Regulation of pituitary and brain enkephalin systems by estrogen

[D-Ala2, D-Leu5] Enkephalin Improves Liver Preservation During Normothermic Ex Vivo Perfusion

BACKGROUND

Meeting the metabolic demands of donor livers using normothermic ex vivo liver perfusion (NEVLP) preservation technology is challenging. The delta opioid agonist [D-Ala2, D-Leu5] enkephalin (DADLE) has been reported to decrease the metabolic demand in models of ischemia and cold preservation. We evaluated the therapeutic potential of DADLE by investigating its ability to protect against oxidative stress and hepatic injury during normothermic perfusion.

MATERIALS AND METHODS

Primary rat hepatocytes were used in an in vitro model of oxidative stress to determine the minimum dose of DADLE needed to induce protection and the mechanisms associated with protection. NEVLP was then used to induce injury in rat livers and determine the effectiveness of DADLE in preventing liver injury.

RESULTS

In hepatocytes, DADLE was protective against oxidative stress and led to a decrease in phosphorylation of JNK and p38. Naltrindole, a δ-opioid receptor antagonist, blocked this effect. DADLE also activated the PI3K/Akt signaling pathway, and PI3K/Akt inhibition decreased the protective effects of DADLE treatment. In addition, DADLE treatment during NEVLP resulted in lower perfusate alanine aminotransferase and tissue malondialdehyde and better tissue adenosine triphosphate and glutathione. Furthermore, perfusion with DADLE compared with perfusate alone preserved tissue architecture.

CONCLUSIONS

DADLE confers protection against oxidative stress in hepatocytes and during NEVLP. These data suggest that the mechanism of protection involved the prevention of mitochondrial dysfunction by opioid receptor signaling and subsequent increased expression of prosurvival/antiapoptotic signaling pathways. Altogether, data suggest that opioid receptor agonism may serve as therapeutic target for improved liver protection during NEVLP.

Chronic estrogenic drug treatment increases preproenkephalin mRNA levels in the rat striatum and nucleus accumbens

Estrogens modulate the expression of preproenkephalin (PPE) in the hypothalamus but little is known for other brain regions. The present study investigated the effect of hormonal withdrawal and replacement therapy on PPE expression in the striatum, nucleus accumbens and cortex. Ovariectomized Sprague-Dawley rats were treated for 2 weeks with estradiol, a specific ligand for estrogen receptor alpha (ERalpha), 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) and estrogen receptor beta (ERbeta) 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN), or the selective estrogen receptor modulators (SERMs) tamoxifen and raloxifene. Brain PPE mRNA levels, measured by in situ hybridization, were high in the striatum and nucleus accumbens compared to the low expression in the cortex. Ovariectomy decreased uterine weights compared to intact uterus, which was corrected by estradiol and PPT. Tamoxifen and raloxifene partially stimulated uterine weights while DPN left it unchanged. In the anterior, median and posterior striatum and in the core and shell of the nucleus accumbens, ovariectomy decreased PPE mRNA levels compared to intact rats, this was corrected by estradiol treatment except for the posterior striatum. PPT, DPN, tamoxifen and raloxifene reproduced the estradiol effect. In the prefrontal and cingulate cortices, neither ovariectomy nor treatments changed PPE mRNA levels. These results show for the first time that estradiol increases PPE mRNA in the striatum and nucleus accumbens. This effect is observed also with estrogen receptor agonists for the ERalpha and ERbeta as well as with SERMs.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Effect of reserpine on catecholamine contents and met-enkephalin and beta-endorphin levels in the hypothalamus and the pituitary

1. The effects of reserpine treatment on the contents of catecholamines and opioid peptides have been studied in the rat hypothalamus and pituitary. 2. Hypothalamic and pituitary catecholamines were drastically depleted following acute reserpine treatment. 3. Reserpine treatment also resulted in a significant decrease in immunoactive met-enkephalin content in both the hypothalamus (25%) and the anterior lobe (50%), but not in the neurointermediate lobe of the pituitary. 4. No changes were observed in immunoactive beta-endorphin levels. 5. These findings suggest that the met-enkephalin contents in the hypothalamus and the anterior pituitary may be under catecholaminergic control. 6. The lack of effect of acute reserpine treatment on immunoactive beta-endorphin contents might be due to the opposing effects of adrenergic and dopaminergic mechanisms.

