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

Thermoregulation in rabbits during fever

1. We have studied the effect of fever on the efficacy of the thermoregulatory control system in conscious rabbits. 2. The control system was challenged by a series of systemic thermal loads produced by the intravenous infusion of hot or cold isotonic solutions. The time integral of the consequent upward or downward displacement of brain temperature was used as an index of the response of the control system. Steady-state fever was induced by intravenous infusion of plasma containing leucocyte pyrogen. 3. With cold loads there was a linear relation between load and response. The regression coefficients were not significantly changed by fever in any of the six rabbits. Wth hot loads given to afebrile rabbits the regression of response on load was generally not statistically significant, but the responses were not demonstrably greater in the febrile state. 4. We were not able to demonstrate impairment in the capacity of the febrile animal to compensate for systemic thermal loads.

Hypothalamic Na+ and Ca++ ions and temperature set-point: new mechanisms of action of a central or peripheral thermal challenge and intrahypothalamic 5-HT, NE, PGEi and pyrogen

The effects of changes in ambient and central temperature, amines, PGEu and pyrogen were investigated with respect to the mechanism of Na+-Ca++ ratio in the posterior hypothalamus of the unrestrained cat. Guide tubes were implanted bilaterally above the posterior hypothalamic area of 23 cats so as to accommodate push-pull cannulae. After a Na+ or Ca++ sensitive site was identified by perfusion at 50 mul/min of an artificial CSF containing 10.4 mM excess Ca++ ions or 13.6 mM excess Na+ ions, several types of experiments were undertaken with the results summarized as follows: if the cat was exposed to a cold or warm environmental temperature as the posterior hypothalamus was perfused with excess cation, the typical hypothermia was produced by Ca++ and hyperthermia by Na+ ions. However, if the cat was exposed to peripheral cooling or warming 30 min prior to the perfusion, the fall or rise produced by Ca++ or Na+ was attenuated or prevented. In other experiments, 1.0 muCi 45Ca++ was injected in the ion sensitive site in the posterior hypothalamus to label stores of the cation. Raising of ambient temperature caused a retention of 45Ca++ in this hypothalmic area, whereas a cold environmental temperature enhanced the efflux of 45Ca++ at the same perfusion site. The magnitude of change in 45Ca++ efflux depended upon the intensity of the thermal challenge. Similarly, warming of the anterior hypothalmic, preoptic area by means of implanted thermodes caused an immediate diminution in 45Ca++ efflux in the posterior hypothalamus, whereas cooling of this anterior region augmented the extrusion of 45Ca++ ions from the posterior area. When substances which produce a temperature change were applied to the same thermosensitive zone, the direction of shift in 45Ca++ flux in the posterior area corresponded to the signal for heat production or heat loss. That is, the microinjection of 5-HT, PGE1 or Salmonella typhosa into the anterior hypothalamus enhanced the efflux of 45Ca++ in the posterior hypothalamus as hyperthermia developed, whereas a similar microinjection of norepinephrine reduced the 45Ca++ output from the same sites. Finally, locally anesthetizing the cells of the anterior hypothalamus by the nerve blocker, procaine, prevented the cold and heat-induced 45Ca++ eflux and retention, respectively. These results suggest that if the Na+-Ca++ ratio in the posterior hypothalamus establishes and maintains the set-point for body temperature of 37 degrees -38 degrees C, the mechanism of lability of Ca++ through changes in binding characteristics, transport, or metabolism of the cation serves two purposes: (1) the active defense of the set-point temperature through gradations in ion shifts; and (2) the upward or downward change in set-point value, pathological or normal, triggered by virtue of impulses relayed from the anterior hypothalamus.

Is prostaglandin fever mediated by the presynaptic release of hypothalamic 5-HT or norepinephrine?

An array of guide tubes to accommodate concentric push-pull cannulae was implanted chronically within the diencephalon of the rhesus or other species of macaque monkey which was accustomed to a primate chair. Colonic and skin temperatures were monitored continuously during each experiment in which a circumscribed site in the monkey's hypothalamus had been labelled by microinjection of 50-100 muCi serotonin (3H-5-HT) or 50-100 muCi or norepinephrine (3H-NE). Consecutive push-pull perfusions with an artificial CSF were carried out for 10 min at a rate of 50 mul/min at 20 min intervals. Under the control condition, a declining washout curve of radioactivity was obtained over 8-10 perfusions. Prostaglandin (PG) E1 in a concentration of 10-20 ng/min was added to the artificial CSF during the third and fifth successive perfusions. Nonlabelled PGE1 failed to exert a precise and consistent effect on the characteristic pattern of efflux of either tritiated 5-HT or NE from perfusion sites distributed widely throughout the hypothalamus and adjacent structures. However, in some experiments, an enhanced efflux of the indoleamine label did occur after the temperature had begun to rise following a perfusion with the PGE. In still other experiments, 15-20 muCi 3H-PGE1 was microinjected to label a perfusion site. Again the addition of either nonlabelled 5-HT or NE to the perfusion fluid produced an unreliable change or no alteration in the efflux of 3H-ge1 from sites in the anterior as well as other parts of the hypothalamus. These findings indicate that prostaglandin injected into the brain does not evoke hyperthemia by way of a pathological disturbance to the balance in the presynaptic release of 5-HT and NE within nerve endings in the rostral hypothalamus of the monkey. Conversely, neither 5-HT nor NE influences the prostaglandin activity within the hypothalamus, at least in so far as a functional change in the body temperature of the primate is concerned.

