Demonstration and characterization of zeta (zeta), a growth-related opioid receptor, in a neuroblastoma cell line

The Opioid Growth Factor in Growth Regulation and Immune Responses in Cancer

It has become apparent that endogenous opioids act not only as neurotransmitters and neuromodulators, but have multiple functions in the body. Activation of the opioid system by opiate drugs is associated with a risk of cancer development through direct stimulation of tumor cell proliferation and through immunosuppression. In contrast, the endogenous peptide opioid [Met5]-enkephalin, now commonly referred to as Opioid Growth Factor (OGF), negatively regulates cell proliferation in a wide number of cells during development, homeostasis, and neoplasia. This action is mediated through the opioid growth factor receptor, originally designated the zeta (ζ) opioid receptor. Further, contrary to the traditional notion of opiates as immunosuppressive, endogenous OGF has been shown to possess a number of positive immunomodulatory properties and may provide a beneficial effect in cancer by augmenting the activity of cells involved in both innate and acquired immunity. Taken together, the evidence supports consideration of opioid peptides such as OGF as new strategies for cancer therapy.

Research progress of opioid growth factor in immune-related diseases and cancer diseases

Methionine enkephalin (MENK) has an important role in both neuroendocrine and immune systems. MENK was known as an opioid growth factor (OGF) for its growth regulatory characteristics. OGF interacts with the OGF receptor (OGFr) to inhibit DNA synthesis by upregulating p16 and/or p21, which delays the cell cycle transition from G0/G1 to S phase, and inhibits cell proliferation. In addition, OGF combines with OGFr in immune cells to exert its immunomodulatory activity and regulate immune function. OGF has been studied as an immunomodulator in a variety of autoimmune diseases, including multiple sclerosis, inflammatory bowel disease, diabetes and viral infections, and has been proven to relieve symptoms of certain diseases in animal and in vitro experiments. Also, OGF and OGFr have various anti-tumor molecular mechanisms. OGF can be used as the primary therapy alone or combined with other drugs to treat tumors. This article summarizes the research progress of OGF in immune-related diseases and cancer diseases.

Prospective oncotarget for gynecological cancer: Opioid growth factor (OGF) - opioid growth factor receptor (OGFr) axis

The standard treatments for neoplasia include surgery, chemotherapy, hormone antagonists and radiotherapy, which can prolong survival, but rarely cure the tumors of gynecological cancer patients. OGF - OGFr expression, in various gynecologic cells and tissues, is an intersection point between cell development, neuroendocrine function and immune modulation. It has been identified that OGF and OGFr expression differs between gynecological tumor and normal cells. Further, exogenous or endogenous OGF and OGFr antagonists have been known to have a role in regulating cell viability and apoptosis. Moreover, the expression of proteins in the OGF - OGFr axis modulate differentiation and membrane expression of immune cells, which can enhance the immune response. In vivo and in vitro assays have shown that OGF and OGFr antagonists inhibit mitosis as well as induce apoptosis in gynecologic cancer cells. Although immune augmentation combination therapies can intensify cytotoxic activity, OGF or OGFr antagonists do not increase toxicities associated with dual-immune regulation. In conclusion, the OGF - OGFr axis provides significant strategies for antitumor efficiency in gynecological cancer.

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.

Immunoelectron microscopic localization of the opioid growth factor receptor (OGFr) and OGF in the cornea

