Pharmacological characterization of EN-9, a novel chimeric peptide of endomorphin-2 and neuropeptide FF that produces potent antinociceptive activity and limited tolerance

G(1-5)-EM2, a multi-targeted agonist to opioid and growth hormone secretagogue receptors exhibited nontolerance forming antinociceptive effects in a mouse model of burn pain

Burn induced-pain (BIP) is one of the most common pain symptoms, which seriously affects the quality of sufferer life. Researches show that multi-targeted drug therapies offer superior efficacy and fewer side effects compared to single-target drug therapies. Consequently, in this study, we developed G(1-5)-EM2, a multi-targeted peptide designed to target μ-opioid receptor and the growth hormone secretagogue receptor 1α (GHS-R1α), and explored its antinociceptive effects on burn injury pain. Calcium mobilization experiments revealed that G(1-5)-EM2 demonstrated weak multi-agonist activities to μ-opioid receptor and κ-opioid receptor as well as GHS-R1α in vitro. Near-infrared fluorescence imaging experiments demonstrated that G(1-5)-EM2 could penetrate the blood-brain barrier (BBB) and access the brain following intravenous injection. The enzymatic stability of G(1-5)-EM2 was significantly enhanced compared to EM2. Our results indicated that intrathecal administration of G(1-5)-EM2 mitigated mechanical allodynia and thermal hyperalgesia in BIP. These antinociceptive effects of G(1-5)-EM2 were partially mediated through μ-opioid receptor and GHS-R1α. Moreover, intrathecal administration of G(1-5)-EM2 significantly decreased burn-induced up-regulation of phosphorylated p38 MAPK, phosphorylated NF-κBp65 and TRPV1 in the ipsilateral spinal cord, reduced the levels of IL-1β, IL-6 and TNF-α in serum, and enhanced wound healing in burned skin. Repeated intrathecal administration of G(1-5)-EM2 produced a non-tolerance-forming antinociception in BIP. These results suggest that the multi-targeted peptide G(1-5)-EM2 exhibits a novel role in alleviating BIP with fewer side effects and may represent a promising strategy for developing new analgesic drugs.

Novel chimeric peptides based on endomorphins and ghrelin receptor antagonist produced supraspinal antinociceptive effects with reduced acute tolerance in mice

It is widely recognized that developing bi- or multifunctional opioid compounds could offer a valuable approach to pain management with fewer side effects compared to single-target compounds. In this study, we designed and characterized two novel chimeric peptides, EM-1-DLS and EM-2-DLS, incorporating endomorphins (EMs) and the ghrelin receptor antagonist [D-Lys3]-GHRP-6 (DLS). Functional assays demonstrated that EM-1-DLS and EM-2-DLS acted as κ-opioid receptor (κ-OR)-preferring agonists, weak μ-opioid receptors (μ-OR) and ghrelin receptor (GHSR) agonists. Upon intracerebroventricular (i.c.v.) administration in mice, both EM-1-DLS and EM-2-DLS exhibited dose- and time-dependent antinociceptive effects in the tail withdrawal test. EM-1-DLS demonstrated the highest antinociceptive potency among the peptides, with an ED50 approximately 8-fold greater than EM-1, while EM-2-DLS showed comparable effects to EM-2. The antinociceptive actions of EM-1-DLS involved activation of GHS-R1α, μ-OR, and κ-OR, whereas EM-2-DLS acted via GHS-R1α, δ-OR, and κ-OR pathways. Additionally, acute antinociceptive tolerance was investigated, revealing that EM-1-DLS induced a tolerance ratio of 2.33-fold, significantly lower than the 5.19-fold ratio induced by EM-1. Cross-tolerance ratios between the chimeric peptides and EMs ranged from 0.92 to 1.76, indicating reduced tolerance compared to EMs alone. These findings highlight the potential of these chimeric peptides to mitigate pain with diminished tolerance development, suggesting a promising strategy for the development of new analgesic therapies with improved safety profiles.