Sorting of signals from thermosensitive areas

Of the several models proposed for the neural regulation of temperature in cold-exposed animals, two have been previously restated in dynamic form using CSMP. Subsequently, computer simulations have led to the design and execution of experiments for selection of the more appropriate model for cold-exposed rats. These experiments, as described in the present paper, have been interpreted as being consistent with the model which sorts signals from thermosensitive areas and channels the selected signals over separate neural pathways to independently control each mode of heat production. Since this model requires multiplication of neural signals, possible neuronal mechanisms which may underline such multiplication are discussed. In addition, parameter variation to account for febril responses and rate sensitivity have been evaluated.

Effects of sodium salicylate, aminopyrine and chlorpromazine on behavioral temperature regulation

To characterize drug actions on thermoregulatory processes it is necessary to know whether compounds which alter body temperature also cause changes in thermoregulatory motivation. In the present experiments the effects of sodium salicylate, aminopyrine and chlorpromazine (CPZ) on rectal temperature (Tre) and behavior were measured in rats trained to escape heat and obtain cooling. All three drugs produced hypothermia in a 23 degree C environment but the effects upon behavior suggest that the compounds have different actions. Sodium salicylate (60-300 mg/kg) increased the amount of time spent responding to escape heat and obtain cooling so that Tre was held below control levels. Aminopyrine (12.5-75 mg/kg) did not alter thermoregulatory motivation even though it caused marked hypothemia. The time spent responding decreased after CPZ (2 and 3 mg/kg) so that drug-induced hypothermias were compensated. The results suggest that sodium salicylate influences the central mechanisms of physiological and behavioral temperature control whereas CPZ affects either peripheral thermoeffectors or central effector pathways without disrupting thermoregulatory motivation. Aminopyrine is presumed to act on central temperature controls to lower body temperature and, at the same time, to reduce the significance of the low body temperature to behavior.

Fever produced by endotoxin injected into the hypothalamus of the monkey and its antagonism by salicylate

1. A suspension of the killed cell bodies of either E. coli, S. dysenteriae or S. typhosa was micro-injected through cannulae implanted chronically at specific sites within the diencephalon and mid-brain of the unanaesthetized monkey. A biphasic, monophasic or an undifferentiated fever could be induced by each type of micro-organism, but the type of response depended solely upon the locus of injection.2. Although little difference in the potency of the three pyrogens was found, the rise in body temperature was in each instance dependent upon the concentration of the endotoxin. A more intense fever was accompanied by shivering, vasoconstriction of the ear vessels, piloerection and huddling behaviour. Tolerance to the pyrexic effect of repeated injections of endotoxin did not develop.3. The febrile response having the shortest latency, greatest maximum rise in temperature and largest 10-hr fever index was evoked by micro-injections into the anterior hypothalamic, preoptic area. The incidence of biphasic fevers was also greater after endotoxin was injected into this same region. Endotoxin given similarly in the posterior hypothalamus or in the mesencephalon had either no effect or produced a smaller elevation in temperature after a longer latency. The distance of an injection site from the coronal plane formed by the optic chiasm and anterior commissure correlated significantly with the latency and magnitude of the temperature change as well as the fever index.4. When given intravenously, endotoxin in a quantity at least 100 times greater was required to evoke a fever similar to that produced when the pyrogen was micro-injected into the anterior hypothalamic, preoptic region. However, a biphasic fever was evoked with a latency of from 3 to 15 min when a larger amount of endotoxin was injected intravenously. Tolerance developed rapidly to the febrile effect of endotoxin administered by this route although toxic reactions were not observed.5. After the fever evoked by the hypothalamic injection of endotoxin had reached a plateau, 300-1200 mg sodium salicylate administered intragastrically produced a dose-dependent fall in temperature, but had no effect on the body temperature of an afebrile monkey.6. It is concluded that in the rhesus monkey, a bacterial pyrogen can evoke a fever which is mediated entirely by an action on the central nervous system, the principal site being the anterior hypothalamic, preoptic area. The first phase of a biphasic fever caused by bacteria acting either by the central or peripheral route seems to be due either to a direct action of the pyrogen on the cells of the anterior hypothalamus, or to the secondary release within this region of an intermediary thermogenic substance such as 5-hydroxytryptamine or prostaglandin. The finding that sodium salicylate counteracts a centrally evoked fever is not compatible with the hypothesis that an antipyretic exerts its action by preventing a pyrogen that is circulating in the blood stream from entering the central nervous system.

