How are neuronal thermosensitivity and lack of thermoreception related in the duck's hypothalamus? A tentative answer

1. 1.|In 15 conscious Pekin ducks, 40 “warm sensitive” hypothalamic neurons were identified according to their discharge rates at 40°C T_hy (F₄₀), local temperature coefficients (Δ/ΔT) and Q₁₀.

2. 2.|Q₁₀ and either F₄₀ or ΔF/ΔT were little or not related.

3. 3.|A positive correlation between F₄₀ and ΔF/ΔT was observed which was particularly close (r = 0.94 and 0.96) when the neurons were classified according to their Q₁₀ of <2 and >2.

4. 4.|The results suggest that neurons with positive temperature coefficients in the duck's hypothalamus mostly exhibit linear to exponential temperature-discharge relationships.

5. 5.|This is an contrast to observations on mammalian hypothalamic thermosensitive neurons and may relate to the absence of the thermosensory function in the duck's rostral brainstem.

A discrete mode of the antipyretic action of AVP, alpha-MSH and ACTH.

The antipyretic effect of AVP, alpha-MSH and ACTH consists in lowering the thermoregulatory threshold and in shortening the time span of the fever. Thus, neuropeptides influence activity of hypothalamic neurones regulating body temperature. This was confirmed by recent experiments of Moravec (this volume) which indicate that spontaneous activity and thermosensitivity of neurones in hypothalamic slices can be influenced, by AVP. Why neuropeptides of different chemical structure such as AVT, on one hand, and alpha-MSH and ACTH, on the other hand, induce the same effect on thermoregulation remains to be elucidated.     

Differences in endogenous pyrogen fevers induced by iv and icv routes in rabbits

We have compared the characteristics of fevers produced by endogenous pyrogen administered by the intravenous (iv) and by the intracerebroventricular (icv) routes in conscious rabbits. Fevers induced by the intracerebroventricular route have a longer latency to onset, a less steep rise in body temperature, and a longer time to peak elevation in body temperature than do fevers induced by the intravenous route. Furthermore, a dose of indomethacin (2 mg/kg) administered intravenously, which is effective in markedly attenuating fevers produced by the intravenous route, was completely without effect on fevers induced by the intracerebroventricular route. On the other hand, when indomethacin (500 micrograms) was infused intracerebroventricularly, it markedly reduced fevers induced by the subsequent injection of endogenous pyrogen into the contralateral cerebral ventricle, but such pretreatment had little effect on fevers elicited by intravenous injections of endogenous pyrogen. It is concluded that the sites of action of endogenous pyrogen in response to intravenous injections of pyrogen are different from those responding to intracerebroventricular injections of pyrogen and that this is manifest in several distinct differences in the characteristics of the two fevers. These results indicate that the intracerebroventricular model of fever production is not appropriate for the study of the normal pathogenesis of fever.     

Is the endogenous antipyretic neuropeptide alpha-MSH responsible for reduced fever in aged rabbits?

The reduced febrile response in aged man has been noted since the beginning of clinical thermometry. Our previous research on aged rabbits and squirrel monkeys disclosed a similar reduced fever, presumably due to a decrease in central receptors for endogenous pyrogen. However, because central alpha-melanocyte stimulating hormone (MSH) appears to have a potent role in physiological control of fever, it may be that increased release of the peptide is responsible for the reduced febrile response in aged animals. To test this idea, antiserum specific to MSH was administered intracerebroventricularly to rabbits of known age. The antiserum given according to three schedules of treatment augmented fever caused by IV injections of interleukin-1 (IL-1) in young (less than 2 years) male and female rabbits. Aged female rabbits (3-5+ years) and females aged 2-3 years showed significant augmentation of fever only after pretreatment plus acute injection of antiserum. A single ICV injection of MSH (200 ng) reduced fever in all groups with the greatest antipyretic effect in the aged females. The results indicate that while aged rabbits have an increased antipyretic response to central MSH, binding of the endogenous peptide does not result in marked increases in fever in these animals. Thus, whereas a change in central MSH sensitivity may contribute to reduced fever in aged homeotherms, a reduction in central pyrogen receptors appears to be the most parsimonious explanation.     

Observations on the central mechanism of acetylsalicylate antipyresis.

