An extensive exploration of the rat brain for sites mediating prostaglandin-induced hyperthermia

Experiments designed to ascertain the site or sites within the brain where prostaglandins act to evoke hyperthermia were carried out. At an ambient temperature of 20 degrees C, microinjections of PGE1 (50-100 ng) or saline vehicle were made in restrained rats. The 272 sites tested formed a matrix encompassing much of the subcortical tissue rostral to the medulla. The sensitivity of each site to PGE1 was evaluated on the basis of the dose required to elicit a criterion level of effect and on the intensity of the effect evoked by a fixed dose. Injections of PGE1 into a sensitive site typically produced a rapidly developing, short lasting, monophasic rise in body temperature. Saline injected at these sites produced no initial hyperthermia. Almost all of the active sites were located within the anterior hypothalamic/preoptic region (AH/PO). The sites of greatest reactivity were found in the ventral aspect of the tissue lying between the anterior commissure and the optic chiasm. These data suggest that the AH/PO region is an important and probably the only supramedullary site of PGE1 action in the rat brain.

Chronic catheterization of the spinal subarachnoid space

To administer drugs into the spinal subarachnoid space of unanesthetized and intact rats and rabbits, a procedure is described whereby a polyethylene catheter (PE-10) may be inserted through a puncture of the atlanto-occipital membrane and secured to the skull. Calibration experiments carried out with bromophenol blue dye, 3H-naloxone and 14C-urea revealed first, that there was little rostro-caudal diffusion of the injectate along the spinal axis and secondly, that even for compounds such as naloxone which can rapidly permeate neural tissues, the levels which do appear in the brain are small following the spinal subarachnoid administration of the drug. Control injections, administered either acutely or repeatedly over a prolonged period of time, had no detectable effect on the animal's behavior. These observations, as well as the lack of pathology in the spinal cords of rats having such catheters for periods of up to 4 months suggests that the implant is well tolerated.

Systematic examination in the rat of brain sites sensitive to the direct application of morphine: observation of differential effects within the periaqueductal gray

An extensive mapping of the rat brain (403 sites) ranging from AP +8 to AP -3 revealed that the region showing maximum sensitivity to the intracerebral administration of morphine in the elevation of the nociceptive threshold lay within the periaqueductal gray. Analysis of the distribution of responsive sites indicated that the most active sites, those having the shortest latency of effect, were located within the ventrolateral aspect of the caudal periaqueductal gray. These antinociceptive actions of morphine were observed to be both dose-dependent and reversible by the administration of naloxone. We observed that microinjections of morphine could produce changes in the pinch withdrawal response which were distributed in a crude somatotopic fashion. Injections into the rostral aspect of the periaqueductal gray yielded a block of the pinch response in the rostral portions of the body, whereas such injections into the caudal periaqueductal gray always yielded a whole body analgesia. In the rostral sites, transient ipsilateral blocks of the pinch response were occasionally seen. A pinch block limited to the hind paws alone was never observed. It is suggested that morphine acting through the periaqueductal gray may actuate a potent supraspinal modulatory system related to the transmission of information derived from behaviorally aversive stimuli.

Analgesia mediated by a direct spinal action of narcotics

Narcotic analgetics administered directly into the spinal subarachnoid space of the rat via a chronically inserted catheter produce a potent analgesia that can be antagonized by naloxone. The narcotics, acting only at the spinal level, changed cord function to block not only spinal reflexes but also the operant response to painful stimuli.

Effects of brain lesions on the antinociceptive properties of morphine in rats

1. Electrolytic lesions were made in various brain regions of the rat, and the effects of these lesions on nociceptive threshold and the antinociceptive actions of morphine were tested using a shock titration technique. 2. Lesions in the medial thalamus, the periaqueductal grey area, or the caudate nucleus, had no effect on the nociceptive threshold; whereas, lesions in the posterior hypothalamus resulted in a small but statistically significant increase in this threshold. 3. Morphine administered intraperitoneally to rats having histologically verified lesions in the posterior hypothalamus, the caudate nucleus or the periaqueductal grey area resulted in a 15-30% increase in the nociceptive threshold. This increase was similar to that observed in unoperated control rats. On the other hand, injections of morphine into animals having greater than 50% of the medial thalamus destroyed produced a highly significant increase of 95% in the threshold. This potentiation of the antinociceptive action of morphine was not observed in rats having less than 50% destruction of the medial thalamus.

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.

