Release of [Met5]enkephalin from the spinal cord by intraventricularly administered endomorphin-2, but not endomorphin-1 in the anesthetized rat

doi:10.1016/S0304-3940(01)02334-5

Neuroscience Letters

Volume 316, Issue 1, 4 December 2001, Pages 1-4

https://doi.org/10.1016/S0304-3940(01)02334-5 Get rights and content

Abstract

Effects of intraventricular injection of endomorphin-1, endomorphin-2 and β-endorphin on the release of immunoreactive [Met⁵]enkephalin from the spinal cord were studied in pentobarbital anesthetized rats. Intraventricular injection of endomorphin-2, but not endomorphin-1, caused an increased release of immunoreactive [Met⁵]enkephalin in the spinal perfusates. β-Endorphin given intraventricularly also increased the release of immunoreactive [Met⁵]enkephalin in an amount 15-fold higher than that produced by endomorphin-2. The increase of the release of immunoreactive [Met⁵]enkephalin induced by endomorphin-2 was blocked by μ-opioid receptor antagonist CTOP. Our result suggests that endomorphin-2 stimulates another subtype of μ-opioid receptor different from that acted by endomorphin-1 at the supraspinal site and subsequently increases the release of [Met⁵]enkephalin from the spinal cord.

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Cited by (11)

