Adenosine inhibition via A(1) receptor of N-type Ca(2+) current and peptide release from isolated neurohypophysial terminals of the rat

Cholinergic and Adenosinergic Modulation of Synaptic Release

In this review we will discuss the effect of two neuromodulatory transmitters, acetylcholine (ACh) and adenosine, on the synaptic release probability and short-term synaptic plasticity. ACh and adenosine differ fundamentally in the way they are released into the extracellular space. ACh is released mostly from synaptic terminals and axonal bouton of cholinergic neurons in the basal forebrain (BF). Its mode of action on synaptic release probability is complex because it activate both ligand-gated ion channels, so-called nicotinic ACh receptors and G-protein coupled muscarinic ACh receptors. In contrast, adenosine is released from both neurons and glia via nucleoside transporters or diffusion over the cell membrane in a non-vesicular, non-synaptic fashion; its receptors are exclusively G-protein coupled receptors. We show that ACh and adenosine effects are highly specific for an identified synaptic connection and depend mostly on the presynaptic but also on the postsynaptic receptor type and discuss the functional implications of these differences.

On the Nature of the Mother-Infant Tie and Its Interaction With Freudian Drives

The affective bond between an infant and its caregiver, the so-called mother-infant tie, was analyzed by various reputable psychologists (e.g., Ainsworth, Clark, Erikson, Anna Freud, Harlow, Klein, Spitz, and Winnicott) but both the basic tenets of the bond and the importance of the trauma of maternal deprivation for personality disorders in adults were introduced by Bowlby. Although Bowlby was a trained psychoanalyst, he rejected central cornerstones of Freudian theory (esp. drive theory) and used concepts promulgated by renowned ethologists (Tinbergen and Lorenz) to establish his framework of "instinctive behavior" that has been developed further into the concept of "attachment theory" under the influence of Mary Ainsworth. However, since any precise experimental facts were lacking when Bowlby formulated his ideas on the concept of instinctive behavior, the whole framework is a descriptive, category-driven approach (like the ones of Freudian drives). In order to connect the mother-infant tie - as propounded by Bowlby - with experimental data, this manuscript undertakes a biochemical analysis of it because this strategy proved somewhat successful in relation to Freudian drives. The analysis unfolded that the neurochemical oxytocin, released by the action of sensory nerves, is of utmost importance for the operation of the mother-infant tie. Furthermore, multiple evidences have been presented to the fact that there is strong interaction between unconsciously operating Freudian drives and the consciously acting mother-infant tie (that is now classified as a drive). The outlined interaction in conjunction with the classification of attachment urges as drives gave a very detailed insight into how a SEEKING-derived reward can be evoked during operation of the mother-infant tie. In summary, there is no need to marginalize either the mother-infant tie or Freudian drives but rather there is need to respect both (principally different) impulses in moving toward a more extensive description.

Purinergic receptor types in the hypothalamic-neurohypophysial system

Many different types of purinergic receptors are present in the Hypothalamic-Neurohypophysial System (HNS), which synthesizes and releases vasopressin and oxytocin. The specific location of purinergic receptor subtypes has important functional repercussions for neuronal activity and synaptic output. Yet, until the advent of receptor KOs, this had been hindered by the low selectivity of the available pharmacological tools. The HNS offers an excellent opportunity to differentiate the functional properties of these purinergic receptors in cell bodies vs. terminals of the same physiological system. P2X2, P2X3, P2X4 and P2X7 receptors are present in vasopressin terminals while oxytocin terminals exclusively express the P2X7 subtype. The latter is not functional in the cell bodies of the HNS. These purinergic receptor subtypes are permeable to sodium vs. calcium in varying amounts and this could play an important role in the release of vasopressin vs. oxytocin during bursting activity. Endogenous ATP and its metabolite, adenosine, have autocrine and paracrine modulatory effects on the release of these neuropeptides during physiological stimulation. Finally, we hypothesize that during such action potential bursts, ATP potentiates the release of vasopressin but not of oxytocin, and that adenosine, via A1 receptors, inhibits the release of both neuropeptides. This article is protected by copyright. All rights reserved.

