Use of selective antagonists and antisense oligonucleotides to evaluate the mechanisms of BUBU antinociception

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors

BACKGROUND AND PURPOSE

Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca²⁺ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the μ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown.

EXPERIMENTAL APPROACH

We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous μ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, μ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors.

KEY RESULTS

Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to μ receptors, suggesting that sigma-1R participate in TRPV1-μ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist.

CONCLUSION AND IMPLICATIONS

Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing μ receptor activity.

Cancer-related bone pain is attenuated by a systemically available delta-opioid receptor agonist

Patients with bone cancer report severe pain and receive mu-opioids. We developed a family of peptidomimetic delta-agonists, one of which H2N-Tyr-dVal-Gly-Phe-Ala-OH ([dVal(L)2,Ala(L)5]E) binds with a 1700x affinity at the delta versus mu receptor. To examine the systemic analgesic efficacy of this delta-agonist versus morphine in osteosarcoma pain, osteosarcoma cells are injected into one femur of the anesthetized mouse. After 10-18 days, a decalcification of the injected femur occurs along with a pronounced tactile allodynia. IP morphine and [dVal(L)2,Ala(L)5]E produced a dose-dependent reversal of allodynia with the respective ED50 values being 5.3+/-1.9 mg/kg for morphine and 1.3+/-0.3 mg/kg for [dVal(L)2,Ala(L)5]E. Plotting peak effect versus area under the analgesic curve for doses of morphine and [dVal(L)2,Ala(L)5]E revealed overlapping curves suggesting that for a given effect, [dVal(L)2,Ala(L)5]E produced a similar duration of action as morphine. These effects were reversed by IP naloxone (3 mg/kg). IP naltrindole (1 mg/kg) preferentially reversed [dVal(L)2,Ala(L)5]E. The upper dose effects of morphine but not [dVal(L)2,Ala(L)5]E were limited by pronounced hyperactivity. No other effects were noted. These results show that IP [dVal(L)2,Ala(L)5]E through a delta receptor produces analgesia equal in efficacy to that of morphine but with a 4.5-fold greater potency. Over the doses examined, morphine actions were side effect limited. The delta side effects were not so limited, suggesting a favorable therapeutic ratio for delta-agonists in this pain model. These studies suggest that a systemically delivered delta-opioid agonist has pronounced analgesic properties on a preclinical cancer pain model.

Synthesis and Biological Evaluation of 14-Alkoxymorphinans. 21. Novel 4-Alkoxy and 14-Phenylpropoxy Derivatives of the μ Opioid Receptor Antagonist Cyprodime

The synthesis, biological, and pharmacological evaluation of novel derivatives of cyprodime are described. Their binding affinities at mu, delta, and kappa opioid receptors were evaluated using receptor binding assay. It was observed that the affinity of these compounds was sensitive to the character and length of the substituent in position 4. Further prolongation of the 4-alkoxy group of cyprodime (1) and its 4-butoxy analogue 2 is detrimental for the mu opioid receptor affinity. Introduction of an arylalkoxy group at C-4 does not increase mu affinity in the case of benzyloxy, while a phenylpropoxy group reduces mu affinity. The delta and kappa affinities were also reduced compared to the reference compounds. A significant increase in the affinity at the mu opioid receptors was achieved by introducing a 14-phenylpropoxy group. Increases in the affinity at delta and kappa receptors were also observed. These findings provide further evidence that the nature of the substituent at position 14 has a major impact on the abilities of morphinans to interact with opioid receptors. In the [(35)S]GTPgammaS binding assay, all tested compounds were partial agonists at mu and delta receptors. Compounds 8 and 17 showed antagonism at kappa receptors, while compound 7 exhibited some partial agonist activity at this receptor. The novel derivatives of cyprodime containing a 14-phenylpropoxy group acted as potent antinociceptives. When tested in vivo, compounds 7, 8, and 17 were considerably more potent than morphine, with phenol 7 showing the highest antinociceptive potency (21-fold in the hot plate test, 38-fold in the tail flick test, and 300-fold in the paraphenylquinone writhing test) in mice. Introduction of a 14-phenylpropoxy substituent leads to a profound alteration in the pharmacological profile of this class of compounds.

Lack of the nociceptin receptor does not affect acute or chronic nociception in mice

The peptide nociceptin/orphanin FQ (N/OFQ) and its receptor ORL-1, also designated opioid receptor 4 (OP(4)) are involved in the modulation of nociception. Using OP(4)-knockout mice, we have studied their response following opioid receptor stimulation and under neuropathic conditions.In vas deferens from wild-type and OP(4)-knockout mice, DAMGO (mu/OP(3) agonist), deltorphine II (delta/OP(1) agonist) and (-)-U-50488 (kappa/OP(2) agonist) induced similar concentration-dependent inhibition of electrically-evoked contractions. Naloxone and naltrindole (delta/OP(1) antagonists) shifted the curves of DAMGO (pA(2)=8.6) and deltorphine II (pA(2)=10.2) to the right, in each group. In the hot-plate assay, N/OFQ (10 nmol per mouse, i.t.) increased baseline latencies two-fold in wild-type mice while morphine (10mg/kg, s.c.), deltorphine II (10 nmol per mouse, i.c.v.) and dynorphin A (20 nmol per mouse, i.c.v.) increased hot-plate latencies by about four- to five-fold with no difference observed between wild-type and knockout mice. Furthermore, no change was evident in the development of the neuropathic condition due to chronic constriction injury (CCI) of the sciatic nerve, after both thermal and mechanical stimulation. Altogether these results suggest that the presence of OP(4) receptor is not crucial for (1) the development of either acute or neuropathic nociceptive responses, and for (2) the regulation of full receptor-mediated responses to opioid agonists, even though compensatory mechanisms could not be excluded.

kappa -opioid receptor agonists modulate visceral nociception at a novel, peripheral site of action

kappa-opioid receptor agonists (kappa-ORAs) have been shown to modulate visceral nociception through an interaction with a peripheral, possibly novel, kappa-opioid-like receptor. We used in the present experiments an antisense strategy to further explore the hypothesis that kappa-ORA effects in the colon are produced at a site different from the cloned kappa-opioid receptor (KOR). An antisense oligodeoxynucleotide (ODN) to the cloned rat KOR was administered intrathecally (12.5 microg, twice daily for 4 d) to specifically knock-down the cloned KOR. Efficacy of the KOR antisense ODN treatment was behaviorally evaluated by assessing the antinociceptive effects of peripherally administered kappa- (EMD 61, 753 and U 69,593), mu- (DAMGO) and delta- (deltorphin) ORAs in the formalin test. Intrathecal antisense, but not mismatch ODN blocked the actions of EMD 61,753 and U 69,593 without affecting the actions of DAMGO or deltorphin; a complete recovery of antinociceptive actions of the kappa-ORA EMD 61,753 was observed 10 d after the termination of antisense ODN treatment. In contrast, the ability of EMD 61,753 to dose-dependently attenuate responses of pelvic nerve afferent fibers to noxious colonic distension was unaffected in the same rats in which the antisense ODN effectively knocked-down the KOR as assessed in the formalin test. Additionally, Western blot analysis demonstrated a significant downregulation of KOR protein in the L4-S1 dorsal root ganglia of antisense, but not mismatch ODN-treated rats. The present results support the existence of a non-kappa-opioid receptor site of action localized in the colon.