Status epilepticus causes a long-lasting redistribution of hippocampal cannabinoid type 1 receptor expression and function in the rat pilocarpine model of acquired epilepsy

Abstract

Activation of the cannabinoid type 1 (CB1) receptor, a major G-protein-coupled receptor in brain, acts to regulate neuronal excitability and has been shown to mediate the anticonvulsant effects of cannabinoids in several animal models of seizure, including the rat pilocarpine model of acquired epilepsy. However, the long-term effects of status epilepticus on the expression and function of the CB1 receptor have not been described. Therefore, this study was initiated to evaluate the effect of status epilepticus on CB1 receptor expression, binding, and G-protein activation in the rat pilocarpine model of acquired epilepsy. Using immunohistochemistry, we demonstrated that status epilepticus causes a unique "redistribution" of hippocampal CB1 receptors, consisting of specific decreases in CB1 immunoreactivity in the dense pyramidal cell layer neuropil and dentate gyrus inner molecular layer, and increases in staining in the CA1-3 strata oriens and radiatum. In addition, this study demonstrates that the redistribution of CB1 receptor expression results in corresponding functional changes in CB1 receptor binding and G-protein activation 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) and agonist-stimulated [35S]GTPgammaS autoradiography, respectively. The redistribution of CB1 receptor-mediated [35S]GTPgammaS binding was 1) attributed to an altered maximal effect (Emax) of WIN55,212-2 to stimulate [35S]GTPgammaS binding, 2) reversed by the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A), 3) confirmed by the use of other CB1 receptor agonists, and 4) not reproduced in other G-protein-coupled receptor systems examined. These results demonstrate that status epilepticus causes a unique and selective reorganization of the CB1 receptor system that persists as a permanent hippocampal neuronal plasticity change associated with the development of acquired epilepsy.

Cannabinoids and Epilepsy

“…CBDV potentiated the effects of phenobarbital, ethosuximide, and valproate in 2 seizure models [181]. These studies suggest that both Δ9-THC and CBD provide significant protection from seizures [147,[154][155][156][157][158][159][160][161][162][163][164][165][166]. GABA = γ-aminobutryic acid in preclinical animal trials, presenting potential targets for human studies.…”

“…Levels of CB 1 R expression in the CA1-3 stratum oriens and radiatum (presumed excitatory inputs) and dentate gyrus steadily increased 1-week post-pilocarpine-induced seizures (Fig. 3, dark green trace) [147,[155][156][157][158]. However, sclerotic and nonsclerotic hippocampal tissue resected from patients with epilepsy displayed a reduction in DAGLα (2-AG biosynthetic enzyme), CB 1 R mRNA, and CB 1 R excitatory terminal immunoreactivity (Fig.…”

“…3, dark red trace). Beginning 4 days following pilocarpine-induced seizures in rats, CB 1 R expression progressively decreased in hippocampal CCK+ inhibitory nerve terminals [161], particularly in the CA1 stratum pyramidale and the dentate gyrus inner molecular layer, unaccounted for by CA1 neuronal cell loss alone [155,156,162]. By reducing CB 1 R expression on inhibitory terminals (and presumed DSI), this homoeostatic process may limit network disinhibtion and restrain elevated excitability during prolonged epileptiform activity.…”

The adenosine kinase hypothesis of epileptogenesis

“…These findings demonstrate that upregulation of ADK in chronic epilepsy is implicated in epileptogenesis by reducing the adenosine tone at the A 1 R. Upregulation of ADK was also found in chronically kindled rats and in the rat pilocarpine model of epilepsy (D. Boison, unpublished observations). Upregulation of ADK in chronic epilepsy is paralleled by changes in G-protein coupled receptor expression such as downregulation of A 1 Rs (Ekonomou et al, 2000;Rebola et al, 2003;Rebola et al, 2005), reorganization of cannabinoid type 1 receptors (Falenski et al, 2007), or increases in GABA B receptor expression (Kokaia et al, 2001;Morimoto et al, 2004).…”

Long-term decrease in calbindin-D28K expression in the hippocampus of epileptic rats following pilocarpine-induced status epilepticus

“…Hippocampal cell loss has been characterized in the pilocarpine model of AE (Mello et al, 1993;Rice and DeLorenzo, 1998;Falenski et al, 2007). To ensure that the decrease in calbindin expression was not a result of hippocampal cell loss associated with this model of AE, we performed cell counts in adjacent Nissl stained sections used for this study using established techniques (Mello et al, 1993) and presented the data as a percent of control (Fig.…”