Thermoregulation around a new set-point' established in the monkey by altering the ratio of sodium to calcium ions within the hypothalamus

1. The cerebral ventricles of the unanaesthetized monkey were perfused at a rate of 50-100 mul./min with artificial cerebrospinal fluid (c.s.f.) containing 11.0-34.0 mM excess sodium or 23.9-47.9 mM excess calcium ions. By repeated perfusions, a marked hyperthermia elicited by excess sodium or deep hypothermia by excess calcium was sustained for up to half a day.2. During the sodium-induced hyperthermia or calcium-induced hypothermia, the monkey thermoregulated adequately around the new level of temperature in response to either 50 or 150 ml. water heated to 58 degrees C or chilled to 0 degrees C and given by the intragastric route. Cold water produced a transient hypothermia after which temperature returned to the pre-load level; hot water given in the same way evoked a short-lasting hyperthermia, and again the temperature returned just to the new setpoint level.3. Perfusions at 50 mul./min of isolated areas of the diencephalon by means of push-pull cannulae with a solution containing 11.0-34.0 mM excess sodium ions or 11.3-47.9 mM excess calcium ions altered the body temperature of the monkey only when the sites of perfusion were located in the mammillary region of the posterior hypothalamus. Following the prolonged hyperthermia produced by excess sodium ions or the deep hypothermia evoked by excess calcium ions in the push-pull perfusion fluid, the monkey thermoregulated around the new temperature level again in response to 150 ml. of either hot or cold water given in the stomach.4. Chelation of calcium ions within the ventricles by perfusing EGTA through the cerebral ventricles caused a long-lasting elevation in temperature. When EGTA was perfused directly in the posterior hypothalamus by means of push-pull cannulae, a profound hyper-pyrexia ensued which could be abolished immediately by perfusing a solution of excess calcium at the same diencephalic site.5. During the calcium-induced hypothermia, the heart rate declined but the electrical activity of the cortex was relatively unchanged. Feeding was also elicited in the hypothermic monkey by noradrenaline microinjected at a site in the hypothalamus at which eating was evoked when the body temperature was normal.6. The evidence supports the hypothesis that in the primate, the inborn mechanism which establishes the set-point for body temperature at 37 degrees C is the constancy in the intrinsic ratio between sodium and calcium ions within the posterior hypothalamus. If the set-point is elevated or lowered by a disturbance of the balance between these two cations, the monkey nevertheless can thermoregulate normally around the new level of body temperature.

Changes in body temperature of the unanaesthetized monkey produced by sodium and calcium ions perfused through the cerebral ventricles

1. In the unanaesthetized Rhesus monkey, solutions containing sodium, calcium, potassium or magnesium in excess of the normal concentration of extracellular fluid were perfused from a lateral to the fourth ventricle through chronically implanted cannulae.2. Sodium (11.0-88.0 mM in excess of the physiological concentration) perfused through the ventricles, caused an immediate rise in body temperature which was accompanied by vasoconstriction, piloerection and shivering. The latency of the hyperthermia was related directly to the rate of perfusion and the concentration of sodium, whereas the magnitude of the response depended upon the concentration only. When the perfusion was terminated, shivering ceased and the temperature of the monkey returned to the base line level.3. When calcium ions were perfused in concentrations 2.5-47.9 mM in excess of that of extracellular fluid, a fall in the temperature of the animal occurred. The magnitude of the decreases depended upon the concentration of calcium in the perfusion fluid. Vasodilatation, sedation and a reduction in withdrawal reflexes accompanied the calcium-induced hypothermia. After the perfusion ended, the temperature continued to fall until the monkey began to shiver and vasoconstriction was observed in many skin areas.4. The perfusion through the cerebral ventricles with modified Krebs solution alone or with the Krebs solution which contained potassium or magnesium ions in concentrations five to ten times normal had virtually no effect on the temperature of the monkey.5. Since the temperature of the monkey was unchanged as long as the physiological ratio of sodium to calcium in the perfusion fluid remained constant, we conclude that the balance between these two essential cations within the brain stem could determine the neural mechanism whereby the set-point for body temperature of the primate is established.

Control of body temperature in the unanaesthetized monkey by cholinergic and aminergic systems in the hypothalamus

1. In the unanaesthetized rhesus monkey, 5-hydroxytryptamine (5-HT), catecholamines, acetylcholine or carbachol were micro-injected in a volume of 1.0 mul. or less through chronically implanted cannulae at eighty-six sites in the hypothalamus.2. 5-HT in doses of 2-10 mug caused a long-lasting elevation in temperature which was dose-dependent. An anatomical ;mapping' of the hypothalamus revealed that the hyperthermic action of 5-HT was localized to the anterior, pre-optic area directly ventral to the anterior commissure.3. Noradrenaline in doses of 1-12 mug produced a dose-dependent fall in temperature of short duration. An anatomical ;mapping' showed that the hypothermic action of this and other catecholamines was again localized to the anterior, pre-optic region.4. Acetylcholine, alone or in a mixture with eserine, or carbachol caused a dose-dependent hyperthermia which was characterized by an intense rise of short duration and vigorous shivering. A ;mapping' of this response revealed a diffuse patterning of sites throughout the hypothalamus which were sensitive to the application of acetylcholine and carbachol. However, in one circumscribed region at the junction between the posterior hypothalamus and mesencephalon, the two cholinomimetic substances caused a marked fall in temperature.5. We conclude that 5-HT activates a cholinergic heat production pathway which projects from the anterior to posterior hypothalamus. Noradrenaline, on the other hand, blocks the hyperthermic action of 5-HT rather than activates the heat loss system. A chemically mediated heat loss pathway apparently does not exist in the hypothalamus.