Using a common model of transient (90-min) focal ischemia that results in reliable infarction but minimal mortality in rats [37
], we show here that stimulation of NMDARs with the partial agonist DCS improves long-term functional outcome. The functional improvement was not due to a reduction in infarction or tissue loss, since pretreatment infarct volume and subsequent tissue loss were similar in DCS- and vehicle-treated animals.
A total of 24 h after MCAO, prior to randomization, all rats included in the study showed signs of infarction and edema in the ipsilateral hemisphere, expressed as an increased signal intensity in the T2
W MR images and increased volume of the ipsilateral hemisphere. As expected in this model as well as in human stroke, the extent of infarction and edema was quite variable, ranging from small, exclusively striatal infarcts to large infarcts with significant cortical involvement [40
]. However, quantitative assessment of the infarction and edema confirmed that the drug- and vehicle-treatment groups were well matched for these two parameters.
Electrical forepaw stimulation resulted in a significant activation of the somatosensory cortex contralateral to the stimulated paw in intact and sham-operated animals, as previously demonstrated [34
]. MCAO drastically reduced activation in the infarcted hemisphere, even when the infarct was restricted to the striatum, so that the functionally compromised region extended beyond the area of infarction, as previously reported in the literature [43
]. Treatment with DCS preserved activation of the ipsilateral somatosensory cortex in animals with striatal and cortical infarcts not involving the primary somatosensory cortex. In animals with infarcts involving the somatosensory cortex treated with DCS, the activation shifted either towards the secondary somatosensory cortex or towards the cingulate, as reported in MCAO animals with spontaneous recovery [44
-cycloserine treatment had an additional beneficial effect on cognitive outcome, which has been previously demonstrated in rodent models of traumatic brain injury [46
]. Using the novel object recognition task [35
], we were able to test the animals several times (days 7, 21 and 30 after stroke), allowing an insight into the time course of the treatment effect. While the nonischemic control rats exhibited significant preference for the novel object at all time points and the vehicle-treated ischemic rats showed a stable deficit, DCS-treated rats had a reduced preference for the novel object, indicative of a memory deficit, in the earliest test (7 days post-MCAO) which improved gradually over time. A total of 30 days after ischemia, DCS-treated animals were no longer different from nonischemic controls. This progressive functional improvement following a single delayed administration of DCS was similar to the one we observed in head injured mice given a single NMDA or DCS injection 24 h after injury [27
Measurement of the infarct and hemispheric volumes of T2
W images obtained just prior to termination revealed no differences between the groups in the size of the infarct or the extent of tissue loss, although absolute infarct volume was reduced over time [48
]. This finding suggests that the mechanisms underlying the effects of DCS are not related to prevention of early neuronal death and infarction, but rather to stimulation of neuronal plasticity and reorganization, which are thought to be the major mechanisms underlying spontaneous recovery from stroke [44
]. A similar dissociation between changes in infarct size and functional outcome has been noted in the past in humans, as well as experimental animals [51
Partial or complete spontaneous resolution of neurological deficits is well documented in the first few weeks or months after stroke [53
], although the extent and rate of recovery can vary greatly, even among patients with identical clinical severity in the acute phase. While various explanations for these variations have been proposed, including reabsorption of perilesional edema and variability in the perfusion territory, it is clear that neural plasticity plays a major role in the recovery process [44
] and is the likely target of DCS.
To elaborate, full and partial agonists of NMDAR, including NMDA, DCS and the naturally occurring amino acid d-serine, promote cell migration during development and improve memory function in adult animals [27
]. DCS was also demonstrated to promote neuroplasticity in humans [56
]. Recently, our group has shown that a single administration of 10 mg/kg DCS 24 h after closed head injury improved memory function as well as neurological deficits in mice, restored long-term potentiation and increased levels of brain-derived neurotrophic factor. Furthermore, we showed the beneficial effects of DCS were blocked by coadministration of the NMDAR antagonist MK801 and that there was no additional benefit derived from multiple administrations [47
To our knowledge, this is the first study demonstrating beneficial effects of NMDAR stimulation in focal ischemic stroke. From the theoretical standpoint, these results support a re-evaluation of the role of glutamate in stroke pathology, emphasizing the importance of considering the dynamic nature of changes in glutamate transmission after stroke and trauma [27
] in the selection of appropriate treatment. From the clinical standpoint, the well-established safety profile of DCS in humans [56
] and the fact that it is already approved for human use as an antimicrobial agent [60
] may facilitate the translation of these findings to the clinical domain, although further experiments are needed to establish the optimal dose and optimal dosing regimen for DCS in stroke.