Respiratory and metabolic response of rats and mice to formalin vapor

The irritant effect of repeated inhalation exposure to vapors of aqueous formalin (formaldehyde concentration, 15 ppm) in male Charles River CD rats and male C57BL6/F1 mice was determined. The data suggest that rats are relatively insensitive to this irritant while mice are substantially more sensitive, the latter showing a comparable reduction in tidal volume, but a greater decrease in respiratory rate and, as a consequence, minute volume. CO₂ production, a reflection of total metabolism, as well as body temperature, were decreased to a greater extent in mice than in rats. The decreased minute ventilation and associated decrease in temperature, responses occurring to a greater extent in the mouse, may decrease both the net formaldehyde dose as well as the toxic actions of formaldehyde that are dependent on systemic metabolic activity.     

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 μg induced dose-related increases in body temperature. Hyperthermic responses to 1 μg of the peptide were greater the warmer the environment. Naloxone given intraventricularly 1 hr after FK 33-824 (1 μg) reduced the hyperthermia. In 12 tests with six cats FK 33-824 (1–25 μg) increased temperature 4.2–4.7°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 μg) 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, ν, receptors, stimulation of which can influence thermoregulation in the cat.     

Gonadal hormones and behavioral regulation of body weight

Gonadal hormones have important effects on the behaviors that determine body weight in laboratory rats (i.e., eating, locomotor activity, and thermoregulatory behavior). These effects are most evident in the female where there are consistent, predictable changes in these behaviors which are correlated with fluctuations in plasma hormone levels during estrous cycles, puberty, pregnancy, or after gonadectomy and replacement therapy. Estradiol, which seems to be the principal ovarian steroid affecting body weight, may act directly on separate neural loci to: (a) inhibit food intake and (b) stimulate locomotor activity, possibly by lowering the set-point of a hypothalamic lipostat. Estradiol does not affect eating and running in prepubertal female rats, perhaps because of influences of pituitary hormone(s) at this age. Ovarian hormones also alter the taste preferences of rats and may be responsible for the changes in self-selection of dietary components during different reproductive states. Some implications of this research are discussed and possible directions for future research suggested.     

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, PGE1, and pyrogen

The effects of changes in ambient and central temperature, amines, PGE₁ 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 μ1/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 μCi ^{4 5}Ca⁺⁺ 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 ^{4 5}Ca⁺⁺ in this hypothalamic area, whereas a cold environmental temperature enhanced the efflux of ^{4 5}Ca⁺⁺ at the same perfusion site. The magnitude of change in ^{4 5}Ca⁺⁺ efflux depended upon the intensity of the thermal challenge. Similarly, warming of the anterior hypothalamic, preoptic area by means of implanted thermodes caused an immediate diminution in ^{4 5}Ca⁺⁺ efflux in the posterior hypothalamus, whereas cooling of this anterior region augmented the extrusion of ^{4 5}Ca⁺⁺ ions from the posterior area. When substances which produce a temperature change were applied to the same thermosensitive zone, the direction of shift in ^{4 5}Ca⁺⁺ 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 ^{4 5}Ca⁺⁺ in the posterior hypothalamus as hyperthermia developed, whereas a similar microinjection of norepinephrine reduced the ^{4 5}Ca⁺⁺ 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 ^{4 5}Ca⁺⁺ efflux and retention, respectively. These results suggest that if the Na⁴−Ca⁺⁺ ratio in the posterior hypothalamus establishes and maintains the set-point for body temperature of 37°–38°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.     

Metabolic Reactions to Changes in Core and Skin Temperature in Man

The metabolic cold response, i.e. the increase in oxygen consumption above that for the given activity in a neutral environment, was measured in 7 subjects during cooling, resting or swimming in cold water (14, 16, 18, 20°C) and during rewarming in air (T_a 20, 30, 40°C), bicycling or resting. Esophageal temperatures varied between 38 and 34°C. Mean skin temperature was considered as equal to water temperature during cooling, and ranged between 25–35°C during rewarming in the different environments. Both central and peripheral cold stimulation induced metabolic cold responses. The skin temperature was the dominating factor in determining the response, especially in transient states. During rewarming a rising skin temperature suppressed the effects of even very low core temperatures.     

