Oxytocin administered intrathecally preferentially increases heart rate rather than arterial pressure in the rat

Oxytocin Receptor Signaling in Vascular Function and Stroke

The oxytocin receptor (OXTR) is a G protein-coupled receptor with a diverse repertoire of intracellular signaling pathways, which are activated in response to binding oxytocin (OXT) and a similar nonapeptide, vasopressin. This review summarizes the cell and molecular biology of the OXTR and its downstream signaling cascades, particularly focusing on the vasoactive functions of OXTR signaling in humans and animal models, as well as the clinical applications of OXTR targeting cerebrovascular accidents.

Transient receptor potential vanilloid type 4 is expressed in vasopressinergic neurons within the magnocellular subdivision of the rat paraventricular nucleus of the hypothalamus

Changes in plasma osmolality can drive changes in the output from brain centres known to control cardiovascular homeostasis, such as the paraventricular nucleus of the hypothalamus (PVN). Within the PVN hypotonicity reduces the firing rate of parvocellular neurons, a neuronal pool known to be involved in modulating sympathetic vasomotor tone. Also present in the PVN is the transient receptor potential vanilloid type 4 (TRPV4) ion channel. Activation of TRPV4 within the PVN mimics the reduction in firing rate of the parvocellular neurons but it is unknown if these neurons express the channel. We used neuronal tracing and immunohistochemistry to investigate which neurons expressed the TRPV4 ion channel protein and its relationship with neurons known to play a role in plasma volume regulation. Spinally projecting preautonomic neurons within the PVN were labelled after spinal cord injection of FluoroGold (FG). This was followed by immunolabelling with anti‐TRPV4 antibody in combination with either anti‐oxytocin (OXT) or anti‐vasopressin (AVP). The TRPV4 ion channel was expressed on 63% of the vasopressinergic magnocellular neurosecretory cells found predominantly within the posterior magnocellular division of the PVN. Oxytocinergic neurons and FG labelled preautonomic neurons were present in the same location, but were distinct from the TRPV4/vasopressin expressing neurons. Vasopressinergic neurons within the supraoptic nucleus (SON) were also found to express TRPV4 and the fibres extending between the SON and PVN. In conclusion within the PVN, TRPV4 is well placed to respond to changes in osmolality by regulating vasopressin secretion, which in turn influences sympathetic output via preautonomic neurons.

Intranasal oxytocin reduces provoked symptoms in female patients with posttraumatic stress disorder despite exerting sympathomimetic and positive chronotropic effects in a randomized controlled trial

BACKGROUND

Posttraumatic stress disorder (PTSD) is a severe psychiatric disease accompanied by neuroendocrine changes such as adrenergic overdrive and hence an elevated cardiovascular morbidity. Current pharmacotherapeutic options for PTSD are less than suboptimal, necessitating the development of PTSD-specific drugs. Although the neuropeptide oxytocin has been repeatedly suggested to be effective in PTSD treatment, there are, to our knowledge, only three studies that have assessed its efficacy on the intensity of PTSD symptoms in PTSD patients - among them one symptom provocation study in male veterans.

METHODS

To evaluate for the first time how oxytocin influences the intensity of provoked PTSD symptoms and, furthermore, cardiac control in female PTSD patients, we assessed their psychic and cardiac response to trauma-script exposure with and without oxytocin pretreatment in a double-blind randomized placebo-controlled study. We used a within-subject design to study 35 female PTSD patients who received oxytocin and placebo in a 2-week interval. Furthermore, we performed a small pilot study to get an idea of the relation of the stress-modulated endogenous oxytocin levels and heart rate - we correlated oxytocin serum levels with the heart rate of 10 healthy individuals before and after exposure to the Trier Social Stress Test (TSST).

RESULTS

Intranasal oxytocin treatment was followed by a reduction of provoked total PTSD symptoms, in particular of avoidance, and by an elevation in baseline and maximum heart rate together with a drop in the pre-ejection period, a marker for sympathetic cardiac control. Furthermore, we found a positive correlation between endogenous oxytocin levels and heart rate both before and after TSST challenge in healthy control subjects.

CONCLUSIONS

This study provides the first evidence that oxytocin treatment reduces the intensity of provoked PTSD symptoms in female PTSD patients. The small size of both samples and the heterogeneity of the patient sample restrict the generalizability of our findings. Future studies have to explore the gender dependency and the tolerability of the oxytocin-mediated increase in heart rate. This randomized controlled trial was retrospectively registered at the German Trials Register (DRKS00009399) on the 02 October 2015.

Coming full circle: contributions of central and peripheral oxytocin actions to energy balance

The neuropeptide oxytocin has emerged as an important anorexigen in the regulation of energy balance. Its effects on food intake have largely been attributed to limiting meal size through interactions in key regulatory brain regions such as the hypothalamus and hindbrain. Pharmacologic and pair-feeding studies indicate that its ability to reduce body mass extends beyond that of food intake, affecting multiple factors that determine energy balance such as energy expenditure, lipolysis, and glucose regulation. Systemic administration of oxytocin recapitulates many of its effects when administered centrally, raising the questions of whether and to what extent circulating oxytocin contributes to energy regulation. Its therapeutic potential to treat metabolic conditions remains to be determined, but data from diet-induced and genetically obese rodent models as well as application of oxytocin in humans in other areas of research have revealed promising results thus far.

