Central and peripheral injections of ACTH (1–24) reduce fever in adrenalectomized rabbits

Central administration of ACTH (1-24) reduces fever in normal rabbits in doses that have no effect on afebrile body temperature. Previous experimental and clinical reports indicate that peripheral administration of both ACTH and corticosteroids reduces fever, and since central injection of corticosteroids can also lower fever it might be that the antipyretic effect of intracerebroventricular (ICV) ACTH (1-24) is due to adrenal stimulation. To learn whether this endogenous central peptide can produce antipyresis independently, ACTH (1-24) was injected ICV in bilaterally adrenalectomized (ADX) rabbits made febrile by IV injections of leukocytic pyrogen (LP). ACTH (250 ng) given ICV reduced fever in these animals and had a slight hypothermic effect when given to the same rabbits when they were afebrile. Doses of 25-75 ng reduced fever without influencing normal body temperature. Intravenous injections of ACTH (2.5 micrograms) also lowered fever caused by IV LP in ADX rabbits. The present findings raise the possibility that release of endogenous central ACTH, and perhaps entry into the brain of circulating ACTH, the release of which is known to increase in fever, limits the magnitude of the febrile response by influencing central temperature controls.     

Antipyretic effect of centrally administered CRF

CRF injected into the third cerebral ventricle (0.5-2.5 micrograms) caused dose-related reductions in fever induced in rabbits by IV administration of leukocytic pyrogen. Control injections of CRF when the same animals were afebrile did not alter normal body temperature. Intravenous injections of 5 and 20 micrograms CRF, doses known to release ACTH and corticosteroids into the bloodstream in other species, did not reduce fever. CRF injected into the cerebral ventricles may be antipyretic per se, or it may reduce fever by virtue of central release of the antipyretic peptides ACTH and alpha-MSH.     

Intravenous alpha-MSH reduces fever in the squirrel monkey

alpha-MSH reduces fever in rabbits when administered IV, ICV, or by gavage; however, the applicability of this finding to higher species, specifically to primates, has not been determined. In this study, we chose the squirrel monkey as an appropriate primate model since it responds reliably to peripheral administration of bacterial endotoxins that cause fever in man. From pilot studies, doses of S. typhosa endotoxin necessary to produce maximum fever and doses of alpha-MSH which did not cause hypothermia were determined for each animal. In the main experiments endotoxin was given via an indwelling catheter in the saphenous vein, followed by alpha-MSH injections when the rectal temperature increased 0.3 degrees C. alpha-MSH (100-400 micrograms) reduced the area under the fever curve an average of 50.0%, but had no effect on afebrile temperature. Molar equivalent amounts of the antipyretic drug acetaminophen had little effect on fever. These findings support the idea, based on research on rabbits, that alpha-MSH has a role in central modulation of fever.     

Lateral cerebral ventricle and preoptic-anterior hypothalamic area infusion and perfusion of beta-endorphin and ACTH to unrestrained rats: core and surface temperature responses

Both beta-endorphin and ACTH have been found in high concentrations within the hypothalami of mammals and each neuropeptide has been proposed to play a physiological role in regulating body temperature. In an attempt to determine how these peptides may alter thermoregulation, small, microgram concentrations of beta-endorphin and ACTH were injected either into lateral cerebral ventricle (ICV) or directly into the preoptic-anterior hypothalamic area (POAH) or perfused into the POAH of unrestrained rats. Core (rectal) and surface (tail) temperatures were recorded before and after ICV and POAH injection of 1 microgram of beta-endorphin or ACTH or perfusion (10 ng/microL) of either neuropeptide. POAH perfusion of naloxone HCl following the neuropeptide perfusion was tested to determine the specificity of the temperature responses. Regardless of the route of central administration, beta-endorphin, in the concentrations used, consistently evoked a hyperthermic core temperature response, that could be antagonized by naloxone. Increased core temperatures may, in part, have been due to peripheral vasoconstriction, as suggested by the decreases seen in tail temperature. The same concentrations of ACTH failed to show any prominent core temperature changes. Results suggest that beta-endorphin is a more potent modulator than ACTH in altering core temperatures of unrestrained rats. Whether beta-endorphin and ACTH act physiologically in an antagonistic manner to maintain a constant body temperature remains to be proven.     

