Endotoxin fever in granulocytopenic rats: evidence that brain cyclooxygenase-2 is more important than circulating prostaglandin E2

PGE(2) is a recognized mediator of many fevers, and cyclooxygenase (COX) is the major therapeutic target for antipyretic therapy. The source, as well as the site of action of PGE(2), as an endogenous pyrogen, is widely accepted as being central, but PGE(2) in the circulation, possibly from leukocytes, may also contribute to the development of fever. However, bacterial infections are important causes of high fever in patients receiving myelosuppressive chemotherapy, and such fevers persist despite the use of COX inhibitors. In the study reported here, the febrile response to bacterial LPS was measured in rats made leukopenic by cyclophosphamide. A striking increase in LPS fever occurred in these granulocytopenic rats when compared with febrile responses in normal animals. Unlike LPS fever in normal rats, fever in granulocytopenic rats was neither accompanied by an increase in blood PGE(2) nor inhibited by ibuprofen. Both leukopenic and normal rats showed LPS-induced COX-2-immunoreactivity in cells associated with brain blood vessels. Furthermore, LPS induced an increase of PGE(2) in cerebrospinal fluid. Induction of COX-2-expression and PGE(2) production was inhibited by ibuprofen in normal but not in leukopenic rats. Although the results presented are, in part, confirmatory, they add new information to this field and open a number of important questions as yet unresolved. Overall, the present results indicate that, in contrast to immunocompetent rats, leukocytes and/or other mechanisms other than PGE(2) are implicated in the mechanisms restricting and reducing the enhanced febrile response to endotoxin in immunosuppressed hosts.

Reactions of Artemisinin and Arteether with Acid: Implications for Stability and Mode of Antimalarial Action

The currently accepted mechanism of trioxane antimalarial action involves generation of free radicals within or near susceptible sites probably arising from the production of distonic radical anions. An alternative mechanistic proposal involving the ionic scission of the peroxide group and consequent generation of a carbocation at C-4 has been suggested to account for antimalarial activity. We have investigated this latter mechanism using DFT (B3LYP/6-31+G* level) and established the preferred Lewis acid protonation sites (artemisinin O5a>>O4a approximately O3a>O2a>O1a; arteether O4a>or=O3a>O5b>>O2a>O1a; Figure 3) and the consequent decomposition pathways and hydrolysis sites. In neither molecule is protonation likely to occur on the peroxide bond O1-O2 and therefore lead to scission. Therefore, the alternative radical pathway remains the likeliest explanation for antimalarial action.

Changes in mGlu5 receptor expression in the basal ganglia of reserpinised rats

Dopamine depletion in Parkinson's disease results in a series of pathophysiological changes in the basal ganglia circuitry. Increased release of glutamate plays an important role in this motor disorder, therefore, agents interacting with glutamatergic transmission may have therapeutic potential. In this study we investigated changes in both mRNA expression and the number of binding sites of the mGlu5 receptor in a reserpinised rat model of Parkinson's disease. The in situ hybridisation demonstrated that acute reserpine treatment caused a significant decrease in the expression of mGlu5 receptor mRNA in the rostral and caudal parts of the rat striatum. At the same time, tritium-labelled 2-ethyl-6-(phenylethynyl)-pyridine ([(3)H]MPEP) ligand binding experiments detected a significant increase in the total number of mGlu5 receptors in the same region of the motor loop. These apparently contradictory data can be explained by mGlu5 receptor turnover being down-regulated in reserpinised rats, due possibly to an imbalance in the rates of synthesis/insertion and internalisation/degradation of the receptor. These findings suggest that changes such as these affecting mGlu5 receptors may be involved in the pathophysiological consequences of dopamine depletion in the brain.

Simultaneous increases in immune-competent cells and nitric oxide in the spleen during Plasmodium berghei infection in mice

BACKGROUND AND PURPOSE

Nitric oxide and other reactive nitrogen intermediates (RNI) are thought to be important mediators of both immunological and pathological responses of the vertebrate host to malaria infection. The role of RNI has been studied most often by assay of stable RNI metabolites (nitrites, nitrates) in blood. This study evaluated the nature of the RNI response of mice to malaria by analyzing the subsets of immune-competent cells within the organ displaying increased RNI in vivo.

METHODS

We measured RNI production indirectly, as stable metabolites of nitric oxide activity in tissue homogenates (brain, liver, spleen) from mice infected with Plasmodium berghei. Only spleen exhibited an RNI concentration response during rising parasitemia. Subsets of immune-competent cells (B cells, CD19+), macrophages/monocytes (MOMA2+) and T cells (CD4+, CD8+) in the spleen were assayed by fluorescence activated cell scan flow cytometry.

RESULTS

The spleen was confirmed as a major source of RNI during mid-phase P. berghei infection. Significant increases in CD19+ and MOMA2+ spleen cells were evident during the mid-phase of P. berghei infection in MF1 mice when RNI are maximally elevated.

CONCLUSIONS

The time courses of the cellular and RNI responses indicate that CD19+ and MOMA2+ cells may be responsible for the increase in RNI in the spleen. However, experiments in vitro are needed to make a definitive identification of the cell type(s) responsible for the increase in RNI in the mouse spleen during P. berghei infection.

Mapping Antimalarial Pharmacophores as a Useful Tool for the Rapid Discovery of Drugs Effective in Vivo: Design, Construction, Characterization, and Pharmacology of Metaquine

Resistant strains of Plasmodium falciparum and the unavailability of useful antimalarial vaccines reinforce the need to develop new efficacious antimalarials. This study details a pharmacophore model that has been used to identify a potent, soluble, orally bioavailable antimalarial bisquinoline, metaquine (N,N'-bis(7-chloroquinolin-4-yl)benzene-1,3-diamine) (dihydrochloride), which is active against Plasmodium berghei in vivo (oral ID(50) of 25 micromol/kg) and multidrug-resistant Plasmodium falciparum K1 in vitro (0.17 microM). Metaquine shows strong affinity for the putative antimalarial receptor, heme at pH 7.4 in aqueous DMSO. Both crystallographic analyses and quantum mechanical calculations (HF/6-31+G) reveal important regions of protonation and bonding thought to persist at parasitic vacuolar pH concordant with our receptor model. Formation of drug-heme adduct in solution was confirmed using high-resolution positive ion electrospray mass spectrometry. Metaquine showed strong binding with the receptor in a 1:1 ratio (log K = 5.7 +/- 0.1) that was predicted by molecular mechanics calculations. This study illustrates a rational multidisciplinary approach for the development of new 4-aminoquinoline antimalarials, with efficacy superior to chloroquine, based on the use of a pharmacophore model.