Neuropeptide FF prevented histamine- or chloroquine-induced acute itch behavior through non-NPFF receptors mechanism in male mice

The neuropeptide FF (NPFF) system regulates various physiological and pharmacological functions, particularly pain modulation. However, the modulatory effect of NPFF system on itch remains unclear. To investigate the modulatory effect and functional mechanism induced by NPFF system on acute itch, we examined the effects of supraspinal administration of NPFF and related peptides on acute itch induced by intradermal (i.d.) injection of histamine or chloroquine in male mice. Our results indicated that intracerebroventricular (i.c.v.) administration of NPFF dose-dependently prevented histamine- or chloroquine-induced acute itch behaviors. In addition, the modulatory effect of NPFF was not affected by the selective NPFF receptor antagonist RF9. Furthermore, we investigated the effects of NPVF and dNPA, the selective agonists of NPFF1 and NPFF2 receptors respectively, on the acute itch. The results demonstrated that both NPFF agonists effectively prevented acute itch induced by histamine or chloroquine in a manner similar to NPFF, and their effects were also not modified by RF9. To further investigate the possible mechanism of the NPFF receptors agonists, the NPFF-derived analogues [Phg8]-NPFF and NPFF(1-7)-NH2 that could not activate NPFF receptors in cAMP assays were subsequently tested in the acute itch model. Interestingly, [Phg8]-NPFF, but not NPFF(1-7)-NH2, prevented acute itch behavior after i.c.v. administration. In conclusion, our findings reveal that NPFF and related peptides prevent histamine- and chloroquine-induced acute itch through a NPFF receptor-independent mechanism. And it was revealed that the C-terminal phenyl structure of NPFF may play a crucial role in these modulatory effects on acute itch.

Multitarget ligands that comprise opioid/nonopioid pharmacophores for pain management: Current state of the science

Chronic pain, which affects more than one-third of the world's population, represents one of the greatest medical challenges of the 21st century, yet its effective management remains sub-optimal. The 'gold standard' for the treatment of moderate to severe pain consists of opioid ligands, such as morphine and fentanyl, that target the µ-opioid receptor (MOP). Paradoxically, these opioids also cause serious side effects, including constipation, respiratory depression, tolerance, and addiction. In addition, the development of opioid-use disorders, such as opioid diversion, misuse, and abuse, has led to the current opioid crisis, with dramatic increases in addiction, overdoses, and ultimately deaths. As pain is a complex, multidimensional experience involving a variety of pathways and mediators, dual or multitarget ligands that can bind to more than one receptor and exert complementary analgesic effects, represent a promising avenue for pain relief. Indeed, unlike monomodal therapeutic approaches, the modulation of several endogenous nociceptive systems can often result in an additive or even synergistic effect, thereby improving the analgesic-to-side-effect ratio. Here, we provide a comprehensive overview of research efforts towards the development of dual- or multi-targeting opioid/nonopioid hybrid ligands for effective and safer pain management. We reflect on the underpinning discovery rationale by discussing the design, medicinal chemistry, and in vivo pharmacological effects of multitarget antinociceptive compounds.

Analgesic tolerance and cross-tolerance to the bifunctional opioid/neuropeptide FF receptors agonist EN-9 and μ-opioid receptor ligands at the supraspinal level in mice

The chimeric peptide EN-9 was reported as a κ-opioid/neuropeptide FF receptors bifunctional agonist that modulated chronic pain with no tolerance. Many lines of evidence have shown that the effect of the κ-opioid receptor is mediated by not only its specific activation but also downstream events participation, especially interaction with the μ-opioid receptor pathway in antinociception and tolerance on most occasions. The present study investigated the acute and chronic cross-tolerance of EN-9 with μ-opioid receptor agonist EM-2, DAMGO, and morphine after intracerebroventricularly (i.c.v) injection in the mouse tail-flick test. In the acute tolerance test, EN-9 showed symmetrical acute cross-tolerance to DAMGO but no cross-tolerance to EM2. In the chronic tolerance test, EN-9 had no tolerance after eight days of repeated administration. However, EN-9 illustrated complete cross-tolerance to morphine and symmetrical cross-tolerance to EM2. In addition, inhibition of NPFF receptor could induce the tolerance development of EN-9. These findings indicated that supraspinal EN-9-induced antinociception contains additional components, which are mediated by the downstream μ-opioid receptor pathway both in acute and chronic treatment, whereas the subtypes of μ-opioid receptor or NPFF system pathway involved in antinociceptive effects induced by EN-9 are complex. Identifying the receptor mechanism could help design preferable bifunctional opioid compounds.