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.

P2X purinergic receptor knockout mice reveal endogenous ATP modulation of both vasopressin and oxytocin release from the intact neurohypophysis

Bursts of action potentials are crucial for neuropeptide release from the hypothalamic neurohypophysial system (HNS). The biophysical properties of the ion channels involved in the release of these neuropeptides, however, cannot explain the efficacy of such bursting patterns on secretion. We have previously shown that ATP, acting via P2X receptors, potentiates only vasopressin (AVP) release from HNS terminals, whereas its metabolite adenosine, via A1 receptors acting on transient Ca(2+) currents, inhibits both AVP and oxytocin (OT) secretion. Thus, purinergic feedback-mechanisms have been proposed to explain bursting efficacy at HNS terminals. Therefore, in the present study, we have used specific P2X receptor knockout (rKO) mice and purportedly selective P2X receptor antagonists to determine the P2X receptor subtype responsible for endogenous ATP induced potentiation of electrically-stimulated neuropeptide release. Intact neurohypophyses (NH) from wild-type (WT), P2X3 rKO, P2X2/3 rKO and P2X7 rKO mice were electrically stimulated with four 25-s bursts (3 V at 39 Hz) separated by 21-s interburst intervals with or without the P2X2 and P2X3 receptor antagonists, suramin or pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). These frequencies, number of bursts, and voltages were determined to maximise both AVP and OT release by electrical stimulations. Treatment of WT mouse NH with suramin/PPADS significantly reduced electrically-stimulated AVP release. A similar inhibition by suramin was observed in electrically-stimulated NH from P2X3 and P2X7 rKO mice but not P2X2/3 rKO mice, indicating that endogenous ATP facilitation of electrically-stimulated AVP release is mediated primarily by the activation of the P2X2 receptor. Unexpectedly, electrically-stimulated OT release from WT, P2X3, P2X2/3 and P2X7 rKO mice was potentiated by suramin, indicating nonpurinergic effects by this 'selective' antagonist. Nevertheless, these results show that sufficient endogenous ATP is released by bursts of action potentials to act at P2X2 receptors in a positive-feedback mechanism to 'differentially' modulate neuropeptide release from central nervous system terminals.

Modulation/physiology of calcium channel sub-types in neurosecretory terminals

The hypothalamic-neurohypophysial system (HNS) controls diuresis and parturition through the release of arginine-vasopressin (AVP) and oxytocin (OT). These neuropeptides are chiefly synthesized in hypothalamic magnocellular somata in the supraoptic and paraventricular nuclei and are released into the blood stream from terminals in the neurohypophysis. These HNS neurons develop specific electrical activity (bursts) in response to various physiological stimuli. The release of AVP and OT at the level of neurohypophysis is directly linked not only to their different burst patterns, but is also regulated by the activity of a number of voltage-dependent channels present in the HNS nerve terminals and by feedback modulators. We found that there is a different complement of voltage-gated Ca(2+) channels (VGCC) in the two types of HNS terminals: L, N, and Q in vasopressinergic terminals vs. L, N, and R in oxytocinergic terminals. These channels, however, do not have sufficiently distinct properties to explain the differences in release efficacy of the specific burst patterns. However, feedback by both opioids and ATP specifically modulate different types of VGCC and hence the amount of AVP and/or OT being released. Opioid receptors have been identified in both AVP and OT terminals. In OT terminals, μ-receptor agonists inhibit all VGCC (particularly R-type), whereas, they induce a limited block of L-, and P/Q-type channels, coupled to an unusual potentiation of the N-type Ca(2+) current in the AVP terminals. In contrast, the N-type Ca(2+) current can be inhibited by adenosine via A(1) receptors leading to the decreased release of both AVP and OT. Furthermore, ATP evokes an inactivating Ca(2+)/Na(+)-current in HNS terminals able to potentiate AVP release through the activation of P2X2, P2X3, P2X4 and P2X7 receptors. In OT terminals, however, only the latter receptor type is probably present. We conclude by proposing a model that can explain how purinergic and/or opioid feedback modulation during bursts can mediate differences in the control of neurohypophysial AVP vs. OT release.