Pharmacological assessment of the role of nitric oxide in mice infected with lethal and nonlethal species of malaria

This pharmacological investigation sought to determine whether nitric oxide (NO) had an antiparasitic effect and/or mediated pathology in mice infected with nonlethal P. chabaudi or lethal P. berghei. Nitric oxide synthase (NOS) inhibitors were evaluated for their ability to inhibit the rise in reactive nitrogen intermediates (RNI) induced by bacterial lipopolysaccharide (LPS) in mice. The more effective compound, aminoguanidine (AG) inhibited the rise in RNI induced by P. chabaudi and increased mortality, but had no effect on parasitaemia. Inducers and donors of NO were screened for their ability to increase RNI and the most effective agents evaluated for their ability to modify P. berghei infection. S-Nitrosoglutathione had little effect, but LPS decreased parasitaemia and mortality. An inconsistent relationship is evident between the abilities of these agents to modify NO activity and their effects on malaria in mice. Increased mortality in mice with P. chabaudi treated with AG indicates a reduction in resistance. The absence of an effect on parasitaemia by a NOS inhibitor or NO donor indicates either RNI have insignificant antimalarial action in vivo or the efficacy of the compounds is inadequade. Resistance to P. berghei in LPS-treated mice demonstrates an antiparasitic effect, but this may be attributable to factors other than NO.

The role of nitric oxide and its up/downstream molecules in malaria: cytotoxic or preventive?

The current study investigated the involvement of nitric oxide (NO) and related molecules in malaria target organs of outbred MF1 mice during lethal Plasmodium berghei and non-lethal P. c. chabaudi infections, in order to evaluate whether changes in NO production are beneficial or detrimental to the host. A number of methods have been applied to test this hypothesis, including Griess microassay, electrochemical assay, RT-PCR and Western blot. The results show that reactive nitrogen intermediate (RNI) accumulation, in vitro levels of endogenous NO production, inducible nitric oxide synthase (iNOS) mRNA induction and NOS protein expression altered during murine malaria. The changes depended upon the tissue, the day of infection, the degree of parasitemia, the strain of Plasmodia and the method of measuring NO biosynthesis. Differences in the pathology of two strains of Plasmodia appear to depend more on the strain of parasite rather than the strain of host. The involvement of NO and its up/downstream molecules in murine malaria are specified to host/parasite combinations and it is influenced by the method used to assess NO. The anti-parasitic function against Plasmodia did not relate only to NO in this study, but a complex process consisting of NO and other immune factors is required to resolve the parasite. Selective delivery of inhibitors and donors of NO synthesis in the tissues of the malarial host is indicated as a potential novel therapy to inhibit the parasite or prevent its pathological symptoms.

Expression of inducible nitric oxide synthase (iNOS) mRNA in target organs of lethal and non-lethal strains of murine malaria

Nitric oxide (NO) is a putative mediator of the immunological and/or pathological responses to malaria, consequently it is a potential target for novel drug therapy. Numerous cell types increase expression of inducible nitric oxide synthase (iNOS) under inflammatory conditions, the most relevant stimuli being cytokines and endotoxins. In this study the expression of iNOS mRNA in several target organs (brain, liver, spleen) of malaria have been investigated in MF1 mice during lethal Plasmodium (P.) berghei and non-lethal P. c. chabaudi infection. In P. berghei malaria, iNOS mRNA decreased in liver and was unchanged in spleen during the period of rising parasitaemia, but increased in both organs late in the infection, when parasitaemia was high and death imminent. In mice infected with P. c. chabaudi, spleen iNOS mRNA increased progressively throughout the early, peak and recovery periods of parasitaemia, but decreased in liver. Brain iNOS mRNA decreased in samples collected throughout the time courses of both infections. Hence it is evident that changes in iNOS mRNA in murine malaria depend upon the tissue, day of infection, degree of parasitaemia and strain of Plasmodium. These data indicate induction of iNOS mRNA in the spleen has a role in combating these strains of Plasmodium in MF1 mice. Failure to clear lethal P. berghei parasitaemia was associated with increased iNOS mRNA expression in the liver, which may contribute to the pathology of this malaria.

Nitric oxide and reactive nitrogen intermediates during lethal and nonlethal strains of murine malaria

The virulence of Plasmodia depends partly on the strain of parasite and partly on the host. In this study, Plasmodium berghei N/13/1A/4/203 caused the death of mice, whereas Plasmodium chabaudi chabaudi AS was not lethal. Current opinion is that nitric oxide (NO) and other reactive nitrogen intermediates (RNI) are produced in several host organs during malaria to resist infection or produce tissue damage. NO and RNI production in blood or plasma, brain, liver and spleen in MF1 mice was investigated during P. berghei and P. c. chabaudi infection, in order to help determine whether changes in NO production are beneficial or detrimental to the host in vivo. NO production was measured both directly and indirectly as nitrites and nitrates, to represent RNI. No changes in blood NO were detected in P. berghei infected mice, but increases were observed in brain, liver and spleen. In P. c. chabaudi infected mice, rises in NO concentration were observed in blood and spleen, whereas a decline in liver NO was seen, but there were no changes in brain. Liver contained the highest concentration of RNI, but increasing concentrations were seen in both plasma and spleen in both P. berghei and P. c. chabaudi infected mice. These results show that NO and RNI production alters during murine malaria. The changes depend upon the tissue, the day of infection, the degree of parasitaemia, the strain of Plasmodia and the method of measuring NO biosynthesis. Lethal P. berghei induced NO production in the mid and late stages of infection in mice when parasitaemia was high, whereas in nonlethal P. c. chabaudi infection, NO production was increased in the early and late stages when parasitaemia was low. These data are consistent with a role for NO in the protection of the MF1 mouse against Plasmodia. Failure to clear the parasite is associated with evidence of increased NO production in brain and liver, which may contribute to the pathology of malaria, but this hypothesis requires confirmation from other experimental approaches.

