Temperature-sensitive neurons in preoptic-anterior hypothalamic region: actions of pyrogen and acetylsalicylate

Current perspectives in NSAID-induced gastropathy

Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most highly prescribed drugs in the world. Their analgesic, anti-inflammatory, and antipyretic actions may be beneficial; however, they are associated with severe side effects including gastrointestinal injury and peptic ulceration. Though several approaches for limiting these side effects have been adopted, like the use of COX-2 specific drugs, comedication of acid suppressants like proton pump inhibitors and prostaglandin analogs, these alternatives have limitations in terms of efficacy and side effects. In this paper, the mechanism of action of NSAIDs and their critical gastrointestinal complications have been reviewed. This paper also provides the information on different preventive measures prescribed to minimize such adverse effects and analyses the new suggested strategies for development of novel drugs to maintain the anti-inflammatory functions of NSAIDs along with effective gastrointestinal protection.

Hypothermic effect of sodium acetysalicylate on afebrile monkeys

1 Afebrile monkeys (Macaca cyclopis) receiving systemic (100-300 mg/kg, i.p.) or central (5-20 mg into the 3rd cerebral ventricle) administration of sodium acetylsalicylate showed a dose-dependent reduction in rectal temperature in a thermoneutral environment (25 degrees C).2 Administration of sodium acetylsalicylate (10 mg) into the 3rd cerebral ventricle produced a hypothermia with a temperature decrement of 1.0 degrees C, ehile an intraperitoneal injection of 300 mg/kg was required for a temperature decrement of 0.9 degrees C. The ratio between the total doses given by the two toutes was 1 to 120.3 Following the administration of sodium acetylsalicylate, a decline in rectal temperature was accompanied by a tail cutaneous vasodilatation.4 The data suggest that sodium acetylsalicylate can lower the normal body temperature by activating heat loss or decreasing the normal (tonic) inhibition of the heat loss mechanism via the central nervous system.

Thermosensitivity is reduced during fever induced by Staphylococcus aureus cells walls in rabbits

Thermosensitivity (TS) and threshold core temperature for metabolic cold defence were determined in six conscious rabbits before, and at seven different times after i.v. injection of killed Staphylococcus aureus (8 x 10(7) or 2 x 10(7) cell walls x kg(-1)) by exposure to short periods (5-10 min) of body cooling. Heat was extracted with a chronically implanted intravascular heat exchanger. TS was calculated by regression of metabolic heat production (M) and core temperature, as indicated by hypothalamic temperature. Threshold for cold defence (shivering threshold) was calculated as the core temperature at which the thermosensitivity line crossed preinjection resting M. The shivering thresholds followed the shape of the fever response. TS was significantly reduced (up to 49%) during the time course of fever induced by the highest dose of pyrogen only. At both high and low doses of pyrogen TS correlated negatively with shivering threshold (r = 0.66 and 0.79 respectively) with similar slopes. The reduction in TS during fever was thus associated with the increase in shivering threshold resulting from the pyrogen injection and not by the dose of pyrogen. Model considerations indicate, however, that changes in sensitivity of the thermosensory input to the hypothalamic controller may affect threshold changes but cause negligible TS changes. It is more likely that the reduction in TS is effected in the specific hypothalamic effector pathways.

Hypothalamic and thalamic sites of action of interleukin-1 beta on food intake, body temperature and pain sensitivity in the rat

Interleukin-1beta (IL-1beta ) has anorectic, hyperthermic, and analgesic or hyperalgesic (depending on the studies) effects in the rat. These effects appear to be mediated by the central nervous system; however, the exact localization of action of IL-1beta in the brain has never been delineated with precision. The purpose of this study was to determine precisely where IL- IO acts in the hypothalamus and in the thalamus to modulate food intake, body temperature, and pain sensitivity. Animals were tested after local intracerebral microinjections of 5 ng of IL-1beta dissolved in 0.3 microl of saline, or of 0.3 microl saline alone. The results show that IL-1beta has anorectic effects in 3 diencephalic sites (the perifornical area, an area above the optic chiasma, and an area internal to the mamillo-thalamic tract), and not in 9 other sites tested. IL-1beta has hyperthermic effects in 7 sites (the media] and lateral preoptic area, the hypothalamic periventricular substance, the dorso-medial and arcuate nuclei of the hypothalamus, and the centro-medial and gelatinosus nuclei of the thalamus), and not in 6 other sites. IL-1beta has analgesic effects in the centro-medial and gelatinosus nuclei of the thalamus, and not in 7 other sites. IL-1beta also increases food intake and decreases pain sensation thresholds in the paraventricular nucleus of the hypothalamus. Therefore IL-1beta has very selective anatomical sites of action in the brain, and the paraventricular nucleus of the hypothalamus appears to have special properties regarding the effects of IL-1beta on food intake and pain sensation regulation.

