Purinergic modulation of synaptic input to Purkinje neurons in rat cerebellar brain slices

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

Introduction to Purinergic Signalling in the Brain

ATP is a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the brain. There is a widespread presence of both adenosine (P1) and P2 nucleotide receptors in the brain on both neurons and glial cells. Adenosine receptors play a major role in presynaptic neuromodulation, while P2X ionotropic receptors are involved in fast synaptic transmission and synaptic plasticity. P2Y G protein-coupled receptors are largely involved in presynaptic activities, as well as mediating long-term (trophic) signalling in cell proliferation, differentiation and death during development and regeneration. Both P1 and P2 receptors participate in neuron-glial interactions. Purinergic signalling is involved in control of cerebral vascular tone and remodelling and has been implicated in learning and memory, locomotor and feeding behaviour and sleep. There is increasing interest in the involvement of purinergic signalling in the pathophysiology of the CNS, including trauma, ischaemia, epilepsy, neurodegenerative diseases, neuropsychiatric and mood disorders, and cancer, including gliomas.

Signaling mechanism for modulation by ATP of glycine receptors on rat retinal ganglion cells

ATP modulates voltage- and ligand-gated channels in the CNS via the activation of ionotropic P2X and metabotropic P2Y receptors. While P2Y receptors are expressed in retinal neurons, the function of these receptors in the retina is largely unknown. Using whole-cell patch-clamp techniques in rat retinal slice preparations, we demonstrated that ATP suppressed glycine receptor-mediated currents of OFF type ganglion cells (OFF-GCs) dose-dependently, and the effect was in part mediated by P2Y1 and P2Y11, but not by P2X. The ATP effect was abolished by intracellular dialysis of a Gq/11 protein inhibitor and phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor, but not phosphatidylcholine (PC)-PLC inhibitor. The ATP effect was accompanied by an increase in [Ca(2+)]i through the IP3-sensitive pathway and was blocked by intracellular Ca(2+)-free solution. Furthermore, the ATP effect was eliminated in the presence of PKC inhibitors. Neither PKA nor PKG system was involved. These results suggest that the ATP-induced suppression may be mediated by a distinct Gq/11/PI-PLC/IP3/Ca(2+)/PKC signaling pathway, following the activation of P2Y1,11 and other P2Y subtypes. Consistently, ATP suppressed glycine receptor-mediated light-evoked inhibitory postsynaptic currents of OFF-GCs. These results suggest that ATP may modify the ON-to-OFF crossover inhibition, thus changing action potential patterns of OFF-GCs.

The inhibitory input to mouse cerebellar Purkinje cells is reciprocally modulated by Bergmann glial P2Y1 and AMPA receptor signaling

Synaptic transmission has been shown to be modulated by glial functions, but the modes of specific glial action may vary in different neural circuits. We have tested the hypothesis, if Bergmann GLIA (BG) are involved in shaping neuronal communication in the mouse cerebellar cortex, using acutely isolated cerebellar slices of wild-type (WT) and of glia-specific receptor knockout mice. Activation of P2Y1 receptors by ADP (100 µM) or glutamatergic receptors by AMPA (0.3 µM) resulted in a robust, reversible and repeatable rise of evoked inhibitory input in Purkinje cells by 80% and 150%, respectively. The ADP-induced response was suppressed by prior application of AMPA, and the AMPA-induced response was suppressed by prior application of ADP. Genetic deletion or pharmacological blockade of either receptor restored the response to the other receptor agonist. Both ADP and AMPA responses were sensitive to Rose Bengal, which blocks vesicular glutamate uptake, and to the NMDA receptor antagonist D-AP5. Our results provide strong evidence that activation of both ADP and AMPA receptors, located on BGs, results in the release of glutamate, which in turn activates inhibitory interneurons via NMDA-type glutamate receptors. This infers that BG cells, by means of metabotropic signaling via their AMPA and P2Y1 receptors, which mutually suppress each other, would interdependently contribute to the fine-tuning of Purkinje cell activity in the cerebellar cortex. GLIA 2016. GLIA 2016;64:1265-1280.

P2Y Receptors in Synaptic Transmission and Plasticity: Therapeutic Potential in Cognitive Dysfunction

ATP released from neurons and astrocytes during neuronal activity or under pathophysiological circumstances is able to influence information flow in neuronal circuits by activation of ionotropic P2X and metabotropic P2Y receptors and subsequent modulation of cellular excitability, synaptic strength, and plasticity. In the present paper we review cellular and network effects of P2Y receptors in the brain. We show that P2Y receptors inhibit the release of neurotransmitters, modulate voltage- and ligand-gated ion channels, and differentially influence the induction of synaptic plasticity in the prefrontal cortex, hippocampus, and cerebellum. The findings discussed here may explain how P2Y₁ receptor activation during brain injury, hypoxia, inflammation, schizophrenia, or Alzheimer's disease leads to an impairment of cognitive processes. Hence, it is suggested that the blockade of P2Y₁ receptors may have therapeutic potential against cognitive disturbances in these states.

