Antipyretic action of centrally administered arginine vasopressin but not oxytocin in the cat

Emerging trends in the delivery of nanoformulated oxytocin across Blood-Brain barrier

Neurological diseases are related to the central nervous system disorders and considered as serious cases. Several drugs are used to treat neurological diseases; however, to date the main issue is to design a therapeutic model which can cross the blood-brain-barrier (BBB) easily. The delivery of neuropeptides into the brain lays as one of the important routes for treating neurological disorders. Neuropeptides have been demonstrated as potential therapeutics for neurological disorders. Among numerous neuropeptides, the oxytocin (OT) hormone is of particular interest as it serves as a neurotransmitter in the brain as well as its role as a hormone. OT has a wide-range of activities in the brain and has a key role in cognitive, neuroendocrine, and social functions. However, OT does not cross the BBB readily coupled with its half-life in the blood being too short. The current literature reveals that the delivery of OT by nanoparticle-based drug delivery system (DDS) improves its efficacy. Nanoparticle based DDS are considered important tools for the targeted delivery of drugs to the brain as they lower toxicity of the drug and improve the drug efficacy. Nanoparticles are advantageous candidates for biomedical applications due to their distinctive characteristics such as quantum effects, large surface area and their ability to carry and transport the drug to its target site. OT can be delivered through oral and intranasal routes, but the bioavailability of OT inside the brain can further be enhanced by the delivery using nanoparticles. The application of nano-based delivery system not only improves the penetration of OT inside brain but also increases its half-life by the application of encapsulation and extended release. The aim of current review is to provide an overview of nanoparticle-based drug-delivery systems for the delivery of OT inside brain.

Oxytocin and the warm outer glow: Thermoregulatory deficits cause huddling abnormalities in oxytocin-deficient mouse pups

Oxytocin is a social and reproductive hormone that also plays critical roles in a range of homeostatic processes, including thermoregulation. Here, we examine the role of oxytocin (OT) as a mediator of brown adipose tissue (BAT) thermogenesis, cold-induced huddling, and thermotaxis in eight-day-old (PD8) OT 'knock out' (OTKO) mouse pups. We tested OTKO and wildtype (WT) pups in single- and mixed-genotype groups of six, exposing these to a period of ambient warmth (~35°C) followed by a period of cold (~21.5°C). Whether huddling exclusively with other OTKO or alongside WT pups, OTKO pups showed reduced BAT thermogenesis and were significantly cooler when cold-challenged. Huddles of OTKO pups were also significantly less cohesive than WT huddles during cooling, suggesting that thermoregulatory deficits contribute to contact abnormalities in OTKO pups. To further explore this issue, we examined thermotaxis in individuals and groups of four OTKO or WT pups placed on the cool end of a thermocline and permitted to freely locomote for 2h. When tested individually, male OTKO pups displayed abnormal thermotaxis, taking significantly longer to move up the thermocline and settling upon significantly lower temperatures than WT pups during the 2h test. OTKO mouse pups thus appear to have deficits in both thermogenesis and thermotaxis-the latter deficit being specific to males. Our results add to a growing body of work indicating that OT plays critical roles in thermoregulation and also highlight the entanglement of social and thermoregulatory processes in small mammals such as mice.

Role of oxytocin in energy metabolism

The basic mechanisms that lead obesity are not fully understood; however, several peptides undoubtedly play a role in regulating body weight. Obesity, a highly complex metabolic disorder, involves central mechanisms that control food intake and energy expenditure. Previous studies have shown that central or peripheral oxytocin administration induces anorexia. Recently, in an apparent discrepancy, rodents that were deficient in oxytocin or the oxytocin receptor were shown to develop late-onset obesity without changing their total food intake, which indicates the physiological importance of oxytocin to body metabolism. Oxytocin is synthesized not only within magnocellular and parvocellular neurons but also in several organs, including the ovary, uterus, placenta, testis, thymus, kidney, heart, blood vessels, and skin. The presence of oxytocin receptors in neurons, the myometrium and myoepithelial cells is well recognized; however, this receptor has also been identified in other tissues, including the pancreas and adipose tissue. The oxytocin receptor is a typical class I G protein-coupled receptor that is primarily linked to phospholipase C-β via Gq proteins but can also be coupled to other G proteins, leading to different functional effects. In this review, we summarize the present knowledge of the effects of oxytocin on controlling energy metabolism, focusing primarily on the role of oxytocin on appetite regulation, thermoregulation, and metabolic homeostasis.

