The combination of opioid and neurotensin receptor agonists improves their analgesic/adverse effect ratio

Abstract

Opioid and neurotensin (NT) receptors are expressed in both central and peripheral nervous systems where they modulate nociceptive responses. Nowadays, opioid analgesics like morphine remain the most prescribed drugs for the treatment of moderate to severe pain. However, despite their daily used, opioids can produce life-threatening side effects, such as constipation or respiratory depression. Besides, NT analogs exert strong opioid-independent analgesia. Here, we thus hypothesized that the combined use of opioid and NT agonists would require lower doses to produce significant analgesic effects, hence decreasing opioid-induced adverse effects. We used isobologram analyses to determine if the combination of a NT brain-penetrant analog, An2-NT(8-13) with morphine results in an inhibitory, synergistic or additive analgesic response. We found that intravenous administration of An2-NT(8-13) reduced by 90% the nocifensive behaviors induced by formalin injection, at the dose of 0.018 mg/kg. Likewise, subcutaneous morphine reduced pain by 90% at 1.8 mg/kg. Importantly, isobologram analyses revealed that the co-injection of An2-NT(8-13) with morphine induced an additive analgesic response. We finally assessed the effects of morphine and An2-NT(8-13) on the gastrointestinal tract motility using the charcoal meal test. As opposed to morphine which significantly reduced the intestinal motility at the analgesic effective dose of 1.8 mg/kg, An2-NT(8-13) did not affect the charcoal meal intestinal transit at 0.018 mg/kg. Interestingly, at the dose providing 90% pain relief, the co-administration of morphine with An2-NT(8-13) had a reduced effect on constipation. Altogether, these results suggest that combining NT agonists with morphine may improve its analgesic/adverse effect ratio.

Introduction

Nowadays, opioids remain the most prescribed analgesics to treat moderate to severe pain (Gomes et al., 2014, Munzing, 2017). Morphine, the prototypical opioid, mediates its actions mainly through the Mu opioid receptor (MOP). In addition to analgesia, morphine also produces a number of unwanted and debilitating effects such as drowsiness, constipation, nausea, respiratory depression, and tolerance (Benyamin et al., 2008). Analgesic tolerance to morphine develops such that the dose needed to maintain a given level of analgesia has to be constantly increased. In order to limit the development of analgesic tolerance and concomitant unwanted effects, drug combination represents a promising alternative to currently used opioids. Indeed, combining an opioid with another analgesic drug might reduce the unwanted effects normally associated to each individual drug. This was indeed found to be true when morphine was intrathecally injected with dexmedetomidine, a selective α2-adrenergic receptor agonist in a rat model of neuropathic pain (Kabalak et al., 2013). Similarly, the combined use of morphine and monoamine reuptake inhibitors was found to potentiate their respective analgesic effects in the rat formalin model (Shen et al., 2013).

We and others have shown that neurotensin (NT) can produce significant pain relief in various animal models of acute and chronic pain (Boules et al., 2006, Demeule et al., 2014, Fanelli et al., 2015, Fantegrossi et al., 2005, Guillemette et al., 2012, Roussy et al., 2008, Tétreault et al., 2013). First identified for its hypotensive effects, NT is also know to participate in a wide variety of physiological functions, including modulation of body temperature, myocardial performance, gastrointestinal tract motility and secretion (Osadchii, 2015, Zhao and Pothoulakis, 2006). According to its wide distribution throughout the central nervous system (Sarret and Beaudet, 2002), NT was also shown to exert brain functions, such as antipsychotic- and psychostimulant-like effects as well as regulation of food intake (Dobner, 2005, Dobner et al., 2003, Mazella et al., 2012). The physiological effects of NT are mediated by the activation of two closely related G protein-coupled receptors (GPCRs), namely NTS1 and NTS2 (Vincent et al., 1999). Importantly, NT-mediated analgesia was found to be independent of the opioid system. There is indeed compelling evidence demonstrating that opioid antagonists do not suppress NT-induced antinociception (Behbehani and Pert, 1984, Clineschmidt et al., 1982, Osbahr et al., 1981). Furthermore, NT also appears to be more potent than morphine in various pain tests (Nemeroff et al., 1979, al-Rodhan et al., 1991).

In the present study, we hypothesized that combining opioid and NT peptides will present some advantages over pure opioid-based therapies. We thus undertook to determine whether systemically administered morphine combined to An2-NT(8-13), consisting of the brain-penetrant peptide Angiopep-2 (An2) conjugated to NT(8-13) (Demeule et al., 2014), can exert additive or synergistic antinociceptive interaction. We therefore evaluated the analgesic effects of combining morphine and An2-NT(8-13) in the rat model of formalin-induced persistent pain, and determined the extent of drug interaction using isobolograms. We further determined whether this drug combination can reduce opioid-induced constipation or NT-induced drop in blood pressure, thus leading to a better analgesic benefit/adverse effect ratio.