Neuromedin S regulates cardiovascular function through the sympathetic nervous system in mice

Intracerebroventricular (icv) injection of neuromedin S (NMS) in mice increased the heart rate in a dose-dependent manner. On the other hand, genetically NMS deficient mice (NMS-KO mice) exhibited a decreased heart rate and significant extension of the QRS and PR interval in the electrocardiogram complex. Although treatment with a parasympathetic nerve blocker, methylscopolamine, and a sympathetic nerve blocker, timolol, respectively increased and decreased the heart rate in both NMS-KO and wild-type mice, the extent of the decrease induced by timolol was smaller in NMS-KO than in wild-type mice. In addition, pretreatment with timolol completely inhibited the NMS-induced heart rate increase in wild-type mice. No expression of mRNA for NMS or the NMS receptor was evident in the heart by RT-PCR analysis. These results suggest that endogenous NMS may regulate cardiovascular function by activating the sympathetic nervous system.

Neurochemistry of the paraventricular nucleus of the hypothalamus: implications for cardiovascular regulation

The paraventricular nucleus of the hypothalamus (PVN) is an important site for autonomic and endocrine homeostasis. The PVN integrates specific afferent stimuli to produce an appropriate differential sympathetic output. The neural circuitry and some of the neurochemical substrates within this circuitry are discussed. The PVN has at least three neural circuits to alter sympathetic activity and cardiovascular regulation. These pathways innervate the vasculature and organs such as the heart, kidney and adrenal medulla. The basal level of sympathetic tone at any given time is dependent upon excitatory and inhibitory inputs. Under normal circumstances the sympathetic nervous system is tonically inhibited. This inhibition is dependent upon GABA and nitric oxide such that nitric oxide potentiates local GABAergic synaptic inputs onto the neurones in the PVN. Excitatory neurotransmitters such as glutamate and angiotensin II modify the tonic inhibitory activity. The neurotransmitters oxytocin, vasopressin and dopamine have been shown to affect cardiovascular function. These neurotransmitters are found in neurones of the PVN and within the spinal cord. Oxytocin and vasopressin terminal fibres are closely associated with sympathetic preganglionic neurones (SPNs). Sympathetic preganglionic neurones have been shown to express receptors for oxytocin, vasopressin and dopamine. Oxytocin causes cardioacceleratory and pressor effects that are greatest in the upper thoracic cord while vasopressin cause these effects but more significant in the lower thoracic cord. Dopaminergic effects on the cardiovascular system include inhibitory or excitatory actions attributed to a direct PVN influence or via interneuronal connections to sympathetic preganglionic neurones.

Cardiac sympatho-excitatory action of PVN-spinal oxytocin neurones

A significant proportion of the spinally projecting neurones in the paraventricular nucleus are immunoreactive for oxytocin. Some of these oxytocin neurones terminate on sympathetic preganglionic neurones in the upper thoracic spinal cord, a region from which cardiac sympathetic neurones originate. No studies have so far identified a cardiac action of the supraspinal oxytocin neurones. The present study was designed to test the hypothesis that these oxytocin neurones excite spinal cardiac sympathetic neurones. This was done by measuring heart rate changes in response to intrathecal oxytocin and a selective agonist, and to stimulation of paraventricular neurones before and during blockade of spinal sites with selective antagonists. Rats were anaesthetised with chloralose and urethane (50 mg and 650 mg/kg) and recordings were made of heart rate and blood pressure. Drugs in a volume of 10 microl were applied to the upper thoracic spinal cord via a catheter placed intrathecally with its tip at T2. The paraventricular nucleus was explored with a glass micropipette, placed stereotaxically, and filled with d,l-homocysteic acid (DLH, 200 mM) for exciting neurones and pontamine sky blue for marking the position. Oxytocin (0.002 mM) applied to the spinal cord elicited increases in heart rate (26+/-5 beats per minute). This was mimicked by a highly selective oxytocin agonist. These heart rate increases were blocked selectively by two different oxytocin antagonists but unaffected by a V(1a) vasopressin antagonist. Excitation of sites in dorsal and medial parvocellular sub-nuclei of the paraventricular nucleus elicited increases in heart rate (36+/-3 bpm) which were significantly reduced by oxytocin antagonists but not affected by V(1a) antagonist. Also these induced increases in heart rate were unaffected by vagotomy or i.v. atropine but were abolished by i.v. esmolol. It is concluded that there is a population of paraventricular-spinal oxytocin neurones that excite cardiac sympathetic preganglionic neurones controlling heart rate.

Landmarks in understanding the central nervous control of the cardiovascular system

In this Paton Lecture I have tried to trace the key experiments that have developed ideas on how the brain regulates the cardiovascular system. It is a personal view and inevitably, owing to constraints on space and time, I have not been able to cover areas such as the nucleus tractus solitarius and cardiac vagal neurones, although I acknowledge that some may consider the story is incomplete without them. Starting with the crucial discovery of vasomotor nerves and 'vasomotor tone', the patterns of activity in sympathetic nerves which led to the important idea of central oscillating networks of neurones are described. I discuss how this knowledge has informed current controversies on the origin of vasomotor activity in presympathetic neurones in the ventral medulla, which identify intrinsic pacemaker activity or synaptic input from multiple oscillators as prime mechanisms. I present an emerging view that the role of other regions of the brain, in particular supramedullary sites, has been underplayed. These regions are pivotal for the non-uniform distribution of cardiac output that is unique to each reflex and behavioural state. I discuss the most recent evidence for 'central command' neurones that offers a plausible explanation for how these patterns of sympathetic activity are achieved. Finally, I stress the importance of these current ideas to the understanding of pathological changes in sympathetic activity in cardiovascular diseases such as hypertension or congestive heart failure.

