Norepinephrine triggers release of glial ATP to increase postsynaptic efficacy

P2 receptors for extracellular nucleotides in the central nervous system: role of P2X7 and P2Y₂ receptor interactions in neuroinflammation

Extracellular nucleotides induce cellular responses in the central nervous system (CNS) through the activation of ionotropic P2X and metabotropic P2Y nucleotide receptors. Activation of these receptors regulates a wide range of physiological and pathological processes. In this review, we present an overview of the current literature regarding P2X and P2Y receptors in the CNS with a focus on the contribution of P2X7 and P2Y(2) receptor-mediated responses to neuroinflammatory and neuroprotective mechanisms.

Glial regulation of neuronal function: from synapse to systems physiology

Classically, glia have been regarded as non-excitable cells that provide nourishment and physical scaffolding for neurones. However, it is now generally accepted that glia are active participants in brain function that can modulate neuronal communication via several mechanisms. Investigations of anatomical plasticity in the magnocellular neuroendocrine system of the hypothalamic paraventricular and supraoptic nuclei led the way in the development of much of our understanding of glial regulation of neuronal activity. In this review, we provide an overview of glial regulation of magnocellular neurone activity from a historical perspective of the development of our knowledge of the morphological changes that are evident in the paraventricular and supraoptic nuclei. We also focus on recent data from the authors' laboratories presented at the 9th World Congress on Neurohypophysial Hormones that have contributed to our understanding of the multiple mechanisms by which glia modulate the activity of neurones, including: gliotransmitter modulation of synaptic transmission; trans-synaptic modulation by glial neurotransmitter transporter regulation of neurotransmitter spillover; and glial neurotransmitter transporter modulation of excitability by regulation of ambient neurotransmitter levels and their action on extrasynaptic receptors. The magnocellular neuroendocrine system secretes oxytocin and vasopressin from the posterior pituitary gland to control birth, lactation and body fluid balance, and we finally speculate as to whether glial regulation of individual magnocellular neurones might co-ordinate population activity to respond appropriately to altered physiological circumstances.

Glial cells in (patho)physiology

Neuroglial cells define brain homeostasis and mount defense against pathological insults. Astroglia regulate neurogenesis and development of brain circuits. In the adult brain, astrocytes enter into intimate dynamic relationship with neurons, especially at synaptic sites where they functionally form the tripartite synapse. At these sites, astrocytes regulate ion and neurotransmitter homeostasis, metabolically support neurons and monitor synaptic activity; one of the readouts of the latter manifests in astrocytic intracellular Ca(2+) signals. This form of astrocytic excitability can lead to release of chemical transmitters via Ca(2+) -dependent exocytosis. Once in the extracellular space, gliotransmitters can modulate synaptic plasticity and cause changes in behavior. Besides these physiological tasks, astrocytes are fundamental for progression and outcome of neurological diseases. In Alzheimer's disease, for example, astrocytes may contribute to the etiology of this disorder. Highly lethal glial-derived tumors use signaling trickery to coerce normal brain cells to assist tumor invasiveness. This review not only sheds new light on the brain operation in health and disease, but also points to many unknowns.

Endothelin-mediated calcium responses in supraoptic nucleus astrocytes influence magnocellular neurosecretory firing activity

In addition to their peripheral vasoactive effects, accumulating evidence supports an important role for endothelins (ETs) in the regulation of the hypothalamic magnocellular neurosecretory system, which produces and releases the neurohormones vasopressin (VP) and oxytocin (OT). Still, the precise cellular substrates, loci and mechanisms underlying the actions of ETs on the magnocellular system are poorly understood. In the present study, we combined patch-clamp electrophysiology, confocal Ca(2+) imaging and immunohistochemistry to study the actions of ETs on supraoptic nucleus (SON) magnocellular neurosecretory neurones and astrocytes. Our studies show that ET-1 evoked rises in [Ca(2+) ](i) levels in SON astrocytes (but not neurones), an effect largely mediated by the activation of ET(B) receptors and mobilisation of thapsigargin-sensitive Ca(2+) stores. The presence of ET(B) receptors in SON astrocytes was also verified immunohistochemically. ET(B) receptor activation either increased (75%) or decreased (25%) SON firing activity, both in VP and putative OT neurones, and these effects were prevented when slices were preincubated in glutamate receptor blockers or nitric oxide synthase blockers, respectively. Moreover, ET(B) -mediated effects in SON neurones were also prevented by a gliotoxin compound, and when changes in [Ca(2+) ](i) were prevented with bath-applied BAPTA-AM or thapsigargin. Conversely, intracellular Ca(2+) chelation in the recorded SON neurones failed to block ET(B) -mediated effects. In summary, our results indicate that ET(B) receptor activation in SON astrocytes induces the mobilisation of [Ca(2+) ](i) , likely resulting in the activation of glutamate and nitric oxide signalling pathways, evoking in turn excitatory and inhibitory SON neuronal responses, respectively. Taken together, our study supports an important role for astrocytes in mediating the actions of ETs on the magnocellular neurosecretory system.

Rod and cone pathway signalling is altered in the P2X7 receptor knock out mouse

The P2X7 receptor (P2X7-R) is expressed in the retina and brain and has been implicated in neurodegenerative diseases. However, whether it is expressed by neurons and plays a role as a neurotransmitter receptor has been the subject of controversy. In this study, we first show that the novel vesicular transporter for ATP, VNUT, is expressed in the retina, verifying the presence of the molecular machinery for ATP to act as neurotransmitter at P2X7-Rs. Secondly we show the presence of P2X7-R mRNA and protein in the retina and cortex and absence of the full length variant 1 of the receptor in the P2X7-R knock out (P2X7-KO) mouse. The role of the P2X7-R in neuronal function of the retina was assessed by comparing the electroretinogram response of P2X7-KO with WT mice. The rod photoreceptor response was found to be similar, while both rod and cone pathway post-photoreceptor responses were significantly larger in P2X7-KO mice. This suggests that activation of P2X7-Rs modulates output of second order retinal neurons. In line with this finding, P2X7-Rs were found in the outer plexiform layer and on inner retinal cell classes, including horizontal, amacrine and ganglion cells. The receptor co-localized with conventional synapses in the IPL and was expressed on amacrine cells post-synaptic to rod bipolar ribbon synapses. In view of the changes in visual function in the P2X7-KO mouse and the immunocytochemical location of the receptor in the normal retina, it is likely the P2X7-R provides excitatory input to photoreceptor terminals or to inhibitory cells that shape both the rod and cone pathway response.

Bidirectional coupling between astrocytes and neurons mediates learning and dynamic coordination in the brain: a multiple modeling approach

In recent years research suggests that astrocyte networks, in addition to nutrient and waste processing functions, regulate both structural and synaptic plasticity. To understand the biological mechanisms that underpin such plasticity requires the development of cell level models that capture the mutual interaction between astrocytes and neurons. This paper presents a detailed model of bidirectional signaling between astrocytes and neurons (the astrocyte-neuron model or AN model) which yields new insights into the computational role of astrocyte-neuronal coupling. From a set of modeling studies we demonstrate two significant findings. Firstly, that spatial signaling via astrocytes can relay a "learning signal" to remote synaptic sites. Results show that slow inward currents cause synchronized postsynaptic activity in remote neurons and subsequently allow Spike-Timing-Dependent Plasticity based learning to occur at the associated synapses. Secondly, that bidirectional communication between neurons and astrocytes underpins dynamic coordination between neuron clusters. Although our composite AN model is presently applied to simplified neural structures and limited to coordination between localized neurons, the principle (which embodies structural, functional and dynamic complexity), and the modeling strategy may be extended to coordination among remote neuron clusters.

