Neural Mechanisms Underlying Opioid-induced Mechanical Hypersensitivity And Tolerance

Chronic pain has a serious impact on people’s lives,and its pathogenesis is complex,so it is a thorny problem in clinical treatment.Morphine and other opiates are the gold standard analgesic drugs and are indispensable for treating moderate-to-severe postoperative and chronic pain.However,the use of opioids can cause a variety of side effects,such as respiratory depression,constipation,nausea,vomiting and so on.The chronic use of opioid drugs is associated with two major problems:opioid-induced hypersensitivity（OIH）and analgesic tolerance.OIH and tolerance are the two primary drivers of diminished pain control and dose escalation.Therapeutic strategies that can resolve OIH and tolerance without disrupting opioid analgesia are urgently required to improve patient safety.To achieve this,a better understanding of the underlying mechanisms of OIH and tolerance is needed.Although opioid analgesia results from binding and signaling throughμ-opioid receptors（MORs）,the opioid receptors and cell types that mediate OIH and tolerance remain unresolved.Currently,the prevalent opinion in pain research is that MORs expressed on transient receptor potential vanilloid type 1（TRPV1）nociceptors initiate the development of OIH and tolerance,both in mechanical and thermal forms.For the first main project of this thesis（Chapter 3 and 4）,we studied the role of peripheral MORs or MOR-expressing neurons in morphine-induced mechanical and thermal OIH and analgesic tolerance.In this study,the loss of peripheral MORs or MOR-expressing neurons rescued morphine thermal tolerance,but did not affect the expression and maintenance of morphine-induced mechanical allodynia and anti-allodynic tolerance.In support of this conclusion,we used multiple approaches to identify repeated administration of morphine-induced mechanical allodynia and anti-allodynia tolerance,including:1)von Frey filaments to identify light pressure-evoked punctate mechanical allodynia;2)gentle brushing to identify brush-evoked dynamic mechanical allodynia;3)dynamic brush-evoked conditional place aversion to identify the affect component of dynamic mechanical allodynia;and 4)recordings of primary afferent low-threshold Aβfiber stimulation-evoked EPSC inputs and AP outputs in the spinal superficial dorsal horn neurons to identify spinal circuits and change of gate control for mechanical allodynia pathways within the SDH.Moreover,to dissect the role of peripheral MORs and MOR-expressing neurons in morphine-induced mechanical allodynia and anti-allodynic tolerance,we used multiple strategies,including:1)conditional knockout of MORs specifically from TRPV1⁺neurons,using TRPV1^Cre/+;Oprm1^flox/flox mice;2)an intersectional genetic strategy to selectively ablate the peripheral TRPV1⁺neurons,using Tau^DTR/+;TRPV1^Cre/+;Advillin^Flpo/+mice;3)conditional knockout of peripheral MORs,using Advillin^Cre/+;Oprm1^flox/flox mice;and 4)an intersectional genetic strategy to perform selective ablation of peripheral MOR-expressing neurons,using Tau^DTR/+;Oprm1^Cre/+;Advillin^Flpo/+mice.Collectively,these results demonstrate that peripheral MORs or MOR-expressing neurons are not required for the expression and maintenance of morphine-mechanical allodynia or anti-allodynic tolerance in mice.For the second project of this thesis（Chapter 5）,we studied the roles of the lateral parabrachial nucleus（LPBN）-projecting spinal CR-expressing neurons in morphine-induced mechanical forms of OIH.Here we demonstrate that the spinal dorsal horn（SDH）-to-LPBN projecting CR neurons control morphine mechanical OIH.We found that repeated administration of morphine caused increased excitability of SDH^CR+neurons.Ablation of SDH^CR+neurons,or retro-blation of SDH-to-LPBN projecting CR⁺neurons,completely prevented the development of morphine mechanical OIH.When morphine mechanical OIH is already established,chemogenetic silencing of the axon terminals of SDH-to-LPBN projecting CR⁺neurons could rescue the established morphine mechanical OIH.Conversely,chemogenetic activation of the axon terminals of SDH-to-LPBN projecting CR⁺neurons alone was sufficient to induce mechanical allodynia in wild type（WT）na?ve mice,in both punctate and dynamic forms.Therefore,targeting the above SDH-to-LPBN projecting CR⁺neurons could offer a new strategy to resolve opioid-induced mechanical forms of analgesic tolerance.Overall,the above two studies suggest that the brainstem projecting spinal CR⁺neuron,but not peripheral MORs,or MOR-expressing neurons,mediates opioid-induced mechanical forms of OIH.Therefore,to dissect the underlying mechanisms for opioid-induced mechanical forms of OIH and analgesic tolerance,future studies should focus more on central than peripheral mechanisms.