Thermal and PGE2 sensitivity of the organum vasculosum lamina terminalis region and preoptic area in rat brain slices

1. The effects of local applications of prostaglandin E2 (PGE2) on the unit activity of fifty-one neurones in the organum vasculosum lamina terminalis (OVLT) region and fifty-eight neurones in the preoptic area (POA) were investigated in small tissue slices from the rat hypothalamus containing the OVLT and POA isolated from each other. 2. Of these, thirty OVLT and twenty-eight POA neurones were warm sensitive and increased their discharge rate in response to a rise in tissue temperature. One OVLT neurone and one POA neurone were cold sensitive and showed the opposite type of responses to changes in temperature. The thermosensitivity of these neurones was still observed in a Ca2+ free-high Mg2+ solution. 3. Perfusion with PGE2 in doses between 1 and 250 nM changed the discharge rate in forty-two of fifty-one OVLT neurones and in thirty-two of fifty-eight POA neurones in a dose-dependent manner. The responses to PGE2 were not lost during synaptic blockade. The threshold dose of PGE2 to alter the discharge rate of the OVLT neurones (4.8 +/- 1.1 (S.E.M.) nM, n = 16) was significantly lower than that of the POA neurones (40.9 +/- 12.2 nM, n = 16). 4. Fifteen of forty-two OVLT neurones exhibited the responses with a slower onset (latency 5-13 min) and a longer duration (20 min to 3 h), but such responses were observed in only one of thirty-two POA neurones. 5. The responses of OVLT and POA neurones to PGE2 (50-250 nM) were reversibly blocked by a concurrent application of AH6809, a prostanoid EP1 and/or a DP receptor antagonist. 6. While there was no clear correlation between the type of thermosensitivity and the type of response to PGE2 among the POA neurones, a significantly higher incidence of inhibitory response to PGE2 was found among the warm-sensitive neurones in the OVLT region. 7. The lower threshold responses to PGE2 and the higher incidence of PGE2 responsiveness among OVLT neurones are consistent with previous findings which showed that the highest density of PGE2 receptor binding and the highest pyrogenic sensitivity to microinjected PGE2 were observed in the OVLT region. The results provide further evidence for the critical involvement of the OVLT region in mediating the febrile responses to blood-borne endogenous pyrogen through the local release of PGE2.

Human recombinant tumor necrosis factor and interferon affect the activity of neurons in the organum vasculosum laminae terminalis

It is generally thought that fever is induced by blood-borne cytokines via an action on hypothalamic thermosensitive neurons. Recent studies suggest that the organum vasculosum laminae terminalis (OVLT), which lacks the blood-brain barrier, may be necessary for fever induction by systemic pyrogens. We have examined the responses of neurons in the OVLT to the pyrogenic cytokines, human recombinant tumor necrosis factor alpha (TNF) and interferon alpha 2 (IFN), by recording extracellular single-units in brain slice preparations from guinea pigs. Of all the OVLT neurons tested with TNF (900-5000 ng/ml perfusate/min) and IFN (1200-8500 units/ml perfusate/min), administered for 2.5 min, 44% increased their firing rates (FR) for 30.6-57.5 min (average, 47.4 min) with onset latencies of 6.1-9.8 min (average, 7.9 min). The remaining 56% of the neurons did not change their FR after TNF or IFN. Carrier vehicles for these cytokines produced no FR change. The results suggest the possibility that the OVLT may be a site where chemical signals of blood-borne cytokines are transduced into neuronal signals.