Differential Modulation of GABAA Receptors Underlies Postsynaptic Depolarization- and Purinoceptor-Mediated Enhancement of Cerebellar Inhibitory Transmission: A Non-Stationary Fluctuation Analysis Study

Cerebellar GABAergic inhibitory transmission between interneurons and Purkinje cells (PCs) undergoes a long-lasting enhancement following different stimulations, such as brief depolarization or activation of purinergic receptors of postsynaptic PCs. The underlying mechanisms, however, are not completely understood. Using a peak-scaled non-stationary fluctuation analysis, we therefore aimed at characterizing changes in the electrophysiological properties of GABA_A receptors in PCs of rat cerebellar cortex during depolarization-induced “rebound potentiation (RP)” and purinoceptor-mediated long-term potentiation (PM-LTP), because both RP and PM-LTP likely depend on postsynaptic mechanisms. Stimulation-evoked inhibitory postsynaptic currents (eIPSCs) were recorded from PCs in neonatal rat cerebellar slices. Our analysis showed that postsynaptic membrane depolarization induced RP of eIPSCs in association with significant increase in the number of synaptic GABA_A receptors without changing the channel conductance. By contrast, bath application of ATP induced PM-LTP of eIPSCs with a significant increase of the channel conductance of GABA_A receptors without affecting the receptor number. Pretreatment with protein kinase A (PKA) inhibitors, H-89 and cAMPS-Rp, completely abolished the PM-LTP. The CaMKII inhibitor KN-62 reported to abolish RP did not alter PM-LTP. These results suggest that the signaling mechanism underlying PM-LTP could involve ATP-induced phosphorylation of synaptic GABA_A receptors, thereby resulting in upregulation of the channel conductance by stimulating adenylyl cyclase-PKA signaling cascade, possibly via activation of P2Y₁₁ purinoceptor. Thus, our findings reveal that postsynaptic GABA_A receptors at the interneuron-PC inhibitory synapses are under the control of two distinct forms of long-term potentiation linked with different second messenger cascades.

Reversal of autism-like behaviors and metabolism in adult mice with single-dose antipurinergic therapy

Autism spectrum disorders (ASDs) now affect 1-2% of the children born in the United States. Hundreds of genetic, metabolic and environmental factors are known to increase the risk of ASD. Similar factors are known to influence the risk of schizophrenia and bipolar disorder; however, a unifying mechanistic explanation has remained elusive. Here we used the maternal immune activation (MIA) mouse model of neurodevelopmental and neuropsychiatric disorders to study the effects of a single dose of the antipurinergic drug suramin on the behavior and metabolism of adult animals. We found that disturbances in social behavior, novelty preference and metabolism are not permanent but are treatable with antipurinergic therapy (APT) in this model of ASD and schizophrenia. A single dose of suramin (20 mg kg(-1) intraperitoneally (i.p.)) given to 6-month-old adults restored normal social behavior, novelty preference and metabolism. Comprehensive metabolomic analysis identified purine metabolism as the key regulatory pathway. Correction of purine metabolism normalized 17 of 18 metabolic pathways that were disturbed in the MIA model. Two days after treatment, the suramin concentration in the plasma and brainstem was 7.64 μM pmol μl(-1) (±0.50) and 5.15 pmol mg(-1) (±0.49), respectively. These data show good uptake of suramin into the central nervous system at the level of the brainstem. Most of the improvements associated with APT were lost after 5 weeks of drug washout, consistent with the 1-week plasma half-life of suramin in mice. Our results show that purine metabolism is a master regulator of behavior and metabolism in the MIA model, and that single-dose APT with suramin acutely reverses these abnormalities, even in adults.

Superior working memory and behavioural habituation but diminished psychomotor coordination in mice lacking the ecto-5'-nucleotidase (CD73) gene

Adenosine is an important neuromodulator in the central nervous system involved in the regulation of wakefulness, sleep, learning and memory, fear and anxiety as well as motor functions. Extracellular adenosine is synthesized by the cell-surface ectoenzyme ecto-5'-nucleotidase (CD73) from 5'-adenosine monophosphate. While CD73 is widely expressed throughout the mammalian brain, its specific role for behaviour is poorly understood. We examined spatial working memory, emotional responses, motor coordination and motor learning as well as behavioural habituation in mice with a targeted deletion of CD73. CD73 knockout (CD73-/-) mice exhibit enhanced spatial working memory in the Y-maze and enhanced long-term behavioural habituation in the open field. Furthermore, impaired psychomotor coordination on the accelerating rotarod was found in CD73-/- mice. No changes in motor learning and/or anxiety-like behaviour were evident in CD73-/- mice. Our data provide evidence for a role of CD73 in the regulation of learning and memory and psychomotor coordination. Our results might be important for the evaluation of adenosine neuromodulators as possible treatments to ameliorate cognitive and motor deficits associated with neurodegenerative diseases.

Developmental expression of P2X5 receptors in the mouse prenatal central and peripheral nervous systems

The functions of P2X purinoceptors (P2X1-7) in the nervous system of adults have been widely studied. However, little is known about their roles during embryonic development. Our previous work has reported an extensive expression of P2X5 receptors in the adult mouse central nervous system. In the present study, we have examined the expression pattern of P2X5 receptor mRNA and protein during prenatal development of the mouse nervous system (from embryonic day E8 to E17). P2X5 receptors appeared in the neural tube as early as E8 and were gradually confined to new-born neurons in the cortical plate and ventral horn of the spinal cord. Heavy signals for P2X5 receptors were also found in dorsal root ganglia (DRG), retina, olfactory epithelium, and nerve fibers in skeletal muscles. In conclusion, P2X5 receptors were strongly represented in the developing mouse nervous system. The transient high expression pattern of P2X5 receptors in epithelium-like structures suggests a role during early neurogenesis.

