Regional distribution of gamma- and beta-endorphin-like peptides in the pituitary and brain of the rat

Topography and characteristics of specific binding sites for non-opioid gamma-type endorphins in the rat brain as studied by autoradiography with [35S]Met-desenkephalin-gamma-endorphin

An in vitro autoradiographic study was performed to characterize specific rat brain binding sites for non-opioid neuroleptic-like gamma-type endorphins, using [35S]Met-des-enkephalin-gamma-endorphin ([35S]Met-DE gamma E; [35]S-beta-endorphins(5-17)) with high specific activity as radioligand. The binding sites appeared to be confined to rat forebrain regions, e.g., orbital cortex, frontal cortex, cingulate cortex, piriform cortex, nucleus accumbens, amygdala, mediodorsal nucleus of the thalamus and arcuate and periventricular nuclei of the hypothalamus. These regions are part of the mesocorticolimbic feedback circuit. Densitometric analysis of the autoradiographs revealed that the density of the binding sites was highest in the mediodorsal nucleus of the thalamus and the amygdala. Concentration-dependent displacement of [35S]Met-DE gamma E (500 pM) with DE gamma E yielded an IC50 of 0.6 nM whereas DE alpha E (beta-endorphin(6-16)) had an IC50 of 210 nM. Various endorphins, sharing the gamma-endorphin C terminus, displaced [35S]Met-DE gamma E to the same extent as non-labelled DE gamma E (at 10(-6) M) whereas non-endorphin peptides did not show displacing capacity. Possible relationships of the binding sites with opioid receptors were investigated. DAMGO (mu) and DPDPE (delta) displaced [35S]Met-DE gamma E to some extent at 10(-6) M whereas U69,593 (kappa) was inactive, suggesting that the binding sites for gamma-type endorphins may resemble mu- and delta-opioid receptors in some aspects. Similarly, relationships with dopamine receptors were investigated. Haloperidol partially displaced [35S]Met-DE gamma E whereas sulpiride, SKF38,393 and 3-PPP at 10(-6) M did not induce significant displacement. Thus, binding sites are distinct from dopamine receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of a Leu-Phe-cleaving endopeptidase activity putatively involved in beta-endorphin metabolism in rat brain

Incubation of beta-endorphin with cytosolic and particulate fractions of rat brain resulted in the formation of several peptides, including gamma-endorphin [beta-endorphin-(1-17)] and beta-endorphin-(18-31), indicating the presence of enzyme activity cleaving the Leu17-Phe18 bond of beta-endorphin. An assay for this Leu-Phe cleaving activity, based on the cleavage of the 14C-labeled substrate acetyl-Val-Thr-Leu-Phe-[epsilon-([14C]CH3)2]Lys-NHCH3, was used to examine the properties of this enzyme activity. beta-Endorphin-(1-31) competitively inhibited the Leu-Phe-cleaving enzyme activity on the pentapeptide substrate. Over 90% of activity was recovered in the cytosolic fraction. Leu-Phe-cleaving activity behaved like a thiol endopeptidase because it was inhibited by low concentrations of N-ethylmaleimide, p-chloromercuribenzoate, p-chloromercuribenzoyl sulfate, and low concentrations of Hg2+. Low concentrations of sulfhydryl compounds stimulated Leu-Phe-cleaving activity. The activity was optimal between pH 8.5 and 9.0. The Km of Leu-Phe-cleaving activity in the cytosolic fraction was 35 microM and in the particulate fraction 88 microM with Vmax values of 193 and 15 nmol mg protein-1 h-1, respectively. The apparent molecular mass of the Leu-Phe-cleaving enzyme was estimated by gel filtration to be approximately 200 kilodaltons. These properties of Leu-Phe-cleaving activity indicate that the Leu-Phe-cleaving enzyme is distinct from any known brain endopeptidase.