This study was conducted to determine the cellular and subcellular location(s) of the opioid growth factor receptor (OGFr), and the opioid growth factor (OGF), [Met(5)]-enkephalin, in the corneal epithelium. Laser scanning confocal microscopy analysis revealed that both OGFr and OGF were colocalized in the paranuclear cytoplasm and cell nuclei in basal, as well as suprabasal, cells of adult rat corneal epithelium. Using a postembedding immunogold procedure for immunoelectron microscopy that included embedding in Unicryl, both single- and double-face labeling studies were performed. Immunogold labeling of OGFr was detected on the outer nuclear envelope, in the paranuclear cytoplasm proximal to the nuclear envelope, perpendicular to the nuclear envelope in a putative nuclear pore complex, and within the nucleus adjacent to heterochromatin. Immunoreactivity for OGF was noted in locations similar to that for OGFr. In addition, aggregates of staining for OGF were found throughout the cytoplasm, including subjacent to the plasma membrane. Double labeling experiments revealed that complexes of OGF-OGFr were colocalized on the outer nuclear envelope, in the paranuclear cytoplasm, extending across the nuclear pore complex, and in the nucleus. Anti-OGFr IgG by itself, but not anti-OGF IgG alone, was associated with the outer nuclear envelope, and uncomplexed OGF immunoreactivity was detected in the cytoplasm in dual labeling experiments. These results based on complementary approaches of confocal microscopy and immunoelectron microscopy, suggest that: (i) OGFr resides on the outer nuclear envelope, (ii) OGF interacts with OGFr at the outer nuclear envelope, (iii) the colocalized receptor and peptide translocates between the cytoplasm and the nucleus at the nuclear pore, and (iv) signal transduction for modulation of cell proliferation necessitates a peptide-receptor complex that interfaces with chromatin in the nucleus.

Differential effects of vascular growth factors on arterial and venous angiogenesis

OBJECTIVE

Angiogenesis, the development of new blood vessels, has become an area of increased interest for both scientific and clinical application purposes. Proangiogenic agents, such as vascular endothelial growth factor (VEGF) and naltrexone, have been shown to effectively induce new blood vessel growth. Other growth factors, such as the endogenous opioid growth factor (OGF; [Met(5)]-enkephalin) and retinoic acid, are inhibitors of angiogenesis. The differential effects on veins and arteries, however, by any vascular growth factor, have not previously been investigated.

METHODS

The chick chorioallantoic membrane (CAM) assay was used for the in vivo quantitation of angiogenesis. After 3 days of incubation, fertilized chick embryos were explanted, and a 3.2-mm methylcellulose disk containing either the known angiogenic stimulators VEGF (0.2 microg, 1.0 microg) or naltrexone (0.1 microg, 5.0 microg), or the angiogenic inhibitors OGF (1.0 microg, 5.0 microg) or retinoic acid (1.0 microg) was placed onto the CAM surface. An equal volume of distilled water served as a control. After 2 days of growth, the CAM arteries and veins were identified, and images were obtained with a digital camera. Quantitative analysis of angiogenesis was performed on a 100-mm(2) area surrounding the applied disk, and the number and length of the veins and arteries were measured.

RESULTS

The angiogenic stimulators VEGF and naltrexone markedly increased both the total number and length of all blood vessels as compared with control values. The mean length of blood vessels decreased, suggesting the induction of new vessel growth. VEGF and naltrexone proportionately increased vein and arterial angiogenesis, maintaining artery/vein ratios for vessel number and length that were unchanged compared with controls. The angiogenic inhibitors, OGF and retinoic acid, notably decreased the total number and length of blood vessels in the CAM preparations. However, these compounds had a disproportionately greater inhibitory effect on arterial angiogenesis as reflected in decreased artery/vein ratios for vessel number and length.

CONCLUSIONS

The angiogenic stimulators VEGF and naltrexone induce development of veins and arteries in a proportional manner. In contrast, the angiogenic inhibitors OGF and retinoic acid demonstrated a greater inhibitory effect on arterial as compared with venous angiogenesis. Such differential effects on angiogenesis may be important in both defining mechanisms of action and designing therapeutic interventions.

Molecular characterization and distribution of the opioid growth factor receptor (OGFr) in mouse

The native opioid growth factor (OGF), [Met(5)]-enkephalin, is a tonic inhibitory peptide that modulates cell proliferation and tissue organization during development, cancer, cellular renewal, wound healing, and angiogenesis. OGF action is mediated by a receptor mechanism. The receptor for OGF, OGFr, has been cloned and sequenced in humans and rats. Using primers based on the rat OGFr cDNA, and a mouse embryo expressed sequence tag, the full-length 2.1 kb mouse OGFr cDNA was sequenced. The open reading frame was found to encode a protein of 634 amino acids, and 14 imperfect repeats of 9 amino acids each were a prominent feature. The molecular weight of OGFr was calculated as 70679, and the isoelectric point was 4.5. Northern blot analysis revealed a 2.1 kb OGFr mRNA transcript in adult mouse brain, heart, lung, liver, kidney, and triceps surae muscle. The amino acids for mouse and rat OGFr were 93% similar and 91% identical, but the mouse and human shared only a 70% similarity and a 58% identity. These results emphasize the molecular validity of OGFr, and explain the interaction of OGF with respect to normal and abnormal growth in mouse cells and tissues.

