Prostaglandin fever in rats throughout the estrous cycle late pregnancy and post parturition.

We have examined the influence of natural variations in endocrine status on the ability to generate a prostaglandin-induced fever in virgin female, pregnant and lactating rats and compared responses to those in male rats. Endocrine status of virgin female rats was assessed from examination of vaginal smears and time of parturition noted to enable accurate dating of pre- and postparturient fevers. Unanesthetized rats, previously prepared with intraventricular guide cannulas and intraperitoneal telemetry thermistors, were given intraventricular injections of prostaglandin E1 (2-100 ng/5 microliters) and temperatures monitored for 3 h after injection. Virgin females developed significantly larger fevers than did males at higher doses. There were no significant alterations in either fever height or duration as a function of the phase of the reproductive cycle in the females. Both pregnant and postparturient rats within the several days around birth displayed significantly lower fevers than did virgin females, but there was no further reduction in the immediate periparturient period. These data indicate that there are sex-, and possibly hormone-dependent differences in the central mechanisms involved in fever generation and antipyresis.

Fever in pregnant, parturient, and lactating rats.

Conscious virgin, pregnant, or lactating rats were given intravenous Escherichia coli endotoxin while their temperatures were monitored telemetrically. Virgin females responded to 10-50 micrograms/kg endotoxin with a slight hypothermia, followed by a fever of nearly 2 degrees C magnitude. In pregnant rats given 25 micrograms/kg of the endotoxin, fevers were reduced between 96 h before and 24 h after parturition compared with those seen in virgins or in lactating rats > 24 h postpartum. In the 24-h period before expected time of parturition, no rat developed a fever and the majority of animals became hypothermic; furthermore, in 80% of such animals given 25 micrograms/kg endotoxin, the hypothermia was accompanied by death within 3-15 h. Some mortality and hypothermia were also seen up to 48 h before birth and up to 24 h after birth. No mortality was observed in virgin, pregnant, or lactating rats outside of this time period. We conclude that, around the time of delivery, there is a suppression of fever in the rat and occasional toxic responses to endotoxin.

Effect of potassium-induced cortical spreading depression on prostaglandin-induced fever in conscious and urethane-anesthetized rats.

Potassium-induced cortical spreading depression (CSD) on prostaglandin E1 (PGE1) induced fever has been investigated in a dose-responsive experimental design in both conscious and urethane-anesthetized adult male Sprague-Dawley rats. While CSD in itself had no effect on nonfebrile body temperature even under cold ambient conditions, CSD significantly suppressed small but not large fevers induced by intracerebroventricular PGE1. The increased oxygen consumption during fever was also reduced. We also explored the possible involvement of the antipyretic peptide arginine vasopressin, in the CSD-induced suppression of fever. Long term castrated rats have significantly reduced ventral septal levels of this peptide, yet CSD was effective in suppressing the initial 40 min of PGE1 fever in these animals. Thus we conclude that increased release of ventral septal arginine vasopressin is probably not involved in the action of CSD on fever.

Spinal and peripheral modulation of gastric acid secretion and arterial pressure by neuropeptide Y, peptide YY, and pancreatic polypeptide in rats.

Spinal and peripheral modulation of pentagastrin-stimulated gastric acid secretion by the pancreatic polypeptide-fold (PP-fold) peptides, neuropeptide Y (NPY), peptide YY (PYY), and pancreatic polypeptide (PP), in urethane-anesthetized rats was evaluated. Neuropeptide Y, PYY, and PP (400 pmol) were administered via intravenous (IV) and intrathecal (IT) injections. The alpha 2 antagonist, yohimbine, was used to evaluate the role of the alpha 2 adrenergic receptors in the modulation of pentagastrin-stimulated gastric acid secretion by NPY, PYY, and PP. Peptide YY and PP (IV) rapidly increased pentagastrin-stimulated gastric acid secretion. Peptide YY and PP (IT) increased pentagastrin-stimulated gastric acid secretion following administration into the thoracic (T8-T10) region of the spinal cord. The alpha 2 adrenergic receptor antagonist, yohimbine, did not modify the increases in pentagastrin-stimulated gastric acid secretion following PYY and PP (IV or IT) administration. Neuropeptide Y (IT) decreased pentagastrin-stimulated gastric acid secretion. However, in the presence of alpha 2 adrenergic receptor blockade, pentagastrin-stimulated gastric acid secretion was potentiated by NPY (IT) administration. Therefore, the inhibitory effect of NPY (IT) on pentagastrin-stimulated gastric acid secretion required the activation of alpha 2 adrenergic receptors in the spinal cord of rats. Mean arterial blood pressure (MAP) was increased immediately following NPY and PYY (IV) administration. During the same time period, PP (IV) decreased MAP in anesthetized rats. Mean arterial blood pressure was rapidly increased by NPY and PYY (IT) in anesthetized rats. The increase in MAP following PYY (IT) was partially attenuated in the presence of yohimbine.(ABSTRACT TRUNCATED AT 250 WORDS)

