Trauma and septic shock initiate simultaneous pro- and anti-inflammatory responses. Based on ongoing laboratory studies, we believe that clinical outcomes can be favorably altered by differential modulation of anti-inflammation over pro-inflammation. While other groups continue to define the complexities involved in the pathophysiologic responses to traumatic and septic shock, we have focused on examining how commonly used ICU interventions such as anesthetics and sedatives can be used to beneficially modulate inflammation. Critically ill patients often have a prolonged hospital course with management in the intensive care unit and may require multiple surgical procedures. As a result, they receive a variety of anesthetic, sedative, and analgesic interventions. Due to its anti-inflammatory properties, ketamine may be a useful therapeutic adjunct in this setting. Moreover, ketamine has been shown to have anti-inflammatory effects in some patient populations, including cardiac surgery and liver transplant, and may be a safe and beneficial adjunct in the brain-injured patient (31
). However, others have suggested that its use in patients with TBI is contraindicated as it could potentially exacerbate TBI due to its effects on cerebral perfusion pressure. While perfusion pressures were not measured in this study, we clearly demonstrated that a 7 mg/kg dose of ketamine does not exacerbate TBI-induced inflammation.
In contrast, ketamine has potent anti-inflammatory effects on LPS induced changes in serum concentrations of IL-1α, IL-1β, IL-6, TNF-α, and IFN-γ and these effects were dose dependent. Indeed the anti-inflammatory effects were found at both anesthetic (70 mg/kg) and sub-anesthetic (7 mg/kg) doses, but were limited at lower doses (1 mg/kg). Ketamine may be most useful as an adjunct in the critical care setting at subanesthetic doses, exploiting its sedative and analgesic properties without producing total intravenous anesthesia. Because the subanesthetic dose of ketamine effectively attenuated LPS induced changes in serum cytokine concentrations and did not anesthetize the rat, this dose of ketamine was examined in a model of TBI induced intracerebral inflammation. However, this dose of ketamine had little effect upon TBI induced cerebral edema formation or changes in cerebral cytokine concentrations.
This study extends our previously published data regarding the effects of ketamine at 70 mg/kg on LPS induced changes in serum cytokines (9
). In that study, we reported that ketamine (70 mg/kg) blunted the LPS induced increases in serum concentrations of IL-1β, IL-6, TNF-α, and IFN-γ. Here we report its effects on IL-1α as well as the effects of ketamine at doses of 7 and 1 mg/kg on LPS induced changes in these cytokines. Taken together, these data indicate that the anti-inflammatory effects of ketamine, with regards to LPS induced changes in serum cytokines, are dose dependent and exist at doses considered sub-anesthetic.
Our results are consistent with the literature. However, these other investigations have been limited to IL-6 and TNF-α (10
). In a cecal ligation and puncture model of rodent sepsis, Yu et al reported that ketamine, given intravenously in a dose of 2.5 mg/kg attenuated the increase in serum concentration of TNF-α, but not IL-6 (12
). Similarly, we found that the lowest dose of ketamine investigated in our study (1 mg/kg IP) was able to significantly blunt the increase in systemic production of TNF-α in response to LPS, but had no significant effects on the other cytokines studied. Thus, these data suggest that the most potent anti-inflammatory effect of ketamine, with regards to LPS induced changes in serum cytokine production, is its ability to attenuate the production of TNF-α. TNF-α is produced by macrophages as a direct response to LPS through a toll-like receptor 4 mediated pathway. That ketamine, at a dose of 1 mg/kg, is able to attenuate the LPS induced production of TNF-α, in the absence of effects upon the LPS induced changes in other cytokines, indicates that ketamine acts, in part, upon the macrophage mediated response to LPS in a manner independent from its effects on other cytokines. Indeed, ketamine has been shown to inhibit macrophage production of TNF-α in vitro
). This effect alone, however, was not sufficient for ketamine to modulate the global cytokine response, as evidenced by the marked reduction of TNF-α production in response to 1 mg/kg of ketamine and the lack of response in the other measured cytokines at this dose. This could reflect an insufficient dose of ketamine to affect other aspects of macrophage function, such as IL-1 production, an insufficient dose of ketamine to affect other cells involved in the inflammatory response, or both. Conversely, ketamine appears to be much less potent with regards to its ability to modulate production of IFN-γ, as neither the 1 mg/kg nor the 7 mg/kg doses of ketamine were able to significantly diminish the LPS induced increases in this cytokine. This finding suggests that the modulation of the inflammatory response by ketamine is not mediated through IFN-γ. Additionally, ketamine had little effect upon the LPS production of IL-10, an anti-inflammatory cytokine. Although ketamine produced a statistically significant attenuation of LPS induced IL-10 production at a dose of 7 mg/kg, this effect is relatively modest and production of IL-10 is largely preserved. That ketamine attenuates the LPS induced production of pro-inflammatory cytokines, while having minimal effect upon the anti-inflammatory cytokine IL-10, suggests that ketamine exerts its effect through selective modulation of the inflammatory response, rather than through a broad non-discriminatory mechanism. Thus, ketamine may modulate the inflammatory response in such a way that the pro-inflammatory response is suppressed, while the anti-inflammatory response is relatively preserved. In addition to the elucidation of the exact mechanism of its action, future studies should also evaluate the effects of ketamine upon LPS induced inflammation and injury when administered in conjunction with and after LPS.