Changes in met-enkephalin and beta-endorphin contents in the hypothalamus and the pituitary in diabetic rats: effects of insulin therapy

1. Immunoreactive (IR)-met-enkephalin and beta-endorphin contents in the hypothalamus and the pituitary were measured in alloxan-diabetic rats with or without insulin treatment. 2. Both IR-met-enkephalin and IR-beta-endorphin in the pituitary were substantially reduced in alloxan-diabetic rats 1 month after treatment. 3. Hypothalamic IR-beta-endorphin content was also significantly lower. 4. Gel-filtration chromatography showed that the peaks co-eluting with met-enkephalin precursor, met-enkephalin and beta-endorphin were lower in the pituitaries from the diabetic rats, whereas the peaks co-eluting with beta-endorphin precursor and beta-lipotropin were not. 5. In another experiment, the IR-beta-endorphin contents of the neuro-intermediate lobe and hypothalamus, but not the anterior lobe were significantly lowered in diabetic rats, whereas IR-met-enkephalin contents were significantly reduced in both the anterior and neuro-intermediate lobe. 6. All these changes were reversed by insulin treatment. 7. As a decrease in general protein synthesis could not explain the recorded changes, these results suggest a possible direct role of insulin in regulating the opioid peptide content of the hypothalamus and pituitary.

Sex-related naloxone influence on growth hormone-releasing hormone-induced growth hormone secretion in normal subjects

The effect of opiate-receptor antagonist naloxone on growth hormone (GH) release after growth hormone-releasing hormone (GHRH) 1-44 administration was investigated in ten normal men and 18 normal women during different phases of their menstrual cycle. Naloxone was infused at a rate of 1.6 mg/h in women and 1.6- and 3.2 mg/h in men, starting one hour before GHRH administration (50 micrograms iv as a bolus). On different day sessions, naloxone, GHRH, or saline were administered as controls. Naloxone infusion reduced the GHRH-induced GH release in normal women. The mean % inhibition of peak GH response was 83% during follicular phase, 46.5% during periovulatory phase, and 77.6% during luteal phase. On the contrary, in normal men, both doses of naloxone infusion were ineffective in blunting the GH response to GHRH. Our studies indicate that naloxone infusion was capable of inhibiting GH release induced by direct stimulation with GHRH in normal women, suggesting an opiate-antagonist action at the anterior pituitary level. The absence of such an effect in normal men strongly indicates a sex dependence of naloxone effects and suggests a role of the sexual steroid environment in opioid modulation of pituitary hormone secretion.

Paracrine interactions in the anterior pituitary

The topographical affinity between certain cell types in rat anterior pituitary as well as the presence of biogenic amines, neuropeptides, growth and tissue factors in specific cell types suggest participation of paracrine control mechanisms in the regulation of anterior pituitary hormone secretion. Due to the recent advances in the separation of pituitary cell types and the development of three-dimensional cell cultures, direct experimental evidence for control by intercellular messengers has become available. The stimulation of PRL release from superfused pituitary cell aggregates by LHRH has been shown to be mediated by gonadotrophs. Gonadotrophs appear to secrete a factor with PRL-releasing activity. Gonadotrophs also modulate the stimulation of PRL release by angiotensin II. Interaction of somatotrophs with an unknown small-sized cell type strongly amplifies the GH response to adrenaline, GRF and VIP. The latter phenomenon requires the permissive action of glucocorticoids. Some of these in vitro observations can be correlated with recently reported in vivo actions of LHRH, PRL and angiotensin II and with pathophysiological changes in the pituitary.

On the mechanism of opioid-oestradiol interactions

Characteristics of opioid binding and possible relationships between oestradiol and opioid binding sites were studied in rat oestrogen sensitive tissues(uterus, preoptic area-anterior hypothalamus, median eminence-basal hypothalamus). Naloxone (Nal) and oestradiol (Oe) bindings were assessed by in vitro saturation analyses. In 800 g supernatants of both uterine and hypothalamic tissues homogenates high affinity (Kd: 2-4 X 10(-9) M) and low capacity [3H]Nal binding sites were found. These binding sites were sedimented from 800 g supernatant by further centrifugation at 10(5) g for 1 h. In competition studies [3H]Nal binding was completely prevented by morphine, while met-enkephalin and leu-enkephalin caused only a partial inhibition. [3H]Nal binding was increased by ovariectomy and decreased by Oe treatment (10 micrograms/100 g b.wt) in both tissues. The cytoplasmic [3H]Oe binding in the studied tissues seems to be affected by the naloxone binding system. After in vitro saturation of naloxone binding sites by naloxone the [3H]Oe binding to low affinity sites (type II) in hypothalamus as well as in uterus has been increased by 8- and 2-fold, respectively. These results indicate the presence of specific [3H]Nal binding in rat uterus with similar properties to those found in the hypothalamus. Furthermore an interaction between opioid and oestradiol receptor systems could be also suggested.

