DAMGO and 6β-glycine substituted 14-O-methyloxymorphone but not morphine show peripheral, preemptive antinociception after systemic administration in a mouse visceral pain model and high intrinsic efficacy in the isolated rat vas deferens

Abstract

Peripheral μ-opioid receptors (MOR) have emerged as important components of inhibitory nociceptive pathways. Here, the antinociceptive effects of MOR agonists, the 6β-glycine derivative of 14-O-methyloxymorphone (HS-731), DAMGO and morphine were evaluated in a mouse model of visceral pain. The abdominal acetic acid-induced writhing test was used to examine the peripheral, preemptive antinociceptive opioid action on visceral nociception. HS-731 administered subcutaneously (s.c.) or intracerebroventricularly (i.c.v.) dose-dependently and completely inhibited writhing, being 24–598-fold more potent, depending on the administration route, than two selective MOR agonists, the enkephalin analogue [d-Ala²,N-Me-Phe⁴,Gly-ol⁵]enkephalin (DAMGO) and morphine. A longer duration of action (2–3 h) was induced by HS-731 given before acetic acid, while shorter effect was produced by morphine (30–60 min) and DAMGO (30–45 min). The antinociceptive effects of systemic opioids were reversed by the s.c. opioid antagonist, naloxone. Blocking of central MOR by the selective MOR antagonist d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP, i.c.v.) resulted in a significant reduction of antinociception of s.c. morphine, whereas it completely failed to antagonize the effects of systemic HS-731 or DAMGO. In in vitro studies, HS-731 and DAMGO, but not morphine showed high intrinsic efficacy, naltrexone-sensitive agonist effect at MOR of the rat vas deferens. These data demonstrate that selective activation of peripheral MOR by systemic s.c. HS-731 or DAMGO produces potent peripheral, preemptive visceral antinociception, while morphine's effects are mediated primarily through central mechanisms. Our findings support the role of peripheral MOR in the pathology of pain states involving sensitization of peripheral nociceptors.

Introduction

Opioids are the cornerstone drugs for the management of moderate to severe pain. Three different receptor classes, μ (MOR), δ (DOR) and κ (KOR), mediate the pharmacological actions of exogenous opioids and interact with endogenous peptides (i.e. enkephalins, dynorphins and endorphins) under physiological conditions [17]. Both, opioid receptors and endogenous opioids are widely distributed throughout the central and peripheral nervous system (CNS and PNS) as well as in peripheral tissues [23].

It is well established that stimulation of central opioid receptors results in analgesia by modulating nociceptive information [13], [17]. However, the central analgesic effects are also associated with a number of adverse actions, including respiratory depression, sedation and physical dependence [26]. Nowadays, an exciting aspect is represented by the substantial evidence emerging from experimental and clinical studies that opioid analgesia is not exclusively mediated by activation of opioid systems within the CNS. Intrinsic pain control can also occur in the periphery [3], [34]. Opioid receptor proteins and their mRNAs are localized in the cell bodies of primary afferent neurons and the occurrence of functional receptors has been demonstrated both on the presynaptic central and peripheral terminals [7], [30], [35]. The exact mechanisms of spinal pain transmission are not very well understood [6], [12], [38], [41]). Activation of peripheral receptors by locally injected opioids has been shown to induce potent and clinically measurable analgesia [36]. All these findings have shed new light on the pharmacological actions of opioid analgesics and the possibility of exploiting these properties.

Medicinal chemistry and opioid pharmacology focused increasingly on exploring the therapeutic potential of the peripheral opioid system by peripheralization of opioid agonists in order to minimize the activation of central opioid receptors [9], [36] aiming for the development of peripheral opioids as novel treatments for pain conditions which are often resistant to conventional drug therapies in human. One strategy to reduce the access of opioids to the CNS includes chemical modifications that increase their hydrophilicity. The 6β-glycine substituted 14-O-methyloxymorphone (HS-731; Fig. 1) is a zwitterionic molecule due to the presence of an amino acid residue (glycine) at C-6 of the morphinan which enhances hydrophilicity and restricts penetration across the blood–brain barrier [31]. Our earlier reports have described HS-731 as a potent opioid agonist with selectivity for the MOR [33]. Systemically (subcutaneously, s.c.) administered HS-731 produces peripherally mediated antinociceptive effect in rat models of acute nociception (tail-flick test) and inflammatory pain (formalin test, carrageenan-induced hyperalgesia) [5], [14]. Furthermore, this is the first peripherally restricted opioid described to be effective after oral administration by reversing mechanical hyperalgesia in rats with carrageenan-induced inflammation [5]. These positive results made HS-731 a good candidate for further in vitro and in vivo pharmacological investigations to delineate its selective peripheral action.

The aim of this study was first to evaluate the potential antinociceptive effect of the MOR agonist HS-731 in a mouse model of visceral pain induced by intraperitoneal (i.p.) administration of acetic acid, and whether these effects are mediated via peripheral opioid receptors. In order to distinguish the central or peripheral site of actions of systemic HS-731, and the two selective MOR agonists, the enkephalin analogue, [d-Ala²,N-Me-Phe⁴,Gly-ol⁵]enkephalin (DAMGO), as well as the classical and clinically relevant opioid analgesic, morphine we examined the sensitivity of their antinociceptive actions to antagonism by intracerebroventricular (i.c.v.) d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP), a highly selective MOR antagonist that should not cross the blood–brain barrier readily [20]. Using the abdominal writhing model, we also planned to examine the preemptive opioid antinociceptive actions on visceral nociception, a clinically significant condition.

Additionally, we tested the activity of HS-731 in rat vas deferens (RVD) containing low opioid-like receptor reserve [32] and compare its effect with DAMGO and morphine.