Lack of analgesic efficacy of spinal ondansetron on thermal and mechanical hypersensitivity following spinal nerve ligation in the rat.

The balance between descending inhibition and facilitation is thought to be disturbed in chronic pain states. Increased facilitation by spinally released serotonin has been suggested by demonstration that mechanically evoked neuronal responses of wide dynamic range neurons are inhibited by 5-HT3 receptor antagonists in rats following spinal nerve ligation (SNL) but not sham operation. Despite these physiologic data, the effects of spinal 5-HT3 receptor blockade on behavioral hypersensitivity and neurochemical alterations in spinal serotonergic system have not been thoroughly investigated following spinal nerve ligation in the rat. To test this, we acutely injected intrathecal ondansetron in rats between 14 and 30 days after SNL and assessed effects on thermal and mechanical hypersensitivity. We also determined the density of serotonergic nerve fibers, serotonin content and the levels of 5-HT3 receptors within the spinal cord at this time point. Intrathecal ondansetron (1, 3, 10, 30, and 100microg) produced no effect on behavioral measures of thermal or mechanical hypersensitivity whereas intrathecal morphine (1microg) and gabapentin (200microg) partially reversed thermal and mechanical hypersensitivity following SNL. In addition, SNL did not alter the density of serotonergic fibers or 5-HT3 receptor immunoreactivity or spinal tissue content of 5-HT within the dorsal horn. These results do not support anatomic plasticity of descending serotonergic pathways or tonic 5-HT3 receptor activity in maintaining hypersensitivity after nerve injury and in contrast to previous studies fail to demonstrate an anti-hypersensitivity effect of intrathecal injection of the 5-HT3 receptor antagonist ondansetron. Importantly, behavioral measures of mechanical hypersensitivity assess threshold responses whereas physiological studies of mechanically evoked neuronal responses involve application of suprathreshold stimuli. Thus, suprathreshold or more intense stimuli may be necessary to recruit descending serotonergic facilitatory drive required to observe the inhibitory effects of ondansetron on spinal neuronal excitability and behavioral hypersensitivity.