Diversity and complexity of roles of granule cells in the cerebellar cortex. Editorial

The cerebellar granule cell, the most numerous neurons in the brain, forms the main excitatory neuron of the cerebellar cortical circuitry. Granule cells are synaptically connected with both mossy fibers and Golgi cells inside specialized structures called glomeruli, and thereby, they are subject to both feed-forward and feed-back inhibition. Their unique architecture with about four dendrites and a single axon ascending in the cerebellar cortex to bifurcate into two parallel fibers making synapses with Purkinje neurons has attracted numerous scientists. Recent advances show that they are much more than just relays of mossy fibers. They perform diverse and complex transformations in the spatiotemporal domain. This special issue highlights novel avenues in our understanding of the roles of this key neuronal population of the cerebellar cortex, ranging from developmental up to physiological and pathological points of view.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

Purinergic signaling in the cerebellum: Bergmann glial cells express functional ionotropic P2X7 receptors

Astrocytes constitute active networks of intercommunicating cells that support the metabolism and the development of neurons and affect synaptic functions via multiple pathways. ATP is one of the major neurotransmitters mediating signaling between neurons and astrocytes. Potentially acting through both purinergic metabotropic P2Y receptors (P2YRs) and ionotropic P2X receptors (P2XRs), up until now ATP has only been shown to activate P2YRs in Bergmann cells, the radial glia of the cerebellar cortex that envelopes Purkinje cell afferent synapses. In this study, using multiple experimental approaches in acute cerebellar slices we demonstrate the existence of functional P2XRs on Bergmann cells. In particular, we show here that Bergmann cells express uniquely P2X7R subtypes: (i) immunohistochemical analysis revealed the presence of P2X7Rs on Bergmann cell processes, (ii) in whole cell recordings P2XR pharmacological agonists induced depolarizing currents that were blocked by specific antagonists of P2X7Rs, and could not be elicited in slices from P2X₇R-deficient mice and finally, (iii) calcium imaging experiments revealed two distinct calcium signals triggered by application of exogenous ATP: a transient signal deriving from release of calcium from intracellular stores, and a persistent one following activation of P2X7Rs. Our data thus reveal a new pathway by which extracellular ATP may affect glial cell function, thus broadening our knowledge on purinergic signaling in the cerebellum.

ERK1/2 activation is involved in the neuroprotective action of P2Y13 and P2X7 receptors against glutamate excitotoxicity in cerebellar granule neurons

Cerebellar granule neurons express several types of nucleotide receptors, with the metabotropic P2Y(13) and the ionotropic P2X7 being the most relevant in this model. In the present study we investigated the role of P2Y(13) and P2X7 nucleotide receptors in ERK1/2 signalling. The nucleotidic agonists 2MeSADP (2-methylthioadenosine-5'-diphosphate) for P2Y(13) and BzATP (2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate) for P2X7 receptors were coupled to ERK1/2 activation in granule neurons, being able to increase around two-fold the levels of ERK1/2 phosphorylation. These effects were sensitive to the inhibitory action of the antagonists MRS-2211 and A-438079, specific for P2Y(13) and P2X7 receptors, respectively. Although both receptor subtypes shared the same pattern of transient ERK1/2 phosphorylation, they differed in the intracellular cascades they triggered, being PI3K-dependent for P2Y(13) and calcium/calmodulin kinase II (CaMKII)-dependent for P2X7. These two different ERK-mediated pathways were involved in the neuroprotective effects displayed by both P2Y(13) and P2X7 receptors against apoptosis induced by an excitotoxic concentration of glutamate, in a similar manner to the neurotrophin, BDNF. In addition, P2Y(13) and P2X7 receptor agonists were also able to phosphorylate and activate the ERK-dependent target CREB, which could be involved in their neuroprotective effect. These results indicate that nucleotide receptors share with trophic factors the same survival routes in neurons, such as the ERK signalling route, and therefore, can contribute to the maintenance of granule neurons in conditions in which survival is being compromised.

Purine receptor-mediated endocannabinoid production and retrograde synaptic signalling in the cerebellar cortex

BACKGROUND AND PURPOSE

Presynaptic CB₁ cannabinoid receptors can be activated by endogenous cannabinoids (endocannabinoids) synthesized by postsynaptic neurones. The hypothesis of the present work was that activation of calcium-permeable transmitter-gated ion channels in postsynaptic neurones, specifically of P2X purine receptors, can lead to endocannabinoid production and retrograde synaptic signalling.

EXPERIMENTAL APPROACH

GABAergic inhibitory postsynaptic currents (IPSCs) were recorded with patch-clamp techniques in Purkinje cells in mouse cerebellar slices. Purine receptors on Purkinje cells were activated by pressure ejection of ATP from a pipette.

KEY RESULTS

ATP evoked an inward current in Purkinje cells, most likely due to P2X receptor activation. The ATP-evoked currents were accompanied by currents via voltage-gated calcium channels. ATP suppressed electrical stimulation-evoked IPSCs and miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin, and these effects were prevented by the CB₁ antagonist rimonabant and the calcium chelator BAPTA (applied into the Purkinje cell). ATP also suppressed mIPSCs when voltage-gated calcium channels were blocked by cadmium, and intracellular calcium stores were depleted by thapsigargin. However, ATP failed to suppress mIPSCs when the extracellular calcium concentration was zero.