Opioid growth factor modulates angiogenesis

OBJECTIVE

Induced angiogenesis has recently been attempted as a therapeutic modality in patients with occlusive arterial atherosclerotic disease. We investigated the possible role of endogenous opioids in the modulation of angiogenesis.

METHODS

Chick chorioallantoic membrane was used as an in vivo model to study angiogenesis. Fertilized chick eggs were incubated for 3 days, explanted, and incubated for an additional 2 days. Three-millimeter methylcellulose disks were placed on the surface of the chorioallantoic membrane; each disk contained opioid growth factor ([Met(5)]-enkephalin; 5 microgram), the short-acting opioid receptor antagonist naloxone (5 microgram), opioid growth factor and naloxone together (5 microgram of each), the long-acting opioid antagonist naltrexone (5 microgram), or distilled water (control). A second series of experiments was performed with distilled water, the angiogenic inhibitor retinoic acid (1 microgram), and vascular endothelial growth factor (1 microgram) to further evaluate our model. The developing vasculature was imaged 2 days later with a digital camera and exported to a computer for image analysis. Total number of blood vessels, total vessel length, and mean vessel length were measured within a 100-mm(2) region surrounding each applied disk. Immunocytochemical analysis was performed with antibodies directed against opioid growth factor and its receptor (OGFr).

RESULTS

Opioid growth factor had a significant inhibitory effect on angiogenesis, both the number of blood vessels and the total vessel length being decreased (by 35% and 20%, respectively) in comparison with control levels (P <.005). The simultaneous addition of naloxone and opioid growth factor had no effect on blood vessel growth, nor did naloxone alone. Chorioallantoic membranes exposed to naltrexone displayed increases of 51% and 24% in blood vessel number and length, respectively, in comparison with control specimens (P <.005). These results indicate that the opioid growth factor effects are receptor mediated and tonically active. Immunocytochemistry demonstrated the presence of both opioid growth factor and OGFr within the endothelial cells and mesenchymal cells of the developing chorioallantoic membrane vessel wall. Retinoic acid significantly reduced the number and the total length of blood vessels, whereas vascular endothelial growth factor increased both the number and the length of blood vessels in comparison with the controls (P <.0001). The magnitude of opioid growth factor's effects were comparable to those seen with retinoic acid, whereas inhibition of opioid growth factor with naltrexone induced an increase in total vessel length comparable to that for vascular endothelial growth factor.

CONCLUSIONS

These results demonstrate for the first time that endogenous opioids modulate in vivo angiogenesis. Opioid growth factor is a tonically active peptide that has a receptor-mediated action in regulating angiogenesis in developing endothelial and mesenchymal vascular cells.

Growth inhibitory effects of a mu opioid on cultured cholinergic neurons from fetal rat ventral forebrain, brainstem, and spinal cord

Cholinergic pathways play a role in respiration in the mammalian brain, and agents that affect respiratory function such as opioid peptides might have positive or negative neurotrophic effects during the development of these cholinergic connections. Rat fetal nerve cell cultures from developmental stages E14-E18 were established in 96-well plates from ventral forebrain (VFB), an area rich in cholinergic neurons, and from brainstem and rostral spinal cord, areas where respiratory control systems and cholinergic neurons co-exist. High affinity 3H-choline uptake was highest in E14 VFB cultures and decreased to 20% of this value by E16 and E18. Choline uptakes in E14 brainstem and spinal cord were only 20% and 13%, respectively, of E14 VFB uptake. A mu opioid receptor agonist, d-ala2-mePhe4-gly(ol)5]-enkephalin (DAMGO), was tested for its effect on somal area and neurite outgrowth in E16 cultures. Cholinergic neurons were identified by immunostaining with choline acetyltransferase antibody. DAMGO (10(-8) M) significantly decreased somal area in VFB cultures and spinal cord, but had no effect on somal area in brainstem. Naltrexone (10(-6) M) reversed this inhibition. Spinal cord cell neurite outgrowth was inhibited by DAMGO, and this inhibition was reversed by naltrexone. DAMGO had no significant effect on neurite length in VFB. Brainstem neurite length was paradoxically increased by both DAMGO and naltrexone. It was concluded that mu-selective opioid peptides inhibit growth of cultured cholinergic neurons in VFB and spinal cord, but not in the brainstem. There was no evidence for endogenous opioid activity in either VFB or spinal cord cultures.

