Neural basis for improgan antinociception

Permanent lesion in rostral ventromedial medulla potentiates swim stress-induced analgesia in formalin test

OBJECTIVE(S)

There are many reports about the role of rostral ventromedial medulla (RVM) in modulating stress-induced analgesia (SIA). In the previous study we demonstrated that temporal inactivation of RVM by lidocaine potentiated stress-induced analgesia. In this study, we investigated the effect of permanent lesion of the RVM on SIA by using formalin test as a model of acute inflammatory pain.

MATERIALS AND METHODS

Three sets of experiments were conducted: (1) Application of stress protocol (2) Formalin injection after exposing the animals to the swim stress (3) Either the relevant vehicle or dopamine receptor 1 (D1) agonist R-SKF38393 was injected into the RVM to cause a lesion. For permanent lesion of RVM, R-SKF38393 was injected into the RVM. Forced swim stress in water was employed in adult male rats. Nociceptive responses were measured by formalin test (50µl injection of formalin 2% subcutaneously into hind paw) and pain related behaviors were monitored for 90 min.

RESULTS

In the unstressed rats, permanent lesion of the RVM by R-SKF38393 decreased formalin-induced nociceptive behaviors in phase 1, while in stressed rats, injection of R-SKF38393 into the RVM potentiated swim stress-induced antinociception in phase 1 and interphase, phase 2A of formalin test. Furthermore, R-SKF38393 had pronociceptive effects in phase2B whereas injections of R-SKF38393 resulted in significant difference in nociceptive bahaviours in all phases of formalin test (P<0.05).

CONCLUSION

The result of the present study demonstrated that permanent inactivation of RVM can potentiate stress-induced analgesia in formalin test.

Antinociceptive activity of CC44, a biotinylated improgan congener

Improgan, a non-opioid, antinociceptive drug, activates descending analgesic circuits following brain administration, but the improgan receptor remains unidentified. Since biotinylation of drugs can enhance drug potency or facilitate discovery of new drug targets, a biotinylated congener of improgan (CC44) and several related compounds were synthesized and tested for antinociceptive activity. In rats and mice, intracerebroventricular (i.c.v.) administration of CC44 produced dose-dependent reductions in thermal nociceptive (tail flick and hot plate) responses, with 5-fold greater potency than improgan. CC44 also robustly attenuated mechanical (tail pinch) nociception in normal rats and mechanical allodynia in a spinal nerve ligation model of neuropathic pain. Similar to the effects of improgan, CC44 antinociception was reversed by the GABAA agonist muscimol (consistent with activation of analgesic circuits), and was resistant to the opioid antagonist naltrexone (implying a non-opioid mechanism). Also like improgan, CC44 produced thermal antinociception when microinjected into the rostral ventromedial medulla (RVM). Unlike improgan, CC44 (i.c.v.) produced antinociception which was resistant to antagonism by the cannabinoid CB1 antagonist/inverse agonist rimonabant. CC44 was inactive in mice following systemic administration, indicating that CC44 does not penetrate the brain. Preliminary findings with other CC44 congeners suggest that the heteroaromatic nucleus (imidazole), but not the biotin moiety, is required for CC44's antinociceptive activity. These findings demonstrate that CC44 is a potent analgesic compound with many improgan-like characteristics. Since powerful techniques are available to characterize and identify the binding partners for biotin-containing ligands, CC44 may be useful in searching for new receptors for analgesic drugs.

Pain-facilitating medullary neurons contribute to opioid-induced respiratory depression

Respiratory depression is a therapy-limiting side effect of opioid analgesics, yet our understanding of the brain circuits mediating this potentially lethal outcome remains incomplete. Here we studied the contribution of the rostral ventromedial medulla (RVM), a region long implicated in pain modulation and homeostatic regulation, to opioid-induced respiratory depression. Microinjection of the μ-opioid agonist DAMGO in the RVM of lightly anesthetized rats produced both analgesia and respiratory depression, showing that neurons in this region can modulate breathing. Blocking opioid action in the RVM by microinjecting the opioid antagonist naltrexone reversed the analgesic and respiratory effects of systemically administered morphine, showing that this region plays a role in both the analgesic and respiratory-depressant properties of systemically administered morphine. The distribution of neurons directly inhibited by RVM opioid microinjection was determined with a fluorescent opioid peptide, dermorphin-Alexa 594, and found to be concentrated in and around the RVM. The non-opioid analgesic improgan, like DAMGO, produced antinociception but, unlike DAMGO, stimulated breathing when microinjected into the RVM. Concurrent recording of RVM neurons during improgan microinjection showed that this agent activated RVM ON-cells, OFF-cells, and NEUTRAL-cells. Since opioids are known to activate OFF-cells but suppress ON-cell firing, the differential respiratory response to these two analgesic drugs is best explained by their opposing effects on the activity of RVM ON-cells. These findings show that pain relief can be separated pharmacologically from respiratory depression and identify RVM OFF-cells as important central targets for continued development of potent analgesics with fewer side effects.