Immunomodulatory effects of pharmacological elevation of cyclic AMP in T lymphocytes proceed via a protein kinase A independent mechanism

The role of the cAMP pathway as an immunomodulatory system has been an area of intensive research. Pharmacological elevation of the cAMP pathway inhibits T lymphocyte proliferation and production of Th1-type cytokines. The effects of cAMP are thought to be mediated via activation of the intracellular receptor, protein kinase A (PKA). We investigated the inhibitory effects of cAMP elevation on human lymphocyte proliferation and function by utilising a range of selective inhibitors of PKA. Elevation of cAMP activity by dbcAMP, Sp-cAMPS and forskolin induced significant decreases of Con A stimulated PBMC proliferation. Co-incubation with the selective PKA inhibitors HA1004, KT5720 and Rp-cAMPS showed these antiproliferative effects to persist, despite measurable PKA activity being inhibited to that of untreated cells or less. IL-2 production was also inhibited by dbcAMP in the presence of HA1004 and Rp-cAMPS. It has been demonstrated that the inhibitory effects of pharmacological elevations in cAMP on human T cell proliferation and IL-2 production do not require PKA activity. These observations indicate that control of lymphocyte proliferation and functional status by cAMP proceeds through PKA-independent events. Identification of the underlying mechanisms behind these effects would increase our understanding of the cAMP cascade and may provide a potentially novel target for immunomodulation.

Nitroglycerine reduces neutrophil activation and acute damage in latissimus dorsi muscle grafts

BACKGROUND

Damage to the latissimus dorsi muscle (LDM) may jeopardize a successful outcome to dynamic cardiomyoplasty. We and others have demonstrated muscle damage in LDM in various species including humans. Ischemia is now recognized to be an important contributory factor. We postulated that glyceryl trinitrate, a nitric oxide donor, might protect against ischemic endothelial dysfunction and so reduce resultant muscle damage.

METHODS

In 20 adult rats the left LDM was mobilized on its thoracodorsal neurovascular pedicle and maintained as an orthotopic graft. Half of the animals received glycerol trinitrate intraoperatively and postoperatively for 24 hours. The other half served as untreated controls. Each group was further subdivided into two groups (n = 5 in each): animals in which the LDM was excised after 4 hours for myeloperoxidase studies, and animals in which the LDM was excised at 24 hours for analysis of muscle damage by histology and enzyme macrohistochemistry. Blood samples were taken at 24 hours for assay of plasma nitrite and nitrate as nitric oxide metabolites.

RESULTS

Glycerol trinitrate-treated animals had higher plasma nitric oxide metabolite levels after 24 hours (after nitrate reductase treatment, total nitrite, 78.3+/-11.8 nmol/mL, mean +/- SEM) than controls (42.1+/-3.7 nmol/ mL, p = 0.008). The proportion of viable LDM in glycerol trinitrate-treated animals was greater than in untreated animals, mainly in the middle and distal regions of the graft (middle region, 96.3%+/-0.5% versus 75.7%+/-4.1%, p<0.001; distal region, 94.4%+/-0.8% versus 40.9%+/-3.1%, p<0.001). Macrohistochemical findings correlated well with the histologic findings. Myeloperoxidase activity (U/g) was markedly lower in glycerol trinitrate-treated LDMs, mainly in the distal part of the graft (glycerol trinitrate versus control, 20.5+/-2.1 versus 40.9+/-3.1 U/g, p<0.001).

CONCLUSIONS

Glycerol trinitrate significantly reduced acute damage to the distal two-thirds of the mobilized LDM, possibly by modifying leukocyte activation and endothelial dysfunction associated with ischemic injury.

Inhibitors of glutathione reductase as potential antimalarial drugs. Kinetic cooperativity and effect of dimethyl sulphoxide on inhibition kinetics

We have developed inhibitors of glutathione reductase that improve on the inhibition of literature lead compounds by up to three orders of magnitude. Thus, analogues of Safranine O and menadione were found to be strong, reversible inhibitors of yeast glutathione reductase. Safranine O exhibited partial, uncompetitive inhibition with Ki and alpha values of 0.5 mM and 0.15, respectively. Thionine O was a partial (hyperbolic) uncompetitive inhibitor with Ki and alpha values of 0.4 microM and 0.15, respectively. LY83583 and 2-anilino-1,4-naphthoquinone also showed (hyperbolic) partial, uncompetitive inhibition with micromolar Ki values. For Nile Blue A a model for two-site binding with (parabolic) uncompetitive inhibition fitted the data with a Ki value of 11 microM and a kinetic cooperativity between the sites of 0.12, increased to 0.46 by preincubation of the enzyme and Nile Blue A in the presence of glutathione disulphide. Analysis of the effects of preincubation on the kinetics and cooperativity indicated the possibility of a slow conformational change in the homodimeric enzyme, the first such indication of kinetic cooperativity in the native enzyme to our knowledge. Further evidence of conformational changes for this enzyme came from studies of the effects of dimethyl sulphoxide which indicated that this co-solvent, which at low concentrations has no apparent effect on initial velocities under normal assay conditions, induced a slow conformational change in the enzyme. Thionine O, Nile Blue A and LY83583 were redox-cycling substrates producing superoxide ion, detectable by means of cytochrome c reduction, but leading to no loss of glutathione reductase activity, under aerobic or anaerobic conditions. The water-soluble Safranine analogues Methylene Blue, Methylene Green, Nile Blue A and Thionine O (5 mg/kg i.p. x 5) were effective antimalarial agents in vivo against P. berghei, but their effect was small and a higher dose (50 mg/kg i.p. x 1) was toxic in mice. Comparison was made with human glutathione reductase and its literature-reported interactions with several tricyclic inhibitors as studied by X-ray diffraction. It is possible that the conformational changes detected in the present study from alterations in detailed kinetic inhibition mechanisms may shed light on information transfer through the glutathione reductase molecule from the dimer interface ligand pocket to the active-site.

Novel aryl-bis-quinolines with antimalarial activity in-vivo

Three rationally designed isomeric aryl-bridged bis-quinolines, N1,Nx-bis(7-chloroquinolin-4-yl)phenylene-1,x-diamines, where x=2, 3 or 4, i.e. o-, m- and p-substituted analogues respectively, were synthesized and evaluated against Plasmodium berghei in-vivo. The compound with x=2 had an ID50 of 30 mg kg(-1), whereas the p-substituted analogue (x=4) was not statistically schizonticidal at either of the two dose levels tested in olive oil-dimethylsulphoxide (5 and 25 mg kg(-1), ID50=60 mg kg(-1) approx.). When the delivery vehicle was changed to saline-DMSO, antimalarial potency increased for the p-substituted compound (ID50 17 mg kg(-1)). In contrast, the m-substituted analogue had marked antimalarial activity (ID50 1.2 mg kg(-1)), which compares favourably with that of chloroquine diphosphate (ID50 = 4.3 mg kg(-1)). The data presented show that the aminomethylene side chain in amodiaquine can be successfully replaced by a 7-halo-4-aminoquinoline, establishing that carbon bridges containing less than four contiguous carbon atoms can be present within highly active aryl-substituted 4-aminoquinoline antimalarials. These results confirm that the presence of an OH group in the aryl bridge is not necessary for antimalarial activity and substantiate the view that, despite the appearance of resistant strains, new and existing aminoquinolines still have an important role in treating malaria.

