABAergic phenotype. These different CIN phenotypes were intermingled in laminas VII and VIII. Our results suggest that glycinergic, GABAergic, and glutamatergic CINs are the principal CIN phenotypes in the CPG region of the lumbar spinal cord in the newborn mouse. We compare these results to descriptions of CIN neurotransmitter phenotypes in other vertebrate species.

The small molecule Mek1/2 inhibitor U0126 disrupts the chordamesoderm to notochord transition in zebrafish

Background

Key molecules involved in notochord differentiation and function have been identified through genetic analysis in zebrafish and mice, but MEK1 and 2 have so far not been implicated in this process due to early lethality (Mek1-/-) and functional redundancy (Mek2-/-) in the knockout animals.

Results

Here, we reveal a potential role for Mek1/2 during notochord development by using the small molecule Mek1/2 inhibitor U0126 which blocks phosphorylation of the Mek1/2 target gene Erk1/2 in vivo. Applying the inhibitor from early gastrulation until the 18-somite stage produces a specific and consistent phenotype with lack of dark pigmentation, shorter tail and an abnormal, undulated notochord. Using morphological analysis, in situ hybridization, immunhistochemistry, TUNEL staining and electron microscopy, we demonstrate that in treated embryos the chordamesoderm to notochord transition is disrupted and identify disorganization in the medial layer of the perinotochordal basement mebrane as the probable cause of the undulations and bulges in the notochord. We also examined and excluded FGF as the upstream signal during this process.

Conclusion

Using the small chemical U0126, we have established a novel link between MAPK-signaling and notochord differentiation. Our phenotypic analysis suggests a potential connection between the MAPK-pathway, the COPI-mediated intracellular transport and/or the copper-dependent posttranslational regulatory processes during notochord differentiation.

Local regulation of [(3)H]-noradrenaline release from the isolated guinea-pig right atrium by P(2X)-receptors located on axon terminals

In this study the regulation of cardiac sympathetic outflow by presynaptic P(2X) receptor-gated ion channels was examined. ATP (30 microM - 1 mM) and other P2-receptor agonists elicited [(3)H]-noradrenaline ([(3)H]-NA) outflow from the isolated guinea-pig right atrium with the potency order of ATP>2-methyl-thioATP>alpha,beta-methylene-ATP=ADP, whereas ss, gamma-methylene-L-ATP was inactive. Ca(2+)-free conditions abolished both electrical field stimulation (EFS)- and ATP-evoked release of tritium. Unlike from EFS-induced outflow, ATP-induced [(3)H]-NA outflow was not reduced by omega-Conotoxin-GVIA (100 nM), Cd(2+) (100 microM) and tetrodotoxin (1 microM). The rapid extracellular decomposition of ATP was revealed by HPLC analysis. However, the effect of ATP to promote [(3)H]-NA release was not prevented by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 250 nM), 3, 7-dimethyl-1-propargylxanthine (DMPX, 250 nM), or by reactive blue 2 (RB2, 10 microM), antagonists of A(1)-, A(2)- and inhibitory P(2) receptors. Zn(2+) (50 microM), the P(2X)-receptor modulator potentiated, and P(2X) receptor antagonists, i.e. suramin (300 microM), pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM) and 2'-o-(trinitrophenyl)-adenosine 5'-triphosphate (TNP-ATP, 30 microM) antagonized the ATP (1 mM)-evoked response. RT - PCR study revealed the expression of P(2X2) and P(2X3) receptor mRNAs in guinea-pig superior cervical ganglion. PPADS (30 microM) significantly reduced the EFS-induced [(3)H]-NA outflow in the presence DPCPX (250 nM) and RB2 (10 microM). In summary a P(2X)-type purinoceptor regulates noradrenaline release from the isolated right atrium of the guinea-pig. The pharmacological profile of the receptor resemble to homo-oligomeric P(2X3) or hetero-oligomeric P(2X2)/P(2X3) complexes, and provide a new target to intervene on sympathetic neuroeffector transmission at the presynaptic site.