The peripheral modulation of duodenal and colonic motility in rats by the pancreatic polypeptide-fold family: neuropeptide Y, peptide YY, and pancreatic polypeptide.

Neuropeptide Y (NPY), peptide YY (PYY), and pancreatic polypeptide (PP) altered intraluminal pressure in the duodenum and colon of fasted anesthetized rats following intravenous bolus administration. There were rapid increases in intraluminal pressure of the duodenum and colon of anesthetized rats following peripheral injections of NPY, PYY and PP. Administration (IV) of NPY, PYY, and PP increased intraduodenal pressure +1.8, +3.2, and +3.7 mmHg compared to saline baseline. Prazosin, an alpha-2 adrenergic antagonist, did not alter the response of the duodenum of urethane-anesthesized rats to any of the PP-fold peptides following peripheral administration. Yohimbine, an alpha 2-adrenergic antagonist, attenuated the excitatory response of rat duodenum following NPY (IV) but did not alter the duodenal response to PP (IV). Intravenous NPY, PYY, and PP increased intracolonic pressure +2.0, +3.3, and +6.2 mmHg compared to saline baseline. In the presence of prazosin, an alpha 1-adrenergic antagonist, the intraluminal pressure of the colon increased +2.6, +2.4, and +8.1 mmHg compared to saline baseline by NPY, PYY, and PP (IV), respectively. In the presence of alpha 2-adrenergic blockade by yohimbine, NPY, PYY, and PP (IV) increased intraluminal pressure of the colon +4.2, +2.9, and +2.5 mmHg compared to saline baseline. The response of the duodenum to the excitatory effect of PYY (IV) was enhanced in the presence of yohimbine. Duodenal and colonic tone were modulated by the PP-fold peptides following peripheral administration. The alpha-adrenergic nervous system played only a minor role in the modulation of GI motility by the PP-fold peptides at peripheral sites.

Spinal modulation of duodenal and colonic motility and arterial pressure by neuropeptide Y, neuropeptide Y fragment 13-36, peptide YY, and pancreatic polypeptide in rats: involvement of the cholinergic nervous system.

The pancreatic polypeptide-fold (PP-fold) peptides, peptide YY (PYY) and pancreatic polypeptide (PP) (200 pmol), increased duodenal intraluminal pressure following intrathecal (IT) administration into the thoracic (T8-T10) spinal cord of urethane-anesthetized rats. Neuropeptide Y (NPY), PPY, and PP (IT) increased colonic intraluminal pressure of rats. The excitatory effects of the PP-fold peptides, NPY and PYY, were accompanied by increases in mean arterial pressure (MAP) during the same time period followed by a decrease to hypotensive levels. There were no further alterations of duodenal or colonic pressure in rats during the hypotensive period. The effect of PP (IT) on MAP was characterized by a pattern of hypotension frequently followed by a hypertensive period. The modulation of duodenal and colonic pressure does not differ between the members of the PP-fold family of peptides; however, the effects of the different members of the PP-fold family of peptides on MAP were varied. The Y2 receptor ligand, NPY (13-36) (200 pmol) (IT), did not alter duodenal and colonic pressure or MAP in rats. Therefore, the effects of PYY and NPY in the thoracic spinal cord on duodenal and colonic motility may be mediated via Y1 (postjunctional) receptors. Atropine, a muscarinic antagonist, attenuated NPY's (IT) excitatory effect on colonic pressure but did not alter the MAP response to this peptide. Atropine did not modify PYY's (IT) regulation of duodenal and colonic intraluminal pressure. However, atropine did attenuate PPY's inhibitory effect on MAP.(ABSTRACT TRUNCATED AT 250 WORDS)

Experimental characterization and applications of an anesthetized animal model for thermoregulatory investigations.

