Beta-endorphin-(1-27) antagonizes beta-endorphin-induced hypothermia in mice

Roles of β-Endorphin in Stress, Behavior, Neuroinflammation, and Brain Energy Metabolism

β-Endorphins are peptides that exert a wide variety of effects throughout the body. Produced through the cleavage pro-opiomelanocortin (POMC), β-endorphins are the primarily agonist of mu opioid receptors, which can be found throughout the body, brain, and cells of the immune system that regulate a diverse set of systems. As an agonist of the body's opioid receptors, β-endorphins are most noted for their potent analgesic effects, but they also have their involvement in reward-centric and homeostasis-restoring behaviors, among other effects. These effects have implicated the peptide in psychiatric and neurodegenerative disorders, making it a research target of interest. This review briefly summarizes the basics of endorphin function, goes over the behaviors and regulatory pathways it governs, and examines the variability of β-endorphin levels observed between normal and disease/disorder affected individuals.

Effects of N-terminal fragments of beta-endorphin on feeding in chicks

It is known that N-terminal fragments of beta-endorphin have biological activities, such as an antagonism effect of beta-endorphin (1-31) on the secretion of hormones or thermoregulation in mammals. We studied the effects of the N-terminal fragments on feeding behavior in male broiler chicks. Intracerebroventricular administration of beta-endorphin (1-27) (0.4nmol) stimulated feeding behavior compared with saline control during the 60-min experimental period. beta-Endorphin (1-17) (2.0nmol) also increased food intake at 30min postinjection. Co-injection of either beta-endorphin (1-27) or (1-17) was effective in reducing full-length beta-endorphin-induced feeding in chicks. These data suggest that the N-terminal fragments of beta-endorphin act as a partial agonist, and may regulate the activity of the central opioidergic system in chicks.

Inhibition of interleukin-1beta and prostaglandin E(2) thermogenesis by glycyl-glutamine, a pro-opiomelanocortin-derived peptide

Interleukin-1beta (IL-1beta) and other cytokines produce fever by stimulating prostaglandin E(2) (PGE(2)) synthesis in thermoregulatory regions of the preoptic area and anterior hypothalamus (POA/AH). Prostaglandin E(2) is thought to raise body temperature, at least in part, by stimulating beta-endorphin release from pro-opiomelanocortin neurons that innervate the POA/AH. In this study, we investigated whether glycyl-glutamine (beta-endorphin(30-31)), an inhibitory dipeptide synthesized from beta-endorphin post-translationally, inhibits IL-1beta and PGE(2)-induced hyperthermia. Hyperthermic sites were identified by microinjecting PGE(2) (3 fmol/1 microl) into the medial preoptic area (mPOA) of conscious, unrestrained rats. Interleukin-1beta (1 U) injection into the same PGE(2) responsive thermogenic sites in the mPOA elicited a prolonged rise in colonic temperature (T(c)) (+1.02+/-0.06 degrees C) that persisted for at least 2 h. Glycyl-glutamine (3 nmol) co-injection into the mPOA inhibited IL-1beta thermogenesis completely (T(c)=-0.18+/-0.22 degrees C). Glycyl-glutamine had no effect on body temperature when given alone to normothermic rats. Co-injection of individual amino acids, glycine and glutamine (3 nmol each amino acid), failed to influence IL-1beta-induced thermogenesis, which indicates that Gly-Gln hydrolysis does not explain its inhibitory activity. Glycyl-glutamine (3 nmol) also prevented the rise in body temperature produced by PGE(2) (PGE(2)=0.89+/-0.05 degrees C; PGE(2) plus Gly-Gln=-0.16+/-0.14 degrees C), consistent with evidence that PGE(2) mediates IL-1beta-induced fever. These findings demonstrate that Gly-Gln inhibits the thermogenic response to endogenous pyrogens.