Our current study also extends our knowledge with respect to TBI and confirms that TBI results in a local inflammatory response as measured by formation of edema and interstitial cytokine concentrations in cortical tissues. The presence of edema at 6 hours after injury, but not at 1 hour after injury, is consistent with previous work as edema formation is expected to be present by 6 hours and to peak at 48 hours (35
). Treatment with ketamine prior to TBI was not able to attenuate or delay the formation of post-TBI cortical edema. The changes in cerebral cytokine concentrations as a result of TBI in this study are expected and consistent with previous results from our laboratory and the work of others (34
). At the site of injury, pro-inflammatory cytokines IL-1α, IL-1β, and TNF-α were found to be elevated at 1 and 6 hours after CCI, while the production of IL-6 was not observed until 6 hours after injury. IL-10, an anti-inflammatory cytokine, was found to be increased at 1 and 6 hours after injury. Despite the dramatic effects seen with ketamine on LPS induced production of serum cytokines, ketamine only blunted the TBI induced increase in cerebral concentrations of TNF-α, and even then only at 1 hour after CCI and only in the penumbra (). With the lone exception of TNF-α, ketamine had no impact on the cytokine responses in our model of TBI. Taken together, the results of our investigations utilizing a model of TBI indicate that ketamine, at the subanesthetic dose of 7 mg/kg, has minimal effects on the early local inflammatory state in the brain that develops in response to TBI.
In our LPS model, the anti-inflammatory effects of ketamine were very potent, especially in regards to its ability to abrogate production of TNF-α. Consequently, we originally hypothesized that the anti-inflammatory effects of ketamine would be present in another model of inflammation, in this case, TBI. However, this was not the case. Nevertheless, it remains possible that higher doses of ketamine may be required to observe any anti-inflammatory effects when using other models of inflammation. That ketamine lacked efficacy during TBI may be secondary to differing mechanisms of the inflammatory insults. Endotoxemia from LPS produces a profound systemic inflammatory response with subsequent organ damage and dysfunction, whereas CCI results in local tissue injury, a localized inflammatory response, and, in more severe cases, a subsequent systemic inflammatory response. Although these two responses, to LPS and to TBI, share many cellular responses, signaling molecules, and molecular pathways, they are clearly the result of two different initial insults and progression. The underlying molecular mechanisms likely differ as well, but remain to be fully elucidated.
The major limitations of this study, specifically in regard to TBI, include the examination of early time-points and the choice of a relatively low dose of ketamine. Previous work in our laboratory indicates that edema can be detected by 6 hours, and persists up to 60 hours (35
). Additionally, local cytokine production varies depending upon the cytokine measured, with increased tissue concentrations observed between 1 to 24 hours. Changes in IL-1α, IL-1β, and TNF-α are detectable up to 12 hours, while changes in IL-6 are found from 6 to 24 hours (34
). In the present study, measurements at the early time-points after CCI evaluate early local inflammatory response. The prolonged course of the local inflammatory response was not evaluated. Thus the progression of edema, the resolution of early cytokine production (IL-1α, IL-1β, and TNF-α), and the sustained production of the pro-inflammatory cytokine IL-6 were not examined. Furthermore, given the differing mechanism responsible for the inflammatory response in TBI when compared to endotoxemia, it is possible that a relatively low dose of ketamine is not sufficient to produce effects. Finally, we have previously shown that pre-treatment with the anesthetic isoflurane is also capable of attenuating the systemic production of IL-1α and IL-1β in response to LPS (9
). In our model of TBI, isoflurane anesthesia is used at the time of CCI, and potential anti-inflammatory effects from the administration of this agent could make any effect that ketamine might produce difficult to detect. Future investigations involving the anti-inflammatory effects of ketamine in TBI are warranted and should incorporate higher doses of ketamine and later time-points.
In conclusion, this study found that ketamine attenuates LPS induced systemic cytokine production, but has little effect on the early local inflammatory response that occurs as a result of CCI induced TBI. However, it is noteworthy that ketamine did not exacerbate this response either. Further studies with this agent to confirm its safety and efficacy in the presence of TBI are warranted. Moreover, the data suggest that ketamine may have utility as a sedative adjunct in septic patients.