Effects of centrally administered prostaglandin EP receptor agonists on febrile and adrenocortical responses in the prepubertal pig

Unraveling the Molecular Details Involved in the Intimate Link between the Immune and Neuroendocrine Systems

During systemic infections, the immune system can signal the brain and act on different neuronal circuits via soluble molecules, such as proinflammatory cytokines, that act on the cells forming the blood-brain barrier and the circumventricular organs. These activated cells release prostaglandin of the E2 type (PGE2), which is the endogenous ligand that triggers the pathways involved in the control of autonomic functions necessary to restore homeostasis and provide inhibitory feedback to innate immunity. Among these neurophysiological functions, activation of the circuits that control the plasma release of glucocorticoids is probably the most critical to the survival of the host in the presence of pathogens. This review revisits this issue and describes in depth the molecular details (including the emerging role of Toll-like receptors during inflammation) underlying the influence of circulating inflammatory molecules on the cerebral tissue, focusing on their contribution in the synthesis and action PGE2 in the brain. We also provide an innovative view supporting the concept of “fast and delayed response” involving the same ligands but different groups of cells, signal transduction pathways, and target genes.

Interactions between the immune and neuroendocrine systems

The growing spark of interest in research concerning the molecular links between the nervous, endocrine and immune systems has caused an explosion of new knowledge concerning the fine mechanisms that orchestrate the integrated response to an immune challenge. For instance, elevation in plasma glucocorticoid (GC) levels is one of the most powerful and well-controlled feedback mechanisms on the proinflammatory signal transduction machinery taking place across the organism. Circulating inflammatory molecules have the ability to target their cognate receptors at the levels of blood-brain barrier, the latter in return produces specific prostaglandins (PGs). This chapter presents the brain circuits involved in the activation of the hypothalamic-pituitary-adrenal (HPA) axis by endogenously produced prostaglandin E(2) (PGE(2)) during systemic innate immune insults.

16,16-Dimethyl prostaglandin E2 efficacy on prevention and protection from bleomycin-induced lung injury and fibrosis

In this study, we evaluated the protective effect and therapeutic potential of the prostaglandin E(2) (PGE(2)) synthetic analog 16,16-dimethyl-PGE(2) (dmPGE(2)) in the animal model of pulmonary fibrosis induced by bleomycin. Mice subjected to intratracheal administration of bleomycin (1 mg/kg) received a dmPGE(2) dose of 30 microg/kg/day by continuous subcutaneous infusion. Bronchoalveolar lavage (BAL); immunohistochemical analysis for IL-1, TNF-alpha, and nitrotyrosine; measurement of fluid content in lung; myeloperoxidase activity assay; and lung histology were performed 1 week later. Lung histology and Sircol assay for collagen deposition were performed 3 weeks after treatments. Changes of body weight and survival rate were also evaluated at 1 and 3 weeks. Compared with bleomycin-treated mice, dmPGE(2) co-treated mice exhibited a reduced degree of body weight loss and mortality rate as well as of lung damage and inflammation, as shown by the significant reduction of: (1) lung infiltration by leukocytes; (2) myeloperoxidase activity; (3) IL-1, TNF-alpha, and nitrotyrosine immunostaining; (4) lung edema; and (5) histologic evidence of lung injury and collagen deposition. In a separate set of experiments, dmPGE(2) treatment was started 3 days after bleomycin administration, and the evaluation of lung damage and inflammation was assessed 4 days later. Importantly, delayed administration of dmPGE(2) also was able to protect from inflammation and lung injury induced by bleomycin. These results, indicating that dmPGE(2) is able to prevent and to reduce bleomycin-induced lung injury through its regulatory and anti-inflammatory properties, encourage further research to find new options for the treatment of pulmonary fibrosis.

Is survival possible without arachidonate metabolites in the brain during systemic infection?

The central nervous system mediates a coordinated set of biological responses during systemic immune stimuli. These responses are essential for the organism to eliminate invading pathogens and restore health. Coincidentally, centrally produced prostaglandins play a determinant role in activating the neuronal circuits involved in the control of autonomic functions.

Contrasting effects of E type prostaglandin (EP) receptor agonists on core body temperature in rats

Prostaglandin E2 (PGE2) is thought to be a principal fever mediator. There are four subtypes of PGE (EP) receptors, EP1-EP4. We investigated which EP receptors mediate PGE2-induced hyperthermia by injecting selective EP receptor agonists into the rat lateral cerebral ventricle under unrestrained condition. ONO-DI-004, an EP1 receptor agonist, increased the core temperature (T(c)) in a dose-dependent manner (1.6+/-0.1 degrees C at 20 nmol, with the peak 30 min after injection) with a time course similar to PGE2-induced hyperthermia. ONO-AE1-259-01 (20 nmol), an EP2 receptor agonist, did not change the T(c). ONO-AE-248 (20 nmol), an EP3 receptor agonist, also increased the T(c). However, the peak effect was delayed (1.2+/-0.2 degrees C, 50 min after injection) compared to PGE2. In contrast, ONO-AE1-329, an EP4 receptor agonist, decreased the T(c). These findings suggest that the EP1, EP3, and EP4 receptors all may contribute to the thermoregulatory response to PGE2, but each may have a different role.