Cyclo(Gly-Gln) inhibits the cardiorespiratory depression produced by beta-endorphin and morphine

Glycyl-L-glutamine (Gly-Gln; beta-endorphin 30-31) is an endogenous dipeptide that is synthesized through the post-translational processing of beta-endorphin. Previously, we showed that Gly-Gln inhibits the hypotension and respiratory depression produced by central beta-endorphin administration. In this study, we tested whether cyclo(Gly-Gln), a non-polar, cyclic Gly-Gln derivative, was similarly effective following intracerebro-ventricular (i.c.v.) or intra-arterial (i.a.) administration to pentobarbital-anesthetized rats pretreated with beta-endorphin (0.5 nmol i.c.v.). Intracerebroventricular cyclo(Gly-Gln) (0.3, 0.6 or 1.0 nmol) injection produced a dose-dependent inhibition of beta-endorphin-induced hypotension, but not bradycardia, with a potency similar to that of Gly-Gln. Cyclo(Gly-Gln) (5 mg/kg) was also effective following i.a. injection and significantly attenuated the fall in arterial pressure elicited by i.c.v. beta-endorphin, consistent with evidence that cyclic dipeptides permeate the blood-brain barrier; i.a. Gly-Gln was ineffective. Intra-arterial cyclo(Gly-Gln) (5 mg/kg) and i.c.v. Gly-Gln (10 nmol) also attenuated the hypotension and respiratory depression induced by morphine (50 or 100 nmol i.c.v.). Cyclo(Gly-Gln) (0.5, 5.0 or 50.0 mg/kg i.a.) had no effect on arterial pressure or heart rate when given alone. These findings indicate that cyclo(Gly-Gln) is a biologically active peptide capable of reversing the cardiorespiratory depression produced by beta-endorphin or morphine.

Dynorphins modulate DNA synthesis in fetal brain cell aggregates

Previously, opioid peptide analogues, beta-endorphin, and synthetic opiates were found to inhibit DNA synthesis in 7-day fetal rat brain cell aggregates via kappa- and mu-opioid receptors. Here dynorphins and other endogenous opioid peptides were investigated for their effect on DNA synthesis in rat and guinea pig brain cell aggregates. At 1 microM, all dynorphins tested and beta-endorphin inhibited [3H]thymidine incorporation into DNA by 20-38% in 7-day rat brain cell aggregates. The putative epsilon-antagonist beta-endorphin (1-27) did not prevent the effect of beta-endorphin, suggesting that the epsilon-receptor is not involved in opioid inhibition of DNA synthesis. The kappa-selective antagonist norbinaltorphimine blocked dynorphin A or B inhibition of DNA synthesis, implicating a kappa-opioid receptor. In dose-dependency studies, dynorphin B was three orders of magnitude more potent than dynorphin A in the attenuation of thymidine incorporation, indicative of the mediation of its action by a discrete kappa-receptor subtype. The IC50 value of 0.1 nM estimated for dynorphin B is in the physiological range for dynorphins in developing brain. In guinea pig brain cell aggregates, the kappa-receptor agonists U50488, U69593, and dynorphin B reduced thymidine incorporation by 40%. When 21-day aggregates were treated with dynorphins, a 33-86% enhancement of thymidine incorporation was observed. Because both 7- and 21-day aggregates correspond to stages in development when glial cell proliferation is prevalent and glia preferentially express kappa-receptors in rat brain, these findings support the hypothesis that dynorphins modulate glial DNA synthesis during brain ontogeny.

