Relevance of Mu-Opioid Receptor Splice Variants and Plasticity of Their Signaling Sequelae to Opioid Analgesic Tolerance

Abstract

Opioid dose escalation to effectively control pain is often linked to the current prescription opioid abuse epidemic. This creates social as well as medical imperatives to better understand the mechanistic underpinnings of opioid tolerance to develop interventions that minimize it, thereby maximizing the analgesic effectiveness of opioids. Profound opioid analgesic tolerance can be observed in the absence of mu-opioid receptor (MOR) downregulation, aggregate MOR G protein uncoupling, and MOR desensitization, in the absence of impaired G protein coupled receptor kinase phosphorylation, arrestin binding, or endocytosis. Thus, we have explored alternative biochemical sequelae that might better account for opioid analgesic tolerance. Our findings indicate that substantial plasticity among upstream and downstream components of opioid receptor signaling and the emergence of alternative signaling pathways are major contributors to opioid analgesic tolerance. An exemplar of this plasticity is our findings that chronic morphine upregulates the MOR variants MOR-1B2 and MOR-1C1 and phosphorylation of their C-terminal sites not present in MOR-1, events causally associated with the chronic morphine-induced shift in MOR G protein coupling from predominantly G_i/G_o inhibitory to G_s-stimulatory adenylyl cyclase signaling. The unique feature(s) of these variants that underlies their susceptibility to adapting to chronic morphine by altering the nature of their G protein coupling reveals the richness and pliability of MOR signaling that is enabled by generating a wide diversity of MOR variants. Furthermore, given differential anatomical expression patterns of MOR variants, MOR splice variant-dependent adaptations to chronic morphine could enable mechanistic underpinnings of tolerance and dependence that are CNS region- and cell-specific.

Fig. 1

Chronic morphine (opioid) upregulates mu-opioid receptor (MOR) splice variants MOR-1B2 and MOR-1C1 in the spinal cord of male rats and Chinese Hamster Ovary cells stably expressing these variants. Chronic morphine also augments protein kinase C (PKC) and protein kinase A (PKA) activities. PKC phosphorylates casein kinase2 (CK2), which phosphorylates S390 in the C-terminal of MOR-1B2. PKA phosphorylates S425 and S428 in the C-terminal of MOR-1C1. MOR-1B2 and MOR-1C1 C-terminal phosphorylation facilitates their functional interaction with Gsα (reflected by increased sufentanil stimulation of GTPγs binding and cAMP formation). Since the above C-terminal phosphorylation sites are not present in MOR-1, consequences of C-terminal phosphorylation of MOR-1B2 and MOR-1C1 on their G protein coupling preferences would likely differ from effects of phosphorylating the C-terminus of MOR-1. This underscores the potentially unique contributions of MOR variant-selective upregulation and phosphorylation to opioid analgesic tolerance