Effects of biogenic amines on central thermoresponsive neurones in the rabbit

1. Single unit activities were recorded with five-barrelled micropipettes from the thermo-responsive neurones in the preoptic area and the mid-brain reticular formation in urethanized rabbits. 5-hydroxytryptamine (5-HT), noradrenaline (NA) and acetylcholine (ACh) were applied micro-iontophoretically to the immediate vicinity of the recording cells.2. Out of seventeen warm-responsive neurones recorded in the preoptic area, fifteen neurones responded to 5-HT with the increase in firing rate and two showed no response. Thirteen out of seventeen warm-units decreased their firing rate in response to application of NA and four were not affected. ACh had no effect on any of the warm-units examined.3. Six out of seven cold-units in the preoptic area were depressed by 5-HT, while NA excited five of six units studied. None of the cold-units were influenced by ACh.4. These results are in good agreement with the changes in rectal temperature produced by 5-HT and NA micro-injected into the hypothalamus in rabbits.5. In the mid-brain reticular formation, 5-HT excited all of fourteen cold-responsive neurones. Of these, eight cold-units were depressed and six were unaffected by NA, while ACh excited six units and had no effect on eight units. All of the five warm-responsive units were inhibited by 5-HT and none were influenced by NA. Thus, the responses of reticular thermoresponsive neurones to 5-HT and NA were opposite to those of the preoptic thermo-responsive neurones.

Salicylate: action on normal body temperature in rats

Rats receiving intraperitoneal injections of sodium salicylate (30 to 300 milligrams per kilogram of body weight) showed a decline in rectal temperature of up to 5.5 degrees C when placed in a 5 degrees C environment. High dosages of salicylate lowered the rectal temperatures of rats kept in a 23 degrees C environment. The finding that salicylate can lower nonfebrile body temperature suggests that this class of antipyretic agents does affect normal temperature regulation.

The role of sodium and calcium ions in the hypothalamus in the control of body temperature of the unanaesthetized cat

1. Isolated regions of the anterior, posterior and other areas of the hypothalamus of the unanaesthetized cat were perfused by means of push-pull cannulae lowered through permanently implanted guide tubes. Each site was perfused for a 30 min interval at a rate of 50 mul./min. Concentrations of sodium, calcium, potassium and magnesium ions in the perfusate were altered selectively.2. Sodium ions in a concentration which varied from 13.6 to 68.0 mM in excess of the level in extracellular fluid caused a steep rise in the temperature of the cat when the solution was perfused at sites located within the posterior hypothalamic area. Shivering, vasoconstriction, and piloerection accompanied the increase in temperature. When the chloride was replaced in the perfusate by the toluene-p-sulphonate salt of sodium, the hyperthermia was equally intense. Solutions containing excess sodium ions perfused within the anterior and other hypothalamic areas produced either a slight fall or rise in temperature as well as other physiological changes.3. Calcium ions in a concentration which varied from 2.6 to 10.4 mM in excess of the physiological level perfused at the same sites within the posterior region of the hypothalamus produced a sharp fall in body temperature, which was accompanied by vasodilatation and a decrease in the activity of the cat. When solutions containing excess calcium were perfused in the anterior and other hypothalamic areas, no consistent change in temperature occurred.4. Potassium or magnesium ions in concentrations which varied from two to ten times the level in extracellular fluid had virtually no effect on the temperature of the cat when they were perfused in the anterior, posterior or other areas of the hypothalamus.5. We conclude that the constancy in the ratio between sodium and calcium ions in the posterior hypothalamus may be the inherent mechanism by which the set-point for body temperature is determined.

Thermosensitivity of neurons in the sensorimotor cortex of the cat

Extracellular action potentials were recorded from 80 neurons in the sensorimotor cortex of the cat as brain temperature was varied by 4 degrees to 8 degrees C. The discharge rate of 37 percent of the neurons studied increased with increasing brain temperature. The discharge rate varied inversely with temperature in 11 percent of the neurons.

The effects of salicylate on temperature regulation in the rabbit

1. A stable pyrexia has been produced in rabbits by intravenous injection of leucocyte pyrogen followed by a continuous infusion.2. Intravenous sodium salicylate, given 4 hr after the start of pyrogen infusion, induced rapid and progressive defervescence. The rate of fall of temperature was dose dependent.3. Intravenous injection of sodium salicylate had no effect on the temperature of afebrile rabbits.4. The intraventricular injection of small doses of sodium salicylate, given 4 hr after the start of a pyrogen infusion, caused a rapid, dose-dependent defervescence, but had no significant effect on the temperature of afebrile rabbits.5. Salicylates exert at least part of their antipyretic activity through an action on the central nervous system.