Spontaneous and ethanol-stimulated in vitro release of beta-endorphin by the hypothalamus of AA and ANA rats

Previous studies demonstrated that both the spontaneous and ethanol-stimulated release of beta-endorphin (beta-EP) like-peptides (beta-EPLPs) by the hypothalami of the ethanol-preferring C57BL/6 mice is more pronounced than by the hypothalami of the ethanol-avoiding DBA/2 mice. The objective of the present studies was to investigate the effects of various concentrations of ethanol on the in vitro release of beta-EP peptides by the hypothalami of the ethanol-preferring Alko-Alcohol (AA) and ethanol-avoiding Alko Non-Alcohol (ANA) lines of rats. Results indicated that although the spontaneous release of hypothalamic beta-EPLPs was higher by the ANA than by the AA rats, the percentage increase following exposure to various concentrations of ethanol was similar in both lines of rats. Furthermore, the release of hypothalamic beta-EPLPs following exposure to 30 mM ethanol was significantly higher than the release following exposure to 10 mM ethanol in the AA, but not the ANA, rats. Analysis of the released beta-EPLPs with Sephadex G-75 and reversed phase HPLC indicated that the nonacetylated beta-EP 1-31 was the major component in the hypothalamic perifusates of the AA rats, whereas the shorter and acetylated forms of beta-EP were the predominant components in the hypothalamic perifusates of the ANA rats. Because the shorter and acetylated forms of beta-EP are devoid of opioid activity, their pronounced release by the hypothalami of the ANA rats may be important in maintaining their low ethanol consumption, even after long-term access to ethanol solutions.

Genetics of alcoholism: role of the endogenous opioid system

At the present time alcoholism is recognized as a metabolic disease exhibiting the clinical features of craving for alcohol, loss of control over drinking, tolerance and physical dependence on alcohol, while both epidemiological and experimental studies have demonstrated that genetic factors may be important in determining whether an individual has a high or low vulnerability to develop alcoholism. Evidence also indicates that alcoholism is not characterized by a single gene single allele inheritance. Instead it seems that multiple genes and environmental factors interact to increase or decrease an individual's vulnerability to become an alcoholic. Current research is aimed at investigating whether certain behavioral, physiological and biochemical markers are highly associated with the incidence of alcoholism. Among the biochemical markers currently under investigation is the endogenous opioid system and its implication in mediating the reinforcing effects of ethanol. It is the objective of this manuscript to review current research on: (a) the interactions of ethanol with the endogenous opioid system at the molecular level; (b) the existence of genetically determined differences in the response of the endogenous opioid system to ethanol between subjects at high and low risk for excessive ethanol consumption, as well as between lines of animals showing preference or aversion for ethanol solutions; (c) the decrease of alcohol consumption following pretreatment with opioid antagonists; and (d) the possible use of specific opioid receptor antagonists together with behavioral therapy to modify drinking behavior, to control craving and to prevent relapse.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Inhibition of spinal opioid antinociception by intrathecal beta-endorphin1-27 in the rat

1. The effects of intrathecal (i.t.) administration of beta-endorphin and two shorter fragments, human and ovine beta-endorphin1-27, were examined for antinociceptive activity in the tail-flick and paw-pressure tests in the rat. Additionally, the ability of ovine beta-endorphin1-27 to influence the action of i.t. beta-endorphin, morphine and [D-Pen2-D-Pen5]enkephalin (DPDPE) was also examined in these tests. 2. After i.t. injection, beta-endorphin produced potent dose-related antinociception in the tail-flick and paw-pressure tests. Shorter endorphins produced much weaker effects. The order of antinociceptive efficacy was beta-endorphin > human beta-endorphin1-27 > ovine beta-endorphin1-27. 3. Administration of ovine beta-endorphin1-27 (0.72, 1.44 nmol, i.t.) significantly attenuated the antinociceptive effect of beta-endorphin (2.88 nmol, i.t.) in the tail-flick and paw-pressure tests. 4. Both i.t. morphine and DPDPE produced dose-related antinociception in the tail-flick and paw-pressure tests. The potency of DPDPE was lower than that of morphine in both tests; however, the effect of DPDPE was weaker in the paw-pressure test. 5. Administration of ovine beta-endorphin1-27 (1.44 nmol, i.t.) significantly attenuated the antinociceptive effect of morphine (14.9 nmol, i.t.) in both tests and the effect of DPDPE (38.7 nmol) in the tail-flick test. 6. The results show that beta-endorphin1-27 acts as an opioid antagonist at the spinal level in the rat. Its ability to inhibit the action of morphine and DPDPE suggests that it may attenuate beta-endorphin action by an interaction with mu- and/or delta-opioid receptors.