Fever during anaesthesia

Fever occurs when pyrogenic stimulation activates thermal control centres. Fever is common during the perioperative period, but rare during anaesthesia. Although only a limited number of studies are available to explain how anaesthesia affects fever, general anaesthesia seems to inhibit fever by decreasing the thermoregulatory-response thresholds to cold. Opioids also inhibit fever; however, the effect is slightly less than that of general anaesthesia. In contrast, epidural anaesthesia does not affect fever. This suggests that hyperthermia, which is often associated with epidural infusions during labour or in the post-operative period, may be a true fever caused by inflammatory activation. Accordingly, this fever might be diminished in patients who receive opioids for pain treatment. Post-operative fever is a normal thermoregulatory response usually of non-infectious aetiology. Fever may be important in the host defence mechanisms and should not be routinely treated lest the associated risks exceed the benefits.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

Vasopressin in the cerebrospinal fluid of febrile children with or without seizures

Immaturity in water and electrolyte balance in the brain has been considered to increase the susceptibility of young animals and children to febrile convulsions (FCs). Arginine-vasopressin (AVP) is involved in the regulation of several centrally mediated events such as modulation of fever and the ease with which water permeates into and out of the brain. To evaluate the possible role of AVP in the control of water balance and susceptibility to convulsions during fever we measured the AVP concentration in the cerebrospinal fluid (CSF) and plasma of febrile children with or without convulsions. The febrile population consisted of 47 children, of whom 29 experienced seizures during fever. Seven children with epileptic symptoms and 18 children without seizures were included as nonfebrile controls. The CSF AVP concentration in febrile children without seizures and in nonfebrile convulsive children was significantly lower (0.60 +/- 0.07 pmol/l, mean +/- SEM, P < 0.01 and 0.65 +/- 0.19 pmol/l, P < 0.05, respectively) than in nonfebrile children without convulsions (0.83 +/- 0.06 pmol/l). However, the levels of CSF AVP were not significantly different in children with FCs (0.71 +/- 0.06 pmol/l) compared with other groups. CSF AVP correlated with the CSF osmolality (r = 0.33, P = 0.02). No statistical differences in plasma AVP levels between the groups could be found. The present data provide support for the hypothesis of synchronous regulation of osmolality and AVP concentration in CSF. During fever the concentration of CSF AVP was lower in nonconvulsive children compared with nonfebrile nonconvulsive children. CSF AVP levels were not affected in febrile children by convulsions.

Does endogenous peripheral arginine vasopressin have a role in the febrile responses of conscious rabbits?

