Background: Intrathecal administration of acetylcholinesterase inhibitors produces antinociception in both animals and humans, but their effect on diabetic neuropathic pain has not been studied. In the current study, we determined the antiallodynic effect of intrathecal injection of an acetylcholinesterase inhibitor, neostigmine, in a rat model of diabetic neuropathic pain. In addition, since acetylcholine can increase release of nitric oxide in the spinal cord, we studied the role of spinal endogenous nitric oxide in the action of intrathecal neostigmine in diabetic neuropathic pain.
Methods: Rats were rendered diabetic with an intraperitoneal 50-mg/kg injection of streptozotocin. Intrathecal catheters were inserted, with tips in the lumbar intrathecal space. Mechanical allodynia was determined by application of von Frey filaments to the hind paw. We first determined the dose-dependent effect of intrathecal neostigmine on allodynia. The role of spinal nitric oxide in the action of intrathecal neostigmine was then examined through intrathecal treatments with a neuronal nitric oxide synthase inhibitor (TRIM), a nitric oxide scavenger (PTIO), L-arginine, or D-arginine.
Results: The diabetic rats developed a sustained tactile allodynia within 4 weeks after streptozotocin injection. Intrathecal injection of 0.1-0.5 microg neostigmine dose-dependently increased the withdrawal threshold in response to application of von Frey filaments. Intrathecal pretreatment with 30 microg TRIM or 30 microg PTIO abolished the antiallodynic effect of intrathecal neostigmine. Furthermore, the inhibitory effect of TRIM on the action of intrathecal neostigmine was reversed by intrathecal injection of 100 microg L-arginine but not D-arginine.
Conclusions: Intrathecal neostigmine produces a profound analgesic effect in a rat model of diabetic neuropathic pain. Spinal endogenous nitric oxide contributes to the analgesic action of intrathecal neostigmine in this rat model of diabetic neuropathic pain.