Biosynthesis of beta-endorphin, beta-lipotropin and the putative ACTH-LPH precursor in the frog pars intermedia

Opioid antinociception in amphibians

Systemic and spinal administration of opioids produces a behaviorally defined antinociception in a variety of mammalian models. Although endogenous opioid peptides and opioid binding sites are ubiquitous throughout phylogeny, little attention has been paid to the function of endogenous opioid system(s) or development of nociceptive models in nonmammalian species. Recent work has shown that the amphibian, Rana pipiens, provides an appropriate model for assessment of opioid antinociception and that endogenous opioid systems may likewise modulate the central processing of noxious information in amphibians as well as mammals.

Spinal antinociceptive action of three representative opioid peptides in frogs

Spinal administration of low doses of dynorphin, beta-endorphin or Met-enkephalin produces a potent, dose-dependent increase in the nociceptive threshold in the unanesthetized frog, Rana pipiens. Nociceptive thresholds were determined by using the acetic acid test, previously shown to be a sensitive indicator of antinociception in this amphibian species. Of particular interest, spinally administered dynorphin produces a potent antinociception in frogs without any signs of motor dysfunction seen after spinally administered dynorphin in mammalian species.

Dermorphin, a new peptide from amphibian skin, inhibits the nociceptive thalamic neurons firing rate evoked by noxious stimuli

Dermorphin is the representative of a new class of potent opioid peptides occurring in amphibian skin and possesses the unique feature of having a D-Ala residue incorporated in the peptide molecule. The effect of dermorphin on the spontaneous and evoked neuronal activity by a nociceptive stimulus was studied in the nucleus lateralis anterior and ventrobasal complex of the rat thalamus. The high firing frequency induced by nociceptive stimuli was blocked when dermorphin was injected intraperitoneally at the dose of 1.5 mg/kg. The action starts about 10 min after injection and lasts on average for 120 min. Naloxone, a specific opioid antagonist, injected i.p. at a dose of 1 mg/kg antagonized the effect of dermorphin. The dermorphin time-course is about twice that of morphine (1.5 mg/kg i.p.) under the same experimental conditions.

Biosynthesis, processing and release of pro-opiomelanocortin related peptides in the intermediate lobe of the pituitary gland of the frog (Rana ridibunda)

The biosynthesis of pro-opiomelanocortin (POMC) and related peptides by the intermediate lobe of the pituitary gland was studied in the frog Rana ridibunda using the pulse-chase technique. Analysis of radioactive proteins by dodecyl sulfate polyacrylamide gel electrophoresis showed that during pulse incubations a 36,000 dalton (36K) glycosylated prohormone was synthesized. It disappeared slowly during chase incubations, giving rise to another glycosylated protein (Mr 18K), identified as the N-terminal fragment of POMC. This latter protein was secreted to the incubation medium. High performance liquid chromatography analysis of peptides synthesized during chase incubations revealed the biosynthesis of two peptides related to gamma-MSH, three peptides related to alpha-MSH, one endorphin-related and one CLIP-related peptides. These newly synthesized peptides were slowly secreted to the incubation medium. Among the alpha-MSH related peptides, only the des-N alpha-acetyl alpha-MSH form of the peptide was found to be present within the cells, in contrast to the incubation medium where the presence of des-N alpha-acetyl alpha-MSH and a modified alpha-MSH was demonstrated.

Stress-induced analgesia in frogs: evidence for the involvement of an opioid system

Immobilization of frogs for 1 h induces analgesia which is blocked but not reversed by low doses of naloxone. After 9 days of daily immobilization for 1 h, the treatment fails to cause analgesia thus indicating that tolerance has developed. Animals tolerant to immobilization-induced analgesia do not show cross-tolerance to the analgesic action of morphine. The development of tolerance to this form of stress-induced analgesia and the ability of naloxone to prevent its occurrence indicate the involvement of opioid pathways. The lack of cross tolerance to morphine suggests that mu receptors are not involved.

