Antidiuretic effects of ATP induced by microinjection into the hypothalamic supraoptic nucleus in water-loaded and ethanol-anesthetized rats

Salt-loading promotes extracellular ATP release mediated by glial cells in the hypothalamic paraventricular nucleus of rats

Previously, we have shown that purinergic signalling is involved in the control of hyperosmotic-induced sympathoexcitation at the level of the PVN, via activation of P2X receptors. However, the source(s) of ATP that drives osmotically-induced increases in sympathetic outflow remained undetermined. Here, we tested the two competing hypotheses that either (1) higher extracellular ATP in PVN during salt loading (SL) is a result of a failure of ectonucleotidases to metabolize ATP; and/or (2) SL can stimulate PVN astrocytes to release ATP. Rats were salt loaded with a 2 % NaCl solution replacing drinking water up to 4 days, an experimental model known to cause a gradual increase in blood pressure and plasma osmolarity. Immunohistochemical assessment of glial-fibrillary acidic protein (GFAP) revealed increased glial cell reactivity in the PVN of rats after 4 days of high salt exposure. ATP and adenosine release measurements via biosensors in hypothalamic slices showed that baseline ATP release was increased 17-fold in the PVN while adenosine remained unchanged. Disruption of Ca²⁺-dependent vesicular release mechanisms in PVN astrocytes by virally-driven expression of a dominant-negative SNARE protein decreased the release of ATP. The activity of ectonucleotidases quantified in vitro by production of adenosine from ATP was increased in SL group. Our results showed that SL stimulates the release of ATP in the PVN, at least in part, from glial cells by a vesicle-mediated route and likely contributes to the neural control of circulation during osmotic challenges.

Somato-dendritic vasopressin and oxytocin secretion in endocrine and autonomic regulation

Somato-dendritic secretion was first demonstrated over 30 years ago. However, although its existence has become widely accepted, the function of somato-dendritic secretion is still not completely understood. Hypothalamic magnocellular neurosecretory cells were among the first neuronal phenotypes in which somato-dendritic secretion was demonstrated and are among the neurones for which the functions of somato-dendritic secretion are best characterised. These neurones secrete the neuropeptides, vasopressin and oxytocin, in an orthograde manner from their axons in the posterior pituitary gland into the blood circulation to regulate body fluid balance and reproductive physiology. Retrograde somato-dendritic secretion of vasopressin and oxytocin modulates the activity of the neurones from which they are secreted, as well as the activity of neighbouring populations of neurones, to provide intra- and inter-population signals that coordinate the endocrine and autonomic responses for the control of peripheral physiology. Somato-dendritic vasopressin and oxytocin have also been proposed to act as hormone-like signals in the brain. There is some evidence that somato-dendritic secretion from magnocellular neurosecretory cells modulates the activity of neurones beyond their local environment where there are no vasopressin- or oxytocin-containing axons but, to date, there is no conclusive evidence for, or against, hormone-like signalling throughout the brain, although it is difficult to imagine that the levels of vasopressin found throughout the brain could be underpinned by release from relatively sparse axon terminal fields. The generation of data to resolve this issue remains a priority for the field.

The role of vasopressin and the vasopressin type V1a receptor agonist selepressin in septic shock

Septic shock remains one of the major causes of morbidity and mortality in the critically ill. Despite early goal therapy and administration of cathecholaminergic agents, up to 30% of patients succumb to the disease. In this manuscript, we first summarize the standard of care of patients with septic shock and current guidelines. We review the physiologic role of vasopressin and its role in septic shock management. We then review the most up-to-date evidence on the potential role of V1a receptor agonists such as Selepressin, in septic shock. Exciting new trials are being completed in order to elucidate the role of V1a receptor agonists as potential first-line vasopressor alternatives in the therapy of circulatory shock in septic patients.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

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.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Bench-to-bedside review: Vasopressin in the management of septic shock