Possible neural mediation of the central effects of oxytocin on uterine motility

The central nervous system contains the nuclei at the origin of autonomic and neuroendocrine pathways to the uterus. Although the anatomical basis of these pathways is known, the conditions of their recruitment and their interactions in the context of copulation remain to be explored. We tested the hypothesis that some central mechanisms could simultaneously recruit both pathways to the uterus. In this aim, we recorded intrauterine pressure changes in anesthetized female rats at the estrus stage after intracerebroventricular (ICV) administration of oxytocin (OT). Doses of 0.3–300 ng elicited increases of frequency and amplitude of uterine contractions. These effects were partly mimicked by the OT agonist [Thr4,Gly7]OT but not by arginine vasopressin. They were blocked by the OT receptor antagonist atosiban delivered either ICV or intravenously. The latter suggests that ICV OT activated the systemic release of OT. The effects of OT were also blocked by hexamethonium, a ganglionic blocking agent, by atropine, a muscarinic receptor antagonist, and by Nω-nitro-l-arginine methyl ester, an inhibitor of nitric oxide synthesis. The results reveal that ICV OT recruits autonomic efferent pathways to the uterus. These results support our hypothesis that the activation of central nuclei can promote uterine contractility, and that OT may be a central coordinator of autonomic and neuroendocrine pathways. The hypothalamus, the source of direct OT-ergic projections to the pituitary, the brain stem, and the spinal cord, may be a target of central OT.

A role for the paraventricular nucleus of the hypothalamus in the autonomic control of heart and kidney

It is now well accepted that the sympathetic nervous system responds to specific afferent stimuli in a unique non-uniform fashion. The means by which the brain transforms the signals from a single type of receptor into an appropriate differential sympathetic output is discussed in this brief review. The detection of and response to venous filling are used for illustration. An expansion of blood volume has been shown in a number of species to increase heart rate reflexly via sympathetic nerves and this effect is primarily an action of volume receptors at the venous-atrial junctions of the heart. Stimulation of these volume receptors also leads to an inhibition of renal sympathetic nerve activity. Thus the reflex response to an increase in plasma volume consists of a distinctive unique pattern of sympathetic activity to maintain fluid balance. This reflex is dependent on neurones in the paraventricular nucleus (PVN). Neurones in the PVN show early gene activation on stimulation of atrial receptors, and a similar differential pattern of cardiac sympathetic excitation and renal inhibition can be evoked by activating PVN neurones. Cardiac atrial afferents selectively cause a PVN GABA neurone-induced inhibition within the PVN of PVN spinally projecting vasopressin-containing neurones that project to renal sympathetic neurones. A lesion of these spinally projecting neurones abolishes the reflex. With regard to the cardiac sympathetics, there is a population of PVN spinally projecting neurones that selectively increase heart rate by the release of oxytocin, a peptide pathway that has no action on renal sympathetic outflow. In heart failure the atrial reflex becomes blunted, and evidence is emerging that there is a downregulation of nitric oxide synthesis and reduced GABA activity in the PVN. How this might give rise to increased sympathetic activity associated with heart failure is briefly discussed.

Spinal effects of oxytocin on uterine motility in anesthetized rats

The rat uterus receives an innervation from the lumbosacral and thoracolumbar segments of the spinal cord. These segments receive descending oxytocinergic projections from the paraventricular nucleus of the hypothalamus. We tested the hypothesis that oxytocin regulates uterine motility through a spinal site of action. Oxytocin was administered in anesthetized female rats either intrathecally at the lumbosacral or thoracolumbar spinal cord levels or intravenously. Uterine activity was revealed by measuring changes of intrauterine pressure using an indwelling balloon placed in one caudal uterine horn. The uterus displayed a spontaneous activity characterized by intrauterine pressure rises, the frequency, amplitude, and duration of which were dependent on the stage of the estrous cycle. Oxytocin delivered at the lumbosacral level affected the frequency (during proestrus, estrus, and diestrus) and amplitude (during proestrus and estrus) of uterine activity. During estrus, oxytocin delivered at the thoracolumbar level affected the frequency, amplitude, and duration of the intrauterine pressure rises. Intravenous oxytocin not only affected intrauterine pressure rises (namely amplitude during proestrus and estrus and frequency and duration during estrus) but also increased the basal tone during estrus. The effects of lumbosacral oxytocin were partly mimicked by the oxytocin agonist [Thr4,Gly7]-oxytocin blocked by the oxytocin receptor antagonist atosiban and by hexamethonium. Arginine vasopressin delivered at the lumbosacral level had no effect. These results support our hypothesis that oxytocin released by descending paraventriculo-spinal pathways and acting on spinal oxytocin receptors modulates the activity of the uterus. This regulation is cycle dependent.

5-HT1A receptor agonist 8-OH-DPAT acts in the hindbrain to reverse the sympatholytic response to severe hemorrhage

Central administration of serotonergic 5-HT1A receptor agonists delays the reflex sympatholytic response to severe hemorrhage in conscious rats. To determine the region where 5-HT1A receptor agonists act to mediate this response, recovery of mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) was compared in hemorrhaged rats after injection of the selective 5-HT1A agonist, (+)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), in various regions of the cerebroventricular system or the systemic circulation. Three minutes after injection of 8-OH-DPAT (48 nmol/kg), MAP and RSNA were higher in hemorrhaged rats given drug in the fourth ventricle (94 +/- 5 mmHg, 82 +/- 18% of baseline) or the systemic circulation (90 +/- 4 mmHg, 113 +/- 15% of baseline) than in rats given drug in the Aqueduct of Sylvius (63 +/- 4 mmHg, 27 +/- 11% of baseline), the lateral ventricle (42 +/- 3 mmHg, -8 +/- 18% of baseline), or in rats given saline in various brain regions (47 +/- 5 mmHg, -42 +/- 10% of baseline). A lower-dose injection of 8-OH-DPAT (10 nmol/kg) also accelerated the recovery of MAP and RSNA in hemorrhaged rats when given in the fourth ventricle (94 +/- 26 mmHg, 72 +/- 33% of baseline 3 min after injection) but not the systemic circulation (46 +/- 4 mmHg, -25 +/- 30% of baseline). These data indicate that 8-OH-DPAT acts on receptors in the hindbrain to reverse the sympatholytic response to hemorrhage in conscious rats.