Rational drug design approach for overcoming drug resistance: application to pyrimethamine resistance in malaria

Pyrimethamine acts by selectively inhibiting malarial dihydrofolate reductase-thymidylate synthase (DHFR-TS). Resistance in the most important human parasite, Plasmodium falciparum, initially results from an S108N mutation in the DHFR domain, with additional mutation (most commonly C59R or N51I or both) imparting much greater resistance. From a homology model of the 3-D structure of DHFR-TS, rational drug design techniques have been used to design and subsequently synthesize inhibitors able to overcome malarial pyrimethamine resistance. Compared to pyrimethamine (Ki 1.5 nM) with purified recombinant DHFR fromP. falciparum, the Ki value of the m-methoxy analogue of pyrimethamine was 1.07 nM, but against the DHFR bearing the double mutation (C59R + S108N), the Ki values for pyrimethamine and the m-methoxy analogue were 71.7 and 14.0 nM, respectively. The m-chloro analogue of pyrimethamine was a stronger inhibitor of both wild-type DHFR (with Ki 0.30 nM) and the doubly mutant (C59R +S108N) purified enzyme (with Ki 2.40 nM). Growth of parasite cultures of P. falciparum in vitro was also strongly inhibited by these compounds with 50% inhibition of growth occurring at 3.7 microM for the m-methoxy and 0.6 microM for the m-chloro compounds with the K1 parasite line bearing the double mutation (S108N + C59R), compared to 10.2 microM for pyrimethamine. These inhibitors were also found in preliminary studies to retain antimalarial activity in vivo in P. berghei-infected mice.

Febrile response to tissue inflammation involves both peripheral and brain IL-1 and TNF-alpha in the rat

We investigated the role and interaction between tumor necrosis factor (TNF)-alpha, interleukin (IL)-1, and IL-6 in the development of fever and their involvement in brain and systemic pathways in response to localized tissue inflammation caused by injection of turpentine (TPS) in the rat. Intramuscular injection of 10 microl TPS caused significant increases in body temperature, of up to 2 degrees C, compared with saline-treated animals. Fevers were maximal 7-8 h after injection and were preceded by significant increases in plasma bioactive IL-6. No changes in circulating bioactive IL-1 or TNF-alpha were detected. Systemic injection of IL-1 receptor antagonist (IL-1ra, 2 mg/kg i.p.) or anti-TNF-alpha antiserum (0.4 ml i.v.) almost completely abolished the febrile responses to TPS over 8 h and markedly inhibited the rise in plasma IL-6 bioactivity measured 6 h after TPS. To test the involvement of brain cytokines, anti-TNF-alpha antiserum and IL-1ra were injected intracerebroventricularly. Injections of anti-TNF-alpha antiserum (3 microl/rat i.c.v.) or IL-1ra (400 microg/kg i.c.v.) significantly (P < 0.01 and P < 0.05, respectively) inhibited fever induced by TPS. These data suggest that both localized peripheral and brain IL-1 and TNF-alpha are involved directly in the pyrogenic response to inflammation. The results indicate that, in the periphery, IL-1 and TNF-alpha cause increased production of IL-6, the most likely candidate as a circulating endogenous pyrogen.

An exploration of the structure-activity relationships of 4-aminoquinolines: novel antimalarials with activity in-vivo

The structure-activity relationships of bisquinolines, a potentially important group of novel antimalarial drugs, were studied. The high-temperature (180-250 degrees C) synthesis of 4-aminoquinolines, including bisquinolines, by nucleophilic displacement was both fast and efficient Several bisquinolines including (+/-)-trans-N1,N2-bis(7-trifluoroquinolin-4-yl)cyclohexane-1, 2-diamine and 1R,2R-(-)-, 1S,2S-(+)-, (+/-)-trans- and cis-N1, N2-bis(7-chloroquinolin-4-yl)cyclohexane-1,2-diamine exhibited potent activity against Plasmodium berghei in mice; (+/-)-trans-N1,N2-bis(7-chloroquinolin-4-yl)cyclohexane-1, 2-diamine was orally active. Our results indicate that these compounds conform to a putative receptor for quinoline antimalarials. In addition, a 7-haloquinoline linked by a heterocyclic bridge, at the 4-position, to another heterocycle (such as an acridine at the 9-position) maximally occupies the active site of our postulated target.

Interleukin-1 receptor antagonist inhibits endotoxin fever and systemic interleukin-6 induction in the rat

Although a number of studies indicate that the pyrogenic activity of lipopolysaccharide (LPS) and/or interleukin (IL)-1 is mediated via induction of IL-6, this has been questioned by recent evidence demonstrating a dissociation between fever and circulating IL-6. The present study reexamines this relationship by use of human recombinant interleukin-1 receptor antagonist (IL-1ra). Injection of LPS (100 micrograms/kg ip) into rats induced fever (2.0 degrees C) that was significantly inhibited (P < 0.05) when IL-1ra (16 mg/kg ip) was given 1 and 2 h after LPS. The rise in plasma IL-6 preceded the febrile response by 1-1.5 h and, although the concentrations of bioactive IL-6 in plasma and cerebrospinal fluid (CSF) were not reduced at 4 h, at 2 h plasma and CSF IL-6 bioactivity was inhibited by 80 and 70%, respectively, after a single injection of IL-1ra (16 mg/kg ip). Intracerebroventricular injection of IL-1ra (200 micrograms/rat) inhibited LPS fever but did not affect the plasma IL-6 bioactivity measured 2 or 4 h after intraperitoneal LPS. These data show that peripheral IL-1 plays a part in the induction of both fever and the rise in plasma IL-6 that precedes it, and that IL-1 within the brain is also important in the induction of fever by LPS.

