Identification of [Met5]-enkephalin in developing, adult, and renewing tissues by reversed-phase high performance liquid chromatography and radioimmunoassay

Targeting the opioid growth factor: Opioid growth factor receptor axis for treatment of human ovarian cancer

The opioid growth factor (OGF) – opioid growth factor receptor (OGFr) axis is a biological pathway that is present in human ovarian cancer cells and tissues. OGF, chemically termed [Met5]-enkephalin, is an endogenous opioid peptide that interfaces with OGFr to delay cells moving through the cell cycle by upregulation of cyclin-dependent inhibitory kinase pathways. OGF inhibitory activity is dose dependent, receptor mediated, reversible, protein and RNA dependent, but not related to apoptosis or necrosis. The OGF-OGFr axis can be targeted for treatment of human ovarian cancer by (i) administration of exogenous OGF, (ii) genetic manipulation to over-express OGFr and (iii) use of low dosages of naltrexone, an opioid antagonist, which stimulates production of OGF and OGFr for subsequent interaction following blockade of the receptor. The OGF-OGFr axis may be a feasible target for treatment of cancer of the ovary (i) in a prophylactic fashion, (ii) following cytoreduction or (iii) in conjunction with standard chemotherapy for additive effectiveness. In summary, preclinical data support the transition of these novel therapies for treatment of human ovarian cancer from the bench to bedside to provide additional targets for treatment of this devastating disease.

Gene expression of OGFr in the developing and adult rat brain and cerebellum

The native opioid peptide, [Met5]-enkephalin (termed opioid growth factor (OGF)), is a tonically active negative growth factor targeted to cell proliferation in the developing nervous system. OGF action is mediated by the OGF receptor (OGFr). The present study investigates gene expression of OGFr in the developing and adult brain and cerebellum of the rat using Northern blot analysis and normalization to GAPDH. OGFr was detected in whole brain at embryonic day 20 and birth, and was at least twofold greater than neonatal levels during the first week of life. From postnatal day 15 onwards to adulthood, levels of OGFr mRNA in the whole brain were detectable but less than those at birth. OGFr mRNA in cerebellum was found on embryonic day 20, and remained relatively constant until postnatal day 12 when a sharp increase was recorded. In the third week of life and continuing into adulthood, cerebellar OGFr mRNA was detected at levels comparable to those in postnatal week 1. These results show that message for OGFr is developmentally regulated prior to and after birth, is ubiquitously expressed during development, and is present in the adult brain and cerebellum even though OGF receptor binding is not recorded.

Transplacental transfer of the opioid growth factor, [Met(5)]-enkephalin, in rats

Placental transfer of the pentapeptide [Met5]-enkephalin, known to function as a growth regulating factor and neuromodulatory agent, was studied in pregnant Sprague-Dawley rats. Using separation by reversed phase high-performance liquid chromatography, and analysis by derivative spectroscopy, [Met5]-enkephalin was detected in 20-day-old fetal tissue including brain, heart, lung, and kidney. Fetal tissues from pregnant rats given an injection of 40 mg/kg [Met5]-enkephalin on gestation day 20 had markedly elevated levels of peptide within 1 h, indicating the transplacental transfer of this opioid. [Met5]-enkephalin levels were increased from control samples at 1, 2, 4, and 14 h post-injection of peptide, but not at 24 h. Evaluation of breakdown products of [Met5]-enkephalin, along with the related peptide [Leu5]-enkephalin, revealed that elution times differed substantially from [Met5]-enkephalin. These data indicate that [Met5]-enkephalin is present in fetal organs, crosses the placenta, does not appear to be restrictive in organ specificity, and is sustained in fetal tissues at detectable levels for at least 14 h. Given that [Met5]-enkephalin tonically inhibits DNA synthesis in the fetus, these results raise the question of whether an elevated level of this peptide (either maternally or from the fetus) may be detrimental to cellular ontogeny in the fetus, and perhaps have long-term implications for postnatal development.