Cardiovascular effects of oxytocin

The well known effects of oxytocin on uterine contraction and milk ejection were found as early as the beginning of the 20th century. Since then many other effects of oxytocin have been found and among them a great number of effects on the cardiovascular system. Oxytocin is released from the neurohypophysis into the circulation and from parvocellular neurons within the paraventricular nucleus (PVN) to many areas within the central nervous system (CNS). Indeed, oxytocin may modify blood pressure as well as heart rate both through effects within the CNS and through effects in other organs, such as the heart, blood vessels and kidney. Oxytocin may also cause cardiovascular effects by affecting other mediators, such as atrial natriuretic peptide (ANP), nitric oxide (NO) and alpha 2-adrenoreceptors.

Negative inotropic and chronotropic effects of oxytocin

We have previously shown that oxytocin receptors are present in the heart and that perfusion of isolated rat hearts with oxytocin results in decreased cardiac flow rate and bradycardia. The mechanisms involved in the negative inotropic and chronotropic effects of oxytocin were investigated in isolated dog right atria in the absence of central mechanisms. Perfusion of atria through the sinus node artery with 10(-6) mol/L oxytocin over 5 minutes (8 mL/min) significantly decreased both beating rate (-14.7+/-4.9% of basal levels, n=5, P<0.004) and force of contraction (-52.4+/-9.1% of basal levels, n=5, P<0.001). Co-perfusion with 10(-6) mol/L oxytocin receptor antagonist (n=3) completely inhibited the effects of oxytocin on frequency (P<0.04) and force of contraction (P<0.004), indicating receptor specificity. The effects of oxytocin were also totally inhibited by co-perfusion with 5x10(-8) mol/L tetrodotoxin (P<0.02) or 10(-6) mol/L atropine (P<0.03) but not by 10(-6) mol/L hexamethonium, which implies that these effects are neurally mediated, primarily by intrinsic parasympathetic postganglionic neurons. Co-perfusion with 10(-6) mol/L NO synthase inhibitor (L-NAME) significantly inhibited oxytocin effects on both beating rate (-1.85+/-1.27% versus -14.7+/-4.9% in oxytocin alone, P<0.05) and force of contraction (-24.9+/-4.4% versus -52.4+/-9.1% in oxytocin alone, n=4, P<0.04). The effect of oxytocin on contractility was further inhibited by L-NAME at 10(-4) mol/L (-8.1+/-1.8%, P<0.01). These studies imply that the negative inotropic and chronotropic effects of oxytocin are mediated by cardiac oxytocin receptors and that intrinsic cardiac cholinergic neurons and NO are involved in these actions.

Neuropeptide expression in rat paraventricular hypothalamic neurons that project to the spinal cord

The paraventricular hypothalamic nucleus (PVH) exerts many of its regulatory functions through projections to spinal cord neurons that control autonomic and sensory functions. By using in situ hybridization histochemistry in combination with retrograde tract tracing, we analyzed the peptide expression among neurons in the rat PVH that send axons to the spinal cord. Projection neurons were labeled by immunohistochemical detection of retrogradely transported cholera toxin subunit B, and radiolabeled long riboprobes were used to identify neurons containing dynorphin, enkephalin, or oxytocin mRNA. Of the spinally projecting neurons in the PVH, approximately 40% expressed dynorphin mRNA, 40% expressed oxytocin mRNA, and 20% expressed enkephalin mRNA. Taken together with our previous findings on the distribution of vasopressin-expressing neurons in the PVH (Hallbeck and Blomqvist [1999] J. Comp. Neurol. 411:201-211), the results demonstrated that the different PVH subdivisions display distinct peptide expression patterns among the spinal cord-projecting neurons. Thus, the lateral parvocellular subdivision contained large numbers of spinal cord-projecting neurons that express any of the four investigated peptides, whereas the ventral part of the medial parvocellular subdivision displayed a strong preponderance for dynorphin- and vasopressin-expressing cells. The dorsal parvocellular subdivision almost exclusively contained dynorphin- and oxytocin-expressing spinal cord-projecting neurons. This parcellation of the peptide-expressing neurons suggested a functional diversity among the spinal cord-projecting subdivisions of the PVH that provide an anatomic basis for its various and distinct influences on autonomic and sensory processing at the spinal level.

Evidence that oxytocin is an endogenous stimulator of autonomic sympathetic preganglionics: the pupillary dilatation response to vaginocervical stimulation in the rat

Vaginocervical mechanostimulation (VS) was shown previously to release oxytocin within the spinal cord and to induce pupillary dilatation. In the present study, (a) injection of oxytocin directly to the spinal cord (10 or 25 microg intrathecally [i.t.] in 5 microl saline) induced pupillary dilatation when observed 1 min after the end of the injection and (b) injection of an oxytocin receptor antagonist ([d(CH2)5-Tyr (Me)2-Orn8]-Vasotocin [OTA]; 25 microg i.t. in 5 microl saline) significantly attenuated the pupillary dilatation response to VS, when VS was applied 3 min after the end of the injection. Since activation of autonomic sympathetic preganglionic neurons in the thoracic spinal cord produces pupillary dilatation, we propose that oxytocin is a central nervous system neurotransmitter that stimulates these neurons directly, or perhaps indirectly, and thus is a mediator of VS-produced pupillary dilatation.