Characterization of beta-endorphin-immunoreactivity in limbic brain structures of rats self-administering heroin or cocaine

The effects of intravenous self-administration of 30 micrograms infusions of either heroin or cocaine, or saline on the concentrations of beta-endorphin-immunoreactivity (beta E-IR) in the anterior part of the rat brain limbic system were studied. Self-administration of heroin and cocaine for 5 daily sessions resulted in a marked reduction of the concentrations of beta E-IR in the nucleus accumbens, rostral striatum, septum and hippocampus at the time of the scheduled next session on day 6. In pooled extracts of these regions from rats receiving saline, combined application of high-pressure liquid chromatography (HPLC) fractionation and specific radioimmunoassays revealed the presence of a number of beta E-related peptides co-chromatographing with synthetic non-acetylated and acetylated alpha, beta- and gamma-type endorphins. Similar profiles were found after HPLC fractionation of extracts of these regions from rats self-administering heroin and cocaine. Rats self-administering heroin or cocaine, however, showed decreased amounts of all detected forms of beta-endorphin as compared to saline rats. These findings indicate that both self-administration of an opiate that induces psychic as well as physical dependence and of a non-opiate stimulant inducing psychic but not physical dependence, results in a significant decrease of beta E and related peptides in limbic brain regions of the rat. All forms of beta E detected after HPLC were equally affected, suggesting an overall effect of the drugs on peptide turnover. These results suggest that beta E and related peptides may be involved in the neurochemical mechanisms underlying psychic dependence to drugs.

Increased concentration of alpha- and gamma-endorphin in post mortem hypothalamic tissue of schizophrenic patients

The concentrations of alpha-, beta- and gamma-endorphin were determined by radioimmunoassay in HPLC fractionated extracts of post mortem hypothalamic tissue obtained from schizophrenic patients and controls. The hypothalamic concentration of alpha- and gamma-endorphin was significantly higher in patients than in controls (+72.9% and +50.5% respectively). No difference was found in the concentration of beta-endorphin, the putative precursor of alpha- and gamma-endorphins. These results suggest a deviant metabolism of beta-endorphin in the brain of schizophrenic patients. Whether this phenomenon is related to the psychopathology, or is a consequence of ante mortem farmacotherapy, remains to be established.

Neonatal monosodium glutamate lesions alter neurosensitivity to ethanol in adult mice

A number of studies have indicated a relationship between brain peptide activity and sensitivity to the behavioral effects of ethanol. Specifically, it has been suggested that ethanol effects are mediated by changes in the endogenous opioid peptides derived from the proopiomelanocortin (POMC) precursor. Most cell bodies containing brain POMC-derived peptides are found in the arcuate nucleus of the hypothalamus. Neonatal administration of monosodium glutamate (MSG) has been reported to destroy cell bodies of the arcuate nucleus. We treated WSC strain mice on postnatal Day 4 with a single SC injection of 4 mg/g MSG or saline. When adult, MSG and control mice were challenged with an IP injection of ethanol and its effect on body temperature, open field activity, or duration of loss of righting reflex was assessed. Blood ethanol concentration (BEC) was measured and the hypothalamic content of beta-endorphin like immunoreactivity (beta-EP) was determined by radioimmunoassay. beta-EP was markedly reduced in both females and males by MSG treatment. MSG-treated animals of both sexes showed significantly less ethanol-induced hypothermia than controls. BEC was higher in MSG-treated animals of both sexes than in controls, so the differences were not due to ethanol pharmacokinetics. beta-EP was generally lower in males. Duration of righting reflex was prolonged in MSG treated animals, and the reduction in open field activity was potentiated. These latter effects may be in part attributable to the higher BECs achieved in lesioned animals. These data suggest that beta-EP cell bodies in the arcuate nucleus of the hypothalamus mediate neurosensitivity to some effects of ethanol in mice, but further experiments will be necessary to implicate beta-EP specifically.

Des-enkephalin-gamma-endorphin: bioavailability in rats following the subcutaneous and intramuscular route of administration

A pharmacokinetic study with [3H]des-enkephalin-gamma-endorphin (3H-DE gamma E) was performed in rats after the intravenous, subcutaneous and intramuscular route of administration. Disappearance of non-metabolized 3H-DE gamma E from blood upon intravenous dosing followed a biphasic decay with half-lives of 0.7 +/- 0.3 (+/- S.D.) min for the initial distribution phase and 6.3 +/- 2.7 min for the terminal elimination phase. The central and peripheral volumes of distribution were strikingly high (0.38 and 0.55 1 X kg-1, respectively). Extensive metabolism occurred already within the first minutes after injection. The blood clearance rate was found to be 0.29 +/- 0.12 1 X min-1 X kg-1, which value points to remarkable extrahepatic elimination of the neuropeptide. As compared to the intravenous route of administration, subcutaneous or intramuscular injection of 3H-DE gamma E resulted in low but longer-lasting peptide levels in blood. These levels reached already peak values at 2 min after both routes of administration and then declined to below the limit of detection at 2-3 h. The absolute bioavailability of DE gamma E after subcutaneous injection amounted to 30.9 +/- 16.3% (range 16.0-46.9%), whereas the bioavailability after intramuscular injection was observed to be 3.5 times lower (8.5 +/- 3.0%; range 4.6-12.0%). These data suggest that subcutaneous dosing of DE gamma E might be more effective in displaying CNS activity than the intramuscular route.

