Protein kinase C-mediated mu-opioid receptor phosphorylation and desensitization in rats, and its prevention during early diabetes

Abstract

Painful diabetic neuropathy is associated with impaired opioid analgesia; however, the precise mechanism in sensory neurons remains unclear. This study aimed to identify putative mechanisms involved in modified opioid responsiveness during early streptozotocin-induced diabetes in rats. In this study, we demonstrate that in diabetic animals, impaired peripheral opioid analgesia is associated with a reduction in functional mu-opioid receptor (MOR) G protein coupling. Mu-opioid receptor immunoreactive neurons colocalized with activated forms of protein kinase C (PKC) and with the receptor for advanced glycation end products (RAGE) during streptozotocin-induced diabetes. Moreover, MOR phosphorylation at Thr370 in sensory neurons of diabetic rats, and thus desensitization, was due to RAGE-dependent PKC activation. Importantly, blocking PKC activation using PKC selective inhibitor, silencing RAGE with intrathecal RAGE siRNA, or inhibiting advanced glycation end product (AGE) formation prevented sensory neuron MOR phosphorylation and, consequently, restored MOR G protein coupling and analgesic efficacy. Thus, our findings give the first in vivo evidence of a RAGE-dependent PKC-mediated heterologous MOR phosphorylation and desensitization in sensory neurons under pathological conditions such as diabetic neuropathy. This may unravel putative mechanisms and suggest possible prevention strategies of impaired opioid responsiveness.

Morphine hyposensitivity in streptozotocin‐diabetic rats: Reversal by dietary l‐arginine treatment

“…However, NO formed from the dietary l-arginine, may have attenuated perineurial hypoxia and the associated oxidative stress, as well as reducing otherwise elevated levels of diacylglycerol and activated protein kinase C (PKC), that are produced as a consequence of persistent hyperglycaemia in diabetes. 14,15 Of particular importance to the present work, PKC mechanisms are implicated in desensitization of the opioid receptor [16][17][18][19][20][21][22] as well as the development of diabetic complications. 16 A role for PKC in the development of opioid hyposensitivity in diabetes is supported by observation that supraspinal pretreatment of STZ-diabetic rats with the PKC inhibitor, calphostin C, prevented the development of opioid hyposensitivity.…”

“…14,15 Of particular importance to the present work, PKC mechanisms are implicated in desensitization of the opioid receptor [16][17][18][19][20][21][22] as well as the development of diabetic complications. 16 A role for PKC in the development of opioid hyposensitivity in diabetes is supported by observation that supraspinal pretreatment of STZ-diabetic rats with the PKC inhibitor, calphostin C, prevented the development of opioid hyposensitivity. 23,24 In recent work by others in STZ-diabetic rats exhibiting opioid hyposensitivity, MOP receptor immunoreactive primary sensory neurons were co-localized with activated PKC isoforms as well as with the receptor for advanced glycation end products (RAGE).…”

“…23,24 In recent work by others in STZ-diabetic rats exhibiting opioid hyposensitivity, MOP receptor immunoreactive primary sensory neurons were co-localized with activated PKC isoforms as well as with the receptor for advanced glycation end products (RAGE). 16 Using a range of measures including administration of a selective PKC inhibitor, intrathecal RAGE siRNA, or inhibition of advanced glycation end product (AGE) formation to prevent both RAGE-dependent PKC activation and desensitization of the MOP receptor, opioid analgesic efficacy was restored. 16 In other work in rats with advanced diabetes at 12 weeks post-STZ, the inhibitory effects of intraplantar bolus doses of morphine on capsaicin-induced nocifensive behaviour were impaired.…”

“…16 Using a range of measures including administration of a selective PKC inhibitor, intrathecal RAGE siRNA, or inhibition of advanced glycation end product (AGE) formation to prevent both RAGE-dependent PKC activation and desensitization of the MOP receptor, opioid analgesic efficacy was restored. 16 In other work in rats with advanced diabetes at 12 weeks post-STZ, the inhibitory effects of intraplantar bolus doses of morphine on capsaicin-induced nocifensive behaviour were impaired. 25 Also, there was a markedly reduced inhibitory effect of morphine on capsaicin-induced TRPV1 current in DRG neurons from these animals.…”

Administrations of thalidomide into the rostral ventromedial medulla alleviates painful diabetic neuropathy in Zucker diabetic fatty rats

“…There are 12 known isoforms of PKC, which are further categorized into three subgroups according to their structure and mechanisms of activation. A recent study has shown that the expression of PKCα and PKCε is increased in the spinal cord of diabetic animals; however, the expression of PKCβ or PKCγ was not altered (Mousa et al, 2016), suggesting subtype specific PKC isoforms are involved in diabetic neuropathy. Thus, future work might be necessary to evaluate the role of specific PKC isoforms in the RVM in modulating neuropathic pain induced by type 2 diabetes.…”