Hypothalamic paraventricular axons projecting to the female rat lumbosacral spinal cord contain oxytocin immunoreactivity

Oxytocin-containing axons project from the hypothalamic paraventricular nucleus to the neurohypophysis and thoracic spinal cord to ultimately influence uterine contractions and autonomic activity, respectively. Whether or not oxytocin-immunoreactive axons project to the female rat lumbosacral spinal cord to influence autonomic outflow to pelvic organs has not been investigated. Thus, the present study was designed to investigate the presence, distribution, and origin of oxytocin-immunoreactive axons in the female rat lumbosacral spinal cord. Immunohistochemistry, spinal cord transections, and axonal tracing with Fluorogold, True Blue, and pseudorabies virus were used. Oxytocin-immunoreactive nerve fibers were present in the L6/S1 segments of the spinal cord. Prominent varicose axons were evident throughout the dorsal horn, along the lateral and medial collateral pathways, in the dorsal intermediate gray area, around the central canal in lamina X, and throughout the sacral parasympathetic nucleus. Injection of retrograde tracer into the L6/S1 spinal cord labeled neurons in the hypothalamic paraventricular nucleus. Transection of the thoracic spinal cord eliminated oxytocin-immunoreactive nerve axons in the L6/S1 spinal cord. In addition, transection of the thoracic spinal cord eliminated transport of retrograde axonal tracer from the L6/S1 spinal cord to the paraventricular nucleus. Pseudorabies virus, a transneuronal retrograde tracer, injected into the uterus and cervix marked uterine-related preganglionic neuronal cell bodies in the sacral parasympathetic nucleus and uterine-related neurons in the hypothalamic paraventricular nucleus. Double immuno-labeling of viral-infected spinal cord sections showed oxytocin-immunoreactive axons closely associated with viral labeled uterine-related preganglionic cell bodies of the sacral parasympathetic nucleus. The results of this study revealed that oxytocin-immunoreactive neurons of the hypothalamic paraventricular nucleus project axons to the lumbosacral spinal cord to areas involved in sensory processing and parasympathetic outflow to the uterus.

Oxytocin antagonist disrupts hypotension-evoked renin secretion and other responses in conscious rats

Previous experiments have indicated that arterial hypotension increases plasma oxytocin (OT) levels in rats and that OT infused intravenously causes an increase in plasma renin activity (PRA). The goal of the present study was to determine whether systemic administration of an OT receptor antagonist would attenuate the increase in PRA that is normally evoked by arterial hypotension in rats. In conscious male rats, intravenous injection of hydralazine or diazoxide produced sustained hypotension and evoked a significant increase in PRA, as expected. Intravenous infusion of an OT receptor antagonist did not alter the hypotension induced by hydralazine or diazoxide, but it did markedly blunt the induced increase in PRA. The OT receptor antagonist also blunted the hypotension-evoked increase in heart rate and plasma vasopressin levels, suggesting that the antagonist may have generally disrupted afferent signaling of hypotension. Thus hypotension-evoked OT secretion may contribute to cardiovascular homeostasis by enhancing baroreceptor signals that stimulate increases in renin secretion, vasopressin secretion, and heart rate during arterial hypotension in rats.

Neural mediation of the cardiovascular responses to intrathecal administration of substance P in the rat: slowing of the cardioacceleration by an adrenal opioid factor

Substance P, given intrathecally at the second (T2) or ninth (T9) thoracic level in the anesthetized rat, increased heart rate, arterial pressure and circulating catecholamines. At T9 in adrenalectomized animals and at T2 in intact animals, the cardioacceleration was more abrupt than in intact animals injected at T9 suggesting that the adrenals are not necessary for the cardiovascular responses and that the adrenals may have released a factor which slows the neurally mediated cardioacceleration. As opioids are co-released with catecholamines from the adrenals, naloxone (10 mg/kg i.v.) or nalorphine HCl (which does not cross the blood-brain barrier; 10 mg/kg s.c.) was given 5 min before administration of substance P at T9 in intact rats. In both groups the cardioacceleration was similar to that elicited in adrenalectomized animals, indicating that the adrenal factor was opioid and that its action was peripheral rather than central. When propranolol (10 mg/kg i.v.) was given 3 or 15 min before, substance P increased arterial pressure but heart rate was unchanged, indicating that the opioid factor was not slowing the cardioacceleration by a direct effect on the heart. The results indicate that intrathecal administration of substance P produces a neurally mediated increase in arterial pressure and heart rate and induces the release of an adrenal opioid factor which slows the neurally-mediated cardioacceleration by an action in the periphery. This indicates a functional interaction between humoral and neural sympathetic mechanisms regulating the cardiovascular system.