Antitussive effect of beta-endorphin is mediated by mu-opioid receptors, but not by kappa- or epsilon-opioid receptors

The present study examined the opioid receptors involved in the antitussive effect of beta-endorphin in mice. beta-Endorphin injected i.c.v. depresses coughs dose dependently in doses from 0.1 to 1 microgram. Blockade of mu-opioid receptors by pretreatment with beta-funaltrexamine significantly reduced the antitussive potency of i.c.v. beta-endorphin. However, the antitussive effect of beta-endorphin was not antagonized by nor-binaltorphimine, a kappa-opioid receptor antagonist. Moreover, i.c.v. injection of beta-endorphin-(1-27), an epsilon-opioid receptor antagonist, did not affect the antitussive effect of beta-endorphin. The results indicate that the antitussive effect of beta-endorphin is mediated by activation of mu-opioid receptors, but not of kappa- or epsilon-opioid receptors.

Differences in the brain and pituitary beta-endorphin system between the alcohol-preferring AA and alcohol-avoiding ANA rats

The content of pro-opiomelanocortin (POMC) mRNA was determined in the hypothalamus, as well as in the anterior and intermediate lobes of the pituitary gland of the alcohol-preferring AA and alcohol-avoiding ANA rats under basal conditions. In addition the content of beta-endorphin-like immunoreactivity (beta-EPLIR) was measured in nine brain regions, the anterior and intermediate lobes of the pituitary gland and the serum. The content of beta-EPLIR was significantly higher in the septum and significantly lower in the amygdala, and periaqueductal gray matter of the AA rats, while there was no significant difference between the AA and ANA rats in the arcuate nucleus plus median eminence, nucleus accumbens, caudate, hippocampus, and cortex. HPLC analysis indicated no significant differences in the relative proportions of non-acetyl and acetyl forms of beta-endorphin peptides in the hypothalamus, distinct brain regions, and anterior and neurointermediate lobes of the pituitary gland, between the AA and ANA rats. The content of POMC mRNA but not of beta-EPLIR was significantly higher in the hypothalamus and neurointermediate lobe of the AA rats, while the content of both beta-EPLIR and POMC mRNA were significantly higher in the anterior pituitary of the AA than of the ANA rats. Thus, there are genetically determined differences in the pituitary and brain beta-endorphin system between the AA and ANA rats, which may be important in controlling the differences in the voluntary ethanol consumption exhibited by these animals.

Administration of slow-release nifedipine does not affect lactate threshold, hormone release during exercise, and quality of life in normal subjects

In a double-blind crossover study of 10 normal healthy subjects, we examined the effects of slow-release nifedipine (nifedipine-SR, 10 mg b.i.d) administration on exercise capacity, hormone levels during exercise, and quality of life (QOL) after a 2-week treatment. Two exercise tests, a progressive exercise test and a constant work-rate exercise test, were performed. Maximal oxygen uptake (VO2max) and blood lactate concentration were measured during the progressive exercise test and the exercise intensity corresponding to half lactate threshold (LT), LT, and 4 mmol/l of lactate concentration was determined. Subjects underwent 20 minutes of constant work-rate exercise at each work load, and blood lactate, plasma epinephrine, plasma norepinephrine, plasma renin activity, plasma aldosterone, atrial natriuretic peptide, plasma beta-endorphin, and met-enkephalin were measured. Taking nifedipine-SR had no effect on the responses of blood pressure, heart rate, VO2max, maximal work load, and LT compared to taking placebo. Blood lactate, plasma catecholamine, plasma renin activity, aldosterone, atrial natriuretic peptide, and beta-endorphin levels increased during exercise, and there was no difference between nifedipine-SR and placebo. Met-enkephalin did not increase with either treatment. In the QOL questionnaires, no differences were noted between the two treatments. These findings suggest nifedipine-SR to be a potentially useful drug in view of the lack of effect on exercise capacity, hormone release, and QOL.