Leukocytic pyrogen and sodium acetylsalicylate (ASA) were microinjected into the anterior hypothalamus (AH) of urethane anesthetized cats under thermoneutral conditions. The responses of single, identified thermoregulatory neurons in the AH to leukocytic pyrogen placed in the contralateral AH were examined initially. The attempt was then made to determine the effect of ASA on the activity of pyrogen challenged neurons when injected into the AH area opposite to that which received the pyrogen. Stereotaxically located AH units were defined as thermoregulatory by their response to body surface and hypothalamic thermal stimulation; both “thermopositive” (warm) and “thermonegative” (cold) types were studied. Administration of the leukocytic pyrogen was always followed by a change in the discharge rate of the distal thermoregulatory neurons. About half of the units were excited and half were depressed regardless of their thermoresponsive behavior. When ASA was injected after the pyrogen into the opposite AH area, it always altered the neuronal activity and the change was invariably in the direction of the unit's thermal response. The findings indicate that the antipyresis produced by ASA is not due to competition with leukocytic pyrogen for a common receptor site.     

Ultrastructural organization of the hypothalamic medial preoptic area and its changes under the effect of pyrogens

In order to study structural bases of central mechanisms of thermoregulation, a comparative electron microscopic analysis of various cellular groups in one of thermosensitive zones of the cat hypothalamic area--the medial preoptic area--have been carried out under conditions of experimental fever. The latter is produced by injection of pyrogen of bacterial origin--pyrogenal--to the animals. Pyrogenal, increasing the body temperature, produces a stimulatory effect on various cellular elements, first of all on leucocytes, monocytes and macrophages in the medial preoptic area, as well as on endotheliocytes of the terminal vessels. Under pyrogenal effect activation of microglial cells and pericytes also takes place, and as a result of the rearrangements, occurring in the structure, they change into macrophages. This is an evidence of their active participation in the immune protection of the brain. According to the data from other investigations, all these activated cells produce peptide interleukin-1, one of the mediators of fever and stimulator of the immune system. Pyrogenal is stated to produce a stimulating effect on the astroglia and on some neurons of the medial preoptic area, that respond with cytoplasm increase and accumulation of numerous organelles. The reactive changes at fever in some neurons of the medial preoptic area can demonstrate that they belong to the thermosensitive pool. A conclusion is made, concerning a complex effect of pyrogenal, that results in a cooperative response of a number of cellular systems of the organism.     

Differences in the mechanisms of the thermoregulatory impairment induced by capsaicin in newborn and adult rats

The function of heat defence was compared in rats pretreated as adults (A-rats) and neonates (N-rats) with capsaicin. The thermoregulatory impairment as tested by whole body heating was similar in A-rats and N-rats. The thermosensitivity and chemosensitivity of the preoptic region (RPO) seemed to be normal in N-rats. A deficiency of RPO mechanisms could be demonstrated in A-rats. It is suggested that the preoptic effects of capsaicin have but a minor significance for the thermoregulatory impairment. It seems that the main cause of decreased heat tolerance is a reduction of peripheral warm sensation due to degeneration of unmyelinated C-fibre primary neurons in both A-rats and N-rats. The results do not support the primary importance of preoptic warm sensation in physiological thermoregulation.     

Antipyretic effect of centrally administered CRF

CRF injected into the third cerebral ventricle (0.5-2.5 micrograms) caused dose-related reductions in fever induced in rabbits by IV administration of leukocytic pyrogen. Control injections of CRF when the same animals were afebrile did not alter normal body temperature. Intravenous injections of 5 and 20 micrograms CRF, doses known to release ACTH and corticosteroids into the bloodstream in other species, did not reduce fever. CRF injected into the cerebral ventricles may be antipyretic per se, or it may reduce fever by virtue of central release of the antipyretic peptides ACTH and alpha-MSH.     

