Our results not only confirm the ability of ceftriaxone to elevate GLT1 expression in cortex and striatum of R6/2 mice, but show that this effect still occurs even after GLT1 levels begin to decline when these mice are 13 weeks of age and severely symptomatic. Thus, it appears that the cellular machinery underlying the ceftriaxone-induced increase in GLT1 expression is operative in late-stage HD.
Although the mechanism by which ceftriaxone increases GLT1 expression is not clear, there is support for activation of nuclear factor-kappa B (NF-kB), a transcription factor that plays a role in regulating immune responses and cell survival . Translocation of the NF-kB complex to the cell nucleus appears to be critical for the action of ceftriaxone , and our results suggest that this mechanism is intact in both cortex and striatum of R6/2 mice regardless of age. Even before the decline in GLT1 expression, moreover, 8-week-old R6/2 mice have a deficit in glutamate uptake, which is reversed by ceftriaxone treatment . Although there is no GLT1 protein deficit at this age, mRNA levels are in decline  and glutamate uptake is reduced , suggesting a loss of transporter function well in advance of protein down-regulation. Thus, ceftriaxone is capable of overcoming a deficit in GLT1 function. It is interesting in this regard that palmitoylation, a process by which proteins are inserted into cellular membranes , is reduced in HD mice, including palmitoylation of GLT1 . Whether ceftriaxone increases GLT1 palmitoylation is the focus of ongoing research.
It is unlikely that other glutamate transporters can account for a ceftriaxone-induced increase in glutamate uptake since ceftriaxone acts selectively on GLT1 . It also is unlikely that loss of other glutamate transporters can account for the decline in uptake since neither mRNA nor protein levels are altered for GLAST and EAAC1 in HD models even at ages when the behavioral phenotype is severe . Post-mortem analysis of HD patients, moreover, shows a selective decline in GLT1 mRNA expression  as well as a loss of glutamate uptake . Nevertheless, we cannot rule out the possibility that ceftriaxone has other actions that may indirectly impact glutamate transmission, including a change in dopamine or GABA dynamics. Although an antibiotic action of ceftriaxone is unlikely in that none of our animals showed signs of sepsis, it would be useful in follow-up studies to determine if non-antibiotics that also up-regulate GLT1, such as GPI-1046 , mimic the effects of ceftriaxone in R6/2 mice.
Increasing GLT1 expression may become an effective HD treatment strategy in that the up-regulation of GLT1 induced by ceftriaxone significantly improves the behavioral phenotype in 8-week-old R6/2 mice . It is unlikely that starting ceftriaxone treatment in 13-week-old R6/2s will result in behavioral improvement given the stage of disease progression in these animals, and in fact, we found that ceftriaxone failed to reverse the decline in body weight, which is evident in R6/2s at this age. But our results suggest that the increase in GLT1 expression that occurs when ceftriaxone treatment is begun earlier will continue to occur even in late-stage HD. Thus, GLT1 expression is likely to be an effective therapeutic target over a relatively long time course.
Glutamate dysregulation, including a possible decline in GLT1 activity, may play a role in several neurodegenerative diseases [5, 24]. In fact, a phase III clinical trial of ceftriaxone for treatment of amyotrophic lateral sclerosis (ALS) is already underway (for review see ). The dose required to increase GLT1 in mice produces comparable levels of ceftriaxone in the central nervous system of patients undergoing treatment for meningitis (0.3-6 μmol/L) , indicating that our treatment protocol is within normal limits for this drug. Nevertheless, it is interesting that ceftriaxone increased cortical and striatal GLT1 expression in both R6/2 and WT mice. WT mice, however, show no discernable behavioral consequences , suggesting that mechanisms are in place to compensate for increased glutamate removal. Whether HD mice lack these mechanisms or simply benefit from an increased rate of glutamate uptake remains to be determined. It appears that within limits increased GLT1 expression is not a problem, but decreased expression, which occurs in HD, is.