Intrathecal lamotrigine attenuates antinociceptive morphine tolerance and suppresses spinal glial cell activation in morphine-tolerant rats

Phenytoin Decreases Pain-like Behaviors and Improves Opioid Analgesia in a Rat Model of Neuropathic Pain

Neuropathic pain remains a clinical challenge due to its complex and not yet fully understood pathomechanism, which result in limited analgesic effectiveness of the management offered, particularly for patients with acute, refractory neuropathic pain states. In addition to the introduction of several modern therapeutic approaches, such as neuromodulation or novel anti-neuropathic drugs, significant efforts have been made in the repurposing of well-known substances such as phenytoin. Although its main mechanism of action occurs at sodium channels in excitable and non-excitable cells and is well documented, how the drug affects the disturbed neuropathic interactions at the spinal cord level and how it influences morphine-induced analgesia have not been clarified, both being crucial from a clinical perspective. We demonstrated that single and repeated systemic administrations of phenytoin decreased tactile and thermal hypersensitivity in an animal model of neuropathic pain. Importantly, we observed an increase in the antinociceptive effect on thermal stimuli with repeated administrations of phenytoin. This is the first study to report that phenytoin improves morphine-induced antinociceptive effects and influences microglia/macrophage activity at the spinal cord and dorsal root ganglion levels in a neuropathic pain model. Our findings support the hypothesis that phenytoin may represent an effective strategy for neuropathic pain management in clinical practice, particularly when combination with opioids is needed.

Modulation of Morphine Analgesia, Antinociceptive Tolerance, and Mu-Opioid Receptor Binding by the Cannabinoid CB2 Receptor Agonist O-1966

Acutely, non-selective cannabinoid (CB) agonists have been shown to increase morphine antinociceptive effects, and we and others have also demonstrated that non-selective CB agonists attenuate morphine antinociceptive tolerance. Activation of cannabinoid CB2 receptors reverses allodynia and hyperalgesia in models of chronic pain, and co-administration of morphine with CB2 receptor selective agonists has been shown to be synergistic. CB2 receptor activation has also been shown to reduce morphine-induced hyperalgesia in rodents, an effect attributed to CB2 receptor modulation of inflammation. In the present set of experiments, we tested both the acute and chronic interactions between morphine and the CB2 receptor selective agonist O-1966 treatments on antinociception and antinociceptive tolerance in C57Bl6 mice. Co-administration of morphine and O-1966 was tested under three dosing regimens: simultaneous administration, morphine pre-treated with O-1966, and O-1966 pre-treated with morphine. The effects of O-1966 on mu-opioid receptor binding were determined using [3H]DAMGO and [35S]GTPγS binding assays, and these interactions were further examined by FRET analysis linked to flow cytometry. Results yielded surprising evidence of interactions between the CB2 receptor selective agonist O-1966 and morphine that were dependent upon the order of administration. When O-1966 was administered prior to or simultaneous with morphine, morphine antinociception was attenuated and antinociceptive tolerance was exacerbated. When O-1966 was administered following morphine, morphine antinociception was not affected and antinociceptive tolerance was attenuated. The [35S]GTPγS results suggest that O-1966 interrupts functional activity of morphine at the mu-opioid receptor, leading to decreased potency of morphine to produce acute thermal antinociceptive effects and potentiation of morphine antinociceptive tolerance. However, O-1966 administered after morphine blocked morphine hyperalgesia and led to an attenuation of morphine tolerance, perhaps due to well-documented anti-inflammatory effects of CB2 receptor agonism.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

The effect of propofol on intrathecal morphine-induced pruritus and its mechanism

BACKGROUND

Previous studies have shown that a low dose of propofol IV bolus had a beneficial effect on intrathecal morphine-induced pruritus in humans. However, its exact mechanism has not been fully understood. In this study, we hypothesized that propofol relieved intrathecal morphine-induced pruritus in rats by upregulating the expression of cannabinoid-1 (CB[1]) receptors in anterior cingulate cortex (ACC).

METHODS

Twenty-four Sprague-Dawley rats were divided into a control group and 20, 40, 80 μg/kg morphine groups to create an intrathecal morphine-induced scratching model. The effects of propofol on intrathecal 40 μg/kg morphine-induced scratching responses were then evaluated. Sixty rats were randomly assigned to control, normal saline, intralipid, and propofol groups, with pruritus behavior observation or killed 8 minutes after venous injection of normal saline, intralipid, or propofol, and brain tissues were then collected for assay. Immunohistochemistry was then performed to identify the expression of CB (1) receptor in ACC, and the concentration of CB(1) receptor in ACC was determined by Western blot analysis.

RESULTS

Compared with the control group, rats in the 20, 40, 80 μg/kg morphine groups had higher mean scratching response rates after intrathecal morphine injection (P =0.020, 0.005, and 0.002, respectively). There was a statistical difference between 20 and 40 μg/kg morphine groups at 10 to 15 and 15 to 20 timepoints after intrathecal morphine injection (P = 0.049 and 0.017, respectively). Propofol almost abolished the scratching response that was induced by 40 μg/kg intrathecal morphine injection (F[2, 15] = 46.87, P < 0.001; F[22, 165] = 2.37, P = 0.001). Compared with the intralipid and normal saline groups, the scratching behavior was significantly attenuated in the propofol group (P < 0.001). Compared with control, normal saline, and intralipid groups, the protein expression of CB(1) receptor in ACC (Western blot) in the propofol group increased (0.86 ± 0.21, 0.94 ± 0.18, 0.86 ± 0.13, and 1.34 ± 0.32, respectively, P < 0.001). There was no significant difference among control, normal saline, and intralipid groups. Compared with the control, normal saline, and intralipid groups, the average number of neurons of CB(1) receptor in the ACC area were higher in the propofol group (21.0 ± 1.4, 19.3 ± 1.8, 24.8 ± 7.7, and 37.2 ± 3.3, respectively, P < 0.001).

CONCLUSIONS

Morphine elicits dose-independent scratching responses after intrathecal injection in rats. Morphine 40 μg/kg intrathecal injection-induced scratching responses can be prevented by propofol. Increased protein expression of CB(1) receptors in ACC may contribute to the reversal of intrathecal morphine-induced scratching.

The Anti-Nociception Effect of Dezocine in a Rat Neuropathic Pain Model

The treatment of neuropathic pain (NP) currently remains clinically challenging. In an attempt to identify novel targets of known opioids, we found that dezocine, a non-addictive opioid, inhibits norepinephrine and serotonin reuptake through their transporter proteins which open the potential for dezocine to manage NP. In the present study, the effect of dezocine on NP was observed in a rat model of chronic constriction injury (CCI). The paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) were used to evaluate thermal hyperalgesia and mechanic allodynia for nociceptive response. PWL and PWT tests were performed at 11:00 AM starting from 1 day before CCI surgery and 1, 3, 7, 10 days after right sciatic nerve ligation in the presence or absence of daily intraperitoneal injection of dezocine. The results demonstrated that the CCI-induced thermal and mechanical pain hypersensitivity was attenuated by dezocine significantly and persistently without sign of tolerance, indicating that dezocine could be an alternative medication for the treatment of NP. Clinical trial to confirm such discovery is warranted.