Journal of the autonomic nervous system最新文献

(1) The methylxanthine caffeine has many pharmacological effects, most of which can be linked to blockade of adenosine receptors, inhibition of phosphodiesterases, and augmentation of calcium-dependent release of calcium from intracellular stores. (2) A variety of xanthines have been developed as potent and/or selective antagonists for adenosine receptors. (3) Several xanthines have been developed that are more potent and more selective inhibitors of cyclic nucleotide phosphodiesterase than caffeine or theophylline. (4) Caffeine remains the xanthine of choice for activation of intracellular calcium-sensitive calcium release channels although millimolar concentrations are required, which can have effects on other aspects of calcium regulation.

Nerve terminals consist of several hundred varicosities or synapses, each with a single active zone. The smooth muscle membrane apposing varicosities within about 50 nm is occupied by a 1-μm diameter cluster of P2X₁ receptors together with a mixture of other P2X subtypes; the rest of the membrane possesses small (0.4 μm diameter) clusters of P2X₁ to P2X₆ subunits. The small P2X clusters appear to form large clusters during development. This is supported by the observation that chimeras of P2X₁ subunits and green fluorescent protein (P2X₁-GFP), when packaged into adenoviruses used to infect excitable cells, initially form a diffuse distribution of small clusters of P2X₁-GFP in the membrane; these can be later observed in real time to form large clusters. Recording the electrical signs of ATP release from single adjacent varicosities, or using antibodies to label the extent of exocytosis from them, shows that they release with quite different probabilities. There are large quantitative differences in the extent of P2X autoreceptors on the membranes of individual varicosities. These will contribute to the differences in the probability of secretion from individual varicosities. The present analysis of NANC transmission at single varicosities indicates that individual synapses possess different probabilities for the secretion of transmitter as well as different complements of autoreceptors and mixtures of postjunctional receptor subunits.

Adenosine 5′-diphosphate (ADP) is a paracrine mediator that activates human blood platelets, causing them to become adhesive and thereby contributing to their role in hemostasis. The actions of ADP were initially thought to be mediated by a unique ADP receptor termed P2_T found only on platelets and antagonized by ATP, but it appears that at least two P2Y receptor subtypes are involved, a P2Y₁ receptor linked in some way to control of intracellular-free calcium levels and another P2Y receptor linked via an inhibitory G protein to adenylate cyclase. In addition, the presence of excitatory P2X₁ receptors that mediate the influx of monovalent and divalent cations in response to both ADP and ATP has been demonstrated. The precise contribution that each of these P2 receptors make to the overall phenomena associated with platelet aggregation, adhesion and hemostasis is yet to be defined. Antithrombotic agents that interfere with the actions of ADP are marketed, and P2 receptor antagonists are entering clinical trials for acute treatments of thrombosis. This review seeks to summarize the present state of knowledge of platelet P2 receptor pharmacology and therapeutics.

ATP-gated P2X ion-channel receptors are localised throughout the mammalian nervous system and have been identified on neurones which participate in conduction of nociceptive information from the periphery to, and within, the CNS. This article briefly reviews recently published research describing the role that ATP and P2X receptors may play in pain perception, highlighting the importance of the P2X₃ receptor in this process. The P2X₃ receptor subunit is almost exclusively expressed on a subset of small and medium diameter sensory neurones innervating cutaneous and visceral tissue. Activation of P2X receptors present on the peripheral terminals of primary afferents results in neuronal depolarisation and, in conscious animals, leads to the manifestation of acute nociceptive behaviour. Recent animal studies have also shown that P2X₃ receptor expression is increased in sensory ganglia following acute neuronal injury, hinting that similar plasticity in the expression of this receptor subtype could underlie the mechanisms involved in a range of conditions characterised by sensory hypersensitivity in man. It is apparent from the evidence available that functional antagonists at specific P2X receptor subtypes could represent an important class of novel analgesic agents.

A role for ATP in nociception and pain induction was proposed on the basis of human psychophysical experiments shortly after the formulation of the purinergic hypothesis. Following the pharmacological definition of distinct P2X and P2Y purinergic receptor subtypes by Burnstock and his collaborators, molecular cloning studies have identified the gene products that underlie the effects of ATP on peripheral sensory neurons. One particular receptor, P2X₃, is of particular interest in the context of pain pathways, because it is relatively selectively expressed at high levels by nociceptive sensory neurons. Evidence that this receptor may play a role in the excitation of sensory neurons has recently been complemented by studies that suggest an additional presynaptic role in the regulation of glutamate release from primary afferent neurons in the dorsal horn of the spinal cord. In this brief review, we discuss the present state of knowledge of the role of ATP in pain induction through its action on peripheral P2X receptors.