Cyclic GMP alters the firing rate and thermosensitivity of hypothalamic neurons

The rostral hypothalamus, especially the preoptic-anterior hypothalamus (POAH), contains temperature-sensitive and -insensitive neurons that form synaptic networks to control thermoregulatory responses. Previous studies suggest that the cyclic nucleotide cGMP is an important mediator in this neuronal network, since hypothalamic microinjections of cGMP analogs produce hypothermia in several species. In the present study, immunohistochemisty showed that rostral hypothalamic neurons contain cGMP, guanylate cyclase (necessary for cGMP synthesis), and CNG A2 (an important cyclic nucleotide-gated channel). Extracellular electrophysiological activity was recorded from different types of neurons in rat hypothalamic tissue slices. Each recorded neuron was classified according to its thermosensitivity as well as its firing rate response to 2-100 microM 8-bromo-cGMP (a membrane-permeable cGMP analog). cGMP has specific effects on different neurons in the rostral hypothalamus. In the POAH, the cGMP analog decreased the spontaneous firing rate in 45% of temperature-sensitive and -insensitive neurons, an effect that is likely due to cGMP-enhanced hyperpolarizing K(+) currents. This decreased POAH activity could attenuate thermoregulatory responses and produce hypothermia during exposures to cool or neutral ambient temperatures. Although 8-bromo-cGMP did not affect the thermosensitivity of most POAH neurons, it did increase the warm sensitivity of neurons in other hypothalamic regions located dorsal, lateral, and posterior to the POAH. This increased thermosensitivity may be due to pacemaker currents that are facilitated by cyclic nucleotides. If some of these non-POAH thermosensitive neurons promote heat loss or inhibit heat production, then their increased thermosensitivity could contribute to cGMP-induced decreases in body temperature.     

α-MSH injected into the septal region reduces fever in rabbits

In previous research the concentration of α-MSH within the septal region of rabbits increased with fever. This finding raises the possibility that the septal concentration of this peptide, which reduces fever when given both peripherally and intracerebroventricularly, is important to limitation of fever. To test this idea, rabbits with cannulas in the septal region were made febrile by IV injections of leukocytic pyrogen (LP). Injection of α-MSH (1 μg bilaterally) into the septal region did reduce fever, consistent with the idea that the increase in septal α-MSH concentration which occurs naturally in fever limits the febrile response. We also noted late rises in body temperature when experimental and control septal injections were given close together in time. These increases in temperature were similar to those known to occur after injections into the primary temperature control in the PO/AH region. This commonality further strengthens the possibility that septal neurons are important to central modulation of body temperature.     

Central arginine vasopressin and endogenous antipyresis.

Arginine vasopressin (AVP) is a centrally synthesized nonapeptide that exerts classical endocrine effects as well as a host of centrally mediated actions. A strong case can be argued in support of a neurotransmitter-neuromodulator role for AVP. Acting within the central nervous system (CNS), AVP has been demonstrated to be involved in the modulation of febrile body temperature. Because AVP acts to reduce pyrogen-induced fevers, but not normal body temperature, its actions are deemed to be antipyretic. However, to demonstrate an endogenous antipyretic function, AVP must be shown to be active during conditions where fever is naturally suppressed. This review will focus on five such conditions where the absence of pyrogen-induced fever can be linked to the endogenous activity of AVP within the brain. In the neonatal rat pup, the use of specific antagonists to the AVP receptor has revealed a role for CNS AVP in the absence of fever following peripheral injections of bacterial endotoxin. These results may help to explain a similar lack of fever in other newborn species. In parturient animals a reduced or absent febrile response has been linked to the increased presence of AVP within the septal area of the brain. The combined use of AVP receptor antagonism as well as immunohistochemistry has shown enhanced AVP activity within the ventral septal area of the rat and guinea pig brain during tolerance to intravenous pyrogens. These results suggest that the mechanism of fever suppression following repeated systemic injections of bacterial pyrogen includes centrally acting AVP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Early development of neuronal hypothalamic thermosensitivity in birds: influence of epigenetic temperature adaptation

The aim of the study was to investigate the prenatal influence of different incubation temperatures on the early postnatal development of neuronal hypothalamic thermosensitivity in birds. The experiments were carried out in brain slices of 1-, 5- and 10-days-old Muscovy ducklings incubated at 35, 37.5 (control) or 38.5 degrees C during the last week of incubation. Firing rate of neuronal activity was recorded extracellularly during sinusoidal temperature changes. The results show that the temperature experienced prenatally has a clear influence on postnatal neuronal hypothalamic thermosensitivity. For instance, at the 10th day post-hatching, exposure to the cooler prenatal incubation temperature resulted in elevated neuronal hypothalamic warm sensitivity through an increased proportion of warm sensitive neurons and a reduced proportion of cold sensitive neurons in comparison with the control group. Exposure to the warmer prenatal incubation temperature induced the opposite effect. In these age group changes in neuronal hypothalamic thermosensitivity occur in relation to the prenatal temperature experienced (proximate adaptive). During the first days of life, prenatal temperature load induced a significant change in the thermosensitivity of hypothalamic neurons which was independent of the direction of change in incubation temperature in comparison with control conditions (proximate non-adaptive). Changes in the thermosensitivity of hypothalamic neurons after prenatal temperature experiences observed in all age groups may be the result of epigenetic temperature adaptation.     