OFP011 Cyclic Peptide as a Multifunctional Agonist for Opioid/Neuropeptide FF Receptors with Improved Blood-Brain Barrier Penetration

Mounting evidence indicates that the neuropeptide FF (NPFF) system is involved in the side effects of opioid usage, including antinociceptive tolerance, hyperalgesia, abuse, constipation, and respiratory depression. Our group recently discovered that the multitarget opioid/NPFF receptor agonist DN-9 exhibits peripheral antinociceptive activity. To improve its metabolic stability, antinociceptive potency, and duration, in this study, we designed and synthesized a novel cyclic disulfide analogue of DN-9, OFP011, and examined its bioactivity through in vitro cyclic adenosine monophosphate (cAMP) functional assays and in vivo behavioral experiments. OFP011 exhibited multifunctional agonistic effects at the μ-opioid and the NPFF₁ and NPFF₂ receptors and partial agonistic effects at the δ- and κ-opioid in vitro, as determined via the cAMP functional assays. Pharmacokinetic and pharmacological experiments revealed improvement in its blood-brain barrier permeability after systemic administration. In addition, subcutaneous OFP011 exhibited potent and long-lasting antinociceptive activity via the central μ- and κ-opioid receptors, as observed in different physiological and pathological pain models. At the highest antinociceptive doses, subcutaneous OFP011 exhibited limited tolerance, gastrointestinal transit, motor coordination, addiction, reward, and respiration depression. Notably, OFP011 exhibited potent oral antinociceptive activities in mouse models of acute, inflammatory, and neuropathic pain. These results suggest that the multifunctional opioid/NPFF receptor agonists with improved blood-brain barrier penetration are a promising strategy for long-term treatment of moderate to severe nociceptive and pathological pain with fewer side effects.

Novel Cyclic Endomorphin Analogues with Multiple Modifications and Oligoarginine Vector Exhibit Potent Antinociception with Reduced Opioid-like Side Effects

Endomorphins (EMs) are potent pharmaceuticals for the treatment of pain. Herein, we investigated several novel EM analogues with multiple modifications and oligoarginine conjugation. Our results showed that analogues 1-6 behaved as potent μ-opioid agonists and enhanced stability and lipophilicity. Analogues 5 and 6 administered centrally and peripherally induced significant and prolonged antinociceptive effects in acute pain. Both analogues also produced long-acting antiallodynic effects against neuropathic and inflammatory pain. Furthermore, they showed a reduced acute antinociceptive tolerance. Analogue 6 decreased the extent of chronic antinociceptive tolerance, and analogue 5 exhibited no tolerance at the supraspinal level. Particularly, they displayed nontolerance-forming antinociception at the peripheral level. In addition, analogues 5 and 6 exhibited reduced or no opioid-like side effects on gastrointestinal transit, conditioned place preference (CPP), and motor impairment. The present investigation established that multiple modifications and oligoarginine-vector conjugation of EMs would be helpful in developing novel analgesics with fewer side effects.

Anti-PD-1 treatment impairs opioid antinociception in rodents and nonhuman primates

Emerging immunotherapies with monoclonal antibodies against programmed cell death protein-1 (PD-1) have shown success in treating cancers. However, PD-1 signaling in neurons is largely unknown. We recently reported that dorsal root ganglion (DRG) primary sensory neurons express PD-1 and activation of PD-1 inhibits neuronal excitability and pain. Opioids are mainstay treatments for cancer pain, and morphine produces antinociception via mu opioid receptor (MOR). Here, we report that morphine antinociception and MOR signaling require neuronal PD-1. Morphine-induced antinociception after systemic or intrathecal injection was compromised in Pd1 ^-/- mice. Morphine antinociception was also diminished in wild-type mice after intravenous or intrathecal administration of nivolumab, a clinically used anti-PD-1 monoclonal antibody. In mouse models of inflammatory, neuropathic, and cancer pain, spinal morphine antinociception was compromised in Pd1 ^-/- mice. MOR and PD-1 are coexpressed in sensory neurons and their axons in mouse and human DRG tissues. Morphine produced antinociception by (i) suppressing calcium currents in DRG neurons, (ii) suppressing excitatory synaptic transmission, and (iii) inducing outward currents in spinal cord neurons; all of these actions were impaired by PD-1 blockade in mice. Loss of PD-1 also enhanced opioid-induced hyperalgesia and tolerance and potentiates opioid-induced microgliosis and long-term potentiation in the spinal cord in mice. Last, intrathecal infusion of nivolumab inhibited intrathecal morphine-induced antinociception in nonhuman primates. Our findings demonstrate that PD-1 regulates opioid receptor signaling in nociceptive neurons, leading to altered opioid-induced antinociception in rodents and nonhuman primates.