Visualization of stress-responsive inhibitory circuits in the GAD65-eGFP transgenic mice

Here, we have revealed that a subset of GABAergic neurons in the mouse brain became activated during systemic stress response. Stress-induced expression of immediate early gene product c-Fos, as a marker of neuronal activation was visualized in a transgenic mouse line expressing enhanced green fluorescent protein (eGFP) under the control of the regulatory region of mouse glutamic acid decarboxylase (GAD) 65 gene. In most GABAergic regions egfp transgene expression corresponded to acknowledged distribution of GABA neurons. Ether inhalation, as a strong systemic stressor induced c-Fos expression throughout the stress-related circuit, and did not affect the distribution and expression of the eGFP-transgene. Stress provoked strong neuronal activation in the piriform cortex, midline thalamic nuclei, lateral septum (LS), bed nucleus of the stria terminalis (BNST), and in parvocellular part of the hypothalamic paraventricular nucleus (PVN) as revealed by c-Fos immunfluorescence. Cells in the LS, BNST, and AHA including the subparaventricular zone (SPVZ) displayed significant eGFP/c-Fos co-localization, revealing stress-responsive GABAergic neurons. None of the stress-activated neurons within the medial parvocellular subdivision of the PVN were GABAergic. Our present results suggest that stress-recruited GABAergic neuron populations are preferentially located in distinct limbic and hypothalamic regions and these neurons might be involved in an inhibitory mechanism that counteract the endocrine, autonomic and behavioral aspects of the stress response. Furthermore, the present GAD65-eGFP transgenic model seems to be a relevant tool to analyze inhibitory control of the central stress circuit at single cell level.

P2X receptor activation elicits transporter-mediated noradrenaline release from rat hippocampal slices

This study was designed to test the hypothesis of whether activation of presynaptic P2X receptor-gated ion channels elicits noradrenaline release from central catecholaminergic terminals. ATP, alpha,beta-methylene-adenosine 5'-triphosphate (alpha,beta-methyleneATP), and ADP elicited concentration-dependent [3H]noradrenaline outflow from superfused rat hippocampal slices with the following rank order of agonist potency: alpha,beta-methyleneATP > ATP > ADP. Among P2 receptor antagonists, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (30 microM), 4,4',4",4"'-[carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino))]tetrakis-1,3-benzenedisulfonic acid (100 nM), and 8,8'-[carbonybis(imino-3,1-phenylenecarbonylimino)]bis1,3,5-naphthalenetrisulphonic acid (10 microM) significantly inhibited the outflow of [3H]noradrenaline, evoked by ATP, whereas Brilliant Blue G (100 nM), 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate tetraammonium (10 microM), the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (250 nM), and the A2A receptor antagonist 3,7-dimethyl-1-propargylxanthine (250 nM) were ineffective. Pretreatment with the Gi protein inhibitor pertussis toxin (2.5 microg/ml) did not change the effect of ATP on [3H]noradrenaline outflow. In contrast, a decrease in extracellular pH from 7.4 to 6.6 significantly attenuated the response by ATP. When extracellular Na+ was replaced by choline chloride and in the presence of the noradrenaline uptake inhibitor desipramine (10 microM), the ATP-evoked [3H]noradrenaline outflow was almost completely abolished, indicating that its underlying mechanism is the sodium-dependent reversal of the noradrenaline transporter. Reverse transcription-polymerase chain reaction analysis revealed that mRNA encoding P2X1, P2X2, P2X3, P2X4, P2X6, P2X7 and P2Y1 receptor subunits were expressed in the brainstem containing catecholaminergic nuclei projecting to the hippocampus, whereas mRNA encoding P2X5, P2Y2, P2Y4, and P2Y6 receptors were absent. Taken together, these results indicate that noradrenergic terminals of the rat hippocampus are equipped with presynaptic facilitatory P2X receptors, displaying a pharmacological profile similar to homomeric P2X1 and P2X3 receptors.