Signaling by purinergic receptors and channels in the pituitary gland

Adenosine 5'-triphosphate is frequently released by cells and acts as an agonist for G protein-coupled P2Y receptors and ligand-gated P2X cationic channels in numerous tissues. 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. In the pituitary gland, adenosine 5'-triphosphate is released from the endings of magnocellular hypothalamic neurons and by anterior pituitary cells through pathway(s) that are still not well characterized. This gland also expresses several members of each family of purinergic receptors. P2X and adenosine receptors are co-expressed in the somata and nerve terminals of vasopressin-releasing neurons as well as in some secretory pituitary cells. P2X receptors stimulate electrical activity and modulate InsP(3)-dependent calcium release from intracellular stores, whereas adenosine receptors terminate electrical activity. Calcium-mobilizing P2Y receptors are expressed in pituicytes, folliculo-stellate cells and some secretory cells of the anterior pituitary.

Purinergic regulation of hypothalamopituitary functions

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

C-Jun N-terminal kinase regulates adenosine A1 receptor-mediated synaptic depression in the rat hippocampus

Adenosine A1 receptors are ubiquitous mediators of presynaptic inhibition of neurotransmission in the central nervous system, yet the signalling pathway linking A1 receptor activation and decreased neurotransmitter release remains poorly resolved. We tested the contribution of c-Jun N-terminal kinase (JNK) to adenosine A1 receptor-mediated depression of field excitatory postsynaptic potentials (fEPSPs) in area CA1 of the rat hippocampus. We found that inhibition of JNK with SP600125 or JNK inhibitor V, but not an inactive analogue, attenuated the depression of fEPSPs induced by adenosine, hypoxia, and the A1 receptor agonist N(6)-cyclopentyladenosine (CPA). In contrast, the JNK inhibitor SP600125 did not inhibit GABA(B)-mediated synaptic depression. In support of our electrophysiological findings, Western blot analysis showed that A1 receptor stimulation resulted in a transient increase in JNK phosphorylation in the membrane fraction of hippocampal lysates. The total amount of JNK in the membrane fraction was unchanged by CPA treatment. The increase in phosphorylated JNK induced by A1 receptor stimulation was blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), indicating that A1 receptors specifically activate JNK in the hippocampus. Together with functional data indicating that JNK inhibition decreased CPA-induced paired pulse facilitation, these results suggest that JNK activation is necessary for adenosine A1 receptor-mediated synaptic depression occurring at a presynaptic locus The adenosine A1 receptor-JNK signalling pathway may represent a novel mechanism underlying inhibition of neurotransmitter release in the CNS.

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.

Endogenous adenosine inhibits CNS terminal Ca(2+) currents and exocytosis

Bursts of action potentials (APs) are crucial for the release of neurotransmitters from dense core granules. This has been most definitively shown for neuropeptide release in the hypothalamic neurohypophysial system (HNS). Why such bursts are necessary, however, is not well understood. Thus far, biophysical characterization of channels involved in depolarization-secretion coupling cannot completely explain this phenomenon at HNS terminals, so purinergic feedback mechanisms have been proposed. We have previously shown that ATP, acting via P2X receptors, potentiates release from HNS terminals, but that its metabolite adenosine, via A(1) receptors acting on transient Ca(2+) currents, inhibit neuropeptide secretion. We now show that endogenous adenosine levels are sufficient to cause tonic inhibition of transient Ca(2+) currents and of stimulated exocytosis in HNS terminals. Initial non-detectable adenosine levels in the static bath increased to 2.9 microM after 40 min. These terminals exhibit an inhibition (39%) of their transient inward Ca(2+) current in a static bath when compared to a constant perfusion stream. CPT, an A(1) adenosine receptor antagonist, greatly reduced this tonic inhibition. An ecto-ATPase antagonist, ARL-67156, similarly reduced tonic inhibition, but CPT had no further effect, suggesting that endogenous adenosine is due to breakdown of released ATP. Finally, stimulated capacitance changes were greatly enhanced (600%) by adding CPT to the static bath. Thus, endogenous adenosine functions at terminals in a negative-feedback mechanism and, therefore, could help terminate peptide release by bursts of APs initiated in HNS cell bodies. This could be a general mechanism for controlling transmitter release in these and other CNS terminals.