Spinal endomorphins attenuate burn-injury pain in male mice by inhibiting p38 MAPK signaling pathway through the mu-opioid receptor

Burn injury is one of the main causes of mortality worldwide and frequently associated with severe and long-lasting pain that compromises the quality of patient life. Several studies have shown that the mu-opioid system plays an important role in burn pain relief. In this study, we investigated the spinal antinociception induced by the endogenous mu-opioid receptor (MOR) agonists endomorphins and explored their mechanisms of actions in burn injury-induced pain model. Our results showed that intrathecal injection of endomorphin-1 and -2 dose-dependently attenuated mechanical allodynia and thermal hyperalgesia via the mu-opioid receptor in mice on day 3 after burn injury, which was consistent with the data obtained from the mu-opioid receptor knockout mice. Western blot showed that the phosphorylation levels of extracellular signal-regulated kinase1/2 (ERK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) in ipsilateral spinal cord tissues were significantly up-regulated after burn injury. Intrathecal injection of endomorphins selectively inhibited the activation of p38 MAPK on day 3 after burn injury via the mu-opioid receptor. Further studies found that repeated application of the specific p38 MAPK inhibitor SB203580 dose-dependently inhibited burn-injury pain, as well as the activation of spinal p38 MAPK. Taken together, our present study demonstrates that intrathecal injection of endomorphins attenuates burn-injury pain in male mice by affecting the spinal activation of p38 MAPK via the mu-opioid receptor.

Chemical similarity assisted search for acetylcholinesterase inhibitors: Molecular modeling and evaluation of their neuroprotective properties

Alzheimer's disease (AD) is an obstinate and progressive neurodegenerative disorder, mainly characterized by cognitive decline. Increasing number of AD patients and the lack of promising treatment strategies demands novel therapeutic agents to combat various disease pathologies in AD. Recent progresses in understanding molecular mechanisms in AD helped researchers to streamline the various therapeutic approaches. Inhibiting acetylcholinesterase (AChE) activity has emerged as one of the potential treatment strategies. The present study discusses the identification of two potent AChE inhibitors (ZINC11709541 and ZINC11996936) from ZINC database through conventional in silico approaches and their in vitro validations. These inhibitors have strong preferences towards AChE than butyrylcholinesterase (BChE) and didn't evoke any significant reduction in the cell viability of HEK-293 cells and primary cortical neurons. Furthermore, promising neuroprotective properties has also been displayed against glutamate induced excitotoxicity in primary cortical neurons. The present study proposes two potential drug lead compounds for the treatment of AD, that can be used for further studies and preclinical evaluation.

Synthesis and Biological Characterization of Cyclic Disulfide-Containing Peptide Analogs of the Multifunctional Opioid/Neuropeptide FF Receptor Agonists That Produce Long-Lasting and Nontolerant Antinociception

In a previously described chimeric peptide, we reported that the multifunctional opioid/neuropeptide FF (NPFF) receptor agonist 0 (BN-9) produced antinociception for 1.5 h after supraspinal administration. Herein, four cyclic disulfide analogs containing l- and/or d-type cysteine at positions 2 and 5 were synthesized. The cyclized analogs and their linear counterparts behaved as multifunctional agonists at both opioid and NPFF receptors in vitro and produced potent analgesia without tolerance development. In comparison to 0, cyclized peptide 6 exhibited sevenfold more potent μ-opioid receptor agonistic activity in vitro. Interestingly, the cyclized analog 6 possessed an improved stability in the brain and an increased blood-brain barrier permeability compared to the parent peptide 0 and produced more potent analgesia after supraspinal or subcutaneous administration with improved duration of action of 4 h. In addition, antinociceptive tolerance of analog 6 was greatly reduced after subcutaneous injection compared to fentanyl, as was the rewarding effect, withdrawal reaction, and gastrointestinal inhibition.