The effect of α-MSH on fever caused by Staphylococcus aureus cell walls in rabbits

The role of endogenous pyrogens induced by gram-positive bacterial pyrogens is not known. Intravenous alpha-MSH (2.5 micrograms) significantly reduced only the first phase of the biphasic thermal response to IV S. aureus cell walls (5 x 10(7)). Intracerebroventricular alpha-MSH (200 ng) had no effect on the fever response. The fall in serum iron concentration was significantly attenuated by the IV alpha-MSH but was not affected by the ICV alpha-MSH. Intravenous alpha-MSH had no effect on fever or the serum iron response caused by muramyl dipeptide (MDP). We conclude that the first phase of the thermal response to S. aureus cell walls is mediated by an endogenous pyrogen (EP) and the second phase of the response by a mechanism not involving EP, but possibly a muramyl peptide.     

Central and peripheral actions of alpha-MSH in the thermoregulation of rats.

The effect of alpha-MSH on thermoregulation in rats at room temperature was examined. alpha-MSH (1 microgram ICV or 30 micrograms IP) alone did not alter temperature. However, this peptide was a potent antipyretic when administered centrally or peripherally in rats treated with pyrogen derived from Salmonella typhi.     

Peripheral administration of α-MSH reduces fever in older and younger rabbits

In these experiments IV, ICV and intra-gastric administration of α-MSH reduced fever caused by injections of leukocytic pyrogen (LP). 2.5 μg α-MSH injected IV reduced fever caused by IV LP, more so in rabbits over 3 yrs old than in those under 2 yrs of age; 5 mg of acetaminophen given IV had no antipyretic effect in either age group. ICV administration of 25 ng α-MSH reduced fever caused by IV LP injection in the older but not in the younger rabbits. α-MSH given IV (2.5 μg) also lowered fever induced by ICV injection of LP in older but not in younger animals. Both older and younger rabbits showed reductions in fever evoked by IV LP after 2.5 mg α-MSH was given by gastric tube. The results indicate that this peptide which occurs naturally within the brain has potent antipyretic properties when given systemically, presumably as a result of a central antipyretic action. Greater sensitivity to central α-MSH in the older rabbits may account for the reduced febrile response seen in the aged. The findings support previous data which suggest that central α-MSH has a physiological role in the limitation of fever.     

beta-Endorphin-induced hyperthermia in the cat

beta-Endorphin was injected into the third cerebral ventricle (ICV) of conscious, unrestrained cats. Hyperthermic response to 50 microgram of this peptide were reduced by 20-100 microgram naloxone given ICV 1 hr later. A dose of 40 microgram beta-endorphin increased body temperature at ambient temperature of 4, 22 and 34 degrees C, with the response being greater the warmer the environment. These results indicate that beta-endorphin acts on a central naloxone-sensitive receptor which is probably the v2 receptor that is activated by low doses of D-Ala2-Met-enkephalinamide to evoke a similar pattern of change in body temperature over a comparable range of ambient temperatures.     

A comparison of grooming behavior potencies of neurohypophyseal nonapeptides

We have previously demonstrated that intracerebroventricular (ICV) administration of oxytocin (OXY) enhanced grooming behaviors in male and female rats at a 1 microgram dose. In the present study female rats were injected ICV with 1 microgram OXY or equimolar doses of other peptides. At this dose arginine-vasopressin (AVP), arginine-vasotocin (AVT) and lysine-vasopressin (LVP), as well as alpha-MSH, were as effective as OXY in increasing grooming behavior. At equimolar doses, ACTH1-10, tocinoic acid (the ring structure of OXY) and Pro-Leu-Gly-NH2 (the tail structure of OXY) had no significant effect on grooming behavior. The potency of AVP and AVT was determined across a 0.05-5 microgram dose range. Grooming scores increased in an apparent linear manner across a similar OXY dose range. Both AVP and AVT, however, manifested an inverted U grooming response curve. Maximum grooming scores resulted from a 0.1 microgram dose of AVT or a 0.5 microgram AVP dose. Analyses of the aspects of grooming separately found that nonapeptides OXY, AVP and AVT all elevated body grooming, washing, and scratching. Because AVT and AVP administration resulted in grooming scores significantly higher than OXY at lower doses, we concluded that the CNS is more sensitive to the effects of AVT and AVP on grooming behavior than OXY.     

