Prostacyclin-induced hypothermia: involvement of central histamine H2-receptors

Prostanoid receptors: structures, properties, and functions

Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Brain histamine mediates the bombesin-induced central activation of sympatho-adrenomedullary outflow

Intracerebroventricular (i.c.v.) administration of bombesin (0.3 nmol) increased plasma levels of both adrenaline and noradrenaline in urethane anesthetized rats. These bombesin-induced increases were inhibited by i.c.v. pretreatment with pyrilamine, an H1-receptor antagonist. Ranitidine, an H2-receptor antagonist also inhibited the increase of adrenaline, however, its effective dose was much larger than that of pyrilamine. Furthermore, the bombesin-induced increase of noradrenaline was not effectively inhibited by ranitidine. In the next series, turnover of histamine was assessed by measuring accumulation of tele-methylhistamine (t-MH), a major metabolite of brain histamine. I.c.v. administration of bombesin (0.3-3 nmol) increased turnover of hypothalamic histamine, while its intravenous administration was without effect. The present results suggest that the bombesin-induced central activation of sympatho-adrenomedullary outflow is probably, at least in part, mediated through brain histaminergic neurons.

Iloprost, a stable analogue of PGI2, potentiates the hyperthermic effect of PGE2 in rats

Centrally mediated effects of iloprost, a stable analogue of PGI2, on rectal temperature have been investigated in conscious rats. ICV administration of iloprost (100-1,000 ng, ICV) produced a dose-dependent, monophasic hyperthermic response that was not inhibited by indomethacin. When injected into the preoptic anterior hypothalamic (POAH) region, iloprost (2-50 ng/POAH) induced a biphasic increase in rectal temperature. While the first phase was inhibited by AH 6809, an E1-type prostaglandin (EP1) receptor antagonist, the second phase was abolished by indomethacin pretreatment. Iloprost was found not to alter rectal temperature when injected into the ventromedial hypothalamic area. Administration of iloprost into the POAH in a dose that had no effect on rectal temperature significantly potentiated the hyperthermic effect of PGE2 (50 ng, ICV). These findings suggest that the pyrogenic effect of iloprost is partly mediated by EP1 receptors located on the POAH. Regarding the similarities of iloprost and PGI2, it is further proposed that endogenous PGI2 might act to modulate hyperthermic effect of PGE2 released during arachidonic acid- or endogenous pyrogen-induced fever.

Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents: II

This survey continues a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published since 1979, but data from many earlier papers are also tabulated.

Role of putative neurotransmitters in the central gastric antisecretory effect of prostaglandin E2 in rats

The role of putative neurotransmitters of the central nervous system in the central gastric antisecretory effect of prostaglandin E2 (PGE2) was investigated in pylorus-ligated rats. Pretreatment of the rats with an intracerebroventricular (i.c.v.) injection of 6-hydroxydopamine (6-OHDA) prevented the antisecretory effect of the i.c.v. administration of PGE2, whereas pretreatment with 5,6-dihydroxytryptamine (5,6-DHT) plus p-chlorophenylalanine (PCPA) had no effect. I.c.v.-administered phentolamine and idazoxan antagonized the inhibition of gastric secretion induced by i.c.v. PGE2, whereas prazosin, propranolol and sulpiride injected via the same route were ineffective. Diphenhydramine, cimetidine, naloxone and theophylline, all administered i.c.v., did not modify the antisecretory effect of i.c.v. PGE2. The results suggest that an activation of alpha 2-adrenoceptors in the brain is involved in the central gastric antisecretory effect of PGE2, whereas neither central 5-hydroxytryptamine receptors, alpha 1- or beta-adrenoceptors, D2-dopamine receptors, histamine or opioid receptors nor adenosine seem to play any role here.

Cholecystokinin-octapeptide-induced hyperthermia in guinea-pigs

Intracerebroventricular administration of cholecystokinin-octapeptide (CCK-8) at room temperature (21 degrees C) induced dose-related hyperthermia in guinea-pigs and also produced hyperthermia at low (10 degrees C) and high (30 degrees C) ambient temperatures. CCK-8-induced hyperthermia was direct and not mediated by prostaglandins, norepinephrine, cyclic AMP or naloxone-sensitive receptors.

Central effects of dibutyryl cyclic AMP and GMP on the temperature in conscious rabbits

Intracerebroventricular (i.c.v.) administration of dibutyryl cyclic AMP (Db-cAMP)- and dibutyryl cyclic GMP (Db-cGMP)-induced hyperthermia in rabbits. Central administration of 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (ZK 62711), a selective inhibitor of cAMP phosphodiesterase, only accentuated the hyperthermia due to Db-cAMP whereas a selective inhibitor of cGMP phosphodiesterase, 2-O-proproxyphenyl-8-azapurin-6-one (M and B 22948), only potentiated the hyperthermia caused by Db-cGMP. The hyperthermia due to Db-cAMP and Db-cGMP was not mediated through prostaglandins (PG). In contrast, central administration of an alpha-adrenergic receptor antagonist, phenoxybenzamine, or a beta-adrenergic receptor antagonist, sotalol, only attenuated the hyperthermic response to Db-cAMP while a cholinergic muscarinic receptor antagonist, atropine, specifically antagonized Db-cGMP-induced hyperthermia. I.c.v. administration of a protein synthesis inhibitor, anisomycin, did inhibit the hyperthermia due to Db-cAMP and Db-cGMP. Opiate antagonist, naloxone, did not antagonize Db-cAMP- and Db-cGMP-induced hyperthermia. These results suggest that a protein mediator is implicated in the induction of hyperthermia by Db-cAMP and Db-cGMP and that cAMP and cGMP may be involved through alpha/beta-adrenergic and cholinergic muscarinic receptors respectively in the central regulation of heat production/conservation in rabbits.