Antipyrogenic properties of oxytocin

Effects of oxytocin on pyrogenal or endogenous pyrogen-induced fever were studied. Intramuscular injection of oxytocin (0.2 micrograms/kg every half an hour) did not significantly affect the pattern of pyrogenal-induced fever. Constant intravenous injection of oxytocin (0.4 and 4 micrograms/kg/h) 2-4.5 h after pyrogenal decreased the rectal temperature, on an average, by 24% and 31%, respectively. Endogenous pyrogen fever was not attenuated by intravenous oxytocin (4 micrograms/kg/h). The antipyrogenic effect of oxytocin is related to inhibition of endogenous pyrogen synthesis rather than to blockade of its action, which is indicated by a decreased second peak of the temperature curve, inhibition of endogenous pyrogen synthesis in vitro, and persistence of the hyperthermic effect of endogenous pyrogen.     

Site of action of calcium channel blockers in inhibiting endogenous pyrogen fever in rats.

We have demonstrated that the Ca2+ channel blocker verapamil, administered intravenously, exerts an antipyretic effect on the febrile responses of rats to intravenously injected endogenous pyrogen (EP). We have also shown that the same intravenous dose of verapamil is ineffective in blocking fevers induced by the microinjection of exogenous prostaglandin E (PGE) into the organum vasculosum laminae terminalis (OVLT) of rats. Experiments were conducted to determine whether the site of this verapamil antipyresis was in the OVLT itself. The febrile responses of six male Sprague-Dawley rats to EP were determined at thermoneutrality. Verapamil (10 micrograms/rat) was microinjected directly into the OVLT, and the febrile responses to the EP dose were redetermined 15-30 min later. In every case the EP fevers were attenuated after verapamil pretreatment. Intra-OVLT injections of verapamil alone were without effect on body temperature. When the same dose of verapamil was injected into the OVLT 15 min before the injection of PGE into the same site, it had no effect on the ensuing PGE-induced fever. In view of the fact that less than 1/250th of the effective systemic dose of verapamil, when injected into the OVLT, was equally effective in blocking the EP fevers, we conclude that verapamil acts within the OVLT to block fever rather than peripherally. Furthermore, because verapamil administered into the OVLT does not block PGE fevers, it is unlikely that PGE produces fever by acting as a Ca2+ ionophore on hypothalamic neurons.     

Arylsulphatase A and acid phosphatase activities in plasma and leucocytes during LPS fever in the ox (Bos taurus)

Intravenous injection of E. coli LPS (0.5 micrograms/kg) produced the biphasic elevation of rectal temperature (TR) in conscious oxen. The fever was accompanied by a significant increase of the arylsulphatase A (AsA) activities in plasma and in mononuclear leucocytes. In polymorphonuclear cells a substantial decrease of the AsA activity after 1 hr fever was observed. After 3.5 hr of fever the polymorphonuclear activity of AsA restored to normal found before LPS administration. In contrast with AsA, the pyrogenic dose of LPS caused negligible changes of the acid phosphatase (AcP) activities in the sampled materials. Daily-repeated injections of pyrogen into the same oxen attenuated magnitudes of fever as well as AsA responses in plasma and granulocytes. Heat-induced hyperthermia provoked only minute changes of the AsA and AcP.     

Observations on the effect of salicylate in fever and the regulation of body temperature against cold.

Prostaglandins appear to be mediators, within the hypothalamus, of heat production and conservation during fever. We have investigated a possible role of prostaglandins in the nonfebrile rabbit during thermoregulation in the cold. Shorn rabbits were placed in an environment of 20 degrees C, and rectal and ear skin temperatures, shivering and respiratory rates were measured. A continuous intravenous infusion of leucocyte pyrogen was given to establish a constant fever of approximately 1 degree C, and after observation of a stable febrile temperature for 90 min, a single injection of 300 mg of sodium salicylate, followed by a 1.5 mg/min infusion was then given. After the salicylate infusion was begun, rectal temperature began to fall, and reached nonfebrile levels within 90 min. Shivering activity ceased, respiratory rates increased, and in two animals, ear skin temperature increased. When these same rabbits were placed in an environment of 10 degrees C, at a time they were not febrile, and an identical amount of salicylate was given, rectal and ear skin temperatures, shivering and respiratory rates did not change. These results indicate that prostagladins do not appear to be involved in heat production and conservation in the nonfebrile rabbit.     