Ethanol-stimulated endorphin and corticotropin secretion in vitro

Although acute administration of ethanol in vivo results in increased plasma glucocorticoid concentration, it is unclear whether this effect is mediated by corticotropin (ACTH) from the anterior pituitary. Secretion of beta-endorphin-like (BE-IR) and corticotropin-like (ACTH-IR) immunoreactivity from perifused, dispersed mouse adenohypophyseal cells was used to evaluate the effect of 17 mM ethanol on secretion of pituitary peptides. Cells were also exposed to 10 nM synthetic corticotropin-releasing factor (CRF), 1 microM vasopressin, 54 mM KCl, 100 nM corticosterone, and calcium-free medium, separately and in combination. Secretion of BE-IR and ACTH-IR were markedly sensitive to low concentrations of ethanol. Exposure to 17 mM ethanol produced 3-fold stimulation of the rate of hormone release. This represented one-third to two-thirds that of the rate of maximum stimulation by CRF. Unlike CRF-stimulated secretion, ethanol-stimulated secretion was transient. Further, a second ethanol exposure 1 h after the first did not stimulate peptide secretion. Similar to CRF-stimulation, ethanol-stimulated peptide secretion required extracellular calcium and was inhibited by the glucocorticoid corticosterone. We suggest that this system is a useful model for investigation of the actions of low concentrations of ethanol at the cellular level.

Inhibition of gamma-endorphin generating endopeptidase activity of rat brain by peptides: structure activity relationship

gamma-Endorphin generating endopeptidase (gamma EGE) activity is an enzyme activity which converts beta-endorphin into gamma-endorphin and beta-endorphin-(18-31). The inhibitory potency on gamma EGE activity of neuropeptides and analogues or fragments of neuropeptides was tested. Dynorphin-(1-13) (IC50: 0.14 microM), human beta-endorphin-(1-31) (IC50: 15.5 microM), porcine ACTH-(1-39) (IC50: 6.3 microM), and substance P (IC50: 26 microM) had an inhibitory activity on gamma EGE activity. beta-Endorphin-(18-31) (IC50: 0.35 microM) but not gamma-endorphin potently inhibited gamma EGE activity. The IC50 of poly (Lys)40-60 was 0.8 microM. It is concluded that 1) gamma EGE activity is strongly inhibited by its product beta-endorphin-(18-31), 2) the enzyme is strongly inhibited by peptides with an aromatic amino acid at the NH2-terminal and/or basic amino acids in the COOH-terminal of the peptide chain.

Light and electron microscopic immunocytochemistry of beta-endorphin/beta-LPH-like immunoreactive neurons in the arcuate nucleus and surrounding areas of the rat hypothalamus

beta-Endorphin/beta-LPH-like immunoreactive neurons in the hypothalamic arcuate nucleus and its surrounding areas were visualized by light and electron microscopic immunocytochemistry. Immunoreactive processes were found in the vicinity of the pia mater, in the lateral part of the external layer of the median eminence and near the lateral wall of the third ventricle. Neuronal perikarya contained immunoreactive dense granules as well as developed cell organellae. They received neuronal inputs from other neurons through axoplasmic and axodendritic synapses. Immunoreactive neuronal processes containing dense granules and mitochondria were found as preterminal elements on non-immunoreactive neuronal soma and dendrites. Immunoreactive processes also make intimate contact with capillaries in the arcuate nucleus near the median eminence.

N alpha-Acetyl-gamma-endorphin is an endogenous non-opioid neuropeptide with biological activity

N alpha-acetyl-gamma-endorphin (Ac gamma E) was identified in the rat neurointermediate pituitary, based on its immunological properties, comigration with synthetic Ac gamma E on HPLC and resistance to aminopeptidase-M degradation. The peptide appeared to be the main form of gamma-endorphin (gamma E) in this tissue and in brain areas remote from the hypothalamus (hippocampus, septum, amygdala). The anterior pituitary, the hypothalamus and the thalamus contained almost exclusively the non-acetylated form of gamma E. In contrast to gamma E, Ac gamma E was completely devoid of specific affinity for brain opiate binding sites. Yet, the peptide mimicked gamma E in that it potently attenuated passive avoidance behaviour in rats, when injected topically into the nucleus accumbens. It is concluded that Ac gamma E is an endogenous neuropeptide with non-opioid biological activity. N alpha-acetylation may not merely represent a mechanism for the inactivation of opioid activities of endorphins, but rather allow the organism to select specific sets of biological activities that reside in the endorphin structure.

