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Mini Review
27 November 2018
The Contribution of the Descending Pain Modulatory Pathway in Opioid Tolerance
Lindsay M. Lueptow
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Erin N. Bobeck
The effects of morphine on neuronal transmission in the descending pain pathway. In the naïve state, GABAergic interneurons in the periaqueductal gray (PAG) fire tonically, thereby producing a steady release of GABA and inhibition of PAG output neurons. Upon administration of acute morphine, postsynaptic mu opioid receptor (MOPr) activate GIRK channels via Gα proteins resulting in K+ release and hyperpolarization of the neuron. Additionally, MOPr activate Gi/o proteins, which result in the inhibition of adenylyl cyclase (AC) and decrease cAMP production. Morphine binding of presynaptic MOPr inhibits voltage dependent calcium (Ca2+) conductances via Gβγ proteins and activated voltage dependent potassium conductances (Kv) via Phospholipase A (PLA). Overall, these two mechanisms block release of the neurotransmitter GABA, therefore suppressing inhibition, increasing output, of the PAG neurons projecting to the rostral ventromedial medulla (RVM). Acute morphine treatment also activates toll-like receptor 4 (TLR4) receptors on astrocytes and microglia in the PAG inducing several signaling cascades.

Opioids remain among the most effective pain-relieving therapeutics. However, their long-term use is limited due to the development of tolerance and potential for addiction. For many years, researchers have explored the underlying mechanisms that lead to this decreased effectiveness of opioids after repeated use, and numerous theories have been proposed to explain these changes. The most widely studied theories involve alterations in receptor trafficking and intracellular signaling. Other possible mechanisms include the recruitment of new structural neuronal and microglia networks. While many of these theories have been developed using molecular and cellular techniques, more recent behavioral data also supports these findings. In this review, we focus on the mechanisms that underlie tolerance within the descending pain modulatory pathway, including alterations in intracellular signaling, neural-glial interactions, and neurotransmission following opioid exposure. Developing a better understanding of the relationship between these various mechanisms, within different parts of this pathway, is vital for the identification of more efficacious, novel therapeutics to treat chronic pain.

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