Electroconvulsive therapy: a novel hypothesis for the involvement of purinergic signalling

It is proposed that ATP is released from both neurons and glia during electroconvulsive therapy (ECT) and that this leads to reduction of depressive behaviour via complex stimulation of neurons and glia directly via P2X and P2Y receptors and also via P1 receptors after extracellular breakdown of ATP to adenosine. In particular, A(1) adenosine receptors inhibit release of excitatory transmitters, and A(2A) and P2Y receptors may modulate the release of dopamine. Sequential ECT may lead to changes in purinoceptor expression in mesolimbic and mesocortical regions of the brain implicated in depression and other mood disorders. In particular, increased expression of P2X7 receptors on glial cells would lead to increased release of cytokines, chemokines and neurotrophins. In summary, we suggest that ATP release following ECT involves neurons, glial cells and neuron-glial interactions acting via both P2 and after breakdown to adenosine via P1 receptors. We suggest that ecto-nucleotidase inhibitors (increasing available amounts of ATP) and purinoceptor agonists may enhance the anti-depressive effect of ECT.

Diacylglycerol signaling underlies astrocytic ATP release

Astrocytes have the ability to modulate neuronal excitability and synaptic transmission by the release of gliotransmitters. The importance of ATP released downstream of the activation of Gq-coupled receptors has been well established, but the mechanisms by which this release is regulated are unclear. The current work reveals that the elevation of diacylglycerol (DAG) in astrocytes induces vesicular ATP release. Unexpectedly, DAG-induced ATP release was found to be independent of PKC activation, but dependent upon activation of a C1 domain-containing protein. Astrocytes express the C1 domain-containing protein Munc13-1, which has been implicated in neuronal transmitter release, and RNAi-targeted downregulation of Munc13-1 inhibits astrocytic ATP release. These studies demonstrate that elevations of DAG induce the exocytotic release of ATP in astrocytes, likely via a Munc13-1-dependent mechanism.

Mitochondrial calcium signaling mediates rhythmic extracellular ATP accumulation in suprachiasmatic nucleus astrocytes

The master circadian pacemaker located within the suprachiasmatic nuclei (SCN) controls neural and neuroendocrine rhythms in the mammalian brain. Astrocytes are abundant in the SCN, and this cell type displays circadian rhythms in clock gene expression and extracellular accumulation of ATP. Still, the intracellular signaling pathways that link the SCN clockworks to circadian rhythms in extracellular ATP accumulation remain unclear. Because ATP release from astrocytes is a calcium-dependent process, we investigated the relationship between intracellular Ca(2+) and ATP accumulation and have demonstrated that intracellular Ca(2+) levels fluctuate in an antiphase relationship with rhythmic ATP accumulation in rat SCN2.2 cell cultures. Furthermore, mitochondrial Ca(2+) levels were rhythmic and maximal in precise antiphase with the peak in cytosolic Ca(2+). In contrast, our finding that peak mitochondrial Ca(2+) occurred during maximal extracellular ATP accumulation suggests a link between these cellular rhythms. Inhibition of the mitochondrial Ca(2+) uniporter disrupted the rhythmic production and extracellular accumulation of ATP. ATP, calcium, and the biological clock affect cell division and have been implicated in cell death processes. Nonetheless, rhythmic extracellular ATP accumulation was not disrupted by cell cycle arrest and was not correlated with caspase activity in SCN2.2 cell cultures. Together, these results demonstrate that mitochondrial Ca(2+) mediates SCN2.2 rhythms in extracellular ATP accumulation and suggest a role for circadian gliotransmission in SCN clock function.

Substantial changes in synaptic firing frequencies induced by glial ATP hysteresis

Recent experimental studies strongly suggest the influence of glial purinergic transmission in the modulation of synaptic dynamics. By releasing adenosine triphosphate (ATP), which accumulates as adenosine, astrocytes tonically suppressed synaptic transmission. The delayed multi-step feedback of the glial ATP with the neuron suggests the existence of a discrete hysteresis phenomena. By integration of this hysteretic behavior into a delayed leaky integrate-and-fire model for the tripartite synapses, a significant sensitivity of the pre- and postsynaptic firing frequency patterns to the adenosine feedback-delays is observed that might be of importance for adenosine-related neurological deficits, such as sleep disorders.

Morphology and properties of astrocytes

Astrocytes were identified about 150 years ago, and, for the longest time, were considered to be supporting cells in the brain providing trophic, metabolic, and structural support for neural networks. Research in the last 2 decades has uncovered many novel molecules in astrocytes and the finding that astrocytes communicate with neurons via Ca2+ signaling, which leads to release of chemical transmitters, termed gliotransmitters, has led to renewed interest in their biology. This chapter will briefly review the unique morphology and molecular properties of astrocytes. The reader will be introduced to the role of astrocytes in blood-brain barrier (BBB) maintenance, in Ca2+ signaling, in synaptic transmission, in CNS synaptogenesis, and as neural progenitor cells. Mention is also made of the diseases in which astrocyte dysfunction has a role.

Morphological plasticity of the rat supraoptic nucleus--cellular consequences

The supraoptic nuclei of the hypothalamus display a remarkable anatomical plasticity during lactation, parturition and chronic dehydration, conditions associated with massive neurohypophysial hormone secretion. This structural remodeling is characterized by a pronounced reduction of the astrocytic coverage of oxytocin neurons, resulting in an increase in the number and extent of directly juxtaposed neuronal surfaces. Although the exact role played by such an anatomical remodeling in the physiology of the hypothalamo-neurohypophysial system is still unknown, several findings obtained over the last decade indicate that synaptic and extrasynaptic transmissions are impacted by these structural changes. We review these data and try to extrapolate how such changes at the cellular level might affect the overall activity of the system. One repercussion of the retraction of glial processes is the accumulation of glutamate in the extracellular space. This build-up of glutamate causes an increased activation of pre-synaptic metabotropic glutamate receptors, which are negatively coupled to neurotransmitter release, and a switch in the mode of action of pre-synaptic kainate receptors that control GABA release. Finally, the range of action of substances released from astrocytes and acting on adjacent magnocellular neurons is also affected during the anatomical remodeling. It thus appears that the structural plasticity of the hypothalamic magnocellular nuclei strongly affects neuron-glial interactions and, as a consequence, induces significant changes in synaptic and extrasynaptic transmission.

