Overexpression of drug efflux pumps at the blood brain barrier (BBB) has been suggested to be one important factor contributing to drug resistance in epilepsy. This would imply that resected brain tissue of drug-resistant patients is drug-sensitive in absence of the BBB. Here we studied the effects of carbamazepine (CBZ) at therapeutically relevant concentration on epileptiform activity electrophysiologically recorded in acute hippocampal slices of patients with mesial temporal lobe epilepsy (MTLE; 28 patients, 49 slices) or extra-hippocampal tumours (tumour; 6 patients, 11 slices). Epileptiform activity was induced by hilar stimulation (0.067 Hz) during elevation of extracellular potassium concentration ([K(+)](o)) and remained self-sustained in presence of 10-12 mM [K(+)](o). Quantitative analysis of data revealed that epileptiform activity in tissue of tumour-patients was predominantly suppressed by CBZ, indicating that the 'epilepsy model' used is CBZ-sensitive. In contrast, epileptiform activity in tissue of drug-resistant MTLE patients was resistant to CBZ in 82% of patients, partially suppressed in 11% and completely suppressed in 7%. The effects of CBZ in tissue of MTLE patients did not depend on the type of activity, hippocampal pathology, excitability of the tissue, or equilibration time of the drug. Considering that CBZ has direct access to all compartments of the slice, our results suggest that CBZ-resistance mechanisms are located within the parenchyma of the dentate gyrus and contribute to drug resistance in the majority of MTLE patients. BBB-located drug-resistance mechanisms per se may play a minor role in this region, because CBZ-sensitivity was only observed in 7% of CBZ-resistant patients.

Soluble oligomers of the amyloid-␤ peptide (A␤Os) accumulate in the brains of Alzheimer disease (AD) patients and are implicated in synapse failure and early memory loss in AD. A␤Os have been shown to impact synapse function by inhibiting long term potentiation, facilitating the induction of long term depression and inducing internalization of both AMPA and NMDA glutamate receptors, critical players in plasticity mechanisms. Because activation of dopamine D1/D5 receptors plays important roles in memory circuits by increasing the insertion of AMPA and NMDA receptors at synapses, we hypothesized that selective activation of D1/D5 receptors could protect synapses from the deleterious action of A␤Os. We show that SKF81297, a selective D1/D5 receptor agonist, prevented the reduction in surface levels of AMPA and NMDA receptors induced by A␤Os in hippocampal neurons in culture. Protection by SKF81297 was abrogated by the specific D1/D5 antagonist, SCH23390. Levels of AMPA receptor subunit GluR1 phosphorylated at Ser 845 , which regulates AMPA receptor association with the plasma membrane, were reduced in a calcineurin-dependent manner in the presence of A␤Os, and treatment with SKF81297 prevented this reduction. Establishing the functional relevance of these findings, SKF81297 blocked the impairment of long term potentiation induced by A␤Os in hippocampal slices. Results suggest that D1/D5 receptors may be relevant targets for development of novel pharmacological approaches to prevent synapse failure in AD. Alzheimer disease (AD)2 is the main cause of dementia among the elderly, and current estimates indicate that it affects around 25 million people worldwide (1, 2). Although much is known about the pathophysiology of AD, there is still no cure or effective treatment capable of slowing the progression of the disease. For this reason, development of novel pharmacological strategies for treatment is of critical importance.Considerable evidence indicates that soluble oligomers of the amyloid-␤ peptide (A␤Os) accumulate in the brains of AD patients and are responsible for synapse dysfunction and memory loss in AD (3-5). Among other deleterious actions, A␤Os impair synaptic plasticity, likely leading to memory loss at early stages of the disease. A␤Os have been shown to inhibit long term potentiation (LTP) (4, 6, 7), facilitate the induction of long term depression (LTD) (8, 9), induce internalization of , and increase activation of protein phosphatases, such as calcineurin and protein phosphatase-1 (9, 10, 15, 16), finally leading to spine loss (14).Dopamine receptors have been grouped into two families: D1-type and D2-type (17). The D1 family comprises D1 and D5 receptor subtypes, which are mostly coupled to G␣ s and stimulate production of the second messenger cyclic AMP, leading to activation of protein kinase A (PKA) (17). D1/D5 receptors play important roles in cognition, mediating plasticity and specific aspects of cognitive function, including working and spatial learning and memory processes (18). Stimulation of D...

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Drug resistant patients undergoing epilepsy surgery have a good chance to become sensitive to anticonvulsant medication, suggesting that the resected brain tissue is responsible for drug resistance. Here, we address the question whether P-glycoprotein (Pgp) and multidrug resistance-associated proteins (MRPs) expressed in the resected tissue contribute to drug resistance in vitro. Effects of anti-epileptic drugs [carbamazepine (CBZ), sodium valproate, phenytoin] and two unspecific inhibitors of Pgp and MRPs [verapamil (VPM) and probenecid (PBN)] on seizure-like events (SLEs) induced in slices from 35 hippocampal and 35 temporal cortex specimens of altogether 51 patients (161 slices) were studied. Although in slice preparations the blood brain barrier is not functional, we found that SLEs predominantly persisted in the presence of anticonvulsant drugs (90%) and also in the presence of VPM and PBN (86%). Following subsequent co-administration of anti-epileptic drugs and drug transport inhibitors, SLEs continued in 63% of 143 slices. Drug sensitivity in slices was recognized either as transition to recurrent epileptiform transients (30%) or as suppression (7%), particularly by perfusion with CBZ in PBN containing solutions (43, 9%). Summarizing responses to co-administration from more than one slice per patient revealed that suppression of seizure-like activity in all slices was only observed in 7% of patients. Patients whose tissue was completely or partially sensitive (65%) presented with higher seizure frequencies than those with resistant tissue (35%). However, corresponding subgroups of patients do not differ with respect to expression rates of drug transporters. Our results imply that parenchymal MRPs and Pgp are not responsible for drug resistance in resected tissue.