Fever and acute-phase response induced in rabbits by intravenous and intracerebroventricular injection of interleukin-6

We investigated the effect of human recombinant interleukin-6 (IL-6) on body temperature and acute-phase response, including changes in plasma levels of iron, zinc, copper, and fibrinogen and in circulating leukocyte count. The intravenous (IV) injection of IL-6 (2 micrograms/kg) produced a monophasic fever. The intracerebroventricular (ICV) injection of IL-6 produced a dose-dependent fever that developed gradually and remained elevated throughout the 5-h recording period. The IV injection of IL-6 decreased the plasma concentration of iron and zinc and increased the circulating leukocyte count. The ICV injection of IL-6 resulted in similar trace metal and leukocyte changes, and increased plasma levels of fibrinogen. These results show that IL-6 can cause fever when injected IV or ICV and induces some acute-phase responses through its action on peripheral target organs and in the central nervous system.     

Dopaminergic modulation of some central actions of angiotensin II in vivo

Studies were performed in 12 conscious sheep of both sexes to determine if a brain dopaminergic pathway is involved in modulating the central actions of angiotensin II (Ang II) in regulating body temperature and plasma renin activity (PRA). Previous data showed that intracerebroventricular (ICV) infusion of Ang II significantly decreased PRA and body temperature. In contrast, converting enzyme inhibitor SQ 20881 (SQ) or dopamine (DA) significantly increased PRA and body temperature of sheep. In the present study, ICV infusion of the DA antagonist metoclopramide (MCP) (20 micrograms/min) significantly decreased PRA to 68 +/- 5% of the basal level. When sheep were pretreated with ICV MCP (20 micrograms/min) for 2 hr and then infused ICV with MCP (20 micrograms/min) plus DA (20 micrograms/min), Ang II (25 ng/min), or SQ (1 microgram/min), the PRA and temperature responses to DA, Ang II, or SQ were all abolished or attenuated significantly. The converse did not hold. Sheep pretreated with SQ (1 microgram/min) still showed a significant increase in body temperature (0.43 +/- 0.05 degree C) when infused with DA (20 micrograms/min). These results support the hypothesis that a central DA pathway is involved in the modulation of the actions of centrally administered Ang II on temperature and PRA.     

α-MSH injected into the septal region reduces fever in rabbits

In previous research the concentration of α-MSH within the septal region of rabbits increased with fever. This finding raises the possibility that the septal concentration of this peptide, which reduces fever when given both peripherally and intracerebroventricularly, is important to limitation of fever. To test this idea, rabbits with cannulas in the septal region were made febrile by IV injections of leukocytic pyrogen (LP). Injection of α-MSH (1 μg bilaterally) into the septal region did reduce fever, consistent with the idea that the increase in septal α-MSH concentration which occurs naturally in fever limits the febrile response. We also noted late rises in body temperature when experimental and control septal injections were given close together in time. These increases in temperature were similar to those known to occur after injections into the primary temperature control in the PO/AH region. This commonality further strengthens the possibility that septal neurons are important to central modulation of body temperature.     

Neuropeptides and thermoregulation: the interactions of bombesin, neurotensin, TRH, somatostatin, naloxone and prostaglandins

In this study we have examined the interactions of bombesin (1 microgram ICV), neurotensin (1 microgram ICV), TRH (10 micrograms ICV), somatostatin (10 micrograms ICV), PGE2 (10 micrograms ICV) and naloxone (10 mg/kg SC) on thermoregulation in the rat at room temperature (20 +/- 1 degree C). Given alone, bombesin, neurotensin, somatostatin and naloxone all produced hypothermia (bombesin greater than neurotensin greater than somatostatin congruent to naloxone). PGE2 was hyperthermic, and TRH had no effect. Bombesin and PGE2 neutralized one another's effects. Neurotensin had no effect on PGE2-induced hyperthermia. Naloxone enhanced the hypothermic effect of bombesin and somatostatin enhanced the rate of onset of hypothermia after bombesin. TRH had no effect on bombesin-induced hypothermia. TRH, somatostatin and naloxone had no effect on neurotensin-induced hypothermia. TRH antagonized the hypothermia due to naloxone and somatostatin.     

