The effect of opioid peptides on ovine pituitary gonadotropin secretion in vitro

Although a hypothalamic site of action has been firmly established for opiate-mediated gonadotropin regulation, there have been several reports which indicate the possibility of a direct influence on the pituitary gland. The objective of this study was to further investigate this possibility in an in vitro pituitary perifusion system utilizing ovine tissue. Treatment with gamma-endorphin (GE) or human beta-endorphin (hBE) resulted in elevated basal LH release (p less than 0.05), followed by an inhibition in the response to a subsequent GnRH challenge (p less than 0.05). The stimulatory effect of hBE was found to be dose-responsive (p less than 0.01). PRL secretion was not similarly stimulated. Ovine beta-endorphin (oBE) had no effect on LH secretion, even though it differs from hBE by only 2 amino acids and contains the active GE sequence. Met-enkephalin also did not influence gonadotropin secretion. Naloxone pretreatment did not reverse the effects of hBE on gonadotropin release. It was found, however, that [D-pGlu1, D-Phe2, D-Trp3,6]-GnRH, a specific GnRH receptor antagonist, did reduce hBE-induced LH and FSH release (p less than 0.05). Naloxone pretreatment alone suppressed the response to GnRH (p less than 0.05). These data indicate that certain opioid peptides can influence ovine gonadotropin secretion in vitro by activating the GnRH receptor. Furthermore, a facilitory role is suggested for endogenous opiates in the local regulation of pituitary gonadotropin secretion.     

D-amphetamine-induced hypothermia and hypermotility in rats: Changes after systemic administration of beta-endorphin

Systemically administered beta-endorphin was tested in rats for its ability to modify the hypothermia and hypermotility induced by d-amphetamine. Colonic temperature and motor activity were measured in a cold (4°C) ambient temperature in animals given IP injections of beta-endorphin (0.1, 1.0, or 3.0 mg/kg), naloxone (10 mg/kg), or morphine (30 mg/kg). The same measurements were taken in animals given beta-endorphin (1.0 mg/kg) in combination with naloxone or saline pretreatment and d-amphetamine (15 mg/kg) or saline post-treatment. Morphine alone had a biphasic effect on thermoregulation, but did not affect d-amphetamine-induced hypothermia. Activity scores were decreased by morphine, in both d-amphetamine and saline treated animals. The thermal response of rats to beta-endorphin alone was variable, depending on dosage, but all 3 dosages partially blocked the hypothermic effect of d-amphetamine. Naloxone blocked the thermal effects of both beta-endorphin and d-amphetamine. Motor activity tended to be decreased by naloxone, regardless of amphetamine treatment, but beta-endorphin tended to increase activity in amphetamine-treated animals and reduce it in saline-treated controls. In their actions on both thermoregulation and activity, naloxone and beta-endorphin appeared to interact independently with d-amphetamine, often producing effects in the same direction, but in combination, they tended to be mutually inhibitory.     

Potentiation of vasopressin analgesia in rats treated neonatally with monosodium glutamate

The analgesic response elicited by central administration of arginine vasopressin (AVP) appears to be dependent upon the integrity of the hypothalamic paraventricular nucleus (PVN), since lesions placed in the PVN eliminate AVP analgesia. A projection to the zona externa of the median eminence constitutes one of the VP-containing efferents of the PVN. Neonatal treatment with monosodium glutamate (MSG) destroys perikarya of the arcuate nucleus and median eminence. The present study examined whether AVP analgesia was affected in the MSG-treated rat and whether these alterations were accompanied by specific changes in VP immunoreactivity in the zona externa of the median eminence. Female rats, neonatally treated with either MSG or a saline control, were tested as adults on the tail-flick test following intracerebroventricular injections of 0, 75, 150 and 500 ng doses of AVP. After testing, selected animals were prepared for AVP and oxytocin immunocytochemistry of the median eminence. Significant potentiations in the magnitude of AVP analgesia were observed in MSG-treated rats. AVP and oxytocin immunoreactivity in the zona interna and oxytocin immunoreactivity in the zona externa of the median eminence were similar in MSG-treated and control rats. In contrast, AVP immunoreactivity in the zona externa of the median eminence was markedly reduced in the MSG-treated rat. These data suggest that VP analgesia may normally be inhibited by those medial-basal hypothalamic neurons affected by neonatal MSG treatment.     

Mediation by gonadal steroids of plasma beta-endorphin and LH in castrated female and male rats

Immunoreactive beta-endorphin (IR-BE) was significantly decreased and luteinizing hormone (LH) significantly increased in female rats castrated for four weeks. Forty eight hours after a single injection of estradiol benzoate (EB), IR-BE levels increased, and LH levels were reduced. On the afternoon following the administration of a second injection of EB given six hours earlier, IR-BE levels were reduced below control values, whereas LH levels were significantly elevated. There was no change in IR-BE levels during the remainder of that afternoon whereas LH levels decreased over time. Similar to female rats, IR-BE was diminished and LH increased in castrated male rats. IR-BE was increased significantly above those values observed in intact animals 24 hr after a single injection of TP and returned to control levels by 48 hr after administration of TP. Injection of TP reduced LH to levels observed prior to castration. These findings suggest that gonadal steroids exert a feedback on the release of IR-BE from the pituitary of female and male rats opposite to their feedback effect on the release of pituitary gonadotropins.     