In astrocytic cultures maintained in vitro, a brief challenge with the ATP analog α,βmethyleneATP (α,βmeATP) results, 3 days later, in marked elongation of astrocytic processes, an event that resembles the astrocytic hypertrophy known to occur in vivo during reactive astrogliosis. α,βmeATP-induced effects were observed in primary astrocytes obtained from both rat striatum and cortex (a brain area highly involved in chronic neurodegenerative pathologies), as well as in human astrocytoma cells (ADF cells). Purine-induced gliosis could be reversed by the non-selective P2X/P2Y receptor antagonist pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS), but not by oxidized ATP (an antagonist of the P2X₇ receptor), in line with previous studies of our laboratory suggesting the involvement of a P2Y receptor subtype. Induction of reactive gliosis was preceded by increased expression of cyclooxygenase-2 (COX-2), an enzyme whose excessive activation has been implicated in both acute and chronic neurodegenerative diseases. The selective COX-2 inhibitor NS-398 prevented both purine-induced astrogliosis and the associated COX-2 induction, suggesting that inhibition of the transcription of the COX-2 gene may also contribute to the anti-inflammatory properties of this agent. Significant blockade of both α,βmeATP-mediated reactive gliosis and COX-2 induction was also observed with PPADS. These data suggest that COX-2 mediates P2Y receptor-induced reactive astrogliosis, and that antagonists selective for this receptor subtype may represent a novel class of anti-inflammatory agents of potential interest in acute and chronic neurological disorders characterized by an inflammatory component and reactive gliosis.

Electrophysiological investigations of autonomic neuromuscular transmission have provided great insights into the role of ATP as a neurotransmitter. Burnstock and Holman made the first recordings of excitatory junction potentials (e.j.p.s) produced by sympathetic nerves innervating the smooth muscle of the guinea-pig vas deferens. This led to the identification of ATP as the mediator of e.j.p.s in this tissue, where ATP acts as a cotransmitter with noradrenaline. The e.j.p.s are mediated solely by ATP acting on P2X₁ receptors leading to action potentials and a rapid phasic contraction, whilst noradrenaline mediates a slower, tonic contraction which is not dependent on membrane depolarisation. Subsequent electrophysiological studies of the autonomic innervation of smooth muscles of the urogenital, gastrointestinal and cardiovascular systems have revealed a similar pattern of response, where ATP mediates a fast electrical and mechanical response, whilst another transmitter such as noradrenaline, acetylcholine, nitric oxide or a peptide mediates a slower response. The modulation of junction potentials by a variety of pre-junctional receptors and the mechanism of inactivation of ATP as a neurotransmitter will also be described.

The pharmacological classification of P2 receptors owes its origin to the pioneering efforts of Geoff Burnstock and those who followed him, research that was conducted primarily in physiological experimental systems. Over recent years, the techniques of molecular biology have been increasingly applied in the study of P2 receptors while, at the same time, advances in their pharmacological analysis have been limited by a lack of potent and selective agonist or antagonist ligands. This has resulted in a classification scheme which is largely structural in nature, with relatively little contribution from pharmacology. Our endeavours in this area have been directed towards the discovery of ligands with which the pharmacological analysis and definition of P2 receptors could be advanced, the ultimate goal being the design of therapeutic agents. This article will describe some of our experiences in this challenging but rewarding area.

ATP has been shown to act as an excitatory neurotransmitter in the central nervous system. In this review, evidence is presented to indicate that when ATP is micro-injected into the ventrolateral medulla (VLM) of the rat, changes in respiratory activity are elicited. These effects, and accompanying changes in heart rate and blood pressure are mediated by P2X purinoreceptors. Immunocytochemistry indicates a prevalence of P2X₂ and P2X₆ purinoreceptors in this region of the medulla. The P2 purinoceptor antagonists, suramin and PPADS blunt the respiratory responses to changes in arterial CO₂ levels when micro-injected into the VLM. This effect is shown electrophysiologically to be mediated by purinoreceptors located primarily on respiratory neurones of the VLM including the Bötzinger complex. As the effects of agonist activation of P2X₂ purinoceptors expressed in HEK293 cells and Xenopus oocytes are potentiated by lowering pH, these data imply that the central respiratory response to CO₂ depends in part on the pH sensitivity of purinoreceptors located on inspiratory neurones. The implications for respiratory activity and control are discussed.