Modulation of the estrogen-regulated proteins cathepsin D and pS2 by opioid agonists in hormone-sensitive breast cancer cell lines (MCF7 and T47D): evidence for an interaction between the two systems

In many cancer cell lines, including breast, prostate, lung, brain, head and neck, retina, and the gastrointestinal tract, opioids decrease cell proliferation in a dose-dependent and reversible manner. Opioid and/or other neuropeptide receptors mediate this decrease. We report that only the steroid-hormone-sensitive cell lines MCF7 and T47D respond to opioid growth inhibition in a dose-dependent manner. Therefore, an interaction of the opioid and steroid receptor system might exist, as is the case with insulin. To investigate this interaction, we have assayed two estrogen-inducible proteins (pS2 and the lysosomal enzyme cathepsin D) in MCF7 and T47D cells. When cells were grown in the presence of FBS (in which case a minimal quantity of estrogens and/or opioids is provided by the serum), we observed either no effect of etorphine or ethylketocyclazocine (EKC) or an increase of secretion and/or production of pS2 and cathepsin D. However, when cells were cultured in charcoal-stripped serum and in the absence of phenol red, the effect of the two opioids is different: EKC decreased the production and/or secretion of pS2 and cathepsin D, whereas etorphine increased their synthesis and/or secretion. The differential effect of the two general opioids was attributed to their different receptor selectivity. Furthermore, the variations of the ratio of secreted/produced protein and the use of cycloheximide indicate that opioids selectively modify the regulatory pathway of each protein discretely. In conclusion, through the interaction with opioid and perhaps other membrane-receptor sites, opioid agonists modify in a dose-dependent manner the production and the secretion of two estrogen-regulated proteins. Opioids may therefore disturb hormonal signals mediated by the estrogen receptors. Hence, these chemicals may have potential endocrine disrupting activities.

Role of opioid peptides in the regulation of DNA synthesis in immature rat uterus

The effects of a single dose of naloxone and of [D-Met2,Pro5]enkephalinamide on the DNA synthesis in the uterus of 7, 14 and 21-day-old rat were studied. After [D-Met2,Pro5]enkephalinamide treatment, an age-dependent decrease in in vitro [3H]thymidine incorporation into DNA was observed in all studied age groups. In the 21-day-old age group a reduced rate of DNA synthesis was detected for 12 h after [D-Met2,Pro5]enkephalinamide treatment followed by the return to control values at 24 h. The rate of inhibition was more marked in the younger age groups. The effect was also more pronounced in younger animals. Specific [3H]naloxone binding was detected both in membrane and nuclear fractions of uterine homogenates. While no age-related changes in binding affinities were found, the number of binding sites varied characteristically during development. Our data suggest the novel involvement of opioid peptides and their receptors in the regulation of uterine development.

Ontogeny of the opioid growth factor, [Met5]-enkephalin, and its binding activity in the rat retina

The endogenous opioid peptide [Met5]-enkephalin is a tonically active opioid growth factor (OGF) with an inhibitory action on DNA synthesis in the developing rat retina. In this study, the ontogeny of the spatial and temporal expression of OGF and its binding activity was examined. OGF-like immunoreactivity was detected in the retina at gestation day (E) 20, but not at E18, and was localized to ganglion cell and neuroblast layers; immunochemical reaction was no longer seen in the retina by postnatal day 6. Native OGF was further identified and characterized by high-performance liquid chromatography (HPLC) studies and immunodot assays, which revealed that [Met5]-enkephalin was present in the neonatal, but not adult, rat retina. OGF binding activity was detected as early as E18 using [125I]-[Met5]-enkephalin and in vitro receptor autoradiography. Little OGF binding activity was noted for prenatal retinas, but appreciable activity was observed from birth to postnatal day 4; no OGF binding could be detected after postnatal day 5 or in the adult. These results reveal the transient appearance of the OGF, [Met5]-enkephalin, and its receptor binding activity in the developing mammalian retina, and show that their ontogeny coincides with the timetable of DNA synthesis of retinal neuroblasts.