Sexual responsiveness and its relationship to vaginal stimulation-produced analgesia in the rat

Increasing amounts of pressure applied to the cervix produce a dose-response-like elevation of pain threshold in rats. This vaginal stimulation-produced analgesia (VSPA) is facilitated in animals given estrogen (E) doses sufficient to induce high levels of sexual receptivity. It has been proposed that enhancement of VSPA may serve to decrease any noxious input associated with multiple intromissions by the male. In this study, the anti-nociceptive effect of VSPA was compared in animals given E doses insufficient to increase receptivity with animals made receptive using subthreshold E levels + progesterone (P) in an attempt to determine if enhancement of VSPA is associated with the receptive state of the animal or the dose of E used. Tail flick latencies and tail shock vocalization thresholds were measured in groups of E, E + P and oil-treated rats during application of 0, 100 and 200 g of force on the cervix. Within oil, E and E + P-treated animals, significant increases in tail flick latencies were observed at 100 and 200 g with respect to baseline (0 g). Moreover, at 100 g of force E treated animals displayed a significant increase in tail flick latency over oil and E + P treated rats. In contrast, tail shock vocalization was increased at 100 and 200 g levels of probing in oil and E + P groups but was not facilitated by E. In the present study, as in previous work, VSPA was potentiated by E; however, this potentiation was not correlated with steroid-induced receptivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Methionine and leucine enkephalin in rat neurohypophysis: different responses to osmotic stimuli and T2 toxin

Specific radioimmunoassays were used to measure the effects of hypertonic saline (salt loading), water deprivation, and trichothecene mycotoxin (T2 toxin) on the content of methionine enkephalin (ME), leucine enkephalin (LE), alpha-neoendorphin, dynorphin A, dynorphin B, vasopressin, and oxytocin in the rat posterior pituitary. Concentrations of vasopressin and oxytocin decreased in response to both osmotic stimuli and treatment with T2 toxin, but the decrease was greater with osmotic stimulations. Similarly, concentrations of LE and dynorphin-related peptides declined after salt loading and water deprivation; LE concentrations also decreased after treatment with T2 toxin. The concentration of ME decreased after water deprivation, did not change after salt loading, and increased after T2 toxin treatment. The differentiating effects of these stimuli on the content of immunoreactive LE and ME are consistent with the hypothesis that LE and ME may be localized in separate populations of nerve endings with different roles in the posterior pituitary.

Differential responses in prolactin levels induced by naloxone in humans

The plasma prolactin (PRL) response to the opiate antagonist naloxone was tested in drug-free healthy volunteers (10 men, 18 regularly menstruating women who were in the late follicular phase of their ovarian cycles, and seven post-menopausal women). Naloxone hydrochloride (2 mg intravenous bolus) and placebo (normal saline) were administered on two different days in a double-blind fashion. Blood samples were collected at -15, 0, 30, 60, 120, 180 and 240 min after the injection. In the women of reproductive age, naloxone reduced plasma PRL concentrations, whereas in the post-menopausal women and the men, naloxone resulted in no significant change. However, in the post-menopausal women treated with estrogen (intramuscular 17-beta-estradiol), the opiate antagonist was able to lower plasma PRL concentrations. Thus, it appears that opiate effects on PRL secretion are influenced by the gonadal steroid environment and that estrogens may play a modulating role.

Exogenous and endogenous opioid-induced enhancements of naloxone's effects on serum luteinizing hormone levels in the male rat

We have previously shown that brief periods of exposure to opiate alkaloid drugs markedly enhance the subsequent effects of the opiate antagonist, naloxone, on serum luteinizing hormone (LH) levels in the male rat. In the present studies, we have found that this phenomenon is not simply a property of opiate drugs, but can be produced by a metabolically stable analog of an endogenously occurring opioid peptide, methionine enkephalin (FK 33-824). These findings suggest that alterations in the sensitivity of those opioid receptors involved in LH-releasing hormone (LHRH) generated in our experimental paradigm may occur under in vivo conditions, particularly since it now appears that endogenous opioids are released in an episodic manner like most neurotransmitter/neuromodulators. We also attempted to more fully characterize the factors responsible for the development of opiate-induced enhancements of naloxone's effects on LH. We found that this effect was produced only by those doses of morphine which initially suppressed serum LH levels, followed by a "rebound" increase in the gonadotropin 6-8 h later. A modest facilitation of LHRH-evoked increases in serum LH was also observed, but our data suggest that this represents only a minor component of opiate-induced enhancements of naloxone's effects. These data indicate that hypothalamic or suprahypothalamic sites are the major loci involved, but no differences in the uptake or regional distribution of naloxone in brain have been previously found, as a function of morphine pretreatment, nor were we able to demonstrate any alterations in opiate binding sites in the hypothalamus or whole brain. Thus, the mechanisms involved in this effect remain unclear.