An immune cell population that responds to beta-endorphin and is responsible for protecting nude mice from the fatal consequences of a virus infection of the central nervous system

Reconstitution of 3- to 4-week-old BALB/c nude (nu/nu) mice with 10(7) syngeneic splenocytes, 48 h before intracerebral inoculation with a temperature-sensitive (ts) mutant of VSV (tsG31 KS5), provided protection from the fatal consequences of clinical disease in 80-90% of the infected animals. Reconstitution of animals with 10(7) splenocytes, first depleted of natural killer (NK) cells with anti-asialo GM1 and complement, also afforded protection against the infectious disease. Depletion of T-lymphocytes with anti-thy-1.2 antibody and complement, however, provided little protection with approximately 40% of the animals succumbing to the virus infection within 30 days post-infection. A single intracerebroventricular injection with 14 pM of beta-endorphin, 24 h prior to viral infection, led to an increased fatality of mice previously reconstituted with T-lymphocytes but not in animals receiving only syngeneic NK cells. The increased fatality caused by the neuropeptide was antagonized by naloxone but not beta-endorphin-(1-27). Separation of splenocyte cell populations by buoyant density centrifugation demonstrated that small race lymphocytes, and not the large granular lymphocytes, were responsible for protection of nude mice from the central nervous system infection with ts-VSV. The beta-endorphin-responsive immune cells were shown to be a minor fraction of the small race T-lymphocyte population that bear the asialo-GM1 marker.

Modulation of the mesolimbic dopaminergic system by beta-endorphin-(1-27) as assessed by microdialysis

In the present study we used in vivo microdialysis to examine the influence of beta-endorphin-(1-27) (beta-EP-(1-27) upon beta-endorphin (beta-EP)-induced dopamine (DA) release in the nucleus accumbens of anesthetized rats. Microdialysis probes were inserted into the nucleus accumbens and perfusates were analyzed for DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using a reversed-phase HPLC system with electrochemical detection. Intracerebroventricular (i.c.v.) administration of beta-EP-(1-27) (5-20 micrograms) resulted in a dose-dependent increase in DA release which was smaller than the beta-EP-induced DA release, whereas metabolite levels were not altered. Pretreatment with beta-EP-(1-27) (5-20 micrograms) significantly altered the beta-EP (5 micrograms)-induced increase in DA release. These results indicate that beta-EP-(1-27) antagonizes the beta-EP-induced release of DA in the nucleus accumbens. In addition to its antagonistic properties at the beta-endorphin binding site, beta-EP-(1-27) appears to be a partial agonist, inducing increased DA release. These findings suggest a regulatory function for this naturally occurring beta-EP fragment within the mesolimbic system.

Beta-endorphin alters the course of central nervous system disease induced by a temperature-sensitive vesicular stomatitis virus in reconstituted nude mice