Alteration of opioid peptide concentrations in the rat pituitary following survivable closed head injury

Concentration changes of methionine enkephalin-like immunoreactivity (ME-li) and beta-endorphin-like immunoreactivity (BE-li) in the rat pituitary following diffuse brain injury were studied. Closed head injury was induced by a weight-drop trauma device (450 g x 2 m). The level of closed head injury used in this study altered the pituitary opioid peptide concentrations. The level of ME-li did not change in the experimental groups 3 hours, 10 hours, 24 hours, and 3 days after the trauma, but significantly increased by 34% 10 days after the trauma. BE-li remained constant 3 hours and 10 hours following the injury, increased by 48% at 24 hours, and remained at this level for 10 days after the trauma (44% at 3 days, and 40% at 10 days). The levels of ME-li and BE-li in the control sham-operated rats did not change during these times. The present measurements of BE-li and ME-li in the pituitary indicate that the opioid peptides that derive from two different neuropeptidergic systems, proopiomelanocortin (POMC) and preproenkephalin A, respectively, may participate in the pathophysiology of a closed head injury.

Differential effects of methionine enkephalin on the growth of brain tumor cells

The effect of methionine enkephalin on the growth of human brain tumor cells was investigated. The results show that this endogenous opioid has dual effects on tumor cell growth. This peptide exerted an inhibitory effect in SK-N-MC human neuroblastoma cell line; in contrast, in U-373 MG human astrocytoma cell line, the peptide showed a stimulatory effect. Treatment with naltrexone, an opioid receptor antagonist, also resulted in a similar alteration of tumor cell growth implicating that its action may be unrelated to opioid receptor blockade. These results suggest that in these tumor cell lines endogenous opioid systems may be involved in cell proliferation. Furthermore, these tumor cell lines may be useful model systems for the study of the signal transduction mechanisms of endogenous opioids.

An opioid growth factor regulates the replication of microorganisms

An opioid growth factor (OGF), [Met5]-enkephalin, interacts with the zeta (zeta) opioid receptor to modulate development of eukaryotes. We have found that [Met5]-enkephalin, an endogenous opioid peptide serves to inhibit the growth of S. aureus. This effect on growth involves cell proliferative events and is under tonic control, since potent opioid antagonists accelerate cell replication. Both the OGF and zeta opioid receptor were associated with these microorganisms. Other opioid receptors (mu, delta and kappa) were not detected. OGF also controlled the growth of other bacteria: P. aeruginosa and S. marcesans. These results indicate that OGF and its receptor, known to be important in the regulation of mammalian development, also function in the growth of simple unicellular organisms. We suggest that the endogenous opioid system related to growth originated billions of years ago.

Glial growth is regulated by agonists selective for multiple opioid receptor types in vitro