Contribution of the μ opioid receptor and enkephalin to the antinociceptive actions of endomorphin-1 analogs with unnatural amino acid modifications in the spinal cord
2021, Peptides
Citation Excerpt :
Effects of antibodies against dynorphin A (1-17), dynorphin B (1-13), [Met5]-enkephalin, and [Leu5]-enkephalin on EM and EM analog induced antinociception Dynorphin and enkephalin have been proposed as endogenous ligands for the κ and δ opioid receptors, respectively [35,36]. Previous studies have indicated that EM-2 antinociception is blocked by antibodies against dynorphin A (1-17) [37].
Endomorphin analogs containing unnatural amino acids have demonstrated potent analgesic effects in our previous studies. In the present study, the differences in antinociception and the mechanisms thereof for analogs 1-3 administered intracerebroventricularly and intrathecally were explored. All analogs at different routes of administration produced potent analgesia compared to the parent peptide endomorphin-1. Multiple antagonists and antibodies were used to explore the mechanisms of action of these analogs, and it was inferred that analogs 1-3 stimulated the μ opioid receptor to induce antinociception. Moreover, the antibody data suggested that analog 2 may induce the release of immunoreactive [Leu⁵]-enkephaline and [Met⁵]-enkephaline to produce a secondary component of antinociception at the spinal level and analog 3 may stimulate the the release of immunoreactive [Met⁵]-enkephaline at the spinal level. Finally, analogs 2 and 3 produced no acute tolerance in the spinal cord. We hypothesize that the unique characteristics of the endomorphin analogs result from their capacities to stimulate the release of endogenous antinociceptive substances.
Structure-constrained endomorphin analogs display differential antinociceptive mechanisms in mice after spinal administration
2017, Peptides
Citation Excerpt :
This finding is in agreement with that of a previous report [22]. In contrast, Sakurada et al. reported that EM-2-induced pain inhibition is blocked by pretreatment with naltriben or antibodies against Met5-enkephalin [28,35]. The reasons for this discrepancy are not clear; however, they may be related to differences in the pain behavior assays used.
We previously reported a series of novel endomorphin analogs with unnatural amino acid modifications. These analogs display good binding affinity and functional activity toward the μ opioid receptor (MOP). In the present study, we further investigated the spinal antinociceptive activity of these compounds. The analogs were potent in several nociceptive models. Opioid antagonists and antibodies against several endogenous opioid peptides were used to determine the mechanisms of action of these peptides. Intrathecal pretreatment with naloxone and β-funaltrexamine (β-FNA) effectively inhibited analog-induced analgesia, demonstrating that activity of the analogs is regulated primarily through MOP. Antinociception induced by analog 2 through 4 was not reversed by δ opioid receptor (DOP) or κ opioid receptor (KOP) antagonist; antibodies against dynorphin-A (1–17), dynorphin-B (1–13), and Leu⁵/Met⁵-enkephalin had no impact on the antinociceptive effects of these analogs. In contrast, antinociceptive effects induced by a spinal injection of the fluorine substituted analog 1 were significantly reversed by KOP antagonism. Furthermore, intrathecal pretreatment with antibodies against dynorphin-B (1–13) attenuated the antinociceptive effect of analog 1. These results indicate that the antinociceptive activity exerted by intrathecally-administered analog 1 is mediated, in part, through KOP with increased release of dynorphin-B (1–13). The chemical modifications used in the present study may serve as a useful tool to gain insight into the mechanisms of endomorphins activity.
Endomorphin-1 and Endomorphin-2: Involvement of Endogenous μ-Opioid Receptor Ligands in Analgesia, Antinociceptive Tolerance, Antianalgesia, and Hyperalgesia
2014, Journal of Experimental and Clinical Medicine (Taiwan)
Citation Excerpt :
Pharmacological findings of EM-2 on the release of Met-enkephalin for producing antinociception are in line with the biochemical finding that EM-2, but not EM-1 given intraventricularly, increases the release of immunoreactive Met-enkephalin in the spinal perfusates in male CD rats. The increased release of Met-enkephalin from the spinal cord induced by EM-2 is blocked by μ-opioid receptor antagonist CTOP.33 Figure 1 illustrates the μ-opioid receptor-mediated spinipetal descending pain control systems activated by EM-1 and EM-2 for producing antinociception.
Endomorphin-1 (EM-1) and endomorphin-2 (EM-2) are endogenous ligands for μ-opioid receptors. Both EM-1 and EM-2, given supraspinally or spinally, produce potent antinociception (analgesia) in mice and rats, measured by the thermal tail-flick response. The antinociception produced by either EM-1 or EM-2 is mediated by the stimulation of μ-opioid receptors, but not by δ- or κ-opioid receptors. EM-1 or EM-2 given supraspinally stimulates primarily μ-opioid receptors and subsequently releases spinipetal noradrenaline and serotonin, acting on α₂-adrenoceptors and serotonin receptors in the spinal cord for producing antinociception. However, the antinociception produced by EM-2, but not by EM-1, also contains an additional component, which is mediated by the release of dynorphin A_1–17 and Met-enkephalin acting on κ-opioid receptors and δ₂-receptors, respectively, in the spinal cord for producing antinociception. Pretreatment with EM-1 or EM-2, given supraspinally or spinally, attenuates the antinociception (antinociceptive tolerance) produced by EM-1 or EM-2, respectively. Pretreatment with EM-2 attenuates the antinociception produced by EM-1; however, pretreatment with EM-1 does not attenuate the antinociception produced by EM-2 (asymmetric cross-tolerance). The antinociception produced by (–)-morphine given into the ventral periaqueductal gray is attenuated by pretreatment with a subanalgesic dose of EM-1 or EM-2 given into the ventral periaqueductal gray in rats (antianalgesia). The antianalgesia produced by EM-2, but not by EM-1, is mediated by the release of dynorphin A_1–17, which antagonizes the analgesic response to (–)-morphine. EM-2, but not EM-1, given into the centromedial amygdala decreases the tail-flick latencies (hyperalgesia) in rats. The hyperalgesia induced by EM-2 from centromedial amygdala is mediated by the release of dynorphin A_1–17 acting on N-methyl-D-aspartate receptors. It is therefore proposed that there are two separate subtypes of μ-opioid receptors: μ and μ′. The μ-opioid receptors are stimulated by both EM-1 and EM-2, (–)-morphine, and [D-Ala²,NMePhe⁴,Gly⁵-ol]enkephalin, and blocked by D-Pro²-endomorphin-1. The μ′-opioid receptors are stimulated by EM-2 but not by EM-1, and blocked by D-Pro²-endomorphin-2, naloxonazine, and 3-methoxynaltrexone. However, both subtypes of μ-opioid receptors are commonly blocked by β-funaltrexamine, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH₂, and (–)-naloxone.
Endomorphin synthesis in rat brain from intracerebroventricularly injected [<sup>3</sup>H]-Tyr-Pro: A possible biosynthetic route for endomorphins
2006, Regulatory Peptides
In spite of concentrated efforts, the biosynthetic route of μ-opioid receptor agonist brain tetrapeptide endomorphins (Tyr-Pro-Trp-Phe-NH₂ and Tyr-Pro-Phe-Phe-NH₂), discovered in 1997, is still obscure. We report presently that 30 min after intracerebroventricular injection of 20 or 200 μCi [³H]Tyr-Pro (49.9 Ci mmol^− 1) the incorporated radioactivity was found in endomorphin-related tetra- and tripeptides in rat brain extracts. As detected by the combination of HPLC with radiodetection, a peak corresponding to endomorphin-2-OH could be identified in two of four extracts of “20 μCi” series. Radioactive peaks in position of Tyr, Tyr-Pro, Tyr-Pro-Phe or Tyr-Pro-Trp appeared regularly in both series and also in the “tetrapeptide cluster” constituted by endomorphins and their free carboxylic forms. In one of the four extracts in the “200 μCi” series a robust active peak in the position of endomorphin 2 could be detected. Intracerebroventricularly injected 100 nmol, but not 10 or 1000 nmol cold Tyr-Pro (devoid of opioid activity in vitro), caused a naloxone-reversible prolongation of tail-flick latency in rats, peaking between 15 and 30 min. We suggest that Tyr-Pro may serve as a biosynthetic precursor to endomorphin synthesis.
A new electrochemical HPLC method for analysis of enkephalins and endomorphins
2006, Journal of Neuroscience Methods
Endogenous opioid peptides, enkephalins and endomorphins, are located in key regions involved in pain transmission and analgesia, including the spinal cord. These endogenous peptides activate opioid receptors to produce analgesia and reduce pain. We describe a new method to measure enkephalin and endomorphins by high performance liquid chromatography with electrochemical detection. This method allows use of a small sample volume to measure met-enkephalin, leu-enkephalin, endomorphin-1 and endomorphin-2 simultaneously. Using push–pull perfusion of the spinal cord, there were detectable concentrations of met-enkephalin, leu-enkephalin, and endomorphin-2. Further infusion of 100 mM potassium chloride evoked release of met-enkephalin and endomorphin-2 but not leu-enkephalin. Thus, we have developed a method to simultaneously measure enkephalins and endomorphins in small sample volume that allows measurement of these opioid peptides in vivo.
Endogenous opiates and behavior: 2001
2002, Peptides
This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular–biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

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Release of [Met5]enkephalin from the spinal cord by intraventricularly administered endomorphin-2, but not endomorphin-1 in the anesthetized rat

Abstract

Neuroscience

Brain Res.

Pharmacological characterization of endomorphin-1 and endomorphin-2 in mouse brain

J. Pharmacol. Exp. Ther.

The distribution of methionine-enkephalin and leucine-enkephalin in the brain and peripheral tissues

Br. J. Pharmacol.

Release of [Met⁵]enkephalin from the spinal cord by intraventricularly administered endomorphin-2, but not endomorphin-1 in the anesthetized rat