In the study of mechanisms of thermoregulation conscious animals are used and in most cases the conscious animal is the preferred choice. In some investigations however an anesthetized animal may be the model necessary for the practical purposes of the experimental design. We have developed, tested, and used an urethane anesthetized animal as a model in thermoregulatory investigations. Experiments have been completed in which we have shown that the adult male rat when anesthetized to a surgical level with urethane (1.5 gm/kg) cannot maintain its body temperature and becomes "poikilothermic-like". By providing a steady level of heat to the animal, body temperature can be stabilized for long periods of time at any basal level. This unique feature is not possible in conscious animals. Although apparently poikilothermic, these anesthetized animals respond with a dose-dependent fever to pyrogenic agents such as bacterial pyrogen, interleukin, and prostaglandin. Accompanying these fevers are physiological changes characteristic of fever including increases in arterial and central venous blood pressure, heart rate, oxygen consumption and shivering all of which are similar to the changes which occur in conscious animals. We have used this model for investigations on intracranial pressure, fever suppression and enhancement, application and recovery (for assay) of neuro-transmitters to and from specific brain loci, microdialysis, osmotic stimulation, and electrophysiological stimulation and recording.

Vasopressin-induced motor effects: localization of a sensitive site in the amygdala.

Arginine vasopressin (AVP) induces motor effects when administered into the cerebral ventricles, the ventral septal area (VSA), or the vestibular cerebellum of the rat brain. Because AVP-like immunoreactivity and AVP-binding sites exist in the central medial amygdala (cmeA), and because the amygdala can be kindled to produce motor effects, we hypothesized that the amygdala might play a role in AVP-induced motor effects. This hypothesis was tested by observing motor behavior in response to injection of AVP into the central medial region of the amygdala. Our results demonstrate that an initial injection of AVP into the cmeA caused minor motor effects, including immobility, prostration and ataxia, whereas a similar injection, given 24 h later, caused severe motor effects including barrel rotations and myoclonic/myotonic-like convulsive behavior. A potential receptor basis for the AVP-induced motor and sensitization effects in the cmeA was investigated using AVP analogues. A V1 antagonist, d(CH2)5Tyr(Me)AVP, blocked both the motor and sensitization effects produced by cmeA AVP injection. A V2 receptor agonist, DDAVP, did not affect motor activity upon cmeA injection, but did, however, sensitize animals to subsequent cmeA AVP injection. These results suggest that the cmeA is a sensitive site for AVP-induced motor effects and that these motor effects are sensitized by prior exposure to AVP. While the motor effects observed after cmeA AVP injection are mediated via AVP receptors that resemble the V1 type, the sensitization effect may be mediated via multiple receptor systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothalamic neuropeptide Y inhibits gastric acid output in rat: role of the autonomic nervous system.

Injection of neuropeptide Y (NPY) into the hypothalamic paraventricular nucleus (PVN) inhibits gastric acid secretion in anesthetized rats. The role of the autonomic nervous system in mediation of this response was investigated. Unilateral microinjection of 200 pmol NPY into the PVN of anesthetized rats inhibited spontaneous and pentagastrin-stimulated gastric acid output. Inhibition was abolished by subdiaphragmatic vagotomy, atropine, and bethanechol but was restored by electrical stimulation of the distal cut end of the vagus in cervically vagotomized rats. Although sympathectomy, phenoxybenzamine, and yohimbine abolished the inhibition, it was not affected by prazosin treatment. Gastric blood flow was not altered by injection of NPY. These results suggest that the antisecretory effect of NPY in the PVN was sympathetically mediated via suppression of gastric vagal cholinergic tone through activation of alpha 2-adrenoceptors.