CONCLUSIONS AND IMPLICATIONS

ATP elicits CB₁ receptor-dependent retrograde synaptic suppression, which is probably mediated by an endocannabinod released by the postsynaptic neurone. An increase in intracellular calcium concentration in the postsynaptic neurone is necessary for this retrograde signalling. We propose that ATP increases the calcium concentration by two mechanisms: calcium enters into the neurone via the P2X receptor ion channel and the ATP-evoked depolarization triggers voltage-gated calcium channels.

Miniature EPSCs in the lateral superior olive before hearing onset: regional and cell-type-specific differences and heterogeneous neuromodulatory effects of ATP

Spontaneous activity occurs in the mammalian auditory system prior to hearing onset and is relevant for neuronal differentiation. Growing evidence indicates that miniature events, i.e., action potential-independent synaptic activity, also have some developmental relevance. An intriguing question is whether these events are purely stochastic or rather display specific characteristics. We addressed this question and studied miniature excitatory postsynaptic currents (mEPSCs) in morphologically defined neurons of the rat lateral superior olive (LSO) during early neonatal life. To do so, whole-cell recordings from neurons in acute slices were combined with Lucifer yellow fillings. mEPSCs were identified by their TTX insensitivity and their blockade by glutamate receptor antagonists. Altogether, 60% of the LSO neurons displayed mEPSCs, and their presence correlated with the cell location and morphology. Their percentage was highest in the medial limb (86%) and lowest in the lateral limb (14%). Seventy-seven percent of the neurons with mEPSCs were bipolar cells, whereas 77% of those without mEPSCs were multipolar cells. The neuromodulator ATP affected the frequency of mEPSCs in 61% of the LSO neurons in a heterogeneous manner: both frequency increases and decreases occurred. These data provide further evidence for the specificity of mEPSCs. Finally, we investigated whether missing cochlear input changes mEPSCs characteristics. Characterizing LSO neurons of Ca(V)1.3(-/-) mice, which lack cochlea-driven nerve activity, we observed higher mEPSC frequencies and peak amplitudes, indicative of a compensatory response to deprivation. Together, our results demonstrate specific, rather than stochastic, characteristics of mEPSCs in the neonatal LSO, in accordance with their potential developmental significance.

Suppression of GABA input by A1 adenosine receptor activation in rat cerebellar granule cells

Synaptic transmission has been shown to be modulated by purinergic receptors. In the cerebellum, spontaneous inhibitory input to Purkinje neurons is enhanced by ATP via P2 receptors, while evoked excitatory input via the granule cell parallel fibers is reduced by presynaptic P1 (A1) adenosine receptors. We have now studied the modulation of the complex GABAergic input to granule cells by the purinergic receptor agonists ATP and adenosine in acute rat cerebellar tissue slices using the whole-cell patch-clamp technique. Our experiments indicate that ATP and adenosine substantially reduce the bicuculline- and gabazine-sensitive GABAergic input to granule cells. Both phasic and tonic inhibitory components were reduced leading to an increased excitability of granule cells. The effect of ATP and adenosine could be blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), but not by other P1 and P2 receptor antagonists, indicating that it was mediated by activation of A1 adenosine receptors. Our results suggest that, in the cerebellar network, A1 receptor activation, known to decrease the excitatory output of granule cells, also increases their excitability by reducing their complex GABAergic input. These findings extend our knowledge on purinergic receptors, mediating multiple modulations at both inhibitory and excitatory input and output sites in the cerebellar network.

Expression of P2X5 receptors in the mouse CNS

P2X receptors are ATP-gated cationic channels composed of seven known subunits (P2X1-7) which are involved in different functions in neural tissue. The present study investigates the P2X5 receptor expression pattern in the mouse CNS using immunohistochemistry and in situ hybridization histochemistry. The specificity of the immunostaining has been verified by pre-absorption, Western blot and in situ hybridization methods. Heavy P2X5 receptor immunostaining was observed in the mitral cells of the olfactory bulb; cerebral cortex; globus pallidum, anterior cortical amygdaloid nucleus, amygdalohippocampal area of subcortical telencephalon; anterior nuclei, anteroventral nucleus, ventrolateral nucleus of thalamus; supraoptic nucleus, ventromedial nucleus, arcuate nucleus of hypothalamus; substantia nigra of midbrain; pontine nuclei, mesencephalic trigeminal nucleus, motor trigeminal nucleus, ambiguous nucleus, inferior olive, hypoglossal nucleus, dorsal motor vagus nucleus, area postrema of hindbrain; Purkinje cells of cerebellum; and spinal cord. The identification of extensive P2X5 receptor immunoreactivity and mRNA distribution within the CNS of the mouse demonstrated here is consistent with a role for extracellular ATP acting as a fast neurotransmitter.

Besides Purkinje cells and granule neurons: an appraisal of the cell biology of the interneurons of the cerebellar cortex

Ever since the groundbreaking work of Ramon y Cajal, the cerebellar cortex has been recognized as one of the most regularly structured and wired parts of the brain formed by a rather limited set of distinct cells. Its rather protracted course of development, which persists well into postnatal life, the availability of multiple natural mutants, and, more recently, the availability of distinct molecular genetic tools to identify and manipulate discrete cell types have suggested the cerebellar cortex as an excellent model to understand the formation and working of the central nervous system. However, the formulation of a unifying model of cerebellar function has so far proven to be a most cantankerous problem, not least because our understanding of the internal cerebellar cortical circuitry is clearly spotty. Recent research has highlighted the fact that cerebellar cortical interneurons are a quite more diverse and heterogeneous class of cells than generally appreciated, and have provided novel insights into the mechanisms that underpin the development and histogenetic integration of these cells. Here, we provide a short overview of cerebellar cortical interneuron diversity, and we summarize some recent results that are hoped to provide a primer on current understanding of cerebellar biology.