p38 mitogen-activated protein kinase contributes to adenosine A1 receptor-mediated synaptic depression in area CA1 of the rat hippocampus

Adenosine is arguably the most potent and widespread presynaptic modulator in the CNS, yet adenosine receptor signal transduction pathways remain unresolved. Here, we demonstrate a novel mechanism in which adenosine A1 receptor stimulation leads to p38 mitogen-activated protein kinase (MAPK) activation and contributes to the inhibition of synaptic transmission. Western blot analysis indicated that selective A1 receptor activation [with N6-cyclopentyladenosine (CPA)] resulted in rapid increases in phosphorylated p38 (phospho-p38) MAPK immunoreactivity in membrane fractions, and decreases in phospho-p38 MAPK in cytosolic fractions. Immunoprecipitation with a phospho-p38 MAPK antibody revealed constitutive association of this phosphoprotein with adenosine A1 receptors. Phospho-p38 MAPK activation by A1 receptor stimulation induced translocation of PP2a (protein phosphatase 2a) to the membrane. We then examined the actions of p38 MAPK activation in A1 receptor-mediated synaptic inhibition. Excitatory postsynaptic field potentials evoked in area CA1 of the rat hippocampus markedly decreased in response to adenosine (10 microM), the A1 receptor agonist CPA (40 nM), or a 5 min exposure to hypoxia. These inhibitory responses were mediated by A1 receptor activation because the selective antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) (100 nM) prevented them. In agreement with the biochemical analysis, the selective p38 MAPK inhibitor SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole] (25 microM) blocked the inhibitory actions of A1 receptor activation, whereas both the inactive analog SB202474 [4-ethyl-2-(p-methoxyphenyl)-5-(4'-pyridyl)-1H-imidazole] (25 microM) and the ERK 1/2 (extracellular signal-regulated kinase 1/2) MAPK inhibitor PD98059 [2'-amino-3'-methoxyflavone] (50 microM) were ineffective. In contrast, the p38 MAPK inhibitors did not inhibit GABA(B)-mediated synaptic depression. These data suggest A1 receptor-mediated p38 MAPK activation is a crucial step underlying the presynaptic inhibitory effect of adenosine on CA3-CA1 synaptic transmission.

Activity-dependent feedback modulation of spike patterning of supraoptic nucleus neurons by endogenous adenosine

Neuropeptide secretion from the dendrites of hypothalamic magnocellular supraoptic nucleus (SON) neurons contributes to the regulation of neuronal activity patterning, which ultimately determines their peptide output from axon terminals in the posterior pituitary gland. SON dendrites also secrete a number of other neuromodulators, including ATP. ATP degrades to adenosine in the extracellular space to complement transported adenosine acting on pre- and postsynaptic SON A1 receptors to reduce neuronal excitability, measured in vitro. To assess adenosine control of electrical activity in vivo, we made extracellular single-unit recordings of the electrical activity of SON neurons in anesthetized male rats. Microdialysis application (retrodialysis) of the A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT) increased phasic vasopressin cell intraburst firing rates progressively over the first 5 s by 4.5 +/- 1.6 Hz (P < 0.05), and increased burst duration by 293 +/- 64% (P < 0.05). Hazard function plots were generated from interval interspike histograms and revealed that these effects were associated with increased postspike excitability. In contrast, CPT had no effect on the firing rates and hazard function plot profiles of continuously active vasopressin and oxytocin cells. However, CPT significantly increased clustering of spikes, as quantified by the index of dispersion, in oxytocin cells and continuously active vasopressin cells (by 267 +/- 113% and 462 +/- 67%, respectively, P < 0.05). Indeed, in 4 of 5 continuously active vasopressin cells, CPT induced a pseudophasic activity pattern. Together, these results indicate that endogenous adenosine is involved in the local control of SON cell activity in vivo.