A discrete mode of the antipyretic action of AVP, alpha-MSH and ACTH.

The antipyretic effect of AVP, alpha-MSH and ACTH consists in lowering the thermoregulatory threshold and in shortening the time span of the fever. Thus, neuropeptides influence activity of hypothalamic neurones regulating body temperature. This was confirmed by recent experiments of Moravec (this volume) which indicate that spontaneous activity and thermosensitivity of neurones in hypothalamic slices can be influenced, by AVP. Why neuropeptides of different chemical structure such as AVT, on one hand, and alpha-MSH and ACTH, on the other hand, induce the same effect on thermoregulation remains to be elucidated.     

Effects of corticotropin-releasing factor, corticotropin and cortisol on gastrointestinal motility in dogs

Gastrointestinal motor activity following intracerebroventricular (ICV) and intravenous (IV) administration of corticotropin releasing factor (CRF), corticotropin (ACTH) and cortisol was investigated in fasted dogs with strain-gauge transducers chronically implanted on the antrum and proximal jejunum. ICV but not IV administration of CRF (20 to 100 ng/kg) suppressed the gastric cyclic migrating motor complex (MMC) for 3 to 6 hours without affecting the jejunum. Similar disruptive effects on the gastric MMC were observed after ICV administration of ACTH (0.5 U/kg) or cortisol (0.1 micrograms/kg) but not after IV administration of 10 times higher doses. These results suggest that in dog CRF may be involved in the central control of the interdigestive gastric motility, these effects were not probably due to the release of ACTH and cortisol the other hormones of the pituitary adrenocortical system change the gastric motility when centrally administered through a possible feed-back mechanism affecting brain CRF level.     

Scrotal heating stimulates panting and reduces body temperature similarly in febrile and non-febrile rams (Ovis aries)

It is known that heating the ram scrotum stimulates heat loss resulting in a decrease in body temperature and that during fever core temperature increases, but local scrotal thermoeffectors operate to maintain normal scrotal temperature. We have investigated whether scrotal warming influences core body temperature and the panting effector during fever generation. We measured rectal temperature, intrascrotal temperature, scrotal skin temperature and respiratory frequency in four adult Merino rams following intravascular injection of saline or lipopolysaccharide at an ambient temperature of 18-20 degrees C while scrotal skin temperature was maintained at 33 degrees C or elevated to 41 degrees C. Compared to maintaining normal scrotal temperature, heating the scrotum increased respiratory frequency and reduced rectal temperature by a similar amount following LPS as following saline. Fever was associated with decreased respiratory frequency compared to saline at both 33 and 41 degrees C scrotal temperature, suggesting that the fever was generated mainly by decreasing respiratory heat loss. We conclude that scrotal thermal afferent stimulation resulted in an offset for the set-point of body temperature regulation in both normothermic and febrile rams.     

Brain CCKA receptors mediate the colonic motor response to feeding in dogs

The influence of central vs. peripheral administration of specific type A and type B CCK receptor antagonists (L364,718 and L365,260, respectively) on colonic motor hyperactivity induced by feeding and CCK8 was investigated in dogs with strain-gauge transducers implanted on the proximal and transverse colon. Intravenous injection of L364,718 (5 and 10 micrograms/kg) reduced by 26.2% and 80.1%, respectively, the 0-4-h postprandial increase in colonic motor index; at similar doses L365,260 had no effect. Intracerebroventricular administration of L364,718, at a dose (1 microgram/kg) not active by the IV route, significantly reduced (p less than 0.01) by 67.5% the feeding-induced colonic hyperactivity. In contrast, L365,260 (1-10 micrograms/kg ICV) injected was inactive. Increase in colonic motility produced by intravenous CCK8 infusion (1 microgram/kg/h) was suppressed by previous ICV and IV administration of L364,718 at doses of 1 and 10 micrograms/kg, respectively, while L365,260 was inactive at similar doses. It is concluded that CCK8 released after a meal is responsible for the postprandial increase in colonic motility and that these effects may be mediated through activation of central CCKA receptors.     

