Prostaglandin E receptor EP1 enhances GABA-mediated inhibition of dopaminergic neurons in the substantia nigra pars compacta and regulates dopamine level in the dorsal striatum.

Dopamine (DA) is a neuromodulator that is critical for sensory-motor, cognitive and emotional functions. We previously found that mice lacking prostaglandin E receptor EP1 showed impulsive emotional behaviors accompanied by enhanced DA turnover in the frontal cortex and striatum. Given that these behavioral phenotypes were corrected by DA receptor antagonists, we hypothesized that EP1 deficiency causes a hyperdopaminergic state for its behavioral phenotype. Here we tested this hypothesis by examining the EP1 action in the nigrostriatal dopaminergic system. We first used microdialysis and found an elevated extracellular DA level in the dorsal striatum of EP1-deficient mice compared with wild-type mice. Despite the EP1 expression in the striatum, neither deficiency nor activation of EP1 altered the intrastriatal control for DA release, uptake or degradation. Immunohistochemistry revealed punctate EP1 signals apposed with dopaminergic neurons in the substantia nigra pars compacta (SNc). Many EP1 signals were colocalized with a marker for GABAergic synapses. Further, an EP1 agonist enhanced GABA(A)-mediated inhibitory inputs to SNc dopaminergic neurons in midbrain slices. Therefore, the prostaglandin E(2)-EP1 signaling directly enhances GABAergic inputs to SNc dopaminergic neurons. The lack of this EP1 action may lead to a hyperdopaminergic state of EP1-deficient mice.

Cataract formation and prevention.

Cataract, a leading cause of blindness worldwide, is a multifactorial eye disease. In developing countries the incidence of cataract among young generations is not uncommon due to malnutrition, excess exposure to ultraviolet radiation and so on. In developed countries, age-related cataract affecting the population over 65 years of age is a major concern. Oxidative stress was suggested to inflict damage to the lens and induce opacification, and a variety of antioxidant nutrients were tested for the prevention or delay of cataract development. Although promising results were obtained in animal studies of various antioxidants, epidemiological studies on human populations do not seem to support their protective effects unequivocally. It is unlikely that age-related cataract in man, similar to the ageing process itself, will be prevented or delayed by therapeutic drugs in the foreseeable future. At present, keeping a health-conscious life style (i.e., no smoking) may be the most effective and least expensive strategy to prevent the onset of age-related cataract.

Immunolocalization of prostanoid EP receptor isotypes in human trabecular meshwork.

PURPOSE: To assess the localization of the EP-type prostanoid receptors in the human trabecular meshwork (TM) and to determine their spatial distribution in relation to the contractile a-smooth muscle actin fibres. METHODS: Cryosections of human anterior segments were obtained from 17 different donors and immunostained with different EP receptor subtype specific antibodies. Double staining for the EP2 receptor and smooth muscle actin was carried out. Western blots of TM protein samples were studied. RESULTS: No specific staining for the EP1 receptor was observed. The antibodies against the EP2 receptor revealed in all donors intense staining of human trabecular cells throughout the meshwork. EP3 receptor specific staining was not detected. EP4 immunostaining was confined to the corneoscleral region near Schwalbe's line. On western blots, the EP2 receptor was detected. In the posterior TM, the EP2 receptor staining was associated with the dense network of actin fibres. CONCLUSIONS: These immunocytochemical results present evidence that the EP2 receptor is the most abundantly expressed isotype of the PGE receptors in the human TM. This conclusion is in agreement with our previous findings at the transcript level. The relaxant responses of the TM to application of EP2 receptor agonists, and flow enhancement evoked by prostaglandin PGE1, may be explained by the close spatial association of the EP2 receptor with actin fibres.

Naphthoquinone cataract in mice: mitochondrial change and protection by superoxide dismutase.

An injection of 1,2-naphthoquinone (NQ) into the anterior chamber of mouse eye produces anterior cortical cataract. It was previously shown by histology that mitochondria in lens epithelial cells are the target of ocular drug toxicity. In this work we investigated NQ-induced cataract by closely examining morphological changes of mitochondria and other cellular organelles in the lens epithelium. Mitochondria exhibited marked swelling in 2 hrs after NQ injection but restored the normal condensed configuration at 4.5 hrs. The nuclear chromatin showed condensation at 2 hrs and returned to the normal appearance at 4.5 hrs. This was unexpected because the lens at 4.5 hrs was cataractous due to vacuole formation in fiber cell layers. The result indicates that, although lens epithelial mitochondria are the target of NQ toxicity, cataract begins to develop before mitochondria and other subcellular organelles become totally dysfunctional. At 1 week after NQ injection, most mitochondria disintegrated and the fragmented chromatin appeared to leak out through the ruptured nuclear membrane. SOD injected with NQ significantly delayed the onset of cataract and protected lens epithelial cells. A second SOD injection further delayed cataract development.

Cellular events preceding acetaminophen cataractogenesis studied by confocal fluorescence microscopy.

Acetaminophen (APAP) is biotransformed by hepatic cytochrome P450 (CYP) enzymes to the cataractogenic metabolite N-acetyl-p-benzoquinone imine (NAPQI). In the previous studies in which NAPQI was injected into the anterior chamber of mouse eye, we observed mitochondrial dysfunction and disturbances in Ca2+ homeostasis in the lens epithelium, and activation of the nonlysosomal neutral protease calpain. In this work we investigated whether intraperitoneal injection of APAP elicits similar cellular responses in the lens epithelium prior to the onset of lens opacity development. Following APAP injection, reactive oxygen species generation, intracellular free Ca2+ increase and calpain activation in the lens epithelium were determined in situ by fluorescence confocal microscopy. It was found that cellular events in the lens prior to the onset of opacification were essentially identical to those elicited by NAPQI. In addition, lens calpain activities were characterized based on their Ca2+ requirement and several calpain inhibitors were shown to prevent cataract development.

Impaired adrenocorticotropic hormone response to bacterial endotoxin in mice deficient in prostaglandin E receptor EP1 and EP3 subtypes.

Sickness evokes various neural responses, one of which is activation of the hypothalamo-pituitary-adrenal (HPA) axis. This response can be induced experimentally by injection of bacterial lipopolysaccharide (LPS) or inflammatory cytokines such as IL-1. Although prostaglandins (PGs) long have been implicated in LPS-induced HPA axis activation, the mechanism downstream of PGs remained unsettled. By using mice lacking each of the four PGE receptors (EP1-EP4) and an EP1-selective antagonist, ONO-8713, we showed that both EP1 and EP3 are required for adrenocorticotropic hormone release in response to LPS. Analysis of c-Fos expression as a marker for neuronal activity indicated that both EP1 and EP3 contribute to activation of neurons in the paraventricular nucleus of the hypothalamus (PVN). This analysis also revealed that EP1, but not EP3, is involved in LPS-induced activation of the central nucleus of the amygdala. EP1 immunostaining in the PVN revealed its localization at synapses on corticotropin-releasing hormone-containing neurons. These findings suggest that EP1- and EP3-mediated neuronal pathways converge at corticotropin-releasing hormone-containing neurons in the PVN to induce HPA axis activation upon sickness.