Purinergic signalling and disorders of the central nervous system

Purines have key roles in neurotransmission and neuromodulation, with their effects being mediated by the purine and pyrimidine receptor subfamilies, P1, P2X and P2Y. Recently, purinergic mechanisms and specific receptor subtypes have been shown to be involved in various pathological conditions including brain trauma and ischaemia, neurodegenerative diseases involving neuroimmune and neuroinflammatory reactions, as well as in neuropsychiatric diseases, including depression and schizophrenia. This article reviews the role of purinergic signalling in CNS disorders, highlighting specific purinergic receptor subtypes, most notably A(2A), P2X(4) and P2X(7), that might be therapeutically targeted for the treatment of these conditions.

GABA release by basket cells onto Purkinje cells, in rat cerebellar slices, is directly controlled by presynaptic purinergic receptors, modulating Ca2+ influx

In many brain regions, Ca(2+) influx through presynaptic P2X receptors influences GABA release from interneurones. In patch-clamp recordings of Purkinje cells (PCs) in rat cerebellar slices, broad spectrum P2 receptor antagonists, PPADS (30microM) or suramin (12microM), result in a decreased amplitude and increased failure rate of minimal evoked GABAergic synaptic currents from basket cells. The effect is mimicked by desensitizing P2X1/3-containing receptors with alpha,beta-methylene ATP. This suggests presynaptic facilitation of GABA release via P2XR-mediated Ca(2+) influx activated by endogenously released ATP. In contrast, activation of P2Y4 receptors (using UTP, 30microM, but not P2Y1 or P2Y6 receptor ligands) results in inhibition of GABA release. Immunological studies reveal the presence of most known P2Rs in >or=20% of GABAergic terminals in the cerebellum. P2X3 receptors and P2Y4 receptors occur in approximately 60% and 50% of GABAergic synaptosomes respectively and are localized presynaptically. Previous studies report that PC output is also influenced by postsynaptic purinergic receptors located on both PCs and interneurones. The high Ca(2+) permeability of the P2X receptor and the ability of ATP to influence intracellular Ca(2+) levels via P2Y receptor-mediated intracellular pathways make ATP the ideal transmitter for the multisite bidirectional modulation of the cerebellar cortical neuronal network.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Glutamatergic and purinergic receptor-mediated calcium transients in Bergmann glial cells

Astrocytes respond to neuronal activity with [Ca2+]i increases after activation of specific receptors. Bergmann glial cells (BGs), astrocytes of the cerebellar molecular layer (ML), express various receptors that can mobilize internal Ca2+. BGs also express Ca2+ permeable AMPA receptors that may be important for maintaining the extensive coverage of Purkinje cell (PC) excitatory synapses by BG processes. Here, we examined Ca2+ signals in single BGs evoked by synaptic activity in cerebellar slices. Short bursts of high-frequency stimulation of the ML elicited Ca2+ transients composed of a small-amplitude fast rising phase, followed by a larger and slower rising phase. The first phase resulted from Ca2+ influx through AMPA receptors, whereas the second phase required release of Ca2+ from internal stores initiated by P2 purinergic receptor activation. We found that such Ca2+ responses could be evoked by direct activation of neurons releasing ATP onto BGs or after activation of metabotropic glutamate receptor 1 on these neurons. Moreover, examination of BG and PC responses to various synaptic stimulation protocols suggested that ML interneurons are likely the cellular source of ATP.

Control of basal extracellular adenosine concentration in rat cerebellum

To re-examine how the basal extracellular concentration of adenosine is regulated in acutely isolated cerebellar slices we have combined electrophysiological and microelectrode biosensor measurements. In almost all cases, synaptic transmission was tonically inhibited by adenosine acting via A₁ receptors. By contrast, in most slices, the biosensors did not measure an adenosine tone but did record a spatially non-uniform extracellular tone of the downstream metabolites (inosine and hypoxanthine). Most of the extracellular hypoxanthine arose from the metabolism of inosine by ecto-purine nucleoside phosphorylase (PNP). Adenosine kinase was the major determinant of adenosine levels, as its inhibition increased both adenosine concentration and A₁ receptor-mediated synaptic inhibition. Breakdown of adenosine by adenosine deaminase was the major source of the inosine/hypoxanthine tone. However adenosine deaminase played a minor role in determining the level of adenosine at synapses, suggesting a distal location. Blockade of adenosine transport (by NBTI/dipyridamole) had inconsistent effects on basal levels of adenosine and synaptic transmission. Unexpectedly, application of NBTI/dipyridamole prevented the efflux of adenosine resulting from block of adenosine kinase at only a subset of synapses. We conclude that there is spatial variation in the functional expression of NBTI/dipyridamole-sensitive transporters. The increased spatial and temporal resolution of the purine biosensor measurements has revealed the complexity of the control of adenosine and purine tone in the cerebellum.