Does conversion of ATP to adenosine terminate ATP-stimulated vasopressin release from hypothalamo-neurohypophyseal explants?

ATP stimulates vasopressin (VP) release from explants of the hypothalamo-neurohypophyseal system (HNS), but the response is not sustained for the duration of exposure to ATP. Since adenosine, a metabolite of ATP, inhibits VP release from neurohypophysial terminals and adenosine receptors (AR) are expressed in supraoptic nucleus (SON) neurons, we postulated that conversion of ATP to adenosine contributed to termination of ATP-stimulated VP release from HNS explants. This was tested using a non-selective AR antagonist, 5-amino-9-chloro-2-(2-furyl)-1, 2, 4-triazolo [1, 5-c] quinazoline (CGS-15943). CGS-15943 did not affect basal VP release and did not alter the initial response to ATP. A selective A1R antagonist, 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), increased basal VP release at 1 microM, without altering the response to ATP. However, at a higher concentration of DPCPX (10 microM), VP release was enhanced by ATP for an extended period of time. Inhibition of the enzymatic conversion of ATP to adenosine using a combination of a potent ecto-5'-nucleotidase inhibitor, alpha,beta-methylene adenosine 5'-diphosphate (AMP-CP), and a competitive substrate for ecto-5'-nucleotidase (guanosine monophosphate, GMP) did not affect basal VP release. Enzymatic inhibition did slightly prolong the response to ATP, but it was not sustained for the duration of exposure to ATP. We conclude that an endogenous inhibitory influence of adenosine decreases basal VP release from HNS explants and that conversion of exogenously applied ATP to adenosine contributes to termination of ATP-induced stimulation of VP release, but additional mechanisms such as receptor desensitization also limit the response to extended exposure to ATP.

Regulation of Oxytocin Secretion

A baby sucks at a mother's breast for comfort and, of course, for milk. Milk is made in specialized cells of the mammary gland, and for a baby to feed, the milk must be released into a collecting chamber from where it can be extracted by sucking. Milk "let-down" is a reflex response to the suckling and kneading of the nipple--and sometimes in response to the sight, smell, and sound of the baby--and is ultimately affected by the secretion of oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down: oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate oxytocin secretion and on the apparent diversity of the roles for oxytocin.

Just the two of us: melatonin and adenosine in rodent pituitary function

The function of the pituitary gland is tightly controlled by neuronal and hormonal afferents of the brain. In this review, the role of the neurohormone melatonin and the neuromodulator adenosine for rodent pituitary function will be elucidated. Adenosine is known as an important paracrine modulator for pituitary endocrine and folliculostellate cells, with availability regulated by local metabolic cellular activity. In general, adenosine inhibits the cyclic adenosine monophosphate (AMP) pathway in pituitary cells by binding to A1-, and A3-adenosinergic receptors, and activates it via A2-adenosinergic receptors. The neurohormone melatonin integrates time-of-day and time-of-year into pituitary function via binding to MT1-melatonin receptors. Melatonin impacts at the hypothalamic level neurons that synthesize releasing and release-inhibiting hormones, and at the pituitary level only cells of the hypophyseal pars tuberalis (PT). Thereby, the daily changes in the duration of the nocturnal melatonin surge are decoded and subsequently relayed to the pars distalis to adapt gonadotropin and prolactin release, respectively, to season. An exciting integration of time within the regulation of pituitary function was deciphered by analysing transmembrane signalling events in cells of the hypophyseal PT: a consecutive daily impact of initially the neurohormone melatonin and later the neuromodulator adenosine on rodent PT cells leads to a circadian rhythm in the transcription of cyclic-AMP-sensitive genes.

