Hélène Beaudry

Increased anxiety-like behaviors in rats experiencing chronic inflammatory pain.

For many patients, chronic pain is often accompanied, and sometimes amplified, by co-morbidities such as anxiety and depression. Although it represents important challenges, the establishment of appropriate preclinical behavioral models contributes to drug development for treating chronic inflammatory pain and associated psychopathologies. In this study, we investigated whether rats experiencing persistent inflammatory pain induced by intraplantar injection of complete Freunds adjuvant (CFA) developed anxiety-like behaviors, and whether clinically used analgesic and anxiolytic drugs were able to reverse CFA-induced anxiety-related phenotypes. These behaviors were evaluated over 28 days in both CFA- and saline-treated groups with a variety of behavioral tests. CFA-induced mechanical allodynia resulted in increased anxiety-like behaviors as evidenced by: (1) a significant decrease in percentage of time spent and number of entries in open arms of the elevated-plus maze (EPM), (2) a decrease in number of central squares visited in the open field (OF), and (3) a reduction in active social interactions in the social interaction test (SI). The number of entries in closed arms in the EPM and the distance traveled in the OF used as indicators of locomotor performance did not differ between treatments. Our results also reveal that in CFA-treated rats, acute administration of morphine (3mg/kg, s.c.) abolished tactile allodynia and anxiety-like behaviors, whereas acute administration of diazepam (1mg/kg, s.c) solely reversed anxiety-like behaviors. Therefore, pharmacological treatment of anxiety-like behaviors induced by chronic inflammatory pain can be objectively evaluated using multiple behavioral tests. Such a model could help identify/validate alternative potential targets that influence pain and cognitive dimensions of anxiety.

Activation of Spinal μ- and δ-Opioid Receptors Potently Inhibits Substance P Release Induced by Peripheral Noxious Stimuli

Over the past few years, δ-opioid receptors (DOPRs) and μ-opioid receptors (MOPRs) have been shown to interact with each other. We have previously seen that expression of MOPR is essential for morphine and inflammation to potentiate the analgesic properties of selective DOPR agonists. In vivo, it is not clear whether MOPRs and DOPRs are expressed in the same neurons. Indeed, it was recently proposed that these receptors are segregated in different populations of nociceptors, with MOPRs and DOPRs expressed by peptidergic and nonpeptidergic fibers, respectively. In the present study, the role and the effects of DOPR- and MOPR-selective agonists in two different pain models were compared. Using preprotachykinin A knock-out mice, we first confirmed that substance P partly mediates intraplantar formalin- and capsaicin-induced pain behaviors. These mice had a significant reduction in pain behavior compared with wild-type mice. We then measured the effects of intrathecal deltorphin II (DOPR agonist) and DAMGO (MOPR agonist) on pain-like behavior, neuronal activation, and substance P release following formalin and capsaicin injection. We found that both agonists were able to decrease formalin- and capsaicin-induced pain, an effect that was correlated with a reduction in the number of c-fos-positive neurons in the superficial laminae of the lumbar spinal cord. Finally, visualization of NK1 (neurokinin 1) receptor internalization revealed that DOPR and MOPR activation strongly reduced formalin- and capsaicin-induced substance P release via direct action on primary afferent fibers. Together, our results indicate that functional MOPRs and DOPRs are both expressed by peptidergic nociceptors.

Differential noxious and motor tolerance of chronic delta opioid receptor agonists in rodents.

In the present study, we asked whether multiple intrathecal injections of deltorphin II, a selective delta opioid receptor (DOPR) agonist, induced DOPR tolerance in three behavioral assays. Unilateral inflammation caused by complete Freunds adjuvant (CFA) injection into the rat or mouse hind paw (CFA model) induced thermal hyperalgesic response that was transiently and dose-dependently reduced by intrathecal administration of deltorphin II or morphine. In both rodent species, the effect of deltorphin II was not modified by a single prior administration of deltorphin II, suggesting an absence of acute tolerance in this paradigm. Repeated administration of intrathecal deltorphin II or s.c. SB-235863 (five consecutive injections over 60 h) also failed to impair the antihyperalgesic response to delta opioid receptor agonist, whereas repeated intrathecal or s.c. injections of morphine induced a significant decrease in the subsequent thermal antihyperalgesic response to morphine. In mice, deltorphin II also induced a rapid, transient motor incoordination/ataxia-like behavior as tested with the accelerating rotarod. In contrast to the antihyperalgesic responses, tolerance to the motoric effect of deltorphin II was evident in mice previously exposed to multiple intrathecal agonist injections, but not multiple saline administrations. Using the tail flick antinociceptive test, we found that DOPR-mediated analgesia was significantly reduced by repeated exposure to deltorphin II. Altogether, these observations suggest that repeated injections of DOPR agonists induce differential tolerance effects on antihyperalgesic, antinociceptive, and motor incoordination/ataxia-like behaviors related to DOPR activation by deltorphin II.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second‐messenger signalling cascades to influence diverse cellular functions in neuronal and non‐neuronal cell types. These receptors activate well‐known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor‐interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

Selective angiotensin II AT2 receptor agonists: Benzamide structure-activity relationships.