A 100 plaque forming unit (pfu) dose of a temperature-sensitive (ts) mutant of vesicular stomatitis virus (VSV), tsG31 KS5, engendered a slowly progressive paralytic central nervous system (CNS) disease that killed all BALB/c nude mice within 28 days. Reconstitution of nude mice with 10(7) syngeneic splenocytes 24 h before intracerebral inoculation with tsG31 KS5 VSV, however, protected 92% of the animals from death. When these reconstituted animals were injected intracerebroventricularly with 14 pmol of beta-endorphin 24 h after reconstitution with splenocytes and 24 h before inoculation with tsG31 KS5 VSV, only 72% of the animals survived. Furthermore, whereas 40% of the afflicted reconstituted nude mice given intracerebroventricular injections of sterile water were able to recover from the symptoms of disease, those surviving animals which received beta-endorphin were unable to do so. A single intravenous injection of 14 pmol beta-endorphin, or repeated postinfection administration of 28 pmol of beta-endorphin intravenously into nude mice reconstituted with syngeneic splenocytes, which were pretreated with beta-endorphin, did not alter the course of CNS disease induced by tsG31 KS5 VSV. The effect induced by intracerebroventricular injection of beta-endorphin was antagonized by naloxone, but not by the neuropeptide fragment beta-endorphin-(1-27). A simultaneous intracerebroventricular injection of reconstituted nude mice with 1220 pmol of naloxone and 14 pmol of beta-endorphin resulted in a 89% survival rate, and 33% of the afflicted animals were able to overcome the symptoms of the disease induced by tsG31 KS5 VSV. Intracerebroventricular injection of reconstituted nude mice with 330 pmol of beta-endorphin-(1-27) and 14 pmol of beta-endorphin resulted in a 72% survival rate and the surviving animals were unable to improve appreciably the clinical status of their disease. Injection of reconstituted nude mice with either 1220 pmol of naloxone or 330 pmol of beta-endorphin-(1-27) alone did not alter the course of the CNS disease in any way. A single intracerebroventricular injection of 29 pmol of another psychoactive peptide, [Des-Tyr]-endorphin, 24 h after reconstitution of nude mice with splenocytes and 24 h prior to infection with virus, resulted in 74% survival; and 39% of the afflicted animals were able to recover from the clinical symptoms.

Characterization of the effects of acute ethanol administration on the release of beta-endorphin peptides by the rat hypothalamus

In the present studies the direct effect of ethanol on the release of beta-endorphin by the rat hypothalamus was investigated. When various concentrations of ethanol (10-120 mM) were added into the incubation medium, it was noticed that though low concentrations of ethanol (10, 20 and 30 mM) induced a pronounced increase in the release of beta-endorphin-like peptides from the hypothalamus, high concentrations of ethanol (40, 60 and 120 mM) induced a less pronounced increase. Exposure of hypothalamus to depolarizing concentrations of potassium chloride (following washing of the ethanol), provoked a significant release of beta-endorphin-like peptides, regardless of the ethanol concentration the tissues were exposed prior to the stimulation with the potassium chloride. Chromatographic analysis of the incubation media with Sephadex-G-75 revealed that the hypothalamus released mainly beta-endorphin-sized peptides. Analysis of the beta-endorphin-sized peptides with reverse-phase high performance liquid chromatography indicated the presence of beta-endorphin-(1-31) as well as non-acetyl and acetyl beta-endorphin-(1-27). Thus ethanol exerts a biphasic effect on the release of beta-endorphin-like peptides by the rat hypothalamus, with low concentrations inducing a dose-dependent increase, reaching maximum at 20 mM ethanol, and with higher concentrations of ethanol inducing a less pronounced increase in the release of beta-endorphin-like peptides, leading to an inverted U-shaped dose response relationship of ethanol and release of beta-endorphin-like peptides from the rat hypothalamus.

Characterization of rat spinal cord receptors to FLFQPQRFamide, a mammalian morphine modulating peptide: a binding study

An in vitro binding assay, using 125I-YLFQPQRFamide, a newly synthetized iodinated analog of FLFQPQRFamide, in which Phe1 (F) has been substituted by a Tyr (Y), was developed to demonstrate and characterize putative binding sites of this brain morphine modulating peptide. This radioligand bound in a time-dependent manner to rat spinal cord membrane preparation. This binding was dose-dependent, saturable and reversible. Both kinetic data and saturation measured at equilibrium lead to the existence of a homogenous population of high affinity binding sites with a Kd value of 0.09-0.1 nM and a maximal capacity Bmax of 14.5 +/- 2 fmol/mg protein. Results of competition experiments show that both FLFQPQRFamide and its analog YLFQPQRFamide had a similar capacity to inhibit the 125I-YLFQPQRFamide binding, suggesting that this radioiodinated analog is a good tool to study binding characteristics of FLFQPQRFamide receptors. The related octadecapeptide AGEGLSSPFWSLAAPQRFamide, another mammalian morphine modulating peptide competes for radioligand binding with similar potency. Our results also show that mu, delta and kappa opiate receptor agonists as well as the antagonist naloxone were not able to affect binding either in presence or in absence of 120 mM NaCl. Together, these data demonstrate that FLFQPQRFamide does not function as an endogenous opiate receptor antagonist and that is capacity to reduce opiate-induced analgesia is supported by specific binding sites.