PGE2 triggers recovery of transmucosal resistance via EP receptor cross talk in porcine ischemia-injured ileum

16,16-Dimethyl-PGE2 (PGE2) may interact with one of four prostaglandin type E (EP) receptors, which signal via cAMP (via EP2 or EP4 receptors) or intracellular Ca(2+) (via EP1 receptors). Furthermore, EP3 receptors have several splice variants, which may signal via cAMP or intracellular Ca(2+). We sought to determine the PGE2 receptor interactions that mediate recovery of transmucosal resistance (R) in ischemia-injured porcine ileum. Porcine ileum was subjected to 45 min of ischemia, after which the mucosa was mounted in Ussing chambers. Tissues were pretreated with indomethacin (5 microM). Treatment with the EP1, EP2, EP3, and EP4 agonist PGE2 (1 microM) elevated R twofold and significantly increased tissue cAMP content, whereas the EP2 and EP4 agonist deoxy-PGE1 (1 microM) or the EP1 and EP3 agonist sulprostone (1 microM) had no effect. However, a combination of deoxy-PGE1 and sulprostone stimulated synergistic elevations in R and tissue cAMP content. Furthermore, treatment of tissues with deoxy-PGE1 and the Ca(2+) ionophore A-23187 stimulated synergistic increases in R and cAMP, indicating that PGE2 triggers recovery of R via EP receptor cross talk mechanisms involving cAMP and intracellular Ca(2+).

Relationship of EP(1-4) prostaglandin receptors with rat hypothalamic cell groups involved in lipopolysaccharide fever responses

The action of prostaglandin E(2) (PGE(2)) in the preoptic area is thought to play an important role in producing fever. Pharmacologic evidence suggests that, among the four subtypes of E-series prostaglandin (EP) receptors, i.e., EP(1), EP(2), EP(3), and EP(4), the EP(1) receptor mediates fever responses. In contrast, evidence from mice with EP receptor gene deletions indicates that the EP(3) receptor is required for the initial (<1 hour) fever after intravenous (i.v.) lipopolysaccharide (LPS). To investigate which subtypes of EP receptors mediate systemic infection-induced fever, we assessed the coexpression of Fos-like immunoreactivity (Fos-IR) and EP(1-4) receptor mRNA in nuclei in the rat hypothalamus that have been shown to be involved in fever responses. Two hours after the administration of i.v. LPS (5 microg/kg), Fos-IR was observed in the ventromedial preoptic nucleus, the median preoptic nucleus, and the paraventricular hypothalamic nucleus. In these nuclei, EP(4) receptor mRNA was strongly expressed and the Fos-IR intensely colocalized with EP(4) receptor mRNA. Strong EP(3) receptor mRNA expression was only seen within the median preoptic nucleus but Fos-IR showed little coexpression with EP(3) receptor mRNA. EP(2) receptor mRNA was not seen in the PGE(2) sensitive parts of the preoptic area. Although approximately half of the Fos-immunoreactive neurons also expressed EP(1) receptor mRNA, EP(1) mRNA expression was weak and its distribution was so diffuse in the preoptic area that it did not represent a specific relationship. In the paraventricular nucleus, EP(4) mRNA was found in most Fos-immunoreactive neurons and levels of EP(4) receptor expression increased after i.v. LPS. Our findings indicate that neurons expressing EP(4) receptor are activated during LPS-induced fever and suggest the involvement of EP(4) receptors in the production of fever.

Distribution of the EP3 prostaglandin E(2) receptor subtype in the rat brain: relationship to sites of interleukin-1-induced cellular responsiveness

The activation of neurosecretory neurons that express corticotropin-releasing hormone (CRH) in response to increased circulating levels of interleukin-1beta (IL-1beta) depends on prostaglandin E(2) (PGE(2)) acting locally within the brain parenchyma. To identify potential central targets for PGE(2) relevant to pituitary-adrenal control, the distribution of mRNA encoding the PGE(2) receptor subtype EP3 (EP3R) was analyzed in rat brain. Hybridization histochemistry revealed prominent labeling of cells in discrete portions of the olfactory system, iso- and hippocampal cortices, and subcortical telencephalic structures in the septal region and amygdala. Labeling over the midline, intralaminar, and anterior thalamic groups was particularly prominent. EP3R expression was enriched in the median preoptic nucleus and adjoining aspects of the medial preoptic area (MPO) implicated in thermoregulatory/febrile responses and sleep induction. EP3R-expressing cells were also prominent in brainstem cell groups involved in nociceptive information processing/modulation (periaqueductal gray, locus coeruleus (LC), parabrachial nucleus (PB), caudal raphé nuclei), arousal and wakefulness (LC, midbrain raphé and tuberomammillary nuclei); and in conveying interoceptive input, including systemic IL-1 signals, to the endocrine hypothalamus (nucleus of the solitary tract (NTS) and rostral ventrolateral medulla [VLM]). Combined hybridization histochemical detection of EP3R mRNA with immunolocalization of IL-1beta-induced Fos protein expression identified cytokine-sensitive, EP3R-positive cells in the medial NTS, rostral VLM, and, to a lesser extent, aspects of the MPO. These findings are consistent with the view that increased circulating IL-1 may stimulate central neural mechanisms, including hypothalamic CRH neurons, through an EP3R-dependent mechanism involving PGE(2)-mediated activation of cells in the caudal medulla and/or preoptic region.