A delayed ATP-elicited K+ current in freshly isolated smooth muscle cells from mouse aorta

Adenosine 5'-triphosphate (ATP) activated two sequential responses in freshly isolated mouse aortic smooth muscle cells. In the first phase, ATP activated Ca(2+)-dependent K(+) or Cl(-) currents and the second phase was the activation of a delayed outward current with a reversal potential of -75.9 +/- 1.4 mV. A high concentration of extracellular K(+) (130 mM) shifted the reversal potential of the delayed ATP-elicited current to -3.5 +/- 1.3 mV. The known K(+)-channel blockers, iberiotoxin, charybdotoxin, glibenclamide, apamin, 4-aminopyridine, Ba(2+) and tetraethylammonium chloride all failed to inhibit the delayed ATP-elicited K(+) current. Removal of ATP did not decrease the amplitude of the ATP-elicited current back to the control values. The simultaneous recording of cytosolic free Ca(2+) and membrane currents revealed that the first phase of the ATP-elicited response is associated with an increase in intracellular Ca(2+), while the second delayed phase develops after the return of cytosolic free Ca(2+) to control levels.ATP did not activate Ca(2+)-dependent K(+) currents, but did elicit Ca(2+)-independent K(+) currents, in cells dialyzed with ethylene glycol-bis (2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). The delay of activation of Ca(2+)-independent currents decreased from 10.5 + 3.4 to 1.27 +/- 0.33 min in the cells dialyzed with 2 mM EGTA. Adenosine alone failed to elicit a Ca(2+)-independent K(+) current but simultaneous application of ATP and adenosine activated the delayed K(+) current. Intracellular dialysis of cells with guanosine 5'-O-(2-thiodiphosphate) transformed the Ca(2+)-independent ATP-elicited response from a sustained to a transient one. A phospholipase C inhibitor, U73122 (1 microM), was shown to abolish the delayed ATP-elicited response. These results indicate that the second phase of the ATP-elicited response was a delayed Ca(2+)-independent K(+) current activated by exogenous ATP. This phase might represent a new vasoregulatory pathway in vascular smooth muscle cells.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Interactions between Purkinje neurones and Bergmann glia

Throughout the development of the cerebellar cortex, Purkinje neurones interact closely with Bergmann glial cells, a specialized form of astrocyte. This review summarizes the intimate developmental, anatomical and functional relationships between these two cell types, with particular emphasis on recent discoveries regarding glutamate release from climbing and parallel fibres as a pathway for signalling synaptic activity to Bergmann glia.

Modulation of synaptic activity in Purkinje neurons by ATP

Adenosine triphosphate (ATP) is a versatile signalling molecule in the central and peripheral nervous system, where it can be released from both neurons and glial cells. In the cerebellum, ATP is released endogenously from the second postnatal week onwards, and is involved in the up-regulation of spontaneous synaptic input to Purkinje neurons by activation of purinergic P2 receptors. In the cerebellar cortex, ATP presumably acts on presynaptic inhibitory interneurons, which are excited by the activation of both P2X and P2Y receptors. P2 receptors have been reported for Purkinje neurons, where they mediate intracellular Ca(2+) responses. The extracellular concentration of ATP is modulated by its enzymatic degradation by ecto-nucleotidases. Adenosine, which modulates evoked transmitter release, does not influence the spontaneous synaptic activity in Purkinje neurons. Some implications of ATP as a tonically active neuromodulator in the cerebellum are discussed.

Inhibition of apoptosis by P2Y2 receptor activation: novel pathways for neuronal survival

Cell survival is an essential function in the development and maintenance of the nervous system. We demonstrate here a previously unappreciated role for extracellular nucleotide signaling through the P2Y2 receptor in the survival of neurons: PC12 (pheochromocytoma 12) cells and dorsal root ganglion neurons are protected from serum starvation-induced apoptosis by ATP, UTP, and ATPgammaS, an effect mediated via P2Y2 receptors, as demonstrated by small interfering RNA and genetic knock-out models. This protection occurs independently of neurophin signaling but requires Src activation of ERK (extracellular signal-regulated kinase) and Akt. Moreover, ATPgammaS and NGF act synergistically to enhance neuronal survival through enhanced TrkA signaling. The results, which define a novel mechanism for inhibition of apoptosis, implicate parallel, interacting systems--extracellular nucleotides/P2Y2 receptors and neurotrophin/TrkA--to sustain neuronal survival.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

alpha1-Adrenergic modulation of synaptic input to Purkinje neurons in rat cerebellar brain slices

The inhibitory activity in the cerebellar network, as investigated in acute brain slices from 14-20 days old rats, is modulated by alpha1-adrenergic stimulation. The specific alpha1-adrenoceptor agonist phenylephrine (PhE; 10 microM) or the alpha-adrenoceptor agonist 6-fluoronoradrenaline (10 microM) increases the frequency and the amplitude of spontaneous postsynaptic currents (sPSC) in Purkinje neurons. The effects are sensitive to the alpha1-adrenoceptor antagonists prazosin (30 microM) and phentolamine (10 microM). The PhE-induced augmentation is suppressed when phospholipase C is blocked by preincubation with U73122 (10 microM) but is not affected by inhibition of protein kinases with H7 (10 microM) or GF109203X (10 microM). Involvement of intracellular Ca(2+) stores was shown by a reduced PhE effect after blocking of SERCA pumps with cyclopiazonic acid (30 microM) and thapsigargin (1 microM). The persistence of the PhE effect on the frequency of miniature postsynaptic currents, as recorded in presence of tetrodotoxin, indicates a presynaptic localization of the alpha1-adrenoceptors. A block of voltage-gated Ca(2+) channels with nifedipine, verapamil, or omega-conotoxin MVIIC did not suppress the PhE-induced increase of the frequency and amplitude of sPSC. The results suggest that alpha1-adrenoceptors at presynaptic terminals mediate an increase of the spontaneous synaptic inhibition of Purkinje neurons in the cerebellar cortex via release of Ca(2+) from intracellular stores.

