Immunoreactivity to P2X(6) receptors in the rat hypothalamo-neurohypophysial system: an ultrastructural study with extravidin and colloidal gold-silver labelling

Different Peas in the Same Pod: The Histaminergic Neuronal Heterogeneity

The histaminergic neuronal system is recently receiving increasing attention, as much has been learned over the past 25 years about histamine role as a neurotransmitter. Indeed, this amine is crucial in maintaining arousal and provides important contributions to regulate circadian rhythms, energy, endocrine homeostasis, motor behavior, and cognition. The extent to which these distinct physiological functions are operated by independent histamine neuronal subpopulation is unclear. In the rat brain histamine neuronal cell bodies are grouped within the tuberomamillary nucleus of the posterior hypothalamus in five clusters, E1-E5, each sending overlapping axons throughout the entire central nervous system with no strict topographical pattern. These features lead to the concept that histamine regulation of a wide range of functions in the central nervous system is achieved by the histaminergic neuronal system as a whole. However, increasing experimental evidence suggesting that the histaminergic system is organized into distinct pathways modulated by selective mechanisms challenges this view. In this review, we summarized experimental evidence supporting the heterogeneity of histamine neurons, and their organization in functionally distinct circuits impinging on separate brain regions and displaying selective control mechanisms. This implies independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections with relevant consequences for the development of specific compounds that affect only subsets of histamine neurons, thus increasing the target specificity.

On P2X receptors in the brain: microvessels. Dedicated to the memory of the late Professor Geoffrey Burnstock (1929-2020)

This tribute article presents selected immunocytochemical and transmission electron microscope data on the location of ATP-gated P2X receptor in the rat brain, as studied in the 1990s in Prof G. Burnstock's laboratory at University College London. There are examples of immuno-ultrastructural findings and introductory information about pre- and post-synaptic location of P2X receptors in the rat cerebellum and endocrine hypothalamus to support the concept of purinergic transmission in the central nervous system. Then findings of diverse immunoreactivity for P2X1, P2X2, P2X4, and P2X6 receptors associated with brain microvessels are shown, including vascular endothelium and pericytes as well as perivascular astrocytes and neuronal components. These findings imply the involvement of P2X receptors and hence purinergic signalling in the neurovascular unit, at least in microvessels in the rat cerebellum and hypothalamic paraventricular and supraoptic nuclei examined here. Various aspects of P2X receptors in brain microvessels are discussed.

Purinergic signaling pathways in endocrine system

Adenosine-5'-triphosphate is released by neuroendocrine, endocrine, and other cell types and acts as an extracellular agonist for ligand-gated P2X cationic channels and G protein-coupled P2Y receptors in numerous organs and tissues, including the endocrine system. The breakdown of ATP by ectonucleotidases not only terminates its extracellular messenger functions, but also provides a pathway for the generation of two additional agonists: adenosine 5'-diphosphate, acting via some P2Y receptors, and adenosine, a native agonist for G protein-coupled adenosine receptors, also expressed in the endocrine system. This article provides a review of purinergic signaling pathways in the hypothalamic magnocellular neurosecretory cells and neurohypophysis, hypothalamic parvocellular neuroendocrine system, adenohypophysis, and effector glands organized in five axes: hypothalamic-pituitary-gonadal, hypothalamic-pituitary-thyroid, hypothalamic-pituitary-adrenal, hypothalamic-pituitary-growth hormone, and hypothalamic-pituitary-prolactin. We attempted to summarize current knowledge of purinergic receptor subtypes expressed in the endocrine system, including their roles in intracellular signaling, hormone secretion, and other cell functions. We also briefly review the release mechanism for adenosine-5'-triphosphate by neuroendocrine, endocrine and surrounding cells, the enzymes involved in adenosine-5'-triphosphate hydrolysis to adenosine-5'-diphosphate and adenosine, and the relevance of this pathway for sequential activation of receptors and termination of signaling.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Potentiation of inhibitory synaptic transmission by extracellular ATP in rat suprachiasmatic nuclei

