Adenylate cyclase inhibition attenuates neuropathic pain but lacks pre-emptive effects in rats

Reduced activity of adenylyl cyclase 1 attenuates morphine induced hyperalgesia and inflammatory pain in mice

Opioid tolerance, opioid-induced hyperalgesia during repeated opioid administration, and chronic pain are associated with upregulation of adenylyl cyclase activity. The objective of this study was to test the hypothesis that a reduction in adenylyl cyclase 1 (AC1) activity or expression would attenuate morphine tolerance and hypersensitivity, and inflammatory pain using murine models. To investigate opioid tolerance and opioid-induced hyperalgesia, mice were subjected to twice daily treatments of saline or morphine using either a static (15 mg/kg, 5 days) or an escalating tolerance paradigm (10–40 mg/kg, 4 days). Systemic treatment with an AC1 inhibitor, ST03437 (2.5–10 mg/kg, IP), reduced morphine-induced hyperalgesia in mice. Lumbar intrathecal administration of a viral vector incorporating a short-hairpin RNA targeting Adcy1 reduced morphine-induced hypersensitivity compared to control mice. In contrast, acute morphine antinociception, along with thermal paw withdrawal latencies, motor performance, exploration in an open field test, and burrowing behaviors were not affected by intrathecal Adcy1 knockdown. Knockdown of Adcy1 by intrathecal injection also decreased inflammatory mechanical hyperalgesia and increased burrowing and nesting activity after intraplantar administration of Complete Freund’s Adjuvant (CFA) one-week post-injection.

Mechanical allodynia

Mechanical allodynia (other pain) is a painful sensation caused by innocuous stimuli like light touch. Unlike inflammatory hyperalgesia that has a protective role, allodynia has no obvious biological utility. Allodynia is associated with nerve damage in conditions such as diabetes, and is likely to become an increasing clinical problem. Unfortunately, the mechanistic basis of this enhanced sensitivity is incompletely understood. In this review, we describe evidence for the involvement of candidate mechanosensitive channels such as Piezo2 and their role in allodynia, as well as the peripheral and central nervous system mechanisms that have also been implicated in this form of pain. Specific treatments that block allodynia could be very useful if the cell and molecular basis of the condition could be determined. There are many potential mechanisms underlying this condition ranging from alterations in mechanotransduction and sensory neuron excitability to the actions of inflammatory mediators and wiring changes in the CNS. As with other pain conditions, it is likely that the range of redundant mechanisms that cause allodynia will make therapeutic intervention problematic.

Involvement of spinal PKA/CREB signaling pathway in the development of bone cancer pain

BACKGROUND

It has been shown that spinal PKA/CREB signaling pathway is involved in neuropathic and inflammatory pain, but its effects on bone cancer pain have not previously been investigated. The aim of this study was to examine the potential role of the spinal PKA/CREB signaling pathway in the development of bone cancer pain.

METHODS

A bone cancer pain model was made by inoculation of Walker 256 cells into the intramedullary space of rat tibia. Western blot analysis examined the expression of PKAca (PKA catalytic subunit) and phospho-CREB (p-CREB) protein levels. The authors further investigated effects of intrathecal treatment with H-89 (a PKA inhibitor, 8 nmol) or forskolin (a PKA agonist, 10 nmol) on nociceptive behavior and the expression of PKAca and p-CREB.

RESULTS

On days 6, 9, and 15 after inoculation, the expression of PKAca and p-CREB protein levels were higher in the bone cancer pain rats compared to the sham rats. On day 9, intrathecal administration of H-89 significantly attenuated bone cancer-induced mechanical allodynia as well as upregulation of PKAca and p-CREB protein levels. These effects were completely abolished by intrathecal pretreatment with the PKA agonist forskolin.

CONCLUSION

The results suggest that the spinal PKA/CREB signaling pathway may participate in the development of bone cancer pain. The findings of this study may provide an evidence for developing novel analgesics to block bone cancer pain.

Activation of spinal TDAG8 and its downstream PKA signaling pathway contribute to bone cancer pain in rats

Bone cancer pain is difficult to treat and has a strong impact on the quality of life of patients. Few therapies have emerged because the molecular mechanisms underlying bone cancer pain are poorly understood. Recently, T-cell death-associated gene 8 (TDAG8) has been shown to participate in complete Freund's adjuvant-induced chronic inflammatory pain. In this study, we aimed to examine whether TDAG8 and its downstream protein kinase A (PKA) pathway are involved in bone cancer pain. A bone cancer pain model was made by inoculation of Walker 256 cells into the intramedullary space of rat tibia. Spinal TDAG8 expression was increased after inoculation with tumor cells. Intrathecal TDAG8 siRNA attenuated bone cancer pain behaviors during the initiation and maintenance phases; there were also concomitant decreases in TDAG8 mRNA and protein levels in spinal cord. Moreover, we found spinal PKA and phosphorylated cAMP response element-binding (pCREB) protein levels were up-regulated in the rat model of bone cancer pain. Knockdown of TDAG8 resulted in reduced bone cancer pain-induced spinal PKA and pCREB protein expression in two procedures. Furthermore, intrathecal H-89 (a PKA inhibitor) significantly attenuated bone cancer pain behaviors in rats. Our results suggest a causal relationship between TDAG8 expression and the initiation and maintenance of bone cancer pain. Activation of spinal TDAG8 contributes to bone cancer pain through the PKA signaling pathway in rats. These findings may lead to novel strategies for the treatment of bone cancer pain.