Cardiac, neuroendocrine, and behavioral effects of central amygdaloid vasopressinergic and oxytocinergic mechanisms under stress-free conditions in rats

The central nucleus of the amygdala (CEA) is considered to play a major role in the expression of behavioral, autonomic, and neuroendocrine components of the stress response. The present study was designed to examine possible modulating effects of the neuropeptides arginine-8-vasopressin (AVP) and oxytocin (OXT) on functioning of the CEA in male Wistar rats. Heart rate, neuroendocrine parameters, and behavioral activity were repeatedly measured before, during, and after local administration of several doses of AVP and OXT under stress-free resting conditions. In comparison with control artificial-CSF infusion, AVP infusion in the lowest dose (20 pg) caused in a part of the animals a long-lasting decrease in heart rate, i.e., bradycardia, without affecting behavioral activity. In contrast, local infusion with high doses of AVP and OXT (2 ng) induced a transient cardioacceleration concomitant with an increase in behavioral activity. Moreover, these latter effects of AVP could effectively be blocked by pretreatment with a selective OXT receptor antagonist. These findings suggest that higher doses of AVP are effective via agonistic action on OXT receptors in the CEA. A strong correlation existed between the magnitudes of the tachycardiac response and behavioral activation. Thus, heart rate increase by OXT receptor stimulation is possibly due to somatic-autonomic coupling rather than genuine autonomic activation. Additionally, plasma corticosterone, but not epinephrine and norepinephrine, concentrations were elevated in response to AVP and OXT infusions. In conclusion, these results suggest that vasopressinergic influences on CEA function involve two receptor mechanisms possibly related to differential output systems.

Glutamate, NMDA and NMDA receptor antagonists: cardiovascular effects of intrathecal administration in the rat

Selected excitatory amino acids and antagonists were tested for their effects on arterial pressure and heart rate when administered intrathecally at the second (T2) or ninth (T9) thoracic spinal levels in urethane-anesthetized Sprague-Dawley rats with spontaneous or artificial respiration. Intrathecal administration of glutamate (1 mumol) and N-methyl-D-aspartic acid (NMDA; 2 nmol) at T9 increased arterial pressure and heart rate. The response began within 1 min, peaked at 2-3 min and persisted for 8-15 min. The maximum changes were 20-25 mm Hg for arterial pressure and 40-50 beats/min for heart rate. These responses were prevented by systemic administration of hexamethonium (10 mg/kg). Responses to administration of NMDA at the two spinal levels were essentially the same. Effects elicited by NMDA but not by glutamate were blocked by pretreatment with the NMDA receptor antagonists, D,L-2-amino-5-phosphonovaleric acid (APV; 10 nmol, intrathecal administration) and ketamine (7 mg/kg, i.v.). Intrathecal administration of APV (10, 50 and 200 nmol) at T2 produced dose-dependent decreases in arterial pressure without changing heart rate. The results support the hypothesis that NMDA receptors are involved in regulation of sympathetic output at the spinal level. They also indicate that in this preparation there is a tonic activation of NMDA receptors in sympathetic pathways to the vessels but not to the heart. Finally, the persistence of the response to glutamate in the presence of NMDA receptor antagonists suggests the involvement of non-NMDA receptors in spinal control of sympathetic output.

NMDA receptor antagonists block cardiovascular responses to intrathecal administration of D-baclofen in the rat

In previous studies we found that D and L-baclofen have different effects on sympathetic output when administered intrathecally, yet the actions of both enantiomers are blocked by intrathecal administration of phaclofen. The present experiments were done to determine the mechanism by which D-baclofen expresses its effects. In urethane-anaesthetized Sprague-Dawley rats, when D-baclofen was given intrathecally at the T9 spinal level following pretreatment with 2 nmol of the NMDA receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV), it increased systolic and diastolic arterial pressures (n = 7), as in the previous studies. However, after intrathecal administration of 10 nmol of APV, administration of D-baclofen had no effect on these parameters (n = 7). Intravenous administration of ketamine (7.5 mg/kg), another NMDA receptor antagonist, also blocked the effect of D-baclofen (n = 6) but it had no effect on the pressor responses produced by intrathecal administration of carbachol (27.4 nmol; n = 6). In additional experiments, L-baclofen (70 nmol) had no effect on the increases in heart rate and arterial pressure produced by N-methyl-D-aspartic acid (NMDA) (2 nmol; n = 8). These results indicate that D-baclofen increases arterial pressure via an NMDA receptor-mediated mechanism, perhaps by provoking the release of an endogenous ligand which activates these receptors.

Oxytocin and sexual behavior

The neurohypophyseal hormone oxytocin has been implicated in many aspects of reproduction including sexual behavior. This review considers the hypotheses that oxytocin and/or the neural events surrounding the release of oxytocin may have behavioral effects during sexual arousal, orgasm, sexual satiety and other aspects of sociosexual interactions.

Differential effects of ACTH4-10, DG-AVP, and DG-OXT on heart rate and passive avoidance behavior in rats

A computerized telemetry system was used to monitor heart rate (HR), core temperature (CT), and gross locomotor activity in rats treated with saline or neuropeptides during a passive avoidance behavior task. Rats were exposed to a single mild footshock (0.15 mA, for 3 s). Retention tests were conducted at 24 and 48 h after the learning trial. One h prior to the 24-h retention test, each rat received one of the following treatments (SC): saline (SAL), desglycinamide [Arg8]-vasopressin (DG-AVP), ACTH4-10, or desglycinamide-oxytocin (DG-OXT), at a dose of 3 micrograms/rat for DG-AVP and DG-OXT, and 50 micrograms/rat for ACTH4-10. Rats treated with SAL showed a modest increase in avoidance latency accompanied by bradycardia at both retention tests. Rats receiving DG-AVP retained the highest avoidance latency among the experimental groups at both the 24- and 48-h retention test. These rats showed a decrease in HR of the same magnitude as the SAL-treated animals at both retention tests. Rats treated with ACTH4-10 showed an increase in avoidance latency during the 24-h but not during the 48-h retention test. In addition, following ACTH4-10 treatment, a tachycardiac response was found during the 24-h retention test. DG-OXT induced both behavioral and cardiac responses opposite to those found in rats given DG-AVP. CT gradually increased while the rats remained on the platform, irrespective of the treatment. Changes in HR and CT were not influenced by somatomotor activity, as no difference in gross locomotor activity was found among the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Phaclofen-reversible effects of GABA in the spinal cord of the rat