Homo- and heteroexchange of adenine nucleotides and nucleosides in rat hippocampal slices by the nucleoside transport system

(1) Here, we investigated how nucleotides and nucleosides affect the release of tritiated purines and endogenous adenosine 5'-triphosphate (ATP) from superfused rat hippocampal slices. (2) ATP elicited concentration-dependent [(3)H]purine efflux from slices preloaded with [(3)H]adenosine. High-performance liquid chromatography analysis of the effluent showed that the tritium label represented the whole set of adenine nucleotides and nucleosides, and ATP significantly increased the outflow of [(3)H]ATP. (3) Adenosine 5'-diphosphate, adenosine, uridine, uridine 5'-triphosphate, alpha,beta-methylene-ATP and 3'-O-(4-benzoylbenzoyl)-ATP were also active in eliciting [(3)H]purine release. Adenosine (300 micro M) also evoked endogenous ATP efflux from the hippocampal slices. (4) Reverse transcription-coupled-polymerase chain reaction analysis revealed that mRNAs encoding a variety of P2X and P2Y receptor proteins are expressed in the rat hippocampus. Nevertheless, neither P2 receptor (i.e. pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid, 30 micro M, suramin, 300 micro M and reactive blue 2, 10 micro M), nor adenosine receptor (8-cyclopentyl-1,3-dipropylxanthine, 250 nM and dimethyl-1-propargylxanthine, 250 nM) antagonists modified the effect of ATP (300 micro M) to evoke [(3)H]purine release. (5) The nucleoside transport inhibitors, dipyridamole (10 micro M), nitrobenzylthioinosine (10 micro M) and adenosine deaminase (2-10 U ml(-1)), but not the ecto-adenylate kinase inhibitor diadenosine pentaphosphate (200 micro M) significantly reduced ATP-evoked [(3)H]purine efflux. (6) In summary, we found that ATP and other nucleotides and nucleosides promote the release of one another and themselves by the nucleoside transport system. This action could have relevance during physiological and pathological elevation of extracellular purine levels high enough to reverse the nucleoside transporter.

Novel interneuronal network in the mouse posterior piriform cortex

The neural circuits of the piriform cortex mediate field potential oscillations and complex functions related to integrating odor cues with behavior, affective states, and multisensory processing. Previous anatomical studies have established major neural pathways linking the piriform cortex to other cortical and subcortical regions and major glutamatergic and GABAergic neuronal subtypes within the piriform circuits. However, the quantitative properties of diverse piriform interneurons are unknown. Using quantitative neural anatomical analysis and electrophysiological recording applied to a GAD65-EGFP transgenic mouse expressing GFP (green fluorescent protein) under the control of the GAD65 promoter, here we report a novel inhibitory network that is composed of neurons positive for GAD65-EGFP in the posterior piriform cortex (PPC). These interneurons had stereotyped dendritic and axonal properties that were distinct from basket cells or interneurons expressing various calcium-binding proteins (parvalbumin, calbindin, and calretinin) within the PPC. The GAD65-GFP neurons are GABAergic and outnumbered any other interneurons (expressing parvalbumin, calbindin, and calretinin) we studied. The firing pattern of these interneurons was highly homogenous and is similar to the regular-spiking nonpyramidal (RSNP) interneurons reported in primary sensory and other neocortical regions. Robust dye coupling among these interneurons and expression of connexin 36 suggested that they form electrically coupled networks. The predominant targets of descending axons of these interneurons were the dendrites of Layer III principal cells. Additionally, synapses were found on dendrites and somata of deep Layer II principal neurons and Layer III basket cells. A similar interneuronal subtype was also found in GAD65-EGFP-negative mouse. The extensive dendritic bifurcation at superficial lamina IA among horizontal afferent fibers and unique axonal targeting pattern suggests that these interneurons may play a role in direct feedforward inhibitory and disinhibitory olfactory processing. We conclude that the GAD65-GFP neurons may play distinct roles in regulating information flow and olfactory-related oscillation within the PPC in vivo.