Response of thermoresponsive neurons of rat's hypothalamus during intracarotid infusion of prostaglandin E2

We investigated the effect of intracarotid infusion of prostaglandin E2 (PGE2) on the single-unit activity of neurons in the preoptic and anterior hypothalamic (PO/AH) region. A total of 35 neurons in the PO/AH region were examined: 10 were warm-responsive neurons, 6 were cold-responsive neurons, and 19 were thermally insensitive neurons. In the warm-responsive neurons, the activities of 4 neurons were facilitated, while 3 neurons were inhibited and 3 neurons were not affected after intracarotid infusion of PGE2. In the cold-responsive neurons, the activities of 2 neurons were facilitated and 4 neurons were not affected by PGE2 infusion. Furthermore, 15 out of 19 thermally insensitive neurons were not affected by PGE2 infusion. These results suggest that thermoresponsive neurons of the PO/AH region respond well to intracarotid infusion of PGE2, compared with thermally insensitive neurons. However, the direction of neuronal response induced by intracarotid infusion of PGE2 could not be generally categorized based on the thermoresponsiveness of individual neurons.

Brain regions involved in the development of acute phase responses accompanying fever in rabbits

1. The effects of microinjection of rabbit endogenous pyrogen and human recombinant interleukin-1 alpha on rectal temperature and acute phase responses were extensively examined in forty different brain regions of rabbits. The acute phase responses that were investigated were the changes in plasma levels of iron, zinc and copper concentration and the changes in circulating leucocyte count. 2. The rostral hypothalamic regions, such as nucleus broca ventralis, preoptic area and anterior hypothalamic region, responded to the microinjection of endogenous pyrogen or interleukin-1 by producing both fever and acute phase responses. 3. The microinjection of endogenous pyrogen or interleukin-1 into the rostral hypothalamic regions significantly decreased the plasma levels of iron and zinc concentration 8 and 24 h after injection. The circulating leucocyte count increased 8 h after injection. However, neither the injections of endogenous pyrogen nor interleukin-1 affected the number of red blood cells. 4. The present results show that the rostral hypothalamic regions respond directly to endogenous pyrogen or interleukin-1 with the consequent development of fever and acute phase responses.

Effect of prostaglandin E2 on thermoresponsive neurones in the preoptic and ventromedial hypothalamic regions of rats

1. We investigated the effect of microinjection of prostaglandin E2 (PGE2) into the preoptic (POA) or the ventromedial hypothalamic (VMH) region on rectal temperature in rats. Fever was induced by microinjection of PGE2 into the POA or the VMH regions. The febrile responses induced by PGE2 injected into the VMH region were significantly greater than those induced by injection into the POA region. 2. The effect of temperature on neuronal activity in the POA and the VMH regions was investigated by using slice preparations from rats. It was revealed that there exist many thermoresponsive neurones in the VMH region as well as in the POA region, and that the proportion of thermoresponsive neurones out of the total neurones examined in the VMH region was almost identical to that in the POA region. In addition, the warm-responsive neurones in the VMH region exhibited larger thermal coefficients than those in the POA region. 3. When PGE2 was applied in a recording chamber where the tissue slice was perfused, most of the neurones in the VMH region which responded to PGE2 showed a decrease in their firing rate, while those in the POA region showed an increase in their firing rate, regardless of their thermoresponsiveness. In the POA region, PGE2 began to affect the activities of the warm-responsive neurones in the range of 5 x 10(-7) to 7 x 10(-6) M, whereas maximum responses were obtained between the concentrations of 5 x 10(-6) and 5 x 10(-5) M. In the VMH region, PGE2 began to change the activities of the warm-responsive neurones in the range of 5 x 10(-8) to 5 x 10(-7) M, and the maximum effect of PGE2 on the VMH warm-responsive neurones occurred between the concentrations of 8 x 10(-7) and 4 x 10(-5) M. 4. The present results show that neurones exhibit different responsiveness to PGE2 and different sensitivity to PGE2 between the POA and the VMH regions. Nevertheless, microinjection of PGE2 into either the POA or the VMH region produces fever. Therefore, it is suggested that fever is produced by complex neuronal networks in the central nervous system.

