Effect On The Nociception And Study On Neurotoxicologic Screening Of Intrathecal Lornoxicam In The Formalin Test In Rats

Background and Objective Non-steroidal anti-inflammatory drug(NSAID) is of importance in pain management by inhibiting function of cyclooxygenase thus reducing synthesis of prostaglandin, a inflammatory pain modulator. It was recently shown that intrathecal administration of NSAID exerts a direct spinal action by blocking the hyperalgesia induced by activation of spinal glutamate , substance P receptors and various noxious stimuli, suggesting the prospective of intrathecal NSAID in pain control. Lornoxicam is a new and injectable NSAID available in China. The aim of the study is to investigate the feasibility of the intrathecal lornoxicam through behavior and neuropathology in the formalin test in rats.Methods(1) Investigation of the ED50 (median effective dose) value of antinociception of preemptive intrathecal lornoxicam in the formalin test in rats. 40 healthy male adult SD rats were randomly divided into experiment group and control group, and the later was further divided into normal saline (NS) group and formalin (F) control group. For NS control group and F group, NS were administered intrathecally followingsubcutaneous administration of NS and formalin respectively in the hind plantar 10 minutes later; For experiment (Lor) group, different (80~~ 160ug) lornoxicam was administered intrathecally following subcutaneous administration of formalin in the hind plantar 10 minutes later. Pain behavior of mean flinching and licking time of the influenced limb was recorded. The dose of the Lor group was determined by Dixon's up-and-down method with more than 50% pain behavior parameter reduced as effective and less than 50% pain behavior parameter reduced as ineffective.(2) Effect of preemptive intrathecal lornoxicam on the nociception of rat in formalin test in rats. 32 healthy male adult SD rats were randomly divided into control groups ( NS, F, LorIM ) and experiment group ( LorIT ). Group NS and F was treated as in experiment (1); For LorIM and LorIT group, 120ug (result of ED50 from experiment above) lornoxicam was administered intramuscularly and intrathecally respectively following subcutaneous administration of formalin in the hind plantar 10 minutes later. Pain behavior of mean flinching and licking time of the influenced limb was recorded.(3) Neurotoxicologic screening of chronically intrathecal lornoxicam in the formalin test in rats. 32 healthy male SD rats were randomly divided into NS control group and experiment groups (Lori 20 , Lor320). Group NS received 20ul of normal saline, and experiment groups received 120ug and 320ug of lornoxicam respectively intrathecally every 4 days for a total number of 4 times. The placing and stepping reflex and the righting reflex were observed during the course of treatment. 10 minutes after the final administration, subcutaneous formalin was injectedinto the hind plantar of the rat. The number of the flinching response was recorded and lumbar/sacral spinal cord was sampled for neuropathological examination through light and electron microscope.Results(1) The ED50 value of antinociception of preemptive intrathecal lomoxicam in the formalin test in rats was 114.6ug, 95%CI (confidence interval) was 103.0-127.4ug.(2) In the experiment part (2), The flinching and licking time of phase II in Groups F, Group LorIT and Group LorIM was much longer than that in Group NS (P< 0.001); The flinching and licking time of phase II in Group LorIT was shorter than that in Group F and Group LorIM (P< 0. 001); The flinching and licking time of phase I and phase II in Group LorIT and in Group LorIM were not different (P>0. 05). The rates of inhibition on phase II flinching and licking time in Group LorIT and in Groups LorIM were 48.15% and 4.8% respectively.(3) The abnormalities of the placing/stepping and righting reflexes were not found during the course of chronically intrathecal lomoxicam treatment. In the formalin test in rats, the number of flinching in both phase I and phase II in group Lori 20 and group Lor320 was less than that in group NS (P<0. 001), however, the number of flinching in both phase I and phase II in group Lorl20 and group Lor320 were not different (P>0. 05). The rates of inhibition on the flinching responses in both phase I and phase II in group Lorl20 were 26.67% and 54.87% respectively, the rates of inhibition in group Lor320 were 27.30% and 51.41% respectively. In group Lorl20, group Lor320 and group NS, macroscopic examination of the lumbar/sacral spinal cord showed no abnormality such hemorrhage ornecrosis. No sign of gliosis, demyelination, fibrosis or inflammation was found under light microscope. The cell bodies of the neurons, including their nuclei, showed normal shapes and intact cell membranes, the cell organelles also displayed normal morphology, the capillaries and astrocytes in spine also demonstrated normal morphology under electron microscope in group Lori20, group Lor320 and group NS. The degeneration and necrosis in cauda equine nerves were not observed through light microscope in three groups.Conclusions(1) The ED50 value of antinociception of preemptive intrathecal lomoxicam in the formalin test in rats was 114.6ug.(2) Preemptive intrathecal lomoxicam reduced the flinching and licking time of phase II in formalin test in rats. Preemptive intramuscular lomoxicam in same dose was not effective in both phase I and phase II, suggesting that intrathecal lomoxicam induced antinociception through directly spinal but not systemic mechanism.(3) Chronic and intrathecal lomoxicam reduced the nociceptive responses in the formalin test in rats. No disorder of movement in behavior and no morphological evidence of neurotoxicity in the rat spinal cord were observed even at large doses, suggesting that intrathecal lomoxicam was safe and effective. However, before clinical trials, lomoxicam intrathecal use remains to be investigated in other species and in other safety respectives.