Characterization of beta-endorphin peptides in the spinal cord of the rat

The objective of the present studies was to estimate the total content of beta-endorphin-like immunoreactivity (beta-EPLIR) and to characterize the beta-endorphin-like peptides in distinct regions of the spinal cord using gel filtration and reverse phase high performance liquid chromatography. The concentration of beta-EPLIR expressed as pg per mg tissue was similar in the four regions of the spinal cord. Sephadex G-75 chromatography demonstrated the presence, in all four regions of the spinal cord, of beta-endorphin (beta-EP) immunoreactive peptides eluting at the positions of standard beta-EP and beta-lipotropin (beta-LPH) peptides as well as a high molecular weight form eluting prior to beta-LPH. High performance liquid chromatography of the beta-EP-sized peptides indicated some differences in the relative proportions of the various beta-EP-sized peptides among the four regions of the spinal cord, which suggest a different origin of the beta-EP fibers terminating in different regions of the spinal cord as well as different physiological importance of the beta-endorphin peptides in the various spinal cord regions.

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Beta-endorphin-(1-27) antagonizes beta-endorphin- but not morphine-, D-Pen2-D-Pen5-enkephalin- and U50, 488H-induced analgesia in mice

beta-Endorphin-(1-27), administered intraventricularly has been previously reported to block the analgesia induced by beta-endorphin injected intraventricularly. The present study was to determine if the blocking effect of beta-endorphin-(1-27) was specific to beta-endorphin which stimulates epsilon receptors, but not to other opioids with activity at different opioid receptors. The antagonistic effects of beta-endorphin-(1-27) on the analgesia induced by beta-endorphin (epsilon-opioid receptor agonist), D-Ala2-NMePhe4-Gly-ol-enkephalin(DAGO) and morphine, (mu-opioid receptor agonists), D-Pen2-D-Pen5-enkephalin(DPDPE) and D-Ala2-D-Leu5-enkephalin(DADLE) (delta-opioid receptor agonists) and U-50, 488H (kappa-opioid receptor agonist) were studied. beta-Endorphin-(1-27) injected intraventricularly, at doses which, when injected alone did not produce analgesia, antagonized the analgesia induced by beta-endorphin given intraventricularly. However, the analgesia induced by DAGO, morphine, DPDPE, DADLE and U-50, 488H given intraventricularly was not antagonized by beta-endorphin-(1-27). The data suggest that beta-endorphin-(1-27) selectively blocks the analgesia induced by the stimulation of epsilon receptors but not by the stimulation of mu, delta, and kappa receptors. The results support the previously proposed hypothesis that beta-endorphin produces its analgesia by stimulating specific epsilon receptors.

Effects of the enkephalin analogue FK33-824 on rectal temperature and respiratory rate in male mice

Subcutaneous administration of the enkephalin analogue FK33-824 (FK) elicited a dose-related decrease in rectal temperature and respiratory rate in male ddY strain mice. Naloxone and 3 days' implantation of morphine pellet decreased the effects of FK, suggesting the involvement of opioid receptors and cross-tolerance with morphine to both effects of FK. A positive correlation was found between the FK-induced decrease in rectal temperature and that in respiratory rate among the 6 strains of inbred mice including BALB/c, C3H, A/J, CBA, C57BL/6 and DBA/2. The degree of hypothermia elicited by FK was different among strains, whereas marginal strain difference was seen in the respiratory depression induced by FK. The strain difference in the FK responses may be due to the difference in the opioid receptor subtypes in the brain.