Recently, minocycline has been shown to have neuroprotective effects in models of central nervous system injury (for review see Yong et al., 2004
). For SCI, it was reported that a significant improvement in functional outcome was obtained with the use of minocycline using a moderate contusion injury similar to that produced in the present replication study (Lee et al., 2003
). In both studies, the force-calibrated weight-drop device developed at New York University was utilized (Gruner, 1992
). Also, a similar anesthetic protocol including chloral hydrate administration was delivered. In the current study, no significant differences at any point in time between the four groups were found in terms of locomotor function as assessed by the BBB test. It should be noted that in the Lee et al., 2003
paper, at 38 days after injury, BBB scores in the vehicle-treated rats were 15 ± 0.5. In our study, the highest average BBB score at the end of the study was 12.1 for the minocycline IP-treated group and the lowest score was for the saline control group with a score of 11.8. Thus, although a similar injury device and injury severity was used in the two studies, there was a difference in BBB scores for the vehicle treated groups. Also, pathological analysis based on area values from horizontal central cord sections showed no significant differences between the different experimental groups in spared tissue and cavity areas when analyzing the epicenter section in the contusion model. Thus, in contrast to published data using this SCI model, minocycline did not improve behavioral or histopathological outcome.
It is important to mention that there were some differences in the present study design with respect to the original study. For example, the chloral hydrate used to anesthetize the animals in the original study was reported to be 50 mg/kg IP (Lee et al., 2003
). Because we found that this concentration of the drug was insufficient for anesthetic induction in our hands, 300 mg/kg IP of chloral hydrate was found to be optimal and used in the present study. Subsequent discussions with the Lee laboratory indicated that the dose of drug was incorrectly reported in the published manuscript (Oh, personal communication). In fact, 500 mg/kg IP was used in the original study but this dose resulted in high mortality in our hands. Whether these differences in the anesthesia dosage explain the different outcomes between the two experiments is a possible consideration. An additional difference between this study and the Lee paper was the animal vendor. The Lee paper obtained their animals from a vendor in Korea while our rats were obtained from Charles River Laboratories. It is difficult to determine what potential effects these different animal sources may have on SCI outcome studies. It is clear however, that we used a similar impact model, similar surgical procedures and injury severity described by Lee and colleagues but obtained different behavioral outcomes even within the nontreated group.
Recently the importance of the method of minocycline treatment has been emphasized. In a study by Fagan and colleagues (2004)
, intravenous administration of minocycline was reported to be a reliable method when neuroprotection was required. These investigators found that the intraperitoneal route led to the drug being incompletely and erratically absorbed. In the present study, we compared IP with IV injections of minocycline and neither treatment route improved outcome after SCI.
In addition to the beneficial effects reported for minocycline treatment, studies have shown recently that minocycline may worsen or not improve outcome under specific experimental conditions (Tsuji et al., 2004
; Yang et al 2003
; Diguet et al 2004
). In a model of MPTP-induced dopaminergic damage to neurons, minocycline treatment was reported to worsen outcome in mice and cynomologus monkeys (Yang et al 2003
; Diguet et al 2004
). Also, in an animal model of Huntington’s disease, minocycline aggravated motor scores and produced neuronal loss in the striatum (Diguet et al 2004
). Finally, in a recent study where minocycline treatment following a balloon-compression SCI was investigated, limited sparing of tissue and no significant effects on BBB locomotor recovery were reported (Saganova et al., 2008
). Thus, it is clear that this treatment may have a wide range of consequences on functional outcome effects depending on the species and model of CNS injury.
In summary, no behavioral or histopathological improvements were observed in the present study within any of the experimental groups administered minocycline in the contusion SCI model. While these results cannot exclude possible beneficial effects of minocycline in other models of CNS injury or with different treatment protocols, the present study does question the benefit of this treatment in traumatic SCI. Thus, the replication of positive findings in independent laboratories should be emphasized before the implementation of clinical trials in SCI are undertaken.