The spinal anti-allodynic effects of endomorphin analogs with C-terminal hydrazide modification in neuropathic pain model

The present study was undertaken to further investigate the spinal anti-allodynic effects of endomorphins (EMs) and their C-terminal hydrazide modified analogs EM-1-NHNH₂ and EM-2-NHNH₂ in the spared nerve injury (SNI) model of neuropathic pain in mice. Our results demonstrated that intrathecal (i.t.) administration of endomorphin-1 (EM-1), endomorphin-2 (EM-2), EM-1-NHNH₂ and EM-2-NHNH₂ produced potent anti-allodynic effects ipsilaterally in neuropathic pain model. Judging from the area under the curve (AUC) values, these two analogs exhibited higher antinociception than their parent peptides. Moreover, they also displayed significant antinociceptive effects in the contralateral paw administered intrathecally. Interestingly, EM-1 and its analog EM-1-NHNH₂ displayed their antinociception probably by μ₂-opioid receptor subtype since the μ₁-opioid receptor antagonist naloxonazine didn't significantly block the anti-allodynia of EM-1 and EM-1-NHNH₂, which implied a same opioid mechanism. However, the anti-allodynia induced by EM-2, but not EM-2-NHNH₂ was significantly reduced by both μ₁-opioid antagonist, naloxonazine and κ-antagonist, nor-binaltorphamine (nor-BNI), indicating multiple opioid receptors were involved in the anti-allodynic effects of EM-2. Most importantly, EM-1-NHNH₂ decreased the antinociceptive tolerance, and EM-2-NHNH₂ displayed non-tolerance-forming antinociception. Therefore, C-terminal amide to hydrazide conversion changed the spinal antinociceptive profiles of EMs in neuropathic pain. The present investigation is of great value in the development of novel opioid therapeutics against neuropathic pain.

Spinal administration of the multi-functional opioid/neuropeptide FF agonist BN-9 produced potent antinociception without development of tolerance and opioid-induced hyperalgesia

Chronic opioids treatment is impeded by the development of analgesic tolerance and opioid-induced hyperalgesia. Recent studies have shown that multi-functional opioid compounds produce analgesic activities with limited side effects. We developed a novel multi-functional peptide targeting opioid and neuropeptide FF receptors named BN-9, which produced potent and non-tolerance forming antinociceptive effect after supraspinal and systemic administrations. In the present study, the analgesic properties and potential side effects of intrathecal BN-9 were investigated in a range of preclinical rodent models. In complete Freund's adjuvant-induced inflammatory pain model, intrathecal BN-9 dose-dependently produced analgesic effect via opioid receptors, and the spinal antinociceptive effect was augmented by the neuropeptide FF receptor antagonist RF9. In contrast, in plantar incision-induced postoperative pain model, BN-9 exhibited potent anti-allodynic effect via opioid receptors and, at least partially, neuropeptide FF receptors. In mouse models of acetic acid-induced visceral pain and formalin pain, BN-9-induced spinal antinociception was mainly mediated by opioid receptors, independent of neuropeptide FF receptors. Furthermore, at the spinal level, chronic treatments with BN-9 did not lead to analgesic tolerance and cross-tolerance to morphine. Moreover, opioid-induced hyperalgesia was observed after repeated administration of morphine, but not BN-9. Taken together, our present study suggests that intrathecal BN-9 produces potent and non-tolerance forming antinociception, and does not cause opioid-induced hyperalgesia. Thus, BN-9 might serve as a promising lead compound in the development of multi-functional opioid analgesics with minimized side effects.