The hypothalamic suprachiasmatic nuclei (SCN), the circadian master clock in mammals, releases ATP in a rhythm, but the role of extracellular ATP in the SCN is still unknown. In this study, we examined the expression and function of ATP-gated P2X receptors (P2XRs) in the SCN neurons of slices isolated from the brain of 16- to 20-day-old rats. Quantitative RT-PCR showed that the SCN contains mRNA for P2X 1-7 receptors and several G-protein-coupled P2Y receptors. Among the P2XR subunits, the P2X2 > P2X7 > P2X4 mRNAs were the most abundant. Whole-cell patch-clamp recordings from SCN neurons revealed that extracellular ATP application increased the frequency of spontaneous GABAergic IPSCs without changes in their amplitudes. The effect of ATP appears to be mediated by presynaptic P2X2Rs because ATPγS and 2MeS-ATP mimics, while the P2XR antagonist PPADS blocks, the observed enhancement of the frequency of GABA currents. There were significant differences between two SCN regions in that the effect of ATP was higher in the ventrolateral subdivision, which is densely innervated from outside the SCN. Little evidence was found for the presence of P2XR channels in somata of SCN neurons as P2X2R immunoreactivity colocalized with synapsin and ATP-induced current was observed in only 7% of cells. In fura-2 AM-loaded slices, BzATP as well as ADP stimulated intracellular Ca(2+) increase, indicating that the SCN cells express functional P2X7 and P2Y receptors. Our data suggest that ATP activates presynaptic P2X2Rs to regulate inhibitory synaptic transmission within the SCN and that this effect varies between regions.

Expression of P2X6 receptors in the enteric nervous system of the rat gastrointestinal tract

Expression of P2X(4) and P2X(6) receptor subunits in the gastrointestinal tract of the rat was studied with double-labeling fluorescence immunohistochemistry. The results showed that P2X(6) receptors were expressed widely in the submucosal and myenteric plexuses. In the myenteric plexus, P2X(6) receptors were expressed mainly in large size neurons which resembled Dogiel type II neurons. These P2X(6) receptor-immunoreactive (ir) neurons also expressed calbindin 28K, calretinin and neuronal nuclei (NeuN), proteins that are markers of intrinsic sensory neurons. In the submucosal plexus, all the calbindin 28K, calretinin and NeuN-ir cells were immunoreactive for P2X(6) receptors. P2X(6) receptors do not form homomultimers, but rather heteromultimers with either P2X(2) or P2X(4) receptors. P2X(4) receptors were not expressed in neurons, but were expressed in macrophages of the rat gastrointestinal tract. These data indicate that P2X(6) receptors are mainly expressed on intrinsic sensory neurons and that ATP, via P2X(6) receptors probably in heteromeric combination with P2X(2) receptors, may be involved in regulating the physiological functions of these neurons.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

P2X receptors are differentially expressed on vasopressin- and oxytocin-containing neurons in the supraoptic and paraventricular nuclei of rat hypothalamus

In the present study, the distribution of P2X receptor protein and colocalization of P2X receptors with vasopressin and oxytocin in the supraoptic and paraventricular nuclei of rat hypothalamus was studied using double-labeling fluorescence immunohistochemistry. The results showed that vasopressin-containing neurons expressed P2X(2), P2X(4), P2X(5) and P2X(6) receptor and oxytocin-containing neurons expressed P2X(2), P2X(4) and P2X(5) receptors in the supraoptic nucleus. In the paraventricular nucleus, vasopressin-containing neurons expressed P2X(4), P2X(5) and P2X(6) receptors, while oxytocin-containing neurons expressed P2X(4) receptors. This study provides the first evidence that P2X receptor subunits are differentially expressed on vasopressin- and oxytocin-containing neurons in the supraoptic and paraventricular nuclei, and hence, provides a substantial neuroanatomical basis for possible functional interactions between the purinergic and vasopressinergic systems, and the purinergic and oxytocinergic systems in the rat hypothalamus.

