Baroreceptor input regulates osmotic control of central vasopressin secretion

Cardiovascular Neuroendocrinology: Emerging Role for Neurohypophyseal Hormones in Pathophysiology

Arginine vasopressin (AVP) and oxytocin (OXY) are released by magnocellular neurosecretory cells that project to the posterior pituitary. While AVP and OXY currently receive more attention for their contributions to affiliative behavior, this mini-review discusses their roles in cardiovascular function broadly defined to include indirect effects that influence cardiovascular function. The traditional view is that neither AVP nor OXY contributes to basal cardiovascular function, although some recent studies suggest that this position might be re-evaluated. More evidence indicates that adaptations and neuroplasticity of AVP and OXY neurons contribute to cardiovascular pathophysiology.

Dendritic Release of Neurotransmitters

Release of neuroactive substances by exocytosis from dendrites is surprisingly widespread and is not confined to a particular class of transmitters: it occurs in multiple brain regions, and includes a range of neuropeptides, classical neurotransmitters, and signaling molecules, such as nitric oxide, carbon monoxide, ATP, and arachidonic acid. This review is focused on hypothalamic neuroendocrine cells that release vasopressin and oxytocin and midbrain neurons that release dopamine. For these two model systems, the stimuli, mechanisms, and physiological functions of dendritic release have been explored in greater detail than is yet available for other neurons and neuroactive substances. © 2017 American Physiological Society. Compr Physiol 7:235-252, 2017.

Efferent pathways in sodium overload-induced renal vasodilation in rats

Hypernatremia stimulates the secretion of oxytocin (OT), but the physiological role of OT remains unclear. The present study sought to determine the involvement of OT and renal nerves in the renal responses to an intravenous infusion of hypertonic saline. Male Wistar rats (280–350 g) were anesthetized with sodium thiopental (40 mg. kg⁻¹, i.v.). A bladder cannula was implanted for collection of urine. Animals were also instrumented for measurement of mean arterial pressure (MAP) and renal blood flow (RBF). Renal vascular conductance (RVC) was calculated as the ratio of RBF by MAP. In anesthetized rats (n = 6), OT infusion (0.03 µg • kg⁻¹, i.v.) induced renal vasodilation. Consistent with this result, exvivo experiments demonstrated that OT caused renal artery relaxation. Blockade of OT receptors (OXTR) reduced these responses to OT, indicating a direct effect of this peptide on OXTR on this artery. Hypertonic saline (3 M NaCl, 1.8 ml • kg⁻¹ b.wt., i.v.) was infused over 60 s. In sham rats (n = 6), hypertonic saline induced renal vasodilation. The OXTR antagonist (AT; atosiban, 40 µg • kg⁻¹ • h⁻¹, i.v.; n = 7) and renal denervation (RX) reduced the renal vasodilation induced by hypernatremia. The combination of atosiban and renal denervation (RX+AT; n = 7) completely abolished the renal vasodilation induced by sodium overload. Intact rats excreted 51% of the injected sodium within 90 min. Natriuresis was slightly blunted by atosiban and renal denervation (42% and 39% of load, respectively), whereas atosiban with renal denervation reduced sodium excretion to 16% of the load. These results suggest that OT and renal nerves are involved in renal vasodilation and natriuresis induced by acute plasma hypernatremia.

Effect of chronic central endothelin-1 on hemodynamics and plasma vasopressin in conscious rats

OBJECTIVES

These studies were designed to test whether chronic central administration of endothelin-1 induces changes in systemic hemodynamics and plasma vasopressin similar to those observed with acute microinjections of endothelin-1.

METHODS

Sprague Dawley rats underwent sham denervation or sinoaortic denervation. Three days later, baseline mean arterial blood pressure, heart rate, and vasopressin were assessed in conscious rats. Then, a cannula was stereotaxically inserted into the lateral ventricle and attached to an osmotic minipump that delivered one of the following: (i) artificial cerebrospinal fluid; (ii) endothelin-1, 10 pmol/hour; (iii) BQ-123, 400 pmol/hour; or (iv) endothelin-1+BQ-123. Mean arterial blood pressure and heart rate were monitored daily and blood was obtained for plasma vasopressin on days 3 and 9. On day 10, the rats were euthanized, the hypothalami were removed, and vasopressin messenger ribonucleic acid content was assessed.

RESULTS

The pressor effect of intracerebroventricular endothelin-1 was similar in intact and sinoaortic denervation rats and was prevented by endothelin receptor A antagonism with BQ-123. Administration of BQ-123 alone resulted in a depressor and bradycardia in sinoaortically denervated rats. Chronic endothelin-1 administration did not change plasma vasopressin but resulted in a significant decrease in hypothalamic vasopressin messenger ribonucleic acid levels, which was reversed by endothelin receptor A inhibition.

DISCUSSION

Although the pressor effect of chronic central endothelin-1 is similar to that reported with acute endothelin-1, plasma vasopressin levels do not increase, at least in part, due to downregulation of hypothalamic vasopressin gene expression. Sinoaortic denervation increases endogenous central endothelin receptor A tone. Furthermore, these observations confirm that the pressor effect of central endothelin-1 is not mediated by plasma vasopressin.

