Abstracts

Rationale: Epileptic seizures constitute a frequent comorbidity of Alzheimer s disease; however, the pathophysiology of seizure-generation in Alzheimer s disease remains poorly defined. A common pathological hallmark of both Alzheimer s disease and epilepsy is astrogliosis-- a cellular process that causes overexpression of adenosine kinase leading to adenosine deficiency and seizures. We hypothesize that astrogliosis and a resulting deficiency of the brain s endogenous neuroprotectant and anticonvulsant adenosine might be the causative agent of seizures in Alzheimer s disease. Here, we developed and characterized a novel mouse model of acute amyloid-beta (A?) induced epileptogenesis that is based on a single high dose injection of A?_1-40 into the dentate gyrus of wild-type mice. Methods: Aggregated human A?_1-40 (200 pmol / 1.5 ?l) or saline (negative control) was unilaterally microinjected in the dentate gyrus of adult C57BL/6 male mice. Acute injury to the hippocampus was assessed 24 hours later by Nissl and TUNEL stains in A?_1-40 and saline controls (N = 5, each). The chronic affects of A?_1-40 or saline (N = 6, each) were assessed 4 weeks after microinjection by (i) unilateral bipolar EEG recordings in the CA1 subregion and (ii) detailed histological analysis. To define the role of adenosine in A?_1-40-related seizures the adenosine kinase inhibitor ITU (3.1 mg / kp, i.p.) and the adenosine A1 receptor agonist CCPA (3.0 mg / kg, i.p.) were administered during EEG recordings. Results: A?_1-40 caused acute neurotoxicity with extensive neuronal apoptosis in the dentate gyrus (247 65 TUNEL positive cells / section) and CA1 (113 33 TUNEL positive cells / section) that was absent in saline injected controls. Four weeks after A?_1-40 injection, all animals displayed prominent hippocampal atrophy (46 % loss of hippocampal volume vs. contralateral side, p < 0.0001) and astrogliosis within the dentate gyrus. Astrogliosis was associated with a 35 % increase in adenosine kinase compared to saline injected controls (p < 0.001). In line with the role of adenosine kinase as a regulator of hippocampal excitability, all animals experienced recurrent electrographic seizures (5.5 0.32 seizures / h) that were recorded from intrahippocampal electrodes. Pharmacologically, seizures could be suppressed by ITU, an adenosine kinase inhibitor (0 seizures / h; efficacy period 3 h) or CCPA, an adenosine A1 receptor agonist (0 seizures / h; efficacy period 16 h), indicating that seizures were related to focal adenosine deficiency. Finally, A?_1-40 mediated hippocampal sclerosis and seizures occurs independent of A? plaque deposition, synthesis of endogenous A?_1-40, or cleavage of amyloid precursor protein into C-terminal fragments. Conclusions: Our data indicate that (i) A? induced neurotoxicity can lead to the development of spontaneous seizures via a gliosis- and adenosine-related mechanism and (ii) that electrographic seizures, related to gliosis, as is commonly observed in Alzheimer s disease, might occur prior to the onset of classic Alzheimer s pathology.