Glutamate receptors in peripheral tissues: current knowledge, future research, and implications for toxicology

We illustrate the specific cellular distribution of different subtypes of glutamate receptors (GluRs) in peripheral neural and non-neural tissues. Some of the noteworthy locations are the heart, kidney, lungs, ovary, testis and endocrine cells. In these tissues the GluRs may be important in mediating cardiorespiratory, endocrine and reproductive functions which include hormone regulation, heart rhythm, blood pressure, circulation and reproduction. Since excitotoxicity of excitatory amino acids (EAAs) in the CNS is intimately associated with the GluRs, the toxic effects may be more generalized than initially assumed. Currently there is not enough evidence to suggest the reassessment of the regulated safety levels for these products in food since little is known on how these receptors work in each of these organs. More research is required to assess the extent that these receptors participate in normal functions and/or in the development of diseases and how they mediate the toxic effects of EAAs. Non-neural GluRs may be involved in normal cellular functions such as excitability and cell to cell communication. This is supported by the wide distribution in plants and animals from invertebrates to primates. The important tasks for the future will be to clarify the multiple biological roles of the GluRs in neural and non-neural tissues and identify the conditions under in which these are up- or down-regulated. Then this could provide new therapeutic strategies to target GluRs outside the CNS.

Preproenkephalin gene expression and [Met5]-enkephalin levels in the developing rat heart

[Met5]-enkephalin, encoded by the preproenkephalin (PPE) gene, serves as a growth factor (opioid growth factor, OGF) during cardiac development in addition to its role as a neuroregulator. This study examined the ontogeny and relationship of gene and peptide expression in the mammalian heart during late embryonic, preweaning, and postweaning periods. Values for PPE mRNA of hearts in rats from embryonic day 16 (E16) to postnatal day 1 were 33 to 50% of levels found in adults. Adult values for the mature heart were comparable to those in the caudate, an area of the rat brain rich in PPE mRNA. Message gradually decreased during the first postnatal week to 10% of adult values and remained so until weaning. PPE mRNA on days 35 and 50 were three- and sevenfold, respectively, higher than at 21 days, and in adults was more than 50% greater than at day 50. Message for PPE in neonatal heart was regulated rapidly and in a sustained fashion by excess opioid agonist (OGF) or blockade of opioid-receptor interaction. [Met5]-enkephalin levels increased sevenfold between E18 and E20, and another 1.6-fold until birth. Having reached a zenith in the neonate, values for enkephalin-like peptide decreased gradually through the 2nd postnatal week, and were extremely low in adulthood. Indeed, a 43-fold difference in peptide levels was detected between neonatal and adult rat heart. These data provide evidence for the expression of a tightly regulated and distinct growth factor (OGF) during the crucial periods of cell proliferation and differentiation in the mammalian heart, and reveal that the source of OGF is autocrine and/or paracrine.

Opioid gene expression in the developing and adult rat heart

Opioid peptides are known to play a role in the function and growth of the mammalian heart. Although some information about gene expression of opioids in the heart is available, there is no data on the cellular location of opioid gene expression during development or in the adult. Using in situ hybridization and rat heart ranging from embryonic day 14 (E14) to adulthood, we have evaluated the distribution of gene expression for proenkephalin, proopiomelanocortin, and prodynorphin. With respect to preproenkephalin mRNA (PPE mRNA), message in the ventricle was abundant from E14 (the first time point examined) until shortly after birth, with a marked reduction noted on postnatal days 5, 10, and 21. Adults displayed considerable message, though less than in preparations of embryonic and neonatal heart. PPE mRNA was detected in epicardial, myocardial, and endocardial cells, as well as the walls of blood vessels, capillaries, and fibroblasts. Preproopiomelanocortin (POMC) mRNA was only found in adults, and was localized to the myocardium. Message for preprodynorphin could not be observed in the ventricles of developing or adult rats. These results are the first to define the temporal and spatial ontogeny of opioid gene expression with regard to the emergence of cardiac architecture. The data suggest that gene expression for proenkephalin is especially prevalent in embryonic and neonatal rats and may be related to the modulatory activity of the opioid growth factor, [Met5]-enkephalin, on cell proliferation and differentiation. The role of PPE and POMC mRNA in adult rat heart requires elucidation.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.