To determine whether one or more opioid receptor types might be preferentially involved in gliogenesis, primary mixed glial cultures derived from mouse cerebra were continuously treated with varying concentrations of opioid agonists selective for mu (mu), i.e., DAGO ([D-Ala2, MePhe4, Gly(ol)5]enkephalin), delta (delta), i.e., DPDPE ([D-PEN2,D-PEN5]enkephalin), or kappa (kappa), i.e., U69,593, opioid receptor types. In addition, a group of cultures was treated with [Met5]-enkephalin, an agonist for delta opioid receptors as well as putative zeta (zeta) opioid receptors. Opioid-dependent changes in growth were assessed by examining alterations in (1) the number of cells in mixed glial cultures at 3, 6, and 8 days in vitro (DIV), (2) [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes at 6 DIV, and (3) the area and form factor of GFAP-immunoreactive, flat (type 1) astrocytes. DPDPE at 10(-8) or 10(-10) M, as well as [Met5]-enkephalin at 10(-6), 10(-8), or 10(-10) M, significantly reduced the total number of glial cells in culture; but this effect was not observed with DAGO or U69,593 (both at 10(-6), 10(-8), or 10(-10) M). Equimolar concentrations (i.e., 10(-6) M) of [Met5]enkephalin or U69,593, but not DPDPE or DAGO, suppressed the rate of [3H]thymidine incorporation by GFAP-immunoreactive, flat (type 1) astrocytes. DAGO had no effect on growth, although in previous studies morphine was found to inhibit glial numbers and astrocyte DNA synthesis. [Met5]enkephalin (10(-6) M) was the only agonist to significantly influence astrocyte area. Collectively, these results indicate that delta (and perhaps mu) opioid receptor agonists reduce the total number of cells in mixed glial cultures; while [Met5]enkephalin-responsive (and perhaps kappa-responsive) opioid receptors mediate DNA synthesis in astrocytes. This implies that delta opioid receptors, as well as [Met5]enkephalin-sensitive, non-delta opioid receptors, mediate opioid-dependent regulation of astrocyte and astrocyte progenitor growth. These data support the concept that opioid-dependent changes in central nervous system growth are the result of endogenous opioid peptides acting through multiple opioid receptor types.

Zeta (zeta), a growth-related opioid receptor in developing rat cerebellum: identification and characterization

Endogenous opioids and opioid receptors (i.e. endogenous opioid systems) are expressed during neural ontogeny, and play a role in the development of the nervous system. Using [3H][Met5]-enkephalin, a potent ligand involved in neural growth, particularly cell proliferation, specific and saturable binding was detected in homogenates of 6-day-old rat cerebellum; the data were consistent with a single binding site. Scatchard analysis yielded a binding affinity (Kd) of 2.2 nM and a binding capacity (Bmax) of 22.3 fmol/mg protein. Binding was linear with protein concentration, dependent on time, temperature, and pH, and was sensitive to Na+, Mg2+, and guanyl nucleotides. Optimal binding required protease inhibitors, and pretreatment of the homogenates with trypsin markedly reduced binding, suggesting that the binding site was proteinaceous in character. The [Met5]-enkephalin binding site was an integral membrane protein located in the nuclear fraction. Competition experiments indicated that [Met5] enkephalin was the most potent displacer of [3H][Met5]-enkephalin, and that binding was stereospecific. In the adult rat cerebellum, non-opioid receptor binding of [3H][Met5]-enkephalin was recorded, mu and kappa receptors were also found in the developing rat cerebellum, while mu, delta, and kappa receptors were recorded in adult cerebellar tissue. The function, pharmacological and biochemical characteristics, subcellular distribution, and temporal expression of the [Met5]-enkephalin binding site suggest the presence of a unique opioid receptor, termed zeta (zeta), in the developing nervous system.

Morphine alters astrocyte growth in primary cultures of mouse glial cells: evidence for a direct effect of opiates on neural maturation

To determine whether exogenous opiate drugs with abuse liability directly modify neural growth, the present study investigated the effects of morphine on astrocyte proliferation and differentiation in primary cultures of murine glial cells. The results indicate that morphine decreases glial cell production in a dose-dependent, naloxone-reversible manner. Most notably, gliogenesis virtually ceased in the presence of 10(-6) M morphine during the first week in culture, whereas 10(-8) M or 10(-10) M morphine caused an intermediate suppression of growth compared to control or 10(-6) M morphine treated cultures. Moreover, morphine treatment inhibited [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes, suggesting that the decrease in glial cell production was due in part to an inhibition of astrocyte proliferation. Morphine also caused significant increases in both cytoplasmic area and process elaboration in flat (type 1) astrocytes indicating greater morphologic differentiation. In the above experiments, morphine-dependent alterations in astrocyte growth were antagonized by naloxone, indicating that morphine action was mediated by specific opioid receptors. These observations suggest that opiate drugs can directly modify neural growth by influencing two critical developmental events in astrocytes, i.e., inhibiting proliferation and inducing morphologic differentiation.