1. The actions of peripheral arginine vasopressin (AVP) on the febrile responses of conscious rabbits induced by peripherally administered polyinosinic:polycytidylic acid (poly(I).poly(C)) have been studied using an AVP V1 receptor antagonist ([deamino-Pen1, O-Me-Tyr2, Arg8]-vasopressin). 2. Temperature responses were monitored continuously using rectal thermistor probes. Test substances were administered intravenously (i.v.). Blood samples were taken at timed intervals from a marginal ear vein and plasma PGE2 and PGF2 alpha levels determined by radioimmunoassay. 3. Poly(I).poly(C) (2.5 micrograms/kg) stimulated a reproducible biphasic rise in body temperature with a lag phase of 45-60 min and peaks at 90 and 225 min. The febrile response was accompanied by a 5-fold rise in circulating immunoreactive (ir) PGE2, which peaked after 90 min and remained elevated up to 300 min. Poly(I).poly(C) also stimulated a 2.5-fold rise in circulating irPGF2 alpha, which peaked after 150 min and was followed by a return to basal levels after 300 min. 4. The overall magnitude of the febrile response to poly(I).poly(C) (2.5 micrograms/kg, i.v.) was significantly antagonized by the AVP V1 receptor antagonist (250 micrograms/kg, i.v.) administered 5 min prior to the pyrogen. 5. The irPGE2 response to poly(I).poly(C) (2.5 micrograms/kg, i.v.) was significantly antagonized by the AVP V1 receptor antagonist (250 micrograms/kg, i.v.) administered 5 min prior to the pyrogen. The irPGF2 alpha response was only reduced at the peak 150 min time point measurement. 6. In conclusion, these results show a modulatory role for a peripherally administered AVP V1 antagonist in the febrile responses to poly(I).poly(C), suggesting a possible propyretic role for endogenous peripheral AVP. This modulatory role appears to be mediated via actions on prostaglandin E2.

Fever: causes and consequences

The present review distinguishes pathogenic, neurogenic, and psychogenic fever, but focuses largely on pathogenic fever, the hallmark of infectious disease. The data presented show that a complex cascade of events underlies pathogenic fever, which in broad outline - and with frank disregard of contradictory data - can be described as follows. An invading microorganism releases endotoxin that stimulates macrophages to synthesize a variety of pyrogenic compounds called cytokines. Carried in blood, these cytokines reach the perivascular spaces of the organum vasculosum laminae terminalis (OVLT) and other regions near the brain where they promote the synthesis and release of prostaglandin (PGE2). This prostaglandin then penetrates the blood-brain barrier to evoke the autonomic and behavioral responses characteristic of fever. But then once expressed, fever does not continue unchecked; endogenous antipyretics likely act on the septum to limit the rise in body temperature. The present review also examines fever-resistance in neonates, the blunting of fever in the aged, and the behaviorally induced rise in body temperature following infection in ectotherms. And finally it takes up the question of whether fever enhances immune responsiveness, and through such enhancement contributes to host survival.

Electrical stimulation of the paraventricular nucleus attenuates pyrogen fever in the rabbit

Arginine vasopressin (AVP) perfused within the ventral septal area (VSA) suppresses fever normally evoked by pyrogenic substances, including Salmonella abortus equi (SAE). Neurons containing AVP and located within the paraventricular nucleus (PVN) or the nearby bed nucleus of the stria terminalis (BnST) are believed to have projections to this septal region. A series of experiments was undertaken to determine whether electrical stimulation of these areas, which might be expected to cause the release of AVP within the VSA, would affect similarly the pathogenesis of fever. A stainless steel cannula was implanted surgically in each of 22 male New Zealand White rabbits and a monopolar electrode was lowered through this guide cannula to the PVN or BnST areas. Electrical stimulation (20 Hz, 10 s on, 10 s off, 2.6-3.2 V) was initiated 30 min prior to and was continued until 90 min after the intravenous (i.v.) administration of 0.1-1.0 micrograms of SAE (1.0 ml carrier vehicle). While afebrile body temperature remained unchanged, electrical stimulation of sites located in the rostral extension of the PVN effectively attenuated the pyrogen-induced fever. Stimulation of sites outside these areas did not affect either the absolute magnitude or the duration of the fever. Although the reduction in fever was most pronounced during the period of electrical stimulation, in some cases the fever remained suppressed beyond the application of the current. These experiments provide the first evidence that electrical stimulation of paraventricular areas with AVP-containing cell bodies is effective in suppressing a fever evoked by systemic administration of a pyrogen. Although untested, it is possible that a stimulus-induced release of AVP within the VSA is responsible for the attenuation of the fever.