Vasopressin during cardiopulmonary resuscitation and different shock states: a review of the literature

Vasopressin administration may be a promising therapy in the management of various shock states. In laboratory models of cardiac arrest, vasopressin improved vital organ blood flow, cerebral oxygen delivery, the rate of return of spontaneous circulation, and neurological recovery compared with epinephrine (adrenaline). In a study of 1219 adult patients with cardiac arrest, the effects of vasopressin were similar to those of epinephrine in the management of ventricular fibrillation and pulseless electrical activity; however, vasopressin was superior to epinephrine in patients with asystole. Furthermore, vasopressin followed by epinephrine resulted in significantly higher rates of survival to hospital admission and hospital discharge. The current cardiopulmonary resuscitation guidelines recommend intravenous vasopressin 40 IU or epinephrine 1mg in adult patients refractory to electrical countershock. Several investigations have demonstrated that vasopressin can successfully stabilize hemodynamic variables in advanced vasodilatory shock. Use of vasopressin in vasodilatory shock should be guided by strict hemodynamic indications, such as hypotension despite norepinephrine (noradrenaline) dosages >0.5 mug/kg/min. Vasopressin must never be used as the sole vasopressor agent. In our institutional routine, a fixed vasopressin dosage of 0.067 IU/min (i.e. 100 IU/50 mL at 2 mL/h) is administered and mean arterial pressure is regulated by adjusting norepinephrine infusion. When norepinephrine dosages decrease to 0.2 microg/kg/min, vasopressin is withdrawn in small steps according to the response in mean arterial pressure. Vasopressin also improved short- and long-term survival in various porcine models of uncontrolled hemorrhagic shock. In the clinical setting, we observed positive effects of vasopressin in some patients with life-threatening hemorrhagic shock, which had no longer responded to adrenergic catecholamines and fluid resuscitation. Clinical employment of vasopressin during hemorrhagic shock is experimental at this point in time.

Antagonistic oxytocin/alpha2-adrenoreceptor interactions in the nucleus tractus solitarii: relevance for central cardiovascular control

The modulation of the central cardiovascular effects of alpha2-adrenoceptor activation by oxytocin in the nucleus tractus solitarii has been evaluated by cardiovascular analysis and by quantitative receptor autoradiography. Microinjections in the nucleus tractus solitarii of a threshold dose of oxytocin effectively and significantly counteracted the vasodepressor and bradycardic actions of an ED50 dose of the alpha2-adrenoceptor agonist clonidine. The coinjection of a threshold dose of oxytocin with a threshold dose of clonidine did not produce any changes in the mean arterial pressure but a tachycardic response was observed. Receptor autoradiographical experiments showed that oxytocin (3 nM) significantly increased the Kd and Bmax values of [3H]p-aminoclonidine binding sites in the nucleus tractus solitarii compatible with a possible antagonistic interaction with the alpha2-adrenoceptors, and this effect was blocked by the presence of the specific oxytocin receptor antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin. These findings suggest the existence of an antagonistic oxytocin/alpha2-adrenoceptor interaction in nucleus tractus solitarii that may be of relevance for the demonstrated modulation of alpha2-adrenoceptor induced cardiovascular responses by oxytocin.

Dendritic release of vasopressin and oxytocin

In addition to the release of neurotransmitters from their axon terminals, several neuronal populations are able to release their products from their dendrites. The cell bodies and dendrites of vasopressin- and oxytocin-producing neurones are mainly located within the hypothalamic supraoptic and paraventricular nuclei and neuropeptide release within the magnocellular nuclei has been shown in vitro and in vivo. Local release is induced by a range of physiological and pharmacological stimuli, and is regulated by a number of brain areas; locally released peptides are mainly involved in pre- and postsynaptic modulation of the electrical activity of magnocellular neurones. Spatial and temporal differences between peptide release within the nuclei and that from the distant axonal varicosities indicate that the release mechanisms are at least partially independent, supporting the hypothesis of locally regulated dendritic release of vasopressin and oxytocin. In this respect, magnocellular neurones show similarities to other neuronal populations and thus autoregulation of neuronal activity by dendritic neuromodulator release may be a general phenomenon within the brain.

Splanchnic and vagal denervation attenuate central Fos but not AVP responses to intragastric salt in rats

We have recently reported that an acute intragastric hypertonic saline load increases plasma arginine vasopressin (PAVP) and Fos immunoreactivity in several central nuclei, including the supraoptic nucleus (SON), paraventricular nucleus (PVN), nucleus of the solitary tract (NTS), area postrema (AP), and lateral parabrachial nucleus (LPBN). We hypothesized that these responses are mediated by stimulation of peripheral osmoreceptors with splanchnic and/or vagal afferent projections. To test this hypothesis, we examined the effect of bilateral subdiaphragmatic vagotomy and bilateral splanchnic denervation on the PAVP and Fos immunoreactivity responses to intragastric hypertonic saline infusion in awake rats. Compared with responses in sham rats, Fos immunoreactivity responses were significantly reduced in vagotomized rats in the AP, SON, and PVN, whereas normal Fos levels were observed in the LPBN. However, vagotomized rats exhibited a normal increase in PAVP. Splanchnic-denervated rats also exhibited similar changes in PAVP in response to intragastric hypertonic saline compared with sham-denervated rats, and no differences were observed in Fos immunoreactivity in the LPBN, SON, and PVN compared with sham rats. However, splanchnic-denervated rats were observed to have significantly lower Fos staining in the NTS and AP compared with sham rats. The inability of splanchnic or vagal denervation alone to block the PAVP response to intragastric hypertonic saline suggests that either peripheral osmoreceptors project via both splanchnic and vagal afferents to mediate AVP release or that the observed response of PAVP is due to the activation of central osmoreceptors in the absence of measurable changes in plasma osmolality.