Opposing actions of dibutyryl cyclic AMP and GMP on temperature in conscious guinea-pigs

Intracerebroventricular administration of dibutyryl cyclic AMP (Db-cAMP), induced hyperthermia in guinea-pigs which was not mediated through prostaglandins (PG) or norepinephrine since a prostaglandin synthesis inhibitor, indomethacin, and an alpha-adrenergic receptor blocking agent, phenoxybenzamine did not antagonize the hyperthermia. In contrast, the hyperthermic response to dibutyryl cyclic AMP was attenuated by central administration of a beta-adrenergic receptor antagonist, sotalol, indicating that cyclic AMP may be involved, through beta-adrenergic receptors, in the central regulation of heat production/conservation. Central administration of dibutyryl cyclic GMP (Db-cGMP) produced hypothermia which was not mediated via histamine H1- or H2-receptors and serotonin since the H1-receptor antagonist, mepyramine, the H2-receptor antagonist, cimetidine, and the serotonin antagonist, methysergide, had no antagonistic effects. The antagonism of hypothermia induced by dibutyryl cyclic GMP and acetylcholine + physostigmine, by central administration of a cholinergic muscarinic receptor antagonist, atropine, and not by a cholinergic nicotinic receptor antagonist, d-tubocurarine, suggests that cholinoceptive neurons and endogenous cyclic GMP may regulate heat loss through cholinergic muscarinic receptors. These results support a regulatory role in thermoregulation provided by a balance between opposing actions of cyclic AMP and cyclic GMP in guinea-pigs.

Role of prostaglandin D2 in the hypothermia of rats caused by bacterial lipopolysaccharide

The intraperitoneal administration of lipopolysaccharide from Salmonella typhimurium (1 mg/kg) caused a fall in the rat colonic temperature of about 2 degrees C at an ambient temperature of 22 +/- 3 degrees C. The hypothermia induced by the lipopolysaccharide was abated in a dose-dependent manner by the administration of indomethacin. Other inhibitors of prostaglandin synthetase such as aspirin, flufenamic acid, and phenylbutazone had effects similar to those of indomethacin. When various prostaglandins were injected intracerebroventricularly, only prostaglandin D2 caused a dose-dependent fall in the colonic temperature at doses between 1.2 and 6 nmol/kg. Microinjection of prostaglandin D2 into the preoptic area caused hypothermia of about 1 degree C. However, injection of prostaglandin D2 into the posterior hypothalamus had little effect on the colonic temperature. The hypothermia caused by prostaglandin D2 was not abated by the administration of indomethacin. The amount of prostaglandin D2 increased significantly in the preoptic/hypothalamic region of rat brain 1 hr after the intraperitoneal administration of the lipopolysaccharide, whereas such increase was not observed in rats pretreated with indomethacin. The in vitro incubation of the preoptic/hypothalamic slices with the lipopolysaccharide also increased the amount of prostaglandin D2. These results suggest that the intraperitoneal administration of the lipopolysaccharide induces the release of prostaglandin D2 in the preoptic/hypothalamic area of rat brain and that the latter compound is involved in the hypothermic response of rats to the lipopolysaccharide.

Prostacyclin-induced hyperthermia: implication of a protein mediator

Intracerebroventricular administration of prostacyclin (PGI2) at room temperature (21 degrees C) induced dose-related hyperthermia in rabbits and also produced hyperthermia at low (4 degrees C) and high (30 degrees C) ambient temperatures. The PGI2-induced hyperthermia was not mediated by its stable metabolite 6-keto prostaglandin F1 alpha. Of the three anion transport systems (iodide, hippurate and liver-like) present in the choroid plexus, only the liver transport system seems to be important to central inactivation of pyrogen, prostaglandin E2 (PGE2) and the PGI2. Iodipamide (an inhibitor of the liver transport system) augmented the hyperthermia produced by PGI2, PGE2 and pyrogen. Phenoxybenzamine and pimozide had no thermolytic effect on PGI2-induced hyperthermia. After norepinephrine (NE) and dopamine levels were depleted by 6-hydroxydopamine, PGI2 still induced hyperthermia. Indomethacin and SC-19220 (a PG antagonist) did not antagonize PGI2-induced hyperthermia. Furthermore, the hyperthermia due to PGI2 was not accentuated by theophylline. In contrast, the hyperthermic response to PGI2 was attenuated by central administration of the protein synthesis inhibitor, anisomycin. These results indicate that PGI2-induced hyperthermia is not mediated by NE, dopamine, PGS, cyclic AMP, but, rather, that a protein mediator is implicated in the induction of fever by PGI2.

Specific binding of [3H]prostaglandin E2 to rat brain membranes and synaptosomes

There is saturable, reversible and specific binding for [3H]prostaglandin E2 (PGE2) to rat brain membranes. This binding is of high affinity, selectively distributed with a maximum in the hypothalamus, the amygdala and the posterior pituitary, and is associated subcellularly with the synaptosomal fraction. This specific PGE2 binding has the characteristics expected for receptors, so opening new perspectives which might clarify the role of PGs in the brain.