Interaction of MIF-I or alpha-MSH with D-amphetamine or chlorpromazine on thermoregulation and motor activity of rats maintained at different ambient temperatures

Administration of several doses of MIF-I or alpha-MSH did not modify colonic temperature or the level of motor activity of rats in ambient temperatures of 4 degree or 20 degrees C. However, the thermoregulatory but not motor effects of the interaction between MIF-I or alpha-MSH with d-amphetamine were dependent upon ambient temperature. At 4 degree C, 1.0 mg/kg of both peptides enhanced the d-amphetamine-induced hypothermia, but at 20 degrees C both peptides blocked the hyperthermic effects of d-amphetamine. The hypothermic effect of chlorpromazine (CPZ) at 4 degree C and 20 degrees C was blocked by 1.0 mg/kg MIF-I but not by 1.0 mg/kg alpha-MSH. No linear dose response relationships between various doses of MIF-I or alpha-MSH and thermal responses were found. Administration of melanin or the use of hypophysectomized rats did not alter the significant interactions observed after peripheral injections.     

Cardiovascular and thermoregulatory responses to vasotocin and angiotensin II in the pigeon.

The cardiovascular and thermoregulatory effects of intrahypothalamically (preoptic/anterior hypothalamus) and intravenously injected arginine vasotocin (AVT) and [Val5]angiotensin II (ANG II) were measured at 2 degrees C in the pigeon (Columba livia). In addition, the effects of intrahypothalamic and intravenous injections of AVT on respiratory rates were measured at 10-15 degrees C. Intrahypothalamic and intravenous AVT (500 ng and 20 micrograms/kg, respectively) reduced shivering and body temperature but had no effects on blood pressure, heart rate or respiratory rate. Intrahypothalamic (500 ng and 1 microgram) and intravenous (3 micrograms/kg) ANG II elevated blood pressure. If the blood pressure increased slowly, the shivering and body temperature also increased, whereas a rapid rise in blood pressure inhibited shivering and lowered body temperature. Intravenous ANG II produced tachycardia but intrahypothalamic ANG II did not affect the heart rate.     

Antipyretic activity of a potent alpha-MSH analog

[Nle4,D-Phe7]-alpha-MSH has exceptional potency in certain biological assays of alpha-MSH activity such as skin darkening in frogs. However, this analog was equipotent to alpha-MSH in induction of grooming in the rat and had opposite effects on the performance of a visual discrimination task. These results led to the suggestion that distinct differences may exist between the melanocyte and CNS receptors for alpha-MSH. We determined the antipyretic and hypothermic potencies of centrally and peripherally administered [Nle4,D-Phe7]-alpha-MSH, relative to those of alpha-MSH, in the rabbit. Central injections of 40 and 80 ng of [Nle4,D-Phe7]-alpha-MSH caused hypothermia in afebrile rabbits, whereas 20 and 10 ng, which had no effect on afebrile body temperature, caused greater than 40% reduction in leukocytic pyrogen-induced fever. These results indicate that this analog is approximately 10 times more potent in reducing fever than alpha-MSH, making it the most potent antipyretic substance yet described. In contrast, IV administration of 16 micrograms of the analog, an extremely large dose relative to established antipyretic doses of alpha-MSH, elicited weak, variable responses. Since this analog is said to be resistant to degradation by serum enzymes, the contrast between the effects of central and peripheral administration may reflect a limited ability of the analog to cross the blood brain barrier when given IV. Our results do not suggest any distinct differences between the melanocyte receptors for alpha-MSH and those involved with CNS control of temperature. The marked central potency of [Nle4,D-Phe7]-alpha-MSH could result from an increased duration of action and/or a greater affinity for central receptor sites relative to alpha-MSH.