Effects of interleukin-1 and arachidonate on the preoptic and anterior hypothalamic neurons

Effects of microelectrophoretic application of ultrapure human interleukin-1 (IL-1), an endogenous pyrogen, on the activity of 80 neurons in the preoptic and anterior hypothalamus (PO/AH) were investigated in the urethane anesthetized rat. IL-1 predominantly decreased the activity of warm-sensitive neurons (15 of 19) and increased the activity of cold-sensitive neurons (10 of 12), but had no effect on 37 of 49 thermally insensitive neurons. The neuronal responses to IL-1 were blocked or attenuated by concurrent application of mepacrine (a phospholipase inhibitor) or sodium salicylate (a cyclooxygenase inhibitor). Local application of sodium arachidonate decreased the activity in 17 of 28 warm-units and excited 12 of 16 cold-units, and the effects of arachidonate were blocked by sodium salicylate. The results are compatible with the view that one or more cyclooxygenase metabolites of arachidonic acid are involved in the IL-1 induced fever.

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.

Effects of preoptic microinjections of alpha-MSH on fever and normal temperature control in rabbits

alpha-MSH within the septal region of the brain has been implicated in fever control; this peptide and ACTH (1-24), which contains the alpha-MSH amino acid sequence, reduce fever when given intracerebroventricularly (ICV) or peripherally. These peptides also cause hypothermia when given in doses larger than those required to reduce fever. Both peptides occur naturally within the preoptic PO region of the brain, the CNS locus of primary temperature control. alpha-MSH (350 ng) injected bilaterally into the PO region via chronic cannulas reduced fever caused in six rabbits by IV injection of IL-1 (interleukin 1, endogenous or leukocyte pyrogen) but had no effect in afebrile animals. A larger dose (1.5 micrograms) not only reduced fever but caused hypothermia in 12 rabbits. In separate experiments PO injections of ACTH (1-24) (1 microgram) reduced normal temperature. In the same six rabbits alpha-MSH (1 microgram) caused slightly smaller hypothermia. alpha-MSH (1.5 micrograms) also had no effect in 8 afebrile rabbits when injected into the septum. The primary conclusion is that alpha-MSH receptors within the PO region can contribute to both the antipyretic and hypothermic actions that are observed after ICV and peripheral administration of the peptide.

The ventral septal area: electrophysiological evidence for putative arginine vasopressin projections onto thermoresponsive neurons

The present experiments were conducted to identify thermoresponsive neurons in the ventral septal area and to characterize such units with respect to their connectivity to potential sources of arginine vasopressin in this area (the paraventricular nucleus and the bed nucleus of the stria terminalis) and to other brain regions (fornix and amygdala). Single unit in vivo microelectrode techniques were used to classify warm responsive, cold responsive, dynamic, biphasic and phasically active thermoresponsive neurons in the ventral septal area which altered their spontaneous activity in response to thermal stimulation of the scrotal skin. The fornix provided a large number of primarily inhibitory afferents to ventral septal thermoresponsive neurons while the amygdala projection consisted of approximately equal excitatory and inhibitory inputs. Electrical stimulation of the paraventricular nucleus resulted in orthodromic inhibition in 9 of 12 thermoresponsive units while projections from the bed nucleus of the stria terminalis appeared to preferentially inhibit warm responsive neurons in this area. These findings implicate the ventral septal area in thermoregulatory pathways by identifying neurons in this area responsive to peripheral thermal stimulation. Further, evidence in support of arginine vasopressin acting in this area to influence thermoregulation is provided by the identification of the paraventricular nucleus and bed nucleus of the stria terminalis as possible sources of afferents to the ventral septal area, and the characterization of the influence of this afferent input on thermoresponsive neurons in this region.

Interaction between the effects of centrally administered arecoline and leucocyte pyrogen on the activity of posterior hypothalamic neurons in the rabbit

In experiments with urethane-anesthetized rabbits, the alteration in the activity of posterior hypothalamic neurons resulting from intracerebroventricular injection of leucocyte pyrogen was attenuated by subsequent administration of arecoline. Atropine failed to alter the neuronal response to leucocyte pyrogen but abolished the effect of arecoline. The neuronal response to arecoline was reversed in the absence of leucocyte pyrogen.