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.

Activity-Dependent Structural and Functional Plasticity of Astrocyte-Neuron Interactions

Observations from different brain areas have established that the adult nervous system can undergo significant experience-related structural changes throughout life. Less familiar is the notion that morphological plasticity affects not only neurons but glial cells as well. Yet there is abundant evidence showing that astrocytes, the most numerous cells in the mammalian brain, are highly mobile. Under physiological conditions as different as reproduction, sensory stimulation, and learning, they display a remarkable structural plasticity, particularly conspicuous at the level of their lamellate distal processes that normally ensheath all portions of neurons. Distal astrocytic processes can undergo morphological changes in a matter of minutes, a remodeling that modifies the geometry and diffusion properties of the extracellular space and relationships with adjacent neuronal elements, especially synapses. Astrocytes respond to neuronal activity via ion channels, neurotransmitter receptors, and transporters on their processes; they transmit information via release of neuroactive substances. Where astrocytic processes are mobile then, astrocytic-neuronal interactions become highly dynamic, a plasticity that has important functional consequences since it modifies extracellular ionic homeostasis, neurotransmission, gliotransmission, and ultimately neuronal function at the cellular and system levels. Although a complete picture of intervening cellular mechanisms is lacking, some have been identified, notably certain permissive molecular factors common to systems capable of remodeling (cell surface and extracellular matrix adhesion molecules, cytoskeletal proteins) and molecules that appear specific to each system (neuropeptides, neurotransmitters, steroids, growth factors) that trigger or reverse the morphological changes.

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.

P2 receptor web: Complexity and fine-tuning

The present review offers a new perspective on a family of receptors, termed P2 receptors, specific for nucleoside tri- and diphosphates of purines/pyrimidines. We emphasize here that while decoding the inputs of various related extracellular ligands, P2 receptors are a clear example of increasing biological complexity. They are represented by 7 ionotropic P2X and 8 metabotropic P2Y receptors; they have very heterogeneous ligands and binding characteristics, molecular properties, transduction mechanisms, cellular localization and protein-protein interactions. While the reason for this sophistication is unknown, a few compelling issues emerge while looking at such a rich variety. We ask, for instance, why so many different receptor subtypes are necessary for triggering biological properties and functions, and if these receptors are more than the sum of their single entities. A first possibility is that newly synthesized P2 proteins are casually located on the cell surface (stochastic hypothesis). Alternatively, distinct subunits are engaged on different cell phenotypes by genetic control (genetic determinism) and/or selective recruitment under physiopathological conditions and epigenetic stimuli (epigenetic determinism). Nevertheless, an appropriate way to both dissect the vast biological scenario and molecular complexity among P2 receptors and to integrate and upgrade their assortment is to regard them as a "combinatorial receptor web", that is, a dynamic architecture of P2 proteins demonstrating economic efficiency and involving a process of "fine-tuning", a mechanism which endorses the dynamic nature of all biological reactions. In the present analysis, we stimulate a scientific query about what contributes to such a vast P2 receptor sophistication.

P2X5 receptors are expressed on neurons containing arginine vasopressin and nitric oxide synthase in the rat hypothalamus

In this study, the P2X(5) receptor was found to be distributed widely in the rat hypothalamus using single and double labeling immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR) methods. The regions of the hypothalamus with the highest expression of P2X(5) receptors in neurons are the paraventricular and supraoptic nuclei. The intensity of P2X(5) immunofluorescence in neurons of the ventromedial nucleus was low. 70-90% of the neurons in the paraventricular nucleus and 46-58% of neurons in the supraoptic and accessory neurosecretory nuclei show colocalization of P2X(5) receptors and arginine vasopressin (AVP). None of the neurons expressing P2X(5) receptors shows colocalization with AVP in the suprachiasmatic and ventromedial nuclei. 87-90% of the neurons in the lateral and ventral paraventricular nucleus and 42-56% of the neurons in the accessory neurosecretory, supraoptic and ventromedial nuclei show colocalization of P2X(5) receptors with neuronal nitric oxide synthase (nNOS). None of the neurons expressing P2X(5) receptors in the suprachiasmatic nucleus shows colocalization with nNOS. These findings provide a morphological basis for possible functional interactions between the purinergic and nitrergic or vasopressinergic neurotransmitter systems.