The effectiveness of arginine vasopressin and sodium salicylate as antipyretics in the Brattleboro rat

The infusion of either 30 micrograms/microliters (approx. 100 micrograms/kg/h) of sodium salicylate or 10 ng/microliters (10(-5) M) arginine vasopressin within the ventral septal area of the Brattleboro rat brain reduced a centrally induced prostaglandin E1 (PGE1) hyperthermia when compared with infusions of artificial cerebrospinal fluid. Conversely, the infusion of a related peptide, oxytocin (10 ng/microliters (10(-5) M), or 33 ng/kg/h) failed to alter the rise in core temperature following the PGE1 injection. These results suggest that the vasopressin receptors reported to be present in the Brattleboro rat may respond normally to exogenously administered vasopressin, thus allowing for the antipyretic action. Moreover, the antipyretic effects of sodium salicylate suggest that aspirin-like drugs may induce the release of alpha-melanocyte-stimulating hormone which, in turn, attenuates the PGE1-evoked fever. Given recent evidence, however, which suggests that the Brattleboro rat may contain vasopressin both peripherally and within the brain, the antipyretic action of sodium salicylate may be alternatively explained through the endogenous release of vasopressin.

Antipyretic doses of centrally administered vasopressin reach physiologically meaningful concentrations in the brain of the rat as evaluated by microdialysis

It was important to determine whether vasopressin (AVP) injected intracerebroventricularly (i.c.v.) in the rat reached the site of action within the ventral septal area (VSA) in sufficient concentrations to account for its physiological effects. Microdialysis was used to evaluate this hypothesis. The exchange rate across the dialysis tubing was determined in vitro to be 0.40%. After placement of the microdialysis cannula in the VSA of the rat the recovery of i.c.v. injected labelled or cold AVP was 0.23 and 0.20%, respectively. Maximum concentrations of AVP in the extracellular fluid of the VSA was determined to be 10.7 nM after 10 ng i.c.v. and hence extrapolated to be 1.07 nM after 1 ng i.c.v. or 2.65 nM after 2.5 ng i.c.v. between which lies the threshold dose of AVP for its antipyretic effects. This can be compared with a reported Kd for these receptors of 1.06 nM as determined by receptor binding assay.

Indomethacin-induced antipyresis in the rat: role of vasopressin receptors

Infusion of 15 micrograms/microliters (approximately 120 micrograms/kg/h) of indomethacin within the ventral septal area of the rat brain significantly reduced a centrally induced prostaglandin E1 (PGE1) hyperthermia when compared with infusions of artificial cerebrospinal fluid. A bolus injection of a V1 receptor antagonist, d(CH2)5Try(Me)AVP, (200, 2000, or 20,000 pmol) within the ventral septal area had no effect of body temperature alone but did suppress the PGE1-induced fever. Similar bolus injections of the V1 receptor antagonist within the ventral septal area failed to alter the antipyretic action of indomethacin on the hyperthermia resulting from centrally administered PGE1. Central injections of a V2 receptor antagonist failed to alter either the PGE1-induced fever or the indomethacin-evoked antipyresis. The results suggest that the V1 receptor antagonist may exert non-specific neurodepressant effects which may interfere with the expression or production of PGE1 hyperthermia and may further mask any contribution of arginine vasopressin to the antipyretic effects of indomethacin.

Criteria for establishing a physiological role for brain peptides. A case in point: the role of vasopressin in thermoregulation during fever and antipyresis

This paper has attempted to present and discuss the criteria necessary for the evaluation of a specific physiological role for a peptide in the CNS. These criteria are based on many experimental approaches to the problem and conclusions must be supported by the weight of the evidence. These criteria were illustrated by examining the hypothesis that AVP is an antipyretic neurotransmitter involved in regulating febrile increases in Tb by release and action in the VSA of the brain. The weight of the evidence in this case implies that this hypothesis is essentially correct. The only serious conflicting evidence comes from the work with Brattleboro rats. It is hoped that further research will resolve these discrepancies or result in a suitably modified hypothesis.