Zinc enhances long-term potentiation through P2X receptor modulation in the hippocampal CA1 region

Zn²(+) is an essential ion that is stored in and co-released from glutamatergic synapses and it modulates neurotransmitter receptors involved in long-term potentiation (LTP). However, the mechanism(s) underlying Zn²(+) -induced modulation of LTP remain(s) unclear. As the purinergic P2X receptors are relevant targets for Zn²(+) action, we have studied their role in LTP modulation by Zn²(+) in the CA1 region of rat hippocampal slices. Induction of LTP in the presence of Zn²(+) revealed a biphasic effect - 5-50 μm enhanced LTP induction, whereas 100-300 μm Zn²(+) inhibited LTP. The involvement of a purinergic mechanism is supported by the fact that application of the P2X receptor antagonists 2',3'-O-(2,4,6-trinitrophenyl) ATP (TNP-ATP) and periodate-oxidized ATP fully abolished the facilitatory effect of Zn²(+) . Notably, application of the P2X₇ receptor-specific antagonist Brilliant Blue G did not modify the Zn²(+) -dependent facilitation of LTP. Exogenous ATP also produced a biphasic effect - 0.1-1 μm ATP facilitated LTP, whereas 5-10 μm inhibited LTP. The facilitatory effect of ATP was abolished by the application of TNP-ATP and was modified in the presence of 5 μm Zn²(+) , suggesting that P2X receptors are involved in LTP induction and that Zn²(+) leads to an increase in the affinity of P2X receptors for ATP. The latter confirms our previous results from heterologous expression systems. Collectively, our results indicate that Zn²(+) at low concentrations enhances LTP by modulating P2X receptors. Although it is not yet clear which purinergic receptor subtype(s) is responsible for these effects on LTP, the data presented here suggest that P2X₄ but not P2X₇ is involved.

Ionotropic ATP receptors in neuronal-glial communication

In the central nervous system ATP is released from both neurones and astroglial cells acting as a homo- and heterocellular neurotransmitter. Glial cells express numerous purinoceptors of both ionotropic (P2X) and metabotropic (P2Y) varieties. Astroglial P2X receptors can be activated by ongoing synaptic transmission and can mediate fast local signalling through elevation in cytoplasmic Ca(2+) and Na(+) concentrations. These ionic signals can be translated into various physiological messages by numerous pathways, including release of gliotransmitters, metabolic support of neurones and regulation of activity of postsynaptic glutamate and GABA receptors. Ionotropic purinoceptors represent a novel pathway of glia-driven modulation of synaptic signalling that involves the release of ATP from neurones and astrocytes followed by activation of P2X receptors which can regulate synaptic activity by variety of mechanisms expressed in both neuronal and glial compartments.

EphrinBs regulate D-serine synthesis and release in astrocytes

There is growing evidence that astrocytes play critical roles in neuron-glial interactions at the synapse. Astrocytes are believed to regulate presynaptic and postsynaptic structures and functions, in part, by the release of gliotransmitters such as glutamate, ATP, and d-serine; however, little is known of how neurons and astrocytes communicate to regulate these processes. Here, we investigated a family of transmembrane proteins called ephrinBs and Eph receptors that are expressed in the synapse and are known to regulate synaptic transmission and plasticity. In addition to their presence on CA1 hippocampal neurons, we determined that ephrins and Eph receptors are also expressed on hippocampal astrocytes. Stimulation of hippocampal astrocytes with soluble ephrinB3, known to be expressed on CA1 postsynaptic dendrites, enhanced d-serine synthesis and release in culture. Conversely, ephrinB3 had no effect on d-serine release from astrocytes deficient in EphB3 and EphA4, which are the primary receptors for ephrinB3. Eph receptors mediate this response through interactions with PICK1 (protein interacting with C-kinase) and by dephosphorylating protein kinase C α to activate the conversion of l-serine to d-serine by serine racemase. These findings are supported in vivo, where reduced d-serine levels and synaptic transmissions are observed in the absence of EphB3 and EphA4. These data support a role for ephrins and Eph receptors in regulating astrocyte gliotransmitters, which may have important implications on synaptic transmission and plasticity.

The effect of glycogen phosphorolysis on basal glutaminergic transmission

Astrocytic glycogen metabolism sustains neuronal activity but its impact on basal glutamatergic synaptic transmission is not clear. To address this issue, we have compared the effect of glycogen breakdown inhibition on miniature excitatory postsynaptic currents (mEPSCs) in rat hippocampal pure neuronal culture (PNC) and in astrocyte-neuronal co-cultures (ANCC). Amplitudes of mEPSC in ANCC were nearly twice as large as in PNC with no difference in current kinetics. Inhibition of glycogen phosphorylase reduced mEPSC amplitude by roughly 40% in ANCC being ineffective in PNC. Altogether, these data indicate that astrocyte-neuronal interaction enhances basal mEPSCs in ANCC mainly due to astrocytic glycogen metabolism.

Neuronal sodium leak channel is responsible for the detection of sodium in the rat median preoptic nucleus

Sodium (Na(+)) ions are of primary importance for hydromineral and cardiovascular homeostasis, and the level of Na(+) in the body fluid compartments [plasma and cerebrospinal fluid (CSF)] is precisely monitored in the hypothalamus. Glial cells seem to play a critical role in the mechanism of Na(+) detection. However, the precise role of neurons in the detection of extracellular Na(+) concentration ([Na(+)](out)) remains unclear. Here we demonstrate that neurons of the median preoptic nucleus (MnPO), a structure in close contact with the CSF, are specific Na(+) sensors. Electrophysiological recordings were performed on dissociated rat MnPO neurons under isotonic [Na(+)] (100 mM NaCl) with local application of hypernatriuric (150, 180 mM NaCl) or hyponatriuric (50 mM NaCl) external solution. The hyper- and hyponatriuric conditions triggered an in- and an outward current, respectively. The reversal potential of the current matched the equilibrium potential of Na(+), indicating that a change in [Na(+)](out) modified the influx of Na(+) in the MnPO neurons. The conductance of the Na(+) current was not affected by either the membrane potential or the [Na(+)](out). Moreover, the channel was highly selective for lithium over guanidinium. Together, these data identified the channel as a Na(+) leak channel. A high correlation between the electrophysiological recordings and immunofluorescent labeling for the Na(X) channel in dissociated MnPO neurons strongly supports this channel as a candidate for the Na(+) leak channel responsible for the Na(+)-sensing ability of rat MnPO neurons. The absence of Na(X) labeling and of a specific current evoked by a change in [Na(+)](out) in mouse MnPO neurons suggests species specificity in the hypothalamus structures participating in central Na(+) detection.

Multiple alpha1-adrenergic receptor subtypes support synergistic stimulation of vasopressin and oxytocin release by ATP and phenylephrine

Simultaneous exposure of explants of the hypothalamo-neurohypophyseal system (HNS) to ATP and the α(1)-adrenergic receptor (α(1)-R) agonist, phenylephrine (ATP+PE) induces a synergistic stimulation of vasopressin and oxytocin (VP/OT) release that is sustained for hours. The current studies confirm that the synergism is dependent upon activation of α(1)-R by demonstrating that an α(1)-R antagonist prevents the response. The role of the α(1)A, B, and D-adrenergic receptor subtypes in the synergistic effect of ATP+PE on intracellular calcium ([Ca(2+)](i)) in supraoptic nucleus (SON) neurons and VP/OT release from neural lobe was evaluated. The increase in [Ca(2+)](i) induced by PE in SON predominantly reflects release from intracellular stores and is mediated by activation of the α(1)A adrenergic receptor subtype. The α(1)A subtype is also required for the sustained elevation in [Ca(2+)](i) induced by ATP+PE. In contrast, although synergistic stimulation of VP/OT release was eliminated by removal of PE and was blunted by benoxathian, an α(1)-R antagonist that is not subtype selective, no single α(1)-R subtype selective antagonist prevented sustained stimulation of VP/OT release by ATP+PE. Thus, sustained activation of α(1)-R is essential for the synergistic VP and OT response to ATP+PE, but multiple α(1)-R subtypes can support the response. Redundancy amongst the α(1)-R subunits in supporting this response is consistent with the predicted importance of the response for sustaining the elevated VP release required to prevent cardiovascular collapse during hemorrhage and sepsis.