Expression of purinergic receptors in the hypothalamus of the rat is modified by reduced food availability

ATP-sensitive P2 receptors are suggested to play an important role in the cerebral signal transduction. We examined the expression of the P2Y1 receptor and the possibly downstream-related neuronal nitric oxide synthase (nNOS) in the hypothalamus of rats food-restricted for 3 or 10 days and rats refed after a restriction of 10 days. The restriction caused a reduction of the body weight and plasma triacylglyceride, an increase of non-esterified fatty acid levels correlating with a decrease of leptin levels and an enhancement of plasma corticosterone. All changes returned to basal levels after refeeding. The restriction induced an enhanced intake within 30 min after food presentation and a reduction in the latency. Interestingly, the latter was not abolished by refeeding. The daily food intake induced by refeeding was enhanced at the first day only. The expression of hypothalamic P2Y1 receptor/nNOS mRNA and protein and of leptin receptor mRNA were enhanced after restricted feeding. These changes were abolished after 3 days of refeeding. Immunofluorescence studies indicated that P2Y1 receptor and nNOS immunoreactivities are present in the dorsomedial, ventromedial and lateral hypothalamus and in the nucleus arcuatus. P2Y1 receptor-positive cells were partially also nNOS-positive. The P2Y1 receptor labeling was restricted to cell bodies of obviously non-glial cells, whereas nNOS labeling could be detected also at cellular processes of these cells. In the nucleus arcuatus, astrocytes were identified, expressing P2Y1 receptors at cell bodies and cellular processes. The data suggest that restricted feeding may enhance the sensitivity of the hypothalamus to extracellular ADP/ATP by regulation of the expression of P2Y1 receptors and possibly of their signal transduction pathway via nitric oxide production.

An Uncharged Region within the N Terminus of the P2X6 Receptor Inhibits Its Assembly and Exit from the Endoplasmic Reticulum

ATP-gated P2X receptors are trimeric complexes formed by the homomeric or heteromeric assembly of seven different subunits. We have shown previously that, unlike all of the other P2X subunits, the P2X6 subunit cannot form homomeric receptors and when expressed alone is retained in the endoplasmic reticulum (ER) in monomeric form (J Biol Chem 280: 107591-10765, 2005). However, other studies have shown that P2X6 can form functional heteromeric receptors with P2X2 and P2X4 subunits. In this study, we used a combination of immunocytochemistry, surface biotinylation, and atomic force microscopy to investigate the assembly and trafficking of the P2X6 subunit, both alone and as part of a heteromer. We show that as a heteromer, it exits the ER and is either stably expressed at the cell surface or constitutively internalized, depending on its partner. Through the use of targeted mutation, we demonstrate that an uncharged region at the N terminus of P2X6 exerts an inhibitory effect on its assembly and export from the ER. When this region is removed, or when charge is added to it, P2X6 forms homotrimeric assemblies, undergoes complex glycosylation and is delivered to the plasma membrane, albeit less efficiently than the P2X2 receptor. The N-terminal mutants were, however, nonfunctional. Substituting the uncharged 14-amino acid N-terminal region for the equivalent region of P2X2 increased ER retention but was not sufficient to prevent the formation of functional homomeric receptors. We propose that the N terminus of the P2X6 subunit contributes to a mechanism that prevents the inappropriate export and plasma membrane expression of nonfunctional P2X receptors.