Adrenocorticotropic hormone as a potential enhancer of T-lymphocyte function in the rat mixed lymphocyte reaction

The effects of adrenocorticotropic hormone (ACTH) (10(-6) to 1 U/ml) on T-lymphocyte proliferation and function were studied in the rat mixed lymphocyte reaction (MLR). ACTH produced a modest increase in lymphocyte proliferation on day 3 in lymph node (LN) cells and on day 5 in spleen cells. In addition, LN MLR cells, activated in the presence of ACTH, showed a higher proliferative response to restimulation on day 5 and on day 11 of the primary culture. Cytotoxic activity and the number of IL-2R- cells were increased in ACTH-treated LN MLR cultures in experiments where control MLR levels were low. ACTH also overcame the generation of low levels of suppressor activity in spleen MLR cells. These findings indicate that ACTH could play a role in increasing the priming of T-lymphocytes and enhancing, in particular, suboptimal primary responses.

Blood-brain barrier damage in experimental African trypanosomiasis

African sleeping sickness is characterized by progressive central nervous system (CNS) involvement, leading to the so-called secondary or late stage in which there are widespread inflammatory changes with lymphoplasmocytic infiltration. A study was made of blood-brain barrier (BBB) integrity in the late stages of a rodent model by assessing the uptake of the fluorescent fluid-phase marker sulphorhodamine B into the brain tissue. Brain oedema was estimated from brain weight, density and electrolyte concentrations. Trypanosome distribution was studied by light and electron microscopy. At 35 days post-infection (p.i.) fluorescent dye penetration occurred in several brain regions, including thalamus and hypothalamus. At 40 days p.i., BBB damage was extensive, with dye penetration throughout both the grey and the white matter of the cortex. Infected rats had significantly higher brain water content than uninfected controls and altered sodium and potassium concentrations characteristic of vasogenic oedema. The morphological studies showed early accumulation of parasites within, and associated damage to the choroid plexus, and, in the late stages, the presence of small numbers of trypansomes scattered in the nerve tissue of the brain and spinal cord, similar to previous descriptions. The findings show that chronic trypanosomiasis in the rat model is accompanied by BBB damage and vasogenic oedema.

Cyclic AMP inhibits macrophage suppressor function and enhances lymphocyte proliferation

The effects of increasing the level of cyclic AMP (cAMP) activity on lymphocyte proliferation in the rat mixed lymphocyte reaction (MLR) were investigated. Dibutyryl cAMP (dbcAMP) and prostaglandin E2 (PGE2) produced a dose-dependent reduction in proliferation in the lymph node (LN) MLR, but produced a substantial increase in proliferation in the spleen MLR at the lower concentrations used (10(-5)-10(-4) M dbcAMP; 10(-7)-10(-6) M PGE2). Enhancement of proliferation was dependent on the presence of macrophages and was probably due to inhibition of macrophage activation. This was based on the following findings: (1) spleen MLR proliferation was lower than that in the LN MLR; (2) depletion of spleen macrophages increased proliferation in the spleen MLR and addition of these macrophages to the LN MLR reduced proliferation; (3) macrophage depletion from the spleen MLR abolished the proliferation-enhancing effect of dbcAMP. In conclusion, cAMP enhances lymphocyte proliferation in this system, apparently as a consequence of suppressing the inhibitory influence of macrophages.

Importance of brain IL-1 type II receptors in fever and thermogenesis in the rat

Interleukin-1 (IL-1) acts centrally to induce fever and thermogenesis in rodents. The central actions of IL-1 alpha and IL-1 beta apparently involve different mechanisms, and the effects of IL-1 beta are not consistent with interaction with a type I (IL-1RI) 80-kDa receptor. In the present study the involvement of the type II IL-1 receptor (IL-1RII) was tested in the rat by examining the effects of central injection of a monoclonal antibody (ALVA-42), which blocks the IL-1RII. Pretreatment of rats with ALVA-42 (6 micrograms icv) inhibited the thermogenic and pyrogenic responses to intracerebroventricular injection of 5 ng (but not 50 ng) of IL-1 beta in conscious rats but did not significantly modify responses to IL-1 alpha. ALVA-42 also failed to modify the responses to peripherally administered IL-1 beta (1 microgram) but significantly attenuated the pyrogenic and thermogenic responses to peripheral (125 micrograms) or central (1 microgram) injection of endotoxin. These data indicate that IL-1RII mediates the central effects of a low dose of IL-1 beta, but not IL-1 alpha, on fever and thermogenesis in the rat. They also imply that responses to endotoxin are due, at least in part, to the activation of IL-1RII by IL-1 beta released within the brain and that effects of peripherally injected IL-1 beta involve different mechanisms, probably associated with IL-1RI.

Chronic effects of interleukin-1 beta on fever, oxygen consumption and food intake in the rat

Chronic subcutaneous infusion (from osmotic minipumps) of IL-1 beta (1 microgram/d) in male rats over seven days caused transient (1-3 d) increases in body temperature and reductions in body weight gain and food intake. By day 3, when colonic temperature was similar for vehicle and IL-1 infused groups, the acute responses (increases in temperature and VO2) to a maximal dose (1 microgram, sc) of IL-1 beta was almost identical in all animals. In a separate study intraperitoneal infusion of the same dose of IL-1 beta (1 microgram/d) increased the duration of changes in body temperature, weight and food intake, compared to subcutaneous infusion. In further groups of rats, pyrogenic responses to daily injections of IL-1 beta (1 microgram ip) were sustained for the entire 7 d period, but this treatment did not affect body weight. These data demonstrate that tolerance to infusion of IL-1 is not accompanied by reduced maximal responses to acute administration of IL-1, and indicate that more sustained effects of IL-1 are achieved by intraperitoneal rather than subcutaneous infusions, or by repetitive daily injections of the cytokine. These observations indicate that low levels of IL-1 release, maintained over periods of several days could be responsible for changes in body temperature and energy balance during chronic infections or inflammation.

Adrenalectomy reverses the impaired pyrogenic responses to interleukin-beta in obese Zucker rats

Interleukin-1 is an important endogenous pyrogen which stimulates thermogenesis in normal animals by a central action which is dependent on release of corticotrophin releasing factor (CRF). Central injection of murine recombinant interleukin-1 beta (IL-1 beta, 5 ng) in conscious lean (+/?) Zucker rats produced significant increases in resting oxygen consumption (VO2, 26 per cent), colonic temperature (1.3 degrees C) and thermogenic activity (mitochondrial GDP binding) of brown adipose tissue (BAT, 24 per cent). In contrast, genetically obese (fa/fa) Zucker rats showed nonsignificant changes in VO2 (4 per cent), temperature (0.5 degrees C) and BAT activity (0 per cent). Bilateral surgical adrenalectomy (ADX) dramatically enhanced the effects of IL-1 beta on VO2 (45 per cent) body temperature (1.8 degrees C) and BAT activity (44 per cent) in obese mutants, but only slightly increased responses in lean rats. These data suggest that impaired responses to IL-1 beta in obese mutants may be due to inhibitory actions of glucocorticoids on either prostaglandin synthesis or CRF release within the hypothalamus.