Integrated brain circuits: neuron-astrocyte interaction in sleep-related rhythmogenesis

Although astrocytes are increasingly recognized as important modulators of neuronal excitability and information transfer at the synapse, whether these cells regulate neuronal network activity has only recently started to be investigated. In this article, we highlight the role of astrocytes in the modulation of circuit function with particular focus on sleep-related rhythmogenesis. We discuss recent data showing that these glial cells regulate slow oscillations, a specific thalamocortical activity that characterizes non-REM sleep, and sleep-associated behaviors. Based on these findings, we predict that our understanding of the genesis and tuning of thalamocortical rhythms will necessarily go through an integrated view of brain circuits in which non-neuronal cells can play important neuromodulatory roles.

Modulation of arterial pressure by P2 purinoceptors in the paraventricular nucleus of the hypothalamus of awake rats

In the present study we evaluated the role of purinergic mechanisms in the PVN on the tonic modulation of the autonomic function to the cardiovascular system as well on the cardiovascular responses to peripheral chemoreflex activation in awake rats. Guide-cannulae were bilaterally implanted in the direction of the PVN of male Wistar rats. Femoral artery and vein were catheterized one day before the experiments. Chemoreflex was activated with KCN (80 μg/0.05 ml, i.v.) before and after microinjections of P2 receptors antagonist into the PVN. Microinjection of PPADS, a non selective P2X antagonist, into the PVN (n=6) produced a significant increase in the baseline MAP (99±2 vs 112±3 mmHg) and HR (332±8 vs 375±8 bpm) but had no effect on the pressor and bradycardic responses to chemoreflex activation. Intravenous injection of vasopressin receptors antagonist after microinjection of PPADS into the PVN produced no effect on the increased baseline MAP. Simultaneous microinjection of PPADS and KYN into the PVN (n=6) had no effect in the baseline MAP, HR or in the pressor and bradycardic responses to chemoreflex activation. We conclude that P2 purinoceptors in the PVN are involved in the modulation of baseline autonomic function to the cardiovascular system but not in the cardiovascular responses to chemoreflex activation in awake rats.

Glutamatergic synaptic transmission in neuroendocrine cells: Basic principles and mechanisms of plasticity

Glutamate synapses drive the output of neuroendocrine cells in the hypothalamus, but until recently, relatively little was known about the fundamental properties of transmission at these synapses. Here we review recent advances in the understanding of glutamate signals in magnocellular neurosecretory cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus that serve as the last step in synaptic integration before neurohormone release. While these synapses exhibit many similarities with other glutamate synapses described throughout the brain, they also exhibit a number of unique properties that are particularly well suited to the physiology of this system and will be discussed here. In addition, a number of recent studies begin to provide insights into new forms of synaptic plasticity that may be common in other brain regions, but in these cells, may serve important adaptive roles.

Bidirectional neuron-glia interactions triggered by deficiency of glutamate uptake at spinal sensory synapses

Bidirectional interactions between neurons and glial cells are crucial to the genesis of pathological pain. The mechanisms regulating these interactions and the role of this process in relaying synaptic input in the spinal dorsal horn remain to be established. We studied the role of glutamate transporters in the regulation of such interactions. On pharmacological blockade of glutamate transporters, slow inward currents (SICs) appeared spontaneously and/or were evoked by peripheral synaptic input in the spinal superficial dorsal horn neurons, including the spinothalamic tract neurons. We showed that the SICs were induced by the release of glutamate from glial cells. On inhibition of glutamate uptake, the stimulation-induced, synaptically released glutamate activated glial cells and caused glial cells to release glutamate. Glial-derived glutamate acted on extrasynaptic N-methyl-d-aspartate (NMDA) receptors mainly composed of NR2B receptors and generated SICs, which led to depolarization and action potential generation in superficial spinal dorsal horn neurons. Thus glutamate transporters regulate glutamatergic neuron-glia interactions at spinal sensory synapses. When glutamate uptake is impaired, glial cells function like excitatory interneurons-they are activated by peripheral synaptic input and release glutamate to activate postsynaptic neurons in spinal pain pathways.

P2Y1 receptors inhibit long-term depression in the prefrontal cortex

Long-term depression (LTD) is a form of synaptic plasticity that may contribute to information storage in the central nervous system. Here we report that LTD can be elicited in layer 5 pyramidal neurons of the rat prefrontal cortex by pairing low frequency stimulation with a modest postsynaptic depolarization. The induction of LTD required the activation of both metabotropic glutamate receptors of the mGlu1 subtype and voltage-sensitive Ca(2+) channels (VSCCs) of the T/R, P/Q and N types, leading to the stimulation of intracellular inositol trisphosphate (IP3) receptors by IP3 and Ca(2+). The subsequent release of Ca(2+) from intracellular stores activated the protein phosphatase cascade involving calcineurin and protein phosphatase 1. The activation of purinergic P2Y(1) receptors blocked LTD. This effect was prevented by P2Y(1) receptor antagonists and was absent in mice lacking P2Y(1) but not P2Y(2) receptors. We also found that activation of P2Y(1) receptors inhibits Ca(2+) transients via VSCCs in the apical dendrites and spines of pyramidal neurons. In addition, we show that the release of ATP under hypoxia is able to inhibit LTD by acting on postsynaptic P2Y(1) receptors. In conclusion, these data suggest that the reduction of Ca(2+) influx via VSCCs caused by the activation of P2Y(1) receptors by ATP is the possible mechanism for the inhibition of LTD in prefrontal cortex.

ErbB receptor signaling in astrocytes: a mediator of neuron-glia communication in the mature central nervous system

Astrocytes are now recognized as active players in the developing and mature central nervous system. Each astrocyte contacts vascular structures and thousands of synapses within discrete territories. These cells receive a myriad of inputs and generate appropriate responses to regulate the function of brain microdomains. Emerging evidence has implicated receptors of the ErbB tyrosine kinase family in the integration and processing of neuronal inputs by astrocytes: ErbB receptors can be activated by a wide range of neuronal stimuli; they control critical steps of glutamate-glutamine metabolism; and they regulate the biosynthesis and release of various glial-derived neurotrophic factors, gliomediators and gliotransmitters. These key properties of astrocytic ErbB signaling in neuron-glia interactions have significance for the physiology of the mature central nervous system, as exemplified by the central control of reproduction within the hypothalamus, and are also likely to contribute to pathological situations, since both dysregulation of ErbB signaling and glial dysfunction occur in many neurological disorders.