Chemical and immunological characterization of salmon endorphins

Five new peptides related to salmon N-acetyl endorphin II (N-Ac-EP II) were isolated from an acid acetone extract of the pituitary. They were identified as EP II and four N-terminal fragments of N-Ac-EP II, namely N-Ac-EP II (1-18), (1-19), (1-20), and (1-29). The sequence comparison with mammalian EPs revealed that N-Ac-EP II (1-18) and (1-19) corresponded to mammalian alpha- and gamma-EP, respectively. The occurrence of peptides similar to mammalian alpha-, beta-, and gamma-EP indicates that further processing of EP takes place in both the teleost and mammalian pituitaries by similar pathways. A radioimmunoassay for salmon EPs has been developed with rabbit antiserum raised against salmon N-Ac-EP II. It was demonstrated that the antiserum showed full cross-reactivity with salmon N-Ac-EP I, N-Ac-EP II (1-20), and N-Ac-EP II (1-18), but none with Met-enkephalin, human beta-EP, and human beta-LPH. Similarly, the salmon EPs did not cross-react with human beta-EP antiserum in the radioimmunoassay.

A spinal site mediates opiate analgesia in frogs

The application of acetic acid to the hind leg of a frog will induce a spinally mediated wiping reflex only if the acetic acid concentration is above a certain threshold. By using this reflex as the basis of a test for nociception, we show that morphine sulfate is a potent analgesic in the frog when injected into the lumbar area of the spinal cord. Significant analgesia is induced within 5 min after injection of as little as 0.0316 microgram of morphine sulfate. Low doses of morphine sulfate (0.0316 or 0.1 microgram) induce analgesia which dissipates within 1 h while for higher doses (0.316, 1.0 or 3.16 micrograms) the analgesia persists for at least 3 h. The analgesic effect of 0.316 micrograms of morphine sulfate is completely blocked by naloxone HCl at either 0.158 or 0.316 micrograms. Animals receiving naloxone alone (0.316 micrograms) appear to be slightly hyperalgesic compared to saline injected controls but this effect is not significant.

Stress induced analgesia in frogs: a naloxone insensitive system

Repeated algesiometric testing of frogs with acetic acid can, under certain circumstances, lead to a pronounced elevation of their nociceptive threshold. Analgesia is reliably induced if algesiometric testing is begun shortly after the animals are placed in the testing cages, but not if the animals are allowed 3 days in which to recover from the stress of transfer and to acclimate to the test environment. Naloxone at doses up to 10 mg/kg neither blocks nor reverses this form of stress-induced analgesia. These data indicate that frogs, like some mammals, can use non-opiate mechanisms to modulate either their perception of or response to noxious stimuli.

In vitro study of frog (Rana ridibunda Pallas) neurointermediate lobe secretion by use of a simplified perifusion system. III. Effect of neuropeptides on alpha-MSH secretion

It has been previously demonstrated that thyrotropin-releasing hormone (TRH) stimulates in vitro the release of alpha-melanocyte-stimulating hormone (alpha-MSH) in frog. In the present study, the effects of various neuropeptides on spontaneous and/or TRH-induced alpha-MSH secretion were investigated, using a well-defined perifusion system technique. Vasoactive intestinal peptide, (VIP) a neurohormone which stimulates TRH target cells in mammals, was totally devoid of effect on frog melanotrophs although VIP-like material could be detected in neurointermediate lobe extracts. Somatostatin-like immunoreactive material was found in high concentrations in the frog neurointermediate lobe complex, but synthetic somatostatin (from 10(-10) to 10(-6) M) did not modify the spontaneous release of alpha-MSH. At doses of 10(-8) and 10(-6) M, synthetic somatostatin did not modify TRH-induced alpha-MSH secretion. Morphine (10(-5) M) and opioid peptides (10(-10) to 10(-6) M) had no effect on spontaneous alpha-MSH secretion. In addition, methionine enkephalin (10(-5) M) did not modify the stimulatory effect of TRH on alpha-MSH secretion. From these results we conclude that, among the neuropeptides which modulate prolactin secretion in mammals, only TRH is involved in alpha-MSH secretion in the frog.

Morphine-induced analgesia and explosive motor behavior in an amphibian

Morphine sulfate is a weak analgesic in the frog Rana pipiens pipiens, causing a slight increase in nociceptive threshold at a dose of 10 mg/kg and a pronounced increase at 100 mg/kg. Morphine-induced analgesia persists for at least 165 min and is significantly attenuated by naloxone. The analgesic doses of morphine are well below the lethal dose and are without noticeable effect on the behavior of the frogs or their responses to non-painful stimuli. Higher doses of morphine (320 and 640 mg/kg) induced a state of hyper-responsiveness to sensory stimuli similar to the explosive motor behavior induced in rats by microinjection of morphine into the periaqueductal gray.