Effects of malaria on O2 consumption and brown adipose tissue activity in mice

Increased energy expenditure often occurs during illness or after injection of endotoxin and can contribute to the generation of fever. In laboratory rats and mice the thermogenic response has been attributed to the sympathetic activation of brown adipose tissue (BAT), although mice often fail to show pyrexia. In this study the effects of malaria on O2 consumption and BAT were studied in mice inoculated with Plasmodium berghei. Parasitemia was maximal (greater than 50% of erythrocytes showing positive Leishman staining) 72 h after inoculation. Up to this time body weight and food intake were similar to values for control mice, although colonic temperatures were slightly depressed in infected mice. Thereafter, infected mice showed marked hypophagia, loss of body weight, and severe hypothermia; colonic temperature was less than 31 degrees C at 96 h when the experiment was terminated. Resting O2 consumption (VO2) measured at 24 degrees C was slightly elevated in infected mice 12 h after inoculation and reached a peak value (31% above controls) at 48 h. VO2 returned to the same value as controls at 96 h. In vitro thermogenic activity of BAT (assessed from binding of guanosine diphosphate to isolated mitochondria) was not significantly altered in infected mice. These data demonstrate a marked thermogenic response to malarial infection, but this is not accompanied by fever in mice and is dissociated from stimulation of BAT activity.

Ergometrine and 5-hydroxytryptamine binding sites in rat brain and myometrium

Functional studies suggest that ergometrine is a partial agonist involving 5-hydroxytryptamine (5-HT) receptors in rat uterus. Ergometrine displaced [3H]5-HT from specific binding sites in rat brain, but did not displace [3H]5-HT at functionally important concentrations in rat myometrium. These binding studies indicate that the agonist and antagonist actions of ergometrine in rat uterus arise from its initial interaction with binding sites other than those for 5-HT.

Central activation of thermogenesis and fever by interleukin-1 beta and interleukin-1 alpha involves different mechanisms

Interleukin-1 exists in two forms (alpha and beta) which are assumed to act on the same receptor. Both forms of the molecule stimulated fever and thermogenesis in the rat when injected into the brain, but interleukin-1 beta was more effective, and combined injection of alpha and beta elicited additive responses. The actions of interleukin-1 beta were inhibited by pretreatment of the animals with either a receptor antagonist or monoclonal antibody to corticotrophin releasing factor. The effects of interleukin-1 alpha were unaltered by these treatments. The results indicate that brain corticotrophin releasing factor mediates thermogenesis and fever induced by interleukin-1 beta but not by interleukin-1 alpha.

Pyrogenic and thermogenic effects of interleukin 1 beta in the rat

Single injections of recombinant human interleukin 1 beta (IL-1 beta) caused large (up to 2 degrees C) and sustained (3 h) increases in body temperature in conscious rats. Intracerebroventricular injections (10-100 ng) were much more effective and elicited greater responses than intravenous injections (0.1-1 microgram). IL-1 beta increased resting oxygen consumption by 25-49% in a dose-dependent manner. The activity of the thermogenic proton conductance pathway in brown adipose tissue (BAT) mitochondria was assessed from purine nucleotide (GDP) binding and was elevated by 40 and 86% 1 h after intravenous (1 microgram) or intracerebroventricular (100 ng) injection of IL-1 beta, respectively. Regional tissue blood flow was determined in anesthetized rats from the distribution of radiolabeled microspheres. Blood flow to liver (hepatic arterial), testes, skin, and white adipose tissue was unaffected by IL-1 beta injection. Blood flow to brain and kidney was increased (142 and 50%) but reduced (58%) to skeletal muscle after intravenous but not intracerebroventricular injection of interleukin. In contrast, blood flow to BAT was markedly elevated after intravenous (288%) or intracerebroventricular (382%) injection of IL-1 beta. Severing the sympathetic nerves supplying the interscapular BAT depot prevented the increase in blood flow. These data indicate that the potent pyrogenic effects of IL-1 beta in the rat are due largely to a central action. Fever is associated with increases in metabolic rate and BAT activity, and these results provide support for the involvement of brown fat in thermogenesis associated with fever.

Impaired effects of interleukin-1 beta on fever and thermogenesis in genetically obese rats

Interleukin-1 beta (IL-1 beta) is an endogenous peptide which induced fever (1.8 degrees C rise in colonic temperature) when injected interacerebroventricularly (i.c.v., 80ng human recombinant IL-1 beta) into conscious lean rats. IL-1 beta also stimulated resting oxygen consumption (VO2) by 38 percent, in vitro thermogenic activity (mitochondrial GDP binding) of brown adipose tissue (BAT) by almost two-fold, and blood flow to brown fat (assessed from the distribution of radiolabelled microspheres) by nine-fold in lean animals. Genetically obese Zucker rats showed only small increases in temperature (0.5 degrees C), VO2 (15 percent) and blood flow to BAT (less than two-fold), and no change in GDP binding, but exhibited normal thermogenic responses to i.c.v. injection of corticotrophin releasing factor (GRF). The results indicate that the obese Zucker is insensitive to the central effects of interleukin; this may explain the reduced febrile responses to endotoxin which have previously been reported.

Evidence against adenosine 3',5'-monophosphate as a mediator of fever in the brain

The aim of this study was to test the hypothesis that increased concentrations of adenosine 3',5'-monophosphate (cyclic AMP) in the pyrogen-sensitive preoptic/anterior hypothalamic region (PO/AH) mediate fever. Micro-injection of N6-2'-O-dibutyryl adenosine 3',5'-monophosphate (db cyclic AMP) into the preoptic/anterior hypothalamic region in rats produced a dose-dependent fall in body temperature which is inconsistent with the proposal that the nucleotide mediates fever. Hyperthermia was observed in some rats in response to large doses of db cyclic AMP, but this response was associated with convulsions. Endogenous concentrations of cyclic AMP in the preoptic/anterior hypothalamic region, as well as in the cerebral cortex, liver, spleen, thymus, white fat and plasma were unaffected by the febrile response to the subcutaneous injection of yeast in rats. A rise in levels of cyclic AMP was observed in the skeletal muscle of rats treated with yeast. The data presented do not indicate that cyclic AMP is involved in the neuronal events mediating fever in the rat.