Presence of the P2X(7) purinergic receptor on immune cells that invade the rat endometrium during oestrus

Eosinophils, macrophages and other leucocytes invade the uterine endometrium during oestrus and play a role in the tissue remodeling and immune responses that occur prior to implantation of the fertilized ovum. Adenosine 5'-triphosphate (ATP) and its metabolites influence uterine function via ATP receptors. In this study, we investigated the presence and localisation of the P2X(7) nucleotide receptor in the cells that infiltrate the uterine endometrium of adult female rats during oestrus at the electron microscope level, using gold-silver pre-embedding immunocytochemical techniques. P2X(7) receptor expression was found in the cytoplasm and the cell membrane of eosinophils, macrophages and fibroblasts in the endometrium during oestrus. These results suggest that ATP-mediated responses may be important in uterine preparation and remodeling before implantation and that this may involve several types of cells. In particular, the presence of P2X(7) receptors on endometrial stromal cells may indicate their involvement in apoptosis and immune and inflammatory responses.

Expression of P2X purinoceptors during rat brain development and their inhibitory role on motor axon outgrowth in neural tube explant cultures

Extracellular ATP is well known as a neurotransmitter and neuromodulator in the CNS of adults. However, little is known about the involvement of ATP during the development of mammalian brain. In the present study, we have examined the expression pattern of P2X receptor subtype mRNA and protein during perinatal rat brain development (from embryonic day (E) 10 to postnatal day (P) 16 brain). While P2X3 receptors appeared early at E11, they declined in the stages that follow. P2X2 and P2X7 receptors were expressed from E14 onwards, while P2X4, P2X5 and P2X6 receptors were expressed from P1 onwards. P2X1 receptor expression was not observed in any of the developmental ages examined. We investigated the effect of 100 microM ATP and alpha,beta-methylene ATP (alpha,beta-meATP; selective agonist for P2X1, P2X2/3 and P2X3 receptors) on motor axon outgrowth in collagen-embedded neural tube explant cultures. Both ATP- and alpha,beta-meATP-treated neural tubes showed a significant reduction in neurite outgrowth compared with the control explants. This inhibitory effect could not be reproduced by uridine triphosphate. In conclusion, all P2X receptor subtypes, except for P2X1, were strongly represented in the developing rat brain. ATP was shown to inhibit motor axon outgrowth during early embryonic neurogenesis, most likely via the P2X3 receptor. It is speculated that P2X7 receptors may be involved in programmed cell death during embryogenesis and that P2X4, P2X(5) and P2X6 receptors might be involved in postnatal neurogenesis.

Molecular physiology of P2 receptors in the central nervous system

Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. To date seven P2X and seven P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X(7) subunits do not form heterooligomeric assemblies and are unique in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X(2), P2X(4), P2X(4)/P2X(6) and P2Y(1) receptors appear to be the predominant neuronal types. The localisation of these receptors may be at the somato-dendritic region (postsynaptic) or at the nerve terminals (presynaptic). Postsynaptic P2 receptors appear to be mostly excitatory, while presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y). Since in the CNS the stimulation of a single neuron may activate multiple networks, a concomitant stimulation of facilitatory and inhibitory circuits as a result of ATP release is also possible. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate via A(1) or A(2A) receptor-activation the ATP effect.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Purinergic Receptors Are Part of a Signaling System for Keratinocyte Proliferation, Differentiation, and Apoptosis in Human Fetal Epidermis

We have investigated the expression of P2X5, P2X7, P2Y1, and P2Y2 receptor subtypes in 8- to 11-wk-old human fetal epidermis in relation to markers of proliferation (proliferating cell nuclear antigen (PCNA) and Ki-67), keratinocyte differentiation (cytokeratin K10 and involucrin), and markers of apoptosis (TdT-mediated dUTP nick end labeling (TUNEL) and anti-caspase-3). Immunohistochemistry showed that each of the four receptors was expressed in spatially distinct zones of the developing epidermis: P2Y1 receptors were found in the basal layer, P2X5 receptors were predominantly in the basal and intermediate layers, and both P2Y2 and P2X7 receptors were in the periderm. Colocalization experiments suggested different functional roles for these receptors. P2Y1 receptors were found in fetal keratinocytes positive for PCNA and Ki-67, suggesting a role in proliferation. P2X5 receptors double labeled with differentiated fetal keratinocytes that were positive for cytokeratin K10, suggesting a role in differentiation. P2X7 receptors colocalized with anti-caspase-3 antibody and were also expressed in periderm cells positive for TUNEL, suggesting a role in periderm cell apoptosis. P2Y2 receptors were found only in periderm cells and may have a role in chloride and fluid secretion into the amniotic fluid.