Age-related changes of beta-endorphin and cholecystokinin in human and rat mononuclear cells

Beta-endorphin (BE) and cholecystokinin (CCK) were measured in fresh PBMC isolated from human subjects and rats. The BE and CCK PBMC contents increased significantly with age both in human and rat models. Moreover, polyclonal stimulation induced a significant decrease of BE but not CCK contents in mononuclear cells from human aged subjects. The time course of changes in BE and CCK concentrations observed in fresh and cultured cells from subjects of different ages did not directly correlate to the time course of age-associated impairment of lectin-induced lymphocyte proliferative response and interleukin-2 synthesis. In fact, the lymphocyte functional defects were significantly observed only in the 71–99 year age group, whereas the neuropeptide changes were already evident in the 31–50 age group. Since BE has been shown to participate in the modulation of the immune system, the age-related modifications of PBMC BE could play a role in the immunodepression observed during aging.     

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).     

Concentrations of free radicals and beta-endorphins in repeat breeder cows

Repeat breeding (RB) is one of the major problems that affect the reproductive efficiency and economy of milk production in dairy animals. So far, the etiopathogenesis of this pathology has not been defined completely.

Stress has been hypothesized to be a cause of impaired reproductive efficiency. Stress may cause an overproduction of beta-endorphins and free radicals; in particular, reactive oxygen species (ROS). The aim of this work is to determine the concentrations of these substances in RB cows and to evaluate the correlation with the serum level of progesterone. The study was performed on 60 dairy cows: 26 RB and 34 control cows. Blood samples were collected on day 12 and day 16, after artificial insemination (AI) in all subjects, in order to assess the concentrations of progesterone, free radicals and beta-endorphins. The stressors, free radicals and beta-endorphins, that we considered, were higher in repeat breeders (day 12, 93.32(±1.91) UCarr and 0.50(±0.03) ng/ml; day 16, 94.42(±1.91) UCarr and 0.61(±0.03) ng/ml), with a lower level of progesterone, which probably is responsible for failure to conceive. The stress factors (free radicals and beta-endorphins) may actually enhance each other and induce an inhibition of progesterone synthesis in repeat breeders.     

Plasma cortisol and beta-endorphin in horses subjected to electro-acupuncture for cutaneous analgesia

Electro-acupuncture (EA) treatment of horses to induce cutaneous analgesia also increased plasma concentrations of beta-endorphin (beta-EP) and cortisol. The magnitude of these increases did not relate consistently to the degree of EA-induced analgesia. Respiration and heart rates were also markedly increased during EA treatment. Intact female horses had higher packed cell volume and plasma beta-EP as well as lower plasma total protein than castrated male horses. Plasma cortisol, heart rate, and respiration rate did not differ significantly between sexes. None of the parameters measured before or during EA treatment provided an explanation for the differential cutaneous analgesia which depended on sex of subject and locus of stimulation as reported elsewhere.     

Further characterization of the major forms of reptile beta-endorphin

Biosynthetically labeled reptile intermediate pituitary beta-endorphin-sized material was fractionated by SP-Sephadex ion exchange chromatography into two major opiate-active forms which eluted at 0.28 M NaCl and 0.32 M NaCl, respectively; the 0.32 M form of reptile β-endorphin (mw=3500), serves as the precursor for the 0.28 M form of reptile β-endorphin (mw=3200), (Dores and Surprenant, 1983). Analysis of tryptic digests of these reptile β-endorphins by paper electrophoresis at pH 3.5 and gel filtration on a Sephadex G-15 column indicated that there are two tyrosine residues, two arginine residues and one methionine residue in reptile β-endorphin. Furthermore, the NH₂-terminal tryptic peptide of both reptile β-endorphins is approximately nine amino acids in size and contains tyrosine, methionine and arginine. Analyses of chymotryptic/protease digests of the [³H]tyrosine-labeled NH₂-terminal tryptic peptide analyzed by descending paper chromatography revealed that the NH₂-terminal tyrosine of reptile β-endorphin is not -N-acetylated. A second tyrosine-containing tryptic peptide was detected in the COOH-terminal region of reptile β-endorphin; however this tryptic peptide differs in the two forms of reptile β-endorphin in terms of size and net charge at pH 3.5. These differences account for the apparent molecular weight differences and distinct ion exchange properties of the 0.28 M and 0.32 M forms of reptile β-endorphin. Thus in the reptile intermediate pituitary the principal post-translational mechanism for modifying β-endorphin is COOH-terminal proteolytic cleavage.     

Effects of intracranial self-stimulation on brain opioid peptides

The neurochemical system(s) underlying brain stimulation reward (ICSS) has been investigated for many years. The catecholamine hypothesis is currently most accepted with predominant emphasis on the role of dopamine. The present report examines the role of three opioid peptides--Methionine and Leucine Enkephalin (ME and LE) and beta-Endorphin (beta-E) in this behavior. Peptide levels from pituitary, hypothalamus and whole brain were determined by independent RIAs and analyzed according to treatment: low, moderate and high ICSS responders, sham controls, animals receiving nonspecific stimulation, and naloxone--with and without ICSS. Not only did naloxone reduce ICSS from high responders by 74%, it also was able to reduce peptide levels--most notably for ME and beta E in most regions. Additionally, the effects of ICSS on endorphin levels was found to be related to the rate category of responding. Since endorphins are known to interact with dopamine systems, it is therefore considered likely that the endogenous opioid peptides play an important role in ICSS either directly or indirectly via their influence on catecholamine systems.