In the investigation of the structure-activity relationship of nonpeptide AT(2) receptor agonists, a series of substituted benzamide analogues of the selective nonpeptide AT(2) receptor agonist M024 have been synthesised. In a second series, the biphenyl scaffold was compared to the thienylphenyl scaffold and the impact of the isobutyl substituent and its position on AT(1)/AT(2) receptor selectivity was also investigated. Both series included several compounds with high affinity and selectivity for the AT(2) receptor. Three of the compounds were also proven to function as agonists at the AT(2) receptor, as deduced from a neurite outgrowth assay, conducted in NG108-15 cells.

Pain after Discontinuation of Morphine Treatment Is Associated with Synaptic Increase of GluA4-Containing AMPAR in the Dorsal Horn of the Spinal Cord

Withdrawal from prescribed opioids results in increased pain sensitivity, which prolongs the treatment. This pain sensitivity is attributed to neuroplastic changes that converge at the spinal cord dorsal horn. We have recently reported that repeated morphine administration triggers an insertion of GluA2-lacking (Ca2+-permeable) α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPAR) in the hippocampus. This finding together with the reported involvement of AMPAR in the mechanisms underlying inflammatory pain led us to hypothesize a role for spinal AMPAR in opioid-induced pain behavior. Mice treated with escalating doses of morphine showed hypersensitivity to mechanical stimulation. Intrathecal administration of a Ca2+-permeable AMPAR selective blocker disrupted morphine-induced mechanical sensitivity. Analysis of the expression and phosphorylation levels of AMPAR subunits (GluA1/2/3/4) in homogenates and in postsynaptic density fractions from spinal cord dorsal horns showed an increase in GluA4 expression and phosphorylation in the postsynaptic density after morphine. Co-immunoprecipitation analyses suggested an increase in GluA4 homomers (Ca2+-permeable AMPAR) and immunohistochemical staining localized the increase in GluA4 levels in laminae III–V. The excitatory postsynaptic currents (EPSCs) recorded in laminae III–V showed enhanced sensitivity to Ca2+-permeable AMPAR blockers in morphine-treated mice. Furthermore, current–voltage relationships of AMPAR-mediated EPSCs showed that rectification index (an indicator of Ca2+-permeable AMPAR contribution) is increased in morphine-treated but not in saline-treated mice. These effects could be reversed by infusion of GluA4 antibody through patch pipette. This is the first direct evidence for a role of GluA4-containing AMPAR in morphine-induced pain and highlights spinal GluA4-containing AMPAR as targets to prevent the morphine-induced pain sensitivity.

Regulation of μ and δ opioid receptor functions: involvement of cyclin-dependent kinase 5

Phosphorylation of δ opioid receptors (DOP receptors) by cyclin‐dependent kinase 5 (CDK5) was shown to regulate the trafficking of this receptor. Therefore, we aimed to determine the role of CDK5 in regulating DOP receptors in rats treated with morphine or with complete Freunds adjuvant (CFA). As μ (MOP) and DOP receptors are known to be co‐regulated, we also sought to determine if CDK5‐mediated regulation of DOP receptors also affects MOP receptor functions.

Altered Morphine-Induced Analgesia in Neurotensin Type 1 Receptor Null Mice

Both neurotensin (NT) and opioid agonists have been shown to induce antinociception in rodents after central administration. Besides, previous studies have revealed the existence of functional interactions between NT and opioid systems in the regulation of pain processing. We recently demonstrated that NTS1 receptors play a key role in the mediation of the analgesic effects of NT in long-lasting pain. In the present study, we therefore investigated whether NTS1 gene deletion affected the antinociceptive action of mu opioid drugs. To this end, pain behavioral responses to formalin were determined following systemic administration of morphine in both male and female NTS1 knockout mice. Acute injection of morphine (2 or 5 mg/kg) produced strong antinociceptive effects in both male and female wild-type littermates, with no significant sex differences. On the other hand, morphine analgesia was considerably reduced in NTS1-deficient mice of both sexes compared to their respective controls, indicating that the NTS1 receptor actively participates in mu opioid alleviating pain. By examining specifically the flinching, licking and biting nociceptive behaviors, we also showed that the functional crosstalk between NTS1 and mu opioid receptors influences the supraspinally-mediated behaviors. Interestingly, sexual dimorphic action of morphine-induced pain inhibition was found in NTS1 null mice in the formalin test, suggesting that the endogenous NT system interacts differently with the opioid network in male and female mice. Altogether, these results demonstrated that NTS1 receptor activation operates downstream to the opioidergic transmission and that NTS1-selective agonists combined with morphine may act synergistically to reduce persistent pain.