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.

Expression of mRNAs for vasopressin, oxytocin and corticotrophin releasing hormone in the hypothalamus, and of cyclooxygenases-1 and -2 in the cerebral vasculature, of endotoxin-challenged pigs

Neuropeptide and cyclooxygenase (Cox) gene expression was examined in the brains of catheterized pigs killed 30 or 120 min after intravenous injection of a low (20 microg) dose of lipopolysaccharide endotoxin (LPS), previously demonstrated to induce fever in this species. In the paraventricular hypothalamic nucleus (PVN), corticotrophin releasing hormone (CRH) mRNA was shown to be present in the pars parvocellularis but was not upregulated 30 or 120 min after 20 microg LPS, or 90 min after 60 microg LPS; there was also no change in proopiomelanocortin (POMC) message in the anterior pituitary (AP). Similarly, expression of mRNAs for lysine vasopressin (LVP) or oxytocin (OT) did not change in the PVN after LPS (20 microg), although LVP message was increased (p<0.05) at 30 min in the hypothalamic supraoptic nucleus (SON). Expression of Cox-1 and Cox-2 genes was quantified in the organum vasculosum lamina terminalis (OVLT) and choroid plexus (CP) in an attempt to determine whether altered expression of prostaglandin (PG) synthetic enzymes in brain vasculature is involved in LPS fever. Although vascular endothelial cells in both structures expressed Cox-1 and Cox-2 mRNAs, neither increased in the OVLT following LPS. However, in the CP, Cox-1 mRNA was enhanced (p<0.05) at 30 and 120 min after LPS injection and Cox-2 showed a similar (NS) change. These results provide the first description of CRH and Cox gene expression in the porcine brain. They also suggest that LPS may influence the activity of genes controlling LVP synthesis in the hypothalamus and PG production by the brain vasculature.

Microinjection of a cyclooxygenase inhibitor into the anteroventral preoptic region attenuates LPS fever

Considerable evidence supports the role of prostaglandins in fever production, but the neuroanatomic sites of prostaglandin synthesis that produce fever remain unknown. With the use of a novel microinjection technique, we injected the cyclooxygenase inhibitor ketorolac into the preoptic area (POA) to determine which preoptic regions produce the prostaglandins required for fever. Initial experiments demonstrated that intravenous ketorolac blocked the fever normally produced by lipopolysaccharide (LPS) 5 micrograms/kg i.v. Microinjection of ketorolac into the POA had no effect on body temperature, and injection of artificial cerebrospinal fluid into the POA did not alter LPS fever. Injection of ketorolac into the anteroventral POA markedly decreased the fever produced by LPS, compared with injections into more rostral, caudal, or dorsal locations. These observations indicate that prostaglandin synthesis in the anteroventral preoptic region is necessary for the production of fever.

A selective inhibitor of cyclooxygenase-2 reverses endotoxin-induced pyretic responses in non-human primates

The anti-pyretic effect of a selective cyclooxygenase-2 inhibitor, DFU (5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulfonyl)phenyl-2(5H)-furano ne), was examined in conscious, un-restrained squirrel monkeys (Saimiri sciureus) using a radio telemetric system. Injection of bacterial endotoxin (lipopolysaccharide, 6 microg kg(-1), i.v.) in squirrel monkeys caused a gradual increase in core body temperature reaching a plateau of 2.07 +/- 0.17 degrees C above baseline at 2 h post-injection. Oral administration of DFU (1 mg kg(-1)) reduced, and DFU (3 mg kg(-1)) completely reversed the lipopolysaccharide-induced pyretic responses. The onset of action of DFU (about 30 min) is in good agreement with the pharmacokinetic profile of this compound in squirrel monkeys. The effect of DFU is comparable to that of a conventional non-selective non-steroidal anti-inflammatory drug (NSAID), diclofenac (3 mg kg(-1)). Since the plasma levels achieved for DFU at the dose employed in the present study are below the threshold required for inhibition of cyclooxygenase-1, it is concluded that the anti-pyretic effect of DFU can be attributed predominantly to an inhibitory action on cyclooxygenase-2. Thus, lipopolysaccharide-induced pyresis in squirrel monkeys can be used as a model for evaluation of anti-pyretic activity of cyclooxygenase inhibitors.