Effects of endotoxin and sodium salicylate on the preoptic thermosensitive neurons in tissue slices

Effects of E. coli endotoxin and sodium salicylate (Sal) on single-unit activity of thermosensitive neurons recorded in slices of preoptic and anterior hypothalamic area (PO/AH) were studied in vitro. Perfusion of endotoxin-containing Krebs-Ringer's solution or local application of endotoxin in the immediate vicinity of recording neurons decreased and increased the firing rate of 31 of 34 warm-sensitive neurons and a cold-sensitive neuron, but had no effect on the majority of thermally insensitive neurons. In about half of warm-sensitive neurons the inhibitory response to endotoxin was preceded by a transient increase in firing rate. The pyrogen-induced decrease in firing rate in warm-sensitive neurons was reversibly blocked or attenuated by local application of Sal in a dose-dependent manner. The results are consistent with the view that pyrogen and Sal act in the PO/AH to produce fever and antipyresis, respectively, by appropriately offsetting the activity of thermosensitive neurons.

Comparison of the action of prostaglandin with endotoxin on thermoregulatory response thresholds

Prostaglandin E2 (PGE2) and lipopolysaccharide (LPS) derived from E. coli were injected into the lateral cerebral ventricle of rabbits at 30 degrees C ambient temperature. The threshold core temperatures for ear cutaneous vasoconstriction (Thv) and shivering (Thsh) were determined by whole-body cooling with an intestinal thermode. Each threshold, as determined at the plateau phase of LPS fever and PGE2 hyperthermia respectively, were compared with the control values before LPS and PGE2 injection. Thsh was not changed by the injection of LPS, while Thv was increased. After PGE2 injection both Thsh and Thv were increased in comparison to their control levels. These changes paralleled the elevation of core temperature. The present study does not exclude prostaglandins as humoral mediators involved in some of the central processes generating fever, but suggest at the same time that there are additional properties of LPS fever for which prostaglandins do not account.

Effects of single cortical spreading depression on metabolic heat production in the rat

The effect of a single cortical spreading depression (CSD), elicited unilaterally by 10% KCI injection at the occipital cortex, was observed on the metabolic heat production in the rat with unilateral lesions in the preoptic and anterior hypothalamus (PO/AH). The metabolic rate increased by maximally 20% above its pre-CSD level for 7 min with a rise in rectal temperature when CSD entered the frontal cortex contralateral to the PO/AH lesion. By contrast, with the same onset, the increase in metabolic rate was lower (maximum 12%) and of shorter duration (3 min) in the ipsilateral CSD trial. Non-CSD trial with 0.9% NaCl injection had no effect on metabolic rate. The results provide further evidence to support the view that the frontal cortex of the rat is involved in the central thermoregulation.

Differential acute-phase responses in febrile and cold- and heat-exposed rabbits

Endotoxin (LPS) characteristically provokes both fever and the acute-phase reaction (APR). The present studies were undertaken to determine whether these two responses are mutually dependent or controlled separately. Rabbits were injected with LPS (2 micrograms/kg, IV) or exposed to heat (37 +/- 1 degree C) such that the patterns and magnitudes of the body temperature (Tco) rises (1.6 +/- 0.5 degrees C) were similar. While the plasma levels of Fe and Zn decreased, and those of Cu and N-acetylneuraminic acid (NANA) increased significantly after LPS-induced fever, no such changes occurred after heat-induced hyperthermia. In subsequent study, different rabbits were exposed to cold (3 +/- 1 degree C), first furred, then sheared. While furred, they thermoregulated by cutaneous vasoconstriction, whereas, while shaved, they additionally increased their metabolic rates. However, in neither case were the plasma levels of Fe, Zn, Cu, or NANA different in comparison to their pre-cooling values (Ta = 22 +/- 1 degree C). It would seem, therefore, that the acute-phase blood responses to LPS (and ultimately, to endogenous pyrogen [EP]) probably are not mediated merely by the rise in the Tco or by the concomitantly induced vascular and/or thermogenic effectors of fever; but rather, the APR appears to be a regulated, separate response to LPS/EP.

