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

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.

Examination of the subdiencephalic rat brain for sites mediating PGE1-induced pyrexia

The subdiencephalic rat brain was mapped for sites capable of mediating prostaglandin-induced pyrexia. In conscious rats, PGE1, 200 ng in a volume of 1 microliter, was injected unilaterally into 412 sites between the midmesencephalon and the caudal medulla. Injections into only 12 sites caused a reproducible, short-latency core temperature increase of at least 0.5 degrees C. None of these was located in the paramedian brainstem, which was considered a likely site of PGE1 action because of the presence there of thermosensitive and pyrogen-sensitive neurons. Rather, the reactive loci were found in the hippocampus (5 sites) and in the vicinity of the cochlear nuclei (7 sites). Injections into only 2 sites in the latter region failed to produce pyrexia. In the hippocampus, however, injections at 31 sites in the same frontal planes as the reactive loci produced no effect. The possibility that the active hippocampal sites were associated with a distribution of injectate to PGE1-sensitive neurons located within hippocampal cleavage planes rather than in a circumscribed region is discussed.

Effects of alcohol on thermoregulation in aged monkeys

Ethanol ingestion has been implicated in accidental hypothermia of the elderly, but there is no prior data on the relative sensitivity to alcohol of aged homeotherms that might account for disproportionate dysthermia in this age group. To assess their sensitivity, ethanol (0.5-2.0 g/kg) or H20 was given by gavage to squirrel monkeys less than 4 yrs old, 4-9 yrs of age and over 9 yrs old. Dose-related decreases in body temperature occurred in all three groups in a 25.5 degrees C environment, with the greatest decreases in the oldest animals. In an 18.5 degrees C environment the hypothermias caused by 0.5 and 1.0 g/kg ethanol were greatly augmented in old monkeys and in some cases their temperatures fell to potentially life-threatening levels. In a 30.5 degrees C environment, 1.0 g/kg ethanol given to monkeys caused approximately equal temperature reductions in the three groups. When determined after 1.0 g/kg ethanol, peripheral vasomotor tone increased significantly in animals of each age group in all three environments. Decreases in rectal temperature were associated with parallel decreases in oxygen consumption. These results indicate that: 1) aging is associated with an increased sensitivity to the hypothermic action of ethanol in thermoneutral and cold environments, 2) ethanol in the doses tested inhibits heat production mechanisms without suppressing compensatory vasoconstriction in response to decreased body temperature.

Extreme hyperthermia induced in cats by the enkephalin analog FK 33-824

FK 33-824 [H2N-Tyr-D-Ala-Gly-MePhe-Met(O)-OH] was injected into the third cerebral ventricle of unrestrained cats. Doses of 0.25-4 microgram induced dose-related increases in body temperature. Hyperthermic responses to 1 microgram of the peptide were greater the warmer the environment. Naloxone given intraventricularly 1 hr after FK 33-824 (1 microgram) reduced the hyperthermia. In 12 tests with six cats FK 33-824 (1-25 microgram) increased temperature 4.2-4.7 degrees C. These marked responses were also inhibited by naloxone, but two cats died when administration of antagonist was delayed for 80 min to 3 hr after attainment of maximal body temperature. Larger doses of FK 33-824 (50-250 microgram) evoked little increase in temperature, indicative of a separate action to depress thermoregulation. Although responses to FK 33-824 were antagonized by naloxone, this peptide, like another enkephalin analog D-Ala2-Met-enkephalinamide, must act on receptors which are not affected by morphine since (1) the hyperthermic response to FK 33-824 varied with environmental temperature, whereas the response to morphine does not, and (2) high doses of FK 33-824 depressed thermoregulation, an activity not shared by morphine in the cat. Furthermore, the maximal increases in temperature after FK 33-824 injection were greater than those evoked by either morphine or D-Ala2-Met-enkephalinamide. This observation provides evidence for an additional subset of naloxone-sensitive, v, receptors, stimulation of which can influence thermoregulation in the cat.