Vasopressin perfusion within the medial amygdaloid nucleus attenuates prostaglandin fever in the urethane-anaesthetized rat.

The antipyretic effect of arginine vasopressin (AVP) introduced into the ventral septal area (VSA) by push-pull perfusion was investigated in the urethane-anaesthetized rat. In addition, experiments were carried out to determine whether AVP could suppress fever when similarly perfused within the medial amygdaloid nucleus (meA). During push-pull perfusion of artificial cerebrospinal fluid within the VSA or meA, PGE1 injected intracerebroventricularly evoked fevers with respective magnitudes of 1.3 +/- 0.2 degrees C and 1.4 +/- 0.3 degrees C above baseline. Perfusion of AVP (6.5 micrograms/ml) within the VSA had significantly reduced the magnitude of PGE1 fever to 0.3 +/- 0.3 degrees C above baseline, while having no significant effect on afebrile colonic temperature. Perfusion of AVP (6.5 micrograms/ml) within the meA had significantly attenuated the magnitude of PGE1 fever to 0.7 +/- 0.2 degrees C above baseline, while having no significant effect on afebrile colonic temperature. These results support further the utility of the urethane-anaesthetized rat model for future investigations of the central control of fever and antipyresis. In addition, these data are consistent with the hypothesis that AVP may act within the meA as an endogenous antipyretic.

Neuropeptide Y, peptide YY, and pancreatic polypeptide modulate duodenal and colonic motility at a thoracic spinal site in rats.

Neuropeptide Y, PYY, and PP (200 pmol) alter intraluminal pressure in the duodenum and colon of rats following their administration into the thoracic (T8-T10) region of the spinal cord. Neuropeptide Y decreases the tone of the duodenum and the colon following intrathecal (T8-T10) administration prior to an increase in tone to baseline or greater. There is no effect on intraluminal pressure of either the duodenum or the colon following intrathecal administration of NPY or PP into the lumbar (L4-L5) region of the spinal cord. Following intrathecal (T8-T10) administration of PYY and PP, increases in intraduodenal pressures are observed (+2.1 and +3.0 mmHg from saline baseline). Phasic contractions of the duodenum are increased following intrathecal administration of PYY into the thoracic spinal cord of rats. Neuropeptide Y, PYY, and PP increase intracolonic pressure +2.2, +3.3, and +3.7 mmHg from saline baseline, respectively. Phasic contractions of the colon are increased following PP intrathecal thoracic administration. Responsiveness of the duodenum or colon to the different ligands of the PP-fold peptide family in the absence of alpha-adrenergic blockade did not vary. The increases in intraluminal pressure of the duodenum and colon following intrathecal administration of the PP-fold peptides are attenuated by both alpha-1 adrenergic (prazosin) and alpha-2 adrenergic (yohimbine) blockade. There is a difference in responsiveness of the colon between the ligands of the PP-fold family in the presence of the alpha-2 adrenergic blockade. The findings of this study indicate that duodenal and colonic motility are modulated by the PP-fold peptides at thoracic spinal sites via alteration of sympathetic outflow.

Vasopressin-induced antipyresis in the medial amygdaloid nucleus of conscious rats.

Experiments were undertaken to characterize a possible receptor mediating antipyretic action of arginine vasopressin (AVP) within the medial amygdaloid nucleus (meA) in the conscious rat. Additional experiments were directed at determining whether the action of endogenously released AVP can be revealed in the meA during fever in the conscious rat. These objectives were achieved using vasopressin analogues directed against vasopressor (V1a) and antidiuretic (V2) receptors. Bilateral injection of AVP (40 pmol) into the meA of conscious rats suppressed fever evoked by intracerebroventricular (icv) administration of prostaglandin E1 (PGE1, 50 ng). The V2 receptor agonist 1-desamino-8-D-AVP (40 pmol) injected into the meA evoked only moderate antipyresis compared with AVP, possibly because of interaction of this agonist with V1a receptors. The antipyretic effect of AVP was blocked when injection of the peptide was preceded by a bilateral injection of the V1a antagonist 1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)-2-(O-methyl)tyrosine AVP [d(CH2)5Tyr(Me)AVP, 400 pmol] into the meA. Injection of d(CH2)5Tyr(Me)AVP alone into the meA was without significant effect on afebrile core temperature. Injection of d(CH2)5Tyr(Me)AVP or 1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)-2-D-valine,4-valine AVP (a V2 antagonist) alone into the meA before icv PGE1 resulted in fevers that were not significantly different from artificial cerebrospinal fluid controls. These data are consistent with the possibility that AVP might act within the meA to evoke antipyresis via receptors that resemble V1a (vasopressor) receptors. However, the action of AVP endogenously released into the meA does not appear to be an absolute requisite in the normal modulation of PGE1 fever.