α-Adrenoceptive Dual Modulation of Inhibitory GABAergic Inputs to Purkinje Cells in the Mouse Cerebellum

Noradrenaline (NA) modulates synaptic transmission in various sites of the CNS. In the cerebellar cortex, several studies have revealed that NA enhances inhibitory synaptic transmission by β-adrenoceptor–and cyclic AMP–dependent pathways. However, the effects of α-adrenoceptor activation on cerebellar inhibitory neurotransmission have not yet been fully elucidated. Therefore we investigated the effects of the α1- or α2-adrenoceptor agonist on inhibitory postsynaptic currents (IPSCs) recorded from mouse Purkinje cells (PCs). We found that the nonselective α-adrenoceptor agonist 6-fluoro-norepinephrine increased both the frequency and amplitude of spontaneous IPSCs (sIPSCs). This enhancement was mostly mimicked by the selective α1-adrenoceptor agonist phenylephrine (PE). PE also enhanced the amplitude of evoked IPSCs (eIPSCs) and increased the frequency but not the amplitude of miniature IPSCs (mIPSCs). Moreover, PE decreased the paired-pulse ratio of eIPSCs and did not change γ-aminobutyric acid (GABA) receptor sensitivity in PCs. Conversely, the selective α2-adrenoceptor agonist clonidine significantly reduced both the frequency and the amplitude of sIPSCs. Neither eIPSCs nor mIPSCs were affected by clonidine. Furthermore, presynaptic cell-attached recordings showed that spontaneous activity of GABAergic interneurons was enhanced by PE but reduced by clonidine. These results suggest that NA enhances inhibitory neurotransmitter release by α1-adrenoceptors, which are expressed in presynaptic terminals and somatodendritic domains, whereas NA suppresses the excitability of interneurons by α2-adrenoceptors, which are expressed in presynaptic somatodendritic domains. Thus cerebellar α-adrenoceptors play roles in a presynaptic dual modulation of GABAergic inputs from interneurons to PCs, thereby providing a likely mechanism for the fine-tuning of information flow in the cerebellar cortex.

P2Y2 receptor activates nerve growth factor/TrkA signaling to enhance neuronal differentiation

Neurotrophins are essential for neuronal differentiation, but the onset and the intensity of neurotrophin signaling within the neuronal microenvironment are poorly understood. We tested the hypothesis that extracellular nucleotides and their cognate receptors regulate neurotrophin-mediated differentiation. We found that 5'-O-(3-thio)triphosphate (ATPgammaS) activation of the G protein-coupled receptor P2Y(2) in the presence of nerve growth factor leads to the colocalization and association of tyrosine receptor kinase A and P2Y(2) receptors and is required for enhanced neuronal differentiation. Consistent with these effects, ATPgammaS promotes phosphorylation of tyrosine receptor kinase A, early response kinase 1/2, and p38, thereby enhancing sensitivity to nerve growth factor and accelerating neurite formation in both PC12 cells and dorsal root ganglion neurons. Genetic or small interfering RNA depletion of P2Y(2) receptors abolished the ATPgammaS-mediated increase in neuronal differentiation. Moreover, in vivo injection of ATPgammaS into the sciatic nerve increased growth-associated protein-43 (GAP-43), a marker for axonal growth, in wild-type but not P2Y(2)(-/-) mice. The interactions of tyrosine kinase- and P2Y(2)-signaling pathways provide a paradigm for the regulation of neuronal differentiation and suggest a role for P2Y(2) as a morphogen receptor that potentiates neurotrophin signaling in neuronal development and regeneration.

Purinergic mediated changes in Ca2+ mobilization and functional responses in microglia: effects of low levels of ATP

Microglia, the immune effector cells of the brain, are stimulated by a diversity of agents to transiently increase levels of intracellular calcium ([Ca2+]i). Changes in [Ca2+]i induced by compounds such as adenosine triphosphate (ATP) serve important roles in cellular signal transduction linking stimuli with cellular functional responses. Purinergic responses in microglia, like that in other cells, are mediated by two families of receptors classified as P2Y and P2X. Activation of metabotropic receptors (P2YR) leads to increased [Ca2+]i due to depletion of intracellular stores, a process that can trigger activation of Ca2+ entry through plasmalemmal store-operated channels (SOC). Activation of ionotropic receptors (P2XR) is associated with influx of Na+ and Ca2+ and efflux of K+ through nonselective cationic channels, leading to cellular depolarization. An intriguing property of purinergic stimulation of microglia is the dependence of cellular responses on agonist concentration. As one example, activation of the subtype P2X7R by higher levels of ATP (millimolar range), leads to a marked enhancement in microglial secretion of inflammatory mediators. Other members of the ionotropic P2XR family sensitive to lower levels of ATP, however, are also important in mediating microglial inflammatory responses in brain. At lower concentrations of ATP (100 microM), activation of SOC in human microglia is not only coupled to P2YR-dependent depletion of internal stores, but is also modulated by ATP binding to a P2XR (not P2X7R). The modulation is consistent with a P2XR-mediated influx of Na+ and inhibition of SOC by depolarization. In this review, a primary focus is placed on the effects of low concentrations of ATP (< or =100 microM) to induce changes in [Ca2+]i and modify functional processes in microglia. In essence, responses mediated by purinergic receptors other than P2X7R are considered.