Control of evaporative heat loss during changes in plasma osmolality in the cat

1. The effects of intravenous infusion of hypertonic saline and distilled water into normally hydrated and dehydrated cats have been examined at both high and neutral ambient temperatures.2. In hydrated cats measurements of body temperature (T(b)) and evaporative heat loss (e.h.l.) show that infusion of 30% saline (1.5 ml./kg) at an ambient temperature of 38 degrees C, lowers e.h.l. by an average of 0.21 W/kg (P < 0.001) and elevates T(b) by 0.43 degrees C (P < 0.01).3. At 25 degrees C alterations in these two parameters were in the same direction, though not statistically different from pre-infusion levels (P > 0.05).4. Infusion of distilled water (15 ml./kg) into dehydrated animals produced significant increases in e.h.l. (+0.35 W/kg, P < 0.001) and reductions in T(b) (-0.45 degrees C, P < 0.001) at 38 degrees C. No significant effects were observed at 25 degrees C.5. Infusion of water into normally hydrated animals at 38 degrees C also significantly increased e.h.l. (+0.13 W/kg, P < 0.05) and insignificantly lowered T(b) (-0.03 degrees C, P > 0.05).6. Local heating of the preoptic hypothalamic area in four animals indicated that hypertonic saline infusion into normally hydrated animals caused a reduction in the slope and displacement to the right of the relationship between hypothalamic temperature and e.h.l.7. Conversely, water infusion into dehydrated animals increased the slope and shifted this relationship to the left.8. These experiments provide evidence for an osmotic interaction in body temperature regulation which acts to alter the responsiveness of the hypothalamus to increasing temperature. This osmotic component may be an important factor in the alterations in thermoregulation seen in dehydrated animals.

Ability of human leukocytic pyrogen to stimulate brain prostaglandin synthesis in vitro

Fever is thought to be mediated by leukocytic pyrogen (LP), a polypeptide synthesized by phagocytic leukocytes and which is responsible for the upwards resetting of the hypothalamic thermostat. In an attempt to study the effects of LP directly on brain tissue, purified human LP was incubated with rabbit brain slices in vitro. Because of the well-documented role of prostaglandin (PG) synthesis in both the production of fever and antipyresis, PGE levels were measured on the supernates of brain slices incubated 30 min with LP. Levels of PGE increased 3- to 4-fold in rabbit anterior and posterior hypothalami. In addition, PGE levels were similarly increased in temporal cortex slices when exposed to LP. In another set of experiments, PGE levels increased 4- to 5-fold when brain tissue was incubated with a highly purified preparation of bacterial endotoxin (ET). The ability of ET to increase brain PGE levels was not affected by moderate heating (56 degrees C, 30 min), whereas this temperature destroyed the PGE-inducing properties of LP. The antipyretic ibuprofen markedly reduced the amount of PGE measured in the brain slice supernates after stimulation with LP, suggesting that LP brings about synthesis of PGE and not the release of preformed PG. The results demonstrate that LP is a potent inducer of PGE synthesis in rabbit brain and that receptors for LP are not restricted to the thermoregulatory center, but rather may be distributed throughout the brain.

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.

Hypothermic and antipyretic effects of centrally administered ACTH (1--24) and alpha-melanotropin

ACTH (1--24) and alpha-melanotropin (alpha-MSH), peptides previously shown to influence body temperature when administered centrally and to occur naturally in brain regions important to temperature control, were injected intracerebroventricularly (ICV) in rabbits. The peptides in doses of 1.25, 2.5 and 5.0 micrograms produced dose-related hypothermias in a 23 degrees C environment, and greater decreases in body temperature when the experiments were repeated in the cold (10 degrees C), but the largest dose had no effect on temperature in the heat (30 degrees C). These results indicate that the peptides do not reduce the central set-point of temperature control. Rather, they appear to selectively inhibit heat conservation and production responses. Five microgram of ACTH reversed vasoconstriction and inhibited rises in temperature caused by leukocytic pyrogen (LP) given IV and ICV. The same dose of alpha-MSH also reduced fever produced by IV and ICV LP, but the reduction was not as great as after ACTH. Both peptides (5 micrograms) also reduced temperature rises and vasoconstriction caused by ICV PGE2. ACTH reduced d-amphetamine-induced hyperthermia without altering vasoconstriction which suggests that this peptide can reduce temperature rises by inhibiting heat production alone. One of the most important findings was that the peptides are antipyretic in that they reduce fever at doses (0.25 microgram, ICV) that do not affect normal temperature. The powerful effects of these peptides on resting body temperature, hyperthermia and fever, together with their presence in brain tissue important to temperature control, suggest that the endogenous central peptides participate in thermoregulation, perhaps by limiting fever and influencing normal temperature.

Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents

This survey, the second in a series, presents extensive tabulations of literature, primarily since 1965, on thermoregulatory effects of cholinergic agonists and antagonists, histamine and H1- and H2-receptor antagonists, narcotic analgesics and antagonists in both non-tolerant and tolerant subjects and of prostaglandins and related agents. 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 drug.