Acute antinociceptive tolerance and partial cross-tolerance to endomorphin-1 and endomorphin-2 given intrathecally in the mouse

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

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.

Endomorphin-1 analogs with oligoarginine-conjugation at C-terminus produce potent antinociception with reduced opioid tolerance in paw withdrawal test

For clinical use, it is essential to develop potent endomorphin (EM) analogs with reduced antinociceptive tolerance. In the present study, the antinociceptive activities and tolerance development of four potent EM-1 analogs with C-terminal oligoarginine-conjugation was evaluated and compared in the radiant heat paw withdrawal test. Following intracerebroventricular (i.c.v.) administration, all analogs 1-4 produced potent and prolonged antinociceptive effects. Notably, analogs 2 and 4 with the introduction of D-Ala in position 2 exhibited relatively higher analgesic potencies than those of analogs 1 and 3 with β-Pro substitution, consistent with their μ-opioid binding characteristic. In addition, at a dose of 50 μmol/kg, endomorphin-1 (EM-1) failed to produce any significant antinociceptive activity after peripheral administration, whereas analogs 1-4 induced potent antinociceptive effects with an increased duration of action. Herein, our results indicated the development of antinociceptive tolerance to EM-1 and morphine at the supraspinal level on day 7. By contrast, analogs 1-4 decreased the antinociceptive tolerance. Furthermore, subcutaneous (s.c.) administration of morphine at 50 μmol/kg also developed the antinociceptive tolerance, whereas the extent of tolerance developed to analogs 1-4 was largely reduced. Especially, analog 4 exhibited non-tolerance-forming antinociception after peripheral administration. The present investigation gave the evidence that C-terminal conjugation of EM-1 with oligoarginine vector will facilitate the development of novel opioid analgesics with reduced opioid tolerance.

C-terminal hydrazide modification changes the spinal antinociceptive profiles of endomorphins in mice

Previously, we have demonstrated that endomorphins (EMs) analogs with C-terminal hydrazide modification retained the μ-opioid receptor affinity and selectivity, and exhibited potent antinociception after intracerebroventricular (i.c.v.) administration. In the present study, we extended our studies to evaluate the antinociceptive profiles of EMs and their analogs EM-1-NHNH₂, EM-2-NHNH₂ given spinally in the radiant heat paw withdrawal test. Following intrathecal (i.t.) administration, EM-1, EM-2, EM-1-NHNH₂ and EM-2-NHNH₂ dose-dependently increased the latency for paw withdrawal response. EM-1-NHNH₂ displayed the highest antinociceptive effects, with the ED₅₀ values being 1.63 nmol, more potent than the parent EM-1 (1.96 nmol), but with no significant difference. By contrast, the analgesic activities of EM-2 and its analog EM-2-NHNH₂ were almost equivalent (P>0.05). Naloxone and β-funaltrexamine (β-FNA) almost completely attenuated the antinociceptive effects of EMs and their analogs EM-1-NHNH₂, EM-2-NHNH₂ (10 nmol, i.t.), indicating the involvement of μ-opioid receptors. Notably, the antinociception of EM-1 was not significantly antagonized by naloxonazine, a selective μ₁-opioid receptor antagonist, but partially reversed the effects of EM-2, suggesting that EM-1 and EM-2 may produce antinociception through distinct μ₁- and μ₂-opioid receptor subtypes. Moreover, naloxonazine didn't significantly block the antinociceptive effects of EM-1-NHNH₂ and EM-2-NHNH₂, and nor-BNI, the κ-opioid receptor antagonist, attenuated the analgesic effects of EM-2, but not EM-1, EM-1-NHNH₂ or EM-2-NHNH₂. These results indicated that C-terminal amide to hydrazide conversion changed the antinociceptive opioid mechanisms of EM-2 but not EM-1 at the spinal level. Herein, the acute antinociceptive tolerance were further determined and compared. EM-1-NHNH₂ and EM-2-NHNH₂ shifted the dose-response curve rightward by only 2.8 and 1.5-fold as determined by tolerance ratio, whereas EM-1 and EM-2 by 3.4 and 4.6-fold, respectively, indicating substantially reduced antinociceptive tolerance. The present study demonstrated that C-terminal hydrazide modification changes the spinal antinociceptive profiles of EMs.

Endomorphin-2 Decreases Excitatory Synaptic Transmission in the Spinal Ventral Horn of the Rat

Motor impairment is one of the serious side-effects of morphine, which is an exogenous agonist of the μ-opioid receptor (MOR) as well as a widely used analgesic drug in clinical practice for chronic pain treatment. Endomorphins (EMs, including EM-1 and EM-2), the most effective and specific endogenous agonists of the MOR, exert more potent analgesia in acute and neuropathic pain than other opiates, such as morphine. Although EMs had fewer side-effects comparing to other opiates, motor impairment was still one unwanted reaction which limited its clinical application. In order to prevent and treat the motor impairment, it is critical to reveal the neural mechanisms underlying such locomotion disorder. The purpose of the present study was to reveal the neural mechanisms underlying the effects of EM-2 on the activity of motoneurons in the spinal ventral horn. First, we examine the distribution of EM-2-immunoreactive (IR) primary afferent fibers and their synaptic connections with the motoneurons innervating the skeletal muscles of the lower limb revealed by sciatic nerve retrograde tracing. The results showed that EM-2-IR fibers and terminals were sparsely observed in lamina IX and they formed symmetric synaptic connections with the motoneurons within lamina IX of the spinal ventral horn. Then, whole-cell patch-clamp technique was used to observe the effects of EM-2 on the spontaneous excitatory postsynaptic current (sEPSC) of motoneurons in lamina IX. The results showed that EM-2 could decrease both the frequency and amplitude of the sEPSC of the motoneurons in lamina IX, which was reversed by the MOR antagonist CTOP. These results indicate that EM-2-IR fibers originated from primary afferent fibers form symmetric synaptic connections with motoneurons innervating skeletal muscles of the lower limbs in lamina IX of the spinal ventral horn and EM-2 might exert inhibitory effects on the activities of these motoneurons through both presynaptic and postsynaptic mechanisms.