The role of vasopressin as an antipyretic in the ventral septal area and its possible involvement in convulsive disorders

Perfusion of the peptide, arginine vasopressin (AVP), within the ventral septal area (VSA) of the brain of a number of species reduces fever but not normal body temperature. This antipyretic response appears to be mediated by AVP receptors of the V1 subtype. Lesions of the VSA with kainic acid are associated with prolonged and enhanced fevers in rats. A role for endogenous AVP in fever suppression within the VSA comes from several types of experiments: (1) AVP release within the VSA is inversely correlated to fever height; (2) AVP antagonists or antiserum injected into the VSA prolong fever; (3) animals lacking endogenous AVP in the VSA (Brattleboro rat, long-term castrated rat) develop enhanced fevers. Electrical stimulation of the AVP-containing cell bodies of the bed nucleus of the stria terminalis (BST) orthodromically inhibits VSA neurons and also suppresses fever; the latter effect can be abolished with application of a V1 antagonist to the VSA. Iontophoretic studies indicate that AVP inhibits glutamate-stimulated activity of thermoresponsive and other VSA neurons. AVP can also act in the VSA to cause severe motor disturbances; this action is receptor mediated and increases in severity upon sequential exposure to AVP. Because sites of action of the antipyretic and convulsive action of AVP are similar, and because animals lacking brain AVP display reduced convulsive activity, it is possible that AVP, released during fever, could be involved in the genesis of convulsive activity.

Antipyresis due to centrally administered vasopressin differentially alters thermoregulatory effectors depending on the ambient temperature

The intracerebroventricular (i.c.v.) administration of arginine vasopressin (AVP), in the febrile rat elicits an antipyresis at cold, warm and neutral ambient temperatures. These experiments were conducted, therefore, to elucidate the thermoregulatory effector mechanisms responsible for this antipyretic effect. At 25 degrees C, AVP-induced antipyresis was mediated by tail skin vasodilation while metabolic rate was unaffected. At 4 degrees C, the antipyresis produced by AVP was approximately double that seen at 25 degrees C. This effect appeared to be mediated exclusively by inhibition of heat production since the metabolic rate decreased markedly following AVP. This antipyresis at 4 degrees C was accompanied by cutaneous vasoconstriction. At 32 degrees C, neither vasomotor tone, metabolic rate nor evaporative heat loss could be shown to contribute to the small antipyretic effect elicited by AVP. We conclude from these data that i.c.v. AVP is producing antipyresis by affecting the febrile body temperature set-point mechanism since the thermoregulatory strategy to lose heat varies at different ambient temperatures and the decrease in body temperature cannot be shown to be due to changes in a single effector mechanism.

Evidence supporting a role for endogenous vasopressin in fever suppression in the rat

1. Infusion of human purified interleukin-1 into a lateral cerebral ventricle of the rat evoked a rise in core temperature which was abolished by heating the interleukin-1. 2. When the intracerebroventricular infusion of interleukin-1 was preceded by a bilateral injection of saline into the ventral septal area, the resulting febrile response was not different from that induced by interleukin-1 alone. However, when the vasopressin V1 antagonist, d(CH2)5Tyr(Me)AVP, was injected into the ventral septal area prior to interleukin-1, a fever was evoked which was significantly greater in magnitude and duration. This enhancement of fever by the V1 antagonist was dose related. 3. Injection of either saline or the V1 antagonist into the ventral septal area, in the absence of interleukin-1, did not evoke any consistent alteration in the core temperature of the rats. 4. The vasopressin V2 antagonist, d(CH2)5-D-ValVAVP, was injected into the ventral septal area to determine the effect of another vasopressin analogue on the fever evoked by interleukin-1. The V2 antagonist did not alter the time course of interleukin-1-induced fever or alter core temperature in the afebrile rat. 5. These data are consistent with the hypothesis that endogenous vasopressin, released in the ventral septal area, may be involved in limiting fever. In addition, these results indicate that the central receptor mediating the antipyretic action of vasopressin may resemble the V1 subtype of peripheral vasopressin receptor.