Because neonatal strokes present acutely with seizures, it is important to identify anticonvulsants that effectively reduces acute seizure and ameliorate ischemic brain damage. We found that gabapentin, given immediately after carotid ligation in P12 CD1 mice, effectively reduced acute seizures and brain atrophy in injured animals. The seizure suppression and neuroprotective effects were only seen at the higher doses. These higher doses of gabapentin did not result in any notable acute behavioral side effects. We further determined that the higher doses decreased the number of epochs with seizure scores over the range of seizure severity and eliminated the most severe seizures (i.e. seizure score of 6).
Similar to prior studies, the severity of acute seizures correlated with the severity of hemispheric brain atrophy in the vehicle-treated mice. The correlation persisted after administration of low doses gabapentin, but was lost after administration of high doses. Loss of correlation at higher doses of gabapentin is likely because the most injured animals drive the correlation that is lost when higher doses suppress the seizures and injury.
The ischemic pathway overlaps greatly with seizure processes and therefore anticonvulsants have been proposed as possible neuroprotective agents (17
). Felbamate was protective in a rat pup model of neonatal hypoxia-ischemia when administered before or up to four hours after injury (18
). Phenytoin pretreatment attenuated hypoxic-ischemic brain injury in neonatal rats (20
) and in utero
hypoxic brain injury in fetal guinea pigs (21
). Zonisamide pretreatment reduced hypoxic-ischemic brain injury in rat pups, but did not decrease acute electrographic seizures (22
). A single dose of lamotrigine reduced hippocampal neuronal damage in the rat neonatal hypoxia-ischemia model (23
). With the unilateral carotid ligation mouse model, it is now possible to determine the impact of anticonvulsants on both the acute seizures and the brain injury.
Gabapentin could reduce the activity of the Na+ or K+ channels, implicated in excitotoxicity (24
). However, the most important mechanism of gabapentin anticonvulsant effects is probably in blocking the influx of calcium into neurons via the α2
δ-1 and α2
δ-2 subunits of voltage-dependent calcium channels. The cortex and hippocampus have been shown to have high densities of the α2
δ-2 subunits. Via this mechanism, gabapentin may reduce synaptic release of excitatory neurotransmitters and post-synaptic neuronal excitation. However the molecular processes involved in the effect of gabapentin upon these calcium channel subunits that results in its anticonvulsant effect are currently unknown (26
). Reduced pre- and post-synaptic excitation would be expected to blunt both acute ischemic seizures and brain injury. The use of gabapentin during pregnancy to reduce brain damage in new-born infants at risk for perinatal asphyxia has been proposed (27
Our data also suggests that female and male mouse pups in this model may respond differently to gabapentin. Compared with mice receiving vehicle or low dose gabapentin, male mice receiving high dose gabapentin after carotid ligation had significantly less severe seizures and hemispheric brain injury. No significant difference in seizures or injury was found in the female mice between those administered vehicle or low dose gabapentin and those administered high dose gabapentin. A caveat in this finding is that the acute seizures and chronic injury were lower in the females compared with males administered the vehicle/low dose of gabapentin, although this difference was not significant. These data suggest that a sex-related difference in response to gabapentin may exist in this model and gabapentin may be acting in a different fashion in females versus males. This could suggest differences in the impact of gabapentin upon the calcium channels in males versus females or a difference in the relative contribution of calcium channel function to the ischemic cascade in the immature male and female mice. Gender differences have been reported in the incidence of cerebral palsy, which can result from perinatal brain injuries, as well as in the cell signaling cascades that mediate cell death in the immature brain (28
). In female neonatal rodents, ischemic injury is mediated predominantly by activation of caspases, while apoptosis inducing factor (AIF) plays a greater role in males (29
). Du et al also reported that in vitro neurons from male rodents are more sensitive to toxicity from glutamate and nitric oxide than female neurons (30
). Renolleau recently reported that the third generation caspase inhibitor Q-VD-OPh preferentially protects female 7 day old rat pups from stroke compared to males (31
). However, the relative contribution of the voltage-dependent calcium channels to these pathways in male and female pups is unclear. Gender specific differences in the voltage gated coronary calcium channel expression and currents (increased in males) (32
), and in mesenteric artery responses to voltage gated coronary calcium channel agonists (also increased in males) (34
) have been reported in adult tissue and linked to sex hormone levels. The relevance of these findings to possible sex-related differences in the immature animal is unknown. Alternatively, there could be a sex-dependent difference in the renal clearance of gabapentin, although this is not the case in adult rodents and humans (35
). This possible sex-dependant difference in the effect of gabapentin upon ischemic seizures and injury should be confirmed in other immature ischemia models.
In conclusion, in this immature model of ischemic seizures and brain injury, gabapentin suppresses seizures and reduces chronic injury at higher doses. These effects may differ according to animal sex. Gabapentin should be further tested in other relevant immature animal models. If further research supports these findings, clinicians could consider giving gabapentin when a neonate presents with a seizure due to a stroke or suspicion of brain asphyxia, particularly in boys. An intravenous preparation of gabapentin would be helpful in this regard. Gabapentin could also be considered as an anticonvulsant for infants with vascular malformations at risk for seizures and stroke. It will be important to determine which anticonvulsants provide the most neuroprotection, the impact of anticonvulsant combinations, and their use optimal with moderate hypothermia. Anticonvulsants could be very promising tools to minimize damage after ischemia in the neonatal or infant brain.