STATUS EPILEPTICUS CAUSES A LONG-LASTING REDISTRIBUTION OF HIPPOCAMPAL CANNABINOID TYPE 1 RECEPTOR EXPRESSION AND FUNCTION IN THE RAT PILOCARPINE MODEL OF ACQUIRED EPILEPSY

Status epilepticus (SE) is a major medical and neurological emergency, and is associated with significant morbidity and mortality (DeLorenzo et al., 1996). The injury from SE has been shown to cause epileptogenesis, resulting in acquired epilepsy (AE) (Delorenzo et al., 2005). A number of changes in long-term neuronal plasticity have been observed following SE, including altered excitatory and inhibitory neurotransmission, changes in neurotrophic factors, and reorganization of hippocampal circuitry (Lothman et al., 1991; Morimoto et al., 2004). Recent studies have shown that the endocannabinoid system plays a regulatory role in the suppression of seizure activity (Wallace et al., 2003; Lutz, 2004); thus, a better understanding of the endocannabinoid system following SE may offer important insights into cannabinoid modulation of neuronal excitability. In the 1970s, cannabinoids were found to be anticonvulsant in several animal models of acute seizure (Karler and Turkanis, 1981), and recently, these anticonvulsant effects have been demonstrated to be cannabinoid type 1 (CB1) receptor-mediated in both in vivo and in vitro models (Wallace et al., 2001, 2002, 2003; Shafaroodi et al., 2004; Blair et al., 2006). In the rat pilocarpine model of AE, endocannabinoid tone was shown to regulate seizure frequency and duration via a CB1 receptor mechanism (Wallace et al., 2003). The CB1 receptor is among the most abundant G-protein-coupled receptors in brain, and is highly expressed in hippocampus (Herkenham et al., 1991; Tsou et al., 1998), a key region involved in limbic seizure generation (Lothman et al., 1991; Morimoto et al., 2004). Evidence exists that SE induces a long-term alteration in the expression of hippocampal CB1 receptors (Wallace et al., 2003); however, further in depth analysis of regional receptor expression and receptor functionality to determine the effects of SE on plasticity of neuronal CB1 receptors is warranted. This study was initiated to characterize the long-term effects of SE on CB1 receptor expression, binding, and G-protein activation in the rat pilocarpine model of AE. This model is well-established and manifests many features similar to partial complex or limbic epilepsy in humans (Mello et al., 1993). Utilizing the rat pilocarpine model of SE-induced AE, hippocampal CB1 receptors were evaluated using immunohistochemical and functional autoradiographic assays on brain sections from control and epileptic rats. Immunohistochemical analysis was performed with antibodies directed at two different epitopes of the CB1 receptor. Furthermore, changes in CB1 receptor binding and G-protein activation were evaluated using [3H] R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl](1-napthalen-yl)methanone mesylate (WIN55,212-2) autoradiography (Jansen et al., 1992) and agonist-stimulated [35S]GTPγS autoradiography (Sim et al., 1995), respectively. The results from this study provide the first evidence that SE causes a long-term reorganization in the expression, receptor binding, and G-protein activation of the hippocampal CB1 receptor.