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

Effect on body temperature in dogs of perfusion of cerebral ventricles with artificiaL CSF deficient in calcium or containing excess of sodium or calcium

In anaesthetized dogs at room temperatures of 28-33 degrees C, the cerebral ventricles were perfused with artificial CSF from the left lateral ventricular to the aqueductal cannulae. The animals' temperatures were recorded from the rectum. Addition of Ca(++) in excess to the artificial CSF perfusing the ventricles produced hyperthermia and addition of Na+ in excess produced hypothermia. Perfusion with medium deficient in Ca(++) and containing sodium edetate produced hypothermia. The temperature effects of Na(+) or Ca(++) in excess were mutually antagonistic.

Brain calcium: role in temperature regulation

Perfusion of the preoptic-anterior hypothalamus with excess calcium ion in ground squirrels produces a drop in core temperature. The magnitude of the drop is directly dependent on ambient temperature. Respiration, heart rate, and oxygen consumption are also reduced during perfusion of calcium ion. It is concluded that the depression of body temperature during calcium ion perfusion is due to generalized depression of the neurons of the preoptic-anterior hypothalamus.

Intermittent hypothermia. Independence of central and reflex thermoregulatory mechanisms

Three patients who had suffered episodes of spontaneous hypothermia are described. Circulatory control was normal for their age but there was evidence of a persistent defect of thermoregulatory mechanisms. In all patients vascular thermoregulatory reflexes dependent upon peripheral receptors were active and in two patients reflex shivering could be obtained. There appeared, however, to be a failure of central thermoregulation as the vascular reflexes were active at abnormally low central temperatures and a fall in central temperature did not initiate shivering. It is concluded that in patients who have suffered spontaneous hypothermia there may be a persistent defect of central mechanisms subserving vasomotion and shivering, although thermoregulatory reflexes dependent upon peripheral receptors can be active. The reflex pathways for thermoregulation in man therefore have a separate integrity, physiologically independent of the central thermoregulatory mechanisms.

The hypothermic effect of tetrodotoxin in the unanaesthetized cat

1. The hypothermic effect on unanaesthetized cats of tetrodotoxin injected I.V. or into the lateral cerebral ventricle was examined.2. At an ambient temperature (T(a)) of 22 degrees C, tetrodotoxin given intraventricularly was over 400 times more potent in lowering body temperature (T(b)) than when given I.V. The magnitude of the hypothermia was dose-dependent for both routes. Decreases in T(b) as great as 6.8 degrees C were induced by infusions or multiple injections of tetrodotoxin into the ventricle.3. Tetrodotoxin also lowered T(b) at T(a) = 13, 30 or 35 degrees C. Tachypnoea, which lasted for longer durations and which became more intense the higher the T(a), accompanied development of hypothermia. Shivering was observed only during recovery from hypothermia at 13 degrees C.4. During the tetrodotoxin-induced hypothermia, animals were still able to regulate against environmental thermal stresses.5. EDTA disodium salt, leucocytic pyrogen and prostaglandin E(1) antagonized the hypothermic effect of tetrodotoxin when they were administered during recovery from tetrodotoxin.6. Activation of heat-loss mechanisms, and the absence of compensatory shivering during development of hypothermia after tetrodotoxin administration, plus lowering of T(b) by tetrodotoxin at T(a) above as well as below the thermoneutral temperature, indicate that lowering of the thermoregulatory set-point is the mechanism by which centrally or peripherally administered tetrodotoxin lowers T(b). Further evidence for set-point lowering after intraventricular administration of tetrodotoxin is provided by persistence of the ability to regulate against both heat and cold stresses during hypothermia. The possibility that the decrease in set-point could be due to the well known action of tetrodotoxin to block transient increases in membrane sodium ion conductance is discussed in terms of a recent hypothesis regarding ionic control of the thermoregulatory set-point.

Differential release of acetylcholine from the hypothalamus and mesencephalon of the monkey during regulation

1. In unanaesthetized monkeys acclimated to primate chairs, 101 isolated sites in the hypothalamus and mesencephalon were perfused at a rate of 30-50 mul./min by means of push-pull cannulae. The perfusate, which contained an anticholinesterase, was assayed for acetylcholine (ACh) activity on the guinea-pig ileum in the presence of neostigmine.2. The body temperature of each animal was monitored continuously during an experiment by colonic and brain thermistors. To alter the ambient temperature by 15-20 degrees C, either a stream of warm air was passed over the monkey's trunk or containers of ice were placed in its chair chamber to cool the same region.3. Assays of the effluent revealed that the release of ACh varied according to the ambient temperature as follows: elevated only during cooling; elevated only during warming; elevated by both thermal stimuli; suppressed only by cooling; suppressed only by warming; suppressed by both thermal stimuli; elevated during cooling but suppressed by warming; and elevated by warming and suppressed by cooling.4. A composite anatomical ;mapping' of all perfusion sites revealed that in response to either peripheral cooling or warming, the output of ACh varied at only 36% of all sites anterior to the mid-hypothalamic plane, but at 65% of those loci caudal to this coronal plane.5. In the anterior, preoptic area, cooling enhanced the output of ACh at 88% of the active releasing sites, whereas warming reduced the release of ACh at 80% of these perfusion loci. Posterior to this region, ACh release was elevated by cooling at about half of the active releasing sites, but lowered by warming at nearly every active perfusion locus. Within the mesencephalon, the ratio of the temperature-induced change in ACh release was similar but in an opposite direction, since the level of ACh in the effluent collected from two out of three sites was augmented by cooling, but diminished by warming.6. These results provide additional evidence for the neurochemical model of Myers & Yaksh (1969), which suggests that a cholinergic pathway originating in the anterior, preoptic region transmits efferent signals for heat production. Further, within the posterior hypothalamic area as well as in the mesencephalon of the monkey, the characteristics of the ACh releasing sites reflect a function delegated primarily to heat gain, although evidence of a cholinergic pathway for the heat loss system is also presented.

Fever: reciprocal shift in brain sodium to calcium ratio as the set-point temperature rises

A bacterial pyrogen acts on the brain by disturbing the natural balance between two essential cations in the cerebral region involved in thermoregulation. After typhoid vaccine is administered to the unanesthetized cat, (45)Ca(2+) efflux into the third cerebral ventricle increases while (22)Na(+) is retained in hypothalamic tissue at the same time that the set-point temperature begins to rise. The subsequent rates of (22)Na(+) and (45)Ca(2+) efflux parallel the course of the bacterial fever but in a reciprocal fashion. This supports the theory that a change in the set-point temperature is determined by an alteration in the inherent ratio of Na(+) to Ca(2+) ions in the hypothalamus.

Feeding produced in the satiated rat by elevating the concentration of calcium in the brain

When the concentration of calcium ions in the cerebral ventricles is elevated, a fully satiated rat eats voraciously. This feeding response is not prevented by prior intraventricular administration of alpha-or beta-adrenergic blocking agents, or other pharmacological antagonists. This supports the concept of an independent ionic mechanism, rather than a neurotransmitter one, for modulating a "set-point" for weight or hunger.