Hypothalamic CNTF volume transmission shapes cortical noradrenergic excitability upon acute stress

Stress-induced cortical alertness is maintained by a heightened excitability of noradrenergic neurons innervating, notably, the prefrontal cortex. However, neither the signaling axis linking hypothalamic activation to delayed and lasting noradrenergic excitability nor the molecular cascade gating noradrenaline synthesis is defined. Here, we show that hypothalamic corticotropin-releasing hormone-releasing neurons innervate ependymal cells of the 3rd ventricle to induce ciliary neurotrophic factor (CNTF) release for transport through the brain's aqueductal system. CNTF binding to its cognate receptors on norepinephrinergic neurons in the locus coeruleus then initiates sequential phosphorylation of extracellular signal-regulated kinase 1 and tyrosine hydroxylase with the Ca2+-sensor secretagogin ensuring activity dependence in both rodent and human brains. Both CNTF and secretagogin ablation occlude stress-induced cortical norepinephrine synthesis, ensuing neuronal excitation and behavioral stereotypes. Cumulatively, we identify a multimodal pathway that is rate-limited by CNTF volume transmission and poised to directly convert hypothalamic activation into long-lasting cortical excitability following acute stress.

Paracellular and transcellular migration of metastatic cells through the cerebral endothelium

Breast cancer and melanoma are among the most frequent cancer types leading to brain metastases. Despite the unquestionable clinical significance, important aspects of the development of secondary tumours of the central nervous system are largely uncharacterized, including extravasation of metastatic cells through the blood-brain barrier. By using transmission electron microscopy, here we followed interactions of cancer cells and brain endothelial cells during the adhesion, intercalation/incorporation and transendothelial migration steps. We observed that brain endothelial cells were actively involved in the initial phases of the extravasation by extending filopodia-like membrane protrusions towards the tumour cells. Melanoma cells tended to intercalate between endothelial cells and to transmigrate by utilizing the paracellular route. On the other hand, breast cancer cells were frequently incorporated into the endothelium and were able to migrate through the transcellular way from the apical to the basolateral side of brain endothelial cells. When co-culturing melanoma cells with cerebral endothelial cells, we observed N-cadherin enrichment at melanoma-melanoma and melanoma-endothelial cell borders. However, for breast cancer cells N-cadherin proved to be dispensable for the transendothelial migration both in vitro and in vivo. Our results indicate that breast cancer cells are more effective in the transcellular type of migration than melanoma cells.

Construction of recombinant pseudorabies viruses optimized for labeling and neurochemical characterization of neural circuitry

In this study we have modified the neuroinvasiveness of pseudorabies virus strain Bartha, a commonly utilized trans-synaptic tract-tracer. In addition, we sought to facilitate detection of cellular mRNAs in neurons infected with the virus. In order to modify spreading characteristics, we inserted the lacZ or the GFP (green fluorescent protein) genes into the genomic loci containing the putative latency-associated transcript promoter (P(LAT2)), resulting in the disruption of the promoter function. Following rat kidney injection, mutant viruses labeled central autonomic neurons in a slower and much more restricted manner than the parent Bartha strain. Since both reporter genes were controlled by the human cytomegalovirus immediate early (IE) 1 promoter, they exhibited IE expression kinetics. This property proved to be important for the co-detection of reporter proteins with neuronal mRNAs, readily detected at early but not at late stage of infection, as shown in tyrosine-hydroxylase expressing A5 catecholaminergic neurons and in serotonin transporter expressing raphe magnus neurons.