Modulation of ATP-induced currents by zinc in acutely isolated hypothalamic neurons of the rat

1. Whole-cell patch-clamp and fast perfusion were used to study the effects of zinc on adenosine 5'-triphosphate (ATP)-induced responses of histaminergic neurons. 2. At 10-30 micro M ATP, Zn(2+) had biphasic effects on ATP responses. Zn(2+) at 3-100 micro M increased the ATP-induced currents, but inhibited them at higher concentrations. 3. At 300 micro M ATP, Zn(2+) predominantly but incompletely inhibited the currents. 4. At 5 and 50 micro M, Zn(2+) shifted to the left the concentration-response curve for ATP-induced currents, without changing the maximal response. At 1 mM, Zn(2+) inhibited ATP-induced currents in a noncompetitive way, reducing the maximal response by 58%. .Zn(2+) increased the decay time of ATP-evoked currents nine fold with an EC(50) of 63 micro M. Upon removal of high concentrations of Zn(2+), there was a rapid increase of the current followed by a slow decline towards the response amplitude seen with ATP alone. The appearance of a tail current is consistent with a Zn(2+)-induced increase of ATP affinity and an inhibition of its efficacy. 6. Thus, Zn(2+) acts as a bidirectional modulator of ATP receptor channels in tuberomamillary neurons, which possess functional P2X(2) receptors. The data are consistent with the existence of two distinct modulatory sites on the P2X receptor, which can be occupied by Zn(2+). 7. Our data suggest that zinc-induced potentiation of ATP-mediated currents is caused by the slowing of ATP dissociation from the receptor, while inhibition of ATP-induced currents is related to the suppression of ATP receptor gating.

Expression and function of P2X purinoceptors in rat histaminergic neurons

(1) The pharmacology of ATP responses and the expression pattern of seven known subunits of the P2X receptor were investigated in individual histaminergic neurons of the tuberomamillary nucleus (TM). (2) ATP (3-1000 micro M) evoked fast non-desensitizing inward currents in TM neurons. 2-methylthioATP (2MeSATP) displayed the same efficacy but a lower potency, EC(50)s 84 micro M versus 48 micro M, when compared with ATP. Adenosine-diphosphate (ADP), uridine-triphosphate (UTP) and alpha beta methylene-ATP (alphabeta-meATP) were inactive. (3) ATP-mediated whole cell currents were potentiated by acidification of the recording solution (pH 7.5 and 6.6 were compared). (4) Single-cell RT-PCR (scRT-PCR) analysis revealed that the P2X(2) receptor is expressed in all PCR-positive neurons. Each of the P2X(1), P2X(3), P2X(4), P2X(5) and P2X(6) mRNAs were detected in less than 35% of the cells. (5) Suramin antagonized ATP responses with an IC(50) of 4.2 micro M and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 micro M) reduced ATP responses to 43% of control, when antagonists were pre-applied 90s before the agonist. Cibacron blue (3 micro M) given together with ATP potentiated control responses by 67%, but inhibited it to 10% after pre-application. (6) 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) antagonized ATP responses with an IC(50) of 7 micro M. (7) Pharmacological properties of ATP responses together with scRT-PCR data suggest that P2X(2) is the major purinoceptor on the soma of TM neurons, however the presence of heteromeric P2X(2/5) receptors in some neurons cannot be excluded.