Glia contribute to the purinergic modulation of inspiratory rhythm-generating networks

Glia modulate neuronal activity by releasing transmitters in a process called gliotransmission. The role of this process in controlling the activity of neuronal networks underlying motor behavior is unknown. ATP features prominently in gliotransmission; it also contributes to the homeostatic ventilatory response evoked by low oxygen through mechanisms that likely include excitation of preBötzinger complex (preBötC) neural networks, brainstem centers critical for breathing. We therefore inhibited glial function in rhythmically active inspiratory networks in vitro to determine whether glia contribute to preBötC ATP sensitivity. Glial toxins markedly reduced preBötC responses to ATP, but not other modulators. Furthermore, since preBötC glia responded to ATP with increased intracellular Ca(2+) and glutamate release, we conclude that glia contribute to the ATP sensitivity of preBötC networks, and possibly the hypoxic ventilatory response. Data reveal a role for glia in signal processing within brainstem motor networks that may be relevant to similar networks throughout the neuraxis.

ATP in neuron-glia bidirectional signalling

ATP accomplishes important roles in brain, where it functions as neurotransmitter or co-transmitter, being stored and released either as single mediator or together with other neuromodulators. In the last years, the purinergic system has emerged as the most relevant mechanism for intercellular signalling in the nervous system, affecting communication between many types of neurons and all types of glia. In this review, we will focus on recently reported data which describe the role of ATP in bidirectional signalling between neurons and different populations of glial cells, in both peripheral and central system.

Integrated brain circuits: astrocytic networks modulate neuronal activity and behavior

The past decade has seen an explosion of research on roles of neuron-astrocyte interactions in the control of brain function. We highlight recent studies performed on the tripartite synapse, the structure consisting of pre- and postsynaptic elements of the synapse and an associated astrocytic process. Astrocytes respond to neuronal activity and neurotransmitters, through the activation of metabotropic receptors, and can release the gliotransmitters ATP, d-serine, and glutamate, which act on neurons. Astrocyte-derived ATP modulates synaptic transmission, either directly or through its metabolic product adenosine. d-serine modulates NMDA receptor function, whereas glia-derived glutamate can play important roles in relapse following withdrawal from drugs of abuse. Cell type-specific molecular genetics has allowed a new level of examination of the function of astrocytes in brain function and has revealed an important role of these glial cells that is mediated by adenosine accumulation in the control of sleep and in cognitive impairments that follow sleep deprivation.

State-dependent plasticity in vasopressin neurones: dehydration-induced changes in activity patterning

Moderate dehydration impairs concentration and co-ordination, whereas severe dehydration can cause seizures, brain damage or death. To slow the progression of dehydration until body fluids can be replenished by drinking, the increased body fluid osmolality associated with dehydration increases vasopressin (antidiuretic hormone) secretion from the posterior pituitary gland. Increased vasopressin secretion reduces water loss in the urine by promoting water reabsorption in the collecting ducts of the kidney. Vasopressin secretion is largely determined by action potential discharge in vasopressin neurones, and depends on both the rate and pattern of discharge. Vasopressin neurone activity depends on intrinsic and extrinsic mechanisms. We review recent advances in our understanding of the physiological regulation of vasopressin neurone activity patterning and the mechanisms by which this is altered to cope with the increased secretory demands of dehydration.

Hydrostatic pressure-induced release of stored calcium in cultured rat optic nerve head astrocytes

PURPOSE

Elevated intraocular pressure is associated with glaucomatous optic nerve damage. Other investigators have shown functional changes in optic nerve head astrocytes subjected to elevated hydrostatic pressure (HP) for 1 to 5 days. Recently, the authors reported ERK1/2, p90(RSK) and NHE1 phosphorylation after 2 hours. Here they examine calcium responses at the onset of HP to determine what precedes ERK1/2 phosphorylation.

METHODS

Cytoplasmic calcium concentration ([Ca(2+)](i)) was measured in cultured rat optic nerve astrocytes loaded with fura-2. The cells were placed in a closed imaging chamber and subjected to an HP increase of 15 mm Hg. Protein phosphorylation was detected by Western blot analysis.

RESULTS

The increase of HP caused an immediate slow increase in [Ca(2+)](i). The response persisted in calcium-free solution and when nickel chloride (4 mM) was added to suppress channel-mediated calcium entry. Previous depletion of the ER calcium stores by cyclopiazonic acid abolished the HP-induced calcium level increase. The HP-induced increase persisted in cells exposed to xestospongin C, an inhibitor of IP3R-mediated calcium release. In contrast, ryanodine receptor (RyR) antagonist ruthenium red (10 microM) or dantrolene (25 microM) inhibited the HP-induced calcium increase. The HP-induced calcium increase was abolished when ryanodine-sensitive calcium stores were pre-depleted with caffeine (3 mM). HP caused ERK1/2 phosphorylation. The magnitude of the ERK1/2 phosphorylation response was reduced by ruthenium red and dantrolene.

CONCLUSIONS

Increasing HP causes calcium release from a ryanodine-sensitive cytoplasmic store and subsequent ERK1/2 activation. Calcium store release appears to be a required early step in the initial astrocyte response to an HP increase.

Astrocyte-mediated distributed plasticity at hypothalamic glutamate synapses

Afferent activity can induce fast, feed-forward changes in synaptic efficacy that are synapse specific. Using combined electrophysiology, caged molecule photolysis, and Ca(2+) imaging, we describe a plasticity in which the recruitment of astrocytes in response to afferent activity causes a fast and feed-forward, yet distributed increase in the amplitude of quantal synaptic currents at multiple glutamate synapses on magnocellular neurosecretory cells in the hypothalamic paraventricular nucleus. The plasticity is largely multiplicative, consistent with a proportional increase or "scaling" in the strength of all synapses on the neuron. This effect requires a metabotropic glutamate receptor-mediated rise in Ca(2+) in the astrocyte processes surrounding the neuron and the release of the gliotransmitter ATP, which acts on postsynaptic purinergic receptors. These data provide evidence for a form of distributed synaptic plasticity that is feed-forward, expressed quickly, and mediated by the synaptic activation of neighboring astrocytes.

Chronic vs. acute interactions between supraoptic oxytocin neurons and astrocytes during lactation: role of glial fibrillary acidic protein plasticity

In this article, we review studies of astrocytic-neuronal interactions and their effects on the activity of oxytocin (OXT) neurons within the magnocellular hypothalamo-neurohypophysial system. Previous work over several decades has shown that withdrawal of astrocyte processes increases OXT neuron excitability in the hypothalamic supraoptic nucleus (SON) during lactation. However, chronically disabling astrocyte withdrawal does not significantly affect the functioning of OXT neurons during suckling. Nevertheless, acute changes in a cytoskeletal element of astrocytes, glial fibrillary acidic protein (GFAP), occur in concert with changes in OXT neuronal activity during suckling. Here, we compare these changes in GFAP and related proteins with chronic changes that persist throughout lactation. During lactation, a decrease in GFAP levels accompanies retraction of astrocyte processes surrounding OXT neurons in the SON, resulting from high extracellular levels of OXT. During the initial stage of suckling, acute increases in OXT levels further strengthen this GFAP reduction and facilitate the retraction of astrocyte processes. This change, in turn, facilitates burst discharges of OXT neurons and leads to a transient increase in excitatory neurochemicals. This transient neurochemical surge acts to reverse GFAP expression and results in postburst inhibition of OXT neurons. The acute changes in astrocyte GFAP levels seen during suckling likely recur periodically, accompanied by rhythmic changes in glutamate metabolism, water transport, gliotransmitter release, and spatial relationships between astrocytes and OXT neurons. In the neurohypophysis, astrocyte retraction and reversal with accompanying GFAP plasticity also likely occur during lactation and suckling, which facilitates OXT release coordinated with its action in the SON. These studies of the dynamic interactions that occur between astrocytes and OXT neurons mediated by GFAP extend our understanding of astrocyte functions within the central nervous system.