Effects of leukocytic pyrogen and sodium salicylate on hypothalamic thermosensitive neurons in vitro

The effects of leukocytic pyrogen and sodium salicylate (Sal) were studied on the single-unit activities of neurons in the preoptic and anterior hypothalamus (PO/AH) recorded in tissue slices in vitro. The pyrogen applied to the immediate vicinity of the recorded neurons decreased the firing rate of the majority of warm-sensitive neurons, but had no effect on the activities of thermally insensitive neurons. The pyrogen-induced inhibition of warm-sensitive neurons was reversibly blocked by local application of Sal. The results support the view that the pyretic and antipyretic effects of intrahypothalamic pyrogen and Sal are caused by their offsetting actions on the PO/AH thermosensitive neurons.

Activity of hypothalamic thermosensitive neurons during cortical spreading depression in the rat

Effects of single waves of cortical spreading depression (CSD) on unit activity of the ipsilateral preoptic and anterior hypothalamic (PO/AH) neurons were studied in urethane-anesthetized rats. When CSD entered the frontal cortex, the firing rate of the majority (85%, 22/26 units) of PO/AH warm-sensitive neurons decreased for 1.4-14.2 min and the remaining 4 neurons were unaffected. In contrast, 10 (77%) of the 13 cold-sensitive neurons increased their firing rates for 1.2-10.4 min after the CSD entrance of the frontal cortex. Of the remaining 3 neurons, two decreased their firing rates and one showed no change during CSD. Thirteen (76%) of the 17 thermally insensitive neurons were not affected by CSD, but the remaining 4 neurons decreased their firing rates slightly for about 1.4 min. The most critical cortical region responsible for the activity change of the PO/AH thermosensitive neurons during CSD was found to lie in the sulcal prefrontal cortex by observing the effect of a single CSD elicited separately from the occipital and the frontal cortex on the same PO/AH neurons (a double-CSD technique). The result, together with the previous ones on thermoregulatory responses during the frontal CSD, suggests that the sulcal prefrontal cortex exerts a tonic influence on the activity of the hypothalamic thermosensitive neurons and is involved in the central control of thermoregulation.

Reduced hypothalamic thermosensitivity following intraventricular injection of pyrogen in conscious rabbits

Hypothalamic temperature thresholds for vasodilatory and respiratory reactions were determined before and after i.c.v. injection of pyrogen in rabbits. During the rising phase of fever the increases in the hypothalamic thresholds for vasodilatory and respiratory reactions differed from those found in the pre-pyrogen preoptic anterior hypothalamic area (POAH) heating by 2.1 +/- 0.2 degrees C and 1.89 +/- 0.31 degrees C, respectively. During the plateau phase of fever the threshold for vasodilatory reaction was further increased (by 0.7 +/- 0.23 degrees C), whereas that for panting remained at the same level. It is concluded that pyrogen exerts a depressive action on POAH thermosensitivity.

Impairment of thermoregulatory cooling behavior by single cortical spreading depression in the rat

Effects of single waves of unilateral cortical spreading depression (CSD) on skin-cooling operant behavior were studied in the rat with and without unilateral lesion in the preoptic and anterior hypothalamus (PO/AH). The skin-cooling behavior was severely suppressed for 6 min, with a resulting rise in rectal temperature (Tre), when CSD entered the frontal cortex contralateral to the PO/AH lesion. However, CSD ipsilateral to the PO/AH lesion slightly suppressed the skin-cooling behavior for 4 min with small rise in Tre. In the PO/AH intact rat, while 0.9% NaCl did not affect the skin-cooling behavior, unilateral CSD slightly suppressed it for 3 min almost in the same way as that of the ipsilateral CSD trial in the PO/AH lesioned rat. The results add further evidence to support the involvement of the frontal cortex in the central control of thermoregulation.

