The α7 nicotinic acetylcholine receptor in neuronal plasticity

A growing body of evidence indicates that neuronal nicotinic acetylcholine receptors (nAChRs), in addition to promoting fast cholinergic transmission, may modulate other neuronal activities within the central nervous system (CNS). In particular, the α7 nAChR is highly permeable to Ca²⁺ and may serve a distinct role in regulating neuronal plasticity. By elevating intracellular Ca²⁺ levels in discrete neuronal locations, these ligand-gated ion channels may influence numerous physiological processes in developing and adult CNS. In this article, we review evidence that both pre- and postsynaptic α7 nAChRs modulate transmitter release in the brain and periphery through Ca²⁺-dependent mechanisms. The possible role of α7 nAChRs in regulating neuronal growth and differentiation in developing CNS is also evaluated. We consider an interaction between cholinergic and glutamatergic transmission and propose a hypothesis on the possible coregulation of intracellular Ca²⁺ byN-methyl-d-aspartate (NMDA) receptors and α7 nAChRs. Finally, the clinical significance of alterations in the normal function of α7 nAChRs is discussed as it pertains to prenatal nicotine exposure, schizophrenia, and epilepsy.     

Recent behavioral findings of pathophysiological involvement of lactate in the central nervous system

Lactate was initially thought of as a fatigue substance. In recent years, however, lactate not only functions as an energy carrier and contributes to ATP production, but also its role as a signal transmitter has been attracting attention due to the identification of lactate receptors. Lactate is synthesized from glucose and glycogen through the glycolytic system. The central nervous system is a major organ of glucose metabolism and is rich in glycogen. Therefore, this review summarizes the recent findings on the contribution of lactate to the pathophysiology of the central nervous system.     

The role of nucleotides in the neuron--glia communication responsible for the brain functions

Accumulating findings indicate that nucleotides play an important role in cell-to-cell communication through P2 purinoceptors, even though ATP is recognized primarily to be a source of free energy and nucleotides are key molecules in cells. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (7 types; P2X(1)-P2X(7)) contain intrinsic pores that open by binding with ATP. P2Y (8 types; P2Y(1, 2, 4, 6, 11, 12, 13,) and (14)) are activated by nucleotides and couple to intracellular second-messenger systems through heteromeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. One of the most exciting cells in non-excitable cells is the glia cells, which are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of P2 purinoceptors and release the 'gliotransmitter' ATP to communicate with neurons, microglia and the vascular walls of capillaries. Microglia also express many types of P2 purinoceptors and are known as resident macrophages in the CNS. ATP and other nucleotides work as 'warning molecules' especially through activating microglia in pathophysiological conditions. Microglia play a key role in neuropathic pain and show phagocytosis through nucleotide-evoked activation of P2X(4) and P2Y(6) receptors, respectively. Such strong molecular, cellular and system-level evidence for extracellular nucleotide signaling places nucleotides in the central stage of cell communications in glia/CNS.     

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.     

P2 Receptors of microglia: sensors for danger signals in the CNS

Following tissue damage or invasion by pathogens a number of soluble signals are generated to alert the immune system of the impending danger and initiate inflammation. Some danger signals are released from injured or dying cells. Once released, danger signals activate a autocrine/paracrine network that recruits inflammatory cells, stimulates cytokine production, promotes dendritic cell maturations and increases the antigen (Ag) presenting efficiency. These events also occurs in the central nervous system (CNS) where cytokines and cytokine-releasing cells have a central role in spreading inflammation. P2 receptors of microglia are the focus of increasing interest, especially after they were shown to mediate chemotaxis, cytokine release and cell death in microglia. We propose that P2 receptors may function in microglia as sensors of the ATP/UTP concentration in the pericellular space, and therefore as sensors of danger signals in the CNS. Furthermore, microglia itself can release ATP when stimulated by inflammatory stimuli. Thus extracellular nucleotides may be included in the family of the early inflammatory mediators acting via P2 receptors to spread inflammation in the CNS.
References 1. Ferrari D., Villalba M., Chiozzi P., Falzoni S., Ricciardi-Castagnoli P. and Di Virgilio F. (1996) Mouse microglia cells express a plasma membrane pore gated by extracellular ATP. J. Immunol. 156 , 1531–1539.
2. Ferrari D., Chiozzi P., Falzoni S., Hanau S. and Di Virgilio F. (1997) Purinergic modulation of interleukin-1B release from microglia cells stimulated with bacterial endotoxin. J. Exp. Med. 185 , 579–582.     

P2 receptor-mediated modulation of neurotransmitter release—an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.     

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.     