Quantitative analysis of cold stress performance after digital replantation

Isolated cold stress testing (ICST) has been used to assess cold stress performance or digital thermoregulation, but statistical analysis of the results has been limited to visual comparisons of data. In this prospective study, 11 patients who underwent complete digital replantation were followed serially with ICST at 6 weeks, 3 months, 6 months, 1 year, and 2 years postoperatively, and the results were analyzed quantitatively. For that analysis, we devised a mathematical method that provided a cooling and a warming coefficient to fit the data. Differences in these cooling and warming coefficients were then regressed against time after replantation, patient age, number of digits replanted, hand dominance, and clinical evidence of sensory recovery. There was a significant correlation between response to isolated cold stress testing (thermoregulation) and degree of sensory recovery (p less than or equal to 0.02). This method for quantitative analysis of isolated cold stress testing data allows objective evaluation of digital blood flow patterns based on temperature, thereby providing a reliable and objective assessment of the recovery of thermoregulation in the replanted human digit.     

Dopamine in the hypothalamus of the cat: Pharmacological characterization and push-pull perfusion analysis of sites mediating hypothermia

Within the rostral diencephalon of the cat, 113 sites were examined for their reactivity to 2.33–14.0 μg dopamine (DA) or 2.33–14.0 μg norepinephrine (NE) microinjected in a volume of 0.75 μl. During each experiment, colonic temperature was monitored and additional physiological measures were recorded continuously. In contrast to CSF controls, an intrahypothalamic injection of either catecholamine at circumscribed sites evoked a dose-dependent fall in the cat's body temperature, with NE ordinarily evoking a more profound hypothermic response. The morphological sites of maximum sensitivity were localized in the anterior hypothalamic, preoptic region. At some but not all sites, a prior microinjection of 3.5–7.0 μg phentollamine attenuated the magnitude of the DA-induced hypothermia and delayed its onset. Conversely, at all loci, the pretreatment by the injection of this alpha-adrenergic antagonist markedly reduced the absolute magnitude of the NE-induced fall in the cat's temperature. Similar pretreatment of a reactive hypothalamic locus with a beta-adrenergic receptor blocking agent, practolol (3.5 μg), failed to alter the hypothermia following a microinjection of DA. Either of two DA receptor antagonists, haloperidol (0.04–7.0 μg) or d-butaclamol (0.48–1.47 μg), when given in a sufficient dose, effictively delayed the onset of the DA-hypothermia and reduced its absolute magnitude; however, the NE-induced decline in the cat's temperature was unaffected by DA receptor blockade. Endogenous stores of DA and/or NE in the cat's hypothalamus were radio-labeled with either ³H- or ¹⁴C-catecholamines or both, microinjected through the implanted guide tube into an identified amine-sensitive site. By using push-pull cannulae, the site was subsequently perfused for 5 min with artificial CSF at a rate of 25 μl/min with samples collected at 15 min intervals. During either the third or fourth perfusion, the ambient temperature of the cat's chamber of 22–24°C was elevated to 35–45°C and maintained at this level for 15 or 30 min. This environmental warming evoked a release of either DA or NE or both amines from certain circumscribed sites within the cat's rosrtal hypothalamus. Overall, these results provide pharmacological, physiological and anatomical evidence for a differential role of DA in the hypothalamic mechanism which mediates the heat loss processes.     