Modulation of glutamatergic synaptic transmission in the bed nucleus of the stria terminalis

Glutamate, catecholamine and neuropeptide signaling within the bed nucleus of the stria terminalis (BNST) have all been identified as key participants in anxiety-like behaviors and behaviors related to withdrawal from exposure to substances of abuse. The BNST is thought to serve as a key relay between limbic cognitive centers and reward, stress and anxiety nuclei. Human studies and animal models have demonstrated that stressors and drugs of abuse can result in long term behavioral modifications that can culminate in psychological diseases such as addiction and post-traumatic stress disorder. The ability of catecholamines and neuropeptides to influence synaptic glutamatergic transmission (stemming from cognitive centers) within the BNST may have profound consequences over these behaviors. In this review we highlight studies examining synaptic plasticity and modulation of excitatory transmission within the BNST, emphasizing how such modulation may result in alterations in anxiety and reward related behavior.

GLIA modulates synaptic transmission

The classical view of glial cells as simple supportive cells for neurons is being replaced by a new vision in which glial cells are active elements involved in the physiology of the nervous system. This new vision is based on the fact that astrocytes, a subtype of glial cells in the CNS, are stimulated by synaptically released neurotransmitters, which increase the astrocyte Ca(2+) levels and stimulate the release of gliotransmitters that regulate synaptic efficacy and plasticity. Consequently, our understanding of synaptic function, previously thought to exclusively result from signaling between neurons, has also changed to include the bidirectional signaling between neurons and astrocytes. Hence, astrocytes have been revealed as integral elements involved in the synaptic physiology, therefore contributing to the processing, transfer and storage of information by the nervous system. Reciprocal communication between astrocytes and neurons is therefore part of the intercellular signaling processes involved in brain function.

Purinergic regulation of hypothalamopituitary functions

The hypothalamus controls the release of hormones by the pituitary and is involved in control of food and water intake, sexual behavior, reproduction and daily cycles in physiological state and behavior, temperature regulation and emotional responses. Adenosine-5'-triphosphate (ATP) and its metabolic products contribute to these functions, acting as agonists for adenosine and P2Y receptors and two-transmembrane domain P2X receptor channels. This review summarizes the recent findings on purinergic receptor expression and their roles in signaling and cellular function in secretory and supporting cells of the hypothalamopituitary system. ATP secretion by these tissues, the enzymes involved in ATP hydrolysis, and the relevance of this pathway for sequential activation of receptors and termination of signaling are also discussed.

The expression of voltage-gated ca2+ channels in pituicytes and the up-regulation of L-type ca2+ channels during water deprivation

The primary components of the neurohypophysis are the neuroendocrine terminals that release vasopressin and oxytocin, and pituicytes, which are astrocytes that normally surround and envelop these terminals. Pituicytes regulate neurohormone release by secreting the inhibitory modulator taurine in an osmotically-regulated fashion and undergo a marked structural reorganisation in response to dehydration as well as during lactation and parturition. Because of these unique functions, and the possibility that Ca2+ influx could regulate their activity, we tested for the expression of voltage-gated Ca2+ channel alpha1 subunits in pituicytes both in situ and in primary culture. Colocalisation studies in neurohypophysial slices show that pituicytes (identified by their expression of the glial marker S100beta), are immunoreactive for antibodies directed against Ca2+ channel alpha1 subunits Ca(V)2.2 and Ca(V)2.3, which mediate N- and R-type Ca2+ currents, respectively. Pituicytes in primary culture express immunoreactivity for Ca(V)1.2, Ca(V)2.1, Ca(V)2.2, Ca(V)2.3 and Ca(V)3.1 (which mediate L-, P/Q-, N-, R- and T-type currents, respectively) and immunoblotting studies confirmed the expression of these Ca2+ channel alpha1 subunits. This increase in Ca2+ channel expression may occur only in pituicytes in culture, or may reflect an inherent capability of pituicytes to initiate the expression of multiple types of Ca2+ channels when stimulated to do so. We therefore performed immunohistochemistry studies on pituitaries obtained from rats that had been deprived of water for 24 h. Pituicytes in these preparations showed a significantly increased immunoreactivity to Ca(V)1.2, suggesting that expression of these channels is up-regulated during the adaptation to long-lasting dehydration. Our results suggest that Ca2+ channels may play important roles in pituicyte function, including a contribution to the adaptation that occurs in pituicytes when the need for hormone release is elevated.

Sustained stimulation of vasopressin and oxytocin release by ATP and phenylephrine requires recruitment of desensitization-resistant P2X purinergic receptors

Coexposure of hypothalamo-neurohypophyseal system explants to ATP and phenylephrine [PE; an alpha1-adrenergic receptor (alpha1-AR) agonist] induces an extended elevation in vasopressin and oxytocin (VP/OT) release. New evidence is presented that this extended response is mediated by recruitment of desensitization-resistant ionotropic purinergic receptor subtypes (P2X-Rs): 1) Antagonists of the P2X2/3 and P2X7-Rs truncated the sustained VP/OT release induced by ATP+PE but did not alter the transient response to ATP alone. 2) The P2X2/3 and P2X7-R antagonists did not alter either ATP or ATP+PE-induced increases in [Ca(2+)](i). 3) P2X2/3 and P2X7-R agonists failed to elevate [Ca(2+)](i), while ATP-gamma-S, an agonist for P2X2-Rs increased [Ca(2+)](i) and induced a transient increase in VP/OT release. 4) A P2Y1-R antagonist did not prevent initiation of the synergistic, sustained stimulation of VP/OT release by ATP+PE but did reduce its duration. Thus, the desensitization-resistant P2X2/3 and P2X7-R subtypes are required for the sustained, synergistic hormone response to ATP+PE, while P2X2-Rs are responsible for the initial activation of Ca(2+)-influx by ATP and ATP stimulation of VP/OT release. Immunohistochemistry, coimmunoprecipitation, and Western blot analysis confirmed the presence of P2X2 and P2X3, P2X2/3, and P2X7-R protein, respectively in SON. These findings support the hypothesis that concurrent activation of P2X2-R and alpha1-AR induces calcium-driven recruitment of P2X2/3 and 7-Rs, allowing sustained activation of a homeostatic circuit. Recruitment of these receptors may provide sustained release of VP during dehydration and may be important for preventing hemorrhagic and septic shock.

Metaplasticity of hypothalamic synapses following in vivo challenge

Neural networks that regulate an organism's internal environment must sense perturbations, respond appropriately, and then reset. These adaptations should be reflected as changes in the efficacy of the synapses that drive the final output of these homeostatic networks. Here we show that hemorrhage, an in vivo challenge to fluid homeostasis, induces LTD at glutamate synapses onto hypothalamic magnocellular neurosecretory cells (MNCs). LTD requires the activation of postsynaptic alpha2-adrenoceptors and the production of endocannabinoids that act in a retrograde fashion to inhibit glutamate release. In addition, both hemorrhage and noradrenaline downregulate presynaptic group III mGluRs. This loss of mGluR function allows high-frequency activity to potentiate these synapses from their depressed state. These findings demonstrate that noradrenaline controls a form of metaplasticity that may underlie the resetting of homeostatic networks following a successful response to an acute physiological challenge.