The role of purinergic signaling in the etiology of migraine and novel antimigraine treatment

Etiopathogenesis of migraine involves different structures of the central nervous system: the trigeminal nerve with nuclei located in the brain stem, vascular system, and the cerebral cortex as well as diverse mechanisms and pathological processes. The multidirectional action of purines in different cell types (blood vessels, neurons, and satellite glial cells) and through different types of purinergic receptors contributes to the etiopathogenesis of migraine pain. Adenosine triphosphate (ATP) and its derivatives are involved in initiation and propagation of migrenogenic signals in several ways: they participate in vasomotor mechanism, cortical spreading depression, and in fast transmission or cross-excitation based on the satellite glial cells in trigeminal ganglion. Contribution of purinergic signaling in the conduction of pain is realized through the activation of P1 and P2 receptors expressed widely in the central nervous system: on the neurons and glial cells as well as on the smooth muscles and endothelium in the vascular system. Therefore, the purinergic receptors can be an excellent target for pharmacologists constructing new antimigraine therapeutics. Moreover, the mechanisms facilitating ATP and adenosine degradation may prevent vasodilatation and thus avoid a secondary central sensitization during a migraine attack. Thus, agonists and antagonists of P receptors as well as ecto-enzymes metabolizing nucleotides/nucleosides could gain the growing attention as therapeutic agents.     

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.     

P2 Receptors of microglia: sensors for danger signals in the CNS

Following tissue damage or invasion by pathogens a number of soluble signals are generated to alert the immune system of the impending danger and initiate inflammation. Some danger signals are released from injured or dying cells. Once released, danger signals activate a autocrine/paracrine network that recruits inflammatory cells, stimulates cytokine production, promotes dendritic cell maturations and increases the antigen (Ag) presenting efficiency. These events also occurs in the central nervous system (CNS) where cytokines and cytokine‐releasing cells have a central role in spreading inflammation. P2 receptors of microglia are the focus of increasing interest, especially after they were shown to mediate chemotaxis, cytokine release and cell death in microglia. We propose that P2 receptors may function in microglia as sensors of the ATP/UTP concentration in the pericellular space, and therefore as sensors of danger signals in the CNS. Furthermore, microglia itself can release ATP when stimulated by inflammatory stimuli. Thus extracellular nucleotides may be included in the family of the early inflammatory mediators acting via P2 receptors to spread inflammation in the CNS. References
1. Ferrari D., Villalba M., Chiozzi P., Falzoni S., Ricciardi‐Castagnoli P. and Di Virgilio F. (1996) Mouse microglia cells express a plasma membrane pore gated by extracellular ATP. J. Immunol. 156 , 1531–1539. 2. Ferrari D., Chiozzi P., Falzoni S., Hanau S. and Di Virgilio F. (1997) Purinergic modulation of interleukin‐1B release from microglia cells stimulated with bacterial endotoxin. J. Exp. Med. 185 , 579–582.     

Isoform specificity of P2X2 purinergic receptor C-terminus binding to tubulin

Adenosine triphosphate (ATP) and other nucleotides can be released in the central and peripheral nervous systems and act as neurotransmitters/neuromodulators. They can activate G-protein coupled receptors and ligand-gated ion channels, which are present throughout the central nervous system (CNS). P2X2 is one of seven known ion channels gated by ATP, and is characterized by having two transmembrane domains, a large extracellular loop and intracellular N- and C-termini. Recently, work from several laboratories has shown that neurotransmitter receptors can interact with other proteins thereby changing their functional attributes. More specifically, it was demonstrated that P2X2 binds beta-tubulin. Our goal was to investigate this interaction, by comparing P2X2 with a naturally occurring splicing variant named P2X2b. These isoforms differ in their C-terminal regions which contain the proposed beta-tubulin-binding domain. Indeed we were able to demonstrate that only the long variant P2X2 binds beta-tubulin I in various biochemical assays. In addition, this interaction can be direct since it also occurred when the P2X2 C-terminus was exposed to purified brain tubulin. When expressed in heterologous cells, P2X2 interacted with beta-tubulin I while present on the outer membrane, as demonstrated by biotinylation of surface proteins. Therefore, the present data strongly support a functional interaction between an ATP-gated channel and the cytoskeleton. Moreover, we show a biochemical difference between the splicing alternatives that might account for novel distinct functional roles.     