Neuropeptide FF and Its Receptors: Therapeutic Applications and Ligand Development

The endogenous neuropeptide FF (NPFF) and its two cognate G protein-coupled receptors, Neuropeptide FF Receptors 1 and 2 (NPFFR1 and NPFFR2), represent a relatively new target system for many therapeutic applications including pain regulation, modulation of opioid side effects, drug reward, anxiety, cardiovascular conditions, and other peripheral effects. Since the cloning of NPFFR1 and NPFFR2 in 2000, significant progress has been made to understand their pharmacological roles and interactions with other receptor systems, notably the opioid receptors. A variety of NPFFR ligands with different mechanisms of action (agonists or antagonists) have been discovered although with limited subtype selectivities. Differential pharmacological effects have been observed for many of these NPFFR ligands, depending on assays/models employed and routes of administration. In this Perspective, we highlight the therapeutic potentials, current knowledge gaps, and latest updates of the development of peptidic and small molecule NPFFR ligands as tool compounds and therapeutic candidates.

Analgesic activities of the mixed opioid and NPFF receptors agonist DN-9 in a mouse model of formalin-induced orofacial inflammatory pain

Orofacial pain is one of the most common pain conditions and compromises the quality of life of the sufferer. Several studies have shown that opioid agonists produced significant analgesia in the orofacial pain, and combination of opioids with drugs belonging to other classes induced synergism in the orofacial pain. However, combination therapy of different analgesic drugs improves the risk of drug-drug interactions. Against this background, we sought to investigate the analgesic effects of the multi-functional opioid peptide DN-9, a mixed opioid and NPFF receptors agonist that produced robust analgesia in acute and inflammatory pain models, on formalin-induced orofacial pain. Our results showed that formalin injection caused significant spontaneous pain behaviors and increased the expressions of the mu-opioid receptor, c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK1/2) in the ipsilateral trigeminal ganglion (TG). In mice pretreated with DN-9, there was a significant reduction in nociceptive behaviors, which was selectively mediated by the mu- and kappa-opioid receptors, independently of the NPFF system. Four hours after formalin injection, the level of c-Fos immunoreactivity in the ipsilateral TG neurons was much lower in mice pretreated with DN-9 or morphine. In addition, DN-9 exhibited a significant inhibition in the expression of p-ERK1/2, which was reversed by the selective antagonists of the mu- and kappa-opioid receptors. In conclusion, our present results demonstrate that central administration of DN-9 produces potential antinociceptive effects via the mu- and kappa-opioid receptors, independently of the NPFF system, and this antinociceptive action is tightly linked with the intracellular ERK activation in TG neurons.

Preemptive intrathecal administration of endomorphins relieves inflammatory pain in male mice via inhibition of p38 MAPK signaling and regulation of inflammatory cytokines

BACKGROUND

Preemptive administration of analgesic drugs reduces perceived pain and prolongs duration of antinociceptive action. Whereas several lines of evidence suggest that endomorphins, the endogenous mu-opioid agonists, attenuate acute and chronic pain at the spinal level, their preemptive analgesic effects remain to be determined. In this study, we evaluated the anti-allodynic activities of endomorphins and explored their mechanisms of action after preemptive administration in a mouse model of inflammatory pain.

METHODS

The anti-allodynic activities of preemptive intrathecal administration of endomorphin-1 and endomorphin-2 were investigated in complete Freund's adjuvant (CFA)-induced inflammatory pain model and paw incision-induced postoperative pain model. The modulating effects of endomorphins on the expression of p38 mitogen-activated protein kinase (p38 MAPK) and inflammatory mediators in dorsal root ganglion (DRG) of CFA-treated mice were assayed by real-time reverse transcription-polymerase chain reaction (RT-PCR), Western blotting, or immunofluorescence staining.

RESULTS

Preemptive intrathecal injection of endomorphins dose-dependently attenuated CFA-induced mechanical allodynia via the mu-opioid receptor and significantly reversed paw incision-induced allodynia. In addition, CFA-caused increase of phosphorylated p38 MAPK in DRG was dramatically reduced by preemptive administration of endomorphins. Repeated intrathecal application of the specific p38 MAPK inhibitor SB203580 reduced CFA-induced mechanical allodynia as well. Further RT-PCR assay showed that endomorphins regulated the mRNA expression of inflammatory cytokines in DRGs induced by peripheral inflammation.

CONCLUSIONS

Our findings reveal a novel mechanism by which preemptive treatment of endomorphins attenuates inflammatory pain through regulating the production of inflammatory cytokines in DRG neurons via inhibition of p38 MAPK phosphorylation.

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.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

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.