Vasopressin: a homeostatic effector in the febrile process

This review compares the physiological changes which accompany infection and fever with the effects of the peptide, arginine vasopressin (AVP). AVP may act as a neuromodulator, a releasing factor, or a hormone to induce responses which are opposite to those homeostatic changes accompanying fever. Since AVP is released into blood and brain during fever, it is hypothesized that AVP contributes to the maintenance of homeostasis in the infected organism.

Changes in body temperature after administration of antipyretics, LSD, delta 9-THC, CNS depressants and stimulants, hormones, inorganic ions, gases, 2,4-DNP and miscellaneous agents

This survey concludes a series of complications of data from the literature, primarily published since 1965, on thermoregulatory effects of antipyretics in afebrile as well as in febrile subjects, LSD and other hallucinogens, cannabinoids, general CNS depressants, CNS stimulants including xanthines, hormones, inorganic ions, gases and fumes, 2,4-dinitrophenol and miscellaneous agents including capsaicin, cardiac glycosides, chemotherapeutic agents, cinchona alkaloids, cyclic nucleotides, cycloheximide, 2-deoxy-D-glucose, dimethylsulfoxide, insecticides, local anesthetics, poly I:poly C, spermidine and spermine, sugars, toxins and transport inhibitors. The information listed includes the species used, route of administration and dose of drug, the environmental temperature at which the experiments were performed, the number of tests, the direction and magnitude of body temperature change and remarks on the presence of special conditions such as age or lesions, or on the influence of other drugs, such as antagonists, on the response to the primary agents.

Effect of calcium removal on thermosensitivity of preoptic neurons in hypothalamic slices

Extracellular single-unit activities of thermosensitive neurons were recorded from the medial preoptic area in rat's hypothalamic slices in vitro. Of 256 preoptic units, 74 were warm-units which increased firing rates in response to a rise in slice temperature and 16 units were cold-units having the opposite type of thermosensitivity. Eighteen of 23 warm-units and 3 of 4 cold-units retained their thermosensitivities during perfusion with Ca2+-free/high Mg2+ salt solution. The thermosensitivities of the remaining 5 warm-units were reversibly abolished in the Ca2+ deficient medium. The results suggest that some warm- and cold-sensitive neurons in the medial preoptic area have an inherent thermosensitivity.

Effect of delta 9-tetrahydrocannabinol on hypothalamic thermosensitive units

Thermosensitive anterior hypothalamic neurons (pre-optic region) were studied in urethane and chloralose anesthetized cats in an attempt to characterize the hypothermic action of delta 9-THC at the neuronal level. One hundred and seventy-eight single neurons were isolated and subjected to thermal challenge, 66 were found to reproducibly alter firing frequency at a significant level (thermosensitivity (T.S.) greater than 0.75). Twenty-one of these units met the criteria for primary thermodetectors, 34 were heat-sensitive interneurons, and 11 were cold-sensitive interneurons. Administration of delta 9-THC (1.0-2.0 mg/kg i.v.) decreased the spontaneous firing and increased the T.S. of the primary thermodetector units. delta 9-THC also increased the spontaneous firing frequency as well as the T.S. of heat-sensitive interneurons, while decreasing both the T.S. and spontaneous firing of cold-sensitive interneurons. The decreased spontaneous firing of primary thermodetectors could result from altered facilitory or inhibitory influences converging on these cells. The increased thermosensitivity is consistent with the hypothesis that the pre-optic region modulates cannabinoid-induced hypothermia.

Effects of prostaglandin E2 on the activity of thermosensitive and insensitive single units in the preoptic/anterior hypothalamus of unanesthetized rabbits

Toe eliminate depressant effects of anesthetics on neuron activity, we recorded the single unit activity of thermoregulatory neurons in the POAH of unanesthetized rabbits. Intraventricularly applied PGE2 induced consistent excitatory effects (190% increase in firing rate) on cold-excitable cells and inhibited the firing rate (50%) of warm-sensitive neurons. Single units that were insensitive or had uncreelatable changes in firing rate with POAH temperature were either facilitated or inhibited by PGE2. The consistent effects of PGE2 on the thermoregulatory neurons found in this study support the proposal of PGE modulation of thermoregulatory neurons during the development of a fever.