Neuropeptide alpha-MSH antagonizes IL-6- and TNF-induced fever.

The pro-opiomelanocortin-derived peptide melanocyte stimulating hormone (alpha-MSH) antagonizes the fever induced by several stimuli including endotoxin, endogenous pyrogen, and certain cytokines. To determine if alpha-MSH can antagonize the pyrogenic action of recombinant IL-6 and TNF directly within the central nervous system, the cytokines were injected with and without alpha-MSH (200 ng) into a lateral cerebral ventricle of rabbits and rectal temperature was monitored continuously. Central administration of both cytokines caused fever. However, when alpha-MSH was injected after cytokine administration, the fevers were markedly reduced. The results are consistent with previous observations on the antipyretic effect of alpha-MSH and they show that the peptide can act within the brain to antagonize pyrogenic actions of specific cytokines believed to be important in CNS mediation of fever.     

Further characterization of high mobility group box 1 (HMGB1) as a proinflammatory cytokine: central nervous system effects

High mobility group box 1 (HMGB1), an abundant, highly conserved cellular protein, is widely known as a nuclear DNA-binding protein. HMGB1 has been recently implicated as a proinflammatory cytokine because of its role as a late mediator of endotoxin lethality and ability to stimulate release of proinflammatory cytokines from monocytes. Production of central cytokines is a critical step in the pathway by which endotoxin and peripheral proinflammatory cytokines, including interleukin-1beta (IL-1) and tumor necrosis factor-alpha (TNF), produce sickness behaviors and fever. Intracerebroventricular (ICV) administration of HMGB1 has been shown to increase TNF expression in mouse brain and induce aphagia and taste aversion. Here we show that ICV injections of HMGB1 induce fever and hypothalamic IL-1 in rats. Furthermore, we show that intrathecal administration of HMGB1 produces mechanical allodynia (lowering of the response threshold to calibrated stimuli). Finally, while endotoxin (lipopolysaccharide, LPS) administration elevates IL-1 and TNF mRNA in various brain regions, HMGB1 mRNA is unchanged. It remains possible that HMGB1 protein is released in brain in response to LPS. Nonetheless, these data suggest that HMGB1 may play a role as an endogenous pyrogen and support the concept that HMGB1 has proinflammatory characteristics within the central nervous system.     

The pyrogenic effect of scarlet fever toxin

Scarlet fever toxin was found to liberate leukocytic pyrogen from granulocytesin vitro. In comparative experiments withSalmonella paratyphi B endotoxin and scarlet fever toxin it was tested whether leukocytes from rabbits tolerant to one of these toxins are able to synthetize and liberate endogenous pyrogen. Leukocytes from rabbits tolerant to endotoxin liberated leukoeytic pyrogen following challenge with endotoxin or with scarlet fever toxin. Leukocytes from animals tolerant to scarlet fever toxin liberated leukocytic pyrogen in the presence of endotoxin, but were insensitive to homologous, i.e. scarlet fever toxin. Similarly, leukocytes from cortisone-treated animals did not liberate leukocytic pyrogen if they were incubated with scarlet fever toxin, but liberation of leukocytic pyrogen did take place under challenge with endotoxin. Leukocytes from normal animals incubated in Hanks solution without toxin did not synthetize endogenous pyrogen.     

Fever in rats during normal and dehydrated conditions

The effect of endogenous pyrogen (EP, from rabbit) and endotoxin (Salmonella typhosa) on rectal temperature (Tre) was investigated in normal and dehydrated rats of both sexes. Intraperitoneal injection of either EP or endotoxin did not affect body temperature. In addition, no changes in Tre were observed when endotoxin was injected intravenously in normally hydrated male rats, but significant falls in Tre occurred in normal female rats. However, intravenous injection of EP produced fever in both sexes, but females generally showed smaller responses. A second intravenous injection of endotoxin, given 3 days after the first injection, always produced fever in normally hydrated rats. The pattern of this febrile response was monophasic. In contrast to the response in normal rats, intravenous endotoxin produced significant fevers with a biphasic pattern in dehydrated rats of either sex, but the febrile responses of male rats were greater than those of female rats. On the other hand, there were no significant differences between febrile responses to intravenous EP exhibited by normal and dehydrated animals. These results show that rats of both sexes possess physiological mechanisms capable of producing a fever following intravenous injections of EP.