Immunological characterization of endorphins, adrenocorticotropin, and melanotropins in frog hypothalamus

To gain more information about the nature and regulation of opiomelanocorticotropic peptides in the frog diencephalon, radioimmunological determinations of alpha- and beta-MSH. ACTH, beta- and gamma-endorphins have been performed in hypothalamic extracts. Sephadex G-50 gel filtration revealed a single peak of alpha-MSH-like immunoreactivity (alpha-MSH-LI) comigrating with synthetic alpha-MSH. Two peaks of ACTH-LI were observed; the major one eluting slightly before human ACTH. Using a porcine beta-endorphin antiserum which exhibited 45% cross-reaction (on a molar basis) with ovine beta-LPH, one major peak of beta-endorphin-LI and two additional components were observed in the elution profile; none of these peaks coeluted with synthetic porcine beta-endorphin. No significant beta-MSH or gamma-endorphin-LI was detected. To investigate a possible role of glucocorticoids on the level of opiomelanocorticotropic peptides in frog hypothalamus, 60 male frogs were treated with dexamethasone (300 micrograms/day) during 8 days. Dexamethasone treatment did not modify the chromatographic distribution and the total amount of alpha-MSH-LI and ACTH-LI in hypothalamic extracts. A slight (15%) increase in beta-endorphin-LI was even observed after 8 days of dexamethasone treatment. From these results it is concluded that three classes of opiomelanocorticotropic peptides are present in the frog hypothalamus in the following order of concentration: beta-endorphin-LI greater than alpha-MSH greater than ACTH. In addition, circulating glucocorticoids which significantly reduce the concentration of opiomelanocorticotropic hormones in the distal lobe of the frog pituitary (S. Jégou, M. C. Tonon, F. Leboulenger, C. Delarue, J. Côté, G. Pelletier, and H. Vaudry (1981a). In "Adv. Physiol. Sci.' E. Stark, G. B. Makara, Zs. Acs, and E. Endröczi, eds., Vol. 13, pp. 129-133. Pergamon, Budapest.) do not modify the amount of these peptides in the hypothalamus.

Localization of multiple forms of ACTH- and beta-endorphin-related substances in the pituitary of the reptile, Anolis carolinensis

Immunohistochemical studies on the pituitary of Anolis carolinensis detected ACTH-like, beta-endorphin-like, and 16K fragment-like immunoreactivity in distinct clusters of cells in the anterior lobe; ACTH-like, alpha MSH-like, beta-endorphin-like, and 16K fragment-like immunoreactivity was detected in all the cells of the intermediate lobe. Crude acid extracts of both lobes, when analyzed by radioimmunoassay, gave displacement curves in ACTH and beta-endorphin assays which were parallel to the appropriate synthetic standard. Only extracts of the intermediate lobe gave parallel displacement curves in an alpha MSH radioimmunoassay. Extracts of both lobes crossreacted with antiserum to 16K fragment, but the displacement curves were not parallel to that of mouse 16K fragment standard. The levels of immunoreactive ACTH and beta-endorphin in the intermediate lobe were approximately 8-fold higher than in the anterior lobe. Fractionation of anterior lobe and intermediate lobe extracts by either gel filtration on Sephadex G-75 in 10% formic acid or sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed multiple forms of ACTH-related and beta-endorphin-related substances in both lobes. In the anterior lobe the major forms of immunoreactivity were, respectively, ACTH-sized and beta-endorphin-sized. In the intermediate lobe the major forms of immunoreactivity were alpha MSH-sized, CLIP-sized, and beta-endorphin-sized. In both lobes, antisera directed against ACTH and beta-endorphin detected high molecular weight material with an apparent molecular weight slightly less than that of mouse pro-ACTH/endorphin; this material probably represents the putative common precursor for ACTH and beta-endorphin in this species.

Evidence for a common precursor for alpha MSH and beta-endorphin in the intermediate lobe of the pituitary of the reptile Anolis carolinensis