Nucleus tractus solitarii lesions alter the metabolic and hyperthermic response to central prostaglandin E1 in the rat.

1. Given that the nucleus tractus solitarii (NTS) may regulate the ability of brown adipose tissue to evoke non-shivering thermogenesis and that brown fat may mediate the rise in whole-body metabolism observed following central pyrogen administration, we assessed whether interruption of baroreceptor afferents coursing though the NTS would interfere with the ability of prostaglandin E1 to evoke a normal fever response profile. 2. Infusion of 150-600 ng of prostaglandin E1 (PGE1) into a lateral cerebral ventricle of the conscious rat resulted in a rise in core temperature, and also an increase in whole-body metabolic rate, brown adipose tissue temperature, arterial blood pressure and heart rate. 3. Following bilateral electrolytic lesions to the NTS, resting core and brown fat temperatures, metabolic rates, blood pressures and heart rates in the NTS-lesioned animals were comparable to control rats. However, the PGE1-evoked increase in metabolic rate, along with the rise in core and brown adipose tissue temperatures and heart rate were attenuated. The pressor response was, however, enhanced, possibly due to the demonstrated interference by the lesions with normal baroreflex control. 4. The findings suggest that the nucleus tractus solitarii region of the rats' brain may be important in mediating the thermogenesis evoked by central PGE1.

Modulation of brown adipose tissue-mediated thermogenesis by lesions to the nucleus tractus solitarius in the rat.

Given that relatively little is known regarding the central control of brown adipose tissue (BAT)-mediated thermogenesis the present study assessed whether the direct pharmacological stimulation of beta- or alpha-adrenergic receptors located on the brown adipocytes would result in a typical thermogenic response following electrolytic lesions to the nucleus tractus solitarius (NTS). Bilateral electrolytic lesions to the NTS in the rat effectively disrupted the baroreceptor reflex arc. It was observed that the metabolic and temperature responses to either norepinephrine (1, 5, or 25 micrograms/kg/min) or to the beta-agonist isoproterenol (0.5 micrograms/kg/min) were significantly attenuated in the NTS-lesioned rats relative to the control animals with an intact baroreflex. Conversely, the cardiovascular effects of norepinephrine or of the alpha-agonist phenylephrine (10 micrograms/kg/min) were enhanced in the NTS-lesioned animals. The results suggest that the functional capacity of the brown adipocytes was reduced following NTS lesions and points to an alteration in the ability of beta-receptors to respond to pharmacological stimulation with a typical thermogenic response.

Contribution of brown adipose tissue to central PGE1-evoked hyperthermia in rats.

The relative contribution of several effector systems to a prostaglandin E1-(PGE1) evoked hyperthermia was examined. Infusion of 150 ng of PGE1 into a lateral cerebral ventricle increased core temperature and whole body metabolic rate, brown adipose tissue temperature, systolic blood pressure, and heart rate. Pretreating the animals with a nonselective beta-antagonist propranolol (1 mg/kg iv in 0.3 ml followed by 3 mg.kg-1.h-1 in 0.3 ml/h) not only attenuated the rise in metabolism observed after the central administration of 150 ng PGE1 but also diminished the elevation in both core and brown fat tissue temperatures as well as the increase in heart rate. Pretreating the animals with the alpha-antagonist prazosin (2 mg/kg im followed by 50 micrograms.kg-1.h-1 iv in 0.3 ml/h) somewhat reduced the rise in whole body metabolism, suppressed the elevation in core temperature, but failed to alter the rise in brown adipose tissue temperature normally seen after the central administration of PGE1. Moreover, both the rise in systolic blood pressure and heart rate were attenuated when the PGE1 administration was preceded by prazosin. These results suggest that brown adipose tissue is an important effector organ responsible for mediating the hyperthermic response observed after the intracerebral injection of PGE1. In addition, the results indicate that alterations in vasomotor tone may also be important in producing or sustaining the elevated core temperature found after a pyrogen administration.