Effects of hypothermia on thyrotropin-releasing hormone content in the rat brain

The thyrotropin-releasing hormone (TRH) content in the brain was determined in normothermic and hypothermic rats subjected to immobilization stress. TRH contents in the hypothalamus, midbrain and cerebral cortex significantly decreased during mild hypothermia (body temperature about 34°C), but not during profound hypothermia (about 24°C). The decreases in the TRH content during mild hypothermia were readily reversed by rewarding the animal. These results indicate that cerebral TRH is involved in the response to a mild body temperature drop when the animal is exposed to a cold environment.     

Hangover hyperthermia in rats: relation to tolerance and external stimuli

The recent finding of rebound hyperthermia in rats on the day after a single IP injection or oral intubation of ethanol was confirmed. In our studies, body temperature measured by rectal probe was significantly decreased for 8 h after 2.5 g/kg ethanol IP and was then significantly elevated 16–24 h after injection; increased vocalization during handling at 24 h was also found. However, rats isolated in a sound-attenuation chamber with remote temperature measurement showed no hyperthermia even though they were hypothermic during intoxication. The results do not support the hypothesis that rebound hyperthermia was caused by either a disruption of circadian rhythms, or by a mild abstinence syndrome alone. Instead, it appears that external stimuli, perhaps related to stress or associated with ethanol administration, are necessary on the day after a moderate dose of ethanol to produce the hyperthermia. Like hangover in humans, hyperthermia was reduced in rats made tolerant to ethanol: both the hypothermia and the rebound hyperthermia were significantly lower on the day after the 12th alternate-day ethanol injection than after the first injection. The aftereffects in rats of acute intoxication are, by definition, hangover signs, and they resemble hangover in humans in several ways, but their relevance as an animal model of hangover remains to be determined.Portions of this material have been presented at meetings previously (Sinclair and Gustafsson 1983; Sinclair et al. 1984; Sinclair et al. 1986). Offprint requests to: J. D. Sinclair     

Comparison of the thermoregulatory responses to intracerebroventricularly injected dopamine,noradrenaline and 5-hydroxytryptamine in the goat

The thermoregulatory responses to dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT), injected into the third cerebral ventricle of goats at 20°C ambient temperature (T_a) or during cold exposure, were compared before and after pretreatment with the DA receptor blocker haloperidol or the 5-HT receptor blocker methysergide. At 20°C T_a, intracerebroventricular (i.c.v.) injection of DA (800 μg), NA (200 μg) or 5-HT (800 μg) induced a decrease in body temperature (T_b) and dilatation of the ear vessels. After DA and 5-HT, but not after NA, panting was observed. During cold exposure both DA and NA caused a suppression of shivering and a fall in T_b. Pretreatment with haloperidol (400 μg, i.c.v.) attenuated the thermoregulatory effects of i.c.v. DA other than the dilatation of the ear vessels at 20°C T_a. Haloperidol did not influence the responses after i.c.v. NA or 5-HT. Methysergide (1.0 mg, i.c.v.) blocked panting and attenuated the decrease in T_b caused by i.c.v. DA and 5-HT at 20°C T_a and blocked the peripheral vasodilatation caused by 5-HT but not the dilatation caused by DA. During cold exposure methysergide antagonized the thermoregulatory effects of DA, but not those of NA. I.c.v. injection of haloperidol (400 μg) or methysergide (1.0 mg) during heat exposure induced a decrease in panting and a rise in body temperature, which suggests that both DA and 5-HT have a physiological role in the mediation of heat loss in the goat. We conclude that in the central thermoregulatory system of the goat there are excitatory DA receptors in the pathway from heat sensors to heat loss effectors. Activation of these receptors by i.c.v. DA or by heat exposure results in a secondary release of 5-HT. Since the vasodilating effect of DA was not influenced by haloperidol or methysergide, this effect could be mediated by haloperidol-insensitive (possibly inhibitory) DA receptors on the pathway controlling peripheral vasomotor tone. The thermoregulatory effects of i.c.v. NA are probably mediated by inhibitory NA receptors on the pathway from cold sensors to heat production effectors.