Enhancement of opiate-induced depressions in serum luteinizing hormone levels by the prior administration of naloxone in the male rat

The effects of naloxone pretreatment on opiate agonist-induced depressions in serum luteinizing hormone (LH) levels were examined in male rats. Our results demonstrated a pronounced enhancement of morphine's actions 6 hours after the administration of naloxone (0.5 mg/kg). This effect was characterized by a 10 fold reduction in the ED50 (1.26 mg/kg versus 0.13 mg/kg in saline- and naloxone-pretreated rats, respectively) and much greater depressions in serum LH levels at each dose of morphine. The actions of naloxone were not confined to morphine, since similar increased potencies were found for opioid agonists with selectivity for a variety of opioid receptor subtypes. Because naloxone did not alter the uptake of subsequently administered morphine into brain, our results cannot be explained on the basis of an increased availability of the agonist. Rather, it appears that naloxone pretreatment induces a change in the sensitivity of those receptors involved in the effects of opioid agonists on LH.

Estrogen-like activity of chlordecone (kepone) on the hypothalamo-pituitary axis: effects on the pituitary enkephalin system

Similarities between the activity of chlordecone and estrogen on the hypothalamo-pituitary axis (HPA) were examined by using the pituitary enkephalin system as a model. A single injection (25 to 75 mg/kg, ip) or repeated injections (2.5 to 10 mg/kg/day for 10 days, ip) of chlordecone caused a time- and dose-related decrease in pituitary [Met5]-enkephalin-like immunoreactivity (ME-LI) in adult rats. Similar to the reported effects of estrogen, chlordecone treatment decreased the levels of ME-LI in the anterior lobe but not in the neurointermediate lobe of male pituitaries. Furthermore, chlordecone failed to alter the pituitary levels of ME-LI in female rats. The level of another pituitary peptide hormone, beta-endorphin, was increased in male rats after chlordecone treatment, indicating some degree of selectivity of this effect. Similarities between chlordecone and estrogen were found with other pituitary hormones, i.e., implantation of diethylstilbestrol or of chlordecone in ovariectomized rats caused qualitatively similar changes in serum prolactin and luteinizing hormone levels. These results suggest that chlordecone qualitatively resembles estrogen in its effect on some HPA peptide systems.

Neuroendocrine mechanisms mediating fluid intake during the estrous cycle

Gonadal steroids appear to influence fluid-electrolyte homeostasis through behavioral as well as renal mechanisms. The marked fluctuations in drinking behavior observed during the estrous cycle of the female rat may be due to an interaction between estrogen and the dipsogenic peptide hormone, angiotensin II, at the level of basal forebrain receptors. The preoptic region in particular may play an important integrative role in the maintenance of extracellular fluid balance in synchrony with the estrous cycle, since it contains receptors for angiotensin and estrogen. Prolactin may also directly participate in mechanisms of extracellular thirst, while an exact role for vasopressin has yet to be established. Recent studies also suggest that estrogens may influence body fluid regulation by interacting with several neurotransmitters, including serotonin, dopamine and noradrenaline.

Testosterone does not influence opiate binding sites in the male rat brain

It has been reported previously that castration produces testosterone-reversible increases in the density of 3H-naltrexone binding sites in the male rat brain. Unfortunately, we were unable to replicate these observations in a comprehensive series of studies. Specifically, we found that castration failed to produce changes in the Kd or Bmax of opiate binding sites in whole male rat brain, or in the hypothalamus, utilizing 3H-dihydromorphine (a mu receptor ligand), 3H-D-alanine, D-leucine enkephalin (delta) or 3H-naltrexone (ubiquitous). Furthermore, we found that the relative proportion of mu and delta binding sites in brain was unchanged by castration. The reasons for the discrepancy between the present results and those previously reported are unclear, but it appears that the provocative hypothesis that testosterone influences opioid receptors in brain must be carefully reevaluated.