Endogenous opioids regulate cell proliferation in the retina of developing rat

The role of endogenous opioids and opioid receptors (endogenous opioid systems) in modulating cell proliferation in the developing mammalian retina was examined in 1-day-old rats. In contrast to a labeling index (LI) of 35.8% in control animals, administration of the opioid peptide [Met5]-enkephalin (100 micrograms/kg) significantly reduced (10.6%) the proportion of cells incorporating [3H]thymidine; concomitant injection of 1 mg/kg naloxone blocked the inhibitory effects of [Met5]-enkephalin on cell division. Naloxone (1 mg/kg) alone did not alter the LI. The interruption of endogenous opioid-opioid receptor interaction by naltrexone (50 mg/kg), a potent opioid antagonist, was accompanied by a significant increase (6.4%) in the LI relative to control levels. Immunocytochemical experiments revealed the presence of enkephalin-like immunoreactivity, with staining of the cortical cytoplasm of proliferating and differentiating retinal cells recorded; no immunoreactivity was noted in the adult retina. In vitro autoradiography using 125I-[Met5]-enkephalin indicated that [Met5]-enkephalin binding sites were localized to the developing retina; no binding of the radiolabeled ligand was recorded in the adult retina. These results demonstrate the presence of growth-related endogenous opioids and opioid receptors in the developing mammalian retina, but not in adult retina, and suggest that endogenous opioids serve as natural inhibitory trophic factors that tonically regulate cell proliferation.

Identification of opioid peptides regulating proliferation of neurons and glia in the developing nervous system

Endogenous opioid systems (i.e. opioids and opioid receptors) play a role in regulating neural development. Using the cerebellar cortex of 6-day-old rats, the most potent opioid peptides involved with cell proliferation were assessed. In both the external germinal (granule) layer (EGL), a germinative matrix giving rise to neurons, and the medullary layer (MED), a pool of cells that are the precursors of glia (astrocytes and oligodendrocytes), [Met5]enkephalin and peptide F were extremely potent in depressing the labeling index (LI) using [3H]thymidine and autoradiographic techniques; concentrations as low as 100 micrograms/kg reduced the LI of EGL cells by 24% and MED cells by 43%. This inhibition of DNA synthesis by opioid peptides was blocked by concomitant exposure to to naloxone, an opioid antagonist. Peptide action was apparent 2 h following drug administration, and concentrations of 80 micrograms/kg but not 1 or 10 micrograms/kg [Met5]enkephalin depressed the LI. These results identify a selective group of opioid peptides, derived from proenkephalin A, as the potent, natural, inhibitory factors targeted to cell proliferation of cells destined to be neurons and glia in the developing nervous system.

Opioid antagonist (naltrexone) stimulation of cell proliferation in human and animal neuroblastoma and human fibrosarcoma cells in culture

Endogenous opioids play a role in carcinogenic events by serving as inhibitory growth factors that alter cell proliferative events by interaction with opioid receptors. The present study addresses the question of whether endogenous opioid systems function tonically in tissue culture. Using S20Y neuroblastoma, a cell line that produces a growth-related opioid peptide (i.e.[Met5]enkephalin) and contains the zeta receptor known to be associated with growth, the effects of opioid receptor blockade by naltrexone, a potent opioid antagonist, was examined. Drug concentrations of 10(-4) to 10(-8) M naltrexone stimulated cell proliferation, with 32-86% more cells found in the naltrexone groups than control from 12 to 48 h after initiating drug exposure; drug concentrations of 10(-9) to 10(-13) M had no effect on growth. Evaluation of labeling and mitotic indices revealed that both DNA synthesis and mitosis were increased by naltrexone, as were the number of cells with process lengths greater than 40 microns. Naltrexone (10(-6) M) also stimulated the growth of N115 murine neuroblastoma, SK-N-MC human neuroblastoma, and HT-1080 human fibrosarcoma. These results indicate that endogenous opioids function in vitro to regulate growth by inhibitory mechanisms, and do so actively. This autocrine mechanism in tissue culture also occurs in other animal neuroblastoma cell lines, as well as for human neuroblastoma and fibrosarcoma cell lines.