Neuroprotective effects of topiramate after hypoxia–ischemia in newborn piglets

Abstract

Background: Perinatal hypoxia–ischemia (HI) is associated with delayed cerebral damage, which involves receptor-mediated excitotoxicity. Until now, successful interventions to reduce excitotoxicity early after HI in experimental settings failed to transform into clinical applications owing to negative side effects. A promising new approach using the anticonvulsant Topiramate (TPM) has shown to be effective to reduce brain damage after early HI in a rodent model of combined TPM-hypothermia. Here, we used TPM solely administered 1 h after HI in a neonatal piglet model in order to verify possible neuroprotection. Methods: Newborn piglets were subjected to HI by transient occlusion of carotid arteries and hypotension (62–65% of baseline). Fifteen minutes later, an additional reduction of the inspired oxygen fraction to 0.06 was performed for 13 min. One cohort (VEHICLE, n = 8) received saline solution i.v. 1 h after HI and then twice a day. Two further cohorts were treated at same times with TPM (HI-TPM10, n = 8, loading dose 20 mg/kg; maintenance dose 10 mg/kg/day; HI-TPM20, n = 8, loading dose 50 mg/kg; maintenance dose 20 mg/kg/day). Untreated animals (CONTROL, n = 8) received all experimental procedures except HI. Animals were monitored 3 days after HI concerning occurrence of seizures as well as neurological and behavioral functions. After 72 h, the brains were perfused and processed to assess neuronal loss and DNA-fragments (TUNEL staining). Results: There was a significant reduction of neuronal cell loss in HI-TPM20 animals. However, apoptosis was increased in the frontal white matter of HI-TPM20 animals. Conclusions: Exclusive TPM treatment shows neuroprotection in newborn piglets after HI.

Introduction

Perinatal hypoxia–ischemia (HI) is the single most important cause of brain injury in the newborn, and has consequences that are potentially devastating and lifelong [5]. HI leads to different neuropathological manifestations, depending on the maturity of the newborn. It has been proposed that neurons connected in already established neuronal circuits appear to be especially vulnerable to excitotoxic damage based on a hyperactivity of the major excitatory glutamatergic input [12]. Therefore, predominant brain damage is seen in the parasagittal region of the cerebral cortex, the basal ganglia, and the hippocampus [21]. The principal mechanism of early brain damage is initiated by energy depletion, which led to depolarization, excessive extracellular glutamate release, and hence prolonged activation of glutamate receptors. Subsequently, intracellular calcium accumulation results were induced predominantly by voltage- and ligand-dependent calcium channels. This activates a variety of calcium-mediated deleterious events leading to secondary energy failure and triggering cellular apoptosis [12].

Until now, concepts of pharmacotherapeutic treatment of early brain damage after HI were focused on single aspects of intervention and remained ineffective, presumably due to the complexity of the ongoing damaging processes. Increasing evidence appears that clinical neuroprotection is more likely to be effective if multiple strategies are employed to interrupt simultaneously several different pathways that promote intracellular calcium accumulation [4]. Recently, it has been shown that early administration of Topiramate (TPM), a neuroprotective agent, in combination with later-onset cooling effectively reduces HI brain injury in a neonatal rat stroke model [15].

However, TPM alone appears to fulfill requirement of multiple interaction with potential cell-injuring pathways: TPM is a structurally novel broad spectrum antiepileptic drug (AED) with several mechanisms of action. These include a negative modulatory effect (use-dependent blockade) on presynaptic voltage-activated neuronal sodium channels. Electrophysiological studies have demonstrated that TPM suppresses the presynaptic voltage-sensitive sodium channel of excitatory synapses, which may reduce excessive glutamate release after HI impact [19]. Furthermore, TMP enhances GABA-mediated chloride flux and GABA-evoked chloride currents in murine brain neurons and increases seizure threshold [22]. It appears possible that TMP induces neuroprotection during HI by GABAergic hyperpolarization in the term neonate. Interestingly, TPM has been shown to attenuate AMPA-kainate receptor-mediated cell death and calcium influx, as well as kainate-evoked currents in developing oligodendrocytes. Therefore, a protective role for TPM was demonstrated in a rat pup model of periventricular leukomalacia [6]. Finally, a negative modulatory effect on some L-type high-voltage-activated calcium channels, and inhibition of the carbonic anhydrase isozymes CA-II and CA IV, has been reported for TPM [18].

Brain maturation in newborn piglets is similar to that in human infants at birth. Therefore, we have developed a model of infant human hypoxia/ischemia using term newborn piglets. We hypothesized that early administration of TPM may decrease seizure activity, improve neurological outcome, and reduce brain damage in newborn piglets subjected to HI.