NADPH oxidase contributes to angiotensin II signaling in the nucleus tractus solitarius

Angiotensin II (AngII), acting through angiotensin type 1 (AT1) receptors, exerts powerful effects on central autonomic networks regulating cardiovascular homeostasis and fluid balance; however, the mechanisms of AngII signaling in functionally defined central autonomic neurons have not been fully elucidated. In vascular cells, reactive oxygen species (ROS) generated by the enzyme NADPH oxidase play a major role in AngII signaling. Thus, we sought to determine whether NADPH oxidase is present in central autonomic neurons and, if so, whether NADPH oxidase-derived ROS are involved in the effects of AngII on these neurons. The present studies focused on the intermediate dorsomedial nucleus of the solitary tract (dmNTS) because this region receives autonomic afferents via the vagus nerve and is an important site of AngII actions. Using double-label immunoelectron microscopy, we found that the essential NADPH oxidase subunit gp91phox is present in somatodendric and axonal profiles containing AT1 receptors. The gp91phox-labeled dendrites received inputs from large axon terminals resembling vagal afferents. In parallel experiments using patch clamp of dissociated NTS neurons anterogradely labeled via the vagus, we found that AngII potentiates the L-type Ca2+ currents, an effect mediated by AT1 receptors and abolished by the ROS scavenger Mn(III) tetrakis (4-benzoic acid) porphyrin chloride. The NADPH oxidase assembly inhibitor apocynin and the peptide inhibitor gp91phox docking sequence, but not its scrambled version, also blocked the potentiation. The results provide evidence that NADPH oxidase-derived ROS are involved in the effects of AngII on Ca2+ influx in NTS neurons receiving vagal afferents and support the notion that ROS are important signaling molecules in central autonomic networks.

Adenosine postsynaptically modulates supraoptic neuronal excitability

Effects of adenosine on the excitability of supraoptic nucleus neurons were investigated in whole cell patch-clamp experiments conducted in horizontal slices of rat hypothalamus. Adenosine (10-100 muM) inhibited all neurons tested by reducing or abolishing spontaneous or evoked discharge. Large hyperpolarizations were seen, averaging -6.08 +/- 0.83 mV below resting membrane potential, and action potential durations were significantly reduced by 134 +/- 41 mus in the presence of 100 muM adenosine. The A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 muM) blocked these effects, whereas the A(1) agonists N(6)-cyclopentyladenosine (CPA) and N(6)-cyclohexyladenosine (CHA) mimicked the actions of adenosine. A(2) receptor contributions to excitability were assessed by application of an A(2) agonist, carboxamidoadenosine (CPCA). This resulted in membrane depolarizations (3.56 +/- 0.65 mV) and maintenance of firing. The presence of endogenous adenosine in the slice was revealed by both the application of the adenosine uptake inhibitor dilazep (1-100 muM), which resulted in a strong inhibition of firing activity, and the application of DPCPX, which induced firing in cells silenced by negative current injection. We tested for postsynaptic actions of adenosine by blocking G protein activation via GDP-beta-S infusion into recorded neurons. Under these conditions, the adenosinergic inhibition of firing and reduction of spike duration were blocked, suggesting the effects were mediated by postsynaptic adenosine receptors. That the effects on excitability could be due to direct activation of adenosine A(1) receptors on supraoptic neurons was further explored immunocytochemically via the co-labeling of magnocellular neurons with polyclonal antibodies raised against the A(1) receptors. It is concluded that adenosine, acting at postsynaptic A(1) receptors, exhibits a powerful inhibitory influence on supraoptic magnocellular activity and is an important endogenous regulator of magnocellular neuroendocrine function.