Our earlier observation that intrathecal administration of L- and D-baclofen had different effects on sympathetic output regulating arterial pressure and heart rate in the rat prompted the present study which was designed to determine whether intrathecal administration of GABA elicits a phaclofen-reversible effect on arterial pressure and/or heart rate and whether this effect mimics that of L-baclofen or that of D-baclofen. Following intrathecal administration of the GABAA antagonist, bicuculline (10 nmol), at the T9 level, administration of GABA at a dose of 5 mumol (n = 8) decreased arterial pressure and heart rate by about 25 mm Hg and 45 bpm, respectively. The responses started at 1-2 min and lasted 3-20 min; comparison was made with rats given NaCl (5 mumol; n = 5), which was without effect on arterial pressure and heart rate. In rats pretreated with both bicuculline and the GABAB antagonist, phaclofen (5 mumol intrathecally; n = 9), the effect of GABA on arterial pressure was attenuated and the effect of GABA on heart rate was absent; comparisons were made with rats given bicuculline, phaclofen and NaCl (n = 5) and with rats given bicuculline, NaCl and GABA (n = 6). These data suggest that there is a phaclofen-reversible effect of GABA in spinal pathways regulating sympathetic output and that this effect of GABA resembles that L-baclofen reported in our earlier study.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular responses to intrathecal administration of L- and D-baclofen in the rat

D- and L-baclofen were given intrathecally at the T2 spinal level in the anaesthetized rat. D-Baclofen, in doses of 7, 35 and 70 nmol produced graded increases in arterial pressure but heart rate remained unaffected. Responses appeared within 30 s, peaked at 2 min and decayed over the next 5 min. Injection i.v. of 70 nmol of D-baclofen failed to alter arterial pressure or heart rate. In contrast, intrathecal administration of L-baclofen decreased both arterial pressure and heart rate. The amplitude and time course of the effects depended on the dose used; 700 nmol of L-baclofen had stronger and longer effects than those induced by 70 nmol, while 7 nmol had no effect. (I.v. injection of 70 nmol of L-baclofen had similar effects to intrathecal administration but with different time course and amplitude.) When given at the T9 level at doses of 70 nmol, D- and L-baclofen had effects similar to those observed at the second thoracic level. Effects of intrathecal administration of D- and L-baclofen at T2 were prevented by pretreatment with either hexamethonium (10 mg/kg i.v.) or lidocaine (25 microliters of a 1% solution, intrathecally). The results suggest that D- and L-baclofen-sensitive receptors in the spinal cord are involved in regulating sympathetic output in pathways to the vessels and/or to the heart. In addition, our results suggest that D- or L-baclofen may not act via classical GABAB receptors or that two types of GABAB receptor exist in spinal sympathetic pathways.

Oxytocin modulation of intrathoracic sympathetic ganglionic neurons regulating the canine heart

Substance P and vasoactive intestinal peptide are known to activate intrathoracic sympathetic neurons which regulate the heart. In the present series of experiments, when 1 I.U. of oxytocin in 0.1 cc of normal saline was administered into the cranial poles of stellate or the middle of middle cervical ganglia cardiac rate and force were augmented. The locations of ganglionic loci which, when injected, resulted in cardiac changes varied between animals. Twenty active sites were identified in the 12 dogs investigated. When the vehicle (0.1 cc of normal saline) was injected into these active sites minimal cardiac responses were induced in one instance. When 1 or 2 I.U. of oxytocin was administered into the superior vena cave of seven animals slight systemic hypotension occurred in two of these animals. Cardiac responses were induced when oxytocin was reinjected into active intrathoracic ganglionic sites after whole body administration of hexamethonium (10 mg/kg IV), but not after local administration of timolol into the ganglia. Thus, it appears that oxytocin can activate intrathoracic ganglionic neurons involved in efferent sympathetic cardiac regulation. That such responses persist in the presence of nicotinic blockade, but not following beta-adrenergic blockade of ganglionic neurons, indicates that oxytocin modifies beta-adrenergic and not nicotinic receptors on neurons in these ganglia.

Spinal mediation of the increases in arterial pressure and heart rate in response to intrathecal administration of bicuculline

The present experiments were designed to determine the mechanisms by which the intrathecal administration of the GABAA antagonist, bicuculline (2.2 and 8.8 nmol), at the second thoracic spinal segment (T2) affects cardiovascular function in the anaesthetized rat. Bicuculline produced a dose-related, transient increase in arterial pressure and heart rate which peaked at 5-7 min and persisted for 30 min or more, depending on dose. There was a mutually reversible interaction between bicuculline and intrathecal administration of the GABAA agonist, muscimol (8.8 nmol), which alone decreased arterial pressure and heart rate. Bicuculline was given intrathecally at the third lumbar spinal level (8.8 nmol) and intravenously (8.8 nmol), but in these cases it failed to affect these cardiovascular parameters. Pretreatment with intrathecal infusion of 15 microliters of 1% lidocaine or with intravenous injection of hexemathonium (10 mg/kg) prevented the responses to intrathecal administration of 8.8 nmol of bicuculline at T2. These results demonstrate that the effects of bicuculline on arterial pressure and heart rate are due to an action in the spinal cord on GABAA receptors, and the data may be interpreted as indicating that there is a tonic GABAergic inhibition of sympathetic outflow at the spinal level.

