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

Developmental changes in the inhibition of cultured rat uterine cell proliferation by opioid peptides

Opioid peptides are negative regulators of cell proliferation in several organs including the uterus. In the present study, the ontogeny of the direct inhibitory action of opioid peptides on the proliferation of cultured rat uterine cells was investigated. Uteri of 7, 14, 21, 28, 35 and 60-day-old rats were removed in a sterile way. Tissue blocks were dispersed by limited digestions with trypsin and collagenase. Cells were cultured in enriched Dulbecco's modified Eagle's medium (DMEM). Treatments were present during the entire culture period. Cell densities of the monolayers were determined by counting the cells following trypsinization and trypan blue exclusion. Rat uterine mixed cell cultures grew to confluence within 10 days. The average population doubling time gradually increased with the age of animals. Epidermal growth factor (EGF) increased cell densities of cultures from all age groups. The oestradiol (E2)-responsiveness appeared at 21 days of age. The effect of [D-Met2-Pro5]-enkephalinamide (ENK) was biphasic. ENK and [Met5]-enkephalin (OGF) decreased cell densities of both unstimulated and EGF-stimulated cultures from 7-day-old rats to the same extent. ENK failed to act in 14-day-old animals. From 21 days of age on, the E2- or EGF-stimulated proliferation was inhibited only by ENK and DAMGO, while 30 nm DPDPE, Dynorhin-A, OGF, [Leu5]-enkephalin, beta-endorphin, and morphiceptin were ineffective. The half-inhibitory concentration of ENK was 0.3 nm. The effects of ENK were prevented by concomitant treatment with naloxone. Our novel data demonstrate two different phases of the inhibitory action of opioid peptides on rat uterine cell proliferation during ontogeny with an insensitive interval in between.

Anti-mitogenic action of opioid peptides on epidermal growth factor-stimulated uterine cells

Endogenous opioid peptides are negative regulators of estradiol-induced uterine cell proliferation. To investigate the possible molecular target site(s) of their anti-mitogenic action, we examined the effect of opioid peptides on epidermal growth factor-induced cell proliferation both in uterine primary cell cultures prepared from adult rats and in human myometrial smooth muscle cell lines. Epidermal growth factor (EGF) significantly increased cell density in both types of cultured monolayers. This EGF-induced stimulation of cell proliferation was blocked by [D-Met(2)-Pro(5)]enkephalinamide in a time-dependent, receptor-mediated manner. The effective concentrations were within the physiological nanomolar range. Enkephalinamide did not have any effect on the basal rate of proliferation of the uterine cells. Our results on this novel physiological cross-talk suggest that shared step(s) of the mechanism of action of estradiol and EGF might be targeted by opioid peptides and not the general machinery of cell proliferation.

Antiestrogenic effect of opioid peptides in rat uterus

The effects of a single injection or continuous infusion of opioid peptide, [D-Met(2),pro(5)]enkephalinamide (ENK) on the hormone binding and transcriptional properties of estrogen receptors were investigated in estradiol (E(2)) treated rat uterus. The level of estrogen- (ER) and progesterone receptor (PR) proteins, the hormone binding of E(2) receptors and the effects of single injection of ENK with or without naltrexone (NAL) on the E(2)-induced changes in the level of Fos and Jun proteins and the binding of AP-1 proteins to DNA were studied. The receptor proteins levels were determined by Western blots and the binding of AP-1 to DNA by electrophoretic mobility shift assay. Both the ER and PR protein concentrations and the [3H]Estradiol binding to the high affinity nuclear receptors decreased after ENK treatment during the first two days. At 72 h the PR concentration decreased further, while no significant changes were found in the level of ER, however, at this time the former competitive E(2) binding turned into positive cooperativity. The E(2)-induced increase in the level of Fos proteins and the binding of AP-1 proteins to DNA was inhibited by a single injection of ENK. We conclude that the endogenous opioid peptides may interact with E(2) in the gene regulation of rat uterus.

Epidermal growth factor influenced by opioid peptides in immature rat uterus

The aim of the present experiment was to investigate the effect of [D-Met2,Pro5] enkephalinamide (ENK) implantation on the development of the uterus during 8-33 days of age and the involvement of epidermal growth factor (EGF) in the effect. Administration of ENK was attained by osmotic minipumps (5 microg/h) implanted intraperitoneally. ENK resulted in a decrease in the EGF content of the uterus, which was already significant after 48 h of the implantation. The DNA content 24 and 48 h after the treatment decreased, no change at 72 h was found, however the protein/DNA ratio on the effect of ENK treatment was significantly decreased at this time in all examined age groups. High affinity and lower capacity competitive naloxone binding sites were demonstrated in the membrane fraction of the uteri. Seventy-two h after ENK treatment the binding capacity of these sites significantly dropped. The present results suggest a novel multiple interaction between estrogen and two probably paracrine hormones, EGF and opioid peptide, in the regulation of growth and development of the uterus.

Opioid peptides inhibit the estradiol-induced proliferation of cultured rat uterine cells

The effect of opioid peptides on estradiol-induced cell proliferation in adult rat uterine primary cell cultures was studied. Estradiol increased cell density by 40%. This estradiol-induced stimulation of cell proliferation was decreased to control values by [D-Met2,Pro5]enkephalinamide. The opioid-induced inhibition of uterine cell proliferation was blocked completely by the specific opiate antagonist naloxone, while naloxone did not have any effect on its own. The inhibition of cell proliferation by enkephalinamide was apparent at each stimulatory estradiol concentration examined. This opioid effect was mediated mainly by the mu opiate receptor. The observed effects occurred within the physiological nanomolar concentration range. Enkephalinamide did not have any effect on the basal proliferation rate of adult rat uterine cells. However, enkephalinamide inhibited the basal rate of cell proliferation in cell cultures prepared from 7-day-old immature rats. In summary, here we present evidence of novel physiological direct cross-talk between the opioid and estrogenic signaling systems in the regulation of normal uterine growth.

Opioids regulate cell proliferation in the developing rat uterus: effects during the period of sexual maturation

The present studies demonstrate, for the first time, that the rate of DNA synthesis in rat uterus of 21-32 days of age is inhibited by opioid peptides [D-Met2, Pro5]enkephalinamide. At around the time of vaginal opening (approximately 33 days) the opioids failed to act. High-affinity nuclear [3H]naloxone binding sites with linear Scatchard plots were detected in the uteri during the opioid-sensitive periods of DNA synthesis. Characteristics of these binding sites and the opioid sensitivity of uterine DNA synthesis are dependent on the age of the animals, the level of circulatory oestradiol and/or the maturity of the nuclear oestrogen receptor system.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. The specific topics covered this year include stress: tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.