Hyperpolarization-activated and cyclic nucleotide-gated cation channel subunit 2 ion channels modulate synaptic transmission from nociceptive primary afferents containing substance P to secondary sensory neurons in laminae I-IIo of the rodent spinal dorsal horn

We have previously demonstrated that hyperpolarization-activated and cyclic nucleotide-gated cation channel subunit 2 (HCN2) is expressed by terminals of peptidergic nociceptive primary afferents in laminae I-IIo of the rat spinal dorsal horn. In this study, we investigated the possible neurotransmitters and postsynaptic targets of these HCN2-expressing primary afferent terminals in the superficial spinal dorsal horn by using immunocytochemical methods. We demonstrated that HCN2 widely colocalizes with substance P (SP), and that HCN2-positive terminals that are also immunoreactive for SP form serial close appositions with dendrites and perikarya of neurokinin 1 receptor-immunoreactive neurons. It was also found that HCN2-immunoreactive terminals are frequently apposed to neurons that are immunoreactive for calbindin, micro-opioid receptor and the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor subunit GluR2, markers for excitatory interneurons. Investigating HCN2 immunoreactivity in glutamic acid decarboxylase 65-green fluorescent protein transgenic mice, we found that HCN2-positive terminals occasionally also contact cells that contain an isoform of glutamic acid decarboxylase (glutamic acid decarboxylase 65), a marker for GABAergic inhibitory neurons. Application of ZD7288, an antagonist of HCN channels, onto neurons that were recorded in spinal cord slices with whole-cell patch-clamp electrodes reduced the number of monosynaptic excitatory postsynaptic potentials evoked by electrical stimulation of primary afferents at nociceptive intensities. The results suggest that HCN2 may contribute to the modulation of membrane excitability of SP-containing nociceptive primary afferent terminals, may increase the reliability of synaptic transmission from primary afferents to secondary sensory neurons and thus may play a role in the fine-tuning of pain transmission from nociceptive primary afferents to neurons in the spinal dorsal horn.

Properties of a population of GABAergic cells in murine auditory cortex weakly excited by thalamic stimulation

Feedforward inhibition triggered by thalamocortical (TC) afferents sharpens onset responses and shapes receptive fields of pyramidal cells in auditory cortex (ACx). Previous studies focused only on interneurons located in and around layer IV in primary ACx, target of the dense thalamic projections from ventral medial geniculate. We investigated a population of feedforward interneurons located throughout layers I-V and activated by both afferents from primary and nonprimary thalamus using recordings from auditory TC brain slices obtained from mice expressing green fluorescent protein under control of the glutamic acid decarboxylase (GAD65) promoter in a subpopulation of cortical GABAergic cells. We studied the responses of these interneurons and of pyramidal cells in ACx to thalamic stimulation and to hyper- and depolarizing current pulses. Most interneurons exhibited monosynaptic responses to thalamic stimulation, but this excitation was weak and subthreshold. Interneurons had multipolar dendritic morphology with widespread and dense axonal projections extending several hundred micrometers from the soma. In pyramidal cells from layers II-IV, thalamic excitatory postsynaptic potentials were significantly larger than in interneurons and were superthreshold in 40% of cells, but in these cells, there was no evidence of feedforward inhibition. By contrast, feedforward inhibition was observed in 12 of 18 layer V pyramidal cells. Thus feedforward inhibition in supragranular layers of ACx is weak, and these interneurons require coincident excitation to be activated by thalamic inputs.

The cryptic gonadotropin-releasing hormone neuronal system of human basal ganglia

Human reproduction is controlled by ~2000 hypothalamic gonadotropin-releasing hormone (GnRH) neurons. Here, we report the discovery and characterization of additional ~150,000–200,000 GnRH-synthesizing cells in the human basal ganglia and basal forebrain. Nearly all extrahypothalamic GnRH neurons expressed the cholinergic marker enzyme choline acetyltransferase. Similarly, hypothalamic GnRH neurons were also cholinergic both in embryonic and adult human brains. Whole-transcriptome analysis of cholinergic interneurons and medium spiny projection neurons laser-microdissected from the human putamen showed selective expression of GNRH1 and GNRHR1 autoreceptors in the cholinergic cell population and uncovered the detailed transcriptome profile and molecular connectome of these two cell types. Higher-order non-reproductive functions regulated by GnRH under physiological conditions in the human basal ganglia and basal forebrain require clarification. The role and changes of GnRH/GnRHR1 signaling in neurodegenerative disorders affecting cholinergic neurocircuitries, including Parkinson’s and Alzheimer’s diseases, need to be explored.