In order to investigate the biosynthesis of alpha MSH and beta-endorphin in a non-mammalian vertebrate, individual lizard intermediate pituitaries were incubated in complete medium containing a radioactive amino acid, using either a steady label or a pulse/chase protocol. Following incubation, acid extracts of the tissue were immunoprecipitated with either an NH2-terminal ACTH antiserum or a beta-endorphin antiserum and analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. After a 24 hr steady label in medium containing [3H]tyrosine, multiple molecular weight forms of beta-endorphin-related and NH2-terminal ACTH-related radioactivity were detected. The major peak of beta-endorphin-related radioactivity co-migrated with synthetic beta-endorphin(1-31); minor peaks of beta LPH-sized material and precursor-sized material were also detected. The major peak of NH2-terminal ACTH-related material co-migrated with synthetic alpha MSH; in addition, smaller amounts of material designated ACTH biosynthetic intermediate 1, ACTH biosynthetic intermediate 2, and precursor-sized material were detected. Sequential immunoprecipitation experiments revealed that the precursor-sized material had antigenic determinants for both alpha MSH and beta-endorphin. Pulse/chase experiments established that this material is the common precursor for alpha MSH and beta-endorphin. Based on gel filtration chromatography in 6 M guanidine HCl, the molecular weights of these various peptides are: common precursor, 23,300 daltons; ACTH biosynthetic intermediate 1, 12,200 daltons; ACTH biosynthetic intermediate 2, 4,200 daltons; alpha MSH, 1,500 daltons; beta LPH, 8000 daltons; beta-endorphin, 3,400 daltons. None of the peptides precipitated with either antiserum incorporated [3H]glucosamine; thus glycosylation does not appear to be involved in this biosynthetic pathway in the lizard. The results of the kinetic experiments and molecular weight determinations indicate that the major biosynthetic pathway involves the following events: common precursor is first cleaved to yield ACTH biosynthetic intermediate 1 plus beta LPH; subsequently, beta LPH is cleaved to produce beta-endorphin; ACTH biosynthetic intermediate 1 is cleaved to produce ACTH biosynthetic intermediate 2 which is subsequently cleaved to produce alpha MSH. The pulse/chase experiments indicate minor pathways exist for cleaving beta-endorphin directly from the common precursor or via a high molecular weight form intermediate in size between the common precursor and beta LPH.

Gamma-melanotropin is not present in an N-terminal peptide of salmon proopiocortin

The N-terminal peptide of salmon proopiocortin has been isolated and the primary structure including two disulfide bonds elucidated. The peptide consisted of 76 amino acid residues, which is 27 residues shorter than the bovine and human peptides. The N-terminal 44 residues of the teleost peptide exhibited significant sequence homology to those of the mammalian peptides. The salmon peptide, however, is lacking in the counterpart of gamma-MSH which is located between residues 51 and 64 in the mammalian peptides.

Chemistry and biosynthesis of pro-opiomelanocortin. ACTH, MSH's, endorphins and their related peptides

Studies of lipotropins, melanotropins and endorphins on one hand, and of adrenocorticotropin on the other, has given rise to the concept of a multipotent precursor molecule recently renamed proopiomelanocortin. The preferential sites of cleavage of the precursor to produce its biologically active components are made of pairs of basic amino acid residues as described for the biosynthesis of beta-MSH and pro-insulin. Such structural feature is also found in other pro-hormone molecules. Pulse chase experiments and secretory studies carried out in both anterior and intermediate lobes of rat pituitary glands revealed the transformation of different forms of the precursor into different end-products, the anterior lobe producing preferentially ACTH and beta-LPH while the intermediate produces mainly the alpha-MSH and beta-endorphin. The multiple forms of precursors seem to differ in their carbohydrate content although at least two different gene products are still possible. The presence of similar peptides in the hypothalamus makes it highly probable that neuropeptides are biosynthesized with similar process. Thus the model of beta-LPH precursor, proposed as early as in 1967, is now applicable to the biosynthesis of all other neuropeptides. Major advances in this field are expected in the 1980s.

Rat beta-LPH, gamma-LPH and beta-endorphin biosynthesized by isolated cells of pars intermedia and pars distalis. Further characterization

Rat pars intermedia cells were incubated for 3 h with the following amino acids: (a) 35S-methionine and 3H-phenylalanine, (b) 3H-valine; and (c) 3H-valine and 3H-lysine. Radioactive gamma-lipotropin, beta-lipotropin and beta-endorphin were purified on carboxymethyl-cellulose and characterized by polyacrylamide disc gel electrophoresis at pH 4.5, molecular weight estimation and micro-sequencing. Rat gamma-lipotropin was shown to differ slightly from ovine gamma-lipotropin in its NH2-terminal amino acid sequence, in containing no methionine and having phenylalanine at position 6, valine at positions 13 and 27, and lysine at position 20. The same variations were observed in the sequence of rat beta-lipotropin, while rat beta-endorphin was shown to be identical to the ovine beta-endorphin. Following a 3-h pulse of rat pars distalis, the cells were extracted with care to avoid beta-lipotropin degradation by proteolytic enzymes, A peptide was purified and identified to be rat beta-endorphin, thus demonstrating that beta-endorphin is biosynthesized in pars distalis and is not an extraction artifact.