A novel, non-invasive method of respiratory monitoring for use with stereotactic procedures

Accurate monitoring of respiration is often needed for neurophysiological studies, as either a dependent experimental variable or an indicator of physiological state. Current options for respiratory monitoring of animals held in a stereotaxic frame include EMG recordings, pneumotachograph measurements, inductance-plethysmography, whole-body plethysmography (WBP), and visual monitoring. While powerful, many of these methods prevent access to the animal's body, interfere with experimental manipulations, or require deep anesthesia and additional surgery. For experiments where these issues may be problematic, we developed a non-invasive method of recording respiratory parameters specifically for use with animals held in a stereotaxic frame. This system, ventilation pressure transduction (VPT), measures variations in pressure at the animal's nostril from inward and outward airflow during breathing. These pressure changes are detected by a sensitive pressure transducer, then filtered and amplified. The output is an analog signal representing each breath. VPT was validated against WBP using 10% carbon dioxide and systemic morphine (4mg/kg) challenges in lightly anesthetized animals. VPT accurately represented breathing rate and tidal volume changes under both baseline and challenge conditions. This novel technique can therefore be used to measure respiratory rate and relative tidal volume when stereotaxic procedures are needed for neuronal manipulations and recording.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Efficacy of improgan, a non-opioid analgesic, in neuropathic pain

Improgan, a non-opioid analgesic, is known to act in the rodent brain stem to produce highly effective antinociception in several acute pain tests. However, improgan has not been studied in any models of chronic pain. To assess the efficacy of improgan in an animal model of neuropathic pain, the effects of this drug were studied on mechanical allodynia following unilateral spinal nerve ligation (SNL) in rats. Intracerebroventricular (icv) improgan (40-80 μg) produced complete, reversible, dose-dependent attenuation of hind paw mechanical allodynia for up to 1h after administration, with no noticeable behavioral or motor side effects. Intracerebral (ic) microinjections of improgan (5-30 μg) into the rostral ventromedial medulla (RVM) also reversed the allodynia, showing this brain area to be an important site for improgan's action. The recently-demonstrated suppression of RVM ON-cell activity by improgan may account for the presently-observed anti-allodynic activity. The present findings suggest that brain-penetrating, improgan-like drugs developed for human use could be effective medications for the treatment of neuropathic pain.

Physiological basis for inhibition of morphine and improgan antinociception by CC12, a P450 epoxygenase inhibitor

Many analgesic drugs, including μ-opioids, cannabinoids, and the novel nonopioid analgesic improgan, produce antinociception by actions in the rostral ventromedial medulla (RVM). There they activate pain-inhibiting neurons, termed "OFF-cells," defined by a nociceptive reflex-related pause in activity. Based on recent functional evidence that neuronal P450 epoxygenases are important for the central antinociceptive actions of morphine and improgan, we explored the convergence of opioid and nonopioid analgesic drug actions in RVM by studying the effects of the P450 epoxygenase inhibitor CC12 on the analgesic drug-induced activation of these OFF-cells and on behavioral antinociception. In rats lightly anesthetized with isoflurane, we recorded the effects of intraventricular morphine and improgan, with and without CC12 pretreatment, on tail flick latency and activity of identified RVM neurons: OFF-cells, ON-cells (pronociceptive neurons), and neutral cells (unresponsive to analgesic drugs). CC12 pretreatment preserved reflex-related changes in OFF-cell firing and blocked the analgesic actions of both drugs, without interfering with the increase in spontaneous firing induced by improgan or morphine. CC12 blocked suppression of evoked ON-cell firing by improgan, but not morphine. CC12 pretreatment had no effect by itself on RVM neurons or behavior. These data show that the epoxygenase inhibitor CC12 works downstream from receptors for both μ-opioid and improgan, at the inhibitory input mediating the OFF-cell pause. This circuit-level analysis thus provides a cellular basis for the convergence of opioid and nonopioid analgesic actions in the RVM. A presynaptic P450 epoxygenase may therefore be an important target for development of clinically useful nonopioid analgesic drugs.