Tripartite synapses: roles for astrocytic purines in the control of synaptic physiology and behavior

Astrocytes are known to release several transmitters to impact neuronal activity. Cell-specific molecular genetic attenuation of vesicular release has shown that ATP is a primary astrocytic transmitter in situ and in vivo. In this review, we discuss the biology of astrocytic ATP release highlighting the exciting discovery that lysosomes might be primary stores for the release of this gliotransmitter. In addition, we discuss the role of ATP and its metabolite adenosine on synaptic transmission and the coordination of synaptic networks. Finally, we discuss the recent elucidation of the involvement of this form of glial signaling in the modulation of mammalian behavior. By controlling neuronal A1-receptor signaling, astrocytes modulate mammalian sleep homeostasis and are essential for mediating the cognitive consequences of sleep deprivation. These discoveries begin to paint a new picture of brain function in which slow-signaling glia modulate fast synaptic transmission and neuronal firing to impact behavioral output. Because these cells have privileged access to synapses, they may be valuable targets for the development of novel therapies for many neurological and psychiatric conditions.

Glial cells: indispensable partners of hypothalamic magnocellular neurones

The hypothalamo-neurohypophysial system is comprised of magnocellular neurones that synthesise the neuropeptides oxytocin or vasopressin. As neurohormones, these peptides intervene in the regulation of vital functions such as parturition, lactation, osmotic and cardiovascular regulation. The release of these peptides in the general circulation depends on the electrical activity of their parent neurones, which in turn is regulated by the activity of their afferent inputs conveying distinct information. Thus, in view of the diversity of information governing the activity of magnocellular neurones, it is crucial that the system adapts the appropriate release of oxytocin and vasopressin upon physiological demand. Until recently, it was considered that only neurones could provide such adaptation and regulation. However, a third partner of the synapse, the astrocyte, has been shown to provide further control. Astrocytic processes are in proximity of the magnocellular neurones and their synapses, well positioned to detect and modulate synaptic signals. For instance, astrocytes detect a synaptic signal owing to their diverse neurotransmitter/neuropeptide receptors. In addition, they release a variety of neuroactive substances (i.e. gliotransmitters), which in turn modulate synaptic activity. An important gliotransmitter is the amino acid, d-serine, which, together with glutamate, activates NMDA receptors. Once activated, NMDA receptors govern the weight of individual inputs on magnocellular neurones and thus the impact of distinct types of information on neuronal activity. As reviewed here, numerous observations show that astrocytes must be considered as key elements in the functioning of the hypothalamo-neurohypophysial system.

Gene expression in the prefrontal cortex during adolescence: implications for the onset of schizophrenia

Background

Many critical maturational processes take place in the human brain during postnatal development. In particular, the prefrontal cortex does not reach maturation until late adolescence and this stage is associated with substantial white matter volume increases. Patients with schizophrenia and other major psychiatric disorders tend to first present with overt symptoms during late adolescence/early adulthood and it has been proposed that this developmental stage represents a "window of vulnerability".

Methods

In this study we used whole genome microarrays to measure gene expression in post mortem prefrontal cortex tissue from human individuals ranging in age from 0 to 49 years. To identify genes specifically altered in the late adolescent period, we applied a template matching procedure. Genes were identified which showed a significant correlation to a template showing a peak of expression between ages 15 and 25.

Results

Approximately 2000 genes displayed an expression pattern that was significantly correlated (positively or negatively) with the template. In the majority of cases, these genes in fact reached a plateau during adolescence with only subtle changes thereafter. These include a number of genes previously associated with schizophrenia including the susceptibility gene neuregulin 1 (NRG1). Functional profiling revealed peak expression in late adolescence for genes associated with energy metabolism and protein and lipid synthesis, together with decreases for genes involved in glutamate and neuropeptide signalling and neuronal development/plasticity. Strikingly, eight myelin-related genes previously found decreased in schizophrenia brain tissue showed a peak in their expression levels in late adolescence, while the single myelin gene reported increased in patients with schizophrenia was decreased in late adolescence.

Conclusion

The observed changes imply that molecular mechanisms critical for adolescent brain development are disturbed in schizophrenia patients.

Ecto-nucleoside triphosphate diphosphohydrolase 3 in the ventral and lateral hypothalamic area of female rats: morphological characterization and functional implications

BACKGROUND

Based on its distribution in the brain, ecto-nucleoside triphosphate diphosphohydrolase 3 (NTPDase3) may play a role in the hypothalamic regulation of homeostatic systems, including feeding, sleep-wake behavior and reproduction. To further characterize the morphological attributes of NTPDase3-immunoreactive (IR) hypothalamic structures in the rat brain, here we investigated: 1.) The cellular and subcellular localization of NTPDase3; 2.) The effects of 17beta-estradiol on the expression level of hypothalamic NTPDase3; and 3.) The effects of NTPDase inhibition in hypothalamic synaptosomal preparations.

METHODS

Combined light- and electron microscopic analyses were carried out to characterize the cellular and subcellular localization of NTPDase3-immunoreactivity. The effects of estrogen on hypothalamic NTPDase3 expression was studied by western blot technique. Finally, the effects of NTPDase inhibition on mitochondrial respiration were investigated using a Clark-type oxygen electrode.

RESULTS

Combined light- and electron microscopic analysis of immunostained hypothalamic slices revealed that NTPDase3-IR is linked to ribosomes and mitochondria, is predominantly present in excitatory axon terminals and in distinct segments of the perikaryal plasma membrane. Immunohistochemical labeling of NTPDase3 and glutamic acid decarboxylase (GAD) indicated that gamma-amino-butyric-acid- (GABA) ergic hypothalamic neurons do not express NTPDase3, further suggesting that in the hypothalamus, NTPDase3 is predominantly present in excitatory neurons. We also investigated whether estrogen influences the expression level of NTPDase3 in the ventrobasal and lateral hypothalamus. A single subcutaneous injection of estrogen differentially increased NTPDase3 expression in the medial and lateral parts of the hypothalamus, indicating that this enzyme likely plays region-specific roles in estrogen-dependent hypothalamic regulatory mechanisms. Determination of mitochondrial respiration rates with and without the inhibition of NTPDases confirmed the presence of NTPDases, including NTPDase3 in neuronal mitochondria and showed that blockade of mitochondrial NTPDase functions decreases state 3 mitochondrial respiration rate and total mitochondrial respiratory capacity.

CONCLUSION

Altogether, these results suggest the possibility that NTPDases, among them NTPDase3, may play an estrogen-dependent modulatory role in the regulation of intracellular availability of ATP needed for excitatory neuronal functions including neurotransmission.