Evidence against putrescine and polyamines as endogenous mediators of fever

Endogenous concentrations of putrescine, spermidine, spermine and related biosynthetic enzymes were not affected by the administration of bacterial endotoxin and the subsequent development of fever in rabbits. In addition, the febrile response to endotoxin was unaffected either by the ornithine decarboxylase inhibitor, DL-alpha-difluoromethylornithine or by putrescine. These data indicate polyamines are not involved in the development of fever.

The pharmacology of fever

The ability to minimise, if not prevent, large variations in deep body temperature that would otherwise result from some environmental conditions is a homeostatic function of unquestioned benefit that is demonstrated only by the more highly evolved animals. Nevertheless, body temperature is raised above normal values in many pathological conditions. This increase in temperature or fever is an active and co-ordinated response, which indicates the involvement of the CNS. Central injection and lesion studies have shown that the brain, in particular the PO/AH, is the site of action of fever-inducing agents, termed pyrogens. Electrophysiological data show that pyrogens modify the activity of central thermosensitive neurones as if to increase heat gain and decrease heat loss. The common response of fever to pyrogens of diverse origins is attributable to fever being mediated by an endogenous pyrogen released by phagocytic cells in the host. The mechanism by which central neuronal function is disturbed by pyrogens present in the periphery is not known. Tracer studies have yet to demonstrate the passage of a pyrogen across the blood-brain barrier. The possible involvement of several putative neurotransmitters and modulators in fever has been reviewed here, but most compounds have not been studied sufficiently to allow firm conclusions to be drawn. Much of the data is limited to the effects of the putative mediators on normal thermoregulation but, even when the effect is hyperthermia, such observations do not necessarily indicate a role for the endogenous material in fever. Dose-response curves for agonists and the effects of antagonists are often undetermined. This shortfall in data is due to some extent to the nature of fever; a central response in vivo over several hours. Although fever may enhance other host reactions to combat infection and inflammation, neither this benefit nor the undesirability of antipyretic therapy has been demonstrated unequivocally in either homeothermic laboratory animals or humans. Consequently, antipyretic drugs continue to be used clinically to alleviate the fever, malaise and/or pain commonly associated with disease. The drugs in common usage are the nonsteroidal antipyretic analgesics, many of which also have an anti-inflammatory effect. The primary mode of action of these drugs as antipyretics appears at present to be the inhibition of cyclo-oxygenase and a consequent reduction of prostanoid material in pyrogen-sensitive areas of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of diflunisal on fever in the rabbit and the rat

The antipyretic efficacy of diflunisal was assessed in rats made febrile by yeast and in rabbits made febrile by bacterial endotoxin. Diflunisal was a more potent antipyretic than aspirin in rats, reducing a maximum fever in doses not producing overt toxic effects. In contrast, submaximal fever in rabbits was not reduced by diflunisal. Fatal hyperthermia of rapid onset was observed in rats and rabbits receiving high doses of diflunisal after administration of microbial pyrogen but not in control animals. These data indicate the toxicity of diflunisal may be potentiated by the presence of pyrogens. It is concluded that the apparent antipyretic efficacy of a drug can depend on the species-pyrogen combination used to screen for antipyresis.

Evidence that cyclic nucleotides are not mediators of fever in rabbits

The N6-2'-O-dibutyryl derivative of adenosine 3',5'-monophosphate (db cyclic AMP) and related compounds have been micro-injected into the preoptic/anterior hypothalamic nuclei (PO/AH) of the unanaesthetized, restrained rabbit and the effects on deep body temperature observed. Db cyclic AMP (100-400 micrograms) produced hypothermia of rapid onset in rabbits at an ambient temperature of 20-23 degrees C. Hypothermia was also produced by N2-2'-O-dibutyryl guanosine 3',5'-monophosphate (db cyclic GMP), but not by saline, sodium n-butyrate, adenosine 3',5'-monophosphate (cyclic AMP), guanosine 3',5'-monophosphate, adenosine 5'-mono-, di- or triphosphate. The initial hypothermic response to db cyclic AMP and db cyclic GMP was followed by a sustained rise in temperature. However, all compounds injected into the PO/AH produced a similar hyperthermia which was attenuated by paracetamol. Development of this tissue-damage fever abolished the hypothermic response to db cyclic AMP in some rabbits. The effects of db cyclic AMP on body temperature and behaviour were not reproduced by the adenylate cyclase activators, cholera toxin (0.125-5 micrograms) and guanyl imidodiphosphate (5-400 micrograms). It is concluded that hypothermia is the principal effect of db cyclic AMP on body temperature when injected into the PO/AH in rabbits. These data do not support the proposal that endogenous cyclic AMP in the rabbit brain mediates pyrexia.

Dissimilar effects on body temperature in the cat produced by guanosine 3',5'-monophosphate, acetylcholine and bacterial endotoxin

1 Guanosine 3',5'-monophosphate (cyclic GMP) and N2-2'-O-dibutyryl guanosine 3',5'-monophosphate (db cyclic GMP) have been injected into the third cerebral ventricle (i.c.v.) of the unanaesthetized cat and the effects of rectal temperature and on behavioural and autonomic activities observed and compared with those of acetylcholine and physostigmine. 2 Acetylcholine (100 nmol) and physostigmine (100 nmol) injected together i.c.v. produced a rise in body temperature in cats at an environmental temperature of 20-24 degrees C, which was abolished by pretreatment i.c.v. with atropine (200 nmol). 3 Cyclic GMP and db cyclic GMP (10--1250 nmol) had no effect on body temperature in cats at an environmental temperature of 20--24 degrees C but produced hypothermia (1250 nmol) in cats at an environmental temperature of 9--11 degrees C. 4 The O-somatic antigen of Shigella dysenteriae (20 microgram/kg i.v.) produced fever in cats which was not potentiated by caffeine (25 mg/kg i.p.). Levels of endogenous cyclic GMP in c.s.f. taken from the cisterna magna during fever induced by bacterial endotoxin in the presence or absence of paracetamol (50 mg/kg i.p.) and/or caffeine were similar to values for afebrile cats. 5 It is concluded that exogenous cyclic GMP and db cyclic GMP can inhibit central events mediating autonomic and behavioural thermoregulation stimulated in cats by exposure to cold environments.