Study on the possible entry of bacterial endotoxin and prostaglandin E2 into the central nervous system from the blood

1 A study has been made of the possible entry of 51Cr-bacterial endotoxin and [5,6,8,11,12,14,15(n)-3H]-prostaglandin E2 ([3H5-PGE2) into the CNS of the anaesthetized cat. 2 No radioactivity was detected in perfusates of the preoptic-anterior hypothalamus or in the cerebrospinal fluid (c.s.f.) in vivo, or in brain tissue post mortem following intracarotid infusion of 51Cr-bacterial endotoxin. 3 Intracarotid administration of [3H]-PGE2 resulted in the entry of radioactivity into the CNS of endotoxin pretreated cats. Chromatographic analysis indicated the radioactivity in c.s.f. to be associated with PGE2 and a metabolite similar to 13, 14-dihydro-15-keto PGE2. 4 Intracarotid administration of 13, 14-dihydro-15-keto [5,6,8,11,12,14(n)-3H]-PGE2 resulted in the presence of the compound in the CNS of the anaesthetized cat after pretreatment with bacterial endotoxin. 5 It is concluded that PGE2 and possibly 13,14-dihydro-15-keto PGE2 but not bacterial endotoxin may enter the CNS from the cerebral circulation to elicit the febrile response to bacterial endotoxin in cats.

Central thermosensitivity during fever produced by intra-PO/AH and intravenous injections of pyrogen

Squirrel monkeys with thermodes implanted in the preoptic/anterior hypothalamic (PO/AH) region and the medulla oblongata were used to examine three questions about central thermoresponsiveness in fever: Does thermoresponsiveness of the PO/AH region and medulla change during fevers caused by injection of bacterial endotoxin IV or directly into the PO/AH region? Does thermosensitivity of these brain regions determine the upper fever limit? Is thermoresponsiveness of the PO/AH region affected by local injections of salicylate? Changes in rectal temperature and oxygen consumption in response to heating and cooling the PO/AH region were reduced during fever caused by intra-PO/AH injections of bacterial endotoxin compared with changes produced during afebrile periods. PO/AH thermosensitivity was also reduced during fever caused by IV administration of bacterial pyrogen. Prolonged cooling of the PO/AH region or the medulla oblongata during fever produced by peripheral and central pyrogen injections did not cause rectal temperature (Tre) to rise above 41.1 degrees C although local heating reduced Tre or limited the fever maximum. From the latter result it is concluded that both pools of central thermoreceptors can limit maximal fever by reacting to local high temperature but that lowered temperature in neither region can raise Tre above a level determined by antagonistic input from thermoreceptors in other parts of the body. Injections of sodium salicylate into the PO/AH region had no effect on thermoresponsiveness of the region. This finding reinforces the idea that salicylates do not produce antipyresis by acting directly on thermosensitive cells of the central temperature control system.

Autonomic response of the fish to pyrogen

Lipopolysaccharide (LPS) applied to the anterior brainstem of the carp caused lightening of body colour. This indicates that an increase in set point temperature is responsible for increased cutaneous autonomic activity following LPS-administration.

Fever in rats after intravenous E. coli endotoxin administration

In conscious unrestrained rats, at an ambient temperature of 22 degrees C, oesophageal temperature was measured and temperature effect of single and repeated intravenous injection of E. coli endotoxin was examined. The first injection of endotoxin in a dose of 10.0 mug/rat did not change the rat body temperature. The second injection of this dose in the same animals repeated after 48 h produced fever. With following injections the fevers observed were less pronounced. The absence of fever after a single injection of endotoxin was accompanied by the rapid loss of pyretic activity of the rat plasma samples (bioassayed in rabbits). When fever was observed (48 h interval between endotoxin injections) the pyretic activity of the rat plasma remained unchanged for 90 min following endotoxin injection. It was concluded that after a single injection endotoxin is rapidly detoxified in the rat circulation while this process does not take place after the second endotoxin injection (48 h interval). The process of endotoxin detoxification can be depressed by the pretreatment with nitrogen mustard. Analysis of changes of skin temperature following endotoxin injections and the influence of aspirin on endotoxin-induced fever suggest that the fever observed was of central origin.