IntroductionPeriventricular leukomalacia (PVL) is a hemispheric white-matter lesion that is often cystic and occurs most frequently in infants born before 32 weeks of gestation (1). PVL is the major cause of cerebral palsy among premature infants (2). Based on epidemiological and experimental studies, the pathophysiology of PVL appears to be multifactorial. It potentially involves the combined toxicity of hypoxic-ischemic insults, excess free radicals, maternal-fetal infection with increased cytokine production, and growth factor deficiency (1,(3)(4)(5)(6)(7)(8). Excess release of glutamate and the subsequent overactivation of the excitotoxic cascade could represent a common molecular pathway for several of these risk factors (6, 9).Findings from several experimental studies support this hypothesis. Newborn mice injected intracerebrally with ibotenate, a glutamate analog acting mainly on N-methyl-D-aspartate (NMDA) receptors, develop periventricular cystic white-matter lesions mimicking several aspects of human PVL (10). In this well characterized murine model, several drugs that interfere with the excitotoxic cascade have been shown to be neuroprotective (7,(11)(12)(13)(14) while proinflammatory cytokines (15) and iron (16) exacerbated ibotenateinduced white-matter lesions. In order to further understand the pathophysiology of the ibotenateinduced cystic lesions, we targeted the NMDA receptors thought to be present on white-matter glial cells. Newborn mice were injected intracerebrally with sense, antisense, or missense oligonucleotides specific for the NMDA R1 receptor subunit (P. Gressens, unpublished work). The sequences of the oligonucleotides were identical to those previously used by Wahlestedt et al. in an in vitro assay (17). In newborn mice, pretreatment with antisense oligonucleotides was neuroprotective while control sense oligonucleotides had no effect on the ibotenate-induced lesion. Unexpectedly, missense oligonucleotides, which were used as another control, induced a threefold reduction in the severity of the excitotoxic whitematter lesions. A search of databases with Basic Local Alignment Search Tool (BLAST) revealed that 12 successive nucleotides of the so-called "NMDA R1 missense oligonucleotide" were actually complementary to a triplicated motif of the murine nociceptin (NC) mRNA, suggesting that the blockade of NC production is neuroprotective in the ibotenate model. Intracerebral administration of the excitotoxin ibotenate to newborn mice induces white-matter lesions, mimicking brain lesions that occur in human preterm infants. Nociceptin (NC), also called orphanin FQ, is the endogenous ligand of the opioid receptor-like 1 (ORL1) receptor and does not bind classical high-affinity opioid receptors. In the present study, administration of NC exacerbated ibotenate-induced white-matter lesions while coadministration of ibotenate with either of two NC antagonists reduced excitotoxic white-matter lesions by up to 64%. Neither ibotenate plus endomorphin I (a selective µ receptor agonist), nor ibot...