Vasopressin and oxytocin in rat brain in response to prostaglandin fever.

Urethan-anesthetized rats were used to identify effective stimuli for the release of the peptides arginine vasopressin (AVP) and oxytocin into the ventral septal area (VSA) of the brain. Febrile responses to intracerebroventricular injection of prostaglandin E1 (PGE1) were observed in rats whose body temperatures were maintained at 35, 37, or 39 degrees C. Microinjection of the AVP antagonist d(CH2)5Tyr(Me)AVP into the VSA enhanced fever only when PGE1 administration was associated with a significant rise in body temperature. Passive elevation ("artificial fever") or reduction of body temperature in the absence of a PGE1 stimulus was not affected by the antagonist. Push-pull perfusion of the VSA and the dorsal hippocampus, followed by radioimmunoassay of perfusates for AVP and oxytocin, revealed enhanced release into the VSA of AVP only when PGE1 administration was followed by a rise in body temperature. Oxytocin was released whenever body temperature was raised. Peptide concentrations in simultaneous perfusates of dorsal hippocampus did not change in response to PGE1 administration or to passive elevation of body temperature. We conclude that AVP is released into the VSA, but not the dorsal hippocampus, of the rat during a fever induced by PGE1. Oxytocin is released into the VSA, but not the hippocampus, when temperature is elevated.

Depletion of brain alpha-MSH alters prostaglandin and interleukin fever in rats.

Alpha-melanocyte stimulating hormone (alpha-MSH), a putative endogenous antipyretic agent, is synthesized largely within neurons in the arcuate nucleus. To test the hypothesis that destruction of this area would increase the febrile response, male Wistar rats, treated as neonates with intraperitoneal injections of monosodium glutamate (MSG) or saline, were given intracerebroventricular (i.c.v.) injections of prostaglandin E1 (20 ng; 200 ng) or purified interleukin-1 (20 U) and body temperature was monitored. The fevers displayed by the MSG-treated animals were significantly greater (P less than 0.05) than those of the controls for the lower dose of PGE1 at 10-30 min and for IL-1 at 3-6 h after the injections. MSG-treated rats showed significant reduction (P less than 0.01) in alpha-MSH content of the medial basal hypothalamus and lateral septum when compared to saline controls. Body temperature response of non-febrile animals to high ambient temperature was not affected by the MSG treatment. These data support the hypothesis that alpha-MSH is an endogenous antipyretic in the rat.

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.

Cerebrospinal fluid pressure in conscious rats during prostaglandin E1 fever.

Intraperitoneal body temperature was monitored from groups of 10 conscious adult male Wistar rats. The daily body temperature rhythm was determined, and an intraperitoneal body temperature dose response relationship was established for prostaglandin E1 (PGE1) delivered into a lateral cerebral ventricle (LCV). Cerebrospinal fluid pressure was measured by direct cannulation of a LCV. Heart rate, arterial and central venous blood pressure, and blood gases were also measured in groups of 10 animals. It was found that there was a daily rhythm in intraperitoneal body temperature that was higher at night than during the day but stable between 0800 and 1600 h. A significant temperature dose (20-2,000 ng)-response relationship was established for administration of PGE1 into the LCV. Cerebrospinal fluid pressure when measured from the LCV increased significantly during the "chill" phase of the PGE1-induced fever. Carotid arterial blood pressure also significantly increased at this time as did central venous pressure, particularly so with vigorous shivering. There was a significant fall in arterial CO2 partial pressure, a slight rise in pH, and no change in arterial O2 partial pressure. These data support the hypothesis that a significant increase in cerebrospinal fluid pressure occurs during the chill phase of a PGE1-induced fever in the conscious rat.