Des-Tyr1-gamma-endorphin (DT gamma E) and des-enkephalin-gamma-endorphin (DE gamma E): plasma profile and brain uptake after systemic administration in the rat

The plasma disappearance, metabolism and uptake in the brain of [3H-Phe4]-DT gamma E and [3H-Lys9]-DE gamma E were investigated following systemic administration of these neuroleptic-like peptides to rats. 3H-DT gamma E, 3H-DE gamma E and their radioactive metabolites in plasma and brain extracts were determined by reversed-phase HPLC. Plasma disappearance of DT gamma E upon intravenous (IV) dosing followed a biphasic pattern with half-lives of 0.7 min (distribution phase) and 5.5 min (elimination phase). For DE gamma E the plasma disappearance curve was best characterized by a one-compartment model since a second elimination phase was hardly detectable by our methods. The corresponding half-life was 0.6 min, probably representative for the initial distribution phase of DE gamma E. Both neuropeptides distributed rapidly over the larger part of the extracellular fluid. Following the IV route of administration, brain uptake of DT gamma E and DE gamma E appeared to be low. Brain levels of DT gamma E decreased from 0.0075% to 0.0031% of the administered dose/g tissue at 2-15.5 min after injection, whereas those of DE gamma E decreased very rapidly from 0.0174% of the dose/g brain tissue to below the detection limit at 2-4.5 min after injection. As compared to the IV route of administration, subcutaneous (SC) injection of DE gamma E resulted into lower but remarkably longer-lasting peptide concentrations in plasma as well as in brain, possibly because of a sustained release from the SC site of injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoreactive-beta-endorphin and LHRH levels in the brains of aged male rats with impaired sex behavior

Levels of immunoreactive beta-endorphin and luteinizing hormone releasing hormone (LHRH) were measured in brain tissue of aged male Long-Evans rats. The animals were tested for sex behavior twice in one week at bimonthly intervals between the 7th and 27th month of life and were sacrificed along with a group of young (5-month old) sexually active rats. Thirty-one of the 89 rats which began the study remained healthy and tumor-free. By month 27, 21 of these had completely ceased to mate and 10 continued to show adequate sexual behavior. Diminished levels of beta-endorphin-like immunoreactivity were measured in the hypothalami and hindbrain of the old animals grouped together as compared to young animals and this reduction was shown to be significantly greater in hypothalamic tissue from the behaviorally inactive subgroup. Hypothalamic LHRH levels were not significantly altered by age in these animals. However, a marked reduction of LHRH content in the septal and midbrain regions of the aged-behaviorally inactive subgroup was evident when compared with the behaviorally active group. The data suggest that altered function of beta-endorphin and LHRH neurons of the aged brain may be involved in the behavioral deterioration observed in aged animals.

Pre-decapitation state of arousal of rats predetermines the effect of des-Tyr1-gamma-endorphin on dopamine release from nucleus accumbens slices in vitro

The non-opiate beta-endorphin fragment des-Tyr1-gamma-endorphin (DT gamma E) had a decreasing effect on K+-induced release of tritiated dopamine from nucleus accumbens slices in vitro, when tissue was used of rats which prior to decapitation were in a state of low arousal. When nucleus accumbens tissue was used of rats which were mildly stressed by exposure to a novel environment prior to decapitation, this effect was absent, while an enhancing effect of DT gamma E became evident on basal dopamine efflux. This latter effect resembled that of haloperidol, which dose-dependently enhanced basal dopamine efflux in vitro. Exposure of rats to ether vapor shortly before decapitation abolished both these in vitro effects of DT gamma E. The results are interpreted as indicating that the quality of the modulating effects of DT gamma E on dopamine release from dopaminergic neurons projecting to the nucleus accumbens is depending on the state of activity of these neurons, which, in its turn, is a reflection of the state of arousal of the rats.