Cardioacceleration provoked by intrathecal administration of vasoactive intestinal peptide (VIP): mediation by a non-central nervous system mechanism

Intrathecal administration of VIP to the thoracic spinal cord in the urethane anaesthetized rat provoked a dose-dependent increase in heart rate without any change in arterial pressure. The cardioacceleration observed following administration of 6.5 nmol of VIP at the T9 level (n = 8) occurred within 1-2 min of administration, with a peak effect of 70-85 bpm, 10-30 min after administration. The magnitude of the maximum change when this dose was given at the T2 level (n = 8) was approximately 100 beats per min, 7-8 min after administration. However, the differences between T2 and T9 administration were not statistically significant. Intravenous administration of 6.5 nmol of VIP (n = 6) mimicked the cardioacceleratory effect of intrathecal administration, and also decreased systolic and diastolic arterial pressure by 9-13 mmHg 6-13 min after administration. The cardioacceleration observed following intrathecal administration at T9 was not blocked by prior systemic administration of the autonomic ganglion blocker hexamethonium (5 mg/kg) or by bilateral vagotomy. Nor was the effect blocked by prior intrathecal administration of the local anaesthetic lidocaine (250 micrograms), although lidocaine did block the tachycardia and hypertension resulting from intrathecal administration of substance P. Considered collectively, the findings that the cardioacceleration observed following intrathecal VIP injection is mimicked by i.v. administration, is not reversed by blockade of nicotinic transmission of autonomic ganglia or by bilateral vagotomy, and is not blocked by lidocaine suggest that VIP's tachycardic effect does not result from a direct action on spinal mechanisms mediating autonomic control of the cardiovascular system, but occurs via diffusion to a site of action outside the central nervous system.

Cardiovascular responses to intrathecal administration of strychnine in the rat

Experiments were done to determine the influence of spinal glycinergic mechanisms in regulating sympathetic output to the heart and vessels in the anaesthetized male Sprague-Dawley rat. Intrathecal administration of 65 (n = 6) and 130 (n = 8) nmol of strychnine, but not of a lower dose (32.5 nmol, n = 8), to the second thoracic segment increased heart rate within one minute (P less than 0.01). Similar administration of artificial cerebrospinal fluid (n = 16) had no effect. The increase in heart rate in response to strychnine peaked at 5-7 min (+35.1 +/- 5.8 bpm with 130 nmol), and slowly returned toward baseline values over the next 25 min. Arterial pressure was unaffected by this treatment. These effects were not mimicked by administration of strychnine (n = 6) at the third lumbar spinal level or by intravenous infusion of strychnine (n = 4) and were abolished by systemic injection of hexamethonium (n = 6). The results suggest that there is a tonic glycine-mediated inhibition of sympathetic output at the spinal level.

Angiotensin II stimulates sympathetic output by a direct spinal action

When administered intrathecally in a dose of 10 mu to the ninth thoracic segment of the spinal cord in the anesthetized rat, angiotensin II produced a transient increase in systolic and diastolic arterial pressures lasting 1-4 min. Heart rate was also increased, but in this case for more than 30 min. Similar administration of 5 micrograms or of CSF had no effect on either arterial pressure or heart rate. Neither the arterial pressure nor the heart rate response to 10 micrograms of angiotensin II was observed in rats given hexamethonium (10 mg/kg, i.v.), suggesting that the effects were mediated by spinal activation of sympathetic output. When the rats were pretreated with 10 micrograms of [Sar1, IIe8]-angiotensin II three min prior to angiotensin II, there was a block of the increase in arterial pressure but not of the increase in heart rate. When the antagonist was given 15 min prior to angiotensin II, the full pressor response appeared, suggesting that the antagonist was effective for less than 15 min; in addition, after the antagonist alone, while arterial pressure remained unaltered, there was a gradual increase in heart rate suggesting that the analogue had agonistic effects on mechanisms regulating heart rate. These results suggest that angiotensin II activates sympathetic mechanisms by a spinal action and that arterial pressure and heart rate are regulated differentially, arterial pressure via a mechanism which is antagonized by [Sar1, IIe8]-angiotensin II, and heart rate via a mechanism in which the analog can act as an agonist.

Brain neuropeptides: actions on central cardiovascular control mechanisms

The many peptides we have not considered (e.g. gastrin, motilin, FMRFamide, carnosine, litorin, dermorphin, casomorphin, eledoisin, prolactin, growth hormone, neuromedin U, proctolin, etc.) were omitted due to lack of information as far as any putative central cardiovascular effects are concerned. However, even for some of these peptide pariahs intriguing snippets of information are available now (e.g. ref. 85), although as we write, the list of possible candidates for investigation grows longer. On an optimistic note, it is becoming clear that many brain neuropeptides may have important effects on cardiovascular regulation. It seems feasible that 'chemically coded' pathways in the brain might be the neuroanatomical correlate of a 'viscerotopic' organization of cardiovascular control mechanisms, whereby the activity of the heart and flows through vascular beds are individually controlled, but in an integrated fashion, utilizing particular combinations of neurotransmitters and neuropeptides within the brain. Such possibilities can only be investigated, properly, by measurement of changes in cardiac output and regional haemodynamics in response to appropriate interventions, in conscious, unrestrained animals.