Tetrapeptide Endomorphin Analogs Require Both Full Length and Truncated Splice Variants of the Mu Opioid Receptor Gene Oprm1 for Analgesia

The mu opioid receptor gene undergoes extensive alternative splicing. Mu opioids can be divided into three classes based on the role of different groups of splice variants. Morphine and methadone require only full length seven transmembrane (7TM) variants for analgesia, whereas IBNtxA (3'-iodobenzyol-6β-naltrexamide) needs only truncated 6TM variants. A set of endomorphin analogs fall into a third group that requires both 6TM and 7TM splice variants. Unlike morphine, endomorphin 1 and 2, DAPP (Dmt,d-Ala-Phe-Phe-NH₂), and IDAPP (3'-iodo-Dmt-d-Ala-Phe-Phe-NH₂) analgesia was lost in an exon 11 knockout mouse lacking 6TM variants. Restoring 6TM variant expression in a knockout mouse lacking both 6TM and 7TM variants failed to rescue DAPP or IDAPP analgesia. However, re-establishing 6TM expression in an exon 11 knockout mouse that still expressed 7TM variants did rescue the response, consistent with the need for both 6TM and 7TM variants. In receptor binding assays, ¹²⁵I-IDAPP labeled more sites (B_max) than ³H-DAMGO ([d-Ala²,N-MePhe⁴,Gly(ol)⁵]-enkephalin) in wild-type mice. In exon 11 knockout mice, ¹²⁵I-IDAPP binding was lowered to levels similar to ³H-DAMGO, which remained relatively unchanged compared to wild-type mice. ¹²⁵I-IDAPP binding was totally lost in an exon 1/exon 11 knockout model lacking all Oprm1 variant expression, confirming that the drug was not cross labeling non-mu opioid receptors. These findings suggested that ¹²⁵I-IDAPP labeled two populations of mu binding sites in wild-type mice, one corresponding to 7TM variants and the second dependent upon 6TM variants. Together, these data indicate that endomorphin analogs represent a unique, genetically defined, and distinct class of mu opioid analgesic.

Downregulation of spinal endomorphin-2 correlates with mechanical allodynia in a rat model of tibia cancer

The endogenous tetrapeptide endomorphin-2 (EM2) participates in pain modulation by binding to pre- and/or post-synaptic μ opioid receptor (MOR). In the present study, pathological expression and antinociceptive effects of EM2 at the spinal level were investigated in a rat model of bone cancer pain. The model was established by introducing Walker 256 mammary gland carcinoma cells into the tibia medullary cavity. Immunohistochemical staining for EM2 showed a markedly reduced EM2-immunoreactivity in the ipsilateral spinal dorsal horn on days 6, 12 and 18 post Walker 256 inoculation (p < 0.05). Intrathecal injection (i.t.) of EM2 significantly attenuated cancer-induced mechanical allodynia (p < 0.05) which could be blocked by β-funaltrexamine (β-FNA), the μ receptor antagonist (p < 0.05). Furthermore, topical application of EM2 dose-dependently inhibited the electrically evoked C-fiber responses and postdischarge of wide dynamic range (WDR) neurons within the spinal cord (p < 0.05), and pretreatment with β-FNA abolished the hyperactivity of these neurons. Compared with the antinociception of morphine which took effect from 40 min to 100 min post application, the analgesic action of EM2 was characterized by quick onset and short-lived efficacy (p < 0.05), being most potent at 10 min and lasting about 20 min. These findings indicate that the down-regulated spinal EM2 is an important contributor to the neuropathological process of bone cancer pain and enhancing activation of EM2/μ receptor signaling might provide a therapeutic alternative to optimizing the treatment of cancer-induced bone pain.

Involvement of mouse μ-opioid receptor splice variants in the spinal antinociception induced by the dermorphin tetrapeptide analog amidino-TAPA

The involvement of the mouse μ-opioid receptor (mMOR-1) splice variants in the antinociceptive effect of intrathecally (i.t.) administered N(α)-amidino-Tyr-D-Arg-Phe-β-Ala (amidino-TAPA) and [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO) was investigated in mice by monitoring the recovery from acute antinociceptive tolerance to amidino-TAPA and DAMGO. A single i.t. pretreatment with DAMGO produced an acute antinociceptive tolerance, which peaked at 2h and disappeared within 5h after the pretreatment. In contrast, a single i.t. pretreatment with amidino-TAPA produced an acute antinociceptive tolerance, which disappeared within 3h after the pretreatment. The concomitant i.t. pretreatment with an antisense oligodeoxynucleotide (ODN) for exon-1, exon-12, exon-13 or exon-14 of mMOR-1 maintained the acute antinociceptive tolerance to amidino-TAPA for 24h after the pretreatment. On the other hand, the concomitant i.t. pretreatment with an antisense ODN for exon-1 of mMOR-1, but not an antisense ODN for exon-12, exon-13 or exon-14 of mMOR-1, maintained the acute antinociceptive tolerance to DAMGO for 24h after the pretreatment. The present results suggest that the spinal antinociception of amidino-TAPA is partially mediated through the activation of the amidino-TAPA-sensitive and DAMGO-insensitive mMOR-1 splice variants MOR-1J, MOR-1K and MOR-1L, which contain the sequence encoded by exon-12, exon-13 and exon-14, respectively.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.