Direct Excitation of Hypocretin/Orexin Cells by Extracellular ATP at P2X Receptors

Hypocretin/orexin (hcrt) neurons play an important role in hypothalamic arousal and energy homeostasis. ATP may be released by neurons or glia or by pathological conditions. Here we studied the effect of extracellular ATP on hypocretin cells using whole cell patch-clamp recording in hypothalamic slices of transgenic mice expressing green fluorescent protein (GFP) exclusively in hcrt-producing cells. Local application of ATP induced a dose-dependent increase in spike frequency. In the presence of TTX, ATP (100 μM) depolarized the cells by 7.8 ± 1.2 mV. In voltage clamp under blockade of synaptic activity with the GABAA receptor antagonist bicuculline, and ionotropic glutamate receptor antagonists dl-2-amino-5-phosphonopentanoic acid (AP-5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), ATP (100 μM) evoked an 18 pA inward current. The inward current was blocked by extracellular choline substitution for Na+, had a reversal potential of −27 mV, and was not affected by nominally Ca2+-free external buffer, suggesting that ATP activated a nonselective cation current. All excitatory effects of ATP showed rapid attenuation. ATP-induced excitatory actions were mimicked by nonhydrolyzable ATP-γ-S but not by α,β-MeATP and inhibited by the purinoceptor antagonists suramin and pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt (PPADS). The current was potentiated by a decrease in bath pH, suggesting P2X2 subunit involvement. Frequency and amplitude of spontaneous and miniature synaptic events were not altered by ATP. Suramin, but not PPADS, caused a small suppression of evoked excitatory synaptic potentials. Together, these results show a depolarizing response to extracellular ATP that would lead to an increased activity of the hypocretin arousal system.

Direct excitation of inhibitory interneurons by extracellular ATP mediated by P2Y1 receptors in the hippocampal slice

ATP is an important cell-to-cell signaling molecule mediating the interactions between astrocytes and neurons in the CNS. In the hippocampal slices, ATP suppresses excitatory transmission mostly through activation of adenosine A1 receptors, because the ectoenzyme activity for the extracellular breakdown of ATP to adenosine is high in slice preparations in contrast to culture environments. Because the hippocampus is also rich in the expression of P2 receptors activated specifically by ATP, we examined whether ATP modulates neuronal excitability in the acute slice preparations independently of adenosine receptors. Although ATP decreased the frequency of spontaneously occurring EPSCs in the CA3 pyramidal neurons through activation of adenosine A1 receptors, ATP concurrently increased the frequency of IPSCs in a manner dependent on action potential generation. This effect was mediated by P2Y1 receptors because (1) 2-methylthio-ATP (2meSATP) was the most potent agonist, (2) 2'-deoxy-N6-methyladenosine-3',5'-bisphosphate diammonium (MRS2179) abolished this effect, and (3) this increase in IPSC frequency was not observed in the transgenic mice lacking P2Y1 receptor proteins. Application of 2meSATP elicited MRS2179-sensitive time- and voltage-dependent inward currents in the interneurons, which depolarized the cell to firing threshold. Also, it increased [Ca2+]i in both astrocytes and interneurons, but, unlike the former effect, the latter was entirely dependent on Ca2+ entry. Thus, in hippocampal slices, in addition to activating A1 receptors of the excitatory terminals after being converted to adenosine, ATP activates P2Y1 receptors in the interneurons, which is linked to activation of unidentified excitatory conductance, through mechanisms distinct from those in the astrocytes.

Enhancement of spontaneous synaptic activity in rat Purkinje neurones by ATP during development

The establishment of functional synaptic connections and activity is a pivotal process in the development of neuronal networks. We have studied the synaptic activity in the developing rat cerebellum, and the contribution mediated by purinergic receptors. The mean frequency of the spontaneous postsynaptic currents (sPSCs) recorded with the whole-cell patch-clamp technique from Purkinje neurones in acute brain slices at room temperature, increased fourfold from 4.4+/-0.8 Hz at postnatal day 9/10 (n=23) to 17.8+/-1.6 Hz at postnatal day 17-20 (p17-p20; n=113; P<0.01). ATP, which increased the frequency of sPSCs by up to 100% (EC50=18 microM) in the third postnatal week, started to modulate the synaptic activity during the second postnatal week, which was determined by three processes: (1) the appearance of functional ATP receptors during p10-p12, (2) the enhancement of the sPSC frequency by endogenous ATP release becoming apparent after inhibition of ecto-ATPases by 6-N,N-diethyl-beta,gamma-dibromomethylene-D-adenosine-5-triphosphate (ARL67156; 50 microM) at p11-p12, and (3) with tonic stimulation of purinoceptors at p14, as revealed by the P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM). ATP had a similar effect at later stages (p24-p27) and at 35 degrees C. Our results suggest that endogenous release of ATP starts to enhance the synaptic activity in Purkinje neurones by the end of the second postnatal week.