This review of vasopressin in septic shock differs from previous reviews by providing more information on the physiology and pathophysiology of vasopressin and vasopressin receptors, particularly because of recent interest in more specific AVPR1a agonists and new information from the Vasopressin and Septic Shock Trial (VASST), a randomized trial of vasopressin versus norepinephrine in septic shock. Relevant literature regarding vasopressin and other AVPR1a agonists was reviewed and synthesized. Vasopressin, a key stress hormone in response to hypotension, stimulates a family of receptors: AVPR1a, AVPR1b, AVPR2, oxytocin receptors and purinergic receptors. Rationales for use of vasopressin in septic shock are as follows: first, a deficiency of vasopressin in septic shock; second, low-dose vasopressin infusion improves blood pressure, decreases requirements for norepinephrine and improves renal function; and third, a recent randomized, controlled, concealed trial of vasopressin versus norepinephrine (VASST) suggests low-dose vasopressin may decrease mortality of less severe septic shock. Previous clinical studies of vasopressin in septic shock were small or not controlled. There was no difference in 28-day mortality between vasopressin-treated versus norepinephrine-treated patients (35% versus 39%, respectively) in VASST. There was potential benefit in the prospectively defined stratum of patients with less severe septic shock (5 to 14 μg/minute norepinephrine at randomization): vasopressin may have lowered mortality compared with norepinephrine (26% versus 36%, respectively, P = 0.04 within stratum). The result was robust: vasopressin also decreased mortality (compared with norepinephrine) if less severe septic shock was defined by the lowest quartile of arterial lactate or by use of one (versus more than one) vasopressor at baseline. Other investigators found greater hemodynamic effects of higher dose of vasopressin (0.06 units/minute) but also unique adverse effects (elevated liver enzymes and serum bilirubin). Use of higher dose vasopressin requires further evaluation of efficacy and safety. There are very few studies of interactions of therapies in critical care - or septic shock - and effects on mortality. Therefore, the interaction of vasopressin infusion, corticosteroid treatment and mortality of septic shock was evaluated in VASST. Low-dose vasopressin infusion plus corticosteroids significantly decreased 28-day mortality compared with corticosteroids plus norepinephrine (44% versus 35%, respectively, P = 0.03; P = 0.008 interaction statistic). Prospective randomized controlled trials would be necessary to confirm this interesting interaction. In conclusion, low-dose vasopressin may be effective in patients who have less severe septic shock already receiving norepinephrine (such as patients with modest norepinephrine infusion (5 to 15 μg/minute) or low serum lactate levels). The interaction of vasopressin infusion and corticosteroid treatment in septic shock requires further study.

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.

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.

Pathophysiology and therapeutic potential of purinergic signaling

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

Physiological pathways regulating the activity of magnocellular neurosecretory cells

Magnocellular oxytocin and vasopressin cells are among the most extensively studied neurons in the brain; their large size and high synthetic capacity, their discrete, homogeneous distribution and the anatomical separation of their terminals from their cell bodies, and the ability to determine their neuronal output readily by measurements of hormone concentration in the plasma, combine to make these systems amenable to a wide range of fundamental investigations. While vasopressin cells have intrinsic burst-generating properties, oxytocin cells are organized within local pattern-generating networks. In this review we consider the rôle played by particular afferent pathways in the regulation of the activity of oxytocin and vasopressin cells. For both cell types, the effects of changes in the activity of synaptic input can be complex.

Localization of ATP-gated P2X2 receptor immunoreactivity in the rat hypothalamus

Previous pharmacological studies have indicated that ATP receptors may be involved in the regulation of physiological functions in hypothalamus. In the present study, the distribution of P2X2 receptor in the rat hypothalamus was studied with immunohistochemistry. It was shown that P2X2 immunoreactivity-positive neurons and nerve fibres were localized in many hypothalamic nuclei. Intense labelling of both neuronal cell bodies and nerve fibres was observed in the paraventricular nucleus, arcuate nucleus, retrochiasmatic area, periventricular nucleus, and the ventral part of tuber cinereum area. In supraoptic, circular, and ventral tuberomammillary nuclei the neuronal cell bodies were strongly positive, but few nerve fibres were positive. Axons with strong P2X2 immunoreactivity were found in the organum vasculosum of the lamina terminalis and median eminence. Some scattered positive neurons and nerve fibres were found in many hypothalamic nuclei including preoptic nucleus. The results of the present study demonstrated the existence of P2X receptors in hypothalamus, as a basis for detailed studies of the roles of P2X receptors in the regulation of hypothalamic functions.