Inhibition of analgesia by C-terminal deletion analogs of human beta-endorphin

Effects of intraventricular injection of morphine and beta-endorphin on serotonin release from the spinal cord in rats

Effects of intraventricular (third ventricle) injection of morphine and beta-endorphin on the release of serotonin (5-HT; 5-hydroxytryptamine) and 5-HIAA (5-hydroxy indolacetic acid) from the spinal cord were studied using urethane anesthetized spinally perfused rats. Intraventricular injection of morphine (25 micrograms) increased the 5-HT level in the perfusate about threefold. The increase of 5-HT release reached at peak between 30 and 60 min after the first injection of morphine. However, the levels of 5-HIAA, a metabolite of 5-HT, was not significantly altered by intraventricular injection of morphine. Furthermore, second intraventricular injection of morphine at the same dose did not increase 5-HT level in the spinal perfusate. In contrast to the results with morphine, beta-endorphin (10 micrograms) administered intraventricularly did not alter the release of 5-HT and 5-HIAA from the spinal cord. In addition, acute antinociceptive tolerance to intraventricular morphine induced by a prior intraventricular injection of morphine was studied in pentobarbital anesthetized rats. Acute tolerance was induced by intraventricular pretreatment with morphine (20 micrograms) for 120 min and the same dose of morphine was injected intraventricularly. The tail-flick test was used as an antinociceptive test. Pretreatment of rats with morphine intraventricularly reduced inhibition of the tail-flick response to intraventricularly injected morphine. The results support our previous hypothesis that beta-endorphin and morphine administered supraspinally activate separate descending systems. Spinopetal serotonergic descending pathway is selectively activated by intraventricularly injected morphine but not beta-endorphin.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal administration of thiorphan, bestatin, desipramine and fluoxetine differentially potentiate the antinociceptive effects induced by beta-endorphin and morphine, administered intracerebroventricularly

The effects of the intrathecal injection of thiorphan (an inhibitor of enkephalinase inhibitor), bestatin (an inhibitor of aminopeptidase), desipramine (an inhibitor of the uptake of noradrenaline) and fluoxetine (an inhibitor of the uptake of serotonin) on the antinociception induced by beta-endorphin and morphine, administered intracerebroventricularly, were studied in male ICR mice. Antinociceptive effects were assessed by the tail-flick and hot-plate tests. Thiorphan (16 micrograms) and bestatin (16 micrograms), injected intrathecally, potentiated inhibition of the tail-flick response, induced by beta-endorphin but not by morphine administered intracerebroventricularly, whereas desipramine (6 micrograms) and fluoxetine (6 micrograms), injected intrathecally potentiated inhibition of the tail-flick response induced by morphine, but not by beta-endorphin, administered intracerebroventricularly. Thiorphan, bestatin, desipramine or fluoxetine, given intrathecally, did not antagonize inhibition of the hot-plate response, induced by beta-endorphin or morphine administered intracerebroventricularly. The results indicate that inhibition of the tail-flick response, induced by beta-endorphin administered intracerebroventricularly, is mediated by the opioid system, but not by noradrenergic and serotonergic systems in the spinal cord. On the other hand, the inhibition of the tail-flick response, induced by morphine given intracerebroventricularly, is mediated by noradrenergic and serotonergic systems, but not by the opioid system in the spinal cord. The lack of effect of enzyme inhibitors and inhibitors of the uptake of biogenic amines intrathecally on beta-endorphin- and morphine-induced inhibition of the hot-plate response, is due to the supraspinal nature of the nociceptive hot-plate response. The present results further support the hypothesis, proposed previously, that intracerebroventricularly injected beta-endorphin and morphine elicit antinociception by activating different descending inhibitory systems.

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.

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.

Secondary processing of neurohormones: intracellular proteolytic cleavage of beta-endorphin generates new active neuropeptides

The following short review considers the physiological significance of naturally occurring peptide fragments derived from intracellular secondary processing of beta-endorphin (beta-EP). Particular attention will be given to the question of whether truncated and/or derivatized forms of beta-EP are true end products subject to release and regulation and might serve important new functions. Since collectively endorphins may play a variety of neurobiological roles which may or may not be mediated through analgesic receptors, most of the material reviewed herein deals with the induction of analgesia, the most commonly used measure of the opiate's acute effects.

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

The effects of beta-endorphin and beta-endorphin-(1-27) on the body temperature of mice was studied at an ambient temperature of 10 degrees C. Intracerebroventricular injection of beta-endorphin (0.25-4 micrograms) and beta-endorphin-(1-27) (0.61 to 10 micrograms) caused a dose-related hypothermia. The duration of hypothermia induced by beta-endorphin-(1-27) was shorter than that induced by beta-endorphin. The hypothermia induced by 2 micrograms of beta-endorphin was attenuated by 5 micrograms of beta-endorphin-(1-27). Our results indicated that beta-endorphin-(1-27) is a partial agonist which produces a small degree of hypothermia and an antagonist which blocks the beta-endorphin-induced hypothermia.

Melanotropin potentiating factor inhibits lipolytic activity of beta-lipotropin but not of melanocyte stimulating hormones

Melanotropin potentiating factor (MPF) potentiates the melanotropic activity of melanocyte stimulating hormones. Although the message sequence for the melanotropic and lipolytic activity are identical for beta-lipotropin, alpha- and beta-MSH, MPF was not able to affect the lipolytic response to alpha- and beta-MSH in rabbit adipocytes. However, MPF at concentrations of 10-5 and 10-6 mol/l inhibited the lipolytic activity of beta-lipotropin. This suggests that the inhibition of the lipolytic response to beta-lipotropin is not connected with the common lipolytic message sequence (beta-LPH 47-53). Since beta-lipotropin has a second lipolytic sequence in its C-terminal part this second lipolytic core of beta-lipotropin might interact with MPF which has no intrinsic lipolytic activity.