Isolation and characterization of alpha-endorphin and gamma-endorphin from single human pituitary glands

alpha-Endorphin and gamma-endorphin, two closely related peptides of the pro-opiomelanocortin family with characteristic biological activities, were purified to homogeneity from single human pituitary glands and chemically identified. Isolation of the peptides was based on size fractionation by Sephadex G-75 chromatography followed by two HPLC steps using reverse-phase and paired-ion reverse-phase systems and was monitored by radioimmunoassay. During the isolation procedure alpha- and gamma-endorphin-sized material behaved chromatographically and immunologically indistinguishably from synthetic alpha- and gamma-endorphin. The amino acid composition and NH2-terminus of isolated peptides demonstrated their identity as authentic alpha-endorphin and gamma-endorphin. Acetylated forms were absent. In addition, evidence is provided that large forms with alpha- and gamma-endorphin immunoreactivity detected during gel filtration are human lipotropin-(1-74) and -(1-75), respectively. The data substantiate that alpha-endorphin and gamma-endorphin exist as endogenous peptides in the human pituitary gland.

Localization and identification of gamma-endorphin and beta-endorphin-like peptides in the hypothalamus and ventral forebrain of the rat

Gamma-endorphin (gamma E) and related peptides were localized in the brain of the rat, and compared to the distribution of beta-endorphin related peptides. gamma E-like immunoreactivity (gamma E-LI) was shown to exist only in brain regions known to receive innervation from POMC neurons but not in their pericaria in the arcuate nucleus. Nuclei of the hypothalamus and ventral forebrain contained the major concentration of gamma E-LI and extracts from these regions were purified and subjected to HPLC in order to identify gamma E-related peptides. gamma E, des-tyrosine1 gamma E, des-enkephalin-gamma E, and alpha-N acetyl gamma E co-elutable peptides as well as various unidentified forms were evident. This study provides evidence of the endogenous presence of pharmacologically active gamma E-related peptides in brain regions where they have been postulated to modulate dopaminergic transmission.

An improved filtration procedure for measuring opiate receptors in small regions of rat brain

A modified filtration method for in vitro receptor binding was used to determine specific binding of [3H]naloxone to small regions of adult rat brain. Reliable determinations of ligand binding were quantified with about 50 micrograms of protein per assay tube. Large differences in [3H]naloxone binding were obtained between various brain nuclei, and these differences were consistent with prior determinations of opiate receptor densities in various rat brain nuclei using autoradiographic techniques.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Gamma-endorphin-like peptides in pituitary tissue: evidence for their existence in vivo

Beta and gamma endorphin-like peptides were measured by radioimmunoassay in whole pituitary. Boiling of acetic acid extracts prior to tissue disruption increased the concentration of both beta E- and gamma E-like peptides. The gamma E-like immunoreactivity from the neurointermediate lobe of the pituitary co-eluted with synthetic gamma E upon gel permeation chromatography. Immunoreactivity for beta E-like and gamma E-like peptides in the intermediate lobe of the pituitary was also shown by immunoperoxidase staining. The results suggest that gamma E-like peptides are present primarily in the pars intermedia in vivo and do not arise as artifacts of acid extraction of pituitary tissue.

Proteolysis of beta-endorphin in brain tissue

The processing of beta-endorphin by brain enzymes into peptides related to the behaviorally active gamma- and alpha-type endorphins and the sequence of proteolytic events in the conversion process are described. Multiple enzyme activities contribute to the generation of the peptides with neurotropic activity. It is proposed that the processing into gamma- and alpha-type neuropeptides is a post-secretional event. The enzymes involved may have a key role in the nature and levels of neurotropic beta-endorphin fragments in the brain.

Regional distribution of alpha- and gamma-type endorphins in rat brain

The regional distribution of Met-enkephalin, beta-endorphin and alpha- and gamma-type endorphins in rat brain was investigated. To that end, brains were dissected into anatomically defined areas. Acetic acid tissue extracts were fractionated using an HPLC system suitable for separation of endorphins and peptide concentrations were subsequently measured by specific radioimmunoassay systems. The distribution of Met-enkephalin and beta-endorphin through the brain was fairly uneven and in accordance with results obtained by others. The peptides alpha-endorphin, gamma-endorphin, des-Tyr-alpha-endorphin (DT alpha E) and des-Tyr-gamma-endorphin (DT gamma E) were detectable in almost all brain areas. Their distribution, however, appeared to be uneven. Hypothalamus and septum showed the highest levels of alpha- and gamma-type endorphins. These regions also contained high amounts of beta-endorphin, underscoring a precursor function of this peptide in the formation of alpha- and gamma-type fragments. In general, levels of alpha-endorphin were higher than those of des-Try-alpha-endorphin, whereas the opposite was found for gamma- and des-Tyr-gamma-endorphin.