ATP in central respiratory control: a three-part signaling system

The landmark demonstrations in 2005 that ATP released centrally during hypoxia and hypercapnia contributes to the respective ventilatory responses validated a decade-old hypothesis and ignited interest in the potential significance of P2 receptor signaling in central respiratory control. Our objective in this review is to provide a non-specialist overview of ATP signaling from the perspective that it is a three-part system where the net effects are determined by an interaction between the signaling actions of ATP and adenosine at P2 and P1 receptors, respectively, and a family of enzymes (ectonucleotidases) that breakdown ATP into adenosine. We review the rationale for the original interest in P2 signaling in respiratory control, the evolution of this hypothesis, and the mechanisms by which ATP might affect respiratory behaviour. The potential significance of P2 receptor, P1 receptor and ectonucleotidase diversity for the different compartments of the respiratory control system is also considered. We conclude with a look to future questions and technical challenges.

Communication between neurons and astrocytes: relevance to the modulation of synaptic and network activity

Neuromodulation is a fundamental process in the brain that regulates synaptic transmission, neuronal network activity and behavior. Emerging evidence demonstrates that astrocytes, a major population of glial cells in the brain, play previously unrecognized functions in neuronal modulation. Astrocytes can detect the level of neuronal activity and release chemical transmitters to influence neuronal function. For example, recent findings show that astrocytes play crucial roles in the control of Hebbian plasticity, the regulation of neuronal excitability and the induction of homeostatic plasticity. This review discusses the importance of astrocyte-to-neuron signaling in different aspects of neuronal function from the activity of single synapses to that of neuronal networks.

Feedback inhibition of action potential discharge by endogenous adenosine enhancement of the medium afterhyperpolarization

Phasic activity in supraoptic nucleus vasopressin neurones is characterized by alternating periods of activity (bursts) and silence. During bursts, activation of a medium afterhyperpolarization induces spike frequency adaptation. Antagonism of A1 adenosine receptors within the supraoptic nucleus decreases spike frequency adaptation and prolongs phasic bursts in vivo, indicating that endogenous adenosine contributes to spike frequency adaptation. Here we used sharp electrode intracellular recordings from supraoptic nucleus neurones in hypothalamic explants to show that endogenous adenosine increases medium afterhyperpolarization amplitude to enhance spike frequency adaptation during phasic bursts. Superfusion of the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT, 10 microM) increased intraburst firing rate of phasic neurones (by 2.0 +/- 0.7 spikes s(-1), P = 0.03) and burst duration (by 141 +/- 113 s, P = 0.03). The CPT-induced increase in intraburst firing rate developed over the first few seconds of firing and persisted thereafter. In a separate series of experiments, CPT reduced the amplitude of the medium afterhyperpolarization evoked by a 1 s 20 Hz spike train (by 0.8 +/- 0.3 mV, P < 0.001) in supraoptic nucleus neurones; this inhibition was not prevented by 3 mM CsCl (0.8 +/- 0.1 mV decrease, P < 0.01) to block the afterdepolarization (which overlaps temporally with the medium afterhyperpolarization). In the presence of apamin to block the medium afterhyperpolarization, CPT did not alter afterdepolarization amplitude. Taken together, these data show that endogenous adenosine enhances medium afterhyperpolarization amplitude to contribute to spike frequency adaptation in phasic supraoptic nucleus neurones.

A putative physiological role of hypothalamic CNTF in the control of energy homeostasis

Administration of CNTF durably reduces food intake and body weight in obese humans and rodent models. However, the involvement of endogenous CNTF in the central regulation of energy homeostasis needs to be elucidated. Here, we demonstrate that CNTF and its receptor are expressed in the arcuate nucleus, a key hypothalamic region controlling food intake, and that CNTF levels are inversely correlated to body weight in rats fed a high-sucrose diet. Thus endogenous CNTF may act, in some individuals, as a protective factor against weight gain during hypercaloric diet and could account for individual differences in the susceptibility to obesity.

Activation of P2X7 receptors in glial satellite cells reduces pain through downregulation of P2X3 receptors in nociceptive neurons

Purinergic ionotropic P2X7 receptors (P2X7Rs) are closely associated with excitotoxicity and nociception. Inhibition of P2X7R activation has been considered as a potentially useful strategy to improve recovery from spinal cord injury and reduce inflammatory damage to trauma. The physiological functions of P2X7Rs, however, are poorly understood, even though such information is essential for making the P2X7R an effective therapeutic target. We show here that P2X7Rs in satellite cells of dorsal root ganglia tonically inhibit the expression of P2X3Rs in neurons. Reducing P2X7R expression using siRNA or blocking P2X7R activity by antagonists elicits P2X3R up-regulation, increases the activity of sensory neurons responding to painful stimuli, and evokes abnormal nociceptive behaviors in rats. Thus, contrary to the notion that P2X7R activation is cytotoxic, P2X7Rs in satellite cells play a crucial role in maintaining proper P2X3R expression in dorsal root ganglia. Studying the mechanism underlying the P2X7R-P2X3R control, we demonstrate that activation of P2X7Rs evokes ATP release from satellite cells. ATP in turn stimulates P2Y1 receptors in neurons. P2Y1 receptor activation appears to be necessary and sufficient for the inhibitory control of P2X3R expression. We further determine the roles of the P2X7R-P2Y1-P2X3R inhibitory control under injurious conditions. Activation of the inhibitory control effectively prevents the development of allodynia and increases the potency of systemically administered P2X7R agonists in inflamed rats. Thus, direct blocking P2X7Rs, as proposed before, may not be the best strategy for reducing pain or lessening neuronal degeneration because it also disrupts the protective function of P2X7Rs.

Noradrenaline triggers muscle tone by amplifying glutamate-driven excitation of somatic motoneurones in anaesthetized rats

Postural muscle tone is potently suppressed during sleep and cataplexy. Since brainstem noradrenergic cell discharge activity is tightly coupled with state-dependent changes in muscle activity, it is assumed that noradrenergic drive on to somatic motoneurones modulates basal muscle tone. However, it has never been determined whether noradrenergic neurotransmission acts to directly regulate motoneurone activity or whether it functions to modulate prevailing synaptic activity. This is an important distinction because noradrenaline regulates cell excitability by both directly depolarizing neurones and by indirectly potentiating glutamate-mediated excitation. We used reverse-microdialysis, electrophysiology, neuro-pharmacological and histological techniques in anaesthetized rats to determine whether strengthening noradrenergic drive (via exogenous noradrenaline application) on to trigeminal motoneurones affects masseter muscle tone by increasing spontaneous motoneurone activity or whether it acts to amplify prevailing glutamate-driven excitation. Although noradrenaline is hypothesized to modulate motor activity, we found that direct stimulation of trigeminal motoneurones by alpha(1)-adrenoceptor activation had no direct effect on basal masseter tone. However, when glutamate-driven excitation was increased at the trigeminal motor pool by either endogenous glutamate release (induced by the monosynaptic masseteric reflex) or exogenous AMPA application, noradrenaline triggered a potent increase in basal masseter tone. The stimulatory effects of noradrenaline were unmasked and rapidly switched on only in the presence of glutamatergic transmission. Blockade of AMPA receptors abolished this excitatory effect, indicating that noradrenergic drive requires ongoing glutamatergic activity. Our data indicate that exogenous noradrenergic drive does not directly affect spontaneous motoneurone discharge activity in anaesthetized rats; rather, it triggers postural muscle tone by amplifying prevailing glutamate-driven excitation.