Chronic pain is a significant healthcare problem. It is disabling and diminishes quality of life. Opioids, such as morphine, remain a primary pharmacologic management for chronic pain. Opioids act at mu opioid receptors (MOPr) in the rostroventral medial medulla (RVM) to produce their analgesic effect. The RVM is a critical relay in pain inhibitory and facilitatory pathways of pain modulation. Furthermore, chronic inflammatory pain, produced by CFA hindpaw injection, leads to adaptive changes in the RVM that change the balance of these pathways and increase the potency of opioids.
MOPr are known to produce their effects via Gi/o proteins.
Pretreatment of several pain modulatory regions with pertussis toxin (PTX) effectively attenuates the antinociceptive effects of MOPr agonists, such as DAMGO. In the RVM, PTX effectively reduced DAMGO stimulated GTPãS binding in uninjured rats. However, despite their effective inactivation of Gi/o proteins, PTX did not diminish the antinociceptive effects of DAMGO in the RVM of uninjured rats. In contrast, in rats with a chronic inflammatory injury, PTX completely abolished the antinociceptive effects of DAMGO. These results suggest a transition from Gi/o independent to Gi/o dependent mechanisms following CFA treatment. In addition, the anti-hyperalgesic effects of DAMGO were not inhibited by PTX, suggesting that DAMGO produces anti-hyperalgesia and antinociception by different mechanisms.
In the RVM, MOPr are present both postsynaptically and presynaptically. Postsynaptic MOPr are thought to produce antinociception by activating GIRK channels, resulting in hyperpolarization and inhibition of pain facilitatory neurons. Indeed, inhibition of GIRK channels in the RVM, via microinjection of tertiapin-Q, attenuated the antinociceptive effects of DAMGO in uninjured rats, providing the first behavioral evidence that MOPr agonists produce analgesia via this proposed mechanism. Interestingly, however, tertiapin-Q did not block the anti-hyperalgesic effects of DAMGO, nor did it diminish the antinociceptive effects of DAMGO in the contralateral hindpaw of CFA treated rats. Furthermore, these differential effects of tertiapin-Q in the uninjured and injured rats are not the result of transcriptional down regulation of GIRK channels in the RVM. Finally, tertiapin-Q alone in the RVM produced a modest antinociception in uninjured rats, providing the first evidence of constitutive GIRK channel activity in the RVM and demonstrating a role for these in pain modulation.
Presynaptic MOPr are thought to produce antinociception by decreasing GABA release onto pain inhibitory neurons. Indeed, microdialysis studies demonstrated that levels of GABA release were decreased in response to DAMGO perfused into the RVM, as well as to high potassium after perfusion of DAMGO. However, they were not decreased in rats after CFA treatment. This suggests that chronic inflammatory injury alters the presynaptic actions of MOPr agonists in the RVM. Interestingly, levels of GLU release where not altered by DAMGO in uninjured or injured rats. Moreover, basal levels of GLU and GABA were also unaltered by CFA treatment.
In conclusion, although MOPr mediate their antinociceptive effects in other pain modulatory regions via Gi/o proteins, this is not the case in the RVM during an uninjured state. However, MOPr-induced antinociception transitions from Gi/o independent to Gi/o dependent mechanisms after CFA treatment. Additionally, these results support both the presynaptic and the postsynaptic postulates by which MOPr agonists are thought to produce their analgesic effects. However, although CFA treatment alters the activity of neurons in the RVM and promotes changes that result in an enhanced anti-hyperalgesic and antinociceptive response to DAMGO in the RVM, neither the postsynaptic nor the presynaptic mechanism, in isolation, seem to account for this enhancement.