Epidermal growth factor and the nervous system

Various growth factors and their receptors are present in the nervous system. This review focuses on the presence of epidermal growth factor (EGF) and its receptors in the central nervous system (CNS). Evidence indicates that EGF in the CNS is the result of local synthesis, by intrinsic and blood-derived macrophages, glial cells and neurons, and uptake from the peripheral blood through the circumventricular organs and probably also through the blood-brain barrier. Evidence is accumulating suggesting that EGF regulates a variety of CNS functions in a specific manner. EGF influences CNS growth, differentiation and maintenance (actions proposed to promote neural regeneration and cell survival following a variety of insults). EGF also induces neuromodulatory actions, affects the neuroendocrine system, and suppresses food intake and gastric acid secretion. Acute and chronic pathological processes, e.g., various cancers, stimulate the production and release of EGF in various cell systems. Monitoring of EGF by the CNS may participate in several neurological manifestations (e.g., appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic disease. EGF and transforming growth factor-alpha (TGF-alpha, a factor that binds to the EGF receptor with high affinity and induces the same biological signals as EGF) also may be involved in the promotion of malignancy in the CNS and in the neuropathogenesis of degenerative disorders. Thus evidence is accumulating concerning the neurophysiological or neuropathophysiological significance of EGF in the nervous system.     

A role for ligand-gated ion channels in rod photoreceptor development

Neurotransmitter receptors are central to communication at synapses. Many components of the machinery for neurotransmission are present prior to synapse formation, suggesting a developmental role. Here, evidence is presented that signaling through glycine receptor alpha2 (GlyRalpha2) and GABA(A) receptors plays a role in photoreceptor development in the vertebrate retina. The signaling is likely mediated by taurine, which is present at high levels throughout the developing central nervous system (CNS). Taurine potentiates the production of rod photoreceptors, and this induction is inhibited by strychnine, an antagonist of glycine receptors, and bicuculline, an antagonist of GABA receptors. Gain-of-function experiments showed that signaling through GlyRalpha2 induced exit from mitosis and an increase in rod photoreceptors. Furthermore, targeted knockdown of GlyRalpha2 decreased the number of photoreceptors while increasing the number of other retinal cell types. These data support a previously undescribed role for these ligand-gated ion channels during the early stages of CNS development.     

P2 receptors and chronic pain

There is abundant evidence that extracellular ATP and other nucleotides have an important role in pain signaling at both the periphery and in the CNS. The focus of attention now is on the possibility that endogenous ATP and its receptor system might be activated in chronic pathological pain states, particularly in neuropathic and inflammatory pain. Neuropathic pain is often a consequence of nerve injury through surgery, bone compression, diabetes or infection. This type of pain can be so severe that even light touching can be intensely painful; unfortunately, this state is generally resistant to currently available treatments. In this review, we summarize the role of ATP receptors, particularly the P2X₄, P2X₃ and P2X₇ receptors, in neuropathic and inflammatory pain. The expression of P2X₄ receptors in the spinal cord is enhanced in spinal microglia after peripheral nerve injury, and blocking pharmacologically and suppressing molecularly P2X₄ receptors produce a reduction of the neuropathic pain behaviour. Understanding the key roles of these ATP receptors may lead to new strategies for the management of intractable chronic pain.     

Regulation of GABAB receptors by histamine and neuronal activity in the isolated spinal cord of neonatal opossum in culture.

The aim of these experiments has been to analyse the properties of receptors for the transmitter gamma-aminobutyric acid (GABA) in developing mammalian nervous system. Changes in responses of GABAB receptors have been measured after alterations of the chemical environment and the level of electrical activity. We have previously shown that when the central nervous system (CNS) of the new-born opossum, Monodelphis domestica, is cultured for three to five days in the presence of histidine, inhibition by baclofen, a GABAB agonist, disappears (Stewart et al. 1991). We have now investigated whether histidine acts indirectly by way of conversion to histamine. As with histidine, culture with 150 microM histamine for five days virtually abolished the inhibition by baclofen. The effects of histidine, as well as histamine, were blocked by mepyramine, a histamine H1-receptor antagonist, and by ranitidine, an H2-antagonist. Tetrodotoxin (TTX), which blocks all electrical activity, protected preparations from the action of histidine but not histamine. Our results suggest that histidine is converted to histamine, which reduces the efficacy of GABAB agonists. We conclude that, in the developing mammalian CNS, transmitter levels and electrical activity can selectively influence the properties of receptors.     

Molecular structure of purinergic P2X receptors and their expression in the hypothalamus and pituitary

Purinergic P2X receptors represent a novel structural type of ligand-gated ion channels activated by extracellular ATP. So far, seven P2X receptor subunits have been found in excitable as well as non-excitable tissues. Little is known about their structure, mechanism of channel opening, localization, and role in the central nervous system. The aim of this work is to summarize recent investigations and describe our contribution to elucidating the structure of the ATP binding site and transmembrane domains of the P2X receptor, we also discuss the expression and physiological roles played by the ATP and P2X receptors in the anterior pituitary and hypothalamus.     

