Up-Regulation of Spinal Muscarinic Receptors and Increased Antinociceptive Effect of Intrathecal Muscarine in Diabetic Rats

Abstract

Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors produce effective pain relief. Intrathecal injection of a small dose of neostigmine produces a profound antiallodynic effect in rats with diabetic neuropathy. However, the mechanisms of increased antinociceptive effect of cholinergic agents on diabetic neuropathic pain are not clear. In the present study, we tested the hypothesis that spinal muscarinic receptors are up-regulated in diabetes. The withdrawal threshold of the hindpaw in response to noxious heat and pressure stimuli was determined in streptozotocin-induced diabetic and age-matched normal rats. 3 H]AF-DX 384 membrane binding was performed to determine the number and affinity of spinal cord M2 muscarinic receptors in normal and diabetic rats. We found that the antinociceptive effect of intrathecal 2 to 12 g muscarine in diabetic animals was potentiated significantly compared with that in normal animals. The maximal muscarine-stimulated [35 S]GTP␥S binding was 112.5 Ϯ 8.3% in normal rats and 168.8 Ϯ 12.1% (P Ͻ 0.05) in diabetic rats. Although the K D value (2.9 nM) was similar in both groups, the B max of [ 3 H]AF-DX 384 membrane binding was significantly higher in diabetic than in normal rats (255.2 Ϯ 5.9 versus 165.9 Ϯ 3.5 fmol/mg protein, P Ͻ 0.05). Collectively, these data strongly suggest that the muscarinic receptor is up-regulated in the dorsal spinal cord in diabetic rats. This finding probably accounts for the increased efficacy of the antinociceptive effect of intrathecal muscarinic agonists in diabetic neuropathic pain.

Modulation of pain transmission by G-protein-coupled receptors

“…Using mAChR subtype knockout mice, it has been shown that the M 2 subtype is abundant in the superficial laminae of the dorsal horn, where the M 4 subtype is expressed only at a low level (Duttaroy et al, 2002;Chen et al, 2005a (Chen et al, 2005a). The mAChRs, especially the M 2 subtype, are upregulated in the dorsal spinal cord in diabetic rats, which accounts for increased analgesic efficacy of mAChR agonists in diabetic neuropathic pain (Chen and Pan, 2003c). Upregulation of the M 2 subtype in small-and medium-sized DRG neurons has also been reported in rats with nerve injury-induced neuropathic pain (Hayashida et al, 2006).…”

“…In this regard, blockade of mAChRs with atropine in the spinal cord causes a large decrease in the nociceptive threshold in rats (Zhuo and Gebhart, 1991). Intrathecal administration of mAChR agonists or acetylcholinesterase inhibitors produces a potent analgesic effect in rats, mice, and humans (Naguib and Yaksh, 1994;Hood et al, 1997;Ellis et al, 1999;Duttaroy et al, 2002;Chen and Pan, 2003c), and this analgesic effect is blocked by atropine (Naguib and Yaksh, 1994). The mAChRs also mediate some of the antinociceptive effects of opioids (Chen and Pan, 2001) and can enhance the analgesic effect of systemic opioids in humans (Hood et al, 1997).…”

“…Also, activation of the M 2 subtype present on cutaneous nociceptors can suppress the transduction of nociceptive information (Bernardini et al, 2002). In a rat model of diabetic neuropathic pain, the antinociceptive effect produced by intrathecal administration of mAChR agonists is largely potentiated by upregulation of the M 2 subtype in the spinal dorsal horn (Chen and Pan, 2003c).…”

Increased Spinal Cord Na+-K+-2Cl− Cotransporter-1 (NKCC1) Activity Contributes to Impairment of Synaptic Inhibition in Paclitaxel-induced Neuropathic Pain

“…A PE-10 catheter (8 cm) was inserted via a small opening made in the atlanto-occipital membrane of the cisterna magna such that the caudal tip reached the lumbar spinal enlargement (19). We then exteriorized the rostral end of the catheter and closed the wound with sutures.…”

“…To activate the device, a foot pedal was pressed to activate a motor that applied a constantly increasing force on a linear scale. When the animal displayed pain by either withdrawing of the paw or vocalizing, the pedal was immediately released, and the animal's nociceptive threshold was read on the scale (19).…”