LONG-TERM ELEVATION OF CYCLOOXYGENASE-2, BUT NOT LIPOXYGENASE, IN REGIONS SYNAPTICALLY DISTANT FROM SPREADING DEPRESSION

Eicosanoids are metabolites of arachidonic acid produced by the cyclooxygenase (COX), lipoxygenase (LIPOX), and cytochrome P450 enzymes (for review, see Needleman et al., 1986). Arachidonic acid (AA) and the eicosanoids have been implicated in many brain functions including the control of sleep-wake cycles (Hayaishi, 1991), temperature and fever (Ueno et al., 1982; Stitt, 1991), pituitary hormone production (Ojeda et al., 1982), and even long-term potentiation (LTP; Luo and Vallano, 1995; for review, see Bliss and Collingridge, 1993). Furthermore, these molecules have a well-established role as mediators of inflammation (Goetzl et al., 1995). Thus, any change in content of these eicosanoid-producing enzymes following stimulation might have pro-found effects on brain function. Cyclooxygenase, also known as prostaglandin endoperoxide H synthase (PGHS), has two known isoforms, a constitutive form (COX-1; Smith et al., 1991) and an inducible form (COX-2; Xie et al., 1991). The distribution of these enzymes in the brain has been characterized (Breder et al., 1992, 1995; Breder, 1996; Yamagata et al., 1993). The distribution of the main LIPOX enzymes and their derivatives in the brain has only begun to be characterized (Miyamoto et al., 1987a,b; Simmet and Peskar, 1990; Cluoet et al., 1991; Nishiyama et al., 1992, 1993). Yamagata et al. (1993) have shown that the expression of COX-2 can be rapidly and transiently elevated from seizures. Furthermore, expression was induced by electrical activity that induced LTP and modulated by systemic administration of glutamate receptor blockers and tetrodotoxin treatment of the optic nerve. These results have led to the suggestion that COX-2 may function as an “immediate” or “early” factor following stimulation and that synaptic activity may have a role in its expression. Spreading depression (SD) is a propagating wave of electrical silence and negative interstitial DC potential (Leao, 1944) significantly different from a seizure, as it is a benign phenomenon that is never associated with irreversible injury of brain cells (Nedergaard and Hansen, 1988; Kraig et al., 1991). Seizures induced by chemical (for review, see Nadler et al., 1986) or electrical (Olney et al., 1983a,b) stimulation can cause characteristic patterns of neuronal cell death. Although it is noninjurious, SD stimulation transforms astrocytes (Kraig et al., 1991) and microglia (Gehrmann et al., 1993; Caggiano and Kraig, 1996) into reactive cells. We have previously shown that the eicosanoids alter the induction of microgliosis from SD (Caggiano and Kraig, 1996). Here we investigate how the expression of the eicosanoid-producing enzymes (i.e., COX-2 and LIPOX) are modulated in the cortex following unilateral cortical SD. Previous work (Kraig et al., 1991) has shown that changes in glial fibrillary acidic protein (GFAP) expression are induced by cortical SD in regions anatomically connected to the cortex but not experiencing SD. Because SD does not travel between gray matter areas (Bures et al., 1974), this observation suggests that some effects of SD may be transmitted by a synaptic mechanism. To determine if COX-2 and/or LIPOX expression are sensitive to these transynaptic effects, we examined COX-2 and LIPOX following cortical SD in regions not experiencing SD, namely the hippocampus. Hippocampal circuitry has been well characterized (Swanson and Cowan, 1977) and provides an ideal location to look at events one, two, and three synapses away from events in the cortex (i.e., dentate gyrus, CA3, and CA1, respectively). SD only travels between gray matter areas under extreme circumstances (e.g., see Largo et al., 1996); thus, cortical SD would not be predicted to travel to the hippocampus, and any changes in the expression of enzymes in the hippocampus following neocortical SD would be the result of hippocampal projections from the cortex. We confirmed a lack of SD in the hippocampus and examined COX-2 and LIPOX expression in the cortex, dentate gyrus (DG), CA3, and CA1 following unilateral neocortical SD. Our results show that COX-2 immunoreactivity (IR) was increased rapidly in the hemisphere experiencing SD and remained elevated for 21 days. This change was reduced by Dex (a glucocorticoid), Pe (an adrenergic agonist), and Snp (a nitric oxide donor; NO) with Pe. Mep (a phospholipase A2 inhibitor), Indo (a COX inhibitor), Lnam (a NO synthase antagonist), and Ndga (a LIPOX inhibitor) did not alter the induction of COX-2. The LIPOX IR was unchanged from SD. Recordings confirmed that SD did not occur in the hippocampus; however, COX-2 IR was significantly elevated in the ipsilateral DG, CA3, and CA1, with the extent of change in the order: cortex > DG > CA3 > CA1. LIPOX IR remained unchanged in the hippocampus.