Benzodiazepine pharmacology of cultured mammalian CNS neurons

Many neurons cultured from the embryonic mammalian central nervous system (CNS) express benzodiazepine receptors while some neurons differentiate specific transmitter phenotypes like glutamic acid decarboxylase (GAD), the synthetic enzyme for gamma-aminobutyric acid (GABA). The benzodiazepine receptors in these cultured neurons are often, if not always coupled to a practically ubiquitous GABA-mediated function, activation of Cl- ion conductance. The transmitter signal serves to inhibit neuronal excitability and is facilitated by clinically important benzodiazepines. Here we review some details regarding the pharmacological actions of benzodiazepines on membrane excitability.     

Purinergic signaling in oligodendrocyte development and function

Myelin, an insulating membrane that enables rapid action potential propagation, is an essential component of an efficient, functional vertebrate nervous system. Oligodendrocytes, the myelinating glia of the central nervous system (CNS), produce myelin throughout the CNS, which requires continuous proliferation, migration, and differentiation of oligodendrocyte progenitor cells. Because myelination is essential for efficient neurotransmission, researchers hypothesize that neuronal signals may regulate the cascade of events necessary for this process. The ability of oligodendrocytes and oligodendrocyte progenitor cells to detect and respond to neuronal activity is becoming increasingly appreciated, although the specific signals involved are still a matter of debate. Recent evidence from multiple studies points to purinergic signaling as a potential regulator of oligodendrocyte development and differentiation. Adenosine triphosphate (ATP) and its derivatives are potent signaling ligands with receptors expressed on many populations of cells in the nervous system, including cells of the oligodendrocyte lineage. Release of ATP into the extracellular space can initiate a multitude of signaling events, and these downstream signals are specific to the particular purinergic receptor (or receptors) expressed, and whether enzymes are present to hydrolyze ATP to its derivatives adenosine diphosphate and adenosine, each of which can activate their own unique downstream signaling cascades. This review will introduce purinergic signaling in the CNS and discuss evidence for its effects on oligodendrocyte proliferation, differentiation, and myelination. We will review sources of extracellular purines in the nervous system and how changes in purinergic receptor expression may be coupled to oligodendrocyte differentiation. We will also briefly discuss purinergic signaling in injury and diseases of the CNS.

     

Purinergic signalling: Its unpopular beginning, its acceptance and its exciting future

Adenosine 5'-triphosphate (ATP) was identified in 1970 as the transmitter responsible for non-adrenergic, non-cholinergic neurotransmission in the gut and bladder and the term 'purinergic' was coined. Purinergic cotransmission was proposed in 1976 and ATP is now recognized as a cotransmitter in all nerves in the peripheral and central nervous systems. P1 (adenosine) and P2 (ATP) receptors were distinguished in 1978. Cloning of these receptors in the early 1990s was a turning point in the acceptance of the purinergic signalling hypothesis. There are both short-term purinergic signalling in neurotransmission, neuromodulation and secretion and long-term (trophic) purinergic signalling of cell proliferation, differentiation and death in development and regeneration. Much is known about the mechanisms of ATP release and its breakdown by ectonucleotidases. Recently, there has been emphasis on purinergic pathophysiology, including neurodegenerative and neuropsychiatric disorders. Purinergic therapeutic strategies are being developed for treatment of gut, kidney, bladder, lung, skeletal and reproductive system disorders, pain and cancer.     

Effects of pentobarbital on purinergic P2X receptors of rat dorsal root ganglion neurons

Purinergic P2X receptors are ligand-gated ion channels that are activated by extracellular adenosine triphosphate (ATP) and are widely expressed not only in the central and peripheral nervous system but also in tissues throughout the body, playing an important role in the transfer of nociceptive information. Since the influence of barbiturates on P2X receptor subtypes is not known, we studied the effects of pentobarbital sodium (PB) on ATP responses in dorsal root ganglion (DRG) neurons. DRG neurons were dissected from 10- to 14-day-old rats and dissociated after enzyme treatment. Electrical measurements were performed using the nystatin-perforated patch recording mode under voltage-clamp conditions. Drugs were applied using the Y-tube method. ATP evoked three types of inward current at -60 mV: fast desensitizing, slow desensitizing, and mixed. The fast-type current was attributed to activation of P2X3 subtype and the slow type to the P2X2 subtype. PB suppressed the fast-type current in a concentration-dependent manner, while the slow type was slightly reduced. A noncompetitive inhibition was suggested by a downward shift of the ATP concentration-response curves. The current-voltage relationships showed inward rectification, and the extent of suppression was not affected by the holding potential. The reduction was greater in external solutions of higher pH. PB had subtype-specific effects on P2X receptors. The ionized form is likely to be responsible for the suppression of the P2X3 receptor current, which may result in a reduction of the excitability of central and peripheral neurons and may contribute to the anesthetic and analgesic actions of the agent.