Thermoregulatory effects of N6-2'-Q-dibutyryl adenosine 3',5'-monophosphate in the restrained mouse

1 The N6-2-O-dibutyryl derivative of adenosine 3',5'-monophosphate (db cyclic AMP) has been micro-injected into the third cerebral ventricle of the unanaesthetized, restrained mouse and the effects on body temperature and thermoregulatory activities observed. 2 Db cyclic AMP (4, 16 and 32 micrograms) injected intracerebroventricularly produced hypothermia when compared with temperature responses to sodium n-butyrate (6.8 micrograms). 3 Hypothermia induced by db cyclic AMP in mice was associated with a fall in oxygen consumption together with behavioural and autonomic heat loss activities but not cutaneous vasodilatation. The effects on rectal temperature and oxygen consumption were dose-dependent. 4 The falls in rectal temperature and oxygen consumption induced by db cyclic AMP (4 micrograms) were decreased by elevation of the environmental temperature from 22 to 32 degrees C and abolished at 36 degrees C. 5 It is concluded db cyclic AMP may inhibit central events mediating the rise in metabolic heat production in mice upon exposure to cold environments.

Study on the possible entry of bacterial endotoxin and prostaglandin E2 into the central nervous system from the blood

1 A study has been made of the possible entry of 51Cr-bacterial endotoxin and [5,6,8,11,12,14,15(n)-3H]-prostaglandin E2 ([3H5-PGE2) into the CNS of the anaesthetized cat. 2 No radioactivity was detected in perfusates of the preoptic-anterior hypothalamus or in the cerebrospinal fluid (c.s.f.) in vivo, or in brain tissue post mortem following intracarotid infusion of 51Cr-bacterial endotoxin. 3 Intracarotid administration of [3H]-PGE2 resulted in the entry of radioactivity into the CNS of endotoxin pretreated cats. Chromatographic analysis indicated the radioactivity in c.s.f. to be associated with PGE2 and a metabolite similar to 13, 14-dihydro-15-keto PGE2. 4 Intracarotid administration of 13, 14-dihydro-15-keto [5,6,8,11,12,14(n)-3H]-PGE2 resulted in the presence of the compound in the CNS of the anaesthetized cat after pretreatment with bacterial endotoxin. 5 It is concluded that PGE2 and possibly 13,14-dihydro-15-keto PGE2 but not bacterial endotoxin may enter the CNS from the cerebral circulation to elicit the febrile response to bacterial endotoxin in cats.

Effects of methylxanthine drugs on pyrogen-induced hyperthermia

The hyperthermic response to pyrogen was not potentiated by caffeine or theophylline administered i.p. into cats or rabbit. Injection of these drugs into the anterior hypothalamus or into the third cerebral ventricle in cats was also without effect on pyrexia. These results support the hypothesis that cyclic AMP in the anterior hypothalamus does not mediate pyrogen-induced hyperthermia in cats.

Cyclic adenosine 3', 5'-monophosphate in cerebrospinal fluid during thermoregulation and fever

1. Samples of cerebrospinal fluid (c.s.f.) have been taken from the cisterna magna of unanaesthetized cats, whilst rectal temperature was recorded, during exposure of the animals to various ambient temperatures and during fever induced by pyrogen. The concentration of adenosine 3', 5'-monophosphate (cyclic AMP) in samples of c.s.f. has been assayed. 2. Cats exposed to low ambient temperatures (-2 to +2 degrees C) for 3 h maintained body temperature by both behavioural and autonomic heat gain activity. Exposure of cats to high ambient temperatures (44 - 45 degrees C) for 3.5 h caused a rise in body temperatures of about 2.5 degrees C, despite behavioural and autonomic heat loss activity. Neither cold nor heat stress had a significant effect on c.s.f. cyclic AMP. 3. Fever induced by intravenous Shigella dysenteriae (2 and 20 mug/kg) was associated with a dose-related increase in the concentration of cyclic AMP in c.s.f. Paracetamol (75 mg/kg) injected I.P. before the onset of fever, suppressed the increase in both temperature and c.s.f. cyclic AMP in response to pyrogen. Paracetamol (50 and 100 mg/kg), injected after the onset of fever, caused a fall in temperature, which was not associated with a decrease in the concentration of cyclic AMP in c.s.f. 4. Fever induced in cats by intravenous Shigella dysenteriae (20 mug/kg) was associated with an increase in the concentration of cyclic AMP in plasma as well as in c.s.f. 5. The sodium salt of cyclic AMP (0.1-10 mg/kg) injected I.V. into unanaesthetized cats caused a dose-related hypothermia, which was associated with autonomic heat loss activity and a dose-related increase in the concentration of cyclic AMP in cisternal c.s.f., which was not mimicked by adenosine. 6. It is concluded that the raised concentrations of cyclic AMP in c.s.f., in response to pyrogen I.V., do not mediate fever in the cat and that the concentration of cyclic AMP in cisternal c.s.f. may be affected by changes in the plasma concentration of the nucleotide.

The effects of cyclic adenosine 3',5'-monophosphate and other adenine nucleotides on body temperature

1. Adenosine 3',5'-monophosphate (cAMP), its dibutyryl derivative (Db-cAMP) and other adenine nucleotides have been micro-injected into the hypothalamic region of the unanaesthetized cat and the effects on body temperature, and on behavioural and autonomic thermoregulatory activities observed. 2. Db-cAMP and cAMP both produced hypothermia when applied to the pre-optic anterior hypothalamus. With Db-cAMP the hypothermia was shown to be dose dependent between 50 and 500 mug (0-096-0-96 mumole). 3. AMP, ADP and ATP also produced hypothermia when injected into the pre-optic anterior hypothalamus. 4. The order of relative potencies of the adenine nucleotides with respect both to the hypothermia produced and to the autonomic thermoregulatory effects observed were similar. Db-cAMP was most potent and cAMP least. 5. Micro-injection into the pre-optic anterior hypothalamus of many substances including saline produced in most cats a non-specific rise in body temperature apparently the result of tissue damage. Intraperitoneal injection of 4-acetamidophenol (paracetamol 50 mg/kg) reduced or abolished this febrile response. 6. The hypothermic effect of the adenine nucleotides has been compared with the effects produced in these same cats by micro-injections of noradrenaline, 5-hydroxytryptamine, a mixture of acetylcholine and physostigmine (1:1), EDTA and excess Ca2+ ions. 7. It is concluded that as Db-cAMP and cAMP both produce hypothermia, it is unlikely that endogenous cAMP in the pre-optic anterior hypothalamus mediates the hyperthermic responses to pyrogens and prostaglandins.